Allow elided ('static) lifetimes in `thread_local!` - #159564

zachs18 · 2026-07-19T13:00:59Z

with or without a const initializer, on all platforms.

Lifetime elision on functions includes named lifetimes and 'static in input lifetime positions, so if we give the macro-generated __rust_std_internal_init_fn function an argument mentioning 'static, then elided lifetimes in the return type default to 'static. This uses PhantomData<&'static ()> so that it should probably compile down to nothing (at least in release mode).

Before this change, elided 'static lifetimes were allowed only with const initializers on the "no-threads" and "native" thread_local! implementations, not on the "os" implementation, and not with non-const initializers.

After this change, they are allowed in all thread_local! implementations, with or without a const initializer (A and B both compile on targets with all three thread_local! implementations.)

// Const initializer
std::thread_local!(static A: &str = const { "" });
// Non-const initializer
std::thread_local!(static B: &str = "");

`thread_local!` implementation	`const` initializer (`A`)	non-`const` initializer (`B`)
no-threads (e.g. `x86_64-unknown-uefi`)	✅️	❌️ -> ✅️
`target_thread_local` (e.g. `x86_64-unknown-linux-gnu`)	✅️	❌️ -> ✅️
os (e.g. `x86_64-pc-windows-gnu`)	❌️ -> ✅️	❌️ -> ✅️

An alternative implementation that would only fix the inconsistency between targets, but not add support for elision with non-const initializers, would be to do this same thing, but only on the os implementation, and only if the initializer is const (i.e. split const initializers to a different macro arm; currently const and non-const initializers generate the same code under the os thread_local! implementation).

Fixes #159538

Fixes #159640 (assuming the non-const-initializer part of this PR is not removed)

with or without a const initializer, on all platforms.

rustbot · 2026-07-19T13:01:04Z

r? @JohnTitor

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zachs18 · 2026-07-19T13:04:14Z

Question for reviewer: (how) should I add a test for this? I tried adding a test to tests/ui/thread-local with revisions for different targets, but ui tests don't seem to build the stdlib for anything but the current target by default, so it failed with "could not find crate core" on the other two non-x86_64-unknown-linux-gnu targets unless I manually ran with --target x86_64-unknown-uefi or --target x86_64-pc-windows-gnu (in which case that target succeeded and the other non-native target still failed).

attempted UI test

// Test each of the three `thread_local!` implementations,
// that it defaults elided lifetimes in the type to `'static`.
// Regression test for 159358.
//@ check-pass

// no-threads
//@ revisions: x86_64-uefi
//@[x86_64-uefi] compile-flags: --target x86_64-unknown-uefi
//@[x86_64-uefi] needs-llvm-components: x86

// target_thread_local
//@ revisions: x86_64-linux
//@[x86_64-linux] compile-flags: --target x86_64-unknown-linux-gnu
//@[x86_64-linux] needs-llvm-components: x86

// os
//@ revisions: x86_64-windows-gnu
//@[x86_64-windows-gnu] compile-flags: --target x86_64-pc-windows-gnu
//@[x86_64-windows-gnu] needs-llvm-components: x86

// Const initializer, non-Drop
std::thread_local!(static A: &str = const { "" });
// Non-const initializer, non-Drop
std::thread_local!(static B: &str = "");

fn main() {}

(Dropness isn't relevant here)

zachs18 · 2026-07-19T13:17:11Z

@rustbot label T-libs-api

I think this needs T-libs-api approval, at least for the non-const-initializer part?

It was testing diagnostics emitted for missing lifetime specifiers in `thread_local!`, but now elided lifetimes in `thread_local!` are `'static`, so there's nothing to test.

zachs18 · 2026-07-21T17:30:13Z

For tests: Hmm, if there's at least one tier-1 target that uses each implementation of thread_local!, then just adding the test with no target revisions should work (i.e. the different impl's will get tested in CI), right? Well, there's no Tier 1 "no-threads" target, but x86_64-unknown-linux-gnu ("native") and x86_64-pc-windows-gnu ("os") are both Tier 1, so those will both be tested.

JohnTitor · 2026-08-15T10:05:14Z

@rustbot reroll

oli-obk · 2026-08-18T13:28:04Z

r? libs

clarfonthey · 2026-08-18T14:34:19Z

I'll nominate for today's libs meeting.

@rustbot label:I-libs-nominated

Amanieu · 2026-08-18T16:49:48Z

@rfcbot merge libs,libs-api

rust-rfcbot · 2026-08-18T16:49:52Z

@Amanieu has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

rust-rfcbot · 2026-09-15T19:10:41Z

🔔 This is now entering its final comment period, as per the review above. 🔔

RalfJung · 2026-09-25T09:56:48Z

+        // We intentionally have an argument-position `'static` lifetime so that elided lifetimes in `$t`
+        // become `'static` like they do for `const`s and `static`s, including in the other two
+        // `thread_local!` implementations.
+        #[allow(mismatched_lifetime_syntaxes)]
        #[inline]


This comment could use a bit more detail, it's quite hard to understand what is actually going on. I'd suggest discussing the concrete example of $t being &str and why the argument matters for that.

View changes since the review

Fair, it took me a bit as a reviewer to understand too. If @zachs18 wants to try and improve the docs before merge I'd be happy to review, otherwise I don't mind improving the docs in a separate PR.

RalfJung · 2026-09-25T09:57:50Z

We still error if the lifetime can't be 'static, right? Is there a test for that?

View changes since the review

Considering the load-bearing:

static __RUST_STD_INTERNAL_VAL: $crate::thread::local_impl::LazyStorage<$t, ()>

uh, yes, if that accepted anything not-'static I would be quite concerned. But I would be happy to add more tests post-merge if it makes you comfortable. Since beta branches today we'll have an entire release cycle to verify there are no issues.

