Unknown ICE

[sgrif]

This code causes Rust to ICE on stable, beta, and nightly.

Playground link

pub trait Reborrow<'a, 'b>
where
    Self: 'a,
    'a: 'b,
{
    type Output: 'b;
    
    fn reborrow(self) -> Self::Output;
}

impl<'a, 'b, T> Reborrow<'a, 'b> for &'a mut T
where
    'a: 'b,
{
    type Output = &'b mut T;
    
    fn reborrow(self) -> Self::Output {
        &mut *self
    }
}

impl<'a, 'b, T: Copy> Reborrow<'a, 'b> for T {
    type Output = Self;
    
    fn reborrow(self) -> Self::Output {
        self
    }
}

fn main() {}

It is specifically the second impl (the one with the Copy bound) that causes the ICE.

Backtrace

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', libcore/option.rs:335:21
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
   1: std::sys_common::backtrace::print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic
   6: std::panicking::begin_panic_fmt
   7: rust_begin_unwind
   8: core::panicking::panic_fmt
   9: core::panicking::panic
  10: <std::thread::local::LocalKey<T>>::with
  11: rustc::ty::context::GlobalCtxt::enter_local
  12: rustc::traits::coherence::overlapping_impls
  13: rustc::traits::specialize::specialization_graph::Graph::insert
  14: rustc::traits::specialize::specialization_graph_provider
  15: rustc::dep_graph::graph::DepGraph::with_task_impl
  16: rustc_errors::Handler::track_diagnostics
  17: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::cycle_check
  18: rustc::ty::maps::<impl rustc::ty::maps::queries::specialization_graph_of<'tcx>>::force
  19: rustc::ty::maps::<impl rustc::ty::maps::queries::specialization_graph_of<'tcx>>::try_get
  20: rustc::ty::maps::TyCtxtAt::specialization_graph_of
  21: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::specialization_graph_of
  22: rustc_typeck::coherence::coherent_trait
  23: rustc::dep_graph::graph::DepGraph::with_task_impl
  24: rustc_errors::Handler::track_diagnostics
  25: rustc::ty::maps::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::cycle_check
  26: rustc::ty::maps::<impl rustc::ty::maps::queries::coherent_trait<'tcx>>::force
  27: rustc::ty::maps::<impl rustc::ty::maps::queries::coherent_trait<'tcx>>::try_get
  28: rustc::ty::maps::TyCtxtAt::coherent_trait
  29: rustc::ty::maps::<impl rustc::ty::maps::queries::coherent_trait<'tcx>>::ensure
  30: rustc_typeck::coherence::check_coherence
  31: rustc_typeck::check_crate
  32: <std::thread::local::LocalKey<T>>::with
  33: <std::thread::local::LocalKey<T>>::with
  34: rustc::ty::context::TyCtxt::create_and_enter
  35: rustc_driver::driver::compile_input
  36: rustc_driver::run_compiler

Tested on rustc 1.23.0 (766bd11c8 2018-01-01), rustc 1.25.0-beta.7 (da64ca931 2018-03-03), and rustc 1.26.0-nightly (e026b59cf 2018-03-03)

[varkor]

This error is coming from a very similar location to #48728. Maybe it's the same bug?

[Mark-Simulacrum]

This code no longer ICEs.

error[E0119]: conflicting implementations of trait `Reborrow<'_, '_>` for type `&mut _`:
  --> src/main.rs:22:1
   |
11 | / impl<'a, 'b, T> Reborrow<'a, 'b> for &'a mut T
12 | | where
13 | |     'a: 'b,
14 | | {
...  |
19 | |     }
20 | | }
   | |_- first implementation here
21 | 
22 |   impl<'a, 'b, T: Copy> Reborrow<'a, 'b> for T {
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&mut _`
   |
   = note: downstream crates may implement trait `std::marker::Copy` for type `&mut _`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0119`.
error: Could not compile `playground`.

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