src/librustc/middle/resolve_lifetime.rs

   1 // Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Name resolution for lifetimes.
  12 //!
  13 //! Name resolution for lifetimes follows MUCH simpler rules than the
  14 //! full resolve. For example, lifetime names are never exported or
  15 //! used between functions, and they operate in a purely top-down
  16 //! way. Therefore we break lifetime name resolution into a separate pass.
  17
  18 pub use self::DefRegion::*;
  19 use self::ScopeChain::*;
  20
  21 use session::Session;
  22 use middle::def::{self, DefMap};
  23 use middle::region;
  24 use middle::subst;
  25 use middle::ty;
  26 use std::fmt;
  27 use syntax::ast;
  28 use syntax::codemap::Span;
  29 use syntax::parse::token::special_idents;
  30 use syntax::parse::token;
  31 use syntax::print::pprust::{lifetime_to_string};
  32 use syntax::visit;
  33 use syntax::visit::Visitor;
  34 use util::nodemap::NodeMap;
  35
  36 #[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Show)]
  37 pub enum DefRegion {
  38     DefStaticRegion,
  39     DefEarlyBoundRegion(/* space */ subst::ParamSpace,
  40                         /* index */ u32,
  41                         /* lifetime decl */ ast::NodeId),
  42     DefLateBoundRegion(ty::DebruijnIndex,
  43                        /* lifetime decl */ ast::NodeId),
  44     DefFreeRegion(/* block scope */ region::CodeExtent,
  45                   /* lifetime decl */ ast::NodeId),
  46 }
  47
  48 // maps the id of each lifetime reference to the lifetime decl
  49 // that it corresponds to
  50 pub type NamedRegionMap = NodeMap<DefRegion>;
  51
  52 struct LifetimeContext<'a> {
  53     sess: &'a Session,
  54     named_region_map: &'a mut NamedRegionMap,
  55     scope: Scope<'a>,
  56     def_map: &'a DefMap,
  57 }
  58
  59 enum ScopeChain<'a> {
  60     /// EarlyScope(i, ['a, 'b, ...], s) extends s with early-bound
  61     /// lifetimes, assigning indexes 'a => i, 'b => i+1, ... etc.
  62     EarlyScope(subst::ParamSpace, &'a Vec<ast::LifetimeDef>, Scope<'a>),
  63     /// LateScope(['a, 'b, ...], s) extends s with late-bound
  64     /// lifetimes introduced by the declaration binder_id.
  65     LateScope(&'a Vec<ast::LifetimeDef>, Scope<'a>),
  66     /// lifetimes introduced by items within a code block are scoped
  67     /// to that block.
  68     BlockScope(region::CodeExtent, Scope<'a>),
  69     RootScope
  70 }
  71
  72 type Scope<'a> = &'a ScopeChain<'a>;
  73
  74 static ROOT_SCOPE: ScopeChain<'static> = RootScope;
  75
  76 pub fn krate(sess: &Session, krate: &ast::Crate, def_map: &DefMap) -> NamedRegionMap {
  77     let mut named_region_map = NodeMap::new();
  78     visit::walk_crate(&mut LifetimeContext {
  79         sess: sess,
  80         named_region_map: &mut named_region_map,
  81         scope: &ROOT_SCOPE,
  82         def_map: def_map,
  83     }, krate);
  84     sess.abort_if_errors();
  85     named_region_map
  86 }
  87
  88 impl<'a, 'v> Visitor<'v> for LifetimeContext<'a> {
  89     fn visit_item(&mut self, item: &ast::Item) {
  90         // Items always introduce a new root scope
  91         self.with(RootScope, |_, this| {
  92             match item.node {
  93                 ast::ItemFn(..) => {
  94                     // Fn lifetimes get added in visit_fn below:
  95                     visit::walk_item(this, item);
  96                 }
  97                 ast::ItemMod(..) |
  98                 ast::ItemMac(..) |
  99                 ast::ItemForeignMod(..) |
 100                 ast::ItemStatic(..) |
 101                 ast::ItemConst(..) => {
 102                     // These sorts of items have no lifetime parameters at all.
