rustc_trait_selection: fix trait solver hang caused by degenerate obligations by InvalidPathException · Pull Request #159774 · rust-lang/rust

InvalidPathException · 2026-07-23T13:06:15Z

Bisection pointed to #142712 but the problem has always been present. Fulfillment can go on forever if a degenerate obligation causes additional obligations from poly_project_and_unify_term to be copies of the degenerate one.

Current fix is checking if the obligation is degenerate, ~~making the degenerate case stall, waiting for other obligations to constrain its inference variables, so it can actually normalize or error instead of propagating this.~~ error immediately since the code will not compile anyways.

I am not sure if this is ideal, might need to check perf due to the added heavy-ish condition check.

rustbot · 2026-07-23T13:06:20Z

r? @mu001999

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mu001999 · 2026-07-23T14:59:52Z

@rustbot reroll

mu001999 · 2026-07-23T15:00:08Z

might need to check perf due to the added heavy-ish condition check.

@bors try @rust-timer queue

…try> rustc_trait_selection: fix trait solver hang caused by degenerate obligations

rust-bors · 2026-07-23T17:08:34Z

☀️ Try build successful (CI)
Build commit: 41450ef (41450ef5e1890fe9aaccef25127708688bc116af)
Base parent: 165cce8 (165cce8d820b229af8f6a8226cf0b910b57600ff)

rust-timer · 2026-07-23T17:48:53Z

Finished benchmarking commit (41450ef): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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)	0.5%	[0.4%, 0.5%]	8
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -8.4%, secondary -2.0%)

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
Improvements ✅ (primary)	-8.4%	[-8.4%, -8.4%]	1
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	-8.4%	[-8.4%, -8.4%]	1

Cycles

Results (primary 2.0%)

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

	mean	range	count
Regressions ❌ (primary)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.0%	[2.0%, 2.0%]	1

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 485.902s -> 492.139s (1.28%)
Artifact size: 387.66 MiB -> 387.65 MiB (-0.00%)

jackh726

I'm curious if there's any code that we expect to start to compile with this change, as opposed to eventually erroring. If the answer is no, then perhaps we can just error immediately, instead of stalling.

View changes since this review

jackh726 · 2026-07-23T22:22:16Z

+                        })
+                    })
+                } {
+                    stalled_on.clear();


This really could be expressed better. Something like

ProjectAndUnifyResult::Holds(os) if os.is_empty() =>ProcessResult::Changed(mk_pending(obligation, os)), ProjectAndUnifyResult::Holds(os) => { let input_projection_term = infcx .resolve_vars_if_possible(project_obligation.predicate) .map_bound(|p| p.projection_term); let all_same_term = os.iter().all(|o| { let Some(proj_clause) = o.predicate.as_projection_clause() else { return false }; infcx.resolve_vars_if_possible(proj_clause).map_bound(|p| p.projection_term) == input }); if all_same_term { ... } else { ProcessResult::Changed(mk_pending(obligation, os)) } }

jackh726 · 2026-07-23T22:24:51Z

+                // Every nested obligation is a copy of the obligation we are processing,
+                // registering would make fulfillment process the same obligation forever.
+                // So, stall until an inference variable in the predicate is constrained.


A concrete example in a comment here would be useful.

This is also slightly stronger than the comment, anyways: every nested obligation has the same projection term.

InvalidPathException · 2026-07-24T01:42:03Z

I'm curious if there's any code that we expect to start to compile with this change, as opposed to eventually erroring. If the answer is no, then perhaps we can just error immediately, instead of stalling.

I did consider this, there should not be anything that compiles because of this today, the stall path is reachable only when

normalization returned NoProgress (ProjectionCandidateSet::None in project)
equating the rigid alias with the term variable failed generalization

At that point it must error regardless of how it resolves later on (as of today). But stalling gives a (maybe) better message as we allow more info to come in, since this path is relatively rare, it seems at least not problematic to stall.

trait Bound {}
impl Bound for u32 {}

trait Driver {
    type Object<Ctx: Bound>: Bound;
}

fn make<T: Driver, C: Bound>(c: C) -> T::Object<C> {
    todo!()
}

fn any<X>() -> X {
    todo!()
}

fn f<T: Driver>() {
    let mut x = any();          // x: ?x
    x = make::<T, _>(x);        // forces <T as Driver>::Object<?x> == ?x
}

fn main() {}

Stall:

error[E0283]: type annotations needed
  --> demo.rs:17:9
   |
17 |     let mut x = any();
   |         ^^^^^
18 |     x = make::<T, _>(x);
   |                   - type must be known at this point
   |
   = note: the type must implement `Bound`
help: the trait `Bound` is implemented for `u32`
  --> demo.rs:2:1
   |
 2 | impl Bound for u32 {}
   | ^^^^^^^^^^^^^^^^^^
note: required by a bound in `make`
  --> demo.rs:8:23
   |
 8 | fn make<T: Driver, C: Bound>(_c: C) -> T::Object<C> {
   |                       ^^^^^ required by this bound in `make`
help: consider giving `x` an explicit type
   |
17 |     let mut x: u32 = any();
   |              +++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0283`.

