ICE on Current Stable

[AldaronLau]

Interestingly, this error only happens when building a binary, and not when building a library.

Code

pub struct RaceFuture<T, F, S, G, U>
    where F: StatefulFuture<Option<T>, S>,
          G: Future<Output = Option<U>>,
{
    inner: F,
    future: G,
    event: fn(&mut S, U) -> T,
}

impl<T, F, S, G, U> StatefulFuture<Option<T>, S> for RaceFuture<T, F, S, G, U>
    where F: StatefulFuture<Option<T>, S>,
          G: Future<Output = Option<U>>,
{
    fn poll(&mut self, cx: &mut Context<'_>, state: &mut S) -> Poll<Option<T>> {
        Poll::Pending // FIXME
    }
}

pub trait StatefulFuture<T, S> {
    fn poll(&mut self, cx: &mut Context<'_>, state: &mut S) -> Poll<T>;
}

pub struct Never<T, S>(PhantomData<(T, S)>);

impl<T, S> StatefulFuture<T, S> for Never<T, S> {
    fn poll(&mut self, cx: &mut Context<'_>, state: &mut S) -> Poll<T> {
        Poll::Pending
    }
}

/// Race builder.  Add tasks by calling `when()`.
pub struct RaceBuilder<T, F, S>
    where F: StatefulFuture<Option<T>, S>
{
    future: F,
    _phantom: PhantomData<(S, T)>,
}

impl<T, F, S> RaceBuilder<T, F, S>
    where F: StatefulFuture<Option<T>, S>
{
    pub fn when<U, G>(self, future: G, event: fn(&mut S, U) -> T)
            -> RaceBuilder<T, RaceFuture<T, F, S, G, U>, S>
        where G: Future<Output = Option<U>>,
    {
        RaceBuilder {
            future: RaceFuture {
                inner: self.future, future, event
            },
            _phantom: PhantomData,
        }
    }
}

/// A future that returns a closure for the first completed future.
#[derive(Debug)]
pub struct Race<'a, T, S, R, F>
    where R: Fn(RaceBuilder<T, Never<T, S>, S>, &mut S) -> RaceBuilder<T, F, S>,
          F: StatefulFuture<Option<T>, S>,
{
    state: &'a mut S,
    race: R,
    _phantom: PhantomData<(T, F)>,
}

impl<'a, T, S, R, F> Race<'a, T, S, R, F>
    where R: Fn(RaceBuilder<T, Never<T, S>, S>, &mut S) -> RaceBuilder<T, F, S>,
          F: StatefulFuture<Option<T>, S>,
{
    pub fn new(state: &'a mut S, race: R) -> Self {
        Self { state, race, _phantom: PhantomData }
    }
}

impl<T, S, R, F> Future for Race<'_, T, S, R, F>
    where R: Fn(RaceBuilder<T, Never<T, S>, S>, &mut S) -> RaceBuilder<T, F, S>,
          F: StatefulFuture<Option<T>, S>,
{
    type Output = T;
    
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
        Poll::Pending // FIXME
    }
}

Meta

rustc --version --verbose:

rustc 1.51.0 (2fd73fabe 2021-03-23)
binary: rustc
commit-hash: 2fd73fabe469357a12c2c974c140f67e7cdd76d0
commit-date: 2021-03-23
host: x86_64-unknown-linux-gnu
release: 1.51.0
LLVM version: 11.0.1

Also happens on nightly:

rustc +nightly --version --verbose

rustc 1.53.0-nightly (07e0e2ec2 2021-03-24)
binary: rustc
commit-hash: 07e0e2ec268c140e607e1ac7f49f145612d0f597
commit-date: 2021-03-24
host: x86_64-unknown-linux-gnu
release: 1.53.0-nightly
LLVM version: 12.0.0

Error output

error: internal compiler error: compiler/rustc_trait_selection/src/traits/select/mod.rs:1818:17: Impl DefId(51:102 ~ pasts[c5c1]::race::{impl#1}) was matchable against Obligation(predicate=Binder(TraitPredicate(<pasts::race::Never<_, State> as pasts::race::StatefulFuture<std::option::Option<_>, State>>)), depth=0) but now is not

thread 'rustc' panicked at 'Box<Any>', /rustc/07e0e2ec268c140e607e1ac7f49f145612d0f597/library/std/src/panic.rs:59:5
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.53.0-nightly (07e0e2ec2 2021-03-24) 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] type-checking `run`
#1 [mir_built] building MIR for `run`
end of query stack
error: aborting due to previous error

