make mount/render optional, error if missing

Author: Rich-Harris

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

@@ -60,6 +60,10 @@
 
 > The `%rune%` rune is only available inside `.svelte` and `.svelte.js/ts` files
 
+## snippet_missing_mount
+
+> Snippets created with `createRawSnippet(...)` and used on the client must specify a `mount` function
+
 ## state_prototype_fixed
 
 > Cannot set prototype of `$state` object

packages/svelte/messages/server-errors/lifecycle.md

@@ -1,3 +1,7 @@
 ## lifecycle_function_unavailable
 
 > `%name%(...)` is not available on the server
+
+## snippet_missing_render
+
+> Snippets created with `createRawSnippet(...)` and used on the server must specify a `render` function

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

@@ -9,6 +9,7 @@ import {
 } from '../../runtime.js';
 import { hydrate_next, hydrate_node, hydrating } from '../hydration.js';
 import { assign_nodes } from '../template.js';
+import * as e from '../../errors.js';
 
 /**
  * @template {(node: TemplateNode, ...args: any[]) => void} SnippetFn
@@ -67,13 +68,17 @@ export function wrap_snippet(component, fn) {
  * Create a snippet imperatively using mount, hydrate and render functions.
  * @template {unknown[]} Params
  * @param {{
- * 	 mount: (...params: Getters<Params>) => Element,
+ *   mount?: (...params: Getters<Params>) => Element,
  *   hydrate?: (element: Element, ...params: Getters<Params>) => void,
- *   render: (...params: Params) => string
+ *   render?: (...params: Params) => string
  * }} options
  * @returns {import('svelte').Snippet<Params>}
  */
 export function createRawSnippet({ mount, hydrate }) {
+	if (mount === undefined) {
+		e.snippet_missing_mount();
+	}
+
 	return add_snippet_symbol(
 		(/** @type {TemplateNode} */ anchor, /** @type {Getters<Params>} */ ...params) => {
 			/** @type {Element} */

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

@@ -262,6 +262,22 @@ export function rune_outside_svelte(rune) {
 	}
 }
 
+/**
+ * Snippets created with `createRawSnippet(...)` and used on the client must specify a `mount` function
+ * @returns {never}
+ */
+export function snippet_missing_mount() {
+	if (DEV) {
+		const error = new Error(`snippet_missing_mount\nSnippets created with \`createRawSnippet(...)\` and used on the client must specify a \`mount\` function`);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		// TODO print a link to the documentation
+		throw new Error("snippet_missing_mount");
+	}
+}
+
 /**
  * Cannot set prototype of `$state` object
  * @returns {never}

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

@@ -8,6 +8,17 @@
 export function lifecycle_function_unavailable(name) {
 	const error = new Error(`lifecycle_function_unavailable\n\`${name}(...)\` is not available on the server`);
 
+	error.name = 'Svelte error';
+	throw error;
+}
+
+/**
+ * Snippets created with `createRawSnippet(...)` and used on the server must specify a `render` function
+ * @returns {never}
+ */
+export function snippet_missing_render() {
+	const error = new Error(`snippet_missing_render\nSnippets created with \`createRawSnippet(...)\` and used on the server must specify a \`render\` function`);
+
 	error.name = 'Svelte error';
 	throw error;
 }

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

@@ -14,6 +14,7 @@ import { DEV } from 'esm-env';
 import { current_component, pop, push } from './context.js';
 import { EMPTY_COMMENT, BLOCK_CLOSE, BLOCK_OPEN } from './hydration.js';
 import { add_snippet_symbol, validate_store } from '../shared/validate.js';
+import * as e from './errors.js';
 
 // https://html.spec.whatwg.org/multipage/syntax.html#attributes-2
 // https://infra.spec.whatwg.org/#noncharacter
@@ -158,12 +159,16 @@ export function head(payload, fn) {
 /**
  * Create a snippet imperatively using mount, hyrdate and render functions.
  * @param {{
- * 	 mount: (...params: any[]) => Element,
+ * 	 mount?: (...params: any[]) => Element,
  *   hydrate?: (element: Element, ...params: any[]) => void,
- *   render: (...params: any[]) => string
+ *   render?: (...params: any[]) => string
  * }} options
  */
 export function createRawSnippet({ render }) {
+	if (render === undefined) {
+		e.snippet_missing_render();
+	}
+
 	const snippet_fn = (/** @type {Payload} */ payload, /** @type {any[]} */ ...args) => {
 		payload.out += render(...args);
 	};

packages/svelte/types/index.d.ts

@@ -369,9 +369,9 @@ declare module 'svelte' {
 	 * Create a snippet imperatively using mount, hydrate and render functions.
 	 * */
 	export function createRawSnippet<Params extends unknown[]>({ mount, hydrate }: {
-		mount: (...params: Getters<Params>) => Element;
+		mount?: (...params: Getters<Params>) => Element;
 		hydrate?: (element: Element, ...params: Getters<Params>) => void;
-		render: (...params: Params) => string;
+		render?: (...params: Params) => string;
 	}): import("svelte").Snippet<Params>;
 	/**
 	 * Mounts a component to the given target and returns the exports and potentially the props (if compiled with `accessors: true`) of the component.