Suggests changing slice-like type instead of index for invalid SliceIndex

[thaliaarchi]

Following #138381, ByteStr now indexes with SliceIndex instead of Index/IndexMut. This worsened some diagnostics for indexing other types.

Since SliceIndex is “backwards”, this leads it to suggest other slice-like types (the LHS), instead of suggesting other index/range types which work for the LHS. When the LHS already indexes with SliceIndex, it's unhelpful to suggest other slice-like types. Furthermore, this leaks ByteStr into stable diagnostics (confirmed with either rust.channel = "stable" in bootstrap.toml or RUSTC_BOOTSTRAP=-1)

The affected diagnostics are shown in commit 9d379e1.

Code

fn main() {
    let x = vec![1];
    x[0i32]; //~ ERROR E0277
}

Current output

error[E0277]: the type `[{integer}]` cannot be indexed by `i32`
  --> tests/ui/indexing/index-help.rs:3:7
   |
LL |     x[0i32];
   |       ^^^^ slice indices are of type `usize` or ranges of `usize`
   |
   = help: the trait `SliceIndex<[{integer}]>` is not implemented for `i32`
   = help: the following other types implement trait `SliceIndex<T>`:
             `usize` implements `SliceIndex<ByteStr>`
             `usize` implements `SliceIndex<[T]>`
   = note: required for `Vec<{integer}>` to implement `Index<i32>`

Desired output

This was the output before that PR:

error[E0277]: the type `[{integer}]` cannot be indexed by `i32`
  --> tests/ui/indexing/index-help.rs:3:7
   |
LL |     x[0i32];
   |       ^^^^ slice indices are of type `usize` or ranges of `usize`
   |
   = help: the trait `SliceIndex<[{integer}]>` is not implemented for `i32`
           but it is implemented for `usize`
   = help: for that trait implementation, expected `usize`, found `i32`
   = note: required for `Vec<{integer}>` to implement `Index<i32>`

Rust Version

rustc 1.88.0 (56ffb4362 2025-04-05)
commit-hash: 56ffb43629bf58996c367073a0fa19e7d422df19
commit-date: 2025-04-05 10:18:03 +0200

[mejrs]

Because of the new implementation, there is no longer just one candidate at

rust/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

Line 1971 in 5e17a2a

if let [single] = &impl_candidates {

and the rest of that function gets to run, which is why the diagnostics changed. Maybe we can do something similar to

rust/compiler/rustc_trait_selection/src/error_reporting/traits/fulfillment_errors.rs

Line 2157 in 5e17a2a

// Check if the trait is the same in all cases. If so, we'll only show the type.

and not start list implementation if they all share the same self type? I'm not familiar enough with this code to know if that'll worsen other diagnostics.