Compiler crashes on impl trait for async fn with unstable feature enabled

[clchiou]

The bug arose when I was implementing a trait for async functions. I tested this code in the playground, and the compiler also crashed.

Code

#![feature(associated_type_bounds)]
#![feature(unboxed_closures)]

use std::future::Future;

use async_trait::async_trait;

#[async_trait]
trait Trait<Arg0>
where
    Arg0: ?Sized,
{
    type Return;

    async fn func(&mut self, arg0: &Arg0) -> Self::Return;
}

#[async_trait]
impl<AsyncFn, Arg0, Return> Trait<Arg0> for AsyncFn
where
    for<'a> AsyncFn: Fn<(&'a Arg0,), Output: Future<Output = Return> + Send + 'a> + Send + Sync,
    Arg0: ?Sized + Sync,
{
    type Return = Return;

    async fn func(&mut self, arg0: &Arg0) -> Self::Return {
        self(arg0).await
    }
}

async fn strlen(x: &str) -> usize {
    x.len()
}

#[tokio::test]
async fn test() {
    assert_eq!(strlen.func("hello world").await, 11);
}

Playground link

Meta

rustc --version --verbose:

rustc 1.72.0-nightly (e6d4725c7 2023-06-05)
binary: rustc
commit-hash: e6d4725c76f3b526c74454bc51afdf6daf133506
commit-date: 2023-06-05
host: x86_64-unknown-linux-gnu
release: 1.72.0-nightly
LLVM version: 16.0.4

Error output

   Compiling playground v0.0.1 (/playground)
warning: function `strlen` is never used
  --> src/lib.rs:31:10
   |
31 | async fn strlen(x: &str) -> usize {
   |          ^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

