libbacktrace allocation is sometimes very slow

originally here: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68239

affects rustc as mentioned here: rust-lang/rust#38984

Basically, if you have Rust via rustup, you can test like this(assuming shell is bash):

$ time RUST_BACKTRACE=1 rustc -vv
error: Option 'verbose' given more than once.


real	0m0.929s
user	0m0.888s
sys	0m0.037s

$ time RUST_BACKTRACE=0 rustc -vv
error: Option 'verbose' given more than once.


real	0m0.161s
user	0m0.139s
sys	0m0.023s

There's a time difference of almost 0.8s, which isn't much, but will be very noticeable(goes up to like 40 seconds?) if rust is compiled with debug-info.

I'm not sure what could be done, but there's a somewhat in-depth explanation here rust-lang/rust#37477 (comment)

Cheers!