Compiler Error: 'rustc' panicked at 'Box<Any>' (w/ async closures)

[gcoakes]

I'm trying to write an implementation of async_std::stream::Stream which takes an async closure and calls/awaits upon it continually. Everything appears to type check, but fails during compilation. It's probably worth metioning that implementing the same code with an async fn instead of an async closure fails type check.

I'm fairly certain my reasoning about pinning and the like is incorrect in this code, but I would expect it to output a type error instead of a panic during compilation.

Code

#![feature(async_closure)]

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

use async_std::prelude::*;

#[async_std::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let s = from_afn(async || Some("foo".to_string()));
    // let s = from_afn(foo);
    while let Some(x) = s.next().await {
        println!("{:?}", x);
    }
    Ok(())
}

async fn foo() -> String {
    "foo".to_string()
}

fn from_afn<F, Fut, Item>(f: F) -> FromAFn<F, Fut>
where
    F: FnMut() -> Fut,
    Fut: Future<Output = Item> + Unpin,
{
    FromAFn::new(f)
}

struct FromAFn<F, Fut> {
    f: F,
    fut: Pin<Box<Fut>>,
}

impl<F, Fut, Item> FromAFn<F, Fut>
where
    F: FnMut() -> Fut,
    Fut: Future<Output = Item> + Unpin,
{
    fn new(f: F) -> Self {
        Self {f, fut: Box::pin((f)())}
    }
}

impl<F, Fut, Item> Stream for FromAFn<F, Fut>
where
    F: FnMut() -> Fut,
    Fut: Future<Output = Item> + Unpin,
{
    type Item = Item;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        match self.fut.as_mut().poll(cx) {
            Poll::Ready(v) => {
                self.fut = Box::pin((self.f)());
                Poll::Ready(Some(v))
            },
            Poll::Pending => Poll::Pending,
        }
    }
}

Meta

rustc --version --verbose:

rustc 1.45.0-nightly (0aa6751c1 2020-05-20)
binary: rustc
commit-hash: 0aa6751c19d3ba80df5b0b02c00bf44e13c97e80
commit-date: 2020-05-20
host: x86_64-unknown-linux-gnu
release: 1.45.0-nightly
LLVM version: 9.0

Error output

   Compiling statusbar v0.1.0 (/home/gcoakes/src/statusbar)
error: internal compiler error: src/librustc_middle/ich/impls_ty.rs:170: ty::TyKind::hash_stable() - can't hash a TyVid _#30t.

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:907:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

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 1.45.0-nightly (0aa6751c1 2020-05-20) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type bin

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

error: aborting due to previous error

error: could not compile `statusbar`.

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

Backtrace

   Compiling statusbar v0.1.0 (/home/gcoakes/src/statusbar)
error: internal compiler error: src/librustc_middle/ich/impls_ty.rs:170: ty::TyKind::hash_stable() - can't hash a TyVid _#30t.

