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, 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, NoPreference, PreferMutLvalue, Ty, TyCtxt};
  19 use middle::ty::adjustment::{AdjustDerefRef, AutoDerefRef, AutoPtr};
  20 use middle::ty::fold::TypeFoldable;
  21 use middle::infer;
  22 use middle::infer::{InferCtxt, TypeOrigin};
  23 use syntax::codemap::Span;
  24 use rustc_front::hir;
  25
  26 struct ConfirmContext<'a, 'tcx:'a> {
  27     fcx: &'a FnCtxt<'a, 'tcx>,
  28     span: Span,
  29     self_expr: &'tcx hir::Expr,
  30     call_expr: &'tcx hir::Expr,
  31 }
  32
  33 struct InstantiatedMethodSig<'tcx> {
  34     /// Function signature of the method being invoked. The 0th
  35     /// argument is the receiver.
  36     method_sig: ty::FnSig<'tcx>,
  37
  38     /// Substitutions for all types/early-bound-regions declared on
  39     /// the method.
  40     all_substs: subst::Substs<'tcx>,
  41
  42     /// Generic bounds on the method's parameters which must be added
  43     /// as pending obligations.
  44     method_predicates: ty::InstantiatedPredicates<'tcx>,
  45 }
  46
  47 pub fn confirm<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
  48                          span: Span,
  49                          self_expr: &'tcx hir::Expr,
  50                          call_expr: &'tcx hir::Expr,
  51                          unadjusted_self_ty: Ty<'tcx>,
  52                          pick: probe::Pick<'tcx>,
  53                          supplied_method_types: Vec<Ty<'tcx>>)
  54                          -> ty::MethodCallee<'tcx>
  55 {
  56     debug!("confirm(unadjusted_self_ty={:?}, pick={:?}, supplied_method_types={:?})",
  57            unadjusted_self_ty,
  58            pick,
  59            supplied_method_types);
  60
  61     let mut confirm_cx = ConfirmContext::new(fcx, span, self_expr, call_expr);
  62     confirm_cx.confirm(unadjusted_self_ty, pick, supplied_method_types)
  63 }
  64
  65 impl<'a,'tcx> ConfirmContext<'a,'tcx> {
  66     fn new(fcx: &'a FnCtxt<'a, 'tcx>,
  67            span: Span,
  68            self_expr: &'tcx hir::Expr,
  69            call_expr: &'tcx hir::Expr)
  70            -> ConfirmContext<'a, 'tcx>
  71     {
  72         ConfirmContext { fcx: fcx, span: span, self_expr: self_expr, call_expr: call_expr }
  73     }
  74
  75     fn confirm(&mut self,
  76                unadjusted_self_ty: Ty<'tcx>,
  77                pick: probe::Pick<'tcx>,
  78                supplied_method_types: Vec<Ty<'tcx>>)
  79                -> ty::MethodCallee<'tcx>
  80     {
  81         // Adjust the self expression the user provided and obtain the adjusted type.
  82         let self_ty = self.adjust_self_ty(unadjusted_self_ty, &pick);
  83
  84         // Make sure nobody calls `drop()` explicitly.
  85         self.enforce_illegal_method_limitations(&pick);
  86
  87         // Create substitutions for the method's type parameters.
  88         let rcvr_substs = self.fresh_receiver_substs(self_ty, &pick);
  89         let all_substs =
  90             self.instantiate_method_substs(
  91                 &pick,
  92                 supplied_method_types,
  93                 rcvr_substs);
  94
  95         debug!("all_substs={:?}", all_substs);
  96
  97         // Create the final signature for the method, replacing late-bound regions.
  98         let InstantiatedMethodSig {
  99             method_sig, all_substs, method_predicates
 100         } = self.instantiate_method_sig(&pick, all_substs);
 101         let all_substs = self.tcx().mk_substs(all_substs);
 102         let method_self_ty = method_sig.inputs[0];
 103
 104         // Unify the (adjusted) self type with what the method expects.
