non-defining existential type use in defining scope with empty lifetime name

[Nemo157]

Really not sure where the error here is coming from, as far as I can tell there should only be a single relevant lifetime, 'a. The same function written as a free function generic over a Write returning impl Future<...> + 'a works fine (included in the playground). I'll try and come up with a reduced example in the next couple of days (playground)

error: non-defining existential type use in defining scope
  --> src/lib.rs:19:5
   |
19 | /     {
20 | |         poll_fn(move |cx| self.reborrow().poll_close(cx))
21 | |     }
   | |_____^ lifetime `` is part of concrete type but not used in parameter list of existential type

#![feature(arbitrary_self_types, existential_type, pin, futures_api)]

use std::future::Future;
use std::marker::Unpin;
use std::mem::PinMut;
use std::task::{self, Poll};

pub trait Write {
    type Error;

    fn poll_close(self: PinMut<Self>, cx: &mut task::Context) -> Poll<Result<(), Self::Error>>;
}

existential type Close<'a, W: Write>: Future<Output = Result<(), W::Error>> + 'a;

trait WriteExt: Write {
    fn close<'a>(self: PinMut<'a, Self>) -> Close<'a, Self>
    where
        Self: Sized,
    {
        poll_fn(move |cx| self.reborrow().poll_close(cx))
    }
}

// ------------
// from futures-util-preview 0.3

pub struct PollFn<F> {
    f: F,
}

impl<F> Unpin for PollFn<F> {}

pub fn poll_fn<T, F>(f: F) -> PollFn<F>
where
    F: FnMut(&mut task::Context) -> Poll<T>,
{
    PollFn { f }
}

impl<T, F> Future for PollFn<F>
where
    F: FnMut(&mut task::Context) -> Poll<T>,
{
    type Output = T;

    fn poll(mut self: PinMut<Self>, cx: &mut task::Context) -> Poll<T> {
        (&mut self.f)(cx)
    }
}

(CC @oli-obk in case you have any hints on where to look)

[Nemo157]

Minimized example (playground)

#![feature(existential_type)]

trait Future {
    fn poll(&self, cx: &mut ());
}

trait Write {
    fn poll_close(&self, cx: &mut ());
}

existential type Close<'a, W: Write>: Future + 'a;

fn broken<'a, W: Write>(w: &'a W) -> Close<'a, W> {
    PollFn(move |cx| w.poll_close(cx))
}

fn working<'a, W: Write>(w: &'a W) -> impl Future + 'a {
    PollFn(move |cx| w.poll_close(cx))
}

pub struct PollFn<F: Fn(&mut ())>(F);

impl<F> Future for PollFn<F> where F: Fn(&mut ()) {
    fn poll(&self, cx: &mut ()) {
        (&self.0)(cx)
    }
}

error: non-defining existential type use in defining scope
  --> src/lib.rs:13:51
   |
13 |   fn broken<'a, W: Write>(w: &'a W) -> Close<'a, W> {
   |  ___________________________________________________^
14 | |     PollFn(move |cx| w.poll_close(cx))
15 | | }
   | |_^ lifetime `` is part of concrete type but not used in parameter list of existential type

Removing the cx parameter everywhere makes this work, at a guess it seems like existential types are incorrectly attributing the lifetime of the &mut () that should be hidden in the closure to the concrete type.

[oli-obk]

This might be because existential types skip (

rust/src/librustc/middle/resolve_lifetime.rs

Line 637 in 6bf6d50

return;

) the lifetime hacks (

rust/src/librustc/middle/resolve_lifetime.rs

Line 656 in 6bf6d50

for lifetime in &path.segments[0].args.as_ref().unwrap().args {

) that impl Trait does

[talchas]

Even more minified:

existential type X: Clone;

fn bar<F: Fn(&i32) + Clone>(f: F) -> F {
    f
}

fn foo() -> X {
    bar(|x| ())
}

or using more stdlib stuff:

existential type X: Iterator<Item = i32>;

fn temp() -> X {
    vec![1].into_iter().filter(|x| *x > 1)
}

[ExpHP]

Also seen in const/static items of type impl Fn(&T): #55272

[matprec]

I've hit this as well, have reduced it further:

#![feature(existential_type)]

existential type Foo<R>: Fn(&R) -> ();

fn bar<R>() -> Foo<R> {
    |r| ()
}

Playground

Changing from Fn(&R) -> () to Fn(R) -> () works

[Lymia]

Has there been progress in fixing this? I'd be willing to see if I can start work on a PR myself since I've been having issues with this.

[oli-obk]

I think it should be enough to add a ReLateBound(_) arm to

rust/src/librustc_typeck/check/writeback.rs

Line 508 in 0576ac1

ty::ReStatic => region,

[oberien]

This seems to have been fixed recently, as the example in #53457 (comment) didn't work on nightly 2019-04-11, but now works on the playpen (if you click the link in that comment).