compiler/rustc_typeck/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};
   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::subst::{GenericArgKind, InternalSubsts, Subst};
  16 use rustc_middle::ty::trait_def::TraitSpecializationKind;
  17 use rustc_middle::ty::{
  18     self, AdtKind, DefIdTree, GenericParamDefKind, ToPredicate, Ty, TyCtxt, TypeFoldable,
  19     TypeSuperVisitable, TypeVisitable, TypeVisitor,
  20 };
  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_trait_selection::autoderef::Autoderef;
  25 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
  26 use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
  27 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt as _;
  28 use rustc_trait_selection::traits::{
  29     self, ObligationCause, ObligationCauseCode, ObligationCtxt, WellFormedLoc,
  30 };
  31
  32 use std::cell::LazyCell;
  33 use std::convert::TryInto;
  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     fn normalize<T>(&self, span: Span, loc: Option<WellFormedLoc>, value: T) -> T
  56     where
  57         T: TypeFoldable<'tcx>,
  58     {
  59         self.ocx.normalize(
  60             ObligationCause::new(span, self.body_id, ObligationCauseCode::WellFormed(loc)),
  61             self.param_env,
  62             value,
  63         )
  64     }
  65
  66     fn register_wf_obligation(
  67         &self,
  68         span: Span,
  69         loc: Option<WellFormedLoc>,
  70         arg: ty::GenericArg<'tcx>,
  71     ) {
  72         let cause =
  73             traits::ObligationCause::new(span, self.body_id, ObligationCauseCode::WellFormed(loc));
  74         // for a type to be WF, we do not need to check if const trait predicates satisfy.
  75         let param_env = self.param_env.without_const();
  76         self.ocx.register_obligation(traits::Obligation::new(
  77             cause,
  78             param_env,
  79             ty::Binder::dummy(ty::PredicateKind::WellFormed(arg)).to_predicate(self.tcx()),
  80         ));
  81     }
  82 }
  83
  84 pub(super) fn enter_wf_checking_ctxt<'tcx, F>(
  85     tcx: TyCtxt<'tcx>,
  86     span: Span,
  87     body_def_id: LocalDefId,
  88     f: F,
  89 ) where
  90     F: for<'a> FnOnce(&WfCheckingCtxt<'a, 'tcx>),
  91 {
  92     let param_env = tcx.param_env(body_def_id);
  93     let body_id = tcx.hir().local_def_id_to_hir_id(body_def_id);
  94     tcx.infer_ctxt().enter(|ref infcx| {
  95         let ocx = ObligationCtxt::new(infcx);
  96
  97         let assumed_wf_types = ocx.assumed_wf_types(param_env, span, body_def_id);
  98
  99         let mut wfcx = WfCheckingCtxt { ocx, span, body_id, param_env };
 100
 101         if !tcx.features().trivial_bounds {
 102             wfcx.check_false_global_bounds()
 103         }
 104         f(&mut wfcx);
 105         let errors = wfcx.select_all_or_error();
 106         if !errors.is_empty() {
 107             infcx.report_fulfillment_errors(&errors, None, false);
 108             return;
 109         }
 110
 111         let implied_bounds = infcx.implied_bounds_tys(param_env, body_id, assumed_wf_types);
 112         let outlives_environment =
 113             OutlivesEnvironment::with_bounds(param_env, Some(infcx), implied_bounds);
 114
 115         infcx.check_region_obligations_and_report_errors(body_def_id, &outlives_environment);
 116     })
 117 }
 118
 119 fn check_well_formed(tcx: TyCtxt<'_>, def_id: LocalDefId) {
 120     let node = tcx.hir().expect_owner(def_id);
 121     match node {
 122         hir::OwnerNode::Crate(_) => {}
 123         hir::OwnerNode::Item(item) => check_item(tcx, item),
 124         hir::OwnerNode::TraitItem(item) => check_trait_item(tcx, item),
 125         hir::OwnerNode::ImplItem(item) => check_impl_item(tcx, item),
 126         hir::OwnerNode::ForeignItem(item) => check_foreign_item(tcx, item),
 127     }
 128
 129     if let Some(generics) = node.generics() {
 130         for param in generics.params {
 131             check_param_wf(tcx, param)
 132         }
 133     }
 134 }
 135
 136 /// Checks that the field types (in a struct def'n) or argument types (in an enum def'n) are
 137 /// well-formed, meaning that they do not require any constraints not declared in the struct
 138 /// definition itself. For example, this definition would be illegal:
 139 ///
 140 /// ```rust
 141 /// struct Ref<'a, T> { x: &'a T }
 142 /// ```
 143 ///
 144 /// because the type did not declare that `T:'a`.
 145 ///
 146 /// We do this check as a pre-pass before checking fn bodies because if these constraints are
 147 /// not included it frequently leads to confusing errors in fn bodies. So it's better to check
 148 /// the types first.
 149 #[instrument(skip(tcx), level = "debug")]
 150 fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) {
 151     let def_id = item.def_id;
 152
 153     debug!(
 154         ?item.def_id,
 155         item.name = ? tcx.def_path_str(def_id.to_def_id())
 156     );
 157
 158     match item.kind {
 159         // Right now we check that every default trait implementation
 160         // has an implementation of itself. Basically, a case like:
 161         //
 162         //     impl Trait for T {}
 163         //
 164         // has a requirement of `T: Trait` which was required for default
 165         // method implementations. Although this could be improved now that
 166         // there's a better infrastructure in place for this, it's being left
 167         // for a follow-up work.
 168         //
 169         // Since there's such a requirement, we need to check *just* positive
 170         // implementations, otherwise things like:
 171         //
 172         //     impl !Send for T {}
 173         //
 174         // won't be allowed unless there's an *explicit* implementation of `Send`
 175         // for `T`
 176         hir::ItemKind::Impl(ref impl_) => {
 177             let is_auto = tcx
 178                 .impl_trait_ref(item.def_id)
 179                 .map_or(false, |trait_ref| tcx.trait_is_auto(trait_ref.def_id));
 180             if let (hir::Defaultness::Default { .. }, true) = (impl_.defaultness, is_auto) {
 181                 let sp = impl_.of_trait.as_ref().map_or(item.span, |t| t.path.span);
 182                 let mut err =
 183                     tcx.sess.struct_span_err(sp, "impls of auto traits cannot be default");
 184                 err.span_labels(impl_.defaultness_span, "default because of this");
 185                 err.span_label(sp, "auto trait");
 186                 err.emit();
 187             }
 188             // We match on both `ty::ImplPolarity` and `ast::ImplPolarity` just to get the `!` span.
 189             match (tcx.impl_polarity(def_id), impl_.polarity) {
 190                 (ty::ImplPolarity::Positive, _) => {
 191                     check_impl(tcx, item, impl_.self_ty, &impl_.of_trait, impl_.constness);
 192                 }
 193                 (ty::ImplPolarity::Negative, ast::ImplPolarity::Negative(span)) => {
 194                     // FIXME(#27579): what amount of WF checking do we need for neg impls?
 195                     if let hir::Defaultness::Default { .. } = impl_.defaultness {
 196                         let mut spans = vec![span];
 197                         spans.extend(impl_.defaultness_span);
 198                         struct_span_err!(
 199                             tcx.sess,
 200                             spans,
 201                             E0750,
 202                             "negative impls cannot be default impls"
 203                         )
 204                         .emit();
 205                     }
 206                 }
 207                 (ty::ImplPolarity::Reservation, _) => {
 208                     // FIXME: what amount of WF checking do we need for reservation impls?
 209                 }
 210                 _ => unreachable!(),
 211             }
 212         }
 213         hir::ItemKind::Fn(ref sig, ..) => {
 214             check_item_fn(tcx, item.def_id, item.ident, item.span, sig.decl);
 215         }
 216         hir::ItemKind::Static(ty, ..) => {
 217             check_item_type(tcx, item.def_id, ty.span, false);
 218         }
 219         hir::ItemKind::Const(ty, ..) => {
 220             check_item_type(tcx, item.def_id, ty.span, false);
 221         }
 222         hir::ItemKind::Struct(ref struct_def, ref ast_generics) => {
 223             check_type_defn(tcx, item, false, |wfcx| vec![wfcx.non_enum_variant(struct_def)]);
 224
 225             check_variances_for_type_defn(tcx, item, ast_generics);
 226         }
 227         hir::ItemKind::Union(ref struct_def, ref ast_generics) => {
 228             check_type_defn(tcx, item, true, |wfcx| vec![wfcx.non_enum_variant(struct_def)]);
 229
 230             check_variances_for_type_defn(tcx, item, ast_generics);
 231         }
 232         hir::ItemKind::Enum(ref enum_def, ref ast_generics) => {
 233             check_type_defn(tcx, item, true, |wfcx| wfcx.enum_variants(enum_def));
 234
 235             check_variances_for_type_defn(tcx, item, ast_generics);
 236         }
 237         hir::ItemKind::Trait(..) => {
 238             check_trait(tcx, item);
 239         }
 240         hir::ItemKind::TraitAlias(..) => {
 241             check_trait(tcx, item);
 242         }
 243         // `ForeignItem`s are handled separately.
