src/librustc_typeck/check/method/confirm.rs

   1 // Copyright 2014 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 use super::probe;
  12
  13 use check::{self, FnCtxt, NoPreference, PreferMutLvalue, callee, demand};
  14 use check::UnresolvedTypeAction;
  15 use middle::def_id::DefId;
  16 use middle::subst::{self};
  17 use middle::traits;
  18 use middle::ty::{self, Ty};
  19 use middle::ty_fold::TypeFoldable;
  20 use middle::infer;
  21 use middle::infer::InferCtxt;
  22 use syntax::ast;
  23 use syntax::codemap::Span;
  24
  25 struct ConfirmContext<'a, 'tcx:'a> {
  26     fcx: &'a FnCtxt<'a, 'tcx>,
  27     span: Span,
  28     self_expr: &'tcx ast::Expr,
  29     call_expr: &'tcx ast::Expr,
  30 }
  31
  32 struct InstantiatedMethodSig<'tcx> {
  33     /// Function signature of the method being invoked. The 0th
  34     /// argument is the receiver.
  35     method_sig: ty::FnSig<'tcx>,
  36
  37     /// Substitutions for all types/early-bound-regions declared on
  38     /// the method.
  39     all_substs: subst::Substs<'tcx>,
  40
  41     /// Generic bounds on the method's parameters which must be added
  42     /// as pending obligations.
  43     method_predicates: ty::InstantiatedPredicates<'tcx>,
  44 }
  45
  46 pub fn confirm<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
  47                          span: Span,
  48                          self_expr: &'tcx ast::Expr,
  49                          call_expr: &'tcx ast::Expr,
  50                          unadjusted_self_ty: Ty<'tcx>,
  51                          pick: probe::Pick<'tcx>,
  52                          supplied_method_types: Vec<Ty<'tcx>>)
  53                          -> ty::MethodCallee<'tcx>
  54 {
  55     debug!("confirm(unadjusted_self_ty={:?}, pick={:?}, supplied_method_types={:?})",
  56            unadjusted_self_ty,
  57            pick,
  58            supplied_method_types);
  59
  60     let mut confirm_cx = ConfirmContext::new(fcx, span, self_expr, call_expr);
  61     confirm_cx.confirm(unadjusted_self_ty, pick, supplied_method_types)
  62 }
  63
  64 impl<'a,'tcx> ConfirmContext<'a,'tcx> {
  65     fn new(fcx: &'a FnCtxt<'a, 'tcx>,
  66            span: Span,
  67            self_expr: &'tcx ast::Expr,
  68            call_expr: &'tcx ast::Expr)
  69            -> ConfirmContext<'a, 'tcx>
  70     {
  71         ConfirmContext { fcx: fcx, span: span, self_expr: self_expr, call_expr: call_expr }
  72     }
  73
  74     fn confirm(&mut self,
  75                unadjusted_self_ty: Ty<'tcx>,
  76                pick: probe::Pick<'tcx>,
  77                supplied_method_types: Vec<Ty<'tcx>>)
  78                -> ty::MethodCallee<'tcx>
  79     {
  80         // Adjust the self expression the user provided and obtain the adjusted type.
  81         let self_ty = self.adjust_self_ty(unadjusted_self_ty, &pick);
  82
  83         // Make sure nobody calls `drop()` explicitly.
  84         self.enforce_illegal_method_limitations(&pick);
  85
  86         // Create substitutions for the method's type parameters.
  87         let rcvr_substs = self.fresh_receiver_substs(self_ty, &pick);
  88         let all_substs =
  89             self.instantiate_method_substs(
  90                 &pick,
  91                 supplied_method_types,
  92                 rcvr_substs);
  93
  94         debug!("all_substs={:?}", all_substs);
  95
  96         // Create the final signature for the method, replacing late-bound regions.
