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 namespace::Namespace;
  20 use rustc::ty::subst::{Subst, Substs};
  21 use rustc::traits::{self, ObligationCause};
  22 use rustc::ty::{self, Ty, ToPolyTraitRef, ToPredicate, TraitRef, TypeFoldable};
  23 use rustc::infer::type_variable::TypeVariableOrigin;
  24 use rustc::util::nodemap::FxHashSet;
  25 use rustc::infer::{self, InferOk};
  26 use syntax::ast;
  27 use syntax::util::lev_distance::{lev_distance, find_best_match_for_name};
  28 use syntax_pos::Span;
  29 use rustc::hir;
  30 use rustc::lint;
  31 use std::mem;
  32 use std::ops::Deref;
  33 use std::rc::Rc;
  34 use std::cmp::max;
  35
  36 use self::CandidateKind::*;
  37 pub use self::PickKind::*;
  38
  39 /// Boolean flag used to indicate if this search is for a suggestion
  40 /// or not.  If true, we can allow ambiguity and so forth.
  41 pub struct IsSuggestion(pub bool);
  42
  43 struct ProbeContext<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
  44     fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
  45     span: Span,
  46     mode: Mode,
  47     method_name: Option<ast::Name>,
  48     return_type: Option<Ty<'tcx>>,
  49     steps: Rc<Vec<CandidateStep<'tcx>>>,
  50     inherent_candidates: Vec<Candidate<'tcx>>,
  51     extension_candidates: Vec<Candidate<'tcx>>,
  52     impl_dups: FxHashSet<DefId>,
  53
  54     /// Collects near misses when the candidate functions are missing a `self` keyword and is only
  55     /// used for error reporting
  56     static_candidates: Vec<CandidateSource>,
  57
  58     /// When probing for names, include names that are close to the
  59     /// requested name (by Levensthein distance)
  60     allow_similar_names: bool,
  61
  62     /// Some(candidate) if there is a private candidate
  63     private_candidate: Option<Def>,
  64
  65     /// Collects near misses when trait bounds for type parameters are unsatisfied and is only used
  66     /// for error reporting
  67     unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  68 }
  69
  70 impl<'a, 'gcx, 'tcx> Deref for ProbeContext<'a, 'gcx, 'tcx> {
  71     type Target = FnCtxt<'a, 'gcx, 'tcx>;
  72     fn deref(&self) -> &Self::Target {
  73         &self.fcx
  74     }
  75 }
  76
  77 #[derive(Debug)]
  78 struct CandidateStep<'tcx> {
  79     self_ty: Ty<'tcx>,
  80     autoderefs: usize,
  81     // true if the type results from a dereference of a raw pointer.
  82     // when assembling candidates, we include these steps, but not when
  83     // picking methods. This so that if we have `foo: *const Foo` and `Foo` has methods
  84     // `fn by_raw_ptr(self: *const Self)` and `fn by_ref(&self)`, then
  85     // `foo.by_raw_ptr()` will work and `foo.by_ref()` won't.
  86     from_unsafe_deref: bool,
  87     unsize: bool,
  88 }
  89
  90 #[derive(Debug)]
  91 struct Candidate<'tcx> {
  92     xform_self_ty: Ty<'tcx>,
  93     xform_ret_ty: Option<Ty<'tcx>>,
  94     item: ty::AssociatedItem,
  95     kind: CandidateKind<'tcx>,
  96     import_id: Option<ast::NodeId>,
  97 }
  98
  99 #[derive(Debug)]
 100 enum CandidateKind<'tcx> {
 101     InherentImplCandidate(&'tcx Substs<'tcx>,
 102                           // Normalize obligations
 103                           Vec<traits::PredicateObligation<'tcx>>),
 104     ObjectCandidate,
 105     TraitCandidate(ty::TraitRef<'tcx>),
 106     WhereClauseCandidate(// Trait
 107                          ty::PolyTraitRef<'tcx>),
 108 }
 109
 110 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 111 enum ProbeResult {
 112     NoMatch,
 113     BadReturnType,
 114     Match,
 115 }
 116
 117 #[derive(Debug, PartialEq, Eq, Clone)]
 118 pub struct Pick<'tcx> {
 119     pub item: ty::AssociatedItem,
 120     pub kind: PickKind<'tcx>,
 121     pub import_id: Option<ast::NodeId>,
 122
 123     // Indicates that the source expression should be autoderef'd N times
 124     //
 125     // A = expr | *expr | **expr | ...
 126     pub autoderefs: usize,
 127
 128     // Indicates that an autoref is applied after the optional autoderefs
 129     //
 130     // B = A | &A | &mut A
 131     pub autoref: Option<hir::Mutability>,
 132
 133     // Indicates that the source expression should be "unsized" to a
 134     // target type. This should probably eventually go away in favor
 135     // of just coercing method receivers.
 136     //
 137     // C = B | unsize(B)
 138     pub unsize: Option<Ty<'tcx>>,
 139 }
 140
 141 #[derive(Clone, Debug, PartialEq, Eq)]
 142 pub enum PickKind<'tcx> {
 143     InherentImplPick,
 144     ObjectPick,
 145     TraitPick,
 146     WhereClausePick(// Trait
 147                     ty::PolyTraitRef<'tcx>),
 148 }
 149
 150 pub type PickResult<'tcx> = Result<Pick<'tcx>, MethodError<'tcx>>;
 151
 152 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 153 pub enum Mode {
 154     // An expression of the form `receiver.method_name(...)`.
 155     // Autoderefs are performed on `receiver`, lookup is done based on the
 156     // `self` argument  of the method, and static methods aren't considered.
 157     MethodCall,
 158     // An expression of the form `Type::item` or `<T>::item`.
 159     // No autoderefs are performed, lookup is done based on the type each
 160     // implementation is for, and static methods are included.
 161     Path,
 162 }
 163
 164 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 165 pub enum ProbeScope {
 166     // Assemble candidates coming only from traits in scope.
 167     TraitsInScope,
 168
 169     // Assemble candidates coming from all traits.
 170     AllTraits,
 171 }
 172
 173 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 174     /// This is used to offer suggestions to users. It returns methods
 175     /// that could have been called which have the desired return
 176     /// type. Some effort is made to rule out methods that, if called,
 177     /// would result in an error (basically, the same criteria we
 178     /// would use to decide if a method is a plausible fit for
 179     /// ambiguity purposes).
