compiler/rustc_typeck/src/astconv/mod.rs

   1 //! Conversion from AST representation of types to the `ty.rs` representation.
   2 //! The main routine here is `ast_ty_to_ty()`; each use is parameterized by an
   3 //! instance of `AstConv`.
   4
   5 mod errors;
   6 mod generics;
   7
   8 use crate::bounds::Bounds;
   9 use crate::collect::HirPlaceholderCollector;
  10 use crate::errors::{
  11     AmbiguousLifetimeBound, MultipleRelaxedDefaultBounds, TraitObjectDeclaredWithNoTraits,
  12     TypeofReservedKeywordUsed, ValueOfAssociatedStructAlreadySpecified,
  13 };
  14 use crate::middle::resolve_lifetime as rl;
  15 use crate::require_c_abi_if_c_variadic;
  16 use rustc_ast::TraitObjectSyntax;
  17 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  18 use rustc_errors::{
  19     struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, FatalError,
  20     MultiSpan,
  21 };
  22 use rustc_hir as hir;
  23 use rustc_hir::def::{CtorOf, DefKind, Namespace, Res};
  24 use rustc_hir::def_id::{DefId, LocalDefId};
  25 use rustc_hir::intravisit::{walk_generics, Visitor as _};
  26 use rustc_hir::lang_items::LangItem;
  27 use rustc_hir::{GenericArg, GenericArgs, OpaqueTyOrigin};
  28 use rustc_middle::middle::stability::AllowUnstable;
  29 use rustc_middle::ty::subst::{self, GenericArgKind, InternalSubsts, Subst, SubstsRef};
  30 use rustc_middle::ty::GenericParamDefKind;
  31 use rustc_middle::ty::{
  32     self, Const, DefIdTree, EarlyBinder, IsSuggestable, Ty, TyCtxt, TypeVisitable,
  33 };
  34 use rustc_session::lint::builtin::{AMBIGUOUS_ASSOCIATED_ITEMS, BARE_TRAIT_OBJECTS};
  35 use rustc_span::edition::Edition;
  36 use rustc_span::lev_distance::find_best_match_for_name;
  37 use rustc_span::symbol::{kw, Ident, Symbol};
  38 use rustc_span::{Span, DUMMY_SP};
  39 use rustc_target::spec::abi;
  40 use rustc_trait_selection::traits;
  41 use rustc_trait_selection::traits::astconv_object_safety_violations;
  42 use rustc_trait_selection::traits::error_reporting::{
  43     report_object_safety_error, suggestions::NextTypeParamName,
  44 };
  45 use rustc_trait_selection::traits::wf::object_region_bounds;
  46
  47 use smallvec::{smallvec, SmallVec};
  48 use std::collections::BTreeSet;
  49 use std::slice;
  50
  51 #[derive(Debug)]
  52 pub struct PathSeg(pub DefId, pub usize);
  53
  54 pub trait AstConv<'tcx> {
  55     fn tcx<'a>(&'a self) -> TyCtxt<'tcx>;
  56
  57     fn item_def_id(&self) -> Option<DefId>;
  58
  59     /// Returns predicates in scope of the form `X: Foo<T>`, where `X`
  60     /// is a type parameter `X` with the given id `def_id` and T
  61     /// matches `assoc_name`. This is a subset of the full set of
  62     /// predicates.
  63     ///
  64     /// This is used for one specific purpose: resolving "short-hand"
  65     /// associated type references like `T::Item`. In principle, we
  66     /// would do that by first getting the full set of predicates in
  67     /// scope and then filtering down to find those that apply to `T`,
  68     /// but this can lead to cycle errors. The problem is that we have
  69     /// to do this resolution *in order to create the predicates in
  70     /// the first place*. Hence, we have this "special pass".
  71     fn get_type_parameter_bounds(
  72         &self,
  73         span: Span,
  74         def_id: DefId,
  75         assoc_name: Ident,
  76     ) -> ty::GenericPredicates<'tcx>;
  77
  78     /// Returns the lifetime to use when a lifetime is omitted (and not elided).
  79     fn re_infer(&self, param: Option<&ty::GenericParamDef>, span: Span)
  80     -> Option<ty::Region<'tcx>>;
  81
  82     /// Returns the type to use when a type is omitted.
  83     fn ty_infer(&self, param: Option<&ty::GenericParamDef>, span: Span) -> Ty<'tcx>;
  84
  85     /// Returns `true` if `_` is allowed in type signatures in the current context.
  86     fn allow_ty_infer(&self) -> bool;
  87
  88     /// Returns the const to use when a const is omitted.
  89     fn ct_infer(
  90         &self,
  91         ty: Ty<'tcx>,
  92         param: Option<&ty::GenericParamDef>,
  93         span: Span,
  94     ) -> Const<'tcx>;
  95
  96     /// Projecting an associated type from a (potentially)
  97     /// higher-ranked trait reference is more complicated, because of
  98     /// the possibility of late-bound regions appearing in the
  99     /// associated type binding. This is not legal in function
 100     /// signatures for that reason. In a function body, we can always
 101     /// handle it because we can use inference variables to remove the
 102     /// late-bound regions.
 103     fn projected_ty_from_poly_trait_ref(
 104         &self,
 105         span: Span,
 106         item_def_id: DefId,
 107         item_segment: &hir::PathSegment<'_>,
 108         poly_trait_ref: ty::PolyTraitRef<'tcx>,
 109     ) -> Ty<'tcx>;
 110
 111     /// Normalize an associated type coming from the user.
 112     fn normalize_ty(&self, span: Span, ty: Ty<'tcx>) -> Ty<'tcx>;
 113
 114     /// Invoked when we encounter an error from some prior pass
 115     /// (e.g., resolve) that is translated into a ty-error. This is
 116     /// used to help suppress derived errors typeck might otherwise
 117     /// report.
 118     fn set_tainted_by_errors(&self);
 119
 120     fn record_ty(&self, hir_id: hir::HirId, ty: Ty<'tcx>, span: Span);
 121 }
 122
 123 #[derive(Debug)]
 124 struct ConvertedBinding<'a, 'tcx> {
 125     hir_id: hir::HirId,
 126     item_name: Ident,
 127     kind: ConvertedBindingKind<'a, 'tcx>,
 128     gen_args: &'a GenericArgs<'a>,
 129     span: Span,
 130 }
 131
 132 #[derive(Debug)]
 133 enum ConvertedBindingKind<'a, 'tcx> {
 134     Equality(ty::Term<'tcx>),
 135     Constraint(&'a [hir::GenericBound<'a>]),
 136 }
 137
 138 /// New-typed boolean indicating whether explicit late-bound lifetimes
 139 /// are present in a set of generic arguments.
 140 ///
 141 /// For example if we have some method `fn f<'a>(&'a self)` implemented
 142 /// for some type `T`, although `f` is generic in the lifetime `'a`, `'a`
 143 /// is late-bound so should not be provided explicitly. Thus, if `f` is
 144 /// instantiated with some generic arguments providing `'a` explicitly,
 145 /// we taint those arguments with `ExplicitLateBound::Yes` so that we
 146 /// can provide an appropriate diagnostic later.
 147 #[derive(Copy, Clone, PartialEq)]
 148 pub enum ExplicitLateBound {
 149     Yes,
 150     No,
 151 }
 152
 153 #[derive(Copy, Clone, PartialEq)]
 154 pub enum IsMethodCall {
 155     Yes,
 156     No,
 157 }
 158
 159 /// Denotes the "position" of a generic argument, indicating if it is a generic type,
 160 /// generic function or generic method call.
 161 #[derive(Copy, Clone, PartialEq)]
 162 pub(crate) enum GenericArgPosition {
 163     Type,
 164     Value, // e.g., functions
 165     MethodCall,
 166 }
 167
 168 /// A marker denoting that the generic arguments that were
 169 /// provided did not match the respective generic parameters.
 170 #[derive(Clone, Default)]
 171 pub struct GenericArgCountMismatch {
 172     /// Indicates whether a fatal error was reported (`Some`), or just a lint (`None`).
 173     pub reported: Option<ErrorGuaranteed>,
 174     /// A list of spans of arguments provided that were not valid.
 175     pub invalid_args: Vec<Span>,
 176 }
 177
 178 /// Decorates the result of a generic argument count mismatch
 179 /// check with whether explicit late bounds were provided.
 180 #[derive(Clone)]
 181 pub struct GenericArgCountResult {
 182     pub explicit_late_bound: ExplicitLateBound,
 183     pub correct: Result<(), GenericArgCountMismatch>,
 184 }
 185
 186 pub trait CreateSubstsForGenericArgsCtxt<'a, 'tcx> {
 187     fn args_for_def_id(&mut self, def_id: DefId) -> (Option<&'a GenericArgs<'a>>, bool);
 188
 189     fn provided_kind(
 190         &mut self,
 191         param: &ty::GenericParamDef,
 192         arg: &GenericArg<'_>,
 193     ) -> subst::GenericArg<'tcx>;
 194
 195     fn inferred_kind(
 196         &mut self,
 197         substs: Option<&[subst::GenericArg<'tcx>]>,
 198         param: &ty::GenericParamDef,
 199         infer_args: bool,
 200     ) -> subst::GenericArg<'tcx>;
 201 }
 202
 203 impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 204     #[tracing::instrument(level = "debug", skip(self))]
 205     pub fn ast_region_to_region(
 206         &self,
 207         lifetime: &hir::Lifetime,
 208         def: Option<&ty::GenericParamDef>,
 209     ) -> ty::Region<'tcx> {
 210         let tcx = self.tcx();
 211         let lifetime_name = |def_id| tcx.hir().name(tcx.hir().local_def_id_to_hir_id(def_id));
 212
 213         let r = match tcx.named_region(lifetime.hir_id) {
 214             Some(rl::Region::Static) => tcx.lifetimes.re_static,
 215
 216             Some(rl::Region::LateBound(debruijn, index, def_id)) => {
 217                 let name = lifetime_name(def_id.expect_local());
 218                 let br = ty::BoundRegion {
 219                     var: ty::BoundVar::from_u32(index),
 220                     kind: ty::BrNamed(def_id, name),
 221                 };
 222                 tcx.mk_region(ty::ReLateBound(debruijn, br))
 223             }
 224
 225             Some(rl::Region::EarlyBound(index, id)) => {
 226                 let name = lifetime_name(id.expect_local());
 227                 tcx.mk_region(ty::ReEarlyBound(ty::EarlyBoundRegion { def_id: id, index, name }))
 228             }
 229
 230             Some(rl::Region::Free(scope, id)) => {
 231                 let name = lifetime_name(id.expect_local());
 232                 tcx.mk_region(ty::ReFree(ty::FreeRegion {
 233                     scope,
 234                     bound_region: ty::BrNamed(id, name),
 235                 }))
 236
 237                 // (*) -- not late-bound, won't change
 238             }
 239
 240             None => {
 241                 self.re_infer(def, lifetime.span).unwrap_or_else(|| {
 242                     debug!(?lifetime, "unelided lifetime in signature");
 243
 244                     // This indicates an illegal lifetime
 245                     // elision. `resolve_lifetime` should have
 246                     // reported an error in this case -- but if
 247                     // not, let's error out.
 248                     tcx.sess.delay_span_bug(lifetime.span, "unelided lifetime in signature");
 249
 250                     // Supply some dummy value. We don't have an
 251                     // `re_error`, annoyingly, so use `'static`.
 252                     tcx.lifetimes.re_static
 253                 })
 254             }
 255         };
 256
 257         debug!("ast_region_to_region(lifetime={:?}) yields {:?}", lifetime, r);
 258
 259         r
 260     }
 261
 262     /// Given a path `path` that refers to an item `I` with the declared generics `decl_generics`,
 263     /// returns an appropriate set of substitutions for this particular reference to `I`.
 264     pub fn ast_path_substs_for_ty(
 265         &self,
 266         span: Span,
 267         def_id: DefId,
 268         item_segment: &hir::PathSegment<'_>,
 269     ) -> SubstsRef<'tcx> {
 270         let (substs, _) = self.create_substs_for_ast_path(
 271             span,
 272             def_id,
 273             &[],
 274             item_segment,
 275             item_segment.args(),
 276             item_segment.infer_args,
 277             None,
 278         );
 279         let assoc_bindings = self.create_assoc_bindings_for_generic_args(item_segment.args());
 280
 281         if let Some(b) = assoc_bindings.first() {
 282             Self::prohibit_assoc_ty_binding(self.tcx(), b.span);
 283         }
 284
 285         substs
 286     }
 287
 288     /// Given the type/lifetime/const arguments provided to some path (along with
 289     /// an implicit `Self`, if this is a trait reference), returns the complete
 290     /// set of substitutions. This may involve applying defaulted type parameters.
 291     /// Constraints on associated types are created from `create_assoc_bindings_for_generic_args`.
 292     ///
 293     /// Example:
 294     ///
 295     /// ```ignore (illustrative)
 296     ///    T: std::ops::Index<usize, Output = u32>
 297     /// // ^1 ^^^^^^^^^^^^^^2 ^^^^3  ^^^^^^^^^^^4
 298     /// ```
 299     ///
 300     /// 1. The `self_ty` here would refer to the type `T`.
 301     /// 2. The path in question is the path to the trait `std::ops::Index`,
 302     ///    which will have been resolved to a `def_id`
 303     /// 3. The `generic_args` contains info on the `<...>` contents. The `usize` type
 304     ///    parameters are returned in the `SubstsRef`, the associated type bindings like
 305     ///    `Output = u32` are returned from `create_assoc_bindings_for_generic_args`.
 306     ///
 307     /// Note that the type listing given here is *exactly* what the user provided.
 308     ///
 309     /// For (generic) associated types
 310     ///
 311     /// ```ignore (illustrative)
 312     /// <Vec<u8> as Iterable<u8>>::Iter::<'a>
 313     /// ```
 314     ///
 315     /// We have the parent substs are the substs for the parent trait:
 316     /// `[Vec<u8>, u8]` and `generic_args` are the arguments for the associated
 317     /// type itself: `['a]`. The returned `SubstsRef` concatenates these two
 318     /// lists: `[Vec<u8>, u8, 'a]`.
 319     #[tracing::instrument(level = "debug", skip(self, span))]
 320     fn create_substs_for_ast_path<'a>(
 321         &self,
 322         span: Span,
 323         def_id: DefId,
 324         parent_substs: &[subst::GenericArg<'tcx>],
 325         seg: &hir::PathSegment<'_>,
 326         generic_args: &'a hir::GenericArgs<'_>,
 327         infer_args: bool,
 328         self_ty: Option<Ty<'tcx>>,
 329     ) -> (SubstsRef<'tcx>, GenericArgCountResult) {
 330         // If the type is parameterized by this region, then replace this
 331         // region with the current anon region binding (in other words,
 332         // whatever & would get replaced with).
 333
 334         let tcx = self.tcx();
 335         let generics = tcx.generics_of(def_id);
 336         debug!("generics: {:?}", generics);
 337
 338         if generics.has_self {
 339             if generics.parent.is_some() {
 340                 // The parent is a trait so it should have at least one subst
 341                 // for the `Self` type.
 342                 assert!(!parent_substs.is_empty())
 343             } else {
 344                 // This item (presumably a trait) needs a self-type.
 345                 assert!(self_ty.is_some());
 346             }
 347         } else {
 348             assert!(self_ty.is_none() && parent_substs.is_empty());
 349         }
 350
 351         let arg_count = Self::check_generic_arg_count(
 352             tcx,
 353             span,
 354             def_id,
 355             seg,
 356             generics,
 357             generic_args,
 358             GenericArgPosition::Type,
 359             self_ty.is_some(),
 360             infer_args,
 361         );
 362
 363         // Skip processing if type has no generic parameters.
 364         // Traits always have `Self` as a generic parameter, which means they will not return early
 365         // here and so associated type bindings will be handled regardless of whether there are any
 366         // non-`Self` generic parameters.
