rust: qom: fix TODO about zeroability of classes

The proposed suggestion is not correct.  First it is not necessary for
*all* classes to be Zeroable, only for Rust-defined ones; classes
defined in C never implement ObjectImpl.

Second, the parent class field need not be Zeroable.  For example,
ChardevClass's chr_write and chr_be_event fields cannot be NULL,
therefore ChardevClass cannot be Zeroable.  However, char_class_init()
initializes them, therefore ChardevClass could be subclassed by Rust code.

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/qom.rs

@@ -534,9 +534,10 @@ pub trait ObjectImpl: ObjectType + IsA<Object> {
     /// While `klass`'s parent class is initialized on entry, the other fields
     /// are all zero; it is therefore assumed that all fields in `T` can be
     /// zeroed, otherwise it would not be possible to provide the class as a
-    /// `&mut T`.  TODO: add a bound of [`Zeroable`](crate::zeroable::Zeroable)
-    /// to T; this is more easily done once Zeroable does not require a manual
-    /// implementation (Rust 1.75.0).
+    /// `&mut T`.  TODO: it may be possible to add an unsafe trait that checks
+    /// that all fields *after the parent class* (but not the parent class
+    /// itself) are Zeroable.  This unsafe trait can be added via a derive
+    /// macro.
     const CLASS_INIT: fn(&mut Self::Class);
 }