Rule suggestion: $derived should not return a function

[dimfeld]

Motivation

AI Coding agents sometimes get a bit confused writing Svelte 5 and create code like this:

let x = $derived(() => values.filter((item) => item.active));

This, of course, makes x into a function instead of the value, and the coding agents often work around this by just changing the uses of x to x(), instead of fixing it properly to use derived.by or a single-expresssion $derived.

It would be helpful to have a lint rule for this.

Description

A $derived rune should not return a function. Either use a single expression with $derived or use $derived.by with a function.

Examples

<script>
// ✓ GOOD 
let x = $derived(expression);
let x = $derived.by(() => expression);
let x = $derived.by(() => {
  return complexExpression;
});
let x = $derived(calculate(value));
let x = $derived.by(calculate);

// ✗ BAD
let x =$derived(() => y);
let x =$derived(function() { return  y });
</script>

Additional comments

No response

[dimfeld]

I had Claude Code create a proof of concept of this. Coudl use some cleanup and types but it seems to work well in my initial testing https://gist.github.com/dimfeld/04d96e8315186f42939ee534b0be05ec