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), 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                     &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 317                     |this| this.lower_ty(ty, &mut 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                     &mut 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                     &mut 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                     &mut 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                     &mut 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                                 &mut ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
 395                             )
 396                         });
 397
 398                         let lowered_ty = this.lower_ty(
 399                             ty,
 400                             &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
 401                         );
 402
 403                         (trait_ref, lowered_ty)
 404                     });
 405
 406                 let new_impl_items = self
 407                     .arena
 408                     .alloc_from_iter(impl_items.iter().map(|item| self.lower_impl_item_ref(item)));
 409
 410                 // `defaultness.has_value()` is never called for an `impl`, always `true` in order
 411                 // to not cause an assertion failure inside the `lower_defaultness` function.
 412                 let has_val = true;
 413                 let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
 414                 let polarity = match polarity {
 415                     ImplPolarity::Positive => ImplPolarity::Positive,
 416                     ImplPolarity::Negative(s) => ImplPolarity::Negative(self.lower_span(s)),
 417                 };
 418                 hir::ItemKind::Impl(self.arena.alloc(hir::Impl {
 419                     unsafety: self.lower_unsafety(unsafety),
 420                     polarity,
 421                     defaultness,
 422                     defaultness_span,
 423                     constness: self.lower_constness(constness),
 424                     generics,
 425                     of_trait: trait_ref,
 426                     self_ty: lowered_ty,
 427                     items: new_impl_items,
 428                 }))
 429             }
 430             ItemKind::Trait(box Trait {
 431                 is_auto,
 432                 unsafety,
 433                 ref generics,
 434                 ref bounds,
 435                 ref items,
 436             }) => {
 437                 let (generics, (unsafety, items, bounds)) = self.lower_generics(
 438                     generics,
 439                     id,
 440                     &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 441                     |this| {
 442                         let bounds = this.lower_param_bounds(
 443                             bounds,
 444                             &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
 445                         );
 446                         let items = this.arena.alloc_from_iter(
 447                             items.iter().map(|item| this.lower_trait_item_ref(item)),
 448                         );
 449                         let unsafety = this.lower_unsafety(unsafety);
 450                         (unsafety, items, bounds)
 451                     },
 452                 );
 453                 hir::ItemKind::Trait(is_auto, unsafety, generics, bounds, items)
 454             }
 455             ItemKind::TraitAlias(ref generics, ref bounds) => {
 456                 let (generics, bounds) = self.lower_generics(
 457                     generics,
 458                     id,
 459                     &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 460                     |this| {
 461                         this.lower_param_bounds(
 462                             bounds,
 463                             &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
 464                         )
 465                     },
 466                 );
 467                 hir::ItemKind::TraitAlias(generics, bounds)
 468             }
 469             ItemKind::MacroDef(MacroDef { ref body, macro_rules }) => {
 470                 let body = P(self.lower_mac_args(body));
 471                 let macro_kind = self.resolver.decl_macro_kind(self.local_def_id(id));
 472                 hir::ItemKind::Macro(ast::MacroDef { body, macro_rules }, macro_kind)
 473             }
 474             ItemKind::MacCall(..) => {
 475                 panic!("`TyMac` should have been expanded by now")
 476             }
 477         }
 478     }
 479
 480     fn lower_const_item(
 481         &mut self,
 482         ty: &Ty,
 483         span: Span,
 484         body: Option<&Expr>,
 485     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 486         let ty = self.lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 487         (ty, self.lower_const_body(span, body))
 488     }
 489
 490     #[instrument(level = "debug", skip(self))]
 491     fn lower_use_tree(
 492         &mut self,
 493         tree: &UseTree,
 494         prefix: &Path,
 495         id: NodeId,
 496         vis_span: Span,
 497         ident: &mut Ident,
 498         attrs: Option<&'hir [Attribute]>,
 499     ) -> hir::ItemKind<'hir> {
 500         let path = &tree.prefix;
 501         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 502
 503         match tree.kind {
 504             UseTreeKind::Simple(rename, id1, id2) => {
 505                 *ident = tree.ident();
 506
 507                 // First, apply the prefix to the path.
 508                 let mut path = Path { segments, span: path.span, tokens: None };
 509
 510                 // Correctly resolve `self` imports.
 511                 if path.segments.len() > 1
 512                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 513                 {
 514                     let _ = path.segments.pop();
 515                     if rename.is_none() {
 516                         *ident = path.segments.last().unwrap().ident;
 517                     }
 518                 }
 519
 520                 let mut resolutions = self.expect_full_res_from_use(id).fuse();
 521                 // We want to return *something* from this function, so hold onto the first item
 522                 // for later.
 523                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 524
 525                 // Here, we are looping over namespaces, if they exist for the definition
 526                 // being imported. We only handle type and value namespaces because we
 527                 // won't be dealing with macros in the rest of the compiler.
 528                 // Essentially a single `use` which imports two names is desugared into
 529                 // two imports.
 530                 for new_node_id in [id1, id2] {
 531                     let new_id = self.local_def_id(new_node_id);
 532                     let Some(res) = resolutions.next() else {
 533                         // Associate an HirId to both ids even if there is no resolution.
