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::rc::Rc;
  27 use std::iter::repeat;
  28 use util::ppaux::Repr;
  29
  30 struct ConfirmContext<'a, 'tcx:'a> {
  31     fcx: &'a FnCtxt<'a, 'tcx>,
  32     span: Span,
  33     self_expr: &'tcx ast::Expr,
  34     call_expr: &'tcx ast::Expr,
  35 }
  36
  37 struct InstantiatedMethodSig<'tcx> {
  38     /// Function signature of the method being invoked. The 0th
  39     /// argument is the receiver.
  40     method_sig: ty::FnSig<'tcx>,
  41
  42     /// Substitutions for all types/early-bound-regions declared on
  43     /// the method.
  44     all_substs: subst::Substs<'tcx>,
  45
  46     /// Generic bounds on the method's parameters which must be added
  47     /// as pending obligations.
  48     method_predicates: ty::InstantiatedPredicates<'tcx>,
  49 }
  50
  51 pub fn confirm<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
  52                          span: Span,
  53                          self_expr: &'tcx ast::Expr,
  54                          call_expr: &'tcx ast::Expr,
  55                          unadjusted_self_ty: Ty<'tcx>,
  56                          pick: probe::Pick<'tcx>,
  57                          supplied_method_types: Vec<Ty<'tcx>>)
  58                          -> MethodCallee<'tcx>
  59 {
  60     debug!("confirm(unadjusted_self_ty={}, pick={}, supplied_method_types={})",
  61            unadjusted_self_ty.repr(fcx.tcx()),
  62            pick.repr(fcx.tcx()),
  63            supplied_method_types.repr(fcx.tcx()));
  64
  65     let mut confirm_cx = ConfirmContext::new(fcx, span, self_expr, call_expr);
  66     confirm_cx.confirm(unadjusted_self_ty, pick, supplied_method_types)
  67 }
  68
  69 impl<'a,'tcx> ConfirmContext<'a,'tcx> {
  70     fn new(fcx: &'a FnCtxt<'a, 'tcx>,
  71            span: Span,
  72            self_expr: &'tcx ast::Expr,
  73            call_expr: &'tcx ast::Expr)
  74            -> ConfirmContext<'a, 'tcx>
  75     {
  76         ConfirmContext { fcx: fcx, span: span, self_expr: self_expr, call_expr: call_expr }
  77     }
  78
  79     fn confirm(&mut self,
  80                unadjusted_self_ty: Ty<'tcx>,
  81                pick: probe::Pick<'tcx>,
  82                supplied_method_types: Vec<Ty<'tcx>>)
  83                -> MethodCallee<'tcx>
  84     {
  85         // Adjust the self expression the user provided and obtain the adjusted type.
  86         let self_ty = self.adjust_self_ty(unadjusted_self_ty, &pick);
  87
  88         // Make sure nobody calls `drop()` explicitly.
  89         self.enforce_illegal_method_limitations(&pick);
  90
  91         // Create substitutions for the method's type parameters.
  92         let (rcvr_substs, method_origin) =
  93             self.fresh_receiver_substs(self_ty, &pick);
  94         let (method_types, method_regions) =
  95             self.instantiate_method_substs(&pick, supplied_method_types);
  96         let all_substs = rcvr_substs.with_method(method_types, method_regions);
  97         debug!("all_substs={}", all_substs.repr(self.tcx()));
  98
  99         // Create the final signature for the method, replacing late-bound regions.
 100         let InstantiatedMethodSig {
 101             method_sig, all_substs, method_predicates
 102         } = self.instantiate_method_sig(&pick, all_substs);
 103         let method_self_ty = method_sig.inputs[0];
 104
 105         // Unify the (adjusted) self type with what the method expects.
 106         self.unify_receivers(self_ty, method_self_ty);
 107
 108         // Add any trait/regions obligations specified on the method's type parameters.
 109         self.add_obligations(&pick, &all_substs, &method_predicates);
 110
 111         // Create the final `MethodCallee`.
