Mastering Rust - Second Edition

Dynamic statics using the lazy_static! macro

As we have seen, global values can only be declared for types that are non-dynamic in their initialization and have a known size on the stack at compile time. For example, you can't create a HashMap as a static value because it requires a heap allocation. Fortunately, we can have HashMap and other dynamic collection types such as Vec as global statics too, using a third-party crate called lazy_static. This crate exposes the lazy_static! macro, which can be used to initialize any dynamic type that's accessible globally from anywhere in the program. Here's a snippet of how to initialize a Vec that can be mutated from multiple threads:

// lazy_static_demouse std::sync::Mutex;lazy_static! { static ...