import std.range;

struct UntilFirst(R, T)
    if(isInputRange!R)
{
    private R _input;
    private T _x;

    this(R input, T x)
    {
        _input = input;
        _x = x;
    }

    bool empty() @property
    {
        return _input.empty || _input.front == _x;
    }

    auto front() @property
    {
        assert(!empty);
        return _input.front;
    }

    void popFront()
    {
        assert(!empty);
        _input.popFront();
    }
}

auto untilFirst(R, T)(R input, T x)
    if(isInputRange!R)
{
    return UntilFirst!(R, T)(input, x);
}

unittest
{
    import std.stdio;
    writeln(untilFirst(iota(20), 13));
    writeln(untilFirst("abcdefghijklmnopqrstuvwxyz", 'j'));
}