format

struct DoeRayMe
{
    string easyAs;
    int oneTwoThree;
}

assert(format("abc").assumeValid == "abc");
assert(format("abc {1:} {0}", 123, "easy as").assumeValid == "abc easy as 123");
assert(format("abc {0}", DoeRayMe("hard as", 321)).assumeValid == `abc DoeRayMe("hard as", 321)`);