compiler/rustc_ast_lowering/src/item.rs

   1 use super::errors::{InvalidAbi, InvalidAbiSuggestion, MisplacedRelaxTraitBound};
   2 use super::ResolverAstLoweringExt;
   3 use super::{Arena, AstOwner, ImplTraitContext, ImplTraitPosition};
   4 use super::{FnDeclKind, LoweringContext, ParamMode};
   5
   6 use rustc_ast::ptr::P;
   7 use rustc_ast::visit::AssocCtxt;
   8 use rustc_ast::*;
   9 use rustc_data_structures::sorted_map::SortedMap;
  10 use rustc_hir as hir;
  11 use rustc_hir::def::{DefKind, Res};
  12 use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
  13 use rustc_hir::PredicateOrigin;
  14 use rustc_index::vec::{Idx, IndexVec};
  15 use rustc_middle::ty::{DefIdTree, ResolverAstLowering, TyCtxt};
  16 use rustc_span::lev_distance::find_best_match_for_name;
  17 use rustc_span::source_map::DesugaringKind;
  18 use rustc_span::symbol::{kw, sym, Ident};
  19 use rustc_span::{Span, Symbol};
  20 use rustc_target::spec::abi;
  21 use smallvec::{smallvec, SmallVec};
  22 use std::iter;
  23 use thin_vec::ThinVec;
  24
  25 pub(super) struct ItemLowerer<'a, 'hir> {
  26     pub(super) tcx: TyCtxt<'hir>,
  27     pub(super) resolver: &'a mut ResolverAstLowering,
  28     pub(super) ast_arena: &'a Arena<'static>,
  29     pub(super) ast_index: &'a IndexVec<LocalDefId, AstOwner<'a>>,
  30     pub(super) owners: &'a mut IndexVec<LocalDefId, hir::MaybeOwner<&'hir hir::OwnerInfo<'hir>>>,
  31 }
  32
  33 /// When we have a ty alias we *may* have two where clauses. To give the best diagnostics, we set the span
  34 /// to the where clause that is preferred, if it exists. Otherwise, it sets the span to the other where
  35 /// clause if it exists.
  36 fn add_ty_alias_where_clause(
  37     generics: &mut ast::Generics,
  38     mut where_clauses: (TyAliasWhereClause, TyAliasWhereClause),
  39     prefer_first: bool,
  40 ) {
  41     if !prefer_first {
  42         where_clauses = (where_clauses.1, where_clauses.0);
  43     }
  44     if where_clauses.0.0 || !where_clauses.1.0 {
  45         generics.where_clause.has_where_token = where_clauses.0.0;
  46         generics.where_clause.span = where_clauses.0.1;
  47     } else {
  48         generics.where_clause.has_where_token = where_clauses.1.0;
  49         generics.where_clause.span = where_clauses.1.1;
  50     }
  51 }
  52
  53 impl<'a, 'hir> ItemLowerer<'a, 'hir> {
  54     fn with_lctx(
  55         &mut self,
  56         owner: NodeId,
  57         f: impl FnOnce(&mut LoweringContext<'_, 'hir>) -> hir::OwnerNode<'hir>,
  58     ) {
  59         let mut lctx = LoweringContext {
  60             // Pseudo-globals.
  61             tcx: self.tcx,
  62             resolver: self.resolver,
  63             arena: self.tcx.hir_arena,
  64             ast_arena: self.ast_arena,
  65
  66             // HirId handling.
  67             bodies: Vec::new(),
  68             attrs: SortedMap::default(),
  69             children: Vec::default(),
  70             current_hir_id_owner: hir::CRATE_OWNER_ID,
  71             item_local_id_counter: hir::ItemLocalId::new(0),
  72             node_id_to_local_id: Default::default(),
  73             local_id_to_def_id: SortedMap::new(),
  74             trait_map: Default::default(),
  75
  76             // Lowering state.
  77             catch_scope: None,
  78             loop_scope: None,
  79             is_in_loop_condition: false,
  80             is_in_trait_impl: false,
  81             is_in_dyn_type: false,
  82             generator_kind: None,
  83             task_context: None,
  84             current_item: None,
  85             impl_trait_defs: Vec::new(),
  86             impl_trait_bounds: Vec::new(),
  87             allow_try_trait: Some([sym::try_trait_v2, sym::yeet_desugar_details][..].into()),
  88             allow_gen_future: Some([sym::gen_future, sym::closure_track_caller][..].into()),
  89             allow_into_future: Some([sym::into_future][..].into()),
  90             generics_def_id_map: Default::default(),
  91         };
  92         lctx.with_hir_id_owner(owner, |lctx| f(lctx));
  93
  94         for (def_id, info) in lctx.children {
  95             self.owners.ensure_contains_elem(def_id, || hir::MaybeOwner::Phantom);
  96             debug_assert!(matches!(self.owners[def_id], hir::MaybeOwner::Phantom));
  97             self.owners[def_id] = info;
  98         }
  99     }
 100
 101     pub(super) fn lower_node(
 102         &mut self,
 103         def_id: LocalDefId,
 104     ) -> hir::MaybeOwner<&'hir hir::OwnerInfo<'hir>> {
 105         self.owners.ensure_contains_elem(def_id, || hir::MaybeOwner::Phantom);
 106         if let hir::MaybeOwner::Phantom = self.owners[def_id] {
 107             let node = self.ast_index[def_id];
 108             match node {
 109                 AstOwner::NonOwner => {}
 110                 AstOwner::Crate(c) => self.lower_crate(c),
 111                 AstOwner::Item(item) => self.lower_item(item),
 112                 AstOwner::AssocItem(item, ctxt) => self.lower_assoc_item(item, ctxt),
 113                 AstOwner::ForeignItem(item) => self.lower_foreign_item(item),
 114             }
 115         }
 116
 117         self.owners[def_id]
 118     }
 119
 120     #[instrument(level = "debug", skip(self, c))]
 121     fn lower_crate(&mut self, c: &Crate) {
 122         debug_assert_eq!(self.resolver.node_id_to_def_id[&CRATE_NODE_ID], CRATE_DEF_ID);
 123         self.with_lctx(CRATE_NODE_ID, |lctx| {
 124             let module = lctx.lower_mod(&c.items, &c.spans);
 125             lctx.lower_attrs(hir::CRATE_HIR_ID, &c.attrs);
 126             hir::OwnerNode::Crate(module)
 127         })
 128     }
 129
 130     #[instrument(level = "debug", skip(self))]
 131     fn lower_item(&mut self, item: &Item) {
 132         self.with_lctx(item.id, |lctx| hir::OwnerNode::Item(lctx.lower_item(item)))
 133     }
 134
 135     fn lower_assoc_item(&mut self, item: &AssocItem, ctxt: AssocCtxt) {
 136         let def_id = self.resolver.node_id_to_def_id[&item.id];
 137
 138         let parent_id = self.tcx.local_parent(def_id);
 139         let parent_hir = self.lower_node(parent_id).unwrap();
 140         self.with_lctx(item.id, |lctx| {
 141             // Evaluate with the lifetimes in `params` in-scope.
 142             // This is used to track which lifetimes have already been defined,
 143             // and which need to be replicated when lowering an async fn.
