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::hir;
  21 use rustc::lint;
  22 use rustc::session::config::nightly_options;
  23 use rustc::ty::subst::{Subst, Substs};
  24 use rustc::traits::{self, ObligationCause};
  25 use rustc::ty::{self, Ty, ToPolyTraitRef, ToPredicate, TraitRef, TypeFoldable};
  26 use rustc::ty::GenericParamDefKind;
  27 use rustc::infer::type_variable::TypeVariableOrigin;
  28 use rustc::util::nodemap::FxHashSet;
  29 use rustc::infer::{self, InferOk};
  30 use rustc::middle::stability;
  31 use syntax::ast;
  32 use syntax::util::lev_distance::{lev_distance, find_best_match_for_name};
  33 use syntax_pos::{Span, symbol::Symbol};
  34 use std::iter;
  35 use std::mem;
  36 use std::ops::Deref;
  37 use std::rc::Rc;
  38 use std::cmp::max;
  39
  40 use self::CandidateKind::*;
  41 pub use self::PickKind::*;
  42
  43 /// Boolean flag used to indicate if this search is for a suggestion
  44 /// or not.  If true, we can allow ambiguity and so forth.
  45 #[derive(Clone, Copy)]
  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     method_name: Option<ast::Ident>,
  53     return_type: Option<Ty<'tcx>>,
  54     steps: Rc<Vec<CandidateStep<'tcx>>>,
  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     /// When probing for names, include names that are close to the
  64     /// requested name (by Levensthein distance)
  65     allow_similar_names: bool,
  66
  67     /// Some(candidate) if there is a private candidate
  68     private_candidate: Option<Def>,
  69
  70     /// Collects near misses when trait bounds for type parameters are unsatisfied and is only used
  71     /// for error reporting
  72     unsatisfied_predicates: Vec<TraitRef<'tcx>>,
  73
  74     is_suggestion: IsSuggestion,
  75 }
  76
  77 impl<'a, 'gcx, 'tcx> Deref for ProbeContext<'a, 'gcx, 'tcx> {
  78     type Target = FnCtxt<'a, 'gcx, 'tcx>;
  79     fn deref(&self) -> &Self::Target {
  80         &self.fcx
  81     }
  82 }
  83
  84 #[derive(Debug)]
  85 struct CandidateStep<'tcx> {
  86     self_ty: Ty<'tcx>,
  87     autoderefs: usize,
  88     // true if the type results from a dereference of a raw pointer.
  89     // when assembling candidates, we include these steps, but not when
  90     // picking methods. This so that if we have `foo: *const Foo` and `Foo` has methods
  91     // `fn by_raw_ptr(self: *const Self)` and `fn by_ref(&self)`, then
  92     // `foo.by_raw_ptr()` will work and `foo.by_ref()` won't.
  93     from_unsafe_deref: bool,
  94     unsize: bool,
  95 }
  96
  97 #[derive(Debug)]
  98 struct Candidate<'tcx> {
  99     xform_self_ty: Ty<'tcx>,
 100     xform_ret_ty: Option<Ty<'tcx>>,
 101     item: ty::AssociatedItem,
 102     kind: CandidateKind<'tcx>,
 103     import_id: Option<ast::NodeId>,
 104 }
 105
 106 #[derive(Debug)]
 107 enum CandidateKind<'tcx> {
 108     InherentImplCandidate(&'tcx Substs<'tcx>,
 109                           // Normalize obligations
 110                           Vec<traits::PredicateObligation<'tcx>>),
 111     ObjectCandidate,
 112     TraitCandidate(ty::TraitRef<'tcx>),
 113     WhereClauseCandidate(// Trait
 114                          ty::PolyTraitRef<'tcx>),
 115 }
 116
 117 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 118 enum ProbeResult {
 119     NoMatch,
 120     BadReturnType,
 121     Match,
 122 }
 123
 124 #[derive(Debug, PartialEq, Clone)]
 125 pub struct Pick<'tcx> {
 126     pub item: ty::AssociatedItem,
 127     pub kind: PickKind<'tcx>,
 128     pub import_id: Option<ast::NodeId>,
 129
 130     // Indicates that the source expression should be autoderef'd N times
 131     //
 132     // A = expr | *expr | **expr | ...
 133     pub autoderefs: usize,
 134
 135     // Indicates that an autoref is applied after the optional autoderefs
 136     //
 137     // B = A | &A | &mut A
 138     pub autoref: Option<hir::Mutability>,
 139
 140     // Indicates that the source expression should be "unsized" to a
 141     // target type. This should probably eventually go away in favor
 142     // of just coercing method receivers.
 143     //
 144     // C = B | unsize(B)
 145     pub unsize: Option<Ty<'tcx>>,
 146 }
 147
 148 #[derive(Clone, Debug, PartialEq, Eq)]
 149 pub enum PickKind<'tcx> {
 150     InherentImplPick,
 151     ObjectPick,
 152     TraitPick,
 153     WhereClausePick(// Trait
 154                     ty::PolyTraitRef<'tcx>),
 155 }
 156
 157 pub type PickResult<'tcx> = Result<Pick<'tcx>, MethodError<'tcx>>;
 158
 159 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 160 pub enum Mode {
 161     // An expression of the form `receiver.method_name(...)`.
 162     // Autoderefs are performed on `receiver`, lookup is done based on the
 163     // `self` argument  of the method, and static methods aren't considered.
 164     MethodCall,
 165     // An expression of the form `Type::item` or `<T>::item`.
 166     // No autoderefs are performed, lookup is done based on the type each
 167     // implementation is for, and static methods are included.
 168     Path,
 169 }
 170
 171 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 172 pub enum ProbeScope {
 173     // Assemble candidates coming only from traits in scope.
 174     TraitsInScope,
 175
 176     // Assemble candidates coming from all traits.
 177     AllTraits,
 178 }
 179
 180 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 181     /// This is used to offer suggestions to users. It returns methods
 182     /// that could have been called which have the desired return
 183     /// type. Some effort is made to rule out methods that, if called,
 184     /// would result in an error (basically, the same criteria we
 185     /// would use to decide if a method is a plausible fit for
 186     /// ambiguity purposes).
