nightly only: internal compiler error: broken MIR (Terminator ..

[FauxFaux]

This (probably incorrect) program causes nightly rustc to panic:

use std::collections::HashMap;
fn main() {
    let things: HashMap<String, Vec<String>> = HashMap::new();
    for src in things.keys() {
        things[src.as_str()].sort();
    }
}

% RUST_BACKTRACE=1 rustc ice.rs
error: internal compiler error: broken MIR (Terminator { source_info: SourceInfo { span: ice.rs:5:9: 5:29, scope: scope4 }, kind: _17 = const std::ops::Index::index(_18, _19) -> [return: bb11, unwind: bb3] }): bad arg #0 (&std::collections::HashMap<std::string::String, std::vec::Vec<std::string::String>> <- &mut std::collections::HashMap<std::string::String, std::vec::Vec<std::string::String>>): Mutability
 --> ice.rs:5:28
  |
5 |         things[src.as_str()].sort();
  |                            ^

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: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:376
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:60
             at /checkout/src/libstd/panicking.rs:355
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:365
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:549
   5: std::panicking::begin_panic
   6: rustc_errors::Handler::span_bug
   7: rustc_mir::transform::type_check::TypeChecker::check_terminator
   8: rustc::infer::InferCtxtBuilder::enter
   9: <rustc_mir::transform::type_check::TypeckMir as rustc::mir::transform::MirPass<'tcx>>::run_pass
  10: <T as rustc::mir::transform::MirMapPass<'tcx>>::run_pass
  11: rustc::mir::transform::Passes::run_passes
  12: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}::{{closure}}
  13: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}
  14: rustc_driver::driver::phase_3_run_analysis_passes
  15: rustc_driver::driver::compile_input
  16: rustc_driver::run_compiler
  17: std::panicking::try::do_call
  18: __rust_maybe_catch_panic
             at /checkout/src/libpanic_unwind/lib.rs:98
  19: <F as alloc::boxed::FnBox<A>>::call_box
  20: std::sys::imp::thread::Thread::new::thread_start
             at /checkout/src/liballoc/boxed.rs:658
             at /checkout/src/libstd/sys_common/thread.rs:21
             at /checkout/src/libstd/sys/unix/thread.rs:84
  21: start_thread
  22: __clone

% rustc --version --verbose
rustc 1.19.0-nightly (6a5fc9eec 2017-05-02)
binary: rustc
commit-hash: 6a5fc9eecec235312755e737fb5b984abe537f2e
commit-date: 2017-05-02
host: x86_64-unknown-linux-gnu
release: 1.19.0-nightly
LLVM version: 4.0

On the beta, instead you get:

error: cannot borrow immutable indexed content as mutable
 --> ice.rs:5:9
  |
5 |         things[src.as_str()].sort();
  |         ^^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable

error: aborting due to previous error

 rustc 1.18.0-beta.1 (4dce67253 2017-04-25)