direnv: add support for nix-direnv

PR #1297

modules/programs/direnv.nix

@@ -70,6 +70,11 @@ in {
         Whether to enable Fish integration.
       '';
     };
+
+    enableNixDirenvIntegration = mkEnableOption ''
+      <link
+          xlink:href="https://github.com/nix-community/nix-direnv">nix-direnv</link>,
+          a fast, persistent use_nix implementation for direnv'';
   };
 
   config = mkIf cfg.enable {
@@ -78,8 +83,11 @@ in {
     xdg.configFile."direnv/config.toml" =
       mkIf (cfg.config != { }) { source = configFile cfg.config; };
 
-    xdg.configFile."direnv/direnvrc" =
-      mkIf (cfg.stdlib != "") { text = cfg.stdlib; };
+    xdg.configFile."direnv/direnvrc" = let
+      text = concatStringsSep "\n" (optional (cfg.stdlib != "") cfg.stdlib
+        ++ optional cfg.enableNixDirenvIntegration
+        "source ${pkgs.nix-direnv}/share/nix-direnv/direnvrc");
+    in mkIf (text != "") { inherit text; };
 
     programs.bash.initExtra = mkIf cfg.enableBashIntegration (
       # Using mkAfter to make it more likely to appear after other

tests/modules/programs/direnv/default.nix

@@ -1,4 +1,6 @@
 {
   direnv-bash = ./bash.nix;
+  direnv-nix-direnv = ./nix-direnv.nix;
   direnv-stdlib = ./stdlib.nix;
+  direnv-stdlib-and-nix-direnv = ./stdlib-and-nix-direnv.nix;
 }

tests/modules/programs/direnv/nix-direnv.nix

@@ -0,0 +1,18 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs.bash.enable = true;
+    programs.direnv.enable = true;
+    programs.direnv.enableNixDirenvIntegration = true;
+
+    nmt.script = ''
+      assertFileExists home-files/.bashrc
+      assertFileRegex \
+        home-files/.config/direnv/direnvrc \
+        'source /nix/store/.*nix-direnv.*/share/nix-direnv/direnvrc'
+    '';
+  };
+}

tests/modules/programs/direnv/stdlib-and-nix-direnv.nix

@@ -0,0 +1,23 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let expectedContent = "something important";
+in {
+  config = {
+    programs.bash.enable = true;
+    programs.direnv.enable = true;
+    programs.direnv.enableNixDirenvIntegration = true;
+    programs.direnv.stdlib = expectedContent;
+
+    nmt.script = ''
+      assertFileExists home-files/.bashrc
+      assertFileRegex \
+        home-files/.config/direnv/direnvrc \
+        'source /nix/store/.*nix-direnv.*/share/nix-direnv/direnvrc'
+      assertFileRegex \
+        home-files/.config/direnv/direnvrc \
+        '${expectedContent}'
+    '';
+  };
+}