expand WF obligations when checking method calls

[compiler-errors]

Don't wrap a bunch of signatures in FnPtr then check their WF; instead, check the WFness of each input/output separately.

This is useful for the new trait solver, since because we stall on root obligations we end up needing to repeatedly recompute the WFness of possibly very large function signature types if it ends up bottoming out in ambiguity.

This may also give us more chances to hit the WF fast path for certain types like built-ins.

Finally, this just seems conceptually correct to do. There's nothing conceptually that suggests that wrapping the function signature in an fn pointer makes sense at all to do; I'm guessing that it was just convenient so that we didn't have to register WF obligations in a loop, but it doesn't affect the readability of this code at all.

[compiler-errors]

@bors2 try @rust-timer queue

[rust-bors]

⌛ Trying commit 170ccbf with merge 732256d…

To cancel the try build, run the command @bors try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 732256d (732256d64d426bee934865a624326e817fab77ca, parent: e5e79f8bd428d0b8d26e8240d718b134ef297459)

[rust-timer]

Finished benchmarking commit (732256d): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.3%]	2
Regressions ❌ (secondary)	0.7%	[0.1%, 1.1%]	10
Improvements ✅ (primary)	-0.2%	[-0.4%, -0.2%]	19
Improvements ✅ (secondary)	-1.2%	[-8.3%, -0.2%]	31
All ❌✅ (primary)	-0.2%	[-0.4%, 0.3%]	21

Max RSS (memory usage)

Results (primary -1.9%, secondary -2.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.9%	[-2.8%, -0.9%]	2
Improvements ✅ (secondary)	-2.2%	[-2.8%, -1.1%]	7
All ❌✅ (primary)	-1.9%	[-2.8%, -0.9%]	2

Cycles

Results (secondary -3.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	4.4%	[4.4%, 4.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.3%	[-7.0%, -2.2%]	6
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 469.342s -> 468.025s (-0.28%)
Artifact size: 376.86 MiB -> 376.85 MiB (-0.00%)

[compiler-errors]

r? lcnr

[compiler-errors]

The only major perf regression here is match-stress which seems somewhat unrelated and might just be noise (#144699 (comment)).

[lcnr]

While these changes here are neutral to positive and I agree using an FnPtr type to represent arbitrary signatures is kinda iffy, it does make me a bit sad.

It seems to indicate that we're leaving a lot of perf on the table by not returning the stalled WellFormed(?leaf_infer) from the trait solver in general. I think that now that the root actually tracks recursion_limit per obligation anyway, it's not that hard to return nested goals. My only other concern is cycle handling/detection 🤔 I do believe that as long as we only ever return nested <infer-ty>: Trait or WellFormed(<infer-ty>) goals we're still gonna properly detect the cycles once that <infer-ty> is resolved, just with a potentially different root

edit: and diagnostics :3

@bors r+

[bors]

📌 Commit 170ccbf has been approved by lcnr

It is now in the queue for this repository.