Panic while trying to cast raw pointer to `Fn`

[xymostech]

I'm getting a rustc panic while trying to compile code like this:

extern crate libc;

fn main() {
    let userdata: *mut libc::c_void;
    let cb: &mut Fn() = unsafe {
        &mut *(userdata as *mut Fn())
    };
}

Here's the error, and the rustc backtrace:

client.rs:6:16: 6:24 error: internal compiler error: expected object type
client.rs:6         &mut *(userdata as *mut Fn())
                           ^~~~~~~~
note: the compiler unexpectedly panicked. 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
thread 'rustc' panicked at 'Box<Any>', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:129

stack backtrace:
   1:     0x7f7130839d60 - sys::backtrace::write::h932393543d112c054sy
   2:     0x7f713085d460 - failure::on_fail::h4947072b7f324f9aVWF
   3:     0x7f71307be800 - rt::unwind::begin_unwind_inner::had7a395ab2928a1fsBF
   4:     0x7f712dbcc920 - rt::unwind::begin_unwind::h6569531574505901365
   5:     0x7f712dbcc8b0 - diagnostic::SpanHandler::span_bug::h7d8e654a08708b434CE
   6:     0x7f712e6dffc0 - session::Session::span_bug::h30c717681b947e2e5aq
   7:     0x7f712ffd3a80 - check::check_cast::h7a31a3aa7ba43ff5aGn
   8:     0x7f7130013ae0 - check::check_expr_with_unifier::h11644655964435566975
   9:     0x7f7130013ae0 - check::check_expr_with_unifier::h11644655964435566975
  10:     0x7f7130013ae0 - check::check_expr_with_unifier::h11644655964435566975
  11:     0x7f7130002810 - check::check_expr_with_unifier::h7412840087484647266
  12:     0x7f712ffce450 - check::check_block_with_expected::h964ea164203d003cLDr
  13:     0x7f7130002810 - check::check_expr_with_unifier::h7412840087484647266
  14:     0x7f7130028dd0 - check::check_decl_local::h83b0a86db28fb770yxr
  15:     0x7f712ffce450 - check::check_block_with_expected::h964ea164203d003cLDr
  16:     0x7f712ffaf2f0 - check::check_fn::h650b0fe39307a8593Qm
  17:     0x7f712ffcb050 - check::check_bare_fn::hf9d06c1c6656a93clGm
  18:     0x7f712ffc2950 - check::check_item::h1e6dbfd08cc874a6CZm
  19:     0x7f713008e9d0 - check_crate::closure.34504
  20:     0x7f71300893b0 - check_crate::h141ffcbde406cd95ZKA
  21:     0x7f7130e2c4e0 - driver::phase_3_run_analysis_passes::hdbd51c532175b9f1pGa
  22:     0x7f7130e12a30 - driver::compile_input::h3abd4b23cefb4912Cba
  23:     0x7f7130ee3f80 - run_compiler::hf6c7680d20cecff97ac
  24:     0x7f7130ee2610 - thunk::F.Invoke<A, R>::invoke::h12458147831130807544
  25:     0x7f7130ee1540 - rt::unwind::try::try_fn::h13767943623378092967
  26:     0x7f71308c93b0 - rust_try_inner
  27:     0x7f71308c93a0 - rust_try
  28:     0x7f7130ee17f0 - thunk::F.Invoke<A, R>::invoke::h15079797001481760000
  29:     0x7f7130849950 - sys::thread::thread_start::h426dd27d44093655UCB
  30:     0x7f712a8d2250 - start_thread
  31:     0x7f713044b219 - clone
  32:                0x0 - <unknown>

I'm using:

$ rustc --version
rustc 1.0.0-nightly (ba2f13ef0 2015-02-04 20:03:55 +0000)