svelte5: while binding this to none-existing property on desctructed object, svelte adds the property

[FoHoOV]

Describe the bug

In JS this:

const user = {name: "test"};
let {name, lastname} =  user;
lastname = "test2";
console.log(user);

will result in:

{name: "test"}

It will not create the lastname property on the user.

In svelte doing something similar:

<script>
	let users = $state([{name: "a"}]);
</script>


<!-- this creates the lastname on users[0] then assigns the value to it -->
<!-- which is not how js works -->
{#each users as {name, lastname}}
	<div bind:this={lastname}></div>
{/each}

<br>
{JSON.stringify(users)}

will create:

[{"name":"a","lastname":{"__svelte_meta":{"loc":{"file":"App.svelte","line":9,"column":1}}}}]

It seems that it doesn't work for other binds tho, just for bind:this. This is a breaking change (bind:value={lastname} works in svelte4 using the each block arguments to bind to), which I didn't find documented, maybe I'm blind, just pointing this out as well.

Reproduction

1. openup this REPL
2. see the output :D

Logs

No response

System Info

REPL 5.0.0-next.200

Severity

annoyance

[brunnerh]

Binding to the each item and destructured properties is a Svelte 3 / 4 feature that is supposed to be gone in runes mode - it outputs an error.

Cannot reassign or bind to each block argument in runes mode. Use the array and index variables instead (e.g. array[i] = value instead of entry = value)

I guess the bind:this was not accounted for.

[FoHoOV]

I guess it should be in the breaking changes then (#11400).

[brunnerh]

I don't think it qualifies as that since it still works in non-runes mode.
If you rewrite to runes, you will have to change quite a few things anyway.

[FoHoOV]

Petition to rename bind:this to start with something else other than bind: :D? I guess there are other issues that gets confusing because bind:this is speciall in a way see (#12401 and this comment).
note: not related to this issue tho