expand.rs

// Copyright 2012-2014 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 ast::{Block, Crate, DeclKind, PatKind};
use ast::{Local, Ident, Mac_, Name};
use ast::{MacStmtStyle, Mrk, Stmt, StmtKind, ItemKind};
use ast::TokenTree;
use ast;
use ext::mtwt;
use ext::build::AstBuilder;
use attr;
use attr::{AttrMetaMethods, WithAttrs};
use codemap;
use codemap::{Span, Spanned, ExpnInfo, NameAndSpan, MacroBang, MacroAttribute};
use ext::base::*;
use feature_gate::{self, Features};
use fold;
use fold::*;
use util::move_map::MoveMap;
use parse;
use parse::token::{fresh_mark, fresh_name, intern};
use ptr::P;
use util::small_vector::SmallVector;
use visit;
use visit::Visitor;
use std_inject;

use std::collections::HashSet;


pub fn expand_expr(e: P<ast::Expr>, fld: &mut MacroExpander) -> P<ast::Expr> {
    let expr_span = e.span;
    return e.and_then(|ast::Expr {id, node, span, attrs}| match node {

        // expr_mac should really be expr_ext or something; it's the
        // entry-point for all syntax extensions.
        ast::ExprKind::Mac(mac) => {

            // Assert that we drop any macro attributes on the floor here
            drop(attrs);

            let expanded_expr = match expand_mac_invoc(mac, span,
                                                       |r| r.make_expr(),
                                                       mark_expr, fld) {
                Some(expr) => expr,
                None => {
                    return DummyResult::raw_expr(span);
                }
            };

            // Keep going, outside-in.
            let fully_expanded = fld.fold_expr(expanded_expr);
            let span = fld.new_span(span);
            fld.cx.bt_pop();

            fully_expanded.map(|e| ast::Expr {
                id: ast::DUMMY_NODE_ID,
                node: e.node,
                span: span,
                attrs: e.attrs,
            })
        }

        ast::ExprKind::InPlace(placer, value_expr) => {
            // Ensure feature-gate is enabled
            feature_gate::check_for_placement_in(
                fld.cx.ecfg.features,
                &fld.cx.parse_sess.span_diagnostic,
                expr_span);

            let placer = fld.fold_expr(placer);
            let value_expr = fld.fold_expr(value_expr);
            fld.cx.expr(span, ast::ExprKind::InPlace(placer, value_expr))
                .with_attrs(fold_thin_attrs(attrs, fld))
        }

        ast::ExprKind::While(cond, body, opt_ident) => {
            let cond = fld.fold_expr(cond);
            let (body, opt_ident) = expand_loop_block(body, opt_ident, fld);
            fld.cx.expr(span, ast::ExprKind::While(cond, body, opt_ident))
                .with_attrs(fold_thin_attrs(attrs, fld))
        }

        ast::ExprKind::WhileLet(pat, expr, body, opt_ident) => {
            let pat = fld.fold_pat(pat);
            let expr = fld.fold_expr(expr);

            // Hygienic renaming of the body.
            let ((body, opt_ident), mut rewritten_pats) =
                rename_in_scope(vec![pat],
                                fld,
                                (body, opt_ident),
                                |rename_fld, fld, (body, opt_ident)| {
                expand_loop_block(rename_fld.fold_block(body), opt_ident, fld)
            });
            assert!(rewritten_pats.len() == 1);

            let wl = ast::ExprKind::WhileLet(rewritten_pats.remove(0), expr, body, opt_ident);
            fld.cx.expr(span, wl).with_attrs(fold_thin_attrs(attrs, fld))
        }

        ast::ExprKind::Loop(loop_block, opt_ident) => {
            let (loop_block, opt_ident) = expand_loop_block(loop_block, opt_ident, fld);
            fld.cx.expr(span, ast::ExprKind::Loop(loop_block, opt_ident))
                .with_attrs(fold_thin_attrs(attrs, fld))
        }

        ast::ExprKind::ForLoop(pat, head, body, opt_ident) => {
            let pat = fld.fold_pat(pat);

            // Hygienic renaming of the for loop body (for loop binds its pattern).
            let ((body, opt_ident), mut rewritten_pats) =
                rename_in_scope(vec![pat],
                                fld,
                                (body, opt_ident),
                                |rename_fld, fld, (body, opt_ident)| {
                expand_loop_block(rename_fld.fold_block(body), opt_ident, fld)
            });
            assert!(rewritten_pats.len() == 1);

            let head = fld.fold_expr(head);
            let fl = ast::ExprKind::ForLoop(rewritten_pats.remove(0), head, body, opt_ident);
            fld.cx.expr(span, fl).with_attrs(fold_thin_attrs(attrs, fld))
        }

        ast::ExprKind::IfLet(pat, sub_expr, body, else_opt) => {
            let pat = fld.fold_pat(pat);

            // Hygienic renaming of the body.
            let (body, mut rewritten_pats) =
                rename_in_scope(vec![pat],
                                fld,
                                body,
                                |rename_fld, fld, body| {
                fld.fold_block(rename_fld.fold_block(body))
            });
            assert!(rewritten_pats.len() == 1);

            let else_opt = else_opt.map(|else_opt| fld.fold_expr(else_opt));
            let sub_expr = fld.fold_expr(sub_expr);
            let il = ast::ExprKind::IfLet(rewritten_pats.remove(0), sub_expr, body, else_opt);
            fld.cx.expr(span, il).with_attrs(fold_thin_attrs(attrs, fld))
        }

        ast::ExprKind::Closure(capture_clause, fn_decl, block) => {
            let (rewritten_fn_decl, rewritten_block)
                = expand_and_rename_fn_decl_and_block(fn_decl, block, fld);
            let new_node = ast::ExprKind::Closure(capture_clause,
                                            rewritten_fn_decl,
                                            rewritten_block);
            P(ast::Expr{id:id, node: new_node, span: fld.new_span(span),
                        attrs: fold_thin_attrs(attrs, fld)})
        }

        _ => {
            P(noop_fold_expr(ast::Expr {
                id: id,
                node: node,
                span: span,
                attrs: attrs
            }, fld))
        }
    });
}

/// Expand a (not-ident-style) macro invocation. Returns the result
/// of expansion and the mark which must be applied to the result.
/// Our current interface doesn't allow us to apply the mark to the
/// result until after calling make_expr, make_items, etc.
fn expand_mac_invoc<T, F, G>(mac: ast::Mac,
                             span: codemap::Span,
                             parse_thunk: F,
                             mark_thunk: G,
                             fld: &mut MacroExpander)
                             -> Option<T> where
    F: for<'a> FnOnce(Box<MacResult+'a>) -> Option<T>,
    G: FnOnce(T, Mrk) -> T,
{
    // it would almost certainly be cleaner to pass the whole
    // macro invocation in, rather than pulling it apart and
    // marking the tts and the ctxt separately. This also goes
    // for the other three macro invocation chunks of code
    // in this file.

    let Mac_ { path: pth, tts, .. } = mac.node;
    if pth.segments.len() > 1 {
        fld.cx.span_err(pth.span,
                        "expected macro name without module \
                        separators");
        // let compilation continue
        return None;
    }
    let extname = pth.segments[0].identifier.name;
    match fld.cx.syntax_env.find(extname) {
        None => {
            let mut err = fld.cx.struct_span_err(
                pth.span,
                &format!("macro undefined: '{}!'",
                        &extname));
            fld.cx.suggest_macro_name(&extname.as_str(), pth.span, &mut err);
            err.emit();

            // let compilation continue
            None
        }
        Some(rc) => match *rc {
            NormalTT(ref expandfun, exp_span, allow_internal_unstable) => {
                fld.cx.bt_push(ExpnInfo {
                        call_site: span,
                        callee: NameAndSpan {
                            format: MacroBang(extname),
                            span: exp_span,
                            allow_internal_unstable: allow_internal_unstable,
                        },
                    });
                let fm = fresh_mark();
                let marked_before = mark_tts(&tts[..], fm);

                // The span that we pass to the expanders we want to
                // be the root of the call stack. That's the most
                // relevant span and it's the actual invocation of
                // the macro.
                let mac_span = fld.cx.original_span();

                let opt_parsed = {
                    let expanded = expandfun.expand(fld.cx,
                                                    mac_span,
                                                    &marked_before[..]);
                    parse_thunk(expanded)
                };
                let parsed = match opt_parsed {
                    Some(e) => e,
                    None => {
                        fld.cx.span_err(
                            pth.span,
                            &format!("non-expression macro in expression position: {}",
                                    extname
                                    ));
                        return None;
                    }
                };
                Some(mark_thunk(parsed,fm))
            }
            _ => {
                fld.cx.span_err(
                    pth.span,
                    &format!("'{}' is not a tt-style macro",
                            extname));
                None
            }
        }
    }
}

