move diagnostics.rs into parser/

5 files changed, 1488 insertions, 4 deletions

diff --git a/src/libsyntax/parse/parser/diagnostics.rs b/src/libsyntax/parse/parser/diagnostics.rs
new file mode 100644
index 00000000000..943838d9dda
--- /dev/null
+++ b/src/libsyntax/parse/parser/diagnostics.rs
@@ -0,0 +1,1483 @@
+use crate::ast::{
+    self, Param, BinOpKind, BindingMode, BlockCheckMode, Expr, ExprKind, Ident, Item, ItemKind,
+    Mutability, Pat, PatKind, PathSegment, QSelf, Ty, TyKind,
+};
+use crate::parse::{SeqSep, PResult, Parser};
+use crate::parse::parser::{BlockMode, PathStyle, SemiColonMode, TokenType, TokenExpectType};
+use crate::parse::token::{self, TokenKind};
+use crate::print::pprust;
+use crate::ptr::P;
+use crate::symbol::{kw, sym};
+use crate::ThinVec;
+use crate::util::parser::AssocOp;
+use errors::{Applicability, DiagnosticBuilder, DiagnosticId, pluralise};
+use rustc_data_structures::fx::FxHashSet;
+use syntax_pos::{Span, DUMMY_SP, MultiSpan, SpanSnippetError};
+use log::{debug, trace};
+use std::mem;
+
+const TURBOFISH: &'static str = "use `::<...>` instead of `<...>` to specify type arguments";
+/// Creates a placeholder argument.
+crate fn dummy_arg(ident: Ident) -> Param {
+    let pat = P(Pat {
+        id: ast::DUMMY_NODE_ID,
+        kind: PatKind::Ident(BindingMode::ByValue(Mutability::Immutable), ident, None),
+        span: ident.span,
+    });
+    let ty = Ty {
+        kind: TyKind::Err,
+        span: ident.span,
+        id: ast::DUMMY_NODE_ID
+    };
+    Param {
+        attrs: ThinVec::default(),
+        id: ast::DUMMY_NODE_ID,
+        pat,
+        span: ident.span,
+        ty: P(ty),
+        is_placeholder: false,
+    }
+}
+
+pub enum Error {
+    FileNotFoundForModule {
+        mod_name: String,
+        default_path: String,
+        secondary_path: String,
+        dir_path: String,
+    },
+    DuplicatePaths {
+        mod_name: String,
+        default_path: String,
+        secondary_path: String,
+    },
+    UselessDocComment,
+    InclusiveRangeWithNoEnd,
+}
+
+impl Error {
+    fn span_err<S: Into<MultiSpan>>(
+        self,
+        sp: S,
+        handler: &errors::Handler,
+    ) -> DiagnosticBuilder<'_> {
+        match self {
+            Error::FileNotFoundForModule {
+                ref mod_name,
+                ref default_path,
+                ref secondary_path,
+                ref dir_path,
+            } => {
+                let mut err = struct_span_err!(
+                    handler,
+                    sp,
+                    E0583,
+                    "file not found for module `{}`",
+                    mod_name,
+                );
+                err.help(&format!(
+                    "name the file either {} or {} inside the directory \"{}\"",
+                    default_path,
+                    secondary_path,
+                    dir_path,
+                ));
+                err
+            }
+            Error::DuplicatePaths { ref mod_name, ref default_path, ref secondary_path } => {
+                let mut err = struct_span_err!(
+                    handler,
+                    sp,
+                    E0584,
+                    "file for module `{}` found at both {} and {}",
+                    mod_name,
+                    default_path,
+                    secondary_path,
+                );
+                err.help("delete or rename one of them to remove the ambiguity");
+                err
+            }
+            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
+            }
+            Error::InclusiveRangeWithNoEnd => {
+                let mut err = struct_span_err!(
+                    handler,
+                    sp,
+                    E0586,
+                    "inclusive range with no end",
+                );
+                err.help("inclusive ranges must be bounded at the end (`..=b` or `a..=b`)");
+                err
+            }
+        }
+    }
+}
+
+pub 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,
+        }
+    }
+}
+
+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: ThinVec::new(),
+            id: ast::DUMMY_NODE_ID,
+        }
+    }
+}
+
+impl<'a> Parser<'a> {
+    pub fn fatal(&self, m: &str) -> DiagnosticBuilder<'a> {
+        self.span_fatal(self.token.span, m)
+    }
+
+    pub fn span_fatal<S: Into<MultiSpan>>(&self, sp: S, m: &str) -> DiagnosticBuilder<'a> {
