Add trait_upcasting related languages changes

[WaffleLapkin]

I applied review comments from #1259 and added updates from rust-lang/rust#120248.

cc @crlf0710 @compiler-errors @RalfJung

[WaffleLapkin]

rust-lang/rust#101336 is still open, did we actually end up committing to something?

[compiler-errors]

@WaffleLapkin: Yes, it's just pending documentation rust-lang/rust#101336 (comment) -- presumably this is (A.) on Niko's list.

[compiler-errors]

You may also want to edit https://doc.rust-lang.org/reference/behavior-considered-undefined.html as Niko mentioned in the issue you linked; you may want to re-read the FCP, but I believe it's UB to conjure up a vtable now since it must be valid for upcasting.

[RalfJung]

You may also want to edit https://doc.rust-lang.org/reference/behavior-considered-undefined.html as Niko mentioned in the issue you linked; you may want to re-read the FCP, but I believe it's UB to conjure up a vtable now since it must be valid for upcasting.

What exactly are you referring to here? I assume the "issue" is rust-lang/rust#101336 (a link would have been good, it took clicking on 4 links to track this down :D ). Niko says a lot of things there, though. :)

[WaffleLapkin]

@compiler-errors doc.rust-lang.org/reference/behavior-considered-undefined.html already mentions invalid metadata in any kind of pointer makes an invalid value, so I don't think we need to add anything on top of that?

• Invalid metadata in a wide reference, Box<T>, or raw pointer. The requirement for the metadata is determined by the type of the unsized tail:
  • dyn Trait metadata is invalid if it is not a pointer to a vtable for Trait.
  • Slice ([T]) metadata is invalid if the length is not a valid usize (i.e., it must not be read from uninitialized memory). Furthermore, for wide references and Box<T>, slice metadata is invalid if it makes the total size of the pointed-to value bigger than isize::MAX.

[compiler-errors]

I believe this is ready and consistent with what is implemented on master. I think all that needs to be done w.r.t. trait upcasting is a fresh stabilization PR?