direnv: enable nushell integration

This enables nushell integration by default for direnv, similar to
bash/zsh/fish. The slightly verbose way of setting this is to ensure
that peoples' existing nushell configuration isn't overwritten, only
appended to, as would be the case if we just used the integration
example from the nushell docs:

  https://www.nushell.sh/cookbook/direnv.html

Closes nix-community#3520

modules/programs/direnv.nix

@@ -80,6 +80,15 @@ in {
       '';
     };
 
+    enableNushellIntegration = mkOption {
+      default = true;
+      type = types.bool;
+      readOnly = true;
+      description = ''
+        Whether to enable Nushell integration.
+      '';
+    };
+
     nix-direnv = {
       enable = mkEnableOption ''
         <link
@@ -119,5 +128,21 @@ in {
       mkAfter ''
         ${pkgs.direnv}/bin/direnv hook fish | source
       '');
+
+    programs.nushell.extraConfig = mkIf cfg.enableNushellIntegration (
+      # Using mkAfter to make it more likely to appear after other
+      # manipulations of the prompt.
+      mkAfter ''
+        let-env config = ($env | default {} config).config
+        let-env config = ($env.config | default {} hooks)
+        let-env config = ($env.config | update hooks ($env.config.hooks | default [] pre_prompt))
+        let-env config = ($env.config | update hooks.pre_prompt ($env.config.hooks.pre_prompt | append {
+          code: "
+            let direnv = (${pkgs.direnv}/bin/direnv export json | from json)
+            let direnv = if ($direnv | length) == 1 { $direnv } else { {} }
+            $direnv | load-env
+            "
+        }))
+      '');
   };
 }

tests/modules/programs/direnv/default.nix

@@ -1,6 +1,7 @@
 {
   direnv-bash = ./bash.nix;
   direnv-nix-direnv = ./nix-direnv.nix;
+  direnv-nushell = ./nushell.nix;
   direnv-stdlib = ./stdlib.nix;
   direnv-stdlib-and-nix-direnv = ./stdlib-and-nix-direnv.nix;
 }

tests/modules/programs/direnv/nushell.nix

@@ -0,0 +1,19 @@
+{ pkgs, ... }:
+
+{
+  programs.nushell.enable = true;
+  programs.direnv.enable = true;
+
+  test.stubs.nushell = { };
+
+  nmt.script = let
+    configFile = if pkgs.stdenv.isDarwin then
+      "home-files/Library/Application Support/nushell/config.nu"
+    else
+      "home-files/.config/nushell/config.nu";
+  in ''
+    assertFileExists "${configFile}"
+    assertFileRegex "${configFile}" \
+      'let direnv = (/nix/store/.*direnv.*/bin/direnv export json | from json)'
+  '';
+}