compiler/rustc_typeck/src/check/method/probe.rs

   1 use super::suggest;
   2 use super::MethodError;
   3 use super::NoMatchData;
   4 use super::{CandidateSource, ImplSource, TraitSource};
   5
   6 use crate::check::FnCtxt;
   7 use crate::errors::MethodCallOnUnknownType;
   8 use crate::hir::def::DefKind;
   9 use crate::hir::def_id::DefId;
  10
  11 use rustc_data_structures::fx::FxHashSet;
  12 use rustc_errors::Applicability;
  13 use rustc_hir as hir;
  14 use rustc_hir::def::Namespace;
  15 use rustc_infer::infer::canonical::OriginalQueryValues;
  16 use rustc_infer::infer::canonical::{Canonical, QueryResponse};
  17 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  18 use rustc_infer::infer::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
  19 use rustc_infer::infer::{self, InferOk, TyCtxtInferExt};
  20 use rustc_middle::middle::stability;
  21 use rustc_middle::ty::subst::{InternalSubsts, Subst, SubstsRef};
  22 use rustc_middle::ty::GenericParamDefKind;
  23 use rustc_middle::ty::{self, ParamEnvAnd, ToPredicate, Ty, TyCtxt, TypeFoldable};
  24 use rustc_session::lint;
  25 use rustc_span::def_id::LocalDefId;
  26 use rustc_span::lev_distance::{find_best_match_for_name, lev_distance};
  27 use rustc_span::{symbol::Ident, Span, Symbol, DUMMY_SP};
  28 use rustc_trait_selection::autoderef::{self, Autoderef};
  29 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  30 use rustc_trait_selection::traits::query::method_autoderef::MethodAutoderefBadTy;
  31 use rustc_trait_selection::traits::query::method_autoderef::{
  32     CandidateStep, MethodAutoderefStepsResult,
  33 };
  34 use rustc_trait_selection::traits::query::CanonicalTyGoal;
  35 use rustc_trait_selection::traits::{self, ObligationCause};
  36 use std::cmp::max;
  37 use std::iter;
  38 use std::mem;
  39 use std::ops::Deref;
  40
  41 use smallvec::{smallvec, SmallVec};
  42
  43 use self::CandidateKind::*;
  44 pub use self::PickKind::*;
  45
  46 /// Boolean flag used to indicate if this search is for a suggestion
  47 /// or not. If true, we can allow ambiguity and so forth.
  48 #[derive(Clone, Copy, Debug)]
  49 pub struct IsSuggestion(pub bool);
  50
  51 struct ProbeContext<'a, 'tcx> {
  52     fcx: &'a FnCtxt<'a, 'tcx>,
  53     span: Span,
  54     mode: Mode,
  55     method_name: Option<Ident>,
  56     return_type: Option<Ty<'tcx>>,
  57
  58     /// This is the OriginalQueryValues for the steps queries
  59     /// that are answered in steps.
  60     orig_steps_var_values: OriginalQueryValues<'tcx>,
  61     steps: &'tcx [CandidateStep<'tcx>],
  62
  63     inherent_candidates: Vec<Candidate<'tcx>>,
  64     extension_candidates: Vec<Candidate<'tcx>>,
  65     impl_dups: FxHashSet<DefId>,
  66
  67     /// Collects near misses when the candidate functions are missing a `self` keyword and is only
  68     /// used for error reporting
  69     static_candidates: Vec<CandidateSource>,
  70
  71     /// When probing for names, include names that are close to the
  72     /// requested name (by Levensthein distance)
  73     allow_similar_names: bool,
  74
  75     /// Some(candidate) if there is a private candidate
  76     private_candidate: Option<(DefKind, DefId)>,
  77
  78     /// Collects near misses when trait bounds for type parameters are unsatisfied and is only used
  79     /// for error reporting
  80     unsatisfied_predicates:
  81         Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>, Option<ObligationCause<'tcx>>)>,
  82
  83     is_suggestion: IsSuggestion,
  84
  85     scope_expr_id: hir::HirId,
  86 }
  87
  88 impl<'a, 'tcx> Deref for ProbeContext<'a, 'tcx> {
  89     type Target = FnCtxt<'a, 'tcx>;
  90     fn deref(&self) -> &Self::Target {
  91         self.fcx
  92     }
  93 }
  94
  95 #[derive(Debug, Clone)]
  96 struct Candidate<'tcx> {
  97     // Candidates are (I'm not quite sure, but they are mostly) basically
  98     // some metadata on top of a `ty::AssocItem` (without substs).
  99     //
 100     // However, method probing wants to be able to evaluate the predicates
 101     // for a function with the substs applied - for example, if a function
 102     // has `where Self: Sized`, we don't want to consider it unless `Self`
 103     // is actually `Sized`, and similarly, return-type suggestions want
 104     // to consider the "actual" return type.
 105     //
 106     // The way this is handled is through `xform_self_ty`. It contains
 107     // the receiver type of this candidate, but `xform_self_ty`,
 108     // `xform_ret_ty` and `kind` (which contains the predicates) have the
 109     // generic parameters of this candidate substituted with the *same set*
 110     // of inference variables, which acts as some weird sort of "query".
 111     //
 112     // When we check out a candidate, we require `xform_self_ty` to be
 113     // a subtype of the passed-in self-type, and this equates the type
 114     // variables in the rest of the fields.
 115     //
 116     // For example, if we have this candidate:
 117     // ```
 118     //    trait Foo {
 119     //        fn foo(&self) where Self: Sized;
 120     //    }
 121     // ```
 122     //
 123     // Then `xform_self_ty` will be `&'erased ?X` and `kind` will contain
 124     // the predicate `?X: Sized`, so if we are evaluating `Foo` for a
 125     // the receiver `&T`, we'll do the subtyping which will make `?X`
 126     // get the right value, then when we evaluate the predicate we'll check
 127     // if `T: Sized`.
 128     xform_self_ty: Ty<'tcx>,
 129     xform_ret_ty: Option<Ty<'tcx>>,
 130     item: ty::AssocItem,
 131     kind: CandidateKind<'tcx>,
 132     import_ids: SmallVec<[LocalDefId; 1]>,
 133 }
 134
 135 #[derive(Debug, Clone)]
 136 enum CandidateKind<'tcx> {
 137     InherentImplCandidate(
 138         SubstsRef<'tcx>,
 139         // Normalize obligations
 140         Vec<traits::PredicateObligation<'tcx>>,
 141     ),
 142     ObjectCandidate,
 143     TraitCandidate(ty::TraitRef<'tcx>),
 144     WhereClauseCandidate(
 145         // Trait
 146         ty::PolyTraitRef<'tcx>,
 147     ),
 148 }
 149
 150 #[derive(Debug, PartialEq, Eq, Copy, Clone)]
 151 enum ProbeResult {
 152     NoMatch,
 153     BadReturnType,
 154     Match,
 155 }
 156
 157 /// When adjusting a receiver we often want to do one of
 158 ///
 159 /// - Add a `&` (or `&mut`), converting the receiver from `T` to `&T` (or `&mut T`)
 160 /// - If the receiver has type `*mut T`, convert it to `*const T`
 161 ///
 162 /// This type tells us which one to do.
 163 ///
 164 /// Note that in principle we could do both at the same time. For example, when the receiver has
 165 /// type `T`, we could autoref it to `&T`, then convert to `*const T`. Or, when it has type `*mut
 166 /// T`, we could convert it to `*const T`, then autoref to `&*const T`. However, currently we do
 167 /// (at most) one of these. Either the receiver has type `T` and we convert it to `&T` (or with
 168 /// `mut`), or it has type `*mut T` and we convert it to `*const T`.
 169 #[derive(Debug, PartialEq, Copy, Clone)]
 170 pub enum AutorefOrPtrAdjustment {
 171     /// Receiver has type `T`, add `&` or `&mut` (it `T` is `mut`), and maybe also "unsize" it.
 172     /// Unsizing is used to convert a `[T; N]` to `[T]`, which only makes sense when autorefing.
 173     Autoref {
 174         mutbl: hir::Mutability,
 175
 176         /// Indicates that the source expression should be "unsized" to a target type.
 177         /// This is special-cased for just arrays unsizing to slices.
 178         unsize: bool,
 179     },
 180     /// Receiver has type `*mut T`, convert to `*const T`
 181     ToConstPtr,
 182 }
 183
 184 impl AutorefOrPtrAdjustment {
 185     fn get_unsize(&self) -> bool {
 186         match self {
 187             AutorefOrPtrAdjustment::Autoref { mutbl: _, unsize } => *unsize,
 188             AutorefOrPtrAdjustment::ToConstPtr => false,
 189         }
 190     }
 191 }
 192
 193 #[derive(Debug, PartialEq, Clone)]
 194 pub struct Pick<'tcx> {
 195     pub item: ty::AssocItem,
 196     pub kind: PickKind<'tcx>,
 197     pub import_ids: SmallVec<[LocalDefId; 1]>,
 198
 199     /// Indicates that the source expression should be autoderef'd N times
 200     ///
 201     ///     A = expr | *expr | **expr | ...
 202     pub autoderefs: usize,
 203
 204     /// Indicates that we want to add an autoref (and maybe also unsize it), or if the receiver is
 205     /// `*mut T`, convert it to `*const T`.
 206     pub autoref_or_ptr_adjustment: Option<AutorefOrPtrAdjustment>,
 207     pub self_ty: Ty<'tcx>,
 208 }
 209
 210 #[derive(Clone, Debug, PartialEq, Eq)]
 211 pub enum PickKind<'tcx> {
 212     InherentImplPick,
 213     ObjectPick,
 214     TraitPick,
 215     WhereClausePick(
 216         // Trait
 217         ty::PolyTraitRef<'tcx>,
 218     ),
 219 }
 220
 221 pub type PickResult<'tcx> = Result<Pick<'tcx>, MethodError<'tcx>>;
 222
 223 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 224 pub enum Mode {
 225     // An expression of the form `receiver.method_name(...)`.
