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<T>(
  30         &mut self,
  31         impl_ref: &Option<TraitRef>,
  32         f: impl FnOnce(&mut Self) -> T,
  33     ) -> T {
  34         let old = self.lctx.is_in_trait_impl;
  35         self.lctx.is_in_trait_impl = impl_ref.is_some();
  36         let ret = f(self);
  37         self.lctx.is_in_trait_impl = old;
  38         ret
  39     }
  40 }
  41
  42 impl<'a> Visitor<'a> for ItemLowerer<'a, '_, '_> {
  43     fn visit_item(&mut self, item: &'a Item) {
  44         let hir_id = self.lctx.with_hir_id_owner(item.id, |lctx| {
  45             lctx.without_in_scope_lifetime_defs(|lctx| {
  46                 let hir_item = lctx.lower_item(item);
  47                 lctx.insert_item(hir_item)
  48             })
  49         });
  50
  51         self.lctx.with_parent_item_lifetime_defs(hir_id, |this| {
  52             let this = &mut ItemLowerer { lctx: this };
  53             match item.kind {
  54                 ItemKind::Mod(..) => {
  55                     let def_id = this.lctx.lower_node_id(item.id).expect_owner();
  56                     let old_current_module = 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     fn visit_fn(&mut self, fk: FnKind<'a>, sp: Span, _: NodeId) {
  69         match fk {
  70             FnKind::Fn(FnCtxt::Foreign, _, sig, _, _) => {
  71                 self.visit_fn_header(&sig.header);
  72                 visit::walk_fn_decl(self, &sig.decl);
  73                 // Don't visit the foreign function body even if it has one, since lowering the
  74                 // body would have no meaning and will have already been caught as a parse error.
  75             }
  76             _ => visit::walk_fn(self, fk, sp),
  77         }
  78     }
  79
  80     fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
  81         self.lctx.with_hir_id_owner(item.id, |lctx| match ctxt {
  82             AssocCtxt::Trait => {
  83                 let hir_item = lctx.lower_trait_item(item);
  84                 lctx.insert_trait_item(hir_item);
  85             }
  86             AssocCtxt::Impl => {
  87                 let hir_item = lctx.lower_impl_item(item);
  88                 lctx.insert_impl_item(hir_item);
  89             }
  90         });
  91
  92         visit::walk_assoc_item(self, item, ctxt);
  93     }
  94
  95     fn visit_foreign_item(&mut self, item: &'a ForeignItem) {
  96         self.lctx.allocate_hir_id_counter(item.id);
  97         self.lctx.with_hir_id_owner(item.id, |lctx| {
  98             let hir_item = lctx.lower_foreign_item(item);
  99             lctx.insert_foreign_item(hir_item);
 100         });
 101
 102         visit::walk_foreign_item(self, item);
 103     }
 104 }
 105
 106 impl<'hir> LoweringContext<'_, 'hir> {
 107     // Same as the method above, but accepts `hir::GenericParam`s
 108     // instead of `ast::GenericParam`s.
 109     // This should only be used with generics that have already had their
 110     // in-band lifetimes added. In practice, this means that this function is
 111     // only used when lowering a child item of a trait or impl.
 112     fn with_parent_item_lifetime_defs<T>(
 113         &mut self,
 114         parent_hir_id: hir::ItemId,
 115         f: impl FnOnce(&mut Self) -> T,
 116     ) -> T {
 117         let old_len = self.in_scope_lifetimes.len();
 118
 119         let parent_generics = match self.owners[parent_hir_id.def_id].unwrap().expect_item().kind {
 120             hir::ItemKind::Impl(hir::Impl { ref generics, .. })
 121             | hir::ItemKind::Trait(_, _, ref generics, ..) => generics.params,
 122             _ => &[],
 123         };
 124         let lt_def_names = parent_generics.iter().filter_map(|param| match param.kind {
 125             hir::GenericParamKind::Lifetime { .. } => Some(param.name.normalize_to_macros_2_0()),
 126             _ => None,
 127         });
 128         self.in_scope_lifetimes.extend(lt_def_names);
 129
 130         let res = f(self);
 131
 132         self.in_scope_lifetimes.truncate(old_len);
 133         res
 134     }
 135
 136     // Clears (and restores) the `in_scope_lifetimes` field. Used when
 137     // visiting nested items, which never inherit in-scope lifetimes
 138     // from their surrounding environment.
 139     fn without_in_scope_lifetime_defs<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
 140         let old_in_scope_lifetimes = mem::replace(&mut self.in_scope_lifetimes, vec![]);
 141
 142         // this vector is only used when walking over impl headers,
 143         // input types, and the like, and should not be non-empty in
 144         // between items
 145         assert!(self.lifetimes_to_define.is_empty());
 146
 147         let res = f(self);
 148
 149         assert!(self.in_scope_lifetimes.is_empty());
 150         self.in_scope_lifetimes = old_in_scope_lifetimes;
 151
 152         res
 153     }
 154
 155     pub(super) fn lower_mod(&mut self, items: &[P<Item>], inner: Span) -> hir::Mod<'hir> {
 156         hir::Mod {
 157             inner: self.lower_span(inner),
 158             item_ids: self.arena.alloc_from_iter(items.iter().flat_map(|x| self.lower_item_id(x))),
 159         }
 160     }
 161
 162     pub(super) fn lower_item_id(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
 163         let node_ids = match i.kind {
 164             ItemKind::Use(ref use_tree) => {
 165                 let mut vec = smallvec![i.id];
 166                 self.lower_item_id_use_tree(use_tree, i.id, &mut vec);
 167                 vec
 168             }
 169             ItemKind::Fn(..) | ItemKind::Impl(box ImplKind { of_trait: None, .. }) => {
 170                 smallvec![i.id]
 171             }
 172             _ => smallvec![i.id],
 173         };
 174
 175         node_ids
 176             .into_iter()
 177             .map(|node_id| hir::ItemId {
 178                 def_id: self.allocate_hir_id_counter(node_id).expect_owner(),
 179             })
 180             .collect()
 181     }
 182
 183     fn lower_item_id_use_tree(
 184         &mut self,
 185         tree: &UseTree,
 186         base_id: NodeId,
 187         vec: &mut SmallVec<[NodeId; 1]>,
 188     ) {
 189         match tree.kind {
 190             UseTreeKind::Nested(ref nested_vec) => {
 191                 for &(ref nested, id) in nested_vec {
 192                     vec.push(id);
 193                     self.lower_item_id_use_tree(nested, id, vec);
 194                 }
 195             }
 196             UseTreeKind::Glob => {}
 197             UseTreeKind::Simple(_, id1, id2) => {
 198                 for (_, &id) in
 199                     iter::zip(self.expect_full_res_from_use(base_id).skip(1), &[id1, id2])
 200                 {
 201                     vec.push(id);
 202                 }
 203             }
 204         }
 205     }
 206
 207     pub fn lower_item(&mut self, i: &Item) -> hir::Item<'hir> {
 208         let mut ident = i.ident;
 209         let mut vis = self.lower_visibility(&i.vis, None);
 210         let hir_id = self.lower_node_id(i.id);
 211         let attrs = self.lower_attrs(hir_id, &i.attrs);
 212         let kind = self.lower_item_kind(i.span, i.id, hir_id, &mut ident, attrs, &mut vis, &i.kind);
 213         hir::Item {
 214             def_id: hir_id.expect_owner(),
 215             ident: self.lower_ident(ident),
 216             kind,
 217             vis,
 218             span: self.lower_span(i.span),
 219         }
 220     }
 221
 222     fn lower_item_kind(
 223         &mut self,
 224         span: Span,
 225         id: NodeId,
 226         hir_id: hir::HirId,
 227         ident: &mut Ident,
 228         attrs: Option<&'hir [Attribute]>,
 229         vis: &mut hir::Visibility<'hir>,
 230         i: &ItemKind,
 231     ) -> hir::ItemKind<'hir> {
 232         match *i {
 233             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
 234             ItemKind::Use(ref use_tree) => {
 235                 // Start with an empty prefix.