/// Rename loop label and expand its loop body
///
/// The renaming procedure for loop is different in the sense that the loop
/// body is in a block enclosed by loop head so the renaming of loop label
/// must be propagated to the enclosed context.
fn expand_loop_block(loop_block: P<Block>,
                     opt_ident: Option<Ident>,
                     fld: &mut MacroExpander) -> (P<Block>, Option<Ident>) {
    match opt_ident {
        Some(label) => {
            let new_label = fresh_name(label);
            let rename = (label, new_label);

            // The rename *must not* be added to the pending list of current
            // syntax context otherwise an unrelated `break` or `continue` in
            // the same context will pick that up in the deferred renaming pass
            // and be renamed incorrectly.
            let mut rename_list = vec!(rename);
            let mut rename_fld = IdentRenamer{renames: &mut rename_list};
            let renamed_ident = rename_fld.fold_ident(label);

            // The rename *must* be added to the enclosed syntax context for
            // `break` or `continue` to pick up because by definition they are
            // in a block enclosed by loop head.
            fld.cx.syntax_env.push_frame();
            fld.cx.syntax_env.info().pending_renames.push(rename);
            let expanded_block = expand_block_elts(loop_block, fld);
            fld.cx.syntax_env.pop_frame();

            (expanded_block, Some(renamed_ident))
        }
        None => (fld.fold_block(loop_block), opt_ident)
    }
}

// eval $e with a new exts frame.
// must be a macro so that $e isn't evaluated too early.
macro_rules! with_exts_frame {
    ($extsboxexpr:expr,$macros_escape:expr,$e:expr) =>
    ({$extsboxexpr.push_frame();
      $extsboxexpr.info().macros_escape = $macros_escape;
      let result = $e;
      $extsboxexpr.pop_frame();
      result
     })
}

// When we enter a module, record it, for the sake of `module!`
pub fn expand_item(it: P<ast::Item>, fld: &mut MacroExpander)
                   -> SmallVector<P<ast::Item>> {
    let it = expand_item_multi_modifier(Annotatable::Item(it), fld);

    expand_annotatable(it, fld)
        .into_iter().map(|i| i.expect_item()).collect()
}

/// Expand item_kind
fn expand_item_kind(item: ast::ItemKind, fld: &mut MacroExpander) -> ast::ItemKind {
    match item {
        ast::ItemKind::Fn(decl, unsafety, constness, abi, generics, body) => {
            let (rewritten_fn_decl, rewritten_body)
                = expand_and_rename_fn_decl_and_block(decl, body, fld);
            let expanded_generics = fold::noop_fold_generics(generics,fld);
            ast::ItemKind::Fn(rewritten_fn_decl, unsafety, constness, abi,
                        expanded_generics, rewritten_body)
        }
        _ => noop_fold_item_kind(item, fld)
    }
}

// does this attribute list contain "macro_use" ?
fn contains_macro_use(fld: &mut MacroExpander, attrs: &[ast::Attribute]) -> bool {
    for attr in attrs {
        let mut is_use = attr.check_name("macro_use");
        if attr.check_name("macro_escape") {
            let mut err =
                fld.cx.struct_span_warn(attr.span,
                                        "macro_escape is a deprecated synonym for macro_use");
            is_use = true;
            if let ast::AttrStyle::Inner = attr.node.style {
                err.fileline_help(attr.span, "consider an outer attribute, \
                                              #[macro_use] mod ...").emit();
            } else {
                err.emit();
            }
        };

        if is_use {
            match attr.node.value.node {
                ast::MetaItemKind::Word(..) => (),
                _ => fld.cx.span_err(attr.span, "arguments to macro_use are not allowed here"),
            }
            return true;
        }
    }
    false
}

// Support for item-position macro invocations, exactly the same
// logic as for expression-position macro invocations.
pub fn expand_item_mac(it: P<ast::Item>,
                       fld: &mut MacroExpander) -> SmallVector<P<ast::Item>> {
    let (extname, path_span, tts, span, attrs, ident) = it.and_then(|it| match it.node {
        ItemKind::Mac(codemap::Spanned { node: Mac_ { path, tts, .. }, .. }) =>
            (path.segments[0].identifier.name, path.span, tts, it.span, it.attrs, it.ident),
        _ => fld.cx.span_bug(it.span, "invalid item macro invocation")
    });

    let fm = fresh_mark();
    let items = {
        let expanded = match fld.cx.syntax_env.find(extname) {
            None => {
                fld.cx.span_err(path_span,
                                &format!("macro undefined: '{}!'",
                                        extname));
                // let compilation continue
                return SmallVector::zero();
            }

            Some(rc) => match *rc {
                NormalTT(ref expander, tt_span, allow_internal_unstable) => {
                    if ident.name != parse::token::special_idents::invalid.name {
                        fld.cx
                            .span_err(path_span,
                                      &format!("macro {}! expects no ident argument, given '{}'",
                                               extname,
                                               ident));
                        return SmallVector::zero();
                    }
                    fld.cx.bt_push(ExpnInfo {
                        call_site: span,
                        callee: NameAndSpan {
                            format: MacroBang(extname),
                            span: tt_span,
                            allow_internal_unstable: allow_internal_unstable,
                        }
                    });
                    // mark before expansion:
                    let marked_before = mark_tts(&tts[..], fm);
                    expander.expand(fld.cx, span, &marked_before[..])
                }
                IdentTT(ref expander, tt_span, allow_internal_unstable) => {
                    if ident.name == parse::token::special_idents::invalid.name {
                        fld.cx.span_err(path_span,
                                        &format!("macro {}! expects an ident argument",
                                                extname));
                        return SmallVector::zero();
                    }
                    fld.cx.bt_push(ExpnInfo {
                        call_site: span,
                        callee: NameAndSpan {
                            format: MacroBang(extname),
                            span: tt_span,
                            allow_internal_unstable: allow_internal_unstable,
                        }
                    });
                    // mark before expansion:
                    let marked_tts = mark_tts(&tts[..], fm);
                    expander.expand(fld.cx, span, ident, marked_tts)
                }
                MacroRulesTT => {
                    if ident.name == parse::token::special_idents::invalid.name {
                        fld.cx.span_err(path_span, "macro_rules! expects an ident argument");
                        return SmallVector::zero();
                    }

                    fld.cx.bt_push(ExpnInfo {
                        call_site: span,
                        callee: NameAndSpan {
                            format: MacroBang(extname),
                            span: None,
                            // `macro_rules!` doesn't directly allow
                            // unstable (this is orthogonal to whether
                            // the macro it creates allows it)
                            allow_internal_unstable: false,
                        }
                    });
                    // DON'T mark before expansion.

                    let allow_internal_unstable = attr::contains_name(&attrs,
                                                                      "allow_internal_unstable");

                    // ensure any #[allow_internal_unstable]s are
                    // detected (including nested macro definitions
                    // etc.)
                    if allow_internal_unstable && !fld.cx.ecfg.enable_allow_internal_unstable() {
                        feature_gate::emit_feature_err(
                            &fld.cx.parse_sess.span_diagnostic,
                            "allow_internal_unstable",
                            span,
                            feature_gate::GateIssue::Language,
                            feature_gate::EXPLAIN_ALLOW_INTERNAL_UNSTABLE)
                    }

                    let export = attr::contains_name(&attrs, "macro_export");
                    let def = ast::MacroDef {
                        ident: ident,
                        attrs: attrs,
                        id: ast::DUMMY_NODE_ID,
                        span: span,
                        imported_from: None,
                        export: export,
                        use_locally: true,
                        allow_internal_unstable: allow_internal_unstable,
                        body: tts,
                    };
                    fld.cx.insert_macro(def);

                    // macro_rules! has a side effect but expands to nothing.
                    fld.cx.bt_pop();
                    return SmallVector::zero();
                }
                _ => {
                    fld.cx.span_err(span,
                                    &format!("{}! is not legal in item position",
                                            extname));
                    return SmallVector::zero();
                }
            }
        };

        expanded.make_items()
    };

    let items = match items {
        Some(items) => {
            items.into_iter()
                .map(|i| mark_item(i, fm))
                .flat_map(|i| fld.fold_item(i).into_iter())
                .collect()
        }
        None => {
            fld.cx.span_err(path_span,
                            &format!("non-item macro in item position: {}",
                                    extname));
            return SmallVector::zero();
        }
    };

    fld.cx.bt_pop();
    items
}

/// Expand a stmt
fn expand_stmt(stmt: Stmt, fld: &mut MacroExpander) -> SmallVector<Stmt> {
    let (mac, style, attrs) = match stmt.node {
        StmtKind::Mac(mac, style, attrs) => (mac, style, attrs),
        _ => return expand_non_macro_stmt(stmt, fld)
    };

