token.rs

pub use BinOpToken::*;
pub use LitKind::*;
pub use Nonterminal::*;
pub use NtExprKind::*;
pub use NtPatKind::*;
pub use TokenKind::*;

use crate::ast;
use crate::ptr::P;
use crate::util::case::Case;

use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
use rustc_data_structures::sync::Lrc;
use rustc_macros::{Decodable, Encodable, HashStable_Generic};
use rustc_span::symbol::{kw, sym};
#[allow(clippy::useless_attribute)] // FIXME: following use of `hidden_glob_reexports` incorrectly triggers `useless_attribute` lint.
#[allow(hidden_glob_reexports)]
use rustc_span::symbol::{Ident, Symbol};
use rustc_span::{edition::Edition, ErrorGuaranteed, Span, DUMMY_SP};
use std::borrow::Cow;
use std::fmt;

#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
pub enum CommentKind {
    Line,
    Block,
}

#[derive(Clone, PartialEq, Encodable, Decodable, Hash, Debug, Copy)]
#[derive(HashStable_Generic)]
pub enum BinOpToken {
    Plus,
    Minus,
    Star,
    Slash,
    Percent,
    Caret,
    And,
    Or,
    Shl,
    Shr,
}

/// Describes how a sequence of token trees is delimited.
/// Cannot use `proc_macro::Delimiter` directly because this
/// structure should implement some additional traits.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
#[derive(Encodable, Decodable, Hash, HashStable_Generic)]
pub enum Delimiter {
    /// `( ... )`
    Parenthesis,
    /// `{ ... }`
    Brace,
    /// `[ ... ]`
    Bracket,
    /// `∅ ... ∅`
    /// An invisible delimiter, that may, for example, appear around tokens coming from a
    /// "macro variable" `$var`. It is important to preserve operator priorities in cases like
    /// `$var * 3` where `$var` is `1 + 2`.
    /// Invisible delimiters might not survive roundtrip of a token stream through a string.
    Invisible,
}

// Note that the suffix is *not* considered when deciding the `LitKind` in this
// type. This means that float literals like `1f32` are classified by this type
// as `Int`. Only upon conversion to `ast::LitKind` will such a literal be
// given the `Float` kind.
#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
pub enum LitKind {
    Bool, // AST only, must never appear in a `Token`
    Byte,
    Char,
    Integer, // e.g. `1`, `1u8`, `1f32`
    Float,   // e.g. `1.`, `1.0`, `1e3f32`
    Str,
    StrRaw(u8), // raw string delimited by `n` hash symbols
    ByteStr,
    ByteStrRaw(u8), // raw byte string delimited by `n` hash symbols
    CStr,
    CStrRaw(u8),
    Err(ErrorGuaranteed),
}

/// A literal token.
#[derive(Clone, Copy, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
pub struct Lit {
    pub kind: LitKind,
    pub symbol: Symbol,
    pub suffix: Option<Symbol>,
}

impl Lit {
    pub fn new(kind: LitKind, symbol: Symbol, suffix: Option<Symbol>) -> Lit {
        Lit { kind, symbol, suffix }
    }

    /// Returns `true` if this is semantically a float literal. This includes
    /// ones like `1f32` that have an `Integer` kind but a float suffix.
    pub fn is_semantic_float(&self) -> bool {
        match self.kind {
            LitKind::Float => true,
            LitKind::Integer => match self.suffix {
                Some(sym) => sym == sym::f32 || sym == sym::f64,
                None => false,
            },
            _ => false,
        }
    }

    /// Keep this in sync with `Token::can_begin_literal_maybe_minus` excluding unary negation.
    pub fn from_token(token: &Token) -> Option<Lit> {
        match token.uninterpolate().kind {
            Ident(name, IdentIsRaw::No) if name.is_bool_lit() => Some(Lit::new(Bool, name, None)),
            Literal(token_lit) => Some(token_lit),
            Interpolated(ref nt)
                if let NtExpr(expr) | NtLiteral(expr) = &**nt
                    && let ast::ExprKind::Lit(token_lit) = expr.kind =>
            {
                Some(token_lit)
            }
            _ => None,
        }
    }
}

impl fmt::Display for Lit {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let Lit { kind, symbol, suffix } = *self;
        match kind {
            Byte => write!(f, "b'{symbol}'")?,
            Char => write!(f, "'{symbol}'")?,
            Str => write!(f, "\"{symbol}\"")?,
            StrRaw(n) => write!(
                f,
                "r{delim}\"{string}\"{delim}",
                delim = "#".repeat(n as usize),
                string = symbol
            )?,
            ByteStr => write!(f, "b\"{symbol}\"")?,
            ByteStrRaw(n) => write!(
                f,
                "br{delim}\"{string}\"{delim}",
                delim = "#".repeat(n as usize),
                string = symbol
            )?,
            CStr => write!(f, "c\"{symbol}\"")?,
            CStrRaw(n) => {
                write!(f, "cr{delim}\"{symbol}\"{delim}", delim = "#".repeat(n as usize))?
            }
            Integer | Float | Bool | Err(_) => write!(f, "{symbol}")?,
        }