 244         hir::ItemKind::ForeignMod { .. } => {}
 245         _ => {}
 246     }
 247 }
 248
 249 fn check_foreign_item(tcx: TyCtxt<'_>, item: &hir::ForeignItem<'_>) {
 250     let def_id = item.def_id;
 251
 252     debug!(
 253         ?item.def_id,
 254         item.name = ? tcx.def_path_str(def_id.to_def_id())
 255     );
 256
 257     match item.kind {
 258         hir::ForeignItemKind::Fn(decl, ..) => {
 259             check_item_fn(tcx, item.def_id, item.ident, item.span, decl)
 260         }
 261         hir::ForeignItemKind::Static(ty, ..) => check_item_type(tcx, item.def_id, ty.span, true),
 262         hir::ForeignItemKind::Type => (),
 263     }
 264 }
 265
 266 fn check_trait_item(tcx: TyCtxt<'_>, trait_item: &hir::TraitItem<'_>) {
 267     let def_id = trait_item.def_id;
 268
 269     let (method_sig, span) = match trait_item.kind {
 270         hir::TraitItemKind::Fn(ref sig, _) => (Some(sig), trait_item.span),
 271         hir::TraitItemKind::Type(_bounds, Some(ty)) => (None, ty.span),
 272         _ => (None, trait_item.span),
 273     };
 274     check_object_unsafe_self_trait_by_name(tcx, trait_item);
 275     check_associated_item(tcx, trait_item.def_id, span, method_sig);
 276
 277     let encl_trait_def_id = tcx.local_parent(def_id);
 278     let encl_trait = tcx.hir().expect_item(encl_trait_def_id);
 279     let encl_trait_def_id = encl_trait.def_id.to_def_id();
 280     let fn_lang_item_name = if Some(encl_trait_def_id) == tcx.lang_items().fn_trait() {
 281         Some("fn")
 282     } else if Some(encl_trait_def_id) == tcx.lang_items().fn_mut_trait() {
 283         Some("fn_mut")
 284     } else {
 285         None
 286     };
 287
 288     if let (Some(fn_lang_item_name), "call") =
 289         (fn_lang_item_name, trait_item.ident.name.to_ident_string().as_str())
 290     {
 291         // We are looking at the `call` function of the `fn` or `fn_mut` lang item.
 292         // Do some rudimentary sanity checking to avoid an ICE later (issue #83471).
 293         if let Some(hir::FnSig { decl, span, .. }) = method_sig {
 294             if let [self_ty, _] = decl.inputs {
 295                 if !matches!(self_ty.kind, hir::TyKind::Rptr(_, _)) {
 296                     tcx.sess
 297                         .struct_span_err(
 298                             self_ty.span,
 299                             &format!(
 300                                 "first argument of `call` in `{fn_lang_item_name}` lang item must be a reference",
 301                             ),
 302                         )
 303                         .emit();
 304                 }
 305             } else {
 306                 tcx.sess
 307                     .struct_span_err(
 308                         *span,
 309                         &format!(
 310                             "`call` function in `{fn_lang_item_name}` lang item takes exactly two arguments",
 311                         ),
 312                     )
 313                     .emit();
 314             }
 315         } else {
 316             tcx.sess
 317                 .struct_span_err(
 318                     trait_item.span,
 319                     &format!(
 320                         "`call` trait item in `{fn_lang_item_name}` lang item must be a function",
 321                     ),
 322                 )
 323                 .emit();
 324         }
 325     }
 326 }
 327
 328 /// Require that the user writes where clauses on GATs for the implicit
 329 /// outlives bounds involving trait parameters in trait functions and
 330 /// lifetimes passed as GAT substs. See `self-outlives-lint` test.
 331 ///
 332 /// We use the following trait as an example throughout this function:
 333 /// ```rust,ignore (this code fails due to this lint)
 334 /// trait IntoIter {
 335 ///     type Iter<'a>: Iterator<Item = Self::Item<'a>>;
 336 ///     type Item<'a>;
 337 ///     fn into_iter<'a>(&'a self) -> Self::Iter<'a>;
 338 /// }
 339 /// ```
 340 fn check_gat_where_clauses(tcx: TyCtxt<'_>, associated_items: &[hir::TraitItemRef]) {
 341     // Associates every GAT's def_id to a list of possibly missing bounds detected by this lint.
 342     let mut required_bounds_by_item = FxHashMap::default();
 343
 344     // Loop over all GATs together, because if this lint suggests adding a where-clause bound
 345     // to one GAT, it might then require us to an additional bound on another GAT.
 346     // In our `IntoIter` example, we discover a missing `Self: 'a` bound on `Iter<'a>`, which
 347     // then in a second loop adds a `Self: 'a` bound to `Item` due to the relationship between
 348     // those GATs.
 349     loop {
 350         let mut should_continue = false;
 351         for gat_item in associated_items {
 352             let gat_def_id = gat_item.id.def_id;
 353             let gat_item = tcx.associated_item(gat_def_id);
 354             // If this item is not an assoc ty, or has no substs, then it's not a GAT
 355             if gat_item.kind != ty::AssocKind::Type {
 356                 continue;
 357             }
 358             let gat_generics = tcx.generics_of(gat_def_id);
 359             // FIXME(jackh726): we can also warn in the more general case
 360             if gat_generics.params.is_empty() {
 361                 continue;
 362             }
 363
 364             // Gather the bounds with which all other items inside of this trait constrain the GAT.
 365             // This is calculated by taking the intersection of the bounds that each item
 366             // constrains the GAT with individually.
 367             let mut new_required_bounds: Option<FxHashSet<ty::Predicate<'_>>> = None;
 368             for item in associated_items {
 369                 let item_def_id = item.id.def_id;
 370                 // Skip our own GAT, since it does not constrain itself at all.
 371                 if item_def_id == gat_def_id {
 372                     continue;
 373                 }
 374
 375                 let item_hir_id = item.id.hir_id();
 376                 let param_env = tcx.param_env(item_def_id);
 377
 378                 let item_required_bounds = match item.kind {
 379                     // In our example, this corresponds to `into_iter` method
 380                     hir::AssocItemKind::Fn { .. } => {
 381                         // For methods, we check the function signature's return type for any GATs
 382                         // to constrain. In the `into_iter` case, we see that the return type
 383                         // `Self::Iter<'a>` is a GAT we want to gather any potential missing bounds from.
 384                         let sig: ty::FnSig<'_> = tcx.liberate_late_bound_regions(
 385                             item_def_id.to_def_id(),
 386                             tcx.fn_sig(item_def_id),
 387                         );
 388                         gather_gat_bounds(
 389                             tcx,
 390                             param_env,
 391                             item_hir_id,
 392                             sig.inputs_and_output,
 393                             // We also assume that all of the function signature's parameter types
 394                             // are well formed.
 395                             &sig.inputs().iter().copied().collect(),
 396                             gat_def_id,
 397                             gat_generics,
 398                         )
 399                     }
 400                     // In our example, this corresponds to the `Iter` and `Item` associated types
 401                     hir::AssocItemKind::Type => {
 402                         // If our associated item is a GAT with missing bounds, add them to
 403                         // the param-env here. This allows this GAT to propagate missing bounds
 404                         // to other GATs.
 405                         let param_env = augment_param_env(
 406                             tcx,
 407                             param_env,
 408                             required_bounds_by_item.get(&item_def_id),
 409                         );
 410                         gather_gat_bounds(
 411                             tcx,
 412                             param_env,
 413                             item_hir_id,
 414                             tcx.explicit_item_bounds(item_def_id)
 415                                 .iter()
 416                                 .copied()
 417                                 .collect::<Vec<_>>(),
 418                             &FxHashSet::default(),
 419                             gat_def_id,
 420                             gat_generics,
 421                         )
 422                     }
 423                     hir::AssocItemKind::Const => None,
 424                 };
 425
 426                 if let Some(item_required_bounds) = item_required_bounds {
 427                     // Take the intersection of the required bounds for this GAT, and
 428                     // the item_required_bounds which are the ones implied by just
 429                     // this item alone.
 430                     // This is why we use an Option<_>, since we need to distinguish
 431                     // the empty set of bounds from the _uninitialized_ set of bounds.
 432                     if let Some(new_required_bounds) = &mut new_required_bounds {
 433                         new_required_bounds.retain(|b| item_required_bounds.contains(b));
 434                     } else {
 435                         new_required_bounds = Some(item_required_bounds);
 436                     }
 437                 }
 438             }
 439
 440             if let Some(new_required_bounds) = new_required_bounds {
 441                 let required_bounds = required_bounds_by_item.entry(gat_def_id).or_default();
 442                 if new_required_bounds.into_iter().any(|p| required_bounds.insert(p)) {
 443                     // Iterate until our required_bounds no longer change
 444                     // Since they changed here, we should continue the loop
 445                     should_continue = true;
 446                 }
 447             }
 448         }
 449         // We know that this loop will eventually halt, since we only set `should_continue` if the
 450         // `required_bounds` for this item grows. Since we are not creating any new region or type
 451         // variables, the set of all region and type bounds that we could ever insert are limited
 452         // by the number of unique types and regions we observe in a given item.
