Rust Programming Cookbook

How it works...

Parameter ranges in Rust's macro system work a little bit like regular expressions. There are several parts to the syntax: $() indicates repetition, the character that follows its separator (,, ;, and => are allowed), and lastly, the qualifier for how often the repetition is expected (+ or *—just like regular expressions, one or more and zero or more respectively).

Step 2 shows the implementation of a set initializer macro akin to vec!. There, we expect a single expression to populate std::collections::HashSet and return the result in a sub-block from the transcriber. This is necessary to allow things such as variable assignments (which are not allowed directly within the transcriber block), but don't hinder the expansion ...