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_errors::struct_span_err;
   7 use rustc_hir as hir;
   8 use rustc_hir::def::{DefKind, Res};
   9 use rustc_hir::def_id::DefId;
  10 use rustc_span::source_map::{respan, DesugaringKind};
  11 use rustc_span::symbol::{kw, sym};
  12 use rustc_span::Span;
  13 use rustc_target::spec::abi;
  14 use syntax::ast::*;
  15 use syntax::attr;
  16 use syntax::node_id::NodeMap;
  17 use syntax::visit::{self, AssocCtxt, Visitor};
  18
  19 use log::debug;
  20 use smallvec::{smallvec, SmallVec};
  21 use std::collections::BTreeSet;
  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 { constness, ref of_trait, .. } = item.kind {
  70                     if let Const::Yes(span) = constness {
  71                         this.lctx
  72                             .diagnostic()
  73                             .struct_span_err(item.span, "const trait impls are not yet implemented")
  74                             .span_label(span, "const because of this")
  75                             .emit();
  76                     }
  77
  78                     this.with_trait_impl_ref(of_trait, |this| visit::walk_item(this, item));
  79                 } else {
  80                     visit::walk_item(this, item);
  81                 }
  82             });
  83         }
  84     }
  85
  86     fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
  87         self.lctx.with_hir_id_owner(item.id, |lctx| match ctxt {
  88             AssocCtxt::Trait => {
  89                 let hir_item = lctx.lower_trait_item(item);
  90                 let id = hir::TraitItemId { hir_id: hir_item.hir_id };
  91                 lctx.trait_items.insert(id, hir_item);
  92                 lctx.modules.get_mut(&lctx.current_module).unwrap().trait_items.insert(id);
  93             }
  94             AssocCtxt::Impl => {
  95                 let hir_item = lctx.lower_impl_item(item);
  96                 let id = hir::ImplItemId { hir_id: hir_item.hir_id };
  97                 lctx.impl_items.insert(id, hir_item);
  98                 lctx.modules.get_mut(&lctx.current_module).unwrap().impl_items.insert(id);
  99             }
 100         });
 101
 102         visit::walk_assoc_item(self, item, ctxt);
 103     }
 104 }
 105
 106 impl<'hir> LoweringContext<'_, 'hir> {
 107     // Same as the method above, but accepts `hir::GenericParam`s
 108     // instead of `ast::GenericParam`s.
 109     // This should only be used with generics that have already had their
 110     // in-band lifetimes added. In practice, this means that this function is
 111     // only used when lowering a child item of a trait or impl.
 112     fn with_parent_item_lifetime_defs<T>(
 113         &mut self,
 114         parent_hir_id: hir::HirId,
 115         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 116     ) -> T {
 117         let old_len = self.in_scope_lifetimes.len();
 118
 119         let parent_generics = match self.items.get(&parent_hir_id).unwrap().kind {
 120             hir::ItemKind::Impl { ref generics, .. }
 121             | hir::ItemKind::Trait(_, _, ref generics, ..) => &generics.params[..],
 122             _ => &[],
 123         };
 124         let lt_def_names = parent_generics.iter().filter_map(|param| match param.kind {
 125             hir::GenericParamKind::Lifetime { .. } => Some(param.name.modern()),
 126             _ => None,
 127         });
 128         self.in_scope_lifetimes.extend(lt_def_names);
 129
 130         let res = f(self);
 131
 132         self.in_scope_lifetimes.truncate(old_len);
 133         res
 134     }
 135
 136     // Clears (and restores) the `in_scope_lifetimes` field. Used when
 137     // visiting nested items, which never inherit in-scope lifetimes
 138     // from their surrounding environment.
 139     fn without_in_scope_lifetime_defs<T>(
 140         &mut self,
 141         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 142     ) -> T {
 143         let old_in_scope_lifetimes = std::mem::replace(&mut self.in_scope_lifetimes, vec![]);
 144
 145         // this vector is only used when walking over impl headers,
 146         // input types, and the like, and should not be non-empty in
 147         // between items
 148         assert!(self.lifetimes_to_define.is_empty());
 149
 150         let res = f(self);
 151
 152         assert!(self.in_scope_lifetimes.is_empty());
 153         self.in_scope_lifetimes = old_in_scope_lifetimes;
 154
 155         res
 156     }
 157
 158     pub(super) fn lower_mod(&mut self, m: &Mod) -> hir::Mod<'hir> {
 159         hir::Mod {
 160             inner: m.inner,
 161             item_ids: self
 162                 .arena
 163                 .alloc_from_iter(m.items.iter().flat_map(|x| self.lower_item_id(x))),
 164         }
 165     }
 166
 167     pub(super) fn lower_item_id(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
 168         let node_ids = match i.kind {
 169             ItemKind::Use(ref use_tree) => {
 170                 let mut vec = smallvec![i.id];
 171                 self.lower_item_id_use_tree(use_tree, i.id, &mut vec);
 172                 vec
 173             }
 174             ItemKind::MacroDef(..) => SmallVec::new(),
 175             ItemKind::Fn(..) | ItemKind::Impl { of_trait: None, .. } => smallvec![i.id],
 176             ItemKind::Static(ref ty, ..) => {
 177                 let mut ids = smallvec![i.id];
 178                 if self.sess.features_untracked().impl_trait_in_bindings {
 179                     let mut visitor = ImplTraitTypeIdVisitor { ids: &mut ids };
 180                     visitor.visit_ty(ty);
 181                 }
 182                 ids
 183             }
 184             ItemKind::Const(ref ty, ..) => {
 185                 let mut ids = smallvec![i.id];
 186                 if self.sess.features_untracked().impl_trait_in_bindings {
 187                     let mut visitor = ImplTraitTypeIdVisitor { ids: &mut ids };
 188                     visitor.visit_ty(ty);
 189                 }
 190                 ids
 191             }
 192             _ => smallvec![i.id],
 193         };
 194
 195         node_ids
 196             .into_iter()
 197             .map(|node_id| hir::ItemId { id: self.allocate_hir_id_counter(node_id) })
 198             .collect()
 199     }
 200
 201     fn lower_item_id_use_tree(
 202         &mut self,
 203         tree: &UseTree,
 204         base_id: NodeId,
 205         vec: &mut SmallVec<[NodeId; 1]>,
 206     ) {
 207         match tree.kind {
 208             UseTreeKind::Nested(ref nested_vec) => {
 209                 for &(ref nested, id) in nested_vec {
 210                     vec.push(id);
 211                     self.lower_item_id_use_tree(nested, id, vec);
 212                 }
 213             }
 214             UseTreeKind::Glob => {}
 215             UseTreeKind::Simple(_, id1, id2) => {
 216                 for (_, &id) in
 217                     self.expect_full_res_from_use(base_id).skip(1).zip([id1, id2].iter())
 218                 {
 219                     vec.push(id);
 220                 }
 221             }
 222         }
 223     }
 224
 225     pub fn lower_item(&mut self, i: &Item) -> Option<hir::Item<'hir>> {
 226         let mut ident = i.ident;
 227         let mut vis = self.lower_visibility(&i.vis, None);
 228         let attrs = self.lower_attrs(&i.attrs);
 229
 230         if let ItemKind::MacroDef(ref def) = i.kind {
 231             if !def.legacy || attr::contains_name(&i.attrs, sym::macro_export) {
 232                 let body = self.lower_token_stream(def.body.inner_tokens());
 233                 let hir_id = self.lower_node_id(i.id);
