no-signal-collection

A signal notifies when you set it. Put a Set or a Map inside one and mutating it in place notifies nobody, so every change means copying the whole collection back through set:

const selected = signal(new Set<string>());

selected.update((s) => {
	const next = new Set(s);
	next.add(id);
	return next;
});

ImplementSet and ImplementMap are real Sets and Maps that are also readables, so their own mutators notify:

const selected = ImplementSet<string>();
selected.add(id); // notifies

The rule reports a signal() holding a collection, whether that is visible from the value (signal(new Set())), the type argument (signal<Map<string, number>>()), or the annotation on the declaration (const x: Signal<Set<string>> = …). The suggestion rewrites the call and adds the import, but only when there is no annotation left describing the old shape.

Opting out

Writing the readonly type is taken as a deliberate statement that the collection is replaced rather than mutated — which is a perfectly good reason to keep it in a signal — and opts out:

// not reported: replace-don't-mutate is the intent, and it is written down
const touched = signal<ReadonlySet<string>>(new Set());