No more FnBox
The book used to have this code in Chapter 20, section 2:
#![allow(unused)] fn main() { trait FnBox { fn call_box(self: Box<Self>); } impl<F: FnOnce()> FnBox for F { fn call_box(self: Box<F>) { (*self)() } } type Job = Box<dyn FnBox + Send + 'static>; }
Here, we define a new trait called FnBox
, and then implement it for all
FnOnce
closures. All the implementation does is call the closure. These
sorts of hacks were needed because a Box<dyn FnOnce>
didn't implement
FnOnce
. This was true for all three posibilities:
Box<dyn Fn>
andFn
Box<dyn FnMut>
andFnMut
Box<dyn FnOnce>
andFnOnce
However, as of Rust 1.35, these traits are implemented for these types,
and so the FnBox
trick is no longer required. In the latest version of
the book, the Job
type looks like this:
#![allow(unused)] fn main() { type Job = Box<dyn FnOnce() + Send + 'static>; }
No need for all that other code.