Fix rustdoc ICE caused by mishandling of ambiguity errors

[ShoyuVanilla]

Fixes #162557

pub trait Service<Request> {
    type Future;
}

pub trait ZebraService<Request>: Service<Request> {}

impl<MaybeVerify, Request> ZebraService<Request> for MaybeVerify where
    MaybeVerify: Service<Request, Future: 'static>
{
}

pub struct Verifier;

impl Service<()> for Verifier {
    type Future = &'static ();
}

In the above minimization of the issue, when we try to check whether the blanket impl can be applied to Verifier,

rust/src/librustdoc/clean/blanket_impl.rs

Lines 42 to 67 in a8a1e6f

	let args = infcx.fresh_args_for_item(DUMMY_SP, item_def_id);
	let impl_ty = ty.instantiate(tcx, args).skip_norm_wip();
	let param_env = ty::ParamEnv::empty();
	let impl_args = infcx.fresh_args_for_item(DUMMY_SP, impl_def_id);
	let impl_trait_ref = trait_ref.instantiate(tcx, impl_args).skip_norm_wip();
	// Require the type the impl is implemented on to match
	// our type, and ignore the impl if there was a mismatch.
	let Ok(eq_result) = infcx.at(&traits::ObligationCause::dummy(), param_env).eq(
	DefineOpaqueTypes::Yes,
	impl_trait_ref.self_ty(),
	impl_ty,
	) else {
	continue;
	};
	let InferOk { value: (), obligations } = eq_result;
	// FIXME(eddyb) ignoring `obligations` might cause false positives.
	drop(obligations);
	let clauses = tcx
	.clauses_of(impl_def_id)
	.instantiate(tcx, impl_args)
	.clauses
	.into_iter()
	.map(Unnormalized::skip_norm_wip)

We make fresh args for the where-clause and skip normalization for it.

So, we have <?MaybeVerify as Service<?Request>>::Future: 'static bound to check. But it immediately evaluated into ambiguity due to stalled on infer vars in the fast path as the obligation contains the infer vars.

But due #162182 we evaluated it directly in the solver without stalled_on and this time it succeeded because we can normalize the alias into a concrete type &'static () and it outlives the static region.

So, I think it's very iffy to call InferCtxt::evaluate_obligation on a non-rigid alias and we should eagerly normalize it in L67 from the above rustdoc code.
But it may break something in rustdoc as normalizations inside it is pretty messy in general 🫠 So, I guess in another PR with crater runs.

r? lcnr

[rustbot]

Some changes occurred to the core trait solver

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

These commits modify tests/rustdoc-json.
rustdoc-json is a public (but unstable) interface.

Please ensure that if you've changed the output:

• It's intentional.
• The FORMAT_VERSION in src/librustdoc-json-types is bumped if necessary.

cc @obi1kenobi

[lcnr]

Oh, the fast path is scuffed here. So, I feel fairly confident that we can encounter cases where stalled_on is wrong due to region vars without there being a bug, as in, your change is correct and desirable, but this specific test is just a bug in the fast path:

<MaybeVerify as Service<?infer>>::Future: 'static should simply not be considered stalled. What's the cost of either entirely removing this trivially_stalled_on fast path or fixing it to bail when encountering non-rigid aliases

This change is correct. Please separately do a PR to fix the type-outlives fastpath 😊

@bors r+ rollup

[rust-bors]

📌 Commit 621507f has been approved by lcnr

It is now in the queue for this repository.

[JonathanBrouwer]

💔 I suspect this PR failed tests as part of a rollup
@bors r-

After fixing the problem, consider running a try job for the failed job before re-approving.

Link to failure: #163595 (comment)

[rust-bors]

This pull request was unapproved.

This PR was contained in a rollup (#163595), which was unapproved.

View changes since this unapproval

[ShoyuVanilla]

💔 I suspect this PR failed tests as part of a rollup @bors r-

After fixing the problem, consider running a try job for the failed job before re-approving.

Link to failure: #163595 (comment)

Sadage. It looks like one of the changed test in the rollup was affected by this again

[lcnr]

@bors r+

[rust-bors]

📌 Commit d85a18b has been tentatively approved by lcnr

It will be put into the queue for this repository once PR CI succeeds.

[rust-timer]

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (4f8baea): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

• If the regression was expected or you think it can be justified,
  please write a comment with sufficient written justification, and add
  @rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
• If you think that you know of a way to resolve the regression, try to create
  a new PR with a fix for the regression.
• If you do not understand the regression or you think that it is just noise,
  you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
  were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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)	0.1%	[0.1%, 0.1%]	3
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	2
Improvements ✅ (secondary)	-0.3%	[-0.5%, -0.1%]	3
All ❌✅ (primary)	-0.0%	[-0.1%, 0.1%]	5

Max RSS (memory usage)

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

Cycles

Results (primary 2.3%, secondary 4.8%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.2%, 2.5%]	2
Regressions ❌ (secondary)	4.8%	[2.7%, 6.9%]	8
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.3%	[2.2%, 2.5%]	2

Binary size

Results (primary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.1%]	6
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.1%]	6

Bootstrap: missing data
Artifact size: 408.61 MiB -> 408.67 MiB (0.02%)