`fixed_map`: `derive(Key)` treats alias as rigid

[lcnr]

https://crater-reports.s3.amazonaws.com/pr-133502-13/try%233cddd79d4be3c8f1e937d09ca618d192abc69373/reg/fixed-map-0.9.5/log.txt

struct Outer;
struct Inner;
trait Id<T> {
    type This;
}
impl<T, U> Id<U> for T {
    type This = T;
}

fn free<T>(x: T) -> <T as Id<Inner>>::This
where
    <T as Id<Outer>>::This: Id<Inner, This = <T as Id<Inner>>::This>,
{
    x
}

compiles with old, errors with new.

[lcnr]

Normalizing <T as Id<Outer>>::This: Id<Inner, This = <T as Id<Inner>>::This> is interesting™

• <T as Id<Outer>>::This always uses the impl and normalizes to T
• <T as Id<Inner>>::This can be normalized by using either the impl or the normalized where-clause

The old solver normalizes it in an unnormalized env, which uses the impl and normalizes it to T.

The new solver properly handles aliases in the environment, so we're able to equate <T as Id<Outer>>::This with T no matter here we normalize <T as Id<Inner>>::This. This means that the alias is treated as rigid with the new solver.

[lcnr]

See https://github.com/rust-lang/trait-system-refactor-initiative/blob/66394e265a9c3671793e042ece4ffdfafe7b9ef0/notes/next-solver/impl-shadowing-via-env.md for my thoughts about this issue