 236                 let prefix = Path { segments: vec![], span: use_tree.span, tokens: None };
 237
 238                 self.lower_use_tree(use_tree, &prefix, id, vis, ident, attrs)
 239             }
 240             ItemKind::Static(ref t, m, ref e) => {
 241                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 242                 hir::ItemKind::Static(ty, m, body_id)
 243             }
 244             ItemKind::Const(_, ref t, ref e) => {
 245                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 246                 hir::ItemKind::Const(ty, body_id)
 247             }
 248             ItemKind::Fn(box FnKind(
 249                 _,
 250                 FnSig { ref decl, header, span: fn_sig_span },
 251                 ref generics,
 252                 ref body,
 253             )) => {
 254                 let fn_def_id = self.resolver.local_def_id(id);
 255                 self.with_new_scopes(|this| {
 256                     this.current_item = Some(ident.span);
 257
 258                     // Note: we don't need to change the return type from `T` to
 259                     // `impl Future<Output = T>` here because lower_body
 260                     // only cares about the input argument patterns in the function
 261                     // declaration (decl), not the return types.
 262                     let asyncness = header.asyncness;
 263                     let body_id =
 264                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 265
 266                     let (generics, decl) = this.add_in_band_defs(
 267                         generics,
 268                         fn_def_id,
 269                         AnonymousLifetimeMode::PassThrough,
 270                         |this, idty| {
 271                             let ret_id = asyncness.opt_return_id();
 272                             this.lower_fn_decl(
 273                                 &decl,
 274                                 Some((fn_def_id.to_def_id(), idty)),
 275                                 true,
 276                                 ret_id,
 277                             )
 278                         },
 279                     );
 280                     let sig = hir::FnSig {
 281                         decl,
 282                         header: this.lower_fn_header(header),
 283                         span: this.lower_span(fn_sig_span),
 284                     };
 285                     hir::ItemKind::Fn(sig, generics, body_id)
 286                 })
 287             }
 288             ItemKind::Mod(_, ref mod_kind) => match mod_kind {
 289                 ModKind::Loaded(items, _, inner_span) => {
 290                     hir::ItemKind::Mod(self.lower_mod(items, *inner_span))
 291                 }
 292                 ModKind::Unloaded => panic!("`mod` items should have been loaded by now"),
 293             },
 294             ItemKind::ForeignMod(ref fm) => hir::ItemKind::ForeignMod {
 295                 abi: fm.abi.map_or(abi::Abi::FALLBACK, |abi| self.lower_abi(abi)),
 296                 items: self
 297                     .arena
 298                     .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
 299             },
 300             ItemKind::GlobalAsm(ref asm) => {
 301                 hir::ItemKind::GlobalAsm(self.lower_inline_asm(span, asm))
 302             }
 303             ItemKind::TyAlias(box TyAliasKind(_, ref gen, _, Some(ref ty))) => {
 304                 // We lower
 305                 //
 306                 // type Foo = impl Trait
 307                 //
 308                 // to
 309                 //
 310                 // type Foo = Foo1
 311                 // opaque type Foo1: Trait
 312                 let ty = self.lower_ty(
 313                     ty,
 314                     ImplTraitContext::TypeAliasesOpaqueTy {
 315                         capturable_lifetimes: &mut FxHashSet::default(),
 316                     },
 317                 );
 318                 let generics = self.lower_generics(gen, ImplTraitContext::disallowed());
 319                 hir::ItemKind::TyAlias(ty, generics)
 320             }
 321             ItemKind::TyAlias(box TyAliasKind(_, ref generics, _, None)) => {
 322                 let ty = self.arena.alloc(self.ty(span, hir::TyKind::Err));
 323                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 324                 hir::ItemKind::TyAlias(ty, generics)
 325             }
 326             ItemKind::Enum(ref enum_definition, ref generics) => hir::ItemKind::Enum(
 327                 hir::EnumDef {
 328                     variants: self.arena.alloc_from_iter(
 329                         enum_definition.variants.iter().map(|x| self.lower_variant(x)),
 330                     ),
 331                 },
 332                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 333             ),
 334             ItemKind::Struct(ref struct_def, ref generics) => {
 335                 let struct_def = self.lower_variant_data(hir_id, struct_def);
 336                 hir::ItemKind::Struct(
 337                     struct_def,
 338                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 339                 )
 340             }
 341             ItemKind::Union(ref vdata, ref generics) => {
 342                 let vdata = self.lower_variant_data(hir_id, vdata);
 343                 hir::ItemKind::Union(
 344                     vdata,
 345                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 346                 )
 347             }
 348             ItemKind::Impl(box ImplKind {
 349                 unsafety,
 350                 polarity,
 351                 defaultness,
 352                 constness,
 353                 generics: ref ast_generics,
 354                 of_trait: ref trait_ref,
 355                 self_ty: ref ty,
 356                 items: ref impl_items,
 357             }) => {
 358                 // Lower the "impl header" first. This ordering is important
 359                 // for in-band lifetimes! Consider `'a` here:
 360                 //
 361                 //     impl Foo<'a> for u32 {
 362                 //         fn method(&'a self) { .. }
 363                 //     }
 364                 //
 365                 // Because we start by lowering the `Foo<'a> for u32`
 366                 // part, we will add `'a` to the list of generics on
 367                 // the impl. When we then encounter it later in the
 368                 // method, it will not be considered an in-band
 369                 // lifetime to be added, but rather a reference to a
 370                 // parent lifetime.