 367         if generics.params.is_empty() {
 368             return (tcx.intern_substs(&[]), arg_count);
 369         }
 370
 371         struct SubstsForAstPathCtxt<'a, 'tcx> {
 372             astconv: &'a (dyn AstConv<'tcx> + 'a),
 373             def_id: DefId,
 374             generic_args: &'a GenericArgs<'a>,
 375             span: Span,
 376             inferred_params: Vec<Span>,
 377             infer_args: bool,
 378         }
 379
 380         impl<'a, 'tcx> CreateSubstsForGenericArgsCtxt<'a, 'tcx> for SubstsForAstPathCtxt<'a, 'tcx> {
 381             fn args_for_def_id(&mut self, did: DefId) -> (Option<&'a GenericArgs<'a>>, bool) {
 382                 if did == self.def_id {
 383                     (Some(self.generic_args), self.infer_args)
 384                 } else {
 385                     // The last component of this tuple is unimportant.
 386                     (None, false)
 387                 }
 388             }
 389
 390             fn provided_kind(
 391                 &mut self,
 392                 param: &ty::GenericParamDef,
 393                 arg: &GenericArg<'_>,
 394             ) -> subst::GenericArg<'tcx> {
 395                 let tcx = self.astconv.tcx();
 396
 397                 let mut handle_ty_args = |has_default, ty: &hir::Ty<'_>| {
 398                     if has_default {
 399                         tcx.check_optional_stability(
 400                             param.def_id,
 401                             Some(arg.id()),
 402                             arg.span(),
 403                             None,
 404                             AllowUnstable::No,
 405                             |_, _| {
 406                                 // Default generic parameters may not be marked
 407                                 // with stability attributes, i.e. when the
 408                                 // default parameter was defined at the same time
 409                                 // as the rest of the type. As such, we ignore missing
 410                                 // stability attributes.
 411                             },
 412                         );
 413                     }
 414                     if let (hir::TyKind::Infer, false) = (&ty.kind, self.astconv.allow_ty_infer()) {
 415                         self.inferred_params.push(ty.span);
 416                         tcx.ty_error().into()
 417                     } else {
 418                         self.astconv.ast_ty_to_ty(ty).into()
 419                     }
 420                 };
 421
 422                 match (&param.kind, arg) {
 423                     (GenericParamDefKind::Lifetime, GenericArg::Lifetime(lt)) => {
 424                         self.astconv.ast_region_to_region(lt, Some(param)).into()
 425                     }
 426                     (&GenericParamDefKind::Type { has_default, .. }, GenericArg::Type(ty)) => {
 427                         handle_ty_args(has_default, ty)
 428                     }
 429                     (&GenericParamDefKind::Type { has_default, .. }, GenericArg::Infer(inf)) => {
 430                         handle_ty_args(has_default, &inf.to_ty())
 431                     }
 432                     (GenericParamDefKind::Const { .. }, GenericArg::Const(ct)) => {
 433                         ty::Const::from_opt_const_arg_anon_const(
 434                             tcx,
 435                             ty::WithOptConstParam {
 436                                 did: tcx.hir().local_def_id(ct.value.hir_id),
 437                                 const_param_did: Some(param.def_id),
 438                             },
 439                         )
 440                         .into()
 441                     }
 442                     (&GenericParamDefKind::Const { .. }, hir::GenericArg::Infer(inf)) => {
 443                         let ty = tcx.at(self.span).type_of(param.def_id);
 444                         if self.astconv.allow_ty_infer() {
 445                             self.astconv.ct_infer(ty, Some(param), inf.span).into()
 446                         } else {
 447                             self.inferred_params.push(inf.span);
 448                             tcx.const_error(ty).into()
 449                         }
 450                     }
 451                     _ => unreachable!(),
 452                 }
 453             }
 454
 455             fn inferred_kind(
 456                 &mut self,
 457                 substs: Option<&[subst::GenericArg<'tcx>]>,
 458                 param: &ty::GenericParamDef,
 459                 infer_args: bool,
 460             ) -> subst::GenericArg<'tcx> {
 461                 let tcx = self.astconv.tcx();
 462                 match param.kind {
 463                     GenericParamDefKind::Lifetime => self
 464                         .astconv
 465                         .re_infer(Some(param), self.span)
 466                         .unwrap_or_else(|| {
 467                             debug!(?param, "unelided lifetime in signature");
 468
 469                             // This indicates an illegal lifetime in a non-assoc-trait position
 470                             tcx.sess.delay_span_bug(self.span, "unelided lifetime in signature");
 471
 472                             // Supply some dummy value. We don't have an
 473                             // `re_error`, annoyingly, so use `'static`.
 474                             tcx.lifetimes.re_static
 475                         })
 476                         .into(),
 477                     GenericParamDefKind::Type { has_default, .. } => {
 478                         if !infer_args && has_default {
 479                             // No type parameter provided, but a default exists.
 480                             let substs = substs.unwrap();
 481                             if substs.iter().any(|arg| match arg.unpack() {
 482                                 GenericArgKind::Type(ty) => ty.references_error(),
 483                                 _ => false,
 484                             }) {
 485                                 // Avoid ICE #86756 when type error recovery goes awry.
 486                                 return tcx.ty_error().into();
 487                             }
 488                             self.astconv
 489                                 .normalize_ty(
 490                                     self.span,
 491                                     EarlyBinder(tcx.at(self.span).type_of(param.def_id))
 492                                         .subst(tcx, substs),
 493                                 )
 494                                 .into()
 495                         } else if infer_args {
 496                             self.astconv.ty_infer(Some(param), self.span).into()
 497                         } else {
 498                             // We've already errored above about the mismatch.
 499                             tcx.ty_error().into()
 500                         }
 501                     }
 502                     GenericParamDefKind::Const { has_default } => {
 503                         let ty = tcx.at(self.span).type_of(param.def_id);
 504                         if !infer_args && has_default {
 505                             tcx.bound_const_param_default(param.def_id)
 506                                 .subst(tcx, substs.unwrap())
 507                                 .into()
 508                         } else {
 509                             if infer_args {
 510                                 self.astconv.ct_infer(ty, Some(param), self.span).into()
 511                             } else {
 512                                 // We've already errored above about the mismatch.
 513                                 tcx.const_error(ty).into()
 514                             }
 515                         }
 516                     }
 517                 }
 518             }
 519         }
 520
 521         let mut substs_ctx = SubstsForAstPathCtxt {
 522             astconv: self,
 523             def_id,
 524             span,
 525             generic_args,
 526             inferred_params: vec![],
 527             infer_args,
 528         };
 529         let substs = Self::create_substs_for_generic_args(
 530             tcx,
 531             def_id,
 532             parent_substs,
 533             self_ty.is_some(),
 534             self_ty,
 535             &arg_count,
 536             &mut substs_ctx,
 537         );
 538
 539         debug!(
 540             "create_substs_for_ast_path(generic_params={:?}, self_ty={:?}) -> {:?}",
 541             generics, self_ty, substs
 542         );
 543
 544         (substs, arg_count)
 545     }
 546
 547     fn create_assoc_bindings_for_generic_args<'a>(
 548         &self,
 549         generic_args: &'a hir::GenericArgs<'_>,
 550     ) -> Vec<ConvertedBinding<'a, 'tcx>> {
 551         // Convert associated-type bindings or constraints into a separate vector.
 552         // Example: Given this:
 553         //
 554         //     T: Iterator<Item = u32>
 555         //
 556         // The `T` is passed in as a self-type; the `Item = u32` is
 557         // not a "type parameter" of the `Iterator` trait, but rather
 558         // a restriction on `<T as Iterator>::Item`, so it is passed
 559         // back separately.
 560         let assoc_bindings = generic_args
 561             .bindings
 562             .iter()
 563             .map(|binding| {
 564                 let kind = match binding.kind {
 565                     hir::TypeBindingKind::Equality { ref term } => match term {
 566                         hir::Term::Ty(ref ty) => {
 567                             ConvertedBindingKind::Equality(self.ast_ty_to_ty(ty).into())
 568                         }
 569                         hir::Term::Const(ref c) => {
 570                             let local_did = self.tcx().hir().local_def_id(c.hir_id);
 571                             let c = Const::from_anon_const(self.tcx(), local_did);
 572                             ConvertedBindingKind::Equality(c.into())
 573                         }
 574                     },
 575                     hir::TypeBindingKind::Constraint { ref bounds } => {
 576                         ConvertedBindingKind::Constraint(bounds)
 577                     }
 578                 };
 579                 ConvertedBinding {
 580                     hir_id: binding.hir_id,
 581                     item_name: binding.ident,
 582                     kind,
 583                     gen_args: binding.gen_args,
 584                     span: binding.span,
 585                 }
 586             })
 587             .collect();
 588
 589         assoc_bindings
 590     }
 591
 592     pub(crate) fn create_substs_for_associated_item(
 593         &self,
 594         tcx: TyCtxt<'tcx>,
 595         span: Span,
 596         item_def_id: DefId,
 597         item_segment: &hir::PathSegment<'_>,
 598         parent_substs: SubstsRef<'tcx>,
 599     ) -> SubstsRef<'tcx> {
 600         debug!(
 601             "create_substs_for_associated_item(span: {:?}, item_def_id: {:?}, item_segment: {:?}",
 602             span, item_def_id, item_segment
 603         );
 604         if tcx.generics_of(item_def_id).params.is_empty() {
 605             self.prohibit_generics(slice::from_ref(item_segment).iter(), |_| {});
 606
 607             parent_substs
 608         } else {
 609             self.create_substs_for_ast_path(
 610                 span,
 611                 item_def_id,
 612                 parent_substs,
 613                 item_segment,
 614                 item_segment.args(),
 615                 item_segment.infer_args,
 616                 None,
 617             )
 618             .0
 619         }
 620     }
 621
 622     /// Instantiates the path for the given trait reference, assuming that it's
 623     /// bound to a valid trait type. Returns the `DefId` of the defining trait.
 624     /// The type _cannot_ be a type other than a trait type.
 625     ///
 626     /// If the `projections` argument is `None`, then assoc type bindings like `Foo<T = X>`
 627     /// are disallowed. Otherwise, they are pushed onto the vector given.
 628     pub fn instantiate_mono_trait_ref(
 629         &self,
 630         trait_ref: &hir::TraitRef<'_>,
 631         self_ty: Ty<'tcx>,
 632     ) -> ty::TraitRef<'tcx> {
 633         self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1.iter(), |_| {});
 634
 635         self.ast_path_to_mono_trait_ref(
 636             trait_ref.path.span,
 637             trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise()),
 638             self_ty,
 639             trait_ref.path.segments.last().unwrap(),
 640             true,
 641         )
 642     }
 643
 644     fn instantiate_poly_trait_ref_inner(
 645         &self,
 646         hir_id: hir::HirId,
 647         span: Span,
 648         binding_span: Option<Span>,
 649         constness: ty::BoundConstness,
 650         bounds: &mut Bounds<'tcx>,
 651         speculative: bool,
 652         trait_ref_span: Span,
 653         trait_def_id: DefId,
 654         trait_segment: &hir::PathSegment<'_>,
 655         args: &GenericArgs<'_>,
 656         infer_args: bool,
 657         self_ty: Ty<'tcx>,
 658     ) -> GenericArgCountResult {
 659         let (substs, arg_count) = self.create_substs_for_ast_path(
 660             trait_ref_span,
 661             trait_def_id,
 662             &[],
 663             trait_segment,
 664             args,
 665             infer_args,
 666             Some(self_ty),
 667         );
 668
 669         let tcx = self.tcx();
 670         let bound_vars = tcx.late_bound_vars(hir_id);
 671         debug!(?bound_vars);
 672
 673         let assoc_bindings = self.create_assoc_bindings_for_generic_args(args);
 674
 675         let poly_trait_ref =
 676             ty::Binder::bind_with_vars(ty::TraitRef::new(trait_def_id, substs), bound_vars);
 677
 678         debug!(?poly_trait_ref, ?assoc_bindings);
 679         bounds.trait_bounds.push((poly_trait_ref, span, constness));
 680
 681         let mut dup_bindings = FxHashMap::default();
 682         for binding in &assoc_bindings {
 683             // Specify type to assert that error was already reported in `Err` case.
 684             let _: Result<_, ErrorGuaranteed> = self.add_predicates_for_ast_type_binding(
 685                 hir_id,
 686                 poly_trait_ref,
 687                 binding,
 688                 bounds,
 689                 speculative,
 690                 &mut dup_bindings,
 691                 binding_span.unwrap_or(binding.span),
 692             );
 693             // Okay to ignore `Err` because of `ErrorGuaranteed` (see above).
 694         }
 695
 696         arg_count
 697     }
 698
 699     /// Given a trait bound like `Debug`, applies that trait bound the given self-type to construct
 700     /// a full trait reference. The resulting trait reference is returned. This may also generate
 701     /// auxiliary bounds, which are added to `bounds`.
 702     ///
 703     /// Example:
 704     ///
 705     /// ```ignore (illustrative)
 706     /// poly_trait_ref = Iterator<Item = u32>
 707     /// self_ty = Foo
 708     /// ```
 709     ///
 710     /// this would return `Foo: Iterator` and add `<Foo as Iterator>::Item = u32` into `bounds`.
 711     ///
 712     /// **A note on binders:** against our usual convention, there is an implied bounder around
 713     /// the `self_ty` and `poly_trait_ref` parameters here. So they may reference bound regions.
 714     /// If for example you had `for<'a> Foo<'a>: Bar<'a>`, then the `self_ty` would be `Foo<'a>`
 715     /// where `'a` is a bound region at depth 0. Similarly, the `poly_trait_ref` would be
 716     /// `Bar<'a>`. The returned poly-trait-ref will have this binder instantiated explicitly,
 717     /// however.
