Support `#[rustc_align_static]` inside `thread_local!` #146281

Jules-Bertholet · 2025-09-06T17:01:11Z

Tracking issue: #146177

thread_local! {
    #[rustc_align_static(64)]
    static SO_ALIGNED: u64 = const { 0 };
}

This increases the amount of recursion the macro performs (once per attribute in addition to the previous once per item), making it easier to hit the recursion limit. I’ve added workarounds to limit the impact in the case of long doc comments, but this still needs a crater run just in case.

r? libs

@rustbot label A-attributes A-macros A-thread-locals F-static_align T-libs

rustbot · 2025-09-06T17:01:16Z

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rustbot · 2025-09-06T17:01:19Z

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

The Miri subtree was changed

cc @rust-lang/miri

jdonszelmann · 2025-09-10T18:59:51Z

@Jules-Bertholet would you mind rebasing? #146178 merged
@rustbot author

rustbot · 2025-09-10T18:59:57Z

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

library/std/src/sys/thread_local/os.rs

Mark-Simulacrum · 2025-09-14T18:58:10Z

library/std/src/thread/local.rs

+    ([$($prev_align_attrs:tt)*] [$($prev_other_attrs:tt)*];
+     #[doc $($doc_rhs_1:tt)*] #[doc $($doc_rhs_2:tt)*] #[doc $($doc_rhs_3:tt)*] #[doc $($doc_rhs_4:tt)*]
+     #[doc $($doc_rhs_5:tt)*] #[doc $($doc_rhs_6:tt)*] #[doc $($doc_rhs_7:tt)*] #[doc $($doc_rhs_8:tt)*]
+     $($rest:tt)*) => (


I don't think we could use a regular proc macro (IIRC, std can't use proc macros), we'd have to make it a part of rustc.

Mark-Simulacrum · 2025-09-14T19:00:41Z

tests/ui/feature-gates/feature-gate-static_align-thread_local.rs

+#![crate_type = "lib"]
+
+thread_local! {
+    //~^ ERROR the `#[rustc_align_static]` attribute is an experimental feature


Are we (implicitly) stabilizing support for cfg_attr on the static in this PR? Can we test other forms of the attribute to verify we didn't allow other attributes (e.g., no_mangle or used(linker), etc.) and what the diagnostics are like?

Are we (implicitly) stabilizing support for cfg_attr on the static in this PR?

It was already stable.

Can we test other forms of the attribute to verify we didn't allow other attributes (e.g., no_mangle or used(linker), etc.) and what the diagnostics are like?

I’ve added some tests for this. The error spans are not great, but that is a pre-existing issue (#146567), this PR doesn’t make it any worse than before.

Another pre-existing issue I just discovered is that unsafe(no_mangle) is erroneously accepted (but has no effect). I am looking into that now… (Edit: #146557)

tests/ui/static/static-align.rs

Jules-Bertholet · 2025-09-19T19:32:00Z

I realized that I had neglected to support cfg_attr invocations where the number of attribute invocations in the right-hand side is not 1. The latest commit fixes this oversight, at the cost of making the macro even more complicated.

jdonszelmann · 2025-09-19T20:04:41Z

This is a T-libs issue but to me this macro looks very scary and I'm not sure it needs to be a decl macro. Maybe a compiler provided macro like format_args!() (though it is obviously much more complex) or line!() would be a better fit? If you need help with that I've worked with that part of the compiler quite a bit. But again, I don't think this is something I can decide, I'm just not very happy with this declmacro

Mark-Simulacrum · 2025-09-21T16:49:27Z

library/std/src/sys/thread_local/no_threads.rs

+        if self.state.get() == State::Alive {
+            self.value.get() as *const T
+        } else {
+            self.initialize(i, f)


This will hurt codegen for thread locals that otherwise had option optimization (e.g., if you store NonNull pointer today, this would optimize to a single load + test, rather than loading from two different memory locations).

I'm not sure how much that matters in practice, but it seems unfortunate to pay the cost for all users. Ideally, we'd only do so for the case where alignof(Option.Some.0) != alignof(Option), but I'm not sure if that's possible to check in macros / const-fn to optimize this.

Does this code get used only in the no_threads case? Or does it end up exported for the actually threaded cases too?

Only the no_threads case. I suppose we could have two impls depending on whether any rustc_align_static is present or not…

Mark-Simulacrum · 2025-09-21T16:52:54Z

library/std/src/sys/thread_local/os.rs

+    (@align) => (1),
+
+    // `rustc_align_static` attributes are present,
+    // translate them into a `const` block that computes the alignment


I wonder if this could be simplified by attaching the attributes to some static and then checking the alignment of that static (via align_of_val)? That way we presumably get most of the error checking from rustc and we don't nee to manually compute the alignment or deal with cfgs.

We would have to translate rustc_align_static(…) into repr(align(…)), so just as much complexity as before.

We do rely on rustc for error checking, via static DUMMY below.

Perhaps this was meant as a comment on the struct idea below? For this comment using a static shouldn't require any translation - presumably align_of_val(&DUMMY) gives us the alignment we need?

With regards to structs, I wonder if we should teach rustc to support rustc_align_static on fields/structs as a forever unstable extension just to simplify the impl here....

presumably align_of_val(&DUMMY) gives us the alignment we need?

No, that would give the alignment of the type of DUMMY, which is 1.

Mark-Simulacrum · 2025-09-21T16:56:59Z

library/std/src/sys/thread_local/os.rs

    // Note that to prevent an infinite loop we reset it back to null right
    // before we return from the destructor ourselves.
    abort_on_dtor_unwind(|| {
-        let ptr = unsafe { Box::from_raw(ptr as *mut Value<T>) };


Similarly to the above thought: is it plausible that for aligned thread locals we could generate a repr(C) struct that bumped the alignment of our stored value to what we need and then we'd alter our code to &struct.0 when actually returning pointers out to userspace? Presumably that would avoid the majority of the internal changes and avoid storing alignments into memory (wastefully).

I realized that we can just use a const generic. Switched to that

Jules-Bertholet · 2025-09-21T22:11:02Z

I have no idea why the Miri floating point test failed in #146281 (comment), or why fca42ed fixes it. @RalfJung do you know what is going on here?

RalfJung · 2025-09-22T07:02:52Z

Well, it's a randomized test, so it always has a small chance of failing.

bors · 2025-09-24T01:56:09Z

☔ The latest upstream changes (presumably #146931) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2025-09-24T02:19:34Z

This PR was rebased onto a different master 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.

rustbot assigned lcnr Sep 6, 2025

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rustbot added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. and removed A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. labels Sep 6, 2025

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Mark-Simulacrum mentioned this pull request Sep 14, 2025

Tracking Issue for static_align - alignment on static items #146177

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Allow &&, ||, and ! in cfg rust-lang/rfcs#3796

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Allow passing expr metavariable to cfg #146961

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Support #[rustc_align_static] inside thread_local!

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Hoist non-platform-specific code out of thread_local_inner!

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thread_local! doesn’t like types whose definitions cannot be written more than once #147103

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Support #[rustc_align_static] inside thread_local! #146281

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Support #[rustc_align_static] inside thread_local! #146281

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Support `#[rustc_align_static]` inside `thread_local!` #146281

Support `#[rustc_align_static]` inside `thread_local!` #146281

Jules-Bertholet commented Sep 6, 2025 •

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