compiler/rustc_ast_lowering/src/item.rs

   1 use super::ResolverAstLoweringExt;
   2 use super::{AstOwner, ImplTraitContext, ImplTraitPosition};
   3 use super::{FnDeclKind, LoweringContext, ParamMode};
   4
   5 use rustc_ast::ptr::P;
   6 use rustc_ast::visit::AssocCtxt;
   7 use rustc_ast::*;
   8 use rustc_data_structures::fx::FxHashMap;
   9 use rustc_data_structures::sorted_map::SortedMap;
  10 use rustc_errors::struct_span_err;
  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             captured_lifetimes: None,
  84             impl_trait_defs: Vec::new(),
  85             impl_trait_bounds: Vec::new(),
  86             allow_try_trait: Some([sym::try_trait_v2, sym::yeet_desugar_details][..].into()),
  87             allow_gen_future: Some([sym::gen_future][..].into()),
  88             allow_into_future: Some([sym::into_future][..].into()),
  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(lctx.arena.alloc(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(&mut self, items: &[P<Item>], spans: &ModSpans) -> hir::Mod<'hir> {
 163         hir::Mod {
 164             spans: hir::ModSpans {
 165                 inner_span: self.lower_span(spans.inner_span),
 166                 inject_use_span: self.lower_span(spans.inject_use_span),
 167             },
 168             item_ids: self.arena.alloc_from_iter(items.iter().flat_map(|x| self.lower_item_ref(x))),
 169         }
 170     }
 171
 172     pub(super) fn lower_item_ref(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
 173         let mut node_ids = smallvec![hir::ItemId { def_id: self.local_def_id(i.id) }];
 174         if let ItemKind::Use(ref use_tree) = &i.kind {
 175             self.lower_item_id_use_tree(use_tree, i.id, &mut node_ids);
 176         }
 177         node_ids
 178     }
 179
 180     fn lower_item_id_use_tree(
 181         &mut self,
 182         tree: &UseTree,
 183         base_id: NodeId,
 184         vec: &mut SmallVec<[hir::ItemId; 1]>,
 185     ) {
 186         match tree.kind {
 187             UseTreeKind::Nested(ref nested_vec) => {
 188                 for &(ref nested, id) in nested_vec {
 189                     vec.push(hir::ItemId { def_id: self.local_def_id(id) });
 190                     self.lower_item_id_use_tree(nested, id, vec);
 191                 }
 192             }
 193             UseTreeKind::Glob => {}
 194             UseTreeKind::Simple(_, id1, id2) => {
 195                 for (_, &id) in
 196                     iter::zip(self.expect_full_res_from_use(base_id).skip(1), &[id1, id2])
 197                 {
 198                     vec.push(hir::ItemId { def_id: self.local_def_id(id) });
 199                 }
 200             }
 201         }
 202     }
 203
 204     fn lower_item(&mut self, i: &Item) -> &'hir hir::Item<'hir> {
 205         let mut ident = i.ident;
 206         let vis_span = self.lower_span(i.vis.span);
 207         let hir_id = self.lower_node_id(i.id);
 208         let attrs = self.lower_attrs(hir_id, &i.attrs);
 209         let kind = self.lower_item_kind(i.span, i.id, hir_id, &mut ident, attrs, vis_span, &i.kind);
 210         let item = hir::Item {
 211             def_id: hir_id.expect_owner(),
 212             ident: self.lower_ident(ident),
 213             kind,
 214             vis_span,
 215             span: self.lower_span(i.span),
 216         };
 217         self.arena.alloc(item)
 218     }
 219
 220     fn lower_item_kind(
 221         &mut self,
 222         span: Span,
 223         id: NodeId,
 224         hir_id: hir::HirId,
 225         ident: &mut Ident,
 226         attrs: Option<&'hir [Attribute]>,
 227         vis_span: Span,
 228         i: &ItemKind,
 229     ) -> hir::ItemKind<'hir> {
 230         match *i {
 231             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
 232             ItemKind::Use(ref use_tree) => {
 233                 // Start with an empty prefix.
 234                 let prefix = Path { segments: vec![], span: use_tree.span, tokens: None };
 235
 236                 self.lower_use_tree(use_tree, &prefix, id, vis_span, ident, attrs)
 237             }
 238             ItemKind::Static(ref t, m, ref e) => {
 239                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 240                 hir::ItemKind::Static(ty, m, body_id)
 241             }
 242             ItemKind::Const(_, ref t, ref e) => {
 243                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 244                 hir::ItemKind::Const(ty, body_id)
 245             }
 246             ItemKind::Fn(box Fn {
 247                 sig: FnSig { ref decl, header, span: fn_sig_span },
 248                 ref generics,
 249                 ref body,
 250                 ..
 251             }) => {
 252                 self.with_new_scopes(|this| {
 253                     this.current_item = Some(ident.span);
 254
 255                     // Note: we don't need to change the return type from `T` to
 256                     // `impl Future<Output = T>` here because lower_body
 257                     // only cares about the input argument patterns in the function
 258                     // declaration (decl), not the return types.
 259                     let asyncness = header.asyncness;
 260                     let body_id =
 261                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 262
 263                     let itctx = ImplTraitContext::Universal;
 264                     let (generics, decl) = this.lower_generics(generics, id, itctx, |this| {
 265                         let ret_id = asyncness.opt_return_id();
 266                         this.lower_fn_decl(&decl, Some(id), FnDeclKind::Fn, ret_id)
 267                     });
 268                     let sig = hir::FnSig {
 269                         decl,
 270                         header: this.lower_fn_header(header),
 271                         span: this.lower_span(fn_sig_span),
 272                     };
 273                     hir::ItemKind::Fn(sig, generics, body_id)
 274                 })
 275             }
 276             ItemKind::Mod(_, ref mod_kind) => match mod_kind {
 277                 ModKind::Loaded(items, _, spans) => {
 278                     hir::ItemKind::Mod(self.lower_mod(items, spans))
 279                 }
 280                 ModKind::Unloaded => panic!("`mod` items should have been loaded by now"),
 281             },
 282             ItemKind::ForeignMod(ref fm) => hir::ItemKind::ForeignMod {
 283                 abi: fm.abi.map_or(abi::Abi::FALLBACK, |abi| self.lower_abi(abi)),
 284                 items: self
 285                     .arena
 286                     .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
 287             },
 288             ItemKind::GlobalAsm(ref asm) => {
 289                 hir::ItemKind::GlobalAsm(self.lower_inline_asm(span, asm))
 290             }
 291             ItemKind::TyAlias(box TyAlias {
 292                 ref generics,
 293                 where_clauses,
 294                 ty: Some(ref ty),
 295                 ..
