nixos/fish: generate autocompletions from man pages

nixos/modules/programs/fish.nix

@@ -169,6 +169,59 @@ in
       end
     '';
 
+    programs.fish.interactiveShellInit = ''
+      # add completions generated by NixOS to $fish_complete_path
+      begin
+        # joins with null byte to acommodate all characters in paths, then respectively gets all paths before (exclusive) / after (inclusive) the first one including "generated_completions",
+        # splits by null byte, and then removes all empty lines produced by using 'string'
+        set -l prev (string join0 $fish_complete_path | string match --regex "^.*?(?=\x00[^\x00]*generated_completions.*)" | string split0 | string match -er ".")
+        set -l post (string join0 $fish_complete_path | string match --regex "[^\x00]*generated_completions.*" | string split0 | string match -er ".")
+        set fish_complete_path $prev "/etc/fish/generated_completions" $post
+      end
+    '';
+
+    environment.etc."fish/generated_completions".source =
+      let
+        patchedGenerator = pkgs.stdenv.mkDerivation {
+          name = "fish_patched-completion-generator";
+          srcs = [
+            "${pkgs.fish}/share/fish/tools/create_manpage_completions.py"
+            "${pkgs.fish}/share/fish/tools/deroff.py"
+          ];
+          unpackCmd = "cp $curSrc $(basename $curSrc)";
+          sourceRoot = ".";
+          patches = [ ./fish_completion-generator.patch ]; # to prevent collisions of identical completion files
+          dontBuild = true;
+          installPhase = ''
+            mkdir -p $out
+            cp * $out/
+          '';
+          preferLocalBuild = true;
+          allowSubstitutes = false;
+        };
+        generateCompletions = package: pkgs.runCommand
+          "${package.name}_fish-completions"
+          (
+            {
+              inherit package;
+              preferLocalBuild = true;
+              allowSubstitutes = false;
+            }
+            // optionalAttrs (package ? meta.priority) { meta.priority = package.meta.priority; }
+          )
+          ''
+            mkdir -p $out
+            if [ -d $package/share/man ]; then
+              find $package/share/man -type f | xargs ${pkgs.python3.interpreter} ${patchedGenerator}/create_manpage_completions.py --directory $out >/dev/null
+            fi
+          '';
+      in
+        pkgs.buildEnv {
+          name = "system_fish-completions";
+          ignoreCollisions = true;
+          paths = map generateCompletions config.environment.systemPackages;
+        };
+
     # include programs that bring their own completions
     environment.pathsToLink = []
       ++ optional cfg.vendor.config.enable "/share/fish/vendor_conf.d"