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