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Merged
rust-bors merged 1 commit into from
Sep 28, 2026
Merged

Implement forced keywords (k#) #161775

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31 changes: 22 additions & 9 deletions compiler/rustc_ast/src/token.rs
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Expand Up @@ -231,7 +231,7 @@ impl Lit {
/// `Parser::eat_token_lit` (excluding unary negation).
pub fn from_token(token: &Token) -> Option<Lit> {
match token.uninterpolate().kind {
Ident(name, IdentKind::Normal) if name.is_bool_lit() => {
Ident(name, IdentKind::Normal | IdentKind::ForcedKeyword) if name.is_bool_lit() => {
Some(Lit::new(Bool, name, None))
}
Literal(token_lit) => Some(token_lit),
Expand Down Expand Up @@ -362,19 +362,23 @@ fn ident_can_begin_type(name: Symbol, span: Span, kind: IdentKind) -> bool {
pub enum IdentKind {
Normal,
Raw,
ForcedKeyword,
}

impl IdentKind {
pub fn to_print_mode_ident(self) -> IdentPrintMode {
match self {
IdentKind::Normal => IdentPrintMode::Normal,
IdentKind::Raw => IdentPrintMode::RawIdent,
IdentKind::ForcedKeyword => IdentPrintMode::ForcedKeywordIdent,
}
}

pub fn to_print_mode_lifetime(self) -> IdentPrintMode {
match self {
IdentKind::Normal => IdentPrintMode::Normal,
IdentKind::Raw => IdentPrintMode::RawLifetime,
IdentKind::ForcedKeyword => unreachable!(),
}
}
}
Expand Down Expand Up @@ -637,7 +641,10 @@ impl Token {
Token::new(TokenKind::Question, DUMMY_SP)
}

/// Recovers a `Token` from an `Ident`. This creates a raw identifier if necessary.
/// Recovers a `Token` from an `Ident`.
///
/// This creates a raw identifier if necessary.
/// It will never create a forced keyword.
pub fn from_ast_ident(ident: sp::Ident) -> Self {
let kind = if ident.is_raw_guess() { IdentKind::Raw } else { IdentKind::Normal };
Token::new(Ident(ident.name, kind), ident.span)
Expand Down Expand Up @@ -763,7 +770,7 @@ impl Token {
pub fn can_begin_const_arg(&self) -> bool {
match self.kind {
OpenBrace | Literal(..) | Minus => true,
Ident(name, IdentKind::Normal) if name.is_bool_lit() => true,
Ident(name, IdentKind::Normal | IdentKind::ForcedKeyword) if name.is_bool_lit() => true,
OpenInvisible(InvisibleOrigin::MetaVar(
MetaVarKind::Expr { .. } | MetaVarKind::Block | MetaVarKind::Literal,
)) => true,
Expand Down Expand Up @@ -812,7 +819,7 @@ impl Token {
pub fn can_begin_literal_maybe_minus(&self) -> bool {
match self.uninterpolate().kind {
Literal(..) | Minus => true,
Ident(name, IdentKind::Normal) if name.is_bool_lit() => true,
Ident(name, IdentKind::Normal | IdentKind::ForcedKeyword) if name.is_bool_lit() => true,
OpenInvisible(InvisibleOrigin::MetaVar(mv_kind)) => match mv_kind {
MetaVarKind::Literal => true,
MetaVarKind::Expr { can_begin_literal_maybe_minus, .. } => {
Expand Down Expand Up @@ -962,23 +969,21 @@ impl Token {

/// Returns `true` if the token is either a special identifier or a keyword.
pub fn is_reserved_ident(&self) -> bool {
self.non_raw_ident().is_some_and(sp::Ident::is_reserved)
self.ident().is_some_and(|(id, kind)| ident_of_kind_is_reserved(id, kind))
}

pub fn is_non_reserved_ident(&self) -> bool {
self.non_reserved_ident().is_some()
}

pub fn non_reserved_ident(&self) -> Option<sp::Ident> {
self.ident()
.filter(|&(id, kind)| kind == IdentKind::Raw || !id.is_reserved())
.map(|(id, _)| id)
self.ident().filter(|&(id, kind)| !ident_of_kind_is_reserved(id, kind)).map(|(id, _)| id)
}

