`-Znext-solver` instantiate predicate binder without recanonicalizing goal #146725

lcnr · 2025-09-18T14:30:03Z

This strengthens the leak check to match the old trait solver. The new trait solver now also instantiates higher ranked goals in the same scope as candidate selection, so the leak check in each candidate detects placeholder errors involving this higher ranked goal.

E.g. let's look at tests/ui/higher-ranked/leak-check/leak-check-in-selection-2.rs

trait Trait<T, U> {}
impl<'a> Trait<&'a str, &'a str> for () {}
impl<'a> Trait<&'a str, String> for () {}
fn impls_trait<T: for<'a> Trait<&'a str, U>, U>() {}

fn main() {
    impls_trait::<(), _>();
}

Here proving (): for<'a> Trait<&'a str, ?u> via impl<'a> Trait<&'a str, &'a str> for () equates ?u with &'!a str which results in a leak check error as ?u cannot name 'a. If this leak check error happens while considering candidates we drop the first impl and infer ?u to String. If not, this remains ambiguous.

This behavior is a bit iffy, see the FCP proposal in #119820 for more details on why this current behavior is somewhat undesirable. However, considering placeholders from higher-ranked goals for candidate selection does allow more code to compile and a lot of the code feels like it should compile. This caused us to revert the change of #119820 in #127568.

I originally expected that we can avoid breakage with the new solver differently here, e.g. by considering OR-region constraints. However, doing so is a significant change and I don't have a great idea for how that should work. Matching the old solver behavior for now should not make this cleaner approach any more difficult in the future, so let's just go with what actually allows us to stabilize the new solver for now.

This PR changing the new solver to match the behavior of the old one wrt the leak check. As the new solver is already used by default in coherence, this allows more code to compile, see tests/ui/higher-ranked/leak-check/leak-check-in-selection-7-coherence.rs:

struct W<T, U>(T, U);

trait Trait<T> {}
// using this impl results in a higher-ranked region error.
impl<'a> Trait<W<&'a str, &'a str>> for () {}
impl<'a> Trait<W<&'a str, String>> for () {}

trait NotString {}
impl NotString for &str {}
impl NotString for u32 {}


trait Overlap<U> {}
impl<T: for<'a> Trait<W<&'a str, U>>, U> Overlap<U> for T {}
impl<U: NotString> Overlap<U> for () {}

fn main() {}

This behavior is quite arbitrary and not something I expect users to rely on in practice, however, it should still go through an FCP imo.

r? @BoxyUwU originally implemented by @compiler-errors in #136997. Closes rust-lang/trait-system-refactor-initiative#120.

rustbot · 2025-09-18T14:30:06Z

changes to inspect_obligations.rs

cc @compiler-errors, @lcnr

Some changes occurred to the core trait solver

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

lcnr · 2025-09-18T14:46:25Z

@rfcbot fcp merge

rust-rfcbot · 2025-09-18T14:46:27Z

Team member @lcnr has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

lcnr · 2025-09-18T14:46:46Z

@bors rollup=never

BoxyUwU · 2025-09-24T17:10:34Z

compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs

-                ty::PredicateKind::Ambiguous => {
-                    self.evaluate_added_goals_and_make_canonical_response(Certainty::AMBIGUOUS)
-                }
+        self.enter_forall(kind, |ecx, kind| match kind {


much cleaner, nice :>

tests/ui/higher-ranked/leak-check/candidate-from-env-universe-err-1.rs

rust-rfcbot · 2025-10-07T21:17:33Z

🔔 This is now entering its final comment period, as per the review above. 🔔

lcnr · 2025-10-20T08:31:25Z

@bors try @rust-timer queeu

`-Znext-solver` instantiate predicate binder without recanonicalizing goal

lcnr · 2025-10-20T08:31:34Z

@rust-timer queue

rust-log-analyzer · 2025-10-20T08:57:54Z

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

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

#4 [auth] library/ubuntu:pull token for registry-1.docker.io
#4 DONE 0.0s

#3 [internal] load metadata for docker.io/library/ubuntu:25.10
#3 ERROR: failed to authorize: failed to fetch oauth token: unexpected status from POST request to https://auth.docker.io/token: 503 Service Unavailable: <html><body><h1>503 Service Unavailable</h1>
No server is available to handle this request.
</body></html>


