only warn if the object is mutated

packages/svelte/src/compiler/phases/2-analyze/visitors/AttachTag.js

@@ -1,7 +1,6 @@
 /** @import { AST } from '#compiler' */
 /** @import { Context } from '../types' */
 
-import { walk } from 'zimmerframe';
 import { mark_subtree_dynamic } from './shared/fragment.js';
 import * as w from '../../../warnings.js';
 
@@ -11,16 +10,15 @@ import * as w from '../../../warnings.js';
  */
 export function AttachTag(node, context) {
 	mark_subtree_dynamic(context.path);
+
+	context.next({ ...context.state, expression: node.metadata.expression });
+
 	if (!context.state.analysis.runes) {
-		walk(
-			node.expression,
-			{},
-			{
-				MemberExpression(node) {
-					w.attachment_legacy_member_access(node);
-				}
+		for (const dep of node.metadata.expression.dependencies) {
+			if (dep.mutated) {
+				w.attachment_legacy_member_access(node.expression);
+				break;
 			}
-		);
+		}
 	}
-	context.next({ ...context.state, expression: node.metadata.expression });
 }

packages/svelte/tests/validator/samples/attachment-legacy-member-access/input.svelte

@@ -1,9 +1,14 @@
 <script>
+	// mutated, so should warn
 	let state = {
 		count: 0,
-		attachment(){
+		attachment(){}
+	};
 
-		}
+	// unmutated, so should not warn
+	let attachments = {
+		foo() {},
+		bar() {}
 	};
 </script>
 
@@ -12,3 +17,6 @@
 }}>{state.count}</button>
 
 <div {@attach state.attachment}></div>
+
+<div {@attach attachments.foo}></div>
+<div {@attach attachments.bar}></div>

packages/svelte/tests/validator/samples/attachment-legacy-member-access/warnings.json

@@ -3,11 +3,11 @@
 		"code": "attachment_legacy_member_access",
 		"message": "Using `@attach` with a function from an object in legacy mode can cause unnecessary reruns of the attachment function if you mutate that object.",
 		"start": {
-			"line": 14,
+			"line": 19,
 			"column": 14
 		},
 		"end": {
-			"line": 14,
+			"line": 19,
 			"column": 30
 		}
 	}