compiler/rustc_ast_lowering/src/item.rs

   1 use super::{AnonymousLifetimeMode, LoweringContext, ParamMode};
   2 use super::{ImplTraitContext, ImplTraitPosition};
   3 use crate::Arena;
   4
   5 use rustc_ast::node_id::NodeMap;
   6 use rustc_ast::ptr::P;
   7 use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
   8 use rustc_ast::*;
   9 use rustc_data_structures::fx::FxHashSet;
  10 use rustc_errors::struct_span_err;
  11 use rustc_hir as hir;
  12 use rustc_hir::def::{DefKind, Res};
  13 use rustc_hir::def_id::LocalDefId;
  14 use rustc_span::source_map::{respan, DesugaringKind};
  15 use rustc_span::symbol::{kw, sym, Ident};
  16 use rustc_span::Span;
  17 use rustc_target::spec::abi;
  18 use smallvec::{smallvec, SmallVec};
  19 use tracing::debug;
  20
  21 use std::mem;
  22
  23 pub(super) struct ItemLowerer<'a, 'lowering, 'hir> {
  24     pub(super) lctx: &'a mut LoweringContext<'lowering, 'hir>,
  25 }
  26
  27 impl ItemLowerer<'_, '_, '_> {
  28     fn with_trait_impl_ref(&mut self, impl_ref: &Option<TraitRef>, f: impl FnOnce(&mut Self)) {
  29         let old = self.lctx.is_in_trait_impl;
  30         self.lctx.is_in_trait_impl = impl_ref.is_some();
  31         f(self);
  32         self.lctx.is_in_trait_impl = old;
  33     }
  34 }
  35
  36 impl<'a> Visitor<'a> for ItemLowerer<'a, '_, '_> {
  37     fn visit_item(&mut self, item: &'a Item) {
  38         let mut item_hir_id = None;
  39         self.lctx.with_hir_id_owner(item.id, |lctx| {
  40             lctx.without_in_scope_lifetime_defs(|lctx| {
  41                 if let Some(hir_item) = lctx.lower_item(item) {
  42                     let id = lctx.insert_item(hir_item);
  43                     item_hir_id = Some(id);
  44                 }
  45             })
  46         });
  47
  48         if let Some(hir_id) = item_hir_id {
  49             self.lctx.with_parent_item_lifetime_defs(hir_id, |this| {
  50                 let this = &mut ItemLowerer { lctx: this };
  51                 match item.kind {
  52                     ItemKind::Mod(..) => {
  53                         let def_id = this.lctx.lower_node_id(item.id).expect_owner();
  54                         let old_current_module =
  55                             mem::replace(&mut this.lctx.current_module, def_id);
  56                         visit::walk_item(this, item);
  57                         this.lctx.current_module = old_current_module;
  58                     }
  59                     ItemKind::Impl(box ImplKind { ref of_trait, .. }) => {
  60                         this.with_trait_impl_ref(of_trait, |this| visit::walk_item(this, item));
  61                     }
  62                     _ => visit::walk_item(this, item),
  63                 }
  64             });
  65         }
  66     }
  67
  68     fn visit_fn(&mut self, fk: FnKind<'a>, sp: Span, _: NodeId) {
  69         match fk {
  70             FnKind::Fn(FnCtxt::Foreign, _, sig, _, _) => {
  71                 self.visit_fn_header(&sig.header);
  72                 visit::walk_fn_decl(self, &sig.decl);
  73                 // Don't visit the foreign function body even if it has one, since lowering the
  74                 // body would have no meaning and will have already been caught as a parse error.
  75             }
  76             _ => visit::walk_fn(self, fk, sp),
  77         }
  78     }
  79
  80     fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
  81         self.lctx.with_hir_id_owner(item.id, |lctx| match ctxt {
  82             AssocCtxt::Trait => {
  83                 let hir_item = lctx.lower_trait_item(item);
  84                 let id = hir_item.trait_item_id();
  85                 lctx.trait_items.insert(id, hir_item);
  86                 lctx.modules.entry(lctx.current_module).or_default().trait_items.insert(id);
  87             }
  88             AssocCtxt::Impl => {
  89                 let hir_item = lctx.lower_impl_item(item);
  90                 let id = hir_item.impl_item_id();
  91                 lctx.impl_items.insert(id, hir_item);
  92                 lctx.modules.entry(lctx.current_module).or_default().impl_items.insert(id);
  93             }
  94         });
  95
  96         visit::walk_assoc_item(self, item, ctxt);
  97     }
  98
  99     fn visit_foreign_item(&mut self, item: &'a ForeignItem) {
 100         self.lctx.allocate_hir_id_counter(item.id);
 101         self.lctx.with_hir_id_owner(item.id, |lctx| {
 102             let hir_item = lctx.lower_foreign_item(item);
 103             let id = hir_item.foreign_item_id();
 104             lctx.foreign_items.insert(id, hir_item);
 105             lctx.modules.entry(lctx.current_module).or_default().foreign_items.insert(id);
 106         });
 107
 108         visit::walk_foreign_item(self, item);
 109     }
 110 }
 111
 112 impl<'hir> LoweringContext<'_, 'hir> {
 113     // Same as the method above, but accepts `hir::GenericParam`s
 114     // instead of `ast::GenericParam`s.
 115     // This should only be used with generics that have already had their
 116     // in-band lifetimes added. In practice, this means that this function is
 117     // only used when lowering a child item of a trait or impl.
 118     fn with_parent_item_lifetime_defs<T>(
 119         &mut self,
 120         parent_hir_id: hir::ItemId,
 121         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 122     ) -> T {
 123         let old_len = self.in_scope_lifetimes.len();
 124
 125         let parent_generics = match self.items.get(&parent_hir_id).unwrap().kind {
 126             hir::ItemKind::Impl(hir::Impl { ref generics, .. })
 127             | hir::ItemKind::Trait(_, _, ref generics, ..) => generics.params,
 128             _ => &[],
 129         };
 130         let lt_def_names = parent_generics.iter().filter_map(|param| match param.kind {
 131             hir::GenericParamKind::Lifetime { .. } => Some(param.name.normalize_to_macros_2_0()),
 132             _ => None,
 133         });
 134         self.in_scope_lifetimes.extend(lt_def_names);
 135
 136         let res = f(self);
 137
 138         self.in_scope_lifetimes.truncate(old_len);
 139         res
 140     }
 141
 142     // Clears (and restores) the `in_scope_lifetimes` field. Used when
 143     // visiting nested items, which never inherit in-scope lifetimes
 144     // from their surrounding environment.
