chore: improve output for <svelte:element>

[Rich-Harris]

• doesn't add spread_dynamic_element_attributes when there are no attributes — Unnecessary spread_dynamic_element_attributes call #9646 (TODO: why are we using this instead of the normal mechanism for setting attributes? Seems weird to always deopt to spread attributes. we can leave that issue open for now)
• skips the child render function altogether if there is nothing to do

Svelte 5 rewrite

Please note that the Svelte codebase is currently being rewritten for Svelte 5. Changes should target Svelte 5, which lives on the default branch (main).

If your PR concerns Svelte 4 (including updates to svelte.dev.docs), please ensure the base branch is svelte-4 and not main.

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• 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]

🦋 Changeset detected

Latest commit: 2ecc0bf

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
svelte-5-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Nov 27, 2023 6:18pm

[dummdidumm]

Re "why always spread" - I thought there was a comment explaining it. The tldr is that we don't know what kind of element this is, and depending on if it's a custom element or not the arguments need to be set differently. Hence the spread which detects this at runtime. The alternative would be to use a different method for single attribute sets that does the check

[Conduitry]

Suggested change

	chore: improve <svelte:element> generated code
	chore: improve `<svelte:element>` generated code

[dummdidumm]

I looked into creating dyamic_element_attr and invoke the custom element path or the regular attribute path, but that didn't catch the class, style and autofocus cases for regular elements, and adding that essentially means replicating the spread behavior, at which point we might as well use the spread function in all cases, anyway. Therefore I vote to keep it as is and not tune this further