How can I capture the name of a variable still to be assigned in R?

Note: This is separate from, though perhaps similar to, the deparse-substitute trick of attaining the name of a passed argument.

Consider the following situation: I have some function to be called, and the return value is to be assigned to some variable, say x. Inside the function, how can I capture that the name to be assigned to the returned value is x?

For example:

nameCapture <- function() {
    # arbitrary code
    captureVarName()
    }

x <- nameCapture()
x
## should return some reference to the name "x"

What in R closest approximates captureVarName() referenced in the example? My intuition was that there would be something in the call stack to do with assign(), where x would be an argument and could be extracted, but sys.call() yielded nothing of the sort; does it then occur internally, and if so, what is a sensible way to attain something like captureVarName()?