This provides auxiliary pieces for a C API to use the hyper library.
The C API of hyper is currently unstable, which means it's not part of the semver contract as the rest of the Rust API is.
Because of that, it's only accessible if --cfg hyper_unstable_ffi is passed to rustc when compiling. The easiest way to do that is setting the RUSTFLAGS environment variable.
The C API is part of the Rust library, but isn't compiled by default. Using a nightly release of cargo, starting with nightly-2022-03-02, it can be compiled with the following command:
RUSTFLAGS="--cfg hyper_unstable_ffi" cargo +nightly rustc --features client,http1,http2,ffi -Z unstable-options --crate-type cdylib