 226     // Autoderefs are performed on `receiver`, lookup is done based on the
 227     // `self` argument  of the method, and static methods aren't considered.
 228     MethodCall,
 229     // An expression of the form `Type::item` or `<T>::item`.
 230     // No autoderefs are performed, lookup is done based on the type each
 231     // implementation is for, and static methods are included.
 232     Path,
 233 }
 234
 235 #[derive(PartialEq, Eq, Copy, Clone, Debug)]
 236 pub enum ProbeScope {
 237     // Assemble candidates coming only from traits in scope.
 238     TraitsInScope,
 239
 240     // Assemble candidates coming from all traits.
 241     AllTraits,
 242 }
 243
 244 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 245     /// This is used to offer suggestions to users. It returns methods
 246     /// that could have been called which have the desired return
 247     /// type. Some effort is made to rule out methods that, if called,
 248     /// would result in an error (basically, the same criteria we
 249     /// would use to decide if a method is a plausible fit for
 250     /// ambiguity purposes).
 251     #[instrument(level = "debug", skip(self, scope_expr_id))]
 252     pub fn probe_for_return_type(
 253         &self,
 254         span: Span,
 255         mode: Mode,
 256         return_type: Ty<'tcx>,
 257         self_ty: Ty<'tcx>,
 258         scope_expr_id: hir::HirId,
 259     ) -> Vec<ty::AssocItem> {
 260         debug!(
 261             "probe(self_ty={:?}, return_type={}, scope_expr_id={})",
 262             self_ty, return_type, scope_expr_id
 263         );
 264         let method_names = self
 265             .probe_op(
 266                 span,
 267                 mode,
 268                 None,
 269                 Some(return_type),
 270                 IsSuggestion(true),
 271                 self_ty,
 272                 scope_expr_id,
 273                 ProbeScope::AllTraits,
 274                 |probe_cx| Ok(probe_cx.candidate_method_names()),
 275             )
 276             .unwrap_or_default();
 277         method_names
 278             .iter()
 279             .flat_map(|&method_name| {
 280                 self.probe_op(
 281                     span,
 282                     mode,
 283                     Some(method_name),
 284                     Some(return_type),
 285                     IsSuggestion(true),
 286                     self_ty,
 287                     scope_expr_id,
 288                     ProbeScope::AllTraits,
 289                     |probe_cx| probe_cx.pick(),
 290                 )
 291                 .ok()
 292                 .map(|pick| pick.item)
 293             })
 294             .collect()
 295     }
 296
 297     #[instrument(level = "debug", skip(self, scope_expr_id))]
 298     pub fn probe_for_name(
 299         &self,
 300         span: Span,
 301         mode: Mode,
 302         item_name: Ident,
 303         is_suggestion: IsSuggestion,
 304         self_ty: Ty<'tcx>,
 305         scope_expr_id: hir::HirId,
 306         scope: ProbeScope,
 307     ) -> PickResult<'tcx> {
 308         debug!(
 309             "probe(self_ty={:?}, item_name={}, scope_expr_id={})",
 310             self_ty, item_name, scope_expr_id
 311         );
 312         self.probe_op(
 313             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.infcx.canonicalize_query(
 342             ParamEnvAnd { param_env: self.param_env, value: self_ty },
 343             &mut orig_values,
 344         );
 345
 346         let steps = if mode == Mode::MethodCall {
 347             self.tcx.method_autoderef_steps(param_env_and_self_ty)
 348         } else {
 349             self.infcx.probe(|_| {
 350                 // Mode::Path - the deref steps is "trivial". This turns
 351                 // our CanonicalQuery into a "trivial" QueryResponse. This
 352                 // is a bit inefficient, but I don't think that writing
 353                 // special handling for this "trivial case" is a good idea.
 354
 355                 let infcx = &self.infcx;
 356                 let (ParamEnvAnd { param_env: _, value: self_ty }, canonical_inference_vars) =
 357                     infcx.instantiate_canonical_with_fresh_inference_vars(
 358                         span,
 359                         &param_env_and_self_ty,
 360                     );
 361                 debug!(
 362                     "probe_op: Mode::Path, param_env_and_self_ty={:?} self_ty={:?}",
 363                     param_env_and_self_ty, self_ty
 364                 );
 365                 MethodAutoderefStepsResult {
 366                     steps: infcx.tcx.arena.alloc_from_iter([CandidateStep {
 367                         self_ty: self.make_query_response_ignoring_pending_obligations(
 368                             canonical_inference_vars,
 369                             self_ty,
 370                         ),
 371                         autoderefs: 0,
 372                         from_unsafe_deref: false,
 373                         unsize: false,
 374                     }]),
 375                     opt_bad_ty: None,
 376                     reached_recursion_limit: false,
 377                 }
 378             })
 379         };
 380
 381         // If our autoderef loop had reached the recursion limit,
 382         // report an overflow error, but continue going on with
 383         // the truncated autoderef list.
 384         if steps.reached_recursion_limit {
 385             self.probe(|_| {
 386                 let ty = &steps
 387                     .steps
 388                     .last()
 389                     .unwrap_or_else(|| span_bug!(span, "reached the recursion limit in 0 steps?"))
 390                     .self_ty;
 391                 let ty = self
 392                     .probe_instantiate_query_response(span, &orig_values, ty)
 393                     .unwrap_or_else(|_| span_bug!(span, "instantiating {:?} failed?", ty));
 394                 autoderef::report_autoderef_recursion_limit_error(self.tcx, span, ty.value);
 395             });
 396         }
 397
 398         // If we encountered an `_` type or an error type during autoderef, this is
 399         // ambiguous.
 400         if let Some(bad_ty) = &steps.opt_bad_ty {
 401             if is_suggestion.0 {
 402                 // Ambiguity was encountered during a suggestion. Just keep going.
 403                 debug!("ProbeContext: encountered ambiguity in suggestion");
 404             } else if bad_ty.reached_raw_pointer && !self.tcx.features().arbitrary_self_types {
 405                 // this case used to be allowed by the compiler,
 406                 // so we do a future-compat lint here for the 2015 edition
 407                 // (see https://github.com/rust-lang/rust/issues/46906)
 408                 if self.tcx.sess.rust_2018() {
 409                     self.tcx.sess.emit_err(MethodCallOnUnknownType { span });
 410                 } else {
 411                     self.tcx.struct_span_lint_hir(
 412                         lint::builtin::TYVAR_BEHIND_RAW_POINTER,
 413                         scope_expr_id,
 414                         span,
 415                         |lint| lint.build("type annotations needed").emit(),
 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::new(
 429                     Vec::new(),
 430                     Vec::new(),
 431                     Vec::new(),
 432                     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                 is_suggestion,
 452                 scope_expr_id,
 453             );
 454
 455             probe_cx.assemble_inherent_candidates();
 456             match scope {
 457                 ProbeScope::TraitsInScope => {
 458                     probe_cx.assemble_extension_candidates_for_traits_in_scope(scope_expr_id)
 459                 }
 460                 ProbeScope::AllTraits => probe_cx.assemble_extension_candidates_for_all_traits(),
 461             };
 462             op(probe_cx)
 463         })
 464     }
 465 }
 466
 467 pub fn provide(providers: &mut ty::query::Providers) {
 468     providers.method_autoderef_steps = method_autoderef_steps;
 469 }
 470
 471 fn method_autoderef_steps<'tcx>(
 472     tcx: TyCtxt<'tcx>,
 473     goal: CanonicalTyGoal<'tcx>,
 474 ) -> MethodAutoderefStepsResult<'tcx> {
 475     debug!("method_autoderef_steps({:?})", goal);
 476
 477     tcx.infer_ctxt().enter_with_canonical(DUMMY_SP, &goal, |ref infcx, goal, inference_vars| {
 478         let ParamEnvAnd { param_env, value: self_ty } = goal;
 479
 480         let mut autoderef =
 481             Autoderef::new(infcx, param_env, hir::CRATE_HIR_ID, DUMMY_SP, self_ty, DUMMY_SP)
 482                 .include_raw_pointers()
 483                 .silence_errors();
 484         let mut reached_raw_pointer = false;
 485         let mut steps: Vec<_> = autoderef
 486             .by_ref()
 487             .map(|(ty, d)| {
 488                 let step = CandidateStep {
 489                     self_ty: infcx.make_query_response_ignoring_pending_obligations(
 490                         inference_vars.clone(),
 491                         ty,
 492                     ),
 493                     autoderefs: d,
 494                     from_unsafe_deref: reached_raw_pointer,
 495                     unsize: false,
 496                 };
 497                 if let ty::RawPtr(_) = ty.kind() {
 498                     // all the subsequent steps will be from_unsafe_deref
 499                     reached_raw_pointer = true;
 500                 }
 501                 step
 502             })
 503             .collect();
 504
 505         let final_ty = autoderef.final_ty(true);
 506         let opt_bad_ty = match final_ty.kind() {
 507             ty::Infer(ty::TyVar(_)) | ty::Error(_) => Some(MethodAutoderefBadTy {
 508                 reached_raw_pointer,
 509                 ty: infcx
 510                     .make_query_response_ignoring_pending_obligations(inference_vars, final_ty),
 511             }),
 512             ty::Array(elem_ty, _) => {
 513                 let dereferences = steps.len() - 1;
 514
 515                 steps.push(CandidateStep {
 516                     self_ty: infcx.make_query_response_ignoring_pending_obligations(
 517                         inference_vars,
 518                         infcx.tcx.mk_slice(*elem_ty),
 519                     ),
 520                     autoderefs: dereferences,
 521                     // this could be from an unsafe deref if we had
