fix: await blockers before initialising const (#17226)

* fix: await blockers before initialising const

Fixes #17224

* ssr + test

.changeset/famous-terms-shake.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: await blockers before initialising const

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

@@ -1,6 +1,7 @@
 /** @import { Pattern } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types' */
+/** @import { ExpressionMetadata } from '../../../nodes.js' */
 import { dev } from '../../../../state.js';
 import { extract_identifiers } from '../../../../utils/ast.js';
 import * as b from '#compiler/builders';
@@ -30,7 +31,7 @@ export function ConstTag(node, context) {
 			context.state,
 			declaration.id,
 			expression,
-			node.metadata.expression.has_await,
+			node.metadata.expression,
 			context.state.scope.get_bindings(declaration)
 		);
 	} else {
@@ -73,7 +74,7 @@ export function ConstTag(node, context) {
 			context.state,
 			tmp,
 			expression,
-			node.metadata.expression.has_await,
+			node.metadata.expression,
 			context.state.scope.get_bindings(declaration)
 		);
 
@@ -89,15 +90,18 @@ export function ConstTag(node, context) {
  * @param {ComponentContext['state']} state
  * @param {import('estree').Identifier} id
  * @param {import('estree').Expression} expression
- * @param {boolean} has_await
+ * @param {ExpressionMetadata} metadata
  * @param {import('#compiler').Binding[]} bindings
  */
-function add_const_declaration(state, id, expression, has_await, bindings) {
+function add_const_declaration(state, id, expression, metadata, bindings) {
 	// we need to eagerly evaluate the expression in order to hit any
 	// 'Cannot access x before initialization' errors
 	const after = dev ? [b.stmt(b.call('$.get', id))] : [];
 
-	if (has_await || state.async_consts) {
+	const has_await = metadata.has_await;
+	const blockers = [...metadata.dependencies].map((dep) => dep.blocker).filter((b) => b !== null);
+
+	if (has_await || state.async_consts || blockers.length > 0) {
 		const run = (state.async_consts ??= {
 			id: b.id(state.scope.generate('promises')),
 			thunks: []
@@ -108,6 +112,8 @@ function add_const_declaration(state, id, expression, has_await, bindings) {
 		const assignment = b.assignment('=', id, expression);
 		const body = after.length === 0 ? assignment : b.block([b.stmt(assignment), ...after]);
 
+		if (blockers.length > 0) run.thunks.push(b.thunk(b.call('Promise.all', b.array(blockers))));
+
 		run.thunks.push(b.thunk(body, has_await));
 
 		const blocker = b.member(run.id, b.literal(run.thunks.length - 1), true);

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

@@ -13,8 +13,11 @@ export function ConstTag(node, context) {
 	const id = /** @type {Pattern} */ (context.visit(declaration.id));
 	const init = /** @type {Expression} */ (context.visit(declaration.init));
 	const has_await = node.metadata.expression.has_await;
+	const blockers = [...node.metadata.expression.dependencies]
+		.map((dep) => dep.blocker)
+		.filter((b) => b !== null);
 
-	if (has_await || context.state.async_consts) {
+	if (has_await || context.state.async_consts || blockers.length > 0) {
 		const run = (context.state.async_consts ??= {
 			id: b.id(context.state.scope.generate('promises')),
 			thunks: []
@@ -27,6 +30,10 @@ export function ConstTag(node, context) {
 			context.state.init.push(b.let(identifier.name));
 		}
 
+		if (blockers.length > 0) {
+			run.thunks.push(b.thunk(b.call('Promise.all', b.array(blockers))));
+		}
+
 		const assignment = b.assignment('=', id, init);
 		run.thunks.push(b.thunk(b.block([b.stmt(assignment)]), has_await));
 

packages/svelte/tests/runtime-runes/samples/async-const/_config.js

@@ -3,11 +3,11 @@ import { test } from '../../test';
 
 export default test({
 	mode: ['async-server', 'client', 'hydrate'],
-	ssrHtml: `<h1>Hello, world!</h1> 5 01234 5 sync 6 5 0`,
+	ssrHtml: `<h1>Hello, world!</h1> 5 01234 5 sync 6 5 0 10`,
 
 	async test({ assert, target }) {
 		await tick();
 
-		assert.htmlEqual(target.innerHTML, `<h1>Hello, world!</h1> 5 01234 5 sync 6 5 0`);
+		assert.htmlEqual(target.innerHTML, `<h1>Hello, world!</h1> 5 01234 5 sync 6 5 0 10`);
 	}
 });

packages/svelte/tests/runtime-runes/samples/async-const/main.svelte

@@ -23,4 +23,9 @@
 
 	{@render greet()}
 	{number} {sync} {after_async} {length} {first}
+
+	{#if sync}
+		{@const double = number * 2}
+		{double}
+	{/if}
 </svelte:boundary>