Assertion failed in erroneous recursive types with methods returning a `impl Trait` type

Summary

• erroneous recursive types (without being wrapped with Box<_>) having methods whose return type is impl Trait causes the assertion failure.

I tried this code:

use std::collections::BTreeSet;

#[derive(Hash)]
pub enum ElemDerived {
    A(ElemDerived)
}

pub enum Elem {
    Derived(ElemDerived)
}

pub struct Set(BTreeSet<Elem>);

impl Set {
    pub fn into_iter(self) -> impl Iterator<Item = Elem> {
        self.0.into_iter()
    }
}

fn main() {}

I expected to see this happen:

• I expected the error message "recursive type ElemDerived has infinite size."
• Indeed, if there is no method with return type impl Trait, we got the following normal error messages:

  error[E0072]: recursive type `ElemDerived` has infinite size
  --> ./bug.rs:4:1
    |
  4 | pub enum ElemDerived {
    | ^^^^^^^^^^^^^^^^^^^^ recursive type has infinite size
  5 |     A(ElemDerived)
    |       ----------- recursive without indirection
    |
    = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `ElemDerived` representable
  
  error: aborting due to previous error
  

Instead, this happened:

• rustc aborted with the following assertion failure.

  % RUST_BACKTRACE=1 rustc ./bug.rs
  error: internal compiler error: src/librustc_infer/traits/codegen/mod.rs:108: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<[type error] as std::marker::Sized>)), depth=2),Ambiguity)]` resolving bounds after type-checking
  
  thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:875:9
  stack backtrace:
    0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
    1: core::fmt::write
    2: std::io::Write::write_fmt
    3: std::panicking::default_hook::{{closure}}
    4: std::panicking::default_hook
    5: rustc_driver::report_ice
    6: std::panicking::rust_panic_with_hook
    7: std::panicking::begin_panic
    8: rustc_errors::HandlerInner::bug
    9: rustc_errors::Handler::bug
    10: rustc::util::bug::opt_span_bug_fmt::{{closure}}
    11: rustc::ty::context::tls::with_opt::{{closure}}
    12: rustc::ty::context::tls::with_opt
    13: rustc::util::bug::opt_span_bug_fmt
    14: rustc::util::bug::bug_fmt
    15: rustc_infer::traits::codegen::<impl rustc_infer::infer::InferCtxt>::drain_fulfillment_cx_or_panic
    16: rustc::ty::context::GlobalCtxt::enter_local
    17: rustc_infer::traits::codegen::codegen_fulfill_obligation
    18: rustc::ty::query::__query_compute::codegen_fulfill_obligation
    19: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::codegen_fulfill_obligation>::compute
    20: rustc::dep_graph::graph::DepGraph::with_task_impl
    21: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
    22: rustc_ty::instance::resolve_instance
    23: rustc::ty::instance::Instance::resolve
    24: rustc_mir_build::lints::check
    25: rustc::ty::context::GlobalCtxt::enter_local
    26: rustc_mir_build::build::mir_built
    27: rustc::ty::query::__query_compute::mir_built
    28: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_built>::compute
    29: rustc::dep_graph::graph::DepGraph::with_task_impl
    30: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
    31: rustc_mir::transform::check_unsafety::unsafety_check_result
    32: rustc::ty::query::__query_compute::unsafety_check_result
    33: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::unsafety_check_result>::compute
    34: rustc::dep_graph::graph::DepGraph::with_task_impl
    35: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
    36: rustc_mir::transform::mir_const
    37: rustc::ty::query::__query_compute::mir_const
    38: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_const>::compute
    39: rustc::dep_graph::graph::DepGraph::with_task_impl
    40: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
    41: rustc_mir::transform::mir_validated
    42: rustc::ty::query::__query_compute::mir_validated
    43: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_validated>::compute
    44: rustc::dep_graph::graph::DepGraph::with_task_impl
    45: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
    46: rustc_mir::borrow_check::mir_borrowck
    47: rustc::ty::query::__query_compute::mir_borrowck
    48: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_borrowck>::compute
    49: rustc::dep_graph::graph::DepGraph::with_task_impl
    50: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
    51: rustc_typeck::collect::type_of::type_of
    52: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::type_of>::compute
    53: rustc::dep_graph::graph::DepGraph::with_task_impl
    54: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
    55: rustc::ty::util::<impl rustc::ty::context::TyCtxt>::try_expand_impl_trait_type::OpaqueTypeExpander::expand_opaque_ty
    56: rustc::ty::util::<impl rustc::ty::context::TyCtxt>::try_expand_impl_trait_type
    57: rustc_typeck::check::check_item_type
    58: rustc::hir::map::Map::visit_item_likes_in_module
    59: rustc_typeck::check::check_mod_item_types
    60: rustc::ty::query::__query_compute::check_mod_item_types
    61: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::check_mod_item_types>::compute
    62: rustc::dep_graph::graph::DepGraph::with_task_impl
    63: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
    64: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::ensure_query
    65: rustc_session::utils::<impl rustc_session::session::Session>::time
    66: rustc_typeck::check_crate
    67: rustc_interface::passes::analysis
    68: rustc::ty::query::__query_compute::analysis
    69: rustc::dep_graph::graph::DepGraph::with_task_impl
    70: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
    71: rustc::ty::context::tls::enter_global
    72: rustc_interface::interface::run_compiler_in_existing_thread_pool
  note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
  
  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 1.43.1 (8d69840ab 2020-05-04) running on x86_64-apple-darwin
  
  query stack during panic:
  #0 [codegen_fulfill_obligation] checking if `std::iter::IntoIterator` fulfills its obligations
  #1 [mir_built] building MIR for
  #2 [unsafety_check_result] unsafety-checking `Set::into_iter`
  #3 [mir_const] processing `Set::into_iter`
  #4 [mir_validated] processing `Set::into_iter`
  #5 [mir_borrowck] borrow-checking `Set::into_iter`
  #6 [type_of] processing `Set::into_iter::{{opaque}}#0`
  #7 [check_mod_item_types] checking item types in top-level module
  #8 [analysis] running analysis passes on this crate
  end of query stack
  error: aborting due to previous error
  

Meta

• rustc --version --verbose:

  % rustc --version --verbose
  rustc 1.43.1 (8d69840ab 2020-05-04)
  binary: rustc
  commit-hash: 8d69840ab92ea7f4d323420088dd8c9775f180cd
  commit-date: 2020-05-04
  host: x86_64-apple-darwin
  release: 1.43.1
  LLVM version: 9.0
  

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