Error immediately:

error[E0284]: type annotations needed for `<T as Driver>::Object<_>`
  --> demo.rs:17:9
   |
17 |     let mut x = any();
   |         ^^^^^
18 |     x = make::<T, _>(x);
   |                      - type must be known at this point
   |
   = note: cannot satisfy `<T as Driver>::Object<_>
help: consider giving `x` an explicit type, where the placeholders `_` are specified
   |
17 |     let mut x: <_ as Driver>::Object<_> = any();
   |              ++++++++++++++++++++++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --

jackh726 · 2026-07-24T13:48:25Z

Given that the new solver is coming soon and completely overhauls normalization, I'm not inclined to potentially introduce more things that could compile, particularly if the reasoning is just better diagnostics.

Even still, the diagnostics in the error immediately case are not really bad. So, let's go that route.

jackh726 · 2026-07-24T21:49:21Z

Sorry, can you squash? And then r=me

…igations

InvalidPathException · 2026-07-25T20:09:39Z

r? @jackh726

rustbot · 2026-07-25T20:09:41Z

Requested reviewer is already assigned to this pull request.

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jackh726 · 2026-07-26T02:08:37Z

@bors r+

rust-bors · 2026-07-26T02:08:39Z

📌 Commit f2e0edf has been approved by jackh726

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

Reason for tree closure: spurious failures

…r-hang, r=jackh726 rustc_trait_selection: fix trait solver hang caused by degenerate obligations Closes rust-lang#157516 and rust-lang#159750. Bisection pointed to rust-lang#142712 but the problem has always been present. Fulfillment can go on forever if a degenerate obligation causes additional obligations from `poly_project_and_unify_term` to be copies of the degenerate one. Current fix is checking if the obligation is degenerate, ~making the degenerate case stall, waiting for other obligations to constrain its inference variables, so it can actually normalize or error instead of propagating this.~ error immediately since the code will not compile anyways. I am not sure if this is ideal, might need to check perf due to the added heavy-ish condition check.

…uwer Rollup of 24 pull requests Successful merges: - #159638 (bootstrap: Split the `Step` trait into multiple traits) - #159774 (rustc_trait_selection: fix trait solver hang caused by degenerate obligations) - #159837 (line-tables-only test: check that the line number matches the function name) - #159946 (Update Enzyme submodule to imporve llvm-cov) - #159617 (Fix up `#[linkage]` target checking) - #159733 (std: Switch implementations of `thread_local!` for WASI) - #159783 (Check unsafe impls on safe EIIs) - #159810 (Add tuple never coercion collection regression test) - #159821 (Update expect message using the recommended style in binary_heap module) - #159826 (Remove redundant `#[rustc_paren_sugar]` feature gate) - #159846 (Implement `str::copy_from_str`) - #159849 (rustc_parse: Stop returning `Option` from statement parsing) - #159853 (Updated expect messages for `CString` struct and method documentation) - #159875 (More cleanup in `rustc_attr_parsing`) - #159882 (Update expect messages in library/alloc/boxed.rs and library/alloc/string.rs to follow the style guide) - #159891 (Split multiline derives into std/rustc macros) - #159893 (Fix `find_attr` hygiene and `rustc_hir` cleanups) - #159895 (rustc-dev-guide subtree update) - #159902 (Clarify that the expected runtime symbols signature is for the current target only) - #159914 (Fix error in diagnostic on_unmatched_args) - #159917 (spare capacity mut constification) - #159918 (rename abort_unwind → abort_on_unwind) - #159936 (Minor `rustc_ast::ast` doc cleanups) - #159945 (Update expect messages in library/core/src/ptr/non_null.rs)

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rust-bors Bot pushed a commit that referenced this pull request Jul 23, 2026

Auto merge of #159774 - InvalidPathException:fix-old-solver-hang, r=<…

41450ef

…try> rustc_trait_selection: fix trait solver hang caused by degenerate obligations

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InvalidPathException requested a review from jackh726 July 24, 2026 03:25

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rustc_trait_selection: fix trait solver hang caused by degenerate obl…

f2e0edf

…igations

InvalidPathException force-pushed the fix-old-solver-hang branch from fab2c18 to f2e0edf Compare July 25, 2026 20:08

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 26, 2026

JonathanBrouwer mentioned this pull request Jul 26, 2026

Rollup of 25 pull requests #159952

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JonathanBrouwer mentioned this pull request Jul 26, 2026

Rollup of 24 pull requests #159957

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rust-timer commented Jul 23, 2026

Overall result: ❌ regressions - no action needed

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