feat: Add Deno.ServeDefaultExport type (#24879)

Closes https://github.com/denoland/deno/issues/23725

cli/tsc/dts/lib.deno.ns.d.ts

@@ -6256,6 +6256,35 @@ declare namespace Deno {
     info: ServeHandlerInfo,
   ) => Response | Promise<Response>;
 
+  /** Interface that module run with `deno serve` subcommand must conform to.
+   *
+   * To ensure your code is type-checked properly, make sure to add `satisfies Deno.ServeDefaultExport`
+   * to the `export default { ... }` like so:
+   *
+   * ```ts
+   * export default {
+   *   fetch(req) {
+   *     return new Response("Hello world");
+   *   }
+   * } satisfies Deno.ServeDefaultExport;
+   * ```
+   *
+   * @category HTTP Server
+   */
+  export interface ServeDefaultExport {
+    /** A handler for HTTP requests. Consumes a request and returns a response.
+     *
+     * If a handler throws, the server calling the handler will assume the impact
+     * of the error is isolated to the individual request. It will catch the error
+     * and if necessary will close the underlying connection.
+     *
+     * @category HTTP Server
+     */
+    fetch: (
+      request: Request,
+    ) => Response | Promise<Response>;
+  }
+
   /** Options which can be set when calling {@linkcode Deno.serve}.
    *
    * @category HTTP Server

tests/specs/serve/type_check/__test__.jsonc

@@ -0,0 +1,6 @@
+{
+  "args": "serve --check --port 12345 main.ts",
+  "output": "main.out",
+  "tempDir": true,
+  "exitCode": 1
+}

tests/specs/serve/type_check/main.out

@@ -0,0 +1,5 @@
+Check [WILDCARD]
+error: TS2353 [ERROR]: Object literal may only specify known properties, and 'bad' does not exist in type 'ServeDefaultExport'.
+  bad() {
+  ~~~
+    at [WILDCARD]main.ts:2:3

tests/specs/serve/type_check/main.ts

@@ -0,0 +1,4 @@
+export default {
+  bad() {
+  },
+} satisfies Deno.ServeDefaultExport;

tests/specs/serve/type_check2/__test__.jsonc

@@ -0,0 +1,6 @@
+{
+  "args": "serve --check --port 12345 main.ts",
+  "output": "main.out",
+  "tempDir": true,
+  "exitCode": 1
+}

tests/specs/serve/type_check2/main.out

@@ -0,0 +1,5 @@
+Check [WILDCARD]
+error: TS2339 [ERROR]: Property 'doesnt_exist' does not exist on type 'Request'.
+    console.log(request.doesnt_exist);
+                        ~~~~~~~~~~~~
+    at [WILDCARD]main.ts:3:25

tests/specs/serve/type_check2/main.ts

@@ -0,0 +1,6 @@
+export default {
+  fetch(request) {
+    console.log(request.doesnt_exist);
+    return new Response("Hello world!");
+  },
+} satisfies Deno.ServeDefaultExport;