 180     pub fn probe_for_return_type(&self,
 181                                  span: Span,
 182                                  mode: Mode,
 183                                  return_type: Ty<'tcx>,
 184                                  self_ty: Ty<'tcx>,
 185                                  scope_expr_id: ast::NodeId)
 186                                  -> Vec<ty::AssociatedItem> {
 187         debug!("probe(self_ty={:?}, return_type={}, scope_expr_id={})",
 188                self_ty,
 189                return_type,
 190                scope_expr_id);
 191         let method_names =
 192             self.probe_op(span, mode, None, Some(return_type), IsSuggestion(true),
 193                           self_ty, scope_expr_id, ProbeScope::AllTraits,
 194                           |probe_cx| Ok(probe_cx.candidate_method_names()))
 195                 .unwrap_or(vec![]);
 196          method_names
 197              .iter()
 198              .flat_map(|&method_name| {
 199                  self.probe_op(
 200                      span, mode, Some(method_name), Some(return_type),
 201                      IsSuggestion(true), self_ty, scope_expr_id,
 202                      ProbeScope::AllTraits, |probe_cx| probe_cx.pick()
 203                  ).ok().map(|pick| pick.item)
 204              })
 205             .collect()
 206     }
 207
 208     pub fn probe_for_name(&self,
 209                           span: Span,
 210                           mode: Mode,
 211                           item_name: ast::Name,
 212                           is_suggestion: IsSuggestion,
 213                           self_ty: Ty<'tcx>,
 214                           scope_expr_id: ast::NodeId,
 215                           scope: ProbeScope)
 216                           -> PickResult<'tcx> {
 217         debug!("probe(self_ty={:?}, item_name={}, scope_expr_id={})",
 218                self_ty,
 219                item_name,
 220                scope_expr_id);
 221         self.probe_op(span,
 222                       mode,
 223                       Some(item_name),
 224                       None,
 225                       is_suggestion,
 226                       self_ty,
 227                       scope_expr_id,
 228                       scope,
 229                       |probe_cx| probe_cx.pick())
 230     }
 231
 232     fn probe_op<OP,R>(&'a self,
 233                       span: Span,
 234                       mode: Mode,
 235                       method_name: Option<ast::Name>,
 236                       return_type: Option<Ty<'tcx>>,
 237                       is_suggestion: IsSuggestion,
 238                       self_ty: Ty<'tcx>,
 239                       scope_expr_id: ast::NodeId,
 240                       scope: ProbeScope,
 241                       op: OP)
 242                       -> Result<R, MethodError<'tcx>>
 243         where OP: FnOnce(ProbeContext<'a, 'gcx, 'tcx>) -> Result<R, MethodError<'tcx>>
 244     {
 245         // FIXME(#18741) -- right now, creating the steps involves evaluating the
 246         // `*` operator, which registers obligations that then escape into
 247         // the global fulfillment context and thus has global
 248         // side-effects. This is a bit of a pain to refactor. So just let
 249         // it ride, although it's really not great, and in fact could I
 250         // think cause spurious errors. Really though this part should
 251         // take place in the `self.probe` below.
 252         let steps = if mode == Mode::MethodCall {
 253             match self.create_steps(span, scope_expr_id, self_ty, is_suggestion) {
 254                 Some(steps) => steps,
 255                 None => {
 256                     return Err(MethodError::NoMatch(NoMatchData::new(Vec::new(),
 257                                                                      Vec::new(),
 258                                                                      Vec::new(),
 259                                                                      None,
 260                                                                      mode)))
 261                 }
 262             }
 263         } else {
 264             vec![CandidateStep {
 265                      self_ty,
 266                      autoderefs: 0,
 267                      from_unsafe_deref: false,
 268                      unsize: false,
 269                  }]
 270         };
 271
 272         debug!("ProbeContext: steps for self_ty={:?} are {:?}",
 273                self_ty,
 274                steps);
 275
 276         // this creates one big transaction so that all type variables etc
 277         // that we create during the probe process are removed later
 278         self.probe(|_| {
 279             let mut probe_cx =
 280                 ProbeContext::new(self, span, mode, method_name, return_type, Rc::new(steps));
 281
 282             probe_cx.assemble_inherent_candidates();
 283             match scope {
 284                 ProbeScope::TraitsInScope =>
 285                     probe_cx.assemble_extension_candidates_for_traits_in_scope(scope_expr_id)?,
 286                 ProbeScope::AllTraits =>
 287                     probe_cx.assemble_extension_candidates_for_all_traits()?,
 288             };
 289             op(probe_cx)
 290         })
 291     }
 292
 293     fn create_steps(&self,
 294                     span: Span,
 295                     scope_expr_id: ast::NodeId,
 296                     self_ty: Ty<'tcx>,
 297                     is_suggestion: IsSuggestion)
 298                     -> Option<Vec<CandidateStep<'tcx>>> {
 299         // FIXME: we don't need to create the entire steps in one pass
 300
 301         let mut autoderef = self.autoderef(span, self_ty).include_raw_pointers();
 302         let mut reached_raw_pointer = false;
 303         let mut steps: Vec<_> = autoderef.by_ref()
 304             .map(|(ty, d)| {
 305                 let step = CandidateStep {
 306                     self_ty: ty,
 307                     autoderefs: d,
 308                     from_unsafe_deref: reached_raw_pointer,
 309                     unsize: false,
 310                 };
 311                 if let ty::TyRawPtr(_) = ty.sty {
 312                     // all the subsequent steps will be from_unsafe_deref
 313                     reached_raw_pointer = true;
 314                 }
 315                 step
 316             })
 317             .collect();
 318
 319         let final_ty = autoderef.maybe_ambiguous_final_ty();
 320         match final_ty.sty {
 321             ty::TyInfer(ty::TyVar(_)) => {
 322                 // Ended in an inference variable. If we are doing
 323                 // a real method lookup, this is a hard error because it's
 324                 // possible that there will be multiple applicable methods.
 325                 if !is_suggestion.0 {
 326                     if reached_raw_pointer
 327                     && !self.tcx.sess.features.borrow().arbitrary_self_types {
 328                         // this case used to be allowed by the compiler,
 329                         // so we do a future-compat lint here for the 2015 epoch
 330                         // (see https://github.com/rust-lang/rust/issues/46906)
 331                         if self.tcx.sess.rust_2018() {
 332                           span_err!(self.tcx.sess, span, E0908,
 333                                     "the type of this value must be known \
 334                                      to call a method on a raw pointer on it");
 335                         } else {
 336                             self.tcx.lint_node(
 337                                 lint::builtin::TYVAR_BEHIND_RAW_POINTER,
 338                                 scope_expr_id,
 339                                 span,
 340                                 &format!("type annotations needed"));
 341                         }
 342                     } else {
 343                         let t = self.structurally_resolved_type(span, final_ty);
 344                         assert_eq!(t, self.tcx.types.err);
 345                         return None
 346                     }
 347                 } else {
 348                     // If we're just looking for suggestions,
 349                     // though, ambiguity is no big thing, we can
 350                     // just ignore it.
