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::iter;
  22 use std::mem;
  23
  24 pub(super) struct ItemLowerer<'a, 'lowering, 'hir> {
  25     pub(super) lctx: &'a mut LoweringContext<'lowering, 'hir>,
  26 }
  27
  28 impl ItemLowerer<'_, '_, '_> {
  29     fn with_trait_impl_ref(&mut self, impl_ref: &Option<TraitRef>, f: impl FnOnce(&mut Self)) {
  30         let old = self.lctx.is_in_trait_impl;
  31         self.lctx.is_in_trait_impl = impl_ref.is_some();
  32         f(self);
  33         self.lctx.is_in_trait_impl = old;
  34     }
  35 }
  36
  37 impl<'a> Visitor<'a> for ItemLowerer<'a, '_, '_> {
  38     fn visit_item(&mut self, item: &'a Item) {
  39         let mut item_hir_id = None;
  40         self.lctx.with_hir_id_owner(item.id, |lctx| {
  41             lctx.without_in_scope_lifetime_defs(|lctx| {
  42                 if let Some(hir_item) = lctx.lower_item(item) {
  43                     let id = lctx.insert_item(hir_item);
  44                     item_hir_id = Some(id);
  45                 }
  46             })
  47         });
  48
  49         if let Some(hir_id) = item_hir_id {
  50             self.lctx.with_parent_item_lifetime_defs(hir_id, |this| {
  51                 let this = &mut ItemLowerer { lctx: this };
  52                 match item.kind {
  53                     ItemKind::Mod(..) => {
  54                         let def_id = this.lctx.lower_node_id(item.id).expect_owner();
  55                         let old_current_module =
  56                             mem::replace(&mut this.lctx.current_module, def_id);
  57                         visit::walk_item(this, item);
  58                         this.lctx.current_module = old_current_module;
  59                     }
  60                     ItemKind::Impl(box ImplKind { ref of_trait, .. }) => {
  61                         this.with_trait_impl_ref(of_trait, |this| visit::walk_item(this, item));
  62                     }
  63                     _ => visit::walk_item(this, item),
  64                 }
  65             });
  66         }
  67     }
  68
  69     fn visit_fn(&mut self, fk: FnKind<'a>, sp: Span, _: NodeId) {
  70         match fk {
  71             FnKind::Fn(FnCtxt::Foreign, _, sig, _, _) => {
  72                 self.visit_fn_header(&sig.header);
  73                 visit::walk_fn_decl(self, &sig.decl);
  74                 // Don't visit the foreign function body even if it has one, since lowering the
  75                 // body would have no meaning and will have already been caught as a parse error.
  76             }
  77             _ => visit::walk_fn(self, fk, sp),
  78         }
  79     }
  80
  81     fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
  82         self.lctx.with_hir_id_owner(item.id, |lctx| match ctxt {
  83             AssocCtxt::Trait => {
  84                 let hir_item = lctx.lower_trait_item(item);
  85                 let id = hir_item.trait_item_id();
  86                 lctx.trait_items.insert(id, hir_item);
  87                 lctx.modules.entry(lctx.current_module).or_default().trait_items.insert(id);
  88             }
  89             AssocCtxt::Impl => {
  90                 let hir_item = lctx.lower_impl_item(item);
  91                 let id = hir_item.impl_item_id();
  92                 lctx.impl_items.insert(id, hir_item);
  93                 lctx.modules.entry(lctx.current_module).or_default().impl_items.insert(id);
  94             }
  95         });
  96
  97         visit::walk_assoc_item(self, item, ctxt);
  98     }
  99
 100     fn visit_foreign_item(&mut self, item: &'a ForeignItem) {
 101         self.lctx.allocate_hir_id_counter(item.id);
 102         self.lctx.with_hir_id_owner(item.id, |lctx| {
 103             let hir_item = lctx.lower_foreign_item(item);
 104             let id = hir_item.foreign_item_id();
 105             lctx.foreign_items.insert(id, hir_item);
 106             lctx.modules.entry(lctx.current_module).or_default().foreign_items.insert(id);
 107         });
 108
 109         visit::walk_foreign_item(self, item);
 110     }
 111 }
 112
 113 impl<'hir> LoweringContext<'_, 'hir> {
 114     // Same as the method above, but accepts `hir::GenericParam`s
 115     // instead of `ast::GenericParam`s.
 116     // This should only be used with generics that have already had their
 117     // in-band lifetimes added. In practice, this means that this function is
 118     // only used when lowering a child item of a trait or impl.
 119     fn with_parent_item_lifetime_defs<T>(
 120         &mut self,
 121         parent_hir_id: hir::ItemId,
 122         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 123     ) -> T {
 124         let old_len = self.in_scope_lifetimes.len();
 125
 126         let parent_generics = match self.items.get(&parent_hir_id).unwrap().kind {
 127             hir::ItemKind::Impl(hir::Impl { ref generics, .. })
 128             | hir::ItemKind::Trait(_, _, ref generics, ..) => generics.params,
 129             _ => &[],
 130         };
 131         let lt_def_names = parent_generics.iter().filter_map(|param| match param.kind {
 132             hir::GenericParamKind::Lifetime { .. } => Some(param.name.normalize_to_macros_2_0()),
 133             _ => None,
 134         });
 135         self.in_scope_lifetimes.extend(lt_def_names);
 136
 137         let res = f(self);
 138
 139         self.in_scope_lifetimes.truncate(old_len);
 140         res
 141     }
 142
 143     // Clears (and restores) the `in_scope_lifetimes` field. Used when
 144     // visiting nested items, which never inherit in-scope lifetimes
 145     // from their surrounding environment.