 534                         let _old = self.children.insert(
 535                             new_id,
 536                             hir::MaybeOwner::NonOwner(hir::HirId::make_owner(new_id)),
 537                         );
 538                         debug_assert!(_old.is_none());
 539                         continue;
 540                     };
 541                     let ident = *ident;
 542                     let mut path = path.clone();
 543                     for seg in &mut path.segments {
 544                         seg.id = self.next_node_id();
 545                     }
 546                     let span = path.span;
 547
 548                     self.with_hir_id_owner(new_node_id, |this| {
 549                         let res = this.lower_res(res);
 550                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit);
 551                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 552                         if let Some(attrs) = attrs {
 553                             this.attrs.insert(hir::ItemLocalId::new(0), attrs);
 554                         }
 555
 556                         let item = hir::Item {
 557                             def_id: new_id,
 558                             ident: this.lower_ident(ident),
 559                             kind,
 560                             vis_span,
 561                             span: this.lower_span(span),
 562                         };
 563                         hir::OwnerNode::Item(this.arena.alloc(item))
 564                     });
 565                 }
 566
 567                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit);
 568                 hir::ItemKind::Use(path, hir::UseKind::Single)
 569             }
 570             UseTreeKind::Glob => {
 571                 let path = self.lower_path(
 572                     id,
 573                     &Path { segments, span: path.span, tokens: None },
 574                     ParamMode::Explicit,
 575                 );
 576                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 577             }
 578             UseTreeKind::Nested(ref trees) => {
 579                 // Nested imports are desugared into simple imports.
 580                 // So, if we start with
 581                 //
 582                 // ```
 583                 // pub(x) use foo::{a, b};
 584                 // ```
 585                 //
 586                 // we will create three items:
 587                 //
 588                 // ```
 589                 // pub(x) use foo::a;
 590                 // pub(x) use foo::b;
 591                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 592                 // ```
 593                 //
 594                 // The first two are produced by recursively invoking
 595                 // `lower_use_tree` (and indeed there may be things
 596                 // like `use foo::{a::{b, c}}` and so forth).  They
 597                 // wind up being directly added to
 598                 // `self.items`. However, the structure of this
 599                 // function also requires us to return one item, and
 600                 // for that we return the `{}` import (called the
 601                 // `ListStem`).
 602
 603                 let prefix = Path { segments, span: prefix.span.to(path.span), tokens: None };
 604
 605                 // Add all the nested `PathListItem`s to the HIR.
 606                 for &(ref use_tree, id) in trees {
 607                     let new_hir_id = self.local_def_id(id);
 608
 609                     let mut prefix = prefix.clone();
 610
 611                     // Give the segments new node-ids since they are being cloned.
 612                     for seg in &mut prefix.segments {
 613                         seg.id = self.next_node_id();
 614                     }
 615
 616                     // Each `use` import is an item and thus are owners of the
 617                     // names in the path. Up to this point the nested import is
 618                     // the current owner, since we want each desugared import to
 619                     // own its own names, we have to adjust the owner before
 620                     // lowering the rest of the import.
 621                     self.with_hir_id_owner(id, |this| {
 622                         let mut ident = *ident;
 623
 624                         let kind =
 625                             this.lower_use_tree(use_tree, &prefix, id, vis_span, &mut ident, attrs);
 626                         if let Some(attrs) = attrs {
 627                             this.attrs.insert(hir::ItemLocalId::new(0), attrs);
 628                         }
 629
 630                         let item = hir::Item {
 631                             def_id: new_hir_id,
 632                             ident: this.lower_ident(ident),
 633                             kind,
 634                             vis_span,
 635                             span: this.lower_span(use_tree.span),
 636                         };
 637                         hir::OwnerNode::Item(this.arena.alloc(item))
 638                     });
 639                 }
 640
 641                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 642                 let res = self.lower_res(res);
 643                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit);
 644                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 645             }
 646         }
 647     }
 648
 649     fn lower_foreign_item(&mut self, i: &ForeignItem) -> &'hir hir::ForeignItem<'hir> {
 650         let hir_id = self.lower_node_id(i.id);
 651         let def_id = hir_id.expect_owner();
 652         self.lower_attrs(hir_id, &i.attrs);
 653         let item = hir::ForeignItem {
 654             def_id,
 655             ident: self.lower_ident(i.ident),
 656             kind: match i.kind {
 657                 ForeignItemKind::Fn(box Fn { ref sig, ref generics, .. }) => {
 658                     let fdec = &sig.decl;
 659                     let mut itctx = ImplTraitContext::Universal;
 660                     let (generics, (fn_dec, fn_args)) =
 661                         self.lower_generics(generics, i.id, &mut itctx, |this| {
 662                             (
 663                                 // Disallow `impl Trait` in foreign items.
