Add next_keyed_account() to instruction_processor_utils (#6309)

* Cleanup KeyedArguments traversal

* Better error message

* Fix clippy warning

* Rename next_arg to next_keyed_account

* Fix clippy warning

* Shorter

sdk/src/instruction.rs

@@ -67,6 +67,9 @@ pub enum InstructionError {
     /// Rent_epoch account changed, but shouldn't have
     RentEpochModified,
 
+    /// The instruction expected additional account keys
+    NotEnoughAccountKeys,
+
     /// CustomError allows on-chain programs to implement program-specific error types and see
     /// them returned by the Solana runtime. A CustomError may be any type that is represented
     /// as or serialized to a u32 integer.

sdk/src/instruction_processor_utils.rs

@@ -38,6 +38,11 @@ where
     }
 }
 
+/// Return the next KeyedAccount or a NotEnoughAccountKeys instruction error
+pub fn next_keyed_account<I: Iterator>(iter: &mut I) -> Result<I::Item, InstructionError> {
+    iter.next().ok_or(InstructionError::NotEnoughAccountKeys)
+}
+
 pub trait DecodeError<E> {
     fn decode_custom_error_to_enum(custom: u32) -> Option<E>
     where