compiler/rustc_typeck/src/collect/type_of.rs

   1 use rustc_errors::{Applicability, ErrorReported, StashKey};
   2 use rustc_hir as hir;
   3 use rustc_hir::def::{DefKind, Res};
   4 use rustc_hir::def_id::{DefId, LocalDefId};
   5 use rustc_hir::intravisit;
   6 use rustc_hir::intravisit::Visitor;
   7 use rustc_hir::{HirId, Node};
   8 use rustc_middle::hir::map::Map;
   9 use rustc_middle::ty::subst::{InternalSubsts, SubstsRef};
  10 use rustc_middle::ty::util::IntTypeExt;
  11 use rustc_middle::ty::{self, DefIdTree, Ty, TyCtxt, TypeFoldable, TypeFolder};
  12 use rustc_span::symbol::Ident;
  13 use rustc_span::{Span, DUMMY_SP};
  14
  15 use super::ItemCtxt;
  16 use super::{bad_placeholder_type, is_suggestable_infer_ty};
  17
  18 /// Computes the relevant generic parameter for a potential generic const argument.
  19 ///
  20 /// This should be called using the query `tcx.opt_const_param_of`.
  21 pub(super) fn opt_const_param_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<DefId> {
  22     // FIXME(generic_arg_infer): allow for returning DefIds of inference of
  23     // GenericArg::Infer below. This may require a change where GenericArg::Infer has some flag
  24     // for const or type.
  25     use hir::*;
  26     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
  27
  28     if let Node::AnonConst(_) = tcx.hir().get(hir_id) {
  29         let parent_node_id = tcx.hir().get_parent_node(hir_id);
  30         let parent_node = tcx.hir().get(parent_node_id);
  31
  32         match parent_node {
  33             // This match arm is for when the def_id appears in a GAT whose
  34             // path can't be resolved without typechecking e.g.
  35             //
  36             // trait Foo {
  37             //   type Assoc<const N: usize>;
  38             //   fn foo() -> Self::Assoc<3>;
  39             // }
  40             //
  41             // In the above code we would call this query with the def_id of 3 and
  42             // the parent_node we match on would be the hir node for Self::Assoc<3>
  43             //
  44             // `Self::Assoc<3>` cant be resolved without typchecking here as we
  45             // didnt write <Self as Foo>::Assoc<3>. If we did then another match
  46             // arm would handle this.
  47             //
  48             // I believe this match arm is only needed for GAT but I am not 100% sure - BoxyUwU
  49             Node::Ty(hir_ty @ Ty { kind: TyKind::Path(QPath::TypeRelative(_, segment)), .. }) => {
  50                 // Find the Item containing the associated type so we can create an ItemCtxt.
  51                 // Using the ItemCtxt convert the HIR for the unresolved assoc type into a
  52                 // ty which is a fully resolved projection.
  53                 // For the code example above, this would mean converting Self::Assoc<3>
  54                 // into a ty::Projection(<Self as Foo>::Assoc<3>)
  55                 let item_hir_id = tcx
  56                     .hir()
  57                     .parent_iter(hir_id)
  58                     .filter(|(_, node)| matches!(node, Node::Item(_)))
  59                     .map(|(id, _)| id)
  60                     .next()
  61                     .unwrap();
  62                 let item_did = tcx.hir().local_def_id(item_hir_id).to_def_id();
  63                 let item_ctxt = &ItemCtxt::new(tcx, item_did) as &dyn crate::astconv::AstConv<'_>;
  64                 let ty = item_ctxt.ast_ty_to_ty(hir_ty);
  65
  66                 // Iterate through the generics of the projection to find the one that corresponds to
  67                 // the def_id that this query was called with. We filter to only const args here as a
  68                 // precaution for if it's ever allowed to elide lifetimes in GAT's. It currently isn't
  69                 // but it can't hurt to be safe ^^
  70                 if let ty::Projection(projection) = ty.kind() {
  71                     let generics = tcx.generics_of(projection.item_def_id);
  72
  73                     let arg_index = segment
  74                         .args
  75                         .and_then(|args| {
  76                             args.args
  77                                 .iter()
  78                                 .filter(|arg| arg.is_const())
  79                                 .position(|arg| arg.id() == hir_id)
  80                         })
  81                         .unwrap_or_else(|| {
  82                             bug!("no arg matching AnonConst in segment");
  83                         });
  84
  85                     return generics
  86                         .params
  87                         .iter()
  88                         .filter(|param| matches!(param.kind, ty::GenericParamDefKind::Const { .. }))
  89                         .nth(arg_index)
  90                         .map(|param| param.def_id);
  91                 }
  92
  93                 // I dont think it's possible to reach this but I'm not 100% sure - BoxyUwU
  94                 tcx.sess.delay_span_bug(
  95                     tcx.def_span(def_id),
  96                     "unexpected non-GAT usage of an anon const",
  97                 );
  98                 return None;
  99             }
 100             Node::Expr(&Expr {
 101                 kind:
 102                     ExprKind::MethodCall(segment, ..) | ExprKind::Path(QPath::TypeRelative(_, segment)),
 103                 ..
