compiler/rustc_hir_analysis/src/check/wfcheck.rs

   1 use crate::constrained_generic_params::{identify_constrained_generic_params, Parameter};
   2 use hir::def::DefKind;
   3 use rustc_ast as ast;
   4 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
   5 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed};
   6 use rustc_hir as hir;
   7 use rustc_hir::def_id::{DefId, LocalDefId};
   8 use rustc_hir::lang_items::LangItem;
   9 use rustc_hir::ItemKind;
  10 use rustc_infer::infer::outlives::env::{OutlivesEnvironment, RegionBoundPairs};
  11 use rustc_infer::infer::outlives::obligations::TypeOutlives;
  12 use rustc_infer::infer::{self, InferCtxt, TyCtxtInferExt};
  13 use rustc_middle::mir::ConstraintCategory;
  14 use rustc_middle::ty::query::Providers;
  15 use rustc_middle::ty::trait_def::TraitSpecializationKind;
  16 use rustc_middle::ty::{
  17     self, AdtKind, DefIdTree, GenericParamDefKind, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable,
  18     TypeVisitable, TypeVisitor,
  19 };
  20 use rustc_middle::ty::{GenericArgKind, InternalSubsts};
  21 use rustc_session::parse::feature_err;
  22 use rustc_span::symbol::{sym, Ident, Symbol};
  23 use rustc_span::{Span, DUMMY_SP};
  24 use rustc_target::spec::abi::Abi;
  25 use rustc_trait_selection::autoderef::Autoderef;
  26 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
  27 use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
  28 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt as _;
  29 use rustc_trait_selection::traits::{
  30     self, ObligationCause, ObligationCauseCode, ObligationCtxt, WellFormedLoc,
  31 };
  32
  33 use std::cell::LazyCell;
  34 use std::iter;
  35 use std::ops::{ControlFlow, Deref};
  36
  37 pub(super) struct WfCheckingCtxt<'a, 'tcx> {
  38     pub(super) ocx: ObligationCtxt<'a, 'tcx>,
  39     span: Span,
  40     body_id: hir::HirId,
  41     param_env: ty::ParamEnv<'tcx>,
  42 }
  43 impl<'a, 'tcx> Deref for WfCheckingCtxt<'a, 'tcx> {
  44     type Target = ObligationCtxt<'a, 'tcx>;
  45     fn deref(&self) -> &Self::Target {
  46         &self.ocx
  47     }
  48 }
  49
  50 impl<'tcx> WfCheckingCtxt<'_, 'tcx> {
  51     fn tcx(&self) -> TyCtxt<'tcx> {
  52         self.ocx.infcx.tcx
  53     }
  54
  55     // Convenience function to normalize during wfcheck. This performs
  56     // `ObligationCtxt::normalize`, but provides a nice `ObligationCauseCode`.
  57     fn normalize<T>(&self, span: Span, loc: Option<WellFormedLoc>, value: T) -> T
  58     where
  59         T: TypeFoldable<'tcx>,
  60     {
  61         self.ocx.normalize(
  62             &ObligationCause::new(span, self.body_id, ObligationCauseCode::WellFormed(loc)),
  63             self.param_env,
  64             value,
  65         )
  66     }
  67
  68     fn register_wf_obligation(
  69         &self,
  70         span: Span,
  71         loc: Option<WellFormedLoc>,
  72         arg: ty::GenericArg<'tcx>,
  73     ) {
  74         let cause =
  75             traits::ObligationCause::new(span, self.body_id, ObligationCauseCode::WellFormed(loc));
  76         // for a type to be WF, we do not need to check if const trait predicates satisfy.
  77         let param_env = self.param_env.without_const();
  78         self.ocx.register_obligation(traits::Obligation::new(
  79             self.tcx(),
  80             cause,
  81             param_env,
  82             ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)),
  83         ));
  84     }
  85 }
  86
  87 pub(super) fn enter_wf_checking_ctxt<'tcx, F>(
  88     tcx: TyCtxt<'tcx>,
  89     span: Span,
  90     body_def_id: LocalDefId,
  91     f: F,
  92 ) where
  93     F: for<'a> FnOnce(&WfCheckingCtxt<'a, 'tcx>),
  94 {
  95     let param_env = tcx.param_env(body_def_id);
  96     let body_id = tcx.hir().local_def_id_to_hir_id(body_def_id);
  97     let infcx = &tcx.infer_ctxt().build();
  98     let ocx = ObligationCtxt::new(infcx);
  99
 100     let mut wfcx = WfCheckingCtxt { ocx, span, body_id, param_env };
 101
 102     if !tcx.features().trivial_bounds {
 103         wfcx.check_false_global_bounds()
 104     }
 105     f(&mut wfcx);
 106
 107     let assumed_wf_types = wfcx.ocx.assumed_wf_types(param_env, span, body_def_id);
 108     let implied_bounds = infcx.implied_bounds_tys(param_env, body_id, assumed_wf_types);
 109
 110     let errors = wfcx.select_all_or_error();
 111     if !errors.is_empty() {
 112         infcx.err_ctxt().report_fulfillment_errors(&errors, None);
 113         return;
 114     }
 115
 116     let outlives_environment =
 117         OutlivesEnvironment::with_bounds(param_env, Some(infcx), implied_bounds);
 118
 119     let _ = infcx
 120         .err_ctxt()
 121         .check_region_obligations_and_report_errors(body_def_id, &outlives_environment);
 122 }
 123
 124 fn check_well_formed(tcx: TyCtxt<'_>, def_id: hir::OwnerId) {
 125     let node = tcx.hir().owner(def_id);
 126     match node {
 127         hir::OwnerNode::Crate(_) => {}
 128         hir::OwnerNode::Item(item) => check_item(tcx, item),
 129         hir::OwnerNode::TraitItem(item) => check_trait_item(tcx, item),
 130         hir::OwnerNode::ImplItem(item) => check_impl_item(tcx, item),
 131         hir::OwnerNode::ForeignItem(item) => check_foreign_item(tcx, item),
 132     }
 133
 134     if let Some(generics) = node.generics() {
 135         for param in generics.params {
 136             check_param_wf(tcx, param)
 137         }
 138     }
 139 }
 140
 141 /// Checks that the field types (in a struct def'n) or argument types (in an enum def'n) are
 142 /// well-formed, meaning that they do not require any constraints not declared in the struct
 143 /// definition itself. For example, this definition would be illegal:
 144 ///
 145 /// ```rust
 146 /// struct Ref<'a, T> { x: &'a T }
 147 /// ```
 148 ///
 149 /// because the type did not declare that `T:'a`.
 150 ///
 151 /// We do this check as a pre-pass before checking fn bodies because if these constraints are
 152 /// not included it frequently leads to confusing errors in fn bodies. So it's better to check
 153 /// the types first.
 154 #[instrument(skip(tcx), level = "debug")]
 155 fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) {
 156     let def_id = item.owner_id.def_id;
 157
 158     debug!(
 159         ?item.owner_id,
 160         item.name = ? tcx.def_path_str(def_id.to_def_id())
 161     );
 162
 163     match item.kind {
 164         // Right now we check that every default trait implementation
 165         // has an implementation of itself. Basically, a case like:
 166         //
 167         //     impl Trait for T {}
 168         //
 169         // has a requirement of `T: Trait` which was required for default
 170         // method implementations. Although this could be improved now that
 171         // there's a better infrastructure in place for this, it's being left
 172         // for a follow-up work.
 173         //
 174         // Since there's such a requirement, we need to check *just* positive
 175         // implementations, otherwise things like:
 176         //
 177         //     impl !Send for T {}
 178         //
 179         // won't be allowed unless there's an *explicit* implementation of `Send`
 180         // for `T`
 181         hir::ItemKind::Impl(ref impl_) => {
 182             let is_auto = tcx
 183                 .impl_trait_ref(def_id)
 184                 .map_or(false, |trait_ref| tcx.trait_is_auto(trait_ref.def_id));
 185             if let (hir::Defaultness::Default { .. }, true) = (impl_.defaultness, is_auto) {
 186                 let sp = impl_.of_trait.as_ref().map_or(item.span, |t| t.path.span);
 187                 let mut err =
 188                     tcx.sess.struct_span_err(sp, "impls of auto traits cannot be default");
 189                 err.span_labels(impl_.defaultness_span, "default because of this");
 190                 err.span_label(sp, "auto trait");
 191                 err.emit();
 192             }
 193             // We match on both `ty::ImplPolarity` and `ast::ImplPolarity` just to get the `!` span.
 194             match (tcx.impl_polarity(def_id), impl_.polarity) {
 195                 (ty::ImplPolarity::Positive, _) => {
 196                     check_impl(tcx, item, impl_.self_ty, &impl_.of_trait, impl_.constness);
 197                 }
 198                 (ty::ImplPolarity::Negative, ast::ImplPolarity::Negative(span)) => {
 199                     // FIXME(#27579): what amount of WF checking do we need for neg impls?
 200                     if let hir::Defaultness::Default { .. } = impl_.defaultness {
 201                         let mut spans = vec![span];
 202                         spans.extend(impl_.defaultness_span);
 203                         struct_span_err!(
 204                             tcx.sess,
 205                             spans,
 206                             E0750,
 207                             "negative impls cannot be default impls"
 208                         )
 209                         .emit();
 210                     }
 211                 }
 212                 (ty::ImplPolarity::Reservation, _) => {
 213                     // FIXME: what amount of WF checking do we need for reservation impls?
 214                 }
 215                 _ => unreachable!(),
 216             }
 217         }
 218         hir::ItemKind::Fn(ref sig, ..) => {
 219             check_item_fn(tcx, def_id, item.ident, item.span, sig.decl);
 220         }
 221         hir::ItemKind::Static(ty, ..) => {
 222             check_item_type(tcx, def_id, ty.span, false);
 223         }
 224         hir::ItemKind::Const(ty, ..) => {
 225             check_item_type(tcx, def_id, ty.span, false);
 226         }
 227         hir::ItemKind::Struct(_, ref ast_generics) => {
 228             check_type_defn(tcx, item, false);
 229             check_variances_for_type_defn(tcx, item, ast_generics);
 230         }
 231         hir::ItemKind::Union(_, ref ast_generics) => {
 232             check_type_defn(tcx, item, true);
 233             check_variances_for_type_defn(tcx, item, ast_generics);
 234         }
 235         hir::ItemKind::Enum(_, ref ast_generics) => {
 236             check_type_defn(tcx, item, true);
 237             check_variances_for_type_defn(tcx, item, ast_generics);
 238         }
 239         hir::ItemKind::Trait(..) => {
 240             check_trait(tcx, item);
 241         }
 242         hir::ItemKind::TraitAlias(..) => {
 243             check_trait(tcx, item);
 244         }
 245         // `ForeignItem`s are handled separately.
