src/librustc_typeck/check/method/probe.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::MethodError;
  12 use super::NoMatchData;
  13 use super::{CandidateSource, ImplSource, TraitSource};
  14 use super::suggest;
  15
  16 use check::FnCtxt;
  17 use hir::def_id::DefId;
  18 use hir::def::Def;
  19 use rustc::ty::subst::{Subst, Substs};
  20 use rustc::traits::{self, ObligationCause};
  21 use rustc::ty::{self, Ty, ToPolyTraitRef, TraitRef, TypeFoldable};
  22 use rustc::infer::type_variable::TypeVariableOrigin;
  23 use rustc::util::nodemap::FxHashSet;
  24 use rustc::infer::{self, InferOk};
  25 use syntax::ast;
  26 use syntax_pos::Span;
  27 use rustc::hir;
  28 use std::mem;
  29 use std::ops::Deref;
  30 use std::rc::Rc;
  31
  32 use self::CandidateKind::*;
  33 pub use self::PickKind::*;
  34
  35 pub enum LookingFor<'tcx> {
  36     /// looking for methods with the given name; this is the normal case
  37     MethodName(ast::Name),
  38
  39     /// looking for methods that return a given type; this is used to
  40     /// assemble suggestions
  41     ReturnType(Ty<'tcx>),
  42 }
  43
  44 /// Boolean flag used to indicate if this search is for a suggestion
  45 /// or not.  If true, we can allow ambiguity and so forth.
  46 pub struct IsSuggestion(pub bool);
  47
  48 struct ProbeContext<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
  49     fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
  50     span: Span,
  51     mode: Mode,
  52     looking_for: LookingFor<'tcx>,
  53     steps: Rc<Vec<CandidateStep<'tcx>>>,
  54     opt_simplified_steps: Option<Vec<ty::fast_reject::SimplifiedType>>,
  55     inherent_candidates: Vec<Candidate<'tcx>>,
  56     extension_candidates: Vec<Candidate<'tcx>>,
  57     impl_dups: FxHashSet<DefId>,
  58
  59     /// Collects near misses when the candidate functions are missing a `self` keyword and is only
  60     /// used for error reporting
  61     static_candidates: Vec<CandidateSource>,
  62
  63     /// Some(candidate) if there is a private candidate
  64     private_candidate: Option<Def>,
  65
  66     /// Collects near misses when trait bounds for type parameters are unsatisfied and is only used
  67     /// for error reporting
  68     unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  69 }
  70
  71 impl<'a, 'gcx, 'tcx> Deref for ProbeContext<'a, 'gcx, 'tcx> {
  72     type Target = FnCtxt<'a, 'gcx, 'tcx>;
  73     fn deref(&self) -> &Self::Target {
  74         &self.fcx
  75     }
  76 }
  77
  78 #[derive(Debug)]
  79 struct CandidateStep<'tcx> {
  80     self_ty: Ty<'tcx>,
  81     autoderefs: usize,
  82     unsize: bool,
  83 }
  84
  85 #[derive(Debug)]
  86 struct Candidate<'tcx> {
  87     xform_self_ty: Ty<'tcx>,
  88     item: ty::AssociatedItem,
  89     kind: CandidateKind<'tcx>,
  90     import_id: Option<ast::NodeId>,
  91 }
  92
  93 #[derive(Debug)]
  94 enum CandidateKind<'tcx> {
  95     InherentImplCandidate(&'tcx Substs<'tcx>,
  96                           // Normalize obligations
  97                           Vec<traits::PredicateObligation<'tcx>>),
  98     ExtensionImplCandidate(// Impl
  99                            DefId,
 100                            &'tcx Substs<'tcx>,
 101                            // Normalize obligations
 102                            Vec<traits::PredicateObligation<'tcx>>),
 103     ObjectCandidate,
 104     TraitCandidate,
 105     WhereClauseCandidate(// Trait
 106                          ty::PolyTraitRef<'tcx>),
 107 }
 108
 109 #[derive(Debug, PartialEq, Eq, Clone)]
 110 pub struct Pick<'tcx> {
 111     pub item: ty::AssociatedItem,
 112     pub kind: PickKind<'tcx>,
 113     pub import_id: Option<ast::NodeId>,
 114
 115     // Indicates that the source expression should be autoderef'd N times
 116     //
 117     // A = expr | *expr | **expr | ...
 118     pub autoderefs: usize,
 119
 120     // Indicates that an autoref is applied after the optional autoderefs
 121     //
 122     // B = A | &A | &mut A
 123     pub autoref: Option<hir::Mutability>,
 124
 125     // Indicates that the source expression should be "unsized" to a
 126     // target type. This should probably eventually go away in favor
 127     // of just coercing method receivers.
 128     //
 129     // C = B | unsize(B)
 130     pub unsize: Option<Ty<'tcx>>,
 131 }
 132
 133 #[derive(Clone, Debug, PartialEq, Eq)]
 134 pub enum PickKind<'tcx> {
 135     InherentImplPick,
 136     ExtensionImplPick(// Impl
 137                       DefId),
 138     ObjectPick,
 139     TraitPick,
 140     WhereClausePick(// Trait
 141                     ty::PolyTraitRef<'tcx>),
 142 }
 143
 144 pub type PickResult<'tcx> = Result<Pick<'tcx>, MethodError<'tcx>>;
 145
 146 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 147 pub enum Mode {
 148     // An expression of the form `receiver.method_name(...)`.
 149     // Autoderefs are performed on `receiver`, lookup is done based on the
 150     // `self` argument  of the method, and static methods aren't considered.
 151     MethodCall,
 152     // An expression of the form `Type::item` or `<T>::item`.
 153     // No autoderefs are performed, lookup is done based on the type each
 154     // implementation is for, and static methods are included.
 155     Path,
 156 }
 157
 158 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 159 pub enum ProbeScope {
 160     // Assemble candidates coming only from traits in scope.
 161     TraitsInScope,
 162
 163     // Assemble candidates coming from all traits.
 164     AllTraits,
 165 }
 166
 167 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 168     /// This is used to offer suggestions to users. It returns methods
 169     /// that could have been called which have the desired return
 170     /// type. Some effort is made to rule out methods that, if called,
 171     /// would result in an error (basically, the same criteria we
 172     /// would use to decide if a method is a plausible fit for
 173     /// ambiguity purposes).
 174     pub fn probe_for_return_type(&self,
 175                                  span: Span,
 176                                  mode: Mode,
 177                                  return_type: Ty<'tcx>,
 178                                  self_ty: Ty<'tcx>,
 179                                  scope_expr_id: ast::NodeId)
 180                                  -> Vec<ty::AssociatedItem> {
 181         debug!("probe(self_ty={:?}, return_type={}, scope_expr_id={})",
 182                self_ty,
 183                return_type,
 184                scope_expr_id);
 185         let method_names =
 186             self.probe_op(span, mode, LookingFor::ReturnType(return_type), IsSuggestion(true),
 187                           self_ty, scope_expr_id, ProbeScope::TraitsInScope,
 188                           |probe_cx| Ok(probe_cx.candidate_method_names()))
 189                 .unwrap_or(vec![]);
 190         method_names
 191             .iter()
 192             .flat_map(|&method_name| {
 193                 match self.probe_for_name(span, mode, method_name, IsSuggestion(true), self_ty,
 194                                           scope_expr_id, ProbeScope::TraitsInScope) {
 195                     Ok(pick) => Some(pick.item),
 196                     Err(_) => None,
 197                 }
 198             })
 199             .collect()
 200     }
 201
 202     pub fn probe_for_name(&self,
 203                           span: Span,
 204                           mode: Mode,
 205                           item_name: ast::Name,
 206                           is_suggestion: IsSuggestion,
 207                           self_ty: Ty<'tcx>,
 208                           scope_expr_id: ast::NodeId,
 209                           scope: ProbeScope)
 210                           -> PickResult<'tcx> {
 211         debug!("probe(self_ty={:?}, item_name={}, scope_expr_id={})",
 212                self_ty,
 213                item_name,
 214                scope_expr_id);
 215         self.probe_op(span,
 216                       mode,
 217                       LookingFor::MethodName(item_name),
 218                       is_suggestion,
 219                       self_ty,
 220                       scope_expr_id,
 221                       scope,
 222                       |probe_cx| probe_cx.pick())
 223     }
 224
 225     fn probe_op<OP,R>(&'a self,
 226                       span: Span,
 227                       mode: Mode,
 228                       looking_for: LookingFor<'tcx>,
 229                       is_suggestion: IsSuggestion,
 230                       self_ty: Ty<'tcx>,
 231                       scope_expr_id: ast::NodeId,
 232                       scope: ProbeScope,
 233                       op: OP)
 234                       -> Result<R, MethodError<'tcx>>
 235         where OP: FnOnce(ProbeContext<'a, 'gcx, 'tcx>) -> Result<R, MethodError<'tcx>>
 236     {
 237         // FIXME(#18741) -- right now, creating the steps involves evaluating the
 238         // `*` operator, which registers obligations that then escape into
 239         // the global fulfillment context and thus has global
 240         // side-effects. This is a bit of a pain to refactor. So just let
 241         // it ride, although it's really not great, and in fact could I
 242         // think cause spurious errors. Really though this part should
 243         // take place in the `self.probe` below.
