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};
   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, ToPredicate, Ty, TyCtxt, TypeFoldable,
  18     TypeSuperVisitable, 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_trait_selection::autoderef::Autoderef;
  25 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
  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     let infcx = &tcx.infer_ctxt().build();
  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.err_ctxt().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 fn check_well_formed(tcx: TyCtxt<'_>, def_id: hir::OwnerId) {
 119     let node = tcx.hir().expect_owner(def_id);
 120     match node {
 121         hir::OwnerNode::Crate(_) => {}
 122         hir::OwnerNode::Item(item) => check_item(tcx, item),
 123         hir::OwnerNode::TraitItem(item) => check_trait_item(tcx, item),
 124         hir::OwnerNode::ImplItem(item) => check_impl_item(tcx, item),
 125         hir::OwnerNode::ForeignItem(item) => check_foreign_item(tcx, item),
 126     }
 127
 128     if let Some(generics) = node.generics() {
 129         for param in generics.params {
 130             check_param_wf(tcx, param)
 131         }
 132     }
 133 }
 134
 135 /// Checks that the field types (in a struct def'n) or argument types (in an enum def'n) are
 136 /// well-formed, meaning that they do not require any constraints not declared in the struct
 137 /// definition itself. For example, this definition would be illegal:
 138 ///
 139 /// ```rust
 140 /// struct Ref<'a, T> { x: &'a T }
 141 /// ```
 142 ///
 143 /// because the type did not declare that `T:'a`.
 144 ///
 145 /// We do this check as a pre-pass before checking fn bodies because if these constraints are
 146 /// not included it frequently leads to confusing errors in fn bodies. So it's better to check
 147 /// the types first.
 148 #[instrument(skip(tcx), level = "debug")]
 149 fn check_item<'tcx>(tcx: TyCtxt<'tcx>, item: &'tcx hir::Item<'tcx>) {
 150     let def_id = item.def_id.def_id;
 151
 152     debug!(
 153         ?item.def_id,
 154         item.name = ? tcx.def_path_str(def_id.to_def_id())
 155     );
 156
 157     match item.kind {
 158         // Right now we check that every default trait implementation
 159         // has an implementation of itself. Basically, a case like:
 160         //
 161         //     impl Trait for T {}
 162         //
 163         // has a requirement of `T: Trait` which was required for default
 164         // method implementations. Although this could be improved now that
 165         // there's a better infrastructure in place for this, it's being left
 166         // for a follow-up work.
 167         //
 168         // Since there's such a requirement, we need to check *just* positive
 169         // implementations, otherwise things like:
 170         //
 171         //     impl !Send for T {}
 172         //
 173         // won't be allowed unless there's an *explicit* implementation of `Send`
 174         // for `T`
 175         hir::ItemKind::Impl(ref impl_) => {
 176             let is_auto = tcx
 177                 .impl_trait_ref(def_id)
 178                 .map_or(false, |trait_ref| tcx.trait_is_auto(trait_ref.def_id));
 179             if let (hir::Defaultness::Default { .. }, true) = (impl_.defaultness, is_auto) {
 180                 let sp = impl_.of_trait.as_ref().map_or(item.span, |t| t.path.span);
 181                 let mut err =
 182                     tcx.sess.struct_span_err(sp, "impls of auto traits cannot be default");
 183                 err.span_labels(impl_.defaultness_span, "default because of this");
 184                 err.span_label(sp, "auto trait");
 185                 err.emit();
 186             }
 187             // We match on both `ty::ImplPolarity` and `ast::ImplPolarity` just to get the `!` span.
 188             match (tcx.impl_polarity(def_id), impl_.polarity) {
 189                 (ty::ImplPolarity::Positive, _) => {
 190                     check_impl(tcx, item, impl_.self_ty, &impl_.of_trait, impl_.constness);
 191                 }
 192                 (ty::ImplPolarity::Negative, ast::ImplPolarity::Negative(span)) => {
 193                     // FIXME(#27579): what amount of WF checking do we need for neg impls?
 194                     if let hir::Defaultness::Default { .. } = impl_.defaultness {
 195                         let mut spans = vec![span];
 196                         spans.extend(impl_.defaultness_span);
 197                         struct_span_err!(
 198                             tcx.sess,
 199                             spans,
 200                             E0750,
 201                             "negative impls cannot be default impls"
 202                         )
 203                         .emit();
 204                     }
 205                 }
 206                 (ty::ImplPolarity::Reservation, _) => {
 207                     // FIXME: what amount of WF checking do we need for reservation impls?
 208                 }
 209                 _ => unreachable!(),
 210             }
 211         }
 212         hir::ItemKind::Fn(ref sig, ..) => {
 213             check_item_fn(tcx, def_id, item.ident, item.span, sig.decl);
 214         }
 215         hir::ItemKind::Static(ty, ..) => {
 216             check_item_type(tcx, def_id, ty.span, false);
 217         }
 218         hir::ItemKind::Const(ty, ..) => {
 219             check_item_type(tcx, def_id, ty.span, false);
 220         }
 221         hir::ItemKind::Struct(ref struct_def, ref ast_generics) => {
 222             check_type_defn(tcx, item, false, |wfcx| vec![wfcx.non_enum_variant(struct_def)]);
 223
 224             check_variances_for_type_defn(tcx, item, ast_generics);
 225         }
 226         hir::ItemKind::Union(ref struct_def, ref ast_generics) => {
 227             check_type_defn(tcx, item, true, |wfcx| vec![wfcx.non_enum_variant(struct_def)]);
 228
 229             check_variances_for_type_defn(tcx, item, ast_generics);
 230         }
 231         hir::ItemKind::Enum(ref enum_def, ref ast_generics) => {
 232             check_type_defn(tcx, item, true, |wfcx| wfcx.enum_variants(enum_def));
 233
 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.def_id.def_id;
 250
 251     debug!(
 252         ?item.def_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.def_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.def_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::Rptr(_, _)) {
 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.def_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.def_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)
 414                                 .iter()
 415                                 .copied()
 416                                 .collect::<Vec<_>>(),
 417                             &FxHashSet::default(),
 418                             gat_def_id.def_id,
 419                             gat_generics,
 420                         )
 421                     }
 422                     hir::AssocItemKind::Const => None,
 423                 };
 424
 425                 if let Some(item_required_bounds) = item_required_bounds {
 426                     // Take the intersection of the required bounds for this GAT, and
 427                     // the item_required_bounds which are the ones implied by just
 428                     // this item alone.
 429                     // This is why we use an Option<_>, since we need to distinguish
 430                     // the empty set of bounds from the _uninitialized_ set of bounds.
 431                     if let Some(new_required_bounds) = &mut new_required_bounds {
 432                         new_required_bounds.retain(|b| item_required_bounds.contains(b));
 433                     } else {
 434                         new_required_bounds = Some(item_required_bounds);
 435                     }
 436                 }
 437             }
 438
 439             if let Some(new_required_bounds) = new_required_bounds {
 440                 let required_bounds = required_bounds_by_item.entry(gat_def_id).or_default();
 441                 if new_required_bounds.into_iter().any(|p| required_bounds.insert(p)) {
 442                     // Iterate until our required_bounds no longer change
 443                     // Since they changed here, we should continue the loop
 444                     should_continue = true;
 445                 }
 446             }
 447         }
 448         // We know that this loop will eventually halt, since we only set `should_continue` if the
 449         // `required_bounds` for this item grows. Since we are not creating any new region or type
 450         // variables, the set of all region and type bounds that we could ever insert are limited
 451         // by the number of unique types and regions we observe in a given item.
