compiler/rustc_hir_typeck/src/method/confirm.rs

   1 use super::{probe, MethodCallee};
   2
   3 use crate::{callee, FnCtxt};
   4 use rustc_hir as hir;
   5 use rustc_hir::def_id::DefId;
   6 use rustc_hir::GenericArg;
   7 use rustc_hir_analysis::astconv::{AstConv, CreateSubstsForGenericArgsCtxt, IsMethodCall};
   8 use rustc_infer::infer::{self, InferOk};
   9 use rustc_middle::traits::{ObligationCauseCode, UnifyReceiverContext};
  10 use rustc_middle::ty::adjustment::{Adjust, Adjustment, PointerCast};
  11 use rustc_middle::ty::adjustment::{AllowTwoPhase, AutoBorrow, AutoBorrowMutability};
  12 use rustc_middle::ty::fold::TypeFoldable;
  13 use rustc_middle::ty::subst::{self, SubstsRef};
  14 use rustc_middle::ty::{self, GenericParamDefKind, Ty};
  15 use rustc_span::Span;
  16 use rustc_trait_selection::traits;
  17
  18 use std::iter;
  19 use std::ops::Deref;
  20
  21 struct ConfirmContext<'a, 'tcx> {
  22     fcx: &'a FnCtxt<'a, 'tcx>,
  23     span: Span,
  24     self_expr: &'tcx hir::Expr<'tcx>,
  25     call_expr: &'tcx hir::Expr<'tcx>,
  26 }
  27
  28 impl<'a, 'tcx> Deref for ConfirmContext<'a, 'tcx> {
  29     type Target = FnCtxt<'a, 'tcx>;
  30     fn deref(&self) -> &Self::Target {
  31         self.fcx
  32     }
  33 }
  34
  35 #[derive(Debug)]
  36 pub struct ConfirmResult<'tcx> {
  37     pub callee: MethodCallee<'tcx>,
  38     pub illegal_sized_bound: Option<Span>,
  39 }
  40
  41 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  42     pub fn confirm_method(
  43         &self,
  44         span: Span,
  45         self_expr: &'tcx hir::Expr<'tcx>,
  46         call_expr: &'tcx hir::Expr<'tcx>,
  47         unadjusted_self_ty: Ty<'tcx>,
  48         pick: probe::Pick<'tcx>,
  49         segment: &hir::PathSegment<'_>,
  50     ) -> ConfirmResult<'tcx> {
  51         debug!(
  52             "confirm(unadjusted_self_ty={:?}, pick={:?}, generic_args={:?})",
  53             unadjusted_self_ty, pick, segment.args,
  54         );
  55
  56         let mut confirm_cx = ConfirmContext::new(self, span, self_expr, call_expr);
  57         confirm_cx.confirm(unadjusted_self_ty, pick, segment)
  58     }
  59 }
  60
  61 impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
  62     fn new(
  63         fcx: &'a FnCtxt<'a, 'tcx>,
  64         span: Span,
  65         self_expr: &'tcx hir::Expr<'tcx>,
  66         call_expr: &'tcx hir::Expr<'tcx>,
  67     ) -> ConfirmContext<'a, 'tcx> {
  68         ConfirmContext { fcx, span, self_expr, call_expr }
  69     }
  70
  71     fn confirm(
  72         &mut self,
  73         unadjusted_self_ty: Ty<'tcx>,
  74         pick: probe::Pick<'tcx>,
  75         segment: &hir::PathSegment<'_>,
  76     ) -> ConfirmResult<'tcx> {
  77         // Adjust the self expression the user provided and obtain the adjusted type.
  78         let self_ty = self.adjust_self_ty(unadjusted_self_ty, &pick);
  79
  80         // Create substitutions for the method's type parameters.
  81         let rcvr_substs = self.fresh_receiver_substs(self_ty, &pick);
  82         let all_substs = self.instantiate_method_substs(&pick, segment, rcvr_substs);
  83
  84         debug!("rcvr_substs={rcvr_substs:?}, all_substs={all_substs:?}");
  85
  86         // Create the final signature for the method, replacing late-bound regions.
  87         let (method_sig, method_predicates) = self.instantiate_method_sig(&pick, all_substs);
  88
  89         // If there is a `Self: Sized` bound and `Self` is a trait object, it is possible that
  90         // something which derefs to `Self` actually implements the trait and the caller
  91         // wanted to make a static dispatch on it but forgot to import the trait.
