Implement `#[rustc_align_static(N)]` on `static`s #146178

folkertdev · 2025-09-03T20:56:42Z

Tracking issue: #146177

#![feature(static_align)]

#[rustc_align_static(64)]
static SO_ALIGNED: u64 = 0;

We need a different attribute than rustc_align because unstable attributes are tied to their feature (we can't have two unstable features use the same unstable attribute). Otherwise this uses all of the same infrastructure as #[rustc_align].

r? @traviscross

rustbot · 2025-09-03T20:56:48Z

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

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

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

RalfJung · 2025-09-03T21:13:33Z

we can't have two unstable features use the same unstable attribute

What do you mean by "use"? How does a feature "use" an attribute?
All rustc_ attributes are unstable anyway...

compiler/rustc_const_eval/src/interpret/memory.rs

compiler/rustc_codegen_gcc/src/consts.rs

folkertdev · 2025-09-03T21:16:31Z

compiler/rustc_feature/src/builtin_attrs.rs

    ),
    // FIXME(#82232, #143834): temporarily renamed to mitigate `#[align]` nameres ambiguity
    gated!(rustc_align, Normal, template!(List: &["alignment"]), DuplicatesOk, EncodeCrossCrate::No, fn_align, experimental!(rustc_align)),
+    gated!(rustc_align_static, Normal, template!(List: &["alignment"]), DuplicatesOk, EncodeCrossCrate::No, static_align, experimental!(rustc_align_static)),


@RalfJung this is where the attribute gets declared, and the feature name on this line (static_align in this case) must be unique

That's very odd, why does it have to be unique? That's not how features work anywhere else in the compiler, AFAIK.

ah, sorry, what I meant by unique is that an attribute must be tied to one unstable feature. #[rustc_align] is already tied to fn_align, so it can't also be used for static_align.

So, one unstable feature can "contain" multiple attributes, but an attribute always is tied to only one unstable feature.

Ah, by "attribute used for feature" you mean "attribute is gated by feature". That's a rather confusing and non-standard way of saying things.^^

So what you want is to have a separate feature gate for aligning functions vs aligning statics, and the attribute registration macro isn't flexible enough to allow that. rustc_align is for functions only, despite its name sounding more general.

Since these are all temporary names anyway, I guess it doesn't matter too much, but when it comes to stabilization we very much might need the ability to stabilize an attribute in some positions while leaving it unstable elsewhere. Otherwise we have no way of unstably extending where an existing stable attribute can be used. So this limitation of attribute feature gating seems like it could become problematic. Cc @jdonszelmann

Jules-Bertholet · 2025-09-03T21:29:12Z

This needs tests for:

Thread locals (#[thread_local] and thread_local!)
extern block statics

folkertdev · 2025-09-03T22:18:19Z

extern

Is there any interesting behavior to test here? (I'll add a test for the attribute being valid).

thread_local!

hmm, the thread_local! macro actually applies the attribute to a constant

rust/library/std/src/sys/thread_local/native/mod.rs

Lines 107 to 110 in fd75a9c

    
           ($(#[$attr:meta])* $vis:vis $name:ident, $t:ty, $($init:tt)*) => { 
        
               $(#[$attr])* $vis const $name: $crate::thread::LocalKey<$t> = 
        
                   $crate::thread::local_impl::thread_local_inner!(@key $t, $($init)*); 
        
           },

compiler/rustc_const_eval/src/interpret/util.rs

Jules-Bertholet · 2025-09-05T03:28:53Z

Here is an implementation for thread_local!: Jules-Bertholet@ad038e6

compiler/rustc_const_eval/src/interpret/memory.rs

rustbot · 2025-09-05T09:51:23Z

Some changes occurred to the CTFE / Miri interpreter

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

folkertdev · 2025-09-05T12:20:27Z

That's odd, the miri tests pass for me locally. Do those somehow cache the standard library even when you don't pass --keep-stage?

RalfJung · 2025-09-05T12:58:54Z

Miri has its own standard library build but it should usually be recompiled automatically when anything changed.

RalfJung · 2025-09-05T13:08:21Z

The error might only occur after merging the branch with latest master (which happens implicitly in CI).

Jules-Bertholet · 2025-09-05T13:27:21Z

which happens implicitly in CI

Doesn’t that apply only to Bors CI? (E.g. Bors try or Bors r+)

compiler/rustc_passes/messages.ftl

jdonszelmann · 2025-09-09T19:34:32Z

some minor questions: @rustbot author

rustbot · 2025-09-09T19:34:37Z

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

We need a different attribute than `rustc_align` because unstable attributes are tied to their feature (we can't have two unstable features use the same unstable attribute). Otherwise this uses all of the same infrastructure as `#[rustc_align]`.

folkertdev · 2025-09-09T20:04:02Z

so it's not entirely clear to me why this is a different attribute from fn_align. I guess it's cause fn align is much further in its stabilization etc. But these feel like the same feature to me. At the risk of bikeshedding, maybe we should rename both to rustc_item_align or something like that?

Keeping them separate seemed most flexible in terms of what/how to stabilize. What we'll do exactly is a bit up in the air, pending a path forward for making #[align] a builtin attribute. Just making that change causes ecosystem breakage, and properly performing name resolution on builtin attributes will require an edition bump.

