If the last line of a large file doesn't end with a newline then we get a discrepancy between GNU and Rust implementations.
Steps to reproduce...
1 - make a large file without a line ending...
$ seq 70000 | head -c -1 >seq_70000

2 - Compare GNU with Rust
GNU...

$head -n -1 seq_70000
...
69998
69999

Rust

$./target/debug/head -n -1 seq_70000
...
69997
69998

Issue only exists for large files, small files are handled fine.

I'm planning to fix this.