 234                 self.exported_macros.push(hir::MacroDef {
 235                     name: ident.name,
 236                     vis,
 237                     attrs,
 238                     hir_id,
 239                     span: i.span,
 240                     body,
 241                     legacy: def.legacy,
 242                 });
 243             } else {
 244                 self.non_exported_macro_attrs.extend(attrs.iter().cloned());
 245             }
 246             return None;
 247         }
 248
 249         let kind = self.lower_item_kind(i.span, i.id, &mut ident, attrs, &mut vis, &i.kind);
 250
 251         Some(hir::Item { hir_id: self.lower_node_id(i.id), ident, attrs, kind, vis, span: i.span })
 252     }
 253
 254     fn lower_item_kind(
 255         &mut self,
 256         span: Span,
 257         id: NodeId,
 258         ident: &mut Ident,
 259         attrs: &'hir [Attribute],
 260         vis: &mut hir::Visibility<'hir>,
 261         i: &ItemKind,
 262     ) -> hir::ItemKind<'hir> {
 263         match *i {
 264             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
 265             ItemKind::Use(ref use_tree) => {
 266                 // Start with an empty prefix.
 267                 let prefix = Path { segments: vec![], span: use_tree.span };
 268
 269                 self.lower_use_tree(use_tree, &prefix, id, vis, ident, attrs)
 270             }
 271             ItemKind::Static(ref t, m, ref e) => {
 272                 let ty = self.lower_ty(
 273                     t,
 274                     if self.sess.features_untracked().impl_trait_in_bindings {
 275                         ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc)
 276                     } else {
 277                         ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
 278                     },
 279                 );
 280                 hir::ItemKind::Static(ty, m, self.lower_const_body(span, Some(e)))
 281             }
 282             ItemKind::Const(ref t, ref e) => {
 283                 let ty = self.lower_ty(
 284                     t,
 285                     if self.sess.features_untracked().impl_trait_in_bindings {
 286                         ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc)
 287                     } else {
 288                         ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
 289                     },
 290                 );
 291                 hir::ItemKind::Const(ty, self.lower_const_body(span, Some(e)))
 292             }
 293             ItemKind::Fn(FnSig { ref decl, header }, ref generics, ref body) => {
 294                 let fn_def_id = self.resolver.definitions().local_def_id(id);
 295                 self.with_new_scopes(|this| {
 296                     this.current_item = Some(ident.span);
 297
 298                     // Note: we don't need to change the return type from `T` to
 299                     // `impl Future<Output = T>` here because lower_body
 300                     // only cares about the input argument patterns in the function
 301                     // declaration (decl), not the return types.
 302                     let asyncness = header.asyncness;
 303                     let body_id =
 304                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 305
 306                     let (generics, decl) = this.add_in_band_defs(
 307                         generics,
 308                         fn_def_id,
 309                         AnonymousLifetimeMode::PassThrough,
 310                         |this, idty| {
 311                             let ret_id = asyncness.opt_return_id();
 312                             this.lower_fn_decl(&decl, Some((fn_def_id, idty)), true, ret_id)
 313                         },
 314                     );
 315                     let sig = hir::FnSig { decl, header: this.lower_fn_header(header) };
 316                     hir::ItemKind::Fn(sig, generics, body_id)
 317                 })
 318             }
 319             ItemKind::Mod(ref m) => hir::ItemKind::Mod(self.lower_mod(m)),
 320             ItemKind::ForeignMod(ref nm) => hir::ItemKind::ForeignMod(self.lower_foreign_mod(nm)),
 321             ItemKind::GlobalAsm(ref ga) => hir::ItemKind::GlobalAsm(self.lower_global_asm(ga)),
 322             ItemKind::TyAlias(ref ty, ref generics) => match ty.kind.opaque_top_hack() {
 323                 None => {
 324                     let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 325                     let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 326                     hir::ItemKind::TyAlias(ty, generics)
 327                 }
 328                 Some(bounds) => {
 329                     let ty = hir::OpaqueTy {
 330                         generics: self.lower_generics(
 331                             generics,
 332                             ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc),
 333                         ),
 334                         bounds: self.lower_param_bounds(
 335                             bounds,
 336                             ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc),
 337                         ),
 338                         impl_trait_fn: None,
 339                         origin: hir::OpaqueTyOrigin::TypeAlias,
 340                     };
 341                     hir::ItemKind::OpaqueTy(ty)
 342                 }
 343             },
 344             ItemKind::Enum(ref enum_definition, ref generics) => hir::ItemKind::Enum(
 345                 hir::EnumDef {
 346                     variants: self.arena.alloc_from_iter(
 347                         enum_definition.variants.iter().map(|x| self.lower_variant(x)),
 348                     ),
 349                 },
 350                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 351             ),
 352             ItemKind::Struct(ref struct_def, ref generics) => {
 353                 let struct_def = self.lower_variant_data(struct_def);
 354                 hir::ItemKind::Struct(
 355                     struct_def,
 356                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 357                 )
 358             }
 359             ItemKind::Union(ref vdata, ref generics) => {
 360                 let vdata = self.lower_variant_data(vdata);
 361                 hir::ItemKind::Union(
 362                     vdata,
 363                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 364                 )
 365             }
 366             ItemKind::Impl {
 367                 unsafety,
 368                 polarity,
 369                 defaultness,
 370                 constness,
 371                 generics: ref ast_generics,
 372                 of_trait: ref trait_ref,
 373                 self_ty: ref ty,
 374                 items: ref impl_items,
 375             } => {
 376                 let def_id = self.resolver.definitions().local_def_id(id);
 377
 378                 // Lower the "impl header" first. This ordering is important
 379                 // for in-band lifetimes! Consider `'a` here:
 380                 //
 381                 //     impl Foo<'a> for u32 {
 382                 //         fn method(&'a self) { .. }
 383                 //     }
 384                 //
 385                 // Because we start by lowering the `Foo<'a> for u32`
 386                 // part, we will add `'a` to the list of generics on
 387                 // the impl. When we then encounter it later in the
 388                 // method, it will not be considered an in-band
 389                 // lifetime to be added, but rather a reference to a
 390                 // parent lifetime.
