if

is an expression.  semicolons are a little weird, but they do make
sense.  (why weird?  because you don't have to write them in the body of
`main`, supposedly because it returns nothing.)

rust/book/src/main.rs

@@ -2,5 +2,9 @@ fn main() {
    let x = 5;
    println!("Hello, World!");
    println!("THE number is {}", x);
    if x == 42 {
        println!("It's THE ANSWER!");
    } else {
        println!("Meh, it's just a number, {}.", x);
    }
}