warning: `playground` (lib) generated 1 warning
thread 'rustc' panicked at 'future has no bound vars', compiler/rustc_trait_selection/src/traits/select/confirmation.rs:751:50
stack backtrace:
   0:     0x7f2d950caba1 - trace
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:     0x7f2d950caba1 - trace_unsynchronized<std::sys_common::backtrace::_print_fmt::{closure_env#1}>
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7f2d950caba1 - _print_fmt
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/sys_common/backtrace.rs:65:5
   3:     0x7f2d950caba1 - fmt
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7f2d9512b18f - fmt
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/core/src/fmt/rt.rs:138:9
   5:     0x7f2d9512b18f - write
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/core/src/fmt/mod.rs:1094:21
   6:     0x7f2d950bde51 - write_fmt<std::sys::unix::stdio::Stderr>
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/io/mod.rs:1713:15
   7:     0x7f2d950ca9b5 - _print
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7f2d950ca9b5 - print
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7f2d950cd677 - {closure#1}
  10:     0x7f2d950cd464 - default_hook
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/panicking.rs:288:9
  11:     0x7f2d983013eb - rustc_driver_impl[19f317ad306b3be2]::install_ice_hook::{closure#0}
  12:     0x7f2d950cddbd - call<(&core::panic::panic_info::PanicInfo), (dyn core::ops::function::Fn<(&core::panic::panic_info::PanicInfo), Output=()> + core::marker::Send + core::marker::Sync), alloc::alloc::Global>
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/alloc/src/boxed.rs:1999:9
  13:     0x7f2d950cddbd - rust_panic_with_hook
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/panicking.rs:709:13
  14:     0x7f2d950cdb57 - {closure#0}
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/panicking.rs:597:13
  15:     0x7f2d950cafd6 - __rust_end_short_backtrace<std::panicking::begin_panic_handler::{closure_env#0}, !>
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/sys_common/backtrace.rs:151:18
  16:     0x7f2d950cd8a2 - begin_panic_handler
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/panicking.rs:593:5
  17:     0x7f2d95127413 - panic_fmt
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/core/src/panicking.rs:67:14
  18:     0x7f2d95127183 - panic_display<&str>
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/core/src/panicking.rs:150:5
  19:     0x7f2d95127183 - panic_str
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/core/src/panicking.rs:134:5
  20:     0x7f2d95127183 - expect_failed
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/core/src/option.rs:1952:5
  21:     0x7f2d968c65ee - <rustc_trait_selection[1af287a7d6dc34a8]::traits::select::SelectionContext>::confirm_candidate
  22:     0x7f2d96b88bc8 - <rustc_trait_selection[1af287a7d6dc34a8]::traits::select::SelectionContext>::evaluate_stack
  23:     0x7f2d96cca1c7 - <rustc_trait_selection[1af287a7d6dc34a8]::traits::select::SelectionContext>::evaluate_predicate_recursively
  24:     0x7f2d96d52719 - rustc_traits[4f6015c626945177]::evaluate_obligation::evaluate_obligation
  25:     0x7f2d963396d2 - rustc_query_impl[1543b9ffeb56b6f4]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[1543b9ffeb56b6f4]::query_impl::evaluate_obligation::dynamic_query::{closure#2}::{closure#0}, rustc_middle[ef323c168d9384f4]::query::erase::Erased<[u8; 2usize]>>
  26:     0x7f2d96832152 - rustc_query_system[494966daf0575a9b]::query::plumbing::try_execute_query::<rustc_query_impl[1543b9ffeb56b6f4]::DynamicConfig<rustc_query_system[494966daf0575a9b]::query::caches::DefaultCache<rustc_middle[ef323c168d9384f4]::infer::canonical::Canonical<rustc_middle[ef323c168d9384f4]::ty::ParamEnvAnd<rustc_middle[ef323c168d9384f4]::ty::Predicate>>, rustc_middle[ef323c168d9384f4]::query::erase::Erased<[u8; 2usize]>>, false, false, false>, rustc_query_impl[1543b9ffeb56b6f4]::plumbing::QueryCtxt, false>
  27:     0x7f2d96831dbe - rustc_query_impl[1543b9ffeb56b6f4]::query_impl::evaluate_obligation::get_query_non_incr::__rust_end_short_backtrace
