fix: silence $inspect errors when the effect is about to be destroyed (#16391)

* fix: silence `$inspect` errors when the effect is about to be destroyed

* changeset

Author: Rich-Harris

.changeset/six-swans-rush.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: silence `$inspect` errors when the effect is about to be destroyed

packages/svelte/src/internal/client/dev/inspect.js

@@ -1,6 +1,6 @@
 import { UNINITIALIZED } from '../../../constants.js';
 import { snapshot } from '../../shared/clone.js';
-import { inspect_effect, validate_effect } from '../reactivity/effects.js';
+import { inspect_effect, render_effect, validate_effect } from '../reactivity/effects.js';
 import { untrack } from '../runtime.js';
 
 /**
@@ -12,29 +12,44 @@ export function inspect(get_value, inspector = console.log) {
 	validate_effect('$inspect');
 
 	let initial = true;
+	let error = /** @type {any} */ (UNINITIALIZED);
 
+	// Inspect effects runs synchronously so that we can capture useful
+	// stack traces. As a consequence, reading the value might result
+	// in an error (an `$inspect(object.property)` will run before the
+	// `{#if object}...{/if}` that contains it)
 	inspect_effect(() => {
-		/** @type {any} */
-		var value = UNINITIALIZED;
-
-		// Capturing the value might result in an exception due to the inspect effect being
-		// sync and thus operating on stale data. In the case we encounter an exception we
-		// can bail-out of reporting the value. Instead we simply console.error the error
-		// so at least it's known that an error occured, but we don't stop execution
 		try {
-			value = get_value();
-		} catch (error) {
-			// eslint-disable-next-line no-console
-			console.error(error);
+			var value = get_value();
+		} catch (e) {
+			error = e;
+			return;
 		}
 
-		if (value !== UNINITIALIZED) {
-			var snap = snapshot(value, true);
-			untrack(() => {
-				inspector(initial ? 'init' : 'update', ...snap);
-			});
-		}
+		var snap = snapshot(value, true);
+		untrack(() => {
+			inspector(initial ? 'init' : 'update', ...snap);
+		});
 
 		initial = false;
 	});
+
+	// If an error occurs, we store it (along with its stack trace).
+	// If the render effect subsequently runs, we log the error,
+	// but if it doesn't run it's because the `$inspect` was
+	// destroyed, meaning we don't need to bother
+	render_effect(() => {
+		try {
+			// call `get_value` so that this runs alongside the inspect effect
+			get_value();
+		} catch {
+			// ignore
+		}
+
+		if (error !== UNINITIALIZED) {
+			// eslint-disable-next-line no-console
+			console.error(error);
+			error = UNINITIALIZED;
+		}
+	});
 }

packages/svelte/tests/runtime-runes/samples/inspect-exception/_config.js

@@ -11,7 +11,7 @@ export default test({
 		b1?.click();
 		flushSync();
 
-		assert.ok(errors.length > 0);
+		assert.equal(errors.length, 0);
 		assert.deepEqual(logs, ['init', 'a', 'init', 'b']);
 	}
 });