syn/

lit.rs

#[cfg(feature = "parsing")]
use crate::ext::TokenStreamExt as _;
#[cfg(feature = "parsing")]
use crate::lookahead;
#[cfg(feature = "parsing")]
use crate::parse::{Parse, Parser};
use crate::{Error, Result};
use alloc::boxed::Box;
use alloc::ffi::CString;
#[cfg(feature = "parsing")]
use alloc::format;
use alloc::string::{String, ToString};
use alloc::vec::Vec;
use core::ffi::CStr;
use core::fmt::{self, Display};
#[cfg(feature = "extra-traits")]
use core::hash::{Hash, Hasher};
use core::str::{self, FromStr};
use proc_macro2::{Ident, Literal, Span};
#[cfg(feature = "parsing")]
use proc_macro2::{TokenStream, TokenTree};

ast_enum_of_structs! {
    /// A Rust literal such as a string or integer or boolean.
    ///
    /// # Syntax tree enum
    ///
    /// This type is a [syntax tree enum].
    ///
    /// [syntax tree enum]: crate::expr::Expr#syntax-tree-enums
    #[non_exhaustive]
    pub enum Lit {
        /// A UTF-8 string literal: `"foo"`.
        Str(LitStr),

        /// A byte string literal: `b"foo"`.
        ByteStr(LitByteStr),

        /// A nul-terminated C-string literal: `c"foo"`.
        CStr(LitCStr),

        /// A byte literal: `b'f'`.
        Byte(LitByte),

        /// A character literal: `'a'`.
        Char(LitChar),

        /// An integer literal: `1` or `1u16`.
        Int(LitInt),

        /// A floating point literal: `1f64` or `1.0e10f64`.
        ///
        /// Must be finite. May not be infinite or NaN.
        Float(LitFloat),

        /// A boolean literal: `true` or `false`.
        Bool(LitBool),

        /// A raw token literal not interpreted by Syn.
        Verbatim(Literal),
    }
}

ast_struct! {
    /// A UTF-8 string literal: `"foo"`.
    pub struct LitStr {
        repr: Box<LitRepr>,
    }
}

ast_struct! {
    /// A byte string literal: `b"foo"`.
    pub struct LitByteStr {
        repr: Box<LitRepr>,
    }
}

ast_struct! {
    /// A nul-terminated C-string literal: `c"foo"`.
    pub struct LitCStr {
        repr: Box<LitRepr>,
    }
}

ast_struct! {
    /// A byte literal: `b'f'`.
    pub struct LitByte {
        repr: Box<LitRepr>,
    }
}

ast_struct! {
    /// A character literal: `'a'`.
    pub struct LitChar {
        repr: Box<LitRepr>,
    }
}

struct LitRepr {
    token: Literal,
    suffix: Box<str>,
}

ast_struct! {
    /// An integer literal: `1` or `1u16`.
    pub struct LitInt {
        repr: Box<LitIntRepr>,
    }
}

struct LitIntRepr {
    token: Literal,
    digits: Box<str>,
    suffix: Box<str>,
}

ast_struct! {
    /// A floating point literal: `1f64` or `1.0e10f64`.
    ///
    /// Must be finite. May not be infinite or NaN.
    pub struct LitFloat {
        repr: Box<LitFloatRepr>,
    }
}

struct LitFloatRepr {
    token: Literal,
    digits: Box<str>,
    suffix: Box<str>,
}

ast_struct! {
    /// A boolean literal: `true` or `false`.
    pub struct LitBool {
        pub value: bool,
        pub span: Span,
    }
}

impl LitStr {
    pub fn new(value: &str, span: Span) -> Self {
        let mut token = Literal::string(value);
        token.set_span(span);
        LitStr {
            repr: Box::new(LitRepr {
                token,
                suffix: Box::<str>::default(),
            }),
        }
    }

    pub fn value(&self) -> String {
        let repr = self.repr.token.to_string();
        let (value, _suffix) = value::parse_lit_str(&repr).unwrap();
        String::from(value)
    }

    /// Parse a syntax tree node from the content of this string literal.
    ///
    /// All spans in the syntax tree will point to the span of this `LitStr`.
    ///
    /// # Example
    ///
    /// ```
    /// use syn::{Attribute, Error, Expr, Lit, Meta, Path, Result};
    ///
    /// // Parses the path from an attribute that looks like:
    /// //
    /// //     #[path = "a::b::c"]
    /// //
    /// // or returns `None` if the input is some other attribute.
    /// fn get_path(attr: &Attribute) -> Result<Option<Path>> {
    ///     if !attr.path().is_ident("path") {
    ///         return Ok(None);
    ///     }
    ///
    ///     if let Meta::NameValue(meta) = &attr.meta {
    ///         if let Expr::Lit(expr) = &meta.value {
    ///             if let Lit::Str(lit_str) = &expr.lit {
    ///                 return lit_str.parse().map(Some);
    ///             }
    ///         }
    ///     }
    ///
    ///     let message = "expected #[path = \"...\"]";
    ///     Err(Error::new_spanned(attr, message))
    /// }
    /// ```
    #[cfg(feature = "parsing")]
    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    pub fn parse<T: Parse>(&self) -> Result<T> {
        self.parse_with(T::parse)
    }

    /// Invoke parser on the content of this string literal.
    ///
    /// All spans in the syntax tree will point to the span of this `LitStr`.
    ///
    /// # Example
    ///
    /// ```
    /// # use proc_macro2::Span;
    /// # use syn::{LitStr, Result};
    /// #
    /// # fn main() -> Result<()> {
    /// #     let lit_str = LitStr::new("a::b::c", Span::call_site());
    /// #
    /// #     const IGNORE: &str = stringify! {
    /// let lit_str: LitStr = /* ... */;
    /// #     };
    ///
    /// // Parse a string literal like "a::b::c" into a Path, not allowing
    /// // generic arguments on any of the path segments.
    /// let basic_path = lit_str.parse_with(syn::Path::parse_mod_style)?;
    /// #
    /// #     Ok(())
    /// # }
    /// ```
    #[cfg(feature = "parsing")]
    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    pub fn parse_with<F: Parser>(&self, parser: F) -> Result<F::Output> {
        use proc_macro2::Group;