  97         let InstantiatedMethodSig {
  98             method_sig, all_substs, method_predicates
  99         } = self.instantiate_method_sig(&pick, all_substs);
 100         let method_self_ty = method_sig.inputs[0];
 101
 102         // Unify the (adjusted) self type with what the method expects.
 103         self.unify_receivers(self_ty, method_self_ty);
 104
 105         // Add any trait/regions obligations specified on the method's type parameters.
 106         self.add_obligations(&pick, &all_substs, &method_predicates);
 107
 108         // Create the final `MethodCallee`.
 109         let method_ty = pick.item.as_opt_method().unwrap();
 110         let fty = self.tcx().mk_fn(None, self.tcx().mk_bare_fn(ty::BareFnTy {
 111             sig: ty::Binder(method_sig),
 112             unsafety: method_ty.fty.unsafety,
 113             abi: method_ty.fty.abi.clone(),
 114         }));
 115         let callee = ty::MethodCallee {
 116             def_id: pick.item.def_id(),
 117             ty: fty,
 118             substs: self.tcx().mk_substs(all_substs)
 119         };
 120
 121         // If this is an `&mut self` method, bias the receiver
 122         // expression towards mutability (this will switch
 123         // e.g. `Deref` to `DerefMut` in overloaded derefs and so on).
 124         self.fixup_derefs_on_method_receiver_if_necessary(&callee);
 125
 126         callee
 127     }
 128
 129     ///////////////////////////////////////////////////////////////////////////
 130     // ADJUSTMENTS
 131
 132     fn adjust_self_ty(&mut self,
 133                       unadjusted_self_ty: Ty<'tcx>,
 134                       pick: &probe::Pick<'tcx>)
 135                       -> Ty<'tcx>
 136     {
 137         let (autoref, unsize) = if let Some(mutbl) = pick.autoref {
 138             let region = self.infcx().next_region_var(infer::Autoref(self.span));
 139             let autoref = ty::AutoPtr(self.tcx().mk_region(region), mutbl);
 140             (Some(autoref), pick.unsize.map(|target| {
 141                 target.adjust_for_autoref(self.tcx(), Some(autoref))
 142             }))
 143         } else {
 144             // No unsizing should be performed without autoref (at
 145             // least during method dispach). This is because we
 146             // currently only unsize `[T;N]` to `[T]`, and naturally
 147             // that must occur being a reference.
 148             assert!(pick.unsize.is_none());
 149             (None, None)
 150         };
 151
 152         // Commit the autoderefs by calling `autoderef again, but this
 153         // time writing the results into the various tables.
 154         let (autoderefd_ty, n, result) = check::autoderef(self.fcx,
 155                                                           self.span,
 156                                                           unadjusted_self_ty,
 157                                                           Some(self.self_expr),
 158                                                           UnresolvedTypeAction::Error,
 159                                                           NoPreference,
 160                                                           |_, n| {
 161             if n == pick.autoderefs {
 162                 Some(())
 163             } else {
 164                 None
 165             }
 166         });
 167         assert_eq!(n, pick.autoderefs);
 168         assert_eq!(result, Some(()));
 169
 170         // Write out the final adjustment.
 171         self.fcx.write_adjustment(self.self_expr.id,
 172                                   ty::AdjustDerefRef(ty::AutoDerefRef {
 173             autoderefs: pick.autoderefs,
 174             autoref: autoref,
 175             unsize: unsize
 176         }));
 177
 178         if let Some(target) = unsize {
 179             target
 180         } else {
 181             autoderefd_ty.adjust_for_autoref(self.tcx(), autoref)
 182         }
 183     }
 184
 185     ///////////////////////////////////////////////////////////////////////////
 186     //
 187
 188     /// Returns a set of substitutions for the method *receiver* where all type and region
 189     /// parameters are instantiated with fresh variables. This substitution does not include any
 190     /// parameters declared on the method itself.
 191     ///
 192     /// Note that this substitution may include late-bound regions from the impl level. If so,
 193     /// these are instantiated later in the `instantiate_method_sig` routine.