 718     #[tracing::instrument(level = "debug", skip(self, span, constness, bounds, speculative))]
 719     pub(crate) fn instantiate_poly_trait_ref(
 720         &self,
 721         trait_ref: &hir::TraitRef<'_>,
 722         span: Span,
 723         constness: ty::BoundConstness,
 724         self_ty: Ty<'tcx>,
 725         bounds: &mut Bounds<'tcx>,
 726         speculative: bool,
 727     ) -> GenericArgCountResult {
 728         let hir_id = trait_ref.hir_ref_id;
 729         let binding_span = None;
 730         let trait_ref_span = trait_ref.path.span;
 731         let trait_def_id = trait_ref.trait_def_id().unwrap_or_else(|| FatalError.raise());
 732         let trait_segment = trait_ref.path.segments.last().unwrap();
 733         let args = trait_segment.args();
 734         let infer_args = trait_segment.infer_args;
 735
 736         self.prohibit_generics(trait_ref.path.segments.split_last().unwrap().1.iter(), |_| {});
 737         self.complain_about_internal_fn_trait(span, trait_def_id, trait_segment, false);
 738
 739         self.instantiate_poly_trait_ref_inner(
 740             hir_id,
 741             span,
 742             binding_span,
 743             constness,
 744             bounds,
 745             speculative,
 746             trait_ref_span,
 747             trait_def_id,
 748             trait_segment,
 749             args,
 750             infer_args,
 751             self_ty,
 752         )
 753     }
 754
 755     pub(crate) fn instantiate_lang_item_trait_ref(
 756         &self,
 757         lang_item: hir::LangItem,
 758         span: Span,
 759         hir_id: hir::HirId,
 760         args: &GenericArgs<'_>,
 761         self_ty: Ty<'tcx>,
 762         bounds: &mut Bounds<'tcx>,
 763     ) {
 764         let binding_span = Some(span);
 765         let constness = ty::BoundConstness::NotConst;
 766         let speculative = false;
 767         let trait_ref_span = span;
 768         let trait_def_id = self.tcx().require_lang_item(lang_item, Some(span));
 769         let trait_segment = &hir::PathSegment::invalid();
 770         let infer_args = false;
 771
 772         self.instantiate_poly_trait_ref_inner(
 773             hir_id,
 774             span,
 775             binding_span,
 776             constness,
 777             bounds,
 778             speculative,
 779             trait_ref_span,
 780             trait_def_id,
 781             trait_segment,
 782             args,
 783             infer_args,
 784             self_ty,
 785         );
 786     }
 787
 788     fn ast_path_to_mono_trait_ref(
 789         &self,
 790         span: Span,
 791         trait_def_id: DefId,
 792         self_ty: Ty<'tcx>,
 793         trait_segment: &hir::PathSegment<'_>,
 794         is_impl: bool,
 795     ) -> ty::TraitRef<'tcx> {
 796         let (substs, _) = self.create_substs_for_ast_trait_ref(
 797             span,
 798             trait_def_id,
 799             self_ty,
 800             trait_segment,
 801             is_impl,
 802         );
 803         let assoc_bindings = self.create_assoc_bindings_for_generic_args(trait_segment.args());
 804         if let Some(b) = assoc_bindings.first() {
 805             Self::prohibit_assoc_ty_binding(self.tcx(), b.span);
 806         }
 807         ty::TraitRef::new(trait_def_id, substs)
 808     }
 809
 810     #[tracing::instrument(level = "debug", skip(self, span))]
 811     fn create_substs_for_ast_trait_ref<'a>(
 812         &self,
 813         span: Span,
 814         trait_def_id: DefId,
 815         self_ty: Ty<'tcx>,
 816         trait_segment: &'a hir::PathSegment<'a>,
 817         is_impl: bool,
 818     ) -> (SubstsRef<'tcx>, GenericArgCountResult) {
 819         self.complain_about_internal_fn_trait(span, trait_def_id, trait_segment, is_impl);
 820
 821         self.create_substs_for_ast_path(
 822             span,
 823             trait_def_id,
 824             &[],
 825             trait_segment,
 826             trait_segment.args(),
 827             trait_segment.infer_args,
 828             Some(self_ty),
 829         )
 830     }
 831
 832     fn trait_defines_associated_type_named(&self, trait_def_id: DefId, assoc_name: Ident) -> bool {
 833         self.tcx()
 834             .associated_items(trait_def_id)
 835             .find_by_name_and_kind(self.tcx(), assoc_name, ty::AssocKind::Type, trait_def_id)
 836             .is_some()
 837     }
 838     fn trait_defines_associated_const_named(&self, trait_def_id: DefId, assoc_name: Ident) -> bool {
 839         self.tcx()
 840             .associated_items(trait_def_id)
 841             .find_by_name_and_kind(self.tcx(), assoc_name, ty::AssocKind::Const, trait_def_id)
 842             .is_some()
 843     }
 844
 845     // Sets `implicitly_sized` to true on `Bounds` if necessary
 846     pub(crate) fn add_implicitly_sized<'hir>(
 847         &self,
 848         bounds: &mut Bounds<'hir>,
 849         ast_bounds: &'hir [hir::GenericBound<'hir>],
 850         self_ty_where_predicates: Option<(hir::HirId, &'hir [hir::WherePredicate<'hir>])>,
 851         span: Span,
 852     ) {
 853         let tcx = self.tcx();
 854
 855         // Try to find an unbound in bounds.
 856         let mut unbound = None;
 857         let mut search_bounds = |ast_bounds: &'hir [hir::GenericBound<'hir>]| {
 858             for ab in ast_bounds {
 859                 if let hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::Maybe) = ab {
 860                     if unbound.is_none() {
 861                         unbound = Some(&ptr.trait_ref);
 862                     } else {
 863                         tcx.sess.emit_err(MultipleRelaxedDefaultBounds { span });
 864                     }
 865                 }
 866             }
 867         };
 868         search_bounds(ast_bounds);
 869         if let Some((self_ty, where_clause)) = self_ty_where_predicates {
 870             let self_ty_def_id = tcx.hir().local_def_id(self_ty).to_def_id();
 871             for clause in where_clause {
 872                 if let hir::WherePredicate::BoundPredicate(pred) = clause {
 873                     if pred.is_param_bound(self_ty_def_id) {
 874                         search_bounds(pred.bounds);
 875                     }
 876                 }
 877             }
 878         }
 879
 880         let sized_def_id = tcx.lang_items().require(LangItem::Sized);
 881         match (&sized_def_id, unbound) {
 882             (Ok(sized_def_id), Some(tpb))
 883                 if tpb.path.res == Res::Def(DefKind::Trait, *sized_def_id) =>
 884             {
 885                 // There was in fact a `?Sized` bound, return without doing anything
 886                 return;
 887             }
 888             (_, Some(_)) => {
 889                 // There was a `?Trait` bound, but it was not `?Sized`; warn.
 890                 tcx.sess.span_warn(
 891                     span,
 892                     "default bound relaxed for a type parameter, but \
 893                         this does nothing because the given bound is not \
 894                         a default; only `?Sized` is supported",
 895                 );
 896                 // Otherwise, add implicitly sized if `Sized` is available.
 897             }
 898             _ => {
 899                 // There was no `?Sized` bound; add implicitly sized if `Sized` is available.
 900             }
 901         }
 902         if sized_def_id.is_err() {
 903             // No lang item for `Sized`, so we can't add it as a bound.
 904             return;
 905         }
 906         bounds.implicitly_sized = Some(span);
 907     }
 908
 909     /// This helper takes a *converted* parameter type (`param_ty`)
 910     /// and an *unconverted* list of bounds:
 911     ///
 912     /// ```text
 913     /// fn foo<T: Debug>
 914     ///        ^  ^^^^^ `ast_bounds` parameter, in HIR form
 915     ///        |
 916     ///        `param_ty`, in ty form
 917     /// ```
 918     ///
 919     /// It adds these `ast_bounds` into the `bounds` structure.
 920     ///
 921     /// **A note on binders:** there is an implied binder around
 922     /// `param_ty` and `ast_bounds`. See `instantiate_poly_trait_ref`
 923     /// for more details.
 924     #[tracing::instrument(level = "debug", skip(self, ast_bounds, bounds))]
 925     pub(crate) fn add_bounds<'hir, I: Iterator<Item = &'hir hir::GenericBound<'hir>>>(
 926         &self,
 927         param_ty: Ty<'tcx>,
 928         ast_bounds: I,
 929         bounds: &mut Bounds<'tcx>,
 930         bound_vars: &'tcx ty::List<ty::BoundVariableKind>,
 931     ) {
 932         for ast_bound in ast_bounds {
 933             match ast_bound {
 934                 hir::GenericBound::Trait(poly_trait_ref, modifier) => {
 935                     let constness = match modifier {
 936                         hir::TraitBoundModifier::MaybeConst => ty::BoundConstness::ConstIfConst,
 937                         hir::TraitBoundModifier::None => ty::BoundConstness::NotConst,
 938                         hir::TraitBoundModifier::Maybe => continue,
 939                     };
 940
 941                     let _ = self.instantiate_poly_trait_ref(
 942                         &poly_trait_ref.trait_ref,
 943                         poly_trait_ref.span,
 944                         constness,
 945                         param_ty,
 946                         bounds,
 947                         false,
 948                     );
 949                 }
 950                 &hir::GenericBound::LangItemTrait(lang_item, span, hir_id, args) => {
 951                     self.instantiate_lang_item_trait_ref(
 952                         lang_item, span, hir_id, args, param_ty, bounds,
 953                     );
 954                 }
 955                 hir::GenericBound::Outlives(lifetime) => {
 956                     let region = self.ast_region_to_region(lifetime, None);
 957                     bounds
 958                         .region_bounds
 959                         .push((ty::Binder::bind_with_vars(region, bound_vars), lifetime.span));
 960                 }
 961             }
 962         }
 963     }
 964
 965     /// Translates a list of bounds from the HIR into the `Bounds` data structure.
 966     /// The self-type for the bounds is given by `param_ty`.
 967     ///
 968     /// Example:
 969     ///
 970     /// ```ignore (illustrative)
 971     /// fn foo<T: Bar + Baz>() { }
 972     /// //     ^  ^^^^^^^^^ ast_bounds
 973     /// //     param_ty
 974     /// ```
 975     ///
 976     /// The `sized_by_default` parameter indicates if, in this context, the `param_ty` should be
 977     /// considered `Sized` unless there is an explicit `?Sized` bound.  This would be true in the
 978     /// example above, but is not true in supertrait listings like `trait Foo: Bar + Baz`.
 979     ///
 980     /// `span` should be the declaration size of the parameter.
 981     pub(crate) fn compute_bounds(
 982         &self,
 983         param_ty: Ty<'tcx>,
 984         ast_bounds: &[hir::GenericBound<'_>],
 985     ) -> Bounds<'tcx> {
 986         self.compute_bounds_inner(param_ty, ast_bounds)
 987     }
 988
 989     /// Convert the bounds in `ast_bounds` that refer to traits which define an associated type
 990     /// named `assoc_name` into ty::Bounds. Ignore the rest.
 991     pub(crate) fn compute_bounds_that_match_assoc_type(
 992         &self,
 993         param_ty: Ty<'tcx>,
 994         ast_bounds: &[hir::GenericBound<'_>],
 995         assoc_name: Ident,
 996     ) -> Bounds<'tcx> {
 997         let mut result = Vec::new();
 998
 999         for ast_bound in ast_bounds {
1000             if let Some(trait_ref) = ast_bound.trait_ref()
1001                 && let Some(trait_did) = trait_ref.trait_def_id()
1002                 && self.tcx().trait_may_define_assoc_type(trait_did, assoc_name)
1003             {
1004                 result.push(ast_bound.clone());
1005             }
1006         }
1007
1008         self.compute_bounds_inner(param_ty, &result)
1009     }
1010
1011     fn compute_bounds_inner(
1012         &self,
1013         param_ty: Ty<'tcx>,
1014         ast_bounds: &[hir::GenericBound<'_>],
1015     ) -> Bounds<'tcx> {
1016         let mut bounds = Bounds::default();
1017
1018         self.add_bounds(param_ty, ast_bounds.iter(), &mut bounds, ty::List::empty());
1019         debug!(?bounds);
1020
1021         bounds
1022     }
1023
1024     /// Given an HIR binding like `Item = Foo` or `Item: Foo`, pushes the corresponding predicates
1025     /// onto `bounds`.
1026     ///
1027     /// **A note on binders:** given something like `T: for<'a> Iterator<Item = &'a u32>`, the
1028     /// `trait_ref` here will be `for<'a> T: Iterator`. The `binding` data however is from *inside*
1029     /// the binder (e.g., `&'a u32`) and hence may reference bound regions.
1030     #[tracing::instrument(
1031         level = "debug",
1032         skip(self, bounds, speculative, dup_bindings, path_span)
1033     )]
1034     fn add_predicates_for_ast_type_binding(
1035         &self,
1036         hir_ref_id: hir::HirId,
1037         trait_ref: ty::PolyTraitRef<'tcx>,
1038         binding: &ConvertedBinding<'_, 'tcx>,
1039         bounds: &mut Bounds<'tcx>,
1040         speculative: bool,
1041         dup_bindings: &mut FxHashMap<DefId, Span>,
1042         path_span: Span,
1043     ) -> Result<(), ErrorGuaranteed> {
1044         // Given something like `U: SomeTrait<T = X>`, we want to produce a
1045         // predicate like `<U as SomeTrait>::T = X`. This is somewhat
1046         // subtle in the event that `T` is defined in a supertrait of
1047         // `SomeTrait`, because in that case we need to upcast.
1048         //
1049         // That is, consider this case:
1050         //
1051         // ```
1052         // trait SubTrait: SuperTrait<i32> { }
1053         // trait SuperTrait<A> { type T; }
1054         //
1055         // ... B: SubTrait<T = foo> ...
1056         // ```
1057         //
1058         // We want to produce `<B as SuperTrait<i32>>::T == foo`.
1059
1060         let tcx = self.tcx();
1061
1062         let candidate =
1063             if self.trait_defines_associated_type_named(trait_ref.def_id(), binding.item_name) {
1064                 // Simple case: X is defined in the current trait.
1065                 trait_ref
1066             } else {
1067                 // Otherwise, we have to walk through the supertraits to find
1068                 // those that do.
1069                 self.one_bound_for_assoc_type(
1070                     || traits::supertraits(tcx, trait_ref),
1071                     || trait_ref.print_only_trait_path().to_string(),
1072                     binding.item_name,
1073                     path_span,
1074                     || match binding.kind {
1075                         ConvertedBindingKind::Equality(ty) => Some(ty.to_string()),
1076                         _ => None,
1077                     },
1078                 )?
1079             };
1080
1081         let (assoc_ident, def_scope) =
1082             tcx.adjust_ident_and_get_scope(binding.item_name, candidate.def_id(), hir_ref_id);
1083
1084         // We have already adjusted the item name above, so compare with `ident.normalize_to_macros_2_0()` instead
1085         // of calling `filter_by_name_and_kind`.
1086         let find_item_of_kind = |kind| {
1087             tcx.associated_items(candidate.def_id())
1088                 .filter_by_name_unhygienic(assoc_ident.name)
1089                 .find(|i| i.kind == kind && i.ident(tcx).normalize_to_macros_2_0() == assoc_ident)
1090         };
1091         let assoc_item = find_item_of_kind(ty::AssocKind::Type)
1092             .or_else(|| find_item_of_kind(ty::AssocKind::Const))
1093             .expect("missing associated type");
1094
1095         if !assoc_item.visibility(tcx).is_accessible_from(def_scope, tcx) {
1096             tcx.sess
1097                 .struct_span_err(
1098                     binding.span,
1099                     &format!("{} `{}` is private", assoc_item.kind, binding.item_name),
1100                 )
1101                 .span_label(binding.span, &format!("private {}", assoc_item.kind))
1102                 .emit();
1103         }
1104         tcx.check_stability(assoc_item.def_id, Some(hir_ref_id), binding.span, None);
1105
1106         if !speculative {
1107             dup_bindings
1108                 .entry(assoc_item.def_id)
1109                 .and_modify(|prev_span| {
1110                     self.tcx().sess.emit_err(ValueOfAssociatedStructAlreadySpecified {
1111                         span: binding.span,
1112                         prev_span: *prev_span,
1113                         item_name: binding.item_name,
1114                         def_path: tcx.def_path_str(assoc_item.container_id(tcx)),
1115                     });
1116                 })
1117                 .or_insert(binding.span);
1118         }
1119
1120         // Include substitutions for generic parameters of associated types
1121         let projection_ty = candidate.map_bound(|trait_ref| {
1122             let ident = Ident::new(assoc_item.name, binding.item_name.span);
1123             let item_segment = hir::PathSegment {
1124                 ident,
1125                 hir_id: Some(binding.hir_id),
1126                 res: None,
1127                 args: Some(binding.gen_args),
1128                 infer_args: false,
1129             };
1130
1131             let substs_trait_ref_and_assoc_item = self.create_substs_for_associated_item(
1132                 tcx,
1133                 path_span,
1134                 assoc_item.def_id,
1135                 &item_segment,
1136                 trait_ref.substs,
1137             );
1138
1139             debug!(
1140                 "add_predicates_for_ast_type_binding: substs for trait-ref and assoc_item: {:?}",
1141                 substs_trait_ref_and_assoc_item
1142             );
1143
1144             ty::ProjectionTy {
1145                 item_def_id: assoc_item.def_id,
1146                 substs: substs_trait_ref_and_assoc_item,
1147             }
1148         });
1149
1150         if !speculative {
1151             // Find any late-bound regions declared in `ty` that are not
1152             // declared in the trait-ref or assoc_item. These are not well-formed.