        // Token stream with every span replaced by the given one.
        fn respan_token_stream(stream: TokenStream, span: Span) -> TokenStream {
            let mut tokens = TokenStream::new();
            for token in stream {
                tokens.append(respan_token_tree(token, span));
            }
            tokens
        }

        // Token tree with every span replaced by the given one.
        fn respan_token_tree(mut token: TokenTree, span: Span) -> TokenTree {
            match &mut token {
                TokenTree::Group(g) => {
                    let stream = respan_token_stream(g.stream(), span);
                    *g = Group::new(g.delimiter(), stream);
                    g.set_span(span);
                }
                other => other.set_span(span),
            }
            token
        }

        // Parse string literal into a token stream with every span equal to the
        // original literal's span.
        let span = self.span();
        let mut tokens =
            TokenStream::from_str(&self.value()).map_err(|err| Error::new(span, err))?;
        tokens = respan_token_stream(tokens, span);

        let result = crate::parse::parse_scoped(parser, span, tokens)?;

        let suffix = self.suffix();
        if !suffix.is_empty() {
            return Err(Error::new(
                self.span(),
                format!("unexpected suffix `{}` on string literal", suffix),
            ));
        }

        Ok(result)
    }

    pub fn span(&self) -> Span {
        self.repr.token.span()
    }

    pub fn set_span(&mut self, span: Span) {
        self.repr.token.set_span(span);
    }

    pub fn suffix(&self) -> &str {
        &self.repr.suffix
    }

    pub fn token(&self) -> Literal {
        self.repr.token.clone()
    }
}

impl LitByteStr {
    pub fn new(value: &[u8], span: Span) -> Self {
        let mut token = Literal::byte_string(value);
        token.set_span(span);
        LitByteStr {
            repr: Box::new(LitRepr {
                token,
                suffix: Box::<str>::default(),
            }),
        }
    }

    pub fn value(&self) -> Vec<u8> {
        let repr = self.repr.token.to_string();
        let (value, _suffix) = value::parse_lit_byte_str(&repr).unwrap();
        value
    }

    pub fn span(&self) -> Span {
        self.repr.token.span()
    }

    pub fn set_span(&mut self, span: Span) {
        self.repr.token.set_span(span);
    }

    pub fn suffix(&self) -> &str {
        &self.repr.suffix
    }

    pub fn token(&self) -> Literal {
        self.repr.token.clone()
    }
}

impl LitCStr {
    pub fn new(value: &CStr, span: Span) -> Self {
        let mut token = Literal::c_string(value);
        token.set_span(span);
        LitCStr {
            repr: Box::new(LitRepr {
                token,
                suffix: Box::<str>::default(),
            }),
        }
    }

    pub fn value(&self) -> CString {
        let repr = self.repr.token.to_string();
        let (value, _suffix) = value::parse_lit_c_str(&repr).unwrap();
        value
    }

    pub fn span(&self) -> Span {
        self.repr.token.span()
    }

    pub fn set_span(&mut self, span: Span) {
        self.repr.token.set_span(span);
    }

    pub fn suffix(&self) -> &str {
        &self.repr.suffix
    }

    pub fn token(&self) -> Literal {
        self.repr.token.clone()
    }
}

impl LitByte {
    pub fn new(value: u8, span: Span) -> Self {
        let mut token = Literal::byte_character(value);
        token.set_span(span);
        LitByte {
            repr: Box::new(LitRepr {
                token,
                suffix: Box::<str>::default(),
            }),
        }
    }

    pub fn value(&self) -> u8 {
        let repr = self.repr.token.to_string();
        let (value, _suffix) = value::parse_lit_byte(&repr).unwrap();
        value
    }

    pub fn span(&self) -> Span {
        self.repr.token.span()
    }

    pub fn set_span(&mut self, span: Span) {
        self.repr.token.set_span(span);
    }

    pub fn suffix(&self) -> &str {
        &self.repr.suffix
    }

    pub fn token(&self) -> Literal {
        self.repr.token.clone()
    }
}

impl LitChar {
    pub fn new(value: char, span: Span) -> Self {
        let mut token = Literal::character(value);
        token.set_span(span);
        LitChar {
            repr: Box::new(LitRepr {
                token,
                suffix: Box::<str>::default(),
            }),
        }
    }

    pub fn value(&self) -> char {
        let repr = self.repr.token.to_string();
        let (value, _suffix) = value::parse_lit_char(&repr).unwrap();
        value
    }

    pub fn span(&self) -> Span {
        self.repr.token.span()
    }

    pub fn set_span(&mut self, span: Span) {
        self.repr.token.set_span(span);
    }

    pub fn suffix(&self) -> &str {
        &self.repr.suffix
    }

    pub fn token(&self) -> Literal {
        self.repr.token.clone()
    }
}

impl LitInt {
    #[track_caller]
    pub fn new(repr: &str, span: Span) -> Self {
        let (digits, suffix) = match value::parse_lit_int(repr) {
            Some(parse) => parse,
            None => panic!("not an integer literal: `{}`", repr),
        };

        let mut token: Literal = repr.parse().unwrap();
        token.set_span(span);
        LitInt {
            repr: Box::new(LitIntRepr {
                token,
                digits,
                suffix,
            }),
        }
    }

    pub fn base10_digits(&self) -> &str {
        &self.repr.digits
    }

    /// Parses the literal into a selected number type.
    ///
    /// This is equivalent to `lit.base10_digits().parse()` except that the
    /// resulting errors will be correctly spanned to point to the literal token
    /// in the macro input.
    ///
    /// ```
    /// use syn::LitInt;
    /// use syn::parse::{Parse, ParseStream, Result};
    ///
    /// struct Port {
    ///     value: u16,
    /// }
    ///
    /// impl Parse for Port {
    ///     fn parse(input: ParseStream) -> Result<Self> {
    ///         let lit: LitInt = input.parse()?;
    ///         let value = lit.base10_parse::<u16>()?;
    ///         Ok(Port { value })
    ///     }
    /// }
    /// ```
    pub fn base10_parse<N>(&self) -> Result<N>
    where
        N: FromStr,
        N::Err: Display,
    {
        self.base10_digits()
            .parse()
            .map_err(|err| Error::new(self.span(), err))
    }

