rust/src/libsyntax/ast_util.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 ast::*;
use ast;
use ast_util;
use codemap::{span, dummy_sp};
use opt_vec;
use parse::token;
use visit::{SimpleVisitor, SimpleVisitorVisitor, Visitor};
use visit;

use std::hashmap::HashMap;
use std::int;
use std::local_data;
use std::num;
use std::option;

pub fn path_name_i(idents: &[ident]) -> ~str {
    // FIXME: Bad copies (#2543 -- same for everything else that says "bad")
    idents.map(|i| token::interner_get(i.name)).connect("::")
}

pub fn path_to_ident(p: &Path) -> ident {
    *p.idents.last()
}

pub fn local_def(id: NodeId) -> def_id {
    ast::def_id { crate: LOCAL_CRATE, node: id }
}

pub fn is_local(did: ast::def_id) -> bool { did.crate == LOCAL_CRATE }

pub fn stmt_id(s: &stmt) -> NodeId {
    match s.node {
      stmt_decl(_, id) => id,
      stmt_expr(_, id) => id,
      stmt_semi(_, id) => id,
      stmt_mac(*) => fail!("attempted to analyze unexpanded stmt")
    }
}

pub fn variant_def_ids(d: def) -> Option<(def_id, def_id)> {
    match d {
      def_variant(enum_id, var_id) => {
          Some((enum_id, var_id))
      }
      _ => None
    }
}

pub fn def_id_of_def(d: def) -> def_id {
    match d {
      def_fn(id, _) | def_static_method(id, _, _) | def_mod(id) |
      def_foreign_mod(id) | def_static(id, _) |
      def_variant(_, id) | def_ty(id) | def_ty_param(id, _) |
      def_use(id) | def_struct(id) | def_trait(id) | def_method(id, _) => {
        id
      }
      def_arg(id, _) | def_local(id, _) | def_self(id, _) | def_self_ty(id)
      | def_upvar(id, _, _, _) | def_binding(id, _) | def_region(id)
      | def_typaram_binder(id) | def_label(id) => {
        local_def(id)
      }

      def_prim_ty(_) => fail!()
    }
}

pub fn binop_to_str(op: binop) -> ~str {
    match op {
      add => return ~"+",
      subtract => return ~"-",
      mul => return ~"*",
      div => return ~"/",
      rem => return ~"%",
      and => return ~"&&",
      or => return ~"||",
      bitxor => return ~"^",
      bitand => return ~"&",
      bitor => return ~"|",
      shl => return ~"<<",
      shr => return ~">>",
      eq => return ~"==",
      lt => return ~"<",
      le => return ~"<=",
      ne => return ~"!=",
      ge => return ~">=",
      gt => return ~">"
    }
}

pub fn binop_to_method_name(op: binop) -> Option<~str> {
    match op {
      add => return Some(~"add"),
      subtract => return Some(~"sub"),
      mul => return Some(~"mul"),
      div => return Some(~"div"),
      rem => return Some(~"rem"),
      bitxor => return Some(~"bitxor"),
      bitand => return Some(~"bitand"),
      bitor => return Some(~"bitor"),
      shl => return Some(~"shl"),
      shr => return Some(~"shr"),
      lt => return Some(~"lt"),
      le => return Some(~"le"),
      ge => return Some(~"ge"),
      gt => return Some(~"gt"),
      eq => return Some(~"eq"),
      ne => return Some(~"ne"),
      and | or => return None
    }
}

pub fn lazy_binop(b: binop) -> bool {
    match b {
      and => true,
      or => true,
      _ => false
    }
}

pub fn is_shift_binop(b: binop) -> bool {
    match b {
      shl => true,
      shr => true,
      _ => false
    }
}

pub fn unop_to_str(op: unop) -> ~str {
    match op {
      box(mt) => if mt == m_mutbl { ~"@mut " } else { ~"@" },
      uniq => ~"~",
      deref => ~"*",
      not => ~"!",
      neg => ~"-"
    }
}

pub fn is_path(e: @expr) -> bool {
    return match e.node { expr_path(_) => true, _ => false };
}

pub fn int_ty_to_str(t: int_ty) -> ~str {
    match t {
      ty_char => ~"u8", // ???
      ty_i => ~"",
      ty_i8 => ~"i8",
      ty_i16 => ~"i16",
      ty_i32 => ~"i32",
      ty_i64 => ~"i64"
    }
}

pub fn int_ty_max(t: int_ty) -> u64 {
    match t {
      ty_i8 => 0x80u64,
      ty_i16 => 0x8000u64,
      ty_i | ty_char | ty_i32 => 0x80000000u64, // actually ni about ty_i
      ty_i64 => 0x8000000000000000u64
    }
}

