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::TyAlias(..) => hir::ForeignItemKind::Type,
 679                 ForeignItemKind::Macro(_) => panic!("macro shouldn't exist here"),
 680             },
 681             vis: self.lower_visibility(&i.vis, None),
 682             span: i.span,
 683         }
 684     }
 685
 686     fn lower_foreign_mod(&mut self, fm: &ForeignMod) -> hir::ForeignMod<'hir> {
 687         hir::ForeignMod {
 688             abi: fm.abi.map_or(abi::Abi::C, |abi| self.lower_abi(abi)),
 689             items: self.arena.alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item(x))),
 690         }
 691     }
 692
 693     fn lower_global_asm(&mut self, ga: &GlobalAsm) -> &'hir hir::GlobalAsm {
 694         self.arena.alloc(hir::GlobalAsm { asm: ga.asm })
 695     }
 696
 697     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 698         hir::Variant {
 699             attrs: self.lower_attrs(&v.attrs),
 700             data: self.lower_variant_data(&v.data),
 701             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 702             id: self.lower_node_id(v.id),
 703             ident: v.ident,
 704             span: v.span,
 705         }
 706     }
 707
 708     fn lower_variant_data(&mut self, vdata: &VariantData) -> hir::VariantData<'hir> {
 709         match *vdata {
 710             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 711                 self.arena
 712                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 713                 recovered,
 714             ),
 715             VariantData::Tuple(ref fields, id) => hir::VariantData::Tuple(
 716                 self.arena
 717                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 718                 self.lower_node_id(id),
 719             ),
 720             VariantData::Unit(id) => hir::VariantData::Unit(self.lower_node_id(id)),
 721         }
 722     }
 723
 724     fn lower_struct_field(&mut self, (index, f): (usize, &StructField)) -> hir::StructField<'hir> {
 725         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 726             let t = self.lower_path_ty(
 727                 &f.ty,
 728                 qself,
 729                 path,
 730                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 731                 ImplTraitContext::disallowed(),
 732             );
 733             self.arena.alloc(t)
 734         } else {
 735             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 736         };
 737         hir::StructField {
 738             span: f.span,
 739             hir_id: self.lower_node_id(f.id),
 740             ident: match f.ident {
 741                 Some(ident) => ident,
 742                 // FIXME(jseyfried): positional field hygiene.
 743                 None => Ident::new(sym::integer(index), f.span),
 744             },
 745             vis: self.lower_visibility(&f.vis, None),
 746             ty,
 747             attrs: self.lower_attrs(&f.attrs),
 748         }
 749     }
 750
 751     fn lower_trait_item(&mut self, i: &AssocItem) -> hir::TraitItem<'hir> {
 752         let trait_item_def_id = self.resolver.definitions().local_def_id(i.id);
 753
 754         let (generics, kind) = match i.kind {
 755             AssocItemKind::Const(_, ref ty, ref default) => {
 756                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 757                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 758                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
 759             }
 760             AssocItemKind::Fn(_, ref sig, ref generics, None) => {
 761                 let names = self.lower_fn_params_to_names(&sig.decl);
 762                 let (generics, sig) =
 763                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
 764                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitMethod::Required(names)))
 765             }
 766             AssocItemKind::Fn(_, ref sig, ref generics, Some(ref body)) => {
 767                 let body_id = self.lower_fn_body_block(i.span, &sig.decl, Some(body));
 768                 let (generics, sig) =
 769                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
 770                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitMethod::Provided(body_id)))
 771             }
 772             AssocItemKind::TyAlias(_, ref generics, ref bounds, ref default) => {
 773                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
 774                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 775                 let kind = hir::TraitItemKind::Type(
 776                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 777                     ty,
 778                 );
 779
 780                 (generics, kind)
 781             }
 782             AssocItemKind::Macro(..) => bug!("macro item shouldn't exist at this point"),
 783         };
 784
 785         hir::TraitItem {
 786             hir_id: self.lower_node_id(i.id),
 787             ident: i.ident,
 788             attrs: self.lower_attrs(&i.attrs),
 789             generics,
 790             kind,
 791             span: i.span,
 792         }
 793     }
 794
 795     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 796         let (kind, has_default) = match &i.kind {
 797             AssocItemKind::Const(_, _, default) => (hir::AssocItemKind::Const, default.is_some()),
 798             AssocItemKind::TyAlias(_, _, _, default) => {
 799                 (hir::AssocItemKind::Type, default.is_some())
 800             }
 801             AssocItemKind::Fn(_, sig, _, default) => {
 802                 (hir::AssocItemKind::Method { has_self: sig.decl.has_self() }, default.is_some())
 803             }
 804             AssocItemKind::Macro(..) => unimplemented!(),
 805         };
 806         let id = hir::TraitItemId { hir_id: self.lower_node_id(i.id) };
 807         let defaultness = hir::Defaultness::Default { has_value: has_default };
 808         hir::TraitItemRef { id, ident: i.ident, span: i.span, defaultness, kind }
 809     }
 810
 811     /// Construct `ExprKind::Err` for the given `span`.
