mv compiler to compiler/

2 files changed, 1125 insertions, 0 deletions

diff --git a/compiler/rustc_parse_format/src/lib.rs b/compiler/rustc_parse_format/src/lib.rs
new file mode 100644
index 00000000000..e07b8b86aef
--- /dev/null
+++ b/compiler/rustc_parse_format/src/lib.rs
@@ -0,0 +1,829 @@
+//! Macro support for format strings
+//!
+//! These structures are used when parsing format strings for the compiler.
+//! Parsing does not happen at runtime: structures of `std::fmt::rt` are
+//! generated instead.
+
+#![doc(
+    html_root_url = "https://doc.rust-lang.org/nightly/",
+    html_playground_url = "https://play.rust-lang.org/",
+    test(attr(deny(warnings)))
+)]
+#![feature(nll)]
+#![feature(or_patterns)]
+#![feature(rustc_private)]
+#![feature(unicode_internals)]
+#![feature(bool_to_option)]
+
+pub use Alignment::*;
+pub use Count::*;
+pub use Flag::*;
+pub use Piece::*;
+pub use Position::*;
+
+use std::iter;
+use std::str;
+use std::string;
+
+use rustc_span::{InnerSpan, Symbol};
+
+/// The type of format string that we are parsing.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum ParseMode {
+    /// A normal format string as per `format_args!`.
+    Format,
+    /// An inline assembly template string for `asm!`.
+    InlineAsm,
+}
+
+#[derive(Copy, Clone)]
+struct InnerOffset(usize);
+
+impl InnerOffset {
+    fn to(self, end: InnerOffset) -> InnerSpan {
+        InnerSpan::new(self.0, end.0)
+    }
+}
+
+/// A piece is a portion of the format string which represents the next part
+/// to emit. These are emitted as a stream by the `Parser` class.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Piece<'a> {
+    /// A literal string which should directly be emitted
+    String(&'a str),
+    /// This describes that formatting should process the next argument (as
+    /// specified inside) for emission.
+    NextArgument(Argument<'a>),
+}
+
+/// Representation of an argument specification.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Argument<'a> {
+    /// Where to find this argument
+    pub position: Position,
+    /// How to format the argument
+    pub format: FormatSpec<'a>,
+}
+
+/// Specification for the formatting of an argument in the format string.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct FormatSpec<'a> {
+    /// Optionally specified character to fill alignment with.
+    pub fill: Option<char>,
+    /// Optionally specified alignment.
+    pub align: Alignment,
+    /// Packed version of various flags provided.
+    pub flags: u32,
+    /// The integer precision to use.
+    pub precision: Count,
+    /// The span of the precision formatting flag (for diagnostics).
+    pub precision_span: Option<InnerSpan>,
+    /// The string width requested for the resulting format.
+    pub width: Count,
+    /// The span of the width formatting flag (for diagnostics).
+    pub width_span: Option<InnerSpan>,
+    /// The descriptor string representing the name of the format desired for
+    /// this argument, this can be empty or any number of characters, although
+    /// it is required to be one word.
+    pub ty: &'a str,
+    /// The span of the descriptor string (for diagnostics).
+    pub ty_span: Option<InnerSpan>,
+}
+
+/// Enum describing where an argument for a format can be located.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Position {
+    /// The argument is implied to be located at an index
+    ArgumentImplicitlyIs(usize),
+    /// The argument is located at a specific index given in the format
+    ArgumentIs(usize),
+    /// The argument has a name.
+    ArgumentNamed(Symbol),
+}
+
+impl Position {
+    pub fn index(&self) -> Option<usize> {
+        match self {
+            ArgumentIs(i) | ArgumentImplicitlyIs(i) => Some(*i),
+            _ => None,
+        }
+    }
+}
+
+/// Enum of alignments which are supported.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Alignment {
+    /// The value will be aligned to the left.
+    AlignLeft,
+    /// The value will be aligned to the right.
+    AlignRight,
+    /// The value will be aligned in the center.
+    AlignCenter,
+    /// The value will take on a default alignment.
+    AlignUnknown,
+}
+
+/// Various flags which can be applied to format strings. The meaning of these
+/// flags is defined by the formatters themselves.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Flag {
+    /// A `+` will be used to denote positive numbers.
+    FlagSignPlus,
+    /// A `-` will be used to denote negative numbers. This is the default.
+    FlagSignMinus,
+    /// An alternate form will be used for the value. In the case of numbers,
+    /// this means that the number will be prefixed with the supplied string.
