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 assumed_wf_types = ocx.assumed_wf_types(param_env, span, body_def_id);
 101
 102     let mut wfcx = WfCheckingCtxt { ocx, span, body_id, param_env };
 103
 104     if !tcx.features().trivial_bounds {
 105         wfcx.check_false_global_bounds()
 106     }
 107     f(&mut wfcx);
 108     let errors = wfcx.select_all_or_error();
 109     if !errors.is_empty() {
 110         infcx.err_ctxt().report_fulfillment_errors(&errors, None);
 111         return;
 112     }
 113
 114     let implied_bounds = infcx.implied_bounds_tys(param_env, body_id, assumed_wf_types);
 115     let outlives_environment =
 116         OutlivesEnvironment::with_bounds(param_env, Some(infcx), implied_bounds);
 117
 118     let _ = infcx.check_region_obligations_and_report_errors(body_def_id, &outlives_environment);
 119 }
 120
 121 fn check_well_formed(tcx: TyCtxt<'_>, def_id: hir::OwnerId) {
 122     let node = tcx.hir().owner(def_id);
 123     match node {
 124         hir::OwnerNode::Crate(_) => {}
 125         hir::OwnerNode::Item(item) => check_item(tcx, item),
 126         hir::OwnerNode::TraitItem(item) => check_trait_item(tcx, item),
 127         hir::OwnerNode::ImplItem(item) => check_impl_item(tcx, item),
 128         hir::OwnerNode::ForeignItem(item) => check_foreign_item(tcx, item),
 129     }
 130
 131     if let Some(generics) = node.generics() {
 132         for param in generics.params {
 133             check_param_wf(tcx, param)
 134         }
 135     }
 136 }
 137
 138 /// Checks that the field types (in a struct def'n) or argument types (in an enum def'n) are
 139 /// well-formed, meaning that they do not require any constraints not declared in the struct
 140 /// definition itself. For example, this definition would be illegal:
 141 ///
 142 /// ```rust
 143 /// struct Ref<'a, T> { x: &'a T }
 144 /// ```
 145 ///
 146 /// because the type did not declare that `T:'a`.
 147 ///
 148 /// We do this check as a pre-pass before checking fn bodies because if these constraints are
 149 /// not included it frequently leads to confusing errors in fn bodies. So it's better to check
 150 /// the types first.
 151 #[instrument(skip(tcx), level = "debug")]
 152 fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) {
 153     let def_id = item.owner_id.def_id;
 154
 155     debug!(
 156         ?item.owner_id,
 157         item.name = ? tcx.def_path_str(def_id.to_def_id())
 158     );
 159
 160     match item.kind {
 161         // Right now we check that every default trait implementation
 162         // has an implementation of itself. Basically, a case like:
 163         //
 164         //     impl Trait for T {}
 165         //
 166         // has a requirement of `T: Trait` which was required for default
 167         // method implementations. Although this could be improved now that
 168         // there's a better infrastructure in place for this, it's being left
 169         // for a follow-up work.
 170         //
 171         // Since there's such a requirement, we need to check *just* positive
 172         // implementations, otherwise things like:
 173         //
 174         //     impl !Send for T {}
 175         //
 176         // won't be allowed unless there's an *explicit* implementation of `Send`
 177         // for `T`
 178         hir::ItemKind::Impl(ref impl_) => {
 179             let is_auto = tcx
 180                 .impl_trait_ref(def_id)
 181                 .map_or(false, |trait_ref| tcx.trait_is_auto(trait_ref.def_id));
 182             if let (hir::Defaultness::Default { .. }, true) = (impl_.defaultness, is_auto) {
 183                 let sp = impl_.of_trait.as_ref().map_or(item.span, |t| t.path.span);
 184                 let mut err =
 185                     tcx.sess.struct_span_err(sp, "impls of auto traits cannot be default");
 186                 err.span_labels(impl_.defaultness_span, "default because of this");
 187                 err.span_label(sp, "auto trait");
 188                 err.emit();
 189             }
 190             // We match on both `ty::ImplPolarity` and `ast::ImplPolarity` just to get the `!` span.
 191             match (tcx.impl_polarity(def_id), impl_.polarity) {
 192                 (ty::ImplPolarity::Positive, _) => {
 193                     check_impl(tcx, item, impl_.self_ty, &impl_.of_trait, impl_.constness);
 194                 }
 195                 (ty::ImplPolarity::Negative, ast::ImplPolarity::Negative(span)) => {
 196                     // FIXME(#27579): what amount of WF checking do we need for neg impls?
 197                     if let hir::Defaultness::Default { .. } = impl_.defaultness {
 198                         let mut spans = vec![span];
 199                         spans.extend(impl_.defaultness_span);
 200                         struct_span_err!(
 201                             tcx.sess,
 202                             spans,
 203                             E0750,
 204                             "negative impls cannot be default impls"
 205                         )
 206                         .emit();
 207                     }
 208                 }
 209                 (ty::ImplPolarity::Reservation, _) => {
 210                     // FIXME: what amount of WF checking do we need for reservation impls?
 211                 }
 212                 _ => unreachable!(),
 213             }
 214         }
 215         hir::ItemKind::Fn(ref sig, ..) => {
 216             check_item_fn(tcx, def_id, item.ident, item.span, sig.decl);
 217         }
 218         hir::ItemKind::Static(ty, ..) => {
 219             check_item_type(tcx, def_id, ty.span, false);
 220         }
 221         hir::ItemKind::Const(ty, ..) => {
 222             check_item_type(tcx, def_id, ty.span, false);
 223         }
 224         hir::ItemKind::Struct(_, ref ast_generics) => {
 225             check_type_defn(tcx, item, false);
 226             check_variances_for_type_defn(tcx, item, ast_generics);
 227         }
 228         hir::ItemKind::Union(_, ref ast_generics) => {
 229             check_type_defn(tcx, item, true);
 230             check_variances_for_type_defn(tcx, item, ast_generics);
 231         }
 232         hir::ItemKind::Enum(_, ref ast_generics) => {
 233             check_type_defn(tcx, item, true);
 234             check_variances_for_type_defn(tcx, item, ast_generics);
 235         }
 236         hir::ItemKind::Trait(..) => {
 237             check_trait(tcx, item);
 238         }
 239         hir::ItemKind::TraitAlias(..) => {
 240             check_trait(tcx, item);
 241         }
 242         // `ForeignItem`s are handled separately.
 243         hir::ItemKind::ForeignMod { .. } => {}
 244         _ => {}
 245     }
 246 }
 247
 248 fn check_foreign_item(tcx: TyCtxt<'_>, item: &hir::ForeignItem<'_>) {
 249     let def_id = item.owner_id.def_id;
 250
 251     debug!(
 252         ?item.owner_id,
 253         item.name = ? tcx.def_path_str(def_id.to_def_id())
 254     );
 255
 256     match item.kind {
 257         hir::ForeignItemKind::Fn(decl, ..) => {
 258             check_item_fn(tcx, def_id, item.ident, item.span, decl)
 259         }
 260         hir::ForeignItemKind::Static(ty, ..) => check_item_type(tcx, def_id, ty.span, true),
 261         hir::ForeignItemKind::Type => (),
 262     }
 263 }
 264
 265 fn check_trait_item(tcx: TyCtxt<'_>, trait_item: &hir::TraitItem<'_>) {
 266     let def_id = trait_item.owner_id.def_id;
 267
 268     let (method_sig, span) = match trait_item.kind {
 269         hir::TraitItemKind::Fn(ref sig, _) => (Some(sig), trait_item.span),
 270         hir::TraitItemKind::Type(_bounds, Some(ty)) => (None, ty.span),
 271         _ => (None, trait_item.span),
 272     };
 273     check_object_unsafe_self_trait_by_name(tcx, trait_item);
 274     check_associated_item(tcx, def_id, span, method_sig);
 275
 276     let encl_trait_def_id = tcx.local_parent(def_id);
 277     let encl_trait = tcx.hir().expect_item(encl_trait_def_id);
 278     let encl_trait_def_id = encl_trait.owner_id.to_def_id();
 279     let fn_lang_item_name = if Some(encl_trait_def_id) == tcx.lang_items().fn_trait() {
 280         Some("fn")
 281     } else if Some(encl_trait_def_id) == tcx.lang_items().fn_mut_trait() {
 282         Some("fn_mut")
 283     } else {
 284         None
 285     };
 286
 287     if let (Some(fn_lang_item_name), "call") =
 288         (fn_lang_item_name, trait_item.ident.name.to_ident_string().as_str())
 289     {
 290         // We are looking at the `call` function of the `fn` or `fn_mut` lang item.
 291         // Do some rudimentary sanity checking to avoid an ICE later (issue #83471).
 292         if let Some(hir::FnSig { decl, span, .. }) = method_sig {
 293             if let [self_ty, _] = decl.inputs {
 294                 if !matches!(self_ty.kind, hir::TyKind::Ref(_, _)) {
 295                     tcx.sess
 296                         .struct_span_err(
 297                             self_ty.span,
 298                             &format!(
 299                                 "first argument of `call` in `{fn_lang_item_name}` lang item must be a reference",
 300                             ),
 301                         )
 302                         .emit();
 303                 }
 304             } else {
 305                 tcx.sess
 306                     .struct_span_err(
 307                         *span,
 308                         &format!(
 309                             "`call` function in `{fn_lang_item_name}` lang item takes exactly two arguments",
 310                         ),
 311                     )
 312                     .emit();
 313             }
 314         } else {
 315             tcx.sess
 316                 .struct_span_err(
 317                     trait_item.span,
 318                     &format!(
 319                         "`call` trait item in `{fn_lang_item_name}` lang item must be a function",
 320                     ),
 321                 )
 322                 .emit();
 323         }
 324     }
 325 }
 326
 327 /// Require that the user writes where clauses on GATs for the implicit
 328 /// outlives bounds involving trait parameters in trait functions and
 329 /// lifetimes passed as GAT substs. See `self-outlives-lint` test.