 351                 }
 352             }
 353             ty::TyArray(elem_ty, _) => {
 354                 let dereferences = steps.len() - 1;
 355
 356                 steps.push(CandidateStep {
 357                     self_ty: self.tcx.mk_slice(elem_ty),
 358                     autoderefs: dereferences,
 359                     // this could be from an unsafe deref if we had
 360                     // a *mut/const [T; N]
 361                     from_unsafe_deref: reached_raw_pointer,
 362                     unsize: true,
 363                 });
 364             }
 365             ty::TyError => return None,
 366             _ => (),
 367         }
 368
 369         debug!("create_steps: steps={:?}", steps);
 370
 371         Some(steps)
 372     }
 373 }
 374
 375 impl<'a, 'gcx, 'tcx> ProbeContext<'a, 'gcx, 'tcx> {
 376     fn new(fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
 377            span: Span,
 378            mode: Mode,
 379            method_name: Option<ast::Name>,
 380            return_type: Option<Ty<'tcx>>,
 381            steps: Rc<Vec<CandidateStep<'tcx>>>)
 382            -> ProbeContext<'a, 'gcx, 'tcx> {
 383         ProbeContext {
 384             fcx,
 385             span,
 386             mode,
 387             method_name,
 388             return_type,
 389             inherent_candidates: Vec::new(),
 390             extension_candidates: Vec::new(),
 391             impl_dups: FxHashSet(),
 392             steps: steps,
 393             static_candidates: Vec::new(),
 394             allow_similar_names: false,
 395             private_candidate: None,
 396             unsatisfied_predicates: Vec::new(),
 397         }
 398     }
 399
 400     fn reset(&mut self) {
 401         self.inherent_candidates.clear();
 402         self.extension_candidates.clear();
 403         self.impl_dups.clear();
 404         self.static_candidates.clear();
 405         self.private_candidate = None;
 406     }
 407
 408     ///////////////////////////////////////////////////////////////////////////
 409     // CANDIDATE ASSEMBLY
 410
 411     fn push_candidate(&mut self,
 412                       candidate: Candidate<'tcx>,
 413                       is_inherent: bool)
 414     {
 415         let is_accessible = if let Some(name) = self.method_name {
 416             let item = candidate.item;
 417             let def_scope = self.tcx.adjust(name, item.container.id(), self.body_id).1;
 418             item.vis.is_accessible_from(def_scope, self.tcx)
 419         } else {
 420             true
 421         };
 422         if is_accessible {
 423             if is_inherent {
 424                 self.inherent_candidates.push(candidate);
 425             } else {
 426                 self.extension_candidates.push(candidate);
 427             }
 428         } else if self.private_candidate.is_none() {
 429             self.private_candidate = Some(candidate.item.def());
 430         }
 431     }
 432
 433     fn assemble_inherent_candidates(&mut self) {
 434         let steps = self.steps.clone();
 435         for step in steps.iter() {
 436             self.assemble_probe(step.self_ty);
 437         }
 438     }
 439
 440     fn assemble_probe(&mut self, self_ty: Ty<'tcx>) {
 441         debug!("assemble_probe: self_ty={:?}", self_ty);
 442         let lang_items = self.tcx.lang_items();
 443
 444         match self_ty.sty {
 445             ty::TyDynamic(ref data, ..) => {
 446                 if let Some(p) = data.principal() {
 447                     self.assemble_inherent_candidates_from_object(self_ty, p);
 448                     self.assemble_inherent_impl_candidates_for_type(p.def_id());
 449                 }
 450             }
 451             ty::TyAdt(def, _) => {
 452                 self.assemble_inherent_impl_candidates_for_type(def.did);
 453             }
 454             ty::TyForeign(did) => {
 455                 self.assemble_inherent_impl_candidates_for_type(did);
 456             }
 457             ty::TyParam(p) => {
 458                 self.assemble_inherent_candidates_from_param(self_ty, p);
 459             }
 460             ty::TyChar => {
 461                 let lang_def_id = lang_items.char_impl();
 462                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 463             }
 464             ty::TyStr => {
 465                 let lang_def_id = lang_items.str_impl();
 466                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 467             }
 468             ty::TySlice(_) => {
 469                 let lang_def_id = lang_items.slice_impl();
 470                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 471
 472                 let lang_def_id = lang_items.slice_u8_impl();
 473                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 474             }
 475             ty::TyRawPtr(ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }) => {
 476                 let lang_def_id = lang_items.const_ptr_impl();
 477                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 478             }
 479             ty::TyRawPtr(ty::TypeAndMut { ty: _, mutbl: hir::MutMutable }) => {
 480                 let lang_def_id = lang_items.mut_ptr_impl();
 481                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 482             }
 483             ty::TyInt(ast::IntTy::I8) => {
 484                 let lang_def_id = lang_items.i8_impl();
 485                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 486             }
 487             ty::TyInt(ast::IntTy::I16) => {
 488                 let lang_def_id = lang_items.i16_impl();
 489                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 490             }
 491             ty::TyInt(ast::IntTy::I32) => {
 492                 let lang_def_id = lang_items.i32_impl();
 493                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 494             }
 495             ty::TyInt(ast::IntTy::I64) => {
 496                 let lang_def_id = lang_items.i64_impl();
 497                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 498             }
 499             ty::TyInt(ast::IntTy::I128) => {
 500                 let lang_def_id = lang_items.i128_impl();
 501                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 502             }
 503             ty::TyInt(ast::IntTy::Isize) => {
 504                 let lang_def_id = lang_items.isize_impl();
 505                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 506             }
 507             ty::TyUint(ast::UintTy::U8) => {
 508                 let lang_def_id = lang_items.u8_impl();
 509                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 510             }
 511             ty::TyUint(ast::UintTy::U16) => {
 512                 let lang_def_id = lang_items.u16_impl();
 513                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 514             }
 515             ty::TyUint(ast::UintTy::U32) => {
 516                 let lang_def_id = lang_items.u32_impl();
 517                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 518             }
 519             ty::TyUint(ast::UintTy::U64) => {
 520                 let lang_def_id = lang_items.u64_impl();
 521                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 522             }
 523             ty::TyUint(ast::UintTy::U128) => {
 524                 let lang_def_id = lang_items.u128_impl();
 525                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 526             }
 527             ty::TyUint(ast::UintTy::Usize) => {
 528                 let lang_def_id = lang_items.usize_impl();
 529                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 530             }
 531             ty::TyFloat(ast::FloatTy::F32) => {
 532                 let lang_def_id = lang_items.f32_impl();
 533                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 534             }
 535             ty::TyFloat(ast::FloatTy::F64) => {
 536                 let lang_def_id = lang_items.f64_impl();
 537                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 538             }
 539             _ => {}
 540         }
 541     }
 542
 543     fn assemble_inherent_impl_for_primitive(&mut self, lang_def_id: Option<DefId>) {
 544         if let Some(impl_def_id) = lang_def_id {
 545             self.assemble_inherent_impl_probe(impl_def_id);
 546         }
 547     }
 548
 549     fn assemble_inherent_impl_candidates_for_type(&mut self, def_id: DefId) {
 550         let impl_def_ids = self.tcx.at(self.span).inherent_impls(def_id);
 551         for &impl_def_id in impl_def_ids.iter() {
 552             self.assemble_inherent_impl_probe(impl_def_id);
 553         }
 554     }
 555
 556     fn assemble_inherent_impl_probe(&mut self, impl_def_id: DefId) {
 557         if !self.impl_dups.insert(impl_def_id) {
 558             return; // already visited
 559         }
 560
 561         debug!("assemble_inherent_impl_probe {:?}", impl_def_id);
 562
 563         for item in self.impl_or_trait_item(impl_def_id) {
 564             if !self.has_applicable_self(&item) {
 565                 // No receiver declared. Not a candidate.
 566                 self.record_static_candidate(ImplSource(impl_def_id));
 567                 continue
 568             }
 569
 570             let (impl_ty, impl_substs) = self.impl_ty_and_substs(impl_def_id);
 571             let impl_ty = impl_ty.subst(self.tcx, impl_substs);
 572
 573             // Determine the receiver type that the method itself expects.
