`let ... else` block must diverge

Why this fires¶

A let PATTERN = expr else { ... } block ran to completion without returning, panicking, breaking, or continuing. If the else block can fall through, execution would continue into the pattern's bindings using a value that did NOT match the pattern — so Zolo requires every path through else to diverge.

struct Point { x: int, y: int }

fn f(p) {
    let Point { x, .. } = p else { print("nope") }
    //                          ^^^^^^^^^^^^^^^^^^ error[TE124]: `else` block of
    //                          `let ... else` must diverge (return, panic, break, or continue)
    print(x)
}

A guard that contains a let clause has the same requirement, even when the pattern is irrefutable or discards its value. Later comma filters may fail.

fn positive(candidate: int?) -> int {
    guard let value = candidate, value > 0 else { print("invalid") } // TE124
    return value
}

Use else { return 0 } here so the continuation can safely use value. The failure block resolves names in the original outer scope.

Fix it¶

Make every path through the else block diverge:

fn f(p) -> int {
    let Point { x, .. } = p else { return 0 }   // ok
    return x
}

fn f(p) {
    let Point { x, .. } = p else { panic("not a point") }   // ok
    print(x)
}

See also¶

• TE125 — a refutable let pattern with no else at all.
• /docs/pattern-matching — let ... else.