Add back Res::matches_ns, implemented in terms of ns

src/librustc_ast_lowering/lib.rs

@@ -1145,7 +1145,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 if let TyKind::Path(ref qself, ref path) = ty.kind {
                     if let Some(partial_res) = self.resolver.get_partial_res(ty.id) {
                         let res = partial_res.base_res();
-                        if res.ns().map(|ns| ns != Namespace::TypeNS).unwrap_or(false) {
+                        if !res.matches_ns(Namespace::TypeNS) {
                             debug!(
                                 "lower_generic_arg: Lowering type argument as const argument: {:?}",
                                 ty,

src/librustc_hir/def.rs

@@ -461,4 +461,9 @@ impl<Id> Res<Id> {
             Res::Err => None,
         }
     }
+
+    /// Always returns `true` if `self` is `Res::Err`
+    pub fn matches_ns(&self, ns: Namespace) -> bool {
+        self.ns().map(|actual_ns| actual_ns == ns).unwrap_or(true)
+    }
 }