chore(ACIR): use u32::MAX for PLACEHOLDER_BRILLIG_INDEX

compiler/noirc_evaluator/src/acir/acir_context/generated_acir/brillig_directive.rs

@@ -9,12 +9,11 @@ use acvm::acir::{
 
 use crate::brillig::brillig_ir::artifact::GeneratedBrillig;
 
-// TODO(https://github.com/noir-lang/noir/issues/10256): Use `Option<BrilligFunctionId>` in place of `PLACEHOLDER_BRILLIG_INDEX`
 /// Brillig calls such as for the Brillig std lib are resolved only after code generation is finished.
 /// This index should be used when adding a Brillig call during code generation.
 /// Code generation should then keep track of that unresolved call opcode which will be resolved with the
 /// correct function index after code generation.
-pub(crate) const PLACEHOLDER_BRILLIG_INDEX: BrilligFunctionId = BrilligFunctionId(0);
+pub(crate) const PLACEHOLDER_BRILLIG_INDEX: BrilligFunctionId = BrilligFunctionId(u32::MAX);
 
 #[derive(Debug, Clone)]
 pub(crate) struct BrilligStdLib<F> {

compiler/noirc_evaluator/src/acir/acir_context/generated_acir/mod.rs

@@ -641,7 +641,10 @@ impl<F: AcirField> GeneratedAcir<F> {
         };
 
         match &mut self.opcodes[acir_index] {
-            AcirOpcode::BrilligCall { id, .. } => *id = brillig_function_index,
+            AcirOpcode::BrilligCall { id, .. } => {
+                assert!(*id == PLACEHOLDER_BRILLIG_INDEX, "expected placeholder brillig index");
+                *id = brillig_function_index;
+            }
             _ => panic!("expected brillig call opcode"),
         }
     }