        if let Some(suffix) = suffix {
            write!(f, "{suffix}")?;
        }

        Ok(())
    }
}

impl LitKind {
    /// An English article for the literal token kind.
    pub fn article(self) -> &'static str {
        match self {
            Integer | Err(_) => "an",
            _ => "a",
        }
    }

    pub fn descr(self) -> &'static str {
        match self {
            Bool => "boolean",
            Byte => "byte",
            Char => "char",
            Integer => "integer",
            Float => "float",
            Str | StrRaw(..) => "string",
            ByteStr | ByteStrRaw(..) => "byte string",
            CStr | CStrRaw(..) => "C string",
            Err(_) => "error",
        }
    }

    pub(crate) fn may_have_suffix(self) -> bool {
        matches!(self, Integer | Float | Err(_))
    }
}

pub fn ident_can_begin_expr(name: Symbol, span: Span, is_raw: IdentIsRaw) -> bool {
    let ident_token = Token::new(Ident(name, is_raw), span);

    !ident_token.is_reserved_ident()
        || ident_token.is_path_segment_keyword()
        || [
            kw::Async,
            kw::Do,
            kw::Box,
            kw::Break,
            kw::Const,
            kw::Continue,
            kw::False,
            kw::For,
            kw::Gen,
            kw::If,
            kw::Let,
            kw::Loop,
            kw::Match,
            kw::Move,
            kw::Return,
            kw::True,
            kw::Try,
            kw::Unsafe,
            kw::While,
            kw::Yield,
            kw::Safe,
            kw::Static,
        ]
        .contains(&name)
}

fn ident_can_begin_type(name: Symbol, span: Span, is_raw: IdentIsRaw) -> bool {
    let ident_token = Token::new(Ident(name, is_raw), span);

    !ident_token.is_reserved_ident()
        || ident_token.is_path_segment_keyword()
        || [kw::Underscore, kw::For, kw::Impl, kw::Fn, kw::Unsafe, kw::Extern, kw::Typeof, kw::Dyn]
            .contains(&name)
}

#[derive(PartialEq, Encodable, Decodable, Debug, Copy, Clone, HashStable_Generic)]
pub enum IdentIsRaw {
    No,
    Yes,
}

impl From<bool> for IdentIsRaw {
    fn from(b: bool) -> Self {
        if b { Self::Yes } else { Self::No }
    }
}

impl From<IdentIsRaw> for bool {
    fn from(is_raw: IdentIsRaw) -> bool {
        matches!(is_raw, IdentIsRaw::Yes)
    }
}

// SAFETY: due to the `Clone` impl below, all fields of all variants other than
// `Interpolated` must impl `Copy`.
#[derive(PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
pub enum TokenKind {
    /* Expression-operator symbols. */
    /// `=`
    Eq,
    /// `<`
    Lt,
    /// `<=`
    Le,
    /// `==`
    EqEq,
    /// `!=`
    Ne,
    /// `>=`
    Ge,
    /// `>`
    Gt,
    /// `&&`
    AndAnd,
    /// `||`
    OrOr,
    /// `!`
    Not,
    /// `~`
    Tilde,
    BinOp(BinOpToken),
    BinOpEq(BinOpToken),

    /* Structural symbols */
    /// `@`
    At,
    /// `.`
    Dot,
    /// `..`
    DotDot,
    /// `...`
    DotDotDot,
    /// `..=`
    DotDotEq,
    /// `,`
    Comma,
    /// `;`
    Semi,
    /// `:`
    Colon,
    /// `::`
    PathSep,
    /// `->`
    RArrow,
    /// `<-`
    LArrow,
    /// `=>`
    FatArrow,
    /// `#`
    Pound,
    /// `$`
    Dollar,
    /// `?`
    Question,
    /// Used by proc macros for representing lifetimes, not generated by lexer right now.
    SingleQuote,
    /// An opening delimiter (e.g., `{`).
    OpenDelim(Delimiter),
    /// A closing delimiter (e.g., `}`).
    CloseDelim(Delimiter),

