-Z show-span crashes rustc

[kaseyc]

Platform: Linux Mint 16, 64-bit
Rustc version: rustc 0.12.0-pre-nightly (09cebc2 2014-09-07 00:31:28 +0000)

Sample Program:

fn main() {
    println!("Hello Rust");
}

When compiled with -Z show-span, the compiler will fail with `task 'rustc' failed at:

visit_mac disabled by default', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/libsyntax/ast_util.rs:776

Backtrace:

   1:     0x7effc43bfee0 - rt::backtrace::imp::write::h0322cc36a3f58594H8q
   2:     0x7effc43c3080 - failure::on_fail::h4244bff77454f4f4pur
   3:     0x7effc4bb8850 - unwind::begin_unwind_inner::h9f77a03cfeff340062d
   4:     0x7effc4f99370 - unwind::begin_unwind::h13888748684028530931
   5:     0x7effc511a510 - visit::Visitor::visit_mac::h5175099642578985137
   6:     0x7effc57f00f0 - visit::walk_block::h15593329261720824210
   7:     0x7effc57f0e50 - visit::walk_fn::h9877552939557075411
   8:     0x7effc57f0770 - visit::walk_item::h5589296153533657905
   9:     0x7effc57f17a0 - front::show_span::run::h345bf25a20bee057xPq
  10:     0x7effc5686d00 - driver::driver::phase_1_parse_input::hb08a34f5b31b9e02qxD
  11:     0x7effc58531d0 - driver::driver::compile_input::hfbabd4a6c7c0be4aIsD
  12:     0x7effc58e4a70 - driver::run_compiler::h5e700b3d5abdd789ZjH
  13:     0x7effc58e4960 - driver::main_args::closure.147495
  14:     0x7effc4fd1310 - task::TaskBuilder<S>::try_future::closure.100103
  15:     0x7effc4fd1110 - task::TaskBuilder<S>::spawn_internal::closure.100074
  16:     0x7effc638e5f0 - task::spawn_opts::closure.8408
  17:     0x7effc4c0e360 - rust_try_inner
  18:     0x7effc4c0e350 - rust_try
  19:     0x7effc4bb5f20 - unwind::try::hd71c364f2ee27fc1gRd
  20:     0x7effc4bb5d80 - task::Task::run::h7964802050e4385692c
  21:     0x7effc638e360 - task::spawn_opts::closure.8348
  22:     0x7effc4bb7900 - thread::thread_start::hdb150e157ee517fagod
  23:     0x7effc3e7cea0 - start_thread
  24:     0x7effc487e999 - __clone
  25:                0x0 - <unknown>