  28:     0x7f2d965ed8bc - <rustc_infer[db35be7103d3010a]::infer::InferCtxt as rustc_trait_selection[1af287a7d6dc34a8]::traits::query::evaluate_obligation::InferCtxtExt>::predicate_must_hold_considering_regions
  29:     0x7f2d9659fa6d - <rustc_trait_selection[1af287a7d6dc34a8]::traits::fulfill::FulfillProcessor as rustc_data_structures[7856cba86f978d37]::obligation_forest::ObligationProcessor>::process_obligation
  30:     0x7f2d96596671 - <rustc_data_structures[7856cba86f978d37]::obligation_forest::ObligationForest<rustc_trait_selection[1af287a7d6dc34a8]::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection[1af287a7d6dc34a8]::traits::fulfill::FulfillProcessor>
  31:     0x7f2d96cd7b75 - <dyn rustc_infer[db35be7103d3010a]::traits::engine::TraitEngine as rustc_infer[db35be7103d3010a]::traits::engine::TraitEngineExt>::select_all_or_error
  32:     0x7f2d96cd6454 - rustc_traits[4f6015c626945177]::codegen::codegen_select_candidate
  33:     0x7f2d96381e4f - rustc_query_impl[1543b9ffeb56b6f4]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[1543b9ffeb56b6f4]::query_impl::codegen_select_candidate::dynamic_query::{closure#2}::{closure#0}, rustc_middle[ef323c168d9384f4]::query::erase::Erased<[u8; 16usize]>>
  34:     0x7f2d97477046 - rustc_query_system[494966daf0575a9b]::query::plumbing::try_execute_query::<rustc_query_impl[1543b9ffeb56b6f4]::DynamicConfig<rustc_query_system[494966daf0575a9b]::query::caches::DefaultCache<(rustc_middle[ef323c168d9384f4]::ty::ParamEnv, rustc_middle[ef323c168d9384f4]::ty::sty::Binder<rustc_middle[ef323c168d9384f4]::ty::sty::TraitRef>), rustc_middle[ef323c168d9384f4]::query::erase::Erased<[u8; 16usize]>>, false, false, false>, rustc_query_impl[1543b9ffeb56b6f4]::plumbing::QueryCtxt, false>
  35:     0x7f2d97476cca - rustc_query_impl[1543b9ffeb56b6f4]::query_impl::codegen_select_candidate::get_query_non_incr::__rust_end_short_backtrace
  36:     0x7f2d970301c1 - rustc_ty_utils[14fb50b748d4fe37]::instance::resolve_instance
  37:     0x7f2d9633f55c - rustc_query_impl[1543b9ffeb56b6f4]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[1543b9ffeb56b6f4]::query_impl::resolve_instance::dynamic_query::{closure#2}::{closure#0}, rustc_middle[ef323c168d9384f4]::query::erase::Erased<[u8; 32usize]>>
  38:     0x7f2d96bda7d3 - rustc_query_system[494966daf0575a9b]::query::plumbing::try_execute_query::<rustc_query_impl[1543b9ffeb56b6f4]::DynamicConfig<rustc_query_system[494966daf0575a9b]::query::caches::DefaultCache<rustc_middle[ef323c168d9384f4]::ty::ParamEnvAnd<(rustc_span[c9fd14582bbb5db9]::def_id::DefId, &rustc_middle[ef323c168d9384f4]::ty::list::List<rustc_middle[ef323c168d9384f4]::ty::subst::GenericArg>)>, rustc_middle[ef323c168d9384f4]::query::erase::Erased<[u8; 32usize]>>, false, false, false>, rustc_query_impl[1543b9ffeb56b6f4]::plumbing::QueryCtxt, false>
  39:     0x7f2d96bda444 - rustc_query_impl[1543b9ffeb56b6f4]::query_impl::resolve_instance::get_query_non_incr::__rust_end_short_backtrace
  40:     0x7f2d96f2ebcc - rustc_monomorphize[93680276fc04688]::collector::collect_used_items
  41:     0x7f2d96f25bf3 - rustc_monomorphize[93680276fc04688]::collector::collect_items_rec
  42:     0x7f2d96f26524 - rustc_monomorphize[93680276fc04688]::collector::collect_items_rec
  43:     0x7f2d96f26524 - rustc_monomorphize[93680276fc04688]::collector::collect_items_rec
  44:     0x7f2d96f26524 - rustc_monomorphize[93680276fc04688]::collector::collect_items_rec
  45:     0x7f2d974f1363 - rustc_data_structures[7856cba86f978d37]::sync::par_for_each_in::<alloc[bb2455db20450995]::vec::Vec<rustc_middle[ef323c168d9384f4]::mir::mono::MonoItem>, rustc_monomorphize[93680276fc04688]::collector::collect_crate_mono_items::{closure#1}::{closure#0}>
  46:     0x7f2d974f1020 - <rustc_session[23023510f27a6956]::session::Session>::time::<(), rustc_monomorphize[93680276fc04688]::collector::collect_crate_mono_items::{closure#1}>
  47:     0x7f2d974f0c68 - rustc_monomorphize[93680276fc04688]::collector::collect_crate_mono_items
  48:     0x7f2d974ef3f7 - rustc_monomorphize[93680276fc04688]::partitioning::collect_and_partition_mono_items
  49:     0x7f2d97900059 - rustc_query_impl[1543b9ffeb56b6f4]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[1543b9ffeb56b6f4]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2}::{closure#0}, rustc_middle[ef323c168d9384f4]::query::erase::Erased<[u8; 24usize]>>
