src/librustc_ast_lowering/item.rs

   1 use super::{AnonymousLifetimeMode, LoweringContext, ParamMode};
   2 use super::{ImplTraitContext, ImplTraitPosition, ImplTraitTypeIdVisitor};
   3
   4 use rustc::arena::Arena;
   5 use rustc::bug;
   6 use rustc_error_codes::*;
   7 use rustc_errors::struct_span_err;
   8 use rustc_hir as hir;
   9 use rustc_hir::def::{DefKind, Res};
  10 use rustc_hir::def_id::DefId;
  11 use rustc_session::node_id::NodeMap;
  12 use rustc_span::source_map::{respan, DesugaringKind};
  13 use rustc_span::symbol::{kw, sym};
  14 use rustc_span::Span;
  15 use rustc_target::spec::abi;
  16 use syntax::ast::*;
  17 use syntax::attr;
  18 use syntax::visit::{self, Visitor};
  19
  20 use log::debug;
  21 use smallvec::{smallvec, SmallVec};
  22 use std::collections::BTreeSet;
  23
  24 pub(super) struct ItemLowerer<'a, 'lowering, 'hir> {
  25     pub(super) lctx: &'a mut LoweringContext<'lowering, 'hir>,
  26 }
  27
  28 impl ItemLowerer<'_, '_, '_> {
  29     fn with_trait_impl_ref(&mut self, impl_ref: &Option<TraitRef>, f: impl FnOnce(&mut Self)) {
  30         let old = self.lctx.is_in_trait_impl;
  31         self.lctx.is_in_trait_impl = if let &None = impl_ref { false } else { true };
  32         f(self);
  33         self.lctx.is_in_trait_impl = old;
  34     }
  35 }
  36
  37 impl<'a> Visitor<'a> for ItemLowerer<'a, '_, '_> {
  38     fn visit_mod(&mut self, m: &'a Mod, _s: Span, _attrs: &[Attribute], n: NodeId) {
  39         let hir_id = self.lctx.lower_node_id(n);
  40
  41         self.lctx.modules.insert(
  42             hir_id,
  43             hir::ModuleItems {
  44                 items: BTreeSet::new(),
  45                 trait_items: BTreeSet::new(),
  46                 impl_items: BTreeSet::new(),
  47             },
  48         );
  49
  50         let old = self.lctx.current_module;
  51         self.lctx.current_module = hir_id;
  52         visit::walk_mod(self, m);
  53         self.lctx.current_module = old;
  54     }
  55
  56     fn visit_item(&mut self, item: &'a Item) {
  57         let mut item_hir_id = None;
  58         self.lctx.with_hir_id_owner(item.id, |lctx| {
  59             lctx.without_in_scope_lifetime_defs(|lctx| {
  60                 if let Some(hir_item) = lctx.lower_item(item) {
  61                     item_hir_id = Some(hir_item.hir_id);
  62                     lctx.insert_item(hir_item);
  63                 }
  64             })
  65         });
  66
  67         if let Some(hir_id) = item_hir_id {
  68             self.lctx.with_parent_item_lifetime_defs(hir_id, |this| {
  69                 let this = &mut ItemLowerer { lctx: this };
  70                 if let ItemKind::Impl(.., ref opt_trait_ref, _, _) = item.kind {
  71                     this.with_trait_impl_ref(opt_trait_ref, |this| visit::walk_item(this, item));
  72                 } else {
  73                     visit::walk_item(this, item);
  74                 }
  75             });
  76         }
  77     }
  78
  79     fn visit_trait_item(&mut self, item: &'a AssocItem) {
  80         self.lctx.with_hir_id_owner(item.id, |lctx| {
  81             let hir_item = lctx.lower_trait_item(item);
  82             let id = hir::TraitItemId { hir_id: hir_item.hir_id };
  83             lctx.trait_items.insert(id, hir_item);
  84             lctx.modules.get_mut(&lctx.current_module).unwrap().trait_items.insert(id);
  85         });
  86
  87         visit::walk_trait_item(self, item);
  88     }
  89
  90     fn visit_impl_item(&mut self, item: &'a AssocItem) {
  91         self.lctx.with_hir_id_owner(item.id, |lctx| {
  92             let hir_item = lctx.lower_impl_item(item);
  93             let id = hir::ImplItemId { hir_id: hir_item.hir_id };
  94             lctx.impl_items.insert(id, hir_item);
  95             lctx.modules.get_mut(&lctx.current_module).unwrap().impl_items.insert(id);
  96         });
  97         visit::walk_impl_item(self, item);
  98     }
  99 }
 100
 101 impl<'hir> LoweringContext<'_, 'hir> {
 102     // Same as the method above, but accepts `hir::GenericParam`s
 103     // instead of `ast::GenericParam`s.
 104     // This should only be used with generics that have already had their
 105     // in-band lifetimes added. In practice, this means that this function is
 106     // only used when lowering a child item of a trait or impl.
 107     fn with_parent_item_lifetime_defs<T>(
 108         &mut self,
 109         parent_hir_id: hir::HirId,
 110         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 111     ) -> T {
 112         let old_len = self.in_scope_lifetimes.len();
 113
 114         let parent_generics = match self.items.get(&parent_hir_id).unwrap().kind {
 115             hir::ItemKind::Impl(_, _, _, ref generics, ..)
 116             | hir::ItemKind::Trait(_, _, ref generics, ..) => &generics.params[..],
 117             _ => &[],
 118         };
 119         let lt_def_names = parent_generics.iter().filter_map(|param| match param.kind {
 120             hir::GenericParamKind::Lifetime { .. } => Some(param.name.modern()),
 121             _ => None,
 122         });
 123         self.in_scope_lifetimes.extend(lt_def_names);
 124
 125         let res = f(self);
 126
 127         self.in_scope_lifetimes.truncate(old_len);
 128         res
 129     }
 130
 131     // Clears (and restores) the `in_scope_lifetimes` field. Used when
 132     // visiting nested items, which never inherit in-scope lifetimes
 133     // from their surrounding environment.
