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 asm) => {
 333                 hir::ItemKind::GlobalAsm(self.lower_inline_asm(span, asm))
 334             }
 335             ItemKind::TyAlias(box TyAliasKind(_, ref gen, _, Some(ref ty))) => {
 336                 // We lower
 337                 //
 338                 // type Foo = impl Trait
 339                 //
 340                 // to
 341                 //
 342                 // type Foo = Foo1
 343                 // opaque type Foo1: Trait
 344                 let ty = self.lower_ty(
 345                     ty,
 346                     ImplTraitContext::TypeAliasesOpaqueTy {
 347                         capturable_lifetimes: &mut FxHashSet::default(),
 348                     },
 349                 );
 350                 let generics = self.lower_generics(gen, ImplTraitContext::disallowed());
 351                 hir::ItemKind::TyAlias(ty, generics)
 352             }
 353             ItemKind::TyAlias(box TyAliasKind(_, ref generics, _, None)) => {
 354                 let ty = self.arena.alloc(self.ty(span, hir::TyKind::Err));
 355                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 356                 hir::ItemKind::TyAlias(ty, generics)
 357             }
 358             ItemKind::Enum(ref enum_definition, ref generics) => hir::ItemKind::Enum(
 359                 hir::EnumDef {
 360                     variants: self.arena.alloc_from_iter(
 361                         enum_definition.variants.iter().map(|x| self.lower_variant(x)),
 362                     ),
 363                 },
 364                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 365             ),
 366             ItemKind::Struct(ref struct_def, ref generics) => {
 367                 let struct_def = self.lower_variant_data(hir_id, struct_def);
 368                 hir::ItemKind::Struct(
 369                     struct_def,
 370                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 371                 )
 372             }
 373             ItemKind::Union(ref vdata, ref generics) => {
 374                 let vdata = self.lower_variant_data(hir_id, vdata);
 375                 hir::ItemKind::Union(
 376                     vdata,
 377                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 378                 )
 379             }
 380             ItemKind::Impl(box ImplKind {
 381                 unsafety,
 382                 polarity,
 383                 defaultness,
 384                 constness,
 385                 generics: ref ast_generics,
 386                 of_trait: ref trait_ref,
 387                 self_ty: ref ty,
 388                 items: ref impl_items,
 389             }) => {
 390                 // Lower the "impl header" first. This ordering is important
 391                 // for in-band lifetimes! Consider `'a` here:
 392                 //
 393                 //     impl Foo<'a> for u32 {
 394                 //         fn method(&'a self) { .. }
 395                 //     }
 396                 //
 397                 // Because we start by lowering the `Foo<'a> for u32`
 398                 // part, we will add `'a` to the list of generics on
 399                 // the impl. When we then encounter it later in the
 400                 // method, it will not be considered an in-band
 401                 // lifetime to be added, but rather a reference to a
 402                 // parent lifetime.
 403                 let lowered_trait_def_id = self.lower_node_id(id).expect_owner();
 404                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 405                     ast_generics,
 406                     lowered_trait_def_id,
 407                     AnonymousLifetimeMode::CreateParameter,
 408                     |this, _| {
 409                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 410                             this.lower_trait_ref(trait_ref, ImplTraitContext::disallowed())
 411                         });
 412
 413                         if let Some(ref trait_ref) = trait_ref {
 414                             if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res {
 415                                 this.trait_impls
 416                                     .entry(def_id)
 417                                     .or_default()
 418                                     .push(lowered_trait_def_id);
 419                             }
 420                         }
 421
 422                         let lowered_ty = this.lower_ty(ty, ImplTraitContext::disallowed());
 423
 424                         (trait_ref, lowered_ty)
 425                     },
 426                 );
 427
 428                 let new_impl_items =
 429                     self.with_in_scope_lifetime_defs(&ast_generics.params, |this| {
 430                         this.arena.alloc_from_iter(
 431                             impl_items.iter().map(|item| this.lower_impl_item_ref(item)),
 432                         )
 433                     });
 434
 435                 // `defaultness.has_value()` is never called for an `impl`, always `true` in order
 436                 // to not cause an assertion failure inside the `lower_defaultness` function.
