Closures in Rust

Today, I want to note down my thoughts on closures. Closures are important in Rust, because they are extensively used in iterator adapters paramount in development highly performant programs. However, to my point of view this topic is not well-covered in The Book. This may be a reason why it is considered among the most difficult parts of the language. In this post, I will try to shed more light on it, hopefully making it more clear to Rust learners. Note that I am still a novice to the language, and my understanding may not be fully correct.