 574             let xform_tys = self.xform_self_ty(&item, impl_ty, impl_substs);
 575
 576             // We can't use normalize_associated_types_in as it will pollute the
 577             // fcx's fulfillment context after this probe is over.
 578             let cause = traits::ObligationCause::misc(self.span, self.body_id);
 579             let selcx = &mut traits::SelectionContext::new(self.fcx);
 580             let traits::Normalized { value: (xform_self_ty, xform_ret_ty), obligations } =
 581                 traits::normalize(selcx, self.param_env, cause, &xform_tys);
 582             debug!("assemble_inherent_impl_probe: xform_self_ty = {:?}/{:?}",
 583                    xform_self_ty, xform_ret_ty);
 584
 585             self.push_candidate(Candidate {
 586                 xform_self_ty, xform_ret_ty, item,
 587                 kind: InherentImplCandidate(impl_substs, obligations),
 588                 import_id: None
 589             }, true);
 590         }
 591     }
 592
 593     fn assemble_inherent_candidates_from_object(&mut self,
 594                                                 self_ty: Ty<'tcx>,
 595                                                 principal: ty::PolyExistentialTraitRef<'tcx>) {
 596         debug!("assemble_inherent_candidates_from_object(self_ty={:?})",
 597                self_ty);
 598
 599         // It is illegal to invoke a method on a trait instance that
 600         // refers to the `Self` type. An error will be reported by
 601         // `enforce_object_limitations()` if the method refers to the
 602         // `Self` type anywhere other than the receiver. Here, we use
 603         // a substitution that replaces `Self` with the object type
 604         // itself. Hence, a `&self` method will wind up with an
 605         // argument type like `&Trait`.
 606         let trait_ref = principal.with_self_ty(self.tcx, self_ty);
 607         self.elaborate_bounds(&[trait_ref], |this, new_trait_ref, item| {
 608             let new_trait_ref = this.erase_late_bound_regions(&new_trait_ref);
 609
 610             let (xform_self_ty, xform_ret_ty) =
 611                 this.xform_self_ty(&item, new_trait_ref.self_ty(), new_trait_ref.substs);
 612             this.push_candidate(Candidate {
 613                 xform_self_ty, xform_ret_ty, item,
 614                 kind: ObjectCandidate,
 615                 import_id: None
 616             }, true);
 617         });
 618     }
 619
 620     fn assemble_inherent_candidates_from_param(&mut self,
 621                                                _rcvr_ty: Ty<'tcx>,
 622                                                param_ty: ty::ParamTy) {
 623         // FIXME -- Do we want to commit to this behavior for param bounds?
 624
 625         let bounds: Vec<_> = self.param_env
 626             .caller_bounds
 627             .iter()
 628             .filter_map(|predicate| {
 629                 match *predicate {
 630                     ty::Predicate::Trait(ref trait_predicate) => {
 631                         match trait_predicate.0.trait_ref.self_ty().sty {
 632                             ty::TyParam(ref p) if *p == param_ty => {
 633                                 Some(trait_predicate.to_poly_trait_ref())
 634                             }
 635                             _ => None,
 636                         }
 637                     }
 638                     ty::Predicate::Equate(..) |
 639                     ty::Predicate::Subtype(..) |
 640                     ty::Predicate::Projection(..) |
 641                     ty::Predicate::RegionOutlives(..) |
 642                     ty::Predicate::WellFormed(..) |
 643                     ty::Predicate::ObjectSafe(..) |
 644                     ty::Predicate::ClosureKind(..) |
 645                     ty::Predicate::TypeOutlives(..) |
 646                     ty::Predicate::ConstEvaluatable(..) => None,
 647                 }
 648             })
 649             .collect();
 650
 651         self.elaborate_bounds(&bounds, |this, poly_trait_ref, item| {
 652             let trait_ref = this.erase_late_bound_regions(&poly_trait_ref);
 653
 654             let (xform_self_ty, xform_ret_ty) =
 655                 this.xform_self_ty(&item, trait_ref.self_ty(), trait_ref.substs);
 656
 657             // Because this trait derives from a where-clause, it
 658             // should not contain any inference variables or other
 659             // artifacts. This means it is safe to put into the
 660             // `WhereClauseCandidate` and (eventually) into the
 661             // `WhereClausePick`.
 662             assert!(!trait_ref.substs.needs_infer());
 663
 664             this.push_candidate(Candidate {
 665                 xform_self_ty, xform_ret_ty, item,
 666                 kind: WhereClauseCandidate(poly_trait_ref),
 667                 import_id: None
 668             }, true);
 669         });
 670     }
 671
 672     // Do a search through a list of bounds, using a callback to actually
 673     // create the candidates.
 674     fn elaborate_bounds<F>(&mut self, bounds: &[ty::PolyTraitRef<'tcx>], mut mk_cand: F)
 675         where F: for<'b> FnMut(&mut ProbeContext<'b, 'gcx, 'tcx>,
 676                                ty::PolyTraitRef<'tcx>,
 677                                ty::AssociatedItem)
 678     {
 679         debug!("elaborate_bounds(bounds={:?})", bounds);
 680
 681         let tcx = self.tcx;
 682         for bound_trait_ref in traits::transitive_bounds(tcx, bounds) {
 683             for item in self.impl_or_trait_item(bound_trait_ref.def_id()) {
 684                 if !self.has_applicable_self(&item) {
 685                     self.record_static_candidate(TraitSource(bound_trait_ref.def_id()));
 686                 } else {
 687                     mk_cand(self, bound_trait_ref, item);
 688                 }
 689             }
 690         }
 691     }
 692
 693     fn assemble_extension_candidates_for_traits_in_scope(&mut self,
 694                                                          expr_id: ast::NodeId)
 695                                                          -> Result<(), MethodError<'tcx>> {
 696         if expr_id == ast::DUMMY_NODE_ID {
 697             return Ok(())
 698         }
 699         let mut duplicates = FxHashSet();
 700         let expr_hir_id = self.tcx.hir.node_to_hir_id(expr_id);
 701         let opt_applicable_traits = self.tcx.in_scope_traits(expr_hir_id);
 702         if let Some(applicable_traits) = opt_applicable_traits {
 703             for trait_candidate in applicable_traits.iter() {
 704                 let trait_did = trait_candidate.def_id;
 705                 if duplicates.insert(trait_did) {
 706                     let import_id = trait_candidate.import_id;
 707                     let result = self.assemble_extension_candidates_for_trait(import_id, trait_did);
 708                     result?;
 709                 }
 710             }
 711         }
 712         Ok(())
 713     }
 714
 715     fn assemble_extension_candidates_for_all_traits(&mut self) -> Result<(), MethodError<'tcx>> {
 716         let mut duplicates = FxHashSet();
 717         for trait_info in suggest::all_traits(self.tcx) {
 718             if duplicates.insert(trait_info.def_id) {
 719                 self.assemble_extension_candidates_for_trait(None, trait_info.def_id)?;
 720             }
 721         }
 722         Ok(())
 723     }
 724
 725     pub fn matches_return_type(&self,
 726                                method: &ty::AssociatedItem,
 727                                self_ty: Option<Ty<'tcx>>,
 728                                expected: Ty<'tcx>) -> bool {
 729         match method.def() {
 730             Def::Method(def_id) => {
 731                 let fty = self.tcx.fn_sig(def_id);
 732                 self.probe(|_| {
 733                     let substs = self.fresh_substs_for_item(ty::UniverseIndex::ROOT,
 734                                                             self.span,
 735                                                             method.def_id);