  92         // See test `src/test/ui/issue-35976.rs`.
  93         //
  94         // In that case, we'll error anyway, but we'll also re-run the search with all traits
  95         // in scope, and if we find another method which can be used, we'll output an
  96         // appropriate hint suggesting to import the trait.
  97         let filler_substs = rcvr_substs
  98             .extend_to(self.tcx, pick.item.def_id, |def, _| self.tcx.mk_param_from_def(def));
  99         let illegal_sized_bound = self.predicates_require_illegal_sized_bound(
 100             &self.tcx.predicates_of(pick.item.def_id).instantiate(self.tcx, filler_substs),
 101         );
 102
 103         // Unify the (adjusted) self type with what the method expects.
 104         //
 105         // SUBTLE: if we want good error messages, because of "guessing" while matching
 106         // traits, no trait system method can be called before this point because they
 107         // could alter our Self-type, except for normalizing the receiver from the
 108         // signature (which is also done during probing).
 109         let method_sig_rcvr = self.normalize_associated_types_in(self.span, method_sig.inputs()[0]);
 110         debug!(
 111             "confirm: self_ty={:?} method_sig_rcvr={:?} method_sig={:?} method_predicates={:?}",
 112             self_ty, method_sig_rcvr, method_sig, method_predicates
 113         );
 114         self.unify_receivers(self_ty, method_sig_rcvr, &pick, all_substs);
 115
 116         let (method_sig, method_predicates) =
 117             self.normalize_associated_types_in(self.span, (method_sig, method_predicates));
 118         let method_sig = ty::Binder::dummy(method_sig);
 119
 120         // Make sure nobody calls `drop()` explicitly.
 121         self.enforce_illegal_method_limitations(&pick);
 122
 123         // Add any trait/regions obligations specified on the method's type parameters.
 124         // We won't add these if we encountered an illegal sized bound, so that we can use
 125         // a custom error in that case.
 126         if illegal_sized_bound.is_none() {
 127             self.add_obligations(
 128                 self.tcx.mk_fn_ptr(method_sig),
 129                 all_substs,
 130                 method_predicates,
 131                 pick.item.def_id,
 132             );
 133         }
 134
 135         // Create the final `MethodCallee`.
 136         let callee = MethodCallee {
 137             def_id: pick.item.def_id,
 138             substs: all_substs,
 139             sig: method_sig.skip_binder(),
 140         };
 141         ConfirmResult { callee, illegal_sized_bound }
 142     }
 143
 144     ///////////////////////////////////////////////////////////////////////////
 145     // ADJUSTMENTS
 146
 147     fn adjust_self_ty(
 148         &mut self,
 149         unadjusted_self_ty: Ty<'tcx>,
 150         pick: &probe::Pick<'tcx>,
 151     ) -> Ty<'tcx> {
 152         // Commit the autoderefs by calling `autoderef` again, but this
 153         // time writing the results into the various typeck results.
 154         let mut autoderef = self.autoderef(self.call_expr.span, unadjusted_self_ty);
 155         let Some((ty, n)) = autoderef.nth(pick.autoderefs) else {
 156             return self.tcx.ty_error_with_message(
 157                 rustc_span::DUMMY_SP,
 158                 &format!("failed autoderef {}", pick.autoderefs),
 159             );
 160         };
 161         assert_eq!(n, pick.autoderefs);
 162
 163         let mut adjustments = self.adjust_steps(&autoderef);
 164         let mut target = self.structurally_resolved_type(autoderef.span(), ty);
 165
 166         match pick.autoref_or_ptr_adjustment {
 167             Some(probe::AutorefOrPtrAdjustment::Autoref { mutbl, unsize }) => {
 168                 let region = self.next_region_var(infer::Autoref(self.span));
 169                 // Type we're wrapping in a reference, used later for unsizing
 170                 let base_ty = target;
 171
 172                 target = self.tcx.mk_ref(region, ty::TypeAndMut { mutbl, ty: target });
 173                 let mutbl = match mutbl {
 174                     hir::Mutability::Not => AutoBorrowMutability::Not,
 175                     hir::Mutability::Mut => AutoBorrowMutability::Mut {
 176                         // Method call receivers are the primary use case
 177                         // for two-phase borrows.
