Reassigning an object to a writable store using $-prefix causes its value to become a proxy in runes mode

[tomoam]

Describe the bug

related #15280

When writing a Svelte component like the following in runes mode...

<script>
	import { writable } from 'svelte/store';

	let obj = writable({ name: 'foo'});

	const clone1 = structuredClone($obj); // OK
	
	$obj = { name: 'bar' };

	const clone2 = structuredClone($obj); // error
</script>

{ name: 'bar' } becomes proxied in the JS output as shown below:

	let obj = writable({ name: 'foo' });
	const clone1 = structuredClone($obj());

	$.store_set(obj, $.proxy({ name: 'bar' })); // here

	const clone2 = structuredClone($obj());

In the following cases, this issue does not occur:

• When assigning a primitive value instead of an object
• When using the writable store’s set method instead of the = operator
• In non-runes mode

Reproduction

https://svelte.dev/playground/77f58c2f61394b4ca4efc5b62ea63d29?version=5.20.0

Logs

System Info

System:
    OS: macOS 14.6.1
    CPU: (12) arm64 Apple M2 Pro
    Memory: 1.45 GB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.18.0 - ~/.nvm/versions/node/v20.18.0/bin/node
    npm: 10.8.2 - ~/.nvm/versions/node/v20.18.0/bin/npm
    pnpm: 9.7.0 - /opt/homebrew/bin/pnpm
  Browsers:
    Chrome: 133.0.6943.54
    Safari: 17.6
    Safari Technology Preview: 18.2

Severity

annoyance

[ginpei]

Is it possible for store to return proxy always?

From us, user sight, store looks to return original object or proxy unpredictably. If it was proxy always, we could avoid "lucky shot" and deal with improper usage from the beginning. How about doing so in dev mode at least?

[tomoam]

I also tested the behavior in Svelte 4 and observed that when changing a store’s value, it was not proxified.

Below are the demo code and a table of the results:

• in Svelte 4
  https://svelte.dev/playground/e7c2002bbfb64a4ebb7f03bfa2a264bd?version=4.2.19
• in non-runes mode of Svelte 5:
  https://svelte.dev/playground/e7c2002bbfb64a4ebb7f03bfa2a264bd?version=5.20.0
• in runes mode of Svelte 5:
  https://svelte.dev/playground/77f58c2f61394b4ca4efc5b62ea63d29?version=5.20.0

Svelte Version	Mode	How to write	Proxified
4	-	store.set(object)	false
4	-	store.update(v => v = object)	false
4	-	$store = object	false
5	Non-runes	store.set(object)	false
5	Non-runes	store.update(v => v = object)	false
5	Non-runes	$store = object	false
5	Runes	store.set(object)	false
5	Runes	store.update(v => v = object)	false
5	Runes	$store = object	true

Considering backwards compatibility with Svelte 4, I believe that having the store’s value proxified when reassigning to the store with $-prefix is unintentional. A store’s value should consistently not be proxified.

[ginpei]

A store’s value should consistently not be proxified

This sounds good to me. It'd be great if the result was either always original or always proxy

Anyway I'm going to replace $store = object with store.set(object) on our migrated code for now. Thanks for hard working, Svelte team!