Implement Encode and RefEncode for Cell

crates/objc2/src/encode.rs

@@ -59,7 +59,7 @@
 //! method argument, but is a very common return type, and hence implements
 //! [`Encode`].
 
-use core::cell::UnsafeCell;
+use core::cell::{Cell, UnsafeCell};
 use core::ffi::c_void;
 use core::mem::{self, ManuallyDrop, MaybeUninit};
 use core::num::{
@@ -492,6 +492,9 @@ encode_impls_transparent! {
     // (e.g. an `Option<UnsafeCell<&u8>>` impl is not available).
     UnsafeCell<T: ?Sized>,
 
+    // SAFETY: Guaranteed to have the same layout as `UnsafeCell<T>`.
+    Cell<T: ?Sized>,
+
     // The inner field is not public, so may not be safe.
     // TODO: Pin<T>,
 
@@ -501,9 +504,6 @@ encode_impls_transparent! {
     // SAFETY: Guaranteed to have the same layout and ABI as `T`.
     Wrapping<T>,
 
-    // It might have requirements that would disourage this impl?
-    // TODO: Cell<T>
-
     // TODO: Types that need to be made repr(transparent) first:
     // - core::cell::Ref?
     // - core::cell::RefCell?
@@ -705,6 +705,9 @@ mod tests {
         assert_eq!(<&ManuallyDrop<Option<&u8>>>::ENCODING, <&&u8>::ENCODING);
 
         assert_eq!(<UnsafeCell<u8>>::ENCODING, u8::ENCODING);
+        assert_eq!(<UnsafeCell<&u8>>::ENCODING, <&u8>::ENCODING);
+        assert_eq!(<Cell<u8>>::ENCODING, u8::ENCODING);
+        assert_eq!(<Cell<&u8>>::ENCODING, <&u8>::ENCODING);
         // assert_eq!(<Pin<u8>>::ENCODING, u8::ENCODING);
         assert_eq!(<MaybeUninit<u8>>::ENCODING, u8::ENCODING);
         assert_eq!(<Wrapping<u8>>::ENCODING, u8::ENCODING);

crates/test-ui/ui/not_encode.rs

@@ -1,5 +1,5 @@
 //! Verify that certain things we don't want to be encode aren't.
-use core::cell::UnsafeCell;
+use core::cell::{Cell, UnsafeCell};
 use core::ffi::c_void;
 
 use objc2::encode::Encode;
@@ -28,4 +28,7 @@ fn main() {
     is_encode::<UnsafeCell<&u8>>();
     // But this mustn't
     is_encode::<Option<UnsafeCell<&u8>>>();
+
+    // Same
+    is_encode::<Option<Cell<&u8>>>();
 }

crates/test-ui/ui/not_encode.stderr

@@ -250,3 +250,26 @@ note: required by a bound in `is_encode`
   |
   | fn is_encode<T: Encode>() {}
   |                 ^^^^^^ required by this bound in `is_encode`
+
+error[E0277]: the trait bound `Cell<&u8>: OptionEncode` is not satisfied
+ --> ui/not_encode.rs
+  |
+  |     is_encode::<Option<Cell<&u8>>>();
+  |                 ^^^^^^^^^^^^^^^^^ the trait `OptionEncode` is not implemented for `Cell<&u8>`
+  |
+  = help: the following other types implement trait `OptionEncode`:
+            &'a T
+            &'a mut T
+            NonNull<T>
+            NonNull<c_void>
+            NonZeroI16
+            NonZeroI32
+            NonZeroI64
+            NonZeroI8
+          and $N others
+  = note: required for `Option<Cell<&u8>>` to implement `Encode`
+note: required by a bound in `is_encode`
+ --> ui/not_encode.rs
+  |
+  | fn is_encode<T: Encode>() {}
+  |                 ^^^^^^ required by this bound in `is_encode`