rustc::traits::trans OutputTypeParameterMismatch ICE with higher-rank trait bounds and associated type

[jturner314]

Given this code:

pub struct Struct {}

pub trait Trait<'a> {
    type Assoc;

    fn method() -> Self::Assoc;
}

impl<'a> Trait<'a> for Struct {
    type Assoc = ();

    fn method() -> Self::Assoc {}
}

pub fn function<F, T>(f: F)
where
    F: for<'a> FnOnce(<T as Trait<'a>>::Assoc),
    T: for<'b> Trait<'b>,
{
    f(T::method());
}

fn main() {
    function::<_, Struct>(|_| {});
}

(Playground)

Trying to build results in an internal compiler error:

   Compiling playground v0.0.1 (file:///playground)
error: internal compiler error: librustc/traits/trans/mod.rs:68: Encountered error `OutputTypeParameterMismatch(Binder(<[closure@src/main.rs:24:27: 24:33] as std::ops::FnOnce<(<Struct as Trait<'_>>::Assoc,)>>), Binder(<[closure@src/main.rs:24:27: 24:33] as std::ops::FnOnce<((),)>>), Sorts(ExpectedFound { expected: (), found: <Struct as Trait<'_>>::Assoc }))` selecting `Binder(<[closure@src/main.rs:24:27: 24:33] as std::ops::FnOnce<((),)>>)` during trans

thread 'rustc' panicked at 'Box<Any>', librustc_errors/lib.rs:543:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: aborting due to previous error


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 1.26.0 (a77568041 2018-05-07) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type bin

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

error: Could not compile `playground`.

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

Note that cargo check finishes without errors; it's only when building that the ICE occurs.

Meta

On my machine, rustc --version --verbose:

rustc 1.26.0 (a77568041 2018-05-07)
binary: rustc
commit-hash: a7756804103447ea4e68a71ccf071e7ad8f7a03e
commit-date: 2018-05-07
host: x86_64-unknown-linux-gnu
release: 1.26.0
LLVM version: 6.0

On the playground, the ICE occurs on stable, beta, and nightly channels in both debug and release mode.

Backtrace generated by running RUST_BACKTRACE=1 cargo build on my machine:

error: internal compiler error: librustc/traits/trans/mod.rs:68: Encountered error `OutputTypeParameterMismatch(Binder(<[closure@src/main.rs:24:27: 24:33] as std::ops::FnOnce<(<Struct as Trait<'_>>::Assoc,)>>), Binder(<[closure@src/main.rs:24:27: 24:33] as std::ops::FnOnce<((),)>>), Sorts(ExpectedFound { expected: (), found: <Struct as Trait<'_>>::Assoc }))` selecting `Binder(<[closure@src/main.rs:24:27: 24:33] as std::ops::FnOnce<((),)>>)` during trans

thread 'rustc' panicked at 'Box<Any>', librustc_errors/lib.rs:543:9
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::print
             at libstd/sys_common/backtrace.rs:71
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at libstd/panicking.rs:207
   3: std::panicking::default_hook
             at libstd/panicking.rs:223
   4: core::ops::function::Fn::call
   5: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:403
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::bug
   8: rustc::session::opt_span_bug_fmt::{{closure}}
   9: rustc::ty::context::tls::with_opt::{{closure}}
  10: <std::thread::local::LocalKey<T>>::try_with
  11: <std::thread::local::LocalKey<T>>::with
  12: rustc::ty::context::tls::with
  13: rustc::ty::context::tls::with_opt
  14: rustc::session::opt_span_bug_fmt
  15: rustc::session::bug_fmt
  16: <std::thread::local::LocalKey<T>>::with
  17: rustc::ty::context::GlobalCtxt::enter_local
  18: rustc::traits::trans::trans_fulfill_obligation
  19: rustc::dep_graph::graph::DepGraph::with_task_impl
  20: rustc_errors::Handler::track_diagnostics
  21: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::cycle_check
  22: rustc::ty::maps::<impl rustc::ty::maps::queries::trans_fulfill_obligation<'tcx>>::force
  23: rustc::ty::maps::<impl rustc::ty::maps::queries::trans_fulfill_obligation<'tcx>>::try_get
  24: rustc::ty::maps::TyCtxtAt::trans_fulfill_obligation
  25: rustc::ty::instance::Instance::resolve
  26: <rustc_mir::monomorphize::collector::MirNeighborCollector<'a, 'tcx> as rustc::mir::visit::Visitor<'tcx>>::visit_terminator_kind
  27: rustc_mir::monomorphize::collector::collect_items_rec
  28: rustc_mir::monomorphize::collector::collect_items_rec
  29: rustc_mir::monomorphize::collector::collect_crate_mono_items
  30: rustc::util::common::time
  31: rustc_trans::base::collect_and_partition_translation_items
  32: rustc::dep_graph::graph::DepGraph::with_task_impl
  33: rustc_errors::Handler::track_diagnostics
  34: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::cycle_check
  35: rustc::ty::maps::<impl rustc::ty::maps::queries::collect_and_partition_translation_items<'tcx>>::force
  36: rustc::ty::maps::<impl rustc::ty::maps::queries::collect_and_partition_translation_items<'tcx>>::try_get
  37: rustc::ty::maps::TyCtxtAt::collect_and_partition_translation_items
  38: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::collect_and_partition_translation_items
  39: rustc_trans::base::trans_crate
  40: <rustc_trans::LlvmTransCrate as rustc_trans_utils::trans_crate::TransCrate>::trans_crate
  41: rustc::util::common::time
  42: rustc_driver::driver::phase_4_translate_to_llvm
  43: rustc_driver::driver::compile_input::{{closure}}
  44: <std::thread::local::LocalKey<T>>::with
  45: <std::thread::local::LocalKey<T>>::with
  46: rustc::ty::context::TyCtxt::create_and_enter
  47: rustc_driver::driver::compile_input
  48: rustc_driver::run_compiler_impl
  49: syntax::with_globals
error: aborting due to previous error

[jturner314]

On further investigation, this looks closely related to #33364 and #34430, if not a duplicate.

[pnkfelix]

closing as duplicate of #62529, which is where I will try to track future instances of this field of ICE.