use types rather than runtime validation to prevent incorrect snippet/component usage

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

@@ -2,14 +2,6 @@
 
 > `%name%(...)` can only be used during component initialisation
 
-## render_tag_invalid_argument
-
-> 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
-
-> A snippet must be rendered with `{@render ...}`
-
 ## store_invalid_shape
 
 > `%name%` is not a store with a `subscribe` method

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

@@ -969,13 +969,7 @@ function serialize_inline_component(node, expression, context) {
 			lets.length === 0 &&
 			children.default.every((node) => node.type !== 'SvelteFragment')
 		) {
-			push_prop(
+			push_prop(b.prop('init', b.id('children'), slot_fn));
-				b.prop(
-					'init',
-					b.id('children'),
-					context.state.options.dev ? b.call('$.add_snippet_symbol', slot_fn) : slot_fn
-				)
-			);
 			// We additionally add the default slot as a boolean, so that the slot render function on the other
 			// side knows it should get the content to render from $$props.children
 			serialized_slots.push(b.init('default', b.true));
@@ -1501,10 +1495,6 @@ const template_visitors = {
 		fn.___snippet = true;
 		// TODO hoist where possible
 		context.state.init.push(fn);
-
-		if (context.state.options.dev) {
-			context.state.init.push(b.stmt(b.call('$.add_snippet_symbol', node.expression)));
-		}
 	},
 	Component(node, context) {
 		serialize_inline_component(node, b.id(node.name), context);

packages/svelte/src/index.d.ts

@@ -1,5 +1,6 @@
 // This should contain all the public interfaces (not all of them are actually importable, check current Svelte for which ones are).
 
+import type { Getters } from '#shared';
 import './ambient.js';
 
 /**
@@ -104,6 +105,10 @@ export class SvelteComponent<
 	$set(props: Partial<Props>): void;
 }
 
+declare const brand: unique symbol;
+type Brand<B> = { [brand]: B };
+type Branded<T, B> = T & Brand<B>;
+
 /**
  * Can be used to create strongly typed Svelte components.
  *
@@ -136,7 +141,8 @@ export interface Component<
 	 * @param props The props passed to the component.
 	 */
 	(
-		internal: unknown,
+		this: void,
+		internal: Branded<{}, 'ComponentInternals'>,
 		props: Props
 	): {
 		/**
@@ -271,13 +277,12 @@ declare const SnippetReturn: unique symbol;
 export interface Snippet<Parameters extends unknown[] = []> {
 	(
 		this: void,
+		internal: Branded<{}, 'SnippetInternals'>,
 		// this conditional allows tuples but not arrays. Arrays would indicate a
 		// rest parameter type, which is not supported. If rest parameters are added
 		// in the future, the condition can be removed.
-		...args: number extends Parameters['length'] ? never : Parameters
+		...args: number extends Parameters['length'] ? never : Getters<Parameters>
-	): typeof SnippetReturn & {
+	): void;
-		_: 'functions passed to {@render ...} tags must use the `Snippet` type imported from "svelte"';
-	};
 }
 
 interface DispatchOptions {

packages/svelte/src/internal/client/dom/blocks/snippet.js

@@ -1,7 +1,6 @@
 /** @import { Snippet } from 'svelte' */
 /** @import { Effect, TemplateNode } from '#client' */
 /** @import { Getters } from '#shared' */
-import { add_snippet_symbol } from '../../../shared/validate.js';
 import { EFFECT_TRANSPARENT } from '../../constants.js';
 import { branch, block, destroy_effect, teardown } from '../../reactivity/effects.js';
 import {
@@ -50,10 +49,11 @@ export function snippet(node, get_snippet, ...args) {
  * In development, wrap the snippet function so that it passes validation, and so that the
  * correct component context is set for ownership checks
  * @param {any} component
- * @param {(node: TemplateNode, ...args: any[]) => void} fn
+ * @param {Snippet} fn
+ * @returns {Snippet}
  */
 export function wrap_snippet(component, fn) {
-	return add_snippet_symbol((/** @type {TemplateNode} */ node, /** @type {any[]} */ ...args) => {
+	return (node, ...args) => {
 		var previous_component_function = dev_current_component_function;
 		set_dev_current_component_function(component);
 
@@ -62,7 +62,7 @@ export function wrap_snippet(component, fn) {
 		} finally {
 			set_dev_current_component_function(previous_component_function);
 		}
-	});
+	};
 }
 
