While

A while loop checks a bool condition before each pass and stops as soon as it fails.

let i = 0;
while i < 5 {
    i += 1;
}
// -> i is 5

Breaking a value

Because a while loop might never run – its condition could be false on the very first check – a value broken out of it is wrapped in an option. The loop yields null if it ends without a break value.

let found: int? = while has_next() {
    let x = next();
    if matches(x) {
        break x; // -> int?, because the loop might not have run at all
    }
};

While let

while let is to while what if let is to if: instead of a bool, it evaluates an expression each pass and keeps looping as long as the result is not null, binding the unwrapped value in the body.

while let job = next_job() {
    // runs as long as `next_job()` returns a value;
    // stops the first time it returns null
    process(job);
}

It’s the idiomatic way to drain a source that signals “nothing left” with null.