 522                     // a *mut/const [T; N]
 523                     from_unsafe_deref: reached_raw_pointer,
 524                     unsize: true,
 525                 });
 526
 527                 None
 528             }
 529             _ => None,
 530         };
 531
 532         debug!("method_autoderef_steps: steps={:?} opt_bad_ty={:?}", steps, opt_bad_ty);
 533
 534         MethodAutoderefStepsResult {
 535             steps: tcx.arena.alloc_from_iter(steps),
 536             opt_bad_ty: opt_bad_ty.map(|ty| &*tcx.arena.alloc(ty)),
 537             reached_recursion_limit: autoderef.reached_recursion_limit(),
 538         }
 539     })
 540 }
 541
 542 impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 543     fn new(
 544         fcx: &'a FnCtxt<'a, 'tcx>,
 545         span: Span,
 546         mode: Mode,
 547         method_name: Option<Ident>,
 548         return_type: Option<Ty<'tcx>>,
 549         orig_steps_var_values: OriginalQueryValues<'tcx>,
 550         steps: &'tcx [CandidateStep<'tcx>],
 551         is_suggestion: IsSuggestion,
 552         scope_expr_id: hir::HirId,
 553     ) -> ProbeContext<'a, 'tcx> {
 554         ProbeContext {
 555             fcx,
 556             span,
 557             mode,
 558             method_name,
 559             return_type,
 560             inherent_candidates: Vec::new(),
 561             extension_candidates: Vec::new(),
 562             impl_dups: FxHashSet::default(),
 563             orig_steps_var_values,
 564             steps,
 565             static_candidates: Vec::new(),
 566             allow_similar_names: false,
 567             private_candidate: None,
 568             unsatisfied_predicates: Vec::new(),
 569             is_suggestion,
 570             scope_expr_id,
 571         }
 572     }
 573
 574     fn reset(&mut self) {
 575         self.inherent_candidates.clear();
 576         self.extension_candidates.clear();
 577         self.impl_dups.clear();
 578         self.static_candidates.clear();
 579         self.private_candidate = None;
 580     }
 581
 582     ///////////////////////////////////////////////////////////////////////////
 583     // CANDIDATE ASSEMBLY
 584
 585     fn push_candidate(&mut self, candidate: Candidate<'tcx>, is_inherent: bool) {
 586         let is_accessible = if let Some(name) = self.method_name {
 587             let item = candidate.item;
 588             let def_scope =
 589                 self.tcx.adjust_ident_and_get_scope(name, item.container.id(), self.body_id).1;
 590             item.vis.is_accessible_from(def_scope, self.tcx)
 591         } else {
 592             true
 593         };
 594         if is_accessible {
 595             if is_inherent {
 596                 self.inherent_candidates.push(candidate);
 597             } else {
 598                 self.extension_candidates.push(candidate);
 599             }
 600         } else if self.private_candidate.is_none() {
 601             self.private_candidate =
 602                 Some((candidate.item.kind.as_def_kind(), candidate.item.def_id));
 603         }
 604     }
 605
 606     fn assemble_inherent_candidates(&mut self) {
 607         for step in self.steps.iter() {
 608             self.assemble_probe(&step.self_ty);
 609         }
 610     }
 611
 612     fn assemble_probe(&mut self, self_ty: &Canonical<'tcx, QueryResponse<'tcx, Ty<'tcx>>>) {
 613         debug!("assemble_probe: self_ty={:?}", self_ty);
 614         let lang_items = self.tcx.lang_items();
 615
 616         match *self_ty.value.value.kind() {
 617             ty::Dynamic(data, ..) if let Some(p) = data.principal() => {
 618                 // Subtle: we can't use `instantiate_query_response` here: using it will
 619                 // commit to all of the type equalities assumed by inference going through
 620                 // autoderef (see the `method-probe-no-guessing` test).
 621                 //
 622                 // However, in this code, it is OK if we end up with an object type that is
 623                 // "more general" than the object type that we are evaluating. For *every*
 624                 // object type `MY_OBJECT`, a function call that goes through a trait-ref
 625                 // of the form `<MY_OBJECT as SuperTraitOf(MY_OBJECT)>::func` is a valid
 626                 // `ObjectCandidate`, and it should be discoverable "exactly" through one
 627                 // of the iterations in the autoderef loop, so there is no problem with it
 628                 // being discoverable in another one of these iterations.
 629                 //
 630                 // Using `instantiate_canonical_with_fresh_inference_vars` on our
 631                 // `Canonical<QueryResponse<Ty<'tcx>>>` and then *throwing away* the
 632                 // `CanonicalVarValues` will exactly give us such a generalization - it
 633                 // will still match the original object type, but it won't pollute our
 634                 // type variables in any form, so just do that!
 635                 let (QueryResponse { value: generalized_self_ty, .. }, _ignored_var_values) =
 636                     self.fcx
 637                         .instantiate_canonical_with_fresh_inference_vars(self.span, self_ty);
 638
 639                 self.assemble_inherent_candidates_from_object(generalized_self_ty);
 640                 self.assemble_inherent_impl_candidates_for_type(p.def_id());
 641             }
 642             ty::Adt(def, _) => {
 643                 self.assemble_inherent_impl_candidates_for_type(def.did);
 644             }
 645             ty::Foreign(did) => {
 646                 self.assemble_inherent_impl_candidates_for_type(did);
 647             }
 648             ty::Param(p) => {
 649                 self.assemble_inherent_candidates_from_param(p);
 650             }
 651             ty::Bool => {
 652                 let lang_def_id = lang_items.bool_impl();
 653                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 654             }
 655             ty::Char => {
 656                 let lang_def_id = lang_items.char_impl();
 657                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 658             }
 659             ty::Str => {
 660                 let lang_def_id = lang_items.str_impl();
 661                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 662
 663                 let lang_def_id = lang_items.str_alloc_impl();
 664                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 665             }
 666             ty::Slice(_) => {
 667                 for lang_def_id in [
 668                     lang_items.slice_impl(),
 669                     lang_items.slice_u8_impl(),
 670                     lang_items.slice_alloc_impl(),
 671                     lang_items.slice_u8_alloc_impl(),
 672                 ] {
 673                     self.assemble_inherent_impl_for_primitive(lang_def_id);
 674                 }
 675             }
 676             ty::Array(_, _) => {
 677                 let lang_def_id = lang_items.array_impl();
 678                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 679             }
 680             ty::RawPtr(ty::TypeAndMut { ty: _, mutbl }) => {
 681                 let (lang_def_id1, lang_def_id2) = match mutbl {
 682                     hir::Mutability::Not => {
 683                         (lang_items.const_ptr_impl(), lang_items.const_slice_ptr_impl())
 684                     }
 685                     hir::Mutability::Mut => {
 686                         (lang_items.mut_ptr_impl(), lang_items.mut_slice_ptr_impl())
 687                     }
 688                 };
 689                 self.assemble_inherent_impl_for_primitive(lang_def_id1);
 690                 self.assemble_inherent_impl_for_primitive(lang_def_id2);
 691             }
 692             ty::Int(i) => {
 693                 let lang_def_id = match i {
 694                     ty::IntTy::I8 => lang_items.i8_impl(),
 695                     ty::IntTy::I16 => lang_items.i16_impl(),
 696                     ty::IntTy::I32 => lang_items.i32_impl(),
 697                     ty::IntTy::I64 => lang_items.i64_impl(),
 698                     ty::IntTy::I128 => lang_items.i128_impl(),
 699                     ty::IntTy::Isize => lang_items.isize_impl(),
 700                 };
 701                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 702             }
 703             ty::Uint(i) => {
 704                 let lang_def_id = match i {
 705                     ty::UintTy::U8 => lang_items.u8_impl(),
 706                     ty::UintTy::U16 => lang_items.u16_impl(),
 707                     ty::UintTy::U32 => lang_items.u32_impl(),
 708                     ty::UintTy::U64 => lang_items.u64_impl(),
 709                     ty::UintTy::U128 => lang_items.u128_impl(),
 710                     ty::UintTy::Usize => lang_items.usize_impl(),
 711                 };
 712                 self.assemble_inherent_impl_for_primitive(lang_def_id);
 713             }
 714             ty::Float(f) => {
 715                 let (lang_def_id1, lang_def_id2) = match f {
 716                     ty::FloatTy::F32 => (lang_items.f32_impl(), lang_items.f32_runtime_impl()),
 717                     ty::FloatTy::F64 => (lang_items.f64_impl(), lang_items.f64_runtime_impl()),
 718                 };
 719                 self.assemble_inherent_impl_for_primitive(lang_def_id1);
 720                 self.assemble_inherent_impl_for_primitive(lang_def_id2);
 721             }
 722             _ => {}
 723         }
 724     }
 725
 726     fn assemble_inherent_impl_for_primitive(&mut self, lang_def_id: Option<DefId>) {
 727         if let Some(impl_def_id) = lang_def_id {
 728             self.assemble_inherent_impl_probe(impl_def_id);
 729         }
 730     }
 731
 732     fn assemble_inherent_impl_candidates_for_type(&mut self, def_id: DefId) {
 733         let impl_def_ids = self.tcx.at(self.span).inherent_impls(def_id);
 734         for &impl_def_id in impl_def_ids.iter() {
 735             self.assemble_inherent_impl_probe(impl_def_id);
 736         }
 737     }
 738
 739     fn assemble_inherent_impl_probe(&mut self, impl_def_id: DefId) {
 740         if !self.impl_dups.insert(impl_def_id) {
 741             return; // already visited
 742         }
 743
 744         debug!("assemble_inherent_impl_probe {:?}", impl_def_id);
 745
 746         for item in self.impl_or_trait_item(impl_def_id) {
 747             if !self.has_applicable_self(&item) {
 748                 // No receiver declared. Not a candidate.