 145     fn without_in_scope_lifetime_defs<T>(
 146         &mut self,
 147         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 148     ) -> T {
 149         let old_in_scope_lifetimes = mem::replace(&mut self.in_scope_lifetimes, vec![]);
 150
 151         // this vector is only used when walking over impl headers,
 152         // input types, and the like, and should not be non-empty in
 153         // between items
 154         assert!(self.lifetimes_to_define.is_empty());
 155
 156         let res = f(self);
 157
 158         assert!(self.in_scope_lifetimes.is_empty());
 159         self.in_scope_lifetimes = old_in_scope_lifetimes;
 160
 161         res
 162     }
 163
 164     pub(super) fn lower_mod(&mut self, items: &[P<Item>], inner: Span) -> hir::Mod<'hir> {
 165         hir::Mod {
 166             inner,
 167             item_ids: self.arena.alloc_from_iter(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(box ImplKind { of_trait: None, .. }) => {
 180                 smallvec![i.id]
 181             }
 182             _ => smallvec![i.id],
 183         };
 184
 185         node_ids
 186             .into_iter()
 187             .map(|node_id| hir::ItemId {
 188                 def_id: self.allocate_hir_id_counter(node_id).expect_owner(),
 189             })
 190             .collect()
 191     }
 192
 193     fn lower_item_id_use_tree(
 194         &mut self,
 195         tree: &UseTree,
 196         base_id: NodeId,
 197         vec: &mut SmallVec<[NodeId; 1]>,
 198     ) {
 199         match tree.kind {
 200             UseTreeKind::Nested(ref nested_vec) => {
 201                 for &(ref nested, id) in nested_vec {
 202                     vec.push(id);
 203                     self.lower_item_id_use_tree(nested, id, vec);
 204                 }
 205             }
 206             UseTreeKind::Glob => {}
 207             UseTreeKind::Simple(_, id1, id2) => {
 208                 for (_, &id) in
 209                     self.expect_full_res_from_use(base_id).skip(1).zip([id1, id2].iter())
 210                 {
 211                     vec.push(id);
 212                 }
 213             }
 214         }
 215     }
 216
 217     pub fn lower_item(&mut self, i: &Item) -> Option<hir::Item<'hir>> {
 218         let mut ident = i.ident;
 219         let mut vis = self.lower_visibility(&i.vis, None);
 220
 221         if let ItemKind::MacroDef(MacroDef { ref body, macro_rules }) = i.kind {
 222             if !macro_rules || self.sess.contains_name(&i.attrs, sym::macro_export) {
 223                 let hir_id = self.lower_node_id(i.id);
 224                 self.lower_attrs(hir_id, &i.attrs);
 225                 let body = P(self.lower_mac_args(body));
 226                 self.exported_macros.push(hir::MacroDef {
 227                     ident,
 228                     vis,
 229                     def_id: hir_id.expect_owner(),
 230                     span: i.span,
 231                     ast: MacroDef { body, macro_rules },
 232                 });
 233             } else {
 234                 for a in i.attrs.iter() {
 235                     let a = self.lower_attr(a);
 236                     self.non_exported_macro_attrs.push(a);
 237                 }
 238             }
 239             return None;
 240         }
 241
 242         let hir_id = self.lower_node_id(i.id);
 243         let attrs = self.lower_attrs(hir_id, &i.attrs);
 244         let kind = self.lower_item_kind(i.span, i.id, hir_id, &mut ident, attrs, &mut vis, &i.kind);
 245         Some(hir::Item { def_id: hir_id.expect_owner(), ident, kind, vis, span: i.span })
 246     }
 247
 248     fn lower_item_kind(
 249         &mut self,
 250         span: Span,
 251         id: NodeId,
 252         hir_id: hir::HirId,
 253         ident: &mut Ident,
 254         attrs: Option<&'hir [Attribute]>,
 255         vis: &mut hir::Visibility<'hir>,
 256         i: &ItemKind,
 257     ) -> hir::ItemKind<'hir> {
 258         match *i {
 259             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
 260             ItemKind::Use(ref use_tree) => {
 261                 // Start with an empty prefix.
 262                 let prefix = Path { segments: vec![], span: use_tree.span, tokens: None };
 263
 264                 self.lower_use_tree(use_tree, &prefix, id, vis, ident, attrs)
 265             }
 266             ItemKind::Static(ref t, m, ref e) => {
 267                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 268                 hir::ItemKind::Static(ty, m, body_id)
 269             }
 270             ItemKind::Const(_, ref t, ref e) => {
 271                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 272                 hir::ItemKind::Const(ty, body_id)
 273             }
 274             ItemKind::Fn(box FnKind(
 275                 _,
 276                 FnSig { ref decl, header, span: fn_sig_span },
 277                 ref generics,
 278                 ref body,
 279             )) => {
 280                 let fn_def_id = self.resolver.local_def_id(id);
 281                 self.with_new_scopes(|this| {
 282                     this.current_item = Some(ident.span);
 283
 284                     // Note: we don't need to change the return type from `T` to
 285                     // `impl Future<Output = T>` here because lower_body
 286                     // only cares about the input argument patterns in the function
 287                     // declaration (decl), not the return types.
