helix: assert languages migration warning

modules/programs/helix.nix

@@ -1,6 +1,7 @@
 {
   config,
   lib,
+  options,
   pkgs,
   ...
 }:
@@ -85,17 +86,9 @@ in
     languages = mkOption {
       type =
         with types;
-        coercedTo (listOf tomlFormat.type) (
-          language:
-          lib.warn ''
-            The syntax of programs.helix.languages has changed.
-            It now generates the whole languages.toml file instead of just the language array in that file.
-
-            Use
-            programs.helix.languages = { language = <languages list>; }
-            instead.
-          '' { inherit language; }
-        ) (addCheck tomlFormat.type builtins.isAttrs);
+        coercedTo (listOf tomlFormat.type) (language: { inherit language; }) (
+          addCheck tomlFormat.type builtins.isAttrs
+        );
       default = { };
       example = literalExpression ''
         {
@@ -210,6 +203,24 @@ in
   };
 
   config = mkIf cfg.enable {
+    warnings = lib.optional (lib.any builtins.isList options.programs.helix.languages.definitions) (
+      lib.hm.deprecations.mkDeprecatedOptionValueWarning {
+        option = [
+          "programs"
+          "helix"
+          "languages"
+        ];
+        old = "a list";
+        replacement = "`programs.helix.languages.language`";
+        details = ''
+          This option now generates the whole languages.toml file instead of just the language array in that file.
+
+          Use:
+            programs.helix.languages = { language = <languages list>; }
+        '';
+      }
+    );
+
     home.packages =
       if cfg.extraPackages != [ ] then
         [

tests/modules/programs/helix/default.nix

@@ -1,4 +1,5 @@
 {
   helix-example-settings = ./example-settings.nix;
+  helix-legacy-languages = ./legacy-languages.nix;
   helix-only-extraconfig = ./only-extraconfig.nix;
 }

tests/modules/programs/helix/legacy-languages.nix

@@ -0,0 +1,57 @@
+{
+  config,
+  pkgs,
+  ...
+}:
+
+{
+  # Exercise merging legacy list definitions with new attrset definitions.
+  imports = [
+    {
+      programs.helix.languages.language-server.nil = {
+        command = "nil";
+      };
+    }
+  ];
+
+  programs.helix = {
+    enable = true;
+    package = config.lib.test.mkStubPackage { outPath = "@helix@"; };
+    languages = [
+      {
+        name = "rust";
+        auto-format = false;
+      }
+    ];
+  };
+
+  test.asserts.warnings.expected = [
+    ''
+      Using `programs.helix.languages` as a list is deprecated and will be
+      removed in a future release. Please use `programs.helix.languages.language` instead.
+
+      This option now generates the whole languages.toml file instead of just the language array in that file.
+
+      Use:
+        programs.helix.languages = { language = <languages list>; }
+
+    ''
+  ];
+
+  nmt.script =
+    let
+      expectedLanguages = pkgs.writeText "helix-languages.expected.toml" ''
+        [[language]]
+        auto-format = false
+        name = "rust"
+
+        [language-server.nil]
+        command = "nil"
+      '';
+    in
+    ''
+      assertFileContent \
+        home-files/.config/helix/languages.toml \
+        ${expectedLanguages}
+    '';
+}