rust/src/libsyntax/ext/expand.rs

// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use core::prelude::*;

use ast::{blk_, attribute_, attr_outer, meta_word};
use ast::{crate, expr_, expr_mac, mac_invoc_tt};
use ast::{item_mac, stmt_, stmt_mac, stmt_expr, stmt_semi};
use ast::{SCTable, illegal_ctxt};
use ast;
use ast_util::{new_rename, new_mark, resolve};
use attr;
use codemap;
use codemap::{span, CallInfo, ExpandedFrom, NameAndSpan, spanned};
use ext::base::*;
use fold::*;
use parse;
use parse::{parse_item_from_source_str};

pub fn expand_expr(extsbox: @mut SyntaxEnv,
                   cx: @ExtCtxt,
                   e: &expr_,
                   s: span,
                   fld: @ast_fold,
                   orig: @fn(&expr_, span, @ast_fold) -> (expr_, span))
                -> (expr_, span) {
    match *e {
        // expr_mac should really be expr_ext or something; it's the
        // entry-point for all syntax extensions.
        expr_mac(ref mac) => {
            match (*mac).node {
                // Token-tree macros:
                mac_invoc_tt(pth, ref tts) => {
                    if (pth.idents.len() > 1u) {
                        cx.span_fatal(
                            pth.span,
                            fmt!("expected macro name without module \
                                  separators"));
                    }
                    /* using idents and token::special_idents would make the
                    the macro names be hygienic */
                    let extname = cx.parse_sess().interner.get(pth.idents[0]);
                    // leaving explicit deref here to highlight unbox op:
                    match (*extsbox).find(&extname) {
                        None => {
                            cx.span_fatal(
                                pth.span,
                                fmt!("macro undefined: '%s'", *extname))
                        }
                        Some(@SE(NormalTT(SyntaxExpanderTT{
                            expander: exp,
                            span: exp_sp
                        }))) => {
                            cx.bt_push(ExpandedFrom(CallInfo {
                                call_site: s,
                                callee: NameAndSpan {
                                    name: copy *extname,
                                    span: exp_sp,
                                },
                            }));

                            let expanded = match exp(cx, mac.span, *tts) {
                                MRExpr(e) => e,
                                MRAny(expr_maker,_,_) => expr_maker(),
                                _ => {
                                    cx.span_fatal(
                                        pth.span,
                                        fmt!(
                                            "non-expr macro in expr pos: %s",
                                            *extname
                                        )
                                    )
                                }
                            };

                            //keep going, outside-in
                            let fully_expanded =
                                copy fld.fold_expr(expanded).node;
                            cx.bt_pop();

                            (fully_expanded, s)
                        }
                        _ => {
                            cx.span_fatal(
                                pth.span,
                                fmt!("'%s' is not a tt-style macro", *extname)
                            )
                        }
                    }
                }
            }
        }
        _ => orig(e, s, fld)
    }
}

// This is a secondary mechanism for invoking syntax extensions on items:
// "decorator" attributes, such as #[auto_encode]. These are invoked by an
// attribute prefixing an item, and are interpreted by feeding the item
// through the named attribute _as a syntax extension_ and splicing in the
// resulting item vec into place in favour of the decorator. Note that
// these do _not_ work for macro extensions, just ItemDecorator ones.
//
// NB: there is some redundancy between this and expand_item, below, and
// they might benefit from some amount of semantic and language-UI merger.
pub fn expand_mod_items(extsbox: @mut SyntaxEnv,
                        cx: @ExtCtxt,
                        module_: &ast::_mod,
                        fld: @ast_fold,
                        orig: @fn(&ast::_mod, @ast_fold) -> ast::_mod)
                     -> ast::_mod {

    // Fold the contents first:
    let module_ = orig(module_, fld);

    // For each item, look through the attributes.  If any of them are
    // decorated with "item decorators", then use that function to transform
    // the item into a new set of items.
    let new_items = do vec::flat_map(module_.items) |item| {
        do vec::foldr(item.attrs, ~[*item]) |attr, items| {
            let mname = attr::get_attr_name(attr);

            match (*extsbox).find(&mname) {
              Some(@SE(ItemDecorator(dec_fn))) => {
                  cx.bt_push(ExpandedFrom(CallInfo {
                      call_site: attr.span,
                      callee: NameAndSpan {
                          name: /*bad*/ copy *mname,
                          span: None
                      }
                  }));
                  let r = dec_fn(cx, attr.span, attr.node.value, items);
                  cx.bt_pop();
                  r
              },
              _ => items,
            }
        }
    };

    ast::_mod { items: new_items, ..module_ }
}


// eval $e with a new exts frame:
macro_rules! with_exts_frame (
    ($extsboxexpr:expr,$e:expr) =>
    ({let extsbox = $extsboxexpr;
      let oldexts = *extsbox;
      *extsbox = oldexts.push_frame();
      let result = $e;
      *extsbox = oldexts;
      result
     })
)