 134     fn without_in_scope_lifetime_defs<T>(
 135         &mut self,
 136         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 137     ) -> T {
 138         let old_in_scope_lifetimes = std::mem::replace(&mut self.in_scope_lifetimes, vec![]);
 139
 140         // this vector is only used when walking over impl headers,
 141         // input types, and the like, and should not be non-empty in
 142         // between items
 143         assert!(self.lifetimes_to_define.is_empty());
 144
 145         let res = f(self);
 146
 147         assert!(self.in_scope_lifetimes.is_empty());
 148         self.in_scope_lifetimes = old_in_scope_lifetimes;
 149
 150         res
 151     }
 152
 153     pub(super) fn lower_mod(&mut self, m: &Mod) -> hir::Mod<'hir> {
 154         hir::Mod {
 155             inner: m.inner,
 156             item_ids: self
 157                 .arena
 158                 .alloc_from_iter(m.items.iter().flat_map(|x| self.lower_item_id(x))),
 159         }
 160     }
 161
 162     pub(super) fn lower_item_id(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
 163         let node_ids = match i.kind {
 164             ItemKind::Use(ref use_tree) => {
 165                 let mut vec = smallvec![i.id];
 166                 self.lower_item_id_use_tree(use_tree, i.id, &mut vec);
 167                 vec
 168             }
 169             ItemKind::MacroDef(..) => SmallVec::new(),
 170             ItemKind::Fn(..) | ItemKind::Impl(.., None, _, _) => smallvec![i.id],
 171             ItemKind::Static(ref ty, ..) => {
 172                 let mut ids = smallvec![i.id];
 173                 if self.sess.features_untracked().impl_trait_in_bindings {
 174                     let mut visitor = ImplTraitTypeIdVisitor { ids: &mut ids };
 175                     visitor.visit_ty(ty);
 176                 }
 177                 ids
 178             }
 179             ItemKind::Const(ref ty, ..) => {
 180                 let mut ids = smallvec![i.id];
 181                 if self.sess.features_untracked().impl_trait_in_bindings {
 182                     let mut visitor = ImplTraitTypeIdVisitor { ids: &mut ids };
 183                     visitor.visit_ty(ty);
 184                 }
 185                 ids
 186             }
 187             _ => smallvec![i.id],
 188         };
 189
 190         node_ids
 191             .into_iter()
 192             .map(|node_id| hir::ItemId { id: self.allocate_hir_id_counter(node_id) })
 193             .collect()
 194     }
 195
 196     fn lower_item_id_use_tree(
 197         &mut self,
 198         tree: &UseTree,
 199         base_id: NodeId,
 200         vec: &mut SmallVec<[NodeId; 1]>,
 201     ) {
 202         match tree.kind {
 203             UseTreeKind::Nested(ref nested_vec) => {
 204                 for &(ref nested, id) in nested_vec {
 205                     vec.push(id);
 206                     self.lower_item_id_use_tree(nested, id, vec);
 207                 }
 208             }
 209             UseTreeKind::Glob => {}
 210             UseTreeKind::Simple(_, id1, id2) => {
 211                 for (_, &id) in
 212                     self.expect_full_res_from_use(base_id).skip(1).zip([id1, id2].iter())
 213                 {
 214                     vec.push(id);
 215                 }
 216             }
 217         }
 218     }
 219
 220     pub fn lower_item(&mut self, i: &Item) -> Option<hir::Item<'hir>> {
 221         let mut ident = i.ident;
 222         let mut vis = self.lower_visibility(&i.vis, None);
 223         let attrs = self.lower_attrs(&i.attrs);
 224
 225         if let ItemKind::MacroDef(ref def) = i.kind {
 226             if !def.legacy || attr::contains_name(&i.attrs, sym::macro_export) {
 227                 let body = self.lower_token_stream(def.body.inner_tokens());
 228                 let hir_id = self.lower_node_id(i.id);
 229                 self.exported_macros.push(hir::MacroDef {
 230                     name: ident.name,
 231                     vis,
 232                     attrs,
 233                     hir_id,
 234                     span: i.span,
 235                     body,
 236                     legacy: def.legacy,
 237                 });
 238             } else {
 239                 self.non_exported_macro_attrs.extend(attrs.iter().cloned());
 240             }
 241             return None;
 242         }
 243
 244         let kind = self.lower_item_kind(i.span, i.id, &mut ident, attrs, &mut vis, &i.kind);
 245
 246         Some(hir::Item { hir_id: self.lower_node_id(i.id), ident, attrs, kind, vis, span: i.span })
 247     }
 248
 249     fn lower_item_kind(
 250         &mut self,
 251         span: Span,
 252         id: NodeId,
 253         ident: &mut Ident,
 254         attrs: &'hir [Attribute],
 255         vis: &mut hir::Visibility<'hir>,
 256         i: &ItemKind,
 257     ) -> hir::ItemKind<'hir> {
 258         match *i {
 259             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
 260             ItemKind::Use(ref use_tree) => {
 261                 // Start with an empty prefix.
 262                 let prefix = Path { segments: vec![], span: use_tree.span };
 263
 264                 self.lower_use_tree(use_tree, &prefix, id, vis, ident, attrs)
 265             }
 266             ItemKind::Static(ref t, m, ref e) => {
 267                 let ty = self.lower_ty(
 268                     t,
 269                     if self.sess.features_untracked().impl_trait_in_bindings {
 270                         ImplTraitContext::OpaqueTy(None)
 271                     } else {
 272                         ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
 273                     },
 274                 );
 275                 hir::ItemKind::Static(ty, m, self.lower_const_body(span, Some(e)))
 276             }
 277             ItemKind::Const(ref t, ref e) => {
 278                 let ty = self.lower_ty(
 279                     t,
 280                     if self.sess.features_untracked().impl_trait_in_bindings {
 281                         ImplTraitContext::OpaqueTy(None)
 282                     } else {
 283                         ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
 284                     },
 285                 );
 286                 hir::ItemKind::Const(ty, self.lower_const_body(span, Some(e)))
 287             }
 288             ItemKind::Fn(FnSig { ref decl, header }, ref generics, ref body) => {
 289                 let fn_def_id = self.resolver.definitions().local_def_id(id);
 290                 self.with_new_scopes(|this| {
 291                     this.current_item = Some(ident.span);
 292
 293                     // Note: we don't need to change the return type from `T` to
 294                     // `impl Future<Output = T>` here because lower_body
 295                     // only cares about the input argument patterns in the function
 296                     // declaration (decl), not the return types.
