error[E0053]: method `call_once` has an incompatible type for trait
  --> $DIR/issue-15094.rs:21:5
   |
LL |     fn call_once(self, _args: ()) {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected "rust-call" fn, found "Rust" fn
   |
   = note: expected type `extern "rust-call" fn(Debuger<T>, ())`
              found type `fn(Debuger<T>, ())`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0053`.