@safe question

[Paul Backus]

On Tuesday, 11 January 2022 at 10:57:28 UTC, forkit wrote:
> On Monday, 10 January 2022 at 03:21:46 UTC, Paul Backus wrote:
>>
>> Taking the address of a local variable is forbidden in @safe 
>> code. Even though str is a ref variable that points to a 
>> heap-allocated string, it is still considered a local variable 
>> because it is declared inside the body of a function.
>
> but strings[] is also a local variable declared in the body of 
> the same function, and yet within the foreach statement, @safe 
> lets me do:
>
> pointers ~= &strings[i]; // safe
>
> ...but not this below, where str is just a reference to the 
> exact same memory as the statement above... is it not? How is 
> this below any more or less safe than the above statement.
>
> pointers ~= &str;  // not safe - ok, but why??

Because the compiler doesn't look at that much context, and it's 
possible to write code where `str` points to memory that's on the 
stack; for example:

     string[3] strings = ["foo", "bar", "baz"];
     foreach (ref str; strings) {
         // ...
     }

If you compile with -preview=dip1000, the compiler will actually 
keep track of which pointers point to stack memory, and will 
allow your original code. But -preview=dip1000 is still somewhat 
experimental, and the documentation for it is pretty sparse, so 
you may have an easier time just working around the limitations 
of the default safety checks.