Question: is it possible to mock functions that belong to external modules?

[rbruggem]

For instance, tokio::time::delay_for.

[asomers-ax]

Sure. You can use automock like this:

#[cfg(test)]
#[automock]
mod time {
    fn delay_for();
}

#[cfg(test)]
use mock_time as time;
#[cfg(not(test))]
use time;

[asomers]

I'm going to close this issue, because I think you're satisfied by that answer. But BTW, the [#double] attribute might help you, too.
https://docs.rs/mockall_double/0.1.0/mockall_double/attr.double.html

[jinq0123]

#[cfg(test)]
#[automock]
mod time {
    fn delay_for();
}

should be

fn delay_for() {}

else

error: custom attribute panicked
   = help: message: Unsupported item.