 187     pub fn probe_for_return_type(&self,
 188                                  span: Span,
 189                                  mode: Mode,
 190                                  return_type: Ty<'tcx>,
 191                                  self_ty: Ty<'tcx>,
 192                                  scope_expr_id: ast::NodeId)
 193                                  -> Vec<ty::AssociatedItem> {
 194         debug!("probe(self_ty={:?}, return_type={}, scope_expr_id={})",
 195                self_ty,
 196                return_type,
 197                scope_expr_id);
 198         let method_names =
 199             self.probe_op(span, mode, None, Some(return_type), IsSuggestion(true),
 200                           self_ty, scope_expr_id, ProbeScope::AllTraits,
 201                           |probe_cx| Ok(probe_cx.candidate_method_names()))
 202                 .unwrap_or(vec![]);
 203          method_names
 204              .iter()
 205              .flat_map(|&method_name| {
 206                  self.probe_op(
 207                      span, mode, Some(method_name), Some(return_type),
 208                      IsSuggestion(true), self_ty, scope_expr_id,
 209                      ProbeScope::AllTraits, |probe_cx| probe_cx.pick()
 210                  ).ok().map(|pick| pick.item)
 211              })
 212             .collect()
 213     }
 214
 215     pub fn probe_for_name(&self,
 216                           span: Span,
 217                           mode: Mode,
 218                           item_name: ast::Ident,
 219                           is_suggestion: IsSuggestion,
 220                           self_ty: Ty<'tcx>,
 221                           scope_expr_id: ast::NodeId,
 222                           scope: ProbeScope)
 223                           -> PickResult<'tcx> {
 224         debug!("probe(self_ty={:?}, item_name={}, scope_expr_id={})",
 225                self_ty,
 226                item_name,
 227                scope_expr_id);
 228         self.probe_op(span,
 229                       mode,
 230                       Some(item_name),
 231                       None,
 232                       is_suggestion,
 233                       self_ty,
 234                       scope_expr_id,
 235                       scope,
 236                       |probe_cx| probe_cx.pick())
 237     }
 238
 239     fn probe_op<OP,R>(&'a self,
 240                       span: Span,
 241                       mode: Mode,
 242                       method_name: Option<ast::Ident>,
 243                       return_type: Option<Ty<'tcx>>,
 244                       is_suggestion: IsSuggestion,
 245                       self_ty: Ty<'tcx>,
 246                       scope_expr_id: ast::NodeId,
 247                       scope: ProbeScope,
 248                       op: OP)
 249                       -> Result<R, MethodError<'tcx>>
 250         where OP: FnOnce(ProbeContext<'a, 'gcx, 'tcx>) -> Result<R, MethodError<'tcx>>
 251     {
 252         // FIXME(#18741) -- right now, creating the steps involves evaluating the
 253         // `*` operator, which registers obligations that then escape into
 254         // the global fulfillment context and thus has global
 255         // side-effects. This is a bit of a pain to refactor. So just let
 256         // it ride, although it's really not great, and in fact could I
 257         // think cause spurious errors. Really though this part should
 258         // take place in the `self.probe` below.
 259         let steps = if mode == Mode::MethodCall {
 260             match self.create_steps(span, scope_expr_id, self_ty, is_suggestion) {
 261                 Some(steps) => steps,
 262                 None => {
 263                     return Err(MethodError::NoMatch(NoMatchData::new(Vec::new(),
 264                                                                      Vec::new(),
 265                                                                      Vec::new(),
 266                                                                      None,
 267                                                                      mode)))
 268                 }
 269             }
 270         } else {
 271             vec![CandidateStep {
 272                      self_ty,
 273                      autoderefs: 0,
 274                      from_unsafe_deref: false,
 275                      unsize: false,
 276                  }]
 277         };
 278
 279         debug!("ProbeContext: steps for self_ty={:?} are {:?}",
 280                self_ty,
 281                steps);
 282
 283         // this creates one big transaction so that all type variables etc
 284         // that we create during the probe process are removed later
 285         self.probe(|_| {
 286             let mut probe_cx = ProbeContext::new(
 287                 self, span, mode, method_name, return_type, Rc::new(steps), is_suggestion,
 288             );
 289
 290             probe_cx.assemble_inherent_candidates();
 291             match scope {
 292                 ProbeScope::TraitsInScope =>
 293                     probe_cx.assemble_extension_candidates_for_traits_in_scope(scope_expr_id)?,
 294                 ProbeScope::AllTraits =>
 295                     probe_cx.assemble_extension_candidates_for_all_traits()?,
 296             };
 297             op(probe_cx)
 298         })
 299     }
 300
 301     fn create_steps(&self,
 302                     span: Span,
 303                     scope_expr_id: ast::NodeId,
 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).include_raw_pointers();
 310         let mut reached_raw_pointer = false;
 311         let mut steps: Vec<_> = autoderef.by_ref()
 312             .map(|(ty, d)| {
 313                 let step = CandidateStep {
 314                     self_ty: ty,
 315                     autoderefs: d,
 316                     from_unsafe_deref: reached_raw_pointer,
 317                     unsize: false,
 318                 };
 319                 if let ty::RawPtr(_) = ty.sty {
 320                     // all the subsequent steps will be from_unsafe_deref
 321                     reached_raw_pointer = true;
 322                 }
 323                 step
 324             })
 325             .collect();
 326
 327         let final_ty = autoderef.maybe_ambiguous_final_ty();
 328         match final_ty.sty {
 329             ty::Infer(ty::TyVar(_)) => {
 330                 // Ended in an inference variable. If we are doing
 331                 // a real method lookup, this is a hard error because it's
 332                 // possible that there will be multiple applicable methods.
 333                 if !is_suggestion.0 {
 334                     if reached_raw_pointer
 335                     && !self.tcx.features().arbitrary_self_types {
 336                         // this case used to be allowed by the compiler,
 337                         // so we do a future-compat lint here for the 2015 edition
 338                         // (see https://github.com/rust-lang/rust/issues/46906)
 339                         if self.tcx.sess.rust_2018() {
 340                           span_err!(self.tcx.sess, span, E0699,
 341                                     "the type of this value must be known \
 342                                      to call a method on a raw pointer on it");
 343                         } else {
 344                             self.tcx.lint_node(
 345                                 lint::builtin::TYVAR_BEHIND_RAW_POINTER,
 346                                 scope_expr_id,
 347                                 span,
 348                                 "type annotations needed");
 349                         }
 350                     } else {
 351                         let t = self.structurally_resolved_type(span, final_ty);
 352                         assert_eq!(t, self.tcx.types.err);
 353                         return None
 354                     }
 355                 } else {
 356                     // If we're just looking for suggestions,
 357                     // though, ambiguity is no big thing, we can
 358                     // just ignore it.