 194     fn fresh_receiver_substs(&mut self,
 195                              self_ty: Ty<'tcx>,
 196                              pick: &probe::Pick<'tcx>)
 197                              -> subst::Substs<'tcx>
 198     {
 199         match pick.kind {
 200             probe::InherentImplPick => {
 201                 let impl_def_id = pick.item.container().id();
 202                 assert!(self.tcx().impl_trait_ref(impl_def_id).is_none(),
 203                         "impl {:?} is not an inherent impl", impl_def_id);
 204                 check::impl_self_ty(self.fcx, self.span, impl_def_id).substs
 205             }
 206
 207             probe::ObjectPick => {
 208                 let trait_def_id = pick.item.container().id();
 209                 self.extract_trait_ref(self_ty, |this, object_ty, data| {
 210                     // The object data has no entry for the Self
 211                     // Type. For the purposes of this method call, we
 212                     // substitute the object type itself. This
 213                     // wouldn't be a sound substitution in all cases,
 214                     // since each instance of the object type is a
 215                     // different existential and hence could match
 216                     // distinct types (e.g., if `Self` appeared as an
 217                     // argument type), but those cases have already
 218                     // been ruled out when we deemed the trait to be
 219                     // "object safe".
 220                     let original_poly_trait_ref =
 221                         data.principal_trait_ref_with_self_ty(this.tcx(), object_ty);
 222                     let upcast_poly_trait_ref =
 223                         this.upcast(original_poly_trait_ref.clone(), trait_def_id);
 224                     let upcast_trait_ref =
 225                         this.replace_late_bound_regions_with_fresh_var(&upcast_poly_trait_ref);
 226                     debug!("original_poly_trait_ref={:?} upcast_trait_ref={:?} target_trait={:?}",
 227                            original_poly_trait_ref,
 228                            upcast_trait_ref,
 229                            trait_def_id);
 230                     upcast_trait_ref.substs.clone()
 231                 })
 232             }
 233
 234             probe::ExtensionImplPick(impl_def_id) => {
 235                 // The method being invoked is the method as defined on the trait,
 236                 // so return the substitutions from the trait. Consider:
 237                 //
 238                 //     impl<A,B,C> Trait<A,B> for Foo<C> { ... }
 239                 //
 240                 // If we instantiate A, B, and C with $A, $B, and $C
 241                 // respectively, then we want to return the type
 242                 // parameters from the trait ([$A,$B]), not those from
 243                 // the impl ([$A,$B,$C]) not the receiver type ([$C]).
 244                 let impl_polytype = check::impl_self_ty(self.fcx, self.span, impl_def_id);
 245                 let impl_trait_ref =
 246                     self.fcx.instantiate_type_scheme(
 247                         self.span,
 248                         &impl_polytype.substs,
 249                         &self.tcx().impl_trait_ref(impl_def_id).unwrap());
 250                 impl_trait_ref.substs.clone()
 251             }
 252
 253             probe::TraitPick => {
 254                 let trait_def_id = pick.item.container().id();
 255                 let trait_def = self.tcx().lookup_trait_def(trait_def_id);
 256
 257                 // Make a trait reference `$0 : Trait<$1...$n>`
 258                 // consisting entirely of type variables. Later on in
 259                 // the process we will unify the transformed-self-type
 260                 // of the method with the actual type in order to
 261                 // unify some of these variables.
 262                 self.infcx().fresh_substs_for_trait(self.span,
 263                                                     &trait_def.generics,
 264                                                     self.infcx().next_ty_var())
 265             }
 266
 267             probe::WhereClausePick(ref poly_trait_ref) => {
 268                 // Where clauses can have bound regions in them. We need to instantiate
 269                 // those to convert from a poly-trait-ref to a trait-ref.
