Unfortunate example in Lifetime Bounds chapter (15.4.6)

[Mingun]

The previous issue: #1686. Created a new one as suggested.

At time of writing the mentioned chapter can be found at this link.

Currently the example that illustrates the Trait + 'a bound looks as this:

use std::fmt::Debug; // Trait to bound with.

#[derive(Debug)]
struct Ref<'a, T: 'a>(&'a T);
// `Ref` contains a reference to a generic type `T` that has
// an unknown lifetime `'a`. `T` is bounded such that any
// *references* in `T` must outlive `'a`. Additionally, the lifetime
// of `Ref` may not exceed `'a`.

// A generic function which prints using the `Debug` trait.
fn print<T>(t: T) where
    T: Debug {
    println!("`print`: t is {:?}", t);
}

// Here a reference to `T` is taken where `T` implements
// `Debug` and all *references* in `T` outlive `'a`. In
// addition, `'a` must outlive the function.
fn print_ref<'a, T>(t: &'a T) where
    T: Debug + 'a {
//    T: Debug {// it is worked even in this case, so `+ 'a` part still stay unclear
    println!("`print_ref`: t is {:?}", t);
}

fn main() {
    let x = 7;
    let ref_x = Ref(&x);

    print_ref(&ref_x);
    print(ref_x);
}

However, as you can easely check, if you just remove + 'a bound the code is still valid. So for newbies it could be still unclear when they should use this construct. I think, that the example should not compile if such bound would be removed to show code that needs that feature.