How to configure fallback value of constant if multiple #[cfg(...)] don't match?

Question

I am trying to set value of constant based on target_os. I want to achieve something like this:

#[cfg(target_os = "linux")]
const MAP_FLAGS: libc::c_int = libc::MAP_POPULATE;

#[cfg(target_os = "macos")]
const MAP_FLAGS: libc::c_int = libc::MAP_NOCACHE;

#[cfg(/* ELSE */)]
const MAP_FLAGS: libc::c_int = 0;

Now, natural idea is to use cfg! macro, but that won't compile because libc::MAP_POPULATE is present only on linux targets and libc::MAP_NOCACHE is present only on macos targets.

Any idea how to solve this?

Answer 1

I think you're very close. If you want a default just for anything that's not linux or macos, you can combine not() with any():

#[cfg(target_os = "linux")]
const MAP_FLAGS: libc::c_int = libc::MAP_POPULATE;

#[cfg(target_os = "macos")]
const MAP_FLAGS: libc::c_int = libc::MAP_NOCACHE;

#[cfg(not(any(target_os = "macos", target_os = "linux")))]
const MAP_FLAGS: libc::c_int = 0;