Padrão refutável em `let` exige `else`

Why this fires¶

A let pattern is structurally refutable (it might not match the value — e.g. an enum-variant pattern, or an array-length pattern) but has no else clause to handle the non-matching case. Without else, a non-matching value would silently bind nonsense (or nil) into the pattern's names.

enum Maybe { Some(int), None }

fn f(m: Maybe) {
    let Maybe::Some(x) = m
    //  ^^^^^^^^^^^^^^^^^^ error[TE125]: refutable pattern in `let` requires `else`
    //  (or use `if let`/`match`)
    print(x)
}

The same applies to parameter patterns:

enum Maybe { Some(int), None }

fn f(Maybe::Some(x)) { print(x) }   // refutable enum pattern in param position

Struct patterns are exempt — a struct's own fields always match, so let Point { x, .. } = p needs no else.

Fix it¶

1. Add else¶

fn f(m: Maybe) {
    let Maybe::Some(x) = m else { return }   // ok
    print(x)
}

2. Use if let or match instead¶

fn f(m: Maybe) {
    if let Maybe::Some(x) = m {
        print(x)
    }
}

See also¶

• TE124 — let ... else's else block must diverge.
• /docs/pattern-matching — refutable vs. irrefutable patterns.