 452         if !should_continue {
 453             break;
 454         }
 455     }
 456
 457     for (gat_def_id, required_bounds) in required_bounds_by_item {
 458         let gat_item_hir = tcx.hir().expect_trait_item(gat_def_id.def_id);
 459         debug!(?required_bounds);
 460         let param_env = tcx.param_env(gat_def_id);
 461         let gat_hir = gat_item_hir.hir_id();
 462
 463         let mut unsatisfied_bounds: Vec<_> = required_bounds
 464             .into_iter()
 465             .filter(|clause| match clause.kind().skip_binder() {
 466                 ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(a, b)) => {
 467                     !region_known_to_outlive(tcx, gat_hir, param_env, &FxHashSet::default(), a, b)
 468                 }
 469                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(a, b)) => {
 470                     !ty_known_to_outlive(tcx, gat_hir, param_env, &FxHashSet::default(), a, b)
 471                 }
 472                 _ => bug!("Unexpected PredicateKind"),
 473             })
 474             .map(|clause| clause.to_string())
 475             .collect();
 476
 477         // We sort so that order is predictable
 478         unsatisfied_bounds.sort();
 479
 480         if !unsatisfied_bounds.is_empty() {
 481             let plural = pluralize!(unsatisfied_bounds.len());
 482             let mut err = tcx.sess.struct_span_err(
 483                 gat_item_hir.span,
 484                 &format!("missing required bound{} on `{}`", plural, gat_item_hir.ident),
 485             );
 486
 487             let suggestion = format!(
 488                 "{} {}",
 489                 gat_item_hir.generics.add_where_or_trailing_comma(),
 490                 unsatisfied_bounds.join(", "),
 491             );
 492             err.span_suggestion(
 493                 gat_item_hir.generics.tail_span_for_predicate_suggestion(),
 494                 &format!("add the required where clause{plural}"),
 495                 suggestion,
 496                 Applicability::MachineApplicable,
 497             );
 498
 499             let bound =
 500                 if unsatisfied_bounds.len() > 1 { "these bounds are" } else { "this bound is" };
 501             err.note(&format!(
 502                 "{} currently required to ensure that impls have maximum flexibility",
 503                 bound
 504             ));
 505             err.note(
 506                 "we are soliciting feedback, see issue #87479 \
 507                  <https://github.com/rust-lang/rust/issues/87479> \
 508                  for more information",
 509             );
 510
 511             err.emit();
 512         }
 513     }
 514 }
 515
 516 /// Add a new set of predicates to the caller_bounds of an existing param_env.
 517 fn augment_param_env<'tcx>(
 518     tcx: TyCtxt<'tcx>,
 519     param_env: ty::ParamEnv<'tcx>,
 520     new_predicates: Option<&FxHashSet<ty::Predicate<'tcx>>>,
 521 ) -> ty::ParamEnv<'tcx> {
 522     let Some(new_predicates) = new_predicates else {
 523         return param_env;
 524     };
 525
 526     if new_predicates.is_empty() {
 527         return param_env;
 528     }
 529
 530     let bounds =
 531         tcx.mk_predicates(param_env.caller_bounds().iter().chain(new_predicates.iter().cloned()));
 532     // FIXME(compiler-errors): Perhaps there is a case where we need to normalize this
 533     // i.e. traits::normalize_param_env_or_error
 534     ty::ParamEnv::new(bounds, param_env.reveal(), param_env.constness())
 535 }
 536
 537 /// We use the following trait as an example throughout this function.
 538 /// Specifically, let's assume that `to_check` here is the return type
 539 /// of `into_iter`, and the GAT we are checking this for is `Iter`.
 540 /// ```rust,ignore (this code fails due to this lint)
 541 /// trait IntoIter {
 542 ///     type Iter<'a>: Iterator<Item = Self::Item<'a>>;
 543 ///     type Item<'a>;
 544 ///     fn into_iter<'a>(&'a self) -> Self::Iter<'a>;
 545 /// }
 546 /// ```
 547 fn gather_gat_bounds<'tcx, T: TypeFoldable<'tcx>>(
 548     tcx: TyCtxt<'tcx>,
 549     param_env: ty::ParamEnv<'tcx>,
 550     item_hir: hir::HirId,
 551     to_check: T,
 552     wf_tys: &FxHashSet<Ty<'tcx>>,
 553     gat_def_id: LocalDefId,
 554     gat_generics: &'tcx ty::Generics,
 555 ) -> Option<FxHashSet<ty::Predicate<'tcx>>> {
 556     // The bounds we that we would require from `to_check`
 557     let mut bounds = FxHashSet::default();
 558
 559     let (regions, types) = GATSubstCollector::visit(gat_def_id.to_def_id(), to_check);
 560
 561     // If both regions and types are empty, then this GAT isn't in the
 562     // set of types we are checking, and we shouldn't try to do clause analysis
 563     // (particularly, doing so would end up with an empty set of clauses,
 564     // since the current method would require none, and we take the
 565     // intersection of requirements of all methods)
 566     if types.is_empty() && regions.is_empty() {
 567         return None;
 568     }
 569
 570     for (region_a, region_a_idx) in &regions {
 571         // Ignore `'static` lifetimes for the purpose of this lint: it's
 572         // because we know it outlives everything and so doesn't give meaningful
 573         // clues
 574         if let ty::ReStatic = **region_a {
 575             continue;
 576         }
 577         // For each region argument (e.g., `'a` in our example), check for a
 578         // relationship to the type arguments (e.g., `Self`). If there is an
 579         // outlives relationship (`Self: 'a`), then we want to ensure that is
 580         // reflected in a where clause on the GAT itself.
 581         for (ty, ty_idx) in &types {
 582             // In our example, requires that `Self: 'a`
 583             if ty_known_to_outlive(tcx, item_hir, param_env, &wf_tys, *ty, *region_a) {
 584                 debug!(?ty_idx, ?region_a_idx);
 585                 debug!("required clause: {ty} must outlive {region_a}");
 586                 // Translate into the generic parameters of the GAT. In
 587                 // our example, the type was `Self`, which will also be
 588                 // `Self` in the GAT.
 589                 let ty_param = gat_generics.param_at(*ty_idx, tcx);
 590                 let ty_param = tcx
 591                     .mk_ty(ty::Param(ty::ParamTy { index: ty_param.index, name: ty_param.name }));
 592                 // Same for the region. In our example, 'a corresponds
 593                 // to the 'me parameter.
 594                 let region_param = gat_generics.param_at(*region_a_idx, tcx);
 595                 let region_param =
 596                     tcx.mk_region(ty::RegionKind::ReEarlyBound(ty::EarlyBoundRegion {
 597                         def_id: region_param.def_id,
 598                         index: region_param.index,
 599                         name: region_param.name,
 600                     }));
 601                 // The predicate we expect to see. (In our example,
 602                 // `Self: 'me`.)
 603                 let clause =
 604                     ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(ty_param, region_param));
 605                 let clause = tcx.mk_predicate(ty::Binder::dummy(clause));
 606                 bounds.insert(clause);
 607             }
 608         }
 609
 610         // For each region argument (e.g., `'a` in our example), also check for a
 611         // relationship to the other region arguments. If there is an outlives
 612         // relationship, then we want to ensure that is reflected in the where clause
 613         // on the GAT itself.
 614         for (region_b, region_b_idx) in &regions {
 615             // Again, skip `'static` because it outlives everything. Also, we trivially
 616             // know that a region outlives itself.
 617             if ty::ReStatic == **region_b || region_a == region_b {
 618                 continue;
 619             }
 620             if region_known_to_outlive(tcx, item_hir, param_env, &wf_tys, *region_a, *region_b) {
 621                 debug!(?region_a_idx, ?region_b_idx);
 622                 debug!("required clause: {region_a} must outlive {region_b}");
 623                 // Translate into the generic parameters of the GAT.
