Rust结构体用dyn trait作字段报错，为何String可正常使用？

原始代码 #

trait Animal {}

struct Sheep {}
struct Cow {}

// 为Sheep实现Animal trait
impl Animal for Sheep {}

// 为Cow实现Animal trait
impl Animal for Cow {}

struct Human {
    name: String,
    pet: dyn Animal,
}

impl Human {
    fn new(name: String, animal: dyn Animal) -> Self {
        Human { name, pet: animal }
    }

    fn say_my_name(&self) {
        println!("{}", self.name);
    }
}

fn main() {
    let sheep = Sheep {};
    let human = Human::new(String::from("alex"), sheep);

    human.say_my_name();
}

编译错误信息 #

error[E0277]: the size for values of type `(dyn Animal + 'static)` cannot be known at compilation time
  --> src/main.rs:18:26
   |
18 |     fn new(name: String, animal: dyn Animal) -> Self {
   |                          ^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `(dyn Animal + 'static)`
help: you can use `impl Trait` as the argument type
   |
18 |     fn new(name: String, animal: impl Animal) -> Self {
   |                                  ~~~~
help: function arguments must have a statically known size, borrowed types always have a known size
   |
18 |     fn new(name: String, animal: &dyn Animal) -> Self {
   |                                  +

error[E0308]: mismatched types
  --> src/main.rs:29:50
   |
29 |     let human = Human::new(String::from("alex"), sheep);
   |                 ----------                       ^^^^^ expected `dyn Animal`, found `Sheep`
   |                 |
   |                 arguments to this function are incorrect
   |
   = note: expected trait object `(dyn Animal + 'static)`
                    found struct `Sheep`
note: associated function defined here
  --> src/main.rs:18:8
   |
18 |     fn new(name: String, animal: dyn Animal) -> Self {
   |        ^^^               ------------------

error[E0277]: the size for values of type `(dyn Animal + 'static)` cannot be known at compilation time
  --> src/main.rs:29:9
   |
29 |     let human = Human::new(String::from("alex"), sheep);
   |         ^^^^^ doesn't have a size known at compile-time
   |
   = help: within `Human`, the trait `Sized` is not implemented for `(dyn Animal + 'static)`
note: required because it appears within the type `Human`
  --> src/main.rs:12:8
   |
12 | struct Human {
   |        ^^^^^
   = note: all local variables must have a statically known size
   = help: unsized locals are gated as an unstable feature

error[E0277]: the size for values of type `(dyn Animal + 'static)` cannot be known at compilation time
  --> src/main.rs:29:50
   |
29 |     let human = Human::new(String::from("alex"), sheep);
   |                                                  ^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `(dyn Animal + 'static)`
   = note: all function arguments must have a statically known size

疑问 #

编译器提示dyn Animal类型的大小在编译时无法确定，但Human结构体的name字段是String类型，同样涉及动态内容却能正常使用，这是为什么？

修复后的代码 #

trait Animal {}

struct Sheep {}
struct Cow {}

// 为Sheep实现Animal trait
impl Animal for Sheep {}

// 为Cow实现Animal trait
impl Animal for Cow {}

struct Human {
    name: String,
    pet: Box<dyn Animal>,
}

impl Human {
    fn new(name: String, animal: Box<dyn Animal>) -> Self {
        Human { name, pet: animal }
    }

    fn say_my_name(&self) {
        println!("{}", self.name);
    }
}

fn main() {
    let sheep = Box::new(Sheep {});
    let human = Human::new(String::from("alex"), sheep);

    human.say_my_name();
}

解答 #

核心差异：String是Sized类型，dyn Animal是非Sized类型 #

Rust中的String本质是一个固定大小的结构体，它包含三个字段：

• 指向堆内存的指针
• 字符串的当前长度
• 堆内存的容量

这三个字段的大小在编译时完全确定（64位系统下每个字段占8字节，总共24字节），所以String实现了Sized trait，完全符合Rust对结构体字段必须是Sized类型的要求（仅结构体最后一个字段可例外为非Sized，但有额外限制）。

而dyn Animal是 trait 对象，它代表任意实现了Animal trait的类型。不同类型（比如Sheep和Cow）的内存大小可能不同，编译器无法在编译时确定dyn Animal的具体大小，因此它属于非Sized类型，不能直接作为结构体字段或函数参数使用——Rust要求局部变量、函数参数、结构体字段（除特殊情况）必须在编译时确定大小，这样才能分配栈内存。

为什么Box<dyn Animal>能解决问题？ #

Box<T>是智能指针，它本身是固定大小的（64位系统下占8字节，就是一个堆内存地址）。它将dyn Animal这个非Sized类型存储在堆上，栈上仅保留指向堆内存的指针，这样整个Box<dyn Animal>的大小在编译时是确定的，满足了Rust的Sized要求。

总结：

• String：栈上固定大小的结构体，指向堆上动态内容，本身是Sized
• dyn Animal：无固定大小，是非Sized类型，不能直接使用
• Box<dyn Animal>：栈上固定大小的指针，指向堆上的trait对象，本身是Sized

内容的提问来源于stack exchange，提问作者alex