Here’s what the Vim manual says in *:syn-include*:
:sy[ntax] include [@{grouplist-name}] {file-name}
All syntax items declared in the included file will have the
"contained" flag added. In addition, if a group list is
specified, all top-level syntax items in the included file will
be added to that list.
We had two rules for `rustModPath`, one `contained` and the other not.
The effect was that the second (now renamed to `rustModPathInUse`) was
being included in the group list, and thus that all identifiers were
being highlighted as `Include`, which is definitely not what we wanted.
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