 664                                 this.lower_fn_decl(fdec, None, FnDeclKind::ExternFn, None),
 665                                 this.lower_fn_params_to_names(fdec),
 666                             )
 667                         });
 668
 669                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 670                 }
 671                 ForeignItemKind::Static(ref t, m, _) => {
 672                     let ty = self
 673                         .lower_ty(t, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 674                     hir::ForeignItemKind::Static(ty, m)
 675                 }
 676                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 677                 ForeignItemKind::MacCall(_) => panic!("macro shouldn't exist here"),
 678             },
 679             vis_span: self.lower_span(i.vis.span),
 680             span: self.lower_span(i.span),
 681         };
 682         self.arena.alloc(item)
 683     }
 684
 685     fn lower_foreign_item_ref(&mut self, i: &ForeignItem) -> hir::ForeignItemRef {
 686         hir::ForeignItemRef {
 687             id: hir::ForeignItemId { def_id: self.local_def_id(i.id) },
 688             ident: self.lower_ident(i.ident),
 689             span: self.lower_span(i.span),
 690         }
 691     }
 692
 693     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 694         let id = self.lower_node_id(v.id);
 695         self.lower_attrs(id, &v.attrs);
 696         hir::Variant {
 697             id,
 698             data: self.lower_variant_data(id, &v.data),
 699             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 700             ident: self.lower_ident(v.ident),
 701             span: self.lower_span(v.span),
 702         }
 703     }
 704
 705     fn lower_variant_data(
 706         &mut self,
 707         parent_id: hir::HirId,
 708         vdata: &VariantData,
 709     ) -> hir::VariantData<'hir> {
 710         match *vdata {
 711             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 712                 self.arena
 713                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f))),
 714                 recovered,
 715             ),
 716             VariantData::Tuple(ref fields, id) => {
 717                 let ctor_id = self.lower_node_id(id);
 718                 self.alias_attrs(ctor_id, parent_id);
 719                 hir::VariantData::Tuple(
 720                     self.arena.alloc_from_iter(
 721                         fields.iter().enumerate().map(|f| self.lower_field_def(f)),
 722                     ),
 723                     ctor_id,
 724                 )
 725             }
 726             VariantData::Unit(id) => {
 727                 let ctor_id = self.lower_node_id(id);
 728                 self.alias_attrs(ctor_id, parent_id);
 729                 hir::VariantData::Unit(ctor_id)
 730             }
 731         }
 732     }
 733
 734     fn lower_field_def(&mut self, (index, f): (usize, &FieldDef)) -> hir::FieldDef<'hir> {
 735         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 736             let t = self.lower_path_ty(
 737                 &f.ty,
 738                 qself,
 739                 path,
 740                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 741                 &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 742             );
 743             self.arena.alloc(t)
 744         } else {
 745             self.lower_ty(&f.ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type))
 746         };
 747         let hir_id = self.lower_node_id(f.id);
 748         self.lower_attrs(hir_id, &f.attrs);
 749         hir::FieldDef {
 750             span: self.lower_span(f.span),
 751             hir_id,
 752             ident: match f.ident {
 753                 Some(ident) => self.lower_ident(ident),
 754                 // FIXME(jseyfried): positional field hygiene.
 755                 None => Ident::new(sym::integer(index), self.lower_span(f.span)),
 756             },
 757             vis_span: self.lower_span(f.vis.span),
 758             ty,
 759         }
 760     }
 761
 762     fn lower_trait_item(&mut self, i: &AssocItem) -> &'hir hir::TraitItem<'hir> {
 763         let hir_id = self.lower_node_id(i.id);
 764         let trait_item_def_id = hir_id.expect_owner();
 765
 766         let (generics, kind, has_default) = match i.kind {
 767             AssocItemKind::Const(_, ref ty, ref default) => {
 768                 let ty =
 769                     self.lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 770                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 771                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body), body.is_some())
 772             }
 773             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: None, .. }) => {
 774                 let names = self.lower_fn_params_to_names(&sig.decl);
 775                 let (generics, sig) =
 776                     self.lower_method_sig(generics, sig, i.id, FnDeclKind::Trait, None);
 777                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)), false)
 778             }
 779             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: Some(ref body), .. }) => {
 780                 let asyncness = sig.header.asyncness;
 781                 let body_id =
 782                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, Some(&body));
 783                 let (generics, sig) = self.lower_method_sig(
 784                     generics,
 785                     sig,
 786                     i.id,
 787                     FnDeclKind::Trait,
 788                     asyncness.opt_return_id(),
 789                 );
 790                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)), true)
 791             }
 792             AssocItemKind::TyAlias(box TyAlias {
 793                 ref generics,
 794                 where_clauses,
 795                 ref bounds,
 796                 ref ty,
 797                 ..
