fix(gnovm): Add debug panic on Deepfill execution on constant type

gnovm/pkg/gnolang/garbage_collector.go

@@ -14,7 +14,7 @@ import (
 // TODO: more accurate benchmark.
 const VisitCpuFactor = 8
 
-// Visit visits all reachable associated values.
+// Visits all reachable associated values.
 // It is used primarily for GC.
 // The caller must provide a callback visitor
 // which knows how to break cycles, otherwise

gnovm/pkg/gnolang/values_fill.go

@@ -1,14 +1,28 @@
 package gnolang
 
+// StringValue, BigintValue, and BigdecValue are used only for constant expressions
+// and are not collected by the GC (see garbage_collector.go).
+// These types should never require DeepFill calls in correct code.
+// In debug builds, we panic to catch potential misuse.
+
 func (sv StringValue) DeepFill(store Store) Value {
+	if debug {
+		panic("StringValue.DeepFill should not be called - StringValue is only used for constant expressions")
+	}
 	return sv
 }
 
 func (biv BigintValue) DeepFill(store Store) Value {
+	if debug {
+		panic("BigintValue.DeepFill should not be called - BigintValue is only used for constant expressions")
+	}
 	return biv
 }
 
 func (bdv BigdecValue) DeepFill(store Store) Value {
+	if debug {
+		panic("BigdecValue.DeepFill should not be called - BigdecValue is only used for constant expressions")
+	}
 	return bdv
 }
 

gnovm/pkg/gnolang/values_fill_test.go

@@ -0,0 +1,22 @@
+package gnolang
+
+import (
+	"testing"
+)
+
+func TestConstantValuesDeepFill(t *testing.T) {
+	sv := StringValue("test")
+	if result := sv.DeepFill(nil); result != sv {
+		t.Errorf("StringValue.DeepFill: expected %v, got %v", sv, result)
+	}
+
+	biv := BigintValue{}
+	if result := biv.DeepFill(nil); result != biv {
+		t.Errorf("BigintValue.DeepFill: expected %v, got %v", biv, result)
+	}
+
+	bdv := BigdecValue{}
+	if result := bdv.DeepFill(nil); result != bdv {
+		t.Errorf("BigdecValue.DeepFill: expected %v, got %v", bdv, result)
+	}
+}