rust: vmstate: add new type safe implementation

The existing translation of the C macros for vmstate does not make
any attempt to type-check vmstate declarations against the struct, so
introduce a new system that computes VMStateField based on the actual
struct declaration.

Macros do not have full access to the type system, therefore a full
implementation of this scheme requires a helper trait to analyze the
type and produce a VMStateField from it; a macro "vmstate_of!" accepts
arguments similar to "offset_of!" and tricks the compiler into looking
up the trait for the right type.

The patch introduces not just vmstate_of!, but also the slightly too
clever enabling macro call_func_with_field!.  The particular trick used
here was proposed on the users.rust-lang.org forum, so I take no merit
and all the blame.

Introduce the trait and some functions to access it; the actual
implementation comes later.

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

rust/qemu-api/src/vmstate.rs

@@ -4,13 +4,114 @@
 
 //! Helper macros to declare migration state for device models.
 //!
-//! Some macros are direct equivalents to the C macros declared in
-//! `include/migration/vmstate.h` while
-//! [`vmstate_subsections`](crate::vmstate_subsections) and
-//! [`vmstate_fields`](crate::vmstate_fields) are meant to be used when
-//! declaring a device model state struct.
+//! This module includes three 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.
+//!
+//! * helper macros to declare a device model state struct, in particular
+//!   [`vmstate_subsections`](crate::vmstate_subsections) and
+//!   [`vmstate_fields`](crate::vmstate_fields).
+//!
+//! * direct equivalents to the C macros declared in
+//!   `include/migration/vmstate.h`. These are not type-safe and should not be
+//!   used if the equivalent functionality is available with `vmstate_of!`.
 
-pub use crate::bindings::VMStateDescription;
+use core::marker::PhantomData;
+
+pub use crate::bindings::{VMStateDescription, VMStateField};
+
+/// This macro is used to call a function with a generic argument bound
+/// to the type of a field.  The function must take a
+/// [`PhantomData`]`<T>` argument; `T` is the type of
+/// field `$field` in the `$typ` type.
+///
+/// # Examples
+///
+/// ```
+/// # use qemu_api::call_func_with_field;
+/// # use core::marker::PhantomData;
+/// const fn size_of_field<T>(_: PhantomData<T>) -> usize {
+///     std::mem::size_of::<T>()
+/// }
+///
+/// struct Foo {
+///     x: u16,
+/// };
+/// // calls size_of_field::<u16>()
+/// assert_eq!(call_func_with_field!(size_of_field, Foo, x), 2);
+/// ```
+#[macro_export]
+macro_rules! call_func_with_field {
+    // Based on the answer by user steffahn (Frank Steffahn) at
+    // https://users.rust-lang.org/t/inferring-type-of-field/122857
+    // and used under MIT license
+    ($func:expr, $typ:ty, $($field:tt).+) => {
+        $func(loop {
+            #![allow(unreachable_code)]
+            const fn phantom__<T>(_: &T) -> ::core::marker::PhantomData<T> { ::core::marker::PhantomData }
+            // Unreachable code is exempt from checks on uninitialized values.
+            // Use that trick to infer the type of this PhantomData.
+            break ::core::marker::PhantomData;
+            break phantom__(&{ let value__: $typ; value__.$($field).+ });
+        })
+    };
+}
+
+/// A trait for types that can be included in a device's migration stream.  It
+/// provides the base contents of a `VMStateField` (minus the name and offset).
+///
+/// # Safety
+///
+/// The contents of this trait go straight into structs that are parsed by C
+/// code and used to introspect into other structs.  Be careful.
+pub unsafe trait VMState {
+    /// The base contents of a `VMStateField` (minus the name and offset) for
+    /// the type that is implementing the trait.
+    const BASE: VMStateField;
+}
+
+/// Internal utility function to retrieve a type's `VMStateField`;
+/// used by [`vmstate_of!`](crate::vmstate_of).
+pub const fn vmstate_base<T: VMState>(_: PhantomData<T>) -> VMStateField {
+    T::BASE
+}
+
+/// Return the `VMStateField` for a field of a struct.  The field must be
+/// visible in the current scope.
+///
+/// In order to support other types, the trait `VMState` must be implemented
+/// for them.
+#[macro_export]
+macro_rules! vmstate_of {
+    ($struct_name:ty, $field_name:ident $(,)?) => {
+        $crate::bindings::VMStateField {
+            name: ::core::concat!(::core::stringify!($field_name), "\0")
+                .as_bytes()
+                .as_ptr() as *const ::std::os::raw::c_char,
+            offset: $crate::offset_of!($struct_name, $field_name),
+            // Compute most of the VMStateField from the type of the field.
+            ..$crate::call_func_with_field!(
+                $crate::vmstate::vmstate_base,
+                $struct_name,
+                $field_name
+            )
+        }
+    };
+}
+
+// Add a couple builder-style methods to VMStateField, allowing
+// easy derivation of VMStateField constants from other types.
+impl VMStateField {
+    #[must_use]
+    pub const fn with_version_id(mut self, version_id: i32) -> Self {
+        assert!(version_id >= 0);
+        self.version_id = version_id;
+        self
+    }
+}
 
 #[doc(alias = "VMSTATE_UNUSED_BUFFER")]
 #[macro_export]