    pub fn suffix(&self) -> &str {
        &self.repr.suffix
    }

    pub fn span(&self) -> Span {
        self.repr.token.span()
    }

    pub fn set_span(&mut self, span: Span) {
        self.repr.token.set_span(span);
    }

    pub fn token(&self) -> Literal {
        self.repr.token.clone()
    }
}

impl Display for LitInt {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        self.repr.token.fmt(formatter)
    }
}

impl LitFloat {
    #[track_caller]
    pub fn new(repr: &str, span: Span) -> Self {
        let (digits, suffix) = match value::parse_lit_float(repr) {
            Some(parse) => parse,
            None => panic!("not a float literal: `{}`", repr),
        };

        let mut token: Literal = repr.parse().unwrap();
        token.set_span(span);
        LitFloat {
            repr: Box::new(LitFloatRepr {
                token,
                digits,
                suffix,
            }),
        }
    }

    pub fn base10_digits(&self) -> &str {
        &self.repr.digits
    }

    pub fn base10_parse<N>(&self) -> Result<N>
    where
        N: FromStr,
        N::Err: Display,
    {
        self.base10_digits()
            .parse()
            .map_err(|err| Error::new(self.span(), err))
    }

    pub fn suffix(&self) -> &str {
        &self.repr.suffix
    }

    pub fn span(&self) -> Span {
        self.repr.token.span()
    }

    pub fn set_span(&mut self, span: Span) {
        self.repr.token.set_span(span);
    }

    pub fn token(&self) -> Literal {
        self.repr.token.clone()
    }
}

impl Display for LitFloat {
    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
        self.repr.token.fmt(formatter)
    }
}

impl LitBool {
    pub fn new(value: bool, span: Span) -> Self {
        LitBool { value, span }
    }

    pub fn value(&self) -> bool {
        self.value
    }

    pub fn span(&self) -> Span {
        self.span
    }

    pub fn set_span(&mut self, span: Span) {
        self.span = span;
    }

    pub fn token(&self) -> Ident {
        let s = if self.value { "true" } else { "false" };
        Ident::new(s, self.span)
    }
}

#[cfg(feature = "extra-traits")]
mod debug_impls {
    use crate::lit::{LitBool, LitByte, LitByteStr, LitCStr, LitChar, LitFloat, LitInt, LitStr};
    use core::fmt::{self, Debug};

    #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
    impl Debug for LitStr {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            self.debug(formatter, "LitStr")
        }
    }

    impl LitStr {
        pub(crate) fn debug(&self, formatter: &mut fmt::Formatter, name: &str) -> fmt::Result {
            formatter
                .debug_struct(name)
                .field("token", &format_args!("{}", self.repr.token))
                .finish()
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
    impl Debug for LitByteStr {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            self.debug(formatter, "LitByteStr")
        }
    }

    impl LitByteStr {
        pub(crate) fn debug(&self, formatter: &mut fmt::Formatter, name: &str) -> fmt::Result {
            formatter
                .debug_struct(name)
                .field("token", &format_args!("{}", self.repr.token))
                .finish()
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
    impl Debug for LitCStr {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            self.debug(formatter, "LitCStr")
        }
    }

    impl LitCStr {
        pub(crate) fn debug(&self, formatter: &mut fmt::Formatter, name: &str) -> fmt::Result {
            formatter
                .debug_struct(name)
                .field("token", &format_args!("{}", self.repr.token))
                .finish()
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
    impl Debug for LitByte {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            self.debug(formatter, "LitByte")
        }
    }

    impl LitByte {
        pub(crate) fn debug(&self, formatter: &mut fmt::Formatter, name: &str) -> fmt::Result {
            formatter
                .debug_struct(name)
                .field("token", &format_args!("{}", self.repr.token))
                .finish()
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
    impl Debug for LitChar {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            self.debug(formatter, "LitChar")
        }
    }

    impl LitChar {
        pub(crate) fn debug(&self, formatter: &mut fmt::Formatter, name: &str) -> fmt::Result {
            formatter
                .debug_struct(name)
                .field("token", &format_args!("{}", self.repr.token))
                .finish()
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
    impl Debug for LitInt {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            self.debug(formatter, "LitInt")
        }
    }

    impl LitInt {
        pub(crate) fn debug(&self, formatter: &mut fmt::Formatter, name: &str) -> fmt::Result {
            formatter
                .debug_struct(name)
                .field("token", &format_args!("{}", self.repr.token))
                .finish()
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
    impl Debug for LitFloat {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            self.debug(formatter, "LitFloat")
        }
    }

    impl LitFloat {
        pub(crate) fn debug(&self, formatter: &mut fmt::Formatter, name: &str) -> fmt::Result {
            formatter
                .debug_struct(name)
                .field("token", &format_args!("{}", self.repr.token))
                .finish()
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
    impl Debug for LitBool {
        fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
            self.debug(formatter, "LitBool")
        }
    }

    impl LitBool {
        pub(crate) fn debug(&self, formatter: &mut fmt::Formatter, name: &str) -> fmt::Result {
            formatter
                .debug_struct(name)
                .field("value", &self.value)
                .finish()
        }
    }
}

#[cfg(feature = "clone-impls")]
#[cfg_attr(docsrs, doc(cfg(feature = "clone-impls")))]
impl Clone for LitRepr {
    fn clone(&self) -> Self {
        LitRepr {
            token: self.token.clone(),
            suffix: self.suffix.clone(),
        }
    }
}

#[cfg(feature = "clone-impls")]
#[cfg_attr(docsrs, doc(cfg(feature = "clone-impls")))]
impl Clone for LitIntRepr {
    fn clone(&self) -> Self {
        LitIntRepr {
            token: self.token.clone(),
            digits: self.digits.clone(),
            suffix: self.suffix.clone(),
        }
    }
}

#[cfg(feature = "clone-impls")]
#[cfg_attr(docsrs, doc(cfg(feature = "clone-impls")))]
impl Clone for LitFloatRepr {
    fn clone(&self) -> Self {
        LitFloatRepr {
            token: self.token.clone(),
            digits: self.digits.clone(),
            suffix: self.suffix.clone(),
        }
    }
}

macro_rules! lit_extra_traits {
    ($ty:ident) => {
        #[cfg(feature = "clone-impls")]
        #[cfg_attr(docsrs, doc(cfg(feature = "clone-impls")))]
        impl Clone for $ty {
            fn clone(&self) -> Self {
                $ty {
                    repr: self.repr.clone(),
                }
            }
        }