    /* Literals */
    Literal(Lit),

    /// Identifier token.
    /// Do not forget about `NtIdent` when you want to match on identifiers.
    /// It's recommended to use `Token::(ident,uninterpolate,uninterpolated_span)` to
    /// treat regular and interpolated identifiers in the same way.
    Ident(Symbol, IdentIsRaw),
    /// This identifier (and its span) is the identifier passed to the
    /// declarative macro. The span in the surrounding `Token` is the span of
    /// the `ident` metavariable in the macro's RHS.
    NtIdent(Ident, IdentIsRaw),

    /// Lifetime identifier token.
    /// Do not forget about `NtLifetime` when you want to match on lifetime identifiers.
    /// It's recommended to use `Token::(lifetime,uninterpolate,uninterpolated_span)` to
    /// treat regular and interpolated lifetime identifiers in the same way.
    Lifetime(Symbol),
    /// This identifier (and its span) is the lifetime passed to the
    /// declarative macro. The span in the surrounding `Token` is the span of
    /// the `lifetime` metavariable in the macro's RHS.
    NtLifetime(Ident),

    /// An embedded AST node, as produced by a macro. This only exists for
    /// historical reasons. We'd like to get rid of it, for multiple reasons.
    /// - It's conceptually very strange. Saying a token can contain an AST
    ///   node is like saying, in natural language, that a word can contain a
    ///   sentence.
    /// - It requires special handling in a bunch of places in the parser.
    /// - It prevents `Token` from implementing `Copy`.
    /// It adds complexity and likely slows things down. Please don't add new
    /// occurrences of this token kind!
    ///
    /// The span in the surrounding `Token` is that of the metavariable in the
    /// macro's RHS. The span within the Nonterminal is that of the fragment
    /// passed to the macro at the call site.
    Interpolated(Lrc<Nonterminal>),

    /// A doc comment token.
    /// `Symbol` is the doc comment's data excluding its "quotes" (`///`, `/**`, etc)
    /// similarly to symbols in string literal tokens.
    DocComment(CommentKind, ast::AttrStyle, Symbol),

    /// End Of File
    Eof,
}

impl Clone for TokenKind {
    fn clone(&self) -> Self {
        // `TokenKind` would impl `Copy` if it weren't for `Interpolated`. So
        // for all other variants, this implementation of `clone` is just like
        // a copy. This is faster than the `derive(Clone)` version which has a
        // separate path for every variant.
        match self {
            Interpolated(nt) => Interpolated(nt.clone()),
            _ => unsafe { std::ptr::read(self) },
        }
    }
}

#[derive(Clone, PartialEq, Encodable, Decodable, Debug, HashStable_Generic)]
pub struct Token {
    pub kind: TokenKind,
    pub span: Span,
}

impl TokenKind {
    pub fn lit(kind: LitKind, symbol: Symbol, suffix: Option<Symbol>) -> TokenKind {
        Literal(Lit::new(kind, symbol, suffix))
    }

    /// An approximation to proc-macro-style single-character operators used by rustc parser.
    /// If the operator token can be broken into two tokens, the first of which is single-character,
    /// then this function performs that operation, otherwise it returns `None`.
    pub fn break_two_token_op(&self) -> Option<(TokenKind, TokenKind)> {
        Some(match *self {
            Le => (Lt, Eq),
            EqEq => (Eq, Eq),
            Ne => (Not, Eq),
            Ge => (Gt, Eq),
            AndAnd => (BinOp(And), BinOp(And)),
            OrOr => (BinOp(Or), BinOp(Or)),
            BinOp(Shl) => (Lt, Lt),
            BinOp(Shr) => (Gt, Gt),
            BinOpEq(Plus) => (BinOp(Plus), Eq),
            BinOpEq(Minus) => (BinOp(Minus), Eq),
            BinOpEq(Star) => (BinOp(Star), Eq),
            BinOpEq(Slash) => (BinOp(Slash), Eq),
            BinOpEq(Percent) => (BinOp(Percent), Eq),
            BinOpEq(Caret) => (BinOp(Caret), Eq),
            BinOpEq(And) => (BinOp(And), Eq),
            BinOpEq(Or) => (BinOp(Or), Eq),
            BinOpEq(Shl) => (Lt, Le),
            BinOpEq(Shr) => (Gt, Ge),
            DotDot => (Dot, Dot),
            DotDotDot => (Dot, DotDot),
            PathSep => (Colon, Colon),
            RArrow => (BinOp(Minus), Gt),
            LArrow => (Lt, BinOp(Minus)),
            FatArrow => (Eq, Gt),
            _ => return None,
        })
    }

