Macro expansion in for trait items

Hi, I see that in `ast.rs` there is no expectation for macro items inside a trait:

``` Rust
pub enum TraitItemKind {
    Const(P<Ty>, Option<P<Expr>>),
    Method(MethodSig, Option<P<Block>>),
    Type(TyParamBounds, Option<P<Ty>>),
}
```

Is there a reason for this? I would like to build up Trait definitions via macros the same way I would build a struct or impl items using macros.