 144             match parent_hir.node().expect_item().kind {
 145                 hir::ItemKind::Impl(hir::Impl { ref of_trait, .. }) => {
 146                     lctx.is_in_trait_impl = of_trait.is_some();
 147                 }
 148                 _ => {}
 149             };
 150
 151             match ctxt {
 152                 AssocCtxt::Trait => hir::OwnerNode::TraitItem(lctx.lower_trait_item(item)),
 153                 AssocCtxt::Impl => hir::OwnerNode::ImplItem(lctx.lower_impl_item(item)),
 154             }
 155         })
 156     }
 157
 158     fn lower_foreign_item(&mut self, item: &ForeignItem) {
 159         self.with_lctx(item.id, |lctx| hir::OwnerNode::ForeignItem(lctx.lower_foreign_item(item)))
 160     }
 161 }
 162
 163 impl<'hir> LoweringContext<'_, 'hir> {
 164     pub(super) fn lower_mod(
 165         &mut self,
 166         items: &[P<Item>],
 167         spans: &ModSpans,
 168     ) -> &'hir hir::Mod<'hir> {
 169         self.arena.alloc(hir::Mod {
 170             spans: hir::ModSpans {
 171                 inner_span: self.lower_span(spans.inner_span),
 172                 inject_use_span: self.lower_span(spans.inject_use_span),
 173             },
 174             item_ids: self.arena.alloc_from_iter(items.iter().flat_map(|x| self.lower_item_ref(x))),
 175         })
 176     }
 177
 178     pub(super) fn lower_item_ref(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
 179         let mut node_ids =
 180             smallvec![hir::ItemId { owner_id: hir::OwnerId { def_id: self.local_def_id(i.id) } }];
 181         if let ItemKind::Use(ref use_tree) = &i.kind {
 182             self.lower_item_id_use_tree(use_tree, i.id, &mut node_ids);
 183         }
 184         node_ids
 185     }
 186
 187     fn lower_item_id_use_tree(
 188         &mut self,
 189         tree: &UseTree,
 190         base_id: NodeId,
 191         vec: &mut SmallVec<[hir::ItemId; 1]>,
 192     ) {
 193         match tree.kind {
 194             UseTreeKind::Nested(ref nested_vec) => {
 195                 for &(ref nested, id) in nested_vec {
 196                     vec.push(hir::ItemId {
 197                         owner_id: hir::OwnerId { def_id: self.local_def_id(id) },
 198                     });
 199                     self.lower_item_id_use_tree(nested, id, vec);
 200                 }
 201             }
 202             UseTreeKind::Glob => {}
 203             UseTreeKind::Simple(_, id1, id2) => {
 204                 for (_, &id) in
 205                     iter::zip(self.expect_full_res_from_use(base_id).skip(1), &[id1, id2])
 206                 {
 207                     vec.push(hir::ItemId {
 208                         owner_id: hir::OwnerId { def_id: self.local_def_id(id) },
 209                     });
 210                 }
 211             }
 212         }
 213     }
 214
 215     fn lower_item(&mut self, i: &Item) -> &'hir hir::Item<'hir> {
 216         let mut ident = i.ident;
 217         let vis_span = self.lower_span(i.vis.span);
 218         let hir_id = self.lower_node_id(i.id);
 219         let attrs = self.lower_attrs(hir_id, &i.attrs);
 220         let kind = self.lower_item_kind(i.span, i.id, hir_id, &mut ident, attrs, vis_span, &i.kind);
 221         let item = hir::Item {
 222             owner_id: hir_id.expect_owner(),
 223             ident: self.lower_ident(ident),
 224             kind,
 225             vis_span,
 226             span: self.lower_span(i.span),
 227         };
 228         self.arena.alloc(item)
 229     }
 230
 231     fn lower_item_kind(
 232         &mut self,
 233         span: Span,
 234         id: NodeId,
 235         hir_id: hir::HirId,
 236         ident: &mut Ident,
 237         attrs: Option<&'hir [Attribute]>,
 238         vis_span: Span,
 239         i: &ItemKind,
 240     ) -> hir::ItemKind<'hir> {
 241         match *i {
 242             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
 243             ItemKind::Use(ref use_tree) => {
 244                 // Start with an empty prefix.
 245                 let prefix = Path { segments: ThinVec::new(), span: use_tree.span, tokens: None };
 246
 247                 self.lower_use_tree(use_tree, &prefix, id, vis_span, ident, attrs)
 248             }
 249             ItemKind::Static(ref t, m, ref e) => {
 250                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 251                 hir::ItemKind::Static(ty, m, body_id)
 252             }
 253             ItemKind::Const(_, ref t, ref e) => {
 254                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 255                 hir::ItemKind::Const(ty, body_id)
 256             }
 257             ItemKind::Fn(box Fn {
 258                 sig: FnSig { ref decl, header, span: fn_sig_span },
 259                 ref generics,
 260                 ref body,
 261                 ..
 262             }) => {
 263                 self.with_new_scopes(|this| {
 264                     this.current_item = Some(ident.span);
 265
 266                     // Note: we don't need to change the return type from `T` to
 267                     // `impl Future<Output = T>` here because lower_body
 268                     // only cares about the input argument patterns in the function
 269                     // declaration (decl), not the return types.
 270                     let asyncness = header.asyncness;
 271                     let body_id =
 272                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 273
 274                     let mut itctx = ImplTraitContext::Universal;
 275                     let (generics, decl) = this.lower_generics(generics, id, &mut itctx, |this| {
 276                         let ret_id = asyncness.opt_return_id();
 277                         this.lower_fn_decl(&decl, Some(id), fn_sig_span, FnDeclKind::Fn, ret_id)
 278                     });
 279                     let sig = hir::FnSig {
 280                         decl,
 281                         header: this.lower_fn_header(header),
 282                         span: this.lower_span(fn_sig_span),
 283                     };
 284                     hir::ItemKind::Fn(sig, generics, body_id)
 285                 })
 286             }
 287             ItemKind::Mod(_, ref mod_kind) => match mod_kind {
 288                 ModKind::Loaded(items, _, spans) => {
 289                     hir::ItemKind::Mod(self.lower_mod(items, spans))
 290                 }
 291                 ModKind::Unloaded => panic!("`mod` items should have been loaded by now"),
 292             },
 293             ItemKind::ForeignMod(ref fm) => hir::ItemKind::ForeignMod {
 294                 abi: fm.abi.map_or(abi::Abi::FALLBACK, |abi| self.lower_abi(abi)),
 295                 items: self
 296                     .arena
 297                     .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
 298             },
 299             ItemKind::GlobalAsm(ref asm) => {
 300                 hir::ItemKind::GlobalAsm(self.lower_inline_asm(span, asm))
 301             }
 302             ItemKind::TyAlias(box TyAlias {
 303                 ref generics,
 304                 where_clauses,
 305                 ty: Some(ref ty),
 306                 ..
 307             }) => {
 308                 // We lower
 309                 //
 310                 // type Foo = impl Trait
 311                 //
 312                 // to
 313                 //
 314                 // type Foo = Foo1
 315                 // opaque type Foo1: Trait
 316                 let mut generics = generics.clone();
 317                 add_ty_alias_where_clause(&mut generics, where_clauses, true);
 318                 let (generics, ty) = self.lower_generics(
 319                     &generics,
 320                     id,
 321                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 322                     |this| this.lower_ty(ty, &ImplTraitContext::TypeAliasesOpaqueTy),
 323                 );
 324                 hir::ItemKind::TyAlias(ty, generics)
 325             }
 326             ItemKind::TyAlias(box TyAlias {
 327                 ref generics, ref where_clauses, ty: None, ..
