18 files changed, 12707 insertions, 0 deletions

diff --git a/compiler/rustc_parse/Cargo.toml b/compiler/rustc_parse/Cargo.toml
new file mode 100644
index 00000000000..52835e5c8a9
--- /dev/null
+++ b/compiler/rustc_parse/Cargo.toml
@@ -0,0 +1,22 @@
+[package]
+authors = ["The Rust Project Developers"]
+name = "rustc_parse"
+version = "0.0.0"
+edition = "2018"
+
+[lib]
+doctest = false
+
+[dependencies]
+bitflags = "1.0"
+tracing = "0.1"
+rustc_ast_pretty = { path = "../rustc_ast_pretty" }
+rustc_data_structures = { path = "../rustc_data_structures" }
+rustc_feature = { path = "../rustc_feature" }
+rustc_lexer = { path = "../rustc_lexer" }
+rustc_errors = { path = "../rustc_errors" }
+rustc_session = { path = "../rustc_session" }
+rustc_span = { path = "../rustc_span" }
+rustc_ast = { path = "../rustc_ast" }
+unicode-normalization = "0.1.11"
+smallvec = { version = "1.0", features = ["union", "may_dangle"] }
diff --git a/compiler/rustc_parse/src/lexer/mod.rs b/compiler/rustc_parse/src/lexer/mod.rs
new file mode 100644
index 00000000000..a65d3446819
--- /dev/null
+++ b/compiler/rustc_parse/src/lexer/mod.rs
@@ -0,0 +1,598 @@
+use rustc_ast::ast::AttrStyle;
+use rustc_ast::token::{self, CommentKind, Token, TokenKind};
+use rustc_data_structures::sync::Lrc;
+use rustc_errors::{error_code, Applicability, DiagnosticBuilder, FatalError};
+use rustc_lexer::Base;
+use rustc_lexer::{unescape, RawStrError};
+use rustc_session::parse::ParseSess;
+use rustc_span::symbol::{sym, Symbol};
+use rustc_span::{BytePos, Pos, Span};
+
+use std::char;
+use tracing::debug;
+
+mod tokentrees;
+mod unescape_error_reporting;
+mod unicode_chars;
+
+use rustc_lexer::{unescape::Mode, DocStyle};
+use unescape_error_reporting::{emit_unescape_error, push_escaped_char};
+
+#[derive(Clone, Debug)]
+pub struct UnmatchedBrace {
+    pub expected_delim: token::DelimToken,
+    pub found_delim: Option<token::DelimToken>,
+    pub found_span: Span,
+    pub unclosed_span: Option<Span>,
+    pub candidate_span: Option<Span>,
+}
+
+pub struct StringReader<'a> {
+    sess: &'a ParseSess,
+    /// Initial position, read-only.
+    start_pos: BytePos,
+    /// The absolute offset within the source_map of the current character.
+    pos: BytePos,
+    /// Stop reading src at this index.
+    end_src_index: usize,
+    /// Source text to tokenize.
+    src: Lrc<String>,
+    override_span: Option<Span>,
+}
+
+impl<'a> StringReader<'a> {
+    pub fn new(
+        sess: &'a ParseSess,
+        source_file: Lrc<rustc_span::SourceFile>,
+        override_span: Option<Span>,
+    ) -> Self {
+        let src = source_file.src.clone().unwrap_or_else(|| {
+            sess.span_diagnostic
+                .bug(&format!("cannot lex `source_file` without source: {}", source_file.name));
+        });
+
+        StringReader {
+            sess,
+            start_pos: source_file.start_pos,
+            pos: source_file.start_pos,
+            end_src_index: src.len(),
+            src,
+            override_span,
+        }
+    }
+
+    fn mk_sp(&self, lo: BytePos, hi: BytePos) -> Span {
+        self.override_span.unwrap_or_else(|| Span::with_root_ctxt(lo, hi))
+    }
+
+    /// Returns the next token, including trivia like whitespace or comments.
+    pub fn next_token(&mut self) -> Token {
+        let start_src_index = self.src_index(self.pos);
+        let text: &str = &self.src[start_src_index..self.end_src_index];
+
+        if text.is_empty() {
+            let span = self.mk_sp(self.pos, self.pos);
+            return Token::new(token::Eof, span);
+        }
+
+        {
+            let is_beginning_of_file = self.pos == self.start_pos;
+            if is_beginning_of_file {
+                if let Some(shebang_len) = rustc_lexer::strip_shebang(text) {
+                    let start = self.pos;
+                    self.pos = self.pos + BytePos::from_usize(shebang_len);
+
+                    let sym = self.symbol_from(start + BytePos::from_usize("#!".len()));
+                    let kind = token::Shebang(sym);
+
+                    let span = self.mk_sp(start, self.pos);
+                    return Token::new(kind, span);
+                }
+            }
+        }
+
+        let token = rustc_lexer::first_token(text);
+
+        let start = self.pos;
+        self.pos = self.pos + BytePos::from_usize(token.len);
+
+        debug!("try_next_token: {:?}({:?})", token.kind, self.str_from(start));
+
+        let kind = self.cook_lexer_token(token.kind, start);
+        let span = self.mk_sp(start, self.pos);
+        Token::new(kind, span)
+    }
+
+    /// Report a fatal lexical error with a given span.
+    fn fatal_span(&self, sp: Span, m: &str) -> FatalError {
+        self.sess.span_diagnostic.span_fatal(sp, m)
+    }
+
+    /// Report a lexical error with a given span.
+    fn err_span(&self, sp: Span, m: &str) {
+        self.sess.span_diagnostic.struct_span_err(sp, m).emit();
+    }
+
+    /// Report a fatal error spanning [`from_pos`, `to_pos`).
+    fn fatal_span_(&self, from_pos: BytePos, to_pos: BytePos, m: &str) -> FatalError {
+        self.fatal_span(self.mk_sp(from_pos, to_pos), m)
+    }
+
+    /// Report a lexical error spanning [`from_pos`, `to_pos`).
+    fn err_span_(&self, from_pos: BytePos, to_pos: BytePos, m: &str) {
+        self.err_span(self.mk_sp(from_pos, to_pos), m)
+    }
+
+    fn struct_fatal_span_char(
+        &self,
+        from_pos: BytePos,
+        to_pos: BytePos,
+        m: &str,
+        c: char,
+    ) -> DiagnosticBuilder<'a> {
+        let mut m = m.to_string();
+        m.push_str(": ");
+        push_escaped_char(&mut m, c);
+
+        self.sess.span_diagnostic.struct_span_fatal(self.mk_sp(from_pos, to_pos), &m[..])
+    }
+
+    /// Turns simple `rustc_lexer::TokenKind` enum into a rich
+    /// `librustc_ast::TokenKind`. This turns strings into interned
+    /// symbols and runs additional validation.
+    fn cook_lexer_token(&self, token: rustc_lexer::TokenKind, start: BytePos) -> TokenKind {
+        match token {
+            rustc_lexer::TokenKind::LineComment { doc_style } => {
+                match doc_style {
+                    Some(doc_style) => {
+                        // Opening delimiter of the length 3 is not included into the symbol.
+                        let content_start = start + BytePos(3);
+                        let content = self.str_from(content_start);
+
+                        self.cook_doc_comment(content_start, content, CommentKind::Line, doc_style)
+                    }
+                    None => token::Comment,
+                }
+            }
+            rustc_lexer::TokenKind::BlockComment { doc_style, terminated } => {
+                if !terminated {
+                    let msg = match doc_style {
+                        Some(_) => "unterminated block doc-comment",
+                        None => "unterminated block comment",
+                    };
+                    let last_bpos = self.pos;
+                    self.sess
+                        .span_diagnostic
+                        .struct_span_fatal_with_code(
+                            self.mk_sp(start, last_bpos),
+                            msg,
+                            error_code!(E0758),
+                        )
+                        .emit();
+                    FatalError.raise();
+                }
+                match doc_style {
+                    Some(doc_style) => {
+                        // Opening delimiter of the length 3 and closing delimiter of the length 2
+                        // are not included into the symbol.
+                        let content_start = start + BytePos(3);
+                        let content_end = self.pos - BytePos(if terminated { 2 } else { 0 });
+                        let content = self.str_from_to(content_start, content_end);
+
+                        self.cook_doc_comment(content_start, content, CommentKind::Block, doc_style)
+                    }
+                    None => token::Comment,
+                }
+            }
+            rustc_lexer::TokenKind::Whitespace => token::Whitespace,
+            rustc_lexer::TokenKind::Ident | rustc_lexer::TokenKind::RawIdent => {
+                let is_raw_ident = token == rustc_lexer::TokenKind::RawIdent;
+                let mut ident_start = start;
+                if is_raw_ident {
+                    ident_start = ident_start + BytePos(2);
+                }
+                let sym = nfc_normalize(self.str_from(ident_start));
+                let span = self.mk_sp(start, self.pos);
+                self.sess.symbol_gallery.insert(sym, span);
+                if is_raw_ident {
+                    if !sym.can_be_raw() {
+                        self.err_span(span, &format!("`{}` cannot be a raw identifier", sym));
+                    }
+                    self.sess.raw_identifier_spans.borrow_mut().push(span);
+                }
+                token::Ident(sym, is_raw_ident)
+            }
+            rustc_lexer::TokenKind::Literal { kind, suffix_start } => {
+                let suffix_start = start + BytePos(suffix_start as u32);
+                let (kind, symbol) = self.cook_lexer_literal(start, suffix_start, kind);
+                let suffix = if suffix_start < self.pos {
+                    let string = self.str_from(suffix_start);
+                    if string == "_" {
+                        self.sess
+                            .span_diagnostic
+                            .struct_span_warn(
+                                self.mk_sp(suffix_start, self.pos),
+                                "underscore literal suffix is not allowed",
+                            )
+                            .warn(
+                                "this was previously accepted by the compiler but is \
+                                   being phased out; it will become a hard error in \
+                                   a future release!",
+                            )
+                            .note(
+                                "see issue #42326 \
+                                 <https://github.com/rust-lang/rust/issues/42326> \
+                                 for more information",
+                            )
+                            .emit();
+                        None
+                    } else {
+                        Some(Symbol::intern(string))
+                    }
+                } else {
+                    None
+                };
+                token::Literal(token::Lit { kind, symbol, suffix })
+            }
+            rustc_lexer::TokenKind::Lifetime { starts_with_number } => {
+                // Include the leading `'` in the real identifier, for macro
+                // expansion purposes. See #12512 for the gory details of why
+                // this is necessary.
+                let lifetime_name = self.str_from(start);
+                if starts_with_number {
+                    self.err_span_(start, self.pos, "lifetimes cannot start with a number");
+                }
+                let ident = Symbol::intern(lifetime_name);
+                token::Lifetime(ident)
+            }
+            rustc_lexer::TokenKind::Semi => token::Semi,
+            rustc_lexer::TokenKind::Comma => token::Comma,
+            rustc_lexer::TokenKind::Dot => token::Dot,
+            rustc_lexer::TokenKind::OpenParen => token::OpenDelim(token::Paren),
+            rustc_lexer::TokenKind::CloseParen => token::CloseDelim(token::Paren),
+            rustc_lexer::TokenKind::OpenBrace => token::OpenDelim(token::Brace),
+            rustc_lexer::TokenKind::CloseBrace => token::CloseDelim(token::Brace),
+            rustc_lexer::TokenKind::OpenBracket => token::OpenDelim(token::Bracket),
+            rustc_lexer::TokenKind::CloseBracket => token::CloseDelim(token::Bracket),
+            rustc_lexer::TokenKind::At => token::At,
+            rustc_lexer::TokenKind::Pound => token::Pound,
+            rustc_lexer::TokenKind::Tilde => token::Tilde,
+            rustc_lexer::TokenKind::Question => token::Question,
+            rustc_lexer::TokenKind::Colon => token::Colon,
+            rustc_lexer::TokenKind::Dollar => token::Dollar,
+            rustc_lexer::TokenKind::Eq => token::Eq,
+            rustc_lexer::TokenKind::Bang => token::Not,
+            rustc_lexer::TokenKind::Lt => token::Lt,
+            rustc_lexer::TokenKind::Gt => token::Gt,
+            rustc_lexer::TokenKind::Minus => token::BinOp(token::Minus),
+            rustc_lexer::TokenKind::And => token::BinOp(token::And),
+            rustc_lexer::TokenKind::Or => token::BinOp(token::Or),
+            rustc_lexer::TokenKind::Plus => token::BinOp(token::Plus),
+            rustc_lexer::TokenKind::Star => token::BinOp(token::Star),
+            rustc_lexer::TokenKind::Slash => token::BinOp(token::Slash),
+            rustc_lexer::TokenKind::Caret => token::BinOp(token::Caret),
+            rustc_lexer::TokenKind::Percent => token::BinOp(token::Percent),
+
+            rustc_lexer::TokenKind::Unknown => {
+                let c = self.str_from(start).chars().next().unwrap();
+                let mut err =
+                    self.struct_fatal_span_char(start, self.pos, "unknown start of token", c);
+                // FIXME: the lexer could be used to turn the ASCII version of unicode homoglyphs,
+                // instead of keeping a table in `check_for_substitution`into the token. Ideally,
+                // this should be inside `rustc_lexer`. However, we should first remove compound
+                // tokens like `<<` from `rustc_lexer`, and then add fancier error recovery to it,
+                // as there will be less overall work to do this way.
+                let token = unicode_chars::check_for_substitution(self, start, c, &mut err)
+                    .unwrap_or_else(|| token::Unknown(self.symbol_from(start)));
+                err.emit();
+                token
+            }
+        }
+    }
+
+    fn cook_doc_comment(
+        &self,
+        content_start: BytePos,
+        content: &str,
+        comment_kind: CommentKind,
+        doc_style: DocStyle,
+    ) -> TokenKind {
+        if content.contains('\r') {
+            for (idx, _) in content.char_indices().filter(|&(_, c)| c == '\r') {
+                self.err_span_(
+                    content_start + BytePos(idx as u32),
+                    content_start + BytePos(idx as u32 + 1),
+                    match comment_kind {
+                        CommentKind::Line => "bare CR not allowed in doc-comment",
+                        CommentKind::Block => "bare CR not allowed in block doc-comment",
+                    },
+                );
+            }
+        }
+
+        let attr_style = match doc_style {
+            DocStyle::Outer => AttrStyle::Outer,
+            DocStyle::Inner => AttrStyle::Inner,
+        };
+
+        token::DocComment(comment_kind, attr_style, Symbol::intern(content))
+    }
+
+    fn cook_lexer_literal(
+        &self,
+        start: BytePos,
+        suffix_start: BytePos,
+        kind: rustc_lexer::LiteralKind,
+    ) -> (token::LitKind, Symbol) {
+        // prefix means `"` or `br"` or `r###"`, ...
+        let (lit_kind, mode, prefix_len, postfix_len) = match kind {
+            rustc_lexer::LiteralKind::Char { terminated } => {
+                if !terminated {
+                    self.sess
+                        .span_diagnostic
+                        .struct_span_fatal_with_code(
+                            self.mk_sp(start, suffix_start),
+                            "unterminated character literal",
+                            error_code!(E0762),
+                        )
+                        .emit();
+                    FatalError.raise();
+                }
+                (token::Char, Mode::Char, 1, 1) // ' '
+            }
+            rustc_lexer::LiteralKind::Byte { terminated } => {
+                if !terminated {
+                    self.sess
+                        .span_diagnostic
+                        .struct_span_fatal_with_code(
+                            self.mk_sp(start + BytePos(1), suffix_start),
+                            "unterminated byte constant",
+                            error_code!(E0763),
+                        )
+                        .emit();
+                    FatalError.raise();
+                }
+                (token::Byte, Mode::Byte, 2, 1) // b' '
+            }
+            rustc_lexer::LiteralKind::Str { terminated } => {
+                if !terminated {
+                    self.sess
+                        .span_diagnostic
+                        .struct_span_fatal_with_code(
+                            self.mk_sp(start, suffix_start),
+                            "unterminated double quote string",
+                            error_code!(E0765),
+                        )
+                        .emit();
+                    FatalError.raise();
+                }
+                (token::Str, Mode::Str, 1, 1) // " "
+            }
+            rustc_lexer::LiteralKind::ByteStr { terminated } => {
+                if !terminated {
+                    self.sess
+                        .span_diagnostic
+                        .struct_span_fatal_with_code(
+                            self.mk_sp(start + BytePos(1), suffix_start),
+                            "unterminated double quote byte string",
+                            error_code!(E0766),
+                        )
+                        .emit();
+                    FatalError.raise();
+                }
+                (token::ByteStr, Mode::ByteStr, 2, 1) // b" "
+            }
+            rustc_lexer::LiteralKind::RawStr { n_hashes, err } => {
+                self.report_raw_str_error(start, err);
+                let n = u32::from(n_hashes);
+                (token::StrRaw(n_hashes), Mode::RawStr, 2 + n, 1 + n) // r##" "##
+            }
+            rustc_lexer::LiteralKind::RawByteStr { n_hashes, err } => {
+                self.report_raw_str_error(start, err);
+                let n = u32::from(n_hashes);
+                (token::ByteStrRaw(n_hashes), Mode::RawByteStr, 3 + n, 1 + n) // br##" "##
+            }
+            rustc_lexer::LiteralKind::Int { base, empty_int } => {
+                return if empty_int {
+                    self.sess
+                        .span_diagnostic
+                        .struct_span_err_with_code(
+                            self.mk_sp(start, suffix_start),
+                            "no valid digits found for number",
+                            error_code!(E0768),
+                        )
+                        .emit();
+                    (token::Integer, sym::integer(0))
+                } else {
+                    self.validate_int_literal(base, start, suffix_start);
+                    (token::Integer, self.symbol_from_to(start, suffix_start))
+                };
+            }
+            rustc_lexer::LiteralKind::Float { base, empty_exponent } => {
+                if empty_exponent {
+                    self.err_span_(start, self.pos, "expected at least one digit in exponent");
+                }
+
+                match base {
+                    Base::Hexadecimal => self.err_span_(
+                        start,
+                        suffix_start,
+                        "hexadecimal float literal is not supported",
+                    ),
+                    Base::Octal => {
+                        self.err_span_(start, suffix_start, "octal float literal is not supported")
+                    }
+                    Base::Binary => {
+                        self.err_span_(start, suffix_start, "binary float literal is not supported")
+                    }
+                    _ => (),
+                }
+
+                let id = self.symbol_from_to(start, suffix_start);
+                return (token::Float, id);
+            }
+        };
+        let content_start = start + BytePos(prefix_len);
+        let content_end = suffix_start - BytePos(postfix_len);
+        let id = self.symbol_from_to(content_start, content_end);
+        self.validate_literal_escape(mode, content_start, content_end);
+        (lit_kind, id)
+    }
+
+    pub fn pos(&self) -> BytePos {
+        self.pos
+    }
+
+    #[inline]
+    fn src_index(&self, pos: BytePos) -> usize {
+        (pos - self.start_pos).to_usize()
+    }
+
+    /// Slice of the source text from `start` up to but excluding `self.pos`,
+    /// meaning the slice does not include the character `self.ch`.
+    fn str_from(&self, start: BytePos) -> &str {
+        self.str_from_to(start, self.pos)
+    }
+
+    /// Creates a Symbol from a given offset to the current offset.
+    fn symbol_from(&self, start: BytePos) -> Symbol {
+        debug!("taking an ident from {:?} to {:?}", start, self.pos);
+        Symbol::intern(self.str_from(start))
+    }
+
+    /// As symbol_from, with an explicit endpoint.
+    fn symbol_from_to(&self, start: BytePos, end: BytePos) -> Symbol {
+        debug!("taking an ident from {:?} to {:?}", start, end);
+        Symbol::intern(self.str_from_to(start, end))
+    }
+
+    /// Slice of the source text spanning from `start` up to but excluding `end`.
+    fn str_from_to(&self, start: BytePos, end: BytePos) -> &str {
+        &self.src[self.src_index(start)..self.src_index(end)]
+    }
+
+    fn report_raw_str_error(&self, start: BytePos, opt_err: Option<RawStrError>) {
+        match opt_err {
+            Some(RawStrError::InvalidStarter { bad_char }) => {
+                self.report_non_started_raw_string(start, bad_char)
+            }
+            Some(RawStrError::NoTerminator { expected, found, possible_terminator_offset }) => self
+                .report_unterminated_raw_string(start, expected, possible_terminator_offset, found),
+            Some(RawStrError::TooManyDelimiters { found }) => {
+                self.report_too_many_hashes(start, found)
+            }
+            None => (),
+        }
+    }
+
+    fn report_non_started_raw_string(&self, start: BytePos, bad_char: char) -> ! {
+        self.struct_fatal_span_char(
+            start,
+            self.pos,
+            "found invalid character; only `#` is allowed in raw string delimitation",
+            bad_char,
+        )
+        .emit();
+        FatalError.raise()
+    }
+
+    fn report_unterminated_raw_string(
+        &self,
+        start: BytePos,
+        n_hashes: usize,
+        possible_offset: Option<usize>,
+        found_terminators: usize,
+    ) -> ! {
+        let mut err = self.sess.span_diagnostic.struct_span_fatal_with_code(
+            self.mk_sp(start, start),
+            "unterminated raw string",
+            error_code!(E0748),
+        );
+
+        err.span_label(self.mk_sp(start, start), "unterminated raw string");
+
+        if n_hashes > 0 {
+            err.note(&format!(
+                "this raw string should be terminated with `\"{}`",
+                "#".repeat(n_hashes)
+            ));
+        }
+
+        if let Some(possible_offset) = possible_offset {
+            let lo = start + BytePos(possible_offset as u32);
+            let hi = lo + BytePos(found_terminators as u32);
+            let span = self.mk_sp(lo, hi);
+            err.span_suggestion(
+                span,
+                "consider terminating the string here",
+                "#".repeat(n_hashes),
+                Applicability::MaybeIncorrect,
+            );
+        }
+
+        err.emit();
+        FatalError.raise()
+    }
+
+    /// Note: It was decided to not add a test case, because it would be to big.
+    /// https://github.com/rust-lang/rust/pull/50296#issuecomment-392135180
+    fn report_too_many_hashes(&self, start: BytePos, found: usize) -> ! {
+        self.fatal_span_(
+            start,
+            self.pos,
+            &format!(
+                "too many `#` symbols: raw strings may be delimited \
+                by up to 65535 `#` symbols, but found {}",
+                found
+            ),
+        )
+        .raise();
+    }
+
+    fn validate_literal_escape(&self, mode: Mode, content_start: BytePos, content_end: BytePos) {
+        let lit_content = self.str_from_to(content_start, content_end);
+        unescape::unescape_literal(lit_content, mode, &mut |range, result| {
+            // Here we only check for errors. The actual unescaping is done later.
+            if let Err(err) = result {
+                let span_with_quotes =
+                    self.mk_sp(content_start - BytePos(1), content_end + BytePos(1));
+                emit_unescape_error(
+                    &self.sess.span_diagnostic,
+                    lit_content,
+                    span_with_quotes,
+                    mode,
+                    range,
+                    err,
+                );
+            }
+        });
+    }
+
+    fn validate_int_literal(&self, base: Base, content_start: BytePos, content_end: BytePos) {
+        let base = match base {
+            Base::Binary => 2,
+            Base::Octal => 8,
+            _ => return,
+        };
+        let s = self.str_from_to(content_start + BytePos(2), content_end);
+        for (idx, c) in s.char_indices() {
+            let idx = idx as u32;
+            if c != '_' && c.to_digit(base).is_none() {
+                let lo = content_start + BytePos(2 + idx);
+                let hi = content_start + BytePos(2 + idx + c.len_utf8() as u32);
+                self.err_span_(lo, hi, &format!("invalid digit for a base {} literal", base));
+            }
+        }
+    }
+}
+
+pub fn nfc_normalize(string: &str) -> Symbol {
+    use unicode_normalization::{is_nfc_quick, IsNormalized, UnicodeNormalization};
+    match is_nfc_quick(string.chars()) {
+        IsNormalized::Yes => Symbol::intern(string),
+        _ => {
+            let normalized_str: String = string.chars().nfc().collect();
+            Symbol::intern(&normalized_str)
+        }
+    }
+}
diff --git a/compiler/rustc_parse/src/lexer/tokentrees.rs b/compiler/rustc_parse/src/lexer/tokentrees.rs
new file mode 100644
index 00000000000..c08659ec9f6
--- /dev/null
+++ b/compiler/rustc_parse/src/lexer/tokentrees.rs
@@ -0,0 +1,313 @@
+use super::{StringReader, UnmatchedBrace};
+
+use rustc_ast::token::{self, DelimToken, Token};
+use rustc_ast::tokenstream::{
+    DelimSpan,
+    IsJoint::{self, *},
+    TokenStream, TokenTree, TreeAndJoint,
+};
+use rustc_ast_pretty::pprust::token_to_string;
+use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::PResult;
+use rustc_span::Span;
+
+impl<'a> StringReader<'a> {
+    crate fn into_token_trees(self) -> (PResult<'a, TokenStream>, Vec<UnmatchedBrace>) {
+        let mut tt_reader = TokenTreesReader {
+            string_reader: self,
+            token: Token::dummy(),
+            joint_to_prev: Joint,
+            open_braces: Vec::new(),
+            unmatched_braces: Vec::new(),
+            matching_delim_spans: Vec::new(),
+            last_unclosed_found_span: None,
+            last_delim_empty_block_spans: FxHashMap::default(),
+            matching_block_spans: Vec::new(),
+        };
+        let res = tt_reader.parse_all_token_trees();
+        (res, tt_reader.unmatched_braces)
+    }
+}
+
+struct TokenTreesReader<'a> {
+    string_reader: StringReader<'a>,
+    token: Token,
+    joint_to_prev: IsJoint,
+    /// Stack of open delimiters and their spans. Used for error message.
+    open_braces: Vec<(token::DelimToken, Span)>,
+    unmatched_braces: Vec<UnmatchedBrace>,
+    /// The type and spans for all braces
+    ///
+    /// Used only for error recovery when arriving to EOF with mismatched braces.
+    matching_delim_spans: Vec<(token::DelimToken, Span, Span)>,
+    last_unclosed_found_span: Option<Span>,
+    /// Collect empty block spans that might have been auto-inserted by editors.
+    last_delim_empty_block_spans: FxHashMap<token::DelimToken, Span>,
+    /// Collect the spans of braces (Open, Close). Used only
+    /// for detecting if blocks are empty and only braces.
+    matching_block_spans: Vec<(Span, Span)>,
+}
+
+impl<'a> TokenTreesReader<'a> {
+    // Parse a stream of tokens into a list of `TokenTree`s, up to an `Eof`.
+    fn parse_all_token_trees(&mut self) -> PResult<'a, TokenStream> {
+        let mut buf = TokenStreamBuilder::default();
+
+        self.real_token();
+        while self.token != token::Eof {
+            buf.push(self.parse_token_tree()?);
+        }
+
+        Ok(buf.into_token_stream())
+    }
+
+    // Parse a stream of tokens into a list of `TokenTree`s, up to a `CloseDelim`.
+    fn parse_token_trees_until_close_delim(&mut self) -> TokenStream {
+        let mut buf = TokenStreamBuilder::default();
+        loop {
+            if let token::CloseDelim(..) = self.token.kind {
+                return buf.into_token_stream();
+            }
+
+            match self.parse_token_tree() {
+                Ok(tree) => buf.push(tree),
+                Err(mut e) => {
+                    e.emit();
+                    return buf.into_token_stream();
+                }
+            }
+        }
+    }
+
+    fn parse_token_tree(&mut self) -> PResult<'a, TreeAndJoint> {
+        let sm = self.string_reader.sess.source_map();
+
+        match self.token.kind {
+            token::Eof => {
+                let msg = "this file contains an unclosed delimiter";
+                let mut err =
+                    self.string_reader.sess.span_diagnostic.struct_span_err(self.token.span, msg);
+                for &(_, sp) in &self.open_braces {
+                    err.span_label(sp, "unclosed delimiter");
+                    self.unmatched_braces.push(UnmatchedBrace {
+                        expected_delim: token::DelimToken::Brace,
+                        found_delim: None,
+                        found_span: self.token.span,
+                        unclosed_span: Some(sp),
+                        candidate_span: None,
+                    });
+                }
+
+                if let Some((delim, _)) = self.open_braces.last() {
+                    if let Some((_, open_sp, close_sp)) =
+                        self.matching_delim_spans.iter().find(|(d, open_sp, close_sp)| {
+                            if let Some(close_padding) = sm.span_to_margin(*close_sp) {
+                                if let Some(open_padding) = sm.span_to_margin(*open_sp) {
+                                    return delim == d && close_padding != open_padding;
+                                }
+                            }
+                            false
+                        })
+                    // these are in reverse order as they get inserted on close, but
+                    {
+                        // we want the last open/first close
+                        err.span_label(*open_sp, "this delimiter might not be properly closed...");
+                        err.span_label(
+                            *close_sp,
+                            "...as it matches this but it has different indentation",
+                        );
+                    }
+                }
+                Err(err)
+            }
+            token::OpenDelim(delim) => {
+                // The span for beginning of the delimited section
+                let pre_span = self.token.span;
+
+                // Parse the open delimiter.
+                self.open_braces.push((delim, self.token.span));
+                self.real_token();
+
+                // Parse the token trees within the delimiters.
+                // We stop at any delimiter so we can try to recover if the user
+                // uses an incorrect delimiter.
+                let tts = self.parse_token_trees_until_close_delim();
+
+                // Expand to cover the entire delimited token tree
+                let delim_span = DelimSpan::from_pair(pre_span, self.token.span);
+
+                match self.token.kind {
+                    // Correct delimiter.
+                    token::CloseDelim(d) if d == delim => {
+                        let (open_brace, open_brace_span) = self.open_braces.pop().unwrap();
+                        let close_brace_span = self.token.span;
+
+                        if tts.is_empty() {
+                            let empty_block_span = open_brace_span.to(close_brace_span);
+                            if !sm.is_multiline(empty_block_span) {
+                                // Only track if the block is in the form of `{}`, otherwise it is
+                                // likely that it was written on purpose.
+                                self.last_delim_empty_block_spans.insert(delim, empty_block_span);
+                            }
+                        }
+
+                        match (open_brace, delim) {
+                            //only add braces
+                            (DelimToken::Brace, DelimToken::Brace) => {
+                                self.matching_block_spans.push((open_brace_span, close_brace_span));
+                            }
+                            _ => {}
+                        }
+
+                        if self.open_braces.is_empty() {
+                            // Clear up these spans to avoid suggesting them as we've found
+                            // properly matched delimiters so far for an entire block.
+                            self.matching_delim_spans.clear();
+                        } else {
+                            self.matching_delim_spans.push((
+                                open_brace,
+                                open_brace_span,
+                                close_brace_span,
+                            ));
+                        }
+                        // Parse the closing delimiter.
+                        self.real_token();
+                    }
+                    // Incorrect delimiter.
+                    token::CloseDelim(other) => {
+                        let mut unclosed_delimiter = None;
+                        let mut candidate = None;
+
+                        if self.last_unclosed_found_span != Some(self.token.span) {
+                            // do not complain about the same unclosed delimiter multiple times
+                            self.last_unclosed_found_span = Some(self.token.span);
+                            // This is a conservative error: only report the last unclosed
+                            // delimiter. The previous unclosed delimiters could actually be
+                            // closed! The parser just hasn't gotten to them yet.
+                            if let Some(&(_, sp)) = self.open_braces.last() {
+                                unclosed_delimiter = Some(sp);
+                            };
+                            if let Some(current_padding) = sm.span_to_margin(self.token.span) {
+                                for (brace, brace_span) in &self.open_braces {
+                                    if let Some(padding) = sm.span_to_margin(*brace_span) {
+                                        // high likelihood of these two corresponding
+                                        if current_padding == padding && brace == &other {
+                                            candidate = Some(*brace_span);
+                                        }
+                                    }
+                                }
+                            }
+                            let (tok, _) = self.open_braces.pop().unwrap();
+                            self.unmatched_braces.push(UnmatchedBrace {
+                                expected_delim: tok,
+                                found_delim: Some(other),
+                                found_span: self.token.span,
+                                unclosed_span: unclosed_delimiter,
+                                candidate_span: candidate,
+                            });
+                        } else {
+                            self.open_braces.pop();
+                        }
+
+                        // If the incorrect delimiter matches an earlier opening
+                        // delimiter, then don't consume it (it can be used to
+                        // close the earlier one). Otherwise, consume it.
+                        // E.g., we try to recover from:
+                        // fn foo() {
+                        //     bar(baz(
+                        // }  // Incorrect delimiter but matches the earlier `{`
+                        if !self.open_braces.iter().any(|&(b, _)| b == other) {
+                            self.real_token();
+                        }
+                    }
+                    token::Eof => {
+                        // Silently recover, the EOF token will be seen again
+                        // and an error emitted then. Thus we don't pop from
+                        // self.open_braces here.
+                    }
+                    _ => {}
+                }
+
+                Ok(TokenTree::Delimited(delim_span, delim, tts).into())
+            }
+            token::CloseDelim(delim) => {
+                // An unexpected closing delimiter (i.e., there is no
+                // matching opening delimiter).
+                let token_str = token_to_string(&self.token);
+                let msg = format!("unexpected closing delimiter: `{}`", token_str);
+                let mut err =
+                    self.string_reader.sess.span_diagnostic.struct_span_err(self.token.span, &msg);
+
+                // Braces are added at the end, so the last element is the biggest block
+                if let Some(parent) = self.matching_block_spans.last() {
+                    if let Some(span) = self.last_delim_empty_block_spans.remove(&delim) {
+                        // Check if the (empty block) is in the last properly closed block
+                        if (parent.0.to(parent.1)).contains(span) {
+                            err.span_label(
+                                span,
+                                "block is empty, you might have not meant to close it",
+                            );
+                        } else {
+                            err.span_label(parent.0, "this opening brace...");
+
+                            err.span_label(parent.1, "...matches this closing brace");
+                        }
+                    } else {
+                        err.span_label(parent.0, "this opening brace...");
+
+                        err.span_label(parent.1, "...matches this closing brace");
+                    }
+                }
+
+                err.span_label(self.token.span, "unexpected closing delimiter");
+                Err(err)
+            }
+            _ => {
+                let tt = TokenTree::Token(self.token.take());
+                self.real_token();
+                let is_joint = self.joint_to_prev == Joint && self.token.is_op();
+                Ok((tt, if is_joint { Joint } else { NonJoint }))
+            }
+        }
+    }
+
+    fn real_token(&mut self) {
+        self.joint_to_prev = Joint;
+        loop {
+            let token = self.string_reader.next_token();
+            match token.kind {
+                token::Whitespace | token::Comment | token::Shebang(_) | token::Unknown(_) => {
+                    self.joint_to_prev = NonJoint;
+                }
+                _ => {
+                    self.token = token;
+                    return;
+                }
+            }
+        }
+    }
+}
+
+#[derive(Default)]
+struct TokenStreamBuilder {
+    buf: Vec<TreeAndJoint>,
+}
+
+impl TokenStreamBuilder {
+    fn push(&mut self, (tree, joint): TreeAndJoint) {
+        if let Some((TokenTree::Token(prev_token), Joint)) = self.buf.last() {
+            if let TokenTree::Token(token) = &tree {
+                if let Some(glued) = prev_token.glue(token) {
+                    self.buf.pop();
+                    self.buf.push((TokenTree::Token(glued), joint));
+                    return;
+                }
+            }
+        }
+        self.buf.push((tree, joint))
+    }
+
+    fn into_token_stream(self) -> TokenStream {
+        TokenStream::new(self.buf)
+    }
+}
diff --git a/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs b/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
new file mode 100644
index 00000000000..6f249f491a6
--- /dev/null
+++ b/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
@@ -0,0 +1,223 @@
+//! Utilities for rendering escape sequence errors as diagnostics.
+
+use std::iter::once;
+use std::ops::Range;
+
+use rustc_errors::{Applicability, Handler};
+use rustc_lexer::unescape::{EscapeError, Mode};
+use rustc_span::{BytePos, Span};
+
+pub(crate) fn emit_unescape_error(
+    handler: &Handler,
+    // interior part of the literal, without quotes
+    lit: &str,
+    // full span of the literal, including quotes
+    span_with_quotes: Span,
+    mode: Mode,
+    // range of the error inside `lit`
+    range: Range<usize>,
+    error: EscapeError,
+) {
+    tracing::debug!(
+        "emit_unescape_error: {:?}, {:?}, {:?}, {:?}, {:?}",
+        lit,
+        span_with_quotes,
+        mode,
+        range,
+        error
+    );
+    let span = {
+        let Range { start, end } = range;
+        let (start, end) = (start as u32, end as u32);
+        let lo = span_with_quotes.lo() + BytePos(start + 1);
+        let hi = lo + BytePos(end - start);
+        span_with_quotes.with_lo(lo).with_hi(hi)
+    };
+    let last_char = || {
+        let c = lit[range.clone()].chars().rev().next().unwrap();
+        let span = span.with_lo(span.hi() - BytePos(c.len_utf8() as u32));
+        (c, span)
+    };
+    match error {
+        EscapeError::LoneSurrogateUnicodeEscape => {
+            handler
+                .struct_span_err(span, "invalid unicode character escape")
+                .help("unicode escape must not be a surrogate")
+                .emit();
+        }
+        EscapeError::OutOfRangeUnicodeEscape => {
+            handler
+                .struct_span_err(span, "invalid unicode character escape")
+                .help("unicode escape must be at most 10FFFF")
+                .emit();
+        }
+        EscapeError::MoreThanOneChar => {
+            let msg = if mode.is_bytes() {
+                "if you meant to write a byte string literal, use double quotes"
+            } else {
+                "if you meant to write a `str` literal, use double quotes"
+            };
+
+            handler
+                .struct_span_err(
+                    span_with_quotes,
+                    "character literal may only contain one codepoint",
+                )
+                .span_suggestion(
+                    span_with_quotes,
+                    msg,
+                    format!("\"{}\"", lit),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        }
+        EscapeError::EscapeOnlyChar => {
+            let (c, _span) = last_char();
+
+            let mut msg = if mode.is_bytes() {
+                "byte constant must be escaped: "
+            } else {
+                "character constant must be escaped: "
+            }
+            .to_string();
+            push_escaped_char(&mut msg, c);
+
+            handler.span_err(span, msg.as_str())
+        }
+        EscapeError::BareCarriageReturn => {
+            let msg = if mode.in_double_quotes() {
+                "bare CR not allowed in string, use \\r instead"
+            } else {
+                "character constant must be escaped: \\r"
+            };
+            handler.span_err(span, msg);
+        }
+        EscapeError::BareCarriageReturnInRawString => {
+            assert!(mode.in_double_quotes());
+            let msg = "bare CR not allowed in raw string";
+            handler.span_err(span, msg);
+        }
+        EscapeError::InvalidEscape => {
+            let (c, span) = last_char();
+
+            let label =
+                if mode.is_bytes() { "unknown byte escape" } else { "unknown character escape" };
+            let mut msg = label.to_string();
+            msg.push_str(": ");
+            push_escaped_char(&mut msg, c);
+
+            let mut diag = handler.struct_span_err(span, msg.as_str());
+            diag.span_label(span, label);
+            if c == '{' || c == '}' && !mode.is_bytes() {
+                diag.help(
+                    "if used in a formatting string, \
+                           curly braces are escaped with `{{` and `}}`",
+                );
+            } else if c == '\r' {
+                diag.help(
+                    "this is an isolated carriage return; \
+                           consider checking your editor and version control settings",
+                );
+            }
+            diag.emit();
+        }
+        EscapeError::TooShortHexEscape => {
+            handler.span_err(span, "numeric character escape is too short")
+        }
+        EscapeError::InvalidCharInHexEscape | EscapeError::InvalidCharInUnicodeEscape => {
+            let (c, span) = last_char();
+
+            let mut msg = if error == EscapeError::InvalidCharInHexEscape {
+                "invalid character in numeric character escape: "
+            } else {
+                "invalid character in unicode escape: "
+            }
+            .to_string();
+            push_escaped_char(&mut msg, c);
+
+            handler.span_err(span, msg.as_str())
+        }
+        EscapeError::NonAsciiCharInByte => {
+            assert!(mode.is_bytes());
+            let (_c, span) = last_char();
+            handler.span_err(
+                span,
+                "byte constant must be ASCII. \
+                                    Use a \\xHH escape for a non-ASCII byte",
+            )
+        }
+        EscapeError::NonAsciiCharInByteString => {
+            assert!(mode.is_bytes());
+            let (_c, span) = last_char();
+            handler.span_err(span, "raw byte string must be ASCII")
+        }
+        EscapeError::OutOfRangeHexEscape => handler.span_err(
+            span,
+            "this form of character escape may only be used \
+                                    with characters in the range [\\x00-\\x7f]",
+        ),
+        EscapeError::LeadingUnderscoreUnicodeEscape => {
+            let (_c, span) = last_char();
+            handler.span_err(span, "invalid start of unicode escape")
+        }
+        EscapeError::OverlongUnicodeEscape => {
+            handler.span_err(span, "overlong unicode escape (must have at most 6 hex digits)")
+        }
+        EscapeError::UnclosedUnicodeEscape => {
+            handler.span_err(span, "unterminated unicode escape (needed a `}`)")
+        }
+        EscapeError::NoBraceInUnicodeEscape => {
+            let msg = "incorrect unicode escape sequence";
+            let mut diag = handler.struct_span_err(span, msg);
+
+            let mut suggestion = "\\u{".to_owned();
+            let mut suggestion_len = 0;
+            let (c, char_span) = last_char();
+            let chars = once(c).chain(lit[range.end..].chars());
+            for c in chars.take(6).take_while(|c| c.is_digit(16)) {
+                suggestion.push(c);
+                suggestion_len += c.len_utf8();
+            }
+
+            if suggestion_len > 0 {
+                suggestion.push('}');
+                let lo = char_span.lo();
+                let hi = lo + BytePos(suggestion_len as u32);
+                diag.span_suggestion(
+                    span.with_lo(lo).with_hi(hi),
+                    "format of unicode escape sequences uses braces",
+                    suggestion,
+                    Applicability::MaybeIncorrect,
+                );
+            } else {
+                diag.span_label(span, msg);
+                diag.help("format of unicode escape sequences is `\\u{...}`");
+            }
+
+            diag.emit();
+        }
+        EscapeError::UnicodeEscapeInByte => handler.span_err(
+            span,
+            "unicode escape sequences cannot be used \
+                                    as a byte or in a byte string",
+        ),
+        EscapeError::EmptyUnicodeEscape => {
+            handler.span_err(span, "empty unicode escape (must have at least 1 hex digit)")
+        }
+        EscapeError::ZeroChars => handler.span_err(span, "empty character literal"),
+        EscapeError::LoneSlash => handler.span_err(span, "invalid trailing slash in literal"),
+    }
+}
+
+/// Pushes a character to a message string for error reporting
+pub(crate) fn push_escaped_char(msg: &mut String, c: char) {
+    match c {
+        '\u{20}'..='\u{7e}' => {
+            // Don't escape \, ' or " for user-facing messages
+            msg.push(c);
+        }
+        _ => {
+            msg.extend(c.escape_default());
+        }
+    }
+}
diff --git a/compiler/rustc_parse/src/lexer/unicode_chars.rs b/compiler/rustc_parse/src/lexer/unicode_chars.rs
new file mode 100644
index 00000000000..ac395f6cbc2
--- /dev/null
+++ b/compiler/rustc_parse/src/lexer/unicode_chars.rs
@@ -0,0 +1,392 @@
+// Characters and their corresponding confusables were collected from
+// http://www.unicode.org/Public/security/10.0.0/confusables.txt
+
+use super::StringReader;
+use crate::token;
+use rustc_errors::{Applicability, DiagnosticBuilder};
+use rustc_span::{symbol::kw, BytePos, Pos, Span};
+
+#[rustfmt::skip] // for line breaks
+const UNICODE_ARRAY: &[(char, &str, char)] = &[
+    ('
', "Line Separator", ' '),
+    ('
', "Paragraph Separator", ' '),
+    (' ', "Ogham Space mark", ' '),
+    (' ', "En Quad", ' '),
+    (' ', "Em Quad", ' '),
+    (' ', "En Space", ' '),
+    (' ', "Em Space", ' '),
+    (' ', "Three-Per-Em Space", ' '),
+    (' ', "Four-Per-Em Space", ' '),
+    (' ', "Six-Per-Em Space", ' '),
+    (' ', "Punctuation Space", ' '),
+    (' ', "Thin Space", ' '),
+    (' ', "Hair Space", ' '),
+    (' ', "Medium Mathematical Space", ' '),
+    (' ', "No-Break Space", ' '),
+    (' ', "Figure Space", ' '),
+    (' ', "Narrow No-Break Space", ' '),
+    ('　', "Ideographic Space", ' '),
+
+    ('ߺ', "Nko Lajanyalan", '_'),
+    ('﹍', "Dashed Low Line", '_'),
+    ('﹎', "Centreline Low Line", '_'),
+    ('﹏', "Wavy Low Line", '_'),
+    ('＿', "Fullwidth Low Line", '_'),
+
+    ('‐', "Hyphen", '-'),
+    ('‑', "Non-Breaking Hyphen", '-'),
+    ('‒', "Figure Dash", '-'),
+    ('–', "En Dash", '-'),
+    ('—', "Em Dash", '-'),
+    ('﹘', "Small Em Dash", '-'),
+    ('۔', "Arabic Full Stop", '-'),
+    ('⁃', "Hyphen Bullet", '-'),
+    ('˗', "Modifier Letter Minus Sign", '-'),
+    ('−', "Minus Sign", '-'),
+    ('➖', "Heavy Minus Sign", '-'),
+    ('Ⲻ', "Coptic Letter Dialect-P Ni", '-'),
+    ('ー', "Katakana-Hiragana Prolonged Sound Mark", '-'),
+    ('－', "Fullwidth Hyphen-Minus", '-'),
+    ('―', "Horizontal Bar", '-'),
+    ('─', "Box Drawings Light Horizontal", '-'),
+    ('━', "Box Drawings Heavy Horizontal", '-'),
+    ('㇐', "CJK Stroke H", '-'),
+    ('ꟷ', "Latin Epigraphic Letter Sideways I", '-'),
+    ('ᅳ', "Hangul Jungseong Eu", '-'),
+    ('ㅡ', "Hangul Letter Eu", '-'),
+    ('一', "CJK Unified Ideograph-4E00", '-'),
+    ('⼀', "Kangxi Radical One", '-'),
+
+    ('؍', "Arabic Date Separator", ','),
+    ('٫', "Arabic Decimal Separator", ','),
+    ('‚', "Single Low-9 Quotation Mark", ','),
+    ('¸', "Cedilla", ','),
+    ('ꓹ', "Lisu Letter Tone Na Po", ','),
+    ('，', "Fullwidth Comma", ','),
+
+    (';', "Greek Question Mark", ';'),
+    ('；', "Fullwidth Semicolon", ';'),
+    ('︔', "Presentation Form For Vertical Semicolon", ';'),
+
+    ('ः', "Devanagari Sign Visarga", ':'),
+    ('ઃ', "Gujarati Sign Visarga", ':'),
+    ('：', "Fullwidth Colon", ':'),
+    ('։', "Armenian Full Stop", ':'),
+    ('܃', "Syriac Supralinear Colon", ':'),
+    ('܄', "Syriac Sublinear Colon", ':'),
+    ('᛬', "Runic Multiple Punctuation", ':'),
+    ('︰', "Presentation Form For Vertical Two Dot Leader", ':'),
+    ('᠃', "Mongolian Full Stop", ':'),
+    ('᠉', "Mongolian Manchu Full Stop", ':'),
+    ('⁚', "Two Dot Punctuation", ':'),
+    ('׃', "Hebrew Punctuation Sof Pasuq", ':'),
+    ('˸', "Modifier Letter Raised Colon", ':'),
+    ('꞉', "Modifier Letter Colon", ':'),
+    ('∶', "Ratio", ':'),
+    ('ː', "Modifier Letter Triangular Colon", ':'),
+    ('ꓽ', "Lisu Letter Tone Mya Jeu", ':'),
+    ('︓', "Presentation Form For Vertical Colon", ':'),
+
+    ('！', "Fullwidth Exclamation Mark", '!'),
+    ('ǃ', "Latin Letter Retroflex Click", '!'),
+    ('ⵑ', "Tifinagh Letter Tuareg Yang", '!'),
+    ('︕', "Presentation Form For Vertical Exclamation Mark", '!'),
+
+    ('ʔ', "Latin Letter Glottal Stop", '?'),
+    ('Ɂ', "Latin Capital Letter Glottal Stop", '?'),
+    ('ॽ', "Devanagari Letter Glottal Stop", '?'),
+    ('Ꭾ', "Cherokee Letter He", '?'),
+    ('ꛫ', "Bamum Letter Ntuu", '?'),
+    ('？', "Fullwidth Question Mark", '?'),
+    ('︖', "Presentation Form For Vertical Question Mark", '?'),
+
+    ('𝅭', "Musical Symbol Combining Augmentation Dot", '.'),
+    ('․', "One Dot Leader", '.'),
+    ('܁', "Syriac Supralinear Full Stop", '.'),
+    ('܂', "Syriac Sublinear Full Stop", '.'),
+    ('꘎', "Vai Full Stop", '.'),
+    ('𐩐', "Kharoshthi Punctuation Dot", '.'),
+    ('٠', "Arabic-Indic Digit Zero", '.'),
+    ('۰', "Extended Arabic-Indic Digit Zero", '.'),
+    ('ꓸ', "Lisu Letter Tone Mya Ti", '.'),
+    ('·', "Middle Dot", '.'),
+    ('・', "Katakana Middle Dot", '.'),
+    ('･', "Halfwidth Katakana Middle Dot", '.'),
+    ('᛫', "Runic Single Punctuation", '.'),
+    ('·', "Greek Ano Teleia", '.'),
+    ('⸱', "Word Separator Middle Dot", '.'),
+    ('𐄁', "Aegean Word Separator Dot", '.'),
+    ('•', "Bullet", '.'),
+    ('‧', "Hyphenation Point", '.'),
+    ('∙', "Bullet Operator", '.'),
+    ('⋅', "Dot Operator", '.'),
+    ('ꞏ', "Latin Letter Sinological Dot", '.'),
+    ('ᐧ', "Canadian Syllabics Final Middle Dot", '.'),
+    ('ᐧ', "Canadian Syllabics Final Middle Dot", '.'),
+    ('．', "Fullwidth Full Stop", '.'),
+    ('。', "Ideographic Full Stop", '.'),
+    ('︒', "Presentation Form For Vertical Ideographic Full Stop", '.'),
+
+    ('՝', "Armenian Comma", '\''),
+    ('＇', "Fullwidth Apostrophe", '\''),
+    ('‘', "Left Single Quotation Mark", '\''),
+    ('’', "Right Single Quotation Mark", '\''),
+    ('‛', "Single High-Reversed-9 Quotation Mark", '\''),
+    ('′', "Prime", '\''),
+    ('‵', "Reversed Prime", '\''),
+    ('՚', "Armenian Apostrophe", '\''),
+    ('׳', "Hebrew Punctuation Geresh", '\''),
+    ('`', "Grave Accent", '\''),
+    ('`', "Greek Varia", '\''),
+    ('｀', "Fullwidth Grave Accent", '\''),
+    ('´', "Acute Accent", '\''),
+    ('΄', "Greek Tonos", '\''),
+    ('´', "Greek Oxia", '\''),
+    ('᾽', "Greek Koronis", '\''),
+    ('᾿', "Greek Psili", '\''),
+    ('῾', "Greek Dasia", '\''),
+    ('ʹ', "Modifier Letter Prime", '\''),
+    ('ʹ', "Greek Numeral Sign", '\''),
+    ('ˈ', "Modifier Letter Vertical Line", '\''),
+    ('ˊ', "Modifier Letter Acute Accent", '\''),
+    ('ˋ', "Modifier Letter Grave Accent", '\''),
+    ('˴', "Modifier Letter Middle Grave Accent", '\''),
+    ('ʻ', "Modifier Letter Turned Comma", '\''),
+    ('ʽ', "Modifier Letter Reversed Comma", '\''),
+    ('ʼ', "Modifier Letter Apostrophe", '\''),
+    ('ʾ', "Modifier Letter Right Half Ring", '\''),
+    ('ꞌ', "Latin Small Letter Saltillo", '\''),
+    ('י', "Hebrew Letter Yod", '\''),
+    ('ߴ', "Nko High Tone Apostrophe", '\''),
+    ('ߵ', "Nko Low Tone Apostrophe", '\''),
+    ('ᑊ', "Canadian Syllabics West-Cree P", '\''),
+    ('ᛌ', "Runic Letter Short-Twig-Sol S", '\''),
+    ('𖽑', "Miao Sign Aspiration", '\''),
+    ('𖽒', "Miao Sign Reformed Voicing", '\''),
+
+    ('᳓', "Vedic Sign Nihshvasa", '"'),
+    ('＂', "Fullwidth Quotation Mark", '"'),
+    ('“', "Left Double Quotation Mark", '"'),
+    ('”', "Right Double Quotation Mark", '"'),
+    ('‟', "Double High-Reversed-9 Quotation Mark", '"'),
+    ('″', "Double Prime", '"'),
+    ('‶', "Reversed Double Prime", '"'),
+    ('〃', "Ditto Mark", '"'),
+    ('״', "Hebrew Punctuation Gershayim", '"'),
+    ('˝', "Double Acute Accent", '"'),
+    ('ʺ', "Modifier Letter Double Prime", '"'),
+    ('˶', "Modifier Letter Middle Double Acute Accent", '"'),
+    ('˵', "Modifier Letter Middle Double Grave Accent", '"'),
+    ('ˮ', "Modifier Letter Double Apostrophe", '"'),
+    ('ײ', "Hebrew Ligature Yiddish Double Yod", '"'),
+    ('❞', "Heavy Double Comma Quotation Mark Ornament", '"'),
+    ('❝', "Heavy Double Turned Comma Quotation Mark Ornament", '"'),
+
+    ('（', "Fullwidth Left Parenthesis", '('),
+    ('❨', "Medium Left Parenthesis Ornament", '('),
+    ('﴾', "Ornate Left Parenthesis", '('),
+
+    ('）', "Fullwidth Right Parenthesis", ')'),
+    ('❩', "Medium Right Parenthesis Ornament", ')'),
+    ('﴿', "Ornate Right Parenthesis", ')'),
+
+    ('［', "Fullwidth Left Square Bracket", '['),
+    ('❲', "Light Left Tortoise Shell Bracket Ornament", '['),
+    ('「', "Left Corner Bracket", '['),
+    ('『', "Left White Corner Bracket", '['),
+    ('【', "Left Black Lenticular Bracket", '['),
+    ('〔', "Left Tortoise Shell Bracket", '['),
+    ('〖', "Left White Lenticular Bracket", '['),
+    ('〘', "Left White Tortoise Shell Bracket", '['),
+    ('〚', "Left White Square Bracket", '['),
+
+    ('］', "Fullwidth Right Square Bracket", ']'),
+    ('❳', "Light Right Tortoise Shell Bracket Ornament", ']'),
+    ('」', "Right Corner Bracket", ']'),
+    ('』', "Right White Corner Bracket", ']'),
+    ('】', "Right Black Lenticular Bracket", ']'),
+    ('〕', "Right Tortoise Shell Bracket", ']'),
+    ('〗', "Right White Lenticular Bracket", ']'),
+    ('〙', "Right White Tortoise Shell Bracket", ']'),
+    ('〛', "Right White Square Bracket", ']'),
+
+    ('❴', "Medium Left Curly Bracket Ornament", '{'),
+    ('𝄔', "Musical Symbol Brace", '{'),
+    ('｛', "Fullwidth Left Curly Bracket", '{'),
+
+    ('❵', "Medium Right Curly Bracket Ornament", '}'),
+    ('｝', "Fullwidth Right Curly Bracket", '}'),
+
+    ('⁎', "Low Asterisk", '*'),
+    ('٭', "Arabic Five Pointed Star", '*'),
+    ('∗', "Asterisk Operator", '*'),
+    ('𐌟', "Old Italic Letter Ess", '*'),
+    ('＊', "Fullwidth Asterisk", '*'),
+
+    ('᜵', "Philippine Single Punctuation", '/'),
+    ('⁁', "Caret Insertion Point", '/'),
+    ('∕', "Division Slash", '/'),
+    ('⁄', "Fraction Slash", '/'),
+    ('╱', "Box Drawings Light Diagonal Upper Right To Lower Left", '/'),
+    ('⟋', "Mathematical Rising Diagonal", '/'),
+    ('⧸', "Big Solidus", '/'),
+    ('𝈺', "Greek Instrumental Notation Symbol-47", '/'),
+    ('㇓', "CJK Stroke Sp", '/'),
+    ('〳', "Vertical Kana Repeat Mark Upper Half", '/'),
+    ('Ⳇ', "Coptic Capital Letter Old Coptic Esh", '/'),
+    ('ノ', "Katakana Letter No", '/'),
+    ('丿', "CJK Unified Ideograph-4E3F", '/'),
+    ('⼃', "Kangxi Radical Slash", '/'),
+    ('／', "Fullwidth Solidus", '/'),
+
+    ('＼', "Fullwidth Reverse Solidus", '\\'),
+    ('﹨', "Small Reverse Solidus", '\\'),
+    ('∖', "Set Minus", '\\'),
+    ('⟍', "Mathematical Falling Diagonal", '\\'),
+    ('⧵', "Reverse Solidus Operator", '\\'),
+    ('⧹', "Big Reverse Solidus", '\\'),
+    ('⧹', "Greek Vocal Notation Symbol-16", '\\'),
+    ('⧹', "Greek Instrumental Symbol-48", '\\'),
+    ('㇔', "CJK Stroke D", '\\'),
+    ('丶', "CJK Unified Ideograph-4E36", '\\'),
+    ('⼂', "Kangxi Radical Dot", '\\'),
+    ('、', "Ideographic Comma", '\\'),
+    ('ヽ', "Katakana Iteration Mark", '\\'),
+
+    ('ꝸ', "Latin Small Letter Um", '&'),
+    ('＆', "Fullwidth Ampersand", '&'),
+
+    ('᛭', "Runic Cross Punctuation", '+'),
+    ('➕', "Heavy Plus Sign", '+'),
+    ('𐊛', "Lycian Letter H", '+'),
+    ('﬩', "Hebrew Letter Alternative Plus Sign", '+'),
+    ('＋', "Fullwidth Plus Sign", '+'),
+
+    ('‹', "Single Left-Pointing Angle Quotation Mark", '<'),
+    ('❮', "Heavy Left-Pointing Angle Quotation Mark Ornament", '<'),
+    ('˂', "Modifier Letter Left Arrowhead", '<'),
+    ('𝈶', "Greek Instrumental Symbol-40", '<'),
+    ('ᐸ', "Canadian Syllabics Pa", '<'),
+    ('ᚲ', "Runic Letter Kauna", '<'),
+    ('❬', "Medium Left-Pointing Angle Bracket Ornament", '<'),
+    ('⟨', "Mathematical Left Angle Bracket", '<'),
+    ('〈', "Left-Pointing Angle Bracket", '<'),
+    ('〈', "Left Angle Bracket", '<'),
+    ('㇛', "CJK Stroke Pd", '<'),
+    ('く', "Hiragana Letter Ku", '<'),
+    ('𡿨', "CJK Unified Ideograph-21FE8", '<'),
+    ('《', "Left Double Angle Bracket", '<'),
+    ('＜', "Fullwidth Less-Than Sign", '<'),
+
+    ('᐀', "Canadian Syllabics Hyphen", '='),
+    ('⹀', "Double Hyphen", '='),
+    ('゠', "Katakana-Hiragana Double Hyphen", '='),
+    ('꓿', "Lisu Punctuation Full Stop", '='),
+    ('＝', "Fullwidth Equals Sign", '='),
+
+    ('›', "Single Right-Pointing Angle Quotation Mark", '>'),
+    ('❯', "Heavy Right-Pointing Angle Quotation Mark Ornament", '>'),
+    ('˃', "Modifier Letter Right Arrowhead", '>'),
+    ('𝈷', "Greek Instrumental Symbol-42", '>'),
+    ('ᐳ', "Canadian Syllabics Po", '>'),
+    ('𖼿', "Miao Letter Archaic Zza", '>'),
+    ('❭', "Medium Right-Pointing Angle Bracket Ornament", '>'),
+    ('⟩', "Mathematical Right Angle Bracket", '>'),
+    ('〉', "Right-Pointing Angle Bracket", '>'),
+    ('〉', "Right Angle Bracket", '>'),
+    ('》', "Right Double Angle Bracket", '>'),
+    ('＞', "Fullwidth Greater-Than Sign", '>'),
+];
+
+// FIXME: the lexer could be used to turn the ASCII version of unicode homoglyphs, instead of
+// keeping the substitution token in this table. Ideally, this should be inside `rustc_lexer`.
+// However, we should first remove compound tokens like `<<` from `rustc_lexer`, and then add
+// fancier error recovery to it, as there will be less overall work to do this way.
+const ASCII_ARRAY: &[(char, &str, Option<token::TokenKind>)] = &[
+    (' ', "Space", Some(token::Whitespace)),
+    ('_', "Underscore", Some(token::Ident(kw::Underscore, false))),
+    ('-', "Minus/Hyphen", Some(token::BinOp(token::Minus))),
+    (',', "Comma", Some(token::Comma)),
+    (';', "Semicolon", Some(token::Semi)),
+    (':', "Colon", Some(token::Colon)),
+    ('!', "Exclamation Mark", Some(token::Not)),
+    ('?', "Question Mark", Some(token::Question)),
+    ('.', "Period", Some(token::Dot)),
+    ('(', "Left Parenthesis", Some(token::OpenDelim(token::Paren))),
+    (')', "Right Parenthesis", Some(token::CloseDelim(token::Paren))),
+    ('[', "Left Square Bracket", Some(token::OpenDelim(token::Bracket))),
+    (']', "Right Square Bracket", Some(token::CloseDelim(token::Bracket))),
+    ('{', "Left Curly Brace", Some(token::OpenDelim(token::Brace))),
+    ('}', "Right Curly Brace", Some(token::CloseDelim(token::Brace))),
+    ('*', "Asterisk", Some(token::BinOp(token::Star))),
+    ('/', "Slash", Some(token::BinOp(token::Slash))),
+    ('\\', "Backslash", None),
+    ('&', "Ampersand", Some(token::BinOp(token::And))),
+    ('+', "Plus Sign", Some(token::BinOp(token::Plus))),
+    ('<', "Less-Than Sign", Some(token::Lt)),
+    ('=', "Equals Sign", Some(token::Eq)),
+    ('>', "Greater-Than Sign", Some(token::Gt)),
+    // FIXME: Literals are already lexed by this point, so we can't recover gracefully just by
+    // spitting the correct token out.
