fish: keep all fish-completions packages around

...even empty ones.

For packages in `home.packages` with no Fish completions, we build an
empty directory that is not included in our runtime closure (because we
build a collection of symlinks to their contents: if there are no
contents, there is nothing to symlink). This means these empty
completion packages get garbage-collected by Nix. They are not that
expensive to rebuild but there can be enough of them it adds up, and any
change to any package in `home.packages` is enough to trigger a rebuild.

Fix this by forcing a dependency on all of them. Since we already
depended on them anyway if they were non-empty, this only adds
dependencies on empty directories (so should not significantly affect
storage space).

Fixes #6157.

modules/programs/fish.nix

@@ -434,7 +434,43 @@ in
     lib.mkMerge [
       { home.packages = [ cfg.package ]; }
 
-      (mkIf cfg.generateCompletions {
+      (mkIf cfg.generateCompletions (
+        let
+          generateCompletions =
+            let
+              getName =
+                attrs: attrs.name or "${attrs.pname or "«pname-missing»"}-${attrs.version or "«version-missing»"}";
+            in
+            package:
+            pkgs.runCommand "${getName package}-fish-completions"
+              {
+                srcs =
+                  [ package ]
+                  ++ lib.filter (p: p != null) (
+                    builtins.map (outName: package.${outName} or null) config.home.extraOutputsToInstall
+                  );
+                nativeBuildInputs = [ pkgs.python3 ];
+                buildInputs = [ cfg.package ];
+                preferLocalBuild = true;
+              }
+              ''
+                mkdir -p $out
+                for src in $srcs; do
+                  if [ -d $src/share/man ]; then
+                    find -L $src/share/man -type f \
+                      -exec python ${cfg.package}/share/fish/tools/create_manpage_completions.py --directory $out {} + \
+                      > /dev/null
+                  fi
+                done
+              '';
+
+          allCompletions =
+            let
+              cmp = (a: b: (a.meta.priority or 0) > (b.meta.priority or 0));
+            in
+            map generateCompletions (lib.sort cmp config.home.packages);
+        in
+        {
           # Support completion for `man` by building a cache for `apropos`.
           programs.man.generateCaches = lib.mkDefault true;
 
@@ -471,43 +507,17 @@ in
                   ${postBuild}
                 '';
 
-            generateCompletions =
-              let
-                getName =
-                  attrs: attrs.name or "${attrs.pname or "«pname-missing»"}-${attrs.version or "«version-missing»"}";
-              in
-              package:
-              pkgs.runCommand "${getName package}-fish-completions"
-                {
-                  srcs =
-                    [ package ]
-                    ++ lib.filter (p: p != null) (
-                      builtins.map (outName: package.${outName} or null) config.home.extraOutputsToInstall
-                    );
-                  nativeBuildInputs = [ pkgs.python3 ];
-                  buildInputs = [ cfg.package ];
-                  preferLocalBuild = true;
-                }
-                ''
-                  mkdir -p $out
-                  for src in $srcs; do
-                    if [ -d $src/share/man ]; then
-                      find -L $src/share/man -type f \
-                        -exec python ${cfg.package}/share/fish/tools/create_manpage_completions.py --directory $out {} + \
-                        > /dev/null
-                    fi
-                  done
-                '';
             in
             destructiveSymlinkJoin {
               name = "${config.home.username}-fish-completions";
-            paths =
+              paths = allCompletions;
-              let
-                cmp = (a: b: (a.meta.priority or 0) > (b.meta.priority or 0));
-              in
-              map generateCompletions (lib.sort cmp config.home.packages);
             };
 
+          # For packages with no Fish completions, generateCompletions will build an empty directory,
+          # which means they will not be in our runtime closure. Force a dependency so these do not get
+          # constantly rebuilt.
+          home.extraDependencies = allCompletions;
+
           programs.fish.interactiveShellInit = ''
             # add completions generated by Home Manager to $fish_complete_path
             begin
@@ -519,7 +529,8 @@ in
               set fish_complete_path $prev "${config.xdg.dataHome}/fish/home-manager_generated_completions" $post
             end
           '';
-      })
+        }
+      ))
 
       {
         xdg.configFile."fish/config.fish".source = fishIndent "config.fish" ''