 437                 let has_val = true;
 438                 let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
 439                 hir::ItemKind::Impl(hir::Impl {
 440                     unsafety: self.lower_unsafety(unsafety),
 441                     polarity,
 442                     defaultness,
 443                     defaultness_span,
 444                     constness: self.lower_constness(constness),
 445                     generics,
 446                     of_trait: trait_ref,
 447                     self_ty: lowered_ty,
 448                     items: new_impl_items,
 449                 })
 450             }
 451             ItemKind::Trait(box TraitKind(
 452                 is_auto,
 453                 unsafety,
 454                 ref generics,
 455                 ref bounds,
 456                 ref items,
 457             )) => {
 458                 let bounds = self.lower_param_bounds(bounds, ImplTraitContext::disallowed());
 459                 let items = self
 460                     .arena
 461                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
 462                 hir::ItemKind::Trait(
 463                     is_auto,
 464                     self.lower_unsafety(unsafety),
 465                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 466                     bounds,
 467                     items,
 468                 )
 469             }
 470             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 471                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 472                 self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 473             ),
 474             ItemKind::MacroDef(..) | ItemKind::MacCall(..) => {
 475                 panic!("`TyMac` should have been expanded by now")
 476             }
 477         }
 478     }
 479
 480     fn lower_const_item(
 481         &mut self,
 482         ty: &Ty,
 483         span: Span,
 484         body: Option<&Expr>,
 485     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 486         let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Binding));
 487         (ty, self.lower_const_body(span, body))
 488     }
 489
 490     fn lower_use_tree(
 491         &mut self,
 492         tree: &UseTree,
 493         prefix: &Path,
 494         id: NodeId,
 495         vis: &mut hir::Visibility<'hir>,
 496         ident: &mut Ident,
 497         attrs: Option<&'hir [Attribute]>,
 498     ) -> hir::ItemKind<'hir> {
 499         debug!("lower_use_tree(tree={:?})", tree);
 500         debug!("lower_use_tree: vis = {:?}", vis);
 501
 502         let path = &tree.prefix;
 503         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 504
 505         match tree.kind {
 506             UseTreeKind::Simple(rename, id1, id2) => {
 507                 *ident = tree.ident();
 508
 509                 // First, apply the prefix to the path.
 510                 let mut path = Path { segments, span: path.span, tokens: None };
 511
 512                 // Correctly resolve `self` imports.
 513                 if path.segments.len() > 1
 514                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 515                 {
 516                     let _ = path.segments.pop();
 517                     if rename.is_none() {
 518                         *ident = path.segments.last().unwrap().ident;
 519                     }
 520                 }
 521
 522                 let mut resolutions = self.expect_full_res_from_use(id);
 523                 // We want to return *something* from this function, so hold onto the first item
 524                 // for later.
 525                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 526
 527                 // Here, we are looping over namespaces, if they exist for the definition
 528                 // being imported. We only handle type and value namespaces because we
 529                 // won't be dealing with macros in the rest of the compiler.
 530                 // Essentially a single `use` which imports two names is desugared into
 531                 // two imports.
 532                 for (res, &new_node_id) in iter::zip(resolutions, &[id1, id2]) {
 533                     let ident = *ident;
 534                     let mut path = path.clone();
 535                     for seg in &mut path.segments {
 536                         seg.id = self.resolver.next_node_id();
 537                     }
 538                     let span = path.span;
 539
 540                     self.with_hir_id_owner(new_node_id, |this| {
 541                         let new_id = this.lower_node_id(new_node_id);
 542                         let res = this.lower_res(res);
 543                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit, None);
 544                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 545                         let vis = this.rebuild_vis(&vis);
 546                         if let Some(attrs) = attrs {
 547                             this.attrs.insert(new_id, attrs);
 548                         }
 549
 550                         this.insert_item(hir::Item {
 551                             def_id: new_id.expect_owner(),
 552                             ident,
 553                             kind,
 554                             vis,
 555                             span,
 556                         });
 557                     });
 558                 }
 559
 560                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit, None);
 561                 hir::ItemKind::Use(path, hir::UseKind::Single)
 562             }
 563             UseTreeKind::Glob => {
 564                 let path = self.lower_path(
 565                     id,
 566                     &Path { segments, span: path.span, tokens: None },
 567                     ParamMode::Explicit,
 568                 );
 569                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 570             }
 571             UseTreeKind::Nested(ref trees) => {
 572                 // Nested imports are desugared into simple imports.
 573                 // So, if we start with
 574                 //
 575                 // ```
 576                 // pub(x) use foo::{a, b};
 577                 // ```
 578                 //
 579                 // we will create three items:
 580                 //
 581                 // ```
 582                 // pub(x) use foo::a;
 583                 // pub(x) use foo::b;
 584                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 585                 // ```
 586                 //
 587                 // The first two are produced by recursively invoking
 588                 // `lower_use_tree` (and indeed there may be things
 589                 // like `use foo::{a::{b, c}}` and so forth).  They
 590                 // wind up being directly added to
 591                 // `self.items`. However, the structure of this
 592                 // function also requires us to return one item, and
 593                 // for that we return the `{}` import (called the
 594                 // `ListStem`).
 595
 596                 let prefix = Path { segments, span: prefix.span.to(path.span), tokens: None };
 597
 598                 // Add all the nested `PathListItem`s to the HIR.
 599                 for &(ref use_tree, id) in trees {
 600                     let new_hir_id = self.lower_node_id(id);
 601
 602                     let mut prefix = prefix.clone();
 603
 604                     // Give the segments new node-ids since they are being cloned.
 605                     for seg in &mut prefix.segments {
 606                         seg.id = self.resolver.next_node_id();
 607                     }
 608
 609                     // Each `use` import is an item and thus are owners of the
 610                     // names in the path. Up to this point the nested import is
 611                     // the current owner, since we want each desugared import to
 612                     // own its own names, we have to adjust the owner before
 613                     // lowering the rest of the import.