Backtrace

error: internal compiler error: compiler/rustc_trait_selection/src/traits/select/mod.rs:1793:17: Impl DefId(51:74 ~ pasts[9c70]::race::{impl#1}) was matchable against Obligation(predicate=Binder(TraitPredicate(<pasts::race::Never<_, State> as pasts::race::StatefulFuture<std::option::Option<_>, State>>)), depth=0) but now is not

thread 'rustc' panicked at 'Box<Any>', /rustc/2fd73fabe469357a12c2c974c140f67e7cdd76d0/library/std/src/panic.rs:59:5
stack backtrace:
   0: std::panicking::begin_panic
   1: std::panic::panic_any
   2: rustc_errors::HandlerInner::bug
   3: rustc_errors::Handler::bug
   4: rustc_middle::ty::context::tls::with_opt
   5: rustc_middle::util::bug::opt_span_bug_fmt
   6: rustc_middle::util::bug::bug_fmt
   7: rustc_infer::infer::InferCtxt::commit_unconditionally
   8: rustc_trait_selection::traits::select::confirmation::<impl rustc_trait_selection::traits::select::SelectionContext>::confirm_candidate
   9: rustc_trait_selection::traits::select::SelectionContext::select
  10: rustc_trait_selection::traits::fulfill::FulfillProcessor::process_trait_obligation
  11: rustc_trait_selection::traits::fulfill::FulfillProcessor::progress_changed_obligations
  12: <rustc_trait_selection::traits::fulfill::FulfillmentContext as rustc_infer::traits::engine::TraitEngine>::select_where_possible
  13: rustc_typeck::check::fn_ctxt::_impl::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::resolve_vars_with_obligations
  14: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_method_argument_types
  15: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  16: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  17: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  18: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  19: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_block_with_expected
  20: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  21: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_return_expr
  22: rustc_typeck::check::check::check_fn
  23: rustc_typeck::check::closure::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_closure
  24: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  25: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  26: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_argument_types
  27: rustc_typeck::check::callee::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::confirm_builtin_call
  28: rustc_typeck::check::callee::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_call
  29: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  30: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  31: rustc_typeck::check::_match::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::demand_scrutinee_type
  32: rustc_typeck::check::_match::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_match
  33: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  34: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  35: rustc_typeck::check::_match::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::demand_scrutinee_type
  36: rustc_typeck::check::_match::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_match
  37: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  38: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  39: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_block_with_expected
  40: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  41: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  42: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_block_with_expected
  43: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  44: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  45: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_block_with_expected
  46: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  47: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_return_expr
  48: rustc_typeck::check::check::check_fn
  49: rustc_typeck::check::closure::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_closure
  50: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  51: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  52: rustc_typeck::check::fn_ctxt::checks::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_argument_types
  53: rustc_typeck::check::callee::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::confirm_builtin_call
  54: rustc_typeck::check::callee::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_call
  55: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_kind
  56: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_expr_with_expectation
  57: rustc_typeck::check::expr::<impl rustc_typeck::check::fn_ctxt::FnCtxt>::check_return_expr
  58: rustc_typeck::check::check::check_fn
  59: rustc_infer::infer::InferCtxtBuilder::enter
  60: rustc_typeck::check::typeck
  61: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  62: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  63: rustc_data_structures::stack::ensure_sufficient_stack
  64: rustc_query_system::query::plumbing::force_query_with_job
  65: rustc_query_system::query::plumbing::get_query_impl
  66: rustc_middle::ty::context::TyCtxt::typeck_opt_const_arg
  67: rustc_infer::infer::InferCtxtBuilder::enter
  68: rustc_mir_build::build::mir_built
  69: 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
  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::force_query_with_job
  74: rustc_query_system::query::plumbing::get_query_impl
  75: rustc_mir::transform::check_unsafety::unsafety_check_result
  76: core::ops::function::FnOnce::call_once
  77: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
  78: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task_impl
  79: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
  80: rustc_data_structures::stack::ensure_sufficient_stack
  81: rustc_query_system::query::plumbing::force_query_with_job
  82: rustc_query_system::query::plumbing::force_query_impl
  83: rustc_middle::dep_graph::dep_node::dep_kind::unsafety_check_result::force_from_dep_node
  84: rustc_query_system::dep_graph::graph::DepGraph<K>::try_mark_previous_green
  85: rustc_query_system::dep_graph::graph::DepGraph<K>::try_mark_previous_green
  86: rustc_query_system::dep_graph::graph::DepGraph<K>::try_mark_previous_green
  87: rustc_query_system::dep_graph::graph::DepGraph<K>::try_mark_green_and_read
  88: rustc_data_structures::stack::ensure_sufficient_stack
  89: rustc_query_system::query::plumbing::get_query_impl
  90: rustc_typeck::collect::type_of::type_of
  91: rustc_middle::ty::query::<impl rustc_query_system::query::config::QueryAccessors<rustc_middle::ty::context::TyCtxt> for rustc_middle::ty::query::queries::type_of>::compute
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.51.0 (2fd73fabe 2021-03-23) 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] type-checking `run`
#1 [mir_built] building MIR for `run`
#2 [unsafety_check_result] unsafety-checking `run`
#3 [mir_borrowck] borrow-checking `run`
#4 [type_of] computing type of `run::{opaque#0}`
#5 [check_mod_item_types] checking item types in top-level module
#6 [analysis] running analysis passes on this crate
end of query stack
error: aborting due to previous error