 798             }) => {
 799                 let mut generics = generics.clone();
 800                 add_ty_alias_where_clause(&mut generics, where_clauses, false);
 801                 let (generics, kind) = self.lower_generics(
 802                     &generics,
 803                     i.id,
 804                     &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 805                     |this| {
 806                         let ty = ty.as_ref().map(|x| {
 807                             this.lower_ty(
 808                                 x,
 809                                 &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
 810                             )
 811                         });
 812                         hir::TraitItemKind::Type(
 813                             this.lower_param_bounds(
 814                                 bounds,
 815                                 &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 816                             ),
 817                             ty,
 818                         )
 819                     },
 820                 );
 821                 (generics, kind, ty.is_some())
 822             }
 823             AssocItemKind::MacCall(..) => panic!("macro item shouldn't exist at this point"),
 824         };
 825
 826         self.lower_attrs(hir_id, &i.attrs);
 827         let item = hir::TraitItem {
 828             def_id: trait_item_def_id,
 829             ident: self.lower_ident(i.ident),
 830             generics,
 831             kind,
 832             span: self.lower_span(i.span),
 833             defaultness: hir::Defaultness::Default { has_value: has_default },
 834         };
 835         self.arena.alloc(item)
 836     }
 837
 838     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 839         let kind = match &i.kind {
 840             AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 841             AssocItemKind::TyAlias(..) => hir::AssocItemKind::Type,
 842             AssocItemKind::Fn(box Fn { sig, .. }) => {
 843                 hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
 844             }
 845             AssocItemKind::MacCall(..) => unimplemented!(),
 846         };
 847         let id = hir::TraitItemId { def_id: self.local_def_id(i.id) };
 848         hir::TraitItemRef {
 849             id,
 850             ident: self.lower_ident(i.ident),
 851             span: self.lower_span(i.span),
 852             kind,
 853         }
 854     }
 855
 856     /// Construct `ExprKind::Err` for the given `span`.
 857     pub(crate) fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 858         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 859     }
 860
 861     fn lower_impl_item(&mut self, i: &AssocItem) -> &'hir hir::ImplItem<'hir> {
 862         // Since `default impl` is not yet implemented, this is always true in impls.
 863         let has_value = true;
 864         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 865
 866         let (generics, kind) = match &i.kind {
 867             AssocItemKind::Const(_, ty, expr) => {
 868                 let ty =
 869                     self.lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 870                 (
 871                     hir::Generics::empty(),
 872                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 873                 )
 874             }
 875             AssocItemKind::Fn(box Fn { sig, generics, body, .. }) => {
 876                 self.current_item = Some(i.span);
 877                 let asyncness = sig.header.asyncness;
 878                 let body_id =
 879                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 880                 let (generics, sig) = self.lower_method_sig(
 881                     generics,
 882                     sig,
 883                     i.id,
 884                     if self.is_in_trait_impl { FnDeclKind::Impl } else { FnDeclKind::Inherent },
 885                     asyncness.opt_return_id(),
 886                 );
 887
 888                 (generics, hir::ImplItemKind::Fn(sig, body_id))
 889             }
 890             AssocItemKind::TyAlias(box TyAlias { generics, where_clauses, ty, .. }) => {
 891                 let mut generics = generics.clone();
 892                 add_ty_alias_where_clause(&mut generics, *where_clauses, false);
 893                 self.lower_generics(
 894                     &generics,
 895                     i.id,
 896                     &mut ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 897                     |this| match ty {
 898                         None => {
 899                             let ty = this.arena.alloc(this.ty(i.span, hir::TyKind::Err));
 900                             hir::ImplItemKind::TyAlias(ty)
 901                         }
 902                         Some(ty) => {
 903                             let ty = this.lower_ty(ty, &mut ImplTraitContext::TypeAliasesOpaqueTy);
 904                             hir::ImplItemKind::TyAlias(ty)
 905                         }
 906                     },
 907                 )
 908             }
 909             AssocItemKind::MacCall(..) => panic!("`TyMac` should have been expanded by now"),
 910         };
 911
 912         let hir_id = self.lower_node_id(i.id);
 913         self.lower_attrs(hir_id, &i.attrs);
 914         let item = hir::ImplItem {
 915             def_id: hir_id.expect_owner(),
 916             ident: self.lower_ident(i.ident),
 917             generics,
 918             kind,
 919             vis_span: self.lower_span(i.vis.span),
 920             span: self.lower_span(i.span),
 921             defaultness,
 922         };
 923         self.arena.alloc(item)
 924     }
 925
 926     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef {
 927         hir::ImplItemRef {
 928             id: hir::ImplItemId { def_id: self.local_def_id(i.id) },
 929             ident: self.lower_ident(i.ident),
 930             span: self.lower_span(i.span),
 931             kind: match &i.kind {
 932                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 933                 AssocItemKind::TyAlias(..) => hir::AssocItemKind::Type,
 934                 AssocItemKind::Fn(box Fn { sig, .. }) => {
 935                     hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
 936                 }
 937                 AssocItemKind::MacCall(..) => unimplemented!(),
 938             },
 939             trait_item_def_id: self.resolver.get_partial_res(i.id).map(|r| r.base_res().def_id()),
 940         }
 941     }
 942
 943     fn lower_defaultness(
 944         &self,
