support non-defining uses of opaques in borrowck

[lcnr]

Reimplements the first part of #139587, but limited to only the new solver. To do so I also entirely rewrite the way we handle opaque types in borrowck even on stable. This should not impact behavior however.

We now support revealing uses during MIR borrowck with the new solver:

fn foo<'a>(x: &'a u32) -> impl Sized + use<'a> {
    let local = 1;
    foo::<'_>(&local);
    x
}

How do opaque types work right now

Whenever we use an opaque type during type checking, we remember this use in the opaque_type_storage of the InferCtxt.

Right now, we collect all member constraints at the end of MIR type check by looking at all uses from the opaque_type_storage. We then apply these constraints while computing the region values for each SCC. This does not add actual region constraints but directly updates the final region values.

This means we need to manually handle any constraints from member constraints for diagnostics. We do this by separately tracking applied_member_constraints in the RegionInferenceContext.

After we've finished computing the region values, it is now immutable and we check whether all member constraints hold. If not, we error.

We now map the hidden types of our defining uses to the defining scope. This assumes that all member constraints apply. To handle non-member regions, we simply map any region in the hidden type we fail to map to a choice region to 'erased

rust/compiler/rustc_borrowck/src/region_infer/opaque_types.rs

Lines 126 to 132 in b1b26b8

	let concrete_type = fold_regions(infcx.tcx, concrete_type, |region, _| {
	arg_regions
	.iter()
	.find(|&&(arg_vid, _)| self.eval_equal(region.as_var(), arg_vid))
	.map(|&(_, arg_named)| arg_named)
	.unwrap_or(infcx.tcx.lifetimes.re_erased)
	});

How do we handle opaque types with this PR

MIR type check still works the same by populating the opaque_type_storage whenever we use an opaque type.

We now have a new step fn handle_opaque_type_uses which happens between MIR type check and compute_regions.

This step looks at all opaque type uses in the storage and first checks whether they are defining: are the arguments of the opaque_type_key unique region parameters. With the new solver we silently ignore any non-defining uses here. The old solver emits an errors.

fn compute_concrete_opaque_types: We then collect all member constraints for the defining uses and apply them just like we did before. However, we do it on a temporary region graph which is only used while computing the concrete opaque types. We then use this region graph to compute the concrete type which we then store in the root_cx.

fn apply_computed_concrete_opaque_types: Now that we know the final concrete type of each opaque type and have mapped them to the definition of the opaque. We iterate over all opaque type uses and equate their hidden type with the instantiated final concrete type. This is the step which actually mutates the region graph.

The actual region checking can now entirely ignores opaque types (outside of the ConstraintCategory from checking the opaque type uses).

Diagnostics issue (chill)

Because we now simply use type equality to "apply member constraints" we get ordinary OutlivesConstraints, even if the regions were already related to another.

This is generally not an issue, expect that it can hide the actual region constraints which resulted in the final value of the opaque. The constraints we get from checking against the final opaque type definition relies on constraints we used to compute that definition.

I mostly handle this by updating find_constraint_path_between_regions to first ignore member constraints in its search and only if that does not find a path, retry while considering member constraints.

Diagnostics issue (not chill)

A separate issue is that find_constraint_paths_between_regions currently looks up member constraints by their scc, not by region value:

rust/compiler/rustc_borrowck/src/region_infer/mod.rs

Lines 1768 to 1775 in 2c1ac85

	// Member constraints can also give rise to `'r: 'x` edges that
	// were not part of the graph initially, so watch out for those.
	// (But they are extremely rare; this loop is very cold.)
	for constraint in self.applied_member_constraints(self.constraint_sccs.scc(r)) {
	let sub = constraint.min_choice;
	let p_c = &self.member_constraints[constraint.member_constraint_index];
	handle_trace(sub, Trace::FromMember(r, sub, p_c.definition_span));
	}

This means that in the borrowck-4 test, the resulting constraint path is currently

('?2: '?5) due to Single(bb0[5]) (None) (Boring) (ConstraintSccIndex(1): ConstraintSccIndex(1)),
('?5: '?3) due to Single(bb0[6]) (None) (Boring) (ConstraintSccIndex(1): ConstraintSccIndex(1)),
('?3: '?0) due to All(src/main.rs:15:5: 15:6 (#0)) (None) (OpaqueType) (ConstraintSccIndex(1): ConstraintSccIndex(1))

Here '?3 is equal to '?4, but the reason why it's in the opaque is that it's related to '?4. With my PR this will be correctly tracked so we end up with

('?2: '?5) due to Single(bb0[5]) (None) (Boring) (ConstraintSccIndex(1): ConstraintSccIndex(1)),
('?5: '?3) due to Single(bb0[6]) (None) (Boring) (ConstraintSccIndex(1): ConstraintSccIndex(1)),
('?3: '?4) due to Single(bb0[6]) (None) (Assignment) (ConstraintSccIndex(1): ConstraintSccIndex(1)),
('?4: '?0) due to All(src/main.rs:15:5: 15:6 (#0)) (None) (OpaqueType) (ConstraintSccIndex(1): ConstraintSccIndex(1)),

This additional Assignment step then worsens the error message as we stop talking about the fact that the closures is returned from the function. Fixing this is hard. I've looked into this and it's making me sad :< Properly handling this requires some deeper changes to MIR borrowck diagnostics and that seems like too much for this PR. Given that this only impacts a single test, it seems acceptable to me.

r? @ghost

[lcnr]

just moved from RegionErrorKind::UnexpectedHiddenRegion

[lcnr]

idk why I've added this FIXME to these 3 tests, only one of them actually failed with the new solver before.

[lcnr]

equiv to existing register_member_constraints

[lcnr]

@bors r=BoxyUwU rollup=never

[bors]

📌 Commit 86063b3 has been approved by BoxyUwU

It is now in the queue for this repository.

[bors]

⌛ Testing commit 86063b3 with merge c57053d...

[bors]

💔 Test failed - checks-actions

[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 r=BoxyUwU

[bors]

📌 Commit c2a0fa8 has been approved by BoxyUwU

It is now in the queue for this repository.