librustc: Implement simple where clauses.

These `where` clauses are accepted everywhere generics are currently
accepted and desugar during type collection to the type parameter bounds
we have today.

A new keyword, `where`, has been added. Therefore, this is a breaking
change. Change uses of `where` to other identifiers.

[breaking-change]

src/libsyntax/ext/deriving/generic/mod.rs

@@ -390,7 +390,7 @@ impl<'a> TraitDef<'a> {
                            methods: Vec<Gc<ast::Method>> ) -> Gc<ast::Item> {
         let trait_path = self.path.to_path(cx, self.span, type_ident, generics);
 
-        let Generics { mut lifetimes, ty_params } =
+        let Generics { mut lifetimes, ty_params, where_clause: _ } =
             self.generics.to_generics(cx, self.span, type_ident, generics);
         let mut ty_params = ty_params.into_vec();
 
@@ -418,7 +418,11 @@ impl<'a> TraitDef<'a> {
         }));
         let trait_generics = Generics {
             lifetimes: lifetimes,
-            ty_params: OwnedSlice::from_vec(ty_params)
+            ty_params: OwnedSlice::from_vec(ty_params),
+            where_clause: ast::WhereClause {
+                id: ast::DUMMY_NODE_ID,
+                predicates: Vec::new(),
+            },
         };
 
         // Create the reference to the trait.

src/libsyntax/ext/deriving/generic/ty.rs

@@ -202,10 +202,15 @@ fn mk_ty_param(cx: &ExtCtxt, span: Span, name: &str,
     cx.typaram(span, cx.ident_of(name), bounds, unbound, None)
 }
 
-fn mk_generics(lifetimes: Vec<ast::LifetimeDef>, ty_params: Vec<ast::TyParam> ) -> Generics {
+fn mk_generics(lifetimes: Vec<ast::LifetimeDef>, ty_params: Vec<ast::TyParam>)
+               -> Generics {
     Generics {
         lifetimes: lifetimes,
-        ty_params: OwnedSlice::from_vec(ty_params)
+        ty_params: OwnedSlice::from_vec(ty_params),
+        where_clause: ast::WhereClause {
+            id: ast::DUMMY_NODE_ID,
+            predicates: Vec::new(),
+        },
     }
 }