On a system like:

OS, ker|rel, machine: Linux, 3.16.0-7-amd64, x86_64
Distribution        : Debian 8.11 (jessie)

in which the kernel is too old for getrandom, the rustc (via cargo) paniced and stopped the compilation. I reported it to rustc folks (github) and they fixed it in the recent nightly build. Currently the rustc toolchain I am using is:
rustc 1.39.0-nightly (34e82a7b7 2019-09-10)
cheers, drl