 146     fn without_in_scope_lifetime_defs<T>(
 147         &mut self,
 148         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 149     ) -> T {
 150         let old_in_scope_lifetimes = mem::replace(&mut self.in_scope_lifetimes, vec![]);
 151
 152         // this vector is only used when walking over impl headers,
 153         // input types, and the like, and should not be non-empty in
 154         // between items
 155         assert!(self.lifetimes_to_define.is_empty());
 156
 157         let res = f(self);
 158
 159         assert!(self.in_scope_lifetimes.is_empty());
 160         self.in_scope_lifetimes = old_in_scope_lifetimes;
 161
 162         res
 163     }
 164
 165     pub(super) fn lower_mod(&mut self, items: &[P<Item>], inner: Span) -> hir::Mod<'hir> {
 166         hir::Mod {
 167             inner,
 168             item_ids: self.arena.alloc_from_iter(items.iter().flat_map(|x| self.lower_item_id(x))),
 169         }
 170     }
 171
 172     pub(super) fn lower_item_id(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
 173         let node_ids = match i.kind {
 174             ItemKind::Use(ref use_tree) => {
 175                 let mut vec = smallvec![i.id];
 176                 self.lower_item_id_use_tree(use_tree, i.id, &mut vec);
 177                 vec
 178             }
 179             ItemKind::MacroDef(..) => SmallVec::new(),
 180             ItemKind::Fn(..) | ItemKind::Impl(box ImplKind { of_trait: None, .. }) => {
 181                 smallvec![i.id]
 182             }
 183             _ => smallvec![i.id],
 184         };
 185
 186         node_ids
 187             .into_iter()
 188             .map(|node_id| hir::ItemId {
 189                 def_id: self.allocate_hir_id_counter(node_id).expect_owner(),
 190             })
 191             .collect()
 192     }
 193
 194     fn lower_item_id_use_tree(
 195         &mut self,
 196         tree: &UseTree,
 197         base_id: NodeId,
 198         vec: &mut SmallVec<[NodeId; 1]>,
 199     ) {
 200         match tree.kind {
 201             UseTreeKind::Nested(ref nested_vec) => {
 202                 for &(ref nested, id) in nested_vec {
 203                     vec.push(id);
 204                     self.lower_item_id_use_tree(nested, id, vec);
 205                 }
 206             }
 207             UseTreeKind::Glob => {}
 208             UseTreeKind::Simple(_, id1, id2) => {
 209                 for (_, &id) in
 210                     iter::zip(self.expect_full_res_from_use(base_id).skip(1), &[id1, id2])
 211                 {
 212                     vec.push(id);
 213                 }
 214             }
 215         }
 216     }
 217
 218     pub fn lower_item(&mut self, i: &Item) -> Option<hir::Item<'hir>> {
 219         let mut ident = i.ident;
 220         let mut vis = self.lower_visibility(&i.vis, None);
 221
 222         if let ItemKind::MacroDef(MacroDef { ref body, macro_rules }) = i.kind {
 223             if !macro_rules || self.sess.contains_name(&i.attrs, sym::macro_export) {
 224                 let hir_id = self.lower_node_id(i.id);
 225                 self.lower_attrs(hir_id, &i.attrs);
 226                 let body = P(self.lower_mac_args(body));
 227                 self.exported_macros.push(hir::MacroDef {
 228                     ident,
 229                     vis,
 230                     def_id: hir_id.expect_owner(),
 231                     span: i.span,
 232                     ast: MacroDef { body, macro_rules },
 233                 });
 234             } else {
 235                 for a in i.attrs.iter() {
 236                     let a = self.lower_attr(a);
 237                     self.non_exported_macro_attrs.push(a);
 238                 }
 239             }
 240             return None;
 241         }
 242
 243         let hir_id = self.lower_node_id(i.id);
 244         let attrs = self.lower_attrs(hir_id, &i.attrs);
 245         let kind = self.lower_item_kind(i.span, i.id, hir_id, &mut ident, attrs, &mut vis, &i.kind);
 246         Some(hir::Item { def_id: hir_id.expect_owner(), ident, kind, vis, span: i.span })
 247     }
 248
 249     fn lower_item_kind(
 250         &mut self,
 251         span: Span,
 252         id: NodeId,
 253         hir_id: hir::HirId,
 254         ident: &mut Ident,
 255         attrs: Option<&'hir [Attribute]>,
 256         vis: &mut hir::Visibility<'hir>,
 257         i: &ItemKind,
 258     ) -> hir::ItemKind<'hir> {
 259         match *i {
 260             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
 261             ItemKind::Use(ref use_tree) => {
 262                 // Start with an empty prefix.
 263                 let prefix = Path { segments: vec![], span: use_tree.span, tokens: None };
 264
 265                 self.lower_use_tree(use_tree, &prefix, id, vis, ident, attrs)
 266             }
 267             ItemKind::Static(ref t, m, ref e) => {
 268                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 269                 hir::ItemKind::Static(ty, m, body_id)
 270             }
 271             ItemKind::Const(_, ref t, ref e) => {
 272                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 273                 hir::ItemKind::Const(ty, body_id)
 274             }
 275             ItemKind::Fn(box FnKind(
 276                 _,
 277                 FnSig { ref decl, header, span: fn_sig_span },
 278                 ref generics,
 279                 ref body,
 280             )) => {
 281                 let fn_def_id = self.resolver.local_def_id(id);
 282                 self.with_new_scopes(|this| {
 283                     this.current_item = Some(ident.span);
 284
 285                     // Note: we don't need to change the return type from `T` to
 286                     // `impl Future<Output = T>` here because lower_body
 287                     // only cares about the input argument patterns in the function
 288                     // declaration (decl), not the return types.
 289                     let asyncness = header.asyncness;
 290                     let body_id =
 291                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 292
 293                     let (generics, decl) = this.add_in_band_defs(
 294                         generics,
 295                         fn_def_id,
 296                         AnonymousLifetimeMode::PassThrough,
 297                         |this, idty| {
 298                             let ret_id = asyncness.opt_return_id();
 299                             this.lower_fn_decl(
 300                                 &decl,
 301                                 Some((fn_def_id.to_def_id(), idty)),
 302                                 true,
 303                                 ret_id,
 304                             )
 305                         },
 306                     );
 307                     let sig = hir::FnSig {
 308                         decl,
 309                         header: this.lower_fn_header(header, fn_sig_span, id),
 310                         span: fn_sig_span,
 311                     };
 312                     hir::ItemKind::Fn(sig, generics, body_id)
 313                 })
 314             }
 315             ItemKind::Mod(_, ref mod_kind) => match mod_kind {
 316                 ModKind::Loaded(items, _, inner_span) => {
 317                     hir::ItemKind::Mod(self.lower_mod(items, *inner_span))
 318                 }
 319                 ModKind::Unloaded => panic!("`mod` items should have been loaded by now"),
 320             },
 321             ItemKind::ForeignMod(ref fm) => {
 322                 if fm.abi.is_none() {
 323                     self.maybe_lint_missing_abi(span, id, abi::Abi::C { unwind: false });
 324                 }
 325                 hir::ItemKind::ForeignMod {
 326                     abi: fm.abi.map_or(abi::Abi::C { unwind: false }, |abi| self.lower_abi(abi)),
 327                     items: self
 328                         .arena
 329                         .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
 330                 }
 331             }
 332             ItemKind::GlobalAsm(ref ga) => hir::ItemKind::GlobalAsm(self.lower_global_asm(ga)),
 333             ItemKind::TyAlias(box TyAliasKind(_, ref gen, _, Some(ref ty))) => {
 334                 // We lower
 335                 //
 336                 // type Foo = impl Trait
 337                 //
 338                 // to
 339                 //
 340                 // type Foo = Foo1
 341                 // opaque type Foo1: Trait
 342                 let ty = self.lower_ty(
 343                     ty,
 344                     ImplTraitContext::OtherOpaqueTy {
 345                         capturable_lifetimes: &mut FxHashSet::default(),
