调试Rust生命周期错误：如何正确指定生命周期参数？

我正在调试以下Rust代码，但无法正确指定生命周期参数：

use std::marker::PhantomData;

#[derive(Clone, Debug, PartialEq)]
pub enum FooBox<'a, T> {
    Ref(&'a Foo<T>),
    Own(Foo<T>),
}

#[derive(Clone, Debug, PartialEq)]
pub struct Foo<T> {
    data: T,
}

impl<T> Foo<T> {
    /// Construct a new Foo
    pub fn new(data: T) -> Self {
        Self { data }
    }
}

pub trait Bar<C, P>
where
    C: Clone,
    P: Clone,
{
    fn iter<'a>(&'a self) -> impl Iterator<Item = (&'a C, FooBox<P>)>
    where
        C: 'a,
        P: 'a;
}

pub struct MyBar<'a, C, F0, F1, P0, P1>
where
    C: Clone,
    F0: Bar<C, P0>,
    F1: Bar<C, P1>,
    P0: Clone,
    P1: Clone,
{
    bar0: &'a F0,
    bar1: &'a F1,

    // Used for type checking at compile time
    c: PhantomData<C>,
    p0: PhantomData<P0>,
    p1: PhantomData<P1>,
}

impl<'a, C, F0, F1, P0, P1> MyBar<'a, C, F0, F1, P0, P1>
where
    C: Clone,
    F0: Bar<C, P0>,
    F1: Bar<C, P1>,
    P0: Clone,
    P1: Clone,
{
    /// Construct a new MyBar
    pub fn new(bar0: &'a F0, bar1: &'a F1) -> Self {
        Self {
            bar0,
            bar1,
            c: PhantomData,
            p0: PhantomData,
            p1: PhantomData,
        }
    }
}

impl<'a, C, F0, F1, P0, P1> Bar<C, (FooBox<'a, P0>, FooBox<'a, P1>)>
    for MyBar<'a, C, F0, F1, P0, P1>
where
    C: Clone + PartialOrd,
    F0: Bar<C, P0>,
    F1: Bar<C, P1>,
    P0: Clone + PartialEq,
    P1: Clone + PartialEq,
{
    fn iter<'b>(&'b self) -> impl Iterator<Item = (&C, FooBox<(FooBox<'a, P0>, FooBox<'a, P1>)>)>
    where
        C: 'b,
        P0: 'b,
        P1: 'b,
    {
        MyBarIter::new(self.bar0.iter(), self.bar1.iter())
    }
}

/// Iterator for an MyBar
pub struct MyBarIter<'a, C, I0, I1, P0, P1>
where
    C: Clone + PartialOrd + 'a,
    I0: Iterator<Item = (&'a C, FooBox<'a, P0>)>,
    I1: Iterator<Item = (&'a C, FooBox<'a, P1>)>,
    P0: Clone + PartialEq + 'a,
    P1: Clone + PartialEq + 'a,
{
    iter0: I0,
    iter1: I1,
}

impl<'a, C, I0, I1, P0, P1> MyBarIter<'a, C, I0, I1, P0, P1>
where
    C: Clone + PartialOrd,
    I0: Iterator<Item = (&'a C, FooBox<'a, P0>)>,
    I1: Iterator<Item = (&'a C, FooBox<'a, P1>)>,
    P0: Clone + PartialEq,
    P1: Clone + PartialEq,
{
    /// Construct a new iterator from the given MyBar
    pub fn new(iter0: I0, iter1: I1) -> Self {
        Self { iter0, iter1 }
    }
}

impl<'a, C, I0, I1, P0, P1> Iterator for MyBarIter<'a, C, I0, I1, P0, P1>
where
    C: Clone + PartialOrd,
    I0: Iterator<Item = (&'a C, FooBox<'a, P0>)>,
    I1: Iterator<Item = (&'a C, FooBox<'a, P1>)>,
    P0: Clone + PartialEq,
    P1: Clone + PartialEq,
{
    type Item = (&'a C, FooBox<'a, (FooBox<'a, P0>, FooBox<'a, P1>)>);

    fn next(&mut self) -> Option<Self::Item> {
        let next0 = self.iter0.next();
        let next1 = self.iter1.next();

        if let (Some((c0, p0)), Some((_, p1))) = (next0, next1) {
            return Some((c0, FooBox::Own(Foo::new((p0, p1)))));
        }

        return None;
    }
}

编译时出现如下错误信息：

error: lifetime may not live long enough
  --> src/lib.rs:84:9
   |
69 | impl<'a, C, F0, F1, P0, P1> Bar<C, (FooBox<'a, P0>, FooBox<'a, P1>)>
   |      -- lifetime `'a` defined here
...
78 |     fn iter<'b>(&'b self) -> impl Iterator<Item = (&C, FooBox<(FooBox<'a, P0>, FooBox<'a, P1>)>)>
   |             -- lifetime `'b` defined here
...
84 |         MyBarIter::new(self.bar0.iter(), self.bar1.iter())
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ method was supposed to return data with lifetime `'a` but it is returning data with lifetime `'b`
   |
   = help: consider adding the following bound: `'b: 'a`