 244         let steps = if mode == Mode::MethodCall {
 245             match self.create_steps(span, self_ty, is_suggestion) {
 246                 Some(steps) => steps,
 247                 None => {
 248                     return Err(MethodError::NoMatch(NoMatchData::new(Vec::new(),
 249                                                                      Vec::new(),
 250                                                                      Vec::new(),
 251                                                                      mode)))
 252                 }
 253             }
 254         } else {
 255             vec![CandidateStep {
 256                      self_ty,
 257                      autoderefs: 0,
 258                      unsize: false,
 259                  }]
 260         };
 261
 262         // Create a list of simplified self types, if we can.
 263         let mut simplified_steps = Vec::new();
 264         for step in &steps {
 265             match ty::fast_reject::simplify_type(self.tcx, step.self_ty, true) {
 266                 None => {
 267                     break;
 268                 }
 269                 Some(simplified_type) => {
 270                     simplified_steps.push(simplified_type);
 271                 }
 272             }
 273         }
 274         let opt_simplified_steps = if simplified_steps.len() < steps.len() {
 275             None // failed to convert at least one of the steps
 276         } else {
 277             Some(simplified_steps)
 278         };
 279
 280         debug!("ProbeContext: steps for self_ty={:?} are {:?}",
 281                self_ty,
 282                steps);
 283
 284         // this creates one big transaction so that all type variables etc
 285         // that we create during the probe process are removed later
 286         self.probe(|_| {
 287             let mut probe_cx =
 288                 ProbeContext::new(self, span, mode, looking_for,
 289                                   steps, opt_simplified_steps);
 290
 291             probe_cx.assemble_inherent_candidates();
 292             match scope {
 293                 ProbeScope::TraitsInScope =>
 294                     probe_cx.assemble_extension_candidates_for_traits_in_scope(scope_expr_id)?,
 295                 ProbeScope::AllTraits =>
 296                     probe_cx.assemble_extension_candidates_for_all_traits()?,
 297             };
 298             op(probe_cx)
 299         })
 300     }
 301
 302     fn create_steps(&self,
 303                     span: Span,
 304                     self_ty: Ty<'tcx>,
 305                     is_suggestion: IsSuggestion)
 306                     -> Option<Vec<CandidateStep<'tcx>>> {
 307         // FIXME: we don't need to create the entire steps in one pass
 308
 309         let mut autoderef = self.autoderef(span, self_ty);
 310         let mut steps: Vec<_> = autoderef.by_ref()
 311             .map(|(ty, d)| {
 312                 CandidateStep {
 313                     self_ty: ty,
 314                     autoderefs: d,
 315                     unsize: false,
 316                 }
 317             })
 318             .collect();
 319
 320         let final_ty = autoderef.maybe_ambiguous_final_ty();
 321         match final_ty.sty {
 322             ty::TyInfer(ty::TyVar(_)) => {
 323                 // Ended in an inference variable. If we are doing
 324                 // a real method lookup, this is a hard error (it's an
 325                 // ambiguity and we can't make progress).
 326                 if !is_suggestion.0 {
 327                     let t = self.structurally_resolved_type(span, final_ty);
 328                     assert_eq!(t, self.tcx.types.err);
 329                     return None
 330                 } else {
 331                     // If we're just looking for suggestions,
 332                     // though, ambiguity is no big thing, we can
 333                     // just ignore it.
 334                 }
 335             }
 336             ty::TyArray(elem_ty, _) => {
 337                 let dereferences = steps.len() - 1;
 338
 339                 steps.push(CandidateStep {
 340                     self_ty: self.tcx.mk_slice(elem_ty),
 341                     autoderefs: dereferences,
 342                     unsize: true,
 343                 });
 344             }
 345             ty::TyError => return None,
 346             _ => (),
 347         }
 348
 349         debug!("create_steps: steps={:?}", steps);
 350
 351         Some(steps)
 352     }
 353 }
 354
 355 impl<'a, 'gcx, 'tcx> ProbeContext<'a, 'gcx, 'tcx> {
 356     fn new(fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
 357            span: Span,
 358            mode: Mode,
 359            looking_for: LookingFor<'tcx>,
 360            steps: Vec<CandidateStep<'tcx>>,
 361            opt_simplified_steps: Option<Vec<ty::fast_reject::SimplifiedType>>)
 362            -> ProbeContext<'a, 'gcx, 'tcx> {
 363         ProbeContext {
 364             fcx,
 365             span,
 366             mode,
 367             looking_for,
 368             inherent_candidates: Vec::new(),
 369             extension_candidates: Vec::new(),
 370             impl_dups: FxHashSet(),
 371             steps: Rc::new(steps),
 372             opt_simplified_steps,
 373             static_candidates: Vec::new(),
 374             private_candidate: None,
 375             unsatisfied_predicates: Vec::new(),
 376         }
 377     }
 378
 379     fn reset(&mut self) {
 380         self.inherent_candidates.clear();
 381         self.extension_candidates.clear();
 382         self.impl_dups.clear();
 383         self.static_candidates.clear();
 384         self.private_candidate = None;
 385     }
 386
 387     ///////////////////////////////////////////////////////////////////////////
 388     // CANDIDATE ASSEMBLY
 389
 390     fn push_inherent_candidate(&mut self, xform_self_ty: Ty<'tcx>, item: ty::AssociatedItem,
 391                                kind: CandidateKind<'tcx>, import_id: Option<ast::NodeId>) {
 392         let is_accessible = if let LookingFor::MethodName(name) = self.looking_for {
 393             let def_scope = self.tcx.adjust(name, item.container.id(), self.body_id).1;
 394             item.vis.is_accessible_from(def_scope, self.tcx)
 395         } else {
 396             true
 397         };
 398         if is_accessible {
 399             self.inherent_candidates.push(Candidate { xform_self_ty, item, kind, import_id });
 400         } else if self.private_candidate.is_none() {
 401             self.private_candidate = Some(item.def());
 402         }
 403     }
 404
 405     fn push_extension_candidate(&mut self, xform_self_ty: Ty<'tcx>, item: ty::AssociatedItem,
 406                                kind: CandidateKind<'tcx>, import_id: Option<ast::NodeId>) {
 407         let is_accessible = if let LookingFor::MethodName(name) = self.looking_for {
 408             let def_scope = self.tcx.adjust(name, item.container.id(), self.body_id).1;
 409             item.vis.is_accessible_from(def_scope, self.tcx)
 410         } else {
 411             true
 412         };
 413         if is_accessible {
 414             self.extension_candidates.push(Candidate { xform_self_ty, item, kind, import_id });
 415         } else if self.private_candidate.is_none() {
 416             self.private_candidate = Some(item.def());
 417         }
 418     }
 419
 420     fn assemble_inherent_candidates(&mut self) {
 421         let steps = self.steps.clone();
 422         for step in steps.iter() {
 423             self.assemble_probe(step.self_ty);
 424         }
 425     }
 426
 427     fn assemble_probe(&mut self, self_ty: Ty<'tcx>) {
 428         debug!("assemble_probe: self_ty={:?}", self_ty);
 429
 430         match self_ty.sty {
 431             ty::TyDynamic(ref data, ..) => {
 432                 if let Some(p) = data.principal() {
 433                     self.assemble_inherent_candidates_from_object(self_ty, p);
 434                     self.assemble_inherent_impl_candidates_for_type(p.def_id());
 435                 }
 436             }
 437             ty::TyAdt(def, _) => {
 438                 self.assemble_inherent_impl_candidates_for_type(def.did);
