compiler/rustc_hir_typeck/src/method/probe.rs

   1 use super::suggest;
   2 use super::CandidateSource;
   3 use super::MethodError;
   4 use super::NoMatchData;
   5
   6 use crate::errors::MethodCallOnUnknownType;
   7 use crate::FnCtxt;
   8 use rustc_data_structures::fx::FxHashSet;
   9 use rustc_errors::Applicability;
  10 use rustc_hir as hir;
  11 use rustc_hir::def::DefKind;
  12 use rustc_infer::infer::canonical::OriginalQueryValues;
  13 use rustc_infer::infer::canonical::{Canonical, QueryResponse};
  14 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  15 use rustc_infer::infer::{self, InferOk, TyCtxtInferExt};
  16 use rustc_middle::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
  17 use rustc_middle::middle::stability;
  18 use rustc_middle::ty::fast_reject::{simplify_type, TreatParams};
  19 use rustc_middle::ty::AssocItem;
  20 use rustc_middle::ty::GenericParamDefKind;
  21 use rustc_middle::ty::ToPredicate;
  22 use rustc_middle::ty::{self, ParamEnvAnd, Ty, TyCtxt, TypeFoldable, TypeVisitable};
  23 use rustc_middle::ty::{InternalSubsts, SubstsRef};
  24 use rustc_session::lint;
  25 use rustc_span::def_id::DefId;
  26 use rustc_span::def_id::LocalDefId;
  27 use rustc_span::lev_distance::{
  28     find_best_match_for_name_with_substrings, lev_distance_with_substrings,
  29 };
  30 use rustc_span::symbol::sym;
  31 use rustc_span::{symbol::Ident, Span, Symbol, DUMMY_SP};
  32 use rustc_trait_selection::autoderef::{self, Autoderef};
  33 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  34 use rustc_trait_selection::traits::query::method_autoderef::MethodAutoderefBadTy;
  35 use rustc_trait_selection::traits::query::method_autoderef::{
  36     CandidateStep, MethodAutoderefStepsResult,
  37 };
  38 use rustc_trait_selection::traits::query::CanonicalTyGoal;
  39 use rustc_trait_selection::traits::NormalizeExt;
  40 use rustc_trait_selection::traits::{self, ObligationCause};
  41 use std::cmp::max;
  42 use std::iter;
  43 use std::mem;
  44 use std::ops::Deref;
  45
  46 use smallvec::{smallvec, SmallVec};
  47
  48 use self::CandidateKind::*;
  49 pub use self::PickKind::*;
  50
  51 /// Boolean flag used to indicate if this search is for a suggestion
  52 /// or not. If true, we can allow ambiguity and so forth.
  53 #[derive(Clone, Copy, Debug)]
  54 pub struct IsSuggestion(pub bool);
  55
  56 struct ProbeContext<'a, 'tcx> {
  57     fcx: &'a FnCtxt<'a, 'tcx>,
  58     span: Span,
  59     mode: Mode,
  60     method_name: Option<Ident>,
  61     return_type: Option<Ty<'tcx>>,
  62
  63     /// This is the OriginalQueryValues for the steps queries
  64     /// that are answered in steps.
  65     orig_steps_var_values: OriginalQueryValues<'tcx>,
  66     steps: &'tcx [CandidateStep<'tcx>],
  67
  68     inherent_candidates: Vec<Candidate<'tcx>>,
  69     extension_candidates: Vec<Candidate<'tcx>>,
  70     impl_dups: FxHashSet<DefId>,
  71
  72     /// Collects near misses when the candidate functions are missing a `self` keyword and is only
  73     /// used for error reporting
  74     static_candidates: Vec<CandidateSource>,
  75
  76     /// When probing for names, include names that are close to the
  77     /// requested name (by Levensthein distance)
  78     allow_similar_names: bool,
  79
  80     /// Some(candidate) if there is a private candidate
  81     private_candidate: Option<(DefKind, DefId)>,
  82
  83     /// Collects near misses when trait bounds for type parameters are unsatisfied and is only used
  84     /// for error reporting
  85     unsatisfied_predicates:
  86         Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>, Option<ObligationCause<'tcx>>)>,
  87
  88     scope_expr_id: hir::HirId,
  89 }
  90
  91 impl<'a, 'tcx> Deref for ProbeContext<'a, 'tcx> {
  92     type Target = FnCtxt<'a, 'tcx>;
  93     fn deref(&self) -> &Self::Target {
  94         self.fcx
  95     }
  96 }
  97
  98 #[derive(Debug, Clone)]
  99 struct Candidate<'tcx> {
 100     // Candidates are (I'm not quite sure, but they are mostly) basically
 101     // some metadata on top of a `ty::AssocItem` (without substs).
 102     //
 103     // However, method probing wants to be able to evaluate the predicates
 104     // for a function with the substs applied - for example, if a function
 105     // has `where Self: Sized`, we don't want to consider it unless `Self`
 106     // is actually `Sized`, and similarly, return-type suggestions want
 107     // to consider the "actual" return type.
 108     //
 109     // The way this is handled is through `xform_self_ty`. It contains
 110     // the receiver type of this candidate, but `xform_self_ty`,
 111     // `xform_ret_ty` and `kind` (which contains the predicates) have the
 112     // generic parameters of this candidate substituted with the *same set*
 113     // of inference variables, which acts as some weird sort of "query".
 114     //
 115     // When we check out a candidate, we require `xform_self_ty` to be
 116     // a subtype of the passed-in self-type, and this equates the type
 117     // variables in the rest of the fields.
 118     //
 119     // For example, if we have this candidate:
 120     // ```
 121     //    trait Foo {
 122     //        fn foo(&self) where Self: Sized;
 123     //    }
 124     // ```
 125     //
 126     // Then `xform_self_ty` will be `&'erased ?X` and `kind` will contain
 127     // the predicate `?X: Sized`, so if we are evaluating `Foo` for a
 128     // the receiver `&T`, we'll do the subtyping which will make `?X`
 129     // get the right value, then when we evaluate the predicate we'll check
 130     // if `T: Sized`.
 131     xform_self_ty: Ty<'tcx>,
 132     xform_ret_ty: Option<Ty<'tcx>>,
 133     item: ty::AssocItem,
 134     kind: CandidateKind<'tcx>,
 135     import_ids: SmallVec<[LocalDefId; 1]>,
 136 }
 137
 138 #[derive(Debug, Clone)]
 139 enum CandidateKind<'tcx> {
 140     InherentImplCandidate(
 141         SubstsRef<'tcx>,
 142         // Normalize obligations
 143         Vec<traits::PredicateObligation<'tcx>>,
 144     ),
 145     ObjectCandidate,
 146     TraitCandidate(ty::TraitRef<'tcx>),
 147     WhereClauseCandidate(
 148         // Trait
 149         ty::PolyTraitRef<'tcx>,
 150     ),
 151 }
 152
 153 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 154 enum ProbeResult {
 155     NoMatch,
 156     BadReturnType,
 157     Match,
 158 }
 159
 160 /// When adjusting a receiver we often want to do one of
 161 ///
 162 /// - Add a `&` (or `&mut`), converting the receiver from `T` to `&T` (or `&mut T`)
 163 /// - If the receiver has type `*mut T`, convert it to `*const T`
 164 ///
 165 /// This type tells us which one to do.
 166 ///
 167 /// Note that in principle we could do both at the same time. For example, when the receiver has
 168 /// type `T`, we could autoref it to `&T`, then convert to `*const T`. Or, when it has type `*mut
 169 /// T`, we could convert it to `*const T`, then autoref to `&*const T`. However, currently we do
 170 /// (at most) one of these. Either the receiver has type `T` and we convert it to `&T` (or with
 171 /// `mut`), or it has type `*mut T` and we convert it to `*const T`.
 172 #[derive(Debug, PartialEq, Copy, Clone)]
 173 pub enum AutorefOrPtrAdjustment {
 174     /// Receiver has type `T`, add `&` or `&mut` (it `T` is `mut`), and maybe also "unsize" it.
 175     /// Unsizing is used to convert a `[T; N]` to `[T]`, which only makes sense when autorefing.
 176     Autoref {
 177         mutbl: hir::Mutability,
 178
 179         /// Indicates that the source expression should be "unsized" to a target type.
 180         /// This is special-cased for just arrays unsizing to slices.
 181         unsize: bool,
 182     },
 183     /// Receiver has type `*mut T`, convert to `*const T`
 184     ToConstPtr,
 185 }
 186
 187 impl AutorefOrPtrAdjustment {
 188     fn get_unsize(&self) -> bool {
 189         match self {
 190             AutorefOrPtrAdjustment::Autoref { mutbl: _, unsize } => *unsize,
 191             AutorefOrPtrAdjustment::ToConstPtr => false,
 192         }
 193     }
 194 }
 195
 196 #[derive(Debug, Clone)]
 197 pub struct Pick<'tcx> {
 198     pub item: ty::AssocItem,
 199     pub kind: PickKind<'tcx>,
 200     pub import_ids: SmallVec<[LocalDefId; 1]>,
 201
 202     /// Indicates that the source expression should be autoderef'd N times
 203     /// ```ignore (not-rust)
 204     /// A = expr | *expr | **expr | ...
 205     /// ```
 206     pub autoderefs: usize,
 207
 208     /// Indicates that we want to add an autoref (and maybe also unsize it), or if the receiver is
 209     /// `*mut T`, convert it to `*const T`.
 210     pub autoref_or_ptr_adjustment: Option<AutorefOrPtrAdjustment>,
 211     pub self_ty: Ty<'tcx>,
 212
 213     /// Unstable candidates alongside the stable ones.
 214     unstable_candidates: Vec<(Candidate<'tcx>, Symbol)>,
 215 }
 216
 217 #[derive(Clone, Debug, PartialEq, Eq)]
 218 pub enum PickKind<'tcx> {
 219     InherentImplPick,
 220     ObjectPick,
 221     TraitPick,
 222     WhereClausePick(
 223         // Trait
 224         ty::PolyTraitRef<'tcx>,
 225     ),
 226 }
 227
 228 pub type PickResult<'tcx> = Result<Pick<'tcx>, MethodError<'tcx>>;
 229
 230 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 231 pub enum Mode {
 232     // An expression of the form `receiver.method_name(...)`.