 453         if !should_continue {
 454             break;
 455         }
 456     }
 457
 458     for (gat_def_id, required_bounds) in required_bounds_by_item {
 459         let gat_item_hir = tcx.hir().expect_trait_item(gat_def_id);
 460         debug!(?required_bounds);
 461         let param_env = tcx.param_env(gat_def_id);
 462         let gat_hir = gat_item_hir.hir_id();
 463
 464         let mut unsatisfied_bounds: Vec<_> = required_bounds
 465             .into_iter()
 466             .filter(|clause| match clause.kind().skip_binder() {
 467                 ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(a, b)) => {
 468                     !region_known_to_outlive(tcx, gat_hir, param_env, &FxHashSet::default(), a, b)
 469                 }
 470                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(a, b)) => {
 471                     !ty_known_to_outlive(tcx, gat_hir, param_env, &FxHashSet::default(), a, b)
 472                 }
 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: &FxHashSet<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 =
 605                     ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_param, region_param));
 606                 let clause = tcx.mk_predicate(ty::Binder::dummy(clause));
 607                 bounds.insert(clause);
 608             }
 609         }
 610
 611         // For each region argument (e.g., `'a` in our example), also check for a
 612         // relationship to the other region arguments. If there is an outlives
 613         // relationship, then we want to ensure that is reflected in the where clause
 614         // on the GAT itself.
 615         for (region_b, region_b_idx) in &regions {
 616             // Again, skip `'static` because it outlives everything. Also, we trivially
 617             // know that a region outlives itself.
 618             if ty::ReStatic == **region_b || region_a == region_b {
 619                 continue;
 620             }
 621             if region_known_to_outlive(tcx, item_hir, param_env, &wf_tys, *region_a, *region_b) {
 622                 debug!(?region_a_idx, ?region_b_idx);
 623                 debug!("required clause: {region_a} must outlive {region_b}");
 624                 // Translate into the generic parameters of the GAT.
 625                 let region_a_param = gat_generics.param_at(*region_a_idx, tcx);
 626                 let region_a_param =
 627                     tcx.mk_region(ty::RegionKind::ReEarlyBound(ty::EarlyBoundRegion {
 628                         def_id: region_a_param.def_id,
 629                         index: region_a_param.index,
 630                         name: region_a_param.name,
 631                     }));
 632                 // Same for the region.
 633                 let region_b_param = gat_generics.param_at(*region_b_idx, tcx);
 634                 let region_b_param =
 635                     tcx.mk_region(ty::RegionKind::ReEarlyBound(ty::EarlyBoundRegion {
 636                         def_id: region_b_param.def_id,
 637                         index: region_b_param.index,
 638                         name: region_b_param.name,
 639                     }));
 640                 // The predicate we expect to see.
 641                 let clause = ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(
 642                     region_a_param,
 643                     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: &FxHashSet<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: &FxHashSet<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: &FxHashSet<Ty<'tcx>>,
 702     add_constraints: impl for<'a> FnOnce(&'a InferCtxt<'a, '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     tcx.infer_ctxt().enter(|infcx| {
 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         let errors = infcx.resolve_regions(&outlives_environment);
 718
 719         debug!(?errors, "errors");
 720
 721         // If we were able to prove that the type outlives the region without
 722         // an error, it must be because of the implied or explicit bounds...
 723         errors.is_empty()
 724     })
 725 }
 726
 727 /// TypeVisitor that looks for uses of GATs like
 728 /// `<P0 as Trait<P1..Pn>>::GAT<Pn..Pm>` and adds the arguments `P0..Pm` into
 729 /// the two vectors, `regions` and `types` (depending on their kind). For each
 730 /// parameter `Pi` also track the index `i`.
 731 struct GATSubstCollector<'tcx> {
 732     gat: DefId,
 733     // Which region appears and which parameter index its substituted for
 734     regions: FxHashSet<(ty::Region<'tcx>, usize)>,
 735     // Which params appears and which parameter index its substituted for
 736     types: FxHashSet<(Ty<'tcx>, usize)>,
 737 }
 738
 739 impl<'tcx> GATSubstCollector<'tcx> {
 740     fn visit<T: TypeFoldable<'tcx>>(
 741         gat: DefId,
 742         t: T,
 743     ) -> (FxHashSet<(ty::Region<'tcx>, usize)>, FxHashSet<(Ty<'tcx>, usize)>) {
 744         let mut visitor =
 745             GATSubstCollector { gat, regions: FxHashSet::default(), types: FxHashSet::default() };
 746         t.visit_with(&mut visitor);
 747         (visitor.regions, visitor.types)
 748     }
 749 }
 750
 751 impl<'tcx> TypeVisitor<'tcx> for GATSubstCollector<'tcx> {
 752     type BreakTy = !;
 753
 754     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 755         match t.kind() {
 756             ty::Projection(p) if p.item_def_id == self.gat => {
 757                 for (idx, subst) in p.substs.iter().enumerate() {
 758                     match subst.unpack() {
 759                         GenericArgKind::Lifetime(lt) if !lt.is_late_bound() => {
 760                             self.regions.insert((lt, idx));
 761                         }
 762                         GenericArgKind::Type(t) => {
 763                             self.types.insert((t, idx));
 764                         }
 765                         _ => {}
 766                     }
 767                 }
 768             }
 769             _ => {}
 770         }
 771         t.super_visit_with(self)
 772     }
 773 }
 774
 775 fn could_be_self(trait_def_id: LocalDefId, ty: &hir::Ty<'_>) -> bool {
 776     match ty.kind {
 777         hir::TyKind::TraitObject([trait_ref], ..) => match trait_ref.trait_ref.path.segments {
 778             [s] => s.res.opt_def_id() == Some(trait_def_id.to_def_id()),
 779             _ => false,
 780         },
 781         _ => false,
 782     }
 783 }
 784
 785 /// Detect when an object unsafe trait is referring to itself in one of its associated items.
 786 /// When this is done, suggest using `Self` instead.
 787 fn check_object_unsafe_self_trait_by_name(tcx: TyCtxt<'_>, item: &hir::TraitItem<'_>) {
 788     let (trait_name, trait_def_id) =
 789         match tcx.hir().get_by_def_id(tcx.hir().get_parent_item(item.hir_id())) {
 790             hir::Node::Item(item) => match item.kind {
 791                 hir::ItemKind::Trait(..) => (item.ident, item.def_id),
 792                 _ => return,
 793             },
 794             _ => return,
 795         };
 796     let mut trait_should_be_self = vec![];
 797     match &item.kind {
 798         hir::TraitItemKind::Const(ty, _) | hir::TraitItemKind::Type(_, Some(ty))
 799             if could_be_self(trait_def_id, ty) =>
 800         {
 801             trait_should_be_self.push(ty.span)
 802         }
 803         hir::TraitItemKind::Fn(sig, _) => {
 804             for ty in sig.decl.inputs {
 805                 if could_be_self(trait_def_id, ty) {
 806                     trait_should_be_self.push(ty.span);
 807                 }
 808             }
 809             match sig.decl.output {
 810                 hir::FnRetTy::Return(ty) if could_be_self(trait_def_id, ty) => {
 811                     trait_should_be_self.push(ty.span);
 812                 }
 813                 _ => {}
 814             }
 815         }
 816         _ => {}
 817     }
 818     if !trait_should_be_self.is_empty() {
 819         if tcx.object_safety_violations(trait_def_id).is_empty() {
 820             return;
 821         }
 822         let sugg = trait_should_be_self.iter().map(|span| (*span, "Self".to_string())).collect();
 823         tcx.sess
 824             .struct_span_err(
 825                 trait_should_be_self,
 826                 "associated item referring to unboxed trait object for its own trait",
 827             )
 828             .span_label(trait_name.span, "in this trait")
 829             .multipart_suggestion(
 830                 "you might have meant to use `Self` to refer to the implementing type",
 831                 sugg,
 832                 Applicability::MachineApplicable,
 833             )
 834             .emit();
 835     }
 836 }
 837
 838 fn check_impl_item(tcx: TyCtxt<'_>, impl_item: &hir::ImplItem<'_>) {
 839     let def_id = impl_item.def_id;
 840
 841     let (method_sig, span) = match impl_item.kind {
 842         hir::ImplItemKind::Fn(ref sig, _) => (Some(sig), impl_item.span),
 843         // Constrain binding and overflow error spans to `<Ty>` in `type foo = <Ty>`.
 844         hir::ImplItemKind::TyAlias(ty) if ty.span != DUMMY_SP => (None, ty.span),
 845         _ => (None, impl_item.span),
 846     };
 847
 848     check_associated_item(tcx, def_id, span, method_sig);
 849 }
 850
 851 fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) {
 852     match param.kind {
 853         // We currently only check wf of const params here.
 854         hir::GenericParamKind::Lifetime { .. } | hir::GenericParamKind::Type { .. } => (),
 855
 856         // Const parameters are well formed if their type is structural match.