 439             }
 440             ty::TyParam(p) => {
 441                 self.assemble_inherent_candidates_from_param(self_ty, p);
 442             }
 443             ty::TyChar => {
 444                 let lang_def_id = self.tcx.lang_items.char_impl();
 445                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 446             }
 447             ty::TyStr => {
 448                 let lang_def_id = self.tcx.lang_items.str_impl();
 449                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 450             }
 451             ty::TySlice(_) => {
 452                 let lang_def_id = self.tcx.lang_items.slice_impl();
 453                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 454             }
 455             ty::TyRawPtr(ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }) => {
 456                 let lang_def_id = self.tcx.lang_items.const_ptr_impl();
 457                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 458             }
 459             ty::TyRawPtr(ty::TypeAndMut { ty: _, mutbl: hir::MutMutable }) => {
 460                 let lang_def_id = self.tcx.lang_items.mut_ptr_impl();
 461                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 462             }
 463             ty::TyInt(ast::IntTy::I8) => {
 464                 let lang_def_id = self.tcx.lang_items.i8_impl();
 465                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 466             }
 467             ty::TyInt(ast::IntTy::I16) => {
 468                 let lang_def_id = self.tcx.lang_items.i16_impl();
 469                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 470             }
 471             ty::TyInt(ast::IntTy::I32) => {
 472                 let lang_def_id = self.tcx.lang_items.i32_impl();
 473                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 474             }
 475             ty::TyInt(ast::IntTy::I64) => {
 476                 let lang_def_id = self.tcx.lang_items.i64_impl();
 477                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 478             }
 479             ty::TyInt(ast::IntTy::I128) => {
 480                 let lang_def_id = self.tcx.lang_items.i128_impl();
 481                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 482             }
 483             ty::TyInt(ast::IntTy::Is) => {
 484                 let lang_def_id = self.tcx.lang_items.isize_impl();
 485                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 486             }
 487             ty::TyUint(ast::UintTy::U8) => {
 488                 let lang_def_id = self.tcx.lang_items.u8_impl();
 489                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 490             }
 491             ty::TyUint(ast::UintTy::U16) => {
 492                 let lang_def_id = self.tcx.lang_items.u16_impl();
 493                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 494             }
 495             ty::TyUint(ast::UintTy::U32) => {
 496                 let lang_def_id = self.tcx.lang_items.u32_impl();
 497                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 498             }
 499             ty::TyUint(ast::UintTy::U64) => {
 500                 let lang_def_id = self.tcx.lang_items.u64_impl();
 501                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 502             }
 503             ty::TyUint(ast::UintTy::U128) => {
 504                 let lang_def_id = self.tcx.lang_items.u128_impl();
 505                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 506             }
 507             ty::TyUint(ast::UintTy::Us) => {
 508                 let lang_def_id = self.tcx.lang_items.usize_impl();
 509                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 510             }
 511             ty::TyFloat(ast::FloatTy::F32) => {
 512                 let lang_def_id = self.tcx.lang_items.f32_impl();
 513                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 514             }
 515             ty::TyFloat(ast::FloatTy::F64) => {
 516                 let lang_def_id = self.tcx.lang_items.f64_impl();
 517                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 518             }
 519             _ => {}
 520         }
 521     }
 522
 523     fn assemble_inherent_impl_for_primitive(&mut self, lang_def_id: Option<DefId>) {
 524         if let Some(impl_def_id) = lang_def_id {
 525             self.assemble_inherent_impl_probe(impl_def_id);
 526         }
 527     }
 528
 529     fn assemble_inherent_impl_candidates_for_type(&mut self, def_id: DefId) {
 530         let impl_def_ids = self.tcx.at(self.span).inherent_impls(def_id);
 531         for &impl_def_id in impl_def_ids.iter() {
 532             self.assemble_inherent_impl_probe(impl_def_id);
 533         }
 534     }
 535
 536     fn assemble_inherent_impl_probe(&mut self, impl_def_id: DefId) {
 537         if !self.impl_dups.insert(impl_def_id) {
 538             return; // already visited
 539         }
 540
 541         debug!("assemble_inherent_impl_probe {:?}", impl_def_id);
 542
 543         for item in self.impl_or_trait_item(impl_def_id) {
 544             if !self.has_applicable_self(&item) {
 545                 // No receiver declared. Not a candidate.
 546                 self.record_static_candidate(ImplSource(impl_def_id));
 547                 continue
 548             }
 549
 550             let (impl_ty, impl_substs) = self.impl_ty_and_substs(impl_def_id);
 551             let impl_ty = impl_ty.subst(self.tcx, impl_substs);
 552
 553             // Determine the receiver type that the method itself expects.
 554             let xform_self_ty = self.xform_self_ty(&item, impl_ty, impl_substs);
 555
 556             // We can't use normalize_associated_types_in as it will pollute the
 557             // fcx's fulfillment context after this probe is over.
 558             let cause = traits::ObligationCause::misc(self.span, self.body_id);
 559             let selcx = &mut traits::SelectionContext::new(self.fcx);
 560             let traits::Normalized { value: xform_self_ty, obligations } =
 561                 traits::normalize(selcx, self.param_env, cause, &xform_self_ty);
 562             debug!("assemble_inherent_impl_probe: xform_self_ty = {:?}",
 563                    xform_self_ty);
 564
 565             self.push_inherent_candidate(xform_self_ty, item,
 566                                          InherentImplCandidate(impl_substs, obligations), None);
 567         }
 568     }
 569
 570     fn assemble_inherent_candidates_from_object(&mut self,
 571                                                 self_ty: Ty<'tcx>,
 572                                                 principal: ty::PolyExistentialTraitRef<'tcx>) {
 573         debug!("assemble_inherent_candidates_from_object(self_ty={:?})",
 574                self_ty);
 575
 576         // It is illegal to invoke a method on a trait instance that
 577         // refers to the `Self` type. An error will be reported by
 578         // `enforce_object_limitations()` if the method refers to the
 579         // `Self` type anywhere other than the receiver. Here, we use
 580         // a substitution that replaces `Self` with the object type
 581         // itself. Hence, a `&self` method will wind up with an
 582         // argument type like `&Trait`.
 583         let trait_ref = principal.with_self_ty(self.tcx, self_ty);
 584         self.elaborate_bounds(&[trait_ref], |this, new_trait_ref, item| {
 585             let new_trait_ref = this.erase_late_bound_regions(&new_trait_ref);
 586
 587             let xform_self_ty =
 588                 this.xform_self_ty(&item, new_trait_ref.self_ty(), new_trait_ref.substs);
 589
 590             this.push_inherent_candidate(xform_self_ty, item, ObjectCandidate, None);
 591         });
 592     }
 593
 594     fn assemble_inherent_candidates_from_param(&mut self,
 595                                                _rcvr_ty: Ty<'tcx>,
 596                                                param_ty: ty::ParamTy) {
 597         // FIXME -- Do we want to commit to this behavior for param bounds?
