Rollup transform of file including sourcemap for vite?

Question

I am attempting to create Rollup plugin to use in my Vite build which will transform a file dynamically during build. Using transform(code: string, id: string) the transformation works as expected. The only issue is that I get a warning in the build that the transformed file's sourcemap does not match because it was transformed. The Rollup documentation says that if I transform the code, I need to manually create the sourcemap. How do you do that?

magic-string provides a simple way to generate such a map for elementary transformations like adding or removing code snippets.

Attempting to use magic-string doesn't seem to be getting me anywhere. Though I create the sourcemap and return it in the way described by the Rollup documentation, the resulting bundle sourcemap does not include the sourcemap for the

My plugin:

export default function () {
  return {
    name: 'exclude-msw',
    transform(code: string, id: string) {
      if (id.match(/main\.ts/)) {
        
        const s = new MagicString(code)
        //...
        //my transformation --- basically, just removes a chunk of code from the file
        //...
        const map = s.generateMap({
          source: id,
          file: id + '.map',
          includeContent: true
        })
        return {
          code: s.toString(),
          map: map.toString()
        }
      }
    }
  }
}

The resulting map still includes the original file's map... Am I doing something wrong? The documentation also says:

If the transformation does not move code, you can preserve existing sourcemaps by returning null:

return {
    code: transformedCode,
    map: null
};

So passing the new map is behaving the same as though I passed null... I don't understand what I am doing wrong. I tried making all kinds of changes but always end up with either the old mapping, or no mapping of the changed file at all.

How can I get the map for the new file's state in the Vite bundle's map?