+    FlagAlternate,
+    /// For numbers, this means that the number will be padded with zeroes,
+    /// and the sign (`+` or `-`) will precede them.
+    FlagSignAwareZeroPad,
+    /// For Debug / `?`, format integers in lower-case hexadecimal.
+    FlagDebugLowerHex,
+    /// For Debug / `?`, format integers in upper-case hexadecimal.
+    FlagDebugUpperHex,
+}
+
+/// A count is used for the precision and width parameters of an integer, and
+/// can reference either an argument or a literal integer.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Count {
+    /// The count is specified explicitly.
+    CountIs(usize),
+    /// The count is specified by the argument with the given name.
+    CountIsName(Symbol),
+    /// The count is specified by the argument at the given index.
+    CountIsParam(usize),
+    /// The count is implied and cannot be explicitly specified.
+    CountImplied,
+}
+
+pub struct ParseError {
+    pub description: string::String,
+    pub note: Option<string::String>,
+    pub label: string::String,
+    pub span: InnerSpan,
+    pub secondary_label: Option<(string::String, InnerSpan)>,
+}
+
+/// The parser structure for interpreting the input format string. This is
+/// modeled as an iterator over `Piece` structures to form a stream of tokens
+/// being output.
+///
+/// This is a recursive-descent parser for the sake of simplicity, and if
+/// necessary there's probably lots of room for improvement performance-wise.
+pub struct Parser<'a> {
+    mode: ParseMode,
+    input: &'a str,
+    cur: iter::Peekable<str::CharIndices<'a>>,
+    /// Error messages accumulated during parsing
+    pub errors: Vec<ParseError>,
+    /// Current position of implicit positional argument pointer
+    pub curarg: usize,
+    /// `Some(raw count)` when the string is "raw", used to position spans correctly
+    style: Option<usize>,
+    /// Start and end byte offset of every successfully parsed argument
+    pub arg_places: Vec<InnerSpan>,
+    /// Characters that need to be shifted
+    skips: Vec<usize>,
+    /// Span of the last opening brace seen, used for error reporting
+    last_opening_brace: Option<InnerSpan>,
+    /// Whether the source string is comes from `println!` as opposed to `format!` or `print!`
+    append_newline: bool,
+    /// Whether this formatting string is a literal or it comes from a macro.
+    pub is_literal: bool,
+    /// Start position of the current line.
+    cur_line_start: usize,
+    /// Start and end byte offset of every line of the format string. Excludes
+    /// newline characters and leading whitespace.
+    pub line_spans: Vec<InnerSpan>,
+}
+
+impl<'a> Iterator for Parser<'a> {
+    type Item = Piece<'a>;
+
+    fn next(&mut self) -> Option<Piece<'a>> {
+        if let Some(&(pos, c)) = self.cur.peek() {
+            match c {
+                '{' => {
+                    let curr_last_brace = self.last_opening_brace;
+                    let byte_pos = self.to_span_index(pos);
+                    self.last_opening_brace = Some(byte_pos.to(InnerOffset(byte_pos.0 + 1)));
+                    self.cur.next();
+                    if self.consume('{') {
+                        self.last_opening_brace = curr_last_brace;
+
+                        Some(String(self.string(pos + 1)))
+                    } else {
+                        let arg = self.argument();
+                        if let Some(end) = self.must_consume('}') {
+                            let start = self.to_span_index(pos);
+                            let end = self.to_span_index(end + 1);
+                            if self.is_literal {
+                                self.arg_places.push(start.to(end));
+                            }
+                        }
+                        Some(NextArgument(arg))
+                    }
+                }
+                '}' => {
+                    self.cur.next();
+                    if self.consume('}') {
+                        Some(String(self.string(pos + 1)))
+                    } else {
+                        let err_pos = self.to_span_index(pos);
+                        self.err_with_note(
+                            "unmatched `}` found",
+                            "unmatched `}`",
+                            "if you intended to print `}`, you can escape it using `}}`",
+                            err_pos.to(err_pos),
+                        );
+                        None
+                    }
+                }
+                _ => Some(String(self.string(pos))),
+            }
+        } else {
+            if self.is_literal {
+                let start = self.to_span_index(self.cur_line_start);
+                let end = self.to_span_index(self.input.len());
+                let span = start.to(end);
+                if self.line_spans.last() != Some(&span) {
+                    self.line_spans.push(span);
+                }
+            }
+            None
+        }
+    }
+}
+
+impl<'a> Parser<'a> {
+    /// Creates a new parser for the given format string
+    pub fn new(
+        s: &'a str,
+        style: Option<usize>,
+        snippet: Option<string::String>,
+        append_newline: bool,
+        mode: ParseMode,
+    ) -> Parser<'a> {
+        let (skips, is_literal) = find_skips_from_snippet(snippet, style);
+        Parser {
+            mode,
+            input: s,
+            cur: s.char_indices().peekable(),
+            errors: vec![],
+            curarg: 0,
+            style,
+            arg_places: vec![],
+            skips,
+            last_opening_brace: None,
+            append_newline,
+            is_literal,
+            cur_line_start: 0,
+            line_spans: vec![],
+        }
+    }
+
+    /// Notifies of an error. The message doesn't actually need to be of type
+    /// String, but I think it does when this eventually uses conditions so it
+    /// might as well start using it now.