 812     fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 813         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 814     }
 815
 816     fn lower_impl_item(&mut self, i: &AssocItem) -> hir::ImplItem<'hir> {
 817         let impl_item_def_id = self.resolver.definitions().local_def_id(i.id);
 818
 819         let (generics, kind) = match &i.kind {
 820             AssocItemKind::Const(_, ty, expr) => {
 821                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 822                 (
 823                     hir::Generics::empty(),
 824                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 825                 )
 826             }
 827             AssocItemKind::Fn(_, sig, generics, body) => {
 828                 self.current_item = Some(i.span);
 829                 let asyncness = sig.header.asyncness;
 830                 let body_id =
 831                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 832                 let impl_trait_return_allow = !self.is_in_trait_impl;
 833                 let (generics, sig) = self.lower_method_sig(
 834                     generics,
 835                     sig,
 836                     impl_item_def_id,
 837                     impl_trait_return_allow,
 838                     asyncness.opt_return_id(),
 839                 );
 840
 841                 (generics, hir::ImplItemKind::Method(sig, body_id))
 842             }
 843             AssocItemKind::TyAlias(_, generics, _, ty) => {
 844                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 845                 let kind = match ty {
 846                     None => {
 847                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 848                         hir::ImplItemKind::TyAlias(ty)
 849                     }
 850                     Some(ty) => match ty.kind.opaque_top_hack() {
 851                         None => {
 852                             let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 853                             hir::ImplItemKind::TyAlias(ty)
 854                         }
 855                         Some(bs) => {
 856                             let bs = self.lower_param_bounds(bs, ImplTraitContext::disallowed());
 857                             hir::ImplItemKind::OpaqueTy(bs)
 858                         }
 859                     },
 860                 };
 861                 (generics, kind)
 862             }
 863             AssocItemKind::Macro(..) => bug!("`TyMac` should have been expanded by now"),
 864         };
 865
 866         hir::ImplItem {
 867             hir_id: self.lower_node_id(i.id),
 868             ident: i.ident,
 869             attrs: self.lower_attrs(&i.attrs),
 870             generics,
 871             vis: self.lower_visibility(&i.vis, None),
 872             defaultness: self.lower_defaultness(i.kind.defaultness(), true /* [1] */),
 873             kind,
 874             span: i.span,
 875         }
 876
 877         // [1] since `default impl` is not yet implemented, this is always true in impls
 878     }
 879
 880     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef<'hir> {
 881         hir::ImplItemRef {
 882             id: hir::ImplItemId { hir_id: self.lower_node_id(i.id) },
 883             ident: i.ident,
 884             span: i.span,
 885             vis: self.lower_visibility(&i.vis, Some(i.id)),
 886             defaultness: self.lower_defaultness(i.kind.defaultness(), true /* [1] */),
 887             kind: match &i.kind {
 888                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 889                 AssocItemKind::TyAlias(.., ty) => {
 890                     match ty.as_deref().and_then(|ty| ty.kind.opaque_top_hack()) {
 891                         None => hir::AssocItemKind::Type,
 892                         Some(_) => hir::AssocItemKind::OpaqueTy,
 893                     }
 894                 }
 895                 AssocItemKind::Fn(_, sig, ..) => {
 896                     hir::AssocItemKind::Method { has_self: sig.decl.has_self() }
 897                 }
 898                 AssocItemKind::Macro(..) => unimplemented!(),
 899             },
 900         }
 901
 902         // [1] since `default impl` is not yet implemented, this is always true in impls
 903     }
 904
 905     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 906     /// the address space of that item instead of the item currently being
 907     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 908     /// lower a `Visibility` value although we haven't lowered the owning
 909     /// `ImplItem` in question yet.
