feat: do not return databus returndata, keep it private. (#5023)

# Description

## Problem\*

Related to  #4974

## Summary\*
Very small PR which avoids returning databus objects, because they need
to be handled as private inputs.
This is incremental work, so it cannot be tested yet. The PR only impact
databus use case.



## Additional Context



## Documentation\*

Check one:
- [X] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [ ] I have tested the changes locally.
- [X] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_evaluator/src/ssa/acir_gen/mod.rs

@@ -1729,13 +1729,16 @@ impl<'a> Context<'a> {
         // will expand the array if there is one.
         let return_acir_vars = self.flatten_value_list(return_values, dfg)?;
         let mut warnings = Vec::new();
-        for acir_var in return_acir_vars {
+        for (acir_var, is_databus) in return_acir_vars {
             if self.acir_context.is_constant(&acir_var) {
                 warnings.push(SsaReport::Warning(InternalWarning::ReturnConstant {
                     call_stack: call_stack.clone(),
                 }));
             }
-            self.acir_context.return_var(acir_var)?;
+            if !is_databus {
+                // We do not return value for the data bus.
+                self.acir_context.return_var(acir_var)?;
+            }
         }
         Ok(warnings)
     }
@@ -2659,12 +2662,22 @@ impl<'a> Context<'a> {
         &mut self,
         arguments: &[ValueId],
         dfg: &DataFlowGraph,
-    ) -> Result<Vec<AcirVar>, InternalError> {
+    ) -> Result<Vec<(AcirVar, bool)>, InternalError> {
         let mut acir_vars = Vec::with_capacity(arguments.len());
         for value_id in arguments {
+            let is_databus = if let Some(return_databus) = self.data_bus.return_data {
+                dfg[*value_id] == dfg[return_databus]
+            } else {
+                false
+            };
             let value = self.convert_value(*value_id, dfg);
             acir_vars.append(
-                &mut self.acir_context.flatten(value)?.iter().map(|(var, _)| *var).collect(),
+                &mut self
+                    .acir_context
+                    .flatten(value)?
+                    .iter()
+                    .map(|(var, _)| (*var, is_databus))
+                    .collect(),
             );
         }
         Ok(acir_vars)

tooling/noirc_abi/src/lib.rs

@@ -471,7 +471,15 @@ impl Abi {
                         .copied()
                 })
             {
-                Some(decode_value(&mut return_witness_values.into_iter(), &return_type.abi_type)?)
+                // We do not return value for the data bus.
+                if return_type.visibility == AbiVisibility::DataBus {
+                    None
+                } else {
+                    Some(decode_value(
+                        &mut return_witness_values.into_iter(),
+                        &return_type.abi_type,
+                    )?)
+                }
             } else {
                 // Unlike for the circuit inputs, we tolerate not being able to find the witness values for the return value.
                 // This is because the user may be decoding a partial witness map for which is hasn't been calculated yet.