WIP

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

@@ -20,6 +20,7 @@ import { ArrowFunctionExpression } from './visitors/ArrowFunctionExpression.js';
 import { AssignmentExpression } from './visitors/AssignmentExpression.js';
 import { Attribute } from './visitors/Attribute.js';
 import { AwaitBlock } from './visitors/AwaitBlock.js';
+import { AwaitExpression } from './visitors/AwaitExpression.js';
 import { BindDirective } from './visitors/BindDirective.js';
 import { CallExpression } from './visitors/CallExpression.js';
 import { ClassBody } from './visitors/ClassBody.js';
@@ -133,6 +134,7 @@ const visitors = {
 	AssignmentExpression,
 	Attribute,
 	AwaitBlock,
+	AwaitExpression,
 	BindDirective,
 	CallExpression,
 	ClassBody,

packages/svelte/src/compiler/phases/2-analyze/visitors/AwaitExpression.js

@@ -0,0 +1,14 @@
+/** @import { AwaitExpression } from 'estree' */
+/** @import { Context } from '../types' */
+
+/**
+ * @param {AwaitExpression} node
+ * @param {Context} context
+ */
+export function AwaitExpression(node, context) {
+	if (context.state.expression) {
+		context.state.expression.is_async = true;
+	}
+
+	context.next();
+}

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

@@ -12,6 +12,7 @@ import { ArrowFunctionExpression } from './visitors/ArrowFunctionExpression.js';
 import { AssignmentExpression } from './visitors/AssignmentExpression.js';
 import { Attribute } from './visitors/Attribute.js';
 import { AwaitBlock } from './visitors/AwaitBlock.js';
+import { AwaitExpression } from './visitors/AwaitExpression.js';
 import { BinaryExpression } from './visitors/BinaryExpression.js';
 import { BindDirective } from './visitors/BindDirective.js';
 import { BlockStatement } from './visitors/BlockStatement.js';
@@ -87,6 +88,7 @@ const visitors = {
 	AssignmentExpression,
 	Attribute,
 	AwaitBlock,
+	AwaitExpression,
 	BinaryExpression,
 	BindDirective,
 	BlockStatement,

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

@@ -0,0 +1,16 @@
+/** @import { AwaitExpression, Expression } from 'estree' */
+/** @import { ComponentContext } from '../types' */
+import * as b from '../../../../utils/builders.js';
+
+/**
+ * @param {AwaitExpression} node
+ * @param {ComponentContext} context
+ */
+export function AwaitExpression(node, context) {
+	return b.await(
+		b.call(
+			'$.preserve_context',
+			node.argument && /** @type {Expression} */ (context.visit(node.argument))
+		)
+	);
+}

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

@@ -69,7 +69,7 @@ export function process_children(nodes, initial, is_element, { visit, state }) {
 
 		state.template.push(' ');
 
-		const { has_state, has_call, value } = build_template_chunk(sequence, visit, state);
+		const { has_state, has_call, is_async, value } = build_template_chunk(sequence, visit, state);
 
 		// if this is a standalone `{expression}`, make sure we handle the case where
 		// no text node was created because the expression was empty during SSR
@@ -79,7 +79,7 @@ export function process_children(nodes, initial, is_element, { visit, state }) {
 		const update = b.stmt(b.call('$.set_text', id, value));
 
 		if (has_call && !within_bound_contenteditable) {
-			state.init.push(build_update(update));
+			state.init.push(build_update(update, is_async));
 		} else if (has_state && !within_bound_contenteditable) {
 			state.update.push(update);
 		} else {

