//! Low-level Rust lexer.
//!
//! The idea with `rustc_lexer` is to make a reusable library,
//! by separating out pure lexing and rustc-specific concerns, like spans,
//! error reporting, and interning.  So, rustc_lexer operates directly on `&str`,
//! produces simple tokens which are a pair of type-tag and a bit of original text,
//! and does not report errors, instead storing them as flags on the token.
//!
//! Tokens produced by this lexer are not yet ready for parsing the Rust syntax.
//! For that see [`rustc_parse::lexer`], which converts this basic token stream
//! into wide tokens used by actual parser.
//!
//! The purpose of this crate is to convert raw sources into a labeled sequence
//! of well-known token types, so building an actual Rust token stream will
//! be easier.
//!
//! The main entity of this crate is the [`TokenKind`] enum which represents common
//! lexeme types.
//!
//! [`rustc_parse::lexer`]: ../rustc_parse/lexer/index.html
#![deny(rustc::untranslatable_diagnostic)]
#![deny(rustc::diagnostic_outside_of_impl)]
// We want to be able to build this crate with a stable compiler, so no
// `#![feature]` attributes should be added.

mod cursor;
pub mod unescape;

#[cfg(test)]
mod tests;

use self::LiteralKind::*;
use self::TokenKind::*;
use crate::cursor::{Cursor, EOF_CHAR};
use std::convert::TryFrom;

/// Parsed token.
/// It doesn't contain information about data that has been parsed,
/// only the type of the token and its size.
#[derive(Debug)]
pub struct Token {
    pub kind: TokenKind,
    pub len: u32,
}

impl Token {
    fn new(kind: TokenKind, len: u32) -> Token {
        Token { kind, len }
    }
}

/// Enum representing common lexeme types.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum TokenKind {
    // Multi-char tokens:
    /// "// comment"
    LineComment { doc_style: Option<DocStyle> },
    /// `/* block comment */`
    ///
    /// Block comments can be recursive, so the sequence like `/* /* */`
    /// will not be considered terminated and will result in a parsing error.
    BlockComment { doc_style: Option<DocStyle>, terminated: bool },
    /// Any whitespace characters sequence.
    Whitespace,
    /// "ident" or "continue"
    /// At this step keywords are also considered identifiers.
    Ident,
    /// Like the above, but containing invalid unicode codepoints.
    InvalidIdent,
    /// "r#ident"
    RawIdent,
    /// An unknown prefix like `foo#`, `foo'`, `foo"`. Note that only the
    /// prefix (`foo`) is included in the token, not the separator (which is
    /// lexed as its own distinct token). In Rust 2021 and later, reserved
    /// prefixes are reported as errors; in earlier editions, they result in a
    /// (allowed by default) lint, and are treated as regular identifier
    /// tokens.
    UnknownPrefix,
    /// "12_u8", "1.0e-40", "b"123"". See `LiteralKind` for more details.
    Literal { kind: LiteralKind, suffix_start: u32 },
    /// "'a"
    Lifetime { starts_with_number: bool },

    // One-char tokens:
    /// ";"
    Semi,
    /// ","
    Comma,
    /// "."
    Dot,
    /// "("
    OpenParen,
    /// ")"
    CloseParen,
    /// "{"
    OpenBrace,
    /// "}"
    CloseBrace,
    /// "["
    OpenBracket,
    /// "]"
    CloseBracket,
    /// "@"
    At,
    /// "#"
    Pound,
    /// "~"
    Tilde,
    /// "?"
    Question,
    /// ":"
    Colon,
    /// "$"
    Dollar,
    /// "="
    Eq,
    /// "!"
    Bang,
    /// "<"
    Lt,
    /// ">"
    Gt,
    /// "-"
    Minus,
    /// "&"
    And,
    /// "|"
    Or,
    /// "+"
    Plus,
    /// "*"
    Star,
    /// "/"
    Slash,
    /// "^"
    Caret,
    /// "%"
    Percent,

