ICE when using 'impl Trait' with an infinity recursive function

[lszxb]

fn main() {
    test();
}

fn test() -> impl TraitA {
    test()
}

trait TraitA {}

Error message

error: internal compiler error: src\librustc_mir\util\elaborate_drops.rs:826: open drop from non-ADT `[type error]`

thread 'rustc' panicked at 'Box<Any>', src\librustc_errors\lib.rs:925:9

Additionally, the compiler warns that the function cannot return.

Compiler version

binary: rustc
commit-hash: 0e8a4b441c5da21a2cb19448728ade5baa299c66
commit-date: 2019-10-16
host: x86_64-pc-windows-msvc
release: 1.40.0-nightly
LLVM version: 9.0

Backtrace

stack backtrace:
   0: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
   1: core::fmt::write
   2: <std::io::IoSliceMut as core::fmt::Debug>::fmt
   3: std::panicking::take_hook
   4: std::panicking::take_hook
   5: rustc_driver::report_ice
   6: std::panicking::rust_panic_with_hook
   7: <rustc_errors::Level as core::fmt::Debug>::fmt
   8: rustc_errors::HandlerInner::abort_if_errors_and_should_abort
   9: rustc_errors::Handler::bug
  10: rustc::util::bug::bug_fmt
  11: rustc::ty::wf::object_region_bounds
  12: rustc::ty::wf::object_region_bounds
  13: rustc::ty::wf::object_region_bounds
  14: rustc::util::bug::bug_fmt
  15: rustc::util::bug::bug_fmt
  16: rustc_mir::util::elaborate_drops::Unwind::is_cleanup
  17: <rustc_mir::borrow_check::nll::constraints::graph::Reverse as rustc_mir::borrow_check::nll::constraints::graph::ConstraintGraphDirecton>::start_region
  18: rustc::ty::print::pretty::<impl core::fmt::Display for rustc::ty::subst::GenericArg>::fmt
  19: rustc::middle::weak_lang_items::<impl rustc::ty::context::TyCtxt>::is_weak_lang_item
  20: rustc::dep_graph::graph::DepGraph::assert_ignored
  21: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::try_print_query_stack
  22: rustc::ty::<impl rustc::ty::context::TyCtxt>::instance_mir
  23: rustc_mir::monomorphize::collector::collect_crate_mono_items
  24: rustc_mir::monomorphize::collector::collect_crate_mono_items
  25: <rustc_mir::interpret::snapshot::InterpSnapshot as core::cmp::PartialEq>::eq
  26: <rustc_mir::interpret::intrinsics::type_name::AbsolutePathPrinter as core::fmt::Write>::write_str
  27: rustc_mir::monomorphize::collector::collect_crate_mono_items
  28: <rustc_mir::interpret::intrinsics::type_name::AbsolutePathPrinter as core::fmt::Write>::write_str
  29: rustc_mir::monomorphize::partitioning::compute_codegen_unit_name
  30: <rustc_codegen_llvm::ModuleLlvm as core::ops::drop::Drop>::drop
  31: <rustc_codegen_llvm::back::lto::ThinBuffer as core::ops::drop::Drop>::drop
  32: <rustc_codegen_llvm::debuginfo::metadata::MemberDescription as core::fmt::Debug>::fmt
  33: <rustc_codegen_llvm::debuginfo::metadata::MemberDescription as core::fmt::Debug>::fmt
  34: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  35: <rustc_interface::util::ReplaceBodyWithLoop as syntax::mut_visit::MutVisitor>::visit_mac
  36: <env_logger::filter::inner::Filter as core::fmt::Debug>::fmt
  37: rustc_interface::passes::BoxedResolver::complete
  38: rustc_interface::interface::Compiler::output_file
  39: rustc_interface::passes::BoxedResolver::to_expansion_result
  40: rustc_interface::queries::<impl rustc_interface::interface::Compiler>::ongoing_codegen
  41: <syntax_pos::symbol::LocalInternedString as core::fmt::Display>::fmt
  42: <rustc_driver::Compilation as core::fmt::Debug>::fmt
  43: <syntax_pos::symbol::LocalInternedString as core::fmt::Display>::fmt
  44: <syntax_pos::symbol::LocalInternedString as core::fmt::Display>::fmt
  45: _rust_maybe_catch_panic
  46: <syntax_pos::symbol::LocalInternedString as core::fmt::Display>::fmt
  47: ZN244_$LT$std..error..$LT$impl$u20$core..convert..From$LT$alloc..string..String$GT$$u20$for$u20$alloc..boxed..Box$LT$dyn$u20$std..error..Error$u2b$core..marker..Send$u2b$core..marker..Sync$GT$$GT$..from..StringError$u20$as$u20$core..fmt..Display$GT$3fmt17
  48: std::sys::windows::thread::Thread::new
  49: BaseThreadInitThunk
  50: RtlUserThreadStart
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

[Centril]

In 1.37.0 this used to say:

error[E0391]: cycle detected when processing `test::{{opaque}}#0`
 --> <source>:7:14
  |
7 | fn test() -> impl TraitA {
  |              ^^^^^^^^^^^
  |

note: ...which requires processing `test`...
 --> <source>:7:26
  |
7 |   fn test() -> impl TraitA {
  |  __________________________^
8 | |     test()
9 | | }
  | |_^
  = note: ...which again requires processing `test::{{opaque}}#0`, completing the cycle

note: cycle used when checking item types in top-level module
 --> <source>:3:1
  |
3 | fn main() {
  | ^^^^^^^^^

error[E0720]: opaque type expands to a recursive type
 --> <source>:7:14
  |
7 | fn test() -> impl TraitA {
  |              ^^^^^^^^^^^ expands to self-referential type
  |
  = note: type resolves to itself


error: aborting due to 2 previous errors

[Centril]

Suggested prio: P-high.

cc @pnkfelix @matthewjasper

[nikomatsakis]

I'm tagging this as E-needs-bisection -- it'd be good to know exactly what broke this!

[lqd]

Even if #65777 will likely fix this, here's the bisection: a5e7bb3, which is the #63043 rollup of:

• Fix cycle error with existential types #62423 (Fix cycle error with existential types)
• Cleanup save-analysis JsonDumper #62979 (Cleanup save-analysis JsonDumper)
• Don't access a static just for its size and alignment #62982 (Don't access a static just for its size and alignment)
• add repr(transparent) to IoSliceMut where missing #63013 (add repr(transparent) to IoSliceMut where missing)
• Stop bare trait lint applying to macro call sites #63014 (Stop bare trait lint applying to macro call sites)
• Add lib section to rustc_lexer's Cargo.toml #63036 (Add lib section to rustc_lexer's Cargo.toml)

where #62423 seems the most probable candidate, as this issue's code used to produce a cycle error indeed before this PR.