Implement Encode and RefEncode for UnsafeCell

Author: madsmtm

crates/objc2/src/encode/mod.rs

@@ -59,6 +59,7 @@
 //! method argument, but is a very common return type, and hence implements
 //! [`Encode`].
 
+use core::cell::UnsafeCell;
 use core::ffi::c_void;
 use core::mem::{self, ManuallyDrop, MaybeUninit};
 use core::num::{
@@ -484,11 +485,12 @@ encode_impls_transparent! {
     // TODO: With specialization: `impl Encode for ManuallyDrop<Box<T>>`
     ManuallyDrop<T: ?Sized>,
 
+    // SAFETY: Guaranteed to have the same in-memory representation `T`.
+    //
     // The fact that this has `repr(no_niche)` has no effect on us, since we
     // don't unconditionally implement `Encode` generically over `Option`.
     // (e.g. an `Option<UnsafeCell<&u8>>` impl is not available).
-    // The inner field is not public, so may not be stable.
-    // TODO: UnsafeCell<T>,
+    UnsafeCell<T: ?Sized>,
 
     // The inner field is not public, so may not be safe.
     // TODO: Pin<T>,
@@ -702,13 +704,10 @@ mod tests {
         assert_eq!(<ManuallyDrop<Option<&u8>>>::ENCODING, u8::ENCODING_REF);
         assert_eq!(<&ManuallyDrop<Option<&u8>>>::ENCODING, <&&u8>::ENCODING);
 
-        // assert_eq!(<UnsafeCell<u8>>::ENCODING, u8::ENCODING);
+        assert_eq!(<UnsafeCell<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);
-
-        // Shouldn't compile
-        // assert_eq!(<Option<UnsafeCell<&u8>>>::ENCODING, <&u8>::ENCODING);
     }
 
     #[test]

crates/test-ui/ui/not_encode.rs

@@ -1,4 +1,5 @@
 //! Verify that certain things we don't want to be encode aren't.
+use core::cell::UnsafeCell;
 use core::ffi::c_void;
 
 use objc2::encode::Encode;
@@ -22,4 +23,9 @@ fn main() {
     is_encode::<fn(i32, ())>();
 
     is_encode::<&Sel>();
+
+    // This should compile
+    is_encode::<UnsafeCell<&u8>>();
+    // But this mustn't
+    is_encode::<Option<UnsafeCell<&u8>>>();
 }

crates/test-ui/ui/not_encode.stderr

@@ -224,6 +224,29 @@ note: required by a bound in `is_encode`
   |                 ^^^^^^ required by this bound in `is_encode`
 help: consider removing the leading `&`-reference
    |
-24 -     is_encode::<&Sel>();
-24 +     is_encode::<Sel>();
+25 -     is_encode::<&Sel>();
+25 +     is_encode::<Sel>();
    |
+
+error[E0277]: the trait bound `UnsafeCell<&u8>: OptionEncode` is not satisfied
+ --> ui/not_encode.rs
+  |
+  |     is_encode::<Option<UnsafeCell<&u8>>>();
+  |                 ^^^^^^^^^^^^^^^^^^^^^^^ the trait `OptionEncode` is not implemented for `UnsafeCell<&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<UnsafeCell<&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`