Rust Programming By Example

Macros

Macro rules, also called macros by example, are a way to avoid code duplication by generating code at compile time. We will implement a simple macro to implement our BitSet trait for integer types:

macro_rules! int_bitset {
    ($ty:ty) => {
        impl BitSet for $ty {
            fn clear(&mut self, index: usize) {
                *self &= !(1 << index);
            }

            fn is_set(&self, index: usize) -> bool {
                (*self >> index) & 1 == 1
            }

            fn set(&mut self, index: usize) {
                *self |= 1 << index;
            }
        }
    };
}

The name of the int_bitset macro is written after macro_rules!. A macro can have multiple rules, similar to match arms, but it matches on Rust syntactic elements instead, with types, expressions, blocks of code, and so on. Here we only have one rule and it matches against a single type ...