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:907:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1069
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1537
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:198
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:218
  10: rustc_driver::report_ice
  11: <alloc::boxed::Box<F> as core::ops::function::Fn<A>>::call
             at /rustc/0aa6751c19d3ba80df5b0b02c00bf44e13c97e80/src/liballoc/boxed.rs:1071
  12: proc_macro::bridge::client::<impl proc_macro::bridge::Bridge>::enter::{{closure}}::{{closure}}
             at /rustc/0aa6751c19d3ba80df5b0b02c00bf44e13c97e80/src/libproc_macro/bridge/client.rs:318
  13: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:490
  14: std::panicking::begin_panic
  15: rustc_errors::HandlerInner::bug
  16: rustc_errors::Handler::bug
  17: rustc_middle::util::bug::opt_span_bug_fmt::{{closure}}
  18: rustc_middle::ty::context::tls::with_opt::{{closure}}
  19: rustc_middle::ty::context::tls::with_opt
  20: rustc_middle::util::bug::opt_span_bug_fmt
  21: rustc_middle::util::bug::bug_fmt
  22: rustc_middle::ich::impls_ty::<impl rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext> for rustc_middle::ty::sty::TyVid>::hash_stable
  23: rustc_middle::ty::sty::_DERIVE_rustc_data_structures_stable_hasher_HashStable_rustc_middle_ich_StableHashingContext_ctx_FOR_InferTy::<impl rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext> for rustc_middle::ty::sty::InferTy>::hash_stable
  24: rustc_middle::ty::sty::_DERIVE_rustc_data_structures_stable_hasher_HashStable_rustc_middle_ich_StableHashingContext_ctx_FOR_TyKind::<impl rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext> for rustc_middle::ty::sty::TyKind>::hash_stable
  25: rustc_middle::ich::impls_ty::<impl rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext> for &rustc_middle::ty::list::List<T>>::hash_stable
  26: rustc_middle::ty::sty::_DERIVE_rustc_data_structures_stable_hasher_HashStable_rustc_middle_ich_StableHashingContext_ctx_FOR_TyKind::<impl rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext> for rustc_middle::ty::sty::TyKind>::hash_stable
  27: rustc_middle::ich::impls_ty::<impl rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext> for &rustc_middle::ty::list::List<T>>::hash_stable
  28: rustc_middle::ty::sty::_DERIVE_rustc_data_structures_stable_hasher_HashStable_rustc_middle_ich_StableHashingContext_ctx_FOR_TyKind::<impl rustc_data_structures::stable_hasher::HashStable<rustc_middle::ich::hcx::StableHashingContext> for rustc_middle::ty::sty::TyKind>::hash_stable
  29: <T as rustc_query_system::dep_graph::dep_node::DepNodeParams<Ctxt>>::to_fingerprint
  30: rustc_query_system::query::plumbing::get_query_impl
  31: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::suggestions::InferCtxtExt>::suggest_await_before_try
  32: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtExt>::report_selection_error
  33: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtExt>::report_fulfillment_errors
  34: rustc_typeck::check::FnCtxt::resolve_vars_with_obligations
  35: rustc_typeck::check::demand::<impl rustc_typeck::check::FnCtxt>::demand_coerce_diag
  36: rustc_typeck::check::FnCtxt::check_argument_types
  37: rustc_typeck::check::callee::<impl rustc_typeck::check::FnCtxt>::confirm_builtin_call
  38: rustc_typeck::check::callee::<impl rustc_typeck::check::FnCtxt>::check_call
  39: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  40: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  41: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_coercable_to_type
  42: rustc_typeck::check::FnCtxt::check_decl_local
  43: rustc_typeck::check::FnCtxt::check_stmt
  44: rustc_typeck::check::FnCtxt::check_block_with_expected
  45: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  46: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  47: rustc_typeck::check::FnCtxt::check_block_with_expected
  48: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  49: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  50: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  51: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  52: rustc_typeck::check::FnCtxt::check_block_with_expected
  53: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  54: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  55: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_return_expr
  56: rustc_typeck::check::check_fn
  57: rustc_typeck::check::closure::<impl rustc_typeck::check::FnCtxt>::check_expr_closure
  58: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  59: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  60: rustc_typeck::check::FnCtxt::check_argument_types
  61: rustc_typeck::check::callee::<impl rustc_typeck::check::FnCtxt>::confirm_builtin_call
  62: rustc_typeck::check::callee::<impl rustc_typeck::check::FnCtxt>::check_call
  63: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_kind
  64: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_expr_with_expectation_and_needs
  65: rustc_typeck::check::expr::<impl rustc_typeck::check::FnCtxt>::check_return_expr
  66: rustc_typeck::check::check_fn
  67: rustc_middle::ty::context::GlobalCtxt::enter_local
  68: rustc_typeck::check::typeck_tables_of
  69: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::typeck_tables_of>::compute
  70: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  71: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  72: rustc_data_structures::stack::ensure_sufficient_stack
  73: rustc_query_system::query::plumbing::get_query_impl
  74: rustc_mir_build::hair::cx::Cx::new
  75: rustc_middle::ty::context::GlobalCtxt::enter_local
  76: rustc_mir_build::build::mir_built
  77: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_built>::compute
  78: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  79: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  80: rustc_query_system::query::plumbing::get_query_impl::{{closure}}
  81: rustc_query_system::query::plumbing::get_query_impl
  82: rustc_mir::transform::check_unsafety::unsafety_check_result
  83: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::unsafety_check_result>::compute
  84: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  85: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  86: rustc_query_system::query::plumbing::get_query_impl::{{closure}}
  87: rustc_query_system::query::plumbing::get_query_impl
  88: rustc_mir::transform::mir_const
  89: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_const>::compute
  90: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  91: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  92: rustc_query_system::query::plumbing::get_query_impl::{{closure}}
  93: rustc_query_system::query::plumbing::get_query_impl
  94: rustc_mir::transform::mir_validated
  95: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_validated>::compute
  96: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  97: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  98: rustc_query_system::query::plumbing::get_query_impl::{{closure}}
  99: rustc_query_system::query::plumbing::get_query_impl
 100: rustc_mir::borrow_check::mir_borrowck
 101: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::mir_borrowck>::compute
 102: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
 103: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
 104: rustc_query_system::query::plumbing::get_query_impl
 105: rustc_typeck::collect::type_of::type_of
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.45.0-nightly (0aa6751c1 2020-05-20) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental --crate-type bin

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

query stack during panic:
#0 [typeck_tables_of] type-checking `main::main`
#1 [mir_built] building MIR for
#2 [unsafety_check_result] unsafety-checking `main::main`
#3 [mir_const] processing `main::main`
#4 [mir_validated] processing `main::main`
#5 [mir_borrowck] borrow-checking `main::main`
#6 [type_of] processing `main::main::{{opaque}}#0`
#7 [check_mod_item_types] checking item types in top-level module
#8 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

error: could not compile `statusbar`.

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

[gcoakes]

I realized it might be relevant that I'm using rustc nightly from nixpkgs-mozilla:

{ pkgs ? import <nixpkgs> {} }:
let
  drv = import ./. { inherit pkgs; };
  mozOverlay = import (pkgs.fetchFromGitHub {
    owner = "mozilla";
    repo = "nixpkgs-mozilla";
    rev = "e912ed483e980dfb4666ae0ed17845c4220e5e7c";
    sha256 = "08fvzb8w80bkkabc1iyhzd15f4sm7ra10jn32kfch5klgl0gj3j3";
  });
  myPkgs = import <nixpkgs> { overlays = [mozOverlay]; };
in pkgs.mkShell {
  name = "env";
  inputsFrom = [ drv ];
  buildInputs = with pkgs; [
    cargo-edit
    myPkgs.latest.rustChannels.nightly.rust
    myPkgs.latest.rustChannels.nightly.cargo
    bashInteractive
  ];
}

[gcoakes]

@jonas-schievink , I'm not certain what E-needs-mcve means. Is that some kind of additional info I need to provide?

[jonas-schievink]

@gcoakes It means that we need a more minimal example that triggers the bug, since your code still depends on async-std. You don't necessarily have to provide that though, someone else can also do it.