dip1000, perhaps annotate with return, and vibe-d

[Paul Backus]

On Wednesday, 24 July 2019 at 12:54:51 UTC, aliak wrote:
> Trying to get dip1000 flag in use. I have this error:
>
> Error: returning Optional(null, false).this(value) escapes a 
> reference to parameter value, perhaps annotate with return
>
> in this function:
>
> public auto some(T)(auto ref T value) {
>     return Optional!T(value); // <-- error on this line
> }
>
> And it's instantiated from here:
>
> static Optional!T fromRepresentation(Json value) {
>   if (value == Json.undefined) {
>     return no!T;
>   }
>   return some(deserializeJson!T(value)); // <-- instantiated 
> from here
> }
>
> I put the qualifier "return" on the parameter to some, but did 
> nothing. And I put it on Optional.this() as well, but I'm not 
> sure where I'm supposed to put it.

It should go on the constructor's parameter; i.e.,

this(auto return ref T value) { /* ... */ }

Under the hood, a constructor actually returns the constructed 
value by reference, so the actual signature of the above 
constructor seen by the lifetime checker is:

ref Optional!T __ctor(auto return ref T value)

You can see this for yourself with something like `pragma(msg, 
typeof(Optional!T.__ctor))`.