Rust: ICE when existential type is exported from one crate and used in another

[Arnavion]

rustc 1.30.0-dev
binary: rustc
commit-hash: b5590423e6ceb048dd7d792382e960d66b7615d2
commit-date: 2018-08-16
host: x86_64-pc-windows-msvc
release: 1.30.0-dev
LLVM version: 7.0

a/Cargo.toml

[package]
name = "a"
version = "0.1.0"

a/src/lib.rs

#![feature(existential_type)]

pub existential type Foo: std::fmt::Debug;

pub fn foo() -> Foo {
    5
}

b/Cargo.toml

[package]
name = "b"
version = "0.1.0"

[dependencies]
a = { path = "../a" }

b/src/main.rs

extern crate a;

fn main() {
    let _ = a::foo();
}

ICE

thread 'main' panicked at 'librustc_resolve\build_reduced_graph.rs:741: unexpected definition: Existential(DefId(9/0:3))', librustc\util\bug.rs:47:26
stack backtrace:
   0: std::sys::windows::backtrace::unwind_backtrace
             at C:\Stuff\Sources\rustsrc\src\libstd\sys\windows\backtrace\mod.rs:95
   1: std::sys_common::backtrace::print
             at C:\Stuff\Sources\rustsrc\src\libstd\sys_common\backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:211
   3: std::panicking::default_hook
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:227
   4: rustc::util::common::panic_hook
             at C:\Stuff\Sources\rustsrc\src\librustc\util\common.rs:51
   5: std::panicking::rust_panic_with_hook
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:479
   6: std::panicking::begin_panic<alloc::string::String>
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:409
   7: rustc::util::bug::opt_span_bug_fmt::{{closure}}<syntax_pos::span_encoding::Span>
             at C:\Stuff\Sources\rustsrc\src\librustc\util\bug.rs:47
   8: rustc::ty::context::tls::with_opt::{{closure}}<closure,!>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:2008
   9: rustc::ty::context::tls::with_context_opt<closure,!>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1939
  10: rustc::ty::context::tls::with_opt<closure,!>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:2008
  11: rustc::util::bug::opt_span_bug_fmt<syntax_pos::span_encoding::Span>
             at C:\Stuff\Sources\rustsrc\src\librustc\util\bug.rs:42
  12: rustc::util::bug::bug_fmt
             at C:\Stuff\Sources\rustsrc\src\librustc\util\bug.rs:22
  13: rustc_resolve::Resolver::populate_module_if_necessary
             at C:\Stuff\Sources\rustsrc\src\librustc_resolve\build_reduced_graph.rs:813
  14: rustc_resolve::Resolver::build_reduced_graph_for_item
             at C:\Stuff\Sources\rustsrc\src\librustc_resolve\build_reduced_graph.rs:441
  15: rustc_resolve::build_reduced_graph::{{impl}}::visit_item
             at C:\Stuff\Sources\rustsrc\src\librustc_resolve\build_reduced_graph.rs:991
  16: syntax::visit::walk_item<rustc_resolve::build_reduced_graph::BuildReducedGraphVisitor>
             at C:\Stuff\Sources\rustsrc\src\libsyntax\visit.rs:245
  17: rustc_resolve::build_reduced_graph::{{impl}}::visit_item
             at C:\Stuff\Sources\rustsrc\src\librustc_resolve\build_reduced_graph.rs:992
  18: syntax::ext::expand::AstFragment::visit_with<rustc_resolve::build_reduced_graph::BuildReducedGraphVisitor>
             at C:\Stuff\Sources\rustsrc\src\libsyntax\ext\expand.rs:120
  19: rustc_resolve::macros::{{impl}}::visit_ast_fragment_with_placeholders
             at C:\Stuff\Sources\rustsrc\src\librustc_resolve\macros.rs:209
  20: syntax::ext::expand::MacroExpander::collect_invocations
             at C:\Stuff\Sources\rustsrc\src\libsyntax\ext\expand.rs:478
  21: syntax::ext::expand::MacroExpander::expand_fragment
             at C:\Stuff\Sources\rustsrc\src\libsyntax\ext\expand.rs:318
  22: syntax::ext::expand::MacroExpander::expand_crate
             at C:\Stuff\Sources\rustsrc\src\libsyntax\ext\expand.rs:292
  23: rustc_driver::driver::phase_2_configure_and_expand_inner::{{closure}}<closure>
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\driver.rs:1006
  24: rustc::util::common::time<syntax::ast::Crate,closure>
             at C:\Stuff\Sources\rustsrc\src\librustc\util\common.rs:157
  25: rustc_driver::driver::phase_2_configure_and_expand<closure>
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\driver.rs:764
  26: rustc_driver::driver::compile_input
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\driver.rs:184
  27: rustc_driver::run_compiler_with_pool
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\lib.rs:562
  28: scoped_tls::ScopedKey<syntax::Globals>::set<syntax::Globals,closure,(core::result::Result<(), rustc::session::CompileIncomplete>, core::option::Option<rustc::session::Session>)>
             at C:\Users\Arnavion\.cargo\registry\src\github.com-1ecc6299db9ec823\scoped-tls-0.1.2\src\lib.rs:155
  29: std::panic::{{impl}}::call_once<(),closure>
             at C:\Stuff\Sources\rustsrc\src\libstd\panic.rs:313
  30: panic_unwind::__rust_maybe_catch_panic
             at C:\Stuff\Sources\rustsrc\src\libpanic_unwind\lib.rs:102
  31: std::panicking::try<(),std::panic::AssertUnwindSafe<closure>>
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:289
  32: rustc_driver::run<closure>
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\lib.rs:187
  33: rustc_driver::main
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\lib.rs:1737
  34: std::rt::lang_start::{{closure}}<()>
             at C:\Stuff\Sources\rustsrc\src\libstd\rt.rs:74
  35: std::panicking::try::do_call<closure,i32>
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:310
  36: panic_unwind::__rust_maybe_catch_panic
             at C:\Stuff\Sources\rustsrc\src\libpanic_unwind\lib.rs:102
  37: std::panicking::try<i32,closure>
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:289
  38: std::rt::lang_start_internal
             at C:\Stuff\Sources\rustsrc\src\libstd\rt.rs:58
  39: main
  40: __scrt_common_main_seh
             at f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:283
  41: BaseThreadInitThunk
  42: RtlUserThreadStart
query stack during panic:
end of query stack