    // Assert that we drop any macro attributes on the floor here
    drop(attrs);

    let maybe_new_items =
        expand_mac_invoc(mac.unwrap(), stmt.span,
                         |r| r.make_stmts(),
                         |stmts, mark| stmts.move_map(|m| mark_stmt(m, mark)),
                         fld);

    let mut fully_expanded = match maybe_new_items {
        Some(stmts) => {
            // Keep going, outside-in.
            let new_items = stmts.into_iter().flat_map(|s| {
                fld.fold_stmt(s).into_iter()
            }).collect();
            fld.cx.bt_pop();
            new_items
        }
        None => SmallVector::zero()
    };

    // If this is a macro invocation with a semicolon, then apply that
    // semicolon to the final statement produced by expansion.
    if style == MacStmtStyle::Semicolon {
        if let Some(stmt) = fully_expanded.pop() {
            let new_stmt = Spanned {
                node: match stmt.node {
                    StmtKind::Expr(e, stmt_id) => StmtKind::Semi(e, stmt_id),
                    _ => stmt.node /* might already have a semi */
                },
                span: stmt.span
            };
            fully_expanded.push(new_stmt);
        }
    }

    fully_expanded
}

// expand a non-macro stmt. this is essentially the fallthrough for
// expand_stmt, above.
fn expand_non_macro_stmt(Spanned {node, span: stmt_span}: Stmt, fld: &mut MacroExpander)
                         -> SmallVector<Stmt> {
    // is it a let?
    match node {
        StmtKind::Decl(decl, node_id) => decl.and_then(|Spanned {node: decl, span}| match decl {
            DeclKind::Local(local) => {
                // take it apart:
                let rewritten_local = local.map(|Local {id, pat, ty, init, span, attrs}| {
                    // expand the ty since TyKind::FixedLengthVec contains an Expr
                    // and thus may have a macro use
                    let expanded_ty = ty.map(|t| fld.fold_ty(t));
                    // expand the pat (it might contain macro uses):
                    let expanded_pat = fld.fold_pat(pat);
                    // find the PatIdents in the pattern:
                    // oh dear heaven... this is going to include the enum
                    // names, as well... but that should be okay, as long as
                    // the new names are gensyms for the old ones.
                    // generate fresh names, push them to a new pending list
                    let idents = pattern_bindings(&expanded_pat);
                    let mut new_pending_renames =
                        idents.iter().map(|ident| (*ident, fresh_name(*ident))).collect();
                    // rewrite the pattern using the new names (the old
                    // ones have already been applied):
                    let rewritten_pat = {
                        // nested binding to allow borrow to expire:
                        let mut rename_fld = IdentRenamer{renames: &mut new_pending_renames};
                        rename_fld.fold_pat(expanded_pat)
                    };
                    // add them to the existing pending renames:
                    fld.cx.syntax_env.info().pending_renames
                          .extend(new_pending_renames);
                    Local {
                        id: id,
                        ty: expanded_ty,
                        pat: rewritten_pat,
                        // also, don't forget to expand the init:
                        init: init.map(|e| fld.fold_expr(e)),
                        span: span,
                        attrs: fold::fold_thin_attrs(attrs, fld),
                    }
                });
                SmallVector::one(Spanned {
                    node: StmtKind::Decl(P(Spanned {
                            node: DeclKind::Local(rewritten_local),
                            span: span
                        }),
                        node_id),
                    span: stmt_span
                })
            }
            _ => {
                noop_fold_stmt(Spanned {
                    node: StmtKind::Decl(P(Spanned {
                            node: decl,
                            span: span
                        }),
                        node_id),
                    span: stmt_span
                }, fld)
            }
        }),
        _ => {
            noop_fold_stmt(Spanned {
                node: node,
                span: stmt_span
            }, fld)
        }
    }
}

// expand the arm of a 'match', renaming for macro hygiene
fn expand_arm(arm: ast::Arm, fld: &mut MacroExpander) -> ast::Arm {
    // expand pats... they might contain macro uses:
    let expanded_pats = arm.pats.move_map(|pat| fld.fold_pat(pat));
    if expanded_pats.is_empty() {
        panic!("encountered match arm with 0 patterns");
    }

    // apply renaming and then expansion to the guard and the body:
    let ((rewritten_guard, rewritten_body), rewritten_pats) =
        rename_in_scope(expanded_pats,
                        fld,
                        (arm.guard, arm.body),
                        |rename_fld, fld, (ag, ab)|{
        let rewritten_guard = ag.map(|g| fld.fold_expr(rename_fld.fold_expr(g)));
        let rewritten_body = fld.fold_expr(rename_fld.fold_expr(ab));
        (rewritten_guard, rewritten_body)
    });

    ast::Arm {
        attrs: fold::fold_attrs(arm.attrs, fld),
        pats: rewritten_pats,
        guard: rewritten_guard,
        body: rewritten_body,
    }
}

fn rename_in_scope<X, F>(pats: Vec<P<ast::Pat>>,
                         fld: &mut MacroExpander,
                         x: X,
                         f: F)
                         -> (X, Vec<P<ast::Pat>>)
    where F: Fn(&mut IdentRenamer, &mut MacroExpander, X) -> X
{
    // all of the pats must have the same set of bindings, so use the
    // first one to extract them and generate new names:
    let idents = pattern_bindings(&pats[0]);
    let new_renames = idents.into_iter().map(|id| (id, fresh_name(id))).collect();
    // apply the renaming, but only to the PatIdents:
    let mut rename_pats_fld = PatIdentRenamer{renames:&new_renames};
    let rewritten_pats = pats.move_map(|pat| rename_pats_fld.fold_pat(pat));

    let mut rename_fld = IdentRenamer{ renames:&new_renames };
    (f(&mut rename_fld, fld, x), rewritten_pats)
}

/// A visitor that extracts the PatKind::Ident (binding) paths
/// from a given thingy and puts them in a mutable
/// array
#[derive(Clone)]
struct PatIdentFinder {
    ident_accumulator: Vec<ast::Ident>
}

impl<'v> Visitor<'v> for PatIdentFinder {
    fn visit_pat(&mut self, pattern: &ast::Pat) {
        match *pattern {
            ast::Pat { id: _, node: PatKind::Ident(_, ref path1, ref inner), span: _ } => {
                self.ident_accumulator.push(path1.node);
                // visit optional subpattern of PatKind::Ident:
                if let Some(ref subpat) = *inner {
                    self.visit_pat(subpat)
                }
            }
            // use the default traversal for non-PatIdents
            _ => visit::walk_pat(self, pattern)
        }
    }
}

/// find the PatKind::Ident paths in a pattern
fn pattern_bindings(pat: &ast::Pat) -> Vec<ast::Ident> {
    let mut name_finder = PatIdentFinder{ident_accumulator:Vec::new()};
    name_finder.visit_pat(pat);
    name_finder.ident_accumulator
}

/// find the PatKind::Ident paths in a
fn fn_decl_arg_bindings(fn_decl: &ast::FnDecl) -> Vec<ast::Ident> {
    let mut pat_idents = PatIdentFinder{ident_accumulator:Vec::new()};
    for arg in &fn_decl.inputs {
        pat_idents.visit_pat(&arg.pat);
    }
    pat_idents.ident_accumulator
}

// expand a block. pushes a new exts_frame, then calls expand_block_elts
pub fn expand_block(blk: P<Block>, fld: &mut MacroExpander) -> P<Block> {
    // see note below about treatment of exts table
    with_exts_frame!(fld.cx.syntax_env,false,
                     expand_block_elts(blk, fld))
}

