fix: prevent null access on tracing_expressions for nested tracing

Author: paoloricciuti

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

@@ -119,7 +119,7 @@ export function trace(label, fn) {
 			console.groupEnd();
 		}
 
-		if (previously_tracing_expressions !== null) {
+		if (previously_tracing_expressions !== null && tracing_expressions !== null) {
 			for (const [signal, entry] of tracing_expressions.entries) {
 				var prev_entry = previously_tracing_expressions.get(signal);
 

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

@@ -0,0 +1,56 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+/**
+ * @param {any[]} logs
+ */
+function normalise_trace_logs(logs) {
+	let normalised = [];
+	for (let i = 0; i < logs.length; i++) {
+		const log = logs[i];
+
+		if (typeof log === 'string' && log.includes('%c')) {
+			const split = log.split('%c');
+			normalised.push({
+				log: (split[0].length !== 0 ? split[0] : split[1]).trim(),
+				highlighted: logs[i + 1] === 'color: CornflowerBlue; font-weight: bold'
+			});
+			i++;
+		} else if (log instanceof Error) {
+			continue;
+		} else {
+			normalised.push({ log });
+		}
+	}
+	return normalised;
+}
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	test({ assert, target, logs }) {
+		// initial log, everything is highlighted
+
+		assert.deepEqual(normalise_trace_logs(logs), [
+			{ log: 'iife', highlighted: false },
+			{ log: '$state', highlighted: true },
+			{ log: 0 },
+			{ log: 'effect', highlighted: false }
+		]);
+
+		logs.length = 0;
+
+		const button = target.querySelector('button');
+		button?.click();
+		flushSync();
+
+		assert.deepEqual(normalise_trace_logs(logs), [
+			{ log: 'iife', highlighted: false },
+			{ log: '$state', highlighted: true },
+			{ log: 1 },
+			{ log: 'effect', highlighted: false }
+		]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/inspect-trace-nested/main.svelte

@@ -0,0 +1,13 @@
+<script>
+	let count = $state(0);
+
+	$effect(() => {
+		$inspect.trace('effect');
+		(()=>{
+			$inspect.trace("iife");
+			count;
+		})();
+	});
+</script>
+
+<button onclick={() => count++}>{count}</button>