 297                     let body_id =
 298                         this.lower_maybe_async_body(span, &decl, header.asyncness.node, Some(body));
 299
 300                     let (generics, decl) = this.add_in_band_defs(
 301                         generics,
 302                         fn_def_id,
 303                         AnonymousLifetimeMode::PassThrough,
 304                         |this, idty| {
 305                             this.lower_fn_decl(
 306                                 &decl,
 307                                 Some((fn_def_id, idty)),
 308                                 true,
 309                                 header.asyncness.node.opt_return_id(),
 310                             )
 311                         },
 312                     );
 313                     let sig = hir::FnSig { decl, header: this.lower_fn_header(header) };
 314                     hir::ItemKind::Fn(sig, generics, body_id)
 315                 })
 316             }
 317             ItemKind::Mod(ref m) => hir::ItemKind::Mod(self.lower_mod(m)),
 318             ItemKind::ForeignMod(ref nm) => hir::ItemKind::ForeignMod(self.lower_foreign_mod(nm)),
 319             ItemKind::GlobalAsm(ref ga) => hir::ItemKind::GlobalAsm(self.lower_global_asm(ga)),
 320             ItemKind::TyAlias(ref ty, ref generics) => match ty.kind.opaque_top_hack() {
 321                 None => {
 322                     let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 323                     let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 324                     hir::ItemKind::TyAlias(ty, generics)
 325                 }
 326                 Some(bounds) => {
 327                     let ty = hir::OpaqueTy {
 328                         generics: self.lower_generics(generics, ImplTraitContext::OpaqueTy(None)),
 329                         bounds: self.lower_param_bounds(bounds, ImplTraitContext::OpaqueTy(None)),
 330                         impl_trait_fn: None,
 331                         origin: hir::OpaqueTyOrigin::TypeAlias,
 332                     };
 333                     hir::ItemKind::OpaqueTy(ty)
 334                 }
 335             },
 336             ItemKind::Enum(ref enum_definition, ref generics) => hir::ItemKind::Enum(
 337                 hir::EnumDef {
 338                     variants: self.arena.alloc_from_iter(
 339                         enum_definition.variants.iter().map(|x| self.lower_variant(x)),
 340                     ),
 341                 },
 342                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 343             ),
 344             ItemKind::Struct(ref struct_def, ref generics) => {
 345                 let struct_def = self.lower_variant_data(struct_def);
 346                 hir::ItemKind::Struct(
 347                     struct_def,
 348                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 349                 )
 350             }
 351             ItemKind::Union(ref vdata, ref generics) => {
 352                 let vdata = self.lower_variant_data(vdata);
 353                 hir::ItemKind::Union(
 354                     vdata,
 355                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 356                 )
 357             }
 358             ItemKind::Impl(
 359                 unsafety,
 360                 polarity,
 361                 defaultness,
 362                 ref ast_generics,
 363                 ref trait_ref,
 364                 ref ty,
 365                 ref impl_items,
 366             ) => {
 367                 let def_id = self.resolver.definitions().local_def_id(id);
 368
 369                 // Lower the "impl header" first. This ordering is important
 370                 // for in-band lifetimes! Consider `'a` here:
 371                 //
 372                 //     impl Foo<'a> for u32 {
 373                 //         fn method(&'a self) { .. }
 374                 //     }
 375                 //
 376                 // Because we start by lowering the `Foo<'a> for u32`
 377                 // part, we will add `'a` to the list of generics on
 378                 // the impl. When we then encounter it later in the
 379                 // method, it will not be considered an in-band
 380                 // lifetime to be added, but rather a reference to a
 381                 // parent lifetime.
 382                 let lowered_trait_impl_id = self.lower_node_id(id);
 383                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 384                     ast_generics,
 385                     def_id,
 386                     AnonymousLifetimeMode::CreateParameter,
 387                     |this, _| {
 388                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 389                             this.lower_trait_ref(trait_ref, ImplTraitContext::disallowed())
 390                         });
 391
 392                         if let Some(ref trait_ref) = trait_ref {
 393                             if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res {
 394                                 this.trait_impls
 395                                     .entry(def_id)
 396                                     .or_default()
 397                                     .push(lowered_trait_impl_id);
 398                             }
 399                         }
 400
 401                         let lowered_ty = this.lower_ty(ty, ImplTraitContext::disallowed());
 402
 403                         (trait_ref, lowered_ty)
 404                     },
 405                 );
 406
 407                 let new_impl_items =
 408                     self.with_in_scope_lifetime_defs(&ast_generics.params, |this| {
 409                         this.arena.alloc_from_iter(
 410                             impl_items.iter().map(|item| this.lower_impl_item_ref(item)),
 411                         )
 412                     });
 413
 414                 hir::ItemKind::Impl(
 415                     unsafety,
 416                     polarity,
 417                     self.lower_defaultness(defaultness, true /* [1] */),
 418                     generics,
 419                     trait_ref,
 420                     lowered_ty,
 421                     new_impl_items,
 422                 )
 423             }
 424             ItemKind::Trait(is_auto, unsafety, ref generics, ref bounds, ref items) => {
 425                 let bounds = self.lower_param_bounds(bounds, ImplTraitContext::disallowed());
 426                 let items = self
 427                     .arena
 428                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
 429                 hir::ItemKind::Trait(
 430                     is_auto,
 431                     unsafety,
 432                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 433                     bounds,
 434                     items,
 435                 )
 436             }
 437             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 438                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 439                 self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 440             ),
 441             ItemKind::MacroDef(..) | ItemKind::Mac(..) => {
 442                 bug!("`TyMac` should have been expanded by now")
 443             }
 444         }
 445
 446         // [1] `defaultness.has_value()` is never called for an `impl`, always `true` in order to
 447         //     not cause an assertion failure inside the `lower_defaultness` function.
 448     }
 449
 450     fn lower_use_tree(
 451         &mut self,
 452         tree: &UseTree,
 453         prefix: &Path,
 454         id: NodeId,
 455         vis: &mut hir::Visibility<'hir>,
 456         ident: &mut Ident,
 457         attrs: &'hir [Attribute],
 458     ) -> hir::ItemKind<'hir> {
 459         debug!("lower_use_tree(tree={:?})", tree);
 460         debug!("lower_use_tree: vis = {:?}", vis);
 461
 462         let path = &tree.prefix;
 463         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 464
 465         match tree.kind {
 466             UseTreeKind::Simple(rename, id1, id2) => {
 467                 *ident = tree.ident();
 468
 469                 // First, apply the prefix to the path.
 470                 let mut path = Path { segments, span: path.span };
 471
 472                 // Correctly resolve `self` imports.
