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, fn_sig_span, id),
 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) => {
 295                 if fm.abi.is_none() {
 296                     self.maybe_lint_missing_abi(span, id, abi::Abi::C { unwind: false });
 297                 }
 298                 hir::ItemKind::ForeignMod {
 299                     abi: fm.abi.map_or(abi::Abi::C { unwind: false }, |abi| self.lower_abi(abi)),
 300                     items: self
 301                         .arena
 302                         .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
 303                 }
 304             }
 305             ItemKind::GlobalAsm(ref asm) => {
 306                 hir::ItemKind::GlobalAsm(self.lower_inline_asm(span, asm))
 307             }
 308             ItemKind::TyAlias(box TyAliasKind(_, ref gen, _, Some(ref ty))) => {
 309                 // We lower
 310                 //
 311                 // type Foo = impl Trait
 312                 //
 313                 // to
 314                 //
 315                 // type Foo = Foo1
 316                 // opaque type Foo1: Trait
 317                 let ty = self.lower_ty(
 318                     ty,
 319                     ImplTraitContext::TypeAliasesOpaqueTy {
 320                         capturable_lifetimes: &mut FxHashSet::default(),
 321                     },
 322                 );
 323                 let generics = self.lower_generics(gen, ImplTraitContext::disallowed());
 324                 hir::ItemKind::TyAlias(ty, generics)
 325             }
 326             ItemKind::TyAlias(box TyAliasKind(_, ref generics, _, None)) => {
 327                 let ty = self.arena.alloc(self.ty(span, hir::TyKind::Err));
 328                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 329                 hir::ItemKind::TyAlias(ty, generics)
 330             }
 331             ItemKind::Enum(ref enum_definition, ref generics) => hir::ItemKind::Enum(
 332                 hir::EnumDef {
 333                     variants: self.arena.alloc_from_iter(
 334                         enum_definition.variants.iter().map(|x| self.lower_variant(x)),
 335                     ),
 336                 },
 337                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 338             ),
 339             ItemKind::Struct(ref struct_def, ref generics) => {
 340                 let struct_def = self.lower_variant_data(hir_id, struct_def);
 341                 hir::ItemKind::Struct(
 342                     struct_def,
 343                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 344                 )
 345             }
 346             ItemKind::Union(ref vdata, ref generics) => {
 347                 let vdata = self.lower_variant_data(hir_id, vdata);
 348                 hir::ItemKind::Union(
 349                     vdata,
 350                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 351                 )
 352             }
 353             ItemKind::Impl(box ImplKind {
 354                 unsafety,
 355                 polarity,
 356                 defaultness,
 357                 constness,
 358                 generics: ref ast_generics,
 359                 of_trait: ref trait_ref,
 360                 self_ty: ref ty,
 361                 items: ref impl_items,
 362             }) => {
 363                 // Lower the "impl header" first. This ordering is important
 364                 // for in-band lifetimes! Consider `'a` here:
 365                 //
 366                 //     impl Foo<'a> for u32 {
 367                 //         fn method(&'a self) { .. }
 368                 //     }
 369                 //
 370                 // Because we start by lowering the `Foo<'a> for u32`
 371                 // part, we will add `'a` to the list of generics on
 372                 // the impl. When we then encounter it later in the
 373                 // method, it will not be considered an in-band
 374                 // lifetime to be added, but rather a reference to a
 375                 // parent lifetime.
 376                 let lowered_trait_def_id = self.lower_node_id(id).expect_owner();
 377                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 378                     ast_generics,
 379                     lowered_trait_def_id,
 380                     AnonymousLifetimeMode::CreateParameter,
 381                     |this, _| {
 382                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 383                             this.lower_trait_ref(trait_ref, ImplTraitContext::disallowed())
 384                         });
 385
 386                         if let Some(ref trait_ref) = trait_ref {
 387                             if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res {
 388                                 this.trait_impls
 389                                     .entry(def_id)
 390                                     .or_default()
 391                                     .push(lowered_trait_def_id);
 392                             }
 393                         }
 394
 395                         let lowered_ty = this.lower_ty(ty, ImplTraitContext::disallowed());
 396
 397                         (trait_ref, lowered_ty)
 398                     },
 399                 );
 400
 401                 let new_impl_items =
 402                     self.with_in_scope_lifetime_defs(&ast_generics.params, |this| {
 403                         this.arena.alloc_from_iter(
 404                             impl_items.iter().map(|item| this.lower_impl_item_ref(item)),
 405                         )
 406                     });
 407
 408                 // `defaultness.has_value()` is never called for an `impl`, always `true` in order
 409                 // to not cause an assertion failure inside the `lower_defaultness` function.
