feat: rune-prefer-let rule

[bfanger]

Svelte 5 allows using both let and const for signals, in regular JavaScript const means:

The const declaration creates an immutable reference to a value.
The variable identifier cannot be reassigned.
You should understand const declarations as "create a variable whose identity remains constant"

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const

But signals do change and are reassigned by Svelte's reactivity system.

This ESLint rule converts:

const { value } = $props()

into

let { value } = $props()

[changeset-bot]

🦋 Changeset detected

Latest commit: 642e4dd

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

This PR includes changesets to release 1 package

Name	Type
eslint-plugin-svelte	Minor

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

[baseballyama]

Thank you for the PR!

This is just small point but return value of $effect.root should use const according to the docs, so I think rune-declaration-kind or something like that is better rule name and we can check $effect.root also.

	const cleanup = $effect.root(() => {
		$effect(() => {
			console.log(count);
		});

		return () => {
			console.log('effect root cleanup');
		};
	});

https://svelte-5-preview.vercel.app/docs/runes#$effect-root

[ota-meshi]

Thank you for the rule suggestion and opening this PR!
However, I think this rule will conflict with the prefer-const rule in ESLint core as it is.
I think we need to consider how to avoid the conflict.
Could you please propose the rule in a new issue first?

[bfanger]

@ota-meshi I was thinking about implementing a svelte/prefer-const rule which would extend prefer-const but ignore signals declarations.

@baseballyama $effect.root is a rune, but not a signal, the svelte/prefer-const rule should convert it to a const

[baseballyama]

$derived is also rune. Not signal. So I think we need to use rune instead of signal for the rule name.

[bfanger]

$derived returns a signal: https://github.com/sveltejs/svelte/blob/8904e0f6cca7c423cd6232ba38870362cf840e63/packages/svelte/src/internal/client/reactivity/deriveds.js#L23

If the svelte compiler injects $.get() when you're using a variable then it's a signal.

[baseballyama]

I know. But we don't use the word "signals" for Svelte developers. We use "runes" instead. We need to use word which mentioned in the docs as much as possible to reduce cognitive cost.