 371                 let lowered_trait_def_id = self.lower_node_id(id).expect_owner();
 372                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 373                     ast_generics,
 374                     lowered_trait_def_id,
 375                     AnonymousLifetimeMode::CreateParameter,
 376                     |this, _| {
 377                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 378                             this.lower_trait_ref(trait_ref, ImplTraitContext::disallowed())
 379                         });
 380
 381                         if let Some(ref trait_ref) = trait_ref {
 382                             if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res {
 383                                 this.trait_impls
 384                                     .entry(def_id)
 385                                     .or_default()
 386                                     .push(lowered_trait_def_id);
 387                             }
 388                         }
 389
 390                         let lowered_ty = this.lower_ty(ty, ImplTraitContext::disallowed());
 391
 392                         (trait_ref, lowered_ty)
 393                     },
 394                 );
 395
 396                 let new_impl_items =
 397                     self.with_in_scope_lifetime_defs(&ast_generics.params, |this| {
 398                         this.arena.alloc_from_iter(
 399                             impl_items.iter().map(|item| this.lower_impl_item_ref(item)),
 400                         )
 401                     });
 402
 403                 // `defaultness.has_value()` is never called for an `impl`, always `true` in order
 404                 // to not cause an assertion failure inside the `lower_defaultness` function.
 405                 let has_val = true;
 406                 let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
 407                 let polarity = match polarity {
 408                     ImplPolarity::Positive => ImplPolarity::Positive,
 409                     ImplPolarity::Negative(s) => ImplPolarity::Negative(self.lower_span(s)),
 410                 };
 411                 hir::ItemKind::Impl(hir::Impl {
 412                     unsafety: self.lower_unsafety(unsafety),
 413                     polarity,
 414                     defaultness,
 415                     defaultness_span,
 416                     constness: self.lower_constness(constness),
 417                     generics,
 418                     of_trait: trait_ref,
 419                     self_ty: lowered_ty,
 420                     items: new_impl_items,
 421                 })
 422             }
 423             ItemKind::Trait(box TraitKind(
 424                 is_auto,
 425                 unsafety,
 426                 ref generics,
 427                 ref bounds,
 428                 ref items,
 429             )) => {
 430                 let bounds = self.lower_param_bounds(bounds, ImplTraitContext::disallowed());
 431                 let items = self
 432                     .arena
 433                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
 434                 hir::ItemKind::Trait(
 435                     is_auto,
 436                     self.lower_unsafety(unsafety),
 437                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 438                     bounds,
 439                     items,
 440                 )
 441             }
 442             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 443                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 444                 self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 445             ),
 446             ItemKind::MacroDef(MacroDef { ref body, macro_rules }) => {
 447                 let body = P(self.lower_mac_args(body));
 448
 449                 hir::ItemKind::Macro(ast::MacroDef { body, macro_rules })
 450             }
 451             ItemKind::MacCall(..) => {
 452                 panic!("`TyMac` should have been expanded by now")
 453             }
 454         }
 455     }
 456
 457     fn lower_const_item(
 458         &mut self,
 459         ty: &Ty,
 460         span: Span,
 461         body: Option<&Expr>,
 462     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 463         let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Binding));
 464         (ty, self.lower_const_body(span, body))
 465     }
 466
 467     fn lower_use_tree(
 468         &mut self,
 469         tree: &UseTree,
 470         prefix: &Path,
 471         id: NodeId,
 472         vis: &mut hir::Visibility<'hir>,
 473         ident: &mut Ident,
 474         attrs: Option<&'hir [Attribute]>,
 475     ) -> hir::ItemKind<'hir> {
 476         debug!("lower_use_tree(tree={:?})", tree);
 477         debug!("lower_use_tree: vis = {:?}", vis);
 478
 479         let path = &tree.prefix;
 480         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 481
 482         match tree.kind {
 483             UseTreeKind::Simple(rename, id1, id2) => {
 484                 *ident = tree.ident();
 485
 486                 // First, apply the prefix to the path.
 487                 let mut path = Path { segments, span: path.span, tokens: None };
 488
 489                 // Correctly resolve `self` imports.
 490                 if path.segments.len() > 1
 491                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 492                 {
 493                     let _ = path.segments.pop();
 494                     if rename.is_none() {
 495                         *ident = path.segments.last().unwrap().ident;
 496                     }
 497                 }
 498
 499                 let mut resolutions = self.expect_full_res_from_use(id);
 500                 // We want to return *something* from this function, so hold onto the first item
 501                 // for later.
 502                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 503
 504                 // Here, we are looping over namespaces, if they exist for the definition
 505                 // being imported. We only handle type and value namespaces because we
 506                 // won't be dealing with macros in the rest of the compiler.
 507                 // Essentially a single `use` which imports two names is desugared into
 508                 // two imports.
 509                 for (res, &new_node_id) in iter::zip(resolutions, &[id1, id2]) {
 510                     let ident = *ident;
 511                     let mut path = path.clone();
 512                     for seg in &mut path.segments {
 513                         seg.id = self.resolver.next_node_id();
 514                     }
 515                     let span = path.span;
 516
 517                     self.with_hir_id_owner(new_node_id, |this| {
 518                         let new_id = this.lower_node_id(new_node_id);
 519                         let res = this.lower_res(res);
 520                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit, None);
 521                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 522                         let vis = this.rebuild_vis(&vis);
 523                         if let Some(attrs) = attrs {
 524                             this.attrs.insert(new_id, attrs);
 525                         }
 526
 527                         this.insert_item(hir::Item {
 528                             def_id: new_id.expect_owner(),
 529                             ident: this.lower_ident(ident),
 530                             kind,
 531                             vis,
 532                             span: this.lower_span(span),
 533                         });
 534                     });
 535                 }
 536
 537                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit, None);
 538                 hir::ItemKind::Use(path, hir::UseKind::Single)
 539             }
 540             UseTreeKind::Glob => {
 541                 let path = self.lower_path(
 542                     id,
 543                     &Path { segments, span: path.span, tokens: None },
 544                     ParamMode::Explicit,
 545                 );
 546                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 547             }
 548             UseTreeKind::Nested(ref trees) => {
 549                 // Nested imports are desugared into simple imports.
