Rustacean

Loop Expressions

3 min read

Loop Expressions

The loop expression repeats a block until control flow exits it, and it can return a value with break value.

What it is

loop { ... } is Rust’s explicit infinite-loop construct. It runs the body repeatedly until break, return, panic, or another diverging operation exits.

Unlike while and for, a loop can evaluate to a non-unit value by using break expression. This makes it useful for retry-until-success logic that computes a result.

continue skips the rest of the current iteration and starts the next one.

How it works

The Reference classifies loop, while, for, and labeled blocks as breakable expressions. A loop without an associated break expression is diverging and has the never type !.

If a loop has break expressions with values, those values must have compatible types because they define the type of the whole loop expression.

Loop labels such as 'outer: loop { ... } let break 'outer or continue 'outer target an outer loop instead of the innermost one.

Example

fn main() {
    let mut counter = 0;
 
    let answer = 'search: loop {
        counter += 1;
 
        if counter < 3 {
            continue;
        }
 
        break 'search counter * 10;
    };
 
    assert_eq!(answer, 30);
}

Common errors

Different break values make the loop expression ill-typed:

fn main() {
    // let value = loop {
    //     if true { break 1; }
    //     break "done";
    // };
}

Typical diagnostic:

error[E0308]: mismatched types

Fix by choosing one result type or returning an enum/Result.

Best practice

  • ✅ Use loop when the exit condition is naturally inside the body.
  • ✅ Use break value for retry or polling code that must produce one final result.
  • ✅ Label nested loops when a break or continue targets anything other than the innermost loop.
  • ✅ Prefer While and For Loops when a condition or iterator clearly describes repetition.

Pitfalls

  • ⚠️ Writing loop without a reachable exit can hang a program unless divergence is intentional.
  • ⚠️ Mixing different break value types makes the loop expression ill-typed.
  • ⚠️ Assuming break exits a function; it exits the targeted loop. Use return to exit the function.
  • ⚠️ Using labels casually can make control flow harder to follow; reserve them for nested-loop clarity.

See also

While and For Loops · Statements vs Expressions · If Expressions · Never Type · Result · panic! · Type Inference · Iterator Method Trio · Basic Concepts & Syntax

Sources

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