 233     // Autoderefs are performed on `receiver`, lookup is done based on the
 234     // `self` argument  of the method, and static methods aren't considered.
 235     MethodCall,
 236     // An expression of the form `Type::item` or `<T>::item`.
 237     // No autoderefs are performed, lookup is done based on the type each
 238     // implementation is for, and static methods are included.
 239     Path,
 240 }
 241
 242 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 243 pub enum ProbeScope {
 244     // Assemble candidates coming only from traits in scope.
 245     TraitsInScope,
 246
 247     // Assemble candidates coming from all traits.
 248     AllTraits,
 249 }
 250
 251 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 252     /// This is used to offer suggestions to users. It returns methods
 253     /// that could have been called which have the desired return
 254     /// type. Some effort is made to rule out methods that, if called,
 255     /// would result in an error (basically, the same criteria we
 256     /// would use to decide if a method is a plausible fit for
 257     /// ambiguity purposes).
 258     #[instrument(level = "debug", skip(self, candidate_filter))]
 259     pub fn probe_for_return_type(
 260         &self,
 261         span: Span,
 262         mode: Mode,
 263         return_type: Ty<'tcx>,
 264         self_ty: Ty<'tcx>,
 265         scope_expr_id: hir::HirId,
 266         candidate_filter: impl Fn(&ty::AssocItem) -> bool,
 267     ) -> Vec<ty::AssocItem> {
 268         let method_names = self
 269             .probe_op(
 270                 span,
 271                 mode,
 272                 None,
 273                 Some(return_type),
 274                 IsSuggestion(true),
 275                 self_ty,
 276                 scope_expr_id,
 277                 ProbeScope::AllTraits,
 278                 |probe_cx| Ok(probe_cx.candidate_method_names(candidate_filter)),
 279             )
 280             .unwrap_or_default();
 281         method_names
 282             .iter()
 283             .flat_map(|&method_name| {
 284                 self.probe_op(
 285                     span,
 286                     mode,
 287                     Some(method_name),
 288                     Some(return_type),
 289                     IsSuggestion(true),
 290                     self_ty,
 291                     scope_expr_id,
 292                     ProbeScope::AllTraits,
 293                     |probe_cx| probe_cx.pick(),
 294                 )
 295                 .ok()
 296                 .map(|pick| pick.item)
 297             })
 298             .collect()
 299     }
 300
 301     #[instrument(level = "debug", skip(self))]
 302     pub fn probe_for_name(
 303         &self,
 304         mode: Mode,
 305         item_name: Ident,
 306         is_suggestion: IsSuggestion,
 307         self_ty: Ty<'tcx>,
 308         scope_expr_id: hir::HirId,
 309         scope: ProbeScope,
 310     ) -> PickResult<'tcx> {
 311         self.probe_op(
 312             item_name.span,
 313             mode,
 314             Some(item_name),
 315             None,
 316             is_suggestion,
 317             self_ty,
 318             scope_expr_id,
 319             scope,
 320             |probe_cx| probe_cx.pick(),
 321         )
 322     }
 323
 324     fn probe_op<OP, R>(
 325         &'a self,
 326         span: Span,
 327         mode: Mode,
 328         method_name: Option<Ident>,
 329         return_type: Option<Ty<'tcx>>,
 330         is_suggestion: IsSuggestion,
 331         self_ty: Ty<'tcx>,
 332         scope_expr_id: hir::HirId,
 333         scope: ProbeScope,
 334         op: OP,
 335     ) -> Result<R, MethodError<'tcx>>
 336     where
 337         OP: FnOnce(ProbeContext<'a, 'tcx>) -> Result<R, MethodError<'tcx>>,
 338     {
 339         let mut orig_values = OriginalQueryValues::default();
 340         let param_env_and_self_ty = self.canonicalize_query(
 341             ParamEnvAnd { param_env: self.param_env, value: self_ty },
 342             &mut orig_values,
 343         );
 344
 345         let steps = match mode {
 346             Mode::MethodCall => self.tcx.method_autoderef_steps(param_env_and_self_ty),
 347             Mode::Path => self.probe(|_| {
 348                 // Mode::Path - the deref steps is "trivial". This turns
 349                 // our CanonicalQuery into a "trivial" QueryResponse. This
 350                 // is a bit inefficient, but I don't think that writing
 351                 // special handling for this "trivial case" is a good idea.
 352
 353                 let infcx = &self.infcx;
 354                 let (ParamEnvAnd { param_env: _, value: self_ty }, canonical_inference_vars) =
 355                     infcx.instantiate_canonical_with_fresh_inference_vars(
 356                         span,
 357                         &param_env_and_self_ty,
 358                     );
 359                 debug!(
 360                     "probe_op: Mode::Path, param_env_and_self_ty={:?} self_ty={:?}",
 361                     param_env_and_self_ty, self_ty
 362                 );
 363                 MethodAutoderefStepsResult {
 364                     steps: infcx.tcx.arena.alloc_from_iter([CandidateStep {
 365                         self_ty: self.make_query_response_ignoring_pending_obligations(
 366                             canonical_inference_vars,
 367                             self_ty,
 368                         ),
 369                         autoderefs: 0,
 370                         from_unsafe_deref: false,
 371                         unsize: false,
 372                     }]),
 373                     opt_bad_ty: None,
 374                     reached_recursion_limit: false,
 375                 }
 376             }),
 377         };
 378
 379         // If our autoderef loop had reached the recursion limit,
 380         // report an overflow error, but continue going on with
 381         // the truncated autoderef list.
 382         if steps.reached_recursion_limit {
 383             self.probe(|_| {
 384                 let ty = &steps
 385                     .steps
 386                     .last()
 387                     .unwrap_or_else(|| span_bug!(span, "reached the recursion limit in 0 steps?"))
 388                     .self_ty;
 389                 let ty = self
 390                     .probe_instantiate_query_response(span, &orig_values, ty)
 391                     .unwrap_or_else(|_| span_bug!(span, "instantiating {:?} failed?", ty));
 392                 autoderef::report_autoderef_recursion_limit_error(self.tcx, span, ty.value);
 393             });
 394         }
 395
 396         // If we encountered an `_` type or an error type during autoderef, this is
 397         // ambiguous.
 398         if let Some(bad_ty) = &steps.opt_bad_ty {
 399             if is_suggestion.0 {
 400                 // Ambiguity was encountered during a suggestion. Just keep going.
 401                 debug!("ProbeContext: encountered ambiguity in suggestion");
 402             } else if bad_ty.reached_raw_pointer && !self.tcx.features().arbitrary_self_types {
 403                 // this case used to be allowed by the compiler,
 404                 // so we do a future-compat lint here for the 2015 edition
 405                 // (see https://github.com/rust-lang/rust/issues/46906)
 406                 if self.tcx.sess.rust_2018() {
 407                     self.tcx.sess.emit_err(MethodCallOnUnknownType { span });
 408                 } else {
 409                     self.tcx.struct_span_lint_hir(
 410                         lint::builtin::TYVAR_BEHIND_RAW_POINTER,
 411                         scope_expr_id,
 412                         span,
 413                         "type annotations needed",
 414                         |lint| lint,
 415                     );
 416                 }
 417             } else {
 418                 // Encountered a real ambiguity, so abort the lookup. If `ty` is not
 419                 // an `Err`, report the right "type annotations needed" error pointing
 420                 // to it.
