1 files changed, 18 insertions, 1 deletions
diff --git a/lib/types.nix b/lib/types.nix
index f47a1f92de7..a0be2ff3a45 100644
--- a/lib/types.nix
+++ b/lib/types.nix
@@ -287,6 +287,13 @@ rec {
       merge = mergeEqualOption;
     };
 
+    nonEmptyStr = mkOptionType {
+      name = "nonEmptyStr";
+      description = "non-empty string";
+      check = x: str.check x && builtins.match "[ \t\n]*" x == null;
+      inherit (str) merge;
+    };
+
     strMatching = pattern: mkOptionType {
       name = "strMatching ${escapeNixString pattern}";
       description = "string matching the pattern ${pattern}";
@@ -581,7 +588,17 @@ rec {
       in
       mkOptionType rec {
         name = "enum";
-        description = "one of ${concatMapStringsSep ", " show values}";
+        description =
+          # Length 0 or 1 enums may occur in a design pattern with type merging
+          # where an "interface" module declares an empty enum and other modules
+          # provide implementations, each extending the enum with their own
+          # identifier.
+          if values == [] then
+            "impossible (empty enum)"
+          else if builtins.length values == 1 then
+            "value ${show (builtins.head values)} (singular enum)"
+          else
+            "one of ${concatMapStringsSep ", " show values}";
         check = flip elem values;
         merge = mergeEqualOption;
         functor = (defaultFunctor name) // { payload = values; binOp = a: b: unique (a ++ b); };