1 error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnMut`
2 --> $DIR/issue-34349.rs:16:17
4 LL | let diary = || { //~ ERROR E0525
5 | ^^ this closure implements `FnMut`, not `Fn`
6 LL | farewell.push_str("!!!");
7 | -------- closure is `FnMut` because it mutates the variable `farewell` here
10 | ----- the requirement to implement `Fn` derives from here
12 error: aborting due to previous error
14 For more information about this error, try `rustc --explain E0525`.