Note that all cryptographic primitives are still going to be in C via an OpenSSL-like API for the next while; the current proposal is to migrate from OpenSSL to one of its forks. Various bits of backend logic that aren't cryptographic primitives (e.g., parsing) have been rewritten in Rust; additionally, https://github.com/ctz/graviola is mentioned near the end as a possible implementation of cryptographic primitives in a combination of Rust and assembly (without any C), but it's not especially mature yet.