 178                         allow_two_phase_borrow: AllowTwoPhase::Yes,
 179                     },
 180                 };
 181                 adjustments.push(Adjustment {
 182                     kind: Adjust::Borrow(AutoBorrow::Ref(region, mutbl)),
 183                     target,
 184                 });
 185
 186                 if unsize {
 187                     let unsized_ty = if let ty::Array(elem_ty, _) = base_ty.kind() {
 188                         self.tcx.mk_slice(*elem_ty)
 189                     } else {
 190                         bug!(
 191                             "AutorefOrPtrAdjustment's unsize flag should only be set for array ty, found {}",
 192                             base_ty
 193                         )
 194                     };
 195                     target = self
 196                         .tcx
 197                         .mk_ref(region, ty::TypeAndMut { mutbl: mutbl.into(), ty: unsized_ty });
 198                     adjustments
 199                         .push(Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target });
 200                 }
 201             }
 202             Some(probe::AutorefOrPtrAdjustment::ToConstPtr) => {
 203                 target = match target.kind() {
 204                     &ty::RawPtr(ty::TypeAndMut { ty, mutbl }) => {
 205                         assert_eq!(mutbl, hir::Mutability::Mut);
 206                         self.tcx.mk_ptr(ty::TypeAndMut { mutbl: hir::Mutability::Not, ty })
 207                     }
 208                     other => panic!("Cannot adjust receiver type {:?} to const ptr", other),
 209                 };
 210
 211                 adjustments.push(Adjustment {
 212                     kind: Adjust::Pointer(PointerCast::MutToConstPointer),
 213                     target,
 214                 });
 215             }
 216             None => {}
 217         }
 218
 219         self.register_predicates(autoderef.into_obligations());
 220
 221         // Write out the final adjustments.
 222         self.apply_adjustments(self.self_expr, adjustments);
 223
 224         target
 225     }
 226
 227     /// Returns a set of substitutions for the method *receiver* where all type and region
 228     /// parameters are instantiated with fresh variables. This substitution does not include any
 229     /// parameters declared on the method itself.
 230     ///
 231     /// Note that this substitution may include late-bound regions from the impl level. If so,
 232     /// these are instantiated later in the `instantiate_method_sig` routine.
 233     fn fresh_receiver_substs(
 234         &mut self,
 235         self_ty: Ty<'tcx>,
 236         pick: &probe::Pick<'tcx>,
 237     ) -> SubstsRef<'tcx> {
 238         match pick.kind {
 239             probe::InherentImplPick => {
 240                 let impl_def_id = pick.item.container_id(self.tcx);
 241                 assert!(
 242                     self.tcx.impl_trait_ref(impl_def_id).is_none(),
 243                     "impl {:?} is not an inherent impl",
 244                     impl_def_id
 245                 );
 246                 self.fresh_substs_for_item(self.span, impl_def_id)
 247             }
 248
 249             probe::ObjectPick => {
 250                 let trait_def_id = pick.item.container_id(self.tcx);
 251                 self.extract_existential_trait_ref(self_ty, |this, object_ty, principal| {
 252                     // The object data has no entry for the Self
 253                     // Type. For the purposes of this method call, we
 254                     // substitute the object type itself. This
 255                     // wouldn't be a sound substitution in all cases,
 256                     // since each instance of the object type is a
 257                     // different existential and hence could match
 258                     // distinct types (e.g., if `Self` appeared as an
 259                     // argument type), but those cases have already
 260                     // been ruled out when we deemed the trait to be
 261                     // "object safe".
 262                     let original_poly_trait_ref = principal.with_self_ty(this.tcx, object_ty);
 263                     let upcast_poly_trait_ref = this.upcast(original_poly_trait_ref, trait_def_id);
 264                     let upcast_trait_ref =
 265                         this.replace_bound_vars_with_fresh_vars(upcast_poly_trait_ref);
 266                     debug!(
 267                         "original_poly_trait_ref={:?} upcast_trait_ref={:?} target_trait={:?}",
 268                         original_poly_trait_ref, upcast_trait_ref, trait_def_id
 269                     );
 270                     upcast_trait_ref.substs
 271                 })
 272             }
 273
 274             probe::TraitPick => {
 275                 let trait_def_id = pick.item.container_id(self.tcx);
 276
 277                 // Make a trait reference `$0 : Trait<$1...$n>`
 278                 // consisting entirely of type variables. Later on in
 279                 // the process we will unify the transformed-self-type
 280                 // of the method with the actual type in order to
 281                 // unify some of these variables.
