fix: correctly validate head snippets on the server (#15755)

* fix: correctly validate head snippets on the server

* put the logic in copy_payload so it gets treeshaken in most cases

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/dirty-zebras-do.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly validate head snippets on the server

packages/svelte/src/internal/server/dev.js

@@ -6,7 +6,7 @@ import {
 } from '../../html-tree-validation.js';
 import { current_component } from './context.js';
 import { invalid_snippet_arguments } from '../shared/errors.js';
-import { Payload } from './payload.js';
+import { HeadPayload, Payload } from './payload.js';
 
 /**
  * @typedef {{
@@ -105,7 +105,11 @@ export function pop_element() {
  * @param {Payload} payload
  */
 export function validate_snippet_args(payload) {
-	if (typeof payload !== 'object' || !(payload instanceof Payload)) {
+	if (
+		typeof payload !== 'object' ||
+		// for some reason typescript consider the type of payload as never after the first instanceof
+		!(payload instanceof Payload || /** @type {any} */ (payload) instanceof HeadPayload)
+	) {
 		invalid_snippet_arguments();
 	}
 }

packages/svelte/src/internal/server/payload.js

@@ -1,16 +1,25 @@
+export class HeadPayload {
+	/** @type {Set<{ hash: string; code: string }>} */
+	css = new Set();
+	out = '';
+	uid = () => '';
+	title = '';
+
+	constructor(css = new Set(), out = '', title = '', uid = () => '') {
+		this.css = css;
+		this.out = out;
+		this.title = title;
+		this.uid = uid;
+	}
+}
+
 export class Payload {
 	/** @type {Set<{ hash: string; code: string }>} */
 	css = new Set();
 	out = '';
 	uid = () => '';
 
-	head = {
-		/** @type {Set<{ hash: string; code: string }>} */
-		css: new Set(),
-		title: '',
-		out: '',
-		uid: () => ''
-	};
+	head = new HeadPayload();
 
 	constructor(id_prefix = '') {
 		this.uid = props_id_generator(id_prefix);
@@ -30,12 +39,11 @@ export function copy_payload({ out, css, head, uid }) {
 	payload.css = new Set(css);
 	payload.uid = uid;
 
-	payload.head = {
-		title: head.title,
-		out: head.out,
-		css: new Set(head.css),
-		uid: head.uid
-	};
+	payload.head = new HeadPayload();
+	payload.head.out = head.out;
+	payload.head.css = new Set(head.css);
+	payload.head.title = head.title;
+	payload.head.uid = head.uid;
 
 	return payload;
 }

packages/svelte/tests/runtime-runes/samples/head-payload-validation/_config.js

@@ -0,0 +1,11 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	mode: ['server'],
+	async test({ errors, assert }) {
+		assert.equal(errors, []);
+	}
+});

packages/svelte/tests/runtime-runes/samples/head-payload-validation/main.svelte

@@ -0,0 +1,7 @@
+{#snippet head()}
+	<title>Cool</title>
+{/snippet}
+
+<svelte:head>
+	{@render head()}
+</svelte:head>