writeText

import std.array : appender;

auto output = appender!string();
output.writeText("The answer is ", 42);

assert(output.data == "The answer is 42");

import std.array : appender;

const color = "red";
auto output = appender!string();
output.writeText(i"My favorite color is $(color)");

assert(output.data == "My favorite color is red");