+    fn err<S1: Into<string::String>, S2: Into<string::String>>(
+        &mut self,
+        description: S1,
+        label: S2,
+        span: InnerSpan,
+    ) {
+        self.errors.push(ParseError {
+            description: description.into(),
+            note: None,
+            label: label.into(),
+            span,
+            secondary_label: None,
+        });
+    }
+
+    /// Notifies of an error. The message doesn't actually need to be of type
+    /// String, but I think it does when this eventually uses conditions so it
+    /// might as well start using it now.
+    fn err_with_note<
+        S1: Into<string::String>,
+        S2: Into<string::String>,
+        S3: Into<string::String>,
+    >(
+        &mut self,
+        description: S1,
+        label: S2,
+        note: S3,
+        span: InnerSpan,
+    ) {
+        self.errors.push(ParseError {
+            description: description.into(),
+            note: Some(note.into()),
+            label: label.into(),
+            span,
+            secondary_label: None,
+        });
+    }
+
+    /// Optionally consumes the specified character. If the character is not at
+    /// the current position, then the current iterator isn't moved and `false` is
+    /// returned, otherwise the character is consumed and `true` is returned.
+    fn consume(&mut self, c: char) -> bool {
+        self.consume_pos(c).is_some()
+    }
+
+    /// Optionally consumes the specified character. If the character is not at
+    /// the current position, then the current iterator isn't moved and `None` is
+    /// returned, otherwise the character is consumed and the current position is
+    /// returned.
+    fn consume_pos(&mut self, c: char) -> Option<usize> {
+        if let Some(&(pos, maybe)) = self.cur.peek() {
+            if c == maybe {
+                self.cur.next();
+                return Some(pos);
+            }
+        }
+        None
+    }
+
+    fn to_span_index(&self, pos: usize) -> InnerOffset {
+        let mut pos = pos;
+        // This handles the raw string case, the raw argument is the number of #
+        // in r###"..."### (we need to add one because of the `r`).
+        let raw = self.style.map(|raw| raw + 1).unwrap_or(0);
+        for skip in &self.skips {
+            if pos > *skip {
+                pos += 1;
+            } else if pos == *skip && raw == 0 {
+                pos += 1;
+            } else {
+                break;
+            }
+        }
+        InnerOffset(raw + pos + 1)
+    }
+
+    /// Forces consumption of the specified character. If the character is not
+    /// found, an error is emitted.
+    fn must_consume(&mut self, c: char) -> Option<usize> {
+        self.ws();
+
+        if let Some(&(pos, maybe)) = self.cur.peek() {
+            if c == maybe {
+                self.cur.next();
+                Some(pos)
+            } else {
+                let pos = self.to_span_index(pos);
+                let description = format!("expected `'}}'`, found `{:?}`", maybe);
+                let label = "expected `}`".to_owned();
+                let (note, secondary_label) = if c == '}' {
+                    (
+                        Some(
+                            "if you intended to print `{`, you can escape it using `{{`".to_owned(),
+                        ),
+                        self.last_opening_brace
+                            .map(|sp| ("because of this opening brace".to_owned(), sp)),
+                    )
+                } else {
+                    (None, None)
+                };
+                self.errors.push(ParseError {
+                    description,
+                    note,
+                    label,
+                    span: pos.to(pos),
+                    secondary_label,
+                });
+                None
+            }
+        } else {
+            let description = format!("expected `{:?}` but string was terminated", c);
+            // point at closing `"`
+            let pos = self.input.len() - if self.append_newline { 1 } else { 0 };
+            let pos = self.to_span_index(pos);
+            if c == '}' {
+                let label = format!("expected `{:?}`", c);
+                let (note, secondary_label) = if c == '}' {
+                    (
+                        Some(
+                            "if you intended to print `{`, you can escape it using `{{`".to_owned(),
+                        ),
+                        self.last_opening_brace
+                            .map(|sp| ("because of this opening brace".to_owned(), sp)),
+                    )
+                } else {
+                    (None, None)
+                };
+                self.errors.push(ParseError {
+                    description,
+                    note,
+                    label,
+                    span: pos.to(pos),
+                    secondary_label,
+                });
+            } else {
+                self.err(description, format!("expected `{:?}`", c), pos.to(pos));
+            }
+            None
+        }
+    }
+
+    /// Consumes all whitespace characters until the first non-whitespace character
+    fn ws(&mut self) {
+        while let Some(&(_, c)) = self.cur.peek() {
+            if c.is_whitespace() {
+                self.cur.next();
+            } else {
+                break;
+            }
+        }
+    }
+
+    /// Parses all of a string which is to be considered a "raw literal" in a
+    /// format string. This is everything outside of the braces.
