feat: detach inert effects

[Rich-Harris]

We create a lot of effects in the course of rendering an application, but many of them don't do any work after the initial execution. In some cases, they shouldn't really be effects in the first place — we're just abusing effects to defer some work until the DOM has been updated, and the actual position in the tree doesn't matter:

svelte/packages/svelte/src/internal/client/dom/elements/misc.js

Lines 15 to 19 in ddb0bc3

	effect(() => {
	if (document.activeElement === body) {
	dom.focus();
	}
	});

In other cases, they do need to be effects because we don't know whether they'll contain anything dynamic (e.g. an expression like <p>{location.href}</p>, or because the position in the tree is important (e.g. onMount). But there's no need to keep them around if they're never going to update.

This PR does two things:

• if an immediate effect contains no dependencies, has no children, contains no DOM and returns no teardown function (hereafter: is "inert"), don't add it to the tree in the first place
• for deferred effects, check if the effect is inert after it executes, and detach from the tree if so

I imagined this would be a fairly modest gain, but it's actually quite substantial. For a simple test like this...

{#each Array(1e4) as _}
  <p>{Math.floor(100)}</p>
{/each}

memory usage in Chrome in prod mode drops 11% from 10MB to 8.9MB.

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• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests and linting

• Run the tests with pnpm test and lint the project with pnpm lint

[changeset-bot]

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Latest commit: 02d2bda

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[dummdidumm]

Is it safe to execute the effect before linking it to its parent? If we have error boundaries in the future, I imagine we have to walk up the effect tree, but it's not constructed yet then.

[trueadm]

That's an interesting point actually. If we have error boundaries in the future then the link will need to be there so we can find the nearest error boundary effect.

[Rich-Harris]

The parent doesn't need to know about the child as long as the child knows about the parent, surely?

[dummdidumm]

oh right, the parent is set - yeah that is probably enough