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    use std::u64;

    /// A `Body` size hint
    ///
    /// The default implementation returns:
    ///
    /// * 0 for `lower`
    /// * `None` for `upper`.
    #[derive(Debug, Default, Clone)]
    pub struct SizeHint {
        lower: u64,
        upper: Option<u64>,
    }

    impl SizeHint {
        /// Returns a new `SizeHint` with default values
        #[inline]
        pub fn new() -> SizeHint {
            SizeHint::default()
        }

        /// Returns a new `SizeHint` with both upper and lower bounds set to the
        /// given value.
        #[inline]
        pub fn with_exact(value: u64) -> SizeHint {
            SizeHint {
                lower: value,
                upper: Some(value),
            }
        }

        /// Returns the lower bound of data that the `Body` will yield before
        /// completing.
        #[inline]
        pub fn lower(&self) -> u64 {
            self.lower
        }

        /// Set the value of the `lower` hint.
        ///
        /// # Panics
        ///
        /// The function panics if `value` is greater than `upper`.
        #[inline]
        pub fn set_lower(&mut self, value: u64) {
            assert!(value <= self.upper.unwrap_or(u64::MAX));
            self.lower = value;
        }

        /// Returns the upper bound of data the `Body` will yield before
        /// completing, or `None` if the value is unknown.
        #[inline]
        pub fn upper(&self) -> Option<u64> {
            self.upper
        }

        /// Set the value of the `upper` hint value.
        ///
        /// # Panics
        ///
        /// This function panics if `value` is less than `lower`.
        #[inline]
        pub fn set_upper(&mut self, value: u64) {
            assert!(value >= self.lower, "`value` is less than than `lower`");

            self.upper = Some(value);
        }

        /// Returns the exact size of data that will be yielded **if** the
        /// `lower` and `upper` bounds are equal.
        #[inline]
        pub fn exact(&self) -> Option<u64> {
            if Some(self.lower) == self.upper {
                self.upper
            } else {
                None
            }
        }

        /// Set the value of the `lower` and `upper` bounds to exactly the same.
        #[inline]
        pub fn set_exact(&mut self, value: u64) {
            self.lower = value;
            self.upper = Some(value);
        }
    }