diagnostic codes / Types

E0206 non-exhaustive-match

A `match` over an enum does not cover every variant.

Enums are closed: the variants are known at the declaration, so a `match` can be checked against the full set. A missing variant is reported with its name. A `_` arm is accepted and is treated as deliberate coverage of everything not listed.

What it looks like

use tier core;

enum E { A, B, C }

fn main() -> i32 {
    let e = E::A;
    match e {
        E::A => { print_str("a"); }
        E::B => { print_str("b"); }
    }
    return 0;
}

ralt check e0206.tp
e0206.tp:7:5: error: match over 'E' misses variant 'C'
  [E0206 non-exhaustive-match] match must cover every enum variant or add a deliberate _ arm

What to do

• Add the missing variant arm, even if its body is empty.
• Or add `_ => { }` if new variants should not force a change here. That is a decision, and the syntax records it.

Why the code exists

Adding a variant should break every `match` that needs to think about it, at compile time. That is the whole reason the enum is closed.

What ralt explain says

ralt explain E0206
E0206
severity: error
name: non-exhaustive-match
hint: match must cover every enum variant or add a deliberate _ arm

Related

• Structs, enums, match /docs/language/data