rust 🚀 - beta regression: ICE on Tried to access field 0 of union Layout

https://github.com/rust-lang/rust/pull/64987 was merged in October, I don't think it's related

oli-obk on 15 Feb 2020

👍1

Does this need an mcve?

hellow554 on 17 Feb 2020

Yeah, but I think at this point a bisection might be more helpful since we're not yet sure what caused this.

jonas-schievink on 17 Feb 2020

Here's a somewhat small-ish MCVE: Haven't managed to remove mio yet, but that's just a matter of time ;)

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=a6b20beddee84373d0ba4f56c6855485

#![allow(deprecated)]
#![feature(never_type)]

extern crate mio;

use mio::deprecated::EventLoop;

pub trait Machine: Sized {
    type Context;
    type Seed: Sized;

    fn create(seed: Self::Seed) -> Response<Self, ()>;
}

pub struct Response<M, N>(std::marker::PhantomData<(M, N)>);

pub struct Loop<M: Machine> {
    mio: EventLoop<Handler<M>>,
    handler: Handler<M>,
}

impl<M: Machine> Loop<M> {
    pub fn new() -> Self {
        todo!()
    }

    pub fn add_machine_with<F>(&mut self, _fun: F)
    where
        F: FnOnce() -> Response<M, !>,
    {
        todo!()
    }

    pub fn run(mut self) {
        let ref mut handler = self.handler;
        let ref mut mio = self.mio;
        mio.run(handler).unwrap();
    }
}

pub struct Handler<M: Machine> {
    _a: std::marker::PhantomData<M>,
}

impl<M: Machine> Handler<M> {
    pub fn new() -> Self {
        todo!()
    }
}

pub fn machine_loop<M>(_handler: &mut Handler<M>)
where
    M: Machine,
{
    let creator = None;
    let mut new = Some(creator.unwrap());
    let _mach = decompose(M::create(new.take().unwrap()));
}

impl<M: Machine> mio::deprecated::Handler for Handler<M> {
    type Message = ();
    type Timeout = ();
    fn timeout(&mut self, _eloop: &mut EventLoop<Self>, _: ()) {
        machine_loop(self)
    }
}

impl<M: Sized, N: Sized> Response<M, N> {
    pub fn map<T, U, S, R>(self, _: S, _: R) -> Response<T, U>
    where
        S: FnOnce(M) -> T,
    {
        todo!()
    }
    pub fn wrap<T, S>(self, _: S) -> Response<T, N>
    where
        S: FnOnce(M) -> T,
    {
        todo!()
    }
}

pub fn decompose<M, N>(_: Response<M, N>) -> M {
    todo!()
}

enum Tcp {}

enum Composed {
    Tcp(Tcp),
}
enum CSeed {
    Tcp(!),
}

impl Machine for Composed {
    type Context = u32;
    type Seed = CSeed;
    fn create(seed: CSeed) -> Response<Self, ()> {
        match seed {
            CSeed::Tcp(x) => todo!(),
        }
    }
}

impl Tcp {
    fn new() -> Response<Tcp, !> {
        todo!()
    }
}

impl Machine for Tcp {
    type Context = u32;
    type Seed = !;
    fn create(_seed: !) -> Response<Self, ()> {
        todo!()
    }
}

fn main() {
    let mut loop_creator = Loop::new();
    loop_creator.add_machine_with(|| Tcp::new().wrap(Composed::Tcp));
    loop_creator.run();
}

Note, that this doesn't depend on the never_type feature. You can use enum Void {} as well.

hellow554 on 17 Feb 2020

Here's a "M"CVE without any dependencies:

pub enum Void {}

pub trait Machine: Sized {
    type Seed: Sized;
    fn create(seed: Self::Seed) -> Response<Self, ()>;
}

pub struct Response<M, N>(std::marker::PhantomData<(M, N)>);
pub struct MyHandler<M>(std::marker::PhantomData<M>);
pub struct Loop<M>(std::marker::PhantomData<M>);

impl<M: Machine> Loop<M> {
    pub fn new() -> Self {
        unimplemented!()
    }

    pub fn add_machine_with<F>(&self, _fun: F)
    where
        F: FnOnce() -> Response<M, Void>,
    {
        unimplemented!()
    }

