feat!: reintroduce custom svelte/ssr resolve as vite 4 had to revert the change we needed to make that step (#532)

Author: dominikg

.changeset/short-clouds-build.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/vite-plugin-svelte': major
+---
+
+reintroduce custom svelte/ssr resolve

packages/vite-plugin-svelte/src/index.ts

@@ -18,6 +18,7 @@ import {
 
 import { ensureWatchedFile, setupWatchers } from './utils/watch';
 import { resolveViaPackageJsonSvelte } from './utils/resolve';
+import { PartialResolvedId } from 'rollup';
 import { toRollupError } from './utils/error';
 import { saveSvelteMetadata } from './utils/optimizer';
 import { svelteInspector } from './ui/inspector/plugin';
@@ -46,7 +47,9 @@ export function svelte(inlineOptions?: Partial<Options>): Plugin[] {
 	let viteConfig: ResolvedConfig;
 	/* eslint-disable no-unused-vars */
 	let compileSvelte: CompileSvelte;
+	/* eslint-enable no-unused-vars */
 
+	let resolvedSvelteSSR: Promise<PartialResolvedId | null>;
 	const api: PluginAPI = {};
 	const plugins: Plugin[] = [
 		{
@@ -133,6 +136,24 @@ export function svelte(inlineOptions?: Partial<Options>): Plugin[] {
 					}
 				}
 
+				if (ssr && importee === 'svelte') {
+					if (!resolvedSvelteSSR) {
+						resolvedSvelteSSR = this.resolve('svelte/ssr', undefined, { skipSelf: true }).then(
+							(svelteSSR) => {
+								log.debug('resolved svelte to svelte/ssr');
+								return svelteSSR;
+							},
+							(err) => {
+								log.debug(
+									'failed to resolve svelte to svelte/ssr. Update svelte to a version that exports it',
+									err
+								);
+								return null; // returning null here leads to svelte getting resolved regularly
+							}
+						);
+					}
+					return resolvedSvelteSSR;
+				}
 				//@ts-expect-error scan
 				const scan = !!opts?.scan; // scanner phase of optimizeDeps
 				const isPrebundled =