The BlockRngCore trait and implementation helpers

The BlockRngCore trait exists to assist in the implementation of RNGs which generate a block of data in a cache instead of returning generated values directly.

Usage of this trait is optional, but provides two advantages: implementations only need to concern themselves with generation of the block, not the various RngCore methods (especially fill_bytes, where the optimal implementations are not trivial), and this allows ReseedingRng (see rand crate) perform periodic reseeding with very low overhead.

Example

use rand_core::{RngCore, SeedableRng};
use rand_core::block::{BlockRngCore, BlockRng};

struct MyRngCore;

impl BlockRngCore for MyRngCore {
    type Item = u32;
    type Results = [u32; 16];

    fn generate(&mut self, results: &mut Self::Results) {
        unimplemented!()
    }
}

impl SeedableRng for MyRngCore {
    type Seed = [u8; 32];
    fn from_seed(seed: Self::Seed) -> Self {
        unimplemented!()
    }
}

// optionally, also implement CryptoBlockRng for MyRngCore

// Final RNG.
let mut rng = BlockRng::<MyRngCore>::seed_from_u64(0);
println!("First value: {}", rng.next_u32());