 103                     visit::walk_item(this, item);
 104                 }
 105                 ast::ItemTy(_, ref generics) |
 106                 ast::ItemEnum(_, ref generics) |
 107                 ast::ItemStruct(_, ref generics) |
 108                 ast::ItemTrait(_, ref generics, _, _) |
 109                 ast::ItemImpl(_, _, ref generics, _, _, _) => {
 110                     // These kinds of items have only early bound lifetime parameters.
 111                     let lifetimes = &generics.lifetimes;
 112                     let early_scope = EarlyScope(subst::TypeSpace, lifetimes, &ROOT_SCOPE);
 113                     this.with(early_scope, |old_scope, this| {
 114                         this.check_lifetime_defs(old_scope, lifetimes);
 115                         visit::walk_item(this, item);
 116                     });
 117                 }
 118             }
 119         });
 120     }
 121
 122     fn visit_fn(&mut self, fk: visit::FnKind<'v>, fd: &'v ast::FnDecl,
 123                 b: &'v ast::Block, s: Span, _: ast::NodeId) {
 124         match fk {
 125             visit::FkItemFn(_, generics, _, _) |
 126             visit::FkMethod(_, generics, _) => {
 127                 self.visit_early_late(subst::FnSpace, generics, |this| {
 128                     visit::walk_fn(this, fk, fd, b, s)
 129                 })
 130             }
 131             visit::FkFnBlock(..) => {
 132                 visit::walk_fn(self, fk, fd, b, s)
 133             }
 134         }
 135     }
 136
 137     fn visit_ty(&mut self, ty: &ast::Ty) {
 138         match ty.node {
 139             ast::TyBareFn(ref c) => {
 140                 visit::walk_lifetime_decls_helper(self, &c.lifetimes);
 141                 self.with(LateScope(&c.lifetimes, self.scope), |old_scope, this| {
 142                     // a bare fn has no bounds, so everything
 143                     // contained within is scoped within its binder.
 144                     this.check_lifetime_defs(old_scope, &c.lifetimes);
 145                     visit::walk_ty(this, ty);
 146                 });
 147             }
 148             ast::TyPath(ref path, id) => {
 149                 // if this path references a trait, then this will resolve to
 150                 // a trait ref, which introduces a binding scope.
 151                 match self.def_map.borrow().get(&id) {
 152                     Some(&def::DefTrait(..)) => {
 153                         self.with(LateScope(&Vec::new(), self.scope), |_, this| {
 154                             this.visit_path(path, id);
 155                         });
 156                     }
 157                     _ => {
 158                         visit::walk_ty(self, ty);
 159                     }
 160                 }
 161             }
 162             _ => {
 163                 visit::walk_ty(self, ty)
 164             }
 165         }
 166     }
 167
 168     fn visit_ty_method(&mut self, m: &ast::TypeMethod) {
 169         self.visit_early_late(
 170             subst::FnSpace, &m.generics,
 171             |this| visit::walk_ty_method(this, m))
 172     }
 173
 174     fn visit_block(&mut self, b: &ast::Block) {
 175         self.with(BlockScope(region::CodeExtent::from_node_id(b.id), self.scope),
 176                   |_, this| visit::walk_block(this, b));
 177     }
 178
 179     fn visit_lifetime_ref(&mut self, lifetime_ref: &ast::Lifetime) {
 180         if lifetime_ref.name == special_idents::static_lifetime.name {
 181             self.insert_lifetime(lifetime_ref, DefStaticRegion);
 182             return;
 183         }
 184         self.resolve_lifetime_ref(lifetime_ref);
 185     }
 186
 187     fn visit_generics(&mut self, generics: &ast::Generics) {
 188         for ty_param in generics.ty_params.iter() {
 189             visit::walk_ty_param_bounds_helper(self, &ty_param.bounds);
 190             match ty_param.default {
 191                 Some(ref ty) => self.visit_ty(&**ty),
 192                 None => {}
 193             }
 194         }
 195         for predicate in generics.where_clause.predicates.iter() {
 196             match predicate {
 197                 &ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate{ ref bounded_ty,
 198                                                                                ref bounds,
 199                                                                                .. }) => {
 200                     self.visit_ty(&**bounded_ty);
 201                     visit::walk_ty_param_bounds_helper(self, bounds);
 202                 }
 203                 &ast::WherePredicate::RegionPredicate(ast::WhereRegionPredicate{ref lifetime,
 204                                                                                 ref bounds,
