src/librustc_ast_lowering/item.rs

   1 use super::{AnonymousLifetimeMode, LoweringContext, ParamMode};
   2 use super::{ImplTraitContext, ImplTraitPosition};
   3 use crate::Arena;
   4
   5 use rustc_ast::node_id::NodeMap;
   6 use rustc_ast::ptr::P;
   7 use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
   8 use rustc_ast::*;
   9 use rustc_data_structures::fx::FxHashSet;
  10 use rustc_errors::struct_span_err;
  11 use rustc_hir as hir;
  12 use rustc_hir::def::{DefKind, Res};
  13 use rustc_hir::def_id::LocalDefId;
  14 use rustc_span::source_map::{respan, DesugaringKind};
  15 use rustc_span::symbol::{kw, sym, Ident};
  16 use rustc_span::Span;
  17 use rustc_target::spec::abi;
  18
  19 use smallvec::{smallvec, SmallVec};
  20 use std::collections::BTreeSet;
  21 use tracing::debug;
  22
  23 pub(super) struct ItemLowerer<'a, 'lowering, 'hir> {
  24     pub(super) lctx: &'a mut LoweringContext<'lowering, 'hir>,
  25 }
  26
  27 impl ItemLowerer<'_, '_, '_> {
  28     fn with_trait_impl_ref(&mut self, impl_ref: &Option<TraitRef>, f: impl FnOnce(&mut Self)) {
  29         let old = self.lctx.is_in_trait_impl;
  30         self.lctx.is_in_trait_impl = if let &None = impl_ref { false } else { true };
  31         f(self);
  32         self.lctx.is_in_trait_impl = old;
  33     }
  34 }
  35
  36 impl<'a> Visitor<'a> for ItemLowerer<'a, '_, '_> {
  37     fn visit_mod(&mut self, m: &'a Mod, _s: Span, _attrs: &[Attribute], n: NodeId) {
  38         let hir_id = self.lctx.lower_node_id(n);
  39
  40         self.lctx.modules.insert(
  41             hir_id,
  42             hir::ModuleItems {
  43                 items: BTreeSet::new(),
  44                 trait_items: BTreeSet::new(),
  45                 impl_items: BTreeSet::new(),
  46             },
  47         );
  48
  49         let old = self.lctx.current_module;
  50         self.lctx.current_module = hir_id;
  51         visit::walk_mod(self, m);
  52         self.lctx.current_module = old;
  53     }
  54
  55     fn visit_item(&mut self, item: &'a Item) {
  56         let mut item_hir_id = None;
  57         self.lctx.with_hir_id_owner(item.id, |lctx| {
  58             lctx.without_in_scope_lifetime_defs(|lctx| {
  59                 if let Some(hir_item) = lctx.lower_item(item) {
  60                     item_hir_id = Some(hir_item.hir_id);
  61                     lctx.insert_item(hir_item);
  62                 }
  63             })
  64         });
  65
  66         if let Some(hir_id) = item_hir_id {
  67             self.lctx.with_parent_item_lifetime_defs(hir_id, |this| {
  68                 let this = &mut ItemLowerer { lctx: this };
  69                 if let ItemKind::Impl { ref of_trait, .. } = item.kind {
  70                     this.with_trait_impl_ref(of_trait, |this| visit::walk_item(this, item));
  71                 } else {
  72                     visit::walk_item(this, item);
  73                 }
  74             });
  75         }
  76     }
  77
  78     fn visit_fn(&mut self, fk: FnKind<'a>, sp: Span, _: NodeId) {
  79         match fk {
  80             FnKind::Fn(FnCtxt::Foreign, _, sig, _, _) => {
  81                 self.visit_fn_header(&sig.header);
  82                 visit::walk_fn_decl(self, &sig.decl);
  83                 // Don't visit the foreign function body even if it has one, since lowering the
  84                 // body would have no meaning and will have already been caught as a parse error.
  85             }
  86             _ => visit::walk_fn(self, fk, sp),
  87         }
  88     }
  89
  90     fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
  91         self.lctx.with_hir_id_owner(item.id, |lctx| match ctxt {
  92             AssocCtxt::Trait => {
  93                 let hir_item = lctx.lower_trait_item(item);
  94                 let id = hir::TraitItemId { hir_id: hir_item.hir_id };
  95                 lctx.trait_items.insert(id, hir_item);
  96                 lctx.modules.get_mut(&lctx.current_module).unwrap().trait_items.insert(id);
  97             }
  98             AssocCtxt::Impl => {
  99                 let hir_item = lctx.lower_impl_item(item);
 100                 let id = hir::ImplItemId { hir_id: hir_item.hir_id };
 101                 lctx.impl_items.insert(id, hir_item);
 102                 lctx.modules.get_mut(&lctx.current_module).unwrap().impl_items.insert(id);
 103             }
 104         });
 105
 106         visit::walk_assoc_item(self, item, ctxt);
 107     }
 108 }
 109
 110 impl<'hir> LoweringContext<'_, 'hir> {
 111     // Same as the method above, but accepts `hir::GenericParam`s
 112     // instead of `ast::GenericParam`s.
 113     // This should only be used with generics that have already had their
 114     // in-band lifetimes added. In practice, this means that this function is
 115     // only used when lowering a child item of a trait or impl.
 116     fn with_parent_item_lifetime_defs<T>(
 117         &mut self,
 118         parent_hir_id: hir::HirId,
 119         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 120     ) -> T {
 121         let old_len = self.in_scope_lifetimes.len();
 122
 123         let parent_generics = match self.items.get(&parent_hir_id).unwrap().kind {
 124             hir::ItemKind::Impl { ref generics, .. }
 125             | hir::ItemKind::Trait(_, _, ref generics, ..) => &generics.params[..],
 126             _ => &[],
 127         };
 128         let lt_def_names = parent_generics.iter().filter_map(|param| match param.kind {
 129             hir::GenericParamKind::Lifetime { .. } => Some(param.name.normalize_to_macros_2_0()),
 130             _ => None,
 131         });
 132         self.in_scope_lifetimes.extend(lt_def_names);
 133
 134         let res = f(self);
 135
 136         self.in_scope_lifetimes.truncate(old_len);
 137         res
 138     }
 139
 140     // Clears (and restores) the `in_scope_lifetimes` field. Used when
 141     // visiting nested items, which never inherit in-scope lifetimes
 142     // from their surrounding environment.
