DataStaticsFields.cpp used to contain identity_traits specializations for
primitive and opaque STL types, as well as macros used for the generated struct
field definitions. Only one of these were actually used in any compilation unit,
and the identity_traits definitions were hidden behind a guard to prevent them
from being compiled at all when building the struct field definitions.
Now, the identity_traits specializations live in their own file,
DataIdentity.cpp (matching their declarations, which live in DataIdentity.h).
This speeds up build times because modifying these specializations no longer
requires rebuilding all of the field definitions unnecessarily.
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