 598
 599         let bounds: Vec<_> = self.param_env
 600             .caller_bounds
 601             .iter()
 602             .filter_map(|predicate| {
 603                 match *predicate {
 604                     ty::Predicate::Trait(ref trait_predicate) => {
 605                         match trait_predicate.0.trait_ref.self_ty().sty {
 606                             ty::TyParam(ref p) if *p == param_ty => {
 607                                 Some(trait_predicate.to_poly_trait_ref())
 608                             }
 609                             _ => None,
 610                         }
 611                     }
 612                     ty::Predicate::Equate(..) |
 613                     ty::Predicate::Subtype(..) |
 614                     ty::Predicate::Projection(..) |
 615                     ty::Predicate::RegionOutlives(..) |
 616                     ty::Predicate::WellFormed(..) |
 617                     ty::Predicate::ObjectSafe(..) |
 618                     ty::Predicate::ClosureKind(..) |
 619                     ty::Predicate::TypeOutlives(..) => None,
 620                 }
 621             })
 622             .collect();
 623
 624         self.elaborate_bounds(&bounds, |this, poly_trait_ref, item| {
 625             let trait_ref = this.erase_late_bound_regions(&poly_trait_ref);
 626
 627             let xform_self_ty = this.xform_self_ty(&item, trait_ref.self_ty(), trait_ref.substs);
 628
 629             // Because this trait derives from a where-clause, it
 630             // should not contain any inference variables or other
 631             // artifacts. This means it is safe to put into the
 632             // `WhereClauseCandidate` and (eventually) into the
 633             // `WhereClausePick`.
 634             assert!(!trait_ref.substs.needs_infer());
 635
 636             this.push_inherent_candidate(xform_self_ty, item,
 637                                          WhereClauseCandidate(poly_trait_ref), None);
 638         });
 639     }
 640
 641     // Do a search through a list of bounds, using a callback to actually
 642     // create the candidates.
 643     fn elaborate_bounds<F>(&mut self, bounds: &[ty::PolyTraitRef<'tcx>], mut mk_cand: F)
 644         where F: for<'b> FnMut(&mut ProbeContext<'b, 'gcx, 'tcx>,
 645                                ty::PolyTraitRef<'tcx>,
 646                                ty::AssociatedItem)
 647     {
 648         debug!("elaborate_bounds(bounds={:?})", bounds);
 649
 650         let tcx = self.tcx;
 651         for bound_trait_ref in traits::transitive_bounds(tcx, bounds) {
 652             for item in self.impl_or_trait_item(bound_trait_ref.def_id()) {
 653                 if !self.has_applicable_self(&item) {
 654                     self.record_static_candidate(TraitSource(bound_trait_ref.def_id()));
 655                 } else {
 656                     mk_cand(self, bound_trait_ref, item);
 657                 }
 658             }
 659         }
 660     }
 661
 662     fn assemble_extension_candidates_for_traits_in_scope(&mut self,
 663                                                          expr_id: ast::NodeId)
 664                                                          -> Result<(), MethodError<'tcx>> {
 665         let mut duplicates = FxHashSet();
 666         let opt_applicable_traits = self.tcx.trait_map.get(&expr_id);
 667         if let Some(applicable_traits) = opt_applicable_traits {
 668             for trait_candidate in applicable_traits {
 669                 let trait_did = trait_candidate.def_id;
 670                 if duplicates.insert(trait_did) {
 671                     let import_id = trait_candidate.import_id;
 672                     let result = self.assemble_extension_candidates_for_trait(import_id, trait_did);
 673                     result?;
 674                 }
 675             }
 676         }
 677         Ok(())
 678     }
 679
 680     fn assemble_extension_candidates_for_all_traits(&mut self) -> Result<(), MethodError<'tcx>> {
 681         let mut duplicates = FxHashSet();
 682         for trait_info in suggest::all_traits(self.tcx) {
 683             if duplicates.insert(trait_info.def_id) {
 684                 self.assemble_extension_candidates_for_trait(None, trait_info.def_id)?;
 685             }
 686         }
 687         Ok(())
 688     }
 689
 690     pub fn matches_return_type(&self, method: &ty::AssociatedItem,
 691                                expected: ty::Ty<'tcx>) -> bool {
 692         match method.def() {
 693             Def::Method(def_id) => {
 694                 let fty = self.tcx.fn_sig(def_id);
 695                 self.probe(|_| {
 696                     let substs = self.fresh_substs_for_item(self.span, method.def_id);
 697                     let output = fty.output().subst(self.tcx, substs);
 698                     let (output, _) = self.replace_late_bound_regions_with_fresh_var(
 699                         self.span, infer::FnCall, &output);
 700                     self.can_sub(self.param_env, output, expected).is_ok()
 701                 })
 702             }
 703             _ => false,
 704         }
 705     }
 706
 707     fn assemble_extension_candidates_for_trait(&mut self,
 708                                                import_id: Option<ast::NodeId>,
 709                                                trait_def_id: DefId)
 710                                                -> Result<(), MethodError<'tcx>> {
 711         debug!("assemble_extension_candidates_for_trait(trait_def_id={:?})",
 712                trait_def_id);
 713
 714         for item in self.impl_or_trait_item(trait_def_id) {
 715             // Check whether `trait_def_id` defines a method with suitable name:
 716             if !self.has_applicable_self(&item) {
 717                 debug!("method has inapplicable self");
 718                 self.record_static_candidate(TraitSource(trait_def_id));
 719                 continue;
 720             }
 721
 722             self.assemble_builtin_candidates(import_id, trait_def_id, item.clone());
 723
 724             self.assemble_extension_candidates_for_trait_impls(import_id, trait_def_id,
 725                                                                item.clone());
 726
 727             self.assemble_closure_candidates(import_id, trait_def_id, item.clone())?;
 728
 729             self.assemble_generator_candidates(import_id, trait_def_id, item.clone())?;
 730
 731             self.assemble_projection_candidates(import_id, trait_def_id, item.clone());
 732
 733             self.assemble_where_clause_candidates(import_id, trait_def_id, item.clone());
 734         }
 735
 736         Ok(())
 737     }
 738
 739     fn assemble_builtin_candidates(&mut self,
 740                                    import_id: Option<ast::NodeId>,
 741                                    trait_def_id: DefId,
 742                                    item: ty::AssociatedItem) {
 743         if Some(trait_def_id) == self.tcx.lang_items.clone_trait() {
 744             self.assemble_builtin_clone_candidates(import_id, trait_def_id, item);
 745         }
 746     }
 747
 748     fn assemble_builtin_clone_candidates(&mut self,
 749                                          import_id: Option<ast::NodeId>,
 750                                          trait_def_id: DefId,
 751                                          item: ty::AssociatedItem) {
 752         for step in Rc::clone(&self.steps).iter() {
 753             match step.self_ty.sty {
 754                 ty::TyInfer(ty::IntVar(_)) | ty::TyInfer(ty::FloatVar(_)) |
 755                 ty::TyUint(_) | ty::TyInt(_) | ty::TyBool | ty::TyFloat(_) |
 756                 ty::TyFnDef(..) | ty::TyFnPtr(_) | ty::TyChar |
 757                 ty::TyRawPtr(..) | ty::TyError | ty::TyNever |
 758                 ty::TyRef(_, ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }) |
 759                 ty::TyArray(..) | ty::TyTuple(..) => {
 760                     ()
 761                 }
 762
 763                 _ => continue,
 764             };
 765
 766             let substs = Substs::for_item(self.tcx,
 767                                           trait_def_id,
 768                                           |def, _| self.region_var_for_def(self.span, def),
 769                                           |def, substs| {
 770                 if def.index == 0 {
 771                     step.self_ty
 772                 } else {
 773                     self.type_var_for_def(self.span, def, substs)
 774                 }
 775             });
 776
 777             let xform_self_ty = self.xform_self_ty(&item, step.self_ty, substs);
 778             self.push_inherent_candidate(xform_self_ty, item, TraitCandidate, import_id);
 779         }
 780     }
 781
 782     fn assemble_extension_candidates_for_trait_impls(&mut self,
 783                                                      import_id: Option<ast::NodeId>,
 784                                                      trait_def_id: DefId,
 785                                                      item: ty::AssociatedItem) {
 786         // FIXME(arielb1): can we use for_each_relevant_impl here?