    /// Returns tokens that are likely to be typed accidentally instead of the current token.
    /// Enables better error recovery when the wrong token is found.
    pub fn similar_tokens(&self) -> Option<Vec<TokenKind>> {
        match *self {
            Comma => Some(vec![Dot, Lt, Semi]),
            Semi => Some(vec![Colon, Comma]),
            Colon => Some(vec![Semi]),
            FatArrow => Some(vec![Eq, RArrow, Ge, Gt]),
            _ => None,
        }
    }

    pub fn should_end_const_arg(&self) -> bool {
        matches!(self, Gt | Ge | BinOp(Shr) | BinOpEq(Shr))
    }
}

impl Token {
    pub fn new(kind: TokenKind, span: Span) -> Self {
        Token { kind, span }
    }

    /// Some token that will be thrown away later.
    pub fn dummy() -> Self {
        Token::new(TokenKind::Question, DUMMY_SP)
    }

    /// Recovers a `Token` from an `Ident`. This creates a raw identifier if necessary.
    pub fn from_ast_ident(ident: Ident) -> Self {
        Token::new(Ident(ident.name, ident.is_raw_guess().into()), ident.span)
    }

    /// For interpolated tokens, returns a span of the fragment to which the interpolated
    /// token refers. For all other tokens this is just a regular span.
    /// It is particularly important to use this for identifiers and lifetimes
    /// for which spans affect name resolution and edition checks.
    /// Note that keywords are also identifiers, so they should use this
    /// if they keep spans or perform edition checks.
    pub fn uninterpolated_span(&self) -> Span {
        match self.kind {
            NtIdent(ident, _) | NtLifetime(ident) => ident.span,
            Interpolated(ref nt) => nt.use_span(),
            _ => self.span,
        }
    }

    pub fn is_range_separator(&self) -> bool {
        [DotDot, DotDotDot, DotDotEq].contains(&self.kind)
    }

    pub fn is_punct(&self) -> bool {
        match self.kind {
            Eq | Lt | Le | EqEq | Ne | Ge | Gt | AndAnd | OrOr | Not | Tilde | BinOp(_)
            | BinOpEq(_) | At | Dot | DotDot | DotDotDot | DotDotEq | Comma | Semi | Colon
            | PathSep | RArrow | LArrow | FatArrow | Pound | Dollar | Question | SingleQuote => {
                true
            }

            OpenDelim(..) | CloseDelim(..) | Literal(..) | DocComment(..) | Ident(..)
            | NtIdent(..) | Lifetime(..) | NtLifetime(..) | Interpolated(..) | Eof => false,
        }
    }

    pub fn is_like_plus(&self) -> bool {
        matches!(self.kind, BinOp(Plus) | BinOpEq(Plus))
    }

    /// Returns `true` if the token can appear at the start of an expression.
    pub fn can_begin_expr(&self) -> bool {
        match self.uninterpolate().kind {
            Ident(name, is_raw)              =>
                ident_can_begin_expr(name, self.span, is_raw), // value name or keyword
            OpenDelim(..)                     | // tuple, array or block
            Literal(..)                       | // literal
            Not                               | // operator not
            BinOp(Minus)                      | // unary minus
            BinOp(Star)                       | // dereference
            BinOp(Or) | OrOr                  | // closure
            BinOp(And)                        | // reference
            AndAnd                            | // double reference
            // DotDotDot is no longer supported, but we need some way to display the error
            DotDot | DotDotDot | DotDotEq     | // range notation
            Lt | BinOp(Shl)                   | // associated path
            PathSep                            | // global path
            Lifetime(..)                      | // labeled loop
            Pound                             => true, // expression attributes
            Interpolated(ref nt) => matches!(&**nt, NtLiteral(..) |
                NtExpr(..)    |
                NtBlock(..)   |
                NtPath(..)),
            _ => false,
        }
    }

    /// Returns `true` if the token can appear at the start of a pattern.
    ///
    /// Shamelessly borrowed from `can_begin_expr`, only used for diagnostics right now.
    pub fn can_begin_pattern(&self) -> bool {
        match self.uninterpolate().kind {
            Ident(name, is_raw)              =>
                ident_can_begin_expr(name, self.span, is_raw), // value name or keyword
            | OpenDelim(Delimiter::Bracket | Delimiter::Parenthesis)  // tuple or array
            | Literal(..)                        // literal
            | BinOp(Minus)                       // unary minus
            | BinOp(And)                         // reference
            | AndAnd                             // double reference
            // DotDotDot is no longer supported
            | DotDot | DotDotDot | DotDotEq      // ranges
            | Lt | BinOp(Shl)                    // associated path
            | PathSep                    => true, // global path
            Interpolated(ref nt) => matches!(&**nt, NtLiteral(..) |
                NtPat(..)     |
                NtBlock(..)   |
                NtPath(..)),
            _ => false,
        }
    }