 296             }) => {
 297                 // We lower
 298                 //
 299                 // type Foo = impl Trait
 300                 //
 301                 // to
 302                 //
 303                 // type Foo = Foo1
 304                 // opaque type Foo1: Trait
 305                 let mut generics = generics.clone();
 306                 add_ty_alias_where_clause(&mut generics, where_clauses, true);
 307                 let (generics, ty) = self.lower_generics(
 308                     &generics,
 309                     id,
 310                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 311                     |this| this.lower_ty(ty, ImplTraitContext::TypeAliasesOpaqueTy),
 312                 );
 313                 hir::ItemKind::TyAlias(ty, generics)
 314             }
 315             ItemKind::TyAlias(box TyAlias {
 316                 ref generics, ref where_clauses, ty: None, ..
 317             }) => {
 318                 let mut generics = generics.clone();
 319                 add_ty_alias_where_clause(&mut generics, *where_clauses, true);
 320                 let (generics, ty) = self.lower_generics(
 321                     &generics,
 322                     id,
 323                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 324                     |this| this.arena.alloc(this.ty(span, hir::TyKind::Err)),
 325                 );
 326                 hir::ItemKind::TyAlias(ty, generics)
 327             }
 328             ItemKind::Enum(ref enum_definition, ref generics) => {
 329                 let (generics, variants) = self.lower_generics(
 330                     generics,
 331                     id,
 332                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 333                     |this| {
 334                         this.arena.alloc_from_iter(
 335                             enum_definition.variants.iter().map(|x| this.lower_variant(x)),
 336                         )
 337                     },
 338                 );
 339                 hir::ItemKind::Enum(hir::EnumDef { variants }, generics)
 340             }
 341             ItemKind::Struct(ref struct_def, ref generics) => {
 342                 let (generics, struct_def) = self.lower_generics(
 343                     generics,
 344                     id,
 345                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 346                     |this| this.lower_variant_data(hir_id, struct_def),
 347                 );
 348                 hir::ItemKind::Struct(struct_def, generics)
 349             }
 350             ItemKind::Union(ref vdata, ref generics) => {
 351                 let (generics, vdata) = self.lower_generics(
 352                     generics,
 353                     id,
 354                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 355                     |this| this.lower_variant_data(hir_id, vdata),
 356                 );
 357                 hir::ItemKind::Union(vdata, generics)
 358             }
 359             ItemKind::Impl(box Impl {
 360                 unsafety,
 361                 polarity,
 362                 defaultness,
 363                 constness,
 364                 generics: ref ast_generics,
 365                 of_trait: ref trait_ref,
 366                 self_ty: ref ty,
 367                 items: ref impl_items,
 368             }) => {
 369                 // Lower the "impl header" first. This ordering is important
 370                 // for in-band lifetimes! Consider `'a` here:
 371                 //
 372                 //     impl Foo<'a> for u32 {
 373                 //         fn method(&'a self) { .. }
 374                 //     }
 375                 //
 376                 // Because we start by lowering the `Foo<'a> for u32`
 377                 // part, we will add `'a` to the list of generics on
 378                 // the impl. When we then encounter it later in the
 379                 // method, it will not be considered an in-band
 380                 // lifetime to be added, but rather a reference to a
 381                 // parent lifetime.
 382                 let itctx = ImplTraitContext::Universal;
 383                 let (generics, (trait_ref, lowered_ty)) =
 384                     self.lower_generics(ast_generics, id, itctx, |this| {
 385                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 386                             this.lower_trait_ref(
 387                                 trait_ref,
 388                                 ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
 389                             )
 390                         });
 391
 392                         let lowered_ty = this
 393                             .lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 394
 395                         (trait_ref, lowered_ty)
 396                     });
 397
 398                 let new_impl_items = self
 399                     .arena
 400                     .alloc_from_iter(impl_items.iter().map(|item| self.lower_impl_item_ref(item)));
 401
 402                 // `defaultness.has_value()` is never called for an `impl`, always `true` in order
 403                 // to not cause an assertion failure inside the `lower_defaultness` function.
 404                 let has_val = true;
 405                 let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
 406                 let polarity = match polarity {
 407                     ImplPolarity::Positive => ImplPolarity::Positive,
 408                     ImplPolarity::Negative(s) => ImplPolarity::Negative(self.lower_span(s)),
 409                 };
 410                 hir::ItemKind::Impl(self.arena.alloc(hir::Impl {
 411                     unsafety: self.lower_unsafety(unsafety),
 412                     polarity,
 413                     defaultness,
 414                     defaultness_span,
 415                     constness: self.lower_constness(constness),
 416                     generics,
 417                     of_trait: trait_ref,
 418                     self_ty: lowered_ty,
 419                     items: new_impl_items,
 420                 }))
 421             }
 422             ItemKind::Trait(box Trait {
 423                 is_auto,
 424                 unsafety,
 425                 ref generics,
 426                 ref bounds,
 427                 ref items,
 428             }) => {
 429                 let (generics, (unsafety, items, bounds)) = self.lower_generics(
 430                     generics,
 431                     id,
 432                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 433                     |this| {
 434                         let bounds = this.lower_param_bounds(
 435                             bounds,
 436                             ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
 437                         );
 438                         let items = this.arena.alloc_from_iter(
 439                             items.iter().map(|item| this.lower_trait_item_ref(item)),
 440                         );
 441                         let unsafety = this.lower_unsafety(unsafety);
 442                         (unsafety, items, bounds)
 443                     },
 444                 );
 445                 hir::ItemKind::Trait(is_auto, unsafety, generics, bounds, items)
 446             }
 447             ItemKind::TraitAlias(ref generics, ref bounds) => {
 448                 let (generics, bounds) = self.lower_generics(
 449                     generics,
 450                     id,
 451                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 452                     |this| {
 453                         this.lower_param_bounds(
 454                             bounds,
 455                             ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
 456                         )
 457                     },
 458                 );
 459                 hir::ItemKind::TraitAlias(generics, bounds)
 460             }
 461             ItemKind::MacroDef(MacroDef { ref body, macro_rules }) => {
 462                 let body = P(self.lower_mac_args(body));
 463                 let macro_kind = self.resolver.decl_macro_kind(self.local_def_id(id));
 464                 hir::ItemKind::Macro(ast::MacroDef { body, macro_rules }, macro_kind)
 465             }
 466             ItemKind::MacCall(..) => {
 467                 panic!("`TyMac` should have been expanded by now")
 468             }
 469         }
 470     }
 471
 472     fn lower_const_item(
 473         &mut self,
 474         ty: &Ty,
 475         span: Span,
 476         body: Option<&Expr>,
 477     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 478         let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 479         (ty, self.lower_const_body(span, body))
 480     }
 481
 482     #[instrument(level = "debug", skip(self))]
 483     fn lower_use_tree(
 484         &mut self,
 485         tree: &UseTree,
 486         prefix: &Path,
 487         id: NodeId,
 488         vis_span: Span,
 489         ident: &mut Ident,
 490         attrs: Option<&'hir [Attribute]>,
 491     ) -> hir::ItemKind<'hir> {
 492         let path = &tree.prefix;
 493         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 494
 495         match tree.kind {
 496             UseTreeKind::Simple(rename, id1, id2) => {
 497                 *ident = tree.ident();
 498
 499                 // First, apply the prefix to the path.
