error[E0596]: cannot borrow `f` as mutable, as it is not declared as mutable
  --> $DIR/issue-84044-drop-non-mut.rs:5:10
   |
LL |     let f = || {};
   |         - help: consider changing this to be mutable: `mut f`
LL |     drop(&mut f);
   |          ^^^^^^ cannot borrow as mutable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.