ref const array error

[Timon Gehr]

On 01/19/2012 03:47 PM, jdrewsen wrote:
> On Wednesday, 18 January 2012 at 23:09:56 UTC, Timon Gehr wrote:
>> On 01/18/2012 10:12 PM, jdrewsen wrote:
>>> On Wednesday, 18 January 2012 at 20:13:04 UTC, Timon Gehr wrote:
>>>> On 01/18/2012 08:59 PM, jdrewsen wrote:
>>>>> On Wednesday, 18 January 2012 at 19:43:52 UTC, Timon Gehr wrote:
>>>>>> On 01/18/2012 08:31 PM, jdrewsen wrote:
>>>>>>> Recently the encoding.safeDecode stopped working for some of my
>>>>>>> existing
>>>>>>> code. This example outlines the issue:
>>>>>>>
>>>>>>> import std.encoding;
>>>>>>>
>>>>>>> void main(string[] args) {
>>>>>>> auto e = EncodingScheme.create("utf-8");
>>>>>>> auto a = new byte[100];
>>>>>>> e.safeDecode(a);
>>>>>>> }
>>>>>>>
>>>>>>> Results in:
>>>>>>>
>>>>>>> Error: function std.encoding.EncodingScheme.safeDecode (ref
>>>>>>> const(ubyte)[] s) const is not callable using argument types
>>>>>>> (byte[])
>>>>>>>
>>>>>>> Isn't this an error in the compiler?
>>>>>>>
>>>>>>> /Jonas
>>>>>>>
>>>>>>
>>>>>> No, this is a bugfix. The operation is unsound:
>>>>>>
>>>>>> immutable(ubyte)[] foo(ref const(ubyte)[] s){
>>>>>> auto r = new immutable(ubyte)[1];
>>>>>> s = r;
>>>>>> return r;
>>>>>> }
>>>>>>
>>>>>> void main() {
>>>>>> ubyte[] x;
>>>>>> immutable(ubyte)[] y = foo(x);
>>>>>> static assert(is(typeof(y[0])==immutable));
>>>>>> auto oldy0 = y[0];
>>>>>> x[0]=oldy0+1;
>>>>>> assert(oldy0 == y[0]); // fail
>>>>>> }
>>>>>>
>>>>>> The functionality is not going away; You will be able to use inout
>>>>>> for
>>>>>> the same purpose once my enhancement request gets implemented:
>>>>>> http://d.puremagic.com/issues/show_bug.cgi?id=7105
>>>>>
>>>>> Wouldn't a nicer solution be to let the compiler ensure that
>>>>> an immutable array cannot escape through a ref const array parameter?
>>>>>
>>>>> /Jonas
>>>>>
>>>>
>>>> That would not suffice.
>>>>
>>>> ubyte[] foo(ref const(ubyte)[] s){
>>>> auto r = new ubyte[1];
>>>> s = r;
>>>> return r;
>>>> }
>>>>
>>>> void main() {
>>>> immutable(ubyte)[] x;
>>>> ubyte[] y = foo(x);
>>>> static assert(is(typeof(x[0])==immutable));
>>>> auto oldx0 = x[0];
>>>> y[0]=oldx0+1;
>>>> assert(oldx0 == x[0]); // fail
>>>> }
>>>
>>> In the example foo is actually using the ref s parameter as an out
>>> parameter. The compiler could catch that you're doing this and show an
>>> error.
>>>
>>> This would force you to let foo look like:
>>>
>>> ubyte[] foo(out const(ubyte)[] s);
>>>
>>> Wouldn't that fix it?
>>>
>>>
>>
>> If it is ref or out is irrelevant for the example, so how would this
>> fix anything? The compiler could, in principle, treat const similarly
>> to inout (just without the context sensitivity and parameter matching
>> etc) for 'ref' parameters and do all the type checking at the call
>> site. However, that would then restrict what the callee can do and
>> introduce a strange special case. inout is the way to go.
>
> But in the example you're using s as an out parameter and that should
> trigger the error I got originally of course. But if ref parameters were
> disallowed to be used as out parameters the compiler would catch the
> error in your example wouldn't it?
>
>

The compiler would catch something in my example, but not the error. The 
error happens at the call site.