chore: add pre- and post-check on array_set optimization pass

compiler/noirc_evaluator/src/ssa/opt/array_set.rs

@@ -20,16 +20,59 @@
     #[tracing::instrument(level = "trace", skip(self))]
     pub(crate) fn array_set_optimization(mut self) -> Self {
         for func in self.functions.values_mut() {
+            #[cfg(debug_assertions)]
+            array_set_optimization_pre_check(func);
+
             func.array_set_optimization();
+
+            #[cfg(debug_assertions)]
+            array_set_optimization_post_check(func);
         }
         self
     }
 }
 
+/// Pre-check condition for [Function::array_set_optimization].
+///
+/// Panics if:
+///   - there already exists a mutable array set instruction.
+#[cfg(debug_assertions)]
+fn array_set_optimization_pre_check(func: &Function) {
+    // There should be no mutable array sets.
+    for block_id in func.reachable_blocks() {
+        let instruction_ids = func.dfg[block_id].instructions();
+        for instruction_id in instruction_ids {
+            if matches!(func.dfg[*instruction_id], Instruction::ArraySet { mutable: true, .. }) {
+                panic!("mutable ArraySet instruction exists before `array_set_optimization` pass");
+            }
+        }
+    }
+}
+
+/// Post-check condition for [Function::array_set_optimization].
+///
+/// Panics if:
+///   - Mutable array_set optimization has been applied to Brillig function
+#[cfg(debug_assertions)]
+fn array_set_optimization_post_check(func: &Function) {
+    // Brillig functions should be not have any mutable array sets.
+    if func.runtime().is_brillig() {
+        for block_id in func.reachable_blocks() {
+            let instruction_ids = func.dfg[block_id].instructions();
+            for instruction_id in instruction_ids {
+                if matches!(func.dfg[*instruction_id], Instruction::ArraySet { mutable: true, .. })
+                {
+                    panic!("Mutable array set instruction in Brillig function");
+                }
+            }
+        }
+    }
+}
+
 impl Function {
     pub(crate) fn array_set_optimization(&mut self) {
         if matches!(self.runtime(), RuntimeType::Brillig(_)) {
             // Brillig is supposed to use refcounting to decide whether to mutate an array;
             // array mutation was only meant for ACIR. We could use it with Brillig as well,
             // but then some of the optimizations that we can do in ACIR around shared
             // references have to be skipped, which makes it more cumbersome.