/// Usually this won't be necessary; covariance is correct for most safe abstractions,
/// such as Box, Rc, Arc, Vec, and LinkedList. This is the case because they
/// provide a public API that follows the normal shared XOR mutable rules of Rust.
+///
+/// Notice that `NonNull<T>` has a `From` instance for `&T`. However, this does
+/// not change the fact that mutating through a (pointer derived from a) shared
+/// reference is undefined behavior unless the mutation happens inside an
+/// [`UnsafeCell<T>`]. When using this `From` instance without an `UnsafeCell<T>`,
+/// it is your responsibility to ensure that `as_mut` is never called, and `as_ptr`
+/// is never used for mutation.
+///
+/// [`UnsafeCell<T>`]: ../cell/struct.UnsafeCell.html
#[stable(feature = "nonnull", since = "1.25.0")]
#[repr(transparent)]
#[rustc_layout_scalar_valid_range_start(1)]