Ensure we match comment minify behavior between terser and swc (#68372)

Currently we are only strip all comments properly when `swcMinify:
false` is set although the default is to use `swcMinify` which can cause
confusion with these two modes not matching comments handling.

This updates the default to match comment stripping between the two and
also adds an experimental config to allow toggling this regardless of
which minifier is being used.

x-ref: [slack
thread](https://vercel.slack.com/archives/C0676QZBWKS/p1722444791037279)
x-ref: NEXT-3642

Author: ijjk

packages/next/src/build/webpack/plugins/terser-webpack-plugin/src/index.ts

@@ -158,6 +158,9 @@ export class TerserPlugin {
                   compress: true,
                   mangle: true,
                   module: 'unknown',
+                  output: {
+                    comments: false,
+                  },
                 }
               )
 

test/production/terser-class-static-blocks/pages/index.tsx

@@ -1,3 +1,10 @@
+/**
+ * My JSDoc comment that should be minified away
+ *
+ * @author Example One <[email protected]>
+ * @author Example Two <[email protected]>
+ * @copyright 2024 Vercel Inc
+ */
 class TestClass {
   static text = 'hello world'
 }

test/production/terser-class-static-blocks/terser-class-static-blocks.test.ts

@@ -1,17 +1,38 @@
-import { createNextDescribe } from 'e2e-utils'
+import glob from 'glob'
+import { nextTestSetup } from 'e2e-utils'
+import path from 'path'
 
-createNextDescribe(
-  'terser-class-static-blocks',
-  {
+describe('terser-class-static-blocks', () => {
+  const { next, isNextDeploy } = nextTestSetup({
     files: __dirname,
-    nextConfig: {
-      swcMinify: false,
-    },
-  },
-  ({ next }) => {
-    it('should work using cheerio', async () => {
-      const $ = await next.render$('/')
-      expect($('p').text()).toBe('hello world')
+    nextConfig: {},
+  })
+
+  it('should work using cheerio', async () => {
+    const $ = await next.render$('/')
+    expect($('p').text()).toBe('hello world')
+  })
+
+  if (!isNextDeploy) {
+    it('should have stripped away all comments', async () => {
+      const chunksDir = path.join(next.testDir, '.next/static')
+      const chunks = glob.sync('**/*.js', {
+        cwd: chunksDir,
+      })
+
+      expect(chunks.length).toBeGreaterThan(0)
+
+      await Promise.all(
+        chunks.map(async (chunk) => {
+          expect(
+            await next.readFile(path.join('.next/static', chunk))
+          ).not.toContain('/*')
+
+          expect(
+            await next.readFile(path.join('.next/static', chunk))
+          ).not.toContain('My JSDoc comment that')
+        })
+      )
     })
   }
-)
+})