fix: disallow mounting a snippet

fixes #11264

Author: dummdidumm

.changeset/itchy-panthers-shave.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: disallow mounting a snippet

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

@@ -21,6 +21,7 @@ import { handle_event_propagation } from './dom/elements/events.js';
 import { reset_head_anchor } from './dom/blocks/svelte-head.js';
 import * as w from './warnings.js';
 import * as e from './errors.js';
+import { validate_component } from '../shared/validate.js';
 
 /** @type {Set<string>} */
 export const all_registered_events = new Set();
@@ -102,6 +103,10 @@ export function stringify(value) {
  * @returns {Exports}
  */
 export function mount(component, options) {
+	if (DEV) {
+		validate_component(component);
+	}
+
 	const anchor = options.anchor ?? options.target.appendChild(empty());
 	// Don't flush previous effects to ensure order of outer effects stays consistent
 	return flush_sync(() => _mount(component, { ...options, anchor }), false);
@@ -125,6 +130,10 @@ export function mount(component, options) {
  * @returns {Exports}
  */
 export function hydrate(component, options) {
+	if (DEV) {
+		validate_component(component);
+	}
+
 	const target = options.target;
 	const previous_hydrate_nodes = hydrate_nodes;
 

packages/svelte/tests/runtime-runes/samples/mount-snippet-error/_config.js

@@ -0,0 +1,12 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	async test({ assert, target }) {
+		const div = target.querySelector('div');
+		assert.htmlEqual(div?.innerHTML || '', '');
+	},
+	runtime_error: 'snippet_used_as_component\nA snippet must be rendered with `{@render ...}`'
+});

packages/svelte/tests/runtime-runes/samples/mount-snippet-error/main.svelte

@@ -0,0 +1,14 @@
+<script>
+    import { onMount, mount } from 'svelte';
+
+    let el;
+
+    onMount(() => {
+        mount(foo, { target: el });
+    });
+</script>
+
+<div bind:this={el}></div>
+{#snippet foo()}
+    shouldnt be rendered
+{/snippet}