Clean up and speed up inference variable resolving code by jdonszelmann · Pull Request #160913 · rust-lang/rust

jdonszelmann · 2026-08-11T14:24:56Z

As part of #162126, I found some small performance wins resulting from the fact that shallow_resolve now returns root vids.

Note

I've not used an LLM for any part of this PR, or any other PR I make. This includes any related work like research.

rustbot · 2026-08-11T14:25:00Z

changes to inspect_obligations.rs

cc @lcnr

changes to the core type system

cc @lcnr

Some changes occurred in need_type_info.rs

cc @lcnr

Some changes occurred to the core trait solver

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

changes to the core type system

cc @lcnr

jdonszelmann · 2026-08-11T14:25:09Z

@bors try @rust-timer queue

Clean up and speed up resolving code

jdonszelmann · 2026-08-11T14:53:08Z

-        match self.inner.borrow_mut().type_variables().probe(vid) {
-            TypeVariableValue::Known { value } => Ok(value),
+        match value {
+            TypeVariableValue::Known { value } => Ok(self.shallow_resolve_non_recursive(value)),


this method (and the one below) now also does the recursive resolving shallow_resolve already did. No tests change here.

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jdonszelmann · 2026-08-11T14:53:33Z

+    // Cold because the case in which a tyvar resolves to an intvar which resolves to a type is
+    // quite rare. It's way more common for `shallow_resolve_non_recursive` to return ty.
+    #[cold]
+    fn shallow_resolve_infer_non_recursive(&self, infer: InferTy, ty: Ty<'tcx>) -> Ty<'tcx> {


the non-recursive caase helps ~0.5% on local benchmarks. Not much, but still a bit.

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jdonszelmann · 2026-08-11T14:59:06Z

+    /// In cases where we do, this can aid performance.
    #[inline(always)]
-    fn shallow_resolve_ty_var(&self, v: TyVid, ty: Ty<'tcx>) -> Ty<'tcx> {
+    fn shallow_resolve_ty_var_with_ty(&self, v: TyVid, ty: Option<Ty<'tcx>>) -> Ty<'tcx> {


By taking an Option here, we can merge more methods' implementations. Doing this has 0 performance overhead, #[inline(always)] makes sure the callsites that always call with Some get optimized properly.

View changes since the review

could we instead change this function to not take a ty and return and Option instead?

rust-bors · 2026-08-11T15:25:32Z

💔 Test for 7e8d4ec failed: CI. Failed job:

try - dist-x86_64-linux-quick (web logs, enhanced plaintext logs)

jdonszelmann · 2026-08-11T15:47:37Z

@bors try (failed due to missing rustdoc changes now applied)

rust-bors · 2026-08-11T15:47:41Z

Unknown argument "(failed". Did you mean to use @bors jobs=<jobs>|parent=<parent>? Run @bors help or go to https://bors.rust-lang.org/help to see available commands.

panstromek · 2026-08-11T16:49:37Z

@bors try

Clean up and speed up resolving code

rust-bors · 2026-08-11T17:49:20Z

☀️ Try build successful (CI)
Build commit: d881e22 (d881e228b79422918a125ef8385170091369309b)
Base parent: a04c7a0 (a04c7a037d59b6dedc9f921e84e391f13a257aa4)

jdonszelmann · 2026-08-12T08:06:52Z

@rustbot review

lcnr · 2026-08-12T09:46:48Z

-                Ty::new_int_var(self.tcx, inner.int_unification_table().find(vid))
-            }
-        }
+    pub fn shallow_resolve_int_var(&self, vid: ty::IntVid) -> Ty<'tcx> {


if we return None if the inner function doesn't make progress, could we always be explicit about reconstructing the type in the caller to make it explicit where we (likely unnecessarily) do so?

