Rust实现Iterator Pattern时遭遇生命周期编译错误求助

问题背景 #

我在Rust中实现迭代器模式，包含两个迭代器：CommitIterator（对应Git迭代器）和ConventionalCommitIterator，其中ConventionalCommitIterator作为CommitIterator的装饰器存在。使用Box和trait模拟接口时，编译出现以下错误：

error: lifetime may not live long enough
--> src/main.rs:99:13
    |
 92 | impl<'repo> ConventionalCommitsRepository<'repo> for ConventionalCommitsRepositoryImpl<'repo> {
    |      ----- lifetime `'repo` defined here
 ...
 99 |             git_iterator,
    |             ^^^^^^^^^^^^ coercion requires that `'repo` must outlive `'static`

原代码 #

use std::error::Error;

#[derive(Clone)]
pub struct Commit {
    pub value: i32,
}

pub trait GitRepository<'repo> {
    fn get_desc_iterator(&'repo self) -> Result<Box<dyn CommitIterator + 'repo>, Box<dyn Error>>;
}

pub struct GitRepositoryImpl {
    commits: Vec<Commit>,
}

impl<'repo> GitRepository<'repo> for GitRepositoryImpl {
    fn get_desc_iterator(&'repo self) -> Result<Box<dyn CommitIterator + 'repo>, Box<dyn Error>> {
        return Ok(Box::new(CommitIteratorImpl {
            commits: self.commits.clone(),
            index: 0,
        }));
    }
}

pub trait CommitIterator {
    fn next(&mut self) -> Option<Commit>;
}

pub struct CommitIteratorImpl {
    commits: Vec<Commit>,
    index: usize,
}

impl CommitIterator for CommitIteratorImpl {
    fn next(&mut self) -> Option<Commit> {
        if self.index >= self.commits.len() {
            return None;
        }

        let commit = self.commits[self.index].clone();
        self.index += 1;

        return Some(commit);
    }
}

pub trait ConventionalCommitsRepository<'repo> {
    fn get_desc_iterator(
        &'repo self,
    ) -> Result<Box<dyn ConventionalCommitIterator + 'repo>, Box<dyn Error>>;
}

pub struct ConventionalCommit {
    pub value: i32,
}

pub trait ConventionalCommitIterator {
    fn has_next(&self) -> bool;
    fn next(&mut self) -> Option<ConventionalCommit>;
}

pub struct ConventionalCommitIteratorImpl {
    git_iterator: Box<dyn CommitIterator>,
}

impl ConventionalCommitIterator for ConventionalCommitIteratorImpl {
    fn has_next(&self) -> bool {
        false
    }

    fn next(&mut self) -> Option<ConventionalCommit> {
        let commit = self.git_iterator.next();
        if commit.is_none() {
            return None;
        }

        let commit = commit.unwrap();
        Some(ConventionalCommit { value: commit.value })
    }
}

pub struct ConventionalCommitsRepositoryImpl<'repo> {
    git_repository: Box<dyn GitRepository<'repo>>,
}

impl<'repo> ConventionalCommitsRepositoryImpl<'repo> {
    pub fn new(git_repository: Box<dyn GitRepository<'repo>>) -> Self {
        Self { git_repository }
    }
}

impl<'repo> ConventionalCommitsRepository<'repo> for ConventionalCommitsRepositoryImpl<'repo> {
    fn get_desc_iterator(
        &'repo self,
    ) -> Result<Box<dyn ConventionalCommitIterator + 'repo>, Box<dyn Error>> {
        let git_iterator = self.git_repository.get_desc_iterator()?;

        Ok(Box::new(ConventionalCommitIteratorImpl {
            git_iterator,
        }))
    }
}

fn main() {
    let mut git = GitRepositoryImpl {
        commits: vec![Commit { value: 1 }, Commit { value: 2 }],
    };

    let mut iterator = git.get_desc_iterator().unwrap();

    let commit = iterator.next().unwrap();
    println!("{}", commit.value);

    let commit = iterator.next().unwrap();
    println!("{}", commit.value);
}

错误原因 #

ConventionalCommitIteratorImpl中定义的git_iterator字段类型为Box<dyn CommitIterator>，在Rust中，未指定生命周期的trait对象默认使用'static生命周期。但我们从git_repository.get_desc_iterator()获取的迭代器是带有'repo生命周期的Box<dyn CommitIterator + 'repo>，将其赋值给要求'static的字段时，编译器会强制要求'repo必须存活到'static，这显然无法满足，因此报错。