 104             }) => {
 105                 let body_owner = tcx.hir().local_def_id(tcx.hir().enclosing_body_owner(hir_id));
 106                 let tables = tcx.typeck(body_owner);
 107                 // This may fail in case the method/path does not actually exist.
 108                 // As there is no relevant param for `def_id`, we simply return
 109                 // `None` here.
 110                 let type_dependent_def = tables.type_dependent_def_id(parent_node_id)?;
 111                 let idx = segment
 112                     .args
 113                     .and_then(|args| {
 114                         args.args
 115                             .iter()
 116                             .filter(|arg| arg.is_const())
 117                             .position(|arg| arg.id() == hir_id)
 118                     })
 119                     .unwrap_or_else(|| {
 120                         bug!("no arg matching AnonConst in segment");
 121                     });
 122
 123                 tcx.generics_of(type_dependent_def)
 124                     .params
 125                     .iter()
 126                     .filter(|param| matches!(param.kind, ty::GenericParamDefKind::Const { .. }))
 127                     .nth(idx)
 128                     .map(|param| param.def_id)
 129             }
 130
 131             Node::Ty(&Ty { kind: TyKind::Path(_), .. })
 132             | Node::Expr(&Expr { kind: ExprKind::Path(_) | ExprKind::Struct(..), .. })
 133             | Node::TraitRef(..)
 134             | Node::Pat(_) => {
 135                 let path = match parent_node {
 136                     Node::Ty(&Ty { kind: TyKind::Path(QPath::Resolved(_, path)), .. })
 137                     | Node::TraitRef(&TraitRef { path, .. }) => &*path,
 138                     Node::Expr(&Expr {
 139                         kind:
 140                             ExprKind::Path(QPath::Resolved(_, path))
 141                             | ExprKind::Struct(&QPath::Resolved(_, path), ..),
 142                         ..
 143                     }) => {
 144                         let body_owner =
 145                             tcx.hir().local_def_id(tcx.hir().enclosing_body_owner(hir_id));
 146                         let _tables = tcx.typeck(body_owner);
 147                         &*path
 148                     }
 149                     Node::Pat(pat) => {
 150                         if let Some(path) = get_path_containing_arg_in_pat(pat, hir_id) {
 151                             path
 152                         } else {
 153                             tcx.sess.delay_span_bug(
 154                                 tcx.def_span(def_id),
 155                                 &format!(
 156                                     "unable to find const parent for {} in pat {:?}",
 157                                     hir_id, pat
 158                                 ),
 159                             );
 160                             return None;
 161                         }
 162                     }
 163                     _ => {
 164                         tcx.sess.delay_span_bug(
 165                             tcx.def_span(def_id),
 166                             &format!("unexpected const parent path {:?}", parent_node),
 167                         );
 168                         return None;
 169                     }
 170                 };
 171
 172                 // We've encountered an `AnonConst` in some path, so we need to
 173                 // figure out which generic parameter it corresponds to and return
 174                 // the relevant type.
 175                 let (arg_index, segment) = path
 176                     .segments
 177                     .iter()
 178                     .filter_map(|seg| seg.args.map(|args| (args.args, seg)))
 179                     .find_map(|(args, seg)| {
 180                         args.iter()
 181                             .filter(|arg| arg.is_const())
 182                             .position(|arg| arg.id() == hir_id)
 183                             .map(|index| (index, seg))
 184                     })
 185                     .unwrap_or_else(|| {
 186                         bug!("no arg matching AnonConst in path");
 187                     });
 188
 189                 // Try to use the segment resolution if it is valid, otherwise we
 190                 // default to the path resolution.