1153             //
1154             // Example:
1155             //
1156             //     for<'a> <T as Iterator>::Item = &'a str // <-- 'a is bad
1157             //     for<'a> <T as FnMut<(&'a u32,)>>::Output = &'a str // <-- 'a is ok
1158             if let ConvertedBindingKind::Equality(ty) = binding.kind {
1159                 let late_bound_in_trait_ref =
1160                     tcx.collect_constrained_late_bound_regions(&projection_ty);
1161                 let late_bound_in_ty =
1162                     tcx.collect_referenced_late_bound_regions(&trait_ref.rebind(ty));
1163                 debug!("late_bound_in_trait_ref = {:?}", late_bound_in_trait_ref);
1164                 debug!("late_bound_in_ty = {:?}", late_bound_in_ty);
1165
1166                 // FIXME: point at the type params that don't have appropriate lifetimes:
1167                 // struct S1<F: for<'a> Fn(&i32, &i32) -> &'a i32>(F);
1168                 //                         ----  ----     ^^^^^^^
1169                 self.validate_late_bound_regions(
1170                     late_bound_in_trait_ref,
1171                     late_bound_in_ty,
1172                     |br_name| {
1173                         struct_span_err!(
1174                             tcx.sess,
1175                             binding.span,
1176                             E0582,
1177                             "binding for associated type `{}` references {}, \
1178                              which does not appear in the trait input types",
1179                             binding.item_name,
1180                             br_name
1181                         )
1182                     },
1183                 );
1184             }
1185         }
1186
1187         match binding.kind {
1188             ConvertedBindingKind::Equality(mut term) => {
1189                 // "Desugar" a constraint like `T: Iterator<Item = u32>` this to
1190                 // the "projection predicate" for:
1191                 //
1192                 // `<T as Iterator>::Item = u32`
1193                 let assoc_item_def_id = projection_ty.skip_binder().item_def_id;
1194                 let def_kind = tcx.def_kind(assoc_item_def_id);
1195                 match (def_kind, term) {
1196                     (hir::def::DefKind::AssocTy, ty::Term::Ty(_))
1197                     | (hir::def::DefKind::AssocConst, ty::Term::Const(_)) => (),
1198                     (_, _) => {
1199                         let got = if let ty::Term::Ty(_) = term { "type" } else { "constant" };
1200                         let expected = def_kind.descr(assoc_item_def_id);
1201                         tcx.sess
1202                             .struct_span_err(
1203                                 binding.span,
1204                                 &format!("expected {expected} bound, found {got}"),
1205                             )
1206                             .span_note(
1207                                 tcx.def_span(assoc_item_def_id),
1208                                 &format!("{expected} defined here"),
1209                             )
1210                             .emit();
1211                         term = match def_kind {
1212                             hir::def::DefKind::AssocTy => tcx.ty_error().into(),
1213                             hir::def::DefKind::AssocConst => tcx
1214                                 .const_error(
1215                                     tcx.bound_type_of(assoc_item_def_id)
1216                                         .subst(tcx, projection_ty.skip_binder().substs),
1217                                 )
1218                                 .into(),
1219                             _ => unreachable!(),
1220                         };
1221                     }
1222                 }
1223                 bounds.projection_bounds.push((
1224                     projection_ty.map_bound(|projection_ty| ty::ProjectionPredicate {
1225                         projection_ty,
1226                         term: term,
1227                     }),
1228                     binding.span,
1229                 ));
1230             }
1231             ConvertedBindingKind::Constraint(ast_bounds) => {
1232                 // "Desugar" a constraint like `T: Iterator<Item: Debug>` to
1233                 //
1234                 // `<T as Iterator>::Item: Debug`
1235                 //
1236                 // Calling `skip_binder` is okay, because `add_bounds` expects the `param_ty`
1237                 // parameter to have a skipped binder.
1238                 let param_ty = tcx.mk_ty(ty::Projection(projection_ty.skip_binder()));
1239                 self.add_bounds(param_ty, ast_bounds.iter(), bounds, candidate.bound_vars());
1240             }
1241         }
1242         Ok(())
1243     }
1244
1245     fn ast_path_to_ty(
1246         &self,
1247         span: Span,
1248         did: DefId,
1249         item_segment: &hir::PathSegment<'_>,
1250     ) -> Ty<'tcx> {
1251         let substs = self.ast_path_substs_for_ty(span, did, item_segment);
1252         self.normalize_ty(
1253             span,
1254             EarlyBinder(self.tcx().at(span).type_of(did)).subst(self.tcx(), substs),
1255         )
1256     }
1257
1258     fn conv_object_ty_poly_trait_ref(
1259         &self,
1260         span: Span,
1261         trait_bounds: &[hir::PolyTraitRef<'_>],
1262         lifetime: &hir::Lifetime,
1263         borrowed: bool,
1264     ) -> Ty<'tcx> {
1265         let tcx = self.tcx();
1266
1267         let mut bounds = Bounds::default();
1268         let mut potential_assoc_types = Vec::new();
1269         let dummy_self = self.tcx().types.trait_object_dummy_self;
1270         for trait_bound in trait_bounds.iter().rev() {
1271             if let GenericArgCountResult {
1272                 correct:
1273                     Err(GenericArgCountMismatch { invalid_args: cur_potential_assoc_types, .. }),
1274                 ..
1275             } = self.instantiate_poly_trait_ref(
1276                 &trait_bound.trait_ref,
1277                 trait_bound.span,
1278                 ty::BoundConstness::NotConst,
1279                 dummy_self,
1280                 &mut bounds,
1281                 false,
1282             ) {
1283                 potential_assoc_types.extend(cur_potential_assoc_types);
1284             }
1285         }
1286
1287         // Expand trait aliases recursively and check that only one regular (non-auto) trait
1288         // is used and no 'maybe' bounds are used.
1289         let expanded_traits =
1290             traits::expand_trait_aliases(tcx, bounds.trait_bounds.iter().map(|&(a, b, _)| (a, b)));
1291         let (mut auto_traits, regular_traits): (Vec<_>, Vec<_>) = expanded_traits
1292             .filter(|i| i.trait_ref().self_ty().skip_binder() == dummy_self)
1293             .partition(|i| tcx.trait_is_auto(i.trait_ref().def_id()));
1294         if regular_traits.len() > 1 {
1295             let first_trait = &regular_traits[0];
1296             let additional_trait = &regular_traits[1];
1297             let mut err = struct_span_err!(
1298                 tcx.sess,
1299                 additional_trait.bottom().1,
1300                 E0225,
1301                 "only auto traits can be used as additional traits in a trait object"
1302             );
1303             additional_trait.label_with_exp_info(
1304                 &mut err,
1305                 "additional non-auto trait",
1306                 "additional use",
1307             );
1308             first_trait.label_with_exp_info(&mut err, "first non-auto trait", "first use");
1309             err.help(&format!(
1310                 "consider creating a new trait with all of these as supertraits and using that \
1311                  trait here instead: `trait NewTrait: {} {{}}`",
1312                 regular_traits
1313                     .iter()
1314                     .map(|t| t.trait_ref().print_only_trait_path().to_string())
1315                     .collect::<Vec<_>>()
1316                     .join(" + "),
1317             ));
1318             err.note(
1319                 "auto-traits like `Send` and `Sync` are traits that have special properties; \
1320                  for more information on them, visit \
1321                  <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>",
1322             );
1323             err.emit();
1324         }
1325
1326         if regular_traits.is_empty() && auto_traits.is_empty() {
1327             let trait_alias_span = bounds
1328                 .trait_bounds
1329                 .iter()
1330                 .map(|&(trait_ref, _, _)| trait_ref.def_id())
1331                 .find(|&trait_ref| tcx.is_trait_alias(trait_ref))
1332                 .map(|trait_ref| tcx.def_span(trait_ref));
1333             tcx.sess.emit_err(TraitObjectDeclaredWithNoTraits { span, trait_alias_span });
1334             return tcx.ty_error();
1335         }
1336
1337         // Check that there are no gross object safety violations;
1338         // most importantly, that the supertraits don't contain `Self`,
1339         // to avoid ICEs.
1340         for item in &regular_traits {
1341             let object_safety_violations =
1342                 astconv_object_safety_violations(tcx, item.trait_ref().def_id());
1343             if !object_safety_violations.is_empty() {
1344                 report_object_safety_error(
1345                     tcx,
1346                     span,
1347                     item.trait_ref().def_id(),
1348                     &object_safety_violations,
1349                 )
1350                 .emit();
1351                 return tcx.ty_error();
1352             }
1353         }
1354
1355         // Use a `BTreeSet` to keep output in a more consistent order.
1356         let mut associated_types: FxHashMap<Span, BTreeSet<DefId>> = FxHashMap::default();
1357
1358         let regular_traits_refs_spans = bounds
1359             .trait_bounds
1360             .into_iter()
1361             .filter(|(trait_ref, _, _)| !tcx.trait_is_auto(trait_ref.def_id()));
1362
1363         for (base_trait_ref, span, constness) in regular_traits_refs_spans {
1364             assert_eq!(constness, ty::BoundConstness::NotConst);
1365
1366             for obligation in traits::elaborate_trait_ref(tcx, base_trait_ref) {
1367                 debug!(
1368                     "conv_object_ty_poly_trait_ref: observing object predicate `{:?}`",
1369                     obligation.predicate
1370                 );
1371
1372                 let bound_predicate = obligation.predicate.kind();
1373                 match bound_predicate.skip_binder() {
1374                     ty::PredicateKind::Trait(pred) => {
1375                         let pred = bound_predicate.rebind(pred);
1376                         associated_types.entry(span).or_default().extend(
1377                             tcx.associated_items(pred.def_id())
1378                                 .in_definition_order()
1379                                 .filter(|item| item.kind == ty::AssocKind::Type)
1380                                 .map(|item| item.def_id),
1381                         );
1382                     }
1383                     ty::PredicateKind::Projection(pred) => {
1384                         let pred = bound_predicate.rebind(pred);
1385                         // A `Self` within the original bound will be substituted with a
1386                         // `trait_object_dummy_self`, so check for that.
1387                         let references_self = match pred.skip_binder().term {
1388                             ty::Term::Ty(ty) => ty.walk().any(|arg| arg == dummy_self.into()),
1389                             ty::Term::Const(c) => c.ty().walk().any(|arg| arg == dummy_self.into()),
1390                         };
1391
1392                         // If the projection output contains `Self`, force the user to
1393                         // elaborate it explicitly to avoid a lot of complexity.
1394                         //
1395                         // The "classically useful" case is the following:
1396                         // ```
1397                         //     trait MyTrait: FnMut() -> <Self as MyTrait>::MyOutput {
1398                         //         type MyOutput;
1399                         //     }
1400                         // ```
1401                         //
1402                         // Here, the user could theoretically write `dyn MyTrait<Output = X>`,
1403                         // but actually supporting that would "expand" to an infinitely-long type
1404                         // `fix $ τ → dyn MyTrait<MyOutput = X, Output = <τ as MyTrait>::MyOutput`.
1405                         //
1406                         // Instead, we force the user to write
1407                         // `dyn MyTrait<MyOutput = X, Output = X>`, which is uglier but works. See
1408                         // the discussion in #56288 for alternatives.
1409                         if !references_self {
1410                             // Include projections defined on supertraits.
1411                             bounds.projection_bounds.push((pred, span));
1412                         }
1413                     }
1414                     _ => (),
1415                 }
1416             }
1417         }
1418
1419         for (projection_bound, _) in &bounds.projection_bounds {
1420             for def_ids in associated_types.values_mut() {
1421                 def_ids.remove(&projection_bound.projection_def_id());
1422             }
1423         }
1424
1425         self.complain_about_missing_associated_types(
1426             associated_types,
1427             potential_assoc_types,
1428             trait_bounds,
1429         );
1430
1431         // De-duplicate auto traits so that, e.g., `dyn Trait + Send + Send` is the same as
1432         // `dyn Trait + Send`.
1433         // We remove duplicates by inserting into a `FxHashSet` to avoid re-ordering
1434         // the bounds
1435         let mut duplicates = FxHashSet::default();
1436         auto_traits.retain(|i| duplicates.insert(i.trait_ref().def_id()));
1437         debug!("regular_traits: {:?}", regular_traits);
1438         debug!("auto_traits: {:?}", auto_traits);
1439
1440         // Erase the `dummy_self` (`trait_object_dummy_self`) used above.
1441         let existential_trait_refs = regular_traits.iter().map(|i| {
1442             i.trait_ref().map_bound(|trait_ref: ty::TraitRef<'tcx>| {
1443                 assert_eq!(trait_ref.self_ty(), dummy_self);
1444
1445                 // Verify that `dummy_self` did not leak inside default type parameters.  This
1446                 // could not be done at path creation, since we need to see through trait aliases.
1447                 let mut missing_type_params = vec![];
1448                 let mut references_self = false;
1449                 let generics = tcx.generics_of(trait_ref.def_id);
1450                 let substs: Vec<_> = trait_ref
1451                     .substs
1452                     .iter()
1453                     .enumerate()
1454                     .skip(1) // Remove `Self` for `ExistentialPredicate`.
1455                     .map(|(index, arg)| {
1456                         if let ty::GenericArgKind::Type(ty) = arg.unpack() {
1457                             debug!(?ty);
1458                             if ty == dummy_self {
1459                                 let param = &generics.params[index];
1460                                 missing_type_params.push(param.name);
1461                                 tcx.ty_error().into()
1462                             } else if ty.walk().any(|arg| arg == dummy_self.into()) {
1463                                 references_self = true;
1464                                 tcx.ty_error().into()
1465                             } else {
1466                                 arg
1467                             }
1468                         } else {
1469                             arg
1470                         }
1471                     })
1472                     .collect();
1473                 let substs = tcx.intern_substs(&substs[..]);
1474
1475                 let span = i.bottom().1;
1476                 let empty_generic_args = trait_bounds.iter().any(|hir_bound| {
1477                     hir_bound.trait_ref.path.res == Res::Def(DefKind::Trait, trait_ref.def_id)
1478                         && hir_bound.span.contains(span)
1479                 });
1480                 self.complain_about_missing_type_params(
1481                     missing_type_params,
1482                     trait_ref.def_id,
1483                     span,
1484                     empty_generic_args,
1485                 );
1486
1487                 if references_self {
1488                     let def_id = i.bottom().0.def_id();
1489                     let mut err = struct_span_err!(
1490                         tcx.sess,
1491                         i.bottom().1,
1492                         E0038,
1493                         "the {} `{}` cannot be made into an object",
1494                         tcx.def_kind(def_id).descr(def_id),
1495                         tcx.item_name(def_id),
1496                     );
1497                     err.note(
1498                         rustc_middle::traits::ObjectSafetyViolation::SupertraitSelf(smallvec![])
1499                             .error_msg(),
1500                     );
1501                     err.emit();
1502                 }
1503
1504                 ty::ExistentialTraitRef { def_id: trait_ref.def_id, substs }
1505             })
1506         });
1507
1508         let existential_projections = bounds.projection_bounds.iter().map(|(bound, _)| {
1509             bound.map_bound(|b| {
1510                 if b.projection_ty.self_ty() != dummy_self {
1511                     tcx.sess.delay_span_bug(
1512                         DUMMY_SP,
1513                         &format!("trait_ref_to_existential called on {:?} with non-dummy Self", b),
1514                     );
1515                 }
1516                 ty::ExistentialProjection::erase_self_ty(tcx, b)
1517             })
1518         });
1519
1520         let regular_trait_predicates = existential_trait_refs
1521             .map(|trait_ref| trait_ref.map_bound(ty::ExistentialPredicate::Trait));
1522         let auto_trait_predicates = auto_traits.into_iter().map(|trait_ref| {
1523             ty::Binder::dummy(ty::ExistentialPredicate::AutoTrait(trait_ref.trait_ref().def_id()))
1524         });
1525         // N.b. principal, projections, auto traits
1526         // FIXME: This is actually wrong with multiple principals in regards to symbol mangling
1527         let mut v = regular_trait_predicates
1528             .chain(
1529                 existential_projections.map(|x| x.map_bound(ty::ExistentialPredicate::Projection)),
1530             )
1531             .chain(auto_trait_predicates)
1532             .collect::<SmallVec<[_; 8]>>();
1533         v.sort_by(|a, b| a.skip_binder().stable_cmp(tcx, &b.skip_binder()));
1534         v.dedup();
1535         let existential_predicates = tcx.mk_poly_existential_predicates(v.into_iter());
1536
1537         // Use explicitly-specified region bound.