 736                     let fty = fty.subst(self.tcx, substs);
 737                     let (fty, _) = self.replace_late_bound_regions_with_fresh_var(
 738                         self.span, infer::FnCall, &fty);
 739
 740                     if let Some(self_ty) = self_ty {
 741                         if let Err(_) = self.at(&ObligationCause::dummy(), self.param_env)
 742                             .sup(fty.inputs()[0], self_ty)
 743                         {
 744                             return false
 745                         }
 746                     }
 747                     self.can_sub(self.param_env, fty.output(), expected).is_ok()
 748                 })
 749             }
 750             _ => false,
 751         }
 752     }
 753
 754     fn assemble_extension_candidates_for_trait(&mut self,
 755                                                import_id: Option<ast::NodeId>,
 756                                                trait_def_id: DefId)
 757                                                -> Result<(), MethodError<'tcx>> {
 758         debug!("assemble_extension_candidates_for_trait(trait_def_id={:?})",
 759                trait_def_id);
 760         let trait_substs = self.fresh_item_substs(trait_def_id);
 761         let trait_ref = ty::TraitRef::new(trait_def_id, trait_substs);
 762
 763         for item in self.impl_or_trait_item(trait_def_id) {
 764             // Check whether `trait_def_id` defines a method with suitable name:
 765             if !self.has_applicable_self(&item) {
 766                 debug!("method has inapplicable self");
 767                 self.record_static_candidate(TraitSource(trait_def_id));
 768                 continue;
 769             }
 770
 771             let (xform_self_ty, xform_ret_ty) =
 772                 self.xform_self_ty(&item, trait_ref.self_ty(), trait_substs);
 773             self.push_candidate(Candidate {
 774                 xform_self_ty, xform_ret_ty, item, import_id,
 775                 kind: TraitCandidate(trait_ref),
 776             }, false);
 777         }
 778         Ok(())
 779     }
 780
 781     fn candidate_method_names(&self) -> Vec<ast::Name> {
 782         let mut set = FxHashSet();
 783         let mut names: Vec<_> = self.inherent_candidates
 784             .iter()
 785             .chain(&self.extension_candidates)
 786             .filter(|candidate| {
 787                 if let Some(return_ty) = self.return_type {
 788                     self.matches_return_type(&candidate.item, None, return_ty)
 789                 } else {
 790                     true
 791                 }
 792             })
 793             .map(|candidate| candidate.item.name)
 794             .filter(|&name| set.insert(name))
 795             .collect();
 796
 797         // sort them by the name so we have a stable result
 798         names.sort_by_key(|n| n.as_str());
 799         names
 800     }
 801
 802     ///////////////////////////////////////////////////////////////////////////
 803     // THE ACTUAL SEARCH
 804
 805     fn pick(mut self) -> PickResult<'tcx> {
 806         assert!(self.method_name.is_some());
 807
 808         if let Some(r) = self.pick_core() {
 809             return r;
 810         }
 811
 812         let static_candidates = mem::replace(&mut self.static_candidates, vec![]);
 813         let private_candidate = mem::replace(&mut self.private_candidate, None);
 814         let unsatisfied_predicates = mem::replace(&mut self.unsatisfied_predicates, vec![]);
 815
 816         // things failed, so lets look at all traits, for diagnostic purposes now:
 817         self.reset();
 818
 819         let span = self.span;
 820         let tcx = self.tcx;
 821
 822         self.assemble_extension_candidates_for_all_traits()?;
 823
 824         let out_of_scope_traits = match self.pick_core() {
 825             Some(Ok(p)) => vec![p.item.container.id()],
 826             //Some(Ok(p)) => p.iter().map(|p| p.item.container().id()).collect(),
 827             Some(Err(MethodError::Ambiguity(v))) => {
 828                 v.into_iter()
 829                     .map(|source| {
 830                         match source {
 831                             TraitSource(id) => id,
 832                             ImplSource(impl_id) => {
 833                                 match tcx.trait_id_of_impl(impl_id) {
 834                                     Some(id) => id,
 835                                     None => {
 836                                         span_bug!(span,
 837                                                   "found inherent method when looking at traits")
 838                                     }
 839                                 }
 840                             }
 841                         }
 842                     })
 843                     .collect()
 844             }
 845             Some(Err(MethodError::NoMatch(NoMatchData { out_of_scope_traits: others, .. }))) => {
 846                 assert!(others.is_empty());
 847                 vec![]
 848             }
 849             _ => vec![],
 850         };
 851
 852         if let Some(def) = private_candidate {
 853             return Err(MethodError::PrivateMatch(def, out_of_scope_traits));
 854         }
 855         let lev_candidate = self.probe_for_lev_candidate()?;
 856
 857         Err(MethodError::NoMatch(NoMatchData::new(static_candidates,
 858                                                   unsatisfied_predicates,
 859                                                   out_of_scope_traits,
 860                                                   lev_candidate,
 861                                                   self.mode)))
 862     }
 863
 864     fn pick_core(&mut self) -> Option<PickResult<'tcx>> {
 865         let steps = self.steps.clone();
 866
 867         // find the first step that works
 868         steps
 869             .iter()
 870             .filter(|step| {
 871                 debug!("pick_core: step={:?}", step);
 872                 // skip types that are from a type error or that would require dereferencing
 873                 // a raw pointer
 874                 !step.self_ty.references_error() && !step.from_unsafe_deref
 875             }).flat_map(|step| {
 876                 self.pick_by_value_method(step).or_else(|| {
 877                 self.pick_autorefd_method(step, hir::MutImmutable).or_else(|| {
 878                 self.pick_autorefd_method(step, hir::MutMutable)
 879             })})})
 880             .next()
 881     }
 882
 883     fn pick_by_value_method(&mut self, step: &CandidateStep<'tcx>) -> Option<PickResult<'tcx>> {
 884         //! For each type `T` in the step list, this attempts to find a
 885         //! method where the (transformed) self type is exactly `T`. We
 886         //! do however do one transformation on the adjustment: if we
 887         //! are passing a region pointer in, we will potentially
 888         //! *reborrow* it to a shorter lifetime. This allows us to
 889         //! transparently pass `&mut` pointers, in particular, without
 890         //! consuming them for their entire lifetime.
 891
 892         if step.unsize {
 893             return None;
 894         }
 895
 896         self.pick_method(step.self_ty).map(|r| {
 897             r.map(|mut pick| {
 898                 pick.autoderefs = step.autoderefs;
 899
 900                 // Insert a `&*` or `&mut *` if this is a reference type:
 901                 if let ty::TyRef(_, mt) = step.self_ty.sty {
 902                     pick.autoderefs += 1;
 903                     pick.autoref = Some(mt.mutbl);
 904                 }
 905
 906                 pick
 907             })
 908         })
 909     }
 910
 911     fn pick_autorefd_method(&mut self, step: &CandidateStep<'tcx>, mutbl: hir::Mutability)
 912                             -> Option<PickResult<'tcx>> {
 913         let tcx = self.tcx;
 914
 915         // In general, during probing we erase regions. See
 916         // `impl_self_ty()` for an explanation.