 282                 self.fresh_substs_for_item(self.span, trait_def_id)
 283             }
 284
 285             probe::WhereClausePick(poly_trait_ref) => {
 286                 // Where clauses can have bound regions in them. We need to instantiate
 287                 // those to convert from a poly-trait-ref to a trait-ref.
 288                 self.replace_bound_vars_with_fresh_vars(poly_trait_ref).substs
 289             }
 290         }
 291     }
 292
 293     fn extract_existential_trait_ref<R, F>(&mut self, self_ty: Ty<'tcx>, mut closure: F) -> R
 294     where
 295         F: FnMut(&mut ConfirmContext<'a, 'tcx>, Ty<'tcx>, ty::PolyExistentialTraitRef<'tcx>) -> R,
 296     {
 297         // If we specified that this is an object method, then the
 298         // self-type ought to be something that can be dereferenced to
 299         // yield an object-type (e.g., `&Object` or `Box<Object>`
 300         // etc).
 301
 302         // FIXME: this feels, like, super dubious
 303         self.fcx
 304             .autoderef(self.span, self_ty)
 305             .include_raw_pointers()
 306             .find_map(|(ty, _)| match ty.kind() {
 307                 ty::Dynamic(data, ..) => Some(closure(
 308                     self,
 309                     ty,
 310                     data.principal().unwrap_or_else(|| {
 311                         span_bug!(self.span, "calling trait method on empty object?")
 312                     }),
 313                 )),
 314                 _ => None,
 315             })
 316             .unwrap_or_else(|| {
 317                 span_bug!(
 318                     self.span,
 319                     "self-type `{}` for ObjectPick never dereferenced to an object",
 320                     self_ty
 321                 )
 322             })
 323     }
 324
 325     fn instantiate_method_substs(
 326         &mut self,
 327         pick: &probe::Pick<'tcx>,
 328         seg: &hir::PathSegment<'_>,
 329         parent_substs: SubstsRef<'tcx>,
 330     ) -> SubstsRef<'tcx> {
 331         // Determine the values for the generic parameters of the method.
 332         // If they were not explicitly supplied, just construct fresh
 333         // variables.
 334         let generics = self.tcx.generics_of(pick.item.def_id);
 335
 336         let arg_count_correct = <dyn AstConv<'_>>::check_generic_arg_count_for_call(
 337             self.tcx,
 338             self.span,
 339             pick.item.def_id,
 340             generics,
 341             seg,
 342             IsMethodCall::Yes,
 343         );
 344
 345         // Create subst for early-bound lifetime parameters, combining
 346         // parameters from the type and those from the method.
 347         assert_eq!(generics.parent_count, parent_substs.len());
 348
 349         struct MethodSubstsCtxt<'a, 'tcx> {
 350             cfcx: &'a ConfirmContext<'a, 'tcx>,
 351             pick: &'a probe::Pick<'tcx>,
 352             seg: &'a hir::PathSegment<'a>,
 353         }
 354         impl<'a, 'tcx> CreateSubstsForGenericArgsCtxt<'a, 'tcx> for MethodSubstsCtxt<'a, 'tcx> {
 355             fn args_for_def_id(
 356                 &mut self,
 357                 def_id: DefId,
 358             ) -> (Option<&'a hir::GenericArgs<'a>>, bool) {
 359                 if def_id == self.pick.item.def_id {
 360                     if let Some(data) = self.seg.args {
 361                         return (Some(data), false);
 362                     }
 363                 }
 364                 (None, false)
 365             }
 366
 367             fn provided_kind(
 368                 &mut self,
 369                 param: &ty::GenericParamDef,
 370                 arg: &GenericArg<'_>,
 371             ) -> subst::GenericArg<'tcx> {
 372                 match (&param.kind, arg) {
 373                     (GenericParamDefKind::Lifetime, GenericArg::Lifetime(lt)) => {
 374                         <dyn AstConv<'_>>::ast_region_to_region(self.cfcx.fcx, lt, Some(param))
 375                             .into()
 376                     }
 377                     (GenericParamDefKind::Type { .. }, GenericArg::Type(ty)) => {
 378                         self.cfcx.to_ty(ty).into()
 379                     }
 380                     (GenericParamDefKind::Const { .. }, GenericArg::Const(ct)) => {
 381                         self.cfcx.const_arg_to_const(&ct.value, param.def_id).into()
 382                     }
