E0638 - Error codes index
Description: Book listing all Rust error codes

# Error code E0638

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.