 288                     let asyncness = header.asyncness;
 289                     let body_id =
 290                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 291
 292                     let (generics, decl) = this.add_in_band_defs(
 293                         generics,
 294                         fn_def_id,
 295                         AnonymousLifetimeMode::PassThrough,
 296                         |this, idty| {
 297                             let ret_id = asyncness.opt_return_id();
 298                             this.lower_fn_decl(
 299                                 &decl,
 300                                 Some((fn_def_id.to_def_id(), idty)),
 301                                 true,
 302                                 ret_id,
 303                             )
 304                         },
 305                     );
 306                     let sig = hir::FnSig {
 307                         decl,
 308                         header: this.lower_fn_header(header, fn_sig_span, id),
 309                         span: fn_sig_span,
 310                     };
 311                     hir::ItemKind::Fn(sig, generics, body_id)
 312                 })
 313             }
 314             ItemKind::Mod(_, ref mod_kind) => match mod_kind {
 315                 ModKind::Loaded(items, _, inner_span) => {
 316                     hir::ItemKind::Mod(self.lower_mod(items, *inner_span))
 317                 }
 318                 ModKind::Unloaded => panic!("`mod` items should have been loaded by now"),
 319             },
 320             ItemKind::ForeignMod(ref fm) => {
 321                 if fm.abi.is_none() {
 322                     self.maybe_lint_missing_abi(span, id, abi::Abi::C { unwind: false });
 323                 }
 324                 hir::ItemKind::ForeignMod {
 325                     abi: fm.abi.map_or(abi::Abi::C { unwind: false }, |abi| self.lower_abi(abi)),
 326                     items: self
 327                         .arena
 328                         .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
 329                 }
 330             }
 331             ItemKind::GlobalAsm(ref ga) => hir::ItemKind::GlobalAsm(self.lower_global_asm(ga)),
 332             ItemKind::TyAlias(box TyAliasKind(_, ref gen, _, Some(ref ty))) => {
 333                 // We lower
 334                 //
 335                 // type Foo = impl Trait
 336                 //
 337                 // to
 338                 //
 339                 // type Foo = Foo1
 340                 // opaque type Foo1: Trait
 341                 let ty = self.lower_ty(
 342                     ty,
 343                     ImplTraitContext::OtherOpaqueTy {
 344                         capturable_lifetimes: &mut FxHashSet::default(),
 345                         origin: hir::OpaqueTyOrigin::TyAlias,
 346                     },
 347                 );
 348                 let generics = self.lower_generics(gen, ImplTraitContext::disallowed());
 349                 hir::ItemKind::TyAlias(ty, generics)
 350             }
 351             ItemKind::TyAlias(box TyAliasKind(_, ref generics, _, None)) => {
 352                 let ty = self.arena.alloc(self.ty(span, hir::TyKind::Err));
 353                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 354                 hir::ItemKind::TyAlias(ty, generics)
 355             }
 356             ItemKind::Enum(ref enum_definition, ref generics) => hir::ItemKind::Enum(
 357                 hir::EnumDef {
 358                     variants: self.arena.alloc_from_iter(
 359                         enum_definition.variants.iter().map(|x| self.lower_variant(x)),
 360                     ),
 361                 },
 362                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 363             ),
 364             ItemKind::Struct(ref struct_def, ref generics) => {
 365                 let struct_def = self.lower_variant_data(hir_id, struct_def);
 366                 hir::ItemKind::Struct(
 367                     struct_def,
 368                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 369                 )
 370             }
 371             ItemKind::Union(ref vdata, ref generics) => {
 372                 let vdata = self.lower_variant_data(hir_id, vdata);
 373                 hir::ItemKind::Union(
 374                     vdata,
 375                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 376                 )
 377             }
 378             ItemKind::Impl(box ImplKind {
 379                 unsafety,
 380                 polarity,
 381                 defaultness,
 382                 constness,
 383                 generics: ref ast_generics,
 384                 of_trait: ref trait_ref,
 385                 self_ty: ref ty,
 386                 items: ref impl_items,
 387             }) => {
 388                 // Lower the "impl header" first. This ordering is important
 389                 // for in-band lifetimes! Consider `'a` here:
 390                 //
 391                 //     impl Foo<'a> for u32 {
 392                 //         fn method(&'a self) { .. }
 393                 //     }
 394                 //
 395                 // Because we start by lowering the `Foo<'a> for u32`
 396                 // part, we will add `'a` to the list of generics on
 397                 // the impl. When we then encounter it later in the
 398                 // method, it will not be considered an in-band
 399                 // lifetime to be added, but rather a reference to a
 400                 // parent lifetime.
 401                 let lowered_trait_def_id = self.lower_node_id(id).expect_owner();
 402                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 403                     ast_generics,
 404                     lowered_trait_def_id,
 405                     AnonymousLifetimeMode::CreateParameter,
 406                     |this, _| {
 407                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 408                             this.lower_trait_ref(trait_ref, ImplTraitContext::disallowed())
 409                         });
 410
 411                         if let Some(ref trait_ref) = trait_ref {
 412                             if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res {
 413                                 this.trait_impls
 414                                     .entry(def_id)
 415                                     .or_default()
 416                                     .push(lowered_trait_def_id);
 417                             }
 418                         }
 419
 420                         let lowered_ty = this.lower_ty(ty, ImplTraitContext::disallowed());
 421
 422                         (trait_ref, lowered_ty)
 423                     },
 424                 );
 425
 426                 let new_impl_items =
 427                     self.with_in_scope_lifetime_defs(&ast_generics.params, |this| {
 428                         this.arena.alloc_from_iter(
 429                             impl_items.iter().map(|item| this.lower_impl_item_ref(item)),
 430                         )
 431                     });
 432
 433                 // `defaultness.has_value()` is never called for an `impl`, always `true` in order
 434                 // to not cause an assertion failure inside the `lower_defaultness` function.
 435                 let has_val = true;
 436                 let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
 437                 hir::ItemKind::Impl(hir::Impl {
 438                     unsafety: self.lower_unsafety(unsafety),
 439                     polarity,
 440                     defaultness,
 441                     defaultness_span,
 442                     constness: self.lower_constness(constness),
 443                     generics,
 444                     of_trait: trait_ref,
 445                     self_ty: lowered_ty,
 446                     items: new_impl_items,
 447                 })
 448             }
 449             ItemKind::Trait(box TraitKind(
 450                 is_auto,
 451                 unsafety,
 452                 ref generics,
 453                 ref bounds,
 454                 ref items,
 455             )) => {
 456                 let bounds = self.lower_param_bounds(bounds, ImplTraitContext::disallowed());
 457                 let items = self
 458                     .arena
 459                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
 460                 hir::ItemKind::Trait(
 461                     is_auto,
 462                     self.lower_unsafety(unsafety),
 463                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 464                     bounds,
 465                     items,
 466                 )
 467             }
 468             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 469                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 470                 self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 471             ),
 472             ItemKind::MacroDef(..) | ItemKind::MacCall(..) => {
 473                 panic!("`TyMac` should have been expanded by now")
 474             }
 475         }
 476     }
 477
 478     fn lower_const_item(
 479         &mut self,
 480         ty: &Ty,
 481         span: Span,
 482         body: Option<&Expr>,
 483     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 484         let mut capturable_lifetimes;
 485         let itctx = if self.sess.features_untracked().impl_trait_in_bindings {
 486             capturable_lifetimes = FxHashSet::default();
 487             ImplTraitContext::OtherOpaqueTy {
 488                 capturable_lifetimes: &mut capturable_lifetimes,
 489                 origin: hir::OpaqueTyOrigin::Misc,
 490             }
 491         } else {
 492             ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
 493         };
 494         let ty = self.lower_ty(ty, itctx);
 495         (ty, self.lower_const_body(span, body))
 496     }
 497
 498     fn lower_use_tree(
 499         &mut self,
 500         tree: &UseTree,
 501         prefix: &Path,
 502         id: NodeId,
 503         vis: &mut hir::Visibility<'hir>,
 504         ident: &mut Ident,
 505         attrs: Option<&'hir [Attribute]>,
 506     ) -> hir::ItemKind<'hir> {
 507         debug!("lower_use_tree(tree={:?})", tree);
 508         debug!("lower_use_tree: vis = {:?}", vis);
 509
 510         let path = &tree.prefix;
 511         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 512
 513         match tree.kind {
 514             UseTreeKind::Simple(rename, id1, id2) => {
 515                 *ident = tree.ident();
 516
 517                 // First, apply the prefix to the path.
 518                 let mut path = Path { segments, span: path.span, tokens: None };
 519
 520                 // Correctly resolve `self` imports.
