fix: `@attach` opt's you into runes mode

.changeset/famous-rocks-poke.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: `@attach` opt's you into runes mode

documentation/docs/03-template-syntax/09-@attach.md

@@ -7,6 +7,9 @@ Attachments are functions that run when an element is mounted to the DOM. Option
 > [!NOTE]
 > Attachments are available in Svelte 5.29 and newer.
 
+> [!NOTE]
+> Attachments also opt you in for runes mode so if you use them in a legacy component you would have to also migrate that component.
+
 ```svelte
 <!--- file: App.svelte --->
 <script>

documentation/docs/99-legacy/00-legacy-overview.md

@@ -9,6 +9,6 @@ The following pages document these features for
 - people still using Svelte 3/4
 - people using Svelte 5, but with components that haven't yet been migrated
 
-Since Svelte 3/4 syntax still works in Svelte 5, we will distinguish between _legacy mode_ and _runes mode_. Once a component is in runes mode (which you can opt into by using runes, or by explicitly setting the `runes: true` compiler option), legacy mode features are no longer available.
+Since Svelte 3/4 syntax still works in Svelte 5, we will distinguish between _legacy mode_ and _runes mode_. Once a component is in runes mode (which you can opt into by using runes or attachments, or by explicitly setting the `runes: true` compiler option), legacy mode features are no longer available.
 
 If you're exclusively interested in the Svelte 3/4 syntax, you can browse its documentation at [v4.svelte.dev](https://v4.svelte.dev).

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -403,7 +403,23 @@ export function analyze_component(root, source, options) {
 
 	const component_name = get_component_name(options.filename);
 
-	const runes = options.runes ?? Array.from(module.scope.references.keys()).some(is_rune);
+	let runes = options.runes ?? Array.from(module.scope.references.keys()).some(is_rune);
+
+	// if we are not in runes mode by the bindings we need to check if there's any attachment
+	if (!runes) {
+		// we need to do it beforehand otherwise we would not be in runes mode until we hit an attachment
+		walk(
+			/** @type {AST.SvelteNode} */ (template.ast),
+			{},
+			{
+				AttachTag(_, context) {
+					runes = true;
+					// once we found one we can stop the traversal
+					context.stop();
+				}
+			}
+		);
+	}
 
 	if (!runes) {
 		for (let check of synthetic_stores_legacy_check) {