I have two grammar books at home, one by Pat O'Connor and the other by Bryan Garner, that say there is such a rule.
Birdeen is right. O'Connor and Garner are wrong, though even the normally infallible Michael Swan writes,"
Like is similar to a preposition. We use it before a noun or pronoun. [...]
As is a conjunction. We use it before a clause, and before an expression beginning with a preposition."
However, Swan later tells us, "Another use of
as is to say what function a person or thing has - what jobs people do, what purpose things are used for, what category they belong to, etc. In this case
as is used like* a preposition, before a noun: [...]
Please don't use that knife as a screwdriver."
* I think this
like should be
as!
Interestingly, even the Oxford ALD is slightly confusing. In its usage advice box it says virtually what Garner and O'Connor say, and informs us that "
AS is a conjunction and an adverb", but in the definitions colums, the first section gives
as as a preposition.
Swan, Michael (1995)
Practical English Usage, Oxford: OUP
Wehmeier, Sally (ed.) (2000)
Oxford Advanced Learner's Dictionary, Oxford: OUP