 328             }) => {
 329                 let mut generics = generics.clone();
 330                 add_ty_alias_where_clause(&mut generics, *where_clauses, true);
 331                 let (generics, ty) = self.lower_generics(
 332                     &generics,
 333                     id,
 334                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 335                     |this| this.arena.alloc(this.ty(span, hir::TyKind::Err)),
 336                 );
 337                 hir::ItemKind::TyAlias(ty, generics)
 338             }
 339             ItemKind::Enum(ref enum_definition, ref generics) => {
 340                 let (generics, variants) = self.lower_generics(
 341                     generics,
 342                     id,
 343                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 344                     |this| {
 345                         this.arena.alloc_from_iter(
 346                             enum_definition.variants.iter().map(|x| this.lower_variant(x)),
 347                         )
 348                     },
 349                 );
 350                 hir::ItemKind::Enum(hir::EnumDef { variants }, generics)
 351             }
 352             ItemKind::Struct(ref struct_def, ref generics) => {
 353                 let (generics, struct_def) = self.lower_generics(
 354                     generics,
 355                     id,
 356                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 357                     |this| this.lower_variant_data(hir_id, struct_def),
 358                 );
 359                 hir::ItemKind::Struct(struct_def, generics)
 360             }
 361             ItemKind::Union(ref vdata, ref generics) => {
 362                 let (generics, vdata) = self.lower_generics(
 363                     generics,
 364                     id,
 365                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 366                     |this| this.lower_variant_data(hir_id, vdata),
 367                 );
 368                 hir::ItemKind::Union(vdata, generics)
 369             }
 370             ItemKind::Impl(box Impl {
 371                 unsafety,
 372                 polarity,
 373                 defaultness,
 374                 constness,
 375                 generics: ref ast_generics,
 376                 of_trait: ref trait_ref,
 377                 self_ty: ref ty,
 378                 items: ref impl_items,
 379             }) => {
 380                 // Lower the "impl header" first. This ordering is important
 381                 // for in-band lifetimes! Consider `'a` here:
 382                 //
 383                 //     impl Foo<'a> for u32 {
 384                 //         fn method(&'a self) { .. }
 385                 //     }
 386                 //
 387                 // Because we start by lowering the `Foo<'a> for u32`
 388                 // part, we will add `'a` to the list of generics on
 389                 // the impl. When we then encounter it later in the
 390                 // method, it will not be considered an in-band
 391                 // lifetime to be added, but rather a reference to a
 392                 // parent lifetime.
 393                 let mut itctx = ImplTraitContext::Universal;
 394                 let (generics, (trait_ref, lowered_ty)) =
 395                     self.lower_generics(ast_generics, id, &mut itctx, |this| {
 396                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 397                             this.lower_trait_ref(
 398                                 trait_ref,
 399                                 &ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
 400                             )
 401                         });
 402
 403                         let lowered_ty = this
 404                             .lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 405
 406                         (trait_ref, lowered_ty)
 407                     });
 408
 409                 let new_impl_items = self
 410                     .arena
 411                     .alloc_from_iter(impl_items.iter().map(|item| self.lower_impl_item_ref(item)));
 412
 413                 // `defaultness.has_value()` is never called for an `impl`, always `true` in order
 414                 // to not cause an assertion failure inside the `lower_defaultness` function.
 415                 let has_val = true;
 416                 let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
 417                 let polarity = match polarity {
 418                     ImplPolarity::Positive => ImplPolarity::Positive,
 419                     ImplPolarity::Negative(s) => ImplPolarity::Negative(self.lower_span(s)),
 420                 };
 421                 hir::ItemKind::Impl(self.arena.alloc(hir::Impl {
 422                     unsafety: self.lower_unsafety(unsafety),
 423                     polarity,
 424                     defaultness,
 425                     defaultness_span,
 426                     constness: self.lower_constness(constness),
 427                     generics,
 428                     of_trait: trait_ref,
 429                     self_ty: lowered_ty,
 430                     items: new_impl_items,
 431                 }))
 432             }
 433             ItemKind::Trait(box Trait {
 434                 is_auto,
 435                 unsafety,
 436                 ref generics,
 437                 ref bounds,
 438                 ref items,
 439             }) => {
 440                 let (generics, (unsafety, items, bounds)) = self.lower_generics(
 441                     generics,
 442                     id,
 443                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 444                     |this| {
 445                         let bounds = this.lower_param_bounds(
 446                             bounds,
 447                             &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
 448                         );
 449                         let items = this.arena.alloc_from_iter(
 450                             items.iter().map(|item| this.lower_trait_item_ref(item)),
 451                         );
 452                         let unsafety = this.lower_unsafety(unsafety);
 453                         (unsafety, items, bounds)
 454                     },
 455                 );
 456                 hir::ItemKind::Trait(is_auto, unsafety, generics, bounds, items)
 457             }
 458             ItemKind::TraitAlias(ref generics, ref bounds) => {
 459                 let (generics, bounds) = self.lower_generics(
 460                     generics,
 461                     id,
 462                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 463                     |this| {
 464                         this.lower_param_bounds(
 465                             bounds,
 466                             &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
 467                         )
 468                     },
 469                 );
 470                 hir::ItemKind::TraitAlias(generics, bounds)
 471             }
 472             ItemKind::MacroDef(MacroDef { ref body, macro_rules }) => {
 473                 let body = P(self.lower_mac_args(body));
 474                 let macro_kind = self.resolver.decl_macro_kind(self.local_def_id(id));
 475                 hir::ItemKind::Macro(ast::MacroDef { body, macro_rules }, macro_kind)
 476             }
 477             ItemKind::MacCall(..) => {
 478                 panic!("`TyMac` should have been expanded by now")
 479             }
 480         }
 481     }
 482
 483     fn lower_const_item(
 484         &mut self,
 485         ty: &Ty,
 486         span: Span,
 487         body: Option<&Expr>,
 488     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 489         let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 490         (ty, self.lower_const_body(span, body))
 491     }
 492
 493     #[instrument(level = "debug", skip(self))]
 494     fn lower_use_tree(
 495         &mut self,
 496         tree: &UseTree,
 497         prefix: &Path,
 498         id: NodeId,
 499         vis_span: Span,
 500         ident: &mut Ident,
 501         attrs: Option<&'hir [Attribute]>,
 502     ) -> hir::ItemKind<'hir> {
 503         let path = &tree.prefix;
 504         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 505
 506         match tree.kind {
 507             UseTreeKind::Simple(rename, id1, id2) => {
 508                 *ident = tree.ident();
 509
 510                 // First, apply the prefix to the path.
 511                 let mut path = Path { segments, span: path.span, tokens: None };
 512
 513                 // Correctly resolve `self` imports.
 514                 if path.segments.len() > 1
 515                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 516                 {
 517                     let _ = path.segments.pop();
 518                     if rename.is_none() {
 519                         *ident = path.segments.last().unwrap().ident;
 520                     }
 521                 }
 522
 523                 let mut resolutions = self.expect_full_res_from_use(id).fuse();
 524                 // We want to return *something* from this function, so hold onto the first item
 525                 // for later.
 526                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 527
 528                 // Here, we are looping over namespaces, if they exist for the definition
 529                 // being imported. We only handle type and value namespaces because we
 530                 // won't be dealing with macros in the rest of the compiler.
