DIP 1022---foreach auto ref---Community Review Round 2

[Andrea Fontana]

On Wednesday, 23 October 2019 at 07:25:40 UTC, Dukc wrote:
> On Saturday, 19 October 2019 at 13:16:17 UTC, Mike Parker wrote:
>> This is the feedback thread for the second round of Community 
>> Review for DIP 1022, "foreach auto ref":
>
> In case someone does not have concentration to see what's 
> changed relative to the first community review, the major 
> things are:
>
> -`static foreach` is no longer supposed to compile with `auto 
> ref`. Rationale of why in description.
>
> -added a simplified example in bottom of rationale section

I'm the author of that pull request. Reading your DIP I think you 
give a misinterpretation of it.

"A pull request[3] was submitted to disallow this behavior.

In every place where foreach (ref <...>) presently iterates by 
value, a compiler error would have resulted after merging that 
pull request."

My patch was not supposed to give any error. And I didn't even 
try to disallow this behaviour.

Before that patch, compiler allowed to iterate a range of structs 
(where front is returned by value) by ref, that probably doesn't 
make any sense. In this case dtor on those structs is never 
called due to a bug in code. This mean that if my struct wrap a 
"malloc" call in struct's ctor and a "free" call in struct's dtor 
(or a refcount like in my case) memory is saturated using a 
foreach (or simply usign phobos "each!T").

My patch simply fixed that missing dtor call, and I left the 
behaviour change for a complex DIP.

Sönke Ludwig summarized [1] this better than me in a comment on 
github:
"Isn't this quite simple? Fixing the failure to call the 
destructor is critical and a clear improvement over the status 
quo. The ref foreach didn't ever do something useful for r-value 
ranges, but that's an orthogonal issue that should simply be 
discussed in the context of a separate ticket (or DIP)."


[1] https://github.com/dlang/dmd/pull/8437#issuecomment-466249392