 143     fn without_in_scope_lifetime_defs<T>(
 144         &mut self,
 145         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 146     ) -> T {
 147         let old_in_scope_lifetimes = std::mem::replace(&mut self.in_scope_lifetimes, vec![]);
 148
 149         // this vector is only used when walking over impl headers,
 150         // input types, and the like, and should not be non-empty in
 151         // between items
 152         assert!(self.lifetimes_to_define.is_empty());
 153
 154         let res = f(self);
 155
 156         assert!(self.in_scope_lifetimes.is_empty());
 157         self.in_scope_lifetimes = old_in_scope_lifetimes;
 158
 159         res
 160     }
 161
 162     pub(super) fn lower_mod(&mut self, m: &Mod) -> hir::Mod<'hir> {
 163         hir::Mod {
 164             inner: m.inner,
 165             item_ids: self
 166                 .arena
 167                 .alloc_from_iter(m.items.iter().flat_map(|x| self.lower_item_id(x))),
 168         }
 169     }
 170
 171     pub(super) fn lower_item_id(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
 172         let node_ids = match i.kind {
 173             ItemKind::Use(ref use_tree) => {
 174                 let mut vec = smallvec![i.id];
 175                 self.lower_item_id_use_tree(use_tree, i.id, &mut vec);
 176                 vec
 177             }
 178             ItemKind::MacroDef(..) => SmallVec::new(),
 179             ItemKind::Fn(..) | ItemKind::Impl { of_trait: None, .. } => smallvec![i.id],
 180             _ => smallvec![i.id],
 181         };
 182
 183         node_ids
 184             .into_iter()
 185             .map(|node_id| hir::ItemId { id: self.allocate_hir_id_counter(node_id) })
 186             .collect()
 187     }
 188
 189     fn lower_item_id_use_tree(
 190         &mut self,
 191         tree: &UseTree,
 192         base_id: NodeId,
 193         vec: &mut SmallVec<[NodeId; 1]>,
 194     ) {
 195         match tree.kind {
 196             UseTreeKind::Nested(ref nested_vec) => {
 197                 for &(ref nested, id) in nested_vec {
 198                     vec.push(id);
 199                     self.lower_item_id_use_tree(nested, id, vec);
 200                 }
 201             }
 202             UseTreeKind::Glob => {}
 203             UseTreeKind::Simple(_, id1, id2) => {
 204                 for (_, &id) in
 205                     self.expect_full_res_from_use(base_id).skip(1).zip([id1, id2].iter())
 206                 {
 207                     vec.push(id);
 208                 }
 209             }
 210         }
 211     }
 212
 213     pub fn lower_item(&mut self, i: &Item) -> Option<hir::Item<'hir>> {
 214         let mut ident = i.ident;
 215         let mut vis = self.lower_visibility(&i.vis, None);
 216         let attrs = self.lower_attrs(&i.attrs);
 217
 218         if let ItemKind::MacroDef(MacroDef { ref body, macro_rules }) = i.kind {
 219             if !macro_rules || self.sess.contains_name(&i.attrs, sym::macro_export) {
 220                 let hir_id = self.lower_node_id(i.id);
 221                 let body = P(self.lower_mac_args(body));
 222                 self.exported_macros.push(hir::MacroDef {
 223                     ident,
 224                     vis,
 225                     attrs,
 226                     hir_id,
 227                     span: i.span,
 228                     ast: MacroDef { body, macro_rules },
 229                 });
 230             } else {
 231                 self.non_exported_macro_attrs.extend(attrs.iter().cloned());
 232             }
 233             return None;
 234         }
 235
 236         let kind = self.lower_item_kind(i.span, i.id, &mut ident, attrs, &mut vis, &i.kind);
 237
 238         Some(hir::Item { hir_id: self.lower_node_id(i.id), ident, attrs, kind, vis, span: i.span })
 239     }
 240
 241     fn lower_item_kind(
 242         &mut self,
 243         span: Span,
 244         id: NodeId,
 245         ident: &mut Ident,
 246         attrs: &'hir [Attribute],
 247         vis: &mut hir::Visibility<'hir>,
 248         i: &ItemKind,
 249     ) -> hir::ItemKind<'hir> {
 250         match *i {
 251             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
 252             ItemKind::Use(ref use_tree) => {
 253                 // Start with an empty prefix.
 254                 let prefix = Path { segments: vec![], span: use_tree.span };
 255
 256                 self.lower_use_tree(use_tree, &prefix, id, vis, ident, attrs)
 257             }
 258             ItemKind::Static(ref t, m, ref e) => {
 259                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 260                 hir::ItemKind::Static(ty, m, body_id)
 261             }
 262             ItemKind::Const(_, ref t, ref e) => {
 263                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 264                 hir::ItemKind::Const(ty, body_id)
 265             }
 266             ItemKind::Fn(_, FnSig { ref decl, header }, ref generics, ref body) => {
 267                 let fn_def_id = self.resolver.local_def_id(id);
 268                 self.with_new_scopes(|this| {
 269                     this.current_item = Some(ident.span);
 270
 271                     // Note: we don't need to change the return type from `T` to
 272                     // `impl Future<Output = T>` here because lower_body
 273                     // only cares about the input argument patterns in the function
 274                     // declaration (decl), not the return types.