+    ('\'', "Single Quote", None),
+    ('"', "Quotation Mark", None),
+];
+
+crate fn check_for_substitution<'a>(
+    reader: &StringReader<'a>,
+    pos: BytePos,
+    ch: char,
+    err: &mut DiagnosticBuilder<'a>,
+) -> Option<token::TokenKind> {
+    let (u_name, ascii_char) = match UNICODE_ARRAY.iter().find(|&&(c, _, _)| c == ch) {
+        Some(&(_u_char, u_name, ascii_char)) => (u_name, ascii_char),
+        None => return None,
+    };
+
+    let span = Span::with_root_ctxt(pos, pos + Pos::from_usize(ch.len_utf8()));
+
+    let (ascii_name, token) = match ASCII_ARRAY.iter().find(|&&(c, _, _)| c == ascii_char) {
+        Some((_ascii_char, ascii_name, token)) => (ascii_name, token),
+        None => {
+            let msg = format!("substitution character not found for '{}'", ch);
+            reader.sess.span_diagnostic.span_bug_no_panic(span, &msg);
+            return None;
+        }
+    };
+
+    // special help suggestion for "directed" double quotes
+    if let Some(s) = peek_delimited(&reader.src[reader.src_index(pos)..], '“', '”') {
+        let msg = format!(
+            "Unicode characters '“' (Left Double Quotation Mark) and \
+             '”' (Right Double Quotation Mark) look like '{}' ({}), but are not",
+            ascii_char, ascii_name
+        );
+        err.span_suggestion(
+            Span::with_root_ctxt(
+                pos,
+                pos + Pos::from_usize('“'.len_utf8() + s.len() + '”'.len_utf8()),
+            ),
+            &msg,
+            format!("\"{}\"", s),
+            Applicability::MaybeIncorrect,
+        );
+    } else {
+        let msg = format!(
+            "Unicode character '{}' ({}) looks like '{}' ({}), but it is not",
+            ch, u_name, ascii_char, ascii_name
+        );
+        err.span_suggestion(span, &msg, ascii_char.to_string(), Applicability::MaybeIncorrect);
+    }
+    token.clone()
+}
+
+/// Extract string if found at current position with given delimiters
+fn peek_delimited(text: &str, from_ch: char, to_ch: char) -> Option<&str> {
+    let mut chars = text.chars();
+    let first_char = chars.next()?;
+    if first_char != from_ch {
+        return None;
+    }
+    let last_char_idx = chars.as_str().find(to_ch)?;
+    Some(&chars.as_str()[..last_char_idx])
+}
diff --git a/compiler/rustc_parse/src/lib.rs b/compiler/rustc_parse/src/lib.rs
new file mode 100644
index 00000000000..bc857c97742
--- /dev/null
+++ b/compiler/rustc_parse/src/lib.rs
@@ -0,0 +1,595 @@
+//! The main parser interface.
+
+#![feature(bool_to_option)]
+#![feature(crate_visibility_modifier)]
+#![feature(bindings_after_at)]
+#![feature(try_blocks)]
+#![feature(or_patterns)]
+
+use rustc_ast as ast;
+use rustc_ast::token::{self, DelimToken, Nonterminal, Token, TokenKind};
+use rustc_ast::tokenstream::{self, IsJoint, TokenStream, TokenTree};
+use rustc_ast_pretty::pprust;
+use rustc_data_structures::sync::Lrc;
+use rustc_errors::{Diagnostic, FatalError, Level, PResult};
+use rustc_session::parse::ParseSess;
+use rustc_span::{symbol::kw, FileName, SourceFile, Span, DUMMY_SP};
+
+use smallvec::SmallVec;
+use std::mem;
+use std::path::Path;
+use std::str;
+
+use tracing::{debug, info};
+
+pub const MACRO_ARGUMENTS: Option<&'static str> = Some("macro arguments");
+
+#[macro_use]
+pub mod parser;
+use parser::{emit_unclosed_delims, make_unclosed_delims_error, Parser};
+pub mod lexer;
+pub mod validate_attr;
+
+// A bunch of utility functions of the form `parse_<thing>_from_<source>`
+// where <thing> includes crate, expr, item, stmt, tts, and one that
+// uses a HOF to parse anything, and <source> includes file and
+// `source_str`.
+
+/// A variant of 'panictry!' that works on a Vec<Diagnostic> instead of a single DiagnosticBuilder.
+macro_rules! panictry_buffer {
+    ($handler:expr, $e:expr) => {{
+        use rustc_errors::FatalError;
+        use std::result::Result::{Err, Ok};
+        match $e {
+            Ok(e) => e,
+            Err(errs) => {
+                for e in errs {
+                    $handler.emit_diagnostic(&e);
+                }
+                FatalError.raise()
+            }
+        }
+    }};
+}
+
+pub fn parse_crate_from_file<'a>(input: &Path, sess: &'a ParseSess) -> PResult<'a, ast::Crate> {
+    let mut parser = new_parser_from_file(sess, input, None);
+    parser.parse_crate_mod()
+}
+
+pub fn parse_crate_attrs_from_file<'a>(
+    input: &Path,
+    sess: &'a ParseSess,
+) -> PResult<'a, Vec<ast::Attribute>> {
+    let mut parser = new_parser_from_file(sess, input, None);
+    parser.parse_inner_attributes()
+}
+
+pub fn parse_crate_from_source_str(
+    name: FileName,
+    source: String,
+    sess: &ParseSess,
+) -> PResult<'_, ast::Crate> {
+    new_parser_from_source_str(sess, name, source).parse_crate_mod()
+}
+
+pub fn parse_crate_attrs_from_source_str(
+    name: FileName,
+    source: String,
+    sess: &ParseSess,
+) -> PResult<'_, Vec<ast::Attribute>> {
+    new_parser_from_source_str(sess, name, source).parse_inner_attributes()
+}
+
+pub fn parse_stream_from_source_str(
+    name: FileName,
+    source: String,
+    sess: &ParseSess,
+    override_span: Option<Span>,
+) -> TokenStream {
+    let (stream, mut errors) =
+        source_file_to_stream(sess, sess.source_map().new_source_file(name, source), override_span);
+    emit_unclosed_delims(&mut errors, &sess);
+    stream
+}
+
+/// Creates a new parser from a source string.
+pub fn new_parser_from_source_str(sess: &ParseSess, name: FileName, source: String) -> Parser<'_> {
+    panictry_buffer!(&sess.span_diagnostic, maybe_new_parser_from_source_str(sess, name, source))
+}
+
+/// Creates a new parser from a source string. Returns any buffered errors from lexing the initial
+/// token stream.
+pub fn maybe_new_parser_from_source_str(
+    sess: &ParseSess,
+    name: FileName,
+    source: String,
+) -> Result<Parser<'_>, Vec<Diagnostic>> {
+    maybe_source_file_to_parser(sess, sess.source_map().new_source_file(name, source))
+}
+
+/// Creates a new parser, handling errors as appropriate if the file doesn't exist.
+/// If a span is given, that is used on an error as the as the source of the problem.
+pub fn new_parser_from_file<'a>(sess: &'a ParseSess, path: &Path, sp: Option<Span>) -> Parser<'a> {
+    source_file_to_parser(sess, file_to_source_file(sess, path, sp))
+}
+
+/// Creates a new parser, returning buffered diagnostics if the file doesn't exist,
+/// or from lexing the initial token stream.
+pub fn maybe_new_parser_from_file<'a>(
+    sess: &'a ParseSess,
+    path: &Path,
+) -> Result<Parser<'a>, Vec<Diagnostic>> {
+    let file = try_file_to_source_file(sess, path, None).map_err(|db| vec![db])?;
+    maybe_source_file_to_parser(sess, file)
+}
+
+/// Given a `source_file` and config, returns a parser.
+fn source_file_to_parser(sess: &ParseSess, source_file: Lrc<SourceFile>) -> Parser<'_> {
+    panictry_buffer!(&sess.span_diagnostic, maybe_source_file_to_parser(sess, source_file))
+}
+
+/// Given a `source_file` and config, return a parser. Returns any buffered errors from lexing the
+/// initial token stream.
+fn maybe_source_file_to_parser(
+    sess: &ParseSess,
+    source_file: Lrc<SourceFile>,
+) -> Result<Parser<'_>, Vec<Diagnostic>> {
+    let end_pos = source_file.end_pos;
+    let (stream, unclosed_delims) = maybe_file_to_stream(sess, source_file, None)?;
+    let mut parser = stream_to_parser(sess, stream, None);
+    parser.unclosed_delims = unclosed_delims;
+    if parser.token == token::Eof {
+        parser.token.span = Span::new(end_pos, end_pos, parser.token.span.ctxt());
+    }
+
+    Ok(parser)
+}
+
+// Must preserve old name for now, because `quote!` from the *existing*
+// compiler expands into it.
+pub fn new_parser_from_tts(sess: &ParseSess, tts: Vec<TokenTree>) -> Parser<'_> {
+    stream_to_parser(sess, tts.into_iter().collect(), crate::MACRO_ARGUMENTS)
+}
+
+// Base abstractions
+
+/// Given a session and a path and an optional span (for error reporting),
+/// add the path to the session's source_map and return the new source_file or
+/// error when a file can't be read.
+fn try_file_to_source_file(
+    sess: &ParseSess,
+    path: &Path,
+    spanopt: Option<Span>,
+) -> Result<Lrc<SourceFile>, Diagnostic> {
+    sess.source_map().load_file(path).map_err(|e| {
+        let msg = format!("couldn't read {}: {}", path.display(), e);
+        let mut diag = Diagnostic::new(Level::Fatal, &msg);
+        if let Some(sp) = spanopt {
+            diag.set_span(sp);
+        }
+        diag
+    })
+}
+
+/// Given a session and a path and an optional span (for error reporting),
+/// adds the path to the session's `source_map` and returns the new `source_file`.
+fn file_to_source_file(sess: &ParseSess, path: &Path, spanopt: Option<Span>) -> Lrc<SourceFile> {
+    match try_file_to_source_file(sess, path, spanopt) {
+        Ok(source_file) => source_file,
+        Err(d) => {
+            sess.span_diagnostic.emit_diagnostic(&d);
+            FatalError.raise();
+        }
+    }
+}
+
+/// Given a `source_file`, produces a sequence of token trees.
+pub fn source_file_to_stream(
+    sess: &ParseSess,
+    source_file: Lrc<SourceFile>,
+    override_span: Option<Span>,
+) -> (TokenStream, Vec<lexer::UnmatchedBrace>) {
+    panictry_buffer!(&sess.span_diagnostic, maybe_file_to_stream(sess, source_file, override_span))
+}
+
+/// Given a source file, produces a sequence of token trees. Returns any buffered errors from
+/// parsing the token stream.
+pub fn maybe_file_to_stream(
+    sess: &ParseSess,
+    source_file: Lrc<SourceFile>,
+    override_span: Option<Span>,
+) -> Result<(TokenStream, Vec<lexer::UnmatchedBrace>), Vec<Diagnostic>> {
+    let srdr = lexer::StringReader::new(sess, source_file, override_span);
+    let (token_trees, unmatched_braces) = srdr.into_token_trees();
+
+    match token_trees {
+        Ok(stream) => Ok((stream, unmatched_braces)),
+        Err(err) => {
+            let mut buffer = Vec::with_capacity(1);
+            err.buffer(&mut buffer);
+            // Not using `emit_unclosed_delims` to use `db.buffer`
+            for unmatched in unmatched_braces {
+                if let Some(err) = make_unclosed_delims_error(unmatched, &sess) {
+                    err.buffer(&mut buffer);
+                }
+            }
+            Err(buffer)
+        }
+    }
+}
+
+/// Given a stream and the `ParseSess`, produces a parser.
+pub fn stream_to_parser<'a>(
+    sess: &'a ParseSess,
+    stream: TokenStream,
+    subparser_name: Option<&'static str>,
+) -> Parser<'a> {
+    Parser::new(sess, stream, false, subparser_name)
+}
+
+/// Runs the given subparser `f` on the tokens of the given `attr`'s item.
+pub fn parse_in<'a, T>(
+    sess: &'a ParseSess,
+    tts: TokenStream,
+    name: &'static str,
+    mut f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
+) -> PResult<'a, T> {
+    let mut parser = Parser::new(sess, tts, false, Some(name));
+    let result = f(&mut parser)?;
+    if parser.token != token::Eof {
+        parser.unexpected()?;
+    }
+    Ok(result)
+}
+
+// NOTE(Centril): The following probably shouldn't be here but it acknowledges the
+// fact that architecturally, we are using parsing (read on below to understand why).
+
+pub fn nt_to_tokenstream(nt: &Nonterminal, sess: &ParseSess, span: Span) -> TokenStream {
+    // A `Nonterminal` is often a parsed AST item. At this point we now
+    // need to convert the parsed AST to an actual token stream, e.g.
+    // un-parse it basically.
+    //
+    // Unfortunately there's not really a great way to do that in a
+    // guaranteed lossless fashion right now. The fallback here is to just
+    // stringify the AST node and reparse it, but this loses all span
+    // information.
+    //
+    // As a result, some AST nodes are annotated with the token stream they
+    // came from. Here we attempt to extract these lossless token streams
+    // before we fall back to the stringification.
+    let tokens = match *nt {
+        Nonterminal::NtItem(ref item) => {
+            prepend_attrs(sess, &item.attrs, item.tokens.as_ref(), span)
+        }
+        Nonterminal::NtPat(ref pat) => pat.tokens.clone(),
+        Nonterminal::NtIdent(ident, is_raw) => {
+            Some(tokenstream::TokenTree::token(token::Ident(ident.name, is_raw), ident.span).into())
+        }
+        Nonterminal::NtLifetime(ident) => {
+            Some(tokenstream::TokenTree::token(token::Lifetime(ident.name), ident.span).into())
+        }
+        Nonterminal::NtTT(ref tt) => Some(tt.clone().into()),
+        Nonterminal::NtExpr(ref expr) => {
+            if expr.tokens.is_none() {
+                debug!("missing tokens for expr {:?}", expr);
+            }
+            prepend_attrs(sess, &expr.attrs, expr.tokens.as_ref(), span)
+        }
+        _ => None,
+    };
+
+    // FIXME(#43081): Avoid this pretty-print + reparse hack
+    let source = pprust::nonterminal_to_string(nt);
+    let filename = FileName::macro_expansion_source_code(&source);
+    let tokens_for_real = parse_stream_from_source_str(filename, source, sess, Some(span));
+
+    // During early phases of the compiler the AST could get modified
+    // directly (e.g., attributes added or removed) and the internal cache
+    // of tokens my not be invalidated or updated. Consequently if the
+    // "lossless" token stream disagrees with our actual stringification
+    // (which has historically been much more battle-tested) then we go
+    // with the lossy stream anyway (losing span information).
+    //
+    // Note that the comparison isn't `==` here to avoid comparing spans,
+    // but it *also* is a "probable" equality which is a pretty weird
+    // definition. We mostly want to catch actual changes to the AST
+    // like a `#[cfg]` being processed or some weird `macro_rules!`
+    // expansion.
+    //
+    // What we *don't* want to catch is the fact that a user-defined
+    // literal like `0xf` is stringified as `15`, causing the cached token
+    // stream to not be literal `==` token-wise (ignoring spans) to the
+    // token stream we got from stringification.
+    //
+    // Instead the "probably equal" check here is "does each token
+    // recursively have the same discriminant?" We basically don't look at
+    // the token values here and assume that such fine grained token stream
+    // modifications, including adding/removing typically non-semantic
+    // tokens such as extra braces and commas, don't happen.
+    if let Some(tokens) = tokens {
+        if tokenstream_probably_equal_for_proc_macro(&tokens, &tokens_for_real, sess) {
+            return tokens;
+        }
+        info!(
+            "cached tokens found, but they're not \"probably equal\", \
+                going with stringified version"
+        );
+        info!("cached tokens: {:?}", tokens);
+        info!("reparsed tokens: {:?}", tokens_for_real);
+    }
+    tokens_for_real
+}
+
+// See comments in `Nonterminal::to_tokenstream` for why we care about
+// *probably* equal here rather than actual equality
+//
+// This is otherwise the same as `eq_unspanned`, only recursing with a
+// different method.
+pub fn tokenstream_probably_equal_for_proc_macro(
+    first: &TokenStream,
+    other: &TokenStream,
+    sess: &ParseSess,
+) -> bool {
+    // When checking for `probably_eq`, we ignore certain tokens that aren't
+    // preserved in the AST. Because they are not preserved, the pretty
+    // printer arbitrarily adds or removes them when printing as token
+    // streams, making a comparison between a token stream generated from an
+    // AST and a token stream which was parsed into an AST more reliable.
+    fn semantic_tree(tree: &TokenTree) -> bool {
+        if let TokenTree::Token(token) = tree {
+            if let
+                // The pretty printer tends to add trailing commas to
+                // everything, and in particular, after struct fields.
+                | token::Comma
+                // The pretty printer emits `NoDelim` as whitespace.
+                | token::OpenDelim(DelimToken::NoDelim)
+                | token::CloseDelim(DelimToken::NoDelim)
+                // The pretty printer collapses many semicolons into one.
+                | token::Semi
+                // The pretty printer collapses whitespace arbitrarily and can
+                // introduce whitespace from `NoDelim`.
+                | token::Whitespace
+                // The pretty printer can turn `$crate` into `::crate_name`
+                | token::ModSep = token.kind {
+                return false;
+            }
+        }
+        true
+    }
+
+    // When comparing two `TokenStream`s, we ignore the `IsJoint` information.
+    //
+    // However, `rustc_parse::lexer::tokentrees::TokenStreamBuilder` will
+    // use `Token.glue` on adjacent tokens with the proper `IsJoint`.
+    // Since we are ignoreing `IsJoint`, a 'glued' token (e.g. `BinOp(Shr)`)
+    // and its 'split'/'unglued' compoenents (e.g. `Gt, Gt`) are equivalent
+    // when determining if two `TokenStream`s are 'probably equal'.
+    //
+    // Therefore, we use `break_two_token_op` to convert all tokens
+    // to the 'unglued' form (if it exists). This ensures that two
+    // `TokenStream`s which differ only in how their tokens are glued
+    // will be considered 'probably equal', which allows us to keep spans.
+    //
+    // This is important when the original `TokenStream` contained
+    // extra spaces (e.g. `f :: < Vec < _ > > ( ) ;'). These extra spaces
+    // will be omitted when we pretty-print, which can cause the original
+    // and reparsed `TokenStream`s to differ in the assignment of `IsJoint`,
+    // leading to some tokens being 'glued' together in one stream but not
+    // the other. See #68489 for more details.
+    fn break_tokens(tree: TokenTree) -> impl Iterator<Item = TokenTree> {
+        // In almost all cases, we should have either zero or one levels
+        // of 'unglueing'. However, in some unusual cases, we may need
+        // to iterate breaking tokens mutliple times. For example:
+        // '[BinOpEq(Shr)] => [Gt, Ge] -> [Gt, Gt, Eq]'
+        let mut token_trees: SmallVec<[_; 2]>;
+        if let TokenTree::Token(token) = &tree {
+            let mut out = SmallVec::<[_; 2]>::new();
+            out.push(token.clone());
+            // Iterate to fixpoint:
+            // * We start off with 'out' containing our initial token, and `temp` empty
+            // * If we are able to break any tokens in `out`, then `out` will have
+            //   at least one more element than 'temp', so we will try to break tokens
+            //   again.
+            // * If we cannot break any tokens in 'out', we are done
+            loop {
+                let mut temp = SmallVec::<[_; 2]>::new();
+                let mut changed = false;
+
+                for token in out.into_iter() {
+                    if let Some((first, second)) = token.kind.break_two_token_op() {
+                        temp.push(Token::new(first, DUMMY_SP));
+                        temp.push(Token::new(second, DUMMY_SP));
+                        changed = true;
+                    } else {
+                        temp.push(token);
+                    }
+                }
+                out = temp;
+                if !changed {
+                    break;
+                }
+            }
+            token_trees = out.into_iter().map(TokenTree::Token).collect();
+        } else {
+            token_trees = SmallVec::new();
+            token_trees.push(tree);
+        }
+        token_trees.into_iter()
+    }
+
+    let expand_nt = |tree: TokenTree| {
+        if let TokenTree::Token(Token { kind: TokenKind::Interpolated(nt), span }) = &tree {
+            // When checking tokenstreams for 'probable equality', we are comparing
+            // a captured (from parsing) `TokenStream` to a reparsed tokenstream.
+            // The reparsed Tokenstream will never have `None`-delimited groups,
+            // since they are only ever inserted as a result of macro expansion.
+            // Therefore, inserting a `None`-delimtied group here (when we
+            // convert a nested `Nonterminal` to a tokenstream) would cause
+            // a mismatch with the reparsed tokenstream.
+            //
+            // Note that we currently do not handle the case where the
+            // reparsed stream has a `Parenthesis`-delimited group
+            // inserted. This will cause a spurious mismatch:
+            // issue #75734 tracks resolving this.
+            nt_to_tokenstream(nt, sess, *span).into_trees()
+        } else {
+            TokenStream::new(vec![(tree, IsJoint::NonJoint)]).into_trees()
+        }
+    };
+
+    // Break tokens after we expand any nonterminals, so that we break tokens
+    // that are produced as a result of nonterminal expansion.
+    let mut t1 = first.trees().filter(semantic_tree).flat_map(expand_nt).flat_map(break_tokens);
+    let mut t2 = other.trees().filter(semantic_tree).flat_map(expand_nt).flat_map(break_tokens);
+    for (t1, t2) in t1.by_ref().zip(t2.by_ref()) {
+        if !tokentree_probably_equal_for_proc_macro(&t1, &t2, sess) {
+            return false;
+        }
+    }
+    t1.next().is_none() && t2.next().is_none()
+}
+
+// See comments in `Nonterminal::to_tokenstream` for why we care about
+// *probably* equal here rather than actual equality
+//
+// This is otherwise the same as `eq_unspanned`, only recursing with a
+// different method.
+pub fn tokentree_probably_equal_for_proc_macro(
+    first: &TokenTree,
+    other: &TokenTree,
+    sess: &ParseSess,
+) -> bool {
+    match (first, other) {
+        (TokenTree::Token(token), TokenTree::Token(token2)) => {
+            token_probably_equal_for_proc_macro(token, token2)
+        }
+        (TokenTree::Delimited(_, delim, tts), TokenTree::Delimited(_, delim2, tts2)) => {
+            delim == delim2 && tokenstream_probably_equal_for_proc_macro(&tts, &tts2, sess)
+        }
+        _ => false,
+    }
+}
+
+// See comments in `Nonterminal::to_tokenstream` for why we care about
+// *probably* equal here rather than actual equality
+fn token_probably_equal_for_proc_macro(first: &Token, other: &Token) -> bool {
+    if mem::discriminant(&first.kind) != mem::discriminant(&other.kind) {
+        return false;
+    }
+    use rustc_ast::token::TokenKind::*;
+    match (&first.kind, &other.kind) {
+        (&Eq, &Eq)
+        | (&Lt, &Lt)
+        | (&Le, &Le)
+        | (&EqEq, &EqEq)
+        | (&Ne, &Ne)
+        | (&Ge, &Ge)
+        | (&Gt, &Gt)
+        | (&AndAnd, &AndAnd)
+        | (&OrOr, &OrOr)
+        | (&Not, &Not)
+        | (&Tilde, &Tilde)
+        | (&At, &At)
+        | (&Dot, &Dot)
+        | (&DotDot, &DotDot)
+        | (&DotDotDot, &DotDotDot)
+        | (&DotDotEq, &DotDotEq)
+        | (&Comma, &Comma)
+        | (&Semi, &Semi)
+        | (&Colon, &Colon)
+        | (&ModSep, &ModSep)
+        | (&RArrow, &RArrow)
+        | (&LArrow, &LArrow)
+        | (&FatArrow, &FatArrow)
+        | (&Pound, &Pound)
+        | (&Dollar, &Dollar)
+        | (&Question, &Question)
+        | (&Whitespace, &Whitespace)
+        | (&Comment, &Comment)
+        | (&Eof, &Eof) => true,
+
+        (&BinOp(a), &BinOp(b)) | (&BinOpEq(a), &BinOpEq(b)) => a == b,
+
+        (&OpenDelim(a), &OpenDelim(b)) | (&CloseDelim(a), &CloseDelim(b)) => a == b,
+
+        (&DocComment(a1, a2, a3), &DocComment(b1, b2, b3)) => a1 == b1 && a2 == b2 && a3 == b3,
+
+        (&Shebang(a), &Shebang(b)) => a == b,
+
+        (&Literal(a), &Literal(b)) => a == b,
+
+        (&Lifetime(a), &Lifetime(b)) => a == b,
+        (&Ident(a, b), &Ident(c, d)) => {
+            b == d && (a == c || a == kw::DollarCrate || c == kw::DollarCrate)
+        }
+
+        (&Interpolated(..), &Interpolated(..)) => panic!("Unexpanded Interpolated!"),
+
+        _ => panic!("forgot to add a token?"),
+    }
+}
+
+fn prepend_attrs(
+    sess: &ParseSess,
+    attrs: &[ast::Attribute],
+    tokens: Option<&tokenstream::TokenStream>,
+    span: rustc_span::Span,
+) -> Option<tokenstream::TokenStream> {
+    let tokens = tokens?;
+    if attrs.is_empty() {
+        return Some(tokens.clone());
+    }
+    let mut builder = tokenstream::TokenStreamBuilder::new();
+    for attr in attrs {
+        assert_eq!(
+            attr.style,
+            ast::AttrStyle::Outer,
+            "inner attributes should prevent cached tokens from existing"
+        );
+
+        let source = pprust::attribute_to_string(attr);
+        let macro_filename = FileName::macro_expansion_source_code(&source);
+
+        let item = match attr.kind {
+            ast::AttrKind::Normal(ref item) => item,
+            ast::AttrKind::DocComment(..) => {
+                let stream = parse_stream_from_source_str(macro_filename, source, sess, Some(span));
+                builder.push(stream);
+                continue;
+            }
+        };
+
+        // synthesize # [ $path $tokens ] manually here
+        let mut brackets = tokenstream::TokenStreamBuilder::new();
+
+        // For simple paths, push the identifier directly
+        if item.path.segments.len() == 1 && item.path.segments[0].args.is_none() {
+            let ident = item.path.segments[0].ident;
+            let token = token::Ident(ident.name, ident.as_str().starts_with("r#"));
+            brackets.push(tokenstream::TokenTree::token(token, ident.span));
+
+        // ... and for more complicated paths, fall back to a reparse hack that
+        // should eventually be removed.
+        } else {
+            let stream = parse_stream_from_source_str(macro_filename, source, sess, Some(span));
+            brackets.push(stream);
+        }
+
+        brackets.push(item.args.outer_tokens());
+
+        // The span we list here for `#` and for `[ ... ]` are both wrong in
+        // that it encompasses more than each token, but it hopefully is "good
+        // enough" for now at least.
+        builder.push(tokenstream::TokenTree::token(token::Pound, attr.span));
+        let delim_span = tokenstream::DelimSpan::from_single(attr.span);
+        builder.push(tokenstream::TokenTree::Delimited(
+            delim_span,
+            token::DelimToken::Bracket,
+            brackets.build(),
+        ));
+    }
+    builder.push(tokens.clone());
+    Some(builder.build())
+}
diff --git a/compiler/rustc_parse/src/parser/attr.rs b/compiler/rustc_parse/src/parser/attr.rs
new file mode 100644
index 00000000000..4e4429e461f
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/attr.rs
@@ -0,0 +1,304 @@
+use super::{Parser, PathStyle};
+use rustc_ast as ast;
+use rustc_ast::attr;
+use rustc_ast::token::{self, Nonterminal};
+use rustc_ast_pretty::pprust;
+use rustc_errors::{error_code, PResult};
+use rustc_span::Span;
+
+use tracing::debug;
+
+#[derive(Debug)]
+pub(super) enum InnerAttrPolicy<'a> {
+    Permitted,
+    Forbidden { reason: &'a str, saw_doc_comment: bool, prev_attr_sp: Option<Span> },
+}
+
+const DEFAULT_UNEXPECTED_INNER_ATTR_ERR_MSG: &str = "an inner attribute is not \
+                                                     permitted in this context";
+
+pub(super) const DEFAULT_INNER_ATTR_FORBIDDEN: InnerAttrPolicy<'_> = InnerAttrPolicy::Forbidden {
+    reason: DEFAULT_UNEXPECTED_INNER_ATTR_ERR_MSG,
+    saw_doc_comment: false,
+    prev_attr_sp: None,
+};
+
+impl<'a> Parser<'a> {
+    /// Parses attributes that appear before an item.
+    pub(super) fn parse_outer_attributes(&mut self) -> PResult<'a, Vec<ast::Attribute>> {
+        let mut attrs: Vec<ast::Attribute> = Vec::new();
+        let mut just_parsed_doc_comment = false;
+        loop {
+            debug!("parse_outer_attributes: self.token={:?}", self.token);
+            if self.check(&token::Pound) {
+                let inner_error_reason = if just_parsed_doc_comment {
+                    "an inner attribute is not permitted following an outer doc comment"
+                } else if !attrs.is_empty() {
+                    "an inner attribute is not permitted following an outer attribute"
+                } else {
+                    DEFAULT_UNEXPECTED_INNER_ATTR_ERR_MSG
+                };
+                let inner_parse_policy = InnerAttrPolicy::Forbidden {
+                    reason: inner_error_reason,
+                    saw_doc_comment: just_parsed_doc_comment,
+                    prev_attr_sp: attrs.last().map(|a| a.span),
+                };
+                let attr = self.parse_attribute_with_inner_parse_policy(inner_parse_policy)?;
+                attrs.push(attr);
+                just_parsed_doc_comment = false;
+            } else if let token::DocComment(comment_kind, attr_style, data) = self.token.kind {
+                let attr = attr::mk_doc_comment(comment_kind, attr_style, data, self.token.span);
+                if attr.style != ast::AttrStyle::Outer {
+                    self.sess
+                        .span_diagnostic
+                        .struct_span_err_with_code(
+                            self.token.span,
+                            "expected outer doc comment",
+                            error_code!(E0753),
+                        )
+                        .note(
+                            "inner doc comments like this (starting with \
+                             `//!` or `/*!`) can only appear before items",
+                        )
+                        .emit();
+                }
+                attrs.push(attr);
+                self.bump();
+                just_parsed_doc_comment = true;
+            } else {
+                break;
+            }
+        }
+        Ok(attrs)
+    }
+
+    /// Matches `attribute = # ! [ meta_item ]`.
+    ///
+    /// If `permit_inner` is `true`, then a leading `!` indicates an inner
+    /// attribute.
+    pub fn parse_attribute(&mut self, permit_inner: bool) -> PResult<'a, ast::Attribute> {
+        debug!("parse_attribute: permit_inner={:?} self.token={:?}", permit_inner, self.token);
+        let inner_parse_policy =
+            if permit_inner { InnerAttrPolicy::Permitted } else { DEFAULT_INNER_ATTR_FORBIDDEN };
+        self.parse_attribute_with_inner_parse_policy(inner_parse_policy)
+    }
+
+    /// The same as `parse_attribute`, except it takes in an `InnerAttrPolicy`
+    /// that prescribes how to handle inner attributes.
+    fn parse_attribute_with_inner_parse_policy(
+        &mut self,
+        inner_parse_policy: InnerAttrPolicy<'_>,
+    ) -> PResult<'a, ast::Attribute> {
+        debug!(
+            "parse_attribute_with_inner_parse_policy: inner_parse_policy={:?} self.token={:?}",
+            inner_parse_policy, self.token
+        );
+        let lo = self.token.span;
+        let (span, item, style) = if self.eat(&token::Pound) {
+            let style =
+                if self.eat(&token::Not) { ast::AttrStyle::Inner } else { ast::AttrStyle::Outer };
+
+            self.expect(&token::OpenDelim(token::Bracket))?;
+            let item = self.parse_attr_item()?;
+            self.expect(&token::CloseDelim(token::Bracket))?;
+            let attr_sp = lo.to(self.prev_token.span);
+
+            // Emit error if inner attribute is encountered and forbidden.
+            if style == ast::AttrStyle::Inner {
+                self.error_on_forbidden_inner_attr(attr_sp, inner_parse_policy);
+            }
+
+            (attr_sp, item, style)
+        } else {
+            let token_str = pprust::token_to_string(&self.token);
+            let msg = &format!("expected `#`, found `{}`", token_str);
+            return Err(self.struct_span_err(self.token.span, msg));
+        };
+
+        Ok(attr::mk_attr_from_item(style, item, span))
+    }
+
+    pub(super) fn error_on_forbidden_inner_attr(&self, attr_sp: Span, policy: InnerAttrPolicy<'_>) {
+        if let InnerAttrPolicy::Forbidden { reason, saw_doc_comment, prev_attr_sp } = policy {
+            let prev_attr_note =
+                if saw_doc_comment { "previous doc comment" } else { "previous outer attribute" };
+
+            let mut diag = self.struct_span_err(attr_sp, reason);
+
+            if let Some(prev_attr_sp) = prev_attr_sp {
+                diag.span_label(attr_sp, "not permitted following an outer attribute")
+                    .span_label(prev_attr_sp, prev_attr_note);
+            }
+
+            diag.note(
+                "inner attributes, like `#![no_std]`, annotate the item enclosing them, \
+                and are usually found at the beginning of source files. \
+                Outer attributes, like `#[test]`, annotate the item following them.",
+            )
+            .emit();
+        }
+    }
+
+    /// Parses an inner part of an attribute (the path and following tokens).
+    /// The tokens must be either a delimited token stream, or empty token stream,
+    /// or the "legacy" key-value form.
+    ///     PATH `(` TOKEN_STREAM `)`
+    ///     PATH `[` TOKEN_STREAM `]`
+    ///     PATH `{` TOKEN_STREAM `}`
+    ///     PATH
+    ///     PATH `=` UNSUFFIXED_LIT
+    /// The delimiters or `=` are still put into the resulting token stream.
+    pub fn parse_attr_item(&mut self) -> PResult<'a, ast::AttrItem> {
+        let item = match self.token.kind {
+            token::Interpolated(ref nt) => match **nt {
+                Nonterminal::NtMeta(ref item) => Some(item.clone().into_inner()),
+                _ => None,
+            },
+            _ => None,
+        };
+        Ok(if let Some(item) = item {
+            self.bump();
+            item
+        } else {
+            let path = self.parse_path(PathStyle::Mod)?;
+            let args = self.parse_attr_args()?;
+            ast::AttrItem { path, args }
+        })
+    }
+
+    /// Parses attributes that appear after the opening of an item. These should
+    /// be preceded by an exclamation mark, but we accept and warn about one
+    /// terminated by a semicolon.
+    ///
+    /// Matches `inner_attrs*`.
+    crate fn parse_inner_attributes(&mut self) -> PResult<'a, Vec<ast::Attribute>> {
+        let mut attrs: Vec<ast::Attribute> = vec![];
+        loop {
+            // Only try to parse if it is an inner attribute (has `!`).
+            if self.check(&token::Pound) && self.look_ahead(1, |t| t == &token::Not) {
+                let attr = self.parse_attribute(true)?;
+                assert_eq!(attr.style, ast::AttrStyle::Inner);
+                attrs.push(attr);
+            } else if let token::DocComment(comment_kind, attr_style, data) = self.token.kind {
+                // We need to get the position of this token before we bump.
+                let attr = attr::mk_doc_comment(comment_kind, attr_style, data, self.token.span);
+                if attr.style == ast::AttrStyle::Inner {
+                    attrs.push(attr);
+                    self.bump();
+                } else {
+                    break;
+                }
+            } else {
+                break;
+            }
+        }
+        Ok(attrs)
+    }
+
+    crate fn parse_unsuffixed_lit(&mut self) -> PResult<'a, ast::Lit> {
+        let lit = self.parse_lit()?;
+        debug!("checking if {:?} is unusuffixed", lit);
+
+        if !lit.kind.is_unsuffixed() {
+            self.struct_span_err(lit.span, "suffixed literals are not allowed in attributes")
+                .help(
+                    "instead of using a suffixed literal (`1u8`, `1.0f32`, etc.), \
+                    use an unsuffixed version (`1`, `1.0`, etc.)",
+                )
+                .emit();
+        }
+
+        Ok(lit)
+    }
+
+    /// Parses `cfg_attr(pred, attr_item_list)` where `attr_item_list` is comma-delimited.
+    pub fn parse_cfg_attr(&mut self) -> PResult<'a, (ast::MetaItem, Vec<(ast::AttrItem, Span)>)> {
+        let cfg_predicate = self.parse_meta_item()?;
+        self.expect(&token::Comma)?;
+
+        // Presumably, the majority of the time there will only be one attr.
+        let mut expanded_attrs = Vec::with_capacity(1);
+        while self.token.kind != token::Eof {
+            let lo = self.token.span;
+            let item = self.parse_attr_item()?;
+            expanded_attrs.push((item, lo.to(self.prev_token.span)));
+            if !self.eat(&token::Comma) {
+                break;
+            }
+        }
+
+        Ok((cfg_predicate, expanded_attrs))
+    }
+
+    /// Matches `COMMASEP(meta_item_inner)`.
+    crate fn parse_meta_seq_top(&mut self) -> PResult<'a, Vec<ast::NestedMetaItem>> {
+        // Presumably, the majority of the time there will only be one attr.
+        let mut nmis = Vec::with_capacity(1);
+        while self.token.kind != token::Eof {
+            nmis.push(self.parse_meta_item_inner()?);
+            if !self.eat(&token::Comma) {
+                break;
+            }
+        }
+        Ok(nmis)
+    }
+
+    /// Matches the following grammar (per RFC 1559).
+    ///
+    ///     meta_item : PATH ( '=' UNSUFFIXED_LIT | '(' meta_item_inner? ')' )? ;
+    ///     meta_item_inner : (meta_item | UNSUFFIXED_LIT) (',' meta_item_inner)? ;
+    pub fn parse_meta_item(&mut self) -> PResult<'a, ast::MetaItem> {
+        let nt_meta = match self.token.kind {
+            token::Interpolated(ref nt) => match **nt {
+                token::NtMeta(ref e) => Some(e.clone()),
+                _ => None,
+            },
+            _ => None,
+        };
+
+        if let Some(item) = nt_meta {
+            return match item.meta(item.path.span) {
+                Some(meta) => {
+                    self.bump();
+                    Ok(meta)
+                }
+                None => self.unexpected(),
+            };
+        }
+
+        let lo = self.token.span;
+        let path = self.parse_path(PathStyle::Mod)?;
+        let kind = self.parse_meta_item_kind()?;
+        let span = lo.to(self.prev_token.span);
+        Ok(ast::MetaItem { path, kind, span })
+    }
+
+    crate fn parse_meta_item_kind(&mut self) -> PResult<'a, ast::MetaItemKind> {
+        Ok(if self.eat(&token::Eq) {
+            ast::MetaItemKind::NameValue(self.parse_unsuffixed_lit()?)
+        } else if self.check(&token::OpenDelim(token::Paren)) {
+            // Matches `meta_seq = ( COMMASEP(meta_item_inner) )`.
+            let (list, _) = self.parse_paren_comma_seq(|p| p.parse_meta_item_inner())?;
+            ast::MetaItemKind::List(list)
+        } else {
+            ast::MetaItemKind::Word
+        })
+    }
+
+    /// Matches `meta_item_inner : (meta_item | UNSUFFIXED_LIT) ;`.
+    fn parse_meta_item_inner(&mut self) -> PResult<'a, ast::NestedMetaItem> {
+        match self.parse_unsuffixed_lit() {
+            Ok(lit) => return Ok(ast::NestedMetaItem::Literal(lit)),
+            Err(ref mut err) => err.cancel(),
+        }
+
+        match self.parse_meta_item() {
+            Ok(mi) => return Ok(ast::NestedMetaItem::MetaItem(mi)),
+            Err(ref mut err) => err.cancel(),
+        }
+
+        let found = pprust::token_to_string(&self.token);
+        let msg = format!("expected unsuffixed literal or identifier, found `{}`", found);
+        Err(self.struct_span_err(self.token.span, &msg))
+    }
+}
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
new file mode 100644
index 00000000000..12efe391fb9
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -0,0 +1,1643 @@
+use super::ty::AllowPlus;
+use super::{BlockMode, Parser, PathStyle, SemiColonMode, SeqSep, TokenExpectType, TokenType};
+
+use rustc_ast::ptr::P;
+use rustc_ast::token::{self, Lit, LitKind, TokenKind};
+use rustc_ast::util::parser::AssocOp;
+use rustc_ast::{
+    self as ast, AngleBracketedArgs, AttrVec, BinOpKind, BindingMode, BlockCheckMode, Expr,
+    ExprKind, Item, ItemKind, Mutability, Param, Pat, PatKind, PathSegment, QSelf, Ty, TyKind,
+};
+use rustc_ast_pretty::pprust;
+use rustc_data_structures::fx::FxHashSet;
+use rustc_errors::{pluralize, struct_span_err};
+use rustc_errors::{Applicability, DiagnosticBuilder, Handler, PResult};
+use rustc_span::source_map::Spanned;
+use rustc_span::symbol::{kw, Ident};
+use rustc_span::{MultiSpan, Span, SpanSnippetError, DUMMY_SP};
+
+use tracing::{debug, trace};
+
+const TURBOFISH: &str = "use `::<...>` instead of `<...>` to specify type arguments";
+
+/// Creates a placeholder argument.
+pub(super) fn dummy_arg(ident: Ident) -> Param {
+    let pat = P(Pat {
+        id: ast::DUMMY_NODE_ID,
+        kind: PatKind::Ident(BindingMode::ByValue(Mutability::Not), ident, None),
+        span: ident.span,
+        tokens: None,
+    });
+    let ty = Ty { kind: TyKind::Err, span: ident.span, id: ast::DUMMY_NODE_ID };
+    Param {
+        attrs: AttrVec::default(),
+        id: ast::DUMMY_NODE_ID,
+        pat,
+        span: ident.span,
+        ty: P(ty),
+        is_placeholder: false,
+    }
+}
+
+pub enum Error {
+    UselessDocComment,
+}
+
+impl Error {
+    fn span_err(self, sp: impl Into<MultiSpan>, handler: &Handler) -> DiagnosticBuilder<'_> {
+        match self {
+            Error::UselessDocComment => {
+                let mut err = struct_span_err!(
+                    handler,
+                    sp,
+                    E0585,
+                    "found a documentation comment that doesn't document anything",
+                );
+                err.help(
+                    "doc comments must come before what they document, maybe a comment was \
+                          intended with `//`?",
+                );
+                err
+            }
+        }
+    }
+}
+
+pub(super) trait RecoverQPath: Sized + 'static {
+    const PATH_STYLE: PathStyle = PathStyle::Expr;
+    fn to_ty(&self) -> Option<P<Ty>>;
+    fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self;
+}
+
+impl RecoverQPath for Ty {
+    const PATH_STYLE: PathStyle = PathStyle::Type;
+    fn to_ty(&self) -> Option<P<Ty>> {
+        Some(P(self.clone()))
+    }
+    fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self {
+        Self { span: path.span, kind: TyKind::Path(qself, path), id: ast::DUMMY_NODE_ID }
+    }
+}
+
+impl RecoverQPath for Pat {
+    fn to_ty(&self) -> Option<P<Ty>> {
+        self.to_ty()
+    }
+    fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self {
+        Self {
+            span: path.span,
+            kind: PatKind::Path(qself, path),
+            id: ast::DUMMY_NODE_ID,
+            tokens: None,
+        }
+    }
+}
+
+impl RecoverQPath for Expr {
+    fn to_ty(&self) -> Option<P<Ty>> {
+        self.to_ty()
+    }
+    fn recovered(qself: Option<QSelf>, path: ast::Path) -> Self {
+        Self {
+            span: path.span,
+            kind: ExprKind::Path(qself, path),
+            attrs: AttrVec::new(),
+            id: ast::DUMMY_NODE_ID,
+            tokens: None,
+        }
+    }
+}
+
+/// Control whether the closing delimiter should be consumed when calling `Parser::consume_block`.
+crate enum ConsumeClosingDelim {
+    Yes,
+    No,
+}
+
+impl<'a> Parser<'a> {
+    pub(super) fn span_fatal_err<S: Into<MultiSpan>>(
+        &self,
+        sp: S,
+        err: Error,
+    ) -> DiagnosticBuilder<'a> {
+        err.span_err(sp, self.diagnostic())
+    }
+
+    pub fn struct_span_err<S: Into<MultiSpan>>(&self, sp: S, m: &str) -> DiagnosticBuilder<'a> {
+        self.sess.span_diagnostic.struct_span_err(sp, m)
+    }
+
+    pub fn span_bug<S: Into<MultiSpan>>(&self, sp: S, m: &str) -> ! {
+        self.sess.span_diagnostic.span_bug(sp, m)
+    }
+
+    pub(super) fn diagnostic(&self) -> &'a Handler {
+        &self.sess.span_diagnostic
+    }
+
+    pub(super) fn span_to_snippet(&self, span: Span) -> Result<String, SpanSnippetError> {
+        self.sess.source_map().span_to_snippet(span)
+    }
+
+    pub(super) fn expected_ident_found(&self) -> DiagnosticBuilder<'a> {
+        let mut err = self.struct_span_err(
+            self.token.span,
+            &format!("expected identifier, found {}", super::token_descr(&self.token)),
+        );
+        let valid_follow = &[
+            TokenKind::Eq,
+            TokenKind::Colon,
+            TokenKind::Comma,
+            TokenKind::Semi,
+            TokenKind::ModSep,
+            TokenKind::OpenDelim(token::DelimToken::Brace),
+            TokenKind::OpenDelim(token::DelimToken::Paren),
+            TokenKind::CloseDelim(token::DelimToken::Brace),
+            TokenKind::CloseDelim(token::DelimToken::Paren),
+        ];
+        match self.token.ident() {
+            Some((ident, false))
+                if ident.is_raw_guess()
+                    && self.look_ahead(1, |t| valid_follow.contains(&t.kind)) =>
+            {
+                err.span_suggestion(
+                    ident.span,
+                    "you can escape reserved keywords to use them as identifiers",
+                    format!("r#{}", ident.name),
+                    Applicability::MaybeIncorrect,
+                );
+            }
+            _ => {}
+        }
+        if let Some(token_descr) = super::token_descr_opt(&self.token) {
+            err.span_label(self.token.span, format!("expected identifier, found {}", token_descr));
+        } else {
+            err.span_label(self.token.span, "expected identifier");
+            if self.token == token::Comma && self.look_ahead(1, |t| t.is_ident()) {
+                err.span_suggestion(
+                    self.token.span,
+                    "remove this comma",
+                    String::new(),
+                    Applicability::MachineApplicable,
+                );
+            }
+        }
+        err
+    }
+
+    pub(super) fn expected_one_of_not_found(
+        &mut self,
+        edible: &[TokenKind],
+        inedible: &[TokenKind],
+    ) -> PResult<'a, bool /* recovered */> {
+        fn tokens_to_string(tokens: &[TokenType]) -> String {
+            let mut i = tokens.iter();
+            // This might be a sign we need a connect method on `Iterator`.
+            let b = i.next().map_or(String::new(), |t| t.to_string());
+            i.enumerate().fold(b, |mut b, (i, a)| {
+                if tokens.len() > 2 && i == tokens.len() - 2 {
+                    b.push_str(", or ");
+                } else if tokens.len() == 2 && i == tokens.len() - 2 {
+                    b.push_str(" or ");
+                } else {
+                    b.push_str(", ");
+                }
+                b.push_str(&a.to_string());
+                b
+            })
+        }
+
+        let mut expected = edible
+            .iter()
+            .map(|x| TokenType::Token(x.clone()))
+            .chain(inedible.iter().map(|x| TokenType::Token(x.clone())))
+            .chain(self.expected_tokens.iter().cloned())
+            .collect::<Vec<_>>();
+        expected.sort_by_cached_key(|x| x.to_string());
+        expected.dedup();
+        let expect = tokens_to_string(&expected[..]);
+        let actual = super::token_descr(&self.token);
+        let (msg_exp, (label_sp, label_exp)) = if expected.len() > 1 {
+            let short_expect = if expected.len() > 6 {
+                format!("{} possible tokens", expected.len())
+            } else {
+                expect.clone()
+            };
+            (
+                format!("expected one of {}, found {}", expect, actual),
+                (self.prev_token.span.shrink_to_hi(), format!("expected one of {}", short_expect)),
+            )
+        } else if expected.is_empty() {
+            (
+                format!("unexpected token: {}", actual),
+                (self.prev_token.span, "unexpected token after this".to_string()),
+            )
+        } else {
+            (
+                format!("expected {}, found {}", expect, actual),
+                (self.prev_token.span.shrink_to_hi(), format!("expected {}", expect)),
+            )
+        };
+        self.last_unexpected_token_span = Some(self.token.span);
+        let mut err = self.struct_span_err(self.token.span, &msg_exp);
+        let sp = if self.token == token::Eof {
+            // This is EOF; don't want to point at the following char, but rather the last token.
+            self.prev_token.span
+        } else {
+            label_sp
+        };
+        match self.recover_closing_delimiter(
+            &expected
+                .iter()
+                .filter_map(|tt| match tt {
+                    TokenType::Token(t) => Some(t.clone()),
+                    _ => None,
+                })
+                .collect::<Vec<_>>(),
+            err,
+        ) {
+            Err(e) => err = e,
+            Ok(recovered) => {
+                return Ok(recovered);
+            }
+        }
+
+        if self.check_too_many_raw_str_terminators(&mut err) {
+            return Err(err);
+        }
+
+        let sm = self.sess.source_map();
+        if self.prev_token.span == DUMMY_SP {
+            // Account for macro context where the previous span might not be
+            // available to avoid incorrect output (#54841).
+            err.span_label(self.token.span, label_exp);
+        } else if !sm.is_multiline(self.token.span.shrink_to_hi().until(sp.shrink_to_lo())) {
+            // When the spans are in the same line, it means that the only content between
+            // them is whitespace, point at the found token in that case:
+            //
+            // X |     () => { syntax error };
+            //   |                    ^^^^^ expected one of 8 possible tokens here
+            //
+            // instead of having:
+            //
+            // X |     () => { syntax error };
+            //   |                   -^^^^^ unexpected token
+            //   |                   |
+            //   |                   expected one of 8 possible tokens here
+            err.span_label(self.token.span, label_exp);
+        } else {
+            err.span_label(sp, label_exp);
+            err.span_label(self.token.span, "unexpected token");
+        }
+        self.maybe_annotate_with_ascription(&mut err, false);
+        Err(err)
+    }
+
+    fn check_too_many_raw_str_terminators(&mut self, err: &mut DiagnosticBuilder<'_>) -> bool {
+        match (&self.prev_token.kind, &self.token.kind) {
+            (
+                TokenKind::Literal(Lit {
+                    kind: LitKind::StrRaw(n_hashes) | LitKind::ByteStrRaw(n_hashes),
+                    ..
+                }),
+                TokenKind::Pound,
+            ) => {
+                err.set_primary_message("too many `#` when terminating raw string");
+                err.span_suggestion(
+                    self.token.span,
+                    "remove the extra `#`",
+                    String::new(),
+                    Applicability::MachineApplicable,
+                );
+                err.note(&format!("the raw string started with {} `#`s", n_hashes));
+                true
+            }
+            _ => false,
+        }
+    }
+
+    pub fn maybe_annotate_with_ascription(
+        &mut self,
+        err: &mut DiagnosticBuilder<'_>,
+        maybe_expected_semicolon: bool,
+    ) {
+        if let Some((sp, likely_path)) = self.last_type_ascription.take() {
+            let sm = self.sess.source_map();
+            let next_pos = sm.lookup_char_pos(self.token.span.lo());
+            let op_pos = sm.lookup_char_pos(sp.hi());
+
+            let allow_unstable = self.sess.unstable_features.is_nightly_build();
+
+            if likely_path {
+                err.span_suggestion(
+                    sp,
+                    "maybe write a path separator here",
+                    "::".to_string(),
+                    if allow_unstable {
+                        Applicability::MaybeIncorrect
+                    } else {
+                        Applicability::MachineApplicable
+                    },
+                );
+                self.sess.type_ascription_path_suggestions.borrow_mut().insert(sp);
+            } else if op_pos.line != next_pos.line && maybe_expected_semicolon {
+                err.span_suggestion(
+                    sp,
+                    "try using a semicolon",
+                    ";".to_string(),
+                    Applicability::MaybeIncorrect,
+                );
+            } else if allow_unstable {
+                err.span_label(sp, "tried to parse a type due to this type ascription");
+            } else {
+                err.span_label(sp, "tried to parse a type due to this");
+            }
+            if allow_unstable {
+                // Give extra information about type ascription only if it's a nightly compiler.
+                err.note(
+                    "`#![feature(type_ascription)]` lets you annotate an expression with a type: \
+                     `<expr>: <type>`",
+                );
+                if !likely_path {
+                    // Avoid giving too much info when it was likely an unrelated typo.
+                    err.note(
+                        "see issue #23416 <https://github.com/rust-lang/rust/issues/23416> \
+                        for more information",
+                    );
+                }
+            }
+        }
+    }
+
+    /// Eats and discards tokens until one of `kets` is encountered. Respects token trees,
+    /// passes through any errors encountered. Used for error recovery.
+    pub(super) fn eat_to_tokens(&mut self, kets: &[&TokenKind]) {
+        if let Err(ref mut err) =
+            self.parse_seq_to_before_tokens(kets, SeqSep::none(), TokenExpectType::Expect, |p| {
+                Ok(p.parse_token_tree())
+            })
+        {
+            err.cancel();
+        }
+    }
+
+    /// This function checks if there are trailing angle brackets and produces
+    /// a diagnostic to suggest removing them.
+    ///
+    /// ```ignore (diagnostic)
+    /// let _ = vec![1, 2, 3].into_iter().collect::<Vec<usize>>>>();
+    ///                                                        ^^ help: remove extra angle brackets
+    /// ```
+    ///
+    /// If `true` is returned, then trailing brackets were recovered, tokens were consumed
+    /// up until one of the tokens in 'end' was encountered, and an error was emitted.
+    pub(super) fn check_trailing_angle_brackets(
+        &mut self,
+        segment: &PathSegment,
+        end: &[&TokenKind],
+    ) -> bool {
+        // This function is intended to be invoked after parsing a path segment where there are two
+        // cases:
+        //
+        // 1. A specific token is expected after the path segment.
+        //    eg. `x.foo(`, `x.foo::<u32>(` (parenthesis - method call),
+        //        `Foo::`, or `Foo::<Bar>::` (mod sep - continued path).
+        // 2. No specific token is expected after the path segment.
+        //    eg. `x.foo` (field access)
+        //
+        // This function is called after parsing `.foo` and before parsing the token `end` (if
+        // present). This includes any angle bracket arguments, such as `.foo::<u32>` or
+        // `Foo::<Bar>`.
+
+        // We only care about trailing angle brackets if we previously parsed angle bracket
+        // arguments. This helps stop us incorrectly suggesting that extra angle brackets be
+        // removed in this case:
+        //
+        // `x.foo >> (3)` (where `x.foo` is a `u32` for example)
+        //
+        // This case is particularly tricky as we won't notice it just looking at the tokens -
+        // it will appear the same (in terms of upcoming tokens) as below (since the `::<u32>` will
+        // have already been parsed):
+        //
+        // `x.foo::<u32>>>(3)`
+        let parsed_angle_bracket_args =
+            segment.args.as_ref().map(|args| args.is_angle_bracketed()).unwrap_or(false);
+
+        debug!(
+            "check_trailing_angle_brackets: parsed_angle_bracket_args={:?}",
+            parsed_angle_bracket_args,
+        );
+        if !parsed_angle_bracket_args {
+            return false;
+        }
+
+        // Keep the span at the start so we can highlight the sequence of `>` characters to be
+        // removed.
+        let lo = self.token.span;
+
+        // We need to look-ahead to see if we have `>` characters without moving the cursor forward
+        // (since we might have the field access case and the characters we're eating are
+        // actual operators and not trailing characters - ie `x.foo >> 3`).
+        let mut position = 0;
+
+        // We can encounter `>` or `>>` tokens in any order, so we need to keep track of how
+        // many of each (so we can correctly pluralize our error messages) and continue to
+        // advance.
+        let mut number_of_shr = 0;
+        let mut number_of_gt = 0;
+        while self.look_ahead(position, |t| {
+            trace!("check_trailing_angle_brackets: t={:?}", t);
+            if *t == token::BinOp(token::BinOpToken::Shr) {
+                number_of_shr += 1;
+                true
+            } else if *t == token::Gt {
+                number_of_gt += 1;
+                true
+            } else {
+                false
+            }
+        }) {
+            position += 1;
+        }
+
+        // If we didn't find any trailing `>` characters, then we have nothing to error about.
+        debug!(
+            "check_trailing_angle_brackets: number_of_gt={:?} number_of_shr={:?}",
+            number_of_gt, number_of_shr,
+        );
+        if number_of_gt < 1 && number_of_shr < 1 {
+            return false;
+        }
+
+        // Finally, double check that we have our end token as otherwise this is the
+        // second case.
+        if self.look_ahead(position, |t| {
+            trace!("check_trailing_angle_brackets: t={:?}", t);
+            end.contains(&&t.kind)
+        }) {
+            // Eat from where we started until the end token so that parsing can continue
+            // as if we didn't have those extra angle brackets.
+            self.eat_to_tokens(end);
+            let span = lo.until(self.token.span);
+
+            let total_num_of_gt = number_of_gt + number_of_shr * 2;
+            self.struct_span_err(
+                span,
+                &format!("unmatched angle bracket{}", pluralize!(total_num_of_gt)),
+            )
+            .span_suggestion(
+                span,
+                &format!("remove extra angle bracket{}", pluralize!(total_num_of_gt)),
+                String::new(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+            return true;
+        }
+        false
+    }
+
+    /// Check if a method call with an intended turbofish has been written without surrounding
+    /// angle brackets.
+    pub(super) fn check_turbofish_missing_angle_brackets(&mut self, segment: &mut PathSegment) {
+        if token::ModSep == self.token.kind && segment.args.is_none() {
+            let snapshot = self.clone();
+            self.bump();
+            let lo = self.token.span;
+            match self.parse_angle_args() {
+                Ok(args) => {
+                    let span = lo.to(self.prev_token.span);
+                    // Detect trailing `>` like in `x.collect::Vec<_>>()`.
+                    let mut trailing_span = self.prev_token.span.shrink_to_hi();
+                    while self.token.kind == token::BinOp(token::Shr)
+                        || self.token.kind == token::Gt
+                    {
+                        trailing_span = trailing_span.to(self.token.span);
+                        self.bump();
+                    }
+                    if self.token.kind == token::OpenDelim(token::Paren) {
+                        // Recover from bad turbofish: `foo.collect::Vec<_>()`.
+                        let args = AngleBracketedArgs { args, span }.into();
+                        segment.args = args;
+
+                        self.struct_span_err(
+                            span,
+                            "generic parameters without surrounding angle brackets",
+                        )
+                        .multipart_suggestion(
+                            "surround the type parameters with angle brackets",
+                            vec![
+                                (span.shrink_to_lo(), "<".to_string()),
+                                (trailing_span, ">".to_string()),
+                            ],
+                            Applicability::MachineApplicable,
+                        )
+                        .emit();
+                    } else {
+                        // This doesn't look like an invalid turbofish, can't recover parse state.
+                        *self = snapshot;
+                    }
+                }
+                Err(mut err) => {
+                    // We could't parse generic parameters, unlikely to be a turbofish. Rely on
+                    // generic parse error instead.
+                    err.cancel();
+                    *self = snapshot;
+                }
+            }
+        }
+    }
+
+    /// Check to see if a pair of chained operators looks like an attempt at chained comparison,
+    /// e.g. `1 < x <= 3`. If so, suggest either splitting the comparison into two, or
+    /// parenthesising the leftmost comparison.
+    fn attempt_chained_comparison_suggestion(
+        &mut self,
+        err: &mut DiagnosticBuilder<'_>,
+        inner_op: &Expr,
+        outer_op: &Spanned<AssocOp>,
+    ) -> bool /* advanced the cursor */ {
+        if let ExprKind::Binary(op, ref l1, ref r1) = inner_op.kind {
+            if let ExprKind::Field(_, ident) = l1.kind {
+                if ident.as_str().parse::<i32>().is_err() && !matches!(r1.kind, ExprKind::Lit(_)) {
+                    // The parser has encountered `foo.bar<baz`, the likelihood of the turbofish
+                    // suggestion being the only one to apply is high.
+                    return false;
+                }
+            }
+            let mut enclose = |left: Span, right: Span| {
+                err.multipart_suggestion(
+                    "parenthesize the comparison",
+                    vec![
+                        (left.shrink_to_lo(), "(".to_string()),
+                        (right.shrink_to_hi(), ")".to_string()),
+                    ],
+                    Applicability::MaybeIncorrect,
+                );
+            };
+            return match (op.node, &outer_op.node) {
+                // `x == y == z`
+                (BinOpKind::Eq, AssocOp::Equal) |
+                // `x < y < z` and friends.
+                (BinOpKind::Lt, AssocOp::Less | AssocOp::LessEqual) |
+                (BinOpKind::Le, AssocOp::LessEqual | AssocOp::Less) |
+                // `x > y > z` and friends.
+                (BinOpKind::Gt, AssocOp::Greater | AssocOp::GreaterEqual) |
+                (BinOpKind::Ge, AssocOp::GreaterEqual | AssocOp::Greater) => {
+                    let expr_to_str = |e: &Expr| {
+                        self.span_to_snippet(e.span)
+                            .unwrap_or_else(|_| pprust::expr_to_string(&e))
+                    };
+                    err.span_suggestion_verbose(
+                        inner_op.span.shrink_to_hi(),
+                        "split the comparison into two",
+                        format!(" && {}", expr_to_str(&r1)),
+                        Applicability::MaybeIncorrect,
+                    );
+                    false // Keep the current parse behavior, where the AST is `(x < y) < z`.
+                }
+                // `x == y < z`
+                (BinOpKind::Eq, AssocOp::Less | AssocOp::LessEqual | AssocOp::Greater | AssocOp::GreaterEqual) => {
+                    // Consume `z`/outer-op-rhs.
+                    let snapshot = self.clone();
+                    match self.parse_expr() {
+                        Ok(r2) => {
+                            // We are sure that outer-op-rhs could be consumed, the suggestion is
+                            // likely correct.
+                            enclose(r1.span, r2.span);
+                            true
+                        }
+                        Err(mut expr_err) => {
+                            expr_err.cancel();
+                            *self = snapshot;
+                            false
+                        }
+                    }
+                }
+                // `x > y == z`
+                (BinOpKind::Lt | BinOpKind::Le | BinOpKind::Gt | BinOpKind::Ge, AssocOp::Equal) => {
+                    let snapshot = self.clone();
+                    // At this point it is always valid to enclose the lhs in parentheses, no
+                    // further checks are necessary.
+                    match self.parse_expr() {
+                        Ok(_) => {
+                            enclose(l1.span, r1.span);
+                            true
+                        }
+                        Err(mut expr_err) => {
+                            expr_err.cancel();
+                            *self = snapshot;
+                            false
+                        }
+                    }
+                }
+                _ => false,
+            };
+        }
+        false
+    }
+
+    /// Produces an error if comparison operators are chained (RFC #558).
+    /// We only need to check the LHS, not the RHS, because all comparison ops have same
+    /// precedence (see `fn precedence`) and are left-associative (see `fn fixity`).
+    ///
+    /// This can also be hit if someone incorrectly writes `foo<bar>()` when they should have used
+    /// the turbofish (`foo::<bar>()`) syntax. We attempt some heuristic recovery if that is the
+    /// case.
+    ///
+    /// Keep in mind that given that `outer_op.is_comparison()` holds and comparison ops are left
+    /// associative we can infer that we have:
+    ///
+    /// ```text
+    ///           outer_op
+    ///           /   \
+    ///     inner_op   r2
+    ///        /  \
+    ///      l1    r1
+    /// ```
+    pub(super) fn check_no_chained_comparison(
+        &mut self,
+        inner_op: &Expr,
+        outer_op: &Spanned<AssocOp>,
+    ) -> PResult<'a, Option<P<Expr>>> {
+        debug_assert!(
+            outer_op.node.is_comparison(),
+            "check_no_chained_comparison: {:?} is not comparison",
+            outer_op.node,
+        );
+
+        let mk_err_expr =
+            |this: &Self, span| Ok(Some(this.mk_expr(span, ExprKind::Err, AttrVec::new())));
+
+        match inner_op.kind {
+            ExprKind::Binary(op, ref l1, ref r1) if op.node.is_comparison() => {
+                let mut err = self.struct_span_err(
+                    vec![op.span, self.prev_token.span],
+                    "comparison operators cannot be chained",
+                );
+
+                let suggest = |err: &mut DiagnosticBuilder<'_>| {
+                    err.span_suggestion_verbose(
+                        op.span.shrink_to_lo(),
+                        TURBOFISH,
+                        "::".to_string(),
+                        Applicability::MaybeIncorrect,
+                    );
+                };
+
+                // Include `<` to provide this recommendation even in a case like
+                // `Foo<Bar<Baz<Qux, ()>>>`
+                if op.node == BinOpKind::Lt && outer_op.node == AssocOp::Less
+                    || outer_op.node == AssocOp::Greater
+                {
+                    if outer_op.node == AssocOp::Less {
+                        let snapshot = self.clone();
+                        self.bump();
+                        // So far we have parsed `foo<bar<`, consume the rest of the type args.
