Keyword if

Evaluate a block if a condition holds.

if is a familiar construct to most programmers, and is the main way you'll often do logic in your code. However, unlike in most languages, if blocks can also act as expressions.

if 1 == 2 {
    println!("whoops, mathematics broke");
} else {
    println!("everything's fine!");
}

let greeting = if rude {
    "sup nerd."
} else {
    "hello, friend!"
};

if let Ok(x) = "123".parse::<i32>() {
    println!("{} double that and you get {}!", greeting, x * 2);
}

Shown above are the three typical forms an if block comes in. First is the usual kind of thing you'd see in many languages, with an optional else block. Second uses if as an expression, which is only possible if all branches return the same type. An if expression can be used everywhere you'd expect. The third kind of if block is an if let block, which behaves similarly to using a match expression:

if let Some(x) = Some(123) {
    // code
} else {
    // something else
}

match Some(123) {
    Some(x) => {
        // code
    },
    _ => {
        // something else
    },
}

Each kind of if expression can be mixed and matched as needed.

if true == false {
    println!("oh no");
} else if "something" == "other thing" {
    println!("oh dear");
} else if let Some(200) = "blarg".parse::<i32>().ok() {
    println!("uh oh");
} else {
    println!("phew, nothing's broken");
}

The if keyword is used in one other place in Rust, namely as a part of pattern matching itself, allowing patterns such as Some(x) if x > 200 to be used.

For more information on if expressions, see the Rust book or the Reference.