------
---
   1 | >>> FROM ubuntu:25.10
   2 |     
   3 |     ARG DEBIAN_FRONTEND=noninteractive
--------------------
ERROR: failed to build: failed to solve: failed to fetch oauth token: unexpected status from POST request to https://auth.docker.io/token: 503 Service Unavailable: <html><body><h1>503 Service Unavailable</h1>
No server is available to handle this request.
</body></html>


Command failed. Attempt 2/5:
---
#2 [auth] library/ubuntu:pull token for registry-1.docker.io
#2 DONE 0.0s

#3 [internal] load metadata for docker.io/library/ubuntu:25.10
#3 ERROR: failed to authorize: failed to fetch oauth token: unexpected status from POST request to https://auth.docker.io/token: 503 Service Unavailable: <html><body><h1>503 Service Unavailable</h1>
No server is available to handle this request.
</body></html>


------
---
   1 | >>> FROM ubuntu:25.10
   2 |     
   3 |     ARG DEBIAN_FRONTEND=noninteractive
--------------------
ERROR: failed to build: failed to solve: failed to fetch oauth token: unexpected status from POST request to https://auth.docker.io/token: 503 Service Unavailable: <html><body><h1>503 Service Unavailable</h1>
No server is available to handle this request.
</body></html>


Command failed. Attempt 3/5:
---
#2 [auth] library/ubuntu:pull token for registry-1.docker.io
#2 DONE 0.0s

#3 [internal] load metadata for docker.io/library/ubuntu:25.10
#3 ERROR: failed to authorize: failed to fetch oauth token: unexpected status from POST request to https://auth.docker.io/token: 503 Service Unavailable: <html><body><h1>503 Service Unavailable</h1>
No server is available to handle this request.
</body></html>


------
---
   1 | >>> FROM ubuntu:25.10
   2 |     
   3 |     ARG DEBIAN_FRONTEND=noninteractive
--------------------
ERROR: failed to build: failed to solve: failed to fetch oauth token: unexpected status from POST request to https://auth.docker.io/token: 503 Service Unavailable: <html><body><h1>503 Service Unavailable</h1>
No server is available to handle this request.
</body></html>


Command failed. Attempt 4/5:
---
#2 [auth] library/ubuntu:pull token for registry-1.docker.io
#2 DONE 0.0s

#3 [internal] load metadata for docker.io/library/ubuntu:25.10
#3 ERROR: failed to authorize: failed to fetch oauth token: unexpected status from POST request to https://auth.docker.io/token: 503 Service Unavailable: <html><body><h1>503 Service Unavailable</h1>
No server is available to handle this request.
</body></html>


------
---
   1 | >>> FROM ubuntu:25.10
   2 |     
   3 |     ARG DEBIAN_FRONTEND=noninteractive
--------------------
ERROR: failed to build: failed to solve: failed to fetch oauth token: unexpected status from POST request to https://auth.docker.io/token: 503 Service Unavailable: <html><body><h1>503 Service Unavailable</h1>
No server is available to handle this request.
</body></html>


Command failed. Attempt 5/5:
---
#2 [auth] library/ubuntu:pull token for registry-1.docker.io
#2 DONE 0.0s

#3 [internal] load metadata for docker.io/library/ubuntu:25.10
#3 ERROR: failed to authorize: failed to fetch oauth token: unexpected status from POST request to https://auth.docker.io/token: 503 Service Unavailable: <html><body><h1>503 Service Unavailable</h1>
No server is available to handle this request.
</body></html>


------
---
   1 | >>> FROM ubuntu:25.10
   2 |     
   3 |     ARG DEBIAN_FRONTEND=noninteractive
--------------------
ERROR: failed to build: failed to solve: failed to fetch oauth token: unexpected status from POST request to https://auth.docker.io/token: 503 Service Unavailable: <html><body><h1>503 Service Unavailable</h1>
No server is available to handle this request.
</body></html>


The command has failed after 5 attempts.

rust-bors · 2025-10-20T10:46:57Z

☀️ Try build successful (CI)
Build commit: e5a41b8 (e5a41b8d8a870a9050679fb33fe54ee3e0e7d81d, parent: ebe145eca703c74525d4a38ed8021d575a23fa30)

rust-timer · 2025-10-20T11:58:18Z

Finished benchmarking commit (e5a41b8): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

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.

