feat: adds legacy mode flag

[trueadm]

This PR makes it so Svelte components that are in legacy mode mark the legacy mode flag and adds the flag to parts of the codebase that do checks for runes. In practice this means that if all your components are in runes mode, Rollup can strip out all the runtime code that handles legacy mode, reducing bundle code-size and improving runtime performance.

I was expecting bigger wins, but some small gains are better than nothing! Notable wins are inside our props logic and await blocks. Some hot paths are also improved as it means less is_runes lookups.

I also considered adding a DEV time console.info message if you have a component that is in legacy mode to help inform the user, but this might be a bit noisy on reflection so I decided against it.

[changeset-bot]

🦋 Changeset detected

Latest commit: 8899263

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

This PR includes changesets to release 1 package

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pnpm add https://pkg.pr.new/svelte@14180

[dummdidumm]

Nice ! Out of curiousity: how much savings are we talking about (in kb, unminified or minified, I don't care)

[Rich-Harris]

What if we move src/internal/feature-flags.js to src/internal/flags/index.js and src/internal/legacy-component.js to src/internal/flags/legacy.js? Keeps things a bit more nicely organised and gives us a place to add other things in future

[trueadm]

@dummdidumm It's variable depending on what you use. For a simple component:

<script>
	let x = $state(0);
	let { y = 0 } = $props();
</script>

<button onclick={() => x++}>{x}</button>
{y}

So a 0.3kb saving on a simple component, but still a saving!

[trueadm]

@Rich-Harris Applied your feedback :)

[dummdidumm]

to not regress, we should add a test similar to the one checking that hydration code is treeshaken out (check-treeshakeability.js)

[trueadm]

@dummdidumm Mind helping me do that? Not sure what we should check for in the code.

[dummdidumm]

it's not an exact science. Basically you pick a string that shouldn't be present anymore after treeshaking, like component_context.l = {

[trueadm]

@dummdidumm Thanks, done that now

[Conduitry]

This changeset needs to be different, IMO. It doesn't describe what's happening to an end user at all. And it should also probably be a fix or chore because there's no visible API changes.

[trueadm]

@Conduitry Is that better?

[Conduitry]

Yup sounds good 👍