DIP77 - Fix unsafe RC pass by 'ref'

[via Digitalmars-d]

On Wednesday, 8 April 2015 at 23:11:08 UTC, Walter Bright wrote:
> http://wiki.dlang.org/DIP77

In general, this is way too much focused on reference counting. 
The DIP does define "RCO" in more general terms, but still, it 
feels like the wrong approach.

You also seem to look at Rust's type system from the same POV 
("only one mutable reference at a time, for efficiency"), but 
that's not the point at all: it's about safety in general, not 
efficiency for RC. It encompasses so much more: memory safety, 
race-free sharing of data, avoiding iterator invalidation... 
DIP77 cannot help with general iterator invalidation at all.

Another thing that just feels wrong is the automagic copying. 
We're passing something by `ref`, and the compiler inserts a copy 
behind our back?! And this isn't even visible to the user...

Because the topic is correctness, @safe/@system seems the correct 
way to approach the problem. The compiler should just treat 
unsafe pass-by-ref as @system, as Martin suggests. Basically, 
instead of inserting a copy, pass-by-ref will be @system, when no 
copy would need to be inserted, it will be @safe.

I also think that the unsafety detection heuristic is too 
simplistic. When scope/return is extended to pointers and other 
kinds of references, there would probably be way too many false 
positives. A more sophisticated analysis is necessary that can 
more realistically determine when mutable aliasing can occur. 
E.g., it can also take uniqueness into account.

Besides, the address and slice operators also need to be taken 
into account, and these can appear outside of function calls.