Attribute non_exhaustive 

Indicates that a type might have more fields or variants added in the future.

Placing #[non_exhaustive] on a struct or enum tells code in other crates not to assume the definition is complete. This lets a library add new fields or variants without breaking existing code.

On an enum, code outside the defining crate must include a wildcard arm when matching:

// in crate `errors`:
#[non_exhaustive]
pub enum ConnectionError {
    Refused,
    Timeout,
}

// in another crate:
use errors::ConnectionError;

match error {
    ConnectionError::Refused => println!("connection refused"),
    ConnectionError::Timeout => println!("timed out"),
    _ => println!("other error"), // required because of #[non_exhaustive]
}

On a struct, code outside the defining crate cannot construct instances using struct literal syntax:

// in crate `config`:
#[non_exhaustive]
pub struct Config {
    pub width: u32,
    pub height: u32,
}

// in another crate:
use config::Config;

let c = Config { width: 800, height: 600 }; // ERROR: cannot construct

Inside the defining crate, exhaustive matching and direct construction are still allowed.

For more information, see the Reference on the non_exhaustive attribute.