src/librustc_ast_lowering/item.rs

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