src/librustc_error_codes/error_codes/E0638.md

   1 This error indicates that the struct, enum or enum variant must be matched
   2 non-exhaustively as it has been marked as `non_exhaustive`.
   3
   4 When applied within a crate, downstream users of the crate will need to use the
   5 `_` pattern when matching enums and use the `..` pattern when matching structs.
   6 Downstream crates cannot match against non-exhaustive enum variants.
   7
   8 For example, in the below example, since the enum is marked as
   9 `non_exhaustive`, it is required that downstream crates match non-exhaustively
  10 on it.
  11
  12 ```rust,ignore (pseudo-Rust)
  13 use std::error::Error as StdError;
  14
  15 #[non_exhaustive] pub enum Error {
  16    Message(String),
  17    Other,
  18 }
  19
  20 impl StdError for Error {
  21    fn description(&self) -> &str {
  22         // This will not error, despite being marked as non_exhaustive, as this
  23         // enum is defined within the current crate, it can be matched
  24         // exhaustively.
  25         match *self {
  26            Message(ref s) => s,
  27            Other => "other or unknown error",
  28         }
  29    }
  30 }
  31 ```
  32
  33 An example of matching non-exhaustively on the above enum is provided below:
  34
  35 ```rust,ignore (pseudo-Rust)
  36 use mycrate::Error;
  37
  38 // This will not error as the non_exhaustive Error enum has been matched with a
  39 // wildcard.
  40 match error {
  41    Message(ref s) => ...,
  42    Other => ...,
  43    _ => ...,
  44 }
  45 ```
  46
  47 Similarly, for structs, match with `..` to avoid this error.