 191                 let res = segment.res.filter(|&r| r != Res::Err).unwrap_or(path.res);
 192                 use def::CtorOf;
 193                 let generics = match res {
 194                     Res::Def(DefKind::Ctor(CtorOf::Variant, _), def_id) => tcx.generics_of(
 195                         tcx.parent(def_id).and_then(|def_id| tcx.parent(def_id)).unwrap(),
 196                     ),
 197                     Res::Def(DefKind::Variant | DefKind::Ctor(CtorOf::Struct, _), def_id) => {
 198                         tcx.generics_of(tcx.parent(def_id).unwrap())
 199                     }
 200                     // Other `DefKind`s don't have generics and would ICE when calling
 201                     // `generics_of`.
 202                     Res::Def(
 203                         DefKind::Struct
 204                         | DefKind::Union
 205                         | DefKind::Enum
 206                         | DefKind::Trait
 207                         | DefKind::OpaqueTy
 208                         | DefKind::TyAlias
 209                         | DefKind::ForeignTy
 210                         | DefKind::TraitAlias
 211                         | DefKind::AssocTy
 212                         | DefKind::Fn
 213                         | DefKind::AssocFn
 214                         | DefKind::AssocConst
 215                         | DefKind::Impl,
 216                         def_id,
 217                     ) => tcx.generics_of(def_id),
 218                     Res::Err => {
 219                         tcx.sess.delay_span_bug(tcx.def_span(def_id), "anon const with Res::Err");
 220                         return None;
 221                     }
 222                     _ => {
 223                         // If the user tries to specify generics on a type that does not take them,
 224                         // e.g. `usize<T>`, we may hit this branch, in which case we treat it as if
 225                         // no arguments have been passed. An error should already have been emitted.
 226                         tcx.sess.delay_span_bug(
 227                             tcx.def_span(def_id),
 228                             &format!("unexpected anon const res {:?} in path: {:?}", res, path),
 229                         );
 230                         return None;
 231                     }
 232                 };
 233
 234                 generics
 235                     .params
 236                     .iter()
 237                     .filter(|param| matches!(param.kind, ty::GenericParamDefKind::Const { .. }))
 238                     .nth(arg_index)
 239                     .map(|param| param.def_id)
 240             }
 241             _ => None,
 242         }
 243     } else {
 244         None
 245     }
 246 }
 247
 248 fn get_path_containing_arg_in_pat<'hir>(
 249     pat: &'hir hir::Pat<'hir>,
 250     arg_id: HirId,
 251 ) -> Option<&'hir hir::Path<'hir>> {
 252     use hir::*;
 253
 254     let is_arg_in_path = |p: &hir::Path<'_>| {
 255         p.segments
 256             .iter()
 257             .filter_map(|seg| seg.args)
 258             .flat_map(|args| args.args)
 259             .any(|arg| arg.id() == arg_id)
 260     };
 261     let mut arg_path = None;
 262     pat.walk(|pat| match pat.kind {
 263         PatKind::Struct(QPath::Resolved(_, path), _, _)
 264         | PatKind::TupleStruct(QPath::Resolved(_, path), _, _)
 265         | PatKind::Path(QPath::Resolved(_, path))
 266             if is_arg_in_path(path) =>
 267         {
 268             arg_path = Some(path);
 269             false
 270         }
 271         _ => true,
 272     });
 273     arg_path
 274 }
 275
 276 pub(super) fn default_anon_const_substs(tcx: TyCtxt<'_>, def_id: DefId) -> SubstsRef<'_> {
 277     let generics = tcx.generics_of(def_id);
 278     if let Some(parent) = generics.parent {
 279         // This is the reason we bother with having optional anon const substs.
 280         //
 281         // In the future the substs of an anon const will depend on its parents predicates
 282         // at which point eagerly looking at them will cause a query cycle.
 283         //
 284         // So for now this is only an assurance that this approach won't cause cycle errors in
 285         // the future.
 286         let _cycle_check = tcx.predicates_of(parent);
 287     }
 288
 289     let substs = InternalSubsts::identity_for_item(tcx, def_id);
 290     // We only expect substs with the following type flags as default substs.
 291     //
 292     // Getting this wrong can lead to ICE and unsoundness, so we assert it here.