 112         let fty = ty::mk_bare_fn(self.tcx(), None, self.tcx().mk_bare_fn(ty::BareFnTy {
 113             sig: ty::Binder(method_sig),
 114             unsafety: pick.method_ty.fty.unsafety,
 115             abi: pick.method_ty.fty.abi.clone(),
 116         }));
 117         let callee = MethodCallee {
 118             origin: method_origin,
 119             ty: fty,
 120             substs: all_substs
 121         };
 122
 123         // If this is an `&mut self` method, bias the receiver
 124         // expression towards mutability (this will switch
 125         // e.g. `Deref` to `DerefMut` in overloaded derefs and so on).
 126         self.fixup_derefs_on_method_receiver_if_necessary(&callee);
 127
 128         callee
 129     }
 130
 131     ///////////////////////////////////////////////////////////////////////////
 132     // ADJUSTMENTS
 133
 134     fn adjust_self_ty(&mut self,
 135                       unadjusted_self_ty: Ty<'tcx>,
 136                       pick: &probe::Pick<'tcx>)
 137                       -> Ty<'tcx>
 138     {
 139         let (autoref, unsize) = if let Some(mutbl) = pick.autoref {
 140             let region = self.infcx().next_region_var(infer::Autoref(self.span));
 141             let autoref = ty::AutoPtr(self.tcx().mk_region(region), mutbl);
 142             (Some(autoref), pick.unsize.map(|target| {
 143                 ty::adjust_ty_for_autoref(self.tcx(), target, Some(autoref))
 144             }))
 145         } else {
 146             // No unsizing should be performed without autoref (at
 147             // least during method dispach). This is because we
 148             // currently only unsize `[T;N]` to `[T]`, and naturally
 149             // that must occur being a reference.
 150             assert!(pick.unsize.is_none());
 151             (None, None)
 152         };
 153
 154         // Commit the autoderefs by calling `autoderef again, but this
 155         // time writing the results into the various tables.
 156         let (autoderefd_ty, n, result) = check::autoderef(self.fcx,
 157                                                           self.span,
 158                                                           unadjusted_self_ty,
 159                                                           Some(self.self_expr),
 160                                                           UnresolvedTypeAction::Error,
 161                                                           NoPreference,
 162                                                           |_, n| {
 163             if n == pick.autoderefs {
 164                 Some(())
 165             } else {
 166                 None
 167             }
 168         });
 169         assert_eq!(n, pick.autoderefs);
 170         assert_eq!(result, Some(()));
 171
 172         // Write out the final adjustment.
 173         self.fcx.write_adjustment(self.self_expr.id,
 174                                   ty::AdjustDerefRef(ty::AutoDerefRef {
 175             autoderefs: pick.autoderefs,
 176             autoref: autoref,
 177             unsize: unsize
 178         }));
 179
 180         if let Some(target) = unsize {
 181             target
 182         } else {
 183             ty::adjust_ty_for_autoref(self.tcx(), autoderefd_ty, autoref)
 184         }
 185     }
 186
 187     ///////////////////////////////////////////////////////////////////////////
 188     //
 189
 190     /// Returns a set of substitutions for the method *receiver* where all type and region
 191     /// parameters are instantiated with fresh variables. This substitution does not include any
 192     /// parameters declared on the method itself.
 193     ///
 194     /// Note that this substitution may include late-bound regions from the impl level. If so,
 195     /// these are instantiated later in the `instantiate_method_sig` routine.
 196     fn fresh_receiver_substs(&mut self,
 197                              self_ty: Ty<'tcx>,
 198                              pick: &probe::Pick<'tcx>)
 199                              -> (subst::Substs<'tcx>, MethodOrigin<'tcx>)
 200     {
 201         match pick.kind {
 202             probe::InherentImplPick(impl_def_id) => {
 203                 assert!(ty::impl_trait_ref(self.tcx(), impl_def_id).is_none(),
 204                         "impl {:?} is not an inherent impl", impl_def_id);
 205                 let impl_polytype = check::impl_self_ty(self.fcx, self.span, impl_def_id);
 206
 207                 (impl_polytype.substs, MethodStatic(pick.method_ty.def_id))
 208             }
 209
 210             probe::ObjectPick(trait_def_id, method_num, vtable_index) => {
 211                 self.extract_trait_ref(self_ty, |this, object_ty, data| {
 212                     // The object data has no entry for the Self
 213                     // Type. For the purposes of this method call, we
 214                     // substitute the object type itself. This
 215                     // wouldn't be a sound substitution in all cases,
 216                     // since each instance of the object type is a
 217                     // different existential and hence could match
 218                     // distinct types (e.g., if `Self` appeared as an
 219                     // argument type), but those cases have already
 220                     // been ruled out when we deemed the trait to be
 221                     // "object safe".
