compiler/rustc_ast_lowering/src/item.rs

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