Don't carry MIR location in ConstraintCategory::CallArgument

[compiler-errors]

It turns out that ConstraintCategory::CallArgument cannot just carry a MIR location in it, since we may bubble them up to totally different MIR bodies.

So instead, revert the commit a6b5f95, and instead just erase regions from the original Option<Ty<'tcx>> that it carried, so that it doesn't ICE with the changes in #103220.

Best reviewed in parts -- the first is just a revert, and the second is where the meaningful changes happen.

Fixes #103624

[rustbot]

r? @cjgillot

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[cjgillot]

Why does erasing regions change diagnostics?

[compiler-errors]

Actually, good question -- I have no idea. My first suspicion when I put this PR up was due to the Ord implementation on ConstraintCategory and something about the sorting of constraints in best_blame_constraint, but I can look into it further.

[compiler-errors]

Yeah, it's because we're not sorting by variance_info, but only the constraint:

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

Line 2230 in 0da281b

categorized_path.sort_by(|p0, p1| p0.category.cmp(&p1.category));

And we have several constraints that differ only in variant info:

1ms DEBUG rustc_borrowck::region_infer sorted_path=[
│     BlameConstraint {
│         category: CallArgument(
│             Some(
│                 fn(fn() -> Type<'_> {foo::<'_>}, <fn() -> Type<'_> {foo::<'_>} as std::ops::FnOnce<()>>::Output) -> <fn() -> Type<'_> {foo::<'_>} as std::ops::FnOnce<()>>::Output {bar::<fn() -> Type<'_> {foo::<'_>}>},
│             ),
│         ),
│         from_closure: false,
│         cause: ObligationCause {
│             span: src/test/ui/associated-types/cache/project-fn-ret-invariant.rs:42:13: 42:22 (#0),
│             body_id: HirId {
│                 owner: OwnerId {
│                     def_id: DefId(0:0 ~ project_fn_ret_invariant[129b]),
│                 },
│                 local_id: 0,
│             },
│             code: InternedObligationCauseCode {
│                 code: None,
│             },
│         },
│         variance_info: Invariant {
│             ty: Type<'_>,
│             param_index: 0,
│         },
│         outlives_constraint: ('_#19r: '_#6r) due to Single(bb1[8]) (Invariant { ty: Type<'_>, param_index: 0 }) (CallArgument(Some(fn(fn() -> Type<'_> {foo::<'_>}, <fn() -> Type<'_> {foo::<'_>} as std::ops::FnOnce<()>>::Output) -> <fn() -> Type<'_> {foo::<'_>} as std::ops::FnOnce<()>>::Output {bar::<fn() -> Type<'_> {foo::<'_>}>}))),
│     },
│     BlameConstraint {
│         category: CallArgument(
│             Some(
│                 fn(fn() -> Type<'_> {foo::<'_>}, <fn() -> Type<'_> {foo::<'_>} as std::ops::FnOnce<()>>::Output) -> <fn() -> Type<'_> {foo::<'_>} as std::ops::FnOnce<()>>::Output {bar::<fn() -> Type<'_> {foo::<'_>}>},
│             ),
│         ),
│         from_closure: false,
│         cause: ObligationCause {
│             span: src/test/ui/associated-types/cache/project-fn-ret-invariant.rs:42:13: 42:22 (#0),
│             body_id: HirId {
│                 owner: OwnerId {
│                     def_id: DefId(0:0 ~ project_fn_ret_invariant[129b]),
│                 },
│                 local_id: 0,
│             },
│             code: InternedObligationCauseCode {
│                 code: None,
│             },
│         },
│         variance_info: Invariant {
│             ty: fn() -> Type<'_> {foo::<'_>},
│             param_index: 0,
│         },
│         outlives_constraint: ('_#6r: '_#18r) due to Single(bb1[8]) (Invariant { ty: fn() -> Type<'_> {foo::<'_>}, param_index: 0 }) (CallArgument(Some(fn(fn() -> Type<'_> {foo::<'_>}, <fn() -> Type<'_> {foo::<'_>} as std::ops::FnOnce<()>>::Output) -> <fn() -> Type<'_> {foo::<'_>} as std::ops::FnOnce<()>>::Output {bar::<fn() -> Type<'_> {foo::<'_>}>}))),
│     },

[...]

-- so erasing regions probably causes hashes to change which causes the order constraints get ordered out to change.

[compiler-errors]

The differing variance_info causes this error message to change slightly in the note, as seen above. I think it's harmless imo. Same with all these other errors changing slightly.

[cjgillot]

Well, if it's just an ordering issue.
@bors r+

[bors]

📌 Commit 4e0c27b has been approved by cjgillot

It is now in the queue for this repository.