Diverging from primitive behavior in overflowing shift

[elichai]

The rust docs say the following about overflowing shift:

If the shift value is too large, then value is masked (N-1) where N is the number of bits, and this value is then used to perform the shift.
(Source: https://doc.rust-lang.org/std/primitive.u64.html#method.overflowing_shl)
And you can see this via the following code:

#[allow(arithmetic_overflow)]
fn main() {
    let a: u64 = 500;
    let b = a << 119;
    println!("{}", b);
}

Which prints 18014398509481984000.

On the other hand in the implementation here zeros will be returned:

crypto-bigint/src/uint/shl.rs

Line 18 in 36dbb69

return Self { limbs };

[tarcieri]

cc @mikelodder7

[elichai]

FYI to achieve the same behavior as libstd we can just add n = n % (Limb::BIT_SIZE * LIMBS) at the start of that function