 945         d: Defaultness,
 946         has_value: bool,
 947     ) -> (hir::Defaultness, Option<Span>) {
 948         match d {
 949             Defaultness::Default(sp) => {
 950                 (hir::Defaultness::Default { has_value }, Some(self.lower_span(sp)))
 951             }
 952             Defaultness::Final => {
 953                 assert!(has_value);
 954                 (hir::Defaultness::Final, None)
 955             }
 956         }
 957     }
 958
 959     fn record_body(
 960         &mut self,
 961         params: &'hir [hir::Param<'hir>],
 962         value: hir::Expr<'hir>,
 963     ) -> hir::BodyId {
 964         let body = hir::Body {
 965             generator_kind: self.generator_kind,
 966             params,
 967             value: self.arena.alloc(value),
 968         };
 969         let id = body.id();
 970         debug_assert_eq!(id.hir_id.owner, self.current_hir_id_owner);
 971         self.bodies.push((id.hir_id.local_id, self.arena.alloc(body)));
 972         id
 973     }
 974
 975     pub(super) fn lower_body(
 976         &mut self,
 977         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
 978     ) -> hir::BodyId {
 979         let prev_gen_kind = self.generator_kind.take();
 980         let task_context = self.task_context.take();
 981         let (parameters, result) = f(self);
 982         let body_id = self.record_body(parameters, result);
 983         self.task_context = task_context;
 984         self.generator_kind = prev_gen_kind;
 985         body_id
 986     }
 987
 988     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
 989         let hir_id = self.lower_node_id(param.id);
 990         self.lower_attrs(hir_id, &param.attrs);
 991         hir::Param {
 992             hir_id,
 993             pat: self.lower_pat(&param.pat),
 994             ty_span: self.lower_span(param.ty.span),
 995             span: self.lower_span(param.span),
 996         }
 997     }
 998
 999     pub(super) fn lower_fn_body(
1000         &mut self,
1001         decl: &FnDecl,
1002         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
1003     ) -> hir::BodyId {
1004         self.lower_body(|this| {
1005             (
1006                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
1007                 body(this),
1008             )
1009         })
1010     }
1011
1012     fn lower_fn_body_block(
1013         &mut self,
1014         span: Span,
1015         decl: &FnDecl,
1016         body: Option<&Block>,
1017     ) -> hir::BodyId {
1018         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1019     }
1020
1021     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1022         match block {
1023             Some(block) => self.lower_block_expr(block),
1024             None => self.expr_err(span),
1025         }
1026     }
1027
1028     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1029         self.lower_body(|this| {
1030             (
1031                 &[],
1032                 match expr {
1033                     Some(expr) => this.lower_expr_mut(expr),
1034                     None => this.expr_err(span),
1035                 },
1036             )
1037         })
1038     }
1039
1040     fn lower_maybe_async_body(
1041         &mut self,
1042         span: Span,
1043         decl: &FnDecl,
1044         asyncness: Async,
1045         body: Option<&Block>,
1046     ) -> hir::BodyId {
1047         let closure_id = match asyncness {
1048             Async::Yes { closure_id, .. } => closure_id,
1049             Async::No => return self.lower_fn_body_block(span, decl, body),
1050         };
1051
1052         self.lower_body(|this| {
1053             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1054             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1055
1056             // Async function parameters are lowered into the closure body so that they are
1057             // captured and so that the drop order matches the equivalent non-async functions.
1058             //
1059             // from:
1060             //
1061             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1062             //         <body>
1063             //     }
1064             //
1065             // into:
1066             //
1067             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1068             //       async move {
1069             //         let __arg2 = __arg2;
1070             //         let <pattern> = __arg2;
1071             //         let __arg1 = __arg1;
1072             //         let <pattern> = __arg1;
1073             //         let __arg0 = __arg0;
1074             //         let <pattern> = __arg0;
1075             //         drop-temps { <body> } // see comments later in fn for details
1076             //       }
1077             //     }
1078             //
1079             // If `<pattern>` is a simple ident, then it is lowered to a single
1080             // `let <pattern> = <pattern>;` statement as an optimization.
1081             //
1082             // Note that the body is embedded in `drop-temps`; an
1083             // equivalent desugaring would be `return { <body>
1084             // };`. The key point is that we wish to drop all the
1085             // let-bound variables and temporaries created in the body
1086             // (and its tail expression!) before we drop the
1087             // parameters (c.f. rust-lang/rust#64512).
1088             for (index, parameter) in decl.inputs.iter().enumerate() {
1089                 let parameter = this.lower_param(parameter);
1090                 let span = parameter.pat.span;
1091
1092                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1093                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1094                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1095                     hir::PatKind::Binding(hir::BindingAnnotation(ByRef::No, _), _, ident, _) => {
1096                         (ident, true)
1097                     }
1098                     // For `ref mut` or wildcard arguments, we can't reuse the binding, but
1099                     // we can keep the same name for the parameter.
1100                     // This lets rustdoc render it correctly in documentation.
1101                     hir::PatKind::Binding(_, _, ident, _) => (ident, false),
1102                     hir::PatKind::Wild => {
1103                         (Ident::with_dummy_span(rustc_span::symbol::kw::Underscore), false)
1104                     }
1105                     _ => {
1106                         // Replace the ident for bindings that aren't simple.
