Well, you could change Rust so that the names of types with interior mutability have to start with an "@" or something like that, and then it would be obvious,

Well, you could change Rust so that the names of types with interior mutability have to start with an "@" or something like that, and then it would be obvious, and could expose a trait to denote that (there is Sync and the internal Freeze, but they don't quite do that). I think it hasn't been done because interior mutability is relatively rarely used, and being able to assert immutability in generic code is not actually that useful (and for things like hash map keys you'd need to implement Hash or other traits manually anyway, so you'll probably notice the issue). In non-generic code, you have to call functions like "borrow()", etc. to access the internally mutable data, so you know it's mutable. Regarding "fearless concurrency", the Sync trait ensures that, allowing "interior mutability" only if thread-safe (i.e. Mutex or Atomic). Obviously you can have race conditions between modifications of different Mutex/Atomics, but that's an unavoidable fundamental problem: even transactional systems will have race conditions if you use two separate transactions where you should have used one, and single-threaded event based systems also have them if you "defer" part of the computation improperly.