 270                 self.replace_late_bound_regions_with_fresh_var(&*poly_trait_ref).substs.clone()
 271             }
 272         }
 273     }
 274
 275     fn extract_trait_ref<R, F>(&mut self, self_ty: Ty<'tcx>, mut closure: F) -> R where
 276         F: FnMut(&mut ConfirmContext<'a, 'tcx>, Ty<'tcx>, &ty::TraitTy<'tcx>) -> R,
 277     {
 278         // If we specified that this is an object method, then the
 279         // self-type ought to be something that can be dereferenced to
 280         // yield an object-type (e.g., `&Object` or `Box<Object>`
 281         // etc).
 282
 283         let (_, _, result) = check::autoderef(self.fcx,
 284                                               self.span,
 285                                               self_ty,
 286                                               None,
 287                                               UnresolvedTypeAction::Error,
 288                                               NoPreference,
 289                                               |ty, _| {
 290             match ty.sty {
 291                 ty::TyTrait(ref data) => Some(closure(self, ty, &**data)),
 292                 _ => None,
 293             }
 294         });
 295
 296         match result {
 297             Some(r) => r,
 298             None => {
 299                 self.tcx().sess.span_bug(
 300                     self.span,
 301                     &format!("self-type `{}` for ObjectPick never dereferenced to an object",
 302                             self_ty))
 303             }
 304         }
 305     }
 306
 307     fn instantiate_method_substs(&mut self,
 308                                  pick: &probe::Pick<'tcx>,
 309                                  supplied_method_types: Vec<Ty<'tcx>>,
 310                                  substs: subst::Substs<'tcx>)
 311                                  -> subst::Substs<'tcx>
 312     {
 313         // Determine the values for the generic parameters of the method.
 314         // If they were not explicitly supplied, just construct fresh
 315         // variables.
 316         let num_supplied_types = supplied_method_types.len();
 317         let method = pick.item.as_opt_method().unwrap();
 318         let method_types = method.generics.types.get_slice(subst::FnSpace);
 319         let num_method_types = method_types.len();
 320
 321
 322         // Create subst for early-bound lifetime parameters, combining
 323         // parameters from the type and those from the method.
 324         //
 325         // FIXME -- permit users to manually specify lifetimes
 326         let method_regions =
 327             self.fcx.infcx().region_vars_for_defs(
 328                 self.span,
 329                 pick.item.as_opt_method().unwrap()
 330                     .generics.regions.get_slice(subst::FnSpace));
 331
 332         let subst::Substs { types, regions } = substs;
 333         let regions = regions.map(|r| r.with_vec(subst::FnSpace, method_regions));
 334         let mut final_substs = subst::Substs { types: types, regions: regions };
 335
 336         if num_supplied_types == 0 {
 337             self.fcx.infcx().type_vars_for_defs(
 338                 self.span,
 339                 subst::FnSpace,
 340                 &mut final_substs,
 341                 method_types);
 342         } else if num_method_types == 0 {
 343             span_err!(self.tcx().sess, self.span, E0035,
 344                 "does not take type parameters");
 345             self.fcx.infcx().type_vars_for_defs(
 346                 self.span,
 347                 subst::FnSpace,
 348                 &mut final_substs,
 349                 method_types);
 350         } else if num_supplied_types != num_method_types {
 351             span_err!(self.tcx().sess, self.span, E0036,
 352                 "incorrect number of type parameters given for this method");
 353             final_substs.types.replace(
 354                 subst::FnSpace,
 355                 vec![self.tcx().types.err; num_method_types]);
 356         } else {
 357             final_substs.types.replace(subst::FnSpace, supplied_method_types);
 358         }
 359
 360         return final_substs;
 361     }
 362
 363     fn unify_receivers(&mut self,
 364                        self_ty: Ty<'tcx>,
 365                        method_self_ty: Ty<'tcx>)
 366     {
 367         match self.fcx.mk_subty(false, infer::Misc(self.span), self_ty, method_self_ty) {
 368             Ok(_) => {}
 369             Err(_) => {
 370                 self.tcx().sess.span_bug(
 371                     self.span,
 372                     &format!("{} was a subtype of {} but now is not?",
 373                              self_ty, method_self_ty));
 374             }
 375         }
 376     }
 377
 378     ///////////////////////////////////////////////////////////////////////////
 379     //
 380
 381     fn instantiate_method_sig(&mut self,
 382                               pick: &probe::Pick<'tcx>,
 383                               all_substs: subst::Substs<'tcx>)
 384                               -> InstantiatedMethodSig<'tcx>
 385     {
 386         debug!("instantiate_method_sig(pick={:?}, all_substs={:?})",
 387                pick,
 388                all_substs);
 389
 390         // Instantiate the bounds on the method with the
 391         // type/early-bound-regions substitutions performed. There can
 392         // be no late-bound regions appearing here.
