Drop 不能在实现扩展特征的泛型结构中使用

Question

博士 https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=99952dfdc8dab353992d2681de6b6f58

完整版 https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=38d0c934cb7e55b868d73bd2dde94454

我不太明白为什么这不起作用：

pub trait State {}
pub trait WithFinal: State {}
pub struct Machine<T: State> {
    pub state: T,
    error: Option<fn(&Event, &T)>,
    transition: Option<fn(&T, &T, Event)>, // fn(&current_state, &previous_state)
}
impl<T: WithFinal> Drop for Machine<T> {
    fn drop(&mut self) {}
}

   Compiling scdlang v0.1.0 (/home/wildan/Projects/OSS/scdlang)
error[E0367]: The requirement `T: statechart::WithFinal` is added only by the Drop impl.
  --> src/main.rs:92:5
   |
92 | /     impl<T: WithFinal> Drop for Machine<T> {
93 | |         fn drop(&mut self) {}
94 | |     }
   | |_____^
   |
note: The same requirement must be part of the struct/enum definition
  --> src/main.rs:74:5
   |
74 | /     pub struct Machine<T: State> {
75 | |         pub state: T,
76 | |         error: Option<fn(&Event, &T)>,
77 | |         transition: Option<fn(&T, &T, Event)>, // fn(&current_state, &previous_state)
78 | |     }
   | |_____^
error: aborting due to previous error
For more information about this error, try `rustc --explain E0367`.
error: Could not compile `scdlang`.
To learn more, run the command again with --verbose.

我认为它应该有效WithFinal因为扩展特质State

但是，这两个impl都可以正常工作：

trait DropLike {
    fn drop(&mut self);
}
impl<T: WithFinal> DropLike for Machine<T> {
    fn drop(&mut self) {}
}
impl<T: State> Drop for Machine<T> {
    fn drop(&mut self) {}
}

Answer 1

简短

的回答是，不允许在专用泛型类型上实现Drop。

您的DropLike特征类似于Drop，但Drop是一个语言项，并且从编译器获得特殊处理。这意味着此错误仅适用于Drop 。

从 rustc 错误索引：

此代码不合法：无法将Drop专用化为 泛型类型的实现。为了使此代码正常工作，MyStruct 还必须要求T实现Foo。

(也可通过rustc --explain E0367看到(

这是似乎促成了这种变化的问题。