@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)	0.7%	[0.7%, 0.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary 3.9%)

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)	3.9%	[2.0%, 5.6%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 4.4%)

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)	4.4%	[4.4%, 4.4%]	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: 473.415s -> 471.736s (-0.35%)
Artifact size: 388.68 MiB -> 388.69 MiB (0.00%)

BoxyUwU · 2025-10-21T14:45:52Z

r=me if CI passes

lcnr · 2025-10-21T20:07:03Z

@bors r=BoxyUwU

bors · 2025-10-21T20:07:06Z

📌 Commit 5508b47 has been approved by BoxyUwU

It is now in the queue for this repository.

bors · 2025-10-21T21:09:50Z

⌛ Testing commit 5508b47 with merge 37ec98f...

bors · 2025-10-22T00:20:00Z

☀️ Test successful - checks-actions
Approved by: BoxyUwU
Pushing 37ec98f to master...

github-actions · 2025-10-22T00:23:47Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 695857b (parent) -> 37ec98f (this PR)

Test differences

Show 21 test diffs

Stage 1

[crashes] tests/crashes/140609.rs: pass -> [missing] (J0)
[ui] tests/ui/const-generics/generic_const_exprs/negative-coherence-ice-140609.rs: [missing] -> pass (J0)
[ui] tests/ui/higher-ranked/leak-check/leak-check-in-selection-7-coherence.rs: [missing] -> pass (J0)

Stage 2

[ui] tests/ui/const-generics/generic_const_exprs/negative-coherence-ice-140609.rs: [missing] -> pass (J1)
[ui] tests/ui/higher-ranked/leak-check/leak-check-in-selection-7-coherence.rs: [missing] -> pass (J1)
[crashes] tests/crashes/140609.rs: pass -> [missing] (J2)

Additionally, 15 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 37ec98f5d33c0876a9ffa5ee17dc0895659efe37 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

pr-check-1: 1480.4s -> 1832.4s (23.8%)
dist-x86_64-windows-gnullvm: 5995.1s -> 4855.7s (-19.0%)
aarch64-apple: 9526.3s -> 8042.2s (-15.6%)
x86_64-gnu-debug: 6548.9s -> 7430.9s (13.5%)
dist-ohos-armv7: 4531.6s -> 3944.3s (-13.0%)
i686-gnu-2: 5514.6s -> 6216.3s (12.7%)
x86_64-gnu-gcc: 3052.9s -> 3421.5s (12.1%)
i686-gnu-nopt-1: 7178.1s -> 7998.6s (11.4%)
dist-loongarch64-musl: 5744.1s -> 5164.1s (-10.1%)
dist-powerpc64le-linux-musl: 5848.7s -> 5303.9s (-9.3%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2025-10-22T01:30:06Z

Finished benchmarking commit (37ec98f): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-0.4%	[-1.1%, -0.1%]	4
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

Max RSS (memory usage)

Results (primary -0.8%)

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)	-0.8%	[-0.8%, -0.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.8%	[-0.8%, -0.8%]	1

Cycles

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

Binary size

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

Bootstrap: 472.072s -> 472.516s (0.09%)
Artifact size: 388.64 MiB -> 388.62 MiB (-0.01%)

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 Sep 18, 2025

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Instantiate predicate binder without recanonicalizing goal in new solver #136997

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lcnr changed the title ~~instantiate predicate binder without recanonicalizing goal~~ -Znext-solver instantiate predicate binder without recanonicalizing goal Sep 18, 2025

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[DO NOT MERGE] bootstrap with -Znext-solver=globally #133502

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Auto merge of #146725 - lcnr:eager-instantiate-binder, r=<try>

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`-Znext-solver` instantiate predicate binder without recanonicalizing goal

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ICE Unevaluated ty::Const in MIR body #140609

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This was referenced Oct 23, 2025

higher-ranked goals in trait goal candidate selection rust-lang/trait-system-refactor-initiative#120

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Next-generation trait solver rust-lang/rust-project-goals#113

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-Znext-solver instantiate predicate binder without recanonicalizing goal #146725

-Znext-solver instantiate predicate binder without recanonicalizing goal #146725

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`-Znext-solver` instantiate predicate binder without recanonicalizing goal #146725

`-Znext-solver` instantiate predicate binder without recanonicalizing goal #146725

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