error: internal compiler error: unexpected panic

https://github.com/rust-lang/rust/blob/b5590423e6ceb048dd7d792382e960d66b7615d2/src/librustc_resolve/build_reduced_graph.rs#L741

Resolver::build_reduced_graph_for_external_crate_def needs an arm for Def::Existential ? @oli-obk

---

Edit: Easy playground repro

[oli-obk]

Oops. Yea. I assumed this would work since impl Trait works.

[Arnavion]

Is the fix just to have self.define(parent, ident, TypeNS, (def, vis, DUMMY_SP, expansion)); ? I added that and it passes the repro in the OP, but my real code triggers another ICE at:

https://github.com/rust-lang/rust/blob/b5590423e6ceb048dd7d792382e960d66b7615d2/src/librustc/middle/resolve_lifetime.rs#L630

where the as_local_node_id returns None

If you think the fix for this ICE is just that one self.define() call, I can start making a minimal repro for this second ICE too.

[oli-obk]

I think they are closely related. We should probably run

{
                            intravisit::walk_ty(self, ty);
                            return;
}

in case as_local_node_id returns None. A repro for that would be nice though. It should probably suffice to add some lifetimes to the existential type or its concrete type.

[Arnavion]

My real code's existential types do not have lifetimes, though they do have 'static bounds.

pub existential type GetResponse: Future<Output = crate::Result<crate::Mod>> + 'static;
pub existential type DownloadResponse: Stream<Item = crate::Result<crate::reqwest_async::Chunk>> + 'static;

Adding 'static bounds to the OP code did not repro the same crash.

The concrete type for the first one is compiler-created because it's the return type of an async block.

pub fn get(&self, mod_name: &factorio_mods_common::ModName) -> GetResponse {
    ...
    async {
        ....
    }
}

I believe this is sugar for std::future::GenFuture, so that should also not have any lifetimes.

The second one's concrete type is my own Stream impl that also does not have lifetimes.

It'll take me some time to get a repro because something is broken with the rustc I built locally (it started hitting #53469 even though it was fine before, not sure why). If you want you can find the real code at https://github.com/Arnavion/fac-rs/tree/3a6afed4. The existential types are in the factorio-mods-web crate and imported by the workspace root crate. It won't compile because I'm still in the process of migrating it to std::future, but atleast it shouldn't ICE.

