unhelpful in gdb, too

gistfile1.txt

(gdb) target exec ./shootout-chameneos-redux
...
(gdb) break upcall_fail
Breakpoint 1 at 0x7ffff71d44f0: file /code/rust/src/rt/rust_upcall.cpp, line 95.
(gdb) run 50
Starting program: /code/chameneos/shootout-chameneos-redux 50
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7ffff7ff7700 (LWP 20173)]
[New Thread 0x7ffff7fd3700 (LWP 20174)]
[New Thread 0x7ffff7fba700 (LWP 20175)]
[New Thread 0x7ffff7fa1700 (LWP 20176)]
[New Thread 0x7ffff7f88700 (LWP 20177)]
[New Thread 0x7ffff7f6f700 (LWP 20178)]
[New Thread 0x7ffff7f56700 (LWP 20179)]
[New Thread 0x7ffff7f3d700 (LWP 20180)]
[New Thread 0x7ffff7f24700 (LWP 20181)]
[New Thread 0x7ffff7f0b700 (LWP 20182)]
blue + blue -> blue
blue + red -> yellow
blue + yellow -> red
red + blue -> yellow
red + red -> red
red + yellow -> blue
yellow + blue -> red
yellow + red -> blue
yellow + yellow -> yellow

[Switching to Thread 0x7ffff7fd3700 (LWP 20174)]

Breakpoint 1, upcall_fail (expr=0x7ffff01059e0 "option none", 
    file=0x439c90 "shootout-chameneos-redux.rs", line=1)
    at /code/rust/src/rt/rust_upcall.cpp:95
95	            size_t line) {
(gdb) bt
#0  upcall_fail (expr=0x7ffff01059e0 "option none", 
    file=0x439c90 "shootout-chameneos-redux.rs", line=1)
    at /code/rust/src/rt/rust_upcall.cpp:95
#1  0x000000000042981b in ?? ()
#2  0x00007ffff0102d50 in ?? ()
#3  0x0000000000000000 in ?? ()
(gdb)