some small doc improvements

[llogiq]

No description provided.

[moxian]

Hi, @llogiq , do you remember in what situations removing the return may run afoul of the borrow checker? Was there an issue to rust-lang/rust repo to get it fixed?
This PR was filed 3 years ago, is it possible that the issue was fixed once NLL landed?

Thanks!

[ebroto]

@moxian for example this code:

#![allow(unused)]

fn read_line() -> String {
    use std::io::BufRead;
    let stdin = ::std::io::stdin();
    return stdin.lock().lines().next().unwrap().unwrap();
}

(playground)

triggers the lint, but applying the suggestion causes this error

error[E0597]: `stdin` does not live long enough

(playground)

This happens because usually temporaries are dropped at the end of the statement, but in a block tail expression they are dropped after the locals of the block have been destroyed, triggering the lifetime error.

I recall seeing multiple issues on the rust-lang repo of users confused by this, but AFAIK this is well-known behavior, not solved by NLL.

BTW you make me think this was fixed in #5680 for the let_and_return lint and I think the same fix can be applied for needless_return. I'll open an issue soon.