Do not implement Copy for types with "&mut self" methods

[newpavlov]

Implicit copies of types with methods accepting &mut self can produce surprising results for library users. Thus it's probably worth to add recommendation to avoid deriving Copy trait for such types.

Related issue.

[dtolnay]

Thanks! We definitely need a guideline for this.

Relevant discussions in the standard library related to Copy for range iterators in the following sequence of PRs:

• remove Copy impls from remaining iterators (merged)
• Revert "remove Copy impls from remaining iterators" (closed)
• Derive Copy for std::ops::Range and RangeFrom (closed)

One of the motivating examples there was:

let mut iter = 0..n;
for i in iter { if i > 2 { break; } }
iter.collect()

With a Copy impl, this iterates over a copy of the range and then collect gets all 0..n. Without a Copy impl, the compiler catches this and makes you use clone() to clone the range or for i in &mut iter to consume elements from the range.