ICE when inferring the type of existential type with trait alias bounds

[matprec]

Minimised, reproducible example:

#![feature(trait_alias)]
#![feature(existential_type)]

trait Foo {
    type Bar: Baz<Self, Self>;

    fn bar(&self) -> Self::Bar;
}

struct X;

impl Foo for X {
    existential type Bar: Baz<Self, Self>;

    fn bar(&self) -> Self::Bar {
        |x| x
    }
}

trait Baz<A, B> = Fn(&A) -> &B;

See playground

Backtrace

   Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
  --> src/lib.rs:13:5
   |
13 |     existential type Bar: Baz<Self, Self>;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected type `std::ops::FnOnce<(&X,)>`
              found type `std::ops::FnOnce<(&X,)>`

thread 'rustc' panicked at 'assertion failed: !ty.needs_infer() && !ty.has_placeholders()', src/librustc_typeck/check/writeback.rs:119:9
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:482
   6: std::panicking::begin_panic
   7: rustc_typeck::check::writeback::WritebackCx::visit_node_id
   8: <rustc_typeck::check::writeback::WritebackCx<'cx, 'gcx, 'tcx> as rustc::hir::intravisit::Visitor<'gcx>>::visit_expr
   9: rustc_typeck::check::writeback::<impl rustc_typeck::check::FnCtxt<'a, 'gcx, 'tcx>>::resolve_type_vars_in_body
  10: rustc::ty::context::GlobalCtxt::enter_local
  11: rustc_typeck::check::typeck_tables_of
  12: rustc::ty::query::__query_compute::typeck_tables_of
  13: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_tables_of<'tcx>>::compute
  14: rustc::dep_graph::graph::DepGraph::with_task_impl
  15: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_with
  16: rustc_typeck::collect::find_existential_constraints::ConstraintLocator::check
  17: rustc::hir::intravisit::Visitor::visit_nested_impl_item
  18: rustc::hir::intravisit::walk_item
  19: rustc_typeck::collect::find_existential_constraints
  20: rustc_typeck::collect::type_of
  21: rustc::ty::query::__query_compute::type_of
  22: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::type_of<'tcx>>::compute
  23: rustc::dep_graph::graph::DepGraph::with_task_impl
  24: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_with
  25: <rustc_typeck::collect::CollectItemTypesVisitor<'a, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_impl_item
  26: rustc::hir::map::Map::visit_item_likes_in_module
  27: rustc_typeck::collect::collect_mod_item_types
  28: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::collect_mod_item_types<'tcx>>::compute
  29: rustc::dep_graph::graph::DepGraph::with_task_impl
  30: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::try_get_with
  31: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::ensure_query
  32: rustc_typeck::collect::collect_item_types
  33: rustc::util::common::time
  34: rustc_typeck::check_crate
  35: <std::thread::local::LocalKey<T>>::with
  36: rustc::ty::context::TyCtxt::create_and_enter
  37: rustc_driver::driver::compile_input
  38: <scoped_tls::ScopedKey<T>>::set
  39: rustc_driver::run_compiler
  40: <scoped_tls::ScopedKey<T>>::set
query stack during panic:
#0 [typeck_tables_of] processing `<X as Foo>::bar`
#1 [type_of] processing `<X as Foo>::Bar`
#2 [collect_mod_item_types] collecting item types in top-level module
end of query stack
error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.

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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.33.0-nightly (2fadb0a16 2019-01-13) running on x86_64-unknown-linux-gnu

note: compiler flags: -C codegen-units=1 -C debuginfo=2 --crate-type lib

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

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

[vorner]

Hello

I'm getting the same ICE, but without any unstable features. I don't have a minimized example, though, but it may be another data point when searching for the problem.

It's this commit: https://github.com/vorner/spirit/tree/rust-crash. The commit happens with

cargo run --package spirit-tokio --example hws-tokio

The code is not correct, it wouldn't compile even if the compiler didn't panic.

rustc 1.33.0-nightly (daa53a52a 2019-01-17)

It doesn't happen on stable, but I guess it's because it bails out sooner on closures taking references:

error[E0631]: type mismatch in closure arguments
   --> spirit-tokio/examples/hws-tokio.rs:100:67
    |
96  |     let init = spirit_tokio::handlers::HandleListenerInit(|_, _| (), |conn, _| handle_connection(conn));
    |                                                                      --------------------------------- found signature of `fn(spirit_tokio::net::limits::LimitedConn<tokio::net::TcpStream>, _) -> _`
...
100 |         .with(Pipeline::new("listen").extract_cfg(Config::listen).transform(init).check())
    |                                                                   ^^^^^^^^^ expected signature of `for<'r> fn(spirit_tokio::net::limits::LimitedConn<tokio::net::TcpStream>, &'r mut _) -> _`
    |

[Aaron1011]

This still produces an error on the latest nightly (not sure if it's supposed to compile or not), but no longer ICEs.