+        self.sess.span_diagnostic.struct_span_fatal(sp, m)
+    }
+
+    pub fn span_fatal_err<S: Into<MultiSpan>>(&self, sp: S, err: Error) -> DiagnosticBuilder<'a> {
+        err.span_err(sp, self.diagnostic())
+    }
+
+    pub fn bug(&self, m: &str) -> ! {
+        self.sess.span_diagnostic.span_bug(self.token.span, m)
+    }
+
+    pub fn span_err<S: Into<MultiSpan>>(&self, sp: S, m: &str) {
+        self.sess.span_diagnostic.span_err(sp, m)
+    }
+
+    crate fn struct_span_err<S: Into<MultiSpan>>(&self, sp: S, m: &str) -> DiagnosticBuilder<'a> {
+        self.sess.span_diagnostic.struct_span_err(sp, m)
+    }
+
+    crate fn span_bug<S: Into<MultiSpan>>(&self, sp: S, m: &str) -> ! {
+        self.sess.span_diagnostic.span_bug(sp, m)
+    }
+
+    crate fn diagnostic(&self) -> &'a errors::Handler {
+        &self.sess.span_diagnostic
+    }
+
+    crate fn span_to_snippet(&self, span: Span) -> Result<String, SpanSnippetError> {
+        self.sess.source_map().span_to_snippet(span)
+    }
+
+    crate fn expected_ident_found(&self) -> DiagnosticBuilder<'a> {
+        let mut err = self.struct_span_err(
+            self.token.span,
+            &format!("expected identifier, found {}", self.this_token_descr()),
+        );
+        if let token::Ident(name, false) = self.token.kind {
+            if Ident::new(name, self.token.span).is_raw_guess() {
+                err.span_suggestion(
+                    self.token.span,
+                    "you can escape reserved keywords to use them as identifiers",
+                    format!("r#{}", name),
+                    Applicability::MaybeIncorrect,
+                );
+            }
+        }
+        if let Some(token_descr) = self.token_descr() {
+            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 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 = self.this_token_to_string();
+        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.sess.source_map().next_point(self.prev_span),
+                format!("expected one of {} here", short_expect)))
+        } else if expected.is_empty() {
+            (format!("unexpected token: `{}`", actual),
+                (self.prev_span, "unexpected token after this".to_string()))
+        } else {
+            (format!("expected {}, found `{}`", expect, actual),
+                (self.sess.source_map().next_point(self.prev_span),
+                format!("expected {} here", expect)))
+        };
+        self.last_unexpected_token_span = Some(self.token.span);
+        let mut err = self.fatal(&msg_exp);
+        if self.token.is_ident_named(sym::and) {
+            err.span_suggestion_short(
+                self.token.span,
+                "use `&&` instead of `and` for the boolean operator",
+                "&&".to_string(),
+                Applicability::MaybeIncorrect,
+            );
+        }
+        if self.token.is_ident_named(sym::or) {
+            err.span_suggestion_short(
+                self.token.span,
+                "use `||` instead of `or` for the boolean operator",
+                "||".to_string(),
+                Applicability::MaybeIncorrect,
+            );
+        }
+        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_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);
+            }
+        }
+
+        let is_semi_suggestable = expected.iter().any(|t| match t {
+            TokenType::Token(token::Semi) => true, // We expect a `;` here.
+            _ => false,
+        }) && ( // A `;` would be expected before the current keyword.
+            self.token.is_keyword(kw::Break) ||
+            self.token.is_keyword(kw::Continue) ||
+            self.token.is_keyword(kw::For) ||
+            self.token.is_keyword(kw::If) ||
+            self.token.is_keyword(kw::Let) ||
+            self.token.is_keyword(kw::Loop) ||
+            self.token.is_keyword(kw::Match) ||
+            self.token.is_keyword(kw::Return) ||
+            self.token.is_keyword(kw::While)
+        );
+        let sm = self.sess.source_map();
+        match (sm.lookup_line(self.token.span.lo()), sm.lookup_line(sp.lo())) {
+            (Ok(ref a), Ok(ref b)) if a.line != b.line && is_semi_suggestable => {
+                // The spans are in different lines, expected `;` and found `let` or `return`.