 614                     self.with_hir_id_owner(id, |this| {
 615                         let mut vis = this.rebuild_vis(&vis);
 616                         let mut ident = *ident;
 617
 618                         let kind =
 619                             this.lower_use_tree(use_tree, &prefix, id, &mut vis, &mut ident, attrs);
 620                         if let Some(attrs) = attrs {
 621                             this.attrs.insert(new_hir_id, attrs);
 622                         }
 623
 624                         this.insert_item(hir::Item {
 625                             def_id: new_hir_id.expect_owner(),
 626                             ident,
 627                             kind,
 628                             vis,
 629                             span: use_tree.span,
 630                         });
 631                     });
 632                 }
 633
 634                 // Subtle and a bit hacky: we lower the privacy level
 635                 // of the list stem to "private" most of the time, but
 636                 // not for "restricted" paths. The key thing is that
 637                 // we don't want it to stay as `pub` (with no caveats)
 638                 // because that affects rustdoc and also the lints
 639                 // about `pub` items. But we can't *always* make it
 640                 // private -- particularly not for restricted paths --
 641                 // because it contains node-ids that would then be
 642                 // unused, failing the check that HirIds are "densely
 643                 // assigned".
 644                 match vis.node {
 645                     hir::VisibilityKind::Public
 646                     | hir::VisibilityKind::Crate(_)
 647                     | hir::VisibilityKind::Inherited => {
 648                         *vis = respan(prefix.span.shrink_to_lo(), hir::VisibilityKind::Inherited);
 649                     }
 650                     hir::VisibilityKind::Restricted { .. } => {
 651                         // Do nothing here, as described in the comment on the match.
 652                     }
 653                 }
 654
 655                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 656                 let res = self.lower_res(res);
 657                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit, None);
 658                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 659             }
 660         }
 661     }
 662
 663     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 664     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 665     /// `NodeId`s. (See, e.g., #56128.)
 666     fn rebuild_use_path(&mut self, path: &hir::Path<'hir>) -> &'hir hir::Path<'hir> {
 667         debug!("rebuild_use_path(path = {:?})", path);
 668         let segments =
 669             self.arena.alloc_from_iter(path.segments.iter().map(|seg| hir::PathSegment {
 670                 ident: seg.ident,
 671                 hir_id: seg.hir_id.map(|_| self.next_id()),
 672                 res: seg.res,
 673                 args: None,
 674                 infer_args: seg.infer_args,
 675             }));
 676         self.arena.alloc(hir::Path { span: path.span, res: path.res, segments })
 677     }
 678
 679     fn rebuild_vis(&mut self, vis: &hir::Visibility<'hir>) -> hir::Visibility<'hir> {
 680         let vis_kind = match vis.node {
 681             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 682             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 683             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 684             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 685                 hir::VisibilityKind::Restricted {
 686                     path: self.rebuild_use_path(path),
 687                     hir_id: self.next_id(),
 688                 }
 689             }
 690         };
 691         respan(vis.span, vis_kind)
 692     }
 693
 694     fn lower_foreign_item(&mut self, i: &ForeignItem) -> hir::ForeignItem<'hir> {
 695         let hir_id = self.lower_node_id(i.id);
 696         let def_id = hir_id.expect_owner();
 697         self.lower_attrs(hir_id, &i.attrs);
 698         hir::ForeignItem {
 699             def_id,
 700             ident: i.ident,
 701             kind: match i.kind {
 702                 ForeignItemKind::Fn(box FnKind(_, ref sig, ref generics, _)) => {
 703                     let fdec = &sig.decl;
 704                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 705                         generics,
 706                         def_id,
 707                         AnonymousLifetimeMode::PassThrough,
 708                         |this, _| {
 709                             (
 710                                 // Disallow `impl Trait` in foreign items.
