[]Keyword break

Exit early from a loop.

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);Run

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, let's outer loop continue.
             break;
         }
         if i >= 2 {
             // breaks from outer loop, and directly to "Bye".
             break 'outer;
         }
     }
 }
 println!("Bye.");Run

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, break; 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);Run

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