consider subtyping when checking if an infer var is sized

[WaffleLapkin]

That is, when checking if an inference variable (say ?0) is sized, with this PR, we now check for obligations like ?1: Sized, where either exists X. (?0 <: X || ?0 :> X) && (?1 <: X || ?1 :> X) holds (previously we were checking for ?0 = ?1). This is sound because subtyping cannot change sized-ness.

This fixes the 4-th point/issue from #155924. Specifically, it makes this code compile:

fn blah(e: !) {
    let source = Box::new(e);
    let _: Box<dyn Send> = source;
}

This also fixes tests/ui/impl-trait/unsized_coercion.rs on new solver, although I'm not exactly sure why.

r? types

[rustbot]

changes to inspect_obligations.rs

cc @lcnr

[WaffleLapkin]

This is not quite true.

View changes since the review