ICE: "unexpected predicate" when using multiple lifetime parameters in existential type

[allen-marshall]

I have encountered an internal compiler error using Rust 1.32.0 nightly. It seems to occur when using multiple lifetime parameters in an existential type.

I can reproduce the error by using the following code in lib.rs. I am not sure if this code should compile or not (I am new to Rust and trying to learn how existential types work), but I know it shouldn't cause an ICE. (The Clone trait bound here is just a placeholder to keep the example simple. The ICE seems to still happen if Clone is replaced by any other trait bound.)

#![feature(existential_type)]

existential type ExType<'a, 'b: 'a>: Clone;

fn new_ex_type<'a, 'b: 'a>() -> ExType<'a, 'b> {
    0usize
}

The output of cargo build --verbose is as follows:

   Compiling rusttest v0.1.0 (/home/allen/IdeaProjects/rusttest)
     Running `rustc --edition=2018 --crate-name rusttest src/lib.rs --color always --crate-type lib --emit=dep-info,link -C debuginfo=2 -C metadata=0e629ce91b3e30a4 -C extra-filename=-0e629ce91b3e30a4 --out-dir /home/allen/IdeaProjects/rusttest/target/debug/deps -C incremental=/home/allen/IdeaProjects/rusttest/target/debug/incremental -L dependency=/home/allen/IdeaProjects/rusttest/target/debug/deps`
error: internal compiler error: librustc_privacy/lib.rs:971: unexpected predicate: Binder(OutlivesPredicate(ReEarlyBound(1, 'b), ReEarlyBound(0, 'a)))

thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:600:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at libstd/sys_common/backtrace.rs:59
             at libstd/panicking.rs:211
   3: std::panicking::default_hook
             at libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at libstd/panicking.rs:480
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::bug
   8: rustc::util::bug::opt_span_bug_fmt::{{closure}}
   9: rustc::ty::context::tls::with_opt::{{closure}}
  10: rustc::ty::context::tls::with_context_opt
  11: rustc::ty::context::tls::with_opt
  12: rustc::util::bug::opt_span_bug_fmt
  13: rustc::util::bug::bug_fmt
  14: <rustc_privacy::TypePrivacyVisitor<'a, 'tcx> as rustc::ty::fold::TypeVisitor<'tcx>>::visit_ty
  15: <rustc_privacy::TypePrivacyVisitor<'a, 'tcx> as rustc::ty::fold::TypeVisitor<'tcx>>::visit_ty
  16: <rustc_privacy::TypePrivacyVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_ty
  17: rustc::hir::intravisit::walk_item
  18: <rustc_privacy::TypePrivacyVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  19: rustc_privacy::privacy_access_levels
  20: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::privacy_access_levels<'tcx>>::compute
  21: rustc::ty::context::tls::with_context
  22: rustc::dep_graph::graph::DepGraph::with_task_impl
  23: rustc::ty::context::tls::with_related_context
  24: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  25: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  26: rustc::util::common::time
  27: rustc::ty::context::tls::enter_context
  28: <std::thread::local::LocalKey<T>>::with
  29: rustc::ty::context::TyCtxt::create_and_enter
  30: rustc_driver::driver::compile_input
  31: rustc_driver::run_compiler_with_pool
  32: <scoped_tls::ScopedKey<T>>::set
  33: rustc_driver::run_compiler
  34: syntax::with_globals
  35: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  36: rustc_driver::run
  37: rustc_driver::main
  38: std::rt::lang_start::{{closure}}
  39: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  40: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  41: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:398
             at libstd/rt.rs:58
  42: main
  43: __libc_start_main
  44: <unknown>
query stack during panic:
#0 [privacy_access_levels] privacy access levels
end of query stack
error: aborting due to previous error


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 1.32.0-nightly (ca79ecd69 2018-11-11) running on x86_64-unknown-linux-gnu

note: compiler flags: -C debuginfo=2 -C incremental --crate-type lib

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

error: Could not compile `rusttest`.

Caused by:
  process didn't exit successfully: `rustc --edition=2018 --crate-name rusttest src/lib.rs --color always --crate-type lib --emit=dep-info,link -C debuginfo=2 -C metadata=0e629ce91b3e30a4 -C extra-filename=-0e629ce91b3e30a4 --out-dir /home/allen/IdeaProjects/rusttest/target/debug/deps -C incremental=/home/allen/IdeaProjects/rusttest/target/debug/incremental -L dependency=/home/allen/IdeaProjects/rusttest/target/debug/deps` (exit code: 101)

If I change the code to only use one lifetime parameter as follows, the ICE doesn't happen and the code compiles successfully:

#![feature(existential_type)]

existential type ExType<'a>: Clone;

fn new_ex_type<'a>() -> ExType<'a> {
    0usize
}

rustc --version --verbose on my system yields:

rustc 1.32.0-nightly (ca79ecd69 2018-11-11)
binary: rustc
commit-hash: ca79ecd6940e30d4b2466bf378632efcdf5745c7
commit-date: 2018-11-11
host: x86_64-unknown-linux-gnu
release: 1.32.0-nightly
LLVM version: 8.0

[oli-obk]

Whoops. I couldn't produce this variant back when I wrote the impl. seems it was as easy as that. I believe we can just make this a nop as privacy is irrelevant for lifetime bounds