Change error message of a failing `assert_eq!`

That could reveal secret information, so it's not an option for the existing assert_eq! macro. Consider an application using assert_eq!(expectedPassword, providedPassword)...

It would also require trait based specialization, to support both types that implement Debug and those that don't. So it can't be implemented at the moment.

I think assertions in production code, and assertions in tests are very different beasts, and it's not possible for one macro to be ideal for both cases. The current implementation seems fine for production assertions, and usable but not okay for test assertions.

Most of the suggestions (pattern matching, debug printing, (re)named arguments) in this thread are for the tests use-case, but those deserve their own macros (first in crates, later perhaps in std).