 293     for arg in substs.iter() {
 294         let allowed_flags = ty::TypeFlags::MAY_NEED_DEFAULT_CONST_SUBSTS
 295             | ty::TypeFlags::STILL_FURTHER_SPECIALIZABLE
 296             | ty::TypeFlags::HAS_ERROR;
 297         assert!(!arg.has_type_flags(!allowed_flags));
 298     }
 299     substs
 300 }
 301
 302 pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
 303     let def_id = def_id.expect_local();
 304     use rustc_hir::*;
 305
 306     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 307
 308     let icx = ItemCtxt::new(tcx, def_id.to_def_id());
 309
 310     match tcx.hir().get(hir_id) {
 311         Node::TraitItem(item) => match item.kind {
 312             TraitItemKind::Fn(..) => {
 313                 let substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
 314                 tcx.mk_fn_def(def_id.to_def_id(), substs)
 315             }
 316             TraitItemKind::Const(ty, body_id) => body_id
 317                 .and_then(|body_id| {
 318                     if is_suggestable_infer_ty(ty) {
 319                         Some(infer_placeholder_type(
 320                             tcx, def_id, body_id, ty.span, item.ident, "constant",
 321                         ))
 322                     } else {
 323                         None
 324                     }
 325                 })
 326                 .unwrap_or_else(|| icx.to_ty(ty)),
 327             TraitItemKind::Type(_, Some(ty)) => icx.to_ty(ty),
 328             TraitItemKind::Type(_, None) => {
 329                 span_bug!(item.span, "associated type missing default");
 330             }
 331         },
 332
 333         Node::ImplItem(item) => match item.kind {
 334             ImplItemKind::Fn(..) => {
 335                 let substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
 336                 tcx.mk_fn_def(def_id.to_def_id(), substs)
 337             }
 338             ImplItemKind::Const(ty, body_id) => {
 339                 if is_suggestable_infer_ty(ty) {
 340                     infer_placeholder_type(tcx, def_id, body_id, ty.span, item.ident, "constant")
 341                 } else {
 342                     icx.to_ty(ty)
 343                 }
 344             }
 345             ImplItemKind::TyAlias(ty) => {
 346                 if tcx.impl_trait_ref(tcx.hir().get_parent_did(hir_id).to_def_id()).is_none() {
 347                     check_feature_inherent_assoc_ty(tcx, item.span);
 348                 }
 349
 350                 icx.to_ty(ty)
 351             }
 352         },
 353
 354         Node::Item(item) => {
 355             match item.kind {
 356                 ItemKind::Static(ty, .., body_id) => {
 357                     if is_suggestable_infer_ty(ty) {
 358                         infer_placeholder_type(
 359                             tcx,
 360                             def_id,
 361                             body_id,
 362                             ty.span,
 363                             item.ident,
 364                             "static variable",
 365                         )
 366                     } else {
 367                         icx.to_ty(ty)
 368                     }
 369                 }
 370                 ItemKind::Const(ty, body_id) => {
 371                     if is_suggestable_infer_ty(ty) {
 372                         infer_placeholder_type(
 373                             tcx, def_id, body_id, ty.span, item.ident, "constant",
 374                         )
 375                     } else {
 376                         icx.to_ty(ty)
 377                     }
 378                 }
 379                 ItemKind::TyAlias(self_ty, _)
 380                 | ItemKind::Impl(hir::Impl { self_ty, .. }) => icx.to_ty(self_ty),
 381                 ItemKind::Fn(..) => {
 382                     let substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
 383                     tcx.mk_fn_def(def_id.to_def_id(), substs)
 384                 }
 385                 ItemKind::Enum(..) | ItemKind::Struct(..) | ItemKind::Union(..) => {
 386                     let def = tcx.adt_def(def_id);
 387                     let substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
 388                     tcx.mk_adt(def, substs)
 389                 }
 390                 ItemKind::OpaqueTy(OpaqueTy { impl_trait_fn: None, .. }) => {
 391                     find_opaque_ty_constraints(tcx, def_id)
 392                 }
 393                 // Opaque types desugared from `impl Trait`.
 394                 ItemKind::OpaqueTy(OpaqueTy { impl_trait_fn: Some(owner), .. }) => {
 395                     let concrete_ty = tcx
 396                         .mir_borrowck(owner.expect_local())
 397                         .concrete_opaque_types
 398                         .get_value_matching(|(key, _)| key.def_id == def_id.to_def_id())
 399                         .copied()
 400                         .unwrap_or_else(|| {
 401                             tcx.sess.delay_span_bug(
 402                                 DUMMY_SP,
 403                                 &format!(
 404                                     "owner {:?} has no opaque type for {:?} in its typeck results",
 405                                     owner, def_id,
 406                                 ),
 407                             );
 408                             if let Some(ErrorReported) =
 409                                 tcx.typeck(owner.expect_local()).tainted_by_errors
 410                             {
 411                                 // Some error in the
 412                                 // owner fn prevented us from populating
 413                                 // the `concrete_opaque_types` table.
