The output of Rng::gen_range is different depending on whether usize is 32 bits or 64 bits on the current platform, even if the requested range is small (0..12 in my case). For example:

    use rand::Rng;
    use rand_core::SeedableRng;

    let mut rng = rand_pcg::Pcg32::seed_from_u64(0);
    for _ in 0..16 {
        let x: usize = rng.gen_range(0..12);
        print!("{} ", x);
    }
    println!();

On a 64-bits system, the output is: 4 8 6 0 5 1 5 8 9 10 11 7 6 11 1 8
On a 32-bits system, the output is: 4 1 8 0 6 1 0 8 5 11 1 9 5 5 8 6

This does not affect SliceRandom::choose, thanks to some special handling in gen_index:

This means that, contrary to my expectations, my_slice.choose(&mut rng) and my_slice[rng.gen_range(0..my_slice.len())] are not equivalent (even ignoring the case of an empty slice).

I'm not sure if this qualifies as a bug, but it definitely surprised me, and took me some time to debug. I'm also not sure if it can be fixed, and if fixing it would constitute a breaking change.

In any case it could be documented, here in the book and here on Rng::gen_range.