// expand the elements of a block.
pub fn expand_block_elts(b: P<Block>, fld: &mut MacroExpander) -> P<Block> {
    b.map(|Block {id, stmts, expr, rules, span}| {
        let new_stmts = stmts.into_iter().flat_map(|x| {
            // perform all pending renames
            let renamed_stmt = {
                let pending_renames = &mut fld.cx.syntax_env.info().pending_renames;
                let mut rename_fld = IdentRenamer{renames:pending_renames};
                rename_fld.fold_stmt(x).expect_one("rename_fold didn't return one value")
            };
            // expand macros in the statement
            fld.fold_stmt(renamed_stmt).into_iter()
        }).collect();
        let new_expr = expr.map(|x| {
            let expr = {
                let pending_renames = &mut fld.cx.syntax_env.info().pending_renames;
                let mut rename_fld = IdentRenamer{renames:pending_renames};
                rename_fld.fold_expr(x)
            };
            fld.fold_expr(expr)
        });
        Block {
            id: fld.new_id(id),
            stmts: new_stmts,
            expr: new_expr,
            rules: rules,
            span: span
        }
    })
}

fn expand_pat(p: P<ast::Pat>, fld: &mut MacroExpander) -> P<ast::Pat> {
    match p.node {
        PatKind::Mac(_) => {}
        _ => return noop_fold_pat(p, fld)
    }
    p.map(|ast::Pat {node, span, ..}| {
        let (pth, tts) = match node {
            PatKind::Mac(mac) => (mac.node.path, mac.node.tts),
            _ => unreachable!()
        };
        if pth.segments.len() > 1 {
            fld.cx.span_err(pth.span, "expected macro name without module separators");
            return DummyResult::raw_pat(span);
        }
        let extname = pth.segments[0].identifier.name;
        let marked_after = match fld.cx.syntax_env.find(extname) {
            None => {
                fld.cx.span_err(pth.span,
                                &format!("macro undefined: '{}!'",
                                        extname));
                // let compilation continue
                return DummyResult::raw_pat(span);
            }

            Some(rc) => match *rc {
                NormalTT(ref expander, tt_span, allow_internal_unstable) => {
                    fld.cx.bt_push(ExpnInfo {
                        call_site: span,
                        callee: NameAndSpan {
                            format: MacroBang(extname),
                            span: tt_span,
                            allow_internal_unstable: allow_internal_unstable,
                        }
                    });

                    let fm = fresh_mark();
                    let marked_before = mark_tts(&tts[..], fm);
                    let mac_span = fld.cx.original_span();
                    let pat = expander.expand(fld.cx,
                                              mac_span,
                                              &marked_before[..]).make_pat();
                    let expanded = match pat {
                        Some(e) => e,
                        None => {
                            fld.cx.span_err(
                                pth.span,
                                &format!(
                                    "non-pattern macro in pattern position: {}",
                                    extname
                                    )
                            );
                            return DummyResult::raw_pat(span);
                        }
                    };

                    // mark after:
                    mark_pat(expanded,fm)
                }
                _ => {
                    fld.cx.span_err(span,
                                    &format!("{}! is not legal in pattern position",
                                            extname));
                    return DummyResult::raw_pat(span);
                }
            }
        };

        let fully_expanded =
            fld.fold_pat(marked_after).node.clone();
        fld.cx.bt_pop();

        ast::Pat {
            id: ast::DUMMY_NODE_ID,
            node: fully_expanded,
            span: span
        }
    })
}

/// A tree-folder that applies every rename in its (mutable) list
/// to every identifier, including both bindings and varrefs
/// (and lots of things that will turn out to be neither)
pub struct IdentRenamer<'a> {
    renames: &'a mtwt::RenameList,
}

impl<'a> Folder for IdentRenamer<'a> {
    fn fold_ident(&mut self, id: Ident) -> Ident {
        Ident::new(id.name, mtwt::apply_renames(self.renames, id.ctxt))
    }
    fn fold_mac(&mut self, mac: ast::Mac) -> ast::Mac {
        fold::noop_fold_mac(mac, self)
    }
}

/// A tree-folder that applies every rename in its list to
/// the idents that are in PatKind::Ident patterns. This is more narrowly
/// focused than IdentRenamer, and is needed for FnDecl,
/// where we want to rename the args but not the fn name or the generics etc.
pub struct PatIdentRenamer<'a> {
    renames: &'a mtwt::RenameList,
}

impl<'a> Folder for PatIdentRenamer<'a> {
    fn fold_pat(&mut self, pat: P<ast::Pat>) -> P<ast::Pat> {
        match pat.node {
            PatKind::Ident(..) => {},
            _ => return noop_fold_pat(pat, self)
        }

        pat.map(|ast::Pat {id, node, span}| match node {
            PatKind::Ident(binding_mode, Spanned{span: sp, node: ident}, sub) => {
                let new_ident = Ident::new(ident.name,
                                           mtwt::apply_renames(self.renames, ident.ctxt));
                let new_node =
                    PatKind::Ident(binding_mode,
                                  Spanned{span: self.new_span(sp), node: new_ident},
                                  sub.map(|p| self.fold_pat(p)));
                ast::Pat {
                    id: id,
                    node: new_node,
                    span: self.new_span(span)
                }
            },
            _ => unreachable!()
        })
    }
    fn fold_mac(&mut self, mac: ast::Mac) -> ast::Mac {
        fold::noop_fold_mac(mac, self)
    }
}

fn expand_annotatable(a: Annotatable,
                      fld: &mut MacroExpander)
                      -> SmallVector<Annotatable> {
    let a = expand_item_multi_modifier(a, fld);

    let mut decorator_items = SmallVector::zero();
    let mut new_attrs = Vec::new();
    expand_decorators(a.clone(), fld, &mut decorator_items, &mut new_attrs);

    let mut new_items: SmallVector<Annotatable> = match a {
        Annotatable::Item(it) => match it.node {
            ast::ItemKind::Mac(..) => {
                expand_item_mac(it, fld).into_iter().map(|i| Annotatable::Item(i)).collect()
            }
            ast::ItemKind::Mod(_) | ast::ItemKind::ForeignMod(_) => {
                let valid_ident =
                    it.ident.name != parse::token::special_idents::invalid.name;

                if valid_ident {
                    fld.cx.mod_push(it.ident);
                }
                let macro_use = contains_macro_use(fld, &new_attrs[..]);
                let result = with_exts_frame!(fld.cx.syntax_env,
                                              macro_use,
                                              noop_fold_item(it, fld));
                if valid_ident {
                    fld.cx.mod_pop();
                }
                result.into_iter().map(|i| Annotatable::Item(i)).collect()
            },
            _ => {
                let it = P(ast::Item {
                    attrs: new_attrs,
                    ..(*it).clone()
                });
                noop_fold_item(it, fld).into_iter().map(|i| Annotatable::Item(i)).collect()
            }
        },

        Annotatable::TraitItem(it) => match it.node {
            ast::TraitItemKind::Method(_, Some(_)) => {
                let ti = it.unwrap();
                SmallVector::one(ast::TraitItem {
                    id: ti.id,
                    ident: ti.ident,
                    attrs: ti.attrs,
                    node: match ti.node  {
                        ast::TraitItemKind::Method(sig, Some(body)) => {
                            let (sig, body) = expand_and_rename_method(sig, body, fld);
                            ast::TraitItemKind::Method(sig, Some(body))
                        }
                        _ => unreachable!()
                    },
                    span: fld.new_span(ti.span)
                })
            }
            _ => fold::noop_fold_trait_item(it.unwrap(), fld)
        }.into_iter().map(|ti| Annotatable::TraitItem(P(ti))).collect(),

        Annotatable::ImplItem(ii) => {
            expand_impl_item(ii.unwrap(), fld).into_iter().
                map(|ii| Annotatable::ImplItem(P(ii))).collect()
        }
    };

    new_items.push_all(decorator_items);
    new_items
}

// Partition a set of attributes into one kind of attribute, and other kinds.
macro_rules! partition {
    ($fn_name: ident, $variant: ident) => {
        #[allow(deprecated)] // The `allow` is needed because the `Modifier` variant might be used.
        fn $fn_name(attrs: &[ast::Attribute],
                    fld: &MacroExpander)
                     -> (Vec<ast::Attribute>, Vec<ast::Attribute>) {
            attrs.iter().cloned().partition(|attr| {
                match fld.cx.syntax_env.find(intern(&attr.name())) {
                    Some(rc) => match *rc {
                        $variant(..) => true,
                        _ => false
                    },
                    _ => false
                }
            })
        }
    }
}

partition!(multi_modifiers, MultiModifier);


fn expand_decorators(a: Annotatable,
                     fld: &mut MacroExpander,
                     decorator_items: &mut SmallVector<Annotatable>,
                     new_attrs: &mut Vec<ast::Attribute>)
{
    for attr in a.attrs() {
        let mname = intern(&attr.name());
        match fld.cx.syntax_env.find(mname) {
            Some(rc) => match *rc {
                MultiDecorator(ref dec) => {
                    attr::mark_used(&attr);

                    fld.cx.bt_push(ExpnInfo {
                        call_site: attr.span,
                        callee: NameAndSpan {
                            format: MacroAttribute(mname),
                            span: Some(attr.span),
                            // attributes can do whatever they like,
                            // for now.
                            allow_internal_unstable: true,
                        }
                    });

