macOS builds are broken with recent rustc #312

thread '<unnamed>' panicked at 'misaligned pointer dereference: address must be a multiple of 0x8 but is 0x7fa07c701059', src/platform/macos/mod.rs:539:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread '<unnamed>' panicked at 'panic in a function that cannot unwind', library/core/src/panicking.rs:126:5
stack backtrace:
   0:        0x104632e43 - std::backtrace_rs::backtrace::libunwind::trace::h8c19cce8ed8ccf51
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/../../backtrace/src/backtrace/libunwind.rs:93:5
   1:        0x104632e43 - std::backtrace_rs::backtrace::trace_unsynchronized::h991e04b43bc79443
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:        0x104632e43 - std::sys_common::backtrace::_print_fmt::h8987ba788c846a85
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/sys_common/backtrace.rs:65:5
   3:        0x104632e43 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h0a694d0e7599586c
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/sys_common/backtrace.rs:44:22
   4:        0x10464f49b - core::fmt::rt::Argument::fmt::h0a8aa1334c46e31c
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/core/src/fmt/rt.rs:138:9
   5:        0x10464f49b - core::fmt::write::h3efe3f464ccee79e
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/core/src/fmt/mod.rs:1094:21
   6:        0x104630148 - std::io::Write::write_fmt::h0e2bb4c22bdace59
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/io/mod.rs:1713:15
   7:        0x104632c3a - std::sys_common::backtrace::_print::h718192d6d9e6c0a5
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/sys_common/backtrace.rs:47:5
   8:        0x104632c3a - std::sys_common::backtrace::print::hbdaa3eeb7c563975
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/sys_common/backtrace.rs:34:9
   9:        0x1046346f5 - std::panicking::default_hook::{{closure}}::hed147903f5cf7c57
  10:        0x1046344ff - std::panicking::default_hook::hfcf98064a6736dfc
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/panicking.rs:288:9
  11:        0x1045b5946 - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h9bbb1f1d9a50e95e
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/alloc/src/boxed.rs:1999:9
  12:        0x1045b5946 - test::test_main::{{closure}}::h253e00dda88e7637
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/test/src/lib.rs:134:21
  13:        0x104634d0a - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::h02bfaba73002fb16
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/alloc/src/boxed.rs:1999:9
  14:        0x104634d0a - std::panicking::rust_panic_with_hook::h07218173ef7e2e36
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/panicking.rs:709:13
  15:        0x104634a72 - std::panicking::begin_panic_handler::{{closure}}::h8f1360464839a927
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/panicking.rs:595:13
  16:        0x104633249 - std::sys_common::backtrace::__rust_end_short_backtrace::hb22d1d0c8f75c030
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/sys_common/backtrace.rs:151:18
  17:        0x1046347ed - rust_begin_unwind
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/panicking.rs:593:5
  18:        0x10465be63 - core::panicking::panic_nounwind_fmt::h3d8629ceefa59b23
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/core/src/panicking.rs:96:14
  19:        0x10465bf0b - core::panicking::panic_nounwind::hd768f4ccdfcb86a4
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/core/src/panicking.rs:126:5
  20:        0x10465c075 - core::panicking::panic_cannot_unwind::h4f579b1494f7e1d8
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/core/src/panicking.rs:188:5
  21:        0x1044b8856 - ipc_channel::platform::macos::OsIpcSender::send::hf10dfb749918bc7a
                               at /Users/jdm/src/ipc-channel/src/platform/macos/mod.rs:480:5
  22:        0x10457c170 - ipc_channel::platform::test::big_data_with_5_fds::{{closure}}::ha84e55d4172d7df0
                               at /Users/jdm/src/ipc-channel/src/platform/test.rs:310:17
  23:        0x10455428d - std::sys_common::backtrace::__rust_begin_short_backtrace::h3841e5dc8595b5b9
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/sys_common/backtrace.rs:135:18
  24:        0x10446e830 - std::thread::Builder::spawn_unchecked_::{{closure}}::{{closure}}::h728edd4a235b5cd5
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/thread/mod.rs:529:17
  25:        0x10456d460 - <core::panic::unwind_safe::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::h1c6f72c1057fbc08
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/core/src/panic/unwind_safe.rs:271:9
  26:        0x104555db4 - std::panicking::try::do_call::h6c132ae3ed4c8cbb
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/panicking.rs:500:40
  27:        0x104556f9d - ___rust_try
  28:        0x104555159 - std::panicking::try::h7fd1d38e673dd57f
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/panicking.rs:464:19
  29:        0x10446b91e - std::panic::catch_unwind::h4e227a1b4a215733
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/panic.rs:142:14
  30:        0x10446b91e - std::thread::Builder::spawn_unchecked_::{{closure}}::h606ea262f5ae4d61
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/thread/mod.rs:528:30
  31:        0x1044d9841 - core::ops::function::FnOnce::call_once{{vtable.shim}}::h0152d430eb975234
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/core/src/ops/function.rs:250:5
  32:        0x1046390b9 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h860f224c375c943e
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/alloc/src/boxed.rs:1985:9
  33:        0x1046390b9 - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::h746ad73385688dbd
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/alloc/src/boxed.rs:1985:9
  34:        0x1046390b9 - std::sys::unix::thread::Thread::new::thread_start::hed672fe9ad457508
                               at /rustc/398fa2187c88de46c13c142f600064483a563c86/library/std/src/sys/unix/thread.rs:108:17
  35:     0x7ff802b4b4e1 - __pthread_start