Don't ignore generator fields in miri

[tmandry]

Right now, saved locals in generators are special-cased in miri, because they might not always be initialized.

With #59897 and #60187, we have the framework to fix this: fields are now only put in generator variants where they are live. The problem remains with aggregate fields, however, e.g.

fn gen() {
  let x: (i32, i32);
  x.0 = 5;
  yield;
  dbg!(x.0);
}

In this case x.1 is never initialized. This still presents a special case, because we cannot have a partially-unitialized field in a regular struct or enum type.

One way to remove the special-casing in miri is to wrap the type of every generator field in MaybeUninitialized when we return it from rustc::ty::layout::field(). This required making MaybeUnitialized a lang item, though, and doesn't actually give us any new features.

Eventually, I think we'd like to handle aggregate fields explicitly, so we can sanitize UB in generators.

[tmandry]

cc @RalfJung @Zoxc @eddyb @cramertj @rust-lang/wg-async-await

[RalfJung]

Right now, saved locals in generators are special-cased in miri, because they might not always be initialized.

In Miri and in layout code, and for the same reason.

[RalfJung]

This required making MaybeUnitialized a lang item, though, and doesn't actually give us any new features.

Well, it gives us a "more correct" layout for generators, for some notion of correctness. Also technically we wouldn't have to do this for every field, if you know which fields can be partially initialized for a particular yield point you can encode that. And that might give us layout optimizations.

[nikomatsakis]

Marking as "deferred" from a stabilization perspective.