 383                     (GenericParamDefKind::Type { .. }, GenericArg::Infer(inf)) => {
 384                         self.cfcx.ty_infer(Some(param), inf.span).into()
 385                     }
 386                     (GenericParamDefKind::Const { .. }, GenericArg::Infer(inf)) => {
 387                         let tcx = self.cfcx.tcx();
 388                         self.cfcx.ct_infer(tcx.type_of(param.def_id), Some(param), inf.span).into()
 389                     }
 390                     _ => unreachable!(),
 391                 }
 392             }
 393
 394             fn inferred_kind(
 395                 &mut self,
 396                 _substs: Option<&[subst::GenericArg<'tcx>]>,
 397                 param: &ty::GenericParamDef,
 398                 _infer_args: bool,
 399             ) -> subst::GenericArg<'tcx> {
 400                 self.cfcx.var_for_def(self.cfcx.span, param)
 401             }
 402         }
 403         <dyn AstConv<'_>>::create_substs_for_generic_args(
 404             self.tcx,
 405             pick.item.def_id,
 406             parent_substs,
 407             false,
 408             None,
 409             &arg_count_correct,
 410             &mut MethodSubstsCtxt { cfcx: self, pick, seg },
 411         )
 412     }
 413
 414     fn unify_receivers(
 415         &mut self,
 416         self_ty: Ty<'tcx>,
 417         method_self_ty: Ty<'tcx>,
 418         pick: &probe::Pick<'tcx>,
 419         substs: SubstsRef<'tcx>,
 420     ) {
 421         debug!(
 422             "unify_receivers: self_ty={:?} method_self_ty={:?} span={:?} pick={:?}",
 423             self_ty, method_self_ty, self.span, pick
 424         );
 425         let cause = self.cause(
 426             self.span,
 427             ObligationCauseCode::UnifyReceiver(Box::new(UnifyReceiverContext {
 428                 assoc_item: pick.item,
 429                 param_env: self.param_env,
 430                 substs,
 431             })),
 432         );
 433         match self.at(&cause, self.param_env).sup(method_self_ty, self_ty) {
 434             Ok(InferOk { obligations, value: () }) => {
 435                 self.register_predicates(obligations);
 436             }
 437             Err(_) => {
 438                 span_bug!(
 439                     self.span,
 440                     "{} was a subtype of {} but now is not?",
 441                     self_ty,
 442                     method_self_ty
 443                 );
 444             }
 445         }
 446     }
 447
 448     // NOTE: this returns the *unnormalized* predicates and method sig. Because of
 449     // inference guessing, the predicates and method signature can't be normalized
 450     // until we unify the `Self` type.
 451     fn instantiate_method_sig(
 452         &mut self,
 453         pick: &probe::Pick<'tcx>,
 454         all_substs: SubstsRef<'tcx>,
 455     ) -> (ty::FnSig<'tcx>, ty::InstantiatedPredicates<'tcx>) {
 456         debug!("instantiate_method_sig(pick={:?}, all_substs={:?})", pick, all_substs);
 457
 458         // Instantiate the bounds on the method with the
 459         // type/early-bound-regions substitutions performed. There can
 460         // be no late-bound regions appearing here.
 461         let def_id = pick.item.def_id;
 462         let method_predicates = self.tcx.predicates_of(def_id).instantiate(self.tcx, all_substs);
 463
 464         debug!("method_predicates after subst = {:?}", method_predicates);
 465
 466         let sig = self.tcx.bound_fn_sig(def_id);
 467
 468         let sig = sig.subst(self.tcx, all_substs);
 469         debug!("type scheme substituted, sig={:?}", sig);
 470
 471         let sig = self.replace_bound_vars_with_fresh_vars(sig);
 472         debug!("late-bound lifetimes from method instantiated, sig={:?}", sig);
 473
 474         (sig, method_predicates)
 475     }
 476
 477     fn add_obligations(
 478         &mut self,
 479         fty: Ty<'tcx>,
 480         all_substs: SubstsRef<'tcx>,
 481         method_predicates: ty::InstantiatedPredicates<'tcx>,
 482         def_id: DefId,
 483     ) {
 484         debug!(
 485             "add_obligations: fty={:?} all_substs={:?} method_predicates={:?} def_id={:?}",
 486             fty, all_substs, method_predicates, def_id
 487         );
 488
 489         // FIXME: could replace with the following, but we already calculated `method_predicates`,
 490         // so we just call `predicates_for_generics` directly to avoid redoing work.
