ICE on stable and nightly: [codegen_fulfill_obligation] checking if `std::ops::FnOnce` fulfills its obligations

[codeflo]

I encountered the following ICE while working on a task queue where the tasks can query for different resources.

Below is a small example. The ICE doesn't seem to be triggered by dead code, which is why I included a main function.

Edit: The following output is from nightly, but I get the same error on stable (rustc 1.48.0 (7eac88a 2020-11-16) running on x86_64-pc-windows-msvc).

Edit 2: I was able to simplify the code quite a bit further. I hope it's close to minimal now.

Code

fn main() {
    enqueue::<(), _>(|_result| {});
}

trait Query<'a> {
    type Result: 'a;
    fn execute() -> Self::Result;
}

impl<'a> Query<'a> for () {
    type Result = ();
    fn execute() -> () {
        ()
    }
}

fn enqueue<Q: for<'q> Query<'q>, F: 'static + for<'r> FnOnce(<Q as Query<'r>>::Result)>(f: F) {
    let _: Box<dyn FnOnce()> = Box::new(move || f(Q::execute()));
}

Meta

rustc --version --verbose:

rustc 1.50.0-nightly (7f9c43cf9 2020-12-23)
binary: rustc
commit-hash: 7f9c43cf98cfe1c369045399929cb098155b8374
commit-date: 2020-12-23
host: x86_64-pc-windows-msvc
release: 1.50.0-nightly

Error output

cargo build:

   Compiling rustc-ice-repro v0.1.0 (C:\***\rustc-ice-repro)
error: internal compiler error: compiler\rustc_trait_selection\src\traits\codegen.rs:78:17: Encountered error `OutputTypeParameterMismatch(Binder(<[closure@src\main.rs:2:22: 2:34] as std::ops::FnOnce<(<() as Query<'_>>::Result,)>>), Binder(<[closure@src\main.rs:2:22: 2:34] as std::ops::FnOnce<((),)>>), Sorts(ExpectedFound { expected: (), found: <() as Query<'_>>::Result }))` selecting `Binder(<[closure@src\main.rs:2:22: 2:34] as std::ops::FnOnce<((),)>>)` during codegen

thread 'rustc' panicked at 'Box<Any>', compiler\rustc_errors\src\lib.rs:958:9
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/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.50.0-nightly (7f9c43cf9 2020-12-23) running on x86_64-pc-windows-msvc

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 [codegen_fulfill_obligation] checking if `std::ops::FnOnce` fulfills its obligations
#1 [resolve_instance] resolving instance `<[closure@src\main.rs:2:22: 2:34] as std::ops::FnOnce<((),)>>::call_once`
end of query stack
error: aborting due to previous error

error: could not compile `rustc-ice-repro`

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

Backtrace

Output with RUST_BACKTRACE=full:

   Compiling rustc-ice-repro v0.1.0 (C:\***\rustc-ice-repro)
error: internal compiler error: compiler\rustc_trait_selection\src\traits\codegen.rs:78:17: Encountered error `OutputTypeParameterMismatch(Binder(<[closure@src\main.rs:2:22: 2:34] as std::ops::FnOnce<(<() as Query<'_>>::Result,)>>), Binder(<[closure@src\main.rs:2:22: 2:34] as std::ops::FnOnce<((),)>>), Sorts(ExpectedFound { expected: (), found: <() as Query<'_>>::Result }))` selecting `Binder(<[closure@src\main.rs:2:22: 2:34] as std::ops::FnOnce<((),)>>)` during codegen

