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