Move placeholder error handling to before region inference

[amandasystems]

This PR supersedes #130227. It removes precise tracking of placeholders in borrowck, moving them to an earlier stage. This leads to better performance, since we track less information. It is also part of the preparations for Polonius by moving specialised logic out of region inference itself and into the region graph or other, common pre-processing steps. Originally, the intention was to do this in multiple steps, but since some of them incur performance penalties won back by later stages, doing them in one go makes the net results clearer.

Error reporting changes

1. Higher-kinded region errors (placeholder outliving a placeholder, etc) are only reported if there are no other borrowck errors.
2. Due to the less precise nature of the revised tracking, there can be multiple overlapping errors involving the same placeholders/existential regions. In that case, not all errors will be reported. Crucially, only one pair per constraint graph SCC will be reported. A common case is Trait<r1, r2>, which frequently gives two error messages, one for each region. Under this logic it only gives one.
3. In some cases, finding named regions to slot into diagnostics is now more difficult, since not all outlived named regions are traced. This degrades some diagnostics by replacing named regions with anonymous ones, or by picking differently named ones.
4. In precisely one test, tests/ui/impl-trait/nested-rpit-hrtb.rs, we now also add r: 'static, which causes an extra error. Being more conservative when rewriting the constraint graph and avoiding it when invalid outlives are already detected breaks existing diagnostics for many, many tests that rely on the extra r: 'static to find out why they error.

Due to reason 2., several UI tests change, a lot, since they rely on diagnostic duplication being turned off to find several duplicate errors.

Code structure changes

• Placeholder errors are now detected earlier, if they happen.
• SCC annotations are dynamically chosen to be less rich for region graphs with no placeholders, opting to skip a lot of the logic. This is crucial for performance; both runtime and memory use, since it allows simple function bodies to skip some allocations and extra computation during SCC construction.
• SCC annotations are now written in an update-style as opposed to the previous, more functional style. It seems to be more performant and it's certainly more succinct.

r? lcnr

[amandasystems]

Wow, a merge conflict with upstream for a commit I JUST PUSHED, that's record time!

[lcnr]

@rustbot blocked

[amandasystems]

@rustbot ready

[amandasystems]

I hate this, but can't find a way around it. My gut feeling says it's another case of an order-dependent extraction procedure somewhere. For what it's worth, it wasn't in the earlier version of this code but it seems to have appeared after the recent changes to the base commit. Presumably the difference is from selecting smallest placeholder by rvid, etc. Help very much appreciated!

[amandasystems]

I think the "checked sub" part is what really broke me here. Why are universes being adjusted? What's a base universe? And why can the subtraction sometimes overflow?!?!

[amandasystems]

See also bound_region_errors, line 502

[amandasystems]

For the record, these are the only reads of base_universe on CanonicalTypeOpDeeplyNormalizeGoal, CanonicalTypeOpAscribeUserTypeGoal and InstantiateOpaqueType.

[amandasystems]

The job pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)
Click to see the possible cause of the failure (guessed by this bot)

 95 | |         constraints.clone(),
 96 | |         &universal_region_relations,
 97 | |         infcx,
 98 | |     );
    | |_____- argument #4 of type `&mut RegionErrors<'_>` is missing
    |
note: function defined here
   --> compiler/rustc_borrowck/src/handle_placeholders.rs:306:15
    |
306 | pub(crate) fn compute_sccs_applying_placeholder_outlives_constraints<'tcx>(
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...
310 |     errors_buffer: &mut RegionErrors<'tcx>,
    |     --------------------------------------
help: provide the argument
    |
 94 -     let lowered_constraints = compute_sccs_applying_placeholder_outlives_constraints(
 95 -         constraints.clone(),
 96 -         &universal_region_relations,
 97 -         infcx,
 98 -     );
 94 +     let lowered_constraints = compute_sccs_applying_placeholder_outlives_constraints(constraints.clone(), &universal_region_relations, infcx, /* &mut RegionErrors<'_> */);
    |

For more information about this error, try `rustc --explain E0061`.
[RUSTC-TIMING] rustc_borrowck test:false 4.478
error: could not compile `rustc_borrowck` (lib) due to 1 previous error

I don't understand where these errors come from. On my branch every single commit passes the ./x check command. What's emitting these? The error refers to a line that doesn't exist in a file that doesn't look like that during any commit in the PR.

[lcnr]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: e7d6bec (e7d6bec114ec0fed7e3ed6ccb2bf98d1385e8992, parent: 7934bbdf84a6b9a30297caf4f4f38286dedf876a)

[rust-timer]

Finished benchmarking commit (e7d6bec): 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)	-0.2%	[-0.4%, -0.1%]	25
Improvements ✅ (secondary)	-0.3%	[-0.9%, -0.1%]	38
All ❌✅ (primary)	-0.2%	[-0.4%, -0.1%]	25

Max RSS (memory usage)

Results (primary -0.4%, secondary -0.4%)

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

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	0.6%	[0.6%, 0.6%]	1
Improvements ✅ (primary)	-1.4%	[-1.4%, -1.4%]	1
Improvements ✅ (secondary)	-1.4%	[-1.4%, -1.4%]	1
All ❌✅ (primary)	-0.4%	[-1.4%, 0.6%]	2

Cycles

Results (secondary -7.1%)

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)	-7.1%	[-7.1%, -7.1%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 472.579s -> 470.881s (-0.36%)
Artifact size: 386.92 MiB -> 386.91 MiB (-0.00%)

[bors]

☔ The latest upstream changes (presumably #149535) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.