Rule to enforce type declaration of $state rune

[AntonioVentilii]

Motivation

Is there a rule that enforces the declaration of the types for $state? If not, is it worth doing?

Description

For consistency, we could have a rule similar to prefer-reduce-type-parameter, that clearly enforces where to define the type of a $state variable.

Examples

<script>

  // BAD
  let title: string | undefined = $state();
  let data: { value?: number } = $state({})
  
  // GOOD
  let title = $state<string | undefined>();
  let data = $state<{ value?: number }>({});
  
  // UNCHANGED
  let label = $state(0)

</script>

Additional comments

No response

[dimfeld]

Is there a reason that passing the type as a generic parameter is preferred? The docs suggest the "bad" way: https://svelte.dev/docs/svelte/typescript#Typing-$state

[baseballyama]

Maybe we can refer to sveltejs/svelte#10694 to understand why we don’t use generics for runes.

[marekdedic]

IMO, Rich's reasoning with inference doesn't apply to $state because unlike $props, the initializer value is a parameter and therefore TS will error out if it's incorrect.

However, IMHO, it's better to be consistent between $state and $props - so my preference would actually be a rule that does the exact opposite from the original proposal :D