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.

It's indeed more common than RefCell, but it's not like you're using them everywhere either (as using multiple locks is a recipe for deadlocks, so you should always be very mindful about how you use shared memory between threads).

I've run a quick analysis on the 1,145,666 lines of Rust code (excluding comments and blank lines) I have on my computer, which belong to several code bases I've worked on over the past few years (professional proprietary code, side projects and open source projects I just cloned to fiddle with), and here are the results:

- unsafe: 8247 matches, 1 every 138 lines (though keep in mind most of those aren't about mutable aliasing at all, like FFI or SIMD intrinsic)

- RefCell: 252 matches, 1 every 4546 lines

- Arc<Mutex<: 199 matches, 1 every 5757 lines

I think that's fair to say that “The typical Rust program has either no refcells, or a very small number, just as it has either no unsafe code or a very small amount”