Break with Value

In Rust, the break keyword can terminate execution of a loop or a labeled block expression and simultaneously yield a value to the surrounding scope. Because these constructs are expressions, they evaluate to a value, and break serves as the mechanism to return that value to the binding context.

Syntax

To return a value, append the expression directly after the break keyword. If breaking from a specific labeled construct, the label must precede the value expression.

When assigning the result of a loop or labeled block to a variable via a let statement, the let statement itself must be terminated with a semicolon. However, if the loop or labeled block is used as a trailing expression (e.g., as the implicit return value of a function), no semicolon is required.

// Standard loop syntax
let binding = loop {
    break expression;
};

// Labeled block syntax (Rust 1.65+)
let binding = 'label: {
    break 'label expression;
};

Mechanics and Compiler Rules

• Type Inference: The type of the loop or labeled block expression is inferred from the type of the expression passed to break.
• Type Homogeneity: If a single construct contains multiple break statements, every break must yield a value of the exact same type.
• Construct Restriction: Yielding a value via break is supported by the loop keyword and labeled blocks. It is a compilation error to attempt to return a value using break inside while, while let, or for loops. Those constructs inherently evaluate to the unit type ().
• Labeled Block Requirement: Breaking out of a labeled block always requires the label to be explicitly specified (e.g., break 'block_name value;). In contrast, breaking from an unnested loop does not require a label (e.g., break value;).
• Unit Type Default: If a construct is broken using break; without an expression, it implicitly yields the unit type ().

Code Examples

Basic Value Yielding In this example, the loop evaluates to an i32, which is bound to result.

let mut counter = 0;

let result = loop {
    counter += 1;

    if counter == 10 {
        break counter * 2; 
    }
};

// result is 20

Combining with Loop Labels When dealing with nested loops, break can be combined with a loop label to yield a value from an inner loop directly to the assignment of an outer loop.

let mut x = 0;

let result = 'outer: loop {
    x += 1;
    let mut y = 0;
    
    loop {
        y += 1;
        
        if x == 2 && y == 3 {
            // Terminates the 'outer loop and yields the value 50
            break 'outer 50; 
        }
        
        if y == 5 {
            // Terminates the inner loop, allowing 'outer to iterate again
            break; 
        }
    }
};

// result is 50

Yielding from a Labeled Block Since Rust 1.65, break can yield a value from a standard block expression annotated with a label, bypassing the need for a loop construct. The label is mandatory when breaking from a block.

let condition = true;

let result = 'block: {
    if condition {
        break 'block 100;
    }
    
    // Default block evaluation if break is not reached
    200 
};

// result is 100

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