 330 ///
 331 /// We use the following trait as an example throughout this function:
 332 /// ```rust,ignore (this code fails due to this lint)
 333 /// trait IntoIter {
 334 ///     type Iter<'a>: Iterator<Item = Self::Item<'a>>;
 335 ///     type Item<'a>;
 336 ///     fn into_iter<'a>(&'a self) -> Self::Iter<'a>;
 337 /// }
 338 /// ```
 339 fn check_gat_where_clauses(tcx: TyCtxt<'_>, associated_items: &[hir::TraitItemRef]) {
 340     // Associates every GAT's def_id to a list of possibly missing bounds detected by this lint.
 341     let mut required_bounds_by_item = FxHashMap::default();
 342
 343     // Loop over all GATs together, because if this lint suggests adding a where-clause bound
 344     // to one GAT, it might then require us to an additional bound on another GAT.
 345     // In our `IntoIter` example, we discover a missing `Self: 'a` bound on `Iter<'a>`, which
 346     // then in a second loop adds a `Self: 'a` bound to `Item` due to the relationship between
 347     // those GATs.
 348     loop {
 349         let mut should_continue = false;
 350         for gat_item in associated_items {
 351             let gat_def_id = gat_item.id.owner_id;
 352             let gat_item = tcx.associated_item(gat_def_id);
 353             // If this item is not an assoc ty, or has no substs, then it's not a GAT
 354             if gat_item.kind != ty::AssocKind::Type {
 355                 continue;
 356             }
 357             let gat_generics = tcx.generics_of(gat_def_id);
 358             // FIXME(jackh726): we can also warn in the more general case
 359             if gat_generics.params.is_empty() {
 360                 continue;
 361             }
 362
 363             // Gather the bounds with which all other items inside of this trait constrain the GAT.
 364             // This is calculated by taking the intersection of the bounds that each item
 365             // constrains the GAT with individually.
 366             let mut new_required_bounds: Option<FxHashSet<ty::Predicate<'_>>> = None;
 367             for item in associated_items {
 368                 let item_def_id = item.id.owner_id;
 369                 // Skip our own GAT, since it does not constrain itself at all.
 370                 if item_def_id == gat_def_id {
 371                     continue;
 372                 }
 373
 374                 let item_hir_id = item.id.hir_id();
 375                 let param_env = tcx.param_env(item_def_id);
 376
 377                 let item_required_bounds = match item.kind {
 378                     // In our example, this corresponds to `into_iter` method
 379                     hir::AssocItemKind::Fn { .. } => {
 380                         // For methods, we check the function signature's return type for any GATs
 381                         // to constrain. In the `into_iter` case, we see that the return type
 382                         // `Self::Iter<'a>` is a GAT we want to gather any potential missing bounds from.
 383                         let sig: ty::FnSig<'_> = tcx.liberate_late_bound_regions(
 384                             item_def_id.to_def_id(),
 385                             tcx.fn_sig(item_def_id),
 386                         );
 387                         gather_gat_bounds(
 388                             tcx,
 389                             param_env,
 390                             item_hir_id,
 391                             sig.inputs_and_output,
 392                             // We also assume that all of the function signature's parameter types
 393                             // are well formed.
 394                             &sig.inputs().iter().copied().collect(),
 395                             gat_def_id.def_id,
 396                             gat_generics,
 397                         )
 398                     }
 399                     // In our example, this corresponds to the `Iter` and `Item` associated types
 400                     hir::AssocItemKind::Type => {
 401                         // If our associated item is a GAT with missing bounds, add them to
 402                         // the param-env here. This allows this GAT to propagate missing bounds
 403                         // to other GATs.
 404                         let param_env = augment_param_env(
 405                             tcx,
 406                             param_env,
 407                             required_bounds_by_item.get(&item_def_id),
 408                         );
 409                         gather_gat_bounds(
 410                             tcx,
 411                             param_env,
 412                             item_hir_id,
 413                             tcx.explicit_item_bounds(item_def_id).to_vec(),
 414                             &FxIndexSet::default(),
 415                             gat_def_id.def_id,
 416                             gat_generics,
 417                         )
 418                     }
 419                     hir::AssocItemKind::Const => None,
 420                 };
 421
 422                 if let Some(item_required_bounds) = item_required_bounds {
 423                     // Take the intersection of the required bounds for this GAT, and
 424                     // the item_required_bounds which are the ones implied by just
 425                     // this item alone.
 426                     // This is why we use an Option<_>, since we need to distinguish
 427                     // the empty set of bounds from the _uninitialized_ set of bounds.
 428                     if let Some(new_required_bounds) = &mut new_required_bounds {
 429                         new_required_bounds.retain(|b| item_required_bounds.contains(b));
 430                     } else {
 431                         new_required_bounds = Some(item_required_bounds);
 432                     }
 433                 }
 434             }
 435
 436             if let Some(new_required_bounds) = new_required_bounds {
 437                 let required_bounds = required_bounds_by_item.entry(gat_def_id).or_default();
 438                 if new_required_bounds.into_iter().any(|p| required_bounds.insert(p)) {
 439                     // Iterate until our required_bounds no longer change
 440                     // Since they changed here, we should continue the loop
 441                     should_continue = true;
 442                 }
 443             }
 444         }
 445         // We know that this loop will eventually halt, since we only set `should_continue` if the
 446         // `required_bounds` for this item grows. Since we are not creating any new region or type
 447         // variables, the set of all region and type bounds that we could ever insert are limited
 448         // by the number of unique types and regions we observe in a given item.
 449         if !should_continue {
 450             break;
 451         }
 452     }
 453
 454     for (gat_def_id, required_bounds) in required_bounds_by_item {
 455         let gat_item_hir = tcx.hir().expect_trait_item(gat_def_id.def_id);
 456         debug!(?required_bounds);
 457         let param_env = tcx.param_env(gat_def_id);
 458         let gat_hir = gat_item_hir.hir_id();
 459
 460         let mut unsatisfied_bounds: Vec<_> = required_bounds
 461             .into_iter()
 462             .filter(|clause| match clause.kind().skip_binder() {
 463                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(ty::OutlivesPredicate(
 464                     a,
 465                     b,
 466                 ))) => {
 467                     !region_known_to_outlive(tcx, gat_hir, param_env, &FxIndexSet::default(), a, b)
 468                 }
 469                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
 470                     a,
 471                     b,
 472                 ))) => !ty_known_to_outlive(tcx, gat_hir, param_env, &FxIndexSet::default(), a, b),
 473                 _ => bug!("Unexpected PredicateKind"),
 474             })
 475             .map(|clause| clause.to_string())
 476             .collect();
 477
 478         // We sort so that order is predictable
 479         unsatisfied_bounds.sort();
 480
 481         if !unsatisfied_bounds.is_empty() {
 482             let plural = pluralize!(unsatisfied_bounds.len());
 483             let mut err = tcx.sess.struct_span_err(
 484                 gat_item_hir.span,
 485                 &format!("missing required bound{} on `{}`", plural, gat_item_hir.ident),
 486             );
 487
 488             let suggestion = format!(
 489                 "{} {}",
 490                 gat_item_hir.generics.add_where_or_trailing_comma(),
 491                 unsatisfied_bounds.join(", "),
 492             );
 493             err.span_suggestion(
 494                 gat_item_hir.generics.tail_span_for_predicate_suggestion(),
 495                 &format!("add the required where clause{plural}"),
 496                 suggestion,
 497                 Applicability::MachineApplicable,
 498             );
 499
 500             let bound =
 501                 if unsatisfied_bounds.len() > 1 { "these bounds are" } else { "this bound is" };
 502             err.note(&format!(
 503                 "{} currently required to ensure that impls have maximum flexibility",
 504                 bound
 505             ));
 506             err.note(
 507                 "we are soliciting feedback, see issue #87479 \
 508                  <https://github.com/rust-lang/rust/issues/87479> \
 509                  for more information",
 510             );
 511
 512             err.emit();
 513         }
 514     }
 515 }
 516
 517 /// Add a new set of predicates to the caller_bounds of an existing param_env.
 518 fn augment_param_env<'tcx>(
 519     tcx: TyCtxt<'tcx>,
 520     param_env: ty::ParamEnv<'tcx>,
 521     new_predicates: Option<&FxHashSet<ty::Predicate<'tcx>>>,
 522 ) -> ty::ParamEnv<'tcx> {
 523     let Some(new_predicates) = new_predicates else {
 524         return param_env;
 525     };
 526
 527     if new_predicates.is_empty() {
 528         return param_env;
 529     }
 530
 531     let bounds =
 532         tcx.mk_predicates(param_env.caller_bounds().iter().chain(new_predicates.iter().cloned()));
 533     // FIXME(compiler-errors): Perhaps there is a case where we need to normalize this
 534     // i.e. traits::normalize_param_env_or_error
 535     ty::ParamEnv::new(bounds, param_env.reveal(), param_env.constness())
 536 }
 537
 538 /// We use the following trait as an example throughout this function.
 539 /// Specifically, let's assume that `to_check` here is the return type
 540 /// of `into_iter`, and the GAT we are checking this for is `Iter`.