 359                 }
 360             }
 361             ty::Array(elem_ty, _) => {
 362                 let dereferences = steps.len() - 1;
 363
 364                 steps.push(CandidateStep {
 365                     self_ty: self.tcx.mk_slice(elem_ty),
 366                     autoderefs: dereferences,
 367                     // this could be from an unsafe deref if we had
 368                     // a *mut/const [T; N]
 369                     from_unsafe_deref: reached_raw_pointer,
 370                     unsize: true,
 371                 });
 372             }
 373             ty::Error => return None,
 374             _ => (),
 375         }
 376
 377         debug!("create_steps: steps={:?}", steps);
 378
 379         Some(steps)
 380     }
 381 }
 382
 383 impl<'a, 'gcx, 'tcx> ProbeContext<'a, 'gcx, 'tcx> {
 384     fn new(fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
 385            span: Span,
 386            mode: Mode,
 387            method_name: Option<ast::Ident>,
 388            return_type: Option<Ty<'tcx>>,
 389            steps: Rc<Vec<CandidateStep<'tcx>>>,
 390            is_suggestion: IsSuggestion)
 391            -> ProbeContext<'a, 'gcx, 'tcx> {
 392         ProbeContext {
 393             fcx,
 394             span,
 395             mode,
 396             method_name,
 397             return_type,
 398             inherent_candidates: Vec::new(),
 399             extension_candidates: Vec::new(),
 400             impl_dups: FxHashSet::default(),
 401             steps: steps,
 402             static_candidates: Vec::new(),
 403             allow_similar_names: false,
 404             private_candidate: None,
 405             unsatisfied_predicates: Vec::new(),
 406             is_suggestion,
 407         }
 408     }
 409
 410     fn reset(&mut self) {
 411         self.inherent_candidates.clear();
 412         self.extension_candidates.clear();
 413         self.impl_dups.clear();
 414         self.static_candidates.clear();
 415         self.private_candidate = None;
 416     }
 417
 418     ///////////////////////////////////////////////////////////////////////////
 419     // CANDIDATE ASSEMBLY
 420
 421     fn push_candidate(&mut self,
 422                       candidate: Candidate<'tcx>,
 423                       is_inherent: bool)
 424     {
 425         let is_accessible = if let Some(name) = self.method_name {
 426             let item = candidate.item;
 427             let def_scope = self.tcx.adjust_ident(name, item.container.id(), self.body_id).1;
 428             item.vis.is_accessible_from(def_scope, self.tcx)
 429         } else {
 430             true
 431         };
 432         if is_accessible {
 433             if is_inherent {
 434                 self.inherent_candidates.push(candidate);
 435             } else {
 436                 self.extension_candidates.push(candidate);
 437             }
 438         } else if self.private_candidate.is_none() {
 439             self.private_candidate = Some(candidate.item.def());
 440         }
 441     }
 442
 443     fn assemble_inherent_candidates(&mut self) {
 444         let steps = self.steps.clone();
 445         for step in steps.iter() {
 446             self.assemble_probe(step.self_ty);
 447         }
 448     }
 449
 450     fn assemble_probe(&mut self, self_ty: Ty<'tcx>) {
 451         debug!("assemble_probe: self_ty={:?}", self_ty);
 452         let lang_items = self.tcx.lang_items();
 453
 454         match self_ty.sty {
 455             ty::Dynamic(ref data, ..) => {
 456                 let p = data.principal();
 457                 self.assemble_inherent_candidates_from_object(self_ty, p);
 458                 self.assemble_inherent_impl_candidates_for_type(p.def_id());
 459             }
 460             ty::Adt(def, _) => {
 461                 self.assemble_inherent_impl_candidates_for_type(def.did);
 462             }
 463             ty::Foreign(did) => {
 464                 self.assemble_inherent_impl_candidates_for_type(did);
 465             }
 466             ty::Param(p) => {
 467                 self.assemble_inherent_candidates_from_param(self_ty, p);
 468             }
 469             ty::Char => {
 470                 let lang_def_id = lang_items.char_impl();
 471                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 472             }
 473             ty::Str => {
 474                 let lang_def_id = lang_items.str_impl();
 475                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 476
 477                 let lang_def_id = lang_items.str_alloc_impl();
 478                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 479             }
 480             ty::Slice(_) => {
 481                 let lang_def_id = lang_items.slice_impl();
 482                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 483
 484                 let lang_def_id = lang_items.slice_u8_impl();
 485                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 486
 487                 let lang_def_id = lang_items.slice_alloc_impl();
 488                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 489
 490                 let lang_def_id = lang_items.slice_u8_alloc_impl();
 491                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 492             }
 493             ty::RawPtr(ty::TypeAndMut { ty: _, mutbl: hir::MutImmutable }) => {
 494                 let lang_def_id = lang_items.const_ptr_impl();
 495                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 496             }
 497             ty::RawPtr(ty::TypeAndMut { ty: _, mutbl: hir::MutMutable }) => {
 498                 let lang_def_id = lang_items.mut_ptr_impl();
 499                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 500             }
 501             ty::Int(ast::IntTy::I8) => {
 502                 let lang_def_id = lang_items.i8_impl();
 503                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 504             }
 505             ty::Int(ast::IntTy::I16) => {
 506                 let lang_def_id = lang_items.i16_impl();
 507                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 508             }
 509             ty::Int(ast::IntTy::I32) => {
 510                 let lang_def_id = lang_items.i32_impl();
 511                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 512             }
 513             ty::Int(ast::IntTy::I64) => {
 514                 let lang_def_id = lang_items.i64_impl();
 515                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 516             }
 517             ty::Int(ast::IntTy::I128) => {
 518                 let lang_def_id = lang_items.i128_impl();
 519                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 520             }
 521             ty::Int(ast::IntTy::Isize) => {
 522                 let lang_def_id = lang_items.isize_impl();
 523                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 524             }
 525             ty::Uint(ast::UintTy::U8) => {
 526                 let lang_def_id = lang_items.u8_impl();
 527                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 528             }
 529             ty::Uint(ast::UintTy::U16) => {
 530                 let lang_def_id = lang_items.u16_impl();
 531                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 532             }
 533             ty::Uint(ast::UintTy::U32) => {
 534                 let lang_def_id = lang_items.u32_impl();
 535                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 536             }
 537             ty::Uint(ast::UintTy::U64) => {
 538                 let lang_def_id = lang_items.u64_impl();
 539                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 540             }
 541             ty::Uint(ast::UintTy::U128) => {
 542                 let lang_def_id = lang_items.u128_impl();
 543                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 544             }
 545             ty::Uint(ast::UintTy::Usize) => {
 546                 let lang_def_id = lang_items.usize_impl();
 547                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 548             }
 549             ty::Float(ast::FloatTy::F32) => {
 550                 let lang_def_id = lang_items.f32_impl();
 551                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 552
 553                 let lang_def_id = lang_items.f32_runtime_impl();
 554                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 555             }
 556             ty::Float(ast::FloatTy::F64) => {
 557                 let lang_def_id = lang_items.f64_impl();
 558                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 559
 560                 let lang_def_id = lang_items.f64_runtime_impl();
 561                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 562             }
 563             _ => {}
 564         }
 565     }
 566
 567     fn assemble_inherent_impl_for_primitive(&mut self, lang_def_id: Option<DefId>) {
 568         if let Some(impl_def_id) = lang_def_id {
 569             self.assemble_inherent_impl_probe(impl_def_id);
 570         }
 571     }
 572
 573     fn assemble_inherent_impl_candidates_for_type(&mut self, def_id: DefId) {
 574         let impl_def_ids = self.tcx.at(self.span).inherent_impls(def_id);
 575         for &impl_def_id in impl_def_ids.iter() {
 576             self.assemble_inherent_impl_probe(impl_def_id);
 577         }
 578     }
 579
 580     fn assemble_inherent_impl_probe(&mut self, impl_def_id: DefId) {
 581         if !self.impl_dups.insert(impl_def_id) {
 582             return; // already visited
 583         }
 584
 585         debug!("assemble_inherent_impl_probe {:?}", impl_def_id);
 586
 587         for item in self.impl_or_trait_item(impl_def_id) {
 588             if !self.has_applicable_self(&item) {
 589                 // No receiver declared. Not a candidate.
 590                 self.record_static_candidate(ImplSource(impl_def_id));
 591                 continue
 592             }
 593
 594             let (impl_ty, impl_substs) = self.impl_ty_and_substs(impl_def_id);
 595             let impl_ty = impl_ty.subst(self.tcx, impl_substs);
 596
 597             // Determine the receiver type that the method itself expects.
 598             let xform_tys = self.xform_self_ty(&item, impl_ty, impl_substs);
 599
 600             // We can't use normalize_associated_types_in as it will pollute the
 601             // fcx's fulfillment context after this probe is over.
 602             let cause = traits::ObligationCause::misc(self.span, self.body_id);
 603             let selcx = &mut traits::SelectionContext::new(self.fcx);
 604             let traits::Normalized { value: (xform_self_ty, xform_ret_ty), obligations } =
 605                 traits::normalize(selcx, self.param_env, cause, &xform_tys);
 606             debug!("assemble_inherent_impl_probe: xform_self_ty = {:?}/{:?}",
 607                    xform_self_ty, xform_ret_ty);
 608
 609             self.push_candidate(Candidate {
 610                 xform_self_ty, xform_ret_ty, item,
 611                 kind: InherentImplCandidate(impl_substs, obligations),
 612                 import_id: None
 613             }, true);
 614         }
 615     }
 616
 617     fn assemble_inherent_candidates_from_object(&mut self,
 618                                                 self_ty: Ty<'tcx>,
 619                                                 principal: ty::PolyExistentialTraitRef<'tcx>) {
 620         debug!("assemble_inherent_candidates_from_object(self_ty={:?})",
 621                self_ty);
 622
 623         // It is illegal to invoke a method on a trait instance that
 624         // refers to the `Self` type. An error will be reported by
 625         // `enforce_object_limitations()` if the method refers to the
 626         // `Self` type anywhere other than the receiver. Here, we use
 627         // a substitution that replaces `Self` with the object type
 628         // itself. Hence, a `&self` method will wind up with an
 629         // argument type like `&Trait`.
