ICE on type mismatch: Index out of bounds

[crumblingstatue]

Test case:

fn main() {
    let mut buffer = Vec::new();
    std::io::stdin().read_line(&mut buffer);
}

Output:

src/main.rs:3:32: 3:43 error: mismatched types:
 expected `&mut collections::string::String`,
    found `&mut collections::vec::Vec<_>`
(expected struct `collections::string::String`,
    found struct `collections::vec::Vec`) [E0308]
src/main.rs:3     std::io::stdin().read_line(&mut buffer);
                                             ^~~~~~~~~~~
src/main.rs:3:32: 3:43 help: run `rustc --explain E0308` to see a detailed explanation
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 4 but the index is 6', ../src/libcollections/vec.rs:1178
stack backtrace:
   1:     0x7f4effa18110 - sys::backtrace::tracing::imp::write::h7af3c3cfd3e65a783bv
   2:     0x7f4effa2146b - panicking::default_handler::_$u7b$$u7b$closure$u7d$$u7d$::closure.44497
   3:     0x7f4effa20fc3 - panicking::default_handler::h75ebcc0c8bcd1085rRz
   4:     0x7f4eff9e585c - sys_common::unwind::begin_unwind_inner::hcec45e84262f1855m0t
   5:     0x7f4eff9e62f8 - sys_common::unwind::begin_unwind_fmt::h62c174e37e54c321sZt
   6:     0x7f4effa15b71 - rust_begin_unwind
   7:     0x7f4effa6856f - panicking::panic_fmt::h5f920bbc722cd548lcM
   8:     0x7f4effa68752 - panicking::panic_bounds_check::hc7552cae568055c9rbM
   9:     0x7f4efcd4069f - middle::infer::InferCtxt<'a, 'tcx>::shallow_resolve::hf6ff75a42cfc2abfCiE
  10:     0x7f4efce10abb - middle::traits::fulfill::process_predicate::hba0cba8efbf8acdawyU
  11:     0x7f4efce0f7e7 - obligation_forest::ObligationForest<O, T>::process_obligations::h16997701126397106394
  12:     0x7f4efce0ae6f - middle::traits::fulfill::FulfillmentContext<'tcx>::select_where_possible::h1b112c81d6b103c5BqU
  13:     0x7f4efefa2ed1 - check::FnCtxt<'a, 'tcx>::select_obligations_where_possible::h3110f11e9a68791ajDq
  14:     0x7f4efefc9c0c - check::check_argument_types::h3e4bc8dcbdfe3e47j3q
  15:     0x7f4efefc8f24 - check::check_method_argument_types::h2b421829214fc9bfK0q
  16:     0x7f4efefdc038 - check::check_expr_with_unifier::check_method_call::h40605e88d809513emFr
  17:     0x7f4eff00523f - check::check_expr_with_unifier::h7435735517896598653
  18:     0x7f4eff018a71 - check::check_stmt::h27f273f9d63167496Vs
  19:     0x7f4efefa9303 - check::check_block_with_expected::h6ffc0ae0594a45f7TZs
  20:     0x7f4efefa0289 - check::check_fn::h2545140b5fe4a17bYPo
  21:     0x7f4efef9d99f - check::check_bare_fn::h92b85a4e7b67d57asFo
  22:     0x7f4efef98846 - check::check_item_body::hf143be9d0498177aY4o
  23:     0x7f4efef8f2fb - check::check_item_bodies::hfdadc35dd9e39914fDo
  24:     0x7f4efef86b83 - check_crate::he92b840a76d6d63cOEC
  25:     0x7f4efff43517 - driver::phase_3_run_analysis_passes::_$u7b$$u7b$closure$u7d$$u7d$::closure.28579
  26:     0x7f4efff41985 - middle::ty::context::ctxt<'tcx>::create_and_enter::h6681449678745636205
  27:     0x7f4efff3e4df - driver::phase_3_run_analysis_passes::h3690321397878926181
  28:     0x7f4efff11845 - driver::compile_input::hf5b4c0cfabf44771Hca
  29:     0x7f4efff00997 - run_compiler::h382da26f627d0d776Oc
  30:     0x7f4effefe111 - sys_common::unwind::try::try_fn::h10497329671291346377
  31:     0x7f4effa15afb - __rust_try
  32:     0x7f4effa0e0ad - sys_common::unwind::inner_try::h27915955daa5bb0coXt
  33:     0x7f4effefe960 - boxed::F.FnBox<A>::call_box::h12722420362853020748
  34:     0x7f4effa1fa39 - sys::thread::Thread::new::thread_start::h47cad50136a0a353LOy
  35:     0x7f4ef81a5423 - start_thread
  36:     0x7f4eff697cbc - clone
  37:                0x0 - <unknown>

[TimNN]

Even smaller test case:

fn main() {
    // each statement produces the same ICE
    let _: &mut String = &mut Vec::new();
    let _: &String = &Vec::new();
    let _: &() = &Vec::new();
}

(Edited with more examples).

[alexcrichton]

Nominating because this seems to be coming up all over the place, I think we've gotten 3-4 separate reports about this ICE (a regression on beta).

Note that an ICE happens on stable, but a worse ICE happens on beta/nightly...

[TimNN]

To be more precise, the regression is that the actual error message about mismatched types only shows up on stable and not on beta or nightly.

[nikomatsakis]

related to #25801

[nikomatsakis]

triage: P-high

This core issue according to @arielb1 is the problem of the OF not doing snapshotting, but method resolution assumes it does. An older fix was #26324.

[arielb1]

Not method resolution, but rather coercion (autoderef in deref-coerce).