 473                 if path.segments.len() > 1
 474                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 475                 {
 476                     let _ = path.segments.pop();
 477                     if rename.is_none() {
 478                         *ident = path.segments.last().unwrap().ident;
 479                     }
 480                 }
 481
 482                 let mut resolutions = self.expect_full_res_from_use(id);
 483                 // We want to return *something* from this function, so hold onto the first item
 484                 // for later.
 485                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 486
 487                 // Here, we are looping over namespaces, if they exist for the definition
 488                 // being imported. We only handle type and value namespaces because we
 489                 // won't be dealing with macros in the rest of the compiler.
 490                 // Essentially a single `use` which imports two names is desugared into
 491                 // two imports.
 492                 for (res, &new_node_id) in resolutions.zip([id1, id2].iter()) {
 493                     let ident = *ident;
 494                     let mut path = path.clone();
 495                     for seg in &mut path.segments {
 496                         seg.id = self.resolver.next_node_id();
 497                     }
 498                     let span = path.span;
 499
 500                     self.with_hir_id_owner(new_node_id, |this| {
 501                         let new_id = this.lower_node_id(new_node_id);
 502                         let res = this.lower_res(res);
 503                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit, None);
 504                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 505                         let vis = this.rebuild_vis(&vis);
 506
 507                         this.insert_item(hir::Item {
 508                             hir_id: new_id,
 509                             ident,
 510                             attrs,
 511                             kind,
 512                             vis,
 513                             span,
 514                         });
 515                     });
 516                 }
 517
 518                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit, None);
 519                 hir::ItemKind::Use(path, hir::UseKind::Single)
 520             }
 521             UseTreeKind::Glob => {
 522                 let path =
 523                     self.lower_path(id, &Path { segments, span: path.span }, ParamMode::Explicit);
 524                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 525             }
 526             UseTreeKind::Nested(ref trees) => {
 527                 // Nested imports are desugared into simple imports.
 528                 // So, if we start with
 529                 //
 530                 // ```
 531                 // pub(x) use foo::{a, b};
 532                 // ```
 533                 //
 534                 // we will create three items:
 535                 //
 536                 // ```
 537                 // pub(x) use foo::a;
 538                 // pub(x) use foo::b;
 539                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 540                 // ```
 541                 //
 542                 // The first two are produced by recursively invoking
 543                 // `lower_use_tree` (and indeed there may be things
 544                 // like `use foo::{a::{b, c}}` and so forth).  They
 545                 // wind up being directly added to
 546                 // `self.items`. However, the structure of this
 547                 // function also requires us to return one item, and
 548                 // for that we return the `{}` import (called the
 549                 // `ListStem`).
 550
 551                 let prefix = Path { segments, span: prefix.span.to(path.span) };
 552
 553                 // Add all the nested `PathListItem`s to the HIR.
 554                 for &(ref use_tree, id) in trees {
 555                     let new_hir_id = self.lower_node_id(id);
 556
 557                     let mut prefix = prefix.clone();
 558
 559                     // Give the segments new node-ids since they are being cloned.
 560                     for seg in &mut prefix.segments {
 561                         seg.id = self.resolver.next_node_id();
 562                     }
 563
 564                     // Each `use` import is an item and thus are owners of the
 565                     // names in the path. Up to this point the nested import is
 566                     // the current owner, since we want each desugared import to
 567                     // own its own names, we have to adjust the owner before
 568                     // lowering the rest of the import.
 569                     self.with_hir_id_owner(id, |this| {
 570                         let mut vis = this.rebuild_vis(&vis);
 571                         let mut ident = *ident;
 572
 573                         let kind =
 574                             this.lower_use_tree(use_tree, &prefix, id, &mut vis, &mut ident, attrs);
 575
 576                         this.insert_item(hir::Item {
 577                             hir_id: new_hir_id,
 578                             ident,
 579                             attrs,
 580                             kind,
 581                             vis,
 582                             span: use_tree.span,
 583                         });
 584                     });
 585                 }
 586
 587                 // Subtle and a bit hacky: we lower the privacy level
 588                 // of the list stem to "private" most of the time, but
 589                 // not for "restricted" paths. The key thing is that
 590                 // we don't want it to stay as `pub` (with no caveats)
 591                 // because that affects rustdoc and also the lints
 592                 // about `pub` items. But we can't *always* make it
 593                 // private -- particularly not for restricted paths --
 594                 // because it contains node-ids that would then be
 595                 // unused, failing the check that HirIds are "densely
 596                 // assigned".
 597                 match vis.node {
 598                     hir::VisibilityKind::Public
 599                     | hir::VisibilityKind::Crate(_)
 600                     | hir::VisibilityKind::Inherited => {
 601                         *vis = respan(prefix.span.shrink_to_lo(), hir::VisibilityKind::Inherited);
 602                     }
 603                     hir::VisibilityKind::Restricted { .. } => {
 604                         // Do nothing here, as described in the comment on the match.
 605                     }
 606                 }
 607
 608                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 609                 let res = self.lower_res(res);
 610                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit, None);
 611                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 612             }
 613         }
 614     }
 615
 616     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 617     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 618     /// `NodeId`s. (See, e.g., #56128.)
 619     fn rebuild_use_path(&mut self, path: &hir::Path<'hir>) -> &'hir hir::Path<'hir> {
 620         debug!("rebuild_use_path(path = {:?})", path);
 621         let segments =
 622             self.arena.alloc_from_iter(path.segments.iter().map(|seg| hir::PathSegment {
 623                 ident: seg.ident,
 624                 hir_id: seg.hir_id.map(|_| self.next_id()),
 625                 res: seg.res,
 626                 args: None,
 627                 infer_args: seg.infer_args,
 628             }));
 629         self.arena.alloc(hir::Path { span: path.span, res: path.res, segments })
 630     }
 631
 632     fn rebuild_vis(&mut self, vis: &hir::Visibility<'hir>) -> hir::Visibility<'hir> {
 633         let vis_kind = match vis.node {
 634             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 635             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 636             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 637             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 638                 hir::VisibilityKind::Restricted {
 639                     path: self.rebuild_use_path(path),
 640                     hir_id: self.next_id(),
 641                 }
 642             }
 643         };
 644         respan(vis.span, vis_kind)
 645     }
 646
 647     fn lower_foreign_item(&mut self, i: &ForeignItem) -> hir::ForeignItem<'hir> {
 648         let def_id = self.resolver.definitions().local_def_id(i.id);
 649         hir::ForeignItem {
 650             hir_id: self.lower_node_id(i.id),
 651             ident: i.ident,
 652             attrs: self.lower_attrs(&i.attrs),
 653             kind: match i.kind {
 654                 ForeignItemKind::Fn(ref fdec, ref generics) => {
 655                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 656                         generics,
 657                         def_id,
 658                         AnonymousLifetimeMode::PassThrough,
 659                         |this, _| {
 660                             (
 661                                 // Disallow `impl Trait` in foreign items.