 630         let trait_ref = principal.with_self_ty(self.tcx, self_ty);
 631         self.elaborate_bounds(iter::once(trait_ref), |this, new_trait_ref, item| {
 632             let new_trait_ref = this.erase_late_bound_regions(&new_trait_ref);
 633
 634             let (xform_self_ty, xform_ret_ty) =
 635                 this.xform_self_ty(&item, new_trait_ref.self_ty(), new_trait_ref.substs);
 636             this.push_candidate(Candidate {
 637                 xform_self_ty, xform_ret_ty, item,
 638                 kind: ObjectCandidate,
 639                 import_id: None
 640             }, true);
 641         });
 642     }
 643
 644     fn assemble_inherent_candidates_from_param(&mut self,
 645                                                _rcvr_ty: Ty<'tcx>,
 646                                                param_ty: ty::ParamTy) {
 647         // FIXME -- Do we want to commit to this behavior for param bounds?
 648
 649         let bounds = self.param_env
 650             .caller_bounds
 651             .iter()
 652             .filter_map(|predicate| {
 653                 match *predicate {
 654                     ty::Predicate::Trait(ref trait_predicate) => {
 655                         match trait_predicate.skip_binder().trait_ref.self_ty().sty {
 656                             ty::Param(ref p) if *p == param_ty => {
 657                                 Some(trait_predicate.to_poly_trait_ref())
 658                             }
 659                             _ => None,
 660                         }
 661                     }
 662                     ty::Predicate::Subtype(..) |
 663                     ty::Predicate::Projection(..) |
 664                     ty::Predicate::RegionOutlives(..) |
 665                     ty::Predicate::WellFormed(..) |
 666                     ty::Predicate::ObjectSafe(..) |
 667                     ty::Predicate::ClosureKind(..) |
 668                     ty::Predicate::TypeOutlives(..) |
 669                     ty::Predicate::ConstEvaluatable(..) => None,
 670                 }
 671             });
 672
 673         self.elaborate_bounds(bounds, |this, poly_trait_ref, item| {
 674             let trait_ref = this.erase_late_bound_regions(&poly_trait_ref);
 675
 676             let (xform_self_ty, xform_ret_ty) =
 677                 this.xform_self_ty(&item, trait_ref.self_ty(), trait_ref.substs);
 678
 679             // Because this trait derives from a where-clause, it
 680             // should not contain any inference variables or other
 681             // artifacts. This means it is safe to put into the
 682             // `WhereClauseCandidate` and (eventually) into the
 683             // `WhereClausePick`.
 684             assert!(!trait_ref.substs.needs_infer());
 685
 686             this.push_candidate(Candidate {
 687                 xform_self_ty, xform_ret_ty, item,
 688                 kind: WhereClauseCandidate(poly_trait_ref),
 689                 import_id: None
 690             }, true);
 691         });
 692     }
 693
 694     // Do a search through a list of bounds, using a callback to actually
 695     // create the candidates.
 696     fn elaborate_bounds<F>(&mut self,
 697                            bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
 698                            mut mk_cand: F)
 699         where F: for<'b> FnMut(&mut ProbeContext<'b, 'gcx, 'tcx>,
 700                                ty::PolyTraitRef<'tcx>,
 701                                ty::AssociatedItem)
 702     {
 703         let tcx = self.tcx;
 704         for bound_trait_ref in traits::transitive_bounds(tcx, bounds) {
 705             debug!("elaborate_bounds(bound_trait_ref={:?})", bound_trait_ref);
 706             for item in self.impl_or_trait_item(bound_trait_ref.def_id()) {
 707                 if !self.has_applicable_self(&item) {
 708                     self.record_static_candidate(TraitSource(bound_trait_ref.def_id()));
 709                 } else {
 710                     mk_cand(self, bound_trait_ref, item);
 711                 }
 712             }
 713         }
 714     }
 715
 716     fn assemble_extension_candidates_for_traits_in_scope(&mut self,
 717                                                          expr_id: ast::NodeId)
 718                                                          -> Result<(), MethodError<'tcx>> {
 719         if expr_id == ast::DUMMY_NODE_ID {
 720             return Ok(())
 721         }
 722         let mut duplicates = FxHashSet::default();
 723         let expr_hir_id = self.tcx.hir.node_to_hir_id(expr_id);
 724         let opt_applicable_traits = self.tcx.in_scope_traits(expr_hir_id);
 725         if let Some(applicable_traits) = opt_applicable_traits {
 726             for trait_candidate in applicable_traits.iter() {
 727                 let trait_did = trait_candidate.def_id;
 728                 if duplicates.insert(trait_did) {
 729                     let import_id = trait_candidate.import_id;
 730                     let result = self.assemble_extension_candidates_for_trait(import_id, trait_did);
 731                     result?;
 732                 }
 733             }
 734         }
 735         Ok(())
 736     }
 737
 738     fn assemble_extension_candidates_for_all_traits(&mut self) -> Result<(), MethodError<'tcx>> {
 739         let mut duplicates = FxHashSet::default();
 740         for trait_info in suggest::all_traits(self.tcx) {
 741             if duplicates.insert(trait_info.def_id) {
 742                 self.assemble_extension_candidates_for_trait(None, trait_info.def_id)?;
 743             }
 744         }
 745         Ok(())
 746     }
 747
 748     pub fn matches_return_type(&self,
 749                                method: &ty::AssociatedItem,
 750                                self_ty: Option<Ty<'tcx>>,
 751                                expected: Ty<'tcx>) -> bool {
 752         match method.def() {
 753             Def::Method(def_id) => {
 754                 let fty = self.tcx.fn_sig(def_id);
 755                 self.probe(|_| {
 756                     let substs = self.fresh_substs_for_item(self.span, method.def_id);
 757                     let fty = fty.subst(self.tcx, substs);
 758                     let (fty, _) = self.replace_bound_vars_with_fresh_vars(
 759                         self.span,
 760                         infer::FnCall,
 761                         &fty
 762                     );
 763
 764                     if let Some(self_ty) = self_ty {
 765                         if self.at(&ObligationCause::dummy(), self.param_env)
 766                                .sup(fty.inputs()[0], self_ty)
 767                                .is_err()
 768                         {
 769                             return false
 770                         }
 771                     }
 772                     self.can_sub(self.param_env, fty.output(), expected).is_ok()
 773                 })
 774             }
 775             _ => false,
 776         }
 777     }
 778
 779     fn assemble_extension_candidates_for_trait(&mut self,
 780                                                import_id: Option<ast::NodeId>,
 781                                                trait_def_id: DefId)
 782                                                -> Result<(), MethodError<'tcx>> {
 783         debug!("assemble_extension_candidates_for_trait(trait_def_id={:?})",
 784                trait_def_id);
 785         let trait_substs = self.fresh_item_substs(trait_def_id);
 786         let trait_ref = ty::TraitRef::new(trait_def_id, trait_substs);
 787
 788         for item in self.impl_or_trait_item(trait_def_id) {
 789             // Check whether `trait_def_id` defines a method with suitable name:
 790             if !self.has_applicable_self(&item) {
 791                 debug!("method has inapplicable self");
 792                 self.record_static_candidate(TraitSource(trait_def_id));
 793                 continue;
 794             }
 795
 796             let (xform_self_ty, xform_ret_ty) =
