toString best practices

[Paolo Invernizzi]

Hello everybody,

Let's assume there's an implementation of a templated struct like 
this:

```
import std.format, std.range.primitives;

struct Point(T)
{
     T x, y;

     void toString(W)(ref W writer, scope const ref 
FormatSpec!char f) const
     if (isOutputRange!(W, char))
     {
         put(writer, "(");
         formatValue(writer, x, f);
         put(writer, ", ");
         formatValue(writer, y, f);
         put(writer, ")");
     }
}

void main(){

         import std.format : format;
         assert( format("%s", Point!int(1,2)) == "(1, 2)");

     	import std.experimental.logger;
     	sharedLog.infof("%s", Point!int(1,2));
/+
  Error: none of the overloads of template 
`std.logger.core.Logger.memLogFunctions!LogLevel.info.logImplf` 
are callable using argument types `!()(string, Point!int) shared`
/Users/pinver/dlang/dmd-2.102.0/osx/bin/../../src/phobos/std/logger/core.d(828,14):        Candidates are: `logImplf(int line = __LINE__, string file = __FILE__, string funcName = __FUNCTION__, string prettyFuncName = __PRETTY_FUNCTION__, string moduleName = __MODULE__, A...)(lazy bool condition, lazy string msg, lazy A args)`
/Users/pinver/dlang/dmd-2.102.0/osx/bin/../../src/phobos/std/logger/core.d(876,14):                        `logImplf(int line = __LINE__, string file = __FILE__, string funcName = __FUNCTION__, string prettyFuncName = __PRETTY_FUNCTION__, string moduleName = __MODULE__, A...)(lazy string msg, lazy A args)`
+/

}
```

What is the best way to handle also shared writers?

Thank you all,
Paolo