Implement forced keywords (`k#`) by fmease · Pull Request #161775 · rust-lang/rust

fmease · 2026-08-25T16:46:21Z

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.

~~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:

implementing forced keyword lifetimes ('k#static)
migrating away from built-in syntax builtin # $ident($($tt)*) / introducing k#-exclusive (weak) keywords
- that'll be done in PR [blocked on 1.101.0 becoming beta & rustfmt stage0] Replace builtin # with k# #162232 instead
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
extending the proc_macro::Ident API to allow users to programmatically create forced keywords (might never be added)

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

fmease · 2026-08-25T16:46:44Z

@bors try @rust-timer queue

Implement forced keywords (`k#`)

fmease · 2026-08-25T17:19:35Z

    /// 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)),


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

No opinion on this.

rust-bors · 2026-08-25T17:48:33Z

☀️ Try build successful (CI)
Build commit: 4eb8eac (4eb8eac3e01e4dbc9d58546df979f51a2ac2e5d5)
Base parent: b751e7a (b751e7a48501be14dcd57b4a430b73a0e51a0c52)

rust-timer · 2026-08-25T18:30:41Z

Finished benchmarking commit (4eb8eac): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.2%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -3.0%, secondary 1.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.8%	[2.5%, 3.1%]	2
Improvements ✅ (primary)	-3.0%	[-4.1%, -1.9%]	3
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-3.0%	[-4.1%, -1.9%]	3

Cycles

Results (primary 2.1%, secondary -1.4%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.1%	[2.1%, 2.1%]	1
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.2%	[-4.1%, -2.3%]	2
All ❌✅ (primary)	2.1%	[2.1%, 2.1%]	1

Binary size

Results (primary 0.0%, secondary -0.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.1%]	4
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-0.0%	[-0.1%, -0.0%]	4
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	2
All ❌✅ (primary)	0.0%	[-0.1%, 0.1%]	8

Bootstrap: 477.576s -> 469.017s (-1.79%)
Artifact size: 400.35 MiB -> 400.38 MiB (0.01%)

fmease · 2026-08-27T14:32:58Z

Ready for review.

Caution

Only review the commits created after ------------------------- BRANCH SEPARATOR -------------------------.
That's because this PR is stacked on top of PR #161792. Hence, it's marked S-blocked Status: Blocked on something else such as an RFC or other implementation work. .

Best reviewed commit by commit.

rustbot · 2026-08-27T14:33:10Z

Some changes occurred in compiler/rustc_builtin_macros/src/autodiff.rs

cc @ZuseZ4

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

rustbot · 2026-09-27T14:43:22Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

fmease · 2026-09-27T14:48:18Z

I've addressed all of your suggestions & added a new commit that drops the abstraction-layer-breaking check from the lexer (can be squashed away).

@rustbot review

petrochenkov · 2026-09-27T20:53:51Z

@bors squash

* Implement forced keywords * Allow any identifier to be forced as a keyword even if not reserved

rust-bors · 2026-09-27T20:54:13Z

🔨 2 commits were squashed into abcef06.

petrochenkov · 2026-09-27T20:54:53Z

@bors r+

rust-bors · 2026-09-27T20:54:57Z

📌 Commit abcef06 has been tentatively approved by petrochenkov

It will be put into the queue for this repository once PR CI succeeds.

rust-bors · 2026-09-28T04:10:34Z

☀️ Test successful - CI
Approved by: petrochenkov
Duration: 3h 3m 33s
Pushing 2269776 to main...

github-actions · 2026-09-28T04:13:50Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing d080e7d (parent) -> 2269776 (this PR)

Test differences

Show 37 test diffs

Stage 1

[ui] tests/ui/parser/forced-keywords/basic.rs: [missing] -> pass (J0)
[ui] tests/ui/parser/forced-keywords/decl-macro-matching.rs: [missing] -> pass (J0)
[ui] tests/ui/parser/forced-keywords/feature-gate-forced-keywords.rs: [missing] -> pass (J0)
[ui] tests/ui/parser/forced-keywords/non-keyword-neg.rs: [missing] -> pass (J0)
[ui] tests/ui/parser/forced-keywords/non-keyword-pos.rs: [missing] -> pass (J0)
[ui] tests/ui/parser/forced-keywords/pre-2021-edition-fail.rs: [missing] -> pass (J0)
[ui] tests/ui/parser/forced-keywords/pre-2021-edition-pass.rs: [missing] -> pass (J0)
[ui (polonius)] tests/ui/parser/forced-keywords/basic.rs: [missing] -> pass (J1)
[ui (polonius)] tests/ui/parser/forced-keywords/decl-macro-matching.rs: [missing] -> pass (J1)
[ui (polonius)] tests/ui/parser/forced-keywords/feature-gate-forced-keywords.rs: [missing] -> pass (J1)
[ui (polonius)] tests/ui/parser/forced-keywords/non-keyword-neg.rs: [missing] -> pass (J1)
[ui (polonius)] tests/ui/parser/forced-keywords/non-keyword-pos.rs: [missing] -> pass (J1)
[ui (polonius)] tests/ui/parser/forced-keywords/pre-2021-edition-fail.rs: [missing] -> pass (J1)
[ui (polonius)] tests/ui/parser/forced-keywords/pre-2021-edition-pass.rs: [missing] -> pass (J1)

