Add rustc_const_unstable attribute

Author: TimDiekmann

src/libcore/ptr.rs

@@ -2892,6 +2892,17 @@ impl<T: ?Sized> !Send for NonNull<T> { }
 impl<T: ?Sized> !Sync for NonNull<T> { }
 
 impl<T: Sized> NonNull<T> {
+    /// No docs here
+    #[stable(feature = "nonnull", since = "1.25.0")]
+    #[inline]
+    #[cfg(stage0)]
+    pub fn dangling() -> Self {
+        unsafe {
+            let ptr = mem::align_of::<T>() as *mut T;
+            NonNull::new_unchecked(ptr)
+        }
+    }
+    
     /// Creates a new `NonNull` that is dangling, but well-aligned.
     ///
     /// This is useful for initializing types which lazily allocate, like
@@ -2903,6 +2914,8 @@ impl<T: Sized> NonNull<T> {
     /// some other means.
     #[stable(feature = "nonnull", since = "1.25.0")]
     #[inline]
+    #[cfg(not(stage0))]
+    #[rustc_const_unstable(feature = "const_ptr_nonnull")]
     pub const fn dangling() -> Self {
         unsafe {
             let ptr = mem::align_of::<T>() as *mut T;
@@ -2963,9 +2976,21 @@ impl<T: ?Sized> NonNull<T> {
         &mut *self.as_ptr()
     }
 
+    /// No docs here
+    #[stable(feature = "nonnull_cast", since = "1.27.0")]
+    #[inline]
+    #[cfg(stage0)]
+    pub fn cast<U>(self) -> NonNull<U> {
+        unsafe {
+            NonNull::new_unchecked(self.as_ptr() as *mut U)
+        }
+    }
+
     /// Cast to a pointer of another type
     #[stable(feature = "nonnull_cast", since = "1.27.0")]
     #[inline]
+    #[cfg(not(stage0))]
+    #[rustc_const_unstable(feature = "const_ptr_nonnull")]
     pub const fn cast<U>(self) -> NonNull<U> {
         unsafe {
             NonNull::new_unchecked(self.as_ptr() as *mut U)