Clippy finds less clear fix for manual_memcpy

[Yuri6037]

I tried this code:

fn main()
{
    let mut block: [u8; 40] = [0; 40];
    let type_ext: [u8; 16] = [0; 16];

    for i in 24..40 {
        block[i] = type_ext[i - 24];
    }
}

I expected to see this happen: Clippy finding fix block[24..40].copy_from_slice(&type_ext)

Instead, this happened: Clippy suggests less clear form: block[24..40].clone_from_slice(&type_ext[..(40 - 24)]);

I suppose clippy did not find that type_ext is 16 u8 and not more.

Meta

Rust version (rustc -Vv):

rustc 1.56.0 (09c42c458 2021-10-18)
binary: rustc
commit-hash: 09c42c45858d5f3aedfa670698275303a3d19afa
commit-date: 2021-10-18
host: x86_64-unknown-linux-gnu
release: 1.56.0
LLVM version: 13.0.0

[ada4a]

this is not really a bug imo.. the suggest is correct in all respects, just somewhat suboptimal

@rustbot label -C-bug C-enhancement