 917         let region = tcx.types.re_erased;
 918
 919         let autoref_ty = tcx.mk_ref(region,
 920                                     ty::TypeAndMut {
 921                                         ty: step.self_ty, mutbl
 922                                     });
 923         self.pick_method(autoref_ty).map(|r| {
 924             r.map(|mut pick| {
 925                 pick.autoderefs = step.autoderefs;
 926                 pick.autoref = Some(mutbl);
 927                 pick.unsize = if step.unsize {
 928                     Some(step.self_ty)
 929                 } else {
 930                     None
 931                 };
 932                 pick
 933             })
 934         })
 935     }
 936
 937     fn pick_method(&mut self, self_ty: Ty<'tcx>) -> Option<PickResult<'tcx>> {
 938         debug!("pick_method(self_ty={})", self.ty_to_string(self_ty));
 939
 940         let mut possibly_unsatisfied_predicates = Vec::new();
 941
 942         debug!("searching inherent candidates");
 943         if let Some(pick) = self.consider_candidates(self_ty,
 944                                                      &self.inherent_candidates,
 945                                                      &mut possibly_unsatisfied_predicates) {
 946             return Some(pick);
 947         }
 948
 949         debug!("searching extension candidates");
 950         let res = self.consider_candidates(self_ty,
 951                                            &self.extension_candidates,
 952                                            &mut possibly_unsatisfied_predicates);
 953         if let None = res {
 954             self.unsatisfied_predicates.extend(possibly_unsatisfied_predicates);
 955         }
 956         res
 957     }
 958
 959     fn consider_candidates(&self,
 960                            self_ty: Ty<'tcx>,
 961                            probes: &[Candidate<'tcx>],
 962                            possibly_unsatisfied_predicates: &mut Vec<TraitRef<'tcx>>)
 963                            -> Option<PickResult<'tcx>> {
 964         let mut applicable_candidates: Vec<_> = probes.iter()
 965             .map(|probe| {
 966                 (probe, self.consider_probe(self_ty, probe, possibly_unsatisfied_predicates))
 967             })
 968             .filter(|&(_, status)| status != ProbeResult::NoMatch)
 969             .collect();
 970
 971         debug!("applicable_candidates: {:?}", applicable_candidates);
 972
 973         if applicable_candidates.len() > 1 {
 974             if let Some(pick) = self.collapse_candidates_to_trait_pick(&applicable_candidates[..]) {
 975                 return Some(Ok(pick));
 976             }
 977         }
 978
 979         if applicable_candidates.len() > 1 {
 980             let sources = probes.iter()
 981                 .map(|p| self.candidate_source(p, self_ty))
 982                 .collect();
 983             return Some(Err(MethodError::Ambiguity(sources)));
 984         }
 985
 986         applicable_candidates.pop().map(|(probe, status)| {
 987             if status == ProbeResult::Match {
 988                 Ok(probe.to_unadjusted_pick())
 989             } else {
 990                 Err(MethodError::BadReturnType)
 991             }
 992         })
 993     }
 994
 995     fn select_trait_candidate(&self, trait_ref: ty::TraitRef<'tcx>)
 996                               -> traits::SelectionResult<'tcx, traits::Selection<'tcx>>
 997     {
 998         let cause = traits::ObligationCause::misc(self.span, self.body_id);
 999         let predicate =
1000             trait_ref.to_poly_trait_ref().to_poly_trait_predicate();
1001         let obligation = traits::Obligation::new(cause, self.param_env, predicate);
1002         traits::SelectionContext::new(self).select(&obligation)
1003     }
1004
1005     fn candidate_source(&self, candidate: &Candidate<'tcx>, self_ty: Ty<'tcx>)
1006                         -> CandidateSource
1007     {
1008         match candidate.kind {
1009             InherentImplCandidate(..) => ImplSource(candidate.item.container.id()),
1010             ObjectCandidate |
1011             WhereClauseCandidate(_) => TraitSource(candidate.item.container.id()),
1012             TraitCandidate(trait_ref) => self.probe(|_| {
1013                 let _ = self.at(&ObligationCause::dummy(), self.param_env)
1014                     .sup(candidate.xform_self_ty, self_ty);
1015                 match self.select_trait_candidate(trait_ref) {
1016                     Ok(Some(traits::Vtable::VtableImpl(ref impl_data))) => {
1017                         // If only a single impl matches, make the error message point
1018                         // to that impl.
1019                         ImplSource(impl_data.impl_def_id)
1020                     }
1021                     _ => {
1022                         TraitSource(candidate.item.container.id())
1023                     }
1024                 }
1025             })
1026         }
1027     }
1028
1029     fn consider_probe(&self,
1030                       self_ty: Ty<'tcx>,
1031                       probe: &Candidate<'tcx>,
1032                       possibly_unsatisfied_predicates: &mut Vec<TraitRef<'tcx>>)
1033                       -> ProbeResult {
1034         debug!("consider_probe: self_ty={:?} probe={:?}", self_ty, probe);
1035
1036         self.probe(|_| {
1037             // First check that the self type can be related.
1038             let sub_obligations = match self.at(&ObligationCause::dummy(), self.param_env)
1039                                             .sup(probe.xform_self_ty, self_ty) {
1040                 Ok(InferOk { obligations, value: () }) => obligations,
1041                 Err(_) => {
1042                     debug!("--> cannot relate self-types");
1043                     return ProbeResult::NoMatch;
1044                 }
1045             };
1046
1047             let mut result = ProbeResult::Match;
1048             let selcx = &mut traits::SelectionContext::new(self);
1049             let cause = traits::ObligationCause::misc(self.span, self.body_id);
1050
1051             // If so, impls may carry other conditions (e.g., where
1052             // clauses) that must be considered. Make sure that those
1053             // match as well (or at least may match, sometimes we
1054             // don't have enough information to fully evaluate).
1055             let candidate_obligations : Vec<_> = match probe.kind {
1056                 InherentImplCandidate(ref substs, ref ref_obligations) => {
1057                     // Check whether the impl imposes obligations we have to worry about.
1058                     let impl_def_id = probe.item.container.id();
1059                     let impl_bounds = self.tcx.predicates_of(impl_def_id);
1060                     let impl_bounds = impl_bounds.instantiate(self.tcx, substs);
1061                     let traits::Normalized { value: impl_bounds, obligations: norm_obligations } =
1062                         traits::normalize(selcx, self.param_env, cause.clone(), &impl_bounds);
1063
1064                     // Convert the bounds into obligations.
1065                     let impl_obligations = traits::predicates_for_generics(
1066                         cause.clone(), self.param_env, &impl_bounds);
1067
1068                     debug!("impl_obligations={:?}", impl_obligations);
1069                     impl_obligations.into_iter()
1070                         .chain(norm_obligations.into_iter())
1071                         .chain(ref_obligations.iter().cloned())
1072                         .collect()
1073                 }
1074
1075                 ObjectCandidate |
1076                 WhereClauseCandidate(..) => {
1077                     // These have no additional conditions to check.
