Fix: Patch structuredClone in dev to correctly clone $state proxies

[Ocean-OS]

Should close #13562. Currently a draft due to lack of tests and other concerns about implementation.

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🦋 Changeset detected

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14599-svelte.vercel.app/

this is an automated message

[Ocean-OS]

Not sure why lint tests are failing, I ran prettier on everything I changed...

[Rich-Harris]

It's erroring because export var structured_clone = structuredClone prevents Svelte from being tree-shaken (I'm not exactly sure why).

Regardless, this seems like a very bad idea — it will mean that code suddenly breaks in production. If we were to monkey-patch structuredClone in dev, the only thing it should be doing is replacing the inevitable error with a more descriptive one.

[Ocean-OS]

Should it throw when attempting to clone a $state proxy instead?

[Rich-Harris]

I mean, it does, the error is just a bit unclear. The easy solution would be to wrap the underlying call to structuredClone in a try-catch block that inspects the error to see if it's a DataCloneError, and prints a message accordingly, but that would have false positives if you did e.g. structuredClone(new Proxy({}, {})) and the message referred to Svelte state.

The more complete solution would basically be to reimplement structuredClone in userland, including handling cycles etc.

[Ocean-OS]

The more complete solution would basically be to reimplement structuredClone in userland, including handling cycles etc.

Would this also clone state proxies, or would it throw an error if attempted?

[Rich-Harris]

It would throw an error. The behaviour would need to be identical to structuredClone except for the very slightly more helpful error message

[Ocean-OS]

@Rich-Harris I've added the more complete option (not completely finished yet) and the basic option. Most of the concerns I have are in the comments in clone.js.
I also wasn't entirely sure as to if I should put the error in shared or client.