ICE when creating a slice of trait objects with DST impls

[sfackler]

trait Foo for Sized? {}
impl Foo for str {}

fn main() {
    let _: &[&Foo] = &["hi"];
}

~ ❯ env RUST_BACKTRACE=1 rustc test.rs
error: internal compiler error: Cannot skolemize an open existential type
note: the compiler hit an unexpected failure path. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
task 'rustc' failed at 'Box<Any>', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/libsyntax/diagnostic.rs:175

stack backtrace:
   1:     0x7f54f43eb2d0 - rt::backtrace::imp::write::h48fa88c972275002Ocq
   2:     0x7f54f43ee460 - failure::on_fail::h74f8e444491ded98byq
   3:     0x7f54f4b807a0 - unwind::begin_unwind_inner::h40c3cf7931984b6cMQd
   4:     0x7f54f2c79110 - unwind::begin_unwind::h3232892288091271644
   5:     0x7f54f2c798c0 - diagnostic::Handler::bug::h03a860c936e83069bVF
   6:     0x7f54f4f61af0 - driver::session::Session::bug::h94a76717ceb404e4K4A
   7:     0x7f54f52e44c0 - middle::typeck::infer::skolemize::TypeSkolemizer<'a, 'tcx>.TypeFolder<'tcx>::fold_ty::h77c93ea94ee9da31L5c
   8:     0x7f54f52cfee0 - iter::Iterator::collect::h9585558730712639288
   9:     0x7f54f52e41c0 - middle::ty_fold::TypeFolder::fold_substs::h3392183712144355477
  10:     0x7f54f52d4f20 - middle::traits::select::SelectionContext<'cx, 'tcx>::select::h17724f317e076ab8nYW
  11:     0x7f54f53694e0 - middle::trans::common::fulfill_obligation::he2dc5420601bb6casG8
  12:     0x7f54f53e9330 - middle::trans::meth::get_vtable::closure.126580
  13:     0x7f54f53eab70 - iter::FlatMap<'a, A, T, U>.Iterator<B>::next::h12179567760356319076
  14:     0x7f54f534d420 - middle::trans::meth::get_vtable::hc1197baede9797a4OVm
  15:     0x7f54f534cd00 - middle::trans::expr::apply_adjustments::unsized_info::h74afa68245e9d941164
  16:     0x7f54f534f260 - middle::trans::expr::apply_adjustments::unsize_expr::closure.122496
  17:     0x7f54f534f410 - middle::trans::expr::apply_adjustments::into_fat_ptr::h831614e550dfdde5ke5
  18:     0x7f54f534baa0 - middle::trans::expr::apply_adjustments::apply_autoref::hf62246e18289fd10YT4
  19:     0x7f54f534baa0 - middle::trans::expr::apply_adjustments::apply_autoref::hf62246e18289fd10YT4
  20:     0x7f54f530d360 - middle::trans::expr::trans::h72b00f77b9eea15f2H4
  21:     0x7f54f530be30 - middle::trans::expr::trans_into::h433dd942d5f4ee578D4
  22:     0x7f54f53e4740 - middle::trans::tvec::write_content::hbf100b1d310b02e8FWl
  23:     0x7f54f53529c0 - middle::trans::tvec::trans_slice_vec::hd0803056b15b46caZLl
  24:     0x7f54f53473b0 - middle::trans::expr::trans_unadjusted::h39ec14a9214b3aef6o5
  25:     0x7f54f530d360 - middle::trans::expr::trans::h72b00f77b9eea15f2H4
  26:     0x7f54f53a9d60 - middle::trans::_match::store_local::hbdc1b7f285db67earuk
  27:     0x7f54f530b410 - middle::trans::base::init_local::h1c6d4245725932e3z9f
  28:     0x7f54f530a7d0 - middle::trans::controlflow::trans_stmt::h0b5493ba18599ed0iP0
  29:     0x7f54f530c250 - middle::trans::controlflow::trans_block::headbddea2186fadfoU0
  30:     0x7f54f53b3540 - middle::trans::base::trans_closure::h0bd7d9b24ff40ba1Z0g
  31:     0x7f54f52fe7a0 - middle::trans::base::trans_fn::hd49d82889958d6fdnch
  32:     0x7f54f52fbdf0 - middle::trans::base::trans_item::ha64567091ae68dc0Gvh
  33:     0x7f54f53be2b0 - middle::trans::base::trans_crate::hc6ddf7cb9b74b3cdxti
  34:     0x7f54f580dda0 - driver::driver::phase_4_translate_to_llvm::h047a754742f6c05fxvA
  35:     0x7f54f5805180 - driver::driver::compile_input::he310fb7caba25623r2z
  36:     0x7f54f588ea30 - driver::run_compiler::hf7bb6d767410da97CPD
  37:     0x7f54f588e8e0 - driver::run::closure.144872
  38:     0x7f54f4f91470 - task::TaskBuilder<S>::try_future::closure.103263
  39:     0x7f54f4f91250 - task::TaskBuilder<S>::spawn_internal::closure.103234
  40:     0x7f54f60f5d90 - task::NativeSpawner.Spawner::spawn::closure.8447
  41:     0x7f54f4bd9a10 - rust_try_inner
  42:     0x7f54f4bd9a00 - rust_try
  43:     0x7f54f4b7dd20 - unwind::try::h91c99068494cbd6cuFd
  44:     0x7f54f4b7dba0 - task::Task::run::h36e17e218abeae3bkLc
  45:     0x7f54f60f5a90 - task::NativeSpawner.Spawner::spawn::closure.8385
  46:     0x7f54f4b7f600 - thread::thread_start::ha76ca183c940617fz6c
  47:     0x7f54f3eb6250 - start_thread
  48:     0x7f54f48583b9 - clone
  49:                0x0 - <unknown>

cc @nick29581

[nrc]

cc @nikomatsakis too, presumably a consequence of the trait reform/new vtable lookup code

[nikomatsakis]

Hmm, yes. Probably we can fix this by just...allowing it to skolemize an open existential type. I just didn't expect this to ever happen.

[nikomatsakis]

@nick29581 actually, while debugging this I remembered that it is not possible to make an object out of an unsized type. We need to add a check for this I guess!

[nikomatsakis]

Explanation from PR: In the general case, at least, it is not possible to make an object out of an unsized type. This is because the object type would have to store the fat pointer information for the self value and the vtable -- meaning it'd have to be a fat pointer with three words -- but for the compiler to know that the object requires three words, it would have to know the self-type of the object (is self a thin or fat pointer?), which of course it doesn't.