 541 /// ```rust,ignore (this code fails due to this lint)
 542 /// trait IntoIter {
 543 ///     type Iter<'a>: Iterator<Item = Self::Item<'a>>;
 544 ///     type Item<'a>;
 545 ///     fn into_iter<'a>(&'a self) -> Self::Iter<'a>;
 546 /// }
 547 /// ```
 548 fn gather_gat_bounds<'tcx, T: TypeFoldable<'tcx>>(
 549     tcx: TyCtxt<'tcx>,
 550     param_env: ty::ParamEnv<'tcx>,
 551     item_hir: hir::HirId,
 552     to_check: T,
 553     wf_tys: &FxIndexSet<Ty<'tcx>>,
 554     gat_def_id: LocalDefId,
 555     gat_generics: &'tcx ty::Generics,
 556 ) -> Option<FxHashSet<ty::Predicate<'tcx>>> {
 557     // The bounds we that we would require from `to_check`
 558     let mut bounds = FxHashSet::default();
 559
 560     let (regions, types) = GATSubstCollector::visit(gat_def_id.to_def_id(), to_check);
 561
 562     // If both regions and types are empty, then this GAT isn't in the
 563     // set of types we are checking, and we shouldn't try to do clause analysis
 564     // (particularly, doing so would end up with an empty set of clauses,
 565     // since the current method would require none, and we take the
 566     // intersection of requirements of all methods)
 567     if types.is_empty() && regions.is_empty() {
 568         return None;
 569     }
 570
 571     for (region_a, region_a_idx) in &regions {
 572         // Ignore `'static` lifetimes for the purpose of this lint: it's
 573         // because we know it outlives everything and so doesn't give meaningful
 574         // clues
 575         if let ty::ReStatic = **region_a {
 576             continue;
 577         }
 578         // For each region argument (e.g., `'a` in our example), check for a
 579         // relationship to the type arguments (e.g., `Self`). If there is an
 580         // outlives relationship (`Self: 'a`), then we want to ensure that is
 581         // reflected in a where clause on the GAT itself.
 582         for (ty, ty_idx) in &types {
 583             // In our example, requires that `Self: 'a`
 584             if ty_known_to_outlive(tcx, item_hir, param_env, &wf_tys, *ty, *region_a) {
 585                 debug!(?ty_idx, ?region_a_idx);
 586                 debug!("required clause: {ty} must outlive {region_a}");
 587                 // Translate into the generic parameters of the GAT. In
 588                 // our example, the type was `Self`, which will also be
 589                 // `Self` in the GAT.
 590                 let ty_param = gat_generics.param_at(*ty_idx, tcx);
 591                 let ty_param = tcx
 592                     .mk_ty(ty::Param(ty::ParamTy { index: ty_param.index, name: ty_param.name }));
 593                 // Same for the region. In our example, 'a corresponds
 594                 // to the 'me parameter.
 595                 let region_param = gat_generics.param_at(*region_a_idx, tcx);
 596                 let region_param =
 597                     tcx.mk_region(ty::RegionKind::ReEarlyBound(ty::EarlyBoundRegion {
 598                         def_id: region_param.def_id,
 599                         index: region_param.index,
 600                         name: region_param.name,
 601                     }));
 602                 // The predicate we expect to see. (In our example,
 603                 // `Self: 'me`.)
 604                 let clause = ty::PredicateKind::Clause(ty::Clause::TypeOutlives(
 605                     ty::OutlivesPredicate(ty_param, region_param),
 606                 ));
 607                 let clause = tcx.mk_predicate(ty::Binder::dummy(clause));
 608                 bounds.insert(clause);
 609             }
 610         }
 611
 612         // For each region argument (e.g., `'a` in our example), also check for a
 613         // relationship to the other region arguments. If there is an outlives
 614         // relationship, then we want to ensure that is reflected in the where clause
 615         // on the GAT itself.
 616         for (region_b, region_b_idx) in &regions {
 617             // Again, skip `'static` because it outlives everything. Also, we trivially
 618             // know that a region outlives itself.
 619             if ty::ReStatic == **region_b || region_a == region_b {
 620                 continue;
 621             }
 622             if region_known_to_outlive(tcx, item_hir, param_env, &wf_tys, *region_a, *region_b) {
 623                 debug!(?region_a_idx, ?region_b_idx);
 624                 debug!("required clause: {region_a} must outlive {region_b}");
 625                 // Translate into the generic parameters of the GAT.
 626                 let region_a_param = gat_generics.param_at(*region_a_idx, tcx);
 627                 let region_a_param =
 628                     tcx.mk_region(ty::RegionKind::ReEarlyBound(ty::EarlyBoundRegion {
 629                         def_id: region_a_param.def_id,
 630                         index: region_a_param.index,
 631                         name: region_a_param.name,
 632                     }));
 633                 // Same for the region.
 634                 let region_b_param = gat_generics.param_at(*region_b_idx, tcx);
 635                 let region_b_param =
 636                     tcx.mk_region(ty::RegionKind::ReEarlyBound(ty::EarlyBoundRegion {
 637                         def_id: region_b_param.def_id,
 638                         index: region_b_param.index,
 639                         name: region_b_param.name,
 640                     }));
 641                 // The predicate we expect to see.
 642                 let clause = ty::PredicateKind::Clause(ty::Clause::RegionOutlives(
 643                     ty::OutlivesPredicate(region_a_param, region_b_param),
 644                 ));
 645                 let clause = tcx.mk_predicate(ty::Binder::dummy(clause));
 646                 bounds.insert(clause);
 647             }
 648         }
 649     }
 650
 651     Some(bounds)
 652 }
 653
 654 /// Given a known `param_env` and a set of well formed types, can we prove that
 655 /// `ty` outlives `region`.
 656 fn ty_known_to_outlive<'tcx>(
 657     tcx: TyCtxt<'tcx>,
 658     id: hir::HirId,
 659     param_env: ty::ParamEnv<'tcx>,
 660     wf_tys: &FxIndexSet<Ty<'tcx>>,
 661     ty: Ty<'tcx>,
 662     region: ty::Region<'tcx>,
 663 ) -> bool {
 664     resolve_regions_with_wf_tys(tcx, id, param_env, &wf_tys, |infcx, region_bound_pairs| {
 665         let origin = infer::RelateParamBound(DUMMY_SP, ty, None);
 666         let outlives = &mut TypeOutlives::new(infcx, tcx, region_bound_pairs, None, param_env);
 667         outlives.type_must_outlive(origin, ty, region, ConstraintCategory::BoringNoLocation);
 668     })
 669 }
 670
 671 /// Given a known `param_env` and a set of well formed types, can we prove that
 672 /// `region_a` outlives `region_b`
 673 fn region_known_to_outlive<'tcx>(
 674     tcx: TyCtxt<'tcx>,
 675     id: hir::HirId,
 676     param_env: ty::ParamEnv<'tcx>,
 677     wf_tys: &FxIndexSet<Ty<'tcx>>,
 678     region_a: ty::Region<'tcx>,
 679     region_b: ty::Region<'tcx>,
 680 ) -> bool {
 681     resolve_regions_with_wf_tys(tcx, id, param_env, &wf_tys, |mut infcx, _| {
 682         use rustc_infer::infer::outlives::obligations::TypeOutlivesDelegate;
 683         let origin = infer::RelateRegionParamBound(DUMMY_SP);
 684         // `region_a: region_b` -> `region_b <= region_a`
 685         infcx.push_sub_region_constraint(
 686             origin,
 687             region_b,
 688             region_a,
 689             ConstraintCategory::BoringNoLocation,
 690         );
 691     })
 692 }
 693
 694 /// Given a known `param_env` and a set of well formed types, set up an
 695 /// `InferCtxt`, call the passed function (to e.g. set up region constraints
 696 /// to be tested), then resolve region and return errors
 697 fn resolve_regions_with_wf_tys<'tcx>(
 698     tcx: TyCtxt<'tcx>,
 699     id: hir::HirId,
 700     param_env: ty::ParamEnv<'tcx>,
 701     wf_tys: &FxIndexSet<Ty<'tcx>>,
 702     add_constraints: impl for<'a> FnOnce(&'a InferCtxt<'tcx>, &'a RegionBoundPairs<'tcx>),
 703 ) -> bool {
 704     // Unfortunately, we have to use a new `InferCtxt` each call, because
 705     // region constraints get added and solved there and we need to test each
 706     // call individually.
 707     let infcx = tcx.infer_ctxt().build();
 708     let outlives_environment = OutlivesEnvironment::with_bounds(
 709         param_env,
 710         Some(&infcx),
 711         infcx.implied_bounds_tys(param_env, id, wf_tys.clone()),
 712     );
 713     let region_bound_pairs = outlives_environment.region_bound_pairs();
 714
 715     add_constraints(&infcx, region_bound_pairs);
 716
 717     infcx.process_registered_region_obligations(
 718         outlives_environment.region_bound_pairs(),
 719         param_env,
 720     );
 721     let errors = infcx.resolve_regions(&outlives_environment);
 722
 723     debug!(?errors, "errors");
 724
 725     // If we were able to prove that the type outlives the region without
 726     // an error, it must be because of the implied or explicit bounds...
 727     errors.is_empty()
 728 }
 729
 730 /// TypeVisitor that looks for uses of GATs like
 731 /// `<P0 as Trait<P1..Pn>>::GAT<Pn..Pm>` and adds the arguments `P0..Pm` into
 732 /// the two vectors, `regions` and `types` (depending on their kind). For each
 733 /// parameter `Pi` also track the index `i`.
 734 struct GATSubstCollector<'tcx> {
 735     gat: DefId,
 736     // Which region appears and which parameter index its substituted for
 737     regions: FxHashSet<(ty::Region<'tcx>, usize)>,
 738     // Which params appears and which parameter index its substituted for
 739     types: FxHashSet<(Ty<'tcx>, usize)>,
 740 }
 741
 742 impl<'tcx> GATSubstCollector<'tcx> {
 743     fn visit<T: TypeFoldable<'tcx>>(
 744         gat: DefId,
 745         t: T,
 746     ) -> (FxHashSet<(ty::Region<'tcx>, usize)>, FxHashSet<(Ty<'tcx>, usize)>) {
 747         let mut visitor =
 748             GATSubstCollector { gat, regions: FxHashSet::default(), types: FxHashSet::default() };
 749         t.visit_with(&mut visitor);
 750         (visitor.regions, visitor.types)
 751     }
 752 }
 753
 754 impl<'tcx> TypeVisitor<'tcx> for GATSubstCollector<'tcx> {
 755     type BreakTy = !;
 756
 757     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 758         match t.kind() {
 759             ty::Alias(ty::Projection, p) if p.def_id == self.gat => {
 760                 for (idx, subst) in p.substs.iter().enumerate() {
 761                     match subst.unpack() {
 762                         GenericArgKind::Lifetime(lt) if !lt.is_late_bound() => {
 763                             self.regions.insert((lt, idx));
 764                         }
 765                         GenericArgKind::Type(t) => {
 766                             self.types.insert((t, idx));
 767                         }
 768                         _ => {}
 769                     }
 770                 }
 771             }
 772             _ => {}
 773         }
 774         t.super_visit_with(self)
 775     }
 776 }
 777
 778 fn could_be_self(trait_def_id: LocalDefId, ty: &hir::Ty<'_>) -> bool {
 779     match ty.kind {
 780         hir::TyKind::TraitObject([trait_ref], ..) => match trait_ref.trait_ref.path.segments {
 781             [s] => s.res.opt_def_id() == Some(trait_def_id.to_def_id()),
 782             _ => false,
 783         },
 784         _ => false,
 785     }
 786 }
 787
 788 /// Detect when an object unsafe trait is referring to itself in one of its associated items.