// When we enter a module, record it, for the sake of `module!`
pub fn expand_item(extsbox: @mut SyntaxEnv,
                   cx: @ExtCtxt,
                   it: @ast::item,
                   fld: @ast_fold,
                   orig: @fn(@ast::item, @ast_fold) -> Option<@ast::item>)
                -> Option<@ast::item> {
    // need to do expansion first... it might turn out to be a module.
    let maybe_it = match it.node {
      ast::item_mac(*) => expand_item_mac(extsbox, cx, it, fld),
      _ => Some(it)
    };
    match maybe_it {
      Some(it) => {
          match it.node {
              ast::item_mod(_) | ast::item_foreign_mod(_) => {
                  cx.mod_push(it.ident);
                  let result =
                      // don't push a macro scope for macro_escape:
                      if contains_macro_escape(it.attrs) {
                      orig(it,fld)
                  } else {
                      // otherwise, push a scope:
                      with_exts_frame!(extsbox,orig(it,fld))
                  };
                  cx.mod_pop();
                  result
              }
              _ => orig(it,fld)
          }
      }
      None => None
    }
}

// does this attribute list contain "macro_escape" ?
pub fn contains_macro_escape (attrs: &[ast::attribute]) -> bool {
    attrs.any(|attr| "macro_escape" == *attr::get_attr_name(attr))
}

// this macro disables (one layer of) macro
// scoping, to allow a block to add macro bindings
// to its parent env
macro_rules! without_macro_scoping(
    ($extsexpr:expr,$exp:expr) =>
    ({
        // only evaluate this once:
        let exts = $extsexpr;
        // capture the existing binding:
        let existingBlockBinding =
            match exts.find(&@~" block"){
                Some(binding) => binding,
                None => cx.bug("expected to find \" block\" binding")
            };
        // this prevents the block from limiting the macros' scope:
        exts.insert(@~" block",@ScopeMacros(false));
        let result = $exp;
        // reset the block binding. Note that since the original
        // one may have been inherited, this procedure may wind
        // up introducing a block binding where one didn't exist
        // before.
        exts.insert(@~" block",existingBlockBinding);
        result
    }))

// Support for item-position macro invocations, exactly the same
// logic as for expression-position macro invocations.
pub fn expand_item_mac(extsbox: @mut SyntaxEnv,
                       cx: @ExtCtxt, it: @ast::item,
                       fld: @ast_fold)
                    -> Option<@ast::item> {
    let (pth, tts) = match it.node {
        item_mac(codemap::spanned { node: mac_invoc_tt(pth, ref tts), _}) => {
            (pth, copy *tts)
        }
        _ => cx.span_bug(it.span, "invalid item macro invocation")
    };

    let extname = cx.parse_sess().interner.get(pth.idents[0]);
    let expanded = match (*extsbox).find(&extname) {
        None => cx.span_fatal(pth.span,
                              fmt!("macro undefined: '%s!'", *extname)),

        Some(@SE(NormalTT(ref expand))) => {
            if it.ident != parse::token::special_idents::invalid {
                cx.span_fatal(pth.span,
                              fmt!("macro %s! expects no ident argument, \
                                    given '%s'", *extname,
                                   *cx.parse_sess().interner.get(it.ident)));
            }
            cx.bt_push(ExpandedFrom(CallInfo {
                call_site: it.span,
                callee: NameAndSpan {
                    name: copy *extname,
                    span: expand.span
                }
            }));
            ((*expand).expander)(cx, it.span, tts)
        }
        Some(@SE(IdentTT(ref expand))) => {
            if it.ident == parse::token::special_idents::invalid {
                cx.span_fatal(pth.span,
                              fmt!("macro %s! expects an ident argument",
                                   *extname));
            }
            cx.bt_push(ExpandedFrom(CallInfo {
                call_site: it.span,
                callee: NameAndSpan {
                    name: copy *extname,
                    span: expand.span
                }
            }));
            ((*expand).expander)(cx, it.span, it.ident, tts)
        }
        _ => cx.span_fatal(
            it.span, fmt!("%s! is not legal in item position", *extname))
    };