 857         hir::GenericParamKind::Const { ty: hir_ty, default: _ } => {
 858             let ty = tcx.type_of(tcx.hir().local_def_id(param.hir_id));
 859
 860             if tcx.features().adt_const_params {
 861                 if let Some(non_structural_match_ty) =
 862                     traits::search_for_adt_const_param_violation(param.span, tcx, ty)
 863                 {
 864                     // We use the same error code in both branches, because this is really the same
 865                     // issue: we just special-case the message for type parameters to make it
 866                     // clearer.
 867                     match non_structural_match_ty.kind() {
 868                         ty::Param(_) => {
 869                             // Const parameters may not have type parameters as their types,
 870                             // because we cannot be sure that the type parameter derives `PartialEq`
 871                             // and `Eq` (just implementing them is not enough for `structural_match`).
 872                             struct_span_err!(
 873                                 tcx.sess,
 874                                 hir_ty.span,
 875                                 E0741,
 876                                 "`{ty}` is not guaranteed to `#[derive(PartialEq, Eq)]`, so may not be \
 877                                 used as the type of a const parameter",
 878                             )
 879                             .span_label(
 880                                 hir_ty.span,
 881                                 format!("`{ty}` may not derive both `PartialEq` and `Eq`"),
 882                             )
 883                             .note(
 884                                 "it is not currently possible to use a type parameter as the type of a \
 885                                 const parameter",
 886                             )
 887                             .emit();
 888                         }
 889                         ty::Float(_) => {
 890                             struct_span_err!(
 891                                 tcx.sess,
 892                                 hir_ty.span,
 893                                 E0741,
 894                                 "`{ty}` is forbidden as the type of a const generic parameter",
 895                             )
 896                             .note("floats do not derive `Eq` or `Ord`, which are required for const parameters")
 897                             .emit();
 898                         }
 899                         ty::FnPtr(_) => {
 900                             struct_span_err!(
 901                                 tcx.sess,
 902                                 hir_ty.span,
 903                                 E0741,
 904                                 "using function pointers as const generic parameters is forbidden",
 905                             )
 906                             .emit();
 907                         }
 908                         ty::RawPtr(_) => {
 909                             struct_span_err!(
 910                                 tcx.sess,
 911                                 hir_ty.span,
 912                                 E0741,
 913                                 "using raw pointers as const generic parameters is forbidden",
 914                             )
 915                             .emit();
 916                         }
 917                         _ => {
 918                             let mut diag = struct_span_err!(
 919                                 tcx.sess,
 920                                 hir_ty.span,
 921                                 E0741,
 922                                 "`{}` must be annotated with `#[derive(PartialEq, Eq)]` to be used as \
 923                                 the type of a const parameter",
 924                                 non_structural_match_ty,
 925                             );
 926
 927                             if ty == non_structural_match_ty {
 928                                 diag.span_label(
 929                                     hir_ty.span,
 930                                     format!("`{ty}` doesn't derive both `PartialEq` and `Eq`"),
 931                                 );
 932                             }
 933
 934                             diag.emit();
 935                         }
 936                     }
 937                 }
 938             } else {
 939                 let err_ty_str;
 940                 let mut is_ptr = true;
 941
 942                 let err = match ty.kind() {
 943                     ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Error(_) => None,
 944                     ty::FnPtr(_) => Some("function pointers"),
 945                     ty::RawPtr(_) => Some("raw pointers"),
 946                     _ => {
 947                         is_ptr = false;
 948                         err_ty_str = format!("`{ty}`");
 949                         Some(err_ty_str.as_str())
 950                     }
 951                 };
 952
 953                 if let Some(unsupported_type) = err {
 954                     if is_ptr {
 955                         tcx.sess.span_err(
 956                             hir_ty.span,
 957                             &format!(
 958                                 "using {unsupported_type} as const generic parameters is forbidden",
 959                             ),
 960                         );
 961                     } else {
 962                         let mut err = tcx.sess.struct_span_err(
 963                             hir_ty.span,
 964                             &format!(
 965                                 "{unsupported_type} is forbidden as the type of a const generic parameter",
 966                             ),
 967                         );
 968                         err.note("the only supported types are integers, `bool` and `char`");
 969                         if tcx.sess.is_nightly_build() {
 970                             err.help(
 971                             "more complex types are supported with `#![feature(adt_const_params)]`",
 972                         );
 973                         }
 974                         err.emit();
 975                     }
 976                 }
 977             }
 978         }
 979     }
 980 }
 981
 982 #[instrument(level = "debug", skip(tcx, span, sig_if_method))]
 983 fn check_associated_item(
 984     tcx: TyCtxt<'_>,
 985     item_id: LocalDefId,
 986     span: Span,
 987     sig_if_method: Option<&hir::FnSig<'_>>,
 988 ) {
 989     let loc = Some(WellFormedLoc::Ty(item_id));
 990     enter_wf_checking_ctxt(tcx, span, item_id, |wfcx| {
 991         let item = tcx.associated_item(item_id);
 992
 993         let self_ty = match item.container {
 994             ty::TraitContainer => tcx.types.self_param,
 995             ty::ImplContainer => tcx.type_of(item.container_id(tcx)),
 996         };
 997
 998         match item.kind {
 999             ty::AssocKind::Const => {
1000                 let ty = tcx.type_of(item.def_id);
1001                 let ty = wfcx.normalize(span, Some(WellFormedLoc::Ty(item_id)), ty);
1002                 wfcx.register_wf_obligation(span, loc, ty.into());
1003             }
1004             ty::AssocKind::Fn => {
1005                 let sig = tcx.fn_sig(item.def_id);
1006                 let hir_sig = sig_if_method.expect("bad signature for method");
1007                 check_fn_or_method(
1008                     wfcx,
1009                     item.ident(tcx).span,
1010                     sig,
1011                     hir_sig.decl,
1012                     item.def_id.expect_local(),
1013                 );
1014                 check_method_receiver(wfcx, hir_sig, item, self_ty);
1015             }
1016             ty::AssocKind::Type => {
1017                 if let ty::AssocItemContainer::TraitContainer = item.container {
1018                     check_associated_type_bounds(wfcx, item, span)
1019                 }
1020                 if item.defaultness(tcx).has_value() {
1021                     let ty = tcx.type_of(item.def_id);
1022                     let ty = wfcx.normalize(span, Some(WellFormedLoc::Ty(item_id)), ty);
1023                     wfcx.register_wf_obligation(span, loc, ty.into());
1024                 }
1025             }
1026         }
1027     })
1028 }
1029
1030 fn item_adt_kind(kind: &ItemKind<'_>) -> Option<AdtKind> {
1031     match kind {
1032         ItemKind::Struct(..) => Some(AdtKind::Struct),
1033         ItemKind::Union(..) => Some(AdtKind::Union),
1034         ItemKind::Enum(..) => Some(AdtKind::Enum),
1035         _ => None,
1036     }
1037 }
1038
1039 /// In a type definition, we check that to ensure that the types of the fields are well-formed.
1040 fn check_type_defn<'tcx, F>(
1041     tcx: TyCtxt<'tcx>,
1042     item: &hir::Item<'tcx>,
1043     all_sized: bool,
1044     mut lookup_fields: F,
1045 ) where
1046     F: FnMut(&WfCheckingCtxt<'_, 'tcx>) -> Vec<AdtVariant<'tcx>>,
1047 {
1048     enter_wf_checking_ctxt(tcx, item.span, item.def_id, |wfcx| {
1049         let variants = lookup_fields(wfcx);
1050         let packed = tcx.adt_def(item.def_id).repr().packed();
1051
1052         for variant in &variants {
1053             // All field types must be well-formed.
1054             for field in &variant.fields {
1055                 wfcx.register_wf_obligation(
1056                     field.span,
1057                     Some(WellFormedLoc::Ty(field.def_id)),
1058                     field.ty.into(),
1059                 )
1060             }
1061
1062             // For DST, or when drop needs to copy things around, all
1063             // intermediate types must be sized.
1064             let needs_drop_copy = || {
1065                 packed && {
1066                     let ty = variant.fields.last().unwrap().ty;
1067                     let ty = tcx.erase_regions(ty);
1068                     if ty.needs_infer() {
1069                         tcx.sess
1070                             .delay_span_bug(item.span, &format!("inference variables in {:?}", ty));
1071                         // Just treat unresolved type expression as if it needs drop.
1072                         true
1073                     } else {
1074                         ty.needs_drop(tcx, tcx.param_env(item.def_id))
1075                     }
1076                 }
1077             };
1078             // All fields (except for possibly the last) should be sized.
1079             let all_sized = all_sized || variant.fields.is_empty() || needs_drop_copy();
1080             let unsized_len = if all_sized { 0 } else { 1 };
1081             for (idx, field) in
1082                 variant.fields[..variant.fields.len() - unsized_len].iter().enumerate()
1083             {
1084                 let last = idx == variant.fields.len() - 1;
1085                 wfcx.register_bound(
1086                     traits::ObligationCause::new(
1087                         field.span,
1088                         wfcx.body_id,
1089                         traits::FieldSized {
1090                             adt_kind: match item_adt_kind(&item.kind) {
1091                                 Some(i) => i,
1092                                 None => bug!(),
1093                             },
1094                             span: field.span,
1095                             last,
1096                         },
1097                     ),
1098                     wfcx.param_env,
1099                     field.ty,
1100                     tcx.require_lang_item(LangItem::Sized, None),
1101                 );
1102             }
1103
1104             // Explicit `enum` discriminant values must const-evaluate successfully.
