no-hanging-unsubscribe

Subscribing to a signal hands you back a function that stops it. Drop that function and the subscription outlives whatever made it — and so does everything its callback closed over.

The rule reports a subscribe, onChange, or watch whose return value is discarded, when the signal being watched outlives the function doing the watching. That last clause is the whole rule, and it is what keeps it quiet on the common harmless case:

import { signal } from "@implementjs/core";
import { mode } from "@/lib/mode";

export function ModeToggle() {
	const preference = signal(mode.userPrefersMode.get());

	// fine — `preference` is created here, so it and the subscription
	// become garbage together when the component goes
	preference.onChange((value) => apply(value));

	// reported — `mode` is imported, so this subscription (and the closure
	// it drags along) lives for as long as the module does
	mode.userPrefersMode.subscribe((value) => preference.set(value));
}

The fix is to give the subscription an owner. ImplementLifecycle calls whatever onMount returns when it unmounts, so returning the unsubscribe is all it takes:

ImplementLifecycle({ onMount: () => mode.userPrefersMode.subscribe(apply) }, Content());

A subscription written at module scope is never reported — living as long as the app is usually the reason it is up there.

Options

"implementjs/no-hanging-unsubscribe": ["error", { checkParameters: false }]

checkParameters (default false) decides whether a signal that arrived as a parameter counts as outliving the function. It is off by default because a parameter's lifetime belongs to the caller, and both answers are common:

// `state` was created by the caller one line before this call, and dies with it
export function MenuRoot(state: MenuState, ...children: Child[]) {
	state.open.subscribe(onOpenChange);
}

// `id` is a router param that outlives every page mounted against it,
// so this one really does leak
export function IssueView(id: Readable<string>) {
	id.onChange(refetch);
}

Nothing in the syntax separates those. Turn checkParameters on if your components are mostly the second kind — routed pages and context consumers — and expect to add a few disable comments for the first kind.