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 core::fmt;

use crate::{
    DoubleEndedLender, ExactSizeLender, FusedLender, Lend, Lender, Lending, try_trait_v2::Try,
};

/// A lender that calls a function with a reference to each element before
/// yielding it.
///
/// This `struct` is created by the
/// [`inspect()`](crate::Lender::inspect) method on [`Lender`].
#[derive(Clone)]
#[must_use = "lenders are lazy and do nothing unless consumed"]
pub struct Inspect<L, F> {
    pub(crate) lender: L,
    pub(crate) f: F,
}

impl<L, F> Inspect<L, F> {
    /// Returns the inner lender.
    #[inline(always)]
    pub fn into_inner(self) -> L {
        self.lender
    }

    /// Returns the inner lender and the inspection function.
    #[inline(always)]
    pub fn into_parts(self) -> (L, F) {
        (self.lender, self.f)
    }
}

impl<L: Lender, F> Inspect<L, F> {
    #[inline(always)]
    pub(crate) fn new(lender: L, f: F) -> Inspect<L, F> {
        let _ = L::__check_covariance(crate::CovariantProof::new());
        Inspect { lender, f }
    }
}

impl<L: fmt::Debug, F> fmt::Debug for Inspect<L, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Inspect")
            .field("lender", &self.lender)
            .finish_non_exhaustive()
    }
}

impl<'lend, L, F> Lending<'lend> for Inspect<L, F>
where
    L: Lender,
    F: FnMut(&Lend<'lend, L>),
{
    type Lend = Lend<'lend, L>;
}

impl<L, F> Lender for Inspect<L, F>
where
    L: Lender,
    F: FnMut(&Lend<'_, L>),
{
    // SAFETY: the lend is that of L
    crate::unsafe_assume_covariance!();
    #[inline]
    fn next(&mut self) -> Option<Lend<'_, Self>> {
        let next = self.lender.next();
        if let Some(ref x) = next {
            (self.f)(x);
        }
        next
    }

    #[inline(always)]
    fn size_hint(&self) -> (usize, Option<usize>) {
        self.lender.size_hint()
    }

    #[inline]
    fn try_fold<B, Fold, R>(&mut self, init: B, mut fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(B, Lend<'_, Self>) -> R,
        R: Try<Output = B>,
    {
        let f = &mut self.f;
        self.lender.try_fold(init, move |acc, x| {
            (f)(&x);
            fold(acc, x)
        })
    }

    #[inline]
    fn fold<B, Fold>(mut self, init: B, mut fold: Fold) -> B
    where
        Self: Sized,
        Fold: FnMut(B, Lend<'_, Self>) -> B,
    {
        self.lender.fold(init, move |acc, x| {
            (self.f)(&x);
            fold(acc, x)
        })
    }
}

impl<L, F> DoubleEndedLender for Inspect<L, F>
where
    L: DoubleEndedLender,
    F: FnMut(&Lend<'_, L>),
{
    #[inline]
    fn next_back(&mut self) -> Option<Lend<'_, Self>> {
        let next = self.lender.next_back();
        if let Some(ref x) = next {
            (self.f)(x);
        }
        next
    }

    #[inline]
    fn try_rfold<B, Fold, R>(&mut self, init: B, mut fold: Fold) -> R
    where
        Self: Sized,
        Fold: FnMut(B, Lend<'_, Self>) -> R,
        R: Try<Output = B>,
    {
        let f = &mut self.f;
        self.lender.try_rfold(init, move |acc, x| {
            (f)(&x);
            fold(acc, x)
        })
    }

    #[inline]
    fn rfold<B, Fold>(mut self, init: B, mut fold: Fold) -> B
    where
        Self: Sized,
        Fold: FnMut(B, Lend<'_, Self>) -> B,
    {
        self.lender.rfold(init, move |acc, x| {
            (self.f)(&x);
            fold(acc, x)
        })
    }
}

impl<L: ExactSizeLender, F> ExactSizeLender for Inspect<L, F>
where
    F: FnMut(&Lend<'_, L>),
{
    #[inline(always)]
    fn len(&self) -> usize {
        self.lender.len()
    }

    #[inline(always)]
    fn is_empty(&self) -> bool {
        self.lender.is_empty()
    }
}

impl<L: FusedLender, F> FusedLender for Inspect<L, F> where F: FnMut(&Lend<'_, L>) {}