Add a missing precondition check

Author: saethlin

core/src/ptr/non_null.rs

@@ -2,6 +2,7 @@ use crate::cmp::Ordering;
 use crate::convert::From;
 use crate::fmt;
 use crate::hash;
+use crate::intrinsics::assert_unsafe_precondition;
 use crate::marker::Unsize;
 use crate::mem::{self, MaybeUninit};
 use crate::num::NonZeroUsize;
@@ -195,7 +196,10 @@ impl<T: ?Sized> NonNull<T> {
     #[inline]
     pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
         // SAFETY: the caller must guarantee that `ptr` is non-null.
-        unsafe { NonNull { pointer: ptr as _ } }
+        unsafe {
+            assert_unsafe_precondition!([T: ?Sized](ptr: *mut T) => !ptr.is_null());
+            NonNull { pointer: ptr as _ }
+        }
     }
 
     /// Creates a new `NonNull` if `ptr` is non-null.