 531                 // Essentially a single `use` which imports two names is desugared into
 532                 // two imports.
 533                 for new_node_id in [id1, id2] {
 534                     let new_id = self.local_def_id(new_node_id);
 535                     let Some(res) = resolutions.next() else {
 536                         debug_assert!(self.children.iter().find(|(id, _)| id == &new_id).is_none());
 537                         // Associate an HirId to both ids even if there is no resolution.
 538                         self.children.push((
 539                             new_id,
 540                             hir::MaybeOwner::NonOwner(hir::HirId::make_owner(new_id))),
 541                         );
 542                         continue;
 543                     };
 544                     let ident = *ident;
 545                     let mut path = path.clone();
 546                     for seg in &mut path.segments {
 547                         // Give the cloned segment the same resolution information
 548                         // as the old one (this is needed for stability checking).
 549                         let new_id = self.next_node_id();
 550                         self.resolver.clone_res(seg.id, new_id);
 551                         seg.id = new_id;
 552                     }
 553                     let span = path.span;
 554
 555                     self.with_hir_id_owner(new_node_id, |this| {
 556                         let res = this.lower_res(res);
 557                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit);
 558                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 559                         if let Some(attrs) = attrs {
 560                             this.attrs.insert(hir::ItemLocalId::new(0), attrs);
 561                         }
 562
 563                         let item = hir::Item {
 564                             owner_id: hir::OwnerId { def_id: new_id },
 565                             ident: this.lower_ident(ident),
 566                             kind,
 567                             vis_span,
 568                             span: this.lower_span(span),
 569                         };
 570                         hir::OwnerNode::Item(this.arena.alloc(item))
 571                     });
 572                 }
 573
 574                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit);
 575                 hir::ItemKind::Use(path, hir::UseKind::Single)
 576             }
 577             UseTreeKind::Glob => {
 578                 let path = self.lower_path(
 579                     id,
 580                     &Path { segments, span: path.span, tokens: None },
 581                     ParamMode::Explicit,
 582                 );
 583                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 584             }
 585             UseTreeKind::Nested(ref trees) => {
 586                 // Nested imports are desugared into simple imports.
 587                 // So, if we start with
 588                 //
 589                 // ```
 590                 // pub(x) use foo::{a, b};
 591                 // ```
 592                 //
 593                 // we will create three items:
 594                 //
 595                 // ```
 596                 // pub(x) use foo::a;
 597                 // pub(x) use foo::b;
 598                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 599                 // ```
 600                 //
 601                 // The first two are produced by recursively invoking
 602                 // `lower_use_tree` (and indeed there may be things
 603                 // like `use foo::{a::{b, c}}` and so forth).  They
 604                 // wind up being directly added to
 605                 // `self.items`. However, the structure of this
 606                 // function also requires us to return one item, and
 607                 // for that we return the `{}` import (called the
 608                 // `ListStem`).
 609
 610                 let prefix = Path { segments, span: prefix.span.to(path.span), tokens: None };
 611
 612                 // Add all the nested `PathListItem`s to the HIR.
 613                 for &(ref use_tree, id) in trees {
 614                     let new_hir_id = self.local_def_id(id);
 615
 616                     let mut prefix = prefix.clone();
 617
 618                     // Give the segments new node-ids since they are being cloned.
 619                     for seg in &mut prefix.segments {
 620                         // Give the cloned segment the same resolution information
 621                         // as the old one (this is needed for stability checking).
 622                         let new_id = self.next_node_id();
 623                         self.resolver.clone_res(seg.id, new_id);
 624                         seg.id = new_id;
 625                     }
 626
 627                     // Each `use` import is an item and thus are owners of the
 628                     // names in the path. Up to this point the nested import is
 629                     // the current owner, since we want each desugared import to
 630                     // own its own names, we have to adjust the owner before
 631                     // lowering the rest of the import.
 632                     self.with_hir_id_owner(id, |this| {
 633                         let mut ident = *ident;
 634
 635                         let kind =
 636                             this.lower_use_tree(use_tree, &prefix, id, vis_span, &mut ident, attrs);
 637                         if let Some(attrs) = attrs {
 638                             this.attrs.insert(hir::ItemLocalId::new(0), attrs);
 639                         }
 640
 641                         let item = hir::Item {
 642                             owner_id: hir::OwnerId { def_id: new_hir_id },
 643                             ident: this.lower_ident(ident),
 644                             kind,
 645                             vis_span,
 646                             span: this.lower_span(use_tree.span),
 647                         };
 648                         hir::OwnerNode::Item(this.arena.alloc(item))
 649                     });
 650                 }
 651
 652                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 653                 let res = self.lower_res(res);
 654                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit);
 655                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 656             }
 657         }
 658     }
 659
 660     fn lower_foreign_item(&mut self, i: &ForeignItem) -> &'hir hir::ForeignItem<'hir> {
 661         let hir_id = self.lower_node_id(i.id);
 662         let owner_id = hir_id.expect_owner();
 663         self.lower_attrs(hir_id, &i.attrs);
 664         let item = hir::ForeignItem {
 665             owner_id,
 666             ident: self.lower_ident(i.ident),
 667             kind: match i.kind {
 668                 ForeignItemKind::Fn(box Fn { ref sig, ref generics, .. }) => {
 669                     let fdec = &sig.decl;
 670                     let mut itctx = ImplTraitContext::Universal;
 671                     let (generics, (fn_dec, fn_args)) =
 672                         self.lower_generics(generics, i.id, &mut itctx, |this| {
 673                             (
 674                                 // Disallow `impl Trait` in foreign items.
 675                                 this.lower_fn_decl(
 676                                     fdec,
 677                                     None,
 678                                     sig.span,
 679                                     FnDeclKind::ExternFn,
 680                                     None,
 681                                 ),
 682                                 this.lower_fn_params_to_names(fdec),
 683                             )
 684                         });
 685
 686                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 687                 }
 688                 ForeignItemKind::Static(ref t, m, _) => {
 689                     let ty =
 690                         self.lower_ty(t, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 691                     hir::ForeignItemKind::Static(ty, m)
 692                 }
 693                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 694                 ForeignItemKind::MacCall(_) => panic!("macro shouldn't exist here"),
 695             },
 696             vis_span: self.lower_span(i.vis.span),
 697             span: self.lower_span(i.span),
 698         };
 699         self.arena.alloc(item)
 700     }
 701
 702     fn lower_foreign_item_ref(&mut self, i: &ForeignItem) -> hir::ForeignItemRef {
 703         hir::ForeignItemRef {
 704             id: hir::ForeignItemId { owner_id: hir::OwnerId { def_id: self.local_def_id(i.id) } },
 705             ident: self.lower_ident(i.ident),
 706             span: self.lower_span(i.span),
 707         }
 708     }
 709
 710     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 711         let hir_id = self.lower_node_id(v.id);
 712         self.lower_attrs(hir_id, &v.attrs);
 713         hir::Variant {
 714             hir_id,
 715             def_id: self.local_def_id(v.id),
 716             data: self.lower_variant_data(hir_id, &v.data),
 717             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 718             ident: self.lower_ident(v.ident),
 719             span: self.lower_span(v.span),
 720         }
 721     }
 722
 723     fn lower_variant_data(
 724         &mut self,
 725         parent_id: hir::HirId,
 726         vdata: &VariantData,
 727     ) -> hir::VariantData<'hir> {
 728         match *vdata {
 729             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 730                 self.arena
 731                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f))),
 732                 recovered,
 733             ),
 734             VariantData::Tuple(ref fields, id) => {
 735                 let ctor_id = self.lower_node_id(id);
 736                 self.alias_attrs(ctor_id, parent_id);
 737                 hir::VariantData::Tuple(
 738                     self.arena.alloc_from_iter(
 739                         fields.iter().enumerate().map(|f| self.lower_field_def(f)),
 740                     ),
 741                     ctor_id,
 742                     self.local_def_id(id),
 743                 )
 744             }
 745             VariantData::Unit(id) => {
 746                 let ctor_id = self.lower_node_id(id);
 747                 self.alias_attrs(ctor_id, parent_id);
 748                 hir::VariantData::Unit(ctor_id, self.local_def_id(id))
 749             }
 750         }
 751     }
 752
 753     fn lower_field_def(&mut self, (index, f): (usize, &FieldDef)) -> hir::FieldDef<'hir> {
 754         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 755             let t = self.lower_path_ty(
 756                 &f.ty,
 757                 qself,
 758                 path,
 759                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 760                 &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 761             );
 762             self.arena.alloc(t)
 763         } else {
 764             self.lower_ty(&f.ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type))
 765         };
 766         let hir_id = self.lower_node_id(f.id);
 767         self.lower_attrs(hir_id, &f.attrs);
 768         hir::FieldDef {
 769             span: self.lower_span(f.span),
 770             hir_id,
 771             def_id: self.local_def_id(f.id),
 772             ident: match f.ident {
 773                 Some(ident) => self.lower_ident(ident),
 774                 // FIXME(jseyfried): positional field hygiene.
