All logic, all state management, all per-device state machines, all command parsing and translation, all data queues, etc. Look at the examples people posted in other comments.

Yep. Its kind of remarkable just how little unsafe code you often need in cases like this.

I ported a C skip list implementation to rust a few years ago. Skip lists are like linked lists, but where instead of a single "next" pointer, each node contains an array of them. As you can imagine, the C code is packed full of fiddly pointer manipulation.

The rust port certainly makes use of a fair bit of unsafe code. But the unsafe blocks still make up a surprisingly small minority of the code.

Porting this package was one of my first experiences working with rust, and it was a big "aha!" moment for me. Debugging the C implementation was a nightmare, because a lot of bugs caused obscure memory corruption problems. They're always a headache to track down. When I first ported the C code to rust, one of my tests segfaulted. At first I was confused - rust doesn't segfault! Then I realised it could only segfault from a bug in an unsafe block. There were only two unsafe functions it could be, and one obvious candidate. I had a read of the code, and spotted the error nearly immediately. The same bug would probably have taken me hours to fix in C because it could have been anywhere. But in rust I found and fixed the problem in a few minutes.