[oli-obk]

factorio-mods-web

Rust + factorio? <3

It'll take me some time to get a repro because something is broken with the rustc I built locally

the issue you are hitting is that a delay_span_bug is hidden from you. I'm fixing that later today

[Arnavion]

@oli-obk The #53469 I started hitting was because of two different versions of libsyntax_pos in stage1 builds, as I described here. So with stage2 builds I don't have that problem,

So I've got a repro for the issue where as_local_node_id returns None. It happens when using the imported existential type in a struct or enum definition, like:

b/src/main.rs

extern crate a;

struct Bar(a::Foo);

fn main() {
}

ICE

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', libcore\option.rs:345:21
stack backtrace:
   0: std::sys::windows::backtrace::unwind_backtrace
             at C:\Stuff\Sources\rustsrc\src\libstd\sys\windows\backtrace\mod.rs:95
   1: std::sys_common::backtrace::print
             at C:\Stuff\Sources\rustsrc\src\libstd\sys_common\backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:211
   3: std::panicking::default_hook
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:227
   4: rustc::util::common::panic_hook
             at C:\Stuff\Sources\rustsrc\src\librustc\util\common.rs:51
   5: std::panicking::rust_panic_with_hook
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:479
   6: std::panicking::continue_panic_fmt
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:390
   7: std::panicking::rust_begin_panic
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:325
   8: core::panicking::panic_fmt
             at C:\Stuff\Sources\rustsrc\src\libcore\panicking.rs:77
   9: core::panicking::panic
             at C:\Stuff\Sources\rustsrc\src\libcore\panicking.rs:52
  10: rustc::middle::resolve_lifetime::{{impl}}::visit_ty
             at C:\Stuff\Sources\rustsrc\src\librustc\middle\resolve_lifetime.rs:626
  11: rustc::hir::intravisit::walk_item<rustc::middle::resolve_lifetime::LifetimeContext>
             at C:\Stuff\Sources\rustsrc\src\librustc\hir\intravisit.rs:539
  12: rustc::middle::resolve_lifetime::{{impl}}::visit_item
             at C:\Stuff\Sources\rustsrc\src\librustc\middle\resolve_lifetime.rs:539
  13: rustc::middle::resolve_lifetime::resolve_lifetimes
             at C:\Stuff\Sources\rustsrc\src\librustc\middle\resolve_lifetime.rs:392
  14: rustc::ty::query::{{impl}}::compute
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:820
  15: rustc::ty::context::tls::with_context<closure,alloc::rc::Rc<rustc::middle::privacy::AccessLevels<syntax::ast::NodeId>>>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1953
  16: rustc::dep_graph::graph::DepGraph::with_task_impl<rustc::ty::context::TyCtxt,rustc::hir::def_id::CrateNum,alloc::rc::Rc<rustc::middle::resolve_lifetime::ResolveLifetimes>>
             at C:\Stuff\Sources\rustsrc\src\librustc\dep_graph\graph.rs:266
  17: rustc::ty::context::tls::with_related_context<closure,(alloc::rc::Rc<rustc::middle::resolve_lifetime::ResolveLifetimes>, rustc::dep_graph::graph::DepNodeIndex)>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1964
  18: rustc::ty::context::TyCtxt::force_query_with_job<rustc::ty::query::queries::resolve_lifetimes>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:543
  19: rustc::ty::context::TyCtxt::get_query<rustc::ty::query::queries::resolve_lifetimes>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:640
  20: core::ops::function::FnOnce::call_once<closure,(rustc::ty::context::TyCtxt, rustc::hir::def_id::DefIndex)>
             at C:\Stuff\Sources\rustsrc\src\libcore\ops\function.rs:238
  21: rustc::ty::query::{{impl}}::compute
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:820
  22: rustc::ty::context::tls::with_context<closure,alloc::rc::Rc<rustc::middle::privacy::AccessLevels<syntax::ast::NodeId>>>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1953
  23: rustc::dep_graph::graph::DepGraph::with_task_impl<rustc::ty::context::TyCtxt,rustc::hir::def_id::DefIndex,core::option::Option<alloc::rc::Rc<std::collections::hash::map::HashMap<rustc::hir::ItemLocalId, alloc::rc::Rc<alloc::vec::Vec<rustc::middle::resolve_lifetime::Set1<rustc::middle::resolve_lifetime::Region>>>, core::hash::BuildHasherDefault<rustc_hash::FxHasher>>>>>
             at C:\Stuff\Sources\rustsrc\src\librustc\dep_graph\graph.rs:266
  24: rustc::ty::context::tls::with_related_context<closure,(core::option::Option<alloc::rc::Rc<std::collections::hash::map::HashMap<rustc::hir::ItemLocalId, alloc::rc::Rc<alloc::vec::Vec<rustc::middle::resolve_lifetime::Set1<rustc::middle::resolve_lifetime::Region>>>, core::hash::BuildHasherDefault<rustc_hash::FxHasher>>>>, rustc::dep_graph::graph::DepNodeIndex)>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1964
  25: rustc::ty::context::TyCtxt::force_query_with_job<rustc::ty::query::queries::object_lifetime_defaults_map>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:543
  26: rustc::ty::context::TyCtxt::get_query<rustc::ty::query::queries::object_lifetime_defaults_map>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:640
  27: rustc::ty::context::TyCtxt::object_lifetime_defaults
