files: support recursive linking of directory

modules/files.nix

@@ -186,6 +186,8 @@ in
     home-files = pkgs.stdenv.mkDerivation {
       name = "home-manager-files";
 
+      nativeBuildInputs = [ pkgs.xlibs.lndir ];
+
       # Symlink directories and files that have the right execute bit.
       # Copy files that need their execute bit changed or use the
       # deprecated 'mode' option.
@@ -197,6 +199,7 @@ in
           local relTarget="$2"
           local executable="$3"
           local mode="$4"     # For backwards compatibility.
+          local recursive="$5"
 
           # Figure out the real absolute path to the target.
           local target
@@ -210,7 +213,12 @@ in
 
           mkdir -p "$(dirname "$target")"
           if [[ -d $source ]]; then
-            ln -s "$source" "$target"
+            if [[ $recursive ]]; then
+              mkdir -p "$target"
+              lndir -silent "$source" "$target"
+            else
+              ln -s "$source" "$target"
+            fi
           elif [[ $mode ]]; then
             install -m "$mode" "$source" "$target"
           else
@@ -234,7 +242,8 @@ in
                      "${if v.executable == null
                         then "symlink"
                         else builtins.toString v.executable}" \
-                     "${builtins.toString v.mode}"
+                     "${builtins.toString v.mode}" \
+                     "${builtins.toString v.recursive}"
         '') cfg
       );
     };

modules/lib/file-type.nix

@@ -89,6 +89,23 @@ in
             for files created through the <varname>text</varname> option.
           '';
         };
+
+        recursive = mkOption {
+          type = types.bool;
+          default = false;
+          description = ''
+            If the file source is a directory, then this option
+            determines whether the directory should be recursively
+            linked to the target location. This option has no effect
+            if the source is a file.
+            </para><para>
+            If <literal>false</literal> (the default) then the target
+            will be a symbolic link to the source directory. If
+            <literal>true</literal> then the target will be a
+            directory structure matching the source's but whose leafs
+            are symbolic links to the files of the source directory.
+          '';
+        };
       };
 
       config = {