[roca]

In six years I wrote a lot of the 200K lines of Rust for Pernosco --- a fairly sophisticated application server for a SaaS debugger. There are 40 instances of the string 'Arc::new(Mutex::new('. A lot of them are shared caches and other innocuous things. There are 22 occurrences of 'RefCell::new('. There are other uses of Arc, and other uses of Mutex, but fundamentally the system has not collapsed into an arbitrary graph of mutable data.

Of course, one's mileage can vary. Most of our system is designed to be stateless at the top level, which helps.