[blocked on 1.101.0 becoming beta & rustfmt stage0] Replace `builtin #` with `k#` by fmease · Pull Request #162232 · rust-lang/rust

fmease · 2026-09-03T12:17:33Z

Part of #153839. Also renames internal feature builtin_syntax to internal_syntax (without tombstoning) and keeps/reuses tracking issue #110680.

Blocked on #161775 reaching beta and getting picked up by stage0 rustfmt.

_{(No LLM was or will be used by me during the entire creation process of this PR)}

@dianne

Implement forced keywords (`k#`) Part of #153839. CC @dianne Introduces a new token kind to Rust >=2021 that looks like `k#ident` and that is called *forced keyword (identifier)*. This is gated behind a new unstable feature called `forced_keywords`. This is backed by [compiler MCP 945](rust-lang/compiler-team#945). ~~The `ident` in `k#ident` *must* be a keyword (from any edition) or a weak/contextual keyword for the token to be valid.~~ The `ident` in `k#ident` needn't be a keyword to be lexically valid (CC #161775 (comment)). --- What **won't** be done in this PR: 1. implementing forced keyword lifetimes (`'k#static`) 2. migrating away from built-in syntax `builtin # $ident($($tt)*)` / introducing `k#`-exclusive (weak) keywords * that'll be done in PR #162232 instead 3. allowing the user to force edition-dependent & context-dependent keywords to be keywords * re. edition-dependent: forcing them to be keywords even in editions where they're usually not a (strong) keyword (today, that only affects `gen` anyway IINM) * re. context-dependent: E.g., `union` is only "active" if it's followed by a non-reserved identifier and under this PR the same rules apply to `k#union` even though that's not necessary; changing it would result in better diagnostics (e.g., for `k#union struct {}`: `` error: expected item, found `k#union` `` => `` expected identifier, found keyword `struct` ``) and maybe also allow for disambiguation for other context-dependent keywords 4. extending the `proc_macro::Ident` API to allow users to programmatically create forced keywords (might never be added) <sub>(No LLM was or will be used by me during the entire creation process of this PR)</sub>

@dianne

Implement forced keywords (`k#`) Part of rust-lang/rust#153839. CC @dianne Introduces a new token kind to Rust >=2021 that looks like `k#ident` and that is called *forced keyword (identifier)*. This is gated behind a new unstable feature called `forced_keywords`. This is backed by [compiler MCP 945](rust-lang/compiler-team#945). ~~The `ident` in `k#ident` *must* be a keyword (from any edition) or a weak/contextual keyword for the token to be valid.~~ The `ident` in `k#ident` needn't be a keyword to be lexically valid (CC rust-lang/rust#161775 (comment)). --- What **won't** be done in this PR: 1. implementing forced keyword lifetimes (`'k#static`) 2. migrating away from built-in syntax `builtin # $ident($($tt)*)` / introducing `k#`-exclusive (weak) keywords * that'll be done in PR rust-lang/rust#162232 instead 3. allowing the user to force edition-dependent & context-dependent keywords to be keywords * re. edition-dependent: forcing them to be keywords even in editions where they're usually not a (strong) keyword (today, that only affects `gen` anyway IINM) * re. context-dependent: E.g., `union` is only "active" if it's followed by a non-reserved identifier and under this PR the same rules apply to `k#union` even though that's not necessary; changing it would result in better diagnostics (e.g., for `k#union struct {}`: `` error: expected item, found `k#union` `` => `` expected identifier, found keyword `struct` ``) and maybe also allow for disambiguation for other context-dependent keywords 4. extending the `proc_macro::Ident` API to allow users to programmatically create forced keywords (might never be added) <sub>(No LLM was or will be used by me during the entire creation process of this PR)</sub>

rust-bors · 2026-10-02T20:26:44Z

☔ The latest upstream changes (presumably #163661) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

fmease added S-blocked Status: Blocked on something else such as an RFC or other implementation work. rla-silenced Silences rust-log-analyzer postings to the PR it's added on. labels Sep 3, 2026

fmease removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Sep 3, 2026

fmease commented Sep 3, 2026

View reviewed changes

Comment thread compiler/rustc_parse/src/parser/expr.rs Outdated

Comment thread compiler/rustc_span/src/symbol.rs

fmease force-pushed the yeet-builtin-hash branch 2 times, most recently from 7415e73 to 1732079 Compare September 3, 2026 15:23

fmease mentioned this pull request Sep 3, 2026

Implement forced keywords (k#) #161775

Merged