[jyn514]

@AldaronLau I can't reproduce: your original code gives errors about import errors, and once I add all the necessary imports it compiles successfully.

use core::pin::Pin;
use core::marker::PhantomData;
use core::task::Poll;
use core::task::Context;
use core::future::Future;

[AldaronLau]

@jyn514 Sorry, I missed some important pieces when I copied. This should work better (at reproducing the bug):

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

mod race {
    use core::future::Future;
    use core::marker::PhantomData;
    use core::pin::Pin;
    use core::task::{Context, Poll};

    pub struct RaceFuture<T, F, S, G, U>
        where F: StatefulFuture<Option<T>, S>,
              G: Future<Output = Option<U>>,
    {
        inner: F,
        future: G,
        event: fn(&mut S, U) -> T,
    }

    impl<T, F, S, G, U> StatefulFuture<Option<T>, S> for RaceFuture<T, F, S, G, U>
        where F: StatefulFuture<Option<T>, S>,
              G: Future<Output = Option<U>>,
    {
        fn poll(&mut self, cx: &mut Context<'_>, state: &mut S) -> Poll<Option<T>> {
            Poll::Pending // FIXME
        }
    }

    pub trait StatefulFuture<T, S> {
        fn poll(&mut self, cx: &mut Context<'_>, state: &mut S) -> Poll<T>;
    }

    pub struct Never<T, S>(PhantomData<(T, S)>);

    impl<T, S> StatefulFuture<T, S> for Never<T, S> {
        fn poll(&mut self, cx: &mut Context<'_>, state: &mut S) -> Poll<T> {
            Poll::Pending
        }
    }

    /// Race builder.  Add tasks by calling `when()`.
    pub struct RaceBuilder<T, F, S>
        where F: StatefulFuture<Option<T>, S>
    {
        future: F,
        _phantom: PhantomData<(S, T)>,
    }

    impl<T, F, S> RaceBuilder<T, F, S>
        where F: StatefulFuture<Option<T>, S>
    {
        pub fn when<U, G>(self, future: G, event: fn(&mut S, U) -> T)
                -> RaceBuilder<T, RaceFuture<T, F, S, G, U>, S>
            where G: Future<Output = Option<U>>,
        {
            RaceBuilder {
                future: RaceFuture {
                    inner: self.future, future, event
                },
                _phantom: PhantomData,
            }
        }
    }

    /// A future that returns a closure for the first completed future.
    #[derive(Debug)]
    pub struct Race<'a, T, S, R, F>
        where R: Fn(RaceBuilder<T, Never<T, S>, S>, &mut S) -> RaceBuilder<T, F, S>,
              F: StatefulFuture<Option<T>, S>,
    {
        state: &'a mut S,
        race: R,
        _phantom: PhantomData<(T, F)>,
    }

    impl<'a, T, S, R, F> Race<'a, T, S, R, F>
        where R: Fn(RaceBuilder<T, Never<T, S>, S>, &mut S) -> RaceBuilder<T, F, S>,
              F: StatefulFuture<Option<T>, S>,
    {
        pub fn new(state: &'a mut S, race: R) -> Self {
            Self { state, race, _phantom: PhantomData }
        }
    }

    impl<T, S, R, F> Future for Race<'_, T, S, R, F>
        where R: Fn(RaceBuilder<T, Never<T, S>, S>, &mut S) -> RaceBuilder<T, F, S>,
              F: StatefulFuture<Option<T>, S>,
    {
        type Output = T;
        
        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
            Poll::Pending // FIXME
        }
    }
}

/// Shared state between tasks on the thread.
struct State {
    counter: usize,
    one: Never,
}

impl State {
    fn one(&mut self, _: ()) -> Option<()> {
        todo!()
    }
}

struct Never();

impl Never {
    fn new() -> Self {}
}

impl Future for Never {
    type Output = ();

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

async fn run() {
    let mut state = State {
        counter: 0,
        one: Never::new(),
    };

    loop {
        if let Some(x) = race::Race::new(&mut state, |race, state| {
            race.when(&mut state.one, State::one)
        }).await {
            break x;
        }
    }
}

[SNCPlay42]

Reduced:

use std::marker::PhantomData;

pub trait StatefulFuture<S> {}
pub struct Never<T>(PhantomData<T>);
impl<T> StatefulFuture<T> for Never<T> {}

pub struct RaceBuilder<F, S> {
    future: F,
    _phantom: PhantomData<S>,
}

impl<T, F> RaceBuilder<T, F>
where
    F: StatefulFuture<Option<T>>,
{
    pub fn when(self) {}
}

pub struct Race<T, R> {
    race: R,
    _phantom: PhantomData<T>,
}

impl<T, R> Race<T, R>
where
    R: Fn(RaceBuilder<T, Never<T>>),
{
    pub fn new(race: R) {}
}

fn main() {
    Race::new(|race| race.when());
}

[fanninpm]

Playground link of above example