 246         hir::ItemKind::ForeignMod { .. } => {}
 247         _ => {}
 248     }
 249 }
 250
 251 fn check_foreign_item(tcx: TyCtxt<'_>, item: &hir::ForeignItem<'_>) {
 252     let def_id = item.owner_id.def_id;
 253
 254     debug!(
 255         ?item.owner_id,
 256         item.name = ? tcx.def_path_str(def_id.to_def_id())
 257     );
 258
 259     match item.kind {
 260         hir::ForeignItemKind::Fn(decl, ..) => {
 261             check_item_fn(tcx, def_id, item.ident, item.span, decl)
 262         }
 263         hir::ForeignItemKind::Static(ty, ..) => check_item_type(tcx, def_id, ty.span, true),
 264         hir::ForeignItemKind::Type => (),
 265     }
 266 }
 267
 268 fn check_trait_item(tcx: TyCtxt<'_>, trait_item: &hir::TraitItem<'_>) {
 269     let def_id = trait_item.owner_id.def_id;
 270
 271     let (method_sig, span) = match trait_item.kind {
 272         hir::TraitItemKind::Fn(ref sig, _) => (Some(sig), trait_item.span),
 273         hir::TraitItemKind::Type(_bounds, Some(ty)) => (None, ty.span),
 274         _ => (None, trait_item.span),
 275     };
 276     check_object_unsafe_self_trait_by_name(tcx, trait_item);
 277     check_associated_item(tcx, def_id, span, method_sig);
 278
 279     let encl_trait_def_id = tcx.local_parent(def_id);
 280     let encl_trait = tcx.hir().expect_item(encl_trait_def_id);
 281     let encl_trait_def_id = encl_trait.owner_id.to_def_id();
 282     let fn_lang_item_name = if Some(encl_trait_def_id) == tcx.lang_items().fn_trait() {
 283         Some("fn")
 284     } else if Some(encl_trait_def_id) == tcx.lang_items().fn_mut_trait() {
 285         Some("fn_mut")
 286     } else {
 287         None
 288     };
 289
 290     if let (Some(fn_lang_item_name), "call") =
 291         (fn_lang_item_name, trait_item.ident.name.to_ident_string().as_str())
 292     {
 293         // We are looking at the `call` function of the `fn` or `fn_mut` lang item.
 294         // Do some rudimentary sanity checking to avoid an ICE later (issue #83471).
 295         if let Some(hir::FnSig { decl, span, .. }) = method_sig {
 296             if let [self_ty, _] = decl.inputs {
 297                 if !matches!(self_ty.kind, hir::TyKind::Ref(_, _)) {
 298                     tcx.sess
 299                         .struct_span_err(
 300                             self_ty.span,
 301                             &format!(
 302                                 "first argument of `call` in `{fn_lang_item_name}` lang item must be a reference",
 303                             ),
 304                         )
 305                         .emit();
 306                 }
 307             } else {
 308                 tcx.sess
 309                     .struct_span_err(
 310                         *span,
 311                         &format!(
 312                             "`call` function in `{fn_lang_item_name}` lang item takes exactly two arguments",
 313                         ),
 314                     )
 315                     .emit();
 316             }
 317         } else {
 318             tcx.sess
 319                 .struct_span_err(
 320                     trait_item.span,
 321                     &format!(
 322                         "`call` trait item in `{fn_lang_item_name}` lang item must be a function",
 323                     ),
 324                 )
 325                 .emit();
 326         }
 327     }
 328 }
 329
 330 /// Require that the user writes where clauses on GATs for the implicit
 331 /// outlives bounds involving trait parameters in trait functions and
 332 /// lifetimes passed as GAT substs. See `self-outlives-lint` test.
 333 ///
 334 /// We use the following trait as an example throughout this function:
 335 /// ```rust,ignore (this code fails due to this lint)
 336 /// trait IntoIter {
 337 ///     type Iter<'a>: Iterator<Item = Self::Item<'a>>;
 338 ///     type Item<'a>;
 339 ///     fn into_iter<'a>(&'a self) -> Self::Iter<'a>;
 340 /// }
 341 /// ```
 342 fn check_gat_where_clauses(tcx: TyCtxt<'_>, associated_items: &[hir::TraitItemRef]) {
 343     // Associates every GAT's def_id to a list of possibly missing bounds detected by this lint.
 344     let mut required_bounds_by_item = FxHashMap::default();
 345
 346     // Loop over all GATs together, because if this lint suggests adding a where-clause bound
 347     // to one GAT, it might then require us to an additional bound on another GAT.
 348     // In our `IntoIter` example, we discover a missing `Self: 'a` bound on `Iter<'a>`, which
 349     // then in a second loop adds a `Self: 'a` bound to `Item` due to the relationship between
 350     // those GATs.
 351     loop {
 352         let mut should_continue = false;
 353         for gat_item in associated_items {
 354             let gat_def_id = gat_item.id.owner_id;
 355             let gat_item = tcx.associated_item(gat_def_id);
 356             // If this item is not an assoc ty, or has no substs, then it's not a GAT
 357             if gat_item.kind != ty::AssocKind::Type {
 358                 continue;
 359             }
 360             let gat_generics = tcx.generics_of(gat_def_id);
 361             // FIXME(jackh726): we can also warn in the more general case
 362             if gat_generics.params.is_empty() {
 363                 continue;
 364             }
 365
 366             // Gather the bounds with which all other items inside of this trait constrain the GAT.
 367             // This is calculated by taking the intersection of the bounds that each item
 368             // constrains the GAT with individually.
 369             let mut new_required_bounds: Option<FxHashSet<ty::Predicate<'_>>> = None;
 370             for item in associated_items {
 371                 let item_def_id = item.id.owner_id;
 372                 // Skip our own GAT, since it does not constrain itself at all.
 373                 if item_def_id == gat_def_id {
 374                     continue;
 375                 }
 376
 377                 let item_hir_id = item.id.hir_id();
 378                 let param_env = tcx.param_env(item_def_id);
 379
 380                 let item_required_bounds = match item.kind {
 381                     // In our example, this corresponds to `into_iter` method
 382                     hir::AssocItemKind::Fn { .. } => {
 383                         // For methods, we check the function signature's return type for any GATs
 384                         // to constrain. In the `into_iter` case, we see that the return type
 385                         // `Self::Iter<'a>` is a GAT we want to gather any potential missing bounds from.
 386                         let sig: ty::FnSig<'_> = tcx.liberate_late_bound_regions(
 387                             item_def_id.to_def_id(),
 388                             tcx.fn_sig(item_def_id),
 389                         );
 390                         gather_gat_bounds(
 391                             tcx,
 392                             param_env,
 393                             item_hir_id,
 394                             sig.inputs_and_output,
 395                             // We also assume that all of the function signature's parameter types
 396                             // are well formed.
 397                             &sig.inputs().iter().copied().collect(),
 398                             gat_def_id.def_id,
 399                             gat_generics,
 400                         )
 401                     }
 402                     // In our example, this corresponds to the `Iter` and `Item` associated types
 403                     hir::AssocItemKind::Type => {
 404                         // If our associated item is a GAT with missing bounds, add them to
 405                         // the param-env here. This allows this GAT to propagate missing bounds
 406                         // to other GATs.
 407                         let param_env = augment_param_env(
 408                             tcx,
 409                             param_env,
 410                             required_bounds_by_item.get(&item_def_id),
 411                         );
 412                         gather_gat_bounds(
 413                             tcx,
 414                             param_env,
 415                             item_hir_id,
 416                             tcx.explicit_item_bounds(item_def_id).to_vec(),
 417                             &FxIndexSet::default(),
 418                             gat_def_id.def_id,
 419                             gat_generics,
 420                         )
 421                     }
 422                     hir::AssocItemKind::Const => None,
 423                 };
 424
 425                 if let Some(item_required_bounds) = item_required_bounds {
 426                     // Take the intersection of the required bounds for this GAT, and
 427                     // the item_required_bounds which are the ones implied by just
 428                     // this item alone.
 429                     // This is why we use an Option<_>, since we need to distinguish
 430                     // the empty set of bounds from the _uninitialized_ set of bounds.
 431                     if let Some(new_required_bounds) = &mut new_required_bounds {
 432                         new_required_bounds.retain(|b| item_required_bounds.contains(b));
 433                     } else {
 434                         new_required_bounds = Some(item_required_bounds);
 435                     }
 436                 }
 437             }
 438
 439             if let Some(new_required_bounds) = new_required_bounds {
 440                 let required_bounds = required_bounds_by_item.entry(gat_def_id).or_default();
 441                 if new_required_bounds.into_iter().any(|p| required_bounds.insert(p)) {
 442                     // Iterate until our required_bounds no longer change
 443                     // Since they changed here, we should continue the loop
 444                     should_continue = true;
 445                 }
 446             }
 447         }
 448         // We know that this loop will eventually halt, since we only set `should_continue` if the
 449         // `required_bounds` for this item grows. Since we are not creating any new region or type
 450         // variables, the set of all region and type bounds that we could ever insert are limited
 451         // by the number of unique types and regions we observe in a given item.