pub fn uint_ty_to_str(t: uint_ty) -> ~str {
    match t {
      ty_u => ~"u",
      ty_u8 => ~"u8",
      ty_u16 => ~"u16",
      ty_u32 => ~"u32",
      ty_u64 => ~"u64"
    }
}

pub fn uint_ty_max(t: uint_ty) -> u64 {
    match t {
      ty_u8 => 0xffu64,
      ty_u16 => 0xffffu64,
      ty_u | ty_u32 => 0xffffffffu64, // actually ni about ty_u
      ty_u64 => 0xffffffffffffffffu64
    }
}

pub fn float_ty_to_str(t: float_ty) -> ~str {
    match t { ty_f => ~"f", ty_f32 => ~"f32", ty_f64 => ~"f64" }
}

pub fn is_call_expr(e: @expr) -> bool {
    match e.node { expr_call(*) => true, _ => false }
}

pub fn block_from_expr(e: @expr) -> Block {
    let mut blk = default_block(~[], option::Some::<@expr>(e), e.id);
    blk.span = e.span;
    return blk;
}

pub fn default_block(
    stmts1: ~[@stmt],
    expr1: Option<@expr>,
    id1: NodeId
) -> Block {
    ast::Block {
        view_items: ~[],
        stmts: stmts1,
        expr: expr1,
        id: id1,
        rules: DefaultBlock,
        span: dummy_sp(),
    }
}

pub fn ident_to_path(s: span, i: ident) -> Path {
    ast::Path { span: s,
                 global: false,
                 idents: ~[i],
                 rp: None,
                 types: ~[] }
}

pub fn ident_to_pat(id: NodeId, s: span, i: ident) -> @pat {
    @ast::pat { id: id,
                node: pat_ident(bind_infer, ident_to_path(s, i), None),
                span: s }
}

pub fn is_unguarded(a: &arm) -> bool {
    match a.guard {
      None => true,
      _    => false
    }
}

pub fn unguarded_pat(a: &arm) -> Option<~[@pat]> {
    if is_unguarded(a) {
        Some(/* FIXME (#2543) */ a.pats.clone())
    } else {
        None
    }
}

pub fn public_methods(ms: ~[@method]) -> ~[@method] {
    do ms.consume_iter().filter |m| {
        match m.vis {
            public => true,
            _   => false
        }
    }.collect()
}

// extract a TypeMethod from a trait_method. if the trait_method is
// a default, pull out the useful fields to make a TypeMethod
pub fn trait_method_to_ty_method(method: &trait_method) -> TypeMethod {
    match *method {
        required(ref m) => (*m).clone(),
        provided(ref m) => {
            TypeMethod {
                ident: m.ident,
                attrs: m.attrs.clone(),
                purity: m.purity,
                decl: m.decl.clone(),
                generics: m.generics.clone(),
                explicit_self: m.explicit_self,
                id: m.id,
                span: m.span,
            }
        }
    }
}

pub fn split_trait_methods(trait_methods: &[trait_method])
    -> (~[TypeMethod], ~[@method]) {
    let mut reqd = ~[];
    let mut provd = ~[];
    for trt_method in trait_methods.iter() {
        match *trt_method {
          required(ref tm) => reqd.push((*tm).clone()),
          provided(m) => provd.push(m)
        }
    };
    (reqd, provd)
}

pub fn struct_field_visibility(field: ast::struct_field) -> visibility {
    match field.node.kind {
        ast::named_field(_, visibility) => visibility,
        ast::unnamed_field => ast::public
    }
}

pub trait inlined_item_utils {
    fn ident(&self) -> ident;
    fn id(&self) -> ast::NodeId;
    fn accept<E: Clone>(&self, e: E, v: @Visitor<E>);
}

impl inlined_item_utils for inlined_item {
    fn ident(&self) -> ident {
        match *self {
            ii_item(i) => i.ident,
            ii_foreign(i) => i.ident,
            ii_method(_, _, m) => m.ident,
        }
    }

    fn id(&self) -> ast::NodeId {
        match *self {
            ii_item(i) => i.id,
            ii_foreign(i) => i.id,
            ii_method(_, _, m) => m.id,
        }
    }

    fn accept<E: Clone>(&self, e: E, v: @Visitor<E>) {
        match *self {
            ii_item(i) => v.visit_item(i, e),
            ii_foreign(i) => v.visit_foreign_item(i, e),
            ii_method(_, _, m) => visit::visit_method_helper(v, m, e),
        }
    }
}

/* True if d is either a def_self, or a chain of def_upvars
 referring to a def_self */
pub fn is_self(d: ast::def) -> bool {
  match d {
    def_self(*)           => true,
    def_upvar(_, d, _, _) => is_self(*d),
    _                     => false
  }
}