/// If this token is a non-raw identifier, return the identifier in question.
pub fn non_raw_ident(&self) -> Option<sp::Ident> {
match self.ident() {
Some((id, IdentKind::Normal)) => Some(id),
Some((id, IdentKind::Normal | IdentKind::ForcedKeyword)) => Some(id),
_ => None,
}
}
Expand Down Expand Up @@ -1102,6 +1107,14 @@ impl PartialEq<TokenKind> for Token {
}
}

pub fn ident_of_kind_is_reserved(id: sp::Ident, kind: IdentKind) -> bool {
match kind {
IdentKind::Normal => id.is_reserved(),
IdentKind::Raw => false,
IdentKind::ForcedKeyword => true,
}
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, Encodable, Decodable, Hash, StableHash)]
pub enum NtPatKind {
// Matches or-patterns. Was written using `pat` in edition 2021 or later.
Expand Down
1 change: 1 addition & 0 deletions compiler/rustc_ast_passes/src/feature_gate.rs
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Expand Up @@ -441,6 +441,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
gate_all!(explicit_tail_calls, "`become` expression is experimental");
gate_all!(final_associated_functions, "`final` on trait functions is experimental");
gate_all!(fn_delegation, "functions delegation is not yet fully implemented");
gate_all!(forced_keywords, "forced keywords are experimental");
gate_all!(frontmatter, "frontmatters are experimental");
gate_all!(gca_min_const_items, "unbraced const blocks as const args are experimental");
gate_all!(gen_blocks, "gen blocks are experimental");
Expand Down
10 changes: 4 additions & 6 deletions compiler/rustc_ast_pretty/src/pprust/state.rs
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Expand Up @@ -370,12 +370,10 @@ fn space_between(tt1: &TokenTree, tt2: &TokenTree) -> bool {

// IDENT|`fn`|`Self`|`pub` + `(`: `f(3)`, `fn(x: u8)`, `Self()`, `pub(crate)`,
// but `let (a, b) = (1, 2)` needs a space after the `let`
(Tok(tk::Token { kind: tk::Ident(sym, kind), span }, _), Del(_, _, Parenthesis, _))
if !Ident::new(*sym, *span).is_reserved()
|| *sym == kw::Fn
|| *sym == kw::SelfUpper
|| *sym == kw::Pub
|| matches!(kind, tk::IdentKind::Raw) =>
(&Tok(tk::Token { kind: tk::Ident(sym, kind), span }, _), Del(_, _, Parenthesis, _))
if kind == tk::IdentKind::Raw
|| matches!(sym, kw::Fn | kw::SelfUpper | kw::Pub)
|| !Ident::new(sym, span).is_reserved() =>
{
false
}
Expand Down
3 changes: 2 additions & 1 deletion compiler/rustc_expand/src/mbe/macro_check.rs
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Expand Up @@ -397,7 +397,8 @@ fn check_nested_occurrences(
(
NestedMacroState::Empty,
&TokenTree::Token(Token {
kind: TokenKind::Ident(name, IdentKind::Normal), ..
kind: TokenKind::Ident(name, IdentKind::Normal | IdentKind::ForcedKeyword),
..
}),
) => {
if name == kw::MacroRules {
Expand Down
6 changes: 3 additions & 3 deletions compiler/rustc_expand/src/mbe/macro_rules.rs
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Expand Up @@ -1750,7 +1750,7 @@ fn is_in_follow(tok: &mbe::TokenTree, kind: NonterminalKind) -> IsInFollow {
match tok {
TokenTree::Token(token) => match token.kind {
FatArrow | Comma | Eq | Or => IsInFollow::Yes,
Ident(name, IdentKind::Normal) if name == kw::If || name == kw::In => {
Ident(kw::If | kw::In, IdentKind::Normal | IdentKind::ForcedKeyword) => {
IsInFollow::Yes
}
_ => IsInFollow::No(TOKENS),
Expand All @@ -1764,7 +1764,7 @@ fn is_in_follow(tok: &mbe::TokenTree, kind: NonterminalKind) -> IsInFollow {
match tok {
TokenTree::Token(token) => match token.kind {
FatArrow | Comma | Eq => IsInFollow::Yes,
Ident(name, IdentKind::Normal) if name == kw::If || name == kw::In => {
Ident(kw::If | kw::In, IdentKind::Normal | IdentKind::ForcedKeyword) => {
IsInFollow::Yes
}
_ => IsInFollow::No(TOKENS),
Expand Down Expand Up @@ -1792,7 +1792,7 @@ fn is_in_follow(tok: &mbe::TokenTree, kind: NonterminalKind) -> IsInFollow {
TokenTree::Token(token) => match token.kind {
OpenBrace | OpenBracket | Comma | FatArrow | Colon | Eq | Gt | Shr
| Semi | Or => IsInFollow::Yes,
Ident(name, IdentKind::Normal) if name == kw::As || name == kw::Where => {
Ident(kw::As | kw::Where, IdentKind::Normal | IdentKind::ForcedKeyword) => {
IsInFollow::Yes
}
_ => IsInFollow::No(TOKENS),
Expand Down
22 changes: 18 additions & 4 deletions compiler/rustc_expand/src/mbe/metavar_expr.rs
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Expand Up @@ -9,7 +9,6 @@ use rustc_span::{Ident, Span, Symbol, sym};