 391                 let lowered_trait_impl_id = self.lower_node_id(id);
 392                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 393                     ast_generics,
 394                     def_id,
 395                     AnonymousLifetimeMode::CreateParameter,
 396                     |this, _| {
 397                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 398                             this.lower_trait_ref(trait_ref, ImplTraitContext::disallowed())
 399                         });
 400
 401                         if let Some(ref trait_ref) = trait_ref {
 402                             if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res {
 403                                 this.trait_impls
 404                                     .entry(def_id)
 405                                     .or_default()
 406                                     .push(lowered_trait_impl_id);
 407                             }
 408                         }
 409
 410                         let lowered_ty = this.lower_ty(ty, ImplTraitContext::disallowed());
 411
 412                         (trait_ref, lowered_ty)
 413                     },
 414                 );
 415
 416                 let new_impl_items =
 417                     self.with_in_scope_lifetime_defs(&ast_generics.params, |this| {
 418                         this.arena.alloc_from_iter(
 419                             impl_items.iter().map(|item| this.lower_impl_item_ref(item)),
 420                         )
 421                     });
 422
 423                 hir::ItemKind::Impl {
 424                     unsafety: self.lower_unsafety(unsafety),
 425                     polarity,
 426                     defaultness: self.lower_defaultness(defaultness, true /* [1] */),
 427                     constness: self.lower_constness(constness),
 428                     generics,
 429                     of_trait: trait_ref,
 430                     self_ty: lowered_ty,
 431                     items: new_impl_items,
 432                 }
 433             }
 434             ItemKind::Trait(is_auto, unsafety, ref generics, ref bounds, ref items) => {
 435                 let bounds = self.lower_param_bounds(bounds, ImplTraitContext::disallowed());
 436                 let items = self
 437                     .arena
 438                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
 439                 hir::ItemKind::Trait(
 440                     is_auto,
 441                     self.lower_unsafety(unsafety),
 442                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 443                     bounds,
 444                     items,
 445                 )
 446             }
 447             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 448                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 449                 self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 450             ),
 451             ItemKind::MacroDef(..) | ItemKind::Mac(..) => {
 452                 bug!("`TyMac` should have been expanded by now")
 453             }
 454         }
 455
 456         // [1] `defaultness.has_value()` is never called for an `impl`, always `true` in order to
 457         //     not cause an assertion failure inside the `lower_defaultness` function.
 458     }
 459
 460     fn lower_use_tree(
 461         &mut self,
 462         tree: &UseTree,
 463         prefix: &Path,
 464         id: NodeId,
 465         vis: &mut hir::Visibility<'hir>,
 466         ident: &mut Ident,
 467         attrs: &'hir [Attribute],
 468     ) -> hir::ItemKind<'hir> {
 469         debug!("lower_use_tree(tree={:?})", tree);
 470         debug!("lower_use_tree: vis = {:?}", vis);
 471
 472         let path = &tree.prefix;
 473         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 474
 475         match tree.kind {
 476             UseTreeKind::Simple(rename, id1, id2) => {
 477                 *ident = tree.ident();
 478
 479                 // First, apply the prefix to the path.
 480                 let mut path = Path { segments, span: path.span };
 481
 482                 // Correctly resolve `self` imports.
 483                 if path.segments.len() > 1
 484                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 485                 {
 486                     let _ = path.segments.pop();
 487                     if rename.is_none() {
 488                         *ident = path.segments.last().unwrap().ident;
 489                     }
 490                 }
 491
 492                 let mut resolutions = self.expect_full_res_from_use(id);
 493                 // We want to return *something* from this function, so hold onto the first item
 494                 // for later.
 495                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 496
 497                 // Here, we are looping over namespaces, if they exist for the definition
 498                 // being imported. We only handle type and value namespaces because we
 499                 // won't be dealing with macros in the rest of the compiler.
 500                 // Essentially a single `use` which imports two names is desugared into
 501                 // two imports.
 502                 for (res, &new_node_id) in resolutions.zip([id1, id2].iter()) {
 503                     let ident = *ident;
 504                     let mut path = path.clone();
 505                     for seg in &mut path.segments {
 506                         seg.id = self.resolver.next_node_id();
 507                     }
 508                     let span = path.span;
 509
 510                     self.with_hir_id_owner(new_node_id, |this| {
 511                         let new_id = this.lower_node_id(new_node_id);
 512                         let res = this.lower_res(res);
 513                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit, None);
 514                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 515                         let vis = this.rebuild_vis(&vis);
 516
 517                         this.insert_item(hir::Item {
 518                             hir_id: new_id,
 519                             ident,
 520                             attrs,
 521                             kind,
 522                             vis,
 523                             span,
 524                         });
 525                     });
 526                 }
 527
 528                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit, None);
 529                 hir::ItemKind::Use(path, hir::UseKind::Single)
 530             }
 531             UseTreeKind::Glob => {
 532                 let path =
 533                     self.lower_path(id, &Path { segments, span: path.span }, ParamMode::Explicit);
 534                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 535             }
 536             UseTreeKind::Nested(ref trees) => {
 537                 // Nested imports are desugared into simple imports.