 346                         origin: hir::OpaqueTyOrigin::TyAlias,
 347                     },
 348                 );
 349                 let generics = self.lower_generics(gen, ImplTraitContext::disallowed());
 350                 hir::ItemKind::TyAlias(ty, generics)
 351             }
 352             ItemKind::TyAlias(box TyAliasKind(_, ref generics, _, None)) => {
 353                 let ty = self.arena.alloc(self.ty(span, hir::TyKind::Err));
 354                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 355                 hir::ItemKind::TyAlias(ty, generics)
 356             }
 357             ItemKind::Enum(ref enum_definition, ref generics) => hir::ItemKind::Enum(
 358                 hir::EnumDef {
 359                     variants: self.arena.alloc_from_iter(
 360                         enum_definition.variants.iter().map(|x| self.lower_variant(x)),
 361                     ),
 362                 },
 363                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 364             ),
 365             ItemKind::Struct(ref struct_def, ref generics) => {
 366                 let struct_def = self.lower_variant_data(hir_id, struct_def);
 367                 hir::ItemKind::Struct(
 368                     struct_def,
 369                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 370                 )
 371             }
 372             ItemKind::Union(ref vdata, ref generics) => {
 373                 let vdata = self.lower_variant_data(hir_id, vdata);
 374                 hir::ItemKind::Union(
 375                     vdata,
 376                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 377                 )
 378             }
 379             ItemKind::Impl(box ImplKind {
 380                 unsafety,
 381                 polarity,
 382                 defaultness,
 383                 constness,
 384                 generics: ref ast_generics,
 385                 of_trait: ref trait_ref,
 386                 self_ty: ref ty,
 387                 items: ref impl_items,
 388             }) => {
 389                 // Lower the "impl header" first. This ordering is important
 390                 // for in-band lifetimes! Consider `'a` here:
 391                 //
 392                 //     impl Foo<'a> for u32 {
 393                 //         fn method(&'a self) { .. }
 394                 //     }
 395                 //
 396                 // Because we start by lowering the `Foo<'a> for u32`
 397                 // part, we will add `'a` to the list of generics on
 398                 // the impl. When we then encounter it later in the
 399                 // method, it will not be considered an in-band
 400                 // lifetime to be added, but rather a reference to a
 401                 // parent lifetime.
 402                 let lowered_trait_def_id = self.lower_node_id(id).expect_owner();
 403                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 404                     ast_generics,
 405                     lowered_trait_def_id,
 406                     AnonymousLifetimeMode::CreateParameter,
 407                     |this, _| {
 408                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 409                             this.lower_trait_ref(trait_ref, ImplTraitContext::disallowed())
 410                         });
 411
 412                         if let Some(ref trait_ref) = trait_ref {
 413                             if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res {
 414                                 this.trait_impls
 415                                     .entry(def_id)
 416                                     .or_default()
 417                                     .push(lowered_trait_def_id);
 418                             }
 419                         }
 420
 421                         let lowered_ty = this.lower_ty(ty, ImplTraitContext::disallowed());
 422
 423                         (trait_ref, lowered_ty)
 424                     },
 425                 );
 426
 427                 let new_impl_items =
 428                     self.with_in_scope_lifetime_defs(&ast_generics.params, |this| {
 429                         this.arena.alloc_from_iter(
 430                             impl_items.iter().map(|item| this.lower_impl_item_ref(item)),
 431                         )
 432                     });
 433
 434                 // `defaultness.has_value()` is never called for an `impl`, always `true` in order
 435                 // to not cause an assertion failure inside the `lower_defaultness` function.
 436                 let has_val = true;
 437                 let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
 438                 hir::ItemKind::Impl(hir::Impl {
 439                     unsafety: self.lower_unsafety(unsafety),
 440                     polarity,
 441                     defaultness,
 442                     defaultness_span,
 443                     constness: self.lower_constness(constness),
 444                     generics,
 445                     of_trait: trait_ref,
 446                     self_ty: lowered_ty,
 447                     items: new_impl_items,
 448                 })
 449             }
 450             ItemKind::Trait(box TraitKind(
 451                 is_auto,
 452                 unsafety,
 453                 ref generics,
 454                 ref bounds,
 455                 ref items,
 456             )) => {
 457                 let bounds = self.lower_param_bounds(bounds, ImplTraitContext::disallowed());
 458                 let items = self
 459                     .arena
 460                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
 461                 hir::ItemKind::Trait(
 462                     is_auto,
 463                     self.lower_unsafety(unsafety),
 464                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 465                     bounds,
 466                     items,
 467                 )
 468             }
 469             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 470                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 471                 self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 472             ),
 473             ItemKind::MacroDef(..) | ItemKind::MacCall(..) => {
 474                 panic!("`TyMac` should have been expanded by now")
 475             }
 476         }
 477     }
 478
 479     fn lower_const_item(
 480         &mut self,
 481         ty: &Ty,
 482         span: Span,
 483         body: Option<&Expr>,
 484     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 485         let mut capturable_lifetimes;
 486         let itctx = if self.sess.features_untracked().impl_trait_in_bindings {
 487             capturable_lifetimes = FxHashSet::default();
 488             ImplTraitContext::OtherOpaqueTy {
 489                 capturable_lifetimes: &mut capturable_lifetimes,
 490                 origin: hir::OpaqueTyOrigin::Misc,
 491             }
 492         } else {
 493             ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
 494         };
 495         let ty = self.lower_ty(ty, itctx);
 496         (ty, self.lower_const_body(span, body))
 497     }
 498
 499     fn lower_use_tree(
 500         &mut self,
 501         tree: &UseTree,
 502         prefix: &Path,
 503         id: NodeId,
 504         vis: &mut hir::Visibility<'hir>,
 505         ident: &mut Ident,
 506         attrs: Option<&'hir [Attribute]>,
 507     ) -> hir::ItemKind<'hir> {
 508         debug!("lower_use_tree(tree={:?})", tree);
 509         debug!("lower_use_tree: vis = {:?}", vis);
 510
 511         let path = &tree.prefix;
 512         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 513
 514         match tree.kind {
 515             UseTreeKind::Simple(rename, id1, id2) => {
 516                 *ident = tree.ident();
 517
 518                 // First, apply the prefix to the path.
 519                 let mut path = Path { segments, span: path.span, tokens: None };
 520
 521                 // Correctly resolve `self` imports.
 522                 if path.segments.len() > 1
 523                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 524                 {
 525                     let _ = path.segments.pop();
 526                     if rename.is_none() {
 527                         *ident = path.segments.last().unwrap().ident;
 528                     }
 529                 }
 530
 531                 let mut resolutions = self.expect_full_res_from_use(id);
 532                 // We want to return *something* from this function, so hold onto the first item
 533                 // for later.