 393         let method_predicates = pick.item.as_opt_method().unwrap()
 394                                     .predicates.instantiate(self.tcx(), &all_substs);
 395         let method_predicates = self.fcx.normalize_associated_types_in(self.span,
 396                                                                        &method_predicates);
 397
 398         debug!("method_predicates after subst = {:?}",
 399                method_predicates);
 400
 401         // Instantiate late-bound regions and substitute the trait
 402         // parameters into the method type to get the actual method type.
 403         //
 404         // NB: Instantiate late-bound regions first so that
 405         // `instantiate_type_scheme` can normalize associated types that
 406         // may reference those regions.
 407         let method_sig = self.replace_late_bound_regions_with_fresh_var(
 408             &pick.item.as_opt_method().unwrap().fty.sig);
 409         debug!("late-bound lifetimes from method instantiated, method_sig={:?}",
 410                method_sig);
 411
 412         let method_sig = self.fcx.instantiate_type_scheme(self.span, &all_substs, &method_sig);
 413         debug!("type scheme substituted, method_sig={:?}",
 414                method_sig);
 415
 416         InstantiatedMethodSig {
 417             method_sig: method_sig,
 418             all_substs: all_substs,
 419             method_predicates: method_predicates,
 420         }
 421     }
 422
 423     fn add_obligations(&mut self,
 424                        pick: &probe::Pick<'tcx>,
 425                        all_substs: &subst::Substs<'tcx>,
 426                        method_predicates: &ty::InstantiatedPredicates<'tcx>) {
 427         debug!("add_obligations: pick={:?} all_substs={:?} method_predicates={:?}",
 428                pick,
 429                all_substs,
 430                method_predicates);
 431
 432         self.fcx.add_obligations_for_parameters(
 433             traits::ObligationCause::misc(self.span, self.fcx.body_id),
 434             method_predicates);
 435
 436         // this is a projection from a trait reference, so we have to
 437         // make sure that the trait reference inputs are well-formed.
 438         self.fcx.add_wf_bounds(
 439             all_substs,
 440             self.call_expr);
 441     }
 442
 443     ///////////////////////////////////////////////////////////////////////////
 444     // RECONCILIATION
 445
 446     /// When we select a method with an `&mut self` receiver, we have to go convert any
 447     /// auto-derefs, indices, etc from `Deref` and `Index` into `DerefMut` and `IndexMut`
 448     /// respectively.
 449     fn fixup_derefs_on_method_receiver_if_necessary(&self,
 450                                                     method_callee: &ty::MethodCallee) {
 451         let sig = match method_callee.ty.sty {
 452             ty::TyBareFn(_, ref f) => f.sig.clone(),
 453             _ => return,
 454         };
 455
 456         match sig.0.inputs[0].sty {
 457             ty::TyRef(_, ty::TypeAndMut {
 458                 ty: _,
 459                 mutbl: ast::MutMutable,
 460             }) => {}
 461             _ => return,
 462         }
 463
 464         // Gather up expressions we want to munge.
