For async closures, cap closure kind, get rid of by_mut_body

[compiler-errors]

Right now we have three AsyncFn* traits, and three corresponding futures that are returned by the call_* functions for them. This is fine, but it is a bit excessive, since the future returned by AsyncFn and AsyncFnMut are identical. Really, the only distinction we need to make with these bodies is "by ref" and "by move".

This PR removes AsyncFn::CallFuture and renames AsyncFnMut::CallMutFuture to AsyncFnMut::CallRefFuture. This simplifies MIR building for async closures, since we don't need to build an extra "by mut" body, but just a "by move" body which is materially different.

We need to do a bit of delicate handling of the ClosureKind for async closures, since we need to "cap" it to AsyncFnMut in some cases when we only care about what body we're looking for.

This also fixes a bug where <{async closure} as Fn>::call was returning a body that takes the async-closure receiver by move.

This also helps align the AsyncFn traits to the LendingFn traits' eventual designs.

[rustbot]

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[bors]

☔ The latest upstream changes (presumably #120712) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #121356) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #121800) made this pull request unmergeable. Please resolve the merge conflicts.

[compiler-errors]

r? oli-obk

This is almost certainly not commented enough; please let me know where I should go into more detail.

[oli-obk]

idk if it's a good time to start documenting how async closures work in the unstable-book, but may be good to have a single central thing to link to

[bors]

☔ The latest upstream changes (presumably #122241) made this pull request unmergeable. Please resolve the merge conflicts.

[oli-obk]

@bors r+

[bors]

📌 Commit 541858e has been approved by oli-obk

It is now in the queue for this repository.

[bors]

⌛ Testing commit 541858e with merge c86f3ac...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing c86f3ac to master...

[rust-timer]

Finished benchmarking commit (c86f3ac): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.2%	[0.1%, 4.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.7%	[3.7%, 3.7%]	1
Regressions ❌ (secondary)	6.1%	[6.1%, 6.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.7%	[3.7%, 3.7%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 670.215s -> 668.111s (-0.31%)
Artifact size: 312.79 MiB -> 312.74 MiB (-0.02%)