                    // we'd ideally decorator_items.push_all(expand_annotatable(ann, fld)),
                    // but that double-mut-borrows fld
                    let mut items: SmallVector<Annotatable> = SmallVector::zero();
                    dec.expand(fld.cx,
                               attr.span,
                               &attr.node.value,
                               &a,
                               &mut |ann| items.push(ann));
                    decorator_items.extend(items.into_iter()
                        .flat_map(|ann| expand_annotatable(ann, fld).into_iter()));

                    fld.cx.bt_pop();
                }
                _ => new_attrs.push((*attr).clone()),
            },
            _ => new_attrs.push((*attr).clone()),
        }
    }
}

fn expand_item_multi_modifier(mut it: Annotatable,
                              fld: &mut MacroExpander)
                              -> Annotatable {
    let (modifiers, other_attrs) = multi_modifiers(it.attrs(), fld);

    // Update the attrs, leave everything else alone. Is this mutation really a good idea?
    it = it.fold_attrs(other_attrs);

    if modifiers.is_empty() {
        return it
    }

    for attr in &modifiers {
        let mname = intern(&attr.name());

        match fld.cx.syntax_env.find(mname) {
            Some(rc) => match *rc {
                MultiModifier(ref mac) => {
                    attr::mark_used(attr);
                    fld.cx.bt_push(ExpnInfo {
                        call_site: attr.span,
                        callee: NameAndSpan {
                            format: MacroAttribute(mname),
                            span: Some(attr.span),
                            // attributes can do whatever they like,
                            // for now
                            allow_internal_unstable: true,
                        }
                    });
                    it = mac.expand(fld.cx, attr.span, &attr.node.value, it);
                    fld.cx.bt_pop();
                }
                _ => unreachable!()
            },
            _ => unreachable!()
        }
    }

    // Expansion may have added new ItemKind::Modifiers.
    expand_item_multi_modifier(it, fld)
}

fn expand_impl_item(ii: ast::ImplItem, fld: &mut MacroExpander)
                 -> SmallVector<ast::ImplItem> {
    match ii.node {
        ast::ImplItemKind::Method(..) => SmallVector::one(ast::ImplItem {
            id: ii.id,
            ident: ii.ident,
            attrs: ii.attrs,
            vis: ii.vis,
            node: match ii.node  {
                ast::ImplItemKind::Method(sig, body) => {
                    let (sig, body) = expand_and_rename_method(sig, body, fld);
                    ast::ImplItemKind::Method(sig, body)
                }
                _ => unreachable!()
            },
            span: fld.new_span(ii.span)
        }),
        ast::ImplItemKind::Macro(_) => {
            let (span, mac) = match ii.node {
                ast::ImplItemKind::Macro(mac) => (ii.span, mac),
                _ => unreachable!()
            };
            let maybe_new_items =
                expand_mac_invoc(mac, span,
                                 |r| r.make_impl_items(),
                                 |meths, mark| meths.move_map(|m| mark_impl_item(m, mark)),
                                 fld);

            match maybe_new_items {
                Some(impl_items) => {
                    // expand again if necessary
                    let new_items = impl_items.into_iter().flat_map(|ii| {
                        expand_impl_item(ii, fld).into_iter()
                    }).collect();
                    fld.cx.bt_pop();
                    new_items
                }
                None => SmallVector::zero()
            }
        }
        _ => fold::noop_fold_impl_item(ii, fld)
    }
}

/// Given a fn_decl and a block and a MacroExpander, expand the fn_decl, then use the
/// PatIdents in its arguments to perform renaming in the FnDecl and
/// the block, returning both the new FnDecl and the new Block.
fn expand_and_rename_fn_decl_and_block(fn_decl: P<ast::FnDecl>, block: P<ast::Block>,
                                       fld: &mut MacroExpander)
                                       -> (P<ast::FnDecl>, P<ast::Block>) {
    let expanded_decl = fld.fold_fn_decl(fn_decl);
    let idents = fn_decl_arg_bindings(&expanded_decl);
    let renames =
        idents.iter().map(|id| (*id,fresh_name(*id))).collect();
    // first, a renamer for the PatIdents, for the fn_decl:
    let mut rename_pat_fld = PatIdentRenamer{renames: &renames};
    let rewritten_fn_decl = rename_pat_fld.fold_fn_decl(expanded_decl);
    // now, a renamer for *all* idents, for the body:
    let mut rename_fld = IdentRenamer{renames: &renames};
    let rewritten_body = fld.fold_block(rename_fld.fold_block(block));
    (rewritten_fn_decl,rewritten_body)
}

fn expand_and_rename_method(sig: ast::MethodSig, body: P<ast::Block>,
                            fld: &mut MacroExpander)
                            -> (ast::MethodSig, P<ast::Block>) {
    let (rewritten_fn_decl, rewritten_body)
        = expand_and_rename_fn_decl_and_block(sig.decl, body, fld);
    (ast::MethodSig {
        generics: fld.fold_generics(sig.generics),
        abi: sig.abi,
        explicit_self: fld.fold_explicit_self(sig.explicit_self),
        unsafety: sig.unsafety,
        constness: sig.constness,
        decl: rewritten_fn_decl
    }, rewritten_body)
}

pub fn expand_type(t: P<ast::Ty>, fld: &mut MacroExpander) -> P<ast::Ty> {
    let t = match t.node.clone() {
        ast::TyKind::Mac(mac) => {
            if fld.cx.ecfg.features.unwrap().type_macros {
                let expanded_ty = match expand_mac_invoc(mac, t.span,
                                                         |r| r.make_ty(),
                                                         mark_ty,
                                                         fld) {
                    Some(ty) => ty,
                    None => {
                        return DummyResult::raw_ty(t.span);
                    }
                };

                // Keep going, outside-in.
                let fully_expanded = fld.fold_ty(expanded_ty);
                fld.cx.bt_pop();

                fully_expanded.map(|t| ast::Ty {
                    id: ast::DUMMY_NODE_ID,
                    node: t.node,
                    span: t.span,
                    })
            } else {
                feature_gate::emit_feature_err(
                    &fld.cx.parse_sess.span_diagnostic,
                    "type_macros",
                    t.span,
                    feature_gate::GateIssue::Language,
                    "type macros are experimental");

                DummyResult::raw_ty(t.span)
            }
        }
        _ => t
    };

    fold::noop_fold_ty(t, fld)
}

/// A tree-folder that performs macro expansion
pub struct MacroExpander<'a, 'b:'a> {
    pub cx: &'a mut ExtCtxt<'b>,
}

impl<'a, 'b> MacroExpander<'a, 'b> {
    pub fn new(cx: &'a mut ExtCtxt<'b>) -> MacroExpander<'a, 'b> {
        MacroExpander { cx: cx }
    }
}

impl<'a, 'b> Folder for MacroExpander<'a, 'b> {
    fn fold_expr(&mut self, expr: P<ast::Expr>) -> P<ast::Expr> {
        expand_expr(expr, self)
    }

    fn fold_pat(&mut self, pat: P<ast::Pat>) -> P<ast::Pat> {
        expand_pat(pat, self)
    }

    fn fold_item(&mut self, item: P<ast::Item>) -> SmallVector<P<ast::Item>> {
        expand_item(item, self)
    }

    fn fold_item_kind(&mut self, item: ast::ItemKind) -> ast::ItemKind {
        expand_item_kind(item, self)
    }

    fn fold_stmt(&mut self, stmt: ast::Stmt) -> SmallVector<ast::Stmt> {
        expand_stmt(stmt, self)
    }

    fn fold_block(&mut self, block: P<Block>) -> P<Block> {
        expand_block(block, self)
    }

    fn fold_arm(&mut self, arm: ast::Arm) -> ast::Arm {
        expand_arm(arm, self)
    }

    fn fold_trait_item(&mut self, i: ast::TraitItem) -> SmallVector<ast::TraitItem> {
        expand_annotatable(Annotatable::TraitItem(P(i)), self)
            .into_iter().map(|i| i.expect_trait_item()).collect()
    }

    fn fold_impl_item(&mut self, i: ast::ImplItem) -> SmallVector<ast::ImplItem> {
        expand_annotatable(Annotatable::ImplItem(P(i)), self)
            .into_iter().map(|i| i.expect_impl_item()).collect()
    }

    fn fold_ty(&mut self, ty: P<ast::Ty>) -> P<ast::Ty> {
        expand_type(ty, self)
    }

    fn new_span(&mut self, span: Span) -> Span {
        new_span(self.cx, span)
    }
}

fn new_span(cx: &ExtCtxt, sp: Span) -> Span {
    /* this discards information in the case of macro-defining macros */
    Span {
        lo: sp.lo,
        hi: sp.hi,
        expn_id: cx.backtrace(),
    }
}