 787         self.tcx.for_each_impl(trait_def_id, |impl_def_id| {
 788             debug!("assemble_extension_candidates_for_trait_impl: trait_def_id={:?} \
 789                                                                   impl_def_id={:?}",
 790                    trait_def_id,
 791                    impl_def_id);
 792
 793             if !self.impl_can_possibly_match(impl_def_id) {
 794                 return;
 795             }
 796
 797             let (_, impl_substs) = self.impl_ty_and_substs(impl_def_id);
 798
 799             debug!("impl_substs={:?}", impl_substs);
 800
 801             let impl_trait_ref = self.tcx.impl_trait_ref(impl_def_id)
 802                 .unwrap() // we know this is a trait impl
 803                 .subst(self.tcx, impl_substs);
 804
 805             debug!("impl_trait_ref={:?}", impl_trait_ref);
 806
 807             // Determine the receiver type that the method itself expects.
 808             let xform_self_ty =
 809                 self.xform_self_ty(&item, impl_trait_ref.self_ty(), impl_trait_ref.substs);
 810
 811             // Normalize the receiver. We can't use normalize_associated_types_in
 812             // as it will pollute the fcx's fulfillment context after this probe
 813             // is over.
 814             let cause = traits::ObligationCause::misc(self.span, self.body_id);
 815             let selcx = &mut traits::SelectionContext::new(self.fcx);
 816             let traits::Normalized { value: xform_self_ty, obligations } =
 817                 traits::normalize(selcx, self.param_env, cause, &xform_self_ty);
 818
 819             debug!("xform_self_ty={:?}", xform_self_ty);
 820
 821             self.push_extension_candidate(xform_self_ty, item,
 822                         ExtensionImplCandidate(impl_def_id, impl_substs, obligations), import_id);
 823         });
 824     }
 825
 826     fn impl_can_possibly_match(&self, impl_def_id: DefId) -> bool {
 827         let simplified_steps = match self.opt_simplified_steps {
 828             Some(ref simplified_steps) => simplified_steps,
 829             None => {
 830                 return true;
 831             }
 832         };
 833
 834         let impl_type = self.tcx.type_of(impl_def_id);
 835         let impl_simplified_type =
 836             match ty::fast_reject::simplify_type(self.tcx, impl_type, false) {
 837                 Some(simplified_type) => simplified_type,
 838                 None => {
 839                     return true;
 840                 }
 841             };
 842
 843         simplified_steps.contains(&impl_simplified_type)
 844     }
 845
 846     fn assemble_closure_candidates(&mut self,
 847                                    import_id: Option<ast::NodeId>,
 848                                    trait_def_id: DefId,
 849                                    item: ty::AssociatedItem)
 850                                    -> Result<(), MethodError<'tcx>> {
 851         // Check if this is one of the Fn,FnMut,FnOnce traits.
 852         let tcx = self.tcx;
 853         let kind = if Some(trait_def_id) == tcx.lang_items.fn_trait() {
 854             ty::ClosureKind::Fn
 855         } else if Some(trait_def_id) == tcx.lang_items.fn_mut_trait() {
 856             ty::ClosureKind::FnMut
 857         } else if Some(trait_def_id) == tcx.lang_items.fn_once_trait() {
 858             ty::ClosureKind::FnOnce
 859         } else {
 860             return Ok(());
 861         };
 862
 863         // Check if there is an unboxed-closure self-type in the list of receivers.
 864         // If so, add "synthetic impls".
 865         let steps = self.steps.clone();
 866         for step in steps.iter() {
 867             let closure_id = match step.self_ty.sty {
 868                 ty::TyClosure(def_id, _) => {
 869                     if let Some(id) = self.tcx.hir.as_local_node_id(def_id) {
 870                         self.tcx.hir.node_to_hir_id(id)
 871                     } else {
 872                         continue;
 873                     }
 874                 }
 875                 _ => continue,
 876             };
 877
 878             let closure_kind = {
 879                 match self.tables.borrow().closure_kinds().get(closure_id) {
 880                     Some(&(k, _)) => k,
 881                     None => {
 882                         return Err(MethodError::ClosureAmbiguity(trait_def_id));
 883                     }
 884                 }
 885             };
 886
 887             // this closure doesn't implement the right kind of `Fn` trait
 888             if !closure_kind.extends(kind) {
 889                 continue;
 890             }
 891
 892             // create some substitutions for the argument/return type;
 893             // for the purposes of our method lookup, we only take
 894             // receiver type into account, so we can just substitute
 895             // fresh types here to use during substitution and subtyping.
 896             let substs = Substs::for_item(self.tcx,
 897                                           trait_def_id,
 898                                           |def, _| self.region_var_for_def(self.span, def),
 899                                           |def, substs| {
 900                 if def.index == 0 {
 901                     step.self_ty
 902                 } else {
 903                     self.type_var_for_def(self.span, def, substs)
 904                 }
 905             });
 906
 907             let xform_self_ty = self.xform_self_ty(&item, step.self_ty, substs);
 908             self.push_inherent_candidate(xform_self_ty, item, TraitCandidate, import_id);
 909         }
 910
 911         Ok(())
 912     }
 913
 914     fn assemble_generator_candidates(&mut self,
 915                                    import_id: Option<ast::NodeId>,
 916                                    trait_def_id: DefId,
 917                                    item: ty::AssociatedItem)
 918                                    -> Result<(), MethodError<'tcx>> {
 919         // Check if this is the Generator trait.
 920         let tcx = self.tcx;
 921         if Some(trait_def_id) != tcx.lang_items.gen_trait() {
 922             return Ok(());
 923         }
 924
 925         // Check if there is an generator self-type in the list of receivers.
 926         // If so, add "synthetic impls".
 927         let steps = self.steps.clone();
 928         for step in steps.iter() {
 929             match step.self_ty.sty {
 930                 ty::TyGenerator(..) => (),
 931                 _ => continue,
 932             };
 933
 934             // create some substitutions for the argument/return type;
 935             // for the purposes of our method lookup, we only take
 936             // receiver type into account, so we can just substitute
 937             // fresh types here to use during substitution and subtyping.
