It is common to use recursion in place of loops. The problem with recursion is that it easily leads to stack overflows. We often rely on tail recursion optimization to avoid stack overflows, but I think this is bad practice unless the compiler warns us when recursive functions which were meant to be tail-recursive aren't.
As a compiler can't trivially decide when a recursive function should be recursive, I suggest the addition of a syntactic construct. An alternative would be to use attributes.

To clarify:

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let findPos items key=
    let trec work items pos =
        match items with
        | [] -> -1, 1
        | head :: rest when head = key -> pos, 2
        | _ :: rest -> work rest (pos+1), 42
   let res, junk = work items 0
   res

trec in the code above would instruct the compiler to generate an error if work isn't tail-recursive.