librustc: Add an intrinsic to retrieve the return pointer of a function.

This is needed for some GC stuff in Servo.

src/test/run-pass/intrinsic-return-address.rs

@@ -0,0 +1,43 @@
+// Copyright 2012-2014 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.
+
+#![feature(intrinsics)];
+
+use std::ptr;
+
+struct Point {
+    x: f32,
+    y: f32,
+    z: f32,
+}
+
+extern "rust-intrinsic" {
+    fn return_address() -> *const u8;
+}
+
+fn f(result: &mut uint) -> Point {
+    unsafe {
+        *result = return_address() as uint;
+        Point {
+            x: 1.0,
+            y: 2.0,
+            z: 3.0,
+        }
+    }
+
+}
+
+fn main() {
+    let mut intrinsic_reported_address = 0;
+    let pt = f(&mut intrinsic_reported_address);
+    let actual_address = &pt as *const Point as uint;
+    assert_eq!(intrinsic_reported_address, actual_address);
+}
+