 797                 self.xform_self_ty(&item, trait_ref.self_ty(), trait_substs);
 798             self.push_candidate(Candidate {
 799                 xform_self_ty, xform_ret_ty, item, import_id,
 800                 kind: TraitCandidate(trait_ref),
 801             }, false);
 802         }
 803         Ok(())
 804     }
 805
 806     fn candidate_method_names(&self) -> Vec<ast::Ident> {
 807         let mut set = FxHashSet::default();
 808         let mut names: Vec<_> = self.inherent_candidates
 809             .iter()
 810             .chain(&self.extension_candidates)
 811             .filter(|candidate| {
 812                 if let Some(return_ty) = self.return_type {
 813                     self.matches_return_type(&candidate.item, None, return_ty)
 814                 } else {
 815                     true
 816                 }
 817             })
 818             .map(|candidate| candidate.item.ident)
 819             .filter(|&name| set.insert(name))
 820             .collect();
 821
 822         // sort them by the name so we have a stable result
 823         names.sort_by_cached_key(|n| n.as_str());
 824         names
 825     }
 826
 827     ///////////////////////////////////////////////////////////////////////////
 828     // THE ACTUAL SEARCH
 829
 830     fn pick(mut self) -> PickResult<'tcx> {
 831         assert!(self.method_name.is_some());
 832
 833         if let Some(r) = self.pick_core() {
 834             return r;
 835         }
 836
 837         let static_candidates = mem::replace(&mut self.static_candidates, vec![]);
 838         let private_candidate = self.private_candidate.take();
 839         let unsatisfied_predicates = mem::replace(&mut self.unsatisfied_predicates, vec![]);
 840
 841         // things failed, so lets look at all traits, for diagnostic purposes now:
 842         self.reset();
 843
 844         let span = self.span;
 845         let tcx = self.tcx;
 846
 847         self.assemble_extension_candidates_for_all_traits()?;
 848
 849         let out_of_scope_traits = match self.pick_core() {
 850             Some(Ok(p)) => vec![p.item.container.id()],
 851             //Some(Ok(p)) => p.iter().map(|p| p.item.container().id()).collect(),
 852             Some(Err(MethodError::Ambiguity(v))) => {
 853                 v.into_iter()
 854                     .map(|source| {
 855                         match source {
 856                             TraitSource(id) => id,
 857                             ImplSource(impl_id) => {
 858                                 match tcx.trait_id_of_impl(impl_id) {
 859                                     Some(id) => id,
 860                                     None => {
 861                                         span_bug!(span,
 862                                                   "found inherent method when looking at traits")
 863                                     }
 864                                 }
 865                             }
 866                         }
 867                     })
 868                     .collect()
 869             }
 870             Some(Err(MethodError::NoMatch(NoMatchData { out_of_scope_traits: others, .. }))) => {
 871                 assert!(others.is_empty());
 872                 vec![]
 873             }
 874             _ => vec![],
 875         };
 876
 877         if let Some(def) = private_candidate {
 878             return Err(MethodError::PrivateMatch(def, out_of_scope_traits));
 879         }
 880         let lev_candidate = self.probe_for_lev_candidate()?;
 881
 882         Err(MethodError::NoMatch(NoMatchData::new(static_candidates,
 883                                                   unsatisfied_predicates,
 884                                                   out_of_scope_traits,
 885                                                   lev_candidate,
 886                                                   self.mode)))
 887     }
 888
 889     fn pick_core(&mut self) -> Option<PickResult<'tcx>> {
 890         let steps = self.steps.clone();
 891
 892         // find the first step that works
 893         steps
 894             .iter()
 895             .filter(|step| {
 896                 debug!("pick_core: step={:?}", step);
 897                 // skip types that are from a type error or that would require dereferencing
 898                 // a raw pointer
 899                 !step.self_ty.references_error() && !step.from_unsafe_deref
 900             }).flat_map(|step| {
 901                 self.pick_by_value_method(step).or_else(|| {
 902                 self.pick_autorefd_method(step, hir::MutImmutable).or_else(|| {
 903                 self.pick_autorefd_method(step, hir::MutMutable)
 904             })})})
 905             .next()
 906     }
 907
 908     fn pick_by_value_method(&mut self, step: &CandidateStep<'tcx>) -> Option<PickResult<'tcx>> {
 909         //! For each type `T` in the step list, this attempts to find a
 910         //! method where the (transformed) self type is exactly `T`. We
 911         //! do however do one transformation on the adjustment: if we
 912         //! are passing a region pointer in, we will potentially
 913         //! *reborrow* it to a shorter lifetime. This allows us to
 914         //! transparently pass `&mut` pointers, in particular, without
 915         //! consuming them for their entire lifetime.
 916
 917         if step.unsize {
 918             return None;
 919         }
 920
 921         self.pick_method(step.self_ty).map(|r| {
 922             r.map(|mut pick| {
 923                 pick.autoderefs = step.autoderefs;
 924
 925                 // Insert a `&*` or `&mut *` if this is a reference type:
 926                 if let ty::Ref(_, _, mutbl) = step.self_ty.sty {
 927                     pick.autoderefs += 1;
 928                     pick.autoref = Some(mutbl);
 929                 }
 930
 931                 pick
 932             })
 933         })
 934     }
 935
 936     fn pick_autorefd_method(&mut self, step: &CandidateStep<'tcx>, mutbl: hir::Mutability)
 937                             -> Option<PickResult<'tcx>> {
 938         let tcx = self.tcx;
 939
 940         // In general, during probing we erase regions. See
 941         // `impl_self_ty()` for an explanation.
 942         let region = tcx.types.re_erased;
 943
 944         let autoref_ty = tcx.mk_ref(region,
 945                                     ty::TypeAndMut {
 946                                         ty: step.self_ty, mutbl
 947                                     });
 948         self.pick_method(autoref_ty).map(|r| {
 949             r.map(|mut pick| {
 950                 pick.autoderefs = step.autoderefs;
 951                 pick.autoref = Some(mutbl);
 952                 pick.unsize = if step.unsize {
 953                     Some(step.self_ty)
 954                 } else {
 955                     None
 956                 };
 957                 pick
 958             })
 959         })
 960     }
 961
 962     fn pick_method(&mut self, self_ty: Ty<'tcx>) -> Option<PickResult<'tcx>> {
 963         debug!("pick_method(self_ty={})", self.ty_to_string(self_ty));
 964
 965         let mut possibly_unsatisfied_predicates = Vec::new();
 966         let mut unstable_candidates = Vec::new();
 967
 968         for (kind, candidates) in &[
 969             ("inherent", &self.inherent_candidates),
 970             ("extension", &self.extension_candidates),
 971         ] {
 972             debug!("searching {} candidates", kind);
 973             let res = self.consider_candidates(
 974                 self_ty,
 975                 candidates.iter(),
 976                 &mut possibly_unsatisfied_predicates,
 977                 Some(&mut unstable_candidates),
 978             );
 979             if let Some(pick) = res {
 980                 if !self.is_suggestion.0 && !unstable_candidates.is_empty() {
 981                     if let Ok(p) = &pick {
 982                         // Emit a lint if there are unstable candidates alongside the stable ones.