 452         if !should_continue {
 453             break;
 454         }
 455     }
 456
 457     for (gat_def_id, required_bounds) in required_bounds_by_item {
 458         let gat_item_hir = tcx.hir().expect_trait_item(gat_def_id.def_id);
 459         debug!(?required_bounds);
 460         let param_env = tcx.param_env(gat_def_id);
 461         let gat_hir = gat_item_hir.hir_id();
 462
 463         let mut unsatisfied_bounds: Vec<_> = required_bounds
 464             .into_iter()
 465             .filter(|clause| match clause.kind().skip_binder() {
 466                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(ty::OutlivesPredicate(
 467                     a,
 468                     b,
 469                 ))) => {
 470                     !region_known_to_outlive(tcx, gat_hir, param_env, &FxIndexSet::default(), a, b)
 471                 }
 472                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
 473                     a,
 474                     b,
 475                 ))) => !ty_known_to_outlive(tcx, gat_hir, param_env, &FxIndexSet::default(), a, b),
 476                 _ => bug!("Unexpected PredicateKind"),
 477             })
 478             .map(|clause| clause.to_string())
 479             .collect();
 480
 481         // We sort so that order is predictable
 482         unsatisfied_bounds.sort();
 483
 484         if !unsatisfied_bounds.is_empty() {
 485             let plural = pluralize!(unsatisfied_bounds.len());
 486             let mut err = tcx.sess.struct_span_err(
 487                 gat_item_hir.span,
 488                 &format!("missing required bound{} on `{}`", plural, gat_item_hir.ident),
 489             );
 490
 491             let suggestion = format!(
 492                 "{} {}",
 493                 gat_item_hir.generics.add_where_or_trailing_comma(),
 494                 unsatisfied_bounds.join(", "),
 495             );
 496             err.span_suggestion(
 497                 gat_item_hir.generics.tail_span_for_predicate_suggestion(),
 498                 &format!("add the required where clause{plural}"),
 499                 suggestion,
 500                 Applicability::MachineApplicable,
 501             );
 502
 503             let bound =
 504                 if unsatisfied_bounds.len() > 1 { "these bounds are" } else { "this bound is" };
 505             err.note(&format!(
 506                 "{} currently required to ensure that impls have maximum flexibility",
 507                 bound
 508             ));
 509             err.note(
 510                 "we are soliciting feedback, see issue #87479 \
 511                  <https://github.com/rust-lang/rust/issues/87479> \
 512                  for more information",
 513             );
 514
 515             err.emit();
 516         }
 517     }
 518 }
 519
 520 /// Add a new set of predicates to the caller_bounds of an existing param_env.
 521 fn augment_param_env<'tcx>(
 522     tcx: TyCtxt<'tcx>,
 523     param_env: ty::ParamEnv<'tcx>,
 524     new_predicates: Option<&FxHashSet<ty::Predicate<'tcx>>>,
 525 ) -> ty::ParamEnv<'tcx> {
 526     let Some(new_predicates) = new_predicates else {
 527         return param_env;
 528     };
 529
 530     if new_predicates.is_empty() {
 531         return param_env;
 532     }
 533
 534     let bounds =
 535         tcx.mk_predicates(param_env.caller_bounds().iter().chain(new_predicates.iter().cloned()));
 536     // FIXME(compiler-errors): Perhaps there is a case where we need to normalize this
 537     // i.e. traits::normalize_param_env_or_error
 538     ty::ParamEnv::new(bounds, param_env.reveal(), param_env.constness())
 539 }
 540
 541 /// We use the following trait as an example throughout this function.
 542 /// Specifically, let's assume that `to_check` here is the return type
 543 /// of `into_iter`, and the GAT we are checking this for is `Iter`.
 544 /// ```rust,ignore (this code fails due to this lint)
 545 /// trait IntoIter {
 546 ///     type Iter<'a>: Iterator<Item = Self::Item<'a>>;
 547 ///     type Item<'a>;
 548 ///     fn into_iter<'a>(&'a self) -> Self::Iter<'a>;
 549 /// }
 550 /// ```
 551 fn gather_gat_bounds<'tcx, T: TypeFoldable<'tcx>>(
 552     tcx: TyCtxt<'tcx>,
 553     param_env: ty::ParamEnv<'tcx>,
 554     item_hir: hir::HirId,
 555     to_check: T,
 556     wf_tys: &FxIndexSet<Ty<'tcx>>,
 557     gat_def_id: LocalDefId,
 558     gat_generics: &'tcx ty::Generics,
 559 ) -> Option<FxHashSet<ty::Predicate<'tcx>>> {
 560     // The bounds we that we would require from `to_check`
 561     let mut bounds = FxHashSet::default();
 562
 563     let (regions, types) = GATSubstCollector::visit(gat_def_id.to_def_id(), to_check);
 564
 565     // If both regions and types are empty, then this GAT isn't in the
 566     // set of types we are checking, and we shouldn't try to do clause analysis
 567     // (particularly, doing so would end up with an empty set of clauses,
 568     // since the current method would require none, and we take the
 569     // intersection of requirements of all methods)
 570     if types.is_empty() && regions.is_empty() {
 571         return None;
 572     }
 573
 574     for (region_a, region_a_idx) in &regions {
 575         // Ignore `'static` lifetimes for the purpose of this lint: it's
 576         // because we know it outlives everything and so doesn't give meaningful
 577         // clues
 578         if let ty::ReStatic = **region_a {
 579             continue;
 580         }
 581         // For each region argument (e.g., `'a` in our example), check for a
 582         // relationship to the type arguments (e.g., `Self`). If there is an
 583         // outlives relationship (`Self: 'a`), then we want to ensure that is
 584         // reflected in a where clause on the GAT itself.
 585         for (ty, ty_idx) in &types {
 586             // In our example, requires that `Self: 'a`
 587             if ty_known_to_outlive(tcx, item_hir, param_env, &wf_tys, *ty, *region_a) {
 588                 debug!(?ty_idx, ?region_a_idx);
 589                 debug!("required clause: {ty} must outlive {region_a}");
 590                 // Translate into the generic parameters of the GAT. In
 591                 // our example, the type was `Self`, which will also be
 592                 // `Self` in the GAT.
 593                 let ty_param = gat_generics.param_at(*ty_idx, tcx);
 594                 let ty_param = tcx
 595                     .mk_ty(ty::Param(ty::ParamTy { index: ty_param.index, name: ty_param.name }));
 596                 // Same for the region. In our example, 'a corresponds
 597                 // to the 'me parameter.
 598                 let region_param = gat_generics.param_at(*region_a_idx, tcx);
 599                 let region_param =
 600                     tcx.mk_region(ty::RegionKind::ReEarlyBound(ty::EarlyBoundRegion {
 601                         def_id: region_param.def_id,
 602                         index: region_param.index,
 603                         name: region_param.name,
 604                     }));
 605                 // The predicate we expect to see. (In our example,
 606                 // `Self: 'me`.)
 607                 let clause = ty::PredicateKind::Clause(ty::Clause::TypeOutlives(
 608                     ty::OutlivesPredicate(ty_param, region_param),
 609                 ));
 610                 let clause = tcx.mk_predicate(ty::Binder::dummy(clause));
 611                 bounds.insert(clause);
 612             }
 613         }
 614
 615         // For each region argument (e.g., `'a` in our example), also check for a
 616         // relationship to the other region arguments. If there is an outlives
 617         // relationship, then we want to ensure that is reflected in the where clause
 618         // on the GAT itself.
 619         for (region_b, region_b_idx) in &regions {
 620             // Again, skip `'static` because it outlives everything. Also, we trivially
 621             // know that a region outlives itself.
 622             if ty::ReStatic == **region_b || region_a == region_b {
 623                 continue;
 624             }
 625             if region_known_to_outlive(tcx, item_hir, param_env, &wf_tys, *region_a, *region_b) {
 626                 debug!(?region_a_idx, ?region_b_idx);
 627                 debug!("required clause: {region_a} must outlive {region_b}");
 628                 // Translate into the generic parameters of the GAT.
 629                 let region_a_param = gat_generics.param_at(*region_a_idx, tcx);
 630                 let region_a_param =
 631                     tcx.mk_region(ty::RegionKind::ReEarlyBound(ty::EarlyBoundRegion {
 632                         def_id: region_a_param.def_id,
 633                         index: region_a_param.index,
 634                         name: region_a_param.name,
 635                     }));
 636                 // Same for the region.
 637                 let region_b_param = gat_generics.param_at(*region_b_idx, tcx);
 638                 let region_b_param =
 639                     tcx.mk_region(ty::RegionKind::ReEarlyBound(ty::EarlyBoundRegion {
 640                         def_id: region_b_param.def_id,
 641                         index: region_b_param.index,
 642                         name: region_b_param.name,
 643                     }));
 644                 // The predicate we expect to see.
 645                 let clause = ty::PredicateKind::Clause(ty::Clause::RegionOutlives(
 646                     ty::OutlivesPredicate(region_a_param, region_b_param),
 647                 ));
 648                 let clause = tcx.mk_predicate(ty::Binder::dummy(clause));
 649                 bounds.insert(clause);
 650             }
 651         }
 652     }
 653
 654     Some(bounds)
 655 }
 656
 657 /// Given a known `param_env` and a set of well formed types, can we prove that
 658 /// `ty` outlives `region`.
 659 fn ty_known_to_outlive<'tcx>(
 660     tcx: TyCtxt<'tcx>,
 661     id: hir::HirId,
 662     param_env: ty::ParamEnv<'tcx>,
 663     wf_tys: &FxIndexSet<Ty<'tcx>>,
 664     ty: Ty<'tcx>,
 665     region: ty::Region<'tcx>,
 666 ) -> bool {
 667     resolve_regions_with_wf_tys(tcx, id, param_env, &wf_tys, |infcx, region_bound_pairs| {
 668         let origin = infer::RelateParamBound(DUMMY_SP, ty, None);
 669         let outlives = &mut TypeOutlives::new(infcx, tcx, region_bound_pairs, None, param_env);
 670         outlives.type_must_outlive(origin, ty, region, ConstraintCategory::BoringNoLocation);
 671     })
 672 }
 673
 674 /// Given a known `param_env` and a set of well formed types, can we prove that
 675 /// `region_a` outlives `region_b`
 676 fn region_known_to_outlive<'tcx>(
 677     tcx: TyCtxt<'tcx>,
 678     id: hir::HirId,
 679     param_env: ty::ParamEnv<'tcx>,
 680     wf_tys: &FxIndexSet<Ty<'tcx>>,
 681     region_a: ty::Region<'tcx>,
 682     region_b: ty::Region<'tcx>,
 683 ) -> bool {
 684     resolve_regions_with_wf_tys(tcx, id, param_env, &wf_tys, |mut infcx, _| {
 685         use rustc_infer::infer::outlives::obligations::TypeOutlivesDelegate;
 686         let origin = infer::RelateRegionParamBound(DUMMY_SP);
 687         // `region_a: region_b` -> `region_b <= region_a`
 688         infcx.push_sub_region_constraint(
 689             origin,
 690             region_b,
 691             region_a,
 692             ConstraintCategory::BoringNoLocation,
 693         );
 694     })
 695 }
 696
 697 /// Given a known `param_env` and a set of well formed types, set up an
 698 /// `InferCtxt`, call the passed function (to e.g. set up region constraints
 699 /// to be tested), then resolve region and return errors
 700 fn resolve_regions_with_wf_tys<'tcx>(
 701     tcx: TyCtxt<'tcx>,
 702     id: hir::HirId,
 703     param_env: ty::ParamEnv<'tcx>,
 704     wf_tys: &FxIndexSet<Ty<'tcx>>,
 705     add_constraints: impl for<'a> FnOnce(&'a InferCtxt<'tcx>, &'a RegionBoundPairs<'tcx>),
 706 ) -> bool {
 707     // Unfortunately, we have to use a new `InferCtxt` each call, because
 708     // region constraints get added and solved there and we need to test each
 709     // call individually.
