rustc: catch `impl X for Y` where X is not a trait in resolve.

src/librustc/middle/resolve.rs

@@ -3894,8 +3894,31 @@ impl<'a> Resolver<'a> {
                 self.resolve_error(trait_reference.path.span, msg.as_slice());
             }
             Some(def) => {
-                debug!("(resolving trait) found trait def: {:?}", def);
-                self.record_def(trait_reference.ref_id, def);
+                match def {
+                    (DefTrait(_), _) => {
+                        debug!("(resolving trait) found trait def: {:?}", def);
+                        self.record_def(trait_reference.ref_id, def);
+                    }
+                    (def, _) => {
+                        self.resolve_error(trait_reference.path.span,
+                                           format!("`{}` is not a trait",
+                                                   self.path_idents_to_str(
+                                                        &trait_reference.path)));
+
+                        // If it's a typedef, give a note
+                        match def {
+                            DefTy(_) => {
+                                self.session.span_note(
+                                                trait_reference.path.span,
+                                                format!("`type` aliases cannot \
+                                                        be used for traits")
+                                                        .as_slice());
+                            }
+                            _ => {}
+                        }
+                    }
+                }
+
             }
         }
     }

src/test/compile-fail/issue-3907-2.rs

@@ -0,0 +1,20 @@
+// Copyright 2013-2014 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.
+
+// aux-build:issue_3907.rs
+extern crate issue_3907;
+
+type Foo = issue_3907::Foo; //~ ERROR: reference to trait
+
+struct S {
+    name: int
+}
+
+fn main() {}

src/test/compile-fail/issue-3907.rs

@@ -1,4 +1,4 @@
-// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.
 //
@@ -11,13 +11,14 @@
 // aux-build:issue_3907.rs
 extern crate issue_3907;
 
-type Foo = issue_3907::Foo; //~ ERROR: reference to trait
+type Foo = issue_3907::Foo;
 
 struct S {
     name: int
 }
 
 impl Foo for S { //~ ERROR: `Foo` is not a trait
+    //~^ NOTE: `type` aliases cannot be used for traits
     fn bar() { }
 }
 

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

@@ -31,7 +31,6 @@ impl NewTrait for Point {
 fn main() {
     let p = Point::new(0.0, 0.0);
     //~^ ERROR unresolved name `Point::new`
-    //~^^ ERROR unresolved name
-    //~^^^ ERROR use of undeclared module `Point`
+    //~^^ ERROR failed to resolve. Use of undeclared module `Point`
     println!("{}", p.a());
 }

src/test/compile-fail/issue-5035-2.rs

@@ -0,0 +1,14 @@
+// Copyright 2013-2014 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.
+
+trait I {}
+type K = I; //~ ERROR: reference to trait
+
+fn main() {}

src/test/compile-fail/issue-5035.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 trait I {}
-type K = I; //~ ERROR: reference to trait
+type K = I;
 impl K for int {} //~ ERROR: `K` is not a trait
+//~^ NOTE: `type` aliases cannot be used for traits
 fn main() {}