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In Rust, a &T is not an immutable reference in the common Functional Programming sense. func(&self) is free to mutate atomics or values wrapped in Mutex through self because any other call through an alias to self is going to synchronize the mutations. &T means "you can have other aliases".

&mut T means "you cannot have other aliases". This contrasts with C++ const where aliases are all over the place and const only exists to help you be suspicious about side-effects.