internal compiler error: unexpected panic on 7 line double ref program

[mwhittaker]

I compiled the following code expecting some sort of error message. Instead, the compiler panicked unexpectedly. (Playground)

fn main() {
    let x = String::from("foo");
    let x_ref = &x;
    let x_ref_ref = &x_ref;
    x_ref.push_str("bar");
    println!("{}", x_ref_ref);
}

cargo run errors:

Compiling hello v0.1.0 (/home/vagrant/hello)
thread 'rustc' panicked at 'no entry found for key', src/libcore/option.rs:1034:5
note: Run with `RUST_BACKTRACE=1` environment variable to display a 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 1.35.0 (3c235d560 2019-05-20) 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

error: Could not compile `hello`.

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

RUST_BACKTRACE=1 cargo run --verbose errors:

   Compiling hello v0.1.0 (/home/vagrant/hello)
     Running `rustc --edition=2018 --crate-name hello src/main.rs --color always --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=3088e465bdea9221 -C extra-filename=-3088e465bdea9221 --out-dir /home/vagrant/hello/ta
rget/debug/deps -C incremental=/home/vagrant/hello/target/debug/incremental -L dependency=/home/vagrant/hello/target/debug/deps`
thread 'rustc' panicked at 'no entry found for key', src/libcore/option.rs:1034:5
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
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print                                                                                                                                                                                        [0/372]
             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:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   4: rustc::util::common::panic_hook
   5: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   6: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:381
   7: rust_begin_unwind
             at src/libstd/panicking.rs:308
   8: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   9: core::option::expect_failed
             at src/libcore/option.rs:1034
  10: rustc_mir::borrow_check::path_utils::each_borrow_involving_path
  11: rustc_mir::borrow_check::MirBorrowckCtxt::access_place
  12: <rustc_mir::borrow_check::MirBorrowckCtxt as rustc_mir::dataflow::DataflowResultsConsumer>::visit_statement_entry
  13: rustc_mir::borrow_check::do_mir_borrowck
  14: rustc::ty::context::GlobalCtxt::enter_local
  15: rustc_mir::borrow_check::mir_borrowck
  16: rustc::ty::query::__query_compute::mir_borrowck
  17: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::mir_borrowck>::compute
  18: rustc::dep_graph::graph::DepGraph::with_task_impl
  19: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  20: rustc::ty::<impl rustc::ty::context::TyCtxt>::par_body_owners
  21: rustc::util::common::time
  22: rustc_interface::passes::analysis
  23: rustc::ty::query::__query_compute::analysis
  24: rustc::ty::query::<impl rustc::ty::query::config::QueryAccessors for rustc::ty::query::queries::analysis>::compute
  25: rustc::dep_graph::graph::DepGraph::with_task_impl
  26: rustc::ty::query::plumbing::<impl rustc::ty::context::TyCtxt>::get_query
  27: rustc_interface::passes::BoxedGlobalCtxt::access::{{closure}}
  28: rustc_interface::passes::create_global_ctxt::{{closure}}
  29: rustc_interface::interface::run_compiler_in_existing_thread_pool
  30: std::thread::local::LocalKey<T>::with
  31: scoped_tls::ScopedKey<T>::set
  32: syntax::with_globals
query stack during panic:
#0 [mir_borrowck] processing `main`
#1 [analysis] running analysis passes on this crate
end of query stack

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 1.35.0 (3c235d560 2019-05-20) 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

error: Could not compile `hello`.

Caused by:
  process didn't exit successfully: `rustc --edition=2018 --crate-name hello src/main.rs --color always --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=3088e465bdea9221 -C extra-filename=-3088e465bdea9221 --out-dir /home/vagrant/hello/target/debug/deps -C incremental=/home/vagrant/hello/target/debug/incremental -L dependency=/home/vagrant/hello/target/debug/deps` (exit code: 101)

rustc --version --verbose:

rustc 1.35.0 (3c235d560 2019-05-20)
binary: rustc
commit-hash: 3c235d5600393dfe6c36eeed34042efad8d4f26e
commit-date: 2019-05-20
host: x86_64-unknown-linux-gnu
release: 1.35.0
LLVM version: 8.0

[Centril]

Triage:

• Likely dupe of rustc panic due to reference passed instead of mut reference #61623. cc @matthewjasper
• Preliminarily assigning P-high before compiler team meeting.