Many rust-related search terms in google return documentation that's out-of-date. The docs do make clear that they are old, but don't link to the newest version. Some examples:

"rust rc" returns this as the 3rd link:

https://doc.rust-lang.org/book/second-edition/ch15-04-rc.html

"rust types of closure" returns this as the first link:

https://doc.rust-lang.org/book/first-edition/closures.html

IMO, the best option is to remove these from the index entirely; if anyone really wants docs for old versions of the language they can probably find them manually.

To do this, I believe you can either put:

<meta name="robots" content="noindex" /> 

per https://support.google.com/webmasters/answer/93710?hl=en, or hide them with robots.txt.