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