starship: add enable_transience for fish (#3975)

Starship has an advanced, experimental feature where fancy stuff in the
prompt can be replaced with something more simple after the command is
ran. This is very helpful for copy and pasting shell history somewhere
else.

docs: https://starship.rs/advanced-config/#transientprompt-and-transientrightprompt-in-fish

Fish is currently the only shell as far as I can tell that both
home-manager and starship support for this feature. Since the function
has to be called after starship is loaded, this seems like the best
place to put it.

format

modules/programs/starship.nix

@@ -77,6 +77,19 @@ in {
     enableNushellIntegration = mkEnableOption "Nushell integration" // {
       default = true;
     };
+
+    enableTransience = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        The TransientPrompt feature of Starship replaces previous prompts with a
+        custom string. This is only a valid option for the Fish shell.
+
+        For documentation on how to change the default replacement string and
+        for more information visit
+        https://starship.rs/advanced-config/#transientprompt-and-transientrightprompt-in-cmd
+      '';
+    };
   };
 
   config = mkIf cfg.enable {
@@ -101,6 +114,7 @@ in {
     programs.fish.interactiveShellInit = mkIf cfg.enableFishIntegration ''
       if test "$TERM" != "dumb"  -a \( -z "$INSIDE_EMACS"  -o "$INSIDE_EMACS" = "vterm" \)
         eval (${starshipCmd} init fish)
+        ${lib.optionalString cfg.enableTransience "enable_transience"}
       end
     '';
 

tests/modules/programs/starship/default.nix

@@ -1 +1,5 @@
-{ starship-settings = ./settings.nix; }
+{
+  starship-settings = ./settings.nix;
+  starship-fish-with-transience = ./fish_with_transience.nix;
+  starship-fish-without-transience = ./fish_without_transience.nix;
+}

tests/modules/programs/starship/fish_with_transience.nix

@@ -0,0 +1,21 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs = {
+      fish.enable = true;
+
+      starship = {
+        enable = true;
+        enableTransience = true;
+      };
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.config/fish/config.fish
+      assertFileRegex home-files/.config/fish/config.fish 'enable_transience'
+    '';
+  };
+}

tests/modules/programs/starship/fish_without_transience.nix

@@ -0,0 +1,17 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs = {
+      fish.enable = true;
+      starship.enable = true;
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.config/fish/config.fish
+      assertFileNotRegex home-files/.config/fish/config.fish 'enable_transience'
+    '';
+  };
+}