+    fn string(&mut self, start: usize) -> &'a str {
+        // we may not consume the character, peek the iterator
+        while let Some(&(pos, c)) = self.cur.peek() {
+            match c {
+                '{' | '}' => {
+                    return &self.input[start..pos];
+                }
+                '\n' if self.is_literal => {
+                    let start = self.to_span_index(self.cur_line_start);
+                    let end = self.to_span_index(pos);
+                    self.line_spans.push(start.to(end));
+                    self.cur_line_start = pos + 1;
+                    self.cur.next();
+                }
+                _ => {
+                    if self.is_literal && pos == self.cur_line_start && c.is_whitespace() {
+                        self.cur_line_start = pos + c.len_utf8();
+                    }
+                    self.cur.next();
+                }
+            }
+        }
+        &self.input[start..self.input.len()]
+    }
+
+    /// Parses an `Argument` structure, or what's contained within braces inside the format string.
+    fn argument(&mut self) -> Argument<'a> {
+        let pos = self.position();
+        let format = match self.mode {
+            ParseMode::Format => self.format(),
+            ParseMode::InlineAsm => self.inline_asm(),
+        };
+
+        // Resolve position after parsing format spec.
+        let pos = match pos {
+            Some(position) => position,
+            None => {
+                let i = self.curarg;
+                self.curarg += 1;
+                ArgumentImplicitlyIs(i)
+            }
+        };
+
+        Argument { position: pos, format }
+    }
+
+    /// Parses a positional argument for a format. This could either be an
+    /// integer index of an argument, a named argument, or a blank string.
+    /// Returns `Some(parsed_position)` if the position is not implicitly
+    /// consuming a macro argument, `None` if it's the case.
+    fn position(&mut self) -> Option<Position> {
+        if let Some(i) = self.integer() {
+            Some(ArgumentIs(i))
+        } else {
+            match self.cur.peek() {
+                Some(&(_, c)) if rustc_lexer::is_id_start(c) => {
+                    Some(ArgumentNamed(Symbol::intern(self.word())))
+                }
+
+                // This is an `ArgumentNext`.
+                // Record the fact and do the resolution after parsing the
+                // format spec, to make things like `{:.*}` work.
+                _ => None,
+            }
+        }
+    }
+
+    /// Parses a format specifier at the current position, returning all of the
+    /// relevant information in the `FormatSpec` struct.
+    fn format(&mut self) -> FormatSpec<'a> {
+        let mut spec = FormatSpec {
+            fill: None,
+            align: AlignUnknown,
+            flags: 0,
+            precision: CountImplied,
+            precision_span: None,
+            width: CountImplied,
+            width_span: None,
+            ty: &self.input[..0],
+            ty_span: None,
+        };
+        if !self.consume(':') {
+            return spec;
+        }
+
+        // fill character
+        if let Some(&(_, c)) = self.cur.peek() {
+            match self.cur.clone().nth(1) {
+                Some((_, '>' | '<' | '^')) => {
+                    spec.fill = Some(c);
+                    self.cur.next();
+                }
+                _ => {}
+            }
+        }
+        // Alignment
+        if self.consume('<') {
+            spec.align = AlignLeft;
+        } else if self.consume('>') {
+            spec.align = AlignRight;
+        } else if self.consume('^') {
+            spec.align = AlignCenter;
+        }
+        // Sign flags
+        if self.consume('+') {
+            spec.flags |= 1 << (FlagSignPlus as u32);
+        } else if self.consume('-') {
+            spec.flags |= 1 << (FlagSignMinus as u32);
+        }
+        // Alternate marker
+        if self.consume('#') {
+            spec.flags |= 1 << (FlagAlternate as u32);
+        }
+        // Width and precision
+        let mut havewidth = false;
+
+        if self.consume('0') {
+            // small ambiguity with '0$' as a format string. In theory this is a
+            // '0' flag and then an ill-formatted format string with just a '$'
+            // and no count, but this is better if we instead interpret this as
+            // no '0' flag and '0$' as the width instead.
