Use From::from fn pointer to convert to boxed errors

[dcechano]

This PR changes the recommended way to convert to a boxed error Box<dyn Error> for types that impl Error. It changes the map_err(|e| e.into()) to map_err(From::from) which I believe is much more concise and idiomatic (and satisfying!).

Motivation

Like a lot of people I bashed my head on this problem of getting the compiler to accept the mapped error as a Box<dyn Error> and not a Box<CustomError>. I eventually found e.into() and then I realized that means I can use From::from.

I have done some searching on the StackOverflow and the Rust user forums to see how everyone else is solving this problem and I have yet to find an example of someone recommending From::from fn pointer. Everyone just suggests |e| e.into() even though the former does the same thing more concisely.

I think it is just an oversight and it doesn't occur to the person helping that if e.into works, that means From::from(e) necessarily works as well. So I figured I would share this solution in the official example book if it is indeed an improvement over the current recommended method.

Any way, let know what you guys think!

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @marioidival (or someone else) some time within the next two weeks.