[WIP] Introduce repr(Swift)

[Lancelotbronner]

Adds the repr(Swift) attribute, allowing direct representation of Swift structs.

Since rebasing on the latest main it probably doesn't compile anymore.
I'd like to gauge interest to see if it's worth my time to continue this.

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

[rustbot]

rustc_codegen_gcc is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_gcc instead.

cc @antoyo, @GuillaumeGomez

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

cc @jdonszelmann, @JonathanBrouwer

rustc_codegen_cranelift is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_cranelift instead.

cc @bjorn3

These commits modify compiler targets.
(See the Target Tier Policy.)

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

⚠️ #[rustc_intrinsic_const_stable_indirect] controls whether intrinsics can be exposed to stable const
code; adding it needs t-lang approval.

cc @rust-lang/wg-const-eval

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

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

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @JohnTitor (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: libs
• libs expanded to 12 candidates
• Random selection from JohnTitor, Mark-Simulacrum, clarfonthey, nia-e

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] jiff test:false 7.416
error[E0308]: mismatched types
   --> compiler/rustc_target/src/callconv/mod.rs:772:21
    |
771 |                 arg.cast_to(CastTarget::prefixed(
    |                             -------------------- arguments to this function are incorrect
772 |                     prefix,
    |                     ^^^^^^ expected `ArrayVec<Reg, 8>`, found `[Option<Reg>; 8]`
    |
    = note: expected struct `ArrayVec<Reg, 8>`
                found array `[std::option::Option<Reg>; 8]`
note: associated function defined here
   --> compiler/rustc_target/src/callconv/mod.rs:297:12
    |
297 |     pub fn prefixed(prefix: ArrayVec<Reg, 8>, rest: Uniform) -> Self {
    |            ^^^^^^^^ ------------------------

For more information about this error, try `rustc --explain E0308`.
[RUSTC-TIMING] rustc_target test:false 2.797
error: could not compile `rustc_target` (lib) due to 1 previous error