 222                     let original_poly_trait_ref =
 223                         data.principal_trait_ref_with_self_ty(this.tcx(), object_ty);
 224                     let upcast_poly_trait_ref =
 225                         this.upcast(original_poly_trait_ref.clone(), trait_def_id);
 226                     let upcast_trait_ref =
 227                         this.replace_late_bound_regions_with_fresh_var(&upcast_poly_trait_ref);
 228                     debug!("original_poly_trait_ref={} upcast_trait_ref={} target_trait={}",
 229                            original_poly_trait_ref.repr(this.tcx()),
 230                            upcast_trait_ref.repr(this.tcx()),
 231                            trait_def_id.repr(this.tcx()));
 232                     let substs = upcast_trait_ref.substs.clone();
 233                     let origin = MethodTraitObject(MethodObject {
 234                         trait_ref: upcast_trait_ref,
 235                         object_trait_id: trait_def_id,
 236                         method_num: method_num,
 237                         vtable_index: vtable_index,
 238                     });
 239                     (substs, origin)
 240                 })
 241             }
 242
 243             probe::ExtensionImplPick(impl_def_id, method_num) => {
 244                 // The method being invoked is the method as defined on the trait,
 245                 // so return the substitutions from the trait. Consider:
 246                 //
 247                 //     impl<A,B,C> Trait<A,B> for Foo<C> { ... }
 248                 //
 249                 // If we instantiate A, B, and C with $A, $B, and $C
 250                 // respectively, then we want to return the type
 251                 // parameters from the trait ([$A,$B]), not those from
 252                 // the impl ([$A,$B,$C]) not the receiver type ([$C]).
 253                 let impl_polytype = check::impl_self_ty(self.fcx, self.span, impl_def_id);
 254                 let impl_trait_ref =
 255                     self.fcx.instantiate_type_scheme(
 256                         self.span,
 257                         &impl_polytype.substs,
 258                         &ty::impl_trait_ref(self.tcx(), impl_def_id).unwrap());
 259                 let origin = MethodTypeParam(MethodParam { trait_ref: impl_trait_ref.clone(),
 260                                                            method_num: method_num,
 261                                                            impl_def_id: Some(impl_def_id) });
 262                 (impl_trait_ref.substs.clone(), origin)
 263             }
 264
 265             probe::TraitPick(trait_def_id, method_num) => {
 266                 let trait_def = ty::lookup_trait_def(self.tcx(), trait_def_id);
 267
 268                 // Make a trait reference `$0 : Trait<$1...$n>`
 269                 // consisting entirely of type variables. Later on in
 270                 // the process we will unify the transformed-self-type
 271                 // of the method with the actual type in order to
 272                 // unify some of these variables.
 273                 let substs = self.infcx().fresh_substs_for_trait(self.span,
 274                                                                  &trait_def.generics,
 275                                                                  self.infcx().next_ty_var());
 276
 277                 let trait_ref =
 278                     Rc::new(ty::TraitRef::new(trait_def_id, self.tcx().mk_substs(substs.clone())));
 279                 let origin = MethodTypeParam(MethodParam { trait_ref: trait_ref,
 280                                                            method_num: method_num,
 281                                                            impl_def_id: None });
 282                 (substs, origin)
 283             }
 284
 285             probe::WhereClausePick(ref poly_trait_ref, method_num) => {
 286                 // Where clauses can have bound regions in them. We need to instantiate
 287                 // those to convert from a poly-trait-ref to a trait-ref.