1105             if let Some(discr_def_id) = variant.explicit_discr {
1106                 let discr_substs = InternalSubsts::identity_for_item(tcx, discr_def_id.to_def_id());
1107
1108                 let cause = traits::ObligationCause::new(
1109                     tcx.def_span(discr_def_id),
1110                     wfcx.body_id,
1111                     traits::MiscObligation,
1112                 );
1113                 wfcx.register_obligation(traits::Obligation::new(
1114                     cause,
1115                     wfcx.param_env,
1116                     ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(ty::Unevaluated::new(
1117                         ty::WithOptConstParam::unknown(discr_def_id.to_def_id()),
1118                         discr_substs,
1119                     )))
1120                     .to_predicate(tcx),
1121                 ));
1122             }
1123         }
1124
1125         check_where_clauses(wfcx, item.span, item.def_id);
1126     });
1127 }
1128
1129 #[instrument(skip(tcx, item))]
1130 fn check_trait(tcx: TyCtxt<'_>, item: &hir::Item<'_>) {
1131     debug!(?item.def_id);
1132
1133     let trait_def = tcx.trait_def(item.def_id);
1134     if trait_def.is_marker
1135         || matches!(trait_def.specialization_kind, TraitSpecializationKind::Marker)
1136     {
1137         for associated_def_id in &*tcx.associated_item_def_ids(item.def_id) {
1138             struct_span_err!(
1139                 tcx.sess,
1140                 tcx.def_span(*associated_def_id),
1141                 E0714,
1142                 "marker traits cannot have associated items",
1143             )
1144             .emit();
1145         }
1146     }
1147
1148     enter_wf_checking_ctxt(tcx, item.span, item.def_id, |wfcx| {
1149         check_where_clauses(wfcx, item.span, item.def_id)
1150     });
1151
1152     // Only check traits, don't check trait aliases
1153     if let hir::ItemKind::Trait(_, _, _, _, items) = item.kind {
1154         check_gat_where_clauses(tcx, items);
1155     }
1156 }
1157
1158 /// Checks all associated type defaults of trait `trait_def_id`.
1159 ///
1160 /// Assuming the defaults are used, check that all predicates (bounds on the
1161 /// assoc type and where clauses on the trait) hold.
1162 fn check_associated_type_bounds(wfcx: &WfCheckingCtxt<'_, '_>, item: &ty::AssocItem, span: Span) {
1163     let bounds = wfcx.tcx().explicit_item_bounds(item.def_id);
1164
1165     debug!("check_associated_type_bounds: bounds={:?}", bounds);
1166     let wf_obligations = bounds.iter().flat_map(|&(bound, bound_span)| {
1167         let normalized_bound = wfcx.normalize(span, None, bound);
1168         traits::wf::predicate_obligations(
1169             wfcx.infcx,
1170             wfcx.param_env,
1171             wfcx.body_id,
1172             normalized_bound,
1173             bound_span,
1174         )
1175     });
1176
1177     wfcx.register_obligations(wf_obligations);
1178 }
1179
1180 fn check_item_fn(
1181     tcx: TyCtxt<'_>,
1182     def_id: LocalDefId,
1183     ident: Ident,
1184     span: Span,
1185     decl: &hir::FnDecl<'_>,
1186 ) {
1187     enter_wf_checking_ctxt(tcx, span, def_id, |wfcx| {
1188         let sig = tcx.fn_sig(def_id);
1189         check_fn_or_method(wfcx, ident.span, sig, decl, def_id);
1190     })
1191 }
1192
1193 fn check_item_type(tcx: TyCtxt<'_>, item_id: LocalDefId, ty_span: Span, allow_foreign_ty: bool) {
1194     debug!("check_item_type: {:?}", item_id);
1195
1196     enter_wf_checking_ctxt(tcx, ty_span, item_id, |wfcx| {
1197         let ty = tcx.type_of(item_id);
1198         let item_ty = wfcx.normalize(ty_span, Some(WellFormedLoc::Ty(item_id)), ty);
1199
1200         let mut forbid_unsized = true;
1201         if allow_foreign_ty {
1202             let tail = tcx.struct_tail_erasing_lifetimes(item_ty, wfcx.param_env);
1203             if let ty::Foreign(_) = tail.kind() {
1204                 forbid_unsized = false;
1205             }
1206         }
1207
1208         wfcx.register_wf_obligation(ty_span, Some(WellFormedLoc::Ty(item_id)), item_ty.into());
1209         if forbid_unsized {
1210             wfcx.register_bound(
1211                 traits::ObligationCause::new(ty_span, wfcx.body_id, traits::WellFormed(None)),
1212                 wfcx.param_env,
1213                 item_ty,
1214                 tcx.require_lang_item(LangItem::Sized, None),
1215             );
1216         }
1217
1218         // Ensure that the end result is `Sync` in a non-thread local `static`.
1219         let should_check_for_sync = tcx.static_mutability(item_id.to_def_id())
1220             == Some(hir::Mutability::Not)
1221             && !tcx.is_foreign_item(item_id.to_def_id())
1222             && !tcx.is_thread_local_static(item_id.to_def_id());
1223
1224         if should_check_for_sync {
1225             wfcx.register_bound(
1226                 traits::ObligationCause::new(ty_span, wfcx.body_id, traits::SharedStatic),
1227                 wfcx.param_env,
1228                 item_ty,
1229                 tcx.require_lang_item(LangItem::Sync, Some(ty_span)),
1230             );
1231         }
1232     });
1233 }
1234
1235 #[instrument(level = "debug", skip(tcx, ast_self_ty, ast_trait_ref))]
1236 fn check_impl<'tcx>(
1237     tcx: TyCtxt<'tcx>,
1238     item: &'tcx hir::Item<'tcx>,
1239     ast_self_ty: &hir::Ty<'_>,
1240     ast_trait_ref: &Option<hir::TraitRef<'_>>,
1241     constness: hir::Constness,
1242 ) {
1243     enter_wf_checking_ctxt(tcx, item.span, item.def_id, |wfcx| {
1244         match *ast_trait_ref {
1245             Some(ref ast_trait_ref) => {
1246                 // `#[rustc_reservation_impl]` impls are not real impls and
1247                 // therefore don't need to be WF (the trait's `Self: Trait` predicate
1248                 // won't hold).
1249                 let trait_ref = tcx.impl_trait_ref(item.def_id).unwrap();
1250                 let trait_ref = wfcx.normalize(ast_trait_ref.path.span, None, trait_ref);
1251                 let trait_pred = ty::TraitPredicate {
1252                     trait_ref,
1253                     constness: match constness {
1254                         hir::Constness::Const => ty::BoundConstness::ConstIfConst,
1255                         hir::Constness::NotConst => ty::BoundConstness::NotConst,
1256                     },
1257                     polarity: ty::ImplPolarity::Positive,
1258                 };
1259                 let obligations = traits::wf::trait_obligations(
1260                     wfcx.infcx,
1261                     wfcx.param_env,
1262                     wfcx.body_id,
1263                     &trait_pred,
1264                     ast_trait_ref.path.span,
1265                     item,
1266                 );
1267                 debug!(?obligations);
1268                 wfcx.register_obligations(obligations);
1269             }
1270             None => {
1271                 let self_ty = tcx.type_of(item.def_id);
1272                 let self_ty = wfcx.normalize(
1273                     item.span,
1274                     Some(WellFormedLoc::Ty(item.hir_id().expect_owner())),
1275                     self_ty,
1276                 );
1277                 wfcx.register_wf_obligation(
1278                     ast_self_ty.span,
1279                     Some(WellFormedLoc::Ty(item.hir_id().expect_owner())),
1280                     self_ty.into(),
1281                 );
1282             }
1283         }
1284
1285         check_where_clauses(wfcx, item.span, item.def_id);
1286     });
1287 }
1288
1289 /// Checks where-clauses and inline bounds that are declared on `def_id`.
1290 #[instrument(level = "debug", skip(wfcx))]
1291 fn check_where_clauses<'tcx>(wfcx: &WfCheckingCtxt<'_, 'tcx>, span: Span, def_id: LocalDefId) {
1292     let infcx = wfcx.infcx;
1293     let tcx = wfcx.tcx();
1294
1295     let predicates = tcx.bound_predicates_of(def_id.to_def_id());
1296     let generics = tcx.generics_of(def_id);
1297
1298     let is_our_default = |def: &ty::GenericParamDef| match def.kind {
1299         GenericParamDefKind::Type { has_default, .. }
1300         | GenericParamDefKind::Const { has_default } => {
1301             has_default && def.index >= generics.parent_count as u32
1302         }
1303         GenericParamDefKind::Lifetime => unreachable!(),
1304     };
1305
1306     // Check that concrete defaults are well-formed. See test `type-check-defaults.rs`.
1307     // For example, this forbids the declaration:
1308     //
1309     //     struct Foo<T = Vec<[u32]>> { .. }
1310     //
1311     // Here, the default `Vec<[u32]>` is not WF because `[u32]: Sized` does not hold.