pub struct ExpansionConfig<'feat> {
    pub crate_name: String,
    pub features: Option<&'feat Features>,
    pub recursion_limit: usize,
    pub trace_mac: bool,
}

macro_rules! feature_tests {
    ($( fn $getter:ident = $field:ident, )*) => {
        $(
            pub fn $getter(&self) -> bool {
                match self.features {
                    Some(&Features { $field: true, .. }) => true,
                    _ => false,
                }
            }
        )*
    }
}

impl<'feat> ExpansionConfig<'feat> {
    pub fn default(crate_name: String) -> ExpansionConfig<'static> {
        ExpansionConfig {
            crate_name: crate_name,
            features: None,
            recursion_limit: 64,
            trace_mac: false,
        }
    }

    feature_tests! {
        fn enable_quotes = allow_quote,
        fn enable_asm = allow_asm,
        fn enable_log_syntax = allow_log_syntax,
        fn enable_concat_idents = allow_concat_idents,
        fn enable_trace_macros = allow_trace_macros,
        fn enable_allow_internal_unstable = allow_internal_unstable,
        fn enable_custom_derive = allow_custom_derive,
        fn enable_pushpop_unsafe = allow_pushpop_unsafe,
    }
}

pub fn expand_crate(mut cx: ExtCtxt,
                    // these are the macros being imported to this crate:
                    imported_macros: Vec<ast::MacroDef>,
                    user_exts: Vec<NamedSyntaxExtension>,
                    c: Crate) -> (Crate, HashSet<Name>) {
    if std_inject::no_core(&c) {
        cx.crate_root = None;
    } else if std_inject::no_std(&c) {
        cx.crate_root = Some("core");
    } else {
        cx.crate_root = Some("std");
    }
    let ret = {
        let mut expander = MacroExpander::new(&mut cx);

        for def in imported_macros {
            expander.cx.insert_macro(def);
        }

        for (name, extension) in user_exts {
            expander.cx.syntax_env.insert(name, extension);
        }

        let err_count = cx.parse_sess.span_diagnostic.err_count();
        let mut ret = expander.fold_crate(c);
        ret.exported_macros = expander.cx.exported_macros.clone();

        if cx.parse_sess.span_diagnostic.err_count() > err_count {
            cx.parse_sess.span_diagnostic.abort_if_errors();
        }

        ret
    };
    return (ret, cx.syntax_env.names);
}

// HYGIENIC CONTEXT EXTENSION:
// all of these functions are for walking over
// ASTs and making some change to the context of every
// element that has one. a CtxtFn is a trait-ified
// version of a closure in (SyntaxContext -> SyntaxContext).
// the ones defined here include:
// Marker - add a mark to a context

// A Marker adds the given mark to the syntax context
struct Marker { mark: Mrk }

impl Folder for Marker {
    fn fold_ident(&mut self, id: Ident) -> Ident {
        ast::Ident::new(id.name, mtwt::apply_mark(self.mark, id.ctxt))
    }
    fn fold_mac(&mut self, Spanned {node, span}: ast::Mac) -> ast::Mac {
        Spanned {
            node: Mac_ {
                path: self.fold_path(node.path),
                tts: self.fold_tts(&node.tts),
                ctxt: mtwt::apply_mark(self.mark, node.ctxt),
            },
            span: span,
        }
    }
}

// apply a given mark to the given token trees. Used prior to expansion of a macro.
fn mark_tts(tts: &[TokenTree], m: Mrk) -> Vec<TokenTree> {
    noop_fold_tts(tts, &mut Marker{mark:m})
}

// apply a given mark to the given expr. Used following the expansion of a macro.
fn mark_expr(expr: P<ast::Expr>, m: Mrk) -> P<ast::Expr> {
    Marker{mark:m}.fold_expr(expr)
}

// apply a given mark to the given pattern. Used following the expansion of a macro.
fn mark_pat(pat: P<ast::Pat>, m: Mrk) -> P<ast::Pat> {
    Marker{mark:m}.fold_pat(pat)
}

// apply a given mark to the given stmt. Used following the expansion of a macro.
fn mark_stmt(stmt: ast::Stmt, m: Mrk) -> ast::Stmt {
    Marker{mark:m}.fold_stmt(stmt)
        .expect_one("marking a stmt didn't return exactly one stmt")
}

// apply a given mark to the given item. Used following the expansion of a macro.
fn mark_item(expr: P<ast::Item>, m: Mrk) -> P<ast::Item> {
    Marker{mark:m}.fold_item(expr)
        .expect_one("marking an item didn't return exactly one item")
}

// apply a given mark to the given item. Used following the expansion of a macro.
fn mark_impl_item(ii: ast::ImplItem, m: Mrk) -> ast::ImplItem {
    Marker{mark:m}.fold_impl_item(ii)
        .expect_one("marking an impl item didn't return exactly one impl item")
}

fn mark_ty(ty: P<ast::Ty>, m: Mrk) -> P<ast::Ty> {
    Marker { mark: m }.fold_ty(ty)
}

/// Check that there are no macro invocations left in the AST:
pub fn check_for_macros(sess: &parse::ParseSess, krate: &ast::Crate) {
    visit::walk_crate(&mut MacroExterminator{sess:sess}, krate);
}

/// A visitor that ensures that no macro invocations remain in an AST.
struct MacroExterminator<'a>{
    sess: &'a parse::ParseSess
}

impl<'a, 'v> Visitor<'v> for MacroExterminator<'a> {
    fn visit_mac(&mut self, mac: &ast::Mac) {
        self.sess.span_diagnostic.span_bug(mac.span,
                                           "macro exterminator: expected AST \
                                           with no macro invocations");
    }
}


#[cfg(test)]
mod tests {
    use super::{pattern_bindings, expand_crate};
    use super::{PatIdentFinder, IdentRenamer, PatIdentRenamer, ExpansionConfig};
    use ast;
    use ast::Name;
    use codemap;
    use ext::base::ExtCtxt;
    use ext::mtwt;
    use fold::Folder;
    use parse;
    use parse::token;
    use util::parser_testing::{string_to_parser};
    use util::parser_testing::{string_to_pat, string_to_crate, strs_to_idents};
    use visit;
    use visit::Visitor;

    // a visitor that extracts the paths
    // from a given thingy and puts them in a mutable
    // array (passed in to the traversal)
    #[derive(Clone)]
    struct PathExprFinderContext {
        path_accumulator: Vec<ast::Path> ,
    }

    impl<'v> Visitor<'v> for PathExprFinderContext {
        fn visit_expr(&mut self, expr: &ast::Expr) {
            if let ast::ExprKind::Path(None, ref p) = expr.node {
                self.path_accumulator.push(p.clone());
            }
            visit::walk_expr(self, expr);
        }
    }

    // find the variable references in a crate
    fn crate_varrefs(the_crate : &ast::Crate) -> Vec<ast::Path> {
        let mut path_finder = PathExprFinderContext{path_accumulator:Vec::new()};
        visit::walk_crate(&mut path_finder, the_crate);
        path_finder.path_accumulator
    }

    /// A Visitor that extracts the identifiers from a thingy.
    // as a side note, I'm starting to want to abstract over these....
    struct IdentFinder {
        ident_accumulator: Vec<ast::Ident>
    }

    impl<'v> Visitor<'v> for IdentFinder {
        fn visit_ident(&mut self, _: codemap::Span, id: ast::Ident){
            self.ident_accumulator.push(id);
        }
    }

    /// Find the idents in a crate
    fn crate_idents(the_crate: &ast::Crate) -> Vec<ast::Ident> {
        let mut ident_finder = IdentFinder{ident_accumulator: Vec::new()};
        visit::walk_crate(&mut ident_finder, the_crate);
        ident_finder.ident_accumulator
    }

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

    fn test_ecfg() -> ExpansionConfig<'static> {
        ExpansionConfig::default("test".to_string())
    }

    // make sure that macros can't escape fns
    #[should_panic]
    #[test] fn macros_cant_escape_fns_test () {
        let src = "fn bogus() {macro_rules! z (() => (3+4));}\
                   fn inty() -> i32 { z!() }".to_string();
        let sess = parse::ParseSess::new();
        let crate_ast = parse::parse_crate_from_source_str(
            "<test>".to_string(),
            src,
            Vec::new(), &sess);
        // should fail:
        let mut gated_cfgs = vec![];
        let ecx = ExtCtxt::new(&sess, vec![], test_ecfg(), &mut gated_cfgs);
        expand_crate(ecx, vec![], vec![], crate_ast);
    }

