diagnostics: account for self type when looking for source of unsolved type variable

[fmease]

Fixes #109905.

When searching for the source of an unsolved infer var inside of a list of generic args, we look through the tcx.generics_of(…).own_substs(…) which skips the self type if present. However, the computed argument_index is later¹ used to index into tcx.generics_of(…).params which may still contain the self type. In such case, we are off by one when indexing into the parameters.

From now on, we account for this immediately after calling own_substs which keeps things local.

This also fixes the wrong output in the preexisting UI test inference/need_type_info/concrete-impl.rs which was overlooked. It used to claim that the type of type parameter Self couldn't be inferred in <Struct as Ambiguous<_>>::method() which of course isn't true: Self equals Struct here, A couldn't be inferred.

@rustbot label A-diagnostics

Footnotes

1. https://github.com/rust-lang/rust/blob/f98a2718141593fbb8dbad10acc537786d748156/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs#L471 ↩

[rustbot]

r? @petrochenkov

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[rustbot]

Some changes occurred in need_type_info.rs

cc @lcnr

[fmease]

Is the situation generics.parent.is_some() && generics.has_self possible?

The CI test failures answer it lol. Going back to checking the parent …

[fmease]

@rustbot review

[fmease]

Is the situation generics.parent.is_some() && generics.has_self possible?

I've looked at the implementation of tcx.generics_of. has_self can also be true if the parent has_self in which case the (own) parameters (generics.params) don't contain the self type and we shouldn't offset the index in this case.

Looking at (vec![1,2,3]).into_iter().sum() in test tests/ui/span/type-annotations-needed-expr.rs for example where we cannot infer param S on method sum, generics.parent is Some(Iterator_def_id) but generics.params only contains param S and notably not the self type. Instead, the parent (trait Iterator) has the self type and thus generics.has_self is true.

relevant definitions

• generics.parent: DefId(2:7268 ~ core[280a]::iter::traits::iterator::Iterator)
• generics.has_self: true
• generics.params: ["S"] (w/o self type)
• substs: [std::vec::IntoIter<i32>, _#20t]
• generics.own_substs_no_defaults(substs): [_#20t]

[petrochenkov]

@bors r+

[bors]

📌 Commit b904ce9 has been approved by petrochenkov

It is now in the queue for this repository.