Panic at `DefId::expect_local` when returning `impl Future` type alias from default trait method using `async move` block

[wxb1ank]

Code

#![feature(type_alias_impl_trait)]

use std::future::Future;

type G<'a, T> = impl Future<Output = ()>;

trait Trait {
    type F: Future<Output = ()>;

    fn f(&self) -> Self::F;

    fn g<'a>(&'a self) -> G<'a, Self>
    where
        Self: Sized,
    {
        async move { self.f().await }
    }
}

fn main() {}

Meta

rustc --version --verbose:

rustc 1.57.0-nightly (0eabf25b9 2021-10-06)
binary: rustc
commit-hash: 0eabf25b90396dead0b2a1aaa275af18a1ae6008
commit-date: 2021-10-06
host: x86_64-unknown-linux-gnu
release: 1.57.0-nightly
LLVM version: 13.0.0

Error output

thread 'rustc' panicked at 'DefId::expect_local: `DefId(2:12723 ~ core[cc79]::future::from_generator::{opaque#0})` isn't local', /rustc/0eabf25b90396dead0b2a1aaa275af18a1ae6008/compiler/rustc_span/src/def_id.rs:234:43
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

error: internal compiler error: unexpected panic

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 1.57.0-nightly (0eabf25b9 2021-10-06) running on x86_64-unknown-linux-gnu

note: compiler flags: -C embed-bitcode=no -C debuginfo=2 -C incremental -C link-arg=-rtlib=compiler-rt -C linker=clang --crate-type bin

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

query stack during panic:
#0 [check_mod_item_types] checking item types in top-level module
#1 [analysis] running analysis passes on this crate
end of query stack

Backtrace

error: internal compiler error: trimmed_def_paths constructed
  |
  = note: delayed at    0: rustc_errors::Handler::delay_good_path_bug
             1: rustc_middle::ty::print::pretty::trimmed_def_paths
             2: rustc_middle::dep_graph::<impl rustc_query_system::dep_graph::DepKind for rustc_middle::dep_graph::dep_node::DepKind>::with_deps
             3: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
             4: rustc_data_structures::stack::ensure_sufficient_stack
             5: rustc_query_system::query::plumbing::try_execute_query
             6: rustc_query_system::query::plumbing::get_query
             7: <rustc_middle::ty::print::pretty::FmtPrinter<F> as rustc_middle::ty::print::Printer>::print_def_path
             8: rustc_middle::ty::print::Printer::default_print_def_path
             9: <rustc_middle::ty::print::pretty::FmtPrinter<F> as rustc_middle::ty::print::Printer>::print_def_path
            10: <rustc_middle::ty::print::pretty::FmtPrinter<F> as rustc_middle::ty::print::pretty::PrettyPrinter>::in_binder
            11: std::thread::local::LocalKey<T>::with
            12: <rustc_middle::ty::print::pretty::FmtPrinter<F> as rustc_middle::ty::print::Printer>::print_type
            13: <rustc_middle::ty::print::pretty::FmtPrinter<F> as rustc_middle::ty::print::Printer>::path_qualified
            14: rustc_middle::ty::print::Printer::default_print_def_path
            15: <rustc_middle::ty::print::pretty::FmtPrinter<F> as rustc_middle::ty::print::Printer>::print_def_path
            16: rustc_middle::ty::print::pretty::<impl rustc_middle::ty::print::Print<P> for rustc_middle::ty::ProjectionPredicate>::print
            17: rustc_middle::ty::print::pretty::<impl rustc_middle::ty::print::Print<P> for rustc_middle::ty::PredicateKind>::print
            18: <rustc_middle::ty::print::pretty::FmtPrinter<F> as rustc_middle::ty::print::pretty::PrettyPrinter>::in_binder
            19: rustc_middle::ty::print::pretty::<impl core::fmt::Display for rustc_middle::ty::Predicate>::fmt
            20: core::fmt::write
                       at /rustc/0eabf25b90396dead0b2a1aaa275af18a1ae6008/library/core/src/fmt/mod.rs:1162:17
            21: core::fmt::Write::write_fmt
                       at /rustc/0eabf25b90396dead0b2a1aaa275af18a1ae6008/library/core/src/fmt/mod.rs:186:9
            22: alloc::fmt::format
                       at /rustc/0eabf25b90396dead0b2a1aaa275af18a1ae6008/library/alloc/src/fmt.rs:579:5
            23: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtPrivExt>::report_projection_error
            24: <rustc_infer::infer::InferCtxt as rustc_trait_selection::traits::error_reporting::InferCtxtExt>::report_fulfillment_errors
            25: rustc_infer::infer::InferCtxtBuilder::enter
            26: rustc_typeck::check::check::check_item_type
            27: rustc_middle::hir::map::Map::visit_item_likes_in_module
            28: rustc_typeck::check::check::check_mod_item_types
            29: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
            30: rustc_data_structures::stack::ensure_sufficient_stack
            31: rustc_query_system::query::plumbing::try_execute_query
            32: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::check_mod_item_types
            33: rustc_middle::hir::map::Map::for_each_module
            34: rustc_session::utils::<impl rustc_session::session::Session>::time
            35: rustc_typeck::check_crate
            36: rustc_interface::passes::analysis
            37: rustc_query_system::dep_graph::graph::DepGraph<K>::with_task
            38: rustc_data_structures::stack::ensure_sufficient_stack
            39: rustc_query_system::query::plumbing::try_execute_query
            40: <rustc_query_impl::Queries as rustc_middle::ty::query::QueryEngine>::analysis
            41: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::enter
            42: rustc_span::with_source_map
            43: scoped_tls::ScopedKey<T>::set
            44: std::sys_common::backtrace::__rust_begin_short_backtrace
            45: core::ops::function::FnOnce::call_once{{vtable.shim}}
            46: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                       at /rustc/0eabf25b90396dead0b2a1aaa275af18a1ae6008/library/alloc/src/boxed.rs:1638:9
            47: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
                       at /rustc/0eabf25b90396dead0b2a1aaa275af18a1ae6008/library/alloc/src/boxed.rs:1638:9
            48: std::sys::unix::thread::Thread::new::thread_start
                       at /rustc/0eabf25b90396dead0b2a1aaa275af18a1ae6008/library/std/src/sys/unix/thread.rs:106:17
            49: start_thread
            50: __GI___clone

[wxb1ank]

If you rewrite the body of Trait::g as simply self.f(), rustc correctly emits:

error[E0271]: type mismatch resolving `<<T as Trait>::F as futures::Future>::Output == u8`
 --> src/main.rs:5:17
  |
5 | type G<'a, T> = impl Future<Output = u8>;
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^ expected `u8`, found associated type
  |
  = note:         expected type `u8`
          found associated type `<<T as Trait>::F as futures::Future>::Output`
  = help: consider constraining the associated type `<<T as Trait>::F as futures::Future>::Output` to `u8`
  = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html

error[E0277]: the trait bound `T: Trait` is not satisfied
 --> src/main.rs:5:17
  |
5 | type G<'a, T> = impl Future<Output = u8>;
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Trait` is not implemented for `T`
  |
help: consider restricting type parameter `T`
  |
5 | type G<'a, T: Trait> = impl Future<Output = u8>;
  |             +++++++

If you accept the second suggestion, the code compiles successfully.