Function display 

pub fn display(
    runtime: Option<&Path>,
    wayland: Option<&str>,
    device: &Path,
) -> Result<Display, NoDisplay>

What this machine offers a window, read from the session qcode was started in.

runtime is the runtime folder of the session and wayland the display it names — the two variables a Wayland session sets. A display named as a bare name is a file in that folder, which is how every compositor writes it; one named as a path is taken as it is, which a few setups do. device is where the graphics device would be, so a test can point elsewhere.

Looks at the disk, which is all it does: everything else here is a value.

Errors

When the session names no compositor, or names one whose socket is not there.