error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnMut`
  --> $DIR/issue-34349.rs:16:17
   |
LL |     let diary = || {
   |                 ^^ this closure implements `FnMut`, not `Fn`
LL |         farewell.push_str("!!!");
   |         -------- closure is `FnMut` because it mutates the variable `farewell` here
...
LL |     apply(diary);
   |     ----- the requirement to implement `Fn` derives from here

error: aborting due to previous error

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