据我分析，self.bar0.iter()和self.bar1.iter()返回的迭代器所引用的数据，生命周期至少与拥有'a生命周期的self.bar0和self.bar1一致，但编译器无法识别这一点。请问如何正确指定生命周期参数以通过编译？

修复方案 #

1. 给iter方法添加生命周期约束

在MyBar的Bar trait实现中，给iter方法的'b生命周期添加'b: 'a约束，确保self的借用周期覆盖MyBar持有的引用周期：

impl<'a, C, F0, F1, P0, P1> Bar<C, (FooBox<'a, P0>, FooBox<'a, P1>)>
    for MyBar<'a, C, F0, F1, P0, P1>
where
    C: Clone + PartialOrd,
    F0: Bar<C, P0>,
    F1: Bar<C, P1>,
    P0: Clone + PartialEq,
    P1: Clone + PartialEq,
{
    fn iter<'b>(&'b self) -> impl Iterator<Item = (&'b C, FooBox<'b, (FooBox<'a, P0>, FooBox<'a, P1>)>)>
    where
        C: 'b,
        P0: 'b,
        P1: 'b,
        'b: 'a, // 添加该约束
    {
        MyBarIter::new(self.bar0.iter(), self.bar1.iter())
    }
}

2. 调整MyBarIter的生命周期参数

将MyBarIter的固定生命周期'a改为与输入迭代器匹配的'b，因为输入迭代器来自&'b self的借用，生命周期为'b：

/// Iterator for an MyBar
pub struct MyBarIter<'b, C, I0, I1, P0, P1>
where
    C: Clone + PartialOrd + 'b,
    I0: Iterator<Item = (&'b C, FooBox<'b, P0>)>,
    I1: Iterator<Item = (&'b C, FooBox<'b, P1>)>,
    P0: Clone + PartialEq + 'b,
    P1: Clone + PartialEq + 'b,
{
    iter0: I0,
    iter1: I1,
}

impl<'b, C, I0, I1, P0, P1> MyBarIter<'b, C, I0, I1, P0, P1>
where
    C: Clone + PartialOrd,
    I0: Iterator<Item = (&'b C, FooBox<'b, P0>)>,
    I1: Iterator<Item = (&'b C, FooBox<'b, P1>)>,
    P0: Clone + PartialEq,
    P1: Clone + PartialEq,
{
    /// Construct a new iterator from the given MyBar
    pub fn new(iter0: I0, iter1: I1) -> Self {
        Self { iter0, iter1 }
    }
}

impl<'b, C, I0, I1, P0, P1> Iterator for MyBarIter<'b, C, I0, I1, P0, P1>
where
    C: Clone + PartialOrd,
    I0: Iterator<Item = (&'b C, FooBox<'b, P0>)>,
    I1: Iterator<Item = (&'b C, FooBox<'b, P1>)>,
    P0: Clone + PartialEq,
    P1: Clone + PartialEq,
{
    type Item = (&'b C, FooBox<'b, (FooBox<'a, P0>, FooBox<'a, P1>)>);

    fn next(&mut self) -> Option<Self::Item> {
        let next0 = self.iter0.next();
        let next1 = self.iter1.next();

        if let (Some((c0, p0)), Some((_, p1))) = (next0, next1) {
            return Some((c0, FooBox::Own(Foo::new((p0, p1)))));
        }

        None
    }
}

核心原因说明 #

• 添加'b: 'a约束后，编译器明确self的借用周期'b不短于MyBar中引用的'a周期，因此从bar0/bar1获取的迭代器数据可以安全满足'a的生命周期要求。
• 调整MyBarIter的生命周期为'b，是因为输入迭代器的生命周期绑定到&'b self，而非MyBar的'a，这样迭代器的实现就能匹配输入的生命周期，消除不匹配错误。

内容的提问来源于stack exchange，提问作者student1868