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borrowck diagnostics: suggest borrowing function inputs in generic positions # Summary This generalizes borrowck's existing suggestions to borrow instead of moving when passing by-value to a function that's generic in that input. Previously, this was special-cased to `AsRef`/`Borrow`-like traits and `Fn`-like traits. This PR changes it to test if, for a moved place with type `T`, that the callee's signature and clauses don't break if you substitute in `&T` or `&mut T`. For instance, it now works with `Read`/`Write`-like traits. Fixes https://github.com/rust-lang/rust/issues/131413 # Incidental changes - No longer spuriously suggests mutable borrows of closures in some situations (see e.g. the tests in [tests/ui/closures/2229_closure_analysis/](https://github.com/rust-lang/rust/compare/master...dianne:rust:suggest-borrow-generic?expand=1#diff-8dfb200c559f0995d0f2ffa2f23bc6f8041b263e264e5c329a1f4171769787c0)). - No longer suggests cloning closures that implement `Fn`, since they can be borrowed (see e.g. [tests/ui/moves/borrow-closures-instead-of-move.stderr](https://github.com/rust-lang/rust/compare/master...dianne:rust:suggest-borrow-generic?expand=1#diff-5db268aac405eec56d099a72d8b58ac46dab523cf013e29008104840168577fb)). This keeps the behavior to suppress suggestions of `fn_once.clone()()`. I think it might make sense to suggest it with a "but this might not be your desired behavior" caveat, as is done when something is used after being consumed as the receiver for a method call. That's probably out of the scope of this PR though. # Limitations and possible improvements - This doesn't work for receivers of method calls. This is a small change, and I have it implemented locally, but I'm not sure it's useful on its own. In most cases I've found borrowing the receiver would change the call's output type (see below). In other cases (e.g. `Iterator::sum`), borrowing the receiver isn't useful since it's consumed. - This doesn't work when it would change the call's output type. In general, I figure inserting references into the output type is an unwanted change. However, this also means it doesn't work in cases where the new output type would be effectively the same as the old one. For example, from the rand crate, the iterator returned by [`Rng::sample_iter`](https://docs.rs/rand/latest/rand/trait.Rng.html#method.sample_iter) is effectively the same (modulo regions) whether you borrow or consume the receiver `Rng`, so common usage involves borrowing it. I'm not sure whether the best approach is to add a more complex check of approximate equivalence, to forego checking the call's output type and give spurious suggestions, or to leave it as-is. - This doesn't work when it would change the call's other input types. Instead, it could suggest borrowing any others that have the same parameter type (but only when suggesting shared borrows). I think this would be a pretty easy change, but I don't think it's very useful so long as the normalized output type can't change. I'm happy to implement any of these (or other potential improvements to this), but I'm not sure which are common enough patterns to justify the added complexity. Please let me know if any sound worthwhile. |
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| COMPILER_TESTS.md | ||