 749                 self.record_static_candidate(ImplSource(impl_def_id));
 750                 continue;
 751             }
 752
 753             let (impl_ty, impl_substs) = self.impl_ty_and_substs(impl_def_id);
 754             let impl_ty = impl_ty.subst(self.tcx, impl_substs);
 755
 756             debug!("impl_ty: {:?}", impl_ty);
 757
 758             // Determine the receiver type that the method itself expects.
 759             let (xform_self_ty, xform_ret_ty) = self.xform_self_ty(&item, impl_ty, impl_substs);
 760             debug!("xform_self_ty: {:?}, xform_ret_ty: {:?}", xform_self_ty, xform_ret_ty);
 761
 762             // We can't use normalize_associated_types_in as it will pollute the
 763             // fcx's fulfillment context after this probe is over.
 764             // Note: we only normalize `xform_self_ty` here since the normalization
 765             // of the return type can lead to inference results that prohibit
 766             // valid canidates from being found, see issue #85671
 767             // FIXME Postponing the normalization of the return type likely only hides a deeper bug,
 768             // which might be caused by the `param_env` itself. The clauses of the `param_env`
 769             // maybe shouldn't include `Param`s, but rather fresh variables or be canonicalized,
 770             // see isssue #89650
 771             let cause = traits::ObligationCause::misc(self.span, self.body_id);
 772             let selcx = &mut traits::SelectionContext::new(self.fcx);
 773             let traits::Normalized { value: xform_self_ty, obligations } =
 774                 traits::normalize(selcx, self.param_env, cause, xform_self_ty);
 775             debug!(
 776                 "assemble_inherent_impl_probe after normalization: xform_self_ty = {:?}/{:?}",
 777                 xform_self_ty, xform_ret_ty
 778             );
 779
 780             self.push_candidate(
 781                 Candidate {
 782                     xform_self_ty,
 783                     xform_ret_ty,
 784                     item,
 785                     kind: InherentImplCandidate(impl_substs, obligations),
 786                     import_ids: smallvec![],
 787                 },
 788                 true,
 789             );
 790         }
 791     }
 792
 793     fn assemble_inherent_candidates_from_object(&mut self, self_ty: Ty<'tcx>) {
 794         debug!("assemble_inherent_candidates_from_object(self_ty={:?})", self_ty);
 795
 796         let principal = match self_ty.kind() {
 797             ty::Dynamic(ref data, ..) => Some(data),
 798             _ => None,
 799         }
 800         .and_then(|data| data.principal())
 801         .unwrap_or_else(|| {
 802             span_bug!(
 803                 self.span,
 804                 "non-object {:?} in assemble_inherent_candidates_from_object",
 805                 self_ty
 806             )
 807         });
 808
 809         // It is illegal to invoke a method on a trait instance that refers to
 810         // the `Self` type. An [`ObjectSafetyViolation::SupertraitSelf`] error
 811         // will be reported by `object_safety.rs` if the method refers to the
 812         // `Self` type anywhere other than the receiver. Here, we use a
 813         // substitution that replaces `Self` with the object type itself. Hence,
 814         // a `&self` method will wind up with an argument type like `&dyn Trait`.
 815         let trait_ref = principal.with_self_ty(self.tcx, self_ty);
 816         self.elaborate_bounds(iter::once(trait_ref), |this, new_trait_ref, item| {
 817             let new_trait_ref = this.erase_late_bound_regions(new_trait_ref);
 818
 819             let (xform_self_ty, xform_ret_ty) =
 820                 this.xform_self_ty(&item, new_trait_ref.self_ty(), new_trait_ref.substs);
 821             this.push_candidate(
 822                 Candidate {
 823                     xform_self_ty,
 824                     xform_ret_ty,
 825                     item,
 826                     kind: ObjectCandidate,
 827                     import_ids: smallvec![],
 828                 },
 829                 true,
 830             );
 831         });
 832     }
 833
 834     fn assemble_inherent_candidates_from_param(&mut self, param_ty: ty::ParamTy) {
 835         // FIXME: do we want to commit to this behavior for param bounds?
 836         debug!("assemble_inherent_candidates_from_param(param_ty={:?})", param_ty);
 837
 838         let bounds = self.param_env.caller_bounds().iter().filter_map(|predicate| {
 839             let bound_predicate = predicate.kind();
 840             match bound_predicate.skip_binder() {
 841                 ty::PredicateKind::Trait(trait_predicate) => {
 842                     match *trait_predicate.trait_ref.self_ty().kind() {
 843                         ty::Param(p) if p == param_ty => {
 844                             Some(bound_predicate.rebind(trait_predicate.trait_ref))
 845                         }
 846                         _ => None,
 847                     }
 848                 }
 849                 ty::PredicateKind::Subtype(..)
 850                 | ty::PredicateKind::Coerce(..)
 851                 | ty::PredicateKind::Projection(..)
 852                 | ty::PredicateKind::RegionOutlives(..)
 853                 | ty::PredicateKind::WellFormed(..)
 854                 | ty::PredicateKind::ObjectSafe(..)
 855                 | ty::PredicateKind::ClosureKind(..)
 856                 | ty::PredicateKind::TypeOutlives(..)
 857                 | ty::PredicateKind::ConstEvaluatable(..)
 858                 | ty::PredicateKind::ConstEquate(..)
 859                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => None,
 860             }
 861         });
 862
 863         self.elaborate_bounds(bounds, |this, poly_trait_ref, item| {
 864             let trait_ref = this.erase_late_bound_regions(poly_trait_ref);
 865
 866             let (xform_self_ty, xform_ret_ty) =
 867                 this.xform_self_ty(&item, trait_ref.self_ty(), trait_ref.substs);
 868
 869             // Because this trait derives from a where-clause, it
 870             // should not contain any inference variables or other
 871             // artifacts. This means it is safe to put into the
 872             // `WhereClauseCandidate` and (eventually) into the
 873             // `WhereClausePick`.
 874             assert!(!trait_ref.substs.needs_infer());
 875
 876             this.push_candidate(
 877                 Candidate {
 878                     xform_self_ty,
 879                     xform_ret_ty,
 880                     item,
 881                     kind: WhereClauseCandidate(poly_trait_ref),
 882                     import_ids: smallvec![],
 883                 },
 884                 true,
 885             );
 886         });
 887     }
 888
 889     // Do a search through a list of bounds, using a callback to actually
 890     // create the candidates.
 891     fn elaborate_bounds<F>(
 892         &mut self,
 893         bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
 894         mut mk_cand: F,
 895     ) where
 896         F: for<'b> FnMut(&mut ProbeContext<'b, 'tcx>, ty::PolyTraitRef<'tcx>, ty::AssocItem),
 897     {
 898         let tcx = self.tcx;
 899         for bound_trait_ref in traits::transitive_bounds(tcx, bounds) {
 900             debug!("elaborate_bounds(bound_trait_ref={:?})", bound_trait_ref);
 901             for item in self.impl_or_trait_item(bound_trait_ref.def_id()) {
 902                 if !self.has_applicable_self(&item) {
 903                     self.record_static_candidate(TraitSource(bound_trait_ref.def_id()));
 904                 } else {
 905                     mk_cand(self, bound_trait_ref, item);
 906                 }
 907             }
 908         }
 909     }
 910
 911     fn assemble_extension_candidates_for_traits_in_scope(&mut self, expr_hir_id: hir::HirId) {
 912         let mut duplicates = FxHashSet::default();
 913         let opt_applicable_traits = self.tcx.in_scope_traits(expr_hir_id);
 914         if let Some(applicable_traits) = opt_applicable_traits {
 915             for trait_candidate in applicable_traits.iter() {
 916                 let trait_did = trait_candidate.def_id;
 917                 if duplicates.insert(trait_did) {
 918                     self.assemble_extension_candidates_for_trait(
 919                         &trait_candidate.import_ids,
 920                         trait_did,
 921                     );
 922                 }
 923             }
 924         }
 925     }
 926
 927     fn assemble_extension_candidates_for_all_traits(&mut self) {
 928         let mut duplicates = FxHashSet::default();
 929         for trait_info in suggest::all_traits(self.tcx) {
 930             if duplicates.insert(trait_info.def_id) {
 931                 self.assemble_extension_candidates_for_trait(&smallvec![], trait_info.def_id);
 932             }
 933         }
 934     }
 935
 936     pub fn matches_return_type(
 937         &self,
 938         method: &ty::AssocItem,
 939         self_ty: Option<Ty<'tcx>>,
 940         expected: Ty<'tcx>,
 941     ) -> bool {
 942         match method.kind {
 943             ty::AssocKind::Fn => {
 944                 let fty = self.tcx.fn_sig(method.def_id);
 945                 self.probe(|_| {
 946                     let substs = self.fresh_substs_for_item(self.span, method.def_id);
 947                     let fty = fty.subst(self.tcx, substs);
 948                     let (fty, _) =
 949                         self.replace_bound_vars_with_fresh_vars(self.span, infer::FnCall, fty);
 950
 951                     if let Some(self_ty) = self_ty {
 952                         if self
 953                             .at(&ObligationCause::dummy(), self.param_env)
 954                             .sup(fty.inputs()[0], self_ty)
 955                             .is_err()
 956                         {
 957                             return false;
 958                         }
 959                     }
 960                     self.can_sub(self.param_env, fty.output(), expected).is_ok()
 961                 })
 962             }
 963             _ => false,
 964         }
 965     }
 966
 967     fn assemble_extension_candidates_for_trait(
 968         &mut self,
 969         import_ids: &SmallVec<[LocalDefId; 1]>,
 970         trait_def_id: DefId,
 971     ) {
 972         debug!("assemble_extension_candidates_for_trait(trait_def_id={:?})", trait_def_id);
 973         let trait_substs = self.fresh_item_substs(trait_def_id);
 974         let trait_ref = ty::TraitRef::new(trait_def_id, trait_substs);
 975
 976         if self.tcx.is_trait_alias(trait_def_id) {
 977             // For trait aliases, assume all supertraits are relevant.
