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, body_id) = self.lower_const_item(t, span, e.as_deref());
 273                 hir::ItemKind::Static(ty, m, body_id)
 274             }
 275             ItemKind::Const(ref t, ref e) => {
 276                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 277                 hir::ItemKind::Const(ty, body_id)
 278             }
 279             ItemKind::Fn(FnSig { ref decl, header }, ref generics, ref body) => {
 280                 let fn_def_id = self.resolver.definitions().local_def_id(id);
 281                 self.with_new_scopes(|this| {
 282                     this.current_item = Some(ident.span);
 283
 284                     // Note: we don't need to change the return type from `T` to
 285                     // `impl Future<Output = T>` here because lower_body
 286                     // only cares about the input argument patterns in the function
 287                     // declaration (decl), not the return types.
 288                     let asyncness = header.asyncness;
 289                     let body_id =
 290                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 291
 292                     let (generics, decl) = this.add_in_band_defs(
 293                         generics,
 294                         fn_def_id,
 295                         AnonymousLifetimeMode::PassThrough,
 296                         |this, idty| {
 297                             let ret_id = asyncness.opt_return_id();
 298                             this.lower_fn_decl(&decl, Some((fn_def_id, idty)), true, ret_id)
 299                         },
 300                     );
 301                     let sig = hir::FnSig { decl, header: this.lower_fn_header(header) };
 302                     hir::ItemKind::Fn(sig, generics, body_id)
 303                 })
 304             }
 305             ItemKind::Mod(ref m) => hir::ItemKind::Mod(self.lower_mod(m)),
 306             ItemKind::ForeignMod(ref nm) => hir::ItemKind::ForeignMod(self.lower_foreign_mod(nm)),
 307             ItemKind::GlobalAsm(ref ga) => hir::ItemKind::GlobalAsm(self.lower_global_asm(ga)),
 308             ItemKind::TyAlias(ref ty, ref generics) => match ty.kind.opaque_top_hack() {
 309                 None => {
 310                     let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 311                     let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 312                     hir::ItemKind::TyAlias(ty, generics)
 313                 }
 314                 Some(bounds) => {
 315                     let ty = hir::OpaqueTy {
 316                         generics: self.lower_generics(
 317                             generics,
 318                             ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc),
 319                         ),
 320                         bounds: self.lower_param_bounds(
 321                             bounds,
 322                             ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc),
 323                         ),
 324                         impl_trait_fn: None,
 325                         origin: hir::OpaqueTyOrigin::TypeAlias,
 326                     };
 327                     hir::ItemKind::OpaqueTy(ty)
 328                 }
 329             },
 330             ItemKind::Enum(ref enum_definition, ref generics) => hir::ItemKind::Enum(
 331                 hir::EnumDef {
 332                     variants: self.arena.alloc_from_iter(
 333                         enum_definition.variants.iter().map(|x| self.lower_variant(x)),
 334                     ),
 335                 },
 336                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 337             ),
 338             ItemKind::Struct(ref struct_def, ref generics) => {
 339                 let struct_def = self.lower_variant_data(struct_def);
 340                 hir::ItemKind::Struct(
 341                     struct_def,
 342                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 343                 )
 344             }
 345             ItemKind::Union(ref vdata, ref generics) => {
 346                 let vdata = self.lower_variant_data(vdata);
 347                 hir::ItemKind::Union(
 348                     vdata,
 349                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 350                 )
 351             }
 352             ItemKind::Impl {
 353                 unsafety,
 354                 polarity,
 355                 defaultness,
 356                 constness,
 357                 generics: ref ast_generics,
 358                 of_trait: ref trait_ref,
 359                 self_ty: ref ty,
 360                 items: ref impl_items,
 361             } => {
 362                 let def_id = self.resolver.definitions().local_def_id(id);
 363
 364                 // Lower the "impl header" first. This ordering is important
 365                 // for in-band lifetimes! Consider `'a` here:
 366                 //
 367                 //     impl Foo<'a> for u32 {
 368                 //         fn method(&'a self) { .. }
 369                 //     }
 370                 //
 371                 // Because we start by lowering the `Foo<'a> for u32`
 372                 // part, we will add `'a` to the list of generics on
 373                 // the impl. When we then encounter it later in the
 374                 // method, it will not be considered an in-band
 375                 // lifetime to be added, but rather a reference to a
 376                 // parent lifetime.
