We don't perform sized elaboration for impl-Trait-in-bindings (trait ascription)

[fmease]

I expected impl …-in-binding to get elaborated to impl Sized + … unless … contains ?Sized because that's what we do for all other kinds of impl-Trait (APIT, RPIT(IT), TAIT, ATPIT). However, we don't seem to do that right now.

I expected the code below to fail but it compiled successfully:

#![feature(impl_trait_in_bindings)]

fn main() {
    let _: &impl std::fmt::Debug = &[0u8][..];
}

The &impl std::fmt::Debug above should actually elaborate to &(impl std::fmt::Debug + Sized) and therefore the code should fail due to an unfulfilled [u8]: Sized obligation/goal.

As in other places, users need to actually write &(impl std::fmt::Debug + ?Sized) if they want the code above to work like on master.

[fmease]

Alternatively — if we dare to diverge in behavior from the other impl-Trait kinds — we ought to reject relaxed bounds like ?Sized inside impl-Trait-in-bindings.

[compiler-errors]

We should probably insert Sized here. Want me to fix it or will you?

[fmease]

You can go ahead, I won't be working on PRs tonight.