 521                 if path.segments.len() > 1
 522                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 523                 {
 524                     let _ = path.segments.pop();
 525                     if rename.is_none() {
 526                         *ident = path.segments.last().unwrap().ident;
 527                     }
 528                 }
 529
 530                 let mut resolutions = self.expect_full_res_from_use(id);
 531                 // We want to return *something* from this function, so hold onto the first item
 532                 // for later.
 533                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 534
 535                 // Here, we are looping over namespaces, if they exist for the definition
 536                 // being imported. We only handle type and value namespaces because we
 537                 // won't be dealing with macros in the rest of the compiler.
 538                 // Essentially a single `use` which imports two names is desugared into
 539                 // two imports.
 540                 for (res, &new_node_id) in resolutions.zip([id1, id2].iter()) {
 541                     let ident = *ident;
 542                     let mut path = path.clone();
 543                     for seg in &mut path.segments {
 544                         seg.id = self.resolver.next_node_id();
 545                     }
 546                     let span = path.span;
 547
 548                     self.with_hir_id_owner(new_node_id, |this| {
 549                         let new_id = this.lower_node_id(new_node_id);
 550                         let res = this.lower_res(res);
 551                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit, None);
 552                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 553                         let vis = this.rebuild_vis(&vis);
 554                         if let Some(attrs) = attrs {
 555                             this.attrs.insert(new_id, attrs);
 556                         }
 557
 558                         this.insert_item(hir::Item {
 559                             def_id: new_id.expect_owner(),
 560                             ident,
 561                             kind,
 562                             vis,
 563                             span,
 564                         });
 565                     });
 566                 }
 567
 568                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit, None);
 569                 hir::ItemKind::Use(path, hir::UseKind::Single)
 570             }
 571             UseTreeKind::Glob => {
 572                 let path = self.lower_path(
 573                     id,
 574                     &Path { segments, span: path.span, tokens: None },
 575                     ParamMode::Explicit,
 576                 );
 577                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 578             }
 579             UseTreeKind::Nested(ref trees) => {
 580                 // Nested imports are desugared into simple imports.
 581                 // So, if we start with
 582                 //
 583                 // ```
 584                 // pub(x) use foo::{a, b};
 585                 // ```
 586                 //
 587                 // we will create three items:
 588                 //
 589                 // ```
 590                 // pub(x) use foo::a;
 591                 // pub(x) use foo::b;
 592                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 593                 // ```
 594                 //
 595                 // The first two are produced by recursively invoking
 596                 // `lower_use_tree` (and indeed there may be things
 597                 // like `use foo::{a::{b, c}}` and so forth).  They
 598                 // wind up being directly added to
 599                 // `self.items`. However, the structure of this
 600                 // function also requires us to return one item, and
 601                 // for that we return the `{}` import (called the
 602                 // `ListStem`).
 603
 604                 let prefix = Path { segments, span: prefix.span.to(path.span), tokens: None };
 605
 606                 // Add all the nested `PathListItem`s to the HIR.
 607                 for &(ref use_tree, id) in trees {
 608                     let new_hir_id = self.lower_node_id(id);
 609
 610                     let mut prefix = prefix.clone();
 611
 612                     // Give the segments new node-ids since they are being cloned.
 613                     for seg in &mut prefix.segments {
 614                         seg.id = self.resolver.next_node_id();
 615                     }
 616
 617                     // Each `use` import is an item and thus are owners of the
 618                     // names in the path. Up to this point the nested import is
 619                     // the current owner, since we want each desugared import to
 620                     // own its own names, we have to adjust the owner before
 621                     // lowering the rest of the import.
 622                     self.with_hir_id_owner(id, |this| {
 623                         let mut vis = this.rebuild_vis(&vis);
 624                         let mut ident = *ident;
 625
 626                         let kind =
 627                             this.lower_use_tree(use_tree, &prefix, id, &mut vis, &mut ident, attrs);
 628                         if let Some(attrs) = attrs {
 629                             this.attrs.insert(new_hir_id, attrs);
 630                         }
 631
 632                         this.insert_item(hir::Item {
 633                             def_id: new_hir_id.expect_owner(),
 634                             ident,
 635                             kind,
 636                             vis,
 637                             span: use_tree.span,
 638                         });
 639                     });
 640                 }
 641
 642                 // Subtle and a bit hacky: we lower the privacy level
 643                 // of the list stem to "private" most of the time, but
 644                 // not for "restricted" paths. The key thing is that
 645                 // we don't want it to stay as `pub` (with no caveats)
 646                 // because that affects rustdoc and also the lints
 647                 // about `pub` items. But we can't *always* make it
 648                 // private -- particularly not for restricted paths --
 649                 // because it contains node-ids that would then be
 650                 // unused, failing the check that HirIds are "densely
 651                 // assigned".
 652                 match vis.node {
 653                     hir::VisibilityKind::Public
 654                     | hir::VisibilityKind::Crate(_)
 655                     | hir::VisibilityKind::Inherited => {
 656                         *vis = respan(prefix.span.shrink_to_lo(), hir::VisibilityKind::Inherited);
 657                     }
 658                     hir::VisibilityKind::Restricted { .. } => {
 659                         // Do nothing here, as described in the comment on the match.
 660                     }
 661                 }
 662
 663                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 664                 let res = self.lower_res(res);
 665                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit, None);
 666                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 667             }
 668         }
 669     }
 670
 671     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 672     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 673     /// `NodeId`s. (See, e.g., #56128.)
 674     fn rebuild_use_path(&mut self, path: &hir::Path<'hir>) -> &'hir hir::Path<'hir> {
 675         debug!("rebuild_use_path(path = {:?})", path);
 676         let segments =
 677             self.arena.alloc_from_iter(path.segments.iter().map(|seg| hir::PathSegment {
 678                 ident: seg.ident,
 679                 hir_id: seg.hir_id.map(|_| self.next_id()),
 680                 res: seg.res,
 681                 args: None,
 682                 infer_args: seg.infer_args,
 683             }));
 684         self.arena.alloc(hir::Path { span: path.span, res: path.res, segments })
 685     }
 686
 687     fn rebuild_vis(&mut self, vis: &hir::Visibility<'hir>) -> hir::Visibility<'hir> {
 688         let vis_kind = match vis.node {
 689             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 690             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 691             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 692             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 693                 hir::VisibilityKind::Restricted {
 694                     path: self.rebuild_use_path(path),
 695                     hir_id: self.next_id(),
 696                 }
 697             }
 698         };
 699         respan(vis.span, vis_kind)
 700     }
 701
 702     fn lower_foreign_item(&mut self, i: &ForeignItem) -> hir::ForeignItem<'hir> {
 703         let hir_id = self.lower_node_id(i.id);
 704         let def_id = hir_id.expect_owner();
 705         self.lower_attrs(hir_id, &i.attrs);
 706         hir::ForeignItem {
 707             def_id,
 708             ident: i.ident,
 709             kind: match i.kind {
 710                 ForeignItemKind::Fn(box FnKind(_, ref sig, ref generics, _)) => {
 711                     let fdec = &sig.decl;
 712                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 713                         generics,
 714                         def_id,
 715                         AnonymousLifetimeMode::PassThrough,
 716                         |this, _| {
 717                             (
 718                                 // Disallow `impl Trait` in foreign items.
