[1.31.0-nightly] internal compiler error: "min_const_fn encountered `Terminator {"

[gpace1]

While experimenting with constant functions I managed to generate an internal compiler error.

The code

enum Foo {
    Prob,
}

impl Foo {
    pub const fn as_val(&self) -> u8 {
        use self::Foo::*;

        match *self {
            Prob => 0x1,
        }
    }
}

fn main() {}

Here is a link to the playground.

Although this code shouldn't compile, I would expect an error along the lines of "error: if, match, && and || are not stable in const fn". The compiler will produce the expected error if enum Foo has more then one variant.

The internal bug with backtrace (from playground):

error: internal compiler error: librustc_mir/transform/qualify_min_const_fn.rs:366: min_const_fn encountered `Terminator {
    source_info: SourceInfo {
        span: src/main.rs:10:13: 10:17,
        scope: scope[0]
    },
    kind: falseEdges -> [real: bb2, imaginary: bb1]
}`
  --> src/main.rs:10:13
   |
10 |             Prob => 0x1,
   |             ^^^^

thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:538:9
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:480
   6: std::panicking::begin_panic
   7: rustc_errors::Handler::span_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::span_bug_fmt
  14: rustc_mir::transform::qualify_min_const_fn::is_min_const_fn
  15: <rustc_mir::transform::qualify_consts::QualifyAndPromoteConstants as rustc_mir::transform::MirPass>::run_pass
  16: rustc_mir::transform::mir_validated::{{closure}}
  17: rustc_mir::transform::mir_validated
  18: rustc::ty::query::__query_compute::mir_validated
  19: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::mir_validated<'tcx>>::compute
  20: rustc::dep_graph::graph::DepGraph::with_task_impl
  21: <rustc::ty::query::plumbing::JobOwner<'a, 'tcx, Q>>::start
  22: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  23: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  24: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::ensure_query
  25: rustc_borrowck::borrowck::borrowck
  26: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors<'tcx> for rustc::ty::query::queries::borrowck<'tcx>>::compute
  27: rustc::dep_graph::graph::DepGraph::with_task_impl
  28: <rustc::ty::query::plumbing::JobOwner<'a, 'tcx, Q>>::start
  29: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::force_query_with_job
  30: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::get_query
  31: rustc::ty::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::par_body_owners
  32: rustc_borrowck::borrowck::check_crate
  33: rustc::util::common::time
  34: rustc::ty::context::tls::enter_context
  35: <std::thread::local::LocalKey<T>>::with
  36: rustc::ty::context::TyCtxt::create_and_enter
  37: rustc_driver::driver::compile_input
  38: rustc_driver::run_compiler_with_pool
  39: rustc_driver::driver::spawn_thread_pool
  40: rustc_driver::run_compiler
  41: <scoped_tls::ScopedKey<T>>::set
  42: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  43: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  44: rustc_driver::run
  45: rustc_driver::main
  46: std::rt::lang_start::{{closure}}
  47: std::panicking::try::do_call
             at libstd/rt.rs:59
             at libstd/panicking.rs:310
  48: __rust_maybe_catch_panic
             at libpanic_unwind/lib.rs:102
  49: std::rt::lang_start_internal
             at libstd/panicking.rs:289
             at libstd/panic.rs:392
             at libstd/rt.rs:58
  50: main
  51: __libc_start_main
  52: <unknown>
query stack during panic:
#0 [mir_validated] processing `Foo::as_val`
#1 [borrowck] processing `Foo::as_val`
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 1.31.0-nightly (4bd4e4130 2018-10-25) running on x86_64-unknown-linux-gnu

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

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

error: Could not compile `playground`.