/// Maps a binary operator to its precedence
pub fn operator_prec(op: ast::binop) -> uint {
  match op {
      // 'as' sits here with 12
      mul | div | rem   => 11u,
      add | subtract    => 10u,
      shl | shr         =>  9u,
      bitand            =>  8u,
      bitxor            =>  7u,
      bitor             =>  6u,
      lt | le | ge | gt =>  4u,
      eq | ne           =>  3u,
      and               =>  2u,
      or                =>  1u
  }
}

/// Precedence of the `as` operator, which is a binary operator
/// not appearing in the prior table.
pub static as_prec: uint = 12u;

pub fn empty_generics() -> Generics {
    Generics {lifetimes: opt_vec::Empty,
              ty_params: opt_vec::Empty}
}

// ______________________________________________________________________
// Enumerating the IDs which appear in an AST

#[deriving(Encodable, Decodable)]
pub struct id_range {
    min: NodeId,
    max: NodeId,
}

impl id_range {
    pub fn max() -> id_range {
        id_range {
            min: int::max_value,
            max: int::min_value,
        }
    }

    pub fn empty(&self) -> bool {
        self.min >= self.max
    }

    pub fn add(&mut self, id: NodeId) {
        self.min = num::min(self.min, id);
        self.max = num::max(self.max, id + 1);
    }
}

struct IdVisitor {
    visit_callback: @fn(NodeId),
    pass_through_items: bool,
    visited_outermost: bool,
}

impl IdVisitor {
    fn visit_generics_helper(@mut self, generics: &Generics) {
        for type_parameter in generics.ty_params.iter() {
            (self.visit_callback)(type_parameter.id)
        }
        for lifetime in generics.lifetimes.iter() {
            (self.visit_callback)(lifetime.id)
        }
    }
}

impl Visitor<()> for IdVisitor {
    fn visit_mod(@mut self,
                 module: &_mod,
                 _span: span,
                 node_id: NodeId,
                 env: ()) {
        (self.visit_callback)(node_id);
        visit::visit_mod(self as @Visitor<()>, module, env)
    }

    fn visit_view_item(@mut self, view_item: &view_item, env: ()) {
        match view_item.node {
            view_item_extern_mod(_, _, node_id) => {
                (self.visit_callback)(node_id)
            }
            view_item_use(ref view_paths) => {
                for view_path in view_paths.iter() {
                    match view_path.node {
                        view_path_simple(_, _, node_id) |
                        view_path_glob(_, node_id) => {
                            (self.visit_callback)(node_id)
                        }
                        view_path_list(_, ref paths, node_id) => {
                            (self.visit_callback)(node_id);
                            for path in paths.iter() {
                                (self.visit_callback)(path.node.id)
                            }
                        }
                    }
                }
            }
        }
        visit::visit_view_item(self as @Visitor<()>, view_item, env)
    }

    fn visit_foreign_item(@mut self, foreign_item: @foreign_item, env: ()) {
        (self.visit_callback)(foreign_item.id);
        visit::visit_foreign_item(self as @Visitor<()>, foreign_item, env)
    }

    fn visit_item(@mut self, item: @item, env: ()) {
        if !self.pass_through_items {
            if self.visited_outermost {
                return
            } else {
                self.visited_outermost = true
            }
        }

        (self.visit_callback)(item.id);
        match item.node {
            item_enum(ref enum_definition, _) => {
                for variant in enum_definition.variants.iter() {
                    (self.visit_callback)(variant.node.id)
                }
            }
            _ => {}
        }

        visit::visit_item(self as @Visitor<()>, item, env);

        self.visited_outermost = false
    }

    fn visit_local(@mut self, local: @Local, env: ()) {
        (self.visit_callback)(local.id);
        visit::visit_local(self as @Visitor<()>, local, env)
    }

    fn visit_block(@mut self, block: &Block, env: ()) {
        (self.visit_callback)(block.id);
        visit::visit_block(self as @Visitor<()>, block, env)
    }

    fn visit_stmt(@mut self, statement: @stmt, env: ()) {
        (self.visit_callback)(ast_util::stmt_id(statement));
        visit::visit_stmt(self as @Visitor<()>, statement, env)
    }

    // XXX: Default
    fn visit_arm(@mut self, arm: &arm, env: ()) {
        visit::visit_arm(self as @Visitor<()>, arm, env)
    }

    fn visit_pat(@mut self, pattern: @pat, env: ()) {
        (self.visit_callback)(pattern.id);
        visit::visit_pat(self as @Visitor<()>, pattern, env)
    }

