fix: handle struct with nested arrays in oracle return values (#5244)

# Description

## Problem\*

Resolves #5238

## Summary\*
Use nested values when the sizes do not match


## 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.

Co-authored-by: Tom French <15848336+TomAFrench@users.noreply.github.com>

acvm-repo/brillig_vm/src/lib.rs

@@ -500,9 +500,14 @@ impl<'a, F: AcirField, B: BlackBoxFunctionSolver<F>> VM<'a, F, B> {
                     match output {
                         ForeignCallParam::Array(values) => {
                             if values.len() != *size {
-                                return Err("Foreign call result array doesn't match expected size".to_string());
+                                // foreign call returning flattened values into a nested type, so the sizes do not match
+                               let destination = self.memory.read_ref(*pointer_index);
+                               let return_type = value_type;
+                               let mut flatten_values_idx = 0; //index of values read from flatten_values
+                               self.write_slice_of_values_to_memory(destination, &output.fields(), &mut flatten_values_idx, return_type)?;
+                            } else {
+                                self.write_values_to_memory_slice(*pointer_index, values, value_types)?;
                             }
-                            self.write_values_to_memory_slice(*pointer_index, values, value_types)?;
                         }
                         _ => {
                             return Err("Function result size does not match brillig bytecode size".to_string());

test_programs/noir_test_success/regression_4561/src/main.nr

@@ -42,3 +42,37 @@ fn two_nested_return() {
     OracleMock::mock("two_nested_return").returns((0, [1, 2, 3, 4, 5, 6], 7, [1, 2, 3, 4, 5, 6]));
     assert_eq(two_nested_return_unconstrained(), (0, [[1, 2, 3], [4, 5, 6]], 7, [[1, 2, 3], [4, 5, 6]]));
 }
+
+#[oracle(foo_return)]
+unconstrained fn foo_return() -> (Field, TReturn, TestTypeFoo) {}
+unconstrained fn foo_return_unconstrained() -> (Field, TReturn, TestTypeFoo) {
+    foo_return()
+}
+
+struct TestTypeFoo {
+    a: Field,
+    b: [[[Field; 3]; 4]; 2],
+    c: [TReturnElem; 2],
+    d: TReturnElem,
+}
+
+#[test]
+fn complexe_struct_return() {
+    OracleMock::mock("foo_return").returns(
+        (
+            0, [1, 2, 3, 4, 5, 6], 7, [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24], [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], [1, 2, 3, 4, 5, 6]
+        )
+    );
+    let foo_x = foo_return_unconstrained();
+    assert_eq((foo_x.0, foo_x.1), (0, [[1, 2, 3], [4, 5, 6]]));
+    assert_eq(foo_x.2.a, 7);
+    assert_eq(
+        foo_x.2.b, [
+        [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]], [[13, 14, 15], [16, 17, 18], [19, 20, 21], [22, 23, 24]]
+    ]
+    );
+    let a: TReturnElem = [1, 2, 3];
+    let b: TReturnElem = [4, 5, 6];
+    assert_eq(foo_x.2.c, [a, b]);
+    assert_eq(foo_x.2.d, a);
+}