Mastering Rust - Second Edition

macro_rules! token types

Before we build more complex macros, it's important to become familiar with the valid inputs that macro_rules! can take. Since macro_rules! work at the syntactic level, it needs to provide users, a handle to these syntactic elements, and distinguish what can and cannot be included within a macro and how we can interact with them.

The following are some important token tree types that you can pass into a macro as input:

• block: This is a sequence of statements. We have already used block in the debugging example. It matches any sequence of statements, delimited by braces, such as what we were using before:

{ silly; things; } 

This block includes the statements silly and things.

• expr: This matches any expression, ...