 662                                 this.lower_fn_decl(fdec, None, false, None),
 663                                 this.lower_fn_params_to_names(fdec),
 664                             )
 665                         },
 666                     );
 667
 668                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 669                 }
 670                 ForeignItemKind::Static(ref t, m) => {
 671                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 672                     hir::ForeignItemKind::Static(ty, m)
 673                 }
 674                 ForeignItemKind::Ty => hir::ForeignItemKind::Type,
 675                 ForeignItemKind::Macro(_) => panic!("macro shouldn't exist here"),
 676             },
 677             vis: self.lower_visibility(&i.vis, None),
 678             span: i.span,
 679         }
 680     }
 681
 682     fn lower_foreign_mod(&mut self, fm: &ForeignMod) -> hir::ForeignMod<'hir> {
 683         hir::ForeignMod {
 684             abi: fm.abi.map_or(abi::Abi::C, |abi| self.lower_abi(abi)),
 685             items: self.arena.alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item(x))),
 686         }
 687     }
 688
 689     fn lower_global_asm(&mut self, ga: &GlobalAsm) -> &'hir hir::GlobalAsm {
 690         self.arena.alloc(hir::GlobalAsm { asm: ga.asm })
 691     }
 692
 693     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 694         hir::Variant {
 695             attrs: self.lower_attrs(&v.attrs),
 696             data: self.lower_variant_data(&v.data),
 697             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 698             id: self.lower_node_id(v.id),
 699             ident: v.ident,
 700             span: v.span,
 701         }
 702     }
 703
 704     fn lower_variant_data(&mut self, vdata: &VariantData) -> hir::VariantData<'hir> {
 705         match *vdata {
 706             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 707                 self.arena
 708                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 709                 recovered,
 710             ),
 711             VariantData::Tuple(ref fields, id) => hir::VariantData::Tuple(
 712                 self.arena
 713                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 714                 self.lower_node_id(id),
 715             ),
 716             VariantData::Unit(id) => hir::VariantData::Unit(self.lower_node_id(id)),
 717         }
 718     }
 719
 720     fn lower_struct_field(&mut self, (index, f): (usize, &StructField)) -> hir::StructField<'hir> {
 721         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 722             let t = self.lower_path_ty(
 723                 &f.ty,
 724                 qself,
 725                 path,
 726                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 727                 ImplTraitContext::disallowed(),
 728             );
 729             self.arena.alloc(t)
 730         } else {
 731             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 732         };
 733         hir::StructField {
 734             span: f.span,
 735             hir_id: self.lower_node_id(f.id),
 736             ident: match f.ident {
 737                 Some(ident) => ident,
 738                 // FIXME(jseyfried): positional field hygiene.
 739                 None => Ident::new(sym::integer(index), f.span),
 740             },
 741             vis: self.lower_visibility(&f.vis, None),
 742             ty,
 743             attrs: self.lower_attrs(&f.attrs),
 744         }
 745     }
 746
 747     fn lower_trait_item(&mut self, i: &AssocItem) -> hir::TraitItem<'hir> {
 748         let trait_item_def_id = self.resolver.definitions().local_def_id(i.id);
 749
 750         let (generics, kind) = match i.kind {
 751             AssocItemKind::Const(ref ty, ref default) => {
 752                 let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
 753                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 754                 (
 755                     generics,
 756                     hir::TraitItemKind::Const(
 757                         ty,
 758                         default.as_ref().map(|x| self.lower_const_body(i.span, Some(x))),
 759                     ),
 760                 )
 761             }
 762             AssocItemKind::Fn(ref sig, None) => {
 763                 let names = self.lower_fn_params_to_names(&sig.decl);
 764                 let (generics, sig) =
 765                     self.lower_method_sig(&i.generics, sig, trait_item_def_id, false, None);
 766                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Required(names)))
 767             }
 768             AssocItemKind::Fn(ref sig, Some(ref body)) => {
 769                 let body_id = self.lower_fn_body_block(i.span, &sig.decl, Some(body));
 770                 let (generics, sig) =
 771                     self.lower_method_sig(&i.generics, sig, trait_item_def_id, false, None);
 772                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Provided(body_id)))
 773             }
 774             AssocItemKind::TyAlias(ref bounds, ref default) => {
 775                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
 776                 let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
 777                 let kind = hir::TraitItemKind::Type(
 778                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 779                     ty,
 780                 );
 781
 782                 (generics, kind)
 783             }
 784             AssocItemKind::Macro(..) => bug!("macro item shouldn't exist at this point"),
 785         };
 786
 787         hir::TraitItem {
 788             hir_id: self.lower_node_id(i.id),
 789             ident: i.ident,
 790             attrs: self.lower_attrs(&i.attrs),
 791             generics,
 792             kind,
 793             span: i.span,
 794         }
 795     }
 796
 797     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 798         let (kind, has_default) = match i.kind {
 799             AssocItemKind::Const(_, ref default) => (hir::AssocItemKind::Const, default.is_some()),
 800             AssocItemKind::TyAlias(_, ref default) => (hir::AssocItemKind::Type, default.is_some()),
 801             AssocItemKind::Fn(ref sig, ref default) => {
 802                 (hir::AssocItemKind::Method { has_self: sig.decl.has_self() }, default.is_some())
 803             }
 804             AssocItemKind::Macro(..) => unimplemented!(),
 805         };
 806         hir::TraitItemRef {
 807             id: hir::TraitItemId { hir_id: self.lower_node_id(i.id) },
 808             ident: i.ident,
 809             span: i.span,
 810             defaultness: self.lower_defaultness(Defaultness::Default, has_default),
 811             kind,
 812         }
 813     }
 814
 815     /// Construct `ExprKind::Err` for the given `span`.
