Add unpolished, experimental support for AFIDT (async fn in dyn trait)

[compiler-errors]

This allows us to begin messing around async fn in dyn Trait. Calling an async fn from a trait object always returns a dyn* Future<Output = ...>.

To make it work, Implementations are currently required to return something that can be coerced to a dyn* Future (see the example in tests/ui/async-await/dyn/works.rs). If it's not the right size, then it'll raise an error at the coercion site (see the example in tests/ui/async-await/dyn/wrong-size.rs). Currently the only practical way of doing this is wrapping the body in Box::pin(async move { .. }).

This PR does not implement a helper type like a "Boxing"¹ adapter, and I'll probably follow-up with another PR to improve the error message for the PointerLike trait (something that explains in just normal prose what is happening here, rather than a trait error).

This PR also does not implement new trait solver support for AFIDT; I'll need to think how best to integrate it into candidate assembly, and that's a bit of a matter of taste, but I don't think it will be difficult to do.

This could also be generalized:

• To work on functions that are -> impl Future (soon).
• To work on functions that are -> impl Iterator and other "dyn rpitit safe" traits. We still need to nail down exactly what is needed for this to be okay (not soon).

Tracking:

• Tracking Issue for async_fn_in_dyn_trait #133119

Footnotes

1. https://rust-lang.github.io/async-fundamentals-initiative/explainer/user_guide_future.html#the-boxing-adapter ↩

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Some changes occurred to MIR optimizations

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[compiler-errors]

r? types

Probably best for a types team review.

[bors]

☔ The latest upstream changes (presumably #132460) made this pull request unmergeable. Please resolve the merge conflicts.