I presume MyNonCopyable() is concretely a non-copyable type, so let y = x would in fact move x into y rather than copy it. When we generalize the generics system to include noncopyable types, then withHandle can't assume Handle is copyable, and it would act as a noncopyable type within the body of the function. So the function would not be able to introduce copies of whatever Handle is, but callers can still pass in copyable types for the Handle that they can copy themselves.
You may also be interested in the @noImplicitCopy attribute, which we're implementing to suppress implicit copying on values that are of copyable type.