Desugar yield in async gen correctly, ensure gen always returns unit

[compiler-errors]

1. Ensure async gen blocks desugar yield $expr to task_context = yield async_gen_ready($expr). Previously we were not assigning the task_context correctly, meaning that yield expressions in async generators returned type ResumeTy instead of (), and that we were not storing the task_context (which is probably unsound if we were reading the old task-context which has an invalidated borrow or something...)
2. Ensure that all (async?) gen blocks and (async?) gen fns return unit. Previously we were only checking this for gen fn, meaning that gen {} and async gen {} and async gen fn were allowed to return values that weren't unit. This is why [ICE]: assertion left == right failed, left: std::future::ResumeTy, right: () #119058 was an ICE rather than an E0308.

Fixes #119058.

[rustbot]

r? @wesleywiser

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[wesleywiser]

@bors r+

[bors]

📌 Commit 0f10acf has been approved by wesleywiser

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0f10acf with merge 59096cd...

[bors]

☀️ Test successful - checks-actions
Approved by: wesleywiser
Pushing 59096cd to master...

[rust-timer]

Finished benchmarking commit (59096cd): comparison URL.

Overall result: ✅ 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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	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)	4.1%	[4.1%, 4.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.7%	[-0.7%, -0.6%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.9%	[-0.7%, 4.1%]	3

Cycles

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)	0.5%	[0.5%, 0.5%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.6%	[-0.6%, -0.5%]	2
Improvements ✅ (secondary)	-2.9%	[-3.1%, -2.7%]	2
All ❌✅ (primary)	-0.2%	[-0.6%, 0.5%]	3

Binary size

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)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.4%, 0.4%]	1

Bootstrap: 673.568s -> 672.298s (-0.19%)
Artifact size: 312.47 MiB -> 312.49 MiB (0.01%)