How to get a range from std.container.array for use with std.format.sformat?

[cc via Digitalmars-d-learn]

        import std.container;
        import std.format;
        Array!char str;
        str.length = 256;
        str.sformat!"%s:%s"("some", "string");
	// Error: template std.format.sformat cannot deduce function 
from argument types !("%s:%s")(Array!char, string, string), 
candidates are:
	//   std.format.sformat(alias fmt, Args...)(char[] buf, Args 
args) if (isSomeString!(typeof(fmt)))
	//   std.format.sformat(Char, Args...)(char[] buf, in Char[] 
fmt, Args args)

        auto rng = cast(char[]) str[];
	// Error: cannot cast expression `str.opSlice()` of type 
`RangeT!(Array!char)` to `char[]`


Additionally, would sformat cause GC allocations even when 
writing to a preallocated buffer?