Make async-to-promises babel plugin experimental (#7006)

* Make async-to-promises babel plugin experimental

* Move excludes for asyncToPromises behind flag too

* Move other configs behind flag

* Re-add original exclude item

packages/next-server/server/config.js

@@ -31,7 +31,8 @@ const defaultConfig = {
     ampBindInitData: false,
     exportTrailingSlash: true,
     profiling: false,
-    flyingShuttle: false
+    flyingShuttle: false,
+    asyncToPromises: false
   }
 }
 

packages/next/build/babel/preset.ts

@@ -58,11 +58,7 @@ module.exports = (api: any, options: NextBabelPresetOptions = {}): BabelPreset =
     // In the test environment `modules` is often needed to be set to true, babel figures that out by itself using the `'auto'` option
     // In production/development this option is set to `false` so that webpack can handle import/export with tree-shaking
     modules: 'auto',
-    exclude: [
-      'transform-typeof-symbol',
-      'transform-regenerator',
-      'transform-async-to-generator'
-    ],
+    exclude: ['transform-typeof-symbol'],
     ...options['preset-env']
   }
   return {
@@ -76,9 +72,6 @@ module.exports = (api: any, options: NextBabelPresetOptions = {}): BabelPreset =
       }]
     ],
     plugins: [
-      ['babel-plugin-transform-async-to-promises', {
-        inlineHelpers: true
-      }],
       require('babel-plugin-react-require'),
       require('@babel/plugin-syntax-dynamic-import'),
       // Transform dynamic import to require
@@ -91,7 +84,7 @@ module.exports = (api: any, options: NextBabelPresetOptions = {}): BabelPreset =
       [require('@babel/plugin-transform-runtime'), {
         corejs: 2,
         helpers: true,
-        regenerator: false,
+        regenerator: true,
         useESModules: supportsESM && presetEnvConfig.modules !== 'commonjs',
         ...options['transform-runtime']
       }],

packages/next/build/webpack-config.ts

@@ -22,7 +22,7 @@ export default function getBaseWebpackConfig (dir: string, {dev = false, debug =
   const defaultLoaders = {
     babel: {
       loader: 'next-babel-loader',
-      options: { isServer, cwd: dir }
+      options: { isServer, cwd: dir, asyncToPromises: config.experimental.asyncToPromises }
     },
     // Backwards compat
     hotSelfAccept: {

packages/next/build/webpack/loaders/next-babel-loader.js

@@ -15,7 +15,8 @@ module.exports = babelLoader.custom(babel => {
   return {
     customOptions (opts) {
       const custom = {
-        isServer: opts.isServer
+        isServer: opts.isServer,
+        asyncToPromises: opts.asyncToPromises
       }
       const filename = join(opts.cwd, 'noop.js')
       const loader = Object.assign({
@@ -31,9 +32,10 @@ module.exports = babelLoader.custom(babel => {
       }, opts)
 
       delete loader.isServer
+      delete loader.asyncToPromises
       return { loader, custom }
     },
-    config (cfg, { source, customOptions: { isServer } }) {
+    config (cfg, { source, customOptions: { isServer, asyncToPromises } }) {
       const filename = this.resourcePath
       const options = Object.assign({}, cfg.options)
       if (cfg.hasFilesystemConfig()) {
@@ -56,6 +58,33 @@ module.exports = babelLoader.custom(babel => {
         options.plugins.push(nextDataPlugin)
       }
 
+      if (asyncToPromises) {
+        const asyncToPromisesPlugin = babel.createConfigItem(['babel-plugin-transform-async-to-promises', {
+          inlineHelpers: true
+        }], { type: 'plugin' })
+        options.plugins = options.plugins || []
+        options.plugins.push(asyncToPromisesPlugin)
+
+        const regeneratorPlugin = options.plugins.find(plugin => {
+          return plugin[0] === require('@babel/plugin-transform-runtime')
+        })
+        if (regeneratorPlugin) {
+          regeneratorPlugin[1].regenerator = false
+        }
+
+        const babelPresetEnv = (options.presets || []).find((preset = []) => {
+          return preset[0] === require('@babel/preset-env').default
+        })
+        if (babelPresetEnv) {
+          babelPresetEnv[1].exclude = (options.presets[0][1].exclude || []).concat([
+            'transform-typeof-symbol',
+            'transform-regenerator',
+            'transform-async-to-generator'
+          ])
+            .filter('transform-typeof-symbol')
+        }
+      }
+
       // If the file has `module.exports` we have to transpile commonjs because Babel adds `import` statements
       // That break webpack, since webpack doesn't support combining commonjs and esmodules
       if (source.indexOf('module.exports') !== -1) {