    // make sure that macros can't escape modules
    #[should_panic]
    #[test] fn macros_cant_escape_mods_test () {
        let src = "mod foo {macro_rules! z (() => (3+4));}\
                   fn inty() -> i32 { z!() }".to_string();
        let sess = parse::ParseSess::new();
        let crate_ast = parse::parse_crate_from_source_str(
            "<test>".to_string(),
            src,
            Vec::new(), &sess);
        let mut gated_cfgs = vec![];
        let ecx = ExtCtxt::new(&sess, vec![], test_ecfg(), &mut gated_cfgs);
        expand_crate(ecx, vec![], vec![], crate_ast);
    }

    // macro_use modules should allow macros to escape
    #[test] fn macros_can_escape_flattened_mods_test () {
        let src = "#[macro_use] mod foo {macro_rules! z (() => (3+4));}\
                   fn inty() -> i32 { z!() }".to_string();
        let sess = parse::ParseSess::new();
        let crate_ast = parse::parse_crate_from_source_str(
            "<test>".to_string(),
            src,
            Vec::new(), &sess);
        let mut gated_cfgs = vec![];
        let ecx = ExtCtxt::new(&sess, vec![], test_ecfg(), &mut gated_cfgs);
        expand_crate(ecx, vec![], vec![], crate_ast);
    }

    fn expand_crate_str(crate_str: String) -> ast::Crate {
        let ps = parse::ParseSess::new();
        let crate_ast = panictry!(string_to_parser(&ps, crate_str).parse_crate_mod());
        // the cfg argument actually does matter, here...
        let mut gated_cfgs = vec![];
        let ecx = ExtCtxt::new(&ps, vec![], test_ecfg(), &mut gated_cfgs);
        expand_crate(ecx, vec![], vec![], crate_ast).0
    }

    // find the pat_ident paths in a crate
    fn crate_bindings(the_crate : &ast::Crate) -> Vec<ast::Ident> {
        let mut name_finder = PatIdentFinder{ident_accumulator:Vec::new()};
        visit::walk_crate(&mut name_finder, the_crate);
        name_finder.ident_accumulator
    }

    #[test] fn macro_tokens_should_match(){
        expand_crate_str(
            "macro_rules! m((a)=>(13)) ;fn main(){m!(a);}".to_string());
    }

    // should be able to use a bound identifier as a literal in a macro definition:
    #[test] fn self_macro_parsing(){
        expand_crate_str(
            "macro_rules! foo ((zz) => (287;));
            fn f(zz: i32) {foo!(zz);}".to_string()
            );
    }

    // renaming tests expand a crate and then check that the bindings match
    // the right varrefs. The specification of the test case includes the
    // text of the crate, and also an array of arrays.  Each element in the
    // outer array corresponds to a binding in the traversal of the AST
    // induced by visit.  Each of these arrays contains a list of indexes,
    // interpreted as the varrefs in the varref traversal that this binding
    // should match.  So, for instance, in a program with two bindings and
    // three varrefs, the array [[1, 2], [0]] would indicate that the first
    // binding should match the second two varrefs, and the second binding
    // should match the first varref.
    //
    // Put differently; this is a sparse representation of a boolean matrix
    // indicating which bindings capture which identifiers.
    //
    // Note also that this matrix is dependent on the implicit ordering of
    // the bindings and the varrefs discovered by the name-finder and the path-finder.
    //
    // The comparisons are done post-mtwt-resolve, so we're comparing renamed
    // names; differences in marks don't matter any more.
    //
    // oog... I also want tests that check "bound-identifier-=?". That is,
    // not just "do these have the same name", but "do they have the same
    // name *and* the same marks"? Understanding this is really pretty painful.
    // in principle, you might want to control this boolean on a per-varref basis,
    // but that would make things even harder to understand, and might not be
    // necessary for thorough testing.
    type RenamingTest = (&'static str, Vec<Vec<usize>>, bool);

    #[test]
    fn automatic_renaming () {
        let tests: Vec<RenamingTest> =
            vec!(// b & c should get new names throughout, in the expr too:
                ("fn a() -> i32 { let b = 13; let c = b; b+c }",
                 vec!(vec!(0,1),vec!(2)), false),
                // both x's should be renamed (how is this causing a bug?)
                ("fn main () {let x: i32 = 13;x;}",
                 vec!(vec!(0)), false),
                // the use of b after the + should be renamed, the other one not:
                ("macro_rules! f (($x:ident) => (b + $x)); fn a() -> i32 { let b = 13; f!(b)}",
                 vec!(vec!(1)), false),
                // the b before the plus should not be renamed (requires marks)
                ("macro_rules! f (($x:ident) => ({let b=9; ($x + b)})); fn a() -> i32 { f!(b)}",
                 vec!(vec!(1)), false),
                // the marks going in and out of letty should cancel, allowing that $x to
                // capture the one following the semicolon.
                // this was an awesome test case, and caught a *lot* of bugs.
                ("macro_rules! letty(($x:ident) => (let $x = 15;));
                  macro_rules! user(($x:ident) => ({letty!($x); $x}));
                  fn main() -> i32 {user!(z)}",
                 vec!(vec!(0)), false)
                );
        for (idx,s) in tests.iter().enumerate() {
            run_renaming_test(s,idx);
        }
    }

    // no longer a fixme #8062: this test exposes a *potential* bug; our system does
    // not behave exactly like MTWT, but a conversation with Matthew Flatt
    // suggests that this can only occur in the presence of local-expand, which
    // we have no plans to support. ... unless it's needed for item hygiene....
    #[ignore]
    #[test]
    fn issue_8062(){
        run_renaming_test(
            &("fn main() {let hrcoo = 19; macro_rules! getx(()=>(hrcoo)); getx!();}",
              vec!(vec!(0)), true), 0)
    }

    // FIXME #6994:
    // the z flows into and out of two macros (g & f) along one path, and one
    // (just g) along the other, so the result of the whole thing should
    // be "let z_123 = 3; z_123"
    #[ignore]
    #[test]
    fn issue_6994(){
        run_renaming_test(
            &("macro_rules! g (($x:ident) =>
              ({macro_rules! f(($y:ident)=>({let $y=3;$x}));f!($x)}));
              fn a(){g!(z)}",
              vec!(vec!(0)),false),
            0)
    }

    // match variable hygiene. Should expand into
    // fn z() {match 8 {x_1 => {match 9 {x_2 | x_2 if x_2 == x_1 => x_2 + x_1}}}}
    #[test]
    fn issue_9384(){
        run_renaming_test(
            &("macro_rules! bad_macro (($ex:expr) => ({match 9 {x | x if x == $ex => x + $ex}}));
              fn z() {match 8 {x => bad_macro!(x)}}",
              // NB: the third "binding" is the repeat of the second one.
              vec!(vec!(1,3),vec!(0,2),vec!(0,2)),
              true),
            0)
    }

    // interpolated nodes weren't getting labeled.
    // should expand into
    // fn main(){let g1_1 = 13; g1_1}}
    #[test]
    fn pat_expand_issue_15221(){
        run_renaming_test(
            &("macro_rules! inner ( ($e:pat ) => ($e));
              macro_rules! outer ( ($e:pat ) => (inner!($e)));
              fn main() { let outer!(g) = 13; g;}",
              vec!(vec!(0)),
              true),
            0)
    }

    // create a really evil test case where a $x appears inside a binding of $x
    // but *shouldn't* bind because it was inserted by a different macro....
    // can't write this test case until we have macro-generating macros.

    // method arg hygiene
    // method expands to fn get_x(&self_0, x_1: i32) {self_0 + self_2 + x_3 + x_1}
    #[test]
    fn method_arg_hygiene(){
        run_renaming_test(
            &("macro_rules! inject_x (()=>(x));
              macro_rules! inject_self (()=>(self));
              struct A;
              impl A{fn get_x(&self, x: i32) {self + inject_self!() + inject_x!() + x;} }",
              vec!(vec!(0),vec!(3)),
              true),
            0)
    }

    // ooh, got another bite?
    // expands to struct A; impl A {fn thingy(&self_1) {self_1;}}
    #[test]
    fn method_arg_hygiene_2(){
        run_renaming_test(
            &("struct A;
              macro_rules! add_method (($T:ty) =>
              (impl $T {  fn thingy(&self) {self;} }));
              add_method!(A);",
              vec!(vec!(0)),
              true),
            0)
    }

    // item fn hygiene
    // expands to fn q(x_1: i32){fn g(x_2: i32){x_2 + x_1};}
    #[test]
    fn issue_9383(){
        run_renaming_test(
            &("macro_rules! bad_macro (($ex:expr) => (fn g(x: i32){ x + $ex }));
              fn q(x: i32) { bad_macro!(x); }",
              vec!(vec!(1),vec!(0)),true),
            0)
    }

