How can you make a safe static singleton in Rust?

It looks like a use case for std::sync::Once:

use std::sync::{Once, ONCE_INIT};
static INIT: Once = ONCE_INIT;

Then in your tests call

INIT.doit(|| unsafe { init(); });

Once guarantees that your init will only be executed once, no matter how many times you call INIT.doit().