 624                 let region_a_param = gat_generics.param_at(*region_a_idx, tcx);
 625                 let region_a_param =
 626                     tcx.mk_region(ty::RegionKind::ReEarlyBound(ty::EarlyBoundRegion {
 627                         def_id: region_a_param.def_id,
 628                         index: region_a_param.index,
 629                         name: region_a_param.name,
 630                     }));
 631                 // Same for the region.
 632                 let region_b_param = gat_generics.param_at(*region_b_idx, tcx);
 633                 let region_b_param =
 634                     tcx.mk_region(ty::RegionKind::ReEarlyBound(ty::EarlyBoundRegion {
 635                         def_id: region_b_param.def_id,
 636                         index: region_b_param.index,
 637                         name: region_b_param.name,
 638                     }));
 639                 // The predicate we expect to see.
 640                 let clause = ty::PredicateKind::RegionOutlives(ty::OutlivesPredicate(
 641                     region_a_param,
 642                     region_b_param,
 643                 ));
 644                 let clause = tcx.mk_predicate(ty::Binder::dummy(clause));
 645                 bounds.insert(clause);
 646             }
 647         }
 648     }
 649
 650     Some(bounds)
 651 }
 652
 653 /// Given a known `param_env` and a set of well formed types, can we prove that
 654 /// `ty` outlives `region`.
 655 fn ty_known_to_outlive<'tcx>(
 656     tcx: TyCtxt<'tcx>,
 657     id: hir::HirId,
 658     param_env: ty::ParamEnv<'tcx>,
 659     wf_tys: &FxHashSet<Ty<'tcx>>,
 660     ty: Ty<'tcx>,
 661     region: ty::Region<'tcx>,
 662 ) -> bool {
 663     resolve_regions_with_wf_tys(tcx, id, param_env, &wf_tys, |infcx, region_bound_pairs| {
 664         let origin = infer::RelateParamBound(DUMMY_SP, ty, None);
 665         let outlives = &mut TypeOutlives::new(infcx, tcx, region_bound_pairs, None, param_env);
 666         outlives.type_must_outlive(origin, ty, region, ConstraintCategory::BoringNoLocation);
 667     })
 668 }
 669
 670 /// Given a known `param_env` and a set of well formed types, can we prove that
 671 /// `region_a` outlives `region_b`
 672 fn region_known_to_outlive<'tcx>(
 673     tcx: TyCtxt<'tcx>,
 674     id: hir::HirId,
 675     param_env: ty::ParamEnv<'tcx>,
 676     wf_tys: &FxHashSet<Ty<'tcx>>,
 677     region_a: ty::Region<'tcx>,
 678     region_b: ty::Region<'tcx>,
 679 ) -> bool {
 680     resolve_regions_with_wf_tys(tcx, id, param_env, &wf_tys, |mut infcx, _| {
 681         use rustc_infer::infer::outlives::obligations::TypeOutlivesDelegate;
 682         let origin = infer::RelateRegionParamBound(DUMMY_SP);
 683         // `region_a: region_b` -> `region_b <= region_a`
 684         infcx.push_sub_region_constraint(
 685             origin,
 686             region_b,
 687             region_a,
 688             ConstraintCategory::BoringNoLocation,
 689         );
 690     })
 691 }
 692
 693 /// Given a known `param_env` and a set of well formed types, set up an
 694 /// `InferCtxt`, call the passed function (to e.g. set up region constraints
 695 /// to be tested), then resolve region and return errors
 696 fn resolve_regions_with_wf_tys<'tcx>(
 697     tcx: TyCtxt<'tcx>,
 698     id: hir::HirId,
 699     param_env: ty::ParamEnv<'tcx>,
 700     wf_tys: &FxHashSet<Ty<'tcx>>,
 701     add_constraints: impl for<'a> FnOnce(&'a InferCtxt<'tcx>, &'a RegionBoundPairs<'tcx>),
 702 ) -> bool {
 703     // Unfortunately, we have to use a new `InferCtxt` each call, because
 704     // region constraints get added and solved there and we need to test each
 705     // call individually.
 706     let infcx = tcx.infer_ctxt().build();
 707     let outlives_environment = OutlivesEnvironment::with_bounds(
 708         param_env,
 709         Some(&infcx),
 710         infcx.implied_bounds_tys(param_env, id, wf_tys.clone()),
 711     );
 712     let region_bound_pairs = outlives_environment.region_bound_pairs();
 713
 714     add_constraints(&infcx, region_bound_pairs);
 715
 716     let errors = infcx.resolve_regions(&outlives_environment);
 717
 718     debug!(?errors, "errors");
 719
 720     // If we were able to prove that the type outlives the region without
 721     // an error, it must be because of the implied or explicit bounds...
 722     errors.is_empty()
 723 }
 724
 725 /// TypeVisitor that looks for uses of GATs like
 726 /// `<P0 as Trait<P1..Pn>>::GAT<Pn..Pm>` and adds the arguments `P0..Pm` into
 727 /// the two vectors, `regions` and `types` (depending on their kind). For each
 728 /// parameter `Pi` also track the index `i`.
 729 struct GATSubstCollector<'tcx> {
 730     gat: DefId,
 731     // Which region appears and which parameter index its substituted for
 732     regions: FxHashSet<(ty::Region<'tcx>, usize)>,
 733     // Which params appears and which parameter index its substituted for
 734     types: FxHashSet<(Ty<'tcx>, usize)>,
 735 }
 736
 737 impl<'tcx> GATSubstCollector<'tcx> {
 738     fn visit<T: TypeFoldable<'tcx>>(
 739         gat: DefId,
 740         t: T,
 741     ) -> (FxHashSet<(ty::Region<'tcx>, usize)>, FxHashSet<(Ty<'tcx>, usize)>) {
 742         let mut visitor =
 743             GATSubstCollector { gat, regions: FxHashSet::default(), types: FxHashSet::default() };
 744         t.visit_with(&mut visitor);
 745         (visitor.regions, visitor.types)
 746     }
 747 }
 748
 749 impl<'tcx> TypeVisitor<'tcx> for GATSubstCollector<'tcx> {
 750     type BreakTy = !;
 751
 752     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 753         match t.kind() {
 754             ty::Projection(p) if p.item_def_id == self.gat => {
 755                 for (idx, subst) in p.substs.iter().enumerate() {
 756                     match subst.unpack() {
 757                         GenericArgKind::Lifetime(lt) if !lt.is_late_bound() => {
 758                             self.regions.insert((lt, idx));
 759                         }
 760                         GenericArgKind::Type(t) => {
 761                             self.types.insert((t, idx));
 762                         }
 763                         _ => {}
 764                     }
 765                 }
 766             }
 767             _ => {}
 768         }
 769         t.super_visit_with(self)
 770     }
 771 }
 772
 773 fn could_be_self(trait_def_id: LocalDefId, ty: &hir::Ty<'_>) -> bool {
 774     match ty.kind {
 775         hir::TyKind::TraitObject([trait_ref], ..) => match trait_ref.trait_ref.path.segments {
 776             [s] => s.res.opt_def_id() == Some(trait_def_id.to_def_id()),
 777             _ => false,
 778         },
 779         _ => false,
 780     }
 781 }
 782
 783 /// Detect when an object unsafe trait is referring to itself in one of its associated items.
 784 /// When this is done, suggest using `Self` instead.
