Implement new layouts system

[Rich-Harris]

How to migration from named layouts

Named layouts have been removed in favor of (groups). See the documentation for an introduction: https://kit.svelte.dev/docs/advanced-routing#advanced-layouts

How to migrate highly depends on your use case:

Use case 1: Rewinding to an earlier layout / "breaking out"

If you used named layouts to rewind the layout inheritance chain to a layout higher up the tree, you can reference the segment directly now:

src/routes/
├ marketing/
│ ├ pricing/
│ │ ├ business/
│ │ │ └ +layout@marketing.svelte // reference the "marketing" segment (i.e. the folder with the name) in your routes
│ │ └ +layout.svelte
- ├ +layout-marketing.svelte
- ├ +layout@marketing.svelte
+ └ +layout.svelte

Use case 2: Rewinding to a blank root layout

If you had +layout-blank.svelte in your root to reset to a blank layout, you need to adjust your actual root layout to be blank and move that functionality into another layout further below in the tree, either using an if-statement in your root layout or using the (group) feature:

src/routes/
+ (main)
+ ├ marketing/
+ │ ├ pricing/
+ │ │ └ +layout@.svelte
+ └ +layout.svelte // this is now what used to be src/routes/+layout.svelte
- marketing/
- ├ pricing/
- │ └ +layout@blank.svelte
- +layout-blank.svelte
└ +layout.svelte // this is now what used to be src/routes/+layout-blank.svelte

Use case 3: Different layouts for routes at the same nesting level

If you used named layouts to specify "these 2 pages get layout X and these other 2 next to them get layout Y", use groups instead.

Before:

src/routes/
├ about/
│ └ +page@unauthed.svelte
├ admin/
│ └ +page@authed.svelte
├ home
│ └ +page@unauthed.svelte
├ user/
│ └ +page@authed.svelte
├ +layout-authed.svelte
└ +layout-unauthed.svelte

After

src/routes/
├ (authed)
│ ├ admin/
│ │ └ +page.svelte
│ ├ user/
│ │ └ +page.svelte
│ └ +layout.svelte
├ (unauthed)
│ ├ about/
│ │ └ +page.svelte
│ ├ home
│ │ └ +page.svelte
│ └ +layout.svelte

Use case 4: arbitrary inheritance chains

If you previously used named layouts to enhance other named layouts, which also enhance layouts, to mix and match them, then that's no longer possible this way. In cases like this - which from reading the layout chains probably got confusing anyway - we advice to use composition and/or if-statements instead. The new documentation has a section with an example for this as inspiration.

PR description

#6124

Closes #6196 (using (groups) and/or composition)
Closes #5763 (root layout guaranteed to always exist + await parent())
Closes #5311 (+page@.svelte)
Closes #4940 (no longer possible to get into this situation)
Closes #2154 (only a single root layout now)

• documentation
• prevent conflicts
• print useful error when encountering a +layout-foo.svelte
• simplify the code that currently has to deal with named layouts
• update write_types
• don't reference root layout/error in dictionary in client-manifest.js
• get it to work in windows

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: 7c74aa9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[maximedupre]

Hi 😊

When will this be available? I'm asking because I did a bunch of modifications based on the docs, but now I just realized that the doc has been deployed before the package 🥹

And I don't remember how to do the previous named layout method (and no more doc on this) 😅

[Rich-Harris]

@maximedupre I just merged the release PR (#6217) so it should hit npm any minute now

[tsukhu]

Excellent change love it

[oneezy]

This PR broke symlink routes

❌ symlinks break 1.0.0-next.432
✅ symlinks work 1.0.0-next.431

Created issue #6303 with repro

---

EDIT:
I believe the issue is located somewhere within:
packages/kit/src/core/sync/create_manifest_data/index.js

Doing a little bit of research I found the symlink issue was fixed back in PR #4957

The Fix (commit eb2ab91)

As per @Conduitry's comment, this would error on symlinks to files, as readdirSync throws when not called on a directory.

There seems to be a problem with Node's fs.readdirSync(dir, { withFileTypes: true }) returning false for isDirectory() on symlinks (open issue at nodejs/node#30646). It doesn't seem likely to get fixed upstream in a timely manner, so reverting to fs.statSync instead of withFileTypes seems like a better solution.

Originally posted by @mrkishi in #4957 (review)