block-buffer: simplify {Lazy,Eager}::split_blocks

[kpp]

Use split_at so you don't need to calculate tail lenght in split_blocks fn. It reduces the amount of ptr arith, and there is no need to create tail from raw ptr, because it's created by split_at itself.

The change is no-panic:

use block_buffer::{EagerBuffer, Error, LazyBuffer, array::sizes::U8};
use no_panic::no_panic;

#[no_panic]
fn check_lazy() -> Result<(), Error> {
    let mut buffer = LazyBuffer::<U8>::try_new(&[0; 8])?;
    buffer.digest_blocks(&[0; 9], |_| {});
    Ok(())
}

#[no_panic]
fn check_eager() -> Result<(), Error> {
    let mut buffer = EagerBuffer::<U8>::try_new(&[0; 8])?;
    buffer.digest_blocks(&[0; 9], |_| {});
    Ok(())
}

fn main() {
    let _ = check_lazy();
    let _ = check_eager();
}

[newpavlov]

This change results in a potential (IIUC unreachable) panic. Compare https://rust.godbolt.org/z/jMoWd1dM6 vs https://rust.godbolt.org/z/qd3MEajPj

[kpp]

Fixed: https://rust.godbolt.org/z/jeMjTPMja. The new code generates no panic and it's shorter in instrs.

[newpavlov]

It's worth to use Array::slice_as_chunks here. It's probably worth to update the Lazy method to it as well.

[newpavlov]

Closing in favor of #1220. It does not use unsafe and compiles into the same number of instructions (though it uses conditional moves instead of branches): https://rust.godbolt.org/z/43xMT3ExP

[kpp]

Thanks!