    pub fn run(self) {
        let handler = MyHandler::<M>::new();
        handler.timeout();
    }
}

impl<M: Machine> MyHandler<M> {
    pub fn new() -> Self {
        unimplemented!()
    }
    fn timeout(self) {
        let creator = None;
        let new = Some(creator.unwrap());
        M::create(new.unwrap());
    }
}

impl<M: Sized, N: Sized> Response<M, N> {
    pub fn map<T, U, S, R>(self, _: S, _: R) -> Response<T, U>
    where
        S: FnOnce(M) -> T,
    {
        unimplemented!()
    }
    pub fn wrap<T, S>(self, _: S) -> Response<T, N>
    where
        S: FnOnce(M) -> T,
    {
        unimplemented!()
    }
}

enum Tcp {}

enum Composed {
    Tcp(Tcp),
}
enum CSeed {
    Tcp(Void),
}

impl Machine for Composed {
    type Seed = CSeed;
    fn create(seed: CSeed) -> Response<Self, ()> {
        match seed {
            CSeed::Tcp(_x) => unimplemented!(),
        }
    }
}

impl Tcp {
    fn new() -> Response<Tcp, Void> {
        unimplemented!()
    }
}

impl Machine for Tcp {
    type Seed = Void;
    fn create(_seed: Void) -> Response<Self, ()> {
        unimplemented!()
    }
}

fn main() {
    let loop_creator = Loop::new();
    loop_creator.add_machine_with(|| Tcp::new().wrap(Composed::Tcp));
    loop_creator.run();
}

regression in nightly-2020-01-11

Backtrace

thread 'rustc' panicked at 'Tried to access field 0 of union TyLayout {
    ty: CSeed,
    details: LayoutDetails {
        variants: Single {
            index: 0,
        },
        fields: Union(
            0,
        ),
        abi: Uninhabited,
        largest_niche: None,
        align: AbiAndPrefAlign {
            abi: Align {
                pow2: 0,
            },
            pref: Align {
                pow2: 0,
            },
        },
        size: Size {
            raw: 0,
        },
    },
} with 0 fields', src/librustc_mir/interpret/place.rs:413:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1052
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1428
   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:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
  12: rust_begin_unwind
             at src/libstd/panicking.rs:378
  13: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:332
  14: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::operand_field
  15: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_place_to_op
  16: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_operand
  17: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place
  18: <rustc_mir::transform::const_prop::ConstPropagator as rustc::mir::visit::MutVisitor>::visit_statement
  19: <rustc_mir::transform::const_prop::ConstProp as rustc_mir::transform::MirPass>::run_pass
  20: rustc_mir::transform::run_passes
  21: rustc_mir::transform::run_optimization_passes
  22: rustc_mir::transform::optimized_mir
  23: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::optimized_mir>::compute
  24: rustc::dep_graph::graph::DepGraph::with_task_impl
  25: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  26: rustc::ty::<impl rustc::ty::context::TyCtxt>::instance_mir
  27: rustc_mir::monomorphize::collector::collect_items_rec
  28: rustc_mir::monomorphize::collector::collect_items_rec
  29: rustc_mir::monomorphize::collector::collect_items_rec
  30: rustc_mir::monomorphize::collector::collect_items_rec
  31: rustc_session::utils::<impl rustc_session::session::Session>::time
  32: rustc_mir::monomorphize::collector::collect_crate_mono_items
  33: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  34: rustc::ty::query::__query_compute::collect_and_partition_mono_items
  35: rustc::dep_graph::graph::DepGraph::with_task_impl
  36: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  37: rustc_codegen_ssa::base::codegen_crate
  38: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  39: rustc_session::utils::<impl rustc_session::session::Session>::time
  40: rustc_interface::passes::QueryContext::enter
  41: rustc_interface::queries::Queries::ongoing_codegen
  42: rustc_interface::interface::run_compiler_in_existing_thread_pool
  43: scoped_tls::ScopedKey<T>::set
  44: syntax::attr::with_globals
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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.43.0-nightly (5e7af4669 2020-02-16) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
#0 [optimized_mir] processing `<Composed as Machine>::create`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: could not compile `foo`.

