Allow generateMetadata to be synchronous (#45696)

The very-newly-released `generateMetadata` function currently does not
allow itself to be synchronous. It leads to situations like this

```ts
export async function generateMetadata() {
  return { title: "Hello, world!" };
}
```

where there's evidently no reason to use `async`, but it's forced by
Next.js.

This PR aims to remove this restriction: `generateMetadata` can be
synchronous now.

## Bug

- [x] Related issues linked using `fixes #45685`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [x]
[e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs)
tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing
doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

Signed-off-by: Vu Van Dung <me@joulev.dev>
Co-authored-by: Jiachi Liu <inbox@huozhi.im>

packages/next/src/build/webpack/plugins/flight-types-plugin.ts

@@ -66,7 +66,7 @@ interface IEntry {
       : ''
   }
   metadata?: any
-  generateMetadata?: (props: PageProps, parent: ResolvingMetadata) => Promise<any>
+  generateMetadata?: (props: PageProps, parent: ResolvingMetadata) => any | Promise<any>
 }
 
 // =============

packages/next/src/lib/metadata/resolve-metadata.ts

@@ -328,7 +328,9 @@ function merge(
   }
 }
 
-type MetadataResolver = (_parent: ResolvingMetadata) => Promise<Metadata>
+type MetadataResolver = (
+  _parent: ResolvingMetadata
+) => Metadata | Promise<Metadata>
 export type MetadataItems = [
   Metadata | MetadataResolver | null,
   Metadata | null
@@ -410,10 +412,11 @@ export async function accumulateMetadata(
 
   for (const item of metadataItems) {
     const [metadataExport, staticFilesMetadata] = item
-    const layerMetadataPromise =
+    const layerMetadataPromise = Promise.resolve(
       typeof metadataExport === 'function'
         ? metadataExport(parentPromise)
-        : Promise.resolve(metadataExport)
+        : metadataExport
+    )
 
     parentPromise = parentPromise.then((resolved) => {
       return layerMetadataPromise.then((exportedMetadata) => {

test/e2e/app-dir/metadata/app/basic/sync-generate-metadata/page.tsx

@@ -0,0 +1,9 @@
+export default function Page() {
+  return 'synchronous generateMetadata'
+}
+
+export function generateMetadata() {
+  return {
+    title: 'synchronous generateMetadata',
+  }
+}

test/e2e/app-dir/metadata/metadata.test.ts

@@ -316,6 +316,11 @@ createNextDescribe(
           /params - blog query - xxx/
         )
       })
+
+      it('should support synchronous generateMetadata export', async () => {
+        const browser = await next.browser('/basic/sync-generate-metadata')
+        expect(await getTitle(browser)).toBe('synchronous generateMetadata')
+      })
     })
 
     describe('opengraph', () => {