Implement vtable support for autoderef.

Closes #13042.

src/test/run-pass/overloaded-autoderef-vtable.rs

@@ -0,0 +1,42 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::ops::Deref;
+
+struct DerefWithHelper<H, T> {
+    helper: H
+}
+
+trait Helper<T> {
+    fn helper_borrow<'a>(&'a self) -> &'a T;
+}
+
+impl<T> Helper<T> for Option<T> {
+    fn helper_borrow<'a>(&'a self) -> &'a T {
+        self.as_ref().unwrap()
+    }
+}
+
+impl<T, H: Helper<T>> Deref<T> for DerefWithHelper<H, T> {
+    fn deref<'a>(&'a self) -> &'a T {
+        self.helper.helper_borrow()
+    }
+}
+
+struct Foo {x: int}
+
+impl Foo {
+    fn foo(&self) -> int {self.x}
+}
+
+pub fn main() {
+    let x: DerefWithHelper<Option<Foo>, Foo> = DerefWithHelper { helper: Some(Foo {x: 5}) };
+    assert!(x.foo() == 5);
+}