[exim] de-taint efficiency

I got caught by the wonderful new de-taint system, fortunately the server
that I updated was not processing live mail at the time.

After a bit of struggling to understand the changes I modified my Exim
config to fix some of the issues, but suspect there may be more hiding.

I was able to get rid of some errors in routers, by doing this:

domains = lsearch,ret=key;/domains/domains.dat

That works, I can then access $domain_data in the rest of the router.

However, it sorta annoys me.

At the start of the Exim configuration file, we already load the local
domains like this:

domainlist local_domains = lsearch;/etc/virtual/domains

How can I just search the local_domains list? I've looked at the various
lookup types but don't see any way to lookup something from the named list
and have it return a clean domain name that I can reference in $domain_data


I've got quite a few routers that are using 'domains =
lsearch,ret=key;/domains/domains.dat' which I assume Exim really is opening
that file and searching every time.

Can I make it more efficient by using the named list?