Change type of async context parameter after state transform.

[cjgillot]

Future::poll expects a &mut std::task::Context<'_>. Meanwhile, async coroutines use std::future::ResumeTy as resume parameter. This is meant to workaround the limitations of borrowck, which cannot prove that coroutines implement for<'a, 'b> CoroutineTrait<&'a mut Context<'b>>.

In the coroutine state transform, we need to change the signature from ResumeTy to the proper &mut Context<'_>. The current code attempts to find locals that have type ResumeTy to change their type. This is needlessly complex and relies on undocumented behaviour of the MIR builder.

Instead, this PR proposes to replace the ResumeTy argument with a new local, with value ResumeTy(transmute(context)).

Based on #156875.

[rustbot]

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[rustbot]

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.