 465         let mut exprs = Vec::new();
 466         exprs.push(self.self_expr);
 467         loop {
 468             let last = exprs[exprs.len() - 1];
 469             match last.node {
 470                 ast::ExprParen(ref expr) |
 471                 ast::ExprField(ref expr, _) |
 472                 ast::ExprTupField(ref expr, _) |
 473                 ast::ExprIndex(ref expr, _) |
 474                 ast::ExprUnary(ast::UnDeref, ref expr) => exprs.push(&**expr),
 475                 _ => break,
 476             }
 477         }
 478
 479         debug!("fixup_derefs_on_method_receiver_if_necessary: exprs={:?}",
 480                exprs);
 481
 482         // Fix up autoderefs and derefs.
 483         for (i, &expr) in exprs.iter().rev().enumerate() {
 484             // Count autoderefs.
 485             let autoderef_count = match self.fcx
 486                                             .inh
 487                                             .tables
 488                                             .borrow()
 489                                             .adjustments
 490                                             .get(&expr.id) {
 491                 Some(&ty::AdjustDerefRef(ref adj)) => adj.autoderefs,
 492                 Some(_) | None => 0,
 493             };
 494
 495             debug!("fixup_derefs_on_method_receiver_if_necessary: i={} expr={:?} \
 496                                                                   autoderef_count={}",
 497                    i, expr, autoderef_count);
 498
 499             if autoderef_count > 0 {
 500                 check::autoderef(self.fcx,
 501                                  expr.span,
 502                                  self.fcx.expr_ty(expr),
 503                                  Some(expr),
 504                                  UnresolvedTypeAction::Error,
 505                                  PreferMutLvalue,
 506                                  |_, autoderefs| {
 507                                      if autoderefs == autoderef_count + 1 {
 508                                          Some(())
 509                                      } else {
 510                                          None
 511                                      }
 512                                  });
 513             }
 514
 515             // Don't retry the first one or we might infinite loop!
 516             if i != 0 {
 517                 match expr.node {
 518                     ast::ExprIndex(ref base_expr, ref index_expr) => {
 519                         // If this is an overloaded index, the
 520                         // adjustment will include an extra layer of
 521                         // autoref because the method is an &self/&mut
 522                         // self method. We have to peel it off to get
 523                         // the raw adjustment that `try_index_step`
 524                         // expects. This is annoying and horrible. We
 525                         // ought to recode this routine so it doesn't
 526                         // (ab)use the normal type checking paths.
 527                         let adj = self.fcx.inh.tables.borrow().adjustments.get(&base_expr.id)
 528                                                                           .cloned();
 529                         let (autoderefs, unsize) = match adj {
 530                             Some(ty::AdjustDerefRef(adr)) => match adr.autoref {
 531                                 None => {
 532                                     assert!(adr.unsize.is_none());
 533                                     (adr.autoderefs, None)
 534                                 }
 535                                 Some(ty::AutoPtr(_, _)) => {
 536                                     (adr.autoderefs, adr.unsize.map(|target| {
 537                                         target.builtin_deref(false)
 538                                               .expect("fixup: AutoPtr is not &T").ty
 539                                     }))
 540                                 }
 541                                 Some(_) => {
 542                                     self.tcx().sess.span_bug(
 543                                         base_expr.span,
 544                                         &format!("unexpected adjustment autoref {:?}",
 545                                                 adr));
 546                                 }
 547                             },
 548                             None => (0, None),
 549                             Some(_) => {
 550                                 self.tcx().sess.span_bug(
 551                                     base_expr.span,
 552                                     "unexpected adjustment type");
 553                             }
 554                         };
 555