 910     fn lower_visibility(
 911         &mut self,
 912         v: &Visibility,
 913         explicit_owner: Option<NodeId>,
 914     ) -> hir::Visibility<'hir> {
 915         let node = match v.node {
 916             VisibilityKind::Public => hir::VisibilityKind::Public,
 917             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 918             VisibilityKind::Restricted { ref path, id } => {
 919                 debug!("lower_visibility: restricted path id = {:?}", id);
 920                 let lowered_id = if let Some(owner) = explicit_owner {
 921                     self.lower_node_id_with_owner(id, owner)
 922                 } else {
 923                     self.lower_node_id(id)
 924                 };
 925                 let res = self.expect_full_res(id);
 926                 let res = self.lower_res(res);
 927                 hir::VisibilityKind::Restricted {
 928                     path: self.lower_path_extra(res, path, ParamMode::Explicit, explicit_owner),
 929                     hir_id: lowered_id,
 930                 }
 931             }
 932             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 933         };
 934         respan(v.span, node)
 935     }
 936
 937     fn lower_defaultness(&self, d: Defaultness, has_value: bool) -> hir::Defaultness {
 938         match d {
 939             Defaultness::Default(_) => hir::Defaultness::Default { has_value },
 940             Defaultness::Final => {
 941                 assert!(has_value);
 942                 hir::Defaultness::Final
 943             }
 944         }
 945     }
 946
 947     fn record_body(
 948         &mut self,
 949         params: &'hir [hir::Param<'hir>],
 950         value: hir::Expr<'hir>,
 951     ) -> hir::BodyId {
 952         let body = hir::Body { generator_kind: self.generator_kind, params, value };
 953         let id = body.id();
 954         self.bodies.insert(id, body);
 955         id
 956     }
 957
 958     fn lower_body(
 959         &mut self,
 960         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
 961     ) -> hir::BodyId {
 962         let prev_gen_kind = self.generator_kind.take();
 963         let (parameters, result) = f(self);
 964         let body_id = self.record_body(parameters, result);
 965         self.generator_kind = prev_gen_kind;
 966         body_id
 967     }
 968
 969     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
 970         hir::Param {
 971             attrs: self.lower_attrs(&param.attrs),
 972             hir_id: self.lower_node_id(param.id),
 973             pat: self.lower_pat(&param.pat),
 974             span: param.span,
 975         }
 976     }
 977
 978     pub(super) fn lower_fn_body(
 979         &mut self,
 980         decl: &FnDecl,
 981         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 982     ) -> hir::BodyId {
 983         self.lower_body(|this| {
 984             (
 985                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
 986                 body(this),
 987             )
 988         })
 989     }
 990
 991     fn lower_fn_body_block(
 992         &mut self,
 993         span: Span,
 994         decl: &FnDecl,
 995         body: Option<&Block>,
 996     ) -> hir::BodyId {
 997         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
 998     }
 999
1000     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1001         match block {
1002             Some(block) => self.lower_block_expr(block),
1003             None => self.expr_err(span),
1004         }
1005     }
1006
1007     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1008         self.lower_body(|this| {
1009             (
1010                 &[],
1011                 match expr {
1012                     Some(expr) => this.lower_expr_mut(expr),
1013                     None => this.expr_err(span),
1014                 },
1015             )
1016         })
1017     }
1018
1019     fn lower_maybe_async_body(
1020         &mut self,
1021         span: Span,
1022         decl: &FnDecl,
1023         asyncness: Async,
1024         body: Option<&Block>,
1025     ) -> hir::BodyId {
1026         let closure_id = match asyncness {
1027             Async::Yes { closure_id, .. } => closure_id,
1028             Async::No => return self.lower_fn_body_block(span, decl, body),
1029         };
1030
1031         self.lower_body(|this| {
1032             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1033             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1034
1035             // Async function parameters are lowered into the closure body so that they are
1036             // captured and so that the drop order matches the equivalent non-async functions.
