DIP 1027---String Interpolation---Format Assessment

[Petar]

On Thursday, 27 February 2020 at 14:58:20 UTC, Adam D. Ruppe 
wrote:
> On Thursday, 27 February 2020 at 14:32:29 UTC, Petar Kirov 
> [ZombineDev] wrote:
>> 2. Have the new type implicitly convert to printf-style args. 
>> I think this is what Adam is proposing. While nice to have, I 
>> don't think it's necessary.
>
> You can read my document for more detail
>
> https://github.com/dlang/DIPs/pull/186
>
> But basically
>
> writefln(i"hi $name, you are visitor ${%2d}(count)");
>
> gets turned into:
>
> writefln(
>    // the format string is represented by this type
>    new_type!("hi ", spec(null), ", you are visitor ", 
> spec("%2d"))(),
>    // then the referenced arguments are passed as a tuple
>    name,
>    count
> )
>
>
> So very, very, very similar to Walter's proposal, just instead 
> of the compiler generating the format string as a plain string, 
> the format string is represented by a new type, defined by the 
> spec and implemented by druntime. As a result, no more guess 
> work - it is clear that this is meant to be interpreted as a 
> format string. It is clear which parts are 
> placeholders/specifiers for which arguments.

Perhaps my assumptions were based on an old version of your 
proposal.

What I want is for:

     auto s = i"hi $name, you are visitor ${%2d}(count)";

to lower to:

     auto s = new_type!(
         "hi ", spec(null), ", you are visitor ", spec("%2d")
     )(name, count);

I.e. the referenced arguments are passed to the constructor of 
new_type.
That way new_type can offer implicit conversion to string, while 
support for zero-allocation printf, write, writeln, writef, 
writefln and so on can be done via function overloading.