 550                 // So, if we start with
 551                 //
 552                 // ```
 553                 // pub(x) use foo::{a, b};
 554                 // ```
 555                 //
 556                 // we will create three items:
 557                 //
 558                 // ```
 559                 // pub(x) use foo::a;
 560                 // pub(x) use foo::b;
 561                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 562                 // ```
 563                 //
 564                 // The first two are produced by recursively invoking
 565                 // `lower_use_tree` (and indeed there may be things
 566                 // like `use foo::{a::{b, c}}` and so forth).  They
 567                 // wind up being directly added to
 568                 // `self.items`. However, the structure of this
 569                 // function also requires us to return one item, and
 570                 // for that we return the `{}` import (called the
 571                 // `ListStem`).
 572
 573                 let prefix = Path { segments, span: prefix.span.to(path.span), tokens: None };
 574
 575                 // Add all the nested `PathListItem`s to the HIR.
 576                 for &(ref use_tree, id) in trees {
 577                     let new_hir_id = self.lower_node_id(id);
 578
 579                     let mut prefix = prefix.clone();
 580
 581                     // Give the segments new node-ids since they are being cloned.
 582                     for seg in &mut prefix.segments {
 583                         seg.id = self.resolver.next_node_id();
 584                     }
 585
 586                     // Each `use` import is an item and thus are owners of the
 587                     // names in the path. Up to this point the nested import is
 588                     // the current owner, since we want each desugared import to
 589                     // own its own names, we have to adjust the owner before
 590                     // lowering the rest of the import.
 591                     self.with_hir_id_owner(id, |this| {
 592                         let mut vis = this.rebuild_vis(&vis);
 593                         let mut ident = *ident;
 594
 595                         let kind =
 596                             this.lower_use_tree(use_tree, &prefix, id, &mut vis, &mut ident, attrs);
 597                         if let Some(attrs) = attrs {
 598                             this.attrs.insert(new_hir_id, attrs);
 599                         }
 600
 601                         this.insert_item(hir::Item {
 602                             def_id: new_hir_id.expect_owner(),
 603                             ident: this.lower_ident(ident),
 604                             kind,
 605                             vis,
 606                             span: this.lower_span(use_tree.span),
 607                         });
 608                     });
 609                 }
 610
 611                 // Subtle and a bit hacky: we lower the privacy level
 612                 // of the list stem to "private" most of the time, but
 613                 // not for "restricted" paths. The key thing is that
 614                 // we don't want it to stay as `pub` (with no caveats)
 615                 // because that affects rustdoc and also the lints
 616                 // about `pub` items. But we can't *always* make it
 617                 // private -- particularly not for restricted paths --
 618                 // because it contains node-ids that would then be
 619                 // unused, failing the check that HirIds are "densely
 620                 // assigned".
 621                 match vis.node {
 622                     hir::VisibilityKind::Public
 623                     | hir::VisibilityKind::Crate(_)
 624                     | hir::VisibilityKind::Inherited => {
 625                         *vis = respan(
 626                             self.lower_span(prefix.span.shrink_to_lo()),
 627                             hir::VisibilityKind::Inherited,
 628                         );
 629                     }
 630                     hir::VisibilityKind::Restricted { .. } => {
 631                         // Do nothing here, as described in the comment on the match.
 632                     }
 633                 }
 634
 635                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 636                 let res = self.lower_res(res);
 637                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit, None);
 638                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 639             }
 640         }
 641     }
 642
 643     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 644     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 645     /// `NodeId`s. (See, e.g., #56128.)
 646     fn rebuild_use_path(&mut self, path: &hir::Path<'hir>) -> &'hir hir::Path<'hir> {
 647         debug!("rebuild_use_path(path = {:?})", path);
 648         let segments =
 649             self.arena.alloc_from_iter(path.segments.iter().map(|seg| hir::PathSegment {
 650                 ident: seg.ident,
 651                 hir_id: seg.hir_id.map(|_| self.next_id()),
 652                 res: seg.res,
 653                 args: None,
 654                 infer_args: seg.infer_args,
 655             }));
 656         self.arena.alloc(hir::Path { span: path.span, res: path.res, segments })
 657     }
 658
 659     fn rebuild_vis(&mut self, vis: &hir::Visibility<'hir>) -> hir::Visibility<'hir> {
 660         let vis_kind = match vis.node {
 661             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 662             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 663             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 664             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 665                 hir::VisibilityKind::Restricted {
 666                     path: self.rebuild_use_path(path),
 667                     hir_id: self.next_id(),
 668                 }
 669             }
 670         };
 671         respan(self.lower_span(vis.span), vis_kind)
 672     }
 673
 674     fn lower_foreign_item(&mut self, i: &ForeignItem) -> hir::ForeignItem<'hir> {
 675         let hir_id = self.lower_node_id(i.id);
 676         let def_id = hir_id.expect_owner();
 677         self.lower_attrs(hir_id, &i.attrs);
 678         hir::ForeignItem {
 679             def_id,
 680             ident: self.lower_ident(i.ident),
 681             kind: match i.kind {
 682                 ForeignItemKind::Fn(box FnKind(_, ref sig, ref generics, _)) => {
 683                     let fdec = &sig.decl;
 684                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 685                         generics,
 686                         def_id,
 687                         AnonymousLifetimeMode::PassThrough,
 688                         |this, _| {
 689                             (
 690                                 // Disallow `impl Trait` in foreign items.