        #[cfg(feature = "extra-traits")]
        #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
        impl PartialEq for $ty {
            fn eq(&self, other: &Self) -> bool {
                self.repr.token.to_string() == other.repr.token.to_string()
            }
        }

        #[cfg(feature = "extra-traits")]
        #[cfg_attr(docsrs, doc(cfg(feature = "extra-traits")))]
        impl Hash for $ty {
            fn hash<H>(&self, state: &mut H)
            where
                H: Hasher,
            {
                self.repr.token.to_string().hash(state);
            }
        }

        #[cfg(feature = "parsing")]
        pub_if_not_doc! {
            #[doc(hidden)]
            #[allow(non_snake_case)]
            pub fn $ty(marker: lookahead::TokenMarker) -> $ty {
                match marker {}
            }
        }
    };
}

lit_extra_traits!(LitStr);
lit_extra_traits!(LitByteStr);
lit_extra_traits!(LitCStr);
lit_extra_traits!(LitByte);
lit_extra_traits!(LitChar);
lit_extra_traits!(LitInt);
lit_extra_traits!(LitFloat);

#[cfg(feature = "parsing")]
pub_if_not_doc! {
    #[doc(hidden)]
    #[allow(non_snake_case)]
    pub fn LitBool(marker: lookahead::TokenMarker) -> LitBool {
        match marker {}
    }
}

#[cfg(feature = "parsing")]
pub_if_not_doc! {
    #[doc(hidden)]
    #[allow(non_snake_case)]
    pub fn Lit(marker: lookahead::TokenMarker) -> Lit {
        match marker {}
    }
}

#[cfg(feature = "parsing")]
pub(crate) mod parsing {
    use crate::buffer::Cursor;
    use crate::error::Result;
    use crate::lit::{
        value, Lit, LitBool, LitByte, LitByteStr, LitCStr, LitChar, LitFloat, LitFloatRepr, LitInt,
        LitIntRepr, LitStr,
    };
    use crate::parse::{Parse, ParseStream, Unexpected};
    use crate::token::{self, Token};
    use alloc::boxed::Box;
    use alloc::rc::Rc;
    use alloc::string::ToString;
    use core::cell::Cell;
    use proc_macro2::{Literal, Punct, Span};

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for Lit {
        fn parse(input: ParseStream) -> Result<Self> {
            input.step(|cursor| {
                if let Some((lit, rest)) = cursor.literal() {
                    return Ok((Lit::new(lit), rest));
                }

                if let Some((ident, rest)) = cursor.ident() {
                    let value = ident == "true";
                    if value || ident == "false" {
                        let lit_bool = LitBool {
                            value,
                            span: ident.span(),
                        };
                        return Ok((Lit::Bool(lit_bool), rest));
                    }
                }

                if let Some((punct, rest)) = cursor.punct() {
                    if punct.as_char() == '-' {
                        if let Some((lit, rest)) = parse_negative_lit(punct, rest) {
                            return Ok((lit, rest));
                        }
                    }
                }

                Err(cursor.error("expected literal"))
            })
        }
    }

    fn parse_negative_lit(neg: Punct, cursor: Cursor) -> Option<(Lit, Cursor)> {
        let (lit, rest) = cursor.literal()?;

        let mut span = neg.span();
        span = span.join(lit.span()).unwrap_or(span);

        let mut repr = lit.to_string();
        repr.insert(0, '-');

        if let Some((digits, suffix)) = value::parse_lit_int(&repr) {
            let mut token: Literal = repr.parse().unwrap();
            token.set_span(span);
            return Some((
                Lit::Int(LitInt {
                    repr: Box::new(LitIntRepr {
                        token,
                        digits,
                        suffix,
                    }),
                }),
                rest,
            ));
        }

        let (digits, suffix) = value::parse_lit_float(&repr)?;
        let mut token: Literal = repr.parse().unwrap();
        token.set_span(span);
        Some((
            Lit::Float(LitFloat {
                repr: Box::new(LitFloatRepr {
                    token,
                    digits,
                    suffix,
                }),
            }),
            rest,
        ))
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for LitStr {
        fn parse(input: ParseStream) -> Result<Self> {
            let head = input.fork();
            match input.parse() {
                Ok(Lit::Str(lit)) => Ok(lit),
                _ => Err(head.error("expected string literal")),
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for LitByteStr {
        fn parse(input: ParseStream) -> Result<Self> {
            let head = input.fork();
            match input.parse() {
                Ok(Lit::ByteStr(lit)) => Ok(lit),
                _ => Err(head.error("expected byte string literal")),
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for LitCStr {
        fn parse(input: ParseStream) -> Result<Self> {
            let head = input.fork();
            match input.parse() {
                Ok(Lit::CStr(lit)) => Ok(lit),
                _ => Err(head.error("expected C string literal")),
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for LitByte {
        fn parse(input: ParseStream) -> Result<Self> {
            let head = input.fork();
            match input.parse() {
                Ok(Lit::Byte(lit)) => Ok(lit),
                _ => Err(head.error("expected byte literal")),
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for LitChar {
        fn parse(input: ParseStream) -> Result<Self> {
            let head = input.fork();
            match input.parse() {
                Ok(Lit::Char(lit)) => Ok(lit),
                _ => Err(head.error("expected character literal")),
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for LitInt {
        fn parse(input: ParseStream) -> Result<Self> {
            let head = input.fork();
            match input.parse() {
                Ok(Lit::Int(lit)) => Ok(lit),
                _ => Err(head.error("expected integer literal")),
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for LitFloat {
        fn parse(input: ParseStream) -> Result<Self> {
            let head = input.fork();
            match input.parse() {
                Ok(Lit::Float(lit)) => Ok(lit),
                _ => Err(head.error("expected floating point literal")),
            }
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "parsing")))]
    impl Parse for LitBool {
        fn parse(input: ParseStream) -> Result<Self> {
            let head = input.fork();
            match input.parse() {
                Ok(Lit::Bool(lit)) => Ok(lit),
                _ => Err(head.error("expected boolean literal")),
            }
        }
    }

