Keyword break

Exit early from a loop or labelled block.

When break is encountered, execution of the associated loop body is immediately terminated.

let mut last = 0;

for x in 1..100 {
    if x > 12 {
        break;
    }
    last = x;
}

assert_eq!(last, 12);
println!("{last}");

A break expression is normally associated with the innermost loop enclosing the break but a label can be used to specify which enclosing loop is affected.

'outer: for i in 1..=5 {
    println!("outer iteration (i): {i}");

    '_inner: for j in 1..=200 {
        println!("    inner iteration (j): {j}");
        if j >= 3 {
            // breaks from inner loop, lets outer loop continue.
            break;
        }
        if i >= 2 {
            // breaks from outer loop, and directly to "Bye".
            break 'outer;
        }
    }
}
println!("Bye.");

When associated with loop, a break expression may be used to return a value from that loop. This is only valid with loop and not with any other type of loop. If no value is specified for break; it returns (). Every break within a loop must return the same type.

let (mut a, mut b) = (1, 1);
let result = loop {
    if b > 10 {
        break b;
    }
    let c = a + b;
    a = b;
    b = c;
};
// first number in Fibonacci sequence over 10:
assert_eq!(result, 13);
println!("{result}");

It is also possible to exit from any labelled block returning the value early. If no value is specified for break; it returns ().

let inputs = vec!["Cow", "Cat", "Dog", "Snake", "Cod"];

let mut results = vec![];
for input in inputs {
    let result = 'filter: {
        if input.len() > 3 {
            break 'filter Err("Too long");
        };

        if !input.contains("C") {
            break 'filter Err("No Cs");
        };

        Ok(input.to_uppercase())
    };

    results.push(result);
}

// [Ok("COW"), Ok("CAT"), Err("No Cs"), Err("Too long"), Ok("COD")]
println!("{:?}", results)

For more details consult the Reference on “break expression” and the Reference on “break and loop values”.