 711                                 this.lower_fn_decl(fdec, None, false, None),
 712                                 this.lower_fn_params_to_names(fdec),
 713                             )
 714                         },
 715                     );
 716
 717                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 718                 }
 719                 ForeignItemKind::Static(ref t, m, _) => {
 720                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 721                     hir::ForeignItemKind::Static(ty, m)
 722                 }
 723                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 724                 ForeignItemKind::MacCall(_) => panic!("macro shouldn't exist here"),
 725             },
 726             vis: self.lower_visibility(&i.vis, None),
 727             span: i.span,
 728         }
 729     }
 730
 731     fn lower_foreign_item_ref(&mut self, i: &ForeignItem) -> hir::ForeignItemRef<'hir> {
 732         hir::ForeignItemRef {
 733             id: hir::ForeignItemId { def_id: self.lower_node_id(i.id).expect_owner() },
 734             ident: i.ident,
 735             span: i.span,
 736             vis: self.lower_visibility(&i.vis, Some(i.id)),
 737         }
 738     }
 739
 740     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 741         let id = self.lower_node_id(v.id);
 742         self.lower_attrs(id, &v.attrs);
 743         hir::Variant {
 744             id,
 745             data: self.lower_variant_data(id, &v.data),
 746             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 747             ident: v.ident,
 748             span: v.span,
 749         }
 750     }
 751
 752     fn lower_variant_data(
 753         &mut self,
 754         parent_id: hir::HirId,
 755         vdata: &VariantData,
 756     ) -> hir::VariantData<'hir> {
 757         match *vdata {
 758             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 759                 self.arena
 760                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f))),
 761                 recovered,
 762             ),
 763             VariantData::Tuple(ref fields, id) => {
 764                 let ctor_id = self.lower_node_id(id);
 765                 self.alias_attrs(ctor_id, parent_id);
 766                 hir::VariantData::Tuple(
 767                     self.arena.alloc_from_iter(
 768                         fields.iter().enumerate().map(|f| self.lower_field_def(f)),
 769                     ),
 770                     ctor_id,
 771                 )
 772             }
 773             VariantData::Unit(id) => {
 774                 let ctor_id = self.lower_node_id(id);
 775                 self.alias_attrs(ctor_id, parent_id);
 776                 hir::VariantData::Unit(ctor_id)
 777             }
 778         }
 779     }
 780
 781     pub(super) fn lower_field_def(
 782         &mut self,
 783         (index, f): (usize, &FieldDef),
 784     ) -> hir::FieldDef<'hir> {
 785         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 786             let t = self.lower_path_ty(
 787                 &f.ty,
 788                 qself,
 789                 path,
 790                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 791                 ImplTraitContext::disallowed(),
 792             );
 793             self.arena.alloc(t)
 794         } else {
 795             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 796         };
 797         let hir_id = self.lower_node_id(f.id);
 798         self.lower_attrs(hir_id, &f.attrs);
 799         hir::FieldDef {
 800             span: f.span,
 801             hir_id,
 802             ident: match f.ident {
 803                 Some(ident) => ident,
 804                 // FIXME(jseyfried): positional field hygiene.
 805                 None => Ident::new(sym::integer(index), f.span),
 806             },
 807             vis: self.lower_visibility(&f.vis, None),
 808             ty,
 809         }
 810     }
 811
 812     fn lower_trait_item(&mut self, i: &AssocItem) -> hir::TraitItem<'hir> {
 813         let hir_id = self.lower_node_id(i.id);
 814         let trait_item_def_id = hir_id.expect_owner();
 815
 816         let (generics, kind) = match i.kind {
 817             AssocItemKind::Const(_, ref ty, ref default) => {
 818                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 819                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 820                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
 821             }
 822             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, None)) => {
 823                 let names = self.lower_fn_params_to_names(&sig.decl);
 824                 let (generics, sig) =
 825                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None, i.id);
 826                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)))
 827             }
 828             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, Some(ref body))) => {
 829                 let asyncness = sig.header.asyncness;
 830                 let body_id =
 831                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, Some(&body));
 832                 let (generics, sig) = self.lower_method_sig(
 833                     generics,
 834                     sig,
 835                     trait_item_def_id,
 836                     false,
 837                     asyncness.opt_return_id(),
 838                     i.id,
 839                 );
 840                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)))
 841             }
 842             AssocItemKind::TyAlias(box TyAliasKind(_, ref generics, ref bounds, ref default)) => {
 843                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
 844                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 845                 let kind = hir::TraitItemKind::Type(
 846                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 847                     ty,
 848                 );
 849
 850                 (generics, kind)
 851             }
 852             AssocItemKind::MacCall(..) => panic!("macro item shouldn't exist at this point"),
 853         };
 854
 855         self.lower_attrs(hir_id, &i.attrs);
 856         hir::TraitItem { def_id: trait_item_def_id, ident: i.ident, generics, kind, span: i.span }
 857     }
 858
 859     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 860         let (kind, has_default) = match &i.kind {
 861             AssocItemKind::Const(_, _, default) => (hir::AssocItemKind::Const, default.is_some()),
 862             AssocItemKind::TyAlias(box TyAliasKind(_, _, _, default)) => {
 863                 (hir::AssocItemKind::Type, default.is_some())
 864             }
 865             AssocItemKind::Fn(box FnKind(_, sig, _, default)) => {
 866                 (hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }, default.is_some())
 867             }
 868             AssocItemKind::MacCall(..) => unimplemented!(),
 869         };
 870         let id = hir::TraitItemId { def_id: self.lower_node_id(i.id).expect_owner() };
 871         let defaultness = hir::Defaultness::Default { has_value: has_default };
 872         hir::TraitItemRef { id, ident: i.ident, span: i.span, defaultness, kind }
 873     }
 874
 875     /// Construct `ExprKind::Err` for the given `span`.