 534                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 535
 536                 // Here, we are looping over namespaces, if they exist for the definition
 537                 // being imported. We only handle type and value namespaces because we
 538                 // won't be dealing with macros in the rest of the compiler.
 539                 // Essentially a single `use` which imports two names is desugared into
 540                 // two imports.
 541                 for (res, &new_node_id) in iter::zip(resolutions, &[id1, id2]) {
 542                     let ident = *ident;
 543                     let mut path = path.clone();
 544                     for seg in &mut path.segments {
 545                         seg.id = self.resolver.next_node_id();
 546                     }
 547                     let span = path.span;
 548
 549                     self.with_hir_id_owner(new_node_id, |this| {
 550                         let new_id = this.lower_node_id(new_node_id);
 551                         let res = this.lower_res(res);
 552                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit, None);
 553                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 554                         let vis = this.rebuild_vis(&vis);
 555                         if let Some(attrs) = attrs {
 556                             this.attrs.insert(new_id, attrs);
 557                         }
 558
 559                         this.insert_item(hir::Item {
 560                             def_id: new_id.expect_owner(),
 561                             ident,
 562                             kind,
 563                             vis,
 564                             span,
 565                         });
 566                     });
 567                 }
 568
 569                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit, None);
 570                 hir::ItemKind::Use(path, hir::UseKind::Single)
 571             }
 572             UseTreeKind::Glob => {
 573                 let path = self.lower_path(
 574                     id,
 575                     &Path { segments, span: path.span, tokens: None },
 576                     ParamMode::Explicit,
 577                 );
 578                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 579             }
 580             UseTreeKind::Nested(ref trees) => {
 581                 // Nested imports are desugared into simple imports.
 582                 // So, if we start with
 583                 //
 584                 // ```
 585                 // pub(x) use foo::{a, b};
 586                 // ```
 587                 //
 588                 // we will create three items:
 589                 //
 590                 // ```
 591                 // pub(x) use foo::a;
 592                 // pub(x) use foo::b;
 593                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 594                 // ```
 595                 //
 596                 // The first two are produced by recursively invoking
 597                 // `lower_use_tree` (and indeed there may be things
 598                 // like `use foo::{a::{b, c}}` and so forth).  They
 599                 // wind up being directly added to
 600                 // `self.items`. However, the structure of this
 601                 // function also requires us to return one item, and
 602                 // for that we return the `{}` import (called the
 603                 // `ListStem`).
 604
 605                 let prefix = Path { segments, span: prefix.span.to(path.span), tokens: None };
 606
 607                 // Add all the nested `PathListItem`s to the HIR.
 608                 for &(ref use_tree, id) in trees {
 609                     let new_hir_id = self.lower_node_id(id);
 610
 611                     let mut prefix = prefix.clone();
 612
 613                     // Give the segments new node-ids since they are being cloned.
 614                     for seg in &mut prefix.segments {
 615                         seg.id = self.resolver.next_node_id();
 616                     }
 617
 618                     // Each `use` import is an item and thus are owners of the
 619                     // names in the path. Up to this point the nested import is
 620                     // the current owner, since we want each desugared import to
 621                     // own its own names, we have to adjust the owner before
 622                     // lowering the rest of the import.
 623                     self.with_hir_id_owner(id, |this| {
 624                         let mut vis = this.rebuild_vis(&vis);
 625                         let mut ident = *ident;
 626
 627                         let kind =
 628                             this.lower_use_tree(use_tree, &prefix, id, &mut vis, &mut ident, attrs);
 629                         if let Some(attrs) = attrs {
 630                             this.attrs.insert(new_hir_id, attrs);
 631                         }
 632
 633                         this.insert_item(hir::Item {
 634                             def_id: new_hir_id.expect_owner(),
 635                             ident,
 636                             kind,
 637                             vis,
 638                             span: use_tree.span,
 639                         });
 640                     });
 641                 }
 642
 643                 // Subtle and a bit hacky: we lower the privacy level
 644                 // of the list stem to "private" most of the time, but
 645                 // not for "restricted" paths. The key thing is that
 646                 // we don't want it to stay as `pub` (with no caveats)
 647                 // because that affects rustdoc and also the lints
 648                 // about `pub` items. But we can't *always* make it
 649                 // private -- particularly not for restricted paths --
 650                 // because it contains node-ids that would then be
 651                 // unused, failing the check that HirIds are "densely
 652                 // assigned".
 653                 match vis.node {
 654                     hir::VisibilityKind::Public
 655                     | hir::VisibilityKind::Crate(_)
 656                     | hir::VisibilityKind::Inherited => {
 657                         *vis = respan(prefix.span.shrink_to_lo(), hir::VisibilityKind::Inherited);
 658                     }
 659                     hir::VisibilityKind::Restricted { .. } => {
 660                         // Do nothing here, as described in the comment on the match.
 661                     }
 662                 }
 663
 664                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 665                 let res = self.lower_res(res);
 666                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit, None);
 667                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 668             }
 669         }
 670     }
 671
 672     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 673     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 674     /// `NodeId`s. (See, e.g., #56128.)
 675     fn rebuild_use_path(&mut self, path: &hir::Path<'hir>) -> &'hir hir::Path<'hir> {
 676         debug!("rebuild_use_path(path = {:?})", path);
 677         let segments =
 678             self.arena.alloc_from_iter(path.segments.iter().map(|seg| hir::PathSegment {
 679                 ident: seg.ident,
 680                 hir_id: seg.hir_id.map(|_| self.next_id()),
 681                 res: seg.res,
 682                 args: None,
 683                 infer_args: seg.infer_args,
 684             }));
 685         self.arena.alloc(hir::Path { span: path.span, res: path.res, segments })
 686     }
 687
 688     fn rebuild_vis(&mut self, vis: &hir::Visibility<'hir>) -> hir::Visibility<'hir> {
 689         let vis_kind = match vis.node {
 690             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 691             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 692             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 693             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 694                 hir::VisibilityKind::Restricted {
 695                     path: self.rebuild_use_path(path),
 696                     hir_id: self.next_id(),
 697                 }
 698             }
 699         };
 700         respan(vis.span, vis_kind)
 701     }
 702
 703     fn lower_foreign_item(&mut self, i: &ForeignItem) -> hir::ForeignItem<'hir> {
 704         let hir_id = self.lower_node_id(i.id);
 705         let def_id = hir_id.expect_owner();
 706         self.lower_attrs(hir_id, &i.attrs);
 707         hir::ForeignItem {
 708             def_id,
 709             ident: i.ident,
 710             kind: match i.kind {
 711                 ForeignItemKind::Fn(box FnKind(_, ref sig, ref generics, _)) => {
 712                     let fdec = &sig.decl;
 713                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 714                         generics,
 715                         def_id,
 716                         AnonymousLifetimeMode::PassThrough,
 717                         |this, _| {
 718                             (
 719                                 // Disallow `impl Trait` in foreign items.
