rust: pl011: Check size of state struct at compile time

The PL011 device's C implementation exposes its PL011State struct to
users of the device, and one common usage pattern is to embed that
struct into the user's own state struct.  (The internals of the
struct are technically visible to the C user of the device, but in
practice are treated as implementation details.)

This means that the Rust version of the state struct must not be
larger than the C version's struct; otherwise it will trip a runtime
assertion in object_initialize_type() when the C user attempts to
in-place initialize the type.

Add a compile-time assertion on the Rust side, so that if we
accidentally make the Rust device state larger we know immediately
that we need to expand the padding in the C version of the struct.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Link: https://lore.kernel.org/r/20250321112523.1774131-4-peter.maydell@linaro.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/hw/char/pl011/src/device.rs

@@ -2,7 +2,7 @@
 // Author(s): Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
 // SPDX-License-Identifier: GPL-2.0-or-later
 
-use std::{ffi::CStr, ptr::addr_of_mut};
+use std::{ffi::CStr, mem::size_of, ptr::addr_of_mut};
 
 use qemu_api::{
     chardev::{CharBackend, Chardev, Event},
@@ -12,6 +12,7 @@
     prelude::*,
     qdev::{Clock, ClockEvent, DeviceImpl, DeviceState, Property, ResetType, ResettablePhasesImpl},
     qom::{ObjectImpl, Owned, ParentField},
+    static_assert,
     sysbus::{SysBusDevice, SysBusDeviceImpl},
     vmstate::VMStateDescription,
 };
@@ -124,6 +125,12 @@ pub struct PL011State {
     pub migrate_clock: bool,
 }
 
+// Some C users of this device embed its state struct into their own
+// structs, so the size of the Rust version must not be any larger
+// than the size of the C one. If this assert triggers you need to
+// expand the padding_for_rust[] array in the C PL011State struct.
+static_assert!(size_of::<PL011State>() <= size_of::<qemu_api::bindings::PL011State>());
+
 qom_isa!(PL011State : SysBusDevice, DeviceState, Object);
 
 #[repr(C)]

rust/wrapper.h

@@ -65,3 +65,4 @@ typedef enum memory_order {
 #include "exec/memattrs.h"
 #include "qemu/timer.h"
 #include "exec/address-spaces.h"
+#include "hw/char/pl011.h"