Broken Mir Resulting From Async Function Pointer Cast

[KSBilodeau]

Code

type AsyncFnPtr = Box<
    dyn Fn() -> std::pin::Pin<Box<dyn std::future::Future<Output = ()>>>,
>;

async fn test() {}

#[allow(unused_must_use)]
fn main() {
    Box::new(test) as AsyncFnPtr;
}

Meta

rustc --version --verbose:

rustc 1.63.0-nightly (fdca237d5 2022-06-24)
binary: rustc
commit-hash: fdca237d5194bf8a1c9b437ebd2114d1c2ba6195
commit-date: 2022-06-24
host: x86_64-apple-darwin
release: 1.63.0-nightly
LLVM version: 14.0.5

Error output

error: internal compiler error: no errors encountered even though `delay_span_bug` issued

error: internal compiler error: broken MIR in DefId(0:7 ~ playground[5af9]::main) (NoSolution): could not prove Binder(TraitPredicate(<std::boxed::Box<fn() -> impl std::future::Future<Output = ()> {test}> as std::ops::CoerceUnsized<std::boxed::Box<dyn std::ops::Fn() -> std::pin::Pin<std::boxed::Box<dyn std::future::Future<Output = ()>>>>>>, polarity:Positive), [])
 --> src/main.rs:9:5
  |
9 |     Box::new(test) as AsyncFnPtr;
  |     ^^^^^^^^^^^^^^
  |
  = note: delayed at compiler/rustc_borrowck/src/type_check/canonical.rs:151:13

