ICE using unboxed closure sugar for FnOnce

[pythonesque]

Sorry if this is a duplicate, I couldn't find anything.

#![feature(unboxed_closures,overloaded_calls)]

fn main() {
  let foo = 0i8;
  let bar = |:|{ foo };
}

---

$ RUST_BACKTRACE=1 rustc foo.rs
error: internal compiler error: unexpected failure
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 'no entry found for key', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/libstd/collections/hashmap/map.rs:1093

stack backtrace:
   1:     0x7f93d1980740 - rt::backtrace::imp::write::ha917ced5a810f66cUVq
   2:     0x7f93d19838d0 - failure::on_fail::h35f292864ffd1035mhr
   3:     0x7f93d21399e0 - unwind::begin_unwind_inner::h7b2733ff05e8f899SUd
   4:     0x7f93d25123c0 - unwind::begin_unwind::h3837541141404774985
   5:     0x7f93d26ed130 - middle::expr_use_visitor::ExprUseVisitor<'d, 't, TYPER>::walk_captures::closure.113283
   6:     0x7f93d26ed010 - middle::ty::with_freevars::h15643820832341047709
   7:     0x7f93d2701150 - middle::expr_use_visitor::ExprUseVisitor<'d, 't, TYPER>::walk_expr::h17509852025542073980
   8:     0x7f93d2700e30 - middle::expr_use_visitor::ExprUseVisitor<'d, 't, TYPER>::walk_block::h7871336635011900049
   9:     0x7f93d26fb0e0 - middle::borrowck::gather_loans::gather_loans_in_fn::h4d158f6362a75ab3P3h
  10:     0x7f93d2721420 - middle::borrowck::build_borrowck_dataflow_data::hf1c03f8dbedd6896XQj
  11:     0x7f93d271e8d0 - middle::borrowck::borrowck_fn::h4dd4dc3f33cb9c337Nj
  12:     0x7f93d27206c0 - visit::walk_item::h977840171384247700
  13:     0x7f93d271f080 - middle::borrowck::check_crate::hc8b4d432f8d78572VGj
  14:     0x7f93d252fc50 - util::common::time::h5222882638945901854
  15:     0x7f93d2d65790 - driver::driver::phase_3_run_analysis_passes::h4c733c82d969baafkPw
  16:     0x7f93d2d60fa0 - driver::driver::compile_input::hd245ec58bbc2ac2b9vw
  17:     0x7f93d2de2da0 - driver::run_compiler::h88fa61952d453b8cdmA
  18:     0x7f93d2de2c80 - driver::main_args::closure.146353
  19:     0x7f93d2548be0 - task::TaskBuilder<S>::try_future::closure.101568
  20:     0x7f93d25489d0 - task::TaskBuilder<S>::spawn_internal::closure.101539
  21:     0x7f93d36f1fa0 - task::spawn_opts::closure.8494
  22:     0x7f93d218eb20 - rust_try_inner
  23:     0x7f93d218eb10 - rust_try
  24:     0x7f93d2136b70 - unwind::try::hef24d636384aaae6AJd
  25:     0x7f93d2136a00 - task::Task::run::hbcbabe3609aa7293UYc
  26:     0x7f93d36f1d10 - task::spawn_opts::closure.8434
  27:     0x7f93d2138a80 - thread::thread_start::h86947667e5e1368fpjd
  28:     0x7f93d144a250 - start_thread
  29:     0x7f93d1e0f3b9 - clone
  30:                0x0 - <unknown>

[bkoropoff]

Taking a look

[bkoropoff]

It appears that walk_captures is expecting to find the free var in the upvar_borrow_map, but regionck did not populate it with anything.

[bkoropoff]

I might have a fix for this, stand by

[bkoropoff]

Well, I fixed the ICE, but upon further inspection it seems that by-ref variable capture in unboxed closures isn't completely baked. I have some patches that seem to get things working in regionck/borrowck, but trans still needs work. I'm not sure if I should continue on this path since @pcwalton may have other plans here.

[bkoropoff]

For some reason github did not mark the PR as merged, but this bug is fixed and can be closed.