 105         self.unify_receivers(self_ty, method_self_ty);
 106
 107         // Create the method type
 108         let def_id = pick.item.def_id();
 109         let method_ty = pick.item.as_opt_method().unwrap();
 110         let fty = self.tcx().mk_fn_def(def_id, all_substs, ty::BareFnTy {
 111             sig: ty::Binder(method_sig),
 112             unsafety: method_ty.fty.unsafety,
 113             abi: method_ty.fty.abi.clone(),
 114         });
 115
 116         // Add any trait/regions obligations specified on the method's type parameters.
 117         self.add_obligations(fty, all_substs, &method_predicates);
 118
 119         // Create the final `MethodCallee`.
 120         let callee = ty::MethodCallee {
 121             def_id: def_id,
 122             ty: fty,
 123             substs: all_substs
 124         };
 125         // If this is an `&mut self` method, bias the receiver
 126         // expression towards mutability (this will switch
 127         // e.g. `Deref` to `DerefMut` in overloaded derefs and so on).
 128         self.fixup_derefs_on_method_receiver_if_necessary(&callee);
 129
 130         callee
 131     }
 132
 133     ///////////////////////////////////////////////////////////////////////////
 134     // ADJUSTMENTS
 135
 136     fn adjust_self_ty(&mut self,
 137                       unadjusted_self_ty: Ty<'tcx>,
 138                       pick: &probe::Pick<'tcx>)
 139                       -> Ty<'tcx>
 140     {
 141         let (autoref, unsize) = if let Some(mutbl) = pick.autoref {
 142             let region = self.infcx().next_region_var(infer::Autoref(self.span));
 143             let autoref = AutoPtr(self.tcx().mk_region(region), mutbl);
 144             (Some(autoref), pick.unsize.map(|target| {
 145                 target.adjust_for_autoref(self.tcx(), Some(autoref))
 146             }))
 147         } else {
 148             // No unsizing should be performed without autoref (at
 149             // least during method dispach). This is because we
 150             // currently only unsize `[T;N]` to `[T]`, and naturally
 151             // that must occur being a reference.
 152             assert!(pick.unsize.is_none());
 153             (None, None)
 154         };
 155
 156         // Commit the autoderefs by calling `autoderef again, but this
 157         // time writing the results into the various tables.
 158         let (autoderefd_ty, n, result) = check::autoderef(self.fcx,
 159                                                           self.span,
 160                                                           unadjusted_self_ty,
 161                                                           || Some(self.self_expr),
 162                                                           UnresolvedTypeAction::Error,
 163                                                           NoPreference,
 164                                                           |_, n| {
 165             if n == pick.autoderefs {
 166                 Some(())
 167             } else {
 168                 None
 169             }
 170         });
 171         assert_eq!(n, pick.autoderefs);
 172         assert_eq!(result, Some(()));
 173
 174         // Write out the final adjustment.
 175         self.fcx.write_adjustment(self.self_expr.id,
 176                                   AdjustDerefRef(AutoDerefRef {
 177             autoderefs: pick.autoderefs,
 178             autoref: autoref,
 179             unsize: unsize
 180         }));
 181
 182         if let Some(target) = unsize {
 183             target
 184         } else {
 185             autoderefd_ty.adjust_for_autoref(self.tcx(), autoref)
 186         }
 187     }
 188
 189     ///////////////////////////////////////////////////////////////////////////
 190     //
 191
 192     /// Returns a set of substitutions for the method *receiver* where all type and region
 193     /// parameters are instantiated with fresh variables. This substitution does not include any
 194     /// parameters declared on the method itself.
 195     ///
 196     /// Note that this substitution may include late-bound regions from the impl level. If so,
 197     /// these are instantiated later in the `instantiate_method_sig` routine.