1538         let region_bound = if !lifetime.is_elided() {
1539             self.ast_region_to_region(lifetime, None)
1540         } else {
1541             self.compute_object_lifetime_bound(span, existential_predicates).unwrap_or_else(|| {
1542                 if tcx.named_region(lifetime.hir_id).is_some() {
1543                     self.ast_region_to_region(lifetime, None)
1544                 } else {
1545                     self.re_infer(None, span).unwrap_or_else(|| {
1546                         let mut err = struct_span_err!(
1547                             tcx.sess,
1548                             span,
1549                             E0228,
1550                             "the lifetime bound for this object type cannot be deduced \
1551                              from context; please supply an explicit bound"
1552                         );
1553                         if borrowed {
1554                             // We will have already emitted an error E0106 complaining about a
1555                             // missing named lifetime in `&dyn Trait`, so we elide this one.
1556                             err.delay_as_bug();
1557                         } else {
1558                             err.emit();
1559                         }
1560                         tcx.lifetimes.re_static
1561                     })
1562                 }
1563             })
1564         };
1565         debug!("region_bound: {:?}", region_bound);
1566
1567         let ty = tcx.mk_dynamic(existential_predicates, region_bound);
1568         debug!("trait_object_type: {:?}", ty);
1569         ty
1570     }
1571
1572     fn report_ambiguous_associated_type(
1573         &self,
1574         span: Span,
1575         type_str: &str,
1576         trait_str: &str,
1577         name: Symbol,
1578     ) -> ErrorGuaranteed {
1579         let mut err = struct_span_err!(self.tcx().sess, span, E0223, "ambiguous associated type");
1580         if self
1581             .tcx()
1582             .resolutions(())
1583             .confused_type_with_std_module
1584             .keys()
1585             .any(|full_span| full_span.contains(span))
1586         {
1587             err.span_suggestion(
1588                 span.shrink_to_lo(),
1589                 "you are looking for the module in `std`, not the primitive type",
1590                 "std::",
1591                 Applicability::MachineApplicable,
1592             );
1593         } else {
1594             err.span_suggestion(
1595                 span,
1596                 "use fully-qualified syntax",
1597                 format!("<{} as {}>::{}", type_str, trait_str, name),
1598                 Applicability::HasPlaceholders,
1599             );
1600         }
1601         err.emit()
1602     }
1603
1604     // Search for a bound on a type parameter which includes the associated item
1605     // given by `assoc_name`. `ty_param_def_id` is the `DefId` of the type parameter
1606     // This function will fail if there are no suitable bounds or there is
1607     // any ambiguity.
1608     fn find_bound_for_assoc_item(
1609         &self,
1610         ty_param_def_id: LocalDefId,
1611         assoc_name: Ident,
1612         span: Span,
1613     ) -> Result<ty::PolyTraitRef<'tcx>, ErrorGuaranteed> {
1614         let tcx = self.tcx();
1615
1616         debug!(
1617             "find_bound_for_assoc_item(ty_param_def_id={:?}, assoc_name={:?}, span={:?})",
1618             ty_param_def_id, assoc_name, span,
1619         );
1620
1621         let predicates = &self
1622             .get_type_parameter_bounds(span, ty_param_def_id.to_def_id(), assoc_name)
1623             .predicates;
1624
1625         debug!("find_bound_for_assoc_item: predicates={:#?}", predicates);
1626
1627         let param_name = tcx.hir().ty_param_name(ty_param_def_id);
1628         self.one_bound_for_assoc_type(
1629             || {
1630                 traits::transitive_bounds_that_define_assoc_type(
1631                     tcx,
1632                     predicates.iter().filter_map(|(p, _)| {
1633                         Some(p.to_opt_poly_trait_pred()?.map_bound(|t| t.trait_ref))
1634                     }),
1635                     assoc_name,
1636                 )
1637             },
1638             || param_name.to_string(),
1639             assoc_name,
1640             span,
1641             || None,
1642         )
1643     }
1644
1645     // Checks that `bounds` contains exactly one element and reports appropriate
1646     // errors otherwise.
1647     fn one_bound_for_assoc_type<I>(
1648         &self,
1649         all_candidates: impl Fn() -> I,
1650         ty_param_name: impl Fn() -> String,
1651         assoc_name: Ident,
1652         span: Span,
1653         is_equality: impl Fn() -> Option<String>,
1654     ) -> Result<ty::PolyTraitRef<'tcx>, ErrorGuaranteed>
1655     where
1656         I: Iterator<Item = ty::PolyTraitRef<'tcx>>,
1657     {
1658         let mut matching_candidates = all_candidates()
1659             .filter(|r| self.trait_defines_associated_type_named(r.def_id(), assoc_name));
1660         let mut const_candidates = all_candidates()
1661             .filter(|r| self.trait_defines_associated_const_named(r.def_id(), assoc_name));
1662
1663         let (bound, next_cand) = match (matching_candidates.next(), const_candidates.next()) {
1664             (Some(bound), _) => (bound, matching_candidates.next()),
1665             (None, Some(bound)) => (bound, const_candidates.next()),
1666             (None, None) => {
1667                 let reported = self.complain_about_assoc_type_not_found(
1668                     all_candidates,
1669                     &ty_param_name(),
1670                     assoc_name,
1671                     span,
1672                 );
1673                 return Err(reported);
1674             }
1675         };
1676         debug!("one_bound_for_assoc_type: bound = {:?}", bound);
1677
1678         if let Some(bound2) = next_cand {
1679             debug!("one_bound_for_assoc_type: bound2 = {:?}", bound2);
1680
1681             let is_equality = is_equality();
1682             let bounds = IntoIterator::into_iter([bound, bound2]).chain(matching_candidates);
1683             let mut err = if is_equality.is_some() {
1684                 // More specific Error Index entry.
1685                 struct_span_err!(
1686                     self.tcx().sess,
1687                     span,
1688                     E0222,
1689                     "ambiguous associated type `{}` in bounds of `{}`",
1690                     assoc_name,
1691                     ty_param_name()
1692                 )
1693             } else {
1694                 struct_span_err!(
1695                     self.tcx().sess,
1696                     span,
1697                     E0221,
1698                     "ambiguous associated type `{}` in bounds of `{}`",
1699                     assoc_name,
1700                     ty_param_name()
1701                 )
1702             };
1703             err.span_label(span, format!("ambiguous associated type `{}`", assoc_name));
1704
1705             let mut where_bounds = vec![];
1706             for bound in bounds {
1707                 let bound_id = bound.def_id();
1708                 let bound_span = self
1709                     .tcx()
1710                     .associated_items(bound_id)
1711                     .find_by_name_and_kind(self.tcx(), assoc_name, ty::AssocKind::Type, bound_id)
1712                     .and_then(|item| self.tcx().hir().span_if_local(item.def_id));
1713
1714                 if let Some(bound_span) = bound_span {
1715                     err.span_label(
1716                         bound_span,
1717                         format!(
1718                             "ambiguous `{}` from `{}`",
1719                             assoc_name,
1720                             bound.print_only_trait_path(),
1721                         ),
1722                     );
1723                     if let Some(constraint) = &is_equality {
1724                         where_bounds.push(format!(
1725                             "        T: {trait}::{assoc} = {constraint}",
1726                             trait=bound.print_only_trait_path(),
1727                             assoc=assoc_name,
1728                             constraint=constraint,
1729                         ));
1730                     } else {
1731                         err.span_suggestion_verbose(
1732                             span.with_hi(assoc_name.span.lo()),
1733                             "use fully qualified syntax to disambiguate",
1734                             format!(
1735                                 "<{} as {}>::",
1736                                 ty_param_name(),
1737                                 bound.print_only_trait_path(),
1738                             ),
1739                             Applicability::MaybeIncorrect,
1740                         );
1741                     }
1742                 } else {
1743                     err.note(&format!(
1744                         "associated type `{}` could derive from `{}`",
1745                         ty_param_name(),
1746                         bound.print_only_trait_path(),
1747                     ));
1748                 }
1749             }
1750             if !where_bounds.is_empty() {
1751                 err.help(&format!(
1752                     "consider introducing a new type parameter `T` and adding `where` constraints:\
1753                      \n    where\n        T: {},\n{}",
1754                     ty_param_name(),
1755                     where_bounds.join(",\n"),
1756                 ));
1757             }
1758             let reported = err.emit();
1759             if !where_bounds.is_empty() {
1760                 return Err(reported);
1761             }
1762         }
1763
1764         Ok(bound)
1765     }
1766
1767     // Create a type from a path to an associated type.
1768     // For a path `A::B::C::D`, `qself_ty` and `qself_def` are the type and def for `A::B::C`
1769     // and item_segment is the path segment for `D`. We return a type and a def for
1770     // the whole path.
1771     // Will fail except for `T::A` and `Self::A`; i.e., if `qself_ty`/`qself_def` are not a type
1772     // parameter or `Self`.
1773     // NOTE: When this function starts resolving `Trait::AssocTy` successfully
1774     // it should also start reporting the `BARE_TRAIT_OBJECTS` lint.
1775     pub fn associated_path_to_ty(
1776         &self,
1777         hir_ref_id: hir::HirId,
1778         span: Span,
1779         qself_ty: Ty<'tcx>,
1780         qself: &hir::Ty<'_>,
1781         assoc_segment: &hir::PathSegment<'_>,
1782         permit_variants: bool,
1783     ) -> Result<(Ty<'tcx>, DefKind, DefId), ErrorGuaranteed> {
1784         let tcx = self.tcx();
1785         let assoc_ident = assoc_segment.ident;
1786         let qself_res = if let hir::TyKind::Path(hir::QPath::Resolved(_, ref path)) = qself.kind {
1787             path.res
1788         } else {
1789             Res::Err
1790         };
1791
1792         debug!("associated_path_to_ty: {:?}::{}", qself_ty, assoc_ident);
1793
1794         // Check if we have an enum variant.
1795         let mut variant_resolution = None;
1796         if let ty::Adt(adt_def, _) = qself_ty.kind() {
1797             if adt_def.is_enum() {
1798                 let variant_def = adt_def
1799                     .variants()
1800                     .iter()
1801                     .find(|vd| tcx.hygienic_eq(assoc_ident, vd.ident(tcx), adt_def.did()));
1802                 if let Some(variant_def) = variant_def {
1803                     if permit_variants {
1804                         tcx.check_stability(variant_def.def_id, Some(hir_ref_id), span, None);
1805                         self.prohibit_generics(slice::from_ref(assoc_segment).iter(), |err| {
1806                             err.note("enum variants can't have type parameters");
1807                             let type_name = tcx.item_name(adt_def.did());
1808                             let msg = format!(
1809                                 "you might have meant to specity type parameters on enum \
1810                                  `{type_name}`"
1811                             );
1812                             let Some(args) = assoc_segment.args else { return; };
1813                             // Get the span of the generics args *including* the leading `::`.
1814                             let args_span = assoc_segment.ident.span.shrink_to_hi().to(args.span_ext);
1815                             if tcx.generics_of(adt_def.did()).count() == 0 {
1816                                 // FIXME(estebank): we could also verify that the arguments being
1817                                 // work for the `enum`, instead of just looking if it takes *any*.
1818                                 err.span_suggestion_verbose(
1819                                     args_span,
1820                                     &format!("{type_name} doesn't have generic parameters"),
1821                                     "",
1822                                     Applicability::MachineApplicable,
1823                                 );
1824                                 return;
1825                             }
1826                             let Ok(snippet) = tcx.sess.source_map().span_to_snippet(args_span) else {
1827                                 err.note(&msg);
1828                                 return;
1829                             };
1830                             let (qself_sugg_span, is_self) = if let hir::TyKind::Path(
1831                                 hir::QPath::Resolved(_, ref path)
1832                             ) = qself.kind {
1833                                 // If the path segment already has type params, we want to overwrite
1834                                 // them.
1835                                 match &path.segments[..] {
1836                                     // `segment` is the previous to last element on the path,
1837                                     // which would normally be the `enum` itself, while the last
1838                                     // `_` `PathSegment` corresponds to the variant.
1839                                     [.., hir::PathSegment {
1840                                         ident,
1841                                         args,
1842                                         res: Some(Res::Def(DefKind::Enum, _)),
1843                                         ..
1844                                     }, _] => (
1845                                         // We need to include the `::` in `Type::Variant::<Args>`
1846                                         // to point the span to `::<Args>`, not just `<Args>`.
1847                                         ident.span.shrink_to_hi().to(args.map_or(
1848                                             ident.span.shrink_to_hi(),
1849                                             |a| a.span_ext)),
1850                                         false,
1851                                     ),
1852                                     [segment] => (
1853                                         // We need to include the `::` in `Type::Variant::<Args>`
1854                                         // to point the span to `::<Args>`, not just `<Args>`.
1855                                         segment.ident.span.shrink_to_hi().to(segment.args.map_or(
1856                                             segment.ident.span.shrink_to_hi(),
1857                                             |a| a.span_ext)),
1858                                         kw::SelfUpper == segment.ident.name,
1859                                     ),
1860                                     _ => {
1861                                         err.note(&msg);
1862                                         return;
1863                                     }
1864                                 }
1865                             } else {
1866                                 err.note(&msg);
1867                                 return;
1868                             };
1869                             let suggestion = vec![
1870                                 if is_self {
1871                                     // Account for people writing `Self::Variant::<Args>`, where
1872                                     // `Self` is the enum, and suggest replacing `Self` with the
1873                                     // appropriate type: `Type::<Args>::Variant`.
1874                                     (qself.span, format!("{type_name}{snippet}"))
1875                                 } else {
1876                                     (qself_sugg_span, snippet)
1877                                 },
1878                                 (args_span, String::new()),
1879                             ];
1880                             err.multipart_suggestion_verbose(
1881                                 &msg,
1882                                 suggestion,
1883                                 Applicability::MaybeIncorrect,
1884                             );
1885                         });
1886                         return Ok((qself_ty, DefKind::Variant, variant_def.def_id));
1887                     } else {
1888                         variant_resolution = Some(variant_def.def_id);
1889                     }
1890                 }
1891             }
1892         }
1893
1894         // Find the type of the associated item, and the trait where the associated
1895         // item is declared.
1896         let bound = match (&qself_ty.kind(), qself_res) {
1897             (_, Res::SelfTy { trait_: Some(_), alias_to: Some((impl_def_id, _)) }) => {
1898                 // `Self` in an impl of a trait -- we have a concrete self type and a
1899                 // trait reference.
1900                 let Some(trait_ref) = tcx.impl_trait_ref(impl_def_id) else {
1901                     // A cycle error occurred, most likely.
1902                     let guar = tcx.sess.delay_span_bug(span, "expected cycle error");
1903                     return Err(guar);
1904                 };
1905
1906                 self.one_bound_for_assoc_type(
1907                     || traits::supertraits(tcx, ty::Binder::dummy(trait_ref)),
1908                     || "Self".to_string(),
1909                     assoc_ident,
1910                     span,
1911                     || None,
1912                 )?
1913             }
1914             (
1915                 &ty::Param(_),
1916                 Res::SelfTy { trait_: Some(param_did), alias_to: None }
1917                 | Res::Def(DefKind::TyParam, param_did),
1918             ) => self.find_bound_for_assoc_item(param_did.expect_local(), assoc_ident, span)?,
1919             _ => {
1920                 let reported = if variant_resolution.is_some() {
1921                     // Variant in type position
1922                     let msg = format!("expected type, found variant `{}`", assoc_ident);
1923                     tcx.sess.span_err(span, &msg)
1924                 } else if qself_ty.is_enum() {
1925                     let mut err = struct_span_err!(
1926                         tcx.sess,
1927                         assoc_ident.span,
1928                         E0599,
1929                         "no variant named `{}` found for enum `{}`",
1930                         assoc_ident,
1931                         qself_ty,
1932                     );
1933
1934                     let adt_def = qself_ty.ty_adt_def().expect("enum is not an ADT");
1935                     if let Some(suggested_name) = find_best_match_for_name(
1936                         &adt_def
1937                             .variants()
1938                             .iter()
1939                             .map(|variant| variant.name)
1940                             .collect::<Vec<Symbol>>(),
1941                         assoc_ident.name,
1942                         None,
1943                     ) {
1944                         err.span_suggestion(
1945                             assoc_ident.span,
1946                             "there is a variant with a similar name",
1947                             suggested_name,
1948                             Applicability::MaybeIncorrect,
1949                         );
1950                     } else {
1951                         err.span_label(
1952                             assoc_ident.span,
1953                             format!("variant not found in `{}`", qself_ty),
1954                         );
1955                     }
1956
1957                     if let Some(sp) = tcx.hir().span_if_local(adt_def.did()) {
1958                         err.span_label(sp, format!("variant `{}` not found here", assoc_ident));
1959                     }
1960
1961                     err.emit()
1962                 } else if let Some(reported) = qself_ty.error_reported() {
1963                     reported
1964                 } else {
1965                     // Don't print `TyErr` to the user.
