diagnostics: Suggest fn binding type for unstable closure for<> binders - #160478
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The parser was modified, potentially altering the grammar of (stable) Rust cc @fmease |
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When `for<'a>` appears on a closure without `closure_lifetime_binder`, suggest rewriting a simple let-bound closure to a stable `for<'a> fn(...)` binding type annotation when that rewrite is safe. Co-authored-by: Cursor <cursoragent@cursor.com>
Cover MachineApplicable rewrites, MaybeIncorrect capture cases, and macro-expanded binders where structured suggestions must be suppressed. Co-authored-by: Cursor <cursoragent@cursor.com>
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Fixes #160431
for<'a> |x: &'a T| -> U { ... }currently just says "consider removing for<...>", which idk is kinda useless when someone landed there while fighting HRTB errors. This rewrites the obvious case tolet cl: for<'a> fn(...) = |x| { ... }instead.Visitor emits the gate so we have AST context for the rewrite, and we still keep a pre-expansion fallback for
#[cfg(false)]etc. MachineApplicable only when we're pretty sure: by-value bindings, lifetime-only unbounded binders, no_in the sig, no macro expansion. Capture checking is a conservative single-segment free-path heuristic since this runs pre-resolution/pre-typeck; when that looks uncertain we fall back to MaybeIncorrect (or just "consider removing"). imo that's the right tradeoff here: better a maybe-wrong help than rustfix auto-applying into E0308. fyi free-fn /None/Somecases also hit MaybeIncorrect for the same reason.ltm if the heuristic feels too conservative and y'all want it tightened later.