[]Keyword else

What expression to evaluate when an if condition evaluates to false.

else expressions are optional. When no else expressions are supplied it is assumed to evaluate to the unit type ().

The type that the else blocks evaluate to must be compatible with the type that the if block evaluates to.

As can be seen below, else must be followed by either: if, if let, or a block {} and it will return the value of that expression.

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

Here's another example but here we do not try and return an expression:

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");
}
Run

The above is still an expression but it will always evaluate to ().

There is possibly no limit to the number of else blocks that could follow an if expression however if you have several then a match expression might be preferable.

Read more about control flow in the Rust Book.