+                // High likelihood that it is only a missing `;`.
+                err.span_suggestion_short(
+                    label_sp,
+                    "a semicolon may be missing here",
+                    ";".to_string(),
+                    Applicability::MaybeIncorrect,
+                );
+                err.emit();
+                return Ok(true);
+            }
+            (Ok(ref a), Ok(ref b)) if a.line == b.line => {
+                // 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);
+            }
+            _ if self.prev_span == syntax_pos::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, "unexpected token");
+            }
+            _ => {
+                err.span_label(sp, label_exp);
+                err.span_label(self.token.span, "unexpected token");
+            }
+        }
+        self.maybe_annotate_with_ascription(&mut err, false);
+        Err(err)
+    }
+
+    pub fn maybe_annotate_with_ascription(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        maybe_expected_semicolon: bool,
+    ) {
+        if let Some((sp, likely_path)) = self.last_type_ascription {
+            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
+                    },
+                );
+            } 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>`");
+                err.note("for more information, see \
+                          https://github.com/rust-lang/rust/issues/23416");
+            }
+        }
+    }
+
+    /// Eats and discards tokens until one of `kets` is encountered. Respects token trees,
+    /// passes through any errors encountered. Used for error recovery.
+    crate 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
+    /// ```
+    crate fn check_trailing_angle_brackets(&mut self, segment: &PathSegment, end: TokenKind) {
+        // 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;
+        }
+
+        // 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;
+        }
+
+        // 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);
+            *t == end
+        }) {
+            // 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.diagnostic()
+                .struct_span_err(
+                    span,
+                    &format!("unmatched angle bracket{}", pluralise!(total_num_of_gt)),
+                )
+                .span_suggestion(
+                    span,
+                    &format!("remove extra angle bracket{}", pluralise!(total_num_of_gt)),
+                    String::new(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        }
+    }
+
+    /// 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:
+    ///
+    ///           outer_op
+    ///           /   \
+    ///     inner_op   r2
+    ///        /  \
+    ///     l1    r1
+    crate fn check_no_chained_comparison(
+        &mut self,
+        lhs: &Expr,
+        outer_op: &AssocOp,
+    ) -> PResult<'a, Option<P<Expr>>> {
+        debug_assert!(
+            outer_op.is_comparison(),
+            "check_no_chained_comparison: {:?} is not comparison",
+            outer_op,
+        );
+
+        let mk_err_expr = |this: &Self, span| {
+            Ok(Some(this.mk_expr(span, ExprKind::Err, ThinVec::new())))
+        };
+
+        match lhs.kind {
+            ExprKind::Binary(op, _, _) if op.node.is_comparison() => {
+                // Respan to include both operators.
+                let op_span = op.span.to(self.prev_span);
+                let mut err = self.struct_span_err(
+                    op_span,
+                    "chained comparison operators require parentheses",
+                );
+
+                let suggest = |err: &mut DiagnosticBuilder<'_>| {
+                    err.span_suggestion_verbose(
+                        op_span.shrink_to_lo(),
+                        TURBOFISH,
+                        "::".to_string(),
+                        Applicability::MaybeIncorrect,
+                    );
+                };
+
+                if op.node == BinOpKind::Lt &&
+                    *outer_op == AssocOp::Less ||  // Include `<` to provide this recommendation
+                    *outer_op == AssocOp::Greater  // even in a case like the following:
+                {                                  //     Foo<Bar<Baz<Qux, ()>>>
+                    if *outer_op == 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.
+                            mem::replace(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, lhs.span.to(self.prev_span))
+                            }
+                            Err(mut expr_err) => {
+                                expr_err.cancel();
+                                // Not entirely sure now, but we bubble the error up with the
+                                // suggestion.
+                                mem::replace(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, lhs.span.to(self.prev_span))
+                            }
+                        }
+                    } else {
+                        // 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");
+                        // These cases cause too many knock-down errors, bail out (#61329).
+                        Err(err)
+                    };
+                }
+                err.emit();
+            }
+            _ => {}
+        }
+        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.
+            mem::replace(self, snapshot);
+            Err(())
+        } else {
+            // 99% certain that the suggestion is correct, continue parsing.
