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diagnostics: account for self type when looking for source of unsolved type variable#109957

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fmease:fix-109905
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diagnostics: account for self type when looking for source of unsolved type variable#109957
bors merged 1 commit intorust-lang:masterfrom
fmease:fix-109905

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@fmease fmease commented Apr 5, 2023 •
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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

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

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r? @petrochenkov

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@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Apr 5, 2023
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Some changes occurred in need_type_info.rs

cc @lcnr

@rustbot rustbot added the A-diagnostics Area: Messages for errors, warnings, and lints label Apr 5, 2023
@fmease fmease changed the title Account for self type when looking for type parameter that couldn't be inferred Account for self type when looking for source of unsolved type variable Apr 5, 2023
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@fmease fmease changed the title Account for self type when looking for source of unsolved type variable diagnostics: account for self type when looking for source of unsolved type variable Apr 5, 2023
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fmease commented Apr 5, 2023 •
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Is the situation generics.parent.is_some() && generics.has_self possible?

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

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fmease commented Apr 5, 2023

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@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 5, 2023
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fmease commented Apr 6, 2023 •
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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]

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petrochenkov commented Apr 6, 2023

@bors r+

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bors commented Apr 6, 2023

📌 Commit b904ce9 has been approved by petrochenkov

It is now in the queue for this repository.

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 6, 2023
@matthiaskrgr matthiaskrgr mentioned this pull request Apr 6, 2023
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bors added a commit to rust-lang-ci/rust that referenced this pull request Apr 7, 2023
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Auto merge of rust-lang#110024 - matthiaskrgr:rollup-mydkufd, r=matth…
c934ce9 
…iaskrgr

Rollup of 6 pull requests

Successful merges:

 - rust-lang#109806 (Workaround rust-lang#109797 on windows-gnu)
 - rust-lang#109957 (diagnostics: account for self type when looking for source of unsolved type variable)
 - rust-lang#109960 (Fix buffer overrun in bootstrap and (test-only) symlink_junction)
 - rust-lang#110013 (Label `non_exhaustive` attribute on privacy errors from non-local items)
 - rust-lang#110016 (Run collapsed GUI test in mobile mode as well)
 - rust-lang#110022 (fix: fix regression in rust-lang#109203)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors merged commit e63586f into rust-lang:master Apr 7, 2023
@rustbot rustbot added this to the 1.70.0 milestone Apr 7, 2023
@fmease fmease deleted the fix-109905 branch April 7, 2023 03:37
@fmease fmease mentioned this pull request Jan 25, 2024
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@fmease fmease mentioned this pull request Dec 28, 2024
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cannot infer type of the type parameter 'a declared on the trait Baz

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