Dubious justification for using `NonNull` in `Arc`

In the page Implementing Arc and Mutex / Arc / Layout, the justification for using Arc is the following:

pub struct Arc<T> { ptr: *mut ArcInner<T>, }

This would compile, however it would be incorrect. First of all, the compiler will give us too strict variance. For example, an Arc<&'static str> couldn't be used where an Arc<&'a str> was expected.

To fix the first problem, we can use NonNull. Note that NonNull is a wrapper around a raw pointer that declares that:

• We are variant over T
• Our pointer is never null

What bugs me here is that ptr wouldn't need to be *mut ArcInner<T>, being *const ArcInner<T> would be enough and wouldn't have the variance problem. The justification for the pointer never being null is right though, but the one about variance is dubious.

Read the Implementing Arc and Mutex / Arc / Layout page and compare its explanation of *mut ArcInner<T>, *const ArcInner<T>, and NonNull<T>. Revise the justification so the variance and non-null claims are technically accurate, then verify that the page clearly explains why the chosen representation is used.