Debug. In Rust Debug is a trait and it can be automatically implemented by using the derive syntax. This is done on a struct and it enables debugging output showing the struct contents.

Typically, it is best to just have the Rust compiler automatically implement Debug. This can speed up development time of Rust programs.

struct

Example code. Consider this example program—the Test struct is decorated with the derive Debug syntax. It is important to start the line with the "#" symbol.

vec

#[derive(Debug)]
struct Test {
    code: usize,
    values: Vec<u8>,
}

fn main() {
    // Step 1: Create new struct.
    let test = Test {
        code: 10,
        values: vec![10, 20, 30],
    };
    // Step 2: Use debug formatting.
    println!("{:?}", test);
    // Step 3: Use pretty-print debug formatting.
    println!("{:#?}", test);
}
Test { code: 10, values: [10, 20, 30] }
Test {
    code: 10,
    values: [
        10,
        20,
        30,
    ],
}

Code size. Does Debug impact code size in Release builds? In a quick test, I compiled a program with the Debug trait on a struct, and then removed it.

println

Summary. Using the Debug trait enables us to output the contents of structs with println. It does not seem to make programs slower unless we add debugging calls.