Rust编译报错：main函数结束时引用仍被借用，求解决方案

问题背景 #

我正在学习《Ray Tracing in One Weekend》并使用Rust实现。代码中定义了Hittable和Material两个trait，Hittable trait为Sphere等对象实现hit方法，每种对象包含对应材质（如Glass）。实际项目中，Sphere结构体和HitRecord结构体（用于hit方法传递结果的可变引用）均包含&dyn Material，因此为相关trait和结构体添加了生命周期参数。但编译时编译器提示main函数结束时引用仍处于借用状态。

问题代码 #

trait Hittable<'a> {
    fn hit(&self);
}

trait Material {
    fn result(&self);
}

struct Glass;

impl Material for Glass {
    fn result(&self) {
        println!("Glass is broken!");
    }
}

struct Sphere<'a> {
    name: String,
    mat_ptr: &'a dyn Material,
}

impl<'a> Hittable<'a> for Sphere<'a> {
    fn hit(&self) {
        println!("Name is {}", self.name);
        self.mat_ptr.result();
    }
}

struct HT<'a> {
    pub objects: Vec<Box<dyn Hittable<'a>>>,
}

fn main() {
    let mut list = HT { objects: vec![] };
    let surface_material = Glass;
    let s = Sphere {
        name: String::from("球"),
        mat_ptr: &surface_material,
    };
    list.objects.push(Box::new(s));
}

错误信息 #

Compiling rust_test v0.1.0 (/home/hnyls2002/rust_test)
error[E0597]: `surface_material` does not live long enough
  --> src/main.rs:38:18
   |
38 |         mat_ptr: &surface_material,
   |                  ^^^^^^^^^^^^^^^^^
   |                  |
   |                  borrowed value does not live long enough
   |                  cast requires that `surface_material` is borrowed for `'static`
...
41 | }
   | - `surface_material` dropped here while still borrowed

For more information about this error, try `rustc --explain E0597`.

问题原因 #

错误核心在于：将Sphere包装进Box<dyn Hittable<'a>>时，Rust默认要求该trait对象满足'static生命周期（除非显式标注）。而surface_material是main函数栈上的局部变量，生命周期仅到main函数结束，但list持有了对它的引用，导致引用生命周期短于被引用值的销毁时机，违反了Rust的借用规则。

解决方案 #

方案1：使用引用计数智能指针（推荐） #

光线追踪场景中多个物体可能共享同一种材质，用Arc<dyn Material>可安全共享材质实例，彻底规避生命周期问题：

use std::sync::Arc;

trait Hittable {
    fn hit(&self);
}

trait Material {
    fn result(&self);
}

struct Glass;

impl Material for Glass {
    fn result(&self) {
        println!("Glass is broken!");
    }
}

struct Sphere {
    name: String,
    mat_ptr: Arc<dyn Material>,
}

impl Hittable for Sphere {
    fn hit(&self) {
        println!("Name is {}", self.name);
        self.mat_ptr.result();
    }
}

struct HT {
    pub objects: Vec<Box<dyn Hittable>>,
}

fn main() {
    let mut list = HT { objects: vec![] };
    let surface_material = Arc::new(Glass);
    let s = Sphere {
        name: String::from("球"),
        mat_ptr: surface_material.clone(),
    };
    list.objects.push(Box::new(s));
}

方案2：调整变量声明顺序 #

利用Rust反向销毁变量的规则，让被引用的材质先声明、后销毁，保证其生命周期长于持有引用的list：

trait Hittable<'a> {
    fn hit(&self);
}

trait Material {
    fn result(&self);
}

struct Glass;

impl Material for Glass {
    fn result(&self) {
        println!("Glass is broken!");
    }
}

struct Sphere<'a> {
    name: String,
    mat_ptr: &'a dyn Material,
}

impl<'a> Hittable<'a> for Sphere<'a> {
    fn hit(&self) {
        println!("Name is {}", self.name);
        self.mat_ptr.result();
    }
}

struct HT<'a> {
    pub objects: Vec<Box<dyn Hittable<'a>>>,
}

fn main() {
    let surface_material = Glass;
    let mut list = HT { objects: vec![] };
    let s = Sphere {
        name: String::from("球"),
        mat_ptr: &surface_material,
    };
    list.objects.push(Box::new(s));
}

方案3：将材质移到堆上获得'static生命周期 #

把材质实例分配到堆上，使其拥有'static生命周期，满足trait对象的约束：

trait Hittable<'a> {
    fn hit(&self);
}

trait Material {
    fn result(&self);
}

struct Glass;

impl Material for Glass {
    fn result(&self) {
        println!("Glass is broken!");
    }
}

struct Sphere<'a> {
    name: String,
    mat_ptr: &'a dyn Material,
}

impl<'a> Hittable<'a> for Sphere<'a> {
    fn hit(&self) {
        println!("Name is {}", self.name);
        self.mat_ptr.result();
    }
}

struct HT<'a> {
    pub objects: Vec<Box<dyn Hittable<'a>>>,
}

fn main() {
    let mut list = HT { objects: vec![] };
    // 将材质分配到堆上，获得'static生命周期
    let surface_material = Box::new(Glass) as Box<dyn Material>;
    let s = Sphere {
        name: String::from("球"),
        mat_ptr: &*surface_material,
    };
    list.objects.push(Box::new(s));
    // 注意：surface_material需与list同生命周期，不能提前销毁
}

内容的提问来源于stack exchange，提问作者hnyls2002