lib/types: Set deprecationMessage for types.loaOf

lib/types.nix

@@ -329,14 +329,12 @@ rec {
     };
 
     # TODO: drop this in the future:
-    loaOf =
-      let msg =
-        ''
-          `types.loaOf` has been removed and mixing lists with attribute values
-          is no longer possible; please use `types.attrsOf` instead.
-          See https://github.com/NixOS/nixpkgs/issues/1800 for the motivation.
-        '';
-      in builtins.trace msg types.attrsOf;
+    loaOf = elemType: types.attrsOf elemType // {
+      name = "loaOf";
+      deprecationMessage = "Mixing lists with attribute values is no longer"
+        + " possible; please use `types.attrsOf` instead. See"
+        + " https://github.com/NixOS/nixpkgs/issues/1800 for the motivation.";
+    };
 
     # Value of given type but with no merging (i.e. `uniq list`s are not concatenated).
     uniq = elemType: mkOptionType rec {