 983                         //
 984                         // We suppress warning if we're picking the method only because it is a
 985                         // suggestion.
 986                         self.emit_unstable_name_collision_hint(p, &unstable_candidates);
 987                     }
 988                 }
 989                 return Some(pick);
 990             }
 991         }
 992
 993         debug!("searching unstable candidates");
 994         let res = self.consider_candidates(
 995             self_ty,
 996             unstable_candidates.into_iter().map(|(c, _)| c),
 997             &mut possibly_unsatisfied_predicates,
 998             None,
 999         );
1000         if res.is_none() {
1001             self.unsatisfied_predicates.extend(possibly_unsatisfied_predicates);
1002         }
1003         res
1004     }
1005
1006     fn consider_candidates<'b, ProbesIter>(
1007         &self,
1008         self_ty: Ty<'tcx>,
1009         probes: ProbesIter,
1010         possibly_unsatisfied_predicates: &mut Vec<TraitRef<'tcx>>,
1011         unstable_candidates: Option<&mut Vec<(&'b Candidate<'tcx>, Symbol)>>,
1012     ) -> Option<PickResult<'tcx>>
1013     where
1014         ProbesIter: Iterator<Item = &'b Candidate<'tcx>> + Clone,
1015     {
1016         let mut applicable_candidates: Vec<_> = probes.clone()
1017             .map(|probe| {
1018                 (probe, self.consider_probe(self_ty, probe, possibly_unsatisfied_predicates))
1019             })
1020             .filter(|&(_, status)| status != ProbeResult::NoMatch)
1021             .collect();
1022
1023         debug!("applicable_candidates: {:?}", applicable_candidates);
1024
1025         if applicable_candidates.len() > 1 {
1026             if let Some(pick) = self.collapse_candidates_to_trait_pick(&applicable_candidates[..]) {
1027                 return Some(Ok(pick));
1028             }
1029         }
1030
1031         if let Some(uc) = unstable_candidates {
1032             applicable_candidates.retain(|&(p, _)| {
1033                 if let stability::EvalResult::Deny { feature, .. } =
1034                     self.tcx.eval_stability(p.item.def_id, None, self.span)
1035                 {
1036                     uc.push((p, feature));
1037                     return false;
1038                 }
1039                 true
1040             });
1041         }
1042
1043         if applicable_candidates.len() > 1 {
1044             let sources = probes
1045                 .map(|p| self.candidate_source(p, self_ty))
1046                 .collect();
1047             return Some(Err(MethodError::Ambiguity(sources)));
1048         }
1049
1050         applicable_candidates.pop().map(|(probe, status)| {
1051             if status == ProbeResult::Match {
1052                 Ok(probe.to_unadjusted_pick())
1053             } else {
1054                 Err(MethodError::BadReturnType)
1055             }
1056         })
1057     }
1058
1059     fn emit_unstable_name_collision_hint(
1060         &self,
1061         stable_pick: &Pick,
1062         unstable_candidates: &[(&Candidate<'tcx>, Symbol)],
1063     ) {
1064         let mut diag = self.tcx.struct_span_lint_node(
1065             lint::builtin::UNSTABLE_NAME_COLLISIONS,
1066             self.fcx.body_id,
1067             self.span,
1068             "a method with this name may be added to the standard library in the future",
1069         );
1070
1071         // FIXME: This should be a `span_suggestion_with_applicability` instead of `help`
1072         // However `self.span` only
1073         // highlights the method name, so we can't use it. Also consider reusing the code from
1074         // `report_method_error()`.
1075         diag.help(&format!(
1076             "call with fully qualified syntax `{}(...)` to keep using the current method",
1077             self.tcx.item_path_str(stable_pick.item.def_id),
1078         ));
1079
1080         if nightly_options::is_nightly_build() {
1081             for (candidate, feature) in unstable_candidates {
1082                 diag.help(&format!(
1083                     "add #![feature({})] to the crate attributes to enable `{}`",
1084                     feature,
1085                     self.tcx.item_path_str(candidate.item.def_id),
1086                 ));
1087             }
1088         }
1089
1090         diag.emit();
1091     }
1092
1093     fn select_trait_candidate(&self, trait_ref: ty::TraitRef<'tcx>)
1094                               -> traits::SelectionResult<'tcx, traits::Selection<'tcx>>
1095     {
1096         let cause = traits::ObligationCause::misc(self.span, self.body_id);
1097         let predicate =
1098             trait_ref.to_poly_trait_ref().to_poly_trait_predicate();
1099         let obligation = traits::Obligation::new(cause, self.param_env, predicate);
1100         traits::SelectionContext::new(self).select(&obligation)
1101     }
1102
1103     fn candidate_source(&self, candidate: &Candidate<'tcx>, self_ty: Ty<'tcx>)
1104                         -> CandidateSource
1105     {
1106         match candidate.kind {
1107             InherentImplCandidate(..) => ImplSource(candidate.item.container.id()),
1108             ObjectCandidate |
1109             WhereClauseCandidate(_) => TraitSource(candidate.item.container.id()),
1110             TraitCandidate(trait_ref) => self.probe(|_| {
1111                 let _ = self.at(&ObligationCause::dummy(), self.param_env)
1112                     .sup(candidate.xform_self_ty, self_ty);
1113                 match self.select_trait_candidate(trait_ref) {
1114                     Ok(Some(traits::Vtable::VtableImpl(ref impl_data))) => {
1115                         // If only a single impl matches, make the error message point
1116                         // to that impl.
1117                         ImplSource(impl_data.impl_def_id)
1118                     }
1119                     _ => {
1120                         TraitSource(candidate.item.container.id())
1121                     }
1122                 }
1123             })
1124         }
1125     }
1126
1127     fn consider_probe(&self,
1128                       self_ty: Ty<'tcx>,
1129                       probe: &Candidate<'tcx>,
1130                       possibly_unsatisfied_predicates: &mut Vec<TraitRef<'tcx>>)
1131                       -> ProbeResult {
1132         debug!("consider_probe: self_ty={:?} probe={:?}", self_ty, probe);
1133
1134         self.probe(|_| {
1135             // First check that the self type can be related.
1136             let sub_obligations = match self.at(&ObligationCause::dummy(), self.param_env)
1137                                             .sup(probe.xform_self_ty, self_ty) {
1138                 Ok(InferOk { obligations, value: () }) => obligations,
1139                 Err(_) => {
1140                     debug!("--> cannot relate self-types");
1141                     return ProbeResult::NoMatch;
1142                 }
1143             };
1144
1145             let mut result = ProbeResult::Match;
1146             let selcx = &mut traits::SelectionContext::new(self);
1147             let cause = traits::ObligationCause::misc(self.span, self.body_id);
1148
1149             // If so, impls may carry other conditions (e.g., where
1150             // clauses) that must be considered. Make sure that those
1151             // match as well (or at least may match, sometimes we
1152             // don't have enough information to fully evaluate).
1153             let candidate_obligations : Vec<_> = match probe.kind {
1154                 InherentImplCandidate(ref substs, ref ref_obligations) => {
1155                     // Check whether the impl imposes obligations we have to worry about.
1156                     let impl_def_id = probe.item.container.id();
1157                     let impl_bounds = self.tcx.predicates_of(impl_def_id);
1158                     let impl_bounds = impl_bounds.instantiate(self.tcx, substs);
1159                     let traits::Normalized { value: impl_bounds, obligations: norm_obligations } =
1160                         traits::normalize(selcx, self.param_env, cause.clone(), &impl_bounds);
1161
1162                     // Convert the bounds into obligations.
