Region inference: split results from RegionInferenceContext by amandasystems · Pull Request #151688 · rust-lang/rust

amandasystems · 2026-01-26T14:43:18Z

What?

This PR turns RegionInferenceContext into a builder that produces the inferred region values, InferredRegions. The resulting struct implements the public API of RegionInferenceContext and replaces it in consumers.rs. RegionInferenceContext::solve() now consumes (moves) the inference context. It is completely private to region inference.

Why?

RegionInferenceContext has become a huge dump for various values people want to access.
region_infer itself is a very large file that's difficult to find your way around.
Polonius wants to skip as much of region inference as possible, since most of it is redundant. This makes that possible
No method now need warnings about being called after region inference or else they'll panic
RegionInferenceContext now takes almost all of its fields by reference
We can probably drop at least some stuff earlier
Data dependencies during region inference are now made more explicit

Knock-on effects

Region constraint (blame) search is now independent of a RegionInferenceContext
Less data is retained after region inference. This may or may not lead to minor improvements in memory use.
RegionInferenceContext now almost exclusively contains references to values, as opposed to owning them. This addresses most of fn compute_closure_requirements_modulo_opaques shouldn't clone all its inputs #146079
This reduces the coupling between region inference and Polonius-next, making it possible to skip region inference if it is not needed to check universal regions or type tests.
region_infer gains two child modules and becomes a lot less of a behemoth.

Detailed overview of changes

new modules: region_infer::constraint_search and region_infer::universal_regions.
handle_placeholders now consumes less input and does constraint rewriting via mutable references
consumers.rs now has a InferenceResults instead of a RegionInferenceContext
The public methods eval_{equal, outlives} are moved to InferredRegions
Constraint (blame) search, MIR dumping, graphviz rendering, and checking universal regions now all get their own builder-style structs, and the methods for their respective tasks are moved onto them
The option for region inference results in ConsumerOptions is now called InferredRegions
calculate_borrows_out_of_scope_at_location of course now takes InferredRegions
LivenessValues for free regions is now initialised as live at all points along with the constraint rewriting during placeholder handling, as opposed to during construction of RegionInferenceContext
Some trace logging, especially during constraint search, is lost since the data required to print the logs was only necessary to print the logs and never used otherwise.

r? @lcnr

Closes #146079.

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

compiler/rustc_borrowck/src/region_infer/mod.rs

compiler/rustc_borrowck/src/diagnostics/find_use.rs

compiler/rustc_borrowck/src/diagnostics/opaque_types.rs

compiler/rustc_borrowck/src/diagnostics/region_name.rs

lcnr · 2026-02-12T17:09:02Z

compiler/rustc_borrowck/src/polonius/dump.rs

+    liveness: &LivenessValues,
    pass_where: PassWhere,
    out: &mut dyn io::Write,
 ) -> io::Result<()> {


pls use a consistent function param ordering between the functions here :<

lcnr · 2026-02-12T17:14:26Z

compiler/rustc_borrowck/src/region_infer/opaque_types/region_ctxt.rs

+        let (definitions, _has_placeholders) = region_definitions(
+            infcx,
+            universal_regions,
+            &mut constraints.liveness_constraints.clone(),


this is... very odd. You give a mutable reference to a clone?

please move the cloned thing into a let binding first

That's a quick hack workaround, I think we should talk about how to not have to do that, where to put region_definitions(), and whether the liveness constarints stuff is necessary in sync because it's pretty much the same thing.

lcnr · 2026-02-12T17:17:03Z

compiler/rustc_borrowck/src/region_infer/graphviz.rs

+    outlives_constraints: &'a OutlivesConstraintSet<'tcx>,
+    mut w: &mut dyn Write,
+) -> io::Result<()> {
+    dot::render(&RawConstraints { tcx, region_definitions, outlives_constraints }, &mut w)


why reference RawConstraints. That type should be Copy

It's because dot::render() takes its arguments by reference (and it wasn't written by me).

