Omitting enum variant fields other than the last doesn't work

[Manish Goregaokar]

Manishearth/rust-gc#142

Essentially, the following code:

#[derive(Finalize, Trace)]
enum Foo {
    Foo(bool, #[unsafe_ignore_trace] Ignore),
    Bar(#[unsafe_ignore_trace] Ignore, bool),
}

using filter() on unsafe_ignore_trace will generate the following function:

            unsafe fn trace(&self) {
                #[allow(dead_code)]
                #[inline]
                unsafe fn mark<T: ::gc::Trace + ?Sized>(it: &T) {
                    ::gc::Trace::trace(it);
                }
                match *self {
                    Foo::Foo(ref __binding_0, ..) => { { mark(__binding_0) } }
                    Foo::Bar(ref __binding_1, ..) => { { mark(__binding_1) } }
                }
            }

Note that the bindings are appropriately numbered, but appropriate gaps are not inserted in the enum pattern match.

[Manish Goregaokar]

#49