Macros that operate on attributes

[nikomatsakis]

There is no way to write a macro that takes a list of attributes as input, like e.g. this one:

macro_rules! test ( ($nm:ident,
                     $a:inner_attr,
                     $i:item) => (mod $nm { $a $i }); )

test!(a,
      #[cfg(qux)],
      pub fn bar() { })

test!(b,
      #[cfg(not(qux))],
      pub fn bar() { })

#[qux]
fn main() {
    a::bar();
    //~^ ERROR use of undeclared module `a`
    //~^^ ERROR unresolved name
    //~^^^ ERROR unresovled name `a::bar`
    b::bar();
}

I need this for #3678, since I have a macro there for generating wrappers around extern fns and it needs attributes. Pull request forthcoming.