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Lifetime explained

fn main() {
    let x = "hi";   // lifetime a.
    let e;       // e should have a lifetime a.

    {
        let y = "you";   // lifetime b.
        e = check(&x, &y); // here we returned lifetime b.
    } // lifetime b ends, y is dropped.

    println!("{}", e) 
} // end of lifetime a.

// The compiler doesn't know which is small lifetime,
// which is big lifetime, we can use `where 'a: 'b` to tell
// the compiler that lifetime a live longer than lifetime b.
fn check<'a, 'b>(s: &'a str, b: &'b str) -> &'b where 'a: 'b {
    println!("{}", b);
     
    // the return signature `-> &'b` means we are returning something to
    // a variable with lifetime b.
    // we returned a content with lifetime a which is bigger than lifetime 
    // b. This is ok.
    return s;
}

// BAD !!!!
fn check<'a, 'b>(s: &'a str, b: &'b str) -> &'a where 'a: 'b {
    // now we returned a content with lifetime b, which is smaller than lifetime a.
    // and the function's return value will be assign to some variable that has lifetime a.
    
    // ---------
    // {
    //   lifetime a 
    // 
    //     {
    //        lifetime b
    //     } // b is dropped, will be not accessable for example.
    //    Since you assign b to a variable with lifetime a,
    //    If I use the variable here, it will cause an error, the variable reference to a content
    //    that is dropped at the ends of lifetime b.
    // }  // end of lifetime a.
    // ---------
    
    return b;
}

// lifetime == scope

let x = 3;     // scope a

{
   let b = 4;  // scope b
}

1. The lifetime is used by the compiler not the user.
2. let a = &b, the lifetime of a must be small than lifetime of b, otherwise it will be invalid when b ends before a. That means we can only assign a smaller lifetime variable to a bigger lifetime variable. small <> big, not big <> small. It is same for the return of function. if the return of function have lifetime big, but the content of the return have lifetime small, than there is a chance that the content will be dropped before lifetime big end, that should not be happend.