Timer is a complex struct, allow adding it to a struct that
uses #[derive(ToMigrationState)]; similar to vmstate_timer, only
the expiration time has to be preserved.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/migration/src/migratable.rs | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/rust/migration/src/migratable.rs b/rust/migration/src/migratable.rs
index fa25317eea8..c4ad4f73d5c 100644
--- a/rust/migration/src/migratable.rs
+++ b/rust/migration/src/migratable.rs
@@ -202,6 +202,37 @@ fn restore_migrated_state(
     ) -> Result<(), InvalidError>;
 }
 
+impl ToMigrationState for util::timer::Timer {
+    type Migrated = i64;
+
+    fn snapshot_migration_state(&self, target: &mut i64) -> Result<(), InvalidError> {
+        // SAFETY: as_ptr() is unsafe to ensure that the caller reasons about
+        // the pinning of the data inside the Opaque<>.  Here all we do is
+        // access a field.
+        *target = unsafe { &*self.as_ptr() }.expire_time;
+        Ok(())
+    }
+
+    fn restore_migrated_state_mut(
+        &mut self,
+        source: Self::Migrated,
+        version_id: u8,
+    ) -> Result<(), InvalidError> {
+        self.restore_migrated_state(source, version_id)
+    }
+}
+
+impl ToMigrationStateShared for util::timer::Timer {
+    fn restore_migrated_state(&self, source: i64, _version_id: u8) -> Result<(), InvalidError> {
+        if source >= 0 {
+            self.modify(source as u64);
+        } else {
+            self.delete();
+        }
+        Ok(())
+    }
+}
+
 impl<T: ToMigrationStateShared, const N: usize> ToMigrationStateShared for [T; N]
 where
     [T::Migrated; N]: Default,
-- 
2.51.0