    // XXX: Default
    fn visit_decl(@mut self, declaration: @decl, env: ()) {
        visit::visit_decl(self as @Visitor<()>, declaration, env)
    }

    fn visit_expr(@mut self, expression: @expr, env: ()) {
        {
            let optional_callee_id = expression.get_callee_id();
            for callee_id in optional_callee_id.iter() {
                (self.visit_callback)(*callee_id)
            }
        }
        (self.visit_callback)(expression.id);
        visit::visit_expr(self as @Visitor<()>, expression, env)
    }

    // XXX: Default
    fn visit_expr_post(@mut self, _: @expr, _: ()) {
        // Empty!
    }

    fn visit_ty(@mut self, typ: &Ty, env: ()) {
        (self.visit_callback)(typ.id);
        match typ.node {
            ty_path(_, _, id) => (self.visit_callback)(id),
            _ => {}
        }
        visit::visit_ty(self as @Visitor<()>, typ, env)
    }

    fn visit_generics(@mut self, generics: &Generics, env: ()) {
        self.visit_generics_helper(generics);
        visit::visit_generics(self as @Visitor<()>, generics, env)
    }

    fn visit_fn(@mut self,
                function_kind: &visit::fn_kind,
                function_declaration: &fn_decl,
                block: &Block,
                span: span,
                node_id: NodeId,
                env: ()) {
        if !self.pass_through_items {
            match *function_kind {
                visit::fk_method(*) if self.visited_outermost => return,
                visit::fk_method(*) => self.visited_outermost = true,
                _ => {}
            }
        }

        (self.visit_callback)(node_id);

        match *function_kind {
            visit::fk_item_fn(_, generics, _, _) => {
                self.visit_generics_helper(generics)
            }
            visit::fk_method(_, generics, method) => {
                (self.visit_callback)(method.self_id);
                self.visit_generics_helper(generics)
            }
            visit::fk_anon(_) | visit::fk_fn_block => {}
        }

        for argument in function_declaration.inputs.iter() {
            (self.visit_callback)(argument.id)
        }

        visit::visit_fn(self as @Visitor<()>,
                        function_kind,
                        function_declaration,
                        block,
                        span,
                        node_id,
                        env);

        if !self.pass_through_items {
            match *function_kind {
                visit::fk_method(*) => self.visited_outermost = false,
                _ => {}
            }
        }
    }

    // XXX: Default
    fn visit_ty_method(@mut self, type_method: &TypeMethod, env: ()) {
        visit::visit_ty_method(self as @Visitor<()>, type_method, env)
    }

    // XXX: Default
    fn visit_trait_method(@mut self, trait_method: &trait_method, env: ()) {
        visit::visit_trait_method(self as @Visitor<()>, trait_method, env)
    }

    // XXX: Default
    fn visit_struct_def(@mut self,
                        struct_definition: @struct_def,
                        identifier: ident,
                        generics: &Generics,
                        node_id: NodeId,
                        env: ()) {
        visit::visit_struct_def(self as @Visitor<()>,
                                struct_definition,
                                identifier,
                                generics,
                                node_id,
                                env)
    }

    fn visit_struct_field(@mut self, struct_field: @struct_field, env: ()) {
        (self.visit_callback)(struct_field.node.id);
        visit::visit_struct_field(self as @Visitor<()>, struct_field, env)
    }
}

pub fn id_visitor(vfn: @fn(NodeId), pass_through_items: bool)
                  -> @Visitor<()> {
    let visitor = @IdVisitor {
        visit_callback: vfn,
        pass_through_items: pass_through_items,
        visited_outermost: false,
    };
    visitor as @Visitor<()>
}

pub fn visit_ids_for_inlined_item(item: &inlined_item, vfn: @fn(NodeId)) {
    item.accept((), id_visitor(|id| vfn(id), true));
}

pub fn compute_id_range(visit_ids_fn: &fn(@fn(NodeId))) -> id_range {
    let result = @mut id_range::max();
    do visit_ids_fn |id| {
        result.add(id);
    }
    *result
}

pub fn compute_id_range_for_inlined_item(item: &inlined_item) -> id_range {
    compute_id_range(|f| visit_ids_for_inlined_item(item, f))
}

pub fn is_item_impl(item: @ast::item) -> bool {
    match item.node {
       item_impl(*) => true,
       _            => false
    }
}

pub fn walk_pat(pat: @pat, it: &fn(@pat) -> bool) -> bool {
    if !it(pat) {
        return false;
    }