thread 'rustc' panicked at 'Box<Any>', compiler\rustc_errors\src\lib.rs:958:9
stack backtrace:
   0:     0x7ffb94b9bce5 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h17841712fc816a42
   1:     0x7ffb94bc871b - core::fmt::write::ha3ea633b18d2da75
   2:     0x7ffb94b8d4cd - <std::io::IoSlice as core::fmt::Debug>::fmt::h9df41d37d9186c48
   3:     0x7ffb94b9fecd - std::panicking::take_hook::h78f8a454f3a4df82
   4:     0x7ffb94b9f9da - std::panicking::take_hook::h78f8a454f3a4df82
   5:     0x7ffb780c9377 - rustc_driver::report_ice::h4f570b5a4fba1dc9
   6:     0x7ffb94ba09af - std::panicking::rust_panic_with_hook::h9fd6ff93f12148ea
   7:     0x7ffb7c9800f0 - <rustc_errors::diagnostic::StringPart as core::fmt::Debug>::fmt::h7af67de8d2c3b61b
   8:     0x7ffb7c980079 - <rustc_errors::diagnostic::StringPart as core::fmt::Debug>::fmt::h7af67de8d2c3b61b
   9:     0x7ffb7c980091 - <rustc_errors::diagnostic::StringPart as core::fmt::Debug>::fmt::h7af67de8d2c3b61b
  10:     0x7ffb7c9b71cb - rustc_errors::HandlerInner::err_count::h50c6ffc480963ac8
  11:     0x7ffb7c9b5312 - rustc_errors::Handler::bug::hb269e1fa812030f1
  12:     0x7ffb7c43696f - rustc_middle::util::bug::bug_fmt::ha0061901ad88904d
  13:     0x7ffb7c4324e0 - rustc_middle::ty::walk::<impl rustc_middle::ty::subst::GenericArg>::walk_shallow::hfcd570d087b90349
  14:     0x7ffb7c432483 - rustc_middle::ty::walk::<impl rustc_middle::ty::subst::GenericArg>::walk_shallow::hfcd570d087b90349
  15:     0x7ffb7c436899 - rustc_middle::util::bug::bug_fmt::ha0061901ad88904d
  16:     0x7ffb7c436807 - rustc_middle::util::bug::bug_fmt::ha0061901ad88904d
  17:     0x7ffb7bf4a884 - unicode_normalization::__test_api::stream_safe::h4a800097b98fe8b6
  18:     0x7ffb7c15ee9c - rustc_trait_selection::traits::codegen::codegen_fulfill_obligation::heb359a89dcda61be
  19:     0x7ffb7a3ad1d9 - regex_syntax::hir::ClassUnicodeRange::end::hbd23d559724b5da5
  20:     0x7ffb7a395725 - regex_syntax::hir::ClassUnicodeRange::end::hbd23d559724b5da5
  21:     0x7ffb7a3a7d02 - regex_syntax::hir::ClassUnicodeRange::end::hbd23d559724b5da5
  22:     0x7ffb7a3b4f4d - rustc_ty_utils::instance::provide::h4b164d63df4db7c9
  23:     0x7ffb7a368f39 - regex_syntax::hir::ClassUnicodeRange::end::hbd23d559724b5da5
  24:     0x7ffb7a3af815 - regex_syntax::hir::ClassUnicodeRange::end::hbd23d559724b5da5
  25:     0x7ffb7a3af3c1 - regex_syntax::hir::ClassUnicodeRange::end::hbd23d559724b5da5
  26:     0x7ffb7c36de8c - <rustc_middle::ty::adjustment::CustomCoerceUnsized as core::fmt::Debug>::fmt::hd5caefa151f67633
  27:     0x7ffb7c647466 - rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::debug_node::h067a466f347afbfa
  28:     0x7ffb7c8a3c90 - <rustc_middle::ty::layout::SizeSkeleton as core::fmt::Debug>::fmt::hdf39e1a062a831fa
  29:     0x7ffb7c4c6877 - rustc_middle::ty::context::TyCtxt::_intern_place_elems::h4e412a0a09631fd5
  30:     0x7ffb7c72080e - <rustc_middle::ty::layout::SizeSkeleton as core::fmt::Debug>::fmt::hdf39e1a062a831fa
  31:     0x7ffb7c35b5fd - rustc_middle::ty::instance::Instance::resolve_opt_const_arg::hd412f8d69c49b383
  32:     0x7ffb7c35b357 - rustc_middle::ty::instance::Instance::resolve::h792afa8d1a698994
  33:     0x7ffb7b2409c8 - <rustc_mir::monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_terminator::h9371f020c44c4369
  34:     0x7ffb7b243639 - <rustc_mir::monomorphize::collector::RootCollector as rustc_hir::itemlikevisit::ItemLikeVisitor>::visit_impl_item::h56d4e421d02eeb1b
  35:     0x7ffb7b23cd52 - rustc_mir::monomorphize::collector::collect_crate_mono_items::hdc49dd9b65a71123
  36:     0x7ffb7b23cfd0 - rustc_mir::monomorphize::collector::collect_crate_mono_items::hdc49dd9b65a71123
  37:     0x7ffb7b23cfd0 - rustc_mir::monomorphize::collector::collect_crate_mono_items::hdc49dd9b65a71123
  38:     0x7ffb7b23cfd0 - rustc_mir::monomorphize::collector::collect_crate_mono_items::hdc49dd9b65a71123
  39:     0x7ffb7b42d283 - <rustc_mir::util::storage::AlwaysLiveLocals as core::fmt::Debug>::fmt::h0c9fc37ba57fd7cd
  40:     0x7ffb7b23bf21 - rustc_mir::monomorphize::collector::collect_crate_mono_items::hdc49dd9b65a71123
  41:     0x7ffb7b2e2d31 - rustc_mir::monomorphize::partitioning::partition::h855dd72ec131e9de
  42:     0x7ffb7851f467 - <rustc_codegen_llvm::back::lto::ThinLTOKeysMap as core::fmt::Debug>::fmt::hbc55bf07d7d5ff94
  43:     0x7ffb7841533e - <rustc_codegen_llvm::llvm_::ffi::PassKind as core::fmt::Debug>::fmt::ha4490398b447c3e3
  44:     0x7ffb784625b7 - <rustc_ast::ast::FloatTy as rustc_codegen_llvm::debuginfo::metadata::MsvcBasicName>::msvc_basic_name::h13038d476c4525fe
  45:     0x7ffb78491351 - rustc_codegen_llvm::type_::<impl rustc_codegen_ssa::traits::type_::LayoutTypeMethods for rustc_codegen_llvm::context::CodegenCx>::reg_backend_type::h90e24b99f24a4f79
  46:     0x7ffb7838e914 - rustc_interface::util::commit_date_str::hd5efa7d3825e0a94
  47:     0x7ffb785382c4 - <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate::h822bcfb8449dad95
  48:     0x7ffb782e21ae - rustc_interface::interface::parse_cfgspecs::h699038d7e71e784d
  49:     0x7ffb7831542a - rustc_interface::passes::BoxedResolver::to_resolver_outputs::h62b13ab93254cba7
  50:     0x7ffb783600f6 - rustc_interface::queries::Queries::ongoing_codegen::ha4c64625b69244aa
  51:     0x7ffb7812fa7a - <rustc_mir::transform::generator::PinArgVisitor as rustc_middle::mir::visit::MutVisitor>::tcx::h7fe0c34315986731
  52:     0x7ffb780eff0c - <rustc_ast::ast::Variant as rustc_ast::attr::HasAttrs>::attrs::h9b5124b2e6e5fe34
  53:     0x7ffb7813118c - <rustc_mir::transform::generator::PinArgVisitor as rustc_middle::mir::visit::MutVisitor>::tcx::h7fe0c34315986731
  54:     0x7ffb78103b54 - <rustc_ast::ast::Variant as rustc_ast::attr::HasAttrs>::attrs::h9b5124b2e6e5fe34
  55:     0x7ffb7811599f - rustc_ast::util::parser::prec_let_scrutinee_needs_par::h0da38a96661182e5
  56:     0x7ffb7808c63d - <tracing_subscriber::util::TryInitError as core::fmt::Display>::fmt::h9f271cb85bcd455c
  57:     0x7ffb94bb00e3 - std::sys::windows::thread::Thread::new::h0de7ecb752bc4a62
  58:     0x7ffc0a247034 - BaseThreadInitThunk
  59:     0x7ffc0bc1d0d1 - RtlUserThreadStart