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:2787
  28: rustc_typeck::collect::generics_of
             at C:\Stuff\Sources\rustsrc\src\librustc_typeck\collect.rs:915
  29: rustc::ty::query::__query_compute::generics_of<closure,rustc::ty::Generics*>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:790
  30: rustc::ty::query::{{impl}}::compute
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:820
  31: rustc::ty::context::tls::with_context<closure,alloc::rc::Rc<rustc::traits::specialize::specialization_graph::Graph>>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1953
  32: rustc::dep_graph::graph::DepGraph::with_task_impl<rustc::ty::context::TyCtxt,rustc::hir::def_id::DefId,rustc::ty::Generics*>
             at C:\Stuff\Sources\rustsrc\src\librustc\dep_graph\graph.rs:266
  33: rustc::ty::context::tls::with_related_context<closure,(rustc::ty::Generics*, rustc::dep_graph::graph::DepNodeIndex)>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1964
  34: rustc::ty::context::TyCtxt::force_query_with_job<rustc::ty::query::queries::generics_of>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:543
  35: rustc::ty::context::TyCtxt::get_query<rustc::ty::query::queries::generics_of>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:640
  36: rustc::ty::context::TyCtxt::generics_of
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:869
  37: rustc_typeck::collect::{{impl}}::visit_item
             at C:\Stuff\Sources\rustsrc\src\librustc_typeck\collect.rs:113
  38: rustc::hir::Crate::visit_all_item_likes<rustc::hir::itemlikevisit::DeepVisitor<rustc_typeck::collect::CollectItemTypesVisitor>>
             at C:\Stuff\Sources\rustsrc\src\librustc\hir\mod.rs:719
  39: rustc::util::common::time<(),closure>
             at C:\Stuff\Sources\rustsrc\src\librustc\util\common.rs:157
  40: rustc_typeck::check_crate
             at C:\Stuff\Sources\rustsrc\src\librustc_typeck\lib.rs:342
  41: rustc::ty::context::tls::enter_context<closure,core::result::Result<core::result::Result<(rustc::session::config::OutputFilenames, alloc::boxed::Box<Any>, rustc::dep_graph::graph::DepGraph), rustc::session::CompileIncomplete>, rustc::session::CompileIncomplete>>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1868
  42: std::thread::local::LocalKey<core::cell::Cell<fn(syntax_pos::span_encoding::Span, mut core::fmt::Formatter*) -> core::result::Result<(), core::fmt::Error>>>::with<core::cell::Cell<fn(syntax_pos::span_encoding::Span, mut core::fmt::Formatter*) -> core::result::Result<(), core::fmt::Error>>,closure,core::result::Result<core::result::Result<(rustc::session::config::OutputFilenames, alloc::boxed
::Box<Any>, rustc::dep_graph::graph::DepGraph), rustc::session::CompileIncomplete>, rustc::session::CompileIncomplete>>
             at C:\Stuff\Sources\rustsrc\src\libstd\thread\local.rs:248
  43: rustc::ty::context::TyCtxt::create_and_enter<closure,core::result::Result<core::result::Result<(rustc::session::config::OutputFilenames, alloc::boxed::Box<Any>, rustc::dep_graph::graph::DepGraph), rustc::session::CompileIncomplete>, rustc::session::CompileIncomplete>>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1186
  44: rustc_driver::driver::compile_input
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\driver.rs:285
  45: rustc_driver::run_compiler_with_pool
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\lib.rs:562
  46: scoped_tls::ScopedKey<syntax::Globals>::set<syntax::Globals,closure,(core::result::Result<(), rustc::session::CompileIncomplete>, core::option::Option<rustc::session::Session>)>
             at C:\Users\Arnavion\.cargo\registry\src\github.com-1ecc6299db9ec823\scoped-tls-0.1.2\src\lib.rs:155
  47: std::panic::{{impl}}::call_once<(),closure>
             at C:\Stuff\Sources\rustsrc\src\libstd\panic.rs:313
  48: panic_unwind::__rust_maybe_catch_panic
             at C:\Stuff\Sources\rustsrc\src\libpanic_unwind\lib.rs:102
  49: std::panicking::try<(),std::panic::AssertUnwindSafe<closure>>
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:289
  50: rustc_driver::run<closure>
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\lib.rs:187
  51: rustc_driver::main
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\lib.rs:1737
  52: std::rt::lang_start::{{closure}}<()>
             at C:\Stuff\Sources\rustsrc\src\libstd\rt.rs:74
  53: std::panicking::try::do_call<closure,i32>
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:310
  54: panic_unwind::__rust_maybe_catch_panic
             at C:\Stuff\Sources\rustsrc\src\libpanic_unwind\lib.rs:102
  55: std::panicking::try<i32,closure>
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:289
  56: std::rt::lang_start_internal
             at C:\Stuff\Sources\rustsrc\src\libstd\rt.rs:58
  57: main
  58: __scrt_common_main_seh
             at f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:288
  59: BaseThreadInitThunk
  60: RtlUserThreadStart
query stack during panic:
#0 [resolve_lifetimes] resolving lifetimes
#1 [object_lifetime_defaults_map] looking up lifetime defaults for a region
#2 [generics_of] processing `Bar`
end of query stack