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1425:13
stack backtrace:
   0:     0x7fd68ac845dd - std::backtrace_rs::backtrace::libunwind::trace::ha6648b7014364bc2
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7fd68ac845dd - std::backtrace_rs::backtrace::trace_unsynchronized::h12a4ddf96a4e2ff5
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7fd68ac845dd - std::sys_common::backtrace::_print_fmt::hf87c549bd1cc89dc
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/std/src/sys_common/backtrace.rs:66:5
   3:     0x7fd68ac845dd - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h4ffedd9ce9224034
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/std/src/sys_common/backtrace.rs:45:22
   4:     0x7fd68acdd91c - core::fmt::write::hf124b4c1c8ae0d2b
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/core/src/fmt/mod.rs:1198:17
   5:     0x7fd68ac75c51 - std::io::Write::write_fmt::hbe26d06d41c58912
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/std/src/io/mod.rs:1672:15
   6:     0x7fd68ac872a5 - std::sys_common::backtrace::_print::h6d5a85ca9b3358b3
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/std/src/sys_common/backtrace.rs:48:5
   7:     0x7fd68ac872a5 - std::sys_common::backtrace::print::h7666fc3156204d9f
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/std/src/sys_common/backtrace.rs:35:9
   8:     0x7fd68ac872a5 - std::panicking::default_hook::{{closure}}::hfa94c116b7086896
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/std/src/panicking.rs:295:22
   9:     0x7fd68ac86fc6 - std::panicking::default_hook::hc8deb5a55c834c81
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/std/src/panicking.rs:314:9
  10:     0x7fd68b481524 - rustc_driver[3a72345c5a3a9278]::DEFAULT_HOOK::{closure#0}::{closure#0}
  11:     0x7fd68ac8797a - std::panicking::rust_panic_with_hook::h16757dfa430f6014
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/std/src/panicking.rs:702:17
  12:     0x7fd68c63a111 - std[a5eff6be7f10e3b3]::panicking::begin_panic::<rustc_errors[8af9c38e420da921]::ExplicitBug>::{closure#0}
  13:     0x7fd68c637cf6 - std[a5eff6be7f10e3b3]::sys_common::backtrace::__rust_end_short_backtrace::<std[a5eff6be7f10e3b3]::panicking::begin_panic<rustc_errors[8af9c38e420da921]::ExplicitBug>::{closure#0}, !>
  14:     0x7fd68c6545f6 - std[a5eff6be7f10e3b3]::panicking::begin_panic::<rustc_errors[8af9c38e420da921]::ExplicitBug>
  15:     0x7fd68c651596 - std[a5eff6be7f10e3b3]::panic::panic_any::<rustc_errors[8af9c38e420da921]::ExplicitBug>
  16:     0x7fd68df2266c - <rustc_errors[8af9c38e420da921]::HandlerInner as core[993276dbfd88d183]::ops::drop::Drop>::drop
  17:     0x7fd68d64d488 - core[993276dbfd88d183]::ptr::drop_in_place::<rustc_session[ba178145ab1da357]::parse::ParseSess>
  18:     0x7fd68d64fda3 - <alloc[40aff515e5175a27]::rc::Rc<rustc_session[ba178145ab1da357]::session::Session> as core[993276dbfd88d183]::ops::drop::Drop>::drop
  19:     0x7fd68d6763bd - core[993276dbfd88d183]::ptr::drop_in_place::<rustc_interface[48a3913523791f79]::interface::Compiler>
  20:     0x7fd68d675da4 - rustc_span[a7833de75ef67468]::with_source_map::<core[993276dbfd88d183]::result::Result<(), rustc_errors[8af9c38e420da921]::ErrorGuaranteed>, rustc_interface[48a3913523791f79]::interface::create_compiler_and_run<core[993276dbfd88d183]::result::Result<(), rustc_errors[8af9c38e420da921]::ErrorGuaranteed>, rustc_driver[3a72345c5a3a9278]::run_compiler::{closure#1}>::{closure#1}>
  21:     0x7fd68d652532 - <scoped_tls[5373c8190146815]::ScopedKey<rustc_span[a7833de75ef67468]::SessionGlobals>>::set::<rustc_interface[48a3913523791f79]::interface::run_compiler<core[993276dbfd88d183]::result::Result<(), rustc_errors[8af9c38e420da921]::ErrorGuaranteed>, rustc_driver[3a72345c5a3a9278]::run_compiler::{closure#1}>::{closure#0}, core[993276dbfd88d183]::result::Result<(), rustc_errors[8af9c38e420da921]::ErrorGuaranteed>>
  22:     0x7fd68d66751f - std[a5eff6be7f10e3b3]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[48a3913523791f79]::util::run_in_thread_pool_with_globals<rustc_interface[48a3913523791f79]::interface::run_compiler<core[993276dbfd88d183]::result::Result<(), rustc_errors[8af9c38e420da921]::ErrorGuaranteed>, rustc_driver[3a72345c5a3a9278]::run_compiler::{closure#1}>::{closure#0}, core[993276dbfd88d183]::result::Result<(), rustc_errors[8af9c38e420da921]::ErrorGuaranteed>>::{closure#0}, core[993276dbfd88d183]::result::Result<(), rustc_errors[8af9c38e420da921]::ErrorGuaranteed>>
  23:     0x7fd68d667659 - <<std[a5eff6be7f10e3b3]::thread::Builder>::spawn_unchecked_<rustc_interface[48a3913523791f79]::util::run_in_thread_pool_with_globals<rustc_interface[48a3913523791f79]::interface::run_compiler<core[993276dbfd88d183]::result::Result<(), rustc_errors[8af9c38e420da921]::ErrorGuaranteed>, rustc_driver[3a72345c5a3a9278]::run_compiler::{closure#1}>::{closure#0}, core[993276dbfd88d183]::result::Result<(), rustc_errors[8af9c38e420da921]::ErrorGuaranteed>>::{closure#0}, core[993276dbfd88d183]::result::Result<(), rustc_errors[8af9c38e420da921]::ErrorGuaranteed>>::{closure#1} as core[993276dbfd88d183]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  24:     0x7fd68ac914f3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h63ee45655a45e444
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/alloc/src/boxed.rs:1951:9
  25:     0x7fd68ac914f3 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::ha9e4bd17f57e4e2f
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/alloc/src/boxed.rs:1951:9
  26:     0x7fd68ac914f3 - std::sys::unix::thread::Thread::new::thread_start::h4bd37e41a2fb87e0
                               at /rustc/2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f/library/std/src/sys/unix/thread.rs:108:17
  27:     0x7fd68abb4609 - start_thread
  28:     0x7fd68aad7133 - clone
  29:                0x0 - <unknown>