 421                 let ty = &bad_ty.ty;
 422                 let ty = self
 423                     .probe_instantiate_query_response(span, &orig_values, ty)
 424                     .unwrap_or_else(|_| span_bug!(span, "instantiating {:?} failed?", ty));
 425                 let ty = self.structurally_resolved_type(span, ty.value);
 426                 assert!(matches!(ty.kind(), ty::Error(_)));
 427                 return Err(MethodError::NoMatch(NoMatchData {
 428                     static_candidates: Vec::new(),
 429                     unsatisfied_predicates: Vec::new(),
 430                     out_of_scope_traits: Vec::new(),
 431                     lev_candidate: None,
 432                     mode,
 433                 }));
 434             }
 435         }
 436
 437         debug!("ProbeContext: steps for self_ty={:?} are {:?}", self_ty, steps);
 438
 439         // this creates one big transaction so that all type variables etc
 440         // that we create during the probe process are removed later
 441         self.probe(|_| {
 442             let mut probe_cx = ProbeContext::new(
 443                 self,
 444                 span,
 445                 mode,
 446                 method_name,
 447                 return_type,
 448                 orig_values,
 449                 steps.steps,
 450                 scope_expr_id,
 451             );
 452
 453             probe_cx.assemble_inherent_candidates();
 454             match scope {
 455                 ProbeScope::TraitsInScope => {
 456                     probe_cx.assemble_extension_candidates_for_traits_in_scope(scope_expr_id)
 457                 }
 458                 ProbeScope::AllTraits => probe_cx.assemble_extension_candidates_for_all_traits(),
 459             };
 460             op(probe_cx)
 461         })
 462     }
 463 }
 464
 465 pub fn provide(providers: &mut ty::query::Providers) {
 466     providers.method_autoderef_steps = method_autoderef_steps;
 467 }
 468
 469 fn method_autoderef_steps<'tcx>(
 470     tcx: TyCtxt<'tcx>,
 471     goal: CanonicalTyGoal<'tcx>,
 472 ) -> MethodAutoderefStepsResult<'tcx> {
 473     debug!("method_autoderef_steps({:?})", goal);
 474
 475     let (ref infcx, goal, inference_vars) = tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &goal);
 476     let ParamEnvAnd { param_env, value: self_ty } = goal;
 477
 478     let mut autoderef = Autoderef::new(infcx, param_env, hir::CRATE_HIR_ID, DUMMY_SP, self_ty)
 479         .include_raw_pointers()
 480         .silence_errors();
 481     let mut reached_raw_pointer = false;
 482     let mut steps: Vec<_> = autoderef
 483         .by_ref()
 484         .map(|(ty, d)| {
 485             let step = CandidateStep {
 486                 self_ty: infcx
 487                     .make_query_response_ignoring_pending_obligations(inference_vars.clone(), ty),
 488                 autoderefs: d,
 489                 from_unsafe_deref: reached_raw_pointer,
 490                 unsize: false,
 491             };
 492             if let ty::RawPtr(_) = ty.kind() {
 493                 // all the subsequent steps will be from_unsafe_deref
 494                 reached_raw_pointer = true;
 495             }
 496             step
 497         })
 498         .collect();
 499
 500     let final_ty = autoderef.final_ty(true);
 501     let opt_bad_ty = match final_ty.kind() {
 502         ty::Infer(ty::TyVar(_)) | ty::Error(_) => Some(MethodAutoderefBadTy {
 503             reached_raw_pointer,
 504             ty: infcx.make_query_response_ignoring_pending_obligations(inference_vars, final_ty),
 505         }),
 506         ty::Array(elem_ty, _) => {
 507             let dereferences = steps.len() - 1;
 508
 509             steps.push(CandidateStep {
 510                 self_ty: infcx.make_query_response_ignoring_pending_obligations(
 511                     inference_vars,
 512                     infcx.tcx.mk_slice(*elem_ty),
 513                 ),
 514                 autoderefs: dereferences,
 515                 // this could be from an unsafe deref if we had
 516                 // a *mut/const [T; N]
 517                 from_unsafe_deref: reached_raw_pointer,
 518                 unsize: true,
 519             });
 520
 521             None
 522         }
 523         _ => None,
 524     };
 525
 526     debug!("method_autoderef_steps: steps={:?} opt_bad_ty={:?}", steps, opt_bad_ty);
 527
 528     MethodAutoderefStepsResult {
 529         steps: tcx.arena.alloc_from_iter(steps),
 530         opt_bad_ty: opt_bad_ty.map(|ty| &*tcx.arena.alloc(ty)),
 531         reached_recursion_limit: autoderef.reached_recursion_limit(),
 532     }
 533 }
 534
 535 impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 536     fn new(
 537         fcx: &'a FnCtxt<'a, 'tcx>,
 538         span: Span,
 539         mode: Mode,
 540         method_name: Option<Ident>,
 541         return_type: Option<Ty<'tcx>>,
 542         orig_steps_var_values: OriginalQueryValues<'tcx>,
 543         steps: &'tcx [CandidateStep<'tcx>],
 544         scope_expr_id: hir::HirId,
 545     ) -> ProbeContext<'a, 'tcx> {
 546         ProbeContext {
 547             fcx,
 548             span,
 549             mode,
 550             method_name,
 551             return_type,
 552             inherent_candidates: Vec::new(),
 553             extension_candidates: Vec::new(),
 554             impl_dups: FxHashSet::default(),
 555             orig_steps_var_values,
 556             steps,
 557             static_candidates: Vec::new(),
 558             allow_similar_names: false,
 559             private_candidate: None,
 560             unsatisfied_predicates: Vec::new(),
 561             scope_expr_id,
 562         }
 563     }
 564
 565     fn reset(&mut self) {
 566         self.inherent_candidates.clear();
 567         self.extension_candidates.clear();
 568         self.impl_dups.clear();
 569         self.static_candidates.clear();
 570         self.private_candidate = None;
 571     }
 572
 573     ///////////////////////////////////////////////////////////////////////////
 574     // CANDIDATE ASSEMBLY
 575
 576     fn push_candidate(&mut self, candidate: Candidate<'tcx>, is_inherent: bool) {
 577         let is_accessible = if let Some(name) = self.method_name {
 578             let item = candidate.item;
 579             let def_scope = self
 580                 .tcx
 581                 .adjust_ident_and_get_scope(name, item.container_id(self.tcx), self.body_id)
 582                 .1;
 583             item.visibility(self.tcx).is_accessible_from(def_scope, self.tcx)
 584         } else {
 585             true
 586         };
 587         if is_accessible {
 588             if is_inherent {
 589                 self.inherent_candidates.push(candidate);
 590             } else {
 591                 self.extension_candidates.push(candidate);
 592             }
 593         } else if self.private_candidate.is_none() {
 594             self.private_candidate =
 595                 Some((candidate.item.kind.as_def_kind(), candidate.item.def_id));
 596         }
 597     }
 598
 599     fn assemble_inherent_candidates(&mut self) {
 600         for step in self.steps.iter() {
 601             self.assemble_probe(&step.self_ty);
 602         }
 603     }
 604
 605     fn assemble_probe(&mut self, self_ty: &Canonical<'tcx, QueryResponse<'tcx, Ty<'tcx>>>) {
 606         debug!("assemble_probe: self_ty={:?}", self_ty);
 607         let raw_self_ty = self_ty.value.value;
 608         match *raw_self_ty.kind() {
 609             ty::Dynamic(data, ..) if let Some(p) = data.principal() => {
 610                 // Subtle: we can't use `instantiate_query_response` here: using it will
 611                 // commit to all of the type equalities assumed by inference going through
 612                 // autoderef (see the `method-probe-no-guessing` test).
 613                 //
 614                 // However, in this code, it is OK if we end up with an object type that is
 615                 // "more general" than the object type that we are evaluating. For *every*
 616                 // object type `MY_OBJECT`, a function call that goes through a trait-ref
 617                 // of the form `<MY_OBJECT as SuperTraitOf(MY_OBJECT)>::func` is a valid
 618                 // `ObjectCandidate`, and it should be discoverable "exactly" through one
 619                 // of the iterations in the autoderef loop, so there is no problem with it
 620                 // being discoverable in another one of these iterations.
 621                 //
 622                 // Using `instantiate_canonical_with_fresh_inference_vars` on our
 623                 // `Canonical<QueryResponse<Ty<'tcx>>>` and then *throwing away* the
 624                 // `CanonicalVarValues` will exactly give us such a generalization - it
 625                 // will still match the original object type, but it won't pollute our
 626                 // type variables in any form, so just do that!
 627                 let (QueryResponse { value: generalized_self_ty, .. }, _ignored_var_values) =
 628                     self.fcx
 629                         .instantiate_canonical_with_fresh_inference_vars(self.span, self_ty);
 630
 631                 self.assemble_inherent_candidates_from_object(generalized_self_ty);
 632                 self.assemble_inherent_impl_candidates_for_type(p.def_id());
 633                 if self.tcx.has_attr(p.def_id(), sym::rustc_has_incoherent_inherent_impls) {
 634                     self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty);
 635                 }
 636             }
 637             ty::Adt(def, _) => {
 638                 let def_id = def.did();
 639                 self.assemble_inherent_impl_candidates_for_type(def_id);
 640                 if self.tcx.has_attr(def_id, sym::rustc_has_incoherent_inherent_impls) {
 641                     self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty);
 642                 }
 643             }
 644             ty::Foreign(did) => {
 645                 self.assemble_inherent_impl_candidates_for_type(did);
 646                 if self.tcx.has_attr(did, sym::rustc_has_incoherent_inherent_impls) {
 647                     self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty);
 648                 }
 649             }
 650             ty::Param(p) => {
 651                 self.assemble_inherent_candidates_from_param(p);
 652             }
 653             ty::Bool
 654             | ty::Char
 655             | ty::Int(_)
 656             | ty::Uint(_)
 657             | ty::Float(_)
 658             | ty::Str
 659             | ty::Array(..)
 660             | ty::Slice(_)
 661             | ty::RawPtr(_)
 662             | ty::Ref(..)
 663             | ty::Never
 664             | ty::Tuple(..) => self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty),
 665             _ => {}
 666         }
 667     }
 668
 669     fn assemble_inherent_candidates_for_incoherent_ty(&mut self, self_ty: Ty<'tcx>) {
 670         let Some(simp) = simplify_type(self.tcx, self_ty, TreatParams::AsInfer) else {
 671             bug!("unexpected incoherent type: {:?}", self_ty)
 672         };
 673         for &impl_def_id in self.tcx.incoherent_impls(simp) {
 674             self.assemble_inherent_impl_probe(impl_def_id);
 675         }
 676     }
 677
 678     fn assemble_inherent_impl_candidates_for_type(&mut self, def_id: DefId) {
 679         let impl_def_ids = self.tcx.at(self.span).inherent_impls(def_id);
 680         for &impl_def_id in impl_def_ids.iter() {
 681             self.assemble_inherent_impl_probe(impl_def_id);
 682         }
 683     }
 684
 685     fn assemble_inherent_impl_probe(&mut self, impl_def_id: DefId) {
 686         if !self.impl_dups.insert(impl_def_id) {
 687             return; // already visited
 688         }
 689
 690         debug!("assemble_inherent_impl_probe {:?}", impl_def_id);
 691
 692         for item in self.impl_or_trait_item(impl_def_id) {
 693             if !self.has_applicable_self(&item) {
 694                 // No receiver declared. Not a candidate.
 695                 self.record_static_candidate(CandidateSource::Impl(impl_def_id));
 696                 continue;
 697             }
 698
 699             let (impl_ty, impl_substs) = self.impl_ty_and_substs(impl_def_id);
 700             let impl_ty = impl_ty.subst(self.tcx, impl_substs);
 701
 702             debug!("impl_ty: {:?}", impl_ty);
 703
 704             // Determine the receiver type that the method itself expects.
 705             let (xform_self_ty, xform_ret_ty) = self.xform_self_ty(&item, impl_ty, impl_substs);
 706             debug!("xform_self_ty: {:?}, xform_ret_ty: {:?}", xform_self_ty, xform_ret_ty);
 707
 708             // We can't use normalize_associated_types_in as it will pollute the
 709             // fcx's fulfillment context after this probe is over.
