compiler/rustc_ast_lowering/src/item.rs

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