 288                 let trait_ref = self.replace_late_bound_regions_with_fresh_var(&*poly_trait_ref);
 289                 let substs = trait_ref.substs.clone();
 290                 let origin = MethodTypeParam(MethodParam { trait_ref: trait_ref,
 291                                                            method_num: method_num,
 292                                                            impl_def_id: None });
 293                 (substs, origin)
 294             }
 295         }
 296     }
 297
 298     fn extract_trait_ref<R, F>(&mut self, self_ty: Ty<'tcx>, mut closure: F) -> R where
 299         F: FnMut(&mut ConfirmContext<'a, 'tcx>, Ty<'tcx>, &ty::TyTrait<'tcx>) -> R,
 300     {
 301         // If we specified that this is an object method, then the
 302         // self-type ought to be something that can be dereferenced to
 303         // yield an object-type (e.g., `&Object` or `Box<Object>`
 304         // etc).
 305
 306         let (_, _, result) = check::autoderef(self.fcx,
 307                                               self.span,
 308                                               self_ty,
 309                                               None,
 310                                               UnresolvedTypeAction::Error,
 311                                               NoPreference,
 312                                               |ty, _| {
 313             match ty.sty {
 314                 ty::ty_trait(ref data) => Some(closure(self, ty, &**data)),
 315                 _ => None,
 316             }
 317         });
 318
 319         match result {
 320             Some(r) => r,
 321             None => {
 322                 self.tcx().sess.span_bug(
 323                     self.span,
 324                     &format!("self-type `{}` for ObjectPick never dereferenced to an object",
 325                             self_ty.repr(self.tcx())))
 326             }
 327         }
 328     }
 329
 330     fn instantiate_method_substs(&mut self,
 331                                  pick: &probe::Pick<'tcx>,
 332                                  supplied_method_types: Vec<Ty<'tcx>>)
 333                                  -> (Vec<Ty<'tcx>>, Vec<ty::Region>)
 334     {
 335         // Determine the values for the generic parameters of the method.
 336         // If they were not explicitly supplied, just construct fresh
 337         // variables.
 338         let num_supplied_types = supplied_method_types.len();
 339         let num_method_types = pick.method_ty.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.method_ty.generics.regions.get_slice(subst::FnSpace));
 364
 365         (method_types, method_regions)
 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, infer::Misc(self.span), self_ty, method_self_ty) {
 373             Ok(_) => {}
 374             Err(_) => {
 375                 self.tcx().sess.span_bug(
 376                     self.span,
 377                     &format!(
 378                         "{} was a subtype of {} but now is not?",
 379                         self_ty.repr(self.tcx()),
 380                         method_self_ty.repr(self.tcx())));
 381             }
 382         }
 383     }
 384
 385     ///////////////////////////////////////////////////////////////////////////
 386     //
 387
 388     fn instantiate_method_sig(&mut self,
 389                               pick: &probe::Pick<'tcx>,
 390                               all_substs: subst::Substs<'tcx>)
 391                               -> InstantiatedMethodSig<'tcx>
 392     {
 393         debug!("instantiate_method_sig(pick={}, all_substs={})",
 394                pick.repr(self.tcx()),
 395                all_substs.repr(self.tcx()));
 396
 397         // Instantiate the bounds on the method with the
 398         // type/early-bound-regions substitutions performed. There can
 399         // be no late-bound regions appearing here.
 400         let method_predicates = pick.method_ty.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.repr(self.tcx()));
 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(&pick.method_ty.fty.sig);
 414         debug!("late-bound lifetimes from method instantiated, method_sig={}",
 415                method_sig.repr(self.tcx()));
 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.repr(self.tcx()));
 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                        pick: &probe::Pick<'tcx>,
 430                        all_substs: &subst::Substs<'tcx>,
 431                        method_predicates: &ty::InstantiatedPredicates<'tcx>) {
 432         debug!("add_obligations: pick={} all_substs={} method_predicates={}",
 433                pick.repr(self.tcx()),
 434                all_substs.repr(self.tcx()),
 435                method_predicates.repr(self.tcx()));
 436
 437         self.fcx.add_obligations_for_parameters(
 438             traits::ObligationCause::misc(self.span, self.fcx.body_id),
 439             method_predicates);
 440
 441         self.fcx.add_default_region_param_bounds(
 442             all_substs,
 443             self.call_expr);
 444     }
 445
 446     ///////////////////////////////////////////////////////////////////////////
 447     // RECONCILIATION
 448
 449     /// When we select a method with an `&mut self` receiver, we have to go convert any
 450     /// auto-derefs, indices, etc from `Deref` and `Index` into `DerefMut` and `IndexMut`
 451     /// respectively.
