Gentoo's Bugzilla –
Attachment 573924 Details for
Bug 684224dev-lang/rust-1.35.0[libressl]: This crate is only compatible with OpenSSL 1.0.1 through 1.1.1, or LibreSSL 2.5 through 2.9.0, but a different version of OpenSSL was found. The build is now aborting due to this version mismatch.