Introduce support for async bound modifier on Fn* traits

[compiler-errors]

Adds async to the list of TraitBoundModifiers, which instructs AST lowering to map the trait to an async flavor of the trait. For now, this is only supported for Fn* to AsyncFn*, and I expect that this manual mapping via lang items will be replaced with a better system in the future.

The motivation for adding these bounds is to separate the users of async closures from the exact trait desugaring of their callable bounds. Instead of users needing to be concerned with the AsyncFn trait, they should be able to write async Fn() and it will desugar to whatever underlying trait we decide is best for the lowering of async closures.

Note: rustfmt support can be done in the rustfmt repo after a subtree sync.

[rustbot]

r? @davidtwco

(rustbot has picked a reviewer for you, use r? to override)

[compiler-errors]

@rustbot label: I-lang-nominated

Hi @rust-lang/lang. Does this warrant a new experiment, or can I consider it to be part of the existing lang team experiment for AsyncFn traits? Any thoughts about this? Feel free to unnominate if you believe this doesn't need further team discussion.

[compiler-errors]

Oh, also cc @rust-lang/wg-async as well.

[rustbot]

Changes to the size of AST and/or HIR nodes.

cc @nnethercote

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit d2bb2cc with merge cb0d84e...

[bors]

☀️ Try build successful - checks-actions
Build commit: cb0d84e (cb0d84e48bea10d939e67f737b1bf3e373c00616)

[rust-timer]

Finished benchmarking commit (cb0d84e): comparison URL.

Overall result: ❌ regressions - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +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)	0.2%	[0.2%, 0.2%]	3
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	3

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)	-	-	0
Regressions ❌ (secondary)	4.9%	[4.9%, 4.9%]	1
Improvements ✅ (primary)	-2.1%	[-3.7%, -0.4%]	2
Improvements ✅ (secondary)	-2.7%	[-5.0%, -0.7%]	3
All ❌✅ (primary)	-2.1%	[-3.7%, -0.4%]	2

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: 660.662s -> 659.815s (-0.13%)
Artifact size: 308.15 MiB -> 308.17 MiB (0.01%)

[compiler-errors]

Perf effect seems unremarkable.

[davidtwco]

Implementation looks good, r=me after relevant approvals, etc.

[fmease]

rustc_ast_pretty still needs to be updated to accommodate the addition.

[compiler-errors]

T-lang went over this in a meeting today saying there is no concern here. I'm gonna approve this as gated under the async_closure feature gate when it's green.

@rustbot label: -I-lang-nominated

[compiler-errors]

@bors r=davidtwco,fmease

[bors]

📌 Commit 3913c9a has been approved by davidtwco,fmease

It is now in the queue for this repository.

[bors]

⌛ Testing commit 3913c9a with merge f3b9d47...

[bors]

☀️ Test successful - checks-actions
Approved by: davidtwco,fmease
Pushing f3b9d47 to master...

[rust-timer]

Finished benchmarking commit (f3b9d47): comparison URL.

Overall result: ❌ regressions - 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)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	0.2%	[0.2%, 0.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	1

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.3%	[-1.3%, -1.3%]	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: 661.246s -> 662.71s (0.22%)
Artifact size: 308.13 MiB -> 308.15 MiB (0.01%)