 816     fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 817         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 818     }
 819
 820     fn lower_impl_item(&mut self, i: &AssocItem) -> hir::ImplItem<'hir> {
 821         let impl_item_def_id = self.resolver.definitions().local_def_id(i.id);
 822
 823         let (generics, kind) = match i.kind {
 824             AssocItemKind::Const(ref ty, ref expr) => {
 825                 let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
 826                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 827                 (
 828                     generics,
 829                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 830                 )
 831             }
 832             AssocItemKind::Fn(ref sig, ref body) => {
 833                 self.current_item = Some(i.span);
 834                 let body_id = self.lower_maybe_async_body(
 835                     i.span,
 836                     &sig.decl,
 837                     sig.header.asyncness.node,
 838                     body.as_deref(),
 839                 );
 840                 let impl_trait_return_allow = !self.is_in_trait_impl;
 841                 let (generics, sig) = self.lower_method_sig(
 842                     &i.generics,
 843                     sig,
 844                     impl_item_def_id,
 845                     impl_trait_return_allow,
 846                     sig.header.asyncness.node.opt_return_id(),
 847                 );
 848
 849                 (generics, hir::ImplItemKind::Method(sig, body_id))
 850             }
 851             AssocItemKind::TyAlias(_, ref ty) => {
 852                 let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
 853                 let kind = match ty {
 854                     None => {
 855                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 856                         hir::ImplItemKind::TyAlias(ty)
 857                     }
 858                     Some(ty) => match ty.kind.opaque_top_hack() {
 859                         None => {
 860                             let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 861                             hir::ImplItemKind::TyAlias(ty)
 862                         }
 863                         Some(bs) => {
 864                             let bs = self.lower_param_bounds(bs, ImplTraitContext::disallowed());
 865                             hir::ImplItemKind::OpaqueTy(bs)
 866                         }
 867                     },
 868                 };
 869                 (generics, kind)
 870             }
 871             AssocItemKind::Macro(..) => bug!("`TyMac` should have been expanded by now"),
 872         };
 873
 874         hir::ImplItem {
 875             hir_id: self.lower_node_id(i.id),
 876             ident: i.ident,
 877             attrs: self.lower_attrs(&i.attrs),
 878             generics,
 879             vis: self.lower_visibility(&i.vis, None),
 880             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
 881             kind,
 882             span: i.span,
 883         }
 884
 885         // [1] since `default impl` is not yet implemented, this is always true in impls
 886     }
 887
 888     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef<'hir> {
 889         hir::ImplItemRef {
 890             id: hir::ImplItemId { hir_id: self.lower_node_id(i.id) },
 891             ident: i.ident,
 892             span: i.span,
 893             vis: self.lower_visibility(&i.vis, Some(i.id)),
 894             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
 895             kind: match &i.kind {
 896                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 897                 AssocItemKind::TyAlias(_, ty) => {
 898                     match ty.as_deref().and_then(|ty| ty.kind.opaque_top_hack()) {
 899                         None => hir::AssocItemKind::Type,
 900                         Some(_) => hir::AssocItemKind::OpaqueTy,
 901                     }
 902                 }
 903                 AssocItemKind::Fn(sig, _) => {
 904                     hir::AssocItemKind::Method { has_self: sig.decl.has_self() }
 905                 }
 906                 AssocItemKind::Macro(..) => unimplemented!(),
 907             },
 908         }
 909
 910         // [1] since `default impl` is not yet implemented, this is always true in impls
 911     }
 912
 913     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 914     /// the address space of that item instead of the item currently being
 915     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 916     /// lower a `Visibility` value although we haven't lowered the owning
 917     /// `ImplItem` in question yet.
 918     fn lower_visibility(
 919         &mut self,
 920         v: &Visibility,
 921         explicit_owner: Option<NodeId>,
 922     ) -> hir::Visibility<'hir> {
 923         let node = match v.node {
 924             VisibilityKind::Public => hir::VisibilityKind::Public,
 925             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 926             VisibilityKind::Restricted { ref path, id } => {
 927                 debug!("lower_visibility: restricted path id = {:?}", id);
 928                 let lowered_id = if let Some(owner) = explicit_owner {
 929                     self.lower_node_id_with_owner(id, owner)
 930                 } else {
 931                     self.lower_node_id(id)
 932                 };
 933                 let res = self.expect_full_res(id);
 934                 let res = self.lower_res(res);
 935                 hir::VisibilityKind::Restricted {
 936                     path: self.lower_path_extra(res, path, ParamMode::Explicit, explicit_owner),
 937                     hir_id: lowered_id,
 938                 }
 939             }
 940             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 941         };
 942         respan(v.span, node)
 943     }
 944
 945     fn lower_defaultness(&self, d: Defaultness, has_value: bool) -> hir::Defaultness {
 946         match d {
 947             Defaultness::Default => hir::Defaultness::Default { has_value: has_value },
 948             Defaultness::Final => {
 949                 assert!(has_value);
 950                 hir::Defaultness::Final
 951             }
 952         }
 953     }
 954
 955     fn record_body(
 956         &mut self,
 957         params: &'hir [hir::Param<'hir>],
 958         value: hir::Expr<'hir>,
 959     ) -> hir::BodyId {
 960         let body = hir::Body { generator_kind: self.generator_kind, params, value };
 961         let id = body.id();
 962         self.bodies.insert(id, body);
 963         id
 964     }
 965
 966     fn lower_body(
 967         &mut self,
 968         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
 969     ) -> hir::BodyId {
 970         let prev_gen_kind = self.generator_kind.take();
 971         let (parameters, result) = f(self);
 972         let body_id = self.record_body(parameters, result);
 973         self.generator_kind = prev_gen_kind;
 974         body_id
 975     }
 976
 977     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
 978         hir::Param {
 979             attrs: self.lower_attrs(&param.attrs),
 980             hir_id: self.lower_node_id(param.id),
 981             pat: self.lower_pat(&param.pat),
 982             span: param.span,
 983         }
 984     }
 985
 986     pub(super) fn lower_fn_body(
 987         &mut self,
 988         decl: &FnDecl,
 989         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 990     ) -> hir::BodyId {
 991         self.lower_body(|this| {
 992             (
 993                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
 994                 body(this),
 995             )
 996         })
 997     }
 998
 999     fn lower_fn_body_block(
1000         &mut self,
1001         span: Span,
1002         decl: &FnDecl,
1003         body: Option<&Block>,
1004     ) -> hir::BodyId {
1005         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1006     }
1007
1008     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1009         match block {
1010             Some(block) => self.lower_block_expr(block),
1011             None => self.expr_err(span),
1012         }
1013     }
1014
1015     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1016         self.lower_body(|this| {
1017             (
1018                 &[],
1019                 match expr {
1020                     Some(expr) => this.lower_expr_mut(expr),
1021                     None => this.expr_err(span),
1022                 },
1023             )
1024         })
1025     }
1026
1027     fn lower_maybe_async_body(
1028         &mut self,
1029         span: Span,
1030         decl: &FnDecl,
1031         asyncness: IsAsync,
1032         body: Option<&Block>,
1033     ) -> hir::BodyId {
1034         let closure_id = match asyncness {
1035             IsAsync::Async { closure_id, .. } => closure_id,
1036             IsAsync::NotAsync => return self.lower_fn_body_block(span, decl, body),
1037         };
1038
1039         self.lower_body(|this| {
1040             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1041             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1042
1043             // Async function parameters are lowered into the closure body so that they are
1044             // captured and so that the drop order matches the equivalent non-async functions.