 789 /// When this is done, suggest using `Self` instead.
 790 fn check_object_unsafe_self_trait_by_name(tcx: TyCtxt<'_>, item: &hir::TraitItem<'_>) {
 791     let (trait_name, trait_def_id) =
 792         match tcx.hir().get_by_def_id(tcx.hir().get_parent_item(item.hir_id()).def_id) {
 793             hir::Node::Item(item) => match item.kind {
 794                 hir::ItemKind::Trait(..) => (item.ident, item.owner_id),
 795                 _ => return,
 796             },
 797             _ => return,
 798         };
 799     let mut trait_should_be_self = vec![];
 800     match &item.kind {
 801         hir::TraitItemKind::Const(ty, _) | hir::TraitItemKind::Type(_, Some(ty))
 802             if could_be_self(trait_def_id.def_id, ty) =>
 803         {
 804             trait_should_be_self.push(ty.span)
 805         }
 806         hir::TraitItemKind::Fn(sig, _) => {
 807             for ty in sig.decl.inputs {
 808                 if could_be_self(trait_def_id.def_id, ty) {
 809                     trait_should_be_self.push(ty.span);
 810                 }
 811             }
 812             match sig.decl.output {
 813                 hir::FnRetTy::Return(ty) if could_be_self(trait_def_id.def_id, ty) => {
 814                     trait_should_be_self.push(ty.span);
 815                 }
 816                 _ => {}
 817             }
 818         }
 819         _ => {}
 820     }
 821     if !trait_should_be_self.is_empty() {
 822         if tcx.object_safety_violations(trait_def_id).is_empty() {
 823             return;
 824         }
 825         let sugg = trait_should_be_self.iter().map(|span| (*span, "Self".to_string())).collect();
 826         tcx.sess
 827             .struct_span_err(
 828                 trait_should_be_self,
 829                 "associated item referring to unboxed trait object for its own trait",
 830             )
 831             .span_label(trait_name.span, "in this trait")
 832             .multipart_suggestion(
 833                 "you might have meant to use `Self` to refer to the implementing type",
 834                 sugg,
 835                 Applicability::MachineApplicable,
 836             )
 837             .emit();
 838     }
 839 }
 840
 841 fn check_impl_item(tcx: TyCtxt<'_>, impl_item: &hir::ImplItem<'_>) {
 842     let (method_sig, span) = match impl_item.kind {
 843         hir::ImplItemKind::Fn(ref sig, _) => (Some(sig), impl_item.span),
 844         // Constrain binding and overflow error spans to `<Ty>` in `type foo = <Ty>`.
 845         hir::ImplItemKind::Type(ty) if ty.span != DUMMY_SP => (None, ty.span),
 846         _ => (None, impl_item.span),
 847     };
 848
 849     check_associated_item(tcx, impl_item.owner_id.def_id, span, method_sig);
 850 }
 851
 852 fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) {
 853     match param.kind {
 854         // We currently only check wf of const params here.
 855         hir::GenericParamKind::Lifetime { .. } | hir::GenericParamKind::Type { .. } => (),
 856
 857         // Const parameters are well formed if their type is structural match.
 858         hir::GenericParamKind::Const { ty: hir_ty, default: _ } => {
 859             let ty = tcx.type_of(param.def_id);
 860
 861             if tcx.features().adt_const_params {
 862                 if let Some(non_structural_match_ty) =
 863                     traits::search_for_adt_const_param_violation(param.span, tcx, ty)
 864                 {
 865                     // We use the same error code in both branches, because this is really the same
 866                     // issue: we just special-case the message for type parameters to make it
 867                     // clearer.
 868                     match non_structural_match_ty.kind() {
 869                         ty::Param(_) => {
 870                             // Const parameters may not have type parameters as their types,
 871                             // because we cannot be sure that the type parameter derives `PartialEq`
 872                             // and `Eq` (just implementing them is not enough for `structural_match`).
 873                             struct_span_err!(
 874                                 tcx.sess,
 875                                 hir_ty.span,
 876                                 E0741,
 877                                 "`{ty}` is not guaranteed to `#[derive(PartialEq, Eq)]`, so may not be \
 878                                 used as the type of a const parameter",
 879                             )
 880                             .span_label(
 881                                 hir_ty.span,
 882                                 format!("`{ty}` may not derive both `PartialEq` and `Eq`"),
 883                             )
 884                             .note(
 885                                 "it is not currently possible to use a type parameter as the type of a \
 886                                 const parameter",
 887                             )
 888                             .emit();
 889                         }
 890                         ty::Float(_) => {
 891                             struct_span_err!(
 892                                 tcx.sess,
 893                                 hir_ty.span,
 894                                 E0741,
 895                                 "`{ty}` is forbidden as the type of a const generic parameter",
 896                             )
 897                             .note("floats do not derive `Eq` or `Ord`, which are required for const parameters")
 898                             .emit();
 899                         }
 900                         ty::FnPtr(_) => {
 901                             struct_span_err!(
 902                                 tcx.sess,
 903                                 hir_ty.span,
 904                                 E0741,
 905                                 "using function pointers as const generic parameters is forbidden",
 906                             )
 907                             .emit();
 908                         }
 909                         ty::RawPtr(_) => {
 910                             struct_span_err!(
 911                                 tcx.sess,
 912                                 hir_ty.span,
 913                                 E0741,
 914                                 "using raw pointers as const generic parameters is forbidden",
 915                             )
 916                             .emit();
 917                         }
 918                         _ => {
 919                             let mut diag = struct_span_err!(
 920                                 tcx.sess,
 921                                 hir_ty.span,
 922                                 E0741,
 923                                 "`{}` must be annotated with `#[derive(PartialEq, Eq)]` to be used as \
 924                                 the type of a const parameter",
 925                                 non_structural_match_ty,
 926                             );
 927
 928                             if ty == non_structural_match_ty {
 929                                 diag.span_label(
 930                                     hir_ty.span,
 931                                     format!("`{ty}` doesn't derive both `PartialEq` and `Eq`"),
 932                                 );
 933                             }
 934
 935                             diag.emit();
 936                         }
 937                     }
 938                 }
 939             } else {
 940                 let err_ty_str;
 941                 let mut is_ptr = true;
 942
 943                 let err = match ty.kind() {
 944                     ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Error(_) => None,
 945                     ty::FnPtr(_) => Some("function pointers"),
 946                     ty::RawPtr(_) => Some("raw pointers"),
 947                     _ => {
 948                         is_ptr = false;
 949                         err_ty_str = format!("`{ty}`");
 950                         Some(err_ty_str.as_str())
 951                     }
 952                 };
 953
 954                 if let Some(unsupported_type) = err {
 955                     if is_ptr {
 956                         tcx.sess.span_err(
 957                             hir_ty.span,
 958                             &format!(
 959                                 "using {unsupported_type} as const generic parameters is forbidden",
 960                             ),
 961                         );
 962                     } else {
 963                         let mut err = tcx.sess.struct_span_err(
 964                             hir_ty.span,
 965                             &format!(
 966                                 "{unsupported_type} is forbidden as the type of a const generic parameter",
 967                             ),
 968                         );
 969                         err.note("the only supported types are integers, `bool` and `char`");
 970                         if tcx.sess.is_nightly_build() {
 971                             err.help(
 972                             "more complex types are supported with `#![feature(adt_const_params)]`",
 973                         );
 974                         }
 975                         err.emit();
 976                     }
 977                 }
 978             }
 979         }
 980     }
 981 }
 982
 983 #[instrument(level = "debug", skip(tcx, span, sig_if_method))]
 984 fn check_associated_item(
 985     tcx: TyCtxt<'_>,
 986     item_id: LocalDefId,
 987     span: Span,
 988     sig_if_method: Option<&hir::FnSig<'_>>,
 989 ) {
 990     let loc = Some(WellFormedLoc::Ty(item_id));
 991     enter_wf_checking_ctxt(tcx, span, item_id, |wfcx| {
 992         let item = tcx.associated_item(item_id);
 993
 994         let self_ty = match item.container {
 995             ty::TraitContainer => tcx.types.self_param,
 996             ty::ImplContainer => tcx.type_of(item.container_id(tcx)),
 997         };
 998
 999         match item.kind {
1000             ty::AssocKind::Const => {
1001                 let ty = tcx.type_of(item.def_id);
1002                 let ty = wfcx.normalize(span, Some(WellFormedLoc::Ty(item_id)), ty);
1003                 wfcx.register_wf_obligation(span, loc, ty.into());
1004             }
1005             ty::AssocKind::Fn => {
1006                 let sig = tcx.fn_sig(item.def_id);
1007                 let hir_sig = sig_if_method.expect("bad signature for method");
1008                 check_fn_or_method(
1009                     wfcx,
1010                     item.ident(tcx).span,
1011                     sig,
1012                     hir_sig.decl,
1013                     item.def_id.expect_local(),
1014                 );
1015                 check_method_receiver(wfcx, hir_sig, item, self_ty);
1016             }
1017             ty::AssocKind::Type => {
1018                 if let ty::AssocItemContainer::TraitContainer = item.container {
1019                     check_associated_type_bounds(wfcx, item, span)
1020                 }
1021                 if item.defaultness(tcx).has_value() {
1022                     let ty = tcx.type_of(item.def_id);
1023                     let ty = wfcx.normalize(span, Some(WellFormedLoc::Ty(item_id)), ty);
1024                     wfcx.register_wf_obligation(span, loc, ty.into());
1025                 }
1026             }
1027         }
1028     })
1029 }
1030
1031 fn item_adt_kind(kind: &ItemKind<'_>) -> Option<AdtKind> {
1032     match kind {
1033         ItemKind::Struct(..) => Some(AdtKind::Struct),
1034         ItemKind::Union(..) => Some(AdtKind::Union),
1035         ItemKind::Enum(..) => Some(AdtKind::Enum),
1036         _ => None,
1037     }
1038 }
1039
1040 /// In a type definition, we check that to ensure that the types of the fields are well-formed.
