Rust中Arith trait泛型实现的生命周期注解问题求助

原始可运行代码 #

pub trait Arith {
    fn square(x: &Self) -> Self;
}

impl Arith for i32 {
     fn square(x: &i32) -> i32 {
         x * x
     }
}

impl Arith for f32 {
    fn square(x: &f32) -> f32 {
        x * x
    }
}

尝试的泛型实现代码 #

pub trait Arith where Self: Sized + Mul<&Self, Output = Self> {
    fn square(x: &Self) -> Self {
        x * x
    }
}

编译错误信息 #

error[E0637]: `&` without an explicit lifetime name cannot be used here
 --> src/main.rs:7:41
  |
7 | pub trait Arith where Self: Sized + Mul<&Self, Output = Self> {
  |                                         ^ explicit lifetime name needed here
  |
help: consider introducing a higher-ranked lifetime here with `for<'a>`
 --> src/main.rs:7:37
  |
7 | pub trait Arith where Self: Sized + Mul<&Self, Output = Self> {
  |                                     ^

error[E0311]: the parameter type `Self` may not live long enough
 --> src/main.rs:7:37
  |
7 | pub trait Arith where Self: Sized + Mul<&Self, Output = Self> {
  |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^ ...so that the reference type `&Self` does not outlive the data it points at

Some errors have detailed explanations: E0311, E0637.
For more information about an error, try `rustc --explain E0311`.

需求 #

保持Arith::square(&x)的调用形式，实现泛版的trait，避免重复为每个数值类型手动写impl。

解决方案 #

问题出在trait约束里的Mul<&Self>缺少明确的生命周期声明，且需要保证Self能和任意生命周期的自身引用相乘。通过更高阶生命周期（HRTB）for<'a>来约束Mul，就能解决这两个错误。

正确代码 #

use std::ops::Mul;

// 使用for<'a>指定：对任意生命周期'a，Self都能和&'a Self相乘得到Self
pub trait Arith: Sized + for<'a> Mul<&'a Self, Output = Self> {
    fn square(x: &Self) -> Self {
        x * x
    }
}

// 现在只需一行就能为类型实现Arith，无需重复写square逻辑
impl Arith for i32 {}
impl Arith for f32 {}

调用示例 #

fn main() {
    let num = 10i32;
    println!("{} squared = {}", num, Arith::square(&num)); // 10 squared = 100

    let float = 2.5f32;
    println!("{} squared = {}", float, Arith::square(&float)); // 2.5 squared = 6.25
}

解释 #

• for<'a>是更高阶生命周期语法，告诉编译器：不管引用的生命周期是什么样的，Self都满足Mul<&'a Self, Output = Self>的约束。这就解决了E0637要求的显式生命周期问题，同时也让编译器确信引用不会超过Self的生命周期（解决E0311）。
• 因为i32、f32等数值类型本身就实现了Mul<&Self, Output = Self>，所以我们只需要简单写impl Arith for 类型 {}就能自动获得square方法。

内容的提问来源于stack exchange，提问作者Yan Zhu