1966                     self.report_ambiguous_associated_type(
1967                         span,
1968                         &qself_ty.to_string(),
1969                         "Trait",
1970                         assoc_ident.name,
1971                     )
1972                 };
1973                 return Err(reported);
1974             }
1975         };
1976
1977         let trait_did = bound.def_id();
1978         let (assoc_ident, def_scope) =
1979             tcx.adjust_ident_and_get_scope(assoc_ident, trait_did, hir_ref_id);
1980
1981         // We have already adjusted the item name above, so compare with `ident.normalize_to_macros_2_0()` instead
1982         // of calling `filter_by_name_and_kind`.
1983         let item = tcx.associated_items(trait_did).in_definition_order().find(|i| {
1984             i.kind.namespace() == Namespace::TypeNS
1985                 && i.ident(tcx).normalize_to_macros_2_0() == assoc_ident
1986         });
1987         // Assume that if it's not matched, there must be a const defined with the same name
1988         // but it was used in a type position.
1989         let Some(item) = item else {
1990             let msg = format!("found associated const `{assoc_ident}` when type was expected");
1991             let guar = tcx.sess.struct_span_err(span, &msg).emit();
1992             return Err(guar);
1993         };
1994
1995         let ty = self.projected_ty_from_poly_trait_ref(span, item.def_id, assoc_segment, bound);
1996         let ty = self.normalize_ty(span, ty);
1997
1998         let kind = DefKind::AssocTy;
1999         if !item.visibility(tcx).is_accessible_from(def_scope, tcx) {
2000             let kind = kind.descr(item.def_id);
2001             let msg = format!("{} `{}` is private", kind, assoc_ident);
2002             tcx.sess
2003                 .struct_span_err(span, &msg)
2004                 .span_label(span, &format!("private {}", kind))
2005                 .emit();
2006         }
2007         tcx.check_stability(item.def_id, Some(hir_ref_id), span, None);
2008
2009         if let Some(variant_def_id) = variant_resolution {
2010             tcx.struct_span_lint_hir(AMBIGUOUS_ASSOCIATED_ITEMS, hir_ref_id, span, |lint| {
2011                 let mut err = lint.build("ambiguous associated item");
2012                 let mut could_refer_to = |kind: DefKind, def_id, also| {
2013                     let note_msg = format!(
2014                         "`{}` could{} refer to the {} defined here",
2015                         assoc_ident,
2016                         also,
2017                         kind.descr(def_id)
2018                     );
2019                     err.span_note(tcx.def_span(def_id), &note_msg);
2020                 };
2021
2022                 could_refer_to(DefKind::Variant, variant_def_id, "");
2023                 could_refer_to(kind, item.def_id, " also");
2024
2025                 err.span_suggestion(
2026                     span,
2027                     "use fully-qualified syntax",
2028                     format!("<{} as {}>::{}", qself_ty, tcx.item_name(trait_did), assoc_ident),
2029                     Applicability::MachineApplicable,
2030                 );
2031
2032                 err.emit();
2033             });
2034         }
2035         Ok((ty, kind, item.def_id))
2036     }
2037
2038     fn qpath_to_ty(
2039         &self,
2040         span: Span,
2041         opt_self_ty: Option<Ty<'tcx>>,
2042         item_def_id: DefId,
2043         trait_segment: &hir::PathSegment<'_>,
2044         item_segment: &hir::PathSegment<'_>,
2045     ) -> Ty<'tcx> {
2046         let tcx = self.tcx();
2047
2048         let trait_def_id = tcx.parent(item_def_id);
2049
2050         debug!("qpath_to_ty: trait_def_id={:?}", trait_def_id);
2051
2052         let Some(self_ty) = opt_self_ty else {
2053             let path_str = tcx.def_path_str(trait_def_id);
2054
2055             let def_id = self.item_def_id();
2056
2057             debug!("qpath_to_ty: self.item_def_id()={:?}", def_id);
2058
2059             let parent_def_id = def_id
2060                 .and_then(|def_id| {
2061                     def_id.as_local().map(|def_id| tcx.hir().local_def_id_to_hir_id(def_id))
2062                 })
2063                 .map(|hir_id| tcx.hir().get_parent_item(hir_id).to_def_id());
2064
2065             debug!("qpath_to_ty: parent_def_id={:?}", parent_def_id);
2066
2067             // If the trait in segment is the same as the trait defining the item,
2068             // use the `<Self as ..>` syntax in the error.
2069             let is_part_of_self_trait_constraints = def_id == Some(trait_def_id);
2070             let is_part_of_fn_in_self_trait = parent_def_id == Some(trait_def_id);
2071
2072             let type_name = if is_part_of_self_trait_constraints || is_part_of_fn_in_self_trait {
2073                 "Self"
2074             } else {
2075                 "Type"
2076             };
2077
2078             self.report_ambiguous_associated_type(
2079                 span,
2080                 type_name,
2081                 &path_str,
2082                 item_segment.ident.name,
2083             );
2084             return tcx.ty_error();
2085         };
2086
2087         debug!("qpath_to_ty: self_type={:?}", self_ty);
2088
2089         let trait_ref =
2090             self.ast_path_to_mono_trait_ref(span, trait_def_id, self_ty, trait_segment, false);
2091
2092         let item_substs = self.create_substs_for_associated_item(
2093             tcx,
2094             span,
2095             item_def_id,
2096             item_segment,
2097             trait_ref.substs,
2098         );
2099
2100         debug!("qpath_to_ty: trait_ref={:?}", trait_ref);
2101
2102         self.normalize_ty(span, tcx.mk_projection(item_def_id, item_substs))
2103     }
2104
2105     pub fn prohibit_generics<'a>(
2106         &self,
2107         segments: impl Iterator<Item = &'a hir::PathSegment<'a>> + Clone,
2108         extend: impl Fn(&mut Diagnostic),
2109     ) -> bool {
2110         let args = segments.clone().flat_map(|segment| segment.args().args);
2111
2112         let (lt, ty, ct, inf) =
2113             args.clone().fold((false, false, false, false), |(lt, ty, ct, inf), arg| match arg {
2114                 hir::GenericArg::Lifetime(_) => (true, ty, ct, inf),
2115                 hir::GenericArg::Type(_) => (lt, true, ct, inf),
2116                 hir::GenericArg::Const(_) => (lt, ty, true, inf),
2117                 hir::GenericArg::Infer(_) => (lt, ty, ct, true),
2118             });
2119         let mut emitted = false;
2120         if lt || ty || ct || inf {
2121             let types_and_spans: Vec<_> = segments
2122                 .clone()
2123                 .flat_map(|segment| {
2124                     segment.res.and_then(|res| {
2125                         if segment.args().args.is_empty() {
2126                             None
2127                         } else {
2128                             Some((
2129                             match res {
2130                                 Res::PrimTy(ty) => format!("{} `{}`", res.descr(), ty.name()),
2131                                 Res::Def(_, def_id)
2132                                 if let Some(name) = self.tcx().opt_item_name(def_id) => {
2133                                     format!("{} `{name}`", res.descr())
2134                                 }
2135                                 Res::Err => "this type".to_string(),
2136                                 _ => res.descr().to_string(),
2137                             },
2138                             segment.ident.span,
2139                         ))
2140                         }
2141                     })
2142                 })
2143                 .collect();
2144             let this_type = match &types_and_spans[..] {
2145                 [.., _, (last, _)] => format!(
2146                     "{} and {last}",
2147                     types_and_spans[..types_and_spans.len() - 1]
2148                         .iter()
2149                         .map(|(x, _)| x.as_str())
2150                         .intersperse(&", ")
2151                         .collect::<String>()
2152                 ),
2153                 [(only, _)] => only.to_string(),
2154                 [] => "this type".to_string(),
2155             };
2156
2157             let arg_spans: Vec<Span> = args.map(|arg| arg.span()).collect();
2158
2159             let mut kinds = Vec::with_capacity(4);
2160             if lt {
2161                 kinds.push("lifetime");
2162             }
2163             if ty {
2164                 kinds.push("type");
2165             }
2166             if ct {
2167                 kinds.push("const");
2168             }
2169             if inf {
2170                 kinds.push("generic");
2171             }
2172             let (kind, s) = match kinds[..] {
2173                 [.., _, last] => (
2174                     format!(
2175                         "{} and {last}",
2176                         kinds[..kinds.len() - 1]
2177                             .iter()
2178                             .map(|&x| x)
2179                             .intersperse(", ")
2180                             .collect::<String>()
2181                     ),
2182                     "s",
2183                 ),
2184                 [only] => (format!("{only}"), ""),
2185                 [] => unreachable!(),
2186             };
2187             let last_span = *arg_spans.last().unwrap();
2188             let span: MultiSpan = arg_spans.into();
2189             let mut err = struct_span_err!(
2190                 self.tcx().sess,
2191                 span,
2192                 E0109,
2193                 "{kind} arguments are not allowed on {this_type}",
2194             );
2195             err.span_label(last_span, format!("{kind} argument{s} not allowed"));
2196             for (what, span) in types_and_spans {
2197                 err.span_label(span, format!("not allowed on {what}"));
2198             }
2199             extend(&mut err);
2200             err.emit();
2201             emitted = true;
2202         }
2203
2204         for segment in segments {
2205             // Only emit the first error to avoid overloading the user with error messages.
2206             if let [binding, ..] = segment.args().bindings {
2207                 Self::prohibit_assoc_ty_binding(self.tcx(), binding.span);
2208                 return true;
2209             }
2210         }
2211         emitted
2212     }
2213
2214     // FIXME(eddyb, varkor) handle type paths here too, not just value ones.
2215     pub fn def_ids_for_value_path_segments(
2216         &self,
2217         segments: &[hir::PathSegment<'_>],
2218         self_ty: Option<Ty<'tcx>>,
2219         kind: DefKind,
2220         def_id: DefId,
2221     ) -> Vec<PathSeg> {
2222         // We need to extract the type parameters supplied by the user in
2223         // the path `path`. Due to the current setup, this is a bit of a
2224         // tricky-process; the problem is that resolve only tells us the
2225         // end-point of the path resolution, and not the intermediate steps.
2226         // Luckily, we can (at least for now) deduce the intermediate steps
2227         // just from the end-point.
2228         //
2229         // There are basically five cases to consider:
2230         //
2231         // 1. Reference to a constructor of a struct:
2232         //
2233         //        struct Foo<T>(...)
2234         //
2235         //    In this case, the parameters are declared in the type space.
2236         //
2237         // 2. Reference to a constructor of an enum variant:
2238         //
2239         //        enum E<T> { Foo(...) }
2240         //
2241         //    In this case, the parameters are defined in the type space,
2242         //    but may be specified either on the type or the variant.
2243         //
2244         // 3. Reference to a fn item or a free constant:
2245         //
2246         //        fn foo<T>() { }
2247         //
2248         //    In this case, the path will again always have the form
2249         //    `a::b::foo::<T>` where only the final segment should have
2250         //    type parameters. However, in this case, those parameters are
2251         //    declared on a value, and hence are in the `FnSpace`.
2252         //
2253         // 4. Reference to a method or an associated constant:
2254         //
2255         //        impl<A> SomeStruct<A> {
2256         //            fn foo<B>(...)
2257         //        }
2258         //
2259         //    Here we can have a path like
2260         //    `a::b::SomeStruct::<A>::foo::<B>`, in which case parameters
2261         //    may appear in two places. The penultimate segment,
2262         //    `SomeStruct::<A>`, contains parameters in TypeSpace, and the
2263         //    final segment, `foo::<B>` contains parameters in fn space.
2264         //
2265         // The first step then is to categorize the segments appropriately.
2266
2267         let tcx = self.tcx();
2268
2269         assert!(!segments.is_empty());
2270         let last = segments.len() - 1;
2271
2272         let mut path_segs = vec![];
2273
2274         match kind {
2275             // Case 1. Reference to a struct constructor.
2276             DefKind::Ctor(CtorOf::Struct, ..) => {
2277                 // Everything but the final segment should have no
2278                 // parameters at all.
2279                 let generics = tcx.generics_of(def_id);
2280                 // Variant and struct constructors use the
2281                 // generics of their parent type definition.
2282                 let generics_def_id = generics.parent.unwrap_or(def_id);
2283                 path_segs.push(PathSeg(generics_def_id, last));
2284             }
2285
2286             // Case 2. Reference to a variant constructor.
2287             DefKind::Ctor(CtorOf::Variant, ..) | DefKind::Variant => {
2288                 let adt_def = self_ty.map(|t| t.ty_adt_def().unwrap());
2289                 let (generics_def_id, index) = if let Some(adt_def) = adt_def {
2290                     debug_assert!(adt_def.is_enum());
2291                     (adt_def.did(), last)
2292                 } else if last >= 1 && segments[last - 1].args.is_some() {
2293                     // Everything but the penultimate segment should have no
2294                     // parameters at all.
2295                     let mut def_id = def_id;
2296
2297                     // `DefKind::Ctor` -> `DefKind::Variant`
2298                     if let DefKind::Ctor(..) = kind {
2299                         def_id = tcx.parent(def_id);
2300                     }
2301
2302                     // `DefKind::Variant` -> `DefKind::Enum`
2303                     let enum_def_id = tcx.parent(def_id);
2304                     (enum_def_id, last - 1)
2305                 } else {
2306                     // FIXME: lint here recommending `Enum::<...>::Variant` form
2307                     // instead of `Enum::Variant::<...>` form.
2308
2309                     // Everything but the final segment should have no
2310                     // parameters at all.
2311                     let generics = tcx.generics_of(def_id);
2312                     // Variant and struct constructors use the
2313                     // generics of their parent type definition.
2314                     (generics.parent.unwrap_or(def_id), last)
2315                 };
2316                 path_segs.push(PathSeg(generics_def_id, index));
2317             }
2318
2319             // Case 3. Reference to a top-level value.
2320             DefKind::Fn | DefKind::Const | DefKind::ConstParam | DefKind::Static(_) => {
2321                 path_segs.push(PathSeg(def_id, last));
2322             }
2323
2324             // Case 4. Reference to a method or associated const.
2325             DefKind::AssocFn | DefKind::AssocConst => {
2326                 if segments.len() >= 2 {
2327                     let generics = tcx.generics_of(def_id);
2328                     path_segs.push(PathSeg(generics.parent.unwrap(), last - 1));
2329                 }
2330                 path_segs.push(PathSeg(def_id, last));
2331             }
2332
2333             kind => bug!("unexpected definition kind {:?} for {:?}", kind, def_id),
2334         }
2335
2336         debug!("path_segs = {:?}", path_segs);
2337
2338         path_segs
2339     }
2340
2341     // Check a type `Path` and convert it to a `Ty`.
2342     pub fn res_to_ty(
2343         &self,
2344         opt_self_ty: Option<Ty<'tcx>>,
2345         path: &hir::Path<'_>,
2346         permit_variants: bool,
2347     ) -> Ty<'tcx> {
2348         let tcx = self.tcx();
2349
2350         debug!(
2351             "res_to_ty(res={:?}, opt_self_ty={:?}, path_segments={:?})",
2352             path.res, opt_self_ty, path.segments
2353         );
2354
2355         let span = path.span;
2356         match path.res {
2357             Res::Def(DefKind::OpaqueTy, did) => {
2358                 // Check for desugared `impl Trait`.
