Wow. that's freaking awesome, thanks! Would you mind moving the postcss package to peerDependencies?

done!

Thanks. I think I can make a release tomorrow

I suspect this code might require refactoring to avoid issues using postcss-modules together with some other PostCSS 8 plugins. You can get the test suite to fail like this:

• in tests/test.js switch the test plugin from Once() to Root(): ludofischer@00b2df8
• three tests fail: only calls plugins once when it preserves comments, only calls plugins once when it composes rules, only calls plugins once when it processes values

I think the only calls plugins once when it preserves comments failure might be serious as the expected lines seem to disappear completely, while the other two failures might just show a different execution order with no impact on the generated CSS.

In https://github.com/ludofischer/postcss-strange-cases there's another reproduction where other plugins do not run correctly with PostCSS 8, if a plugin creates anotherpostcss() instance inside a visitor (just like postcss-modules).
You probably can't detetct the issue if you only test with autoprefixer since it uses Once().

I don't think it's possible for this plugin to ensure compatibility with all plugins out there - I assume most plugins will be using Once anyway. I tested this with the plugins Vue's SFC compilers are using and they do work fine.

Either way, I think it's better to get a new version out so people can test it and then fix potential issues if there are any.

@ludofischer good point, this stuff has definitely to be checked twice.

I’m going to do this later this or next week. I wouldn’t like to release something that breaks existing code if it’s possible.

I suspect any plugins that would normally run after postcss-modules don't get called properly, because postcss-modules runs on a different tree than the other plugins.
If you need a release ASAP, the cheapest improvement might be switching to OnceExit() ludofischer@ce553b7, then other plugins will run correctly before postcss-modules 'breaks' the AST traversal at the end. You need to update two test snapshots, but I hope those are not meaningful changes, just plugin order getting swapped with the same end result.

I've had similar problems working on cssnano, so here's more detail about how I see the issue (but I haven't tested all the ideas below yet):

1. PostCSS 7 would call plugins in order and they would perform their own AST traversal one after the other, but PostCSS 8 can also register plugins as visitors during a single AST traversal. Since postcss-modules starts its own AST traversal by calling postcss().process(), plugins that expect to be called in the main AST traversal can't 'see' the changes generated by postcss-modules. That could probably be roughly solved with OnceExit(). The 'best' solution (I haven't tried yet) might be to run the plugins in postcss-modules on the same tree as all other plugins. Can maybe be achieved by exposing the internal postcss instance directly, since a plugin can also be postcss instance (see API docs).

2. postcss-modules seems interested in what plugins run earlier and how many times plugins get called (for example here and in tests), but in PostCSS 8 you can't guarantee any order unless everything uses Once(). But I hope we don't have to force everyone to use Once(): if we ensure the postcss-modules plugins run in the correct order all together at the end, the ordering of plugins that run before should not matter.

I agree the best refactor would be completely overhauling this plugin to avoid the "sub processor" thing which seems hacky in the first place. But I also think first getting a version out that is usable with Postcss 8 for those who don't have conflicting plugins now would be beneficial. Mostly, this module's release is blocking @vue/compiler-sfc to release a new version supporting Postcss 8, which in turn blocks Vue users from using Tailwind 2 without duplicated versions of postcss.

P.S switched to OnceExit and updated snapshots

Merging, @yyx990803 thank you!

I would like to make a total refactoring of the plugin to match the new PostCSS architecture and ditch out all the deps. Hope I will have some time in the future for this.