In the "Spawning a task" section of the tutorial, the text refers to thechild_task variable in this snippet:

let some_value = 22;
task::spawn {||
    std::io::println("This executes in the child task.");
    std::io::println(#fmt("%d", some_value));
}

No child_task is mentioned. From subsequent examples, I assume it should be let child_task = task::spawn {||.