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standback 0.4.3

New standard library, old compiler.
Documentation 
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pub(crate) mod f32 {
    pub(crate) mod consts {
        pub const LOG10_2: f32 = 0.301029995663981195213738894724493027_f32;
        pub const LOG2_10: f32 = 3.32192809488736234787031942948939018_f32;
    }
}

pub(crate) mod f64 {
    pub(crate) mod consts {
        pub const LOG10_2: f64 = 0.301029995663981195213738894724493027_f64;
        pub const LOG2_10: f64 = 3.32192809488736234787031942948939018_f64;
    }
}

pub(crate) mod iter {
    use core::iter::FusedIterator;

    pub fn once_with<A, F: FnOnce() -> A>(gen: F) -> OnceWith<F> {
        OnceWith { gen: Some(gen) }
    }

    #[derive(Copy, Clone, Debug)]
    pub struct OnceWith<F> {
        gen: Option<F>,
    }

    impl<A, F: FnOnce() -> A> Iterator for OnceWith<F> {
        type Item = A;

        fn next(&mut self) -> Option<A> {
            let f = self.gen.take()?;
            Some(f())
        }

        fn size_hint(&self) -> (usize, Option<usize>) {
            self.gen.iter().size_hint()
        }
    }

    impl<A, F: FnOnce() -> A> DoubleEndedIterator for OnceWith<F> {
        fn next_back(&mut self) -> Option<A> {
            self.next()
        }
    }

    impl<A, F: FnOnce() -> A> ExactSizeIterator for OnceWith<F> {
        fn len(&self) -> usize {
            self.gen.iter().len()
        }
    }

    impl<A, F: FnOnce() -> A> FusedIterator for OnceWith<F> {}
}