fix: check that snippet is not rendered as a component (#9423)

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

.changeset/good-pianos-jump.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: check that snippet is not rendered as a component

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

@@ -850,7 +850,14 @@ function serialize_inline_component(node, component_name, context) {
 					b.thunk(b.array(props_and_spreads.map((p) => (Array.isArray(p) ? b.object(p) : p))))
 			  );
 	/** @param {import('estree').Identifier} node_id */
-	let fn = (node_id) => b.call(component_name, node_id, props_expression);
+	let fn = (node_id) =>
+		b.call(
+			context.state.options.dev
+				? b.call('$.validate_component', b.id(component_name))
+				: component_name,
+			node_id,
+			props_expression
+		);
 
 	if (bind_this !== null) {
 		const prev = fn;

packages/svelte/src/compiler/phases/3-transform/server/transform-server.js

@@ -882,7 +882,12 @@ function serialize_inline_component(node, component_name, context) {
 	/** @type {import('estree').Statement} */
 	let statement = b.stmt(
 		(typeof component_name === 'string' ? b.call : b.maybe_call)(
-			component_name,
+			context.state.options.dev
+				? b.call(
+						'$.validate_component',
+						typeof component_name === 'string' ? b.id(component_name) : component_name
+				  )
+				: component_name,
 			b.id('$$payload'),
 			props_expression
 		)

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

@@ -102,13 +102,13 @@ export function loop_guard(timeout) {
 	};
 }
 
-const symbol = Symbol.for('svelte.snippet');
+const snippet_symbol = Symbol.for('svelte.snippet');
 
 /**
  * @param {any} fn
  */
 export function add_snippet_symbol(fn) {
-	fn[symbol] = true;
+	fn[snippet_symbol] = true;
 	return fn;
 }
 
@@ -117,7 +117,7 @@ export function add_snippet_symbol(fn) {
  * @param {any} snippet_fn
  */
 export function validate_snippet(snippet_fn) {
-	if (snippet_fn[symbol] !== true) {
+	if (snippet_fn[snippet_symbol] !== true) {
 		throw new Error(
 			'The argument to `{@render ...}` must be a snippet function, not a component or some other kind of function. ' +
 				'If you want to dynamically render one snippet or another, use `$derived` and pass its result to `{@render ...}`.'
@@ -125,3 +125,14 @@ export function validate_snippet(snippet_fn) {
 	}
 	return snippet_fn;
 }
+
+/**
+ * Validate that the function behind `<Component />` isn't a snippet.
+ * @param {any} component_fn
+ */
+export function validate_component(component_fn) {
+	if (component_fn?.[snippet_symbol] === true) {
+		throw new Error('A snippet must be rendered with `{@render ...}`');
+	}
+	return component_fn;
+}

packages/svelte/tests/runtime-runes/samples/snippet-validation-error-2/_config.js

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	error: 'A snippet must be rendered with `{@render ...}`'
+});

packages/svelte/tests/runtime-runes/samples/snippet-validation-error-2/main.svelte

@@ -0,0 +1,5 @@
+{#snippet Foo()}
+	<p>hello</p>
+{/snippet}
+
+<Foo />