feat: allow state/derived/props to be explicitly exported from components

[Rich-Harris]

I think it's fine to export state/derived/props from components, which means reverting #10430.

Note that export let is still forbidden in runes mode, since it's much more likely to be unintentional than not. To export state, you must do it in two parts:

let x = $state(...);
export { x };

We could potentially allow export let x = $state(...) in future, once we go runes-only.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 6104318

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

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[benmccann]

Will the exported state be reactive in this case though? That's usually how I see people try to do this and I think it could cause confusion if they were allowed to do it and then it didn't work

[Rich-Harris]

Yes, it will — though if you try and mutate it from outside the component Svelte will yell at you (#10464)

[dummdidumm]

This isn't working correctly yet.

• The REPL in Signal is exposed when creating $derived value accessor #10293 returns the signal as-is, not a getter of it (preview playground link)
• The REPL in Error when creating an accessor for an prop #10311 is broken, $.get of the prop is not called in the right places (preview playground link). I'm still unsure if the use case "create a getter/setter from a prop" is a thing we should allow.

[Rich-Harris]

the first of those is fixed, working on the second

[Rich-Harris]

should be all set now

[dummdidumm]

Code looks good now, though I slightly prefered the changed error messages because they gave you instructions on how to resolve the error. Thoughts?

[Rich-Harris]

merged the changes from both

resolved