Improve comments and asserts

ConflictSet.cpp

@@ -518,7 +518,7 @@ struct Iterator {
 };
 
 // Call Stepwise::step for each element of remaining until it returns true.
-// Applies a permutation to remaining as a side effect.
+// Applies a permutation to `remaining` as a side effect.
 template <class Stepwise>
 void runInterleaved(std::span<Stepwise> remaining, int stepLimit = -1) {
   while (remaining.size() > 0) {
@@ -939,7 +939,7 @@ struct __attribute__((__visibility__("hidden"))) ConflictSet::Impl {
   }
 
   struct StepwiseInsert {
-    // After this phase, the heap invariant may be violated for
+    // After this phase, the heap invariant may be violated for *current and
     // (*current)->parent.
     Node **current;
     Node *parent;
@@ -964,6 +964,7 @@ struct __attribute__((__visibility__("hidden"))) ConflictSet::Impl {
         // We could interleave the iteration in ::next, but we'd need a careful
         // analysis for correctness and it's unlikely to be worthwhile.
         auto *prev = ::next(newNode, false);
+        // The empty key always exists. If *key is empty then we won't reach here.
         assert(prev != nullptr);
         assert(prev->rangeVersion <= writeVersion);
         newNode->rangeVersion = prev->rangeVersion;
@@ -1010,6 +1011,8 @@ struct __attribute__((__visibility__("hidden"))) ConflictSet::Impl {
         ArenaAlloc<Node *>(&arena)};
     workList.reserve(count);
     for (int i = 0; i < count; ++i) {
+      Node *node = *stepwiseInserts[i].current;
+      assert(node != nullptr);
       workList.push_back(*stepwiseInserts[i].current);
     }