+            if self.consume('$') {
+                spec.width = CountIsParam(0);
+                havewidth = true;
+            } else {
+                spec.flags |= 1 << (FlagSignAwareZeroPad as u32);
+            }
+        }
+        if !havewidth {
+            let width_span_start = if let Some((pos, _)) = self.cur.peek() { *pos } else { 0 };
+            let (w, sp) = self.count(width_span_start);
+            spec.width = w;
+            spec.width_span = sp;
+        }
+        if let Some(start) = self.consume_pos('.') {
+            if let Some(end) = self.consume_pos('*') {
+                // Resolve `CountIsNextParam`.
+                // We can do this immediately as `position` is resolved later.
+                let i = self.curarg;
+                self.curarg += 1;
+                spec.precision = CountIsParam(i);
+                spec.precision_span =
+                    Some(self.to_span_index(start).to(self.to_span_index(end + 1)));
+            } else {
+                let (p, sp) = self.count(start);
+                spec.precision = p;
+                spec.precision_span = sp;
+            }
+        }
+        let ty_span_start = self.cur.peek().map(|(pos, _)| *pos);
+        // Optional radix followed by the actual format specifier
+        if self.consume('x') {
+            if self.consume('?') {
+                spec.flags |= 1 << (FlagDebugLowerHex as u32);
+                spec.ty = "?";
+            } else {
+                spec.ty = "x";
+            }
+        } else if self.consume('X') {
+            if self.consume('?') {
+                spec.flags |= 1 << (FlagDebugUpperHex as u32);
+                spec.ty = "?";
+            } else {
+                spec.ty = "X";
+            }
+        } else if self.consume('?') {
+            spec.ty = "?";
+        } else {
+            spec.ty = self.word();
+            let ty_span_end = self.cur.peek().map(|(pos, _)| *pos);
+            if !spec.ty.is_empty() {
+                spec.ty_span = ty_span_start
+                    .and_then(|s| ty_span_end.map(|e| (s, e)))
+                    .map(|(start, end)| self.to_span_index(start).to(self.to_span_index(end)));
+            }
+        }
+        spec
+    }
+
+    /// Parses an inline assembly template modifier at the current position, returning the modifier
+    /// in the `ty` field of the `FormatSpec` struct.
+    fn inline_asm(&mut self) -> FormatSpec<'a> {
+        let mut spec = FormatSpec {
+            fill: None,
+            align: AlignUnknown,
+            flags: 0,
+            precision: CountImplied,
+            precision_span: None,
+            width: CountImplied,
+            width_span: None,
+            ty: &self.input[..0],
+            ty_span: None,
+        };
+        if !self.consume(':') {
+            return spec;
+        }
+
+        let ty_span_start = self.cur.peek().map(|(pos, _)| *pos);
+        spec.ty = self.word();
+        let ty_span_end = self.cur.peek().map(|(pos, _)| *pos);
+        if !spec.ty.is_empty() {
+            spec.ty_span = ty_span_start
+                .and_then(|s| ty_span_end.map(|e| (s, e)))
+                .map(|(start, end)| self.to_span_index(start).to(self.to_span_index(end)));
+        }
+
+        spec
+    }
+
+    /// Parses a `Count` parameter at the current position. This does not check
+    /// for 'CountIsNextParam' because that is only used in precision, not
+    /// width.
+    fn count(&mut self, start: usize) -> (Count, Option<InnerSpan>) {
+        if let Some(i) = self.integer() {
+            if let Some(end) = self.consume_pos('$') {
+                let span = self.to_span_index(start).to(self.to_span_index(end + 1));
+                (CountIsParam(i), Some(span))
+            } else {
+                (CountIs(i), None)
+            }
+        } else {
+            let tmp = self.cur.clone();
+            let word = self.word();
+            if word.is_empty() {
+                self.cur = tmp;
+                (CountImplied, None)
+            } else if self.consume('$') {
+                (CountIsName(Symbol::intern(word)), None)
+            } else {
+                self.cur = tmp;
+                (CountImplied, None)
+            }
+        }
+    }
+
+    /// Parses a word starting at the current position. A word is the same as
+    /// Rust identifier, except that it can't start with `_` character.