    fn peek_impl(cursor: Cursor, peek: fn(ParseStream) -> bool) -> bool {
        let scope = Span::call_site();
        let unexpected = Rc::new(Cell::new(Unexpected::None));
        let buffer = crate::parse::new_parse_buffer(scope, cursor, unexpected);
        peek(&buffer)
    }

    macro_rules! impl_token {
        ($display:literal $name:ty) => {
            impl Token for $name {
                fn peek(cursor: Cursor) -> bool {
                    fn peek(input: ParseStream) -> bool {
                        <$name as Parse>::parse(input).is_ok()
                    }
                    peek_impl(cursor, peek)
                }

                fn display() -> &'static str {
                    $display
                }
            }

            impl token::private::Sealed for $name {}
        };
    }

    impl_token!("literal" Lit);
    impl_token!("string literal" LitStr);
    impl_token!("byte string literal" LitByteStr);
    impl_token!("C-string literal" LitCStr);
    impl_token!("byte literal" LitByte);
    impl_token!("character literal" LitChar);
    impl_token!("integer literal" LitInt);
    impl_token!("floating point literal" LitFloat);
    impl_token!("boolean literal" LitBool);
}

#[cfg(feature = "printing")]
mod printing {
    use crate::lit::{LitBool, LitByte, LitByteStr, LitCStr, LitChar, LitFloat, LitInt, LitStr};
    use proc_macro2::TokenStream;
    use quote::{ToTokens, TokenStreamExt as _};

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for LitStr {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            self.repr.token.to_tokens(tokens);
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for LitByteStr {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            self.repr.token.to_tokens(tokens);
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for LitCStr {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            self.repr.token.to_tokens(tokens);
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for LitByte {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            self.repr.token.to_tokens(tokens);
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for LitChar {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            self.repr.token.to_tokens(tokens);
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for LitInt {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            self.repr.token.to_tokens(tokens);
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for LitFloat {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            self.repr.token.to_tokens(tokens);
        }
    }

    #[cfg_attr(docsrs, doc(cfg(feature = "printing")))]
    impl ToTokens for LitBool {
        fn to_tokens(&self, tokens: &mut TokenStream) {
            tokens.append(self.token());
        }
    }
}

mod value {
    use crate::bigint::BigInt;
    use crate::lit::{
        Lit, LitBool, LitByte, LitByteStr, LitCStr, LitChar, LitFloat, LitFloatRepr, LitInt,
        LitIntRepr, LitRepr, LitStr,
    };
    use alloc::borrow::ToOwned;
    use alloc::boxed::Box;
    use alloc::ffi::CString;
    use alloc::string::{String, ToString};
    use alloc::vec::Vec;
    use core::char;
    use core::ops::{Index, RangeFrom};
    use proc_macro2::{Literal, Span};

    impl Lit {
        /// Interpret a Syn literal from a proc-macro2 literal.
        pub fn new(token: Literal) -> Self {
            let repr = token.to_string();
            Lit::from_str(token, &repr)
        }

        #[cfg(fuzzing)]
        #[doc(hidden)]
        pub fn from_str_for_fuzzing(repr: &str) -> Self {
            let token = Literal::u8_unsuffixed(0);
            Lit::from_str(token, repr)
        }

        fn from_str(token: Literal, repr: &str) -> Self {
            match byte(repr, 0) {
                // "...", r"...", r#"..."#
                b'"' | b'r' => {
                    if let Some((_, suffix)) = parse_lit_str(repr) {
                        return Lit::Str(LitStr {
                            repr: Box::new(LitRepr { token, suffix }),
                        });
                    }
                }
                b'b' => match byte(repr, 1) {
                    // b"...", br"...", br#"...#"
                    b'"' | b'r' => {
                        if let Some((_, suffix)) = parse_lit_byte_str(repr) {
                            return Lit::ByteStr(LitByteStr {
                                repr: Box::new(LitRepr { token, suffix }),
                            });
                        }
                    }
                    // b'...'
                    b'\'' => {
                        if let Some((_, suffix)) = parse_lit_byte(repr) {
                            return Lit::Byte(LitByte {
                                repr: Box::new(LitRepr { token, suffix }),
                            });
                        }
                    }
                    _ => {}
                },
                b'c' => match byte(repr, 1) {
                    // c"...", cr"...", cr#"..."#
                    b'"' | b'r' => {
                        if let Some((_, suffix)) = parse_lit_c_str(repr) {
                            return Lit::CStr(LitCStr {
                                repr: Box::new(LitRepr { token, suffix }),
                            });
                        }
                    }
                    _ => {}
                },
                // '...'
                b'\'' => {
                    if let Some((_, suffix)) = parse_lit_char(repr) {
                        return Lit::Char(LitChar {
                            repr: Box::new(LitRepr { token, suffix }),
                        });
                    }
                }
                b'0'..=b'9' | b'-' => {
                    // 0, 123, 0xFF, 0o77, 0b11
                    if let Some((digits, suffix)) = parse_lit_int(repr) {
                        return Lit::Int(LitInt {
                            repr: Box::new(LitIntRepr {
                                token,
                                digits,
                                suffix,
                            }),
                        });
                    }
                    // 1.0, 1e-1, 1e+1
                    if let Some((digits, suffix)) = parse_lit_float(repr) {
                        return Lit::Float(LitFloat {
                            repr: Box::new(LitFloatRepr {
                                token,
                                digits,
                                suffix,
                            }),
                        });
                    }
                }
                // true, false
                b't' | b'f' if repr == "true" || repr == "false" => {
                    return Lit::Bool(LitBool {
                        value: repr == "true",
                        span: token.span(),
                    });
                }
                b'(' if repr == "(/*ERROR*/)" => return Lit::Verbatim(token),
                _ => {}
            }