1107                         let name = format!("__arg{}", index);
1108                         let ident = Ident::from_str(&name);
1109
1110                         (ident, false)
1111                     }
1112                 };
1113
1114                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1115
1116                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1117                 // async function.
1118                 //
1119                 // If this is the simple case, this parameter will end up being the same as the
1120                 // original parameter, but with a different pattern id.
1121                 let stmt_attrs = this.attrs.get(&parameter.hir_id.local_id).copied();
1122                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1123                 let new_parameter = hir::Param {
1124                     hir_id: parameter.hir_id,
1125                     pat: new_parameter_pat,
1126                     ty_span: this.lower_span(parameter.ty_span),
1127                     span: this.lower_span(parameter.span),
1128                 };
1129
1130                 if is_simple_parameter {
1131                     // If this is the simple case, then we only insert one statement that is
1132                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1133                     // `HirId`s are densely assigned.
1134                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1135                     let stmt = this.stmt_let_pat(
1136                         stmt_attrs,
1137                         desugared_span,
1138                         Some(expr),
1139                         parameter.pat,
1140                         hir::LocalSource::AsyncFn,
1141                     );
1142                     statements.push(stmt);
1143                 } else {
1144                     // If this is not the simple case, then we construct two statements:
1145                     //
1146                     // ```
1147                     // let __argN = __argN;
1148                     // let <pat> = __argN;
1149                     // ```
1150                     //
1151                     // The first statement moves the parameter into the closure and thus ensures
1152                     // that the drop order is correct.
1153                     //
1154                     // The second statement creates the bindings that the user wrote.
1155
1156                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1157                     // because the user may have specified a `ref mut` binding in the next
1158                     // statement.
1159                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1160                         desugared_span,
1161                         ident,
1162                         hir::BindingAnnotation::MUT,
1163                     );
1164                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1165                     let move_stmt = this.stmt_let_pat(
1166                         None,
1167                         desugared_span,
1168                         Some(move_expr),
1169                         move_pat,
1170                         hir::LocalSource::AsyncFn,
1171                     );
1172
1173                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1174                     // parameter's pattern so that `HirId`s are densely assigned.
1175                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1176                     let pattern_stmt = this.stmt_let_pat(
1177                         stmt_attrs,
1178                         desugared_span,
1179                         Some(pattern_expr),
1180                         parameter.pat,
1181                         hir::LocalSource::AsyncFn,
1182                     );
1183
1184                     statements.push(move_stmt);
1185                     statements.push(pattern_stmt);
1186                 };
1187
1188                 parameters.push(new_parameter);
1189             }
1190
1191             let body_span = body.map_or(span, |b| b.span);
1192             let async_expr = this.make_async_expr(
1193                 CaptureBy::Value,
1194                 closure_id,
1195                 None,
1196                 body_span,
1197                 hir::AsyncGeneratorKind::Fn,
1198                 |this| {
1199                     // Create a block from the user's function body:
1200                     let user_body = this.lower_block_expr_opt(body_span, body);
1201
1202                     // Transform into `drop-temps { <user-body> }`, an expression:
1203                     let desugared_span =
1204                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1205                     let user_body = this.expr_drop_temps(
1206                         desugared_span,
1207                         this.arena.alloc(user_body),
1208                         AttrVec::new(),
1209                     );
1210
1211                     // As noted above, create the final block like
1212                     //
1213                     // ```
1214                     // {
1215                     //   let $param_pattern = $raw_param;
1216                     //   ...
1217                     //   drop-temps { <user-body> }
1218                     // }
1219                     // ```
1220                     let body = this.block_all(
1221                         desugared_span,
1222                         this.arena.alloc_from_iter(statements),
1223                         Some(user_body),
1224                     );
1225
1226                     this.expr_block(body, AttrVec::new())
1227                 },
1228             );
1229
1230             (
1231                 this.arena.alloc_from_iter(parameters),
1232                 this.expr(body_span, async_expr, AttrVec::new()),
1233             )
1234         })
1235     }
1236
1237     fn lower_method_sig(
1238         &mut self,
1239         generics: &Generics,
1240         sig: &FnSig,
1241         id: NodeId,
1242         kind: FnDeclKind,
1243         is_async: Option<NodeId>,
1244     ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
1245         let header = self.lower_fn_header(sig.header);
1246         let mut itctx = ImplTraitContext::Universal;
1247         let (generics, decl) = self.lower_generics(generics, id, &mut itctx, |this| {
1248             this.lower_fn_decl(&sig.decl, Some(id), kind, is_async)
1249         });
1250         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
1251     }
1252
1253     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1254         hir::FnHeader {
1255             unsafety: self.lower_unsafety(h.unsafety),
1256             asyncness: self.lower_asyncness(h.asyncness),
1257             constness: self.lower_constness(h.constness),
1258             abi: self.lower_extern(h.ext),
1259         }
1260     }
1261
1262     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1263         abi::lookup(abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1264             self.error_on_invalid_abi(abi);
1265             abi::Abi::Rust
1266         })
1267     }
1268
1269     pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
1270         match ext {
1271             Extern::None => abi::Abi::Rust,
1272             Extern::Implicit(_) => abi::Abi::FALLBACK,
1273             Extern::Explicit(abi, _) => self.lower_abi(abi),
1274         }
1275     }
1276
1277     fn error_on_invalid_abi(&self, abi: StrLit) {
1278         self.tcx.sess.emit_err(InvalidAbi {
1279             span: abi.span,
1280             abi: abi.symbol,
1281             valid_abis: abi::all_names().join(", "),
1282         });
1283     }
1284
1285     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1286         match a {
1287             Async::Yes { .. } => hir::IsAsync::Async,
1288             Async::No => hir::IsAsync::NotAsync,
1289         }
1290     }
1291
1292     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1293         match c {
1294             Const::Yes(_) => hir::Constness::Const,
1295             Const::No => hir::Constness::NotConst,
1296         }
1297     }
1298
1299     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1300         match u {
1301             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1302             Unsafe::No => hir::Unsafety::Normal,
1303         }
1304     }
1305
1306     /// Return the pair of the lowered `generics` as `hir::Generics` and the evaluation of `f` with
1307     /// the carried impl trait definitions and bounds.