 500                 let mut path = Path { segments, span: path.span, tokens: None };
 501
 502                 // Correctly resolve `self` imports.
 503                 if path.segments.len() > 1
 504                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 505                 {
 506                     let _ = path.segments.pop();
 507                     if rename.is_none() {
 508                         *ident = path.segments.last().unwrap().ident;
 509                     }
 510                 }
 511
 512                 let mut resolutions = self.expect_full_res_from_use(id).fuse();
 513                 // We want to return *something* from this function, so hold onto the first item
 514                 // for later.
 515                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 516
 517                 // Here, we are looping over namespaces, if they exist for the definition
 518                 // being imported. We only handle type and value namespaces because we
 519                 // won't be dealing with macros in the rest of the compiler.
 520                 // Essentially a single `use` which imports two names is desugared into
 521                 // two imports.
 522                 for new_node_id in [id1, id2] {
 523                     let new_id = self.local_def_id(new_node_id);
 524                     let Some(res) = resolutions.next() else {
 525                         // Associate an HirId to both ids even if there is no resolution.
 526                         let _old = self.children.insert(
 527                             new_id,
 528                             hir::MaybeOwner::NonOwner(hir::HirId::make_owner(new_id)),
 529                         );
 530                         debug_assert!(_old.is_none());
 531                         continue;
 532                     };
 533                     let ident = *ident;
 534                     let mut path = path.clone();
 535                     for seg in &mut path.segments {
 536                         seg.id = self.next_node_id();
 537                     }
 538                     let span = path.span;
 539
 540                     self.with_hir_id_owner(new_node_id, |this| {
 541                         let res = this.lower_res(res);
 542                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit);
 543                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 544                         if let Some(attrs) = attrs {
 545                             this.attrs.insert(hir::ItemLocalId::new(0), attrs);
 546                         }
 547
 548                         let item = hir::Item {
 549                             def_id: new_id,
 550                             ident: this.lower_ident(ident),
 551                             kind,
 552                             vis_span,
 553                             span: this.lower_span(span),
 554                         };
 555                         hir::OwnerNode::Item(this.arena.alloc(item))
 556                     });
 557                 }
 558
 559                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit);
 560                 hir::ItemKind::Use(path, hir::UseKind::Single)
 561             }
 562             UseTreeKind::Glob => {
 563                 let path = self.lower_path(
 564                     id,
 565                     &Path { segments, span: path.span, tokens: None },
 566                     ParamMode::Explicit,
 567                 );
 568                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 569             }
 570             UseTreeKind::Nested(ref trees) => {
 571                 // Nested imports are desugared into simple imports.
 572                 // So, if we start with
 573                 //
 574                 // ```
 575                 // pub(x) use foo::{a, b};
 576                 // ```
 577                 //
 578                 // we will create three items:
 579                 //
 580                 // ```
 581                 // pub(x) use foo::a;
 582                 // pub(x) use foo::b;
 583                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 584                 // ```
 585                 //
 586                 // The first two are produced by recursively invoking
 587                 // `lower_use_tree` (and indeed there may be things
 588                 // like `use foo::{a::{b, c}}` and so forth).  They
 589                 // wind up being directly added to
 590                 // `self.items`. However, the structure of this
 591                 // function also requires us to return one item, and
 592                 // for that we return the `{}` import (called the
 593                 // `ListStem`).
 594
 595                 let prefix = Path { segments, span: prefix.span.to(path.span), tokens: None };
 596
 597                 // Add all the nested `PathListItem`s to the HIR.
 598                 for &(ref use_tree, id) in trees {
 599                     let new_hir_id = self.local_def_id(id);
 600
 601                     let mut prefix = prefix.clone();
 602
 603                     // Give the segments new node-ids since they are being cloned.
 604                     for seg in &mut prefix.segments {
 605                         seg.id = self.next_node_id();
 606                     }
 607
 608                     // Each `use` import is an item and thus are owners of the
 609                     // names in the path. Up to this point the nested import is
 610                     // the current owner, since we want each desugared import to
 611                     // own its own names, we have to adjust the owner before
 612                     // lowering the rest of the import.
