#[plugin] #[no_link] extern crate bleh;
becomes a crate attribute
#![plugin(bleh)]
The feature gate is still required.
It's almost never correct to link a plugin into the resulting library /
executable, because it will bring all of libsyntax and librustc with it.
However if you really want this behavior, you can get it with a separate
`extern crate` item in addition to the `plugin` attribute.
Fixes#21043.
Fixes#20769.
[breaking-change]
