Compiler panics when using trait alias

[realcr]

I tried to compile the following code:

use core::pin::Pin;
use futures::channel::mpsc;
use futures::Future;

pub type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;
pub type ConnPair<SendItem, RecvItem> = (mpsc::Sender<SendItem>,mpsc::Receiver<RecvItem>);

/// ...

/// Transform a connection into another connection
pub trait ConnTransform {
    type OldSendItem;
    type OldRecvItem;
    type NewSendItem;
    type NewRecvItem;
    type Arg;

    fn transform(&mut self, arg: Self::Arg, conn_pair: ConnPair<Self::OldSendItem, Self::OldRecvItem>) 
        -> BoxFuture<'_, Option<ConnPair<Self::NewSendItem, Self::NewRecvItem>>>;
}

pub trait BytesConnTransform<Arg> = ConnTransform<OldSendItem=Vec<u8>,OldRecvItem=Vec<u8>,
                                                 NewSendItem=Vec<u8>,NewRecvItem=Vec<u8>,
                                                 Arg=Arg>;

Compilation resulted in the following panic:

$ RUST_BACKTRACE=1 cargo test -p offst-common
   Compiling offst-common v0.1.0 (/home/real/projects/d/offst/components/common)
warning: unused import: `FutureExt`
 --> components/common/src/dummy_connector.rs:3:23
  |
3 | use futures::{Future, FutureExt, SinkExt};
  |                       ^^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

error: internal compiler error: src/librustc_metadata/encoder.rs:1158: cannot encode info for item Item { name: BytesConnTransform, id: NodeId(2464), hir_id: HirId { owner: DefIndex(0:155), local_id: 0 }, attrs: [], node: TraitAlias(Generics { params: [GenericParam { id: NodeId(2465), name: Plain(Arg#0), attrs: [], bounds: [], span: components/common/src/conn.rs:41:30: 41:33, pure_wrt_drop: false, kind: Type { default: None, synthetic: None } }], where_clause: WhereClause { id: NodeId(2466), predicates: [] }, span: components/common/src/conn.rs:41:29: 41:34 }, [Trait(PolyTraitRef { bound_generic_params: [], trait_ref: TraitRef { path: path(ConnTransform<OldSendItem = Vec<u8>, OldRecvItem = Vec<u8>, NewSendItem =
Vec<u8>, NewRecvItem = Vec<u8>, Arg = Arg>), ref_id: NodeId(2467), hir_ref_id: HirId { owner: DefIndex(0:155), local_id: 27 } }, span: components/common/src/conn.rs:41:37: 43:58 }, None)]), vis: Spanned { node: Public, span: components/common/src/conn.rs:41:1: 41:4 }, span: components/common/src/conn.rs:41:1: 43:59 }

thread 'main' panicked at 'Box<Any>', src/librustc_errors/lib.rs:600:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:211
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/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_metadata::encoder::<impl rustc_metadata::isolated_encoder::IsolatedEncoder<'a, 'b, 'tcx>>::encode_info_for_item
  15: rustc::ty::context::tls::with_context
  16: <rustc_metadata::encoder::EncodeVisitor<'a, 'b, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  17: rustc::hir::Crate::visit_all_item_likes
  18: rustc_metadata::encoder::encode_metadata
  19: rustc_metadata::cstore_impl::<impl rustc::middle::cstore::CrateStore for rustc_metadata::cstore::CStore>::encode_metadata
  20: rustc::ty::context::TyCtxt::encode_metadata
  21: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::ExtraBackendMethods>::write_metadata
  22: rustc::util::common::time
  23: rustc_codegen_ssa::base::codegen_crate
  24: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  25: rustc::util::common::time
  26: rustc_driver::driver::phase_4_codegen
  27: rustc_driver::driver::compile_input::{{closure}}
  28: rustc::ty::context::tls::enter_context
  29: <std::thread::local::LocalKey<T>>::with
  30: rustc::ty::context::TyCtxt::create_and_enter
  31: rustc_driver::driver::compile_input
  32: rustc_driver::run_compiler_with_pool
  33: <scoped_tls::ScopedKey<T>>::set
  34: rustc_driver::run_compiler
  35: syntax::with_globals
  36: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:102
  37: rustc_driver::run
  38: rustc_driver::main
  39: std::rt::lang_start::{{closure}}
  40: std::panicking::try::do_call
             at src/libstd/rt.rs:59
             at src/libstd/panicking.rs:310
  41: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:102
  42: std::rt::lang_start_internal
             at src/libstd/panicking.rs:289
             at src/libstd/panic.rs:398
             at src/libstd/rt.rs:58
  43: main
  44: __libc_start_main
  45: <unknown>
query stack during panic:
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 (6bfb46e4a 2018-11-26) 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 `offst-common`.

