rust - 如何统一结构和特征之间的生命周期？

Question

我有一个在 trait 方法上指定生命周期的特征，并且我有一个包含也需要生命周期的值的结构。我想让结构实现特征，这意味着生命周期需要匹配。但是，我不确定如何表达。

struct Context<'d> {
    name: &'d str,
}

struct Value<'d> {
    name: &'d str,
}

trait Expression {
    fn evaluate<'d>(&self, context: &Context<'d>) -> Value<'d>;
}

struct Constant<'d> {
    value: Value<'d>,
}

尝试 1 - 指定方法的生命周期:

impl<'d> Expression for Constant<'d> {
    fn evaluate<'d>(&self, _unused: &Context<'d>) -> Value<'d> {
        self.value
    }
}

这会导致 impl 的生命周期被隐藏，并且我会收到如下错误:

$ rustc x.rs
x.rs:18:5: 20:6 help: consider using an explicit lifetime parameter as shown: fn evaluate<'d>(&self, _unused: &Context<'d>) -> Value<'d>
x.rs:18     fn evaluate<'d>(&self, _unused: &Context<'d>) -> Value<'d> {
x.rs:19         self.value
x.rs:20     }
x.rs:19:9: 19:19 error: mismatched types: expected `Value<'d>`, found `Value<'d>` (lifetime mismatch)
x.rs:19         self.value
                ^~~~~~~~~~

尝试 2 - 未指定生命周期:

impl<'d> Expression for Constant<'d> {
    fn evaluate(&self, _unused: &Context<'d>) -> Value<'d> {
        self.value
    }
}

这导致我实际上没有实现该方法:

$ rustc x.rs
x.rs:18:5: 20:6 error: method `evaluate` has an incompatible type for trait: expected concrete lifetime, found bound lifetime parameter 'd [E0053]
x.rs:18     fn evaluate(&self, _unused: &Context<'d>) -> Value<'d> {
x.rs:19         self.value
x.rs:20     }
x.rs:18:60: 20:6 note: expected concrete lifetime is the lifetime 'd as defined on the block at 18:59
x.rs:18     fn evaluate(&self, _unused: &Context<'d>) -> Value<'d> {
x.rs:19         self.value
x.rs:20     }

Answer 1

你需要给特征本身一个生命周期参数:

trait Expression<'d> {
    fn evaluate(&self, context: &Context<'d>) -> Value<'d>;
}

impl<'d> Expression<'d> for Constant<'d> {
    fn evaluate(&self, _unused: &Context<'d>) -> Value<'d> {
        self.value
    }
}

Playpen

你得到错误 #1(除了阴影)的原因是你的特征限制 context 有返回类型的生命周期，而不是 self .要约束 self，您需要在特征本身上设置生命周期参数。

出现错误 #2 的原因很简单:生命周期是类型系统的一部分，不能不匹配。您编写的任何通用代码都应该适用于特征的所有实现者——如果生命周期在每个实现中的绑定(bind)不同，那将不起作用。