Rust使用dyn Domainable报trait无法构造对象错误如何解决

期望实现一套约束满足问题相关的类型结构，核心代码骨架初始设计如下：

use std::collections::HashSet;
use std::hash::Hash;

pub trait Domainable: Eq + Hash {}
impl<T: Eq + Hash> Domainable for T {}

pub struct Variable<T: Domainable> {
    domain: HashSet<T>,
}

pub struct Assignment<'a, T: Domainable> {
    variable: &'a Variable<T>,
    assignment: T,
}

pub struct State<'a, T: Domainable> {
    assignments: Vec<&'a Assignment<'a, dyn Domainable>>,
}

核心需求是在State结构体内存储一个Assignment向量，向量内的元素可对应不同的、均实现Domainable trait的类型，使用示例如下：

let var1 = Variable::new(HashSet::from([1, 2, 3, 4]));
let var2 = Variable::new(HashSet::from([true, false]));

let assignment1 = Assignment::new(&var1, 2);
let assignment2 = Assignment::new(&var2, false);

let state = State::new(vec![assignment1, assignment2]);

编写过程中dyn Domainable持续抛出编译错误：

error[E0038]: the trait `Domainable` cannot be made into an object
   --> src/main.rs:17:41
    |
17  |     assignments: Vec<&'a Assignment<'a, dyn Domainable>>,
    |                                         ^^^^^^^^^^^^^^ `Domainable` cannot be made into an object
    |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically
    |
   ::: src/main.rs:4:11
    |
4   | pub trait Domainable: Eq + Hash {}
    |           ---------- this trait cannot be made into an object...

尝试按照异构集合资料使用&dyn Trait写法仍无法通过编译。

编译错误核心是**Domainable trait不满足对象安全要求**，无法被构造为dyn Domainable trait对象：

• 你的Domainable继承了Eq和Hash两个trait，其中Eq依赖的PartialEq定义了fn eq(&self, other: &Self) -> bool方法，这类方法签名中出现具体Self类型、依赖Self: Sized约束的trait，无法生成动态分发需要的vtable，自然不支持trait对象。
• 额外逻辑错误：Assignment<'a, i32>和Assignment<'a, bool>是完全不同的具体类型，就算Domainable满足对象安全，你也无法把这两个不同类型的引用直接强转为&Assignment<'a, dyn Domainable>，二者内存布局不兼容。

不要尝试把内部的Domainable泛型参数直接擦除为trait对象，而是给Assignment本身定义一套对象安全的公共操作trait，把不同类型的Assignment统一擦除为这个trait的对象存入Vec即可，参考实现如下：

use std::collections::HashSet;
use std::hash::Hash;
use std::any::Any;

// 保留原Domainable定义，不需要修改
pub trait Domainable: Eq + Hash {}
impl<T: Eq + Hash> Domainable for T {}

pub struct Variable<T: Domainable> {
    domain: HashSet<T>,
}

impl<T: Domainable> Variable<T> {
    pub fn new(domain: HashSet<T>) -> Self {
        Self { domain }
    }
}

pub struct Assignment<'a, T: Domainable> {
    variable: &'a Variable<T>,
    assignment: T,
}

impl<'a, T: Domainable> Assignment<'a, T> {
    pub fn new(variable: &'a Variable<T>, assignment: T) -> Self {
        // 可按需添加赋值合法性校验
        debug_assert!(variable.domain.contains(&assignment));
        Self { variable, assignment }
    }
}

// 定义对象安全的Assignment公共trait，所有需要对Assignment做的操作都在这里声明
pub trait AnyAssignment {
    // 如需运行时按类型获取内部赋值，可通过Any做安全类型转换
    fn as_any(&self) -> &dyn Any;
}

// 为所有符合要求的Assignment实现该trait
impl<'a, T: Domainable + 'static> AnyAssignment for Assignment<'a, T> {
    fn as_any(&self) -> &dyn Any {
        self
    }
}

pub struct State<'a> {
    // 存储擦除类型后的Assignment trait对象
    assignments: Vec<&'a dyn AnyAssignment>,
}

impl<'a> State<'a> {
    pub fn new(assignments: Vec<&'a dyn AnyAssignment>) -> Self {
        Self { assignments }
    }

    // 示例：按类型获取对应变量的赋值
    pub fn get_assignment<T: Domainable + 'static>(&self) -> Option<&T> {
        for assn in &self.assignments {
            if let Some(assn) = assn.as_any().downcast_ref::<Assignment<T>>() {
                return Some(&assn.assignment);
            }
        }
        None
    }
}

fn main() {
    let var1 = Variable::new(HashSet::from([1, 2, 3, 4]));
    let var2 = Variable::new(HashSet::from([true, false]));

    let assignment1 = Assignment::new(&var1, 2);
    let assignment2 = Assignment::new(&var2, false);

    let state = State::new(vec![&assignment1, &assignment2]);
    // 功能验证
    assert_eq!(state.get_assignment::<i32>(), Some(&2));
    assert_eq!(state.get_assignment::<bool>(), Some(&false));
}

如果不需要运行时按类型取具体赋值，可以去掉Any相关逻辑，直接在AnyAssignment trait里定义无泛型的业务方法即可，运行时性能会更好。如果需要State持有Assignment的所有权而非借用，只需要把State内的字段类型改为Vec<Box<dyn AnyAssignment>>，创建时传入vec![Box::new(assignment1), Box::new(assignment2)]即可，逻辑和借用版本完全一致。

内容的提问来源于stack exchange，提问作者Mike