    /// Unknown token, not expected by the lexer, e.g. "№"
    Unknown,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum DocStyle {
    Outer,
    Inner,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub enum LiteralKind {
    /// "12_u8", "0o100", "0b120i99"
    Int { base: Base, empty_int: bool },
    /// "12.34f32", "0b100.100"
    Float { base: Base, empty_exponent: bool },
    /// "'a'", "'\\'", "'''", "';"
    Char { terminated: bool },
    /// "b'a'", "b'\\'", "b'''", "b';"
    Byte { terminated: bool },
    /// ""abc"", ""abc"
    Str { terminated: bool },
    /// "b"abc"", "b"abc"
    ByteStr { terminated: bool },
    /// "r"abc"", "r#"abc"#", "r####"ab"###"c"####", "r#"a". `None` indicates
    /// an invalid literal.
    RawStr { n_hashes: Option<u8> },
    /// "br"abc"", "br#"abc"#", "br####"ab"###"c"####", "br#"a". `None`
    /// indicates an invalid literal.
    RawByteStr { n_hashes: Option<u8> },
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub enum RawStrError {
    /// Non `#` characters exist between `r` and `"`, e.g. `r##~"abcde"##`
    InvalidStarter { bad_char: char },
    /// The string was not terminated, e.g. `r###"abcde"##`.
    /// `possible_terminator_offset` is the number of characters after `r` or
    /// `br` where they may have intended to terminate it.
    NoTerminator { expected: u32, found: u32, possible_terminator_offset: Option<u32> },
    /// More than 255 `#`s exist.
    TooManyDelimiters { found: u32 },
}

/// Base of numeric literal encoding according to its prefix.
#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
pub enum Base {
    /// Literal starts with "0b".
    Binary,
    /// Literal starts with "0o".
    Octal,
    /// Literal starts with "0x".
    Hexadecimal,
    /// Literal doesn't contain a prefix.
    Decimal,
}

/// `rustc` allows files to have a shebang, e.g. "#!/usr/bin/rustrun",
/// but shebang isn't a part of rust syntax.
pub fn strip_shebang(input: &str) -> Option<usize> {
    // Shebang must start with `#!` literally, without any preceding whitespace.
    // For simplicity we consider any line starting with `#!` a shebang,
    // regardless of restrictions put on shebangs by specific platforms.
    if let Some(input_tail) = input.strip_prefix("#!") {
        // Ok, this is a shebang but if the next non-whitespace token is `[`,
        // then it may be valid Rust code, so consider it Rust code.
        let next_non_whitespace_token = tokenize(input_tail).map(|tok| tok.kind).find(|tok| {
            !matches!(
                tok,
                TokenKind::Whitespace
                    | TokenKind::LineComment { doc_style: None }
                    | TokenKind::BlockComment { doc_style: None, .. }
            )
        });
        if next_non_whitespace_token != Some(TokenKind::OpenBracket) {
            // No other choice than to consider this a shebang.
            return Some(2 + input_tail.lines().next().unwrap_or_default().len());
        }
    }
    None
}

/// Parses the first token from the provided input string.
#[inline]
pub fn first_token(input: &str) -> Token {
    debug_assert!(!input.is_empty());
    Cursor::new(input).advance_token()
}

/// Validates a raw string literal. Used for getting more information about a
/// problem with a `RawStr`/`RawByteStr` with a `None` field.
#[inline]
pub fn validate_raw_str(input: &str, prefix_len: u32) -> Result<(), RawStrError> {
    debug_assert!(!input.is_empty());
    let mut cursor = Cursor::new(input);
    // Move past the leading `r` or `br`.
    for _ in 0..prefix_len {
        cursor.bump().unwrap();
    }
    cursor.raw_double_quoted_string(prefix_len).map(|_| ())
}

/// Creates an iterator that produces tokens from the input string.
pub fn tokenize(input: &str) -> impl Iterator<Item = Token> + '_ {
    let mut cursor = Cursor::new(input);
    std::iter::from_fn(move || {
        if cursor.is_eof() {
            None
        } else {
            cursor.reset_len_consumed();
            Some(cursor.advance_token())
        }
    })
}

/// True if `c` is considered a whitespace according to Rust language definition.
/// See [Rust language reference](https://doc.rust-lang.org/reference/whitespace.html)
/// for definitions of these classes.
pub fn is_whitespace(c: char) -> bool {
    // This is Pattern_White_Space.
    //
    // Note that this set is stable (ie, it doesn't change with different
    // Unicode versions), so it's ok to just hard-code the values.

    matches!(
        c,
        // Usual ASCII suspects
        '\u{0009}'   // \t
        | '\u{000A}' // \n
        | '\u{000B}' // vertical tab
        | '\u{000C}' // form feed
        | '\u{000D}' // \r
        | '\u{0020}' // space

