rustc hangs when RUST_BACKTRACE is on

[oconnor663]

Rustc hangs for almost 20 seconds when trying to compile an empty file, but only when RUST_BACKTRACE is on.

$ touch /tmp/empty.rs

$ export RUST_BACKTRACE=0
$ time rustc /tmp/empty.rs
error: main function not found

error: aborting due to previous error


real    0m0.073s
user    0m0.057s
sys     0m0.010s


$ export RUST_BACKTRACE=1
$ time rustc /tmp/empty.rs
error: main function not found

error: aborting due to previous error


real    0m18.557s
user    0m18.497s
sys     0m0.057s

$ rustc --version
rustc 1.14.0 (e8a012324 2016-12-16)

[retep998]

Dupe of #37477?

[sfackler]

Closing as dup