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