 538                 // So, if we start with
 539                 //
 540                 // ```
 541                 // pub(x) use foo::{a, b};
 542                 // ```
 543                 //
 544                 // we will create three items:
 545                 //
 546                 // ```
 547                 // pub(x) use foo::a;
 548                 // pub(x) use foo::b;
 549                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 550                 // ```
 551                 //
 552                 // The first two are produced by recursively invoking
 553                 // `lower_use_tree` (and indeed there may be things
 554                 // like `use foo::{a::{b, c}}` and so forth).  They
 555                 // wind up being directly added to
 556                 // `self.items`. However, the structure of this
 557                 // function also requires us to return one item, and
 558                 // for that we return the `{}` import (called the
 559                 // `ListStem`).
 560
 561                 let prefix = Path { segments, span: prefix.span.to(path.span) };
 562
 563                 // Add all the nested `PathListItem`s to the HIR.
 564                 for &(ref use_tree, id) in trees {
 565                     let new_hir_id = self.lower_node_id(id);
 566
 567                     let mut prefix = prefix.clone();
 568
 569                     // Give the segments new node-ids since they are being cloned.
 570                     for seg in &mut prefix.segments {
 571                         seg.id = self.resolver.next_node_id();
 572                     }
 573
 574                     // Each `use` import is an item and thus are owners of the
 575                     // names in the path. Up to this point the nested import is
 576                     // the current owner, since we want each desugared import to
 577                     // own its own names, we have to adjust the owner before
 578                     // lowering the rest of the import.
 579                     self.with_hir_id_owner(id, |this| {
 580                         let mut vis = this.rebuild_vis(&vis);
 581                         let mut ident = *ident;
 582
 583                         let kind =
 584                             this.lower_use_tree(use_tree, &prefix, id, &mut vis, &mut ident, attrs);
 585
 586                         this.insert_item(hir::Item {
 587                             hir_id: new_hir_id,
 588                             ident,
 589                             attrs,
 590                             kind,
 591                             vis,
 592                             span: use_tree.span,
 593                         });
 594                     });
 595                 }
 596
 597                 // Subtle and a bit hacky: we lower the privacy level
 598                 // of the list stem to "private" most of the time, but
 599                 // not for "restricted" paths. The key thing is that
 600                 // we don't want it to stay as `pub` (with no caveats)
 601                 // because that affects rustdoc and also the lints
 602                 // about `pub` items. But we can't *always* make it
 603                 // private -- particularly not for restricted paths --
 604                 // because it contains node-ids that would then be
 605                 // unused, failing the check that HirIds are "densely
 606                 // assigned".
 607                 match vis.node {
 608                     hir::VisibilityKind::Public
 609                     | hir::VisibilityKind::Crate(_)
 610                     | hir::VisibilityKind::Inherited => {
 611                         *vis = respan(prefix.span.shrink_to_lo(), hir::VisibilityKind::Inherited);
 612                     }
 613                     hir::VisibilityKind::Restricted { .. } => {
 614                         // Do nothing here, as described in the comment on the match.
 615                     }
 616                 }
 617
 618                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 619                 let res = self.lower_res(res);
 620                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit, None);
 621                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 622             }
 623         }
 624     }
 625
 626     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 627     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 628     /// `NodeId`s. (See, e.g., #56128.)
 629     fn rebuild_use_path(&mut self, path: &hir::Path<'hir>) -> &'hir hir::Path<'hir> {
 630         debug!("rebuild_use_path(path = {:?})", path);
 631         let segments =
 632             self.arena.alloc_from_iter(path.segments.iter().map(|seg| hir::PathSegment {
 633                 ident: seg.ident,
 634                 hir_id: seg.hir_id.map(|_| self.next_id()),
 635                 res: seg.res,
 636                 args: None,
 637                 infer_args: seg.infer_args,
 638             }));
 639         self.arena.alloc(hir::Path { span: path.span, res: path.res, segments })
 640     }
 641
 642     fn rebuild_vis(&mut self, vis: &hir::Visibility<'hir>) -> hir::Visibility<'hir> {
 643         let vis_kind = match vis.node {
 644             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 645             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 646             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 647             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 648                 hir::VisibilityKind::Restricted {
 649                     path: self.rebuild_use_path(path),
 650                     hir_id: self.next_id(),
 651                 }
 652             }
 653         };
 654         respan(vis.span, vis_kind)
 655     }
 656
 657     fn lower_foreign_item(&mut self, i: &ForeignItem) -> hir::ForeignItem<'hir> {
 658         let def_id = self.resolver.definitions().local_def_id(i.id);
 659         hir::ForeignItem {
 660             hir_id: self.lower_node_id(i.id),
 661             ident: i.ident,
 662             attrs: self.lower_attrs(&i.attrs),
 663             kind: match i.kind {
 664                 ForeignItemKind::Fn(ref sig, ref generics, _) => {
 665                     let fdec = &sig.decl;
 666                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 667                         generics,
 668                         def_id,
 669                         AnonymousLifetimeMode::PassThrough,
 670                         |this, _| {
 671                             (
 672                                 // Disallow `impl Trait` in foreign items.