 613                     self.with_hir_id_owner(id, |this| {
 614                         let mut ident = *ident;
 615
 616                         let kind =
 617                             this.lower_use_tree(use_tree, &prefix, id, vis_span, &mut ident, attrs);
 618                         if let Some(attrs) = attrs {
 619                             this.attrs.insert(hir::ItemLocalId::new(0), attrs);
 620                         }
 621
 622                         let item = hir::Item {
 623                             def_id: new_hir_id,
 624                             ident: this.lower_ident(ident),
 625                             kind,
 626                             vis_span,
 627                             span: this.lower_span(use_tree.span),
 628                         };
 629                         hir::OwnerNode::Item(this.arena.alloc(item))
 630                     });
 631                 }
 632
 633                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 634                 let res = self.lower_res(res);
 635                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit);
 636                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 637             }
 638         }
 639     }
 640
 641     fn lower_foreign_item(&mut self, i: &ForeignItem) -> &'hir hir::ForeignItem<'hir> {
 642         let hir_id = self.lower_node_id(i.id);
 643         let def_id = hir_id.expect_owner();
 644         self.lower_attrs(hir_id, &i.attrs);
 645         let item = hir::ForeignItem {
 646             def_id,
 647             ident: self.lower_ident(i.ident),
 648             kind: match i.kind {
 649                 ForeignItemKind::Fn(box Fn { ref sig, ref generics, .. }) => {
 650                     let fdec = &sig.decl;
 651                     let itctx = ImplTraitContext::Universal;
 652                     let (generics, (fn_dec, fn_args)) =
 653                         self.lower_generics(generics, i.id, itctx, |this| {
 654                             (
 655                                 // Disallow `impl Trait` in foreign items.
 656                                 this.lower_fn_decl(fdec, None, FnDeclKind::ExternFn, None),
 657                                 this.lower_fn_params_to_names(fdec),
 658                             )
 659                         });
 660
 661                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 662                 }
 663                 ForeignItemKind::Static(ref t, m, _) => {
 664                     let ty =
 665                         self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 666                     hir::ForeignItemKind::Static(ty, m)
 667                 }
 668                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 669                 ForeignItemKind::MacCall(_) => panic!("macro shouldn't exist here"),
 670             },
 671             vis_span: self.lower_span(i.vis.span),
 672             span: self.lower_span(i.span),
 673         };
 674         self.arena.alloc(item)
 675     }
 676
 677     fn lower_foreign_item_ref(&mut self, i: &ForeignItem) -> hir::ForeignItemRef {
 678         hir::ForeignItemRef {
 679             id: hir::ForeignItemId { def_id: self.local_def_id(i.id) },
 680             ident: self.lower_ident(i.ident),
 681             span: self.lower_span(i.span),
 682         }
 683     }
 684
 685     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 686         let id = self.lower_node_id(v.id);
 687         self.lower_attrs(id, &v.attrs);
 688         hir::Variant {
 689             id,
 690             data: self.lower_variant_data(id, &v.data),
 691             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 692             ident: self.lower_ident(v.ident),
 693             span: self.lower_span(v.span),
 694         }
 695     }
 696
 697     fn lower_variant_data(
 698         &mut self,
 699         parent_id: hir::HirId,
 700         vdata: &VariantData,
 701     ) -> hir::VariantData<'hir> {
 702         match *vdata {
 703             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 704                 self.arena
 705                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f))),
 706                 recovered,
 707             ),
 708             VariantData::Tuple(ref fields, id) => {
 709                 let ctor_id = self.lower_node_id(id);
 710                 self.alias_attrs(ctor_id, parent_id);
 711                 hir::VariantData::Tuple(
 712                     self.arena.alloc_from_iter(
 713                         fields.iter().enumerate().map(|f| self.lower_field_def(f)),
 714                     ),
 715                     ctor_id,
 716                 )
 717             }
 718             VariantData::Unit(id) => {
 719                 let ctor_id = self.lower_node_id(id);
 720                 self.alias_attrs(ctor_id, parent_id);
 721                 hir::VariantData::Unit(ctor_id)
 722             }
 723         }
 724     }
 725
 726     fn lower_field_def(&mut self, (index, f): (usize, &FieldDef)) -> hir::FieldDef<'hir> {
 727         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 728             let t = self.lower_path_ty(
 729                 &f.ty,
 730                 qself,
 731                 path,
 732                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 733                 ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 734             );
 735             self.arena.alloc(t)
 736         } else {
 737             self.lower_ty(&f.ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type))
 738         };
 739         let hir_id = self.lower_node_id(f.id);
 740         self.lower_attrs(hir_id, &f.attrs);
 741         hir::FieldDef {
 742             span: self.lower_span(f.span),
 743             hir_id,
 744             ident: match f.ident {
 745                 Some(ident) => self.lower_ident(ident),
 746                 // FIXME(jseyfried): positional field hygiene.
 747                 None => Ident::new(sym::integer(index), self.lower_span(f.span)),
 748             },
 749             vis_span: self.lower_span(f.vis.span),
 750             ty,
 751         }
 752     }
 753
 754     fn lower_trait_item(&mut self, i: &AssocItem) -> &'hir hir::TraitItem<'hir> {
 755         let hir_id = self.lower_node_id(i.id);
 756         let trait_item_def_id = hir_id.expect_owner();
 757
 758         let (generics, kind) = match i.kind {
 759             AssocItemKind::Const(_, ref ty, ref default) => {
 760                 let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 761                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 762                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
 763             }
 764             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: None, .. }) => {
 765                 let names = self.lower_fn_params_to_names(&sig.decl);
 766                 let (generics, sig) =
 767                     self.lower_method_sig(generics, sig, i.id, FnDeclKind::Trait, None);
 768                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)))
 769             }
 770             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: Some(ref body), .. }) => {
 771                 let asyncness = sig.header.asyncness;
 772                 let body_id =
 773                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, Some(&body));
 774                 let (generics, sig) = self.lower_method_sig(
 775                     generics,
 776                     sig,
 777                     i.id,
 778                     FnDeclKind::Trait,
 779                     asyncness.opt_return_id(),
 780                 );
 781                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)))
 782             }
 783             AssocItemKind::TyAlias(box TyAlias {
 784                 ref generics,
 785                 where_clauses,
 786                 ref bounds,
 787                 ref ty,
 788                 ..
