The Clojure dialect of Lisp comes with a software transactional memory (STM). This basically provides relational database atomicity semantics around in-memory variable access. 99% of the pain of concurrent programming goes away.
At a high level, you provide a pure function that takes the current value. The system re-runs it as many times as needed to sort out collisions with other threads, then stores the new value -- which then becomes the argument for the pure update functions running in all the other threads.
There have been some vague discussions, but STM is (as you mention), tough in an impure language. It is also not clear that it achieves Rust's efficiency goals.
At a high level, you provide a pure function that takes the current value. The system re-runs it as many times as needed to sort out collisions with other threads, then stores the new value -- which then becomes the argument for the pure update functions running in all the other threads.
Has anyone considered an STM for Rust?