add attachments symbol

packages/svelte/src/attachments/index.js

@@ -0,0 +1 @@
+export { ATTACHMENTS_SYMBOL as AttachmentsKey } from '../internal/client/constants.js';

packages/svelte/src/attachments/public.d.ts

@@ -0,0 +1,4 @@
+/**
+ * A unique symbol used for defining the attachments to be applied to an element or component.
+ */
+export const AttachmentsKey: unique symbol;

packages/svelte/src/internal/client/constants.js

@@ -25,3 +25,4 @@ export const STATE_SYMBOL = Symbol('$state');
 export const STATE_SYMBOL_METADATA = Symbol('$state metadata');
 export const LEGACY_PROPS = Symbol('legacy props');
 export const LOADING_ATTR_SYMBOL = Symbol('');
+export const ATTACHMENTS_SYMBOL = Symbol.for('svelte.attachments');

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -4,7 +4,7 @@ import { get_descriptors, get_prototype_of } from '../../../shared/utils.js';
 import { create_event, delegate } from './events.js';
 import { add_form_reset_listener, autofocus } from './misc.js';
 import * as w from '../../warnings.js';
-import { LOADING_ATTR_SYMBOL } from '../../constants.js';
+import { ATTACHMENTS_SYMBOL, LOADING_ATTR_SYMBOL } from '../../constants.js';
 import { queue_idle_task } from '../task.js';
 import { is_capture_event, is_delegated, normalize_attribute } from '../../../../utils.js';
 import {
@@ -416,8 +416,11 @@ export function set_attributes(
 		}
 	}
 
-	for (let symbol of Object.getOwnPropertySymbols(next)) {
+	const attachments = next[ATTACHMENTS_SYMBOL];
-		attach(element, () => next[symbol]);
+	if (attachments) {
+		for (let attachment of attachments) {
+			attach(element, () => attachment);
+		}
 	}
 
 	return current;

packages/svelte/types/index.d.ts

@@ -593,6 +593,13 @@ declare module 'svelte/action' {
 	export {};
 }
 
+declare module 'svelte/attachments' {
+	/**
+	 * A unique symbol used for defining the attachments to be applied to an element or component.
+	 */
+	export const AttachmentsKey: unique symbol;
+}
+
 declare module 'svelte/animate' {
 	// todo: same as Transition, should it be shared?
 	export interface AnimationConfig {