A deeply reactive derived variable

[aster-void]

Describe the problem

As written in the svelte doc, variable declared with $derived are not converted to deeply reactive proxies:
https://svelte.dev/docs/svelte/$derived#Deriveds-and-reactivity

The only way there currently seems to be is using $state to create the derived variable and $effect to reflect the changes.

related discussion: #16187

Describe the proposed solution

I want something that allows me to natively do that.

maybe something like

const thisIsDeeplyReactive = $derived($state(source));

Importance

nice to have

[paoloricciuti]

What would be the advantage? Just syntactic sugar on top of

$derived.by(()=>{
    let val = $state(data);
    return val;
});

?

[Conduitry]

Or

function proxify(value) {
    const proxified = $state(value);
    return proxified;
}

let foo = $derived(proxify(...));

One of the other bits of syntax that was suggested internally (by Ben, in the 'writable deriveds' thread in the maintainers channel) was $derived($state(...)).

I do think it would be somewhat nice to have a simpler-looking way to do this. Having it in the docs might at least remind people further that $derived()s by themselves are not proxies.

[paoloricciuti]

Dominic also wanted to make $derived($state()) work but that was before writable deriveds...I think a new rune is an overkill for this tho

[aster-void]

I didn't know those were possible, in that case it's no more than a nice-to-have feature.