 452     fn fixup_derefs_on_method_receiver_if_necessary(&self,
 453                                                     method_callee: &MethodCallee) {
 454         let sig = match method_callee.ty.sty {
 455             ty::ty_bare_fn(_, ref f) => f.sig.clone(),
 456             _ => return,
 457         };
 458
 459         match sig.0.inputs[0].sty {
 460             ty::ty_rptr(_, ty::mt {
 461                 ty: _,
 462                 mutbl: ast::MutMutable,
 463             }) => {}
 464             _ => return,
 465         }
 466
 467         // Gather up expressions we want to munge.
 468         let mut exprs = Vec::new();
 469         exprs.push(self.self_expr);
 470         loop {
 471             let last = exprs[exprs.len() - 1];
 472             match last.node {
 473                 ast::ExprParen(ref expr) |
 474                 ast::ExprField(ref expr, _) |
 475                 ast::ExprTupField(ref expr, _) |
 476                 ast::ExprIndex(ref expr, _) |
 477                 ast::ExprUnary(ast::UnDeref, ref expr) => exprs.push(&**expr),
 478                 _ => break,
 479             }
 480         }
 481
 482         debug!("fixup_derefs_on_method_receiver_if_necessary: exprs={}",
 483                exprs.repr(self.tcx()));
 484
 485         // Fix up autoderefs and derefs.
 486         for (i, &expr) in exprs.iter().rev().enumerate() {
 487             // Count autoderefs.
 488             let autoderef_count = match self.fcx
 489                                             .inh
 490                                             .adjustments
 491                                             .borrow()
 492                                             .get(&expr.id) {
 493                 Some(&ty::AdjustDerefRef(ref adj)) => adj.autoderefs,
 494                 Some(_) | None => 0,
 495             };
 496
 497             debug!("fixup_derefs_on_method_receiver_if_necessary: i={} expr={} autoderef_count={}",
 498                    i, expr.repr(self.tcx()), autoderef_count);
 499
 500             if autoderef_count > 0 {
 501                 check::autoderef(self.fcx,
 502                                  expr.span,
 503                                  self.fcx.expr_ty(expr),
 504                                  Some(expr),
 505                                  UnresolvedTypeAction::Error,
 506                                  PreferMutLvalue,
 507                                  |_, autoderefs| {
 508                                      if autoderefs == autoderef_count + 1 {
 509                                          Some(())
 510                                      } else {
 511                                          None
 512                                      }
 513                                  });
 514             }
 515
 516             // Don't retry the first one or we might infinite loop!
 517             if i != 0 {
 518                 match expr.node {
 519                     ast::ExprIndex(ref base_expr, ref index_expr) => {
 520                         // If this is an overloaded index, the
 521                         // adjustment will include an extra layer of
 522                         // autoref because the method is an &self/&mut
 523                         // self method. We have to peel it off to get
 524                         // the raw adjustment that `try_index_step`
 525                         // expects. This is annoying and horrible. We
 526                         // ought to recode this routine so it doesn't
 527                         // (ab)use the normal type checking paths.
 528                         let adj = self.fcx.inh.adjustments.borrow().get(&base_expr.id).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                                         ty::deref(target, 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.repr(self.tcx())));
 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                             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 = MethodCall::expr(expr.id);
 590                         if self.fcx.inh.method_map.borrow().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.method_ty.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.method_ty.def_id));
 629             }
 630         }
 631     }
 632
 633     fn upcast(&mut self,
 634               source_trait_ref: ty::PolyTraitRef<'tcx>,
 635               target_trait_def_id: ast::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.repr(self.tcx()),
 648                          target_trait_def_id.repr(self.tcx()),
 649                          upcast_trait_refs.repr(self.tcx())));
 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> + Repr<'tcx>
 657     {
 658         self.infcx().replace_late_bound_regions_with_fresh_var(
 659             self.span, infer::FnCall, value).0
 660     }
 661 }