 978             let bounds = iter::once(ty::Binder::dummy(trait_ref));
 979             self.elaborate_bounds(bounds, |this, new_trait_ref, item| {
 980                 let new_trait_ref = this.erase_late_bound_regions(new_trait_ref);
 981
 982                 let (xform_self_ty, xform_ret_ty) =
 983                     this.xform_self_ty(&item, new_trait_ref.self_ty(), new_trait_ref.substs);
 984                 this.push_candidate(
 985                     Candidate {
 986                         xform_self_ty,
 987                         xform_ret_ty,
 988                         item,
 989                         import_ids: import_ids.clone(),
 990                         kind: TraitCandidate(new_trait_ref),
 991                     },
 992                     false,
 993                 );
 994             });
 995         } else {
 996             debug_assert!(self.tcx.is_trait(trait_def_id));
 997             for item in self.impl_or_trait_item(trait_def_id) {
 998                 // Check whether `trait_def_id` defines a method with suitable name.
 999                 if !self.has_applicable_self(&item) {
1000                     debug!("method has inapplicable self");
1001                     self.record_static_candidate(TraitSource(trait_def_id));
1002                     continue;
1003                 }
1004
1005                 let (xform_self_ty, xform_ret_ty) =
1006                     self.xform_self_ty(&item, trait_ref.self_ty(), trait_substs);
1007                 self.push_candidate(
1008                     Candidate {
1009                         xform_self_ty,
1010                         xform_ret_ty,
1011                         item,
1012                         import_ids: import_ids.clone(),
1013                         kind: TraitCandidate(trait_ref),
1014                     },
1015                     false,
1016                 );
1017             }
1018         }
1019     }
1020
1021     fn candidate_method_names(&self) -> Vec<Ident> {
1022         let mut set = FxHashSet::default();
1023         let mut names: Vec<_> = self
1024             .inherent_candidates
1025             .iter()
1026             .chain(&self.extension_candidates)
1027             .filter(|candidate| {
1028                 if let Some(return_ty) = self.return_type {
1029                     self.matches_return_type(&candidate.item, None, return_ty)
1030                 } else {
1031                     true
1032                 }
1033             })
1034             .map(|candidate| candidate.item.ident(self.tcx))
1035             .filter(|&name| set.insert(name))
1036             .collect();
1037
1038         // Sort them by the name so we have a stable result.
1039         names.sort_by(|a, b| a.as_str().partial_cmp(b.as_str()).unwrap());
1040         names
1041     }
1042
1043     ///////////////////////////////////////////////////////////////////////////
1044     // THE ACTUAL SEARCH
1045
1046     fn pick(mut self) -> PickResult<'tcx> {
1047         assert!(self.method_name.is_some());
1048
1049         if let Some(r) = self.pick_core() {
1050             return r;
1051         }
1052
1053         debug!("pick: actual search failed, assemble diagnostics");
1054
1055         let static_candidates = mem::take(&mut self.static_candidates);
1056         let private_candidate = self.private_candidate.take();
1057         let unsatisfied_predicates = mem::take(&mut self.unsatisfied_predicates);
1058
1059         // things failed, so lets look at all traits, for diagnostic purposes now:
1060         self.reset();
1061
1062         let span = self.span;
1063         let tcx = self.tcx;
1064
1065         self.assemble_extension_candidates_for_all_traits();
1066
1067         let out_of_scope_traits = match self.pick_core() {
1068             Some(Ok(p)) => vec![p.item.container.id()],
1069             //Some(Ok(p)) => p.iter().map(|p| p.item.container().id()).collect(),
1070             Some(Err(MethodError::Ambiguity(v))) => v
1071                 .into_iter()
1072                 .map(|source| match source {
1073                     TraitSource(id) => id,
1074                     ImplSource(impl_id) => match tcx.trait_id_of_impl(impl_id) {
1075                         Some(id) => id,
1076                         None => span_bug!(span, "found inherent method when looking at traits"),
1077                     },
1078                 })
1079                 .collect(),
1080             Some(Err(MethodError::NoMatch(NoMatchData {
1081                 out_of_scope_traits: others, ..
1082             }))) => {
1083                 assert!(others.is_empty());
1084                 vec![]
1085             }
1086             _ => vec![],
1087         };
1088
1089         if let Some((kind, def_id)) = private_candidate {
1090             return Err(MethodError::PrivateMatch(kind, def_id, out_of_scope_traits));
1091         }
1092         let lev_candidate = self.probe_for_lev_candidate()?;
1093
1094         Err(MethodError::NoMatch(NoMatchData::new(
1095             static_candidates,
1096             unsatisfied_predicates,
1097             out_of_scope_traits,
1098             lev_candidate,
1099             self.mode,
1100         )))
1101     }
1102
1103     fn pick_core(&mut self) -> Option<PickResult<'tcx>> {
1104         let mut unstable_candidates = Vec::new();
1105         let pick = self.pick_all_method(Some(&mut unstable_candidates));
1106
1107         // In this case unstable picking is done by `pick_method`.
1108         if !self.tcx.sess.opts.debugging_opts.pick_stable_methods_before_any_unstable {
1109             return pick;
1110         }
1111
1112         match pick {
1113             // Emit a lint if there are unstable candidates alongside the stable ones.
1114             //
1115             // We suppress warning if we're picking the method only because it is a
1116             // suggestion.
1117             Some(Ok(ref p)) if !self.is_suggestion.0 && !unstable_candidates.is_empty() => {
1118                 self.emit_unstable_name_collision_hint(p, &unstable_candidates);
1119                 pick
1120             }
1121             Some(_) => pick,
1122             None => self.pick_all_method(None),
1123         }
1124     }
1125
1126     fn pick_all_method(
1127         &mut self,
1128         mut unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1129     ) -> Option<PickResult<'tcx>> {
1130         let steps = self.steps.clone();
1131         steps
1132             .iter()
1133             .filter(|step| {
1134                 debug!("pick_all_method: step={:?}", step);
1135                 // skip types that are from a type error or that would require dereferencing
1136                 // a raw pointer
1137                 !step.self_ty.references_error() && !step.from_unsafe_deref
1138             })
1139             .flat_map(|step| {
1140                 let InferOk { value: self_ty, obligations: _ } = self
1141                     .fcx
1142                     .probe_instantiate_query_response(
1143                         self.span,
1144                         &self.orig_steps_var_values,
1145                         &step.self_ty,
1146                     )
1147                     .unwrap_or_else(|_| {
1148                         span_bug!(self.span, "{:?} was applicable but now isn't?", step.self_ty)
1149                     });
1150                 self.pick_by_value_method(step, self_ty, unstable_candidates.as_deref_mut())
1151                     .or_else(|| {
1152                         self.pick_autorefd_method(
1153                             step,
1154                             self_ty,
1155                             hir::Mutability::Not,
1156                             unstable_candidates.as_deref_mut(),
1157                         )
1158                         .or_else(|| {
1159                             self.pick_autorefd_method(
1160                                 step,
1161                                 self_ty,
1162                                 hir::Mutability::Mut,
1163                                 unstable_candidates.as_deref_mut(),
1164                             )
1165                         })
1166                         .or_else(|| {
1167                             self.pick_const_ptr_method(
1168                                 step,
1169                                 self_ty,
1170                                 unstable_candidates.as_deref_mut(),
1171                             )
1172                         })
1173                     })
1174             })
1175             .next()
1176     }
1177
1178     /// For each type `T` in the step list, this attempts to find a method where
1179     /// the (transformed) self type is exactly `T`. We do however do one
1180     /// transformation on the adjustment: if we are passing a region pointer in,
1181     /// we will potentially *reborrow* it to a shorter lifetime. This allows us
1182     /// to transparently pass `&mut` pointers, in particular, without consuming
1183     /// them for their entire lifetime.
1184     fn pick_by_value_method(
1185         &mut self,
1186         step: &CandidateStep<'tcx>,
1187         self_ty: Ty<'tcx>,
1188         unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1189     ) -> Option<PickResult<'tcx>> {
1190         if step.unsize {
1191             return None;
1192         }
1193
1194         self.pick_method(self_ty, unstable_candidates).map(|r| {
1195             r.map(|mut pick| {
1196                 pick.autoderefs = step.autoderefs;
1197
1198                 // Insert a `&*` or `&mut *` if this is a reference type:
1199                 if let ty::Ref(_, _, mutbl) = *step.self_ty.value.value.kind() {
1200                     pick.autoderefs += 1;
1201                     pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::Autoref {
1202                         mutbl,
1203                         unsize: pick.autoref_or_ptr_adjustment.map_or(false, |a| a.get_unsize()),
1204                     })
1205                 }
1206
1207                 pick
1208             })
1209         })
1210     }
1211
1212     fn pick_autorefd_method(
1213         &mut self,
1214         step: &CandidateStep<'tcx>,
1215         self_ty: Ty<'tcx>,
1216         mutbl: hir::Mutability,
1217         unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1218     ) -> Option<PickResult<'tcx>> {
1219         let tcx = self.tcx;
1220
1221         // In general, during probing we erase regions.
