This defines an extension type used in requests for the client that is used to setup a callback for receipt of informational (1xx) responses. The type isn't currently public, and is only usable in the C API.
C API for hyper
This provides auxiliary pieces for a C API to use the hyper library.
Unstable
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.
Building
The C API is part of the Rust library, but isn't compiled by default. Using cargo, it can be compiled with the following command:
RUSTFLAGS="--cfg hyper_unstable_ffi" cargo build --features client,http1,http2,ffi