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

@rustbot modify labels: -E-needs-bisection -E-needs-mcve

hellow554 on 18 Feb 2020

searched nightlies: from nightly-2019-11-01 to nightly-2020-02-16
regressed nightly: nightly-2020-01-11
searched commits: from https://github.com/rust-lang/rust/commit/72b2bd55edbb1e63a930c5ddd08b25e4f9044786 to https://github.com/rust-lang/rust/commit/175631311716d7dfeceec40d2587cde7142ffa8c
regressed commit: https://github.com/rust-lang/rust/commit/2d8d559bbecf6272eb41f8a800e319238aa9d621
source code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=b9ff885bb9e3de044e629f93bdbd81fd

Instructions

Reproduce bisection with:

$ cargo bisect-rustc --start 2020-01-10 --end 2020-01-11 --prompt

Error

COLLAPSIBLE ERROR STACKTRACE

warning: variant is never constructed: `Tcp`
  --> src/main.rs:62:5
   |
62 |     Tcp(Void),
   |     ^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

thread 'rustc' panicked at 'Tried to access field 0 of union TyLayout {
    ty: CSeed,
    details: LayoutDetails {
        variants: Single {
            index: 0,
        },
        fields: Union(
            0,
        ),
        abi: Uninhabited,
        largest_niche: None,
        align: AbiAndPrefAlign {
            abi: Align {
                pow2: 0,
            },
            pref: Align {
                pow2: 0,
            },
        },
        size: Size {
            raw: 0,
        },
    },
} with 0 fields', src/librustc_mir/interpret/place.rs:413:17
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/libunwind.rs:86
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.44/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:78
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1052
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1428
   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:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: rustc_driver::report_ice
  11: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
  12: rust_begin_unwind
             at src/libstd/panicking.rs:378
  13: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:332
  14: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::operand_field
  15: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_place_to_op
  16: rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_operand
  17: rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place
  18: <rustc_mir::transform::const_prop::ConstPropagator as rustc::mir::visit::MutVisitor>::visit_statement
  19: <rustc_mir::transform::const_prop::ConstProp as rustc_mir::transform::MirPass>::run_pass
  20: rustc_mir::transform::run_passes
  21: rustc_mir::transform::run_optimization_passes
  22: rustc_mir::transform::optimized_mir
  23: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::optimized_mir>::compute
  24: rustc::dep_graph::graph::DepGraph::with_task_impl
  25: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  26: rustc::ty::<impl rustc::ty::context::TyCtxt>::instance_mir
  27: rustc_mir::monomorphize::collector::collect_items_rec
  28: rustc_mir::monomorphize::collector::collect_items_rec
  29: rustc_mir::monomorphize::collector::collect_items_rec
  30: rustc_mir::monomorphize::collector::collect_items_rec
  31: rustc_session::utils::<impl rustc_session::session::Session>::time
  32: rustc_mir::monomorphize::collector::collect_crate_mono_items
  33: rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items
  34: rustc::ty::query::__query_compute::collect_and_partition_mono_items
  35: rustc::dep_graph::graph::DepGraph::with_task_impl
  36: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  37: rustc_codegen_ssa::base::codegen_crate
  38: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate
  39: rustc_session::utils::<impl rustc_session::session::Session>::time
  40: rustc_interface::passes::QueryContext::enter
  41: rustc_interface::queries::Queries::ongoing_codegen
  42: rustc_interface::interface::run_compiler_in_existing_thread_pool
  43: scoped_tls::ScopedKey<T>::set
  44: syntax::attr::with_globals
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/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.43.0-nightly (0176a9eef 2020-02-17) running on x86_64-unknown-linux-gnu

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

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

query stack during panic:
#0 [optimized_mir] processing `<Composed as Machine>::create`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

ERROR BEFORE ICE REGRESSION

warning: variant is never constructed: `Tcp`
  --> src/main.rs:62:5
   |
62 |     Tcp(Void),
   |     ^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.23s


nightly-2020-01-10 finished with exit code Some(0).

chrissimpkins on 19 Feb 2020

It would be good to continue the bisection into the rollup PR, among the following PRs:

#66463 (Point at opaque and closure type definitions in type errors)
#67501 (Reduce special treatment for zsts)
#67820 (Parse the syntax described in RFC 2632)
#67922 (rustc_ast_lowering: misc cleanup & rustc dep reductions)
#68071 (Extend support of _ in type parameters)
#68073 (expect fn after const unsafe / const extern)

pnkfelix on 20 Feb 2020

👍1

triage: P-high, removing nomination, assigning to self.