1045             //
1046             // from:
1047             //
1048             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1049             //         <body>
1050             //     }
1051             //
1052             // into:
1053             //
1054             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1055             //       async move {
1056             //         let __arg2 = __arg2;
1057             //         let <pattern> = __arg2;
1058             //         let __arg1 = __arg1;
1059             //         let <pattern> = __arg1;
1060             //         let __arg0 = __arg0;
1061             //         let <pattern> = __arg0;
1062             //         drop-temps { <body> } // see comments later in fn for details
1063             //       }
1064             //     }
1065             //
1066             // If `<pattern>` is a simple ident, then it is lowered to a single
1067             // `let <pattern> = <pattern>;` statement as an optimization.
1068             //
1069             // Note that the body is embedded in `drop-temps`; an
1070             // equivalent desugaring would be `return { <body>
1071             // };`. The key point is that we wish to drop all the
1072             // let-bound variables and temporaries created in the body
1073             // (and its tail expression!) before we drop the
1074             // parameters (c.f. rust-lang/rust#64512).
1075             for (index, parameter) in decl.inputs.iter().enumerate() {
1076                 let parameter = this.lower_param(parameter);
1077                 let span = parameter.pat.span;
1078
1079                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1080                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1081                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1082                     hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, ident, _) => {
1083                         (ident, true)
1084                     }
1085                     _ => {
1086                         // Replace the ident for bindings that aren't simple.
1087                         let name = format!("__arg{}", index);
1088                         let ident = Ident::from_str(&name);
1089
1090                         (ident, false)
1091                     }
1092                 };
1093
1094                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1095
1096                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1097                 // async function.
1098                 //
1099                 // If this is the simple case, this parameter will end up being the same as the
1100                 // original parameter, but with a different pattern id.
1101                 let mut stmt_attrs = AttrVec::new();
1102                 stmt_attrs.extend(parameter.attrs.iter().cloned());
1103                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1104                 let new_parameter = hir::Param {
1105                     attrs: parameter.attrs,
1106                     hir_id: parameter.hir_id,
1107                     pat: new_parameter_pat,
1108                     span: parameter.span,
1109                 };
1110
1111                 if is_simple_parameter {
1112                     // If this is the simple case, then we only insert one statement that is
1113                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1114                     // `HirId`s are densely assigned.
1115                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1116                     let stmt = this.stmt_let_pat(
1117                         stmt_attrs,
1118                         desugared_span,
1119                         Some(expr),
1120                         parameter.pat,
1121                         hir::LocalSource::AsyncFn,
1122                     );
1123                     statements.push(stmt);
1124                 } else {
1125                     // If this is not the simple case, then we construct two statements:
1126                     //
1127                     // ```
1128                     // let __argN = __argN;
1129                     // let <pat> = __argN;
1130                     // ```
1131                     //
1132                     // The first statement moves the parameter into the closure and thus ensures
1133                     // that the drop order is correct.
1134                     //
1135                     // The second statement creates the bindings that the user wrote.
1136
1137                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1138                     // because the user may have specified a `ref mut` binding in the next
1139                     // statement.
1140                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1141                         desugared_span,
1142                         ident,
1143                         hir::BindingAnnotation::Mutable,
1144                     );
1145                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1146                     let move_stmt = this.stmt_let_pat(
1147                         AttrVec::new(),
1148                         desugared_span,
1149                         Some(move_expr),
1150                         move_pat,
1151                         hir::LocalSource::AsyncFn,
1152                     );
1153
1154                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1155                     // parameter's pattern so that `HirId`s are densely assigned.
1156                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1157                     let pattern_stmt = this.stmt_let_pat(
1158                         stmt_attrs,
1159                         desugared_span,
1160                         Some(pattern_expr),
1161                         parameter.pat,
1162                         hir::LocalSource::AsyncFn,
1163                     );
1164
1165                     statements.push(move_stmt);
1166                     statements.push(pattern_stmt);
1167                 };
1168
1169                 parameters.push(new_parameter);
1170             }
1171
1172             let body_span = body.map_or(span, |b| b.span);
1173             let async_expr = this.make_async_expr(
1174                 CaptureBy::Value,
1175                 closure_id,
1176                 None,
1177                 body_span,
1178                 hir::AsyncGeneratorKind::Fn,
1179                 |this| {
1180                     // Create a block from the user's function body:
1181                     let user_body = this.lower_block_expr_opt(body_span, body);
1182
1183                     // Transform into `drop-temps { <user-body> }`, an expression:
1184                     let desugared_span =
1185                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1186                     let user_body = this.expr_drop_temps(
1187                         desugared_span,
1188                         this.arena.alloc(user_body),
1189                         AttrVec::new(),
1190                     );
1191
1192                     // As noted above, create the final block like
1193                     //
1194                     // ```
1195                     // {
1196                     //   let $param_pattern = $raw_param;
1197                     //   ...
