class directive generates update code for unreactive dependencies

[pushkine]

<script>
	const o = { v: true }
</script>

<div class:test={o.v}>
	{o.v}
</div>

p(ctx, [dirty]) {
	if (dirty & /*o*/ 1) {
		toggle_class(div, "test", /*o*/ ctx[0].v);
	}
}

https://svelte.dev/repl/5b14ec60637b43a184460475790105c0?version=3.31.2

Given that <div>{o.v}</div> does not generate update code, neither should the class directive
Expected noop at fragment.p

[j-delaney]

I've been thinking a lot about this myself!

Because of how JavaScript const works, I think doing this optimization with all consts would be pretty hard. In your example you can change the value of o.v because const only guarantees that o refers to the same object, not that it doesn't change (which is pretty silly, but it's what they chose to do). See https://svelte.dev/repl/b435e75ce57f440b84993eb809eb7575?version=3 for an example. To get around that you would have to do some pretty gnarly code analysis (see if that variable is ever written to, including understanding if it can be passed into a function that writes to it).

I think a simpler, more direct optimization would be to limit this optimization to const primitives (e.g. only const o = true) because in those cases o can't be changed.

[Conduitry]

As of 3.32.1, the update method is now a no-op https://svelte.dev/repl/5b14ec60637b43a184460475790105c0?version=3.32.1