1078                     vec![]
1079                 }
1080
1081                 TraitCandidate(trait_ref) => {
1082                     let predicate = trait_ref.to_predicate();
1083                     let obligation =
1084                         traits::Obligation::new(cause.clone(), self.param_env, predicate);
1085                     if !selcx.evaluate_obligation(&obligation) {
1086                         if self.probe(|_| self.select_trait_candidate(trait_ref).is_err()) {
1087                             // This candidate's primary obligation doesn't even
1088                             // select - don't bother registering anything in
1089                             // `potentially_unsatisfied_predicates`.
1090                             return ProbeResult::NoMatch;
1091                         } else {
1092                             // Some nested subobligation of this predicate
1093                             // failed.
1094                             //
1095                             // FIXME: try to find the exact nested subobligation
1096                             // and point at it rather than reporting the entire
1097                             // trait-ref?
1098                             result = ProbeResult::NoMatch;
1099                             let trait_ref = self.resolve_type_vars_if_possible(&trait_ref);
1100                             possibly_unsatisfied_predicates.push(trait_ref);
1101                         }
1102                     }
1103                     vec![]
1104                 }
1105             };
1106
1107             debug!("consider_probe - candidate_obligations={:?} sub_obligations={:?}",
1108                    candidate_obligations, sub_obligations);
1109
1110             // Evaluate those obligations to see if they might possibly hold.
1111             for o in candidate_obligations.into_iter().chain(sub_obligations) {
1112                 let o = self.resolve_type_vars_if_possible(&o);
1113                 if !selcx.evaluate_obligation(&o) {
1114                     result = ProbeResult::NoMatch;
1115                     if let &ty::Predicate::Trait(ref pred) = &o.predicate {
1116                         possibly_unsatisfied_predicates.push(pred.0.trait_ref);
1117                     }
1118                 }
1119             }
1120
1121             if let ProbeResult::Match = result {
1122                 if let (Some(return_ty), Some(xform_ret_ty)) =
1123                     (self.return_type, probe.xform_ret_ty)
1124                 {
1125                     let xform_ret_ty = self.resolve_type_vars_if_possible(&xform_ret_ty);
1126                     debug!("comparing return_ty {:?} with xform ret ty {:?}",
1127                            return_ty,
1128                            probe.xform_ret_ty);
1129                     if self.at(&ObligationCause::dummy(), self.param_env)
1130                         .sup(return_ty, xform_ret_ty)
1131                         .is_err()
1132                     {
1133                         return ProbeResult::BadReturnType;
1134                     }
1135                 }
1136             }
1137
1138             result
1139         })
1140     }
1141
1142     /// Sometimes we get in a situation where we have multiple probes that are all impls of the
1143     /// same trait, but we don't know which impl to use. In this case, since in all cases the
1144     /// external interface of the method can be determined from the trait, it's ok not to decide.
1145     /// We can basically just collapse all of the probes for various impls into one where-clause
1146     /// probe. This will result in a pending obligation so when more type-info is available we can
1147     /// make the final decision.
1148     ///
1149     /// Example (`src/test/run-pass/method-two-trait-defer-resolution-1.rs`):
1150     ///
1151     /// ```
1152     /// trait Foo { ... }
1153     /// impl Foo for Vec<int> { ... }
1154     /// impl Foo for Vec<usize> { ... }
1155     /// ```
1156     ///
1157     /// Now imagine the receiver is `Vec<_>`. It doesn't really matter at this time which impl we
1158     /// use, so it's ok to just commit to "using the method from the trait Foo".
1159     fn collapse_candidates_to_trait_pick(&self, probes: &[(&Candidate<'tcx>, ProbeResult)])
1160                                          -> Option<Pick<'tcx>>
1161     {
1162         // Do all probes correspond to the same trait?
1163         let container = probes[0].0.item.container;
1164         match container {
1165             ty::TraitContainer(_) => {}
1166             ty::ImplContainer(_) => return None,
1167         }
1168         if probes[1..].iter().any(|&(p, _)| p.item.container != container) {
1169             return None;
1170         }
1171
1172         // FIXME: check the return type here somehow.
1173         // If so, just use this trait and call it a day.
1174         Some(Pick {
1175             item: probes[0].0.item.clone(),
1176             kind: TraitPick,
1177             import_id: probes[0].0.import_id,
1178             autoderefs: 0,
1179             autoref: None,
1180             unsize: None,
1181         })
1182     }
1183
1184     /// Similarly to `probe_for_return_type`, this method attempts to find the best matching
1185     /// candidate method where the method name may have been misspelt. Similarly to other
1186     /// Levenshtein based suggestions, we provide at most one such suggestion.
1187     fn probe_for_lev_candidate(&mut self) -> Result<Option<ty::AssociatedItem>, MethodError<'tcx>> {
1188         debug!("Probing for method names similar to {:?}",
1189                self.method_name);
1190
1191         let steps = self.steps.clone();
1192         self.probe(|_| {
1193             let mut pcx = ProbeContext::new(self.fcx, self.span, self.mode, self.method_name,
1194                                             self.return_type, steps);
1195             pcx.allow_similar_names = true;
1196             pcx.assemble_inherent_candidates();
1197             pcx.assemble_extension_candidates_for_traits_in_scope(ast::DUMMY_NODE_ID)?;
1198
1199             let method_names = pcx.candidate_method_names();
1200             pcx.allow_similar_names = false;
1201             let applicable_close_candidates: Vec<ty::AssociatedItem> = method_names
1202                 .iter()
1203                 .filter_map(|&method_name| {
1204                     pcx.reset();
1205                     pcx.method_name = Some(method_name);
1206                     pcx.assemble_inherent_candidates();
1207                     pcx.assemble_extension_candidates_for_traits_in_scope(ast::DUMMY_NODE_ID)
1208                         .ok().map_or(None, |_| {
1209                             pcx.pick_core()
1210                                 .and_then(|pick| pick.ok())
1211                                 .and_then(|pick| Some(pick.item))
1212                         })
1213                 })
1214                .collect();
1215
1216             if applicable_close_candidates.is_empty() {
1217                 Ok(None)
1218             } else {
1219                 let best_name = {
1220                     let names = applicable_close_candidates.iter().map(|cand| &cand.name);
1221                     find_best_match_for_name(names,
1222                                              &self.method_name.unwrap().as_str(),
1223                                              None)
1224                 }.unwrap();
1225                 Ok(applicable_close_candidates
1226                    .into_iter()
1227                    .find(|method| method.name == best_name))
1228             }
1229         })
1230     }
1231
1232     ///////////////////////////////////////////////////////////////////////////
1233     // MISCELLANY
1234     fn has_applicable_self(&self, item: &ty::AssociatedItem) -> bool {
1235         // "Fast track" -- check for usage of sugar when in method call
1236         // mode.
1237         //
1238         // In Path mode (i.e., resolving a value like `T::next`), consider any
1239         // associated value (i.e., methods, constants) but not types.
1240         match self.mode {
1241             Mode::MethodCall => item.method_has_self_argument,
1242             Mode::Path => match item.kind {
1243                 ty::AssociatedKind::Type => false,
1244                 ty::AssociatedKind::Method | ty::AssociatedKind::Const => true
1245             },
1246         }
1247         // FIXME -- check for types that deref to `Self`,
1248         // like `Rc<Self>` and so on.