+                        let modifiers =
+                            [(token::Lt, 1), (token::Gt, -1), (token::BinOp(token::Shr), -2)];
+                        self.consume_tts(1, &modifiers[..]);
+
+                        if !&[token::OpenDelim(token::Paren), token::ModSep]
+                            .contains(&self.token.kind)
+                        {
+                            // We don't have `foo< bar >(` or `foo< bar >::`, so we rewind the
+                            // parser and bail out.
+                            *self = snapshot.clone();
+                        }
+                    }
+                    return if token::ModSep == self.token.kind {
+                        // We have some certainty that this was a bad turbofish at this point.
+                        // `foo< bar >::`
+                        suggest(&mut err);
+
+                        let snapshot = self.clone();
+                        self.bump(); // `::`
+
+                        // Consume the rest of the likely `foo<bar>::new()` or return at `foo<bar>`.
+                        match self.parse_expr() {
+                            Ok(_) => {
+                                // 99% certain that the suggestion is correct, continue parsing.
+                                err.emit();
+                                // FIXME: actually check that the two expressions in the binop are
+                                // paths and resynthesize new fn call expression instead of using
+                                // `ExprKind::Err` placeholder.
+                                mk_err_expr(self, inner_op.span.to(self.prev_token.span))
+                            }
+                            Err(mut expr_err) => {
+                                expr_err.cancel();
+                                // Not entirely sure now, but we bubble the error up with the
+                                // suggestion.
+                                *self = snapshot;
+                                Err(err)
+                            }
+                        }
+                    } else if token::OpenDelim(token::Paren) == self.token.kind {
+                        // We have high certainty that this was a bad turbofish at this point.
+                        // `foo< bar >(`
+                        suggest(&mut err);
+                        // Consume the fn call arguments.
+                        match self.consume_fn_args() {
+                            Err(()) => Err(err),
+                            Ok(()) => {
+                                err.emit();
+                                // FIXME: actually check that the two expressions in the binop are
+                                // paths and resynthesize new fn call expression instead of using
+                                // `ExprKind::Err` placeholder.
+                                mk_err_expr(self, inner_op.span.to(self.prev_token.span))
+                            }
+                        }
+                    } else {
+                        if !matches!(l1.kind, ExprKind::Lit(_))
+                            && !matches!(r1.kind, ExprKind::Lit(_))
+                        {
+                            // All we know is that this is `foo < bar >` and *nothing* else. Try to
+                            // be helpful, but don't attempt to recover.
+                            err.help(TURBOFISH);
+                            err.help("or use `(...)` if you meant to specify fn arguments");
+                        }
+
+                        // If it looks like a genuine attempt to chain operators (as opposed to a
+                        // misformatted turbofish, for instance), suggest a correct form.
+                        if self.attempt_chained_comparison_suggestion(&mut err, inner_op, outer_op)
+                        {
+                            err.emit();
+                            mk_err_expr(self, inner_op.span.to(self.prev_token.span))
+                        } else {
+                            // These cases cause too many knock-down errors, bail out (#61329).
+                            Err(err)
+                        }
+                    };
+                }
+                let recover =
+                    self.attempt_chained_comparison_suggestion(&mut err, inner_op, outer_op);
+                err.emit();
+                if recover {
+                    return mk_err_expr(self, inner_op.span.to(self.prev_token.span));
+                }
+            }
+            _ => {}
+        }
+        Ok(None)
+    }
+
+    fn consume_fn_args(&mut self) -> Result<(), ()> {
+        let snapshot = self.clone();
+        self.bump(); // `(`
+
+        // Consume the fn call arguments.
+        let modifiers =
+            [(token::OpenDelim(token::Paren), 1), (token::CloseDelim(token::Paren), -1)];
+        self.consume_tts(1, &modifiers[..]);
+
+        if self.token.kind == token::Eof {
+            // Not entirely sure that what we consumed were fn arguments, rollback.
+            *self = snapshot;
+            Err(())
+        } else {
+            // 99% certain that the suggestion is correct, continue parsing.
+            Ok(())
+        }
+    }
+
+    pub(super) fn maybe_report_ambiguous_plus(
+        &mut self,
+        allow_plus: AllowPlus,
+        impl_dyn_multi: bool,
+        ty: &Ty,
+    ) {
+        if matches!(allow_plus, AllowPlus::No) && impl_dyn_multi {
+            let sum_with_parens = format!("({})", pprust::ty_to_string(&ty));
+            self.struct_span_err(ty.span, "ambiguous `+` in a type")
+                .span_suggestion(
+                    ty.span,
+                    "use parentheses to disambiguate",
+                    sum_with_parens,
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        }
+    }
+
+    pub(super) fn maybe_recover_from_bad_type_plus(
+        &mut self,
+        allow_plus: AllowPlus,
+        ty: &Ty,
+    ) -> PResult<'a, ()> {
+        // Do not add `+` to expected tokens.
+        if matches!(allow_plus, AllowPlus::No) || !self.token.is_like_plus() {
+            return Ok(());
+        }
+
+        self.bump(); // `+`
+        let bounds = self.parse_generic_bounds(None)?;
+        let sum_span = ty.span.to(self.prev_token.span);
+
+        let mut err = struct_span_err!(
+            self.sess.span_diagnostic,
+            sum_span,
+            E0178,
+            "expected a path on the left-hand side of `+`, not `{}`",
+            pprust::ty_to_string(ty)
+        );
+
+        match ty.kind {
+            TyKind::Rptr(ref lifetime, ref mut_ty) => {
+                let sum_with_parens = pprust::to_string(|s| {
+                    s.s.word("&");
+                    s.print_opt_lifetime(lifetime);
+                    s.print_mutability(mut_ty.mutbl, false);
+                    s.popen();
+                    s.print_type(&mut_ty.ty);
+                    s.print_type_bounds(" +", &bounds);
+                    s.pclose()
+                });
+                err.span_suggestion(
+                    sum_span,
+                    "try adding parentheses",
+                    sum_with_parens,
+                    Applicability::MachineApplicable,
+                );
+            }
+            TyKind::Ptr(..) | TyKind::BareFn(..) => {
+                err.span_label(sum_span, "perhaps you forgot parentheses?");
+            }
+            _ => {
+                err.span_label(sum_span, "expected a path");
+            }
+        }
+        err.emit();
+        Ok(())
+    }
+
+    /// Tries to recover from associated item paths like `[T]::AssocItem` / `(T, U)::AssocItem`.
+    /// Attempts to convert the base expression/pattern/type into a type, parses the `::AssocItem`
+    /// tail, and combines them into a `<Ty>::AssocItem` expression/pattern/type.
+    pub(super) fn maybe_recover_from_bad_qpath<T: RecoverQPath>(
+        &mut self,
+        base: P<T>,
+        allow_recovery: bool,
+    ) -> PResult<'a, P<T>> {
+        // Do not add `::` to expected tokens.
+        if allow_recovery && self.token == token::ModSep {
+            if let Some(ty) = base.to_ty() {
+                return self.maybe_recover_from_bad_qpath_stage_2(ty.span, ty);
+            }
+        }
+        Ok(base)
+    }
+
+    /// Given an already parsed `Ty`, parses the `::AssocItem` tail and
+    /// combines them into a `<Ty>::AssocItem` expression/pattern/type.
+    pub(super) fn maybe_recover_from_bad_qpath_stage_2<T: RecoverQPath>(
+        &mut self,
+        ty_span: Span,
+        ty: P<Ty>,
+    ) -> PResult<'a, P<T>> {
+        self.expect(&token::ModSep)?;
+
+        let mut path = ast::Path { segments: Vec::new(), span: DUMMY_SP };
+        self.parse_path_segments(&mut path.segments, T::PATH_STYLE)?;
+        path.span = ty_span.to(self.prev_token.span);
+
+        let ty_str = self.span_to_snippet(ty_span).unwrap_or_else(|_| pprust::ty_to_string(&ty));
+        self.struct_span_err(path.span, "missing angle brackets in associated item path")
+            .span_suggestion(
+                // This is a best-effort recovery.
+                path.span,
+                "try",
+                format!("<{}>::{}", ty_str, pprust::path_to_string(&path)),
+                Applicability::MaybeIncorrect,
+            )
+            .emit();
+
+        let path_span = ty_span.shrink_to_hi(); // Use an empty path since `position == 0`.
+        Ok(P(T::recovered(Some(QSelf { ty, path_span, position: 0 }), path)))
+    }
+
+    pub(super) fn maybe_consume_incorrect_semicolon(&mut self, items: &[P<Item>]) -> bool {
+        if self.eat(&token::Semi) {
+            let mut err = self.struct_span_err(self.prev_token.span, "expected item, found `;`");
+            err.span_suggestion_short(
+                self.prev_token.span,
+                "remove this semicolon",
+                String::new(),
+                Applicability::MachineApplicable,
+            );
+            if !items.is_empty() {
+                let previous_item = &items[items.len() - 1];
+                let previous_item_kind_name = match previous_item.kind {
+                    // Say "braced struct" because tuple-structs and
+                    // braceless-empty-struct declarations do take a semicolon.
+                    ItemKind::Struct(..) => Some("braced struct"),
+                    ItemKind::Enum(..) => Some("enum"),
+                    ItemKind::Trait(..) => Some("trait"),
+                    ItemKind::Union(..) => Some("union"),
+                    _ => None,
+                };
+                if let Some(name) = previous_item_kind_name {
+                    err.help(&format!("{} declarations are not followed by a semicolon", name));
+                }
+            }
+            err.emit();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Creates a `DiagnosticBuilder` for an unexpected token `t` and tries to recover if it is a
+    /// closing delimiter.
+    pub(super) fn unexpected_try_recover(
+        &mut self,
+        t: &TokenKind,
+    ) -> PResult<'a, bool /* recovered */> {
+        let token_str = pprust::token_kind_to_string(t);
+        let this_token_str = super::token_descr(&self.token);
+        let (prev_sp, sp) = match (&self.token.kind, self.subparser_name) {
+            // Point at the end of the macro call when reaching end of macro arguments.
+            (token::Eof, Some(_)) => {
+                let sp = self.sess.source_map().next_point(self.token.span);
+                (sp, sp)
+            }
+            // We don't want to point at the following span after DUMMY_SP.
+            // This happens when the parser finds an empty TokenStream.
+            _ if self.prev_token.span == DUMMY_SP => (self.token.span, self.token.span),
+            // EOF, don't want to point at the following char, but rather the last token.
+            (token::Eof, None) => (self.prev_token.span, self.token.span),
+            _ => (self.prev_token.span.shrink_to_hi(), self.token.span),
+        };
+        let msg = format!(
+            "expected `{}`, found {}",
+            token_str,
+            match (&self.token.kind, self.subparser_name) {
+                (token::Eof, Some(origin)) => format!("end of {}", origin),
+                _ => this_token_str,
+            },
+        );
+        let mut err = self.struct_span_err(sp, &msg);
+        let label_exp = format!("expected `{}`", token_str);
+        match self.recover_closing_delimiter(&[t.clone()], err) {
+            Err(e) => err = e,
+            Ok(recovered) => {
+                return Ok(recovered);
+            }
+        }
+        let sm = self.sess.source_map();
+        if !sm.is_multiline(prev_sp.until(sp)) {
+            // When the spans are in the same line, it means that the only content
+            // between them is whitespace, point only at the found token.
+            err.span_label(sp, label_exp);
+        } else {
+            err.span_label(prev_sp, label_exp);
+            err.span_label(sp, "unexpected token");
+        }
+        Err(err)
+    }
+
+    pub(super) fn expect_semi(&mut self) -> PResult<'a, ()> {
+        if self.eat(&token::Semi) {
+            return Ok(());
+        }
+        let sm = self.sess.source_map();
+        let msg = format!("expected `;`, found {}", super::token_descr(&self.token));
+        let appl = Applicability::MachineApplicable;
+        if self.token.span == DUMMY_SP || self.prev_token.span == DUMMY_SP {
+            // Likely inside a macro, can't provide meaningful suggestions.
+            return self.expect(&token::Semi).map(drop);
+        } else if !sm.is_multiline(self.prev_token.span.until(self.token.span)) {
+            // The current token is in the same line as the prior token, not recoverable.
+        } else if [token::Comma, token::Colon].contains(&self.token.kind)
+            && self.prev_token.kind == token::CloseDelim(token::Paren)
+        {
+            // Likely typo: The current token is on a new line and is expected to be
+            // `.`, `;`, `?`, or an operator after a close delimiter token.
+            //
+            // let a = std::process::Command::new("echo")
+            //         .arg("1")
+            //         ,arg("2")
+            //         ^
+            // https://github.com/rust-lang/rust/issues/72253
+            self.expect(&token::Semi)?;
+            return Ok(());
+        } else if self.look_ahead(1, |t| {
+            t == &token::CloseDelim(token::Brace) || t.can_begin_expr() && t.kind != token::Colon
+        }) && [token::Comma, token::Colon].contains(&self.token.kind)
+        {
+            // Likely typo: `,` → `;` or `:` → `;`. This is triggered if the current token is
+            // either `,` or `:`, and the next token could either start a new statement or is a
+            // block close. For example:
+            //
+            //   let x = 32:
+            //   let y = 42;
+            self.bump();
+            let sp = self.prev_token.span;
+            self.struct_span_err(sp, &msg)
+                .span_suggestion_short(sp, "change this to `;`", ";".to_string(), appl)
+                .emit();
+            return Ok(());
+        } else if self.look_ahead(0, |t| {
+            t == &token::CloseDelim(token::Brace)
+                || (
+                    t.can_begin_expr() && t != &token::Semi && t != &token::Pound
+                    // Avoid triggering with too many trailing `#` in raw string.
+                )
+        }) {
+            // Missing semicolon typo. This is triggered if the next token could either start a
+            // new statement or is a block close. For example:
+            //
+            //   let x = 32
+            //   let y = 42;
+            let sp = self.prev_token.span.shrink_to_hi();
+            self.struct_span_err(sp, &msg)
+                .span_label(self.token.span, "unexpected token")
+                .span_suggestion_short(sp, "add `;` here", ";".to_string(), appl)
+                .emit();
+            return Ok(());
+        }
+        self.expect(&token::Semi).map(drop) // Error unconditionally
+    }
+
+    /// Consumes alternative await syntaxes like `await!(<expr>)`, `await <expr>`,
+    /// `await? <expr>`, `await(<expr>)`, and `await { <expr> }`.
+    pub(super) fn recover_incorrect_await_syntax(
+        &mut self,
+        lo: Span,
+        await_sp: Span,
+        attrs: AttrVec,
+    ) -> PResult<'a, P<Expr>> {
+        let (hi, expr, is_question) = if self.token == token::Not {
+            // Handle `await!(<expr>)`.
+            self.recover_await_macro()?
+        } else {
+            self.recover_await_prefix(await_sp)?
+        };
+        let sp = self.error_on_incorrect_await(lo, hi, &expr, is_question);
+        let expr = self.mk_expr(lo.to(sp), ExprKind::Await(expr), attrs);
+        self.maybe_recover_from_bad_qpath(expr, true)
+    }
+
+    fn recover_await_macro(&mut self) -> PResult<'a, (Span, P<Expr>, bool)> {
+        self.expect(&token::Not)?;
+        self.expect(&token::OpenDelim(token::Paren))?;
+        let expr = self.parse_expr()?;
+        self.expect(&token::CloseDelim(token::Paren))?;
+        Ok((self.prev_token.span, expr, false))
+    }
+
+    fn recover_await_prefix(&mut self, await_sp: Span) -> PResult<'a, (Span, P<Expr>, bool)> {
+        let is_question = self.eat(&token::Question); // Handle `await? <expr>`.
+        let expr = if self.token == token::OpenDelim(token::Brace) {
+            // Handle `await { <expr> }`.
+            // This needs to be handled separatedly from the next arm to avoid
+            // interpreting `await { <expr> }?` as `<expr>?.await`.
+            self.parse_block_expr(None, self.token.span, BlockCheckMode::Default, AttrVec::new())
+        } else {
+            self.parse_expr()
+        }
+        .map_err(|mut err| {
+            err.span_label(await_sp, "while parsing this incorrect await expression");
+            err
+        })?;
+        Ok((expr.span, expr, is_question))
+    }
+
+    fn error_on_incorrect_await(&self, lo: Span, hi: Span, expr: &Expr, is_question: bool) -> Span {
+        let expr_str =
+            self.span_to_snippet(expr.span).unwrap_or_else(|_| pprust::expr_to_string(&expr));
+        let suggestion = format!("{}.await{}", expr_str, if is_question { "?" } else { "" });
+        let sp = lo.to(hi);
+        let app = match expr.kind {
+            ExprKind::Try(_) => Applicability::MaybeIncorrect, // `await <expr>?`
+            _ => Applicability::MachineApplicable,
+        };
+        self.struct_span_err(sp, "incorrect use of `await`")
+            .span_suggestion(sp, "`await` is a postfix operation", suggestion, app)
+            .emit();
+        sp
+    }
+
+    /// If encountering `future.await()`, consumes and emits an error.
+    pub(super) fn recover_from_await_method_call(&mut self) {
+        if self.token == token::OpenDelim(token::Paren)
+            && self.look_ahead(1, |t| t == &token::CloseDelim(token::Paren))
+        {
+            // future.await()
+            let lo = self.token.span;
+            self.bump(); // (
+            let sp = lo.to(self.token.span);
+            self.bump(); // )
+            self.struct_span_err(sp, "incorrect use of `await`")
+                .span_suggestion(
+                    sp,
+                    "`await` is not a method call, remove the parentheses",
+                    String::new(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        }
+    }
+
+    pub(super) fn try_macro_suggestion(&mut self) -> PResult<'a, P<Expr>> {
+        let is_try = self.token.is_keyword(kw::Try);
+        let is_questionmark = self.look_ahead(1, |t| t == &token::Not); //check for !
+        let is_open = self.look_ahead(2, |t| t == &token::OpenDelim(token::Paren)); //check for (
+
+        if is_try && is_questionmark && is_open {
+            let lo = self.token.span;
+            self.bump(); //remove try
+            self.bump(); //remove !
+            let try_span = lo.to(self.token.span); //we take the try!( span
+            self.bump(); //remove (
+            let is_empty = self.token == token::CloseDelim(token::Paren); //check if the block is empty
+            self.consume_block(token::Paren, ConsumeClosingDelim::No); //eat the block
+            let hi = self.token.span;
+            self.bump(); //remove )
+            let mut err = self.struct_span_err(lo.to(hi), "use of deprecated `try` macro");
+            err.note("in the 2018 edition `try` is a reserved keyword, and the `try!()` macro is deprecated");
+            let prefix = if is_empty { "" } else { "alternatively, " };
+            if !is_empty {
+                err.multipart_suggestion(
+                    "you can use the `?` operator instead",
+                    vec![(try_span, "".to_owned()), (hi, "?".to_owned())],
+                    Applicability::MachineApplicable,
+                );
+            }
+            err.span_suggestion(lo.shrink_to_lo(), &format!("{}you can still access the deprecated `try!()` macro using the \"raw identifier\" syntax", prefix), "r#".to_string(), Applicability::MachineApplicable);
+            err.emit();
+            Ok(self.mk_expr_err(lo.to(hi)))
+        } else {
+            Err(self.expected_expression_found()) // The user isn't trying to invoke the try! macro
+        }
+    }
+
+    /// Recovers a situation like `for ( $pat in $expr )`
+    /// and suggest writing `for $pat in $expr` instead.
+    ///
+    /// This should be called before parsing the `$block`.
+    pub(super) fn recover_parens_around_for_head(
+        &mut self,
+        pat: P<Pat>,
+        expr: &Expr,
+        begin_paren: Option<Span>,
+    ) -> P<Pat> {
+        match (&self.token.kind, begin_paren) {
+            (token::CloseDelim(token::Paren), Some(begin_par_sp)) => {
+                self.bump();
+
+                let pat_str = self
+                    // Remove the `(` from the span of the pattern:
+                    .span_to_snippet(pat.span.trim_start(begin_par_sp).unwrap())
+                    .unwrap_or_else(|_| pprust::pat_to_string(&pat));
+
+                self.struct_span_err(self.prev_token.span, "unexpected closing `)`")
+                    .span_label(begin_par_sp, "opening `(`")
+                    .span_suggestion(
+                        begin_par_sp.to(self.prev_token.span),
+                        "remove parenthesis in `for` loop",
+                        format!("{} in {}", pat_str, pprust::expr_to_string(&expr)),
+                        // With e.g. `for (x) in y)` this would replace `(x) in y)`
+                        // with `x) in y)` which is syntactically invalid.
+                        // However, this is prevented before we get here.
+                        Applicability::MachineApplicable,
+                    )
+                    .emit();
+
+                // Unwrap `(pat)` into `pat` to avoid the `unused_parens` lint.
+                pat.and_then(|pat| match pat.kind {
+                    PatKind::Paren(pat) => pat,
+                    _ => P(pat),
+                })
+            }
+            _ => pat,
+        }
+    }
+
+    pub(super) fn could_ascription_be_path(&self, node: &ast::ExprKind) -> bool {
+        (self.token == token::Lt && // `foo:<bar`, likely a typoed turbofish.
+            self.look_ahead(1, |t| t.is_ident() && !t.is_reserved_ident()))
+            || self.token.is_ident() &&
+            match node {
+                // `foo::` → `foo:` or `foo.bar::` → `foo.bar:`
+                ast::ExprKind::Path(..) | ast::ExprKind::Field(..) => true,
+                _ => false,
+            } &&
+            !self.token.is_reserved_ident() &&           // v `foo:bar(baz)`
+            self.look_ahead(1, |t| t == &token::OpenDelim(token::Paren))
+            || self.look_ahead(1, |t| t == &token::OpenDelim(token::Brace)) // `foo:bar {`
+            || self.look_ahead(1, |t| t == &token::Colon) &&     // `foo:bar::<baz`
+            self.look_ahead(2, |t| t == &token::Lt) &&
+            self.look_ahead(3, |t| t.is_ident())
+            || self.look_ahead(1, |t| t == &token::Colon) &&  // `foo:bar:baz`
+            self.look_ahead(2, |t| t.is_ident())
+            || self.look_ahead(1, |t| t == &token::ModSep)
+                && (self.look_ahead(2, |t| t.is_ident()) ||   // `foo:bar::baz`
+            self.look_ahead(2, |t| t == &token::Lt)) // `foo:bar::<baz>`
+    }
+
+    pub(super) fn recover_seq_parse_error(
+        &mut self,
+        delim: token::DelimToken,
+        lo: Span,
+        result: PResult<'a, P<Expr>>,
+    ) -> P<Expr> {
+        match result {
+            Ok(x) => x,
+            Err(mut err) => {
+                err.emit();
+                // Recover from parse error, callers expect the closing delim to be consumed.
+                self.consume_block(delim, ConsumeClosingDelim::Yes);
+                self.mk_expr(lo.to(self.prev_token.span), ExprKind::Err, AttrVec::new())
+            }
+        }
+    }
+
+    pub(super) fn recover_closing_delimiter(
+        &mut self,
+        tokens: &[TokenKind],
+        mut err: DiagnosticBuilder<'a>,
+    ) -> PResult<'a, bool> {
+        let mut pos = None;
+        // We want to use the last closing delim that would apply.
+        for (i, unmatched) in self.unclosed_delims.iter().enumerate().rev() {
+            if tokens.contains(&token::CloseDelim(unmatched.expected_delim))
+                && Some(self.token.span) > unmatched.unclosed_span
+            {
+                pos = Some(i);
+            }
+        }
+        match pos {
+            Some(pos) => {
+                // Recover and assume that the detected unclosed delimiter was meant for
+                // this location. Emit the diagnostic and act as if the delimiter was
+                // present for the parser's sake.
+
+                // Don't attempt to recover from this unclosed delimiter more than once.
+                let unmatched = self.unclosed_delims.remove(pos);
+                let delim = TokenType::Token(token::CloseDelim(unmatched.expected_delim));
+                if unmatched.found_delim.is_none() {
+                    // We encountered `Eof`, set this fact here to avoid complaining about missing
+                    // `fn main()` when we found place to suggest the closing brace.
+                    *self.sess.reached_eof.borrow_mut() = true;
+                }
+
+                // We want to suggest the inclusion of the closing delimiter where it makes
+                // the most sense, which is immediately after the last token:
+                //
+                //  {foo(bar {}}
+                //      -      ^
+                //      |      |
+                //      |      help: `)` may belong here
+                //      |
+                //      unclosed delimiter
+                if let Some(sp) = unmatched.unclosed_span {
+                    err.span_label(sp, "unclosed delimiter");
+                }
+                // Backticks should be removed to apply suggestions.
+                let mut delim = delim.to_string();
+                delim.retain(|c| c != '`');
+                err.span_suggestion_short(
+                    self.prev_token.span.shrink_to_hi(),
+                    &format!("`{}` may belong here", delim),
+                    delim,
+                    Applicability::MaybeIncorrect,
+                );
+                if unmatched.found_delim.is_none() {
+                    // Encountered `Eof` when lexing blocks. Do not recover here to avoid knockdown
+                    // errors which would be emitted elsewhere in the parser and let other error
+                    // recovery consume the rest of the file.
+                    Err(err)
+                } else {
+                    err.emit();
+                    self.expected_tokens.clear(); // Reduce the number of errors.
+                    Ok(true)
+                }
+            }
+            _ => Err(err),
+        }
+    }
+
+    /// Eats tokens until we can be relatively sure we reached the end of the
+    /// statement. This is something of a best-effort heuristic.
+    ///
+    /// We terminate when we find an unmatched `}` (without consuming it).
+    pub(super) fn recover_stmt(&mut self) {
+        self.recover_stmt_(SemiColonMode::Ignore, BlockMode::Ignore)
+    }
+
+    /// If `break_on_semi` is `Break`, then we will stop consuming tokens after
+    /// finding (and consuming) a `;` outside of `{}` or `[]` (note that this is
+    /// approximate -- it can mean we break too early due to macros, but that
+    /// should only lead to sub-optimal recovery, not inaccurate parsing).
+    ///
+    /// If `break_on_block` is `Break`, then we will stop consuming tokens
+    /// after finding (and consuming) a brace-delimited block.
+    pub(super) fn recover_stmt_(
+        &mut self,
+        break_on_semi: SemiColonMode,
+        break_on_block: BlockMode,
+    ) {
+        let mut brace_depth = 0;
+        let mut bracket_depth = 0;
+        let mut in_block = false;
+        debug!("recover_stmt_ enter loop (semi={:?}, block={:?})", break_on_semi, break_on_block);
+        loop {
+            debug!("recover_stmt_ loop {:?}", self.token);
+            match self.token.kind {
+                token::OpenDelim(token::DelimToken::Brace) => {
+                    brace_depth += 1;
+                    self.bump();
+                    if break_on_block == BlockMode::Break && brace_depth == 1 && bracket_depth == 0
+                    {
+                        in_block = true;
+                    }
+                }
+                token::OpenDelim(token::DelimToken::Bracket) => {
+                    bracket_depth += 1;
+                    self.bump();
+                }
+                token::CloseDelim(token::DelimToken::Brace) => {
+                    if brace_depth == 0 {
+                        debug!("recover_stmt_ return - close delim {:?}", self.token);
+                        break;
+                    }
+                    brace_depth -= 1;
+                    self.bump();
+                    if in_block && bracket_depth == 0 && brace_depth == 0 {
+                        debug!("recover_stmt_ return - block end {:?}", self.token);
+                        break;
+                    }
+                }
+                token::CloseDelim(token::DelimToken::Bracket) => {
+                    bracket_depth -= 1;
+                    if bracket_depth < 0 {
+                        bracket_depth = 0;
+                    }
+                    self.bump();
+                }
+                token::Eof => {
+                    debug!("recover_stmt_ return - Eof");
+                    break;
+                }
+                token::Semi => {
+                    self.bump();
+                    if break_on_semi == SemiColonMode::Break
+                        && brace_depth == 0
+                        && bracket_depth == 0
+                    {
+                        debug!("recover_stmt_ return - Semi");
+                        break;
+                    }
+                }
+                token::Comma
+                    if break_on_semi == SemiColonMode::Comma
+                        && brace_depth == 0
+                        && bracket_depth == 0 =>
+                {
+                    debug!("recover_stmt_ return - Semi");
+                    break;
+                }
+                _ => self.bump(),
+            }
+        }
+    }
+
+    pub(super) fn check_for_for_in_in_typo(&mut self, in_span: Span) {
+        if self.eat_keyword(kw::In) {
+            // a common typo: `for _ in in bar {}`
+            self.struct_span_err(self.prev_token.span, "expected iterable, found keyword `in`")
+                .span_suggestion_short(
+                    in_span.until(self.prev_token.span),
+                    "remove the duplicated `in`",
+                    String::new(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        }
+    }
+
+    pub(super) fn expected_semi_or_open_brace<T>(&mut self) -> PResult<'a, T> {
+        let token_str = super::token_descr(&self.token);
+        let msg = &format!("expected `;` or `{{`, found {}", token_str);
+        let mut err = self.struct_span_err(self.token.span, msg);
+        err.span_label(self.token.span, "expected `;` or `{`");
+        Err(err)
+    }
+
+    pub(super) fn eat_incorrect_doc_comment_for_param_type(&mut self) {
+        if let token::DocComment(..) = self.token.kind {
+            self.struct_span_err(
+                self.token.span,
+                "documentation comments cannot be applied to a function parameter's type",
+            )
+            .span_label(self.token.span, "doc comments are not allowed here")
+            .emit();
+            self.bump();
+        } else if self.token == token::Pound
+            && self.look_ahead(1, |t| *t == token::OpenDelim(token::Bracket))
+        {
+            let lo = self.token.span;
+            // Skip every token until next possible arg.
+            while self.token != token::CloseDelim(token::Bracket) {
+                self.bump();
+            }
+            let sp = lo.to(self.token.span);
+            self.bump();
+            self.struct_span_err(sp, "attributes cannot be applied to a function parameter's type")
+                .span_label(sp, "attributes are not allowed here")
+                .emit();
+        }
+    }
+
+    pub(super) fn parameter_without_type(
+        &mut self,
+        err: &mut DiagnosticBuilder<'_>,
+        pat: P<ast::Pat>,
+        require_name: bool,
+        first_param: bool,
+    ) -> Option<Ident> {
+        // If we find a pattern followed by an identifier, it could be an (incorrect)
+        // C-style parameter declaration.
+        if self.check_ident()
+            && self.look_ahead(1, |t| *t == token::Comma || *t == token::CloseDelim(token::Paren))
+        {
+            // `fn foo(String s) {}`
+            let ident = self.parse_ident().unwrap();
+            let span = pat.span.with_hi(ident.span.hi());
+
+            err.span_suggestion(
+                span,
+                "declare the type after the parameter binding",
+                String::from("<identifier>: <type>"),
+                Applicability::HasPlaceholders,
+            );
+            return Some(ident);
+        } else if let PatKind::Ident(_, ident, _) = pat.kind {
+            if require_name
+                && (self.token == token::Comma
+                    || self.token == token::Lt
+                    || self.token == token::CloseDelim(token::Paren))
+            {
+                // `fn foo(a, b) {}`, `fn foo(a<x>, b<y>) {}` or `fn foo(usize, usize) {}`
+                if first_param {
+                    err.span_suggestion(
+                        pat.span,
+                        "if this is a `self` type, give it a parameter name",
+                        format!("self: {}", ident),
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+                // Avoid suggesting that `fn foo(HashMap<u32>)` is fixed with a change to
+                // `fn foo(HashMap: TypeName<u32>)`.
+                if self.token != token::Lt {
+                    err.span_suggestion(
+                        pat.span,
+                        "if this is a parameter name, give it a type",
+                        format!("{}: TypeName", ident),
+                        Applicability::HasPlaceholders,
+                    );
+                }
+                err.span_suggestion(
+                    pat.span,
+                    "if this is a type, explicitly ignore the parameter name",
+                    format!("_: {}", ident),
+                    Applicability::MachineApplicable,
+                );
+                err.note("anonymous parameters are removed in the 2018 edition (see RFC 1685)");
+
+                // Don't attempt to recover by using the `X` in `X<Y>` as the parameter name.
+                return if self.token == token::Lt { None } else { Some(ident) };
+            }
+        }
+        None
+    }
+
+    pub(super) fn recover_arg_parse(&mut self) -> PResult<'a, (P<ast::Pat>, P<ast::Ty>)> {
+        let pat = self.parse_pat(Some("argument name"))?;
+        self.expect(&token::Colon)?;
+        let ty = self.parse_ty()?;
+
+        struct_span_err!(
+            self.diagnostic(),
+            pat.span,
+            E0642,
+            "patterns aren't allowed in methods without bodies",
+        )
+        .span_suggestion_short(
+            pat.span,
+            "give this argument a name or use an underscore to ignore it",
+            "_".to_owned(),
+            Applicability::MachineApplicable,
+        )
+        .emit();
+
+        // Pretend the pattern is `_`, to avoid duplicate errors from AST validation.
+        let pat =
+            P(Pat { kind: PatKind::Wild, span: pat.span, id: ast::DUMMY_NODE_ID, tokens: None });
+        Ok((pat, ty))
+    }
+
+    pub(super) fn recover_bad_self_param(&mut self, mut param: Param) -> PResult<'a, Param> {
+        let sp = param.pat.span;
+        param.ty.kind = TyKind::Err;
+        self.struct_span_err(sp, "unexpected `self` parameter in function")
+            .span_label(sp, "must be the first parameter of an associated function")
+            .emit();
+        Ok(param)
+    }
+
+    pub(super) fn consume_block(
+        &mut self,
+        delim: token::DelimToken,
+        consume_close: ConsumeClosingDelim,
+    ) {
+        let mut brace_depth = 0;
+        loop {
+            if self.eat(&token::OpenDelim(delim)) {
+                brace_depth += 1;
+            } else if self.check(&token::CloseDelim(delim)) {
+                if brace_depth == 0 {
+                    if let ConsumeClosingDelim::Yes = consume_close {
+                        // Some of the callers of this method expect to be able to parse the
+                        // closing delimiter themselves, so we leave it alone. Otherwise we advance
+                        // the parser.
+                        self.bump();
+                    }
+                    return;
+                } else {
+                    self.bump();
+                    brace_depth -= 1;
+                    continue;
+                }
+            } else if self.token == token::Eof || self.eat(&token::CloseDelim(token::NoDelim)) {
+                return;
+            } else {
+                self.bump();
+            }
+        }
+    }
+
+    pub(super) fn expected_expression_found(&self) -> DiagnosticBuilder<'a> {
+        let (span, msg) = match (&self.token.kind, self.subparser_name) {
+            (&token::Eof, Some(origin)) => {
+                let sp = self.sess.source_map().next_point(self.token.span);
+                (sp, format!("expected expression, found end of {}", origin))
+            }
+            _ => (
+                self.token.span,
+                format!("expected expression, found {}", super::token_descr(&self.token),),
+            ),
+        };
+        let mut err = self.struct_span_err(span, &msg);
+        let sp = self.sess.source_map().start_point(self.token.span);
+        if let Some(sp) = self.sess.ambiguous_block_expr_parse.borrow().get(&sp) {
+            self.sess.expr_parentheses_needed(&mut err, *sp, None);
+        }
+        err.span_label(span, "expected expression");
+        err
+    }
+
+    fn consume_tts(
+        &mut self,
+        mut acc: i64, // `i64` because malformed code can have more closing delims than opening.
+        // Not using `FxHashMap` due to `token::TokenKind: !Eq + !Hash`.
+        modifier: &[(token::TokenKind, i64)],
+    ) {
+        while acc > 0 {
+            if let Some((_, val)) = modifier.iter().find(|(t, _)| *t == self.token.kind) {
+                acc += *val;
+            }
+            if self.token.kind == token::Eof {
+                break;
+            }
+            self.bump();
+        }
+    }
+
+    /// Replace duplicated recovered parameters with `_` pattern to avoid unnecessary errors.
+    ///
+    /// This is necessary because at this point we don't know whether we parsed a function with
+    /// anonymous parameters or a function with names but no types. In order to minimize
+    /// unnecessary errors, we assume the parameters are in the shape of `fn foo(a, b, c)` where
+    /// the parameters are *names* (so we don't emit errors about not being able to find `b` in
+    /// the local scope), but if we find the same name multiple times, like in `fn foo(i8, i8)`,
+    /// we deduplicate them to not complain about duplicated parameter names.
+    pub(super) fn deduplicate_recovered_params_names(&self, fn_inputs: &mut Vec<Param>) {
+        let mut seen_inputs = FxHashSet::default();
+        for input in fn_inputs.iter_mut() {
+            let opt_ident = if let (PatKind::Ident(_, ident, _), TyKind::Err) =
+                (&input.pat.kind, &input.ty.kind)
+            {
+                Some(*ident)
+            } else {
+                None
+            };
+            if let Some(ident) = opt_ident {
+                if seen_inputs.contains(&ident) {
+                    input.pat.kind = PatKind::Wild;
+                }
+                seen_inputs.insert(ident);
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
new file mode 100644
index 00000000000..f022c628fe2
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -0,0 +1,2293 @@
+use super::pat::{GateOr, PARAM_EXPECTED};
+use super::ty::{AllowPlus, RecoverQPath};
+use super::{BlockMode, Parser, PathStyle, Restrictions, TokenType};
+use super::{SemiColonMode, SeqSep, TokenExpectType};
+use crate::maybe_recover_from_interpolated_ty_qpath;
+
+use rustc_ast::ptr::P;
+use rustc_ast::token::{self, Token, TokenKind};
+use rustc_ast::util::classify;
+use rustc_ast::util::literal::LitError;
+use rustc_ast::util::parser::{prec_let_scrutinee_needs_par, AssocOp, Fixity};
+use rustc_ast::{self as ast, AttrStyle, AttrVec, CaptureBy, Field, Lit, UnOp, DUMMY_NODE_ID};
+use rustc_ast::{AnonConst, BinOp, BinOpKind, FnDecl, FnRetTy, MacCall, Param, Ty, TyKind};
+use rustc_ast::{Arm, Async, BlockCheckMode, Expr, ExprKind, Label, Movability, RangeLimits};
+use rustc_ast_pretty::pprust;
+use rustc_errors::{Applicability, DiagnosticBuilder, PResult};
+use rustc_span::source_map::{self, Span, Spanned};
+use rustc_span::symbol::{kw, sym, Ident, Symbol};
+use std::mem;
+use tracing::debug;
+
+/// Possibly accepts an `token::Interpolated` expression (a pre-parsed expression
+/// dropped into the token stream, which happens while parsing the result of
+/// macro expansion). Placement of these is not as complex as I feared it would
+/// be. The important thing is to make sure that lookahead doesn't balk at
+/// `token::Interpolated` tokens.
+macro_rules! maybe_whole_expr {
+    ($p:expr) => {
+        if let token::Interpolated(nt) = &$p.token.kind {
+            match &**nt {
+                token::NtExpr(e) | token::NtLiteral(e) => {
+                    let e = e.clone();
+                    $p.bump();
+                    return Ok(e);
+                }
+                token::NtPath(path) => {
+                    let path = path.clone();
+                    $p.bump();
+                    return Ok($p.mk_expr(
+                        $p.token.span,
+                        ExprKind::Path(None, path),
+                        AttrVec::new(),
+                    ));
+                }
+                token::NtBlock(block) => {
+                    let block = block.clone();
+                    $p.bump();
+                    return Ok($p.mk_expr(
+                        $p.token.span,
+                        ExprKind::Block(block, None),
+                        AttrVec::new(),
+                    ));
+                }
+                _ => {}
+            };
+        }
+    };
+}
+
+#[derive(Debug)]
+pub(super) enum LhsExpr {
+    NotYetParsed,
+    AttributesParsed(AttrVec),
+    AlreadyParsed(P<Expr>),
+}
+
+impl From<Option<AttrVec>> for LhsExpr {
+    /// Converts `Some(attrs)` into `LhsExpr::AttributesParsed(attrs)`
+    /// and `None` into `LhsExpr::NotYetParsed`.
+    ///
+    /// This conversion does not allocate.
+    fn from(o: Option<AttrVec>) -> Self {
+        if let Some(attrs) = o { LhsExpr::AttributesParsed(attrs) } else { LhsExpr::NotYetParsed }
+    }
+}
+
+impl From<P<Expr>> for LhsExpr {
+    /// Converts the `expr: P<Expr>` into `LhsExpr::AlreadyParsed(expr)`.
+    ///
+    /// This conversion does not allocate.
+    fn from(expr: P<Expr>) -> Self {
+        LhsExpr::AlreadyParsed(expr)
+    }
+}
+
+impl<'a> Parser<'a> {
+    /// Parses an expression.
+    #[inline]
+    pub fn parse_expr(&mut self) -> PResult<'a, P<Expr>> {
+        self.parse_expr_res(Restrictions::empty(), None)
+    }
+
+    pub(super) fn parse_anon_const_expr(&mut self) -> PResult<'a, AnonConst> {
+        self.parse_expr().map(|value| AnonConst { id: DUMMY_NODE_ID, value })
+    }
+
+    fn parse_expr_catch_underscore(&mut self) -> PResult<'a, P<Expr>> {
+        match self.parse_expr() {
+            Ok(expr) => Ok(expr),
+            Err(mut err) => match self.token.ident() {
+                Some((Ident { name: kw::Underscore, .. }, false))
+                    if self.look_ahead(1, |t| t == &token::Comma) =>
+                {
+                    // Special-case handling of `foo(_, _, _)`
+                    err.emit();
+                    self.bump();
+                    Ok(self.mk_expr(self.prev_token.span, ExprKind::Err, AttrVec::new()))
+                }
+                _ => Err(err),
+            },
+        }
+    }
+
+    /// Parses a sequence of expressions delimited by parentheses.
+    fn parse_paren_expr_seq(&mut self) -> PResult<'a, Vec<P<Expr>>> {
+        self.parse_paren_comma_seq(|p| p.parse_expr_catch_underscore()).map(|(r, _)| r)
+    }
+
+    /// Parses an expression, subject to the given restrictions.
+    #[inline]
+    pub(super) fn parse_expr_res(
+        &mut self,
+        r: Restrictions,
+        already_parsed_attrs: Option<AttrVec>,
+    ) -> PResult<'a, P<Expr>> {
+        self.with_res(r, |this| this.parse_assoc_expr(already_parsed_attrs))
+    }
+
+    /// Parses an associative expression.
+    ///
+    /// This parses an expression accounting for associativity and precedence of the operators in
+    /// the expression.
+    #[inline]
+    fn parse_assoc_expr(&mut self, already_parsed_attrs: Option<AttrVec>) -> PResult<'a, P<Expr>> {
+        self.parse_assoc_expr_with(0, already_parsed_attrs.into())
+    }
+
+    /// Parses an associative expression with operators of at least `min_prec` precedence.
+    pub(super) fn parse_assoc_expr_with(
+        &mut self,
+        min_prec: usize,
+        lhs: LhsExpr,
+    ) -> PResult<'a, P<Expr>> {
+        let mut lhs = if let LhsExpr::AlreadyParsed(expr) = lhs {
+            expr
+        } else {
+            let attrs = match lhs {
+                LhsExpr::AttributesParsed(attrs) => Some(attrs),
+                _ => None,
+            };
+            if [token::DotDot, token::DotDotDot, token::DotDotEq].contains(&self.token.kind) {
+                return self.parse_prefix_range_expr(attrs);
+            } else {
+                self.parse_prefix_expr(attrs)?
+            }
+        };
+        let last_type_ascription_set = self.last_type_ascription.is_some();
+
+        if !self.should_continue_as_assoc_expr(&lhs) {
+            self.last_type_ascription = None;
+            return Ok(lhs);
+        }
+
+        self.expected_tokens.push(TokenType::Operator);
+        while let Some(op) = self.check_assoc_op() {
+            // Adjust the span for interpolated LHS to point to the `$lhs` token
+            // and not to what it refers to.
+            let lhs_span = match self.prev_token.kind {
+                TokenKind::Interpolated(..) => self.prev_token.span,
+                _ => lhs.span,
+            };
+
+            let cur_op_span = self.token.span;
+            let restrictions = if op.node.is_assign_like() {
+                self.restrictions & Restrictions::NO_STRUCT_LITERAL
+            } else {
+                self.restrictions
+            };
+            let prec = op.node.precedence();
+            if prec < min_prec {
+                break;
+            }
+            // Check for deprecated `...` syntax
+            if self.token == token::DotDotDot && op.node == AssocOp::DotDotEq {
+                self.err_dotdotdot_syntax(self.token.span);
+            }
+
+            if self.token == token::LArrow {
+                self.err_larrow_operator(self.token.span);
+            }
+
+            self.bump();
+            if op.node.is_comparison() {
+                if let Some(expr) = self.check_no_chained_comparison(&lhs, &op)? {
+                    return Ok(expr);
+                }
+            }
+
+            if (op.node == AssocOp::Equal || op.node == AssocOp::NotEqual)
+                && self.token.kind == token::Eq
+                && self.prev_token.span.hi() == self.token.span.lo()
+            {
+                // Look for JS' `===` and `!==` and recover 😇
+                let sp = op.span.to(self.token.span);
+                let sugg = match op.node {
+                    AssocOp::Equal => "==",
+                    AssocOp::NotEqual => "!=",
+                    _ => unreachable!(),
+                };
+                self.struct_span_err(sp, &format!("invalid comparison operator `{}=`", sugg))
+                    .span_suggestion_short(
+                        sp,
+                        &format!("`{s}=` is not a valid comparison operator, use `{s}`", s = sugg),
+                        sugg.to_string(),
+                        Applicability::MachineApplicable,
+                    )
+                    .emit();
+                self.bump();
+            }
+
+            let op = op.node;
+            // Special cases:
+            if op == AssocOp::As {
+                lhs = self.parse_assoc_op_cast(lhs, lhs_span, ExprKind::Cast)?;
+                continue;
+            } else if op == AssocOp::Colon {
+                lhs = self.parse_assoc_op_ascribe(lhs, lhs_span)?;
+                continue;
+            } else if op == AssocOp::DotDot || op == AssocOp::DotDotEq {
+                // If we didn’t have to handle `x..`/`x..=`, it would be pretty easy to
+                // generalise it to the Fixity::None code.
+                lhs = self.parse_range_expr(prec, lhs, op, cur_op_span)?;
+                break;
+            }
+
+            let fixity = op.fixity();
+            let prec_adjustment = match fixity {
+                Fixity::Right => 0,
+                Fixity::Left => 1,
+                // We currently have no non-associative operators that are not handled above by
+                // the special cases. The code is here only for future convenience.
+                Fixity::None => 1,
+            };
+            let rhs = self.with_res(restrictions - Restrictions::STMT_EXPR, |this| {
+                this.parse_assoc_expr_with(prec + prec_adjustment, LhsExpr::NotYetParsed)
+            })?;
+
+            // Make sure that the span of the parent node is larger than the span of lhs and rhs,
+            // including the attributes.
+            let lhs_span =
+                lhs.attrs.iter().find(|a| a.style == AttrStyle::Outer).map_or(lhs_span, |a| a.span);
+            let span = lhs_span.to(rhs.span);
+            lhs = match op {
+                AssocOp::Add
+                | AssocOp::Subtract
+                | AssocOp::Multiply
+                | AssocOp::Divide
+                | AssocOp::Modulus
+                | AssocOp::LAnd
+                | AssocOp::LOr
+                | AssocOp::BitXor
+                | AssocOp::BitAnd
+                | AssocOp::BitOr
+                | AssocOp::ShiftLeft
+                | AssocOp::ShiftRight
+                | AssocOp::Equal
+                | AssocOp::Less
+                | AssocOp::LessEqual
+                | AssocOp::NotEqual
+                | AssocOp::Greater
+                | AssocOp::GreaterEqual => {
+                    let ast_op = op.to_ast_binop().unwrap();
+                    let binary = self.mk_binary(source_map::respan(cur_op_span, ast_op), lhs, rhs);
+                    self.mk_expr(span, binary, AttrVec::new())
+                }
+                AssocOp::Assign => {
+                    self.mk_expr(span, ExprKind::Assign(lhs, rhs, cur_op_span), AttrVec::new())
+                }
+                AssocOp::AssignOp(k) => {
+                    let aop = match k {
+                        token::Plus => BinOpKind::Add,
+                        token::Minus => BinOpKind::Sub,
+                        token::Star => BinOpKind::Mul,
+                        token::Slash => BinOpKind::Div,
+                        token::Percent => BinOpKind::Rem,
+                        token::Caret => BinOpKind::BitXor,
+                        token::And => BinOpKind::BitAnd,
+                        token::Or => BinOpKind::BitOr,
+                        token::Shl => BinOpKind::Shl,
+                        token::Shr => BinOpKind::Shr,
+                    };
+                    let aopexpr = self.mk_assign_op(source_map::respan(cur_op_span, aop), lhs, rhs);
+                    self.mk_expr(span, aopexpr, AttrVec::new())
+                }
+                AssocOp::As | AssocOp::Colon | AssocOp::DotDot | AssocOp::DotDotEq => {
+                    self.span_bug(span, "AssocOp should have been handled by special case")
+                }
+            };
+
+            if let Fixity::None = fixity {
+                break;
+            }
+        }
+        if last_type_ascription_set {
+            self.last_type_ascription = None;
+        }
+        Ok(lhs)
+    }
+
+    fn should_continue_as_assoc_expr(&mut self, lhs: &Expr) -> bool {
+        match (self.expr_is_complete(lhs), AssocOp::from_token(&self.token)) {
+            // Semi-statement forms are odd:
+            // See https://github.com/rust-lang/rust/issues/29071
+            (true, None) => false,
+            (false, _) => true, // Continue parsing the expression.
+            // An exhaustive check is done in the following block, but these are checked first
+            // because they *are* ambiguous but also reasonable looking incorrect syntax, so we
+            // want to keep their span info to improve diagnostics in these cases in a later stage.
+            (true, Some(AssocOp::Multiply)) | // `{ 42 } *foo = bar;` or `{ 42 } * 3`
+            (true, Some(AssocOp::Subtract)) | // `{ 42 } -5`
+            (true, Some(AssocOp::Add)) // `{ 42 } + 42
+            // If the next token is a keyword, then the tokens above *are* unambiguously incorrect:
+            // `if x { a } else { b } && if y { c } else { d }`
+            if !self.look_ahead(1, |t| t.is_used_keyword()) => {
+                // These cases are ambiguous and can't be identified in the parser alone.
+                let sp = self.sess.source_map().start_point(self.token.span);
+                self.sess.ambiguous_block_expr_parse.borrow_mut().insert(sp, lhs.span);
+                false
+            }
+            (true, Some(AssocOp::LAnd)) => {
+                // `{ 42 } &&x` (#61475) or `{ 42 } && if x { 1 } else { 0 }`. Separated from the
+                // above due to #74233.
+                // These cases are ambiguous and can't be identified in the parser alone.
+                let sp = self.sess.source_map().start_point(self.token.span);
+                self.sess.ambiguous_block_expr_parse.borrow_mut().insert(sp, lhs.span);
+                false
+            }
+            (true, Some(ref op)) if !op.can_continue_expr_unambiguously() => false,
+            (true, Some(_)) => {
+                self.error_found_expr_would_be_stmt(lhs);
+                true
+            }
+        }
+    }
+
+    /// We've found an expression that would be parsed as a statement,
+    /// but the next token implies this should be parsed as an expression.
+    /// For example: `if let Some(x) = x { x } else { 0 } / 2`.
+    fn error_found_expr_would_be_stmt(&self, lhs: &Expr) {
+        let mut err = self.struct_span_err(
+            self.token.span,
+            &format!("expected expression, found `{}`", pprust::token_to_string(&self.token),),
+        );
+        err.span_label(self.token.span, "expected expression");
+        self.sess.expr_parentheses_needed(&mut err, lhs.span, Some(pprust::expr_to_string(&lhs)));
+        err.emit();
+    }
+
+    /// Possibly translate the current token to an associative operator.
+    /// The method does not advance the current token.
+    ///
+    /// Also performs recovery for `and` / `or` which are mistaken for `&&` and `||` respectively.
+    fn check_assoc_op(&self) -> Option<Spanned<AssocOp>> {
+        let (op, span) = match (AssocOp::from_token(&self.token), self.token.ident()) {
+            (Some(op), _) => (op, self.token.span),
+            (None, Some((Ident { name: sym::and, span }, false))) => {
+                self.error_bad_logical_op("and", "&&", "conjunction");
+                (AssocOp::LAnd, span)
+            }
+            (None, Some((Ident { name: sym::or, span }, false))) => {
+                self.error_bad_logical_op("or", "||", "disjunction");
+                (AssocOp::LOr, span)
+            }
+            _ => return None,
+        };
+        Some(source_map::respan(span, op))
+    }
+
+    /// Error on `and` and `or` suggesting `&&` and `||` respectively.
+    fn error_bad_logical_op(&self, bad: &str, good: &str, english: &str) {
+        self.struct_span_err(self.token.span, &format!("`{}` is not a logical operator", bad))
+            .span_suggestion_short(
+                self.token.span,
+                &format!("use `{}` to perform logical {}", good, english),
+                good.to_string(),
+                Applicability::MachineApplicable,
+            )
+            .note("unlike in e.g., python and PHP, `&&` and `||` are used for logical operators")
+            .emit();
+    }
+
+    /// Checks if this expression is a successfully parsed statement.
+    fn expr_is_complete(&self, e: &Expr) -> bool {
+        self.restrictions.contains(Restrictions::STMT_EXPR)
+            && !classify::expr_requires_semi_to_be_stmt(e)
+    }
+
+    /// Parses `x..y`, `x..=y`, and `x..`/`x..=`.
+    /// The other two variants are handled in `parse_prefix_range_expr` below.
+    fn parse_range_expr(
+        &mut self,
+        prec: usize,
+        lhs: P<Expr>,
+        op: AssocOp,
+        cur_op_span: Span,
+    ) -> PResult<'a, P<Expr>> {
+        let rhs = if self.is_at_start_of_range_notation_rhs() {
+            Some(self.parse_assoc_expr_with(prec + 1, LhsExpr::NotYetParsed)?)
+        } else {
+            None
+        };
+        let rhs_span = rhs.as_ref().map_or(cur_op_span, |x| x.span);
+        let span = lhs.span.to(rhs_span);
+        let limits =
+            if op == AssocOp::DotDot { RangeLimits::HalfOpen } else { RangeLimits::Closed };
+        Ok(self.mk_expr(span, self.mk_range(Some(lhs), rhs, limits)?, AttrVec::new()))
+    }
+
+    fn is_at_start_of_range_notation_rhs(&self) -> bool {
+        if self.token.can_begin_expr() {
+            // Parse `for i in 1.. { }` as infinite loop, not as `for i in (1..{})`.
+            if self.token == token::OpenDelim(token::Brace) {
+                return !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL);
+            }
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Parses prefix-forms of range notation: `..expr`, `..`, `..=expr`.
+    fn parse_prefix_range_expr(&mut self, attrs: Option<AttrVec>) -> PResult<'a, P<Expr>> {
+        // Check for deprecated `...` syntax.
+        if self.token == token::DotDotDot {
+            self.err_dotdotdot_syntax(self.token.span);
+        }
+
+        debug_assert!(
+            [token::DotDot, token::DotDotDot, token::DotDotEq].contains(&self.token.kind),
+            "parse_prefix_range_expr: token {:?} is not DotDot/DotDotEq",
+            self.token
+        );
+
+        let limits = match self.token.kind {
+            token::DotDot => RangeLimits::HalfOpen,
+            _ => RangeLimits::Closed,
+        };
+        let op = AssocOp::from_token(&self.token);
+        let attrs = self.parse_or_use_outer_attributes(attrs)?;
+        let lo = self.token.span;
+        self.bump();
+        let (span, opt_end) = if self.is_at_start_of_range_notation_rhs() {
+            // RHS must be parsed with more associativity than the dots.
+            self.parse_assoc_expr_with(op.unwrap().precedence() + 1, LhsExpr::NotYetParsed)
+                .map(|x| (lo.to(x.span), Some(x)))?
+        } else {
+            (lo, None)
+        };
+        Ok(self.mk_expr(span, self.mk_range(None, opt_end, limits)?, attrs))
+    }
+
+    /// Parses a prefix-unary-operator expr.
+    fn parse_prefix_expr(&mut self, attrs: Option<AttrVec>) -> PResult<'a, P<Expr>> {
+        let attrs = self.parse_or_use_outer_attributes(attrs)?;
+        self.maybe_collect_tokens(!attrs.is_empty(), |this| {
+            let lo = this.token.span;
+            // Note: when adding new unary operators, don't forget to adjust TokenKind::can_begin_expr()
+            let (hi, ex) = match this.token.uninterpolate().kind {
+                token::Not => this.parse_unary_expr(lo, UnOp::Not), // `!expr`
+                token::Tilde => this.recover_tilde_expr(lo),        // `~expr`
+                token::BinOp(token::Minus) => this.parse_unary_expr(lo, UnOp::Neg), // `-expr`
+                token::BinOp(token::Star) => this.parse_unary_expr(lo, UnOp::Deref), // `*expr`
+                token::BinOp(token::And) | token::AndAnd => this.parse_borrow_expr(lo),
+                token::Ident(..) if this.token.is_keyword(kw::Box) => this.parse_box_expr(lo),
+                token::Ident(..) if this.is_mistaken_not_ident_negation() => {
+                    this.recover_not_expr(lo)
+                }
+                _ => return this.parse_dot_or_call_expr(Some(attrs)),
+            }?;
+            Ok(this.mk_expr(lo.to(hi), ex, attrs))
+        })
+    }
+
+    fn parse_prefix_expr_common(&mut self, lo: Span) -> PResult<'a, (Span, P<Expr>)> {
+        self.bump();
+        let expr = self.parse_prefix_expr(None);
+        let (span, expr) = self.interpolated_or_expr_span(expr)?;
+        Ok((lo.to(span), expr))
+    }
+
+    fn parse_unary_expr(&mut self, lo: Span, op: UnOp) -> PResult<'a, (Span, ExprKind)> {
+        let (span, expr) = self.parse_prefix_expr_common(lo)?;
+        Ok((span, self.mk_unary(op, expr)))
+    }
+
+    // Recover on `!` suggesting for bitwise negation instead.
+    fn recover_tilde_expr(&mut self, lo: Span) -> PResult<'a, (Span, ExprKind)> {
+        self.struct_span_err(lo, "`~` cannot be used as a unary operator")
+            .span_suggestion_short(
+                lo,
+                "use `!` to perform bitwise not",
+                "!".to_owned(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+
+        self.parse_unary_expr(lo, UnOp::Not)
+    }
+
+    /// Parse `box expr`.
+    fn parse_box_expr(&mut self, lo: Span) -> PResult<'a, (Span, ExprKind)> {
+        let (span, expr) = self.parse_prefix_expr_common(lo)?;
+        self.sess.gated_spans.gate(sym::box_syntax, span);
+        Ok((span, ExprKind::Box(expr)))
+    }
+
+    fn is_mistaken_not_ident_negation(&self) -> bool {
+        let token_cannot_continue_expr = |t: &Token| match t.uninterpolate().kind {
+            // These tokens can start an expression after `!`, but
+            // can't continue an expression after an ident
+            token::Ident(name, is_raw) => token::ident_can_begin_expr(name, t.span, is_raw),
+            token::Literal(..) | token::Pound => true,
+            _ => t.is_whole_expr(),
+        };
+        self.token.is_ident_named(sym::not) && self.look_ahead(1, token_cannot_continue_expr)
+    }
+
+    /// Recover on `not expr` in favor of `!expr`.
+    fn recover_not_expr(&mut self, lo: Span) -> PResult<'a, (Span, ExprKind)> {
+        // Emit the error...
+        let not_token = self.look_ahead(1, |t| t.clone());
+        self.struct_span_err(
+            not_token.span,
+            &format!("unexpected {} after identifier", super::token_descr(&not_token)),
+        )
+        .span_suggestion_short(
+            // Span the `not` plus trailing whitespace to avoid
+            // trailing whitespace after the `!` in our suggestion
+            self.sess.source_map().span_until_non_whitespace(lo.to(not_token.span)),
+            "use `!` to perform logical negation",
+            "!".to_owned(),
+            Applicability::MachineApplicable,
+        )
+        .emit();
+
+        // ...and recover!
+        self.parse_unary_expr(lo, UnOp::Not)
+    }
+
+    /// Returns the span of expr, if it was not interpolated or the span of the interpolated token.
+    fn interpolated_or_expr_span(
+        &self,
+        expr: PResult<'a, P<Expr>>,
+    ) -> PResult<'a, (Span, P<Expr>)> {
+        expr.map(|e| {
+            (
+                match self.prev_token.kind {
+                    TokenKind::Interpolated(..) => self.prev_token.span,
+                    _ => e.span,
+                },
+                e,
+            )
+        })
+    }
+
+    fn parse_assoc_op_cast(
+        &mut self,
+        lhs: P<Expr>,
+        lhs_span: Span,
+        expr_kind: fn(P<Expr>, P<Ty>) -> ExprKind,
+    ) -> PResult<'a, P<Expr>> {
+        let mk_expr = |this: &mut Self, rhs: P<Ty>| {
+            this.mk_expr(lhs_span.to(rhs.span), expr_kind(lhs, rhs), AttrVec::new())
+        };
+
+        // Save the state of the parser before parsing type normally, in case there is a
+        // LessThan comparison after this cast.
+        let parser_snapshot_before_type = self.clone();
+        let cast_expr = match self.parse_ty_no_plus() {
+            Ok(rhs) => mk_expr(self, rhs),
+            Err(mut type_err) => {
+                // Rewind to before attempting to parse the type with generics, to recover
+                // from situations like `x as usize < y` in which we first tried to parse
+                // `usize < y` as a type with generic arguments.
+                let parser_snapshot_after_type = mem::replace(self, parser_snapshot_before_type);
+
+                match self.parse_path(PathStyle::Expr) {
+                    Ok(path) => {
+                        let (op_noun, op_verb) = match self.token.kind {
+                            token::Lt => ("comparison", "comparing"),
+                            token::BinOp(token::Shl) => ("shift", "shifting"),
+                            _ => {
+                                // We can end up here even without `<` being the next token, for
+                                // example because `parse_ty_no_plus` returns `Err` on keywords,
+                                // but `parse_path` returns `Ok` on them due to error recovery.
+                                // Return original error and parser state.
+                                *self = parser_snapshot_after_type;
+                                return Err(type_err);
+                            }
+                        };
+
+                        // Successfully parsed the type path leaving a `<` yet to parse.
+                        type_err.cancel();
+
+                        // Report non-fatal diagnostics, keep `x as usize` as an expression
+                        // in AST and continue parsing.
+                        let msg = format!(
+                            "`<` is interpreted as a start of generic arguments for `{}`, not a {}",
+                            pprust::path_to_string(&path),
+                            op_noun,
+                        );
+                        let span_after_type = parser_snapshot_after_type.token.span;
+                        let expr = mk_expr(self, self.mk_ty(path.span, TyKind::Path(None, path)));
+
+                        let expr_str = self
+                            .span_to_snippet(expr.span)
+                            .unwrap_or_else(|_| pprust::expr_to_string(&expr));
+
+                        self.struct_span_err(self.token.span, &msg)
+                            .span_label(
+                                self.look_ahead(1, |t| t.span).to(span_after_type),
+                                "interpreted as generic arguments",
+                            )
+                            .span_label(self.token.span, format!("not interpreted as {}", op_noun))
+                            .span_suggestion(
+                                expr.span,
+                                &format!("try {} the cast value", op_verb),
+                                format!("({})", expr_str),
+                                Applicability::MachineApplicable,
+                            )
+                            .emit();
+
+                        expr
+                    }
+                    Err(mut path_err) => {
+                        // Couldn't parse as a path, return original error and parser state.