        // NEXT LINE from latin1
        | '\u{0085}'

        // Bidi markers
        | '\u{200E}' // LEFT-TO-RIGHT MARK
        | '\u{200F}' // RIGHT-TO-LEFT MARK

        // Dedicated whitespace characters from Unicode
        | '\u{2028}' // LINE SEPARATOR
        | '\u{2029}' // PARAGRAPH SEPARATOR
    )
}

/// True if `c` is valid as a first character of an identifier.
/// See [Rust language reference](https://doc.rust-lang.org/reference/identifiers.html) for
/// a formal definition of valid identifier name.
pub fn is_id_start(c: char) -> bool {
    // This is XID_Start OR '_' (which formally is not a XID_Start).
    c == '_' || unicode_xid::UnicodeXID::is_xid_start(c)
}

/// True if `c` is valid as a non-first character of an identifier.
/// See [Rust language reference](https://doc.rust-lang.org/reference/identifiers.html) for
/// a formal definition of valid identifier name.
pub fn is_id_continue(c: char) -> bool {
    unicode_xid::UnicodeXID::is_xid_continue(c)
}

/// The passed string is lexically an identifier.
pub fn is_ident(string: &str) -> bool {
    let mut chars = string.chars();
    if let Some(start) = chars.next() {
        is_id_start(start) && chars.all(is_id_continue)
    } else {
        false
    }
}

impl Cursor<'_> {
    /// Parses a token from the input string.
    fn advance_token(&mut self) -> Token {
        let first_char = self.bump().unwrap();
        let token_kind = match first_char {
            // Slash, comment or block comment.
            '/' => match self.first() {
                '/' => self.line_comment(),
                '*' => self.block_comment(),
                _ => Slash,
            },

            // Whitespace sequence.
            c if is_whitespace(c) => self.whitespace(),

            // Raw identifier, raw string literal or identifier.
            'r' => match (self.first(), self.second()) {
                ('#', c1) if is_id_start(c1) => self.raw_ident(),
                ('#', _) | ('"', _) => {
                    let res = self.raw_double_quoted_string(1);
                    let suffix_start = self.len_consumed();
                    if res.is_ok() {
                        self.eat_literal_suffix();
                    }
                    let kind = RawStr { n_hashes: res.ok() };
                    Literal { kind, suffix_start }
                }
                _ => self.ident_or_unknown_prefix(),
            },

            // Byte literal, byte string literal, raw byte string literal or identifier.
            'b' => match (self.first(), self.second()) {
                ('\'', _) => {
                    self.bump();
                    let terminated = self.single_quoted_string();
                    let suffix_start = self.len_consumed();
                    if terminated {
                        self.eat_literal_suffix();
                    }
                    let kind = Byte { terminated };
                    Literal { kind, suffix_start }
                }
                ('"', _) => {
                    self.bump();
                    let terminated = self.double_quoted_string();
                    let suffix_start = self.len_consumed();
                    if terminated {
                        self.eat_literal_suffix();
                    }
                    let kind = ByteStr { terminated };
                    Literal { kind, suffix_start }
                }
                ('r', '"') | ('r', '#') => {
                    self.bump();
                    let res = self.raw_double_quoted_string(2);
                    let suffix_start = self.len_consumed();
                    if res.is_ok() {
                        self.eat_literal_suffix();
                    }
                    let kind = RawByteStr { n_hashes: res.ok() };
                    Literal { kind, suffix_start }
                }
                _ => self.ident_or_unknown_prefix(),
            },

            // Identifier (this should be checked after other variant that can
            // start as identifier).
            c if is_id_start(c) => self.ident_or_unknown_prefix(),

            // Numeric literal.
            c @ '0'..='9' => {
                let literal_kind = self.number(c);
                let suffix_start = self.len_consumed();
                self.eat_literal_suffix();
                TokenKind::Literal { kind: literal_kind, suffix_start }
            }