1163                     let impl_obligations = traits::predicates_for_generics(
1164                         cause, self.param_env, &impl_bounds);
1165
1166                     debug!("impl_obligations={:?}", impl_obligations);
1167                     impl_obligations.into_iter()
1168                         .chain(norm_obligations.into_iter())
1169                         .chain(ref_obligations.iter().cloned())
1170                         .collect()
1171                 }
1172
1173                 ObjectCandidate |
1174                 WhereClauseCandidate(..) => {
1175                     // These have no additional conditions to check.
1176                     vec![]
1177                 }
1178
1179                 TraitCandidate(trait_ref) => {
1180                     let predicate = trait_ref.to_predicate();
1181                     let obligation =
1182                         traits::Obligation::new(cause, self.param_env, predicate);
1183                     if !self.predicate_may_hold(&obligation) {
1184                         if self.probe(|_| self.select_trait_candidate(trait_ref).is_err()) {
1185                             // This candidate's primary obligation doesn't even
1186                             // select - don't bother registering anything in
1187                             // `potentially_unsatisfied_predicates`.
1188                             return ProbeResult::NoMatch;
1189                         } else {
1190                             // Some nested subobligation of this predicate
1191                             // failed.
1192                             //
1193                             // FIXME: try to find the exact nested subobligation
1194                             // and point at it rather than reporting the entire
1195                             // trait-ref?
1196                             result = ProbeResult::NoMatch;
1197                             let trait_ref = self.resolve_type_vars_if_possible(&trait_ref);
1198                             possibly_unsatisfied_predicates.push(trait_ref);
1199                         }
1200                     }
1201                     vec![]
1202                 }
1203             };
1204
1205             debug!("consider_probe - candidate_obligations={:?} sub_obligations={:?}",
1206                    candidate_obligations, sub_obligations);
1207
1208             // Evaluate those obligations to see if they might possibly hold.
1209             for o in candidate_obligations.into_iter().chain(sub_obligations) {
1210                 let o = self.resolve_type_vars_if_possible(&o);
1211                 if !self.predicate_may_hold(&o) {
1212                     result = ProbeResult::NoMatch;
1213                     if let &ty::Predicate::Trait(ref pred) = &o.predicate {
1214                         possibly_unsatisfied_predicates.push(pred.skip_binder().trait_ref);
1215                     }
1216                 }
1217             }
1218
1219             if let ProbeResult::Match = result {
1220                 if let (Some(return_ty), Some(xform_ret_ty)) =
1221                     (self.return_type, probe.xform_ret_ty)
1222                 {
1223                     let xform_ret_ty = self.resolve_type_vars_if_possible(&xform_ret_ty);
1224                     debug!("comparing return_ty {:?} with xform ret ty {:?}",
1225                            return_ty,
1226                            probe.xform_ret_ty);
1227                     if self.at(&ObligationCause::dummy(), self.param_env)
1228                         .sup(return_ty, xform_ret_ty)
1229                         .is_err()
1230                     {
1231                         return ProbeResult::BadReturnType;
1232                     }
1233                 }
1234             }
1235
1236             result
1237         })
1238     }
1239
1240     /// Sometimes we get in a situation where we have multiple probes that are all impls of the
1241     /// same trait, but we don't know which impl to use. In this case, since in all cases the
1242     /// external interface of the method can be determined from the trait, it's ok not to decide.
1243     /// We can basically just collapse all of the probes for various impls into one where-clause
1244     /// probe. This will result in a pending obligation so when more type-info is available we can
1245     /// make the final decision.
1246     ///
1247     /// Example (`src/test/run-pass/method-two-trait-defer-resolution-1.rs`):
1248     ///
1249     /// ```
1250     /// trait Foo { ... }
1251     /// impl Foo for Vec<int> { ... }
1252     /// impl Foo for Vec<usize> { ... }
1253     /// ```
1254     ///
1255     /// Now imagine the receiver is `Vec<_>`. It doesn't really matter at this time which impl we
1256     /// use, so it's ok to just commit to "using the method from the trait Foo".
1257     fn collapse_candidates_to_trait_pick(&self, probes: &[(&Candidate<'tcx>, ProbeResult)])
1258                                          -> Option<Pick<'tcx>>
1259     {
1260         // Do all probes correspond to the same trait?
1261         let container = probes[0].0.item.container;
1262         if let ty::ImplContainer(_) = container {
1263             return None
1264         }
1265         if probes[1..].iter().any(|&(p, _)| p.item.container != container) {
1266             return None;
1267         }
1268
1269         // FIXME: check the return type here somehow.
1270         // If so, just use this trait and call it a day.
1271         Some(Pick {
1272             item: probes[0].0.item.clone(),
1273             kind: TraitPick,
1274             import_id: probes[0].0.import_id,
1275             autoderefs: 0,
1276             autoref: None,
1277             unsize: None,
1278         })
1279     }
1280
1281     /// Similarly to `probe_for_return_type`, this method attempts to find the best matching
1282     /// candidate method where the method name may have been misspelt. Similarly to other
1283     /// Levenshtein based suggestions, we provide at most one such suggestion.
1284     fn probe_for_lev_candidate(&mut self) -> Result<Option<ty::AssociatedItem>, MethodError<'tcx>> {
1285         debug!("Probing for method names similar to {:?}",
1286                self.method_name);
1287
1288         let steps = self.steps.clone();
1289         self.probe(|_| {
1290             let mut pcx = ProbeContext::new(self.fcx, self.span, self.mode, self.method_name,
1291                                             self.return_type, steps, IsSuggestion(true));
1292             pcx.allow_similar_names = true;
1293             pcx.assemble_inherent_candidates();
1294             pcx.assemble_extension_candidates_for_traits_in_scope(ast::DUMMY_NODE_ID)?;
1295
1296             let method_names = pcx.candidate_method_names();
1297             pcx.allow_similar_names = false;
1298             let applicable_close_candidates: Vec<ty::AssociatedItem> = method_names
1299                 .iter()
1300                 .filter_map(|&method_name| {
1301                     pcx.reset();
1302                     pcx.method_name = Some(method_name);
1303                     pcx.assemble_inherent_candidates();
1304                     pcx.assemble_extension_candidates_for_traits_in_scope(ast::DUMMY_NODE_ID)
1305                         .ok().map_or(None, |_| {
1306                             pcx.pick_core()
1307                                 .and_then(|pick| pick.ok())
1308                                 .and_then(|pick| Some(pick.item))
1309                         })
1310                 })
1311                .collect();
1312
1313             if applicable_close_candidates.is_empty() {
1314                 Ok(None)
1315             } else {
1316                 let best_name = {
1317                     let names = applicable_close_candidates.iter().map(|cand| &cand.ident.name);
1318                     find_best_match_for_name(names,
1319                                              &self.method_name.unwrap().as_str(),
1320                                              None)
1321                 }.unwrap();
1322                 Ok(applicable_close_candidates
1323                    .into_iter()
1324                    .find(|method| method.ident.name == best_name))
1325             }
1326         })
1327     }
1328
1329     ///////////////////////////////////////////////////////////////////////////
1330     // MISCELLANY
1331     fn has_applicable_self(&self, item: &ty::AssociatedItem) -> bool {
1332         // "Fast track" -- check for usage of sugar when in method call
1333         // mode.
1334         //
1335         // In Path mode (i.e., resolving a value like `T::next`), consider any
1336         // associated value (i.e., methods, constants) but not types.
