ENH: add List::resize_unsafe(label)

- changes the addressed list size without affecting list allocation.
  Can be useful for the following type of coding pattern:

  - pre-allocate a List with some max content length
  - populate with some content (likely not the entire pre-allocated size)
  - truncate the list to the length of valid content
  - process the List
  - discard the List

  Since the List is being discarded, using resize_unsafe() instead of
  resize() avoids an additional allocation with the new size and
  copying/moving of the elements.

  This programming pattern can also be used when the List is being
  returned from a subroutine, and carrying about a bit of unused memory
  is less important than avoiding reallocation + copy/move.

  If used improperly, it can obviously result in addressing into
  unmanaged memory regions (ie, 'unsafe').

applications/test/DynamicList/Test-DynamicList.C

@@ -348,6 +348,32 @@ int main(int argc, char *argv[])
             Info<< "input = " << flatOutput(input1) << nl
                 << "remove " << range << " = " << flatOutput(list1) << nl;
         }
+
+        {
+            input1 = identity(5);
+            list1 = identity(4, 5);
+            list1.reserve(10);
+
+            Info<< nl << "test swap(List)" << nl;
+            Info<< "  input: " << input1.size()
+                << '/' << input1.capacity() << ' '
+                << flatOutput(input1) << nl;
+            Info<< "  list:  " << list1.size() << '/'
+                << list1.capacity() << ' '
+                << flatOutput(list1) << nl;
+
+
+            // input1.swap(list1);   // This is wrong!
+            list1.swap(input1);   // Correct
+
+            Info<< "after swap:" << nl;
+            Info<< "  input: " << input1.size()
+                << '/' << input1.capacity() << ' '
+                << flatOutput(input1) << nl;
+            Info<< "  list:  " << list1.size() << '/'
+                << list1.capacity() << ' '
+                << flatOutput(list1) << nl;
+        }
     }
 
     Info<< "\nEnd\n";

applications/test/List/Test-List.C

@@ -169,8 +169,17 @@ int main(int argc, char *argv[])
             Info<<" " << *iter;
         }
         Info<< nl;
-    }
 
+        Info<< "data:" << Foam::name(ident.cdata())
+            << " size:" << ident.size() << nl;
+
+
+        Info<< "resize_unsafe(10)" << nl;
+        ident.resize_unsafe(10);
+
+        Info<< "data:" << Foam::name(ident.cdata())
+            << " size:" << ident.size() << nl;
+    }
 
     if (false)
     {

src/OpenFOAM/containers/Lists/List/List.H

@@ -227,6 +227,13 @@ public:
         //  Otherwise the contents will be uninitialized.
         inline void resize_nocopy(const label len);
 
+        //- Change the addressed list size directly without affecting
+        //- any memory management (advanced usage).
+        //
+        //  It is left to the caller to avoid \em unsafe lengthening beyond
+        //  the allocated memory region.
+        inline void resize_unsafe(const label len) noexcept;
+
         //- Alias for resize()
         void setSize(const label n) { this->resize(n); }
 

src/OpenFOAM/containers/Lists/List/ListI.H

@@ -185,6 +185,13 @@ inline void Foam::List<T>::resize_nocopy(const label len)
 }
 
 
+template<class T>
+inline void Foam::List<T>::resize_unsafe(const label len) noexcept
+{
+    UList<T>::setAddressableSize(len);
+}
+
+
 template<class T>
 inline T& Foam::List<T>::newElmt(const label i)
 {