+                        path_err.cancel();
+                        *self = parser_snapshot_after_type;
+                        return Err(type_err);
+                    }
+                }
+            }
+        };
+
+        self.parse_and_disallow_postfix_after_cast(cast_expr)
+    }
+
+    /// Parses a postfix operators such as `.`, `?`, or index (`[]`) after a cast,
+    /// then emits an error and returns the newly parsed tree.
+    /// The resulting parse tree for `&x as T[0]` has a precedence of `((&x) as T)[0]`.
+    fn parse_and_disallow_postfix_after_cast(
+        &mut self,
+        cast_expr: P<Expr>,
+    ) -> PResult<'a, P<Expr>> {
+        // Save the memory location of expr before parsing any following postfix operators.
+        // This will be compared with the memory location of the output expression.
+        // If they different we can assume we parsed another expression because the existing expression is not reallocated.
+        let addr_before = &*cast_expr as *const _ as usize;
+        let span = cast_expr.span;
+        let with_postfix = self.parse_dot_or_call_expr_with_(cast_expr, span)?;
+        let changed = addr_before != &*with_postfix as *const _ as usize;
+
+        // Check if an illegal postfix operator has been added after the cast.
+        // If the resulting expression is not a cast, or has a different memory location, it is an illegal postfix operator.
+        if !matches!(with_postfix.kind, ExprKind::Cast(_, _) | ExprKind::Type(_, _)) || changed {
+            let msg = format!(
+                "casts cannot be followed by {}",
+                match with_postfix.kind {
+                    ExprKind::Index(_, _) => "indexing",
+                    ExprKind::Try(_) => "?",
+                    ExprKind::Field(_, _) => "a field access",
+                    ExprKind::MethodCall(_, _, _) => "a method call",
+                    ExprKind::Call(_, _) => "a function call",
+                    ExprKind::Await(_) => "`.await`",
+                    ExprKind::Err => return Ok(with_postfix),
+                    _ => unreachable!("parse_dot_or_call_expr_with_ shouldn't produce this"),
+                }
+            );
+            let mut err = self.struct_span_err(span, &msg);
+            // If type ascription is "likely an error", the user will already be getting a useful
+            // help message, and doesn't need a second.
+            if self.last_type_ascription.map_or(false, |last_ascription| last_ascription.1) {
+                self.maybe_annotate_with_ascription(&mut err, false);
+            } else {
+                let suggestions = vec![
+                    (span.shrink_to_lo(), "(".to_string()),
+                    (span.shrink_to_hi(), ")".to_string()),
+                ];
+                err.multipart_suggestion(
+                    "try surrounding the expression in parentheses",
+                    suggestions,
+                    Applicability::MachineApplicable,
+                );
+            }
+            err.emit();
+        };
+        Ok(with_postfix)
+    }
+
+    fn parse_assoc_op_ascribe(&mut self, lhs: P<Expr>, lhs_span: Span) -> PResult<'a, P<Expr>> {
+        let maybe_path = self.could_ascription_be_path(&lhs.kind);
+        self.last_type_ascription = Some((self.prev_token.span, maybe_path));
+        let lhs = self.parse_assoc_op_cast(lhs, lhs_span, ExprKind::Type)?;
+        self.sess.gated_spans.gate(sym::type_ascription, lhs.span);
+        Ok(lhs)
+    }
+
+    /// Parse `& mut? <expr>` or `& raw [ const | mut ] <expr>`.
+    fn parse_borrow_expr(&mut self, lo: Span) -> PResult<'a, (Span, ExprKind)> {
+        self.expect_and()?;
+        let has_lifetime = self.token.is_lifetime() && self.look_ahead(1, |t| t != &token::Colon);
+        let lifetime = has_lifetime.then(|| self.expect_lifetime()); // For recovery, see below.
+        let (borrow_kind, mutbl) = self.parse_borrow_modifiers(lo);
+        let expr = self.parse_prefix_expr(None);
+        let (hi, expr) = self.interpolated_or_expr_span(expr)?;
+        let span = lo.to(hi);
+        if let Some(lt) = lifetime {
+            self.error_remove_borrow_lifetime(span, lt.ident.span);
+        }
+        Ok((span, ExprKind::AddrOf(borrow_kind, mutbl, expr)))
+    }
+
+    fn error_remove_borrow_lifetime(&self, span: Span, lt_span: Span) {
+        self.struct_span_err(span, "borrow expressions cannot be annotated with lifetimes")
+            .span_label(lt_span, "annotated with lifetime here")
+            .span_suggestion(
+                lt_span,
+                "remove the lifetime annotation",
+                String::new(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+    }
+
+    /// Parse `mut?` or `raw [ const | mut ]`.
+    fn parse_borrow_modifiers(&mut self, lo: Span) -> (ast::BorrowKind, ast::Mutability) {
+        if self.check_keyword(kw::Raw) && self.look_ahead(1, Token::is_mutability) {
+            // `raw [ const | mut ]`.
+            let found_raw = self.eat_keyword(kw::Raw);
+            assert!(found_raw);
+            let mutability = self.parse_const_or_mut().unwrap();
+            self.sess.gated_spans.gate(sym::raw_ref_op, lo.to(self.prev_token.span));
+            (ast::BorrowKind::Raw, mutability)
+        } else {
+            // `mut?`
+            (ast::BorrowKind::Ref, self.parse_mutability())
+        }
+    }
+
+    /// Parses `a.b` or `a(13)` or `a[4]` or just `a`.
+    fn parse_dot_or_call_expr(&mut self, attrs: Option<AttrVec>) -> PResult<'a, P<Expr>> {
+        let attrs = self.parse_or_use_outer_attributes(attrs)?;
+        let base = self.parse_bottom_expr();
+        let (span, base) = self.interpolated_or_expr_span(base)?;
+        self.parse_dot_or_call_expr_with(base, span, attrs)
+    }
+
+    pub(super) fn parse_dot_or_call_expr_with(
+        &mut self,
+        e0: P<Expr>,
+        lo: Span,
+        mut attrs: AttrVec,
+    ) -> PResult<'a, P<Expr>> {
+        // Stitch the list of outer attributes onto the return value.
+        // A little bit ugly, but the best way given the current code
+        // structure
+        self.parse_dot_or_call_expr_with_(e0, lo).map(|expr| {
+            expr.map(|mut expr| {
+                attrs.extend::<Vec<_>>(expr.attrs.into());
+                expr.attrs = attrs;
+                expr
+            })
+        })
+    }
+
+    fn parse_dot_or_call_expr_with_(&mut self, mut e: P<Expr>, lo: Span) -> PResult<'a, P<Expr>> {
+        loop {
+            if self.eat(&token::Question) {
+                // `expr?`
+                e = self.mk_expr(lo.to(self.prev_token.span), ExprKind::Try(e), AttrVec::new());
+                continue;
+            }
+            if self.eat(&token::Dot) {
+                // expr.f
+                e = self.parse_dot_suffix_expr(lo, e)?;
+                continue;
+            }
+            if self.expr_is_complete(&e) {
+                return Ok(e);
+            }
+            e = match self.token.kind {
+                token::OpenDelim(token::Paren) => self.parse_fn_call_expr(lo, e),
+                token::OpenDelim(token::Bracket) => self.parse_index_expr(lo, e)?,
+                _ => return Ok(e),
+            }
+        }
+    }
+
+    fn parse_dot_suffix_expr(&mut self, lo: Span, base: P<Expr>) -> PResult<'a, P<Expr>> {
+        match self.token.uninterpolate().kind {
+            token::Ident(..) => self.parse_dot_suffix(base, lo),
+            token::Literal(token::Lit { kind: token::Integer, symbol, suffix }) => {
+                Ok(self.parse_tuple_field_access_expr(lo, base, symbol, suffix, None))
+            }
+            token::Literal(token::Lit { kind: token::Float, symbol, suffix }) => {
+                Ok(self.parse_tuple_field_access_expr_float(lo, base, symbol, suffix))
+            }
+            _ => {
+                self.error_unexpected_after_dot();
+                Ok(base)
+            }
+        }
+    }
+
+    fn error_unexpected_after_dot(&self) {
+        // FIXME Could factor this out into non_fatal_unexpected or something.
+        let actual = pprust::token_to_string(&self.token);
+        self.struct_span_err(self.token.span, &format!("unexpected token: `{}`", actual)).emit();
+    }
+
+    // We need and identifier or integer, but the next token is a float.
+    // Break the float into components to extract the identifier or integer.
+    // FIXME: With current `TokenCursor` it's hard to break tokens into more than 2
+    // parts unless those parts are processed immediately. `TokenCursor` should either
+    // support pushing "future tokens" (would be also helpful to `break_and_eat`), or
+    // we should break everything including floats into more basic proc-macro style
+    // tokens in the lexer (probably preferable).
+    fn parse_tuple_field_access_expr_float(
+        &mut self,
+        lo: Span,
+        base: P<Expr>,
+        float: Symbol,
+        suffix: Option<Symbol>,
+    ) -> P<Expr> {
+        #[derive(Debug)]
+        enum FloatComponent {
+            IdentLike(String),
+            Punct(char),
+        }
+        use FloatComponent::*;
+
+        let mut components = Vec::new();
+        let mut ident_like = String::new();
+        for c in float.as_str().chars() {
+            if c == '_' || c.is_ascii_alphanumeric() {
+                ident_like.push(c);
+            } else if matches!(c, '.' | '+' | '-') {
+                if !ident_like.is_empty() {
+                    components.push(IdentLike(mem::take(&mut ident_like)));
+                }
+                components.push(Punct(c));
+            } else {
+                panic!("unexpected character in a float token: {:?}", c)
+            }
+        }
+        if !ident_like.is_empty() {
+            components.push(IdentLike(ident_like));
+        }
+
+        // FIXME: Make the span more precise.
+        let span = self.token.span;
+        match &*components {
+            // 1e2
+            [IdentLike(i)] => {
+                self.parse_tuple_field_access_expr(lo, base, Symbol::intern(&i), suffix, None)
+            }
+            // 1.
+            [IdentLike(i), Punct('.')] => {
+                assert!(suffix.is_none());
+                let symbol = Symbol::intern(&i);
+                self.token = Token::new(token::Ident(symbol, false), span);
+                let next_token = Token::new(token::Dot, span);
+                self.parse_tuple_field_access_expr(lo, base, symbol, None, Some(next_token))
+            }
+            // 1.2 | 1.2e3
+            [IdentLike(i1), Punct('.'), IdentLike(i2)] => {
+                let symbol1 = Symbol::intern(&i1);
+                self.token = Token::new(token::Ident(symbol1, false), span);
+                let next_token1 = Token::new(token::Dot, span);
+                let base1 =
+                    self.parse_tuple_field_access_expr(lo, base, symbol1, None, Some(next_token1));
+                let symbol2 = Symbol::intern(&i2);
+                let next_token2 = Token::new(token::Ident(symbol2, false), span);
+                self.bump_with(next_token2); // `.`
+                self.parse_tuple_field_access_expr(lo, base1, symbol2, suffix, None)
+            }
+            // 1e+ | 1e- (recovered)
+            [IdentLike(_), Punct('+' | '-')] |
+            // 1e+2 | 1e-2
+            [IdentLike(_), Punct('+' | '-'), IdentLike(_)] |
+            // 1.2e+3 | 1.2e-3
+            [IdentLike(_), Punct('.'), IdentLike(_), Punct('+' | '-'), IdentLike(_)] => {
+                // See the FIXME about `TokenCursor` above.
+                self.error_unexpected_after_dot();
+                base
+            }
+            _ => panic!("unexpected components in a float token: {:?}", components),
+        }
+    }
+
+    fn parse_tuple_field_access_expr(
+        &mut self,
+        lo: Span,
+        base: P<Expr>,
+        field: Symbol,
+        suffix: Option<Symbol>,
+        next_token: Option<Token>,
+    ) -> P<Expr> {
+        match next_token {
+            Some(next_token) => self.bump_with(next_token),
+            None => self.bump(),
+        }
+        let span = self.prev_token.span;
+        let field = ExprKind::Field(base, Ident::new(field, span));
+        self.expect_no_suffix(span, "a tuple index", suffix);
+        self.mk_expr(lo.to(span), field, AttrVec::new())
+    }
+
+    /// Parse a function call expression, `expr(...)`.
+    fn parse_fn_call_expr(&mut self, lo: Span, fun: P<Expr>) -> P<Expr> {
+        let seq = self.parse_paren_expr_seq().map(|args| {
+            self.mk_expr(lo.to(self.prev_token.span), self.mk_call(fun, args), AttrVec::new())
+        });
+        self.recover_seq_parse_error(token::Paren, lo, seq)
+    }
+
+    /// Parse an indexing expression `expr[...]`.
+    fn parse_index_expr(&mut self, lo: Span, base: P<Expr>) -> PResult<'a, P<Expr>> {
+        self.bump(); // `[`
+        let index = self.parse_expr()?;
+        self.expect(&token::CloseDelim(token::Bracket))?;
+        Ok(self.mk_expr(lo.to(self.prev_token.span), self.mk_index(base, index), AttrVec::new()))
+    }
+
+    /// Assuming we have just parsed `.`, continue parsing into an expression.
+    fn parse_dot_suffix(&mut self, self_arg: P<Expr>, lo: Span) -> PResult<'a, P<Expr>> {
+        if self.token.uninterpolated_span().rust_2018() && self.eat_keyword(kw::Await) {
+            return self.mk_await_expr(self_arg, lo);
+        }
+
+        let fn_span_lo = self.token.span;
+        let mut segment = self.parse_path_segment(PathStyle::Expr)?;
+        self.check_trailing_angle_brackets(&segment, &[&token::OpenDelim(token::Paren)]);
+        self.check_turbofish_missing_angle_brackets(&mut segment);
+
+        if self.check(&token::OpenDelim(token::Paren)) {
+            // Method call `expr.f()`
+            let mut args = self.parse_paren_expr_seq()?;
+            args.insert(0, self_arg);
+
+            let fn_span = fn_span_lo.to(self.prev_token.span);
+            let span = lo.to(self.prev_token.span);
+            Ok(self.mk_expr(span, ExprKind::MethodCall(segment, args, fn_span), AttrVec::new()))
+        } else {
+            // Field access `expr.f`
+            if let Some(args) = segment.args {
+                self.struct_span_err(
+                    args.span(),
+                    "field expressions cannot have generic arguments",
+                )
+                .emit();
+            }
+
+            let span = lo.to(self.prev_token.span);
+            Ok(self.mk_expr(span, ExprKind::Field(self_arg, segment.ident), AttrVec::new()))
+        }
+    }
+
+    /// At the bottom (top?) of the precedence hierarchy,
+    /// Parses things like parenthesized exprs, macros, `return`, etc.
+    ///
+    /// N.B., this does not parse outer attributes, and is private because it only works
+    /// correctly if called from `parse_dot_or_call_expr()`.
+    fn parse_bottom_expr(&mut self) -> PResult<'a, P<Expr>> {
+        maybe_recover_from_interpolated_ty_qpath!(self, true);
+        maybe_whole_expr!(self);
+
+        // Outer attributes are already parsed and will be
+        // added to the return value after the fact.
+        //
+        // Therefore, prevent sub-parser from parsing
+        // attributes by giving them a empty "already-parsed" list.
+        let attrs = AttrVec::new();
+
+        // Note: when adding new syntax here, don't forget to adjust `TokenKind::can_begin_expr()`.
+        let lo = self.token.span;
+        if let token::Literal(_) = self.token.kind {
+            // This match arm is a special-case of the `_` match arm below and
+            // could be removed without changing functionality, but it's faster
+            // to have it here, especially for programs with large constants.
+            self.parse_lit_expr(attrs)
+        } else if self.check(&token::OpenDelim(token::Paren)) {
+            self.parse_tuple_parens_expr(attrs)
+        } else if self.check(&token::OpenDelim(token::Brace)) {
+            self.parse_block_expr(None, lo, BlockCheckMode::Default, attrs)
+        } else if self.check(&token::BinOp(token::Or)) || self.check(&token::OrOr) {
+            self.parse_closure_expr(attrs)
+        } else if self.check(&token::OpenDelim(token::Bracket)) {
+            self.parse_array_or_repeat_expr(attrs)
+        } else if self.eat_lt() {
+            let (qself, path) = self.parse_qpath(PathStyle::Expr)?;
+            Ok(self.mk_expr(lo.to(path.span), ExprKind::Path(Some(qself), path), attrs))
+        } else if self.check_path() {
+            self.parse_path_start_expr(attrs)
+        } else if self.check_keyword(kw::Move) || self.check_keyword(kw::Static) {
+            self.parse_closure_expr(attrs)
+        } else if self.eat_keyword(kw::If) {
+            self.parse_if_expr(attrs)
+        } else if self.check_keyword(kw::For) {
+            if self.choose_generics_over_qpath(1) {
+                // NOTE(Centril, eddyb): DO NOT REMOVE! Beyond providing parser recovery,
+                // this is an insurance policy in case we allow qpaths in (tuple-)struct patterns.
+                // When `for <Foo as Bar>::Proj in $expr $block` is wanted,
+                // you can disambiguate in favor of a pattern with `(...)`.
+                self.recover_quantified_closure_expr(attrs)
+            } else {
+                assert!(self.eat_keyword(kw::For));
+                self.parse_for_expr(None, self.prev_token.span, attrs)
+            }
+        } else if self.eat_keyword(kw::While) {
+            self.parse_while_expr(None, self.prev_token.span, attrs)
+        } else if let Some(label) = self.eat_label() {
+            self.parse_labeled_expr(label, attrs)
+        } else if self.eat_keyword(kw::Loop) {
+            self.parse_loop_expr(None, self.prev_token.span, attrs)
+        } else if self.eat_keyword(kw::Continue) {
+            let kind = ExprKind::Continue(self.eat_label());
+            Ok(self.mk_expr(lo.to(self.prev_token.span), kind, attrs))
+        } else if self.eat_keyword(kw::Match) {
+            let match_sp = self.prev_token.span;
+            self.parse_match_expr(attrs).map_err(|mut err| {
+                err.span_label(match_sp, "while parsing this match expression");
+                err
+            })
+        } else if self.eat_keyword(kw::Unsafe) {
+            self.parse_block_expr(None, lo, BlockCheckMode::Unsafe(ast::UserProvided), attrs)
+        } else if self.is_do_catch_block() {
+            self.recover_do_catch(attrs)
+        } else if self.is_try_block() {
+            self.expect_keyword(kw::Try)?;
+            self.parse_try_block(lo, attrs)
+        } else if self.eat_keyword(kw::Return) {
+            self.parse_return_expr(attrs)
+        } else if self.eat_keyword(kw::Break) {
+            self.parse_break_expr(attrs)
+        } else if self.eat_keyword(kw::Yield) {
+            self.parse_yield_expr(attrs)
+        } else if self.eat_keyword(kw::Let) {
+            self.parse_let_expr(attrs)
+        } else if !self.unclosed_delims.is_empty() && self.check(&token::Semi) {
+            // Don't complain about bare semicolons after unclosed braces
+            // recovery in order to keep the error count down. Fixing the
+            // delimiters will possibly also fix the bare semicolon found in
+            // expression context. For example, silence the following error:
+            //
+            //     error: expected expression, found `;`
+            //      --> file.rs:2:13
+            //       |
+            //     2 |     foo(bar(;
+            //       |             ^ expected expression
+            self.bump();
+            Ok(self.mk_expr_err(self.token.span))
+        } else if self.token.uninterpolated_span().rust_2018() {
+            // `Span::rust_2018()` is somewhat expensive; don't get it repeatedly.
+            if self.check_keyword(kw::Async) {
+                if self.is_async_block() {
+                    // Check for `async {` and `async move {`.
+                    self.parse_async_block(attrs)
+                } else {
+                    self.parse_closure_expr(attrs)
+                }
+            } else if self.eat_keyword(kw::Await) {
+                self.recover_incorrect_await_syntax(lo, self.prev_token.span, attrs)
+            } else {
+                self.parse_lit_expr(attrs)
+            }
+        } else {
+            self.parse_lit_expr(attrs)
+        }
+    }
+
+    fn maybe_collect_tokens(
+        &mut self,
+        has_outer_attrs: bool,
+        f: impl FnOnce(&mut Self) -> PResult<'a, P<Expr>>,
+    ) -> PResult<'a, P<Expr>> {
+        if has_outer_attrs {
+            let (mut expr, tokens) = self.collect_tokens(f)?;
+            debug!("maybe_collect_tokens: Collected tokens for {:?} (tokens {:?}", expr, tokens);
+            expr.tokens = Some(tokens);
+            Ok(expr)
+        } else {
+            f(self)
+        }
+    }
+
+    fn parse_lit_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.token.span;
+        match self.parse_opt_lit() {
+            Some(literal) => {
+                let expr = self.mk_expr(lo.to(self.prev_token.span), ExprKind::Lit(literal), attrs);
+                self.maybe_recover_from_bad_qpath(expr, true)
+            }
+            None => self.try_macro_suggestion(),
+        }
+    }
+
+    fn parse_tuple_parens_expr(&mut self, mut attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.token.span;
+        self.expect(&token::OpenDelim(token::Paren))?;
+        attrs.extend(self.parse_inner_attributes()?); // `(#![foo] a, b, ...)` is OK.
+        let (es, trailing_comma) = match self.parse_seq_to_end(
+            &token::CloseDelim(token::Paren),
+            SeqSep::trailing_allowed(token::Comma),
+            |p| p.parse_expr_catch_underscore(),
+        ) {
+            Ok(x) => x,
+            Err(err) => return Ok(self.recover_seq_parse_error(token::Paren, lo, Err(err))),
+        };
+        let kind = if es.len() == 1 && !trailing_comma {
+            // `(e)` is parenthesized `e`.
+            ExprKind::Paren(es.into_iter().next().unwrap())
+        } else {
+            // `(e,)` is a tuple with only one field, `e`.
+            ExprKind::Tup(es)
+        };
+        let expr = self.mk_expr(lo.to(self.prev_token.span), kind, attrs);
+        self.maybe_recover_from_bad_qpath(expr, true)
+    }
+
+    fn parse_array_or_repeat_expr(&mut self, mut attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.token.span;
+        self.bump(); // `[`
+
+        attrs.extend(self.parse_inner_attributes()?);
+
+        let close = &token::CloseDelim(token::Bracket);
+        let kind = if self.eat(close) {
+            // Empty vector
+            ExprKind::Array(Vec::new())
+        } else {
+            // Non-empty vector
+            let first_expr = self.parse_expr()?;
+            if self.eat(&token::Semi) {
+                // Repeating array syntax: `[ 0; 512 ]`
+                let count = self.parse_anon_const_expr()?;
+                self.expect(close)?;
+                ExprKind::Repeat(first_expr, count)
+            } else if self.eat(&token::Comma) {
+                // Vector with two or more elements.
+                let sep = SeqSep::trailing_allowed(token::Comma);
+                let (remaining_exprs, _) = self.parse_seq_to_end(close, sep, |p| p.parse_expr())?;
+                let mut exprs = vec![first_expr];
+                exprs.extend(remaining_exprs);
+                ExprKind::Array(exprs)
+            } else {
+                // Vector with one element
+                self.expect(close)?;
+                ExprKind::Array(vec![first_expr])
+            }
+        };
+        let expr = self.mk_expr(lo.to(self.prev_token.span), kind, attrs);
+        self.maybe_recover_from_bad_qpath(expr, true)
+    }
+
+    fn parse_path_start_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let path = self.parse_path(PathStyle::Expr)?;
+        let lo = path.span;
+
+        // `!`, as an operator, is prefix, so we know this isn't that.
+        let (hi, kind) = if self.eat(&token::Not) {
+            // MACRO INVOCATION expression
+            let mac = MacCall {
+                path,
+                args: self.parse_mac_args()?,
+                prior_type_ascription: self.last_type_ascription,
+            };
+            (self.prev_token.span, ExprKind::MacCall(mac))
+        } else if self.check(&token::OpenDelim(token::Brace)) {
+            if let Some(expr) = self.maybe_parse_struct_expr(&path, &attrs) {
+                return expr;
+            } else {
+                (path.span, ExprKind::Path(None, path))
+            }
+        } else {
+            (path.span, ExprKind::Path(None, path))
+        };
+
+        let expr = self.mk_expr(lo.to(hi), kind, attrs);
+        self.maybe_recover_from_bad_qpath(expr, true)
+    }
+
+    /// Parse `'label: $expr`. The label is already parsed.
+    fn parse_labeled_expr(&mut self, label: Label, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = label.ident.span;
+        let label = Some(label);
+        let ate_colon = self.eat(&token::Colon);
+        let expr = if self.eat_keyword(kw::While) {
+            self.parse_while_expr(label, lo, attrs)
+        } else if self.eat_keyword(kw::For) {
+            self.parse_for_expr(label, lo, attrs)
+        } else if self.eat_keyword(kw::Loop) {
+            self.parse_loop_expr(label, lo, attrs)
+        } else if self.check(&token::OpenDelim(token::Brace)) || self.token.is_whole_block() {
+            self.parse_block_expr(label, lo, BlockCheckMode::Default, attrs)
+        } else {
+            let msg = "expected `while`, `for`, `loop` or `{` after a label";
+            self.struct_span_err(self.token.span, msg).span_label(self.token.span, msg).emit();
+            // Continue as an expression in an effort to recover on `'label: non_block_expr`.
+            self.parse_expr()
+        }?;
+
+        if !ate_colon {
+            self.error_labeled_expr_must_be_followed_by_colon(lo, expr.span);
+        }
+
+        Ok(expr)
+    }
+
+    fn error_labeled_expr_must_be_followed_by_colon(&self, lo: Span, span: Span) {
+        self.struct_span_err(span, "labeled expression must be followed by `:`")
+            .span_label(lo, "the label")
+            .span_suggestion_short(
+                lo.shrink_to_hi(),
+                "add `:` after the label",
+                ": ".to_string(),
+                Applicability::MachineApplicable,
+            )
+            .note("labels are used before loops and blocks, allowing e.g., `break 'label` to them")
+            .emit();
+    }
+
+    /// Recover on the syntax `do catch { ... }` suggesting `try { ... }` instead.
+    fn recover_do_catch(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.token.span;
+
+        self.bump(); // `do`
+        self.bump(); // `catch`
+
+        let span_dc = lo.to(self.prev_token.span);
+        self.struct_span_err(span_dc, "found removed `do catch` syntax")
+            .span_suggestion(
+                span_dc,
+                "replace with the new syntax",
+                "try".to_string(),
+                Applicability::MachineApplicable,
+            )
+            .note("following RFC #2388, the new non-placeholder syntax is `try`")
+            .emit();
+
+        self.parse_try_block(lo, attrs)
+    }
+
+    /// Parse an expression if the token can begin one.
+    fn parse_expr_opt(&mut self) -> PResult<'a, Option<P<Expr>>> {
+        Ok(if self.token.can_begin_expr() { Some(self.parse_expr()?) } else { None })
+    }
+
+    /// Parse `"return" expr?`.
+    fn parse_return_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.prev_token.span;
+        let kind = ExprKind::Ret(self.parse_expr_opt()?);
+        let expr = self.mk_expr(lo.to(self.prev_token.span), kind, attrs);
+        self.maybe_recover_from_bad_qpath(expr, true)
+    }
+
+    /// Parse `"('label ":")? break expr?`.
+    fn parse_break_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.prev_token.span;
+        let label = self.eat_label();
+        let kind = if self.token != token::OpenDelim(token::Brace)
+            || !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL)
+        {
+            self.parse_expr_opt()?
+        } else {
+            None
+        };
+        let expr = self.mk_expr(lo.to(self.prev_token.span), ExprKind::Break(label, kind), attrs);
+        self.maybe_recover_from_bad_qpath(expr, true)
+    }
+
+    /// Parse `"yield" expr?`.
+    fn parse_yield_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.prev_token.span;
+        let kind = ExprKind::Yield(self.parse_expr_opt()?);
+        let span = lo.to(self.prev_token.span);
+        self.sess.gated_spans.gate(sym::generators, span);
+        let expr = self.mk_expr(span, kind, attrs);
+        self.maybe_recover_from_bad_qpath(expr, true)
+    }
+
+    /// Returns a string literal if the next token is a string literal.
+    /// In case of error returns `Some(lit)` if the next token is a literal with a wrong kind,
+    /// and returns `None` if the next token is not literal at all.
+    pub fn parse_str_lit(&mut self) -> Result<ast::StrLit, Option<Lit>> {
+        match self.parse_opt_lit() {
+            Some(lit) => match lit.kind {
+                ast::LitKind::Str(symbol_unescaped, style) => Ok(ast::StrLit {
+                    style,
+                    symbol: lit.token.symbol,
+                    suffix: lit.token.suffix,
+                    span: lit.span,
+                    symbol_unescaped,
+                }),
+                _ => Err(Some(lit)),
+            },
+            None => Err(None),
+        }
+    }
+
+    pub(super) fn parse_lit(&mut self) -> PResult<'a, Lit> {
+        self.parse_opt_lit().ok_or_else(|| {
+            let msg = format!("unexpected token: {}", super::token_descr(&self.token));
+            self.struct_span_err(self.token.span, &msg)
+        })
+    }
+
+    /// Matches `lit = true | false | token_lit`.
+    /// Returns `None` if the next token is not a literal.
+    pub(super) fn parse_opt_lit(&mut self) -> Option<Lit> {
+        let mut recovered = None;
+        if self.token == token::Dot {
+            // Attempt to recover `.4` as `0.4`. We don't currently have any syntax where
+            // dot would follow an optional literal, so we do this unconditionally.
+            recovered = self.look_ahead(1, |next_token| {
+                if let token::Literal(token::Lit { kind: token::Integer, symbol, suffix }) =
+                    next_token.kind
+                {
+                    if self.token.span.hi() == next_token.span.lo() {
+                        let s = String::from("0.") + &symbol.as_str();
+                        let kind = TokenKind::lit(token::Float, Symbol::intern(&s), suffix);
+                        return Some(Token::new(kind, self.token.span.to(next_token.span)));
+                    }
+                }
+                None
+            });
+            if let Some(token) = &recovered {
+                self.bump();
+                self.error_float_lits_must_have_int_part(&token);
+            }
+        }
+
+        let token = recovered.as_ref().unwrap_or(&self.token);
+        match Lit::from_token(token) {
+            Ok(lit) => {
+                self.bump();
+                Some(lit)
+            }
+            Err(LitError::NotLiteral) => None,
+            Err(err) => {
+                let span = token.span;
+                let lit = match token.kind {
+                    token::Literal(lit) => lit,
+                    _ => unreachable!(),
+                };
+                self.bump();
+                self.report_lit_error(err, lit, span);
+                // Pack possible quotes and prefixes from the original literal into
+                // the error literal's symbol so they can be pretty-printed faithfully.
+                let suffixless_lit = token::Lit::new(lit.kind, lit.symbol, None);
+                let symbol = Symbol::intern(&suffixless_lit.to_string());
+                let lit = token::Lit::new(token::Err, symbol, lit.suffix);
+                Some(Lit::from_lit_token(lit, span).unwrap_or_else(|_| unreachable!()))
+            }
+        }
+    }
+
+    fn error_float_lits_must_have_int_part(&self, token: &Token) {
+        self.struct_span_err(token.span, "float literals must have an integer part")
+            .span_suggestion(
+                token.span,
+                "must have an integer part",
+                pprust::token_to_string(token),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+    }
+
+    fn report_lit_error(&self, err: LitError, lit: token::Lit, span: Span) {
+        // Checks if `s` looks like i32 or u1234 etc.
+        fn looks_like_width_suffix(first_chars: &[char], s: &str) -> bool {
+            s.len() > 1 && s.starts_with(first_chars) && s[1..].chars().all(|c| c.is_ascii_digit())
+        }
+
+        let token::Lit { kind, suffix, .. } = lit;
+        match err {
+            // `NotLiteral` is not an error by itself, so we don't report
+            // it and give the parser opportunity to try something else.
+            LitError::NotLiteral => {}
+            // `LexerError` *is* an error, but it was already reported
+            // by lexer, so here we don't report it the second time.
+            LitError::LexerError => {}
+            LitError::InvalidSuffix => {
+                self.expect_no_suffix(
+                    span,
+                    &format!("{} {} literal", kind.article(), kind.descr()),
+                    suffix,
+                );
+            }
+            LitError::InvalidIntSuffix => {
+                let suf = suffix.expect("suffix error with no suffix").as_str();
+                if looks_like_width_suffix(&['i', 'u'], &suf) {
+                    // If it looks like a width, try to be helpful.
+                    let msg = format!("invalid width `{}` for integer literal", &suf[1..]);
+                    self.struct_span_err(span, &msg)
+                        .help("valid widths are 8, 16, 32, 64 and 128")
+                        .emit();
+                } else {
+                    let msg = format!("invalid suffix `{}` for integer literal", suf);
+                    self.struct_span_err(span, &msg)
+                        .span_label(span, format!("invalid suffix `{}`", suf))
+                        .help("the suffix must be one of the integral types (`u32`, `isize`, etc)")
+                        .emit();
+                }
+            }
+            LitError::InvalidFloatSuffix => {
+                let suf = suffix.expect("suffix error with no suffix").as_str();
+                if looks_like_width_suffix(&['f'], &suf) {
+                    // If it looks like a width, try to be helpful.
+                    let msg = format!("invalid width `{}` for float literal", &suf[1..]);
+                    self.struct_span_err(span, &msg).help("valid widths are 32 and 64").emit();
+                } else {
+                    let msg = format!("invalid suffix `{}` for float literal", suf);
+                    self.struct_span_err(span, &msg)
+                        .span_label(span, format!("invalid suffix `{}`", suf))
+                        .help("valid suffixes are `f32` and `f64`")
+                        .emit();
+                }
+            }
+            LitError::NonDecimalFloat(base) => {
+                let descr = match base {
+                    16 => "hexadecimal",
+                    8 => "octal",
+                    2 => "binary",
+                    _ => unreachable!(),
+                };
+                self.struct_span_err(span, &format!("{} float literal is not supported", descr))
+                    .span_label(span, "not supported")
+                    .emit();
+            }
+            LitError::IntTooLarge => {
+                self.struct_span_err(span, "integer literal is too large").emit();
+            }
+        }
+    }
+
+    pub(super) fn expect_no_suffix(&self, sp: Span, kind: &str, suffix: Option<Symbol>) {
+        if let Some(suf) = suffix {
+            let mut err = if kind == "a tuple index"
+                && [sym::i32, sym::u32, sym::isize, sym::usize].contains(&suf)
+            {
+                // #59553: warn instead of reject out of hand to allow the fix to percolate
+                // through the ecosystem when people fix their macros
+                let mut err = self
+                    .sess
+                    .span_diagnostic
+                    .struct_span_warn(sp, &format!("suffixes on {} are invalid", kind));
+                err.note(&format!(
+                    "`{}` is *temporarily* accepted on tuple index fields as it was \
+                        incorrectly accepted on stable for a few releases",
+                    suf,
+                ));
+                err.help(
+                    "on proc macros, you'll want to use `syn::Index::from` or \
+                        `proc_macro::Literal::*_unsuffixed` for code that will desugar \
+                        to tuple field access",
+                );
+                err.note(
+                    "see issue #60210 <https://github.com/rust-lang/rust/issues/60210> \
+                     for more information",
+                );
+                err
+            } else {
+                self.struct_span_err(sp, &format!("suffixes on {} are invalid", kind))
+            };
+            err.span_label(sp, format!("invalid suffix `{}`", suf));
+            err.emit();
+        }
+    }
+
+    /// Matches `'-' lit | lit` (cf. `ast_validation::AstValidator::check_expr_within_pat`).
+    /// Keep this in sync with `Token::can_begin_literal_maybe_minus`.
+    pub(super) fn parse_literal_maybe_minus(&mut self) -> PResult<'a, P<Expr>> {
+        maybe_whole_expr!(self);
+
+        let lo = self.token.span;
+        let minus_present = self.eat(&token::BinOp(token::Minus));
+        let lit = self.parse_lit()?;
+        let expr = self.mk_expr(lit.span, ExprKind::Lit(lit), AttrVec::new());
+
+        if minus_present {
+            Ok(self.mk_expr(
+                lo.to(self.prev_token.span),
+                self.mk_unary(UnOp::Neg, expr),
+                AttrVec::new(),
+            ))
+        } else {
+            Ok(expr)
+        }
+    }
+
+    /// Parses a block or unsafe block.
+    pub(super) fn parse_block_expr(
+        &mut self,
+        opt_label: Option<Label>,
+        lo: Span,
+        blk_mode: BlockCheckMode,
+        mut attrs: AttrVec,
+    ) -> PResult<'a, P<Expr>> {
+        if let Some(label) = opt_label {
+            self.sess.gated_spans.gate(sym::label_break_value, label.ident.span);
+        }
+
+        if self.token.is_whole_block() {
+            self.struct_span_err(self.token.span, "cannot use a `block` macro fragment here")
+                .span_label(lo.to(self.token.span), "the `block` fragment is within this context")
+                .emit();
+        }
+
+        let (inner_attrs, blk) = self.parse_block_common(lo, blk_mode)?;
+        attrs.extend(inner_attrs);
+        Ok(self.mk_expr(blk.span, ExprKind::Block(blk, opt_label), attrs))
+    }
+
+    /// Recover on an explicitly quantified closure expression, e.g., `for<'a> |x: &'a u8| *x + 1`.
+    fn recover_quantified_closure_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.token.span;
+        let _ = self.parse_late_bound_lifetime_defs()?;
+        let span_for = lo.to(self.prev_token.span);
+        let closure = self.parse_closure_expr(attrs)?;
+
+        self.struct_span_err(span_for, "cannot introduce explicit parameters for a closure")
+            .span_label(closure.span, "the parameters are attached to this closure")
+            .span_suggestion(
+                span_for,
+                "remove the parameters",
+                String::new(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+
+        Ok(self.mk_expr_err(lo.to(closure.span)))
+    }
+
+    /// Parses a closure expression (e.g., `move |args| expr`).
+    fn parse_closure_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.token.span;
+
+        let movability =
+            if self.eat_keyword(kw::Static) { Movability::Static } else { Movability::Movable };
+
+        let asyncness = if self.token.uninterpolated_span().rust_2018() {
+            self.parse_asyncness()
+        } else {
+            Async::No
+        };
+        if let Async::Yes { span, .. } = asyncness {
+            // Feature-gate `async ||` closures.
+            self.sess.gated_spans.gate(sym::async_closure, span);
+        }
+
+        let capture_clause = self.parse_capture_clause();
+        let decl = self.parse_fn_block_decl()?;
+        let decl_hi = self.prev_token.span;
+        let body = match decl.output {
+            FnRetTy::Default(_) => {
+                let restrictions = self.restrictions - Restrictions::STMT_EXPR;
+                self.parse_expr_res(restrictions, None)?
+            }
+            _ => {
+                // If an explicit return type is given, require a block to appear (RFC 968).
+                let body_lo = self.token.span;
+                self.parse_block_expr(None, body_lo, BlockCheckMode::Default, AttrVec::new())?
+            }
+        };
+
+        Ok(self.mk_expr(
+            lo.to(body.span),
+            ExprKind::Closure(capture_clause, asyncness, movability, decl, body, lo.to(decl_hi)),
+            attrs,
+        ))
+    }
+
+    /// Parses an optional `move` prefix to a closure-like construct.
+    fn parse_capture_clause(&mut self) -> CaptureBy {
+        if self.eat_keyword(kw::Move) { CaptureBy::Value } else { CaptureBy::Ref }
+    }
+
+    /// Parses the `|arg, arg|` header of a closure.
+    fn parse_fn_block_decl(&mut self) -> PResult<'a, P<FnDecl>> {
+        let inputs = if self.eat(&token::OrOr) {
+            Vec::new()
+        } else {
+            self.expect(&token::BinOp(token::Or))?;
+            let args = self
+                .parse_seq_to_before_tokens(
+                    &[&token::BinOp(token::Or), &token::OrOr],
+                    SeqSep::trailing_allowed(token::Comma),
+                    TokenExpectType::NoExpect,
+                    |p| p.parse_fn_block_param(),
+                )?
+                .0;
+            self.expect_or()?;
+            args
+        };
+        let output = self.parse_ret_ty(AllowPlus::Yes, RecoverQPath::Yes)?;
+
+        Ok(P(FnDecl { inputs, output }))
+    }
+
+    /// Parses a parameter in a closure header (e.g., `|arg, arg|`).
+    fn parse_fn_block_param(&mut self) -> PResult<'a, Param> {
+        let lo = self.token.span;
+        let attrs = self.parse_outer_attributes()?;
+        let pat = self.parse_pat(PARAM_EXPECTED)?;
+        let ty = if self.eat(&token::Colon) {
+            self.parse_ty()?
+        } else {
+            self.mk_ty(self.prev_token.span, TyKind::Infer)
+        };
+        Ok(Param {
+            attrs: attrs.into(),
+            ty,
+            pat,
+            span: lo.to(self.token.span),
+            id: DUMMY_NODE_ID,
+            is_placeholder: false,
+        })
+    }
+
+    /// Parses an `if` expression (`if` token already eaten).
+    fn parse_if_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.prev_token.span;
+        let cond = self.parse_cond_expr()?;
+
+        // Verify that the parsed `if` condition makes sense as a condition. If it is a block, then
+        // verify that the last statement is either an implicit return (no `;`) or an explicit
+        // return. This won't catch blocks with an explicit `return`, but that would be caught by
+        // the dead code lint.
+        let thn = if self.eat_keyword(kw::Else) || !cond.returns() {
+            self.error_missing_if_cond(lo, cond.span)
+        } else {
+            let attrs = self.parse_outer_attributes()?; // For recovery.
+            let not_block = self.token != token::OpenDelim(token::Brace);
+            let block = self.parse_block().map_err(|mut err| {
+                if not_block {
+                    err.span_label(lo, "this `if` expression has a condition, but no block");
+                    if let ExprKind::Binary(_, _, ref right) = cond.kind {
+                        if let ExprKind::Block(_, _) = right.kind {
+                            err.help("maybe you forgot the right operand of the condition?");
+                        }
+                    }
+                }
+                err
+            })?;
+            self.error_on_if_block_attrs(lo, false, block.span, &attrs);
+            block
+        };
+        let els = if self.eat_keyword(kw::Else) { Some(self.parse_else_expr()?) } else { None };
+        Ok(self.mk_expr(lo.to(self.prev_token.span), ExprKind::If(cond, thn, els), attrs))
+    }
+
+    fn error_missing_if_cond(&self, lo: Span, span: Span) -> P<ast::Block> {
+        let sp = self.sess.source_map().next_point(lo);
+        self.struct_span_err(sp, "missing condition for `if` expression")
+            .span_label(sp, "expected if condition here")
+            .emit();
+        self.mk_block_err(span)
+    }
+
+    /// Parses the condition of a `if` or `while` expression.
+    fn parse_cond_expr(&mut self) -> PResult<'a, P<Expr>> {
+        let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
+
+        if let ExprKind::Let(..) = cond.kind {
+            // Remove the last feature gating of a `let` expression since it's stable.
+            self.sess.gated_spans.ungate_last(sym::let_chains, cond.span);
+        }
+
+        Ok(cond)
+    }
+
+    /// Parses a `let $pat = $expr` pseudo-expression.
+    /// The `let` token has already been eaten.
+    fn parse_let_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.prev_token.span;
+        let pat = self.parse_top_pat(GateOr::No)?;
+        self.expect(&token::Eq)?;
+        let expr = self.with_res(Restrictions::NO_STRUCT_LITERAL, |this| {
+            this.parse_assoc_expr_with(1 + prec_let_scrutinee_needs_par(), None.into())
+        })?;
+        let span = lo.to(expr.span);
+        self.sess.gated_spans.gate(sym::let_chains, span);
+        Ok(self.mk_expr(span, ExprKind::Let(pat, expr), attrs))
+    }
+
+    /// Parses an `else { ... }` expression (`else` token already eaten).
+    fn parse_else_expr(&mut self) -> PResult<'a, P<Expr>> {
+        let ctx_span = self.prev_token.span; // `else`
+        let attrs = self.parse_outer_attributes()?; // For recovery.
+        let expr = if self.eat_keyword(kw::If) {
+            self.parse_if_expr(AttrVec::new())?
+        } else {
+            let blk = self.parse_block()?;
+            self.mk_expr(blk.span, ExprKind::Block(blk, None), AttrVec::new())
+        };
+        self.error_on_if_block_attrs(ctx_span, true, expr.span, &attrs);
+        Ok(expr)
+    }
+
+    fn error_on_if_block_attrs(
+        &self,
+        ctx_span: Span,
+        is_ctx_else: bool,
+        branch_span: Span,
+        attrs: &[ast::Attribute],
+    ) {
+        let (span, last) = match attrs {
+            [] => return,
+            [x0 @ xn] | [x0, .., xn] => (x0.span.to(xn.span), xn.span),
+        };
+        let ctx = if is_ctx_else { "else" } else { "if" };
+        self.struct_span_err(last, "outer attributes are not allowed on `if` and `else` branches")
+            .span_label(branch_span, "the attributes are attached to this branch")
+            .span_label(ctx_span, format!("the branch belongs to this `{}`", ctx))
+            .span_suggestion(
+                span,
+                "remove the attributes",
+                String::new(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+    }
+
+    /// Parses `for <src_pat> in <src_expr> <src_loop_block>` (`for` token already eaten).
+    fn parse_for_expr(
+        &mut self,
+        opt_label: Option<Label>,
+        lo: Span,
+        mut attrs: AttrVec,
+    ) -> PResult<'a, P<Expr>> {
+        // Record whether we are about to parse `for (`.
+        // This is used below for recovery in case of `for ( $stuff ) $block`
+        // in which case we will suggest `for $stuff $block`.
+        let begin_paren = match self.token.kind {
+            token::OpenDelim(token::Paren) => Some(self.token.span),
+            _ => None,
+        };
+
+        let pat = self.parse_top_pat(GateOr::Yes)?;
+        if !self.eat_keyword(kw::In) {
+            self.error_missing_in_for_loop();
+        }
+        self.check_for_for_in_in_typo(self.prev_token.span);
+        let expr = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
+
+        let pat = self.recover_parens_around_for_head(pat, &expr, begin_paren);
+
+        let (iattrs, loop_block) = self.parse_inner_attrs_and_block()?;
+        attrs.extend(iattrs);
+
+        let kind = ExprKind::ForLoop(pat, expr, loop_block, opt_label);
+        Ok(self.mk_expr(lo.to(self.prev_token.span), kind, attrs))
+    }
+
+    fn error_missing_in_for_loop(&mut self) {
+        let (span, msg, sugg) = if self.token.is_ident_named(sym::of) {
+            // Possibly using JS syntax (#75311).
+            let span = self.token.span;
+            self.bump();
+            (span, "try using `in` here instead", "in")
+        } else {
+            (self.prev_token.span.between(self.token.span), "try adding `in` here", " in ")
+        };
+        self.struct_span_err(span, "missing `in` in `for` loop")
+            .span_suggestion_short(
+                span,
+                msg,
+                sugg.into(),
+                // Has been misleading, at least in the past (closed Issue #48492).
+                Applicability::MaybeIncorrect,
+            )
+            .emit();
+    }
+
+    /// Parses a `while` or `while let` expression (`while` token already eaten).
+    fn parse_while_expr(
+        &mut self,
+        opt_label: Option<Label>,
+        lo: Span,
+        mut attrs: AttrVec,
+    ) -> PResult<'a, P<Expr>> {
+        let cond = self.parse_cond_expr()?;
+        let (iattrs, body) = self.parse_inner_attrs_and_block()?;
+        attrs.extend(iattrs);
+        Ok(self.mk_expr(lo.to(self.prev_token.span), ExprKind::While(cond, body, opt_label), attrs))
+    }
+
+    /// Parses `loop { ... }` (`loop` token already eaten).
+    fn parse_loop_expr(
+        &mut self,
+        opt_label: Option<Label>,
+        lo: Span,
+        mut attrs: AttrVec,
+    ) -> PResult<'a, P<Expr>> {
+        let (iattrs, body) = self.parse_inner_attrs_and_block()?;
+        attrs.extend(iattrs);
+        Ok(self.mk_expr(lo.to(self.prev_token.span), ExprKind::Loop(body, opt_label), attrs))
+    }
+
+    fn eat_label(&mut self) -> Option<Label> {
+        self.token.lifetime().map(|ident| {
+            self.bump();
+            Label { ident }
+        })
+    }
+
+    /// Parses a `match ... { ... }` expression (`match` token already eaten).
+    fn parse_match_expr(&mut self, mut attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let match_span = self.prev_token.span;
+        let lo = self.prev_token.span;
+        let scrutinee = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
+        if let Err(mut e) = self.expect(&token::OpenDelim(token::Brace)) {
+            if self.token == token::Semi {
+                e.span_suggestion_short(
+                    match_span,
+                    "try removing this `match`",
+                    String::new(),
+                    Applicability::MaybeIncorrect, // speculative
+                );
+            }
+            return Err(e);
+        }
+        attrs.extend(self.parse_inner_attributes()?);
+
+        let mut arms: Vec<Arm> = Vec::new();
+        while self.token != token::CloseDelim(token::Brace) {
+            match self.parse_arm() {
+                Ok(arm) => arms.push(arm),
+                Err(mut e) => {
+                    // Recover by skipping to the end of the block.
+                    e.emit();
+                    self.recover_stmt();
+                    let span = lo.to(self.token.span);
+                    if self.token == token::CloseDelim(token::Brace) {
+                        self.bump();
+                    }
+                    return Ok(self.mk_expr(span, ExprKind::Match(scrutinee, arms), attrs));
+                }
+            }
+        }
+        let hi = self.token.span;
+        self.bump();
+        Ok(self.mk_expr(lo.to(hi), ExprKind::Match(scrutinee, arms), attrs))
+    }
+
+    pub(super) fn parse_arm(&mut self) -> PResult<'a, Arm> {
+        let attrs = self.parse_outer_attributes()?;
+        let lo = self.token.span;
+        let pat = self.parse_top_pat(GateOr::No)?;
+        let guard = if self.eat_keyword(kw::If) {
+            let if_span = self.prev_token.span;
+            let cond = self.parse_expr()?;
+            if let ExprKind::Let(..) = cond.kind {
+                // Remove the last feature gating of a `let` expression since it's stable.
+                self.sess.gated_spans.ungate_last(sym::let_chains, cond.span);
+                let span = if_span.to(cond.span);
+                self.sess.gated_spans.gate(sym::if_let_guard, span);
+            }
+            Some(cond)
+        } else {
+            None
+        };
+        let arrow_span = self.token.span;
+        self.expect(&token::FatArrow)?;
+        let arm_start_span = self.token.span;
+
+        let expr = self.parse_expr_res(Restrictions::STMT_EXPR, None).map_err(|mut err| {
+            err.span_label(arrow_span, "while parsing the `match` arm starting here");
+            err
+        })?;
+
+        let require_comma = classify::expr_requires_semi_to_be_stmt(&expr)
+            && self.token != token::CloseDelim(token::Brace);
+
+        let hi = self.prev_token.span;
+
+        if require_comma {
+            let sm = self.sess.source_map();
+            self.expect_one_of(&[token::Comma], &[token::CloseDelim(token::Brace)]).map_err(
+                |mut err| {
+                    match (sm.span_to_lines(expr.span), sm.span_to_lines(arm_start_span)) {
+                        (Ok(ref expr_lines), Ok(ref arm_start_lines))
+                            if arm_start_lines.lines[0].end_col == expr_lines.lines[0].end_col
+                                && expr_lines.lines.len() == 2
+                                && self.token == token::FatArrow =>
+                        {
+                            // We check whether there's any trailing code in the parse span,
+                            // if there isn't, we very likely have the following:
+                            //
+                            // X |     &Y => "y"
+                            //   |        --    - missing comma
+                            //   |        |
+                            //   |        arrow_span
+                            // X |     &X => "x"
+                            //   |      - ^^ self.token.span
+                            //   |      |
+                            //   |      parsed until here as `"y" & X`
+                            err.span_suggestion_short(
+                                arm_start_span.shrink_to_hi(),
+                                "missing a comma here to end this `match` arm",
+                                ",".to_owned(),
+                                Applicability::MachineApplicable,
+                            );
+                        }
+                        _ => {
+                            err.span_label(
+                                arrow_span,
+                                "while parsing the `match` arm starting here",
+                            );
+                        }
+                    }
+                    err
+                },
+            )?;
+        } else {
+            self.eat(&token::Comma);
+        }
+
+        Ok(ast::Arm {
+            attrs,
+            pat,
+            guard,
+            body: expr,
+            span: lo.to(hi),
+            id: DUMMY_NODE_ID,
+            is_placeholder: false,
+        })
+    }
+
+    /// Parses a `try {...}` expression (`try` token already eaten).
+    fn parse_try_block(&mut self, span_lo: Span, mut attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let (iattrs, body) = self.parse_inner_attrs_and_block()?;
+        attrs.extend(iattrs);
+        if self.eat_keyword(kw::Catch) {
+            let mut error = self.struct_span_err(
+                self.prev_token.span,
+                "keyword `catch` cannot follow a `try` block",
+            );
+            error.help("try using `match` on the result of the `try` block instead");
+            error.emit();
+            Err(error)
+        } else {
+            let span = span_lo.to(body.span);
+            self.sess.gated_spans.gate(sym::try_blocks, span);
+            Ok(self.mk_expr(span, ExprKind::TryBlock(body), attrs))
+        }
+    }
+
+    fn is_do_catch_block(&self) -> bool {
+        self.token.is_keyword(kw::Do)
+            && self.is_keyword_ahead(1, &[kw::Catch])
+            && self.look_ahead(2, |t| *t == token::OpenDelim(token::Brace))
+            && !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL)
+    }
+
+    fn is_try_block(&self) -> bool {
+        self.token.is_keyword(kw::Try)
+            && self.look_ahead(1, |t| *t == token::OpenDelim(token::Brace))
+            && self.token.uninterpolated_span().rust_2018()
+    }
+
+    /// Parses an `async move? {...}` expression.
+    fn parse_async_block(&mut self, mut attrs: AttrVec) -> PResult<'a, P<Expr>> {
+        let lo = self.token.span;
+        self.expect_keyword(kw::Async)?;
+        let capture_clause = self.parse_capture_clause();
+        let (iattrs, body) = self.parse_inner_attrs_and_block()?;
+        attrs.extend(iattrs);
+        let kind = ExprKind::Async(capture_clause, DUMMY_NODE_ID, body);
+        Ok(self.mk_expr(lo.to(self.prev_token.span), kind, attrs))
+    }
+
+    fn is_async_block(&self) -> bool {
+        self.token.is_keyword(kw::Async)
+            && ((
+                // `async move {`
+                self.is_keyword_ahead(1, &[kw::Move])
+                    && self.look_ahead(2, |t| *t == token::OpenDelim(token::Brace))
+            ) || (
+                // `async {`
+                self.look_ahead(1, |t| *t == token::OpenDelim(token::Brace))
+            ))
+    }
+
+    fn is_certainly_not_a_block(&self) -> bool {
+        self.look_ahead(1, |t| t.is_ident())
+            && (
+                // `{ ident, ` cannot start a block.
+                self.look_ahead(2, |t| t == &token::Comma)
+                    || self.look_ahead(2, |t| t == &token::Colon)
+                        && (
+                            // `{ ident: token, ` cannot start a block.
+                            self.look_ahead(4, |t| t == &token::Comma) ||
+                // `{ ident: ` cannot start a block unless it's a type ascription `ident: Type`.
+                self.look_ahead(3, |t| !t.can_begin_type())
+                        )
+            )
+    }
+
+    fn maybe_parse_struct_expr(
+        &mut self,
+        path: &ast::Path,
+        attrs: &AttrVec,
+    ) -> Option<PResult<'a, P<Expr>>> {
+        let struct_allowed = !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL);
+        if struct_allowed || self.is_certainly_not_a_block() {
+            // This is a struct literal, but we don't can't accept them here.
+            let expr = self.parse_struct_expr(path.clone(), attrs.clone());
+            if let (Ok(expr), false) = (&expr, struct_allowed) {
+                self.error_struct_lit_not_allowed_here(path.span, expr.span);
+            }
+            return Some(expr);
+        }
+        None
+    }
+
+    fn error_struct_lit_not_allowed_here(&self, lo: Span, sp: Span) {
+        self.struct_span_err(sp, "struct literals are not allowed here")
+            .multipart_suggestion(
+                "surround the struct literal with parentheses",
+                vec![(lo.shrink_to_lo(), "(".to_string()), (sp.shrink_to_hi(), ")".to_string())],
+                Applicability::MachineApplicable,
+            )
+            .emit();
+    }
+
+    pub(super) fn parse_struct_expr(
+        &mut self,
+        pth: ast::Path,
+        mut attrs: AttrVec,
+    ) -> PResult<'a, P<Expr>> {
+        self.bump();
+        let mut fields = Vec::new();
+        let mut base = None;
+        let mut recover_async = false;
+
+        attrs.extend(self.parse_inner_attributes()?);
+
+        let mut async_block_err = |e: &mut DiagnosticBuilder<'_>, span: Span| {
+            recover_async = true;
+            e.span_label(span, "`async` blocks are only allowed in the 2018 edition");
+            e.help("set `edition = \"2018\"` in `Cargo.toml`");
+            e.note("for more on editions, read https://doc.rust-lang.org/edition-guide");
+        };
+
+        while self.token != token::CloseDelim(token::Brace) {
+            if self.eat(&token::DotDot) {
+                let exp_span = self.prev_token.span;
+                match self.parse_expr() {
+                    Ok(e) => base = Some(e),
+                    Err(mut e) => {
+                        e.emit();
+                        self.recover_stmt();
+                    }
+                }
+                self.recover_struct_comma_after_dotdot(exp_span);
+                break;
+            }
+
+            let recovery_field = self.find_struct_error_after_field_looking_code();
+            let parsed_field = match self.parse_field() {
+                Ok(f) => Some(f),
+                Err(mut e) => {
+                    if pth == kw::Async {
+                        async_block_err(&mut e, pth.span);
+                    } else {
+                        e.span_label(pth.span, "while parsing this struct");
+                    }
+                    e.emit();
+
+                    // If the next token is a comma, then try to parse
+                    // what comes next as additional fields, rather than
+                    // bailing out until next `}`.
+                    if self.token != token::Comma {
+                        self.recover_stmt_(SemiColonMode::Comma, BlockMode::Ignore);
+                        if self.token != token::Comma {
+                            break;
+                        }
+                    }
+                    None
+                }
+            };
+
+            match self.expect_one_of(&[token::Comma], &[token::CloseDelim(token::Brace)]) {
+                Ok(_) => {
+                    if let Some(f) = parsed_field.or(recovery_field) {
+                        // Only include the field if there's no parse error for the field name.
+                        fields.push(f);
+                    }
+                }
+                Err(mut e) => {
+                    if pth == kw::Async {
+                        async_block_err(&mut e, pth.span);
+                    } else {
+                        e.span_label(pth.span, "while parsing this struct");
+                        if let Some(f) = recovery_field {
+                            fields.push(f);
+                            e.span_suggestion(
+                                self.prev_token.span.shrink_to_hi(),
+                                "try adding a comma",
+                                ",".into(),
+                                Applicability::MachineApplicable,
+                            );
+                        }
+                    }
+                    e.emit();
+                    self.recover_stmt_(SemiColonMode::Comma, BlockMode::Ignore);
+                    self.eat(&token::Comma);
+                }
+            }
+        }
+
+        let span = pth.span.to(self.token.span);
+        self.expect(&token::CloseDelim(token::Brace))?;
+        let expr = if recover_async { ExprKind::Err } else { ExprKind::Struct(pth, fields, base) };
+        Ok(self.mk_expr(span, expr, attrs))
+    }
+
+    /// Use in case of error after field-looking code: `S { foo: () with a }`.
+    fn find_struct_error_after_field_looking_code(&self) -> Option<Field> {
+        match self.token.ident() {
+            Some((ident, is_raw))
+                if (is_raw || !ident.is_reserved())
+                    && self.look_ahead(1, |t| *t == token::Colon) =>
+            {
+                Some(ast::Field {
+                    ident,
+                    span: self.token.span,
+                    expr: self.mk_expr_err(self.token.span),
+                    is_shorthand: false,
+                    attrs: AttrVec::new(),
+                    id: DUMMY_NODE_ID,
+                    is_placeholder: false,
+                })
+            }
+            _ => None,
+        }
+    }
+
+    fn recover_struct_comma_after_dotdot(&mut self, span: Span) {
+        if self.token != token::Comma {
+            return;
+        }
+        self.struct_span_err(
+            span.to(self.prev_token.span),
+            "cannot use a comma after the base struct",
+        )
+        .span_suggestion_short(
+            self.token.span,
+            "remove this comma",
+            String::new(),
+            Applicability::MachineApplicable,
+        )
+        .note("the base struct must always be the last field")
+        .emit();
+        self.recover_stmt();
+    }
+
+    /// Parses `ident (COLON expr)?`.
+    fn parse_field(&mut self) -> PResult<'a, Field> {
+        let attrs = self.parse_outer_attributes()?.into();
+        let lo = self.token.span;
+
+        // Check if a colon exists one ahead. This means we're parsing a fieldname.
+        let is_shorthand = !self.look_ahead(1, |t| t == &token::Colon || t == &token::Eq);
+        let (ident, expr) = if is_shorthand {
+            // Mimic `x: x` for the `x` field shorthand.
+            let ident = self.parse_ident_common(false)?;
+            let path = ast::Path::from_ident(ident);
+            (ident, self.mk_expr(ident.span, ExprKind::Path(None, path), AttrVec::new()))
+        } else {
+            let ident = self.parse_field_name()?;
+            self.error_on_eq_field_init(ident);
+            self.bump(); // `:`
+            (ident, self.parse_expr()?)
+        };
+        Ok(ast::Field {
+            ident,
+            span: lo.to(expr.span),
+            expr,
+            is_shorthand,
+            attrs,
+            id: DUMMY_NODE_ID,
+            is_placeholder: false,
+        })
+    }
+
+    /// Check for `=`. This means the source incorrectly attempts to
+    /// initialize a field with an eq rather than a colon.