 673                                 this.lower_fn_decl(fdec, None, false, None),
 674                                 this.lower_fn_params_to_names(fdec),
 675                             )
 676                         },
 677                     );
 678
 679                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 680                 }
 681                 ForeignItemKind::Static(ref t, m) => {
 682                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 683                     hir::ForeignItemKind::Static(ty, m)
 684                 }
 685                 ForeignItemKind::Ty => hir::ForeignItemKind::Type,
 686                 ForeignItemKind::Macro(_) => panic!("macro shouldn't exist here"),
 687             },
 688             vis: self.lower_visibility(&i.vis, None),
 689             span: i.span,
 690         }
 691     }
 692
 693     fn lower_foreign_mod(&mut self, fm: &ForeignMod) -> hir::ForeignMod<'hir> {
 694         hir::ForeignMod {
 695             abi: fm.abi.map_or(abi::Abi::C, |abi| self.lower_abi(abi)),
 696             items: self.arena.alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item(x))),
 697         }
 698     }
 699
 700     fn lower_global_asm(&mut self, ga: &GlobalAsm) -> &'hir hir::GlobalAsm {
 701         self.arena.alloc(hir::GlobalAsm { asm: ga.asm })
 702     }
 703
 704     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 705         hir::Variant {
 706             attrs: self.lower_attrs(&v.attrs),
 707             data: self.lower_variant_data(&v.data),
 708             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 709             id: self.lower_node_id(v.id),
 710             ident: v.ident,
 711             span: v.span,
 712         }
 713     }
 714
 715     fn lower_variant_data(&mut self, vdata: &VariantData) -> hir::VariantData<'hir> {
 716         match *vdata {
 717             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 718                 self.arena
 719                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 720                 recovered,
 721             ),
 722             VariantData::Tuple(ref fields, id) => hir::VariantData::Tuple(
 723                 self.arena
 724                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 725                 self.lower_node_id(id),
 726             ),
 727             VariantData::Unit(id) => hir::VariantData::Unit(self.lower_node_id(id)),
 728         }
 729     }
 730
 731     fn lower_struct_field(&mut self, (index, f): (usize, &StructField)) -> hir::StructField<'hir> {
 732         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 733             let t = self.lower_path_ty(
 734                 &f.ty,
 735                 qself,
 736                 path,
 737                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 738                 ImplTraitContext::disallowed(),
 739             );
 740             self.arena.alloc(t)
 741         } else {
 742             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 743         };
 744         hir::StructField {
 745             span: f.span,
 746             hir_id: self.lower_node_id(f.id),
 747             ident: match f.ident {
 748                 Some(ident) => ident,
 749                 // FIXME(jseyfried): positional field hygiene.
 750                 None => Ident::new(sym::integer(index), f.span),
 751             },
 752             vis: self.lower_visibility(&f.vis, None),
 753             ty,
 754             attrs: self.lower_attrs(&f.attrs),
 755         }
 756     }
 757
 758     fn lower_trait_item(&mut self, i: &AssocItem) -> hir::TraitItem<'hir> {
 759         let trait_item_def_id = self.resolver.definitions().local_def_id(i.id);
 760
 761         let (generics, kind) = match i.kind {
 762             AssocItemKind::Const(ref ty, ref default) => {
 763                 let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
 764                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 765                 (
 766                     generics,
 767                     hir::TraitItemKind::Const(
 768                         ty,
 769                         default.as_ref().map(|x| self.lower_const_body(i.span, Some(x))),
 770                     ),
 771                 )
 772             }
 773             AssocItemKind::Fn(ref sig, None) => {
 774                 let names = self.lower_fn_params_to_names(&sig.decl);
 775                 let (generics, sig) =
 776                     self.lower_method_sig(&i.generics, sig, trait_item_def_id, false, None);
 777                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Required(names)))
 778             }
 779             AssocItemKind::Fn(ref sig, Some(ref body)) => {
 780                 let body_id = self.lower_fn_body_block(i.span, &sig.decl, Some(body));
 781                 let (generics, sig) =
 782                     self.lower_method_sig(&i.generics, sig, trait_item_def_id, false, None);
 783                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Provided(body_id)))
 784             }
 785             AssocItemKind::TyAlias(ref bounds, ref default) => {
 786                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
 787                 let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
 788                 let kind = hir::TraitItemKind::Type(
 789                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 790                     ty,
 791                 );
 792
 793                 (generics, kind)
 794             }
 795             AssocItemKind::Macro(..) => bug!("macro item shouldn't exist at this point"),
 796         };
 797
 798         hir::TraitItem {
 799             hir_id: self.lower_node_id(i.id),
 800             ident: i.ident,
 801             attrs: self.lower_attrs(&i.attrs),
 802             generics,
 803             kind,
 804             span: i.span,
 805         }
 806     }
 807
 808     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 809         let (kind, has_default) = match i.kind {
 810             AssocItemKind::Const(_, ref default) => (hir::AssocItemKind::Const, default.is_some()),
 811             AssocItemKind::TyAlias(_, ref default) => (hir::AssocItemKind::Type, default.is_some()),
 812             AssocItemKind::Fn(ref sig, ref default) => {
 813                 (hir::AssocItemKind::Method { has_self: sig.decl.has_self() }, default.is_some())
 814             }
 815             AssocItemKind::Macro(..) => unimplemented!(),
 816         };
 817         hir::TraitItemRef {
 818             id: hir::TraitItemId { hir_id: self.lower_node_id(i.id) },
 819             ident: i.ident,
 820             span: i.span,
 821             defaultness: self.lower_defaultness(Defaultness::Default, has_default),
 822             kind,
 823         }
 824     }
 825
 826     /// Construct `ExprKind::Err` for the given `span`.
