What does it mean when we let a trait inherits 'static?

Rust supports trait inheritance, as follows:

pub trait A {}
pub trait B: A {}

B: A means that if some type T implements B, it also needs to implement all the methods in A.

But today I see the following code:

trait Display: 'static {
    fn print(&self);
}

What does it mean? It doesn't seem to be trait inheritance.

Rust doesn't have inheritance.

What it has is a way to define constraints. For example a trait may be constrained to only be implemented by types which implement another trait.

In your case the constraint is a lifetime bound.

To implement your Display trait, an object may contain references but in this case their lifetime must respect this constraint.

Let's suppose you have this type:

struct S<'a> {
    s: &'a str,
}

Then you can't implement the trait for any lifetime, but only 'static.

impl Display for S<'static> {
    fn print(&self){}
}

fn main() {
    let s1 = "test";
    let a = S { s: s1 };
    a.print(); // compiles

    let s2 = "test".to_string();
    let a = S { s: &s2 };
    a.print(); // doesn't compile because s doesn't live long enough
}