Folks should first learn to correctly use "its" vs. "it's" and "lets" vs. "let's". Then graduate to correct past-perfect usage :-)
—Oxford University Press, Edpress News
it is => it's
| If you are using this as a mnemonic about its/it's,
| this is fine; but your statement of it is as a rule
If you need a general rule, how about:
when a possessive and a contraction collide,
the contraction winsThe only reason I posted at all is because linguistics is an area where a lot of quite intelligent people hold some extremely unexamined (and incorrect) beliefs, and there is furthermore a common tendency to propagate those beliefs as if they were fact. As a result, whenever I see someone articulating anything that is formulated like a general rule about a language (or about language in general), I try to make corrections where I can.
Even if that statement is taken as a general rule, I can't think of any contractions that don't use an apostrophe, and it certainly doesn't state that contractions are the only place where apostrophes are used.