 719                                 this.lower_fn_decl(fdec, None, false, None),
 720                                 this.lower_fn_params_to_names(fdec),
 721                             )
 722                         },
 723                     );
 724
 725                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 726                 }
 727                 ForeignItemKind::Static(ref t, m, _) => {
 728                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 729                     hir::ForeignItemKind::Static(ty, m)
 730                 }
 731                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 732                 ForeignItemKind::MacCall(_) => panic!("macro shouldn't exist here"),
 733             },
 734             vis: self.lower_visibility(&i.vis, None),
 735             span: i.span,
 736         }
 737     }
 738
 739     fn lower_foreign_item_ref(&mut self, i: &ForeignItem) -> hir::ForeignItemRef<'hir> {
 740         hir::ForeignItemRef {
 741             id: hir::ForeignItemId { def_id: self.lower_node_id(i.id).expect_owner() },
 742             ident: i.ident,
 743             span: i.span,
 744             vis: self.lower_visibility(&i.vis, Some(i.id)),
 745         }
 746     }
 747
 748     fn lower_global_asm(&mut self, ga: &GlobalAsm) -> &'hir hir::GlobalAsm {
 749         self.arena.alloc(hir::GlobalAsm { asm: ga.asm })
 750     }
 751
 752     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 753         let id = self.lower_node_id(v.id);
 754         self.lower_attrs(id, &v.attrs);
 755         hir::Variant {
 756             id,
 757             data: self.lower_variant_data(id, &v.data),
 758             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 759             ident: v.ident,
 760             span: v.span,
 761         }
 762     }
 763
 764     fn lower_variant_data(
 765         &mut self,
 766         parent_id: hir::HirId,
 767         vdata: &VariantData,
 768     ) -> hir::VariantData<'hir> {
 769         match *vdata {
 770             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 771                 self.arena
 772                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f))),
 773                 recovered,
 774             ),
 775             VariantData::Tuple(ref fields, id) => {
 776                 let ctor_id = self.lower_node_id(id);
 777                 self.alias_attrs(ctor_id, parent_id);
 778                 hir::VariantData::Tuple(
 779                     self.arena.alloc_from_iter(
 780                         fields.iter().enumerate().map(|f| self.lower_field_def(f)),
 781                     ),
 782                     ctor_id,
 783                 )
 784             }
 785             VariantData::Unit(id) => {
 786                 let ctor_id = self.lower_node_id(id);
 787                 self.alias_attrs(ctor_id, parent_id);
 788                 hir::VariantData::Unit(ctor_id)
 789             }
 790         }
 791     }
 792
 793     fn lower_field_def(&mut self, (index, f): (usize, &FieldDef)) -> hir::FieldDef<'hir> {
 794         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 795             let t = self.lower_path_ty(
 796                 &f.ty,
 797                 qself,
 798                 path,
 799                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 800                 ImplTraitContext::disallowed(),
 801             );
 802             self.arena.alloc(t)
 803         } else {
 804             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 805         };
 806         let hir_id = self.lower_node_id(f.id);
 807         self.lower_attrs(hir_id, &f.attrs);
 808         hir::FieldDef {
 809             span: f.span,
 810             hir_id,
 811             ident: match f.ident {
 812                 Some(ident) => ident,
 813                 // FIXME(jseyfried): positional field hygiene.
 814                 None => Ident::new(sym::integer(index), f.span),
 815             },
 816             vis: self.lower_visibility(&f.vis, None),
 817             ty,
 818         }
 819     }
 820
 821     fn lower_trait_item(&mut self, i: &AssocItem) -> hir::TraitItem<'hir> {
 822         let hir_id = self.lower_node_id(i.id);
 823         let trait_item_def_id = hir_id.expect_owner();
 824
 825         let (generics, kind) = match i.kind {
 826             AssocItemKind::Const(_, ref ty, ref default) => {
 827                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 828                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 829                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
 830             }
 831             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, None)) => {
 832                 let names = self.lower_fn_params_to_names(&sig.decl);
 833                 let (generics, sig) =
 834                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None, i.id);
 835                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)))
 836             }
 837             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, Some(ref body))) => {
 838                 let body_id = self.lower_fn_body_block(i.span, &sig.decl, Some(body));
 839                 let (generics, sig) =
 840                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None, i.id);
 841                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)))
 842             }
 843             AssocItemKind::TyAlias(box TyAliasKind(_, ref generics, ref bounds, ref default)) => {
 844                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
 845                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 846                 let kind = hir::TraitItemKind::Type(
 847                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 848                     ty,
 849                 );
 850
 851                 (generics, kind)
 852             }
 853             AssocItemKind::MacCall(..) => panic!("macro item shouldn't exist at this point"),
 854         };
 855
 856         self.lower_attrs(hir_id, &i.attrs);
 857         hir::TraitItem { def_id: trait_item_def_id, ident: i.ident, generics, kind, span: i.span }
 858     }
 859
 860     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 861         let (kind, has_default) = match &i.kind {
 862             AssocItemKind::Const(_, _, default) => (hir::AssocItemKind::Const, default.is_some()),
 863             AssocItemKind::TyAlias(box TyAliasKind(_, _, _, default)) => {
 864                 (hir::AssocItemKind::Type, default.is_some())
 865             }
 866             AssocItemKind::Fn(box FnKind(_, sig, _, default)) => {
 867                 (hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }, default.is_some())
 868             }
 869             AssocItemKind::MacCall(..) => unimplemented!(),
 870         };
 871         let id = hir::TraitItemId { def_id: self.lower_node_id(i.id).expect_owner() };
 872         let defaultness = hir::Defaultness::Default { has_value: has_default };
 873         hir::TraitItemRef { id, ident: i.ident, span: i.span, defaultness, kind }
 874     }
 875
 876     /// Construct `ExprKind::Err` for the given `span`.
