fish: autogenerate completions from man pages

modules/programs/fish.nix

@@ -21,6 +21,15 @@ in
     programs.fish = {
       enable = mkEnableOption "fish friendly interactive shell";
 
+      package = mkOption {
+        default = pkgs.fish;
+        defaultText = "pkgs.fish";
+        description = ''
+          The fish package to install. May be used to change the version.
+        '';
+        type = types.package;
+      };
+
       shellAliases = mkOption {
         default = {};
         description = ''
@@ -74,7 +83,70 @@ in
   };
 
   config = mkIf cfg.enable {
-    home.packages = [ pkgs.fish ];
+    home.packages = [ cfg.package ];
+
+    xdg.dataFile."fish/home-manager_generated_completions".source =
+      let
+        # paths later in the list will overwrite those already linked
+        destructiveSymlinkJoin =
+          args_@{ name
+              , paths
+              , preferLocalBuild ? true
+              , allowSubstitutes ? false
+              , postBuild ? ""
+              , ...
+              }:
+          let
+            args = removeAttrs args_ [ "name" "postBuild" ]
+              // { inherit preferLocalBuild allowSubstitutes; }; # pass the defaults
+          in pkgs.runCommand name args
+            ''
+              mkdir -p $out
+              for i in $paths; do
+                if [ -z "$(find $i -prune -empty)" ]; then
+                  cp -srf $i/* $out
+                fi
+              done
+              ${postBuild}
+            '';
+        generateCompletions = package: pkgs.runCommand
+          "${package.name}-fish-completions"
+          {
+            src = package;
+            nativeBuildInputs = [ pkgs.python2 ];
+            buildInputs = [ cfg.package ];
+            preferLocalBuild = true;
+            allowSubstitutes = false;
+          }
+          ''
+            mkdir -p $out
+            if [ -d $src/share/man ]; then
+              find $src/share/man -type f \
+                | xargs python ${cfg.package}/share/fish/tools/create_manpage_completions.py --directory $out \
+                > /dev/null
+            fi
+          '';
+      in
+        destructiveSymlinkJoin {
+          name = "${config.home.username}-fish-completions";
+          paths =
+            let
+              cmp = (a: b: (a.meta.priority or 0) > (b.meta.priority or 0));
+            in
+              map generateCompletions (sort cmp config.home.packages);
+        };
+
+    programs.fish.interactiveShellInit = ''
+      # add completions generated by Home Manager to $fish_complete_path
+      begin
+        set -l joined (string join " " $fish_complete_path)
+        set -l prev_joined (string replace --regex "[^\s]*generated_completions.*" "" $joined)
+        set -l post_joined (string replace $prev_joined "" $joined)
+        set -l prev (string split " " (string trim $prev_joined))
+        set -l post (string split " " (string trim $post_joined))
+        set fish_complete_path $prev "${config.xdg.dataHome}/fish/home-manager_generated_completions" $post
+      end
+    '';
 
     xdg.configFile."fish/config.fish".text = ''
       # ~/.config/fish/config.fish: DO NOT EDIT -- this file has been generated automatically.