Compiler error when an async function is store in a async struct

[jfortin42]

Hello,

I was trying differents things with Future and callbacks - to be more precise, I am trying to store an asynchronous callback inside an asynchronous struct.
In my attempts, I found an unexpected compiler crash - I have taken the care to create a minimal reproducible example, which you can see, and test for yourself - note that the crash occurs both on the latest "stable" and "nightly" versions:

Code

use std::{
    future::Future,
    pin::Pin,
    task::{Context, Poll},
};

pub struct StructAsync<F: Fn() -> Pin<Box<dyn Future<Output = ()>>>> {
    pub callback: F,
}

impl<F> Future for StructAsync<F>
where
    F: Fn() -> Pin<Box<dyn Future<Output = ()>>>,
{
    type Output = ();

    fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
        Poll::Pending
    }
}

async fn callback() {}

#[tokio::main]
async fn main() {
    StructAsync { callback }.await;
}

Meta

rustc --version --verbose:

rustc 1.61.0 (fe5b13d68 2022-05-18)
binary: rustc
commit-hash: fe5b13d681f25ee6474be29d748c65adcd91f69e
commit-date: 2022-05-18
host: x86_64-apple-darwin
release: 1.61.0
LLVM version: 14.0.0

Error output

warning: Error finalizing incremental compilation session directory `/Users/jonathan/Documents/rust/compiler_error/target/debug/incremental/compiler_error-236kbd4gmvetj/s-gb5nhhpzbs-1n5kt82-working`: No such file or directory (os error 2)

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

error: internal compiler error: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(ProjectionPredicate(ProjectionTy { substs: [fn() -> impl std::future::Future<Output = ()> {callback}, ()], item_def_id: DefId(2:3311 ~ core[5293]::ops::function::FnOnce::Output) }, Ty(std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = ()> + 'static)>>)), []), depth=2),MismatchedProjectionTypes(Sorts(ExpectedFound { expected: std::future::from_generator::GenFuture<[static generator@src/main.rs:22:21: 22:23]>, found: std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = ()> + 'static)>> })))]` resolving bounds outside of type inference
  |
  = note: delayed at compiler/rustc_trait_selection/src/traits/codegen.rs:125:24

error: internal compiler error: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(ProjectionPredicate(ProjectionTy { substs: [fn() -> impl std::future::Future<Output = ()> {callback}, ()], item_def_id: DefId(2:3311 ~ core[5293]::ops::function::FnOnce::Output) }, Ty(std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = ()> + 'static)>>)), []), depth=1),MismatchedProjectionTypes(Sorts(ExpectedFound { expected: std::future::from_generator::GenFuture<[static generator@src/main.rs:22:21: 22:23]>, found: std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = ()> + 'static)>> })))]` resolving bounds outside of type inference
  |
  = note: delayed at compiler/rustc_trait_selection/src/traits/codegen.rs:125:24

thread 'rustc' panicked at 'Box<dyn Any>', compiler/rustc_errors/src/lib.rs:1309:13

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: rustc_interface::interface::create_compiler_and_run::<core::result::Result<(), rustc_errors::ErrorGuaranteed>, rustc_driver::run_compiler::{closure#1}>
   8: <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>>

On nightly

Meta

rustc +nightly --version --verbose:

rustc 1.64.0-nightly (2f3ddd9f5 2022-06-27)
binary: rustc
commit-hash: 2f3ddd9f594adf9773547aa7cedb43c4ac8ffd2f
commit-date: 2022-06-27
host: x86_64-apple-darwin
release: 1.64.0-nightly
LLVM version: 14.0.6

Error output

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', compiler/rustc_monomorphize/src/collector.rs:958:93

Backtrace

stack backtrace:
   0: _rust_begin_unwind
   1: core::panicking::panic_fmt
   2: core::panicking::panic
   3: <rustc_monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_terminator
   4: rustc_monomorphize::collector::collect_neighbours
   5: rustc_monomorphize::collector::collect_items_rec
   6: rustc_monomorphize::collector::collect_items_rec
   7: rustc_monomorphize::collector::collect_items_rec
   8: rustc_monomorphize::collector::collect_items_rec
   9: rustc_monomorphize::collector::collect_items_rec
  10: rustc_monomorphize::collector::collect_items_rec
  11: rustc_monomorphize::collector::collect_items_rec
  12: rustc_monomorphize::collector::collect_items_rec
  13: rustc_monomorphize::collector::collect_items_rec
  14: rustc_monomorphize::collector::collect_items_rec
  15: rustc_monomorphize::collector::collect_items_rec
  16: rustc_monomorphize::collector::collect_items_rec
  17: rustc_monomorphize::collector::collect_items_rec
  18: <rustc_session::session::Session>::time::<(), rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}>
  19: rustc_monomorphize::collector::collect_crate_mono_items
  20: rustc_monomorphize::partitioning::collect_and_partition_mono_items
  21: <rustc_query_system::dep_graph::graph::DepGraph<rustc_middle::dep_graph::dep_node::DepKind>>::with_task::<rustc_middle::ty::context::TyCtxt, (), (&std::collections::hash::set::HashSet<rustc_span::def_id::DefId, core::hash::BuildHasherDefault<rustc_hash::FxHasher>>, &[rustc_middle::mir::mono::CodegenUnit])>
  22: rustc_query_system::query::plumbing::try_execute_query::<rustc_query_impl::plumbing::QueryCtxt, rustc_query_system::query::caches::DefaultCache<(), (&std::collections::hash::set::HashSet<rustc_span::def_id::DefId, core::hash::BuildHasherDefault<rustc_hash::FxHasher>>, &[rustc_middle::mir::mono::CodegenUnit])>>
  23: rustc_query_system::query::plumbing::get_query::<rustc_query_impl::queries::collect_and_partition_mono_items, rustc_query_impl::plumbing::QueryCtxt>
  24: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::collect_and_partition_mono_items
  25: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  26: <rustc_session::session::Session>::time::<alloc::boxed::Box<dyn core::any::Any>, rustc_interface::passes::start_codegen::{closure#0}>
  27: <rustc_interface::passes::QueryContext>::enter::<<rustc_interface::queries::Queries>::ongoing_codegen::{closure#0}::{closure#0}, core::result::Result<alloc::boxed::Box<dyn core::any::Any>, rustc_errors::ErrorGuaranteed>>
  28: <rustc_interface::queries::Queries>::ongoing_codegen
  29: <rustc_interface::interface::Compiler>::enter::<rustc_driver::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_errors::ErrorGuaranteed>>
  30: 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}>
  31: <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>>

[Rageking8]

Just to add this also ICEs on current latest beta version: 1.62.0-beta.7 (2022-06-26 747075d)

[eggyal]

And to further add, it does not ICE on stable.

@rustbot label: +regression-from-stable-to-beta

[eggyal]

Apologies, as mentioned in the OP it does ICE on stable too (but, at least on the Playground, only when running not just building; same as for other channels).

@rustbot label: -I-prioritize -regression-from-stable-to-beta

[ouz-a]

This no longer ICEs https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=b4e963b54d6500b7a16110150cce7d9e

[Rageking8]

@rustbot claim
For the test