            // One-symbol tokens.
            ';' => Semi,
            ',' => Comma,
            '.' => Dot,
            '(' => OpenParen,
            ')' => CloseParen,
            '{' => OpenBrace,
            '}' => CloseBrace,
            '[' => OpenBracket,
            ']' => CloseBracket,
            '@' => At,
            '#' => Pound,
            '~' => Tilde,
            '?' => Question,
            ':' => Colon,
            '$' => Dollar,
            '=' => Eq,
            '!' => Bang,
            '<' => Lt,
            '>' => Gt,
            '-' => Minus,
            '&' => And,
            '|' => Or,
            '+' => Plus,
            '*' => Star,
            '^' => Caret,
            '%' => Percent,

            // Lifetime or character literal.
            '\'' => self.lifetime_or_char(),

            // String literal.
            '"' => {
                let terminated = self.double_quoted_string();
                let suffix_start = self.len_consumed();
                if terminated {
                    self.eat_literal_suffix();
                }
                let kind = Str { terminated };
                Literal { kind, suffix_start }
            }
            // Identifier starting with an emoji. Only lexed for graceful error recovery.
            c if !c.is_ascii() && unic_emoji_char::is_emoji(c) => {
                self.fake_ident_or_unknown_prefix()
            }
            _ => Unknown,
        };
        Token::new(token_kind, self.len_consumed())
    }

    fn line_comment(&mut self) -> TokenKind {
        debug_assert!(self.prev() == '/' && self.first() == '/');
        self.bump();

        let doc_style = match self.first() {
            // `//!` is an inner line doc comment.
            '!' => Some(DocStyle::Inner),
            // `////` (more than 3 slashes) is not considered a doc comment.
            '/' if self.second() != '/' => Some(DocStyle::Outer),
            _ => None,
        };

        self.eat_while(|c| c != '\n');
        LineComment { doc_style }
    }

    fn block_comment(&mut self) -> TokenKind {
        debug_assert!(self.prev() == '/' && self.first() == '*');
        self.bump();

        let doc_style = match self.first() {
            // `/*!` is an inner block doc comment.
            '!' => Some(DocStyle::Inner),
            // `/***` (more than 2 stars) is not considered a doc comment.
            // `/**/` is not considered a doc comment.
            '*' if !matches!(self.second(), '*' | '/') => Some(DocStyle::Outer),
            _ => None,
        };

        let mut depth = 1usize;
        while let Some(c) = self.bump() {
            match c {
                '/' if self.first() == '*' => {
                    self.bump();
                    depth += 1;
                }
                '*' if self.first() == '/' => {
                    self.bump();
                    depth -= 1;
                    if depth == 0 {
                        // This block comment is closed, so for a construction like "/* */ */"
                        // there will be a successfully parsed block comment "/* */"
                        // and " */" will be processed separately.
                        break;
                    }
                }
                _ => (),
            }
        }

        BlockComment { doc_style, terminated: depth == 0 }
    }

    fn whitespace(&mut self) -> TokenKind {
        debug_assert!(is_whitespace(self.prev()));
        self.eat_while(is_whitespace);
        Whitespace
    }

    fn raw_ident(&mut self) -> TokenKind {
        debug_assert!(self.prev() == 'r' && self.first() == '#' && is_id_start(self.second()));
        // Eat "#" symbol.
        self.bump();
        // Eat the identifier part of RawIdent.
        self.eat_identifier();
        RawIdent
    }

    fn ident_or_unknown_prefix(&mut self) -> TokenKind {
        debug_assert!(is_id_start(self.prev()));
        // Start is already eaten, eat the rest of identifier.
        self.eat_while(is_id_continue);
        // Known prefixes must have been handled earlier. So if
        // we see a prefix here, it is definitely an unknown prefix.
        match self.first() {
            '#' | '"' | '\'' => UnknownPrefix,
            c if !c.is_ascii() && unic_emoji_char::is_emoji(c) => {
                self.fake_ident_or_unknown_prefix()
            }
            _ => Ident,
        }
    }

    fn fake_ident_or_unknown_prefix(&mut self) -> TokenKind {
        // Start is already eaten, eat the rest of identifier.
        self.eat_while(|c| {
            unicode_xid::UnicodeXID::is_xid_continue(c)
                || (!c.is_ascii() && unic_emoji_char::is_emoji(c))
                || c == '\u{200d}'
        });
        // Known prefixes must have been handled earlier. So if
        // we see a prefix here, it is definitely an unknown prefix.
        match self.first() {
            '#' | '"' | '\'' => UnknownPrefix,
            _ => InvalidIdent,
        }
    }

