ICE: unexpected concrete region in borrowck: ReStatic

[ehuss]

Trying to build warp 0.2.5 with latest nightly results in an ICE.

Bisected the regression to #87287, cc @oli-obk.

I have tried to minimize it. This still requires futures 0.3, I'm not sure how to minimize if further.

use futures::{Stream, TryStream};
use std::error::Error as StdError;
use std::pin::Pin;
use std::task::{Context, Poll};

pub trait ServerSentEvent: Sized + Send + Sync + 'static {}

impl<T: Send + Sync + 'static> ServerSentEvent for T {}

struct SseKeepAlive<S> {
    event_stream: S,
}

struct SseComment<T>(T);

impl<S> Stream for SseKeepAlive<S>
where
    S: TryStream + Send + 'static,
    S::Ok: ServerSentEvent,
    S::Error: StdError + Send + Sync + 'static,
{
    type Item = Result<SseComment<&'static str>, ()>;
    fn poll_next(self: Pin<&mut Self>, _cx: &mut Context) -> Poll<Option<Self::Item>> {
        unimplemented!()
    }
}

pub fn keep<S>(
    event_stream: S,
) -> impl TryStream<Ok = impl ServerSentEvent + Send + 'static, Error = ()> + Send + 'static
where
    S: TryStream + Send + 'static,
    S::Ok: ServerSentEvent + Send,
    S::Error: StdError + Send + Sync + 'static,
{
    SseKeepAlive { event_stream }
}

Meta

rustc --version --verbose:

rustc 1.56.0-nightly (d9aa28767 2021-07-24)
binary: rustc
commit-hash: d9aa28767287670df6cf823b94629122e04442c0
commit-date: 2021-07-24
host: x86_64-apple-darwin
release: 1.56.0-nightly
LLVM version: 12.0.1

Error output

warning: field is never read: `event_stream`
  --> src/lib.rs:11:5
   |
11 |     event_stream: S,
   |     ^^^^^^^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: Error finalizing incremental compilation session directory `/Users/eric/Temp/z50/warp-0.2.5/eric/target/debug/incremental/eric-3a2ytbduf2rnx/s-g0sxn41dw4-19a86sm-working`: No such file or directory (os error 2)

error: internal compiler error: unexpected concrete region in borrowck: ReStatic
  --> src/lib.rs:28:1
   |
28 | / pub fn keep<S>(
29 | |     event_stream: S,
30 | | ) -> impl TryStream<Ok = impl ServerSentEvent + Send + 'static, Error = ()> + Send + 'static
31 | | where
...  |
36 | |     SseKeepAlive { event_stream }
37 | | }
   | |_^
   |
   = note: delayed at compiler/rustc_mir/src/borrow_check/region_infer/opaque_types.rs:87:44

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:1050:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.56.0-nightly (d9aa28767 2021-07-24) running on x86_64-apple-darwin

note: compiler flags: -C embed-bitcode=no -C split-debuginfo=unpacked -C debuginfo=2 -C incremental --crate-type lib

note: some of the compiler flags provided by cargo are hidden

query stack during panic:
end of query stack
warning: `eric` (lib) generated 2 warnings
error: could not compile `eric`; 2 warnings emitted

Backtrace

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', compiler/rustc_errors/src/lib.rs:1050:13
stack backtrace:
   0: _rust_begin_unwind
   1: std::panicking::begin_panic_fmt
   2: rustc_errors::HandlerInner::flush_delayed
   3: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
   4: core::ptr::drop_in_place<rustc_session::parse::ParseSess>
   5: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
   6: core::ptr::drop_in_place<rustc_interface::interface::Compiler>
   7: rustc_span::with_source_map
   8: rustc_interface::interface::create_compiler_and_run
   9: scoped_tls::ScopedKey<T>::set

[oli-obk]

Seems like a duplicate of #79590

I don't think that is related, neither the error matches nor is it a recent regression. Did you link the wrong issue?

#87483 fixes this ICE

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-high

[oli-obk]

Still needs a backport

[pnkfelix]

(assigning self to ensure backport happens.)

[tmandry]

Also seen in the latest crater run (#87749):

• zebra-consensus: https://crater-reports.s3.amazonaws.com/beta-1.55-1/beta-2021-08-01/gh/andrcmdr.zcash-zebra/log.txt
• warp v0.3.1: https://crater-reports.s3.amazonaws.com/beta-1.55-1/beta-2021-08-01/gh/apiraino.warp-mobc-tokio/log.txt
• linkerd: https://crater-reports.s3.amazonaws.com/beta-1.55-1/beta-2021-08-01/gh/linkerd.linkerd2-proxy/log.txt

Along with a number of others.

[wesleywiser]

Backport (#88190) was merged.