`#snippet` rest args and `@render` spread?

[brunnerh]

Describe the problem

Rest args and spreads are not supported, the latter of which has a specific error message:

cannot use spread arguments in {@render ...} tags

Is there a reason why these patterns are not supported?

{#snippet test(...items)}
	{#each items as item}
		<p>{item}</p>
	{/each}
{/snippet}

{@render test(1, 2, 3)}

REPL

<script>
	const args = [1, 2, 3];
</script>

{#snippet test(a, b, c)}
	{a} {b} {c}
{/snippet}

{@render test(...args)}

REPL

Describe the proposed solution

Support rest/spread analogous to how they work with functions.

Importance

nice to have

[Rich-Harris]

Is there a reason why these patterns are not supported?

Yeah, it doesn't work

[7nik]

That PR is turning

{@render foo(...[1, 2, count])} 

into

foo(node, ...[1, 2, $.get(count)].map(val => () => val) );

so of course, it isn't reactive. But shouldn't it be

const args = $.derived(() => [1, 2, $.get(count)]);
foo(node, ...args.map((_, i, arr) => arr[i]) );

Though, it still won't work well if the array size will change.
Edit: maybe it should just error out when the array size changes (plus mentioning in the docs) because snippets do not support resting anyway.