Working in HR for many years has left me with a conviction that I can only help a person if I know what they really want.  Lying or being economical with the truth is the surest pathway to poor decisions.

So when I discuss my career needs and decisions with a Line Manager, if I'm unhappy I'm inclined to be honest about it.  But if I know that my current employer can't meet my needs and, as as result, I'm looking for work elsewhere, should I tell them that?

I've always had very open conversations about this sort of thing with my line managers and subordinates, but others I know - outside HR but in a wide range of industries and roles - consider this to be an absolute no-no and are horrified by the ease with which I talk about career aspirations within my circle.  When I've pressed them to explain why they consider it to be such a red-line issue, they're struggled to articulate anything concrete but there seems to be a wide belief that telling your employer that you're looking elsewhere is career suicide, that you'll be the first to be made redundant and that you'll be sidelined from anything important.  Of course, I can imagine that an employer would want to exclude a potential leaver from commercially sensitive work, but isn't it more professional to say "I'm probably leaving, so don't put me on that thing because I won't finish it" than to lie about it then leave your employer in the lurch?

I can't decide if I'm hopelessly naive and trusting about this or if my friends are unnecessarily cynical.

Any thoughts?  What sort of risk might you expose yourself to by being honest about your career plans with your boss?