    match pat.node {
        pat_ident(_, _, Some(p)) => walk_pat(p, it),
        pat_struct(_, ref fields, _) => {
            fields.iter().advance(|f| walk_pat(f.pat, |p| it(p)))
        }
        pat_enum(_, Some(ref s)) | pat_tup(ref s) => {
            s.iter().advance(|&p| walk_pat(p, |p| it(p)))
        }
        pat_box(s) | pat_uniq(s) | pat_region(s) => {
            walk_pat(s, it)
        }
        pat_vec(ref before, ref slice, ref after) => {
            before.iter().advance(|&p| walk_pat(p, |p| it(p))) &&
                slice.iter().advance(|&p| walk_pat(p, |p| it(p))) &&
                after.iter().advance(|&p| walk_pat(p, |p| it(p)))
        }
        pat_wild | pat_lit(_) | pat_range(_, _) | pat_ident(_, _, _) |
        pat_enum(_, _) => {
            true
        }
    }
}

pub trait EachViewItem {
    pub fn each_view_item(&self, f: @fn(&ast::view_item) -> bool) -> bool;
}

struct EachViewItemData {
    callback: @fn(&ast::view_item) -> bool,
}

impl SimpleVisitor for EachViewItemData {
    fn visit_mod(@mut self, _: &_mod, _: span, _: NodeId) {
        // XXX: Default method.
    }
    fn visit_view_item(@mut self, view_item: &view_item) {
        let _ = (self.callback)(view_item);
    }
    fn visit_foreign_item(@mut self, _: @foreign_item) {
        // XXX: Default method.
    }
    fn visit_item(@mut self, _: @item) {
        // XXX: Default method.
    }
    fn visit_local(@mut self, _: @Local) {
        // XXX: Default method.
    }
    fn visit_block(@mut self, _: &Block) {
        // XXX: Default method.
    }
    fn visit_stmt(@mut self, _: @stmt) {
        // XXX: Default method.
    }
    fn visit_arm(@mut self, _: &arm) {
        // XXX: Default method.
    }
    fn visit_pat(@mut self, _: @pat) {
        // XXX: Default method.
    }
    fn visit_decl(@mut self, _: @decl) {
        // XXX: Default method.
    }
    fn visit_expr(@mut self, _: @expr) {
        // XXX: Default method.
    }
    fn visit_expr_post(@mut self, _: @expr) {
        // XXX: Default method.
    }
    fn visit_ty(@mut self, _: &Ty) {
        // XXX: Default method.
    }
    fn visit_generics(@mut self, _: &Generics) {
        // XXX: Default method.
    }
    fn visit_fn(@mut self,
                _: &visit::fn_kind,
                _: &fn_decl,
                _: &Block,
                _: span,
                _: NodeId) {
        // XXX: Default method.
    }
    fn visit_ty_method(@mut self, _: &TypeMethod) {
        // XXX: Default method.
    }
    fn visit_trait_method(@mut self, _: &trait_method) {
        // XXX: Default method.
    }
    fn visit_struct_def(@mut self,
                        _: @struct_def,
                        _: ident,
                        _: &Generics,
                        _: NodeId) {
        // XXX: Default method.
    }
    fn visit_struct_field(@mut self, _: @struct_field) {
        // XXX: Default method.
    }
    fn visit_struct_method(@mut self, _: @method) {
        // XXX: Default method.
    }
}

impl EachViewItem for ast::Crate {
    fn each_view_item(&self, f: @fn(&ast::view_item) -> bool) -> bool {
        let data = @mut EachViewItemData {
            callback: f,
        };
        let visitor = @mut SimpleVisitorVisitor {
            simple_visitor: data as @SimpleVisitor,
        };
        visit::visit_crate(visitor as @Visitor<()>, self, ());
        true
    }
}

pub fn view_path_id(p: &view_path) -> NodeId {
    match p.node {
      view_path_simple(_, _, id) |
      view_path_glob(_, id) |
      view_path_list(_, _, id) => id
    }
}

/// Returns true if the given struct def is tuple-like; i.e. that its fields
/// are unnamed.
pub fn struct_def_is_tuple_like(struct_def: &ast::struct_def) -> bool {
    struct_def.ctor_id.is_some()
}

pub fn visibility_to_privacy(visibility: visibility) -> Privacy {
    match visibility {
        public => Public,
        inherited | private => Private
    }
}

pub fn variant_visibility_to_privacy(visibility: visibility,
                                     enclosing_is_public: bool)
                                  -> Privacy {
    if enclosing_is_public {
        match visibility {
            public | inherited => Public,
            private => Private
        }
    } else {
        visibility_to_privacy(visibility)
    }
}

#[deriving(Eq)]
pub enum Privacy {
    Private,
    Public
}

/// Returns true if the given pattern consists solely of an identifier
/// and false otherwise.
pub fn pat_is_ident(pat: @ast::pat) -> bool {
    match pat.node {
        ast::pat_ident(*) => true,
        _ => false,
    }
}

// HYGIENE FUNCTIONS

/// Construct an identifier with the given name and an empty context:
pub fn new_ident(name: Name) -> ident { ident {name: name, ctxt: 0}}

