fix(frontend): Override to allow empty array input

compiler/noirc_frontend/src/hir_def/types.rs

@@ -1299,7 +1299,7 @@
             }
 
             Type::Array(length, element) => {
-                self.array_or_string_len_is_not_zero()
+                (Self::should_allow_zero_sized_input() || self.array_or_string_len_is_not_zero())
                     && length.is_valid_for_program_input()
                     && element.is_valid_for_program_input()
             }
@@ -1322,6 +1322,14 @@
         }
     }
 
+    /// Check if it is okay to allow for zero-sized input (e..g zero-sized arrays) to a program.
+    /// This behavior is not well defined and is banned by default.
+    /// However, this ban is a breaking change for some dependent projects so this override
+    /// is provided until those projects migrate away from using zero-sized array input (e.g. <https://github.com/AztecProtocol/aztec-packages/issues/14388>).
+    fn should_allow_zero_sized_input() -> bool {
+        std::env::var("ALLOW_EMPTY_INPUT").ok().map(|v| v == "1" || v == "true").unwrap_or_default()
+    }
+
     /// Empty arrays and strings (which are arrays under the hood) are disallowed
     /// as input to program entry points.
     ///
@@ -2423,7 +2431,7 @@
         }
 
         let recur_on_binding = |id, replacement: &Type| {
             // Prevent recuring forever if there's a `T := T` binding
             if replacement.type_variable_id() == Some(id) {
                 replacement.clone()
             } else {
@@ -2509,7 +2517,7 @@
                 Type::Tuple(fields)
             }
             Type::Forall(typevars, typ) => {
                 // Trying to substitute_helper a variable de, substitute_bound_typevarsfined within a nested Forall
                 // is usually impossible and indicative of an error in the type checker somewhere.
                 for var in typevars {
                     assert!(!type_bindings.contains_key(&var.id()));
@@ -2676,7 +2684,7 @@
         }
     }
 
     /// Follow bindings if this is a type variable or generic to the first non-typevariable
     /// type. Unlike `follow_bindings`, this won't recursively follow any bindings on any
     /// fields or arguments of this type.
     pub fn follow_bindings_shallow(&self) -> Cow<Type> {
@@ -2701,7 +2709,7 @@
 
     /// Replace any `Type::NamedGeneric` in this type with a `Type::TypeVariable`
     /// using to the same inner `TypeVariable`. This is used during monomorphization
     /// to bind to named generics since they are unbindable during type checking.
     pub fn replace_named_generics_with_type_variables(&mut self) {
         match self {
             Type::FieldElement
@@ -3247,7 +3255,7 @@
                 len.hash(state);
                 env.hash(state);
             }
             Type::Tuple(elems) => elems.hash(state),
             Type::DataType(def, args) => {
                 def.hash(state);
                 args.hash(state);