Provide `Display` implementation for types that can be used in diagnostic messages

[celinval]

In order to make it easier for generating user friendly messages, it would be great if we could provide a Display implementation for things like type, function definition, instance, static variables, and everything that implements CrateDef. The implementation would print their definition to the final user.

[momvart]

@rustbot claim

[momvart]

@celinval
Just to double-check, by definition you mean the full information, right? Like for AdtDef to include all variants and fields, for FnDef the body, etc.

[celinval]

Hi @momvart, that's a good question. I was thinking about just the name including any generic / instantiation parameter. For example, for things that implement CrateDef, I believe we can just invoke the name() function. Instance has its own name function that can be used.

For types, I think the closest we have is pretty_ty(), but there's a PR open to finish implementing it: rust-lang/rust#118364.

For types, I think we should probably implement Display for Ty, and replace pretty_ty usages.

[momvart]

Thanks for the answer. So overall I think long and fine details are not required and some identifiers are enough for this case.