/// Extend a syntax context with a given mark
pub fn new_mark(m:Mrk, tail:SyntaxContext) -> SyntaxContext {
    new_mark_internal(m,tail,get_sctable())
}

// Extend a syntax context with a given mark and table
// FIXME #4536 : currently pub to allow testing
pub fn new_mark_internal(m:Mrk, tail:SyntaxContext,table:&mut SCTable)
    -> SyntaxContext {
    let key = (tail,m);
    // FIXME #5074 : can't use more natural style because we're missing
    // flow-sensitivity. Results in two lookups on a hash table hit.
    // also applies to new_rename_internal, below.
    // let try_lookup = table.mark_memo.find(&key);
    match table.mark_memo.contains_key(&key) {
        false => {
            let new_idx = idx_push(&mut table.table,Mark(m,tail));
            table.mark_memo.insert(key,new_idx);
            new_idx
        }
        true => {
            match table.mark_memo.find(&key) {
                None => fail!(~"internal error: key disappeared 2013042901"),
                Some(idxptr) => {*idxptr}
            }
        }
    }
}

/// Extend a syntax context with a given rename
pub fn new_rename(id:ident, to:Name, tail:SyntaxContext) -> SyntaxContext {
    new_rename_internal(id, to, tail, get_sctable())
}

// Extend a syntax context with a given rename and sctable
// FIXME #4536 : currently pub to allow testing
pub fn new_rename_internal(id:ident, to:Name, tail:SyntaxContext, table: &mut SCTable)
    -> SyntaxContext {
    let key = (tail,id,to);
    // FIXME #5074
    //let try_lookup = table.rename_memo.find(&key);
    match table.rename_memo.contains_key(&key) {
        false => {
            let new_idx = idx_push(&mut table.table,Rename(id,to,tail));
            table.rename_memo.insert(key,new_idx);
            new_idx
        }
        true => {
            match table.rename_memo.find(&key) {
                None => fail!(~"internal error: key disappeared 2013042902"),
                Some(idxptr) => {*idxptr}
            }
        }
    }
}

/// Make a fresh syntax context table with EmptyCtxt in slot zero
/// and IllegalCtxt in slot one.
// FIXME #4536 : currently pub to allow testing
pub fn new_sctable_internal() -> SCTable {
    SCTable {
        table: ~[EmptyCtxt,IllegalCtxt],
        mark_memo: HashMap::new(),
        rename_memo: HashMap::new()
    }
}

// fetch the SCTable from TLS, create one if it doesn't yet exist.
pub fn get_sctable() -> @mut SCTable {
    static sctable_key: local_data::Key<@@mut SCTable> = &local_data::Key;
    match local_data::get(sctable_key, |k| k.map(|&k| *k)) {
        None => {
            let new_table = @@mut new_sctable_internal();
            local_data::set(sctable_key,new_table);
            *new_table
        },
        Some(intr) => *intr
    }
}

/// Add a value to the end of a vec, return its index
fn idx_push<T>(vec: &mut ~[T], val: T) -> uint {
    vec.push(val);
    vec.len() - 1
}

/// Resolve a syntax object to a name, per MTWT.
pub fn resolve(id : ident) -> Name {
    resolve_internal(id, get_sctable())
}

// Resolve a syntax object to a name, per MTWT.
// FIXME #4536 : currently pub to allow testing
pub fn resolve_internal(id : ident, table : &mut SCTable) -> Name {
    match table.table[id.ctxt] {
        EmptyCtxt => id.name,
        // ignore marks here:
        Mark(_,subctxt) => resolve_internal(ident{name:id.name, ctxt: subctxt},table),
        // do the rename if necessary:
        Rename(ident{name,ctxt},toname,subctxt) => {
            // this could be cached or computed eagerly:
            let resolvedfrom = resolve_internal(ident{name:name,ctxt:ctxt},table);
            let resolvedthis = resolve_internal(ident{name:id.name,ctxt:subctxt},table);
            if ((resolvedthis == resolvedfrom)
                && (marksof(ctxt,resolvedthis,table)
                    == marksof(subctxt,resolvedthis,table))) {
                toname
            } else {
                resolvedthis
            }
        }
        IllegalCtxt() => fail!(~"expected resolvable context, got IllegalCtxt")
    }
}

/// Compute the marks associated with a syntax context.
// it's not clear to me whether it's better to use a [] mutable
// vector or a cons-list for this.
pub fn marksof(ctxt: SyntaxContext, stopname: Name, table: &SCTable) -> ~[Mrk] {
    let mut result = ~[];
    let mut loopvar = ctxt;
    loop {
        match table.table[loopvar] {
            EmptyCtxt => {return result;},
            Mark(mark,tl) => {
                xorPush(&mut result,mark);
                loopvar = tl;
            },
            Rename(_,name,tl) => {
                // see MTWT for details on the purpose of the stopname.
                // short version: it prevents duplication of effort.
                if (name == stopname) {
                    return result;
                } else {
                    loopvar = tl;
                }
            }
            IllegalCtxt => fail!(~"expected resolvable context, got IllegalCtxt")
        }
    }
}

