Compiler panic on invalid reference type

[ninewise]

Context: I was playing around with trees for a bit, when I decided the order of the children didn't matter and swapped Vecs for HashSets. However, I forgot to substitute them everywhere, which resulted in the following (minimized) code and a compiler panic.

I tried this code:

use std::collections::HashMap;
use std::collections::HashSet;

fn main() {
    let tree: HashMap<usize, HashSet<usize>> = HashMap::new();
    let size_of_1 = tree.get(&1).unwrap_or(&Vec::new()).len();
}

I expected to see this happen: the rust compiler complaining about incorrect types. After all, the &Vec::new() should've been a &HashSet::new().

Instead, this happened:

$ rustc test.rs 
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
thread 'rustc' panicked at 'index out of bounds: the len is 18 but the index is 20', src/libcollections/vec.rs:1166
note: Run with `RUST_BACKTRACE=1` for a backtrace.

Note that using the correct intializer works, and using

use std::collections::HashMap;
use std::collections::HashSet;

struct A { a: usize }

fn main() {
    let tree: HashMap<usize, HashSet<usize>> = HashMap::new();
    let size_of_1 = tree.get(&1).unwrap_or(&A {a: 1}).len();
}

complains as expected:

$ rustc test.rs
test.rs:9:44: 9:53 error: mismatched types:
 expected `&std::collections::HashSet<usize>`,
    found `&A`
(expected struct `std::collections::HashSet`,
    found struct `A`) [E0308]
test.rs:9     let size_of_1 = tree.get(&1).unwrap_or(&A {a: 1}).len();
                                                     ^~~~~~~~~
test.rs:9:44: 9:53 help: run `rustc --explain E0308` to see a detailed explanation
error: aborting due to previous error

Meta

$ rustc --version --verbose
rustc 1.9.0
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.9.0

Backtrace:

$ RUST_BACKTRACE=1 rustc test.rs
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: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'index out of bounds: the len is 18 but the index is 20', src/libcollections/vec.rs:1166
stack backtrace:
   1:     0x7fedb86d7310 - <unknown>
   2:     0x7fedb86e4a0b - <unknown>
   3:     0x7fedb86e45ac - <unknown>
   4:     0x7fedb86a953f - std::sys_common::unwind::begin_unwind_inner::h39d40f52add53ef7
   5:     0x7fedb86ab628 - std::sys_common::unwind::begin_unwind_fmt::h64c0ff793199cc1b
   6:     0x7fedb86d4b21 - rust_begin_unwind
   7:     0x7fedb872730f - core::panicking::panic_fmt::h73bf9d7e8e891a73
   8:     0x7fedb87274f2 - core::panicking::panic_bounds_check::hc30e971884568c27
   9:     0x7fedb2d52bcd - <unknown>
  10:     0x7fedb2ce89e0 - rustc::infer::InferCtxt::shallow_resolve::he47cdea6677f40d2
  11:     0x7fedb2e49eb0 - <unknown>
  12:     0x7fedb2e48c27 - <unknown>
  13:     0x7fedb2e4424f - rustc::traits::fulfill::FulfillmentContext::select_where_possible::hf3fc6484c301b5bb
  14:     0x7fedb3abbc91 - <unknown>
  15:     0x7fedb3acd423 - rustc_typeck::check::FnCtxt::resolve_type_vars_if_possible::ha2a2b0468a33b7be
  16:     0x7fedb3ae1ef6 - rustc_typeck::check::demand::coerce::hd43013069440cf8a
  17:     0x7fedb3adf606 - <unknown>
  18:     0x7fedb3ade63f - <unknown>
  19:     0x7fedb3ae4b49 - <unknown>
  20:     0x7fedb3ae2ba2 - <unknown>
  21:     0x7fedb3b1a40a - rustc_typeck::check::check_decl_initializer::h130cfba0c1b76b7a
  22:     0x7fedb3b1a4d1 - rustc_typeck::check::check_decl_local::he651fa13100c1d0f
  23:     0x7fedb3b1a7c1 - rustc_typeck::check::check_stmt::h90d31dcbe73b93a2
  24:     0x7fedb3ac15a8 - <unknown>
  25:     0x7fedb3ab9239 - <unknown>
  26:     0x7fedb3ab6aec - <unknown>
  27:     0x7fedb3ab19c6 - rustc_typeck::check::check_item_body::h1895344155d5b2b2
  28:     0x7fedb3aa9881 - rustc_typeck::check::check_item_bodies::h525c1aa0e1abb529
  29:     0x7fedb3aa0def - rustc_typeck::check_crate::h0ef96f4043e1e69a
  30:     0x7fedb8c21960 - <unknown>
  31:     0x7fedb8c1fa48 - <unknown>
  32:     0x7fedb8c1c45e - <unknown>
  33:     0x7fedb8bee90f - rustc_driver::driver::compile_input::h6491aaddd9e61258
  34:     0x7fedb8bd4e74 - rustc_driver::run_compiler::h80b2ba5e4d787c5f
  35:     0x7fedb8bd22d1 - <unknown>
  36:     0x7fedb86d4aab - <unknown>
  37:     0x7fedb86d4a3d - std::sys_common::unwind::inner_try::h9eebd8dc83f388a6
  38:     0x7fedb8bd2b1a - <unknown>
  39:     0x7fedb86e2ba4 - <unknown>
  40:     0x7fedb120b3f3 - start_thread
                        at /builddir/glibc-2.23/nptl/pthread_create.c:333
  41:     0x7fedb833fbac - clone
                        at ../sysdeps/unix/sysv/linux/x86_64/clone.S:109
  42:                0x0 - <unknown>

[jonas-schievink]

Probably autoderef, might be fixed on nightly

[retep998]

Still occurs on nightly according to playpen.

[jonas-schievink]

I think playpen's nightly is outdated for some reason. The current nightly gives the expected error and no ICE.

[TimNN]

The nightly on playpen is 2016-05-18, see rust-lang/rust-playpen#215

[nagisa]

Same as #34428, I think. The code now works on nightly, and the other issue has better backtrace.