To correctly implement this, it must be known on instantiation whether E is
copy-constructible, which is not the case if only a forward declaration is
available. This can be resolved either by making sure a full definition of E is
available, which is preferable. But in cases where this is not (easily) possible,
the information can be explicitly provided by the MOZ_DECLARE_COPY_CONSTRUCTIBLE
and MOZ_DECLARE_NON_COPY_CONSTRUCTIBLE macros. In particular, declarations for
IPDL-declared types are added to nsTArray.h itself, like it was already done
for MOZ_DECLARE_RELOCATE_USING_MOVE_CONSTRUCTOR.
Differential Revision: https://phabricator.services.mozilla.com/D66244
5431b0ef4b