Scalar DiffRules which use the output

[willtebbutt]

Noting that stuff like

y = exp(x)
∂/∂x exp(x) = y

happens, it might make sense to augment the set of scalar diffrules with extra code that optionally allows a user to provide a symbol for the output of the call. This is potentially useful for forwards-mode and reverse-mode stuff. That said, I can imagine that a lot instances of this kind of thing might be possible to optimise away via a compiler pass, so maybe this isn't such a valuable thing to provide?

[willtebbutt]

Now irrelevant because ChainRules.jl