If a struct definition has fields that are reference to any type, we need to explicitly specify how long those references will live. The syntax is similar to that of function signatures: we first declare the lifetime names on the struct line, and then use them in the fields.

Here's what the syntax looks like in its simplest form:

//  lifetime_struct.rs

struct Number<'a> { 
    num: &'a u8 
}

fn main() {
    let _n = Number {num: &545}; 
}

The definition of Number lives as long as the reference for num.