 877     crate fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 878         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 879     }
 880
 881     fn lower_impl_item(&mut self, i: &AssocItem) -> hir::ImplItem<'hir> {
 882         let impl_item_def_id = self.resolver.local_def_id(i.id);
 883
 884         let (generics, kind) = match &i.kind {
 885             AssocItemKind::Const(_, ty, expr) => {
 886                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 887                 (
 888                     hir::Generics::empty(),
 889                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 890                 )
 891             }
 892             AssocItemKind::Fn(box FnKind(_, sig, generics, body)) => {
 893                 self.current_item = Some(i.span);
 894                 let asyncness = sig.header.asyncness;
 895                 let body_id =
 896                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 897                 let impl_trait_return_allow = !self.is_in_trait_impl;
 898                 let (generics, sig) = self.lower_method_sig(
 899                     generics,
 900                     sig,
 901                     impl_item_def_id,
 902                     impl_trait_return_allow,
 903                     asyncness.opt_return_id(),
 904                     i.id,
 905                 );
 906
 907                 (generics, hir::ImplItemKind::Fn(sig, body_id))
 908             }
 909             AssocItemKind::TyAlias(box TyAliasKind(_, generics, _, ty)) => {
 910                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 911                 let kind = match ty {
 912                     None => {
 913                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 914                         hir::ImplItemKind::TyAlias(ty)
 915                     }
 916                     Some(ty) => {
 917                         let ty = self.lower_ty(
 918                             ty,
 919                             ImplTraitContext::OtherOpaqueTy {
 920                                 capturable_lifetimes: &mut FxHashSet::default(),
 921                                 origin: hir::OpaqueTyOrigin::TyAlias,
 922                             },
 923                         );
 924                         hir::ImplItemKind::TyAlias(ty)
 925                     }
 926                 };
 927                 (generics, kind)
 928             }
 929             AssocItemKind::MacCall(..) => panic!("`TyMac` should have been expanded by now"),
 930         };
 931
 932         // Since `default impl` is not yet implemented, this is always true in impls.
 933         let has_value = true;
 934         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 935         let hir_id = self.lower_node_id(i.id);
 936         self.lower_attrs(hir_id, &i.attrs);
 937         hir::ImplItem {
 938             def_id: hir_id.expect_owner(),
 939             ident: i.ident,
 940             generics,
 941             vis: self.lower_visibility(&i.vis, None),
 942             defaultness,
 943             kind,
 944             span: i.span,
 945         }
 946     }
 947
 948     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef<'hir> {
 949         // Since `default impl` is not yet implemented, this is always true in impls.
 950         let has_value = true;
 951         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 952         hir::ImplItemRef {
 953             id: hir::ImplItemId { def_id: self.lower_node_id(i.id).expect_owner() },
 954             ident: i.ident,
 955             span: i.span,
 956             vis: self.lower_visibility(&i.vis, Some(i.id)),
 957             defaultness,
 958             kind: match &i.kind {
 959                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 960                 AssocItemKind::TyAlias(..) => hir::AssocItemKind::Type,
 961                 AssocItemKind::Fn(box FnKind(_, sig, ..)) => {
 962                     hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
 963                 }
 964                 AssocItemKind::MacCall(..) => unimplemented!(),
 965             },
 966         }
 967     }
 968
 969     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 970     /// the address space of that item instead of the item currently being
 971     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 972     /// lower a `Visibility` value although we haven't lowered the owning
 973     /// `ImplItem` in question yet.
 974     fn lower_visibility(
 975         &mut self,
 976         v: &Visibility,
 977         explicit_owner: Option<NodeId>,
 978     ) -> hir::Visibility<'hir> {
 979         let node = match v.kind {
 980             VisibilityKind::Public => hir::VisibilityKind::Public,
 981             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 982             VisibilityKind::Restricted { ref path, id } => {
 983                 debug!("lower_visibility: restricted path id = {:?}", id);
 984                 let lowered_id = if let Some(owner) = explicit_owner {
 985                     self.lower_node_id_with_owner(id, owner)
 986                 } else {
 987                     self.lower_node_id(id)
 988                 };
 989                 let res = self.expect_full_res(id);
 990                 let res = self.lower_res(res);
 991                 hir::VisibilityKind::Restricted {
 992                     path: self.lower_path_extra(res, path, ParamMode::Explicit, explicit_owner),
 993                     hir_id: lowered_id,
 994                 }
 995             }
 996             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 997         };
 998         respan(v.span, node)
 999     }
1000
1001     fn lower_defaultness(
1002         &self,
1003         d: Defaultness,
1004         has_value: bool,
1005     ) -> (hir::Defaultness, Option<Span>) {
1006         match d {
1007             Defaultness::Default(sp) => (hir::Defaultness::Default { has_value }, Some(sp)),
1008             Defaultness::Final => {
1009                 assert!(has_value);
1010                 (hir::Defaultness::Final, None)
1011             }
1012         }
1013     }
1014
1015     fn record_body(
1016         &mut self,
1017         params: &'hir [hir::Param<'hir>],
1018         value: hir::Expr<'hir>,
1019     ) -> hir::BodyId {
1020         let body = hir::Body { generator_kind: self.generator_kind, params, value };
1021         let id = body.id();
1022         self.bodies.insert(id, body);
1023         id
1024     }
1025
1026     pub(super) fn lower_body(
1027         &mut self,
1028         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
1029     ) -> hir::BodyId {
1030         let prev_gen_kind = self.generator_kind.take();
1031         let task_context = self.task_context.take();
1032         let (parameters, result) = f(self);
1033         let body_id = self.record_body(parameters, result);
1034         self.task_context = task_context;
1035         self.generator_kind = prev_gen_kind;
1036         body_id
1037     }
1038
1039     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
1040         let hir_id = self.lower_node_id(param.id);
1041         self.lower_attrs(hir_id, &param.attrs);
1042         hir::Param {
1043             hir_id,
1044             pat: self.lower_pat(&param.pat),
1045             ty_span: param.ty.span,
1046             span: param.span,
1047         }
1048     }
1049
1050     pub(super) fn lower_fn_body(
1051         &mut self,
1052         decl: &FnDecl,
1053         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
1054     ) -> hir::BodyId {
1055         self.lower_body(|this| {
1056             (
1057                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
1058                 body(this),
1059             )
1060         })
1061     }
1062
1063     fn lower_fn_body_block(
1064         &mut self,
1065         span: Span,
1066         decl: &FnDecl,
1067         body: Option<&Block>,
1068     ) -> hir::BodyId {
1069         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1070     }
1071
1072     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1073         match block {
1074             Some(block) => self.lower_block_expr(block),
1075             None => self.expr_err(span),
1076         }
1077     }
1078
1079     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1080         self.lower_body(|this| {
1081             (
1082                 &[],
1083                 match expr {
1084                     Some(expr) => this.lower_expr_mut(expr),
1085                     None => this.expr_err(span),
1086                 },
1087             )
1088         })
1089     }
1090
1091     fn lower_maybe_async_body(
1092         &mut self,
1093         span: Span,
1094         decl: &FnDecl,
1095         asyncness: Async,
1096         body: Option<&Block>,
1097     ) -> hir::BodyId {
1098         let closure_id = match asyncness {
1099             Async::Yes { closure_id, .. } => closure_id,
1100             Async::No => return self.lower_fn_body_block(span, decl, body),
1101         };
1102
1103         self.lower_body(|this| {
1104             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1105             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1106
1107             // Async function parameters are lowered into the closure body so that they are
1108             // captured and so that the drop order matches the equivalent non-async functions.