+    fn word(&mut self) -> &'a str {
+        let start = match self.cur.peek() {
+            Some(&(pos, c)) if rustc_lexer::is_id_start(c) => {
+                self.cur.next();
+                pos
+            }
+            _ => {
+                return "";
+            }
+        };
+        let mut end = None;
+        while let Some(&(pos, c)) = self.cur.peek() {
+            if rustc_lexer::is_id_continue(c) {
+                self.cur.next();
+            } else {
+                end = Some(pos);
+                break;
+            }
+        }
+        let end = end.unwrap_or(self.input.len());
+        let word = &self.input[start..end];
+        if word == "_" {
+            self.err_with_note(
+                "invalid argument name `_`",
+                "invalid argument name",
+                "argument name cannot be a single underscore",
+                self.to_span_index(start).to(self.to_span_index(end)),
+            );
+        }
+        word
+    }
+
+    /// Optionally parses an integer at the current position. This doesn't deal
+    /// with overflow at all, it's just accumulating digits.
+    fn integer(&mut self) -> Option<usize> {
+        let mut cur = 0;
+        let mut found = false;
+        while let Some(&(_, c)) = self.cur.peek() {
+            if let Some(i) = c.to_digit(10) {
+                cur = cur * 10 + i as usize;
+                found = true;
+                self.cur.next();
+            } else {
+                break;
+            }
+        }
+        found.then_some(cur)
+    }
+}
+
+/// Finds the indices of all characters that have been processed and differ between the actual
+/// written code (code snippet) and the `InternedString` that gets processed in the `Parser`
+/// in order to properly synthethise the intra-string `Span`s for error diagnostics.
+fn find_skips_from_snippet(
+    snippet: Option<string::String>,
+    str_style: Option<usize>,
+) -> (Vec<usize>, bool) {
+    let snippet = match snippet {
+        Some(ref s) if s.starts_with('"') || s.starts_with("r#") => s,
+        _ => return (vec![], false),
+    };
+
+    fn find_skips(snippet: &str, is_raw: bool) -> Vec<usize> {
+        let mut eat_ws = false;
+        let mut s = snippet.chars().enumerate().peekable();
+        let mut skips = vec![];
+        while let Some((pos, c)) = s.next() {
+            match (c, s.peek()) {
+                // skip whitespace and empty lines ending in '\\'
+                ('\\', Some((next_pos, '\n'))) if !is_raw => {
+                    eat_ws = true;
+                    skips.push(pos);
+                    skips.push(*next_pos);
+                    let _ = s.next();
+                }
+                ('\\', Some((next_pos, '\n' | 'n' | 't'))) if eat_ws => {
+                    skips.push(pos);
+                    skips.push(*next_pos);
+                    let _ = s.next();
+                }
+                (' ' | '\n' | '\t', _) if eat_ws => {
+                    skips.push(pos);
+                }
+                ('\\', Some((next_pos, 'n' | 't' | 'r' | '0' | '\\' | '\'' | '\"'))) => {
+                    skips.push(*next_pos);
+                    let _ = s.next();
+                }
+                ('\\', Some((_, 'x'))) if !is_raw => {
+                    for _ in 0..3 {
+                        // consume `\xAB` literal
+                        if let Some((pos, _)) = s.next() {
+                            skips.push(pos);
+                        } else {
+                            break;
+                        }
+                    }
+                }
+                ('\\', Some((_, 'u'))) if !is_raw => {
+                    if let Some((pos, _)) = s.next() {
+                        skips.push(pos);
+                    }
+                    if let Some((next_pos, next_c)) = s.next() {
+                        if next_c == '{' {
+                            skips.push(next_pos);
+                            let mut i = 0; // consume up to 6 hexanumeric chars + closing `}`
+                            while let (Some((next_pos, c)), true) = (s.next(), i < 7) {
+                                if c.is_digit(16) {
+                                    skips.push(next_pos);
+                                } else if c == '}' {
+                                    skips.push(next_pos);
+                                    break;
+                                } else {
+                                    break;
+                                }
+                                i += 1;
+                            }
+                        } else if next_c.is_digit(16) {
+                            skips.push(next_pos);
+                            // We suggest adding `{` and `}` when appropriate, accept it here as if
+                            // it were correct