  50:     0x7f2d9790002b - <rustc_query_impl[1543b9ffeb56b6f4]::query_impl::collect_and_partition_mono_items::dynamic_query::{closure#2} as core[89b1b7d1b31a1e7d]::ops::function::FnOnce<(rustc_middle[ef323c168d9384f4]::ty::context::TyCtxt, ())>>::call_once
  51:     0x7f2d97a0ed4c - rustc_query_system[494966daf0575a9b]::query::plumbing::try_execute_query::<rustc_query_impl[1543b9ffeb56b6f4]::DynamicConfig<rustc_query_system[494966daf0575a9b]::query::caches::SingleCache<rustc_middle[ef323c168d9384f4]::query::erase::Erased<[u8; 24usize]>>, false, false, false>, rustc_query_impl[1543b9ffeb56b6f4]::plumbing::QueryCtxt, false>
  52:     0x7f2d97c8e444 - rustc_query_impl[1543b9ffeb56b6f4]::query_impl::collect_and_partition_mono_items::get_query_non_incr::__rust_end_short_backtrace
  53:     0x7f2d97967546 - rustc_codegen_ssa[3134fc578afcf247]::base::codegen_crate::<rustc_codegen_llvm[7c63b95a4e813ac8]::LlvmCodegenBackend>
  54:     0x7f2d9796732f - <rustc_codegen_llvm[7c63b95a4e813ac8]::LlvmCodegenBackend as rustc_codegen_ssa[3134fc578afcf247]::traits::backend::CodegenBackend>::codegen_crate
  55:     0x7f2d97541d32 - <rustc_session[23023510f27a6956]::session::Session>::time::<alloc[bb2455db20450995]::boxed::Box<dyn core[89b1b7d1b31a1e7d]::any::Any>, rustc_interface[e4001d1c190a5888]::passes::start_codegen::{closure#0}>
  56:     0x7f2d9754187b - rustc_interface[e4001d1c190a5888]::passes::start_codegen
  57:     0x7f2d9753d06d - <rustc_middle[ef323c168d9384f4]::ty::context::GlobalCtxt>::enter::<<rustc_interface[e4001d1c190a5888]::queries::Queries>::ongoing_codegen::{closure#0}::{closure#0}, core[89b1b7d1b31a1e7d]::result::Result<alloc[bb2455db20450995]::boxed::Box<dyn core[89b1b7d1b31a1e7d]::any::Any>, rustc_span[c9fd14582bbb5db9]::ErrorGuaranteed>>
  58:     0x7f2d9753b7bd - <rustc_interface[e4001d1c190a5888]::queries::Queries>::ongoing_codegen
  59:     0x7f2d9753ad94 - <rustc_interface[e4001d1c190a5888]::interface::Compiler>::enter::<rustc_driver_impl[19f317ad306b3be2]::run_compiler::{closure#1}::{closure#2}, core[89b1b7d1b31a1e7d]::result::Result<core[89b1b7d1b31a1e7d]::option::Option<rustc_interface[e4001d1c190a5888]::queries::Linker>, rustc_span[c9fd14582bbb5db9]::ErrorGuaranteed>>
  60:     0x7f2d97538370 - std[4cc418835f9904b9]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[e4001d1c190a5888]::util::run_in_thread_pool_with_globals<rustc_interface[e4001d1c190a5888]::interface::run_compiler<core[89b1b7d1b31a1e7d]::result::Result<(), rustc_span[c9fd14582bbb5db9]::ErrorGuaranteed>, rustc_driver_impl[19f317ad306b3be2]::run_compiler::{closure#1}>::{closure#0}, core[89b1b7d1b31a1e7d]::result::Result<(), rustc_span[c9fd14582bbb5db9]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[89b1b7d1b31a1e7d]::result::Result<(), rustc_span[c9fd14582bbb5db9]::ErrorGuaranteed>>
  61:     0x7f2d97537b15 - <<std[4cc418835f9904b9]::thread::Builder>::spawn_unchecked_<rustc_interface[e4001d1c190a5888]::util::run_in_thread_pool_with_globals<rustc_interface[e4001d1c190a5888]::interface::run_compiler<core[89b1b7d1b31a1e7d]::result::Result<(), rustc_span[c9fd14582bbb5db9]::ErrorGuaranteed>, rustc_driver_impl[19f317ad306b3be2]::run_compiler::{closure#1}>::{closure#0}, core[89b1b7d1b31a1e7d]::result::Result<(), rustc_span[c9fd14582bbb5db9]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[89b1b7d1b31a1e7d]::result::Result<(), rustc_span[c9fd14582bbb5db9]::ErrorGuaranteed>>::{closure#1} as core[89b1b7d1b31a1e7d]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  62:     0x7f2d950d82b5 - call_once<(), dyn core::ops::function::FnOnce<(), Output=()>, alloc::alloc::Global>
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/alloc/src/boxed.rs:1985:9
  63:     0x7f2d950d82b5 - call_once<(), alloc::boxed::Box<dyn core::ops::function::FnOnce<(), Output=()>, alloc::alloc::Global>, alloc::alloc::Global>
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/alloc/src/boxed.rs:1985:9
  64:     0x7f2d950d82b5 - thread_start
                               at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/sys/unix/thread.rs:108:17
  65:     0x7f2d94fa6609 - start_thread
  66:     0x7f2d94ec9133 - clone
  67:                0x0 - <unknown>

