feat: allow returning nested arrays from brillig (#2047)

* feat: allow returning nested arrays from brillig

* style: remove extraneous println

* style: restore whitespace

* chore: update json and witness

* style: whitespacing

* docs: added doc comment

crates/nargo_cli/tests/test_data_ssa_refactor/nested_arrays_from_brillig/Nargo.toml

@@ -0,0 +1,5 @@
+[package]
+authors = [""]
+compiler_version = "0.6.0"
+
+[dependencies]

crates/nargo_cli/tests/test_data_ssa_refactor/nested_arrays_from_brillig/Prover.toml

@@ -0,0 +1 @@
+values = [1,2,3,4,5,6]

crates/nargo_cli/tests/test_data_ssa_refactor/nested_arrays_from_brillig/src/main.nr

@@ -0,0 +1,26 @@
+struct Header {
+    params: [Field; 3],
+}
+
+struct MyNote {
+    plain: Field,
+    array: [Field; 2],
+    header: Header,
+}
+
+fn access_nested(notes: [MyNote; 2]) -> Field {
+    notes[0].array[1] + notes[1].array[0] + notes[0].plain + notes[1].header.params[0]
+}
+
+unconstrained fn create_inside_brillig(values: [Field; 6]) -> [MyNote; 2] {
+    let header = Header { params: [values[0], values[1], values[2]]};
+    let note0 = MyNote { array: [values[0], values[1]], plain : values[2], header };
+    let note1 = MyNote { array: [values[3], values[4]], plain : values[5], header };
+    [note0, note1]
+}
+
+fn main(values: [Field; 6]) {
+    let notes = create_inside_brillig(values);
+    assert(access_nested(notes) == (2 + 4 + 3 + 1));
+}
+

crates/nargo_cli/tests/test_data_ssa_refactor/nested_arrays_from_brillig/target/main.json

@@ -0,0 +1 @@
+{"backend":"acvm-backend-barretenberg","abi":{"parameters":[{"name":"values","type":{"kind":"array","length":6,"type":{"kind":"field"}},"visibility":"private"}],"param_witnesses":{"values":[1,2,3,4,5,6]},"return_type":null,"return_witnesses":[]},"bytecode":"H4sIAAAAAAAA/92a21LiQBCGh1MiBCOCGE6+Q44Q7niVdRcv9pn3YTYzpsufqSGidrtkp2oqCRO6vz5mjKyUUkP1Om7qo1cfO9XsVrNXzX41B/WaV6/RPXS/gs/0ONTH+Gsj6fDJiqUYuy1g7LWAsd8CxkELGD1GRqz1cX3uq9d+oXvHqJpBvXZbzbCad9WcVPO+mlP1Nv7UR/29rjod2EcOPOxbj9+3sY7/b2Ae8Mr/qX3Tt3zTsa4PcD4AFp/ZVi3zhlmmUm/PHD0oD4h9pN5yAe/Dc/IPsQWw7sGRmT0R8HFCNtv+uAF/DOGcfDGyfNGx5Bzg2gefjGGdgd/kyC2zTC0jBFvI/jH4IajP8T48J9uJLYD1APzBzJ4I+Dg554/bT/hj/I4/mPM7FajDDH2gh6uHYK3Qkc6xZlDOQbn3w22sGTvel+SInVf/U83Y/eAj/nivZpjZUwEfp+f8IZEf3PUu0Je2aI8erufupfsQH3zg2ocws+cCPt6RzbY/sKd+dR+CPZW7/0n0aS3ju5673P1Pok+f84fEvow5vwuBOtyjD/Rw9ZCv7kNw797GmmnRc/dbaqZF+7JUYm9zzh/c+YHvmPrW9xSshbXv+rx2mjoYOPT1wc5efY7vcDxeDpN/voPDAw7Sj72Guf+m5/yPvZL04x5KIi6ufQ7Gxd7nSMUlcHBgXEi/4HPRxGXs4MC4kP4xcDD3GvM3fejgwL5A+kPguOPlyLWMiYPjDjhI/wQ47nk5Ci1j6uC4Bw7SPwWOGS/Hdmhx6NH0PnoGLA+8LKZ256CLuEhPAOtYM3NeDvM/yQfLfrqeg95HXr3pUJ3ar0dTLB6BJeJlMbFYgC7iisD/tI79fMHLYWIRWfbTNfJdyjq6Ala8x5XjkWWHQHwbczyS81E2VKf269GU4wtgWfKymBxfgS7iIj0BrPeAY8XLYWKxtOyna+S7lHXQIlb/CljxHlc9Li07BHKxsR6Xn2Cd/GNWgXia5yPar0dT71gBy5qXxfSODegiLtITwDruqze8HCYWa8t+uka+S1nDK2DFe1w5vrbsEIhvY46v5fR++PmILMzxMjn+BLqIi/QEsI7PxydeDhOLjWU/XSPfpayDFrH6V8CK97jqcWPZIdU7ztUj6RrCZ1P4rGsxa5/aNcX64odGj1n2BGRl8TbPj7v0mGTJjzjdP5dFnBfP2zIpk6IsfqVllh3LvNztn/e7eJ/k2TF5KfbZSy1sxigrUrxNR8p/PqOsIaOsBUMsji9m7CT9FzLKihjzb8HHFeOPSnFw/4h3xsiMvNgk6aV315ETnoBNytJj+zFUwo1XIkgPAnLnii/5peye88fopLlfu09x/AWj8PM1SjMAAA==","proving_key":null,"verification_key":null}

