Lightweight error value in TryFrom<i32> for enums

Add a new UnknownEnumValue error type to prost,
change the TryFrom<i32> conversions generated for enums to use that
as the associated Error type. The error value carries the original
integer, making it also more informative than DecodeError.

prost-derive/src/lib.rs

@@ -350,12 +350,12 @@ fn try_enumeration(input: TokenStream) -> Result<TokenStream, Error> {
         }
 
         impl #impl_generics ::core::convert::TryFrom::<i32> for #ident #ty_generics #where_clause {
-            type Error = ::prost::DecodeError;
+            type Error = ::prost::UnknownEnumValue;
 
-            fn try_from(value: i32) -> ::core::result::Result<#ident, ::prost::DecodeError> {
+            fn try_from(value: i32) -> ::core::result::Result<#ident, Self::Error> {
                 match value {
                     #(#try_from,)*
-                    _ => ::core::result::Result::Err(::prost::DecodeError::new("invalid enumeration value")),
+                    _ => ::core::result::Result::Err(::prost::UnknownEnumValue(value)),
                 }
             }
         }

src/error.rs

@@ -129,3 +129,20 @@ impl From<EncodeError> for std::io::Error {
         std::io::Error::new(std::io::ErrorKind::InvalidInput, error)
     }
 }
+
+/// An error indicating that an unknown enumeration value was encountered.
+///
+/// The Protobuf spec mandates that enumeration value sets are ‘open’, so this
+/// error's value represents an integer value unrecognized by the
+/// presently used enum definition.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct UnknownEnumValue(pub i32);
+
+impl fmt::Display for UnknownEnumValue {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "unknown enumeration value {}", self.0)
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for UnknownEnumValue {}

src/lib.rs

@@ -17,7 +17,7 @@ mod types;
 #[doc(hidden)]
 pub mod encoding;
 
-pub use crate::error::{DecodeError, EncodeError};
+pub use crate::error::{DecodeError, EncodeError, UnknownEnumValue};
 pub use crate::message::Message;
 pub use crate::name::Name;
 

tests/src/lib.rs

@@ -627,10 +627,7 @@ mod tests {
             Ok(PrivacyLevel::PrivacyLevelprivacyLevelFour),
             PrivacyLevel::try_from(4)
         );
-        assert_eq!(
-            Err(prost::DecodeError::new("invalid enumeration value")),
-            PrivacyLevel::try_from(5)
-        );
+        assert_eq!(Err(prost::UnknownEnumValue(5)), PrivacyLevel::try_from(5));
 
         assert_eq!(
             Ok(ERemoteClientBroadcastMsg::KERemoteClientBroadcastMsgDiscovery),