 377                 let lowered_trait_impl_id = self.lower_node_id(id);
 378                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 379                     ast_generics,
 380                     def_id,
 381                     AnonymousLifetimeMode::CreateParameter,
 382                     |this, _| {
 383                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 384                             this.lower_trait_ref(trait_ref, ImplTraitContext::disallowed())
 385                         });
 386
 387                         if let Some(ref trait_ref) = trait_ref {
 388                             if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res {
 389                                 this.trait_impls
 390                                     .entry(def_id)
 391                                     .or_default()
 392                                     .push(lowered_trait_impl_id);
 393                             }
 394                         }
 395
 396                         let lowered_ty = this.lower_ty(ty, ImplTraitContext::disallowed());
 397
 398                         (trait_ref, lowered_ty)
 399                     },
 400                 );
 401
 402                 let new_impl_items =
 403                     self.with_in_scope_lifetime_defs(&ast_generics.params, |this| {
 404                         this.arena.alloc_from_iter(
 405                             impl_items.iter().map(|item| this.lower_impl_item_ref(item)),
 406                         )
 407                     });
 408
 409                 hir::ItemKind::Impl {
 410                     unsafety: self.lower_unsafety(unsafety),
 411                     polarity,
 412                     defaultness: self.lower_defaultness(defaultness, true /* [1] */),
 413                     constness: self.lower_constness(constness),
 414                     generics,
 415                     of_trait: trait_ref,
 416                     self_ty: lowered_ty,
 417                     items: new_impl_items,
 418                 }
 419             }
 420             ItemKind::Trait(is_auto, unsafety, ref generics, ref bounds, ref items) => {
 421                 let bounds = self.lower_param_bounds(bounds, ImplTraitContext::disallowed());
 422                 let items = self
 423                     .arena
 424                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
 425                 hir::ItemKind::Trait(
 426                     is_auto,
 427                     self.lower_unsafety(unsafety),
 428                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 429                     bounds,
 430                     items,
 431                 )
 432             }
 433             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 434                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 435                 self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 436             ),
 437             ItemKind::MacroDef(..) | ItemKind::Mac(..) => {
 438                 bug!("`TyMac` should have been expanded by now")
 439             }
 440         }
 441
 442         // [1] `defaultness.has_value()` is never called for an `impl`, always `true` in order to
 443         //     not cause an assertion failure inside the `lower_defaultness` function.
 444     }
 445
 446     fn lower_const_item(
 447         &mut self,
 448         ty: &Ty,
 449         span: Span,
 450         body: Option<&Expr>,
 451     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 452         let itctx = if self.sess.features_untracked().impl_trait_in_bindings {
 453             ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc)
 454         } else {
 455             ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
 456         };
 457         let ty = self.lower_ty(ty, itctx);
 458         (ty, self.lower_const_body(span, body))
 459     }
 460
 461     fn lower_use_tree(
 462         &mut self,
 463         tree: &UseTree,
 464         prefix: &Path,
 465         id: NodeId,
 466         vis: &mut hir::Visibility<'hir>,
 467         ident: &mut Ident,
 468         attrs: &'hir [Attribute],
 469     ) -> hir::ItemKind<'hir> {
 470         debug!("lower_use_tree(tree={:?})", tree);
 471         debug!("lower_use_tree: vis = {:?}", vis);
 472
 473         let path = &tree.prefix;
 474         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 475
 476         match tree.kind {
 477             UseTreeKind::Simple(rename, id1, id2) => {
 478                 *ident = tree.ident();
 479
 480                 // First, apply the prefix to the path.
 481                 let mut path = Path { segments, span: path.span };
 482
 483                 // Correctly resolve `self` imports.
 484                 if path.segments.len() > 1
 485                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 486                 {
 487                     let _ = path.segments.pop();
 488                     if rename.is_none() {
 489                         *ident = path.segments.last().unwrap().ident;
 490                     }
 491                 }
 492
 493                 let mut resolutions = self.expect_full_res_from_use(id);
 494                 // We want to return *something* from this function, so hold onto the first item
 495                 // for later.
 496                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 497
 498                 // Here, we are looping over namespaces, if they exist for the definition
 499                 // being imported. We only handle type and value namespaces because we
 500                 // won't be dealing with macros in the rest of the compiler.
 501                 // Essentially a single `use` which imports two names is desugared into
 502                 // two imports.
 503                 for (res, &new_node_id) in resolutions.zip([id1, id2].iter()) {
 504                     let ident = *ident;
 505                     let mut path = path.clone();
 506                     for seg in &mut path.segments {
 507                         seg.id = self.resolver.next_node_id();
 508                     }
 509                     let span = path.span;
 510
 511                     self.with_hir_id_owner(new_node_id, |this| {
 512                         let new_id = this.lower_node_id(new_node_id);
 513                         let res = this.lower_res(res);
 514                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit, None);
 515                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 516                         let vis = this.rebuild_vis(&vis);
 517
 518                         this.insert_item(hir::Item {
 519                             hir_id: new_id,
 520                             ident,
 521                             attrs,
 522                             kind,
 523                             vis,
 524                             span,
 525                         });
 526                     });
 527                 }
 528
 529                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit, None);
 530                 hir::ItemKind::Use(path, hir::UseKind::Single)
 531             }
 532             UseTreeKind::Glob => {
 533                 let path =
 534                     self.lower_path(id, &Path { segments, span: path.span }, ParamMode::Explicit);
 535                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 536             }
 537             UseTreeKind::Nested(ref trees) => {
 538                 // Nested imports are desugared into simple imports.
