Is it expected that postcss-loader does not work with parallel: true?

[mjames-c]

https://rspack.rs/config/module-rules#rulesuseparallel

postcss-loader@8.2.1
postcss@8.5.2
css-loader@5.2.6
@rspack/core@1.7.9
@rspack/dev-server@1.2.1
@rspack/plugin-react-refresh@1.6.1

the weird thing is that our current postcss-loader setup works with thread loader when used with webpack...

[mjames-c]

Circling back, postcss-loader DOES work with parallel: true!

The problem with our setup was actually that postcss-loader was running in parallel but css-loader was not. Running css-loader in the worker thread as well resolved the issue.

{
  exclude: [/node_modules/],
  test: /\.css$/,
  enforce: 'pre',
  use: [
    {
      loader: require.resolve('css-loader'),
      options: { ... },
      ident: 'css',
      parallel: true, // parallel: true must be set css-loader if also set on postcss-loader!
    },
    {
      loader: require.resolve('postcss-loader'),
      options: { ... },
      ident: 'postcss',
      parallel: true,
    },
  ],
}