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));

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
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

🦋 Changeset detected

Latest commit: aebef7d

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

This PR includes changesets to release 1 package

Name	Type
svelte	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

[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