React tree-shakable component library with Rollup and Sass - reactjs

Question

I have a requirement which I'm not sure if can be achieved with Rollup (or with Webpack?)
I've written a React component library. Each component imports it's stylesheet. Something like
import "./button.scss"
export default function Button() {
...
}
Now, these scss files use sass variables of all sorts that I would like to define in a global file.
I want the consumer of my library to be able to import Button as so:
import { Button } from "mylib" or even better:
import Button from "mylib/button"
and have only the code required by Button be added to my library.
I can't for the life of me figure/google out how to achieve this.
Is there a good reference to a tree-shakable React component library that uses sass and sass variables?
Bonus question:
I have some third party css that only some of the component require.
So, if each of the component's scss files imports:
#import "~some-third-party-css-lib.css"
Will Rollup duplicate that css? Or is there some kind of deduping mechanism?
Thanks!

Answer 1

From what I know now (Feb 2020) the only options to achieve good tree-shaking results for a component library are:
When using css modules
You need to rely on the consumer of your library to handle the (s)css imports (Create React App for example is doing this pretty well)
Make sure to not transpile/bundle the css in your package and that the import paths are correct
Make the consumers import each component separately (import Button from "mylib/button"). This is because css imports are considered side-effects by most bundlers and it's petty hard (maybe even impossible) to tell which classes are actually used. So if you have an index in your library that re-exports all components, this causes ALL css to be imported and probably bundled.
When using a css-in-js system with a runtime
also make sure that you do not pre-bundle/export the css in the library build-step but rather have that leave that to the consuming app.
This has some implications that should be considered.
The consumer of your components must ship a css-in-js runtime alongside their app
If the consumer is trying to do server-side pre-rendering in some sort that might require custom additional steps depending of the css-in-js library you're using
Conclusion
As of now I don't know a solution that supports full tree-shaking (including css) while also not imposing additional complexity to the consuming app. But I'm currently still investigating this topic and will post updates here if I find something interesting.

Answer 2

Look at the library tailwindcss. It automatically purges the unused css variables.