# Type system attributes

The following attributes are used for changing how a type can be used.

## The `non_exhaustive` attribute

The `non_exhaustive` attribute indicates that a type or variant may have more fields or variants added in the future. It can be applied to `struct`s, `enum`s, and `enum` variants.

The `non_exhaustive` attribute uses the MetaWord syntax and thus does not take any inputs.

Within the defining crate, `non_exhaustive` has no effect.

    #![allow(unused_variables)]
    fn main() {
    #[non_exhaustive]
    pub struct Config {
        pub window_width: u16,
        pub window_height: u16,
    }

    #[non_exhaustive]
    pub enum Error {
        Message(String),
        Other,
    }

    pub enum Message {
        #[non_exhaustive] Send { from: u32, to: u32, contents: String },
        #[non_exhaustive] Reaction(u32),
        #[non_exhaustive] Quit,
    }

    // Non-exhaustive structs can be constructed as normal within the defining crate.
    let config = Config { window_width: 640, window_height: 480 };

    // Non-exhaustive structs can be matched on exhaustively within the defining crate.
    if let Config { window_width, window_height } = config {
        // ...
    }

    let error = Error::Other;
    let message = Message::Reaction(3);

    // Non-exhaustive enums can be matched on exhaustively within the defining crate.
    match error {
        Error::Message(ref s) => { },
        Error::Other => { },
    }

    match message {
        // Non-exhaustive variants can be matched on exhaustively within the defining crate.
        Message::Send { from, to, contents } => { },
        Message::Reaction(id) => { },
        Message::Quit => { },
    }
    }

Outside of the defining crate, types annotated with `non_exhaustive` have limitations that preserve backwards compatibility when new fields or variants are added.

Non-exhaustive types cannot be constructed outside of the defining crate:

- Non-exhaustive variants (`struct` or `enum` variant) cannot be constructed with a StructExpression (including with functional update syntax).
- `enum` instances can be constructed in an EnumerationVariantExpression.

    // `Config`, `Error`, and `Message` are types defined in an upstream crate that have been
    // annotated as `#[non_exhaustive]`.
    use upstream::{Config, Error, Message};

    // Cannot construct an instance of `Config`, if new fields were added in
    // a new version of `upstream` then this would fail to compile, so it is
    // disallowed.
    let config = Config { window_width: 640, window_height: 480 };

    // Can construct an instance of `Error`, new variants being introduced would
    // not result in this failing to compile.
    let error = Error::Message("foo".to_string());

    // Cannot construct an instance of `Message::Send` or `Message::Reaction`,
    // if new fields were added in a new version of `upstream` then this would
    // fail to compile, so it is disallowed.
    let message = Message::Send { from: 0, to: 1, contents: "foo".to_string(), };
    let message = Message::Reaction(0);

    // Cannot construct an instance of `Message::Quit`, if this were converted to
    // a tuple-variant `upstream` then this would fail to compile.
    let message = Message::Quit;

There are limitations when matching on non-exhaustive types outside of the defining crate:

- When pattern matching on a non-exhaustive variant (`struct` or `enum` variant), a StructPattern must be used which must include a `..`. Tuple variant constructor visibility is lowered to `min($vis, pub(crate))`.
- When pattern matching on a non-exhaustive `enum`, matching on a variant does not contribute towards the exhaustiveness of the arms.

    // `Config`, `Error`, and `Message` are types defined in an upstream crate that have been
    // annotated as `#[non_exhaustive]`.
    use upstream::{Config, Error, Message};

    // Cannot match on a non-exhaustive enum without including a wildcard arm.
    match error {
      Error::Message(ref s) => { },
      Error::Other => { },
      // would compile with: `_ => {},`
    }

    // Cannot match on a non-exhaustive struct without a wildcard.
    if let Ok(Config { window_width, window_height }) = config {
        // would compile with: `..`
    }

    match message {
      // Cannot match on a non-exhaustive struct enum variant without including a wildcard.
      Message::Send { from, to, contents } => { },
      // Cannot match on a non-exhaustive tuple or unit enum variant.
      Message::Reaction(type) => { },
      Message::Quit => { },
    }

Non-exhaustive types are always considered inhabited in downstream crates.