Add assertions and warnings to the pre-commit module

modules/pre-commit.nix

@@ -33,23 +33,25 @@ let
     mapAttrsToList (id: value: value.raw // { inherit id; }) enabledHooks;
 
   configFile =
-    runCommand "pre-commit-config.json"
-      {
-        buildInputs = [
-          pkgs.jq
-          # needs to be an input so we regenerate the config and reinstall the hooks
-          # when the package changes
-          cfg.package
-        ];
-        passAsFile = [ "rawJSON" ];
-        rawJSON = builtins.toJSON cfg.rawConfig;
-      } ''
-      {
-        echo '# DO NOT MODIFY';
-        echo '# This file was generated by pre-commit-hooks.nix';
-        jq . <"$rawJSONPath"
-      } >$out
-    '';
+    performAssertions (
+      runCommand "pre-commit-config.json"
+        {
+          buildInputs = [
+            pkgs.jq
+            # needs to be an input so we regenerate the config and reinstall the hooks
+            # when the package changes
+            cfg.package
+          ];
+          passAsFile = [ "rawJSON" ];
+          rawJSON = builtins.toJSON cfg.rawConfig;
+        } ''
+        {
+          echo '# DO NOT MODIFY';
+          echo '# This file was generated by pre-commit-hooks.nix';
+          jq . <"$rawJSONPath"
+        } >$out
+      ''
+    );
 
   run =
     runCommand "pre-commit-run" { buildInputs = [ git ]; } ''
@@ -80,6 +82,24 @@ let
       touch $out
       [ $? -eq 0 ] && exit $exitcode
     '';
+
+  failedAssertions = builtins.map (x: x.message) (builtins.filter (x: !x.assertion) config.assertions);
+
+  performAssertions =
+    let
+      formatAssertionMessage = message:
+        let
+          lines = lib.splitString "\n" message;
+        in
+        "- ${lib.concatStringsSep "\n  " lines}";
+    in
+    if failedAssertions != [ ]
+    then
+      throw ''
+        Failed assertions:
+        ${lib.concatStringsSep "\n" (builtins.map formatAssertionMessage failedAssertions)}
+      ''
+    else lib.trivial.showWarnings config.warnings;
 in
 {
   options =
@@ -243,6 +263,30 @@ in
             '';
           internal = true;
         };
+
+      assertions = lib.mkOption {
+        type = types.listOf types.unspecified;
+        internal = true;
+        default = [ ];
+        example = [{ assertion = false; message = "you can't enable this for that reason"; }];
+        description = ''
+          This option allows modules to express conditions that must
+          hold for the evaluation of the configuration to succeed,
+          along with associated error messages for the user.
+        '';
+      };
+
+      warnings = lib.mkOption {
+        type = types.listOf types.str;
+        internal = true;
+        default = [ ];
+        example = [ "you should fix this or that" ];
+        description = ''
+          This option allows modules to express warnings about the
+          configuration. For example, `lib.mkRenamedOptionModule` uses this to
+          display a warning message when a renamed option is used.
+        '';
+      };
     };
 
   config =