pnkfelix on 20 Feb 2020

Given the backtrace, I think it's probably

#67501 (Reduce special treatment for zsts)

wesleywiser on 21 Feb 2020

I can indeed reproduce the ICE at c5c4fa8e7633c28464e3b27a57f2f175cc6700fd, #67501 seems to be the culprit.
cc @oli-obk @pnkfelix

Backtrace

thread 'rustc' panicked at 'Tried to access field 0 of union TyLayout {
    ty: CSeed,
    details: LayoutDetails {
        variants: Single {
            index: 0,
        },
        fields: Union(
            0,
        ),
        abi: Uninhabited,
        largest_niche: None,
        align: AbiAndPrefAlign {
            abi: Align {
                pow2: 0,
            },
            pref: Align {
                pow2: 0,
            },
        },
        size: Size {
            raw: 0,
        },
    },
} with 0 fields', src/librustc_mir/interpret/place.rs:414:17
stack backtrace:
   0:        0x110b37025 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h69a1e3d2ca9c1e11
   1:        0x110b535f1 - core::fmt::write::hf3458d798296d204
   2:        0x110b2bf79 - std::io::Write::write_fmt::h66a47365b38681ee
   3:        0x110b36ddb - std::sys_common::backtrace::print::h02498af503ed0c92
   4:        0x110b04a0e - std::panicking::default_hook::{{closure}}::h34e746f1db241fc9
   5:        0x110b04791 - std::panicking::default_hook::h2e4333a70604ecb2
   6:        0x106060dd4 - rustc_driver::report_ice::he4e28a13284da972
   7:        0x110b05040 - std::panicking::rust_panic_with_hook::h877c0d796963b6c1
   8:        0x110b04b6a - rust_begin_unwind
   9:        0x110b729d8 - std::panicking::begin_panic_fmt::h71f3a4b90a95f3d3
  10:        0x108f0858c - rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::operand_field::h87b529a0597d5280
  11:        0x108f09aa1 - rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_place_to_op::h0badc7f16a061eb7
  12:        0x108f0aad3 - rustc_mir::interpret::operand::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_operand::h388bdbfc5c47bf0c
  13:        0x108f2ea3a - rustc_mir::interpret::step::<impl rustc_mir::interpret::eval_context::InterpCx<M>>::eval_rvalue_into_place::h4f24d1525a1f30d8
  14:        0x108fcea9d - rustc_mir::transform::const_prop::ConstPropagator::use_ecx::h9e82b3a972541897
  15:        0x108ec9f11 - <rustc_mir::transform::const_prop::ConstPropagator as rustc::mir::visit::MutVisitor>::visit_statement::hbcbb76349a1b2af8
  16:        0x108fcc172 - rustc::mir::visit::MutVisitor::visit_body::hed185033fa2d801d
  17:        0x108ec820c - <rustc_mir::transform::const_prop::ConstProp as rustc_mir::transform::MirPass>::run_pass::h3e923c7c053214b4
  18:        0x10910c911 - rustc_mir::transform::run_passes::hb67a5bf194d0c50b
  19:        0x10910d456 - rustc_mir::transform::run_optimization_passes::h4d95f5ca8063715d
  20:        0x10910d733 - rustc_mir::transform::optimized_mir::h2eea86349c4420ec
  21:        0x109dd673b - rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::optimized_mir>::compute::h3776afa08b9955c7
  22:        0x109eaf462 - rustc::dep_graph::graph::DepGraph::with_task_impl::h3bcd274945700fd0
  23:        0x109ca6d3d - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::h1f6b8e5f14eef5bb
  24:        0x109e5dfee - rustc::ty::<impl rustc::ty::context::TyCtxt>::instance_mir::hf27ba3a3da6bb109
  25:        0x108ddf208 - rustc_mir::monomorphize::collector::collect_items_rec::h3d134124ea30f5b1
  26:        0x108ddf449 - rustc_mir::monomorphize::collector::collect_items_rec::h3d134124ea30f5b1
  27:        0x108ddf449 - rustc_mir::monomorphize::collector::collect_items_rec::h3d134124ea30f5b1
  28:        0x108ddf449 - rustc_mir::monomorphize::collector::collect_items_rec::h3d134124ea30f5b1
