PostCSS PreviousMap missing

[SassNinja]

🐛 bug report

I expect to have access to the source map (PreviousMap) within my PostCSS plugin (similar to webpack)

module.exports = postcss.plugin('postcss-extract-media-query', opts => {
  return (root, result) => {
    // expect root.source.input.map to contain the PreviousMap
  };
});

🎛 Configuration (.babelrc, package.json, cli command)

package.json

"devDependencies": {
  "parcel-bundler": "^1.12.4",
  "parcel-plugin-nuke-dist": "^1.0.1",
  "postcss-extract-media-query": "2.x",
  "sass": "^1.24.0"
}

parcel.js

const parcel = require('parcel-bundler');
const path = require('path');
const file = path.join(__dirname, 'src/example.js');
const options = {
  watch: false,
  sourceMaps: true,
  cache: false
};
const bundler = new parcel(file, options);
bundler.bundle();

🤔 Expected Behavior

root.source.input.map should contain the PreviousMap so I expect

root: {
  source: {
    input: {
      css: ".foo {\n color: red...",
      file: "/Users/...",
      from: "/Users/...",
      map: PreviousMap
    }
  }
}

😯 Current Behavior

I'm getting

root: {
  source: {
    input: {
      css: ".foo {\n color: red...",
      file: "/Users/...",
      from: "/Users/..."
    }
  }
}

💁 Possible Solution

Since it's working with webpack as expected and the CSS file emitted by parcel contains the source map I assume something is missing in the postcss implementation of parcel.

🔦 Context

I've built my PostCSS plugin with the goal to support as many bundlers & task runners as possible. But without access to the source map I cannot provide 100% compatibility to parcel.

💻 Code Sample

You can find the whole code example here:
https://github.com/SassNinja/postcss-extract-media-query/tree/feature/source-map/examples/parcel

🌍 Your Environment

Software	Version(s)
Parcel	1.12.4
Node	10.16.0
npm	6.11.3
Operating System	Mac OS X