Right-shifting an primitive integer in Rust (and most other languages AFAIK) is equivalent to dividing by a power of 2 and rounding towards -∞. But right-shifting a BigInt rounds towards 0 instead.

You can see the difference by pasting the following into https://play.rust-lang.org/:

extern crate num;

fn main() {
    println!("-1 >> 1 == {} (primitive)", -1 >> 1);
    println!("-1 >> 1 == {} (BigInt)", num::BigInt::from(-1) >> 1);
}

This prints

-1 >> 1 == -1 (primitive)
-1 >> 1 == 0 (BigInt)

If this behavior is intentional it should probably be documented and unit-tested.