1041 fn check_type_defn<'tcx>(tcx: TyCtxt<'tcx>, item: &hir::Item<'tcx>, all_sized: bool) {
1042     let _ = tcx.representability(item.owner_id.def_id);
1043     let adt_def = tcx.adt_def(item.owner_id);
1044
1045     enter_wf_checking_ctxt(tcx, item.span, item.owner_id.def_id, |wfcx| {
1046         let variants = adt_def.variants();
1047         let packed = adt_def.repr().packed();
1048
1049         for variant in variants.iter() {
1050             // All field types must be well-formed.
1051             for field in &variant.fields {
1052                 let field_id = field.did.expect_local();
1053                 let hir::Node::Field(hir::FieldDef { ty: hir_ty, .. }) = tcx.hir().get_by_def_id(field_id)
1054                 else { bug!() };
1055                 let ty = wfcx.normalize(hir_ty.span, None, tcx.type_of(field.did));
1056                 wfcx.register_wf_obligation(
1057                     hir_ty.span,
1058                     Some(WellFormedLoc::Ty(field_id)),
1059                     ty.into(),
1060                 )
1061             }
1062
1063             // For DST, or when drop needs to copy things around, all
1064             // intermediate types must be sized.
1065             let needs_drop_copy = || {
1066                 packed && {
1067                     let ty = tcx.type_of(variant.fields.last().unwrap().did);
1068                     let ty = tcx.erase_regions(ty);
1069                     if ty.needs_infer() {
1070                         tcx.sess
1071                             .delay_span_bug(item.span, &format!("inference variables in {:?}", ty));
1072                         // Just treat unresolved type expression as if it needs drop.
1073                         true
1074                     } else {
1075                         ty.needs_drop(tcx, tcx.param_env(item.owner_id))
1076                     }
1077                 }
1078             };
1079             // All fields (except for possibly the last) should be sized.
1080             let all_sized = all_sized || variant.fields.is_empty() || needs_drop_copy();
1081             let unsized_len = if all_sized { 0 } else { 1 };
1082             for (idx, field) in
1083                 variant.fields[..variant.fields.len() - unsized_len].iter().enumerate()
1084             {
1085                 let last = idx == variant.fields.len() - 1;
1086                 let field_id = field.did.expect_local();
1087                 let hir::Node::Field(hir::FieldDef { ty: hir_ty, .. }) = tcx.hir().get_by_def_id(field_id)
1088                 else { bug!() };
1089                 let ty = wfcx.normalize(hir_ty.span, None, tcx.type_of(field.did));
1090                 wfcx.register_bound(
1091                     traits::ObligationCause::new(
1092                         hir_ty.span,
1093                         wfcx.body_id,
1094                         traits::FieldSized {
1095                             adt_kind: match item_adt_kind(&item.kind) {
1096                                 Some(i) => i,
1097                                 None => bug!(),
1098                             },
1099                             span: hir_ty.span,
1100                             last,
1101                         },
1102                     ),
1103                     wfcx.param_env,
1104                     ty,
1105                     tcx.require_lang_item(LangItem::Sized, None),
1106                 );
1107             }
1108
1109             // Explicit `enum` discriminant values must const-evaluate successfully.
1110             if let ty::VariantDiscr::Explicit(discr_def_id) = variant.discr {
1111                 let cause = traits::ObligationCause::new(
1112                     tcx.def_span(discr_def_id),
1113                     wfcx.body_id,
1114                     traits::MiscObligation,
1115                 );
1116                 wfcx.register_obligation(traits::Obligation::new(
1117                     tcx,
1118                     cause,
1119                     wfcx.param_env,
1120                     ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(
1121                         ty::Const::from_anon_const(tcx, discr_def_id.expect_local()),
1122                     )),
1123                 ));
1124             }
1125         }
1126
1127         check_where_clauses(wfcx, item.span, item.owner_id.def_id);
1128     });
1129 }
1130
1131 #[instrument(skip(tcx, item))]
1132 fn check_trait(tcx: TyCtxt<'_>, item: &hir::Item<'_>) {
1133     debug!(?item.owner_id);
1134
1135     let def_id = item.owner_id.def_id;
1136     let trait_def = tcx.trait_def(def_id);
1137     if trait_def.is_marker
1138         || matches!(trait_def.specialization_kind, TraitSpecializationKind::Marker)
1139     {
1140         for associated_def_id in &*tcx.associated_item_def_ids(def_id) {
1141             struct_span_err!(
1142                 tcx.sess,
1143                 tcx.def_span(*associated_def_id),
1144                 E0714,
1145                 "marker traits cannot have associated items",
1146             )
1147             .emit();
1148         }
1149     }
1150
1151     enter_wf_checking_ctxt(tcx, item.span, def_id, |wfcx| {
1152         check_where_clauses(wfcx, item.span, def_id)
1153     });
1154
1155     // Only check traits, don't check trait aliases
1156     if let hir::ItemKind::Trait(_, _, _, _, items) = item.kind {
1157         check_gat_where_clauses(tcx, items);
1158     }
1159 }
1160
1161 /// Checks all associated type defaults of trait `trait_def_id`.
1162 ///
1163 /// Assuming the defaults are used, check that all predicates (bounds on the
1164 /// assoc type and where clauses on the trait) hold.
1165 fn check_associated_type_bounds(wfcx: &WfCheckingCtxt<'_, '_>, item: &ty::AssocItem, span: Span) {
1166     let bounds = wfcx.tcx().explicit_item_bounds(item.def_id);
1167
1168     debug!("check_associated_type_bounds: bounds={:?}", bounds);
1169     let wf_obligations = bounds.iter().flat_map(|&(bound, bound_span)| {
1170         let normalized_bound = wfcx.normalize(span, None, bound);
1171         traits::wf::predicate_obligations(
1172             wfcx.infcx,
1173             wfcx.param_env,
1174             wfcx.body_id,
1175             normalized_bound,
1176             bound_span,
1177         )
1178     });
1179
1180     wfcx.register_obligations(wf_obligations);
1181 }
1182
1183 fn check_item_fn(
1184     tcx: TyCtxt<'_>,
1185     def_id: LocalDefId,
1186     ident: Ident,
1187     span: Span,
1188     decl: &hir::FnDecl<'_>,
1189 ) {
1190     enter_wf_checking_ctxt(tcx, span, def_id, |wfcx| {
1191         let sig = tcx.fn_sig(def_id);
1192         check_fn_or_method(wfcx, ident.span, sig, decl, def_id);
1193     })
1194 }
1195
1196 fn check_item_type(tcx: TyCtxt<'_>, item_id: LocalDefId, ty_span: Span, allow_foreign_ty: bool) {
1197     debug!("check_item_type: {:?}", item_id);
1198
1199     enter_wf_checking_ctxt(tcx, ty_span, item_id, |wfcx| {
1200         let ty = tcx.type_of(item_id);
1201         let item_ty = wfcx.normalize(ty_span, Some(WellFormedLoc::Ty(item_id)), ty);
1202
1203         let mut forbid_unsized = true;
1204         if allow_foreign_ty {
1205             let tail = tcx.struct_tail_erasing_lifetimes(item_ty, wfcx.param_env);
1206             if let ty::Foreign(_) = tail.kind() {
1207                 forbid_unsized = false;
1208             }
1209         }
1210
1211         wfcx.register_wf_obligation(ty_span, Some(WellFormedLoc::Ty(item_id)), item_ty.into());
1212         if forbid_unsized {
1213             wfcx.register_bound(
1214                 traits::ObligationCause::new(ty_span, wfcx.body_id, traits::WellFormed(None)),
1215                 wfcx.param_env,
1216                 item_ty,
1217                 tcx.require_lang_item(LangItem::Sized, None),
1218             );
1219         }
1220
1221         // Ensure that the end result is `Sync` in a non-thread local `static`.
1222         let should_check_for_sync = tcx.static_mutability(item_id.to_def_id())
1223             == Some(hir::Mutability::Not)
1224             && !tcx.is_foreign_item(item_id.to_def_id())
1225             && !tcx.is_thread_local_static(item_id.to_def_id());
1226
1227         if should_check_for_sync {
1228             wfcx.register_bound(
1229                 traits::ObligationCause::new(ty_span, wfcx.body_id, traits::SharedStatic),
1230                 wfcx.param_env,
1231                 item_ty,
1232                 tcx.require_lang_item(LangItem::Sync, Some(ty_span)),
1233             );
1234         }
1235     });
1236 }
1237
1238 #[instrument(level = "debug", skip(tcx, ast_self_ty, ast_trait_ref))]
1239 fn check_impl<'tcx>(
1240     tcx: TyCtxt<'tcx>,
1241     item: &'tcx hir::Item<'tcx>,
1242     ast_self_ty: &hir::Ty<'_>,
1243     ast_trait_ref: &Option<hir::TraitRef<'_>>,
1244     constness: hir::Constness,
1245 ) {
1246     enter_wf_checking_ctxt(tcx, item.span, item.owner_id.def_id, |wfcx| {
1247         match *ast_trait_ref {
1248             Some(ref ast_trait_ref) => {
1249                 // `#[rustc_reservation_impl]` impls are not real impls and
1250                 // therefore don't need to be WF (the trait's `Self: Trait` predicate
1251                 // won't hold).