 205                                                                                 .. }) => {
 206
 207                     self.visit_lifetime_ref(lifetime);
 208                     for bound in bounds.iter() {
 209                         self.visit_lifetime_ref(bound);
 210                     }
 211                 }
 212                 &ast::WherePredicate::EqPredicate(ast::WhereEqPredicate{ id,
 213                                                                          ref path,
 214                                                                          ref ty,
 215                                                                          .. }) => {
 216                     self.visit_path(path, id);
 217                     self.visit_ty(&**ty);
 218                 }
 219             }
 220         }
 221     }
 222
 223     fn visit_poly_trait_ref(&mut self, trait_ref:
 224                             &ast::PolyTraitRef,
 225                             _modifier: &ast::TraitBoundModifier) {
 226         debug!("visit_poly_trait_ref trait_ref={:?}", trait_ref);
 227
 228         self.with(LateScope(&trait_ref.bound_lifetimes, self.scope), |old_scope, this| {
 229             this.check_lifetime_defs(old_scope, &trait_ref.bound_lifetimes);
 230             for lifetime in trait_ref.bound_lifetimes.iter() {
 231                 this.visit_lifetime_def(lifetime);
 232             }
 233             this.visit_trait_ref(&trait_ref.trait_ref)
 234         })
 235     }
 236
 237     fn visit_trait_ref(&mut self, trait_ref: &ast::TraitRef) {
 238         self.visit_path(&trait_ref.path, trait_ref.ref_id);
 239     }
 240 }
 241
 242 impl<'a> LifetimeContext<'a> {
 243     fn with<F>(&mut self, wrap_scope: ScopeChain, f: F) where
 244         F: FnOnce(Scope, &mut LifetimeContext),
 245     {
 246         let LifetimeContext {sess, ref mut named_region_map, ..} = *self;
 247         let mut this = LifetimeContext {
 248             sess: sess,
 249             named_region_map: *named_region_map,
 250             scope: &wrap_scope,
 251             def_map: self.def_map,
 252         };
 253         debug!("entering scope {:?}", this.scope);
 254         f(self.scope, &mut this);
 255         debug!("exiting scope {:?}", this.scope);
 256     }
 257
 258     /// Visits self by adding a scope and handling recursive walk over the contents with `walk`.
 259     ///
 260     /// Handles visiting fns and methods. These are a bit complicated because we must distinguish
 261     /// early- vs late-bound lifetime parameters. We do this by checking which lifetimes appear
 262     /// within type bounds; those are early bound lifetimes, and the rest are late bound.
 263     ///
 264     /// For example:
 265     ///
 266     ///    fn foo<'a,'b,'c,T:Trait<'b>>(...)
 267     ///
 268     /// Here `'a` and `'c` are late bound but `'b` is early bound. Note that early- and late-bound
 269     /// lifetimes may be interspersed together.
 270     ///
 271     /// If early bound lifetimes are present, we separate them into their own list (and likewise
 272     /// for late bound). They will be numbered sequentially, starting from the lowest index that is
 273     /// already in scope (for a fn item, that will be 0, but for a method it might not be). Late
 274     /// bound lifetimes are resolved by name and associated with a binder id (`binder_id`), so the
 275     /// ordering is not important there.
 276     fn visit_early_late<F>(&mut self,
 277                            early_space: subst::ParamSpace,
 278                            generics: &ast::Generics,
 279                            walk: F) where
 280         F: FnOnce(&mut LifetimeContext),
 281     {
 282         let referenced_idents = early_bound_lifetime_names(generics);
 283
 284         debug!("visit_early_late: referenced_idents={:?}",
 285                referenced_idents);
 286
 287         let (early, late): (Vec<_>, _) = generics.lifetimes.iter().cloned().partition(
 288             |l| referenced_idents.iter().any(|&i| i == l.lifetime.name));
 289
 290         self.with(EarlyScope(early_space, &early, self.scope), move |old_scope, this| {
 291             this.with(LateScope(&late, this.scope), move |_, this| {
 292                 this.check_lifetime_defs(old_scope, &generics.lifetimes);
 293                 walk(this);
 294             });
 295         });
 296     }
 297
 298     fn resolve_lifetime_ref(&mut self, lifetime_ref: &ast::Lifetime) {
 299         // Walk up the scope chain, tracking the number of fn scopes
 300         // that we pass through, until we find a lifetime with the
 301         // given name or we run out of scopes. If we encounter a code
 302         // block, then the lifetime is not bound but free, so switch
 303         // over to `resolve_free_lifetime_ref()` to complete the
 304         // search.