 710     let infcx = tcx.infer_ctxt().build();
 711     let outlives_environment = OutlivesEnvironment::with_bounds(
 712         param_env,
 713         Some(&infcx),
 714         infcx.implied_bounds_tys(param_env, id, wf_tys.clone()),
 715     );
 716     let region_bound_pairs = outlives_environment.region_bound_pairs();
 717
 718     add_constraints(&infcx, region_bound_pairs);
 719
 720     infcx.process_registered_region_obligations(
 721         outlives_environment.region_bound_pairs(),
 722         param_env,
 723     );
 724     let errors = infcx.resolve_regions(&outlives_environment);
 725
 726     debug!(?errors, "errors");
 727
 728     // If we were able to prove that the type outlives the region without
 729     // an error, it must be because of the implied or explicit bounds...
 730     errors.is_empty()
 731 }
 732
 733 /// TypeVisitor that looks for uses of GATs like
 734 /// `<P0 as Trait<P1..Pn>>::GAT<Pn..Pm>` and adds the arguments `P0..Pm` into
 735 /// the two vectors, `regions` and `types` (depending on their kind). For each
 736 /// parameter `Pi` also track the index `i`.
 737 struct GATSubstCollector<'tcx> {
 738     gat: DefId,
 739     // Which region appears and which parameter index its substituted for
 740     regions: FxHashSet<(ty::Region<'tcx>, usize)>,
 741     // Which params appears and which parameter index its substituted for
 742     types: FxHashSet<(Ty<'tcx>, usize)>,
 743 }
 744
 745 impl<'tcx> GATSubstCollector<'tcx> {
 746     fn visit<T: TypeFoldable<'tcx>>(
 747         gat: DefId,
 748         t: T,
 749     ) -> (FxHashSet<(ty::Region<'tcx>, usize)>, FxHashSet<(Ty<'tcx>, usize)>) {
 750         let mut visitor =
 751             GATSubstCollector { gat, regions: FxHashSet::default(), types: FxHashSet::default() };
 752         t.visit_with(&mut visitor);
 753         (visitor.regions, visitor.types)
 754     }
 755 }
 756
 757 impl<'tcx> TypeVisitor<'tcx> for GATSubstCollector<'tcx> {
 758     type BreakTy = !;
 759
 760     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 761         match t.kind() {
 762             ty::Alias(ty::Projection, p) if p.def_id == self.gat => {
 763                 for (idx, subst) in p.substs.iter().enumerate() {
 764                     match subst.unpack() {
 765                         GenericArgKind::Lifetime(lt) if !lt.is_late_bound() => {
 766                             self.regions.insert((lt, idx));
 767                         }
 768                         GenericArgKind::Type(t) => {
 769                             self.types.insert((t, idx));
 770                         }
 771                         _ => {}
 772                     }
 773                 }
 774             }
 775             _ => {}
 776         }
 777         t.super_visit_with(self)
 778     }
 779 }
 780
 781 fn could_be_self(trait_def_id: LocalDefId, ty: &hir::Ty<'_>) -> bool {
 782     match ty.kind {
 783         hir::TyKind::TraitObject([trait_ref], ..) => match trait_ref.trait_ref.path.segments {
 784             [s] => s.res.opt_def_id() == Some(trait_def_id.to_def_id()),
 785             _ => false,
 786         },
 787         _ => false,
 788     }
 789 }
 790
 791 /// Detect when an object unsafe trait is referring to itself in one of its associated items.
 792 /// When this is done, suggest using `Self` instead.
 793 fn check_object_unsafe_self_trait_by_name(tcx: TyCtxt<'_>, item: &hir::TraitItem<'_>) {
 794     let (trait_name, trait_def_id) =
 795         match tcx.hir().get_by_def_id(tcx.hir().get_parent_item(item.hir_id()).def_id) {
 796             hir::Node::Item(item) => match item.kind {
 797                 hir::ItemKind::Trait(..) => (item.ident, item.owner_id),
 798                 _ => return,
 799             },
 800             _ => return,
 801         };
 802     let mut trait_should_be_self = vec![];
 803     match &item.kind {
 804         hir::TraitItemKind::Const(ty, _) | hir::TraitItemKind::Type(_, Some(ty))
 805             if could_be_self(trait_def_id.def_id, ty) =>
 806         {
 807             trait_should_be_self.push(ty.span)
 808         }
 809         hir::TraitItemKind::Fn(sig, _) => {
 810             for ty in sig.decl.inputs {
 811                 if could_be_self(trait_def_id.def_id, ty) {
 812                     trait_should_be_self.push(ty.span);
 813                 }
 814             }
 815             match sig.decl.output {
 816                 hir::FnRetTy::Return(ty) if could_be_self(trait_def_id.def_id, ty) => {
 817                     trait_should_be_self.push(ty.span);
 818                 }
 819                 _ => {}
 820             }
 821         }
 822         _ => {}
 823     }
 824     if !trait_should_be_self.is_empty() {
 825         if tcx.object_safety_violations(trait_def_id).is_empty() {
 826             return;
 827         }
 828         let sugg = trait_should_be_self.iter().map(|span| (*span, "Self".to_string())).collect();
 829         tcx.sess
 830             .struct_span_err(
 831                 trait_should_be_self,
 832                 "associated item referring to unboxed trait object for its own trait",
 833             )
 834             .span_label(trait_name.span, "in this trait")
 835             .multipart_suggestion(
 836                 "you might have meant to use `Self` to refer to the implementing type",
 837                 sugg,
 838                 Applicability::MachineApplicable,
 839             )
 840             .emit();
 841     }
 842 }
 843
 844 fn check_impl_item(tcx: TyCtxt<'_>, impl_item: &hir::ImplItem<'_>) {
 845     let (method_sig, span) = match impl_item.kind {
 846         hir::ImplItemKind::Fn(ref sig, _) => (Some(sig), impl_item.span),
 847         // Constrain binding and overflow error spans to `<Ty>` in `type foo = <Ty>`.
 848         hir::ImplItemKind::Type(ty) if ty.span != DUMMY_SP => (None, ty.span),
 849         _ => (None, impl_item.span),
 850     };
 851
 852     check_associated_item(tcx, impl_item.owner_id.def_id, span, method_sig);
 853 }
 854
 855 fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) {
 856     match param.kind {
 857         // We currently only check wf of const params here.
 858         hir::GenericParamKind::Lifetime { .. } | hir::GenericParamKind::Type { .. } => (),
 859
 860         // Const parameters are well formed if their type is structural match.
 861         hir::GenericParamKind::Const { ty: hir_ty, default: _ } => {
 862             let ty = tcx.type_of(param.def_id);
 863
 864             if tcx.features().adt_const_params {
 865                 if let Some(non_structural_match_ty) =
 866                     traits::search_for_adt_const_param_violation(param.span, tcx, ty)
 867                 {
 868                     // We use the same error code in both branches, because this is really the same
 869                     // issue: we just special-case the message for type parameters to make it
 870                     // clearer.
 871                     match non_structural_match_ty.kind() {
 872                         ty::Param(_) => {
 873                             // Const parameters may not have type parameters as their types,
 874                             // because we cannot be sure that the type parameter derives `PartialEq`
 875                             // and `Eq` (just implementing them is not enough for `structural_match`).