    /// Returns `true` if the token can appear at the start of a type.
    pub fn can_begin_type(&self) -> bool {
        match self.uninterpolate().kind {
            Ident(name, is_raw)        =>
                ident_can_begin_type(name, self.span, is_raw), // type name or keyword
            OpenDelim(Delimiter::Parenthesis) | // tuple
            OpenDelim(Delimiter::Bracket)     | // array
            Not                         | // never
            BinOp(Star)                 | // raw pointer
            BinOp(And)                  | // reference
            AndAnd                      | // double reference
            Question                    | // maybe bound in trait object
            Lifetime(..)                | // lifetime bound in trait object
            Lt | BinOp(Shl)             | // associated path
            PathSep                      => true, // global path
            Interpolated(ref nt) => matches!(&**nt, NtTy(..) | NtPath(..)),
            // For anonymous structs or unions, which only appear in specific positions
            // (type of struct fields or union fields), we don't consider them as regular types
            _ => false,
        }
    }

    /// Returns `true` if the token can appear at the start of a const param.
    pub fn can_begin_const_arg(&self) -> bool {
        match self.kind {
            OpenDelim(Delimiter::Brace) | Literal(..) | BinOp(Minus) => true,
            Ident(name, IdentIsRaw::No) if name.is_bool_lit() => true,
            Interpolated(ref nt) => matches!(&**nt, NtExpr(..) | NtBlock(..) | NtLiteral(..)),
            _ => false,
        }
    }

    /// Returns `true` if the token can appear at the start of an item.
    pub fn can_begin_item(&self) -> bool {
        match self.kind {
            Ident(name, _) => [
                kw::Fn,
                kw::Use,
                kw::Struct,
                kw::Enum,
                kw::Pub,
                kw::Trait,
                kw::Extern,
                kw::Impl,
                kw::Unsafe,
                kw::Const,
                kw::Safe,
                kw::Static,
                kw::Union,
                kw::Macro,
                kw::Mod,
                kw::Type,
            ]
            .contains(&name),
            _ => false,
        }
    }

    /// Returns `true` if the token is any literal.
    pub fn is_lit(&self) -> bool {
        matches!(self.kind, Literal(..))
    }

    /// Returns `true` if the token is any literal, a minus (which can prefix a literal,
    /// for example a '-42', or one of the boolean idents).
    ///
    /// In other words, would this token be a valid start of `parse_literal_maybe_minus`?
    ///
    /// Keep this in sync with and `Lit::from_token`, excluding unary negation.
    pub fn can_begin_literal_maybe_minus(&self) -> bool {
        match self.uninterpolate().kind {
            Literal(..) | BinOp(Minus) => true,
            Ident(name, IdentIsRaw::No) if name.is_bool_lit() => true,
            Interpolated(ref nt) => match &**nt {
                NtLiteral(_) => true,
                NtExpr(e) => match &e.kind {
                    ast::ExprKind::Lit(_) => true,
                    ast::ExprKind::Unary(ast::UnOp::Neg, e) => {
                        matches!(&e.kind, ast::ExprKind::Lit(_))
                    }
                    _ => false,
                },
                _ => false,
            },
            _ => false,
        }
    }

    pub fn can_begin_string_literal(&self) -> bool {
        match self.uninterpolate().kind {
            Literal(..) => true,
            Interpolated(ref nt) => match &**nt {
                NtLiteral(_) => true,
                NtExpr(e) => match &e.kind {
                    ast::ExprKind::Lit(_) => true,
                    _ => false,
                },
                _ => false,
            },
            _ => false,
        }
    }

    /// A convenience function for matching on identifiers during parsing.
    /// Turns interpolated identifier (`$i: ident`) or lifetime (`$l: lifetime`) token
    /// into the regular identifier or lifetime token it refers to,
    /// otherwise returns the original token.
    pub fn uninterpolate(&self) -> Cow<'_, Token> {
        match self.kind {
            NtIdent(ident, is_raw) => Cow::Owned(Token::new(Ident(ident.name, is_raw), ident.span)),
            NtLifetime(ident) => Cow::Owned(Token::new(Lifetime(ident.name), ident.span)),
            _ => Cow::Borrowed(self),
        }
    }

    /// Returns an identifier if this token is an identifier.
    #[inline]
    pub fn ident(&self) -> Option<(Ident, IdentIsRaw)> {
        // We avoid using `Token::uninterpolate` here because it's slow.
        match self.kind {
            Ident(name, is_raw) => Some((Ident::new(name, self.span), is_raw)),
            NtIdent(ident, is_raw) => Some((ident, is_raw)),
            _ => None,
        }
    }

    /// Returns a lifetime identifier if this token is a lifetime.
    #[inline]
    pub fn lifetime(&self) -> Option<Ident> {
        // We avoid using `Token::uninterpolate` here because it's slow.
        match self.kind {
            Lifetime(name) => Some(Ident::new(name, self.span)),
            NtLifetime(ident) => Some(ident),
            _ => None,
        }
    }

