Rustls is a TLS backend written in Rust. Curl can be built to use it as an alternative to OpenSSL or other TLS backends. We use the rustls-ffi C bindings. This version of curl depends on version v0.8.2 of rustls-ffi.
First, install Rust.
Next, check out, build, and install the appropriate version of rustls-ffi:
% cargo install cbindgen
% git clone https://github.com/rustls/rustls-ffi -b v0.8.2
% cd rustls-ffi
% make
% make DESTDIR=${HOME}/rustls-ffi-built/ install
Now configure and build curl with rustls:
% git clone https://github.com/curl/curl
% cd curl
% autoreconf -fi
% ./configure --with-rustls=${HOME}/rustls-ffi-built
% make