special case nix3 flake completions

flake.nix

@@ -57,8 +57,26 @@
           imports = [ ./nix/module.nix ];
           programs.inshellah.package = self.packages.${pkgs.stdenv.hostPlatform.system}.default;
           programs.inshellah.snippet = ''
-            let inshellah_complete = {|spans|
-              inshellah complete ...$spans | from json
+            let inshellah_complete = { |spans|
+              let completions = inshellah complete ...$spans | from json
+              # special case nix completions, for `nix run nixpkgs#..` type input
+              let additional = if ($completions == null and
+                ($spans | length) > 0 and $spans.0 == "nix") {
+                $env.NIX_GET_COMPLETIONS = ($spans | length) - 1
+                let nix_output = $spans | run-external $in | split row -r '\n' | str trim | skip 1
+                let entries = $nix_output |
+                  if (($in | length) < 6 and
+                    ($spans | last) =~ "[a-zA-Z][a-zA-Z0-9_-]*#[a-zA-Z][a-zA-Z0-9_-]*") { 
+                    hide-env NIX_GET_COMPLETIONS
+                    $in | par-each { |e| { value: $e, description: (^nix eval $e --apply "f: f.meta.description") }}
+                } else {
+                  $in | each { |e| { value: $e, description: "" } }
+                }
+                $entries
+              } else {
+                null
+              }
+              $completions | append $additional
             }
             $env.config.completions.external = {
               enable: true