openpgp: Mark enum MessageParserError non-exhaustive.

  - See #405.

1 files changed, 8 insertions, 0 deletions

diff --git a/openpgp/src/message/mod.rs b/openpgp/src/message/mod.rs
index 0839d482..5e1c2fba 100644
--- a/openpgp/src/message/mod.rs
+++ b/openpgp/src/message/mod.rs
@@ -36,12 +36,19 @@ use lalrpop_util::ParseError;
 use self::grammar::MessageParser;
 
 /// Errors that MessageValidator::check may return.
+///
+/// Note: This enum cannot be exhaustively matched to allow future
+/// extensions.
 #[derive(Debug, Clone)]
 pub enum MessageParserError {
     /// A parser error.
     Parser(ParseError<usize, Token, LexicalError>),
     /// An OpenPGP error.
     OpenPGP(Error),
+
+    /// This marks this enum as non-exhaustive.  Do not use this
+    /// variant.
+    #[doc(hidden)] __Nonexhaustive,
 }
 
 impl From<MessageParserError> for failure::Error {
@@ -49,6 +56,7 @@ impl From<MessageParserError> for failure::Error {
         match err {
             MessageParserError::Parser(p) => p.into(),
             MessageParserError::OpenPGP(p) => p.into(),
+            MessageParserError::__Nonexhaustive => unreachable!(),
         }
     }
 }