 691                                 this.lower_fn_decl(fdec, None, false, None),
 692                                 this.lower_fn_params_to_names(fdec),
 693                             )
 694                         },
 695                     );
 696
 697                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 698                 }
 699                 ForeignItemKind::Static(ref t, m, _) => {
 700                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 701                     hir::ForeignItemKind::Static(ty, m)
 702                 }
 703                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 704                 ForeignItemKind::MacCall(_) => panic!("macro shouldn't exist here"),
 705             },
 706             vis: self.lower_visibility(&i.vis, None),
 707             span: self.lower_span(i.span),
 708         }
 709     }
 710
 711     fn lower_foreign_item_ref(&mut self, i: &ForeignItem) -> hir::ForeignItemRef<'hir> {
 712         hir::ForeignItemRef {
 713             id: hir::ForeignItemId { def_id: self.lower_node_id(i.id).expect_owner() },
 714             ident: self.lower_ident(i.ident),
 715             span: self.lower_span(i.span),
 716             vis: self.lower_visibility(&i.vis, Some(i.id)),
 717         }
 718     }
 719
 720     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 721         let id = self.lower_node_id(v.id);
 722         self.lower_attrs(id, &v.attrs);
 723         hir::Variant {
 724             id,
 725             data: self.lower_variant_data(id, &v.data),
 726             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 727             ident: self.lower_ident(v.ident),
 728             span: self.lower_span(v.span),
 729         }
 730     }
 731
 732     fn lower_variant_data(
 733         &mut self,
 734         parent_id: hir::HirId,
 735         vdata: &VariantData,
 736     ) -> hir::VariantData<'hir> {
 737         match *vdata {
 738             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 739                 self.arena
 740                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f))),
 741                 recovered,
 742             ),
 743             VariantData::Tuple(ref fields, id) => {
 744                 let ctor_id = self.lower_node_id(id);
 745                 self.alias_attrs(ctor_id, parent_id);
 746                 hir::VariantData::Tuple(
 747                     self.arena.alloc_from_iter(
 748                         fields.iter().enumerate().map(|f| self.lower_field_def(f)),
 749                     ),
 750                     ctor_id,
 751                 )
 752             }
 753             VariantData::Unit(id) => {
 754                 let ctor_id = self.lower_node_id(id);
 755                 self.alias_attrs(ctor_id, parent_id);
 756                 hir::VariantData::Unit(ctor_id)
 757             }
 758         }
 759     }
 760
 761     pub(super) fn lower_field_def(
 762         &mut self,
 763         (index, f): (usize, &FieldDef),
 764     ) -> hir::FieldDef<'hir> {
 765         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 766             let t = self.lower_path_ty(
 767                 &f.ty,
 768                 qself,
 769                 path,
 770                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 771                 ImplTraitContext::disallowed(),
 772             );
 773             self.arena.alloc(t)
 774         } else {
 775             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 776         };
 777         let hir_id = self.lower_node_id(f.id);
 778         self.lower_attrs(hir_id, &f.attrs);
 779         hir::FieldDef {
 780             span: self.lower_span(f.span),
 781             hir_id,
 782             ident: match f.ident {
 783                 Some(ident) => self.lower_ident(ident),
 784                 // FIXME(jseyfried): positional field hygiene.
 785                 None => Ident::new(sym::integer(index), self.lower_span(f.span)),
 786             },
 787             vis: self.lower_visibility(&f.vis, None),
 788             ty,
 789         }
 790     }
 791
 792     fn lower_trait_item(&mut self, i: &AssocItem) -> hir::TraitItem<'hir> {
 793         let hir_id = self.lower_node_id(i.id);
 794         let trait_item_def_id = hir_id.expect_owner();
 795
 796         let (generics, kind) = match i.kind {
 797             AssocItemKind::Const(_, ref ty, ref default) => {
 798                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 799                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 800                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
 801             }
 802             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, None)) => {
 803                 let names = self.lower_fn_params_to_names(&sig.decl);
 804                 let (generics, sig) =
 805                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
 806                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)))
 807             }
 808             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, Some(ref body))) => {
 809                 let asyncness = sig.header.asyncness;
 810                 let body_id =
 811                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, Some(&body));
 812                 let (generics, sig) = self.lower_method_sig(
 813                     generics,
 814                     sig,
 815                     trait_item_def_id,
 816                     false,
 817                     asyncness.opt_return_id(),
 818                 );
 819                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)))
 820             }
 821             AssocItemKind::TyAlias(box TyAliasKind(_, ref generics, ref bounds, ref default)) => {
 822                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
 823                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 824                 let kind = hir::TraitItemKind::Type(
 825                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 826                     ty,
 827                 );
 828
 829                 (generics, kind)
 830             }
 831             AssocItemKind::MacCall(..) => panic!("macro item shouldn't exist at this point"),
 832         };
 833
 834         self.lower_attrs(hir_id, &i.attrs);
 835         hir::TraitItem {
 836             def_id: trait_item_def_id,
 837             ident: self.lower_ident(i.ident),
 838             generics,
 839             kind,
 840             span: self.lower_span(i.span),
 841         }
 842     }
 843
 844     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 845         let (kind, has_default) = match &i.kind {
 846             AssocItemKind::Const(_, _, default) => (hir::AssocItemKind::Const, default.is_some()),
 847             AssocItemKind::TyAlias(box TyAliasKind(_, _, _, default)) => {
 848                 (hir::AssocItemKind::Type, default.is_some())
 849             }
 850             AssocItemKind::Fn(box FnKind(_, sig, _, default)) => {
 851                 (hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }, default.is_some())
 852             }
 853             AssocItemKind::MacCall(..) => unimplemented!(),
 854         };
 855         let id = hir::TraitItemId { def_id: self.lower_node_id(i.id).expect_owner() };
 856         let defaultness = hir::Defaultness::Default { has_value: has_default };
 857         hir::TraitItemRef {
 858             id,
 859             ident: self.lower_ident(i.ident),
 860             span: self.lower_span(i.span),
 861             defaultness,
 862             kind,
 863         }
 864     }
 865
 866     /// Construct `ExprKind::Err` for the given `span`.
