src/librustc/hir/lowering/item.rs

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