 876     crate fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 877         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 878     }
 879
 880     fn lower_impl_item(&mut self, i: &AssocItem) -> hir::ImplItem<'hir> {
 881         let impl_item_def_id = self.resolver.local_def_id(i.id);
 882
 883         let (generics, kind) = match &i.kind {
 884             AssocItemKind::Const(_, ty, expr) => {
 885                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 886                 (
 887                     hir::Generics::empty(),
 888                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 889                 )
 890             }
 891             AssocItemKind::Fn(box FnKind(_, sig, generics, body)) => {
 892                 self.current_item = Some(i.span);
 893                 let asyncness = sig.header.asyncness;
 894                 let body_id =
 895                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 896                 let impl_trait_return_allow = !self.is_in_trait_impl;
 897                 let (generics, sig) = self.lower_method_sig(
 898                     generics,
 899                     sig,
 900                     impl_item_def_id,
 901                     impl_trait_return_allow,
 902                     asyncness.opt_return_id(),
 903                     i.id,
 904                 );
 905
 906                 (generics, hir::ImplItemKind::Fn(sig, body_id))
 907             }
 908             AssocItemKind::TyAlias(box TyAliasKind(_, generics, _, ty)) => {
 909                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 910                 let kind = match ty {
 911                     None => {
 912                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 913                         hir::ImplItemKind::TyAlias(ty)
 914                     }
 915                     Some(ty) => {
 916                         let ty = self.lower_ty(
 917                             ty,
 918                             ImplTraitContext::TypeAliasesOpaqueTy {
 919                                 capturable_lifetimes: &mut FxHashSet::default(),
 920                             },
 921                         );
 922                         hir::ImplItemKind::TyAlias(ty)
 923                     }
 924                 };
 925                 (generics, kind)
 926             }
 927             AssocItemKind::MacCall(..) => panic!("`TyMac` should have been expanded by now"),
 928         };
 929
 930         // Since `default impl` is not yet implemented, this is always true in impls.
 931         let has_value = true;
 932         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 933         let hir_id = self.lower_node_id(i.id);
 934         self.lower_attrs(hir_id, &i.attrs);
 935         hir::ImplItem {
 936             def_id: hir_id.expect_owner(),
 937             ident: i.ident,
 938             generics,
 939             vis: self.lower_visibility(&i.vis, None),
 940             defaultness,
 941             kind,
 942             span: i.span,
 943         }
 944     }
 945
 946     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef<'hir> {
 947         // Since `default impl` is not yet implemented, this is always true in impls.
 948         let has_value = true;
 949         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 950         hir::ImplItemRef {
 951             id: hir::ImplItemId { def_id: self.lower_node_id(i.id).expect_owner() },
 952             ident: i.ident,
 953             span: i.span,
 954             vis: self.lower_visibility(&i.vis, Some(i.id)),
 955             defaultness,
 956             kind: match &i.kind {
 957                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 958                 AssocItemKind::TyAlias(..) => hir::AssocItemKind::Type,
 959                 AssocItemKind::Fn(box FnKind(_, sig, ..)) => {
 960                     hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
 961                 }
 962                 AssocItemKind::MacCall(..) => unimplemented!(),
 963             },
 964         }
 965     }
 966
 967     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 968     /// the address space of that item instead of the item currently being
 969     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 970     /// lower a `Visibility` value although we haven't lowered the owning
 971     /// `ImplItem` in question yet.
 972     fn lower_visibility(
 973         &mut self,
 974         v: &Visibility,
 975         explicit_owner: Option<NodeId>,
 976     ) -> hir::Visibility<'hir> {
 977         let node = match v.kind {
 978             VisibilityKind::Public => hir::VisibilityKind::Public,
 979             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 980             VisibilityKind::Restricted { ref path, id } => {
 981                 debug!("lower_visibility: restricted path id = {:?}", id);
 982                 let lowered_id = if let Some(owner) = explicit_owner {
 983                     self.lower_node_id_with_owner(id, owner)
 984                 } else {
 985                     self.lower_node_id(id)
 986                 };
 987                 let res = self.expect_full_res(id);
 988                 let res = self.lower_res(res);
 989                 hir::VisibilityKind::Restricted {
 990                     path: self.lower_path_extra(res, path, ParamMode::Explicit, explicit_owner),
 991                     hir_id: lowered_id,
 992                 }
 993             }
 994             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 995         };
 996         respan(v.span, node)
 997     }
 998
 999     fn lower_defaultness(
1000         &self,
1001         d: Defaultness,
1002         has_value: bool,
1003     ) -> (hir::Defaultness, Option<Span>) {
1004         match d {
1005             Defaultness::Default(sp) => (hir::Defaultness::Default { has_value }, Some(sp)),
1006             Defaultness::Final => {
1007                 assert!(has_value);
1008                 (hir::Defaultness::Final, None)
1009             }
1010         }
1011     }
1012
1013     fn record_body(
1014         &mut self,
1015         params: &'hir [hir::Param<'hir>],
1016         value: hir::Expr<'hir>,
1017     ) -> hir::BodyId {
1018         let body = hir::Body { generator_kind: self.generator_kind, params, value };
1019         let id = body.id();
1020         self.bodies.insert(id, body);
1021         id
1022     }
1023
1024     pub(super) fn lower_body(
1025         &mut self,
1026         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
1027     ) -> hir::BodyId {
1028         let prev_gen_kind = self.generator_kind.take();
1029         let task_context = self.task_context.take();
