Add xfailed test for #3973

src/test/compile-fail/issue-3973.rs

+// Copyright 2012 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.
+
+// xfail-test
+
+struct Point {
+    mut x: float,
+    mut y: float,
+}
+
+impl Point : ToStr { //~ ERROR implements a method not defined in the trait
+    static fn new(x: float, y: float) -> Point {
+        Point { x: x, y: y }
+    }
+
+    pure fn to_str() -> ~str {
+        fmt!("(%f, %f)", self.x, self.y)
+    }
+}
+
+fn main() {
+    let p = Point::new(0.0f, 0.0f);
+    io::println(p.to_str());
+}