 275                     let asyncness = header.asyncness;
 276                     let body_id =
 277                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 278
 279                     let (generics, decl) = this.add_in_band_defs(
 280                         generics,
 281                         fn_def_id,
 282                         AnonymousLifetimeMode::PassThrough,
 283                         |this, idty| {
 284                             let ret_id = asyncness.opt_return_id();
 285                             this.lower_fn_decl(
 286                                 &decl,
 287                                 Some((fn_def_id.to_def_id(), idty)),
 288                                 true,
 289                                 ret_id,
 290                             )
 291                         },
 292                     );
 293                     let sig = hir::FnSig { decl, header: this.lower_fn_header(header) };
 294                     hir::ItemKind::Fn(sig, generics, body_id)
 295                 })
 296             }
 297             ItemKind::Mod(ref m) => hir::ItemKind::Mod(self.lower_mod(m)),
 298             ItemKind::ForeignMod(ref nm) => hir::ItemKind::ForeignMod(self.lower_foreign_mod(nm)),
 299             ItemKind::GlobalAsm(ref ga) => hir::ItemKind::GlobalAsm(self.lower_global_asm(ga)),
 300             ItemKind::TyAlias(_, ref gen, _, Some(ref ty)) => {
 301                 // We lower
 302                 //
 303                 // type Foo = impl Trait
 304                 //
 305                 // to
 306                 //
 307                 // type Foo = Foo1
 308                 // opaque type Foo1: Trait
 309                 let ty = self.lower_ty(
 310                     ty,
 311                     ImplTraitContext::OtherOpaqueTy {
 312                         capturable_lifetimes: &mut FxHashSet::default(),
 313                         origin: hir::OpaqueTyOrigin::Misc,
 314                     },
 315                 );
 316                 let generics = self.lower_generics(gen, ImplTraitContext::disallowed());
 317                 hir::ItemKind::TyAlias(ty, generics)
 318             }
 319             ItemKind::TyAlias(_, ref generics, _, None) => {
 320                 let ty = self.arena.alloc(self.ty(span, hir::TyKind::Err));
 321                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 322                 hir::ItemKind::TyAlias(ty, generics)
 323             }
 324             ItemKind::Enum(ref enum_definition, ref generics) => hir::ItemKind::Enum(
 325                 hir::EnumDef {
 326                     variants: self.arena.alloc_from_iter(
 327                         enum_definition.variants.iter().map(|x| self.lower_variant(x)),
 328                     ),
 329                 },
 330                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 331             ),
 332             ItemKind::Struct(ref struct_def, ref generics) => {
 333                 let struct_def = self.lower_variant_data(struct_def);
 334                 hir::ItemKind::Struct(
 335                     struct_def,
 336                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 337                 )
 338             }
 339             ItemKind::Union(ref vdata, ref generics) => {
 340                 let vdata = self.lower_variant_data(vdata);
 341                 hir::ItemKind::Union(
 342                     vdata,
 343                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 344                 )
 345             }
 346             ItemKind::Impl {
 347                 unsafety,
 348                 polarity,
 349                 defaultness,
 350                 constness,
 351                 generics: ref ast_generics,
 352                 of_trait: ref trait_ref,
 353                 self_ty: ref ty,
 354                 items: ref impl_items,
 355             } => {
 356                 let def_id = self.resolver.local_def_id(id);
 357
 358                 // Lower the "impl header" first. This ordering is important
 359                 // for in-band lifetimes! Consider `'a` here:
 360                 //
 361                 //     impl Foo<'a> for u32 {
 362                 //         fn method(&'a self) { .. }
 363                 //     }
 364                 //
 365                 // Because we start by lowering the `Foo<'a> for u32`
 366                 // part, we will add `'a` to the list of generics on
 367                 // the impl. When we then encounter it later in the
 368                 // method, it will not be considered an in-band
 369                 // lifetime to be added, but rather a reference to a
 370                 // parent lifetime.
 371                 let lowered_trait_impl_id = self.lower_node_id(id);
 372                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 373                     ast_generics,
 374                     def_id,
 375                     AnonymousLifetimeMode::CreateParameter,
 376                     |this, _| {
 377                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 378                             this.lower_trait_ref(trait_ref, ImplTraitContext::disallowed())
 379                         });
 380
 381                         if let Some(ref trait_ref) = trait_ref {
 382                             if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res {
 383                                 this.trait_impls
 384                                     .entry(def_id)
 385                                     .or_default()
 386                                     .push(lowered_trait_impl_id);
 387                             }
 388                         }
 389
 390                         let lowered_ty = this.lower_ty(ty, ImplTraitContext::disallowed());
 391
 392                         (trait_ref, lowered_ty)
 393                     },
 394                 );
 395
 396                 let new_impl_items =
 397                     self.with_in_scope_lifetime_defs(&ast_generics.params, |this| {
 398                         this.arena.alloc_from_iter(
 399                             impl_items.iter().map(|item| this.lower_impl_item_ref(item)),
 400                         )
 401                     });
 402
 403                 // `defaultness.has_value()` is never called for an `impl`, always `true` in order
 404                 // to not cause an assertion failure inside the `lower_defaultness` function.
 405                 let has_val = true;
 406                 let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
 407                 hir::ItemKind::Impl {
 408                     unsafety: self.lower_unsafety(unsafety),
 409                     polarity,
 410                     defaultness,
 411                     defaultness_span,
 412                     constness: self.lower_constness(constness),
 413                     generics,
 414                     of_trait: trait_ref,
 415                     self_ty: lowered_ty,
 416                     items: new_impl_items,
 417                 }
 418             }
 419             ItemKind::Trait(is_auto, unsafety, ref generics, ref bounds, ref items) => {
 420                 let bounds = self.lower_param_bounds(bounds, ImplTraitContext::disallowed());
 421                 let items = self
 422                     .arena
 423                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
 424                 hir::ItemKind::Trait(
 425                     is_auto,
 426                     self.lower_unsafety(unsafety),
 427                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 428                     bounds,
 429                     items,
 430                 )
 431             }
 432             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 433                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 434                 self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 435             ),
 436             ItemKind::MacroDef(..) | ItemKind::MacCall(..) => {
 437                 panic!("`TyMac` should have been expanded by now")
 438             }
 439         }
 440     }
 441
 442     fn lower_const_item(
 443         &mut self,
 444         ty: &Ty,
 445         span: Span,
 446         body: Option<&Expr>,
 447     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 448         let mut capturable_lifetimes;
 449         let itctx = if self.sess.features_untracked().impl_trait_in_bindings {
 450             capturable_lifetimes = FxHashSet::default();
 451             ImplTraitContext::OtherOpaqueTy {
 452                 capturable_lifetimes: &mut capturable_lifetimes,
 453                 origin: hir::OpaqueTyOrigin::Misc,
 454             }
 455         } else {
 456             ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
 457         };
 458         let ty = self.lower_ty(ty, itctx);
 459         (ty, self.lower_const_body(span, body))
 460     }
 461
 462     fn lower_use_tree(
 463         &mut self,
 464         tree: &UseTree,
 465         prefix: &Path,
 466         id: NodeId,
 467         vis: &mut hir::Visibility<'hir>,
 468         ident: &mut Ident,
 469         attrs: &'hir [Attribute],
 470     ) -> hir::ItemKind<'hir> {
 471         debug!("lower_use_tree(tree={:?})", tree);
 472         debug!("lower_use_tree: vis = {:?}", vis);
 473
 474         let path = &tree.prefix;
 475         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 476
 477         match tree.kind {
 478             UseTreeKind::Simple(rename, id1, id2) => {
 479                 *ident = tree.ident();
 480
 481                 // First, apply the prefix to the path.