2359                 assert!(ty::is_impl_trait_defn(tcx, did).is_none());
2360                 let item_segment = path.segments.split_last().unwrap();
2361                 self.prohibit_generics(item_segment.1.iter(), |err| {
2362                     err.note("`impl Trait` types can't have type parameters");
2363                 });
2364                 let substs = self.ast_path_substs_for_ty(span, did, item_segment.0);
2365                 self.normalize_ty(span, tcx.mk_opaque(did, substs))
2366             }
2367             Res::Def(
2368                 DefKind::Enum
2369                 | DefKind::TyAlias
2370                 | DefKind::Struct
2371                 | DefKind::Union
2372                 | DefKind::ForeignTy,
2373                 did,
2374             ) => {
2375                 assert_eq!(opt_self_ty, None);
2376                 self.prohibit_generics(path.segments.split_last().unwrap().1.iter(), |_| {});
2377                 self.ast_path_to_ty(span, did, path.segments.last().unwrap())
2378             }
2379             Res::Def(kind @ DefKind::Variant, def_id) if permit_variants => {
2380                 // Convert "variant type" as if it were a real type.
2381                 // The resulting `Ty` is type of the variant's enum for now.
2382                 assert_eq!(opt_self_ty, None);
2383
2384                 let path_segs =
2385                     self.def_ids_for_value_path_segments(path.segments, None, kind, def_id);
2386                 let generic_segs: FxHashSet<_> =
2387                     path_segs.iter().map(|PathSeg(_, index)| index).collect();
2388                 self.prohibit_generics(
2389                     path.segments.iter().enumerate().filter_map(|(index, seg)| {
2390                         if !generic_segs.contains(&index) { Some(seg) } else { None }
2391                     }),
2392                     |err| {
2393                         err.note("enum variants can't have type parameters");
2394                     },
2395                 );
2396
2397                 let PathSeg(def_id, index) = path_segs.last().unwrap();
2398                 self.ast_path_to_ty(span, *def_id, &path.segments[*index])
2399             }
2400             Res::Def(DefKind::TyParam, def_id) => {
2401                 assert_eq!(opt_self_ty, None);
2402                 self.prohibit_generics(path.segments.iter(), |err| {
2403                     if let Some(span) = tcx.def_ident_span(def_id) {
2404                         let name = tcx.item_name(def_id);
2405                         err.span_note(span, &format!("type parameter `{name}` defined here"));
2406                     }
2407                 });
2408
2409                 let def_id = def_id.expect_local();
2410                 let item_def_id = tcx.hir().ty_param_owner(def_id);
2411                 let generics = tcx.generics_of(item_def_id);
2412                 let index = generics.param_def_id_to_index[&def_id.to_def_id()];
2413                 tcx.mk_ty_param(index, tcx.hir().ty_param_name(def_id))
2414             }
2415             Res::SelfTy { trait_: Some(_), alias_to: None } => {
2416                 // `Self` in trait or type alias.
2417                 assert_eq!(opt_self_ty, None);
2418                 self.prohibit_generics(path.segments.iter(), |err| {
2419                     if let [hir::PathSegment { args: Some(args), ident, .. }] = &path.segments[..] {
2420                         err.span_suggestion_verbose(
2421                             ident.span.shrink_to_hi().to(args.span_ext),
2422                             "the `Self` type doesn't accept type parameters",
2423                             "",
2424                             Applicability::MaybeIncorrect,
2425                         );
2426                     }
2427                 });
2428                 tcx.types.self_param
2429             }
2430             Res::SelfTy { trait_: _, alias_to: Some((def_id, forbid_generic)) } => {
2431                 // `Self` in impl (we know the concrete type).
2432                 assert_eq!(opt_self_ty, None);
2433                 // Try to evaluate any array length constants.
2434                 let ty = tcx.at(span).type_of(def_id);
2435                 let span_of_impl = tcx.span_of_impl(def_id);
2436                 self.prohibit_generics(path.segments.iter(), |err| {
2437                     let def_id = match *ty.kind() {
2438                         ty::Adt(self_def, _) => self_def.did(),
2439                         _ => return,
2440                     };
2441
2442                     let type_name = tcx.item_name(def_id);
2443                     let span_of_ty = tcx.def_ident_span(def_id);
2444                     let generics = tcx.generics_of(def_id).count();
2445
2446                     let msg = format!("`Self` is of type `{ty}`");
2447                     if let (Ok(i_sp), Some(t_sp)) = (span_of_impl, span_of_ty) {
2448                         let mut span: MultiSpan = vec![t_sp].into();
2449                         span.push_span_label(
2450                             i_sp,
2451                             &format!("`Self` is on type `{type_name}` in this `impl`"),
2452                         );
2453                         let mut postfix = "";
2454                         if generics == 0 {
2455                             postfix = ", which doesn't have generic parameters";
2456                         }
2457                         span.push_span_label(
2458                             t_sp,
2459                             &format!("`Self` corresponds to this type{postfix}"),
2460                         );
2461                         err.span_note(span, &msg);
2462                     } else {
2463                         err.note(&msg);
2464                     }
2465                     for segment in path.segments {
2466                         if let Some(args) = segment.args && segment.ident.name == kw::SelfUpper {
2467                             if generics == 0 {
2468                                 // FIXME(estebank): we could also verify that the arguments being
2469                                 // work for the `enum`, instead of just looking if it takes *any*.
2470                                 err.span_suggestion_verbose(
2471                                     segment.ident.span.shrink_to_hi().to(args.span_ext),
2472                                     "the `Self` type doesn't accept type parameters",
2473                                     "",
2474                                     Applicability::MachineApplicable,
2475                                 );
2476                                 return;
2477                             } else {
2478                                 err.span_suggestion_verbose(
2479                                     segment.ident.span,
2480                                     format!(
2481                                         "the `Self` type doesn't accept type parameters, use the \
2482                                         concrete type's name `{type_name}` instead if you want to \
2483                                         specify its type parameters"
2484                                     ),
2485                                     type_name,
2486                                     Applicability::MaybeIncorrect,
2487                                 );
2488                             }
2489                         }
2490                     }
2491                 });
2492                 // HACK(min_const_generics): Forbid generic `Self` types
2493                 // here as we can't easily do that during nameres.
2494                 //
2495                 // We do this before normalization as we otherwise allow
2496                 // ```rust
2497                 // trait AlwaysApplicable { type Assoc; }
2498                 // impl<T: ?Sized> AlwaysApplicable for T { type Assoc = usize; }
2499                 //
2500                 // trait BindsParam<T> {
2501                 //     type ArrayTy;
2502                 // }
2503                 // impl<T> BindsParam<T> for <T as AlwaysApplicable>::Assoc {
2504                 //    type ArrayTy = [u8; Self::MAX];
2505                 // }
2506                 // ```
2507                 // Note that the normalization happens in the param env of
2508                 // the anon const, which is empty. This is why the
2509                 // `AlwaysApplicable` impl needs a `T: ?Sized` bound for
2510                 // this to compile if we were to normalize here.
2511                 if forbid_generic && ty.needs_subst() {
2512                     let mut err = tcx.sess.struct_span_err(
2513                         path.span,
2514                         "generic `Self` types are currently not permitted in anonymous constants",
2515                     );
2516                     if let Some(hir::Node::Item(&hir::Item {
2517                         kind: hir::ItemKind::Impl(ref impl_),
2518                         ..
2519                     })) = tcx.hir().get_if_local(def_id)
2520                     {
2521                         err.span_note(impl_.self_ty.span, "not a concrete type");
2522                     }
2523                     err.emit();
2524                     tcx.ty_error()
2525                 } else {
2526                     self.normalize_ty(span, ty)
2527                 }
2528             }
2529             Res::Def(DefKind::AssocTy, def_id) => {
2530                 debug_assert!(path.segments.len() >= 2);
2531                 self.prohibit_generics(path.segments[..path.segments.len() - 2].iter(), |_| {});
2532                 self.qpath_to_ty(
2533                     span,
2534                     opt_self_ty,
2535                     def_id,
2536                     &path.segments[path.segments.len() - 2],
2537                     path.segments.last().unwrap(),
2538                 )
2539             }
2540             Res::PrimTy(prim_ty) => {
2541                 assert_eq!(opt_self_ty, None);
2542                 self.prohibit_generics(path.segments.iter(), |err| {
2543                     let name = prim_ty.name_str();
2544                     for segment in path.segments {
2545                         if let Some(args) = segment.args {
2546                             err.span_suggestion_verbose(
2547                                 segment.ident.span.shrink_to_hi().to(args.span_ext),
2548                                 &format!("primitive type `{name}` doesn't have generic parameters"),
2549                                 "",
2550                                 Applicability::MaybeIncorrect,
2551                             );
2552                         }
2553                     }
2554                 });
2555                 match prim_ty {
2556                     hir::PrimTy::Bool => tcx.types.bool,
2557                     hir::PrimTy::Char => tcx.types.char,
2558                     hir::PrimTy::Int(it) => tcx.mk_mach_int(ty::int_ty(it)),
2559                     hir::PrimTy::Uint(uit) => tcx.mk_mach_uint(ty::uint_ty(uit)),
2560                     hir::PrimTy::Float(ft) => tcx.mk_mach_float(ty::float_ty(ft)),
2561                     hir::PrimTy::Str => tcx.types.str_,
2562                 }
2563             }
2564             Res::Err => {
2565                 self.set_tainted_by_errors();
2566                 self.tcx().ty_error()
2567             }
2568             _ => span_bug!(span, "unexpected resolution: {:?}", path.res),
2569         }
2570     }
2571
2572     /// Parses the programmer's textual representation of a type into our
2573     /// internal notion of a type.
2574     pub fn ast_ty_to_ty(&self, ast_ty: &hir::Ty<'_>) -> Ty<'tcx> {
2575         self.ast_ty_to_ty_inner(ast_ty, false, false)
2576     }
2577
2578     /// Parses the programmer's textual representation of a type into our
2579     /// internal notion of a type.  This is meant to be used within a path.
2580     pub fn ast_ty_to_ty_in_path(&self, ast_ty: &hir::Ty<'_>) -> Ty<'tcx> {
2581         self.ast_ty_to_ty_inner(ast_ty, false, true)
2582     }
2583
2584     /// Turns a `hir::Ty` into a `Ty`. For diagnostics' purposes we keep track of whether trait
2585     /// objects are borrowed like `&dyn Trait` to avoid emitting redundant errors.
2586     #[tracing::instrument(level = "debug", skip(self))]
2587     fn ast_ty_to_ty_inner(&self, ast_ty: &hir::Ty<'_>, borrowed: bool, in_path: bool) -> Ty<'tcx> {
2588         let tcx = self.tcx();
2589
2590         let result_ty = match ast_ty.kind {
2591             hir::TyKind::Slice(ref ty) => tcx.mk_slice(self.ast_ty_to_ty(ty)),
2592             hir::TyKind::Ptr(ref mt) => {
2593                 tcx.mk_ptr(ty::TypeAndMut { ty: self.ast_ty_to_ty(mt.ty), mutbl: mt.mutbl })
2594             }
2595             hir::TyKind::Rptr(ref region, ref mt) => {
2596                 let r = self.ast_region_to_region(region, None);
2597                 debug!(?r);
2598                 let t = self.ast_ty_to_ty_inner(mt.ty, true, false);
2599                 tcx.mk_ref(r, ty::TypeAndMut { ty: t, mutbl: mt.mutbl })
2600             }
2601             hir::TyKind::Never => tcx.types.never,
2602             hir::TyKind::Tup(fields) => tcx.mk_tup(fields.iter().map(|t| self.ast_ty_to_ty(t))),
2603             hir::TyKind::BareFn(bf) => {
2604                 require_c_abi_if_c_variadic(tcx, bf.decl, bf.abi, ast_ty.span);
2605
2606                 tcx.mk_fn_ptr(self.ty_of_fn(
2607                     ast_ty.hir_id,
2608                     bf.unsafety,
2609                     bf.abi,
2610                     bf.decl,
2611                     None,
2612                     Some(ast_ty),
2613                 ))
2614             }
2615             hir::TyKind::TraitObject(bounds, ref lifetime, _) => {
2616                 self.maybe_lint_bare_trait(ast_ty, in_path);
2617                 self.conv_object_ty_poly_trait_ref(ast_ty.span, bounds, lifetime, borrowed)
2618             }
2619             hir::TyKind::Path(hir::QPath::Resolved(ref maybe_qself, ref path)) => {
2620                 debug!(?maybe_qself, ?path);
2621                 let opt_self_ty = maybe_qself.as_ref().map(|qself| self.ast_ty_to_ty(qself));
2622                 self.res_to_ty(opt_self_ty, path, false)
2623             }
2624             hir::TyKind::OpaqueDef(item_id, lifetimes) => {
2625                 let opaque_ty = tcx.hir().item(item_id);
2626                 let def_id = item_id.def_id.to_def_id();
2627
2628                 match opaque_ty.kind {
2629                     hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => {
2630                         self.impl_trait_ty_to_ty(def_id, lifetimes, origin)
2631                     }
2632                     ref i => bug!("`impl Trait` pointed to non-opaque type?? {:#?}", i),
2633                 }
2634             }
2635             hir::TyKind::Path(hir::QPath::TypeRelative(ref qself, ref segment)) => {
2636                 debug!(?qself, ?segment);
2637                 let ty = self.ast_ty_to_ty_inner(qself, false, true);
2638                 self.associated_path_to_ty(ast_ty.hir_id, ast_ty.span, ty, qself, segment, false)
2639                     .map(|(ty, _, _)| ty)
2640                     .unwrap_or_else(|_| tcx.ty_error())
2641             }
2642             hir::TyKind::Path(hir::QPath::LangItem(lang_item, span, _)) => {
2643                 let def_id = tcx.require_lang_item(lang_item, Some(span));
2644                 let (substs, _) = self.create_substs_for_ast_path(
2645                     span,
2646                     def_id,
2647                     &[],
2648                     &hir::PathSegment::invalid(),
2649                     &GenericArgs::none(),
2650                     true,
2651                     None,
2652                 );
2653                 EarlyBinder(self.normalize_ty(span, tcx.at(span).type_of(def_id)))
2654                     .subst(tcx, substs)
2655             }
2656             hir::TyKind::Array(ref ty, ref length) => {
2657                 let length = match length {
2658                     &hir::ArrayLen::Infer(_, span) => self.ct_infer(tcx.types.usize, None, span),
2659                     hir::ArrayLen::Body(constant) => {
2660                         let length_def_id = tcx.hir().local_def_id(constant.hir_id);
2661                         ty::Const::from_anon_const(tcx, length_def_id)
2662                     }
2663                 };
2664
2665                 let array_ty = tcx.mk_ty(ty::Array(self.ast_ty_to_ty(ty), length));
2666                 self.normalize_ty(ast_ty.span, array_ty)
2667             }
2668             hir::TyKind::Typeof(ref e) => {
2669                 let ty = tcx.type_of(tcx.hir().local_def_id(e.hir_id));
2670                 let span = ast_ty.span;
2671                 tcx.sess.emit_err(TypeofReservedKeywordUsed {
2672                     span,
2673                     ty,
2674                     opt_sugg: Some((span, Applicability::MachineApplicable))
2675                         .filter(|_| ty.is_suggestable(tcx, false)),
2676                 });
2677
2678                 ty
2679             }
2680             hir::TyKind::Infer => {
2681                 // Infer also appears as the type of arguments or return
2682                 // values in an ExprKind::Closure, or as
2683                 // the type of local variables. Both of these cases are
2684                 // handled specially and will not descend into this routine.
2685                 self.ty_infer(None, ast_ty.span)
2686             }
2687             hir::TyKind::Err => tcx.ty_error(),
2688         };
2689
2690         debug!(?result_ty);
2691
2692         self.record_ty(ast_ty.hir_id, result_ty, ast_ty.span);
2693         result_ty
2694     }
2695
2696     fn impl_trait_ty_to_ty(
2697         &self,
2698         def_id: DefId,
2699         lifetimes: &[hir::GenericArg<'_>],
2700         origin: OpaqueTyOrigin,
2701     ) -> Ty<'tcx> {
2702         debug!("impl_trait_ty_to_ty(def_id={:?}, lifetimes={:?})", def_id, lifetimes);
2703         let tcx = self.tcx();
2704
2705         let generics = tcx.generics_of(def_id);
2706
2707         debug!("impl_trait_ty_to_ty: generics={:?}", generics);
2708         let substs = InternalSubsts::for_item(tcx, def_id, |param, _| {
2709             if let Some(i) = (param.index as usize).checked_sub(generics.parent_count) {
2710                 // Our own parameters are the resolved lifetimes.