  29:        0x108c2159e - rustc_session::utils::<impl rustc_session::session::Session>::time::h3d5518a7f37fd9d1
  30:        0x108dde585 - rustc_mir::monomorphize::collector::collect_crate_mono_items::h0d91d5b14a52cb4b
  31:        0x108c21a80 - rustc_session::utils::<impl rustc_session::session::Session>::time::hb1832d4b48f9a79c
  32:        0x10909de3f - rustc_mir::monomorphize::partitioning::collect_and_partition_mono_items::h96d8f9e8b1696162
  33:        0x10639ab5c - rustc::ty::query::__query_compute::collect_and_partition_mono_items::h8b7e8e7a35df0e0f
  34:        0x10649fe56 - rustc::dep_graph::graph::DepGraph::with_task_impl::hbb6e03c47e6ffedc
  35:        0x1063d3267 - rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query::ha0e0a8abfcf6451c
  36:        0x10638c0cd - rustc_codegen_ssa::base::codegen_crate::h3768de7321d0405f
  37:        0x106342ab9 - <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate::hd0d92b55e53ebbcb
  38:        0x106337090 - rustc_session::utils::<impl rustc_session::session::Session>::time::hdb245d4656aad9d6
  39:        0x106329c23 - rustc_interface::passes::start_codegen::h21caada137f8cf2e
  40:        0x10632a0ae - rustc::ty::context::tls::enter_global::h125d9d08f1491fc8
  41:        0x1061eb87f - rustc_interface::queries::Query<T>::compute::h8afa9a79dfe3203f
  42:        0x1061e7edc - rustc_interface::queries::Queries::ongoing_codegen::h8f7e07e1dac4f6c6
  43:        0x1060a8cae - rustc_interface::interface::run_compiler_in_existing_thread_pool::h424d11116d6dbfb6
  44:        0x1060935dd - scoped_tls::ScopedKey<T>::set::h56349b514de6b8d6
  45:        0x10606fb45 - syntax::with_globals::hd4aec9efb871578a
  46:        0x1060a616e - std::sys_common::backtrace::__rust_begin_short_backtrace::h9eeb9c817fb78bb0
  47:        0x1060e8b62 - std::panicking::try::do_call::h0d69788ad8d7e60a
  48:        0x110b37aca - __rust_maybe_catch_panic
  49:        0x1060e8919 - std::panicking::try::h0817a7b1e46e110f
  50:        0x10607c8ec - core::ops::function::FnOnce::call_once{{vtable.shim}}::h138312dfe0ed3c99
  51:        0x110b0c61e - <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once::hf14a404fd9d2e584
  52:        0x110b0c5a6 - <alloc::boxed::Box<F> as core::ops::function::FnOnce<A>>::call_once::h8607d8336ffda5c8
  53:        0x110b2ce3a - std::sys_common::thread::start_thread::h83b37fb852a26050
  54:        0x110b20529 - std::sys::unix::thread::Thread::new::thread_start::h843727c91d6328f4
  55:     0x7fff6b060d76 - _pthread_start

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 1.42.0-dev running on x86_64-apple-darwin

query stack during panic:
#0 [optimized_mir] processing `<Composed as Machine>::create`
#1 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack

LeSeulArtichaut on 4 Mar 2020

Here is a more minimal MCVE:

pub enum Void {}

enum UninhabitedUnivariant {
    Variant(Void),
}

fn main() {
    let seed: UninhabitedUnivariant = None.unwrap();
    match seed {
        UninhabitedUnivariant::Variant(_x) => {}
    }
}

/me needs to invest some time in tools like rust-reduce or creduce...

pnkfelix on 5 Mar 2020

👍2

Okay so now we have two different PR's to fix this.

PR #69768 is almost certainly the right call for nightly itself.

My only Question is whether we should also land PR #69753 and also then backport that to beta. My argument for why is that it is "obviously" less risky, which is important because the beta is going to be imminently promoted to stable.

pnkfelix on 6 Mar 2020

Rust: beta regression: ICE on Tried to access field 0 of union Layout

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