 938             let substs = Substs::for_item(self.tcx,
 939                                           trait_def_id,
 940                                           |def, _| self.region_var_for_def(self.span, def),
 941                                           |def, substs| {
 942                 if def.index == 0 {
 943                     step.self_ty
 944                 } else {
 945                     self.type_var_for_def(self.span, def, substs)
 946                 }
 947             });
 948
 949             let xform_self_ty = self.xform_self_ty(&item, step.self_ty, substs);
 950             self.push_inherent_candidate(xform_self_ty, item, TraitCandidate, import_id);
 951         }
 952
 953         Ok(())
 954     }
 955
 956     fn assemble_projection_candidates(&mut self,
 957                                       import_id: Option<ast::NodeId>,
 958                                       trait_def_id: DefId,
 959                                       item: ty::AssociatedItem) {
 960         debug!("assemble_projection_candidates(\
 961                trait_def_id={:?}, \
 962                item={:?})",
 963                trait_def_id,
 964                item);
 965
 966         for step in Rc::clone(&self.steps).iter() {
 967             debug!("assemble_projection_candidates: step={:?}", step);
 968
 969             let (def_id, substs) = match step.self_ty.sty {
 970                 ty::TyProjection(ref data) => {
 971                     let trait_ref = data.trait_ref(self.tcx);
 972                     (trait_ref.def_id, trait_ref.substs)
 973                 },
 974                 ty::TyAnon(def_id, substs) => (def_id, substs),
 975                 _ => continue,
 976             };
 977
 978             debug!("assemble_projection_candidates: def_id={:?} substs={:?}",
 979                    def_id,
 980                    substs);
 981
 982             let trait_predicates = self.tcx.predicates_of(def_id);
 983             let bounds = trait_predicates.instantiate(self.tcx, substs);
 984             let predicates = bounds.predicates;
 985             debug!("assemble_projection_candidates: predicates={:?}",
 986                    predicates);
 987             for poly_bound in traits::elaborate_predicates(self.tcx, predicates)
 988                 .filter_map(|p| p.to_opt_poly_trait_ref())
 989                 .filter(|b| b.def_id() == trait_def_id) {
 990                 let bound = self.erase_late_bound_regions(&poly_bound);
 991
 992                 debug!("assemble_projection_candidates: def_id={:?} substs={:?} bound={:?}",
 993                        def_id,
 994                        substs,
 995                        bound);
 996
 997                 if self.can_eq(self.param_env, step.self_ty, bound.self_ty()).is_ok() {
 998                     let xform_self_ty = self.xform_self_ty(&item, bound.self_ty(), bound.substs);
 999
1000                     debug!("assemble_projection_candidates: bound={:?} xform_self_ty={:?}",
1001                            bound,
1002                            xform_self_ty);
1003
1004                     self.push_extension_candidate(xform_self_ty, item, TraitCandidate, import_id);
1005                 }
1006             }
1007         }
1008     }
1009
1010     fn assemble_where_clause_candidates(&mut self,
1011                                         import_id: Option<ast::NodeId>,
1012                                         trait_def_id: DefId,
1013                                         item: ty::AssociatedItem) {
1014         debug!("assemble_where_clause_candidates(trait_def_id={:?})",
1015                trait_def_id);
1016
1017         let caller_predicates = self.param_env.caller_bounds.to_vec();
1018         for poly_bound in traits::elaborate_predicates(self.tcx, caller_predicates)
1019             .filter_map(|p| p.to_opt_poly_trait_ref())
1020             .filter(|b| b.def_id() == trait_def_id) {
1021             let bound = self.erase_late_bound_regions(&poly_bound);
1022             let xform_self_ty = self.xform_self_ty(&item, bound.self_ty(), bound.substs);
1023
1024             debug!("assemble_where_clause_candidates: bound={:?} xform_self_ty={:?}",
1025                    bound,
1026                    xform_self_ty);
1027
1028             self.push_extension_candidate(xform_self_ty, item,
1029                                           WhereClauseCandidate(poly_bound), import_id);
1030         }
1031     }
1032
1033     fn candidate_method_names(&self) -> Vec<ast::Name> {
1034         let mut set = FxHashSet();
1035         let mut names: Vec<_> =
1036             self.inherent_candidates
1037                 .iter()
1038                 .chain(&self.extension_candidates)
1039                 .map(|candidate| candidate.item.name)
1040                 .filter(|&name| set.insert(name))
1041                 .collect();
1042
1043         // sort them by the name so we have a stable result
1044         names.sort_by_key(|n| n.as_str());
1045         names
1046     }
1047
1048     ///////////////////////////////////////////////////////////////////////////
1049     // THE ACTUAL SEARCH
1050
1051     fn pick(mut self) -> PickResult<'tcx> {
1052         assert!(match self.looking_for {
1053             LookingFor::MethodName(_) => true,
1054             LookingFor::ReturnType(_) => false,
1055         });
1056
1057         if let Some(r) = self.pick_core() {
1058             return r;
1059         }
1060
1061         let static_candidates = mem::replace(&mut self.static_candidates, vec![]);
1062         let private_candidate = mem::replace(&mut self.private_candidate, None);
1063         let unsatisfied_predicates = mem::replace(&mut self.unsatisfied_predicates, vec![]);
1064
1065         // things failed, so lets look at all traits, for diagnostic purposes now:
1066         self.reset();
1067
1068         let span = self.span;
1069         let tcx = self.tcx;
1070
1071         self.assemble_extension_candidates_for_all_traits()?;
1072
1073         let out_of_scope_traits = match self.pick_core() {
1074             Some(Ok(p)) => vec![p.item.container.id()],
1075             //Some(Ok(p)) => p.iter().map(|p| p.item.container().id()).collect(),
1076             Some(Err(MethodError::Ambiguity(v))) => {
1077                 v.into_iter()
1078                     .map(|source| {
1079                         match source {
1080                             TraitSource(id) => id,
1081                             ImplSource(impl_id) => {
1082                                 match tcx.trait_id_of_impl(impl_id) {
1083                                     Some(id) => id,
1084                                     None => {
1085                                         span_bug!(span,
1086                                                   "found inherent method when looking at traits")
1087                                     }
1088                                 }
1089                             }
1090                         }
1091                     })
1092                     .collect()
1093             }
1094             Some(Err(MethodError::NoMatch(NoMatchData { out_of_scope_traits: others, .. }))) => {
1095                 assert!(others.is_empty());
1096                 vec![]
1097             }
1098             Some(Err(MethodError::ClosureAmbiguity(..))) => {
1099                 // this error only occurs when assembling candidates
1100                 span_bug!(span, "encountered ClosureAmbiguity from pick_core");
1101             }
1102             _ => vec![],
1103         };
1104
1105         if let Some(def) = private_candidate {
1106             return Err(MethodError::PrivateMatch(def, out_of_scope_traits));
1107         }
1108
1109         Err(MethodError::NoMatch(NoMatchData::new(static_candidates,
1110                                                   unsatisfied_predicates,
1111                                                   out_of_scope_traits,
1112                                                   self.mode)))
1113     }
1114
1115     fn pick_core(&mut self) -> Option<PickResult<'tcx>> {
1116         let steps = self.steps.clone();
1117
1118         // find the first step that works
1119         steps
1120             .iter()
1121             .filter(|step| {
1122                 debug!("pick_core: step={:?}", step);
1123                 !step.self_ty.references_error()
1124             }).flat_map(|step| {
1125                 self.pick_by_value_method(step).or_else(|| {
1126                 self.pick_autorefd_method(step, hir::MutImmutable).or_else(|| {
1127                 self.pick_autorefd_method(step, hir::MutMutable)
1128             })})})
1129             .next()
1130     }
1131
1132     fn pick_by_value_method(&mut self, step: &CandidateStep<'tcx>) -> Option<PickResult<'tcx>> {
1133         //! For each type `T` in the step list, this attempts to find a
1134         //! method where the (transformed) self type is exactly `T`. We
1135         //! do however do one transformation on the adjustment: if we
1136         //! are passing a region pointer in, we will potentially
1137         //! *reborrow* it to a shorter lifetime. This allows us to
1138         //! transparently pass `&mut` pointers, in particular, without
1139         //! consuming them for their entire lifetime.
1140
1141         if step.unsize {
1142             return None;
1143         }
1144
1145         self.pick_method(step.self_ty).map(|r| {
1146             r.map(|mut pick| {
1147                 pick.autoderefs = step.autoderefs;
1148
1149                 // Insert a `&*` or `&mut *` if this is a reference type:
1150                 if let ty::TyRef(_, mt) = step.self_ty.sty {
1151                     pick.autoderefs += 1;
1152                     pick.autoref = Some(mt.mutbl);
1153                 }
1154
1155                 pick
1156             })
1157         })
1158     }
1159
1160     fn pick_autorefd_method(&mut self, step: &CandidateStep<'tcx>, mutbl: hir::Mutability)
1161                             -> Option<PickResult<'tcx>> {
1162         let tcx = self.tcx;
1163
1164         // In general, during probing we erase regions. See
1165         // `impl_self_ty()` for an explanation.
