Add test for short-circuiting #[deriving(Eq,Ord,TotalEq,TotalOrd)].

src/test/run-pass/deriving-cmp-shortcircuit.rs

+// Copyright 2013 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.
+
+// check that the derived impls for the comparison traits shortcircuit
+// where possible, by having a type that fails when compared as the
+// second element, so this passes iff the instances shortcircuit.
+
+pub struct FailCmp;
+impl Eq for FailCmp {
+    fn eq(&self, _: &FailCmp) -> bool { fail!("eq") }
+}
+
+impl Ord for FailCmp {
+    fn lt(&self, _: &FailCmp) -> bool { fail!("lt") }
+}
+
+impl TotalEq for FailCmp {
+    fn equals(&self, _: &FailCmp) -> bool { fail!("equals") }
+}
+
+impl TotalOrd for FailCmp {
+    fn cmp(&self, _: &FailCmp) -> Ordering { fail!("cmp") }
+}
+
+#[deriving(Eq,Ord,TotalEq,TotalOrd)]
+struct ShortCircuit {
+    x: int,
+    y: FailCmp
+}
+
+fn main() {
+    let a = ShortCircuit { x: 1, y: FailCmp };
+    let b = ShortCircuit { x: 2, y: FailCmp };
+
+    assert!(a != b);
+    assert!(a < b);
+    assert!(!a.equals(&b));
+    assert_eq!(a.cmp(&b), ::std::cmp::Less);
+}