            Lit::Verbatim(token)
        }

        pub fn suffix(&self) -> &str {
            match self {
                Lit::Str(lit) => lit.suffix(),
                Lit::ByteStr(lit) => lit.suffix(),
                Lit::CStr(lit) => lit.suffix(),
                Lit::Byte(lit) => lit.suffix(),
                Lit::Char(lit) => lit.suffix(),
                Lit::Int(lit) => lit.suffix(),
                Lit::Float(lit) => lit.suffix(),
                Lit::Bool(_) | Lit::Verbatim(_) => "",
            }
        }

        pub fn span(&self) -> Span {
            match self {
                Lit::Str(lit) => lit.span(),
                Lit::ByteStr(lit) => lit.span(),
                Lit::CStr(lit) => lit.span(),
                Lit::Byte(lit) => lit.span(),
                Lit::Char(lit) => lit.span(),
                Lit::Int(lit) => lit.span(),
                Lit::Float(lit) => lit.span(),
                Lit::Bool(lit) => lit.span,
                Lit::Verbatim(lit) => lit.span(),
            }
        }

        pub fn set_span(&mut self, span: Span) {
            match self {
                Lit::Str(lit) => lit.set_span(span),
                Lit::ByteStr(lit) => lit.set_span(span),
                Lit::CStr(lit) => lit.set_span(span),
                Lit::Byte(lit) => lit.set_span(span),
                Lit::Char(lit) => lit.set_span(span),
                Lit::Int(lit) => lit.set_span(span),
                Lit::Float(lit) => lit.set_span(span),
                Lit::Bool(lit) => lit.span = span,
                Lit::Verbatim(lit) => lit.set_span(span),
            }
        }
    }

    /// Get the byte at offset idx, or a default of `b'\0'` if we're looking
    /// past the end of the input buffer.
    pub(crate) fn byte<S: AsRef<[u8]> + ?Sized>(s: &S, idx: usize) -> u8 {
        let s = s.as_ref();
        if idx < s.len() {
            s[idx]
        } else {
            0
        }
    }

    fn next_chr(s: &str) -> char {
        s.chars().next().unwrap_or('\0')
    }

    // Returns (content, suffix).
    pub(crate) fn parse_lit_str(s: &str) -> Option<(Box<str>, Box<str>)> {
        match byte(s, 0) {
            b'"' => parse_lit_str_cooked(s),
            b'r' => parse_lit_str_raw(s),
            _ => unreachable!(),
        }
    }

    fn parse_lit_str_cooked(mut s: &str) -> Option<(Box<str>, Box<str>)> {
        assert_eq!(byte(s, 0), b'"');
        s = &s[1..];

        let mut content = String::new();
        'outer: loop {
            let ch = match byte(s, 0) {
                b'"' => break,
                b'\\' => {
                    let b = byte(s, 1);
                    s = s.get(2..)?;
                    match b {
                        b'x' => {
                            let (byte, rest) = backslash_x(s)?;
                            s = rest;
                            if byte > 0x7F {
                                // invalid \x byte in string literal
                                return None;
                            }
                            char::from(byte)
                        }
                        b'u' => {
                            let (ch, rest) = backslash_u(s)?;
                            s = rest;
                            ch
                        }
                        b'n' => '\n',
                        b'r' => '\r',
                        b't' => '\t',
                        b'\\' => '\\',
                        b'0' => '\0',
                        b'\'' => '\'',
                        b'"' => '"',
                        b'\r' | b'\n' => loop {
                            let b = byte(s, 0);
                            match b {
                                b' ' | b'\t' | b'\n' | b'\r' => s = &s[1..],
                                _ => continue 'outer,
                            }
                        },
                        _ => {
                            // unexpected byte after backslash
                            return None;
                        }
                    }
                }
                b'\r' => {
                    if byte(s, 1) != b'\n' {
                        // bare carriage return not allowed in string
                        return None;
                    }
                    s = &s[2..];
                    '\n'
                }
                _ => {
                    let ch = next_chr(s);
                    s = s.get(ch.len_utf8()..)?;
                    ch
                }
            };
            content.push(ch);
        }

        assert!(s.starts_with('"'));
        let content = content.into_boxed_str();
        let suffix = s[1..].to_owned().into_boxed_str();
        Some((content, suffix))
    }

    fn parse_lit_str_raw(mut s: &str) -> Option<(Box<str>, Box<str>)> {
        assert_eq!(byte(s, 0), b'r');
        s = &s[1..];

        let mut pounds = 0;
        loop {
            match byte(s, pounds) {
                b'#' => pounds += 1,
                b'"' => break,
                _ => return None,
            }
        }
        let close = s.rfind('"').unwrap();
        for end in s.get(close + 1..close + 1 + pounds)?.bytes() {
            if end != b'#' {
                return None;
            }
        }

        let content = s.get(pounds + 1..close)?.to_owned().into_boxed_str();
        let suffix = s[close + 1 + pounds..].to_owned().into_boxed_str();
        Some((content, suffix))
    }

    // Returns (content, suffix).
    pub(crate) fn parse_lit_byte_str(s: &str) -> Option<(Vec<u8>, Box<str>)> {
        assert_eq!(byte(s, 0), b'b');
        match byte(s, 1) {
            b'"' => parse_lit_byte_str_cooked(s),
            b'r' => parse_lit_byte_str_raw(s),
            _ => unreachable!(),
        }
    }

    fn parse_lit_byte_str_cooked(mut s: &str) -> Option<(Vec<u8>, Box<str>)> {
        assert_eq!(byte(s, 0), b'b');
        assert_eq!(byte(s, 1), b'"');
        s = &s[2..];

        // We're going to want to have slices which don't respect codepoint boundaries.
        let mut v = s.as_bytes();

        let mut out = Vec::new();
        'outer: loop {
            let byte = match byte(v, 0) {
                b'"' => break,
                b'\\' => {
                    let b = byte(v, 1);
                    v = v.get(2..)?;
                    match b {
                        b'x' => {
                            let (b, rest) = backslash_x(v)?;
                            v = rest;
                            b
                        }
                        b'n' => b'\n',
                        b'r' => b'\r',
                        b't' => b'\t',
                        b'\\' => b'\\',
                        b'0' => b'\0',
                        b'\'' => b'\'',
                        b'"' => b'"',
                        b'\r' | b'\n' => loop {
                            let byte = byte(v, 0);
                            if matches!(byte, b' ' | b'\t' | b'\n' | b'\r') {
                                v = &v[1..];
                            } else {
                                continue 'outer;
                            }
                        },
                        _ => {
                            // unexpected byte after backslash
                            return None;
                        }
                    }
                }
                b'\r' => {
                    if byte(v, 1) != b'\n' {
                        // bare carriage return not allowed in string
                        return None;
                    }
                    v = &v[2..];
                    b'\n'
                }
                b => {
                    v = v.get(1..)?;
                    b
                }
            };
            out.push(byte);
        }

        assert_eq!(byte(v, 0), b'"');
        let suffix = s[s.len() - v.len() + 1..].to_owned().into_boxed_str();
        Some((out, suffix))
    }

    fn parse_lit_byte_str_raw(s: &str) -> Option<(Vec<u8>, Box<str>)> {
        assert_eq!(byte(s, 0), b'b');
        let (value, suffix) = parse_lit_str_raw(&s[1..])?;
        Some((String::from(value).into_bytes(), suffix))
    }