 720                                 this.lower_fn_decl(fdec, None, false, None),
 721                                 this.lower_fn_params_to_names(fdec),
 722                             )
 723                         },
 724                     );
 725
 726                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 727                 }
 728                 ForeignItemKind::Static(ref t, m, _) => {
 729                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 730                     hir::ForeignItemKind::Static(ty, m)
 731                 }
 732                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 733                 ForeignItemKind::MacCall(_) => panic!("macro shouldn't exist here"),
 734             },
 735             vis: self.lower_visibility(&i.vis, None),
 736             span: i.span,
 737         }
 738     }
 739
 740     fn lower_foreign_item_ref(&mut self, i: &ForeignItem) -> hir::ForeignItemRef<'hir> {
 741         hir::ForeignItemRef {
 742             id: hir::ForeignItemId { def_id: self.lower_node_id(i.id).expect_owner() },
 743             ident: i.ident,
 744             span: i.span,
 745             vis: self.lower_visibility(&i.vis, Some(i.id)),
 746         }
 747     }
 748
 749     fn lower_global_asm(&mut self, ga: &GlobalAsm) -> &'hir hir::GlobalAsm {
 750         self.arena.alloc(hir::GlobalAsm { asm: ga.asm })
 751     }
 752
 753     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 754         let id = self.lower_node_id(v.id);
 755         self.lower_attrs(id, &v.attrs);
 756         hir::Variant {
 757             id,
 758             data: self.lower_variant_data(id, &v.data),
 759             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 760             ident: v.ident,
 761             span: v.span,
 762         }
 763     }
 764
 765     fn lower_variant_data(
 766         &mut self,
 767         parent_id: hir::HirId,
 768         vdata: &VariantData,
 769     ) -> hir::VariantData<'hir> {
 770         match *vdata {
 771             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 772                 self.arena
 773                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f))),
 774                 recovered,
 775             ),
 776             VariantData::Tuple(ref fields, id) => {
 777                 let ctor_id = self.lower_node_id(id);
 778                 self.alias_attrs(ctor_id, parent_id);
 779                 hir::VariantData::Tuple(
 780                     self.arena.alloc_from_iter(
 781                         fields.iter().enumerate().map(|f| self.lower_field_def(f)),
 782                     ),
 783                     ctor_id,
 784                 )
 785             }
 786             VariantData::Unit(id) => {
 787                 let ctor_id = self.lower_node_id(id);
 788                 self.alias_attrs(ctor_id, parent_id);
 789                 hir::VariantData::Unit(ctor_id)
 790             }
 791         }
 792     }
 793
 794     fn lower_field_def(&mut self, (index, f): (usize, &FieldDef)) -> hir::FieldDef<'hir> {
 795         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 796             let t = self.lower_path_ty(
 797                 &f.ty,
 798                 qself,
 799                 path,
 800                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 801                 ImplTraitContext::disallowed(),
 802             );
 803             self.arena.alloc(t)
 804         } else {
 805             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 806         };
 807         let hir_id = self.lower_node_id(f.id);
 808         self.lower_attrs(hir_id, &f.attrs);
 809         hir::FieldDef {
 810             span: f.span,
 811             hir_id,
 812             ident: match f.ident {
 813                 Some(ident) => ident,
 814                 // FIXME(jseyfried): positional field hygiene.
 815                 None => Ident::new(sym::integer(index), f.span),
 816             },
 817             vis: self.lower_visibility(&f.vis, None),
 818             ty,
 819         }
 820     }
 821
 822     fn lower_trait_item(&mut self, i: &AssocItem) -> hir::TraitItem<'hir> {
 823         let hir_id = self.lower_node_id(i.id);
 824         let trait_item_def_id = hir_id.expect_owner();
 825
 826         let (generics, kind) = match i.kind {
 827             AssocItemKind::Const(_, ref ty, ref default) => {
 828                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 829                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 830                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
 831             }
 832             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, None)) => {
 833                 let names = self.lower_fn_params_to_names(&sig.decl);
 834                 let (generics, sig) =
 835                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None, i.id);
 836                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)))
 837             }
 838             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, Some(ref body))) => {
 839                 let body_id = self.lower_fn_body_block(i.span, &sig.decl, Some(body));
 840                 let (generics, sig) =
 841                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None, i.id);
 842                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)))
 843             }
 844             AssocItemKind::TyAlias(box TyAliasKind(_, ref generics, ref bounds, ref default)) => {
 845                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
 846                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 847                 let kind = hir::TraitItemKind::Type(
 848                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 849                     ty,
 850                 );
 851
 852                 (generics, kind)
 853             }
 854             AssocItemKind::MacCall(..) => panic!("macro item shouldn't exist at this point"),
 855         };
 856
 857         self.lower_attrs(hir_id, &i.attrs);
 858         hir::TraitItem { def_id: trait_item_def_id, ident: i.ident, generics, kind, span: i.span }
 859     }
 860
 861     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 862         let (kind, has_default) = match &i.kind {
 863             AssocItemKind::Const(_, _, default) => (hir::AssocItemKind::Const, default.is_some()),
 864             AssocItemKind::TyAlias(box TyAliasKind(_, _, _, default)) => {
 865                 (hir::AssocItemKind::Type, default.is_some())
 866             }
 867             AssocItemKind::Fn(box FnKind(_, sig, _, default)) => {
 868                 (hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }, default.is_some())
 869             }
 870             AssocItemKind::MacCall(..) => unimplemented!(),
 871         };
 872         let id = hir::TraitItemId { def_id: self.lower_node_id(i.id).expect_owner() };
 873         let defaultness = hir::Defaultness::Default { has_value: has_default };
 874         hir::TraitItemRef { id, ident: i.ident, span: i.span, defaultness, kind }
 875     }
 876
 877     /// Construct `ExprKind::Err` for the given `span`.
