Discussion Thread: DIP 1035-- at system Variables--Final Review

[Dennis]

On Monday, 21 February 2022 at 21:50:31 UTC, Paul Backus wrote:
> If the goal is being able to define custom pointer types, then 
> the DIP should use that as an example instead of talking about 
> file descriptors, and it should explain *exactly* which part of 
> the example depends on this feature for memory safety (as the 
> other examples do).

A double `fclose` on a `FILE*` is basically a double free. I 
thought the same would apply to raw file descriptors, but I just 
read that a double `close` simply results in an `EBADF` error, so 
maybe it's not a good example.

> I still don't think it's a compelling use-case, though.
> [`TailUnqual`][1] does something very similar, using the 
> `union` workaround, and it would not benefit from having access 
> to `scope`-checked integers because (a) it stores a `size_t`, 
> so eliminating the `union` wouldn't save any space; and (b) it 
> needs the `union` for correct GC scanning regardless.

Yes, TailUnqual doesn't need `scope`-checked integers, but that 
doesn't mean other code doesn't need it. I added the rule for two 
reasons:

- The compiler currently has a notion of a type that 
`hasPointers`. The extra complexity of adding a notion 
`hasSystemVariables` was daunting, but then I thought we could 
just make them the same. I think that would not only simplify the 
implementation, but also the feature in general. It makes it easy 
to draw a parallel to a pointer and a `@system size_t`.
- Some people asked for the feature (see links in the rationale 
section)

I can improve the DIP text, but I'm not yet convinced the rule 
should be scrapped.