Only suggest reborrowing a moved value where the reborrow is valid#157196
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Only suggest reborrowing a moved value where the reborrow is valid#157196onehr wants to merge 1 commit into
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When a `&mut`-typed value is moved out of a closure via a `for` loop's implicit `into_iter`, borrowck suggests "consider creating a fresh reborrow" (`&mut *place`) with `Applicability::MachineApplicable`. For a value captured by an `Fn` closure -- which holds its captures through `&self` -- that place cannot be borrowed mutably (E0596), so applying the machine-applicable suggestion produces code that does not compile. Gate the suggestion on borrowck's own `is_mutable` check for the reborrowed place, so it is only offered where `&mut *place` is valid: suppressed for `Fn`-closure captures, still offered for `FnMut` closures and ordinary `&mut` locals.
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Fixes #150175.
Moving a
&mut-typed value out of a closure (e.g.for foo in foos, whose implicitinto_iterconsumesfoos) reportsE0507. borrowck then suggests reborrowing the move site as&mut *foos, markedApplicability::MachineApplicable— but anFnclosure only holds its captures through&self, so the reborrow does not compile:The previous code only checked that the moved value was a
&mut, not whether it could actually be reborrowed mutably in the current context. This gates the suggestion on borrowck's ownis_mutablecheck for the reborrowed place (*moved_place) — the same predicate that decides whether&mut *moved_placewould emitE0596— so the suggestion is offered exactly when it compiles:Fn-closure capture (reached through&self) → suppressed;FnMut-closure capture (reached through&mut self) → still offered (the reborrow is valid there);&mutlocal/param → still offered (unchanged).The regression test exercises both the
Fncase (no suggestion) and theFnMutcase (suggestion kept), pinning down the precise behavior.