+    fn error_on_eq_field_init(&self, field_name: Ident) {
+        if self.token != token::Eq {
+            return;
+        }
+
+        self.struct_span_err(self.token.span, "expected `:`, found `=`")
+            .span_suggestion(
+                field_name.span.shrink_to_hi().to(self.token.span),
+                "replace equals symbol with a colon",
+                ":".to_string(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+    }
+
+    fn err_dotdotdot_syntax(&self, span: Span) {
+        self.struct_span_err(span, "unexpected token: `...`")
+            .span_suggestion(
+                span,
+                "use `..` for an exclusive range",
+                "..".to_owned(),
+                Applicability::MaybeIncorrect,
+            )
+            .span_suggestion(
+                span,
+                "or `..=` for an inclusive range",
+                "..=".to_owned(),
+                Applicability::MaybeIncorrect,
+            )
+            .emit();
+    }
+
+    fn err_larrow_operator(&self, span: Span) {
+        self.struct_span_err(span, "unexpected token: `<-`")
+            .span_suggestion(
+                span,
+                "if you meant to write a comparison against a negative value, add a \
+             space in between `<` and `-`",
+                "< -".to_string(),
+                Applicability::MaybeIncorrect,
+            )
+            .emit();
+    }
+
+    fn mk_assign_op(&self, binop: BinOp, lhs: P<Expr>, rhs: P<Expr>) -> ExprKind {
+        ExprKind::AssignOp(binop, lhs, rhs)
+    }
+
+    fn mk_range(
+        &self,
+        start: Option<P<Expr>>,
+        end: Option<P<Expr>>,
+        limits: RangeLimits,
+    ) -> PResult<'a, ExprKind> {
+        if end.is_none() && limits == RangeLimits::Closed {
+            self.error_inclusive_range_with_no_end(self.prev_token.span);
+            Ok(ExprKind::Err)
+        } else {
+            Ok(ExprKind::Range(start, end, limits))
+        }
+    }
+
+    fn mk_unary(&self, unop: UnOp, expr: P<Expr>) -> ExprKind {
+        ExprKind::Unary(unop, expr)
+    }
+
+    fn mk_binary(&self, binop: BinOp, lhs: P<Expr>, rhs: P<Expr>) -> ExprKind {
+        ExprKind::Binary(binop, lhs, rhs)
+    }
+
+    fn mk_index(&self, expr: P<Expr>, idx: P<Expr>) -> ExprKind {
+        ExprKind::Index(expr, idx)
+    }
+
+    fn mk_call(&self, f: P<Expr>, args: Vec<P<Expr>>) -> ExprKind {
+        ExprKind::Call(f, args)
+    }
+
+    fn mk_await_expr(&mut self, self_arg: P<Expr>, lo: Span) -> PResult<'a, P<Expr>> {
+        let span = lo.to(self.prev_token.span);
+        let await_expr = self.mk_expr(span, ExprKind::Await(self_arg), AttrVec::new());
+        self.recover_from_await_method_call();
+        Ok(await_expr)
+    }
+
+    crate fn mk_expr(&self, span: Span, kind: ExprKind, attrs: AttrVec) -> P<Expr> {
+        P(Expr { kind, span, attrs, id: DUMMY_NODE_ID, tokens: None })
+    }
+
+    pub(super) fn mk_expr_err(&self, span: Span) -> P<Expr> {
+        self.mk_expr(span, ExprKind::Err, AttrVec::new())
+    }
+}
diff --git a/compiler/rustc_parse/src/parser/generics.rs b/compiler/rustc_parse/src/parser/generics.rs
new file mode 100644
index 00000000000..dd99a7587dd
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/generics.rs
@@ -0,0 +1,292 @@
+use super::Parser;
+
+use rustc_ast::token;
+use rustc_ast::{
+    self as ast, Attribute, GenericBounds, GenericParam, GenericParamKind, WhereClause,
+};
+use rustc_errors::PResult;
+use rustc_span::symbol::{kw, sym};
+
+impl<'a> Parser<'a> {
+    /// Parses bounds of a lifetime parameter `BOUND + BOUND + BOUND`, possibly with trailing `+`.
+    ///
+    /// ```text
+    /// BOUND = LT_BOUND (e.g., `'a`)
+    /// ```
+    fn parse_lt_param_bounds(&mut self) -> GenericBounds {
+        let mut lifetimes = Vec::new();
+        while self.check_lifetime() {
+            lifetimes.push(ast::GenericBound::Outlives(self.expect_lifetime()));
+
+            if !self.eat_plus() {
+                break;
+            }
+        }
+        lifetimes
+    }
+
+    /// Matches `typaram = IDENT (`?` unbound)? optbounds ( EQ ty )?`.
+    fn parse_ty_param(&mut self, preceding_attrs: Vec<Attribute>) -> PResult<'a, GenericParam> {
+        let ident = self.parse_ident()?;
+
+        // Parse optional colon and param bounds.
+        let bounds = if self.eat(&token::Colon) {
+            self.parse_generic_bounds(Some(self.prev_token.span))?
+        } else {
+            Vec::new()
+        };
+
+        let default = if self.eat(&token::Eq) { Some(self.parse_ty()?) } else { None };
+
+        Ok(GenericParam {
+            ident,
+            id: ast::DUMMY_NODE_ID,
+            attrs: preceding_attrs.into(),
+            bounds,
+            kind: GenericParamKind::Type { default },
+            is_placeholder: false,
+        })
+    }
+
+    fn parse_const_param(&mut self, preceding_attrs: Vec<Attribute>) -> PResult<'a, GenericParam> {
+        let const_span = self.token.span;
+
+        self.expect_keyword(kw::Const)?;
+        let ident = self.parse_ident()?;
+        self.expect(&token::Colon)?;
+        let ty = self.parse_ty()?;
+
+        self.sess.gated_spans.gate(sym::min_const_generics, const_span.to(self.prev_token.span));
+
+        Ok(GenericParam {
+            ident,
+            id: ast::DUMMY_NODE_ID,
+            attrs: preceding_attrs.into(),
+            bounds: Vec::new(),
+            kind: GenericParamKind::Const { ty, kw_span: const_span },
+            is_placeholder: false,
+        })
+    }
+
+    /// Parses a (possibly empty) list of lifetime and type parameters, possibly including
+    /// a trailing comma and erroneous trailing attributes.
+    pub(super) fn parse_generic_params(&mut self) -> PResult<'a, Vec<ast::GenericParam>> {
+        let mut params = Vec::new();
+        loop {
+            let attrs = self.parse_outer_attributes()?;
+            if self.check_lifetime() {
+                let lifetime = self.expect_lifetime();
+                // Parse lifetime parameter.
+                let bounds =
+                    if self.eat(&token::Colon) { self.parse_lt_param_bounds() } else { Vec::new() };
+                params.push(ast::GenericParam {
+                    ident: lifetime.ident,
+                    id: lifetime.id,
+                    attrs: attrs.into(),
+                    bounds,
+                    kind: ast::GenericParamKind::Lifetime,
+                    is_placeholder: false,
+                });
+            } else if self.check_keyword(kw::Const) {
+                // Parse const parameter.
+                params.push(self.parse_const_param(attrs)?);
+            } else if self.check_ident() {
+                // Parse type parameter.
+                params.push(self.parse_ty_param(attrs)?);
+            } else if self.token.can_begin_type() {
+                // Trying to write an associated type bound? (#26271)
+                let snapshot = self.clone();
+                match self.parse_ty_where_predicate() {
+                    Ok(where_predicate) => {
+                        self.struct_span_err(
+                            where_predicate.span(),
+                            "bounds on associated types do not belong here",
+                        )
+                        .span_label(where_predicate.span(), "belongs in `where` clause")
+                        .emit();
+                    }
+                    Err(mut err) => {
+                        err.cancel();
+                        *self = snapshot;
+                        break;
+                    }
+                }
+            } else {
+                // Check for trailing attributes and stop parsing.
+                if !attrs.is_empty() {
+                    if !params.is_empty() {
+                        self.struct_span_err(
+                            attrs[0].span,
+                            "trailing attribute after generic parameter",
+                        )
+                        .span_label(attrs[0].span, "attributes must go before parameters")
+                        .emit();
+                    } else {
+                        self.struct_span_err(attrs[0].span, "attribute without generic parameters")
+                            .span_label(
+                                attrs[0].span,
+                                "attributes are only permitted when preceding parameters",
+                            )
+                            .emit();
+                    }
+                }
+                break;
+            }
+
+            if !self.eat(&token::Comma) {
+                break;
+            }
+        }
+        Ok(params)
+    }
+
+    /// Parses a set of optional generic type parameter declarations. Where
+    /// clauses are not parsed here, and must be added later via
+    /// `parse_where_clause()`.
+    ///
+    /// matches generics = ( ) | ( < > ) | ( < typaramseq ( , )? > ) | ( < lifetimes ( , )? > )
+    ///                  | ( < lifetimes , typaramseq ( , )? > )
+    /// where   typaramseq = ( typaram ) | ( typaram , typaramseq )
+    pub(super) fn parse_generics(&mut self) -> PResult<'a, ast::Generics> {
+        let span_lo = self.token.span;
+        let (params, span) = if self.eat_lt() {
+            let params = self.parse_generic_params()?;
+            self.expect_gt()?;
+            (params, span_lo.to(self.prev_token.span))
+        } else {
+            (vec![], self.prev_token.span.shrink_to_hi())
+        };
+        Ok(ast::Generics {
+            params,
+            where_clause: WhereClause {
+                has_where_token: false,
+                predicates: Vec::new(),
+                span: self.prev_token.span.shrink_to_hi(),
+            },
+            span,
+        })
+    }
+
+    /// Parses an optional where-clause and places it in `generics`.
+    ///
+    /// ```ignore (only-for-syntax-highlight)
+    /// where T : Trait<U, V> + 'b, 'a : 'b
+    /// ```
+    pub(super) fn parse_where_clause(&mut self) -> PResult<'a, WhereClause> {
+        let mut where_clause = WhereClause {
+            has_where_token: false,
+            predicates: Vec::new(),
+            span: self.prev_token.span.shrink_to_hi(),
+        };
+
+        if !self.eat_keyword(kw::Where) {
+            return Ok(where_clause);
+        }
+        where_clause.has_where_token = true;
+        let lo = self.prev_token.span;
+
+        // We are considering adding generics to the `where` keyword as an alternative higher-rank
+        // parameter syntax (as in `where<'a>` or `where<T>`. To avoid that being a breaking
+        // change we parse those generics now, but report an error.
+        if self.choose_generics_over_qpath(0) {
+            let generics = self.parse_generics()?;
+            self.struct_span_err(
+                generics.span,
+                "generic parameters on `where` clauses are reserved for future use",
+            )
+            .span_label(generics.span, "currently unsupported")
+            .emit();
+        }
+
+        loop {
+            let lo = self.token.span;
+            if self.check_lifetime() && self.look_ahead(1, |t| !t.is_like_plus()) {
+                let lifetime = self.expect_lifetime();
+                // Bounds starting with a colon are mandatory, but possibly empty.
+                self.expect(&token::Colon)?;
+                let bounds = self.parse_lt_param_bounds();
+                where_clause.predicates.push(ast::WherePredicate::RegionPredicate(
+                    ast::WhereRegionPredicate {
+                        span: lo.to(self.prev_token.span),
+                        lifetime,
+                        bounds,
+                    },
+                ));
+            } else if self.check_type() {
+                where_clause.predicates.push(self.parse_ty_where_predicate()?);
+            } else {
+                break;
+            }
+
+            if !self.eat(&token::Comma) {
+                break;
+            }
+        }
+
+        where_clause.span = lo.to(self.prev_token.span);
+        Ok(where_clause)
+    }
+
+    fn parse_ty_where_predicate(&mut self) -> PResult<'a, ast::WherePredicate> {
+        let lo = self.token.span;
+        // Parse optional `for<'a, 'b>`.
+        // This `for` is parsed greedily and applies to the whole predicate,
+        // the bounded type can have its own `for` applying only to it.
+        // Examples:
+        // * `for<'a> Trait1<'a>: Trait2<'a /* ok */>`
+        // * `(for<'a> Trait1<'a>): Trait2<'a /* not ok */>`
+        // * `for<'a> for<'b> Trait1<'a, 'b>: Trait2<'a /* ok */, 'b /* not ok */>`
+        let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
+
+        // Parse type with mandatory colon and (possibly empty) bounds,
+        // or with mandatory equality sign and the second type.
+        let ty = self.parse_ty()?;
+        if self.eat(&token::Colon) {
+            let bounds = self.parse_generic_bounds(Some(self.prev_token.span))?;
+            Ok(ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate {
+                span: lo.to(self.prev_token.span),
+                bound_generic_params: lifetime_defs,
+                bounded_ty: ty,
+                bounds,
+            }))
+        // FIXME: Decide what should be used here, `=` or `==`.
+        // FIXME: We are just dropping the binders in lifetime_defs on the floor here.
+        } else if self.eat(&token::Eq) || self.eat(&token::EqEq) {
+            let rhs_ty = self.parse_ty()?;
+            Ok(ast::WherePredicate::EqPredicate(ast::WhereEqPredicate {
+                span: lo.to(self.prev_token.span),
+                lhs_ty: ty,
+                rhs_ty,
+                id: ast::DUMMY_NODE_ID,
+            }))
+        } else {
+            self.unexpected()
+        }
+    }
+
+    pub(super) fn choose_generics_over_qpath(&self, start: usize) -> bool {
+        // There's an ambiguity between generic parameters and qualified paths in impls.
+        // If we see `<` it may start both, so we have to inspect some following tokens.
+        // The following combinations can only start generics,
+        // but not qualified paths (with one exception):
+        //     `<` `>` - empty generic parameters
+        //     `<` `#` - generic parameters with attributes
+        //     `<` (LIFETIME|IDENT) `>` - single generic parameter
+        //     `<` (LIFETIME|IDENT) `,` - first generic parameter in a list
+        //     `<` (LIFETIME|IDENT) `:` - generic parameter with bounds
+        //     `<` (LIFETIME|IDENT) `=` - generic parameter with a default
+        //     `<` const                - generic const parameter
+        // The only truly ambiguous case is
+        //     `<` IDENT `>` `::` IDENT ...
+        // we disambiguate it in favor of generics (`impl<T> ::absolute::Path<T> { ... }`)
+        // because this is what almost always expected in practice, qualified paths in impls
+        // (`impl <Type>::AssocTy { ... }`) aren't even allowed by type checker at the moment.
+        self.look_ahead(start, |t| t == &token::Lt)
+            && (self.look_ahead(start + 1, |t| t == &token::Pound || t == &token::Gt)
+                || self.look_ahead(start + 1, |t| t.is_lifetime() || t.is_ident())
+                    && self.look_ahead(start + 2, |t| {
+                        matches!(t.kind, token::Gt | token::Comma | token::Colon | token::Eq)
+                    })
+                || self.is_keyword_ahead(start + 1, &[kw::Const]))
+    }
+}
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
new file mode 100644
index 00000000000..9143af651df
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -0,0 +1,1843 @@
+use super::diagnostics::{dummy_arg, ConsumeClosingDelim, Error};
+use super::ty::{AllowPlus, RecoverQPath};
+use super::{FollowedByType, Parser, PathStyle};
+
+use crate::maybe_whole;
+
+use rustc_ast::ptr::P;
+use rustc_ast::token::{self, TokenKind};
+use rustc_ast::tokenstream::{DelimSpan, TokenStream, TokenTree};
+use rustc_ast::{self as ast, AttrStyle, AttrVec, Attribute, DUMMY_NODE_ID};
+use rustc_ast::{AssocItem, AssocItemKind, ForeignItemKind, Item, ItemKind, Mod};
+use rustc_ast::{Async, Const, Defaultness, IsAuto, Mutability, Unsafe, UseTree, UseTreeKind};
+use rustc_ast::{BindingMode, Block, FnDecl, FnSig, Param, SelfKind};
+use rustc_ast::{EnumDef, Generics, StructField, TraitRef, Ty, TyKind, Variant, VariantData};
+use rustc_ast::{FnHeader, ForeignItem, Path, PathSegment, Visibility, VisibilityKind};
+use rustc_ast::{MacArgs, MacCall, MacDelimiter};
+use rustc_ast_pretty::pprust;
+use rustc_errors::{struct_span_err, Applicability, PResult, StashKey};
+use rustc_span::edition::Edition;
+use rustc_span::source_map::{self, Span};
+use rustc_span::symbol::{kw, sym, Ident, Symbol};
+
+use std::convert::TryFrom;
+use std::mem;
+use tracing::debug;
+
+impl<'a> Parser<'a> {
+    /// Parses a source module as a crate. This is the main entry point for the parser.
+    pub fn parse_crate_mod(&mut self) -> PResult<'a, ast::Crate> {
+        let lo = self.token.span;
+        let (module, attrs) = self.parse_mod(&token::Eof)?;
+        let span = lo.to(self.token.span);
+        let proc_macros = Vec::new(); // Filled in by `proc_macro_harness::inject()`.
+        Ok(ast::Crate { attrs, module, span, proc_macros })
+    }
+
+    /// Parses a `mod <foo> { ... }` or `mod <foo>;` item.
+    fn parse_item_mod(&mut self, attrs: &mut Vec<Attribute>) -> PResult<'a, ItemInfo> {
+        let id = self.parse_ident()?;
+        let (module, mut inner_attrs) = if self.eat(&token::Semi) {
+            Default::default()
+        } else {
+            self.expect(&token::OpenDelim(token::Brace))?;
+            self.parse_mod(&token::CloseDelim(token::Brace))?
+        };
+        attrs.append(&mut inner_attrs);
+        Ok((id, ItemKind::Mod(module)))
+    }
+
+    /// Parses the contents of a module (inner attributes followed by module items).
+    pub fn parse_mod(&mut self, term: &TokenKind) -> PResult<'a, (Mod, Vec<Attribute>)> {
+        let lo = self.token.span;
+        let attrs = self.parse_inner_attributes()?;
+        let module = self.parse_mod_items(term, lo)?;
+        Ok((module, attrs))
+    }
+
+    /// Given a termination token, parses all of the items in a module.
+    fn parse_mod_items(&mut self, term: &TokenKind, inner_lo: Span) -> PResult<'a, Mod> {
+        let mut items = vec![];
+        while let Some(item) = self.parse_item()? {
+            items.push(item);
+            self.maybe_consume_incorrect_semicolon(&items);
+        }
+
+        if !self.eat(term) {
+            let token_str = super::token_descr(&self.token);
+            if !self.maybe_consume_incorrect_semicolon(&items) {
+                let msg = &format!("expected item, found {}", token_str);
+                let mut err = self.struct_span_err(self.token.span, msg);
+                err.span_label(self.token.span, "expected item");
+                return Err(err);
+            }
+        }
+
+        let hi = if self.token.span.is_dummy() { inner_lo } else { self.prev_token.span };
+
+        Ok(Mod { inner: inner_lo.to(hi), items, inline: true })
+    }
+}
+
+pub(super) type ItemInfo = (Ident, ItemKind);
+
+impl<'a> Parser<'a> {
+    pub fn parse_item(&mut self) -> PResult<'a, Option<P<Item>>> {
+        self.parse_item_(|_| true).map(|i| i.map(P))
+    }
+
+    fn parse_item_(&mut self, req_name: ReqName) -> PResult<'a, Option<Item>> {
+        let attrs = self.parse_outer_attributes()?;
+        self.parse_item_common(attrs, true, false, req_name)
+    }
+
+    pub(super) fn parse_item_common(
+        &mut self,
+        mut attrs: Vec<Attribute>,
+        mac_allowed: bool,
+        attrs_allowed: bool,
+        req_name: ReqName,
+    ) -> PResult<'a, Option<Item>> {
+        maybe_whole!(self, NtItem, |item| {
+            let mut item = item;
+            mem::swap(&mut item.attrs, &mut attrs);
+            item.attrs.extend(attrs);
+            Some(item.into_inner())
+        });
+
+        let mut unclosed_delims = vec![];
+        let has_attrs = !attrs.is_empty();
+        let parse_item = |this: &mut Self| {
+            let item = this.parse_item_common_(attrs, mac_allowed, attrs_allowed, req_name);
+            unclosed_delims.append(&mut this.unclosed_delims);
+            item
+        };
+
+        let (mut item, tokens) = if has_attrs {
+            let (item, tokens) = self.collect_tokens(parse_item)?;
+            (item, Some(tokens))
+        } else {
+            (parse_item(self)?, None)
+        };
+
+        self.unclosed_delims.append(&mut unclosed_delims);
+
+        // Once we've parsed an item and recorded the tokens we got while
+        // parsing we may want to store `tokens` into the item we're about to
+        // return. Note, though, that we specifically didn't capture tokens
+        // related to outer attributes. The `tokens` field here may later be
+        // used with procedural macros to convert this item back into a token
+        // stream, but during expansion we may be removing attributes as we go
+        // along.
+        //
+        // If we've got inner attributes then the `tokens` we've got above holds
+        // these inner attributes. If an inner attribute is expanded we won't
+        // actually remove it from the token stream, so we'll just keep yielding
+        // it (bad!). To work around this case for now we just avoid recording
+        // `tokens` if we detect any inner attributes. This should help keep
+        // expansion correct, but we should fix this bug one day!
+        if let Some(tokens) = tokens {
+            if let Some(item) = &mut item {
+                if !item.attrs.iter().any(|attr| attr.style == AttrStyle::Inner) {
+                    item.tokens = Some(tokens);
+                }
+            }
+        }
+        Ok(item)
+    }
+
+    fn parse_item_common_(
+        &mut self,
+        mut attrs: Vec<Attribute>,
+        mac_allowed: bool,
+        attrs_allowed: bool,
+        req_name: ReqName,
+    ) -> PResult<'a, Option<Item>> {
+        let lo = self.token.span;
+        let vis = self.parse_visibility(FollowedByType::No)?;
+        let mut def = self.parse_defaultness();
+        let kind = self.parse_item_kind(&mut attrs, mac_allowed, lo, &vis, &mut def, req_name)?;
+        if let Some((ident, kind)) = kind {
+            self.error_on_unconsumed_default(def, &kind);
+            let span = lo.to(self.prev_token.span);
+            let id = DUMMY_NODE_ID;
+            let item = Item { ident, attrs, id, kind, vis, span, tokens: None };
+            return Ok(Some(item));
+        }
+
+        // At this point, we have failed to parse an item.
+        self.error_on_unmatched_vis(&vis);
+        self.error_on_unmatched_defaultness(def);
+        if !attrs_allowed {
+            self.recover_attrs_no_item(&attrs)?;
+        }
+        Ok(None)
+    }
+
+    /// Error in-case a non-inherited visibility was parsed but no item followed.
+    fn error_on_unmatched_vis(&self, vis: &Visibility) {
+        if let VisibilityKind::Inherited = vis.node {
+            return;
+        }
+        let vs = pprust::vis_to_string(&vis);
+        let vs = vs.trim_end();
+        self.struct_span_err(vis.span, &format!("visibility `{}` is not followed by an item", vs))
+            .span_label(vis.span, "the visibility")
+            .help(&format!("you likely meant to define an item, e.g., `{} fn foo() {{}}`", vs))
+            .emit();
+    }
+
+    /// Error in-case a `default` was parsed but no item followed.
+    fn error_on_unmatched_defaultness(&self, def: Defaultness) {
+        if let Defaultness::Default(sp) = def {
+            self.struct_span_err(sp, "`default` is not followed by an item")
+                .span_label(sp, "the `default` qualifier")
+                .note("only `fn`, `const`, `type`, or `impl` items may be prefixed by `default`")
+                .emit();
+        }
+    }
+
+    /// Error in-case `default` was parsed in an in-appropriate context.
+    fn error_on_unconsumed_default(&self, def: Defaultness, kind: &ItemKind) {
+        if let Defaultness::Default(span) = def {
+            let msg = format!("{} {} cannot be `default`", kind.article(), kind.descr());
+            self.struct_span_err(span, &msg)
+                .span_label(span, "`default` because of this")
+                .note("only associated `fn`, `const`, and `type` items can be `default`")
+                .emit();
+        }
+    }
+
+    /// Parses one of the items allowed by the flags.
+    fn parse_item_kind(
+        &mut self,
+        attrs: &mut Vec<Attribute>,
+        macros_allowed: bool,
+        lo: Span,
+        vis: &Visibility,
+        def: &mut Defaultness,
+        req_name: ReqName,
+    ) -> PResult<'a, Option<ItemInfo>> {
+        let mut def = || mem::replace(def, Defaultness::Final);
+
+        let info = if self.eat_keyword(kw::Use) {
+            // USE ITEM
+            let tree = self.parse_use_tree()?;
+            self.expect_semi()?;
+            (Ident::invalid(), ItemKind::Use(P(tree)))
+        } else if self.check_fn_front_matter() {
+            // FUNCTION ITEM
+            let (ident, sig, generics, body) = self.parse_fn(attrs, req_name, lo)?;
+            (ident, ItemKind::Fn(def(), sig, generics, body))
+        } else if self.eat_keyword(kw::Extern) {
+            if self.eat_keyword(kw::Crate) {
+                // EXTERN CRATE
+                self.parse_item_extern_crate()?
+            } else {
+                // EXTERN BLOCK
+                self.parse_item_foreign_mod(attrs)?
+            }
+        } else if self.is_static_global() {
+            // STATIC ITEM
+            self.bump(); // `static`
+            let m = self.parse_mutability();
+            let (ident, ty, expr) = self.parse_item_global(Some(m))?;
+            (ident, ItemKind::Static(ty, m, expr))
+        } else if let Const::Yes(const_span) = self.parse_constness() {
+            // CONST ITEM
+            self.recover_const_mut(const_span);
+            let (ident, ty, expr) = self.parse_item_global(None)?;
+            (ident, ItemKind::Const(def(), ty, expr))
+        } else if self.check_keyword(kw::Trait) || self.check_auto_or_unsafe_trait_item() {
+            // TRAIT ITEM
+            self.parse_item_trait(attrs, lo)?
+        } else if self.check_keyword(kw::Impl)
+            || self.check_keyword(kw::Unsafe) && self.is_keyword_ahead(1, &[kw::Impl])
+        {
+            // IMPL ITEM
+            self.parse_item_impl(attrs, def())?
+        } else if self.eat_keyword(kw::Mod) {
+            // MODULE ITEM
+            self.parse_item_mod(attrs)?
+        } else if self.eat_keyword(kw::Type) {
+            // TYPE ITEM
+            self.parse_type_alias(def())?
+        } else if self.eat_keyword(kw::Enum) {
+            // ENUM ITEM
+            self.parse_item_enum()?
+        } else if self.eat_keyword(kw::Struct) {
+            // STRUCT ITEM
+            self.parse_item_struct()?
+        } else if self.is_kw_followed_by_ident(kw::Union) {
+            // UNION ITEM
+            self.bump(); // `union`
+            self.parse_item_union()?
+        } else if self.eat_keyword(kw::Macro) {
+            // MACROS 2.0 ITEM
+            self.parse_item_decl_macro(lo)?
+        } else if self.is_macro_rules_item() {
+            // MACRO_RULES ITEM
+            self.parse_item_macro_rules(vis)?
+        } else if vis.node.is_pub() && self.isnt_macro_invocation() {
+            self.recover_missing_kw_before_item()?;
+            return Ok(None);
+        } else if macros_allowed && self.check_path() {
+            // MACRO INVOCATION ITEM
+            (Ident::invalid(), ItemKind::MacCall(self.parse_item_macro(vis)?))
+        } else {
+            return Ok(None);
+        };
+        Ok(Some(info))
+    }
+
+    /// When parsing a statement, would the start of a path be an item?
+    pub(super) fn is_path_start_item(&mut self) -> bool {
+        self.is_crate_vis() // no: `crate::b`, yes: `crate $item`
+        || self.is_kw_followed_by_ident(kw::Union) // no: `union::b`, yes: `union U { .. }`
+        || self.check_auto_or_unsafe_trait_item() // no: `auto::b`, yes: `auto trait X { .. }`
+        || self.is_async_fn() // no(2015): `async::b`, yes: `async fn`
+        || self.is_macro_rules_item() // no: `macro_rules::b`, yes: `macro_rules! mac`
+    }
+
+    /// Are we sure this could not possibly be a macro invocation?
+    fn isnt_macro_invocation(&mut self) -> bool {
+        self.check_ident() && self.look_ahead(1, |t| *t != token::Not && *t != token::ModSep)
+    }
+
+    /// Recover on encountering a struct or method definition where the user
+    /// forgot to add the `struct` or `fn` keyword after writing `pub`: `pub S {}`.
+    fn recover_missing_kw_before_item(&mut self) -> PResult<'a, ()> {
+        // Space between `pub` keyword and the identifier
+        //
+        //     pub   S {}
+        //        ^^^ `sp` points here
+        let sp = self.prev_token.span.between(self.token.span);
+        let full_sp = self.prev_token.span.to(self.token.span);
+        let ident_sp = self.token.span;
+        if self.look_ahead(1, |t| *t == token::OpenDelim(token::Brace)) {
+            // possible public struct definition where `struct` was forgotten
+            let ident = self.parse_ident().unwrap();
+            let msg = format!("add `struct` here to parse `{}` as a public struct", ident);
+            let mut err = self.struct_span_err(sp, "missing `struct` for struct definition");
+            err.span_suggestion_short(
+                sp,
+                &msg,
+                " struct ".into(),
+                Applicability::MaybeIncorrect, // speculative
+            );
+            Err(err)
+        } else if self.look_ahead(1, |t| *t == token::OpenDelim(token::Paren)) {
+            let ident = self.parse_ident().unwrap();
+            self.bump(); // `(`
+            let kw_name = self.recover_first_param();
+            self.consume_block(token::Paren, ConsumeClosingDelim::Yes);
+            let (kw, kw_name, ambiguous) = if self.check(&token::RArrow) {
+                self.eat_to_tokens(&[&token::OpenDelim(token::Brace)]);
+                self.bump(); // `{`
+                ("fn", kw_name, false)
+            } else if self.check(&token::OpenDelim(token::Brace)) {
+                self.bump(); // `{`
+                ("fn", kw_name, false)
+            } else if self.check(&token::Colon) {
+                let kw = "struct";
+                (kw, kw, false)
+            } else {
+                ("fn` or `struct", "function or struct", true)
+            };
+
+            let msg = format!("missing `{}` for {} definition", kw, kw_name);
+            let mut err = self.struct_span_err(sp, &msg);
+            if !ambiguous {
+                self.consume_block(token::Brace, ConsumeClosingDelim::Yes);
+                let suggestion =
+                    format!("add `{}` here to parse `{}` as a public {}", kw, ident, kw_name);
+                err.span_suggestion_short(
+                    sp,
+                    &suggestion,
+                    format!(" {} ", kw),
+                    Applicability::MachineApplicable,
+                );
+            } else {
+                if let Ok(snippet) = self.span_to_snippet(ident_sp) {
+                    err.span_suggestion(
+                        full_sp,
+                        "if you meant to call a macro, try",
+                        format!("{}!", snippet),
+                        // this is the `ambiguous` conditional branch
+                        Applicability::MaybeIncorrect,
+                    );
+                } else {
+                    err.help(
+                        "if you meant to call a macro, remove the `pub` \
+                                  and add a trailing `!` after the identifier",
+                    );
+                }
+            }
+            Err(err)
+        } else if self.look_ahead(1, |t| *t == token::Lt) {
+            let ident = self.parse_ident().unwrap();
+            self.eat_to_tokens(&[&token::Gt]);
+            self.bump(); // `>`
+            let (kw, kw_name, ambiguous) = if self.eat(&token::OpenDelim(token::Paren)) {
+                ("fn", self.recover_first_param(), false)
+            } else if self.check(&token::OpenDelim(token::Brace)) {
+                ("struct", "struct", false)
+            } else {
+                ("fn` or `struct", "function or struct", true)
+            };
+            let msg = format!("missing `{}` for {} definition", kw, kw_name);
+            let mut err = self.struct_span_err(sp, &msg);
+            if !ambiguous {
+                err.span_suggestion_short(
+                    sp,
+                    &format!("add `{}` here to parse `{}` as a public {}", kw, ident, kw_name),
+                    format!(" {} ", kw),
+                    Applicability::MachineApplicable,
+                );
+            }
+            Err(err)
+        } else {
+            Ok(())
+        }
+    }
+
+    /// Parses an item macro, e.g., `item!();`.
+    fn parse_item_macro(&mut self, vis: &Visibility) -> PResult<'a, MacCall> {
+        let path = self.parse_path(PathStyle::Mod)?; // `foo::bar`
+        self.expect(&token::Not)?; // `!`
+        let args = self.parse_mac_args()?; // `( .. )` or `[ .. ]` (followed by `;`), or `{ .. }`.
+        self.eat_semi_for_macro_if_needed(&args);
+        self.complain_if_pub_macro(vis, false);
+        Ok(MacCall { path, args, prior_type_ascription: self.last_type_ascription })
+    }
+
+    /// Recover if we parsed attributes and expected an item but there was none.
+    fn recover_attrs_no_item(&mut self, attrs: &[Attribute]) -> PResult<'a, ()> {
+        let (start, end) = match attrs {
+            [] => return Ok(()),
+            [x0 @ xn] | [x0, .., xn] => (x0, xn),
+        };
+        let msg = if end.is_doc_comment() {
+            "expected item after doc comment"
+        } else {
+            "expected item after attributes"
+        };
+        let mut err = self.struct_span_err(end.span, msg);
+        if end.is_doc_comment() {
+            err.span_label(end.span, "this doc comment doesn't document anything");
+        }
+        if let [.., penultimate, _] = attrs {
+            err.span_label(start.span.to(penultimate.span), "other attributes here");
+        }
+        Err(err)
+    }
+
+    fn is_async_fn(&self) -> bool {
+        self.token.is_keyword(kw::Async) && self.is_keyword_ahead(1, &[kw::Fn])
+    }
+
+    fn parse_polarity(&mut self) -> ast::ImplPolarity {
+        // Disambiguate `impl !Trait for Type { ... }` and `impl ! { ... }` for the never type.
+        if self.check(&token::Not) && self.look_ahead(1, |t| t.can_begin_type()) {
+            self.bump(); // `!`
+            ast::ImplPolarity::Negative(self.prev_token.span)
+        } else {
+            ast::ImplPolarity::Positive
+        }
+    }
+
+    /// Parses an implementation item.
+    ///
+    /// ```
+    /// impl<'a, T> TYPE { /* impl items */ }
+    /// impl<'a, T> TRAIT for TYPE { /* impl items */ }
+    /// impl<'a, T> !TRAIT for TYPE { /* impl items */ }
+    /// impl<'a, T> const TRAIT for TYPE { /* impl items */ }
+    /// ```
+    ///
+    /// We actually parse slightly more relaxed grammar for better error reporting and recovery.
+    /// ```
+    /// "impl" GENERICS "const"? "!"? TYPE "for"? (TYPE | "..") ("where" PREDICATES)? "{" BODY "}"
+    /// "impl" GENERICS "const"? "!"? TYPE ("where" PREDICATES)? "{" BODY "}"
+    /// ```
+    fn parse_item_impl(
+        &mut self,
+        attrs: &mut Vec<Attribute>,
+        defaultness: Defaultness,
+    ) -> PResult<'a, ItemInfo> {
+        let unsafety = self.parse_unsafety();
+        self.expect_keyword(kw::Impl)?;
+
+        // First, parse generic parameters if necessary.
+        let mut generics = if self.choose_generics_over_qpath(0) {
+            self.parse_generics()?
+        } else {
+            let mut generics = Generics::default();
+            // impl A for B {}
+            //    /\ this is where `generics.span` should point when there are no type params.
+            generics.span = self.prev_token.span.shrink_to_hi();
+            generics
+        };
+
+        let constness = self.parse_constness();
+        if let Const::Yes(span) = constness {
+            self.sess.gated_spans.gate(sym::const_trait_impl, span);
+        }
+
+        let polarity = self.parse_polarity();
+
+        // Parse both types and traits as a type, then reinterpret if necessary.
+        let err_path = |span| ast::Path::from_ident(Ident::new(kw::Invalid, span));
+        let ty_first = if self.token.is_keyword(kw::For) && self.look_ahead(1, |t| t != &token::Lt)
+        {
+            let span = self.prev_token.span.between(self.token.span);
+            self.struct_span_err(span, "missing trait in a trait impl").emit();
+            P(Ty { kind: TyKind::Path(None, err_path(span)), span, id: DUMMY_NODE_ID })
+        } else {
+            self.parse_ty()?
+        };
+
+        // If `for` is missing we try to recover.
+        let has_for = self.eat_keyword(kw::For);
+        let missing_for_span = self.prev_token.span.between(self.token.span);
+
+        let ty_second = if self.token == token::DotDot {
+            // We need to report this error after `cfg` expansion for compatibility reasons
+            self.bump(); // `..`, do not add it to expected tokens
+            Some(self.mk_ty(self.prev_token.span, TyKind::Err))
+        } else if has_for || self.token.can_begin_type() {
+            Some(self.parse_ty()?)
+        } else {
+            None
+        };
+
+        generics.where_clause = self.parse_where_clause()?;
+
+        let impl_items = self.parse_item_list(attrs, |p| p.parse_impl_item())?;
+
+        let item_kind = match ty_second {
+            Some(ty_second) => {
+                // impl Trait for Type
+                if !has_for {
+                    self.struct_span_err(missing_for_span, "missing `for` in a trait impl")
+                        .span_suggestion_short(
+                            missing_for_span,
+                            "add `for` here",
+                            " for ".to_string(),
+                            Applicability::MachineApplicable,
+                        )
+                        .emit();
+                }
+
+                let ty_first = ty_first.into_inner();
+                let path = match ty_first.kind {
+                    // This notably includes paths passed through `ty` macro fragments (#46438).
+                    TyKind::Path(None, path) => path,
+                    _ => {
+                        self.struct_span_err(ty_first.span, "expected a trait, found type").emit();
+                        err_path(ty_first.span)
+                    }
+                };
+                let trait_ref = TraitRef { path, ref_id: ty_first.id };
+
+                ItemKind::Impl {
+                    unsafety,
+                    polarity,
+                    defaultness,
+                    constness,
+                    generics,
+                    of_trait: Some(trait_ref),
+                    self_ty: ty_second,
+                    items: impl_items,
+                }
+            }
+            None => {
+                // impl Type
+                ItemKind::Impl {
+                    unsafety,
+                    polarity,
+                    defaultness,
+                    constness,
+                    generics,
+                    of_trait: None,
+                    self_ty: ty_first,
+                    items: impl_items,
+                }
+            }
+        };
+
+        Ok((Ident::invalid(), item_kind))
+    }
+
+    fn parse_item_list<T>(
+        &mut self,
+        attrs: &mut Vec<Attribute>,
+        mut parse_item: impl FnMut(&mut Parser<'a>) -> PResult<'a, Option<Option<T>>>,
+    ) -> PResult<'a, Vec<T>> {
+        let open_brace_span = self.token.span;
+        self.expect(&token::OpenDelim(token::Brace))?;
+        attrs.append(&mut self.parse_inner_attributes()?);
+
+        let mut items = Vec::new();
+        while !self.eat(&token::CloseDelim(token::Brace)) {
+            if self.recover_doc_comment_before_brace() {
+                continue;
+            }
+            match parse_item(self) {
+                Ok(None) => {
+                    // We have to bail or we'll potentially never make progress.
+                    let non_item_span = self.token.span;
+                    self.consume_block(token::Brace, ConsumeClosingDelim::Yes);
+                    self.struct_span_err(non_item_span, "non-item in item list")
+                        .span_label(open_brace_span, "item list starts here")
+                        .span_label(non_item_span, "non-item starts here")
+                        .span_label(self.prev_token.span, "item list ends here")
+                        .emit();
+                    break;
+                }
+                Ok(Some(item)) => items.extend(item),
+                Err(mut err) => {
+                    self.consume_block(token::Brace, ConsumeClosingDelim::Yes);
+                    err.span_label(open_brace_span, "while parsing this item list starting here")
+                        .span_label(self.prev_token.span, "the item list ends here")
+                        .emit();
+                    break;
+                }
+            }
+        }
+        Ok(items)
+    }
+
+    /// Recover on a doc comment before `}`.
+    fn recover_doc_comment_before_brace(&mut self) -> bool {
+        if let token::DocComment(..) = self.token.kind {
+            if self.look_ahead(1, |tok| tok == &token::CloseDelim(token::Brace)) {
+                struct_span_err!(
+                    self.diagnostic(),
+                    self.token.span,
+                    E0584,
+                    "found a documentation comment that doesn't document anything",
+                )
+                .span_label(self.token.span, "this doc comment doesn't document anything")
+                .help(
+                    "doc comments must come before what they document, maybe a \
+                    comment was intended with `//`?",
+                )
+                .emit();
+                self.bump();
+                return true;
+            }
+        }
+        false
+    }
+
+    /// Parses defaultness (i.e., `default` or nothing).
+    fn parse_defaultness(&mut self) -> Defaultness {
+        // We are interested in `default` followed by another identifier.
+        // However, we must avoid keywords that occur as binary operators.
+        // Currently, the only applicable keyword is `as` (`default as Ty`).
+        if self.check_keyword(kw::Default)
+            && self.look_ahead(1, |t| t.is_non_raw_ident_where(|i| i.name != kw::As))
+        {
+            self.bump(); // `default`
+            Defaultness::Default(self.prev_token.uninterpolated_span())
+        } else {
+            Defaultness::Final
+        }
+    }
+
+    /// Is this an `(unsafe auto? | auto) trait` item?
+    fn check_auto_or_unsafe_trait_item(&mut self) -> bool {
+        // auto trait
+        self.check_keyword(kw::Auto) && self.is_keyword_ahead(1, &[kw::Trait])
+            // unsafe auto trait
+            || self.check_keyword(kw::Unsafe) && self.is_keyword_ahead(1, &[kw::Trait, kw::Auto])
+    }
+
+    /// Parses `unsafe? auto? trait Foo { ... }` or `trait Foo = Bar;`.
+    fn parse_item_trait(&mut self, attrs: &mut Vec<Attribute>, lo: Span) -> PResult<'a, ItemInfo> {
+        let unsafety = self.parse_unsafety();
+        // Parse optional `auto` prefix.
+        let is_auto = if self.eat_keyword(kw::Auto) { IsAuto::Yes } else { IsAuto::No };
+
+        self.expect_keyword(kw::Trait)?;
+        let ident = self.parse_ident()?;
+        let mut tps = self.parse_generics()?;
+
+        // Parse optional colon and supertrait bounds.
+        let had_colon = self.eat(&token::Colon);
+        let span_at_colon = self.prev_token.span;
+        let bounds = if had_colon {
+            self.parse_generic_bounds(Some(self.prev_token.span))?
+        } else {
+            Vec::new()
+        };
+
+        let span_before_eq = self.prev_token.span;
+        if self.eat(&token::Eq) {
+            // It's a trait alias.
+            if had_colon {
+                let span = span_at_colon.to(span_before_eq);
+                self.struct_span_err(span, "bounds are not allowed on trait aliases").emit();
+            }
+
+            let bounds = self.parse_generic_bounds(None)?;
+            tps.where_clause = self.parse_where_clause()?;
+            self.expect_semi()?;
+
+            let whole_span = lo.to(self.prev_token.span);
+            if is_auto == IsAuto::Yes {
+                let msg = "trait aliases cannot be `auto`";
+                self.struct_span_err(whole_span, msg).span_label(whole_span, msg).emit();
+            }
+            if let Unsafe::Yes(_) = unsafety {
+                let msg = "trait aliases cannot be `unsafe`";
+                self.struct_span_err(whole_span, msg).span_label(whole_span, msg).emit();
+            }
+
+            self.sess.gated_spans.gate(sym::trait_alias, whole_span);
+
+            Ok((ident, ItemKind::TraitAlias(tps, bounds)))
+        } else {
+            // It's a normal trait.
+            tps.where_clause = self.parse_where_clause()?;
+            let items = self.parse_item_list(attrs, |p| p.parse_trait_item())?;
+            Ok((ident, ItemKind::Trait(is_auto, unsafety, tps, bounds, items)))
+        }
+    }
+
+    pub fn parse_impl_item(&mut self) -> PResult<'a, Option<Option<P<AssocItem>>>> {
+        self.parse_assoc_item(|_| true)
+    }
+
+    pub fn parse_trait_item(&mut self) -> PResult<'a, Option<Option<P<AssocItem>>>> {
+        self.parse_assoc_item(|edition| edition >= Edition::Edition2018)
+    }
+
+    /// Parses associated items.
+    fn parse_assoc_item(&mut self, req_name: ReqName) -> PResult<'a, Option<Option<P<AssocItem>>>> {
+        Ok(self.parse_item_(req_name)?.map(|Item { attrs, id, span, vis, ident, kind, tokens }| {
+            let kind = match AssocItemKind::try_from(kind) {
+                Ok(kind) => kind,
+                Err(kind) => match kind {
+                    ItemKind::Static(a, _, b) => {
+                        self.struct_span_err(span, "associated `static` items are not allowed")
+                            .emit();
+                        AssocItemKind::Const(Defaultness::Final, a, b)
+                    }
+                    _ => return self.error_bad_item_kind(span, &kind, "`trait`s or `impl`s"),
+                },
+            };
+            Some(P(Item { attrs, id, span, vis, ident, kind, tokens }))
+        }))
+    }
+
+    /// Parses a `type` alias with the following grammar:
+    /// ```
+    /// TypeAlias = "type" Ident Generics {":" GenericBounds}? {"=" Ty}? ";" ;
+    /// ```
+    /// The `"type"` has already been eaten.
+    fn parse_type_alias(&mut self, def: Defaultness) -> PResult<'a, ItemInfo> {
+        let ident = self.parse_ident()?;
+        let mut generics = self.parse_generics()?;
+
+        // Parse optional colon and param bounds.
+        let bounds =
+            if self.eat(&token::Colon) { self.parse_generic_bounds(None)? } else { Vec::new() };
+        generics.where_clause = self.parse_where_clause()?;
+
+        let default = if self.eat(&token::Eq) { Some(self.parse_ty()?) } else { None };
+        self.expect_semi()?;
+
+        Ok((ident, ItemKind::TyAlias(def, generics, bounds, default)))
+    }
+
+    /// Parses a `UseTree`.
+    ///
+    /// ```text
+    /// USE_TREE = [`::`] `*` |
+    ///            [`::`] `{` USE_TREE_LIST `}` |
+    ///            PATH `::` `*` |
+    ///            PATH `::` `{` USE_TREE_LIST `}` |
+    ///            PATH [`as` IDENT]
+    /// ```
+    fn parse_use_tree(&mut self) -> PResult<'a, UseTree> {
+        let lo = self.token.span;
+
+        let mut prefix = ast::Path { segments: Vec::new(), span: lo.shrink_to_lo() };
+        let kind = if self.check(&token::OpenDelim(token::Brace))
+            || self.check(&token::BinOp(token::Star))
+            || self.is_import_coupler()
+        {
+            // `use *;` or `use ::*;` or `use {...};` or `use ::{...};`
+            let mod_sep_ctxt = self.token.span.ctxt();
+            if self.eat(&token::ModSep) {
+                prefix
+                    .segments
+                    .push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
+            }
+
+            self.parse_use_tree_glob_or_nested()?
+        } else {
+            // `use path::*;` or `use path::{...};` or `use path;` or `use path as bar;`
+            prefix = self.parse_path(PathStyle::Mod)?;
+
+            if self.eat(&token::ModSep) {
+                self.parse_use_tree_glob_or_nested()?
+            } else {
+                UseTreeKind::Simple(self.parse_rename()?, DUMMY_NODE_ID, DUMMY_NODE_ID)
+            }
+        };
+
+        Ok(UseTree { prefix, kind, span: lo.to(self.prev_token.span) })
+    }
+
+    /// Parses `*` or `{...}`.
+    fn parse_use_tree_glob_or_nested(&mut self) -> PResult<'a, UseTreeKind> {
+        Ok(if self.eat(&token::BinOp(token::Star)) {
+            UseTreeKind::Glob
+        } else {
+            UseTreeKind::Nested(self.parse_use_tree_list()?)
+        })
+    }
+
+    /// Parses a `UseTreeKind::Nested(list)`.
+    ///
+    /// ```text
+    /// USE_TREE_LIST = Ø | (USE_TREE `,`)* USE_TREE [`,`]
+    /// ```
+    fn parse_use_tree_list(&mut self) -> PResult<'a, Vec<(UseTree, ast::NodeId)>> {
+        self.parse_delim_comma_seq(token::Brace, |p| Ok((p.parse_use_tree()?, DUMMY_NODE_ID)))
+            .map(|(r, _)| r)
+    }
+
+    fn parse_rename(&mut self) -> PResult<'a, Option<Ident>> {
+        if self.eat_keyword(kw::As) { self.parse_ident_or_underscore().map(Some) } else { Ok(None) }
+    }
+
+    fn parse_ident_or_underscore(&mut self) -> PResult<'a, Ident> {
+        match self.token.ident() {
+            Some((ident @ Ident { name: kw::Underscore, .. }, false)) => {
+                self.bump();
+                Ok(ident)
+            }
+            _ => self.parse_ident(),
+        }
+    }
+
+    /// Parses `extern crate` links.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// extern crate foo;
+    /// extern crate bar as foo;
+    /// ```
+    fn parse_item_extern_crate(&mut self) -> PResult<'a, ItemInfo> {
+        // Accept `extern crate name-like-this` for better diagnostics
+        let orig_name = self.parse_crate_name_with_dashes()?;
+        let (item_name, orig_name) = if let Some(rename) = self.parse_rename()? {
+            (rename, Some(orig_name.name))
+        } else {
+            (orig_name, None)
+        };
+        self.expect_semi()?;
+        Ok((item_name, ItemKind::ExternCrate(orig_name)))
+    }
+
+    fn parse_crate_name_with_dashes(&mut self) -> PResult<'a, Ident> {
+        let error_msg = "crate name using dashes are not valid in `extern crate` statements";
+        let suggestion_msg = "if the original crate name uses dashes you need to use underscores \
+                              in the code";
+        let mut ident = if self.token.is_keyword(kw::SelfLower) {
+            self.parse_path_segment_ident()
+        } else {
+            self.parse_ident()
+        }?;
+        let mut idents = vec![];
+        let mut replacement = vec![];
+        let mut fixed_crate_name = false;
+        // Accept `extern crate name-like-this` for better diagnostics.
+        let dash = token::BinOp(token::BinOpToken::Minus);
+        if self.token == dash {
+            // Do not include `-` as part of the expected tokens list.
+            while self.eat(&dash) {
+                fixed_crate_name = true;
+                replacement.push((self.prev_token.span, "_".to_string()));
+                idents.push(self.parse_ident()?);
+            }
+        }
+        if fixed_crate_name {
+            let fixed_name_sp = ident.span.to(idents.last().unwrap().span);
+            let mut fixed_name = format!("{}", ident.name);
+            for part in idents {
+                fixed_name.push_str(&format!("_{}", part.name));
+            }
+            ident = Ident::from_str_and_span(&fixed_name, fixed_name_sp);
+
+            self.struct_span_err(fixed_name_sp, error_msg)
+                .span_label(fixed_name_sp, "dash-separated idents are not valid")
+                .multipart_suggestion(suggestion_msg, replacement, Applicability::MachineApplicable)
+                .emit();
+        }
+        Ok(ident)
+    }
+
+    /// Parses `extern` for foreign ABIs modules.
+    ///
+    /// `extern` is expected to have been consumed before calling this method.
+    ///
+    /// # Examples
+    ///
+    /// ```ignore (only-for-syntax-highlight)
+    /// extern "C" {}
+    /// extern {}
+    /// ```
+    fn parse_item_foreign_mod(&mut self, attrs: &mut Vec<Attribute>) -> PResult<'a, ItemInfo> {
+        let abi = self.parse_abi(); // ABI?
+        let items = self.parse_item_list(attrs, |p| p.parse_foreign_item())?;
+        let module = ast::ForeignMod { abi, items };
+        Ok((Ident::invalid(), ItemKind::ForeignMod(module)))
+    }
+
+    /// Parses a foreign item (one in an `extern { ... }` block).
+    pub fn parse_foreign_item(&mut self) -> PResult<'a, Option<Option<P<ForeignItem>>>> {
+        Ok(self.parse_item_(|_| true)?.map(|Item { attrs, id, span, vis, ident, kind, tokens }| {
+            let kind = match ForeignItemKind::try_from(kind) {
+                Ok(kind) => kind,
+                Err(kind) => match kind {
+                    ItemKind::Const(_, a, b) => {
+                        self.error_on_foreign_const(span, ident);
+                        ForeignItemKind::Static(a, Mutability::Not, b)
+                    }
+                    _ => return self.error_bad_item_kind(span, &kind, "`extern` blocks"),
+                },
+            };
+            Some(P(Item { attrs, id, span, vis, ident, kind, tokens }))
+        }))
+    }
+
+    fn error_bad_item_kind<T>(&self, span: Span, kind: &ItemKind, ctx: &str) -> Option<T> {
+        let span = self.sess.source_map().guess_head_span(span);
+        let descr = kind.descr();
+        self.struct_span_err(span, &format!("{} is not supported in {}", descr, ctx))
+            .help(&format!("consider moving the {} out to a nearby module scope", descr))
+            .emit();
+        None
+    }
+
+    fn error_on_foreign_const(&self, span: Span, ident: Ident) {
+        self.struct_span_err(ident.span, "extern items cannot be `const`")
+            .span_suggestion(
+                span.with_hi(ident.span.lo()),
+                "try using a static value",
+                "static ".to_string(),
+                Applicability::MachineApplicable,
+            )
+            .note("for more information, visit https://doc.rust-lang.org/std/keyword.extern.html")
+            .emit();
+    }
+
+    fn is_static_global(&mut self) -> bool {
+        if self.check_keyword(kw::Static) {
+            // Check if this could be a closure.
+            !self.look_ahead(1, |token| {
+                if token.is_keyword(kw::Move) {
+                    return true;
+                }
+                match token.kind {
+                    token::BinOp(token::Or) | token::OrOr => true,
+                    _ => false,
+                }
+            })
+        } else {
+            false
+        }
+    }
+
+    /// Recover on `const mut` with `const` already eaten.
+    fn recover_const_mut(&mut self, const_span: Span) {
+        if self.eat_keyword(kw::Mut) {
+            let span = self.prev_token.span;
+            self.struct_span_err(span, "const globals cannot be mutable")
+                .span_label(span, "cannot be mutable")
+                .span_suggestion(
+                    const_span,
+                    "you might want to declare a static instead",
+                    "static".to_owned(),
+                    Applicability::MaybeIncorrect,
+                )
+                .emit();
+        }
+    }
+
+    /// Parse `["const" | ("static" "mut"?)] $ident ":" $ty (= $expr)?` with
+    /// `["const" | ("static" "mut"?)]` already parsed and stored in `m`.
+    ///
+    /// When `m` is `"const"`, `$ident` may also be `"_"`.
+    fn parse_item_global(
+        &mut self,
+        m: Option<Mutability>,
+    ) -> PResult<'a, (Ident, P<Ty>, Option<P<ast::Expr>>)> {
+        let id = if m.is_none() { self.parse_ident_or_underscore() } else { self.parse_ident() }?;
+
+        // Parse the type of a `const` or `static mut?` item.
+        // That is, the `":" $ty` fragment.
+        let ty = if self.eat(&token::Colon) {
+            self.parse_ty()?
+        } else {
+            self.recover_missing_const_type(id, m)
+        };
+
+        let expr = if self.eat(&token::Eq) { Some(self.parse_expr()?) } else { None };
+        self.expect_semi()?;
+        Ok((id, ty, expr))
+    }
+
+    /// We were supposed to parse `:` but the `:` was missing.
+    /// This means that the type is missing.
+    fn recover_missing_const_type(&mut self, id: Ident, m: Option<Mutability>) -> P<Ty> {
+        // Construct the error and stash it away with the hope
+        // that typeck will later enrich the error with a type.
+        let kind = match m {
+            Some(Mutability::Mut) => "static mut",
+            Some(Mutability::Not) => "static",
+            None => "const",
+        };
+        let mut err = self.struct_span_err(id.span, &format!("missing type for `{}` item", kind));
+        err.span_suggestion(
+            id.span,
+            "provide a type for the item",
+            format!("{}: <type>", id),
+            Applicability::HasPlaceholders,
+        );
+        err.stash(id.span, StashKey::ItemNoType);
+
+        // The user intended that the type be inferred,
+        // so treat this as if the user wrote e.g. `const A: _ = expr;`.
+        P(Ty { kind: TyKind::Infer, span: id.span, id: ast::DUMMY_NODE_ID })
+    }
+
+    /// Parses an enum declaration.
+    fn parse_item_enum(&mut self) -> PResult<'a, ItemInfo> {
+        let id = self.parse_ident()?;
+        let mut generics = self.parse_generics()?;
+        generics.where_clause = self.parse_where_clause()?;
+
+        let (variants, _) =
+            self.parse_delim_comma_seq(token::Brace, |p| p.parse_enum_variant()).map_err(|e| {
+                self.recover_stmt();
+                e
+            })?;
+
+        let enum_definition =
+            EnumDef { variants: variants.into_iter().filter_map(|v| v).collect() };
+        Ok((id, ItemKind::Enum(enum_definition, generics)))
+    }
+
+    fn parse_enum_variant(&mut self) -> PResult<'a, Option<Variant>> {
+        let variant_attrs = self.parse_outer_attributes()?;
+        let vlo = self.token.span;
+
+        let vis = self.parse_visibility(FollowedByType::No)?;
+        if !self.recover_nested_adt_item(kw::Enum)? {
+            return Ok(None);
+        }
+        let ident = self.parse_ident()?;
+
+        let struct_def = if self.check(&token::OpenDelim(token::Brace)) {
+            // Parse a struct variant.
+            let (fields, recovered) = self.parse_record_struct_body()?;
+            VariantData::Struct(fields, recovered)
+        } else if self.check(&token::OpenDelim(token::Paren)) {
+            VariantData::Tuple(self.parse_tuple_struct_body()?, DUMMY_NODE_ID)
+        } else {
+            VariantData::Unit(DUMMY_NODE_ID)
+        };
+
+        let disr_expr =
+            if self.eat(&token::Eq) { Some(self.parse_anon_const_expr()?) } else { None };
+
+        let vr = ast::Variant {
+            ident,
+            vis,
+            id: DUMMY_NODE_ID,
+            attrs: variant_attrs,
+            data: struct_def,
+            disr_expr,
+            span: vlo.to(self.prev_token.span),
+            is_placeholder: false,
+        };
+
+        Ok(Some(vr))
+    }
+
+    /// Parses `struct Foo { ... }`.
+    fn parse_item_struct(&mut self) -> PResult<'a, ItemInfo> {
+        let class_name = self.parse_ident()?;
+
+        let mut generics = self.parse_generics()?;
+
+        // There is a special case worth noting here, as reported in issue #17904.
+        // If we are parsing a tuple struct it is the case that the where clause
+        // should follow the field list. Like so:
+        //
+        // struct Foo<T>(T) where T: Copy;
+        //
+        // If we are parsing a normal record-style struct it is the case
+        // that the where clause comes before the body, and after the generics.
+        // So if we look ahead and see a brace or a where-clause we begin
+        // parsing a record style struct.
+        //
+        // Otherwise if we look ahead and see a paren we parse a tuple-style
+        // struct.
+
+        let vdata = if self.token.is_keyword(kw::Where) {
+            generics.where_clause = self.parse_where_clause()?;
+            if self.eat(&token::Semi) {
+                // If we see a: `struct Foo<T> where T: Copy;` style decl.
+                VariantData::Unit(DUMMY_NODE_ID)
+            } else {
+                // If we see: `struct Foo<T> where T: Copy { ... }`
+                let (fields, recovered) = self.parse_record_struct_body()?;
+                VariantData::Struct(fields, recovered)
+            }
+        // No `where` so: `struct Foo<T>;`
+        } else if self.eat(&token::Semi) {
+            VariantData::Unit(DUMMY_NODE_ID)
+        // Record-style struct definition
+        } else if self.token == token::OpenDelim(token::Brace) {
+            let (fields, recovered) = self.parse_record_struct_body()?;
+            VariantData::Struct(fields, recovered)
+        // Tuple-style struct definition with optional where-clause.
+        } else if self.token == token::OpenDelim(token::Paren) {
+            let body = VariantData::Tuple(self.parse_tuple_struct_body()?, DUMMY_NODE_ID);
+            generics.where_clause = self.parse_where_clause()?;
+            self.expect_semi()?;
+            body
+        } else {
+            let token_str = super::token_descr(&self.token);
+            let msg = &format!(
+                "expected `where`, `{{`, `(`, or `;` after struct name, found {}",
+                token_str
+            );
+            let mut err = self.struct_span_err(self.token.span, msg);
+            err.span_label(self.token.span, "expected `where`, `{`, `(`, or `;` after struct name");
+            return Err(err);
+        };
+
+        Ok((class_name, ItemKind::Struct(vdata, generics)))
+    }
+
+    /// Parses `union Foo { ... }`.
+    fn parse_item_union(&mut self) -> PResult<'a, ItemInfo> {
+        let class_name = self.parse_ident()?;
+
+        let mut generics = self.parse_generics()?;
+
+        let vdata = if self.token.is_keyword(kw::Where) {
+            generics.where_clause = self.parse_where_clause()?;
+            let (fields, recovered) = self.parse_record_struct_body()?;
+            VariantData::Struct(fields, recovered)
+        } else if self.token == token::OpenDelim(token::Brace) {
+            let (fields, recovered) = self.parse_record_struct_body()?;
+            VariantData::Struct(fields, recovered)
+        } else {
+            let token_str = super::token_descr(&self.token);
+            let msg = &format!("expected `where` or `{{` after union name, found {}", token_str);
+            let mut err = self.struct_span_err(self.token.span, msg);
+            err.span_label(self.token.span, "expected `where` or `{` after union name");
+            return Err(err);
+        };
+
+        Ok((class_name, ItemKind::Union(vdata, generics)))
+    }
+
+    fn parse_record_struct_body(
+        &mut self,
+    ) -> PResult<'a, (Vec<StructField>, /* recovered */ bool)> {
+        let mut fields = Vec::new();
+        let mut recovered = false;
+        if self.eat(&token::OpenDelim(token::Brace)) {
+            while self.token != token::CloseDelim(token::Brace) {
+                let field = self.parse_struct_decl_field().map_err(|e| {
+                    self.consume_block(token::Brace, ConsumeClosingDelim::No);
+                    recovered = true;
+                    e
+                });
+                match field {
+                    Ok(field) => fields.push(field),
+                    Err(mut err) => {
+                        err.emit();
+                        break;
+                    }
+                }
+            }
+            self.eat(&token::CloseDelim(token::Brace));
+        } else {
+            let token_str = super::token_descr(&self.token);
+            let msg = &format!("expected `where`, or `{{` after struct name, found {}", token_str);
+            let mut err = self.struct_span_err(self.token.span, msg);
+            err.span_label(self.token.span, "expected `where`, or `{` after struct name");
+            return Err(err);
+        }
+
+        Ok((fields, recovered))
+    }
+
+    fn parse_tuple_struct_body(&mut self) -> PResult<'a, Vec<StructField>> {
+        // This is the case where we find `struct Foo<T>(T) where T: Copy;`
+        // Unit like structs are handled in parse_item_struct function
+        self.parse_paren_comma_seq(|p| {
+            let attrs = p.parse_outer_attributes()?;
+            let lo = p.token.span;
+            let vis = p.parse_visibility(FollowedByType::Yes)?;
+            let ty = p.parse_ty()?;
+            Ok(StructField {
+                span: lo.to(ty.span),
+                vis,
+                ident: None,
+                id: DUMMY_NODE_ID,
+                ty,
+                attrs,
+                is_placeholder: false,
+            })
+        })
+        .map(|(r, _)| r)
+    }
+
+    /// Parses an element of a struct declaration.
+    fn parse_struct_decl_field(&mut self) -> PResult<'a, StructField> {
+        let attrs = self.parse_outer_attributes()?;
+        let lo = self.token.span;
+        let vis = self.parse_visibility(FollowedByType::No)?;
+        self.parse_single_struct_field(lo, vis, attrs)
+    }
+
+    /// Parses a structure field declaration.