use crate::diagnostics;

pub(crate) const RAW_IDENT_ERR: &str = "`${concat(..)}` currently does not support raw identifiers";
pub(crate) const UNSUPPORTED_CONCAT_ELEM_ERR: &str = "expected identifier or string literal";

/// A meta-variable expression, for expansions based on properties of meta-variables.
Expand Down Expand Up @@ -197,6 +196,10 @@ fn parse_concat<'psess>(
} else {
match parse_ident_from_token(psess, token) {
Err(err) => {
// FIXME: Canceling this error means we emit a worse message when encountering
// raw identifiers (`r#ident`). However, we also don't want to forward
// (the current version of) this error as is since we also want to
// mentioning string literals as a valid token kind.
err.cancel();
return Err(psess
.dcx()
Expand Down Expand Up @@ -281,9 +284,7 @@ fn parse_ident_from_token<'psess>(
token: &Token,
) -> PResult<'psess, Ident> {
if let Some((elem, kind)) = token.ident() {
if let IdentKind::Raw = kind {
return Err(psess.dcx().struct_span_err(elem.span, RAW_IDENT_ERR));
}
validate_ident_kind(psess.dcx(), kind, elem.span)?;
return Ok(elem);
}
let token_str = pprust::token_to_string(token);
Expand Down Expand Up @@ -347,3 +348,16 @@ fn eat_dollar<'psess>(
"meta-variables within meta-variable expressions must be referenced using a dollar sign",
))
}

pub(crate) fn validate_ident_kind<'a>(
dcx: rustc_errors::DiagCtxtHandle<'a>,
kind: IdentKind,
span: Span,
) -> PResult<'a, ()> {
let kind = match kind {
IdentKind::Normal => return Ok(()),
IdentKind::Raw => "raw identifiers",
IdentKind::ForcedKeyword => "forced keywords",
};
Err(dcx.struct_span_err(span, format!("`${{concat(..)}}` currently does not support {kind}")))
}
9 changes: 6 additions & 3 deletions compiler/rustc_expand/src/mbe/quoted.rs
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Expand Up @@ -337,10 +337,13 @@ fn parse_tree<'a>(

// `tree` is followed by an `ident`. This could be `$meta_var` or the `$crate`
// special metavariable that names the crate of the invocation.
Some(tokenstream::TokenTree::Token(token, _)) if token.is_ident() => {
let (ident, kind) = token.ident().unwrap();
Some(tokenstream::TokenTree::Token(token, _))
if let Some((ident, kind)) = token.ident() =>
{
let span = ident.span.with_lo(dollar_span.lo());
if ident.name == kw::Crate && matches!(kind, IdentKind::Normal) {
if let kw::Crate = ident.name
&& let IdentKind::Normal | IdentKind::ForcedKeyword = kind
{
TokenTree::token(token::Ident(kw::DollarCrate, kind), span)
} else {
TokenTree::MetaVar(span, ident)
Expand Down
16 changes: 5 additions & 11 deletions compiler/rustc_expand/src/mbe/transcribe.rs
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Expand Up @@ -25,7 +25,7 @@ use crate::diagnostics::{
};
use crate::mbe::macro_parser::NamedMatch;
use crate::mbe::macro_parser::NamedMatch::*;
use crate::mbe::metavar_expr::{MetaVarExprConcatElem, RAW_IDENT_ERR};
use crate::mbe::metavar_expr::{MetaVarExprConcatElem, validate_ident_kind};
use crate::mbe::{self, KleeneOp, MetaVarExpr};