@rustbot ready

jdonszelmann · 2025-09-09T20:17:28Z

@bors r=jdonszelmann,ralfjung

bors · 2025-09-09T20:17:31Z

📌 Commit cbacd00 has been approved by jdonszelmann,ralfjung

It is now in the queue for this repository.

…lmann,ralfjung Implement `#[rustc_align_static(N)]` on `static`s Tracking issue: rust-lang#146177 ```rust #![feature(static_align)] #[rustc_align_static(64)] static SO_ALIGNED: u64 = 0; ``` We need a different attribute than `rustc_align` because unstable attributes are tied to their feature (we can't have two unstable features use the same unstable attribute). Otherwise this uses all of the same infrastructure as `#[rustc_align]`. r? `@traviscross`

…lmann,ralfjung Implement `#[rustc_align_static(N)]` on `static`s Tracking issue: rust-lang#146177 ```rust #![feature(static_align)] #[rustc_align_static(64)] static SO_ALIGNED: u64 = 0; ``` We need a different attribute than `rustc_align` because unstable attributes are tied to their feature (we can't have two unstable features use the same unstable attribute). Otherwise this uses all of the same infrastructure as `#[rustc_align]`. r? ``@traviscross``

traviscross · 2025-09-10T05:46:28Z

@bors r=jdonszelmann,ralfjung,traviscross

bors · 2025-09-10T05:46:31Z

💡 This pull request was already approved, no need to approve it again.

There's another pull request that is currently being tested, blocking this pull request: [beta] backports #146350

bors · 2025-09-10T05:46:32Z

📌 Commit cbacd00 has been approved by jdonszelmann,ralfjung,traviscross

It is now in the queue for this repository.

RalfJung · 2025-09-10T06:51:13Z

Keeping them separate seemed most flexible in terms of what/how to stabilize. What we'll do exactly is a bit up in the air, pending a path forward for making #[align] a builtin attribute. Just making that change causes ecosystem breakage, and properly performing name resolution on builtin attributes will require an edition bump.

Note that a stabilization PR should just swap the feature gate, not rename anything. (That makes it easier to review, easier to revert, and ensures the thing we had on nightly is actually what we are stabilizing.) So if in the stable form, the attributes should have the same name, then that rename should happen separately from stabilization.

But IIUC, in this case the name itself is still subject to bikeshedding.

Rollup of 6 pull requests Successful merges: - #144765 (inclusive `Range`s: change `end` to `last`) - #145177 (std: move `thread` into `sys`) - #146178 (Implement `#[rustc_align_static(N)]` on `static`s) - #146368 (CI: rfl: move job forward to Linux v6.17-rc5 to remove temporary commits) - #146378 (Update wasm-component-ld to 0.5.17) - #146391 (Trim paths less in MIR dumping) r? `@ghost` `@rustbot` modify labels: rollup

Rollup of 5 pull requests Successful merges: - #144765 (inclusive `Range`s: change `end` to `last`) - #146178 (Implement `#[rustc_align_static(N)]` on `static`s) - #146368 (CI: rfl: move job forward to Linux v6.17-rc5 to remove temporary commits) - #146378 (Update wasm-component-ld to 0.5.17) - #146391 (Trim paths less in MIR dumping) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #146178 - folkertdev:static-align, r=jdonszelmann,ralfjung,traviscross Implement `#[rustc_align_static(N)]` on `static`s Tracking issue: #146177 ```rust #![feature(static_align)] #[rustc_align_static(64)] static SO_ALIGNED: u64 = 0; ``` We need a different attribute than `rustc_align` because unstable attributes are tied to their feature (we can't have two unstable features use the same unstable attribute). Otherwise this uses all of the same infrastructure as `#[rustc_align]`. r? `@traviscross`

Rollup of 5 pull requests Successful merges: - rust-lang/rust#144765 (inclusive `Range`s: change `end` to `last`) - rust-lang/rust#146178 (Implement `#[rustc_align_static(N)]` on `static`s) - rust-lang/rust#146368 (CI: rfl: move job forward to Linux v6.17-rc5 to remove temporary commits) - rust-lang/rust#146378 (Update wasm-component-ld to 0.5.17) - rust-lang/rust#146391 (Trim paths less in MIR dumping) r? `@ghost` `@rustbot` modify labels: rollup

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folkertdev force-pushed the static-align branch from 190ad9c to 07f4381 Compare September 3, 2025 23:07

traviscross changed the title ~~implement #[rustc_align_static(N)] on statics~~ Implement #[rustc_align_static(N)] on statics Sep 3, 2025

traviscross added the F-static_align #![feature(static_align)] label Sep 3, 2025

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folkertdev force-pushed the static-align branch from 07f4381 to d66847e Compare September 4, 2025 20:17

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folkertdev force-pushed the static-align branch from d66847e to 6f0825e Compare September 5, 2025 09:51

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folkertdev force-pushed the static-align branch from 389dad1 to 602e757 Compare September 5, 2025 12:46

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folkertdev force-pushed the static-align branch from af834f3 to cbacd00 Compare September 9, 2025 19:55

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matthiaskrgr mentioned this pull request Sep 10, 2025

Rollup of 6 pull requests #146395

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Rollup of 6 pull requests #146396

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Rollup of 5 pull requests #146409

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bors merged commit 422c76a into rust-lang:master Sep 10, 2025
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Implement #[rustc_align_static(N)] on statics #146178

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Implement `#[rustc_align_static(N)]` on `static`s #146178

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