[RFC PATCH 6/9] rust: vmstate: add public utility macros to implement VMState

[Paolo Bonzini <pbonzini@redhat.com>]

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/qemu-api/src/vmstate.rs | 61 ++++++++++++++++++++++++++++++++++--
 1 file changed, 58 insertions(+), 3 deletions(-)

diff --git a/rust/qemu-api/src/vmstate.rs b/rust/qemu-api/src/vmstate.rs
index edd0cbff162..b59a4b66339 100644
--- a/rust/qemu-api/src/vmstate.rs
+++ b/rust/qemu-api/src/vmstate.rs
@@ -4,13 +4,18 @@
 
 //! Helper macros to declare migration state for device models.
 //!
-//! This module includes three families of macros:
+//! This module includes four families of macros:
 //!
 //! * [`vmstate_unused!`](crate::vmstate_unused) and
 //!   [`vmstate_of!`](crate::vmstate_of), which are used to express the
 //!   migration format for a struct.  This is based on the [`VMState`] trait,
 //!   which is defined by all migrateable types.
 //!
+//! * [`impl_vmstate_forward`](crate::impl_vmstate_forward) and
+//!   [`impl_vmstate_bitsized`](crate::impl_vmstate_bitsized), which help with
+//!   the definition of the [`VMState`] trait (respectively for transparent
+//!   structs and for `bilge`-defined types)
+//!
 //! * helper macros to declare a device model state struct, in particular
 //!   [`vmstate_subsections`](crate::vmstate_subsections) and
 //!   [`vmstate_fields`](crate::vmstate_fields).
@@ -131,7 +136,9 @@ macro_rules! info_enum_to_ref {
 /// # Safety
 ///
 /// The contents of this trait go straight into structs that are parsed by C
-/// code and used to introspect into other structs.  Be careful.
+/// code and used to introspect into other structs.  Generally, you don't need
+/// to implement it except via macros that do it for you, such as
+/// `impl_vmstate_bitsized!`.
 pub unsafe trait VMState {
     /// The `info` member of a `VMStateField` is a pointer and as such cannot
     /// yet be included in the [`BASE`](VMState::BASE) associated constant;
@@ -185,7 +192,9 @@ pub const fn vmstate_varray_flag<T: VMState>(_: PhantomData<T>) -> VMStateField
 /// * an array of any of the above
 ///
 /// In order to support other types, the trait `VMState` must be implemented
-/// for them.
+/// for them.  The macros
+/// [`impl_vmstate_bitsized!`](crate::impl_vmstate_bitsized)
+/// and [`impl_vmstate_forward!`](crate::impl_vmstate_forward) help with this.
 #[macro_export]
 macro_rules! vmstate_of {
     ($struct_name:ty, $field_name:ident $([0 .. $num:tt $(* $factor:expr)?])? $(,)?) => {
@@ -263,6 +272,32 @@ pub const fn with_varray_multiply(mut self, num: u32) -> VMStateField {
     }
 }
 
+/// This macro can be used (by just passing it a type) to forward the `VMState`
+/// trait to the first field of a tuple.  This is a workaround for lack of
+/// support of nested [`offset_of`](core::mem::offset_of) until Rust 1.82.0.
+///
+/// # Examples
+///
+/// ```
+/// # use qemu_api::vmstate::impl_vmstate_forward;
+/// pub struct Fifo([u8; 16]);
+/// impl_vmstate_forward!(Fifo);
+/// ```
+#[macro_export]
+macro_rules! impl_vmstate_forward {
+    // This is similar to impl_vmstate_transparent below, but it
+    // uses the same trick as vmstate_of! to obtain the type of
+    // the first field of the tuple
+    ($tuple:ty) => {
+        unsafe impl $crate::vmstate::VMState for $tuple {
+            const SCALAR_TYPE: $crate::vmstate::VMStateFieldType =
+                $crate::call_func_with_field!($crate::vmstate::vmstate_scalar_type, $tuple, 0);
+            const BASE: $crate::bindings::VMStateField =
+                $crate::call_func_with_field!($crate::vmstate::vmstate_base, $tuple, 0);
+        }
+    };
+}
+
 // Transparent wrappers: just use the internal type
 
 macro_rules! impl_vmstate_transparent {
@@ -283,6 +318,26 @@ unsafe impl<$base> VMState for $type where $base: VMState $($where)* {
 impl_vmstate_transparent!(crate::cell::BqlCell<T> where T: VMState);
 impl_vmstate_transparent!(crate::cell::BqlRefCell<T> where T: VMState);
 
+#[macro_export]
+macro_rules! impl_vmstate_bitsized {
+    ($type:ty) => {
+        unsafe impl $crate::vmstate::VMState for $type {
+            const SCALAR_TYPE: $crate::vmstate::VMStateFieldType =
+                                        <<<$type as ::bilge::prelude::Bitsized>::ArbitraryInt
+                                          as ::bilge::prelude::Number>::UnderlyingType
+                                         as $crate::vmstate::VMState>::SCALAR_TYPE;
+            const BASE: $crate::bindings::VMStateField =
+                                        <<<$type as ::bilge::prelude::Bitsized>::ArbitraryInt
+                                          as ::bilge::prelude::Number>::UnderlyingType
+                                         as $crate::vmstate::VMState>::BASE;
+            const VARRAY_FLAG: $crate::bindings::VMStateFlags =
+                                        <<<$type as ::bilge::prelude::Bitsized>::ArbitraryInt
+                                          as ::bilge::prelude::Number>::UnderlyingType
+                                         as $crate::vmstate::VMState>::VARRAY_FLAG;
+        }
+    };
+}
+
 // Scalar types using predefined VMStateInfos
 
 macro_rules! impl_vmstate_scalar {
-- 
2.47.1