 775                 None => Ident::new(sym::integer(index), self.lower_span(f.span)),
 776             },
 777             vis_span: self.lower_span(f.vis.span),
 778             ty,
 779         }
 780     }
 781
 782     fn lower_trait_item(&mut self, i: &AssocItem) -> &'hir hir::TraitItem<'hir> {
 783         let hir_id = self.lower_node_id(i.id);
 784         let trait_item_def_id = hir_id.expect_owner();
 785
 786         let (generics, kind, has_default) = match i.kind {
 787             AssocItemKind::Const(_, ref ty, ref default) => {
 788                 let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 789                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 790                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body), body.is_some())
 791             }
 792             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: None, .. }) => {
 793                 let asyncness = sig.header.asyncness;
 794                 let names = self.lower_fn_params_to_names(&sig.decl);
 795                 let (generics, sig) = self.lower_method_sig(
 796                     generics,
 797                     sig,
 798                     i.id,
 799                     FnDeclKind::Trait,
 800                     asyncness.opt_return_id(),
 801                 );
 802                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)), false)
 803             }
 804             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: Some(ref body), .. }) => {
 805                 let asyncness = sig.header.asyncness;
 806                 let body_id =
 807                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, Some(&body));
 808                 let (generics, sig) = self.lower_method_sig(
 809                     generics,
 810                     sig,
 811                     i.id,
 812                     FnDeclKind::Trait,
 813                     asyncness.opt_return_id(),
 814                 );
 815                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)), true)
 816             }
 817             AssocItemKind::Type(box TyAlias {
 818                 ref generics,
 819                 where_clauses,
 820                 ref bounds,
 821                 ref ty,
 822                 ..
 823             }) => {
 824                 let mut generics = generics.clone();
 825                 add_ty_alias_where_clause(&mut generics, where_clauses, false);
 826                 let (generics, kind) = self.lower_generics(
 827                     &generics,
 828                     i.id,
 829                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 830                     |this| {
 831                         let ty = ty.as_ref().map(|x| {
 832                             this.lower_ty(x, &ImplTraitContext::Disallowed(ImplTraitPosition::Type))
 833                         });
 834                         hir::TraitItemKind::Type(
 835                             this.lower_param_bounds(
 836                                 bounds,
 837                                 &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 838                             ),
 839                             ty,
 840                         )
 841                     },
 842                 );
 843                 (generics, kind, ty.is_some())
 844             }
 845             AssocItemKind::MacCall(..) => panic!("macro item shouldn't exist at this point"),
 846         };
 847
 848         self.lower_attrs(hir_id, &i.attrs);
 849         let item = hir::TraitItem {
 850             owner_id: trait_item_def_id,
 851             ident: self.lower_ident(i.ident),
 852             generics,
 853             kind,
 854             span: self.lower_span(i.span),
 855             defaultness: hir::Defaultness::Default { has_value: has_default },
 856         };
 857         self.arena.alloc(item)
 858     }
 859
 860     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 861         let kind = match &i.kind {
 862             AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 863             AssocItemKind::Type(..) => hir::AssocItemKind::Type,
 864             AssocItemKind::Fn(box Fn { sig, .. }) => {
 865                 hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
 866             }
 867             AssocItemKind::MacCall(..) => unimplemented!(),
 868         };
 869         let id = hir::TraitItemId { owner_id: hir::OwnerId { def_id: self.local_def_id(i.id) } };
 870         hir::TraitItemRef {
 871             id,
 872             ident: self.lower_ident(i.ident),
 873             span: self.lower_span(i.span),
 874             kind,
 875         }
 876     }
 877
 878     /// Construct `ExprKind::Err` for the given `span`.
 879     pub(crate) fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 880         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 881     }
 882
 883     fn lower_impl_item(&mut self, i: &AssocItem) -> &'hir hir::ImplItem<'hir> {
 884         // Since `default impl` is not yet implemented, this is always true in impls.
