Lifetime inference and Pin

[withoutboats]

There appears to be some lifetime inference issue with how Deref is implemented for Pin, so that I get issues about how the pin does not live long in code that should clearly work. For example, this gives an error about how the pin does not live long enough:

fn foo<'a>(x: Pin<&'a i32>) -> &'a i32 {
    &*x
}

https://play.rust-lang.org/?gist=5749b0645d17308d62501924e618e49d&version=nightly&mode=debug&edition=2015

cc #49150

[Spoonbender]

Triage: no change