I like the subprocess library. What are we missing in the library? The maintainer is very responsive so we should consider just asking him to add what we need.

maybe due to platform compatibility like _posixsubprocess?

Yeah, it's generally more difficult to implement an API with a large surface area than it is to implement the low-level primitives it's based on. This also guarantees parity in the implementation, since there could be a subtle disparity between cpython's _posixsubprocess or windows subprocess implementation and the subprocess crate that causes some weird bug. This way we know that as long as the fork_exec or CreateProcess code (which are pretty small) are equivalent, then so should the subprocess module.