fix: snapshot destructured values in #each blocks to prevent mutation…

[AyushCoder9]

… issues

Fixes #17227

When using destructuring in #each blocks (e.g., {#each gen() as [item]}), if the generator mutates the yielded object, all items would end up with the same final value because they all reference the same object.

This fix adds snapshotting for destructured patterns in each blocks, ensuring that each iteration gets a snapshot of the value at the time it was yielded, matching the behavior of a standard for...of loop.

• Add snapshot_each_value, snapshot_array, and snapshot_object utilities
• Apply snapshotting in both client and server EachBlock visitors
• Add test case to prevent regression

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Tests and linting

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

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Latest commit: 8348132

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[Rich-Harris]

Why would you want this behaviour? It's wrong. If you don't want the array to be mutated, don't mutate it.

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@17232

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17232-svelte.vercel.app/

[AyushCoder9]

Why would you want this behaviour? It's wrong. If you don't want the array to be mutated, don't mutate it.

I've updated the fix to match for...of behavior. Instead of snapshotting, array destructuring now happens immediately during iteration, capturing values at the time they are yielded.
Changes Made

1. New to_array_destructured utility function
   Destructures array elements immediately during iteration (not after)
   When a generator yields the same array object multiple times (mutating it), we extract the values at iteration time
   Returns an array of arrays so extract_paths can process them correctly
2. Updated EachBlock visitors (client & server)
   Detects when array destructuring patterns are used (e.g., [item])
   Calls to_array_destructured to destructure immediately during collection conversion
   This ensures each iteration gets the value at that moment, matching for...of behavior
3. Object destructuring unchanged
   Object destructuring still uses snapshotting (shallow copy) since objects need to be copied
   Only array destructuring uses immediate destructuring
   How it works
   Before (incorrect behavior):
   // Generator yields same array object, mutating itgen() → [arr, arr, arr, arr, arr] // all reference same object// Later destructuring reads from same object → all get final value (4)
   After (matches for...of):
   // Destructure immediately during iterationgen() → extract [0] at yield 0, [1] at yield 1, etc.// Result: [0, 1, 2, 3, 4] captured at iteration time
   This matches for...of behavior where destructuring happens immediately, not by preventing mutation. The fix ensures values are captured at the time they're yielded, regardless of later mutations.
   Test case added to prevent regression. Ready for review.