make lazy the exception

Author: Rich-Harris

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

@@ -2,7 +2,7 @@
 /** @import { Binding } from '#compiler' */
 /** @import { Context } from '../types.js' */
 /** @import { Scope } from '../../../scope.js' */
-import { build_fallback, extract_paths, is_expression_async } from '../../../../utils/ast.js';
+import { build_fallback, extract_paths } from '../../../../utils/ast.js';
 import * as b from '../../../../utils/builders.js';
 import { get_rune } from '../../../scope.js';
 import { walk } from 'zimmerframe';

packages/svelte/src/compiler/utils/ast.js

@@ -551,7 +551,15 @@ export function is_expression_async(expression) {
  * @param {ESTree.Expression} fallback
  */
 export function build_fallback(expression, fallback) {
+	if (is_simple_expression(fallback)) {
+		return b.call('$.fallback', expression, fallback);
+	}
+
+	if (fallback.type === 'AwaitExpression' && is_simple_expression(fallback.argument)) {
+		return b.await(b.call('$.fallback', expression, fallback.argument));
+	}
+
 	return is_expression_async(fallback)
-		? b.await(b.call('$.fallback', expression, b.thunk(fallback, true)))
-		: b.call('$.fallback', expression, b.thunk(fallback));
+		? b.await(b.call('$.fallback', expression, b.thunk(fallback, true), b.true))
+		: b.call('$.fallback', expression, b.thunk(fallback), b.true);
 }

packages/svelte/src/internal/shared/utils.js

@@ -50,9 +50,14 @@ export function run_all(arr) {
 /**
  * @template V
  * @param {V} value
- * @param {() => V} fallback lazy because could contain side effects
+ * @param {V | (() => V)} fallback
+ * @param {boolean} [lazy]
  * @returns {V}
  */
-export function fallback(value, fallback) {
-	return value === undefined ? fallback() : value;
+export function fallback(value, fallback, lazy = false) {
+	return value === undefined
+		? lazy
+			? /** @type {() => V} */ (fallback)()
+			: /** @type {V} */ (fallback)
+		: value;
 }