 789             }) => {
 790                 let mut generics = generics.clone();
 791                 add_ty_alias_where_clause(&mut generics, where_clauses, false);
 792                 self.lower_generics(
 793                     &generics,
 794                     i.id,
 795                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 796                     |this| {
 797                         let ty = ty.as_ref().map(|x| {
 798                             this.lower_ty(x, ImplTraitContext::Disallowed(ImplTraitPosition::Type))
 799                         });
 800                         hir::TraitItemKind::Type(
 801                             this.lower_param_bounds(
 802                                 bounds,
 803                                 ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 804                             ),
 805                             ty,
 806                         )
 807                     },
 808                 )
 809             }
 810             AssocItemKind::MacCall(..) => panic!("macro item shouldn't exist at this point"),
 811         };
 812
 813         self.lower_attrs(hir_id, &i.attrs);
 814         let item = hir::TraitItem {
 815             def_id: trait_item_def_id,
 816             ident: self.lower_ident(i.ident),
 817             generics,
 818             kind,
 819             span: self.lower_span(i.span),
 820         };
 821         self.arena.alloc(item)
 822     }
 823
 824     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 825         let (kind, has_default) = match &i.kind {
 826             AssocItemKind::Const(_, _, default) => (hir::AssocItemKind::Const, default.is_some()),
 827             AssocItemKind::TyAlias(box TyAlias { ty, .. }) => {
 828                 (hir::AssocItemKind::Type, ty.is_some())
 829             }
 830             AssocItemKind::Fn(box Fn { sig, body, .. }) => {
 831                 (hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }, body.is_some())
 832             }
 833             AssocItemKind::MacCall(..) => unimplemented!(),
 834         };
 835         let id = hir::TraitItemId { def_id: self.local_def_id(i.id) };
 836         let defaultness = hir::Defaultness::Default { has_value: has_default };
 837         hir::TraitItemRef {
 838             id,
 839             ident: self.lower_ident(i.ident),
 840             span: self.lower_span(i.span),
 841             defaultness,
 842             kind,
 843         }
 844     }
 845
 846     /// Construct `ExprKind::Err` for the given `span`.
 847     pub(crate) fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 848         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 849     }
 850
 851     fn lower_impl_item(&mut self, i: &AssocItem) -> &'hir hir::ImplItem<'hir> {
 852         let (generics, kind) = match &i.kind {
 853             AssocItemKind::Const(_, ty, expr) => {
 854                 let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 855                 (
 856                     hir::Generics::empty(),
 857                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 858                 )
 859             }
 860             AssocItemKind::Fn(box Fn { sig, generics, body, .. }) => {
 861                 self.current_item = Some(i.span);
 862                 let asyncness = sig.header.asyncness;
 863                 let body_id =
 864                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 865                 let (generics, sig) = self.lower_method_sig(
 866                     generics,
 867                     sig,
 868                     i.id,
 869                     if self.is_in_trait_impl { FnDeclKind::Impl } else { FnDeclKind::Inherent },
 870                     asyncness.opt_return_id(),
 871                 );
 872
 873                 (generics, hir::ImplItemKind::Fn(sig, body_id))
 874             }
 875             AssocItemKind::TyAlias(box TyAlias { generics, where_clauses, ty, .. }) => {
 876                 let mut generics = generics.clone();
 877                 add_ty_alias_where_clause(&mut generics, *where_clauses, false);
 878                 self.lower_generics(
 879                     &generics,
 880                     i.id,
 881                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 882                     |this| match ty {
 883                         None => {
 884                             let ty = this.arena.alloc(this.ty(i.span, hir::TyKind::Err));
 885                             hir::ImplItemKind::TyAlias(ty)
 886                         }
 887                         Some(ty) => {
 888                             let ty = this.lower_ty(ty, ImplTraitContext::TypeAliasesOpaqueTy);
 889                             hir::ImplItemKind::TyAlias(ty)
 890                         }
 891                     },
 892                 )
 893             }
 894             AssocItemKind::MacCall(..) => panic!("`TyMac` should have been expanded by now"),
 895         };
 896
 897         let hir_id = self.lower_node_id(i.id);
 898         self.lower_attrs(hir_id, &i.attrs);
 899         let item = hir::ImplItem {
 900             def_id: hir_id.expect_owner(),
 901             ident: self.lower_ident(i.ident),
 902             generics,
 903             kind,
 904             vis_span: self.lower_span(i.vis.span),
 905             span: self.lower_span(i.span),
 906         };
 907         self.arena.alloc(item)
 908     }
 909
 910     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef {
 911         // Since `default impl` is not yet implemented, this is always true in impls.
 912         let has_value = true;
 913         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 914         hir::ImplItemRef {
 915             id: hir::ImplItemId { def_id: self.local_def_id(i.id) },
 916             ident: self.lower_ident(i.ident),
 917             span: self.lower_span(i.span),
 918             defaultness,
 919             kind: match &i.kind {
 920                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 921                 AssocItemKind::TyAlias(..) => hir::AssocItemKind::Type,
 922                 AssocItemKind::Fn(box Fn { sig, .. }) => {
 923                     hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
 924                 }
 925                 AssocItemKind::MacCall(..) => unimplemented!(),
 926             },
 927             trait_item_def_id: self.resolver.get_partial_res(i.id).map(|r| r.base_res().def_id()),
 928         }
 929     }
 930
 931     fn lower_defaultness(
 932         &self,
 933         d: Defaultness,
 934         has_value: bool,
 935     ) -> (hir::Defaultness, Option<Span>) {
 936         match d {
 937             Defaultness::Default(sp) => {
 938                 (hir::Defaultness::Default { has_value }, Some(self.lower_span(sp)))
 939             }
 940             Defaultness::Final => {
 941                 assert!(has_value);
 942                 (hir::Defaultness::Final, None)
 943             }
 944         }
 945     }
 946
 947     fn record_body(
 948         &mut self,
 949         params: &'hir [hir::Param<'hir>],
 950         value: hir::Expr<'hir>,
 951     ) -> hir::BodyId {
 952         let body = hir::Body { generator_kind: self.generator_kind, params, value };
 953         let id = body.id();
 954         debug_assert_eq!(id.hir_id.owner, self.current_hir_id_owner);
 955         self.bodies.push((id.hir_id.local_id, self.arena.alloc(body)));
 956         id
 957     }
 958
 959     pub(super) fn lower_body(
 960         &mut self,
 961         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
 962     ) -> hir::BodyId {
 963         let prev_gen_kind = self.generator_kind.take();
 964         let task_context = self.task_context.take();
 965         let (parameters, result) = f(self);
 966         let body_id = self.record_body(parameters, result);
 967         self.task_context = task_context;
 968         self.generator_kind = prev_gen_kind;
 969         body_id
 970     }
 971
 972     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
 973         let hir_id = self.lower_node_id(param.id);
 974         self.lower_attrs(hir_id, &param.attrs);
 975         hir::Param {
 976             hir_id,
 977             pat: self.lower_pat(&param.pat),
 978             ty_span: self.lower_span(param.ty.span),
 979             span: self.lower_span(param.span),
 980         }
 981     }
 982
 983     pub(super) fn lower_fn_body(
 984         &mut self,
 985         decl: &FnDecl,
 986         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 987     ) -> hir::BodyId {
 988         self.lower_body(|this| {
 989             (
 990                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
 991                 body(this),
 992             )
 993         })
 994     }
 995
 996     fn lower_fn_body_block(
 997         &mut self,
 998         span: Span,
 999         decl: &FnDecl,
1000         body: Option<&Block>,
1001     ) -> hir::BodyId {
1002         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1003     }
1004
1005     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1006         match block {
1007             Some(block) => self.lower_block_expr(block),
1008             None => self.expr_err(span),
1009         }
1010     }
1011
1012     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1013         self.lower_body(|this| {
1014             (
1015                 &[],
1016                 match expr {
1017                     Some(expr) => this.lower_expr_mut(expr),
1018                     None => this.expr_err(span),
1019                 },
1020             )
1021         })
1022     }
1023
1024     fn lower_maybe_async_body(
1025         &mut self,
1026         span: Span,
1027         decl: &FnDecl,
1028         asyncness: Async,
1029         body: Option<&Block>,
1030     ) -> hir::BodyId {
1031         let closure_id = match asyncness {
1032             Async::Yes { closure_id, .. } => closure_id,
1033             Async::No => return self.lower_fn_body_block(span, decl, body),
1034         };
1035
1036         self.lower_body(|this| {
1037             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1038             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1039
1040             // Async function parameters are lowered into the closure body so that they are
1041             // captured and so that the drop order matches the equivalent non-async functions.