1222         let region = tcx.lifetimes.re_erased;
1223
1224         let autoref_ty = tcx.mk_ref(region, ty::TypeAndMut { ty: self_ty, mutbl });
1225         self.pick_method(autoref_ty, unstable_candidates).map(|r| {
1226             r.map(|mut pick| {
1227                 pick.autoderefs = step.autoderefs;
1228                 pick.autoref_or_ptr_adjustment =
1229                     Some(AutorefOrPtrAdjustment::Autoref { mutbl, unsize: step.unsize });
1230                 pick
1231             })
1232         })
1233     }
1234
1235     /// If `self_ty` is `*mut T` then this picks `*const T` methods. The reason why we have a
1236     /// special case for this is because going from `*mut T` to `*const T` with autoderefs and
1237     /// autorefs would require dereferencing the pointer, which is not safe.
1238     fn pick_const_ptr_method(
1239         &mut self,
1240         step: &CandidateStep<'tcx>,
1241         self_ty: Ty<'tcx>,
1242         unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1243     ) -> Option<PickResult<'tcx>> {
1244         // Don't convert an unsized reference to ptr
1245         if step.unsize {
1246             return None;
1247         }
1248
1249         let ty = match self_ty.kind() {
1250             &ty::RawPtr(ty::TypeAndMut { ty, mutbl: hir::Mutability::Mut }) => ty,
1251             _ => return None,
1252         };
1253
1254         let const_self_ty = ty::TypeAndMut { ty, mutbl: hir::Mutability::Not };
1255         let const_ptr_ty = self.tcx.mk_ptr(const_self_ty);
1256         self.pick_method(const_ptr_ty, unstable_candidates).map(|r| {
1257             r.map(|mut pick| {
1258                 pick.autoderefs = step.autoderefs;
1259                 pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::ToConstPtr);
1260                 pick
1261             })
1262         })
1263     }
1264
1265     fn pick_method_with_unstable(&mut self, self_ty: Ty<'tcx>) -> Option<PickResult<'tcx>> {
1266         debug!("pick_method_with_unstable(self_ty={})", self.ty_to_string(self_ty));
1267
1268         let mut possibly_unsatisfied_predicates = Vec::new();
1269         let mut unstable_candidates = Vec::new();
1270
1271         for (kind, candidates) in
1272             &[("inherent", &self.inherent_candidates), ("extension", &self.extension_candidates)]
1273         {
1274             debug!("searching {} candidates", kind);
1275             let res = self.consider_candidates(
1276                 self_ty,
1277                 candidates.iter(),
1278                 &mut possibly_unsatisfied_predicates,
1279                 Some(&mut unstable_candidates),
1280             );
1281             if let Some(pick) = res {
1282                 if !self.is_suggestion.0 && !unstable_candidates.is_empty() {
1283                     if let Ok(p) = &pick {
1284                         // Emit a lint if there are unstable candidates alongside the stable ones.
1285                         //
1286                         // We suppress warning if we're picking the method only because it is a
1287                         // suggestion.
1288                         self.emit_unstable_name_collision_hint(p, &unstable_candidates);
1289                     }
1290                 }
1291                 return Some(pick);
1292             }
1293         }
1294
1295         debug!("searching unstable candidates");
1296         let res = self.consider_candidates(
1297             self_ty,
1298             unstable_candidates.iter().map(|(c, _)| c),
1299             &mut possibly_unsatisfied_predicates,
1300             None,
1301         );
1302         if res.is_none() {
1303             self.unsatisfied_predicates.extend(possibly_unsatisfied_predicates);
1304         }
1305         res
1306     }
1307
1308     fn pick_method(
1309         &mut self,
1310         self_ty: Ty<'tcx>,
1311         mut unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1312     ) -> Option<PickResult<'tcx>> {
1313         if !self.tcx.sess.opts.debugging_opts.pick_stable_methods_before_any_unstable {
1314             return self.pick_method_with_unstable(self_ty);
1315         }
1316
1317         debug!("pick_method(self_ty={})", self.ty_to_string(self_ty));
1318
1319         let mut possibly_unsatisfied_predicates = Vec::new();
1320
1321         for (kind, candidates) in
1322             &[("inherent", &self.inherent_candidates), ("extension", &self.extension_candidates)]
1323         {
1324             debug!("searching {} candidates", kind);
1325             let res = self.consider_candidates(
1326                 self_ty,
1327                 candidates.iter(),
1328                 &mut possibly_unsatisfied_predicates,
1329                 unstable_candidates.as_deref_mut(),
1330             );
1331             if let Some(pick) = res {
1332                 return Some(pick);
1333             }
1334         }
1335
1336         // `pick_method` may be called twice for the same self_ty if no stable methods
1337         // match. Only extend once.
1338         if unstable_candidates.is_some() {
1339             self.unsatisfied_predicates.extend(possibly_unsatisfied_predicates);
1340         }
1341         None
1342     }
1343
1344     fn consider_candidates<'b, ProbesIter>(
1345         &self,
1346         self_ty: Ty<'tcx>,
1347         probes: ProbesIter,
1348         possibly_unsatisfied_predicates: &mut Vec<(
1349             ty::Predicate<'tcx>,
1350             Option<ty::Predicate<'tcx>>,
1351             Option<ObligationCause<'tcx>>,
1352         )>,
1353         unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
1354     ) -> Option<PickResult<'tcx>>
1355     where
1356         ProbesIter: Iterator<Item = &'b Candidate<'tcx>> + Clone,
1357         'tcx: 'b,
1358     {
1359         let mut applicable_candidates: Vec<_> = probes
1360             .clone()
1361             .map(|probe| {
1362                 (probe, self.consider_probe(self_ty, probe, possibly_unsatisfied_predicates))
1363             })
1364             .filter(|&(_, status)| status != ProbeResult::NoMatch)
1365             .collect();
1366
1367         debug!("applicable_candidates: {:?}", applicable_candidates);
1368
1369         if applicable_candidates.len() > 1 {
1370             if let Some(pick) =
1371                 self.collapse_candidates_to_trait_pick(self_ty, &applicable_candidates)
1372             {
1373                 return Some(Ok(pick));
1374             }
1375         }
1376
1377         if let Some(uc) = unstable_candidates {
1378             applicable_candidates.retain(|&(p, _)| {
1379                 if let stability::EvalResult::Deny { feature, .. } =
1380                     self.tcx.eval_stability(p.item.def_id, None, self.span, None)
1381                 {
1382                     uc.push((p.clone(), feature));
1383                     return false;
1384                 }
1385                 true
1386             });
1387         }
1388
1389         if applicable_candidates.len() > 1 {
1390             let sources = probes.map(|p| self.candidate_source(p, self_ty)).collect();
1391             return Some(Err(MethodError::Ambiguity(sources)));
1392         }
1393
1394         applicable_candidates.pop().map(|(probe, status)| {
1395             if status == ProbeResult::Match {
1396                 Ok(probe.to_unadjusted_pick(self_ty))
1397             } else {
1398                 Err(MethodError::BadReturnType)
1399             }
1400         })
1401     }
1402
1403     fn emit_unstable_name_collision_hint(
1404         &self,
1405         stable_pick: &Pick<'_>,
1406         unstable_candidates: &[(Candidate<'tcx>, Symbol)],
1407     ) {
1408         self.tcx.struct_span_lint_hir(
1409             lint::builtin::UNSTABLE_NAME_COLLISIONS,
1410             self.scope_expr_id,
1411             self.span,
1412             |lint| {
1413                 let def_kind = stable_pick.item.kind.as_def_kind();
1414                 let mut diag = lint.build(&format!(
1415                     "{} {} with this name may be added to the standard library in the future",
1416                     def_kind.article(),
1417                     def_kind.descr(stable_pick.item.def_id),
1418                 ));
1419                 match (stable_pick.item.kind, stable_pick.item.container) {
1420                     (ty::AssocKind::Fn, _) => {
1421                         // FIXME: This should be a `span_suggestion` instead of `help`
1422                         // However `self.span` only
1423                         // highlights the method name, so we can't use it. Also consider reusing
1424                         // the code from `report_method_error()`.
1425                         diag.help(&format!(
1426                             "call with fully qualified syntax `{}(...)` to keep using the current \
1427                              method",
1428                             self.tcx.def_path_str(stable_pick.item.def_id),
1429                         ));
1430                     }
1431                     (ty::AssocKind::Const, ty::AssocItemContainer::TraitContainer(def_id)) => {
1432                         diag.span_suggestion(
1433                             self.span,
1434                             "use the fully qualified path to the associated const",
1435                             format!(
1436                                 "<{} as {}>::{}",
1437                                 stable_pick.self_ty,
1438                                 self.tcx.def_path_str(def_id),
1439                                 stable_pick.item.name
1440                             ),
1441                             Applicability::MachineApplicable,
1442                         );
1443                     }
1444                     _ => {}
1445                 }
1446                 if self.tcx.sess.is_nightly_build() {
1447                     for (candidate, feature) in unstable_candidates {
1448                         diag.help(&format!(
1449                             "add `#![feature({})]` to the crate attributes to enable `{}`",
1450                             feature,
1451                             self.tcx.def_path_str(candidate.item.def_id),
1452                         ));
1453                     }
1454                 }
1455
1456                 diag.emit();
1457             },
1458         );
1459     }
1460
1461     fn select_trait_candidate(
1462         &self,
1463         trait_ref: ty::TraitRef<'tcx>,
1464     ) -> traits::SelectionResult<'tcx, traits::Selection<'tcx>> {
1465         let cause = traits::ObligationCause::misc(self.span, self.body_id);
1466         let predicate = ty::Binder::dummy(trait_ref).to_poly_trait_predicate();
1467         let obligation = traits::Obligation::new(cause, self.param_env, predicate);
1468         traits::SelectionContext::new(self).select(&obligation)
1469     }
1470
1471     fn candidate_source(&self, candidate: &Candidate<'tcx>, self_ty: Ty<'tcx>) -> CandidateSource {
1472         match candidate.kind {
1473             InherentImplCandidate(..) => ImplSource(candidate.item.container.id()),
1474             ObjectCandidate | WhereClauseCandidate(_) => TraitSource(candidate.item.container.id()),
1475             TraitCandidate(trait_ref) => self.probe(|_| {
1476                 let _ = self
1477                     .at(&ObligationCause::dummy(), self.param_env)
1478                     .sup(candidate.xform_self_ty, self_ty);
1479                 match self.select_trait_candidate(trait_ref) {
1480                     Ok(Some(traits::ImplSource::UserDefined(ref impl_data))) => {
1481                         // If only a single impl matches, make the error message point
1482                         // to that impl.