 885         let has_value = true;
 886         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 887
 888         let (generics, kind) = match &i.kind {
 889             AssocItemKind::Const(_, ty, expr) => {
 890                 let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 891                 (
 892                     hir::Generics::empty(),
 893                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 894                 )
 895             }
 896             AssocItemKind::Fn(box Fn { sig, generics, body, .. }) => {
 897                 self.current_item = Some(i.span);
 898                 let asyncness = sig.header.asyncness;
 899                 let body_id =
 900                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 901                 let (generics, sig) = self.lower_method_sig(
 902                     generics,
 903                     sig,
 904                     i.id,
 905                     if self.is_in_trait_impl { FnDeclKind::Impl } else { FnDeclKind::Inherent },
 906                     asyncness.opt_return_id(),
 907                 );
 908
 909                 (generics, hir::ImplItemKind::Fn(sig, body_id))
 910             }
 911             AssocItemKind::Type(box TyAlias { generics, where_clauses, ty, .. }) => {
 912                 let mut generics = generics.clone();
 913                 add_ty_alias_where_clause(&mut generics, *where_clauses, false);
 914                 self.lower_generics(
 915                     &generics,
 916                     i.id,
 917                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 918                     |this| match ty {
 919                         None => {
 920                             let ty = this.arena.alloc(this.ty(i.span, hir::TyKind::Err));
 921                             hir::ImplItemKind::Type(ty)
 922                         }
 923                         Some(ty) => {
 924                             let ty = this.lower_ty(ty, &ImplTraitContext::TypeAliasesOpaqueTy);
 925                             hir::ImplItemKind::Type(ty)
 926                         }
 927                     },
 928                 )
 929             }
 930             AssocItemKind::MacCall(..) => panic!("`TyMac` should have been expanded by now"),
 931         };
 932
 933         let hir_id = self.lower_node_id(i.id);
 934         self.lower_attrs(hir_id, &i.attrs);
 935         let item = hir::ImplItem {
 936             owner_id: hir_id.expect_owner(),
 937             ident: self.lower_ident(i.ident),
 938             generics,
 939             kind,
 940             vis_span: self.lower_span(i.vis.span),
 941             span: self.lower_span(i.span),
 942             defaultness,
 943         };
 944         self.arena.alloc(item)
 945     }
 946
 947     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef {
 948         hir::ImplItemRef {
 949             id: hir::ImplItemId { owner_id: hir::OwnerId { def_id: self.local_def_id(i.id) } },
 950             ident: self.lower_ident(i.ident),
 951             span: self.lower_span(i.span),
 952             kind: match &i.kind {
 953                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 954                 AssocItemKind::Type(..) => hir::AssocItemKind::Type,
 955                 AssocItemKind::Fn(box Fn { sig, .. }) => {
 956                     hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
 957                 }
 958                 AssocItemKind::MacCall(..) => unimplemented!(),
 959             },
 960             trait_item_def_id: self
 961                 .resolver
 962                 .get_partial_res(i.id)
 963                 .map(|r| r.expect_full_res().def_id()),
 964         }
 965     }
 966
 967     fn lower_defaultness(
 968         &self,
 969         d: Defaultness,
 970         has_value: bool,
 971     ) -> (hir::Defaultness, Option<Span>) {
 972         match d {
 973             Defaultness::Default(sp) => {
 974                 (hir::Defaultness::Default { has_value }, Some(self.lower_span(sp)))
 975             }
 976             Defaultness::Final => {
 977                 assert!(has_value);
 978                 (hir::Defaultness::Final, None)
 979             }
 980         }
 981     }
 982
 983     fn record_body(
 984         &mut self,
 985         params: &'hir [hir::Param<'hir>],
 986         value: hir::Expr<'hir>,
 987     ) -> hir::BodyId {
 988         let body = hir::Body {
 989             generator_kind: self.generator_kind,
 990             params,
 991             value: self.arena.alloc(value),
 992         };
 993         let id = body.id();
 994         debug_assert_eq!(id.hir_id.owner, self.current_hir_id_owner);
 995         self.bodies.push((id.hir_id.local_id, self.arena.alloc(body)));
 996         id
 997     }
 998
 999     pub(super) fn lower_body(
1000         &mut self,
1001         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
1002     ) -> hir::BodyId {
1003         let prev_gen_kind = self.generator_kind.take();
1004         let task_context = self.task_context.take();
1005         let (parameters, result) = f(self);
1006         let body_id = self.record_body(parameters, result);
1007         self.task_context = task_context;
1008         self.generator_kind = prev_gen_kind;
1009         body_id
1010     }
1011
1012     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
1013         let hir_id = self.lower_node_id(param.id);
1014         self.lower_attrs(hir_id, &param.attrs);
1015         hir::Param {
1016             hir_id,
1017             pat: self.lower_pat(&param.pat),
1018             ty_span: self.lower_span(param.ty.span),
1019             span: self.lower_span(param.span),
1020         }
1021     }
1022
1023     pub(super) fn lower_fn_body(
1024         &mut self,
1025         decl: &FnDecl,
1026         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
1027     ) -> hir::BodyId {
1028         self.lower_body(|this| {
1029             (
1030                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
1031                 body(this),
1032             )
1033         })
1034     }
1035
1036     fn lower_fn_body_block(
1037         &mut self,
1038         span: Span,
1039         decl: &FnDecl,
1040         body: Option<&Block>,
1041     ) -> hir::BodyId {
1042         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1043     }
1044
1045     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1046         match block {
1047             Some(block) => self.lower_block_expr(block),
1048             None => self.expr_err(span),
1049         }
1050     }
1051
1052     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1053         self.lower_body(|this| {
1054             (
1055                 &[],
1056                 match expr {
1057                     Some(expr) => this.lower_expr_mut(expr),
1058                     None => this.expr_err(span),
1059                 },
1060             )
1061         })
1062     }
1063
1064     fn lower_maybe_async_body(
1065         &mut self,
1066         span: Span,
1067         decl: &FnDecl,
1068         asyncness: Async,
1069         body: Option<&Block>,
1070     ) -> hir::BodyId {
1071         let (closure_id, body) = match (asyncness, body) {
1072             (Async::Yes { closure_id, .. }, Some(body)) => (closure_id, body),
1073             _ => return self.lower_fn_body_block(span, decl, body),
1074         };
1075
1076         self.lower_body(|this| {
1077             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1078             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1079
1080             // Async function parameters are lowered into the closure body so that they are
1081             // captured and so that the drop order matches the equivalent non-async functions.
1082             //
1083             // from:
1084             //
1085             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1086             //         <body>
1087             //     }
1088             //
1089             // into:
1090             //
1091             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1092             //       async move {
1093             //         let __arg2 = __arg2;
1094             //         let <pattern> = __arg2;
1095             //         let __arg1 = __arg1;
1096             //         let <pattern> = __arg1;
1097             //         let __arg0 = __arg0;
1098             //         let <pattern> = __arg0;
1099             //         drop-temps { <body> } // see comments later in fn for details
1100             //       }
1101             //     }
1102             //
1103             // If `<pattern>` is a simple ident, then it is lowered to a single
1104             // `let <pattern> = <pattern>;` statement as an optimization.
1105             //
1106             // Note that the body is embedded in `drop-temps`; an
1107             // equivalent desugaring would be `return { <body>
1108             // };`. The key point is that we wish to drop all the
1109             // let-bound variables and temporaries created in the body
1110             // (and its tail expression!) before we drop the
1111             // parameters (c.f. rust-lang/rust#64512).
1112             for (index, parameter) in decl.inputs.iter().enumerate() {
1113                 let parameter = this.lower_param(parameter);
1114                 let span = parameter.pat.span;
1115
1116                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1117                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1118                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1119                     hir::PatKind::Binding(hir::BindingAnnotation(ByRef::No, _), _, ident, _) => {
1120                         (ident, true)
1121                     }
1122                     // For `ref mut` or wildcard arguments, we can't reuse the binding, but
1123                     // we can keep the same name for the parameter.
1124                     // This lets rustdoc render it correctly in documentation.
1125                     hir::PatKind::Binding(_, _, ident, _) => (ident, false),
1126                     hir::PatKind::Wild => {
1127                         (Ident::with_dummy_span(rustc_span::symbol::kw::Underscore), false)
1128                     }
1129                     _ => {
1130                         // Replace the ident for bindings that aren't simple.
1131                         let name = format!("__arg{}", index);
1132                         let ident = Ident::from_str(&name);
1133
1134                         (ident, false)
1135                     }
1136                 };
1137
1138                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1139
1140                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1141                 // async function.
1142                 //
1143                 // If this is the simple case, this parameter will end up being the same as the
1144                 // original parameter, but with a different pattern id.
1145                 let stmt_attrs = this.attrs.get(&parameter.hir_id.local_id).copied();
1146                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1147                 let new_parameter = hir::Param {
1148                     hir_id: parameter.hir_id,
1149                     pat: new_parameter_pat,
1150                     ty_span: this.lower_span(parameter.ty_span),
1151                     span: this.lower_span(parameter.span),
1152                 };
1153
1154                 if is_simple_parameter {
1155                     // If this is the simple case, then we only insert one statement that is
1156                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1157                     // `HirId`s are densely assigned.
