Make sure to account for the right item universal regions in borrowck

[compiler-errors]

Fixes #144608.

The ICE comes from a mismatch between the liberated late bound regions (i.e. "ReLateParam"s) that come from promoting closure outlives, and the regions we have in our region vid mapping from UniversalRegions.

When building UniversalRegions, we end up using the liberated regions from the binder of the closure's signature:

rust/compiler/rustc_borrowck/src/universal_regions.rs

Line 521 in c8bb4e8

for (idx, bound_var) in bound_inputs_and_output.bound_vars().iter().enumerate() {

Notably, this signature may be anonymized if the closure signature being deduced comes from an external constraints:

rust/compiler/rustc_hir_typeck/src/closure.rs

Lines 759 to 762 in c8bb4e8

	// `deduce_expectations_from_expected_type` introduces
	// late-bound lifetimes defined elsewhere, which we now
	// anonymize away, so as not to confuse the user.
	let bound_sig = self.tcx.anonymize_bound_vars(bound_sig);

This is true in the test file I committed, where the signature is influenced by the impl FnMut(&mut ()) RPIT.

However, when promoting a type outlives constraint we end up creating a late bound lifetime mapping that disagrees with those liberated late bound regions we constructed in UniversalRegions:

rust/compiler/rustc_borrowck/src/universal_regions.rs

Line 299 in c8bb4e8

for_each_late_bound_region_in_recursive_scope(tcx, tcx.local_parent(closure_def_id), |r| {

Specifically, in for_each_late_bound_region_in_item (which is called by for_each_late_bound_region_in_recursive_scope), we were using tcx.late_bound_vars which uses the late bound regions from the HIR. This query both undercounts the late bound regions (e.g. those that end up being deduced from bounds), and also doesn't account for the fact that we anonymize them in the signature as mentioned above.

rust/compiler/rustc_borrowck/src/universal_regions.rs

Line 977 in c8bb4e8

tcx.late_bound_vars(tcx.local_def_id_to_hir_id(mir_def_id)).iter().enumerate()

This PR fixes that function to use the correct signature, which properly considers the bound vars that come from deducing the signature of the closure, and which comes from the closure's args from the type_of query.

[rustbot]

r? @davidtwco

rustbot has assigned @davidtwco.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[lqd]

chef's kiss at the PR description.

r? lqd @bors r+

[bors]

📌 Commit 98d08ff has been approved by lqd

It is now in the queue for this repository.