error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnMut`
  --> $DIR/issue-26046-fn-mut.rs:4:19
   |
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

error: aborting due to previous error

For more information about this error, try `rustc --explain E0525`.