 539                 // So, if we start with
 540                 //
 541                 // ```
 542                 // pub(x) use foo::{a, b};
 543                 // ```
 544                 //
 545                 // we will create three items:
 546                 //
 547                 // ```
 548                 // pub(x) use foo::a;
 549                 // pub(x) use foo::b;
 550                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 551                 // ```
 552                 //
 553                 // The first two are produced by recursively invoking
 554                 // `lower_use_tree` (and indeed there may be things
 555                 // like `use foo::{a::{b, c}}` and so forth).  They
 556                 // wind up being directly added to
 557                 // `self.items`. However, the structure of this
 558                 // function also requires us to return one item, and
 559                 // for that we return the `{}` import (called the
 560                 // `ListStem`).
 561
 562                 let prefix = Path { segments, span: prefix.span.to(path.span) };
 563
 564                 // Add all the nested `PathListItem`s to the HIR.
 565                 for &(ref use_tree, id) in trees {
 566                     let new_hir_id = self.lower_node_id(id);
 567
 568                     let mut prefix = prefix.clone();
 569
 570                     // Give the segments new node-ids since they are being cloned.
 571                     for seg in &mut prefix.segments {
 572                         seg.id = self.resolver.next_node_id();
 573                     }
 574
 575                     // Each `use` import is an item and thus are owners of the
 576                     // names in the path. Up to this point the nested import is
 577                     // the current owner, since we want each desugared import to
 578                     // own its own names, we have to adjust the owner before
 579                     // lowering the rest of the import.
 580                     self.with_hir_id_owner(id, |this| {
 581                         let mut vis = this.rebuild_vis(&vis);
 582                         let mut ident = *ident;
 583
 584                         let kind =
 585                             this.lower_use_tree(use_tree, &prefix, id, &mut vis, &mut ident, attrs);
 586
 587                         this.insert_item(hir::Item {
 588                             hir_id: new_hir_id,
 589                             ident,
 590                             attrs,
 591                             kind,
 592                             vis,
 593                             span: use_tree.span,
 594                         });
 595                     });
 596                 }
 597
 598                 // Subtle and a bit hacky: we lower the privacy level
 599                 // of the list stem to "private" most of the time, but
 600                 // not for "restricted" paths. The key thing is that
 601                 // we don't want it to stay as `pub` (with no caveats)
 602                 // because that affects rustdoc and also the lints
 603                 // about `pub` items. But we can't *always* make it
 604                 // private -- particularly not for restricted paths --
 605                 // because it contains node-ids that would then be
 606                 // unused, failing the check that HirIds are "densely
 607                 // assigned".
 608                 match vis.node {
 609                     hir::VisibilityKind::Public
 610                     | hir::VisibilityKind::Crate(_)
 611                     | hir::VisibilityKind::Inherited => {
 612                         *vis = respan(prefix.span.shrink_to_lo(), hir::VisibilityKind::Inherited);
 613                     }
 614                     hir::VisibilityKind::Restricted { .. } => {
 615                         // Do nothing here, as described in the comment on the match.
 616                     }
 617                 }
 618
 619                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 620                 let res = self.lower_res(res);
 621                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit, None);
 622                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 623             }
 624         }
 625     }
 626
 627     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 628     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 629     /// `NodeId`s. (See, e.g., #56128.)
 630     fn rebuild_use_path(&mut self, path: &hir::Path<'hir>) -> &'hir hir::Path<'hir> {
 631         debug!("rebuild_use_path(path = {:?})", path);
 632         let segments =
 633             self.arena.alloc_from_iter(path.segments.iter().map(|seg| hir::PathSegment {
 634                 ident: seg.ident,
 635                 hir_id: seg.hir_id.map(|_| self.next_id()),
 636                 res: seg.res,
 637                 args: None,
 638                 infer_args: seg.infer_args,
 639             }));
 640         self.arena.alloc(hir::Path { span: path.span, res: path.res, segments })
 641     }
 642
 643     fn rebuild_vis(&mut self, vis: &hir::Visibility<'hir>) -> hir::Visibility<'hir> {
 644         let vis_kind = match vis.node {
 645             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 646             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 647             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 648             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 649                 hir::VisibilityKind::Restricted {
 650                     path: self.rebuild_use_path(path),
 651                     hir_id: self.next_id(),
 652                 }
 653             }
 654         };
 655         respan(vis.span, vis_kind)
 656     }
 657
 658     fn lower_foreign_item(&mut self, i: &ForeignItem) -> hir::ForeignItem<'hir> {
 659         let def_id = self.resolver.definitions().local_def_id(i.id);
 660         hir::ForeignItem {
 661             hir_id: self.lower_node_id(i.id),
 662             ident: i.ident,
 663             attrs: self.lower_attrs(&i.attrs),
 664             kind: match i.kind {
 665                 ForeignItemKind::Fn(ref sig, ref generics, _) => {
 666                     let fdec = &sig.decl;
 667                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 668                         generics,
 669                         def_id,
 670                         AnonymousLifetimeMode::PassThrough,
 671                         |this, _| {
 672                             (
 673                                 // Disallow `impl Trait` in foreign items.
 674                                 this.lower_fn_decl(fdec, None, false, None),
 675                                 this.lower_fn_params_to_names(fdec),
 676                             )
 677                         },
 678                     );
 679
 680                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 681                 }
 682                 ForeignItemKind::Static(ref t, m, _) => {
 683                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 684                     hir::ForeignItemKind::Static(ty, m)
 685                 }
 686                 ForeignItemKind::Const(ref t, _) => {
 687                     // For recovery purposes.
