fix: ensure inserted code is preserved during migration (#13617)

* fix: moving deriveds during migration deletes part of the inserted code

* fix: use update instead of remove

* Update .changeset/cool-apes-confess.md

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/cool-apes-confess.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure inserted code is preserved during migration 

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

@@ -273,7 +273,7 @@ export function migrate(source, { filename } = {}) {
 				const { start, end } = get_node_range(source, node);
 				str.appendLeft(end, '\n');
 				str.move(start, end, /** @type {number} */ (parsed.instance?.content.end));
-				str.remove(start - (source[start - 2] === '\r' ? 2 : 1), start);
+				str.update(start - (source[start - 2] === '\r' ? 2 : 1), start, '');
 			}
 		}
 

packages/svelte/tests/migrate/samples/reactive-statements-reorder-not-deleting-additions/input.svelte

@@ -0,0 +1,12 @@
+<script>
+	export let data
+	$: ({ foo } = data)
+
+	import { blah } from './blah.js'
+
+	let bar
+	$: {
+		bar = []
+		let baz
+	}
+</script>

packages/svelte/tests/migrate/samples/reactive-statements-reorder-not-deleting-additions/output.svelte

@@ -0,0 +1,15 @@
+<script>
+	import { run } from 'svelte/legacy';
+
+
+	import { blah } from './blah.js'
+	/** @type {{data: any}} */
+	let { data } = $props();
+
+	let bar = $state()
+	let { foo } = $derived(data)
+	run(() => {
+		bar = []
+		let baz
+	});
+</script>