1312     for param in &generics.params {
1313         match param.kind {
1314             GenericParamDefKind::Type { .. } => {
1315                 if is_our_default(param) {
1316                     let ty = tcx.type_of(param.def_id);
1317                     // Ignore dependent defaults -- that is, where the default of one type
1318                     // parameter includes another (e.g., `<T, U = T>`). In those cases, we can't
1319                     // be sure if it will error or not as user might always specify the other.
1320                     if !ty.needs_subst() {
1321                         wfcx.register_wf_obligation(
1322                             tcx.def_span(param.def_id),
1323                             Some(WellFormedLoc::Ty(param.def_id.expect_local())),
1324                             ty.into(),
1325                         );
1326                     }
1327                 }
1328             }
1329             GenericParamDefKind::Const { .. } => {
1330                 if is_our_default(param) {
1331                     // FIXME(const_generics_defaults): This
1332                     // is incorrect when dealing with unused substs, for example
1333                     // for `struct Foo<const N: usize, const M: usize = { 1 - 2 }>`
1334                     // we should eagerly error.
1335                     let default_ct = tcx.const_param_default(param.def_id);
1336                     if !default_ct.needs_subst() {
1337                         wfcx.register_wf_obligation(
1338                             tcx.def_span(param.def_id),
1339                             None,
1340                             default_ct.into(),
1341                         );
1342                     }
1343                 }
1344             }
1345             // Doesn't have defaults.
1346             GenericParamDefKind::Lifetime => {}
1347         }
1348     }
1349
1350     // Check that trait predicates are WF when params are substituted by their defaults.
1351     // We don't want to overly constrain the predicates that may be written but we want to
1352     // catch cases where a default my never be applied such as `struct Foo<T: Copy = String>`.
1353     // Therefore we check if a predicate which contains a single type param
1354     // with a concrete default is WF with that default substituted.
1355     // For more examples see tests `defaults-well-formedness.rs` and `type-check-defaults.rs`.
1356     //
1357     // First we build the defaulted substitution.
1358     let substs = InternalSubsts::for_item(tcx, def_id.to_def_id(), |param, _| {
1359         match param.kind {
1360             GenericParamDefKind::Lifetime => {
1361                 // All regions are identity.
1362                 tcx.mk_param_from_def(param)
1363             }
1364
1365             GenericParamDefKind::Type { .. } => {
1366                 // If the param has a default, ...
1367                 if is_our_default(param) {
1368                     let default_ty = tcx.type_of(param.def_id);
1369                     // ... and it's not a dependent default, ...
1370                     if !default_ty.needs_subst() {
1371                         // ... then substitute it with the default.
1372                         return default_ty.into();
1373                     }
1374                 }
1375
1376                 tcx.mk_param_from_def(param)
1377             }
1378             GenericParamDefKind::Const { .. } => {
1379                 // If the param has a default, ...
1380                 if is_our_default(param) {
1381                     let default_ct = tcx.const_param_default(param.def_id);
1382                     // ... and it's not a dependent default, ...
1383                     if !default_ct.needs_subst() {
1384                         // ... then substitute it with the default.
1385                         return default_ct.into();
1386                     }
1387                 }
1388
1389                 tcx.mk_param_from_def(param)
1390             }
1391         }
1392     });
1393
1394     // Now we build the substituted predicates.
1395     let default_obligations = predicates
1396         .0
1397         .predicates
1398         .iter()
1399         .flat_map(|&(pred, sp)| {
1400             #[derive(Default)]
1401             struct CountParams {
1402                 params: FxHashSet<u32>,
1403             }
1404             impl<'tcx> ty::visit::TypeVisitor<'tcx> for CountParams {
1405                 type BreakTy = ();
1406
1407                 fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
1408                     if let ty::Param(param) = t.kind() {
1409                         self.params.insert(param.index);
1410                     }
1411                     t.super_visit_with(self)
1412                 }
1413
1414                 fn visit_region(&mut self, _: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
1415                     ControlFlow::BREAK
1416                 }
1417
1418                 fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
1419                     if let ty::ConstKind::Param(param) = c.kind() {
1420                         self.params.insert(param.index);
1421                     }
1422                     c.super_visit_with(self)
1423                 }
1424             }
1425             let mut param_count = CountParams::default();
1426             let has_region = pred.visit_with(&mut param_count).is_break();
1427             let substituted_pred = predicates.rebind(pred).subst(tcx, substs);
1428             // Don't check non-defaulted params, dependent defaults (including lifetimes)
1429             // or preds with multiple params.
1430             if substituted_pred.has_param_types_or_consts()
1431                 || param_count.params.len() > 1
1432                 || has_region
1433             {
1434                 None
1435             } else if predicates.0.predicates.iter().any(|&(p, _)| p == substituted_pred) {
1436                 // Avoid duplication of predicates that contain no parameters, for example.
1437                 None
1438             } else {
1439                 Some((substituted_pred, sp))
1440             }
1441         })
1442         .map(|(pred, sp)| {
1443             // Convert each of those into an obligation. So if you have
1444             // something like `struct Foo<T: Copy = String>`, we would
1445             // take that predicate `T: Copy`, substitute to `String: Copy`
1446             // (actually that happens in the previous `flat_map` call),
1447             // and then try to prove it (in this case, we'll fail).
1448             //
1449             // Note the subtle difference from how we handle `predicates`
1450             // below: there, we are not trying to prove those predicates
1451             // to be *true* but merely *well-formed*.
1452             let pred = wfcx.normalize(sp, None, pred);
1453             let cause = traits::ObligationCause::new(
1454                 sp,
1455                 wfcx.body_id,
1456                 traits::ItemObligation(def_id.to_def_id()),
1457             );
1458             traits::Obligation::new(cause, wfcx.param_env, pred)
1459         });
1460
1461     let predicates = predicates.0.instantiate_identity(tcx);
1462
1463     let predicates = wfcx.normalize(span, None, predicates);
1464
1465     debug!(?predicates.predicates);
1466     assert_eq!(predicates.predicates.len(), predicates.spans.len());
1467     let wf_obligations =
1468         iter::zip(&predicates.predicates, &predicates.spans).flat_map(|(&p, &sp)| {
1469             traits::wf::predicate_obligations(
1470                 infcx,
1471                 wfcx.param_env.without_const(),
1472                 wfcx.body_id,
1473                 p,
1474                 sp,
1475             )
1476         });
1477
1478     let obligations: Vec<_> = wf_obligations.chain(default_obligations).collect();
1479     wfcx.register_obligations(obligations);
1480 }
1481
1482 #[instrument(level = "debug", skip(wfcx, span, hir_decl))]
1483 fn check_fn_or_method<'tcx>(
1484     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1485     span: Span,
1486     sig: ty::PolyFnSig<'tcx>,
1487     hir_decl: &hir::FnDecl<'_>,
1488     def_id: LocalDefId,
1489 ) {
1490     let tcx = wfcx.tcx();
1491     let sig = tcx.liberate_late_bound_regions(def_id.to_def_id(), sig);
1492
1493     // Normalize the input and output types one at a time, using a different
1494     // `WellFormedLoc` for each. We cannot call `normalize_associated_types`
1495     // on the entire `FnSig`, since this would use the same `WellFormedLoc`
1496     // for each type, preventing the HIR wf check from generating
1497     // a nice error message.
1498     let ty::FnSig { mut inputs_and_output, c_variadic, unsafety, abi } = sig;
1499     inputs_and_output = tcx.mk_type_list(inputs_and_output.iter().enumerate().map(|(i, ty)| {
1500         wfcx.normalize(
1501             span,
1502             Some(WellFormedLoc::Param {
1503                 function: def_id,
1504                 // Note that the `param_idx` of the output type is
1505                 // one greater than the index of the last input type.
1506                 param_idx: i.try_into().unwrap(),
1507             }),
1508             ty,
1509         )
1510     }));
1511     // Manually call `normalize_associated_types_in` on the other types
1512     // in `FnSig`. This ensures that if the types of these fields
1513     // ever change to include projections, we will start normalizing
1514     // them automatically.
1515     let sig = ty::FnSig {
1516         inputs_and_output,
1517         c_variadic: wfcx.normalize(span, None, c_variadic),
1518         unsafety: wfcx.normalize(span, None, unsafety),
1519         abi: wfcx.normalize(span, None, abi),
1520     };
1521
1522     for (i, (&input_ty, ty)) in iter::zip(sig.inputs(), hir_decl.inputs).enumerate() {
1523         wfcx.register_wf_obligation(
1524             ty.span,
1525             Some(WellFormedLoc::Param { function: def_id, param_idx: i.try_into().unwrap() }),
1526             input_ty.into(),
1527         );
1528     }
1529
1530     wfcx.register_wf_obligation(
1531         hir_decl.output.span(),
1532         Some(WellFormedLoc::Param {
1533             function: def_id,
1534             param_idx: sig.inputs().len().try_into().unwrap(),
1535         }),
1536         sig.output().into(),
1537     );
1538
1539     check_where_clauses(wfcx, span, def_id);
1540
1541     check_return_position_impl_trait_in_trait_bounds(
1542         tcx,
1543         wfcx,
1544         def_id,
1545         sig.output(),
1546         hir_decl.output.span(),
1547     );
1548 }
1549
1550 /// Basically `check_associated_type_bounds`, but separated for now and should be
1551 /// deduplicated when RPITITs get lowered into real associated items.
