Crash (null ptr) in recv_opt

[gavinb]

A simple I/O test sending messages across channels between tasks causes a crash.

Tested on OS X 10.9 with rev c84890 (a very recent build from master).

Source here: https://gist.github.com/gavinb/9064658

The output is:

Sending...
Sending...
Joe 0
Ann 0
Receiving...
Receiving...

But it then crashes with:

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000
[Switching to process 3210 thread 0x1003]
0x0000000100001841 in sync::atomics::atomic_load::h0eb835f964245badDqa::v0.0 ()
(gdb) bt
#0  0x0000000100001841 in sync::atomics::atomic_load::h0eb835f964245badDqa::v0.0 ()
#1  0x00000001000a5b50 in io::stdio::println_args::closure ()
#2  0x00000001000017d4 in sync::atomics::AtomicUint::load::h630f72316818459ejqa::v0.0 ()
#3  0x0000000100014004 in sync::arc::UnsafeArc$LT$T$GT$::get::hcae23af7d89013bbIsc::v0.0 ()
#4  0x0000000100026de0 in comm::Port$LT$T$GT$::recv_opt::hdfac7690a2025dc4J3d::v0.0 ()
#5  0x0000000100026cc6 in comm::Port$LT$T$GT$::recv::h93835721c191a171gSe::v0.0 () at boring09.rs:55
#6  0x00000001000267e7 in fn4474 () at boring09.rs:55
#7  0x000000010011dcec in task::bootstrap_green_task::closure ()
#8  0x00000001000faa78 in rt::task::Task::run::closure ()
#9  0x000000010010479c in rust_try ()
#10 0x000000010011dba5 in task::bootstrap_green_task::h763de3b86e4a2837Sbc::v0.10.pre ()

[alexcrichton]

Sadly this is just #12041 striking again, so closing as a dupe of that bug.

You're moving out of wait_chan and wait_port in a loop, and the compiler shouldn't let you do that.