[Arnavion]

Your suggestion did fix that error by the way.

--- a/src/librustc/middle/resolve_lifetime.rs
+++ b/src/librustc/middle/resolve_lifetime.rs
@@ -623,7 +623,13 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
             }
             hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) => {
                 if let Def::Existential(exist_ty_did) = path.def {
-                    let id = self.tcx.hir.as_local_node_id(exist_ty_did).unwrap();
+                    let id = match self.tcx.hir.as_local_node_id(exist_ty_did) {
+                        Some(id) => id,
+                        None => {
+                            intravisit::walk_ty(self, ty);
+                            return;
+                        },
+                    };

                     // Resolve the lifetimes in the bounds to the lifetime defs in the generics.
                     // `fn foo<'a>() -> impl MyTrait<'a> { ... }` desugars to

[Arnavion]

With the above patch, there is a new ICE when using the field inside an inherent method.

extern crate a;

struct Bar(a::Foo);

impl Bar {
    fn zero(&self) -> &a::Foo {
        &self.0
    }
}

fn main() {
}

ICE

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', libcore\option.rs:345:21
stack backtrace:
   0: std::sys::windows::backtrace::unwind_backtrace
             at C:\Stuff\Sources\rustsrc\src\libstd\sys\windows\backtrace\mod.rs:95
   1: std::sys_common::backtrace::print
             at C:\Stuff\Sources\rustsrc\src\libstd\sys_common\backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:211
   3: std::panicking::default_hook
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:227
   4: rustc::util::common::panic_hook
             at C:\Stuff\Sources\rustsrc\src\librustc\util\common.rs:51
   5: std::panicking::rust_panic_with_hook
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:479
   6: std::panicking::continue_panic_fmt
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:390
   7: std::panicking::rust_begin_panic
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:325
   8: core::panicking::panic_fmt
             at C:\Stuff\Sources\rustsrc\src\libcore\panicking.rs:77
   9: core::panicking::panic
             at C:\Stuff\Sources\rustsrc\src\libcore\panicking.rs:52
  10: rustc::ty::fold::{{impl}}::fold_ty<closure,closure>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\fold.rs:221
  11: rustc::ty::structural_impls::{{impl}}::super_fold_with<rustc::ty::fold::BottomUpFolder<closure, closure>>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\structural_impls.rs:851
  12: rustc::ty::fold::{{impl}}::fold_ty<closure,closure>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\fold.rs:219
  13: rustc_typeck::check::wfcheck::check_where_clauses
             at C:\Stuff\Sources\rustsrc\src\librustc_typeck\check\wfcheck.rs:503
  14: rustc_typeck::check::wfcheck::check_fn_or_method
             at C:\Stuff\Sources\rustsrc\src\librustc_typeck\check\wfcheck.rs:544
  15: rustc::ty::context::tls::with_related_context<closure,()>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1964
  16: rustc::infer::InferCtxtBuilder::enter<closure,()>
             at C:\Stuff\Sources\rustsrc\src\librustc\infer\mod.rs:469
  17: rustc_typeck::check::wfcheck::check_associated_item
             at C:\Stuff\Sources\rustsrc\src\librustc_typeck\check\wfcheck.rs:182
  18: rustc_typeck::check::wfcheck::check_impl_item
             at C:\Stuff\Sources\rustsrc\src\librustc_typeck\check\wfcheck.rs:174
  19: rustc::ty::query::{{impl}}::compute
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:820
  20: rustc::ty::context::tls::with_context<closure,()>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1953
  21: rustc::dep_graph::graph::DepGraph::with_task_impl<rustc::ty::context::TyCtxt,rustc::hir::def_id::DefId,()>
             at C:\Stuff\Sources\rustsrc\src\librustc\dep_graph\graph.rs:266
  22: rustc::ty::context::tls::with_related_context<closure,((), rustc::dep_graph::graph::DepNodeIndex)>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1964
  23: rustc::ty::context::TyCtxt::force_query_with_job<rustc::ty::query::queries::check_impl_item_well_formed>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:543
