Simplify array length mismatch error reporting (to not try to turn consts into target usizes)

[compiler-errors]

This changes TypeError::FixedArrayLen to use ExpectedFound<ty::Const<'tcx>> (instead of ExpectedFound<u64>), and renames it to TypeError::ArrayLen. This allows us to avoid a try_to_target_usize call in the type relation, which ICEs when we have a scalar of the wrong bit length (i.e. u8).

This also makes structurally_relate_tys to always use this type error kind any time we have a const mismatch resulting from relating the array-len part of [T; N].

This has the effect of changing the error message we issue for array length mismatches involving non-valtree consts. I actually quite like the change, though, since before:

LL | fn test<const N: usize, const M: usize>() -> [u8; M] {
   |                                              ------- expected `[u8; M]` because of return type
LL |     [0; N]
   |     ^^^^^^ expected `M`, found `N`
   |
   = note: expected array `[u8; M]`
              found array `[u8; N]`

and after, which I think is far less verbose:

LL | fn test<const N: usize, const M: usize>() -> [u8; M] {
   |                                              ------- expected `[u8; M]` because of return type
LL |     [0; N]
   |     ^^^^^^ expected an array with a size of M, found one with a size of N

The only questions I have are:

1. Should we do something about backticks here? Right now we don't backtick either fully evaluated consts like 2, or rigid consts like Foo::BAR.... but maybe we should? It seems kinda verbose to do for numbers -- maybe we could intercept those specifically.
2. I guess we may still run the risk of leaking unevaluated consts into error reporting like 2 + 1...?

r? @BoxyUwU

Fixes #126359
Fixes #131101

[bors]

☔ The latest upstream changes (presumably #133377) made this pull request unmergeable. Please resolve the merge conflicts.

[BoxyUwU]

@bors r+ rollup

I think putting backticks around non-literal array lengths would make sense. It probably also makes sense to normalize somewhere in diagnostics to ensure we don't talk about arrays with size 1 + 2 🤔 Can be done in a follow up PR if you want

[bors]

📌 Commit 28970a2 has been approved by BoxyUwU

It is now in the queue for this repository.