 878     crate fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 879         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 880     }
 881
 882     fn lower_impl_item(&mut self, i: &AssocItem) -> hir::ImplItem<'hir> {
 883         let impl_item_def_id = self.resolver.local_def_id(i.id);
 884
 885         let (generics, kind) = match &i.kind {
 886             AssocItemKind::Const(_, ty, expr) => {
 887                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 888                 (
 889                     hir::Generics::empty(),
 890                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 891                 )
 892             }
 893             AssocItemKind::Fn(box FnKind(_, sig, generics, body)) => {
 894                 self.current_item = Some(i.span);
 895                 let asyncness = sig.header.asyncness;
 896                 let body_id =
 897                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 898                 let impl_trait_return_allow = !self.is_in_trait_impl;
 899                 let (generics, sig) = self.lower_method_sig(
 900                     generics,
 901                     sig,
 902                     impl_item_def_id,
 903                     impl_trait_return_allow,
 904                     asyncness.opt_return_id(),
 905                     i.id,
 906                 );
 907
 908                 (generics, hir::ImplItemKind::Fn(sig, body_id))
 909             }
 910             AssocItemKind::TyAlias(box TyAliasKind(_, generics, _, ty)) => {
 911                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 912                 let kind = match ty {
 913                     None => {
 914                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 915                         hir::ImplItemKind::TyAlias(ty)
 916                     }
 917                     Some(ty) => {
 918                         let ty = self.lower_ty(
 919                             ty,
 920                             ImplTraitContext::OtherOpaqueTy {
 921                                 capturable_lifetimes: &mut FxHashSet::default(),
 922                                 origin: hir::OpaqueTyOrigin::TyAlias,
 923                             },
 924                         );
 925                         hir::ImplItemKind::TyAlias(ty)
 926                     }
 927                 };
 928                 (generics, kind)
 929             }
 930             AssocItemKind::MacCall(..) => panic!("`TyMac` should have been expanded by now"),
 931         };
 932
 933         // Since `default impl` is not yet implemented, this is always true in impls.
 934         let has_value = true;
 935         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 936         let hir_id = self.lower_node_id(i.id);
 937         self.lower_attrs(hir_id, &i.attrs);
 938         hir::ImplItem {
 939             def_id: hir_id.expect_owner(),
 940             ident: i.ident,
 941             generics,
 942             vis: self.lower_visibility(&i.vis, None),
 943             defaultness,
 944             kind,
 945             span: i.span,
 946         }
 947     }
 948
 949     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef<'hir> {
 950         // Since `default impl` is not yet implemented, this is always true in impls.
 951         let has_value = true;
 952         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 953         hir::ImplItemRef {
 954             id: hir::ImplItemId { def_id: self.lower_node_id(i.id).expect_owner() },
 955             ident: i.ident,
 956             span: i.span,
 957             vis: self.lower_visibility(&i.vis, Some(i.id)),
 958             defaultness,
 959             kind: match &i.kind {
 960                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 961                 AssocItemKind::TyAlias(..) => hir::AssocItemKind::Type,
 962                 AssocItemKind::Fn(box FnKind(_, sig, ..)) => {
 963                     hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
 964                 }
 965                 AssocItemKind::MacCall(..) => unimplemented!(),
 966             },
 967         }
 968     }
 969
 970     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 971     /// the address space of that item instead of the item currently being
 972     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 973     /// lower a `Visibility` value although we haven't lowered the owning
 974     /// `ImplItem` in question yet.
 975     fn lower_visibility(
 976         &mut self,
 977         v: &Visibility,
 978         explicit_owner: Option<NodeId>,
 979     ) -> hir::Visibility<'hir> {
 980         let node = match v.kind {
 981             VisibilityKind::Public => hir::VisibilityKind::Public,
 982             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 983             VisibilityKind::Restricted { ref path, id } => {
 984                 debug!("lower_visibility: restricted path id = {:?}", id);
 985                 let lowered_id = if let Some(owner) = explicit_owner {
 986                     self.lower_node_id_with_owner(id, owner)
 987                 } else {
 988                     self.lower_node_id(id)
 989                 };
 990                 let res = self.expect_full_res(id);
 991                 let res = self.lower_res(res);
 992                 hir::VisibilityKind::Restricted {
 993                     path: self.lower_path_extra(res, path, ParamMode::Explicit, explicit_owner),
 994                     hir_id: lowered_id,
 995                 }
 996             }
 997             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 998         };
 999         respan(v.span, node)
1000     }
1001
1002     fn lower_defaultness(
1003         &self,
1004         d: Defaultness,
1005         has_value: bool,
1006     ) -> (hir::Defaultness, Option<Span>) {
1007         match d {
1008             Defaultness::Default(sp) => (hir::Defaultness::Default { has_value }, Some(sp)),
1009             Defaultness::Final => {
1010                 assert!(has_value);
1011                 (hir::Defaultness::Final, None)
1012             }
1013         }
1014     }
1015
1016     fn record_body(
1017         &mut self,
1018         params: &'hir [hir::Param<'hir>],
1019         value: hir::Expr<'hir>,
1020     ) -> hir::BodyId {
1021         let body = hir::Body { generator_kind: self.generator_kind, params, value };
1022         let id = body.id();
1023         self.bodies.insert(id, body);
1024         id
1025     }
1026
1027     pub(super) fn lower_body(
1028         &mut self,
1029         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
1030     ) -> hir::BodyId {
1031         let prev_gen_kind = self.generator_kind.take();
1032         let task_context = self.task_context.take();
1033         let (parameters, result) = f(self);
1034         let body_id = self.record_body(parameters, result);
1035         self.task_context = task_context;
1036         self.generator_kind = prev_gen_kind;
1037         body_id
1038     }
1039
1040     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
1041         let hir_id = self.lower_node_id(param.id);
1042         self.lower_attrs(hir_id, &param.attrs);
1043         hir::Param {
1044             hir_id,
1045             pat: self.lower_pat(&param.pat),
1046             ty_span: param.ty.span,
1047             span: param.span,
1048         }
1049     }
1050
1051     pub(super) fn lower_fn_body(
1052         &mut self,
1053         decl: &FnDecl,
1054         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
1055     ) -> hir::BodyId {
1056         self.lower_body(|this| {
1057             (
1058                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
1059                 body(this),
1060             )
1061         })
1062     }
1063
1064     fn lower_fn_body_block(
1065         &mut self,
1066         span: Span,
1067         decl: &FnDecl,
1068         body: Option<&Block>,
1069     ) -> hir::BodyId {
1070         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1071     }
1072
1073     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1074         match block {
1075             Some(block) => self.lower_block_expr(block),
1076             None => self.expr_err(span),
1077         }
1078     }
1079
1080     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1081         self.lower_body(|this| {
1082             (
1083                 &[],
1084                 match expr {
1085                     Some(expr) => this.lower_expr_mut(expr),
1086                     None => this.expr_err(span),
1087                 },
1088             )
1089         })
1090     }
1091
1092     fn lower_maybe_async_body(
1093         &mut self,
1094         span: Span,
1095         decl: &FnDecl,
1096         asyncness: Async,
1097         body: Option<&Block>,
1098     ) -> hir::BodyId {
1099         let closure_id = match asyncness {
1100             Async::Yes { closure_id, .. } => closure_id,
1101             Async::No => return self.lower_fn_body_block(span, decl, body),
1102         };
1103
1104         self.lower_body(|this| {
1105             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1106             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1107
1108             // Async function parameters are lowered into the closure body so that they are
1109             // captured and so that the drop order matches the equivalent non-async functions.
