Add no-anon-default-export Babel lint rule (#14519)

Fixes #12291 

![image](https://user-images.githubusercontent.com/616428/85499698-176b4900-b5b0-11ea-8a5d-a7f0b4c20307.png)

packages/next/build/babel/plugins/no-anonymous-default-export.ts

+import { PluginObj, types as BabelTypes } from '@babel/core'
+import chalk from 'next/dist/compiled/chalk'
+
+export default function NoAnonymousDefaultExport({
+  types: t,
+  ...babel
+}: {
+  types: typeof BabelTypes
+  caller: (callerCallback: (caller: any) => any) => any
+}): PluginObj<any> {
+  let onWarning: ((reason: string | Error) => void) | null = null
+  babel.caller((caller) => {
+    onWarning = caller.onWarning
+    return '' // Intentionally empty to not invalidate cache
+  })
+
+  if (typeof onWarning !== 'function') {
+    return { visitor: {} }
+  }
+
+  const warn = onWarning!
+  return {
+    visitor: {
+      ExportDefaultDeclaration(path) {
+        const def = path.node.declaration
+
+        if (
+          !(
+            def.type === 'ArrowFunctionExpression' ||
+            def.type === 'FunctionDeclaration'
+          )
+        ) {
+          return
+        }
+
+        switch (def.type) {
+          case 'ArrowFunctionExpression': {
+            warn(
+              [
+                chalk.yellow.bold(
+                  'Anonymous arrow functions cause Fast Refresh to not preserve local component state.'
+                ),
+                'Please add a name to your function, for example:',
+                '',
+                chalk.bold('Before'),
+                chalk.cyan('export default () => <div />;'),
+                '',
+                chalk.bold('After'),
+                chalk.cyan('const Named = () => <div />;'),
+                chalk.cyan('export default Named;'),
+              ].join('\n')
+            )
+            break
+          }
+          case 'FunctionDeclaration': {
+            const isAnonymous = !Boolean(def.id)
+            if (isAnonymous) {
+              warn(
+                [
+                  chalk.yellow.bold(
+                    'Anonymous function declarations cause Fast Refresh to not preserve local component state.'
+                  ),
+                  'Please add a name to your function, for example:',
+                  '',
+                  chalk.bold('Before'),
+                  chalk.cyan('export default function () { /* ... */ }'),
+                  '',
+                  chalk.bold('After'),
+                  chalk.cyan('export default function Named() { /* ... */ }'),
+                ].join('\n')
+              )
+            }
+            break
+          }
+          default: {
+            // eslint-disable-next-line @typescript-eslint/no-unused-vars
+            const _: never = def
+          }
+        }
+      },
+    },
+  }
+}

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

@@ -137,6 +137,18 @@ module.exports = babelLoader.custom((babel) => {
       options.caller.isModern = isModern
       options.caller.isDev = development
 
+      const emitWarning = this.emitWarning.bind(this)
+      Object.defineProperty(options.caller, 'onWarning', {
+        enumerable: false,
+        writable: false,
+        value: (options.caller.onWarning = function (reason) {
+          if (!(reason instanceof Error)) {
+            reason = new Error(reason)
+          }
+          emitWarning(reason)
+        }),
+      })
+
       options.plugins = options.plugins || []
 
       if (hasReactRefresh) {
@@ -145,6 +157,13 @@ module.exports = babelLoader.custom((babel) => {
           { type: 'plugin' }
         )
         options.plugins.unshift(reactRefreshPlugin)
+        if (!isServer) {
+          const noAnonymousDefaultExportPlugin = babel.createConfigItem(
+            [require('../../babel/plugins/no-anonymous-default-export'), {}],
+            { type: 'plugin' }
+          )
+          options.plugins.unshift(noAnonymousDefaultExportPlugin)
+        }
       }
 
       if (!isServer && isPageFile) {

test/integration/hydration/pages/_app.js

-export default ({ Component, pageProps }) => <Component {...pageProps} />
+export default function CustomApp({ Component, pageProps }) {
+  return <Component {...pageProps} />
+}

test/integration/no-anon-default-export/components/Child.js

+export default () => {
+  return <div />
+}

test/integration/no-anon-default-export/pages/both.js

+import Child from '../components/Child'
+export default function () {
+  return <Child />
+}

test/integration/no-anon-default-export/pages/child.js

+import Child from '../components/Child'
+export default function A() {
+  return <Child />
+}

test/integration/no-anon-default-export/pages/page.js

+export default function () {
+  return <div />
+}

test/integration/no-anon-default-export/test/index.test.js

+/* eslint-env jest */
+
+import fs from 'fs-extra'
+import { check, findPort, killApp, launchApp } from 'next-test-utils'
+import webdriver from 'next-webdriver'
+import { join } from 'path'
+
+jest.setTimeout(1000 * 60 * 3)
+
+const appDir = join(__dirname, '../')
+
+describe('no anonymous default export warning', () => {
+  function getRegexCount(text, regex) {
+    return (text.match(regex) || []).length
+  }
+
+  beforeEach(async () => {
+    await fs.remove(join(appDir, '.next'))
+  })
+
+  it('show correct warnings for page', async () => {
+    let stdout = ''
+
+    const appPort = await findPort()
+    const app = await launchApp(appDir, appPort, {
+      env: { __NEXT_TEST_WITH_DEVTOOL: true },
+      onStdout(msg) {
+        stdout += msg || ''
+      },
+    })
+
+    const browser = await webdriver(appPort, '/page')
+
+    const found = await check(() => stdout, /anonymous/i, false)
+    expect(found).toBeTruthy()
+    await browser.close()
+
+    expect(getRegexCount(stdout, /not preserve local component state/g)).toBe(1)
+
+    await killApp(app)
+  })
+
+  it('show correct warnings for child', async () => {
+    let stdout = ''
+
+    const appPort = await findPort()
+    const app = await launchApp(appDir, appPort, {
+      env: { __NEXT_TEST_WITH_DEVTOOL: true },
+      onStdout(msg) {
+        stdout += msg || ''
+      },
+    })
+
+    const browser = await webdriver(appPort, '/child')
+
+    const found = await check(() => stdout, /anonymous/i, false)
+    expect(found).toBeTruthy()
+    await browser.close()
+
+    expect(getRegexCount(stdout, /not preserve local component state/g)).toBe(1)
+
+    await killApp(app)
+  })
+
+  it('show correct warnings for both', async () => {
+    let stdout = ''
+
+    const appPort = await findPort()
+    const app = await launchApp(appDir, appPort, {
+      env: { __NEXT_TEST_WITH_DEVTOOL: true },
+      onStdout(msg) {
+        stdout += msg || ''
+      },
+    })
+
+    const browser = await webdriver(appPort, '/both')
+
+    const found = await check(() => stdout, /anonymous/i, false)
+    expect(found).toBeTruthy()
+    await browser.close()
+
+    expect(getRegexCount(stdout, /not preserve local component state/g)).toBe(2)
+
+    await killApp(app)
+  })
+})

test/unit/next-babel-loader.test.js

@@ -38,6 +38,7 @@ const babel = async (
         return callback
       },
       callback,
+      emitWarning() {},
       query: {
         // babel opts
         babelrc: false,