Add helper macro for AddAssigning with saturating arithmetic

sdk/src/lib.rs

@@ -103,10 +103,34 @@ macro_rules! program_stubs {
     () => {};
 }
 
+/// Convenience macro for `AddAssign` with saturating arithmetic.
+/// Replace by `std::num::Saturating` once stable
+#[macro_export]
+macro_rules! saturating_add_assign {
+    ($i:expr, $v:expr) => {{
+        $i = $i.saturating_add($v)
+    }};
+}
+
 #[macro_use]
 extern crate serde_derive;
 pub extern crate bs58;
 extern crate log as logger;
 
 #[macro_use]
 extern crate solana_frozen_abi_macro;
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_saturating_add_assign() {
+        let mut i = 0u64;
+        let v = 1;
+        saturating_add_assign!(i, v);
+        assert_eq!(i, 1);
+
+        i = u64::MAX;
+        saturating_add_assign!(i, v);
+        assert_eq!(i, u64::MAX);
+    }
+}