1110             //
1111             // from:
1112             //
1113             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1114             //         <body>
1115             //     }
1116             //
1117             // into:
1118             //
1119             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1120             //       async move {
1121             //         let __arg2 = __arg2;
1122             //         let <pattern> = __arg2;
1123             //         let __arg1 = __arg1;
1124             //         let <pattern> = __arg1;
1125             //         let __arg0 = __arg0;
1126             //         let <pattern> = __arg0;
1127             //         drop-temps { <body> } // see comments later in fn for details
1128             //       }
1129             //     }
1130             //
1131             // If `<pattern>` is a simple ident, then it is lowered to a single
1132             // `let <pattern> = <pattern>;` statement as an optimization.
1133             //
1134             // Note that the body is embedded in `drop-temps`; an
1135             // equivalent desugaring would be `return { <body>
1136             // };`. The key point is that we wish to drop all the
1137             // let-bound variables and temporaries created in the body
1138             // (and its tail expression!) before we drop the
1139             // parameters (c.f. rust-lang/rust#64512).
1140             for (index, parameter) in decl.inputs.iter().enumerate() {
1141                 let parameter = this.lower_param(parameter);
1142                 let span = parameter.pat.span;
1143
1144                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1145                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1146                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1147                     hir::PatKind::Binding(
1148                         hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable,
1149                         _,
1150                         ident,
1151                         _,
1152                     ) => (ident, true),
1153                     // For `ref mut` or wildcard arguments, we can't reuse the binding, but
1154                     // we can keep the same name for the parameter.
1155                     // This lets rustdoc render it correctly in documentation.
1156                     hir::PatKind::Binding(_, _, ident, _) => (ident, false),
1157                     hir::PatKind::Wild => {
1158                         (Ident::with_dummy_span(rustc_span::symbol::kw::Underscore), false)
1159                     }
1160                     _ => {
1161                         // Replace the ident for bindings that aren't simple.
1162                         let name = format!("__arg{}", index);
1163                         let ident = Ident::from_str(&name);
1164
1165                         (ident, false)
1166                     }
1167                 };
1168
1169                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1170
1171                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1172                 // async function.
1173                 //
1174                 // If this is the simple case, this parameter will end up being the same as the
1175                 // original parameter, but with a different pattern id.
1176                 let stmt_attrs = this.attrs.get(&parameter.hir_id).copied();
1177                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1178                 let new_parameter = hir::Param {
1179                     hir_id: parameter.hir_id,
1180                     pat: new_parameter_pat,
1181                     ty_span: parameter.ty_span,
1182                     span: parameter.span,
1183                 };
1184
1185                 if is_simple_parameter {
1186                     // If this is the simple case, then we only insert one statement that is
1187                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1188                     // `HirId`s are densely assigned.
1189                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1190                     let stmt = this.stmt_let_pat(
1191                         stmt_attrs,
1192                         desugared_span,
1193                         Some(expr),
1194                         parameter.pat,
1195                         hir::LocalSource::AsyncFn,
1196                     );
1197                     statements.push(stmt);
1198                 } else {
1199                     // If this is not the simple case, then we construct two statements:
1200                     //
1201                     // ```
1202                     // let __argN = __argN;
1203                     // let <pat> = __argN;
1204                     // ```
1205                     //
1206                     // The first statement moves the parameter into the closure and thus ensures
1207                     // that the drop order is correct.
1208                     //
1209                     // The second statement creates the bindings that the user wrote.
1210
1211                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1212                     // because the user may have specified a `ref mut` binding in the next
1213                     // statement.
1214                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1215                         desugared_span,
1216                         ident,
1217                         hir::BindingAnnotation::Mutable,
1218                     );
1219                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1220                     let move_stmt = this.stmt_let_pat(
1221                         None,
1222                         desugared_span,
1223                         Some(move_expr),
1224                         move_pat,
1225                         hir::LocalSource::AsyncFn,
1226                     );
1227
1228                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1229                     // parameter's pattern so that `HirId`s are densely assigned.
1230                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1231                     let pattern_stmt = this.stmt_let_pat(
1232                         stmt_attrs,
1233                         desugared_span,
1234                         Some(pattern_expr),
1235                         parameter.pat,
1236                         hir::LocalSource::AsyncFn,
1237                     );
1238
1239                     statements.push(move_stmt);
1240                     statements.push(pattern_stmt);
1241                 };
1242
1243                 parameters.push(new_parameter);
1244             }
1245
1246             let body_span = body.map_or(span, |b| b.span);
1247             let async_expr = this.make_async_expr(
1248                 CaptureBy::Value,
1249                 closure_id,
1250                 None,
1251                 body_span,
1252                 hir::AsyncGeneratorKind::Fn,
1253                 |this| {
1254                     // Create a block from the user's function body:
1255                     let user_body = this.lower_block_expr_opt(body_span, body);
1256
1257                     // Transform into `drop-temps { <user-body> }`, an expression:
1258                     let desugared_span =
1259                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1260                     let user_body = this.expr_drop_temps(
1261                         desugared_span,
1262                         this.arena.alloc(user_body),
1263                         AttrVec::new(),
1264                     );
1265
1266                     // As noted above, create the final block like
1267                     //
1268                     // ```
1269                     // {
1270                     //   let $param_pattern = $raw_param;
1271                     //   ...