1166         let region = tcx.types.re_erased;
1167
1168         let autoref_ty = tcx.mk_ref(region,
1169                                     ty::TypeAndMut {
1170                                         ty: step.self_ty, mutbl
1171                                     });
1172         self.pick_method(autoref_ty).map(|r| {
1173             r.map(|mut pick| {
1174                 pick.autoderefs = step.autoderefs;
1175                 pick.autoref = Some(mutbl);
1176                 pick.unsize = if step.unsize {
1177                     Some(step.self_ty)
1178                 } else {
1179                     None
1180                 };
1181                 pick
1182             })
1183         })
1184     }
1185
1186     fn pick_method(&mut self, self_ty: Ty<'tcx>) -> Option<PickResult<'tcx>> {
1187         debug!("pick_method(self_ty={})", self.ty_to_string(self_ty));
1188
1189         let mut possibly_unsatisfied_predicates = Vec::new();
1190
1191         debug!("searching inherent candidates");
1192         if let Some(pick) = self.consider_candidates(self_ty,
1193                                                      &self.inherent_candidates,
1194                                                      &mut possibly_unsatisfied_predicates) {
1195             return Some(pick);
1196         }
1197
1198         debug!("searching extension candidates");
1199         let res = self.consider_candidates(self_ty,
1200                                            &self.extension_candidates,
1201                                            &mut possibly_unsatisfied_predicates);
1202         if let None = res {
1203             self.unsatisfied_predicates.extend(possibly_unsatisfied_predicates);
1204         }
1205         res
1206     }
1207
1208     fn consider_candidates(&self,
1209                            self_ty: Ty<'tcx>,
1210                            probes: &[Candidate<'tcx>],
1211                            possibly_unsatisfied_predicates: &mut Vec<TraitRef<'tcx>>)
1212                            -> Option<PickResult<'tcx>> {
1213         let mut applicable_candidates: Vec<_> = probes.iter()
1214             .filter(|&probe| self.consider_probe(self_ty, probe, possibly_unsatisfied_predicates))
1215             .collect();
1216
1217         debug!("applicable_candidates: {:?}", applicable_candidates);
1218
1219         if applicable_candidates.len() > 1 {
1220             match self.collapse_candidates_to_trait_pick(&applicable_candidates[..]) {
1221                 Some(pick) => {
1222                     return Some(Ok(pick));
1223                 }
1224                 None => {}
1225             }
1226         }
1227
1228         if applicable_candidates.len() > 1 {
1229             let sources = probes.iter().map(|p| p.to_source()).collect();
1230             return Some(Err(MethodError::Ambiguity(sources)));
1231         }
1232
1233         applicable_candidates.pop().map(|probe| Ok(probe.to_unadjusted_pick()))
1234     }
1235
1236     fn consider_probe(&self,
1237                       self_ty: Ty<'tcx>,
1238                       probe: &Candidate<'tcx>,
1239                       possibly_unsatisfied_predicates: &mut Vec<TraitRef<'tcx>>)
1240                       -> bool {
1241         debug!("consider_probe: self_ty={:?} probe={:?}", self_ty, probe);
1242
1243         self.probe(|_| {
1244             // First check that the self type can be related.
1245             let sub_obligations = match self.at(&ObligationCause::dummy(), self.param_env)
1246                                             .sup(probe.xform_self_ty, self_ty) {
1247                 Ok(InferOk { obligations, value: () }) => obligations,
1248                 Err(_) => {
1249                     debug!("--> cannot relate self-types");
1250                     return false;
1251                 }
1252             };
1253
1254             // If so, impls may carry other conditions (e.g., where
1255             // clauses) that must be considered. Make sure that those
1256             // match as well (or at least may match, sometimes we
1257             // don't have enough information to fully evaluate).
1258             let (impl_def_id, substs, ref_obligations) = match probe.kind {
1259                 InherentImplCandidate(ref substs, ref ref_obligations) => {
1260                     (probe.item.container.id(), substs, ref_obligations)
1261                 }
1262
1263                 ExtensionImplCandidate(impl_def_id, ref substs, ref ref_obligations) => {
1264                     (impl_def_id, substs, ref_obligations)
1265                 }
1266
1267                 ObjectCandidate |
1268                 TraitCandidate |
1269                 WhereClauseCandidate(..) => {
1270                     // These have no additional conditions to check.
1271                     return true;
1272                 }
1273             };
1274
1275             let selcx = &mut traits::SelectionContext::new(self);
1276             let cause = traits::ObligationCause::misc(self.span, self.body_id);
1277
1278             // Check whether the impl imposes obligations we have to worry about.
1279             let impl_bounds = self.tcx.predicates_of(impl_def_id);
1280             let impl_bounds = impl_bounds.instantiate(self.tcx, substs);
1281             let traits::Normalized { value: impl_bounds, obligations: norm_obligations } =
1282                 traits::normalize(selcx, self.param_env, cause.clone(), &impl_bounds);
1283
1284             // Convert the bounds into obligations.
1285             let obligations = traits::predicates_for_generics(cause.clone(),
1286                                                               self.param_env,
1287                                                               &impl_bounds);
1288             debug!("impl_obligations={:?}", obligations);
1289
1290             // Evaluate those obligations to see if they might possibly hold.
1291             let mut all_true = true;
1292             for o in obligations.iter()
1293                 .chain(sub_obligations.iter())
1294                 .chain(norm_obligations.iter())
1295                 .chain(ref_obligations.iter()) {
1296                 if !selcx.evaluate_obligation(o) {
1297                     all_true = false;
1298                     if let &ty::Predicate::Trait(ref pred) = &o.predicate {
1299                         possibly_unsatisfied_predicates.push(pred.0.trait_ref);
1300                     }
1301                 }
1302             }
1303             all_true
1304         })
1305     }
1306
1307     /// Sometimes we get in a situation where we have multiple probes that are all impls of the
1308     /// same trait, but we don't know which impl to use. In this case, since in all cases the
1309     /// external interface of the method can be determined from the trait, it's ok not to decide.
1310     /// We can basically just collapse all of the probes for various impls into one where-clause
1311     /// probe. This will result in a pending obligation so when more type-info is available we can
1312     /// make the final decision.
1313     ///
1314     /// Example (`src/test/run-pass/method-two-trait-defer-resolution-1.rs`):
1315     ///
1316     /// ```
1317     /// trait Foo { ... }
1318     /// impl Foo for Vec<int> { ... }
1319     /// impl Foo for Vec<usize> { ... }
1320     /// ```
1321     ///
1322     /// Now imagine the receiver is `Vec<_>`. It doesn't really matter at this time which impl we
1323     /// use, so it's ok to just commit to "using the method from the trait Foo".
1324     fn collapse_candidates_to_trait_pick(&self, probes: &[&Candidate<'tcx>]) -> Option<Pick<'tcx>> {
1325         // Do all probes correspond to the same trait?
1326         let container = probes[0].item.container;
1327         match container {
1328             ty::TraitContainer(_) => {}
1329             ty::ImplContainer(_) => return None,
1330         }
1331         if probes[1..].iter().any(|p| p.item.container != container) {
1332             return None;
1333         }
1334
1335         // If so, just use this trait and call it a day.
1336         Some(Pick {
1337             item: probes[0].item.clone(),
1338             kind: TraitPick,
1339             import_id: probes[0].import_id,
1340             autoderefs: 0,
1341             autoref: None,
1342             unsize: None,
1343         })
1344     }
1345
1346     ///////////////////////////////////////////////////////////////////////////
1347     // MISCELLANY
1348     fn has_applicable_self(&self, item: &ty::AssociatedItem) -> bool {
1349         // "Fast track" -- check for usage of sugar when in method call
1350         // mode.