 785 fn check_object_unsafe_self_trait_by_name(tcx: TyCtxt<'_>, item: &hir::TraitItem<'_>) {
 786     let (trait_name, trait_def_id) =
 787         match tcx.hir().get_by_def_id(tcx.hir().get_parent_item(item.hir_id()).def_id) {
 788             hir::Node::Item(item) => match item.kind {
 789                 hir::ItemKind::Trait(..) => (item.ident, item.def_id),
 790                 _ => return,
 791             },
 792             _ => return,
 793         };
 794     let mut trait_should_be_self = vec![];
 795     match &item.kind {
 796         hir::TraitItemKind::Const(ty, _) | hir::TraitItemKind::Type(_, Some(ty))
 797             if could_be_self(trait_def_id.def_id, ty) =>
 798         {
 799             trait_should_be_self.push(ty.span)
 800         }
 801         hir::TraitItemKind::Fn(sig, _) => {
 802             for ty in sig.decl.inputs {
 803                 if could_be_self(trait_def_id.def_id, ty) {
 804                     trait_should_be_self.push(ty.span);
 805                 }
 806             }
 807             match sig.decl.output {
 808                 hir::FnRetTy::Return(ty) if could_be_self(trait_def_id.def_id, ty) => {
 809                     trait_should_be_self.push(ty.span);
 810                 }
 811                 _ => {}
 812             }
 813         }
 814         _ => {}
 815     }
 816     if !trait_should_be_self.is_empty() {
 817         if tcx.object_safety_violations(trait_def_id).is_empty() {
 818             return;
 819         }
 820         let sugg = trait_should_be_self.iter().map(|span| (*span, "Self".to_string())).collect();
 821         tcx.sess
 822             .struct_span_err(
 823                 trait_should_be_self,
 824                 "associated item referring to unboxed trait object for its own trait",
 825             )
 826             .span_label(trait_name.span, "in this trait")
 827             .multipart_suggestion(
 828                 "you might have meant to use `Self` to refer to the implementing type",
 829                 sugg,
 830                 Applicability::MachineApplicable,
 831             )
 832             .emit();
 833     }
 834 }
 835
 836 fn check_impl_item(tcx: TyCtxt<'_>, impl_item: &hir::ImplItem<'_>) {
 837     let (method_sig, span) = match impl_item.kind {
 838         hir::ImplItemKind::Fn(ref sig, _) => (Some(sig), impl_item.span),
 839         // Constrain binding and overflow error spans to `<Ty>` in `type foo = <Ty>`.
 840         hir::ImplItemKind::TyAlias(ty) if ty.span != DUMMY_SP => (None, ty.span),
 841         _ => (None, impl_item.span),
 842     };
 843
 844     check_associated_item(tcx, impl_item.def_id.def_id, span, method_sig);
 845 }
 846
 847 fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) {
 848     match param.kind {
 849         // We currently only check wf of const params here.
 850         hir::GenericParamKind::Lifetime { .. } | hir::GenericParamKind::Type { .. } => (),
 851
 852         // Const parameters are well formed if their type is structural match.
 853         hir::GenericParamKind::Const { ty: hir_ty, default: _ } => {
 854             let ty = tcx.type_of(tcx.hir().local_def_id(param.hir_id));
 855
 856             if tcx.features().adt_const_params {
 857                 if let Some(non_structural_match_ty) =
 858                     traits::search_for_adt_const_param_violation(param.span, tcx, ty)
 859                 {
 860                     // We use the same error code in both branches, because this is really the same
 861                     // issue: we just special-case the message for type parameters to make it
 862                     // clearer.
 863                     match non_structural_match_ty.kind() {
 864                         ty::Param(_) => {
 865                             // Const parameters may not have type parameters as their types,
 866                             // because we cannot be sure that the type parameter derives `PartialEq`
 867                             // and `Eq` (just implementing them is not enough for `structural_match`).
 868                             struct_span_err!(
 869                                 tcx.sess,
 870                                 hir_ty.span,
 871                                 E0741,
 872                                 "`{ty}` is not guaranteed to `#[derive(PartialEq, Eq)]`, so may not be \
 873                                 used as the type of a const parameter",
 874                             )
 875                             .span_label(
 876                                 hir_ty.span,
 877                                 format!("`{ty}` may not derive both `PartialEq` and `Eq`"),
 878                             )
 879                             .note(
 880                                 "it is not currently possible to use a type parameter as the type of a \
 881                                 const parameter",
 882                             )
 883                             .emit();
 884                         }
 885                         ty::Float(_) => {
 886                             struct_span_err!(
 887                                 tcx.sess,
 888                                 hir_ty.span,
 889                                 E0741,
 890                                 "`{ty}` is forbidden as the type of a const generic parameter",
 891                             )
 892                             .note("floats do not derive `Eq` or `Ord`, which are required for const parameters")
 893                             .emit();
 894                         }
 895                         ty::FnPtr(_) => {
 896                             struct_span_err!(
 897                                 tcx.sess,
 898                                 hir_ty.span,
 899                                 E0741,
 900                                 "using function pointers as const generic parameters is forbidden",
 901                             )
 902                             .emit();
 903                         }
 904                         ty::RawPtr(_) => {
 905                             struct_span_err!(
 906                                 tcx.sess,
 907                                 hir_ty.span,
 908                                 E0741,
 909                                 "using raw pointers as const generic parameters is forbidden",
 910                             )
 911                             .emit();
 912                         }
 913                         _ => {
 914                             let mut diag = struct_span_err!(
 915                                 tcx.sess,
 916                                 hir_ty.span,
 917                                 E0741,
 918                                 "`{}` must be annotated with `#[derive(PartialEq, Eq)]` to be used as \
 919                                 the type of a const parameter",
 920                                 non_structural_match_ty,
 921                             );
 922
 923                             if ty == non_structural_match_ty {
 924                                 diag.span_label(
 925                                     hir_ty.span,
 926                                     format!("`{ty}` doesn't derive both `PartialEq` and `Eq`"),
 927                                 );
 928                             }
 929
 930                             diag.emit();
 931                         }
 932                     }
 933                 }
 934             } else {
 935                 let err_ty_str;
 936                 let mut is_ptr = true;
 937
 938                 let err = match ty.kind() {
 939                     ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Error(_) => None,
 940                     ty::FnPtr(_) => Some("function pointers"),
 941                     ty::RawPtr(_) => Some("raw pointers"),
 942                     _ => {
 943                         is_ptr = false;
 944                         err_ty_str = format!("`{ty}`");
 945                         Some(err_ty_str.as_str())
 946                     }
 947                 };
 948
 949                 if let Some(unsupported_type) = err {
 950                     if is_ptr {
 951                         tcx.sess.span_err(
 952                             hir_ty.span,
 953                             &format!(
 954                                 "using {unsupported_type} as const generic parameters is forbidden",
 955                             ),
 956                         );
 957                     } else {
 958                         let mut err = tcx.sess.struct_span_err(
 959                             hir_ty.span,
 960                             &format!(
 961                                 "{unsupported_type} is forbidden as the type of a const generic parameter",
 962                             ),
 963                         );
 964                         err.note("the only supported types are integers, `bool` and `char`");
 965                         if tcx.sess.is_nightly_build() {
 966                             err.help(
 967                             "more complex types are supported with `#![feature(adt_const_params)]`",
 968                         );
 969                         }
 970                         err.emit();
 971                     }
 972                 }
 973             }
 974         }
 975     }
 976 }
 977
 978 #[instrument(level = "debug", skip(tcx, span, sig_if_method))]
 979 fn check_associated_item(
 980     tcx: TyCtxt<'_>,
 981     item_id: LocalDefId,
 982     span: Span,
 983     sig_if_method: Option<&hir::FnSig<'_>>,
 984 ) {
 985     let loc = Some(WellFormedLoc::Ty(item_id));
 986     enter_wf_checking_ctxt(tcx, span, item_id, |wfcx| {
 987         let item = tcx.associated_item(item_id);
 988
 989         let self_ty = match item.container {
 990             ty::TraitContainer => tcx.types.self_param,
 991             ty::ImplContainer => tcx.type_of(item.container_id(tcx)),
 992         };
 993
 994         match item.kind {
 995             ty::AssocKind::Const => {
 996                 let ty = tcx.type_of(item.def_id);
 997                 let ty = wfcx.normalize(span, Some(WellFormedLoc::Ty(item_id)), ty);
 998                 wfcx.register_wf_obligation(span, loc, ty.into());
 999             }
1000             ty::AssocKind::Fn => {
1001                 let sig = tcx.fn_sig(item.def_id);
1002                 let hir_sig = sig_if_method.expect("bad signature for method");
1003                 check_fn_or_method(
1004                     wfcx,
1005                     item.ident(tcx).span,
1006                     sig,
1007                     hir_sig.decl,
1008                     item.def_id.expect_local(),
1009                 );
1010                 check_method_receiver(wfcx, hir_sig, item, self_ty);
1011             }
1012             ty::AssocKind::Type => {
1013                 if let ty::AssocItemContainer::TraitContainer = item.container {
1014                     check_associated_type_bounds(wfcx, item, span)
1015                 }
1016                 if item.defaultness(tcx).has_value() {
1017                     let ty = tcx.type_of(item.def_id);
1018                     let ty = wfcx.normalize(span, Some(WellFormedLoc::Ty(item_id)), ty);
1019                     wfcx.register_wf_obligation(span, loc, ty.into());
1020                 }
1021             }
1022         }
1023     })
1024 }
1025
1026 fn item_adt_kind(kind: &ItemKind<'_>) -> Option<AdtKind> {
1027     match kind {
1028         ItemKind::Struct(..) => Some(AdtKind::Struct),
1029         ItemKind::Union(..) => Some(AdtKind::Union),
1030         ItemKind::Enum(..) => Some(AdtKind::Enum),
1031         _ => None,
1032     }
1033 }
1034
1035 /// In a type definition, we check that to ensure that the types of the fields are well-formed.
