fix: add warning when you read object property within attachments in legacy mode

.changeset/breezy-pears-punch.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: add warning when you read object property within attachments in legacy mode

documentation/docs/98-reference/.generated/compile-warnings.md

@@ -556,6 +556,12 @@ Elements with ARIA roles must use a valid, non-abstract ARIA role. A reference t
 <div role="toooltip"></div>
 ```
 
+### attachment_legacy_member_access
+
+```
+Using `@attach` with a function from an object in legacy mode can cause unnecessary reruns of the attachment function if you mutate that object.
+```
+
 ### attribute_avoid_is
 
 ```

packages/svelte/messages/compile-warnings/template.md

@@ -1,3 +1,7 @@
+## attachment_legacy_member_access
+
+> Using `@attach` with a function from an object in legacy mode can cause unnecessary reruns of the attachment function if you mutate that object.
+
 ## attribute_avoid_is
 
 > The "is" attribute is not supported cross-browser and should be avoided

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

@@ -1,7 +1,9 @@
 /** @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';
 
 /**
  * @param {AST.AttachTag} node
@@ -9,5 +11,16 @@ import { mark_subtree_dynamic } from './shared/fragment.js';
  */
 export function AttachTag(node, context) {
 	mark_subtree_dynamic(context.path);
+	if (!context.state.analysis.runes) {
+		walk(
+			node.expression,
+			{},
+			{
+				MemberExpression(node) {
+					w.attachment_legacy_member_access(node);
+				}
+			}
+		);
+	}
 	context.next({ ...context.state, expression: node.metadata.expression });
 }

packages/svelte/src/compiler/warnings.js

@@ -106,6 +106,7 @@ export const codes = [
 	'state_referenced_locally',
 	'store_rune_conflict',
 	'css_unused_selector',
+	'attachment_legacy_member_access',
 	'attribute_avoid_is',
 	'attribute_global_event_reference',
 	'attribute_illegal_colon',
@@ -677,6 +678,14 @@ export function css_unused_selector(node, name) {
 	w(node, 'css_unused_selector', `Unused CSS selector "${name}"\nhttps://svelte.dev/e/css_unused_selector`);
 }
 
+/**
+ * Using `@attach` with a function from an object in legacy mode can cause unnecessary reruns of the attachment function if you mutate that object.
+ * @param {null | NodeLike} node
+ */
+export function attachment_legacy_member_access(node) {
+	w(node, 'attachment_legacy_member_access', `Using \`@attach\` with a function from an object can cause unnecessary reruns of the attachment function if you mutate that object in legacy mode.\nhttps://svelte.dev/e/attachment_legacy_member_access`);
+}
+
 /**
  * The "is" attribute is not supported cross-browser and should be avoided
  * @param {null | NodeLike} node

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

@@ -0,0 +1,14 @@
+<script>
+	let state = {
+		count: 0,
+		attachment(){
+			
+		}
+	};
+</script>
+
+<button onclick={()=>{
+	state.count++;
+}}>{state.count}</button>
+
+<div {@attach state.attachment}></div>

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

@@ -0,0 +1,14 @@
+[
+	{
+		"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,
+			"column": 14
+		},
+		"end": {
+			"line": 14,
+			"column": 30
+		}
+	}
+]