Only compute specializes query if (min)specialization is enabled in the crate of the specializing impl

[compiler-errors]

Fixes (after backport) #125197

What

#122791 makes it so that inductive cycles are no longer hard errors. That means that when we are testing, for example, whether these impls overlap:

impl PartialEq<Self> for AnyId {
    fn eq(&self, _: &Self) -> bool {
        todo!()
    }
}

impl<T: Identifier> PartialEq<T> for AnyId {
    fn eq(&self, _: &T) -> bool {
        todo!()
    }
}

...given...

pub trait Identifier: Display + 'static {}

impl<T> Identifier for T where T: PartialEq + Display + 'static {}

Then we try to see if the second impl holds given T = AnyId. That requires AnyId: Identifier, which requires that AnyId: PartialEq, which is satisfied by these two impl candidates... The PartialEq<T> impl is a cycle, and we used to winnow it when we used to treat inductive cycles as errors.

However, now that we don't winnow it, this means that we now try calling candidate_should_be_dropped_in_favor_of, which tries to check whether one of the impls specializes the other: the specializes query. In that query, we currently bail early if the impl is local.

However, in a foreign crate, we try to compute if the two impls specialize each other by doing trait solving. This may itself lead to the same situation where we call specializes, which will lead to a query cycle.

How does this fix the problem

We now record whether specialization is enabled in foreign crates, and extend this early-return behavior to foreign impls too. This means that we can only encounter these cycles if we truly have a specializing impl from a crate with specialization enabled.

---

r? @oli-obk or @lcnr

[compiler-errors]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 4b188d9 with merge 20182a9...

[bors]

☀️ Try build successful - checks-actions
Build commit: 20182a9 (20182a9ce3a4c9bba5a4d4df53c3b8777f2006cc)

[rust-timer]

Finished benchmarking commit (20182a9): comparison URL.

Overall result: ❌ regressions - no action needed

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

Cycles

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

Binary size

Results (secondary 0.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: missing data
Artifact size: 319.56 MiB -> 318.87 MiB (-0.21%)

[lcnr]

@bors r+

[bors]

📌 Commit 4b188d9 has been approved by lcnr

It is now in the queue for this repository.

[lqd]

As 1.79 artifacts are being prepared right now, this PR could be stable-nominated in the future. Maybe we'll just keep a look at open issues to see whether people encounter #125197 or similar in the wild. There wasn't many crater regressions (nor did it seem issues were opened about this on nightly?), and maybe a stable backport won't be needed.

[bors]

⌛ Testing commit 4b188d9 with merge 336e6ab...

[bors]

☀️ Test successful - checks-actions
Approved by: lcnr
Pushing 336e6ab to master...

[rust-timer]

Finished benchmarking commit (336e6ab): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

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

Cycles

Results (secondary -4.8%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.8%	[-6.4%, -2.2%]	6
All ❌✅ (primary)	-	-	0

Binary size

Results (secondary 0.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 677.913s -> 675.77s (-0.32%)
Artifact size: 320.07 MiB -> 319.45 MiB (-0.19%)

[apiraino]

Beta backport accepted as per compiler team on Zulip. A backport PR will be authored by the release team at the end of the current development cycle.

@rustbot label +beta-accepted

[apiraino]

Also suitable for a stable backport in case a point release is planned.

@rustbot label +stable-accepted

[cuviper]

Adding the nomination so it comes up in our queries.

@rustbot label +stable-nominated

[cuviper]

Clearing stable backport labels since we didn't ship a 1.79 point release, and this is already in current stable 1.80.0.

@rustbot label -stable-nominated -stable-accepted