1198                     //   drop-temps { <user-body> }
1199                     // }
1200                     // ```
1201                     let body = this.block_all(
1202                         desugared_span,
1203                         this.arena.alloc_from_iter(statements),
1204                         Some(user_body),
1205                     );
1206
1207                     this.expr_block(body, AttrVec::new())
1208                 },
1209             );
1210
1211             (
1212                 this.arena.alloc_from_iter(parameters),
1213                 this.expr(body_span, async_expr, AttrVec::new()),
1214             )
1215         })
1216     }
1217
1218     fn lower_method_sig(
1219         &mut self,
1220         generics: &Generics,
1221         sig: &FnSig,
1222         fn_def_id: DefId,
1223         impl_trait_return_allow: bool,
1224         is_async: Option<NodeId>,
1225     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1226         let header = self.lower_fn_header(sig.header);
1227         let (generics, decl) = self.add_in_band_defs(
1228             generics,
1229             fn_def_id,
1230             AnonymousLifetimeMode::PassThrough,
1231             |this, idty| {
1232                 this.lower_fn_decl(
1233                     &sig.decl,
1234                     Some((fn_def_id, idty)),
1235                     impl_trait_return_allow,
1236                     is_async,
1237                 )
1238             },
1239         );
1240         (generics, hir::FnSig { header, decl })
1241     }
1242
1243     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1244         hir::FnHeader {
1245             unsafety: h.unsafety,
1246             asyncness: self.lower_asyncness(h.asyncness.node),
1247             constness: h.constness.node,
1248             abi: self.lower_extern(h.ext),
1249         }
1250     }
1251
1252     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1253         abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1254             self.error_on_invalid_abi(abi);
1255             abi::Abi::Rust
1256         })
1257     }
1258
1259     pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
1260         match ext {
1261             Extern::None => abi::Abi::Rust,
1262             Extern::Implicit => abi::Abi::C,
1263             Extern::Explicit(abi) => self.lower_abi(abi),
1264         }
1265     }
1266
1267     fn error_on_invalid_abi(&self, abi: StrLit) {
1268         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1269             .span_label(abi.span, "invalid ABI")
1270             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1271             .emit();
1272     }
1273
1274     fn lower_asyncness(&mut self, a: IsAsync) -> hir::IsAsync {
1275         match a {
1276             IsAsync::Async { .. } => hir::IsAsync::Async,
1277             IsAsync::NotAsync => hir::IsAsync::NotAsync,
1278         }
1279     }
1280
1281     pub(super) fn lower_generics_mut(
1282         &mut self,
1283         generics: &Generics,
1284         itctx: ImplTraitContext<'_, 'hir>,
1285     ) -> GenericsCtor<'hir> {
1286         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1287         // FIXME: this could probably be done with less rightward drift. It also looks like two
1288         // control paths where `report_error` is called are the only paths that advance to after the
1289         // match statement, so the error reporting could probably just be moved there.
1290         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1291         for pred in &generics.where_clause.predicates {
1292             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1293                 'next_bound: for bound in &bound_pred.bounds {
1294                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1295                         let report_error = |this: &mut Self| {
1296                             this.diagnostic().span_err(
1297                                 bound_pred.bounded_ty.span,
1298                                 "`?Trait` bounds are only permitted at the \
1299                                  point where a type parameter is declared",
1300                             );
1301                         };
1302                         // Check if the where clause type is a plain type parameter.
1303                         match bound_pred.bounded_ty.kind {
1304                             TyKind::Path(None, ref path)
1305                                 if path.segments.len() == 1
1306                                     && bound_pred.bound_generic_params.is_empty() =>
1307                             {
1308                                 if let Some(Res::Def(DefKind::TyParam, def_id)) = self
1309                                     .resolver
1310                                     .get_partial_res(bound_pred.bounded_ty.id)
1311                                     .map(|d| d.base_res())
1312                                 {
1313                                     if let Some(node_id) =
1314                                         self.resolver.definitions().as_local_node_id(def_id)
1315                                     {
1316                                         for param in &generics.params {
1317                                             match param.kind {
1318                                                 GenericParamKind::Type { .. } => {
1319                                                     if node_id == param.id {
1320                                                         add_bounds
1321                                                             .entry(param.id)
1322                                                             .or_default()
1323                                                             .push(bound.clone());
1324                                                         continue 'next_bound;
1325                                                     }
1326                                                 }
1327                                                 _ => {}
1328                                             }
1329                                         }
1330                                     }
1331                                 }
1332                                 report_error(self)
1333                             }
1334                             _ => report_error(self),
1335                         }
1336                     }
1337                 }
1338             }
1339         }
1340
1341         GenericsCtor {
1342             params: self.lower_generic_params_mut(&generics.params, &add_bounds, itctx).collect(),
1343             where_clause: self.lower_where_clause(&generics.where_clause),
1344             span: generics.span,
1345         }
1346     }
1347
1348     pub(super) fn lower_generics(
1349         &mut self,
1350         generics: &Generics,
1351         itctx: ImplTraitContext<'_, 'hir>,
1352     ) -> hir::Generics<'hir> {
1353         let generics_ctor = self.lower_generics_mut(generics, itctx);
1354         generics_ctor.into_generics(self.arena)
1355     }
1356
1357     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1358         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1359             hir::WhereClause {
1360                 predicates: this.arena.alloc_from_iter(
1361                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1362                 ),
1363                 span: wc.span,
1364             }
1365         })
1366     }
1367
1368     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1369         match *pred {
1370             WherePredicate::BoundPredicate(WhereBoundPredicate {
1371                 ref bound_generic_params,
1372                 ref bounded_ty,
1373                 ref bounds,
1374                 span,
1375             }) => {
1376                 self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1377                     hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1378                         bound_generic_params: this.lower_generic_params(
1379                             bound_generic_params,
1380                             &NodeMap::default(),
1381                             ImplTraitContext::disallowed(),
1382                         ),
1383                         bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1384                         bounds: this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| {
1385                             match *bound {
1386                                 // Ignore `?Trait` bounds.
1387                                 // They were copied into type parameters already.
1388                                 GenericBound::Trait(_, TraitBoundModifier::Maybe) => None,
1389                                 _ => Some(
1390                                     this.lower_param_bound(bound, ImplTraitContext::disallowed()),
1391                                 ),
1392                             }
1393                         })),
1394                         span,
1395                     })
1396                 })
1397             }
1398             WherePredicate::RegionPredicate(WhereRegionPredicate {
1399                 ref lifetime,
1400                 ref bounds,
1401                 span,
1402             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1403                 span,
1404                 lifetime: self.lower_lifetime(lifetime),
1405                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1406             }),
1407             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1408                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1409                     hir_id: self.lower_node_id(id),
1410                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1411                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1412                     span,
1413                 })
1414             }
1415         }
1416     }
1417 }
1418
1419 /// Helper struct for delayed construction of Generics.
1420 pub(super) struct GenericsCtor<'hir> {
1421     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1422     where_clause: hir::WhereClause<'hir>,
1423     span: Span,
1424 }
1425
1426 impl<'hir> GenericsCtor<'hir> {
1427     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1428         hir::Generics {
1429             params: arena.alloc_from_iter(self.params),
1430             where_clause: self.where_clause,
1431             span: self.span,
1432         }
1433     }
1434 }