1272                     //   drop-temps { <user-body> }
1273                     // }
1274                     // ```
1275                     let body = this.block_all(
1276                         desugared_span,
1277                         this.arena.alloc_from_iter(statements),
1278                         Some(user_body),
1279                     );
1280
1281                     this.expr_block(body, AttrVec::new())
1282                 },
1283             );
1284
1285             (
1286                 this.arena.alloc_from_iter(parameters),
1287                 this.expr(body_span, async_expr, AttrVec::new()),
1288             )
1289         })
1290     }
1291
1292     fn lower_method_sig(
1293         &mut self,
1294         generics: &Generics,
1295         sig: &FnSig,
1296         fn_def_id: LocalDefId,
1297         impl_trait_return_allow: bool,
1298         is_async: Option<NodeId>,
1299         id: NodeId,
1300     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1301         let header = self.lower_fn_header(sig.header, sig.span, id);
1302         let (generics, decl) = self.add_in_band_defs(
1303             generics,
1304             fn_def_id,
1305             AnonymousLifetimeMode::PassThrough,
1306             |this, idty| {
1307                 this.lower_fn_decl(
1308                     &sig.decl,
1309                     Some((fn_def_id.to_def_id(), idty)),
1310                     impl_trait_return_allow,
1311                     is_async,
1312                 )
1313             },
1314         );
1315         (generics, hir::FnSig { header, decl, span: sig.span })
1316     }
1317
1318     fn lower_fn_header(&mut self, h: FnHeader, span: Span, id: NodeId) -> hir::FnHeader {
1319         hir::FnHeader {
1320             unsafety: self.lower_unsafety(h.unsafety),
1321             asyncness: self.lower_asyncness(h.asyncness),
1322             constness: self.lower_constness(h.constness),
1323             abi: self.lower_extern(h.ext, span, id),
1324         }
1325     }
1326
1327     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1328         abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1329             self.error_on_invalid_abi(abi);
1330             abi::Abi::Rust
1331         })
1332     }
1333
1334     pub(super) fn lower_extern(&mut self, ext: Extern, span: Span, id: NodeId) -> abi::Abi {
1335         match ext {
1336             Extern::None => abi::Abi::Rust,
1337             Extern::Implicit => {
1338                 self.maybe_lint_missing_abi(span, id, abi::Abi::C { unwind: false });
1339                 abi::Abi::C { unwind: false }
1340             }
1341             Extern::Explicit(abi) => self.lower_abi(abi),
1342         }
1343     }
1344
1345     fn error_on_invalid_abi(&self, abi: StrLit) {
1346         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1347             .span_label(abi.span, "invalid ABI")
1348             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1349             .emit();
1350     }
1351
1352     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1353         match a {
1354             Async::Yes { .. } => hir::IsAsync::Async,
1355             Async::No => hir::IsAsync::NotAsync,
1356         }
1357     }
1358
1359     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1360         match c {
1361             Const::Yes(_) => hir::Constness::Const,
1362             Const::No => hir::Constness::NotConst,
1363         }
1364     }
1365
1366     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1367         match u {
1368             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1369             Unsafe::No => hir::Unsafety::Normal,
1370         }
1371     }
1372
1373     pub(super) fn lower_generics_mut(
1374         &mut self,
1375         generics: &Generics,
1376         itctx: ImplTraitContext<'_, 'hir>,
1377     ) -> GenericsCtor<'hir> {
1378         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1379         // FIXME: this could probably be done with less rightward drift. It also looks like two
1380         // control paths where `report_error` is called are the only paths that advance to after the
1381         // match statement, so the error reporting could probably just be moved there.
1382         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1383         for pred in &generics.where_clause.predicates {
1384             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1385                 'next_bound: for bound in &bound_pred.bounds {
1386                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1387                         let report_error = |this: &mut Self| {
1388                             this.diagnostic().span_err(
1389                                 bound_pred.bounded_ty.span,
1390                                 "`?Trait` bounds are only permitted at the \
1391                                  point where a type parameter is declared",
1392                             );
1393                         };
1394                         // Check if the where clause type is a plain type parameter.
1395                         match bound_pred.bounded_ty.kind {
1396                             TyKind::Path(None, ref path)
1397                                 if path.segments.len() == 1
1398                                     && bound_pred.bound_generic_params.is_empty() =>
1399                             {
1400                                 if let Some(Res::Def(DefKind::TyParam, def_id)) = self
1401                                     .resolver
1402                                     .get_partial_res(bound_pred.bounded_ty.id)
1403                                     .map(|d| d.base_res())
1404                                 {
1405                                     if let Some(def_id) = def_id.as_local() {
1406                                         for param in &generics.params {
1407                                             if let GenericParamKind::Type { .. } = param.kind {
1408                                                 if def_id == self.resolver.local_def_id(param.id) {
1409                                                     add_bounds
1410                                                         .entry(param.id)
1411                                                         .or_default()
1412                                                         .push(bound.clone());
1413                                                     continue 'next_bound;
1414                                                 }
1415                                             }
1416                                         }
1417                                     }
1418                                 }
1419                                 report_error(self)
1420                             }
1421                             _ => report_error(self),
1422                         }
1423                     }
1424                 }
1425             }
1426         }
1427
1428         GenericsCtor {
1429             params: self.lower_generic_params_mut(&generics.params, &add_bounds, itctx).collect(),
1430             where_clause: self.lower_where_clause(&generics.where_clause),
1431             span: generics.span,
1432         }
1433     }
1434
1435     pub(super) fn lower_generics(
1436         &mut self,
1437         generics: &Generics,
1438         itctx: ImplTraitContext<'_, 'hir>,
1439     ) -> hir::Generics<'hir> {
1440         let generics_ctor = self.lower_generics_mut(generics, itctx);
1441         generics_ctor.into_generics(self.arena)
1442     }
1443
1444     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1445         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1446             hir::WhereClause {
1447                 predicates: this.arena.alloc_from_iter(
1448                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1449                 ),
1450                 span: wc.span,
1451             }
1452         })
1453     }
1454
1455     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1456         match *pred {
1457             WherePredicate::BoundPredicate(WhereBoundPredicate {
1458                 ref bound_generic_params,
1459                 ref bounded_ty,
1460                 ref bounds,
1461                 span,
1462             }) => {
1463                 self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1464                     hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1465                         bound_generic_params: this.lower_generic_params(
1466                             bound_generic_params,
1467                             &NodeMap::default(),
1468                             ImplTraitContext::disallowed(),
1469                         ),
1470                         bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1471                         bounds: this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| {
1472                             match *bound {
1473                                 // Ignore `?Trait` bounds.
1474                                 // They were copied into type parameters already.
1475                                 GenericBound::Trait(_, TraitBoundModifier::Maybe) => None,
1476                                 _ => Some(
1477                                     this.lower_param_bound(bound, ImplTraitContext::disallowed()),
1478                                 ),
1479                             }
1480                         })),
1481                         span,
1482                     })
1483                 })
1484             }
1485             WherePredicate::RegionPredicate(WhereRegionPredicate {
1486                 ref lifetime,
1487                 ref bounds,
1488                 span,
1489             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1490                 span,
1491                 lifetime: self.lower_lifetime(lifetime),
1492                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1493             }),
1494             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1495                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1496                     hir_id: self.lower_node_id(id),
1497                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1498                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1499                     span,
1500                 })
1501             }
1502         }
1503     }
1504 }
1505
1506 /// Helper struct for delayed construction of Generics.
1507 pub(super) struct GenericsCtor<'hir> {
1508     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1509     where_clause: hir::WhereClause<'hir>,
1510     span: Span,
1511 }
1512
1513 impl<'hir> GenericsCtor<'hir> {
1514     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1515         hir::Generics {
1516             params: arena.alloc_from_iter(self.params),
1517             where_clause: self.where_clause,
1518             span: self.span,
1519         }
1520     }
1521 }