make return statement optional

zokrates_cli/examples/compile_errors/no_return.zok

@@ -1,5 +0,0 @@
-def foo() {}
-
-def main() {
-    return;
-}

zokrates_core/src/semantics.rs

@@ -1186,11 +1186,23 @@ impl<'ast, T: Field> Checker<'ast, T> {
                 }
 
                 if !found_return {
+                    match &s.output {
+                        box DeclarationType::Tuple(tuple_type)
+                            if tuple_type.elements.is_empty() =>
+                        {
+                            statements_checked.push(TypedStatement::Return(TypedExpression::Tuple(
+                                TupleExpressionInner::Value(vec![])
+                                    .annotate(TupleType::new(vec![])),
+                            )))
+                        }
+                        _ => {
                             errors.push(ErrorInner {
                                 pos: Some(pos),
                                 message: "Expected a return statement".to_string(),
                             });
                         }
+                    }
+                }
 
                 signature = Some(s);
             }

zokrates_core_test/tests/tests/no_return.json

@@ -0,0 +1,5 @@
+{
+	"entry_point": "./tests/tests/no_return.zok",
+	"tests": [
+	]
+}

zokrates_core_test/tests/tests/no_return.zok

@@ -0,0 +1,9 @@
+def foo() {
+    return;
+}
+
+def bar() {
+    return foo();
+}
+
+def main() {}