Don't suggest nonsense suggestions for unconstrained type vars in note_source_of_type_mismatch_constraint

[compiler-errors]

The way we do type inference for suggestions in note_source_of_type_mismatch_constraint is a bit strange. We compute the "ideal" method signature, which takes the receiver that we want and uses it to compute the types of the arguments that would have given us that receiver via type inference, and use that to suggest how to change an argument to make sure our receiver type is inferred correctly.

The problem is that sometimes we have totally unconstrained arguments (well, they're constrained by things outside of the type checker per se, like associated types), and therefore type suggestions are happy to coerce anything to that unconstrained argument. This leads to bogus suggestions, like #116155. This is partly due to above, and partly due to the fact that emit_type_mismatch_suggestions doesn't double check that its suggestions are actually compatible with the program other than trying to satisfy the type mismatch.

This adds a hack to make sure that at least the types are fully constrained, but I guess I could also rip out this logic altogether. There would be some sad diagnostics regressions though, such as tests/ui/type/type-check/point-at-inference-4.rs.

Fixes #116155

[rustbot]

r? @wesleywiser

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[wesleywiser]

@compiler-errors I think you just need to rebase & bless the test output.

r=me when CI is green 🙂

[compiler-errors]

@bors r=wesleywiser

just needed to bless past the {closure} change that happened recently

[bors]

📌 Commit ac5aa8c has been approved by wesleywiser

It is now in the queue for this repository.