Tracking issue for existential lifetimes

[alexreg]

This is a tracking issue for existential lifetimes.

Description:

Allow hiding a type (via impl Trait) that is invariant over some lifetime without explicitly mentioning the invariant lifetime.

Consider the following:

impl Trait<'b> for Cell<&'a u32> { }

fn foo(x: Cell<&'x u32>) -> impl Trait<'y> where 'x: 'y { x }

There is no reason this cannot be legal, although it is not permitted at present. We would want to translate the function signature internally into something like:

fn foo(x: Cell<&'x u32>) -> impl exists<'x: 'y> Trait<'y> where 'x: 'y { x }

Although it be noted there is no need for user-facing exists<...> syntax; only HIR and ty representations probably. The concrete type corresponding to impl exists<'x: 'y> Trait<'y> can this be soundly checked by the compiler at use site.

Note, we still need to be careful to ban situations like those mentioned by @matthewjasper in #59402. By actually embedding the existential lifetime in the type rather than simply doing a check when resolving the opaque type, we should be able to resolve these issues, however. One can view this solution as a "compiler-internalised" version of the Captures marker trait solution, in some sense.

Steps:

• Decide on exact semantics of existential lifetimes. Perhaps @nikomatsakis can briefly write up his thoughts here.
• Implement the RFC (cc previous attempts Allow hidden lifetimes in impl Trait #57870 and Allow hidden lifetimes in impl Trait, take 2 #59402, @nikomatsakis for mentoring instructions?)
• Stabilization PR (see instructions on rustc-guide)

CC @Centril @nikomatsakis @matthewjasper @cramertj

[Centril]

cc https://rust-lang.zulipchat.com/#narrow/stream/122657-t-compiler.2Fwg-nll/topic/impl.20Trait.20and.20union.20regions/near/167389453

[alexreg]

I think @nikomatsakis is talking about a different issue there?

[steveklabnik]

Triage: not aware of any changes