Since Rust 1.0, we have had a great macro system. Macros allow us to apply some code to multiple types or expressions, as they work by expanding themselves at compile time. This means that when you use a macro, you are effectively writing a lot of code before the actual compilation starts. This has two main benefits, first, the codebase can be easier to maintain by being smaller and reusing code, and second, since macros expand before starting the creation of object code, you can abstract at the syntactic level.

For example, you can have a function like this one:

fn add_one(input: u32) -> u32 {
    input + 1
}

This function restricts the input to u32 types and the return type to u32. We could add some more accepted types by using generics, which may accept &u32 if we use the Add trait. Macros allow us to create this kind of code for any element that can be written to the left of the + sign and it will be expanded differently for each type of element, creating different code for each case.

To create a macro, you will need to use a macro built in to the language, the macro_rules!{} macro. This macro receives the name of the new macro as a first parameter and a block with the macro code as a second element. The syntax can be a bit complex the first time you see it, but it can be learned quickly. Let's start with a macro that does just the same as the function we saw before:

macro_rules! add_one {
    ($input:expr) => {
        $input + 1
    }
}

You can now call that macro from your main() function by calling add_one!(integer);. Note that the macro needs to be defined before the first call, even if it's in the same file. It will work with any integer, which wasn't possible with functions.

Let's analyze how the syntax works. In the block after the name of the new macro (add_one), we can see two sections. In the first part, on the left of the =>, we see $input:expr inside parentheses. Then, to the right, we see a Rust block where we do the actual addition.

The left part works similarly (in some ways) to a pattern match. You can add any combination of characters and then some variables, all of them starting with a dollar sign ($) and showing the type of variable after a colon. In this case, the only variable is the $input variable and it's an expression. This means that you can insert any kind of expression there and it will be written in the code to the right, substituting the variable with the expression.