[metadata] Move metadata rendering adjacent to page component

[huozhi]

What

This work prepares for streaming metadata by moving metadata from the top-level head component into the body, rendering it just before and adjacent to the page component. In the future, we’ll wrap it with a suspense boundary, depending on whether the request's user agent supports streaming rendering.

Minor Changes

• Move charset into viewport at early top as according to spec: <meta> elements which declare a character encoding must be located entirely within the first 1024 bytes of the document.
• Replace the previous metadata data in the HeadData to null. It will be changed back to single ReactNode later in [metadata] Change the array head to single node in flight data #74299

[huozhi]

• [metadata] add option of configuring ua of async metadata #74594
• [metadata] Change the array head to single node in flight data #74299
• [metadata] Move metadata rendering adjacent to page component #74262 👈 (View in Graphite)
• canary

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[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js huozhi/12-23-_move_metadata_render	Change
buildDuration	25s	22s	N/A
buildDurationCached	20.7s	17.8s	N/A
nodeModulesSize	417 MB	417 MB	⚠️ +24 kB
nextStartRea..uration (ms)	580ms	570ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js huozhi/12-23-_move_metadata_render	Change
1187-HASH.js gzip	52.6 kB	52.6 kB	N/A
8276.HASH.js gzip	169 B	168 B	N/A
8377-HASH.js gzip	5.44 kB	5.44 kB	N/A
bccd1874-HASH.js gzip	52.9 kB	52.9 kB	N/A
framework-HASH.js gzip	57.5 kB	57.5 kB	N/A
main-app-HASH.js gzip	233 B	235 B	N/A
main-HASH.js gzip	34.1 kB	34.1 kB	N/A
webpack-HASH.js gzip	1.71 kB	1.71 kB	N/A
Overall change	0 B	0 B	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js huozhi/12-23-_move_metadata_render	Change
polyfills-HASH.js gzip	39.4 kB	39.4 kB	✓
Overall change	39.4 kB	39.4 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js huozhi/12-23-_move_metadata_render	Change
_app-HASH.js gzip	193 B	193 B	✓
_error-HASH.js gzip	193 B	193 B	✓
amp-HASH.js gzip	512 B	510 B	N/A
css-HASH.js gzip	343 B	342 B	N/A
dynamic-HASH.js gzip	1.84 kB	1.84 kB	✓
edge-ssr-HASH.js gzip	265 B	265 B	✓
head-HASH.js gzip	363 B	362 B	N/A
hooks-HASH.js gzip	393 B	392 B	N/A
image-HASH.js gzip	4.57 kB	4.57 kB	N/A
index-HASH.js gzip	268 B	268 B	✓
link-HASH.js gzip	2.35 kB	2.34 kB	N/A
routerDirect..HASH.js gzip	328 B	328 B	✓
script-HASH.js gzip	397 B	397 B	✓
withRouter-HASH.js gzip	323 B	326 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.59 kB	3.59 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js huozhi/12-23-_move_metadata_render	Change
_buildManifest.js gzip	749 B	747 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js huozhi/12-23-_move_metadata_render	Change
index.html gzip	524 B	523 B	N/A
link.html gzip	540 B	537 B	N/A
withRouter.html gzip	521 B	520 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js huozhi/12-23-_move_metadata_render	Change
edge-ssr.js gzip	128 kB	128 kB	N/A
page.js gzip	206 kB	206 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js huozhi/12-23-_move_metadata_render	Change
middleware-b..fest.js gzip	672 B	669 B	N/A
middleware-r..fest.js gzip	155 B	156 B	N/A
middleware.js gzip	31.2 kB	31.2 kB	N/A
edge-runtime..pack.js gzip	844 B	844 B	✓
Overall change	844 B	844 B	✓

Next Runtimes

	vercel/next.js canary	vercel/next.js huozhi/12-23-_move_metadata_render	Change
274-experime...dev.js gzip	322 B	322 B	✓
274.runtime.dev.js gzip	314 B	314 B	✓
app-page-exp...dev.js gzip	363 kB	363 kB	N/A
app-page-exp..prod.js gzip	129 kB	129 kB	N/A
app-page-tur..prod.js gzip	142 kB	142 kB	N/A
app-page-tur..prod.js gzip	138 kB	138 kB	N/A
app-page.run...dev.js gzip	352 kB	352 kB	N/A
app-page.run..prod.js gzip	125 kB	125 kB	N/A
app-route-ex...dev.js gzip	37.5 kB	37.5 kB	✓
app-route-ex..prod.js gzip	25.5 kB	25.5 kB	✓
app-route-tu..prod.js gzip	25.5 kB	25.5 kB	✓
app-route-tu..prod.js gzip	25.4 kB	25.4 kB	✓
app-route.ru...dev.js gzip	39.2 kB	39.2 kB	✓
app-route.ru..prod.js gzip	25.4 kB	25.4 kB	✓
pages-api-tu..prod.js gzip	9.69 kB	9.69 kB	✓
pages-api.ru...dev.js gzip	11.6 kB	11.6 kB	✓
pages-api.ru..prod.js gzip	9.68 kB	9.68 kB	✓
pages-turbo...prod.js gzip	21.7 kB	21.7 kB	✓
pages.runtim...dev.js gzip	27.5 kB	27.5 kB	✓
pages.runtim..prod.js gzip	21.7 kB	21.7 kB	✓
server.runti..prod.js gzip	916 kB	916 kB	✓
Overall change	1.2 MB	1.2 MB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	vercel/next.js huozhi/12-23-_move_metadata_render	Change
0.pack gzip	2.09 MB	2.09 MB	⚠️ +770 B
index.pack gzip	75.5 kB	75 kB	N/A
Overall change	2.09 MB	2.09 MB	⚠️ +770 B

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Commit: 9be477c

[ztanner]

could you clarify this a bit more? the default segment could be what's SSRed when you load a page that has no page.tsx in a particular slot, but has a default.tsx

[huozhi]

Ideally it shouldn't render.
When the page is missing it renders 404, when the default of slot is rendered, it shouldn't overide the page metadata. However I found this is not running as expected right now. Created another task to track this issue and we can fix separately, as it's same between this branch and canary branch. I can follow up later