If this is indeed Diggory's point, it's not very pointy (sorry). The compiler should be able to safely omit the postblit on 's' when it passes it to foo (and the possible destructor of foo's argument inside foo when foo exits).
If the postblit is omitted, and the function or code it calls accesses the const variable through a global mutable reference then that function will be able to see those changes despite the parameter being passed by value. Without this optimisation it would not be able to see the changes.
Also, the function needs to be strongly pure, not just weakly pure, otherwise the calling code could pass in both the const variable and a non-const reference to that variable. If the callee modifies the non-const reference it would see the changes to the const parameter which it shouldn't be able to do.