    // Returns (content, suffix).
    pub(crate) fn parse_lit_c_str(s: &str) -> Option<(CString, Box<str>)> {
        assert_eq!(byte(s, 0), b'c');
        match byte(s, 1) {
            b'"' => parse_lit_c_str_cooked(s),
            b'r' => parse_lit_c_str_raw(s),
            _ => unreachable!(),
        }
    }

    fn parse_lit_c_str_cooked(mut s: &str) -> Option<(CString, Box<str>)> {
        assert_eq!(byte(s, 0), b'c');
        assert_eq!(byte(s, 1), b'"');
        s = &s[2..];

        // We're going to want to have slices which don't respect codepoint boundaries.
        let mut v = s.as_bytes();

        let mut out = Vec::new();
        'outer: loop {
            let byte = match byte(v, 0) {
                b'"' => break,
                b'\\' => {
                    let b = byte(v, 1);
                    v = v.get(2..)?;
                    match b {
                        b'x' => {
                            let (b, rest) = backslash_x(v)?;
                            if b == 0 {
                                // \x00 is not allowed in C-string literal
                                return None;
                            }
                            v = rest;
                            b
                        }
                        b'u' => {
                            let (ch, rest) = backslash_u(v)?;
                            if ch == '\0' {
                                // \u{0} is not allowed in C-string literal
                                return None;
                            }
                            v = rest;
                            out.extend_from_slice(ch.encode_utf8(&mut [0u8; 4]).as_bytes());
                            continue 'outer;
                        }
                        b'n' => b'\n',
                        b'r' => b'\r',
                        b't' => b'\t',
                        b'\\' => b'\\',
                        b'\'' => b'\'',
                        b'"' => b'"',
                        b'\r' | b'\n' => loop {
                            let byte = byte(v, 0);
                            if matches!(byte, b' ' | b'\t' | b'\n' | b'\r') {
                                v = &v[1..];
                            } else {
                                continue 'outer;
                            }
                        },
                        _ => {
                            // unexpected byte after backslash
                            return None;
                        }
                    }
                }
                b'\r' => {
                    if byte(v, 1) != b'\n' {
                        // bare carriage return not allowed in string
                        return None;
                    }
                    v = &v[2..];
                    b'\n'
                }
                b => {
                    v = v.get(1..)?;
                    b
                }
            };
            out.push(byte);
        }

        assert_eq!(byte(v, 0), b'"');
        let suffix = s[s.len() - v.len() + 1..].to_owned().into_boxed_str();
        let cstring = CString::new(out).ok()?;
        Some((cstring, suffix))
    }

    fn parse_lit_c_str_raw(s: &str) -> Option<(CString, Box<str>)> {
        assert_eq!(byte(s, 0), b'c');
        let (value, suffix) = parse_lit_str_raw(&s[1..])?;
        let cstring = CString::new(String::from(value)).ok()?;
        Some((cstring, suffix))
    }

    // Returns (value, suffix).
    pub(crate) fn parse_lit_byte(s: &str) -> Option<(u8, Box<str>)> {
        assert_eq!(byte(s, 0), b'b');
        assert_eq!(byte(s, 1), b'\'');

        // We're going to want to have slices which don't respect codepoint boundaries.
        let mut v = &s.as_bytes()[2..];

        let b = match byte(v, 0) {
            b'\\' => {
                let b = byte(v, 1);
                v = v.get(2..)?;
                match b {
                    b'x' => {
                        let (b, rest) = backslash_x(v)?;
                        v = rest;
                        b
                    }
                    b'n' => b'\n',
                    b'r' => b'\r',
                    b't' => b'\t',
                    b'\\' => b'\\',
                    b'0' => b'\0',
                    b'\'' => b'\'',
                    b'"' => b'"',
                    _ => {
                        // unexpected byte after backslash
                        return None;
                    }
                }
            }
            b => {
                v = v.get(1..)?;
                b
            }
        };

        if byte(v, 0) != b'\'' {
            return None;
        }

        let suffix = s[s.len() - v.len() + 1..].to_owned().into_boxed_str();
        Some((b, suffix))
    }

    // Returns (value, suffix).
    pub(crate) fn parse_lit_char(mut s: &str) -> Option<(char, Box<str>)> {
        assert_eq!(byte(s, 0), b'\'');
        s = &s[1..];

        let ch = match byte(s, 0) {
            b'\\' => {
                let b = byte(s, 1);
                s = s.get(2..)?;
                match b {
                    b'x' => {
                        let (byte, rest) = backslash_x(s)?;
                        s = rest;
                        if byte > 0x7F {
                            // invalid \x byte in character literal
                            return None;
                        }
                        char::from(byte)
                    }
                    b'u' => {
                        let (ch, rest) = backslash_u(s)?;
                        s = rest;
                        ch
                    }
                    b'n' => '\n',
                    b'r' => '\r',
                    b't' => '\t',
                    b'\\' => '\\',
                    b'0' => '\0',
                    b'\'' => '\'',
                    b'"' => '"',
                    _ => {
                        // unexpected byte after backslash
                        return None;
                    }
                }
            }
            _ => {
                let ch = next_chr(s);
                s = s.get(ch.len_utf8()..)?;
                ch
            }
        };

        if byte(s, 0) != b'\'' {
            return None;
        }

        let suffix = s[1..].to_owned().into_boxed_str();
        Some((ch, suffix))
    }

