uniquify root goals during HIR typeck #144405

lcnr · 2025-07-24T14:58:56Z

We need to rely on region identity to deal with hangs such as rust-lang/trait-system-refactor-initiative#210 and to keep the current behavior of fn try_merge_responses.

This is a problem as borrowck starts by replacing each occurrence of a region with a unique inference variable. This frequently splits a single region during HIR typeck into multiple distinct regions. As we assume goals to always succeed during borrowck, relying on two occurances of a region being identical during HIR typeck causes ICE. See the now fixed examples in rust-lang/trait-system-refactor-initiative#27 and #139409.

We've previously tried to avoid this issue by always uniquifying regions when canonicalizing goals. This prevents caching subtrees during canonicalization which resulted in hangs for very large types. People rely on such types in practice, which caused us to revert our attempt to reinstate #[type_length_limit] in #127670. The complete list of changes here:

After more consideration, all occurrences of such large types need to happen outside of typeck/borrowck. We know this as we already walk over all types in the MIR body when replacing their regions with nll vars.

This PR therefore enables us to rely on region identity inside of the trait solver by exclusively uniquifying root goals during HIR typeck. These are the only goals we assume to hold during borrowck. This is insufficient as type inference variables may "hide" regions we later uniquify. Because of this, we now stash proven goals which depend on inference variables in HIR typeck and reprove them after writeback. This closes rust-lang/trait-system-refactor-initiative#127.

This was originally part of #144258 but I've moved it into a separate PR. While I believe we need to rely on region identity to fix the performance issues in some way, I don't know whether #144258 is the best approach to actually do so. Regardless of how we deal with the hangs however, this change is necessary and desirable regardless.

r? @compiler-errors or @BoxyUwU

rustbot · 2025-07-24T14:59:02Z

compiler-errors is not on the review rotation at the moment.
They may take a while to respond.

rustbot · 2025-07-24T14:59:04Z

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

compiler/rustc_infer/src/infer/mod.rs

BoxyUwU · 2025-07-24T19:21:24Z

compiler/rustc_next_trait_solver/src/canonicalizer.rs

+    /// When canonicalizing the `param_env`, we keep `'static` as merging
+    /// trait candidates relies on it when deciding whether a where-bound
+    /// is trivial.
+    ParamEnv,


new solver special cases 'static? I thought we weren't supposed to do this 🤔

We need to do so for the param_env to detect "global where-bounds". See #133643 / the FCP in #132325

ah yh that makes sense

BoxyUwU · 2025-07-24T19:30:16Z

compiler/rustc_next_trait_solver/src/canonicalizer.rs

+        if let CanonicalizeMode::Input(CanonicalizeInputKind::Predicate {
+            is_hir_typeck_root_goal: true,
+        }) = self.canonicalize_mode
+        {
+            self.cached_fold_ty(t)
+        } else if let Some(&ty) = self.cache.get(&(self.binder_index, t)) {


I find this somewhat confusing, it reads like "if its a root goal use the cache" which seems wrong, and then we follow it up with "otherwise use the cache" which I thought we just did :>

but cached_fold_ty is about "caching" in the sense that we want to map multiple occurrences of ?x to the same canonical var? whereas self.cache is a proper ty -> canonical ty cache?

cached_fold_ty is the part of fn fold_ty which was cached :3

renamed it to inner_fold_ty and added a comment

BoxyUwU · 2025-07-24T19:43:59Z

compiler/rustc_infer/src/infer/mod.rs

+    ///
+    /// See the tests in trait-system-refactor-initiative#27 for concrete examples.
+    ///
+    /// FIXME(-Znext-solver): This is insufficient in theory as a goal `T: Trait<?x, ?x>`


why not do this at the same time :3 uniquifying regions seems somewhat meh if we aren't handling T: Trait<?x, ?x> correctly, and if it turns out that we cant "just" handle it correctly then we'd probably want to revert this (again again xd)

because it's annoying :3

BoxyUwU · 2025-07-24T19:44:13Z

r? BoxyUwU

rustbot · 2025-07-25T12:40:12Z

Some changes occurred to the core trait solver

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

lcnr · 2025-07-25T12:47:18Z

expect this to result in a minor perf hit

@bors2 try @rust-timer queue

uniquify root goals during HIR typeck

rust-bors · 2025-07-25T12:47:23Z

⌛ Trying commit 5082eab with merge b91de35…

To cancel the try build, run the command @bors try cancel.

rust-bors · 2025-07-25T15:02:30Z

☀️ Try build successful (CI)
Build commit: b91de35 (b91de35e69e61422874a93fd8135dd731eb4baa3, parent: b56aaec52bc0fa35591a872fb4aac81f606e265c)

rust-timer · 2025-07-25T16:23:46Z

Finished benchmarking commit (b91de35): 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
Regressions ❌ (secondary)	1.9%	[0.1%, 7.6%]	22
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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%	[1.3%, 3.7%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 2.2%, secondary 3.8%)

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

	mean	range	count
Regressions ❌ (primary)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	3.8%	[2.1%, 5.3%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.2%	[2.2%, 2.2%]	1

Binary size

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

Bootstrap: 469.891s -> 470.268s (0.08%)
Artifact size: 374.63 MiB -> 374.53 MiB (-0.03%)

rustbot assigned compiler-errors Jul 24, 2025

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 Jul 24, 2025

BoxyUwU reviewed Jul 24, 2025

View reviewed changes

rustbot assigned BoxyUwU and unassigned compiler-errors Jul 24, 2025

BoxyUwU added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 24, 2025

uniquify root goals during HIR typeck

0b323ea

rust-cloud-vms bot force-pushed the hir-typeck-uniquify branch from 53a905a to 0cb0427 Compare July 25, 2025 12:40

rust-cloud-vms bot force-pushed the hir-typeck-uniquify branch 2 times, most recently from 5df011f to 5082eab Compare July 25, 2025 12:44

This comment has been minimized.

Sign in to view

rust-bors bot added a commit that referenced this pull request Jul 25, 2025

Auto merge of #144405 - lcnr:hir-typeck-uniquify, r=<try>

b91de35

uniquify root goals during HIR typeck

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 25, 2025

This comment has been minimized.

Sign in to view

handle region dependent goals due to infer vars

c55b6af

rust-cloud-vms bot force-pushed the hir-typeck-uniquify branch from 5082eab to c55b6af Compare July 25, 2025 15:28

rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jul 25, 2025

uniquify root goals during HIR typeck #144405

Are you sure you want to change the base?

uniquify root goals during HIR typeck #144405

Conversation

lcnr commented Jul 24, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Jul 24, 2025

Uh oh!

rustbot commented Jul 24, 2025

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

BoxyUwU commented Jul 24, 2025

Uh oh!

rustbot commented Jul 25, 2025

Uh oh!

lcnr commented Jul 25, 2025

Uh oh!

This comment has been minimized.

rust-bors bot commented Jul 25, 2025

Uh oh!

This comment has been minimized.

rust-bors bot commented Jul 25, 2025

Uh oh!

This comment has been minimized.

rust-timer commented Jul 25, 2025

Overall result: ❌ regressions - please read the text below

Uh oh!

Uh oh!

lcnr commented Jul 24, 2025 •

edited

Loading