People coming from other languages might wonder what lazy_static offers that can't already be done by a normal static object. The difference between them is as follows.
In Rust, a static variable is a variable that lives for the entire duration of the program, which is why they get their own, special lifetime, 'static. The catch is that the variable has to be built in a constant way, that is, a way that is known at compile time. In our example, we cannot replace CURRENCIES [11] with a normal static because HashMap::new() returns a newly constructed HashMap sitting somewhere in the memory during runtime. As this requires it to live in memory, it's impossible to build a HashMap during compile time, so its constructor is not constant.
Another catch with static variables is that, because they have a global lifetime, the borrow checker cannot make sure that their access is thread-safe. As a consequence, any access on a static mut variable will always...