fix: reinstate reactivity loss tracking

.changeset/twelve-stars-serve.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: reinstate reactivity loss tracking

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

@@ -19,10 +19,10 @@ import {
 import { Batch, current_batch } from './batch.js';
 import {
 	async_derived,
-	current_async_effect,
+	active_async_effect,
 	derived,
 	derived_safe_equal,
-	set_from_async_derived
+	set_active_async_effect
 } from './deriveds.js';
 import { aborted } from './effects.js';
 
@@ -120,7 +120,7 @@ export function capture() {
 		if (activate_batch) previous_batch?.activate();
 
 		if (DEV) {
-			set_from_async_derived(null);
+			set_active_async_effect(null);
 			set_dev_stack(previous_dev_stack);
 		}
 	};
@@ -152,11 +152,11 @@ export async function save(promise) {
  * @returns {Promise<() => T>}
  */
 export async function track_reactivity_loss(promise) {
-	var previous_async_effect = current_async_effect;
+	var previous_async_effect = active_async_effect;
 	var value = await promise;
 
 	return () => {
-		set_from_async_derived(previous_async_effect);
+		set_active_async_effect(previous_async_effect);
 		return value;
 	};
 }
@@ -213,7 +213,7 @@ export function unset_context(deactivate_batch = true) {
 	if (deactivate_batch) current_batch?.deactivate();
 
 	if (DEV) {
-		set_from_async_derived(null);
+		set_active_async_effect(null);
 		set_dev_stack(null);
 	}
 }

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

@@ -36,7 +36,6 @@ import {
 import { eager_effects, internal_set, set_eager_effects, source } from './sources.js';
 import { get_error } from '../../shared/dev.js';
 import { async_mode_flag, tracing_mode_flag } from '../../flags/index.js';
-import { Boundary } from '../dom/blocks/boundary.js';
 import { component_context } from '../context.js';
 import { UNINITIALIZED } from '../../../constants.js';
 import { batch_values, current_batch } from './batch.js';
@@ -45,11 +44,11 @@ import { deferred, includes, noop } from '../../shared/utils.js';
 import { set_signal_status, update_derived_status } from './status.js';
 
 /** @type {Effect | null} */
-export let current_async_effect = null;
+export let active_async_effect = null;
 
 /** @param {Effect | null} v */
-export function set_from_async_derived(v) {
-	current_async_effect = v;
+export function set_active_async_effect(v) {
+	active_async_effect = v;
 }
 
 export const recent_async_deriveds = new Set();
@@ -123,7 +122,7 @@ export function async_derived(fn, label, location) {
 	var deferreds = new Map();
 
 	async_effect(() => {
-		if (DEV) current_async_effect = active_effect;
+		if (DEV) active_async_effect = active_effect;
 
 		/** @type {ReturnType<typeof deferred<V>>} */
 		var d = deferred();
@@ -139,7 +138,7 @@ export function async_derived(fn, label, location) {
 			unset_context();
 		}
 
-		if (DEV) current_async_effect = null;
+		if (DEV) active_async_effect = null;
 
 		var batch = /** @type {Batch} */ (current_batch);
 
@@ -156,7 +155,7 @@ export function async_derived(fn, label, location) {
 		 * @param {unknown} error
 		 */
 		const handler = (value, error = undefined) => {
-			current_async_effect = null;
+			if (DEV) active_async_effect = null;
 
 			batch.activate();
 

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

@@ -27,7 +27,7 @@ import {
 } from './constants.js';
 import { old_values } from './reactivity/sources.js';
 import {
-	destroy_derived_effects,
+	active_async_effect,
 	execute_derived,
 	freeze_derived_effects,
 	recent_async_deriveds,
@@ -58,6 +58,7 @@ import { UNINITIALIZED } from '../../constants.js';
 import { captured_signals } from './legacy.js';
 import { without_reactive_context } from './dom/elements/bindings/shared.js';
 import { set_signal_status, update_derived_status } from './reactivity/status.js';
+import * as w from './warnings.js';
 
 let is_updating_effect = false;
 
@@ -568,19 +569,17 @@ export function get(signal) {
 	}
 
 	if (DEV) {
-		// TODO reinstate this, but make it actually work
-		// if (current_async_effect) {
-		// 	var tracking = (current_async_effect.f & REACTION_IS_UPDATING) !== 0;
-		// 	var was_read = current_async_effect.deps?.includes(signal);
-
-		// 	if (!tracking && !untracking && !was_read) {
-		// 		w.await_reactivity_loss(/** @type {string} */ (signal.label));
+		if (
+			!untracking &&
+			active_async_effect &&
+			(active_async_effect.f & REACTION_IS_UPDATING) === 0
+		) {
+			w.await_reactivity_loss(/** @type {string} */ (signal.label));
 
-		// 		var trace = get_error('traced at');
-		// 		// eslint-disable-next-line no-console
-		// 		if (trace) console.warn(trace);
-		// 	}
-		// }
+			var trace = get_error('traced at');
+			// eslint-disable-next-line no-console
+			if (trace) console.warn(trace);
+		}
 
 		recent_async_deriveds.delete(signal);
 
@@ -595,7 +594,7 @@ export function get(signal) {
 			if (signal.trace) {
 				signal.trace();
 			} else {
-				var trace = get_error('traced at');
+				trace = get_error('traced at');
 
 				if (trace) {
 					var entry = tracing_expressions.entries.get(signal);