allow nominal types to be declared non-copyable

[nikomatsakis]

There are many times when, for correctness or performance, we would like to guarantee that instances of a class are not copyable. This can be synthesized now by including a drop() method or by embedding a resource, but that's a hack that adds unnecessary overhead.

[nikomatsakis]

My plan would be to use an annotation. #[non_copyable] class or something.

[eholk]

@pcwalton and some of the rest of us were talking about this yesterday. I think Patrick's ideas were more like having a copy interface that types could explicitly declare that they do not implement. #[no_copy] seems like a reasonable idea too, although I feel like attributes are usually used for things that are somewhat orthogonal to semantics.

[nikomatsakis]

Not critical for 0.6; removing milestone

[catamorphism]

Nominating for milestone 1, well-defined.

[graydon]

closing as WONTFIX, the existing workaround works well enough.