View changes since the review

lcnr · 2026-08-12T09:47:29Z

                .borrow_mut()
                .unwrap_region_constraints()
-                .opportunistic_resolve_var(canonicalizer.tcx, vid);
+                .shallow_resolve_region_var(canonicalizer.tcx, vid);


we don't reuse r here if the root doesn't change 🤔 feels like doing so would be good for perf 🤔

View changes since the review

lcnr

reviewed most of the files.

can you split this PR into multiple separate ones. Reviewing that many changes in this PR feels kind of iffy

View changes since this review

lcnr · 2026-08-24T16:27:41Z

        &mut self,
        tcx: TyCtxt<'tcx>,
        vid: ty::RegionVid,
    ) -> ty::Region<'tcx> {


also separate: I feel like shallow_resolve_X should return Option and None if the vid doesn't change 🤔

there are a bunch of callers which shouldn't rebuild the ty/region here

lcnr · 2026-08-24T16:28:37Z

-            Err(_) => Ty::new_var(self.tcx, self.root_var(vid)),
-        }
+    fn shallow_resolve_ty_var(&self, vid: ty::TyVid) -> Ty<'tcx> {
+        self.shallow_resolve_ty_var_with_ty(vid, None)


I dislike this function. Imo we should instead have shallow_resolve_ty_var_with_ty return Option<Ty> and then do the unwrap_or(ty) in the caller

lcnr · 2026-08-24T16:31:30Z

+    /// Resolve a type variable to a type, if known.
+    /// Otherwise return a type with the root vid in it.
+    #[inline(always)]
+    fn shallow_resolve_ty_var_with_ty(&self, v: TyVid, ty: Option<Ty<'tcx>>) -> Ty<'tcx> {


yeah, with_ty is :<

… tyvar.

…er a shallow_resolve

rustbot · 2026-09-14T09:30:53Z

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.

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. WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) labels Aug 11, 2026

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rust-bors Bot pushed a commit that referenced this pull request Aug 11, 2026

Auto merge of #160913 - jdonszelmann:cleanup-resolving, r=<try>

7e8d4ec

Clean up and speed up resolving code

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jdonszelmann force-pushed the cleanup-resolving branch from 3dde145 to b5fe24d Compare August 11, 2026 14:45

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jdonszelmann commented Aug 11, 2026

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Comment thread compiler/rustc_infer/src/infer/canonical/canonicalizer.rs Outdated

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jdonszelmann force-pushed the cleanup-resolving branch from b5fe24d to 6b88c26 Compare August 11, 2026 15:47

rustbot added the T-rustdoc-frontend Relevant to the rustdoc-frontend team, which will review and decide on the web UI/UX output. label Aug 11, 2026

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rust-bors Bot pushed a commit that referenced this pull request Aug 11, 2026

Auto merge of #160913 - jdonszelmann:cleanup-resolving, r=<try>

d881e22

Clean up and speed up resolving code

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rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 12, 2026

lcnr changed the title ~~Clean up and speed up resolving code~~ Clean up and speed up inference variable resolving code Aug 12, 2026

lcnr reviewed Aug 12, 2026

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Comment thread compiler/rustc_infer/src/infer/mod.rs Outdated

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jdonszelmann mentioned this pull request Aug 18, 2026

Optimize probing in shallow_resolve #160885

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jdonszelmann force-pushed the cleanup-resolving branch from 06dd748 to 9ddd601 Compare August 21, 2026 09:15

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jdonszelmann force-pushed the cleanup-resolving branch from 9ddd601 to 68fded2 Compare August 21, 2026 09:17

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jdonszelmann force-pushed the cleanup-resolving branch from 68fded2 to aad9f36 Compare August 24, 2026 08:11

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lcnr reviewed Aug 24, 2026

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jdonszelmann force-pushed the cleanup-resolving branch from aad9f36 to 25e4653 Compare September 1, 2026 13:18

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jdonszelmann added 4 commits September 14, 2026 11:29

optimize resolve, and properly document the related methods

5bf41db

use probe_key_value for shallow_resolve_region

3973dd5

Make const methods consistent and expose methods to shallow resolve a…

cf4cb9d

… tyvar.

fixup various sites that use root_{ty,const}_var that can skip it aft…

5ca7e19

…er a shallow_resolve

jdonszelmann force-pushed the cleanup-resolving branch from 25e4653 to 5ca7e19 Compare September 14, 2026 09:30

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