 198     fn fresh_receiver_substs(&mut self,
 199                              self_ty: Ty<'tcx>,
 200                              pick: &probe::Pick<'tcx>)
 201                              -> subst::Substs<'tcx>
 202     {
 203         match pick.kind {
 204             probe::InherentImplPick => {
 205                 let impl_def_id = pick.item.container().id();
 206                 assert!(self.tcx().impl_trait_ref(impl_def_id).is_none(),
 207                         "impl {:?} is not an inherent impl", impl_def_id);
 208                 check::impl_self_ty(self.fcx, self.span, impl_def_id).substs
 209             }
 210
 211             probe::ObjectPick => {
 212                 let trait_def_id = pick.item.container().id();
 213                 self.extract_trait_ref(self_ty, |this, object_ty, data| {
 214                     // The object data has no entry for the Self
 215                     // Type. For the purposes of this method call, we
 216                     // substitute the object type itself. This
 217                     // wouldn't be a sound substitution in all cases,
 218                     // since each instance of the object type is a
 219                     // different existential and hence could match
 220                     // distinct types (e.g., if `Self` appeared as an
 221                     // argument type), but those cases have already
 222                     // been ruled out when we deemed the trait to be
 223                     // "object safe".
 224                     let original_poly_trait_ref =
 225                         data.principal_trait_ref_with_self_ty(this.tcx(), object_ty);
 226                     let upcast_poly_trait_ref =
 227                         this.upcast(original_poly_trait_ref.clone(), trait_def_id);
 228                     let upcast_trait_ref =
 229                         this.replace_late_bound_regions_with_fresh_var(&upcast_poly_trait_ref);
 230                     debug!("original_poly_trait_ref={:?} upcast_trait_ref={:?} target_trait={:?}",
 231                            original_poly_trait_ref,
 232                            upcast_trait_ref,
 233                            trait_def_id);
 234                     upcast_trait_ref.substs.clone()
 235                 })
 236             }
 237
 238             probe::ExtensionImplPick(impl_def_id) => {
 239                 // The method being invoked is the method as defined on the trait,
 240                 // so return the substitutions from the trait. Consider:
 241                 //
 242                 //     impl<A,B,C> Trait<A,B> for Foo<C> { ... }
 243                 //
 244                 // If we instantiate A, B, and C with $A, $B, and $C
 245                 // respectively, then we want to return the type
 246                 // parameters from the trait ([$A,$B]), not those from
 247                 // the impl ([$A,$B,$C]) not the receiver type ([$C]).
 248                 let impl_polytype = check::impl_self_ty(self.fcx, self.span, impl_def_id);
 249                 let impl_trait_ref =
 250                     self.fcx.instantiate_type_scheme(
 251                         self.span,
 252                         &impl_polytype.substs,
 253                         &self.tcx().impl_trait_ref(impl_def_id).unwrap());
 254                 impl_trait_ref.substs.clone()
 255             }
 256
 257             probe::TraitPick => {
 258                 let trait_def_id = pick.item.container().id();
 259                 let trait_def = self.tcx().lookup_trait_def(trait_def_id);
 260
 261                 // Make a trait reference `$0 : Trait<$1...$n>`
 262                 // consisting entirely of type variables. Later on in
 263                 // the process we will unify the transformed-self-type
 264                 // of the method with the actual type in order to
 265                 // unify some of these variables.
 266                 self.infcx().fresh_substs_for_trait(self.span,
 267                                                     &trait_def.generics,
 268                                                     self.infcx().next_ty_var())
 269             }
 270
 271             probe::WhereClausePick(ref poly_trait_ref) => {
 272                 // Where clauses can have bound regions in them. We need to instantiate
 273                 // those to convert from a poly-trait-ref to a trait-ref.