1109             //
1110             // from:
1111             //
1112             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1113             //         <body>
1114             //     }
1115             //
1116             // into:
1117             //
1118             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1119             //       async move {
1120             //         let __arg2 = __arg2;
1121             //         let <pattern> = __arg2;
1122             //         let __arg1 = __arg1;
1123             //         let <pattern> = __arg1;
1124             //         let __arg0 = __arg0;
1125             //         let <pattern> = __arg0;
1126             //         drop-temps { <body> } // see comments later in fn for details
1127             //       }
1128             //     }
1129             //
1130             // If `<pattern>` is a simple ident, then it is lowered to a single
1131             // `let <pattern> = <pattern>;` statement as an optimization.
1132             //
1133             // Note that the body is embedded in `drop-temps`; an
1134             // equivalent desugaring would be `return { <body>
1135             // };`. The key point is that we wish to drop all the
1136             // let-bound variables and temporaries created in the body
1137             // (and its tail expression!) before we drop the
1138             // parameters (c.f. rust-lang/rust#64512).
1139             for (index, parameter) in decl.inputs.iter().enumerate() {
1140                 let parameter = this.lower_param(parameter);
1141                 let span = parameter.pat.span;
1142
1143                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1144                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1145                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1146                     hir::PatKind::Binding(
1147                         hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable,
1148                         _,
1149                         ident,
1150                         _,
1151                     ) => (ident, true),
1152                     // For `ref mut` or wildcard arguments, we can't reuse the binding, but
1153                     // we can keep the same name for the parameter.
1154                     // This lets rustdoc render it correctly in documentation.
1155                     hir::PatKind::Binding(_, _, ident, _) => (ident, false),
1156                     hir::PatKind::Wild => {
1157                         (Ident::with_dummy_span(rustc_span::symbol::kw::Underscore), false)
1158                     }
1159                     _ => {
1160                         // Replace the ident for bindings that aren't simple.
1161                         let name = format!("__arg{}", index);
1162                         let ident = Ident::from_str(&name);
1163
1164                         (ident, false)
1165                     }
1166                 };
1167
1168                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1169
1170                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1171                 // async function.
1172                 //
1173                 // If this is the simple case, this parameter will end up being the same as the
1174                 // original parameter, but with a different pattern id.
1175                 let stmt_attrs = this.attrs.get(&parameter.hir_id).copied();
1176                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1177                 let new_parameter = hir::Param {
1178                     hir_id: parameter.hir_id,
1179                     pat: new_parameter_pat,
1180                     ty_span: parameter.ty_span,
1181                     span: parameter.span,
1182                 };
1183
1184                 if is_simple_parameter {
1185                     // If this is the simple case, then we only insert one statement that is
1186                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1187                     // `HirId`s are densely assigned.
1188                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1189                     let stmt = this.stmt_let_pat(
1190                         stmt_attrs,
1191                         desugared_span,
1192                         Some(expr),
1193                         parameter.pat,
1194                         hir::LocalSource::AsyncFn,
1195                     );
1196                     statements.push(stmt);
1197                 } else {
1198                     // If this is not the simple case, then we construct two statements:
1199                     //
1200                     // ```
1201                     // let __argN = __argN;
1202                     // let <pat> = __argN;
1203                     // ```
1204                     //
1205                     // The first statement moves the parameter into the closure and thus ensures
1206                     // that the drop order is correct.
1207                     //
1208                     // The second statement creates the bindings that the user wrote.
1209
1210                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1211                     // because the user may have specified a `ref mut` binding in the next
1212                     // statement.
1213                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1214                         desugared_span,
1215                         ident,
1216                         hir::BindingAnnotation::Mutable,
1217                     );
1218                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1219                     let move_stmt = this.stmt_let_pat(
1220                         None,
1221                         desugared_span,
1222                         Some(move_expr),
1223                         move_pat,
1224                         hir::LocalSource::AsyncFn,
1225                     );
1226
1227                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1228                     // parameter's pattern so that `HirId`s are densely assigned.
1229                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1230                     let pattern_stmt = this.stmt_let_pat(
1231                         stmt_attrs,
1232                         desugared_span,
1233                         Some(pattern_expr),
1234                         parameter.pat,
1235                         hir::LocalSource::AsyncFn,
1236                     );
1237
1238                     statements.push(move_stmt);
1239                     statements.push(pattern_stmt);
1240                 };
1241
1242                 parameters.push(new_parameter);
1243             }
1244
1245             let body_span = body.map_or(span, |b| b.span);
1246             let async_expr = this.make_async_expr(
1247                 CaptureBy::Value,
1248                 closure_id,
1249                 None,
1250                 body_span,
1251                 hir::AsyncGeneratorKind::Fn,
1252                 |this| {
1253                     // Create a block from the user's function body:
1254                     let user_body = this.lower_block_expr_opt(body_span, body);
1255
1256                     // Transform into `drop-temps { <user-body> }`, an expression:
1257                     let desugared_span =
1258                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1259                     let user_body = this.expr_drop_temps(
1260                         desugared_span,
1261                         this.arena.alloc(user_body),
1262                         AttrVec::new(),
1263                     );
1264
1265                     // As noted above, create the final block like
1266                     //
1267                     // ```
1268                     // {
1269                     //   let $param_pattern = $raw_param;
1270                     //   ...