 710             // Note: we only normalize `xform_self_ty` here since the normalization
 711             // of the return type can lead to inference results that prohibit
 712             // valid candidates from being found, see issue #85671
 713             // FIXME Postponing the normalization of the return type likely only hides a deeper bug,
 714             // which might be caused by the `param_env` itself. The clauses of the `param_env`
 715             // maybe shouldn't include `Param`s, but rather fresh variables or be canonicalized,
 716             // see issue #89650
 717             let cause = traits::ObligationCause::misc(self.span, self.body_id);
 718             let InferOk { value: xform_self_ty, obligations } =
 719                 self.fcx.at(&cause, self.param_env).normalize(xform_self_ty);
 720
 721             debug!(
 722                 "assemble_inherent_impl_probe after normalization: xform_self_ty = {:?}/{:?}",
 723                 xform_self_ty, xform_ret_ty
 724             );
 725
 726             self.push_candidate(
 727                 Candidate {
 728                     xform_self_ty,
 729                     xform_ret_ty,
 730                     item,
 731                     kind: InherentImplCandidate(impl_substs, obligations),
 732                     import_ids: smallvec![],
 733                 },
 734                 true,
 735             );
 736         }
 737     }
 738
 739     fn assemble_inherent_candidates_from_object(&mut self, self_ty: Ty<'tcx>) {
 740         debug!("assemble_inherent_candidates_from_object(self_ty={:?})", self_ty);
 741
 742         let principal = match self_ty.kind() {
 743             ty::Dynamic(ref data, ..) => Some(data),
 744             _ => None,
 745         }
 746         .and_then(|data| data.principal())
 747         .unwrap_or_else(|| {
 748             span_bug!(
 749                 self.span,
 750                 "non-object {:?} in assemble_inherent_candidates_from_object",
 751                 self_ty
 752             )
 753         });
 754
 755         // It is illegal to invoke a method on a trait instance that refers to
 756         // the `Self` type. An [`ObjectSafetyViolation::SupertraitSelf`] error
 757         // will be reported by `object_safety.rs` if the method refers to the
 758         // `Self` type anywhere other than the receiver. Here, we use a
 759         // substitution that replaces `Self` with the object type itself. Hence,
 760         // a `&self` method will wind up with an argument type like `&dyn Trait`.
 761         let trait_ref = principal.with_self_ty(self.tcx, self_ty);
 762         self.elaborate_bounds(iter::once(trait_ref), |this, new_trait_ref, item| {
 763             let new_trait_ref = this.erase_late_bound_regions(new_trait_ref);
 764
 765             let (xform_self_ty, xform_ret_ty) =
 766                 this.xform_self_ty(&item, new_trait_ref.self_ty(), new_trait_ref.substs);
 767             this.push_candidate(
 768                 Candidate {
 769                     xform_self_ty,
 770                     xform_ret_ty,
 771                     item,
 772                     kind: ObjectCandidate,
 773                     import_ids: smallvec![],
 774                 },
 775                 true,
 776             );
 777         });
 778     }
 779
 780     fn assemble_inherent_candidates_from_param(&mut self, param_ty: ty::ParamTy) {
 781         // FIXME: do we want to commit to this behavior for param bounds?
 782         debug!("assemble_inherent_candidates_from_param(param_ty={:?})", param_ty);
 783
 784         let bounds = self.param_env.caller_bounds().iter().filter_map(|predicate| {
 785             let bound_predicate = predicate.kind();
 786             match bound_predicate.skip_binder() {
 787                 ty::PredicateKind::Clause(ty::Clause::Trait(trait_predicate)) => {
 788                     match *trait_predicate.trait_ref.self_ty().kind() {
 789                         ty::Param(p) if p == param_ty => {
 790                             Some(bound_predicate.rebind(trait_predicate.trait_ref))
 791                         }
 792                         _ => None,
 793                     }
 794                 }
 795                 ty::PredicateKind::Subtype(..)
 796                 | ty::PredicateKind::Coerce(..)
 797                 | ty::PredicateKind::Clause(ty::Clause::Projection(..))
 798                 | ty::PredicateKind::Clause(ty::Clause::RegionOutlives(..))
 799                 | ty::PredicateKind::WellFormed(..)
 800                 | ty::PredicateKind::ObjectSafe(..)
 801                 | ty::PredicateKind::ClosureKind(..)
 802                 | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(..))
 803                 | ty::PredicateKind::ConstEvaluatable(..)
 804                 | ty::PredicateKind::ConstEquate(..)
 805                 | ty::PredicateKind::Ambiguous
 806                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 807             }
 808         });
 809
 810         self.elaborate_bounds(bounds, |this, poly_trait_ref, item| {
 811             let trait_ref = this.erase_late_bound_regions(poly_trait_ref);
 812
 813             let (xform_self_ty, xform_ret_ty) =
 814                 this.xform_self_ty(&item, trait_ref.self_ty(), trait_ref.substs);
 815
 816             // Because this trait derives from a where-clause, it
 817             // should not contain any inference variables or other
 818             // artifacts. This means it is safe to put into the
 819             // `WhereClauseCandidate` and (eventually) into the
 820             // `WhereClausePick`.
 821             assert!(!trait_ref.substs.needs_infer());
 822
 823             this.push_candidate(
 824                 Candidate {
 825                     xform_self_ty,
 826                     xform_ret_ty,
 827                     item,
 828                     kind: WhereClauseCandidate(poly_trait_ref),
 829                     import_ids: smallvec![],
 830                 },
 831                 true,
 832             );
 833         });
 834     }
 835
 836     // Do a search through a list of bounds, using a callback to actually
 837     // create the candidates.
 838     fn elaborate_bounds<F>(
 839         &mut self,
 840         bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
 841         mut mk_cand: F,
 842     ) where
 843         F: for<'b> FnMut(&mut ProbeContext<'b, 'tcx>, ty::PolyTraitRef<'tcx>, ty::AssocItem),
 844     {
 845         let tcx = self.tcx;
 846         for bound_trait_ref in traits::transitive_bounds(tcx, bounds) {
 847             debug!("elaborate_bounds(bound_trait_ref={:?})", bound_trait_ref);
 848             for item in self.impl_or_trait_item(bound_trait_ref.def_id()) {
 849                 if !self.has_applicable_self(&item) {
 850                     self.record_static_candidate(CandidateSource::Trait(bound_trait_ref.def_id()));
 851                 } else {
 852                     mk_cand(self, bound_trait_ref, item);
 853                 }
 854             }
 855         }
 856     }
 857
 858     fn assemble_extension_candidates_for_traits_in_scope(&mut self, expr_hir_id: hir::HirId) {
 859         let mut duplicates = FxHashSet::default();
 860         let opt_applicable_traits = self.tcx.in_scope_traits(expr_hir_id);
 861         if let Some(applicable_traits) = opt_applicable_traits {
 862             for trait_candidate in applicable_traits.iter() {
 863                 let trait_did = trait_candidate.def_id;
 864                 if duplicates.insert(trait_did) {
 865                     self.assemble_extension_candidates_for_trait(
 866                         &trait_candidate.import_ids,
 867                         trait_did,
 868                     );
 869                 }
 870             }
 871         }
 872     }
 873
 874     fn assemble_extension_candidates_for_all_traits(&mut self) {
 875         let mut duplicates = FxHashSet::default();
 876         for trait_info in suggest::all_traits(self.tcx) {
 877             if duplicates.insert(trait_info.def_id) {
 878                 self.assemble_extension_candidates_for_trait(&smallvec![], trait_info.def_id);
 879             }
 880         }
 881     }
 882
 883     fn matches_return_type(
 884         &self,
 885         method: &ty::AssocItem,
 886         self_ty: Option<Ty<'tcx>>,
 887         expected: Ty<'tcx>,
 888     ) -> bool {
 889         match method.kind {
 890             ty::AssocKind::Fn => {
 891                 let fty = self.tcx.bound_fn_sig(method.def_id);
 892                 self.probe(|_| {
 893                     let substs = self.fresh_substs_for_item(self.span, method.def_id);
 894                     let fty = fty.subst(self.tcx, substs);
 895                     let fty =
 896                         self.replace_bound_vars_with_fresh_vars(self.span, infer::FnCall, fty);
 897
 898                     if let Some(self_ty) = self_ty {
 899                         if self
 900                             .at(&ObligationCause::dummy(), self.param_env)
 901                             .sup(fty.inputs()[0], self_ty)
 902                             .is_err()
 903                         {
 904                             return false;
 905                         }
 906                     }
 907                     self.can_sub(self.param_env, fty.output(), expected).is_ok()
 908                 })
 909             }
 910             _ => false,
 911         }
 912     }
 913
 914     fn assemble_extension_candidates_for_trait(
 915         &mut self,
 916         import_ids: &SmallVec<[LocalDefId; 1]>,
 917         trait_def_id: DefId,
 918     ) {
 919         debug!("assemble_extension_candidates_for_trait(trait_def_id={:?})", trait_def_id);
 920         let trait_substs = self.fresh_item_substs(trait_def_id);
 921         let trait_ref = ty::TraitRef::new(trait_def_id, trait_substs);
 922
 923         if self.tcx.is_trait_alias(trait_def_id) {
 924             // For trait aliases, assume all supertraits are relevant.
 925             let bounds = iter::once(ty::Binder::dummy(trait_ref));
 926             self.elaborate_bounds(bounds, |this, new_trait_ref, item| {
 927                 let new_trait_ref = this.erase_late_bound_regions(new_trait_ref);
 928
 929                 let (xform_self_ty, xform_ret_ty) =
 930                     this.xform_self_ty(&item, new_trait_ref.self_ty(), new_trait_ref.substs);
 931                 this.push_candidate(
 932                     Candidate {
 933                         xform_self_ty,
 934                         xform_ret_ty,
 935                         item,
 936                         import_ids: import_ids.clone(),
 937                         kind: TraitCandidate(new_trait_ref),
 938                     },
 939                     false,
 940                 );
 941             });
 942         } else {
 943             debug_assert!(self.tcx.is_trait(trait_def_id));
 944             for item in self.impl_or_trait_item(trait_def_id) {
 945                 // Check whether `trait_def_id` defines a method with suitable name.