    let maybe_it = match expanded {
        MRItem(it) => fld.fold_item(it),
        MRExpr(_) => cx.span_fatal(pth.span,
                                    ~"expr macro in item position: "
                                    + *extname),
        MRAny(_, item_maker, _) => item_maker().chain(|i| {fld.fold_item(i)}),
        MRDef(ref mdef) => {
            extsbox.insert(@/*bad*/ copy mdef.name, @SE((*mdef).ext));
            None
        }
    };
    cx.bt_pop();
    return maybe_it;
}

// expand a stmt
pub fn expand_stmt(extsbox: @mut SyntaxEnv,
                   cx: @ExtCtxt,
                   s: &stmt_,
                   sp: span,
                   fld: @ast_fold,
                   orig: @fn(&stmt_, span, @ast_fold) -> (stmt_, span))
                -> (stmt_, span) {
    let (mac, pth, tts, semi) = match *s {
        stmt_mac(ref mac, semi) => {
            match mac.node {
                mac_invoc_tt(pth, ref tts) => {
                    (copy *mac, pth, copy *tts, semi)
                }
            }
        }
        _ => return orig(s, sp, fld)
    };
    if (pth.idents.len() > 1u) {
        cx.span_fatal(
            pth.span,
            fmt!("expected macro name without module \
                  separators"));
    }
    let extname = cx.parse_sess().interner.get(pth.idents[0]);
    let (fully_expanded, sp) = match (*extsbox).find(&extname) {
        None =>
            cx.span_fatal(pth.span, fmt!("macro undefined: '%s'", *extname)),

        Some(@SE(NormalTT(
            SyntaxExpanderTT{expander: exp, span: exp_sp}))) => {
            cx.bt_push(ExpandedFrom(CallInfo {
                call_site: sp,
                callee: NameAndSpan { name: copy *extname, span: exp_sp }
            }));
            let expanded = match exp(cx, mac.span, tts) {
                MRExpr(e) =>
                    @codemap::spanned { node: stmt_expr(e, cx.next_id()),
                                    span: e.span},
                MRAny(_,_,stmt_mkr) => stmt_mkr(),
                _ => cx.span_fatal(
                    pth.span,
                    fmt!("non-stmt macro in stmt pos: %s", *extname))
            };

            //keep going, outside-in
            let fully_expanded = copy fld.fold_stmt(expanded).node;
            cx.bt_pop();

            (fully_expanded, sp)
        }

        _ => {
            cx.span_fatal(pth.span,
                          fmt!("'%s' is not a tt-style macro", *extname))
        }
    };

    (match fully_expanded {
        stmt_expr(e, stmt_id) if semi => stmt_semi(e, stmt_id),
        _ => { fully_expanded } /* might already have a semi */
    }, sp)

}



pub fn expand_block(extsbox: @mut SyntaxEnv,
                    cx: @ExtCtxt,
                    blk: &blk_,
                    sp: span,
                    fld: @ast_fold,
                    orig: @fn(&blk_, span, @ast_fold) -> (blk_, span))
                 -> (blk_, span) {
    match (*extsbox).find(&@~" block") {
        // no scope limit on macros in this block, no need
        // to push an exts frame:
        Some(@ScopeMacros(false)) => {
            orig (blk,sp,fld)
        },
        // this block should limit the scope of its macros:
        Some(@ScopeMacros(true)) => {
            // see note below about treatment of exts table
            with_exts_frame!(extsbox,orig(blk,sp,fld))
        },
        _ => cx.span_bug(sp, "expected ScopeMacros binding for \" block\"")
    }
}

pub fn new_span(cx: @ExtCtxt, sp: span) -> span {
    /* this discards information in the case of macro-defining macros */
    return span {lo: sp.lo, hi: sp.hi, expn_info: cx.backtrace()};
}

// FIXME (#2247): this is a moderately bad kludge to inject some macros into
// the default compilation environment. It would be much nicer to use
// a mechanism like syntax_quote to ensure hygiene.

