Struct Pass 

pub struct Pass(/* private fields */);

A key for reading/writing to RwData

This key is necessary in order to prevent breakage of the number one rule of Rust: any number of shared references, or one exclusive reference.

When you call RwData::read, any call to RwData::write may end up breaking this rule, and vice-versa, which is why this struct is necessary.

One downside of this approach is that it is even more restrictive than Rust’s rule of thumb, since that one is enforced on individual instances, while this one is enforced on all RwDatas. This (as far as i know) cannot be circumvented, as a more advanced compile time checker (that distinguishes RwData<T>s of different Ts, for example) does not seem feasible without the use of unfinished features, which I am not willing to use.