 946                 if !self.has_applicable_self(&item) {
 947                     debug!("method has inapplicable self");
 948                     self.record_static_candidate(CandidateSource::Trait(trait_def_id));
 949                     continue;
 950                 }
 951
 952                 let (xform_self_ty, xform_ret_ty) =
 953                     self.xform_self_ty(&item, trait_ref.self_ty(), trait_substs);
 954                 self.push_candidate(
 955                     Candidate {
 956                         xform_self_ty,
 957                         xform_ret_ty,
 958                         item,
 959                         import_ids: import_ids.clone(),
 960                         kind: TraitCandidate(trait_ref),
 961                     },
 962                     false,
 963                 );
 964             }
 965         }
 966     }
 967
 968     fn candidate_method_names(
 969         &self,
 970         candidate_filter: impl Fn(&ty::AssocItem) -> bool,
 971     ) -> Vec<Ident> {
 972         let mut set = FxHashSet::default();
 973         let mut names: Vec<_> = self
 974             .inherent_candidates
 975             .iter()
 976             .chain(&self.extension_candidates)
 977             .filter(|candidate| candidate_filter(&candidate.item))
 978             .filter(|candidate| {
 979                 if let Some(return_ty) = self.return_type {
 980                     self.matches_return_type(&candidate.item, None, return_ty)
 981                 } else {
 982                     true
 983                 }
 984             })
 985             .map(|candidate| candidate.item.ident(self.tcx))
 986             .filter(|&name| set.insert(name))
 987             .collect();
 988
 989         // Sort them by the name so we have a stable result.
 990         names.sort_by(|a, b| a.as_str().partial_cmp(b.as_str()).unwrap());
 991         names
 992     }
 993
 994     ///////////////////////////////////////////////////////////////////////////
 995     // THE ACTUAL SEARCH
 996
 997     fn pick(mut self) -> PickResult<'tcx> {
 998         assert!(self.method_name.is_some());
 999
1000         if let Some(r) = self.pick_core() {
1001             return r;
1002         }
1003
1004         debug!("pick: actual search failed, assemble diagnostics");
1005
1006         let static_candidates = mem::take(&mut self.static_candidates);
1007         let private_candidate = self.private_candidate.take();
1008         let unsatisfied_predicates = mem::take(&mut self.unsatisfied_predicates);
1009
1010         // things failed, so lets look at all traits, for diagnostic purposes now:
1011         self.reset();
1012
1013         let span = self.span;
1014         let tcx = self.tcx;
1015
1016         self.assemble_extension_candidates_for_all_traits();
1017
1018         let out_of_scope_traits = match self.pick_core() {
1019             Some(Ok(p)) => vec![p.item.container_id(self.tcx)],
1020             Some(Err(MethodError::Ambiguity(v))) => v
1021                 .into_iter()
1022                 .map(|source| match source {
1023                     CandidateSource::Trait(id) => id,
1024                     CandidateSource::Impl(impl_id) => match tcx.trait_id_of_impl(impl_id) {
1025                         Some(id) => id,
1026                         None => span_bug!(span, "found inherent method when looking at traits"),
1027                     },
1028                 })
1029                 .collect(),
1030             Some(Err(MethodError::NoMatch(NoMatchData {
1031                 out_of_scope_traits: others, ..
1032             }))) => {
1033                 assert!(others.is_empty());
1034                 vec![]
1035             }
1036             _ => vec![],
1037         };
1038
1039         if let Some((kind, def_id)) = private_candidate {
1040             return Err(MethodError::PrivateMatch(kind, def_id, out_of_scope_traits));
1041         }
1042         let lev_candidate = self.probe_for_lev_candidate()?;
1043
1044         Err(MethodError::NoMatch(NoMatchData {
1045             static_candidates,
1046             unsatisfied_predicates,
1047             out_of_scope_traits,
1048             lev_candidate,
1049             mode: self.mode,
1050         }))
1051     }
1052
1053     fn pick_core(&mut self) -> Option<PickResult<'tcx>> {
1054         let pick = self.pick_all_method(Some(&mut vec![]));
1055
1056         // In this case unstable picking is done by `pick_method`.
1057         if !self.tcx.sess.opts.unstable_opts.pick_stable_methods_before_any_unstable {
1058             return pick;
1059         }
1060
1061         if pick.is_none() {
1062             return self.pick_all_method(None);
1063         }
1064         pick
1065     }
1066
1067     fn pick_all_method(
1068         &mut self,
1069         mut unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1070     ) -> Option<PickResult<'tcx>> {
1071         let steps = self.steps.clone();
1072         steps
1073             .iter()
1074             .filter(|step| {
1075                 debug!("pick_all_method: step={:?}", step);
1076                 // skip types that are from a type error or that would require dereferencing
1077                 // a raw pointer
1078                 !step.self_ty.references_error() && !step.from_unsafe_deref
1079             })
1080             .flat_map(|step| {
1081                 let InferOk { value: self_ty, obligations: _ } = self
1082                     .fcx
1083                     .probe_instantiate_query_response(
1084                         self.span,
1085                         &self.orig_steps_var_values,
1086                         &step.self_ty,
1087                     )
1088                     .unwrap_or_else(|_| {
1089                         span_bug!(self.span, "{:?} was applicable but now isn't?", step.self_ty)
1090                     });
1091                 self.pick_by_value_method(step, self_ty, unstable_candidates.as_deref_mut())
1092                     .or_else(|| {
1093                         self.pick_autorefd_method(
1094                             step,
1095                             self_ty,
1096                             hir::Mutability::Not,
1097                             unstable_candidates.as_deref_mut(),
1098                         )
1099                         .or_else(|| {
1100                             self.pick_autorefd_method(
1101                                 step,
1102                                 self_ty,
1103                                 hir::Mutability::Mut,
1104                                 unstable_candidates.as_deref_mut(),
1105                             )
1106                         })
1107                         .or_else(|| {
1108                             self.pick_const_ptr_method(
1109                                 step,
1110                                 self_ty,
1111                                 unstable_candidates.as_deref_mut(),
1112                             )
1113                         })
1114                     })
1115             })
1116             .next()
1117     }
1118
1119     /// For each type `T` in the step list, this attempts to find a method where
1120     /// the (transformed) self type is exactly `T`. We do however do one
1121     /// transformation on the adjustment: if we are passing a region pointer in,
1122     /// we will potentially *reborrow* it to a shorter lifetime. This allows us
1123     /// to transparently pass `&mut` pointers, in particular, without consuming
1124     /// them for their entire lifetime.
1125     fn pick_by_value_method(
1126         &mut self,
1127         step: &CandidateStep<'tcx>,
1128         self_ty: Ty<'tcx>,
1129         unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1130     ) -> Option<PickResult<'tcx>> {
1131         if step.unsize {
1132             return None;
1133         }
1134
1135         self.pick_method(self_ty, unstable_candidates).map(|r| {
1136             r.map(|mut pick| {
1137                 pick.autoderefs = step.autoderefs;
1138
1139                 // Insert a `&*` or `&mut *` if this is a reference type:
1140                 if let ty::Ref(_, _, mutbl) = *step.self_ty.value.value.kind() {
1141                     pick.autoderefs += 1;
1142                     pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::Autoref {
1143                         mutbl,
1144                         unsize: pick.autoref_or_ptr_adjustment.map_or(false, |a| a.get_unsize()),
1145                     })
1146                 }
1147
1148                 pick
1149             })
1150         })
1151     }
1152
1153     fn pick_autorefd_method(
1154         &mut self,
1155         step: &CandidateStep<'tcx>,
1156         self_ty: Ty<'tcx>,
1157         mutbl: hir::Mutability,
1158         unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1159     ) -> Option<PickResult<'tcx>> {
1160         let tcx = self.tcx;
1161
1162         // In general, during probing we erase regions.
1163         let region = tcx.lifetimes.re_erased;
1164
1165         let autoref_ty = tcx.mk_ref(region, ty::TypeAndMut { ty: self_ty, mutbl });
1166         self.pick_method(autoref_ty, unstable_candidates).map(|r| {
1167             r.map(|mut pick| {
1168                 pick.autoderefs = step.autoderefs;
1169                 pick.autoref_or_ptr_adjustment =
1170                     Some(AutorefOrPtrAdjustment::Autoref { mutbl, unsize: step.unsize });
1171                 pick
1172             })
1173         })
1174     }
1175
1176     /// If `self_ty` is `*mut T` then this picks `*const T` methods. The reason why we have a
1177     /// special case for this is because going from `*mut T` to `*const T` with autoderefs and
1178     /// autorefs would require dereferencing the pointer, which is not safe.
