fix: disallow using `let:` directives with component render tags (#12400)

closes #12087

.changeset/grumpy-insects-sleep.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: disallow using `let:` directives with component render tags

packages/svelte/messages/shared-errors/errors.md

@@ -4,7 +4,7 @@
 
 ## render_tag_invalid_argument
 
-> 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 `$derived` and pass its result to `{@render ...}`
+> The argument to `{@render ...}` must be a snippet function, not a component or a slot with a `let:` directive or some other kind of function. If you want to dynamically render one snippet or another, use `$derived` and pass its result to `{@render ...}`
 
 ## snippet_used_as_component
 

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

@@ -1864,7 +1864,11 @@ export const template_visitors = {
 
 		let snippet_function = /** @type {import('estree').Expression} */ (context.visit(callee));
 		if (context.state.options.dev) {
-			snippet_function = b.call('$.validate_snippet', snippet_function);
+			snippet_function = b.call(
+				'$.validate_snippet',
+				snippet_function,
+				args.length ? b.id('$$props') : undefined
+			);
 		}
 
 		if (node.metadata.dynamic) {

packages/svelte/src/internal/shared/errors.js

@@ -20,12 +20,12 @@ export function lifecycle_outside_component(name) {
 }
 
 /**
- * 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 `$derived` and pass its result to `{@render ...}`
+ * The argument to `{@render ...}` must be a snippet function, not a component or a slot with a `let:` directive or some other kind of function. If you want to dynamically render one snippet or another, use `$derived` and pass its result to `{@render ...}`
  * @returns {never}
  */
 export function render_tag_invalid_argument() {
 	if (DEV) {
-		const error = new Error(`render_tag_invalid_argument\nThe 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 \`$derived\` and pass its result to \`{@render ...}\``);
+		const error = new Error(`render_tag_invalid_argument\nThe argument to \`{@render ...}\` must be a snippet function, not a component or a slot with a \`let:\` directive or some other kind of function. If you want to dynamically render one snippet or another, use \`$derived\` and pass its result to \`{@render ...}\``);
 
 		error.name = 'Svelte error';
 		throw error;

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

@@ -15,9 +15,10 @@ export function add_snippet_symbol(fn) {
 /**
  * Validate that the function handed to `{@render ...}` is a snippet function, and not some other kind of function.
  * @param {any} snippet_fn
+ * @param {Record<string, any> | undefined} $$props Only passed if render tag receives arguments
  */
-export function validate_snippet(snippet_fn) {
+export function validate_snippet(snippet_fn, $$props) {
-	if (snippet_fn && snippet_fn[snippet_symbol] !== true) {
+	if ($$props?.$$slots?.default || (snippet_fn && snippet_fn[snippet_symbol] !== true)) {
 		e.render_tag_invalid_argument();
 	}
 

packages/svelte/tests/runtime-legacy/shared.ts

@@ -117,7 +117,9 @@ export function runtime_suite(runes: boolean) {
 					(!config.test_ssr &&
 						config.html === undefined &&
 						config.ssrHtml === undefined &&
-						config.error === undefined)
+						config.error === undefined &&
+						config.runtime_error === undefined &&
+						!config.mode?.includes('server'))
 				) {
 					return 'no-test';
 				}

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

@@ -0,0 +1,8 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+	runtime_error: 'render_tag_invalid_argument'
+});

packages/svelte/tests/runtime-runes/samples/snippet-slot-let-error/inner.svelte

@@ -0,0 +1,5 @@
+<script>
+    let { children } = $props();
+</script>
+
+{@render children(true)}

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

@@ -0,0 +1,7 @@
+<script>
+	import Inner from './inner.svelte';
+</script>
+
+<Inner let:foo>
+	{foo}
+</Inner>

packages/svelte/tests/runtime-runes/samples/snippet-validation-error-1/_config.js

@@ -4,8 +4,5 @@ export default test({
 	compileOptions: {
 		dev: true
 	},
-	error:
+	error: 'render_tag_invalid_argument'
-		'render_tag_invalid_argument\n' +
-		'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 ...}`'
 });