[Segment Cache] No data on tree prefetch if no PPR

[acdlite]

When the client Segment Cache attempts to prefetch a route tree, and it the route does not have PPR enabled, the server falls through to the old implementation, which sends back a FlightRouterState.

This is fine, since the client can use the FlightRouterState to recreate the non-PPR behavior.

However, when loading.tsx is present in the route, the server will not just send back the FlightRouterState, but also render the page up to the first loading boundary. This is not what we want in the Segment Cache implementation — we want the route tree only, so we can determine which parts the client already has before sending a second request for the rest.

So this PR disables the loading.tsx mechanism when prefetching the route tree, by checking the Next-Router-Segment-Prefetch header.

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	acdlite/next.js segment-cache-no-data-on-tree-prefetch	Change
buildDuration	17.8s	16.2s	N/A
buildDurationCached	15.3s	13s	N/A
nodeModulesSize	409 MB	409 MB	⚠️ +17 kB
nextStartRea..uration (ms)	461ms	485ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	acdlite/next.js segment-cache-no-data-on-tree-prefetch	Change
1187-HASH.js gzip	50.7 kB	50.7 kB	N/A
8276.HASH.js gzip	169 B	168 B	N/A
8377-HASH.js gzip	5.36 kB	5.36 kB	N/A
bccd1874-HASH.js gzip	53 kB	53 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 kB	34 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	acdlite/next.js segment-cache-no-data-on-tree-prefetch	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	acdlite/next.js segment-cache-no-data-on-tree-prefetch	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	4.49 kB	4.49 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	acdlite/next.js segment-cache-no-data-on-tree-prefetch	Change
_buildManifest.js gzip	747 B	746 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	acdlite/next.js segment-cache-no-data-on-tree-prefetch	Change
index.html gzip	524 B	522 B	N/A
link.html gzip	539 B	537 B	N/A
withRouter.html gzip	520 B	519 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

	vercel/next.js canary	acdlite/next.js segment-cache-no-data-on-tree-prefetch	Change
edge-ssr.js gzip	128 kB	128 kB	N/A
page.js gzip	203 kB	203 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	acdlite/next.js segment-cache-no-data-on-tree-prefetch	Change
middleware-b..fest.js gzip	671 B	667 B	N/A
middleware-r..fest.js gzip	155 B	156 B	N/A
middleware.js gzip	31.1 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	acdlite/next.js segment-cache-no-data-on-tree-prefetch	Change
523-experime...dev.js gzip	322 B	322 B	✓
523.runtime.dev.js gzip	314 B	314 B	✓
app-page-exp...dev.js gzip	323 kB	323 kB	N/A
app-page-exp..prod.js gzip	127 kB	127 kB	N/A
app-page-tur..prod.js gzip	140 kB	140 kB	N/A
app-page-tur..prod.js gzip	135 kB	135 kB	N/A
app-page.run...dev.js gzip	313 kB	313 kB	N/A
app-page.run..prod.js gzip	123 kB	123 kB	N/A
app-route-ex...dev.js gzip	37.3 kB	37.3 kB	✓
app-route-ex..prod.js gzip	25.4 kB	25.4 kB	✓
app-route-tu..prod.js gzip	25.4 kB	25.4 kB	✓
app-route-tu..prod.js gzip	25.2 kB	25.2 kB	✓
app-route.ru...dev.js gzip	38.9 kB	38.9 kB	✓
app-route.ru..prod.js gzip	25.2 kB	25.2 kB	✓
pages-api-tu..prod.js gzip	9.67 kB	9.67 kB	✓
pages-api.ru...dev.js gzip	11.6 kB	11.6 kB	✓
pages-api.ru..prod.js gzip	9.66 kB	9.66 kB	✓
pages-turbo...prod.js gzip	21.7 kB	21.7 kB	✓
pages.runtim...dev.js gzip	27.4 kB	27.4 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	acdlite/next.js segment-cache-no-data-on-tree-prefetch	Change
0.pack gzip	2.05 MB	2.05 MB	N/A
index.pack gzip	71.6 kB	72.2 kB	⚠️ +548 B
Overall change	71.6 kB	72.2 kB	⚠️ +548 B

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Commit: 847d503