 482                 let mut path = Path { segments, span: path.span };
 483
 484                 // Correctly resolve `self` imports.
 485                 if path.segments.len() > 1
 486                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 487                 {
 488                     let _ = path.segments.pop();
 489                     if rename.is_none() {
 490                         *ident = path.segments.last().unwrap().ident;
 491                     }
 492                 }
 493
 494                 let mut resolutions = self.expect_full_res_from_use(id);
 495                 // We want to return *something* from this function, so hold onto the first item
 496                 // for later.
 497                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 498
 499                 // Here, we are looping over namespaces, if they exist for the definition
 500                 // being imported. We only handle type and value namespaces because we
 501                 // won't be dealing with macros in the rest of the compiler.
 502                 // Essentially a single `use` which imports two names is desugared into
 503                 // two imports.
 504                 for (res, &new_node_id) in resolutions.zip([id1, id2].iter()) {
 505                     let ident = *ident;
 506                     let mut path = path.clone();
 507                     for seg in &mut path.segments {
 508                         seg.id = self.resolver.next_node_id();
 509                     }
 510                     let span = path.span;
 511
 512                     self.with_hir_id_owner(new_node_id, |this| {
 513                         let new_id = this.lower_node_id(new_node_id);
 514                         let res = this.lower_res(res);
 515                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit, None);
 516                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 517                         let vis = this.rebuild_vis(&vis);
 518
 519                         this.insert_item(hir::Item {
 520                             hir_id: new_id,
 521                             ident,
 522                             attrs,
 523                             kind,
 524                             vis,
 525                             span,
 526                         });
 527                     });
 528                 }
 529
 530                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit, None);
 531                 hir::ItemKind::Use(path, hir::UseKind::Single)
 532             }
 533             UseTreeKind::Glob => {
 534                 let path =
 535                     self.lower_path(id, &Path { segments, span: path.span }, ParamMode::Explicit);
 536                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 537             }
 538             UseTreeKind::Nested(ref trees) => {
 539                 // Nested imports are desugared into simple imports.
 540                 // So, if we start with
 541                 //
 542                 // ```
 543                 // pub(x) use foo::{a, b};
 544                 // ```
 545                 //
 546                 // we will create three items:
 547                 //
 548                 // ```
 549                 // pub(x) use foo::a;
 550                 // pub(x) use foo::b;
 551                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 552                 // ```
 553                 //
 554                 // The first two are produced by recursively invoking
 555                 // `lower_use_tree` (and indeed there may be things
 556                 // like `use foo::{a::{b, c}}` and so forth).  They
 557                 // wind up being directly added to
 558                 // `self.items`. However, the structure of this
 559                 // function also requires us to return one item, and
 560                 // for that we return the `{}` import (called the
 561                 // `ListStem`).
 562
 563                 let prefix = Path { segments, span: prefix.span.to(path.span) };
 564
 565                 // Add all the nested `PathListItem`s to the HIR.
 566                 for &(ref use_tree, id) in trees {
 567                     let new_hir_id = self.lower_node_id(id);
 568
 569                     let mut prefix = prefix.clone();
 570
 571                     // Give the segments new node-ids since they are being cloned.
 572                     for seg in &mut prefix.segments {
 573                         seg.id = self.resolver.next_node_id();
 574                     }
 575
 576                     // Each `use` import is an item and thus are owners of the
 577                     // names in the path. Up to this point the nested import is
 578                     // the current owner, since we want each desugared import to
 579                     // own its own names, we have to adjust the owner before
 580                     // lowering the rest of the import.
 581                     self.with_hir_id_owner(id, |this| {
 582                         let mut vis = this.rebuild_vis(&vis);
 583                         let mut ident = *ident;
 584
 585                         let kind =
 586                             this.lower_use_tree(use_tree, &prefix, id, &mut vis, &mut ident, attrs);
 587
 588                         this.insert_item(hir::Item {
 589                             hir_id: new_hir_id,
 590                             ident,
 591                             attrs,
 592                             kind,
 593                             vis,
 594                             span: use_tree.span,
 595                         });
 596                     });
 597                 }
 598
 599                 // Subtle and a bit hacky: we lower the privacy level
 600                 // of the list stem to "private" most of the time, but
 601                 // not for "restricted" paths. The key thing is that
 602                 // we don't want it to stay as `pub` (with no caveats)
 603                 // because that affects rustdoc and also the lints
 604                 // about `pub` items. But we can't *always* make it
 605                 // private -- particularly not for restricted paths --
 606                 // because it contains node-ids that would then be
 607                 // unused, failing the check that HirIds are "densely
 608                 // assigned".
 609                 match vis.node {
 610                     hir::VisibilityKind::Public
 611                     | hir::VisibilityKind::Crate(_)
 612                     | hir::VisibilityKind::Inherited => {
 613                         *vis = respan(prefix.span.shrink_to_lo(), hir::VisibilityKind::Inherited);
 614                     }
 615                     hir::VisibilityKind::Restricted { .. } => {
 616                         // Do nothing here, as described in the comment on the match.
 617                     }
 618                 }
 619
 620                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 621                 let res = self.lower_res(res);
 622                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit, None);
 623                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 624             }
 625         }
 626     }
 627
 628     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 629     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 630     /// `NodeId`s. (See, e.g., #56128.)
 631     fn rebuild_use_path(&mut self, path: &hir::Path<'hir>) -> &'hir hir::Path<'hir> {
 632         debug!("rebuild_use_path(path = {:?})", path);
 633         let segments =
 634             self.arena.alloc_from_iter(path.segments.iter().map(|seg| hir::PathSegment {
 635                 ident: seg.ident,
 636                 hir_id: seg.hir_id.map(|_| self.next_id()),
 637                 res: seg.res,
 638                 args: None,
 639                 infer_args: seg.infer_args,
 640             }));
 641         self.arena.alloc(hir::Path { span: path.span, res: path.res, segments })
 642     }
 643
 644     fn rebuild_vis(&mut self, vis: &hir::Visibility<'hir>) -> hir::Visibility<'hir> {
 645         let vis_kind = match vis.node {
 646             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 647             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 648             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 649             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 650                 hir::VisibilityKind::Restricted {
 651                     path: self.rebuild_use_path(path),
 652                     hir_id: self.next_id(),
 653                 }
 654             }
 655         };
 656         respan(vis.span, vis_kind)
 657     }
 658
 659     fn lower_foreign_item(&mut self, i: &ForeignItem) -> hir::ForeignItem<'hir> {
 660         let def_id = self.resolver.local_def_id(i.id);
 661         hir::ForeignItem {
 662             hir_id: self.lower_node_id(i.id),
 663             ident: i.ident,
 664             attrs: self.lower_attrs(&i.attrs),
 665             kind: match i.kind {
 666                 ForeignItemKind::Fn(_, ref sig, ref generics, _) => {
 667                     let fdec = &sig.decl;
 668                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 669                         generics,
 670                         def_id,
 671                         AnonymousLifetimeMode::PassThrough,
 672                         |this, _| {
 673                             (
 674                                 // Disallow `impl Trait` in foreign items.
