fix: capture infinite_loop_guard in error boundary

.changeset/hip-forks-exercise.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: capture infinite_loop_guard in error boundary

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

@@ -48,6 +48,9 @@ let scheduler_mode = FLUSH_MICROTASK;
 // Used for handling scheduling
 let is_micro_task_queued = false;
 
+/** @type {Effect | null} */
+let last_scheduled_effect = null;
+
 export let is_flushing_effect = false;
 export let is_destroying_effect = false;
 
@@ -532,27 +535,47 @@ export function update_effect(effect) {
 	}
 }
 
+function log_effect_stack() {
+	// eslint-disable-next-line no-console
+	console.error(
+		'Last ten effects were: ',
+		dev_effect_stack.slice(-10).map((d) => d.fn)
+	);
+	dev_effect_stack = [];
+}
+
 function infinite_loop_guard() {
 	if (flush_count > 1000) {
 		flush_count = 0;
-		if (DEV) {
-			try {
-				e.effect_update_depth_exceeded();
-			} catch (error) {
+		try {
+			e.effect_update_depth_exceeded();
+		} catch (error) {
+			if (DEV) {
 				// stack is garbage, ignore. Instead add a console.error message.
 				define_property(error, 'stack', {
 					value: ''
 				});
-				// eslint-disable-next-line no-console
-				console.error(
-					'Last ten effects were: ',
-					dev_effect_stack.slice(-10).map((d) => d.fn)
-				);
-				dev_effect_stack = [];
+			}
+			// Try and handle the error so it can be caught at a boundary, that's
+			// if there's an effect available from when it was last scheduled
+			if (last_scheduled_effect !== null) {
+				if (DEV) {
+					try {
+						handle_error(error, last_scheduled_effect, null, null);
+					} catch (e) {
+						// Only log the effect stack if the error is re-thrown
+						log_effect_stack();
+						throw e;
+					}
+				} else {
+					handle_error(error, last_scheduled_effect, null, null);
+				}
+			} else {
+				if (DEV) {
+					log_effect_stack();
+				}
 				throw error;
 			}
-		} else {
-			e.effect_update_depth_exceeded();
 		}
 	}
 	flush_count++;
@@ -637,8 +660,10 @@ function process_deferred() {
 	const previous_queued_root_effects = queued_root_effects;
 	queued_root_effects = [];
 	flush_queued_root_effects(previous_queued_root_effects);
+
 	if (!is_micro_task_queued) {
 		flush_count = 0;
+		last_scheduled_effect = null;
 		if (DEV) {
 			dev_effect_stack = [];
 		}
@@ -657,6 +682,8 @@ export function schedule_effect(signal) {
 		}
 	}
 
+	last_scheduled_effect = signal;
+
 	var effect = signal;
 
 	while (effect.parent !== null) {
@@ -776,6 +803,7 @@ export function flush_sync(fn) {
 		}
 
 		flush_count = 0;
+		last_scheduled_effect = null;
 		if (DEV) {
 			dev_effect_stack = [];
 		}

packages/svelte/tests/runtime-runes/samples/error-boundary-20/Child.svelte

@@ -0,0 +1,17 @@
+<script>
+	let count = $state(0);
+	let clicked = $state(false);
+
+	function increment() {
+		clicked = true;
+		count++;
+	}
+
+	$effect(() => {
+		if (clicked) {
+			count++;
+		}
+	});
+</script>
+
+<button onclick={increment}>{count}</button>

packages/svelte/tests/runtime-runes/samples/error-boundary-20/_config.js

@@ -0,0 +1,13 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		let btn = target.querySelector('button');
+
+		btn?.click();
+		flushSync();
+
+		assert.htmlEqual(target.innerHTML, `<div class="error">An error occurred!</div>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/error-boundary-20/main.svelte

@@ -0,0 +1,10 @@
+<script>
+	import Child from "./Child.svelte"
+</script>
+<svelte:boundary>
+	<Child />
+
+	{#snippet failed()}
+		<div class="error">An error occurred!</div>
+	{/snippet}
+</svelte:boundary>