我正在使用一个早于1.31的Rust实现一个引用类型的特性.Why does Rust want an explicit lifetime when I tell it what reference type I'm implementing the trait for?

下面是一个简单的例子.struct Inches是

最初的例子

(Rust playground link)

use std::ops::Add;

struct Inches(i32);

// this would work: impl<'b> Add for &'b Inches
impl Add for &Inches {
    type Output = Inches;

    fn add(self, other: &Inches) -> Inches {
        let &Inches(x) = self;
        let &Inches(y) = other;

        Inches(x + y)
    }
}

// lifetime specifier needed here because otherwise 
// `total = hilt + blade` doesn't know whether `total` should live
// as long as `hilt`, or as long as `blade`.
fn add_inches<'a>(hilt: &'a Inches, blade: &'a Inches) {
    let total = hilt + blade;
    let Inches(t) = total;
    println!("length {}", t);
}

fn main() {
    let hilt = Inches(10);
    let blade = Inches(20);

    add_inches(&hilt, &blade);
}

编译失败，出现以下错误:

error: missing lifetime specifier [E0106]
    impl Add for &Inches {
                 ^~~~~~~

我添加了缺少的生存期说明符(仍然无法编译)

// was: impl Add for &Inches {
impl Add for &'b Inches {
    ...
}

编译错误:

error: use of undeclared lifetime name `'b` [E0261]
    impl Add for &'b Inches {

I declare the lifetime on the impl (now it compiles)

(Rust playground link)

// was: impl Add for &'b Inches {
impl<'b> Add for &'b Inches {
    ...
}

最后，它可以正确编译.

我的问题

为什么impl Add for &Inches人中有&Inches人被认为缺乏