Add const initializer to argon2::Block

[rlee287]

For #![no_std] environments, adding the const initializer makes it possible to write

static mut ARGON2_MEMBLOCK: [argon2::Block; MEM_BLOCK_COUNT] = [argon2::Block::new(); MEM_BLOCK_COUNT];

Previously, without a const initializer for argon2::Block, doing the same would require code like the following:

static mut MEM_BLOCKS: [MaybeUninit<argon2::Block>; MEM_BLOCK_COUNT] = unsafe {MaybeUninit::uninit().assume_init()};

// Initialize blocks when they are actually used

Unfortunately, the compiler is not able to optimize the initialization properly, resulting in an extra 1KB of space wasted on the stack, which becomes an issue in memory-constrained embedded systems. The following program is an illustration of this:

#![no_std]

use core::mem::MaybeUninit;
use core::panic::PanicInfo;

// Arbitrary values
const SALTED_PWD_LEN: usize = 32;
const MEM_BLOCK_COUNT: usize = 81;

static mut MEM_BLOCKS: [MaybeUninit<argon2::Block>; MEM_BLOCK_COUNT] = unsafe {MaybeUninit::uninit().assume_init()};

#[panic_handler]
fn panic_handler(_panic: &PanicInfo<'_>) -> ! {
    loop {}
}

pub fn argon2_wrapper(pwd: &[u8], salt: &[u8]) {
    let argon2 = argon2::Argon2::new(
        argon2::Algorithm::Argon2id,
        argon2::Version::V0x13,
        argon2::Params::new(MEM_BLOCK_COUNT.try_into().unwrap(), 47, 1, Some(SALTED_PWD_LEN)).unwrap()
    );
    let mut out = [0x00; SALTED_PWD_LEN];
    let mem_blocks_ref = unsafe {
        for maybeuninit_block in &mut MEM_BLOCKS {
            maybeuninit_block.write(argon2::Block::default());
        }
        // We are transmuting a live reference (-> NonNull), so as_mut() is Some()
        (&mut MEM_BLOCKS as *mut [MaybeUninit<argon2::Block>; MEM_BLOCK_COUNT]
            as *mut [argon2::Block; MEM_BLOCK_COUNT]).as_mut().unwrap()
    };
    argon2.hash_password_into_with_memory(pwd, salt, &mut out, mem_blocks_ref).unwrap();
}

When compiled with cargo build --release --target thumbv7em-none-eabi, the resulting assembly initializes each argon2::Block on the stack and them memcpys it into the global array:

Thus, adding a const initializer for argon2::Block makes using the crate in embedded environments easier, as well as saving stack space. The particular project that I extracted this code from was very tight on memory space, almost needing to account for every single available RAM byte that the particular microcontroller we were using had available.

The argon2 crate was the only instance I found of a custom argon2::Block type, but if there are others, I think they should all get a const initializer as well.