Trait upcasting is not implemented yet

[ShoyuVanilla]

Miri also hits this incorrect diagnostics, in this line.

Originally posted by @RalfJung in #18047 (comment)

[Veykril]

this is going to be stabilized soon rust-lang/rust#134367 and we trigger type mismatch diagnostics on it so we should do something about this, example tests to check against are here https://github.com/WaffleLapkin/rust/tree/trait_upcasting_as_a_treat/tests/ui/traits/trait-upcasting

[ChayimFriedman2]

I believe this needs to be fixed in Chalk, but I will check to be sure.

Edit: Yes, this needs to be fixed in the builtin Unsize impls, aka. here.