你不能（yet）自己在稳定的Rust上实现Fn特性，你应该定义自己的特性来进行计算，然后你可以为你的结构体和任何实现Fn（或FnMut）的类型实现这些特性：

// I'm opting for `&mut self` here as it's more general
// but this might not be necessary for your use case
trait Calculate<T, U> {
    fn calculate(&mut self, t: T) -> U;
}

// Because `Fn: FnMut` this will also implement `Calculate` for `Fn`.
impl<F, T, U> Calculate<T, U> for F
where F: FnMut(T) -> U {
    fn calculate(&mut self, t: T) -> U {
        (self)(t)
    }
}

struct State {
    flag: bool,
}
impl<T, U> Calculate<T, U> for State
where T: Into<U> {
    fn calculate(&mut self, t: T) -> U {
        self.flag = true;
        // or whatever calculation you actually need to do here
        t.into()
    }
}