dip1000 and preview in combine to cause extra safety errors

[John Colvin]

On Wednesday, 8 June 2022 at 15:35:56 UTC, Steven Schveighoffer 
wrote:
> On 6/8/22 11:10 AM, Dukc wrote:
>> On Wednesday, 8 June 2022 at 14:52:53 UTC, Steven 
>> Schveighoffer wrote:
>>> ```d
>>> string foo(in string s)
>>> {
>>>     return s;
>>> }
>>>
>>> void main()
>>> {
>>>     import std.stdio;
>>>     string[] result;
>>>     foreach(c; "hello")
>>>     {
>>>         result ~= foo([c]);
>>>     }
>>>     writeln(result);
>>> }
>>> ```
>>>
>>> Thoughts?
>> 
>> This is simply the result of using `in` wrong. `in` means 
>> `const scope`. `scope` (without preceeding `return`) means you 
>> won't return a reference to the address to the argument 
>> (unless the function can reach it via some other channel). 
>> Result: undefined behaviour.
>
> So silently changing behavior to create new dangling pointers 
> with a preview switch is ok?
>
> Remember, there is already code that does this. It's not trying 
> to be clever via scope, it's not trying to be `@safe`, it's 
> expecting that an array literal is allocated on the GC (as has 
> always been the case).
>
> -Steve

The preview switch is changing the meaning of `in` which changes 
the signature of `foo` (which is then inconsistent with the 
implementation), which in turn will affect the call sites. This 
seems roughly as expected, no?