Add nix shell detection to tide

home/fish/tide/nix-shell.nix

@@ -0,0 +1,21 @@
+{ config, pkgs, lib, ... }:
+
+with lib;
+let
+  cfg = config.programs.fish.tide.nix-shell;
+in
+{
+  options.programs.fish.tide.nix-shell = {
+    enable = mkEnableOption "An indicator of having a `nix shell` environment";
+  };
+
+  config.programs.fish = mkIf cfg.enable {
+    functions._tide_item_nix_shell = ''
+      if string match -q "/nix/store/*" $PATH
+        set -U tide_nix_shell_color blue
+        set -U tide_nix_shell_bg_color normal
+        _tide_print_item nix_shell "❄"
+      end
+    '';
+  };
+}