2711                 if let GenericParamDefKind::Lifetime = param.kind {
2712                     if let hir::GenericArg::Lifetime(lifetime) = &lifetimes[i] {
2713                         self.ast_region_to_region(lifetime, None).into()
2714                     } else {
2715                         bug!()
2716                     }
2717                 } else {
2718                     bug!()
2719                 }
2720             } else {
2721                 match param.kind {
2722                     // For RPIT (return position impl trait), only lifetimes
2723                     // mentioned in the impl Trait predicate are captured by
2724                     // the opaque type, so the lifetime parameters from the
2725                     // parent item need to be replaced with `'static`.
2726                     //
2727                     // For `impl Trait` in the types of statics, constants,
2728                     // locals and type aliases. These capture all parent
2729                     // lifetimes, so they can use their identity subst.
2730                     GenericParamDefKind::Lifetime
2731                         if matches!(
2732                             origin,
2733                             hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..)
2734                         ) =>
2735                     {
2736                         tcx.lifetimes.re_static.into()
2737                     }
2738                     _ => tcx.mk_param_from_def(param),
2739                 }
2740             }
2741         });
2742         debug!("impl_trait_ty_to_ty: substs={:?}", substs);
2743
2744         let ty = tcx.mk_opaque(def_id, substs);
2745         debug!("impl_trait_ty_to_ty: {}", ty);
2746         ty
2747     }
2748
2749     pub fn ty_of_arg(&self, ty: &hir::Ty<'_>, expected_ty: Option<Ty<'tcx>>) -> Ty<'tcx> {
2750         match ty.kind {
2751             hir::TyKind::Infer if expected_ty.is_some() => {
2752                 self.record_ty(ty.hir_id, expected_ty.unwrap(), ty.span);
2753                 expected_ty.unwrap()
2754             }
2755             _ => self.ast_ty_to_ty(ty),
2756         }
2757     }
2758
2759     pub fn ty_of_fn(
2760         &self,
2761         hir_id: hir::HirId,
2762         unsafety: hir::Unsafety,
2763         abi: abi::Abi,
2764         decl: &hir::FnDecl<'_>,
2765         generics: Option<&hir::Generics<'_>>,
2766         hir_ty: Option<&hir::Ty<'_>>,
2767     ) -> ty::PolyFnSig<'tcx> {
2768         debug!("ty_of_fn");
2769
2770         let tcx = self.tcx();
2771         let bound_vars = tcx.late_bound_vars(hir_id);
2772         debug!(?bound_vars);
2773
2774         // We proactively collect all the inferred type params to emit a single error per fn def.
2775         let mut visitor = HirPlaceholderCollector::default();
2776         let mut infer_replacements = vec![];
2777
2778         if let Some(generics) = generics {
2779             walk_generics(&mut visitor, generics);
2780         }
2781
2782         let input_tys: Vec<_> = decl
2783             .inputs
2784             .iter()
2785             .enumerate()
2786             .map(|(i, a)| {
2787                 if let hir::TyKind::Infer = a.kind && !self.allow_ty_infer() {
2788                     if let Some(suggested_ty) =
2789                         self.suggest_trait_fn_ty_for_impl_fn_infer(hir_id, Some(i))
2790                     {
2791                         infer_replacements.push((a.span, suggested_ty.to_string()));
2792                         return suggested_ty;
2793                     }
2794                 }
2795
2796                 // Only visit the type looking for `_` if we didn't fix the type above
2797                 visitor.visit_ty(a);
2798                 self.ty_of_arg(a, None)
2799             })
2800             .collect();
2801
2802         let output_ty = match decl.output {
2803             hir::FnRetTy::Return(output) => {
2804                 if let hir::TyKind::Infer = output.kind
2805                     && !self.allow_ty_infer()
2806                     && let Some(suggested_ty) =
2807                         self.suggest_trait_fn_ty_for_impl_fn_infer(hir_id, None)
2808                 {
2809                     infer_replacements.push((output.span, suggested_ty.to_string()));
2810                     suggested_ty
2811                 } else {
2812                     visitor.visit_ty(output);
2813                     self.ast_ty_to_ty(output)
2814                 }
2815             }
2816             hir::FnRetTy::DefaultReturn(..) => tcx.mk_unit(),
2817         };
2818
2819         debug!("ty_of_fn: output_ty={:?}", output_ty);
2820
2821         let fn_ty = tcx.mk_fn_sig(input_tys.into_iter(), output_ty, decl.c_variadic, unsafety, abi);
2822         let bare_fn_ty = ty::Binder::bind_with_vars(fn_ty, bound_vars);
2823
2824         if !self.allow_ty_infer() && !(visitor.0.is_empty() && infer_replacements.is_empty()) {
2825             // We always collect the spans for placeholder types when evaluating `fn`s, but we
2826             // only want to emit an error complaining about them if infer types (`_`) are not
2827             // allowed. `allow_ty_infer` gates this behavior. We check for the presence of
2828             // `ident_span` to not emit an error twice when we have `fn foo(_: fn() -> _)`.
2829
2830             let mut diag = crate::collect::placeholder_type_error_diag(
2831                 tcx,
2832                 generics,
2833                 visitor.0,
2834                 infer_replacements.iter().map(|(s, _)| *s).collect(),
2835                 true,
2836                 hir_ty,
2837                 "function",
2838             );
2839
2840             if !infer_replacements.is_empty() {
2841                 diag.multipart_suggestion(&format!(
2842                     "try replacing `_` with the type{} in the corresponding trait method signature",
2843                     rustc_errors::pluralize!(infer_replacements.len()),
2844                 ), infer_replacements, Applicability::MachineApplicable);
2845             }
2846
2847             diag.emit();
2848         }
2849
2850         // Find any late-bound regions declared in return type that do
2851         // not appear in the arguments. These are not well-formed.
2852         //
2853         // Example:
2854         //     for<'a> fn() -> &'a str <-- 'a is bad
2855         //     for<'a> fn(&'a String) -> &'a str <-- 'a is ok
2856         let inputs = bare_fn_ty.inputs();
2857         let late_bound_in_args =
2858             tcx.collect_constrained_late_bound_regions(&inputs.map_bound(|i| i.to_owned()));
2859         let output = bare_fn_ty.output();
2860         let late_bound_in_ret = tcx.collect_referenced_late_bound_regions(&output);
2861
2862         self.validate_late_bound_regions(late_bound_in_args, late_bound_in_ret, |br_name| {
2863             struct_span_err!(
2864                 tcx.sess,
2865                 decl.output.span(),
2866                 E0581,
2867                 "return type references {}, which is not constrained by the fn input types",
2868                 br_name
2869             )
2870         });
2871
2872         bare_fn_ty
2873     }
2874
2875     /// Given a fn_hir_id for a impl function, suggest the type that is found on the
2876     /// corresponding function in the trait that the impl implements, if it exists.
2877     /// If arg_idx is Some, then it corresponds to an input type index, otherwise it
2878     /// corresponds to the return type.
2879     fn suggest_trait_fn_ty_for_impl_fn_infer(
2880         &self,
2881         fn_hir_id: hir::HirId,
2882         arg_idx: Option<usize>,
2883     ) -> Option<Ty<'tcx>> {
2884         let tcx = self.tcx();
2885         let hir = tcx.hir();
2886
2887         let hir::Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), ident, .. }) =
2888             hir.get(fn_hir_id) else { return None };
2889         let hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(i), .. }) =
2890                 hir.get(hir.get_parent_node(fn_hir_id)) else { bug!("ImplItem should have Impl parent") };
2891
2892         let trait_ref =
2893             self.instantiate_mono_trait_ref(i.of_trait.as_ref()?, self.ast_ty_to_ty(i.self_ty));
2894
2895         let assoc = tcx.associated_items(trait_ref.def_id).find_by_name_and_kind(
2896             tcx,
2897             *ident,
2898             ty::AssocKind::Fn,
2899             trait_ref.def_id,
2900         )?;
2901
2902         let fn_sig = tcx.bound_fn_sig(assoc.def_id).subst(
2903             tcx,
2904             trait_ref.substs.extend_to(tcx, assoc.def_id, |param, _| tcx.mk_param_from_def(param)),
2905         );
2906
2907         let ty = if let Some(arg_idx) = arg_idx { fn_sig.input(arg_idx) } else { fn_sig.output() };
2908
2909         Some(tcx.liberate_late_bound_regions(fn_hir_id.expect_owner().to_def_id(), ty))
2910     }
2911
2912     fn validate_late_bound_regions(
2913         &self,
2914         constrained_regions: FxHashSet<ty::BoundRegionKind>,
2915         referenced_regions: FxHashSet<ty::BoundRegionKind>,
2916         generate_err: impl Fn(&str) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>,
2917     ) {
2918         for br in referenced_regions.difference(&constrained_regions) {
2919             let br_name = match *br {
2920                 ty::BrNamed(_, kw::UnderscoreLifetime) | ty::BrAnon(_) | ty::BrEnv => {
2921                     "an anonymous lifetime".to_string()
2922                 }
2923                 ty::BrNamed(_, name) => format!("lifetime `{}`", name),
2924             };
2925
2926             let mut err = generate_err(&br_name);
2927
2928             if let ty::BrNamed(_, kw::UnderscoreLifetime) | ty::BrAnon(_) = *br {
2929                 // The only way for an anonymous lifetime to wind up
2930                 // in the return type but **also** be unconstrained is
2931                 // if it only appears in "associated types" in the
2932                 // input. See #47511 and #62200 for examples. In this case,
2933                 // though we can easily give a hint that ought to be
2934                 // relevant.
2935                 err.note(
2936                     "lifetimes appearing in an associated type are not considered constrained",
2937                 );
2938             }
2939
2940             err.emit();
2941         }
2942     }
2943
2944     /// Given the bounds on an object, determines what single region bound (if any) we can
2945     /// use to summarize this type. The basic idea is that we will use the bound the user
2946     /// provided, if they provided one, and otherwise search the supertypes of trait bounds
2947     /// for region bounds. It may be that we can derive no bound at all, in which case
2948     /// we return `None`.
2949     fn compute_object_lifetime_bound(
2950         &self,
2951         span: Span,
2952         existential_predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
2953     ) -> Option<ty::Region<'tcx>> // if None, use the default
2954     {
2955         let tcx = self.tcx();
2956
2957         debug!("compute_opt_region_bound(existential_predicates={:?})", existential_predicates);
2958
2959         // No explicit region bound specified. Therefore, examine trait
2960         // bounds and see if we can derive region bounds from those.
2961         let derived_region_bounds = object_region_bounds(tcx, existential_predicates);
2962
2963         // If there are no derived region bounds, then report back that we
2964         // can find no region bound. The caller will use the default.
2965         if derived_region_bounds.is_empty() {
2966             return None;
2967         }
2968
2969         // If any of the derived region bounds are 'static, that is always
2970         // the best choice.
2971         if derived_region_bounds.iter().any(|r| r.is_static()) {
2972             return Some(tcx.lifetimes.re_static);
2973         }
2974
2975         // Determine whether there is exactly one unique region in the set
2976         // of derived region bounds. If so, use that. Otherwise, report an
2977         // error.
2978         let r = derived_region_bounds[0];
2979         if derived_region_bounds[1..].iter().any(|r1| r != *r1) {
2980             tcx.sess.emit_err(AmbiguousLifetimeBound { span });
2981         }
2982         Some(r)
2983     }
2984
2985     /// Make sure that we are in the condition to suggest the blanket implementation.
2986     fn maybe_lint_blanket_trait_impl(&self, self_ty: &hir::Ty<'_>, diag: &mut Diagnostic) {
2987         let tcx = self.tcx();
2988         let parent_id = tcx.hir().get_parent_item(self_ty.hir_id);
2989         if let hir::Node::Item(hir::Item {
2990             kind:
2991                 hir::ItemKind::Impl(hir::Impl {
2992                     self_ty: impl_self_ty, of_trait: Some(of_trait_ref), generics, ..
2993                 }),
2994             ..
2995         }) = tcx.hir().get_by_def_id(parent_id) && self_ty.hir_id == impl_self_ty.hir_id
2996         {
2997             if !of_trait_ref.trait_def_id().map_or(false, |def_id| def_id.is_local()) {
2998                 return;
2999             }
3000             let of_trait_span = of_trait_ref.path.span;
3001             // make sure that we are not calling unwrap to abort during the compilation
3002             let Ok(impl_trait_name) = tcx.sess.source_map().span_to_snippet(self_ty.span) else { return; };
3003             let Ok(of_trait_name) = tcx.sess.source_map().span_to_snippet(of_trait_span) else { return; };
3004             // check if the trait has generics, to make a correct suggestion
3005             let param_name = generics.params.next_type_param_name(None);
3006
3007             let add_generic_sugg = if let Some(span) = generics.span_for_param_suggestion() {
3008                 (span, format!(", {}: {}", param_name, impl_trait_name))
3009             } else {
3010                 (generics.span, format!("<{}: {}>", param_name, impl_trait_name))
3011             };
3012             diag.multipart_suggestion(
3013             format!("alternatively use a blanket \
3014                      implementation to implement `{of_trait_name}` for \
3015                      all types that also implement `{impl_trait_name}`"),
3016                 vec![
3017                     (self_ty.span, param_name),
3018                     add_generic_sugg,
3019                 ],
3020                 Applicability::MaybeIncorrect,
3021             );
3022         }
3023     }
3024
3025     fn maybe_lint_bare_trait(&self, self_ty: &hir::Ty<'_>, in_path: bool) {
3026         let tcx = self.tcx();
3027         if let hir::TyKind::TraitObject([poly_trait_ref, ..], _, TraitObjectSyntax::None) =
3028             self_ty.kind
3029         {
3030             let needs_bracket = in_path
3031                 && !tcx
3032                     .sess
3033                     .source_map()
3034                     .span_to_prev_source(self_ty.span)
3035                     .ok()
3036                     .map_or(false, |s| s.trim_end().ends_with('<'));
3037
3038             let is_global = poly_trait_ref.trait_ref.path.is_global();
3039             let sugg = Vec::from_iter([
3040                 (
3041                     self_ty.span.shrink_to_lo(),
3042                     format!(
3043                         "{}dyn {}",
3044                         if needs_bracket { "<" } else { "" },
3045                         if is_global { "(" } else { "" },
3046                     ),
3047                 ),
3048                 (
3049                     self_ty.span.shrink_to_hi(),
3050                     format!(
3051                         "{}{}",
3052                         if is_global { ")" } else { "" },
3053                         if needs_bracket { ">" } else { "" },
3054                     ),
3055                 ),
3056             ]);
3057             if self_ty.span.edition() >= Edition::Edition2021 {
3058                 let msg = "trait objects must include the `dyn` keyword";
3059                 let label = "add `dyn` keyword before this trait";
3060                 let mut diag =
3061                     rustc_errors::struct_span_err!(tcx.sess, self_ty.span, E0782, "{}", msg);
3062                 diag.multipart_suggestion_verbose(label, sugg, Applicability::MachineApplicable);
3063                 // check if the impl trait that we are considering is a impl of a local trait
3064                 self.maybe_lint_blanket_trait_impl(&self_ty, &mut diag);
3065                 diag.emit();
3066             } else {
3067                 let msg = "trait objects without an explicit `dyn` are deprecated";
3068                 tcx.struct_span_lint_hir(
3069                     BARE_TRAIT_OBJECTS,
3070                     self_ty.hir_id,
3071                     self_ty.span,
3072                     |lint| {
3073                         let mut diag = lint.build(msg);
3074                         diag.multipart_suggestion_verbose(
3075                             "use `dyn`",
3076                             sugg,
3077                             Applicability::MachineApplicable,
3078                         );
3079                         self.maybe_lint_blanket_trait_impl(&self_ty, &mut diag);
3080                         diag.emit();
3081                     },
3082                 );
3083             }
3084         }
3085     }
3086 }