#[allow_internal_unstable] plugin #2

foo_macro.rs

#![crate_type="dylib"]
#![feature(plugin_registrar, rustc_private, slice_patterns)]

extern crate syntax;
extern crate rustc;

use syntax::codemap::Span;
use syntax::ast;
use syntax::ext::base::*;
use syntax::ext::build::AstBuilder;
use syntax::parse::token;
use rustc::plugin::Registry;

fn expand_foo(cx: &mut ExtCtxt, sp: Span, tts: &[ast::TokenTree])
        -> Box<MacResult + 'static> {
    let mut p = cx.new_parser_from_tts(tts);
    let arg1 = p.parse_expr();
    let unsafe_expr = cx.expr_path(cx.path_global(sp, vec![
        cx.ident_of("std"),
        cx.ident_of("rt"),
        cx.ident_of("DEFAULT_ERROR_CODE"),
    ]));
    let expr = cx.expr_tuple(sp, vec![arg1, unsafe_expr]);

    MacEager::expr(expr)
}

#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
    let expander: MacroExpanderFn = expand_foo;
    reg.syntax_exts.push((token::intern("foo"),
                          NormalTT(Box::new(expander), None, true)));
}

main.rs

#![feature(allow_internal_unstable, plugin)]
#![plugin(foo_macro)]

macro_rules! nested1 { () => (foo!(1)) }

#[allow_internal_unstable]
macro_rules! nested2 { () => (foo!(1)) }

#[allow_internal_unstable]
macro_rules! ident { ($arg:expr) => ({ let x = $arg; x }) }

fn main() {
    // GOOD: no stability error
    let x = foo!(1);
    println!("{:?}", x);

    // GOOD: stability error
    foo!(std::rt::DEFAULT_ERROR_CODE);
    //   ^~~~~~~~~~~~~~~~~~~~~~~~~~~

    // BAD: stability errors:
    nested1!();
    ident!(nested1!());
    nested2!();

    // BAD: stability error for the println! macro range.
    let x = println!("{:?}", foo!(1)) ;
    //      ^~~~~~~~~~~~~~~~~~~~~~~~~~~

    // BAD: stability error for the range of the macros.
    let x = ident!(ident!(nested2!())) ;
    //      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
}