What does it mean for a trait to have a lifetime parameter?

Question

I understand how lifetime parameters apply to functions and structs, but what does it mean for a trait to have a lifetime parameter? Is it a shortcut to introduce a lifetime parameter to its methods, or is it something else?

Answer 1

If you have a trait with a lifetime bound, then implementors of the trait can participate in the same lifetime. Concretely, this allows you to store references with that lifetime. It is not a shortcut for specifying lifetimes on member methods, and difficulty and confusing error messages lie that way!

trait Keeper<'a> {
    fn save(&mut self, v: &'a u8);
    fn restore(&self) -> &'a u8;
}

struct SimpleKeeper<'a> {
    val: &'a u8,
}

impl<'a> Keeper<'a> for SimpleKeeper<'a> {
    fn save(&mut self, v: &'a u8) {
        self.val = v
    }
    fn restore(&self) -> &'a u8 {
        self.val
    }
}

Note how both the struct and the trait are parameterized on a lifetime, and that lifetime is the same.

What would the non-trait versions of save() and restore() look like for SimpleKeeper<'a>?

Very similar, actually. The important part is that the struct stores the reference itself, so it needs to have a lifetime parameter for the values inside.

struct SimpleKeeper<'a> {
    val: &'a u8,
}

impl<'a> SimpleKeeper<'a> {
    fn save(&mut self, v: &'a u8) {
        self.val = v
    }
    fn restore(&self) -> &'a u8 {
        self.val
    }
}

And would they mean exactly the same thing as the the trait version?

Yep!