 274                 self.replace_late_bound_regions_with_fresh_var(&poly_trait_ref).substs.clone()
 275             }
 276         }
 277     }
 278
 279     fn extract_trait_ref<R, F>(&mut self, self_ty: Ty<'tcx>, mut closure: F) -> R where
 280         F: FnMut(&mut ConfirmContext<'a, 'tcx>, Ty<'tcx>, &ty::TraitTy<'tcx>) -> R,
 281     {
 282         // If we specified that this is an object method, then the
 283         // self-type ought to be something that can be dereferenced to
 284         // yield an object-type (e.g., `&Object` or `Box<Object>`
 285         // etc).
 286
 287         let (_, _, result) = check::autoderef(self.fcx,
 288                                               self.span,
 289                                               self_ty,
 290                                               || None,
 291                                               UnresolvedTypeAction::Error,
 292                                               NoPreference,
 293                                               |ty, _| {
 294             match ty.sty {
 295                 ty::TyTrait(ref data) => Some(closure(self, ty, &data)),
 296                 _ => None,
 297             }
 298         });
 299
 300         match result {
 301             Some(r) => r,
 302             None => {
 303                 self.tcx().sess.span_bug(
 304                     self.span,
 305                     &format!("self-type `{}` for ObjectPick never dereferenced to an object",
 306                             self_ty))
 307             }
 308         }
 309     }
 310
 311     fn instantiate_method_substs(&mut self,
 312                                  pick: &probe::Pick<'tcx>,
 313                                  supplied_method_types: Vec<Ty<'tcx>>,
 314                                  substs: subst::Substs<'tcx>)
 315                                  -> subst::Substs<'tcx>
 316     {
 317         // Determine the values for the generic parameters of the method.
 318         // If they were not explicitly supplied, just construct fresh
 319         // variables.
 320         let num_supplied_types = supplied_method_types.len();
 321         let method = pick.item.as_opt_method().unwrap();
 322         let method_types = method.generics.types.get_slice(subst::FnSpace);
 323         let num_method_types = method_types.len();
 324
 325
 326         // Create subst for early-bound lifetime parameters, combining
 327         // parameters from the type and those from the method.
 328         //
 329         // FIXME -- permit users to manually specify lifetimes
 330         let method_regions =
 331             self.fcx.infcx().region_vars_for_defs(
 332                 self.span,
 333                 pick.item.as_opt_method().unwrap()
 334                     .generics.regions.get_slice(subst::FnSpace));
 335
 336         let subst::Substs { types, regions } = substs;
 337         let regions = regions.with_slice(subst::FnSpace, &method_regions);
 338         let mut final_substs = subst::Substs { types: types, regions: regions };
 339
 340         if num_supplied_types == 0 {
 341             self.fcx.infcx().type_vars_for_defs(
 342                 self.span,
 343                 subst::FnSpace,
 344                 &mut final_substs,
 345                 method_types);
 346         } else if num_method_types == 0 {
 347             span_err!(self.tcx().sess, self.span, E0035,
 348                 "does not take type parameters");
 349             self.fcx.infcx().type_vars_for_defs(
 350                 self.span,
 351                 subst::FnSpace,
 352                 &mut final_substs,
 353                 method_types);
 354         } else if num_supplied_types != num_method_types {
 355             span_err!(self.tcx().sess, self.span, E0036,
 356                 "incorrect number of type parameters given for this method: expected {}, found {}",
 357                 num_method_types, num_supplied_types);
 358             final_substs.types.replace(
 359                 subst::FnSpace,
 360                 vec![self.tcx().types.err; num_method_types]);
 361         } else {
 362             final_substs.types.replace(subst::FnSpace, supplied_method_types);
 363         }
 364
 365         return final_substs;
 366     }
 367
 368     fn unify_receivers(&mut self,
 369                        self_ty: Ty<'tcx>,
 370                        method_self_ty: Ty<'tcx>)
 371     {
 372         match self.fcx.mk_subty(false, TypeOrigin::Misc(self.span), self_ty, method_self_ty) {
 373             Ok(_) => {}
 374             Err(_) => {
 375                 self.tcx().sess.span_bug(
 376                     self.span,
 377                     &format!("{} was a subtype of {} but now is not?",
 378                              self_ty, method_self_ty));
 379             }
 380         }
 381     }
 382
 383     ///////////////////////////////////////////////////////////////////////////
 384     //
 385
 386     fn instantiate_method_sig(&mut self,
 387                               pick: &probe::Pick<'tcx>,
 388                               all_substs: subst::Substs<'tcx>)
 389                               -> InstantiatedMethodSig<'tcx>
 390     {
 391         debug!("instantiate_method_sig(pick={:?}, all_substs={:?})",
 392                pick,
 393                all_substs);
 394
 395         // Instantiate the bounds on the method with the
 396         // type/early-bound-regions substitutions performed. There can
 397         // be no late-bound regions appearing here.
