hyper uses libc in ffi feature for the C language's type aliases. As hyper's ffi feature is an unstable feature, hyper could use these types via std::ffi which require Rust 1.64, except for c_size_t which is a nightly only API. As libc::size_t and core::ffi::c_size_t are both defined as usize, this type might be able to be defined as type alias in hyper until it stabilizes.