Reland "Prefer module fields for RSC server layer""

packages/next/src/build/webpack-config.ts

@@ -1941,6 +1941,10 @@ export default async function getBaseWebpackConfig(
                   ],
                 },
                 resolve: {
+                  mainFields: isEdgeServer
+                    ? mainFieldsPerCompiler[COMPILER_NAMES.edgeServer]
+                    : // Prefer module fields over main fields for isomorphic packages on server layer
+                      ['module', 'main'],
                   conditionNames: reactServerCondition,
                   alias: {
                     // If missing the alias override here, the default alias will be used which aliases

test/e2e/app-dir/app-external/app-external.test.ts

@@ -88,24 +88,23 @@ createNextDescribe(
     })
 
     it('should resolve 3rd party package exports based on the react-server condition', async () => {
-      await next
-        .fetch('/react-server/3rd-party-package')
-        .then(async (response) => {
-          const result = await resolveStreamResponse(response)
-
-          // Package should be resolved based on the react-server condition,
-          // as well as package's internal & external dependencies.
-          expect(result).toContain(
-            'Server: index.react-server:react.subset:dep.server'
-          )
-          expect(result).toContain(
-            'Client: index.default:react.full:dep.default'
-          )
+      const $ = await next.render$('/react-server/3rd-party-package')
+
+      const result = $('body').text()
+
+      // Package should be resolved based on the react-server condition,
+      // as well as package's internal & external dependencies.
+      expect(result).toContain(
+        'Server: index.react-server:react.subset:dep.server'
+      )
+      expect(result).toContain('Client: index.default:react.full:dep.default')
+
+      // Subpath exports should be resolved based on the condition too.
+      expect(result).toContain('Server subpath: subpath.react-server')
+      expect(result).toContain('Client subpath: subpath.default')
 
-          // Subpath exports should be resolved based on the condition too.
-          expect(result).toContain('Server subpath: subpath.react-server')
-          expect(result).toContain('Client subpath: subpath.default')
-        })
+      // Prefer `module` field for isomorphic packages.
+      expect($('#main-field').text()).toContain('server-module-field:module')
     })
 
     it('should correctly collect global css imports and mark them as side effects', async () => {

test/e2e/app-dir/app-external/app/react-server/3rd-party-package/page.js

@@ -1,5 +1,6 @@
 import v from 'conditional-exports'
 import v1 from 'conditional-exports/subpath'
+import { name as serverFieldName } from 'server-module-field'
 
 import Client from './client'
 
@@ -11,6 +12,8 @@ export default function Page() {
       {`Server subpath: ${v1}`}
       <br />
       <Client />
+      <br />
+      <div id="main-field">{`Server module field: ${serverFieldName}`}</div>
     </div>
   )
 }

test/e2e/app-dir/app-external/node_modules_bak/server-module-field/index.cjs

@@ -0,0 +1 @@
+exports.name = 'server-module-field:main'

test/e2e/app-dir/app-external/node_modules_bak/server-module-field/index.esm.js

@@ -0,0 +1 @@
+export const name = 'server-module-field:module'

test/e2e/app-dir/app-external/node_modules_bak/server-module-field/package.json

@@ -0,0 +1,4 @@
+{
+  "main": "./index.cjs",
+  "module": "./index.esm.js"
+}