1483                         ImplSource(impl_data.impl_def_id)
1484                     }
1485                     _ => TraitSource(candidate.item.container.id()),
1486                 }
1487             }),
1488         }
1489     }
1490
1491     fn consider_probe(
1492         &self,
1493         self_ty: Ty<'tcx>,
1494         probe: &Candidate<'tcx>,
1495         possibly_unsatisfied_predicates: &mut Vec<(
1496             ty::Predicate<'tcx>,
1497             Option<ty::Predicate<'tcx>>,
1498             Option<ObligationCause<'tcx>>,
1499         )>,
1500     ) -> ProbeResult {
1501         debug!("consider_probe: self_ty={:?} probe={:?}", self_ty, probe);
1502
1503         self.probe(|_| {
1504             // First check that the self type can be related.
1505             let sub_obligations = match self
1506                 .at(&ObligationCause::dummy(), self.param_env)
1507                 .sup(probe.xform_self_ty, self_ty)
1508             {
1509                 Ok(InferOk { obligations, value: () }) => obligations,
1510                 Err(err) => {
1511                     debug!("--> cannot relate self-types {:?}", err);
1512                     return ProbeResult::NoMatch;
1513                 }
1514             };
1515
1516             let mut result = ProbeResult::Match;
1517             let mut xform_ret_ty = probe.xform_ret_ty;
1518             debug!(?xform_ret_ty);
1519
1520             let selcx = &mut traits::SelectionContext::new(self);
1521             let cause = traits::ObligationCause::misc(self.span, self.body_id);
1522
1523             // If so, impls may carry other conditions (e.g., where
1524             // clauses) that must be considered. Make sure that those
1525             // match as well (or at least may match, sometimes we
1526             // don't have enough information to fully evaluate).
1527             match probe.kind {
1528                 InherentImplCandidate(ref substs, ref ref_obligations) => {
1529                     // `xform_ret_ty` hasn't been normalized yet, only `xform_self_ty`,
1530                     // see the reasons mentioned in the comments in `assemble_inherent_impl_probe`
1531                     // for why this is necessary
1532                     let traits::Normalized {
1533                         value: normalized_xform_ret_ty,
1534                         obligations: normalization_obligations,
1535                     } = traits::normalize(selcx, self.param_env, cause.clone(), probe.xform_ret_ty);
1536                     xform_ret_ty = normalized_xform_ret_ty;
1537                     debug!("xform_ret_ty after normalization: {:?}", xform_ret_ty);
1538
1539                     // Check whether the impl imposes obligations we have to worry about.
1540                     let impl_def_id = probe.item.container.id();
1541                     let impl_bounds = self.tcx.predicates_of(impl_def_id);
1542                     let impl_bounds = impl_bounds.instantiate(self.tcx, substs);
1543                     let traits::Normalized { value: impl_bounds, obligations: norm_obligations } =
1544                         traits::normalize(selcx, self.param_env, cause.clone(), impl_bounds);
1545
1546                     // Convert the bounds into obligations.
1547                     let impl_obligations =
1548                         traits::predicates_for_generics(cause, self.param_env, impl_bounds);
1549
1550                     let candidate_obligations = impl_obligations
1551                         .chain(norm_obligations.into_iter())
1552                         .chain(ref_obligations.iter().cloned())
1553                         .chain(normalization_obligations.into_iter());
1554
1555                     // Evaluate those obligations to see if they might possibly hold.
1556                     for o in candidate_obligations {
1557                         let o = self.resolve_vars_if_possible(o);
1558                         if !self.predicate_may_hold(&o) {
1559                             result = ProbeResult::NoMatch;
1560                             possibly_unsatisfied_predicates.push((
1561                                 o.predicate,
1562                                 None,
1563                                 Some(o.cause),
1564                             ));
1565                         }
1566                     }
1567                 }
1568
1569                 ObjectCandidate | WhereClauseCandidate(..) => {
1570                     // These have no additional conditions to check.
1571                 }
1572
1573                 TraitCandidate(trait_ref) => {
1574                     if let Some(method_name) = self.method_name {
1575                         // Some trait methods are excluded for arrays before 2021.
1576                         // (`array.into_iter()` wants a slice iterator for compatibility.)
1577                         if self_ty.is_array() && !method_name.span.rust_2021() {
1578                             let trait_def = self.tcx.trait_def(trait_ref.def_id);
1579                             if trait_def.skip_array_during_method_dispatch {
1580                                 return ProbeResult::NoMatch;
1581                             }
1582                         }
1583                     }
1584                     let predicate =
1585                         ty::Binder::dummy(trait_ref).without_const().to_predicate(self.tcx);
1586                     let obligation = traits::Obligation::new(cause, self.param_env, predicate);
1587                     if !self.predicate_may_hold(&obligation) {
1588                         result = ProbeResult::NoMatch;
1589                         if self.probe(|_| {
1590                             match self.select_trait_candidate(trait_ref) {
1591                                 Err(_) => return true,
1592                                 Ok(Some(impl_source))
1593                                     if !impl_source.borrow_nested_obligations().is_empty() =>
1594                                 {
1595                                     for obligation in impl_source.borrow_nested_obligations() {
1596                                         // Determine exactly which obligation wasn't met, so
1597                                         // that we can give more context in the error.
1598                                         if !self.predicate_may_hold(obligation) {
1599                                             let nested_predicate =
1600                                                 self.resolve_vars_if_possible(obligation.predicate);
1601                                             let predicate =
1602                                                 self.resolve_vars_if_possible(predicate);
1603                                             let p = if predicate == nested_predicate {
1604                                                 // Avoid "`MyStruct: Foo` which is required by
1605                                                 // `MyStruct: Foo`" in E0599.
1606                                                 None
1607                                             } else {
1608                                                 Some(predicate)
1609                                             };
1610                                             possibly_unsatisfied_predicates.push((
1611                                                 nested_predicate,
1612                                                 p,
1613                                                 Some(obligation.cause.clone()),
1614                                             ));
1615                                         }
1616                                     }
1617                                 }
1618                                 _ => {
1619                                     // Some nested subobligation of this predicate
1620                                     // failed.
1621                                     let predicate = self.resolve_vars_if_possible(predicate);
1622                                     possibly_unsatisfied_predicates.push((predicate, None, None));
1623                                 }
1624                             }
1625                             false
1626                         }) {
1627                             // This candidate's primary obligation doesn't even
1628                             // select - don't bother registering anything in
1629                             // `potentially_unsatisfied_predicates`.
1630                             return ProbeResult::NoMatch;
1631                         }
1632                     }
1633                 }
1634             }
1635
1636             // Evaluate those obligations to see if they might possibly hold.
1637             for o in sub_obligations {
1638                 let o = self.resolve_vars_if_possible(o);
1639                 if !self.predicate_may_hold(&o) {
1640                     result = ProbeResult::NoMatch;
1641                     possibly_unsatisfied_predicates.push((o.predicate, None, Some(o.cause)));
1642                 }
1643             }
1644
1645             if let ProbeResult::Match = result {
1646                 if let (Some(return_ty), Some(xform_ret_ty)) = (self.return_type, xform_ret_ty) {
1647                     let xform_ret_ty = self.resolve_vars_if_possible(xform_ret_ty);
1648                     debug!(
1649                         "comparing return_ty {:?} with xform ret ty {:?}",
1650                         return_ty, probe.xform_ret_ty
1651                     );
1652                     if self
1653                         .at(&ObligationCause::dummy(), self.param_env)
1654                         .sup(return_ty, xform_ret_ty)
1655                         .is_err()
1656                     {
1657                         return ProbeResult::BadReturnType;
1658                     }
1659                 }
1660             }
1661
1662             result
1663         })
1664     }
1665
1666     /// Sometimes we get in a situation where we have multiple probes that are all impls of the
1667     /// same trait, but we don't know which impl to use. In this case, since in all cases the
1668     /// external interface of the method can be determined from the trait, it's ok not to decide.
1669     /// We can basically just collapse all of the probes for various impls into one where-clause
1670     /// probe. This will result in a pending obligation so when more type-info is available we can
1671     /// make the final decision.
1672     ///
1673     /// Example (`src/test/ui/method-two-trait-defer-resolution-1.rs`):
1674     ///
1675     /// ```
1676     /// trait Foo { ... }
1677     /// impl Foo for Vec<i32> { ... }
1678     /// impl Foo for Vec<usize> { ... }
1679     /// ```
1680     ///
1681     /// Now imagine the receiver is `Vec<_>`. It doesn't really matter at this time which impl we
1682     /// use, so it's ok to just commit to "using the method from the trait Foo".
1683     fn collapse_candidates_to_trait_pick(
1684         &self,
1685         self_ty: Ty<'tcx>,
1686         probes: &[(&Candidate<'tcx>, ProbeResult)],
1687     ) -> Option<Pick<'tcx>> {
1688         // Do all probes correspond to the same trait?
1689         let container = probes[0].0.item.container;
1690         if let ty::ImplContainer(_) = container {
1691             return None;
1692         }
1693         if probes[1..].iter().any(|&(p, _)| p.item.container != container) {
1694             return None;
1695         }
1696
1697         // FIXME: check the return type here somehow.
1698         // If so, just use this trait and call it a day.