 827     fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 828         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 829     }
 830
 831     fn lower_impl_item(&mut self, i: &AssocItem) -> hir::ImplItem<'hir> {
 832         let impl_item_def_id = self.resolver.definitions().local_def_id(i.id);
 833
 834         let (generics, kind) = match i.kind {
 835             AssocItemKind::Const(ref ty, ref expr) => {
 836                 let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
 837                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 838                 (
 839                     generics,
 840                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 841                 )
 842             }
 843             AssocItemKind::Fn(ref sig, ref body) => {
 844                 self.current_item = Some(i.span);
 845                 let asyncness = sig.header.asyncness;
 846                 let body_id =
 847                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 848                 let impl_trait_return_allow = !self.is_in_trait_impl;
 849                 let (generics, sig) = self.lower_method_sig(
 850                     &i.generics,
 851                     sig,
 852                     impl_item_def_id,
 853                     impl_trait_return_allow,
 854                     asyncness.opt_return_id(),
 855                 );
 856
 857                 (generics, hir::ImplItemKind::Method(sig, body_id))
 858             }
 859             AssocItemKind::TyAlias(_, ref ty) => {
 860                 let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
 861                 let kind = match ty {
 862                     None => {
 863                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 864                         hir::ImplItemKind::TyAlias(ty)
 865                     }
 866                     Some(ty) => match ty.kind.opaque_top_hack() {
 867                         None => {
 868                             let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 869                             hir::ImplItemKind::TyAlias(ty)
 870                         }
 871                         Some(bs) => {
 872                             let bs = self.lower_param_bounds(bs, ImplTraitContext::disallowed());
 873                             hir::ImplItemKind::OpaqueTy(bs)
 874                         }
 875                     },
 876                 };
 877                 (generics, kind)
 878             }
 879             AssocItemKind::Macro(..) => bug!("`TyMac` should have been expanded by now"),
 880         };
 881
 882         hir::ImplItem {
 883             hir_id: self.lower_node_id(i.id),
 884             ident: i.ident,
 885             attrs: self.lower_attrs(&i.attrs),
 886             generics,
 887             vis: self.lower_visibility(&i.vis, None),
 888             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
 889             kind,
 890             span: i.span,
 891         }
 892
 893         // [1] since `default impl` is not yet implemented, this is always true in impls
 894     }
 895
 896     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef<'hir> {
 897         hir::ImplItemRef {
 898             id: hir::ImplItemId { hir_id: self.lower_node_id(i.id) },
 899             ident: i.ident,
 900             span: i.span,
 901             vis: self.lower_visibility(&i.vis, Some(i.id)),
 902             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
 903             kind: match &i.kind {
 904                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 905                 AssocItemKind::TyAlias(_, ty) => {
 906                     match ty.as_deref().and_then(|ty| ty.kind.opaque_top_hack()) {
 907                         None => hir::AssocItemKind::Type,
 908                         Some(_) => hir::AssocItemKind::OpaqueTy,
 909                     }
 910                 }
 911                 AssocItemKind::Fn(sig, _) => {
 912                     hir::AssocItemKind::Method { has_self: sig.decl.has_self() }
 913                 }
 914                 AssocItemKind::Macro(..) => unimplemented!(),
 915             },
 916         }
 917
 918         // [1] since `default impl` is not yet implemented, this is always true in impls
 919     }
 920
 921     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 922     /// the address space of that item instead of the item currently being
 923     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 924     /// lower a `Visibility` value although we haven't lowered the owning
 925     /// `ImplItem` in question yet.
 926     fn lower_visibility(
 927         &mut self,
 928         v: &Visibility,
 929         explicit_owner: Option<NodeId>,
 930     ) -> hir::Visibility<'hir> {
 931         let node = match v.node {
 932             VisibilityKind::Public => hir::VisibilityKind::Public,
 933             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 934             VisibilityKind::Restricted { ref path, id } => {
 935                 debug!("lower_visibility: restricted path id = {:?}", id);
 936                 let lowered_id = if let Some(owner) = explicit_owner {
 937                     self.lower_node_id_with_owner(id, owner)
 938                 } else {
 939                     self.lower_node_id(id)
 940                 };
 941                 let res = self.expect_full_res(id);
 942                 let res = self.lower_res(res);
 943                 hir::VisibilityKind::Restricted {
 944                     path: self.lower_path_extra(res, path, ParamMode::Explicit, explicit_owner),
 945                     hir_id: lowered_id,
 946                 }
 947             }
 948             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 949         };
 950         respan(v.span, node)
 951     }
 952
 953     fn lower_defaultness(&self, d: Defaultness, has_value: bool) -> hir::Defaultness {
 954         match d {
 955             Defaultness::Default => hir::Defaultness::Default { has_value: has_value },
 956             Defaultness::Final => {
 957                 assert!(has_value);
 958                 hir::Defaultness::Final
 959             }
 960         }
 961     }
 962
 963     fn record_body(
 964         &mut self,
 965         params: &'hir [hir::Param<'hir>],
 966         value: hir::Expr<'hir>,
 967     ) -> hir::BodyId {
 968         let body = hir::Body { generator_kind: self.generator_kind, params, value };
 969         let id = body.id();
 970         self.bodies.insert(id, body);
 971         id
 972     }
 973
 974     fn lower_body(
 975         &mut self,
 976         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
 977     ) -> hir::BodyId {
 978         let prev_gen_kind = self.generator_kind.take();
 979         let (parameters, result) = f(self);
 980         let body_id = self.record_body(parameters, result);
 981         self.generator_kind = prev_gen_kind;
 982         body_id
 983     }
 984
 985     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
 986         hir::Param {
 987             attrs: self.lower_attrs(&param.attrs),
 988             hir_id: self.lower_node_id(param.id),
 989             pat: self.lower_pat(&param.pat),
 990             span: param.span,
 991         }
 992     }
 993
 994     pub(super) fn lower_fn_body(
 995         &mut self,
 996         decl: &FnDecl,
 997         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 998     ) -> hir::BodyId {
 999         self.lower_body(|this| {
1000             (
1001                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
1002                 body(this),
1003             )
1004         })
1005     }
1006
1007     fn lower_fn_body_block(
1008         &mut self,
1009         span: Span,
1010         decl: &FnDecl,
1011         body: Option<&Block>,
1012     ) -> hir::BodyId {
1013         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1014     }
1015
1016     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1017         match block {
1018             Some(block) => self.lower_block_expr(block),
1019             None => self.expr_err(span),
1020         }
1021     }
1022
1023     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1024         self.lower_body(|this| {
1025             (
1026                 &[],
1027                 match expr {
1028                     Some(expr) => this.lower_expr_mut(expr),
1029                     None => this.expr_err(span),
1030                 },
1031             )
1032         })
1033     }
1034
1035     fn lower_maybe_async_body(
1036         &mut self,
1037         span: Span,
1038         decl: &FnDecl,
1039         asyncness: Async,
1040         body: Option<&Block>,
1041     ) -> hir::BodyId {
1042         let closure_id = match asyncness {
1043             Async::Yes { closure_id, .. } => closure_id,
1044             Async::No => return self.lower_fn_body_block(span, decl, body),
1045         };
1046
1047         self.lower_body(|this| {
1048             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1049             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1050
1051             // Async function parameters are lowered into the closure body so that they are
1052             // captured and so that the drop order matches the equivalent non-async functions.
