macros.md

Macros

Macros must be surrounded by double brackets. Optional trailing commas should be added when possible.

Example

Don't do:

macro_rules! print_user_success {
    ($($tts:tt)*) => {
        eprint!("✅ ");
        eprintln!($($tts)*);
    }
}

Do instead:

macro_rules! print_user_success {
    ($($tts:tt)* $(,)?) => {{
        eprint!("✅ ");
        eprintln!($($tts)*);
    }}
}

Explanation

When trying to use macros as values, it can cause errors when the macros don't include their own block in the macro itself. Using two brackets ensures the macro can be used in match arms without extra brackets.

Having an optional trailing comma makes it possible to use and format macros like any other Rust function. This is especially important when multiple arguments are taken and a linebreak is not unlikely.