 398         let method_predicates = pick.item.as_opt_method().unwrap()
 399                                     .predicates.instantiate(self.tcx(), &all_substs);
 400         let method_predicates = self.fcx.normalize_associated_types_in(self.span,
 401                                                                        &method_predicates);
 402
 403         debug!("method_predicates after subst = {:?}",
 404                method_predicates);
 405
 406         // Instantiate late-bound regions and substitute the trait
 407         // parameters into the method type to get the actual method type.
 408         //
 409         // NB: Instantiate late-bound regions first so that
 410         // `instantiate_type_scheme` can normalize associated types that
 411         // may reference those regions.
 412         let method_sig = self.replace_late_bound_regions_with_fresh_var(
 413             &pick.item.as_opt_method().unwrap().fty.sig);
 414         debug!("late-bound lifetimes from method instantiated, method_sig={:?}",
 415                method_sig);
 416
 417         let method_sig = self.fcx.instantiate_type_scheme(self.span, &all_substs, &method_sig);
 418         debug!("type scheme substituted, method_sig={:?}",
 419                method_sig);
 420
 421         InstantiatedMethodSig {
 422             method_sig: method_sig,
 423             all_substs: all_substs,
 424             method_predicates: method_predicates,
 425         }
 426     }
 427
 428     fn add_obligations(&mut self,
 429                        fty: Ty<'tcx>,
 430                        all_substs: &subst::Substs<'tcx>,
 431                        method_predicates: &ty::InstantiatedPredicates<'tcx>) {
 432         debug!("add_obligations: fty={:?} all_substs={:?} method_predicates={:?}",
 433                fty,
 434                all_substs,
 435                method_predicates);
 436
 437         self.fcx.add_obligations_for_parameters(
 438             traits::ObligationCause::misc(self.span, self.fcx.body_id),
 439             method_predicates);
 440
 441         // this is a projection from a trait reference, so we have to
 442         // make sure that the trait reference inputs are well-formed.
 443         self.fcx.add_wf_bounds(
 444             all_substs,
 445             self.call_expr);
 446
 447         // the function type must also be well-formed (this is not
 448         // implied by the substs being well-formed because of inherent
 449         // impls and late-bound regions - see issue #28609).
 450         self.fcx.register_wf_obligation(fty, self.span, traits::MiscObligation);
 451     }
 452
 453     ///////////////////////////////////////////////////////////////////////////
 454     // RECONCILIATION
 455
 456     /// When we select a method with an `&mut self` receiver, we have to go convert any
 457     /// auto-derefs, indices, etc from `Deref` and `Index` into `DerefMut` and `IndexMut`
 458     /// respectively.
 459     fn fixup_derefs_on_method_receiver_if_necessary(&self,
 460                                                     method_callee: &ty::MethodCallee) {
 461         let sig = match method_callee.ty.sty {
 462             ty::TyFnDef(_, _, ref f) => f.sig.clone(),
 463             _ => return,
 464         };
 465
 466         match sig.0.inputs[0].sty {
 467             ty::TyRef(_, ty::TypeAndMut {
 468                 ty: _,
 469                 mutbl: hir::MutMutable,
 470             }) => {}
 471             _ => return,
 472         }
 473
 474         // Gather up expressions we want to munge.