1252                 let trait_ref = tcx.impl_trait_ref(item.owner_id).unwrap();
1253                 let trait_ref = wfcx.normalize(ast_trait_ref.path.span, None, trait_ref);
1254                 let trait_pred = ty::TraitPredicate {
1255                     trait_ref,
1256                     constness: match constness {
1257                         hir::Constness::Const => ty::BoundConstness::ConstIfConst,
1258                         hir::Constness::NotConst => ty::BoundConstness::NotConst,
1259                     },
1260                     polarity: ty::ImplPolarity::Positive,
1261                 };
1262                 let obligations = traits::wf::trait_obligations(
1263                     wfcx.infcx,
1264                     wfcx.param_env,
1265                     wfcx.body_id,
1266                     &trait_pred,
1267                     ast_trait_ref.path.span,
1268                     item,
1269                 );
1270                 debug!(?obligations);
1271                 wfcx.register_obligations(obligations);
1272             }
1273             None => {
1274                 let self_ty = tcx.type_of(item.owner_id);
1275                 let self_ty = wfcx.normalize(
1276                     item.span,
1277                     Some(WellFormedLoc::Ty(item.hir_id().expect_owner().def_id)),
1278                     self_ty,
1279                 );
1280                 wfcx.register_wf_obligation(
1281                     ast_self_ty.span,
1282                     Some(WellFormedLoc::Ty(item.hir_id().expect_owner().def_id)),
1283                     self_ty.into(),
1284                 );
1285             }
1286         }
1287
1288         check_where_clauses(wfcx, item.span, item.owner_id.def_id);
1289     });
1290 }
1291
1292 /// Checks where-clauses and inline bounds that are declared on `def_id`.
1293 #[instrument(level = "debug", skip(wfcx))]
1294 fn check_where_clauses<'tcx>(wfcx: &WfCheckingCtxt<'_, 'tcx>, span: Span, def_id: LocalDefId) {
1295     let infcx = wfcx.infcx;
1296     let tcx = wfcx.tcx();
1297
1298     let predicates = tcx.bound_predicates_of(def_id.to_def_id());
1299     let generics = tcx.generics_of(def_id);
1300
1301     let is_our_default = |def: &ty::GenericParamDef| match def.kind {
1302         GenericParamDefKind::Type { has_default, .. }
1303         | GenericParamDefKind::Const { has_default } => {
1304             has_default && def.index >= generics.parent_count as u32
1305         }
1306         GenericParamDefKind::Lifetime => unreachable!(),
1307     };
1308
1309     // Check that concrete defaults are well-formed. See test `type-check-defaults.rs`.
1310     // For example, this forbids the declaration:
1311     //
1312     //     struct Foo<T = Vec<[u32]>> { .. }
1313     //
1314     // Here, the default `Vec<[u32]>` is not WF because `[u32]: Sized` does not hold.
1315     for param in &generics.params {
1316         match param.kind {
1317             GenericParamDefKind::Type { .. } => {
1318                 if is_our_default(param) {
1319                     let ty = tcx.type_of(param.def_id);
1320                     // Ignore dependent defaults -- that is, where the default of one type
1321                     // parameter includes another (e.g., `<T, U = T>`). In those cases, we can't
1322                     // be sure if it will error or not as user might always specify the other.
1323                     if !ty.needs_subst() {
1324                         wfcx.register_wf_obligation(
1325                             tcx.def_span(param.def_id),
1326                             Some(WellFormedLoc::Ty(param.def_id.expect_local())),
1327                             ty.into(),
1328                         );
1329                     }
1330                 }
1331             }
1332             GenericParamDefKind::Const { .. } => {
1333                 if is_our_default(param) {
1334                     // FIXME(const_generics_defaults): This
1335                     // is incorrect when dealing with unused substs, for example
1336                     // for `struct Foo<const N: usize, const M: usize = { 1 - 2 }>`
1337                     // we should eagerly error.
1338                     let default_ct = tcx.const_param_default(param.def_id);
1339                     if !default_ct.needs_subst() {
1340                         wfcx.register_wf_obligation(
1341                             tcx.def_span(param.def_id),
1342                             None,
1343                             default_ct.into(),
1344                         );
1345                     }
1346                 }
1347             }
1348             // Doesn't have defaults.
1349             GenericParamDefKind::Lifetime => {}
1350         }
1351     }
1352
1353     // Check that trait predicates are WF when params are substituted by their defaults.
1354     // We don't want to overly constrain the predicates that may be written but we want to
1355     // catch cases where a default my never be applied such as `struct Foo<T: Copy = String>`.
1356     // Therefore we check if a predicate which contains a single type param
1357     // with a concrete default is WF with that default substituted.
1358     // For more examples see tests `defaults-well-formedness.rs` and `type-check-defaults.rs`.
1359     //
1360     // First we build the defaulted substitution.
1361     let substs = InternalSubsts::for_item(tcx, def_id.to_def_id(), |param, _| {
1362         match param.kind {
1363             GenericParamDefKind::Lifetime => {
1364                 // All regions are identity.
1365                 tcx.mk_param_from_def(param)
1366             }
1367
1368             GenericParamDefKind::Type { .. } => {
1369                 // If the param has a default, ...
1370                 if is_our_default(param) {
1371                     let default_ty = tcx.type_of(param.def_id);
1372                     // ... and it's not a dependent default, ...
1373                     if !default_ty.needs_subst() {
1374                         // ... then substitute it with the default.
1375                         return default_ty.into();
1376                     }
1377                 }
1378
1379                 tcx.mk_param_from_def(param)
1380             }
1381             GenericParamDefKind::Const { .. } => {
1382                 // If the param has a default, ...
1383                 if is_our_default(param) {
1384                     let default_ct = tcx.const_param_default(param.def_id);
1385                     // ... and it's not a dependent default, ...
1386                     if !default_ct.needs_subst() {
1387                         // ... then substitute it with the default.
1388                         return default_ct.into();
1389                     }
1390                 }
1391
1392                 tcx.mk_param_from_def(param)
1393             }
1394         }
1395     });
1396
1397     // Now we build the substituted predicates.
1398     let default_obligations = predicates
1399         .0
1400         .predicates
1401         .iter()
1402         .flat_map(|&(pred, sp)| {
1403             #[derive(Default)]
1404             struct CountParams {
1405                 params: FxHashSet<u32>,
1406             }
1407             impl<'tcx> ty::visit::TypeVisitor<'tcx> for CountParams {
1408                 type BreakTy = ();
1409
1410                 fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
1411                     if let ty::Param(param) = t.kind() {
1412                         self.params.insert(param.index);
1413                     }
1414                     t.super_visit_with(self)
1415                 }
1416
1417                 fn visit_region(&mut self, _: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
1418                     ControlFlow::BREAK
1419                 }
1420
1421                 fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
1422                     if let ty::ConstKind::Param(param) = c.kind() {
1423                         self.params.insert(param.index);
1424                     }
1425                     c.super_visit_with(self)
1426                 }
1427             }
1428             let mut param_count = CountParams::default();
1429             let has_region = pred.visit_with(&mut param_count).is_break();
1430             let substituted_pred = predicates.rebind(pred).subst(tcx, substs);
1431             // Don't check non-defaulted params, dependent defaults (including lifetimes)
1432             // or preds with multiple params.
1433             if substituted_pred.has_non_region_param() || param_count.params.len() > 1 || has_region
1434             {
1435                 None
1436             } else if predicates.0.predicates.iter().any(|&(p, _)| p == substituted_pred) {
1437                 // Avoid duplication of predicates that contain no parameters, for example.
1438                 None
1439             } else {
1440                 Some((substituted_pred, sp))
1441             }
1442         })
1443         .map(|(pred, sp)| {
1444             // Convert each of those into an obligation. So if you have
1445             // something like `struct Foo<T: Copy = String>`, we would
1446             // take that predicate `T: Copy`, substitute to `String: Copy`
1447             // (actually that happens in the previous `flat_map` call),
1448             // and then try to prove it (in this case, we'll fail).
1449             //
1450             // Note the subtle difference from how we handle `predicates`
1451             // below: there, we are not trying to prove those predicates
1452             // to be *true* but merely *well-formed*.
1453             let pred = wfcx.normalize(sp, None, pred);
1454             let cause = traits::ObligationCause::new(
1455                 sp,
1456                 wfcx.body_id,
1457                 traits::ItemObligation(def_id.to_def_id()),
1458             );
1459             traits::Obligation::new(tcx, cause, wfcx.param_env, pred)
1460         });
1461
1462     let predicates = predicates.0.instantiate_identity(tcx);
1463
1464     let predicates = wfcx.normalize(span, None, predicates);
1465
1466     debug!(?predicates.predicates);
1467     assert_eq!(predicates.predicates.len(), predicates.spans.len());
1468     let wf_obligations =
1469         iter::zip(&predicates.predicates, &predicates.spans).flat_map(|(&p, &sp)| {
1470             traits::wf::predicate_obligations(
1471                 infcx,
1472                 wfcx.param_env.without_const(),
1473                 wfcx.body_id,
1474                 p,
1475                 sp,
1476             )
1477         });
1478
1479     let obligations: Vec<_> = wf_obligations.chain(default_obligations).collect();
1480     wfcx.register_obligations(obligations);
1481 }
1482
1483 #[instrument(level = "debug", skip(wfcx, span, hir_decl))]
1484 fn check_fn_or_method<'tcx>(
1485     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1486     span: Span,
1487     sig: ty::PolyFnSig<'tcx>,
1488     hir_decl: &hir::FnDecl<'_>,
1489     def_id: LocalDefId,
1490 ) {
1491     let tcx = wfcx.tcx();
1492     let sig = tcx.liberate_late_bound_regions(def_id.to_def_id(), sig);
1493
1494     // Normalize the input and output types one at a time, using a different
1495     // `WellFormedLoc` for each. We cannot call `normalize_associated_types`
1496     // on the entire `FnSig`, since this would use the same `WellFormedLoc`
1497     // for each type, preventing the HIR wf check from generating
1498     // a nice error message.
