如何创建需要覆盖 Drop 的特征？

Question

考虑一个需要实现特定清理方法的特征，并确保在删除实例时实际执行此清理。我们可以写这样的东西：

trait MyTrait {
    fn cleanup(&self);
}

struct MyStruct;

impl MyTrait for MyStruct {
    fn cleanup(&self) {
        println!("Cleaning up");
    }
}

impl Drop for MyStruct {
    fn drop(&mut self) {
        <Self as MyTrait>::cleanup(self);
    }
}

fn main() {
    {
        let x = MyStruct;
    }
    println!("done");
}

并且它会起作用，但它不会强制在

Drop

上实现

MyStruct

，并且可能很容易被忽略。 有没有一种方法可以确保或提供

MyTrait

调用

drop()

的默认实现？

Answer 1

cleanup()

用户可以通过 

mod library {
    pub trait MyTraitImpls {
        fn cleanup(&self);
    }

    /// Users are forbidden from implementing this.
    #[doc(hidden)]
    pub trait __Private {}
    
    /// Implement via `MyTraitImpls` and `impl_my_trait!(...)`
    pub trait MyTrait: MyTraitImpls + __Private {}
    
    impl<T: MyTraitImpls + __Private> MyTrait for T {}
    
    #[macro_export]
    macro_rules! impl_my_trait {
        ($name: ident) => {
            impl library::__Private for $name {}
        
            impl Drop for $name {
                fn drop(&mut self) {
                    <Self as library::MyTraitImpls>::cleanup(self);
                }
            }
        }
    }
}

use crate::library::*;

struct MyStruct;
impl MyTraitImpls for MyStruct {
    fn cleanup(&self) {
        println!("Cleaning up");
    }
}
impl_my_trait!(MyStruct);

绕过这个，但至少你不会意外地这样做。