1036 fn check_type_defn<'tcx, F>(
1037     tcx: TyCtxt<'tcx>,
1038     item: &hir::Item<'tcx>,
1039     all_sized: bool,
1040     mut lookup_fields: F,
1041 ) where
1042     F: FnMut(&WfCheckingCtxt<'_, 'tcx>) -> Vec<AdtVariant<'tcx>>,
1043 {
1044     enter_wf_checking_ctxt(tcx, item.span, item.def_id.def_id, |wfcx| {
1045         let variants = lookup_fields(wfcx);
1046         let packed = tcx.adt_def(item.def_id).repr().packed();
1047
1048         for variant in &variants {
1049             // All field types must be well-formed.
1050             for field in &variant.fields {
1051                 wfcx.register_wf_obligation(
1052                     field.span,
1053                     Some(WellFormedLoc::Ty(field.def_id)),
1054                     field.ty.into(),
1055                 )
1056             }
1057
1058             // For DST, or when drop needs to copy things around, all
1059             // intermediate types must be sized.
1060             let needs_drop_copy = || {
1061                 packed && {
1062                     let ty = variant.fields.last().unwrap().ty;
1063                     let ty = tcx.erase_regions(ty);
1064                     if ty.needs_infer() {
1065                         tcx.sess
1066                             .delay_span_bug(item.span, &format!("inference variables in {:?}", ty));
1067                         // Just treat unresolved type expression as if it needs drop.
1068                         true
1069                     } else {
1070                         ty.needs_drop(tcx, tcx.param_env(item.def_id))
1071                     }
1072                 }
1073             };
1074             // All fields (except for possibly the last) should be sized.
1075             let all_sized = all_sized || variant.fields.is_empty() || needs_drop_copy();
1076             let unsized_len = if all_sized { 0 } else { 1 };
1077             for (idx, field) in
1078                 variant.fields[..variant.fields.len() - unsized_len].iter().enumerate()
1079             {
1080                 let last = idx == variant.fields.len() - 1;
1081                 wfcx.register_bound(
1082                     traits::ObligationCause::new(
1083                         field.span,
1084                         wfcx.body_id,
1085                         traits::FieldSized {
1086                             adt_kind: match item_adt_kind(&item.kind) {
1087                                 Some(i) => i,
1088                                 None => bug!(),
1089                             },
1090                             span: field.span,
1091                             last,
1092                         },
1093                     ),
1094                     wfcx.param_env,
1095                     field.ty,
1096                     tcx.require_lang_item(LangItem::Sized, None),
1097                 );
1098             }
1099
1100             // Explicit `enum` discriminant values must const-evaluate successfully.
1101             if let Some(discr_def_id) = variant.explicit_discr {
1102                 let discr_substs = InternalSubsts::identity_for_item(tcx, discr_def_id.to_def_id());
1103
1104                 let cause = traits::ObligationCause::new(
1105                     tcx.def_span(discr_def_id),
1106                     wfcx.body_id,
1107                     traits::MiscObligation,
1108                 );
1109                 wfcx.register_obligation(traits::Obligation::new(
1110                     cause,
1111                     wfcx.param_env,
1112                     ty::Binder::dummy(ty::PredicateKind::ConstEvaluatable(
1113                         ty::UnevaluatedConst::new(
1114                             ty::WithOptConstParam::unknown(discr_def_id.to_def_id()),
1115                             discr_substs,
1116                         ),
1117                     ))
1118                     .to_predicate(tcx),
1119                 ));
1120             }
1121         }
1122
1123         check_where_clauses(wfcx, item.span, item.def_id.def_id);
1124     });
1125 }
1126
1127 #[instrument(skip(tcx, item))]
1128 fn check_trait(tcx: TyCtxt<'_>, item: &hir::Item<'_>) {
1129     debug!(?item.def_id);
1130
1131     let def_id = item.def_id.def_id;
1132     let trait_def = tcx.trait_def(def_id);
1133     if trait_def.is_marker
1134         || matches!(trait_def.specialization_kind, TraitSpecializationKind::Marker)
1135     {
1136         for associated_def_id in &*tcx.associated_item_def_ids(def_id) {
1137             struct_span_err!(
1138                 tcx.sess,
1139                 tcx.def_span(*associated_def_id),
1140                 E0714,
1141                 "marker traits cannot have associated items",
1142             )
1143             .emit();
1144         }
1145     }
1146
1147     enter_wf_checking_ctxt(tcx, item.span, def_id, |wfcx| {
1148         check_where_clauses(wfcx, item.span, def_id)
1149     });
1150
1151     // Only check traits, don't check trait aliases
1152     if let hir::ItemKind::Trait(_, _, _, _, items) = item.kind {
1153         check_gat_where_clauses(tcx, items);
1154     }
1155 }
1156
1157 /// Checks all associated type defaults of trait `trait_def_id`.
1158 ///
1159 /// Assuming the defaults are used, check that all predicates (bounds on the
1160 /// assoc type and where clauses on the trait) hold.
