AI review feedback: createGuards

- Refactor the Guard decision in withTxnType into createGuards, which
  lives in Rules.cpp. It is physically located near
  setCurrentTransactionRules, and documented to explain that changes
  need to be synchronized.

include/xrpl/basics/Number.h

@@ -73,7 +73,7 @@ isPowerOfTen(T value)
  * should be left in place, but the "Small" scale option should be removed. This
  * will allow for future expansion beyond 64-bits if it is ever needed.
  */
-struct MantissaRange
+struct MantissaRange final
 {
     using rep = std::uint64_t;
     enum class MantissaScale {
@@ -250,7 +250,7 @@ concept Integral64 = std::is_same_v<T, std::int64_t> || std::is_same_v<T, std::u
  * amendments are enabled to determine which result to expect.
  *
  */
-class Number
+class Number final
 {
     using rep = std::int64_t;
     using internalrep = MantissaRange::rep;

include/xrpl/protocol/Rules.h

@@ -122,4 +122,13 @@ private:
     std::optional<Rules> saved_;
 };
 
+class NumberSO;
+
+void
+createGuards(
+    Rules const& rules,
+    std::optional<NumberSO>& stNumberSO,
+    std::optional<CurrentTransactionRulesGuard>& rulesGuard,
+    std::optional<NumberMantissaScaleGuard>& mantissaScaleGuard);
+
 }  // namespace xrpl

src/libxrpl/protocol/Rules.cpp

@@ -7,6 +7,7 @@
 #include <xrpl/beast/hash/uhash.h>
 #include <xrpl/beast/utility/instrumentation.h>
 #include <xrpl/protocol/Feature.h>
+#include <xrpl/protocol/IOUAmount.h>
 #include <xrpl/protocol/STVector256.h>
 
 #include <memory>
@@ -46,6 +47,9 @@ setCurrentTransactionRules(std::optional<Rules> r)
     // Because fixCleanup3_2_0 fixes a separate bug related to the large
     // mantissas, that can take precedence and activate the large
     // mantissas even in the absence of the other two amendments.
+    //
+    // If any new conditions with new amendments are added, those amendments must also be added to
+    // createGuards.
     bool const enableCuspRoundingFix = !r || r->enabled(fixCleanup3_2_0);
     bool const enableVaultNumbers = enableCuspRoundingFix ||
         (r->enabled(featureSingleAssetVault) || r->enabled(featureLendingProtocol));
@@ -57,6 +61,34 @@ setCurrentTransactionRules(std::optional<Rules> r)
     *getCurrentTransactionRulesRef() = std::move(r);
 }
 
+void
+createGuards(
+    Rules const& rules,
+    std::optional<NumberSO>& stNumberSO,
+    std::optional<CurrentTransactionRulesGuard>& rulesGuard,
+    std::optional<NumberMantissaScaleGuard>& mantissaScaleGuard)
+{
+    // The list amendments used to decide which guard(s) to create must be a superset of the list
+    // used to figure out which mantissa scale to use in setCurrentTransactionRules. Additional
+    // amendments can be added if desired.
+    //
+    // As soon as any one of these amendments is retired, this whole function can be removed, and
+    // the first set of guards can be created directly at the call site, without using optional.
+    if (rules.enabled(fixCleanup3_2_0) || rules.enabled(featureSingleAssetVault) ||
+        rules.enabled(featureLendingProtocol))
+    {
+        // raii classes for the current ledger rules.
+        // fixUniversalNumber predates the rulesGuard and should be replaced.
+        stNumberSO.emplace(rules.enabled(fixUniversalNumber));
+        rulesGuard.emplace(rules);
+    }
+    else
+    {
+        // Without those features enabled, always use the old number rules.
+        mantissaScaleGuard.emplace(MantissaRange::MantissaScale::Small);
+    }
+}
+
 class Rules::Impl
 {
 private:

src/libxrpl/tx/applySteps.cpp

@@ -66,26 +66,15 @@ withTxnType(Rules const& rules, TxType txnType, F&& f)
     // so these need to be more global.
     //
     // To prevent unintentional side effects on existing checks, they will be
-    // set for every operation only once SingleAssetVault (or later
-    // LendingProtocol) are enabled.
+    // set for every operation only once at least one of the relevant amendments
+    // are enabled.
     //
     // See also Transactor::operator().
     //
     std::optional<NumberSO> stNumberSO;
     std::optional<CurrentTransactionRulesGuard> rulesGuard;
     std::optional<NumberMantissaScaleGuard> mantissaScaleGuard;
-    if (rules.enabled(featureSingleAssetVault) || rules.enabled(featureLendingProtocol))
-    {
-        // raii classes for the current ledger rules.
-        // fixUniversalNumber predates the rulesGuard and should be replaced.
-        stNumberSO.emplace(rules.enabled(fixUniversalNumber));
-        rulesGuard.emplace(rules);
-    }
-    else
-    {
-        // Without those features enabled, always use the old number rules.
-        mantissaScaleGuard.emplace(MantissaRange::MantissaScale::Small);
-    }
+    createGuards(rules, stNumberSO, rulesGuard, mantissaScaleGuard);
 
     switch (txnType)
     {