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    use crate::ser::part::Sink;
    use crate::ser::Error;
    use serde::Serialize;
    use std::borrow::Cow;
    use std::ops::Deref;

    pub enum Key<'key> {
        Static(&'static str),
        Dynamic(Cow<'key, str>),
    }

    impl<'key> Deref for Key<'key> {
        type Target = str;

        fn deref(&self) -> &str {
            match *self {
                Key::Static(key) => key,
                Key::Dynamic(ref key) => key,
            }
        }
    }

    impl<'key> From<Key<'key>> for Cow<'static, str> {
        fn from(key: Key<'key>) -> Self {
            match key {
                Key::Static(key) => key.into(),
                Key::Dynamic(key) => key.into_owned().into(),
            }
        }
    }

    pub struct KeySink<End> {
        end: End,
    }

    impl<End, Ok> KeySink<End>
    where
        End: for<'key> FnOnce(Key<'key>) -> Result<Ok, Error>,
    {
        pub fn new(end: End) -> Self {
            KeySink { end }
        }
    }

    impl<End, Ok> Sink for KeySink<End>
    where
        End: for<'key> FnOnce(Key<'key>) -> Result<Ok, Error>,
    {
        type Ok = Ok;

        fn serialize_static_str(self, value: &'static str) -> Result<Ok, Error> {
            (self.end)(Key::Static(value))
        }

        fn serialize_str(self, value: &str) -> Result<Ok, Error> {
            (self.end)(Key::Dynamic(value.into()))
        }

        fn serialize_string(self, value: String) -> Result<Ok, Error> {
            (self.end)(Key::Dynamic(value.into()))
        }

        fn serialize_none(self) -> Result<Ok, Error> {
            Err(self.unsupported())
        }

        fn serialize_some<T: ?Sized + Serialize>(
            self,
            _value: &T,
        ) -> Result<Ok, Error> {
            Err(self.unsupported())
        }

        fn unsupported(self) -> Error {
            Error::Custom("unsupported key".into())
        }
    }