lender 0.8.0

A lending-iterator trait based on higher-rank trait bounds, with full std::iter::Iterator functionality
Documentation 
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use core::ops::ControlFlow;

use crate::{
    FallibleLend, FallibleLender, FallibleLending, FusedFallibleLender, TakeWhile,
    try_trait_v2::Try,
};

impl<L: FallibleLender, P> TakeWhile<L, P> {
    #[inline]
    pub(crate) fn new_fallible(lender: L, predicate: P) -> TakeWhile<L, P> {
        crate::__check_fallible_lender_covariance::<L>();
        TakeWhile {
            lender,
            predicate,
            flag: false,
        }
    }
}

impl<'lend, L, P> FallibleLending<'lend> for TakeWhile<L, P>
where
    P: FnMut(&FallibleLend<'lend, L>) -> Result<bool, L::Error>,
    L: FallibleLender,
{
    type Lend = FallibleLend<'lend, L>;
}

impl<L, P> FallibleLender for TakeWhile<L, P>
where
    P: FnMut(&FallibleLend<'_, L>) -> Result<bool, L::Error>,
    L: FallibleLender,
{
    type Error = L::Error;
    // SAFETY: the lend is that of L
    crate::unsafe_assume_covariance_fallible!();

    #[inline]
    fn next(&mut self) -> Result<Option<FallibleLend<'_, Self>>, Self::Error> {
        if !self.flag {
            let x = match self.lender.next()? {
                Some(x) => x,
                None => {
                    self.flag = true;
                    return Ok(None);
                }
            };
            if (self.predicate)(&x)? {
                return Ok(Some(x));
            }
            self.flag = true;
        }
        Ok(None)
    }

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

    #[inline]
    fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> Result<R, Self::Error>
    where
        Self: Sized,
        F: FnMut(B, FallibleLend<'_, Self>) -> Result<R, Self::Error>,
        R: Try<Output = B>,
    {
        if self.flag {
            return Ok(R::from_output(init));
        }
        let flag = &mut self.flag;
        let predicate = &mut self.predicate;
        match self.lender.try_fold(init, move |acc, x| {
            if (predicate)(&x)? {
                match f(acc, x)?.branch() {
                    ControlFlow::Continue(b) => Ok(ControlFlow::Continue(b)),
                    ControlFlow::Break(residual) => {
                        Ok(ControlFlow::Break(R::from_residual(residual)))
                    }
                }
            } else {
                *flag = true;
                Ok(ControlFlow::Break(R::from_output(acc)))
            }
        })? {
            ControlFlow::Continue(acc) => Ok(R::from_output(acc)),
            ControlFlow::Break(r) => Ok(r),
        }
    }

    #[inline]
    fn fold<B, F>(mut self, init: B, mut f: F) -> Result<B, Self::Error>
    where
        Self: Sized,
        F: FnMut(B, FallibleLend<'_, Self>) -> Result<B, Self::Error>,
    {
        if self.flag {
            return Ok(init);
        }
        let flag = &mut self.flag;
        let predicate = &mut self.predicate;
        match self.lender.try_fold(init, move |acc, x| {
            if (predicate)(&x)? {
                match f(acc, x) {
                    Ok(b) => Ok(ControlFlow::Continue(b)),
                    Err(e) => Err(e),
                }
            } else {
                *flag = true;
                Ok(ControlFlow::Break(acc))
            }
        })? {
            ControlFlow::Continue(acc) | ControlFlow::Break(acc) => Ok(acc),
        }
    }
}

impl<L, P> FusedFallibleLender for TakeWhile<L, P>
where
    P: FnMut(&FallibleLend<'_, L>) -> Result<bool, L::Error>,
    L: FallibleLender,
{
}

#[cfg(test)]
mod test {
    use core::convert::Infallible;

    use crate::prelude::*;

    // A non-fused fallible lender: yields 10, then None, then 30 if polled again.
    struct Resurrect {
        step: usize,
    }
    impl<'l> crate::FallibleLending<'l> for Resurrect {
        type Lend = i32;
    }
    impl crate::FallibleLender for Resurrect {
        type Error = Infallible;
        crate::check_covariance_fallible!();
        fn next(&mut self) -> Result<Option<crate::FallibleLend<'_, Self>>, Self::Error> {
            let s = self.step;
            self.step += 1;
            Ok(match s {
                0 => Some(10),
                2 => Some(30),
                _ => None,
            })
        }
    }

    #[test]
    fn test_none_is_terminal() -> Result<(), Infallible> {
        let mut tw = (Resurrect { step: 0 }).take_while(|_| Ok(true));
        assert_eq!(tw.next()?, Some(10));
        assert_eq!(tw.next()?, None); // inner yields None -> TakeWhile is done
        assert_eq!(tw.next()?, None); // must not resurrect 30 (TakeWhile is FusedFallibleLender)
        Ok(())
    }
}