 475         let mut exprs = Vec::new();
 476         exprs.push(self.self_expr);
 477         loop {
 478             let last = exprs[exprs.len() - 1];
 479             match last.node {
 480                 hir::ExprField(ref expr, _) |
 481                 hir::ExprTupField(ref expr, _) |
 482                 hir::ExprIndex(ref expr, _) |
 483                 hir::ExprUnary(hir::UnDeref, ref expr) => exprs.push(&expr),
 484                 _ => break,
 485             }
 486         }
 487
 488         debug!("fixup_derefs_on_method_receiver_if_necessary: exprs={:?}",
 489                exprs);
 490
 491         // Fix up autoderefs and derefs.
 492         for (i, &expr) in exprs.iter().rev().enumerate() {
 493             // Count autoderefs.
 494             let autoderef_count = match self.fcx
 495                                             .inh
 496                                             .tables
 497                                             .borrow()
 498                                             .adjustments
 499                                             .get(&expr.id) {
 500                 Some(&AdjustDerefRef(ref adj)) => adj.autoderefs,
 501                 Some(_) | None => 0,
 502             };
 503
 504             debug!("fixup_derefs_on_method_receiver_if_necessary: i={} expr={:?} \
 505                                                                   autoderef_count={}",
 506                    i, expr, autoderef_count);
 507
 508             if autoderef_count > 0 {
 509                 check::autoderef(self.fcx,
 510                                  expr.span,
 511                                  self.fcx.expr_ty(expr),
 512                                  || Some(expr),
 513                                  UnresolvedTypeAction::Error,
 514                                  PreferMutLvalue,
 515                                  |_, autoderefs| {
 516                                      if autoderefs == autoderef_count + 1 {
 517                                          Some(())
 518                                      } else {
 519                                          None
 520                                      }
 521                                  });
 522             }
 523
 524             // Don't retry the first one or we might infinite loop!
 525             if i == 0 {
 526                 continue;
 527             }
 528             match expr.node {
 529                 hir::ExprIndex(ref base_expr, ref index_expr) => {
 530                     // If this is an overloaded index, the
 531                     // adjustment will include an extra layer of
 532                     // autoref because the method is an &self/&mut
 533                     // self method. We have to peel it off to get
 534                     // the raw adjustment that `try_index_step`
 535                     // expects. This is annoying and horrible. We
 536                     // ought to recode this routine so it doesn't
 537                     // (ab)use the normal type checking paths.
 538                     let adj = self.fcx.inh.tables.borrow().adjustments.get(&base_expr.id)
 539                                                                         .cloned();
 540                     let (autoderefs, unsize) = match adj {
 541                         Some(AdjustDerefRef(adr)) => match adr.autoref {
 542                             None => {
 543                                 assert!(adr.unsize.is_none());
 544                                 (adr.autoderefs, None)
 545                             }
 546                             Some(AutoPtr(_, _)) => {
 547                                 (adr.autoderefs, adr.unsize.map(|target| {
 548                                     target.builtin_deref(false, NoPreference)
 549                                             .expect("fixup: AutoPtr is not &T").ty
 550                                 }))
 551                             }
 552                             Some(_) => {
 553                                 self.tcx().sess.span_bug(
 554                                     base_expr.span,
 555                                     &format!("unexpected adjustment autoref {:?}",
 556                                             adr));
 557                             }
 558                         },
 559                         None => (0, None),
 560                         Some(_) => {
 561                             self.tcx().sess.span_bug(
 562                                 base_expr.span,
