restrict which symbols are recognised as attachment keys

packages/svelte/package.json

@@ -33,6 +33,10 @@
       "types": "./types/index.d.ts",
       "default": "./src/animate/index.js"
     },
+    "./attachments": {
+      "types": "./types/index.d.ts",
+      "default": "./src/attachments/index.js"
+    },
     "./compiler": {
       "types": "./types/index.d.ts",
       "require": "./compiler/index.js",

packages/svelte/scripts/generate-types.js

@@ -30,6 +30,7 @@ await createBundle({
 		[pkg.name]: `${dir}/src/index.d.ts`,
 		[`${pkg.name}/action`]: `${dir}/src/action/public.d.ts`,
 		[`${pkg.name}/animate`]: `${dir}/src/animate/public.d.ts`,
+		[`${pkg.name}/attachments`]: `${dir}/src/attachments/public.d.ts`,
 		[`${pkg.name}/compiler`]: `${dir}/src/compiler/public.d.ts`,
 		[`${pkg.name}/easing`]: `${dir}/src/easing/index.js`,
 		[`${pkg.name}/legacy`]: `${dir}/src/legacy/legacy-client.js`,

packages/svelte/src/attachments/index.js

@@ -0,0 +1,4 @@
+export {
+	create_attachment_key as createAttachmentKey,
+	is_attachment_key as isAttachmentKey
+} from '../internal/client/dom/elements/attachments.js';

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/component.js

@@ -257,7 +257,7 @@ export function build_component(node, component_name, context, anchor = context.
 			push_prop(
 				b.prop(
 					'get',
-					b.call('Symbol', b.literal('@attach')),
+					b.call('$.create_attachment_key'),
 					/** @type {Expression} */ (context.visit(attribute.expression)),
 					true
 				)

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

@@ -1,5 +1,24 @@
 import { effect } from '../../reactivity/effects.js';
 
+const key = `@attach-${Math.random().toString(36).slice(2)}`;
+const name = `Symbol(${key})`;
+
+// TODO this feels a bit belt-and-braces to me, tbh — are we sure we need it?
+/**
+ * Creates a `Symbol` that Svelte recognises as an attachment key
+ */
+export function create_attachment_key() {
+	return Symbol(key);
+}
+
+/**
+ * Returns `true` if the symbol was created with `createAttachmentKey`
+ * @param {string | symbol} key
+ */
+export function is_attachment_key(key) {
+	return typeof key === 'symbol' && key.toString() === name;
+}
+
 /**
  * @param {Element} node
  * @param {() => (node: Element) => void} get_fn

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

@@ -13,7 +13,7 @@ import {
 	set_active_effect,
 	set_active_reaction
 } from '../../runtime.js';
-import { attach } from './attachments.js';
+import { attach, is_attachment_key } from './attachments.js';
 
 /**
  * The value/checked attribute in the template actually corresponds to the defaultValue property, so we need
@@ -242,8 +242,8 @@ export function set_custom_element_data(node, prop, value) {
 /**
  * Spreads attributes onto a DOM element, taking into account the currently set attributes
  * @param {Element & ElementCSSInlineStyle} element
- * @param {Record<string, any> | undefined} prev
- * @param {Record<string, any>} next New attributes - this function mutates this object
+ * @param {Record<string | symbol, any> | undefined} prev
+ * @param {Record<string | symbol, any>} next New attributes - this function mutates this object
  * @param {string} [css_hash]
  * @param {boolean} [preserve_attribute_case]
  * @param {boolean} [is_custom_element]
@@ -409,7 +409,9 @@ export function set_attributes(
 	}
 
 	for (let symbol of Object.getOwnPropertySymbols(next)) {
-		attach(element, () => next[symbol]);
+		if (is_attachment_key(symbol)) {
+			attach(element, () => next[symbol]);
+		}
 	}
 
 	return current;

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

@@ -27,7 +27,7 @@ export { element } from './dom/blocks/svelte-element.js';
 export { head } from './dom/blocks/svelte-head.js';
 export { append_styles } from './dom/css.js';
 export { action } from './dom/elements/actions.js';
-export { attach } from './dom/elements/attachments.js';
+export { attach, create_attachment_key, is_attachment_key } from './dom/elements/attachments.js';
 export {
 	remove_input_defaults,
 	set_attribute,

packages/svelte/tests/runtime-runes/samples/attachment-spread/main.svelte

@@ -1,6 +1,8 @@
 <script>
+	import { createAttachmentKey } from 'svelte/attachments';
+
 	let stuff = $state({
-		[Symbol()]: () => console.log('hello')
+		[createAttachmentKey()]: () => console.log('hello')
 	});
 </script>