 410                 let has_val = true;
 411                 let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
 412                 let polarity = match polarity {
 413                     ImplPolarity::Positive => ImplPolarity::Positive,
 414                     ImplPolarity::Negative(s) => ImplPolarity::Negative(self.lower_span(s)),
 415                 };
 416                 hir::ItemKind::Impl(hir::Impl {
 417                     unsafety: self.lower_unsafety(unsafety),
 418                     polarity,
 419                     defaultness,
 420                     defaultness_span,
 421                     constness: self.lower_constness(constness),
 422                     generics,
 423                     of_trait: trait_ref,
 424                     self_ty: lowered_ty,
 425                     items: new_impl_items,
 426                 })
 427             }
 428             ItemKind::Trait(box TraitKind(
 429                 is_auto,
 430                 unsafety,
 431                 ref generics,
 432                 ref bounds,
 433                 ref items,
 434             )) => {
 435                 let bounds = self.lower_param_bounds(bounds, ImplTraitContext::disallowed());
 436                 let items = self
 437                     .arena
 438                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
 439                 hir::ItemKind::Trait(
 440                     is_auto,
 441                     self.lower_unsafety(unsafety),
 442                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 443                     bounds,
 444                     items,
 445                 )
 446             }
 447             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 448                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 449                 self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 450             ),
 451             ItemKind::MacroDef(MacroDef { ref body, macro_rules }) => {
 452                 let body = P(self.lower_mac_args(body));
 453
 454                 hir::ItemKind::Macro(ast::MacroDef { body, macro_rules })
 455             }
 456             ItemKind::MacCall(..) => {
 457                 panic!("`TyMac` should have been expanded by now")
 458             }
 459         }
 460     }
 461
 462     fn lower_const_item(
 463         &mut self,
 464         ty: &Ty,
 465         span: Span,
 466         body: Option<&Expr>,
 467     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 468         let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Binding));
 469         (ty, self.lower_const_body(span, body))
 470     }
 471
 472     fn lower_use_tree(
 473         &mut self,
 474         tree: &UseTree,
 475         prefix: &Path,
 476         id: NodeId,
 477         vis: &mut hir::Visibility<'hir>,
 478         ident: &mut Ident,
 479         attrs: Option<&'hir [Attribute]>,
 480     ) -> hir::ItemKind<'hir> {
 481         debug!("lower_use_tree(tree={:?})", tree);
 482         debug!("lower_use_tree: vis = {:?}", vis);
 483
 484         let path = &tree.prefix;
 485         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 486
 487         match tree.kind {
 488             UseTreeKind::Simple(rename, id1, id2) => {
 489                 *ident = tree.ident();
 490
 491                 // First, apply the prefix to the path.
 492                 let mut path = Path { segments, span: path.span, tokens: None };
 493
 494                 // Correctly resolve `self` imports.
 495                 if path.segments.len() > 1
 496                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 497                 {
 498                     let _ = path.segments.pop();
 499                     if rename.is_none() {
 500                         *ident = path.segments.last().unwrap().ident;
 501                     }
 502                 }
 503
 504                 let mut resolutions = self.expect_full_res_from_use(id);
 505                 // We want to return *something* from this function, so hold onto the first item
 506                 // for later.
 507                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 508
 509                 // Here, we are looping over namespaces, if they exist for the definition
 510                 // being imported. We only handle type and value namespaces because we
 511                 // won't be dealing with macros in the rest of the compiler.
 512                 // Essentially a single `use` which imports two names is desugared into
 513                 // two imports.
