error: internal compiler error: broken MIR in DefId ...

[rajivr]

I tried this code:

#![feature(async_await)]
#![feature(impl_trait_in_bindings)]

async fn a() {}

async fn b() {}

async fn start() {
    // ICE
    let a_boxed_fut: Box<impl std::future::Future> = Box::new(a());

    // NO ICE
    // let a_boxed_fut = Box::new(a());

    b().await;
}

fn main() {
    println!("Hello, world!");
}

This happened:

error: internal compiler error: broken MIR in DefId(0:20 ~ async_ice[a52a]::start[0]::{{closure}}[0]) (Terminator { source_info: SourceInfo { span: src/main.rs:10:54: 10:67, scope: scope[0] }, kind: _2 = const std::boxed::Box::<impl std::future::Future>::new(move _3) -> [return: bb6, unwind: bb8] }): call dest mismatch (std::boxed::Box<impl std::future::Future> <- std::boxed::Box<impl std::future::Future>): NoSolution
  --> src/main.rs:8:18
   |
8  |   async fn start() {
   |  __________________^
9  | |     // ICE
10 | |     let a_boxed_fut: Box<impl std::future::Future> = Box::new(a());
11 | |
...  |
15 | |     b().await;
16 | | }
   | |_^

thread 'rustc' panicked at 'no errors encountered even though `delay_span_bug` issued', src/librustc_errors/lib.rs:357:17
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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.37.0-nightly (5f3656ce9 2019-06-11) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type bin

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

error: Could not compile `async-ice`.

To learn more, run the command again with --verbose.

Meta

rustc --version --verbose:

rustc 1.37.0-nightly (5f3656ce9 2019-06-11)
binary: rustc
commit-hash: 5f3656ce9a2212fad872605b7a4ee103a155e9f3
commit-date: 2019-06-11
host: x86_64-unknown-linux-gnu
release: 1.37.0-nightly
LLVM version: 8.0

Backtrace:

stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.29/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:47
   3: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:36
   4: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   5: std::panicking::default_hook
             at src/libstd/panicking.rs:212
   6: rustc::util::common::panic_hook
   7: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:479
   8: std::panicking::begin_panic
   9: <rustc_errors::Handler as core::ops::drop::Drop>::drop
  10: core::ptr::real_drop_in_place
  11: core::ptr::real_drop_in_place
  12: <alloc::rc::Rc<T> as core::ops::drop::Drop>::drop
  13: core::ptr::real_drop_in_place
  14: rustc_interface::interface::run_compiler_in_existing_thread_pool
  15: std::thread::local::LocalKey<T>::with
  16: scoped_tls::ScopedKey<T>::set
  17: syntax::with_globals
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
query stack during panic:
end of query stack

This is my first encounter with a rustc bug. I asked for guidance on #wg-async-foundation discord, and I was requested to tag issue #61834 as it might be related. Also, thank you for Rust. :-)

[cramertj]

I'm imagining this is a dup of #61834 and would also be closed by #61872