Make trailingSlash a page/endpoint option, like prerender

[Rich-Harris]

How to migrate

1. Remove the option from from svelte.config.js
2. Add the option to src/routes/+layout.js:

export const trailingSlash = 'never'; // or 'always' or 'ignore', depending on what you want

PR description

Closes #7302. This moves trailingSlash configuration from svelte.config.js and into pages. Benefits:

• Moving stuff out of config and into code is always good
• We can now have e.g. /blog/ and /blog/my-post
• We can now control trailing slashes on endpoints (niche, but will probably be useful one day)

It has very limited impact on client-side bundle size or data transfer.

The downside is that on the server, we need to load code for the route before we can return a redirect response. In practice the impact is negligible (since we don't need to render anything, just import/evaluate it) and in any case it's not the most important thing to optimise.

TODO:

• documentation
• delete packages/kit/test/prerendering/trailing-slash etc — no longer necessary, the tests can be moved into other apps

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: 0b5da1b

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

This PR includes changesets to release 2 packages

Name	Type
@sveltejs/adapter-vercel	Patch
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[dummdidumm]

Looks good code-wise. Question I have though: How likely is it that people are pissed off by this because they now have to add export const trailingSlash = ".." to all of their +server.js files? Previously, you had it to set it only once in the config, and it applied to endpoints as well.

[Rich-Harris]

Previously, you had it to set it only once in the config, and it applied to endpoints as well.

Not quite — trailing slashes were completely ignored for endpoints. Now they'll be treated as never by default, which I think is what people will want in the overwhelming majority of cases (it's only really pages where a trailing slash ever makes sense), but in the exceptional cases where they do want to enforce a trailing slash they can now do so much more easily than before, where they had to implement the redirect logic themselves