  24: rustc::ty::context::TyCtxt::get_query<rustc::ty::query::queries::check_impl_item_well_formed>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:640
  25: rustc::ty::context::TyCtxt::ensure_query<rustc::ty::query::queries::check_impl_item_well_formed>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\query\plumbing.rs:602
  26: rustc_typeck::check::wfcheck::{{impl}}::visit_impl_item
             at C:\Stuff\Sources\rustsrc\src\librustc_typeck\check\wfcheck.rs:934
  27: rustc::hir::Crate::visit_all_item_likes<rustc::hir::itemlikevisit::DeepVisitor<rustc_typeck::check::wfcheck::CheckTypeWellFormedVisitor>>
             at C:\Stuff\Sources\rustsrc\src\librustc\hir\mod.rs:727
  28: rustc::session::Session::track_errors<closure,()>
             at C:\Stuff\Sources\rustsrc\src\librustc\session\mod.rs:320
  29: rustc::util::common::time<core::result::Result<(), rustc::util::common::ErrorReported>,closure>
             at C:\Stuff\Sources\rustsrc\src\librustc\util\common.rs:157
  30: rustc_typeck::check_crate
             at C:\Stuff\Sources\rustsrc\src\librustc_typeck\lib.rs:368
  31: rustc::ty::context::tls::enter_context<closure,core::result::Result<core::result::Result<(rustc::session::config::OutputFilenames, alloc::boxed::Box<Any>, rustc::dep_graph::graph::DepGraph), rustc::session::CompileIncomplete>, rustc::session::CompileIncomplete>>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1868
  32: std::thread::local::LocalKey<core::cell::Cell<fn(syntax_pos::span_encoding::Span, mut core::fmt::Formatter*) -> core::result::Result<(), core::fmt::Error>>>::with<core::cell::Cell<fn(syntax_pos::span_encoding::Span, mut core::fmt::Formatter*) -> core::result::Result<(), core::fmt::Error>>,closure,core::result::Result<core::result::Result<(rustc::session::config::OutputFilenames, alloc::boxed::Box<Any>, rustc::dep_graph::graph::DepGraph), rustc::session::CompileIncomplete>, rustc::session::CompileIncomplete>>
             at C:\Stuff\Sources\rustsrc\src\libstd\thread\local.rs:248
  33: rustc::ty::context::TyCtxt::create_and_enter<closure,core::result::Result<core::result::Result<(rustc::session::config::OutputFilenames, alloc::boxed::Box<Any>, rustc::dep_graph::graph::DepGraph), rustc::session::CompileIncomplete>, rustc::session::CompileIncomplete>>
             at C:\Stuff\Sources\rustsrc\src\librustc\ty\context.rs:1186
  34: rustc_driver::driver::compile_input
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\driver.rs:285
  35: rustc_driver::run_compiler_with_pool
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\lib.rs:562
  36: scoped_tls::ScopedKey<syntax::Globals>::set<syntax::Globals,closure,(core::result::Result<(), rustc::session::CompileIncomplete>, core::option::Option<rustc::session::Session>)>
             at C:\Users\Arnavion\.cargo\registry\src\github.com-1ecc6299db9ec823\scoped-tls-0.1.2\src\lib.rs:155
  37: std::panic::{{impl}}::call_once<(),closure>
             at C:\Stuff\Sources\rustsrc\src\libstd\panic.rs:313
  38: panic_unwind::__rust_maybe_catch_panic
             at C:\Stuff\Sources\rustsrc\src\libpanic_unwind\lib.rs:102
  39: std::panicking::try<(),std::panic::AssertUnwindSafe<closure>>
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:289
  40: rustc_driver::run<closure>
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\lib.rs:187
  41: rustc_driver::main
             at C:\Stuff\Sources\rustsrc\src\librustc_driver\lib.rs:1737
  42: std::rt::lang_start::{{closure}}<()>
             at C:\Stuff\Sources\rustsrc\src\libstd\rt.rs:74
  43: std::panicking::try::do_call<closure,i32>
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:310
  44: panic_unwind::__rust_maybe_catch_panic
             at C:\Stuff\Sources\rustsrc\src\libpanic_unwind\lib.rs:102
  45: std::panicking::try<i32,closure>
             at C:\Stuff\Sources\rustsrc\src\libstd\panicking.rs:289
  46: std::rt::lang_start_internal
             at C:\Stuff\Sources\rustsrc\src\libstd\rt.rs:58
  47: main
  48: __scrt_common_main_seh
             at f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:288
  49: BaseThreadInitThunk
  50: RtlUserThreadStart
query stack during panic:
#0 [check_impl_item_well_formed] processing `Bar::zero`
end of query stack

