Clarify prevector::erase and avoid swap-to-clear

backports bitcoin/bitcoin@e241a63

src/prevector.h

@@ -387,6 +387,12 @@ class prevector {
     }
 
     iterator erase(iterator first, iterator last) {
+        // Erase is not allowed to the change the object's capacity. That means
+        // that when starting with an indirectly allocated prevector with
+        // size and capacity > N, the result may be a still indirectly allocated
+        // prevector with size <= N and capacity > N. A shrink_to_fit() call is
+        // necessary to switch to the (more efficient) directly allocated
+        // representation (with capacity N and size <= N).
         iterator p = first;
         char* endp = (char*)&(*end());
         if (!std::is_trivially_destructible<T>::value) {

src/script/script.h

@@ -643,8 +643,9 @@ class CScript : public CScriptBase
     std::string ToString() const;
     void clear()
     {
-        // The default std::vector::clear() does not release memory.
-        CScriptBase().swap(*this);
+        // The default prevector::clear() does not release memory
+        CScriptBase::clear();
+        shrink_to_fit();
     }
 };