ptr-meta: DynMetadata and Pointee::Metadata are not UnwindSafe

[nwsharp]

DynMetadata is only UnwindSafe if its Dyn type argument is as well. This is a bit strange as DynMetadata is, data-wise, just a &'static to an immutable virtual table. There's not a way to observe broken invariant involving such a reference unless somehow the shared library containing the vtable is unloaded -- which violates 'static, would be super-mega-bad, and probably shouldn't be happening during unwind.

https://rust-lang.github.io/rfcs/2580-ptr-meta.html (#81513) states that Pointee::Metadata is Copy + Send + Sync + Ord + Hash + Unpin. Given these constraints, as well as the current types being (), usize, and DynMetadata, it seems reasonable to me that Pointee::Metadata also therefore be UnwindSafe.

I understand that UnwindSafe is currently in std, but with #84662 it will move into libcore. Provided that we indeed want it as a bound of Pointee::Metadata, we should definitely land this before exposing the ability for users to create custom metadata types.

Changing the bounds on Pointee::Metadata is explicitly mentioned as a concern in these meeting minutes.

(Also, the shared library unloading thing may indicate that we may want to drop the &'static in favor of *const.)

[nwsharp]

@rustbot label +A-dst +A-lang-item +A-raw-pointers +C-enhancement +requires-nightly +T-libs-api