 876                             struct_span_err!(
 877                                 tcx.sess,
 878                                 hir_ty.span,
 879                                 E0741,
 880                                 "`{ty}` is not guaranteed to `#[derive(PartialEq, Eq)]`, so may not be \
 881                                 used as the type of a const parameter",
 882                             )
 883                             .span_label(
 884                                 hir_ty.span,
 885                                 format!("`{ty}` may not derive both `PartialEq` and `Eq`"),
 886                             )
 887                             .note(
 888                                 "it is not currently possible to use a type parameter as the type of a \
 889                                 const parameter",
 890                             )
 891                             .emit();
 892                         }
 893                         ty::Float(_) => {
 894                             struct_span_err!(
 895                                 tcx.sess,
 896                                 hir_ty.span,
 897                                 E0741,
 898                                 "`{ty}` is forbidden as the type of a const generic parameter",
 899                             )
 900                             .note("floats do not derive `Eq` or `Ord`, which are required for const parameters")
 901                             .emit();
 902                         }
 903                         ty::FnPtr(_) => {
 904                             struct_span_err!(
 905                                 tcx.sess,
 906                                 hir_ty.span,
 907                                 E0741,
 908                                 "using function pointers as const generic parameters is forbidden",
 909                             )
 910                             .emit();
 911                         }
 912                         ty::RawPtr(_) => {
 913                             struct_span_err!(
 914                                 tcx.sess,
 915                                 hir_ty.span,
 916                                 E0741,
 917                                 "using raw pointers as const generic parameters is forbidden",
 918                             )
 919                             .emit();
 920                         }
 921                         _ => {
 922                             let mut diag = struct_span_err!(
 923                                 tcx.sess,
 924                                 hir_ty.span,
 925                                 E0741,
 926                                 "`{}` must be annotated with `#[derive(PartialEq, Eq)]` to be used as \
 927                                 the type of a const parameter",
 928                                 non_structural_match_ty,
 929                             );
 930
 931                             if ty == non_structural_match_ty {
 932                                 diag.span_label(
 933                                     hir_ty.span,
 934                                     format!("`{ty}` doesn't derive both `PartialEq` and `Eq`"),
 935                                 );
 936                             }
 937
 938                             diag.emit();
 939                         }
 940                     }
 941                 }
 942             } else {
 943                 let err_ty_str;
 944                 let mut is_ptr = true;
 945
 946                 let err = match ty.kind() {
 947                     ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Error(_) => None,
 948                     ty::FnPtr(_) => Some("function pointers"),
 949                     ty::RawPtr(_) => Some("raw pointers"),
 950                     _ => {
 951                         is_ptr = false;
 952                         err_ty_str = format!("`{ty}`");
 953                         Some(err_ty_str.as_str())
 954                     }
 955                 };
 956
 957                 if let Some(unsupported_type) = err {
 958                     if is_ptr {
 959                         tcx.sess.span_err(
 960                             hir_ty.span,
 961                             &format!(
 962                                 "using {unsupported_type} as const generic parameters is forbidden",
 963                             ),
 964                         );
 965                     } else {
 966                         let mut err = tcx.sess.struct_span_err(
 967                             hir_ty.span,
 968                             &format!(
 969                                 "{unsupported_type} is forbidden as the type of a const generic parameter",
 970                             ),
 971                         );
 972                         err.note("the only supported types are integers, `bool` and `char`");
 973                         if tcx.sess.is_nightly_build() {
 974                             err.help(
 975                             "more complex types are supported with `#![feature(adt_const_params)]`",
 976                         );
 977                         }
 978                         err.emit();
 979                     }
 980                 }
 981             }
 982         }
 983     }
 984 }
 985
 986 #[instrument(level = "debug", skip(tcx, span, sig_if_method))]
 987 fn check_associated_item(
 988     tcx: TyCtxt<'_>,
 989     item_id: LocalDefId,
 990     span: Span,
 991     sig_if_method: Option<&hir::FnSig<'_>>,
 992 ) {
 993     let loc = Some(WellFormedLoc::Ty(item_id));
 994     enter_wf_checking_ctxt(tcx, span, item_id, |wfcx| {
 995         let item = tcx.associated_item(item_id);
 996
 997         let self_ty = match item.container {
 998             ty::TraitContainer => tcx.types.self_param,
 999             ty::ImplContainer => tcx.type_of(item.container_id(tcx)),
1000         };
1001
1002         match item.kind {
1003             ty::AssocKind::Const => {
1004                 let ty = tcx.type_of(item.def_id);
1005                 let ty = wfcx.normalize(span, Some(WellFormedLoc::Ty(item_id)), ty);
1006                 wfcx.register_wf_obligation(span, loc, ty.into());
1007             }
1008             ty::AssocKind::Fn => {
1009                 let sig = tcx.fn_sig(item.def_id);
1010                 let hir_sig = sig_if_method.expect("bad signature for method");
1011                 check_fn_or_method(
1012                     wfcx,
1013                     item.ident(tcx).span,
1014                     sig,
1015                     hir_sig.decl,
1016                     item.def_id.expect_local(),
1017                 );
1018                 check_method_receiver(wfcx, hir_sig, item, self_ty);
1019             }
1020             ty::AssocKind::Type => {
1021                 if let ty::AssocItemContainer::TraitContainer = item.container {
1022                     check_associated_type_bounds(wfcx, item, span)
1023                 }
1024                 if item.defaultness(tcx).has_value() {
1025                     let ty = tcx.type_of(item.def_id);
1026                     let ty = wfcx.normalize(span, Some(WellFormedLoc::Ty(item_id)), ty);
1027                     wfcx.register_wf_obligation(span, loc, ty.into());
1028                 }
1029             }
1030         }
1031     })
1032 }
1033
1034 fn item_adt_kind(kind: &ItemKind<'_>) -> Option<AdtKind> {
1035     match kind {
1036         ItemKind::Struct(..) => Some(AdtKind::Struct),
1037         ItemKind::Union(..) => Some(AdtKind::Union),
1038         ItemKind::Enum(..) => Some(AdtKind::Enum),
1039         _ => None,
1040     }
1041 }
1042
1043 /// In a type definition, we check that to ensure that the types of the fields are well-formed.
1044 fn check_type_defn<'tcx>(tcx: TyCtxt<'tcx>, item: &hir::Item<'tcx>, all_sized: bool) {
1045     let _ = tcx.representability(item.owner_id.def_id);
1046     let adt_def = tcx.adt_def(item.owner_id);
1047
1048     enter_wf_checking_ctxt(tcx, item.span, item.owner_id.def_id, |wfcx| {
1049         let variants = adt_def.variants();
1050         let packed = adt_def.repr().packed();
1051
1052         for variant in variants.iter() {
1053             // All field types must be well-formed.
1054             for field in &variant.fields {
1055                 let field_id = field.did.expect_local();
1056                 let hir::Node::Field(hir::FieldDef { ty: hir_ty, .. }) = tcx.hir().get_by_def_id(field_id)
1057                 else { bug!() };
1058                 let ty = wfcx.normalize(hir_ty.span, None, tcx.type_of(field.did));
1059                 wfcx.register_wf_obligation(
1060                     hir_ty.span,
1061                     Some(WellFormedLoc::Ty(field_id)),
1062                     ty.into(),
1063                 )
1064             }
1065
1066             // For DST, or when drop needs to copy things around, all
1067             // intermediate types must be sized.
1068             let needs_drop_copy = || {
1069                 packed && {
1070                     let ty = tcx.type_of(variant.fields.last().unwrap().did);
1071                     let ty = tcx.erase_regions(ty);
1072                     if ty.needs_infer() {
1073                         tcx.sess
1074                             .delay_span_bug(item.span, &format!("inference variables in {:?}", ty));
1075                         // Just treat unresolved type expression as if it needs drop.
1076                         true
1077                     } else {
1078                         ty.needs_drop(tcx, tcx.param_env(item.owner_id))
1079                     }
1080                 }
1081             };
1082             // All fields (except for possibly the last) should be sized.
1083             let all_sized = all_sized || variant.fields.is_empty() || needs_drop_copy();
1084             let unsized_len = if all_sized { 0 } else { 1 };
1085             for (idx, field) in
1086                 variant.fields[..variant.fields.len() - unsized_len].iter().enumerate()
1087             {
1088                 let last = idx == variant.fields.len() - 1;
1089                 let field_id = field.did.expect_local();
1090                 let hir::Node::Field(hir::FieldDef { ty: hir_ty, .. }) = tcx.hir().get_by_def_id(field_id)
1091                 else { bug!() };
1092                 let ty = wfcx.normalize(hir_ty.span, None, tcx.type_of(field.did));
1093                 wfcx.register_bound(
1094                     traits::ObligationCause::new(
1095                         hir_ty.span,
1096                         wfcx.body_id,
1097                         traits::FieldSized {
1098                             adt_kind: match item_adt_kind(&item.kind) {
1099                                 Some(i) => i,
1100                                 None => bug!(),
1101                             },
1102                             span: hir_ty.span,
1103                             last,
1104                         },
1105                     ),
1106                     wfcx.param_env,
1107                     ty,
1108                     tcx.require_lang_item(LangItem::Sized, None),
1109                 );
1110             }
1111
1112             // Explicit `enum` discriminant values must const-evaluate successfully.
1113             if let ty::VariantDiscr::Explicit(discr_def_id) = variant.discr {
1114                 let cause = traits::ObligationCause::new(
1115                     tcx.def_span(discr_def_id),
1116                     wfcx.body_id,
1117                     traits::MiscObligation,
1118                 );
1119                 wfcx.register_obligation(traits::Obligation::new(
1120                     tcx,
1121                     cause,
1122                     wfcx.param_env,
1123                     ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(
1124                         ty::Const::from_anon_const(tcx, discr_def_id.expect_local()),
1125                     )),
1126                 ));
1127             }
1128         }
1129
1130         check_where_clauses(wfcx, item.span, item.owner_id.def_id);
1131     });
1132 }
1133
1134 #[instrument(skip(tcx, item))]
1135 fn check_trait(tcx: TyCtxt<'_>, item: &hir::Item<'_>) {
1136     debug!(?item.owner_id);
1137
1138     let def_id = item.owner_id.def_id;
1139     let trait_def = tcx.trait_def(def_id);
1140     if trait_def.is_marker
1141         || matches!(trait_def.specialization_kind, TraitSpecializationKind::Marker)
1142     {
1143         for associated_def_id in &*tcx.associated_item_def_ids(def_id) {
1144             struct_span_err!(
1145                 tcx.sess,
1146                 tcx.def_span(*associated_def_id),
1147                 E0714,
1148                 "marker traits cannot have associated items",
1149             )
1150             .emit();
1151         }
1152     }
1153
1154     enter_wf_checking_ctxt(tcx, item.span, def_id, |wfcx| {
1155         check_where_clauses(wfcx, item.span, def_id)
1156     });
1157
1158     // Only check traits, don't check trait aliases
1159     if let hir::ItemKind::Trait(_, _, _, _, items) = item.kind {
1160         check_gat_where_clauses(tcx, items);
1161     }
1162 }
1163
1164 /// Checks all associated type defaults of trait `trait_def_id`.
1165 ///
1166 /// Assuming the defaults are used, check that all predicates (bounds on the
1167 /// assoc type and where clauses on the trait) hold.
1168 fn check_associated_type_bounds(wfcx: &WfCheckingCtxt<'_, '_>, item: &ty::AssocItem, span: Span) {
1169     let bounds = wfcx.tcx().explicit_item_bounds(item.def_id);
1170
1171     debug!("check_associated_type_bounds: bounds={:?}", bounds);
1172     let wf_obligations = bounds.iter().flat_map(|&(bound, bound_span)| {
1173         let normalized_bound = wfcx.normalize(span, None, bound);
1174         traits::wf::predicate_obligations(
1175             wfcx.infcx,
1176             wfcx.param_env,
1177             wfcx.body_id,
1178             normalized_bound,
1179             bound_span,
1180         )
1181     });
1182
1183     wfcx.register_obligations(wf_obligations);
1184 }
1185
1186 fn check_item_fn(
1187     tcx: TyCtxt<'_>,
1188     def_id: LocalDefId,
1189     ident: Ident,
1190     span: Span,
1191     decl: &hir::FnDecl<'_>,
1192 ) {
1193     enter_wf_checking_ctxt(tcx, span, def_id, |wfcx| {
1194         let sig = tcx.fn_sig(def_id);
1195         check_fn_or_method(wfcx, ident.span, sig, decl, def_id);
1196     })
1197 }
1198
1199 fn check_item_type(tcx: TyCtxt<'_>, item_id: LocalDefId, ty_span: Span, allow_foreign_ty: bool) {
1200     debug!("check_item_type: {:?}", item_id);
1201
1202     enter_wf_checking_ctxt(tcx, ty_span, item_id, |wfcx| {
1203         let ty = tcx.type_of(item_id);
1204         let item_ty = wfcx.normalize(ty_span, Some(WellFormedLoc::Ty(item_id)), ty);
1205
1206         let mut forbid_unsized = true;
1207         if allow_foreign_ty {
1208             let tail = tcx.struct_tail_erasing_lifetimes(item_ty, wfcx.param_env);
1209             if let ty::Foreign(_) = tail.kind() {
1210                 forbid_unsized = false;
1211             }
1212         }
1213
1214         wfcx.register_wf_obligation(ty_span, Some(WellFormedLoc::Ty(item_id)), item_ty.into());
1215         if forbid_unsized {
1216             wfcx.register_bound(
1217                 traits::ObligationCause::new(ty_span, wfcx.body_id, traits::WellFormed(None)),
1218                 wfcx.param_env,
1219                 item_ty,
1220                 tcx.require_lang_item(LangItem::Sized, None),
1221             );
1222         }
1223
1224         // Ensure that the end result is `Sync` in a non-thread local `static`.
