Improving DIP74: functions borrow by default, retain only if needed

[Andrei Alexandrescu via Digitalmars-d]

DIP74's function call protocol for RCOs has the caller insert opAddRef 
for each RCO passed by value. Then the callee has the responsibility to 
call opRelease (or defer that to another entity). This choice of 
protocol mimics the constructor/destructor protocol and probably shows 
our C++ bias.

However, ARC does not do that. Instead, it implicitly assumes the callee 
is a borrower of the reference. Only if the callee wants to copy the 
parameter to a member or a global (i.e. save it beyond the duration of 
the call), a new call to retain() (= opAddRef) is inserted. That way, 
functions that only need to look at the object but not store it incur no 
reference call overhead.

So I was thinking of changing DIP74 as follows:

* Caller does NOT insert an opAddRef for byval RCOs

* Callee does NOT insert an opRelease for its byval RCO parameters

It seems everything will just work with this change (including all move 
scenarios), but it is simple enough to make me worry I'm missing 
something. Thoughts?


Andrei