error[E0499]: cannot borrow `*self` as mutable more than once at a time
  --> $DIR/unboxed-closures-recursive-fn-using-fn-mut.rs:22:21
   |
LL |         (self.func)(self, arg)
   |         ----------- ^^^^ second mutable borrow occurs here
   |         |
   |         first mutable borrow occurs here
   |         first borrow later used by call

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0499`.