readline: optionally place config file in XDG dir

Based on PR #4189

modules/home-environment.nix

@@ -473,6 +473,12 @@ in
         will be printed when the user configuration is being built.
       '';
     };
+
+    home.preferXdgDirectories = mkEnableOption "" // {
+    description = ''
+      Whether to make programs use XDG directories whenever supported.
+    '';
+};
   };
 
   config = {

modules/programs/readline.nix

@@ -68,8 +68,8 @@ in {
     };
   };
 
-  config = mkIf cfg.enable {
-    home.file.".inputrc".text = let
+  config = mkIf cfg.enable (let
+    finalConfig = let
       configStr = concatStringsSep "\n"
         (optional cfg.includeSystemConfig "$include /etc/inputrc"
           ++ mapAttrsToList mkSetVariableStr cfg.variables
@@ -80,5 +80,13 @@ in {
       ${configStr}
       ${cfg.extraConfig}
     '';
-  };
+  in mkMerge [
+    (mkIf (!config.home.preferXdgDirectories) {
+      home.file.".inputrc".text = finalConfig;
+    })
+    (mkIf config.home.preferXdgDirectories {
+      xdg.configFile.inputrc.text = finalConfig;
+      home.sessionVariables.INPUTRC = "${config.xdg.configHome}/inputrc";
+    })
+  ]);
 }

tests/modules/programs/readline/default.nix

@@ -1 +1,4 @@
-{ readline-using-all-options = ./using-all-options.nix; }
+{
+  readline-using-all-options = ./using-all-options.nix;
+  readline-prefer-xdg-dirs = ./prefer-xdg-dirs.nix;
+}

tests/modules/programs/readline/prefer-xdg-dirs.nix

@@ -0,0 +1,28 @@
+{ ... }:
+
+{
+  home.preferXdgDirectories = true;
+
+  programs.readline = {
+    enable = true;
+    variables.bell-style = "audible";
+  };
+
+  nmt.script = ''
+    assertFileContent \
+      home-files/.config/inputrc \
+      ${
+        builtins.toFile "readline-expected" ''
+          # Generated by Home Manager.
+
+          $include /etc/inputrc
+          set bell-style audible
+
+        ''
+      }
+
+    assertFileContains \
+      home-path/etc/profile.d/hm-session-vars.sh \
+      'export INPUTRC="/home/hm-user/.config/inputrc"'
+  '';
+}