 /**
@@ -75,29 +75,27 @@ export function wrap_snippet(component, fn) {
  * @returns {Snippet<Params>}
  */
 export function createRawSnippet(fn) {
-	return add_snippet_symbol(
+	return (anchor, ...params) => {
-		(/** @type {TemplateNode} */ anchor, /** @type {Getters<Params>} */ ...params) => {
+		var snippet = fn(...params);
-			var snippet = fn(...params);
 
-			/** @type {Element} */
+		/** @type {Element} */
-			var element;
+		var element;
 
-			if (hydrating) {
+		if (hydrating) {
-				element = /** @type {Element} */ (hydrate_node);
+			element = /** @type {Element} */ (hydrate_node);
-				hydrate_next();
+			hydrate_next();
-			} else {
+		} else {
-				var html = snippet.render().trim();
+			var html = snippet.render().trim();
-				var fragment = create_fragment_from_html(html);
+			var fragment = create_fragment_from_html(html);
-				element = /** @type {Element} */ (fragment.firstChild);
+			element = /** @type {Element} */ (fragment.firstChild);
-				anchor.before(element);
+			/** @type {TemplateNode} */ (/** @type {unknown} */ (anchor)).before(element);
-			}
+		}
 
-			const result = snippet.setup?.(element);
+		const result = snippet.setup?.(element);
-			assign_nodes(element, element);
+		assign_nodes(element, element);
 
-			if (typeof result === 'function') {
+		if (typeof result === 'function') {
-				teardown(result);
+			teardown(result);
-			}
 		}
-	);
+	};
 }

packages/svelte/src/internal/server/blocks/snippet.js

@@ -1,7 +1,6 @@
 /** @import { Snippet } from 'svelte' */
 /** @import { Payload } from '#server' */
 /** @import { Getters } from '#shared' */
-import { add_snippet_symbol } from '../../shared/validate.js';
 
 /**
  * Create a snippet programmatically
@@ -13,10 +12,10 @@ import { add_snippet_symbol } from '../../shared/validate.js';
  * @returns {Snippet<Params>}
  */
 export function createRawSnippet(fn) {
-	return add_snippet_symbol((/** @type {Payload} */ payload, /** @type {Params} */ ...args) => {
+	return (payload, ...args) => {
 		var getters = /** @type {Getters<Params>} */ (args.map((value) => () => value));
-		payload.out += fn(...getters)
+		/** @type {Payload} */ (/** @type {unknown} */ (payload)).out += fn(...getters)
 			.render()
 			.trim();
-	});
+	};
 }

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

@@ -555,7 +555,6 @@ export { push_element, pop_element } from './dev.js';
 export { snapshot } from '../shared/clone.js';
 
 export {
-	add_snippet_symbol,
 	validate_component,
 	validate_dynamic_element_tag,
 	validate_snippet,

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

@@ -4,27 +4,13 @@ import { is_void } from '../../constants.js';
 import * as w from './warnings.js';
 import * as e from './errors.js';
 
-const snippet_symbol = Symbol.for('svelte.snippet');
-
-/**
- * @param {any} fn
- * @returns {import('svelte').Snippet}
- */
-export function add_snippet_symbol(fn) {
-	fn[snippet_symbol] = true;
-	return 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} $$props Only passed if render tag receives arguments and is for the children prop
  */
 export function validate_snippet(snippet_fn, $$props) {
-	if (
+	if ($$props?.$$slots?.default && typeof $$props.$$slots.default !== 'boolean') {
-		($$props?.$$slots?.default && typeof $$props.$$slots.default !== 'boolean') ||
-		(snippet_fn && snippet_fn[snippet_symbol] !== true)
-	) {
 		e.render_tag_invalid_argument();
 	}
 
@@ -36,11 +22,7 @@ export function validate_snippet(snippet_fn, $$props) {
  * @param {any} component_fn
  */
 export function validate_component(component_fn) {
-	if (component_fn?.[snippet_symbol] === true) {
+	return component_fn; // TODO get rid
-		e.snippet_used_as_component();
-	}
-
-	return component_fn;
 }
 
 /**

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

@@ -1,12 +0,0 @@
-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

@@ -1,14 +0,0 @@
-<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}

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

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

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

@@ -1,7 +0,0 @@
-<script>
-	function not_a_snippet() {
-		console.log('hello');
-	}
-</script>
-
-{@render not_a_snippet()}

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

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

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

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