error: 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 1.72.0-nightly (e6d4725c7 2023-06-05) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2

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

query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `for<'a> [async fn body@src/lib.rs:31:35: 33:2]: core::future::future::Future`
#1 [codegen_select_candidate] computing candidate for `<strlen as Trait<str>>`
#2 [resolve_instance] resolving instance `<strlen as Trait<str>>::func::<'_, '_, '_>`
#3 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: could not compile `playground` (lib test)

Backtrace

RUST_BACKTRACE=1 cargo build

thread 'rustc' panicked at 'future has no bound vars', compiler/rustc_trait_selection/src/traits/select/confirmation.rs:751:50
stack backtrace:
   0: begin_panic_handler
             at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/std/src/panicking.rs:593:5
   1: panic_fmt
             at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/core/src/panicking.rs:67:14
   2: panic_display<&str>
             at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/core/src/panicking.rs:150:5
   3: panic_str
             at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/core/src/panicking.rs:134:5
   4: expect_failed
             at /rustc/e6d4725c76f3b526c74454bc51afdf6daf133506/library/core/src/option.rs:1952:5
   5: <rustc_trait_selection::traits::select::SelectionContext>::confirm_candidate
   6: <rustc_trait_selection::traits::select::SelectionContext>::evaluate_stack
   7: <rustc_middle::dep_graph::dep_node::DepKind as rustc_query_system::dep_graph::DepKind>::with_deps::<<rustc_trait_selection::traits::select::SelectionContext>::in_task<<rustc_trait_selection::traits::select::SelectionContext>::evaluate_trait_predicate_recursively::{closure#0}::{closure#2}, core::result::Result<rustc_middle::traits::select::EvaluationResult, rustc_middle::traits::select::OverflowError>>::{closure#0}, core::result::Result<rustc_middle::traits::select::EvaluationResult, rustc_middle::traits::select::OverflowError>>
   8: <rustc_trait_selection::traits::select::SelectionContext>::evaluate_predicate_recursively
   9: rustc_traits::evaluate_obligation::evaluate_obligation
  10: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt>::predicate_must_hold_considering_regions
  11: <rustc_trait_selection::traits::fulfill::FulfillProcessor as rustc_data_structures::obligation_forest::ObligationProcessor>::process_obligation
  12: <rustc_data_structures::obligation_forest::ObligationForest<rustc_trait_selection::traits::fulfill::PendingPredicateObligation>>::process_obligations::<rustc_trait_selection::traits::fulfill::FulfillProcessor>
  13: <dyn rustc_infer::traits::engine::TraitEngine as rustc_infer::traits::engine::TraitEngineExt>::select_all_or_error
  14: rustc_traits::codegen::codegen_select_candidate
  15: rustc_ty_utils::instance::resolve_instance
  16: rustc_monomorphize::collector::collect_used_items
  17: rustc_monomorphize::collector::collect_items_rec
  18: rustc_monomorphize::collector::collect_items_rec
  19: rustc_monomorphize::collector::collect_items_rec
  20: rustc_monomorphize::collector::collect_items_rec
  21: rustc_monomorphize::collector::collect_items_rec
  22: rustc_monomorphize::collector::collect_items_rec
  23: rustc_monomorphize::collector::collect_items_rec
  24: rustc_monomorphize::collector::collect_items_rec
  25: rustc_monomorphize::collector::collect_items_rec
  26: rustc_data_structures::sync::par_for_each_in::<alloc::vec::Vec<rustc_middle::mir::mono::MonoItem>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure#0}>
  27: <rustc_session::session::Session>::time::<(), rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}>
  28: rustc_monomorphize::collector::collect_crate_mono_items
  29: rustc_monomorphize::partitioning::collect_and_partition_mono_items
  30: rustc_codegen_ssa::base::codegen_crate::<rustc_codegen_llvm::LlvmCodegenBackend>
  31: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  32: <rustc_session::session::Session>::time::<alloc::boxed::Box<dyn core::any::Any>, rustc_interface::passes::start_codegen::{closure#0}>
  33: rustc_interface::passes::start_codegen
  34: <rustc_middle::ty::context::GlobalCtxt>::enter::<<rustc_interface::queries::Queries>::ongoing_codegen::{closure#0}::{closure#0}, core::result::Result<alloc::boxed::Box<dyn core::any::Any>, rustc_span::ErrorGuaranteed>>
  35: <rustc_interface::queries::Queries>::ongoing_codegen
  36: <rustc_interface::interface::Compiler>::enter::<rustc_driver_impl::run_compiler::{closure#1}::{closure#2}, core::result::Result<core::option::Option<rustc_interface::queries::Linker>, rustc_span::ErrorGuaranteed>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

