A more involved macro – writing a DSL for HashMap initialization

Armed with the knowledge of repetitions and token tree types, let's build something practical using repetitions in macro_rules!. In this section, we'll build a crate that exposes a macro that allows you to create HashMaps such as the following:

let my_map = map! {
    1 => 2,
    2 => 3
};

This is more concise and readable compared to manually calling HashMap::new(), followed by one or more insert calls. Let's  create a new cargo project by running cargo new macro_map --lib with the initial block for macro_rules!:

// macro_map/lib.rs

#[macro_export]
macro_rules! map {
    // todo
}

Since we want the users to use our macros, we need to add a #[macro_export] attribute on this macro definition. Macros are private by default in a module, which is similar to other items. We'll call our macro map! and since we are building our own syntax to initialize HashMap, we'll go with the k => v syntax, where k is the key and v is the...