1037             //
1038             // from:
1039             //
1040             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1041             //         <body>
1042             //     }
1043             //
1044             // into:
1045             //
1046             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1047             //       async move {
1048             //         let __arg2 = __arg2;
1049             //         let <pattern> = __arg2;
1050             //         let __arg1 = __arg1;
1051             //         let <pattern> = __arg1;
1052             //         let __arg0 = __arg0;
1053             //         let <pattern> = __arg0;
1054             //         drop-temps { <body> } // see comments later in fn for details
1055             //       }
1056             //     }
1057             //
1058             // If `<pattern>` is a simple ident, then it is lowered to a single
1059             // `let <pattern> = <pattern>;` statement as an optimization.
1060             //
1061             // Note that the body is embedded in `drop-temps`; an
1062             // equivalent desugaring would be `return { <body>
1063             // };`. The key point is that we wish to drop all the
1064             // let-bound variables and temporaries created in the body
1065             // (and its tail expression!) before we drop the
1066             // parameters (c.f. rust-lang/rust#64512).
1067             for (index, parameter) in decl.inputs.iter().enumerate() {
1068                 let parameter = this.lower_param(parameter);
1069                 let span = parameter.pat.span;
1070
1071                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1072                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1073                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1074                     hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, ident, _) => {
1075                         (ident, true)
1076                     }
1077                     _ => {
1078                         // Replace the ident for bindings that aren't simple.
1079                         let name = format!("__arg{}", index);
1080                         let ident = Ident::from_str(&name);
1081
1082                         (ident, false)
1083                     }
1084                 };
1085
1086                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1087
1088                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1089                 // async function.
1090                 //
1091                 // If this is the simple case, this parameter will end up being the same as the
1092                 // original parameter, but with a different pattern id.
1093                 let mut stmt_attrs = AttrVec::new();
1094                 stmt_attrs.extend(parameter.attrs.iter().cloned());
1095                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1096                 let new_parameter = hir::Param {
1097                     attrs: parameter.attrs,
1098                     hir_id: parameter.hir_id,
1099                     pat: new_parameter_pat,
1100                     span: parameter.span,
1101                 };
1102
1103                 if is_simple_parameter {
1104                     // If this is the simple case, then we only insert one statement that is
1105                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1106                     // `HirId`s are densely assigned.
1107                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1108                     let stmt = this.stmt_let_pat(
1109                         stmt_attrs,
1110                         desugared_span,
1111                         Some(expr),
1112                         parameter.pat,
1113                         hir::LocalSource::AsyncFn,
1114                     );
1115                     statements.push(stmt);
1116                 } else {
1117                     // If this is not the simple case, then we construct two statements:
1118                     //
1119                     // ```
1120                     // let __argN = __argN;
1121                     // let <pat> = __argN;
1122                     // ```
1123                     //
1124                     // The first statement moves the parameter into the closure and thus ensures
1125                     // that the drop order is correct.
1126                     //
1127                     // The second statement creates the bindings that the user wrote.
1128
1129                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1130                     // because the user may have specified a `ref mut` binding in the next
1131                     // statement.
1132                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1133                         desugared_span,
1134                         ident,
1135                         hir::BindingAnnotation::Mutable,
1136                     );
1137                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1138                     let move_stmt = this.stmt_let_pat(
1139                         AttrVec::new(),
1140                         desugared_span,
1141                         Some(move_expr),
1142                         move_pat,
1143                         hir::LocalSource::AsyncFn,
1144                     );
1145
1146                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1147                     // parameter's pattern so that `HirId`s are densely assigned.