 688                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 689                     hir::ForeignItemKind::Static(ty, Mutability::Not)
 690                 }
 691                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 692                 ForeignItemKind::Macro(_) => panic!("macro shouldn't exist here"),
 693             },
 694             vis: self.lower_visibility(&i.vis, None),
 695             span: i.span,
 696         }
 697     }
 698
 699     fn lower_foreign_mod(&mut self, fm: &ForeignMod) -> hir::ForeignMod<'hir> {
 700         hir::ForeignMod {
 701             abi: fm.abi.map_or(abi::Abi::C, |abi| self.lower_abi(abi)),
 702             items: self.arena.alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item(x))),
 703         }
 704     }
 705
 706     fn lower_global_asm(&mut self, ga: &GlobalAsm) -> &'hir hir::GlobalAsm {
 707         self.arena.alloc(hir::GlobalAsm { asm: ga.asm })
 708     }
 709
 710     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 711         hir::Variant {
 712             attrs: self.lower_attrs(&v.attrs),
 713             data: self.lower_variant_data(&v.data),
 714             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 715             id: self.lower_node_id(v.id),
 716             ident: v.ident,
 717             span: v.span,
 718         }
 719     }
 720
 721     fn lower_variant_data(&mut self, vdata: &VariantData) -> hir::VariantData<'hir> {
 722         match *vdata {
 723             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 724                 self.arena
 725                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 726                 recovered,
 727             ),
 728             VariantData::Tuple(ref fields, id) => hir::VariantData::Tuple(
 729                 self.arena
 730                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 731                 self.lower_node_id(id),
 732             ),
 733             VariantData::Unit(id) => hir::VariantData::Unit(self.lower_node_id(id)),
 734         }
 735     }
 736
 737     fn lower_struct_field(&mut self, (index, f): (usize, &StructField)) -> hir::StructField<'hir> {
 738         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 739             let t = self.lower_path_ty(
 740                 &f.ty,
 741                 qself,
 742                 path,
 743                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 744                 ImplTraitContext::disallowed(),
 745             );
 746             self.arena.alloc(t)
 747         } else {
 748             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 749         };
 750         hir::StructField {
 751             span: f.span,
 752             hir_id: self.lower_node_id(f.id),
 753             ident: match f.ident {
 754                 Some(ident) => ident,
 755                 // FIXME(jseyfried): positional field hygiene.
 756                 None => Ident::new(sym::integer(index), f.span),
 757             },
 758             vis: self.lower_visibility(&f.vis, None),
 759             ty,
 760             attrs: self.lower_attrs(&f.attrs),
 761         }
 762     }
 763
 764     fn lower_trait_item(&mut self, i: &AssocItem) -> hir::TraitItem<'hir> {
 765         let trait_item_def_id = self.resolver.definitions().local_def_id(i.id);
 766
 767         let (generics, kind) = match i.kind {
 768             AssocItemKind::Static(ref ty, _, ref default) // Let's pretend this is a `const`.
 769             | AssocItemKind::Const(ref ty, ref default) => {
 770                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 771                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 772                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
 773             }
 774             AssocItemKind::Fn(ref sig, ref generics, None) => {
 775                 let names = self.lower_fn_params_to_names(&sig.decl);
 776                 let (generics, sig) =
 777                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
 778                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Required(names)))
 779             }
 780             AssocItemKind::Fn(ref sig, ref generics, Some(ref body)) => {
 781                 let body_id = self.lower_fn_body_block(i.span, &sig.decl, Some(body));
 782                 let (generics, sig) =
 783                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
 784                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Provided(body_id)))
 785             }
 786             AssocItemKind::TyAlias(ref generics, ref bounds, ref default) => {
 787                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
 788                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 789                 let kind = hir::TraitItemKind::Type(
 790                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 791                     ty,
 792                 );
 793
 794                 (generics, kind)
 795             }
 796             AssocItemKind::Macro(..) => bug!("macro item shouldn't exist at this point"),
 797         };
 798
 799         hir::TraitItem {
 800             hir_id: self.lower_node_id(i.id),
 801             ident: i.ident,
 802             attrs: self.lower_attrs(&i.attrs),
 803             generics,
 804             kind,
 805             span: i.span,
 806         }
 807     }
 808
 809     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 810         let (kind, has_default) = match &i.kind {
 811             AssocItemKind::Static(_, _, default) // Let's pretend this is a `const` for recovery.
 812             | AssocItemKind::Const(_, default) => {
 813                 (hir::AssocItemKind::Const, default.is_some())
 814             }
 815             AssocItemKind::TyAlias(_, _, default) => (hir::AssocItemKind::Type, default.is_some()),
 816             AssocItemKind::Fn(sig, _, default) => {
 817                 (hir::AssocItemKind::Method { has_self: sig.decl.has_self() }, default.is_some())
 818             }
 819             AssocItemKind::Macro(..) => unimplemented!(),
 820         };
 821         hir::TraitItemRef {
 822             id: hir::TraitItemId { hir_id: self.lower_node_id(i.id) },
 823             ident: i.ident,
 824             span: i.span,
 825             defaultness: self.lower_defaultness(Defaultness::Default, has_default),
 826             kind,
 827         }
 828     }
 829
 830     /// Construct `ExprKind::Err` for the given `span`.
