import std.range: put;

import std.array: appender;

import std.format: formattedWrite;

​

struct Foo

{

    void toString(Writer)(ref Writer writer)

    {

        put(writer, "Foo");

    }

}

​

void main()

{    

    auto buffer = appender!string;

    Foo foo;

    formattedWrite(buffer, "%s", foo);

    assert(buffer[] == "Foo");

}