1271                     //   drop-temps { <user-body> }
1272                     // }
1273                     // ```
1274                     let body = this.block_all(
1275                         desugared_span,
1276                         this.arena.alloc_from_iter(statements),
1277                         Some(user_body),
1278                     );
1279
1280                     this.expr_block(body, AttrVec::new())
1281                 },
1282             );
1283
1284             (
1285                 this.arena.alloc_from_iter(parameters),
1286                 this.expr(body_span, async_expr, AttrVec::new()),
1287             )
1288         })
1289     }
1290
1291     fn lower_method_sig(
1292         &mut self,
1293         generics: &Generics,
1294         sig: &FnSig,
1295         fn_def_id: LocalDefId,
1296         impl_trait_return_allow: bool,
1297         is_async: Option<NodeId>,
1298         id: NodeId,
1299     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1300         let header = self.lower_fn_header(sig.header, sig.span, id);
1301         let (generics, decl) = self.add_in_band_defs(
1302             generics,
1303             fn_def_id,
1304             AnonymousLifetimeMode::PassThrough,
1305             |this, idty| {
1306                 this.lower_fn_decl(
1307                     &sig.decl,
1308                     Some((fn_def_id.to_def_id(), idty)),
1309                     impl_trait_return_allow,
1310                     is_async,
1311                 )
1312             },
1313         );
1314         (generics, hir::FnSig { header, decl, span: sig.span })
1315     }
1316
1317     fn lower_fn_header(&mut self, h: FnHeader, span: Span, id: NodeId) -> hir::FnHeader {
1318         hir::FnHeader {
1319             unsafety: self.lower_unsafety(h.unsafety),
1320             asyncness: self.lower_asyncness(h.asyncness),
1321             constness: self.lower_constness(h.constness),
1322             abi: self.lower_extern(h.ext, span, id),
1323         }
1324     }
1325
1326     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1327         abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1328             self.error_on_invalid_abi(abi);
1329             abi::Abi::Rust
1330         })
1331     }
1332
1333     pub(super) fn lower_extern(&mut self, ext: Extern, span: Span, id: NodeId) -> abi::Abi {
1334         match ext {
1335             Extern::None => abi::Abi::Rust,
1336             Extern::Implicit => {
1337                 self.maybe_lint_missing_abi(span, id, abi::Abi::C { unwind: false });
1338                 abi::Abi::C { unwind: false }
1339             }
1340             Extern::Explicit(abi) => self.lower_abi(abi),
1341         }
1342     }
1343
1344     fn error_on_invalid_abi(&self, abi: StrLit) {
1345         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1346             .span_label(abi.span, "invalid ABI")
1347             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1348             .emit();
1349     }
1350
1351     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1352         match a {
1353             Async::Yes { .. } => hir::IsAsync::Async,
1354             Async::No => hir::IsAsync::NotAsync,
1355         }
1356     }
1357
1358     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1359         match c {
1360             Const::Yes(_) => hir::Constness::Const,
1361             Const::No => hir::Constness::NotConst,
1362         }
1363     }
1364
1365     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1366         match u {
1367             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1368             Unsafe::No => hir::Unsafety::Normal,
1369         }
1370     }
1371
1372     pub(super) fn lower_generics_mut(
1373         &mut self,
1374         generics: &Generics,
1375         itctx: ImplTraitContext<'_, 'hir>,
1376     ) -> GenericsCtor<'hir> {
1377         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1378         // FIXME: this could probably be done with less rightward drift. It also looks like two
1379         // control paths where `report_error` is called are the only paths that advance to after the
1380         // match statement, so the error reporting could probably just be moved there.
1381         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1382         for pred in &generics.where_clause.predicates {
1383             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1384                 'next_bound: for bound in &bound_pred.bounds {
1385                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1386                         let report_error = |this: &mut Self| {
1387                             this.diagnostic().span_err(
1388                                 bound_pred.bounded_ty.span,
1389                                 "`?Trait` bounds are only permitted at the \
1390                                  point where a type parameter is declared",
1391                             );
1392                         };
1393                         // Check if the where clause type is a plain type parameter.
1394                         match bound_pred.bounded_ty.kind {
1395                             TyKind::Path(None, ref path)
1396                                 if path.segments.len() == 1
1397                                     && bound_pred.bound_generic_params.is_empty() =>
1398                             {
1399                                 if let Some(Res::Def(DefKind::TyParam, def_id)) = self
1400                                     .resolver
1401                                     .get_partial_res(bound_pred.bounded_ty.id)
1402                                     .map(|d| d.base_res())
1403                                 {
1404                                     if let Some(def_id) = def_id.as_local() {
1405                                         for param in &generics.params {
1406                                             if let GenericParamKind::Type { .. } = param.kind {
1407                                                 if def_id == self.resolver.local_def_id(param.id) {
1408                                                     add_bounds
1409                                                         .entry(param.id)
1410                                                         .or_default()
1411                                                         .push(bound.clone());
1412                                                     continue 'next_bound;
1413                                                 }
1414                                             }
1415                                         }
1416                                     }
1417                                 }
1418                                 report_error(self)
1419                             }
1420                             _ => report_error(self),
1421                         }
1422                     }
1423                 }
1424             }
1425         }
1426
1427         GenericsCtor {
1428             params: self.lower_generic_params_mut(&generics.params, &add_bounds, itctx).collect(),
1429             where_clause: self.lower_where_clause(&generics.where_clause),
1430             span: generics.span,
1431         }
1432     }
1433
1434     pub(super) fn lower_generics(
1435         &mut self,
1436         generics: &Generics,
1437         itctx: ImplTraitContext<'_, 'hir>,
1438     ) -> hir::Generics<'hir> {
1439         let generics_ctor = self.lower_generics_mut(generics, itctx);
1440         generics_ctor.into_generics(self.arena)
1441     }
1442
1443     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1444         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1445             hir::WhereClause {
1446                 predicates: this.arena.alloc_from_iter(
1447                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1448                 ),
1449                 span: wc.span,
1450             }
1451         })
1452     }
1453
1454     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1455         match *pred {
1456             WherePredicate::BoundPredicate(WhereBoundPredicate {
1457                 ref bound_generic_params,
1458                 ref bounded_ty,
1459                 ref bounds,
1460                 span,
1461             }) => {
1462                 self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1463                     hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1464                         bound_generic_params: this.lower_generic_params(
1465                             bound_generic_params,
1466                             &NodeMap::default(),
1467                             ImplTraitContext::disallowed(),
1468                         ),
1469                         bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1470                         bounds: this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| {
1471                             match *bound {
1472                                 // Ignore `?Trait` bounds.
1473                                 // They were copied into type parameters already.
1474                                 GenericBound::Trait(_, TraitBoundModifier::Maybe) => None,
1475                                 _ => Some(
1476                                     this.lower_param_bound(bound, ImplTraitContext::disallowed()),
1477                                 ),
1478                             }
1479                         })),
1480                         span,
1481                     })
1482                 })
1483             }
1484             WherePredicate::RegionPredicate(WhereRegionPredicate {
1485                 ref lifetime,
1486                 ref bounds,
1487                 span,
1488             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1489                 span,
1490                 lifetime: self.lower_lifetime(lifetime),
1491                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1492             }),
1493             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1494                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1495                     hir_id: self.lower_node_id(id),
1496                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1497                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1498                     span,
1499                 })
1500             }
1501         }
1502     }
1503 }
1504
1505 /// Helper struct for delayed construction of Generics.
1506 pub(super) struct GenericsCtor<'hir> {
1507     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1508     where_clause: hir::WhereClause<'hir>,
1509     span: Span,
1510 }
1511
1512 impl<'hir> GenericsCtor<'hir> {
1513     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1514         hir::Generics {
1515             params: arena.alloc_from_iter(self.params),
1516             where_clause: self.where_clause,
1517             span: self.span,
1518         }
1519     }
1520 }