 491         // `self.add_required_obligations(self.span, def_id, &all_substs);`
 492         for obligation in traits::predicates_for_generics(
 493             |idx, span| {
 494                 let code = if span.is_dummy() {
 495                     ObligationCauseCode::ExprItemObligation(def_id, self.call_expr.hir_id, idx)
 496                 } else {
 497                     ObligationCauseCode::ExprBindingObligation(
 498                         def_id,
 499                         span,
 500                         self.call_expr.hir_id,
 501                         idx,
 502                     )
 503                 };
 504                 traits::ObligationCause::new(self.span, self.body_id, code)
 505             },
 506             self.param_env,
 507             method_predicates,
 508         ) {
 509             self.register_predicate(obligation);
 510         }
 511
 512         // this is a projection from a trait reference, so we have to
 513         // make sure that the trait reference inputs are well-formed.
 514         self.add_wf_bounds(all_substs, self.call_expr);
 515
 516         // the function type must also be well-formed (this is not
 517         // implied by the substs being well-formed because of inherent
 518         // impls and late-bound regions - see issue #28609).
 519         self.register_wf_obligation(fty.into(), self.span, traits::WellFormed(None));
 520     }
 521
 522     ///////////////////////////////////////////////////////////////////////////
 523     // MISCELLANY
 524
 525     fn predicates_require_illegal_sized_bound(
 526         &self,
 527         predicates: &ty::InstantiatedPredicates<'tcx>,
 528     ) -> Option<Span> {
 529         let sized_def_id = self.tcx.lang_items().sized_trait()?;
 530
 531         traits::elaborate_predicates(self.tcx, predicates.predicates.iter().copied())
 532             // We don't care about regions here.
 533             .filter_map(|obligation| match obligation.predicate.kind().skip_binder() {
 534                 ty::PredicateKind::Trait(trait_pred) if trait_pred.def_id() == sized_def_id => {
 535                     let span = iter::zip(&predicates.predicates, &predicates.spans)
 536                         .find_map(
 537                             |(p, span)| {
 538                                 if *p == obligation.predicate { Some(*span) } else { None }
 539                             },
 540                         )
 541                         .unwrap_or(rustc_span::DUMMY_SP);
 542                     Some((trait_pred, span))
 543                 }
 544                 _ => None,
 545             })
 546             .find_map(|(trait_pred, span)| match trait_pred.self_ty().kind() {
 547                 ty::Dynamic(..) => Some(span),
 548                 _ => None,
 549             })
 550     }
 551
 552     fn enforce_illegal_method_limitations(&self, pick: &probe::Pick<'_>) {
 553         // Disallow calls to the method `drop` defined in the `Drop` trait.
 554         if let Some(trait_def_id) = pick.item.trait_container(self.tcx) {
 555             callee::check_legal_trait_for_method_call(
 556                 self.tcx,
 557                 self.span,
 558                 Some(self.self_expr.span),
 559                 self.call_expr.span,
 560                 trait_def_id,
 561             )
 562         }
 563     }
 564
 565     fn upcast(
 566         &mut self,
 567         source_trait_ref: ty::PolyTraitRef<'tcx>,
 568         target_trait_def_id: DefId,
 569     ) -> ty::PolyTraitRef<'tcx> {
 570         let upcast_trait_refs =
 571             traits::upcast_choices(self.tcx, source_trait_ref, target_trait_def_id);
 572
 573         // must be exactly one trait ref or we'd get an ambig error etc
 574         if upcast_trait_refs.len() != 1 {
 575             span_bug!(
 576                 self.span,
 577                 "cannot uniquely upcast `{:?}` to `{:?}`: `{:?}`",
 578                 source_trait_ref,
 579                 target_trait_def_id,
 580                 upcast_trait_refs
 581             );
 582         }
 583
 584         upcast_trait_refs.into_iter().next().unwrap()
 585     }
 586
 587     fn replace_bound_vars_with_fresh_vars<T>(&self, value: ty::Binder<'tcx, T>) -> T
 588     where
 589         T: TypeFoldable<'tcx> + Copy,
 590     {
 591         self.fcx.replace_bound_vars_with_fresh_vars(self.span, infer::FnCall, value)
 592     }
 593 }