A `rustc`-internal compile error on future and task API

[luojia65]

Today when I tried this piece of code:

#![feature(async_await, gen_future)]

use core::task::Context;
use core::task::Poll;
use core::pin::Pin;
use core::future::Future;
use std::future::poll_with_tls_context;

struct CountdownFuture(u32);

impl Future for CountdownFuture {
    type Output = ();

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
        if self.0 == 0 {
            Poll::Ready(())
        } else {
            self.0 -= 1;
            Poll::Pending
        }
    }
}

fn main() {
    let mut fut = CountdownFuture(3);
    let pin = Pin::new(&mut fut);
    let res = poll_with_tls_context(pin);
    println!("{:?}", res);
}

I expected to see a line of debug formatted output representing a Poll::Pending state is set for variable res. However, when I execute this piece of code, a compile error occurred:

   Finished dev [unoptimized + debuginfo] target(s) in 0.32s
     Running `target\debug\test-futures.exe`
thread 'main' panicked at 'TLS Context not set. This is a rustc bug. Please file an issue on https://github.com/rust-lang/rust.', src\libcore\option.rs:1036:5
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
error: process didn't exit successfully: `target\debug\test-futures.exe` (exit code: 101)

And with RUST_BACKTRACE=1:

    Finished dev [unoptimized + debuginfo] target(s) in 0.04s
     Running `target\debug\test-futures.exe`
thread 'main' panicked at 'TLS Context not set. This is a rustc bug. Please file an issue on https://github.com/rust-lang/rust.', src\libcore\option.rs:1036:5
stack backtrace:
   0: backtrace::backtrace::trace_unsynchronized
             at C:\Users\appveyor\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.25\src\backtrace\mod.rs:66
   1: std::sys_common::backtrace::_print
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\sys_common\backtrace.rs:47
   2: std::sys_common::backtrace::print
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\sys_common\backtrace.rs:36
   3: std::panicking::default_hook::{{closure}}
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\panicking.rs:197
   4: std::panicking::default_hook
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\panicking.rs:211
   5: std::panicking::rust_panic_with_hook
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\panicking.rs:474
   6: std::panicking::continue_panic_fmt
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\panicking.rs:381
   7: std::panicking::rust_begin_panic
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\panicking.rs:308
   8: core::panicking::panic_fmt
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libcore\panicking.rs:85
   9: core::option::expect_failed
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libcore\option.rs:1036
  10: core::option::Option<core::ptr::non_null::NonNull<core::task::wake::Context>>::expect<core::ptr::non_null::NonNull<core::task::wake::Context>>
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\src\libcore\option.rs:314
  11: std::future::get_task_context<closure,core::task::poll::Poll<()>>
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\src\libstd\future.rs:95
  12: std::future::poll_with_tls_context<test_futures::CountdownFuture>
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\src\libstd\future.rs:114
  13: test_futures::main
             at .\src\main.rs:27
  14: std::rt::lang_start::{{closure}}<()>
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\src\libstd\rt.rs:64
  15: std::rt::lang_start_internal::{{closure}}
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\rt.rs:49
  16: std::panicking::try::do_call<closure,i32>
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\panicking.rs:293
  17: panic_unwind::__rust_maybe_catch_panic
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libpanic_unwind\lib.rs:85
  18: std::panicking::try
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\panicking.rs:272
  19: std::panic::catch_unwind
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\panic.rs:388
  20: std::rt::lang_start_internal
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\/src\libstd\rt.rs:48
  21: std::rt::lang_start<()>
             at /rustc/627486af15d222bcba336b12ea92a05237cc9ab1\src\libstd\rt.rs:64
  22: main
  23: invoke_main
             at d:\agent\_work\3\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:78
  24: __scrt_common_main_seh
             at d:\agent\_work\3\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288
  25: BaseThreadInitThunk
  26: RtlUserThreadStart
error: process didn't exit successfully: `target\debug\test-futures.exe` (exit code: 101)

It seems that there is an internal issue with rustc itself. Does anyone met the same problem or similar issues as me? Or is there something wrong with my code? BTW, I doubt that I may need to change a executor function other than poll_with_tls_context.
Here's my rustc version:

PS D:\RustProjects\test-futures> rustc -V
rustc 1.37.0-nightly (627486af1 2019-06-02)

I'm using rustc target nightly-x86_64-pc-windows-msvc on Windows 10.

[sfackler]

That's just a bad panic message. You need to set the thread local context with https://doc.rust-lang.org/std/future/fn.set_task_context.html before you can use poll_with_tls_context.

[luojia65]

@sfackler Thank you! I'd try by creating a Context and set it using that function.

[nikomatsakis]

Discussed in the async-await-foundations meeting -- doesn't block stabilization, but would be a good thing to have clearer documentation.

[withoutboats]

poll_with_tls_context should be made doc(hidden) - its a part of how await is currently implemented and not intended to ever be stabilized or to be used by end users

[Arnavion]

#61523 marked it doc(hidden). Is there anything else to do here?