+                            let mut i = 0; // consume up to 6 hexanumeric chars
+                            while let (Some((next_pos, c)), _) = (s.next(), i < 6) {
+                                if c.is_digit(16) {
+                                    skips.push(next_pos);
+                                } else {
+                                    break;
+                                }
+                                i += 1;
+                            }
+                        }
+                    }
+                }
+                _ if eat_ws => {
+                    // `take_while(|c| c.is_whitespace())`
+                    eat_ws = false;
+                }
+                _ => {}
+            }
+        }
+        skips
+    }
+
+    let r_start = str_style.map(|r| r + 1).unwrap_or(0);
+    let r_end = str_style.unwrap_or(0);
+    let s = &snippet[r_start + 1..snippet.len() - r_end - 1];
+    (find_skips(s, str_style.is_some()), true)
+}
+
+#[cfg(test)]
+mod tests;
diff --git a/compiler/rustc_parse_format/src/tests.rs b/compiler/rustc_parse_format/src/tests.rs
new file mode 100644
index 00000000000..9fd0497fffe
--- /dev/null
+++ b/compiler/rustc_parse_format/src/tests.rs
@@ -0,0 +1,296 @@
+use super::*;
+
+fn same(fmt: &'static str, p: &[Piece<'static>]) {
+    let parser = Parser::new(fmt, None, None, false, ParseMode::Format);
+    assert_eq!(parser.collect::<Vec<Piece<'static>>>(), p);
+}
+
+fn fmtdflt() -> FormatSpec<'static> {
+    return FormatSpec {
+        fill: None,
+        align: AlignUnknown,
+        flags: 0,
+        precision: CountImplied,
+        width: CountImplied,
+        precision_span: None,
+        width_span: None,
+        ty: "",
+        ty_span: None,
+    };
+}
+
+fn musterr(s: &str) {
+    let mut p = Parser::new(s, None, None, false, ParseMode::Format);
+    p.next();
+    assert!(!p.errors.is_empty());
+}
+
+#[test]
+fn simple() {
+    same("asdf", &[String("asdf")]);
+    same("a{{b", &[String("a"), String("{b")]);
+    same("a}}b", &[String("a"), String("}b")]);
+    same("a}}", &[String("a"), String("}")]);
+    same("}}", &[String("}")]);
+    same("\\}}", &[String("\\"), String("}")]);
+}
+
+#[test]
+fn invalid01() {
+    musterr("{")
+}
+#[test]
+fn invalid02() {
+    musterr("}")
+}
+#[test]
+fn invalid04() {
+    musterr("{3a}")
+}
+#[test]
+fn invalid05() {
+    musterr("{:|}")
+}
+#[test]
+fn invalid06() {
+    musterr("{:>>>}")
+}
+
+#[test]
+fn format_nothing() {
+    same("{}", &[NextArgument(Argument { position: ArgumentImplicitlyIs(0), format: fmtdflt() })]);
+}
+#[test]
+fn format_position() {
+    same("{3}", &[NextArgument(Argument { position: ArgumentIs(3), format: fmtdflt() })]);
+}
+#[test]
+fn format_position_nothing_else() {
+    same("{3:}", &[NextArgument(Argument { position: ArgumentIs(3), format: fmtdflt() })]);
+}
+#[test]
+fn format_type() {
+    same(
+        "{3:x}",
+        &[NextArgument(Argument {
+            position: ArgumentIs(3),
+            format: FormatSpec {
+                fill: None,
+                align: AlignUnknown,
+                flags: 0,
+                precision: CountImplied,
+                width: CountImplied,
+                precision_span: None,
+                width_span: None,
+                ty: "x",
+                ty_span: None,
+            },
+        })],
+    );
+}
+#[test]
+fn format_align_fill() {
+    same(
+        "{3:>}",
+        &[NextArgument(Argument {
+            position: ArgumentIs(3),
+            format: FormatSpec {
+                fill: None,
+                align: AlignRight,
+                flags: 0,
+                precision: CountImplied,
+                width: CountImplied,
+                precision_span: None,
+                width_span: None,
+                ty: "",
+                ty_span: None,
+            },
+        })],
+    );
+    same(
+        "{3:0<}",
+        &[NextArgument(Argument {
+            position: ArgumentIs(3),
+            format: FormatSpec {
+                fill: Some('0'),
+                align: AlignLeft,
+                flags: 0,
+                precision: CountImplied,
+                width: CountImplied,
+                precision_span: None,
+                width_span: None,
+                ty: "",
+                ty_span: None,
+            },
+        })],
+    );
+    same(
+        "{3:*<abcd}",
+        &[NextArgument(Argument {
+            position: ArgumentIs(3),
+            format: FormatSpec {
+                fill: Some('*'),
+                align: AlignLeft,
+                flags: 0,
+                precision: CountImplied,
+                width: CountImplied,
+                precision_span: None,
+                width_span: None,
+                ty: "abcd",
+                ty_span: Some(InnerSpan::new(6, 10)),
+            },
+        })],