1161 fn check_associated_type_bounds(wfcx: &WfCheckingCtxt<'_, '_>, item: &ty::AssocItem, span: Span) {
1162     let bounds = wfcx.tcx().explicit_item_bounds(item.def_id);
1163
1164     debug!("check_associated_type_bounds: bounds={:?}", bounds);
1165     let wf_obligations = bounds.iter().flat_map(|&(bound, bound_span)| {
1166         let normalized_bound = wfcx.normalize(span, None, bound);
1167         traits::wf::predicate_obligations(
1168             wfcx.infcx,
1169             wfcx.param_env,
1170             wfcx.body_id,
1171             normalized_bound,
1172             bound_span,
1173         )
1174     });
1175
1176     wfcx.register_obligations(wf_obligations);
1177 }
1178
1179 fn check_item_fn(
1180     tcx: TyCtxt<'_>,
1181     def_id: LocalDefId,
1182     ident: Ident,
1183     span: Span,
1184     decl: &hir::FnDecl<'_>,
1185 ) {
1186     enter_wf_checking_ctxt(tcx, span, def_id, |wfcx| {
1187         let sig = tcx.fn_sig(def_id);
1188         check_fn_or_method(wfcx, ident.span, sig, decl, def_id);
1189     })
1190 }
1191
1192 fn check_item_type(tcx: TyCtxt<'_>, item_id: LocalDefId, ty_span: Span, allow_foreign_ty: bool) {
1193     debug!("check_item_type: {:?}", item_id);
1194
1195     enter_wf_checking_ctxt(tcx, ty_span, item_id, |wfcx| {
1196         let ty = tcx.type_of(item_id);
1197         let item_ty = wfcx.normalize(ty_span, Some(WellFormedLoc::Ty(item_id)), ty);
1198
1199         let mut forbid_unsized = true;
1200         if allow_foreign_ty {
1201             let tail = tcx.struct_tail_erasing_lifetimes(item_ty, wfcx.param_env);
1202             if let ty::Foreign(_) = tail.kind() {
1203                 forbid_unsized = false;
1204             }
1205         }
1206
1207         wfcx.register_wf_obligation(ty_span, Some(WellFormedLoc::Ty(item_id)), item_ty.into());
1208         if forbid_unsized {
1209             wfcx.register_bound(
1210                 traits::ObligationCause::new(ty_span, wfcx.body_id, traits::WellFormed(None)),
1211                 wfcx.param_env,
1212                 item_ty,
1213                 tcx.require_lang_item(LangItem::Sized, None),
1214             );
1215         }
1216
1217         // Ensure that the end result is `Sync` in a non-thread local `static`.
1218         let should_check_for_sync = tcx.static_mutability(item_id.to_def_id())
1219             == Some(hir::Mutability::Not)
1220             && !tcx.is_foreign_item(item_id.to_def_id())
1221             && !tcx.is_thread_local_static(item_id.to_def_id());
1222
1223         if should_check_for_sync {
1224             wfcx.register_bound(
1225                 traits::ObligationCause::new(ty_span, wfcx.body_id, traits::SharedStatic),
1226                 wfcx.param_env,
1227                 item_ty,
1228                 tcx.require_lang_item(LangItem::Sync, Some(ty_span)),
1229             );
1230         }
1231     });
1232 }
1233
1234 #[instrument(level = "debug", skip(tcx, ast_self_ty, ast_trait_ref))]
1235 fn check_impl<'tcx>(
1236     tcx: TyCtxt<'tcx>,
1237     item: &'tcx hir::Item<'tcx>,
1238     ast_self_ty: &hir::Ty<'_>,
1239     ast_trait_ref: &Option<hir::TraitRef<'_>>,
1240     constness: hir::Constness,
1241 ) {
1242     enter_wf_checking_ctxt(tcx, item.span, item.def_id.def_id, |wfcx| {
1243         match *ast_trait_ref {
1244             Some(ref ast_trait_ref) => {
1245                 // `#[rustc_reservation_impl]` impls are not real impls and
1246                 // therefore don't need to be WF (the trait's `Self: Trait` predicate
1247                 // won't hold).
1248                 let trait_ref = tcx.impl_trait_ref(item.def_id).unwrap();
1249                 let trait_ref = wfcx.normalize(ast_trait_ref.path.span, None, trait_ref);
1250                 let trait_pred = ty::TraitPredicate {
1251                     trait_ref,
1252                     constness: match constness {
1253                         hir::Constness::Const => ty::BoundConstness::ConstIfConst,
1254                         hir::Constness::NotConst => ty::BoundConstness::NotConst,
1255                     },
1256                     polarity: ty::ImplPolarity::Positive,
1257                 };
1258                 let obligations = traits::wf::trait_obligations(
1259                     wfcx.infcx,
1260                     wfcx.param_env,
1261                     wfcx.body_id,
1262                     &trait_pred,
1263                     ast_trait_ref.path.span,
1264                     item,
1265                 );
1266                 debug!(?obligations);
1267                 wfcx.register_obligations(obligations);
1268             }
1269             None => {
1270                 let self_ty = tcx.type_of(item.def_id);
1271                 let self_ty = wfcx.normalize(
1272                     item.span,
1273                     Some(WellFormedLoc::Ty(item.hir_id().expect_owner().def_id)),
1274                     self_ty,
1275                 );
1276                 wfcx.register_wf_obligation(
1277                     ast_self_ty.span,
1278                     Some(WellFormedLoc::Ty(item.hir_id().expect_owner().def_id)),
1279                     self_ty.into(),
1280                 );
1281             }
1282         }
1283
1284         check_where_clauses(wfcx, item.span, item.def_id.def_id);
1285     });
1286 }
1287
1288 /// Checks where-clauses and inline bounds that are declared on `def_id`.
1289 #[instrument(level = "debug", skip(wfcx))]
1290 fn check_where_clauses<'tcx>(wfcx: &WfCheckingCtxt<'_, 'tcx>, span: Span, def_id: LocalDefId) {
1291     let infcx = wfcx.infcx;
1292     let tcx = wfcx.tcx();
1293
1294     let predicates = tcx.bound_predicates_of(def_id.to_def_id());
1295     let generics = tcx.generics_of(def_id);
1296
1297     let is_our_default = |def: &ty::GenericParamDef| match def.kind {
1298         GenericParamDefKind::Type { has_default, .. }
1299         | GenericParamDefKind::Const { has_default } => {
1300             has_default && def.index >= generics.parent_count as u32
1301         }
1302         GenericParamDefKind::Lifetime => unreachable!(),
1303     };
1304
1305     // Check that concrete defaults are well-formed. See test `type-check-defaults.rs`.
1306     // For example, this forbids the declaration:
1307     //
1308     //     struct Foo<T = Vec<[u32]>> { .. }
1309     //
1310     // Here, the default `Vec<[u32]>` is not WF because `[u32]: Sized` does not hold.
1311     for param in &generics.params {
1312         match param.kind {
1313             GenericParamDefKind::Type { .. } => {
1314                 if is_our_default(param) {
1315                     let ty = tcx.type_of(param.def_id);
1316                     // Ignore dependent defaults -- that is, where the default of one type
1317                     // parameter includes another (e.g., `<T, U = T>`). In those cases, we can't
1318                     // be sure if it will error or not as user might always specify the other.
1319                     if !ty.needs_subst() {
1320                         wfcx.register_wf_obligation(
1321                             tcx.def_span(param.def_id),
1322                             Some(WellFormedLoc::Ty(param.def_id.expect_local())),
1323                             ty.into(),
1324                         );
1325                     }
1326                 }
1327             }
1328             GenericParamDefKind::Const { .. } => {
1329                 if is_our_default(param) {
1330                     // FIXME(const_generics_defaults): This
1331                     // is incorrect when dealing with unused substs, for example
1332                     // for `struct Foo<const N: usize, const M: usize = { 1 - 2 }>`
1333                     // we should eagerly error.
1334                     let default_ct = tcx.const_param_default(param.def_id);
1335                     if !default_ct.needs_subst() {
1336                         wfcx.register_wf_obligation(
1337                             tcx.def_span(param.def_id),
1338                             None,
1339                             default_ct.into(),
1340                         );
1341                     }
1342                 }
1343             }
1344             // Doesn't have defaults.
