[PATCH v3 10/15] rust/vmstate: Support vmstate_validate

[Zhao Liu <zhao1.liu@intel.com>]

In C version, VMSTATE_VALIDATE accepts the function pointer, which is
used to check if some conditions of structure could meet, although the
C version macro doesn't accept any structure as the opaque type.

But it's hard to integrate VMSTATE_VALIDAE into vmstate_struct, a new
macro has to be introduced to specifically handle the case corresponding
to VMSTATE_VALIDATE.

One of the difficulties is inferring the type of a callback by its name
`test_fn`. We can't directly use `test_fn` as a parameter of
test_cb_builder__() to get its type "F", because in this way, Rust
compiler will be too conservative on drop check and complain "the
destructor for this type cannot be evaluated in constant functions".

Fortunately, PhantomData<T> could help in this case, because it is
considered to never have a destructor, no matter its field type [*].

The `phantom__()` in the `call_func_with_field` macro provides a good
example of using PhantomData to infer type. So copy this idea and apply
it to the `vmstate_validate` macro.

[*]: https://doc.rust-lang.org/std/ops/trait.Drop.html#drop-check

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
---
Changes since v1:
 * Rename `with_exist_check()` to `with_validate_flag()`.
 * Add a `with_exist_check()` method to help add field_exists callback,
   though it can't be used in static VMStateDescription definition but
   will be useful for vmstate builder.

Changes since v2:
 * Drop `with_exist_check()` and `with_validate_flag()`.
---
 rust/qemu-api/src/vmstate.rs | 52 +++++++++++++++++++++++++++++++++++-
 1 file changed, 51 insertions(+), 1 deletion(-)

diff --git a/rust/qemu-api/src/vmstate.rs b/rust/qemu-api/src/vmstate.rs
index 01f06ed7379e..9740931fb16a 100644
--- a/rust/qemu-api/src/vmstate.rs
+++ b/rust/qemu-api/src/vmstate.rs
@@ -25,9 +25,12 @@
 //!   functionality that is missing from `vmstate_of!`.
 
 use core::{marker::PhantomData, mem, ptr::NonNull};
+use std::os::raw::{c_int, c_void};
 
 pub use crate::bindings::{VMStateDescription, VMStateField};
-use crate::{bindings::VMStateFlags, prelude::*, qom::Owned, zeroable::Zeroable};
+use crate::{
+    bindings::VMStateFlags, callbacks::FnCall, prelude::*, qom::Owned, zeroable::Zeroable,
+};
 
 /// This macro is used to call a function with a generic argument bound
 /// to the type of a field.  The function must take a
@@ -508,6 +511,53 @@ macro_rules! vmstate_fields {
     }}
 }
 
+pub extern "C" fn rust_vms_test_field_exists<T, F: for<'a> FnCall<(&'a T, u8), bool>>(
+    opaque: *mut c_void,
+    version_id: c_int,
+) -> bool {
+    let owner: &T = unsafe { &*(opaque.cast::<T>()) };
+    let version: u8 = version_id.try_into().unwrap();
+    // SAFETY: the opaque was passed as a reference to `T`.
+    F::call((owner, version))
+}
+
+pub type VMSFieldExistCb = unsafe extern "C" fn(
+    opaque: *mut std::os::raw::c_void,
+    version_id: std::os::raw::c_int,
+) -> bool;
+
+#[doc(alias = "VMSTATE_VALIDATE")]
+#[macro_export]
+macro_rules! vmstate_validate {
+    ($struct_name:ty, $test_name:expr, $test_fn:expr $(,)?) => {
+        $crate::bindings::VMStateField {
+            name: ::std::ffi::CStr::as_ptr($test_name),
+            field_exists: {
+                const fn test_cb_builder__<
+                    T,
+                    F: for<'a> $crate::callbacks::FnCall<(&'a T, u8), bool>,
+                >(
+                    _phantom: ::core::marker::PhantomData<F>,
+                ) -> $crate::vmstate::VMSFieldExistCb {
+                    let _: () = F::ASSERT_IS_SOME;
+                    $crate::vmstate::rust_vms_test_field_exists::<T, F>
+                }
+
+                const fn phantom__<T>(_: &T) -> ::core::marker::PhantomData<T> {
+                    ::core::marker::PhantomData
+                }
+                Some(test_cb_builder__::<$struct_name, _>(phantom__(&$test_fn)))
+            },
+            flags: $crate::bindings::VMStateFlags(
+                $crate::bindings::VMStateFlags::VMS_MUST_EXIST.0
+                    | $crate::bindings::VMStateFlags::VMS_ARRAY.0,
+            ),
+            num: 0, // 0 elements: no data, only run test_fn callback
+            ..$crate::zeroable::Zeroable::ZERO
+        }
+    };
+}
+
 /// A transparent wrapper type for the `subsections` field of
 /// [`VMStateDescription`].
 ///
-- 
2.34.1