crates/noirc_evaluator/src/ssa_refactor/acir_gen/acir_ir/acir_variable.rs

@@ -855,18 +855,9 @@ impl AcirContext {
                 AcirValue::Var(var, output.clone())
             }
             AcirType::Array(element_types, size) => {
-                let mut witnesses = Vec::new();
-                let mut array_values = im::Vector::new();
-                for _ in 0..size {
-                    for element_type in &element_types {
-                        let witness_index = self.acir_ir.next_witness_index();
-                        witnesses.push(witness_index);
-                        let var = self.add_data(AcirVarData::Witness(witness_index));
-                        array_values.push_back(AcirValue::Var(var, element_type.clone()));
-                    }
-                }
+                let (acir_value, witnesses) = self.brillig_array_output(&element_types, size);
                 b_outputs.push(BrilligOutputs::Array(witnesses));
-                AcirValue::Array(array_values)
+                acir_value
             }
         });
         let predicate = self.vars[&predicate].to_expression().into_owned();
@@ -906,6 +897,36 @@ impl AcirContext {
         }
     }
 
+    /// Recursively create acir values for returned arrays. This is necessary because a brillig returned array can have nested arrays as elements.
+    /// A singular array of witnesses is collected for a top level array, by deflattening the assigned witnesses at each level.
+    fn brillig_array_output(
+        &mut self,
+        element_types: &[AcirType],
+        size: usize,
+    ) -> (AcirValue, Vec<Witness>) {
+        let mut witnesses = Vec::new();
+        let mut array_values = im::Vector::new();
+        for _ in 0..size {
+            for element_type in element_types {
+                match element_type {
+                    AcirType::Array(nested_element_types, nested_size) => {
+                        let (nested_acir_value, mut nested_witnesses) =
+                            self.brillig_array_output(nested_element_types, *nested_size);
+                        witnesses.append(&mut nested_witnesses);
+                        array_values.push_back(nested_acir_value);
+                    }
+                    AcirType::NumericType(_) => {
+                        let witness_index = self.acir_ir.next_witness_index();
+                        witnesses.push(witness_index);
+                        let var = self.add_data(AcirVarData::Witness(witness_index));
+                        array_values.push_back(AcirValue::Var(var, element_type.clone()));
+                    }
+                }
+            }
+        }
+        (AcirValue::Array(array_values), witnesses)
+    }
+
     /// Generate output variables that are constrained to be the sorted inputs
     /// The outputs are the sorted inputs iff
     /// outputs are sorted and