使用Rayon并行迭代树对象时遇Sized trait不满足问题求助

问题描述 #

我持有一个Vec<ValidRelationshipTree>集合，希望通过Rayon的并行迭代器对每棵树做并行处理，但编译时触发错误，提示ValidRelationshipTree未满足std::marker::Sized trait约束。可实际上这个结构体的大小在编译时是明确可知的（标注为16字节，对齐8字节）。

相关结构体定义 #

ValidRelationshipTree #

// size = 16 (0x10), align = 0x8
struct ValidRelationshipTree {
    root: RefCell<Rc<ValidRelationshipNode>>,
}

ValidRelationshipNode #

struct ValidRelationshipNode {
    payload: String,
    extended: String,                             
    parent: RefCell<Weak<ValidRelationshipNode>>, // 使用Weak<T>避免引用循环
    children: RefCell<Vec<Rc<ValidRelationshipNode>>>,
}

编译错误详情 #

the method `into_par_iter` exists for struct `Vec<ValidRelationshipTree>`, but its trait bounds were not satisfied
the following trait bounds were not satisfied:
`[metadata::relationships::ValidRelationshipTree]: std::marker::Sized`
which is required by `[metadata::relationships::ValidRelationshipTree]: rayon::iter::IntoParallelIterator`
`[metadata::relationships::ValidRelationshipTree]: rayon::iter::ParallelIterator`
which is required by `[metadata::relationships::ValidRelationshipTree]: rayon::iter::IntoParallelIterator`

可正常运行的顺序迭代代码 #

let forest: Vec<ValidRelationshipTree> = self.generate_relationship_trees(n_tables, true);

let mut extended: Vec<String> = forest
    .into_iter()
    .flat_map(|tree: ValidRelationshipTree| {
        tree.nodes_with_length(n_tables)
            .into_iter()
            .map(|node: Rc<ValidRelationshipNode>| node.extended().clone())
    })
    .collect();

触发错误的并行迭代代码 #

let forest: Vec<ValidRelationshipTree> = self.generate_relationship_trees(n_tables, true);

let extended_par: Vec<String> = forest
    .into_par_iter() // ERROR: 以下 trait 约束未满足:
                     // [metadata::relationships::ValidRelationshipTree]: std::marker::Sized`
    .map(|node| node.extended().clone())
    .collect();

我的疑问 #

明明ValidRelationshipTree的编译时大小是确定的，为什么会被判定未实现Sized？我查阅过Sized trait文档、Rayon相关问题，甚至尝试为Vec<ValidRelationshipTree>创建包装结构体，都无法解决这个问题，希望有人能填补我的知识盲区，给出解决方案。

内容的提问来源于stack exchange，提问作者ejovo13