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