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enum Message {
    Quit,
    Move { x: i32, y: i32 },
    Write(String),
    ChangeColor(i32, i32, i32),
}

fn main() {
    let m1 = Message::Quit;
    let m2 = Message::Move{x:1,y:1};
    let m3 = Message::ChangeColor(255,255,0);
}

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enum Option<T> {
    Some(T),
    None,
}

impl<T> Option<T> {
    pub fn as_ref(&self) -> Option<&T> //从`&Option<T>`转换为`Option<&T>`
    pub fn as_mut(&mut self) -> Option<&mut T> //从&mut Option<T>转换为Option<&mut T> 

    // unwrap: 当遇到None值时会panic
    fn unwrap(self) -> T {
        match self {
            Option::Some(val) => val,
            Option::None => panic!("unwrap option None"),
        }
    }
}

//
fn unwrap_or<T>(option: Option<T>, default: T) -> T {
    match option {
        None => default,
        Some(value) => value,
    }
}

//
fn and_then<F, T, A>(option: Option<T>, f: F) -> Option<A>
        where F: FnOnce(T) -> Option<A> {
    match option {
        None => None,
        Some(value) => f(value),
    }
}

fn ok_or<T, E>(option: Option<T>, err: E) -> Result<T, E> {
    match option {
        Some(val) => Ok(val),
        None => Err(err),
    }
}

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pub enum Cow<'a, B> 
where
    B: 'a + ToOwned + ?Sized, 
 {
    Borrowed(&'a B),
    Owned(<B as ToOwned>::Owned),
}