deref_trait.rs

use std::ops::Deref;

trait Foo { fn foo(&self); }

struct Bar { }
impl Foo for Bar {
  fn foo(&self) { }
}

fn baz<T: Foo> (_: &T) { }

struct WrapBar { wbar: Bar }

impl Deref for WrapBar {
  type Target = Bar;
  fn deref(&self) -> &Self::Target { &self.wbar }
}

fn main() {
  let b = WrapBar { wbar: Bar {} };
  // explicit call via &Bar
  &b.wbar.foo();
  // implicit Deref to &Bar before call
  &b.foo();
  // succeeds because WrapBar can be Deref'd to Bar because we are aware of baz::<Bar> having the signature &Bar -> ()
  baz::<Bar>(&b);
  // fails because WrapBar does not implement Foo because baz has signature &(T: Foo) -> ()
  // baz(&b);
}