pub fn core_macros() -> ~str {
    return
~"pub mod macros {
    macro_rules! ignore (($($x:tt)*) => (()))

    macro_rules! error (
        ($arg:expr) => (
            __log(1u32, fmt!( \"%?\", $arg ))
        );
        ($( $arg:expr ),+) => (
            __log(1u32, fmt!( $($arg),+ ))
        )
    )

    macro_rules! warn (
        ($arg:expr) => (
            __log(2u32, fmt!( \"%?\", $arg ))
        );
        ($( $arg:expr ),+) => (
            __log(2u32, fmt!( $($arg),+ ))
        )
    )

    macro_rules! info (
        ($arg:expr) => (
            __log(3u32, fmt!( \"%?\", $arg ))
        );
        ($( $arg:expr ),+) => (
            __log(3u32, fmt!( $($arg),+ ))
        )
    )

    macro_rules! debug (
        ($arg:expr) => (
            __log(4u32, fmt!( \"%?\", $arg ))
        );
        ($( $arg:expr ),+) => (
            __log(4u32, fmt!( $($arg),+ ))
        )
    )

    macro_rules! fail(
        () => (
            fail!(\"explicit failure\")
        );
        ($msg:expr) => (
            ::std::sys::FailWithCause::fail_with($msg, file!(), line!())
        );
        ($( $arg:expr ),+) => (
            ::std::sys::FailWithCause::fail_with(fmt!( $($arg),+ ), file!(), line!())
        )
    )

    macro_rules! assert(
        ($cond:expr) => {
            if !$cond {
                ::std::sys::FailWithCause::fail_with(
                    ~\"assertion failed: \" + stringify!($cond), file!(), line!())
            }
        };
        ($cond:expr, $msg:expr) => {
            if !$cond {
                ::std::sys::FailWithCause::fail_with($msg, file!(), line!())
            }
        };
        ($cond:expr, $( $arg:expr ),+) => {
            if !$cond {
                ::std::sys::FailWithCause::fail_with(fmt!( $($arg),+ ), file!(), line!())
            }
        }
    )

    macro_rules! assert_eq (
        ($given:expr , $expected:expr) => (
            {
                let given_val = $given;
                let expected_val = $expected;
                // check both directions of equality....
                if !((given_val == expected_val) && (expected_val == given_val)) {
                    fail!(\"left: %? does not equal right: %?\", given_val, expected_val);
                }
            }
        )
    )

    macro_rules! assert_approx_eq (
        ($given:expr , $expected:expr) => (
            {
                use std::cmp::ApproxEq;

                let given_val = $given;
                let expected_val = $expected;
                // check both directions of equality....
                if !(
                    given_val.approx_eq(&expected_val) &&
                    expected_val.approx_eq(&given_val)
                ) {
                    fail!(\"left: %? does not approximately equal right: %?\",
                          given_val, expected_val);
                }
            }
        );
        ($given:expr , $expected:expr , $epsilon:expr) => (
            {
                use std::cmp::ApproxEq;

                let given_val = $given;
                let expected_val = $expected;
                let epsilon_val = $epsilon;
                // check both directions of equality....
                if !(
                    given_val.approx_eq_eps(&expected_val, &epsilon_val) &&
                    expected_val.approx_eq_eps(&given_val, &epsilon_val)
                ) {
                    fail!(\"left: %? does not approximately equal right: %? with epsilon: %?\",
                          given_val, expected_val, epsilon_val);
                }
            }
        )
    )

    macro_rules! condition (

        { pub $c:ident: $in:ty -> $out:ty; } => {

            pub mod $c {
                fn key(_x: @::std::condition::Handler<$in,$out>) { }

                pub static cond :
                    ::std::condition::Condition<'static,$in,$out> =
                    ::std::condition::Condition {
                        name: stringify!($c),
                        key: key
                    };
            }
        };

        { $c:ident: $in:ty -> $out:ty; } => {

            // FIXME (#6009): remove mod's `pub` below once variant above lands.
            pub mod $c {
                fn key(_x: @::std::condition::Handler<$in,$out>) { }

                pub static cond :
                    ::std::condition::Condition<'static,$in,$out> =
                    ::std::condition::Condition {
                        name: stringify!($c),
                        key: key
                    };
            }
        }
    )