 867     crate fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 868         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 869     }
 870
 871     fn lower_impl_item(&mut self, i: &AssocItem) -> hir::ImplItem<'hir> {
 872         let impl_item_def_id = self.resolver.local_def_id(i.id);
 873
 874         let (generics, kind) = match &i.kind {
 875             AssocItemKind::Const(_, ty, expr) => {
 876                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 877                 (
 878                     hir::Generics::empty(),
 879                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 880                 )
 881             }
 882             AssocItemKind::Fn(box FnKind(_, sig, generics, body)) => {
 883                 self.current_item = Some(i.span);
 884                 let asyncness = sig.header.asyncness;
 885                 let body_id =
 886                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 887                 let impl_trait_return_allow = !self.is_in_trait_impl;
 888                 let (generics, sig) = self.lower_method_sig(
 889                     generics,
 890                     sig,
 891                     impl_item_def_id,
 892                     impl_trait_return_allow,
 893                     asyncness.opt_return_id(),
 894                 );
 895
 896                 (generics, hir::ImplItemKind::Fn(sig, body_id))
 897             }
 898             AssocItemKind::TyAlias(box TyAliasKind(_, generics, _, ty)) => {
 899                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 900                 let kind = match ty {
 901                     None => {
 902                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 903                         hir::ImplItemKind::TyAlias(ty)
 904                     }
 905                     Some(ty) => {
 906                         let ty = self.lower_ty(
 907                             ty,
 908                             ImplTraitContext::TypeAliasesOpaqueTy {
 909                                 capturable_lifetimes: &mut FxHashSet::default(),
 910                             },
 911                         );
 912                         hir::ImplItemKind::TyAlias(ty)
 913                     }
 914                 };
 915                 (generics, kind)
 916             }
 917             AssocItemKind::MacCall(..) => panic!("`TyMac` should have been expanded by now"),
 918         };
 919
 920         // Since `default impl` is not yet implemented, this is always true in impls.
 921         let has_value = true;
 922         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 923         let hir_id = self.lower_node_id(i.id);
 924         self.lower_attrs(hir_id, &i.attrs);
 925         hir::ImplItem {
 926             def_id: hir_id.expect_owner(),
 927             ident: self.lower_ident(i.ident),
 928             generics,
 929             vis: self.lower_visibility(&i.vis, None),
 930             defaultness,
 931             kind,
 932             span: self.lower_span(i.span),
 933         }
 934     }
 935
 936     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef<'hir> {
 937         // Since `default impl` is not yet implemented, this is always true in impls.
 938         let has_value = true;
 939         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 940         hir::ImplItemRef {
 941             id: hir::ImplItemId { def_id: self.lower_node_id(i.id).expect_owner() },
 942             ident: self.lower_ident(i.ident),
 943             span: self.lower_span(i.span),
 944             vis: self.lower_visibility(&i.vis, Some(i.id)),
 945             defaultness,
 946             kind: match &i.kind {
 947                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 948                 AssocItemKind::TyAlias(..) => hir::AssocItemKind::Type,
 949                 AssocItemKind::Fn(box FnKind(_, sig, ..)) => {
 950                     hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
 951                 }
 952                 AssocItemKind::MacCall(..) => unimplemented!(),
 953             },
 954         }
 955     }
 956
 957     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 958     /// the address space of that item instead of the item currently being
 959     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 960     /// lower a `Visibility` value although we haven't lowered the owning
 961     /// `ImplItem` in question yet.
 962     fn lower_visibility(
 963         &mut self,
 964         v: &Visibility,
 965         explicit_owner: Option<NodeId>,
 966     ) -> hir::Visibility<'hir> {
 967         let node = match v.kind {
 968             VisibilityKind::Public => hir::VisibilityKind::Public,
 969             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 970             VisibilityKind::Restricted { ref path, id } => {
 971                 debug!("lower_visibility: restricted path id = {:?}", id);
 972                 let lowered_id = if let Some(owner) = explicit_owner {
 973                     self.lower_node_id_with_owner(id, owner)
 974                 } else {
 975                     self.lower_node_id(id)
 976                 };
 977                 let res = self.expect_full_res(id);
 978                 let res = self.lower_res(res);
 979                 hir::VisibilityKind::Restricted {
 980                     path: self.lower_path_extra(res, path, ParamMode::Explicit, explicit_owner),
 981                     hir_id: lowered_id,
 982                 }
 983             }
 984             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 985         };
 986         respan(self.lower_span(v.span), node)
 987     }
 988
 989     fn lower_defaultness(
 990         &self,
 991         d: Defaultness,
 992         has_value: bool,
 993     ) -> (hir::Defaultness, Option<Span>) {
 994         match d {
 995             Defaultness::Default(sp) => {
 996                 (hir::Defaultness::Default { has_value }, Some(self.lower_span(sp)))
 997             }
 998             Defaultness::Final => {
 999                 assert!(has_value);
1000                 (hir::Defaultness::Final, None)
1001             }
1002         }
1003     }
1004
1005     fn record_body(
1006         &mut self,
1007         params: &'hir [hir::Param<'hir>],
1008         value: hir::Expr<'hir>,
1009     ) -> hir::BodyId {
1010         let body = hir::Body { generator_kind: self.generator_kind, params, value };
1011         let id = body.id();
1012         self.bodies.insert(id, body);
1013         id
1014     }
1015
1016     pub(super) fn lower_body(
1017         &mut self,
1018         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
1019     ) -> hir::BodyId {
1020         let prev_gen_kind = self.generator_kind.take();
1021         let task_context = self.task_context.take();
1022         let (parameters, result) = f(self);
1023         let body_id = self.record_body(parameters, result);
1024         self.task_context = task_context;
1025         self.generator_kind = prev_gen_kind;
1026         body_id
1027     }
1028
1029     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
1030         let hir_id = self.lower_node_id(param.id);
1031         self.lower_attrs(hir_id, &param.attrs);
1032         hir::Param {
1033             hir_id,
1034             pat: self.lower_pat(&param.pat),
1035             ty_span: self.lower_span(param.ty.span),
1036             span: self.lower_span(param.span),
1037         }
1038     }
1039
1040     pub(super) fn lower_fn_body(
1041         &mut self,
1042         decl: &FnDecl,
1043         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
1044     ) -> hir::BodyId {
1045         self.lower_body(|this| {
1046             (
1047                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
1048                 body(this),
1049             )
1050         })
1051     }
1052
1053     fn lower_fn_body_block(
1054         &mut self,
1055         span: Span,
1056         decl: &FnDecl,
1057         body: Option<&Block>,
1058     ) -> hir::BodyId {
1059         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1060     }
1061
1062     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1063         match block {
1064             Some(block) => self.lower_block_expr(block),
1065             None => self.expr_err(span),
1066         }
1067     }
1068
1069     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1070         self.lower_body(|this| {
1071             (
1072                 &[],
1073                 match expr {
1074                     Some(expr) => this.lower_expr_mut(expr),
1075                     None => this.expr_err(span),
1076                 },
1077             )
1078         })
1079     }
1080
1081     fn lower_maybe_async_body(
1082         &mut self,
1083         span: Span,
1084         decl: &FnDecl,
1085         asyncness: Async,
1086         body: Option<&Block>,
1087     ) -> hir::BodyId {
1088         let closure_id = match asyncness {
1089             Async::Yes { closure_id, .. } => closure_id,
1090             Async::No => return self.lower_fn_body_block(span, decl, body),
1091         };
1092
1093         self.lower_body(|this| {
1094             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1095             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1096
1097             // Async function parameters are lowered into the closure body so that they are
1098             // captured and so that the drop order matches the equivalent non-async functions.
