parallel routes: support multi-slot layouts

[ztanner]

What?

When using layouts in multiple parallel route slots, only 1 of the layouts would render.

Why?

The resolveParallelSegments logic responsible for populating the loader tree was incorrectly bailing if it found another parallel route that matched a page component.

How?

I did my best to update this loader code with some more comments to make it a bit easier to reason about, and also made some slight refactors. But the gist of the fix is just ensuring that each parallel route (that isn't a direct match on the children slot) is resolved as an array, so that when the subtree is created, it doesn't skip over the slot.

Fixes #58506
Fixes #59463

Closes NEXT-2222

[ztanner]

• canary
  • parallel routes: support multi-slot layouts #61115 👈

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[ijjk]

Stats from current PR

Default Build

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js 01-24-parallel_routes_support_multi-slot_layouts	Change
buildDuration	11.6s	11.8s	⚠️ +173ms
buildDurationCached	6.1s	4.8s	N/A
nodeModulesSize	200 MB	200 MB	⚠️ +6.95 kB
nextStartRea..uration (ms)	426ms	427ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js 01-24-parallel_routes_support_multi-slot_layouts	Change
3f784ff6-HASH.js gzip	53.4 kB	53.4 kB	✓
423.HASH.js gzip	185 B	181 B	N/A
68-HASH.js gzip	29.8 kB	29.8 kB	N/A
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	238 B	239 B	N/A
main-HASH.js gzip	31.8 kB	31.8 kB	N/A
webpack-HASH.js gzip	1.7 kB	1.7 kB	✓
Overall change	100 kB	100 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js 01-24-parallel_routes_support_multi-slot_layouts	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js 01-24-parallel_routes_support_multi-slot_layouts	Change
_app-HASH.js gzip	194 B	195 B	N/A
_error-HASH.js gzip	182 B	181 B	N/A
amp-HASH.js gzip	502 B	502 B	✓
css-HASH.js gzip	320 B	322 B	N/A
dynamic-HASH.js gzip	2.5 kB	2.5 kB	N/A
edge-ssr-HASH.js gzip	255 B	256 B	N/A
head-HASH.js gzip	350 B	349 B	N/A
hooks-HASH.js gzip	368 B	369 B	N/A
image-HASH.js gzip	4.18 kB	4.18 kB	N/A
index-HASH.js gzip	257 B	256 B	N/A
link-HASH.js gzip	2.61 kB	2.61 kB	N/A
routerDirect..HASH.js gzip	310 B	311 B	N/A
script-HASH.js gzip	384 B	383 B	N/A
withRouter-HASH.js gzip	306 B	308 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	608 B	608 B	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js 01-24-parallel_routes_support_multi-slot_layouts	Change
_buildManifest.js gzip	484 B	484 B	✓
Overall change	484 B	484 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js 01-24-parallel_routes_support_multi-slot_layouts	Change
index.html gzip	527 B	529 B	N/A
link.html gzip	539 B	541 B	N/A
withRouter.html gzip	521 B	524 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js 01-24-parallel_routes_support_multi-slot_layouts	Change
edge-ssr.js gzip	94 kB	94 kB	N/A
page.js gzip	149 kB	149 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js 01-24-parallel_routes_support_multi-slot_layouts	Change
middleware-b..fest.js gzip	625 B	622 B	N/A
middleware-r..fest.js gzip	151 B	149 B	N/A
middleware.js gzip	37.6 kB	37.6 kB	N/A
edge-runtime..pack.js gzip	1.92 kB	1.92 kB	✓
Overall change	1.92 kB	1.92 kB	✓

Next Runtimes

	vercel/next.js canary	vercel/next.js 01-24-parallel_routes_support_multi-slot_layouts	Change
app-page-exp...dev.js gzip	170 kB	170 kB	✓
app-page-exp..prod.js gzip	95.7 kB	95.7 kB	✓
app-page-tur..prod.js gzip	96.3 kB	96.3 kB	✓
app-page-tur..prod.js gzip	90.9 kB	90.9 kB	✓
app-page.run...dev.js gzip	142 kB	142 kB	✓
app-page.run..prod.js gzip	90.2 kB	90.2 kB	✓
app-route-ex...dev.js gzip	22.2 kB	22.2 kB	✓
app-route-ex..prod.js gzip	14.9 kB	14.9 kB	✓
app-route-tu..prod.js gzip	14.9 kB	14.9 kB	✓
app-route-tu..prod.js gzip	14.5 kB	14.5 kB	✓
app-route.ru...dev.js gzip	21.6 kB	21.6 kB	✓
app-route.ru..prod.js gzip	14.5 kB	14.5 kB	✓
pages-api-tu..prod.js gzip	9.42 kB	9.42 kB	✓
pages-api.ru...dev.js gzip	9.69 kB	9.69 kB	✓
pages-api.ru..prod.js gzip	9.42 kB	9.42 kB	✓
pages-turbo...prod.js gzip	22 kB	22 kB	✓
pages.runtim...dev.js gzip	22.6 kB	22.6 kB	✓
pages.runtim..prod.js gzip	22 kB	22 kB	✓
server.runti..prod.js gzip	49.7 kB	49.7 kB	✓
Overall change	932 kB	932 kB	✓

Commit: 770a36b

[ztanner]

afaict this was always operating on a single element and I'm not sure why it was being normalized into an array, so I refactored this.

[ijjk]

Tests Passed