    // closure arg hygiene (ExprKind::Closure)
    // expands to fn f(){(|x_1 : i32| {(x_2 + x_1)})(3);}
    #[test]
    fn closure_arg_hygiene(){
        run_renaming_test(
            &("macro_rules! inject_x (()=>(x));
            fn f(){(|x : i32| {(inject_x!() + x)})(3);}",
              vec!(vec!(1)),
              true),
            0)
    }

    // macro_rules in method position. Sadly, unimplemented.
    #[test]
    fn macro_in_method_posn(){
        expand_crate_str(
            "macro_rules! my_method (() => (fn thirteen(&self) -> i32 {13}));
            struct A;
            impl A{ my_method!(); }
            fn f(){A.thirteen;}".to_string());
    }

    // another nested macro
    // expands to impl Entries {fn size_hint(&self_1) {self_1;}
    #[test]
    fn item_macro_workaround(){
        run_renaming_test(
            &("macro_rules! item { ($i:item) => {$i}}
              struct Entries;
              macro_rules! iterator_impl {
              () => { item!( impl Entries { fn size_hint(&self) { self;}});}}
              iterator_impl! { }",
              vec!(vec!(0)), true),
            0)
    }

    // run one of the renaming tests
    fn run_renaming_test(t: &RenamingTest, test_idx: usize) {
        let invalid_name = token::special_idents::invalid.name;
        let (teststr, bound_connections, bound_ident_check) = match *t {
            (ref str,ref conns, bic) => (str.to_string(), conns.clone(), bic)
        };
        let cr = expand_crate_str(teststr.to_string());
        let bindings = crate_bindings(&cr);
        let varrefs = crate_varrefs(&cr);

        // must be one check clause for each binding:
        assert_eq!(bindings.len(),bound_connections.len());
        for (binding_idx,shouldmatch) in bound_connections.iter().enumerate() {
            let binding_name = mtwt::resolve(bindings[binding_idx]);
            let binding_marks = mtwt::marksof(bindings[binding_idx].ctxt, invalid_name);
            // shouldmatch can't name varrefs that don't exist:
            assert!((shouldmatch.is_empty()) ||
                    (varrefs.len() > *shouldmatch.iter().max().unwrap()));
            for (idx,varref) in varrefs.iter().enumerate() {
                let print_hygiene_debug_info = || {
                    // good lord, you can't make a path with 0 segments, can you?
                    let final_varref_ident = match varref.segments.last() {
                        Some(pathsegment) => pathsegment.identifier,
                        None => panic!("varref with 0 path segments?")
                    };
                    let varref_name = mtwt::resolve(final_varref_ident);
                    let varref_idents : Vec<ast::Ident>
                        = varref.segments.iter().map(|s| s.identifier)
                        .collect();
                    println!("varref #{}: {:?}, resolves to {}",idx, varref_idents, varref_name);
                    println!("varref's first segment's string: \"{}\"", final_varref_ident);
                    println!("binding #{}: {}, resolves to {}",
                             binding_idx, bindings[binding_idx], binding_name);
                    mtwt::with_sctable(|x| mtwt::display_sctable(x));
                };
                if shouldmatch.contains(&idx) {
                    // it should be a path of length 1, and it should
                    // be free-identifier=? or bound-identifier=? to the given binding
                    assert_eq!(varref.segments.len(),1);
                    let varref_name = mtwt::resolve(varref.segments[0].identifier);
                    let varref_marks = mtwt::marksof(varref.segments[0]
                                                           .identifier
                                                           .ctxt,
                                                     invalid_name);
                    if !(varref_name==binding_name) {
                        println!("uh oh, should match but doesn't:");
                        print_hygiene_debug_info();
                    }
                    assert_eq!(varref_name,binding_name);
                    if bound_ident_check {
                        // we're checking bound-identifier=?, and the marks
                        // should be the same, too:
                        assert_eq!(varref_marks,binding_marks.clone());
                    }
                } else {
                    let varref_name = mtwt::resolve(varref.segments[0].identifier);
                    let fail = (varref.segments.len() == 1)
                        && (varref_name == binding_name);
                    // temp debugging:
                    if fail {
                        println!("failure on test {}",test_idx);
                        println!("text of test case: \"{}\"", teststr);
                        println!("");
                        println!("uh oh, matches but shouldn't:");
                        print_hygiene_debug_info();
                    }
                    assert!(!fail);
                }
            }
        }
    }

    #[test]
    fn fmt_in_macro_used_inside_module_macro() {
        let crate_str = "macro_rules! fmt_wrap(($b:expr)=>($b.to_string()));
macro_rules! foo_module (() => (mod generated { fn a() { let xx = 147; fmt_wrap!(xx);}}));
foo_module!();
".to_string();
        let cr = expand_crate_str(crate_str);
        // find the xx binding
        let bindings = crate_bindings(&cr);
        let cxbinds: Vec<&ast::Ident> =
            bindings.iter().filter(|b| b.name.as_str() == "xx").collect();
        let cxbinds: &[&ast::Ident] = &cxbinds[..];
        let cxbind = match (cxbinds.len(), cxbinds.get(0)) {
            (1, Some(b)) => *b,
            _ => panic!("expected just one binding for ext_cx")
        };
        let resolved_binding = mtwt::resolve(*cxbind);
        let varrefs = crate_varrefs(&cr);

        // the xx binding should bind all of the xx varrefs:
        for (idx,v) in varrefs.iter().filter(|p| {
            p.segments.len() == 1
            && p.segments[0].identifier.name.as_str() == "xx"
        }).enumerate() {
            if mtwt::resolve(v.segments[0].identifier) != resolved_binding {
                println!("uh oh, xx binding didn't match xx varref:");
                println!("this is xx varref \\# {}", idx);
                println!("binding: {}", cxbind);
                println!("resolves to: {}", resolved_binding);
                println!("varref: {}", v.segments[0].identifier);
                println!("resolves to: {}",
                         mtwt::resolve(v.segments[0].identifier));
                mtwt::with_sctable(|x| mtwt::display_sctable(x));
            }
            assert_eq!(mtwt::resolve(v.segments[0].identifier),
                       resolved_binding);
        };
    }

    #[test]
    fn pat_idents(){
        let pat = string_to_pat(
            "(a,Foo{x:c @ (b,9),y:Bar(4,d)})".to_string());
        let idents = pattern_bindings(&pat);
        assert_eq!(idents, strs_to_idents(vec!("a","c","b","d")));
    }

    // test the list of identifier patterns gathered by the visitor. Note that
    // 'None' is listed as an identifier pattern because we don't yet know that
    // it's the name of a 0-ary variant, and that 'i' appears twice in succession.
    #[test]
    fn crate_bindings_test(){
        let the_crate = string_to_crate("fn main (a: i32) -> i32 {|b| {
        match 34 {None => 3, Some(i) | i => j, Foo{k:z,l:y} => \"banana\"}} }".to_string());
        let idents = crate_bindings(&the_crate);
        assert_eq!(idents, strs_to_idents(vec!("a","b","None","i","i","z","y")));
    }

    // test the IdentRenamer directly
    #[test]
    fn ident_renamer_test () {
        let the_crate = string_to_crate("fn f(x: i32){let x = x; x}".to_string());
        let f_ident = token::str_to_ident("f");
        let x_ident = token::str_to_ident("x");
        let int_ident = token::str_to_ident("i32");
        let renames = vec!((x_ident,Name(16)));
        let mut renamer = IdentRenamer{renames: &renames};
        let renamed_crate = renamer.fold_crate(the_crate);
        let idents = crate_idents(&renamed_crate);
        let resolved : Vec<ast::Name> = idents.iter().map(|id| mtwt::resolve(*id)).collect();
        assert_eq!(resolved, [f_ident.name,Name(16),int_ident.name,Name(16),Name(16),Name(16)]);
    }

    // test the PatIdentRenamer; only PatIdents get renamed
    #[test]
    fn pat_ident_renamer_test () {
        let the_crate = string_to_crate("fn f(x: i32){let x = x; x}".to_string());
        let f_ident = token::str_to_ident("f");
        let x_ident = token::str_to_ident("x");
        let int_ident = token::str_to_ident("i32");
        let renames = vec!((x_ident,Name(16)));
        let mut renamer = PatIdentRenamer{renames: &renames};
        let renamed_crate = renamer.fold_crate(the_crate);
        let idents = crate_idents(&renamed_crate);
        let resolved : Vec<ast::Name> = idents.iter().map(|id| mtwt::resolve(*id)).collect();
        let x_name = x_ident.name;
        assert_eq!(resolved, [f_ident.name,Name(16),int_ident.name,Name(16),x_name,x_name]);
    }
}