Error about "different enum" could be more helpful

[viraptor]

I ran into an error saying:

src/database.rs:141:28: 141:52 note: expansion site
src/db_pool.rs:20:55: 20:68 error: mismatched types:
 expected `postgres::SslMode`,
    found `postgres::SslMode`
(expected enum `postgres::SslMode`,
    found a different enum `postgres::SslMode`) [E0308]
src/db_pool.rs:20     let manager = PostgresConnectionManager::new(url, SslMode::None).unwrap();

Which is not very useful for debugging. I get where the problem could come from - different library versions used in different dependencies... but this doesn't really help me see where the different versions come from.

[sfackler]

This looks like somewhat of a dup of #22750, but we definitely do need better handling of this kind of situation. The current message is pretty awful.

[steveklabnik]

Yes, this is a dup of #22750, and agreed that it's particularly terrible :(