1053             //
1054             // from:
1055             //
1056             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1057             //         <body>
1058             //     }
1059             //
1060             // into:
1061             //
1062             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1063             //       async move {
1064             //         let __arg2 = __arg2;
1065             //         let <pattern> = __arg2;
1066             //         let __arg1 = __arg1;
1067             //         let <pattern> = __arg1;
1068             //         let __arg0 = __arg0;
1069             //         let <pattern> = __arg0;
1070             //         drop-temps { <body> } // see comments later in fn for details
1071             //       }
1072             //     }
1073             //
1074             // If `<pattern>` is a simple ident, then it is lowered to a single
1075             // `let <pattern> = <pattern>;` statement as an optimization.
1076             //
1077             // Note that the body is embedded in `drop-temps`; an
1078             // equivalent desugaring would be `return { <body>
1079             // };`. The key point is that we wish to drop all the
1080             // let-bound variables and temporaries created in the body
1081             // (and its tail expression!) before we drop the
1082             // parameters (c.f. rust-lang/rust#64512).
1083             for (index, parameter) in decl.inputs.iter().enumerate() {
1084                 let parameter = this.lower_param(parameter);
1085                 let span = parameter.pat.span;
1086
1087                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1088                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1089                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1090                     hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, ident, _) => {
1091                         (ident, true)
1092                     }
1093                     _ => {
1094                         // Replace the ident for bindings that aren't simple.
1095                         let name = format!("__arg{}", index);
1096                         let ident = Ident::from_str(&name);
1097
1098                         (ident, false)
1099                     }
1100                 };
1101
1102                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1103
1104                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1105                 // async function.
1106                 //
1107                 // If this is the simple case, this parameter will end up being the same as the
1108                 // original parameter, but with a different pattern id.
1109                 let mut stmt_attrs = AttrVec::new();
1110                 stmt_attrs.extend(parameter.attrs.iter().cloned());
1111                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1112                 let new_parameter = hir::Param {
1113                     attrs: parameter.attrs,
1114                     hir_id: parameter.hir_id,
1115                     pat: new_parameter_pat,
1116                     span: parameter.span,
1117                 };
1118
1119                 if is_simple_parameter {
1120                     // If this is the simple case, then we only insert one statement that is
1121                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1122                     // `HirId`s are densely assigned.
1123                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1124                     let stmt = this.stmt_let_pat(
1125                         stmt_attrs,
1126                         desugared_span,
1127                         Some(expr),
1128                         parameter.pat,
1129                         hir::LocalSource::AsyncFn,
1130                     );
1131                     statements.push(stmt);
1132                 } else {
1133                     // If this is not the simple case, then we construct two statements:
1134                     //
1135                     // ```
1136                     // let __argN = __argN;
1137                     // let <pat> = __argN;
1138                     // ```
1139                     //
1140                     // The first statement moves the parameter into the closure and thus ensures
1141                     // that the drop order is correct.
1142                     //
1143                     // The second statement creates the bindings that the user wrote.
1144
1145                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1146                     // because the user may have specified a `ref mut` binding in the next
1147                     // statement.
1148                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1149                         desugared_span,
1150                         ident,
1151                         hir::BindingAnnotation::Mutable,
1152                     );
1153                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1154                     let move_stmt = this.stmt_let_pat(
1155                         AttrVec::new(),
1156                         desugared_span,
1157                         Some(move_expr),
1158                         move_pat,
1159                         hir::LocalSource::AsyncFn,
1160                     );
1161
1162                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1163                     // parameter's pattern so that `HirId`s are densely assigned.
1164                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1165                     let pattern_stmt = this.stmt_let_pat(
1166                         stmt_attrs,
1167                         desugared_span,
1168                         Some(pattern_expr),
1169                         parameter.pat,
1170                         hir::LocalSource::AsyncFn,
1171                     );
1172
1173                     statements.push(move_stmt);
1174                     statements.push(pattern_stmt);
1175                 };
1176
1177                 parameters.push(new_parameter);
1178             }
1179
1180             let body_span = body.map_or(span, |b| b.span);
1181             let async_expr = this.make_async_expr(
1182                 CaptureBy::Value,
1183                 closure_id,
1184                 None,
1185                 body_span,
1186                 hir::AsyncGeneratorKind::Fn,
1187                 |this| {
1188                     // Create a block from the user's function body:
1189                     let user_body = this.lower_block_expr_opt(body_span, body);
1190
1191                     // Transform into `drop-temps { <user-body> }`, an expression:
1192                     let desugared_span =
1193                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1194                     let user_body = this.expr_drop_temps(
1195                         desugared_span,
1196                         this.arena.alloc(user_body),
1197                         AttrVec::new(),
1198                     );
1199
1200                     // As noted above, create the final block like
1201                     //
1202                     // ```
1203                     // {
1204                     //   let $param_pattern = $raw_param;
1205                     //   ...
