Rust Backtraces?

[rslawson]

In PR #690 I introduced a GDB script that should be able to grab backtraces when Rust code panics in the TCP test. However, despite my best efforts, those backtraces are limited to a depth of 2, and one of the displayed functions is invariably core::panicking::panic (or related). I'm not sure if this is because something is ignoring panic = "abort", or if the -C force-unwind-tables=yes -C force-frame-pointer=yes options are being ignored somehow, but this is definitely worth investigating.