 556                         let (adjusted_base_ty, unsize) = if let Some(target) = unsize {
 557                             (target, true)
 558                         } else {
 559                             (self.fcx.adjust_expr_ty(base_expr,
 560                                 Some(&ty::AdjustDerefRef(ty::AutoDerefRef {
 561                                     autoderefs: autoderefs,
 562                                     autoref: None,
 563                                     unsize: None
 564                                 }))), false)
 565                         };
 566                         let index_expr_ty = self.fcx.expr_ty(&**index_expr);
 567
 568                         let result = check::try_index_step(
 569                             self.fcx,
 570                             ty::MethodCall::expr(expr.id),
 571                             expr,
 572                             &**base_expr,
 573                             adjusted_base_ty,
 574                             autoderefs,
 575                             unsize,
 576                             PreferMutLvalue,
 577                             index_expr_ty);
 578
 579                         if let Some((input_ty, return_ty)) = result {
 580                             demand::suptype(self.fcx, index_expr.span, input_ty, index_expr_ty);
 581
 582                             let expr_ty = self.fcx.expr_ty(&*expr);
 583                             demand::suptype(self.fcx, expr.span, expr_ty, return_ty);
 584                         }
 585                     }
 586                     ast::ExprUnary(ast::UnDeref, ref base_expr) => {
 587                         // if this is an overloaded deref, then re-evaluate with
 588                         // a preference for mut
 589                         let method_call = ty::MethodCall::expr(expr.id);
 590                         if self.fcx.inh.tables.borrow().method_map.contains_key(&method_call) {
 591                             check::try_overloaded_deref(
 592                                 self.fcx,
 593                                 expr.span,
 594                                 Some(method_call),
 595                                 Some(&**base_expr),
 596                                 self.fcx.expr_ty(&**base_expr),
 597                                 PreferMutLvalue);
 598                         }
 599                     }
 600                     _ => {}
 601                 }
 602             }
 603         }
 604     }
 605
 606     ///////////////////////////////////////////////////////////////////////////
 607     // MISCELLANY
 608
 609     fn tcx(&self) -> &'a ty::ctxt<'tcx> {
 610         self.fcx.tcx()
 611     }
 612
 613     fn infcx(&self) -> &'a InferCtxt<'a, 'tcx> {
 614         self.fcx.infcx()
 615     }
 616
 617     fn enforce_illegal_method_limitations(&self, pick: &probe::Pick) {
 618         // Disallow calls to the method `drop` defined in the `Drop` trait.
 619         match pick.item.container() {
 620             ty::TraitContainer(trait_def_id) => {
 621                 callee::check_legal_trait_for_method_call(self.fcx.ccx, self.span, trait_def_id)
 622             }
 623             ty::ImplContainer(..) => {
 624                 // Since `drop` is a trait method, we expect that any
 625                 // potential calls to it will wind up in the other
 626                 // arm. But just to be sure, check that the method id
 627                 // does not appear in the list of destructors.
 628                 assert!(!self.tcx().destructors.borrow().contains(&pick.item.def_id()));
 629             }
 630         }
 631     }
 632
 633     fn upcast(&mut self,
 634               source_trait_ref: ty::PolyTraitRef<'tcx>,
 635               target_trait_def_id: DefId)
 636               -> ty::PolyTraitRef<'tcx>
 637     {
 638         let upcast_trait_refs = traits::upcast(self.tcx(),
 639                                                source_trait_ref.clone(),
 640                                                target_trait_def_id);
 641
 642         // must be exactly one trait ref or we'd get an ambig error etc
 643         if upcast_trait_refs.len() != 1 {
 644             self.tcx().sess.span_bug(
 645                 self.span,
 646                 &format!("cannot uniquely upcast `{:?}` to `{:?}`: `{:?}`",
 647                          source_trait_ref,
 648                          target_trait_def_id,
 649                          upcast_trait_refs));
 650         }
 651
 652         upcast_trait_refs.into_iter().next().unwrap()
 653     }
 654
 655     fn replace_late_bound_regions_with_fresh_var<T>(&self, value: &ty::Binder<T>) -> T
 656         where T : TypeFoldable<'tcx>
 657     {
 658         self.infcx().replace_late_bound_regions_with_fresh_var(
 659             self.span, infer::FnCall, value).0
 660     }
 661 }