1042             //
1043             // from:
1044             //
1045             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1046             //         <body>
1047             //     }
1048             //
1049             // into:
1050             //
1051             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1052             //       async move {
1053             //         let __arg2 = __arg2;
1054             //         let <pattern> = __arg2;
1055             //         let __arg1 = __arg1;
1056             //         let <pattern> = __arg1;
1057             //         let __arg0 = __arg0;
1058             //         let <pattern> = __arg0;
1059             //         drop-temps { <body> } // see comments later in fn for details
1060             //       }
1061             //     }
1062             //
1063             // If `<pattern>` is a simple ident, then it is lowered to a single
1064             // `let <pattern> = <pattern>;` statement as an optimization.
1065             //
1066             // Note that the body is embedded in `drop-temps`; an
1067             // equivalent desugaring would be `return { <body>
1068             // };`. The key point is that we wish to drop all the
1069             // let-bound variables and temporaries created in the body
1070             // (and its tail expression!) before we drop the
1071             // parameters (c.f. rust-lang/rust#64512).
1072             for (index, parameter) in decl.inputs.iter().enumerate() {
1073                 let parameter = this.lower_param(parameter);
1074                 let span = parameter.pat.span;
1075
1076                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1077                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1078                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1079                     hir::PatKind::Binding(
1080                         hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable,
1081                         _,
1082                         ident,
1083                         _,
1084                     ) => (ident, true),
1085                     // For `ref mut` or wildcard arguments, we can't reuse the binding, but
1086                     // we can keep the same name for the parameter.
1087                     // This lets rustdoc render it correctly in documentation.
1088                     hir::PatKind::Binding(_, _, ident, _) => (ident, false),
1089                     hir::PatKind::Wild => {
1090                         (Ident::with_dummy_span(rustc_span::symbol::kw::Underscore), false)
1091                     }
1092                     _ => {
1093                         // Replace the ident for bindings that aren't simple.
1094                         let name = format!("__arg{}", index);
1095                         let ident = Ident::from_str(&name);
1096
1097                         (ident, false)
1098                     }
1099                 };
1100
1101                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1102
1103                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1104                 // async function.
1105                 //
1106                 // If this is the simple case, this parameter will end up being the same as the
1107                 // original parameter, but with a different pattern id.
1108                 let stmt_attrs = this.attrs.get(&parameter.hir_id.local_id).copied();
1109                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1110                 let new_parameter = hir::Param {
1111                     hir_id: parameter.hir_id,
1112                     pat: new_parameter_pat,
1113                     ty_span: this.lower_span(parameter.ty_span),
1114                     span: this.lower_span(parameter.span),
1115                 };
1116
1117                 if is_simple_parameter {
1118                     // If this is the simple case, then we only insert one statement that is
1119                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1120                     // `HirId`s are densely assigned.
1121                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1122                     let stmt = this.stmt_let_pat(
1123                         stmt_attrs,
1124                         desugared_span,
1125                         Some(expr),
1126                         parameter.pat,
1127                         hir::LocalSource::AsyncFn,
1128                     );
1129                     statements.push(stmt);
1130                 } else {
1131                     // If this is not the simple case, then we construct two statements:
1132                     //
1133                     // ```
1134                     // let __argN = __argN;
1135                     // let <pat> = __argN;
1136                     // ```
1137                     //
1138                     // The first statement moves the parameter into the closure and thus ensures
1139                     // that the drop order is correct.
1140                     //
1141                     // The second statement creates the bindings that the user wrote.
1142
1143                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1144                     // because the user may have specified a `ref mut` binding in the next
1145                     // statement.
1146                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1147                         desugared_span,
1148                         ident,
1149                         hir::BindingAnnotation::Mutable,
1150                     );
1151                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1152                     let move_stmt = this.stmt_let_pat(
1153                         None,
1154                         desugared_span,
1155                         Some(move_expr),
1156                         move_pat,
1157                         hir::LocalSource::AsyncFn,
1158                     );
1159
1160                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1161                     // parameter's pattern so that `HirId`s are densely assigned.
