compiler/rustc_error_codes/src/error_codes/E0638.md

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.