dns/bind916 builds rust unexpectedly

Hi all, I've just upgraded bind916 which brought half my system down since
it suddenly required a mountain of python packages and rust which needed
around 13GB (and hours) to build - space which I didn't have nor have ever
remotely expected to need for a ports build.

My bind configuration options are basically the defaults:

# grep OPTIONS_FILE_SET /var/db/ports/dns_bind916/options
OPTIONS_FILE_SET+=IDN
OPTIONS_FILE_SET+=JSON
OPTIONS_FILE_SET+=LMDB
OPTIONS_FILE_SET+=MANPAGES
OPTIONS_FILE_SET+=TCP_FASTOPEN
OPTIONS_FILE_SET+=GSSAPI_NONE
OPTIONS_FILE_SET+=DLZ_FILESYSTEM

These are the top level dependencies:

# make -C /usr/ports/dns/bind916 build-depends-list
/usr/ports/ports-mgmt/pkg
/usr/ports/textproc/py-sphinx
/usr/ports/devel/pkgconf
/usr/ports/security/openssl
/usr/ports/converters/libiconv
/usr/ports/devel/libuv
/usr/ports/textproc/libxml2
/usr/ports/dns/libidn2
/usr/ports/devel/json-c
/usr/ports/databases/lmdb
/usr/ports/devel/libedit

Does anyone know which option/dependency is causing this? I suspect
MANPAGES -> py-sphinx since it has 'py' but who knows. Which itself would
be crazy that just a manpage would trigger this kind of intense build.