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Rollup merge of #109957 - fmease:fix-109905, r=petrochenkov

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

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 later[^1] 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

[^1]: f98a271814/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs (L471)
This commit is contained in:
Matthias Krüger 2023-04-07 00:00:24 +02:00 • committed by GitHub
commit e63586f386
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5 changed files with 52 additions and 5 deletions
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5
compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs
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@ -1191,11 +1191,14 @@ impl<'a, 'tcx> Visitor<'tcx> for FindInferSourceVisitor<'a, 'tcx> {
have_turbofish,
} = args;
let generics = tcx.generics_of(generics_def_id);
if let Some(argument_index) = generics
if let Some(mut argument_index) = generics
.own_substs(substs)
.iter()
.position(|&arg| self.generic_arg_contains_target(arg))
{
if generics.parent.is_none() && generics.has_self {
argument_index += 1;
}
let substs = self.infcx.resolve_vars_if_possible(substs);
let generic_args = &generics.own_substs_no_defaults(tcx, substs)
[generics.own_counts().lifetimes..];