1099             //
1100             // from:
1101             //
1102             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1103             //         <body>
1104             //     }
1105             //
1106             // into:
1107             //
1108             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1109             //       async move {
1110             //         let __arg2 = __arg2;
1111             //         let <pattern> = __arg2;
1112             //         let __arg1 = __arg1;
1113             //         let <pattern> = __arg1;
1114             //         let __arg0 = __arg0;
1115             //         let <pattern> = __arg0;
1116             //         drop-temps { <body> } // see comments later in fn for details
1117             //       }
1118             //     }
1119             //
1120             // If `<pattern>` is a simple ident, then it is lowered to a single
1121             // `let <pattern> = <pattern>;` statement as an optimization.
1122             //
1123             // Note that the body is embedded in `drop-temps`; an
1124             // equivalent desugaring would be `return { <body>
1125             // };`. The key point is that we wish to drop all the
1126             // let-bound variables and temporaries created in the body
1127             // (and its tail expression!) before we drop the
1128             // parameters (c.f. rust-lang/rust#64512).
1129             for (index, parameter) in decl.inputs.iter().enumerate() {
1130                 let parameter = this.lower_param(parameter);
1131                 let span = parameter.pat.span;
1132
1133                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1134                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1135                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1136                     hir::PatKind::Binding(
1137                         hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable,
1138                         _,
1139                         ident,
1140                         _,
1141                     ) => (ident, true),
1142                     // For `ref mut` or wildcard arguments, we can't reuse the binding, but
1143                     // we can keep the same name for the parameter.
1144                     // This lets rustdoc render it correctly in documentation.
1145                     hir::PatKind::Binding(_, _, ident, _) => (ident, false),
1146                     hir::PatKind::Wild => {
1147                         (Ident::with_dummy_span(rustc_span::symbol::kw::Underscore), false)
1148                     }
1149                     _ => {
1150                         // Replace the ident for bindings that aren't simple.
1151                         let name = format!("__arg{}", index);
1152                         let ident = Ident::from_str(&name);
1153
1154                         (ident, false)
1155                     }
1156                 };
1157
1158                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1159
1160                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1161                 // async function.
1162                 //
1163                 // If this is the simple case, this parameter will end up being the same as the
1164                 // original parameter, but with a different pattern id.
1165                 let stmt_attrs = this.attrs.get(&parameter.hir_id).copied();
1166                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1167                 let new_parameter = hir::Param {
1168                     hir_id: parameter.hir_id,
1169                     pat: new_parameter_pat,
1170                     ty_span: this.lower_span(parameter.ty_span),
1171                     span: this.lower_span(parameter.span),
1172                 };
1173
1174                 if is_simple_parameter {
1175                     // If this is the simple case, then we only insert one statement that is
1176                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1177                     // `HirId`s are densely assigned.
1178                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1179                     let stmt = this.stmt_let_pat(
1180                         stmt_attrs,
1181                         desugared_span,
1182                         Some(expr),
1183                         parameter.pat,
1184                         hir::LocalSource::AsyncFn,
1185                     );
1186                     statements.push(stmt);
1187                 } else {
1188                     // If this is not the simple case, then we construct two statements:
1189                     //
1190                     // ```
1191                     // let __argN = __argN;
1192                     // let <pat> = __argN;
1193                     // ```
1194                     //
1195                     // The first statement moves the parameter into the closure and thus ensures
1196                     // that the drop order is correct.
1197                     //
1198                     // The second statement creates the bindings that the user wrote.
1199
1200                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1201                     // because the user may have specified a `ref mut` binding in the next
1202                     // statement.
1203                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1204                         desugared_span,
1205                         ident,
1206                         hir::BindingAnnotation::Mutable,
1207                     );
1208                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1209                     let move_stmt = this.stmt_let_pat(
1210                         None,
1211                         desugared_span,
1212                         Some(move_expr),
1213                         move_pat,
1214                         hir::LocalSource::AsyncFn,
1215                     );
1216
1217                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1218                     // parameter's pattern so that `HirId`s are densely assigned.
1219                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1220                     let pattern_stmt = this.stmt_let_pat(
1221                         stmt_attrs,
1222                         desugared_span,
1223                         Some(pattern_expr),
1224                         parameter.pat,
1225                         hir::LocalSource::AsyncFn,
1226                     );
1227
1228                     statements.push(move_stmt);
1229                     statements.push(pattern_stmt);
1230                 };
1231
1232                 parameters.push(new_parameter);
1233             }
1234
1235             let body_span = body.map_or(span, |b| b.span);
1236             let async_expr = this.make_async_expr(
1237                 CaptureBy::Value,
1238                 closure_id,
1239                 None,
1240                 body_span,
1241                 hir::AsyncGeneratorKind::Fn,
1242                 |this| {
1243                     // Create a block from the user's function body:
1244                     let user_body = this.lower_block_expr_opt(body_span, body);
1245
1246                     // Transform into `drop-temps { <user-body> }`, an expression:
1247                     let desugared_span =
1248                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1249                     let user_body = this.expr_drop_temps(
1250                         desugared_span,
1251                         this.arena.alloc(user_body),
1252                         AttrVec::new(),
1253                     );
1254
1255                     // As noted above, create the final block like
1256                     //
1257                     // ```
1258                     // {
1259                     //   let $param_pattern = $raw_param;
1260                     //   ...
