Function set_engine 

pub fn set_engine(
    engine: &'static dyn Engine,
) -> Result<(), &'static dyn Engine>

Installs engine for every load in this process that does not name one.

Call it before the first init(). A load that names its own engine — builder(..).engine(..) — uses that one whatever is installed here.

Errors

If one is already installed. The rejected engine is returned, so a caller can tell “already set” from “failed”.