1162                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1163                     let pattern_stmt = this.stmt_let_pat(
1164                         stmt_attrs,
1165                         desugared_span,
1166                         Some(pattern_expr),
1167                         parameter.pat,
1168                         hir::LocalSource::AsyncFn,
1169                     );
1170
1171                     statements.push(move_stmt);
1172                     statements.push(pattern_stmt);
1173                 };
1174
1175                 parameters.push(new_parameter);
1176             }
1177
1178             let body_span = body.map_or(span, |b| b.span);
1179             let async_expr = this.make_async_expr(
1180                 CaptureBy::Value,
1181                 closure_id,
1182                 None,
1183                 body_span,
1184                 hir::AsyncGeneratorKind::Fn,
1185                 |this| {
1186                     // Create a block from the user's function body:
1187                     let user_body = this.lower_block_expr_opt(body_span, body);
1188
1189                     // Transform into `drop-temps { <user-body> }`, an expression:
1190                     let desugared_span =
1191                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1192                     let user_body = this.expr_drop_temps(
1193                         desugared_span,
1194                         this.arena.alloc(user_body),
1195                         AttrVec::new(),
1196                     );
1197
1198                     // As noted above, create the final block like
1199                     //
1200                     // ```
1201                     // {
1202                     //   let $param_pattern = $raw_param;
1203                     //   ...
1204                     //   drop-temps { <user-body> }
1205                     // }
1206                     // ```
1207                     let body = this.block_all(
1208                         desugared_span,
1209                         this.arena.alloc_from_iter(statements),
1210                         Some(user_body),
1211                     );
1212
1213                     this.expr_block(body, AttrVec::new())
1214                 },
1215             );
1216
1217             (
1218                 this.arena.alloc_from_iter(parameters),
1219                 this.expr(body_span, async_expr, AttrVec::new()),
1220             )
1221         })
1222     }
1223
1224     fn lower_method_sig(
1225         &mut self,
1226         generics: &Generics,
1227         sig: &FnSig,
1228         id: NodeId,
1229         kind: FnDeclKind,
1230         is_async: Option<NodeId>,
1231     ) -> (&'hir hir::Generics<'hir>, hir::FnSig<'hir>) {
1232         let header = self.lower_fn_header(sig.header);
1233         let itctx = ImplTraitContext::Universal;
1234         let (generics, decl) = self.lower_generics(generics, id, itctx, |this| {
1235             this.lower_fn_decl(&sig.decl, Some(id), kind, is_async)
1236         });
1237         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
1238     }
1239
1240     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1241         hir::FnHeader {
1242             unsafety: self.lower_unsafety(h.unsafety),
1243             asyncness: self.lower_asyncness(h.asyncness),
1244             constness: self.lower_constness(h.constness),
1245             abi: self.lower_extern(h.ext),
1246         }
1247     }
1248
1249     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1250         abi::lookup(abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1251             self.error_on_invalid_abi(abi);
1252             abi::Abi::Rust
1253         })
1254     }
1255
1256     pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
1257         match ext {
1258             Extern::None => abi::Abi::Rust,
1259             Extern::Implicit(_) => abi::Abi::FALLBACK,
1260             Extern::Explicit(abi, _) => self.lower_abi(abi),
1261         }
1262     }
1263
1264     fn error_on_invalid_abi(&self, abi: StrLit) {
1265         struct_span_err!(self.tcx.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1266             .span_label(abi.span, "invalid ABI")
1267             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1268             .emit();
1269     }
1270
1271     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1272         match a {
1273             Async::Yes { .. } => hir::IsAsync::Async,
1274             Async::No => hir::IsAsync::NotAsync,
1275         }
1276     }
1277
1278     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1279         match c {
1280             Const::Yes(_) => hir::Constness::Const,
1281             Const::No => hir::Constness::NotConst,
1282         }
1283     }
1284
1285     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1286         match u {
1287             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1288             Unsafe::No => hir::Unsafety::Normal,
1289         }
1290     }
1291
1292     /// Return the pair of the lowered `generics` as `hir::Generics` and the evaluation of `f` with
1293     /// the carried impl trait definitions and bounds.
1294     #[instrument(level = "debug", skip(self, f))]
1295     fn lower_generics<T>(
1296         &mut self,
1297         generics: &Generics,
1298         parent_node_id: NodeId,
1299         itctx: ImplTraitContext,
1300         f: impl FnOnce(&mut Self) -> T,
1301     ) -> (&'hir hir::Generics<'hir>, T) {
1302         debug_assert!(self.impl_trait_defs.is_empty());
1303         debug_assert!(self.impl_trait_bounds.is_empty());
1304
1305         // Error if `?Trait` bounds in where clauses don't refer directly to type parameters.
1306         // Note: we used to clone these bounds directly onto the type parameter (and avoid lowering
1307         // these into hir when we lower thee where clauses), but this makes it quite difficult to
1308         // keep track of the Span info. Now, `add_implicitly_sized` in `AstConv` checks both param bounds and
1309         // where clauses for `?Sized`.
1310         for pred in &generics.where_clause.predicates {
1311             let WherePredicate::BoundPredicate(ref bound_pred) = *pred else {
1312                 continue;
1313             };
1314             let compute_is_param = || {
1315                 // Check if the where clause type is a plain type parameter.
1316                 match self
1317                     .resolver
1318                     .get_partial_res(bound_pred.bounded_ty.id)
1319                     .map(|d| (d.base_res(), d.unresolved_segments()))
1320                 {
1321                     Some((Res::Def(DefKind::TyParam, def_id), 0))
1322                         if bound_pred.bound_generic_params.is_empty() =>
1323                     {
1324                         generics
1325                             .params
1326                             .iter()
1327                             .any(|p| def_id == self.local_def_id(p.id).to_def_id())
1328                     }
1329                     // Either the `bounded_ty` is not a plain type parameter, or
1330                     // it's not found in the generic type parameters list.
1331                     _ => false,
1332                 }
1333             };
1334             // We only need to compute this once per `WherePredicate`, but don't
1335             // need to compute this at all unless there is a Maybe bound.