1499     let ty::FnSig { mut inputs_and_output, c_variadic, unsafety, abi } = sig;
1500     inputs_and_output = tcx.mk_type_list(inputs_and_output.iter().enumerate().map(|(i, ty)| {
1501         wfcx.normalize(
1502             span,
1503             Some(WellFormedLoc::Param {
1504                 function: def_id,
1505                 // Note that the `param_idx` of the output type is
1506                 // one greater than the index of the last input type.
1507                 param_idx: i.try_into().unwrap(),
1508             }),
1509             ty,
1510         )
1511     }));
1512     // Manually call `normalize_associated_types_in` on the other types
1513     // in `FnSig`. This ensures that if the types of these fields
1514     // ever change to include projections, we will start normalizing
1515     // them automatically.
1516     let sig = ty::FnSig {
1517         inputs_and_output,
1518         c_variadic: wfcx.normalize(span, None, c_variadic),
1519         unsafety: wfcx.normalize(span, None, unsafety),
1520         abi: wfcx.normalize(span, None, abi),
1521     };
1522
1523     for (i, (&input_ty, ty)) in iter::zip(sig.inputs(), hir_decl.inputs).enumerate() {
1524         wfcx.register_wf_obligation(
1525             ty.span,
1526             Some(WellFormedLoc::Param { function: def_id, param_idx: i.try_into().unwrap() }),
1527             input_ty.into(),
1528         );
1529     }
1530
1531     wfcx.register_wf_obligation(
1532         hir_decl.output.span(),
1533         Some(WellFormedLoc::Param {
1534             function: def_id,
1535             param_idx: sig.inputs().len().try_into().unwrap(),
1536         }),
1537         sig.output().into(),
1538     );
1539
1540     check_where_clauses(wfcx, span, def_id);
1541
1542     check_return_position_impl_trait_in_trait_bounds(
1543         wfcx,
1544         def_id,
1545         sig.output(),
1546         hir_decl.output.span(),
1547     );
1548
1549     if sig.abi == Abi::RustCall {
1550         let span = tcx.def_span(def_id);
1551         let has_implicit_self = hir_decl.implicit_self != hir::ImplicitSelfKind::None;
1552         let mut inputs = sig.inputs().iter().skip(if has_implicit_self { 1 } else { 0 });
1553         // Check that the argument is a tuple
1554         if let Some(ty) = inputs.next() {
1555             wfcx.register_bound(
1556                 ObligationCause::new(span, wfcx.body_id, ObligationCauseCode::RustCall),
1557                 wfcx.param_env,
1558                 *ty,
1559                 tcx.require_lang_item(hir::LangItem::Tuple, Some(span)),
1560             );
1561         } else {
1562             tcx.sess.span_err(
1563                 hir_decl.inputs.last().map_or(span, |input| input.span),
1564                 "functions with the \"rust-call\" ABI must take a single non-self tuple argument",
1565             );
1566         }
1567         // No more inputs other than the `self` type and the tuple type
1568         if inputs.next().is_some() {
1569             tcx.sess.span_err(
1570                 hir_decl.inputs.last().map_or(span, |input| input.span),
1571                 "functions with the \"rust-call\" ABI must take a single non-self tuple argument",
1572             );
1573         }
1574     }
1575 }
1576
1577 /// Basically `check_associated_type_bounds`, but separated for now and should be
1578 /// deduplicated when RPITITs get lowered into real associated items.
1579 #[tracing::instrument(level = "trace", skip(wfcx))]
1580 fn check_return_position_impl_trait_in_trait_bounds<'tcx>(
1581     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1582     fn_def_id: LocalDefId,
1583     fn_output: Ty<'tcx>,
1584     span: Span,
1585 ) {
1586     let tcx = wfcx.tcx();
1587     if let Some(assoc_item) = tcx.opt_associated_item(fn_def_id.to_def_id())
1588         && assoc_item.container == ty::AssocItemContainer::TraitContainer
1589     {
1590         for arg in fn_output.walk() {
1591             if let ty::GenericArgKind::Type(ty) = arg.unpack()
1592                 && let ty::Alias(ty::Projection, proj) = ty.kind()
1593                 && tcx.def_kind(proj.def_id) == DefKind::ImplTraitPlaceholder
1594                 && tcx.impl_trait_in_trait_parent(proj.def_id) == fn_def_id.to_def_id()
1595             {
1596                 let span = tcx.def_span(proj.def_id);
1597                 let bounds = wfcx.tcx().explicit_item_bounds(proj.def_id);
1598                 let wf_obligations = bounds.iter().flat_map(|&(bound, bound_span)| {
1599                     let bound = ty::EarlyBinder(bound).subst(tcx, proj.substs);
1600                     let normalized_bound = wfcx.normalize(span, None, bound);
1601                     traits::wf::predicate_obligations(
1602                         wfcx.infcx,
1603                         wfcx.param_env,
1604                         wfcx.body_id,
1605                         normalized_bound,
1606                         bound_span,
1607                     )
1608                 });
1609                 wfcx.register_obligations(wf_obligations);
1610             }
1611         }
1612     }
1613 }
1614
1615 const HELP_FOR_SELF_TYPE: &str = "consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, \
1616      `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one \
1617      of the previous types except `Self`)";
1618
1619 #[instrument(level = "debug", skip(wfcx))]
1620 fn check_method_receiver<'tcx>(
1621     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1622     fn_sig: &hir::FnSig<'_>,
1623     method: &ty::AssocItem,
1624     self_ty: Ty<'tcx>,
1625 ) {
1626     let tcx = wfcx.tcx();
1627
1628     if !method.fn_has_self_parameter {
1629         return;
1630     }
1631
1632     let span = fn_sig.decl.inputs[0].span;
1633
1634     let sig = tcx.fn_sig(method.def_id);
1635     let sig = tcx.liberate_late_bound_regions(method.def_id, sig);
1636     let sig = wfcx.normalize(span, None, sig);
1637
1638     debug!("check_method_receiver: sig={:?}", sig);
1639
1640     let self_ty = wfcx.normalize(span, None, self_ty);
1641
1642     let receiver_ty = sig.inputs()[0];
1643     let receiver_ty = wfcx.normalize(span, None, receiver_ty);
1644
1645     if tcx.features().arbitrary_self_types {
1646         if !receiver_is_valid(wfcx, span, receiver_ty, self_ty, true) {
1647             // Report error; `arbitrary_self_types` was enabled.
1648             e0307(tcx, span, receiver_ty);
1649         }
1650     } else {
1651         if !receiver_is_valid(wfcx, span, receiver_ty, self_ty, false) {
1652             if receiver_is_valid(wfcx, span, receiver_ty, self_ty, true) {
1653                 // Report error; would have worked with `arbitrary_self_types`.
1654                 feature_err(
1655                     &tcx.sess.parse_sess,
1656                     sym::arbitrary_self_types,
1657                     span,
1658                     &format!(
1659                         "`{receiver_ty}` cannot be used as the type of `self` without \
1660                          the `arbitrary_self_types` feature",
1661                     ),
1662                 )
1663                 .help(HELP_FOR_SELF_TYPE)
1664                 .emit();
1665             } else {
1666                 // Report error; would not have worked with `arbitrary_self_types`.
1667                 e0307(tcx, span, receiver_ty);
1668             }
1669         }
1670     }
1671 }
1672
1673 fn e0307(tcx: TyCtxt<'_>, span: Span, receiver_ty: Ty<'_>) {
1674     struct_span_err!(
1675         tcx.sess.diagnostic(),
1676         span,
1677         E0307,
1678         "invalid `self` parameter type: {receiver_ty}"
1679     )
1680     .note("type of `self` must be `Self` or a type that dereferences to it")
1681     .help(HELP_FOR_SELF_TYPE)
1682     .emit();
1683 }
1684
1685 /// Returns whether `receiver_ty` would be considered a valid receiver type for `self_ty`. If
1686 /// `arbitrary_self_types` is enabled, `receiver_ty` must transitively deref to `self_ty`, possibly
1687 /// through a `*const/mut T` raw pointer. If the feature is not enabled, the requirements are more
1688 /// strict: `receiver_ty` must implement `Receiver` and directly implement
1689 /// `Deref<Target = self_ty>`.
1690 ///
1691 /// N.B., there are cases this function returns `true` but causes an error to be emitted,
1692 /// particularly when `receiver_ty` derefs to a type that is the same as `self_ty` but has the
1693 /// wrong lifetime. Be careful of this if you are calling this function speculatively.
1694 fn receiver_is_valid<'tcx>(
1695     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1696     span: Span,
1697     receiver_ty: Ty<'tcx>,
1698     self_ty: Ty<'tcx>,
1699     arbitrary_self_types_enabled: bool,
1700 ) -> bool {
1701     let infcx = wfcx.infcx;
1702     let tcx = wfcx.tcx();
1703     let cause =
1704         ObligationCause::new(span, wfcx.body_id, traits::ObligationCauseCode::MethodReceiver);
1705
1706     let can_eq_self = |ty| infcx.can_eq(wfcx.param_env, self_ty, ty).is_ok();
1707
1708     // `self: Self` is always valid.
1709     if can_eq_self(receiver_ty) {
1710         if let Err(err) = wfcx.eq(&cause, wfcx.param_env, self_ty, receiver_ty) {
1711             infcx.err_ctxt().report_mismatched_types(&cause, self_ty, receiver_ty, err).emit();
1712         }
1713         return true;
1714     }
1715
1716     let mut autoderef = Autoderef::new(infcx, wfcx.param_env, wfcx.body_id, span, receiver_ty);
1717
1718     // The `arbitrary_self_types` feature allows raw pointer receivers like `self: *const Self`.
