fix: rewrite destructuring logic to handle iterators

[Ocean-OS]

Currently, the compiler assumes that whenever you use array destructuring that the object you're destructuring is an array, and can be interpreted as this:

let [a, b] = object; // svelte thinks this is equivalent: let a = object[0]; let b = object[1];

However, due to iterators, and anything else using Symbol.iterator, this breaks things. This fixes that by reusing the original destructuring structure, like so:

let [a, b] = object; // becomes let [$$1, $$2] = object; let a = $.state($.proxy($$1)); let b = $.state($.proxy($$2));

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[changeset-bot]

🦋 Changeset detected

Latest commit: aebef7d

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

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[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15813 

[7nik]

This may produce multiple spreadings: example

 // source let [first, second, third] = $state(array); // output let tmp = array, first = $.state($.proxy([...tmp][0])), second = $.state($.proxy([...tmp][1])), third = $.state($.proxy([...tmp][2]));

while it would be better

 let tmp = [...array], first = $.state($.proxy(tmp[0])), second = $.state($.proxy(tmp[1])), third = $.state($.proxy(tmp[2]));

or even

 function to_array(iterable) { return Array.isArray(iterable) ? iterable : [...iterable] } let tmp = to_array(array), ...

though I'm not sure about TypedArrays

[Ocean-OS]

I'll try the second one, and see how well it works. The only issue is assignments, which don't always result in a temporary variable being made.

[7nik]

Still the same behavior 🤔

[Ocean-OS]

Just realized that since generator functions are iterators, then this sort of thing (which I use very often) would break.
I'll look into doing something completely different. 🫠

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15813-svelte.vercel.app/

[Rich-Harris]

this is a great fix! slight shame we need to reintroduce IIFEs but it's such a rare case that I don't think it matters