Allow customising ty::TraitRef's printing behavior.

[eddyb]

(Originally discussed in #58140 (comment))
Right now a TraitRef prints as <T as Trait<U>> via "debug" ({:?}) and Trait<U> via "display" ({}) - this is used all over rustc to indicate what printed form of the trait is desired.

However, it would be overall cleaner for "display" and "debug" printing to match (or even for fmt::Display to not be implemented at all, perhaps?), and provide all the information that exists (i.e. including the Self type).

We would then need a replacement for printing "just the trait path with its parameters", and I think we can have a method, .print_only_trait_path(), that returns a wrapper which has the different formatting behavior.

[eddyb]

Blocked on #58140.

Also, in order to do this thoroughly, fmt::Display would need to be removed from TraitRef (comment-out its section in src/librustc/ty/print/pretty.rs) and all uses of formatting it with {} would have to be put through the new .print_only_trait_path() method instead.

Only after that is done and everything compiles again, can fmt::Display be re-enabled for TraitRef (and it can be given the same behavior as {:?} today).
Some simplification of ty::print may be possible afterwards (right now associated items of a trait have to print <T as Trait<U>> themselves, without the special-casing Trait<U> would be printed).

cc @nikomatsakis

[stepnivlk]

hey @eddyp #58140 got merged does it mean you're happy with proceeding forward with this one? If so I think I can tackle it.

[nathankleyn]

Hey @eddyb! I would love to pick this one up and got a bit of the way into exploring this, but wondering — what did you have in mind for where .print_only_trait_path would be implemented?

As in — is this method added to the existing Print / PrettyPrinter traits somehow? Using a new wrapper trait with that method which is impld only for ty::TraitRef? A fn at the top level somewhere? Somehow else?

[eddyp]

@stepnivlk you probably meant @eddyb

[eddyb]

TraitRef::print_only_trait_path would only construct a trivial wrapper around self (and that wrapper would be e.g. pub struct TraitRefPrintOnlyTraitPath<'tcx>(ty::TraitRef<'tcx>);).
We can also have the same method on PolyTraitRef (i.e. Binder<TraitRef>) that returns Binder<TraitRefPrintOnlyTraitPath> (using map_bound).

Then, in ty::print::pretty, we'd have change this to be on TraitRefPrintOnlyTraitPath:

rust/src/librustc/ty/print/pretty.rs

Lines 1530 to 1532 in 316a391

	ty::TraitRef<'tcx> {
	p!(print_def_path(self.def_id, self.substs));
	}

and also here:

rust/src/librustc/ty/print/pretty.rs

Line 1449 in 316a391

ty::Binder<ty::TraitRef<'tcx>>,

(since that is the behavior for printing with {}, i.e. fmt::Display, it's the one we want to force to go through print_only_trait_path)

[nathankleyn]

Thanks @eddyb — that's really helpful! I'll try to make a start on this and see how far I get, it's a really great way to explore my way around the compiler internals some more! 🤞

[basil-cow]

@nathankleyn would you mind me snatching this?

[nathankleyn]

@Areredify Feel free! Some partial work here if it helps: nathankleyn@f5ec9d8

However it was, when I was last working on it, giving me some confusing issues and I couldn't untangle it.