This error indicates that the struct, enum or enum variant must be matched
non-exhaustively as it has been marked as `non_exhaustive`.
When applied within a crate, downstream users of the crate will need to use the
`_` pattern when matching enums and use the `..` pattern when matching structs.
Downstream crates cannot match against non-exhaustive enum variants.
For example, in the below example, since the enum is marked as
`non_exhaustive`, it is required that downstream crates match non-exhaustively
on it.
```rust,ignore (pseudo-Rust)
#[non_exhaustive]
pub enum Error {
Message(String),
Other,
}
impl Display for Error {
fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
// This will not error, despite being marked as non_exhaustive, as this
// enum is defined within the current crate, it can be matched
// exhaustively.
let display = match self {
Message(s) => s,
Other => "other or unknown error",
};
formatter.write_str(display)
}
}
```
An example of matching non-exhaustively on the above enum is provided below:
```rust,ignore (pseudo-Rust)
use mycrate::Error;
// This will not error as the non_exhaustive Error enum has been matched with a
// wildcard.
match error {
Message(s) => ...,
Other => ...,
_ => ...,
}
```
Similarly, for structs, match with `..` to avoid this error.