I built this branch and installed it into my project.
Can confirm that main usecase does work: using composes on .scss file, containing line-comments and mixins, doesn't cause any errors, and resulting styles working as expected.

Importing values via @value directive from .scss file does work too.

Using composes on .css file containing nested selectors (powered by postcss-nested plugin) does work too.

More sophisticated cases which don't work (idk whether they should):

• using composes on .css file containing mixins (via postcss-mixins plugin): no errors produced, but mixins are not processed. Output styles is like this:

._mix_css_1n1sl_5 {
  @mixin test: ;
}

• using composes on .css file containing line-comments (with syntax: postcss-scss option enabled in postcss config): causes build crash: CssSyntaxError: [...]/css.module.css:1:1: Unexpected '/'. Escaping special characters with \ may help.

Thanks for the PR. We discussed about this PR in the last meeting. We think we can have this feature. The remaining things that this PR needs is:

1. Someone needs to check how to handle the sophisticated cases described in the comment above (fix(css modules): apply preprocessor to composed file if needed (fix #10340) #12076 (comment))
2. Avoid using the internals of postcss-modules (e.g. postcss-modules/build/FileSystemLoader)
3. Check if this can be supported by lightningcss as well in future
4. Add source map support

@Sec32fun32 we'll need to ban you from the org if you continue sending these approvals, they are generating notifications for everyone in watching the repository.