1552 fn check_return_position_impl_trait_in_trait_bounds<'tcx>(
1553     tcx: TyCtxt<'tcx>,
1554     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1555     fn_def_id: LocalDefId,
1556     fn_output: Ty<'tcx>,
1557     span: Span,
1558 ) {
1559     if let Some(assoc_item) = tcx.opt_associated_item(fn_def_id.to_def_id())
1560         && assoc_item.container == ty::AssocItemContainer::TraitContainer
1561     {
1562         for arg in fn_output.walk() {
1563             if let ty::GenericArgKind::Type(ty) = arg.unpack()
1564                 && let ty::Projection(proj) = ty.kind()
1565                 && tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder
1566                 && tcx.impl_trait_in_trait_parent(proj.item_def_id) == fn_def_id.to_def_id()
1567             {
1568                 let bounds = wfcx.tcx().explicit_item_bounds(proj.item_def_id);
1569                 let wf_obligations = bounds.iter().flat_map(|&(bound, bound_span)| {
1570                     let normalized_bound = wfcx.normalize(span, None, bound);
1571                     traits::wf::predicate_obligations(
1572                         wfcx.infcx,
1573                         wfcx.param_env,
1574                         wfcx.body_id,
1575                         normalized_bound,
1576                         bound_span,
1577                     )
1578                 });
1579                 wfcx.register_obligations(wf_obligations);
1580             }
1581         }
1582     }
1583 }
1584
1585 const HELP_FOR_SELF_TYPE: &str = "consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, \
1586      `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one \
1587      of the previous types except `Self`)";
1588
1589 #[instrument(level = "debug", skip(wfcx))]
1590 fn check_method_receiver<'tcx>(
1591     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1592     fn_sig: &hir::FnSig<'_>,
1593     method: &ty::AssocItem,
1594     self_ty: Ty<'tcx>,
1595 ) {
1596     let tcx = wfcx.tcx();
1597
1598     if !method.fn_has_self_parameter {
1599         return;
1600     }
1601
1602     let span = fn_sig.decl.inputs[0].span;
1603
1604     let sig = tcx.fn_sig(method.def_id);
1605     let sig = tcx.liberate_late_bound_regions(method.def_id, sig);
1606     let sig = wfcx.normalize(span, None, sig);
1607
1608     debug!("check_method_receiver: sig={:?}", sig);
1609
1610     let self_ty = wfcx.normalize(span, None, self_ty);
1611
1612     let receiver_ty = sig.inputs()[0];
1613     let receiver_ty = wfcx.normalize(span, None, receiver_ty);
1614
1615     if tcx.features().arbitrary_self_types {
1616         if !receiver_is_valid(wfcx, span, receiver_ty, self_ty, true) {
1617             // Report error; `arbitrary_self_types` was enabled.
1618             e0307(tcx, span, receiver_ty);
1619         }
1620     } else {
1621         if !receiver_is_valid(wfcx, span, receiver_ty, self_ty, false) {
1622             if receiver_is_valid(wfcx, span, receiver_ty, self_ty, true) {
1623                 // Report error; would have worked with `arbitrary_self_types`.
1624                 feature_err(
1625                     &tcx.sess.parse_sess,
1626                     sym::arbitrary_self_types,
1627                     span,
1628                     &format!(
1629                         "`{receiver_ty}` cannot be used as the type of `self` without \
1630                          the `arbitrary_self_types` feature",
1631                     ),
1632                 )
1633                 .help(HELP_FOR_SELF_TYPE)
1634                 .emit();
1635             } else {
1636                 // Report error; would not have worked with `arbitrary_self_types`.
1637                 e0307(tcx, span, receiver_ty);
1638             }
1639         }
1640     }
1641 }
1642
1643 fn e0307<'tcx>(tcx: TyCtxt<'tcx>, span: Span, receiver_ty: Ty<'_>) {
1644     struct_span_err!(
1645         tcx.sess.diagnostic(),
1646         span,
1647         E0307,
1648         "invalid `self` parameter type: {receiver_ty}"
1649     )
1650     .note("type of `self` must be `Self` or a type that dereferences to it")
1651     .help(HELP_FOR_SELF_TYPE)
1652     .emit();
1653 }
1654
1655 /// Returns whether `receiver_ty` would be considered a valid receiver type for `self_ty`. If
1656 /// `arbitrary_self_types` is enabled, `receiver_ty` must transitively deref to `self_ty`, possibly
1657 /// through a `*const/mut T` raw pointer. If the feature is not enabled, the requirements are more
1658 /// strict: `receiver_ty` must implement `Receiver` and directly implement
1659 /// `Deref<Target = self_ty>`.
1660 ///
1661 /// N.B., there are cases this function returns `true` but causes an error to be emitted,
1662 /// particularly when `receiver_ty` derefs to a type that is the same as `self_ty` but has the
1663 /// wrong lifetime. Be careful of this if you are calling this function speculatively.
1664 fn receiver_is_valid<'tcx>(
1665     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1666     span: Span,
1667     receiver_ty: Ty<'tcx>,
1668     self_ty: Ty<'tcx>,
1669     arbitrary_self_types_enabled: bool,
1670 ) -> bool {
1671     let infcx = wfcx.infcx;
1672     let tcx = wfcx.tcx();
1673     let cause =
1674         ObligationCause::new(span, wfcx.body_id, traits::ObligationCauseCode::MethodReceiver);
1675
1676     let can_eq_self = |ty| infcx.can_eq(wfcx.param_env, self_ty, ty).is_ok();
1677
1678     // `self: Self` is always valid.
1679     if can_eq_self(receiver_ty) {
1680         if let Err(err) = wfcx.equate_types(&cause, wfcx.param_env, self_ty, receiver_ty) {
1681             infcx.report_mismatched_types(&cause, self_ty, receiver_ty, err).emit();
1682         }
1683         return true;
1684     }
1685
1686     let mut autoderef =
1687         Autoderef::new(infcx, wfcx.param_env, wfcx.body_id, span, receiver_ty, span);
1688
1689     // The `arbitrary_self_types` feature allows raw pointer receivers like `self: *const Self`.
1690     if arbitrary_self_types_enabled {
1691         autoderef = autoderef.include_raw_pointers();
1692     }
1693
1694     // The first type is `receiver_ty`, which we know its not equal to `self_ty`; skip it.
1695     autoderef.next();
1696
1697     let receiver_trait_def_id = tcx.require_lang_item(LangItem::Receiver, None);
1698
1699     // Keep dereferencing `receiver_ty` until we get to `self_ty`.
1700     loop {
1701         if let Some((potential_self_ty, _)) = autoderef.next() {
1702             debug!(
1703                 "receiver_is_valid: potential self type `{:?}` to match `{:?}`",
1704                 potential_self_ty, self_ty
1705             );
1706
1707             if can_eq_self(potential_self_ty) {
1708                 wfcx.register_obligations(autoderef.into_obligations());
1709
1710                 if let Err(err) =
1711                     wfcx.equate_types(&cause, wfcx.param_env, self_ty, potential_self_ty)
1712                 {
1713                     infcx.report_mismatched_types(&cause, self_ty, potential_self_ty, err).emit();
1714                 }
1715
1716                 break;
1717             } else {
1718                 // Without `feature(arbitrary_self_types)`, we require that each step in the
1719                 // deref chain implement `receiver`
1720                 if !arbitrary_self_types_enabled
1721                     && !receiver_is_implemented(
1722                         wfcx,
1723                         receiver_trait_def_id,
1724                         cause.clone(),
1725                         potential_self_ty,
1726                     )
1727                 {
1728                     return false;
1729                 }
1730             }
1731         } else {
1732             debug!("receiver_is_valid: type `{:?}` does not deref to `{:?}`", receiver_ty, self_ty);
1733             // If the receiver already has errors reported due to it, consider it valid to avoid
1734             // unnecessary errors (#58712).
1735             return receiver_ty.references_error();
1736         }
1737     }
1738
1739     // Without `feature(arbitrary_self_types)`, we require that `receiver_ty` implements `Receiver`.