How's that load-bearing? Seems worth a comment then.

rust-rfcbot · 2026-09-25T19:11:58Z

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

clarfonthey · 2026-09-25T19:36:51Z

@bors try

Allow elided ('static) lifetimes in `thread_local!`

rust-bors · 2026-09-25T20:36:36Z

☀️ Try build successful (CI)
Build commit: 5986ef5 (5986ef50c8f1a4e25072bd37f245d83f0e1e04f3)
Base parent: 5ceaf66 (5ceaf6608eb354c2f5bbb3b8d974caa367dac81c)

clarfonthey · 2026-09-26T16:47:36Z

@bors r+ rollup

Hooray! This is finally merged. I will follow up with a PR adding some more docs/tests for @RalfJung since we have a whole release cycle to clarify that.

rust-bors · 2026-09-26T16:47:39Z

📌 Commit e423836 has been approved by clarfonthey

It is now in the queue for this repository.

…uwer Rollup of 6 pull requests Successful merges: - #150824 (Document platform-specific behavior of `current_exe`, including that Linux can add `" (deleted)"`) - #159564 (Allow elided ('static) lifetimes in `thread_local!`) - #163026 (hir_typeck: simplify `upvar::determine_capture_info` impl) - #163256 (Document `rustc_abi::VariantLayout`) - #163366 (Stabilize SyncView) - #163376 (Option, Result: not all arguments passed to map_or are eagerly evaluated)

Rollup merge of #159564 - zachs18:fix-159358, r=clarfonthey Allow elided ('static) lifetimes in `thread_local!` with or without a const initializer, on all platforms. Lifetime elision on functions includes named lifetimes and `'static` in input lifetime positions, so if we give the macro-generated `__rust_std_internal_init_fn` function an argument mentioning `'static`, then elided lifetimes in the return type default to `'static`. This uses `PhantomData<&'static ()>` so that it should probably compile down to nothing (at least in release mode). Before this change, elided `'static` lifetimes were allowed only with `const` initializers on the "no-threads" and "native" `thread_local!` implementations, not on the "os" implementation, and not with non-`const` initializers. After this change, they are allowed in all `thread_local!` implementations, with or without a `const` initializer (`A` and `B` both compile on targets with all three `thread_local!` implementations.) ```rs // Const initializer std::thread_local!(static A: &str = const { "" }); // Non-const initializer std::thread_local!(static B: &str = ""); ``` | `thread_local!` implementation | `const` initializer (`A`) | non-`const` initializer (`B`) | | ------------- | ------------- | ---- | | no-threads (e.g. `x86_64-unknown-uefi`) | ✅️ | ❌️ -> ✅️ | | `target_thread_local` (e.g. `x86_64-unknown-linux-gnu`) | ✅️ | ❌️ -> ✅️ | | os (e.g. `x86_64-pc-windows-gnu`) | ❌️ -> ✅️ | ❌️ -> ✅️ | An alternative implementation that would only fix the inconsistency between targets, but not add support for elision with non-`const` initializers, would be to do this same thing, but only on the `os` implementation, and only if the initializer is `const` (i.e. split const initializers to a different macro arm; currently const and non-const initializers generate the same code under the `os` `thread_local!` implementation). Fixes #159538 Fixes #159640 (assuming the non-`const`-initializer part of this PR is not removed)

Allow elided ('static) lifetimes in #[thread_local]

ea48bea

with or without a const initializer, on all platforms.

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jul 19, 2026

rustbot assigned JohnTitor Jul 19, 2026

zachs18 changed the title ~~Allow elided ('static) lifetimes in #[thread_local]~~ Allow elided ('static) lifetimes in thread_local! Jul 19, 2026

rustbot added the T-libs-api [DEPRECATED; DO NOT USE] label Jul 19, 2026

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Remove tests/ui/suggestions/missing-lifetime-specifier.rs

e3532e0

It was testing diagnostics emitted for missing lifetime specifiers in `thread_local!`, but now elided lifetimes in `thread_local!` are `'static`, so there's nothing to test.

zachs18 mentioned this pull request Jul 21, 2026

Inconsistent lifetime elision for thread_local! #159640

Closed

Add test for thread_local! lifetime elision.

e423836

clarfonthey removed the T-libs-api [DEPRECATED; DO NOT USE] label Aug 12, 2026

rustbot assigned oli-obk and unassigned JohnTitor Aug 15, 2026

rustbot assigned clarfonthey and unassigned oli-obk Aug 18, 2026

rustbot added the I-libs-nominated Nominated for discussion during a libs team meeting. label Aug 18, 2026

rust-rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Aug 18, 2026

nia-e removed the I-libs-nominated Nominated for discussion during a libs team meeting. label Aug 24, 2026

rust-rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Sep 15, 2026

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RalfJung reviewed Sep 25, 2026

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rustbot mentioned this pull request Sep 25, 2026

Tracking issue for release notes of #159564: Allow elided ('static) lifetimes in thread_local! #163353

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

Auto merge of #159564 - zachs18:fix-159358, r=<try>

5986ef5

Allow elided ('static) lifetimes in `thread_local!`

rust-bors Bot 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 26, 2026

JonathanBrouwer mentioned this pull request Sep 26, 2026

Rollup of 6 pull requests #163386

Merged

clarfonthey mentioned this pull request Sep 26, 2026

thread_local! doc improvements + tests for operational pedantics #163387

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rust-bors Bot merged commit d8fb820 into rust-lang:main Sep 26, 2026
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rustbot added this to the 1.101.0 milestone Sep 26, 2026

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