add rustfmt as external formatter

home/modules/zed/default.nix

@@ -63,6 +63,15 @@ in
               arguments = [ "-q" ];
             };
           };
+          Rust = {
+            formatter.external = {
+              command = lib.getExe pkgs.rustfmt;
+              arguments = [
+                "--edition"
+                "2018"
+              ];
+            };
+          };
         };
         lsp = with pkgs; {
           nixd.binary.path = lib.getExe nixd;