1345             GenericParamDefKind::Lifetime => {}
1346         }
1347     }
1348
1349     // Check that trait predicates are WF when params are substituted by their defaults.
1350     // We don't want to overly constrain the predicates that may be written but we want to
1351     // catch cases where a default my never be applied such as `struct Foo<T: Copy = String>`.
1352     // Therefore we check if a predicate which contains a single type param
1353     // with a concrete default is WF with that default substituted.
1354     // For more examples see tests `defaults-well-formedness.rs` and `type-check-defaults.rs`.
1355     //
1356     // First we build the defaulted substitution.
1357     let substs = InternalSubsts::for_item(tcx, def_id.to_def_id(), |param, _| {
1358         match param.kind {
1359             GenericParamDefKind::Lifetime => {
1360                 // All regions are identity.
1361                 tcx.mk_param_from_def(param)
1362             }
1363
1364             GenericParamDefKind::Type { .. } => {
1365                 // If the param has a default, ...
1366                 if is_our_default(param) {
1367                     let default_ty = tcx.type_of(param.def_id);
1368                     // ... and it's not a dependent default, ...
1369                     if !default_ty.needs_subst() {
1370                         // ... then substitute it with the default.
1371                         return default_ty.into();
1372                     }
1373                 }
1374
1375                 tcx.mk_param_from_def(param)
1376             }
1377             GenericParamDefKind::Const { .. } => {
1378                 // If the param has a default, ...
1379                 if is_our_default(param) {
1380                     let default_ct = tcx.const_param_default(param.def_id);
1381                     // ... and it's not a dependent default, ...
1382                     if !default_ct.needs_subst() {
1383                         // ... then substitute it with the default.
1384                         return default_ct.into();
1385                     }
1386                 }
1387
1388                 tcx.mk_param_from_def(param)
1389             }
1390         }
1391     });
1392
1393     // Now we build the substituted predicates.
1394     let default_obligations = predicates
1395         .0
1396         .predicates
1397         .iter()
1398         .flat_map(|&(pred, sp)| {
1399             #[derive(Default)]
1400             struct CountParams {
1401                 params: FxHashSet<u32>,
1402             }
1403             impl<'tcx> ty::visit::TypeVisitor<'tcx> for CountParams {
1404                 type BreakTy = ();
1405
1406                 fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
1407                     if let ty::Param(param) = t.kind() {
1408                         self.params.insert(param.index);
1409                     }
1410                     t.super_visit_with(self)
1411                 }
1412
1413                 fn visit_region(&mut self, _: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
1414                     ControlFlow::BREAK
1415                 }
1416
1417                 fn visit_const(&mut self, c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
1418                     if let ty::ConstKind::Param(param) = c.kind() {
1419                         self.params.insert(param.index);
1420                     }
1421                     c.super_visit_with(self)
1422                 }
1423             }
1424             let mut param_count = CountParams::default();
1425             let has_region = pred.visit_with(&mut param_count).is_break();
1426             let substituted_pred = predicates.rebind(pred).subst(tcx, substs);
1427             // Don't check non-defaulted params, dependent defaults (including lifetimes)
1428             // or preds with multiple params.
1429             if substituted_pred.has_non_region_param() || param_count.params.len() > 1 || has_region
1430             {
1431                 None
1432             } else if predicates.0.predicates.iter().any(|&(p, _)| p == substituted_pred) {
1433                 // Avoid duplication of predicates that contain no parameters, for example.
1434                 None
1435             } else {
1436                 Some((substituted_pred, sp))
1437             }
1438         })
1439         .map(|(pred, sp)| {
1440             // Convert each of those into an obligation. So if you have
1441             // something like `struct Foo<T: Copy = String>`, we would
1442             // take that predicate `T: Copy`, substitute to `String: Copy`
1443             // (actually that happens in the previous `flat_map` call),
1444             // and then try to prove it (in this case, we'll fail).
1445             //
1446             // Note the subtle difference from how we handle `predicates`
1447             // below: there, we are not trying to prove those predicates
1448             // to be *true* but merely *well-formed*.
1449             let pred = wfcx.normalize(sp, None, pred);
1450             let cause = traits::ObligationCause::new(
1451                 sp,
1452                 wfcx.body_id,
1453                 traits::ItemObligation(def_id.to_def_id()),
1454             );
1455             traits::Obligation::new(cause, wfcx.param_env, pred)
1456         });
1457
1458     let predicates = predicates.0.instantiate_identity(tcx);
1459
1460     let predicates = wfcx.normalize(span, None, predicates);
1461
1462     debug!(?predicates.predicates);
1463     assert_eq!(predicates.predicates.len(), predicates.spans.len());
1464     let wf_obligations =
1465         iter::zip(&predicates.predicates, &predicates.spans).flat_map(|(&p, &sp)| {
1466             traits::wf::predicate_obligations(
1467                 infcx,
1468                 wfcx.param_env.without_const(),
1469                 wfcx.body_id,
1470                 p,
1471                 sp,
1472             )
1473         });
1474
1475     let obligations: Vec<_> = wf_obligations.chain(default_obligations).collect();
1476     wfcx.register_obligations(obligations);
1477 }
1478
1479 #[instrument(level = "debug", skip(wfcx, span, hir_decl))]
1480 fn check_fn_or_method<'tcx>(
1481     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1482     span: Span,
1483     sig: ty::PolyFnSig<'tcx>,
1484     hir_decl: &hir::FnDecl<'_>,
1485     def_id: LocalDefId,
1486 ) {
1487     let tcx = wfcx.tcx();
1488     let sig = tcx.liberate_late_bound_regions(def_id.to_def_id(), sig);
1489
1490     // Normalize the input and output types one at a time, using a different
1491     // `WellFormedLoc` for each. We cannot call `normalize_associated_types`
1492     // on the entire `FnSig`, since this would use the same `WellFormedLoc`
1493     // for each type, preventing the HIR wf check from generating
1494     // a nice error message.
1495     let ty::FnSig { mut inputs_and_output, c_variadic, unsafety, abi } = sig;
1496     inputs_and_output = tcx.mk_type_list(inputs_and_output.iter().enumerate().map(|(i, ty)| {
1497         wfcx.normalize(
1498             span,
1499             Some(WellFormedLoc::Param {
1500                 function: def_id,
1501                 // Note that the `param_idx` of the output type is
1502                 // one greater than the index of the last input type.
1503                 param_idx: i.try_into().unwrap(),
1504             }),
1505             ty,
1506         )
1507     }));
1508     // Manually call `normalize_associated_types_in` on the other types
1509     // in `FnSig`. This ensures that if the types of these fields
1510     // ever change to include projections, we will start normalizing
1511     // them automatically.
1512     let sig = ty::FnSig {
1513         inputs_and_output,
1514         c_variadic: wfcx.normalize(span, None, c_variadic),
1515         unsafety: wfcx.normalize(span, None, unsafety),
1516         abi: wfcx.normalize(span, None, abi),
1517     };
1518
1519     for (i, (&input_ty, ty)) in iter::zip(sig.inputs(), hir_decl.inputs).enumerate() {
1520         wfcx.register_wf_obligation(
1521             ty.span,
1522             Some(WellFormedLoc::Param { function: def_id, param_idx: i.try_into().unwrap() }),
1523             input_ty.into(),
1524         );
1525     }
1526
1527     wfcx.register_wf_obligation(
1528         hir_decl.output.span(),
1529         Some(WellFormedLoc::Param {
1530             function: def_id,
1531             param_idx: sig.inputs().len().try_into().unwrap(),
1532         }),
1533         sig.output().into(),
1534     );
1535
1536     check_where_clauses(wfcx, span, def_id);
1537
1538     check_return_position_impl_trait_in_trait_bounds(
1539         tcx,
1540         wfcx,
1541         def_id,
1542         sig.output(),
1543         hir_decl.output.span(),
1544     );
1545 }
1546
1547 /// Basically `check_associated_type_bounds`, but separated for now and should be
1548 /// deduplicated when RPITITs get lowered into real associated items.
