Rust for C-Programmers

21.4 Plain Variable Assignment as a Pattern

Every let x = something; statement in Rust is effectively a pattern match. By default, x itself is the pattern. However, you can make this more elaborate:

fn main() {
    let (width, height) = (20, 10);
    println!("Width = {}, Height = {}", width, height);
}

Here, (width, height) is an irrefutable tuple pattern. It always matches (20, 10). Any attempt to use a refutable pattern—something that might fail—would be disallowed in a plain let.