arbitrary_self_types don't support lifetime elision

[Nemo157]

I mentioned this on the RFC recently, but since this is being hit by futures 0.3 I thought it's worth having an issue for the current implementation in case it's an easy fix.

#![feature(arbitrary_self_types)]

struct Foo;
struct Bar<'a>(&'a Foo);

impl std::ops::Deref for Bar<'_> {
    type Target = Foo;
    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl Foo {
    fn a(&self) -> Bar<'_> {
        Bar(self)
    }
    
    fn b(c: &Self) -> Bar<'_> {
        Bar(c)
    }

    fn c(self: Bar<'_>) -> Bar<'_> {
        self
    }
    
    fn d(e: Bar<'_>) -> Bar<'_> {
        e
    }
}

fn main() {
    let foo = Foo;
    { foo.a() };
    { Foo::b(&foo) };
    { Bar(&foo).c() };
    { Foo::d(Bar(&foo)) };
}

error[E0106]: missing lifetime specifier
  --> src/main.rs:15:32
   |
15 |     fn c(self: Bar<'_>) -> Bar<'_> {
   |                                ^^ expected lifetime parameter
   |
   = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
   = help: consider giving it a 'static lifetime

(playground)

[nox]

Any progress on that? I need this feature in Servo and the absence of lifetime elision is a huge papercut.

[Nemo157]

This also interacts with the just added suggestion in #58919, the suggested fix won't work:

impl Foo {
    fn c(self: Bar<'_>) -> impl std::fmt::Debug {
        self
    }
}

suggests adding + '_ to the return type, but that gives

error[E0106]: missing lifetime specifier