 414                                 tcx.ty_error()
 415                             } else {
 416                                 // We failed to resolve the opaque type or it
 417                                 // resolves to itself. Return the non-revealed
 418                                 // type, which should result in E0720.
 419                                 tcx.mk_opaque(
 420                                     def_id.to_def_id(),
 421                                     InternalSubsts::identity_for_item(tcx, def_id.to_def_id()),
 422                                 )
 423                             }
 424                         });
 425                     debug!("concrete_ty = {:?}", concrete_ty);
 426                     concrete_ty
 427                 }
 428                 ItemKind::Trait(..)
 429                 | ItemKind::TraitAlias(..)
 430                 | ItemKind::Macro(..)
 431                 | ItemKind::Mod(..)
 432                 | ItemKind::ForeignMod { .. }
 433                 | ItemKind::GlobalAsm(..)
 434                 | ItemKind::ExternCrate(..)
 435                 | ItemKind::Use(..) => {
 436                     span_bug!(
 437                         item.span,
 438                         "compute_type_of_item: unexpected item type: {:?}",
 439                         item.kind
 440                     );
 441                 }
 442             }
 443         }
 444
 445         Node::ForeignItem(foreign_item) => match foreign_item.kind {
 446             ForeignItemKind::Fn(..) => {
 447                 let substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
 448                 tcx.mk_fn_def(def_id.to_def_id(), substs)
 449             }
 450             ForeignItemKind::Static(t, _) => icx.to_ty(t),
 451             ForeignItemKind::Type => tcx.mk_foreign(def_id.to_def_id()),
 452         },
 453
 454         Node::Ctor(&ref def) | Node::Variant(Variant { data: ref def, .. }) => match *def {
 455             VariantData::Unit(..) | VariantData::Struct(..) => {
 456                 tcx.type_of(tcx.hir().get_parent_did(hir_id).to_def_id())
 457             }
 458             VariantData::Tuple(..) => {
 459                 let substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
 460                 tcx.mk_fn_def(def_id.to_def_id(), substs)
 461             }
 462         },
 463
 464         Node::Field(field) => icx.to_ty(field.ty),
 465
 466         Node::Expr(&Expr { kind: ExprKind::Closure(.., gen), .. }) => {
 467             let substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
 468             if let Some(movability) = gen {
 469                 tcx.mk_generator(def_id.to_def_id(), substs, movability)
 470             } else {
 471                 tcx.mk_closure(def_id.to_def_id(), substs)
 472             }
 473         }
 474
 475         Node::AnonConst(_) if let Some(param) = tcx.opt_const_param_of(def_id) => {
 476             // We defer to `type_of` of the corresponding parameter
 477             // for generic arguments.
 478             tcx.type_of(param)
 479         }
 480
 481         Node::AnonConst(_) => {
 482             let parent_node = tcx.hir().get(tcx.hir().get_parent_node(hir_id));
 483             match parent_node {
 484                 Node::Ty(&Ty { kind: TyKind::Array(_, ref constant), .. })
 485                 | Node::Expr(&Expr { kind: ExprKind::Repeat(_, ref constant), .. })
 486                     if constant.hir_id == hir_id =>
 487                 {
 488                     tcx.types.usize
 489                 }
 490                 Node::Ty(&Ty { kind: TyKind::Typeof(ref e), .. }) if e.hir_id == hir_id => {
 491                     tcx.typeck(def_id).node_type(e.hir_id)
 492                 }
 493
 494                 Node::Expr(&Expr { kind: ExprKind::ConstBlock(ref anon_const), .. })
 495                     if anon_const.hir_id == hir_id =>
 496                 {
 497                     let substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
 498                     substs.as_inline_const().ty()
 499                 }
 500
 501                 Node::Expr(&Expr { kind: ExprKind::InlineAsm(asm), .. })
 502                 | Node::Item(&Item { kind: ItemKind::GlobalAsm(asm), .. })
 503                     if asm.operands.iter().any(|(op, _op_sp)| match op {
 504                         hir::InlineAsmOperand::Const { anon_const } => anon_const.hir_id == hir_id,
 505                         _ => false,
 506                     }) =>
 507                 {
 508                     tcx.typeck(def_id).node_type(hir_id)
 509                 }
 510
 511                 Node::Variant(Variant { disr_expr: Some(ref e), .. }) if e.hir_id == hir_id => tcx
 512                     .adt_def(tcx.hir().get_parent_did(hir_id).to_def_id())
 513                     .repr
 514                     .discr_type()
 515                     .to_ty(tcx),
 516
 517                 Node::GenericParam(&GenericParam {
 518                     hir_id: param_hir_id,
 519                     kind: GenericParamKind::Const { default: Some(ct), .. },
 520                     ..
