contracts: add increment_left_and_call to double counter

The function is intended to test calls to contracts that do not exist,
whilst allowing for an easy check on the state.

contracts/double_counter/src/lib.rs

@@ -10,6 +10,7 @@
 #![no_std]
 
 use piecrust_uplink as uplink;
+use uplink::{ContractError, ContractId};
 
 /// Struct that describes the state of the DoubleCounter contract
 pub struct DoubleCounter {
@@ -40,6 +41,20 @@ impl DoubleCounter {
         let value = self.right_value + 1;
         self.right_value = value;
     }
+
+    /// Increment the counter by 1 and call the given contract, with the given
+    /// arguments.
+    ///
+    /// This is intended to test the behavior of the contract on calling a
+    /// contract that doesn't exist.
+    pub fn increment_left_and_call(
+        &mut self,
+        contract: ContractId,
+    ) -> Result<(), ContractError> {
+        let value = self.left_value + 1;
+        self.left_value = value;
+        uplink::call(contract, "hello", &())
+    }
 }
 
 /// Expose `Counter::read_value()` to the host
@@ -59,3 +74,11 @@ unsafe fn increment_left(arg_len: u32) -> u32 {
 unsafe fn increment_right(arg_len: u32) -> u32 {
     uplink::wrap_call(arg_len, |_: ()| STATE.increment_right())
 }
+
+/// Expose `Counter::increment_and_call()` to the host
+#[no_mangle]
+unsafe fn increment_left_and_call(arg_len: u32) -> u32 {
+    uplink::wrap_call_unchecked(arg_len, |arg| {
+        STATE.increment_left_and_call(arg)
+    })
+}