 514                 for (res, &new_node_id) in iter::zip(resolutions, &[id1, id2]) {
 515                     let ident = *ident;
 516                     let mut path = path.clone();
 517                     for seg in &mut path.segments {
 518                         seg.id = self.resolver.next_node_id();
 519                     }
 520                     let span = path.span;
 521
 522                     self.with_hir_id_owner(new_node_id, |this| {
 523                         let new_id = this.lower_node_id(new_node_id);
 524                         let res = this.lower_res(res);
 525                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit, None);
 526                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 527                         let vis = this.rebuild_vis(&vis);
 528                         if let Some(attrs) = attrs {
 529                             this.attrs.insert(new_id, attrs);
 530                         }
 531
 532                         this.insert_item(hir::Item {
 533                             def_id: new_id.expect_owner(),
 534                             ident: this.lower_ident(ident),
 535                             kind,
 536                             vis,
 537                             span: this.lower_span(span),
 538                         });
 539                     });
 540                 }
 541
 542                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit, None);
 543                 hir::ItemKind::Use(path, hir::UseKind::Single)
 544             }
 545             UseTreeKind::Glob => {
 546                 let path = self.lower_path(
 547                     id,
 548                     &Path { segments, span: path.span, tokens: None },
 549                     ParamMode::Explicit,
 550                 );
 551                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 552             }
 553             UseTreeKind::Nested(ref trees) => {
 554                 // Nested imports are desugared into simple imports.
 555                 // So, if we start with
 556                 //
 557                 // ```
 558                 // pub(x) use foo::{a, b};
 559                 // ```
 560                 //
 561                 // we will create three items:
 562                 //
 563                 // ```
 564                 // pub(x) use foo::a;
 565                 // pub(x) use foo::b;
 566                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 567                 // ```
 568                 //
 569                 // The first two are produced by recursively invoking
 570                 // `lower_use_tree` (and indeed there may be things
 571                 // like `use foo::{a::{b, c}}` and so forth).  They
 572                 // wind up being directly added to
 573                 // `self.items`. However, the structure of this
 574                 // function also requires us to return one item, and
 575                 // for that we return the `{}` import (called the
 576                 // `ListStem`).
 577
 578                 let prefix = Path { segments, span: prefix.span.to(path.span), tokens: None };
 579
 580                 // Add all the nested `PathListItem`s to the HIR.
 581                 for &(ref use_tree, id) in trees {
 582                     let new_hir_id = self.lower_node_id(id);
 583
 584                     let mut prefix = prefix.clone();
 585
 586                     // Give the segments new node-ids since they are being cloned.
 587                     for seg in &mut prefix.segments {
 588                         seg.id = self.resolver.next_node_id();
 589                     }
 590
 591                     // Each `use` import is an item and thus are owners of the
 592                     // names in the path. Up to this point the nested import is
 593                     // the current owner, since we want each desugared import to
 594                     // own its own names, we have to adjust the owner before
 595                     // lowering the rest of the import.
 596                     self.with_hir_id_owner(id, |this| {
 597                         let mut vis = this.rebuild_vis(&vis);
 598                         let mut ident = *ident;
 599
 600                         let kind =
 601                             this.lower_use_tree(use_tree, &prefix, id, &mut vis, &mut ident, attrs);
 602                         if let Some(attrs) = attrs {
 603                             this.attrs.insert(new_hir_id, attrs);
 604                         }
 605
 606                         this.insert_item(hir::Item {
 607                             def_id: new_hir_id.expect_owner(),
 608                             ident: this.lower_ident(ident),
 609                             kind,
 610                             vis,
 611                             span: this.lower_span(use_tree.span),
 612                         });
 613                     });
 614                 }
 615
 616                 // Subtle and a bit hacky: we lower the privacy level
 617                 // of the list stem to "private" most of the time, but
 618                 // not for "restricted" paths. The key thing is that
 619                 // we don't want it to stay as `pub` (with no caveats)
 620                 // because that affects rustdoc and also the lints
 621                 // about `pub` items. But we can't *always* make it
 622                 // private -- particularly not for restricted paths --
 623                 // because it contains node-ids that would then be
 624                 // unused, failing the check that HirIds are "densely
 625                 // assigned".
