This coherence failure...

pub(crate) trait Drive {}

trait DerefAndDrive {}

impl<T> Drive for T
where
    T: 'static,
    for<'a> &'a T: IntoIterator,
    for<'a> <&'a T as IntoIterator>::Item: DerefAndDrive,
{
}

impl Drive for () {}

fn main() {}

... is because the goal <&'a () as IntoIterator>::Item: DerefAndDrive is considered ambiguous when unifying the two impls above. That's because we try normalizing the self type of the goal, and &'a (): IntoIterator is treated as unknowable.

I think this is problematic, because even if we were to add an upstream implementation to be able to normalize <&'a () as IntoIterator>::Item, it couldn't implement DerefAndDrive since that's a trait local to the crate.