1249         //
1250         // Note also that the current code will break if this type
1251         // includes any of the type parameters defined on the method
1252         // -- but this could be overcome.
1253     }
1254
1255     fn record_static_candidate(&mut self, source: CandidateSource) {
1256         self.static_candidates.push(source);
1257     }
1258
1259     fn xform_self_ty(&self,
1260                      item: &ty::AssociatedItem,
1261                      impl_ty: Ty<'tcx>,
1262                      substs: &Substs<'tcx>)
1263                      -> (Ty<'tcx>, Option<Ty<'tcx>>) {
1264         if item.kind == ty::AssociatedKind::Method && self.mode == Mode::MethodCall {
1265             let sig = self.xform_method_sig(item.def_id, substs);
1266             (sig.inputs()[0], Some(sig.output()))
1267         } else {
1268             (impl_ty, None)
1269         }
1270     }
1271
1272     fn xform_method_sig(&self,
1273                         method: DefId,
1274                         substs: &Substs<'tcx>)
1275                         -> ty::FnSig<'tcx>
1276     {
1277         let fn_sig = self.tcx.fn_sig(method);
1278         debug!("xform_self_ty(fn_sig={:?}, substs={:?})",
1279                fn_sig,
1280                substs);
1281
1282         assert!(!substs.has_escaping_regions());
1283
1284         // It is possible for type parameters or early-bound lifetimes
1285         // to appear in the signature of `self`. The substitutions we
1286         // are given do not include type/lifetime parameters for the
1287         // method yet. So create fresh variables here for those too,
1288         // if there are any.
1289         let generics = self.tcx.generics_of(method);
1290         assert_eq!(substs.types().count(), generics.parent_types as usize);
1291         assert_eq!(substs.regions().count(), generics.parent_regions as usize);
1292
1293         // Erase any late-bound regions from the method and substitute
1294         // in the values from the substitution.
1295         let xform_fn_sig = self.erase_late_bound_regions(&fn_sig);
1296
1297         if generics.types.is_empty() && generics.regions.is_empty() {
1298             xform_fn_sig.subst(self.tcx, substs)
1299         } else {
1300             let substs = Substs::for_item(self.tcx, method, |def, _| {
1301                 let i = def.index as usize;
1302                 if i < substs.len() {
1303                     substs.region_at(i)
1304                 } else {
1305                     // In general, during probe we erase regions. See
1306                     // `impl_self_ty()` for an explanation.
1307                     self.tcx.types.re_erased
1308                 }
1309             }, |def, _cur_substs| {
1310                 let i = def.index as usize;
1311                 if i < substs.len() {
1312                     substs.type_at(i)
1313                 } else {
1314                     self.type_var_for_def(ty::UniverseIndex::ROOT, self.span, def)
1315                 }
1316             });
1317             xform_fn_sig.subst(self.tcx, substs)
1318         }
1319     }
1320
1321     /// Get the type of an impl and generate substitutions with placeholders.
1322     fn impl_ty_and_substs(&self, impl_def_id: DefId) -> (Ty<'tcx>, &'tcx Substs<'tcx>) {
1323         (self.tcx.type_of(impl_def_id), self.fresh_item_substs(impl_def_id))
1324     }
1325
1326     fn fresh_item_substs(&self, def_id: DefId) -> &'tcx Substs<'tcx> {
1327         Substs::for_item(self.tcx,
1328                          def_id,
1329                          |_, _| self.tcx.types.re_erased,
1330                          |_, _| self.next_ty_var(
1331                              ty::UniverseIndex::ROOT,
1332                              TypeVariableOrigin::SubstitutionPlaceholder(
1333                                  self.tcx.def_span(def_id))))
1334     }
1335
1336     /// Replace late-bound-regions bound by `value` with `'static` using
1337     /// `ty::erase_late_bound_regions`.
1338     ///
1339     /// This is only a reasonable thing to do during the *probe* phase, not the *confirm* phase, of
1340     /// method matching. It is reasonable during the probe phase because we don't consider region
1341     /// relationships at all. Therefore, we can just replace all the region variables with 'static
1342     /// rather than creating fresh region variables. This is nice for two reasons:
1343     ///
1344     /// 1. Because the numbers of the region variables would otherwise be fairly unique to this
1345     ///    particular method call, it winds up creating fewer types overall, which helps for memory
1346     ///    usage. (Admittedly, this is a rather small effect, though measureable.)
1347     ///
1348     /// 2. It makes it easier to deal with higher-ranked trait bounds, because we can replace any
1349     ///    late-bound regions with 'static. Otherwise, if we were going to replace late-bound
1350     ///    regions with actual region variables as is proper, we'd have to ensure that the same
1351     ///    region got replaced with the same variable, which requires a bit more coordination
1352     ///    and/or tracking the substitution and
1353     ///    so forth.
1354     fn erase_late_bound_regions<T>(&self, value: &ty::Binder<T>) -> T
1355         where T: TypeFoldable<'tcx>
1356     {
1357         self.tcx.erase_late_bound_regions(value)
1358     }
1359
1360     /// Find the method with the appropriate name (or return type, as the case may be). If
1361     /// `allow_similar_names` is set, find methods with close-matching names.
1362     fn impl_or_trait_item(&self, def_id: DefId) -> Vec<ty::AssociatedItem> {
1363         if let Some(name) = self.method_name {
1364             if self.allow_similar_names {
1365                 let max_dist = max(name.as_str().len(), 3) / 3;
1366                 self.tcx.associated_items(def_id)
1367                     .filter(|x| {
1368                         let dist = lev_distance(&*name.as_str(), &x.name.as_str());
1369                         Namespace::from(x.kind) == Namespace::Value && dist > 0
1370                         && dist <= max_dist
1371                     })
1372                     .collect()
1373             } else {
1374                 self.fcx
1375                     .associated_item(def_id, name, Namespace::Value)
1376                     .map_or(Vec::new(), |x| vec![x])
1377             }
1378         } else {
1379             self.tcx.associated_items(def_id).collect()
1380         }
1381     }
1382 }
1383
1384 impl<'tcx> Candidate<'tcx> {
1385     fn to_unadjusted_pick(&self) -> Pick<'tcx> {
1386         Pick {
1387             item: self.item.clone(),
1388             kind: match self.kind {
1389                 InherentImplCandidate(..) => InherentImplPick,
1390                 ObjectCandidate => ObjectPick,
1391                 TraitCandidate(_) => TraitPick,
1392                 WhereClauseCandidate(ref trait_ref) => {
1393                     // Only trait derived from where-clauses should
1394                     // appear here, so they should not contain any
1395                     // inference variables or other artifacts. This
1396                     // means they are safe to put into the
1397                     // `WhereClausePick`.
1398                     assert!(!trait_ref.substs().needs_infer());
1399
1400                     WhereClausePick(trait_ref.clone())
1401                 }
1402             },
1403             import_id: self.import_id,
1404             autoderefs: 0,
1405             autoref: None,
1406             unsize: None,
1407         }
1408     }
1409 }