1179     fn pick_const_ptr_method(
1180         &mut self,
1181         step: &CandidateStep<'tcx>,
1182         self_ty: Ty<'tcx>,
1183         unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1184     ) -> Option<PickResult<'tcx>> {
1185         // Don't convert an unsized reference to ptr
1186         if step.unsize {
1187             return None;
1188         }
1189
1190         let &ty::RawPtr(ty::TypeAndMut { ty, mutbl: hir::Mutability::Mut }) = self_ty.kind() else {
1191             return None;
1192         };
1193
1194         let const_self_ty = ty::TypeAndMut { ty, mutbl: hir::Mutability::Not };
1195         let const_ptr_ty = self.tcx.mk_ptr(const_self_ty);
1196         self.pick_method(const_ptr_ty, unstable_candidates).map(|r| {
1197             r.map(|mut pick| {
1198                 pick.autoderefs = step.autoderefs;
1199                 pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::ToConstPtr);
1200                 pick
1201             })
1202         })
1203     }
1204
1205     fn pick_method_with_unstable(&mut self, self_ty: Ty<'tcx>) -> Option<PickResult<'tcx>> {
1206         debug!("pick_method_with_unstable(self_ty={})", self.ty_to_string(self_ty));
1207
1208         let mut possibly_unsatisfied_predicates = Vec::new();
1209
1210         for (kind, candidates) in
1211             &[("inherent", &self.inherent_candidates), ("extension", &self.extension_candidates)]
1212         {
1213             debug!("searching {} candidates", kind);
1214             let res = self.consider_candidates(
1215                 self_ty,
1216                 candidates.iter(),
1217                 &mut possibly_unsatisfied_predicates,
1218                 Some(&mut vec![]),
1219             );
1220             if res.is_some() {
1221                 return res;
1222             }
1223         }
1224
1225         debug!("searching unstable candidates");
1226         let res = self.consider_candidates(
1227             self_ty,
1228             self.inherent_candidates.iter().chain(&self.extension_candidates),
1229             &mut possibly_unsatisfied_predicates,
1230             None,
1231         );
1232         if res.is_none() {
1233             self.unsatisfied_predicates.extend(possibly_unsatisfied_predicates);
1234         }
1235         res
1236     }
1237
1238     fn pick_method(
1239         &mut self,
1240         self_ty: Ty<'tcx>,
1241         mut unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1242     ) -> Option<PickResult<'tcx>> {
1243         if !self.tcx.sess.opts.unstable_opts.pick_stable_methods_before_any_unstable {
1244             return self.pick_method_with_unstable(self_ty);
1245         }
1246
1247         debug!("pick_method(self_ty={})", self.ty_to_string(self_ty));
1248
1249         let mut possibly_unsatisfied_predicates = Vec::new();
1250
1251         for (kind, candidates) in
1252             &[("inherent", &self.inherent_candidates), ("extension", &self.extension_candidates)]
1253         {
1254             debug!("searching {} candidates", kind);
1255             let res = self.consider_candidates(
1256                 self_ty,
1257                 candidates.iter(),
1258                 &mut possibly_unsatisfied_predicates,
1259                 unstable_candidates.as_deref_mut(),
1260             );
1261             if let Some(pick) = res {
1262                 return Some(pick);
1263             }
1264         }
1265
1266         // `pick_method` may be called twice for the same self_ty if no stable methods
1267         // match. Only extend once.
1268         if unstable_candidates.is_some() {
1269             self.unsatisfied_predicates.extend(possibly_unsatisfied_predicates);
1270         }
1271         None
1272     }
1273
1274     fn consider_candidates<'b, ProbesIter>(
1275         &self,
1276         self_ty: Ty<'tcx>,
1277         probes: ProbesIter,
1278         possibly_unsatisfied_predicates: &mut Vec<(
1279             ty::Predicate<'tcx>,
1280             Option<ty::Predicate<'tcx>>,
1281             Option<ObligationCause<'tcx>>,
1282         )>,
1283         mut unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1284     ) -> Option<PickResult<'tcx>>
1285     where
1286         ProbesIter: Iterator<Item = &'b Candidate<'tcx>> + Clone,
1287         'tcx: 'b,
1288     {
1289         let mut applicable_candidates: Vec<_> = probes
1290             .clone()
1291             .map(|probe| {
1292                 (probe, self.consider_probe(self_ty, probe, possibly_unsatisfied_predicates))
1293             })
1294             .filter(|&(_, status)| status != ProbeResult::NoMatch)
1295             .collect();
1296
1297         debug!("applicable_candidates: {:?}", applicable_candidates);
1298
1299         if applicable_candidates.len() > 1 {
1300             if let Some(pick) =
1301                 self.collapse_candidates_to_trait_pick(self_ty, &applicable_candidates)
1302             {
1303                 return Some(Ok(pick));
1304             }
1305         }
1306
1307         if let Some(uc) = &mut unstable_candidates {
1308             applicable_candidates.retain(|&(p, _)| {
1309                 if let stability::EvalResult::Deny { feature, .. } =
1310                     self.tcx.eval_stability(p.item.def_id, None, self.span, None)
1311                 {
1312                     uc.push((p.clone(), feature));
1313                     return false;
1314                 }
1315                 true
1316             });
1317         }
1318
1319         if applicable_candidates.len() > 1 {
1320             let sources = probes.map(|p| self.candidate_source(p, self_ty)).collect();
1321             return Some(Err(MethodError::Ambiguity(sources)));
1322         }
1323
1324         applicable_candidates.pop().map(|(probe, status)| {
1325             if status == ProbeResult::Match {
1326                 Ok(probe
1327                     .to_unadjusted_pick(self_ty, unstable_candidates.cloned().unwrap_or_default()))
1328             } else {
1329                 Err(MethodError::BadReturnType)
1330             }
1331         })
1332     }
1333 }
1334
1335 impl<'tcx> Pick<'tcx> {
1336     /// In case there were unstable name collisions, emit them as a lint.
1337     /// Checks whether two picks do not refer to the same trait item for the same `Self` type.
1338     /// Only useful for comparisons of picks in order to improve diagnostics.
1339     /// Do not use for type checking.
1340     pub fn differs_from(&self, other: &Self) -> bool {
1341         let Self {
1342             item:
1343                 AssocItem {
1344                     def_id,
1345                     name: _,
1346                     kind: _,
1347                     container: _,
1348                     trait_item_def_id: _,
1349                     fn_has_self_parameter: _,
1350                 },
1351             kind: _,
1352             import_ids: _,
1353             autoderefs: _,
1354             autoref_or_ptr_adjustment: _,
1355             self_ty,
1356             unstable_candidates: _,
1357         } = *self;
1358         self_ty != other.self_ty || def_id != other.item.def_id
1359     }
1360
1361     /// In case there were unstable name collisions, emit them as a lint.
1362     pub fn maybe_emit_unstable_name_collision_hint(
1363         &self,
1364         tcx: TyCtxt<'tcx>,
1365         span: Span,
1366         scope_expr_id: hir::HirId,
1367     ) {
1368         if self.unstable_candidates.is_empty() {
1369             return;
1370         }
1371         let def_kind = self.item.kind.as_def_kind();
1372         tcx.struct_span_lint_hir(
1373             lint::builtin::UNSTABLE_NAME_COLLISIONS,
1374             scope_expr_id,
1375             span,
1376             format!(
1377                 "{} {} with this name may be added to the standard library in the future",
1378                 def_kind.article(),
1379                 def_kind.descr(self.item.def_id),
1380             ),
1381             |lint| {
1382                 match (self.item.kind, self.item.container) {
1383                     (ty::AssocKind::Fn, _) => {
1384                         // FIXME: This should be a `span_suggestion` instead of `help`
1385                         // However `self.span` only
1386                         // highlights the method name, so we can't use it. Also consider reusing
1387                         // the code from `report_method_error()`.
1388                         lint.help(&format!(
1389                             "call with fully qualified syntax `{}(...)` to keep using the current \
1390                              method",
1391                             tcx.def_path_str(self.item.def_id),
1392                         ));
1393                     }
1394                     (ty::AssocKind::Const, ty::AssocItemContainer::TraitContainer) => {
1395                         let def_id = self.item.container_id(tcx);
1396                         lint.span_suggestion(
1397                             span,
1398                             "use the fully qualified path to the associated const",
1399                             format!(
1400                                 "<{} as {}>::{}",
1401                                 self.self_ty,
1402                                 tcx.def_path_str(def_id),
1403                                 self.item.name
1404                             ),
1405                             Applicability::MachineApplicable,
1406                         );
1407                     }
1408                     _ => {}
1409                 }
1410                 if tcx.sess.is_nightly_build() {
1411                     for (candidate, feature) in &self.unstable_candidates {
1412                         lint.help(&format!(
1413                             "add `#![feature({})]` to the crate attributes to enable `{}`",
1414                             feature,
1415                             tcx.def_path_str(candidate.item.def_id),
1416                         ));
1417                     }
1418                 }
1419
1420                 lint
1421             },
1422         );
1423     }
1424 }
1425
1426 impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
1427     fn select_trait_candidate(
1428         &self,
1429         trait_ref: ty::TraitRef<'tcx>,
1430     ) -> traits::SelectionResult<'tcx, traits::Selection<'tcx>> {
1431         let cause = traits::ObligationCause::misc(self.span, self.body_id);
1432         let predicate = ty::Binder::dummy(trait_ref);
1433         let obligation = traits::Obligation::new(self.tcx, cause, self.param_env, predicate);
1434         traits::SelectionContext::new(self).select(&obligation)
1435     }
1436
1437     fn candidate_source(&self, candidate: &Candidate<'tcx>, self_ty: Ty<'tcx>) -> CandidateSource {
1438         match candidate.kind {
1439             InherentImplCandidate(..) => {
1440                 CandidateSource::Impl(candidate.item.container_id(self.tcx))
1441             }
1442             ObjectCandidate | WhereClauseCandidate(_) => {
1443                 CandidateSource::Trait(candidate.item.container_id(self.tcx))
1444             }
1445             TraitCandidate(trait_ref) => self.probe(|_| {
1446                 let _ = self
1447                     .at(&ObligationCause::dummy(), self.param_env)
1448                     .define_opaque_types(false)
1449                     .sup(candidate.xform_self_ty, self_ty);
1450                 match self.select_trait_candidate(trait_ref) {
1451                     Ok(Some(traits::ImplSource::UserDefined(ref impl_data))) => {
1452                         // If only a single impl matches, make the error message point
1453                         // to that impl.
1454                         CandidateSource::Impl(impl_data.impl_def_id)
1455                     }
1456                     _ => CandidateSource::Trait(candidate.item.container_id(self.tcx)),
1457                 }
1458             }),
1459         }
1460     }
1461
1462     fn consider_probe(
1463         &self,
1464         self_ty: Ty<'tcx>,
1465         probe: &Candidate<'tcx>,
1466         possibly_unsatisfied_predicates: &mut Vec<(
1467             ty::Predicate<'tcx>,
1468             Option<ty::Predicate<'tcx>>,
1469             Option<ObligationCause<'tcx>>,
1470         )>,
1471     ) -> ProbeResult {
1472         debug!("consider_probe: self_ty={:?} probe={:?}", self_ty, probe);
1473
1474         self.probe(|_| {
1475             // First check that the self type can be related.
