opt into runes mode when using blocking await

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -203,9 +203,9 @@ function js(script, root, allow_reactive_declarations, parent) {
 		body: []
 	};
 
-	const { scope, scopes } = create_scopes(ast, root, allow_reactive_declarations, parent);
+	const { scope, scopes, is_async } = create_scopes(ast, root, allow_reactive_declarations, parent);
 
-	return { ast, scope, scopes };
+	return { ast, scope, scopes, is_async };
 }
 
 /**
@@ -230,7 +230,7 @@ const RESERVED = ['$$props', '$$restProps', '$$slots'];
  * @returns {Analysis}
  */
 export function analyze_module(ast, options) {
-	const { scope, scopes } = create_scopes(ast, new ScopeRoot(), false, null);
+	const { scope, scopes, is_async } = create_scopes(ast, new ScopeRoot(), false, null);
 
 	for (const [name, references] of scope.references) {
 		if (name[0] !== '$' || RESERVED.includes(name)) continue;
@@ -259,7 +259,7 @@ export function analyze_module(ast, options) {
 	);
 
 	return {
-		module: { ast, scope, scopes },
+		module: { ast, scope, scopes, is_async },
 		name: options.filename,
 		accessors: false,
 		runes: true,
@@ -282,7 +282,12 @@ export function analyze_component(root, source, options) {
 	const module = js(root.module, scope_root, false, null);
 	const instance = js(root.instance, scope_root, true, module.scope);
 
-	const { scope, scopes } = create_scopes(root.fragment, scope_root, false, instance.scope);
+	const { scope, scopes, is_async } = create_scopes(
+		root.fragment,
+		scope_root,
+		false,
+		instance.scope
+	);
 
 	/** @type {Template} */
 	const template = { ast: root.fragment, scope, scopes };
@@ -390,7 +395,9 @@ export function analyze_component(root, source, options) {
 
 	const component_name = get_component_name(options.filename);
 
-	const runes = options.runes ?? Array.from(module.scope.references.keys()).some(is_rune);
+	const runes =
+		options.runes ??
+		(is_async || instance.is_async || Array.from(module.scope.references.keys()).some(is_rune));
 
 	if (!runes) {
 		for (let check of synthetic_stores_legacy_check) {

packages/svelte/src/compiler/phases/scope.js

@@ -345,7 +345,24 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 		}
 	};
 
+	let is_async = false;
+
 	walk(ast, state, {
+		AwaitExpression(node, context) {
+			// this doesn't _really_ belong here, but it allows us to
+			// automatically opt into runes mode on encountering
+			// blocking awaits, without doing an additional walk
+			// before the analysis occurs
+			is_async ||= context.path.every(
+				({ type }) =>
+					type !== 'ArrowFunctionExpression' &&
+					type !== 'FunctionExpression' &&
+					type !== 'FunctionDeclaration'
+			);
+
+			context.next();
+		},
+
 		// references
 		Identifier(node, { path, state }) {
 			const parent = path.at(-1);
@@ -713,6 +730,7 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 	}
 
 	return {
+		is_async,
 		scope,
 		scopes
 	};

packages/svelte/src/compiler/phases/types.d.ts

@@ -13,6 +13,7 @@ export interface Js {
 	ast: Program;
 	scope: Scope;
 	scopes: Map<AST.SvelteNode, Scope>;
+	is_async: boolean;
 }
 
 export interface Template {