/// Context needed to perform transcription of metavariable expressions.
Expand Down Expand Up @@ -1033,21 +1033,15 @@ fn extract_symbol_from_pnr<'a>(
) -> PResult<'a, Symbol> {
match pnr {
ParseNtResult::Ident(nt_ident, kind) => {
if let IdentKind::Raw = kind {
Err(dcx.struct_span_err(span_err, RAW_IDENT_ERR))
} else {
Ok(nt_ident.name)
}
validate_ident_kind(dcx, *kind, span_err)?;
Ok(nt_ident.name)
}
ParseNtResult::Tt(TokenTree::Token(
Token { kind: TokenKind::Ident(symbol, kind), .. },
_,
)) => {
if let IdentKind::Raw = kind {
Err(dcx.struct_span_err(span_err, RAW_IDENT_ERR))
} else {
Ok(*symbol)
}
validate_ident_kind(dcx, *kind, span_err)?;
Ok(*symbol)
}
ParseNtResult::Tt(TokenTree::Token(
Token {
Expand Down
52 changes: 36 additions & 16 deletions compiler/rustc_expand/src/proc_macro_server.rs
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Expand Up @@ -11,7 +11,8 @@ use rustc_parse::lexer::{StripTokens, nfc_normalize};
use rustc_parse::parser::Parser;
use rustc_parse::{exp, new_parser_from_source_str, source_str_to_stream};
use rustc_proc_macro::bridge::{
DelimSpan, Diagnostic, ExpnGlobals, Group, Ident, LitKind, Literal, Punct, TokenTree, server,
DelimSpan, Diagnostic, ExpnGlobals, Group, Ident, IdentKind, LitKind, Literal, Punct,
TokenTree, server,
};
use rustc_proc_macro::{Delimiter, Level};
use rustc_session::Session;
Expand Down Expand Up @@ -103,6 +104,26 @@ impl ToInternal<tk::LitKind> for LitKind {
}
}

Comment thread
petrochenkov marked this conversation as resolved.
Resolved
impl FromInternal<tk::IdentKind> for IdentKind {
fn from_internal(kind: tk::IdentKind) -> Self {
match kind {
tk::IdentKind::Normal => IdentKind::Normal,
tk::IdentKind::Raw => IdentKind::Raw,
tk::IdentKind::ForcedKeyword => IdentKind::ForcedKeyword,
}
}
}

impl ToInternal<tk::IdentKind> for IdentKind {
fn to_internal(self) -> tk::IdentKind {
match self {
IdentKind::Normal => tk::IdentKind::Normal,
IdentKind::Raw => tk::IdentKind::Raw,
IdentKind::ForcedKeyword => tk::IdentKind::ForcedKeyword,
}
}
}

impl FromInternal<TokenStream> for Vec<TokenTree<TokenStream, Span, Symbol>> {
fn from_internal(stream: TokenStream) -> Self {
// Estimate the capacity as `stream.len()` rounded up to the next power
Expand All @@ -127,7 +148,7 @@ impl FromInternal<TokenStream> for Vec<TokenTree<TokenStream, Span, Symbol>> {
}

trees.push(TokenTree::Group(Group {
delimiter: rustc_proc_macro::Delimiter::from_internal(delim),
delimiter: Delimiter::from_internal(delim),
stream: Some(stream),
span: DelimSpan {
open: span.open,
Expand Down Expand Up @@ -230,12 +251,12 @@ impl FromInternal<TokenStream> for Vec<TokenTree<TokenStream, Span, Symbol>> {

tk::Ident(sym, kind) => trees.push(TokenTree::Ident(Ident {
sym,
is_raw: matches!(kind, tk::IdentKind::Raw),
kind: IdentKind::from_internal(kind),
span,
})),
tk::NtIdent(ident, kind) => trees.push(TokenTree::Ident(Ident {
sym: ident.name,
is_raw: matches!(kind, tk::IdentKind::Raw),
kind: IdentKind::from_internal(kind),
span: ident.span,
})),

