see rationale in https://issues.dlang.org/show_bug.cgi?id=18529

On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour wrote:
> see rationale in https://issues.dlang.org/show_bug.cgi?id=18529

It looks like the actual deprecation was made with this PR:  https://github.com/dlang/dmd/pull/5860  Meaning it's been deprecated for more than a year and a half.

I think you're going to have to take it up with Walter.

Mike

On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour wrote:
> see rationale in https://issues.dlang.org/show_bug.cgi?id=18529

All I see is a rationale for how it can't be replaced, but not a rationale for actually doing it.

I can't imagine a "safe" situation where you want to do it.

Tbh. just put your code in @trusted

On Tuesday, 27 February 2018 at 09:23:19 UTC, bauss wrote:
> On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour wrote:
>> see rationale in https://issues.dlang.org/show_bug.cgi?id=18529
>
> All I see is a rationale for how it can't be replaced, but not a rationale for actually doing it.
>
> I can't imagine a "safe" situation where you want to do it.
>
> Tbh. just put your code in @trusted

Checking if an array is the slice of another.

On Tuesday, 27 February 2018 at 09:47:51 UTC, Stefan Koch wrote:
> On Tuesday, 27 February 2018 at 09:23:19 UTC, bauss wrote:
>> On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour wrote:
>>> see rationale in https://issues.dlang.org/show_bug.cgi?id=18529
>>
>> All I see is a rationale for how it can't be replaced, but not a rationale for actually doing it.
>>
>> I can't imagine a "safe" situation where you want to do it.
>>
>> Tbh. just put your code in @trusted
>
> Checking if an array is the slice of another.

Like?

assert(a.ptr != b.ptr);

Which already works in @safe.

On Tuesday, 27 February 2018 at 09:58:00 UTC, bauss wrote:
> On Tuesday, 27 February 2018 at 09:47:51 UTC, Stefan Koch wrote:
>> On Tuesday, 27 February 2018 at 09:23:19 UTC, bauss wrote:
>>> On Tuesday, 27 February 2018 at 08:43:32 UTC, Timothee Cour wrote:
>>>> see rationale in https://issues.dlang.org/show_bug.cgi?id=18529
>>>
>>> All I see is a rationale for how it can't be replaced, but not a rationale for actually doing it.
>>>
>>> I can't imagine a "safe" situation where you want to do it.
>>>
>>> Tbh. just put your code in @trusted
>>
>> Checking if an array is the slice of another.
>
> Like?
>
> assert(a.ptr != b.ptr);
>
> Which already works in @safe.

That only checks if the first element is the same. For a full 'is slice' check you'd need something like this:

    assert(a.ptr <= b.ptr && b.ptr + b.length <= a.ptr + a.length);

// Or:

    auto c = b.ptr - a.ptr;
    assert(c >= 0 && c + b.length <= a.length);

And trust me, the compiler complains about both of these. Possibly rightfully in the first example, but the latter never does anything scary with the given pointers.

--
  Simen

On Tuesday, February 27, 2018 11:33:04 Simen Kjærås via Digitalmars-d wrote:
> And trust me, the compiler complains about both of these. Possibly rightfully in the first example, but the latter never does anything scary with the given pointers.

As I understand it, the way that @safety checks generally work is they check whether a particular operation is @safe or not. They don't usually care about what is then done with the result. So, if you do something like take the address of something, that's immediately @system regardless of what you do with the result. That changes on some level with DIP 1000 and scope, because then it uses scope to ensure that the lifetime of stuff like pointers doesn't exceed the lifetime of what they point to so that it can then know that taking the address is @safe, but without DIP 1000, it takes very little for something to become @system. e.g. this is compiles with -dip1000 but otherwise doesn't:

void main() @safe
{
    int i;
    assert(&i !is null);
}

Now, the compiler does seem to be a bit smarter with dynamic arrays and ptr given that this compiles without -dip1000

void main() @safe
{
    int[] i;
    assert(i.ptr !is null);
}

However, this doesn't compile with -dip1000:

void main() @safe
{
    int[] i;
    auto j = i.ptr;
    assert(j !is null);
}

and not even this compiles with -dip1000:

void main() @safe
{
    int[] i;
    scope j = i.ptr;
    assert(j !is null);
}

though I'm inclined to think that that's a bug from what I understand of -dip1000.

In any case, @safety checks tend to be fairly primitive, so once you start mucking around with pointers, it's not hard to write code that gets treated as @system because of a single expression in the code that is clearly @safe within the context of the function, but the compiler can't see it.

And for better or worse, accessing a dynamic array's ptr member is now @system, because it's not @safe in all circumstances. If the compiler were smarter, then a number of uses of ptr would probably be @safe, but its analysis for stuff like that is usually pretty primitive, in part because making it sophisticated requires stuff like code flow analysis, which the compiler doesn't do a lot of, precisely because it is complicated and easy to get wrong. Walter is particularly leery about making it so that stuff is an error or not based on code flow analysis, and @safe falls into that camp. Clearly, some of that is going on with DIP 1000, but that seems to be largely by using the type system to solve the problem rather than doing much in the way of code flow analysis.