1699         Some(Pick {
1700             item: probes[0].0.item,
1701             kind: TraitPick,
1702             import_ids: probes[0].0.import_ids.clone(),
1703             autoderefs: 0,
1704             autoref_or_ptr_adjustment: None,
1705             self_ty,
1706         })
1707     }
1708
1709     /// Similarly to `probe_for_return_type`, this method attempts to find the best matching
1710     /// candidate method where the method name may have been misspelt. Similarly to other
1711     /// Levenshtein based suggestions, we provide at most one such suggestion.
1712     fn probe_for_lev_candidate(&mut self) -> Result<Option<ty::AssocItem>, MethodError<'tcx>> {
1713         debug!("probing for method names similar to {:?}", self.method_name);
1714
1715         let steps = self.steps.clone();
1716         self.probe(|_| {
1717             let mut pcx = ProbeContext::new(
1718                 self.fcx,
1719                 self.span,
1720                 self.mode,
1721                 self.method_name,
1722                 self.return_type,
1723                 self.orig_steps_var_values.clone(),
1724                 steps,
1725                 IsSuggestion(true),
1726                 self.scope_expr_id,
1727             );
1728             pcx.allow_similar_names = true;
1729             pcx.assemble_inherent_candidates();
1730
1731             let method_names = pcx.candidate_method_names();
1732             pcx.allow_similar_names = false;
1733             let applicable_close_candidates: Vec<ty::AssocItem> = method_names
1734                 .iter()
1735                 .filter_map(|&method_name| {
1736                     pcx.reset();
1737                     pcx.method_name = Some(method_name);
1738                     pcx.assemble_inherent_candidates();
1739                     pcx.pick_core().and_then(|pick| pick.ok()).map(|pick| pick.item)
1740                 })
1741                 .collect();
1742
1743             if applicable_close_candidates.is_empty() {
1744                 Ok(None)
1745             } else {
1746                 let best_name = {
1747                     let names = applicable_close_candidates
1748                         .iter()
1749                         .map(|cand| cand.name)
1750                         .collect::<Vec<Symbol>>();
1751                     find_best_match_for_name(&names, self.method_name.unwrap().name, None)
1752                 }
1753                 .unwrap();
1754                 Ok(applicable_close_candidates.into_iter().find(|method| method.name == best_name))
1755             }
1756         })
1757     }
1758
1759     ///////////////////////////////////////////////////////////////////////////
1760     // MISCELLANY
1761     fn has_applicable_self(&self, item: &ty::AssocItem) -> bool {
1762         // "Fast track" -- check for usage of sugar when in method call
1763         // mode.
1764         //
1765         // In Path mode (i.e., resolving a value like `T::next`), consider any
1766         // associated value (i.e., methods, constants) but not types.
1767         match self.mode {
1768             Mode::MethodCall => item.fn_has_self_parameter,
1769             Mode::Path => match item.kind {
1770                 ty::AssocKind::Type => false,
1771                 ty::AssocKind::Fn | ty::AssocKind::Const => true,
1772             },
1773         }
1774         // FIXME -- check for types that deref to `Self`,
1775         // like `Rc<Self>` and so on.
1776         //
1777         // Note also that the current code will break if this type
1778         // includes any of the type parameters defined on the method
1779         // -- but this could be overcome.
1780     }
1781
1782     fn record_static_candidate(&mut self, source: CandidateSource) {
1783         self.static_candidates.push(source);
1784     }
1785
1786     #[instrument(level = "debug", skip(self))]
1787     fn xform_self_ty(
1788         &self,
1789         item: &ty::AssocItem,
1790         impl_ty: Ty<'tcx>,
1791         substs: SubstsRef<'tcx>,
1792     ) -> (Ty<'tcx>, Option<Ty<'tcx>>) {
1793         if item.kind == ty::AssocKind::Fn && self.mode == Mode::MethodCall {
1794             let sig = self.xform_method_sig(item.def_id, substs);
1795             (sig.inputs()[0], Some(sig.output()))
1796         } else {
1797             (impl_ty, None)
1798         }
1799     }
1800
1801     #[instrument(level = "debug", skip(self))]
1802     fn xform_method_sig(&self, method: DefId, substs: SubstsRef<'tcx>) -> ty::FnSig<'tcx> {
1803         let fn_sig = self.tcx.fn_sig(method);
1804         debug!(?fn_sig);
1805
1806         assert!(!substs.has_escaping_bound_vars());
1807
1808         // It is possible for type parameters or early-bound lifetimes
1809         // to appear in the signature of `self`. The substitutions we
1810         // are given do not include type/lifetime parameters for the
1811         // method yet. So create fresh variables here for those too,
1812         // if there are any.
1813         let generics = self.tcx.generics_of(method);
1814         assert_eq!(substs.len(), generics.parent_count as usize);
1815
1816         // Erase any late-bound regions from the method and substitute
1817         // in the values from the substitution.
1818         let xform_fn_sig = self.erase_late_bound_regions(fn_sig);
1819
1820         if generics.params.is_empty() {
1821             xform_fn_sig.subst(self.tcx, substs)
1822         } else {
1823             let substs = InternalSubsts::for_item(self.tcx, method, |param, _| {
1824                 let i = param.index as usize;
1825                 if i < substs.len() {
1826                     substs[i]
1827                 } else {
1828                     match param.kind {
1829                         GenericParamDefKind::Lifetime => {
1830                             // In general, during probe we erase regions.
1831                             self.tcx.lifetimes.re_erased.into()
1832                         }
1833                         GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
1834                             self.var_for_def(self.span, param)
1835                         }
1836                     }
1837                 }
1838             });
1839             xform_fn_sig.subst(self.tcx, substs)
1840         }
1841     }
1842
1843     /// Gets the type of an impl and generate substitutions with placeholders.
1844     fn impl_ty_and_substs(&self, impl_def_id: DefId) -> (Ty<'tcx>, SubstsRef<'tcx>) {
1845         (self.tcx.type_of(impl_def_id), self.fresh_item_substs(impl_def_id))
1846     }
1847
1848     fn fresh_item_substs(&self, def_id: DefId) -> SubstsRef<'tcx> {
1849         InternalSubsts::for_item(self.tcx, def_id, |param, _| match param.kind {
1850             GenericParamDefKind::Lifetime => self.tcx.lifetimes.re_erased.into(),
1851             GenericParamDefKind::Type { .. } => self
1852                 .next_ty_var(TypeVariableOrigin {
1853                     kind: TypeVariableOriginKind::SubstitutionPlaceholder,
1854                     span: self.tcx.def_span(def_id),
1855                 })
1856                 .into(),
1857             GenericParamDefKind::Const { .. } => {
1858                 let span = self.tcx.def_span(def_id);
1859                 let origin = ConstVariableOrigin {
1860                     kind: ConstVariableOriginKind::SubstitutionPlaceholder,
1861                     span,
1862                 };
1863                 self.next_const_var(self.tcx.type_of(param.def_id), origin).into()
1864             }
1865         })
1866     }
1867
1868     /// Replaces late-bound-regions bound by `value` with `'static` using
1869     /// `ty::erase_late_bound_regions`.
1870     ///
1871     /// This is only a reasonable thing to do during the *probe* phase, not the *confirm* phase, of
1872     /// method matching. It is reasonable during the probe phase because we don't consider region
1873     /// relationships at all. Therefore, we can just replace all the region variables with 'static
1874     /// rather than creating fresh region variables. This is nice for two reasons:
1875     ///
1876     /// 1. Because the numbers of the region variables would otherwise be fairly unique to this
1877     ///    particular method call, it winds up creating fewer types overall, which helps for memory
1878     ///    usage. (Admittedly, this is a rather small effect, though measurable.)
1879     ///
1880     /// 2. It makes it easier to deal with higher-ranked trait bounds, because we can replace any
1881     ///    late-bound regions with 'static. Otherwise, if we were going to replace late-bound
1882     ///    regions with actual region variables as is proper, we'd have to ensure that the same
1883     ///    region got replaced with the same variable, which requires a bit more coordination
1884     ///    and/or tracking the substitution and
1885     ///    so forth.
1886     fn erase_late_bound_regions<T>(&self, value: ty::Binder<'tcx, T>) -> T
1887     where
1888         T: TypeFoldable<'tcx>,
1889     {
1890         self.tcx.erase_late_bound_regions(value)
1891     }
1892
1893     /// Finds the method with the appropriate name (or return type, as the case may be). If
1894     /// `allow_similar_names` is set, find methods with close-matching names.
1895     // The length of the returned iterator is nearly always 0 or 1 and this
1896     // method is fairly hot.
1897     fn impl_or_trait_item(&self, def_id: DefId) -> SmallVec<[ty::AssocItem; 1]> {
1898         if let Some(name) = self.method_name {
1899             if self.allow_similar_names {
1900                 let max_dist = max(name.as_str().len(), 3) / 3;
1901                 self.tcx
1902                     .associated_items(def_id)
1903                     .in_definition_order()
1904                     .filter(|x| {
1905                         if x.kind.namespace() != Namespace::ValueNS {
1906                             return false;
1907                         }
1908                         match lev_distance(name.as_str(), x.name.as_str(), max_dist) {
1909                             Some(d) => d > 0,
1910                             None => false,
1911                         }
1912                     })
1913                     .copied()
1914                     .collect()
1915             } else {
1916                 self.fcx
1917                     .associated_value(def_id, name)
1918                     .map_or_else(SmallVec::new, |x| SmallVec::from_buf([x]))
1919             }
1920         } else {
1921             self.tcx.associated_items(def_id).in_definition_order().copied().collect()
1922         }
1923     }
1924 }
1925
1926 impl<'tcx> Candidate<'tcx> {
1927     fn to_unadjusted_pick(&self, self_ty: Ty<'tcx>) -> 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         }
1953     }
1954 }