[andreytkachenko]

For me another variant of this issues is blocker:

a/Cargo.toml and b/Cargo.toml - is same
a/src/lib.rs:

#![feature(existential_type)]

pub trait View {
    type Tmp: Iterator<Item = u32>;

    fn test(&self) -> Self::Tmp;
}

pub struct X;

impl View for X {
    existential type Tmp: Iterator<Item = u32>;

    fn test(&self) -> Self::Tmp {
        vec![1,2,3].into_iter()
    }
}

and b/src/main.rs:

extern crate a;
use a::View;

fn main() {
    let v = a::X;
    v.test();
}

ICE

error: internal compiler error: librustc_metadata/decoder.rs:834: cannot get associated-item of `DefKey { parent: Some(DefIndex(0:7)), disambiguated_data: DisambiguatedDefPathData { data: AssocExistentialInImpl("Tmp"), disambiguator: 0 } }`

thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:586:9
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
             at libstd/sys_common/backtrace.rs:59
   2: std::panicking::default_hook::{{closure}}
             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:481
   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::decoder::<impl rustc_metadata::cstore::CrateMetadata>::get_associated_item
  15: rustc_metadata::cstore_impl::provide_extern::associated_item
  16: rustc::ty::query::__query_compute::associated_item
  17: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::associated_item<'tcx>>::compute
  18: rustc::ty::context::tls::with_context
  19: rustc::dep_graph::graph::DepGraph::with_task_impl
  20: <rustc::ty::query::plumbing::JobOwner<'a, 'tcx, Q>>::start
  21: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  22: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  23: <core::iter::Map<I, F> as core::iter::iterator::Iterator>::next
  24: rustc::traits::project::assoc_ty_def
  25: rustc::infer::InferCtxt::commit_if_ok
  26: rustc::traits::project::opt_normalize_projection_type
  27: rustc::traits::project::normalize_projection_type
  28: <rustc::traits::project::AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> as rustc::ty::fold::TypeFolder<'gcx, 'tcx>>::fold_ty
  29: <rustc_data_structures::accumulate_vec::AccumulateVec<A> as core::iter::traits::FromIterator<<A as rustc_data_structures::array_vec::Array>::Element>>::from_iter
  30: rustc::ty::fold::TypeFoldable::fold_with
  31: rustc::infer::InferCtxt::partially_normalize_associated_types_in
  32: rustc_typeck::check::method::confirm::ConfirmContext::confirm
  33: rustc_typeck::check::FnCtxt::check_expr_kind
  34: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
  35: rustc_typeck::check::FnCtxt::check_block_with_expected
  36: rustc_typeck::check::FnCtxt::check_expr_kind
  37: rustc_typeck::check::FnCtxt::check_expr_with_expectation_and_needs
  38: rustc_typeck::check::FnCtxt::check_return_expr
  39: rustc_typeck::check::check_fn
  40: rustc::ty::context::tls::with_related_context
  41: rustc::infer::InferCtxtBuilder::enter
  42: rustc_typeck::check::typeck_tables_of
  43: rustc::ty::query::__query_compute::typeck_tables_of
  44: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_tables_of<'tcx>>::compute
  45: rustc::ty::context::tls::with_context
  46: rustc::dep_graph::graph::DepGraph::with_task_impl
  47: <rustc::ty::query::plumbing::JobOwner<'a, 'tcx, Q>>::start
  48: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  49: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  50: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::ensure_query
  51: rustc_typeck::check::typeck_item_bodies
  52: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::typeck_item_bodies<'tcx>>::compute
  53: rustc::ty::context::tls::with_context
  54: rustc::dep_graph::graph::DepGraph::with_task_impl
  55: <rustc::ty::query::plumbing::JobOwner<'a, 'tcx, Q>>::start
  56: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  57: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  58: rustc_typeck::check_crate
  59: rustc::ty::context::tls::enter_context
  60: <std::thread::local::LocalKey<T>>::with
  61: rustc::ty::context::TyCtxt::create_and_enter
  62: rustc_driver::driver::compile_input
  63: rustc_driver::run_compiler_with_pool
  64: <scoped_tls::ScopedKey<T>>::set
  65: <scoped_tls::ScopedKey<T>>::set
  66: syntax::with_globals
  67: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  68: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  69: rustc_driver::run
  70: rustc_driver::main
  71: std::rt::lang_start::{{closure}}
  72: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  73: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  74: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  75: main
  76: __libc_start_main
  77: <unknown>
query stack during panic:
#0 [associated_item] processing `<a::X as a::View>::Tmp`
#1 [typeck_tables_of] processing `main`
#2 [typeck_item_bodies] type-checking all item bodies
end of query stack