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

@@ -14,7 +14,7 @@ import { locator } from '../../../../../state.js';
  * @param {Array<AST.Text | AST.ExpressionTag>} values
  * @param {(node: AST.SvelteNode, state: any) => any} visit
  * @param {ComponentClientTransformState} state
- * @returns {{ value: Expression, has_state: boolean, has_call: boolean }}
+ * @returns {{ value: Expression, has_state: boolean, has_call: boolean, is_async: boolean }}
  */
 export function build_template_chunk(values, visit, state) {
 	/** @type {Expression[]} */
@@ -25,6 +25,7 @@ export function build_template_chunk(values, visit, state) {
 
 	let has_call = false;
 	let has_state = false;
+	let is_async = false;
 	let contains_multiple_call_expression = false;
 
 	for (const node of values) {
@@ -34,6 +35,7 @@ export function build_template_chunk(values, visit, state) {
 			contains_multiple_call_expression ||= has_call && metadata.has_call;
 			has_call ||= metadata.has_call;
 			has_state ||= metadata.has_state;
+			is_async ||= metadata.is_async;
 		}
 	}
 
@@ -68,7 +70,7 @@ export function build_template_chunk(values, visit, state) {
 			} else if (values.length === 1) {
 				// If we have a single expression, then pass that in directly to possibly avoid doing
 				// extra work in the template_effect (instead we do the work in set_text).
-				return { value: visit(node.expression, state), has_state, has_call };
+				return { value: visit(node.expression, state), has_state, has_call, is_async };
 			} else {
 				expressions.push(b.logical('??', visit(node.expression, state), b.literal('')));
 			}
@@ -84,17 +86,18 @@ export function build_template_chunk(values, visit, state) {
 
 	const value = b.template(quasis, expressions);
 
-	return { value, has_state, has_call };
+	return { value, has_state, has_call, is_async };
 }
 
 /**
  * @param {Statement} statement
+ * @param {boolean} is_async
  */
-export function build_update(statement) {
+export function build_update(statement, is_async) {
 	const body =
 		statement.type === 'ExpressionStatement' ? statement.expression : b.block([statement]);
 
-	return b.stmt(b.call('$.template_effect', b.thunk(body)));
+	return b.stmt(b.call('$.template_effect', b.thunk(body, is_async)));
 }
 
 /**

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

@@ -58,6 +58,7 @@ export function create_expression_metadata() {
 	return {
 		dependencies: new Set(),
 		has_state: false,
-		has_call: false
+		has_call: false,
+		is_async: false
 	};
 }

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

@@ -318,6 +318,8 @@ export interface ExpressionMetadata {
 	has_state: boolean;
 	/** True if the expression involves a call expression (often, it will need to be wrapped in a derived) */
 	has_call: boolean;
+	/** True if the expression contains `await` */
+	is_async: boolean;
 }
 
 export * from './template.js';

packages/svelte/src/index-client.js

@@ -191,5 +191,3 @@ export {
 } from './internal/client/runtime.js';
 
 export { createRawSnippet } from './internal/client/dom/blocks/snippet.js';
-
-export { create_suspense } from './internal/client/dom/blocks/boundary.js';

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

@@ -248,3 +248,30 @@ export function create_suspense() {
 
 	return [suspend, unsuspend];
 }
+
+/**
+ * @template T
+ * @param {Promise<T>} promise
+ * @returns {Promise<T>}
+ */
+export async function preserve_context(promise) {
+	if (!active_effect) {
+		return promise;
+	}
+
+	var previous_effect = active_effect;
+	var previous_reaction = active_reaction;
+	var previous_component_context = component_context;
+
+	const [suspend, unsuspend] = create_suspense();
+
+	try {
+		suspend();
+		return await promise;
+	} finally {
+		set_active_effect(previous_effect);
+		set_active_reaction(previous_reaction);
+		set_component_context(previous_component_context);
+		unsuspend();
+	}
+}

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

@@ -129,7 +129,7 @@ export {
 	update_store,
 	mark_store_binding
 } from './reactivity/store.js';
-export { boundary } from './dom/blocks/boundary.js';
+export { boundary, preserve_context } from './dom/blocks/boundary.js';
 export { set_text } from './render.js';
 export {
 	get,

packages/svelte/types/index.d.ts

@@ -419,6 +419,7 @@ declare module 'svelte' {
 		render: () => string;
 		setup?: (element: Element) => void | (() => void);
 	}): Snippet<Params>;
+	export function create_suspense(): (() => void)[];
 	/** Anything except a function */
 	type NotFunction<T> = T extends Function ? never : T;
 	/**