error: 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 1.72.0-nightly (e6d4725c7 2023-06-05) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
#0 [evaluate_obligation] evaluating trait selection obligation `for<'a> [async fn body@src/bin/compiler-crash.rs:31:35: 33:2]: core::future::future::Future`
#1 [codegen_select_candidate] computing candidate for `<strlen as Trait<str>>`
#2 [resolve_instance] resolving instance `<strlen as Trait<str>>::func::<'_, '_, '_>`
#3 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

[clchiou]

In the playground, Build does not crash. You will have to click Test to make it crash.

[compiler-errors]

Not related to associated_type_bounds:

#![feature(unboxed_closures)]

use std::future::Future;

use async_trait::async_trait;

#[async_trait]
trait Trait<Arg0>
where
    Arg0: ?Sized,
{
    type Return;

    async fn func(&mut self, arg0: &Arg0) -> Self::Return;
}

#[async_trait]
impl<AsyncFn, Arg0, Return> Trait<Arg0> for AsyncFn
where
    for<'a> AsyncFn: Fn<(&'a Arg0,)> + Send + Sync,
    for<'a> <AsyncFn as FnOnce<(&'a Arg0,)>>::Output: Future<Output = Return> + Send + 'a,
    Arg0: ?Sized + Sync,
{
    type Return = Return;

    async fn func(&mut self, arg0: &Arg0) -> Self::Return {
        self(arg0).await
    }
}

async fn strlen(x: &str) -> usize {
    x.len()
}

#[tokio::main]
async fn main() {
    assert_eq!(strlen.func("hello world").await, 11);
}

Could be further minimized, I think.

[clchiou]

Thanks for the improvement.

I did a small experiment. I found that type Return can be safely removed. However, when I changed Arg0 from a reference to a non-reference, the compiler passed. In other words, async f<T>(T) passes, but async f<T>(&T) crashes.

[dswij]

@rustbot claim

[dswij]

Can be further minimized to:

#![feature(unboxed_closures)]

use std::future::Future;

trait Trait {
    fn func(&self, _: &str);
}

impl<T> Trait for T
where
    for<'a> T: Fn<(&'a str,)> + Send + Sync,
    for<'a> <T as FnOnce<(&'a str,)>>::Output: Future<Output = usize> + Send,
{
    fn func(&self, _: &str) {
        println!("hello!");
    }
}

async fn strlen(x: &str) -> usize {
    x.len()
}

fn main() {
    strlen.func("hi");
}

The problem is because Futures doesn't expect bounds when resolving, but is given here with HRTB.

[compiler-errors]

Here's an even more minimal example, that only relies on type_alias_impl_trait which I assume we want to stabilize sooner than unboxed_closures 😄

#![feature(type_alias_impl_trait)]

use std::future::Future;

type Fut<'a> = impl Future<Output = ()> + 'a;

fn foo<'a>(_: &()) -> Fut<'_> {
    async {}
}

trait Test {
    fn hello();
}

impl Test for ()
where
    for<'a> Fut<'a>: Future<Output = ()>,
{
    fn hello() {}
}

fn main() {
    <()>::hello();
}

[albertlarsan68]

@compiler-errors Your minimisation doesn't work on latest nightly (2023-07-12), but @dswij's still works