implement "lifetime juggling" methods from chalk integration trait

[nikomatsakis]

The chalk integration trait includes a few methods whose job is to do "lifetime juggling", in terms of converting values between the "type inference" arena lifetime ('tcx) and the "global arena" lifetime ('gcx) (see the rustc guide for more details about these lifetimes, if you are not familiar with them).

Case 1 is the LiftExClause trait:

rust/src/librustc_traits/chalk_context.rs

Lines 473 to 482 in 4f9b581

	impl ExClauseLift<'gcx> for ChalkArenas<'a> {
	type LiftedExClause = ChalkExClause<'gcx>;
	fn lift_ex_clause_to_tcx(
	_ex_clause: &ChalkExClause<'a>,
	_tcx: TyCtxt<'_, '_, 'tcx>,
	) -> Option<Self::LiftedExClause> {
	panic!()
	}
	}

This trait exists for coherence reasons -- on the one hand, the ExClause type is defined in chalk. The Lift trait is defined in rustc. Therefore, the impl of Lift for ExClause has to be in librustc -- but librustc doesn't know enough to do the implementation. So we add an auxiliary trait that is implemented over in librustc_traits. And this is the impl!

We need to implement the fn body here. It's probably just a matter of invoking tcx.lift_to_global, but I'm not sure.

For this case, we probably just need to invoke tcx.lift_to_global(value) -- but we may need to add some impls of Lift for the DelayedLiteral type.

Case 2 is the lift_delayed_literal method:

rust/src/librustc_traits/chalk_context.rs

Lines 445 to 450 in 4f9b581

	fn lift_delayed_literal(
	&self,
	_value: DelayedLiteral<ChalkArenas<'tcx>>,
	) -> DelayedLiteral<ChalkArenas<'gcx>> {
	panic!("lift")
	}

This probably needs an "auxiliary" trait like LiftExClause above, for the same reason.

[nikomatsakis]

Mentoring instructions

To start, it's a good idea to read the rustc guide chapter on the tcx and how it uses lifetimes.

I will try to write up some docs on the Lift impls.

[nikomatsakis]

(@uberjay is interested in working on this)

[nikomatsakis]

Created a Zulip topic

[nikomatsakis]

Removing @uberjay since they mentioned they'll be pretty busy for the next little while.