    //
    // A scheme-style conditional that helps to improve code clarity in some instances when
    // the `if`, `else if`, and `else` keywords obscure predicates undesirably.
    //
    // # Example
    //
    // ~~~
    // let clamped =
    //     if x > mx { mx }
    //     else if x < mn { mn }
    //     else { x };
    // ~~~
    //
    // Using `cond!`, the above could be written as:
    //
    // ~~~
    // let clamped = cond!(
    //     (x > mx) { mx }
    //     (x < mn) { mn }
    //     _        { x  }
    // );
    // ~~~
    //
    // The optional default case is denoted by `_`.
    //
    macro_rules! cond (
        ( $(($pred:expr) $body:block)+ _ $default:block ) => (
            $(if $pred $body else)+
            $default
        );
        // for if the default case was ommitted
        ( $(($pred:expr) $body:block)+ ) => (
            $(if $pred $body)else+
        );
    )
}";
}

pub fn expand_crate(parse_sess: @mut parse::ParseSess,
                    cfg: ast::crate_cfg, c: @crate) -> @crate {
    // adding *another* layer of indirection here so that the block
    // visitor can swap out one exts table for another for the duration
    // of the block.  The cleaner alternative would be to thread the
    // exts table through the fold, but that would require updating
    // every method/element of AstFoldFns in fold.rs.
    let extsbox = @mut syntax_expander_table();
    let afp = default_ast_fold();
    let cx = ExtCtxt::new(parse_sess, copy cfg);
    let f_pre = @AstFoldFns {
        fold_expr: |expr,span,recur|
            expand_expr(extsbox, cx, expr, span, recur, afp.fold_expr),
        fold_mod: |modd,recur|
            expand_mod_items(extsbox, cx, modd, recur, afp.fold_mod),
        fold_item: |item,recur|
            expand_item(extsbox, cx, item, recur, afp.fold_item),
        fold_stmt: |stmt,span,recur|
            expand_stmt(extsbox, cx, stmt, span, recur, afp.fold_stmt),
        fold_block: |blk,span,recur|
            expand_block(extsbox, cx, blk, span, recur, afp.fold_block),
        new_span: |a| new_span(cx, a),
        .. *afp};
    let f = make_fold(f_pre);
    // add a bunch of macros as though they were placed at the
    // head of the program (ick).
    let attrs = ~[
        spanned {
            span: codemap::dummy_sp(),
            node: attribute_ {
                style: attr_outer,
                value: @spanned {
                    node: meta_word(@~"macro_escape"),
                    span: codemap::dummy_sp(),
                },
                is_sugared_doc: false,
            }
        }
    ];

    let cm = match parse_item_from_source_str(~"<core-macros>",
                                              @core_macros(),
                                              copy cfg,
                                              attrs,
                                              parse_sess) {
        Some(item) => item,
        None => cx.bug("expected core macros to parse correctly")
    };
    // This is run for its side-effects on the expander env,
    // as it registers all the core macros as expanders.
    f.fold_item(cm);

    @f.fold_crate(&*c)
}

// given a function from idents to idents, produce
// an ast_fold that applies that function:
pub fn fun_to_ident_folder(f: @fn(ast::ident)->ast::ident) -> @ast_fold{
    let afp = default_ast_fold();
    let f_pre = @AstFoldFns{
        fold_ident : |id, _| f(id),
        .. *afp
    };
    make_fold(f_pre)
}

// update the ctxts in a path to get a rename node
pub fn new_ident_renamer(from: ast::ident,
                      to: ast::Name,
                      table: @mut SCTable) ->
    @fn(ast::ident)->ast::ident {
    |id : ast::ident|
    ast::ident{
        repr: id.repr,
        ctxt: new_rename(from,to,id.ctxt,table)
    }
}


// update the ctxts in a path to get a mark node
pub fn new_ident_marker(mark: uint,
                        table: @mut SCTable) ->
    @fn(ast::ident)->ast::ident {
    |id : ast::ident|
    ast::ident{
        repr: id.repr,
        ctxt: new_mark(mark,id.ctxt,table)
    }
}

// perform resolution (in the MTWT sense) on all of the
// idents in the tree. This is the final step in expansion.
pub fn new_ident_resolver(table: @mut SCTable) ->
    @fn(ast::ident)->ast::ident {
    |id : ast::ident|
    ast::ident {
        repr : resolve(id,table),
        ctxt : illegal_ctxt
    }
}


#[cfg(test)]
mod test {
    use super::*;
    use ast;
    use ast::{attribute_, attr_outer, meta_word, empty_ctxt};
    use ast_util::{new_sctable};
    use codemap;
    use codemap::spanned;
    use parse;
    use core::io;
    use core::option::{None, Some};
    use util::parser_testing::{string_to_item_and_sess};

    // make sure that fail! is present
    #[test] fn fail_exists_test () {
        let src = ~"fn main() { fail!(\"something appropriately gloomy\");}";
        let sess = parse::new_parse_sess(None);
        let crate_ast = parse::parse_crate_from_source_str(
            ~"<test>",
            @src,
            ~[],sess);
        expand_crate(sess,~[],crate_ast);
    }

    // these following tests are quite fragile, in that they don't test what
    // *kind* of failure occurs.