1148                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1149                     let pattern_stmt = this.stmt_let_pat(
1150                         stmt_attrs,
1151                         desugared_span,
1152                         Some(pattern_expr),
1153                         parameter.pat,
1154                         hir::LocalSource::AsyncFn,
1155                     );
1156
1157                     statements.push(move_stmt);
1158                     statements.push(pattern_stmt);
1159                 };
1160
1161                 parameters.push(new_parameter);
1162             }
1163
1164             let body_span = body.map_or(span, |b| b.span);
1165             let async_expr = this.make_async_expr(
1166                 CaptureBy::Value,
1167                 closure_id,
1168                 None,
1169                 body_span,
1170                 hir::AsyncGeneratorKind::Fn,
1171                 |this| {
1172                     // Create a block from the user's function body:
1173                     let user_body = this.lower_block_expr_opt(body_span, body);
1174
1175                     // Transform into `drop-temps { <user-body> }`, an expression:
1176                     let desugared_span =
1177                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1178                     let user_body = this.expr_drop_temps(
1179                         desugared_span,
1180                         this.arena.alloc(user_body),
1181                         AttrVec::new(),
1182                     );
1183
1184                     // As noted above, create the final block like
1185                     //
1186                     // ```
1187                     // {
1188                     //   let $param_pattern = $raw_param;
1189                     //   ...
1190                     //   drop-temps { <user-body> }
1191                     // }
1192                     // ```
1193                     let body = this.block_all(
1194                         desugared_span,
1195                         this.arena.alloc_from_iter(statements),
1196                         Some(user_body),
1197                     );
1198
1199                     this.expr_block(body, AttrVec::new())
1200                 },
1201             );
1202
1203             (
1204                 this.arena.alloc_from_iter(parameters),
1205                 this.expr(body_span, async_expr, AttrVec::new()),
1206             )
1207         })
1208     }
1209
1210     fn lower_method_sig(
1211         &mut self,
1212         generics: &Generics,
1213         sig: &FnSig,
1214         fn_def_id: DefId,
1215         impl_trait_return_allow: bool,
1216         is_async: Option<NodeId>,
1217     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1218         let header = self.lower_fn_header(sig.header);
1219         let (generics, decl) = self.add_in_band_defs(
1220             generics,
1221             fn_def_id,
1222             AnonymousLifetimeMode::PassThrough,
1223             |this, idty| {
1224                 this.lower_fn_decl(
1225                     &sig.decl,
1226                     Some((fn_def_id, idty)),
1227                     impl_trait_return_allow,
1228                     is_async,
1229                 )
1230             },
1231         );
1232         (generics, hir::FnSig { header, decl })
1233     }
1234
1235     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1236         hir::FnHeader {
1237             unsafety: self.lower_unsafety(h.unsafety),
1238             asyncness: self.lower_asyncness(h.asyncness),
1239             constness: self.lower_constness(h.constness),
1240             abi: self.lower_extern(h.ext),
1241         }
1242     }
1243
1244     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1245         abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1246             self.error_on_invalid_abi(abi);
1247             abi::Abi::Rust
1248         })
1249     }
1250
1251     pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
1252         match ext {
1253             Extern::None => abi::Abi::Rust,
1254             Extern::Implicit => abi::Abi::C,
1255             Extern::Explicit(abi) => self.lower_abi(abi),
1256         }
1257     }
1258
1259     fn error_on_invalid_abi(&self, abi: StrLit) {
1260         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1261             .span_label(abi.span, "invalid ABI")
1262             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1263             .emit();
1264     }
1265
1266     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1267         match a {
1268             Async::Yes { .. } => hir::IsAsync::Async,
1269             Async::No => hir::IsAsync::NotAsync,
1270         }
1271     }
1272
1273     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1274         match c {
1275             Const::Yes(_) => hir::Constness::Const,
1276             Const::No => hir::Constness::NotConst,
1277         }
1278     }
1279
1280     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1281         match u {
1282             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1283             Unsafe::No => hir::Unsafety::Normal,
1284         }
1285     }
1286
1287     pub(super) fn lower_generics_mut(
1288         &mut self,
1289         generics: &Generics,
1290         itctx: ImplTraitContext<'_, 'hir>,
1291     ) -> GenericsCtor<'hir> {
1292         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1293         // FIXME: this could probably be done with less rightward drift. It also looks like two
1294         // control paths where `report_error` is called are the only paths that advance to after the
1295         // match statement, so the error reporting could probably just be moved there.
1296         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1297         for pred in &generics.where_clause.predicates {
1298             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1299                 'next_bound: for bound in &bound_pred.bounds {
1300                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1301                         let report_error = |this: &mut Self| {
1302                             this.diagnostic().span_err(
1303                                 bound_pred.bounded_ty.span,
1304                                 "`?Trait` bounds are only permitted at the \
1305                                  point where a type parameter is declared",
1306                             );
1307                         };
1308                         // Check if the where clause type is a plain type parameter.