 675                                 this.lower_fn_decl(fdec, None, false, None),
 676                                 this.lower_fn_params_to_names(fdec),
 677                             )
 678                         },
 679                     );
 680
 681                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 682                 }
 683                 ForeignItemKind::Static(ref t, m, _) => {
 684                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 685                     hir::ForeignItemKind::Static(ty, m)
 686                 }
 687                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 688                 ForeignItemKind::MacCall(_) => panic!("macro shouldn't exist here"),
 689             },
 690             vis: self.lower_visibility(&i.vis, None),
 691             span: i.span,
 692         }
 693     }
 694
 695     fn lower_foreign_mod(&mut self, fm: &ForeignMod) -> hir::ForeignMod<'hir> {
 696         hir::ForeignMod {
 697             abi: fm.abi.map_or(abi::Abi::C, |abi| self.lower_abi(abi)),
 698             items: self.arena.alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item(x))),
 699         }
 700     }
 701
 702     fn lower_global_asm(&mut self, ga: &GlobalAsm) -> &'hir hir::GlobalAsm {
 703         self.arena.alloc(hir::GlobalAsm { asm: ga.asm })
 704     }
 705
 706     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 707         hir::Variant {
 708             attrs: self.lower_attrs(&v.attrs),
 709             data: self.lower_variant_data(&v.data),
 710             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 711             id: self.lower_node_id(v.id),
 712             ident: v.ident,
 713             span: v.span,
 714         }
 715     }
 716
 717     fn lower_variant_data(&mut self, vdata: &VariantData) -> hir::VariantData<'hir> {
 718         match *vdata {
 719             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 720                 self.arena
 721                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 722                 recovered,
 723             ),
 724             VariantData::Tuple(ref fields, id) => hir::VariantData::Tuple(
 725                 self.arena
 726                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 727                 self.lower_node_id(id),
 728             ),
 729             VariantData::Unit(id) => hir::VariantData::Unit(self.lower_node_id(id)),
 730         }
 731     }
 732
 733     fn lower_struct_field(&mut self, (index, f): (usize, &StructField)) -> hir::StructField<'hir> {
 734         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 735             let t = self.lower_path_ty(
 736                 &f.ty,
 737                 qself,
 738                 path,
 739                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 740                 ImplTraitContext::disallowed(),
 741             );
 742             self.arena.alloc(t)
 743         } else {
 744             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 745         };
 746         hir::StructField {
 747             span: f.span,
 748             hir_id: self.lower_node_id(f.id),
 749             ident: match f.ident {
 750                 Some(ident) => ident,
 751                 // FIXME(jseyfried): positional field hygiene.
 752                 None => Ident::new(sym::integer(index), f.span),
 753             },
 754             vis: self.lower_visibility(&f.vis, None),
 755             ty,
 756             attrs: self.lower_attrs(&f.attrs),
 757         }
 758     }
 759
 760     fn lower_trait_item(&mut self, i: &AssocItem) -> hir::TraitItem<'hir> {
 761         let trait_item_def_id = self.resolver.local_def_id(i.id);
 762
 763         let (generics, kind) = match i.kind {
 764             AssocItemKind::Const(_, ref ty, ref default) => {
 765                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 766                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 767                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
 768             }
 769             AssocItemKind::Fn(_, ref sig, ref generics, None) => {
 770                 let names = self.lower_fn_params_to_names(&sig.decl);
 771                 let (generics, sig) =
 772                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
 773                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)))
 774             }
 775             AssocItemKind::Fn(_, ref sig, ref generics, Some(ref body)) => {
 776                 let body_id = self.lower_fn_body_block(i.span, &sig.decl, Some(body));
 777                 let (generics, sig) =
 778                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
 779                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)))
 780             }
 781             AssocItemKind::TyAlias(_, ref generics, ref bounds, ref default) => {
 782                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
 783                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 784                 let kind = hir::TraitItemKind::Type(
 785                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 786                     ty,
 787                 );
 788
 789                 (generics, kind)
 790             }
 791             AssocItemKind::MacCall(..) => panic!("macro item shouldn't exist at this point"),
 792         };
 793
 794         hir::TraitItem {
 795             hir_id: self.lower_node_id(i.id),
 796             ident: i.ident,
 797             attrs: self.lower_attrs(&i.attrs),
 798             generics,
 799             kind,
 800             span: i.span,
 801         }
 802     }
 803
 804     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 805         let (kind, has_default) = match &i.kind {
 806             AssocItemKind::Const(_, _, default) => (hir::AssocItemKind::Const, default.is_some()),
 807             AssocItemKind::TyAlias(_, _, _, default) => {
 808                 (hir::AssocItemKind::Type, default.is_some())
 809             }
 810             AssocItemKind::Fn(_, sig, _, default) => {
 811                 (hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }, default.is_some())
 812             }
 813             AssocItemKind::MacCall(..) => unimplemented!(),
 814         };
 815         let id = hir::TraitItemId { hir_id: self.lower_node_id(i.id) };
 816         let defaultness = hir::Defaultness::Default { has_value: has_default };
 817         hir::TraitItemRef { id, ident: i.ident, span: i.span, defaultness, kind }
 818     }
 819
 820     /// Construct `ExprKind::Err` for the given `span`.