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 1.50.0-nightly (7f9c43cf9 2020-12-23) running on x86_64-pc-windows-msvc

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 [codegen_fulfill_obligation] checking if `std::ops::FnOnce` fulfills its obligations
#1 [resolve_instance] resolving instance `<[closure@src\main.rs:2:22: 2:34] as std::ops::FnOnce<((),)>>::call_once`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to previous error

error: could not compile `rustc-ice-repro`

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

[camelid]

Thanks for minimizing and checking on both stable and nightly! It's very helpful :)

[steffahn]

Looks like this might be a duplicate of #80291.

[camelid]

Yeah, seems likely.

[codeflo]

Interesting. Yeah, that looks surprisingly similar, although my usecase is maybe a bit different. I want to box and store the closure instead of executing it directly. For that reason, I don't think shifting around the lifetime bounds as mentioned in that discussion would work for me.

In case it helps, some additional context why this came up. My actual code is closer to this definition:

struct Source {
    // essentially AnyMap
}

trait Query<'a> {
    type Result: 'a;
    fn execute(source: &'a Source) -> Self::Result;
}

fn enqueue<Q: for<'q> Query<'q>, F: 'static + for<'r> FnOnce(<Q as Query<'r>>::Result)>(f: F) {
    let _box: Box<dyn FnOnce(&Source)> = Box::new(move |source| f(Q::execute(source)));
    // Store _box in queue
}

As you can see, the Query result is supposed to reference into the Source. At least intuitively, it seems like the higher ranked bounds are necessary to make this sound: Since the boxed Fn is only executed much later, neither Q nor F are allowed to care about the concrete lifetime of the Source parameter.

BTW, I didn't write the "Query" trait myself. It's actually this trait from the Legion crate, which is an entity-component system I use.

[apiraino]

Assigning P-medium as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

[apiraino]

At a cursory look it seems to this ICE started after 1.26
https://rust.godbolt.org/z/6sjTY6

[camelid]

@apiraino If I use bare dyn Trait syntax (i.e. remove the dyn), I get an ICE as far back as 1.9. Not sure if it's the same ICE though:

thread 'rustc' panicked at 'Box<Any>', ../src/libsyntax/errors/mod.rs:536
note: Run with `RUST_BACKTRACE=1` for a backtrace.

error: internal compiler error: ../src/librustc_trans/common.rs:1102: Encountered error `OutputTypeParameterMismatch(Binder(<[closure@<source>:2:22: 2:34] as std::ops::FnOnce<(<() as Query<'_>>::Result,)>>), Binder(<[closure@<source>:2:22: 2:34] as std::ops::FnOnce<((),)>>), Sorts(ExpectedFound { expected: (), found: <() as Query<'_>>::Result }))` selecting `Binder(<[closure@<source>:2:22: 2:34] as std::ops::FnOnce<((),)>>)` during trans
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