1719     if arbitrary_self_types_enabled {
1720         autoderef = autoderef.include_raw_pointers();
1721     }
1722
1723     // The first type is `receiver_ty`, which we know its not equal to `self_ty`; skip it.
1724     autoderef.next();
1725
1726     let receiver_trait_def_id = tcx.require_lang_item(LangItem::Receiver, Some(span));
1727
1728     // Keep dereferencing `receiver_ty` until we get to `self_ty`.
1729     loop {
1730         if let Some((potential_self_ty, _)) = autoderef.next() {
1731             debug!(
1732                 "receiver_is_valid: potential self type `{:?}` to match `{:?}`",
1733                 potential_self_ty, self_ty
1734             );
1735
1736             if can_eq_self(potential_self_ty) {
1737                 wfcx.register_obligations(autoderef.into_obligations());
1738
1739                 if let Err(err) = wfcx.eq(&cause, wfcx.param_env, self_ty, potential_self_ty) {
1740                     infcx
1741                         .err_ctxt()
1742                         .report_mismatched_types(&cause, self_ty, potential_self_ty, err)
1743                         .emit();
1744                 }
1745
1746                 break;
1747             } else {
1748                 // Without `feature(arbitrary_self_types)`, we require that each step in the
1749                 // deref chain implement `receiver`
1750                 if !arbitrary_self_types_enabled
1751                     && !receiver_is_implemented(
1752                         wfcx,
1753                         receiver_trait_def_id,
1754                         cause.clone(),
1755                         potential_self_ty,
1756                     )
1757                 {
1758                     return false;
1759                 }
1760             }
1761         } else {
1762             debug!("receiver_is_valid: type `{:?}` does not deref to `{:?}`", receiver_ty, self_ty);
1763             // If the receiver already has errors reported due to it, consider it valid to avoid
1764             // unnecessary errors (#58712).
1765             return receiver_ty.references_error();
1766         }
1767     }
1768
1769     // Without `feature(arbitrary_self_types)`, we require that `receiver_ty` implements `Receiver`.
1770     if !arbitrary_self_types_enabled
1771         && !receiver_is_implemented(wfcx, receiver_trait_def_id, cause.clone(), receiver_ty)
1772     {
1773         return false;
1774     }
1775
1776     true
1777 }
1778
1779 fn receiver_is_implemented<'tcx>(
1780     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1781     receiver_trait_def_id: DefId,
1782     cause: ObligationCause<'tcx>,
1783     receiver_ty: Ty<'tcx>,
1784 ) -> bool {
1785     let tcx = wfcx.tcx();
1786     let trait_ref = ty::Binder::dummy(tcx.mk_trait_ref(receiver_trait_def_id, [receiver_ty]));
1787
1788     let obligation = traits::Obligation::new(tcx, cause, wfcx.param_env, trait_ref);
1789
1790     if wfcx.infcx.predicate_must_hold_modulo_regions(&obligation) {
1791         true
1792     } else {
1793         debug!(
1794             "receiver_is_implemented: type `{:?}` does not implement `Receiver` trait",
1795             receiver_ty
1796         );
1797         false
1798     }
1799 }
1800
1801 fn check_variances_for_type_defn<'tcx>(
1802     tcx: TyCtxt<'tcx>,
1803     item: &hir::Item<'tcx>,
1804     hir_generics: &hir::Generics<'_>,
1805 ) {
1806     let ty = tcx.type_of(item.owner_id);
1807     if tcx.has_error_field(ty) {
1808         return;
1809     }
1810
1811     let ty_predicates = tcx.predicates_of(item.owner_id);
1812     assert_eq!(ty_predicates.parent, None);
1813     let variances = tcx.variances_of(item.owner_id);
1814
1815     let mut constrained_parameters: FxHashSet<_> = variances
1816         .iter()
1817         .enumerate()
1818         .filter(|&(_, &variance)| variance != ty::Bivariant)
1819         .map(|(index, _)| Parameter(index as u32))
1820         .collect();
1821
1822     identify_constrained_generic_params(tcx, ty_predicates, None, &mut constrained_parameters);
1823
1824     // Lazily calculated because it is only needed in case of an error.
1825     let explicitly_bounded_params = LazyCell::new(|| {
1826         let icx = crate::collect::ItemCtxt::new(tcx, item.owner_id.to_def_id());
1827         hir_generics
1828             .predicates
1829             .iter()
1830             .filter_map(|predicate| match predicate {
1831                 hir::WherePredicate::BoundPredicate(predicate) => {
1832                     match icx.to_ty(predicate.bounded_ty).kind() {
1833                         ty::Param(data) => Some(Parameter(data.index)),
1834                         _ => None,
1835                     }
1836                 }
1837                 _ => None,
1838             })
1839             .collect::<FxHashSet<_>>()
1840     });
1841
1842     for (index, _) in variances.iter().enumerate() {
1843         let parameter = Parameter(index as u32);
1844
1845         if constrained_parameters.contains(&parameter) {
1846             continue;
1847         }
1848
1849         let param = &hir_generics.params[index];
1850
1851         match param.name {
1852             hir::ParamName::Error => {}
1853             _ => {
1854                 let has_explicit_bounds = explicitly_bounded_params.contains(&parameter);
1855                 report_bivariance(tcx, param, has_explicit_bounds);
1856             }
1857         }
1858     }
1859 }
1860
1861 fn report_bivariance(
1862     tcx: TyCtxt<'_>,
1863     param: &rustc_hir::GenericParam<'_>,
1864     has_explicit_bounds: bool,
1865 ) -> ErrorGuaranteed {
1866     let span = param.span;
1867     let param_name = param.name.ident().name;
1868     let mut err = error_392(tcx, span, param_name);
1869
1870     let suggested_marker_id = tcx.lang_items().phantom_data();
1871     // Help is available only in presence of lang items.
1872     let msg = if let Some(def_id) = suggested_marker_id {
1873         format!(
1874             "consider removing `{}`, referring to it in a field, or using a marker such as `{}`",
1875             param_name,
1876             tcx.def_path_str(def_id),
1877         )
1878     } else {
1879         format!("consider removing `{param_name}` or referring to it in a field")
1880     };
1881     err.help(&msg);
1882
1883     if matches!(param.kind, hir::GenericParamKind::Type { .. }) && !has_explicit_bounds {
1884         err.help(&format!(
1885             "if you intended `{0}` to be a const parameter, use `const {0}: usize` instead",
1886             param_name
1887         ));
1888     }
1889     err.emit()
1890 }
1891
1892 impl<'tcx> WfCheckingCtxt<'_, 'tcx> {
1893     /// Feature gates RFC 2056 -- trivial bounds, checking for global bounds that
1894     /// aren't true.
1895     #[instrument(level = "debug", skip(self))]
1896     fn check_false_global_bounds(&mut self) {
1897         let tcx = self.ocx.infcx.tcx;
1898         let mut span = self.span;
1899         let empty_env = ty::ParamEnv::empty();
1900
1901         let def_id = tcx.hir().local_def_id(self.body_id);
1902         let predicates_with_span = tcx.predicates_of(def_id).predicates.iter().copied();
1903         // Check elaborated bounds.
1904         let implied_obligations = traits::elaborate_predicates_with_span(tcx, predicates_with_span);
1905
1906         for obligation in implied_obligations {
1907             // We lower empty bounds like `Vec<dyn Copy>:` as
1908             // `WellFormed(Vec<dyn Copy>)`, which will later get checked by
1909             // regular WF checking
1910             if let ty::PredicateKind::WellFormed(..) = obligation.predicate.kind().skip_binder() {
1911                 continue;
1912             }
1913             let pred = obligation.predicate;
1914             // Match the existing behavior.
1915             if pred.is_global() && !pred.has_late_bound_vars() {
1916                 let pred = self.normalize(span, None, pred);
1917                 let hir_node = tcx.hir().find(self.body_id);
1918
1919                 // only use the span of the predicate clause (#90869)
1920
1921                 if let Some(hir::Generics { predicates, .. }) =
1922                     hir_node.and_then(|node| node.generics())
1923                 {
1924                     let obligation_span = obligation.cause.span();
1925
1926                     span = predicates
1927                         .iter()
1928                         // There seems to be no better way to find out which predicate we are in
1929                         .find(|pred| pred.span().contains(obligation_span))
1930                         .map(|pred| pred.span())
1931                         .unwrap_or(obligation_span);
1932                 }
1933
1934                 let obligation = traits::Obligation::new(
1935                     tcx,
1936                     traits::ObligationCause::new(span, self.body_id, traits::TrivialBound),
1937                     empty_env,
1938                     pred,
1939                 );
1940                 self.ocx.register_obligation(obligation);
1941             }
1942         }
1943     }
1944 }
1945
1946 fn check_mod_type_wf(tcx: TyCtxt<'_>, module: LocalDefId) {
1947     let items = tcx.hir_module_items(module);
1948     items.par_items(|item| tcx.ensure().check_well_formed(item.owner_id));
1949     items.par_impl_items(|item| tcx.ensure().check_well_formed(item.owner_id));
1950     items.par_trait_items(|item| tcx.ensure().check_well_formed(item.owner_id));
1951     items.par_foreign_items(|item| tcx.ensure().check_well_formed(item.owner_id));
1952 }
1953
1954 fn error_392(
1955     tcx: TyCtxt<'_>,
1956     span: Span,
1957     param_name: Symbol,
1958 ) -> DiagnosticBuilder<'_, ErrorGuaranteed> {
1959     let mut err = struct_span_err!(tcx.sess, span, E0392, "parameter `{param_name}` is never used");
1960     err.span_label(span, "unused parameter");
1961     err
1962 }
1963
1964 pub fn provide(providers: &mut Providers) {
1965     *providers = Providers { check_mod_type_wf, check_well_formed, ..*providers };
1966 }