 521                 }) if ct.hir_id == hir_id => tcx.type_of(tcx.hir().local_def_id(param_hir_id)),
 522
 523                 x => tcx.ty_error_with_message(
 524                     DUMMY_SP,
 525                     &format!("unexpected const parent in type_of(): {:?}", x),
 526                 ),
 527             }
 528         }
 529
 530         Node::GenericParam(param) => match &param.kind {
 531             GenericParamKind::Type { default: Some(ty), .. }
 532             | GenericParamKind::Const { ty, .. } => icx.to_ty(ty),
 533             x => bug!("unexpected non-type Node::GenericParam: {:?}", x),
 534         },
 535
 536         x => {
 537             bug!("unexpected sort of node in type_of(): {:?}", x);
 538         }
 539     }
 540 }
 541
 542 #[instrument(skip(tcx), level = "debug")]
 543 /// Checks "defining uses" of opaque `impl Trait` types to ensure that they meet the restrictions
 544 /// laid for "higher-order pattern unification".
 545 /// This ensures that inference is tractable.
 546 /// In particular, definitions of opaque types can only use other generics as arguments,
 547 /// and they cannot repeat an argument. Example:
 548 ///
 549 /// ```rust
 550 /// type Foo<A, B> = impl Bar<A, B>;
 551 ///
 552 /// // Okay -- `Foo` is applied to two distinct, generic types.
 553 /// fn a<T, U>() -> Foo<T, U> { .. }
 554 ///
 555 /// // Not okay -- `Foo` is applied to `T` twice.
 556 /// fn b<T>() -> Foo<T, T> { .. }
 557 ///
 558 /// // Not okay -- `Foo` is applied to a non-generic type.
 559 /// fn b<T>() -> Foo<T, u32> { .. }
 560 /// ```
 561 ///
 562 fn find_opaque_ty_constraints(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Ty<'_> {
 563     use rustc_hir::{Expr, ImplItem, Item, TraitItem};
 564
 565     struct ConstraintLocator<'tcx> {
 566         tcx: TyCtxt<'tcx>,
 567
 568         /// def_id of the opaque type whose defining uses are being checked
 569         def_id: DefId,
 570
 571         /// as we walk the defining uses, we are checking that all of them
 572         /// define the same hidden type. This variable is set to `Some`
 573         /// with the first type that we find, and then later types are
 574         /// checked against it (we also carry the span of that first
 575         /// type).
 576         found: Option<(Span, Ty<'tcx>)>,
 577     }
 578
 579     impl ConstraintLocator<'_> {
 580         #[instrument(skip(self), level = "debug")]
 581         fn check(&mut self, def_id: LocalDefId) {
 582             // Don't try to check items that cannot possibly constrain the type.
 583             if !self.tcx.has_typeck_results(def_id) {
 584                 debug!("no constraint: no typeck results");
 585                 return;
 586             }
 587             // Calling `mir_borrowck` can lead to cycle errors through
 588             // const-checking, avoid calling it if we don't have to.
 589             if !self.tcx.typeck(def_id).concrete_opaque_types.contains(&self.def_id) {
 590                 debug!("no constraints in typeck results");
 591                 return;
 592             }
 593             // Use borrowck to get the type with unerased regions.
 594             let concrete_opaque_types = &self.tcx.mir_borrowck(def_id).concrete_opaque_types;
 595             debug!(?concrete_opaque_types);
 596             for (opaque_type_key, concrete_type) in concrete_opaque_types {
 597                 if opaque_type_key.def_id != self.def_id {
 598                     // Ignore constraints for other opaque types.
 599                     continue;
 600                 }
 601
 602                 debug!(?concrete_type, ?opaque_type_key.substs, "found constraint");
 603
 604                 // FIXME(oli-obk): trace the actual span from inference to improve errors.