    // make sure that macros can leave scope
    #[should_fail]
    #[test] fn macros_cant_escape_fns_test () {
        let src = ~"fn bogus() {macro_rules! z (() => (3+4))}\
                    fn inty() -> int { z!() }";
        let sess = parse::new_parse_sess(None);
        let crate_ast = parse::parse_crate_from_source_str(
            ~"<test>",
            @src,
            ~[],sess);
        // should fail:
        expand_crate(sess,~[],crate_ast);
    }

    // make sure that macros can leave scope for modules
    #[should_fail]
    #[test] fn macros_cant_escape_mods_test () {
        let src = ~"mod foo {macro_rules! z (() => (3+4))}\
                    fn inty() -> int { z!() }";
        let sess = parse::new_parse_sess(None);
        let crate_ast = parse::parse_crate_from_source_str(
            ~"<test>",
            @src,
            ~[],sess);
        // should fail:
        expand_crate(sess,~[],crate_ast);
    }

    // macro_escape modules shouldn't cause macros to leave scope
    #[test] fn macros_can_escape_flattened_mods_test () {
        let src = ~"#[macro_escape] mod foo {macro_rules! z (() => (3+4))}\
                    fn inty() -> int { z!() }";
        let sess = parse::new_parse_sess(None);
        let crate_ast = parse::parse_crate_from_source_str(
            ~"<test>",
            @src,
            ~[], sess);
        // should fail:
        expand_crate(sess,~[],crate_ast);
    }

    #[test] fn core_macros_must_parse () {
        let src = ~"
  pub mod macros {
    macro_rules! ignore (($($x:tt)*) => (()))

    macro_rules! error ( ($( $arg:expr ),+) => (
        log(::core::error, fmt!( $($arg),+ )) ))
}";
        let sess = parse::new_parse_sess(None);
        let cfg = ~[];
        let item_ast = parse::parse_item_from_source_str(
            ~"<test>",
            @src,
            cfg,~[make_dummy_attr (@~"macro_escape")],sess);
        match item_ast {
            Some(_) => (), // success
            None => fail!("expected this to parse")
        }
    }

    #[test] fn test_contains_flatten (){
        let attr1 = make_dummy_attr (@~"foo");
        let attr2 = make_dummy_attr (@~"bar");
        let escape_attr = make_dummy_attr (@~"macro_escape");
        let attrs1 = ~[attr1, escape_attr, attr2];
        assert_eq!(contains_macro_escape (attrs1),true);
        let attrs2 = ~[attr1,attr2];
        assert_eq!(contains_macro_escape (attrs2),false);
    }

    // make a "meta_word" outer attribute with the given name
    fn make_dummy_attr(s: @~str) -> ast::attribute {
        spanned {
            span:codemap::dummy_sp(),
            node: attribute_ {
                style: attr_outer,
                value: @spanned {
                    node: meta_word(s),
                    span: codemap::dummy_sp(),
                },
                is_sugared_doc: false,
            }
        }
    }

    #[test]
    fn renaming () {
        let (maybe_item_ast,sess) = string_to_item_and_sess(@~"fn a() -> int { let b = 13; b} ");
        let item_ast = match maybe_item_ast {
            Some(x) => x,
            None => fail!("test case fail")
        };
        let table = @mut new_sctable();
        let a_name = 100; // enforced by testing_interner
        let a2_name = sess.interner.gensym("a2").repr;
        let renamer = new_ident_renamer(ast::ident{repr:a_name,ctxt:empty_ctxt},
                                        a2_name,table);
        let renamed_ast = fun_to_ident_folder(renamer).fold_item(item_ast).get();
        let resolver = new_ident_resolver(table);
        let resolved_ast = fun_to_ident_folder(resolver).fold_item(renamed_ast).get();
        io::print(fmt!("ast: %?\n",resolved_ast))
    }

}