Support attributes on macro invocations

[pnkfelix]

Attempting to compile the following example code fails with an error:

macro_rules! b3{ ( band $x:ident ) => ( static $x : int = 3i; ) }
macro_rules! b4{ ( bind $x:ident ) => ( static $x : int = 4i; ) }

#[cfg(stage0)]
b3!(band x)

#[cfg(not(stage0))]
b4!(bind x)

fn main() { io::println(fmt!("Hello world: %?", x)); }

The error:

% rustc /tmp/m.rs
/tmp/m.rs:5:0: 5:2 error: attrs on macros are not yet supported
/tmp/m.rs:5 b3!(band x)
            ^~

We should support such attributes. Here are some motivations:

• As illustrated by the above example, it is difficult to migrate from one macro definition form in the snapshot rustc to a newer form in the development rustc, because you cannot annotate macro invocations with the usual #[cfg(stage0)] and #[cfg(not(stage0)].
• One cannot put documentation comments on macros (which I believe are represented as attributes within the compiler internally; if this is not true, then this needs to be a separate issue ticket).

[catamorphism]

Nominating for milestone 5, production-ready

[graydon]

accepted for feature-complete milestone

[chris-morgan]

This also includes things like attempting to document macros:

/// This macro does X, Y and even Z.
macro_rules! foo(()=>())

I'm not sure if this would do what I want or not, but I do want to be able to document macros when using #[macro_escape];

[huonw]

Fixed by #9753.