 821     crate fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 822         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 823     }
 824
 825     fn lower_impl_item(&mut self, i: &AssocItem) -> hir::ImplItem<'hir> {
 826         let impl_item_def_id = self.resolver.local_def_id(i.id);
 827
 828         let (generics, kind) = match &i.kind {
 829             AssocItemKind::Const(_, ty, expr) => {
 830                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 831                 (
 832                     hir::Generics::empty(),
 833                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 834                 )
 835             }
 836             AssocItemKind::Fn(_, sig, generics, body) => {
 837                 self.current_item = Some(i.span);
 838                 let asyncness = sig.header.asyncness;
 839                 let body_id =
 840                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 841                 let impl_trait_return_allow = !self.is_in_trait_impl;
 842                 let (generics, sig) = self.lower_method_sig(
 843                     generics,
 844                     sig,
 845                     impl_item_def_id,
 846                     impl_trait_return_allow,
 847                     asyncness.opt_return_id(),
 848                 );
 849
 850                 (generics, hir::ImplItemKind::Fn(sig, body_id))
 851             }
 852             AssocItemKind::TyAlias(_, generics, _, ty) => {
 853                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 854                 let kind = match ty {
 855                     None => {
 856                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 857                         hir::ImplItemKind::TyAlias(ty)
 858                     }
 859                     Some(ty) => {
 860                         let ty = self.lower_ty(
 861                             ty,
 862                             ImplTraitContext::OtherOpaqueTy {
 863                                 capturable_lifetimes: &mut FxHashSet::default(),
 864                                 origin: hir::OpaqueTyOrigin::Misc,
 865                             },
 866                         );
 867                         hir::ImplItemKind::TyAlias(ty)
 868                     }
 869                 };
 870                 (generics, kind)
 871             }
 872             AssocItemKind::MacCall(..) => panic!("`TyMac` should have been expanded by now"),
 873         };
 874
 875         // Since `default impl` is not yet implemented, this is always true in impls.
 876         let has_value = true;
 877         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 878         hir::ImplItem {
 879             hir_id: self.lower_node_id(i.id),
 880             ident: i.ident,
 881             attrs: self.lower_attrs(&i.attrs),
 882             generics,
 883             vis: self.lower_visibility(&i.vis, None),
 884             defaultness,
 885             kind,
 886             span: i.span,
 887         }
 888     }
 889
 890     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef<'hir> {
 891         // Since `default impl` is not yet implemented, this is always true in impls.
 892         let has_value = true;
 893         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 894         hir::ImplItemRef {
 895             id: hir::ImplItemId { hir_id: self.lower_node_id(i.id) },
 896             ident: i.ident,
 897             span: i.span,
 898             vis: self.lower_visibility(&i.vis, Some(i.id)),
 899             defaultness,
 900             kind: match &i.kind {
 901                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 902                 AssocItemKind::TyAlias(..) => hir::AssocItemKind::Type,
 903                 AssocItemKind::Fn(_, sig, ..) => {
 904                     hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
 905                 }
 906                 AssocItemKind::MacCall(..) => unimplemented!(),
 907             },
 908         }
 909     }
 910
 911     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 912     /// the address space of that item instead of the item currently being
 913     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 914     /// lower a `Visibility` value although we haven't lowered the owning
 915     /// `ImplItem` in question yet.
 916     fn lower_visibility(
 917         &mut self,
 918         v: &Visibility,
 919         explicit_owner: Option<NodeId>,
 920     ) -> hir::Visibility<'hir> {
 921         let node = match v.node {
 922             VisibilityKind::Public => hir::VisibilityKind::Public,
 923             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 924             VisibilityKind::Restricted { ref path, id } => {
 925                 debug!("lower_visibility: restricted path id = {:?}", id);
 926                 let lowered_id = if let Some(owner) = explicit_owner {
 927                     self.lower_node_id_with_owner(id, owner)
 928                 } else {
 929                     self.lower_node_id(id)
 930                 };
 931                 let res = self.expect_full_res(id);
 932                 let res = self.lower_res(res);
 933                 hir::VisibilityKind::Restricted {
 934                     path: self.lower_path_extra(res, path, ParamMode::Explicit, explicit_owner),
 935                     hir_id: lowered_id,
 936                 }
 937             }
 938             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 939         };
 940         respan(v.span, node)
 941     }
 942
 943     fn lower_defaultness(
 944         &self,
 945         d: Defaultness,
 946         has_value: bool,
 947     ) -> (hir::Defaultness, Option<Span>) {
 948         match d {
 949             Defaultness::Default(sp) => (hir::Defaultness::Default { has_value }, Some(sp)),
 950             Defaultness::Final => {
 951                 assert!(has_value);
 952                 (hir::Defaultness::Final, None)
 953             }
 954         }
 955     }
 956
 957     fn record_body(
 958         &mut self,
 959         params: &'hir [hir::Param<'hir>],
 960         value: hir::Expr<'hir>,
 961     ) -> hir::BodyId {
 962         let body = hir::Body { generator_kind: self.generator_kind, params, value };
 963         let id = body.id();
 964         self.bodies.insert(id, body);
 965         id
 966     }
 967
 968     pub(super) fn lower_body(
 969         &mut self,
 970         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
 971     ) -> hir::BodyId {
 972         let prev_gen_kind = self.generator_kind.take();
 973         let task_context = self.task_context.take();
 974         let (parameters, result) = f(self);
 975         let body_id = self.record_body(parameters, result);
 976         self.task_context = task_context;
 977         self.generator_kind = prev_gen_kind;
 978         body_id
 979     }
 980
 981     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
 982         hir::Param {
 983             attrs: self.lower_attrs(&param.attrs),
 984             hir_id: self.lower_node_id(param.id),
 985             pat: self.lower_pat(&param.pat),
 986             ty_span: param.ty.span,
 987             span: param.span,
 988         }
 989     }
 990
 991     pub(super) fn lower_fn_body(
 992         &mut self,
 993         decl: &FnDecl,
 994         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 995     ) -> hir::BodyId {
 996         self.lower_body(|this| {
 997             (
 998                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
 999                 body(this),
1000             )
1001         })
1002     }
1003
1004     fn lower_fn_body_block(
1005         &mut self,
1006         span: Span,
1007         decl: &FnDecl,
1008         body: Option<&Block>,
1009     ) -> hir::BodyId {
1010         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1011     }
1012
1013     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1014         match block {
1015             Some(block) => self.lower_block_expr(block),
1016             None => self.expr_err(span),
1017         }
1018     }
1019
1020     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1021         self.lower_body(|this| {
1022             (
1023                 &[],
1024                 match expr {
1025                     Some(expr) => this.lower_expr_mut(expr),
1026                     None => this.expr_err(span),
1027                 },
1028             )
1029         })
1030     }
1031
1032     fn lower_maybe_async_body(
1033         &mut self,
1034         span: Span,
1035         decl: &FnDecl,
1036         asyncness: Async,
1037         body: Option<&Block>,
1038     ) -> hir::BodyId {
1039         let closure_id = match asyncness {
1040             Async::Yes { closure_id, .. } => closure_id,
1041             Async::No => return self.lower_fn_body_block(span, decl, body),
1042         };
1043
1044         self.lower_body(|this| {
1045             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1046             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1047
1048             // Async function parameters are lowered into the closure body so that they are
1049             // captured and so that the drop order matches the equivalent non-async functions.
