Yes, in the standard library, an immutable object can hide mutable state in e.g. a 'Mutex', and effects to the external system aren't wired through anything like monads or unique objects.
I see those as compromises Rust makes in the name of pragmatism and being a system'ey language. I don't necessarily like all of them, but I find the general uniqueness typing based approach interesting.
See articles comparing Clean and Haskell for an interesting historical perspective, including how both approaches could be used to model side-effects in a purely functional language. Haskell "won", possibly because it was seen as more generic and composable. I always felt Clean's approach had merit too, so I was really glad to see Rust bring the idea, or a closely related idea, to prominence.
Passing a unique world object around is effectively the same as composing with the IO monad, and borrowing 'f(&mut world)' is basically equivalent to 'let world = f(world)'.
Maybe someone will one day write a standard library in that style.