 831     fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 832         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 833     }
 834
 835     fn lower_impl_item(&mut self, i: &AssocItem) -> hir::ImplItem<'hir> {
 836         let impl_item_def_id = self.resolver.definitions().local_def_id(i.id);
 837
 838         let (generics, kind) = match i.kind {
 839             AssocItemKind::Static(ref ty, _, ref expr) | AssocItemKind::Const(ref ty, ref expr) => {
 840                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 841                 (
 842                     hir::Generics::empty(),
 843                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 844                 )
 845             }
 846             AssocItemKind::Fn(ref sig, ref generics, ref body) => {
 847                 self.current_item = Some(i.span);
 848                 let asyncness = sig.header.asyncness;
 849                 let body_id =
 850                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 851                 let impl_trait_return_allow = !self.is_in_trait_impl;
 852                 let (generics, sig) = self.lower_method_sig(
 853                     generics,
 854                     sig,
 855                     impl_item_def_id,
 856                     impl_trait_return_allow,
 857                     asyncness.opt_return_id(),
 858                 );
 859
 860                 (generics, hir::ImplItemKind::Method(sig, body_id))
 861             }
 862             AssocItemKind::TyAlias(ref generics, _, ref ty) => {
 863                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 864                 let kind = match ty {
 865                     None => {
 866                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 867                         hir::ImplItemKind::TyAlias(ty)
 868                     }
 869                     Some(ty) => match ty.kind.opaque_top_hack() {
 870                         None => {
 871                             let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 872                             hir::ImplItemKind::TyAlias(ty)
 873                         }
 874                         Some(bs) => {
 875                             let bs = self.lower_param_bounds(bs, ImplTraitContext::disallowed());
 876                             hir::ImplItemKind::OpaqueTy(bs)
 877                         }
 878                     },
 879                 };
 880                 (generics, kind)
 881             }
 882             AssocItemKind::Macro(..) => bug!("`TyMac` should have been expanded by now"),
 883         };
 884
 885         hir::ImplItem {
 886             hir_id: self.lower_node_id(i.id),
 887             ident: i.ident,
 888             attrs: self.lower_attrs(&i.attrs),
 889             generics,
 890             vis: self.lower_visibility(&i.vis, None),
 891             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
 892             kind,
 893             span: i.span,
 894         }
 895
 896         // [1] since `default impl` is not yet implemented, this is always true in impls
 897     }
 898
 899     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef<'hir> {
 900         hir::ImplItemRef {
 901             id: hir::ImplItemId { hir_id: self.lower_node_id(i.id) },
 902             ident: i.ident,
 903             span: i.span,
 904             vis: self.lower_visibility(&i.vis, Some(i.id)),
 905             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
 906             kind: match &i.kind {
 907                 AssocItemKind::Static(..) // Let's pretend this is a `const` for recovery.
 908                 | AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 909                 AssocItemKind::TyAlias(_, _, ty) => {
 910                     match ty.as_deref().and_then(|ty| ty.kind.opaque_top_hack()) {
 911                         None => hir::AssocItemKind::Type,
 912                         Some(_) => hir::AssocItemKind::OpaqueTy,
 913                     }
 914                 }
 915                 AssocItemKind::Fn(sig, _, _) => {
 916                     hir::AssocItemKind::Method { has_self: sig.decl.has_self() }
 917                 }
 918                 AssocItemKind::Macro(..) => unimplemented!(),
 919             },
 920         }
 921
 922         // [1] since `default impl` is not yet implemented, this is always true in impls
 923     }
 924
 925     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 926     /// the address space of that item instead of the item currently being
 927     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 928     /// lower a `Visibility` value although we haven't lowered the owning
 929     /// `ImplItem` in question yet.