Stage 2

[ui] tests/ui/parser/forced-keywords/basic.rs: [missing] -> pass (J2)
[ui] tests/ui/parser/forced-keywords/decl-macro-matching.rs: [missing] -> pass (J2)
[ui] tests/ui/parser/forced-keywords/feature-gate-forced-keywords.rs: [missing] -> pass (J2)
[ui] tests/ui/parser/forced-keywords/non-keyword-neg.rs: [missing] -> pass (J2)
[ui] tests/ui/parser/forced-keywords/non-keyword-pos.rs: [missing] -> pass (J2)
[ui] tests/ui/parser/forced-keywords/pre-2021-edition-fail.rs: [missing] -> pass (J2)
[ui] tests/ui/parser/forced-keywords/pre-2021-edition-pass.rs: [missing] -> pass (J2)

Additionally, 16 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 22697761c6a01e8d12adcd91e4a5bc755596767a --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

test-x86_64-gnu-distcheck: 1h 35m -> 2h 24m (+52.2%)
test-x86_64-gnu-aux: 1h 54m -> 2h 41m (+41.3%)
dist-x86_64-msvc: 3h 27m -> 2h 3m (-40.4%)
dist-ohos-x86_64: 58m 4s -> 1h 21m (+40.0%)
test-x86_64-msvc-ext3: 1h 25m -> 1h 58m (+39.8%)
test-x86_64-gnu: 2h 37m -> 1h 38m (-37.5%)
dist-x86_64-solaris: 1h 15m -> 1h 43m (+36.2%)
dist-loongarch64-musl: 2h 9m -> 1h 23m (-35.7%)
dist-riscv64-linux-musl: 1h 7m -> 1h 30m (+32.8%)
dist-loongarch64-linux: 2h 5m -> 1h 27m (-30.0%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-09-28T04:54:51Z

Finished benchmarking commit (2269776): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.2%]	11
Improvements ✅ (secondary)	-0.4%	[-0.6%, -0.2%]	23
All ❌✅ (primary)	-0.2%	[-0.3%, -0.2%]	11

Max RSS (memory usage)

Results (primary 0.8%, secondary 2.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.8%	[0.8%, 0.8%]	1
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.8%	[0.8%, 0.8%]	1

Cycles

Results (primary -2.2%, secondary -3.6%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-2.2%	[-2.2%, -2.2%]	1
Improvements ✅ (secondary)	-5.6%	[-7.1%, -3.4%]	3
All ❌✅ (primary)	-2.2%	[-2.2%, -2.2%]	1

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 488.289s -> 489.806s (0.31%)
Artifact size: 407.14 MiB -> 406.32 MiB (-0.20%)

@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>

panstromek · 2026-09-29T05:44:54Z

perf triage:

This improvement looks real, but it didn't show up before merge. Do we know where this could come from?

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 25, 2026

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rust-bors Bot pushed a commit that referenced this pull request Aug 25, 2026

Auto merge of #161775 - fmease:kay-hash, r=<try>

4eb8eac

Implement forced keywords (`k#`)

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fmease force-pushed the kay-hash branch from 421efcd to d0c5d34 Compare August 25, 2026 17:12

fmease commented Aug 25, 2026

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fmease force-pushed the kay-hash branch from d0c5d34 to 4aee2b5 Compare August 25, 2026 17:54

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 25, 2026

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fmease force-pushed the kay-hash branch 2 times, most recently from 9e590de to 1de8681 Compare August 27, 2026 14:25

fmease added the S-blocked Status: Blocked on something else such as an RFC or other implementation work. label Aug 27, 2026

fmease changed the title ~~Implement forced keywords (k#)~~ [STACKED] Implement forced keywords (k#) Aug 27, 2026

fmease marked this pull request as ready for review August 27, 2026 14:33

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 27, 2026

rustbot assigned jackh726 Aug 27, 2026

petrochenkov added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Sep 25, 2026

fmease force-pushed the kay-hash branch from 450b77a to 14f9ced Compare September 27, 2026 14:43

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 27, 2026

fmease force-pushed the kay-hash branch from 14f9ced to 628d0f7 Compare September 27, 2026 14:49

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Implement forced keywords (k#)

abcef06

* Implement forced keywords * Allow any identifier to be forced as a keyword even if not reserved

rust-bors Bot force-pushed the kay-hash branch from 628d0f7 to abcef06 Compare September 27, 2026 20:54

petrochenkov added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 27, 2026

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rust-bors Bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Sep 28, 2026

rust-bors Bot merged commit 2269776 into rust-lang:main Sep 28, 2026
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rustbot added this to the 1.101.0 milestone Sep 28, 2026

rust-bors Bot mentioned this pull request Sep 28, 2026

Allow : after pat_param #162264

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fmease deleted the kay-hash branch September 28, 2026 04:12

fmease mentioned this pull request Sep 28, 2026

Tracking issue for MCP 945: Parse unstable keywords for experimental syntax #153839

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1 of 8 tasks

DanielEScherzer mentioned this pull request Sep 29, 2026

Grammar check: update rustc for forced keywords rust-lang/reference#2369

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rust-timer commented Aug 25, 2026

Overall result: ✅ improvements - no action needed

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Test differences

Stage 1

Stage 2

Job duration changes

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Overall result: ✅ improvements - no action needed

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