Discussion Thread: DIP 1035-- at system Variables--Community Review Round 2

[Steven Schveighoffer]

On 2/27/21 10:55 AM, Paul Backus wrote:
> On Saturday, 27 February 2021 at 15:21:41 UTC, Steven Schveighoffer wrote:
>> On 2/27/21 8:32 AM, Paul Backus wrote:
>>> You *can* get the compiler to treat any value as unsafe by 
>>> overlapping it with a pointer; e.g.,
>>>
>>> struct Unsafe(T)
>>> {
>>>      union Storage { T value; void* dummy; }
>>>      Storage storage;
>>>      ref T get() { return Storage.value; }
>>>      /* ... */
>>> }
>>>
>>> But this introduces memory overhead for any T smaller than a pointer, 
>>> which is not ideal.
>>
>> Better mark that get as @system, or it can be used in @safe code.
> 
> It's in a template, so it will be inferred as @system.

D considers that @safe. It was a whole section of my Dconf online talk. 
I filed a bug report on it (in which you seem to think this is OK): 
https://issues.dlang.org/show_bug.cgi?id=21565

Even without templates:

struct DThinksThisIsSafe
{
    union Storage { int value; void *dummy; }
    Storage storage;
    @safe ref int get() return { return storage.value; } // compiles 
just fine
}

-Steve