Fix where-bound suggestion with legacy const generics

[IsaiahCoroama]

Summary

During lowering of legacy const generics in lower_legacy_const_generics, the function's span was passed when creating the associated LocalDefId instead of the const argument span. This later caused diagnostics to pull the function's name instead of the const argument for suggestions when emitting an unconstrained generic constant error.

Also adds regression tests for all basic integer types. The full set isn't redundant: non-usize types exercise the as usize cast in the suggestion, while usize exercises the no-cast path. The tests use a cross-crate aux crate rather than a target-specific intrinsic (the rewrite only fires cross-crate), so they run on all platforms.

Sample

#![allow(incomplete_features)]
#![feature(generic_const_exprs)]

use std::arch::x86_64::*;
fn uwu<const N: i32>() {
    unsafe {
        let a = _mm_setzero_ps();
        let b = _mm_setzero_ps();
        _mm_shuffle_ps(a, b, N + 1);
    }
}

Previous Output

error: unconstrained generic constant
 --> src/lib.rs:9:30
  |
9 |         _mm_shuffle_ps(a, b, N + 1);
  |                              ^^^^^
  |
help: try adding a `where` bound
  |
5 | fn uwu<const N: i32>() where [(); _mm_shuffle_ps as usize]: {
  |                        ++++++++++++++++++++++++++++++++++++

Current Output

error: unconstrained generic constant
 --> src/lib.rs:9:30
  |
9 |         _mm_shuffle_ps(a, b, N + 1);
  |                              ^^^^^
  |
help: try adding a `where` bound
  |
5 | fn uwu<const N: i32>() where [(); N + 1 as usize]: {
  |                        +++++++++++++++++++++++++++

AI Assistance Disclosure

This is my first time contributing to the compiler, and I used Claude Opus 4.8 to assist with finding the root cause and implementing the fix, and to navigate the contributing docs, build system, and debugging tooling. The regression tests were designed and written entirely by me. I used Grok to proofread the written materials (commit bodies and this PR) for spelling and grammar. I reviewed and verified all changes myself.

Related Issues

Closes #108382

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[khyperia]

thanks for looking into this! as noted by @asquared31415 in the issue, generic_const_exprs is on its way out, to be replaced by generic_const_args (min_generic_const_args to start with). It would be nice if we could get a repro with min_generic_const_args instead if at all possible, but it's fine to keep this testing generic_const_exprs as well I suppose.

Some feedback:

• the suggestions for the non-usize ones don't actually compile - where [(); N + 1 as usize] should be where [(); (N + 1) as usize]. This feels like potentially an unrelated issue though, and if you'd like, it's fine to leave it unresolved IMO as the current output is miles better than the previous, especially if you can find a repro of bad paren suggestions unrelated to this issue (making sure it's tracked with an issue would be nice!)
• could you remove #![feature(generic_const_exprs)] (and #![allow(incomplete_features)]) from the aux crate? the aux crate is pretending to be std/core, which doesn't have GCE enabled, and removing it doesn't affect the test case, just makes it a little closer to the real world I suppose.

@rustbot author

[rustbot]

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[rustbot]

This PR was rebased onto a different main 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.