    fn number(&mut self, first_digit: char) -> LiteralKind {
        debug_assert!('0' <= self.prev() && self.prev() <= '9');
        let mut base = Base::Decimal;
        if first_digit == '0' {
            // Attempt to parse encoding base.
            let has_digits = match self.first() {
                'b' => {
                    base = Base::Binary;
                    self.bump();
                    self.eat_decimal_digits()
                }
                'o' => {
                    base = Base::Octal;
                    self.bump();
                    self.eat_decimal_digits()
                }
                'x' => {
                    base = Base::Hexadecimal;
                    self.bump();
                    self.eat_hexadecimal_digits()
                }
                // Not a base prefix.
                '0'..='9' | '_' | '.' | 'e' | 'E' => {
                    self.eat_decimal_digits();
                    true
                }
                // Just a 0.
                _ => return Int { base, empty_int: false },
            };
            // Base prefix was provided, but there were no digits
            // after it, e.g. "0x".
            if !has_digits {
                return Int { base, empty_int: true };
            }
        } else {
            // No base prefix, parse number in the usual way.
            self.eat_decimal_digits();
        };

        match self.first() {
            // Don't be greedy if this is actually an
            // integer literal followed by field/method access or a range pattern
            // (`0..2` and `12.foo()`)
            '.' if self.second() != '.' && !is_id_start(self.second()) => {
                // might have stuff after the ., and if it does, it needs to start
                // with a number
                self.bump();
                let mut empty_exponent = false;
                if self.first().is_digit(10) {
                    self.eat_decimal_digits();
                    match self.first() {
                        'e' | 'E' => {
                            self.bump();
                            empty_exponent = !self.eat_float_exponent();
                        }
                        _ => (),
                    }
                }
                Float { base, empty_exponent }
            }
            'e' | 'E' => {
                self.bump();
                let empty_exponent = !self.eat_float_exponent();
                Float { base, empty_exponent }
            }
            _ => Int { base, empty_int: false },
        }
    }

    fn lifetime_or_char(&mut self) -> TokenKind {
        debug_assert!(self.prev() == '\'');

        let can_be_a_lifetime = if self.second() == '\'' {
            // It's surely not a lifetime.
            false
        } else {
            // If the first symbol is valid for identifier, it can be a lifetime.
            // Also check if it's a number for a better error reporting (so '0 will
            // be reported as invalid lifetime and not as unterminated char literal).
            is_id_start(self.first()) || self.first().is_digit(10)
        };

        if !can_be_a_lifetime {
            let terminated = self.single_quoted_string();
            let suffix_start = self.len_consumed();
            if terminated {
                self.eat_literal_suffix();
            }
            let kind = Char { terminated };
            return Literal { kind, suffix_start };
        }

        // Either a lifetime or a character literal with
        // length greater than 1.

        let starts_with_number = self.first().is_digit(10);

        // Skip the literal contents.
        // First symbol can be a number (which isn't a valid identifier start),
        // so skip it without any checks.
        self.bump();
        self.eat_while(is_id_continue);

        // Check if after skipping literal contents we've met a closing
        // single quote (which means that user attempted to create a
        // string with single quotes).
        if self.first() == '\'' {
            self.bump();
            let kind = Char { terminated: true };
            Literal { kind, suffix_start: self.len_consumed() }
        } else {
            Lifetime { starts_with_number }
        }
    }

    fn single_quoted_string(&mut self) -> bool {
        debug_assert!(self.prev() == '\'');
        // Check if it's a one-symbol literal.
        if self.second() == '\'' && self.first() != '\\' {
            self.bump();
            self.bump();
            return true;
        }

        // Literal has more than one symbol.

        // Parse until either quotes are terminated or error is detected.
        loop {
            match self.first() {
                // Quotes are terminated, finish parsing.
                '\'' => {
                    self.bump();
                    return true;
                }
                // Probably beginning of the comment, which we don't want to include
                // to the error report.
                '/' => break,
                // Newline without following '\'' means unclosed quote, stop parsing.
                '\n' if self.second() != '\'' => break,
                // End of file, stop parsing.
                EOF_CHAR if self.is_eof() => break,
                // Escaped slash is considered one character, so bump twice.
                '\\' => {
                    self.bump();
                    self.bump();
                }
                // Skip the character.
                _ => {
                    self.bump();
                }
            }
        }
        // String was not terminated.
        false
    }