1308     #[instrument(level = "debug", skip(self, f))]
1309     fn lower_generics<T>(
1310         &mut self,
1311         generics: &Generics,
1312         parent_node_id: NodeId,
1313         itctx: &mut ImplTraitContext,
1314         f: impl FnOnce(&mut Self) -> T,
1315     ) -> (&'hir hir::Generics<'hir>, T) {
1316         debug_assert!(self.impl_trait_defs.is_empty());
1317         debug_assert!(self.impl_trait_bounds.is_empty());
1318
1319         // Error if `?Trait` bounds in where clauses don't refer directly to type parameters.
1320         // Note: we used to clone these bounds directly onto the type parameter (and avoid lowering
1321         // these into hir when we lower thee where clauses), but this makes it quite difficult to
1322         // keep track of the Span info. Now, `add_implicitly_sized` in `AstConv` checks both param bounds and
1323         // where clauses for `?Sized`.
1324         for pred in &generics.where_clause.predicates {
1325             let WherePredicate::BoundPredicate(ref bound_pred) = *pred else {
1326                 continue;
1327             };
1328             let compute_is_param = || {
1329                 // Check if the where clause type is a plain type parameter.
1330                 match self
1331                     .resolver
1332                     .get_partial_res(bound_pred.bounded_ty.id)
1333                     .map(|d| (d.base_res(), d.unresolved_segments()))
1334                 {
1335                     Some((Res::Def(DefKind::TyParam, def_id), 0))
1336                         if bound_pred.bound_generic_params.is_empty() =>
1337                     {
1338                         generics
1339                             .params
1340                             .iter()
1341                             .any(|p| def_id == self.local_def_id(p.id).to_def_id())
1342                     }
1343                     // Either the `bounded_ty` is not a plain type parameter, or
1344                     // it's not found in the generic type parameters list.
1345                     _ => false,
1346                 }
1347             };
1348             // We only need to compute this once per `WherePredicate`, but don't
1349             // need to compute this at all unless there is a Maybe bound.
1350             let mut is_param: Option<bool> = None;
1351             for bound in &bound_pred.bounds {
1352                 if !matches!(*bound, GenericBound::Trait(_, TraitBoundModifier::Maybe)) {
1353                     continue;
1354                 }
1355                 let is_param = *is_param.get_or_insert_with(compute_is_param);
1356                 if !is_param {
1357                     self.tcx.sess.emit_err(MisplacedRelaxTraitBound { span: bound.span() });
1358                 }
1359             }
1360         }
1361
1362         let mut predicates: SmallVec<[hir::WherePredicate<'hir>; 4]> = SmallVec::new();
1363         predicates.extend(generics.params.iter().filter_map(|param| {
1364             self.lower_generic_bound_predicate(
1365                 param.ident,
1366                 param.id,
1367                 &param.kind,
1368                 &param.bounds,
1369                 itctx,
1370                 PredicateOrigin::GenericParam,
1371             )
1372         }));
1373         predicates.extend(
1374             generics
1375                 .where_clause
1376                 .predicates
1377                 .iter()
1378                 .map(|predicate| self.lower_where_predicate(predicate)),
1379         );
1380
1381         let mut params: SmallVec<[hir::GenericParam<'hir>; 4]> =
1382             self.lower_generic_params_mut(&generics.params).collect();
1383
1384         // Introduce extra lifetimes if late resolution tells us to.