+            Ok(())
+        }
+    }
+
+    crate fn maybe_report_ambiguous_plus(
+        &mut self,
+        allow_plus: bool,
+        impl_dyn_multi: bool,
+        ty: &Ty,
+    ) {
+        if !allow_plus && 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: bool,
+        ty: &Ty,
+    ) -> PResult<'a, ()> {
+        // Do not add `+` to expected tokens.
+        if !allow_plus || !self.token.is_like_plus() {
+            return Ok(());
+        }
+
+        self.bump(); // `+`
+        let bounds = self.parse_generic_bounds(None)?;
+        let sum_span = ty.span.to(self.prev_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);
+                    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.
+    crate 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.
+    crate 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_span);
+
+        let ty_str = self
+            .span_to_snippet(ty_span)
+            .unwrap_or_else(|_| pprust::ty_to_string(&ty));
+        self.diagnostic()
+            .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,
+        )))
+    }
+
+    crate 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_span, "expected item, found `;`");
+            err.span_suggestion_short(
+                self.prev_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 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 = self.this_token_descr();
+        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_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_span, self.token.span),
+            _ => (self.sess.source_map().next_point(self.prev_span), 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();
+        match (sm.lookup_line(prev_sp.lo()), sm.lookup_line(sp.lo())) {
+            (Ok(ref a), Ok(ref b)) if a.line == b.line => {
+                // 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);
+            }
+            _ => {
+                err.span_label(prev_sp, label_exp);
+                err.span_label(sp, "unexpected token");
+            }
+        }
+        Err(err)
+    }
+
+    crate fn parse_semi_or_incorrect_foreign_fn_body(
+        &mut self,
+        ident: &Ident,
+        extern_sp: Span,
+    ) -> PResult<'a, ()> {
+        if self.token != token::Semi {
+            // This might be an incorrect fn definition (#62109).
+            let parser_snapshot = self.clone();
+            match self.parse_inner_attrs_and_block() {
+                Ok((_, body)) => {
+                    self.struct_span_err(ident.span, "incorrect `fn` inside `extern` block")
+                        .span_label(ident.span, "can't have a body")
+                        .span_label(body.span, "this body is invalid here")
+                        .span_label(
+                            extern_sp,
+                            "`extern` blocks define existing foreign functions and `fn`s \
+                             inside of them cannot have a body")
+                        .help("you might have meant to write a function accessible through ffi, \
+                               which can be done by writing `extern fn` outside of the \
+                               `extern` block")
+                        .note("for more information, visit \
+                               https://doc.rust-lang.org/std/keyword.extern.html")
+                        .emit();
+                }
+                Err(mut err) => {
+                    err.cancel();
+                    mem::replace(self, parser_snapshot);
+                    self.expect(&token::Semi)?;
+                }
+            }
+        } else {
+            self.bump();
+        }
+        Ok(())
+    }
+
+    /// Consumes alternative await syntaxes like `await!(<expr>)`, `await <expr>`,
+    /// `await? <expr>`, `await(<expr>)`, and `await { <expr> }`.
+    crate fn parse_incorrect_await_syntax(
+        &mut self,
+        lo: Span,
+        await_sp: Span,
+    ) -> PResult<'a, (Span, ExprKind)> {
+        if self.token == token::Not {
+            // Handle `await!(<expr>)`.
+            self.expect(&token::Not)?;
+            self.expect(&token::OpenDelim(token::Paren))?;
+            let expr = self.parse_expr()?;
+            self.expect(&token::CloseDelim(token::Paren))?;
+            let sp = self.error_on_incorrect_await(lo, self.prev_span, &expr, false);
+            return Ok((sp, ExprKind::Await(expr)))
+        }
+
+        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,
+                ThinVec::new(),
+            )
+        } else {
+            self.parse_expr()
+        }.map_err(|mut err| {
+            err.span_label(await_sp, "while parsing this incorrect await expression");
+            err
+        })?;
+        let sp = self.error_on_incorrect_await(lo, expr.span, &expr, is_question);
+        Ok((sp, ExprKind::Await(expr)))
+    }
+
+    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.
+    crate 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()
+        }
+    }
+
+    /// Recovers a situation like `for ( $pat in $expr )`
+    /// and suggest writing `for $pat in $expr` instead.
+    ///
+    /// This should be called before parsing the `$block`.
