Is there any hope for "lazy" and @nogc?

[Iain Buclaw]

On 1 August 2018 at 18:52, Shachar Shemesh via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
> On 01/08/18 17:13, Steven Schveighoffer wrote:
>>
>> On 8/1/18 3:59 AM, Shachar Shemesh wrote:
>>>
>>> Thank you! Finally!
>>>
>>> Let me just state, for the record, that having *yet another* syntax
>>> special case is just appalling.
>>
>>
>> The lazy variadic thing is a distinction between specifying variadic lazy
>> parameters and a lazy variadic array. The distinction is so miniscule, but
>> necessary to have a disambiguous syntax.
>>
>> But I had actually thought for a while, that you could simply specify a
>> delegate, and it would be treated as a lazy parameter, which would probably
>> solve your problem. I really think this syntax should be available.
>>
>>> With that said, I was hoping that specifying it explicitly as a delegate
>>> would allow me to scope it. Apparently, that doesn't work :-(
>>>
>>
>> I guess you mean you can't scope the delegates? I'm surprised if that
>> doesn't work.
>>
>> -Steve
>
>
>
>
> import std.string;
>
> alias Dg = string delegate() @nogc nothrow;
>
> void myAssert(bool cond, scope Dg[1] msg_dg ...) @nogc nothrow
> {
>     import core.stdc.stdio;
>     if (!cond)
>     {
>         string msg = msg_dg[0]();
>         printf("%*s\n", msg.length, msg.ptr);
>     }
> }
>
> void main() @nogc {
>     string msg = "Hello";
>     myAssert(true, msg); // <- errors on this line
> }
>
> It errors out: complains it needs to allocate main's frame on the GC, but
> main is @nogc. The same happens if I move the scope to the alias.

My first thought was to have a look at enforce(), but on closer
observation it is neither @nogc or nothrow.

Maybe you should raise a bug report?

It's certainly worth an attempt to bridge these two features together.
I think it makes sense enough that lazy parameters should infer
attributes from the function, and that it should be an error to pass a
parameter that does not meet those constraints.

i.e:
---
// Signatures.
void myAssert(bool cond, lazy string msg) @nogc nothrow;
string mayAlloc() nothrow;
string mayThrow() @nogc;

// Code
myAssert(cond, mayAlloc());    // violates @nogc
myAssert(cond, mayThrow());    // violates nothrow
---

Iain.