#[deriving] should copy lint attributes from the original item

[lifthrasiir]

Otherwise it can be very annoying to use lint-affected items selectively. For example, the following code tries to use an unstable item X from a stable item A, but boilerplates generated from #[deriving(Clone)] do not have #[allow(unstable)] attached, so it fails to compile. Currently the only way available is to manually add impls with the proper lint attributes.

#[deny(unstable)];

#[allow(unstable)]
#[unstable]
#[deriving(Clone)]
struct X;

#[allow(unstable)]
#[deriving(Clone)]
struct A { x: X }

fn main() { }

[petrochenkov]

It was resolved by #29152