The D Programming Language page 161.

[Andrej Mitrovic]

On 11/13/13, Adam D. Ruppe <destructionator at gmail.com> wrote:
> On Wednesday, 13 November 2013 at 08:16:34 UTC, Tor Einar
> Tønnessen wrote:
>> void writeln(string a0, int a1, string a2, int[new] a3) {
>
> Around the time the book was written, there was a debate as to if
> we should make dynamic arrays and slices two different types.
> T[new] was the proposed syntax for a dynamic array, leaving T[]
> to be a slice.

I think it was just an oversight that it was left in the book, I don't
recall reading about that syntax on any other page.