Rollup merge of rust-lang#93236 - woppopo:const_nonnull_new, r=oli-obk

Make `NonNull::new` `const`

Tracking issue: rust-lang#93235

library/core/src/ptr/non_null.rs

@@ -211,8 +211,9 @@ impl<T: ?Sized> NonNull<T> {
     /// }
     /// ```
     #[stable(feature = "nonnull", since = "1.25.0")]
+    #[rustc_const_unstable(feature = "const_nonnull_new", issue = "93235")]
     #[inline]
-    pub fn new(ptr: *mut T) -> Option<Self> {
+    pub const fn new(ptr: *mut T) -> Option<Self> {
         if !ptr.is_null() {
             // SAFETY: The pointer is already checked and is not null
             Some(unsafe { Self::new_unchecked(ptr) })

library/core/tests/lib.rs

@@ -16,7 +16,9 @@
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_assume_init)]
 #![feature(const_maybe_uninit_assume_init_read)]
+#![feature(const_nonnull_new)]
 #![feature(const_num_from_num)]
+#![feature(const_ptr_as_ref)]
 #![feature(const_ptr_read)]
 #![feature(const_ptr_write)]
 #![feature(const_ptr_offset)]

library/core/tests/ptr.rs

@@ -274,6 +274,21 @@ fn test_unsized_nonnull() {
     assert!(ys == zs);
 }
 
+#[test]
+fn test_const_nonnull_new() {
+    const {
+        assert!(NonNull::new(core::ptr::null_mut::<()>()).is_none());
+
+        let value = &mut 0u32;
+        let mut ptr = NonNull::new(value).unwrap();
+        unsafe { *ptr.as_mut() = 42 };
+
+        let reference = unsafe { &*ptr.as_ref() };
+        assert!(*reference == *value);
+        assert!(*reference == 42);
+    };
+}
+
 #[test]
 #[allow(warnings)]
 // Have a symbol for the test below. It doesn’t need to be an actual variadic function, match the