Move placeholder error handling to before region inference

[amandasystems]

This PR breaks out yet another part of #130227. It prepares for the removal of precise placeholder tracking by colocating detecting placeholder errors for higher-ranked relations (placeholders outliving existentials that can't name them, other placeholders) with the preprocessing step that adds outlives-static constraints. This also relieves that step from the burden of ensuring these errors are detected via propagation, and fully splits the logic into one part that rewrites the constraint graph to add edges to 'static and one that detects hard errors.

For stepping-on-toes reasons it is currently based on #140737, but can be rebased on regular master and merged separately nope that's a terrible idea actually, it relies on some of the tracking introduced there.

This step also makes region errors more fine-grained, which is useful to simplify the error reporting pipeline, which is currently based on a very brittle method of reconstructing an error event after the fact to figure out what went wrong. Doing that is left as future work, though.

This changes the logic of error reporting slightly to not emit errors for placeholder/higher kinded constraints if there are other borrowck errors. This somewhat lessens the flood of errors and was described as if anything an improvement by @nikomatsakis in informal review.

r? lcnr

Fixes: #146467

[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.

[rustbot]

This PR was rebased onto a different master 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.

[lcnr]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 242f8ac (242f8ac02ecefa54679e01596e7e4764d5a32f64, parent: 35456985fab3419baf4bf111b8bbbd5e8565615d)

[rust-timer]

Finished benchmarking commit (242f8ac): comparison URL.

Overall result: ❌ regressions - 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)	0.1%	[0.1%, 0.2%]	12
Regressions ❌ (secondary)	0.2%	[0.0%, 0.4%]	19
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.1%, 0.2%]	12

Max RSS (memory usage)

Results (primary -4.1%, secondary -0.3%)

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)	0.8%	[0.8%, 0.8%]	1
Improvements ✅ (primary)	-4.1%	[-6.3%, -1.8%]	2
Improvements ✅ (secondary)	-1.4%	[-1.4%, -1.4%]	1
All ❌✅ (primary)	-4.1%	[-6.3%, -1.8%]	2

Cycles

Results (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
Regressions ❌ (secondary)	4.3%	[4.3%, 4.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
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: 474.753s -> 473.062s (-0.36%)
Artifact size: 388.13 MiB -> 388.15 MiB (0.00%)

[lcnr]

do we need these two origins? i feel like we should be able to just recompute them

[amandasystems]

Yep, all we need is the SCC annotation and the defintions.

[amandasystems]

Actually, the definitions are of course enough.

[lcnr]

this split seems odd to me. In a sense we'd want a single "min (max nameable universe) < placeholder universe" and then check the source of that "min (max nameable universe)" to figure out whether its

• placeholder: placeholder
• existential: placeholder
• location: placeholder

Do we even care about location: placeholder? I

[amandasystems]

Do we even care about location: placeholder? I

Some experiments I did suggested we do (otherwise we miss errors) but uh from what I've had explained to me we probably shouldn't? It's strange and I don't like it.

[amandasystems]

But this points towards a potentially much nicer variant that's a state machine tracking precisely this.

[lcnr]

Option<()>?

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

-        placeholder_to_region: _
+        placeholder_to_region: _,
     } = constraints;
 
     let fr_static = universal_regions.fr_static;
fmt: checked 6478 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:00:48
  local time: Wed Oct 15 11:19:35 UTC 2025
  network time: Wed, 15 Oct 2025 11:19:36 GMT