- Jonathan M Davis

this would be more bearable if there was a standard @trusted method to
get array `.ptr`, eg:
in `object.d` (so that it's indeed standard)

```
@trusted @nogc pure nothrow
auto pointer(T)(T a){
  return a.ptr;
}
```

again, the deprecation message is misleading because `&a[0]` isn't
equivalent to `a.ptr`
having something like `pointer` (and making deprecation msg use that)
would be a better mitigation



On Tue, Feb 27, 2018 at 3:56 AM, Jonathan M Davis via Digitalmars-d <digitalmars-d@puremagic.com> wrote:
> On Tuesday, February 27, 2018 11:33:04 Simen Kjærås via Digitalmars-d wrote:
>> And trust me, the compiler complains about both of these. Possibly rightfully in the first example, but the latter never does anything scary with the given pointers.
>
> As I understand it, the way that @safety checks generally work is they check whether a particular operation is @safe or not. They don't usually care about what is then done with the result. So, if you do something like take the address of something, that's immediately @system regardless of what you do with the result. That changes on some level with DIP 1000 and scope, because then it uses scope to ensure that the lifetime of stuff like pointers doesn't exceed the lifetime of what they point to so that it can then know that taking the address is @safe, but without DIP 1000, it takes very little for something to become @system. e.g. this is compiles with -dip1000 but otherwise doesn't:
>
> void main() @safe
> {
>     int i;
>     assert(&i !is null);
> }
>
> Now, the compiler does seem to be a bit smarter with dynamic arrays and ptr given that this compiles without -dip1000
>
> void main() @safe
> {
>     int[] i;
>     assert(i.ptr !is null);
> }
>
> However, this doesn't compile with -dip1000:
>
> void main() @safe
> {
>     int[] i;
>     auto j = i.ptr;
>     assert(j !is null);
> }
>
> and not even this compiles with -dip1000:
>
> void main() @safe
> {
>     int[] i;
>     scope j = i.ptr;
>     assert(j !is null);
> }
>
> though I'm inclined to think that that's a bug from what I understand of -dip1000.
>
> In any case, @safety checks tend to be fairly primitive, so once you start mucking around with pointers, it's not hard to write code that gets treated as @system because of a single expression in the code that is clearly @safe within the context of the function, but the compiler can't see it.
>
> And for better or worse, accessing a dynamic array's ptr member is now @system, because it's not @safe in all circumstances. If the compiler were smarter, then a number of uses of ptr would probably be @safe, but its analysis for stuff like that is usually pretty primitive, in part because making it sophisticated requires stuff like code flow analysis, which the compiler doesn't do a lot of, precisely because it is complicated and easy to get wrong. Walter is particularly leery about making it so that stuff is an error or not based on code flow analysis, and @safe falls into that camp. Clearly, some of that is going on with DIP 1000, but that seems to be largely by using the type system to solve the problem rather than doing much in the way of code flow analysis.
>
> - Jonathan M Davis
>
>

On Tuesday, February 27, 2018 04:20:38 Timothee Cour via Digitalmars-d wrote:
> this would be more bearable if there was a standard @trusted method to
> get array `.ptr`, eg:
> in `object.d` (so that it's indeed standard)
>
> ```
> @trusted @nogc pure nothrow
> auto pointer(T)(T a){
>   return a.ptr;
> }
> ```

Except that that's really not how @trusted is supposed to be used. The programmer needs to verify that the caller is using a.ptr in a manner that is actually @safe, because the compiler is not smart enough to determine that for you. Wrapping it in an @trusted function means that the caller won't get an error and that the programmer won't necessarily know that they need to verify the calling code. It's the code that's using ptr that needs to be verified, not the actual accessing of ptr.

Hiding the access to ptr within an @trusted function goes against that entire idea of @trusted and makes it easy to use ptr without realizing that you need to be checking the code that's using it, since you just called a wrapper function to silence the compiler instead of listening the compiler and studying the code using ptr to verify its @safety.

>
> again, the deprecation message is misleading because `&a[0]` isn't
> equivalent to `a.ptr`
> having something like `pointer` (and making deprecation msg use that)
> would be a better mitigation

In almost all cases, &a[0] is equivalent to a.ptr except that it does bounds checking, so it's actually @safe and thus doesn't need to be manually verified by the programmer, unlike your pointer function suggestion.

If you have a use case where you need a.ptr rather than &a[0], then that just means that you're going to have to verify that your code is @safe in spite of using an @system operation and mark it @trusted if it is. Simply treating a.ptr as @safe when it isn't or wrapping it in an @trusted function just defeats the purpose of the whole @safety system.

Now, maybe in some simple cases, the compiler can be improved to detect that what you're doing with a.ptr is actually @safe and not give an error, but as long as it can't, any use of a.ptr needs to be verified by the programmer.

- Jonathan M Davis

On 2/27/18 7:20 AM, Timothee Cour wrote:
> this would be more bearable if there was a standard @trusted method to
> get array `.ptr`, eg:
> in `object.d` (so that it's indeed standard)
> 
> ```
> @trusted @nogc pure nothrow
> auto pointer(T)(T a){
>    return a.ptr;
> }
> ```

That completely defeats the purpose of the restriction! Of course we aren't going to do that :)

This is the standard way (and it's on you to make sure you don't mess it up):

auto x = (() @trusted => a.ptr)();

> again, the deprecation message is misleading because `&a[0]` isn't
> equivalent to `a.ptr`

It isn't equivalent, but it is a @safe mechanism to do it. If you want to do unsafe things, then you need to use @trusted escapes, druntime/phobos is not going to help you there.

-Steve