Fix feature gate for repr(simd)

[JonathanBrouwer]

The following code currently compiles on stable rust:

#[repr(simd)]
println!()

This has the following behavior:

• 1.90 and earlier give an "unused attribute warning"
• 1.91 ..= beta give no diagnostic output
• nightly gives the following:

warning: `#[repr(simd)]` attribute cannot be used on macro calls
 --> src/main.rs:4:5
  |
4 |     #[repr(simd)]
  |     ^^^^^^^^^^^^^
  |
  = help: `#[repr(simd)]` can only be applied to structs
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: `#[warn(unused_attributes)]` (part of `#[warn(unused)]`) on by default

This PR changes this to a feature gate error.
r? @mejrs

[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

[mejrs]

Please amend the PR description to state that this compiles without warning on rust 1.91 ..= current beta; this warning is only shown on nightly.

View changes since this review

[JonathanBrouwer]

@rustbot ready

[mejrs]

r=me after lang decision

View changes since this review

[traviscross]

Makes sense to me. Thanks for cleaning this up.

@rfcbot fcp merge lang

[rust-rfcbot]

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

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

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!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[tmandry]

@rfcbot reviewed

[rust-rfcbot]

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

[nikomatsakis]

@rfcbot reviewed

[joshtriplett]

Took a moment to realize that this is both no longer allowing repr(simd) on a macro call and moving the feature gate logic (which is also why the test output gets reordered).

Looks fine, though.

[scottmcm]

I was pondering whether this is clearly a bug and thus we should just skip the 10-day wait since the branch is Friday(?). Mark pointed out that it's already on stable, though, so if it waits another release it's no big deal.

[mejrs]

Took a moment to realize that this is both no longer allowing repr(simd) on a macro call and moving the feature gate logic

We do currently allow the other repr variants on macro calls

#[repr(C)] 
println!();

with the same "1.91 ..= beta give no diagnostic output" problem.

I'm currently running crater on that (#158816), any breakage from this PR should also show up in it.