+    fn parse_single_struct_field(
+        &mut self,
+        lo: Span,
+        vis: Visibility,
+        attrs: Vec<Attribute>,
+    ) -> PResult<'a, StructField> {
+        let mut seen_comma: bool = false;
+        let a_var = self.parse_name_and_ty(lo, vis, attrs)?;
+        if self.token == token::Comma {
+            seen_comma = true;
+        }
+        match self.token.kind {
+            token::Comma => {
+                self.bump();
+            }
+            token::CloseDelim(token::Brace) => {}
+            token::DocComment(..) => {
+                let previous_span = self.prev_token.span;
+                let mut err = self.span_fatal_err(self.token.span, Error::UselessDocComment);
+                self.bump(); // consume the doc comment
+                let comma_after_doc_seen = self.eat(&token::Comma);
+                // `seen_comma` is always false, because we are inside doc block
+                // condition is here to make code more readable
+                if !seen_comma && comma_after_doc_seen {
+                    seen_comma = true;
+                }
+                if comma_after_doc_seen || self.token == token::CloseDelim(token::Brace) {
+                    err.emit();
+                } else {
+                    if !seen_comma {
+                        let sp = self.sess.source_map().next_point(previous_span);
+                        err.span_suggestion(
+                            sp,
+                            "missing comma here",
+                            ",".into(),
+                            Applicability::MachineApplicable,
+                        );
+                    }
+                    return Err(err);
+                }
+            }
+            _ => {
+                let sp = self.prev_token.span.shrink_to_hi();
+                let mut err = self.struct_span_err(
+                    sp,
+                    &format!("expected `,`, or `}}`, found {}", super::token_descr(&self.token)),
+                );
+
+                // Try to recover extra trailing angle brackets
+                let mut recovered = false;
+                if let TyKind::Path(_, Path { segments, .. }) = &a_var.ty.kind {
+                    if let Some(last_segment) = segments.last() {
+                        recovered = self.check_trailing_angle_brackets(
+                            last_segment,
+                            &[&token::Comma, &token::CloseDelim(token::Brace)],
+                        );
+                        if recovered {
+                            // Handle a case like `Vec<u8>>,` where we can continue parsing fields
+                            // after the comma
+                            self.eat(&token::Comma);
+                            // `check_trailing_angle_brackets` already emitted a nicer error
+                            err.cancel();
+                        }
+                    }
+                }
+
+                if self.token.is_ident() {
+                    // This is likely another field; emit the diagnostic and keep going
+                    err.span_suggestion(
+                        sp,
+                        "try adding a comma",
+                        ",".into(),
+                        Applicability::MachineApplicable,
+                    );
+                    err.emit();
+                    recovered = true;
+                }
+
+                if recovered {
+                    // Make sure an error was emitted (either by recovering an angle bracket,
+                    // or by finding an identifier as the next token), since we're
+                    // going to continue parsing
+                    assert!(self.sess.span_diagnostic.has_errors());
+                } else {
+                    return Err(err);
+                }
+            }
+        }
+        Ok(a_var)
+    }
+
+    /// Parses a structure field.
+    fn parse_name_and_ty(
+        &mut self,
+        lo: Span,
+        vis: Visibility,
+        attrs: Vec<Attribute>,
+    ) -> PResult<'a, StructField> {
+        let name = self.parse_ident_common(false)?;
+        self.expect(&token::Colon)?;
+        let ty = self.parse_ty()?;
+        Ok(StructField {
+            span: lo.to(self.prev_token.span),
+            ident: Some(name),
+            vis,
+            id: DUMMY_NODE_ID,
+            ty,
+            attrs,
+            is_placeholder: false,
+        })
+    }
+
+    /// Parses a declarative macro 2.0 definition.
+    /// The `macro` keyword has already been parsed.
+    /// ```
+    /// MacBody = "{" TOKEN_STREAM "}" ;
+    /// MacParams = "(" TOKEN_STREAM ")" ;
+    /// DeclMac = "macro" Ident MacParams? MacBody ;
+    /// ```
+    fn parse_item_decl_macro(&mut self, lo: Span) -> PResult<'a, ItemInfo> {
+        let ident = self.parse_ident()?;
+        let body = if self.check(&token::OpenDelim(token::Brace)) {
+            self.parse_mac_args()? // `MacBody`
+        } else if self.check(&token::OpenDelim(token::Paren)) {
+            let params = self.parse_token_tree(); // `MacParams`
+            let pspan = params.span();
+            if !self.check(&token::OpenDelim(token::Brace)) {
+                return self.unexpected();
+            }
+            let body = self.parse_token_tree(); // `MacBody`
+            // Convert `MacParams MacBody` into `{ MacParams => MacBody }`.
+            let bspan = body.span();
+            let arrow = TokenTree::token(token::FatArrow, pspan.between(bspan)); // `=>`
+            let tokens = TokenStream::new(vec![params.into(), arrow.into(), body.into()]);
+            let dspan = DelimSpan::from_pair(pspan.shrink_to_lo(), bspan.shrink_to_hi());
+            P(MacArgs::Delimited(dspan, MacDelimiter::Brace, tokens))
+        } else {
+            return self.unexpected();
+        };
+
+        self.sess.gated_spans.gate(sym::decl_macro, lo.to(self.prev_token.span));
+        Ok((ident, ItemKind::MacroDef(ast::MacroDef { body, macro_rules: false })))
+    }
+
+    /// Is this unambiguously the start of a `macro_rules! foo` item defnition?
+    fn is_macro_rules_item(&mut self) -> bool {
+        self.check_keyword(kw::MacroRules)
+            && self.look_ahead(1, |t| *t == token::Not)
+            && self.look_ahead(2, |t| t.is_ident())
+    }
+
+    /// Parses a `macro_rules! foo { ... }` declarative macro.
+    fn parse_item_macro_rules(&mut self, vis: &Visibility) -> PResult<'a, ItemInfo> {
+        self.expect_keyword(kw::MacroRules)?; // `macro_rules`
+        self.expect(&token::Not)?; // `!`
+
+        let ident = self.parse_ident()?;
+        let body = self.parse_mac_args()?;
+        self.eat_semi_for_macro_if_needed(&body);
+        self.complain_if_pub_macro(vis, true);
+
+        Ok((ident, ItemKind::MacroDef(ast::MacroDef { body, macro_rules: true })))
+    }
+
+    /// Item macro invocations or `macro_rules!` definitions need inherited visibility.
+    /// If that's not the case, emit an error.
+    fn complain_if_pub_macro(&self, vis: &Visibility, macro_rules: bool) {
+        if let VisibilityKind::Inherited = vis.node {
+            return;
+        }
+
+        let vstr = pprust::vis_to_string(vis);
+        let vstr = vstr.trim_end();
+        if macro_rules {
+            let msg = format!("can't qualify macro_rules invocation with `{}`", vstr);
+            self.struct_span_err(vis.span, &msg)
+                .span_suggestion(
+                    vis.span,
+                    "try exporting the macro",
+                    "#[macro_export]".to_owned(),
+                    Applicability::MaybeIncorrect, // speculative
+                )
+                .emit();
+        } else {
+            self.struct_span_err(vis.span, "can't qualify macro invocation with `pub`")
+                .span_suggestion(
+                    vis.span,
+                    "remove the visibility",
+                    String::new(),
+                    Applicability::MachineApplicable,
+                )
+                .help(&format!("try adjusting the macro to put `{}` inside the invocation", vstr))
+                .emit();
+        }
+    }
+
+    fn eat_semi_for_macro_if_needed(&mut self, args: &MacArgs) {
+        if args.need_semicolon() && !self.eat(&token::Semi) {
+            self.report_invalid_macro_expansion_item(args);
+        }
+    }
+
+    fn report_invalid_macro_expansion_item(&self, args: &MacArgs) {
+        let span = args.span().expect("undelimited macro call");
+        let mut err = self.struct_span_err(
+            span,
+            "macros that expand to items must be delimited with braces or followed by a semicolon",
+        );
+        if self.unclosed_delims.is_empty() {
+            let DelimSpan { open, close } = match args {
+                MacArgs::Empty | MacArgs::Eq(..) => unreachable!(),
+                MacArgs::Delimited(dspan, ..) => *dspan,
+            };
+            err.multipart_suggestion(
+                "change the delimiters to curly braces",
+                vec![(open, "{".to_string()), (close, '}'.to_string())],
+                Applicability::MaybeIncorrect,
+            );
+        } else {
+            err.span_suggestion(
+                span,
+                "change the delimiters to curly braces",
+                " { /* items */ }".to_string(),
+                Applicability::HasPlaceholders,
+            );
+        }
+        err.span_suggestion(
+            span.shrink_to_hi(),
+            "add a semicolon",
+            ';'.to_string(),
+            Applicability::MaybeIncorrect,
+        );
+        err.emit();
+    }
+
+    /// Checks if current token is one of tokens which cannot be nested like `kw::Enum`. In case
+    /// it is, we try to parse the item and report error about nested types.
+    fn recover_nested_adt_item(&mut self, keyword: Symbol) -> PResult<'a, bool> {
+        if (self.token.is_keyword(kw::Enum)
+            || self.token.is_keyword(kw::Struct)
+            || self.token.is_keyword(kw::Union))
+            && self.look_ahead(1, |t| t.is_ident())
+        {
+            let kw_token = self.token.clone();
+            let kw_str = pprust::token_to_string(&kw_token);
+            let item = self.parse_item()?;
+
+            self.struct_span_err(
+                kw_token.span,
+                &format!("`{}` definition cannot be nested inside `{}`", kw_str, keyword),
+            )
+            .span_suggestion(
+                item.unwrap().span,
+                &format!("consider creating a new `{}` definition instead of nesting", kw_str),
+                String::new(),
+                Applicability::MaybeIncorrect,
+            )
+            .emit();
+            // We successfully parsed the item but we must inform the caller about nested problem.
+            return Ok(false);
+        }
+        Ok(true)
+    }
+}
+
+/// The parsing configuration used to parse a parameter list (see `parse_fn_params`).
+///
+/// The function decides if, per-parameter `p`, `p` must have a pattern or just a type.
+type ReqName = fn(Edition) -> bool;
+
+/// Parsing of functions and methods.
+impl<'a> Parser<'a> {
+    /// Parse a function starting from the front matter (`const ...`) to the body `{ ... }` or `;`.
+    fn parse_fn(
+        &mut self,
+        attrs: &mut Vec<Attribute>,
+        req_name: ReqName,
+        sig_lo: Span,
+    ) -> PResult<'a, (Ident, FnSig, Generics, Option<P<Block>>)> {
+        let header = self.parse_fn_front_matter()?; // `const ... fn`
+        let ident = self.parse_ident()?; // `foo`
+        let mut generics = self.parse_generics()?; // `<'a, T, ...>`
+        let decl = self.parse_fn_decl(req_name, AllowPlus::Yes)?; // `(p: u8, ...)`
+        generics.where_clause = self.parse_where_clause()?; // `where T: Ord`
+
+        let mut sig_hi = self.prev_token.span;
+        let body = self.parse_fn_body(attrs, &mut sig_hi)?; // `;` or `{ ... }`.
+        let fn_sig_span = sig_lo.to(sig_hi);
+        Ok((ident, FnSig { header, decl, span: fn_sig_span }, generics, body))
+    }
+
+    /// Parse the "body" of a function.
+    /// This can either be `;` when there's no body,
+    /// or e.g. a block when the function is a provided one.
+    fn parse_fn_body(
+        &mut self,
+        attrs: &mut Vec<Attribute>,
+        sig_hi: &mut Span,
+    ) -> PResult<'a, Option<P<Block>>> {
+        let (inner_attrs, body) = if self.check(&token::Semi) {
+            // Include the trailing semicolon in the span of the signature
+            *sig_hi = self.token.span;
+            self.bump(); // `;`
+            (Vec::new(), None)
+        } else if self.check(&token::OpenDelim(token::Brace)) || self.token.is_whole_block() {
+            self.parse_inner_attrs_and_block().map(|(attrs, body)| (attrs, Some(body)))?
+        } else if self.token.kind == token::Eq {
+            // Recover `fn foo() = $expr;`.
+            self.bump(); // `=`
+            let eq_sp = self.prev_token.span;
+            let _ = self.parse_expr()?;
+            self.expect_semi()?; // `;`
+            let span = eq_sp.to(self.prev_token.span);
+            self.struct_span_err(span, "function body cannot be `= expression;`")
+                .multipart_suggestion(
+                    "surround the expression with `{` and `}` instead of `=` and `;`",
+                    vec![(eq_sp, "{".to_string()), (self.prev_token.span, " }".to_string())],
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+            (Vec::new(), Some(self.mk_block_err(span)))
+        } else {
+            return self.expected_semi_or_open_brace();
+        };
+        attrs.extend(inner_attrs);
+        Ok(body)
+    }
+
+    /// Is the current token the start of an `FnHeader` / not a valid parse?
+    pub(super) fn check_fn_front_matter(&mut self) -> bool {
+        // We use an over-approximation here.
+        // `const const`, `fn const` won't parse, but we're not stepping over other syntax either.
+        const QUALS: [Symbol; 4] = [kw::Const, kw::Async, kw::Unsafe, kw::Extern];
+        self.check_keyword(kw::Fn) // Definitely an `fn`.
+            // `$qual fn` or `$qual $qual`:
+            || QUALS.iter().any(|&kw| self.check_keyword(kw))
+                && self.look_ahead(1, |t| {
+                    // ...qualified and then `fn`, e.g. `const fn`.
+                    t.is_keyword(kw::Fn)
+                    // Two qualifiers. This is enough. Due `async` we need to check that it's reserved.
+                    || t.is_non_raw_ident_where(|i| QUALS.contains(&i.name) && i.is_reserved())
+                })
+            // `extern ABI fn`
+            || self.check_keyword(kw::Extern)
+                && self.look_ahead(1, |t| t.can_begin_literal_maybe_minus())
+                && self.look_ahead(2, |t| t.is_keyword(kw::Fn))
+    }
+
+    /// Parses all the "front matter" (or "qualifiers") for a `fn` declaration,
+    /// up to and including the `fn` keyword. The formal grammar is:
+    ///
+    /// ```
+    /// Extern = "extern" StringLit ;
+    /// FnQual = "const"? "async"? "unsafe"? Extern? ;
+    /// FnFrontMatter = FnQual? "fn" ;
+    /// ```
+    pub(super) fn parse_fn_front_matter(&mut self) -> PResult<'a, FnHeader> {
+        let constness = self.parse_constness();
+        let asyncness = self.parse_asyncness();
+        let unsafety = self.parse_unsafety();
+        let ext = self.parse_extern()?;
+
+        if let Async::Yes { span, .. } = asyncness {
+            self.ban_async_in_2015(span);
+        }
+
+        if !self.eat_keyword(kw::Fn) {
+            // It is possible for `expect_one_of` to recover given the contents of
+            // `self.expected_tokens`, therefore, do not use `self.unexpected()` which doesn't
+            // account for this.
+            if !self.expect_one_of(&[], &[])? {
+                unreachable!()
+            }
+        }
+
+        Ok(FnHeader { constness, unsafety, asyncness, ext })
+    }
+
+    /// We are parsing `async fn`. If we are on Rust 2015, emit an error.
+    fn ban_async_in_2015(&self, span: Span) {
+        if span.rust_2015() {
+            let diag = self.diagnostic();
+            struct_span_err!(diag, span, E0670, "`async fn` is not permitted in the 2015 edition")
+                .span_label(span, "to use `async fn`, switch to Rust 2018")
+                .help("set `edition = \"2018\"` in `Cargo.toml`")
+                .note("for more on editions, read https://doc.rust-lang.org/edition-guide")
+                .emit();
+        }
+    }
+
+    /// Parses the parameter list and result type of a function declaration.
+    pub(super) fn parse_fn_decl(
+        &mut self,
+        req_name: ReqName,
+        ret_allow_plus: AllowPlus,
+    ) -> PResult<'a, P<FnDecl>> {
+        Ok(P(FnDecl {
+            inputs: self.parse_fn_params(req_name)?,
+            output: self.parse_ret_ty(ret_allow_plus, RecoverQPath::Yes)?,
+        }))
+    }
+
+    /// Parses the parameter list of a function, including the `(` and `)` delimiters.
+    fn parse_fn_params(&mut self, req_name: ReqName) -> PResult<'a, Vec<Param>> {
+        let mut first_param = true;
+        // Parse the arguments, starting out with `self` being allowed...
+        let (mut params, _) = self.parse_paren_comma_seq(|p| {
+            let param = p.parse_param_general(req_name, first_param).or_else(|mut e| {
+                e.emit();
+                let lo = p.prev_token.span;
+                // Skip every token until next possible arg or end.
+                p.eat_to_tokens(&[&token::Comma, &token::CloseDelim(token::Paren)]);
+                // Create a placeholder argument for proper arg count (issue #34264).
+                Ok(dummy_arg(Ident::new(kw::Invalid, lo.to(p.prev_token.span))))
+            });
+            // ...now that we've parsed the first argument, `self` is no longer allowed.
+            first_param = false;
+            param
+        })?;
+        // Replace duplicated recovered params with `_` pattern to avoid unnecessary errors.
+        self.deduplicate_recovered_params_names(&mut params);
+        Ok(params)
+    }
+
+    /// Parses a single function parameter.
+    ///
+    /// - `self` is syntactically allowed when `first_param` holds.
+    fn parse_param_general(&mut self, req_name: ReqName, first_param: bool) -> PResult<'a, Param> {
+        let lo = self.token.span;
+        let attrs = self.parse_outer_attributes()?;
+
+        // Possibly parse `self`. Recover if we parsed it and it wasn't allowed here.
+        if let Some(mut param) = self.parse_self_param()? {
+            param.attrs = attrs.into();
+            return if first_param { Ok(param) } else { self.recover_bad_self_param(param) };
+        }
+
+        let is_name_required = match self.token.kind {
+            token::DotDotDot => false,
+            _ => req_name(self.token.span.edition()),
+        };
+        let (pat, ty) = if is_name_required || self.is_named_param() {
+            debug!("parse_param_general parse_pat (is_name_required:{})", is_name_required);
+
+            let pat = self.parse_fn_param_pat()?;
+            if let Err(mut err) = self.expect(&token::Colon) {
+                return if let Some(ident) =
+                    self.parameter_without_type(&mut err, pat, is_name_required, first_param)
+                {
+                    err.emit();
+                    Ok(dummy_arg(ident))
+                } else {
+                    Err(err)
+                };
+            }
+
+            self.eat_incorrect_doc_comment_for_param_type();
+            (pat, self.parse_ty_for_param()?)
+        } else {
+            debug!("parse_param_general ident_to_pat");
+            let parser_snapshot_before_ty = self.clone();
+            self.eat_incorrect_doc_comment_for_param_type();
+            let mut ty = self.parse_ty_for_param();
+            if ty.is_ok()
+                && self.token != token::Comma
+                && self.token != token::CloseDelim(token::Paren)
+            {
+                // This wasn't actually a type, but a pattern looking like a type,
+                // so we are going to rollback and re-parse for recovery.
+                ty = self.unexpected();
+            }
+            match ty {
+                Ok(ty) => {
+                    let ident = Ident::new(kw::Invalid, self.prev_token.span);
+                    let bm = BindingMode::ByValue(Mutability::Not);
+                    let pat = self.mk_pat_ident(ty.span, bm, ident);
+                    (pat, ty)
+                }
+                // If this is a C-variadic argument and we hit an error, return the error.
+                Err(err) if self.token == token::DotDotDot => return Err(err),
+                // Recover from attempting to parse the argument as a type without pattern.
+                Err(mut err) => {
+                    err.cancel();
+                    *self = parser_snapshot_before_ty;
+                    self.recover_arg_parse()?
+                }
+            }
+        };
+
+        let span = lo.to(self.token.span);
+
+        Ok(Param {
+            attrs: attrs.into(),
+            id: ast::DUMMY_NODE_ID,
+            is_placeholder: false,
+            pat,
+            span,
+            ty,
+        })
+    }
+
+    /// Returns the parsed optional self parameter and whether a self shortcut was used.
+    fn parse_self_param(&mut self) -> PResult<'a, Option<Param>> {
+        // Extract an identifier *after* having confirmed that the token is one.
+        let expect_self_ident = |this: &mut Self| match this.token.ident() {
+            Some((ident, false)) => {
+                this.bump();
+                ident
+            }
+            _ => unreachable!(),
+        };
+        // Is `self` `n` tokens ahead?
+        let is_isolated_self = |this: &Self, n| {
+            this.is_keyword_ahead(n, &[kw::SelfLower])
+                && this.look_ahead(n + 1, |t| t != &token::ModSep)
+        };
+        // Is `mut self` `n` tokens ahead?
+        let is_isolated_mut_self =
+            |this: &Self, n| this.is_keyword_ahead(n, &[kw::Mut]) && is_isolated_self(this, n + 1);
+        // Parse `self` or `self: TYPE`. We already know the current token is `self`.
+        let parse_self_possibly_typed = |this: &mut Self, m| {
+            let eself_ident = expect_self_ident(this);
+            let eself_hi = this.prev_token.span;
+            let eself = if this.eat(&token::Colon) {
+                SelfKind::Explicit(this.parse_ty()?, m)
+            } else {
+                SelfKind::Value(m)
+            };
+            Ok((eself, eself_ident, eself_hi))
+        };
+        // Recover for the grammar `*self`, `*const self`, and `*mut self`.
+        let recover_self_ptr = |this: &mut Self| {
+            let msg = "cannot pass `self` by raw pointer";
+            let span = this.token.span;
+            this.struct_span_err(span, msg).span_label(span, msg).emit();
+
+            Ok((SelfKind::Value(Mutability::Not), expect_self_ident(this), this.prev_token.span))
+        };
+
+        // Parse optional `self` parameter of a method.
+        // Only a limited set of initial token sequences is considered `self` parameters; anything
+        // else is parsed as a normal function parameter list, so some lookahead is required.
+        let eself_lo = self.token.span;
+        let (eself, eself_ident, eself_hi) = match self.token.uninterpolate().kind {
+            token::BinOp(token::And) => {
+                let eself = if is_isolated_self(self, 1) {
+                    // `&self`
+                    self.bump();
+                    SelfKind::Region(None, Mutability::Not)
+                } else if is_isolated_mut_self(self, 1) {
+                    // `&mut self`
+                    self.bump();
+                    self.bump();
+                    SelfKind::Region(None, Mutability::Mut)
+                } else if self.look_ahead(1, |t| t.is_lifetime()) && is_isolated_self(self, 2) {
+                    // `&'lt self`
+                    self.bump();
+                    let lt = self.expect_lifetime();
+                    SelfKind::Region(Some(lt), Mutability::Not)
+                } else if self.look_ahead(1, |t| t.is_lifetime()) && is_isolated_mut_self(self, 2) {
+                    // `&'lt mut self`
+                    self.bump();
+                    let lt = self.expect_lifetime();
+                    self.bump();
+                    SelfKind::Region(Some(lt), Mutability::Mut)
+                } else {
+                    // `&not_self`
+                    return Ok(None);
+                };
+                (eself, expect_self_ident(self), self.prev_token.span)
+            }
+            // `*self`
+            token::BinOp(token::Star) if is_isolated_self(self, 1) => {
+                self.bump();
+                recover_self_ptr(self)?
+            }
+            // `*mut self` and `*const self`
+            token::BinOp(token::Star)
+                if self.look_ahead(1, |t| t.is_mutability()) && is_isolated_self(self, 2) =>
+            {
+                self.bump();
+                self.bump();
+                recover_self_ptr(self)?
+            }
+            // `self` and `self: TYPE`
+            token::Ident(..) if is_isolated_self(self, 0) => {
+                parse_self_possibly_typed(self, Mutability::Not)?
+            }
+            // `mut self` and `mut self: TYPE`
+            token::Ident(..) if is_isolated_mut_self(self, 0) => {
+                self.bump();
+                parse_self_possibly_typed(self, Mutability::Mut)?
+            }
+            _ => return Ok(None),
+        };
+
+        let eself = source_map::respan(eself_lo.to(eself_hi), eself);
+        Ok(Some(Param::from_self(AttrVec::default(), eself, eself_ident)))
+    }
+
+    fn is_named_param(&self) -> bool {
+        let offset = match self.token.kind {
+            token::Interpolated(ref nt) => match **nt {
+                token::NtPat(..) => return self.look_ahead(1, |t| t == &token::Colon),
+                _ => 0,
+            },
+            token::BinOp(token::And) | token::AndAnd => 1,
+            _ if self.token.is_keyword(kw::Mut) => 1,
+            _ => 0,
+        };
+
+        self.look_ahead(offset, |t| t.is_ident())
+            && self.look_ahead(offset + 1, |t| t == &token::Colon)
+    }
+
+    fn recover_first_param(&mut self) -> &'static str {
+        match self
+            .parse_outer_attributes()
+            .and_then(|_| self.parse_self_param())
+            .map_err(|mut e| e.cancel())
+        {
+            Ok(Some(_)) => "method",
+            _ => "function",
+        }
+    }
+}
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
new file mode 100644
index 00000000000..d67ed74bc99
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -0,0 +1,1270 @@
+pub mod attr;
+mod diagnostics;
+mod expr;
+mod generics;
+mod item;
+mod nonterminal;
+mod pat;
+mod path;
+mod stmt;
+mod ty;
+
+use crate::lexer::UnmatchedBrace;
+use diagnostics::Error;
+pub use path::PathStyle;
+
+use rustc_ast::ptr::P;
+use rustc_ast::token::{self, DelimToken, Token, TokenKind};
+use rustc_ast::tokenstream::{self, DelimSpan, TokenStream, TokenTree, TreeAndJoint};
+use rustc_ast::DUMMY_NODE_ID;
+use rustc_ast::{self as ast, AttrStyle, AttrVec, Const, CrateSugar, Extern, Unsafe};
+use rustc_ast::{Async, MacArgs, MacDelimiter, Mutability, StrLit, Visibility, VisibilityKind};
+use rustc_ast_pretty::pprust;
+use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder, FatalError, PResult};
+use rustc_session::parse::ParseSess;
+use rustc_span::source_map::{respan, Span, DUMMY_SP};
+use rustc_span::symbol::{kw, sym, Ident, Symbol};
+use tracing::debug;
+
+use std::{cmp, mem, slice};
+
+bitflags::bitflags! {
+    struct Restrictions: u8 {
+        const STMT_EXPR         = 1 << 0;
+        const NO_STRUCT_LITERAL = 1 << 1;
+    }
+}
+
+#[derive(Clone, Copy, PartialEq, Debug)]
+enum SemiColonMode {
+    Break,
+    Ignore,
+    Comma,
+}
+
+#[derive(Clone, Copy, PartialEq, Debug)]
+enum BlockMode {
+    Break,
+    Ignore,
+}
+
+/// Like `maybe_whole_expr`, but for things other than expressions.
+#[macro_export]
+macro_rules! maybe_whole {
+    ($p:expr, $constructor:ident, |$x:ident| $e:expr) => {
+        if let token::Interpolated(nt) = &$p.token.kind {
+            if let token::$constructor(x) = &**nt {
+                let $x = x.clone();
+                $p.bump();
+                return Ok($e);
+            }
+        }
+    };
+}
+
+/// If the next tokens are ill-formed `$ty::` recover them as `<$ty>::`.
+#[macro_export]
+macro_rules! maybe_recover_from_interpolated_ty_qpath {
+    ($self: expr, $allow_qpath_recovery: expr) => {
+        if $allow_qpath_recovery && $self.look_ahead(1, |t| t == &token::ModSep) {
+            if let token::Interpolated(nt) = &$self.token.kind {
+                if let token::NtTy(ty) = &**nt {
+                    let ty = ty.clone();
+                    $self.bump();
+                    return $self.maybe_recover_from_bad_qpath_stage_2($self.prev_token.span, ty);
+                }
+            }
+        }
+    };
+}
+
+#[derive(Clone)]
+pub struct Parser<'a> {
+    pub sess: &'a ParseSess,
+    /// The current token.
+    pub token: Token,
+    /// The previous token.
+    pub prev_token: Token,
+    restrictions: Restrictions,
+    expected_tokens: Vec<TokenType>,
+    token_cursor: TokenCursor,
+    desugar_doc_comments: bool,
+    /// This field is used to keep track of how many left angle brackets we have seen. This is
+    /// required in order to detect extra leading left angle brackets (`<` characters) and error
+    /// appropriately.
+    ///
+    /// See the comments in the `parse_path_segment` function for more details.
+    unmatched_angle_bracket_count: u32,
+    max_angle_bracket_count: u32,
+    /// A list of all unclosed delimiters found by the lexer. If an entry is used for error recovery
+    /// it gets removed from here. Every entry left at the end gets emitted as an independent
+    /// error.
+    pub(super) unclosed_delims: Vec<UnmatchedBrace>,
+    last_unexpected_token_span: Option<Span>,
+    /// Span pointing at the `:` for the last type ascription the parser has seen, and whether it
+    /// looked like it could have been a mistyped path or literal `Option:Some(42)`).
+    pub last_type_ascription: Option<(Span, bool /* likely path typo */)>,
+    /// If present, this `Parser` is not parsing Rust code but rather a macro call.
+    subparser_name: Option<&'static str>,
+}
+
+impl<'a> Drop for Parser<'a> {
+    fn drop(&mut self) {
+        emit_unclosed_delims(&mut self.unclosed_delims, &self.sess);
+    }
+}
+
+#[derive(Clone)]
+struct TokenCursor {
+    frame: TokenCursorFrame,
+    stack: Vec<TokenCursorFrame>,
+    cur_token: Option<TreeAndJoint>,
+    collecting: Option<Collecting>,
+}
+
+#[derive(Clone)]
+struct TokenCursorFrame {
+    delim: token::DelimToken,
+    span: DelimSpan,
+    open_delim: bool,
+    tree_cursor: tokenstream::Cursor,
+    close_delim: bool,
+}
+
+/// Used to track additional state needed by `collect_tokens`
+#[derive(Clone, Debug)]
+struct Collecting {
+    /// Holds the current tokens captured during the most
+    /// recent call to `collect_tokens`
+    buf: Vec<TreeAndJoint>,
+    /// The depth of the `TokenCursor` stack at the time
+    /// collection was started. When we encounter a `TokenTree::Delimited`,
+    /// we want to record the `TokenTree::Delimited` itself,
+    /// but *not* any of the inner tokens while we are inside
+    /// the new frame (this would cause us to record duplicate tokens).
+    ///
+    /// This `depth` fields tracks stack depth we are recording tokens.
+    /// Only tokens encountered at this depth will be recorded. See
+    /// `TokenCursor::next` for more details.
+    depth: usize,
+}
+
+impl TokenCursorFrame {
+    fn new(span: DelimSpan, delim: DelimToken, tts: &TokenStream) -> Self {
+        TokenCursorFrame {
+            delim,
+            span,
+            open_delim: delim == token::NoDelim,
+            tree_cursor: tts.clone().into_trees(),
+            close_delim: delim == token::NoDelim,
+        }
+    }
+}
+
+impl TokenCursor {
+    fn next(&mut self) -> Token {
+        loop {
+            let tree = if !self.frame.open_delim {
+                self.frame.open_delim = true;
+                TokenTree::open_tt(self.frame.span, self.frame.delim).into()
+            } else if let Some(tree) = self.frame.tree_cursor.next_with_joint() {
+                tree
+            } else if !self.frame.close_delim {
+                self.frame.close_delim = true;
+                TokenTree::close_tt(self.frame.span, self.frame.delim).into()
+            } else if let Some(frame) = self.stack.pop() {
+                self.frame = frame;
+                continue;
+            } else {
+                return Token::new(token::Eof, DUMMY_SP);
+            };
+
+            // Don't set an open delimiter as our current token - we want
+            // to leave it as the full `TokenTree::Delimited` from the previous
+            // iteration of this loop
+            if !matches!(tree.0, TokenTree::Token(Token { kind: TokenKind::OpenDelim(_), .. })) {
+                self.cur_token = Some(tree.clone());
+            }
+
+            if let Some(collecting) = &mut self.collecting {
+                if collecting.depth == self.stack.len() {
+                    debug!(
+                        "TokenCursor::next():  collected {:?} at depth {:?}",
+                        tree,
+                        self.stack.len()
+                    );
+                    collecting.buf.push(tree.clone())
+                }
+            }
+
+            match tree.0 {
+                TokenTree::Token(token) => return token,
+                TokenTree::Delimited(sp, delim, tts) => {
+                    let frame = TokenCursorFrame::new(sp, delim, &tts);
+                    self.stack.push(mem::replace(&mut self.frame, frame));
+                }
+            }
+        }
+    }
+
+    fn next_desugared(&mut self) -> Token {
+        let (data, attr_style, sp) = match self.next() {
+            Token { kind: token::DocComment(_, attr_style, data), span } => {
+                (data, attr_style, span)
+            }
+            tok => return tok,
+        };
+
+        // Searches for the occurrences of `"#*` and returns the minimum number of `#`s
+        // required to wrap the text.
+        let mut num_of_hashes = 0;
+        let mut count = 0;
+        for ch in data.as_str().chars() {
+            count = match ch {
+                '"' => 1,
+                '#' if count > 0 => count + 1,
+                _ => 0,
+            };
+            num_of_hashes = cmp::max(num_of_hashes, count);
+        }
+
+        let delim_span = DelimSpan::from_single(sp);
+        let body = TokenTree::Delimited(
+            delim_span,
+            token::Bracket,
+            [
+                TokenTree::token(token::Ident(sym::doc, false), sp),
+                TokenTree::token(token::Eq, sp),
+                TokenTree::token(TokenKind::lit(token::StrRaw(num_of_hashes), data, None), sp),
+            ]
+            .iter()
+            .cloned()
+            .collect::<TokenStream>(),
+        );
+
+        self.stack.push(mem::replace(
+            &mut self.frame,
+            TokenCursorFrame::new(
+                delim_span,
+                token::NoDelim,
+                &if attr_style == AttrStyle::Inner {
+                    [TokenTree::token(token::Pound, sp), TokenTree::token(token::Not, sp), body]
+                        .iter()
+                        .cloned()
+                        .collect::<TokenStream>()
+                } else {
+                    [TokenTree::token(token::Pound, sp), body]
+                        .iter()
+                        .cloned()
+                        .collect::<TokenStream>()
+                },
+            ),
+        ));
+
+        self.next()
+    }
+}
+
+#[derive(Clone, PartialEq)]
+enum TokenType {
+    Token(TokenKind),
+    Keyword(Symbol),
+    Operator,
+    Lifetime,
+    Ident,
+    Path,
+    Type,
+    Const,
+}
+
+impl TokenType {
+    fn to_string(&self) -> String {
+        match *self {
+            TokenType::Token(ref t) => format!("`{}`", pprust::token_kind_to_string(t)),
+            TokenType::Keyword(kw) => format!("`{}`", kw),
+            TokenType::Operator => "an operator".to_string(),
+            TokenType::Lifetime => "lifetime".to_string(),
+            TokenType::Ident => "identifier".to_string(),
+            TokenType::Path => "path".to_string(),
+            TokenType::Type => "type".to_string(),
+            TokenType::Const => "const".to_string(),
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+enum TokenExpectType {
+    Expect,
+    NoExpect,
+}
+
+/// A sequence separator.
+struct SeqSep {
+    /// The separator token.
+    sep: Option<TokenKind>,
+    /// `true` if a trailing separator is allowed.
+    trailing_sep_allowed: bool,
+}
+
+impl SeqSep {
+    fn trailing_allowed(t: TokenKind) -> SeqSep {
+        SeqSep { sep: Some(t), trailing_sep_allowed: true }
+    }
+
+    fn none() -> SeqSep {
+        SeqSep { sep: None, trailing_sep_allowed: false }
+    }
+}
+
+pub enum FollowedByType {
+    Yes,
+    No,
+}
+
+fn token_descr_opt(token: &Token) -> Option<&'static str> {
+    Some(match token.kind {
+        _ if token.is_special_ident() => "reserved identifier",
+        _ if token.is_used_keyword() => "keyword",
+        _ if token.is_unused_keyword() => "reserved keyword",
+        token::DocComment(..) => "doc comment",
+        _ => return None,
+    })
+}
+
+pub(super) fn token_descr(token: &Token) -> String {
+    let token_str = pprust::token_to_string(token);
+    match token_descr_opt(token) {
+        Some(prefix) => format!("{} `{}`", prefix, token_str),
+        _ => format!("`{}`", token_str),
+    }
+}
+
+impl<'a> Parser<'a> {
+    pub fn new(
+        sess: &'a ParseSess,
+        tokens: TokenStream,
+        desugar_doc_comments: bool,
+        subparser_name: Option<&'static str>,
+    ) -> Self {
+        let mut parser = Parser {
+            sess,
+            token: Token::dummy(),
+            prev_token: Token::dummy(),
+            restrictions: Restrictions::empty(),
+            expected_tokens: Vec::new(),
+            token_cursor: TokenCursor {
+                frame: TokenCursorFrame::new(DelimSpan::dummy(), token::NoDelim, &tokens),
+                stack: Vec::new(),
+                cur_token: None,
+                collecting: None,
+            },
+            desugar_doc_comments,
+            unmatched_angle_bracket_count: 0,
+            max_angle_bracket_count: 0,
+            unclosed_delims: Vec::new(),
+            last_unexpected_token_span: None,
+            last_type_ascription: None,
+            subparser_name,
+        };
+
+        // Make parser point to the first token.
+        parser.bump();
+
+        parser
+    }
+
+    fn next_tok(&mut self, fallback_span: Span) -> Token {
+        let mut next = if self.desugar_doc_comments {
+            self.token_cursor.next_desugared()
+        } else {
+            self.token_cursor.next()
+        };
+        if next.span.is_dummy() {
+            // Tweak the location for better diagnostics, but keep syntactic context intact.
+            next.span = fallback_span.with_ctxt(next.span.ctxt());
+        }
+        next
+    }
+
+    crate fn unexpected<T>(&mut self) -> PResult<'a, T> {
+        match self.expect_one_of(&[], &[]) {
+            Err(e) => Err(e),
+            // We can get `Ok(true)` from `recover_closing_delimiter`
+            // which is called in `expected_one_of_not_found`.
+            Ok(_) => FatalError.raise(),
+        }
+    }
+
+    /// Expects and consumes the token `t`. Signals an error if the next token is not `t`.
+    pub fn expect(&mut self, t: &TokenKind) -> PResult<'a, bool /* recovered */> {
+        if self.expected_tokens.is_empty() {
+            if self.token == *t {
+                self.bump();
+                Ok(false)
+            } else {
+                self.unexpected_try_recover(t)
+            }
+        } else {
+            self.expect_one_of(slice::from_ref(t), &[])
+        }
+    }
+
+    /// Expect next token to be edible or inedible token.  If edible,
+    /// then consume it; if inedible, then return without consuming
+    /// anything.  Signal a fatal error if next token is unexpected.
+    pub fn expect_one_of(
+        &mut self,
+        edible: &[TokenKind],
+        inedible: &[TokenKind],
+    ) -> PResult<'a, bool /* recovered */> {
+        if edible.contains(&self.token.kind) {
+            self.bump();
+            Ok(false)
+        } else if inedible.contains(&self.token.kind) {
+            // leave it in the input
+            Ok(false)
+        } else if self.last_unexpected_token_span == Some(self.token.span) {
+            FatalError.raise();
+        } else {
+            self.expected_one_of_not_found(edible, inedible)
+        }
+    }
+
+    // Public for rustfmt usage.
+    pub fn parse_ident(&mut self) -> PResult<'a, Ident> {
+        self.parse_ident_common(true)
+    }
+
+    fn parse_ident_common(&mut self, recover: bool) -> PResult<'a, Ident> {
+        match self.token.ident() {
+            Some((ident, is_raw)) => {
+                if !is_raw && ident.is_reserved() {
+                    let mut err = self.expected_ident_found();
+                    if recover {
+                        err.emit();
+                    } else {
+                        return Err(err);
+                    }
+                }
+                self.bump();
+                Ok(ident)
+            }
+            _ => Err(match self.prev_token.kind {
+                TokenKind::DocComment(..) => {
+                    self.span_fatal_err(self.prev_token.span, Error::UselessDocComment)
+                }
+                _ => self.expected_ident_found(),
+            }),
+        }
+    }
+
+    /// Checks if the next token is `tok`, and returns `true` if so.
+    ///
+    /// This method will automatically add `tok` to `expected_tokens` if `tok` is not
+    /// encountered.
+    fn check(&mut self, tok: &TokenKind) -> bool {
+        let is_present = self.token == *tok;
+        if !is_present {
+            self.expected_tokens.push(TokenType::Token(tok.clone()));
+        }
+        is_present
+    }
+
+    /// Consumes a token 'tok' if it exists. Returns whether the given token was present.
+    pub fn eat(&mut self, tok: &TokenKind) -> bool {
+        let is_present = self.check(tok);
+        if is_present {
+            self.bump()
+        }
+        is_present
+    }
+
+    /// If the next token is the given keyword, returns `true` without eating it.
+    /// An expectation is also added for diagnostics purposes.
+    fn check_keyword(&mut self, kw: Symbol) -> bool {
+        self.expected_tokens.push(TokenType::Keyword(kw));
+        self.token.is_keyword(kw)
+    }
+
+    /// If the next token is the given keyword, eats it and returns `true`.
+    /// Otherwise, returns `false`. An expectation is also added for diagnostics purposes.
+    // Public for rustfmt usage.
+    pub fn eat_keyword(&mut self, kw: Symbol) -> bool {
+        if self.check_keyword(kw) {
+            self.bump();
+            true
+        } else {
+            false
+        }
+    }
+
+    fn eat_keyword_noexpect(&mut self, kw: Symbol) -> bool {
+        if self.token.is_keyword(kw) {
+            self.bump();
+            true
+        } else {
+            false
+        }
+    }
+
+    /// If the given word is not a keyword, signals an error.
+    /// If the next token is not the given word, signals an error.
+    /// Otherwise, eats it.
+    fn expect_keyword(&mut self, kw: Symbol) -> PResult<'a, ()> {
+        if !self.eat_keyword(kw) { self.unexpected() } else { Ok(()) }
+    }
+
+    /// Is the given keyword `kw` followed by a non-reserved identifier?
+    fn is_kw_followed_by_ident(&self, kw: Symbol) -> bool {
+        self.token.is_keyword(kw) && self.look_ahead(1, |t| t.is_ident() && !t.is_reserved_ident())
+    }
+
+    fn check_or_expected(&mut self, ok: bool, typ: TokenType) -> bool {
+        if ok {
+            true
+        } else {
+            self.expected_tokens.push(typ);
+            false
+        }
+    }
+
+    fn check_ident(&mut self) -> bool {
+        self.check_or_expected(self.token.is_ident(), TokenType::Ident)
+    }
+
+    fn check_path(&mut self) -> bool {
+        self.check_or_expected(self.token.is_path_start(), TokenType::Path)
+    }
+
+    fn check_type(&mut self) -> bool {
+        self.check_or_expected(self.token.can_begin_type(), TokenType::Type)
+    }
+
+    fn check_const_arg(&mut self) -> bool {
+        self.check_or_expected(self.token.can_begin_const_arg(), TokenType::Const)
+    }
+
+    /// Checks to see if the next token is either `+` or `+=`.
+    /// Otherwise returns `false`.
+    fn check_plus(&mut self) -> bool {
+        self.check_or_expected(
+            self.token.is_like_plus(),
+            TokenType::Token(token::BinOp(token::Plus)),
+        )
+    }
+
+    /// Eats the expected token if it's present possibly breaking
+    /// compound tokens like multi-character operators in process.
+    /// Returns `true` if the token was eaten.
+    fn break_and_eat(&mut self, expected: TokenKind) -> bool {
+        if self.token.kind == expected {
+            self.bump();
+            return true;
+        }
+        match self.token.kind.break_two_token_op() {
+            Some((first, second)) if first == expected => {
+                let first_span = self.sess.source_map().start_point(self.token.span);
+                let second_span = self.token.span.with_lo(first_span.hi());
+                self.token = Token::new(first, first_span);
+                self.bump_with(Token::new(second, second_span));
+                true
+            }
+            _ => {
+                self.expected_tokens.push(TokenType::Token(expected));
+                false
+            }
+        }
+    }
+
+    /// Eats `+` possibly breaking tokens like `+=` in process.
+    fn eat_plus(&mut self) -> bool {
+        self.break_and_eat(token::BinOp(token::Plus))
+    }
+
+    /// Eats `&` possibly breaking tokens like `&&` in process.
+    /// Signals an error if `&` is not eaten.
+    fn expect_and(&mut self) -> PResult<'a, ()> {
+        if self.break_and_eat(token::BinOp(token::And)) { Ok(()) } else { self.unexpected() }
+    }
+
+    /// Eats `|` possibly breaking tokens like `||` in process.
+    /// Signals an error if `|` was not eaten.
+    fn expect_or(&mut self) -> PResult<'a, ()> {
+        if self.break_and_eat(token::BinOp(token::Or)) { Ok(()) } else { self.unexpected() }
+    }
+
+    /// Eats `<` possibly breaking tokens like `<<` in process.
+    fn eat_lt(&mut self) -> bool {
+        let ate = self.break_and_eat(token::Lt);
+        if ate {
+            // See doc comment for `unmatched_angle_bracket_count`.
+            self.unmatched_angle_bracket_count += 1;
+            self.max_angle_bracket_count += 1;
+            debug!("eat_lt: (increment) count={:?}", self.unmatched_angle_bracket_count);
+        }
+        ate
+    }
+
+    /// Eats `<` possibly breaking tokens like `<<` in process.
+    /// Signals an error if `<` was not eaten.
+    fn expect_lt(&mut self) -> PResult<'a, ()> {
+        if self.eat_lt() { Ok(()) } else { self.unexpected() }
+    }
+
+    /// Eats `>` possibly breaking tokens like `>>` in process.
+    /// Signals an error if `>` was not eaten.
+    fn expect_gt(&mut self) -> PResult<'a, ()> {
+        if self.break_and_eat(token::Gt) {
+            // See doc comment for `unmatched_angle_bracket_count`.
+            if self.unmatched_angle_bracket_count > 0 {
+                self.unmatched_angle_bracket_count -= 1;
+                debug!("expect_gt: (decrement) count={:?}", self.unmatched_angle_bracket_count);
+            }
+            Ok(())
+        } else {
+            self.unexpected()
+        }
+    }
+
+    fn expect_any_with_type(&mut self, kets: &[&TokenKind], expect: TokenExpectType) -> bool {
+        kets.iter().any(|k| match expect {
+            TokenExpectType::Expect => self.check(k),
+            TokenExpectType::NoExpect => self.token == **k,
+        })
+    }
+
+    fn parse_seq_to_before_tokens<T>(
+        &mut self,
+        kets: &[&TokenKind],
+        sep: SeqSep,
+        expect: TokenExpectType,
+        mut f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
+    ) -> PResult<'a, (Vec<T>, bool /* trailing */, bool /* recovered */)> {
+        let mut first = true;
+        let mut recovered = false;
+        let mut trailing = false;
+        let mut v = vec![];
+        while !self.expect_any_with_type(kets, expect) {
+            if let token::CloseDelim(..) | token::Eof = self.token.kind {
+                break;
+            }
+            if let Some(ref t) = sep.sep {
+                if first {
+                    first = false;
+                } else {
+                    match self.expect(t) {
+                        Ok(false) => {}
+                        Ok(true) => {
+                            recovered = true;
+                            break;
+                        }
+                        Err(mut expect_err) => {
+                            let sp = self.prev_token.span.shrink_to_hi();
+                            let token_str = pprust::token_kind_to_string(t);
+
+                            // Attempt to keep parsing if it was a similar separator.
+                            if let Some(ref tokens) = t.similar_tokens() {
+                                if tokens.contains(&self.token.kind) {
+                                    self.bump();
+                                }
+                            }
+
+                            // If this was a missing `@` in a binding pattern
+                            // bail with a suggestion
+                            // https://github.com/rust-lang/rust/issues/72373
+                            if self.prev_token.is_ident() && self.token.kind == token::DotDot {
+                                let msg = format!(
+                                    "if you meant to bind the contents of \
+                                    the rest of the array pattern into `{}`, use `@`",
+                                    pprust::token_to_string(&self.prev_token)
+                                );
+                                expect_err
+                                    .span_suggestion_verbose(
+                                        self.prev_token.span.shrink_to_hi().until(self.token.span),
+                                        &msg,
+                                        " @ ".to_string(),
+                                        Applicability::MaybeIncorrect,
+                                    )
+                                    .emit();
+                                break;
+                            }
+
+                            // Attempt to keep parsing if it was an omitted separator.
+                            match f(self) {
+                                Ok(t) => {
+                                    // Parsed successfully, therefore most probably the code only
+                                    // misses a separator.
+                                    expect_err
+                                        .span_suggestion_short(
+                                            self.sess.source_map().next_point(sp),
+                                            &format!("missing `{}`", token_str),
+                                            token_str,
+                                            Applicability::MaybeIncorrect,
+                                        )
+                                        .emit();
+
+                                    v.push(t);
+                                    continue;
+                                }
+                                Err(mut e) => {
+                                    // Parsing failed, therefore it must be something more serious
+                                    // than just a missing separator.
+                                    expect_err.emit();
+
+                                    e.cancel();
+                                    break;
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+            if sep.trailing_sep_allowed && self.expect_any_with_type(kets, expect) {
+                trailing = true;
+                break;
+            }
+
+            let t = f(self)?;
+            v.push(t);
+        }
+
+        Ok((v, trailing, recovered))
+    }
+
+    /// Parses a sequence, not including the closing delimiter. The function
+    /// `f` must consume tokens until reaching the next separator or
+    /// closing bracket.
+    fn parse_seq_to_before_end<T>(
+        &mut self,
+        ket: &TokenKind,
+        sep: SeqSep,
+        f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
+    ) -> PResult<'a, (Vec<T>, bool, bool)> {
+        self.parse_seq_to_before_tokens(&[ket], sep, TokenExpectType::Expect, f)
+    }
+
+    /// Parses a sequence, including the closing delimiter. The function
+    /// `f` must consume tokens until reaching the next separator or
+    /// closing bracket.
+    fn parse_seq_to_end<T>(
+        &mut self,
+        ket: &TokenKind,
+        sep: SeqSep,
+        f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
+    ) -> PResult<'a, (Vec<T>, bool /* trailing */)> {
+        let (val, trailing, recovered) = self.parse_seq_to_before_end(ket, sep, f)?;
+        if !recovered {
+            self.eat(ket);
+        }
+        Ok((val, trailing))
+    }
+
+    /// Parses a sequence, including the closing delimiter. The function
+    /// `f` must consume tokens until reaching the next separator or
+    /// closing bracket.
+    fn parse_unspanned_seq<T>(
+        &mut self,
+        bra: &TokenKind,
+        ket: &TokenKind,
+        sep: SeqSep,
+        f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
+    ) -> PResult<'a, (Vec<T>, bool)> {
+        self.expect(bra)?;
+        self.parse_seq_to_end(ket, sep, f)
+    }
+
+    fn parse_delim_comma_seq<T>(
+        &mut self,
+        delim: DelimToken,
+        f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
+    ) -> PResult<'a, (Vec<T>, bool)> {
+        self.parse_unspanned_seq(
+            &token::OpenDelim(delim),
+            &token::CloseDelim(delim),
+            SeqSep::trailing_allowed(token::Comma),
+            f,
+        )
+    }
+
+    fn parse_paren_comma_seq<T>(
+        &mut self,
+        f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
+    ) -> PResult<'a, (Vec<T>, bool)> {
+        self.parse_delim_comma_seq(token::Paren, f)
+    }
+
+    /// Advance the parser by one token using provided token as the next one.
+    fn bump_with(&mut self, next_token: Token) {
+        // Bumping after EOF is a bad sign, usually an infinite loop.
+        if self.prev_token.kind == TokenKind::Eof {
+            let msg = "attempted to bump the parser past EOF (may be stuck in a loop)";
+            self.span_bug(self.token.span, msg);
+        }
+
+        // Update the current and previous tokens.
+        self.prev_token = mem::replace(&mut self.token, next_token);
+
+        // Diagnostics.
+        self.expected_tokens.clear();
+    }
+
+    /// Advance the parser by one token.
+    pub fn bump(&mut self) {
+        let next_token = self.next_tok(self.token.span);
+        self.bump_with(next_token);
+    }
+
+    /// Look-ahead `dist` tokens of `self.token` and get access to that token there.
+    /// When `dist == 0` then the current token is looked at.
+    pub fn look_ahead<R>(&self, dist: usize, looker: impl FnOnce(&Token) -> R) -> R {
+        if dist == 0 {
+            return looker(&self.token);
+        }
+
+        let frame = &self.token_cursor.frame;
+        looker(&match frame.tree_cursor.look_ahead(dist - 1) {
+            Some(tree) => match tree {
+                TokenTree::Token(token) => token,
+                TokenTree::Delimited(dspan, delim, _) => {
+                    Token::new(token::OpenDelim(delim), dspan.open)
+                }
+            },
+            None => Token::new(token::CloseDelim(frame.delim), frame.span.close),
+        })
+    }
+
+    /// Returns whether any of the given keywords are `dist` tokens ahead of the current one.
+    fn is_keyword_ahead(&self, dist: usize, kws: &[Symbol]) -> bool {
+        self.look_ahead(dist, |t| kws.iter().any(|&kw| t.is_keyword(kw)))
+    }
+
+    /// Parses asyncness: `async` or nothing.
+    fn parse_asyncness(&mut self) -> Async {
+        if self.eat_keyword(kw::Async) {
+            let span = self.prev_token.uninterpolated_span();
+            Async::Yes { span, closure_id: DUMMY_NODE_ID, return_impl_trait_id: DUMMY_NODE_ID }
+        } else {
+            Async::No
+        }
+    }
+
+    /// Parses unsafety: `unsafe` or nothing.
+    fn parse_unsafety(&mut self) -> Unsafe {
+        if self.eat_keyword(kw::Unsafe) {
+            Unsafe::Yes(self.prev_token.uninterpolated_span())
+        } else {
+            Unsafe::No
+        }
+    }
+
+    /// Parses constness: `const` or nothing.
+    fn parse_constness(&mut self) -> Const {
+        if self.eat_keyword(kw::Const) {
+            Const::Yes(self.prev_token.uninterpolated_span())
+        } else {
+            Const::No
+        }
+    }
+
+    /// Parses mutability (`mut` or nothing).
+    fn parse_mutability(&mut self) -> Mutability {
+        if self.eat_keyword(kw::Mut) { Mutability::Mut } else { Mutability::Not }
+    }
+
+    /// Possibly parses mutability (`const` or `mut`).
+    fn parse_const_or_mut(&mut self) -> Option<Mutability> {
+        if self.eat_keyword(kw::Mut) {
+            Some(Mutability::Mut)
+        } else if self.eat_keyword(kw::Const) {
+            Some(Mutability::Not)
+        } else {
+            None
+        }
+    }
+
+    fn parse_field_name(&mut self) -> PResult<'a, Ident> {
+        if let token::Literal(token::Lit { kind: token::Integer, symbol, suffix }) = self.token.kind
+        {
+            self.expect_no_suffix(self.token.span, "a tuple index", suffix);
+            self.bump();
+            Ok(Ident::new(symbol, self.prev_token.span))
+        } else {
+            self.parse_ident_common(false)
+        }
+    }
+
+    fn parse_mac_args(&mut self) -> PResult<'a, P<MacArgs>> {
+        self.parse_mac_args_common(true).map(P)
+    }
+
+    fn parse_attr_args(&mut self) -> PResult<'a, MacArgs> {
+        self.parse_mac_args_common(false)
+    }
+
+    fn parse_mac_args_common(&mut self, delimited_only: bool) -> PResult<'a, MacArgs> {
+        Ok(
+            if self.check(&token::OpenDelim(DelimToken::Paren))
+                || self.check(&token::OpenDelim(DelimToken::Bracket))
+                || self.check(&token::OpenDelim(DelimToken::Brace))
+            {
+                match self.parse_token_tree() {
+                    TokenTree::Delimited(dspan, delim, tokens) =>
+                    // We've confirmed above that there is a delimiter so unwrapping is OK.
+                    {
+                        MacArgs::Delimited(dspan, MacDelimiter::from_token(delim).unwrap(), tokens)
+                    }
+                    _ => unreachable!(),
+                }
+            } else if !delimited_only {
+                if self.eat(&token::Eq) {
+                    let eq_span = self.prev_token.span;
+                    let mut is_interpolated_expr = false;
+                    if let token::Interpolated(nt) = &self.token.kind {
+                        if let token::NtExpr(..) = **nt {
+                            is_interpolated_expr = true;
+                        }
+                    }
+                    let token_tree = if is_interpolated_expr {
+                        // We need to accept arbitrary interpolated expressions to continue
+                        // supporting things like `doc = $expr` that work on stable.
+                        // Non-literal interpolated expressions are rejected after expansion.
+                        self.parse_token_tree()
+                    } else {
+                        self.parse_unsuffixed_lit()?.token_tree()
+                    };
+
+                    MacArgs::Eq(eq_span, token_tree.into())
+                } else {
+                    MacArgs::Empty
+                }
+            } else {
+                return self.unexpected();
+            },
+        )
+    }
+
+    fn parse_or_use_outer_attributes(
+        &mut self,
+        already_parsed_attrs: Option<AttrVec>,
+    ) -> PResult<'a, AttrVec> {
+        if let Some(attrs) = already_parsed_attrs {
+            Ok(attrs)
+        } else {
+            self.parse_outer_attributes().map(|a| a.into())
+        }
+    }
+
+    /// Parses a single token tree from the input.
+    pub(crate) fn parse_token_tree(&mut self) -> TokenTree {
+        match self.token.kind {
+            token::OpenDelim(..) => {
+                let frame = mem::replace(
+                    &mut self.token_cursor.frame,
+                    self.token_cursor.stack.pop().unwrap(),
+                );
+                self.token = Token::new(TokenKind::CloseDelim(frame.delim), frame.span.close);
+                self.bump();
+                TokenTree::Delimited(frame.span, frame.delim, frame.tree_cursor.stream)
+            }
+            token::CloseDelim(_) | token::Eof => unreachable!(),
+            _ => {
+                self.bump();
+                TokenTree::Token(self.prev_token.clone())
+            }
+        }
+    }
+
+    /// Parses a stream of tokens into a list of `TokenTree`s, up to EOF.
+    pub fn parse_all_token_trees(&mut self) -> PResult<'a, Vec<TokenTree>> {
+        let mut tts = Vec::new();
+        while self.token != token::Eof {
+            tts.push(self.parse_token_tree());
+        }
+        Ok(tts)
+    }
+
+    pub fn parse_tokens(&mut self) -> TokenStream {
+        let mut result = Vec::new();
+        loop {
+            match self.token.kind {
+                token::Eof | token::CloseDelim(..) => break,
+                _ => result.push(self.parse_token_tree().into()),
+            }
+        }
+        TokenStream::new(result)
+    }
+
+    /// Evaluates the closure with restrictions in place.
+    ///
+    /// Afters the closure is evaluated, restrictions are reset.
+    fn with_res<T>(&mut self, res: Restrictions, f: impl FnOnce(&mut Self) -> T) -> T {
+        let old = self.restrictions;
+        self.restrictions = res;
+        let res = f(self);
+        self.restrictions = old;
+        res
+    }
+
+    fn is_crate_vis(&self) -> bool {
+        self.token.is_keyword(kw::Crate) && self.look_ahead(1, |t| t != &token::ModSep)
+    }
+
+    /// Parses `pub`, `pub(crate)` and `pub(in path)` plus shortcuts `crate` for `pub(crate)`,
+    /// `pub(self)` for `pub(in self)` and `pub(super)` for `pub(in super)`.
+    /// If the following element can't be a tuple (i.e., it's a function definition), then
+    /// it's not a tuple struct field), and the contents within the parentheses isn't valid,
+    /// so emit a proper diagnostic.
+    pub(crate) fn parse_visibility(&mut self, fbt: FollowedByType) -> PResult<'a, Visibility> {
+        maybe_whole!(self, NtVis, |x| x);
+
+        self.expected_tokens.push(TokenType::Keyword(kw::Crate));
+        if self.is_crate_vis() {
+            self.bump(); // `crate`
+            self.sess.gated_spans.gate(sym::crate_visibility_modifier, self.prev_token.span);
+            return Ok(respan(self.prev_token.span, VisibilityKind::Crate(CrateSugar::JustCrate)));
+        }
+
+        if !self.eat_keyword(kw::Pub) {
+            // We need a span for our `Spanned<VisibilityKind>`, but there's inherently no
+            // keyword to grab a span from for inherited visibility; an empty span at the
+            // beginning of the current token would seem to be the "Schelling span".
+            return Ok(respan(self.token.span.shrink_to_lo(), VisibilityKind::Inherited));
+        }
+        let lo = self.prev_token.span;
+
+        if self.check(&token::OpenDelim(token::Paren)) {
+            // We don't `self.bump()` the `(` yet because this might be a struct definition where
+            // `()` or a tuple might be allowed. For example, `struct Struct(pub (), pub (usize));`.
+            // Because of this, we only `bump` the `(` if we're assured it is appropriate to do so
+            // by the following tokens.
+            if self.is_keyword_ahead(1, &[kw::Crate]) && self.look_ahead(2, |t| t != &token::ModSep)
+            // account for `pub(crate::foo)`
+            {
+                // Parse `pub(crate)`.
+                self.bump(); // `(`
+                self.bump(); // `crate`
+                self.expect(&token::CloseDelim(token::Paren))?; // `)`
+                let vis = VisibilityKind::Crate(CrateSugar::PubCrate);
+                return Ok(respan(lo.to(self.prev_token.span), vis));
+            } else if self.is_keyword_ahead(1, &[kw::In]) {
+                // Parse `pub(in path)`.
+                self.bump(); // `(`
+                self.bump(); // `in`
+                let path = self.parse_path(PathStyle::Mod)?; // `path`
+                self.expect(&token::CloseDelim(token::Paren))?; // `)`
+                let vis = VisibilityKind::Restricted { path: P(path), id: ast::DUMMY_NODE_ID };
+                return Ok(respan(lo.to(self.prev_token.span), vis));
+            } else if self.look_ahead(2, |t| t == &token::CloseDelim(token::Paren))
+                && self.is_keyword_ahead(1, &[kw::Super, kw::SelfLower])
+            {
+                // Parse `pub(self)` or `pub(super)`.
+                self.bump(); // `(`
+                let path = self.parse_path(PathStyle::Mod)?; // `super`/`self`
+                self.expect(&token::CloseDelim(token::Paren))?; // `)`
+                let vis = VisibilityKind::Restricted { path: P(path), id: ast::DUMMY_NODE_ID };
+                return Ok(respan(lo.to(self.prev_token.span), vis));
+            } else if let FollowedByType::No = fbt {
+                // Provide this diagnostic if a type cannot follow;
+                // in particular, if this is not a tuple struct.
+                self.recover_incorrect_vis_restriction()?;
+                // Emit diagnostic, but continue with public visibility.
+            }
+        }
+
+        Ok(respan(lo, VisibilityKind::Public))
+    }
+
+    /// Recovery for e.g. `pub(something) fn ...` or `struct X { pub(something) y: Z }`
+    fn recover_incorrect_vis_restriction(&mut self) -> PResult<'a, ()> {
+        self.bump(); // `(`
+        let path = self.parse_path(PathStyle::Mod)?;
+        self.expect(&token::CloseDelim(token::Paren))?; // `)`
+
+        let msg = "incorrect visibility restriction";
+        let suggestion = r##"some possible visibility restrictions are:
+`pub(crate)`: visible only on the current crate
+`pub(super)`: visible only in the current module's parent
+`pub(in path::to::module)`: visible only on the specified path"##;
+
+        let path_str = pprust::path_to_string(&path);
+
+        struct_span_err!(self.sess.span_diagnostic, path.span, E0704, "{}", msg)
+            .help(suggestion)
+            .span_suggestion(
+                path.span,
+                &format!("make this visible only to module `{}` with `in`", path_str),
+                format!("in {}", path_str),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+
+        Ok(())
+    }
+
+    /// Parses `extern string_literal?`.
+    fn parse_extern(&mut self) -> PResult<'a, Extern> {
+        Ok(if self.eat_keyword(kw::Extern) {
+            Extern::from_abi(self.parse_abi())
+        } else {
+            Extern::None
+        })
+    }
+
+    /// Parses a string literal as an ABI spec.