1309                         match bound_pred.bounded_ty.kind {
1310                             TyKind::Path(None, ref path)
1311                                 if path.segments.len() == 1
1312                                     && bound_pred.bound_generic_params.is_empty() =>
1313                             {
1314                                 if let Some(Res::Def(DefKind::TyParam, def_id)) = self
1315                                     .resolver
1316                                     .get_partial_res(bound_pred.bounded_ty.id)
1317                                     .map(|d| d.base_res())
1318                                 {
1319                                     if let Some(node_id) =
1320                                         self.resolver.definitions().as_local_node_id(def_id)
1321                                     {
1322                                         for param in &generics.params {
1323                                             match param.kind {
1324                                                 GenericParamKind::Type { .. } => {
1325                                                     if node_id == param.id {
1326                                                         add_bounds
1327                                                             .entry(param.id)
1328                                                             .or_default()
1329                                                             .push(bound.clone());
1330                                                         continue 'next_bound;
1331                                                     }
1332                                                 }
1333                                                 _ => {}
1334                                             }
1335                                         }
1336                                     }
1337                                 }
1338                                 report_error(self)
1339                             }
1340                             _ => report_error(self),
1341                         }
1342                     }
1343                 }
1344             }
1345         }
1346
1347         GenericsCtor {
1348             params: self.lower_generic_params_mut(&generics.params, &add_bounds, itctx).collect(),
1349             where_clause: self.lower_where_clause(&generics.where_clause),
1350             span: generics.span,
1351         }
1352     }
1353
1354     pub(super) fn lower_generics(
1355         &mut self,
1356         generics: &Generics,
1357         itctx: ImplTraitContext<'_, 'hir>,
1358     ) -> hir::Generics<'hir> {
1359         let generics_ctor = self.lower_generics_mut(generics, itctx);
1360         generics_ctor.into_generics(self.arena)
1361     }
1362
1363     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1364         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1365             hir::WhereClause {
1366                 predicates: this.arena.alloc_from_iter(
1367                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1368                 ),
1369                 span: wc.span,
1370             }
1371         })
1372     }
1373
1374     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1375         match *pred {
1376             WherePredicate::BoundPredicate(WhereBoundPredicate {
1377                 ref bound_generic_params,
1378                 ref bounded_ty,
1379                 ref bounds,
1380                 span,
1381             }) => {
1382                 self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1383                     hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1384                         bound_generic_params: this.lower_generic_params(
1385                             bound_generic_params,
1386                             &NodeMap::default(),
1387                             ImplTraitContext::disallowed(),
1388                         ),
1389                         bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1390                         bounds: this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| {
1391                             match *bound {
1392                                 // Ignore `?Trait` bounds.
1393                                 // They were copied into type parameters already.
1394                                 GenericBound::Trait(_, TraitBoundModifier::Maybe) => None,
1395                                 _ => Some(
1396                                     this.lower_param_bound(bound, ImplTraitContext::disallowed()),
1397                                 ),
1398                             }
1399                         })),
1400                         span,
1401                     })
1402                 })
1403             }
1404             WherePredicate::RegionPredicate(WhereRegionPredicate {
1405                 ref lifetime,
1406                 ref bounds,
1407                 span,
1408             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1409                 span,
1410                 lifetime: self.lower_lifetime(lifetime),
1411                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1412             }),
1413             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1414                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1415                     hir_id: self.lower_node_id(id),
1416                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1417                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1418                     span,
1419                 })
1420             }
1421         }
1422     }
1423 }
1424
1425 /// Helper struct for delayed construction of Generics.
1426 pub(super) struct GenericsCtor<'hir> {
1427     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1428     where_clause: hir::WhereClause<'hir>,
1429     span: Span,
1430 }
1431
1432 impl<'hir> GenericsCtor<'hir> {
1433     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1434         hir::Generics {
1435             params: arena.alloc_from_iter(self.params),
1436             where_clause: self.where_clause,
1437             span: self.span,
1438         }
1439     }
1440 }