+    );
+}
+#[test]
+fn format_counts() {
+    use rustc_span::{edition, SessionGlobals, SESSION_GLOBALS};
+    SESSION_GLOBALS.set(&SessionGlobals::new(edition::DEFAULT_EDITION), || {
+        same(
+            "{:10x}",
+            &[NextArgument(Argument {
+                position: ArgumentImplicitlyIs(0),
+                format: FormatSpec {
+                    fill: None,
+                    align: AlignUnknown,
+                    flags: 0,
+                    precision: CountImplied,
+                    width: CountIs(10),
+                    precision_span: None,
+                    width_span: None,
+                    ty: "x",
+                    ty_span: None,
+                },
+            })],
+        );
+        same(
+            "{:10$.10x}",
+            &[NextArgument(Argument {
+                position: ArgumentImplicitlyIs(0),
+                format: FormatSpec {
+                    fill: None,
+                    align: AlignUnknown,
+                    flags: 0,
+                    precision: CountIs(10),
+                    width: CountIsParam(10),
+                    precision_span: None,
+                    width_span: Some(InnerSpan::new(3, 6)),
+                    ty: "x",
+                    ty_span: None,
+                },
+            })],
+        );
+        same(
+            "{:.*x}",
+            &[NextArgument(Argument {
+                position: ArgumentImplicitlyIs(1),
+                format: FormatSpec {
+                    fill: None,
+                    align: AlignUnknown,
+                    flags: 0,
+                    precision: CountIsParam(0),
+                    width: CountImplied,
+                    precision_span: Some(InnerSpan::new(3, 5)),
+                    width_span: None,
+                    ty: "x",
+                    ty_span: None,
+                },
+            })],
+        );
+        same(
+            "{:.10$x}",
+            &[NextArgument(Argument {
+                position: ArgumentImplicitlyIs(0),
+                format: FormatSpec {
+                    fill: None,
+                    align: AlignUnknown,
+                    flags: 0,
+                    precision: CountIsParam(10),
+                    width: CountImplied,
+                    precision_span: Some(InnerSpan::new(3, 7)),
+                    width_span: None,
+                    ty: "x",
+                    ty_span: None,
+                },
+            })],
+        );
+        same(
+            "{:a$.b$?}",
+            &[NextArgument(Argument {
+                position: ArgumentImplicitlyIs(0),
+                format: FormatSpec {
+                    fill: None,
+                    align: AlignUnknown,
+                    flags: 0,
+                    precision: CountIsName(Symbol::intern("b")),
+                    width: CountIsName(Symbol::intern("a")),
+                    precision_span: None,
+                    width_span: None,
+                    ty: "?",
+                    ty_span: None,
+                },
+            })],
+        );
+    });
+}
+#[test]
+fn format_flags() {
+    same(
+        "{:-}",
+        &[NextArgument(Argument {
+            position: ArgumentImplicitlyIs(0),
+            format: FormatSpec {
+                fill: None,
+                align: AlignUnknown,
+                flags: (1 << FlagSignMinus as u32),
+                precision: CountImplied,
+                width: CountImplied,
+                precision_span: None,
+                width_span: None,
+                ty: "",
+                ty_span: None,
+            },
+        })],
+    );
+    same(
+        "{:+#}",
+        &[NextArgument(Argument {
+            position: ArgumentImplicitlyIs(0),
+            format: FormatSpec {
+                fill: None,
+                align: AlignUnknown,
+                flags: (1 << FlagSignPlus as u32) | (1 << FlagAlternate as u32),
+                precision: CountImplied,
+                width: CountImplied,
+                precision_span: None,
+                width_span: None,
+                ty: "",
+                ty_span: None,
+            },
+        })],
+    );
+}
+#[test]
+fn format_mixture() {
+    same(
+        "abcd {3:x} efg",
+        &[
+            String("abcd "),
+            NextArgument(Argument {
+                position: ArgumentIs(3),
+                format: FormatSpec {
+                    fill: None,
+                    align: AlignUnknown,
+                    flags: 0,
+                    precision: CountImplied,
+                    width: CountImplied,
+                    precision_span: None,
+                    width_span: None,
+                    ty: "x",
+                    ty_span: None,
+                },
+            }),
+            String(" efg"),
+        ],
+    );
+}