什么时候关联常量使用“static”以外的生命周期有用？

Question

RFC 1623，在 Rust 1.17.0 中稳定下来，这样我们就不必在 static 或 const 中明确指定 'static 生命周期：

const MY_DEFAULT_NAME: &str = "Anna";
//                      ^ Look, ma! No 'static!

RFC 195 定义了 相关常量，在 Rust 1.20.0 中得到稳定：

struct A;

impl A {
    const B: i32 = 42;
}

尝试将这两者结合时，出现错误：

struct A;

impl A {
    const MY_DEFAULT_NAME: &str = "Anna";
}

error[E0106]: missing lifetime specifier
 --> src/main.rs:4:28
  |
4 |     const MY_DEFAULT_NAME: &str = "Anna";
  |                            ^ expected lifetime parameter

related GitHub issue #38831 has a comment:

We decided against it because, in that case, there might be other lifetimes you would want. For example:
trait Foo<'a> {
    const T: &'a str;
}
that said, revisiting this explanation, it feel a bit weak, since the value of an associated constant would still have to be all consisting of static data. So it seems like 'static is a pretty good default if you don't say otherwise.

具有非 'static 生命周期的关联常量的示例是什么？提供非'static生命周期有什么好处？

Answer 1

人们可能会考虑作为函数的常量：

trait Foo<'a> {
    const BAR: fn(&Self) -> &'a str;
}

struct MyFoo<'a> {
    x: &'a str,
}

impl<'a> Foo<'a> for MyFoo<'a> {
    const BAR: fn(&Self) -> &'a str = my_bar;
}

fn my_bar<'a>(a: &MyFoo<'a>) -> &'a str {
    &a.x
}

现在，我想不出这比方法更有用。

什么时候关联常量使用“static”以外的生命周期有用？

When is it useful for an associated constant to use a lifetime other than 'static?

constants

rust

associated-types