    /// Returns `true` if the token is an identifier.
    pub fn is_ident(&self) -> bool {
        self.ident().is_some()
    }

    /// Returns `true` if the token is a lifetime.
    pub fn is_lifetime(&self) -> bool {
        self.lifetime().is_some()
    }

    /// Returns `true` if the token is an identifier whose name is the given
    /// string slice.
    pub fn is_ident_named(&self, name: Symbol) -> bool {
        self.ident().is_some_and(|(ident, _)| ident.name == name)
    }

    /// Returns `true` if the token is an interpolated path.
    fn is_whole_path(&self) -> bool {
        if let Interpolated(nt) = &self.kind
            && let NtPath(..) = &**nt
        {
            return true;
        }

        false
    }

    /// Would `maybe_whole_expr` in `parser.rs` return `Ok(..)`?
    /// That is, is this a pre-parsed expression dropped into the token stream
    /// (which happens while parsing the result of macro expansion)?
    pub fn is_whole_expr(&self) -> bool {
        if let Interpolated(nt) = &self.kind
            && let NtExpr(_) | NtLiteral(_) | NtPath(_) | NtBlock(_) = &**nt
        {
            return true;
        }

        false
    }

    /// Is the token an interpolated block (`$b:block`)?
    pub fn is_whole_block(&self) -> bool {
        if let Interpolated(nt) = &self.kind
            && let NtBlock(..) = &**nt
        {
            return true;
        }

        false
    }

    /// Returns `true` if the token is either the `mut` or `const` keyword.
    pub fn is_mutability(&self) -> bool {
        self.is_keyword(kw::Mut) || self.is_keyword(kw::Const)
    }

    pub fn is_qpath_start(&self) -> bool {
        self == &Lt || self == &BinOp(Shl)
    }

    pub fn is_path_start(&self) -> bool {
        self == &PathSep
            || self.is_qpath_start()
            || self.is_whole_path()
            || self.is_path_segment_keyword()
            || self.is_ident() && !self.is_reserved_ident()
    }

    /// Returns `true` if the token is a given keyword, `kw`.
    pub fn is_keyword(&self, kw: Symbol) -> bool {
        self.is_non_raw_ident_where(|id| id.name == kw)
    }

    /// Returns `true` if the token is a given keyword, `kw` or if `case` is `Insensitive` and this token is an identifier equal to `kw` ignoring the case.
    pub fn is_keyword_case(&self, kw: Symbol, case: Case) -> bool {
        self.is_keyword(kw)
            || (case == Case::Insensitive
                && self.is_non_raw_ident_where(|id| {
                    id.name.as_str().to_lowercase() == kw.as_str().to_lowercase()
                }))
    }

    pub fn is_path_segment_keyword(&self) -> bool {
        self.is_non_raw_ident_where(Ident::is_path_segment_keyword)
    }

    /// Returns true for reserved identifiers used internally for elided lifetimes,
    /// unnamed method parameters, crate root module, error recovery etc.
    pub fn is_special_ident(&self) -> bool {
        self.is_non_raw_ident_where(Ident::is_special)
    }

    /// Returns `true` if the token is a keyword used in the language.
    pub fn is_used_keyword(&self) -> bool {
        self.is_non_raw_ident_where(Ident::is_used_keyword)
    }

    /// Returns `true` if the token is a keyword reserved for possible future use.
    pub fn is_unused_keyword(&self) -> bool {
        self.is_non_raw_ident_where(Ident::is_unused_keyword)
    }

    /// Returns `true` if the token is either a special identifier or a keyword.
    pub fn is_reserved_ident(&self) -> bool {
        self.is_non_raw_ident_where(Ident::is_reserved)
    }

    /// Returns `true` if the token is the identifier `true` or `false`.
    pub fn is_bool_lit(&self) -> bool {
        self.is_non_raw_ident_where(|id| id.name.is_bool_lit())
    }

    pub fn is_numeric_lit(&self) -> bool {
        matches!(
            self.kind,
            Literal(Lit { kind: LitKind::Integer, .. }) | Literal(Lit { kind: LitKind::Float, .. })
        )
    }

    /// Returns `true` if the token is the integer literal.
    pub fn is_integer_lit(&self) -> bool {
        matches!(self.kind, Literal(Lit { kind: LitKind::Integer, .. }))
    }

