read_line panics on non-empty buffer

[nanoqsh]

I found some weird behavior of read_line
It panics in debug on non-empty buffer:

use futures_lite::{future, AsyncBufReadExt};

future::block_on(async {
    let mut bytes = "foo".as_bytes();
    let mut buf = String::from("bar");
    _ = bytes.read_line(&mut buf).await;
});

This code prints

thread 'main' panicked at /home/nano/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/futures-lite-2.6.0/src/io.rs:1909:13:
assertion failed: buf.is_empty()
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Although this is a bit of unexpected behavior since read_line from std appends a string.

It is also written in both documentations:

All bytes up to, and including, the newline delimiter (if found) will be appended to buf.

However, nothing is said about you can't use a non-empty string. I think it would be better to repeat std semantic and to append the buffer, at least when it's non empty