fix(plugin-legacy): fix regression introduced in #4536 (#4861)

packages/playground/legacy/__tests__/ssr/legacy-ssr.spec.ts

@@ -0,0 +1,20 @@
+import { isBuild } from '../../../testUtils'
+import { port } from './serve'
+
+const url = `http://localhost:${port}`
+
+if (isBuild) {
+  test('should work', async () => {
+    await page.goto(url)
+    expect(await page.textContent('#app')).toMatch('Hello')
+  })
+
+  test('import.meta.env.LEGACY', async () => {
+    // SSR build is always modern
+    expect(await page.textContent('#env')).toMatch('false')
+  })
+} else {
+  // this test doesn't support serve mode
+  // must contain at least one test
+  test('should work', () => void 0)
+}

packages/playground/legacy/__tests__/ssr/serve.js

@@ -0,0 +1,52 @@
+// @ts-check
+// this is automtically detected by scripts/jestPerTestSetup.ts and will replace
+// the default e2e test serve behavior
+const path = require('path')
+
+const port = (exports.port = 9527)
+
+/**
+ * @param {string} root
+ * @param {boolean} _isProd
+ */
+exports.serve = async function serve(root, _isProd) {
+  const { build } = require('vite')
+  await build({
+    root,
+    logLevel: 'silent',
+    build: {
+      target: 'esnext',
+      ssr: 'entry-server.js',
+      outDir: 'dist/server'
+    }
+  })
+
+  const express = require('express')
+  const app = express()
+
+  app.use('/', async (_req, res) => {
+    const { render } = require(path.resolve(
+      root,
+      './dist/server/entry-server.js'
+    ))
+    const html = await render()
+    res.status(200).set({ 'Content-Type': 'text/html' }).end(html)
+  })
+
+  return new Promise((resolve, reject) => {
+    try {
+      const server = app.listen(port, () => {
+        resolve({
+          // for test teardown
+          async close() {
+            await new Promise((resolve) => {
+              server.close(resolve)
+            })
+          }
+        })
+      })
+    } catch (e) {
+      reject(e)
+    }
+  })
+}

packages/playground/legacy/entry-server.js

@@ -0,0 +1,7 @@
+// This counts as 'server-side' rendering, yes?
+export async function render() {
+  return /* html */ `
+    <div id="app">Hello</div>
+    <div id="env">${import.meta.env.LEGACY}</div>
+  `
+}

packages/plugin-legacy/index.js

@@ -451,12 +451,14 @@ function viteLegacyPlugin(options = {}) {
   const legacyEnvPlugin = {
     name: 'legacy-env',
 
-    config(_, env) {
+    config(config, env) {
       if (env) {
         return {
           define: {
             'import.meta.env.LEGACY':
-              env.command === 'serve' ? false : legacyEnvVarMarker
+              env.command === 'serve' || config.build.ssr
+                ? false
+                : legacyEnvVarMarker
           }
         }
       } else {