    /// Returns `true` if the token is a non-raw identifier for which `pred` holds.
    pub fn is_non_raw_ident_where(&self, pred: impl FnOnce(Ident) -> bool) -> bool {
        match self.ident() {
            Some((id, IdentIsRaw::No)) => pred(id),
            _ => false,
        }
    }

    pub fn glue(&self, joint: &Token) -> Option<Token> {
        let kind = match self.kind {
            Eq => match joint.kind {
                Eq => EqEq,
                Gt => FatArrow,
                _ => return None,
            },
            Lt => match joint.kind {
                Eq => Le,
                Lt => BinOp(Shl),
                Le => BinOpEq(Shl),
                BinOp(Minus) => LArrow,
                _ => return None,
            },
            Gt => match joint.kind {
                Eq => Ge,
                Gt => BinOp(Shr),
                Ge => BinOpEq(Shr),
                _ => return None,
            },
            Not => match joint.kind {
                Eq => Ne,
                _ => return None,
            },
            BinOp(op) => match joint.kind {
                Eq => BinOpEq(op),
                BinOp(And) if op == And => AndAnd,
                BinOp(Or) if op == Or => OrOr,
                Gt if op == Minus => RArrow,
                _ => return None,
            },
            Dot => match joint.kind {
                Dot => DotDot,
                DotDot => DotDotDot,
                _ => return None,
            },
            DotDot => match joint.kind {
                Dot => DotDotDot,
                Eq => DotDotEq,
                _ => return None,
            },
            Colon => match joint.kind {
                Colon => PathSep,
                _ => return None,
            },
            SingleQuote => match joint.kind {
                Ident(name, IdentIsRaw::No) => Lifetime(Symbol::intern(&format!("'{name}"))),
                _ => return None,
            },

            Le | EqEq | Ne | Ge | AndAnd | OrOr | Tilde | BinOpEq(..) | At | DotDotDot
            | DotDotEq | Comma | Semi | PathSep | RArrow | LArrow | FatArrow | Pound | Dollar
            | Question | OpenDelim(..) | CloseDelim(..) | Literal(..) | Ident(..) | NtIdent(..)
            | Lifetime(..) | NtLifetime(..) | Interpolated(..) | DocComment(..) | Eof => {
                return None;
            }
        };

        Some(Token::new(kind, self.span.to(joint.span)))
    }
}

impl PartialEq<TokenKind> for Token {
    #[inline]
    fn eq(&self, rhs: &TokenKind) -> bool {
        self.kind == *rhs
    }
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, Encodable, Decodable)]
pub enum NtPatKind {
    // Matches or-patterns. Was written using `pat` in edition 2021 or later.
    PatWithOr,
    // Doesn't match or-patterns.
    // - `inferred`: was written using `pat` in edition 2015 or 2018.
    // - `!inferred`: was written using `pat_param`.
    PatParam { inferred: bool },
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, Encodable, Decodable)]
pub enum NtExprKind {
    // Matches expressions using the post-edition 2024. Was written using
    // `expr` in edition 2024 or later.
    Expr,
    // Matches expressions using the pre-edition 2024 rules.
    // - `inferred`: was written using `expr` in edition 2021 or earlier.
    // - `!inferred`: was written using `expr_2021`.
    Expr2021 { inferred: bool },
}

#[derive(Clone, Encodable, Decodable)]
/// For interpolation during macro expansion.
pub enum Nonterminal {
    NtItem(P<ast::Item>),
    NtBlock(P<ast::Block>),
    NtStmt(P<ast::Stmt>),
    NtPat(P<ast::Pat>),
    NtExpr(P<ast::Expr>),
    NtTy(P<ast::Ty>),
    NtLiteral(P<ast::Expr>),
    /// Stuff inside brackets for attributes
    NtMeta(P<ast::AttrItem>),
    NtPath(P<ast::Path>),
    NtVis(P<ast::Visibility>),
}

#[derive(Debug, Copy, Clone, PartialEq, Encodable, Decodable)]
pub enum NonterminalKind {
    Item,
    Block,
    Stmt,
    Pat(NtPatKind),
    Expr(NtExprKind),
    Ty,
    Ident,
    Lifetime,
    Literal,
    Meta,
    Path,
    Vis,
    TT,
}

