如何使从对象到其孙子的生命周期约束 "pass through"？

Question

我正在对生命周期和借用检查器进行一些实验。想象一下第一个结构：

struct First {}

impl First {
    fn new() -> Self {
        Self {}
    }

    fn second(&self) -> Second {
        Second::new(self)
    }

    fn hello(&self) {
        println!("Hello");
    }
}

第二个，其生命周期限制取决于 First:

struct Second<'a> {
    owner: &'a First,
}

impl<'a> Second<'a> {
    fn new(owner: &'a First) -> Self {
        Self { owner }
    }

    fn hello(&self) {
        self.owner.hello();
    }
}

上面的代码工作得很好：Second 是由 First 创建的，它不能超过 First。

问题

现在让我们修改 Second 以便它可以创建第三个结构，Third:

struct Second<'a> {
    owner: &'a First,
}

impl<'a> Second<'a> {
    fn new(owner: &'a First) -> Self {
        Self { owner }
    }

    fn third(&self) -> Third {
        Third::new(self.owner)
    }

    fn hello(&self) {
        self.owner.hello();
    }
}

和Third本身，这也取决于First:

struct Third<'a> {
    owner: &'a First,
}

impl<'a> Third<'a> {
    fn new(owner: &'a First) -> Self {
        Self { owner }
    }

    fn hello(&self) {
        self.owner.hello();
    }
}

我想，在创建 Third 的实例时，它会依赖于 First，但事实并非如此。实际上 Third 取决于 Second:

fn main() {
    let f = First::new();
    let t = {
        let sss = f.second();
        sss.third() // error: sss does not live long enough
    };
}

那么，我怎样才能使生命周期约束从 First“通过”到 Third？

Full playground.

Answer 1

创建Third时，只需要在Second中指定与内部引用相同的生命周期即可：

impl<'a> Second<'a> {
    fn new(owner: &'a First) -> Self {
        Self { owner }
    }
    fn third(&self) -> Third<'a> {
        Third::new(self.owner)
    }
    fn hello(&self) {
        self.owner.hello();
    }
}

Playground

和第一个创建Second时一样：

impl First {
...
    fn second(&self) -> Second {
        Second::new(self)
    }
...
}

在这里，second 被绑定到 'self 生命周期，也就是说，它可以与 &self First 实例一样长。但在这种情况下，编译器会为您指定它。

Answer 2

Rust 有如何推断函数生命周期的规则：lifetime elision rules.

这些规则规定：

Each elided lifetime (i.e. a type that should had have a lifetime, but doesn't, like &T that is actually &'a T) in the parameters becomes a distinct lifetime parameter.

If there is exactly one lifetime used in the parameters (elided or not), that lifetime is assigned to all elided output lifetimes.

In method signatures there is another rule

If the receiver has type &Self or &mut Self, then the lifetime of that reference to Self is assigned to all elided output lifetime parameters.

我们以First::second()为例。它的签名是：

fn second(&self) -> Second

或者，显式省略所有生命周期（顺便说一下，显式省略所有不在引用上的生命周期被认为是一种很好的做法，例如本例中的 Second<'_>）：

fn second(&'_ self) -> Second<'_>

所以根据规则 #1 我们分配一个新的生命周期，我们称它为 'a，到 &self:

fn second<'a>(&'a self) -> Second<'_>

现在，根据规则 #3，我们为 Second<'_> 选择 'a:

fn second<'a>(&'a self) -> Second<'a>

也就是说，我们 return Second 与对 self 的引用具有相同的生命周期。

现在让我们将它应用到 Second::third()...

fn third(&self) -> Third
// Becomes
fn third<'b>(&'b self) -> Third<'b> // Lifetime `'a` is already used

但这不是我们想要的！我们希望生成的 Third 取决于我们包含的 First 实例的生命周期，而不是 &self 的生命周期！所以我们真正需要的是使用 Third<'a>:

fn third(&self) -> Third<'a> { ... }

现在it works beautifully.

如何使从对象到其孙子的生命周期约束 "pass through"？

How to make a lifetime constraint "pass through" from object to its grandchild?

constraints

lifetime

rust

borrow-checker

问题