prevent effects being created in unowned deriveds

Author: Rich-Harris

packages/svelte/messages/client-errors/errors.md

@@ -20,6 +20,10 @@
 
 > `%rune%` cannot be used inside an effect cleanup function
 
+## effect_in_unowned_derived
+
+> Effect cannot be created inside a `$derived` value that was not itself created inside an effect
+
 ## effect_orphan
 
 > `%rune%` can only be used inside an effect (e.g. during component initialisation)

packages/svelte/src/internal/client/errors.js

@@ -92,6 +92,22 @@ export function effect_in_teardown(rune) {
 	}
 }
 
+/**
+ * Effect cannot be created inside a `$derived` value that was not itself created inside an effect
+ * @returns {never}
+ */
+export function effect_in_unowned_derived() {
+	if (DEV) {
+		const error = new Error(`${"effect_in_unowned_derived"}\n${"Effect cannot be created inside a `$derived` value that was not itself created inside an effect"}`);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		// TODO print a link to the documentation
+		throw new Error("effect_in_unowned_derived");
+	}
+}
+
 /**
  * `%rune%` can only be used inside an effect (e.g. during component initialisation)
  * @param {string} rune

packages/svelte/src/internal/client/reactivity/effects.js

@@ -93,6 +93,10 @@ function create_effect(type, fn, sync) {
 	}
 
 	if (current_reaction !== null && !is_root) {
+		if ((current_reaction.f & (DERIVED | UNOWNED)) === (DERIVED | UNOWNED)) {
+			e.effect_in_unowned_derived();
+		}
+
 		push_effect(effect, current_reaction);
 	}