hir_typeck: Propagate async closure output into its body - #161708
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Async closure bodies are checked as coroutines, but their expected return type was lost before body checking. Reuse the matching coroutine signature. Keep this path limited to async closures because gen closures have a different input shape.
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This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed. Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers. |
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Fixes #161619
Rustc already knows the output type it expects from an async closure, but that type wasn't reaching the closure body during type checking. When the body returned the wrong thing, rustc compared the generated async types and suggested pinning. That sends you in completely the wrong direction.
The fix carries the coroutine signature into the body check, so the regular type error points at
String::new()and says it expected().My first pass matched any coroutine with the same def id. It looked fine, then the iterator suite caught the problem:
iter!comes through the same path but its body has no resume input. I narrowed the match to async closure bodies. I prefer that version because it uses type information rustc already has and leaves the gen path alone. The test runs with both trait solvers, and the existing iterator tests cover the case that bit me.