1030         let (parameters, result) = f(self);
1031         let body_id = self.record_body(parameters, result);
1032         self.task_context = task_context;
1033         self.generator_kind = prev_gen_kind;
1034         body_id
1035     }
1036
1037     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
1038         let hir_id = self.lower_node_id(param.id);
1039         self.lower_attrs(hir_id, &param.attrs);
1040         hir::Param {
1041             hir_id,
1042             pat: self.lower_pat(&param.pat),
1043             ty_span: param.ty.span,
1044             span: param.span,
1045         }
1046     }
1047
1048     pub(super) fn lower_fn_body(
1049         &mut self,
1050         decl: &FnDecl,
1051         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
1052     ) -> hir::BodyId {
1053         self.lower_body(|this| {
1054             (
1055                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
1056                 body(this),
1057             )
1058         })
1059     }
1060
1061     fn lower_fn_body_block(
1062         &mut self,
1063         span: Span,
1064         decl: &FnDecl,
1065         body: Option<&Block>,
1066     ) -> hir::BodyId {
1067         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1068     }
1069
1070     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1071         match block {
1072             Some(block) => self.lower_block_expr(block),
1073             None => self.expr_err(span),
1074         }
1075     }
1076
1077     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1078         self.lower_body(|this| {
1079             (
1080                 &[],
1081                 match expr {
1082                     Some(expr) => this.lower_expr_mut(expr),
1083                     None => this.expr_err(span),
1084                 },
1085             )
1086         })
1087     }
1088
1089     fn lower_maybe_async_body(
1090         &mut self,
1091         span: Span,
1092         decl: &FnDecl,
1093         asyncness: Async,
1094         body: Option<&Block>,
1095     ) -> hir::BodyId {
1096         let closure_id = match asyncness {
1097             Async::Yes { closure_id, .. } => closure_id,
1098             Async::No => return self.lower_fn_body_block(span, decl, body),
1099         };
1100
1101         self.lower_body(|this| {
1102             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1103             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1104
1105             // Async function parameters are lowered into the closure body so that they are
1106             // captured and so that the drop order matches the equivalent non-async functions.
1107             //
1108             // from:
1109             //
1110             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1111             //         <body>
1112             //     }
1113             //
1114             // into:
1115             //
1116             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1117             //       async move {
1118             //         let __arg2 = __arg2;
1119             //         let <pattern> = __arg2;
1120             //         let __arg1 = __arg1;
1121             //         let <pattern> = __arg1;
1122             //         let __arg0 = __arg0;
1123             //         let <pattern> = __arg0;
1124             //         drop-temps { <body> } // see comments later in fn for details
1125             //       }
1126             //     }
1127             //
1128             // If `<pattern>` is a simple ident, then it is lowered to a single
1129             // `let <pattern> = <pattern>;` statement as an optimization.
1130             //
1131             // Note that the body is embedded in `drop-temps`; an
1132             // equivalent desugaring would be `return { <body>
1133             // };`. The key point is that we wish to drop all the
1134             // let-bound variables and temporaries created in the body
1135             // (and its tail expression!) before we drop the
1136             // parameters (c.f. rust-lang/rust#64512).
1137             for (index, parameter) in decl.inputs.iter().enumerate() {
1138                 let parameter = this.lower_param(parameter);
1139                 let span = parameter.pat.span;
1140
1141                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1142                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1143                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1144                     hir::PatKind::Binding(
1145                         hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable,
1146                         _,
1147                         ident,
1148                         _,
1149                     ) => (ident, true),
1150                     // For `ref mut` or wildcard arguments, we can't reuse the binding, but
1151                     // we can keep the same name for the parameter.
1152                     // This lets rustdoc render it correctly in documentation.
1153                     hir::PatKind::Binding(_, _, ident, _) => (ident, false),
1154                     hir::PatKind::Wild => {
1155                         (Ident::with_dummy_span(rustc_span::symbol::kw::Underscore), false)
1156                     }
1157                     _ => {
1158                         // Replace the ident for bindings that aren't simple.
1159                         let name = format!("__arg{}", index);
1160                         let ident = Ident::from_str(&name);
1161
1162                         (ident, false)
1163                     }
1164                 };
1165
1166                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1167
1168                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1169                 // async function.
1170                 //
1171                 // If this is the simple case, this parameter will end up being the same as the
1172                 // original parameter, but with a different pattern id.
1173                 let stmt_attrs = this.attrs.get(&parameter.hir_id).copied();
1174                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1175                 let new_parameter = hir::Param {
1176                     hir_id: parameter.hir_id,
1177                     pat: new_parameter_pat,
1178                     ty_span: parameter.ty_span,
1179                     span: parameter.span,
1180                 };
1181
1182                 if is_simple_parameter {
1183                     // If this is the simple case, then we only insert one statement that is
1184                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1185                     // `HirId`s are densely assigned.
