ubyte[] -> immutable(ubyte)[]

[Andrej Mitrovic]

This is from TDPL page 407:

import std.algorithm,
       std.concurrency,
       std.stdio;

void main()
{
    enum bufferSize = 1024 * 100;
    auto tid = spawn(&fileWriter);
    
    // Read loop
    foreach (immutable(ubyte)[] buffer; stdin.byChunk(bufferSize))
    {
        send(tid, buffer);
    }
}

void fileWriter()
{
    // write loop
    while (true)
    {
        auto buffer = receiveOnly!(immutable(ubyte)[])();
        tgt.write(buffer);
    }
}

Error:
C:\DMD\dmd2\windows\bin\..\..\src\phobos\std\stdio.d(1943):
Error: cannot implicitly convert expression (buffer) of type ubyte[] 
to immutable(ubyte)[]

Yet interestingly I can't use type inference:

    foreach (buffer; stdin.byChunk(bufferSize))
    {
        send(tid, buffer);
    }

Error: stdin_stdout_copy.d(11): Error: cannot infer type for buffer

But in the original code I get back a mutable ubyte[] which I can't implicitly convert to immutable (I'd need a copy first). There's no .dup or .idup property for stdin.byChunk. So what am I supossed to do here?