+    crate 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_span, "unexpected closing `)`")
+                    .span_label(begin_par_sp, "opening `(`")
+                    .span_suggestion(
+                        begin_par_sp.to(self.prev_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,
+        }
+    }
+
+    crate fn could_ascription_be_path(&self, node: &ast::ExprKind) -> bool {
+        self.token.is_ident() &&
+            if let ast::ExprKind::Path(..) = node { true } else { 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::Lt) &&     // `foo:bar<baz`
+            self.look_ahead(2, |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>`
+    }
+
+    crate 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.
+                self.consume_block(delim);
+                self.mk_expr(lo.to(self.prev_span), ExprKind::Err, ThinVec::new())
+            }
+        }
+    }
+
+    crate 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));
+
+                // 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");
+                }
+                err.span_suggestion_short(
+                    self.sess.source_map().next_point(self.prev_span),
+                    &format!("{} may belong here", delim.to_string()),
+                    delim.to_string(),
+                    Applicability::MaybeIncorrect,
+                );
+                err.emit();
+                self.expected_tokens.clear();  // reduce errors
+                Ok(true)
+            }
+            _ => Err(err),
+        }
+    }
+
+    /// Recovers from `pub` keyword in places where it seems _reasonable_ but isn't valid.
+    crate fn eat_bad_pub(&mut self) {
+        if self.token.is_keyword(kw::Pub) {
+            match self.parse_visibility(false) {
+                Ok(vis) => {
+                    self.diagnostic()
+                        .struct_span_err(vis.span, "unnecessary visibility qualifier")
+                        .span_label(vis.span, "`pub` not permitted here")
+                        .emit();
+                }
+                Err(mut err) => err.emit(),
+            }
+        }
+    }
+
+    /// 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).
+    crate 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.
+    crate 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()
+                }
+            }
+        }
+    }
+
+    crate 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_span, "expected iterable, found keyword `in`")
+                .span_suggestion_short(
+                    in_span.until(self.prev_span),
+                    "remove the duplicated `in`",
+                    String::new(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+        }
+    }
+
+    crate fn expected_semi_or_open_brace<T>(&mut self) -> PResult<'a, T> {
+        let token_str = self.this_token_descr();
+        let mut err = self.fatal(&format!("expected `;` or `{{`, found {}", token_str));
+        err.span_label(self.token.span, "expected `;` or `{`");
+        Err(err)
+    }
+
+    crate 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();
+        }
+    }
+
+    crate fn parameter_without_type(
+        &mut self,
+        err: &mut DiagnosticBuilder<'_>,
+        pat: P<ast::Pat>,
+        require_name: bool,
+        is_self_allowed: bool,
+        is_trait_item: 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 && (
+                is_trait_item ||
+                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 is_self_allowed {
+                    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 was 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
+    }
+
+    crate 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()?;
+
+        self.diagnostic()
+            .struct_span_err_with_code(
+                pat.span,
+                "patterns aren't allowed in methods without bodies",
+                DiagnosticId::Error("E0642".into()),
+            )
+            .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
+        });
+        Ok((pat, ty))
+    }
+
+    crate fn recover_bad_self_param(
+        &mut self,
+        mut param: ast::Param,
+        is_trait_item: bool,
+    ) -> PResult<'a, ast::Param> {
+        let sp = param.pat.span;
+        param.ty.kind = TyKind::Err;
+        let mut err = self.struct_span_err(sp, "unexpected `self` parameter in function");
+        if is_trait_item {
+            err.span_label(sp, "must be the first associated function parameter");
+        } else {
+            err.span_label(sp, "not valid as function parameter");
+            err.note("`self` is only valid as the first parameter of an associated function");
+        }
+        err.emit();
+        Ok(param)
+    }
+
+    crate fn consume_block(&mut self, delim: token::DelimToken) {
+        let mut brace_depth = 0;
+        loop {
+            if self.eat(&token::OpenDelim(delim)) {
+                brace_depth += 1;
+            } else if self.eat(&token::CloseDelim(delim)) {
+                if brace_depth == 0 {
+                    return;
+                } else {
+                    brace_depth -= 1;
+                    continue;
+                }
+            } else if self.token == token::Eof || self.eat(&token::CloseDelim(token::NoDelim)) {
+                return;
+            } else {
+                self.bump();
+            }
+        }
+    }
+
+    crate 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 {}",
+                self.this_token_descr(),
+            )),
+        };
+        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 unecessary 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
+    /// unecessary 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.
