Ensure original variable names are shown when debugging node runtime

Using `eval-source-map` also allows us to revert the disabling of
`preloadEntries` when debugging.

Author: unstubbable

contributing/core/vscode-debugger.md

@@ -18,8 +18,10 @@ Any Next.js app inside the monorepo can be debugged with these configurations.
 2. Enter the app dirname, e.g. `examples/hello-world` or `test/e2e/app-dir/app`.
 3. Select the `next` command from the presented options (`dev`, `build`, or `start`).
 
-To see the changes you make to the Next.js codebase during development, you can run `pnpm dev` in the root directory, which will watch for file changes in `packages/next` and recompile the Next.js source code on any file saves.
+To see the changes you make to the Next.js codebase during development, you can run `NEXT_SERVER_EVAL_SOURCE_MAPS=1 pnpm dev` in the root directory, which will watch for file changes in `packages/next` and recompile the Next.js source code on any file saves.
 
 ## Breakpoints
 
 When developing/debugging Next.js, you can set breakpoints anywhere in the `packages/next` source code that will stop the debugger at certain locations so you can examine the behavior. Read more about [breakpoints in the VS Code documentation](https://code.visualstudio.com/docs/nodejs/nodejs-debugging#_breakpoints).
+
+To ensure that the original names are displayed in the "Variables" section, build the Next.js source code with `NEXT_SERVER_EVAL_SOURCE_MAPS=1` (see above).

packages/next/src/server/next-server.ts

@@ -231,7 +231,6 @@ export default class NextNodeServer extends BaseServer<
 
     if (
       !options.dev &&
-      !options.isNodeDebugging &&
       !this.minimalMode &&
       this.nextConfig.experimental.preloadEntriesOnStart
     ) {

packages/next/webpack.config.js

@@ -152,7 +152,9 @@ module.exports = ({ dev, turbo, bundleType, experimental }) => {
       }.runtime.${dev ? 'dev' : 'prod'}.js`,
       libraryTarget: 'commonjs2',
     },
-    devtool: 'source-map',
+    devtool: process.env.NEXT_SERVER_EVAL_SOURCE_MAPS
+      ? 'eval-source-map'
+      : 'source-map',
     optimization: {
       moduleIds: 'named',
       minimize: true,