catch unnecessary pass by values

[Manishearth]

If we're just accessing fields and methods, pass by reference might make more sense. Maybe.

[llogiq]

The problem here is that some functions actually should pass by value. Those functions tend to be named into_-something, drop or transmute, among others. Perhaps using a whitelist of name patterns would suffice to weed out most false positives – we could then also advise to either access by ref or rename the function.

[birkenfeld]

Well, I would assume that the into_ methods generally move out at least some parts of self.

But it's a good point that into_ should always take by value, kind of a complement lint. (And as_ and to_ should take a reference.)

[llogiq]

@birkenfeld added an issue.