My understanding is that MesaLink uses the low-level crypto from BoringSSL (transitively via ring) and the high-level protocol code from ring, which is written in Rust.

Most memory safety problems in TLS implementations come from the higher-level protocol code, not the implementations of the raw crypto primitives. In fact, the latter benefit from being written in as low-level of a style as possible to mitigate timing attacks.

Edit: Brian Smith has an authoritative comment here: https://www.reddit.com/r/rust/comments/89aiyw/mesalink_a_mem...

MesaLink relies on ring for its crypto operations. Ring is written in a mix of Rust, C and assembly, with most of the C and assembly code coming from (Boring|Open)SSL.