fix: resolve MISSING_EXPORT warnings for optional hooks exports

Spread namespace imports into plain objects so that Rollup/Rolldown
no longer warns about accessing potentially missing exports like
reroute, transport, handleError, or init from hooks files.

This removes the onwarn suppression workaround that was added in #13687
and fixes the root cause of the issue.

Closes #13508

Author: FrankFMY

.changeset/fix-optional-hooks-import.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: avoid MISSING_EXPORT warnings for optional hooks by spreading namespace imports into plain objects

packages/kit/src/core/sync/write_client_manifest.js

@@ -142,12 +142,12 @@ export function write_client_manifest(kit, manifest_data, output, metadata) {
 		dedent`
 			${
 				client_hooks_file
-					? `import * as client_hooks from '${relative_path(output, client_hooks_file)}';`
+					? `import * as _client_hooks from '${relative_path(output, client_hooks_file)}';\nconst client_hooks = { ..._client_hooks };`
 					: ''
 			}
 			${
 				universal_hooks_file
-					? `import * as universal_hooks from '${relative_path(output, universal_hooks_file)}';`
+					? `import * as _universal_hooks from '${relative_path(output, universal_hooks_file)}';\nconst universal_hooks = { ..._universal_hooks };`
 					: ''
 			}
 

packages/kit/src/exports/vite/index.js

@@ -190,9 +190,6 @@ async function kit({ svelte_config }) {
 	/** @type {import('vite')} */
 	const vite = await import_peer('vite');
 
-	// @ts-ignore `vite.rolldownVersion` only exists in `rolldown-vite`
-	const isRolldown = !!vite.rolldownVersion;
-
 	const { kit } = svelte_config;
 	const out = `${kit.outDir}/output`;
 
@@ -917,21 +914,7 @@ async function kit({ svelte_config }) {
 								sourcemapIgnoreList,
 								inlineDynamicImports: !split
 							},
-							preserveEntrySignatures: 'strict',
-							onwarn(warning, handler) {
-								if (
-									(isRolldown
-										? warning.code === 'IMPORT_IS_UNDEFINED'
-										: warning.code === 'MISSING_EXPORT') &&
-									warning.id === `${kit.outDir}/generated/client-optimized/app.js`
-								) {
-									// ignore e.g. undefined `handleError` hook when
-									// referencing `client_hooks.handleError`
-									return;
-								}
-
-								handler(warning);
-							}
+							preserveEntrySignatures: 'strict'
 						},
 						ssrEmitAssets: true,
 						target: ssr ? 'node18.13' : undefined