+    crate 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/src/libsyntax/parse/parser/expr.rs b/src/libsyntax/parse/parser/expr.rs
index dd0fd834fb0..6e52a07c61d 100644
--- a/src/libsyntax/parse/parser/expr.rs
+++ b/src/libsyntax/parse/parser/expr.rs
@@ -1,6 +1,7 @@
 use super::{Parser, PResult, Restrictions, PrevTokenKind, TokenType, PathStyle, BlockMode};
 use super::{SemiColonMode, SeqSep, TokenExpectType};
 use super::pat::{GateOr, PARAM_EXPECTED};
+use super::diagnostics::Error;
 
 use crate::parse::literal::LitError;
 
@@ -12,7 +13,6 @@ use crate::ast::{
 use crate::maybe_recover_from_interpolated_ty_qpath;
 use crate::parse::classify;
 use crate::parse::token::{self, Token, TokenKind};
-use crate::parse::diagnostics::Error;
 use crate::print::pprust;
 use crate::ptr::P;
 use crate::source_map::{self, Span};
diff --git a/src/libsyntax/parse/parser/item.rs b/src/libsyntax/parse/parser/item.rs
index 08c624b5539..90517d4d770 100644
--- a/src/libsyntax/parse/parser/item.rs
+++ b/src/libsyntax/parse/parser/item.rs
@@ -1,4 +1,6 @@
 use super::{Parser, PResult, PathStyle, SemiColonMode, BlockMode};
+use super::diagnostics::{Error, dummy_arg};
+
 use crate::maybe_whole;
 use crate::ptr::P;
 use crate::ast::{self, DUMMY_NODE_ID, Ident, Attribute, AttrStyle, AnonConst, Item, ItemKind};
@@ -10,7 +12,6 @@ use crate::ast::{Mac, MacDelimiter, Block, BindingMode, FnDecl, MethodSig, SelfK
 use crate::ext::base::DummyResult;
 use crate::parse::token;
 use crate::parse::parser::maybe_append;
-use crate::parse::diagnostics::{Error, dummy_arg};
 use crate::tokenstream::{TokenTree, TokenStream};
 use crate::symbol::{kw, sym};
 use crate::source_map::{self, respan, Span};
diff --git a/src/libsyntax/parse/parser/module.rs b/src/libsyntax/parse/parser/module.rs
index 2d2fb487d7d..20bd189cf8f 100644
--- a/src/libsyntax/parse/parser/module.rs
+++ b/src/libsyntax/parse/parser/module.rs
@@ -1,11 +1,11 @@
 use super::{Parser, PResult};
 use super::item::ItemInfo;
+use super::diagnostics::Error;
 
 use crate::attr;
 use crate::ast::{self, Ident, Attribute, ItemKind, Mod, Crate};
 use crate::parse::{new_sub_parser_from_file, DirectoryOwnership};
 use crate::parse::token::{self, TokenKind};
-use crate::parse::diagnostics::{Error};
 use crate::source_map::{SourceMap, Span, DUMMY_SP, FileName};
 use crate::symbol::sym;
 
diff --git a/src/libsyntax/parse/parser/stmt.rs b/src/libsyntax/parse/parser/stmt.rs
index 855b03ddd6f..c8d6b6ff9dd 100644
--- a/src/libsyntax/parse/parser/stmt.rs
+++ b/src/libsyntax/parse/parser/stmt.rs
@@ -2,6 +2,7 @@ use super::{Parser, PResult, Restrictions, PrevTokenKind, SemiColonMode, BlockMo
 use super::expr::LhsExpr;
 use super::path::PathStyle;
 use super::pat::GateOr;
+use super::diagnostics::Error;
 
 use crate::ptr::P;
 use crate::{maybe_whole, ThinVec};
@@ -9,7 +10,6 @@ use crate::ast::{self, DUMMY_NODE_ID, Stmt, StmtKind, Local, Block, BlockCheckMo
 use crate::ast::{Attribute, AttrStyle, VisibilityKind, MacStmtStyle, Mac, MacDelimiter};
 use crate::ext::base::DummyResult;
 use crate::parse::{classify, DirectoryOwnership};
-use crate::parse::diagnostics::Error;
 use crate::parse::token;
 use crate::source_map::{respan, Span};
 use crate::symbol::{kw, sym};