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error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnMut`
--> $DIR/issue-26046-fn-mut.rs:4:19
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LL | let closure = || {
| ^^ this closure implements `FnMut`, not `Fn`
LL | num += 1;
| --- closure is `FnMut` because it mutates the variable `num` here
...
LL | Box::new(closure)
| ----------------- the requirement to implement `Fn` derives from here
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= note: required for the cast from `Box<{closure@$DIR/issue-26046-fn-mut.rs:4:19: 4:21}>` to `Box<(dyn Fn() + 'static)>`
error: aborting due to 1 previous error
For more information about this error, try `rustc --explain E0525`.
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