Rust error E0638: this error indicates that the struct, enum or enum variant must be matched non-exhaustively as it has been marked as `non_exhaustive`

The compiler team's explanation

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.

#[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:

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.

From rustc --explain E0638 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.