VTK-m has a deprecation method that supports API changes in minor
releases. When an API change is made, the old API is marked with the
VTKM_DEPRECATED macro. If code attempts to use the old API, it still
works, but the compiler issues a warning that the thing is deprecated
and where to find the new API.
We have recently run into an issue when the API changes have a header
file renamed or moved. We still keep the old header file with the old
API, so code including that file will still work. However, sometimes
code expected the contents of that header file without directly
including that header file. In these cases, the code could get an error
about missing classes.
As an example, consider the change from `DynamicCellSet` to
`UnknownCellSet`/`UncertainCellSet`. The deprecated `DynamicCellSet` is
still around. But there is a lot of code that did not directly include
DynamicCellSet.h. This header file was necessarily included by
DataSet.h. Now, when this code uses `vtkm::cont::DynamicCellSet`, you
get a confusing error that the class does not exist. Backward
compatibility broken.
In response to this, we should be more careful about where we put the
deprecated API. Instead of containing the deprecated API, moved headers
should be empty except for a warning and an inclusion of the new header
file. The deprecated API should be moved to the new header file. For
example, in the case of `DynamicCellSet`, the implementation for the
deprecated `DynamicCellSet` is moved to UnknownCellSet.h, which is
included by anything that was including DynamicCellSet.h before.
This filter adds a pair of fields to a `DataSet` which mirror the
indices of the points and cells, respectively. These fields are useful
for tracking the provenance of the elements of a `DataSet` as it gets
manipulated by the filters. It is also convenient for adding indices to
operations designed for fields and for testing purposes.
