rust: qom: allow initializing interface vtables

Unlike regular classes, interface vtables can only be obtained via
object_class_dynamic_cast.  Provide a wrapper that allows accessing
the vtable and pass it to a ClassInitImpl implementation, for example
ClassInitImpl<ResettableClass>.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/src/prelude.rs

@@ -9,6 +9,7 @@ pub use crate::cell::BqlRefCell;
 
 pub use crate::qdev::DeviceMethods;
 
+pub use crate::qom::InterfaceType;
 pub use crate::qom::IsA;
 pub use crate::qom::Object;
 pub use crate::qom::ObjectCast;

rust/qemu-api/src/qom.rs

@@ -66,8 +66,8 @@ pub use bindings::{Object, ObjectClass};
 
 use crate::{
     bindings::{
-        self, object_dynamic_cast, object_get_class, object_get_typename, object_new, object_ref,
+        self, object_class_dynamic_cast, object_dynamic_cast, object_get_class,
-        object_unref, TypeInfo,
+        object_get_typename, object_new, object_ref, object_unref, TypeInfo,
     },
     cell::bql_locked,
 };
@@ -263,6 +263,47 @@ pub unsafe trait ObjectType: Sized {
     }
 }
 
+/// Trait exposed by all structs corresponding to QOM interfaces.
+/// Unlike `ObjectType`, it is implemented on the class type (which provides
+/// the vtable for the interfaces).
+///
+/// # Safety
+///
+/// `TYPE` must match the contents of the `TypeInfo` as found in the C code;
+/// right now, interfaces can only be declared in C.
+pub unsafe trait InterfaceType: Sized {
+    /// The name of the type, which can be passed to
+    /// `object_class_dynamic_cast()` to obtain the pointer to the vtable
+    /// for this interface.
+    const TYPE_NAME: &'static CStr;
+
+    /// Initialize the vtable for the interface; the generic argument `T` is the
+    /// type being initialized, while the generic argument `U` is the type that
+    /// lists the interface in its `TypeInfo`.
+    ///
+    /// # Panics
+    ///
+    /// Panic if the incoming argument if `T` does not implement the interface.
+    fn interface_init<
+        T: ObjectType + ClassInitImpl<Self> + ClassInitImpl<U::Class>,
+        U: ObjectType,
+    >(
+        klass: &mut U::Class,
+    ) {
+        unsafe {
+            // SAFETY: upcasting to ObjectClass is always valid, and the
+            // return type is either NULL or the argument itself
+            let result: *mut Self = object_class_dynamic_cast(
+                (klass as *mut U::Class).cast(),
+                Self::TYPE_NAME.as_ptr(),
+            )
+            .cast();
+
+            <T as ClassInitImpl<Self>>::class_init(result.as_mut().unwrap())
+        }
+    }
+}
+
 /// This trait provides safe casting operations for QOM objects to raw pointers,
 /// to be used for example for FFI. The trait can be applied to any kind of
 /// reference or smart pointers, and enforces correctness through the [`IsA`]