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    use std::any::{Any, TypeId};
    use std::collections::HashMap;
    use std::fmt;
    use std::hash::{BuildHasherDefault, Hasher};

    type AnyMap = HashMap<TypeId, Box<dyn Any + Send + Sync>, BuildHasherDefault<IdHasher>>;

    // With TypeIds as keys, there's no need to hash them. They are already hashes
    // themselves, coming from the compiler. The IdHasher just holds the u64 of
    // the TypeId, and then returns it, instead of doing any bit fiddling.
    #[derive(Default)]
    struct IdHasher(u64);

    impl Hasher for IdHasher {
        fn write(&mut self, _: &[u8]) {
            unreachable!("TypeId calls write_u64");
        }

        #[inline]
        fn write_u64(&mut self, id: u64) {
            self.0 = id;
        }

        #[inline]
        fn finish(&self) -> u64 {
            self.0
        }
    }

    /// A type map of protocol extensions.
    ///
    /// `Extensions` can be used by `Request` and `Response` to store
    /// extra data derived from the underlying protocol.
    #[derive(Default)]
    pub struct Extensions {
        // If extensions are never used, no need to carry around an empty HashMap.
        // That's 3 words. Instead, this is only 1 word.
        map: Option<Box<AnyMap>>,
    }

    impl Extensions {
        /// Create an empty `Extensions`.
        #[inline]
        pub fn new() -> Extensions {
            Extensions { map: None }
        }

        /// Insert a type into this `Extensions`.
        ///
        /// If a extension of this type already existed, it will
        /// be returned.
        ///
        /// # Example
        ///
        /// ```
        /// # use http::Extensions;
        /// let mut ext = Extensions::new();
        /// assert!(ext.insert(5i32).is_none());
        /// assert!(ext.insert(4u8).is_none());
        /// assert_eq!(ext.insert(9i32), Some(5i32));
        /// ```
        pub fn insert<T: Send + Sync + 'static>(&mut self, val: T) -> Option<T> {
            self.map
                .get_or_insert_with(|| Box::new(HashMap::default()))
                .insert(TypeId::of::<T>(), Box::new(val))
                .and_then(|boxed| {
                    (boxed as Box<dyn Any + 'static>)
                        .downcast()
                        .ok()
                        .map(|boxed| *boxed)
                })
        }

        /// Get a reference to a type previously inserted on this `Extensions`.
        ///
        /// # Example
        ///
        /// ```
        /// # use http::Extensions;
        /// let mut ext = Extensions::new();
        /// assert!(ext.get::<i32>().is_none());
        /// ext.insert(5i32);
        ///
        /// assert_eq!(ext.get::<i32>(), Some(&5i32));
        /// ```
        pub fn get<T: Send + Sync + 'static>(&self) -> Option<&T> {
            self.map
                .as_ref()
                .and_then(|map| map.get(&TypeId::of::<T>()))
                .and_then(|boxed| (&**boxed as &(dyn Any + 'static)).downcast_ref())
        }

        /// Get a mutable reference to a type previously inserted on this `Extensions`.
        ///
        /// # Example
        ///
        /// ```
        /// # use http::Extensions;
        /// let mut ext = Extensions::new();
        /// ext.insert(String::from("Hello"));
        /// ext.get_mut::<String>().unwrap().push_str(" World");
        ///
        /// assert_eq!(ext.get::<String>().unwrap(), "Hello World");
        /// ```
        pub fn get_mut<T: Send + Sync + 'static>(&mut self) -> Option<&mut T> {
            self.map
                .as_mut()
                .and_then(|map| map.get_mut(&TypeId::of::<T>()))
                .and_then(|boxed| (&mut **boxed as &mut (dyn Any + 'static)).downcast_mut())
        }

        /// Remove a type from this `Extensions`.
        ///
        /// If a extension of this type existed, it will be returned.
        ///
        /// # Example
        ///
        /// ```
        /// # use http::Extensions;
        /// let mut ext = Extensions::new();
        /// ext.insert(5i32);
        /// assert_eq!(ext.remove::<i32>(), Some(5i32));
        /// assert!(ext.get::<i32>().is_none());
        /// ```
        pub fn remove<T: Send + Sync + 'static>(&mut self) -> Option<T> {
            self.map
                .as_mut()
                .and_then(|map| map.remove(&TypeId::of::<T>()))
                .and_then(|boxed| {
                    (boxed as Box<dyn Any + 'static>)
                        .downcast()
                        .ok()
                        .map(|boxed| *boxed)
                })
        }

        /// Clear the `Extensions` of all inserted extensions.
        ///
        /// # Example
        ///
        /// ```
        /// # use http::Extensions;
        /// let mut ext = Extensions::new();
        /// ext.insert(5i32);
        /// ext.clear();
        ///
        /// assert!(ext.get::<i32>().is_none());
        /// ```
        #[inline]
        pub fn clear(&mut self) {
            if let Some(ref mut map) = self.map {
                map.clear();
            }
        }
    }

    impl fmt::Debug for Extensions {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Extensions").finish()
        }
    }

    #[test]
    fn test_extensions() {
        #[derive(Debug, PartialEq)]
        struct MyType(i32);

        let mut extensions = Extensions::new();

        extensions.insert(5i32);
        extensions.insert(MyType(10));

        assert_eq!(extensions.get(), Some(&5i32));
        assert_eq!(extensions.get_mut(), Some(&mut 5i32));

        assert_eq!(extensions.remove::<i32>(), Some(5i32));
        assert!(extensions.get::<i32>().is_none());

        assert_eq!(extensions.get::<bool>(), None);
        assert_eq!(extensions.get(), Some(&MyType(10)));
    }