1225         let should_check_for_sync = tcx.static_mutability(item_id.to_def_id())
1226             == Some(hir::Mutability::Not)
1227             && !tcx.is_foreign_item(item_id.to_def_id())
1228             && !tcx.is_thread_local_static(item_id.to_def_id());
1229
1230         if should_check_for_sync {
1231             wfcx.register_bound(
1232                 traits::ObligationCause::new(ty_span, wfcx.body_id, traits::SharedStatic),
1233                 wfcx.param_env,
1234                 item_ty,
1235                 tcx.require_lang_item(LangItem::Sync, Some(ty_span)),
1236             );
1237         }
1238     });
1239 }
1240
1241 #[instrument(level = "debug", skip(tcx, ast_self_ty, ast_trait_ref))]
1242 fn check_impl<'tcx>(
1243     tcx: TyCtxt<'tcx>,
1244     item: &'tcx hir::Item<'tcx>,
1245     ast_self_ty: &hir::Ty<'_>,
1246     ast_trait_ref: &Option<hir::TraitRef<'_>>,
1247     constness: hir::Constness,
1248 ) {
1249     enter_wf_checking_ctxt(tcx, item.span, item.owner_id.def_id, |wfcx| {
1250         match *ast_trait_ref {
1251             Some(ref ast_trait_ref) => {
1252                 // `#[rustc_reservation_impl]` impls are not real impls and
1253                 // therefore don't need to be WF (the trait's `Self: Trait` predicate
1254                 // won't hold).
1255                 let trait_ref = tcx.impl_trait_ref(item.owner_id).unwrap();
1256                 let trait_ref = wfcx.normalize(ast_trait_ref.path.span, None, trait_ref);
1257                 let trait_pred = ty::TraitPredicate {
1258                     trait_ref,
1259                     constness: match constness {
1260                         hir::Constness::Const => ty::BoundConstness::ConstIfConst,
1261                         hir::Constness::NotConst => ty::BoundConstness::NotConst,
1262                     },
1263                     polarity: ty::ImplPolarity::Positive,
1264                 };
1265                 let obligations = traits::wf::trait_obligations(
1266                     wfcx.infcx,
1267                     wfcx.param_env,
1268                     wfcx.body_id,
1269                     &trait_pred,
1270                     ast_trait_ref.path.span,
1271                     item,
1272                 );
1273                 debug!(?obligations);
1274                 wfcx.register_obligations(obligations);
1275             }
1276             None => {
1277                 let self_ty = tcx.type_of(item.owner_id);
1278                 let self_ty = wfcx.normalize(
1279                     item.span,
1280                     Some(WellFormedLoc::Ty(item.hir_id().expect_owner().def_id)),
1281                     self_ty,
1282                 );
1283                 wfcx.register_wf_obligation(
1284                     ast_self_ty.span,
1285                     Some(WellFormedLoc::Ty(item.hir_id().expect_owner().def_id)),
1286                     self_ty.into(),
1287                 );
1288             }
1289         }
1290
1291         check_where_clauses(wfcx, item.span, item.owner_id.def_id);
1292     });
1293 }
1294
1295 /// Checks where-clauses and inline bounds that are declared on `def_id`.
1296 #[instrument(level = "debug", skip(wfcx))]
1297 fn check_where_clauses<'tcx>(wfcx: &WfCheckingCtxt<'_, 'tcx>, span: Span, def_id: LocalDefId) {
1298     let infcx = wfcx.infcx;
1299     let tcx = wfcx.tcx();
1300
1301     let predicates = tcx.bound_predicates_of(def_id.to_def_id());
1302     let generics = tcx.generics_of(def_id);
1303
1304     let is_our_default = |def: &ty::GenericParamDef| match def.kind {
1305         GenericParamDefKind::Type { has_default, .. }
1306         | GenericParamDefKind::Const { has_default } => {
1307             has_default && def.index >= generics.parent_count as u32
1308         }
1309         GenericParamDefKind::Lifetime => unreachable!(),
1310     };
1311
1312     // Check that concrete defaults are well-formed. See test `type-check-defaults.rs`.
1313     // For example, this forbids the declaration:
1314     //
1315     //     struct Foo<T = Vec<[u32]>> { .. }
1316     //
1317     // Here, the default `Vec<[u32]>` is not WF because `[u32]: Sized` does not hold.
1318     for param in &generics.params {
1319         match param.kind {
1320             GenericParamDefKind::Type { .. } => {
1321                 if is_our_default(param) {
1322                     let ty = tcx.type_of(param.def_id);
1323                     // Ignore dependent defaults -- that is, where the default of one type
1324                     // parameter includes another (e.g., `<T, U = T>`). In those cases, we can't
1325                     // be sure if it will error or not as user might always specify the other.
1326                     if !ty.needs_subst() {
1327                         wfcx.register_wf_obligation(
1328                             tcx.def_span(param.def_id),
1329                             Some(WellFormedLoc::Ty(param.def_id.expect_local())),
1330                             ty.into(),
1331                         );
1332                     }
1333                 }
1334             }
1335             GenericParamDefKind::Const { .. } => {
1336                 if is_our_default(param) {
1337                     // FIXME(const_generics_defaults): This
1338                     // is incorrect when dealing with unused substs, for example
1339                     // for `struct Foo<const N: usize, const M: usize = { 1 - 2 }>`
1340                     // we should eagerly error.
1341                     let default_ct = tcx.const_param_default(param.def_id);
1342                     if !default_ct.needs_subst() {
1343                         wfcx.register_wf_obligation(
1344                             tcx.def_span(param.def_id),
1345                             None,
1346                             default_ct.into(),
1347                         );
1348                     }
1349                 }
1350             }
1351             // Doesn't have defaults.
1352             GenericParamDefKind::Lifetime => {}
1353         }
1354     }
1355
1356     // Check that trait predicates are WF when params are substituted by their defaults.
1357     // We don't want to overly constrain the predicates that may be written but we want to
1358     // catch cases where a default my never be applied such as `struct Foo<T: Copy = String>`.
1359     // Therefore we check if a predicate which contains a single type param
1360     // with a concrete default is WF with that default substituted.
1361     // For more examples see tests `defaults-well-formedness.rs` and `type-check-defaults.rs`.
1362     //
1363     // First we build the defaulted substitution.
1364     let substs = InternalSubsts::for_item(tcx, def_id.to_def_id(), |param, _| {
1365         match param.kind {
1366             GenericParamDefKind::Lifetime => {
1367                 // All regions are identity.
1368                 tcx.mk_param_from_def(param)
1369             }
1370
1371             GenericParamDefKind::Type { .. } => {
1372                 // If the param has a default, ...
1373                 if is_our_default(param) {
1374                     let default_ty = tcx.type_of(param.def_id);
1375                     // ... and it's not a dependent default, ...
1376                     if !default_ty.needs_subst() {
1377                         // ... then substitute it with the default.
1378                         return default_ty.into();
1379                     }
1380                 }
1381
1382                 tcx.mk_param_from_def(param)
1383             }
1384             GenericParamDefKind::Const { .. } => {
1385                 // If the param has a default, ...
1386                 if is_our_default(param) {
1387                     let default_ct = tcx.const_param_default(param.def_id);
1388                     // ... and it's not a dependent default, ...
1389                     if !default_ct.needs_subst() {
1390                         // ... then substitute it with the default.
1391                         return default_ct.into();
1392                     }
1393                 }
1394
1395                 tcx.mk_param_from_def(param)
1396             }
1397         }
1398     });
1399
1400     // Now we build the substituted predicates.
1401     let default_obligations = predicates
1402         .0
1403         .predicates
1404         .iter()
1405         .flat_map(|&(pred, sp)| {
1406             #[derive(Default)]
1407             struct CountParams {
1408                 params: FxHashSet<u32>,
1409             }
1410             impl<'tcx> ty::visit::TypeVisitor<'tcx> for CountParams {
1411                 type BreakTy = ();
1412
1413                 fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
1414                     if let ty::Param(param) = t.kind() {
1415                         self.params.insert(param.index);
1416                     }
1417                     t.super_visit_with(self)
1418                 }
1419
1420                 fn visit_region(&mut self, _: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
1421                     ControlFlow::BREAK
1422                 }
1423
1424                 fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
1425                     if let ty::ConstKind::Param(param) = c.kind() {
1426                         self.params.insert(param.index);
1427                     }
1428                     c.super_visit_with(self)
1429                 }
1430             }
1431             let mut param_count = CountParams::default();
1432             let has_region = pred.visit_with(&mut param_count).is_break();
1433             let substituted_pred = predicates.rebind(pred).subst(tcx, substs);
1434             // Don't check non-defaulted params, dependent defaults (including lifetimes)
1435             // or preds with multiple params.
1436             if substituted_pred.has_non_region_param() || param_count.params.len() > 1 || has_region
1437             {
1438                 None
1439             } else if predicates.0.predicates.iter().any(|&(p, _)| p == substituted_pred) {
1440                 // Avoid duplication of predicates that contain no parameters, for example.
1441                 None
1442             } else {
1443                 Some((substituted_pred, sp))
1444             }
1445         })
1446         .map(|(pred, sp)| {
1447             // Convert each of those into an obligation. So if you have
1448             // something like `struct Foo<T: Copy = String>`, we would
1449             // take that predicate `T: Copy`, substitute to `String: Copy`
1450             // (actually that happens in the previous `flat_map` call),
1451             // and then try to prove it (in this case, we'll fail).