 305         let mut late_depth = 0;
 306         let mut scope = self.scope;
 307         loop {
 308             match *scope {
 309                 BlockScope(blk_scope, s) => {
 310                     return self.resolve_free_lifetime_ref(blk_scope, lifetime_ref, s);
 311                 }
 312
 313                 RootScope => {
 314                     break;
 315                 }
 316
 317                 EarlyScope(space, lifetimes, s) => {
 318                     match search_lifetimes(lifetimes, lifetime_ref) {
 319                         Some((index, lifetime_def)) => {
 320                             let decl_id = lifetime_def.id;
 321                             let def = DefEarlyBoundRegion(space, index, decl_id);
 322                             self.insert_lifetime(lifetime_ref, def);
 323                             return;
 324                         }
 325                         None => {
 326                             scope = s;
 327                         }
 328                     }
 329                 }
 330
 331                 LateScope(lifetimes, s) => {
 332                     match search_lifetimes(lifetimes, lifetime_ref) {
 333                         Some((_index, lifetime_def)) => {
 334                             let decl_id = lifetime_def.id;
 335                             let debruijn = ty::DebruijnIndex::new(late_depth + 1);
 336                             let def = DefLateBoundRegion(debruijn, decl_id);
 337                             self.insert_lifetime(lifetime_ref, def);
 338                             return;
 339                         }
 340
 341                         None => {
 342                             late_depth += 1;
 343                             scope = s;
 344                         }
 345                     }
 346                 }
 347             }
 348         }
 349
 350         self.unresolved_lifetime_ref(lifetime_ref);
 351     }
 352
 353     fn resolve_free_lifetime_ref(&mut self,
 354                                  scope_data: region::CodeExtent,
 355                                  lifetime_ref: &ast::Lifetime,
 356                                  scope: Scope) {
 357         // Walk up the scope chain, tracking the outermost free scope,
 358         // until we encounter a scope that contains the named lifetime
 359         // or we run out of scopes.
 360         let mut scope_data = scope_data;
 361         let mut scope = scope;
 362         let mut search_result = None;
 363         loop {
 364             match *scope {
 365                 BlockScope(blk_scope_data, s) => {
 366                     scope_data = blk_scope_data;
 367                     scope = s;
 368                 }
 369
 370                 RootScope => {
 371                     break;
 372                 }
 373
 374                 EarlyScope(_, lifetimes, s) |
 375                 LateScope(lifetimes, s) => {
 376                     search_result = search_lifetimes(lifetimes, lifetime_ref);
 377                     if search_result.is_some() {
 378                         break;
 379                     }
 380                     scope = s;
 381                 }
 382             }
 383         }
 384
 385         match search_result {
 386             Some((_depth, lifetime)) => {
 387                 let def = DefFreeRegion(scope_data, lifetime.id);
 388                 self.insert_lifetime(lifetime_ref, def);
 389             }
 390
 391             None => {
 392                 self.unresolved_lifetime_ref(lifetime_ref);
 393             }
 394         }
 395
 396     }
 397
 398     fn unresolved_lifetime_ref(&self, lifetime_ref: &ast::Lifetime) {
 399         self.sess.span_err(
 400             lifetime_ref.span,
 401             &format!("use of undeclared lifetime name `{}`",
 402                     token::get_name(lifetime_ref.name))[]);
 403     }
 404
 405     fn check_lifetime_defs(&mut self, old_scope: Scope, lifetimes: &Vec<ast::LifetimeDef>) {
 406         for i in range(0, lifetimes.len()) {
 407             let lifetime_i = &lifetimes[i];
 408
 409             let special_idents = [special_idents::static_lifetime];
 410             for lifetime in lifetimes.iter() {
 411                 if special_idents.iter().any(|&i| i.name == lifetime.lifetime.name) {
 412                     self.sess.span_err(
 413                         lifetime.lifetime.span,
 414                         &format!("illegal lifetime parameter name: `{}`",
 415                                 token::get_name(lifetime.lifetime.name))
 416                         []);
 417                 }
 418             }
 419
 420             // It is a hard error to shadow a lifetime within the same scope.
