ICE when implementing an extern function for a struct

[michaelwu]

This seems to ICE rustc with:

task 'rustc' panicked at 'assertion failed: f.abi == Rust || f.abi == RustCall', /home/mikew/moz/rust/src/librustc/middle/trans/base.rs:2354

stack backtrace:
   1:     0x7fa499f9e9d0 - rt::backtrace::imp::write::h0cab0bbb58bba71escq
   2:     0x7fa499fa1a50 - failure::on_fail::h6ac545c6bf7ea5263xq
   3:     0x7fa49a73c600 - unwind::begin_unwind_inner::he5d38a771f1206897Rd
   4:     0x7fa49ab1a3b0 - unwind::begin_unwind::h1086155491838850086
   5:     0x7fa49af441e0 - middle::trans::base::register_fn::hfd170b590ed3a02fmii
   6:     0x7fa49af46cd0 - middle::trans::base::register_method::hfe10e0da5709cf90LYi
   7:     0x7fa49ae8fe30 - middle::trans::base::get_item_val::h6d62bf177e2d4e6aTGi
   8:     0x7fa49af42540 - middle::trans::meth::trans_impl::hd263ff9d7dcb7badB0m
   9:     0x7fa49ae8eb10 - middle::trans::base::trans_item::h2513dbab928ac05aSai
  10:     0x7fa49af49450 - middle::trans::base::trans_crate::h573d15cff926c8d7T8i
  11:     0x7fa49b39afe0 - driver::driver::phase_4_translate_to_llvm::h61f5ea2b09bc4eebAyC
  12:     0x7fa49b391e80 - driver::driver::compile_input::h20774442580da6c5u5B
  13:     0x7fa49b415230 - driver::run_compiler::h32117b7363daaea3LVF
  14:     0x7fa49b415120 - driver::run::closure.146540
  15:     0x7fa49ab4f4a0 - task::TaskBuilder<S>::try_future::closure.104852
  16:     0x7fa49ab4f290 - task::TaskBuilder<S>::spawn_internal::closure.104823
  17:     0x7fa49bc9f160 - task::NativeSpawner.Spawner::spawn::closure.8456
  18:     0x7fa49a791650 - rust_try_inner
  19:     0x7fa49a791640 - rust_try
  20:     0x7fa49a739f80 - unwind::try::h3154e4b14627d719PGd
  21:     0x7fa49a739e10 - task::Task::run::h554324f28003d18dFMc
  22:     0x7fa49bc9eea0 - task::NativeSpawner.Spawner::spawn::closure.8394
  23:     0x7fa49a73b620 - thread::thread_start::h4297c8c4ec78b606U7c
  24:     0x7fa499a6cd40 - start_thread
  25:     0x7fa49a40c299 - clone
  26:                0x0 - <unknown>

Testcase:

struct Foo {
        Bar: i32,
}

impl Foo {
        extern fn X() -> i32 {
                0
        }
}

fn main() {
}

[huonw]

Dupe of #19047; thanks for filing!