error[E0599]: no method named `f` found for fn pointer `fn(&u8)` in the current scope
  --> $DIR/issue-57362-1.rs:20:7
   |
LL |     a.f();
   |       ^ method not found in `fn(&u8)`
   |
   = note: a is a function, perhaps you wish to call it
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `f`, perhaps you need to implement it:
           candidate #1: `Trait`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0599`.