Region inference: Use outlives-static constraints in constraint search

[amandasystems]

Revise the extra r: 'static constraints added upon universe issues to add an explanation, and use that explanation during constraint blame search. This greatly simplifies the region inference logic, which now does not need to reverse-engineer the event that caused a region to outlive 'static.

This cosmetically changes the output of two UI tests. I blessed them i separate commits with separate motivations, but that can of course be squashed as desired. We probably want that.

The PR was extracted out of #130227 and consists of one-third of its functional payload.

r? lcnr

[bors]

☔ The latest upstream changes (presumably #140466) made this pull request unmergeable. Please resolve the merge conflicts.

[amandasystems]

(We may also want a perf run to see if I messed something up badly)

[lcnr]

@bors try @rust-timer queue

[bors]

⌛ Trying commit a209255 with merge 0d3d480...

[bors]

☀️ Try build successful - checks-actions
Build commit: 0d3d480 (0d3d48082efe47a20c953415945cc721105e5f85)

[rust-timer]

Finished benchmarking commit (0d3d480): 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)	0.2%	[0.1%, 0.3%]	12
Regressions ❌ (secondary)	0.2%	[0.1%, 0.3%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.6%, -0.5%]	6
All ❌✅ (primary)	0.2%	[0.1%, 0.3%]	12

Max RSS (memory usage)

Results (primary 0.4%, secondary -1.0%)

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

	mean	range	count
Regressions ❌ (primary)	1.4%	[1.3%, 1.5%]	2
Regressions ❌ (secondary)	3.0%	[3.0%, 3.0%]	1
Improvements ✅ (primary)	-1.7%	[-1.7%, -1.7%]	1
Improvements ✅ (secondary)	-1.9%	[-2.4%, -0.6%]	4
All ❌✅ (primary)	0.4%	[-1.7%, 1.5%]	3

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: 753.257s -> 756.017s (0.37%)
Artifact size: 372.34 MiB -> 372.33 MiB (-0.00%)

[lcnr]

why can we not use the region from max_nameable_universe here?

[amandasystems]

That sometimes slightly worsens the error messages (or at least causes diagnostics regressions) because previous logic picked the first smallest region. It's very annoying.

[lcnr]

can you dump the changes caused by this somewhere? I personally believe that some amount of diagnostics regression is acceptable if it cleans up the code/removes subtly different variations of the same concept.

[amandasystems]

Here's the an output from the UI tests attached when doing that!

$ ./x test ui --keep-stage 1 > simpler-choice-blame-ui-tests.log

simpler-choice-blame-ui-tests.log

There's a bunch of squiggly lines getting longer, which I think is mostly harmless:

• tests/ui/higher-ranked/higher-ranked-lifetime-error.rs: I'd say fully harmless, a slightly longer squiggly line
• tests/ui/higher-ranked/trait-bounds/due-to-where-clause.rs same as above
• tests/ui/mismatched_types/hr-projection-mismatch.rs#current same as above

Then there is a duplicate of a diagnostic due to a longer span which we may or may not care about, in tests/ui/associated-inherent-types/issue-111404-1.rs. This should be very fixable in the error reporting phase, but that part of the code is absolutely incomprehensible (though I've tried), and I've simply given up on it.

I remember this being much, much worse! Maybe this is acceptable?

[amandasystems]

If you think this is ok, say the word and I'll dump like half the file to get rid of this extra logic!

[lcnr]

from placeholder to an existential should error, shouldn't it?

we only add outlives-static whenever we've got "scc_which_cannot_name reaches(transitively outlives) placeholder" constraints 🤔

My question above was, why does

rust/compiler/rustc_borrowck/src/region_infer/mod.rs

Lines 1796 to 1810 in b03635d

	// If we are passing through a constraint added because `'lt: 'unnameable`,
	// where cannot name `'unnameable`, redirect search towards `'unnameable`.
	let due_to_placeholder_outlives = path.iter().find_map(|c| {
	if let ConstraintCategory::OutlivesUnnameablePlaceholder(lt, unnameable) = c.category {
	Some((lt, unnameable))
	} else {
	None
	}
	});
	let path = if let Some((lt, unnameable)) = due_to_placeholder_outlives {
	// This the `false` argument is what prevents circular reasoning here!
	self.constraint_path_to(lt, |r| r == unnameable, false).unwrap().0
	} else {
	path
	};

check constraint_path_to(lt, ..) instead of constraint_path_to(from_region, ..)

[amandasystems]

My question above was [...]

I think this was a misthought/optimisation I did at some point (since it should shorten the path by starting from somewhere in the middle as opposed to restarting all of the search). The general “complete context-less redirect” model also appealed to me. Changing the behaviour does restore some of the original output for tests/ui/nll/ice-106874.stderr, as far as I can tell, creates no other regressions, and generally simplifies everything so I’ll uh just go with that.

You’re very good at spotting my bullshit!

[amandasystems]

I would love for u to look into why that other constraint exists.

This is my next order of business, because if I can show that then we can bless the note-less output and have an Even Simpler Implementation™

[amandasystems]

from placeholder to an existential should error, shouldn't it?

It should, and it's handled separately in the next PR after this one! While writing this I discovered universes on random regions so I've resorted to partially mindlessly reimplementing the previous logic (which, among other things, deals with a member of an SCC being unable to name the representative, which is tricky to express without adding tons of extra outlives-static constraints -- a thing we may want to do!).

[amandasystems]

Interesting update: I dug into this, and it turns out the Complicated Logic I had before is actually essential to discovering placeholders outliving existentials. I'm not sure how exactly, which is an indication it was way, way too clever.

This is somewhat good news, because it means I must fold in the next PR, which does this logic much clearer and more straightforward.

[rustbot]

⚠️ Warning ⚠️

• This PR is based on an upstream commit that is 28 days old.

  It's recommended to update your branch according to the rustc-dev-guide.

[bors]

☔ The latest upstream changes (presumably #144469) made this pull request unmergeable. Please resolve the merge conflicts.