Add trait name to conflicting implementation error message

Author: davidhalperin

src/librustc/middle/typeck/coherence.rs

@@ -1,4 +1,4 @@
-// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -395,8 +395,11 @@ impl CoherenceChecker {
 
                     if self.polytypes_unify(polytype_a, polytype_b) {
                         let session = self.crate_context.tcx.sess;
-                        session.span_err(self.span_of_impl(implementation_b),
-                                         "conflicting implementations for a trait");
+                        session.span_err(
+                            self.span_of_impl(implementation_b),
+                            fmt!("conflicting implementations for trait %s",
+                                 ty::item_path_str(self.crate_context.tcx,
+                                                   trait_def_id)));
                         session.span_note(self.span_of_impl(implementation_a),
                                           "note conflicting implementation here");
                     }

src/test/compile-fail/conflicting-implementations.rs

@@ -0,0 +1,24 @@
+// 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.
+
+// error-pattern: conflicting implementations for trait Foo
+trait Foo {
+}
+
+impl Foo for int {
+
+}
+
+impl<A> Foo for A {
+
+}
+
+fn main() {
+}