/// Push a name... unless it matches the one on top, in which
/// case pop and discard (so two of the same marks cancel)
pub fn xorPush(marks: &mut ~[uint], mark: uint) {
    if ((marks.len() > 0) && (getLast(marks) == mark)) {
        marks.pop();
    } else {
        marks.push(mark);
    }
}

// get the last element of a mutable array.
// FIXME #4903: , must be a separate procedure for now.
pub fn getLast(arr: &~[Mrk]) -> uint {
    *arr.last()
}


#[cfg(test)]
mod test {
    use ast::*;
    use super::*;
    use std::io;

    #[test] fn xorpush_test () {
        let mut s = ~[];
        xorPush(&mut s,14);
        assert_eq!(s.clone(),~[14]);
        xorPush(&mut s,14);
        assert_eq!(s.clone(),~[]);
        xorPush(&mut s,14);
        assert_eq!(s.clone(),~[14]);
        xorPush(&mut s,15);
        assert_eq!(s.clone(),~[14,15]);
        xorPush (&mut s,16);
        assert_eq!(s.clone(),~[14,15,16]);
        xorPush (&mut s,16);
        assert_eq!(s.clone(),~[14,15]);
        xorPush (&mut s,15);
        assert_eq!(s.clone(),~[14]);
    }

    // convert a list of uints to an @[ident]
    // (ignores the interner completely)
    fn uints_to_idents (uints: &~[uint]) -> @~[ident] {
        @uints.map(|u| ident {name:*u, ctxt: empty_ctxt})
    }

    fn id (u : uint, s: SyntaxContext) -> ident {
        ident{name:u, ctxt: s}
    }

    // because of the SCTable, I now need a tidy way of
    // creating syntax objects. Sigh.
    #[deriving(Clone, Eq)]
    enum TestSC {
        M(Mrk),
        R(ident,Name)
    }

    // unfold a vector of TestSC values into a SCTable,
    // returning the resulting index
    fn unfold_test_sc(tscs : ~[TestSC], tail: SyntaxContext, table : &mut SCTable)
        -> SyntaxContext {
        tscs.rev_iter().fold(tail, |tail : SyntaxContext, tsc : &TestSC|
                  {match *tsc {
                      M(mrk) => new_mark_internal(mrk,tail,table),
                      R(ident,name) => new_rename_internal(ident,name,tail,table)}})
    }

    // gather a SyntaxContext back into a vector of TestSCs
    fn refold_test_sc(mut sc: SyntaxContext, table : &SCTable) -> ~[TestSC] {
        let mut result = ~[];
        loop {
            match table.table[sc] {
                EmptyCtxt => {return result;},
                Mark(mrk,tail) => {
                    result.push(M(mrk));
                    sc = tail;
                    loop;
                },
                Rename(id,name,tail) => {
                    result.push(R(id,name));
                    sc = tail;
                    loop;
                }
                IllegalCtxt => fail!("expected resolvable context, got IllegalCtxt")
            }
        }
    }

    #[test] fn test_unfold_refold(){
        let mut t = new_sctable_internal();

        let test_sc = ~[M(3),R(id(101,0),14),M(9)];
        assert_eq!(unfold_test_sc(test_sc.clone(),empty_ctxt,&mut t),4);
        assert_eq!(t.table[2],Mark(9,0));
        assert_eq!(t.table[3],Rename(id(101,0),14,2));
        assert_eq!(t.table[4],Mark(3,3));
        assert_eq!(refold_test_sc(4,&t),test_sc);
    }

    // extend a syntax context with a sequence of marks given
    // in a vector. v[0] will be the outermost mark.
    fn unfold_marks(mrks:~[Mrk],tail:SyntaxContext,table: &mut SCTable) -> SyntaxContext {
        mrks.rev_iter().fold(tail, |tail:SyntaxContext, mrk:&Mrk|
                   {new_mark_internal(*mrk,tail,table)})
    }

    #[test] fn unfold_marks_test() {
        let mut t = new_sctable_internal();

        assert_eq!(unfold_marks(~[3,7],empty_ctxt,&mut t),3);
        assert_eq!(t.table[2],Mark(7,0));
        assert_eq!(t.table[3],Mark(3,2));
    }