 605                 let span = self.tcx.def_span(def_id);
 606
 607                 if let Some((prev_span, prev_ty)) = self.found {
 608                     if *concrete_type != prev_ty && !(*concrete_type, prev_ty).references_error() {
 609                         debug!(?span);
 610                         // Found different concrete types for the opaque type.
 611                         let mut err = self.tcx.sess.struct_span_err(
 612                             span,
 613                             "concrete type differs from previous defining opaque type use",
 614                         );
 615                         err.span_label(
 616                             span,
 617                             format!("expected `{}`, got `{}`", prev_ty, concrete_type),
 618                         );
 619                         err.span_note(prev_span, "previous use here");
 620                         err.emit();
 621                     }
 622                 } else {
 623                     self.found = Some((span, concrete_type));
 624                 }
 625             }
 626         }
 627     }
 628
 629     impl<'tcx> intravisit::Visitor<'tcx> for ConstraintLocator<'tcx> {
 630         type Map = Map<'tcx>;
 631
 632         fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
 633             intravisit::NestedVisitorMap::All(self.tcx.hir())
 634         }
 635         fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
 636             if let hir::ExprKind::Closure(..) = ex.kind {
 637                 let def_id = self.tcx.hir().local_def_id(ex.hir_id);
 638                 self.check(def_id);
 639             }
 640             intravisit::walk_expr(self, ex);
 641         }
 642         fn visit_item(&mut self, it: &'tcx Item<'tcx>) {
 643             debug!("find_existential_constraints: visiting {:?}", it);
 644             // The opaque type itself or its children are not within its reveal scope.
 645             if it.def_id.to_def_id() != self.def_id {
 646                 self.check(it.def_id);
 647                 intravisit::walk_item(self, it);
 648             }
 649         }
 650         fn visit_impl_item(&mut self, it: &'tcx ImplItem<'tcx>) {
 651             debug!("find_existential_constraints: visiting {:?}", it);
 652             // The opaque type itself or its children are not within its reveal scope.
 653             if it.def_id.to_def_id() != self.def_id {
 654                 self.check(it.def_id);
 655                 intravisit::walk_impl_item(self, it);
 656             }
 657         }
 658         fn visit_trait_item(&mut self, it: &'tcx TraitItem<'tcx>) {
 659             debug!("find_existential_constraints: visiting {:?}", it);
 660             self.check(it.def_id);
 661             intravisit::walk_trait_item(self, it);
 662         }
 663     }
 664
 665     let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 666     let scope = tcx.hir().get_defining_scope(hir_id);
 667     let mut locator = ConstraintLocator { def_id: def_id.to_def_id(), tcx, found: None };
 668
 669     debug!("find_opaque_ty_constraints: scope={:?}", scope);
 670
 671     if scope == hir::CRATE_HIR_ID {
 672         tcx.hir().walk_toplevel_module(&mut locator);
 673     } else {
 674         debug!("find_opaque_ty_constraints: scope={:?}", tcx.hir().get(scope));
 675         match tcx.hir().get(scope) {
 676             // We explicitly call `visit_*` methods, instead of using `intravisit::walk_*` methods
 677             // This allows our visitor to process the defining item itself, causing
 678             // it to pick up any 'sibling' defining uses.
 679             //
 680             // For example, this code:
 681             // ```
 682             // fn foo() {
 683             //     type Blah = impl Debug;
 684             //     let my_closure = || -> Blah { true };
 685             // }
 686             // ```
 687             //
 688             // requires us to explicitly process `foo()` in order
 689             // to notice the defining usage of `Blah`.
 690             Node::Item(it) => locator.visit_item(it),
 691             Node::ImplItem(it) => locator.visit_impl_item(it),
 692             Node::TraitItem(it) => locator.visit_trait_item(it),
 693             other => bug!("{:?} is not a valid scope for an opaque type item", other),
 694         }
 695     }
 696
 697     match locator.found {
 698         Some((_, ty)) => ty,
 699         None => {
 700             let span = tcx.def_span(def_id);
 701             tcx.sess.span_err(span, "could not find defining uses");
 702             tcx.ty_error()
 703         }
 704     }
 705 }
 706
 707 fn infer_placeholder_type<'a>(
 708     tcx: TyCtxt<'a>,
 709     def_id: LocalDefId,
 710     body_id: hir::BodyId,
 711     span: Span,
 712     item_ident: Ident,
 713     kind: &'static str,
 714 ) -> Ty<'a> {
 715     // Attempts to make the type nameable by turning FnDefs into FnPtrs.
