liballoc/RawVec: constify non_null, restore ptr impl

Author: mammothbane

library/alloc/src/raw_vec.rs

@@ -489,12 +489,12 @@ impl<A: Allocator> RawVecInner<A> {
 
     #[inline]
     const fn ptr<T>(&self) -> *mut T {
-        self.ptr.as_ptr() as _
+        self.non_null::<T>().as_ptr()
     }
 
     #[inline]
-    fn non_null<T>(&self) -> NonNull<T> {
-        self.ptr.cast().into()
+    const fn non_null<T>(&self) -> NonNull<T> {
+        self.ptr.cast().as_non_null_ptr()
     }
 
     #[inline]