1336             let mut is_param: Option<bool> = None;
1337             for bound in &bound_pred.bounds {
1338                 if !matches!(*bound, GenericBound::Trait(_, TraitBoundModifier::Maybe)) {
1339                     continue;
1340                 }
1341                 let is_param = *is_param.get_or_insert_with(compute_is_param);
1342                 if !is_param {
1343                     self.diagnostic().span_err(
1344                         bound.span(),
1345                         "`?Trait` bounds are only permitted at the \
1346                         point where a type parameter is declared",
1347                     );
1348                 }
1349             }
1350         }
1351
1352         let mut predicates: SmallVec<[hir::WherePredicate<'hir>; 4]> = SmallVec::new();
1353         predicates.extend(generics.params.iter().filter_map(|param| {
1354             let bounds = self.lower_param_bounds(&param.bounds, itctx);
1355             self.lower_generic_bound_predicate(
1356                 param.ident,
1357                 param.id,
1358                 &param.kind,
1359                 bounds,
1360                 PredicateOrigin::GenericParam,
1361             )
1362         }));
1363         predicates.extend(
1364             generics
1365                 .where_clause
1366                 .predicates
1367                 .iter()
1368                 .map(|predicate| self.lower_where_predicate(predicate)),
1369         );
1370
1371         let mut params: SmallVec<[hir::GenericParam<'hir>; 4]> =
1372             self.lower_generic_params_mut(&generics.params).collect();
1373
1374         // Introduce extra lifetimes if late resolution tells us to.
1375         let extra_lifetimes = self.resolver.take_extra_lifetime_params(parent_node_id);
1376         params.extend(extra_lifetimes.into_iter().filter_map(|(ident, node_id, res)| {
1377             self.lifetime_res_to_generic_param(ident, node_id, res)
1378         }));
1379
1380         let has_where_clause_predicates = !generics.where_clause.predicates.is_empty();
1381         let where_clause_span = self.lower_span(generics.where_clause.span);
1382         let span = self.lower_span(generics.span);
1383         let res = f(self);
1384
1385         let impl_trait_defs = std::mem::take(&mut self.impl_trait_defs);
1386         params.extend(impl_trait_defs.into_iter());
1387
1388         let impl_trait_bounds = std::mem::take(&mut self.impl_trait_bounds);
1389         predicates.extend(impl_trait_bounds.into_iter());
1390
1391         let lowered_generics = self.arena.alloc(hir::Generics {
1392             params: self.arena.alloc_from_iter(params),
1393             predicates: self.arena.alloc_from_iter(predicates),
1394             has_where_clause_predicates,
1395             where_clause_span,
1396             span,
1397         });
1398
1399         (lowered_generics, res)
1400     }
1401
1402     pub(super) fn lower_generic_bound_predicate(
1403         &mut self,
1404         ident: Ident,
1405         id: NodeId,
1406         kind: &GenericParamKind,
1407         bounds: &'hir [hir::GenericBound<'hir>],
1408         origin: PredicateOrigin,
1409     ) -> Option<hir::WherePredicate<'hir>> {
1410         // Do not create a clause if we do not have anything inside it.
1411         if bounds.is_empty() {
1412             return None;
1413         }
1414         let ident = self.lower_ident(ident);
1415         let param_span = ident.span;
1416         let span = bounds
1417             .iter()
1418             .fold(Some(param_span.shrink_to_hi()), |span: Option<Span>, bound| {
1419                 let bound_span = bound.span();
1420                 // We include bounds that come from a `#[derive(_)]` but point at the user's code,
1421                 // as we use this method to get a span appropriate for suggestions.
1422                 if !bound_span.can_be_used_for_suggestions() {
1423                     None
1424                 } else if let Some(span) = span {
1425                     Some(span.to(bound_span))
1426                 } else {
1427                     Some(bound_span)
1428                 }
1429             })
1430             .unwrap_or(param_span.shrink_to_hi());
1431         match kind {
1432             GenericParamKind::Const { .. } => None,
1433             GenericParamKind::Type { .. } => {
1434                 let def_id = self.local_def_id(id).to_def_id();
1435                 let ty_path = self.arena.alloc(hir::Path {
1436                     span: param_span,
1437                     res: Res::Def(DefKind::TyParam, def_id),
1438                     segments: self.arena.alloc_from_iter([hir::PathSegment::from_ident(ident)]),
1439                 });
1440                 let ty_id = self.next_id();
1441                 let bounded_ty =
1442                     self.ty_path(ty_id, param_span, hir::QPath::Resolved(None, ty_path));
1443                 Some(hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1444                     bounded_ty: self.arena.alloc(bounded_ty),
1445                     bounds,
1446                     span,
1447                     bound_generic_params: &[],
1448                     origin,
1449                 }))
1450             }
1451             GenericParamKind::Lifetime => {
1452                 let ident_span = self.lower_span(ident.span);
1453                 let ident = self.lower_ident(ident);
1454                 let res = self.resolver.get_lifetime_res(id).unwrap_or_else(|| {
1455                     panic!("Missing resolution for lifetime {:?} at {:?}", id, ident.span)
1456                 });
1457                 let lt_id = self.next_node_id();
1458                 let lifetime = self.new_named_lifetime_with_res(lt_id, ident_span, ident, res);
1459                 Some(hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1460                     lifetime,
1461                     span,
1462                     bounds,
1463                     in_where_clause: false,
1464                 }))
1465             }
1466         }
1467     }
1468
1469     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1470         match *pred {
1471             WherePredicate::BoundPredicate(WhereBoundPredicate {
1472                 ref bound_generic_params,
1473                 ref bounded_ty,
1474                 ref bounds,
1475                 span,
1476             }) => hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1477                 bound_generic_params: self.lower_generic_params(bound_generic_params),
1478                 bounded_ty: self
1479                     .lower_ty(bounded_ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
1480                 bounds: self.arena.alloc_from_iter(bounds.iter().map(|bound| {
1481                     self.lower_param_bound(
1482                         bound,
1483                         ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
1484                     )
1485                 })),
1486                 span: self.lower_span(span),
1487                 origin: PredicateOrigin::WhereClause,
1488             }),
1489             WherePredicate::RegionPredicate(WhereRegionPredicate {
1490                 ref lifetime,
1491                 ref bounds,
1492                 span,
1493             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1494                 span: self.lower_span(span),
1495                 lifetime: self.lower_lifetime(lifetime),
1496                 bounds: self.lower_param_bounds(
1497                     bounds,
1498                     ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
1499                 ),
1500                 in_where_clause: true,
1501             }),
1502             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1503                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1504                     hir_id: self.lower_node_id(id),
1505                     lhs_ty: self
1506                         .lower_ty(lhs_ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
1507                     rhs_ty: self
1508                         .lower_ty(rhs_ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
1509                     span: self.lower_span(span),
1510                 })
1511             }
1512         }
1513     }
1514 }