This rearranges the deriving code so that #[deriving] a trait on a field
that doesn't implement that trait will point to the field in question,
e.g.
struct NotEq; // doesn't implement Eq
#[deriving(Eq)]
struct Foo {
ok: int,
also_ok: ~str,
bad: NotEq // error points here.
}
Unfortunately, this means the error is disconnected from the `deriving`
itself but there's no current way to pass that information through to
rustc except via the spans, at the moment.
Fixes#7724.