    /// Eats double-quoted string and returns true
    /// if string is terminated.
    fn double_quoted_string(&mut self) -> bool {
        debug_assert!(self.prev() == '"');
        while let Some(c) = self.bump() {
            match c {
                '"' => {
                    return true;
                }
                '\\' if self.first() == '\\' || self.first() == '"' => {
                    // Bump again to skip escaped character.
                    self.bump();
                }
                _ => (),
            }
        }
        // End of file reached.
        false
    }

    /// Eats the double-quoted string and returns `n_hashes` and an error if encountered.
    fn raw_double_quoted_string(&mut self, prefix_len: u32) -> Result<u8, RawStrError> {
        // Wrap the actual function to handle the error with too many hashes.
        // This way, it eats the whole raw string.
        let n_hashes = self.raw_string_unvalidated(prefix_len)?;
        // Only up to 255 `#`s are allowed in raw strings
        match u8::try_from(n_hashes) {
            Ok(num) => Ok(num),
            Err(_) => Err(RawStrError::TooManyDelimiters { found: n_hashes }),
        }
    }

    fn raw_string_unvalidated(&mut self, prefix_len: u32) -> Result<u32, RawStrError> {
        debug_assert!(self.prev() == 'r');
        let start_pos = self.len_consumed();
        let mut possible_terminator_offset = None;
        let mut max_hashes = 0;

        // Count opening '#' symbols.
        let mut eaten = 0;
        while self.first() == '#' {
            eaten += 1;
            self.bump();
        }
        let n_start_hashes = eaten;

        // Check that string is started.
        match self.bump() {
            Some('"') => (),
            c => {
                let c = c.unwrap_or(EOF_CHAR);
                return Err(RawStrError::InvalidStarter { bad_char: c });
            }
        }

        // Skip the string contents and on each '#' character met, check if this is
        // a raw string termination.
        loop {
            self.eat_while(|c| c != '"');

            if self.is_eof() {
                return Err(RawStrError::NoTerminator {
                    expected: n_start_hashes,
                    found: max_hashes,
                    possible_terminator_offset,
                });
            }

            // Eat closing double quote.
            self.bump();

            // Check that amount of closing '#' symbols
            // is equal to the amount of opening ones.
            // Note that this will not consume extra trailing `#` characters:
            // `r###"abcde"####` is lexed as a `RawStr { n_hashes: 3 }`
            // followed by a `#` token.
            let mut n_end_hashes = 0;
            while self.first() == '#' && n_end_hashes < n_start_hashes {
                n_end_hashes += 1;
                self.bump();
            }

            if n_end_hashes == n_start_hashes {
                return Ok(n_start_hashes);
            } else if n_end_hashes > max_hashes {
                // Keep track of possible terminators to give a hint about
                // where there might be a missing terminator
                possible_terminator_offset =
                    Some(self.len_consumed() - start_pos - n_end_hashes + prefix_len);
                max_hashes = n_end_hashes;
            }
        }
    }

    fn eat_decimal_digits(&mut self) -> bool {
        let mut has_digits = false;
        loop {
            match self.first() {
                '_' => {
                    self.bump();
                }
                '0'..='9' => {
                    has_digits = true;
                    self.bump();
                }
                _ => break,
            }
        }
        has_digits
    }

    fn eat_hexadecimal_digits(&mut self) -> bool {
        let mut has_digits = false;
        loop {
            match self.first() {
                '_' => {
                    self.bump();
                }
                '0'..='9' | 'a'..='f' | 'A'..='F' => {
                    has_digits = true;
                    self.bump();
                }
                _ => break,
            }
        }
        has_digits
    }

    /// Eats the float exponent. Returns true if at least one digit was met,
    /// and returns false otherwise.
    fn eat_float_exponent(&mut self) -> bool {
        debug_assert!(self.prev() == 'e' || self.prev() == 'E');
        if self.first() == '-' || self.first() == '+' {
            self.bump();
        }
        self.eat_decimal_digits()
    }

    // Eats the suffix of the literal, e.g. "_u8".
    fn eat_literal_suffix(&mut self) {
        self.eat_identifier();
    }

    // Eats the identifier.
    fn eat_identifier(&mut self) {
        if !is_id_start(self.first()) {
            return;
        }
        self.bump();

        self.eat_while(is_id_continue);
    }
}