[tikue]

Is there any progress on this?

[tikue]

@Arnavion how do you feel about making a PR with your patch that has a partial fix? It looks like that would be enough to unblock me from using existential types in my project.

[Arnavion]

A bunch of stuff changed with oli-obk's recent change so the second patch won't apply directly. I'll retest and put up a fork.

[Arnavion]

@tikue

For the first ICE (function that returns existential type in crate A called from crate B), the patch (on top of 46880f41b7aeb897b8245474196bba9dc11f0e88) is

diff --git a/src/librustc_resolve/build_reduced_graph.rs b/src/librustc_resolve/build_reduced_graph.rs
index 25a7ff9cd3..02a28a1e2f 100644
--- a/src/librustc_resolve/build_reduced_graph.rs
+++ b/src/librustc_resolve/build_reduced_graph.rs
@@ -736,6 +736,9 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
             Def::Macro(..) => {
                 self.define(parent, ident, MacroNS, (def, vis, DUMMY_SP, expansion));
             }
+            Def::Existential(..) => {
+                self.define(parent, ident, TypeNS, (def, vis, DUMMY_SP, expansion));
+            }
             _ => bug!("unexpected definition: {:?}", def)
         }
     }

The second ICE (existential type exported from crate A used as struct member in crate B) is already fixed.

The third ICE (existential type exported from crate A used in return type of a function in crate B) still exists.

And, while I'm at it, @andreytkachenko's ICE also still exists.

[tikue]

@Arnavion thanks so much for following up on this! Do you want to open a PR, since you've done all the work? 😁

[Arnavion]

The patch is just guesswork on my part, and the whole thing is still broken anyway, so there's no point to me making a PR. If you want to try it you can apply that patch locally.

[jimmycuadra]

Just encountered an ICE and searching led me here. Is this just another instance of the same bug or should I report it separately?

Run cargo test on rustin@a100f20 using nightly rustc and you get:

error: internal compiler error: src/librustc_metadata/decoder.rs:893: cannot get associated-item of `DefKey { parent: Some(DefIndex(0:47)), disambiguated_data: DisambiguatedDefPathData { data: AssocExistentialInImpl("Success"), disambiguator: 0 } }`

thread 'main' panicked at 'Box<Any>', src/librustc_errors/lib.rs:600:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: aborting due to previous error

[Arnavion]

@jonhoo Yes. If you see the "Easy playground repro" link in the OP, that happens because the doctest is a separate crate.

[jonhoo]

@oli-obk any thoughts on the further ICEs that @Arnavion discovered when trying to fix this? This ICE sadly makes existential types pretty unusable in libraries.

[oli-obk]

Sorry about taking so long, it was just a few small fixes in the end. I think https://github.com/rust-lang/rust/pull/57836 fixes all the ICEs mentioned in this issue.

[Arnavion]

Thanks. I confirmed this fixed all the issues with my original project too.

[andreytkachenko]

@oli-obk could you also take a look at #53598, I suppose it is very close to that one. Thank you.