Python cryptography, Rust, and Gentoo

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.

Bitfields have some downsides, the biggest one being the lack of a standard, portable memory layout. The placement of individual bitfields within a structure is implementation-defined and varies in practice according to the target architecture, especially with respect to byte order. As a result, it is generally considered best practice to avoid directly sharing structures with bitfields between programs which may not share the same code-generation settings—for example, between user-space and kernel-space, or anything related to the layout of bits in a hardware register or persistent storage. Even for local data within the same process there are some ergonomic issues, such as the fact that one cannot take the address of a bitfield, make use of atomic operations, or easily set, clear, or test multiple bitfields within the same structure in the same operation in the same way that one can use bitwise AND/OR operations with a bitmask. The compiler can paper over some of these issues but this relies more heavily on the optimizer to merge together a series of one-bit read-modify-write sequences.