Using output-range overloads of SysTime.toISO{Ext}String with formatting code

[Joseph Rushton Wakeling]

On Sunday, 7 July 2019 at 20:12:30 UTC, drug wrote:
> 07.07.2019 17:49, Joseph Rushton Wakeling пишет:
>> it's possible to do something like 
>> `writefln!"%s"(now.toISOExtString)` and have it automatically 
>> use the output range overload rather than allocating a new 
>> string instance.
>
> This is exactly how it is intended to work: 
> https://run.dlang.io/is/ATjAkx

Thanks for taking the time to answer, but I don't think this 
really addresses my question.

Your example shows a struct with `toString` overloads.  However, 
SysTime.toISOExtString does not work like this: it is a method 
with two explicit overloads, one of which just returns a newly 
allocated `string`, the other of which returns nothing but 
accepts an output range as input:
https://dlang.org/phobos/std_datetime_systime.html#.SysTime.toISOExtString

I want to know if there's an easy way to work with that in 
`format` and `writefln` statements.

Note that while SysTime does also have `toString` methods, these 
give no control over the kind of datetime string that results:
https://dlang.org/phobos/std_datetime_systime.html#.SysTime.toString

Since I explicitly need the extended ISO format, I need to use 
`toISOExtString` directly.