ICE when calling mutation method on map value indexed with mutable reference

[saghm]

When calling a mutation method on an value obtained from a HashMap with a mutable reference index, an ICE occurs.

Code:

use std::collections::HashMap;

fn main() {
    let mut map: HashMap<i32, Vec<i32>> = HashMap::new();
    let mut i = 1;
    map.insert(i, Vec::new());

    map[&mut i].push(3); // ICE occurs here
}

Compiler output:

error: internal compiler error: re-trying op failed
 --> src/main.rs:8:5
  |
8 |     map[&mut i].push(3);
  |     ^^^^^^^^^^^

note: the compiler unexpectedly panicked. this is a bug.

Backtrace:

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:437:8
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
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:380
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:390
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:611
   5: std::panicking::begin_panic_new
   6: rustc_errors::Handler::abort_if_errors
   7: rustc_passes::consts::check_crate
   8: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}
   9: rustc_driver::driver::phase_3_run_analysis_passes
  10: rustc_driver::driver::compile_input
  11: rustc_driver::run_compiler

I believe that the correct behavior here would be for the compiler to report an error to the user on the indicated line due to IndexMut not being implemented for HashMap, which is what occurs when replacing &mut i with just &i.

rustc version:

rustc 1.20.0 (f3d6973f4 2017-08-27)
binary: rustc
commit-hash: f3d6973f41a7d1fb83029c9c0ceaf0f5d4fd7208
commit-date: 2017-08-27
host: x86_64-unknown-linux-gnu
release: 1.20.0
LLVM version: 4.0

[Aaron1011]

Here's a self-contained version of the snippet:

use std::ops::Index;

struct Test;
struct Container(Test);

impl Test {
    fn test(&mut self) {
    }
}

impl<'a> Index<&'a bool> for Container {
    type Output = Test;

    fn index(&self, index: &'a bool) -> &Test {
        &self.0
    }

}


fn main() {
    let mut container = Container(Test);
    let mut val = true;
    container[&mut val].test();
}

Playground link

[jakubadamw]

This doesn't ICE any more.