1050             //
1051             // from:
1052             //
1053             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1054             //         <body>
1055             //     }
1056             //
1057             // into:
1058             //
1059             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1060             //       async move {
1061             //         let __arg2 = __arg2;
1062             //         let <pattern> = __arg2;
1063             //         let __arg1 = __arg1;
1064             //         let <pattern> = __arg1;
1065             //         let __arg0 = __arg0;
1066             //         let <pattern> = __arg0;
1067             //         drop-temps { <body> } // see comments later in fn for details
1068             //       }
1069             //     }
1070             //
1071             // If `<pattern>` is a simple ident, then it is lowered to a single
1072             // `let <pattern> = <pattern>;` statement as an optimization.
1073             //
1074             // Note that the body is embedded in `drop-temps`; an
1075             // equivalent desugaring would be `return { <body>
1076             // };`. The key point is that we wish to drop all the
1077             // let-bound variables and temporaries created in the body
1078             // (and its tail expression!) before we drop the
1079             // parameters (c.f. rust-lang/rust#64512).
1080             for (index, parameter) in decl.inputs.iter().enumerate() {
1081                 let parameter = this.lower_param(parameter);
1082                 let span = parameter.pat.span;
1083
1084                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1085                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1086                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1087                     hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, ident, _) => {
1088                         (ident, true)
1089                     }
1090                     _ => {
1091                         // Replace the ident for bindings that aren't simple.
1092                         let name = format!("__arg{}", index);
1093                         let ident = Ident::from_str(&name);
1094
1095                         (ident, false)
1096                     }
1097                 };
1098
1099                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1100
1101                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1102                 // async function.
1103                 //
1104                 // If this is the simple case, this parameter will end up being the same as the
1105                 // original parameter, but with a different pattern id.
1106                 let mut stmt_attrs = AttrVec::new();
1107                 stmt_attrs.extend(parameter.attrs.iter().cloned());
1108                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1109                 let new_parameter = hir::Param {
1110                     attrs: parameter.attrs,
1111                     hir_id: parameter.hir_id,
1112                     pat: new_parameter_pat,
1113                     ty_span: parameter.ty_span,
1114                     span: parameter.span,
1115                 };
1116
1117                 if is_simple_parameter {
1118                     // If this is the simple case, then we only insert one statement that is
1119                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1120                     // `HirId`s are densely assigned.
1121                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1122                     let stmt = this.stmt_let_pat(
1123                         stmt_attrs,
1124                         desugared_span,
1125                         Some(expr),
1126                         parameter.pat,
1127                         hir::LocalSource::AsyncFn,
1128                     );
1129                     statements.push(stmt);
1130                 } else {
1131                     // If this is not the simple case, then we construct two statements:
1132                     //
1133                     // ```
1134                     // let __argN = __argN;
1135                     // let <pat> = __argN;
1136                     // ```
1137                     //
1138                     // The first statement moves the parameter into the closure and thus ensures
1139                     // that the drop order is correct.
1140                     //
1141                     // The second statement creates the bindings that the user wrote.
1142
1143                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1144                     // because the user may have specified a `ref mut` binding in the next
1145                     // statement.
1146                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1147                         desugared_span,
1148                         ident,
1149                         hir::BindingAnnotation::Mutable,
1150                     );
1151                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1152                     let move_stmt = this.stmt_let_pat(
1153                         AttrVec::new(),
1154                         desugared_span,
1155                         Some(move_expr),
1156                         move_pat,
1157                         hir::LocalSource::AsyncFn,
1158                     );
1159
1160                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1161                     // parameter's pattern so that `HirId`s are densely assigned.
1162                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1163                     let pattern_stmt = this.stmt_let_pat(
1164                         stmt_attrs,
1165                         desugared_span,
1166                         Some(pattern_expr),
1167                         parameter.pat,
1168                         hir::LocalSource::AsyncFn,
1169                     );
1170
1171                     statements.push(move_stmt);
1172                     statements.push(pattern_stmt);
1173                 };
1174
1175                 parameters.push(new_parameter);
1176             }
1177
1178             let body_span = body.map_or(span, |b| b.span);
1179             let async_expr = this.make_async_expr(
1180                 CaptureBy::Value,
1181                 closure_id,
1182                 None,
1183                 body_span,
1184                 hir::AsyncGeneratorKind::Fn,
1185                 |this| {
1186                     // Create a block from the user's function body:
1187                     let user_body = this.lower_block_expr_opt(body_span, body);
1188
1189                     // Transform into `drop-temps { <user-body> }`, an expression:
1190                     let desugared_span =
1191                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1192                     let user_body = this.expr_drop_temps(
1193                         desugared_span,
1194                         this.arena.alloc(user_body),
1195                         AttrVec::new(),
1196                     );
1197
1198                     // As noted above, create the final block like
1199                     //
1200                     // ```
1201                     // {
1202                     //   let $param_pattern = $raw_param;
1203                     //   ...
