What is the best way to determine the version of an `.rds`?

Hi,

I am writing a test that consults the serialization version of an `.rds`
file.

An attractive way to get this is:

tools:::get_serialization_version() # reports just version

which calls

.Internal(serializeInfoFromConn() # reports much more

but neither is truly exported for public use.

Is there an official, exported way to get the serialization version? It is
possible to get this information with R code yourself, but it doesn't feel
very elegant.

If not, could we have this? It's pretty easy these days to acquire a
version 3 file, without real intent, which risks making the package require
R >= 3.5.

Thanks,
Jenny

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