1337         match self.mode {
1338             Mode::MethodCall => item.method_has_self_argument,
1339             Mode::Path => match item.kind {
1340                 ty::AssociatedKind::Existential |
1341                 ty::AssociatedKind::Type => false,
1342                 ty::AssociatedKind::Method | ty::AssociatedKind::Const => true
1343             },
1344         }
1345         // FIXME -- check for types that deref to `Self`,
1346         // like `Rc<Self>` and so on.
1347         //
1348         // Note also that the current code will break if this type
1349         // includes any of the type parameters defined on the method
1350         // -- but this could be overcome.
1351     }
1352
1353     fn record_static_candidate(&mut self, source: CandidateSource) {
1354         self.static_candidates.push(source);
1355     }
1356
1357     fn xform_self_ty(&self,
1358                      item: &ty::AssociatedItem,
1359                      impl_ty: Ty<'tcx>,
1360                      substs: &Substs<'tcx>)
1361                      -> (Ty<'tcx>, Option<Ty<'tcx>>) {
1362         if item.kind == ty::AssociatedKind::Method && self.mode == Mode::MethodCall {
1363             let sig = self.xform_method_sig(item.def_id, substs);
1364             (sig.inputs()[0], Some(sig.output()))
1365         } else {
1366             (impl_ty, None)
1367         }
1368     }
1369
1370     fn xform_method_sig(&self,
1371                         method: DefId,
1372                         substs: &Substs<'tcx>)
1373                         -> ty::FnSig<'tcx>
1374     {
1375         let fn_sig = self.tcx.fn_sig(method);
1376         debug!("xform_self_ty(fn_sig={:?}, substs={:?})",
1377                fn_sig,
1378                substs);
1379
1380         assert!(!substs.has_escaping_bound_vars());
1381
1382         // It is possible for type parameters or early-bound lifetimes
1383         // to appear in the signature of `self`. The substitutions we
1384         // are given do not include type/lifetime parameters for the
1385         // method yet. So create fresh variables here for those too,
1386         // if there are any.
1387         let generics = self.tcx.generics_of(method);
1388         assert_eq!(substs.len(), generics.parent_count as usize);
1389
1390         // Erase any late-bound regions from the method and substitute
1391         // in the values from the substitution.
1392         let xform_fn_sig = self.erase_late_bound_regions(&fn_sig);
1393
1394         if generics.params.is_empty() {
1395             xform_fn_sig.subst(self.tcx, substs)
1396         } else {
1397             let substs = Substs::for_item(self.tcx, method, |param, _| {
1398                 let i = param.index as usize;
1399                 if i < substs.len() {
1400                     substs[i]
1401                 } else {
1402                     match param.kind {
1403                         GenericParamDefKind::Lifetime => {
1404                             // In general, during probe we erase regions. See
1405                             // `impl_self_ty()` for an explanation.
1406                             self.tcx.types.re_erased.into()
1407                         }
1408                         GenericParamDefKind::Type {..} => self.var_for_def(self.span, param),
1409                     }
1410                 }
1411             });
1412             xform_fn_sig.subst(self.tcx, substs)
1413         }
1414     }
1415
1416     /// Get the type of an impl and generate substitutions with placeholders.
1417     fn impl_ty_and_substs(&self, impl_def_id: DefId) -> (Ty<'tcx>, &'tcx Substs<'tcx>) {
1418         (self.tcx.type_of(impl_def_id), self.fresh_item_substs(impl_def_id))
1419     }
1420
1421     fn fresh_item_substs(&self, def_id: DefId) -> &'tcx Substs<'tcx> {
1422         Substs::for_item(self.tcx, def_id, |param, _| {
1423             match param.kind {
1424                 GenericParamDefKind::Lifetime => self.tcx.types.re_erased.into(),
1425                 GenericParamDefKind::Type {..} => {
1426                     self.next_ty_var(TypeVariableOrigin::SubstitutionPlaceholder(
1427                         self.tcx.def_span(def_id))).into()
1428                 }
1429             }
1430         })
1431     }
1432
1433     /// Replace late-bound-regions bound by `value` with `'static` using
1434     /// `ty::erase_late_bound_regions`.
1435     ///
1436     /// This is only a reasonable thing to do during the *probe* phase, not the *confirm* phase, of
1437     /// method matching. It is reasonable during the probe phase because we don't consider region
1438     /// relationships at all. Therefore, we can just replace all the region variables with 'static
1439     /// rather than creating fresh region variables. This is nice for two reasons:
1440     ///
1441     /// 1. Because the numbers of the region variables would otherwise be fairly unique to this
1442     ///    particular method call, it winds up creating fewer types overall, which helps for memory
1443     ///    usage. (Admittedly, this is a rather small effect, though measurable.)
1444     ///
1445     /// 2. It makes it easier to deal with higher-ranked trait bounds, because we can replace any
1446     ///    late-bound regions with 'static. Otherwise, if we were going to replace late-bound
1447     ///    regions with actual region variables as is proper, we'd have to ensure that the same
1448     ///    region got replaced with the same variable, which requires a bit more coordination
1449     ///    and/or tracking the substitution and
1450     ///    so forth.
1451     fn erase_late_bound_regions<T>(&self, value: &ty::Binder<T>) -> T
1452         where T: TypeFoldable<'tcx>
1453     {
1454         self.tcx.erase_late_bound_regions(value)
1455     }
1456
1457     /// Find the method with the appropriate name (or return type, as the case may be). If
1458     /// `allow_similar_names` is set, find methods with close-matching names.
1459     fn impl_or_trait_item(&self, def_id: DefId) -> Vec<ty::AssociatedItem> {
1460         if let Some(name) = self.method_name {
1461             if self.allow_similar_names {
1462                 let max_dist = max(name.as_str().len(), 3) / 3;
1463                 self.tcx.associated_items(def_id)
1464                     .filter(|x| {
1465                         let dist = lev_distance(&*name.as_str(), &x.ident.as_str());
1466                         Namespace::from(x.kind) == Namespace::Value && dist > 0
1467                             && dist <= max_dist
1468                     })
1469                     .collect()
1470             } else {
1471                 self.fcx
1472                     .associated_item(def_id, name, Namespace::Value)
1473                     .map_or(Vec::new(), |x| vec![x])
1474             }
1475         } else {
1476             self.tcx.associated_items(def_id).collect()
1477         }
1478     }
1479 }
1480
1481 impl<'tcx> Candidate<'tcx> {
1482     fn to_unadjusted_pick(&self) -> Pick<'tcx> {
1483         Pick {
1484             item: self.item.clone(),
1485             kind: match self.kind {
1486                 InherentImplCandidate(..) => InherentImplPick,
1487                 ObjectCandidate => ObjectPick,
1488                 TraitCandidate(_) => TraitPick,
1489                 WhereClauseCandidate(ref trait_ref) => {
1490                     // Only trait derived from where-clauses should
1491                     // appear here, so they should not contain any
1492                     // inference variables or other artifacts. This
1493                     // means they are safe to put into the
1494                     // `WhereClausePick`.
1495                     assert!(
1496                         !trait_ref.skip_binder().substs.needs_infer()
1497                             && !trait_ref.skip_binder().substs.has_skol()
1498                     );
1499
1500                     WhereClausePick(trait_ref.clone())
1501                 }
1502             },
1503             import_id: self.import_id,
1504             autoderefs: 0,
1505             autoref: None,
1506             unsize: None,
1507         }
1508     }
1509 }