 563                                 "unexpected adjustment type");
 564                         }
 565                     };
 566
 567                     let (adjusted_base_ty, unsize) = if let Some(target) = unsize {
 568                         (target, true)
 569                     } else {
 570                         (self.fcx.adjust_expr_ty(base_expr,
 571                             Some(&AdjustDerefRef(AutoDerefRef {
 572                                 autoderefs: autoderefs,
 573                                 autoref: None,
 574                                 unsize: None
 575                             }))), false)
 576                     };
 577                     let index_expr_ty = self.fcx.expr_ty(&index_expr);
 578
 579                     let result = check::try_index_step(
 580                         self.fcx,
 581                         ty::MethodCall::expr(expr.id),
 582                         expr,
 583                         &base_expr,
 584                         adjusted_base_ty,
 585                         autoderefs,
 586                         unsize,
 587                         PreferMutLvalue,
 588                         index_expr_ty);
 589
 590                     if let Some((input_ty, return_ty)) = result {
 591                         demand::suptype(self.fcx, index_expr.span, input_ty, index_expr_ty);
 592
 593                         let expr_ty = self.fcx.expr_ty(&expr);
 594                         demand::suptype(self.fcx, expr.span, expr_ty, return_ty);
 595                     }
 596                 }
 597                 hir::ExprUnary(hir::UnDeref, ref base_expr) => {
 598                     // if this is an overloaded deref, then re-evaluate with
 599                     // a preference for mut
 600                     let method_call = ty::MethodCall::expr(expr.id);
 601                     if self.fcx.inh.tables.borrow().method_map.contains_key(&method_call) {
 602                         let method = check::try_overloaded_deref(
 603                             self.fcx,
 604                             expr.span,
 605                             Some(&base_expr),
 606                             self.fcx.expr_ty(&base_expr),
 607                             PreferMutLvalue);
 608                         let method = method.expect("re-trying deref failed");
 609                         self.fcx.inh.tables.borrow_mut().method_map.insert(method_call, method);
 610                     }
 611                 }
 612                 _ => {}
 613             }
 614         }
 615     }
 616
 617     ///////////////////////////////////////////////////////////////////////////
 618     // MISCELLANY
 619
 620     fn tcx(&self) -> &'a TyCtxt<'tcx> {
 621         self.fcx.tcx()
 622     }
 623
 624     fn infcx(&self) -> &'a InferCtxt<'a, 'tcx> {
 625         self.fcx.infcx()
 626     }
 627
 628     fn enforce_illegal_method_limitations(&self, pick: &probe::Pick) {
 629         // Disallow calls to the method `drop` defined in the `Drop` trait.
 630         match pick.item.container() {
 631             ty::TraitContainer(trait_def_id) => {
 632                 callee::check_legal_trait_for_method_call(self.fcx.ccx, self.span, trait_def_id)
 633             }
 634             ty::ImplContainer(..) => {}
 635         }
 636     }
 637
 638     fn upcast(&mut self,
 639               source_trait_ref: ty::PolyTraitRef<'tcx>,
 640               target_trait_def_id: DefId)
 641               -> ty::PolyTraitRef<'tcx>
 642     {
 643         let upcast_trait_refs = traits::upcast(self.tcx(),
 644                                                source_trait_ref.clone(),
 645                                                target_trait_def_id);
 646
 647         // must be exactly one trait ref or we'd get an ambig error etc
 648         if upcast_trait_refs.len() != 1 {
 649             self.tcx().sess.span_bug(
 650                 self.span,
 651                 &format!("cannot uniquely upcast `{:?}` to `{:?}`: `{:?}`",
 652                          source_trait_ref,
 653                          target_trait_def_id,
 654                          upcast_trait_refs));
 655         }
 656
 657         upcast_trait_refs.into_iter().next().unwrap()
 658     }
 659
 660     fn replace_late_bound_regions_with_fresh_var<T>(&self, value: &ty::Binder<T>) -> T
 661         where T : TypeFoldable<'tcx>
 662     {
 663         self.infcx().replace_late_bound_regions_with_fresh_var(
 664             self.span, infer::FnCall, value).0
 665     }
 666 }