There are lots of people doing awesome stuff with cryptography in Rust, including (but not limited to) the people who maintain Mundane.

You are right that ring has a much-reduced set of build dependencies compared to Mundane/BoringSSL. However, it is also true that Google implemented some build system support (to support Mundane?) that will help ring's custom build system a lot too, once I find some time to make use of it.