error: failed to find an overloaded call trait for closure call
  --> $DIR/fn-fn_mut-call-ill-formed.rs:39:5
   |
LL |     a();
   |     ^^^
   |
   = help: make sure the `fn`/`fn_mut`/`fn_once` lang items are defined and have correctly defined `call`/`call_mut`/`call_once` methods

error: failed to find an overloaded call trait for closure call
  --> $DIR/fn-fn_mut-call-ill-formed.rs:43:5
   |
LL |     b();
   |     ^^^
   |
   = help: make sure the `fn`/`fn_mut`/`fn_once` lang items are defined and have correctly defined `call`/`call_mut`/`call_once` methods

error: aborting due to 2 previous errors