Add experimental unnamed_enum_variants feature gate

[kupiakos]

This parses the basic syntax and defines a feature gate for the unnamed enum variants lang experiment, tracking issue #156628.

cc @joshtriplett, @scottmcm, @tmandry

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

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]

r? @Kivooeo

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They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 73 candidates
• Random selection from 17 candidates

[ChayimFriedman2]

Please separate the rust-analyzer changes into a PR for rust-lang/rust-analyzer.

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[kupiakos]

I've opened a PR for the rust-analyzer bits and will rebase this PR once those changes are pushed over to rust-lang/rust.

In the meantime, the rest of the PR should be able to receive feedback as-is.

@rustbot ready

[ChayimFriedman2]

You can remove the r-a changes from this PR. The other changes are not dependent on them. And also, I don't want to merge the r-a PR until the rustc PR is merged.

[kupiakos]

@ChayimFriedman2 Ah, I'd mistakenly thought that the r-a parser was actually being utilized by rustc here based on the rustc dev guide. But indeed this change isn't dependent on r-a. I've removed the r-a bits from this PR entirely as requested.

[Kivooeo]

(wrong PR ^^)

[Kivooeo]

Quick question: is there a discussion on Zulip or anywhere else I can found input from t-lang about this experiment?

[kupiakos]

@Kivooeo The tracking issue references the design meeting, where a lang experiment was approved. It's associated with an accepted 2026 project goal with its own tracking issue. There's some input on the RFC as well.

[Kivooeo]

Thanks! I will review today

[Kivooeo]

Looks good to me. Scope is just parsing the syntax + the feature gate, with lowering intentionally bailing via an error until the real implementation lands

View changes since this review

[Kivooeo]

@bors r+ rollup

[rust-bors]

📌 Commit e6b4102 has been approved by Kivooeo

It is now in the queue for this repository.

[rust-bors]

☀️ Test successful - CI
Approved by: Kivooeo
Duration: 3h 18m 12s
Pushing 0cf9681 to main...

[github-actions]

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 49b19d3 (parent) -> 0cf9681 (this PR)

Test differences

Show 6 test diffs

Stage 1

• [ui] tests/ui/feature-gates/feature-gate-unnamed-enum-variants.rs: [missing] -> pass (J0)
• [ui] tests/ui/unnamed-enum-variants/unimplemented.rs: [missing] -> pass (J0)

Stage 2

• [ui] tests/ui/feature-gates/feature-gate-unnamed-enum-variants.rs: [missing] -> pass (J1)
• [ui] tests/ui/unnamed-enum-variants/unimplemented.rs: [missing] -> pass (J1)

Additionally, 2 doctest diffs were found. These are ignored, as they are noisy.

Job group index

• J0: x86_64-gnu-llvm-21-3, x86_64-gnu-llvm-22-3
• J1: aarch64-apple, aarch64-gnu, aarch64-gnu-llvm-21-1, aarch64-msvc-1, arm-android, armhf-gnu, dist-i586-gnu-i586-i686-musl, i686-gnu-1, i686-gnu-nopt-1, i686-msvc-1, optional-x86_64-gnu-parallel-frontend, test-various, x86_64-gnu, x86_64-gnu-debug, x86_64-gnu-gcc, x86_64-gnu-llvm-21, x86_64-gnu-llvm-21-2, x86_64-gnu-llvm-22-2, x86_64-gnu-nopt, x86_64-gnu-stable, x86_64-mingw-1, x86_64-msvc-1

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 0cf96812614b8250543d749629867b36f2757f4d --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

1. x86_64-gnu-gcc-core-tests: 7m 48s -> 14m 49s (+90.1%)
2. dist-sparcv9-solaris: 57m 39s -> 1h 33m (+62.1%)
3. dist-various-1: 49m 11s -> 1h 17m (+56.8%)
4. dist-x86_64-msvc-alt: 2h 10m -> 2h 55m (+34.3%)
5. pr-check-1: 26m 12s -> 34m 32s (+31.8%)
6. dist-ohos-aarch64: 1h 1m -> 1h 17m (+25.9%)
7. dist-x86_64-apple: 2h 13m -> 1h 39m (-25.8%)
8. dist-loongarch64-musl: 1h 31m -> 1h 53m (+23.8%)
9. dist-arm-linux-musl: 1h 21m -> 1h 41m (+23.4%)
10. dist-i686-msvc: 1h 50m -> 2h 15m (+22.8%)

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]

Finished benchmarking commit (0cf9681): comparison URL.

Overall result: ❌✅ regressions and 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)	0.1%	[0.0%, 0.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 0.8%, secondary -2.4%)

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

	mean	range	count
Regressions ❌ (primary)	3.1%	[3.1%, 3.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.5%	[-1.5%, -1.5%]	1
Improvements ✅ (secondary)	-2.4%	[-4.2%, -1.6%]	8
All ❌✅ (primary)	0.8%	[-1.5%, 3.1%]	2

Cycles

Results (secondary 0.1%)

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)	3.8%	[2.2%, 5.7%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.1%	[-7.6%, -5.0%]	3
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 511.14s -> 512.762s (0.32%)
Artifact size: 398.59 MiB -> 400.63 MiB (0.51%)