1204                     //   drop-temps { <user-body> }
1205                     // }
1206                     // ```
1207                     let body = this.block_all(
1208                         desugared_span,
1209                         this.arena.alloc_from_iter(statements),
1210                         Some(user_body),
1211                     );
1212
1213                     this.expr_block(body, AttrVec::new())
1214                 },
1215             );
1216
1217             (
1218                 this.arena.alloc_from_iter(parameters),
1219                 this.expr(body_span, async_expr, AttrVec::new()),
1220             )
1221         })
1222     }
1223
1224     fn lower_method_sig(
1225         &mut self,
1226         generics: &Generics,
1227         sig: &FnSig,
1228         fn_def_id: LocalDefId,
1229         impl_trait_return_allow: bool,
1230         is_async: Option<NodeId>,
1231     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1232         let header = self.lower_fn_header(sig.header);
1233         let (generics, decl) = self.add_in_band_defs(
1234             generics,
1235             fn_def_id,
1236             AnonymousLifetimeMode::PassThrough,
1237             |this, idty| {
1238                 this.lower_fn_decl(
1239                     &sig.decl,
1240                     Some((fn_def_id.to_def_id(), idty)),
1241                     impl_trait_return_allow,
1242                     is_async,
1243                 )
1244             },
1245         );
1246         (generics, hir::FnSig { header, decl })
1247     }
1248
1249     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1250         hir::FnHeader {
1251             unsafety: self.lower_unsafety(h.unsafety),
1252             asyncness: self.lower_asyncness(h.asyncness),
1253             constness: self.lower_constness(h.constness),
1254             abi: self.lower_extern(h.ext),
1255         }
1256     }
1257
1258     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1259         abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1260             self.error_on_invalid_abi(abi);
1261             abi::Abi::Rust
1262         })
1263     }
1264
1265     pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
1266         match ext {
1267             Extern::None => abi::Abi::Rust,
1268             Extern::Implicit => abi::Abi::C,
1269             Extern::Explicit(abi) => self.lower_abi(abi),
1270         }
1271     }
1272
1273     fn error_on_invalid_abi(&self, abi: StrLit) {
1274         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1275             .span_label(abi.span, "invalid ABI")
1276             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1277             .emit();
1278     }
1279
1280     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1281         match a {
1282             Async::Yes { .. } => hir::IsAsync::Async,
1283             Async::No => hir::IsAsync::NotAsync,
1284         }
1285     }
1286
1287     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1288         match c {
1289             Const::Yes(_) => hir::Constness::Const,
1290             Const::No => hir::Constness::NotConst,
1291         }
1292     }
1293
1294     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1295         match u {
1296             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1297             Unsafe::No => hir::Unsafety::Normal,
1298         }
1299     }
1300
1301     pub(super) fn lower_generics_mut(
1302         &mut self,
1303         generics: &Generics,
1304         itctx: ImplTraitContext<'_, 'hir>,
1305     ) -> GenericsCtor<'hir> {
1306         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1307         // FIXME: this could probably be done with less rightward drift. It also looks like two
1308         // control paths where `report_error` is called are the only paths that advance to after the
1309         // match statement, so the error reporting could probably just be moved there.
1310         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1311         for pred in &generics.where_clause.predicates {
1312             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1313                 'next_bound: for bound in &bound_pred.bounds {
1314                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1315                         let report_error = |this: &mut Self| {
1316                             this.diagnostic().span_err(
1317                                 bound_pred.bounded_ty.span,
1318                                 "`?Trait` bounds are only permitted at the \
1319                                  point where a type parameter is declared",
1320                             );
1321                         };
1322                         // Check if the where clause type is a plain type parameter.
1323                         match bound_pred.bounded_ty.kind {
1324                             TyKind::Path(None, ref path)
1325                                 if path.segments.len() == 1
1326                                     && bound_pred.bound_generic_params.is_empty() =>
1327                             {
1328                                 if let Some(Res::Def(DefKind::TyParam, def_id)) = self
1329                                     .resolver
1330                                     .get_partial_res(bound_pred.bounded_ty.id)
1331                                     .map(|d| d.base_res())
1332                                 {
1333                                     if let Some(def_id) = def_id.as_local() {
1334                                         for param in &generics.params {
1335                                             if let GenericParamKind::Type { .. } = param.kind {
1336                                                 if def_id == self.resolver.local_def_id(param.id) {
1337                                                     add_bounds
1338                                                         .entry(param.id)
1339                                                         .or_default()
1340                                                         .push(bound.clone());
1341                                                     continue 'next_bound;
1342                                                 }
1343                                             }
1344                                         }
1345                                     }
1346                                 }
1347                                 report_error(self)
1348                             }
1349                             _ => report_error(self),
1350                         }
1351                     }
1352                 }
1353             }
1354         }
1355
1356         GenericsCtor {
1357             params: self.lower_generic_params_mut(&generics.params, &add_bounds, itctx).collect(),
1358             where_clause: self.lower_where_clause(&generics.where_clause),
1359             span: generics.span,
1360         }
1361     }
1362
1363     pub(super) fn lower_generics(
1364         &mut self,
1365         generics: &Generics,
1366         itctx: ImplTraitContext<'_, 'hir>,
1367     ) -> hir::Generics<'hir> {
1368         let generics_ctor = self.lower_generics_mut(generics, itctx);
1369         generics_ctor.into_generics(self.arena)
1370     }
1371
1372     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1373         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1374             hir::WhereClause {
1375                 predicates: this.arena.alloc_from_iter(
1376                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1377                 ),
1378                 span: wc.span,
1379             }
1380         })
1381     }
1382
1383     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1384         match *pred {
1385             WherePredicate::BoundPredicate(WhereBoundPredicate {
1386                 ref bound_generic_params,
1387                 ref bounded_ty,
1388                 ref bounds,
1389                 span,
1390             }) => {
1391                 self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1392                     hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1393                         bound_generic_params: this.lower_generic_params(
1394                             bound_generic_params,
1395                             &NodeMap::default(),
1396                             ImplTraitContext::disallowed(),
1397                         ),
1398                         bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1399                         bounds: this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| {
1400                             match *bound {
1401                                 // Ignore `?Trait` bounds.
1402                                 // They were copied into type parameters already.
1403                                 GenericBound::Trait(_, TraitBoundModifier::Maybe) => None,
1404                                 _ => Some(
1405                                     this.lower_param_bound(bound, ImplTraitContext::disallowed()),
1406                                 ),
1407                             }
1408                         })),
1409                         span,
1410                     })
1411                 })
1412             }
1413             WherePredicate::RegionPredicate(WhereRegionPredicate {
1414                 ref lifetime,
1415                 ref bounds,
1416                 span,
1417             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1418                 span,
1419                 lifetime: self.lower_lifetime(lifetime),
1420                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1421             }),
1422             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1423                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1424                     hir_id: self.lower_node_id(id),
1425                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1426                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1427                     span,
1428                 })
1429             }
1430         }
1431     }
1432 }
1433
1434 /// Helper struct for delayed construction of Generics.
1435 pub(super) struct GenericsCtor<'hir> {
1436     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1437     where_clause: hir::WhereClause<'hir>,
1438     span: Span,
1439 }
1440
1441 impl<'hir> GenericsCtor<'hir> {
1442     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1443         hir::Generics {
1444             params: arena.alloc_from_iter(self.params),
1445             where_clause: self.where_clause,
1446             span: self.span,
1447         }
1448     }
1449 }