break and continue

break leaves a loop at once. continue ends the current pass and moves on to the next one.

break-statement    = "break" [label] ";"
continue-statement = "continue" [label] ";"

for v in values {
    if v == 0 {
        break;
    }
    if v < 0 {
        continue;
    }
    sum += v;
}

Without a label, both act on the innermost loop around them. Outside every loop they are errors: error: 'break' can only be used inside 'while', 'for', or 'loop', and the same for continue. break takes no value; a loop does not produce one.

Labels

A label names a loop. It is written before the loop keyword, followed by :, and any of the four loops may carry one:

outer: for row in 0..3 {
    for col in 0..3 {
        if col == 2 {
            continue outer;
        }
        if row == 2 {
            break outer;
        }
        Print("({} {}) ", row, col);
    }
}

continue outer abandons the inner loop and starts the next pass of outer; break outer leaves both. This prints (0 0) (0 1) (1 0) (1 1).

A label names exactly one loop at a time. An inner loop may not reuse the label of a loop around it, because break outer inside it would then read as either one:

error: loop label 'outer' shadows an enclosing loop label
  help: give the inner loop a different label

Loops that follow one another, rather than nest, may reuse a label. A label must belong to a loop that encloses the statement: continue missing; is error: 'continue' refers to unknown loop label 'missing'. Only loops take labels.

Inside expressions

break and continue, with or without a label, may also stand where an expression would leave rather than produce a value: as a whole match arm, or as the fallback of ??:

for v in values {
    let step = match v {
        0 => break,
        -1 => continue,
        else => v
    };
    sum += step;
}

for v in values {
    let x = maybe ?? continue;
    PrintLine("{}", x);
}

They cannot be used as an ordinary value: n > 0 ? n : break is error: 'break' cannot be used as a value here.

See also

• Loops — the four loops
• return — leaving the whole function
• Coalescing — ?? break and ?? continue
• Learn: Break, Continue, Label