feat: Add brillig array index check (#4127)

# Description

## Problem\*

Adds a runtime length check to avoid out of bounds accesses, to emulate
the behavior with ACIR dynamic array accesses.

## Summary\*



## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** 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/brillig/brillig_gen/brillig_block.rs

@@ -564,6 +564,7 @@ impl<'block> BrilligBlock<'block> {
                 };
 
                 let index_register = self.convert_ssa_register_value(*index, dfg);
+                self.validate_array_index(array_variable, index_register);
                 self.retrieve_variable_from_array(
                     array_pointer,
                     index_register,
@@ -582,6 +583,7 @@ impl<'block> BrilligBlock<'block> {
                     result_ids[0],
                     dfg,
                 );
+                self.validate_array_index(source_variable, index_register);
 
                 self.convert_ssa_array_set(
                     source_variable,
@@ -690,6 +692,37 @@ impl<'block> BrilligBlock<'block> {
             .post_call_prep_returns_load_registers(&returned_registers, &saved_registers);
     }
 
+    fn validate_array_index(
+        &mut self,
+        array_variable: BrilligVariable,
+        index_register: RegisterIndex,
+    ) {
+        let (size_as_register, should_deallocate_size) = match array_variable {
+            BrilligVariable::BrilligArray(BrilligArray { size, .. }) => {
+                (self.brillig_context.make_constant(size.into()), true)
+            }
+            BrilligVariable::BrilligVector(BrilligVector { size, .. }) => (size, false),
+            _ => unreachable!("ICE: validate array index on non-array"),
+        };
+
+        let condition = self.brillig_context.allocate_register();
+
+        self.brillig_context.memory_op(
+            index_register,
+            size_as_register,
+            condition,
+            BinaryIntOp::LessThan,
+        );
+
+        self.brillig_context
+            .constrain_instruction(condition, Some("Array index out of bounds".to_owned()));
+
+        if should_deallocate_size {
+            self.brillig_context.deallocate_register(size_as_register);
+        }
+        self.brillig_context.deallocate_register(condition);
+    }
+
     pub(crate) fn retrieve_variable_from_array(
         &mut self,
         array_pointer: RegisterIndex,

test_programs/noir_test_success/out_of_bounds_alignment/Nargo.toml

@@ -0,0 +1,5 @@
+[package]
+name = "out_of_bounds_alignment"
+type = "bin"
+authors = [""]
+[dependencies]

test_programs/noir_test_success/out_of_bounds_alignment/src/main.nr

@@ -0,0 +1,17 @@
+fn out_of_bounds(arr_1: [Field; 50]) -> Field {
+    arr_1[50 + 1]
+}
+
+unconstrained fn out_of_bounds_unconstrained_wrapper(arr_1: [Field; 50], arr_2: [Field; 50]) -> Field {
+    out_of_bounds(arr_1)
+}
+
+#[test(should_fail)]
+fn test_acir() {
+    assert_eq(out_of_bounds([0; 50]), 0);
+}
+
+#[test(should_fail)]
+fn test_brillig() {
+    assert_eq(out_of_bounds_unconstrained_wrapper([0; 50], [0; 50]), 0);
+}