 930     fn lower_visibility(
 931         &mut self,
 932         v: &Visibility,
 933         explicit_owner: Option<NodeId>,
 934     ) -> hir::Visibility<'hir> {
 935         let node = match v.node {
 936             VisibilityKind::Public => hir::VisibilityKind::Public,
 937             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 938             VisibilityKind::Restricted { ref path, id } => {
 939                 debug!("lower_visibility: restricted path id = {:?}", id);
 940                 let lowered_id = if let Some(owner) = explicit_owner {
 941                     self.lower_node_id_with_owner(id, owner)
 942                 } else {
 943                     self.lower_node_id(id)
 944                 };
 945                 let res = self.expect_full_res(id);
 946                 let res = self.lower_res(res);
 947                 hir::VisibilityKind::Restricted {
 948                     path: self.lower_path_extra(res, path, ParamMode::Explicit, explicit_owner),
 949                     hir_id: lowered_id,
 950                 }
 951             }
 952             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 953         };
 954         respan(v.span, node)
 955     }
 956
 957     fn lower_defaultness(&self, d: Defaultness, has_value: bool) -> hir::Defaultness {
 958         match d {
 959             Defaultness::Default => hir::Defaultness::Default { has_value: has_value },
 960             Defaultness::Final => {
 961                 assert!(has_value);
 962                 hir::Defaultness::Final
 963             }
 964         }
 965     }
 966
 967     fn record_body(
 968         &mut self,
 969         params: &'hir [hir::Param<'hir>],
 970         value: hir::Expr<'hir>,
 971     ) -> hir::BodyId {
 972         let body = hir::Body { generator_kind: self.generator_kind, params, value };
 973         let id = body.id();
 974         self.bodies.insert(id, body);
 975         id
 976     }
 977
 978     fn lower_body(
 979         &mut self,
 980         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
 981     ) -> hir::BodyId {
 982         let prev_gen_kind = self.generator_kind.take();
 983         let (parameters, result) = f(self);
 984         let body_id = self.record_body(parameters, result);
 985         self.generator_kind = prev_gen_kind;
 986         body_id
 987     }
 988
 989     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
 990         hir::Param {
 991             attrs: self.lower_attrs(&param.attrs),
 992             hir_id: self.lower_node_id(param.id),
 993             pat: self.lower_pat(&param.pat),
 994             span: param.span,
 995         }
 996     }
 997
 998     pub(super) fn lower_fn_body(
 999         &mut self,
1000         decl: &FnDecl,
1001         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
1002     ) -> hir::BodyId {
1003         self.lower_body(|this| {
1004             (
1005                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
1006                 body(this),
1007             )
1008         })
1009     }
1010
1011     fn lower_fn_body_block(
1012         &mut self,
1013         span: Span,
1014         decl: &FnDecl,
1015         body: Option<&Block>,
1016     ) -> hir::BodyId {
1017         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1018     }
1019
1020     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1021         match block {
1022             Some(block) => self.lower_block_expr(block),
1023             None => self.expr_err(span),
1024         }
1025     }
1026
1027     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1028         self.lower_body(|this| {
1029             (
1030                 &[],
1031                 match expr {
1032                     Some(expr) => this.lower_expr_mut(expr),
1033                     None => this.expr_err(span),
1034                 },
1035             )
1036         })
1037     }
1038
1039     fn lower_maybe_async_body(
1040         &mut self,
1041         span: Span,
1042         decl: &FnDecl,
1043         asyncness: Async,
1044         body: Option<&Block>,
1045     ) -> hir::BodyId {
1046         let closure_id = match asyncness {
1047             Async::Yes { closure_id, .. } => closure_id,
1048             Async::No => return self.lower_fn_body_block(span, decl, body),
1049         };
1050
1051         self.lower_body(|this| {
1052             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1053             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1054
1055             // Async function parameters are lowered into the closure body so that they are
1056             // captured and so that the drop order matches the equivalent non-async functions.
1057             //
1058             // from:
1059             //
1060             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1061             //         <body>
1062             //     }
1063             //
1064             // into:
1065             //
1066             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1067             //       async move {
1068             //         let __arg2 = __arg2;
1069             //         let <pattern> = __arg2;
1070             //         let __arg1 = __arg1;
1071             //         let <pattern> = __arg1;
1072             //         let __arg0 = __arg0;
1073             //         let <pattern> = __arg0;
1074             //         drop-temps { <body> } // see comments later in fn for details
1075             //       }
1076             //     }
1077             //
1078             // If `<pattern>` is a simple ident, then it is lowered to a single
1079             // `let <pattern> = <pattern>;` statement as an optimization.
1080             //
1081             // Note that the body is embedded in `drop-temps`; an
1082             // equivalent desugaring would be `return { <body>
1083             // };`. The key point is that we wish to drop all the
1084             // let-bound variables and temporaries created in the body
1085             // (and its tail expression!) before we drop the
1086             // parameters (c.f. rust-lang/rust#64512).
1087             for (index, parameter) in decl.inputs.iter().enumerate() {
1088                 let parameter = this.lower_param(parameter);
1089                 let span = parameter.pat.span;
1090
1091                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1092                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1093                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1094                     hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, ident, _) => {
1095                         (ident, true)
1096                     }
1097                     _ => {
1098                         // Replace the ident for bindings that aren't simple.
1099                         let name = format!("__arg{}", index);
1100                         let ident = Ident::from_str(&name);
1101
1102                         (ident, false)
1103                     }
1104                 };
1105
1106                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1107
1108                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1109                 // async function.
1110                 //
1111                 // If this is the simple case, this parameter will end up being the same as the
1112                 // original parameter, but with a different pattern id.
1113                 let mut stmt_attrs = AttrVec::new();
1114                 stmt_attrs.extend(parameter.attrs.iter().cloned());
1115                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1116                 let new_parameter = hir::Param {
1117                     attrs: parameter.attrs,
1118                     hir_id: parameter.hir_id,
1119                     pat: new_parameter_pat,
1120                     span: parameter.span,
1121                 };
1122
1123                 if is_simple_parameter {
1124                     // If this is the simple case, then we only insert one statement that is
1125                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1126                     // `HirId`s are densely assigned.