1158                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1159                     let stmt = this.stmt_let_pat(
1160                         stmt_attrs,
1161                         desugared_span,
1162                         Some(expr),
1163                         parameter.pat,
1164                         hir::LocalSource::AsyncFn,
1165                     );
1166                     statements.push(stmt);
1167                 } else {
1168                     // If this is not the simple case, then we construct two statements:
1169                     //
1170                     // ```
1171                     // let __argN = __argN;
1172                     // let <pat> = __argN;
1173                     // ```
1174                     //
1175                     // The first statement moves the parameter into the closure and thus ensures
1176                     // that the drop order is correct.
1177                     //
1178                     // The second statement creates the bindings that the user wrote.
1179
1180                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1181                     // because the user may have specified a `ref mut` binding in the next
1182                     // statement.
1183                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1184                         desugared_span,
1185                         ident,
1186                         hir::BindingAnnotation::MUT,
1187                     );
1188                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1189                     let move_stmt = this.stmt_let_pat(
1190                         None,
1191                         desugared_span,
1192                         Some(move_expr),
1193                         move_pat,
1194                         hir::LocalSource::AsyncFn,
1195                     );
1196
1197                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1198                     // parameter's pattern so that `HirId`s are densely assigned.
1199                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1200                     let pattern_stmt = this.stmt_let_pat(
1201                         stmt_attrs,
1202                         desugared_span,
1203                         Some(pattern_expr),
1204                         parameter.pat,
1205                         hir::LocalSource::AsyncFn,
1206                     );
1207
1208                     statements.push(move_stmt);
1209                     statements.push(pattern_stmt);
1210                 };
1211
1212                 parameters.push(new_parameter);
1213             }
1214
1215             let async_expr = this.make_async_expr(
1216                 CaptureBy::Value,
1217                 closure_id,
1218                 None,
1219                 body.span,
1220                 hir::AsyncGeneratorKind::Fn,
1221                 |this| {
1222                     // Create a block from the user's function body:
1223                     let user_body = this.lower_block_expr(body);
1224
1225                     // Transform into `drop-temps { <user-body> }`, an expression:
1226                     let desugared_span =
1227                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1228                     let user_body = this.expr_drop_temps(
1229                         desugared_span,
1230                         this.arena.alloc(user_body),
1231                         AttrVec::new(),
1232                     );
1233
1234                     // As noted above, create the final block like
1235                     //
1236                     // ```
1237                     // {
1238                     //   let $param_pattern = $raw_param;
1239                     //   ...
1240                     //   drop-temps { <user-body> }
1241                     // }
1242                     // ```
1243                     let body = this.block_all(
1244                         desugared_span,
1245                         this.arena.alloc_from_iter(statements),
1246                         Some(user_body),
1247                     );
1248
1249                     this.expr_block(body, AttrVec::new())
1250                 },
1251             );
1252
1253             (
1254                 this.arena.alloc_from_iter(parameters),
1255                 this.expr(body.span, async_expr, AttrVec::new()),
1256             )
1257         })
1258     }
1259
1260     fn lower_method_sig(
1261         &mut self,
1262         generics: &Generics,
1263         sig: &FnSig,
1264         id: NodeId,
1265         kind: FnDeclKind,
1266         is_async: Option<(NodeId, Span)>,
1267     ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
1268         let header = self.lower_fn_header(sig.header);
1269         let mut itctx = ImplTraitContext::Universal;
1270         let (generics, decl) = self.lower_generics(generics, id, &mut itctx, |this| {
1271             this.lower_fn_decl(&sig.decl, Some(id), sig.span, kind, is_async)
1272         });
1273         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
1274     }
1275
1276     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1277         hir::FnHeader {
1278             unsafety: self.lower_unsafety(h.unsafety),
1279             asyncness: self.lower_asyncness(h.asyncness),
1280             constness: self.lower_constness(h.constness),
1281             abi: self.lower_extern(h.ext),
1282         }
1283     }
1284
1285     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1286         abi::lookup(abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1287             self.error_on_invalid_abi(abi);
1288             abi::Abi::Rust
1289         })
1290     }
1291
1292     pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
1293         match ext {
1294             Extern::None => abi::Abi::Rust,
1295             Extern::Implicit(_) => abi::Abi::FALLBACK,
1296             Extern::Explicit(abi, _) => self.lower_abi(abi),
1297         }
1298     }
1299
1300     fn error_on_invalid_abi(&self, abi: StrLit) {
1301         let abi_names = abi::enabled_names(self.tcx.features(), abi.span)
1302             .iter()
1303             .map(|s| Symbol::intern(s))
1304             .collect::<Vec<_>>();
1305         let suggested_name = find_best_match_for_name(&abi_names, abi.symbol_unescaped, None);
1306         self.tcx.sess.emit_err(InvalidAbi {
1307             abi: abi.symbol_unescaped,
1308             span: abi.span,
1309             suggestion: suggested_name.map(|suggested_name| InvalidAbiSuggestion {
1310                 span: abi.span,
1311                 suggestion: format!("\"{suggested_name}\""),
1312             }),
1313             command: "rustc --print=calling-conventions".to_string(),
1314         });
1315     }
1316
1317     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1318         match a {
1319             Async::Yes { .. } => hir::IsAsync::Async,
1320             Async::No => hir::IsAsync::NotAsync,
1321         }
1322     }
1323
1324     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1325         match c {
1326             Const::Yes(_) => hir::Constness::Const,
1327             Const::No => hir::Constness::NotConst,
1328         }
1329     }
1330
1331     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1332         match u {
1333             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1334             Unsafe::No => hir::Unsafety::Normal,
1335         }
1336     }
1337
1338     /// Return the pair of the lowered `generics` as `hir::Generics` and the evaluation of `f` with
1339     /// the carried impl trait definitions and bounds.
1340     #[instrument(level = "debug", skip(self, f))]
1341     fn lower_generics<T>(
1342         &mut self,
1343         generics: &Generics,
1344         parent_node_id: NodeId,
1345         itctx: &ImplTraitContext,
1346         f: impl FnOnce(&mut Self) -> T,
1347     ) -> (&'hir hir::Generics<'hir>, T) {
1348         debug_assert!(self.impl_trait_defs.is_empty());
1349         debug_assert!(self.impl_trait_bounds.is_empty());
1350
1351         // Error if `?Trait` bounds in where clauses don't refer directly to type parameters.
1352         // Note: we used to clone these bounds directly onto the type parameter (and avoid lowering
1353         // these into hir when we lower thee where clauses), but this makes it quite difficult to
1354         // keep track of the Span info. Now, `add_implicitly_sized` in `AstConv` checks both param bounds and
1355         // where clauses for `?Sized`.
1356         for pred in &generics.where_clause.predicates {
1357             let WherePredicate::BoundPredicate(ref bound_pred) = *pred else {
1358                 continue;
1359             };
1360             let compute_is_param = || {
1361                 // Check if the where clause type is a plain type parameter.