1452             //
1453             // Note the subtle difference from how we handle `predicates`
1454             // below: there, we are not trying to prove those predicates
1455             // to be *true* but merely *well-formed*.
1456             let pred = wfcx.normalize(sp, None, pred);
1457             let cause = traits::ObligationCause::new(
1458                 sp,
1459                 wfcx.body_id,
1460                 traits::ItemObligation(def_id.to_def_id()),
1461             );
1462             traits::Obligation::new(tcx, cause, wfcx.param_env, pred)
1463         });
1464
1465     let predicates = predicates.0.instantiate_identity(tcx);
1466
1467     let predicates = wfcx.normalize(span, None, predicates);
1468
1469     debug!(?predicates.predicates);
1470     assert_eq!(predicates.predicates.len(), predicates.spans.len());
1471     let wf_obligations =
1472         iter::zip(&predicates.predicates, &predicates.spans).flat_map(|(&p, &sp)| {
1473             traits::wf::predicate_obligations(
1474                 infcx,
1475                 wfcx.param_env.without_const(),
1476                 wfcx.body_id,
1477                 p,
1478                 sp,
1479             )
1480         });
1481
1482     let obligations: Vec<_> = wf_obligations.chain(default_obligations).collect();
1483     wfcx.register_obligations(obligations);
1484 }
1485
1486 #[instrument(level = "debug", skip(wfcx, span, hir_decl))]
1487 fn check_fn_or_method<'tcx>(
1488     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1489     span: Span,
1490     sig: ty::PolyFnSig<'tcx>,
1491     hir_decl: &hir::FnDecl<'_>,
1492     def_id: LocalDefId,
1493 ) {
1494     let tcx = wfcx.tcx();
1495     let mut sig = tcx.liberate_late_bound_regions(def_id.to_def_id(), sig);
1496
1497     // Normalize the input and output types one at a time, using a different
1498     // `WellFormedLoc` for each. We cannot call `normalize_associated_types`
1499     // on the entire `FnSig`, since this would use the same `WellFormedLoc`
1500     // for each type, preventing the HIR wf check from generating
1501     // a nice error message.
1502     let arg_span =
1503         |idx| hir_decl.inputs.get(idx).map_or(hir_decl.output.span(), |arg: &hir::Ty<'_>| arg.span);
1504
1505     sig.inputs_and_output =
1506         tcx.mk_type_list(sig.inputs_and_output.iter().enumerate().map(|(idx, ty)| {
1507             wfcx.normalize(
1508                 arg_span(idx),
1509                 Some(WellFormedLoc::Param {
1510                     function: def_id,
1511                     // Note that the `param_idx` of the output type is
1512                     // one greater than the index of the last input type.
1513                     param_idx: idx.try_into().unwrap(),
1514                 }),
1515                 ty,
1516             )
1517         }));
1518
1519     for (idx, ty) in sig.inputs_and_output.iter().enumerate() {
1520         wfcx.register_wf_obligation(
1521             arg_span(idx),
1522             Some(WellFormedLoc::Param { function: def_id, param_idx: idx.try_into().unwrap() }),
1523             ty.into(),
1524         );
1525     }
1526
1527     check_where_clauses(wfcx, span, def_id);
1528
1529     check_return_position_impl_trait_in_trait_bounds(
1530         wfcx,
1531         def_id,
1532         sig.output(),
1533         hir_decl.output.span(),
1534     );
1535
1536     if sig.abi == Abi::RustCall {
1537         let span = tcx.def_span(def_id);
1538         let has_implicit_self = hir_decl.implicit_self != hir::ImplicitSelfKind::None;
1539         let mut inputs = sig.inputs().iter().skip(if has_implicit_self { 1 } else { 0 });
1540         // Check that the argument is a tuple
1541         if let Some(ty) = inputs.next() {
1542             wfcx.register_bound(
1543                 ObligationCause::new(span, wfcx.body_id, ObligationCauseCode::RustCall),
1544                 wfcx.param_env,
1545                 *ty,
1546                 tcx.require_lang_item(hir::LangItem::Tuple, Some(span)),
1547             );
1548         } else {
1549             tcx.sess.span_err(
1550                 hir_decl.inputs.last().map_or(span, |input| input.span),
1551                 "functions with the \"rust-call\" ABI must take a single non-self tuple argument",
1552             );
1553         }
1554         // No more inputs other than the `self` type and the tuple type
1555         if inputs.next().is_some() {
1556             tcx.sess.span_err(
1557                 hir_decl.inputs.last().map_or(span, |input| input.span),
1558                 "functions with the \"rust-call\" ABI must take a single non-self tuple argument",
1559             );
1560         }
1561     }
1562 }
1563
1564 /// Basically `check_associated_type_bounds`, but separated for now and should be
1565 /// deduplicated when RPITITs get lowered into real associated items.
1566 #[tracing::instrument(level = "trace", skip(wfcx))]
1567 fn check_return_position_impl_trait_in_trait_bounds<'tcx>(
1568     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1569     fn_def_id: LocalDefId,
1570     fn_output: Ty<'tcx>,
1571     span: Span,
1572 ) {
1573     let tcx = wfcx.tcx();
1574     if let Some(assoc_item) = tcx.opt_associated_item(fn_def_id.to_def_id())
1575         && assoc_item.container == ty::AssocItemContainer::TraitContainer
1576     {
1577         for arg in fn_output.walk() {
1578             if let ty::GenericArgKind::Type(ty) = arg.unpack()
1579                 && let ty::Alias(ty::Projection, proj) = ty.kind()
1580                 && tcx.def_kind(proj.def_id) == DefKind::ImplTraitPlaceholder
1581                 && tcx.impl_trait_in_trait_parent(proj.def_id) == fn_def_id.to_def_id()
1582             {
1583                 let span = tcx.def_span(proj.def_id);
1584                 let bounds = wfcx.tcx().explicit_item_bounds(proj.def_id);
1585                 let wf_obligations = bounds.iter().flat_map(|&(bound, bound_span)| {
1586                     let bound = ty::EarlyBinder(bound).subst(tcx, proj.substs);
1587                     let normalized_bound = wfcx.normalize(span, None, bound);
1588                     traits::wf::predicate_obligations(
1589                         wfcx.infcx,
1590                         wfcx.param_env,
1591                         wfcx.body_id,
1592                         normalized_bound,
1593                         bound_span,
1594                     )
1595                 });
1596                 wfcx.register_obligations(wf_obligations);
1597             }
1598         }
1599     }
1600 }
1601
1602 const HELP_FOR_SELF_TYPE: &str = "consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, \
1603      `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one \
1604      of the previous types except `Self`)";
1605
1606 #[instrument(level = "debug", skip(wfcx))]
1607 fn check_method_receiver<'tcx>(
1608     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1609     fn_sig: &hir::FnSig<'_>,
1610     method: &ty::AssocItem,
1611     self_ty: Ty<'tcx>,
1612 ) {
1613     let tcx = wfcx.tcx();
1614
1615     if !method.fn_has_self_parameter {
1616         return;
1617     }
1618
1619     let span = fn_sig.decl.inputs[0].span;
1620
1621     let sig = tcx.fn_sig(method.def_id);
1622     let sig = tcx.liberate_late_bound_regions(method.def_id, sig);
1623     let sig = wfcx.normalize(span, None, sig);
1624
1625     debug!("check_method_receiver: sig={:?}", sig);
1626
1627     let self_ty = wfcx.normalize(span, None, self_ty);
1628
1629     let receiver_ty = sig.inputs()[0];
1630     let receiver_ty = wfcx.normalize(span, None, receiver_ty);
1631
1632     if tcx.features().arbitrary_self_types {
1633         if !receiver_is_valid(wfcx, span, receiver_ty, self_ty, true) {
1634             // Report error; `arbitrary_self_types` was enabled.
1635             e0307(tcx, span, receiver_ty);
1636         }
1637     } else {
1638         if !receiver_is_valid(wfcx, span, receiver_ty, self_ty, false) {
1639             if receiver_is_valid(wfcx, span, receiver_ty, self_ty, true) {
1640                 // Report error; would have worked with `arbitrary_self_types`.
1641                 feature_err(
1642                     &tcx.sess.parse_sess,
1643                     sym::arbitrary_self_types,
1644                     span,
1645                     &format!(
1646                         "`{receiver_ty}` cannot be used as the type of `self` without \
1647                          the `arbitrary_self_types` feature",
1648                     ),
1649                 )
1650                 .help(HELP_FOR_SELF_TYPE)
1651                 .emit();
1652             } else {
1653                 // Report error; would not have worked with `arbitrary_self_types`.
1654                 e0307(tcx, span, receiver_ty);
1655             }
1656         }
1657     }
1658 }
1659
1660 fn e0307(tcx: TyCtxt<'_>, span: Span, receiver_ty: Ty<'_>) {
1661     struct_span_err!(
1662         tcx.sess.diagnostic(),
1663         span,
1664         E0307,
1665         "invalid `self` parameter type: {receiver_ty}"
1666     )
1667     .note("type of `self` must be `Self` or a type that dereferences to it")
1668     .help(HELP_FOR_SELF_TYPE)
1669     .emit();
1670 }
1671
1672 /// Returns whether `receiver_ty` would be considered a valid receiver type for `self_ty`. If
1673 /// `arbitrary_self_types` is enabled, `receiver_ty` must transitively deref to `self_ty`, possibly
1674 /// through a `*const/mut T` raw pointer. If the feature is not enabled, the requirements are more
1675 /// strict: `receiver_ty` must implement `Receiver` and directly implement
1676 /// `Deref<Target = self_ty>`.
1677 ///
1678 /// N.B., there are cases this function returns `true` but causes an error to be emitted,
1679 /// particularly when `receiver_ty` derefs to a type that is the same as `self_ty` but has the
1680 /// wrong lifetime. Be careful of this if you are calling this function speculatively.
1681 fn receiver_is_valid<'tcx>(
1682     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1683     span: Span,
1684     receiver_ty: Ty<'tcx>,
1685     self_ty: Ty<'tcx>,
1686     arbitrary_self_types_enabled: bool,
1687 ) -> bool {
1688     let infcx = wfcx.infcx;
1689     let tcx = wfcx.tcx();
1690     let cause =
1691         ObligationCause::new(span, wfcx.body_id, traits::ObligationCauseCode::MethodReceiver);
1692
1693     let can_eq_self = |ty| infcx.can_eq(wfcx.param_env, self_ty, ty).is_ok();
1694
1695     // `self: Self` is always valid.