1127                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1128                     let stmt = this.stmt_let_pat(
1129                         stmt_attrs,
1130                         desugared_span,
1131                         Some(expr),
1132                         parameter.pat,
1133                         hir::LocalSource::AsyncFn,
1134                     );
1135                     statements.push(stmt);
1136                 } else {
1137                     // If this is not the simple case, then we construct two statements:
1138                     //
1139                     // ```
1140                     // let __argN = __argN;
1141                     // let <pat> = __argN;
1142                     // ```
1143                     //
1144                     // The first statement moves the parameter into the closure and thus ensures
1145                     // that the drop order is correct.
1146                     //
1147                     // The second statement creates the bindings that the user wrote.
1148
1149                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1150                     // because the user may have specified a `ref mut` binding in the next
1151                     // statement.
1152                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1153                         desugared_span,
1154                         ident,
1155                         hir::BindingAnnotation::Mutable,
1156                     );
1157                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1158                     let move_stmt = this.stmt_let_pat(
1159                         AttrVec::new(),
1160                         desugared_span,
1161                         Some(move_expr),
1162                         move_pat,
1163                         hir::LocalSource::AsyncFn,
1164                     );
1165
1166                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1167                     // parameter's pattern so that `HirId`s are densely assigned.
1168                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1169                     let pattern_stmt = this.stmt_let_pat(
1170                         stmt_attrs,
1171                         desugared_span,
1172                         Some(pattern_expr),
1173                         parameter.pat,
1174                         hir::LocalSource::AsyncFn,
1175                     );
1176
1177                     statements.push(move_stmt);
1178                     statements.push(pattern_stmt);
1179                 };
1180
1181                 parameters.push(new_parameter);
1182             }
1183
1184             let body_span = body.map_or(span, |b| b.span);
1185             let async_expr = this.make_async_expr(
1186                 CaptureBy::Value,
1187                 closure_id,
1188                 None,
1189                 body_span,
1190                 hir::AsyncGeneratorKind::Fn,
1191                 |this| {
1192                     // Create a block from the user's function body:
1193                     let user_body = this.lower_block_expr_opt(body_span, body);
1194
1195                     // Transform into `drop-temps { <user-body> }`, an expression:
1196                     let desugared_span =
1197                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1198                     let user_body = this.expr_drop_temps(
1199                         desugared_span,
1200                         this.arena.alloc(user_body),
1201                         AttrVec::new(),
1202                     );
1203
1204                     // As noted above, create the final block like
1205                     //
1206                     // ```
1207                     // {
1208                     //   let $param_pattern = $raw_param;
1209                     //   ...
1210                     //   drop-temps { <user-body> }
1211                     // }
1212                     // ```
1213                     let body = this.block_all(
1214                         desugared_span,
1215                         this.arena.alloc_from_iter(statements),
1216                         Some(user_body),
1217                     );
1218
1219                     this.expr_block(body, AttrVec::new())
1220                 },
1221             );
1222
1223             (
1224                 this.arena.alloc_from_iter(parameters),
1225                 this.expr(body_span, async_expr, AttrVec::new()),
1226             )
1227         })
1228     }
1229
1230     fn lower_method_sig(
1231         &mut self,
1232         generics: &Generics,
1233         sig: &FnSig,
1234         fn_def_id: DefId,
1235         impl_trait_return_allow: bool,
1236         is_async: Option<NodeId>,
1237     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1238         let header = self.lower_fn_header(sig.header);
1239         let (generics, decl) = self.add_in_band_defs(
1240             generics,
1241             fn_def_id,
1242             AnonymousLifetimeMode::PassThrough,
1243             |this, idty| {
1244                 this.lower_fn_decl(
1245                     &sig.decl,
1246                     Some((fn_def_id, idty)),
1247                     impl_trait_return_allow,
1248                     is_async,
1249                 )
1250             },
1251         );
1252         (generics, hir::FnSig { header, decl })
1253     }
1254
1255     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1256         hir::FnHeader {
1257             unsafety: self.lower_unsafety(h.unsafety),
1258             asyncness: self.lower_asyncness(h.asyncness),
1259             constness: self.lower_constness(h.constness),
1260             abi: self.lower_extern(h.ext),
1261         }
1262     }
1263
1264     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1265         abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1266             self.error_on_invalid_abi(abi);
1267             abi::Abi::Rust
1268         })
1269     }
1270
1271     pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
1272         match ext {
1273             Extern::None => abi::Abi::Rust,
1274             Extern::Implicit => abi::Abi::C,
1275             Extern::Explicit(abi) => self.lower_abi(abi),
1276         }
1277     }
1278
1279     fn error_on_invalid_abi(&self, abi: StrLit) {
1280         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1281             .span_label(abi.span, "invalid ABI")
1282             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1283             .emit();
1284     }
1285
1286     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1287         match a {
1288             Async::Yes { .. } => hir::IsAsync::Async,
1289             Async::No => hir::IsAsync::NotAsync,
1290         }
1291     }
1292
1293     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1294         match c {
1295             Const::Yes(_) => hir::Constness::Const,
1296             Const::No => hir::Constness::NotConst,
1297         }
1298     }
1299
1300     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1301         match u {
1302             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1303             Unsafe::No => hir::Unsafety::Normal,
1304         }
1305     }
1306
1307     pub(super) fn lower_generics_mut(
1308         &mut self,
1309         generics: &Generics,
1310         itctx: ImplTraitContext<'_, 'hir>,
1311     ) -> GenericsCtor<'hir> {
1312         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1313         // FIXME: this could probably be done with less rightward drift. It also looks like two
1314         // control paths where `report_error` is called are the only paths that advance to after the
1315         // match statement, so the error reporting could probably just be moved there.
