I wrote a long and probably dull chapter on closures and first-class and higher-order functions in Rust. It goes into some detail on the implementation and some of the subtleties like higher-ranked lifetime bounds.

I was going to post it here too, but it is really too long. Instead, pop over to the 'Rust for C++ programmers' repo and read it there.

http://www.ncameron.org/blog/closures-and-first-class-functions/