Compiler crashes when wrapping Rust callbacks in C callbacks

[manuels]

I want to implement a Rust interface for libevent (because an external library I want to use depends on it).
Note that there is an example with working code, code that issues the compiler error and a diff between these two in this gist:
https://gist.github.com/manuels/aaaf1ef5d8a072c19f11

When I define a C callback function for a libevent event:

extern "C"
fn printme(ev: *const c_int, flags: c_short, args: *const c_int) {
    // using this as a libevent callback works just fine
    println!("hello");
}

the code works fine. Note that args is a userdefined argument that will be used in the next step.

Relying on callbacks to be defined as "extern C" is a bit clumsy, so I'd like to define an "extern C" wrapper function that calls the rust callback function:

type watch_fd_callback_t = fn(*const c_int, c_short);

extern "C"
fn c_watch_fd_callback(ev:*const c_int, type_:c_short, cb: watch_fd_callback_t) {
    // using this as a libevent callback crashes the compiler.
    // probably because this "extern C" function indirectly 
    // calls the non "extern C" function printme() (==cb)
    cb(ev, type_)
}

fn printme(ev: *const c_int, flags: i16) {
    println!("hello");
}

So when I use the "extern C" wrapper callback c_watch_fd_callback() that calls the rust callback printme() the compiler crashes with this backtrace:

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 'assertion failed: f.abi == Rust || f.abi == RustCall', /home/rustbuild/src/rust-buildbot/slave/nightly-linux/build/src/librustc/middle/trans/base.rs:2337

stack backtrace:
   1:     0x7fcf4ff31bf0 - rt::backtrace::imp::write::h37a3af22082b7094FKq
   2:     0x7fcf4ff34d20 - failure::on_fail::hf01d9f1e1bdb6e5425q
   3:     0x7fcf5449db30 - unwind::begin_unwind_inner::h2ec64a9d433f6b868yd
   4:     0x7fcf54ae2f20 - unwind::begin_unwind::h16532647071079331256
   5:     0x7fcf54f55770 - middle::trans::base::register_fn::habe93b484695cbab1kh
   6:     0x7fcf54f58680 - middle::trans::base::register_method::hd6b6a60e17289fdea1h
   7:     0x7fcf54e998e0 - middle::trans::base::get_item_val::h87cb1de1e8e4455aeJh
   8:     0x7fcf54f53a60 - middle::trans::meth::trans_impl::h061539a782350394U1l
   9:     0x7fcf54e98400 - middle::trans::base::trans_item::h8e6f687fd31b992bxdh
  10:     0x7fcf54e98400 - middle::trans::base::trans_item::h8e6f687fd31b992bxdh
  11:     0x7fcf54f5b030 - middle::trans::base::trans_crate::h1e2bed8778c95c8dkbi
  12:     0x7fcf553b43b0 - driver::driver::phase_4_translate_to_llvm::hbdeb13ca358e159eD9z
  13:     0x7fcf553ab450 - driver::driver::compile_input::h8a818e073bdb900cyGz
  14:     0x7fcf55436950 - driver::run_compiler::h7e400358677f3066StD
  15:     0x7fcf55436800 - driver::run::closure.145782
  16:     0x7fcf54b1bb70 - task::TaskBuilder<S>::try_future::closure.103164
  17:     0x7fcf54b1b950 - task::TaskBuilder<S>::spawn_internal::closure.103135
  18:     0x7fcf547e8a30 - task::spawn_opts::closure.8464
  19:     0x7fcf544f7aa0 - rust_try_inner
  20:     0x7fcf544f7a90 - rust_try
  21:     0x7fcf5449b3f0 - unwind::try::h0bc5cc5739b8d3c3Qnd
  22:     0x7fcf5449b270 - task::Task::run::h517e4360aedfbf45uDc
  23:     0x7fcf547e8770 - task::spawn_opts::closure.8404
  24:     0x7fcf5449cb40 - thread::thread_start::h43c1915a7e23212cFXc
  25:     0x7fcf4f2ddfe0 - start_thread
  26:     0x7fcf5416dbf9 - __clone
  27:                0x0 - <unknown>

I'm running Linux 3.16-2-amd64 #1 SMP Debian 3.16.3-2 (2014-09-20) x86_64 GNU/Linux with rustc 0.13.0-nightly (222ae8b9b 2014-10-18 00:47:22 +0000).

[tamird]

@manuels this code no longer compiles. Perhaps you could reduce the test case while you're looking at it again?

[manuels]

Sure, I'm gonna have a look at it when in a week when I'm back from vacations.

[manuels]

I just updated the file (https://gist.github.com/manuels/8c264818252cabe3081c) and with the latest rustc 1.0.0-beta2 the bug vanished ;)