1476             let sub_obligations = match self
1477                 .at(&ObligationCause::dummy(), self.param_env)
1478                 .define_opaque_types(false)
1479                 .sup(probe.xform_self_ty, self_ty)
1480             {
1481                 Ok(InferOk { obligations, value: () }) => obligations,
1482                 Err(err) => {
1483                     debug!("--> cannot relate self-types {:?}", err);
1484                     return ProbeResult::NoMatch;
1485                 }
1486             };
1487
1488             let mut result = ProbeResult::Match;
1489             let mut xform_ret_ty = probe.xform_ret_ty;
1490             debug!(?xform_ret_ty);
1491
1492             let cause = traits::ObligationCause::misc(self.span, self.body_id);
1493
1494             let mut parent_pred = None;
1495
1496             // If so, impls may carry other conditions (e.g., where
1497             // clauses) that must be considered. Make sure that those
1498             // match as well (or at least may match, sometimes we
1499             // don't have enough information to fully evaluate).
1500             match probe.kind {
1501                 InherentImplCandidate(ref substs, ref ref_obligations) => {
1502                     // `xform_ret_ty` hasn't been normalized yet, only `xform_self_ty`,
1503                     // see the reasons mentioned in the comments in `assemble_inherent_impl_probe`
1504                     // for why this is necessary
1505                     let InferOk {
1506                         value: normalized_xform_ret_ty,
1507                         obligations: normalization_obligations,
1508                     } = self.fcx.at(&cause, self.param_env).normalize(probe.xform_ret_ty);
1509                     xform_ret_ty = normalized_xform_ret_ty;
1510                     debug!("xform_ret_ty after normalization: {:?}", xform_ret_ty);
1511
1512                     // Check whether the impl imposes obligations we have to worry about.
1513                     let impl_def_id = probe.item.container_id(self.tcx);
1514                     let impl_bounds = self.tcx.predicates_of(impl_def_id);
1515                     let impl_bounds = impl_bounds.instantiate(self.tcx, substs);
1516
1517                     let InferOk { value: impl_bounds, obligations: norm_obligations } =
1518                         self.fcx.at(&cause, self.param_env).normalize(impl_bounds);
1519
1520                     // Convert the bounds into obligations.
1521                     let impl_obligations = traits::predicates_for_generics(
1522                         move |_, _| cause.clone(),
1523                         self.param_env,
1524                         impl_bounds,
1525                     );
1526
1527                     let candidate_obligations = impl_obligations
1528                         .chain(norm_obligations.into_iter())
1529                         .chain(ref_obligations.iter().cloned())
1530                         .chain(normalization_obligations.into_iter());
1531
1532                     // Evaluate those obligations to see if they might possibly hold.
1533                     for o in candidate_obligations {
1534                         let o = self.resolve_vars_if_possible(o);
1535                         if !self.predicate_may_hold(&o) {
1536                             result = ProbeResult::NoMatch;
1537                             possibly_unsatisfied_predicates.push((
1538                                 o.predicate,
1539                                 None,
1540                                 Some(o.cause),
1541                             ));
1542                         }
1543                     }
1544                 }
1545
1546                 ObjectCandidate | WhereClauseCandidate(..) => {
1547                     // These have no additional conditions to check.
1548                 }
1549
1550                 TraitCandidate(trait_ref) => {
1551                     if let Some(method_name) = self.method_name {
1552                         // Some trait methods are excluded for arrays before 2021.
1553                         // (`array.into_iter()` wants a slice iterator for compatibility.)
1554                         if self_ty.is_array() && !method_name.span.rust_2021() {
1555                             let trait_def = self.tcx.trait_def(trait_ref.def_id);
1556                             if trait_def.skip_array_during_method_dispatch {
1557                                 return ProbeResult::NoMatch;
1558                             }
1559                         }
1560                     }
1561                     let predicate =
1562                         ty::Binder::dummy(trait_ref).without_const().to_predicate(self.tcx);
1563                     parent_pred = Some(predicate);
1564                     let obligation =
1565                         traits::Obligation::new(self.tcx, cause, self.param_env, predicate);
1566                     if !self.predicate_may_hold(&obligation) {
1567                         result = ProbeResult::NoMatch;
1568                         if self.probe(|_| {
1569                             match self.select_trait_candidate(trait_ref) {
1570                                 Err(_) => return true,
1571                                 Ok(Some(impl_source))
1572                                     if !impl_source.borrow_nested_obligations().is_empty() =>
1573                                 {
1574                                     for obligation in impl_source.borrow_nested_obligations() {
1575                                         // Determine exactly which obligation wasn't met, so
1576                                         // that we can give more context in the error.
1577                                         if !self.predicate_may_hold(obligation) {
1578                                             let nested_predicate =
1579                                                 self.resolve_vars_if_possible(obligation.predicate);
1580                                             let predicate =
1581                                                 self.resolve_vars_if_possible(predicate);
1582                                             let p = if predicate == nested_predicate {
1583                                                 // Avoid "`MyStruct: Foo` which is required by
1584                                                 // `MyStruct: Foo`" in E0599.
1585                                                 None
1586                                             } else {
1587                                                 Some(predicate)
1588                                             };
1589                                             possibly_unsatisfied_predicates.push((
1590                                                 nested_predicate,
1591                                                 p,
1592                                                 Some(obligation.cause.clone()),
1593                                             ));
1594                                         }
1595                                     }
1596                                 }
1597                                 _ => {
1598                                     // Some nested subobligation of this predicate
1599                                     // failed.
1600                                     let predicate = self.resolve_vars_if_possible(predicate);
1601                                     possibly_unsatisfied_predicates.push((predicate, None, None));
1602                                 }
1603                             }
1604                             false
1605                         }) {
1606                             // This candidate's primary obligation doesn't even
1607                             // select - don't bother registering anything in
1608                             // `potentially_unsatisfied_predicates`.
1609                             return ProbeResult::NoMatch;
1610                         }
1611                     }
1612                 }
1613             }
1614
1615             // Evaluate those obligations to see if they might possibly hold.
1616             for o in sub_obligations {
1617                 let o = self.resolve_vars_if_possible(o);
1618                 if !self.predicate_may_hold(&o) {
1619                     result = ProbeResult::NoMatch;
1620                     possibly_unsatisfied_predicates.push((o.predicate, parent_pred, Some(o.cause)));
1621                 }
1622             }
1623
1624             if let ProbeResult::Match = result {
1625                 if let (Some(return_ty), Some(xform_ret_ty)) = (self.return_type, xform_ret_ty) {
1626                     let xform_ret_ty = self.resolve_vars_if_possible(xform_ret_ty);
1627                     debug!(
1628                         "comparing return_ty {:?} with xform ret ty {:?}",
1629                         return_ty, probe.xform_ret_ty
1630                     );
1631                     if self
1632                         .at(&ObligationCause::dummy(), self.param_env)
1633                         .define_opaque_types(false)
1634                         .sup(return_ty, xform_ret_ty)
1635                         .is_err()
1636                     {
1637                         return ProbeResult::BadReturnType;
1638                     }
1639                 }
1640             }
1641
1642             result
1643         })
1644     }
1645
1646     /// Sometimes we get in a situation where we have multiple probes that are all impls of the
1647     /// same trait, but we don't know which impl to use. In this case, since in all cases the
1648     /// external interface of the method can be determined from the trait, it's ok not to decide.
1649     /// We can basically just collapse all of the probes for various impls into one where-clause
1650     /// probe. This will result in a pending obligation so when more type-info is available we can
1651     /// make the final decision.
1652     ///
1653     /// Example (`src/test/ui/method-two-trait-defer-resolution-1.rs`):
1654     ///
1655     /// ```ignore (illustrative)
1656     /// trait Foo { ... }
1657     /// impl Foo for Vec<i32> { ... }
1658     /// impl Foo for Vec<usize> { ... }
1659     /// ```
1660     ///
1661     /// Now imagine the receiver is `Vec<_>`. It doesn't really matter at this time which impl we
1662     /// use, so it's ok to just commit to "using the method from the trait Foo".
1663     fn collapse_candidates_to_trait_pick(
1664         &self,
1665         self_ty: Ty<'tcx>,
1666         probes: &[(&Candidate<'tcx>, ProbeResult)],
1667     ) -> Option<Pick<'tcx>> {
1668         // Do all probes correspond to the same trait?
1669         let container = probes[0].0.item.trait_container(self.tcx)?;
1670         for (p, _) in &probes[1..] {
1671             let p_container = p.item.trait_container(self.tcx)?;
1672             if p_container != container {
1673                 return None;
1674             }
1675         }
1676
1677         // FIXME: check the return type here somehow.
1678         // If so, just use this trait and call it a day.
1679         Some(Pick {
1680             item: probes[0].0.item,
1681             kind: TraitPick,
1682             import_ids: probes[0].0.import_ids.clone(),
1683             autoderefs: 0,
1684             autoref_or_ptr_adjustment: None,
1685             self_ty,
1686             unstable_candidates: vec![],
1687         })
1688     }
1689
1690     /// Similarly to `probe_for_return_type`, this method attempts to find the best matching
1691     /// candidate method where the method name may have been misspelled. Similarly to other
1692     /// Levenshtein based suggestions, we provide at most one such suggestion.
1693     fn probe_for_lev_candidate(&mut self) -> Result<Option<ty::AssocItem>, MethodError<'tcx>> {
1694         debug!("probing for method names similar to {:?}", self.method_name);
1695
1696         let steps = self.steps.clone();
1697         self.probe(|_| {
1698             let mut pcx = ProbeContext::new(
1699                 self.fcx,
1700                 self.span,
1701                 self.mode,
1702                 self.method_name,
1703                 self.return_type,
1704                 self.orig_steps_var_values.clone(),
1705                 steps,
1706                 self.scope_expr_id,
1707             );
1708             pcx.allow_similar_names = true;
1709             pcx.assemble_inherent_candidates();
1710
1711             let method_names = pcx.candidate_method_names(|_| true);
1712             pcx.allow_similar_names = false;
1713             let applicable_close_candidates: Vec<ty::AssocItem> = method_names
1714                 .iter()
1715                 .filter_map(|&method_name| {
1716                     pcx.reset();
1717                     pcx.method_name = Some(method_name);
1718                     pcx.assemble_inherent_candidates();
1719                     pcx.pick_core().and_then(|pick| pick.ok()).map(|pick| pick.item)
1720                 })
1721                 .collect();
1722
1723             if applicable_close_candidates.is_empty() {
1724                 Ok(None)
1725             } else {
1726                 let best_name = {
1727                     let names = applicable_close_candidates
1728                         .iter()
1729                         .map(|cand| cand.name)
1730                         .collect::<Vec<Symbol>>();
1731                     find_best_match_for_name_with_substrings(
1732                         &names,
1733                         self.method_name.unwrap().name,
1734                         None,
1735                     )
1736                 }
1737                 .unwrap();
1738                 Ok(applicable_close_candidates.into_iter().find(|method| method.name == best_name))
1739             }
1740         })
1741     }
1742
1743     ///////////////////////////////////////////////////////////////////////////
1744     // MISCELLANY
1745     fn has_applicable_self(&self, item: &ty::AssocItem) -> bool {
1746         // "Fast track" -- check for usage of sugar when in method call
1747         // mode.