impl NonterminalKind {
    /// The `edition` closure is used to get the edition for the given symbol. Doing
    /// `span.edition()` is expensive, so we do it lazily.
    pub fn from_symbol(
        symbol: Symbol,
        edition: impl FnOnce() -> Edition,
    ) -> Option<NonterminalKind> {
        Some(match symbol {
            sym::item => NonterminalKind::Item,
            sym::block => NonterminalKind::Block,
            sym::stmt => NonterminalKind::Stmt,
            sym::pat => {
                if edition().at_least_rust_2021() {
                    NonterminalKind::Pat(PatWithOr)
                } else {
                    NonterminalKind::Pat(PatParam { inferred: true })
                }
            }
            sym::pat_param => NonterminalKind::Pat(PatParam { inferred: false }),
            sym::expr => {
                if edition().at_least_rust_2024() {
                    NonterminalKind::Expr(Expr)
                } else {
                    NonterminalKind::Expr(Expr2021 { inferred: true })
                }
            }
            sym::expr_2021 => NonterminalKind::Expr(Expr2021 { inferred: false }),
            sym::ty => NonterminalKind::Ty,
            sym::ident => NonterminalKind::Ident,
            sym::lifetime => NonterminalKind::Lifetime,
            sym::literal => NonterminalKind::Literal,
            sym::meta => NonterminalKind::Meta,
            sym::path => NonterminalKind::Path,
            sym::vis => NonterminalKind::Vis,
            sym::tt => NonterminalKind::TT,
            _ => return None,
        })
    }

    fn symbol(self) -> Symbol {
        match self {
            NonterminalKind::Item => sym::item,
            NonterminalKind::Block => sym::block,
            NonterminalKind::Stmt => sym::stmt,
            NonterminalKind::Pat(PatParam { inferred: true } | PatWithOr) => sym::pat,
            NonterminalKind::Pat(PatParam { inferred: false }) => sym::pat_param,
            NonterminalKind::Expr(Expr2021 { inferred: true } | Expr) => sym::expr,
            NonterminalKind::Expr(Expr2021 { inferred: false }) => sym::expr_2021,
            NonterminalKind::Ty => sym::ty,
            NonterminalKind::Ident => sym::ident,
            NonterminalKind::Lifetime => sym::lifetime,
            NonterminalKind::Literal => sym::literal,
            NonterminalKind::Meta => sym::meta,
            NonterminalKind::Path => sym::path,
            NonterminalKind::Vis => sym::vis,
            NonterminalKind::TT => sym::tt,
        }
    }
}

impl fmt::Display for NonterminalKind {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.symbol())
    }
}

impl Nonterminal {
    pub fn use_span(&self) -> Span {
        match self {
            NtItem(item) => item.span,
            NtBlock(block) => block.span,
            NtStmt(stmt) => stmt.span,
            NtPat(pat) => pat.span,
            NtExpr(expr) | NtLiteral(expr) => expr.span,
            NtTy(ty) => ty.span,
            NtMeta(attr_item) => attr_item.span(),
            NtPath(path) => path.span,
            NtVis(vis) => vis.span,
        }
    }

    pub fn descr(&self) -> &'static str {
        match self {
            NtItem(..) => "item",
            NtBlock(..) => "block",
            NtStmt(..) => "statement",
            NtPat(..) => "pattern",
            NtExpr(..) => "expression",
            NtLiteral(..) => "literal",
            NtTy(..) => "type",
            NtMeta(..) => "attribute",
            NtPath(..) => "path",
            NtVis(..) => "visibility",
        }
    }
}

impl PartialEq for Nonterminal {
    fn eq(&self, _rhs: &Self) -> bool {
        // FIXME: Assume that all nonterminals are not equal, we can't compare them
        // correctly based on data from AST. This will prevent them from matching each other
        // in macros. The comparison will become possible only when each nonterminal has an
        // attached token stream from which it was parsed.
        false
    }
}

impl fmt::Debug for Nonterminal {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match *self {
            NtItem(..) => f.pad("NtItem(..)"),
            NtBlock(..) => f.pad("NtBlock(..)"),
            NtStmt(..) => f.pad("NtStmt(..)"),
            NtPat(..) => f.pad("NtPat(..)"),
            NtExpr(..) => f.pad("NtExpr(..)"),
            NtTy(..) => f.pad("NtTy(..)"),
            NtLiteral(..) => f.pad("NtLiteral(..)"),
            NtMeta(..) => f.pad("NtMeta(..)"),
            NtPath(..) => f.pad("NtPath(..)"),
            NtVis(..) => f.pad("NtVis(..)"),
        }
    }
}

impl<CTX> HashStable<CTX> for Nonterminal
where
    CTX: crate::HashStableContext,
{
    fn hash_stable(&self, _hcx: &mut CTX, _hasher: &mut StableHasher) {
        panic!("interpolated tokens should not be present in the HIR")
    }
}

// Some types are used a lot. Make sure they don't unintentionally get bigger.
#[cfg(target_pointer_width = "64")]
mod size_asserts {
    use super::*;
    use rustc_data_structures::static_assert_size;
    // tidy-alphabetical-start
    static_assert_size!(Lit, 12);
    static_assert_size!(LitKind, 2);
    static_assert_size!(Nonterminal, 16);
    static_assert_size!(Token, 24);
    static_assert_size!(TokenKind, 16);
    // tidy-alphabetical-end
}