WIP

Author: Rich-Harris

packages/svelte/src/compiler/phases/3-transform/client/visitors/AwaitExpression.js

@@ -24,7 +24,7 @@ export function AwaitExpression(node, context) {
 	// in dev, note which values are read inside a reactive expression,
 	// but don't track them
 	else if (dev && !is_ignored(node, 'await_reactivity_loss')) {
-		return b.call(b.await(b.call('$.save', argument, b.false)));
+		return b.call(b.await(b.call('$.track_reactivity_loss', argument, b.false)));
 	}
 
 	return argument === node.argument ? node : { ...node, argument };

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

@@ -98,7 +98,7 @@ export {
 	props_id,
 	with_script
 } from './dom/template.js';
-export { save } from './reactivity/async.js';
+export { save, track_reactivity_loss } from './reactivity/async.js';
 export { flushSync as flush, suspend } from './reactivity/batch.js';
 export {
 	async_derived,

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

@@ -65,33 +65,21 @@ export function flatten(sync, async, fn) {
 
 /**
  * Captures the current effect context so that we can restore it after
- * some asynchronous work has happened if `track` is true (so that e.g.
- * `await a + b` causes `b` to be registered as a dependency).
- *
- * If `track` is false, we just take a note of which async derived
- * brought us here, so that we can emit a `async_reactivity_loss`
- * warning when it's appropriate to do so.
- *
- * @param {boolean} track
+ * some asynchronous work has happened (so that e.g. `await a + b`
+ * causes `b` to be registered as a dependency).
  */
-export function capture(track = true) {
+function capture() {
 	var previous_effect = active_effect;
 	var previous_reaction = active_reaction;
 	var previous_component_context = component_context;
 
-	if (DEV && !track) {
-		var previous_async_effect = current_async_effect;
-	}
-
 	return function restore() {
-		if (track) {
-			set_active_effect(previous_effect);
-			set_active_reaction(previous_reaction);
-			set_component_context(previous_component_context);
-		}
+		set_active_effect(previous_effect);
+		set_active_reaction(previous_reaction);
+		set_component_context(previous_component_context);
 
 		if (DEV) {
-			set_from_async_derived(track ? null : previous_async_effect);
+			set_from_async_derived(null);
 		}
 
 		// prevent the active effect from outstaying its welcome
@@ -106,11 +94,10 @@ export function capture(track = true) {
  * `await a + b` becomes `(await $.save(a))() + b`
  * @template T
  * @param {Promise<T>} promise
- * @param {boolean} [track]
  * @returns {Promise<() => T>}
  */
-export async function save(promise, track = true) {
-	var restore = capture(track);
+export async function save(promise) {
+	var restore = capture();
 	var value = await promise;
 
 	return () => {
@@ -119,6 +106,23 @@ export async function save(promise, track = true) {
 	};
 }
 
+/**
+ * Reset `current_async_effect` after the `promise` resolves, so
+ * that we can emit `await_reactivity_loss` warnings
+ * @template T
+ * @param {Promise<T>} promise
+ * @returns {Promise<() => T>}
+ */
+export async function track_reactivity_loss(promise) {
+	var previous_async_effect = current_async_effect;
+	var value = await promise;
+
+	return () => {
+		set_from_async_derived(previous_async_effect);
+		return value;
+	};
+}
+
 function unset_context() {
 	set_active_effect(null);
 	set_active_reaction(null);