 716     struct MakeNameable<'tcx> {
 717         success: bool,
 718         tcx: TyCtxt<'tcx>,
 719     }
 720
 721     impl<'tcx> MakeNameable<'tcx> {
 722         fn new(tcx: TyCtxt<'tcx>) -> Self {
 723             MakeNameable { success: true, tcx }
 724         }
 725     }
 726
 727     impl TypeFolder<'tcx> for MakeNameable<'tcx> {
 728         fn tcx(&self) -> TyCtxt<'tcx> {
 729             self.tcx
 730         }
 731
 732         fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 733             if !self.success {
 734                 return ty;
 735             }
 736
 737             match ty.kind() {
 738                 ty::FnDef(def_id, _) => self.tcx.mk_fn_ptr(self.tcx.fn_sig(*def_id)),
 739                 // FIXME: non-capturing closures should also suggest a function pointer
 740                 ty::Closure(..) | ty::Generator(..) => {
 741                     self.success = false;
 742                     ty
 743                 }
 744                 _ => ty.super_fold_with(self),
 745             }
 746         }
 747     }
 748
 749     let ty = tcx.diagnostic_only_typeck(def_id).node_type(body_id.hir_id);
 750
 751     // If this came from a free `const` or `static mut?` item,
 752     // then the user may have written e.g. `const A = 42;`.
 753     // In this case, the parser has stashed a diagnostic for
 754     // us to improve in typeck so we do that now.
 755     match tcx.sess.diagnostic().steal_diagnostic(span, StashKey::ItemNoType) {
 756         Some(mut err) => {
 757             if !ty.references_error() {
 758                 // The parser provided a sub-optimal `HasPlaceholders` suggestion for the type.
 759                 // We are typeck and have the real type, so remove that and suggest the actual type.
 760                 err.suggestions.clear();
 761
 762                 // Suggesting unnameable types won't help.
 763                 let mut mk_nameable = MakeNameable::new(tcx);
 764                 let ty = mk_nameable.fold_ty(ty);
 765                 let sugg_ty = if mk_nameable.success { Some(ty) } else { None };
 766                 if let Some(sugg_ty) = sugg_ty {
 767                     err.span_suggestion(
 768                         span,
 769                         &format!("provide a type for the {item}", item = kind),
 770                         format!("{}: {}", item_ident, sugg_ty),
 771                         Applicability::MachineApplicable,
 772                     );
 773                 } else {
 774                     err.span_note(
 775                         tcx.hir().body(body_id).value.span,
 776                         &format!("however, the inferred type `{}` cannot be named", ty),
 777                     );
 778                 }
 779             }
 780
 781             err.emit();
 782         }
 783         None => {
 784             let mut diag = bad_placeholder_type(tcx, vec![span], kind);
 785
 786             if !ty.references_error() {
 787                 let mut mk_nameable = MakeNameable::new(tcx);
 788                 let ty = mk_nameable.fold_ty(ty);
 789                 let sugg_ty = if mk_nameable.success { Some(ty) } else { None };
 790                 if let Some(sugg_ty) = sugg_ty {
 791                     diag.span_suggestion(
 792                         span,
 793                         "replace with the correct type",
 794                         sugg_ty.to_string(),
 795                         Applicability::MaybeIncorrect,
 796                     );
 797                 } else {
 798                     diag.span_note(
 799                         tcx.hir().body(body_id).value.span,
 800                         &format!("however, the inferred type `{}` cannot be named", ty),
 801                     );
 802                 }
 803             }
 804
 805             diag.emit();
 806         }
 807     }
 808
 809     // Typeck doesn't expect erased regions to be returned from `type_of`.
 810     tcx.fold_regions(ty, &mut false, |r, _| match r {
 811         ty::ReErased => tcx.lifetimes.re_static,
 812         _ => r,
 813     })
 814 }
 815
 816 fn check_feature_inherent_assoc_ty(tcx: TyCtxt<'_>, span: Span) {
 817     if !tcx.features().inherent_associated_types {
 818         use rustc_session::parse::feature_err;
 819         use rustc_span::symbol::sym;
 820         feature_err(
 821             &tcx.sess.parse_sess,
 822             sym::inherent_associated_types,
 823             span,
 824             "inherent associated types are unstable",
 825         )
 826         .emit();
 827     }
 828 }