Deserialization error loses information after being passed through the erased API

[fjarri]

I noticed it when using postcard with erased-serde, but other formats may be susceptible to the same problem.

postcard::Error is an enum with a bunch of variants. It implements serde::de::Error, but the custom() method just ignores its msg argument and creates a postcard::Error::SerdeSerCustom.

Now, in erased-serde if there's an error during deserialization, it's passed through erase_de which calls serde::de::Error::custom(). So whatever information there was in the error is now lost.

My question is: is it technically a bug in postcard, and it should implement custom() without ignoring the message, even though it never uses this method itself? Or can it be fixed in erased-serde? The latter can be done by storing Box<dyn Any> in erased_serde::Error instead of Box<ErrorImpl>, and then downcasting it back in unerase_de(), but that would require either an .expect() (which I think should be safe, but does not look good in the code), or making unerase_de() fallible, passing the downcasting error to the deserializer (something that will never happen unless there's a bug in the code).