1385         let extra_lifetimes = self.resolver.take_extra_lifetime_params(parent_node_id);
1386         params.extend(extra_lifetimes.into_iter().filter_map(|(ident, node_id, res)| {
1387             self.lifetime_res_to_generic_param(ident, node_id, res)
1388         }));
1389
1390         let has_where_clause_predicates = !generics.where_clause.predicates.is_empty();
1391         let where_clause_span = self.lower_span(generics.where_clause.span);
1392         let span = self.lower_span(generics.span);
1393         let res = f(self);
1394
1395         let impl_trait_defs = std::mem::take(&mut self.impl_trait_defs);
1396         params.extend(impl_trait_defs.into_iter());
1397
1398         let impl_trait_bounds = std::mem::take(&mut self.impl_trait_bounds);
1399         predicates.extend(impl_trait_bounds.into_iter());
1400
1401         let lowered_generics = self.arena.alloc(hir::Generics {
1402             params: self.arena.alloc_from_iter(params),
1403             predicates: self.arena.alloc_from_iter(predicates),
1404             has_where_clause_predicates,
1405             where_clause_span,
1406             span,
1407         });
1408
1409         (lowered_generics, res)
1410     }
1411
1412     pub(super) fn lower_generic_bound_predicate(
1413         &mut self,
1414         ident: Ident,
1415         id: NodeId,
1416         kind: &GenericParamKind,
1417         bounds: &[GenericBound],
1418         itctx: &mut ImplTraitContext,
1419         origin: PredicateOrigin,
1420     ) -> Option<hir::WherePredicate<'hir>> {
1421         // Do not create a clause if we do not have anything inside it.
1422         if bounds.is_empty() {
1423             return None;
1424         }
1425
1426         let bounds = self.lower_param_bounds(bounds, itctx);
1427
1428         let ident = self.lower_ident(ident);
1429         let param_span = ident.span;
1430         let span = bounds
1431             .iter()
1432             .fold(Some(param_span.shrink_to_hi()), |span: Option<Span>, bound| {
1433                 let bound_span = bound.span();
1434                 // We include bounds that come from a `#[derive(_)]` but point at the user's code,
1435                 // as we use this method to get a span appropriate for suggestions.
1436                 if !bound_span.can_be_used_for_suggestions() {
1437                     None
1438                 } else if let Some(span) = span {
1439                     Some(span.to(bound_span))
1440                 } else {
1441                     Some(bound_span)
1442                 }
1443             })
1444             .unwrap_or(param_span.shrink_to_hi());
1445         match kind {
1446             GenericParamKind::Const { .. } => None,
1447             GenericParamKind::Type { .. } => {
1448                 let def_id = self.local_def_id(id).to_def_id();
1449                 let hir_id = self.next_id();
1450                 let res = Res::Def(DefKind::TyParam, def_id);
1451                 let ty_path = self.arena.alloc(hir::Path {
1452                     span: param_span,
1453                     res,
1454                     segments: self
1455                         .arena
1456                         .alloc_from_iter([hir::PathSegment::new(ident, hir_id, res)]),
1457                 });
1458                 let ty_id = self.next_id();
1459                 let bounded_ty =
1460                     self.ty_path(ty_id, param_span, hir::QPath::Resolved(None, ty_path));
1461                 Some(hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1462                     bounded_ty: self.arena.alloc(bounded_ty),
1463                     bounds,
1464                     span,
1465                     bound_generic_params: &[],
1466                     origin,
1467                 }))
1468             }
1469             GenericParamKind::Lifetime => {
1470                 let ident_span = self.lower_span(ident.span);
1471                 let ident = self.lower_ident(ident);
1472                 let lt_id = self.next_node_id();
1473                 let lifetime = self.new_named_lifetime(id, lt_id, ident_span, ident);
1474                 Some(hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1475                     lifetime,
1476                     span,
1477                     bounds,
1478                     in_where_clause: false,
1479                 }))
1480             }
1481         }
1482     }
1483
1484     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1485         match *pred {
1486             WherePredicate::BoundPredicate(WhereBoundPredicate {
1487                 ref bound_generic_params,
1488                 ref bounded_ty,
1489                 ref bounds,
1490                 span,
1491             }) => hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1492                 bound_generic_params: self.lower_generic_params(bound_generic_params),
1493                 bounded_ty: self.lower_ty(
1494                     bounded_ty,
1495                     &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
1496                 ),
1497                 bounds: self.arena.alloc_from_iter(bounds.iter().map(|bound| {
1498                     self.lower_param_bound(
1499                         bound,
1500                         &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
1501                     )
1502                 })),
1503                 span: self.lower_span(span),
1504                 origin: PredicateOrigin::WhereClause,
1505             }),
1506             WherePredicate::RegionPredicate(WhereRegionPredicate {
1507                 ref lifetime,
1508                 ref bounds,
1509                 span,
1510             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1511                 span: self.lower_span(span),
1512                 lifetime: self.lower_lifetime(lifetime),
1513                 bounds: self.lower_param_bounds(
1514                     bounds,
1515                     &mut ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
1516                 ),
1517                 in_where_clause: true,
1518             }),
1519             WherePredicate::EqPredicate(WhereEqPredicate { ref lhs_ty, ref rhs_ty, span }) => {
1520                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1521                     lhs_ty: self.lower_ty(
1522                         lhs_ty,
1523                         &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
1524                     ),
1525                     rhs_ty: self.lower_ty(
1526                         rhs_ty,
1527                         &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type),
1528                     ),
1529                     span: self.lower_span(span),
1530                 })
1531             }
1532         }
1533     }
1534 }