`Failed to normalize` `async_fn_in_trait` ICE for indirect recursion of async trait method calls

[rkusa]

The following happens for the async_fn_in_trait feature. I guess it is caused by the indirect recursion (First::first() -> Second::second() -> First::first() -> Second::second() -> First::first() -> ...).

Code

#![feature(async_fn_in_trait)]

fn main() {
    let _ = async {
        A.first().await.second().await;
    };
}

pub trait First {
    type Second: Second;
    async fn first(self) -> Self::Second;
}

struct A;

impl First for A {
    type Second = A;
    async fn first(self) -> Self::Second {
        A
    }
}

pub trait Second {
    async fn second(self);
}

impl<C> Second for C
where
    C: First,
{
    async fn second(self) {
        self.first().await.second().await;
    }
}

Meta

rustc --version --verbose:

rustc 1.71.0-nightly (cca7ee581 2023-05-27)
binary: rustc
commit-hash: cca7ee58110726983951a19d5fb7316d9243925d
commit-date: 2023-05-27
host: aarch64-apple-darwin
release: 1.71.0-nightly
LLVM version: 16.0.4

Error output

error: internal compiler error: compiler/rustc_middle/src/ty/normalize_erasing_regions.rs:195:90: Failed to normalize [async block@src/main.rs:4:13: 6:6], maybe try to call `try_normalize_erasing_regions` instead

Backtrace

thread 'rustc' panicked at 'Box<dyn Any>', /rustc/cca7ee58110726983951a19d5fb7316d9243925d/compiler/rustc_errors/src/lib.rs:1651:9
stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: <rustc_errors::HandlerInner>::bug::<alloc::string::String>
   2: <rustc_errors::Handler>::bug::<alloc::string::String>
   3: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
   4: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
   5: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
   6: rustc_middle::util::bug::bug_fmt
   7: <rustc_middle::ty::normalize_erasing_regions::NormalizeAfterErasingRegionsFolder as rustc_type_ir::fold::TypeFolder<rustc_middle::ty::context::TyCtxt>>::fold_ty
   8: <rustc_monomorphize::collector::MirNeighborCollector as rustc_middle::mir::visit::Visitor>::visit_terminator
   9: rustc_monomorphize::collector::collect_neighbours
  10: rustc_monomorphize::collector::collect_items_rec
  11: <core::panic::unwind_safe::AssertUnwindSafe<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}>::{closure#0}::{closure#0}> as core::ops::function::FnOnce<()>>::call_once
  12: 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}>
  13: <rustc_session::session::Session>::time::<(), rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}>
  14: rustc_monomorphize::collector::collect_crate_mono_items
  15: rustc_monomorphize::partitioning::collect_and_partition_mono_items
  16: rustc_codegen_ssa::base::codegen_crate::<rustc_codegen_llvm::LlvmCodegenBackend>
  17: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  18: rustc_interface::passes::start_codegen
  19: <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>>
  20: <rustc_interface::queries::Queries>::ongoing_codegen
  21: <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>>
  22: rustc_span::set_source_map::<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}::{closure#0}>
  23: <scoped_tls::ScopedKey<rustc_span::SessionGlobals>>::set::<rustc_interface::interface::run_compiler<core::result::Result<(), rustc_span::ErrorGuaranteed>, rustc_driver_impl::run_compiler::{closure#1}>::{closure#0}, core::result::Result<(), rustc_span::ErrorGuaranteed>>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

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 1.71.0-nightly (cca7ee581 2023-05-27) running on aarch64-apple-darwin

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

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

query stack during panic:
#0 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

[matthiaskrgr]

Regression in nightly-2022-09-24

[compiler-errors]

This is reproducible with type alias impl trait (well, impl trait in assoc ty) similarly:

#![feature(impl_trait_in_assoc_type)]

use core::future::Future;

trait Recur {
    type Recur: Future<Output = ()>;
    
    fn recur(self) -> Self::Recur;
}

async fn recur(t: impl Recur) {
    t.recur().await;
}

impl Recur for () {
    type Recur = impl Future<Output = ()>;
    
    fn recur(self) -> Self::Recur {
        async move { recur(self).await; }
    }
}

fn main() {
    recur(());
}

I consider this to be somewhat concerning, because both of these features introduce a new kind of post-monomorphization error that (afaict) can't be reproduced with associated types on stable rust.