Rustfmt/pretty-print types in error messages

[eira-fransham]

For any highly generic library (futures is the one that I hit this issue with a lot) it's easy for types to become unreadable very fast. Even impl Trait only helps so much (we're using Box<Trait> a lot in our code so impl Trait won't get us better error messages). A good solution would be to rustfmt-style pretty-print types in the error messages so that it's possible for humans to parse them. It might be useful to make this optional or otherwise limit the maximum size because I can imagine this causing diesel's types to take hundreds of lines to print. Probably some kind of overrideable heuristic would be useful to implement here.

[olivia-fl]

This would be very nice. One of the things that I really liked about rust initially was the high quality error messages. I feel like with a lot of these combinator based libraries, the error messages get pretty terrible. I don't think we're at C++ template level yet, but it definitely reminds me of them.

I also think it might be useful to also optionally leave out irrelevant stuff or provide a short summary at the top that is easy to parse for a human in cases in which just pretty-printing things isn't enough.

[Centril]

Closing in favor of rust-lang/rust#54923 as this is a diagnostics issue.