+    fn parse_abi(&mut self) -> Option<StrLit> {
+        match self.parse_str_lit() {
+            Ok(str_lit) => Some(str_lit),
+            Err(Some(lit)) => match lit.kind {
+                ast::LitKind::Err(_) => None,
+                _ => {
+                    self.struct_span_err(lit.span, "non-string ABI literal")
+                        .span_suggestion(
+                            lit.span,
+                            "specify the ABI with a string literal",
+                            "\"C\"".to_string(),
+                            Applicability::MaybeIncorrect,
+                        )
+                        .emit();
+                    None
+                }
+            },
+            Err(None) => None,
+        }
+    }
+
+    /// Records all tokens consumed by the provided callback,
+    /// including the current token. These tokens are collected
+    /// into a `TokenStream`, and returned along with the result
+    /// of the callback.
+    ///
+    /// Note: If your callback consumes an opening delimiter
+    /// (including the case where you call `collect_tokens`
+    /// when the current token is an opening delimeter),
+    /// you must also consume the corresponding closing delimiter.
+    ///
+    /// That is, you can consume
+    /// `something ([{ }])` or `([{}])`, but not `([{}]`
+    ///
+    /// This restriction shouldn't be an issue in practice,
+    /// since this function is used to record the tokens for
+    /// a parsed AST item, which always has matching delimiters.
+    pub fn collect_tokens<R>(
+        &mut self,
+        f: impl FnOnce(&mut Self) -> PResult<'a, R>,
+    ) -> PResult<'a, (R, TokenStream)> {
+        // Record all tokens we parse when parsing this item.
+        let tokens: Vec<TreeAndJoint> = self.token_cursor.cur_token.clone().into_iter().collect();
+        debug!("collect_tokens: starting with {:?}", tokens);
+
+        // We need special handling for the case where `collect_tokens` is called
+        // on an opening delimeter (e.g. '('). At this point, we have already pushed
+        // a new frame - however, we want to record the original `TokenTree::Delimited`,
+        // for consistency with the case where we start recording one token earlier.
+        // See `TokenCursor::next` to see how `cur_token` is set up.
+        let prev_depth =
+            if matches!(self.token_cursor.cur_token, Some((TokenTree::Delimited(..), _))) {
+                if self.token_cursor.stack.is_empty() {
+                    // There is nothing below us in the stack that
+                    // the function could consume, so the only thing it can legally
+                    // capture is the entire contents of the current frame.
+                    return Ok((f(self)?, TokenStream::new(tokens)));
+                }
+                // We have already recorded the full `TokenTree::Delimited` when we created
+                // our `tokens` vector at the start of this function. We are now inside
+                // a new frame corresponding to the `TokenTree::Delimited` we already recoreded.
+                // We don't want to record any of the tokens inside this frame, since they
+                // will be duplicates of the tokens nested inside the `TokenTree::Delimited`.
+                // Therefore, we set our recording depth to the *previous* frame. This allows
+                // us to record a sequence like: `(foo).bar()`: the `(foo)` will be recored
+                // as our initial `cur_token`, while the `.bar()` will be recored after we
+                // pop the `(foo)` frame.
+                self.token_cursor.stack.len() - 1
+            } else {
+                self.token_cursor.stack.len()
+            };
+        let prev_collecting =
+            self.token_cursor.collecting.replace(Collecting { buf: tokens, depth: prev_depth });
+
+        let ret = f(self);
+
+        let mut collected_tokens = if let Some(collecting) = self.token_cursor.collecting.take() {
+            collecting.buf
+        } else {
+            let msg = "our vector went away?";
+            debug!("collect_tokens: {}", msg);
+            self.sess.span_diagnostic.delay_span_bug(self.token.span, &msg);
+            // This can happen due to a bad interaction of two unrelated recovery mechanisms
+            // with mismatched delimiters *and* recovery lookahead on the likely typo
+            // `pub ident(` (#62895, different but similar to the case above).
+            return Ok((ret?, TokenStream::default()));
+        };
+
+        debug!("collect_tokens: got raw tokens {:?}", collected_tokens);
+
+        // If we're not at EOF our current token wasn't actually consumed by
+        // `f`, but it'll still be in our list that we pulled out. In that case
+        // put it back.
+        let extra_token = if self.token != token::Eof { collected_tokens.pop() } else { None };
+
+        if let Some(mut collecting) = prev_collecting {
+            // If we were previously collecting at the same depth,
+            // then the previous call to `collect_tokens` needs to see
+            // the tokens we just recorded.
+            //
+            // If we were previously recording at an lower `depth`,
+            // then the previous `collect_tokens` call already recorded
+            // this entire frame in the form of a `TokenTree::Delimited`,
+            // so there is nothing else for us to do.
+            if collecting.depth == prev_depth {
+                collecting.buf.extend(collected_tokens.iter().cloned());
+                collecting.buf.extend(extra_token);
+                debug!("collect_tokens: updating previous buf to {:?}", collecting);
+            }
+            self.token_cursor.collecting = Some(collecting)
+        }
+
+        Ok((ret?, TokenStream::new(collected_tokens)))
+    }
+
+    /// `::{` or `::*`
+    fn is_import_coupler(&mut self) -> bool {
+        self.check(&token::ModSep)
+            && self.look_ahead(1, |t| {
+                *t == token::OpenDelim(token::Brace) || *t == token::BinOp(token::Star)
+            })
+    }
+}
+
+crate fn make_unclosed_delims_error(
+    unmatched: UnmatchedBrace,
+    sess: &ParseSess,
+) -> Option<DiagnosticBuilder<'_>> {
+    // `None` here means an `Eof` was found. We already emit those errors elsewhere, we add them to
+    // `unmatched_braces` only for error recovery in the `Parser`.
+    let found_delim = unmatched.found_delim?;
+    let mut err = sess.span_diagnostic.struct_span_err(
+        unmatched.found_span,
+        &format!(
+            "mismatched closing delimiter: `{}`",
+            pprust::token_kind_to_string(&token::CloseDelim(found_delim)),
+        ),
+    );
+    err.span_label(unmatched.found_span, "mismatched closing delimiter");
+    if let Some(sp) = unmatched.candidate_span {
+        err.span_label(sp, "closing delimiter possibly meant for this");
+    }
+    if let Some(sp) = unmatched.unclosed_span {
+        err.span_label(sp, "unclosed delimiter");
+    }
+    Some(err)
+}
+
+pub fn emit_unclosed_delims(unclosed_delims: &mut Vec<UnmatchedBrace>, sess: &ParseSess) {
+    *sess.reached_eof.borrow_mut() |=
+        unclosed_delims.iter().any(|unmatched_delim| unmatched_delim.found_delim.is_none());
+    for unmatched in unclosed_delims.drain(..) {
+        if let Some(mut e) = make_unclosed_delims_error(unmatched, sess) {
+            e.emit();
+        }
+    }
+}
diff --git a/compiler/rustc_parse/src/parser/nonterminal.rs b/compiler/rustc_parse/src/parser/nonterminal.rs
new file mode 100644
index 00000000000..f40cd1131d2
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/nonterminal.rs
@@ -0,0 +1,170 @@
+use rustc_ast::ptr::P;
+use rustc_ast::token::{self, Nonterminal, NonterminalKind, Token};
+use rustc_ast_pretty::pprust;
+use rustc_errors::PResult;
+use rustc_span::symbol::{kw, Ident};
+
+use crate::parser::{FollowedByType, Parser, PathStyle};
+
+impl<'a> Parser<'a> {
+    /// Checks whether a non-terminal may begin with a particular token.
+    ///
+    /// Returning `false` is a *stability guarantee* that such a matcher will *never* begin with that
+    /// token. Be conservative (return true) if not sure.
+    pub fn nonterminal_may_begin_with(kind: NonterminalKind, token: &Token) -> bool {
+        /// Checks whether the non-terminal may contain a single (non-keyword) identifier.
+        fn may_be_ident(nt: &token::Nonterminal) -> bool {
+            match *nt {
+                token::NtItem(_) | token::NtBlock(_) | token::NtVis(_) | token::NtLifetime(_) => {
+                    false
+                }
+                _ => true,
+            }
+        }
+
+        match kind {
+            NonterminalKind::Expr => {
+                token.can_begin_expr()
+                // This exception is here for backwards compatibility.
+                && !token.is_keyword(kw::Let)
+            }
+            NonterminalKind::Ty => token.can_begin_type(),
+            NonterminalKind::Ident => get_macro_ident(token).is_some(),
+            NonterminalKind::Literal => token.can_begin_literal_maybe_minus(),
+            NonterminalKind::Vis => match token.kind {
+                // The follow-set of :vis + "priv" keyword + interpolated
+                token::Comma | token::Ident(..) | token::Interpolated(..) => true,
+                _ => token.can_begin_type(),
+            },
+            NonterminalKind::Block => match token.kind {
+                token::OpenDelim(token::Brace) => true,
+                token::Interpolated(ref nt) => match **nt {
+                    token::NtItem(_)
+                    | token::NtPat(_)
+                    | token::NtTy(_)
+                    | token::NtIdent(..)
+                    | token::NtMeta(_)
+                    | token::NtPath(_)
+                    | token::NtVis(_) => false, // none of these may start with '{'.
+                    _ => true,
+                },
+                _ => false,
+            },
+            NonterminalKind::Path | NonterminalKind::Meta => match token.kind {
+                token::ModSep | token::Ident(..) => true,
+                token::Interpolated(ref nt) => match **nt {
+                    token::NtPath(_) | token::NtMeta(_) => true,
+                    _ => may_be_ident(&nt),
+                },
+                _ => false,
+            },
+            NonterminalKind::Pat => match token.kind {
+                token::Ident(..) |                  // box, ref, mut, and other identifiers (can stricten)
+                token::OpenDelim(token::Paren) |    // tuple pattern
+                token::OpenDelim(token::Bracket) |  // slice pattern
+                token::BinOp(token::And) |          // reference
+                token::BinOp(token::Minus) |        // negative literal
+                token::AndAnd |                     // double reference
+                token::Literal(..) |                // literal
+                token::DotDot |                     // range pattern (future compat)
+                token::DotDotDot |                  // range pattern (future compat)
+                token::ModSep |                     // path
+                token::Lt |                         // path (UFCS constant)
+                token::BinOp(token::Shl) => true,   // path (double UFCS)
+                token::Interpolated(ref nt) => may_be_ident(nt),
+                _ => false,
+            },
+            NonterminalKind::Lifetime => match token.kind {
+                token::Lifetime(_) => true,
+                token::Interpolated(ref nt) => match **nt {
+                    token::NtLifetime(_) | token::NtTT(_) => true,
+                    _ => false,
+                },
+                _ => false,
+            },
+            NonterminalKind::TT | NonterminalKind::Item | NonterminalKind::Stmt => match token.kind
+            {
+                token::CloseDelim(_) => false,
+                _ => true,
+            },
+        }
+    }
+
+    pub fn parse_nonterminal(&mut self, kind: NonterminalKind) -> PResult<'a, Nonterminal> {
+        // Any `Nonterminal` which stores its tokens (currently `NtItem` and `NtExpr`)
+        // needs to have them force-captured here.
+        // A `macro_rules!` invocation may pass a captured item/expr to a proc-macro,
+        // which requires having captured tokens available. Since we cannot determine
+        // in advance whether or not a proc-macro will be (transitively) invoked,
+        // we always capture tokens for any `Nonterminal` which needs them.
+        Ok(match kind {
+            NonterminalKind::Item => match self.collect_tokens(|this| this.parse_item())? {
+                (Some(mut item), tokens) => {
+                    // If we captured tokens during parsing (due to outer attributes),
+                    // use those.
+                    if item.tokens.is_none() {
+                        item.tokens = Some(tokens);
+                    }
+                    token::NtItem(item)
+                }
+                (None, _) => {
+                    return Err(self.struct_span_err(self.token.span, "expected an item keyword"));
+                }
+            },
+            NonterminalKind::Block => token::NtBlock(self.parse_block()?),
+            NonterminalKind::Stmt => match self.parse_stmt()? {
+                Some(s) => token::NtStmt(s),
+                None => return Err(self.struct_span_err(self.token.span, "expected a statement")),
+            },
+            NonterminalKind::Pat => {
+                let (mut pat, tokens) = self.collect_tokens(|this| this.parse_pat(None))?;
+                // We have have eaten an NtPat, which could already have tokens
+                if pat.tokens.is_none() {
+                    pat.tokens = Some(tokens);
+                }
+                token::NtPat(pat)
+            }
+            NonterminalKind::Expr => {
+                let (mut expr, tokens) = self.collect_tokens(|this| this.parse_expr())?;
+                // If we captured tokens during parsing (due to outer attributes),
+                // use those.
+                if expr.tokens.is_none() {
+                    expr.tokens = Some(tokens);
+                }
+                token::NtExpr(expr)
+            }
+            NonterminalKind::Literal => token::NtLiteral(self.parse_literal_maybe_minus()?),
+            NonterminalKind::Ty => token::NtTy(self.parse_ty()?),
+            // this could be handled like a token, since it is one
+            NonterminalKind::Ident => {
+                if let Some((ident, is_raw)) = get_macro_ident(&self.token) {
+                    self.bump();
+                    token::NtIdent(ident, is_raw)
+                } else {
+                    let token_str = pprust::token_to_string(&self.token);
+                    let msg = &format!("expected ident, found {}", &token_str);
+                    return Err(self.struct_span_err(self.token.span, msg));
+                }
+            }
+            NonterminalKind::Path => token::NtPath(self.parse_path(PathStyle::Type)?),
+            NonterminalKind::Meta => token::NtMeta(P(self.parse_attr_item()?)),
+            NonterminalKind::TT => token::NtTT(self.parse_token_tree()),
+            NonterminalKind::Vis => token::NtVis(self.parse_visibility(FollowedByType::Yes)?),
+            NonterminalKind::Lifetime => {
+                if self.check_lifetime() {
+                    token::NtLifetime(self.expect_lifetime().ident)
+                } else {
+                    let token_str = pprust::token_to_string(&self.token);
+                    let msg = &format!("expected a lifetime, found `{}`", &token_str);
+                    return Err(self.struct_span_err(self.token.span, msg));
+                }
+            }
+        })
+    }
+}
+
+/// The token is an identifier, but not `_`.
+/// We prohibit passing `_` to macros expecting `ident` for now.
+fn get_macro_ident(token: &Token) -> Option<(Ident, bool)> {
+    token.ident().filter(|(ident, _)| ident.name != kw::Underscore)
+}
diff --git a/compiler/rustc_parse/src/parser/pat.rs b/compiler/rustc_parse/src/parser/pat.rs
new file mode 100644
index 00000000000..2c0133a24dc
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/pat.rs
@@ -0,0 +1,1012 @@
+use super::{Parser, PathStyle};
+use crate::{maybe_recover_from_interpolated_ty_qpath, maybe_whole};
+use rustc_ast::mut_visit::{noop_visit_mac, noop_visit_pat, MutVisitor};
+use rustc_ast::ptr::P;
+use rustc_ast::token;
+use rustc_ast::{self as ast, AttrVec, Attribute, FieldPat, MacCall, Pat, PatKind, RangeEnd};
+use rustc_ast::{BindingMode, Expr, ExprKind, Mutability, Path, QSelf, RangeSyntax};
+use rustc_ast_pretty::pprust;
+use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder, PResult};
+use rustc_span::source_map::{respan, Span, Spanned};
+use rustc_span::symbol::{kw, sym, Ident};
+
+type Expected = Option<&'static str>;
+
+/// `Expected` for function and lambda parameter patterns.
+pub(super) const PARAM_EXPECTED: Expected = Some("parameter name");
+
+const WHILE_PARSING_OR_MSG: &str = "while parsing this or-pattern starting here";
+
+/// Whether or not an or-pattern should be gated when occurring in the current context.
+#[derive(PartialEq)]
+pub(super) enum GateOr {
+    Yes,
+    No,
+}
+
+/// Whether or not to recover a `,` when parsing or-patterns.
+#[derive(PartialEq, Copy, Clone)]
+enum RecoverComma {
+    Yes,
+    No,
+}
+
+impl<'a> Parser<'a> {
+    /// Parses a pattern.
+    ///
+    /// Corresponds to `pat<no_top_alt>` in RFC 2535 and does not admit or-patterns
+    /// at the top level. Used when parsing the parameters of lambda expressions,
+    /// functions, function pointers, and `pat` macro fragments.
+    pub fn parse_pat(&mut self, expected: Expected) -> PResult<'a, P<Pat>> {
+        self.parse_pat_with_range_pat(true, expected)
+    }
+
+    /// Entry point to the main pattern parser.
+    /// Corresponds to `top_pat` in RFC 2535 and allows or-pattern at the top level.
+    pub(super) fn parse_top_pat(&mut self, gate_or: GateOr) -> PResult<'a, P<Pat>> {
+        // Allow a '|' before the pats (RFCs 1925, 2530, and 2535).
+        let gated_leading_vert = self.eat_or_separator(None) && gate_or == GateOr::Yes;
+        let leading_vert_span = self.prev_token.span;
+
+        // Parse the possibly-or-pattern.
+        let pat = self.parse_pat_with_or(None, gate_or, RecoverComma::Yes)?;
+
+        // If we parsed a leading `|` which should be gated,
+        // and no other gated or-pattern has been parsed thus far,
+        // then we should really gate the leading `|`.
+        // This complicated procedure is done purely for diagnostics UX.
+        if gated_leading_vert && self.sess.gated_spans.is_ungated(sym::or_patterns) {
+            self.sess.gated_spans.gate(sym::or_patterns, leading_vert_span);
+        }
+
+        Ok(pat)
+    }
+
+    /// Parse the pattern for a function or function pointer parameter.
+    /// Special recovery is provided for or-patterns and leading `|`.
+    pub(super) fn parse_fn_param_pat(&mut self) -> PResult<'a, P<Pat>> {
+        self.recover_leading_vert(None, "not allowed in a parameter pattern");
+        let pat = self.parse_pat_with_or(PARAM_EXPECTED, GateOr::No, RecoverComma::No)?;
+
+        if let PatKind::Or(..) = &pat.kind {
+            self.ban_illegal_fn_param_or_pat(&pat);
+        }
+
+        Ok(pat)
+    }
+
+    /// Ban `A | B` immediately in a parameter pattern and suggest wrapping in parens.
+    fn ban_illegal_fn_param_or_pat(&self, pat: &Pat) {
+        let msg = "wrap the pattern in parenthesis";
+        let fix = format!("({})", pprust::pat_to_string(pat));
+        self.struct_span_err(pat.span, "an or-pattern parameter must be wrapped in parenthesis")
+            .span_suggestion(pat.span, msg, fix, Applicability::MachineApplicable)
+            .emit();
+    }
+
+    /// Parses a pattern, that may be a or-pattern (e.g. `Foo | Bar` in `Some(Foo | Bar)`).
+    /// Corresponds to `pat<allow_top_alt>` in RFC 2535.
+    fn parse_pat_with_or(
+        &mut self,
+        expected: Expected,
+        gate_or: GateOr,
+        rc: RecoverComma,
+    ) -> PResult<'a, P<Pat>> {
+        // Parse the first pattern (`p_0`).
+        let first_pat = self.parse_pat(expected)?;
+        self.maybe_recover_unexpected_comma(first_pat.span, rc)?;
+
+        // If the next token is not a `|`,
+        // this is not an or-pattern and we should exit here.
+        if !self.check(&token::BinOp(token::Or)) && self.token != token::OrOr {
+            return Ok(first_pat);
+        }
+
+        // Parse the patterns `p_1 | ... | p_n` where `n > 0`.
+        let lo = first_pat.span;
+        let mut pats = vec![first_pat];
+        while self.eat_or_separator(Some(lo)) {
+            let pat = self.parse_pat(expected).map_err(|mut err| {
+                err.span_label(lo, WHILE_PARSING_OR_MSG);
+                err
+            })?;
+            self.maybe_recover_unexpected_comma(pat.span, rc)?;
+            pats.push(pat);
+        }
+        let or_pattern_span = lo.to(self.prev_token.span);
+
+        // Feature gate the or-pattern if instructed:
+        if gate_or == GateOr::Yes {
+            self.sess.gated_spans.gate(sym::or_patterns, or_pattern_span);
+        }
+
+        Ok(self.mk_pat(or_pattern_span, PatKind::Or(pats)))
+    }
+
+    /// Eat the or-pattern `|` separator.
+    /// If instead a `||` token is encountered, recover and pretend we parsed `|`.
+    fn eat_or_separator(&mut self, lo: Option<Span>) -> bool {
+        if self.recover_trailing_vert(lo) {
+            return false;
+        }
+
+        match self.token.kind {
+            token::OrOr => {
+                // Found `||`; Recover and pretend we parsed `|`.
+                self.ban_unexpected_or_or(lo);
+                self.bump();
+                true
+            }
+            _ => self.eat(&token::BinOp(token::Or)),
+        }
+    }
+
+    /// Recover if `|` or `||` is the current token and we have one of the
+    /// tokens `=>`, `if`, `=`, `:`, `;`, `,`, `]`, `)`, or `}` ahead of us.
+    ///
+    /// These tokens all indicate that we reached the end of the or-pattern
+    /// list and can now reliably say that the `|` was an illegal trailing vert.
+    /// Note that there are more tokens such as `@` for which we know that the `|`
+    /// is an illegal parse. However, the user's intent is less clear in that case.
+    fn recover_trailing_vert(&mut self, lo: Option<Span>) -> bool {
+        let is_end_ahead = self.look_ahead(1, |token| match &token.uninterpolate().kind {
+            token::FatArrow // e.g. `a | => 0,`.
+            | token::Ident(kw::If, false) // e.g. `a | if expr`.
+            | token::Eq // e.g. `let a | = 0`.
+            | token::Semi // e.g. `let a |;`.
+            | token::Colon // e.g. `let a | :`.
+            | token::Comma // e.g. `let (a |,)`.
+            | token::CloseDelim(token::Bracket) // e.g. `let [a | ]`.
+            | token::CloseDelim(token::Paren) // e.g. `let (a | )`.
+            | token::CloseDelim(token::Brace) => true, // e.g. `let A { f: a | }`.
+            _ => false,
+        });
+        match (is_end_ahead, &self.token.kind) {
+            (true, token::BinOp(token::Or) | token::OrOr) => {
+                self.ban_illegal_vert(lo, "trailing", "not allowed in an or-pattern");
+                self.bump();
+                true
+            }
+            _ => false,
+        }
+    }
+
+    /// We have parsed `||` instead of `|`. Error and suggest `|` instead.
+    fn ban_unexpected_or_or(&mut self, lo: Option<Span>) {
+        let mut err = self.struct_span_err(self.token.span, "unexpected token `||` after pattern");
+        err.span_suggestion(
+            self.token.span,
+            "use a single `|` to separate multiple alternative patterns",
+            "|".to_owned(),
+            Applicability::MachineApplicable,
+        );
+        if let Some(lo) = lo {
+            err.span_label(lo, WHILE_PARSING_OR_MSG);
+        }
+        err.emit();
+    }
+
+    /// Some special error handling for the "top-level" patterns in a match arm,
+    /// `for` loop, `let`, &c. (in contrast to subpatterns within such).
+    fn maybe_recover_unexpected_comma(&mut self, lo: Span, rc: RecoverComma) -> PResult<'a, ()> {
+        if rc == RecoverComma::No || self.token != token::Comma {
+            return Ok(());
+        }
+
+        // An unexpected comma after a top-level pattern is a clue that the
+        // user (perhaps more accustomed to some other language) forgot the
+        // parentheses in what should have been a tuple pattern; return a
+        // suggestion-enhanced error here rather than choking on the comma later.
+        let comma_span = self.token.span;
+        self.bump();
+        if let Err(mut err) = self.skip_pat_list() {
+            // We didn't expect this to work anyway; we just wanted to advance to the
+            // end of the comma-sequence so we know the span to suggest parenthesizing.
+            err.cancel();
+        }
+        let seq_span = lo.to(self.prev_token.span);
+        let mut err = self.struct_span_err(comma_span, "unexpected `,` in pattern");
+        if let Ok(seq_snippet) = self.span_to_snippet(seq_span) {
+            err.span_suggestion(
+                seq_span,
+                "try adding parentheses to match on a tuple...",
+                format!("({})", seq_snippet),
+                Applicability::MachineApplicable,
+            )
+            .span_suggestion(
+                seq_span,
+                "...or a vertical bar to match on multiple alternatives",
+                seq_snippet.replace(",", " |"),
+                Applicability::MachineApplicable,
+            );
+        }
+        Err(err)
+    }
+
+    /// Parse and throw away a parentesized comma separated
+    /// sequence of patterns until `)` is reached.
+    fn skip_pat_list(&mut self) -> PResult<'a, ()> {
+        while !self.check(&token::CloseDelim(token::Paren)) {
+            self.parse_pat(None)?;
+            if !self.eat(&token::Comma) {
+                return Ok(());
+            }
+        }
+        Ok(())
+    }
+
+    /// Recursive possibly-or-pattern parser with recovery for an erroneous leading `|`.
+    /// See `parse_pat_with_or` for details on parsing or-patterns.
+    fn parse_pat_with_or_inner(&mut self) -> PResult<'a, P<Pat>> {
+        self.recover_leading_vert(None, "only allowed in a top-level pattern");
+        self.parse_pat_with_or(None, GateOr::Yes, RecoverComma::No)
+    }
+
+    /// Recover if `|` or `||` is here.
+    /// The user is thinking that a leading `|` is allowed in this position.
+    fn recover_leading_vert(&mut self, lo: Option<Span>, ctx: &str) {
+        if let token::BinOp(token::Or) | token::OrOr = self.token.kind {
+            self.ban_illegal_vert(lo, "leading", ctx);
+            self.bump();
+        }
+    }
+
+    /// A `|` or possibly `||` token shouldn't be here. Ban it.
+    fn ban_illegal_vert(&mut self, lo: Option<Span>, pos: &str, ctx: &str) {
+        let span = self.token.span;
+        let mut err = self.struct_span_err(span, &format!("a {} `|` is {}", pos, ctx));
+        err.span_suggestion(
+            span,
+            &format!("remove the `{}`", pprust::token_to_string(&self.token)),
+            String::new(),
+            Applicability::MachineApplicable,
+        );
+        if let Some(lo) = lo {
+            err.span_label(lo, WHILE_PARSING_OR_MSG);
+        }
+        if let token::OrOr = self.token.kind {
+            err.note("alternatives in or-patterns are separated with `|`, not `||`");
+        }
+        err.emit();
+    }
+
+    /// Parses a pattern, with a setting whether modern range patterns (e.g., `a..=b`, `a..b` are
+    /// allowed).
+    fn parse_pat_with_range_pat(
+        &mut self,
+        allow_range_pat: bool,
+        expected: Expected,
+    ) -> PResult<'a, P<Pat>> {
+        maybe_recover_from_interpolated_ty_qpath!(self, true);
+        maybe_whole!(self, NtPat, |x| x);
+
+        let lo = self.token.span;
+
+        let pat = if self.check(&token::BinOp(token::And)) || self.token.kind == token::AndAnd {
+            self.parse_pat_deref(expected)?
+        } else if self.check(&token::OpenDelim(token::Paren)) {
+            self.parse_pat_tuple_or_parens()?
+        } else if self.check(&token::OpenDelim(token::Bracket)) {
+            // Parse `[pat, pat,...]` as a slice pattern.
+            let (pats, _) =
+                self.parse_delim_comma_seq(token::Bracket, |p| p.parse_pat_with_or_inner())?;
+            PatKind::Slice(pats)
+        } else if self.check(&token::DotDot) && !self.is_pat_range_end_start(1) {
+            // A rest pattern `..`.
+            self.bump(); // `..`
+            PatKind::Rest
+        } else if self.check(&token::DotDotDot) && !self.is_pat_range_end_start(1) {
+            self.recover_dotdotdot_rest_pat(lo)
+        } else if let Some(form) = self.parse_range_end() {
+            self.parse_pat_range_to(form)? // `..=X`, `...X`, or `..X`.
+        } else if self.eat_keyword(kw::Underscore) {
+            // Parse _
+            PatKind::Wild
+        } else if self.eat_keyword(kw::Mut) {
+            self.parse_pat_ident_mut()?
+        } else if self.eat_keyword(kw::Ref) {
+            // Parse ref ident @ pat / ref mut ident @ pat
+            let mutbl = self.parse_mutability();
+            self.parse_pat_ident(BindingMode::ByRef(mutbl))?
+        } else if self.eat_keyword(kw::Box) {
+            // Parse `box pat`
+            let pat = self.parse_pat_with_range_pat(false, None)?;
+            self.sess.gated_spans.gate(sym::box_patterns, lo.to(self.prev_token.span));
+            PatKind::Box(pat)
+        } else if self.can_be_ident_pat() {
+            // Parse `ident @ pat`
+            // This can give false positives and parse nullary enums,
+            // they are dealt with later in resolve.
+            self.parse_pat_ident(BindingMode::ByValue(Mutability::Not))?
+        } else if self.is_start_of_pat_with_path() {
+            // Parse pattern starting with a path
+            let (qself, path) = if self.eat_lt() {
+                // Parse a qualified path
+                let (qself, path) = self.parse_qpath(PathStyle::Expr)?;
+                (Some(qself), path)
+            } else {
+                // Parse an unqualified path
+                (None, self.parse_path(PathStyle::Expr)?)
+            };
+            let span = lo.to(self.prev_token.span);
+
+            if qself.is_none() && self.check(&token::Not) {
+                self.parse_pat_mac_invoc(path)?
+            } else if let Some(form) = self.parse_range_end() {
+                let begin = self.mk_expr(span, ExprKind::Path(qself, path), AttrVec::new());
+                self.parse_pat_range_begin_with(begin, form)?
+            } else if self.check(&token::OpenDelim(token::Brace)) {
+                self.parse_pat_struct(qself, path)?
+            } else if self.check(&token::OpenDelim(token::Paren)) {
+                self.parse_pat_tuple_struct(qself, path)?
+            } else {
+                PatKind::Path(qself, path)
+            }
+        } else {
+            // Try to parse everything else as literal with optional minus
+            match self.parse_literal_maybe_minus() {
+                Ok(begin) => match self.parse_range_end() {
+                    Some(form) => self.parse_pat_range_begin_with(begin, form)?,
+                    None => PatKind::Lit(begin),
+                },
+                Err(err) => return self.fatal_unexpected_non_pat(err, expected),
+            }
+        };
+
+        let pat = self.mk_pat(lo.to(self.prev_token.span), pat);
+        let pat = self.maybe_recover_from_bad_qpath(pat, true)?;
+        let pat = self.recover_intersection_pat(pat)?;
+
+        if !allow_range_pat {
+            self.ban_pat_range_if_ambiguous(&pat)
+        }
+
+        Ok(pat)
+    }
+
+    /// Recover from a typoed `...` pattern that was encountered
+    /// Ref: Issue #70388
+    fn recover_dotdotdot_rest_pat(&mut self, lo: Span) -> PatKind {
+        // A typoed rest pattern `...`.
+        self.bump(); // `...`
+
+        // The user probably mistook `...` for a rest pattern `..`.
+        self.struct_span_err(lo, "unexpected `...`")
+            .span_label(lo, "not a valid pattern")
+            .span_suggestion_short(
+                lo,
+                "for a rest pattern, use `..` instead of `...`",
+                "..".to_owned(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+        PatKind::Rest
+    }
+
+    /// Try to recover the more general form `intersect ::= $pat_lhs @ $pat_rhs`.
+    ///
+    /// Allowed binding patterns generated by `binding ::= ref? mut? $ident @ $pat_rhs`
+    /// should already have been parsed by now  at this point,
+    /// if the next token is `@` then we can try to parse the more general form.
+    ///
+    /// Consult `parse_pat_ident` for the `binding` grammar.
+    ///
+    /// The notion of intersection patterns are found in
+    /// e.g. [F#][and] where they are called AND-patterns.
+    ///
+    /// [and]: https://docs.microsoft.com/en-us/dotnet/fsharp/language-reference/pattern-matching
+    fn recover_intersection_pat(&mut self, lhs: P<Pat>) -> PResult<'a, P<Pat>> {
+        if self.token.kind != token::At {
+            // Next token is not `@` so it's not going to be an intersection pattern.
+            return Ok(lhs);
+        }
+
+        // At this point we attempt to parse `@ $pat_rhs` and emit an error.
+        self.bump(); // `@`
+        let mut rhs = self.parse_pat(None)?;
+        let sp = lhs.span.to(rhs.span);
+
+        if let PatKind::Ident(_, _, ref mut sub @ None) = rhs.kind {
+            // The user inverted the order, so help them fix that.
+            let mut applicability = Applicability::MachineApplicable;
+            // FIXME(bindings_after_at): Remove this code when stabilizing the feature.
+            lhs.walk(&mut |p| match p.kind {
+                // `check_match` is unhappy if the subpattern has a binding anywhere.
+                PatKind::Ident(..) => {
+                    applicability = Applicability::MaybeIncorrect;
+                    false // Short-circuit.
+                }
+                _ => true,
+            });
+
+            let lhs_span = lhs.span;
+            // Move the LHS into the RHS as a subpattern.
+            // The RHS is now the full pattern.
+            *sub = Some(lhs);
+
+            self.struct_span_err(sp, "pattern on wrong side of `@`")
+                .span_label(lhs_span, "pattern on the left, should be on the right")
+                .span_label(rhs.span, "binding on the right, should be on the left")
+                .span_suggestion(sp, "switch the order", pprust::pat_to_string(&rhs), applicability)
+                .emit();
+        } else {
+            // The special case above doesn't apply so we may have e.g. `A(x) @ B(y)`.
+            rhs.kind = PatKind::Wild;
+            self.struct_span_err(sp, "left-hand side of `@` must be a binding")
+                .span_label(lhs.span, "interpreted as a pattern, not a binding")
+                .span_label(rhs.span, "also a pattern")
+                .note("bindings are `x`, `mut x`, `ref x`, and `ref mut x`")
+                .emit();
+        }
+
+        rhs.span = sp;
+        Ok(rhs)
+    }
+
+    /// Ban a range pattern if it has an ambiguous interpretation.
+    fn ban_pat_range_if_ambiguous(&self, pat: &Pat) {
+        match pat.kind {
+            PatKind::Range(
+                ..,
+                Spanned { node: RangeEnd::Included(RangeSyntax::DotDotDot), .. },
+            ) => return,
+            PatKind::Range(..) => {}
+            _ => return,
+        }
+
+        self.struct_span_err(pat.span, "the range pattern here has ambiguous interpretation")
+            .span_suggestion(
+                pat.span,
+                "add parentheses to clarify the precedence",
+                format!("({})", pprust::pat_to_string(&pat)),
+                // "ambiguous interpretation" implies that we have to be guessing
+                Applicability::MaybeIncorrect,
+            )
+            .emit();
+    }
+
+    /// Parse `&pat` / `&mut pat`.
+    fn parse_pat_deref(&mut self, expected: Expected) -> PResult<'a, PatKind> {
+        self.expect_and()?;
+        self.recover_lifetime_in_deref_pat();
+        let mutbl = self.parse_mutability();
+        let subpat = self.parse_pat_with_range_pat(false, expected)?;
+        Ok(PatKind::Ref(subpat, mutbl))
+    }
+
+    fn recover_lifetime_in_deref_pat(&mut self) {
+        if let token::Lifetime(name) = self.token.kind {
+            self.bump(); // `'a`
+
+            let span = self.prev_token.span;
+            self.struct_span_err(span, &format!("unexpected lifetime `{}` in pattern", name))
+                .span_suggestion(
+                    span,
+                    "remove the lifetime",
+                    String::new(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        }
+    }
+
+    /// Parse a tuple or parenthesis pattern.
+    fn parse_pat_tuple_or_parens(&mut self) -> PResult<'a, PatKind> {
+        let (fields, trailing_comma) =
+            self.parse_paren_comma_seq(|p| p.parse_pat_with_or_inner())?;
+
+        // Here, `(pat,)` is a tuple pattern.
+        // For backward compatibility, `(..)` is a tuple pattern as well.
+        Ok(if fields.len() == 1 && !(trailing_comma || fields[0].is_rest()) {
+            PatKind::Paren(fields.into_iter().next().unwrap())
+        } else {
+            PatKind::Tuple(fields)
+        })
+    }
+
+    /// Parse a mutable binding with the `mut` token already eaten.
+    fn parse_pat_ident_mut(&mut self) -> PResult<'a, PatKind> {
+        let mut_span = self.prev_token.span;
+
+        if self.eat_keyword(kw::Ref) {
+            return self.recover_mut_ref_ident(mut_span);
+        }
+
+        self.recover_additional_muts();
+
+        // Make sure we don't allow e.g. `let mut $p;` where `$p:pat`.
+        if let token::Interpolated(ref nt) = self.token.kind {
+            if let token::NtPat(_) = **nt {
+                self.expected_ident_found().emit();
+            }
+        }
+
+        // Parse the pattern we hope to be an identifier.
+        let mut pat = self.parse_pat(Some("identifier"))?;
+
+        // If we don't have `mut $ident (@ pat)?`, error.
+        if let PatKind::Ident(BindingMode::ByValue(m @ Mutability::Not), ..) = &mut pat.kind {
+            // Don't recurse into the subpattern.
+            // `mut` on the outer binding doesn't affect the inner bindings.
+            *m = Mutability::Mut;
+        } else {
+            // Add `mut` to any binding in the parsed pattern.
+            let changed_any_binding = Self::make_all_value_bindings_mutable(&mut pat);
+            self.ban_mut_general_pat(mut_span, &pat, changed_any_binding);
+        }
+
+        Ok(pat.into_inner().kind)
+    }
+
+    /// Recover on `mut ref? ident @ pat` and suggest
+    /// that the order of `mut` and `ref` is incorrect.
+    fn recover_mut_ref_ident(&mut self, lo: Span) -> PResult<'a, PatKind> {
+        let mutref_span = lo.to(self.prev_token.span);
+        self.struct_span_err(mutref_span, "the order of `mut` and `ref` is incorrect")
+            .span_suggestion(
+                mutref_span,
+                "try switching the order",
+                "ref mut".into(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+
+        self.parse_pat_ident(BindingMode::ByRef(Mutability::Mut))
+    }
+
+    /// Turn all by-value immutable bindings in a pattern into mutable bindings.
+    /// Returns `true` if any change was made.
+    fn make_all_value_bindings_mutable(pat: &mut P<Pat>) -> bool {
+        struct AddMut(bool);
+        impl MutVisitor for AddMut {
+            fn visit_mac(&mut self, mac: &mut MacCall) {
+                noop_visit_mac(mac, self);
+            }
+
+            fn visit_pat(&mut self, pat: &mut P<Pat>) {
+                if let PatKind::Ident(BindingMode::ByValue(m @ Mutability::Not), ..) = &mut pat.kind
+                {
+                    self.0 = true;
+                    *m = Mutability::Mut;
+                }
+                noop_visit_pat(pat, self);
+            }
+        }
+
+        let mut add_mut = AddMut(false);
+        add_mut.visit_pat(pat);
+        add_mut.0
+    }
+
+    /// Error on `mut $pat` where `$pat` is not an ident.
+    fn ban_mut_general_pat(&self, lo: Span, pat: &Pat, changed_any_binding: bool) {
+        let span = lo.to(pat.span);
+        let fix = pprust::pat_to_string(&pat);
+        let (problem, suggestion) = if changed_any_binding {
+            ("`mut` must be attached to each individual binding", "add `mut` to each binding")
+        } else {
+            ("`mut` must be followed by a named binding", "remove the `mut` prefix")
+        };
+        self.struct_span_err(span, problem)
+            .span_suggestion(span, suggestion, fix, Applicability::MachineApplicable)
+            .note("`mut` may be followed by `variable` and `variable @ pattern`")
+            .emit();
+    }
+
+    /// Eat any extraneous `mut`s and error + recover if we ate any.
+    fn recover_additional_muts(&mut self) {
+        let lo = self.token.span;
+        while self.eat_keyword(kw::Mut) {}
+        if lo == self.token.span {
+            return;
+        }
+
+        let span = lo.to(self.prev_token.span);
+        self.struct_span_err(span, "`mut` on a binding may not be repeated")
+            .span_suggestion(
+                span,
+                "remove the additional `mut`s",
+                String::new(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+    }
+
+    /// Parse macro invocation
+    fn parse_pat_mac_invoc(&mut self, path: Path) -> PResult<'a, PatKind> {
+        self.bump();
+        let args = self.parse_mac_args()?;
+        let mac = MacCall { path, args, prior_type_ascription: self.last_type_ascription };
+        Ok(PatKind::MacCall(mac))
+    }
+
+    fn fatal_unexpected_non_pat(
+        &mut self,
+        mut err: DiagnosticBuilder<'a>,
+        expected: Expected,
+    ) -> PResult<'a, P<Pat>> {
+        err.cancel();
+
+        let expected = expected.unwrap_or("pattern");
+        let msg = format!("expected {}, found {}", expected, super::token_descr(&self.token));
+
+        let mut err = self.struct_span_err(self.token.span, &msg);
+        err.span_label(self.token.span, format!("expected {}", expected));
+
+        let sp = self.sess.source_map().start_point(self.token.span);
+        if let Some(sp) = self.sess.ambiguous_block_expr_parse.borrow().get(&sp) {
+            self.sess.expr_parentheses_needed(&mut err, *sp, None);
+        }
+
+        Err(err)
+    }
+
+    /// Parses the range pattern end form `".." | "..." | "..=" ;`.
+    fn parse_range_end(&mut self) -> Option<Spanned<RangeEnd>> {
+        let re = if self.eat(&token::DotDotDot) {
+            RangeEnd::Included(RangeSyntax::DotDotDot)
+        } else if self.eat(&token::DotDotEq) {
+            RangeEnd::Included(RangeSyntax::DotDotEq)
+        } else if self.eat(&token::DotDot) {
+            self.sess.gated_spans.gate(sym::exclusive_range_pattern, self.prev_token.span);
+            RangeEnd::Excluded
+        } else {
+            return None;
+        };
+        Some(respan(self.prev_token.span, re))
+    }
+
+    /// Parse a range pattern `$begin $form $end?` where `$form = ".." | "..." | "..=" ;`.
+    /// `$begin $form` has already been parsed.
+    fn parse_pat_range_begin_with(
+        &mut self,
+        begin: P<Expr>,
+        re: Spanned<RangeEnd>,
+    ) -> PResult<'a, PatKind> {
+        let end = if self.is_pat_range_end_start(0) {
+            // Parsing e.g. `X..=Y`.
+            Some(self.parse_pat_range_end()?)
+        } else {
+            // Parsing e.g. `X..`.
+            self.sess.gated_spans.gate(sym::half_open_range_patterns, begin.span.to(re.span));
+            if let RangeEnd::Included(_) = re.node {
+                // FIXME(Centril): Consider semantic errors instead in `ast_validation`.
+                // Possibly also do this for `X..=` in *expression* contexts.
+                self.error_inclusive_range_with_no_end(re.span);
+            }
+            None
+        };
+        Ok(PatKind::Range(Some(begin), end, re))
+    }
+
+    pub(super) fn error_inclusive_range_with_no_end(&self, span: Span) {
+        struct_span_err!(self.sess.span_diagnostic, span, E0586, "inclusive range with no end")
+            .span_suggestion_short(
+                span,
+                "use `..` instead",
+                "..".to_string(),
+                Applicability::MachineApplicable,
+            )
+            .note("inclusive ranges must be bounded at the end (`..=b` or `a..=b`)")
+            .emit();
+    }
+
+    /// Parse a range-to pattern, `..X` or `..=X` where `X` remains to be parsed.
+    ///
+    /// The form `...X` is prohibited to reduce confusion with the potential
+    /// expression syntax `...expr` for splatting in expressions.
+    fn parse_pat_range_to(&mut self, mut re: Spanned<RangeEnd>) -> PResult<'a, PatKind> {
+        let end = self.parse_pat_range_end()?;
+        self.sess.gated_spans.gate(sym::half_open_range_patterns, re.span.to(self.prev_token.span));
+        if let RangeEnd::Included(ref mut syn @ RangeSyntax::DotDotDot) = &mut re.node {
+            *syn = RangeSyntax::DotDotEq;
+            self.struct_span_err(re.span, "range-to patterns with `...` are not allowed")
+                .span_suggestion_short(
+                    re.span,
+                    "use `..=` instead",
+                    "..=".to_string(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        }
+        Ok(PatKind::Range(None, Some(end), re))
+    }
+
+    /// Is the token `dist` away from the current suitable as the start of a range patterns end?
+    fn is_pat_range_end_start(&self, dist: usize) -> bool {
+        self.look_ahead(dist, |t| {
+            t.is_path_start() // e.g. `MY_CONST`;
+                || t.kind == token::Dot // e.g. `.5` for recovery;
+                || t.can_begin_literal_maybe_minus() // e.g. `42`.
+                || t.is_whole_expr()
+        })
+    }
+
+    fn parse_pat_range_end(&mut self) -> PResult<'a, P<Expr>> {
+        if self.check_path() {
+            let lo = self.token.span;
+            let (qself, path) = if self.eat_lt() {
+                // Parse a qualified path
+                let (qself, path) = self.parse_qpath(PathStyle::Expr)?;
+                (Some(qself), path)
+            } else {
+                // Parse an unqualified path
+                (None, self.parse_path(PathStyle::Expr)?)
+            };
+            let hi = self.prev_token.span;
+            Ok(self.mk_expr(lo.to(hi), ExprKind::Path(qself, path), AttrVec::new()))
+        } else {
+            self.parse_literal_maybe_minus()
+        }
+    }
+
+    /// Is this the start of a pattern beginning with a path?
+    fn is_start_of_pat_with_path(&mut self) -> bool {
+        self.check_path()
+        // Just for recovery (see `can_be_ident`).
+        || self.token.is_ident() && !self.token.is_bool_lit() && !self.token.is_keyword(kw::In)
+    }
+
+    /// Would `parse_pat_ident` be appropriate here?
+    fn can_be_ident_pat(&mut self) -> bool {
+        self.check_ident()
+        && !self.token.is_bool_lit() // Avoid `true` or `false` as a binding as it is a literal.
+        && !self.token.is_path_segment_keyword() // Avoid e.g. `Self` as it is a path.
+        // Avoid `in`. Due to recovery in the list parser this messes with `for ( $pat in $expr )`.
+        && !self.token.is_keyword(kw::In)
+        && self.look_ahead(1, |t| match t.kind { // Try to do something more complex?
+            token::OpenDelim(token::Paren) // A tuple struct pattern.
+            | token::OpenDelim(token::Brace) // A struct pattern.
+            | token::DotDotDot | token::DotDotEq | token::DotDot // A range pattern.
+            | token::ModSep // A tuple / struct variant pattern.
+            | token::Not => false, // A macro expanding to a pattern.
+            _ => true,
+        })
+    }
+
+    /// Parses `ident` or `ident @ pat`.
+    /// Used by the copy foo and ref foo patterns to give a good
+    /// error message when parsing mistakes like `ref foo(a, b)`.
+    fn parse_pat_ident(&mut self, binding_mode: BindingMode) -> PResult<'a, PatKind> {
+        let ident = self.parse_ident()?;
+        let sub = if self.eat(&token::At) {
+            Some(self.parse_pat(Some("binding pattern"))?)
+        } else {
+            None
+        };
+
+        // Just to be friendly, if they write something like `ref Some(i)`,
+        // we end up here with `(` as the current token.
+        // This shortly leads to a parse error. Note that if there is no explicit
+        // binding mode then we do not end up here, because the lookahead
+        // will direct us over to `parse_enum_variant()`.
+        if self.token == token::OpenDelim(token::Paren) {
+            return Err(self
+                .struct_span_err(self.prev_token.span, "expected identifier, found enum pattern"));
+        }
+
+        Ok(PatKind::Ident(binding_mode, ident, sub))
+    }
+
+    /// Parse a struct ("record") pattern (e.g. `Foo { ... }` or `Foo::Bar { ... }`).
+    fn parse_pat_struct(&mut self, qself: Option<QSelf>, path: Path) -> PResult<'a, PatKind> {
+        if qself.is_some() {
+            return self.error_qpath_before_pat(&path, "{");
+        }
+        self.bump();
+        let (fields, etc) = self.parse_pat_fields().unwrap_or_else(|mut e| {
+            e.emit();
+            self.recover_stmt();
+            (vec![], true)
+        });
+        self.bump();
+        Ok(PatKind::Struct(path, fields, etc))
+    }
+
+    /// Parse tuple struct or tuple variant pattern (e.g. `Foo(...)` or `Foo::Bar(...)`).
+    fn parse_pat_tuple_struct(&mut self, qself: Option<QSelf>, path: Path) -> PResult<'a, PatKind> {
+        if qself.is_some() {
+            return self.error_qpath_before_pat(&path, "(");
+        }
+        let (fields, _) = self.parse_paren_comma_seq(|p| p.parse_pat_with_or_inner())?;
+        Ok(PatKind::TupleStruct(path, fields))
+    }
+
+    /// Error when there's a qualified path, e.g. `<Foo as Bar>::Baz`
+    /// as the path of e.g., a tuple or record struct pattern.
+    fn error_qpath_before_pat(&mut self, path: &Path, token: &str) -> PResult<'a, PatKind> {
+        let msg = &format!("unexpected `{}` after qualified path", token);
+        let mut err = self.struct_span_err(self.token.span, msg);
+        err.span_label(self.token.span, msg);
+        err.span_label(path.span, "the qualified path");
+        Err(err)
+    }
+
+    /// Parses the fields of a struct-like pattern.
+    fn parse_pat_fields(&mut self) -> PResult<'a, (Vec<FieldPat>, bool)> {
+        let mut fields = Vec::new();
+        let mut etc = false;
+        let mut ate_comma = true;
+        let mut delayed_err: Option<DiagnosticBuilder<'a>> = None;
+        let mut etc_span = None;
+
+        while self.token != token::CloseDelim(token::Brace) {
+            let attrs = match self.parse_outer_attributes() {
+                Ok(attrs) => attrs,
+                Err(err) => {
+                    if let Some(mut delayed) = delayed_err {
+                        delayed.emit();
+                    }
+                    return Err(err);
+                }
+            };
+            let lo = self.token.span;
+
+            // check that a comma comes after every field
+            if !ate_comma {
+                let err = self.struct_span_err(self.prev_token.span, "expected `,`");
+                if let Some(mut delayed) = delayed_err {
+                    delayed.emit();
+                }
+                return Err(err);
+            }
+            ate_comma = false;
+
+            if self.check(&token::DotDot) || self.token == token::DotDotDot {
+                etc = true;
+                let mut etc_sp = self.token.span;
+
+                self.recover_one_fewer_dotdot();
+                self.bump(); // `..` || `...`
+
+                if self.token == token::CloseDelim(token::Brace) {
+                    etc_span = Some(etc_sp);
+                    break;
+                }
+                let token_str = super::token_descr(&self.token);
+                let msg = &format!("expected `}}`, found {}", token_str);
+                let mut err = self.struct_span_err(self.token.span, msg);
+
+                err.span_label(self.token.span, "expected `}`");
+                let mut comma_sp = None;
+                if self.token == token::Comma {
+                    // Issue #49257
+                    let nw_span = self.sess.source_map().span_until_non_whitespace(self.token.span);
+                    etc_sp = etc_sp.to(nw_span);
+                    err.span_label(
+                        etc_sp,
+                        "`..` must be at the end and cannot have a trailing comma",
+                    );
+                    comma_sp = Some(self.token.span);
+                    self.bump();
+                    ate_comma = true;
+                }
+
+                etc_span = Some(etc_sp.until(self.token.span));
+                if self.token == token::CloseDelim(token::Brace) {
+                    // If the struct looks otherwise well formed, recover and continue.
+                    if let Some(sp) = comma_sp {
+                        err.span_suggestion_short(
+                            sp,
+                            "remove this comma",
+                            String::new(),
+                            Applicability::MachineApplicable,
+                        );
+                    }
+                    err.emit();
+                    break;
+                } else if self.token.is_ident() && ate_comma {
+                    // Accept fields coming after `..,`.
+                    // This way we avoid "pattern missing fields" errors afterwards.
+                    // We delay this error until the end in order to have a span for a
+                    // suggested fix.
+                    if let Some(mut delayed_err) = delayed_err {
+                        delayed_err.emit();
+                        return Err(err);
+                    } else {
+                        delayed_err = Some(err);
+                    }
+                } else {
+                    if let Some(mut err) = delayed_err {
+                        err.emit();
+                    }
+                    return Err(err);
+                }
+            }
+
+            fields.push(match self.parse_pat_field(lo, attrs) {
+                Ok(field) => field,
+                Err(err) => {
+                    if let Some(mut delayed_err) = delayed_err {
+                        delayed_err.emit();
+                    }
+                    return Err(err);
+                }
+            });
+            ate_comma = self.eat(&token::Comma);
+        }
+
+        if let Some(mut err) = delayed_err {
+            if let Some(etc_span) = etc_span {
+                err.multipart_suggestion(
+                    "move the `..` to the end of the field list",
+                    vec![
+                        (etc_span, String::new()),
+                        (self.token.span, format!("{}.. }}", if ate_comma { "" } else { ", " })),
+                    ],
+                    Applicability::MachineApplicable,
+                );
+            }
+            err.emit();
+        }
+        Ok((fields, etc))
+    }
+
+    /// Recover on `...` as if it were `..` to avoid further errors.
+    /// See issue #46718.
+    fn recover_one_fewer_dotdot(&self) {
+        if self.token != token::DotDotDot {
+            return;
+        }
+
+        self.struct_span_err(self.token.span, "expected field pattern, found `...`")
+            .span_suggestion(
+                self.token.span,
+                "to omit remaining fields, use one fewer `.`",
+                "..".to_owned(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+    }
+
+    fn parse_pat_field(&mut self, lo: Span, attrs: Vec<Attribute>) -> PResult<'a, FieldPat> {
+        // Check if a colon exists one ahead. This means we're parsing a fieldname.
+        let hi;
+        let (subpat, fieldname, is_shorthand) = if self.look_ahead(1, |t| t == &token::Colon) {
+            // Parsing a pattern of the form `fieldname: pat`.
+            let fieldname = self.parse_field_name()?;
+            self.bump();
+            let pat = self.parse_pat_with_or_inner()?;
+            hi = pat.span;
+            (pat, fieldname, false)
+        } else {
+            // Parsing a pattern of the form `(box) (ref) (mut) fieldname`.
+            let is_box = self.eat_keyword(kw::Box);
+            let boxed_span = self.token.span;
+            let is_ref = self.eat_keyword(kw::Ref);
+            let is_mut = self.eat_keyword(kw::Mut);
+            let fieldname = self.parse_ident()?;
+            hi = self.prev_token.span;
+
+            let bind_type = match (is_ref, is_mut) {
+                (true, true) => BindingMode::ByRef(Mutability::Mut),
+                (true, false) => BindingMode::ByRef(Mutability::Not),
+                (false, true) => BindingMode::ByValue(Mutability::Mut),
+                (false, false) => BindingMode::ByValue(Mutability::Not),
+            };
+
+            let fieldpat = self.mk_pat_ident(boxed_span.to(hi), bind_type, fieldname);
+            let subpat =
+                if is_box { self.mk_pat(lo.to(hi), PatKind::Box(fieldpat)) } else { fieldpat };
+            (subpat, fieldname, true)
+        };
+
+        Ok(FieldPat {
+            ident: fieldname,
+            pat: subpat,
+            is_shorthand,
+            attrs: attrs.into(),
+            id: ast::DUMMY_NODE_ID,
+            span: lo.to(hi),
+            is_placeholder: false,
+        })
+    }
+
+    pub(super) fn mk_pat_ident(&self, span: Span, bm: BindingMode, ident: Ident) -> P<Pat> {
+        self.mk_pat(span, PatKind::Ident(bm, ident, None))
+    }
+
+    fn mk_pat(&self, span: Span, kind: PatKind) -> P<Pat> {
+        P(Pat { kind, span, id: ast::DUMMY_NODE_ID, tokens: None })
+    }
+}
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
new file mode 100644
index 00000000000..54b4df8613f
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -0,0 +1,516 @@
+use super::ty::{AllowPlus, RecoverQPath};
+use super::{Parser, TokenType};
+use crate::maybe_whole;
+use rustc_ast::ptr::P;
+use rustc_ast::token::{self, Token};
+use rustc_ast::{
+    self as ast, AngleBracketedArg, AngleBracketedArgs, GenericArg, ParenthesizedArgs,
+};
+use rustc_ast::{AnonConst, AssocTyConstraint, AssocTyConstraintKind, BlockCheckMode};
+use rustc_ast::{Path, PathSegment, QSelf};
+use rustc_errors::{pluralize, Applicability, PResult};
+use rustc_span::source_map::{BytePos, Span};
+use rustc_span::symbol::{kw, sym, Ident};
+
+use std::mem;
+use tracing::debug;
+
+/// Specifies how to parse a path.
+#[derive(Copy, Clone, PartialEq)]
+pub enum PathStyle {
+    /// In some contexts, notably in expressions, paths with generic arguments are ambiguous
+    /// with something else. For example, in expressions `segment < ....` can be interpreted
+    /// as a comparison and `segment ( ....` can be interpreted as a function call.
+    /// In all such contexts the non-path interpretation is preferred by default for practical
+    /// reasons, but the path interpretation can be forced by the disambiguator `::`, e.g.
+    /// `x<y>` - comparisons, `x::<y>` - unambiguously a path.
+    Expr,
+    /// In other contexts, notably in types, no ambiguity exists and paths can be written
+    /// without the disambiguator, e.g., `x<y>` - unambiguously a path.
+    /// Paths with disambiguators are still accepted, `x::<Y>` - unambiguously a path too.
+    Type,
+    /// A path with generic arguments disallowed, e.g., `foo::bar::Baz`, used in imports,
+    /// visibilities or attributes.
+    /// Technically, this variant is unnecessary and e.g., `Expr` can be used instead
+    /// (paths in "mod" contexts have to be checked later for absence of generic arguments
+    /// anyway, due to macros), but it is used to avoid weird suggestions about expected
+    /// tokens when something goes wrong.
+    Mod,
+}
+
+impl<'a> Parser<'a> {
+    /// Parses a qualified path.
+    /// Assumes that the leading `<` has been parsed already.
+    ///
+    /// `qualified_path = <type [as trait_ref]>::path`
+    ///
+    /// # Examples
+    /// `<T>::default`
+    /// `<T as U>::a`
+    /// `<T as U>::F::a<S>` (without disambiguator)
+    /// `<T as U>::F::a::<S>` (with disambiguator)
+    pub(super) fn parse_qpath(&mut self, style: PathStyle) -> PResult<'a, (QSelf, Path)> {
+        let lo = self.prev_token.span;
+        let ty = self.parse_ty()?;
+
+        // `path` will contain the prefix of the path up to the `>`,
+        // if any (e.g., `U` in the `<T as U>::*` examples
+        // above). `path_span` has the span of that path, or an empty
+        // span in the case of something like `<T>::Bar`.
+        let (mut path, path_span);
+        if self.eat_keyword(kw::As) {
+            let path_lo = self.token.span;
+            path = self.parse_path(PathStyle::Type)?;
+            path_span = path_lo.to(self.prev_token.span);
+        } else {
+            path_span = self.token.span.to(self.token.span);
+            path = ast::Path { segments: Vec::new(), span: path_span };
+        }
+
+        // See doc comment for `unmatched_angle_bracket_count`.
+        self.expect(&token::Gt)?;
+        if self.unmatched_angle_bracket_count > 0 {
+            self.unmatched_angle_bracket_count -= 1;
+            debug!("parse_qpath: (decrement) count={:?}", self.unmatched_angle_bracket_count);
+        }
+
+        if !self.recover_colon_before_qpath_proj() {
+            self.expect(&token::ModSep)?;
+        }
+
+        let qself = QSelf { ty, path_span, position: path.segments.len() };
+        self.parse_path_segments(&mut path.segments, style)?;
+
+        Ok((qself, Path { segments: path.segments, span: lo.to(self.prev_token.span) }))
+    }
+
+    /// Recover from an invalid single colon, when the user likely meant a qualified path.
+    /// We avoid emitting this if not followed by an identifier, as our assumption that the user
+    /// intended this to be a qualified path may not be correct.
+    ///
+    /// ```ignore (diagnostics)
+    /// <Bar as Baz<T>>:Qux
+    ///                ^ help: use double colon
+    /// ```
+    fn recover_colon_before_qpath_proj(&mut self) -> bool {
+        if self.token.kind != token::Colon
+            || self.look_ahead(1, |t| !t.is_ident() || t.is_reserved_ident())
+        {
+            return false;
+        }
+
+        self.bump(); // colon
+
+        self.diagnostic()
+            .struct_span_err(
+                self.prev_token.span,
+                "found single colon before projection in qualified path",
+            )
+            .span_suggestion(
+                self.prev_token.span,
+                "use double colon",
+                "::".to_string(),
+                Applicability::MachineApplicable,
+            )
+            .emit();
+
+        true
+    }
+
+    /// Parses simple paths.
+    ///
+    /// `path = [::] segment+`
+    /// `segment = ident | ident[::]<args> | ident[::](args) [-> type]`
+    ///
+    /// # Examples
+    /// `a::b::C<D>` (without disambiguator)
+    /// `a::b::C::<D>` (with disambiguator)
+    /// `Fn(Args)` (without disambiguator)
+    /// `Fn::(Args)` (with disambiguator)
+    pub(super) fn parse_path(&mut self, style: PathStyle) -> PResult<'a, Path> {
+        maybe_whole!(self, NtPath, |path| {
+            if style == PathStyle::Mod && path.segments.iter().any(|segment| segment.args.is_some())
+            {
+                self.struct_span_err(path.span, "unexpected generic arguments in path").emit();
+            }
+            path
+        });
+
+        let lo = self.token.span;
+        let mut segments = Vec::new();
+        let mod_sep_ctxt = self.token.span.ctxt();
+        if self.eat(&token::ModSep) {
+            segments.push(PathSegment::path_root(lo.shrink_to_lo().with_ctxt(mod_sep_ctxt)));
+        }
+        self.parse_path_segments(&mut segments, style)?;
+
+        Ok(Path { segments, span: lo.to(self.prev_token.span) })
+    }
+
+    pub(super) fn parse_path_segments(
+        &mut self,
+        segments: &mut Vec<PathSegment>,
+        style: PathStyle,
+    ) -> PResult<'a, ()> {
+        loop {
+            let segment = self.parse_path_segment(style)?;
+            if style == PathStyle::Expr {
+                // In order to check for trailing angle brackets, we must have finished
+                // recursing (`parse_path_segment` can indirectly call this function),
+                // that is, the next token must be the highlighted part of the below example:
+                //
+                // `Foo::<Bar as Baz<T>>::Qux`
+                //                      ^ here
+                //
+                // As opposed to the below highlight (if we had only finished the first
+                // recursion):
+                //
+                // `Foo::<Bar as Baz<T>>::Qux`
+                //                     ^ here
+                //
+                // `PathStyle::Expr` is only provided at the root invocation and never in
+                // `parse_path_segment` to recurse and therefore can be checked to maintain
+                // this invariant.
+                self.check_trailing_angle_brackets(&segment, &[&token::ModSep]);
+            }
+            segments.push(segment);
+
+            if self.is_import_coupler() || !self.eat(&token::ModSep) {
+                return Ok(());
+            }
+        }
+    }
+
+    pub(super) fn parse_path_segment(&mut self, style: PathStyle) -> PResult<'a, PathSegment> {
+        let ident = self.parse_path_segment_ident()?;
+
+        let is_args_start = |token: &Token| match token.kind {
+            token::Lt
+            | token::BinOp(token::Shl)
+            | token::OpenDelim(token::Paren)
+            | token::LArrow => true,
+            _ => false,
+        };
+        let check_args_start = |this: &mut Self| {
+            this.expected_tokens.extend_from_slice(&[
+                TokenType::Token(token::Lt),
+                TokenType::Token(token::OpenDelim(token::Paren)),
+            ]);
+            is_args_start(&this.token)
+        };
+
+        Ok(
+            if style == PathStyle::Type && check_args_start(self)
+                || style != PathStyle::Mod
+                    && self.check(&token::ModSep)
+                    && self.look_ahead(1, |t| is_args_start(t))
+            {
+                // We use `style == PathStyle::Expr` to check if this is in a recursion or not. If
+                // it isn't, then we reset the unmatched angle bracket count as we're about to start
+                // parsing a new path.