    #[test] fn test_marksof () {
        let stopname = 242;
        let name1 = 243;
        let mut t = new_sctable_internal();
        assert_eq!(marksof (empty_ctxt,stopname,&t),~[]);
        // FIXME #5074: ANF'd to dodge nested calls
        { let ans = unfold_marks(~[4,98],empty_ctxt,&mut t);
         assert_eq! (marksof (ans,stopname,&t),~[4,98]);}
        // does xoring work?
        { let ans = unfold_marks(~[5,5,16],empty_ctxt,&mut t);
         assert_eq! (marksof (ans,stopname,&t), ~[16]);}
        // does nested xoring work?
        { let ans = unfold_marks(~[5,10,10,5,16],empty_ctxt,&mut t);
         assert_eq! (marksof (ans, stopname,&t), ~[16]);}
        // rename where stop doesn't match:
        { let chain = ~[M(9),
                        R(id(name1,
                             new_mark_internal (4, empty_ctxt,&mut t)),
                          100101102),
                        M(14)];
         let ans = unfold_test_sc(chain,empty_ctxt,&mut t);
         assert_eq! (marksof (ans, stopname, &t), ~[9,14]);}
        // rename where stop does match
        { let name1sc = new_mark_internal(4, empty_ctxt, &mut t);
         let chain = ~[M(9),
                       R(id(name1, name1sc),
                         stopname),
                       M(14)];
         let ans = unfold_test_sc(chain,empty_ctxt,&mut t);
         assert_eq! (marksof (ans, stopname, &t), ~[9]); }
    }


    #[test] fn resolve_tests () {
        let a = 40;
        let mut t = new_sctable_internal();
        // - ctxt is MT
        assert_eq!(resolve_internal(id(a,empty_ctxt),&mut t),a);
        // - simple ignored marks
        { let sc = unfold_marks(~[1,2,3],empty_ctxt,&mut t);
         assert_eq!(resolve_internal(id(a,sc),&mut t),a);}
        // - orthogonal rename where names don't match
        { let sc = unfold_test_sc(~[R(id(50,empty_ctxt),51),M(12)],empty_ctxt,&mut t);
         assert_eq!(resolve_internal(id(a,sc),&mut t),a);}
        // - rename where names do match, but marks don't
        { let sc1 = new_mark_internal(1,empty_ctxt,&mut t);
         let sc = unfold_test_sc(~[R(id(a,sc1),50),
                                   M(1),
                                   M(2)],
                                 empty_ctxt,&mut t);
        assert_eq!(resolve_internal(id(a,sc),&mut t), a);}
        // - rename where names and marks match
        { let sc1 = unfold_test_sc(~[M(1),M(2)],empty_ctxt,&mut t);
         let sc = unfold_test_sc(~[R(id(a,sc1),50),M(1),M(2)],empty_ctxt,&mut t);
         assert_eq!(resolve_internal(id(a,sc),&mut t), 50); }
        // - rename where names and marks match by literal sharing
        { let sc1 = unfold_test_sc(~[M(1),M(2)],empty_ctxt,&mut t);
         let sc = unfold_test_sc(~[R(id(a,sc1),50)],sc1,&mut t);
         assert_eq!(resolve_internal(id(a,sc),&mut t), 50); }
        // - two renames of the same var.. can only happen if you use
        // local-expand to prevent the inner binding from being renamed
        // during the rename-pass caused by the first:
        io::println("about to run bad test");
        { let sc = unfold_test_sc(~[R(id(a,empty_ctxt),50),
                                    R(id(a,empty_ctxt),51)],
                                  empty_ctxt,&mut t);
         assert_eq!(resolve_internal(id(a,sc),&mut t), 51); }
        // the simplest double-rename:
        { let a_to_a50 = new_rename_internal(id(a,empty_ctxt),50,empty_ctxt,&mut t);
         let a50_to_a51 = new_rename_internal(id(a,a_to_a50),51,a_to_a50,&mut t);
         assert_eq!(resolve_internal(id(a,a50_to_a51),&mut t),51);
         // mark on the outside doesn't stop rename:
         let sc = new_mark_internal(9,a50_to_a51,&mut t);
         assert_eq!(resolve_internal(id(a,sc),&mut t),51);
         // but mark on the inside does:
         let a50_to_a51_b = unfold_test_sc(~[R(id(a,a_to_a50),51),
                                              M(9)],
                                           a_to_a50,
                                           &mut t);
         assert_eq!(resolve_internal(id(a,a50_to_a51_b),&mut t),50);}
    }

    #[test] fn hashing_tests () {
        let mut t = new_sctable_internal();
        assert_eq!(new_mark_internal(12,empty_ctxt,&mut t),2);
        assert_eq!(new_mark_internal(13,empty_ctxt,&mut t),3);
        // using the same one again should result in the same index:
        assert_eq!(new_mark_internal(12,empty_ctxt,&mut t),2);
        // I'm assuming that the rename table will behave the same....
    }

}