Make the error messages that result from referencing

nonexistent traits consistent, and add a test.

src/test/compile-fail/resolve-unknown-trait.rs

@@ -0,0 +1,20 @@
+// 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.
+
+
+trait NewTrait : SomeNonExistentTrait {}
+//~^ ERROR attempt to derive a nonexistent trait `SomeNonExistentTrait`
+
+impl SomeNonExistentTrait for int {}
+//~^ ERROR attempt to implement a nonexistent trait `SomeNonExistentTrait`
+
+fn f<T:SomeNonExistentTrait>() {}
+//~^ ERROR attempt to bound type parameter to a nonexistent trait `SomeNonExistentTrait`
+