解决方案 #

给ConventionalCommitIteratorImpl添加生命周期参数，让其持有的迭代器生命周期与传入的迭代器保持一致：

1. 修改ConventionalCommitIteratorImpl的定义，添加生命周期参数：

pub struct ConventionalCommitIteratorImpl<'a> {
    git_iterator: Box<dyn CommitIterator + 'a>,
}

2. 实现ConventionalCommitIterator时带上对应的生命周期：

impl<'a> ConventionalCommitIterator for ConventionalCommitIteratorImpl<'a> {
    fn has_next(&self) -> bool {
        false
    }

    fn next(&mut self) -> Option<ConventionalCommit> {
        self.git_iterator.next().map(|commit| ConventionalCommit { value: commit.value })
    }
}

3. 在ConventionalCommitsRepositoryImpl的get_desc_iterator方法中，返回的Box<dyn ConventionalCommitIterator + 'repo>会自动匹配ConventionalCommitIteratorImpl<'repo>的生命周期，此时类型约束满足，编译通过。

修改后的完整代码 #

use std::error::Error;

#[derive(Clone)]
pub struct Commit {
    pub value: i32,
}

pub trait GitRepository<'repo> {
    fn get_desc_iterator(&'repo self) -> Result<Box<dyn CommitIterator + 'repo>, Box<dyn Error>>;
}

pub struct GitRepositoryImpl {
    commits: Vec<Commit>,
}

impl<'repo> GitRepository<'repo> for GitRepositoryImpl {
    fn get_desc_iterator(&'repo self) -> Result<Box<dyn CommitIterator + 'repo>, Box<dyn Error>> {
        Ok(Box::new(CommitIteratorImpl {
            commits: self.commits.clone(),
            index: 0,
        }))
    }
}

pub trait CommitIterator {
    fn next(&mut self) -> Option<Commit>;
}

pub struct CommitIteratorImpl {
    commits: Vec<Commit>,
    index: usize,
}

impl CommitIterator for CommitIteratorImpl {
    fn next(&mut self) -> Option<Commit> {
        if self.index >= self.commits.len() {
            return None;
        }

        let commit = self.commits[self.index].clone();
        self.index += 1;

        Some(commit)
    }
}

pub trait ConventionalCommitsRepository<'repo> {
    fn get_desc_iterator(
        &'repo self,
    ) -> Result<Box<dyn ConventionalCommitIterator + 'repo>, Box<dyn Error>>;
}

pub struct ConventionalCommit {
    pub value: i32,
}

pub trait ConventionalCommitIterator {
    fn has_next(&self) -> bool;
    fn next(&mut self) -> Option<ConventionalCommit>;
}

pub struct ConventionalCommitIteratorImpl<'a> {
    git_iterator: Box<dyn CommitIterator + 'a>,
}

impl<'a> ConventionalCommitIterator for ConventionalCommitIteratorImpl<'a> {
    fn has_next(&self) -> bool {
        false
    }

    fn next(&mut self) -> Option<ConventionalCommit> {
        self.git_iterator.next().map(|commit| ConventionalCommit { value: commit.value })
    }
}

pub struct ConventionalCommitsRepositoryImpl<'repo> {
    git_repository: Box<dyn GitRepository<'repo>>,
}

impl<'repo> ConventionalCommitsRepositoryImpl<'repo> {
    pub fn new(git_repository: Box<dyn GitRepository<'repo>>) -> Self {
        Self { git_repository }
    }
}

impl<'repo> ConventionalCommitsRepository<'repo> for ConventionalCommitsRepositoryImpl<'repo> {
    fn get_desc_iterator(
        &'repo self,
    ) -> Result<Box<dyn ConventionalCommitIterator + 'repo>, Box<dyn Error>> {
        let git_iterator = self.git_repository.get_desc_iterator()?;

        Ok(Box::new(ConventionalCommitIteratorImpl {
            git_iterator,
        }))
    }
}

fn main() {
    let mut git = GitRepositoryImpl {
        commits: vec![Commit { value: 1 }, Commit { value: 2 }],
    };

    let mut iterator = git.get_desc_iterator().unwrap();

    let commit = iterator.next().unwrap();
    println!("{}", commit.value);

    let commit = iterator.next().unwrap();
    println!("{}", commit.value);
}

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