-Znext-solver instantiate predicate binder without recanonicalizing goal

[lcnr]

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]

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]

@rfcbot fcp merge

[rust-rfcbot]

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

• @BoxyUwU
• @compiler-errors
• @jackh726
• @lcnr
• @nikomatsakis
• @oli-obk
• @spastorino

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]

@bors rollup=never

[BoxyUwU]

much cleaner, nice :>

[rust-rfcbot]

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

[rust-rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[rustbot]

This PR was rebased onto a different master 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.

[lcnr]

@bors try @rust-timer queeu

[lcnr]

@rust-timer queue

[rust-log-analyzer]

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]

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

[rust-timer]

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%)