 421             for j in range(i + 1, lifetimes.len()) {
 422                 let lifetime_j = &lifetimes[j];
 423
 424                 if lifetime_i.lifetime.name == lifetime_j.lifetime.name {
 425                     self.sess.span_err(
 426                         lifetime_j.lifetime.span,
 427                         &format!("lifetime name `{}` declared twice in \
 428                                 the same scope",
 429                                 token::get_name(lifetime_j.lifetime.name))
 430                         []);
 431                 }
 432             }
 433
 434             // It is a soft error to shadow a lifetime within a parent scope.
 435             self.check_lifetime_def_for_shadowing(old_scope, &lifetime_i.lifetime);
 436
 437             for bound in lifetime_i.bounds.iter() {
 438                 self.resolve_lifetime_ref(bound);
 439             }
 440         }
 441     }
 442
 443     fn check_lifetime_def_for_shadowing(&self,
 444                                         mut old_scope: Scope,
 445                                         lifetime: &ast::Lifetime)
 446     {
 447         loop {
 448             match *old_scope {
 449                 BlockScope(_, s) => {
 450                     old_scope = s;
 451                 }
 452
 453                 RootScope => {
 454                     return;
 455                 }
 456
 457                 EarlyScope(_, lifetimes, s) |
 458                 LateScope(lifetimes, s) => {
 459                     if let Some((_, lifetime_def)) = search_lifetimes(lifetimes, lifetime) {
 460                         self.sess.span_warn(
 461                             lifetime.span,
 462                             format!("lifetime name `{}` shadows another \
 463                                     lifetime name that is already in scope",
 464                                     token::get_name(lifetime.name)).as_slice());
 465                         self.sess.span_help(
 466                             lifetime_def.span,
 467                             format!("shadowed lifetime `{}` declared here",
 468                                     token::get_name(lifetime.name)).as_slice());
 469                         self.sess.span_help(
 470                             lifetime.span,
 471                             "shadowed lifetimes are deprecated \
 472                              and will become a hard error before 1.0");
 473                         return;
 474                     }
 475
 476                     old_scope = s;
 477                 }
 478             }
 479         }
 480     }
 481
 482     fn insert_lifetime(&mut self,
 483                        lifetime_ref: &ast::Lifetime,
 484                        def: DefRegion) {
 485         if lifetime_ref.id == ast::DUMMY_NODE_ID {
 486             self.sess.span_bug(lifetime_ref.span,
 487                                "lifetime reference not renumbered, \
 488                                probably a bug in syntax::fold");
 489         }
 490
 491         debug!("lifetime_ref={:?} id={:?} resolved to {:?}",
 492                 lifetime_to_string(lifetime_ref),
 493                 lifetime_ref.id,
 494                 def);
 495         self.named_region_map.insert(lifetime_ref.id, def);
 496     }
 497 }
 498
 499 fn search_lifetimes<'a>(lifetimes: &'a Vec<ast::LifetimeDef>,
 500                     lifetime_ref: &ast::Lifetime)
 501                     -> Option<(u32, &'a ast::Lifetime)> {
 502     for (i, lifetime_decl) in lifetimes.iter().enumerate() {
 503         if lifetime_decl.lifetime.name == lifetime_ref.name {
 504             return Some((i as u32, &lifetime_decl.lifetime));
 505         }
 506     }
 507     return None;
 508 }
 509
 510 ///////////////////////////////////////////////////////////////////////////
 511
 512 pub fn early_bound_lifetimes<'a>(generics: &'a ast::Generics) -> Vec<ast::LifetimeDef> {
 513     let referenced_idents = early_bound_lifetime_names(generics);
 514     if referenced_idents.is_empty() {
 515         return Vec::new();
 516     }
 517
 518     generics.lifetimes.iter()
 519         .filter(|l| referenced_idents.iter().any(|&i| i == l.lifetime.name))
 520         .map(|l| (*l).clone())
 521         .collect()
 522 }
 523
 524 /// Given a set of generic declarations, returns a list of names containing all early bound
 525 /// lifetime names for those generics. (In fact, this list may also contain other names.)