1362                 match self
1363                     .resolver
1364                     .get_partial_res(bound_pred.bounded_ty.id)
1365                     .and_then(|r| r.full_res())
1366                 {
1367                     Some(Res::Def(DefKind::TyParam, def_id))
1368                         if bound_pred.bound_generic_params.is_empty() =>
1369                     {
1370                         generics
1371                             .params
1372                             .iter()
1373                             .any(|p| def_id == self.local_def_id(p.id).to_def_id())
1374                     }
1375                     // Either the `bounded_ty` is not a plain type parameter, or
1376                     // it's not found in the generic type parameters list.
1377                     _ => false,
1378                 }
1379             };
1380             // We only need to compute this once per `WherePredicate`, but don't
1381             // need to compute this at all unless there is a Maybe bound.
1382             let mut is_param: Option<bool> = None;
1383             for bound in &bound_pred.bounds {
1384                 if !matches!(*bound, GenericBound::Trait(_, TraitBoundModifier::Maybe)) {
1385                     continue;
1386                 }
1387                 let is_param = *is_param.get_or_insert_with(compute_is_param);
1388                 if !is_param {
1389                     self.tcx.sess.emit_err(MisplacedRelaxTraitBound { span: bound.span() });
1390                 }
1391             }
1392         }
1393
1394         let mut predicates: SmallVec<[hir::WherePredicate<'hir>; 4]> = SmallVec::new();
1395         predicates.extend(generics.params.iter().filter_map(|param| {
1396             self.lower_generic_bound_predicate(
1397                 param.ident,
1398                 param.id,
1399                 &param.kind,
1400                 &param.bounds,
1401                 itctx,
1402                 PredicateOrigin::GenericParam,
1403             )
1404         }));
1405         predicates.extend(
1406             generics
1407                 .where_clause
1408                 .predicates
1409                 .iter()
1410                 .map(|predicate| self.lower_where_predicate(predicate)),
1411         );
1412
1413         let mut params: SmallVec<[hir::GenericParam<'hir>; 4]> =
1414             self.lower_generic_params_mut(&generics.params).collect();
1415
1416         // Introduce extra lifetimes if late resolution tells us to.
1417         let extra_lifetimes = self.resolver.take_extra_lifetime_params(parent_node_id);
1418         params.extend(extra_lifetimes.into_iter().filter_map(|(ident, node_id, res)| {
1419             self.lifetime_res_to_generic_param(ident, node_id, res)
1420         }));
1421
1422         let has_where_clause_predicates = !generics.where_clause.predicates.is_empty();
1423         let where_clause_span = self.lower_span(generics.where_clause.span);
1424         let span = self.lower_span(generics.span);
1425         let res = f(self);
1426
1427         let impl_trait_defs = std::mem::take(&mut self.impl_trait_defs);
1428         params.extend(impl_trait_defs.into_iter());
1429
1430         let impl_trait_bounds = std::mem::take(&mut self.impl_trait_bounds);
1431         predicates.extend(impl_trait_bounds.into_iter());
1432
1433         let lowered_generics = self.arena.alloc(hir::Generics {
1434             params: self.arena.alloc_from_iter(params),
1435             predicates: self.arena.alloc_from_iter(predicates),
1436             has_where_clause_predicates,
1437             where_clause_span,
1438             span,
1439         });
1440
1441         (lowered_generics, res)
1442     }
1443
1444     pub(super) fn lower_generic_bound_predicate(
1445         &mut self,
1446         ident: Ident,
1447         id: NodeId,
1448         kind: &GenericParamKind,
1449         bounds: &[GenericBound],
1450         itctx: &ImplTraitContext,
1451         origin: PredicateOrigin,
1452     ) -> Option<hir::WherePredicate<'hir>> {
1453         // Do not create a clause if we do not have anything inside it.
1454         if bounds.is_empty() {
1455             return None;
1456         }
1457
1458         let bounds = self.lower_param_bounds(bounds, itctx);
1459
1460         let ident = self.lower_ident(ident);
1461         let param_span = ident.span;
1462         let span = bounds
1463             .iter()
1464             .fold(Some(param_span.shrink_to_hi()), |span: Option<Span>, bound| {
1465                 let bound_span = bound.span();
1466                 // We include bounds that come from a `#[derive(_)]` but point at the user's code,
1467                 // as we use this method to get a span appropriate for suggestions.
1468                 if !bound_span.can_be_used_for_suggestions() {
1469                     None
1470                 } else if let Some(span) = span {
1471                     Some(span.to(bound_span))
1472                 } else {
1473                     Some(bound_span)
1474                 }
1475             })
1476             .unwrap_or(param_span.shrink_to_hi());
1477         match kind {
1478             GenericParamKind::Const { .. } => None,
1479             GenericParamKind::Type { .. } => {
1480                 let def_id = self.local_def_id(id).to_def_id();
1481                 let hir_id = self.next_id();
1482                 let res = Res::Def(DefKind::TyParam, def_id);
1483                 let ty_path = self.arena.alloc(hir::Path {
1484                     span: param_span,
1485                     res,
1486                     segments: self
1487                         .arena
1488                         .alloc_from_iter([hir::PathSegment::new(ident, hir_id, res)]),
1489                 });
1490                 let ty_id = self.next_id();
1491                 let bounded_ty =
1492                     self.ty_path(ty_id, param_span, hir::QPath::Resolved(None, ty_path));
1493                 Some(hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1494                     hir_id: self.next_id(),
1495                     bounded_ty: self.arena.alloc(bounded_ty),
1496                     bounds,
1497                     span,
1498                     bound_generic_params: &[],
1499                     origin,
1500                 }))
1501             }
1502             GenericParamKind::Lifetime => {
1503                 let ident_span = self.lower_span(ident.span);
1504                 let ident = self.lower_ident(ident);
1505                 let lt_id = self.next_node_id();
1506                 let lifetime = self.new_named_lifetime(id, lt_id, ident_span, ident);
1507                 Some(hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1508                     lifetime,
1509                     span,
1510                     bounds,
1511                     in_where_clause: false,
1512                 }))
1513             }
1514         }
1515     }
1516
1517     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1518         match *pred {
1519             WherePredicate::BoundPredicate(WhereBoundPredicate {
1520                 ref bound_generic_params,
1521                 ref bounded_ty,
1522                 ref bounds,
1523                 span,
1524             }) => hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1525                 hir_id: self.next_id(),
1526                 bound_generic_params: self.lower_generic_params(bound_generic_params),
1527                 bounded_ty: self
1528                     .lower_ty(bounded_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
1529                 bounds: self.arena.alloc_from_iter(bounds.iter().map(|bound| {
1530                     self.lower_param_bound(
1531                         bound,
1532                         &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
1533                     )
1534                 })),
1535                 span: self.lower_span(span),
1536                 origin: PredicateOrigin::WhereClause,
1537             }),
1538             WherePredicate::RegionPredicate(WhereRegionPredicate {
1539                 ref lifetime,
1540                 ref bounds,
1541                 span,
1542             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1543                 span: self.lower_span(span),
1544                 lifetime: self.lower_lifetime(lifetime),
1545                 bounds: self.lower_param_bounds(
1546                     bounds,
1547                     &ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
1548                 ),
1549                 in_where_clause: true,
1550             }),
1551             WherePredicate::EqPredicate(WhereEqPredicate { ref lhs_ty, ref rhs_ty, span }) => {
1552                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1553                     lhs_ty: self
1554                         .lower_ty(lhs_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
1555                     rhs_ty: self
1556                         .lower_ty(rhs_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
1557                     span: self.lower_span(span),
1558                 })
1559             }
1560         }
1561     }
1562 }