1206                     //   drop-temps { <user-body> }
1207                     // }
1208                     // ```
1209                     let body = this.block_all(
1210                         desugared_span,
1211                         this.arena.alloc_from_iter(statements),
1212                         Some(user_body),
1213                     );
1214
1215                     this.expr_block(body, AttrVec::new())
1216                 },
1217             );
1218
1219             (
1220                 this.arena.alloc_from_iter(parameters),
1221                 this.expr(body_span, async_expr, AttrVec::new()),
1222             )
1223         })
1224     }
1225
1226     fn lower_method_sig(
1227         &mut self,
1228         generics: &Generics,
1229         sig: &FnSig,
1230         fn_def_id: DefId,
1231         impl_trait_return_allow: bool,
1232         is_async: Option<NodeId>,
1233     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1234         let header = self.lower_fn_header(sig.header);
1235         let (generics, decl) = self.add_in_band_defs(
1236             generics,
1237             fn_def_id,
1238             AnonymousLifetimeMode::PassThrough,
1239             |this, idty| {
1240                 this.lower_fn_decl(
1241                     &sig.decl,
1242                     Some((fn_def_id, idty)),
1243                     impl_trait_return_allow,
1244                     is_async,
1245                 )
1246             },
1247         );
1248         (generics, hir::FnSig { header, decl })
1249     }
1250
1251     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1252         hir::FnHeader {
1253             unsafety: self.lower_unsafety(h.unsafety),
1254             asyncness: self.lower_asyncness(h.asyncness),
1255             constness: self.lower_constness(h.constness),
1256             abi: self.lower_extern(h.ext),
1257         }
1258     }
1259
1260     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1261         abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1262             self.error_on_invalid_abi(abi);
1263             abi::Abi::Rust
1264         })
1265     }
1266
1267     pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
1268         match ext {
1269             Extern::None => abi::Abi::Rust,
1270             Extern::Implicit => abi::Abi::C,
1271             Extern::Explicit(abi) => self.lower_abi(abi),
1272         }
1273     }
1274
1275     fn error_on_invalid_abi(&self, abi: StrLit) {
1276         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1277             .span_label(abi.span, "invalid ABI")
1278             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1279             .emit();
1280     }
1281
1282     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1283         match a {
1284             Async::Yes { .. } => hir::IsAsync::Async,
1285             Async::No => hir::IsAsync::NotAsync,
1286         }
1287     }
1288
1289     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1290         match c {
1291             Const::Yes(_) => hir::Constness::Const,
1292             Const::No => hir::Constness::NotConst,
1293         }
1294     }
1295
1296     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1297         match u {
1298             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1299             Unsafe::No => hir::Unsafety::Normal,
1300         }
1301     }
1302
1303     pub(super) fn lower_generics_mut(
1304         &mut self,
1305         generics: &Generics,
1306         itctx: ImplTraitContext<'_, 'hir>,
1307     ) -> GenericsCtor<'hir> {
1308         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1309         // FIXME: this could probably be done with less rightward drift. It also looks like two
1310         // control paths where `report_error` is called are the only paths that advance to after the
1311         // match statement, so the error reporting could probably just be moved there.
1312         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1313         for pred in &generics.where_clause.predicates {
1314             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1315                 'next_bound: for bound in &bound_pred.bounds {
1316                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1317                         let report_error = |this: &mut Self| {
1318                             this.diagnostic().span_err(
1319                                 bound_pred.bounded_ty.span,
1320                                 "`?Trait` bounds are only permitted at the \
1321                                  point where a type parameter is declared",
1322                             );
1323                         };
1324                         // Check if the where clause type is a plain type parameter.
1325                         match bound_pred.bounded_ty.kind {
1326                             TyKind::Path(None, ref path)
1327                                 if path.segments.len() == 1
1328                                     && bound_pred.bound_generic_params.is_empty() =>
1329                             {
1330                                 if let Some(Res::Def(DefKind::TyParam, def_id)) = self
1331                                     .resolver
1332                                     .get_partial_res(bound_pred.bounded_ty.id)
1333                                     .map(|d| d.base_res())
1334                                 {
1335                                     if let Some(node_id) =
1336                                         self.resolver.definitions().as_local_node_id(def_id)
1337                                     {
1338                                         for param in &generics.params {
1339                                             match param.kind {
1340                                                 GenericParamKind::Type { .. } => {
1341                                                     if node_id == param.id {
1342                                                         add_bounds
1343                                                             .entry(param.id)
1344                                                             .or_default()
1345                                                             .push(bound.clone());
1346                                                         continue 'next_bound;
1347                                                     }
1348                                                 }
1349                                                 _ => {}
1350                                             }
1351                                         }
1352                                     }
1353                                 }
1354                                 report_error(self)
1355                             }
1356                             _ => report_error(self),
1357                         }
1358                     }
1359                 }
1360             }
1361         }
1362
1363         GenericsCtor {
1364             params: self.lower_generic_params_mut(&generics.params, &add_bounds, itctx).collect(),
1365             where_clause: self.lower_where_clause(&generics.where_clause),
1366             span: generics.span,
1367         }
1368     }
1369
1370     pub(super) fn lower_generics(
1371         &mut self,
1372         generics: &Generics,
1373         itctx: ImplTraitContext<'_, 'hir>,
1374     ) -> hir::Generics<'hir> {
1375         let generics_ctor = self.lower_generics_mut(generics, itctx);
1376         generics_ctor.into_generics(self.arena)
1377     }
1378
1379     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1380         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1381             hir::WhereClause {
1382                 predicates: this.arena.alloc_from_iter(
1383                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1384                 ),
1385                 span: wc.span,
1386             }
1387         })
1388     }
1389
1390     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1391         match *pred {
1392             WherePredicate::BoundPredicate(WhereBoundPredicate {
1393                 ref bound_generic_params,
1394                 ref bounded_ty,
1395                 ref bounds,
1396                 span,
1397             }) => {
1398                 self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1399                     hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1400                         bound_generic_params: this.lower_generic_params(
1401                             bound_generic_params,
1402                             &NodeMap::default(),
1403                             ImplTraitContext::disallowed(),
1404                         ),
1405                         bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1406                         bounds: this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| {
1407                             match *bound {
1408                                 // Ignore `?Trait` bounds.
1409                                 // They were copied into type parameters already.
1410                                 GenericBound::Trait(_, TraitBoundModifier::Maybe) => None,
1411                                 _ => Some(
1412                                     this.lower_param_bound(bound, ImplTraitContext::disallowed()),
1413                                 ),
1414                             }
1415                         })),
1416                         span,
1417                     })
1418                 })
1419             }
1420             WherePredicate::RegionPredicate(WhereRegionPredicate {
1421                 ref lifetime,
1422                 ref bounds,
1423                 span,
1424             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1425                 span,
1426                 lifetime: self.lower_lifetime(lifetime),
1427                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1428             }),
1429             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1430                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1431                     hir_id: self.lower_node_id(id),
1432                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1433                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1434                     span,
1435                 })
1436             }
1437         }
1438     }
1439 }
1440
1441 /// Helper struct for delayed construction of Generics.
1442 pub(super) struct GenericsCtor<'hir> {
1443     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1444     where_clause: hir::WhereClause<'hir>,
1445     span: Span,
1446 }
1447
1448 impl<'hir> GenericsCtor<'hir> {
1449     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1450         hir::Generics {
1451             params: arena.alloc_from_iter(self.params),
1452             where_clause: self.where_clause,
1453             span: self.span,
1454         }
1455     }
1456 }