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 1.64.0-nightly (2f3ddd9f5 2022-06-27) running on x86_64-unknown-linux-gnu

note: compiler flags: --crate-type bin -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

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

query stack during panic:
end of query stack
error: could not compile `playground`

Backtrace

stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: std::panic::panic_any::<rustc_errors::ExplicitBug>
   2: <rustc_errors::HandlerInner as core::ops::drop::Drop>::drop
   3: core::ptr::drop_in_place::<rustc_session::parse::ParseSess>
   4: <alloc::rc::Rc<rustc_session::session::Session> as core::ops::drop::Drop>::drop
   5: core::ptr::drop_in_place::<rustc_interface::interface::Compiler>
   6: rustc_span::with_source_map::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_interface::interface::create_compiler_and_run<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>::{closure#1}>
   7: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_errors::ErrorGuaranteed>>

[KSBilodeau]

This ICE is also triggered by unsizing coercion such as follows:

type AsyncFnPtr = Box<
    dyn Fn() -> std::pin::Pin<Box<dyn std::future::Future<Output = ()>>>,
>;

async fn test() {}

fn main() {
    let _: AsyncFnPtr = Box::new(test);
}

[Noratrieb]

Borrowck is correct in rejecting this code, since the cast is not allowed like this. Looks like some earlier pass didn't check it it correctly.

[compiler-errors]

cc @oli-obk this regressed in #94081

[oli-obk]

Yea regular typeck should already reject that cast/coercion

[compiler-errors]

Strangely, inlining the type alias's definition gives us:

error[[E0271]](https://doc.rust-lang.org/nightly/error-index.html#E0271): type mismatch resolving `<fn() -> impl Future<Output = ()> {test} as FnOnce<()>>::Output == Pin<Box<dyn Future<Output = ()>>>`
 --> src/main.rs:5:5
  |
5 |     Box::new(test) as Box<
  |     ^^^^^^^^^^^^^^ expected struct `Pin`, found opaque type
  |
note: while checking the return type of the `async fn`
 --> src/main.rs:1:17
  |
1 | async fn test() {}
  |                 ^ checked the `Output` of this `async fn`, found opaque type
  = note:   expected struct `Pin<Box<dyn Future<Output = ()>>>`
          found opaque type `impl Future<Output = ()>`
  = note: required for the cast to the object type `dyn Fn() -> Pin<Box<dyn Future<Output = ()>>>`

For more information about this error, try `rustc --explain E0271`.
error: could not compile `playground` due to previous error

[Noratrieb]

I investigated this closer and.. found a soundness bug! #98608

[Noratrieb]

Seems like the soundness bug is caused by the same issue, so fixing that should fix this as well. It's just that here borrowck managed to find the broken code and bail out, but there, no one was there to catch it.

[dtolnay]

Correct error in 1.60.0, and nightly prior to f132bcf:

error[E0271]: type mismatch resolving `<fn() -> impl Future<Output = ()> {test} as FnOnce<()>>::Output == Pin<Box<(dyn Future<Output = ()> + 'static)>>`
 --> src/main.rs:9:5
  |
9 |     Box::new(test) as AsyncFnPtr;
  |     ^^^^^^^^^^^^^^ expected struct `Pin`, found opaque type
  |
note: while checking the return type of the `async fn`
 --> src/main.rs:5:17
  |
5 | async fn test() {}
  |                 ^ checked the `Output` of this `async fn`, found opaque type
  = note:   expected struct `Pin<Box<(dyn Future<Output = ()> + 'static)>>`
          found opaque type `impl Future<Output = ()>`
  = note: required for the cast to the object type `dyn Fn() -> Pin<Box<(dyn Future<Output = ()> + 'static)>>`

For more information about this error, try `rustc --explain E0271`.

[compiler-errors]

just for the record, oli figured out the bug, so other ppl don't waste too much time investigating

[Dylan-DPC]

Marked p-medium as per zulip triage discussion

[apiraino]

Link for WG-prioritization Zulip discussion