In ty::impl_traits, treat structs properly

Treat structs just like impls: use their associated list of
trait refs to get the list of traits that one of them implements.
I don't understand what was happening before, but it was wrong.

Closes #2936

src/rustc/middle/ty.rs

@@ -2826,22 +2826,11 @@ fn impl_traits(cx: ctxt, id: ast::def_id) -> ~[t] {
                     node_id_to_type(cx, trait_ref.ref_id)
                 }
            }
-           Some(ast_map::node_item(@{node: ast::item_class(*),
+           Some(ast_map::node_item(@{node: ast::item_class(sd,_),
                            _},_)) => {
-             match cx.def_map.find(id.node) {
-               Some(def_ty(trait_id)) => {
-                   // XXX: Doesn't work cross-crate.
-                   debug!("(impl_traits) found trait id %?", trait_id);
-                   ~[node_id_to_type(cx, trait_id.node)]
-               }
-               Some(x) => {
-                 cx.sess.bug(fmt!("impl_traits: trait ref is in trait map \
-                                   but is bound to %?", x));
-               }
-               None => {
-                 ~[]
-               }
-             }
+               do vec::map(sd.traits) |trait_ref| {
+                    node_id_to_type(cx, trait_ref.ref_id)
+                }
            }
            _ => ~[]
         }

src/test/run-pass/issue-2936.rs

@@ -0,0 +1,20 @@
+trait bar<T> {
+    fn get_bar() -> T;
+}
+
+fn foo<T, U: bar<T>>(b: U) -> T {
+    b.get_bar()
+}
+
+struct cbar : bar<int> {
+    x: int;
+    new(x: int) { self.x = x; }
+    fn get_bar() -> int {
+        self.x
+    }
+}
+
+fn main() {
+    let x: int = foo::<int, cbar>(cbar(5));
+    assert x == 5;
+}