Well Zircon (Fuchsia's kernel) doesn't have any Rust code inside of it; it's C++. Parts of rust code used in Fuchsia can be found in the Garnet layer and it is just one of the languages that can interface with the OS via the FIDL (Fuchsia Interface Definition Language) [0][1]. An example of this is a tool written in Rust that communicates with parts of the bluetooth stack[2]. The network stack is also using it and is written in Go. [3]