1549 fn check_return_position_impl_trait_in_trait_bounds<'tcx>(
1550     tcx: TyCtxt<'tcx>,
1551     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1552     fn_def_id: LocalDefId,
1553     fn_output: Ty<'tcx>,
1554     span: Span,
1555 ) {
1556     if let Some(assoc_item) = tcx.opt_associated_item(fn_def_id.to_def_id())
1557         && assoc_item.container == ty::AssocItemContainer::TraitContainer
1558     {
1559         for arg in fn_output.walk() {
1560             if let ty::GenericArgKind::Type(ty) = arg.unpack()
1561                 && let ty::Projection(proj) = ty.kind()
1562                 && tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder
1563                 && tcx.impl_trait_in_trait_parent(proj.item_def_id) == fn_def_id.to_def_id()
1564             {
1565                 let bounds = wfcx.tcx().explicit_item_bounds(proj.item_def_id);
1566                 let wf_obligations = bounds.iter().flat_map(|&(bound, bound_span)| {
1567                     let normalized_bound = wfcx.normalize(span, None, bound);
1568                     traits::wf::predicate_obligations(
1569                         wfcx.infcx,
1570                         wfcx.param_env,
1571                         wfcx.body_id,
1572                         normalized_bound,
1573                         bound_span,
1574                     )
1575                 });
1576                 wfcx.register_obligations(wf_obligations);
1577             }
1578         }
1579     }
1580 }
1581
1582 const HELP_FOR_SELF_TYPE: &str = "consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, \
1583      `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one \
1584      of the previous types except `Self`)";
1585
1586 #[instrument(level = "debug", skip(wfcx))]
1587 fn check_method_receiver<'tcx>(
1588     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1589     fn_sig: &hir::FnSig<'_>,
1590     method: &ty::AssocItem,
1591     self_ty: Ty<'tcx>,
1592 ) {
1593     let tcx = wfcx.tcx();
1594
1595     if !method.fn_has_self_parameter {
1596         return;
1597     }
1598
1599     let span = fn_sig.decl.inputs[0].span;
1600
1601     let sig = tcx.fn_sig(method.def_id);
1602     let sig = tcx.liberate_late_bound_regions(method.def_id, sig);
1603     let sig = wfcx.normalize(span, None, sig);
1604
1605     debug!("check_method_receiver: sig={:?}", sig);
1606
1607     let self_ty = wfcx.normalize(span, None, self_ty);
1608
1609     let receiver_ty = sig.inputs()[0];
1610     let receiver_ty = wfcx.normalize(span, None, receiver_ty);
1611
1612     if tcx.features().arbitrary_self_types {
1613         if !receiver_is_valid(wfcx, span, receiver_ty, self_ty, true) {
1614             // Report error; `arbitrary_self_types` was enabled.
1615             e0307(tcx, span, receiver_ty);
1616         }
1617     } else {
1618         if !receiver_is_valid(wfcx, span, receiver_ty, self_ty, false) {
1619             if receiver_is_valid(wfcx, span, receiver_ty, self_ty, true) {
1620                 // Report error; would have worked with `arbitrary_self_types`.
1621                 feature_err(
1622                     &tcx.sess.parse_sess,
1623                     sym::arbitrary_self_types,
1624                     span,
1625                     &format!(
1626                         "`{receiver_ty}` cannot be used as the type of `self` without \
1627                          the `arbitrary_self_types` feature",
1628                     ),
1629                 )
1630                 .help(HELP_FOR_SELF_TYPE)
1631                 .emit();
1632             } else {
1633                 // Report error; would not have worked with `arbitrary_self_types`.
1634                 e0307(tcx, span, receiver_ty);
1635             }
1636         }
1637     }
1638 }
1639
1640 fn e0307<'tcx>(tcx: TyCtxt<'tcx>, span: Span, receiver_ty: Ty<'_>) {
1641     struct_span_err!(
1642         tcx.sess.diagnostic(),
1643         span,
1644         E0307,
1645         "invalid `self` parameter type: {receiver_ty}"
1646     )
1647     .note("type of `self` must be `Self` or a type that dereferences to it")
1648     .help(HELP_FOR_SELF_TYPE)
1649     .emit();
1650 }
1651
1652 /// Returns whether `receiver_ty` would be considered a valid receiver type for `self_ty`. If
1653 /// `arbitrary_self_types` is enabled, `receiver_ty` must transitively deref to `self_ty`, possibly
1654 /// through a `*const/mut T` raw pointer. If the feature is not enabled, the requirements are more
1655 /// strict: `receiver_ty` must implement `Receiver` and directly implement
1656 /// `Deref<Target = self_ty>`.
1657 ///
1658 /// N.B., there are cases this function returns `true` but causes an error to be emitted,
1659 /// particularly when `receiver_ty` derefs to a type that is the same as `self_ty` but has the
1660 /// wrong lifetime. Be careful of this if you are calling this function speculatively.
1661 fn receiver_is_valid<'tcx>(
1662     wfcx: &WfCheckingCtxt<'_, 'tcx>,
1663     span: Span,
1664     receiver_ty: Ty<'tcx>,
1665     self_ty: Ty<'tcx>,
1666     arbitrary_self_types_enabled: bool,
1667 ) -> bool {
1668     let infcx = wfcx.infcx;
1669     let tcx = wfcx.tcx();
1670     let cause =
1671         ObligationCause::new(span, wfcx.body_id, traits::ObligationCauseCode::MethodReceiver);
1672
1673     let can_eq_self = |ty| infcx.can_eq(wfcx.param_env, self_ty, ty).is_ok();
1674
1675     // `self: Self` is always valid.
1676     if can_eq_self(receiver_ty) {
1677         if let Err(err) = wfcx.equate_types(&cause, wfcx.param_env, self_ty, receiver_ty) {
1678             infcx.err_ctxt().report_mismatched_types(&cause, self_ty, receiver_ty, err).emit();
1679         }
1680         return true;
1681     }
1682
1683     let mut autoderef =
1684         Autoderef::new(infcx, wfcx.param_env, wfcx.body_id, span, receiver_ty, span);
1685
1686     // The `arbitrary_self_types` feature allows raw pointer receivers like `self: *const Self`.
1687     if arbitrary_self_types_enabled {
1688         autoderef = autoderef.include_raw_pointers();
1689     }
1690
1691     // The first type is `receiver_ty`, which we know its not equal to `self_ty`; skip it.
1692     autoderef.next();
1693
1694     let receiver_trait_def_id = tcx.require_lang_item(LangItem::Receiver, None);
1695
1696     // Keep dereferencing `receiver_ty` until we get to `self_ty`.
1697     loop {
1698         if let Some((potential_self_ty, _)) = autoderef.next() {
1699             debug!(
1700                 "receiver_is_valid: potential self type `{:?}` to match `{:?}`",
1701                 potential_self_ty, self_ty
1702             );
1703
1704             if can_eq_self(potential_self_ty) {
1705                 wfcx.register_obligations(autoderef.into_obligations());
1706
1707                 if let Err(err) =
1708                     wfcx.equate_types(&cause, wfcx.param_env, self_ty, potential_self_ty)
1709                 {
1710                     infcx
1711                         .err_ctxt()
1712                         .report_mismatched_types(&cause, self_ty, potential_self_ty, err)
1713                         .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 }