feat: ssrBuild flag in config env

docs/config/index.md

@@ -61,10 +61,10 @@ Vite also directly supports TS config files. You can use `vite.config.ts` with t
 
 ## Conditional Config
 
-If the config needs to conditional determine options based on the command (`dev`/`serve` or `build`) or the [mode](/guide/env-and-mode) being used, it can export a function instead:
+If the config needs to conditional determine options based on the command (`dev`/`serve` or `build`), the [mode](/guide/env-and-mode) being used, or if the build is for a SSR entry, it can export a function instead:
 
 ```js
-export default defineConfig(({ command, mode }) => {
+export default defineConfig(({ command, mode, ssrBuild }) => {
   if (command === 'serve') {
     return {
       // dev specific config

packages/vite/src/node/config.ts

@@ -62,9 +62,11 @@ export type { RenderBuiltAssetUrl } from './build'
 
 // NOTE: every export in this file is re-exported from ./index.ts so it will
 // be part of the public API.
+
 export interface ConfigEnv {
   command: 'build' | 'serve'
   mode: string
+  ssrBuild: boolean | string
 }
 
 /**
@@ -375,7 +377,8 @@ export async function resolveConfig(
 
   const configEnv = {
     mode,
-    command
+    command,
+    ssrBuild: config.build?.ssr ?? false
   }
 
   let { configFile } = config

playground/ssr-vue/vite.config.js

@@ -10,7 +10,7 @@ const nestedVirtualId = '\0' + nestedVirtualFile
 
 const base = '/test/'
 
-export default defineConfig({
+export default defineConfig(({ command, ssrBuild }) => ({
   base,
   plugins: [
     vuePlugin(),
@@ -22,9 +22,15 @@ export default defineConfig({
           return id
         }
       },
-      load(id) {
+      load(id, options) {
+        const ssrFromOptions = options?.ssr ?? false
         if (id === '@foo') {
-          return `export default { msg: 'hi' }`
+          // Force a mismatch error if ssrBuild is different from ssrFromOptions
+          return `export default { msg: '${
+            command === 'build' && !!ssrBuild !== ssrFromOptions
+              ? `defineConfig ssrBuild !== ssr from load options`
+              : 'hi'
+          }' }`
         }
       }
     },
@@ -111,4 +117,4 @@ export default defineConfig({
   optimizeDeps: {
     exclude: ['example-external-component']
   }
-})
+}))