Everything seems to compile correctly if I remove the trait alias part:

pub trait BytesConnTransform<Arg> = ...

Thank you for making my life great by developing Rust!

[hellow554]

Can you please provide a full MCVE? If I copy your code it won't compile due to missing use and features.

Just to clarify: I really love, that you opened the bug and help making rust a better language! Just for easier fixing and reproducing it is important to create short code snippets that will trigger the error.

There is huge test code base, where (nearly) each error that will get reported gets an own testcase so it won't happen in the future again. That's why it's so important to be able to reproduce the error and make the testcase as small as possible.

As said again, thanks for reporting it! ❤️

[hellow554]

Just as an example, this also triggers the error:

#![feature(trait_alias)]

pub trait Foo {}
pub trait FooAlias = Foo;

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error: internal compiler error: src/librustc_metadata/encoder.rs:1158: cannot encode info for item Item { name: FooAlias, id: NodeId(13), hir_id: HirId { owner: DefIndex(0:4), local_id: 0 }, attrs: [], node: TraitAlias(Generics { params: [], where_clause: WhereClause { id: NodeId(14), predicates: [] }, span: src/lib.rs:1:1: 1:1 }, [Trait(PolyTraitRef { bound_generic_params: [], trait_ref: TraitRef { path: path(Foo), ref_id: NodeId(15), hir_ref_id: HirId { owner: DefIndex(0:4), local_id: 3 } }, span: src/lib.rs:4:22: 4:25 }, None)]), vis: Spanned { node: Public, span: src/lib.rs:4:1: 4:4 }, span: src/lib.rs:4:1: 4:26 }

thread 'main' panicked at 'Box<Any>', src/librustc_errors/lib.rs:600:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:211
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:495
   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_metadata::encoder::<impl rustc_metadata::isolated_encoder::IsolatedEncoder<'a, 'b, 'tcx>>::encode_info_for_item
  15: rustc::ty::context::tls::with_context
  16: <rustc_metadata::encoder::EncodeVisitor<'a, 'b, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  17: rustc::hir::Crate::visit_all_item_likes
  18: rustc_metadata::encoder::encode_metadata
  19: rustc_metadata::cstore_impl::<impl rustc::middle::cstore::CrateStore for rustc_metadata::cstore::CStore>::encode_metadata
  20: rustc::ty::context::TyCtxt::encode_metadata
  21: rustc_codegen_llvm::base::write_metadata
  22: rustc::util::common::time
  23: rustc_codegen_ssa::base::codegen_crate
  24: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  25: rustc::util::common::time
  26: rustc_driver::driver::phase_4_codegen
  27: rustc_driver::driver::compile_input::{{closure}}
  28: rustc::ty::context::tls::enter_context
  29: <std::thread::local::LocalKey<T>>::with
  30: rustc::ty::context::TyCtxt::create_and_enter
  31: rustc_driver::driver::compile_input
  32: rustc_driver::run_compiler_with_pool
  33: <scoped_tls::ScopedKey<T>>::set
  34: rustc_driver::run_compiler
  35: rustc_driver::monitor::{{closure}}
  36: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:102
  37: rustc_driver::run
  38: rustc_driver::main
  39: std::rt::lang_start::{{closure}}
  40: std::panicking::try::do_call
             at src/libstd/rt.rs:59
             at src/libstd/panicking.rs:310
  41: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:102
  42: std::rt::lang_start_internal
             at src/libstd/panicking.rs:289
             at src/libstd/panic.rs:398
             at src/libstd/rt.rs:58
  43: main
  44: __libc_start_main
  45: <unknown>
query stack during panic:
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 (14997d56a 2018-12-05) 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.

[realcr]

Hi, sorry for the hasty report! I think that I managed to have a self contained example of the compiler crash.