1696     if can_eq_self(receiver_ty) {
1697         if let Err(err) = wfcx.eq(&cause, wfcx.param_env, self_ty, receiver_ty) {
1698             infcx.err_ctxt().report_mismatched_types(&cause, self_ty, receiver_ty, err).emit();
1699         }
1700         return true;
1701     }
1702
1703     let mut autoderef = Autoderef::new(infcx, wfcx.param_env, wfcx.body_id, span, receiver_ty);
1704
1705     // The `arbitrary_self_types` feature allows raw pointer receivers like `self: *const Self`.
1706     if arbitrary_self_types_enabled {
1707         autoderef = autoderef.include_raw_pointers();
1708     }
1709
1710     // The first type is `receiver_ty`, which we know its not equal to `self_ty`; skip it.
1711     autoderef.next();
1712
1713     let receiver_trait_def_id = tcx.require_lang_item(LangItem::Receiver, Some(span));
1714
1715     // Keep dereferencing `receiver_ty` until we get to `self_ty`.
1716     loop {
1717         if let Some((potential_self_ty, _)) = autoderef.next() {
1718             debug!(
1719                 "receiver_is_valid: potential self type `{:?}` to match `{:?}`",
1720                 potential_self_ty, self_ty
1721             );
1722
1723             if can_eq_self(potential_self_ty) {
1724                 wfcx.register_obligations(autoderef.into_obligations());
1725
1726                 if let Err(err) = wfcx.eq(&cause, wfcx.param_env, self_ty, potential_self_ty) {
1727                     infcx
1728                         .err_ctxt()
1729                         .report_mismatched_types(&cause, self_ty, potential_self_ty, err)
1730                         .emit();
1731                 }
1732
1733                 break;
1734             } else {
1735                 // Without `feature(arbitrary_self_types)`, we require that each step in the
1736                 // deref chain implement `receiver`
1737                 if !arbitrary_self_types_enabled
1738                     && !receiver_is_implemented(
1739                         wfcx,
1740                         receiver_trait_def_id,
1741                         cause.clone(),
1742                         potential_self_ty,
1743                     )
1744                 {
1745                     return false;
1746                 }
1747             }
1748         } else {
1749             debug!("receiver_is_valid: type `{:?}` does not deref to `{:?}`", receiver_ty, self_ty);
1750             // If the receiver already has errors reported due to it, consider it valid to avoid
1751             // unnecessary errors (#58712).
1752             return receiver_ty.references_error();
1753         }
1754     }
1755
1756     // Without `feature(arbitrary_self_types)`, we require that `receiver_ty` implements `Receiver`.
1757     if !arbitrary_self_types_enabled
1758         && !receiver_is_implemented(wfcx, receiver_trait_def_id, cause.clone(), receiver_ty)
1759     {
1760         return false;
1761     }
1762
1763     true
1764 }
1765
1766 fn receiver_is_implemented<'tcx>(
1767     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1768     receiver_trait_def_id: DefId,
1769     cause: ObligationCause<'tcx>,
1770     receiver_ty: Ty<'tcx>,
1771 ) -> bool {
1772     let tcx = wfcx.tcx();
1773     let trait_ref = ty::Binder::dummy(tcx.mk_trait_ref(receiver_trait_def_id, [receiver_ty]));
1774
1775     let obligation = traits::Obligation::new(tcx, cause, wfcx.param_env, trait_ref);
1776
1777     if wfcx.infcx.predicate_must_hold_modulo_regions(&obligation) {
1778         true
1779     } else {
1780         debug!(
1781             "receiver_is_implemented: type `{:?}` does not implement `Receiver` trait",
1782             receiver_ty
1783         );
1784         false
1785     }
1786 }
1787
1788 fn check_variances_for_type_defn<'tcx>(
1789     tcx: TyCtxt<'tcx>,
1790     item: &hir::Item<'tcx>,
1791     hir_generics: &hir::Generics<'_>,
1792 ) {
1793     let ty = tcx.type_of(item.owner_id);
1794     if tcx.has_error_field(ty) {
1795         return;
1796     }
1797
1798     let ty_predicates = tcx.predicates_of(item.owner_id);
1799     assert_eq!(ty_predicates.parent, None);
1800     let variances = tcx.variances_of(item.owner_id);
1801
1802     let mut constrained_parameters: FxHashSet<_> = variances
1803         .iter()
1804         .enumerate()
1805         .filter(|&(_, &variance)| variance != ty::Bivariant)
1806         .map(|(index, _)| Parameter(index as u32))
1807         .collect();
1808
1809     identify_constrained_generic_params(tcx, ty_predicates, None, &mut constrained_parameters);
1810
1811     // Lazily calculated because it is only needed in case of an error.
1812     let explicitly_bounded_params = LazyCell::new(|| {
1813         let icx = crate::collect::ItemCtxt::new(tcx, item.owner_id.to_def_id());
1814         hir_generics
1815             .predicates
1816             .iter()
1817             .filter_map(|predicate| match predicate {
1818                 hir::WherePredicate::BoundPredicate(predicate) => {
1819                     match icx.to_ty(predicate.bounded_ty).kind() {
1820                         ty::Param(data) => Some(Parameter(data.index)),
1821                         _ => None,
1822                     }
1823                 }
1824                 _ => None,
1825             })
1826             .collect::<FxHashSet<_>>()
1827     });
1828
1829     for (index, _) in variances.iter().enumerate() {
1830         let parameter = Parameter(index as u32);
1831
1832         if constrained_parameters.contains(&parameter) {
1833             continue;
1834         }
1835
1836         let param = &hir_generics.params[index];
1837
1838         match param.name {
1839             hir::ParamName::Error => {}
1840             _ => {
1841                 let has_explicit_bounds = explicitly_bounded_params.contains(&parameter);
1842                 report_bivariance(tcx, param, has_explicit_bounds);
1843             }
1844         }
1845     }
1846 }
1847
1848 fn report_bivariance(
1849     tcx: TyCtxt<'_>,
1850     param: &rustc_hir::GenericParam<'_>,
1851     has_explicit_bounds: bool,
1852 ) -> ErrorGuaranteed {
1853     let span = param.span;
1854     let param_name = param.name.ident().name;
1855     let mut err = error_392(tcx, span, param_name);
1856
1857     let suggested_marker_id = tcx.lang_items().phantom_data();
1858     // Help is available only in presence of lang items.
1859     let msg = if let Some(def_id) = suggested_marker_id {
1860         format!(
1861             "consider removing `{}`, referring to it in a field, or using a marker such as `{}`",
1862             param_name,
1863             tcx.def_path_str(def_id),
1864         )
1865     } else {
1866         format!("consider removing `{param_name}` or referring to it in a field")
1867     };
1868     err.help(&msg);
1869
1870     if matches!(param.kind, hir::GenericParamKind::Type { .. }) && !has_explicit_bounds {
1871         err.help(&format!(
1872             "if you intended `{0}` to be a const parameter, use `const {0}: usize` instead",
1873             param_name
1874         ));
1875     }
1876     err.emit()
1877 }
1878
1879 impl<'tcx> WfCheckingCtxt<'_, 'tcx> {
1880     /// Feature gates RFC 2056 -- trivial bounds, checking for global bounds that
1881     /// aren't true.
1882     #[instrument(level = "debug", skip(self))]
1883     fn check_false_global_bounds(&mut self) {
1884         let tcx = self.ocx.infcx.tcx;
1885         let mut span = self.span;
1886         let empty_env = ty::ParamEnv::empty();
1887
1888         let def_id = tcx.hir().local_def_id(self.body_id);
1889         let predicates_with_span = tcx.predicates_of(def_id).predicates.iter().copied();
1890         // Check elaborated bounds.
1891         let implied_obligations = traits::elaborate_predicates_with_span(tcx, predicates_with_span);
1892
1893         for obligation in implied_obligations {
1894             // We lower empty bounds like `Vec<dyn Copy>:` as
1895             // `WellFormed(Vec<dyn Copy>)`, which will later get checked by
1896             // regular WF checking
1897             if let ty::PredicateKind::WellFormed(..) = obligation.predicate.kind().skip_binder() {
1898                 continue;
1899             }
1900             let pred = obligation.predicate;
1901             // Match the existing behavior.
1902             if pred.is_global() && !pred.has_late_bound_vars() {
1903                 let pred = self.normalize(span, None, pred);
1904                 let hir_node = tcx.hir().find(self.body_id);
1905
1906                 // only use the span of the predicate clause (#90869)
1907
1908                 if let Some(hir::Generics { predicates, .. }) =
1909                     hir_node.and_then(|node| node.generics())
1910                 {
1911                     let obligation_span = obligation.cause.span();
1912
1913                     span = predicates
1914                         .iter()
1915                         // There seems to be no better way to find out which predicate we are in
1916                         .find(|pred| pred.span().contains(obligation_span))
1917                         .map(|pred| pred.span())
1918                         .unwrap_or(obligation_span);
1919                 }
1920
1921                 let obligation = traits::Obligation::new(
1922                     tcx,
1923                     traits::ObligationCause::new(span, self.body_id, traits::TrivialBound),
1924                     empty_env,
1925                     pred,
1926                 );
1927                 self.ocx.register_obligation(obligation);
1928             }
1929         }
1930     }
1931 }
1932
1933 fn check_mod_type_wf(tcx: TyCtxt<'_>, module: LocalDefId) {
1934     let items = tcx.hir_module_items(module);
1935     items.par_items(|item| tcx.ensure().check_well_formed(item.owner_id));
1936     items.par_impl_items(|item| tcx.ensure().check_well_formed(item.owner_id));
1937     items.par_trait_items(|item| tcx.ensure().check_well_formed(item.owner_id));
1938     items.par_foreign_items(|item| tcx.ensure().check_well_formed(item.owner_id));
1939 }
1940
1941 fn error_392(
1942     tcx: TyCtxt<'_>,
1943     span: Span,
1944     param_name: Symbol,
1945 ) -> DiagnosticBuilder<'_, ErrorGuaranteed> {
1946     let mut err = struct_span_err!(tcx.sess, span, E0392, "parameter `{param_name}` is never used");
1947     err.span_label(span, "unused parameter");
1948     err
1949 }
1950
1951 pub fn provide(providers: &mut Providers) {
1952     *providers = Providers { check_mod_type_wf, check_well_formed, ..*providers };
1953 }