fish: use babelfish for `hm-session-vars.sh` (#4012)

* home-environment: add `home.sessionVariablesPackage`

Allow the `hm-session-vars.sh` derivation to be referenced from other
modules, e.g. to translate it to fish with babelfish at build time.

* fish: use babelfish for `hm-session-vars.sh`

Translate `hm-session-vars.sh` to fish at system build time,
significantly decreasing shell startup time.

Based on https://github.com/NixOS/nixpkgs/pull/108947 by @kevingriffin.

docs/release-notes/rl-2305.adoc

@@ -38,6 +38,14 @@ with Nix flakes uses this new command.
 The standard installation method remains the same but uses the new command internally.
 See <<sec-flakes-standalone>> for more.
 
+* When using <<opt-programs.fish.enable>>, the setup code
+for <<opt-home.sessionVariables>> is now translated
+with https://github.com/bouk/babelfish[babelfish].
+This should result in significantly faster shell startup times
+but could theoretically break
+if you have very complex bash expressions in a session variable.
+Please report any issues you experience.
+
 [[sec-release-23.05-state-version-changes]]
 === State Version Changes
 

modules/home-environment.nix

@@ -291,6 +291,15 @@ in
       '';
     };
 
+    home.sessionVariablesPackage = mkOption {
+      type = types.package;
+      internal = true;
+      description = ''
+        The package containing the
+        <filename>hm-session-vars.sh</filename> file.
+      '';
+    };
+
     home.sessionPath = mkOption {
       type = with types; listOf str;
       default = [ ];
@@ -544,24 +553,22 @@ in
         //
         (maybeSet "LC_MEASUREMENT" cfg.language.measurement);
 
-    home.packages = [
-      # Provide a file holding all session variables.
-      (
-        pkgs.writeTextFile {
-          name = "hm-session-vars.sh";
-          destination = "/etc/profile.d/hm-session-vars.sh";
-          text = ''
-            # Only source this once.
-            if [ -n "$__HM_SESS_VARS_SOURCED" ]; then return; fi
-            export __HM_SESS_VARS_SOURCED=1
+    # Provide a file holding all session variables.
+    home.sessionVariablesPackage = pkgs.writeTextFile {
+      name = "hm-session-vars.sh";
+      destination = "/etc/profile.d/hm-session-vars.sh";
+      text = ''
+        # Only source this once.
+        if [ -n "$__HM_SESS_VARS_SOURCED" ]; then return; fi
+        export __HM_SESS_VARS_SOURCED=1
 
-            ${config.lib.shell.exportAll cfg.sessionVariables}
-          '' + lib.optionalString (cfg.sessionPath != [ ]) ''
-            export PATH="$PATH''${PATH:+:}${concatStringsSep ":" cfg.sessionPath}"
-          '' + cfg.sessionVariablesExtra;
-        }
-      )
-    ];
+        ${config.lib.shell.exportAll cfg.sessionVariables}
+      '' + lib.optionalString (cfg.sessionPath != [ ]) ''
+        export PATH="$PATH''${PATH:+:}${concatStringsSep ":" cfg.sessionPath}"
+      '' + cfg.sessionVariablesExtra;
+    };
+
+    home.packages = [ config.home.sessionVariablesPackage ];
 
     # A dummy entry acting as a boundary between the activation
     # script's "check" and the "write" phases.

modules/programs/fish.nix

@@ -152,6 +152,13 @@ let
       passAsFile = [ "text" ];
     } "env HOME=$(mktemp -d) fish_indent < $textPath > $out";
 
+  translatedSessionVariables =
+    pkgs.runCommandLocal "hm-session-vars.fish" { } ''
+      ${pkgs.babelfish}/bin/babelfish \
+        <${config.home.sessionVariablesPackage}/etc/profile.d/hm-session-vars.sh \
+        >$out
+    '';
+
 in {
   imports = [
     (mkRemovedOptionModule [ "programs" "fish" "promptInit" ] ''
@@ -354,9 +361,7 @@ in {
         set -q __fish_home_manager_config_sourced; and exit
         set -g __fish_home_manager_config_sourced 1
 
-        set --prepend fish_function_path ${pkgs.fishPlugins.foreign-env}/share/fish/vendor_functions.d
-        fenv source ${config.home.profileDirectory}/etc/profile.d/hm-session-vars.sh > /dev/null
-        set -e fish_function_path[1]
+        source ${translatedSessionVariables}
 
         ${cfg.shellInit}