From d178106a73c42f28a1eee2e9424c62fdd8430535 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 24 Oct 2025 09:25:03 -0400
Subject: [PATCH] fix: better optimization of await expressions

---
 .changeset/odd-plants-lead.md                        |  5 +++++
 .../svelte/src/compiler/phases/2-analyze/index.js    |  2 +-
 .../3-transform/server/visitors/shared/utils.js      |  4 ++--
 packages/svelte/src/compiler/utils/ast.js            | 12 ++++++++----
 packages/svelte/src/compiler/utils/builders.js       |  4 ++--
 5 files changed, 18 insertions(+), 9 deletions(-)
 create mode 100644 .changeset/odd-plants-lead.md

diff --git a/.changeset/odd-plants-lead.md b/.changeset/odd-plants-lead.md
new file mode 100644
index 000000000000..1df2236c2a4b
--- /dev/null
+++ b/.changeset/odd-plants-lead.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: better optimization of await expressions
diff --git a/packages/svelte/src/compiler/phases/2-analyze/index.js b/packages/svelte/src/compiler/phases/2-analyze/index.js
index 52be9973748e..b4c704c34d81 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/index.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/index.js
@@ -6,7 +6,7 @@ import { walk } from 'zimmerframe';
 import { parse } from '../1-parse/acorn.js';
 import * as e from '../../errors.js';
 import * as w from '../../warnings.js';
-import { extract_identifiers } from '../../utils/ast.js';
+import { extract_identifiers, has_await_expression } from '../../utils/ast.js';
 import * as b from '#compiler/builders';
 import { Scope, ScopeRoot, create_scopes, get_rune, set_scope } from '../scope.js';
 import check_graph_for_cycles from './utils/check_graph_for_cycles.js';
diff --git a/packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/utils.js b/packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/utils.js
index f5132c1cf81d..92653ed73c90 100644
--- a/packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/utils.js
+++ b/packages/svelte/src/compiler/phases/3-transform/server/visitors/shared/utils.js
@@ -12,7 +12,7 @@ import {
 import * as b from '#compiler/builders';
 import { sanitize_template_string } from '../../../../../utils/sanitize_template_string.js';
 import { regex_whitespaces_strict } from '../../../../patterns.js';
-import { has_await } from '../../../../../utils/ast.js';
+import { has_await_expression } from '../../../../../utils/ast.js';
 
 /** Opens an if/each block, so that we can remove nodes in the case of a mismatch */
 export const block_open = b.literal(BLOCK_OPEN);
@@ -315,7 +315,7 @@ export class PromiseOptimiser {
 
 		const promises = b.array(
 			this.expressions.map((expression) => {
-				return expression.type === 'AwaitExpression' && !has_await(expression.argument)
+				return expression.type === 'AwaitExpression' && !has_await_expression(expression.argument)
 					? expression.argument
 					: b.call(b.thunk(expression, true));
 			})
diff --git a/packages/svelte/src/compiler/utils/ast.js b/packages/svelte/src/compiler/utils/ast.js
index 541921befbce..bd92dda5d968 100644
--- a/packages/svelte/src/compiler/utils/ast.js
+++ b/packages/svelte/src/compiler/utils/ast.js
@@ -611,16 +611,20 @@ export function build_assignment_value(operator, left, right) {
 }
 
 /**
- * @param {ESTree.Expression} expression
+ * @param {ESTree.Node} node
  */
-export function has_await(expression) {
+export function has_await_expression(node) {
 	let has_await = false;
 
-	walk(expression, null, {
+	walk(node, null, {
 		AwaitExpression(_node, context) {
 			has_await = true;
 			context.stop();
-		}
+		},
+		// don't traverse into these
+		FunctionDeclaration() {},
+		FunctionExpression() {},
+		ArrowFunctionExpression() {}
 	});
 
 	return has_await;
diff --git a/packages/svelte/src/compiler/utils/builders.js b/packages/svelte/src/compiler/utils/builders.js
index 99306ce4d966..f21b0dc8b43f 100644
--- a/packages/svelte/src/compiler/utils/builders.js
+++ b/packages/svelte/src/compiler/utils/builders.js
@@ -2,7 +2,7 @@
 import { walk } from 'zimmerframe';
 import { regex_is_valid_identifier } from '../phases/patterns.js';
 import { sanitize_template_string } from './sanitize_template_string.js';
-import { has_await } from './ast.js';
+import { has_await_expression } from './ast.js';
 
 /**
  * @param {Array<ESTree.Expression | ESTree.SpreadElement | null>} elements
@@ -451,7 +451,7 @@ export function thunk(expression, async = false) {
 export function unthunk(expression) {
 	// optimize `async () => await x()`, but not `async () => await x(await y)`
 	if (expression.async && expression.body.type === 'AwaitExpression') {
-		if (!has_await(expression.body.argument)) {
+		if (!has_await_expression(expression.body.argument)) {
 			return unthunk(arrow(expression.params, expression.body.argument));
 		}
 	}