lender 0.6.2

A lending-iterator trait based on higher-rank trait bounds, with full std::iter::Iterator functionality
Documentation 
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use crate::{
    Covar, FallibleLend, FallibleLender, FallibleLending, Scan, higher_order::FnMutHKAResOpt,
};

impl<L: FallibleLender, St, F> Scan<L, St, F> {
    #[inline(always)]
    pub(crate) fn new_fallible(lender: L, state: St, f: Covar<F>) -> Scan<L, St, F> {
        let _ = L::__check_covariance(crate::CovariantProof::new());
        Scan { lender, state, f }
    }
}

impl<'lend, L, St, F> FallibleLending<'lend> for Scan<L, St, F>
where
    F: for<'all> FnMutHKAResOpt<'all, (&'all mut St, FallibleLend<'all, L>), L::Error>,
    L: FallibleLender,
{
    type Lend = <F as FnMutHKAResOpt<'lend, (&'lend mut St, FallibleLend<'lend, L>), L::Error>>::B;
}

impl<L, St, F> FallibleLender for Scan<L, St, F>
where
    for<'all> F: FnMutHKAResOpt<'all, (&'all mut St, FallibleLend<'all, L>), L::Error>,
    L: FallibleLender,
{
    type Error = L::Error;
    // SAFETY: the lend is the return type of F, whose covariance
    // has been checked at Covar construction time.
    crate::unsafe_assume_covariance_fallible!();

    #[inline]
    fn next(&mut self) -> Result<Option<FallibleLend<'_, Self>>, Self::Error> {
        match self.lender.next()? {
            Some(next) => (self.f.as_inner_mut())((&mut self.state, next)),
            None => Ok(None),
        }
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        let (_, upper) = self.lender.size_hint();
        (0, upper)
    }
}