lib/options: Introduce showDefs

For pretty-printing definitions, including file and values

lib/options.nix

@@ -213,6 +213,24 @@ rec {
          else escaped;
     in (concatStringsSep ".") (map escapeOptionPart parts);
   showFiles = files: concatStringsSep " and " (map (f: "`${f}'") files);
+
+  showDefs = defs: concatMapStrings (def:
+    let
+      # Pretty print the value for display, if successful
+      prettyEval = builtins.tryEval (lib.generators.toPretty {} def.value);
+      # Split it into its lines
+      lines = filter (v: ! isList v) (builtins.split "\n" prettyEval.value);
+      # Only display the first 5 lines, and indent them for better visibility
+      value = concatStringsSep "\n    " (take 5 lines ++ optional (length lines > 5) "...");
+      result =
+        # Don't print any value if evaluating the value strictly fails
+        if ! prettyEval.success then ""
+        # Put it on a new line if it consists of multiple
+        else if length lines > 1 then ":\n    " + value
+        else ": " + value;
+    in "\n- In `${def.file}'${result}"
+  ) defs;
+
   unknownModule = "<unknown-file>";
 
 }