make internal NonNull methods available to the alloc crate

Author: the8472

library/core/src/ptr/non_null.rs

@@ -452,7 +452,9 @@ impl<T: ?Sized> NonNull<T> {
 
     /// See [`pointer::add`] for semantics and safety requirements.
     #[inline]
-    pub(crate) const unsafe fn add(self, delta: usize) -> Self
+    #[doc(hidden)]
+    #[unstable(feature = "std_internals", issue = "none")]
+    pub const unsafe fn add(self, delta: usize) -> Self
     where
         T: Sized,
     {
@@ -465,7 +467,9 @@ impl<T: ?Sized> NonNull<T> {
 
     /// See [`pointer::sub`] for semantics and safety requirements.
     #[inline]
-    pub(crate) const unsafe fn sub(self, delta: usize) -> Self
+    #[doc(hidden)]
+    #[unstable(feature = "std_internals", issue = "none")]
+    pub const unsafe fn sub(self, delta: usize) -> Self
     where
         T: Sized,
     {
@@ -478,7 +482,9 @@ impl<T: ?Sized> NonNull<T> {
 
     /// See [`pointer::sub_ptr`] for semantics and safety requirements.
     #[inline]
-    pub(crate) const unsafe fn sub_ptr(self, subtrahend: Self) -> usize
+    #[doc(hidden)]
+    #[unstable(feature = "std_internals", issue = "none")]
+    pub const unsafe fn sub_ptr(self, subtrahend: Self) -> usize
     where
         T: Sized,
     {