errors:

documentation/docs/98-reference/.generated/server-errors.md

@@ -1,5 +1,13 @@
 <!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->
 
+### async_in_sync
+
+```
+Encountered asynchronous work while rendering synchronously.
+```
+
+You (or the framework you're using) used `render` with an async component. Either use `renderAsync` or wrap the async component in a `svelte:boundary` with a `pending` snippet.
+
 ### lifecycle_function_unavailable
 
 ```

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

@@ -0,0 +1,5 @@
+## async_in_sync
+
+> Encountered asynchronous work while rendering synchronously.
+
+You (or the framework you're using) used `render` with an async component. Either use `renderAsync` or wrap the async component in a `svelte:boundary` with a `pending` snippet.

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

@@ -2,6 +2,18 @@
 
 export *  from '../shared/errors.js';
 
+/**
+ * Encountered asynchronous work while rendering synchronously.
+ * @returns {never}
+ */
+export function async_in_sync() {
+	const error = new Error(`async_in_sync\nEncountered asynchronous work while rendering synchronously.\nhttps://svelte.dev/e/async_in_sync`);
+
+	error.name = 'Svelte error';
+
+	throw error;
+}
+
 /**
  * `%name%(...)` is not available on the server
  * @param {string} name

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

@@ -1,3 +1,6 @@
+import { pop, push, set_ssr_context, ssr_context } from './context.js';
+import * as e from './errors.js';
+
 /** @typedef {'head' | 'body'} PayloadType */
 /** @typedef {{ [key in PayloadType]: string }} AccumulatedContent */
 /** @typedef {{ start: number, end: number, fn: (content: AccumulatedContent) => AccumulatedContent | Promise<AccumulatedContent> }} Compaction */
@@ -9,8 +12,6 @@
  * @typedef {string | Payload} PayloadItem
  */
 
-import { pop, push, set_ssr_context, ssr_context } from './context.js';
-
 /**
  * Payloads are basically a tree of `string | Payload`s, where each `Payload` in the tree represents
  * work that may or may not have completed. A payload can be {@link collect}ed to aggregate the
@@ -198,8 +199,9 @@ export class Payload {
 	 */
 	subsume(other) {
 		if (this.global.mode !== other.global.mode) {
-			// TODO message - this should be impossible though
+			throw new Error(
-			throw new Error('invariant: a payload cannot switch modes');
+				"invariant: A payload cannot switch modes. If you're seeing this, there's a compiler bug. File an issue!"
+			);
 		}
 
 		this.global.subsume(other.global);
@@ -228,7 +230,9 @@ export class Payload {
 		const content = Payload.#collect_content(to_compact, type);
 		const transformed_content = fn(content);
 		if (transformed_content instanceof Promise) {
-			throw new Error('invariant: should never reach this');
+			throw new Error(
+				"invariant: Somehow you've encountered asynchronous work while rendering synchronously. If you're seeing this, there's a compiler bug. File an issue!"
+			);
 		} else {
 			Payload.#push_accumulated_content(child, transformed_content);
 		}
@@ -281,8 +285,7 @@ export class Payload {
 		const result = render(child_payload);
 		if (result instanceof Promise) {
 			if (this.global.mode === 'sync') {
-				// TODO more-proper error
+				e.async_in_sync();
-				throw new Error('Encountered an asynchronous component while rendering synchronously');
 			}
 			// just to avoid unhandled promise rejections -- we'll end up throwing in `collect_async` if something fails
 			result.catch(() => {});