Add Id::retain_null helper function

Author: madsmtm

objc2/CHANGELOG.md

@@ -11,7 +11,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
   upgrading easier.
 * Allow using `From`/`TryFrom` to convert between `rc::Id` and `rc::WeakId`.
 * Added `Bool::as_bool` (more descriptive name than `Bool::is_true`).
-* Added convenience methods `Id::new_null` and `Id::as_ptr`.
+* Added convenience methods `Id::new_null`, `Id::as_ptr` and
+  `Id::retain_null`.
 
 
 ## 0.3.0-alpha.6 - 2022-01-03

objc2/src/rc/id.rs

@@ -251,6 +251,21 @@ impl<T: Message, O: Ownership> Id<T, O> {
         unsafe { Self::new(res) }
     }
 
+    /// Retains an object pointer that may be null.
+    ///
+    /// This is just a convenience wrapper over [`Id::retain`] so that you
+    /// don't need to construct a [`NonNull`] when you know the pointer may
+    /// be null.
+    ///
+    /// # Safety
+    ///
+    /// Same as [`Id::retain`].
+    #[inline]
+    pub unsafe fn retain_null(ptr: *mut T) -> Option<Id<T, O>> {
+        // SAFETY: Upheld by the caller
+        NonNull::new(ptr).map(|ptr| unsafe { Id::retain(ptr) })
+    }
+
     #[cfg_attr(not(debug_assertions), inline)]
     fn autorelease_inner(self) -> *mut T {
         // Note that this (and the actual `autorelease`) is not an associated