1351         //
1352         // In Path mode (i.e., resolving a value like `T::next`), consider any
1353         // associated value (i.e., methods, constants) but not types.
1354         match self.mode {
1355             Mode::MethodCall => item.method_has_self_argument,
1356             Mode::Path => match item.kind {
1357                 ty::AssociatedKind::Type => false,
1358                 ty::AssociatedKind::Method | ty::AssociatedKind::Const => true
1359             },
1360         }
1361         // FIXME -- check for types that deref to `Self`,
1362         // like `Rc<Self>` and so on.
1363         //
1364         // Note also that the current code will break if this type
1365         // includes any of the type parameters defined on the method
1366         // -- but this could be overcome.
1367     }
1368
1369     fn record_static_candidate(&mut self, source: CandidateSource) {
1370         self.static_candidates.push(source);
1371     }
1372
1373     fn xform_self_ty(&self,
1374                      item: &ty::AssociatedItem,
1375                      impl_ty: Ty<'tcx>,
1376                      substs: &Substs<'tcx>)
1377                      -> Ty<'tcx> {
1378         if item.kind == ty::AssociatedKind::Method && self.mode == Mode::MethodCall {
1379             self.xform_method_self_ty(item.def_id, impl_ty, substs)
1380         } else {
1381             impl_ty
1382         }
1383     }
1384
1385     fn xform_method_self_ty(&self,
1386                             method: DefId,
1387                             impl_ty: Ty<'tcx>,
1388                             substs: &Substs<'tcx>)
1389                             -> Ty<'tcx> {
1390         let self_ty = self.tcx.fn_sig(method).input(0);
1391         debug!("xform_self_ty(impl_ty={:?}, self_ty={:?}, substs={:?})",
1392                impl_ty,
1393                self_ty,
1394                substs);
1395
1396         assert!(!substs.has_escaping_regions());
1397
1398         // It is possible for type parameters or early-bound lifetimes
1399         // to appear in the signature of `self`. The substitutions we
1400         // are given do not include type/lifetime parameters for the
1401         // method yet. So create fresh variables here for those too,
1402         // if there are any.
1403         let generics = self.tcx.generics_of(method);
1404         assert_eq!(substs.types().count(), generics.parent_types as usize);
1405         assert_eq!(substs.regions().count(), generics.parent_regions as usize);
1406
1407         // Erase any late-bound regions from the method and substitute
1408         // in the values from the substitution.
1409         let xform_self_ty = self.erase_late_bound_regions(&self_ty);
1410
1411         if generics.types.is_empty() && generics.regions.is_empty() {
1412             xform_self_ty.subst(self.tcx, substs)
1413         } else {
1414             let substs = Substs::for_item(self.tcx, method, |def, _| {
1415                 let i = def.index as usize;
1416                 if i < substs.len() {
1417                     substs.region_at(i)
1418                 } else {
1419                     // In general, during probe we erase regions. See
1420                     // `impl_self_ty()` for an explanation.
1421                     self.tcx.types.re_erased
1422                 }
1423             }, |def, cur_substs| {
1424                 let i = def.index as usize;
1425                 if i < substs.len() {
1426                     substs.type_at(i)
1427                 } else {
1428                     self.type_var_for_def(self.span, def, cur_substs)
1429                 }
1430             });
1431             xform_self_ty.subst(self.tcx, substs)
1432         }
1433     }
1434
1435     /// Get the type of an impl and generate substitutions with placeholders.
1436     fn impl_ty_and_substs(&self, impl_def_id: DefId) -> (Ty<'tcx>, &'tcx Substs<'tcx>) {
1437         let impl_ty = self.tcx.type_of(impl_def_id);
1438
1439         let substs = Substs::for_item(self.tcx,
1440                                       impl_def_id,
1441                                       |_, _| self.tcx.types.re_erased,
1442                                       |_, _| self.next_ty_var(
1443                                         TypeVariableOrigin::SubstitutionPlaceholder(
1444                                             self.tcx.def_span(impl_def_id))));
1445
1446         (impl_ty, substs)
1447     }
1448
1449     /// Replace late-bound-regions bound by `value` with `'static` using
1450     /// `ty::erase_late_bound_regions`.
1451     ///
1452     /// This is only a reasonable thing to do during the *probe* phase, not the *confirm* phase, of
1453     /// method matching. It is reasonable during the probe phase because we don't consider region
1454     /// relationships at all. Therefore, we can just replace all the region variables with 'static
1455     /// rather than creating fresh region variables. This is nice for two reasons:
1456     ///
1457     /// 1. Because the numbers of the region variables would otherwise be fairly unique to this
1458     ///    particular method call, it winds up creating fewer types overall, which helps for memory
1459     ///    usage. (Admittedly, this is a rather small effect, though measureable.)
1460     ///
1461     /// 2. It makes it easier to deal with higher-ranked trait bounds, because we can replace any
1462     ///    late-bound regions with 'static. Otherwise, if we were going to replace late-bound
1463     ///    regions with actual region variables as is proper, we'd have to ensure that the same
1464     ///    region got replaced with the same variable, which requires a bit more coordination
1465     ///    and/or tracking the substitution and
1466     ///    so forth.
1467     fn erase_late_bound_regions<T>(&self, value: &ty::Binder<T>) -> T
1468         where T: TypeFoldable<'tcx>
1469     {
1470         self.tcx.erase_late_bound_regions(value)
1471     }
1472
1473     /// Find the method with the appropriate name (or return type, as the case may be).
1474     fn impl_or_trait_item(&self, def_id: DefId) -> Vec<ty::AssociatedItem> {
1475         match self.looking_for {
1476             LookingFor::MethodName(name) => {
1477                 self.fcx.associated_item(def_id, name).map_or(Vec::new(), |x| vec![x])
1478             }
1479             LookingFor::ReturnType(return_ty) => {
1480                 self.tcx
1481                     .associated_items(def_id)
1482                     .map(|did| self.tcx.associated_item(did.def_id))
1483                     .filter(|m| self.matches_return_type(m, return_ty))
1484                     .collect()
1485             }
1486         }
1487     }
1488 }
1489
1490 impl<'tcx> Candidate<'tcx> {
1491     fn to_unadjusted_pick(&self) -> Pick<'tcx> {
1492         Pick {
1493             item: self.item.clone(),
1494             kind: match self.kind {
1495                 InherentImplCandidate(..) => InherentImplPick,
1496                 ExtensionImplCandidate(def_id, ..) => ExtensionImplPick(def_id),
1497                 ObjectCandidate => ObjectPick,
1498                 TraitCandidate => TraitPick,
1499                 WhereClauseCandidate(ref trait_ref) => {
1500                     // Only trait derived from where-clauses should
1501                     // appear here, so they should not contain any
1502                     // inference variables or other artifacts. This
1503                     // means they are safe to put into the
1504                     // `WhereClausePick`.
1505                     assert!(!trait_ref.substs().needs_infer());
1506
1507                     WhereClausePick(trait_ref.clone())
1508                 }
1509             },
1510             import_id: self.import_id,
1511             autoderefs: 0,
1512             autoref: None,
1513             unsize: None,
1514         }
1515     }
1516
1517     fn to_source(&self) -> CandidateSource {
1518         match self.kind {
1519             InherentImplCandidate(..) => ImplSource(self.item.container.id()),
1520             ExtensionImplCandidate(def_id, ..) => ImplSource(def_id),
1521             ObjectCandidate |
1522             TraitCandidate |
1523             WhereClauseCandidate(_) => TraitSource(self.item.container.id()),
1524         }
1525     }
1526 }