Return error if the return value is missing in execution

tooling/nargo_cli/src/cli/execute_cmd.rs

@@ -67,6 +67,7 @@ pub(crate) fn run(args: ExecuteCommand, config: NargoConfig) -> Result<(), CliEr
         let program_artifact_path = workspace.package_build_path(package);
         let program: CompiledProgram =
             read_program_from_file(program_artifact_path.clone())?.into();
+        let abi = program.abi.clone();
 
         let results = execute_program_and_decode(
             program,
@@ -93,6 +94,14 @@ pub(crate) fn run(args: ExecuteCommand, config: NargoConfig) -> Result<(), CliEr
                 return Err(CliError::UnexpectedReturn { expected, actual: results.actual_return });
             }
         }
+        // Abi::decode returns None if the WitnessMap is empty in places corresponding to the return,
+        // positing that the caller should decide whether a result should be present.
+        // Arguably at the end of circuit execution it should be.
+        if let Some(ref expected) = abi.return_type {
+            if results.actual_return.is_none() {
+                return Err(CliError::MissingReturn { expected: expected.clone() });
+            }
+        }
     }
     Ok(())
 }

tooling/nargo_cli/src/errors.rs

@@ -5,6 +5,7 @@ use noir_debugger::errors::DapError;
 use noirc_abi::{
     errors::{AbiError, InputParserError},
     input_parser::InputValue,
+    AbiReturnType,
 };
 use std::path::PathBuf;
 use thiserror::Error;
@@ -70,4 +71,7 @@ pub(crate) enum CliError {
 
     #[error("Unexpected return value: expected {expected:?}; got {actual:?}")]
     UnexpectedReturn { expected: InputValue, actual: Option<InputValue> },
+
+    #[error("Missing return witnesses; expected {expected:?}")]
+    MissingReturn { expected: AbiReturnType },
 }