dip1000 and preview in combine to cause extra safety errors

[Steven Schveighoffer]

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