This code for recalculating `mcdc_bitmap_bytes` doesn't provide any benefit,
because its result won't have changed from the value in `FunctionCoverageInfo`
that was computed during the MIR instrumentation pass.

These comments appear to be inspired by the similar comments on
`CounterIncrement` and `ExpressionUsed`. But those comments refer to specific
simplification steps performed during coverage codegen, and there is no
corresponding step for the MC/DC coverage statements.

If these statements do not survive optimization, they will simply not
participate in code generation, just like any other statement.

…cts, r=compiler-errors

Also expand weak alias tys inside consts inside `expand_weak_alias_tys`

Ever since rust-lang#121344 has been merged, I couldn't let go of the fear that I might've slipped a tiny bug into rustc (:P).

Checking the type flags of the `Const` is strictly more correct than only checking the ones of the `Const`'s `Ty`. I don't think it's possible to trigger an ICE rn (i.e., one of the two `bug!("unexpected weak alias type")` I added in branches where `expand_weak_alias_tys` should've expanded *all* weak alias tys) because presently const exprs aren't allowed to capture late-bound vars. To be future-proof however, we should iron this out.

A possible reproducer would be the following if I'm not mistaken (currently fails to compile due to the aforementioned restriction):

```rs
#![feature(lazy_type_alias, adt_const_params, generic_const_exprs)]

type F = for<'a> fn(A<{ S::<Weak<'a>>(loop {}) }>) -> &'a ();

type A<const N: S<Weak<'static>>> = ();

#[derive(PartialEq, Eq, std::marker::ConstParamTy)]
struct S<T>(T);

type Weak<'a> = &'a ();
```

Whether a late-bound region should actually be considered constrained by a const expr is a separate question — one which we don't need to answer until / unless we actually allow them in such contexts (probable answer: only inside the return exprs of a block but not inside the stmts).

r? oli-obk (he's not available rn but that's fine) or types or compiler

…hercote

coverage: `CoverageIdsInfo::mcdc_bitmap_bytes` is never needed

This code for recalculating `mcdc_bitmap_bytes` in a query doesn't provide any benefit, because its result won't have changed from the value in `FunctionCoverageInfo` that was computed during the MIR instrumentation pass.

Extracted from rust-lang#124571, to avoid having this held up by unrelated issues with condition count checks.

`@rustbot` label +A-code-coverage

Add `on_unimplemented" typo suggestions

…mpiler-errors

Fix the dedup error because of spans from suggestion

Fixes rust-lang#116502

I believe this kind of issue is supposed resolved by rust-lang#118057, but the `==` in `span` respect syntax context, here we should only care that they point to the same bytes of source text, so should use `source_equal`.