1316         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1317         for pred in &generics.where_clause.predicates {
1318             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1319                 'next_bound: for bound in &bound_pred.bounds {
1320                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1321                         let report_error = |this: &mut Self| {
1322                             this.diagnostic().span_err(
1323                                 bound_pred.bounded_ty.span,
1324                                 "`?Trait` bounds are only permitted at the \
1325                                  point where a type parameter is declared",
1326                             );
1327                         };
1328                         // Check if the where clause type is a plain type parameter.
1329                         match bound_pred.bounded_ty.kind {
1330                             TyKind::Path(None, ref path)
1331                                 if path.segments.len() == 1
1332                                     && bound_pred.bound_generic_params.is_empty() =>
1333                             {
1334                                 if let Some(Res::Def(DefKind::TyParam, def_id)) = self
1335                                     .resolver
1336                                     .get_partial_res(bound_pred.bounded_ty.id)
1337                                     .map(|d| d.base_res())
1338                                 {
1339                                     if let Some(node_id) =
1340                                         self.resolver.definitions().as_local_node_id(def_id)
1341                                     {
1342                                         for param in &generics.params {
1343                                             match param.kind {
1344                                                 GenericParamKind::Type { .. } => {
1345                                                     if node_id == param.id {
1346                                                         add_bounds
1347                                                             .entry(param.id)
1348                                                             .or_default()
1349                                                             .push(bound.clone());
1350                                                         continue 'next_bound;
1351                                                     }
1352                                                 }
1353                                                 _ => {}
1354                                             }
1355                                         }
1356                                     }
1357                                 }
1358                                 report_error(self)
1359                             }
1360                             _ => report_error(self),
1361                         }
1362                     }
1363                 }
1364             }
1365         }
1366
1367         GenericsCtor {
1368             params: self.lower_generic_params_mut(&generics.params, &add_bounds, itctx).collect(),
1369             where_clause: self.lower_where_clause(&generics.where_clause),
1370             span: generics.span,
1371         }
1372     }
1373
1374     pub(super) fn lower_generics(
1375         &mut self,
1376         generics: &Generics,
1377         itctx: ImplTraitContext<'_, 'hir>,
1378     ) -> hir::Generics<'hir> {
1379         let generics_ctor = self.lower_generics_mut(generics, itctx);
1380         generics_ctor.into_generics(self.arena)
1381     }
1382
1383     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1384         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1385             hir::WhereClause {
1386                 predicates: this.arena.alloc_from_iter(
1387                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1388                 ),
1389                 span: wc.span,
1390             }
1391         })
1392     }
1393
1394     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1395         match *pred {
1396             WherePredicate::BoundPredicate(WhereBoundPredicate {
1397                 ref bound_generic_params,
1398                 ref bounded_ty,
1399                 ref bounds,
1400                 span,
1401             }) => {
1402                 self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1403                     hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1404                         bound_generic_params: this.lower_generic_params(
1405                             bound_generic_params,
1406                             &NodeMap::default(),
1407                             ImplTraitContext::disallowed(),
1408                         ),
1409                         bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1410                         bounds: this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| {
1411                             match *bound {
1412                                 // Ignore `?Trait` bounds.
1413                                 // They were copied into type parameters already.
1414                                 GenericBound::Trait(_, TraitBoundModifier::Maybe) => None,
1415                                 _ => Some(
1416                                     this.lower_param_bound(bound, ImplTraitContext::disallowed()),
1417                                 ),
1418                             }
1419                         })),
1420                         span,
1421                     })
1422                 })
1423             }
1424             WherePredicate::RegionPredicate(WhereRegionPredicate {
1425                 ref lifetime,
1426                 ref bounds,
1427                 span,
1428             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1429                 span,
1430                 lifetime: self.lower_lifetime(lifetime),
1431                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1432             }),
1433             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1434                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1435                     hir_id: self.lower_node_id(id),
1436                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1437                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1438                     span,
1439                 })
1440             }
1441         }
1442     }
1443 }
1444
1445 /// Helper struct for delayed construction of Generics.
1446 pub(super) struct GenericsCtor<'hir> {
1447     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1448     where_clause: hir::WhereClause<'hir>,
1449     span: Span,
1450 }
1451
1452 impl<'hir> GenericsCtor<'hir> {
1453     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1454         hir::Generics {
1455             params: arena.alloc_from_iter(self.params),
1456             where_clause: self.where_clause,
1457             span: self.span,
1458         }
1459     }
1460 }