    fn backslash_x<S>(s: &S) -> Option<(u8, &S)>
    where
        S: Index<RangeFrom<usize>, Output = S> + AsRef<[u8]> + ?Sized,
    {
        let mut ch = 0;
        let b0 = byte(s, 0);
        let b1 = byte(s, 1);
        ch += 0x10
            * match b0 {
                b'0'..=b'9' => b0 - b'0',
                b'a'..=b'f' => 10 + (b0 - b'a'),
                b'A'..=b'F' => 10 + (b0 - b'A'),
                _ => return None,
            };
        ch += match b1 {
            b'0'..=b'9' => b1 - b'0',
            b'a'..=b'f' => 10 + (b1 - b'a'),
            b'A'..=b'F' => 10 + (b1 - b'A'),
            _ => return None,
        };
        Some((ch, &s[2..]))
    }

    fn backslash_u<S>(mut s: &S) -> Option<(char, &S)>
    where
        S: Index<RangeFrom<usize>, Output = S> + AsRef<[u8]> + ?Sized,
    {
        if byte(s, 0) != b'{' {
            return None;
        }
        s = &s[1..];

        let mut ch = 0;
        let mut digits = 0;
        loop {
            let b = byte(s, 0);
            let digit = match b {
                b'0'..=b'9' => b - b'0',
                b'a'..=b'f' => 10 + b - b'a',
                b'A'..=b'F' => 10 + b - b'A',
                b'_' if digits > 0 => {
                    s = &s[1..];
                    continue;
                }
                b'}' if digits == 0 => return None,
                b'}' => break,
                _ => return None,
            };
            if digits == 6 {
                return None;
            }
            ch *= 0x10;
            ch += u32::from(digit);
            digits += 1;
            s = &s[1..];
        }
        if byte(s, 0) != b'}' {
            return None;
        }
        s = &s[1..];

        let ch = char::from_u32(ch)?;
        Some((ch, s))
    }

    // Returns base 10 digits and suffix.
    pub(crate) fn parse_lit_int(mut s: &str) -> Option<(Box<str>, Box<str>)> {
        let negative = byte(s, 0) == b'-';
        if negative {
            s = &s[1..];
        }

        let base = match (byte(s, 0), byte(s, 1)) {
            (b'0', b'x') => {
                s = &s[2..];
                16
            }
            (b'0', b'o') => {
                s = &s[2..];
                8
            }
            (b'0', b'b') => {
                s = &s[2..];
                2
            }
            (b'0'..=b'9', _) => 10,
            _ => return None,
        };

        let mut value = BigInt::new();
        let mut has_digit = false;
        'outer: loop {
            let b = byte(s, 0);
            let digit = match b {
                b'0'..=b'9' => b - b'0',
                b'a'..=b'f' if base > 10 => b - b'a' + 10,
                b'A'..=b'F' if base > 10 => b - b'A' + 10,
                b'_' => {
                    s = &s[1..];
                    continue;
                }
                // If looking at a floating point literal, we don't want to
                // consider it an integer.
                b'.' if base == 10 => return None,
                b'e' | b'E' if base == 10 => {
                    let mut has_exp = false;
                    for (i, b) in s[1..].bytes().enumerate() {
                        match b {
                            b'_' => {}
                            b'-' | b'+' => return None,
                            b'0'..=b'9' => has_exp = true,
                            _ => {
                                let suffix = &s[1 + i..];
                                if has_exp && crate::ident::xid_ok(suffix) {
                                    return None;
                                } else {
                                    break 'outer;
                                }
                            }
                        }
                    }
                    if has_exp {
                        return None;
                    } else {
                        break;
                    }
                }
                _ => break,
            };

            if digit >= base {
                return None;
            }

            has_digit = true;
            value *= base;
            value += digit;
            s = &s[1..];
        }

        if !has_digit {
            return None;
        }

        let suffix = s;
        if suffix.is_empty() || crate::ident::xid_ok(suffix) {
            let mut repr = value.to_string();
            if negative {
                repr.insert(0, '-');
            }
            Some((repr.into_boxed_str(), suffix.to_owned().into_boxed_str()))
        } else {
            None
        }
    }

    // Returns base 10 digits and suffix.
    pub(crate) fn parse_lit_float(input: &str) -> Option<(Box<str>, Box<str>)> {
        // Rust's floating point literals are very similar to the ones parsed by
        // the standard library, except that rust's literals can contain
        // ignorable underscores. Let's remove those underscores.

        let mut bytes = input.to_owned().into_bytes();

        let start = (*bytes.first()? == b'-') as usize;
        match bytes.get(start)? {
            b'0'..=b'9' => {}
            _ => return None,
        }

        let mut read = start;
        let mut write = start;
        let mut has_dot = false;
        let mut has_e = false;
        let mut has_sign = false;
        let mut has_exponent = false;
        while read < bytes.len() {
            match bytes[read] {
                b'_' => {
                    // Don't increase write
                    read += 1;
                    continue;
                }
                b'0'..=b'9' => {
                    if has_e {
                        has_exponent = true;
                    }
                    bytes[write] = bytes[read];
                }
                b'.' => {
                    if has_e || has_dot {
                        return None;
                    }
                    has_dot = true;
                    bytes[write] = b'.';
                }
                b'e' | b'E' => {
                    match bytes[read + 1..]
                        .iter()
                        .find(|b| **b != b'_')
                        .unwrap_or(&b'\0')
                    {
                        b'-' | b'+' | b'0'..=b'9' => {}
                        _ => break,
                    }
                    if has_e {
                        if has_exponent {
                            break;
                        } else {
                            return None;
                        }
                    }
                    has_e = true;
                    bytes[write] = b'e';
                }
                b'-' | b'+' => {
                    if has_sign || has_exponent || !has_e {
                        return None;
                    }
                    has_sign = true;
                    if bytes[read] == b'-' {
                        bytes[write] = bytes[read];
                    } else {
                        // Omit '+'
                        read += 1;
                        continue;
                    }
                }
                _ => break,
            }
            read += 1;
            write += 1;
        }

        if has_e && !has_exponent {
            return None;
        }

        let mut digits = String::from_utf8(bytes).unwrap();
        let suffix = digits.split_off(read);
        digits.truncate(write);
        if suffix.is_empty() || crate::ident::xid_ok(&suffix) {
            Some((digits.into_boxed_str(), suffix.into_boxed_str()))
        } else {
            None
        }
    }
}