+                if style == PathStyle::Expr {
+                    self.unmatched_angle_bracket_count = 0;
+                    self.max_angle_bracket_count = 0;
+                }
+
+                // Generic arguments are found - `<`, `(`, `::<` or `::(`.
+                self.eat(&token::ModSep);
+                let lo = self.token.span;
+                let args = if self.eat_lt() {
+                    // `<'a, T, A = U>`
+                    let args =
+                        self.parse_angle_args_with_leading_angle_bracket_recovery(style, lo)?;
+                    self.expect_gt()?;
+                    let span = lo.to(self.prev_token.span);
+                    AngleBracketedArgs { args, span }.into()
+                } else {
+                    // `(T, U) -> R`
+                    let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
+                    let span = ident.span.to(self.prev_token.span);
+                    let output = self.parse_ret_ty(AllowPlus::No, RecoverQPath::No)?;
+                    ParenthesizedArgs { inputs, output, span }.into()
+                };
+
+                PathSegment { ident, args, id: ast::DUMMY_NODE_ID }
+            } else {
+                // Generic arguments are not found.
+                PathSegment::from_ident(ident)
+            },
+        )
+    }
+
+    pub(super) fn parse_path_segment_ident(&mut self) -> PResult<'a, Ident> {
+        match self.token.ident() {
+            Some((ident, false)) if ident.is_path_segment_keyword() => {
+                self.bump();
+                Ok(ident)
+            }
+            _ => self.parse_ident(),
+        }
+    }
+
+    /// Parses generic args (within a path segment) with recovery for extra leading angle brackets.
+    /// For the purposes of understanding the parsing logic of generic arguments, this function
+    /// can be thought of being the same as just calling `self.parse_angle_args()` if the source
+    /// had the correct amount of leading angle brackets.
+    ///
+    /// ```ignore (diagnostics)
+    /// bar::<<<<T as Foo>::Output>();
+    ///      ^^ help: remove extra angle brackets
+    /// ```
+    fn parse_angle_args_with_leading_angle_bracket_recovery(
+        &mut self,
+        style: PathStyle,
+        lo: Span,
+    ) -> PResult<'a, Vec<AngleBracketedArg>> {
+        // We need to detect whether there are extra leading left angle brackets and produce an
+        // appropriate error and suggestion. This cannot be implemented by looking ahead at
+        // upcoming tokens for a matching `>` character - if there are unmatched `<` tokens
+        // then there won't be matching `>` tokens to find.
+        //
+        // To explain how this detection works, consider the following example:
+        //
+        // ```ignore (diagnostics)
+        // bar::<<<<T as Foo>::Output>();
+        //      ^^ help: remove extra angle brackets
+        // ```
+        //
+        // Parsing of the left angle brackets starts in this function. We start by parsing the
+        // `<` token (incrementing the counter of unmatched angle brackets on `Parser` via
+        // `eat_lt`):
+        //
+        // *Upcoming tokens:* `<<<<T as Foo>::Output>;`
+        // *Unmatched count:* 1
+        // *`parse_path_segment` calls deep:* 0
+        //
+        // This has the effect of recursing as this function is called if a `<` character
+        // is found within the expected generic arguments:
+        //
+        // *Upcoming tokens:* `<<<T as Foo>::Output>;`
+        // *Unmatched count:* 2
+        // *`parse_path_segment` calls deep:* 1
+        //
+        // Eventually we will have recursed until having consumed all of the `<` tokens and
+        // this will be reflected in the count:
+        //
+        // *Upcoming tokens:* `T as Foo>::Output>;`
+        // *Unmatched count:* 4
+        // `parse_path_segment` calls deep:* 3
+        //
+        // The parser will continue until reaching the first `>` - this will decrement the
+        // unmatched angle bracket count and return to the parent invocation of this function
+        // having succeeded in parsing:
+        //
+        // *Upcoming tokens:* `::Output>;`
+        // *Unmatched count:* 3
+        // *`parse_path_segment` calls deep:* 2
+        //
+        // This will continue until the next `>` character which will also return successfully
+        // to the parent invocation of this function and decrement the count:
+        //
+        // *Upcoming tokens:* `;`
+        // *Unmatched count:* 2
+        // *`parse_path_segment` calls deep:* 1
+        //
+        // At this point, this function will expect to find another matching `>` character but
+        // won't be able to and will return an error. This will continue all the way up the
+        // call stack until the first invocation:
+        //
+        // *Upcoming tokens:* `;`
+        // *Unmatched count:* 2
+        // *`parse_path_segment` calls deep:* 0
+        //
+        // In doing this, we have managed to work out how many unmatched leading left angle
+        // brackets there are, but we cannot recover as the unmatched angle brackets have
+        // already been consumed. To remedy this, we keep a snapshot of the parser state
+        // before we do the above. We can then inspect whether we ended up with a parsing error
+        // and unmatched left angle brackets and if so, restore the parser state before we
+        // consumed any `<` characters to emit an error and consume the erroneous tokens to
+        // recover by attempting to parse again.
+        //
+        // In practice, the recursion of this function is indirect and there will be other
+        // locations that consume some `<` characters - as long as we update the count when
+        // this happens, it isn't an issue.
+
+        let is_first_invocation = style == PathStyle::Expr;
+        // Take a snapshot before attempting to parse - we can restore this later.
+        let snapshot = if is_first_invocation { Some(self.clone()) } else { None };
+
+        debug!("parse_generic_args_with_leading_angle_bracket_recovery: (snapshotting)");
+        match self.parse_angle_args() {
+            Ok(args) => Ok(args),
+            Err(ref mut e) if is_first_invocation && self.unmatched_angle_bracket_count > 0 => {
+                // Cancel error from being unable to find `>`. We know the error
+                // must have been this due to a non-zero unmatched angle bracket
+                // count.
+                e.cancel();
+
+                // Swap `self` with our backup of the parser state before attempting to parse
+                // generic arguments.
+                let snapshot = mem::replace(self, snapshot.unwrap());
+
+                debug!(
+                    "parse_generic_args_with_leading_angle_bracket_recovery: (snapshot failure) \
+                     snapshot.count={:?}",
+                    snapshot.unmatched_angle_bracket_count,
+                );
+
+                // Eat the unmatched angle brackets.
+                for _ in 0..snapshot.unmatched_angle_bracket_count {
+                    self.eat_lt();
+                }
+
+                // Make a span over ${unmatched angle bracket count} characters.
+                let span = lo.with_hi(lo.lo() + BytePos(snapshot.unmatched_angle_bracket_count));
+                self.struct_span_err(
+                    span,
+                    &format!(
+                        "unmatched angle bracket{}",
+                        pluralize!(snapshot.unmatched_angle_bracket_count)
+                    ),
+                )
+                .span_suggestion(
+                    span,
+                    &format!(
+                        "remove extra angle bracket{}",
+                        pluralize!(snapshot.unmatched_angle_bracket_count)
+                    ),
+                    String::new(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+
+                // Try again without unmatched angle bracket characters.
+                self.parse_angle_args()
+            }
+            Err(e) => Err(e),
+        }
+    }
+
+    /// Parses (possibly empty) list of generic arguments / associated item constraints,
+    /// possibly including trailing comma.
+    pub(super) fn parse_angle_args(&mut self) -> PResult<'a, Vec<AngleBracketedArg>> {
+        let mut args = Vec::new();
+        while let Some(arg) = self.parse_angle_arg()? {
+            args.push(arg);
+            if !self.eat(&token::Comma) {
+                break;
+            }
+        }
+        Ok(args)
+    }
+
+    /// Parses a single argument in the angle arguments `<...>` of a path segment.
+    fn parse_angle_arg(&mut self) -> PResult<'a, Option<AngleBracketedArg>> {
+        if self.check_ident() && self.look_ahead(1, |t| matches!(t.kind, token::Eq | token::Colon))
+        {
+            // Parse associated type constraint.
+            let lo = self.token.span;
+            let ident = self.parse_ident()?;
+            let kind = if self.eat(&token::Eq) {
+                let ty = self.parse_assoc_equality_term(ident, self.prev_token.span)?;
+                AssocTyConstraintKind::Equality { ty }
+            } else if self.eat(&token::Colon) {
+                let bounds = self.parse_generic_bounds(Some(self.prev_token.span))?;
+                AssocTyConstraintKind::Bound { bounds }
+            } else {
+                unreachable!();
+            };
+
+            let span = lo.to(self.prev_token.span);
+
+            // Gate associated type bounds, e.g., `Iterator<Item: Ord>`.
+            if let AssocTyConstraintKind::Bound { .. } = kind {
+                self.sess.gated_spans.gate(sym::associated_type_bounds, span);
+            }
+
+            let constraint = AssocTyConstraint { id: ast::DUMMY_NODE_ID, ident, kind, span };
+            Ok(Some(AngleBracketedArg::Constraint(constraint)))
+        } else {
+            Ok(self.parse_generic_arg()?.map(AngleBracketedArg::Arg))
+        }
+    }
+
+    /// Parse the term to the right of an associated item equality constraint.
+    /// That is, parse `<term>` in `Item = <term>`.
+    /// Right now, this only admits types in `<term>`.
+    fn parse_assoc_equality_term(&mut self, ident: Ident, eq: Span) -> PResult<'a, P<ast::Ty>> {
+        let arg = self.parse_generic_arg()?;
+        let span = ident.span.to(self.prev_token.span);
+        match arg {
+            Some(GenericArg::Type(ty)) => return Ok(ty),
+            Some(GenericArg::Const(expr)) => {
+                self.struct_span_err(span, "cannot constrain an associated constant to a value")
+                    .span_label(ident.span, "this associated constant...")
+                    .span_label(expr.value.span, "...cannot be constrained to this value")
+                    .emit();
+            }
+            Some(GenericArg::Lifetime(lt)) => {
+                self.struct_span_err(span, "associated lifetimes are not supported")
+                    .span_label(lt.ident.span, "the lifetime is given here")
+                    .help("if you meant to specify a trait object, write `dyn Trait + 'lifetime`")
+                    .emit();
+            }
+            None => {
+                let after_eq = eq.shrink_to_hi();
+                let before_next = self.token.span.shrink_to_lo();
+                self.struct_span_err(after_eq.to(before_next), "missing type to the right of `=`")
+                    .span_suggestion(
+                        self.sess.source_map().next_point(eq).to(before_next),
+                        "to constrain the associated type, add a type after `=`",
+                        " TheType".to_string(),
+                        Applicability::HasPlaceholders,
+                    )
+                    .span_suggestion(
+                        eq.to(before_next),
+                        &format!("remove the `=` if `{}` is a type", ident),
+                        String::new(),
+                        Applicability::MaybeIncorrect,
+                    )
+                    .emit();
+            }
+        }
+        Ok(self.mk_ty(span, ast::TyKind::Err))
+    }
+
+    /// Parse a generic argument in a path segment.
+    /// This does not include constraints, e.g., `Item = u8`, which is handled in `parse_angle_arg`.
+    fn parse_generic_arg(&mut self) -> PResult<'a, Option<GenericArg>> {
+        let arg = if self.check_lifetime() && self.look_ahead(1, |t| !t.is_like_plus()) {
+            // Parse lifetime argument.
+            GenericArg::Lifetime(self.expect_lifetime())
+        } else if self.check_const_arg() {
+            // Parse const argument.
+            let expr = if let token::OpenDelim(token::Brace) = self.token.kind {
+                self.parse_block_expr(
+                    None,
+                    self.token.span,
+                    BlockCheckMode::Default,
+                    ast::AttrVec::new(),
+                )?
+            } else if self.token.is_ident() {
+                // FIXME(const_generics): to distinguish between idents for types and consts,
+                // we should introduce a GenericArg::Ident in the AST and distinguish when
+                // lowering to the HIR. For now, idents for const args are not permitted.
+                if self.token.is_bool_lit() {
+                    self.parse_literal_maybe_minus()?
+                } else {
+                    let span = self.token.span;
+                    let msg = "identifiers may currently not be used for const generics";
+                    self.struct_span_err(span, msg).emit();
+                    let block = self.mk_block_err(span);
+                    self.mk_expr(span, ast::ExprKind::Block(block, None), ast::AttrVec::new())
+                }
+            } else {
+                self.parse_literal_maybe_minus()?
+            };
+            GenericArg::Const(AnonConst { id: ast::DUMMY_NODE_ID, value: expr })
+        } else if self.check_type() {
+            // Parse type argument.
+            GenericArg::Type(self.parse_ty()?)
+        } else {
+            return Ok(None);
+        };
+        Ok(Some(arg))
+    }
+}
diff --git a/compiler/rustc_parse/src/parser/stmt.rs b/compiler/rustc_parse/src/parser/stmt.rs
new file mode 100644
index 00000000000..ac067cb0eab
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/stmt.rs
@@ -0,0 +1,427 @@
+use super::attr::DEFAULT_INNER_ATTR_FORBIDDEN;
+use super::diagnostics::Error;
+use super::expr::LhsExpr;
+use super::pat::GateOr;
+use super::path::PathStyle;
+use super::{BlockMode, Parser, Restrictions, SemiColonMode};
+use crate::maybe_whole;
+
+use rustc_ast as ast;
+use rustc_ast::ptr::P;
+use rustc_ast::token::{self, TokenKind};
+use rustc_ast::util::classify;
+use rustc_ast::{AttrStyle, AttrVec, Attribute, MacCall, MacStmtStyle};
+use rustc_ast::{Block, BlockCheckMode, Expr, ExprKind, Local, Stmt, StmtKind, DUMMY_NODE_ID};
+use rustc_errors::{Applicability, PResult};
+use rustc_span::source_map::{BytePos, Span};
+use rustc_span::symbol::{kw, sym};
+
+use std::mem;
+
+impl<'a> Parser<'a> {
+    /// Parses a statement. This stops just before trailing semicolons on everything but items.
+    /// e.g., a `StmtKind::Semi` parses to a `StmtKind::Expr`, leaving the trailing `;` unconsumed.
+    pub(super) fn parse_stmt(&mut self) -> PResult<'a, Option<Stmt>> {
+        Ok(self.parse_stmt_without_recovery().unwrap_or_else(|mut e| {
+            e.emit();
+            self.recover_stmt_(SemiColonMode::Break, BlockMode::Ignore);
+            None
+        }))
+    }
+
+    fn parse_stmt_without_recovery(&mut self) -> PResult<'a, Option<Stmt>> {
+        maybe_whole!(self, NtStmt, |x| Some(x));
+
+        let attrs = self.parse_outer_attributes()?;
+        let lo = self.token.span;
+
+        let stmt = if self.eat_keyword(kw::Let) {
+            self.parse_local_mk(lo, attrs.into())?
+        } else if self.is_kw_followed_by_ident(kw::Mut) {
+            self.recover_stmt_local(lo, attrs.into(), "missing keyword", "let mut")?
+        } else if self.is_kw_followed_by_ident(kw::Auto) {
+            self.bump(); // `auto`
+            let msg = "write `let` instead of `auto` to introduce a new variable";
+            self.recover_stmt_local(lo, attrs.into(), msg, "let")?
+        } else if self.is_kw_followed_by_ident(sym::var) {
+            self.bump(); // `var`
+            let msg = "write `let` instead of `var` to introduce a new variable";
+            self.recover_stmt_local(lo, attrs.into(), msg, "let")?
+        } else if self.check_path() && !self.token.is_qpath_start() && !self.is_path_start_item() {
+            // We have avoided contextual keywords like `union`, items with `crate` visibility,
+            // or `auto trait` items. We aim to parse an arbitrary path `a::b` but not something
+            // that starts like a path (1 token), but it fact not a path.
+            // Also, we avoid stealing syntax from `parse_item_`.
+            self.parse_stmt_path_start(lo, attrs)?
+        } else if let Some(item) = self.parse_item_common(attrs.clone(), false, true, |_| true)? {
+            // FIXME: Bad copy of attrs
+            self.mk_stmt(lo.to(item.span), StmtKind::Item(P(item)))
+        } else if self.eat(&token::Semi) {
+            // Do not attempt to parse an expression if we're done here.
+            self.error_outer_attrs(&attrs);
+            self.mk_stmt(lo, StmtKind::Empty)
+        } else if self.token != token::CloseDelim(token::Brace) {
+            // Remainder are line-expr stmts.
+            let e = self.parse_expr_res(Restrictions::STMT_EXPR, Some(attrs.into()))?;
+            self.mk_stmt(lo.to(e.span), StmtKind::Expr(e))
+        } else {
+            self.error_outer_attrs(&attrs);
+            return Ok(None);
+        };
+        Ok(Some(stmt))
+    }
+
+    fn parse_stmt_path_start(&mut self, lo: Span, attrs: Vec<Attribute>) -> PResult<'a, Stmt> {
+        let path = self.parse_path(PathStyle::Expr)?;
+
+        if self.eat(&token::Not) {
+            return self.parse_stmt_mac(lo, attrs.into(), path);
+        }
+
+        let expr = if self.check(&token::OpenDelim(token::Brace)) {
+            self.parse_struct_expr(path, AttrVec::new())?
+        } else {
+            let hi = self.prev_token.span;
+            self.mk_expr(lo.to(hi), ExprKind::Path(None, path), AttrVec::new())
+        };
+
+        let expr = self.with_res(Restrictions::STMT_EXPR, |this| {
+            let expr = this.parse_dot_or_call_expr_with(expr, lo, attrs.into())?;
+            this.parse_assoc_expr_with(0, LhsExpr::AlreadyParsed(expr))
+        })?;
+        Ok(self.mk_stmt(lo.to(self.prev_token.span), StmtKind::Expr(expr)))
+    }
+
+    /// Parses a statement macro `mac!(args)` provided a `path` representing `mac`.
+    /// At this point, the `!` token after the path has already been eaten.
+    fn parse_stmt_mac(&mut self, lo: Span, attrs: AttrVec, path: ast::Path) -> PResult<'a, Stmt> {
+        let args = self.parse_mac_args()?;
+        let delim = args.delim();
+        let hi = self.prev_token.span;
+
+        let style =
+            if delim == token::Brace { MacStmtStyle::Braces } else { MacStmtStyle::NoBraces };
+
+        let mac = MacCall { path, args, prior_type_ascription: self.last_type_ascription };
+
+        let kind = if delim == token::Brace || self.token == token::Semi || self.token == token::Eof
+        {
+            StmtKind::MacCall(P((mac, style, attrs)))
+        } else {
+            // Since none of the above applied, this is an expression statement macro.
+            let e = self.mk_expr(lo.to(hi), ExprKind::MacCall(mac), AttrVec::new());
+            let e = self.maybe_recover_from_bad_qpath(e, true)?;
+            let e = self.parse_dot_or_call_expr_with(e, lo, attrs)?;
+            let e = self.parse_assoc_expr_with(0, LhsExpr::AlreadyParsed(e))?;
+            StmtKind::Expr(e)
+        };
+        Ok(self.mk_stmt(lo.to(hi), kind))
+    }
+
+    /// Error on outer attributes in this context.
+    /// Also error if the previous token was a doc comment.
+    fn error_outer_attrs(&self, attrs: &[Attribute]) {
+        if let [.., last] = attrs {
+            if last.is_doc_comment() {
+                self.span_fatal_err(last.span, Error::UselessDocComment).emit();
+            } else if attrs.iter().any(|a| a.style == AttrStyle::Outer) {
+                self.struct_span_err(last.span, "expected statement after outer attribute").emit();
+            }
+        }
+    }
+
+    fn recover_stmt_local(
+        &mut self,
+        lo: Span,
+        attrs: AttrVec,
+        msg: &str,
+        sugg: &str,
+    ) -> PResult<'a, Stmt> {
+        let stmt = self.parse_local_mk(lo, attrs)?;
+        self.struct_span_err(lo, "invalid variable declaration")
+            .span_suggestion(lo, msg, sugg.to_string(), Applicability::MachineApplicable)
+            .emit();
+        Ok(stmt)
+    }
+
+    fn parse_local_mk(&mut self, lo: Span, attrs: AttrVec) -> PResult<'a, Stmt> {
+        let local = self.parse_local(attrs)?;
+        Ok(self.mk_stmt(lo.to(self.prev_token.span), StmtKind::Local(local)))
+    }
+
+    /// Parses a local variable declaration.
+    fn parse_local(&mut self, attrs: AttrVec) -> PResult<'a, P<Local>> {
+        let lo = self.prev_token.span;
+        let pat = self.parse_top_pat(GateOr::Yes)?;
+
+        let (err, ty) = if self.eat(&token::Colon) {
+            // Save the state of the parser before parsing type normally, in case there is a `:`
+            // instead of an `=` typo.
+            let parser_snapshot_before_type = self.clone();
+            let colon_sp = self.prev_token.span;
+            match self.parse_ty() {
+                Ok(ty) => (None, Some(ty)),
+                Err(mut err) => {
+                    if let Ok(snip) = self.span_to_snippet(pat.span) {
+                        err.span_label(pat.span, format!("while parsing the type for `{}`", snip));
+                    }
+                    let err = if self.check(&token::Eq) {
+                        err.emit();
+                        None
+                    } else {
+                        // Rewind to before attempting to parse the type and continue parsing.
+                        let parser_snapshot_after_type =
+                            mem::replace(self, parser_snapshot_before_type);
+                        Some((parser_snapshot_after_type, colon_sp, err))
+                    };
+                    (err, None)
+                }
+            }
+        } else {
+            (None, None)
+        };
+        let init = match (self.parse_initializer(err.is_some()), err) {
+            (Ok(init), None) => {
+                // init parsed, ty parsed
+                init
+            }
+            (Ok(init), Some((_, colon_sp, mut err))) => {
+                // init parsed, ty error
+                // Could parse the type as if it were the initializer, it is likely there was a
+                // typo in the code: `:` instead of `=`. Add suggestion and emit the error.
+                err.span_suggestion_short(
+                    colon_sp,
+                    "use `=` if you meant to assign",
+                    " =".to_string(),
+                    Applicability::MachineApplicable,
+                );
+                err.emit();
+                // As this was parsed successfully, continue as if the code has been fixed for the
+                // rest of the file. It will still fail due to the emitted error, but we avoid
+                // extra noise.
+                init
+            }
+            (Err(mut init_err), Some((snapshot, _, ty_err))) => {
+                // init error, ty error
+                init_err.cancel();
+                // Couldn't parse the type nor the initializer, only raise the type error and
+                // return to the parser state before parsing the type as the initializer.
+                // let x: <parse_error>;
+                *self = snapshot;
+                return Err(ty_err);
+            }
+            (Err(err), None) => {
+                // init error, ty parsed
+                // Couldn't parse the initializer and we're not attempting to recover a failed
+                // parse of the type, return the error.
+                return Err(err);
+            }
+        };
+        let hi = if self.token == token::Semi { self.token.span } else { self.prev_token.span };
+        Ok(P(ast::Local { ty, pat, init, id: DUMMY_NODE_ID, span: lo.to(hi), attrs }))
+    }
+
+    /// Parses the RHS of a local variable declaration (e.g., '= 14;').
+    fn parse_initializer(&mut self, eq_optional: bool) -> PResult<'a, Option<P<Expr>>> {
+        let eq_consumed = match self.token.kind {
+            token::BinOpEq(..) => {
+                // Recover `let x <op>= 1` as `let x = 1`
+                self.struct_span_err(
+                    self.token.span,
+                    "can't reassign to an uninitialized variable",
+                )
+                .span_suggestion_short(
+                    self.token.span,
+                    "initialize the variable",
+                    "=".to_string(),
+                    Applicability::MaybeIncorrect,
+                )
+                .emit();
+                self.bump();
+                true
+            }
+            _ => self.eat(&token::Eq),
+        };
+
+        Ok(if eq_consumed || eq_optional { Some(self.parse_expr()?) } else { None })
+    }
+
+    /// Parses a block. No inner attributes are allowed.
+    pub(super) fn parse_block(&mut self) -> PResult<'a, P<Block>> {
+        let (attrs, block) = self.parse_inner_attrs_and_block()?;
+        if let [.., last] = &*attrs {
+            self.error_on_forbidden_inner_attr(last.span, DEFAULT_INNER_ATTR_FORBIDDEN);
+        }
+        Ok(block)
+    }
+
+    fn error_block_no_opening_brace<T>(&mut self) -> PResult<'a, T> {
+        let sp = self.token.span;
+        let tok = super::token_descr(&self.token);
+        let mut e = self.struct_span_err(sp, &format!("expected `{{`, found {}", tok));
+        let do_not_suggest_help = self.token.is_keyword(kw::In) || self.token == token::Colon;
+
+        // Check to see if the user has written something like
+        //
+        //    if (cond)
+        //      bar;
+        //
+        // which is valid in other languages, but not Rust.
+        match self.parse_stmt_without_recovery() {
+            // If the next token is an open brace (e.g., `if a b {`), the place-
+            // inside-a-block suggestion would be more likely wrong than right.
+            Ok(Some(_))
+                if self.look_ahead(1, |t| t == &token::OpenDelim(token::Brace))
+                    || do_not_suggest_help => {}
+            Ok(Some(stmt)) => {
+                let stmt_own_line = self.sess.source_map().is_line_before_span_empty(sp);
+                let stmt_span = if stmt_own_line && self.eat(&token::Semi) {
+                    // Expand the span to include the semicolon.
+                    stmt.span.with_hi(self.prev_token.span.hi())
+                } else {
+                    stmt.span
+                };
+                if let Ok(snippet) = self.span_to_snippet(stmt_span) {
+                    e.span_suggestion(
+                        stmt_span,
+                        "try placing this code inside a block",
+                        format!("{{ {} }}", snippet),
+                        // Speculative; has been misleading in the past (#46836).
+                        Applicability::MaybeIncorrect,
+                    );
+                }
+            }
+            Err(mut e) => {
+                self.recover_stmt_(SemiColonMode::Break, BlockMode::Ignore);
+                e.cancel();
+            }
+            _ => {}
+        }
+        e.span_label(sp, "expected `{`");
+        Err(e)
+    }
+
+    /// Parses a block. Inner attributes are allowed.
+    pub(super) fn parse_inner_attrs_and_block(
+        &mut self,
+    ) -> PResult<'a, (Vec<Attribute>, P<Block>)> {
+        self.parse_block_common(self.token.span, BlockCheckMode::Default)
+    }
+
+    /// Parses a block. Inner attributes are allowed.
+    pub(super) fn parse_block_common(
+        &mut self,
+        lo: Span,
+        blk_mode: BlockCheckMode,
+    ) -> PResult<'a, (Vec<Attribute>, P<Block>)> {
+        maybe_whole!(self, NtBlock, |x| (Vec::new(), x));
+
+        if !self.eat(&token::OpenDelim(token::Brace)) {
+            return self.error_block_no_opening_brace();
+        }
+
+        Ok((self.parse_inner_attributes()?, self.parse_block_tail(lo, blk_mode)?))
+    }
+
+    /// Parses the rest of a block expression or function body.
+    /// Precondition: already parsed the '{'.
+    fn parse_block_tail(&mut self, lo: Span, s: BlockCheckMode) -> PResult<'a, P<Block>> {
+        let mut stmts = vec![];
+        while !self.eat(&token::CloseDelim(token::Brace)) {
+            if self.token == token::Eof {
+                break;
+            }
+            let stmt = match self.parse_full_stmt() {
+                Err(mut err) => {
+                    self.maybe_annotate_with_ascription(&mut err, false);
+                    err.emit();
+                    self.recover_stmt_(SemiColonMode::Ignore, BlockMode::Ignore);
+                    Some(self.mk_stmt_err(self.token.span))
+                }
+                Ok(stmt) => stmt,
+            };
+            if let Some(stmt) = stmt {
+                stmts.push(stmt);
+            } else {
+                // Found only `;` or `}`.
+                continue;
+            };
+        }
+        Ok(self.mk_block(stmts, s, lo.to(self.prev_token.span)))
+    }
+
+    /// Parses a statement, including the trailing semicolon.
+    pub fn parse_full_stmt(&mut self) -> PResult<'a, Option<Stmt>> {
+        // Skip looking for a trailing semicolon when we have an interpolated statement.
+        maybe_whole!(self, NtStmt, |x| Some(x));
+
+        let mut stmt = match self.parse_stmt_without_recovery()? {
+            Some(stmt) => stmt,
+            None => return Ok(None),
+        };
+
+        let mut eat_semi = true;
+        match stmt.kind {
+            // Expression without semicolon.
+            StmtKind::Expr(ref expr)
+                if self.token != token::Eof && classify::expr_requires_semi_to_be_stmt(expr) =>
+            {
+                // Just check for errors and recover; do not eat semicolon yet.
+                if let Err(mut e) =
+                    self.expect_one_of(&[], &[token::Semi, token::CloseDelim(token::Brace)])
+                {
+                    if let TokenKind::DocComment(..) = self.token.kind {
+                        if let Ok(snippet) = self.span_to_snippet(self.token.span) {
+                            let sp = self.token.span;
+                            let marker = &snippet[..3];
+                            let (comment_marker, doc_comment_marker) = marker.split_at(2);
+
+                            e.span_suggestion(
+                                sp.with_hi(sp.lo() + BytePos(marker.len() as u32)),
+                                &format!(
+                                    "add a space before `{}` to use a regular comment",
+                                    doc_comment_marker,
+                                ),
+                                format!("{} {}", comment_marker, doc_comment_marker),
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
+                    }
+                    e.emit();
+                    self.recover_stmt();
+                    // Don't complain about type errors in body tail after parse error (#57383).
+                    let sp = expr.span.to(self.prev_token.span);
+                    stmt.kind = StmtKind::Expr(self.mk_expr_err(sp));
+                }
+            }
+            StmtKind::Local(..) => {
+                self.expect_semi()?;
+                eat_semi = false;
+            }
+            StmtKind::Empty => eat_semi = false,
+            _ => {}
+        }
+
+        if eat_semi && self.eat(&token::Semi) {
+            stmt = stmt.add_trailing_semicolon();
+        }
+        stmt.span = stmt.span.to(self.prev_token.span);
+        Ok(Some(stmt))
+    }
+
+    pub(super) fn mk_block(&self, stmts: Vec<Stmt>, rules: BlockCheckMode, span: Span) -> P<Block> {
+        P(Block { stmts, id: DUMMY_NODE_ID, rules, span })
+    }
+
+    pub(super) fn mk_stmt(&self, span: Span, kind: StmtKind) -> Stmt {
+        Stmt { id: DUMMY_NODE_ID, kind, span }
+    }
+
+    fn mk_stmt_err(&self, span: Span) -> Stmt {
+        self.mk_stmt(span, StmtKind::Expr(self.mk_expr_err(span)))
+    }
+
+    pub(super) fn mk_block_err(&self, span: Span) -> P<Block> {
+        self.mk_block(vec![self.mk_stmt_err(span)], BlockCheckMode::Default, span)
+    }
+}
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
new file mode 100644
index 00000000000..4356850818e
--- /dev/null
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -0,0 +1,631 @@
+use super::{Parser, PathStyle, TokenType};
+
+use crate::{maybe_recover_from_interpolated_ty_qpath, maybe_whole};
+
+use rustc_ast::ptr::P;
+use rustc_ast::token::{self, Token, TokenKind};
+use rustc_ast::{self as ast, BareFnTy, FnRetTy, GenericParam, Lifetime, MutTy, Ty, TyKind};
+use rustc_ast::{GenericBound, GenericBounds, MacCall, Mutability};
+use rustc_ast::{PolyTraitRef, TraitBoundModifier, TraitObjectSyntax};
+use rustc_errors::{pluralize, struct_span_err, Applicability, PResult};
+use rustc_span::source_map::Span;
+use rustc_span::symbol::{kw, sym};
+
+/// Any `?` or `?const` modifiers that appear at the start of a bound.
+struct BoundModifiers {
+    /// `?Trait`.
+    maybe: Option<Span>,
+
+    /// `?const Trait`.
+    maybe_const: Option<Span>,
+}
+
+impl BoundModifiers {
+    fn to_trait_bound_modifier(&self) -> TraitBoundModifier {
+        match (self.maybe, self.maybe_const) {
+            (None, None) => TraitBoundModifier::None,
+            (Some(_), None) => TraitBoundModifier::Maybe,
+            (None, Some(_)) => TraitBoundModifier::MaybeConst,
+            (Some(_), Some(_)) => TraitBoundModifier::MaybeConstMaybe,
+        }
+    }
+}
+
+#[derive(Copy, Clone, PartialEq)]
+pub(super) enum AllowPlus {
+    Yes,
+    No,
+}
+
+#[derive(PartialEq)]
+pub(super) enum RecoverQPath {
+    Yes,
+    No,
+}
+
+// Is `...` (`CVarArgs`) legal at this level of type parsing?
+#[derive(PartialEq)]
+enum AllowCVariadic {
+    Yes,
+    No,
+}
+
+/// Returns `true` if `IDENT t` can start a type -- `IDENT::a::b`, `IDENT<u8, u8>`,
+/// `IDENT<<u8 as Trait>::AssocTy>`.
+///
+/// Types can also be of the form `IDENT(u8, u8) -> u8`, however this assumes
+/// that `IDENT` is not the ident of a fn trait.
+fn can_continue_type_after_non_fn_ident(t: &Token) -> bool {
+    t == &token::ModSep || t == &token::Lt || t == &token::BinOp(token::Shl)
+}
+
+impl<'a> Parser<'a> {
+    /// Parses a type.
+    pub fn parse_ty(&mut self) -> PResult<'a, P<Ty>> {
+        self.parse_ty_common(AllowPlus::Yes, RecoverQPath::Yes, AllowCVariadic::No)
+    }
+
+    /// Parse a type suitable for a function or function pointer parameter.
+    /// The difference from `parse_ty` is that this version allows `...`
+    /// (`CVarArgs`) at the top level of the the type.
+    pub(super) fn parse_ty_for_param(&mut self) -> PResult<'a, P<Ty>> {
+        self.parse_ty_common(AllowPlus::Yes, RecoverQPath::Yes, AllowCVariadic::Yes)
+    }
+
+    /// Parses a type in restricted contexts where `+` is not permitted.
+    ///
+    /// Example 1: `&'a TYPE`
+    ///     `+` is prohibited to maintain operator priority (P(+) < P(&)).
+    /// Example 2: `value1 as TYPE + value2`
+    ///     `+` is prohibited to avoid interactions with expression grammar.
+    pub(super) fn parse_ty_no_plus(&mut self) -> PResult<'a, P<Ty>> {
+        self.parse_ty_common(AllowPlus::No, RecoverQPath::Yes, AllowCVariadic::No)
+    }
+
+    /// Parses an optional return type `[ -> TY ]` in a function declaration.
+    pub(super) fn parse_ret_ty(
+        &mut self,
+        allow_plus: AllowPlus,
+        recover_qpath: RecoverQPath,
+    ) -> PResult<'a, FnRetTy> {
+        Ok(if self.eat(&token::RArrow) {
+            // FIXME(Centril): Can we unconditionally `allow_plus`?
+            let ty = self.parse_ty_common(allow_plus, recover_qpath, AllowCVariadic::No)?;
+            FnRetTy::Ty(ty)
+        } else {
+            FnRetTy::Default(self.token.span.shrink_to_lo())
+        })
+    }
+
+    fn parse_ty_common(
+        &mut self,
+        allow_plus: AllowPlus,
+        recover_qpath: RecoverQPath,
+        allow_c_variadic: AllowCVariadic,
+    ) -> PResult<'a, P<Ty>> {
+        let allow_qpath_recovery = recover_qpath == RecoverQPath::Yes;
+        maybe_recover_from_interpolated_ty_qpath!(self, allow_qpath_recovery);
+        maybe_whole!(self, NtTy, |x| x);
+
+        let lo = self.token.span;
+        let mut impl_dyn_multi = false;
+        let kind = if self.check(&token::OpenDelim(token::Paren)) {
+            self.parse_ty_tuple_or_parens(lo, allow_plus)?
+        } else if self.eat(&token::Not) {
+            // Never type `!`
+            TyKind::Never
+        } else if self.eat(&token::BinOp(token::Star)) {
+            self.parse_ty_ptr()?
+        } else if self.eat(&token::OpenDelim(token::Bracket)) {
+            self.parse_array_or_slice_ty()?
+        } else if self.check(&token::BinOp(token::And)) || self.check(&token::AndAnd) {
+            // Reference
+            self.expect_and()?;
+            self.parse_borrowed_pointee()?
+        } else if self.eat_keyword_noexpect(kw::Typeof) {
+            self.parse_typeof_ty()?
+        } else if self.eat_keyword(kw::Underscore) {
+            // A type to be inferred `_`
+            TyKind::Infer
+        } else if self.check_fn_front_matter() {
+            // Function pointer type
+            self.parse_ty_bare_fn(lo, Vec::new())?
+        } else if self.check_keyword(kw::For) {
+            // Function pointer type or bound list (trait object type) starting with a poly-trait.
+            //   `for<'lt> [unsafe] [extern "ABI"] fn (&'lt S) -> T`
+            //   `for<'lt> Trait1<'lt> + Trait2 + 'a`
+            let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
+            if self.check_fn_front_matter() {
+                self.parse_ty_bare_fn(lo, lifetime_defs)?
+            } else {
+                let path = self.parse_path(PathStyle::Type)?;
+                let parse_plus = allow_plus == AllowPlus::Yes && self.check_plus();
+                self.parse_remaining_bounds_path(lifetime_defs, path, lo, parse_plus)?
+            }
+        } else if self.eat_keyword(kw::Impl) {
+            self.parse_impl_ty(&mut impl_dyn_multi)?
+        } else if self.is_explicit_dyn_type() {
+            self.parse_dyn_ty(&mut impl_dyn_multi)?
+        } else if self.eat_lt() {
+            // Qualified path
+            let (qself, path) = self.parse_qpath(PathStyle::Type)?;
+            TyKind::Path(Some(qself), path)
+        } else if self.check_path() {
+            self.parse_path_start_ty(lo, allow_plus)?
+        } else if self.can_begin_bound() {
+            self.parse_bare_trait_object(lo, allow_plus)?
+        } else if self.eat(&token::DotDotDot) {
+            if allow_c_variadic == AllowCVariadic::Yes {
+                TyKind::CVarArgs
+            } else {
+                // FIXME(Centril): Should we just allow `...` syntactically
+                // anywhere in a type and use semantic restrictions instead?
+                self.error_illegal_c_varadic_ty(lo);
+                TyKind::Err
+            }
+        } else {
+            let msg = format!("expected type, found {}", super::token_descr(&self.token));
+            let mut err = self.struct_span_err(self.token.span, &msg);
+            err.span_label(self.token.span, "expected type");
+            self.maybe_annotate_with_ascription(&mut err, true);
+            return Err(err);
+        };
+
+        let span = lo.to(self.prev_token.span);
+        let ty = self.mk_ty(span, kind);
+
+        // Try to recover from use of `+` with incorrect priority.
+        self.maybe_report_ambiguous_plus(allow_plus, impl_dyn_multi, &ty);
+        self.maybe_recover_from_bad_type_plus(allow_plus, &ty)?;
+        self.maybe_recover_from_bad_qpath(ty, allow_qpath_recovery)
+    }
+
+    /// Parses either:
+    /// - `(TYPE)`, a parenthesized type.
+    /// - `(TYPE,)`, a tuple with a single field of type TYPE.
+    fn parse_ty_tuple_or_parens(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
+        let mut trailing_plus = false;
+        let (ts, trailing) = self.parse_paren_comma_seq(|p| {
+            let ty = p.parse_ty()?;
+            trailing_plus = p.prev_token.kind == TokenKind::BinOp(token::Plus);
+            Ok(ty)
+        })?;
+
+        if ts.len() == 1 && !trailing {
+            let ty = ts.into_iter().next().unwrap().into_inner();
+            let maybe_bounds = allow_plus == AllowPlus::Yes && self.token.is_like_plus();
+            match ty.kind {
+                // `(TY_BOUND_NOPAREN) + BOUND + ...`.
+                TyKind::Path(None, path) if maybe_bounds => {
+                    self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
+                }
+                TyKind::TraitObject(bounds, TraitObjectSyntax::None)
+                    if maybe_bounds && bounds.len() == 1 && !trailing_plus =>
+                {
+                    self.parse_remaining_bounds(bounds, true)
+                }
+                // `(TYPE)`
+                _ => Ok(TyKind::Paren(P(ty))),
+            }
+        } else {
+            Ok(TyKind::Tup(ts))
+        }
+    }
+
+    fn parse_bare_trait_object(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
+        let lt_no_plus = self.check_lifetime() && !self.look_ahead(1, |t| t.is_like_plus());
+        let bounds = self.parse_generic_bounds_common(allow_plus, None)?;
+        if lt_no_plus {
+            self.struct_span_err(lo, "lifetime in trait object type must be followed by `+`").emit()
+        }
+        Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
+    }
+
+    fn parse_remaining_bounds_path(
+        &mut self,
+        generic_params: Vec<GenericParam>,
+        path: ast::Path,
+        lo: Span,
+        parse_plus: bool,
+    ) -> PResult<'a, TyKind> {
+        let poly_trait_ref = PolyTraitRef::new(generic_params, path, lo.to(self.prev_token.span));
+        let bounds = vec![GenericBound::Trait(poly_trait_ref, TraitBoundModifier::None)];
+        self.parse_remaining_bounds(bounds, parse_plus)
+    }
+
+    /// Parse the remainder of a bare trait object type given an already parsed list.
+    fn parse_remaining_bounds(
+        &mut self,
+        mut bounds: GenericBounds,
+        plus: bool,
+    ) -> PResult<'a, TyKind> {
+        assert_ne!(self.token, token::Question);
+        if plus {
+            self.eat_plus(); // `+`, or `+=` gets split and `+` is discarded
+            bounds.append(&mut self.parse_generic_bounds(Some(self.prev_token.span))?);
+        }
+        Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
+    }
+
+    /// Parses a raw pointer type: `*[const | mut] $type`.
+    fn parse_ty_ptr(&mut self) -> PResult<'a, TyKind> {
+        let mutbl = self.parse_const_or_mut().unwrap_or_else(|| {
+            let span = self.prev_token.span;
+            let msg = "expected mut or const in raw pointer type";
+            self.struct_span_err(span, msg)
+                .span_label(span, msg)
+                .help("use `*mut T` or `*const T` as appropriate")
+                .emit();
+            Mutability::Not
+        });
+        let ty = self.parse_ty_no_plus()?;
+        Ok(TyKind::Ptr(MutTy { ty, mutbl }))
+    }
+
+    /// Parses an array (`[TYPE; EXPR]`) or slice (`[TYPE]`) type.
+    /// The opening `[` bracket is already eaten.
+    fn parse_array_or_slice_ty(&mut self) -> PResult<'a, TyKind> {
+        let elt_ty = self.parse_ty()?;
+        let ty = if self.eat(&token::Semi) {
+            TyKind::Array(elt_ty, self.parse_anon_const_expr()?)
+        } else {
+            TyKind::Slice(elt_ty)
+        };
+        self.expect(&token::CloseDelim(token::Bracket))?;
+        Ok(ty)
+    }
+
+    fn parse_borrowed_pointee(&mut self) -> PResult<'a, TyKind> {
+        let opt_lifetime = if self.check_lifetime() { Some(self.expect_lifetime()) } else { None };
+        let mutbl = self.parse_mutability();
+        let ty = self.parse_ty_no_plus()?;
+        Ok(TyKind::Rptr(opt_lifetime, MutTy { ty, mutbl }))
+    }
+
+    // Parses the `typeof(EXPR)`.
+    // To avoid ambiguity, the type is surrounded by parenthesis.
+    fn parse_typeof_ty(&mut self) -> PResult<'a, TyKind> {
+        self.expect(&token::OpenDelim(token::Paren))?;
+        let expr = self.parse_anon_const_expr()?;
+        self.expect(&token::CloseDelim(token::Paren))?;
+        Ok(TyKind::Typeof(expr))
+    }
+
+    /// Parses a function pointer type (`TyKind::BareFn`).
+    /// ```
+    /// [unsafe] [extern "ABI"] fn (S) -> T
+    ///  ^~~~~^          ^~~~^     ^~^    ^
+    ///    |               |        |     |
+    ///    |               |        |   Return type
+    /// Function Style    ABI  Parameter types
+    /// ```
+    /// We actually parse `FnHeader FnDecl`, but we error on `const` and `async` qualifiers.
+    fn parse_ty_bare_fn(&mut self, lo: Span, params: Vec<GenericParam>) -> PResult<'a, TyKind> {
+        let ast::FnHeader { ext, unsafety, constness, asyncness } = self.parse_fn_front_matter()?;
+        let decl = self.parse_fn_decl(|_| false, AllowPlus::No)?;
+        let whole_span = lo.to(self.prev_token.span);
+        if let ast::Const::Yes(span) = constness {
+            self.error_fn_ptr_bad_qualifier(whole_span, span, "const");
+        }
+        if let ast::Async::Yes { span, .. } = asyncness {
+            self.error_fn_ptr_bad_qualifier(whole_span, span, "async");
+        }
+        Ok(TyKind::BareFn(P(BareFnTy { ext, unsafety, generic_params: params, decl })))
+    }
+
+    /// Emit an error for the given bad function pointer qualifier.
+    fn error_fn_ptr_bad_qualifier(&self, span: Span, qual_span: Span, qual: &str) {
+        self.struct_span_err(span, &format!("an `fn` pointer type cannot be `{}`", qual))
+            .span_label(qual_span, format!("`{}` because of this", qual))
+            .span_suggestion_short(
+                qual_span,
+                &format!("remove the `{}` qualifier", qual),
+                String::new(),
+                Applicability::MaybeIncorrect,
+            )
+            .emit();
+    }
+
+    /// Parses an `impl B0 + ... + Bn` type.
+    fn parse_impl_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
+        // Always parse bounds greedily for better error recovery.
+        let bounds = self.parse_generic_bounds(None)?;
+        *impl_dyn_multi = bounds.len() > 1 || self.prev_token.kind == TokenKind::BinOp(token::Plus);
+        Ok(TyKind::ImplTrait(ast::DUMMY_NODE_ID, bounds))
+    }
+
+    /// Is a `dyn B0 + ... + Bn` type allowed here?
+    fn is_explicit_dyn_type(&mut self) -> bool {
+        self.check_keyword(kw::Dyn)
+            && (self.token.uninterpolated_span().rust_2018()
+                || self.look_ahead(1, |t| {
+                    t.can_begin_bound() && !can_continue_type_after_non_fn_ident(t)
+                }))
+    }
+
+    /// Parses a `dyn B0 + ... + Bn` type.
+    ///
+    /// Note that this does *not* parse bare trait objects.
+    fn parse_dyn_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
+        self.bump(); // `dyn`
+        // Always parse bounds greedily for better error recovery.
+        let bounds = self.parse_generic_bounds(None)?;
+        *impl_dyn_multi = bounds.len() > 1 || self.prev_token.kind == TokenKind::BinOp(token::Plus);
+        Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::Dyn))
+    }
+
+    /// Parses a type starting with a path.
+    ///
+    /// This can be:
+    /// 1. a type macro, `mac!(...)`,
+    /// 2. a bare trait object, `B0 + ... + Bn`,
+    /// 3. or a path, `path::to::MyType`.
+    fn parse_path_start_ty(&mut self, lo: Span, allow_plus: AllowPlus) -> PResult<'a, TyKind> {
+        // Simple path
+        let path = self.parse_path(PathStyle::Type)?;
+        if self.eat(&token::Not) {
+            // Macro invocation in type position
+            Ok(TyKind::MacCall(MacCall {
+                path,
+                args: self.parse_mac_args()?,
+                prior_type_ascription: self.last_type_ascription,
+            }))
+        } else if allow_plus == AllowPlus::Yes && self.check_plus() {
+            // `Trait1 + Trait2 + 'a`
+            self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
+        } else {
+            // Just a type path.
+            Ok(TyKind::Path(None, path))
+        }
+    }
+
+    fn error_illegal_c_varadic_ty(&self, lo: Span) {
+        struct_span_err!(
+            self.sess.span_diagnostic,
+            lo.to(self.prev_token.span),
+            E0743,
+            "C-variadic type `...` may not be nested inside another type",
+        )
+        .emit();
+    }
+
+    pub(super) fn parse_generic_bounds(
+        &mut self,
+        colon_span: Option<Span>,
+    ) -> PResult<'a, GenericBounds> {
+        self.parse_generic_bounds_common(AllowPlus::Yes, colon_span)
+    }
+
+    /// Parses bounds of a type parameter `BOUND + BOUND + ...`, possibly with trailing `+`.
+    ///
+    /// See `parse_generic_bound` for the `BOUND` grammar.
+    fn parse_generic_bounds_common(
+        &mut self,
+        allow_plus: AllowPlus,
+        colon_span: Option<Span>,
+    ) -> PResult<'a, GenericBounds> {
+        let mut bounds = Vec::new();
+        let mut negative_bounds = Vec::new();
+        while self.can_begin_bound() {
+            match self.parse_generic_bound()? {
+                Ok(bound) => bounds.push(bound),
+                Err(neg_sp) => negative_bounds.push(neg_sp),
+            }
+            if allow_plus == AllowPlus::No || !self.eat_plus() {
+                break;
+            }
+        }
+
+        if !negative_bounds.is_empty() {
+            self.error_negative_bounds(colon_span, &bounds, negative_bounds);
+        }
+
+        Ok(bounds)
+    }
+
+    /// Can the current token begin a bound?
+    fn can_begin_bound(&mut self) -> bool {
+        // This needs to be synchronized with `TokenKind::can_begin_bound`.
+        self.check_path()
+        || self.check_lifetime()
+        || self.check(&token::Not) // Used for error reporting only.
+        || self.check(&token::Question)
+        || self.check_keyword(kw::For)
+        || self.check(&token::OpenDelim(token::Paren))
+    }
+
+    fn error_negative_bounds(
+        &self,
+        colon_span: Option<Span>,
+        bounds: &[GenericBound],
+        negative_bounds: Vec<Span>,
+    ) {
+        let negative_bounds_len = negative_bounds.len();
+        let last_span = *negative_bounds.last().expect("no negative bounds, but still error?");
+        let mut err = self.struct_span_err(negative_bounds, "negative bounds are not supported");
+        err.span_label(last_span, "negative bounds are not supported");
+        if let Some(bound_list) = colon_span {
+            let bound_list = bound_list.to(self.prev_token.span);
+            let mut new_bound_list = String::new();
+            if !bounds.is_empty() {
+                let mut snippets = bounds.iter().map(|bound| self.span_to_snippet(bound.span()));
+                while let Some(Ok(snippet)) = snippets.next() {
+                    new_bound_list.push_str(" + ");
+                    new_bound_list.push_str(&snippet);
+                }
+                new_bound_list = new_bound_list.replacen(" +", ":", 1);
+            }
+            err.tool_only_span_suggestion(
+                bound_list,
+                &format!("remove the bound{}", pluralize!(negative_bounds_len)),
+                new_bound_list,
+                Applicability::MachineApplicable,
+            );
+        }
+        err.emit();
+    }
+
+    /// Parses a bound according to the grammar:
+    /// ```
+    /// BOUND = TY_BOUND | LT_BOUND
+    /// ```
+    fn parse_generic_bound(&mut self) -> PResult<'a, Result<GenericBound, Span>> {
+        let anchor_lo = self.prev_token.span;
+        let lo = self.token.span;
+        let has_parens = self.eat(&token::OpenDelim(token::Paren));
+        let inner_lo = self.token.span;
+        let is_negative = self.eat(&token::Not);
+
+        let modifiers = self.parse_ty_bound_modifiers();
+        let bound = if self.token.is_lifetime() {
+            self.error_lt_bound_with_modifiers(modifiers);
+            self.parse_generic_lt_bound(lo, inner_lo, has_parens)?
+        } else {
+            self.parse_generic_ty_bound(lo, has_parens, modifiers)?
+        };
+
+        Ok(if is_negative { Err(anchor_lo.to(self.prev_token.span)) } else { Ok(bound) })
+    }
+
+    /// Parses a lifetime ("outlives") bound, e.g. `'a`, according to:
+    /// ```
+    /// LT_BOUND = LIFETIME
+    /// ```
+    fn parse_generic_lt_bound(
+        &mut self,
+        lo: Span,
+        inner_lo: Span,
+        has_parens: bool,
+    ) -> PResult<'a, GenericBound> {
+        let bound = GenericBound::Outlives(self.expect_lifetime());
+        if has_parens {
+            // FIXME(Centril): Consider not erroring here and accepting `('lt)` instead,
+            // possibly introducing `GenericBound::Paren(P<GenericBound>)`?
+            self.recover_paren_lifetime(lo, inner_lo)?;
+        }
+        Ok(bound)
+    }
+
+    /// Emits an error if any trait bound modifiers were present.
+    fn error_lt_bound_with_modifiers(&self, modifiers: BoundModifiers) {
+        if let Some(span) = modifiers.maybe_const {
+            self.struct_span_err(
+                span,
+                "`?const` may only modify trait bounds, not lifetime bounds",
+            )
+            .emit();
+        }
+
+        if let Some(span) = modifiers.maybe {
+            self.struct_span_err(span, "`?` may only modify trait bounds, not lifetime bounds")
+                .emit();
+        }
+    }
+
+    /// Recover on `('lifetime)` with `(` already eaten.
+    fn recover_paren_lifetime(&mut self, lo: Span, inner_lo: Span) -> PResult<'a, ()> {
+        let inner_span = inner_lo.to(self.prev_token.span);
+        self.expect(&token::CloseDelim(token::Paren))?;
+        let mut err = self.struct_span_err(
+            lo.to(self.prev_token.span),
+            "parenthesized lifetime bounds are not supported",
+        );
+        if let Ok(snippet) = self.span_to_snippet(inner_span) {
+            err.span_suggestion_short(
+                lo.to(self.prev_token.span),
+                "remove the parentheses",
+                snippet,
+                Applicability::MachineApplicable,
+            );
+        }
+        err.emit();
+        Ok(())
+    }
+
+    /// Parses the modifiers that may precede a trait in a bound, e.g. `?Trait` or `?const Trait`.
+    ///
+    /// If no modifiers are present, this does not consume any tokens.
+    ///
+    /// ```
+    /// TY_BOUND_MODIFIERS = "?" ["const" ["?"]]
+    /// ```
+    fn parse_ty_bound_modifiers(&mut self) -> BoundModifiers {
+        if !self.eat(&token::Question) {
+            return BoundModifiers { maybe: None, maybe_const: None };
+        }
+
+        // `? ...`
+        let first_question = self.prev_token.span;
+        if !self.eat_keyword(kw::Const) {
+            return BoundModifiers { maybe: Some(first_question), maybe_const: None };
+        }
+
+        // `?const ...`
+        let maybe_const = first_question.to(self.prev_token.span);
+        self.sess.gated_spans.gate(sym::const_trait_bound_opt_out, maybe_const);
+        if !self.eat(&token::Question) {
+            return BoundModifiers { maybe: None, maybe_const: Some(maybe_const) };
+        }
+
+        // `?const ? ...`
+        let second_question = self.prev_token.span;
+        BoundModifiers { maybe: Some(second_question), maybe_const: Some(maybe_const) }
+    }
+
+    /// Parses a type bound according to:
+    /// ```
+    /// TY_BOUND = TY_BOUND_NOPAREN | (TY_BOUND_NOPAREN)
+    /// TY_BOUND_NOPAREN = [TY_BOUND_MODIFIERS] [for<LT_PARAM_DEFS>] SIMPLE_PATH
+    /// ```
+    ///
+    /// For example, this grammar accepts `?const ?for<'a: 'b> m::Trait<'a>`.
+    fn parse_generic_ty_bound(
+        &mut self,
+        lo: Span,
+        has_parens: bool,
+        modifiers: BoundModifiers,
+    ) -> PResult<'a, GenericBound> {
+        let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
+        let path = self.parse_path(PathStyle::Type)?;
+        if has_parens {
+            self.expect(&token::CloseDelim(token::Paren))?;
+        }
+
+        let modifier = modifiers.to_trait_bound_modifier();
+        let poly_trait = PolyTraitRef::new(lifetime_defs, path, lo.to(self.prev_token.span));
+        Ok(GenericBound::Trait(poly_trait, modifier))
+    }
+
+    /// Optionally parses `for<$generic_params>`.
+    pub(super) fn parse_late_bound_lifetime_defs(&mut self) -> PResult<'a, Vec<GenericParam>> {
+        if self.eat_keyword(kw::For) {
+            self.expect_lt()?;
+            let params = self.parse_generic_params()?;
+            self.expect_gt()?;
+            // We rely on AST validation to rule out invalid cases: There must not be type
+            // parameters, and the lifetime parameters must not have bounds.
+            Ok(params)
+        } else {
+            Ok(Vec::new())
+        }
+    }
+
+    pub(super) fn check_lifetime(&mut self) -> bool {
+        self.expected_tokens.push(TokenType::Lifetime);
+        self.token.is_lifetime()
+    }
+
+    /// Parses a single lifetime `'a` or panics.
+    pub(super) fn expect_lifetime(&mut self) -> Lifetime {
+        if let Some(ident) = self.token.lifetime() {
+            self.bump();
+            Lifetime { ident, id: ast::DUMMY_NODE_ID }
+        } else {
+            self.span_bug(self.token.span, "not a lifetime")
+        }
+    }
+
+    pub(super) fn mk_ty(&self, span: Span, kind: TyKind) -> P<Ty> {
+        P(Ty { kind, span, id: ast::DUMMY_NODE_ID })
+    }
+}
diff --git a/compiler/rustc_parse/src/validate_attr.rs b/compiler/rustc_parse/src/validate_attr.rs
new file mode 100644
index 00000000000..f4bb9610940
--- /dev/null
+++ b/compiler/rustc_parse/src/validate_attr.rs
@@ -0,0 +1,163 @@
+//! Meta-syntax validation logic of attributes for post-expansion.
+
+use crate::parse_in;
+
+use rustc_ast::tokenstream::DelimSpan;
+use rustc_ast::{self as ast, Attribute, MacArgs, MacDelimiter, MetaItem, MetaItemKind};
+use rustc_errors::{Applicability, PResult};
+use rustc_feature::{AttributeTemplate, BUILTIN_ATTRIBUTE_MAP};
+use rustc_session::lint::builtin::ILL_FORMED_ATTRIBUTE_INPUT;
+use rustc_session::parse::ParseSess;
+use rustc_span::{sym, Symbol};
+
+pub fn check_meta(sess: &ParseSess, attr: &Attribute) {
+    if attr.is_doc_comment() {
+        return;
+    }
+
+    let attr_info =
+        attr.ident().and_then(|ident| BUILTIN_ATTRIBUTE_MAP.get(&ident.name)).map(|a| **a);
+
+    // Check input tokens for built-in and key-value attributes.
+    match attr_info {
+        // `rustc_dummy` doesn't have any restrictions specific to built-in attributes.
+        Some((name, _, template, _)) if name != sym::rustc_dummy => {
+            check_builtin_attribute(sess, attr, name, template)
+        }
+        _ => {
+            if let MacArgs::Eq(..) = attr.get_normal_item().args {
+                // All key-value attributes are restricted to meta-item syntax.
+                parse_meta(sess, attr)
+                    .map_err(|mut err| {
+                        err.emit();
+                    })
+                    .ok();
+            }
+        }
+    }
+}
+
+pub fn parse_meta<'a>(sess: &'a ParseSess, attr: &Attribute) -> PResult<'a, MetaItem> {
+    let item = attr.get_normal_item();
+    Ok(MetaItem {
+        span: attr.span,
+        path: item.path.clone(),
+        kind: match &item.args {
+            MacArgs::Empty => MetaItemKind::Word,
+            MacArgs::Eq(_, t) => {
+                let v = parse_in(sess, t.clone(), "name value", |p| p.parse_unsuffixed_lit())?;
+                MetaItemKind::NameValue(v)
+            }
+            MacArgs::Delimited(dspan, delim, t) => {
+                check_meta_bad_delim(sess, *dspan, *delim, "wrong meta list delimiters");
+                let nmis = parse_in(sess, t.clone(), "meta list", |p| p.parse_meta_seq_top())?;
+                MetaItemKind::List(nmis)
+            }
+        },
+    })
+}
+
+pub fn check_meta_bad_delim(sess: &ParseSess, span: DelimSpan, delim: MacDelimiter, msg: &str) {
+    if let ast::MacDelimiter::Parenthesis = delim {
+        return;
+    }
+
+    sess.span_diagnostic
+        .struct_span_err(span.entire(), msg)
+        .multipart_suggestion(
+            "the delimiters should be `(` and `)`",
+            vec![(span.open, "(".to_string()), (span.close, ")".to_string())],
+            Applicability::MachineApplicable,
+        )
+        .emit();
+}
+
+/// Checks that the given meta-item is compatible with this `AttributeTemplate`.
+fn is_attr_template_compatible(template: &AttributeTemplate, meta: &ast::MetaItemKind) -> bool {
+    match meta {
+        MetaItemKind::Word => template.word,
+        MetaItemKind::List(..) => template.list.is_some(),
+        MetaItemKind::NameValue(lit) if lit.kind.is_str() => template.name_value_str.is_some(),
+        MetaItemKind::NameValue(..) => false,
+    }
+}
+
+pub fn check_builtin_attribute(
+    sess: &ParseSess,
+    attr: &Attribute,
+    name: Symbol,
+    template: AttributeTemplate,
+) {
+    // Some special attributes like `cfg` must be checked
+    // before the generic check, so we skip them here.
+    let should_skip = |name| name == sym::cfg;
+    // Some of previously accepted forms were used in practice,
+    // report them as warnings for now.
+    let should_warn = |name| {
+        name == sym::doc
+            || name == sym::ignore
+            || name == sym::inline
+            || name == sym::link
+            || name == sym::test
+            || name == sym::bench
+    };
+
+    match parse_meta(sess, attr) {
+        Ok(meta) => {
+            if !should_skip(name) && !is_attr_template_compatible(&template, &meta.kind) {
+                let error_msg = format!("malformed `{}` attribute input", name);
+                let mut msg = "attribute must be of the form ".to_owned();
+                let mut suggestions = vec![];
+                let mut first = true;
+                if template.word {
+                    first = false;
+                    let code = format!("#[{}]", name);
+                    msg.push_str(&format!("`{}`", &code));
+                    suggestions.push(code);
+                }
+                if let Some(descr) = template.list {
+                    if !first {
+                        msg.push_str(" or ");
+                    }
+                    first = false;
+                    let code = format!("#[{}({})]", name, descr);
+                    msg.push_str(&format!("`{}`", &code));
+                    suggestions.push(code);
+                }
+                if let Some(descr) = template.name_value_str {
+                    if !first {
+                        msg.push_str(" or ");
+                    }
+                    let code = format!("#[{} = \"{}\"]", name, descr);
+                    msg.push_str(&format!("`{}`", &code));
+                    suggestions.push(code);
+                }
+                if should_warn(name) {
+                    sess.buffer_lint(
+                        &ILL_FORMED_ATTRIBUTE_INPUT,
+                        meta.span,
+                        ast::CRATE_NODE_ID,
+                        &msg,
+                    );
+                } else {
+                    sess.span_diagnostic
+                        .struct_span_err(meta.span, &error_msg)
+                        .span_suggestions(
+                            meta.span,
+                            if suggestions.len() == 1 {
+                                "must be of the form"
+                            } else {
+                                "the following are the possible correct uses"
+                            },
+                            suggestions.into_iter(),
+                            Applicability::HasPlaceholders,
+                        )
+                        .emit();
+                }
+            }
+        }
+        Err(mut err) => {
+            err.emit();
+        }
+    }
+}