fix: more accurately detect $derived migration use cases

packages/svelte/src/compiler/migrate/index.js

@@ -739,7 +739,7 @@ const instance_script = {
 			);
 			const bindings = ids.map((id) => state.scope.get(id.name));
 			const reassigned_bindings = bindings.filter((b) => b?.reassigned);
-			if (reassigned_bindings.length === 0 && !bindings.some((b) => b?.kind === 'store_sub')) {
+			if (reassigned_bindings.length === 0 && !bindings.some((b) => b?.kind === 'store_sub') && node.body.expression.left.type === "Identifier") {
 				let { start, end } = /** @type {{ start: number, end: number }} */ (
 					node.body.expression.right
 				);

packages/svelte/tests/migrate/samples/derivations/input.svelte

@@ -6,6 +6,11 @@
 	// no semicolon at the end
 	$: time_8 = count * 8
 	$: ({ time_16 } = { time_16: count * 16 })
+	// preceeding let that doesn't do anything
+	let time_32;
+	$: time_32 = count * doubled;
+	let very_high;
+	$: very_high = time_32 * count;
 </script>
 
 {count} / {doubled} / {quadrupled} / {time_8} / {time_16}

packages/svelte/tests/migrate/samples/effects/input.svelte

@@ -10,4 +10,7 @@
 		console.log('bar');
 	}
 	$: $count = 1;
+	$: $count.x = count;
 </script>
+
+<button onclick={() => count++}>increment</button>

packages/svelte/tests/migrate/samples/effects/output.svelte

@@ -18,4 +18,6 @@
 	run(() => {
 		$count = 1;
 	});
-</script>
+</script>
+
+<button onclick={() => count++}>increment</button>