1748         //
1749         // In Path mode (i.e., resolving a value like `T::next`), consider any
1750         // associated value (i.e., methods, constants) but not types.
1751         match self.mode {
1752             Mode::MethodCall => item.fn_has_self_parameter,
1753             Mode::Path => match item.kind {
1754                 ty::AssocKind::Type => false,
1755                 ty::AssocKind::Fn | ty::AssocKind::Const => true,
1756             },
1757         }
1758         // FIXME -- check for types that deref to `Self`,
1759         // like `Rc<Self>` and so on.
1760         //
1761         // Note also that the current code will break if this type
1762         // includes any of the type parameters defined on the method
1763         // -- but this could be overcome.
1764     }
1765
1766     fn record_static_candidate(&mut self, source: CandidateSource) {
1767         self.static_candidates.push(source);
1768     }
1769
1770     #[instrument(level = "debug", skip(self))]
1771     fn xform_self_ty(
1772         &self,
1773         item: &ty::AssocItem,
1774         impl_ty: Ty<'tcx>,
1775         substs: SubstsRef<'tcx>,
1776     ) -> (Ty<'tcx>, Option<Ty<'tcx>>) {
1777         if item.kind == ty::AssocKind::Fn && self.mode == Mode::MethodCall {
1778             let sig = self.xform_method_sig(item.def_id, substs);
1779             (sig.inputs()[0], Some(sig.output()))
1780         } else {
1781             (impl_ty, None)
1782         }
1783     }
1784
1785     #[instrument(level = "debug", skip(self))]
1786     fn xform_method_sig(&self, method: DefId, substs: SubstsRef<'tcx>) -> ty::FnSig<'tcx> {
1787         let fn_sig = self.tcx.bound_fn_sig(method);
1788         debug!(?fn_sig);
1789
1790         assert!(!substs.has_escaping_bound_vars());
1791
1792         // It is possible for type parameters or early-bound lifetimes
1793         // to appear in the signature of `self`. The substitutions we
1794         // are given do not include type/lifetime parameters for the
1795         // method yet. So create fresh variables here for those too,
1796         // if there are any.
1797         let generics = self.tcx.generics_of(method);
1798         assert_eq!(substs.len(), generics.parent_count as usize);
1799
1800         let xform_fn_sig = if generics.params.is_empty() {
1801             fn_sig.subst(self.tcx, substs)
1802         } else {
1803             let substs = InternalSubsts::for_item(self.tcx, method, |param, _| {
1804                 let i = param.index as usize;
1805                 if i < substs.len() {
1806                     substs[i]
1807                 } else {
1808                     match param.kind {
1809                         GenericParamDefKind::Lifetime => {
1810                             // In general, during probe we erase regions.
1811                             self.tcx.lifetimes.re_erased.into()
1812                         }
1813                         GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
1814                             self.var_for_def(self.span, param)
1815                         }
1816                     }
1817                 }
1818             });
1819             fn_sig.subst(self.tcx, substs)
1820         };
1821
1822         self.erase_late_bound_regions(xform_fn_sig)
1823     }
1824
1825     /// Gets the type of an impl and generate substitutions with inference vars.
1826     fn impl_ty_and_substs(
1827         &self,
1828         impl_def_id: DefId,
1829     ) -> (ty::EarlyBinder<Ty<'tcx>>, SubstsRef<'tcx>) {
1830         (self.tcx.bound_type_of(impl_def_id), self.fresh_item_substs(impl_def_id))
1831     }
1832
1833     fn fresh_item_substs(&self, def_id: DefId) -> SubstsRef<'tcx> {
1834         InternalSubsts::for_item(self.tcx, def_id, |param, _| match param.kind {
1835             GenericParamDefKind::Lifetime => self.tcx.lifetimes.re_erased.into(),
1836             GenericParamDefKind::Type { .. } => self
1837                 .next_ty_var(TypeVariableOrigin {
1838                     kind: TypeVariableOriginKind::SubstitutionPlaceholder,
1839                     span: self.tcx.def_span(def_id),
1840                 })
1841                 .into(),
1842             GenericParamDefKind::Const { .. } => {
1843                 let span = self.tcx.def_span(def_id);
1844                 let origin = ConstVariableOrigin {
1845                     kind: ConstVariableOriginKind::SubstitutionPlaceholder,
1846                     span,
1847                 };
1848                 self.next_const_var(self.tcx.type_of(param.def_id), origin).into()
1849             }
1850         })
1851     }
1852
1853     /// Replaces late-bound-regions bound by `value` with `'static` using
1854     /// `ty::erase_late_bound_regions`.
1855     ///
1856     /// This is only a reasonable thing to do during the *probe* phase, not the *confirm* phase, of
1857     /// method matching. It is reasonable during the probe phase because we don't consider region
1858     /// relationships at all. Therefore, we can just replace all the region variables with 'static
1859     /// rather than creating fresh region variables. This is nice for two reasons:
1860     ///
1861     /// 1. Because the numbers of the region variables would otherwise be fairly unique to this
1862     ///    particular method call, it winds up creating fewer types overall, which helps for memory
1863     ///    usage. (Admittedly, this is a rather small effect, though measurable.)
1864     ///
1865     /// 2. It makes it easier to deal with higher-ranked trait bounds, because we can replace any
1866     ///    late-bound regions with 'static. Otherwise, if we were going to replace late-bound
1867     ///    regions with actual region variables as is proper, we'd have to ensure that the same
1868     ///    region got replaced with the same variable, which requires a bit more coordination
1869     ///    and/or tracking the substitution and
1870     ///    so forth.
1871     fn erase_late_bound_regions<T>(&self, value: ty::Binder<'tcx, T>) -> T
1872     where
1873         T: TypeFoldable<'tcx>,
1874     {
1875         self.tcx.erase_late_bound_regions(value)
1876     }
1877
1878     /// Determine if the given associated item type is relevant in the current context.
1879     fn is_relevant_kind_for_mode(&self, kind: ty::AssocKind) -> bool {
1880         match (self.mode, kind) {
1881             (Mode::MethodCall, ty::AssocKind::Fn) => true,
1882             (Mode::Path, ty::AssocKind::Const | ty::AssocKind::Fn) => true,
1883             _ => false,
1884         }
1885     }
1886
1887     /// Finds the method with the appropriate name (or return type, as the case may be). If
1888     /// `allow_similar_names` is set, find methods with close-matching names.
1889     // The length of the returned iterator is nearly always 0 or 1 and this
1890     // method is fairly hot.
1891     fn impl_or_trait_item(&self, def_id: DefId) -> SmallVec<[ty::AssocItem; 1]> {
1892         if let Some(name) = self.method_name {
1893             if self.allow_similar_names {
1894                 let max_dist = max(name.as_str().len(), 3) / 3;
1895                 self.tcx
1896                     .associated_items(def_id)
1897                     .in_definition_order()
1898                     .filter(|x| {
1899                         if !self.is_relevant_kind_for_mode(x.kind) {
1900                             return false;
1901                         }
1902                         match lev_distance_with_substrings(name.as_str(), x.name.as_str(), max_dist)
1903                         {
1904                             Some(d) => d > 0,
1905                             None => false,
1906                         }
1907                     })
1908                     .copied()
1909                     .collect()
1910             } else {
1911                 self.fcx
1912                     .associated_value(def_id, name)
1913                     .filter(|x| self.is_relevant_kind_for_mode(x.kind))
1914                     .map_or_else(SmallVec::new, |x| SmallVec::from_buf([x]))
1915             }
1916         } else {
1917             self.tcx
1918                 .associated_items(def_id)
1919                 .in_definition_order()
1920                 .filter(|x| self.is_relevant_kind_for_mode(x.kind))
1921                 .copied()
1922                 .collect()
1923         }
1924     }
1925 }
1926
1927 impl<'tcx> Candidate<'tcx> {
1928     fn to_unadjusted_pick(
1929         &self,
1930         self_ty: Ty<'tcx>,
1931         unstable_candidates: Vec<(Candidate<'tcx>, Symbol)>,
1932     ) -> Pick<'tcx> {
1933         Pick {
1934             item: self.item,
1935             kind: match self.kind {
1936                 InherentImplCandidate(..) => InherentImplPick,
1937                 ObjectCandidate => ObjectPick,
1938                 TraitCandidate(_) => TraitPick,
1939                 WhereClauseCandidate(ref trait_ref) => {
1940                     // Only trait derived from where-clauses should
1941                     // appear here, so they should not contain any
1942                     // inference variables or other artifacts. This
1943                     // means they are safe to put into the
1944                     // `WhereClausePick`.
1945                     assert!(
1946                         !trait_ref.skip_binder().substs.needs_infer()
1947                             && !trait_ref.skip_binder().substs.has_placeholders()
1948                     );
1949
1950                     WhereClausePick(*trait_ref)
1951                 }
1952             },
1953             import_ids: self.import_ids.clone(),
1954             autoderefs: 0,
1955             autoref_or_ptr_adjustment: None,
1956             self_ty,
1957             unstable_candidates,
1958         }
1959     }
1960 }