No? Adding a Rust backend does not change the API. It just calls into the Rust backend via C bindings. If all you care is ABI stability from the user side none

No? Adding a Rust backend does not change the API. It just calls into the Rust backend via C bindings. If all you care is ABI stability from the user side none of this would break. cURL already supports other Rust-written backends as the article pointed out. See https://everything.curl.dev/internals/backends.html https://everything.curl.dev/internals/backends.html.