1186                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1187                     let stmt = this.stmt_let_pat(
1188                         stmt_attrs,
1189                         desugared_span,
1190                         Some(expr),
1191                         parameter.pat,
1192                         hir::LocalSource::AsyncFn,
1193                     );
1194                     statements.push(stmt);
1195                 } else {
1196                     // If this is not the simple case, then we construct two statements:
1197                     //
1198                     // ```
1199                     // let __argN = __argN;
1200                     // let <pat> = __argN;
1201                     // ```
1202                     //
1203                     // The first statement moves the parameter into the closure and thus ensures
1204                     // that the drop order is correct.
1205                     //
1206                     // The second statement creates the bindings that the user wrote.
1207
1208                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1209                     // because the user may have specified a `ref mut` binding in the next
1210                     // statement.
1211                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1212                         desugared_span,
1213                         ident,
1214                         hir::BindingAnnotation::Mutable,
1215                     );
1216                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1217                     let move_stmt = this.stmt_let_pat(
1218                         None,
1219                         desugared_span,
1220                         Some(move_expr),
1221                         move_pat,
1222                         hir::LocalSource::AsyncFn,
1223                     );
1224
1225                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1226                     // parameter's pattern so that `HirId`s are densely assigned.
1227                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1228                     let pattern_stmt = this.stmt_let_pat(
1229                         stmt_attrs,
1230                         desugared_span,
1231                         Some(pattern_expr),
1232                         parameter.pat,
1233                         hir::LocalSource::AsyncFn,
1234                     );
1235
1236                     statements.push(move_stmt);
1237                     statements.push(pattern_stmt);
1238                 };
1239
1240                 parameters.push(new_parameter);
1241             }
1242
1243             let body_span = body.map_or(span, |b| b.span);
1244             let async_expr = this.make_async_expr(
1245                 CaptureBy::Value,
1246                 closure_id,
1247                 None,
1248                 body_span,
1249                 hir::AsyncGeneratorKind::Fn,
1250                 |this| {
1251                     // Create a block from the user's function body:
1252                     let user_body = this.lower_block_expr_opt(body_span, body);
1253
1254                     // Transform into `drop-temps { <user-body> }`, an expression:
1255                     let desugared_span =
1256                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1257                     let user_body = this.expr_drop_temps(
1258                         desugared_span,
1259                         this.arena.alloc(user_body),
1260                         AttrVec::new(),
1261                     );
1262
1263                     // As noted above, create the final block like
1264                     //
1265                     // ```
1266                     // {
1267                     //   let $param_pattern = $raw_param;
1268                     //   ...
1269                     //   drop-temps { <user-body> }
1270                     // }
1271                     // ```
1272                     let body = this.block_all(
1273                         desugared_span,
1274                         this.arena.alloc_from_iter(statements),
1275                         Some(user_body),
1276                     );
1277
1278                     this.expr_block(body, AttrVec::new())
1279                 },
1280             );
1281
1282             (
1283                 this.arena.alloc_from_iter(parameters),
1284                 this.expr(body_span, async_expr, AttrVec::new()),
1285             )
1286         })
1287     }
1288
1289     fn lower_method_sig(
1290         &mut self,
1291         generics: &Generics,
1292         sig: &FnSig,
1293         fn_def_id: LocalDefId,
1294         impl_trait_return_allow: bool,
1295         is_async: Option<NodeId>,
1296         id: NodeId,
1297     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1298         let header = self.lower_fn_header(sig.header, sig.span, id);
1299         let (generics, decl) = self.add_in_band_defs(
1300             generics,
1301             fn_def_id,
1302             AnonymousLifetimeMode::PassThrough,
1303             |this, idty| {
1304                 this.lower_fn_decl(
1305                     &sig.decl,
1306                     Some((fn_def_id.to_def_id(), idty)),
1307                     impl_trait_return_allow,
1308                     is_async,
1309                 )
1310             },
1311         );
1312         (generics, hir::FnSig { header, decl, span: sig.span })
1313     }
1314
1315     fn lower_fn_header(&mut self, h: FnHeader, span: Span, id: NodeId) -> hir::FnHeader {
1316         hir::FnHeader {
1317             unsafety: self.lower_unsafety(h.unsafety),
1318             asyncness: self.lower_asyncness(h.asyncness),
1319             constness: self.lower_constness(h.constness),
1320             abi: self.lower_extern(h.ext, span, id),
1321         }
1322     }
1323
1324     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1325         abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1326             self.error_on_invalid_abi(abi);
1327             abi::Abi::Rust
1328         })
1329     }
1330
1331     pub(super) fn lower_extern(&mut self, ext: Extern, span: Span, id: NodeId) -> abi::Abi {
1332         match ext {
1333             Extern::None => abi::Abi::Rust,
1334             Extern::Implicit => {
1335                 self.maybe_lint_missing_abi(span, id, abi::Abi::C { unwind: false });
1336                 abi::Abi::C { unwind: false }
1337             }
1338             Extern::Explicit(abi) => self.lower_abi(abi),
1339         }
1340     }
1341
1342     fn error_on_invalid_abi(&self, abi: StrLit) {
1343         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1344             .span_label(abi.span, "invalid ABI")
1345             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1346             .emit();
1347     }
1348
1349     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1350         match a {
1351             Async::Yes { .. } => hir::IsAsync::Async,
1352             Async::No => hir::IsAsync::NotAsync,
1353         }
1354     }
1355
1356     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1357         match c {
1358             Const::Yes(_) => hir::Constness::Const,
1359             Const::No => hir::Constness::NotConst,
1360         }
1361     }
1362
1363     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1364         match u {
1365             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1366             Unsafe::No => hir::Unsafety::Normal,
1367         }
1368     }
1369
1370     pub(super) fn lower_generics_mut(
1371         &mut self,
1372         generics: &Generics,
1373         itctx: ImplTraitContext<'_, 'hir>,
1374     ) -> GenericsCtor<'hir> {
1375         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1376         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1377         for pred in &generics.where_clause.predicates {
1378             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1379                 'next_bound: for bound in &bound_pred.bounds {
1380                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1381                         // Check if the where clause type is a plain type parameter.
