In `impl Trait`, the description of typing is incorrect.

[elfsternberg]

The text says that B in the section on .map() is of type Self; this is incorrect.

Consider the following demonstration (this code compiles under Rust 2018 as a drop in for the contents of main.rs after cargo new mapdemo --bin; cargo run):

fn main() {
        let x = [1, 2, 3, 4, 5];
        let z = x.iter().map(|&x| format!("{}", x)).collect::<Vec<String>>();
        println!("{:?}", z);
}

The self argument to map will be an Iter<u64>. The Self::Item type is therefore u64. The return type of the function being passed to map is String. This is what the B type variable indicates: the return type of the function, as can be clearly seen in the second line of the where clause in your example:

    F: FnMut(Self::Item) -> B

I have left a pull request: #42