The ObjectDeref trait now provides all the magic that is required to fake
inheritance.  Replace the "impl SysBusDevice" block of qemu_api::sysbus
with a trait, so that sysbus_init_irq() can be invoked as "self.init_irq()"
without any intermediate upcast.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/hw/char/pl011/src/device.rs |  6 ++----
 rust/qemu-api/src/irq.rs         |  3 +--
 rust/qemu-api/src/prelude.rs     |  2 ++
 rust/qemu-api/src/sysbus.rs      | 17 +++++++++--------
 4 files changed, 14 insertions(+), 14 deletions(-)

diff --git a/rust/hw/char/pl011/src/device.rs b/rust/hw/char/pl011/src/device.rs
index 6792d13fb77..994c2fc0593 100644
--- a/rust/hw/char/pl011/src/device.rs
+++ b/rust/hw/char/pl011/src/device.rs
@@ -207,11 +207,9 @@ unsafe fn init(&mut self) {
     }
 
     fn post_init(&self) {
-        let sbd: &SysBusDevice = self.upcast();
-
-        sbd.init_mmio(&self.iomem);
+        self.init_mmio(&self.iomem);
         for irq in self.interrupts.iter() {
-            sbd.init_irq(irq);
+            self.init_irq(irq);
         }
     }
 
diff --git a/rust/qemu-api/src/irq.rs b/rust/qemu-api/src/irq.rs
index 6258141bdf0..378e5202951 100644
--- a/rust/qemu-api/src/irq.rs
+++ b/rust/qemu-api/src/irq.rs
@@ -24,8 +24,7 @@
 ///
 /// Interrupts are implemented as a pointer to the interrupt "sink", which has
 /// type [`IRQState`].  A device exposes its source as a QOM link property using
-/// a function such as
-/// [`SysBusDevice::init_irq`](crate::sysbus::SysBusDevice::init_irq), and
+/// a function such as [`SysBusDeviceMethods::init_irq`], and
 /// initially leaves the pointer to a NULL value, representing an unconnected
 /// interrupt. To connect it, whoever creates the device fills the pointer with
 /// the sink's `IRQState *`, for example using `sysbus_connect_irq`.  Because
diff --git a/rust/qemu-api/src/prelude.rs b/rust/qemu-api/src/prelude.rs
index 6f32deeb2ed..4ea70b9c823 100644
--- a/rust/qemu-api/src/prelude.rs
+++ b/rust/qemu-api/src/prelude.rs
@@ -16,3 +16,5 @@
 pub use crate::qom::ObjectType;
 
 pub use crate::qom_isa;
+
+pub use crate::sysbus::SysBusDeviceMethods;
diff --git a/rust/qemu-api/src/sysbus.rs b/rust/qemu-api/src/sysbus.rs
index b96eaaf25f2..e6762b5c145 100644
--- a/rust/qemu-api/src/sysbus.rs
+++ b/rust/qemu-api/src/sysbus.rs
@@ -32,18 +32,17 @@ fn class_init(sdc: &mut SysBusDeviceClass) {
     }
 }
 
-impl SysBusDevice {
-    /// Return `self` cast to a mutable pointer, for use in calls to C code.
-    const fn as_mut_ptr(&self) -> *mut SysBusDevice {
-        addr_of!(*self) as *mut _
-    }
-
+/// Trait for methods of [`SysBusDevice`] and its subclasses.
+pub trait SysBusDeviceMethods: ObjectDeref
+where
+    Self::Target: IsA<SysBusDevice>,
+{
     /// Expose a memory region to the board so that it can give it an address
     /// in guest memory.  Note that the ordering of calls to `init_mmio` is
     /// important, since whoever creates the sysbus device will refer to the
     /// region with a number that corresponds to the order of calls to
     /// `init_mmio`.
-    pub fn init_mmio(&self, iomem: &bindings::MemoryRegion) {
+    fn init_mmio(&self, iomem: &bindings::MemoryRegion) {
         assert!(bql_locked());
         unsafe {
             bindings::sysbus_init_mmio(self.as_mut_ptr(), addr_of!(*iomem) as *mut _);
@@ -54,10 +53,12 @@ pub fn init_mmio(&self, iomem: &bindings::MemoryRegion) {
     /// Note that the ordering of calls to `init_irq` is important, since
     /// whoever creates the sysbus device will refer to the interrupts with
     /// a number that corresponds to the order of calls to `init_irq`.
-    pub fn init_irq(&self, irq: &InterruptSource) {
+    fn init_irq(&self, irq: &InterruptSource) {
         assert!(bql_locked());
         unsafe {
             bindings::sysbus_init_irq(self.as_mut_ptr(), irq.as_ptr());
         }
     }
 }
+
+impl<R: ObjectDeref> SysBusDeviceMethods for R where R::Target: IsA<SysBusDevice> {}
-- 
2.47.1