1740     if !arbitrary_self_types_enabled
1741         && !receiver_is_implemented(wfcx, receiver_trait_def_id, cause.clone(), receiver_ty)
1742     {
1743         return false;
1744     }
1745
1746     true
1747 }
1748
1749 fn receiver_is_implemented<'tcx>(
1750     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1751     receiver_trait_def_id: DefId,
1752     cause: ObligationCause<'tcx>,
1753     receiver_ty: Ty<'tcx>,
1754 ) -> bool {
1755     let tcx = wfcx.tcx();
1756     let trait_ref = ty::Binder::dummy(ty::TraitRef {
1757         def_id: receiver_trait_def_id,
1758         substs: tcx.mk_substs_trait(receiver_ty, &[]),
1759     });
1760
1761     let obligation =
1762         traits::Obligation::new(cause, wfcx.param_env, trait_ref.without_const().to_predicate(tcx));
1763
1764     if wfcx.infcx.predicate_must_hold_modulo_regions(&obligation) {
1765         true
1766     } else {
1767         debug!(
1768             "receiver_is_implemented: type `{:?}` does not implement `Receiver` trait",
1769             receiver_ty
1770         );
1771         false
1772     }
1773 }
1774
1775 fn check_variances_for_type_defn<'tcx>(
1776     tcx: TyCtxt<'tcx>,
1777     item: &hir::Item<'tcx>,
1778     hir_generics: &hir::Generics<'_>,
1779 ) {
1780     let ty = tcx.type_of(item.def_id);
1781     if tcx.has_error_field(ty) {
1782         return;
1783     }
1784
1785     let ty_predicates = tcx.predicates_of(item.def_id);
1786     assert_eq!(ty_predicates.parent, None);
1787     let variances = tcx.variances_of(item.def_id);
1788
1789     let mut constrained_parameters: FxHashSet<_> = variances
1790         .iter()
1791         .enumerate()
1792         .filter(|&(_, &variance)| variance != ty::Bivariant)
1793         .map(|(index, _)| Parameter(index as u32))
1794         .collect();
1795
1796     identify_constrained_generic_params(tcx, ty_predicates, None, &mut constrained_parameters);
1797
1798     // Lazily calculated because it is only needed in case of an error.
1799     let explicitly_bounded_params = LazyCell::new(|| {
1800         let icx = crate::collect::ItemCtxt::new(tcx, item.def_id.to_def_id());
1801         hir_generics
1802             .predicates
1803             .iter()
1804             .filter_map(|predicate| match predicate {
1805                 hir::WherePredicate::BoundPredicate(predicate) => {
1806                     match icx.to_ty(predicate.bounded_ty).kind() {
1807                         ty::Param(data) => Some(Parameter(data.index)),
1808                         _ => None,
1809                     }
1810                 }
1811                 _ => None,
1812             })
1813             .collect::<FxHashSet<_>>()
1814     });
1815
1816     for (index, _) in variances.iter().enumerate() {
1817         let parameter = Parameter(index as u32);
1818
1819         if constrained_parameters.contains(&parameter) {
1820             continue;
1821         }
1822
1823         let param = &hir_generics.params[index];
1824
1825         match param.name {
1826             hir::ParamName::Error => {}
1827             _ => {
1828                 let has_explicit_bounds = explicitly_bounded_params.contains(&parameter);
1829                 report_bivariance(tcx, param, has_explicit_bounds);
1830             }
1831         }
1832     }
1833 }
1834
1835 fn report_bivariance(
1836     tcx: TyCtxt<'_>,
1837     param: &rustc_hir::GenericParam<'_>,
1838     has_explicit_bounds: bool,
1839 ) -> ErrorGuaranteed {
1840     let span = param.span;
1841     let param_name = param.name.ident().name;
1842     let mut err = error_392(tcx, span, param_name);
1843
1844     let suggested_marker_id = tcx.lang_items().phantom_data();
1845     // Help is available only in presence of lang items.
1846     let msg = if let Some(def_id) = suggested_marker_id {
1847         format!(
1848             "consider removing `{}`, referring to it in a field, or using a marker such as `{}`",
1849             param_name,
1850             tcx.def_path_str(def_id),
1851         )
1852     } else {
1853         format!("consider removing `{param_name}` or referring to it in a field")
1854     };
1855     err.help(&msg);
1856
1857     if matches!(param.kind, hir::GenericParamKind::Type { .. }) && !has_explicit_bounds {
1858         err.help(&format!(
1859             "if you intended `{0}` to be a const parameter, use `const {0}: usize` instead",
1860             param_name
1861         ));
1862     }
1863     err.emit()
1864 }
1865
1866 impl<'tcx> WfCheckingCtxt<'_, 'tcx> {
1867     /// Feature gates RFC 2056 -- trivial bounds, checking for global bounds that
1868     /// aren't true.
1869     #[instrument(level = "debug", skip(self))]
1870     fn check_false_global_bounds(&mut self) {
1871         let tcx = self.ocx.infcx.tcx;
1872         let mut span = self.span;
1873         let empty_env = ty::ParamEnv::empty();
1874
1875         let def_id = tcx.hir().local_def_id(self.body_id);
1876         let predicates_with_span = tcx.predicates_of(def_id).predicates.iter().copied();
1877         // Check elaborated bounds.
1878         let implied_obligations = traits::elaborate_predicates_with_span(tcx, predicates_with_span);
1879
1880         for obligation in implied_obligations {
1881             // We lower empty bounds like `Vec<dyn Copy>:` as
1882             // `WellFormed(Vec<dyn Copy>)`, which will later get checked by
1883             // regular WF checking
1884             if let ty::PredicateKind::WellFormed(..) = obligation.predicate.kind().skip_binder() {
1885                 continue;
1886             }
1887             let pred = obligation.predicate;
1888             // Match the existing behavior.
1889             if pred.is_global() && !pred.has_late_bound_regions() {
1890                 let pred = self.normalize(span, None, pred);
1891                 let hir_node = tcx.hir().find(self.body_id);
1892
1893                 // only use the span of the predicate clause (#90869)
1894
1895                 if let Some(hir::Generics { predicates, .. }) =
1896                     hir_node.and_then(|node| node.generics())
1897                 {
1898                     let obligation_span = obligation.cause.span();
1899
1900                     span = predicates
1901                         .iter()
1902                         // There seems to be no better way to find out which predicate we are in
1903                         .find(|pred| pred.span().contains(obligation_span))
1904                         .map(|pred| pred.span())
1905                         .unwrap_or(obligation_span);
1906                 }
1907
1908                 let obligation = traits::Obligation::new(
1909                     traits::ObligationCause::new(span, self.body_id, traits::TrivialBound),
1910                     empty_env,
1911                     pred,
1912                 );
1913                 self.ocx.register_obligation(obligation);
1914             }
1915         }
1916     }
1917 }
1918
1919 fn check_mod_type_wf(tcx: TyCtxt<'_>, module: LocalDefId) {
1920     let items = tcx.hir_module_items(module);
1921     items.par_items(|item| tcx.ensure().check_well_formed(item.def_id));
1922     items.par_impl_items(|item| tcx.ensure().check_well_formed(item.def_id));
1923     items.par_trait_items(|item| tcx.ensure().check_well_formed(item.def_id));
1924     items.par_foreign_items(|item| tcx.ensure().check_well_formed(item.def_id));
1925 }
1926
1927 ///////////////////////////////////////////////////////////////////////////
1928 // ADT
1929
1930 // FIXME(eddyb) replace this with getting fields/discriminants through `ty::AdtDef`.
1931 struct AdtVariant<'tcx> {
1932     /// Types of fields in the variant, that must be well-formed.
1933     fields: Vec<AdtField<'tcx>>,
1934
1935     /// Explicit discriminant of this variant (e.g. `A = 123`),
1936     /// that must evaluate to a constant value.
1937     explicit_discr: Option<LocalDefId>,
1938 }
1939
1940 struct AdtField<'tcx> {
1941     ty: Ty<'tcx>,
1942     def_id: LocalDefId,
1943     span: Span,
1944 }
1945
1946 impl<'a, 'tcx> WfCheckingCtxt<'a, 'tcx> {
1947     // FIXME(eddyb) replace this with getting fields through `ty::AdtDef`.
1948     fn non_enum_variant(&self, struct_def: &hir::VariantData<'_>) -> AdtVariant<'tcx> {
1949         let fields = struct_def
1950             .fields()
1951             .iter()
1952             .map(|field| {
1953                 let def_id = self.tcx().hir().local_def_id(field.hir_id);
1954                 let field_ty = self.tcx().type_of(def_id);
1955                 let field_ty = self.normalize(field.ty.span, None, field_ty);
1956                 debug!("non_enum_variant: type of field {:?} is {:?}", field, field_ty);
1957                 AdtField { ty: field_ty, span: field.ty.span, def_id }
1958             })
1959             .collect();
1960         AdtVariant { fields, explicit_discr: None }
1961     }
1962
1963     fn enum_variants(&self, enum_def: &hir::EnumDef<'_>) -> Vec<AdtVariant<'tcx>> {
1964         enum_def
1965             .variants
1966             .iter()
1967             .map(|variant| AdtVariant {
1968                 fields: self.non_enum_variant(&variant.data).fields,
1969                 explicit_discr: variant
1970                     .disr_expr
1971                     .map(|explicit_discr| self.tcx().hir().local_def_id(explicit_discr.hir_id)),
1972             })
1973             .collect()
1974     }
1975 }
1976
1977 fn error_392(
1978     tcx: TyCtxt<'_>,
1979     span: Span,
1980     param_name: Symbol,
1981 ) -> DiagnosticBuilder<'_, ErrorGuaranteed> {
1982     let mut err = struct_span_err!(tcx.sess, span, E0392, "parameter `{param_name}` is never used");
1983     err.span_label(span, "unused parameter");
1984     err
1985 }
1986
1987 pub fn provide(providers: &mut Providers) {
1988     *providers = Providers { check_mod_type_wf, check_well_formed, ..*providers };
1989 }