Expand All @@ -245,7 +266,7 @@ impl FromInternal<TokenStream> for Vec<TokenTree<TokenStream, Span, Symbol>> {
TokenTree::Punct(Punct { ch: b'\'', joint: true, span }),
TokenTree::Ident(Ident {
sym: ident.name,
is_raw: matches!(kind, tk::IdentKind::Raw),
kind: IdentKind::from_internal(kind),
span,
}),
]);
Expand All @@ -254,7 +275,7 @@ impl FromInternal<TokenStream> for Vec<TokenTree<TokenStream, Span, Symbol>> {
let stream =
TokenStream::token_alone(tk::Lifetime(ident.name, kind), ident.span);
trees.push(TokenTree::Group(Group {
delimiter: rustc_proc_macro::Delimiter::None,
delimiter: Delimiter::None,
stream: Some(stream),
span: DelimSpan::from_single(span),
}))
Expand Down Expand Up @@ -286,7 +307,7 @@ impl FromInternal<TokenStream> for Vec<TokenTree<TokenStream, Span, Symbol>> {
trees.push(TokenTree::Punct(Punct { ch: b'!', joint: false, span }));
}
trees.push(TokenTree::Group(Group {
delimiter: rustc_proc_macro::Delimiter::Bracket,
delimiter: Delimiter::Bracket,
stream: Some(stream),
span: DelimSpan::from_single(span),
}));
Expand Down Expand Up @@ -364,17 +385,16 @@ impl ToInternal<SmallVec<[tokenstream::TokenTree; 2]>>
stream.unwrap_or_default(),
)]
}
TokenTree::Ident(self::Ident { sym, is_raw, span }) => {
TokenTree::Ident(Ident { sym, kind, span }) => {
rustc.psess().symbol_gallery.insert(sym, span);
let kind = if is_raw { tk::IdentKind::Raw } else { tk::IdentKind::Normal };
smallvec![tokenstream::TokenTree::token_alone(tk::Ident(sym, kind), span)]
smallvec![tokenstream::TokenTree::token_alone(
tk::Ident(sym, kind.to_internal()),
span
)]
}
TokenTree::Literal(self::Literal {
kind: self::LitKind::Integer,
symbol,
suffix,
span,
}) if let Some(symbol) = symbol.as_str().strip_prefix('-') => {
TokenTree::Literal(self::Literal { kind: LitKind::Integer, symbol, suffix, span })
if let Some(symbol) = symbol.as_str().strip_prefix('-') =>
{
let symbol = Symbol::intern(symbol);
let integer = tk::TokenKind::lit(tk::Integer, symbol, suffix);
let a = tokenstream::TokenTree::token_joint_hidden(tk::Minus, span);
Expand Down
2 changes: 2 additions & 0 deletions compiler/rustc_feature/src/unstable.rs
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Expand Up @@ -542,6 +542,8 @@ declare_features! (
(incomplete, fn_delegation, "1.76.0", Some(118212)),
/// Traits for function pointers and items
(unstable, fn_static, "CURRENT_RUSTC_VERSION", Some(148768)),
/// Allows using forced keywords `k#fn`.
(unstable, forced_keywords, "CURRENT_RUSTC_VERSION", Some(153839)),

@fmease fmease Aug 25, 2026 •
edited
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I've intentionally not marked it internal even though this feature is only backed by a T-compiler MCP and not by a T-lang RFC or in-tree experiment:

I would find it a bit weird if we (in a hypothetical future) told users to "please try out" "k#only bounds" or "&k#own types & exprs" (features that would obviously be non-internal) but they'd have to enable an internal feature in tandem (could be perceived as off-putting).

However, I'm okay with switching it to incomplete or even internal if requested.

View changes since the review

@petrochenkov petrochenkov Sep 25, 2026

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No opinion on this.

/// Allows impls for the Freeze trait.
(internal, freeze_impls, "1.78.0", Some(121675)),
/// Frontmatter `---` blocks for use by external tools.
Expand Down
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