What is the meaning of 'static as a function constraint?

Question

What is the meaning of static in this context?

fn foo<F: Human + 'static>(param: F) {}

fn main() {
    let kate = Kate { age: 30 };
    foo(kate);
}

I understand marking global_variable static is similar to what static means ie. in c#, space for the variable is allocated in a separate segment of the memory, existing for the whole execution of the program.

static global_variable: i32 = 5;

But what I don't understand what giving 'static constraint means. Is kate somehow promoted, and her lifetime extended so it now lives for the whole execution of program too?

Or does it simply mean it'll be deallocated as soon as foo stops using it?

Answer 1

Putting a constraint like T: 'a means that all lifetime parameters of the type T must satisfy the lifetime constraint 'a (thus, must outlive it).

For example, if I have this struct:

struct Kate<'a, 'b> {
    address: &'a str,
    lastname: &'b str
}

Kate<'a, 'b> will satisfy the constraint F: Human + 'static only if 'a == 'static and 'b == 'static.

However, a struct without any lifetime parameter will always satisfy any lifetime constraint.

So as a summary, a constraint like F: 'static means that either:

• F has no lifetime parameter
• all lifetime parameters of F are 'static