Broken MIR: generator contains type X but typeck does not know it

[manuels]

I am running into a compiler error when trying to compile this bit of code with rustc 1.22.0-nightly (eba374f 2017-09-11) running on x86_64-unknown-linux-gnu and the current futures-await crate (579a98bb):

#![feature(proc_macro, conservative_impl_trait, generators)]

extern crate futures_await as futures;

use futures::prelude::*;
use std::io::Error;

#[async]
fn foo(x: i8) -> Result<i8, (i8, Error)> {
  Ok(x)
}

#[async]
fn bar() -> Result<(),()> {
    let mut x = Ok(1);
    loop {
        let y = match x {
           Ok(a) => a,
           Err((b, _e)) => b,
           //Err((b, _)) => b, // Compilation works fine if we use this line instead of the one above!
        };

        x = await!(foo(y));
    }
}

fn main() {
    bar().wait().unwrap();
}

The compiler error reads:

$ RUST_BACKTRACE=1 cargo +nightly run --verbose
       Fresh futures v0.1.15
       Fresh futures-await-macro v0.1.0 (https://github.com/alexcrichton/futures-await#579a98bb)
       Fresh unicode-xid v0.1.0
       Fresh proc-macro2 v0.1.3
       Fresh quote v0.3.15 (https://github.com/dtolnay/quote#873c0d0e)
       Fresh synom v0.11.3 (https://github.com/dtolnay/syn#d5e98599)
       Fresh syn v0.11.11 (https://github.com/dtolnay/syn#d5e98599)
       Fresh futures-async-macro v0.1.0 (https://github.com/alexcrichton/futures-await#579a98bb)
       Fresh futures-await v0.1.0 (https://github.com/alexcrichton/futures-await#579a98bb)
   Compiling foo v0.1.0 (file:///home/manuel/tmp/await-bug)
     Running `rustc --crate-name foo src/main.rs --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=68d8605c99341a0a -C extra-filename=-68d8605c99341a0a --out-dir /home/manuel/tmp/await-bug/target/debug/deps -L dependency=/home/manuel/tmp/await-bug/target/debug/deps --extern futures_await=/home/manuel/tmp/await-bug/target/debug/deps/libfutures_await-5aec48bc7b80f434.rlib`
error: internal compiler error: /checkout/src/librustc_mir/transform/generator.rs:352: Broken MIR: generator contains type std::io::Error in MIR, but typeck only knows about (std::result::Result<(), ()>, std::result::Result<i8, (i8, std::io::Error)>, i8, (), impl futures::__rt::MyFuture<std::result::Result<i8, (i8, std::io::Error)>>)
 --> src/main.rs:14:27
  |
1 | | #![feature(proc_macro, conservative_impl_trait, generators)]
  | |____^
...
14|   fn bar() -> Result<(),()> {
  |  ___________________________^

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

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.22.0-nightly (eba374fb2 2017-09-11) running on x86_64-unknown-linux-gnu

note: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:439:8
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:60
             at /checkout/src/libstd/panicking.rs:381
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:391
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:577
   5: std::panicking::begin_panic
   6: rustc_errors::Handler::span_bug
   7: rustc::session::opt_span_bug_fmt::{{closure}}
   8: rustc::session::span_bug_fmt
   9: <rustc_mir::transform::generator::StateTransform as rustc::mir::transform::MirPass>::run_pass
  10: rustc_mir::transform::run_suite
  11: rustc_mir::transform::optimized_mir
  12: rustc::dep_graph::graph::DepGraph::with_task
  13: rustc::ty::maps::<impl rustc::ty::maps::queries::optimized_mir<'tcx>>::try_get
  14: rustc::ty::maps::TyCtxtAt::optimized_mir
  15: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::optimized_mir
  16: rustc_mir::shim::make_shim
  17: rustc::dep_graph::graph::DepGraph::with_task
  18: rustc::ty::maps::<impl rustc::ty::maps::queries::mir_shims<'tcx>>::try_get
  19: rustc::ty::maps::TyCtxtAt::mir_shims
  20: rustc::ty::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::instance_mir
  21: rustc_trans::collector::collect_items_rec
  22: rustc_trans::collector::collect_items_rec
  23: rustc_trans::collector::collect_items_rec
  24: rustc_trans::collector::collect_items_rec
  25: rustc_trans::collector::collect_items_rec
  26: rustc_trans::base::collect_and_partition_translation_items::{{closure}}
  27: rustc_trans::base::trans_crate
  28: rustc_driver::driver::phase_4_translate_to_llvm
  29: rustc_driver::driver::compile_input::{{closure}}
  30: rustc::ty::context::TyCtxt::create_and_enter
  31: rustc_driver::driver::compile_input
  32: rustc_driver::run_compiler

error: Could not compile `foo`.

Caused by:
  process didn't exit successfully: `rustc --crate-name foo src/main.rs --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=68d8605c99341a0a -C extra-filename=-68d8605c99341a0a --out-dir /home/manuel/tmp/await-bug/target/debug/deps -L dependency=/home/manuel/tmp/await-bug/target/debug/deps --extern futures_await=/home/manuel/tmp/await-bug/target/debug/deps/libfutures_await-5aec48bc7b80f434.rlib` (exit code: 101)

Note, however, that if I use Err((b, _)) in the match statement instead of Err((b, _e)) the compilation succeeds.

If you need any other information to fix this issue, please let me know!

[manuels]

Might be a duplicate of #44184

[Mark-Simulacrum]

Appears to have been a duplicated, now fixed, closing.