My lib.rs:

#![feature(futures_api, pin, async_await, await_macro, arbitrary_self_types)]
#![feature(trait_alias)]

use core::pin::Pin;
use futures::channel::mpsc;
use futures::{future, Future};

pub type BoxFuture<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;

pub type ConnPair<SendItem, RecvItem> = (mpsc::Sender<SendItem>,mpsc::Receiver<RecvItem>);

/// Transform a connection into another connection
pub trait ConnTransform {
    type OldSendItem;
    type OldRecvItem;
    type NewSendItem;
    type NewRecvItem;
    type Arg;

    fn transform(&mut self, arg: Self::Arg, conn_pair: ConnPair<Self::OldSendItem, Self::OldRecvItem>) 
        -> BoxFuture<'_, Option<ConnPair<Self::NewSendItem, Self::NewRecvItem>>>;
}

pub trait BytesConnTransform<Arg> = ConnTransform<OldSendItem=Vec<u8>,OldRecvItem=Vec<u8>,
                                                 NewSendItem=Vec<u8>,NewRecvItem=Vec<u8>,
                                                 Arg=Arg>;

My Cargo.toml:

[package]
name = "offst-common"
version = "0.1.0"
authors = ["real <real@freedomlayer.org>"]
edition = "2018"

[dependencies]

futures-preview = "0.3.0-alpha.10"

[dev-dependencies]

Compilation attempt:

$ RUST_BACKTRACE=1 cargo build
   Compiling offst-common v0.1.0 (/home/real/temp/trait_alias_crash/common)
warning: unused import: `future`
 --> src/lib.rs:6:15
  |
6 | use futures::{future, Future};
  |               ^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

error: internal compiler error: src/librustc_metadata/encoder.rs:1158: cannot encode info for item Item { name: BytesConnTransform, id: NodeId(113), hir_id: HirId { owner: DefIndex(0:17), local_id: 0 }, attrs: [], node: TraitAlias(Generics { params: [GenericParam { id: NodeId(114), name: Plain(Arg#0), attrs: [], bounds: [], span: src/lib.rs:24:30: 24:33, pure_wrt_drop: false, kind: Type { default: None, synthetic: None } }], where_clause: WhereClause { id: NodeId(115), predicates: [] }, span: src/lib.rs:24:29: 24:34 }, [Trait(PolyTraitRef { bound_generic_params: [], trait_ref: TraitRef { path: path(ConnTransform<OldSendItem = Vec<u8>, OldRecvItem = Vec<u8>, NewSendItem =
Vec<u8>, NewRecvItem = Vec<u8>, Arg = Arg>), ref_id: NodeId(116), hir_ref_id: HirId { owner: DefIndex(0:17), local_id: 27 } }, span: src/lib.rs:24:37: 26:58 }, None)]), vis: Spanned { node: Public, span: src/lib.rs:24:1: 24:4 }, span: src/lib.rs:24:1: 26:59 }

thread 'main' panicked at 'Box<Any>', src/librustc_errors/lib.rs:600:9
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:211
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:227
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/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_metadata::encoder::<impl rustc_metadata::isolated_encoder::IsolatedEncoder<'a, 'b, 'tcx>>::encode_info_for_item
  15: rustc::ty::context::tls::with_context
  16: <rustc_metadata::encoder::EncodeVisitor<'a, 'b, 'tcx> as rustc::hir::intravisit::Visitor<'tcx>>::visit_item
  17: rustc::hir::Crate::visit_all_item_likes
  18: rustc_metadata::encoder::encode_metadata
  19: rustc_metadata::cstore_impl::<impl rustc::middle::cstore::CrateStore for rustc_metadata::cstore::CStore>::encode_metadata
  20: rustc::ty::context::TyCtxt::encode_metadata
  21: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::ExtraBackendMethods>::write_metadata
  22: rustc::util::common::time
  23: rustc_codegen_ssa::base::codegen_crate
  24: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  25: rustc::util::common::time
  26: rustc_driver::driver::phase_4_codegen
  27: rustc_driver::driver::compile_input::{{closure}}
  28: rustc::ty::context::tls::enter_context
  29: <std::thread::local::LocalKey<T>>::with
  30: rustc::ty::context::TyCtxt::create_and_enter
  31: rustc_driver::driver::compile_input
  32: rustc_driver::run_compiler_with_pool
  33: <scoped_tls::ScopedKey<T>>::set
  34: rustc_driver::run_compiler
  35: syntax::with_globals
  36: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:102
  37: rustc_driver::run
  38: rustc_driver::main
  39: std::rt::lang_start::{{closure}}
  40: std::panicking::try::do_call
             at src/libstd/rt.rs:59
             at src/libstd/panicking.rs:310
  41: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:102
  42: std::rt::lang_start_internal
             at src/libstd/panicking.rs:289
             at src/libstd/panic.rs:398
             at src/libstd/rt.rs:58
  43: main
  44: __libc_start_main
  45: <unknown>
query stack during panic:
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 (6bfb46e4a 2018-11-26) 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 `offst-common`.

If you can not reproduce this please contact me, I might be able to put a repository with this code.
Thank you for your work!

[estebank]

This is a regression, trying the shortened case out in rustc 1.32.0-nightly (15d7704 2018-11-06) yields no error.