lcnr · 2026-02-12T17:17:29Z

compiler/rustc_borrowck/src/region_infer/graphviz.rs

+    dot::render(&SccConstraints { tcx, region_definitions, constraint_sccs, nodes_per_scc }, &mut w)
 }

 struct RawConstraints<'a, 'tcx> {


make this copy and take it by value

lcnr · 2026-02-12T17:17:41Z

compiler/rustc_borrowck/src/region_infer/graphviz.rs

    nodes_per_scc: IndexVec<ConstraintSccIndex, Vec<RegionVid>>,
+    region_definitions: &'a IndexVec<RegionVid, RegionDefinition<'tcx>>,
+    constraint_sccs: &'a ConstraintSccs,
 }


lcnr · 2026-02-12T17:21:57Z

compiler/rustc_borrowck/src/region_infer/mod.rs

+            outlives_requirements.as_mut(),
+            &mut errors_buffer,
+            type_tests,
+        );


hmm, I would prefer check_type_tests to stay on the region inference context 🤔

they are a part of region inference to me

or like, the RegionInferenceCtxt should either not exist at all anymore, or still be used for the region computations 🤔 maybe chat about it in sync

lcnr · 2026-02-12T17:25:53Z

compiler/rustc_borrowck/src/handle_placeholders.rs

+        if let NllRegionVariableOrigin::FreeRegion = origin {
+            // Add all nodes in the CFG to liveness constraints for free regions
+            liveness_constraints.add_all_points(rvid);
+        }


this is a functional change? 🤔

It should not be, this always happens during region inference, I just moved it slightly earlier.

lcnr

scc_values: InferredRegions feels wrong. Maybe just computed_regions: ComputedRegions, regions: InferredRegions

region_definitions why mutable liveness constraints

MirDumper::dump_mir why is scc_values an fn param instead of a field?

View changes since this review

lcnr · 2026-02-12T17:29:27Z

compiler/rustc_borrowck/src/nll.rs

+    // These clones can more or less be avoided,
+    // since they are probably idempotent.


what exactly does this comment mean? It's confusing to me

It's a grammar error; what I meant was that the clones make the operations local since it operates on cloned values as opposed to shared ones, but the operations being performed are probably idempotent so doing them twice (in case of shared values) is likely fine. In other words, just dropping the clones will probably do the right thing.

lcnr · 2026-02-12T17:33:21Z

Can you rebase, I will then do a perf run and locally look at the docs after this PR as that should make it clearer how the API has changed

compiler/rustc_borrowck/src/region_infer/mod.rs

rustbot · 2026-03-03T15:44:22Z

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.

amandasystems · 2026-03-03T15:50:26Z

Can you rebase, I will then do a perf run and locally look at the docs after this PR as that should make it clearer how the API has changed

Phew! That was quite a rebase! Fixed it now, but I had to squash. I suspect we will want to land this in stages anyway.

This ensures all of region inference is immutable, and makes every operation that requires region inference to have been executed to run explicitly require the results.

lcnr · 2026-03-09T13:50:58Z

@bors try @rust-timer queue

Region inference: split results from RegionInferenceContext

rust-bors · 2026-03-09T16:06:16Z

☀️ Try build successful (CI)
Build commit: 1ee78dc (1ee78dc33b57da0d3c9e4cb23d402532bafd397b, parent: 655a7d20fefe23757ca9ecbbea01e4fe80208aaf)

rust-timer · 2026-03-09T16:46:39Z

Finished benchmarking commit (1ee78dc): 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)	0.3%	[0.1%, 0.7%]	11
Regressions ❌ (secondary)	0.3%	[0.2%, 0.5%]	15
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.1%, 0.7%]	11

Max RSS (memory usage)

Results (secondary 2.5%)

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)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (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)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 479.555s -> 479.91s (0.07%)
Artifact size: 395.03 MiB -> 394.99 MiB (-0.01%)

rustbot assigned lcnr Jan 26, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 26, 2026

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amandasystems force-pushed the split-region-inference branch from 6befd69 to 13a57f6 Compare January 26, 2026 14:50

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amandasystems force-pushed the split-region-inference branch from 13a57f6 to 00e8121 Compare January 26, 2026 16:12

lcnr reviewed Jan 27, 2026

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amandasystems force-pushed the split-region-inference branch from 00e8121 to 454f98a Compare January 28, 2026 22:21

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lcnr reviewed Feb 3, 2026

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amandasystems force-pushed the split-region-inference branch from 4c01f95 to 174023c Compare February 6, 2026 14:58

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