1382                         match self
1383                             .resolver
1384                             .get_partial_res(bound_pred.bounded_ty.id)
1385                             .map(|d| (d.base_res(), d.unresolved_segments()))
1386                         {
1387                             Some((Res::Def(DefKind::TyParam, def_id), 0))
1388                                 if bound_pred.bound_generic_params.is_empty() =>
1389                             {
1390                                 for param in &generics.params {
1391                                     if def_id == self.resolver.local_def_id(param.id).to_def_id() {
1392                                         add_bounds.entry(param.id).or_default().push(bound.clone());
1393                                         continue 'next_bound;
1394                                     }
1395                                 }
1396                             }
1397                             _ => {}
1398                         }
1399                         self.diagnostic().span_err(
1400                             bound_pred.bounded_ty.span,
1401                             "`?Trait` bounds are only permitted at the \
1402                                  point where a type parameter is declared",
1403                         );
1404                     }
1405                 }
1406             }
1407         }
1408
1409         GenericsCtor {
1410             params: self.lower_generic_params_mut(&generics.params, &add_bounds, itctx).collect(),
1411             where_clause: self.lower_where_clause(&generics.where_clause),
1412             span: generics.span,
1413         }
1414     }
1415
1416     pub(super) fn lower_generics(
1417         &mut self,
1418         generics: &Generics,
1419         itctx: ImplTraitContext<'_, 'hir>,
1420     ) -> hir::Generics<'hir> {
1421         let generics_ctor = self.lower_generics_mut(generics, itctx);
1422         generics_ctor.into_generics(self.arena)
1423     }
1424
1425     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1426         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1427             hir::WhereClause {
1428                 predicates: this.arena.alloc_from_iter(
1429                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1430                 ),
1431                 span: wc.span,
1432             }
1433         })
1434     }
1435
1436     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1437         match *pred {
1438             WherePredicate::BoundPredicate(WhereBoundPredicate {
1439                 ref bound_generic_params,
1440                 ref bounded_ty,
1441                 ref bounds,
1442                 span,
1443             }) => {
1444                 self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1445                     hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1446                         bound_generic_params: this.lower_generic_params(
1447                             bound_generic_params,
1448                             &NodeMap::default(),
1449                             ImplTraitContext::disallowed(),
1450                         ),
1451                         bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1452                         bounds: this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| {
1453                             match *bound {
1454                                 // Ignore `?Trait` bounds.
1455                                 // They were copied into type parameters already.
1456                                 GenericBound::Trait(_, TraitBoundModifier::Maybe) => None,
1457                                 _ => Some(
1458                                     this.lower_param_bound(bound, ImplTraitContext::disallowed()),
1459                                 ),
1460                             }
1461                         })),
1462                         span,
1463                     })
1464                 })
1465             }
1466             WherePredicate::RegionPredicate(WhereRegionPredicate {
1467                 ref lifetime,
1468                 ref bounds,
1469                 span,
1470             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1471                 span,
1472                 lifetime: self.lower_lifetime(lifetime),
1473                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1474             }),
1475             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1476                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1477                     hir_id: self.lower_node_id(id),
1478                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1479                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1480                     span,
1481                 })
1482             }
1483         }
1484     }
1485 }
1486
1487 /// Helper struct for delayed construction of Generics.
1488 pub(super) struct GenericsCtor<'hir> {
1489     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1490     where_clause: hir::WhereClause<'hir>,
1491     span: Span,
1492 }
1493
1494 impl<'hir> GenericsCtor<'hir> {
1495     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1496         hir::Generics {
1497             params: arena.alloc_from_iter(self.params),
1498             where_clause: self.where_clause,
1499             span: self.span,
1500         }
1501     }
1502 }