 526 fn early_bound_lifetime_names(generics: &ast::Generics) -> Vec<ast::Name> {
 527     // Create two lists, dividing the lifetimes into early/late bound.
 528     // Initially, all of them are considered late, but we will move
 529     // things from late into early as we go if we find references to
 530     // them.
 531     let mut early_bound = Vec::new();
 532     let mut late_bound = generics.lifetimes.iter()
 533                                            .map(|l| l.lifetime.name)
 534                                            .collect();
 535
 536     // Any lifetime that appears in a type bound is early.
 537     {
 538         let mut collector =
 539             FreeLifetimeCollector { early_bound: &mut early_bound,
 540                                     late_bound: &mut late_bound };
 541         for ty_param in generics.ty_params.iter() {
 542             visit::walk_ty_param_bounds_helper(&mut collector, &ty_param.bounds);
 543         }
 544         for predicate in generics.where_clause.predicates.iter() {
 545             match predicate {
 546                 &ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate{ref bounds,
 547                                                                               ref bounded_ty,
 548                                                                               ..}) => {
 549                     collector.visit_ty(&**bounded_ty);
 550                     visit::walk_ty_param_bounds_helper(&mut collector, bounds);
 551                 }
 552                 &ast::WherePredicate::RegionPredicate(ast::WhereRegionPredicate{ref lifetime,
 553                                                                                 ref bounds,
 554                                                                                 ..}) => {
 555                     collector.visit_lifetime_ref(lifetime);
 556
 557                     for bound in bounds.iter() {
 558                         collector.visit_lifetime_ref(bound);
 559                     }
 560                 }
 561                 &ast::WherePredicate::EqPredicate(_) => unimplemented!()
 562             }
 563         }
 564     }
 565
 566     // Any lifetime that either has a bound or is referenced by a
 567     // bound is early.
 568     for lifetime_def in generics.lifetimes.iter() {
 569         if !lifetime_def.bounds.is_empty() {
 570             shuffle(&mut early_bound, &mut late_bound,
 571                     lifetime_def.lifetime.name);
 572             for bound in lifetime_def.bounds.iter() {
 573                 shuffle(&mut early_bound, &mut late_bound,
 574                         bound.name);
 575             }
 576         }
 577     }
 578     return early_bound;
 579
 580     struct FreeLifetimeCollector<'a> {
 581         early_bound: &'a mut Vec<ast::Name>,
 582         late_bound: &'a mut Vec<ast::Name>,
 583     }
 584
 585     impl<'a, 'v> Visitor<'v> for FreeLifetimeCollector<'a> {
 586         fn visit_lifetime_ref(&mut self, lifetime_ref: &ast::Lifetime) {
 587             shuffle(self.early_bound, self.late_bound,
 588                     lifetime_ref.name);
 589         }
 590     }
 591
 592     fn shuffle(early_bound: &mut Vec<ast::Name>,
 593                late_bound: &mut Vec<ast::Name>,
 594                name: ast::Name) {
 595         match late_bound.iter().position(|n| *n == name) {
 596             Some(index) => {
 597                 late_bound.swap_remove(index);
 598                 early_bound.push(name);
 599             }
 600             None => { }
 601         }
 602     }
 603 }
 604
 605 impl<'a> fmt::Show for ScopeChain<'a> {
 606     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
 607         match *self {
 608             EarlyScope(space, defs, _) => write!(fmt, "EarlyScope({:?}, {:?})", space, defs),
 609             LateScope(defs, _) => write!(fmt, "LateScope({:?})", defs),
 610             BlockScope(id, _) => write!(fmt, "BlockScope({:?})", id),
 611             RootScope => write!(fmt, "RootScope"),
 612         }
 613     }
 614 }