Unconstrained lifetimes allowed with member_constraints/type_alias_impl_trait

Code

#![feature(member_constraints)]
#![feature(type_alias_impl_trait)]

pub trait A {
  type B;
  fn f(&self) -> Self::B;
}
impl<'a, 'b> A for () {
  type B = impl core::fmt::Debug;

  fn f(&self) -> Self::B {}
}

fn main() {}

Playground

Error output

   Compiling playground v0.0.1 (/playground)
thread 'rustc' panicked at 'assertion failed: self.universal_regions.is_universal_region(shorter)', src/librustc_mir/borrow_check/type_check/free_region_relations.rs:377:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I had first seen:

thread 'rustc' panicked at 'assertion failed: self.universal_regions.is_universal_region(longer)', src/librustc_mir/borrow_check/type_check/free_region_relations.rs:379:9

Removing the second lifetime (impl<'a> A for () {) gives the correct error,

error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
 --> src/main.rs:8:6
  |
8 | impl<'a> A for () {
  |      ^^ unconstrained lifetime parameter

Additionally, disabling either type_alias_impl_trait or member_constraints still panics (and prompts to enable those features), except in stable where it prompts to enable the features, but does not panic.

Perhaps similar to #57200

Backtrace

``` Compiling playground v0.0.1 (/playground) thread 'rustc' panicked at 'assertion failed: self.universal_regions.is_universal_region(shorter)', src/librustc_mir/borrow_check/type_check/free_region_relations.rs:377:9 stack backtrace: 0: backtrace::backtrace::libunwind::trace at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/libunwind.rs:86 1: backtrace::backtrace::trace_unsynchronized at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.46/src/backtrace/mod.rs:66 2: std::sys_common::backtrace::_print_fmt at src/libstd/sys_common/backtrace.rs:78 3: ::fmt at src/libstd/sys_common/backtrace.rs:59 4: core::fmt::write at src/libcore/fmt/mod.rs:1117 5: std::io::Write::write_fmt at src/libstd/io/mod.rs:1508 6: std::sys_common::backtrace::_print at src/libstd/sys_common/backtrace.rs:62 7: std::sys_common::backtrace::print at src/libstd/sys_common/backtrace.rs:49 8: std::panicking::default_hook::{{closure}} at src/libstd/panicking.rs:198 9: std::panicking::default_hook at src/libstd/panicking.rs:217 10: rustc_driver::report_ice 11: std::panicking::rust_panic_with_hook at src/libstd/panicking.rs:530 12: std::panicking::begin_panic 13: ::sub_free_regions 14: rustc_infer::infer::InferCtxt::commit_if_ok 15: rustc_mir::borrow_check::type_check::TypeChecker::eq_opaque_type_and_type 16: rustc_mir::borrow_check::type_check::type_check 17: rustc_mir::borrow_check::nll::compute_regions 18: rustc_mir::borrow_check::do_mir_borrowck 19: rustc_infer::infer::InferCtxtBuilder::enter 20: rustc_mir::borrow_check::mir_borrowck 21: core::ops::function::FnOnce::call_once 22: rustc_middle::ty::query:: for rustc_middle::ty::query::queries::mir_borrowck>::compute 23: rustc_query_system::dep_graph::graph::DepGraph::with_task_impl 24: rustc_data_structures::stack::ensure_sufficient_stack 25: rustc_query_system::query::plumbing::get_query_impl 26: rustc_typeck::collect::type_of::find_opaque_ty_constraints::ConstraintLocator::check 27: rustc_hir::intravisit::walk_impl_item_ref 28: rustc_hir::intravisit::walk_item 29: rustc_typeck::collect::type_of::type_of 30: rustc_middle::ty::query:: for rustc_middle::ty::query::queries::type_of>::compute 31: rustc_query_system::dep_graph::graph::DepGraph::with_task_impl 32: rustc_data_structures::stack::ensure_sufficient_stack 33: rustc_query_system::query::plumbing::get_query_impl 34: rustc_query_system::query::plumbing::ensure_query_impl 35: ::visit_item 36: rustc_middle::hir::map::Map::visit_item_likes_in_module 37: rustc_typeck::collect::collect_mod_item_types 38: rustc_middle::ty::query:: for rustc_middle::ty::query::queries::collect_mod_item_types>::compute 39: rustc_query_system::dep_graph::graph::DepGraph::with_task_impl 40: rustc_data_structures::stack::ensure_sufficient_stack 41: rustc_query_system::query::plumbing::get_query_impl 42: rustc_query_system::query::plumbing::ensure_query_impl 43: rustc_session::session::Session::track_errors 44: rustc_typeck::check_crate 45: rustc_interface::passes::analysis 46: rustc_middle::ty::query:: for rustc_middle::ty::query::queries::analysis>::compute 47: rustc_query_system::dep_graph::graph::DepGraph::with_task_impl 48: rustc_data_structures::stack::ensure_sufficient_stack 49: rustc_query_system::query::plumbing::get_query_impl 50: rustc_middle::ty::context::tls::enter_global 51: rustc_interface::queries::::enter 52: rustc_span::with_source_map 53: rustc_interface::interface::create_compiler_and_run 54: scoped_tls::ScopedKey::set note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose 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 1.47.0-nightly (5ef299eb9 2020-07-24) running on x86_64-unknown-linux-gnu note: compiler flags: -C embed-bitcode=no -C codegen-units=1 -C debuginfo=2 --crate-type bin note: some of the compiler flags provided by cargo are hidden query stack during panic: #0 [mir_borrowck] borrow-checking `<() as A>::f` #1 [type_of] computing type of `<() as A>::B::{{opaque}}#0` #2 [collect_mod_item_types] collecting item types in top-level module #3 [analysis] running analysis passes on this crate end of query stack error: could not compile `playground`. To learn more, run the command again with --verbose. ```

rust-lang / rust

Unconstrained lifetimes allowed with member_constraints/type_alias_impl_trait #74761

Code

Meta

Error output