non_exhaustive

An attribute that says more variants or fields may be added later, so adding them isn't a breaking change.

#[non_exhaustive] on an enum or struct forces other crates to include a wildcard arm when matching and forbids them from constructing the struct with a literal.

Examples

With #[non_exhaustive] pub enum Event { Watered, Alarm }, a match in another crate that lists both variants still fails with E0004 until it adds a _ => arm.