obligations_for_self_ty: skip irrelevant goals

[lcnr]

Reduces the compile time of wg-grammar from more than 70s to about 40s. So a >30% perf improvement for that crate.

r? @BoxyUwU @compiler-errors

[rustbot]

Some changes occurred to the core trait solver

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

changes to inspect_obligations.rs

cc @compiler-errors, @lcnr

[lcnr]

@bors try @rust-timer queue

[lcnr]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: ba651ad (ba651ad1746c1435192a5568cd8732d36f8536ce, parent: 2f4dfc753fd86c672aa4145940db075a8a149f17)

[rust-timer]

Finished benchmarking commit (ba651ad): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.6%	[-1.0%, -0.3%]	7
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.0%	[-3.8%, -2.2%]	2
All ❌✅ (primary)	-	-	0

Binary size

Results (secondary 0.0%)

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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 470.95s -> 473.304s (0.50%)
Artifact size: 389.99 MiB -> 390.01 MiB (0.01%)

[lcnr]

comment

[BoxyUwU]

Is this like, important, or is it "just" a small perf win?

[lcnr]

@rustbot ready

see pr descr

[BoxyUwU]

I feel relatively uneasy about this. I don't like that this optimization makes obligations_for_self_ty wrong if try_evaluate_obligations hasn't been previously called in order to update all the stalled_on vars.

If this were only a theoretical issue it'd still be pretty bad since I don't think we'd be able to expect it to not happen in practice in the long term. In practice it should be possible for unsize coercion to hit this with its custom obligation evaluation loop and inability to call try_evaluate_goals.

I think the only way I could approve this if I thought it was ok to have obligations_for_self_ty have incorrect behaviour in some edge cases and I think that is not the case.

I'm not sure what the right fix is here given that unsize coercion does actually hit this in theory, so asserting that we're always calling obligations_for_self_ty in "good" cases doesn't actually work ^^'

View changes since this review

[lcnr]

I think the only way I could approve this if I thought it was ok to have obligations_for_self_ty have incorrect behaviour in some edge cases and I think that is not the case.

It is totally okay for obligations_for_self_ty to have incorrect behavior, e.g. we currently bail at depth 4 or 8 and don't recur any deeper.