Guard types should implement more traits

[huonw]

Types like std::sync::MutexGuard are semantically just a &mut with a destructor, and so it makes sense for them to implement the same sort of traits, e.g. Display and Debug.

[steveklabnik]

Triage: MutexGuard does not implement those traits today.

[Mark-Simulacrum]

Debug is implemented today, but not Display.

@rust-lang/libs: This seems relevant to libz blitz, we probably want to work out some guidelines for wrapper types like this (what traits they should "forward"). For example, I can't think of a good Display implementation for MutexGuard, but perhaps there is one.

To an extent, it could be argued that most if not all std traits should be forwarded through types like MutexGuard, but this feels like it gets painful very quickly.

[sfackler]

The Display impl for MutexGuard would just delegate to the inner value's Display.

[brson]

Smart pointers and guards should eagerly implement common traits. Debug at least should be transparent, custom implemented to defer to the inner type. Not clear whether Eq/Ord should be implemented against the inner type.

Next step here is to identify such types in std and file a PR.

Here's an issue for the guidelines: rust-lang/api-guidelines#94

[ChrisMacNaughton]

I'd like to have a go at this one

[ChrisMacNaughton]

As a first pass, I'm looking at ensuring that MutexGuard, RwLockReadGuard, and RwLockWriteGuard impl Display and Debug. How does that sound?