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