How would a modern OS implement this?

In libc, you can use setvbuf to change the buffering mode.

> Surprisingly, Rust, as of now, uses line buffering for both TTYs and non-TTYs.

> The FIXME comment shows the Rust team acknowledges that ideally they should check if something is executed in TTYs or not and use LineWriter or BufWriter accordingly, but I guess this was not on their priority list.

This does not inspire confidence.