Turbopack loaders don't work with middleware

[dlehmhus]

Link to the code that reproduces this issue

https://github.com/dlehmhus/turbopack-loader-middleware-bug

To Reproduce

1. Run npm run dev
2. Open website and see error stacktrace

Current vs. Expected behavior

When using a loader in the turbopack rules like this:

// next.config.ts
  turbopack: {
    rules: {
      "*.ts": {
        loaders: [
          {
            loader: join(__dirname, "./custom-loader.js"),
            options: {},
          },
        ],
      },
    },
  },

// custom-loader.js
module.exports = (code) => {
  return code;
};

and having a middleware at the same time, the app crashes with the following error:

Error: [turbopack-node]/transforms/transforms.ts:53:10
lint TP1006 path.join(???*0*, (???*2* ? ???*5* : ???*7*)) is very dynamic
  51 | }
  52 | export const fromPath = (path: string) => {
> 53 |   return join(contextDir, sep !== '/' ? path.replaceAll('/', sep) : path)
     |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  54 | }
  55 |
  56 | // Patch process.env to track which env vars are read

- *0* process.cwd*1*()
  ⚠️  process.cwd is not specified in the environment
  ⚠️  This value might have side effects
- *1* process.cwd: The Node.js process.cwd method: https://nodejs.org/api/process.html#processcwd
- *2* (???*3* !== "/")
  ⚠️  nested operation
- *3* path*4*["sep"]
  ⚠️  unsupported property on Node.js path module
  ⚠️  This value might have side effects
- *4* path: The Node.js path module: https://nodejs.org/api/path.html
- *5* ???*6*["replaceAll"]("/", module<path, {}>["sep"])
  ⚠️  unknown callee object
- *6* arguments[0]
  ⚠️  function calls are not analysed yet
- *7* arguments[0]
  ⚠️  function calls are not analysed yet


    at <unknown> (Error: [turbopack-node]/transforms/transforms.ts:53:10)
    at Object.getCompilationErrors (file:///path/to/project/turbopack-loader-middleware-bug/node_modules/next/dist/server/dev/hot-reloader-turbopack.js:717:55)
    at DevBundlerService.getCompilationError (file:///path/to/project/turbopack-loader-middleware-bug/node_modules/next/dist/server/lib/dev-bundler-service.js:39:55)
    at DevServer.getCompilationError (file:///path/to/project/turbopack-loader-middleware-bug/node_modules/next/dist/server/dev/next-dev-server.js:663:42)
    at DevServer.findPageComponents (file:///path/to/project/turbopack-loader-middleware-bug/node_modules/next/dist/server/dev/next-dev-server.js:633:43)
    at async DevServer.renderErrorToResponseImpl (file:///path/to/project/turbopack-loader-middleware-bug/node_modules/next/dist/server/base-server.js:2645:26)

Provide environment information

Node.js v22.9.0

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.4.0: Fri Apr 11 18:33:46 PDT 2025; root:xnu-11417.101.15~117/RELEASE_ARM64_T8112
  Available memory (MB): 24576
  Available CPU cores: 8
Binaries:
  Node: 22.9.0
  npm: 10.8.3
  Yarn: 1.22.22
  pnpm: N/A
Relevant Packages:
  next: 15.4.0-canary.50 // Latest available version is detected (15.4.0-canary.50).
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.8.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Turbopack, Middleware

Which stage(s) are affected? (Select all that apply)

next dev (local)

Additional context

No response

_PACK-4684

[zh-lx]

@timneutkens @unstubbable Is there any process for this issue? Or could you please provide some advice how we should modify our loader?

[17hz]

@zh-lx Like this issue #83613 , waiting for PR merge