Python cryptography, Rust, and Gentoo

> Unfortunately, it's not the same for all POSIX platforms - just about every platform has extensions to POSIX that aren't the same as other platforms :-(

Sure there are some extensions, but that's not much compared to the common Posix API.

Porting rust to a system that provides the Posix interface should just be a matter of running a script that looks for the ABI of the Posix interface (even if only to record it in the source tree). Like perl has been doing for decades.