1261                     //   drop-temps { <user-body> }
1262                     // }
1263                     // ```
1264                     let body = this.block_all(
1265                         desugared_span,
1266                         this.arena.alloc_from_iter(statements),
1267                         Some(user_body),
1268                     );
1269
1270                     this.expr_block(body, AttrVec::new())
1271                 },
1272             );
1273
1274             (
1275                 this.arena.alloc_from_iter(parameters),
1276                 this.expr(body_span, async_expr, AttrVec::new()),
1277             )
1278         })
1279     }
1280
1281     fn lower_method_sig(
1282         &mut self,
1283         generics: &Generics,
1284         sig: &FnSig,
1285         fn_def_id: LocalDefId,
1286         impl_trait_return_allow: bool,
1287         is_async: Option<NodeId>,
1288     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1289         let header = self.lower_fn_header(sig.header);
1290         let (generics, decl) = self.add_in_band_defs(
1291             generics,
1292             fn_def_id,
1293             AnonymousLifetimeMode::PassThrough,
1294             |this, idty| {
1295                 this.lower_fn_decl(
1296                     &sig.decl,
1297                     Some((fn_def_id.to_def_id(), idty)),
1298                     impl_trait_return_allow,
1299                     is_async,
1300                 )
1301             },
1302         );
1303         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
1304     }
1305
1306     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1307         hir::FnHeader {
1308             unsafety: self.lower_unsafety(h.unsafety),
1309             asyncness: self.lower_asyncness(h.asyncness),
1310             constness: self.lower_constness(h.constness),
1311             abi: self.lower_extern(h.ext),
1312         }
1313     }
1314
1315     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1316         abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1317             self.error_on_invalid_abi(abi);
1318             abi::Abi::Rust
1319         })
1320     }
1321
1322     pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
1323         match ext {
1324             Extern::None => abi::Abi::Rust,
1325             Extern::Implicit => abi::Abi::FALLBACK,
1326             Extern::Explicit(abi) => self.lower_abi(abi),
1327         }
1328     }
1329
1330     fn error_on_invalid_abi(&self, abi: StrLit) {
1331         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1332             .span_label(abi.span, "invalid ABI")
1333             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1334             .emit();
1335     }
1336
1337     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1338         match a {
1339             Async::Yes { .. } => hir::IsAsync::Async,
1340             Async::No => hir::IsAsync::NotAsync,
1341         }
1342     }
1343
1344     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1345         match c {
1346             Const::Yes(_) => hir::Constness::Const,
1347             Const::No => hir::Constness::NotConst,
1348         }
1349     }
1350
1351     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1352         match u {
1353             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1354             Unsafe::No => hir::Unsafety::Normal,
1355         }
1356     }
1357
1358     pub(super) fn lower_generics_mut(
1359         &mut self,
1360         generics: &Generics,
1361         itctx: ImplTraitContext<'_, 'hir>,
1362     ) -> GenericsCtor<'hir> {
1363         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1364         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1365         for pred in &generics.where_clause.predicates {
1366             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1367                 'next_bound: for bound in &bound_pred.bounds {
1368                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1369                         // Check if the where clause type is a plain type parameter.
1370                         match self
1371                             .resolver
1372                             .get_partial_res(bound_pred.bounded_ty.id)
1373                             .map(|d| (d.base_res(), d.unresolved_segments()))
1374                         {
1375                             Some((Res::Def(DefKind::TyParam, def_id), 0))
1376                                 if bound_pred.bound_generic_params.is_empty() =>
1377                             {
1378                                 for param in &generics.params {
1379                                     if def_id == self.resolver.local_def_id(param.id).to_def_id() {
1380                                         add_bounds.entry(param.id).or_default().push(bound.clone());
1381                                         continue 'next_bound;
1382                                     }
1383                                 }
1384                             }
1385                             _ => {}
1386                         }
1387                         self.diagnostic().span_err(
1388                             bound_pred.bounded_ty.span,
1389                             "`?Trait` bounds are only permitted at the \
1390                                  point where a type parameter is declared",
1391                         );
1392                     }
1393                 }
1394             }
1395         }
1396
1397         GenericsCtor {
1398             params: self.lower_generic_params_mut(&generics.params, &add_bounds, itctx).collect(),
1399             where_clause: self.lower_where_clause(&generics.where_clause),
1400             span: self.lower_span(generics.span),
1401         }
1402     }
1403
1404     pub(super) fn lower_generics(
1405         &mut self,
1406         generics: &Generics,
1407         itctx: ImplTraitContext<'_, 'hir>,
1408     ) -> hir::Generics<'hir> {
1409         let generics_ctor = self.lower_generics_mut(generics, itctx);
1410         generics_ctor.into_generics(self.arena)
1411     }
1412
1413     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1414         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1415             hir::WhereClause {
1416                 predicates: this.arena.alloc_from_iter(
1417                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1418                 ),
1419                 span: this.lower_span(wc.span),
1420             }
1421         })
1422     }
1423
1424     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1425         match *pred {
1426             WherePredicate::BoundPredicate(WhereBoundPredicate {
1427                 ref bound_generic_params,
1428                 ref bounded_ty,
1429                 ref bounds,
1430                 span,
1431             }) => {
1432                 self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1433                     hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1434                         bound_generic_params: this.lower_generic_params(
1435                             bound_generic_params,
1436                             &NodeMap::default(),
1437                             ImplTraitContext::disallowed(),
1438                         ),
1439                         bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1440                         bounds: this.arena.alloc_from_iter(bounds.iter().map(
1441                             |bound| match bound {
1442                                 // We used to ignore `?Trait` bounds, as they were copied into type
1443                                 // parameters already, but we need to keep them around only for
1444                                 // diagnostics when we suggest removal of `?Sized` bounds. See
1445                                 // `suggest_constraining_type_param`. This will need to change if
1446                                 // we ever allow something *other* than `?Sized`.
1447                                 GenericBound::Trait(p, TraitBoundModifier::Maybe) => {
1448                                     hir::GenericBound::Unsized(this.lower_span(p.span))
1449                                 }
1450                                 _ => this.lower_param_bound(bound, ImplTraitContext::disallowed()),
1451                             },
1452                         )),
1453                         span: this.lower_span(span),
1454                     })
1455                 })
1456             }
1457             WherePredicate::RegionPredicate(WhereRegionPredicate {
1458                 ref lifetime,
1459                 ref bounds,
1460                 span,
1461             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1462                 span: self.lower_span(span),
1463                 lifetime: self.lower_lifetime(lifetime),
1464                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1465             }),
1466             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1467                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1468                     hir_id: self.lower_node_id(id),
1469                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1470                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1471                     span: self.lower_span(span),
1472                 })
1473             }
1474         }
1475     }
1476 }
1477
1478 /// Helper struct for delayed construction of Generics.
1479 pub(super) struct GenericsCtor<'hir> {
1480     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1481     where_clause: hir::WhereClause<'hir>,
1482     span: Span,
1483 }
1484
1485 impl<'hir> GenericsCtor<'hir> {
1486     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1487         hir::Generics {
1488             params: arena.alloc_from_iter(self.params),
1489             where_clause: self.where_clause,
1490             span: self.span,
1491         }
1492     }
1493 }