fix: Initialise databus using return values (noir-lang#6074)

# Description

## Problem\*

Resolves noir-lang#5715 

## Summary\*
Use ABI's return values for the Databus return_data


## 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\*

- [X] 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/acir_ir/acir_variable.rs

@@ -1944,6 +1944,15 @@ impl<F: AcirField> AcirContext<F> {
         Ok(())
     }
 
+    /// Insert the MemoryInit for the Return Data array, using the provided witnesses
+    pub(crate) fn initialize_return_data(&mut self, block_id: BlockId, init: Vec<Witness>) {
+        self.acir_ir.push_opcode(Opcode::MemoryInit {
+            block_id,
+            init,
+            block_type: BlockType::ReturnData,
+        });
+    }
+
     /// Initializes an array in memory with the given values `optional_values`.
     /// If `optional_values` is empty, then the array is initialized with zeros.
     pub(crate) fn initialize_array(

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

@@ -434,16 +434,9 @@ impl<'a> Context<'a> {
         for instruction_id in entry_block.instructions() {
             warnings.extend(self.convert_ssa_instruction(*instruction_id, dfg, ssa, brillig)?);
         }
-
         let (return_vars, return_warnings) =
             self.convert_ssa_return(entry_block.unwrap_terminator(), dfg)?;
 
-        let call_data_arrays: Vec<ValueId> =
-            self.data_bus.call_data.iter().map(|cd| cd.array_id).collect();
-        for call_data_array in call_data_arrays {
-            self.ensure_array_is_initialized(call_data_array, dfg)?;
-        }
-
         // TODO: This is a naive method of assigning the return values to their witnesses as
         // we're likely to get a number of constraints which are asserting one witness to be equal to another.
         //
@@ -452,13 +445,42 @@ impl<'a> Context<'a> {
             self.acir_context.assert_eq_var(*witness_var, return_var, None)?;
         }
 
+        self.initialize_databus(&return_witnesses, dfg)?;
         warnings.extend(return_warnings);
         warnings.extend(self.acir_context.warnings.clone());
 
         // Add the warnings from the alter Ssa passes
         Ok(self.acir_context.finish(input_witness, return_witnesses, warnings))
     }
 
+    fn initialize_databus(
+        &mut self,
+        witnesses: &Vec<Witness>,
+        dfg: &DataFlowGraph,
+    ) -> Result<(), RuntimeError> {
+        // Initialize return_data using provided witnesses
+        if let Some(return_data) = self.data_bus.return_data {
+            let block_id = self.block_id(&return_data);
+            let already_initialized = self.initialized_arrays.contains(&block_id);
+            if !already_initialized {
+                // We hijack ensure_array_is_initialized() because we want the return data to use the return value witnesses,
+                // but the databus contains the computed values instead, that have just been asserted to be equal to the return values.
+                // We do not use initialize_array either for the case where a constant value is returned.
+                // In that case, the constant value has already been assigned a witness and the returned acir vars will be
+                // converted to it, instead of the corresponding return value witness.
+                self.acir_context.initialize_return_data(block_id, witnesses.to_owned());
+            }
+        }
+
+        // Initialize call_data
+        let call_data_arrays: Vec<ValueId> =
+            self.data_bus.call_data.iter().map(|cd| cd.array_id).collect();
+        for call_data_array in call_data_arrays {
+            self.ensure_array_is_initialized(call_data_array, dfg)?;
+        }
+        Ok(())
+    }
+
     fn convert_brillig_main(
         mut self,
         main_func: &Function,
@@ -1792,19 +1814,9 @@ impl<'a> Context<'a> {
             _ => unreachable!("ICE: Program must have a singular return"),
         };
 
-        return_values.iter().fold(0, |acc, value_id| {
-            let is_databus = self
-                .data_bus
-                .return_data
-                .map_or(false, |return_databus| dfg[*value_id] == dfg[return_databus]);
-
-            if is_databus {
-                // We do not return value for the data bus.
-                acc
-            } else {
-                acc + dfg.type_of_value(*value_id).flattened_size()
-            }
-        })
+        return_values
+            .iter()
+            .fold(0, |acc, value_id| acc + dfg.type_of_value(*value_id).flattened_size())
     }
 
     /// Converts an SSA terminator's return values into their ACIR representations
@@ -1824,27 +1836,13 @@ impl<'a> Context<'a> {
         let mut has_constant_return = false;
         let mut return_vars: Vec<AcirVar> = Vec::new();
         for value_id in return_values {
-            let is_databus = self
-                .data_bus
-                .return_data
-                .map_or(false, |return_databus| dfg[*value_id] == dfg[return_databus]);
             let value = self.convert_value(*value_id, dfg);
 
             // `value` may or may not be an array reference. Calling `flatten` will expand the array if there is one.
             let acir_vars = self.acir_context.flatten(value)?;
             for (acir_var, _) in acir_vars {
                 has_constant_return |= self.acir_context.is_constant(&acir_var);
-                if is_databus {
-                    // We do not return value for the data bus.
-                    self.ensure_array_is_initialized(
-                        self.data_bus.return_data.expect(
-                            "`is_databus == true` implies `data_bus.return_data` is `Some`",
-                        ),
-                        dfg,
-                    )?;
-                } else {
-                    return_vars.push(acir_var);
-                }
+                return_vars.push(acir_var);
             }
         }