compiler/rustc_ast_lowering/src/item.rs

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