 626                 match vis.node {
 627                     hir::VisibilityKind::Public
 628                     | hir::VisibilityKind::Crate(_)
 629                     | hir::VisibilityKind::Inherited => {
 630                         *vis = respan(
 631                             self.lower_span(prefix.span.shrink_to_lo()),
 632                             hir::VisibilityKind::Inherited,
 633                         );
 634                     }
 635                     hir::VisibilityKind::Restricted { .. } => {
 636                         // Do nothing here, as described in the comment on the match.
 637                     }
 638                 }
 639
 640                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 641                 let res = self.lower_res(res);
 642                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit, None);
 643                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 644             }
 645         }
 646     }
 647
 648     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 649     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 650     /// `NodeId`s. (See, e.g., #56128.)
 651     fn rebuild_use_path(&mut self, path: &hir::Path<'hir>) -> &'hir hir::Path<'hir> {
 652         debug!("rebuild_use_path(path = {:?})", path);
 653         let segments =
 654             self.arena.alloc_from_iter(path.segments.iter().map(|seg| hir::PathSegment {
 655                 ident: seg.ident,
 656                 hir_id: seg.hir_id.map(|_| self.next_id()),
 657                 res: seg.res,
 658                 args: None,
 659                 infer_args: seg.infer_args,
 660             }));
 661         self.arena.alloc(hir::Path { span: path.span, res: path.res, segments })
 662     }
 663
 664     fn rebuild_vis(&mut self, vis: &hir::Visibility<'hir>) -> hir::Visibility<'hir> {
 665         let vis_kind = match vis.node {
 666             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 667             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 668             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 669             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 670                 hir::VisibilityKind::Restricted {
 671                     path: self.rebuild_use_path(path),
 672                     hir_id: self.next_id(),
 673                 }
 674             }
 675         };
 676         respan(self.lower_span(vis.span), vis_kind)
 677     }
 678
 679     fn lower_foreign_item(&mut self, i: &ForeignItem) -> hir::ForeignItem<'hir> {
 680         let hir_id = self.lower_node_id(i.id);
 681         let def_id = hir_id.expect_owner();
 682         self.lower_attrs(hir_id, &i.attrs);
 683         hir::ForeignItem {
 684             def_id,
 685             ident: self.lower_ident(i.ident),
 686             kind: match i.kind {
 687                 ForeignItemKind::Fn(box FnKind(_, ref sig, ref generics, _)) => {
 688                     let fdec = &sig.decl;
 689                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 690                         generics,
 691                         def_id,
 692                         AnonymousLifetimeMode::PassThrough,
 693                         |this, _| {
 694                             (
 695                                 // Disallow `impl Trait` in foreign items.
 696                                 this.lower_fn_decl(fdec, None, false, None),
 697                                 this.lower_fn_params_to_names(fdec),
 698                             )
 699                         },
 700                     );
 701
 702                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 703                 }
 704                 ForeignItemKind::Static(ref t, m, _) => {
 705                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 706                     hir::ForeignItemKind::Static(ty, m)
 707                 }
 708                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 709                 ForeignItemKind::MacCall(_) => panic!("macro shouldn't exist here"),
 710             },
 711             vis: self.lower_visibility(&i.vis, None),
 712             span: self.lower_span(i.span),
 713         }
 714     }
 715
 716     fn lower_foreign_item_ref(&mut self, i: &ForeignItem) -> hir::ForeignItemRef<'hir> {
 717         hir::ForeignItemRef {
 718             id: hir::ForeignItemId { def_id: self.lower_node_id(i.id).expect_owner() },
 719             ident: self.lower_ident(i.ident),
 720             span: self.lower_span(i.span),
 721             vis: self.lower_visibility(&i.vis, Some(i.id)),
 722         }
 723     }
 724
 725     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 726         let id = self.lower_node_id(v.id);
 727         self.lower_attrs(id, &v.attrs);
 728         hir::Variant {
 729             id,
 730             data: self.lower_variant_data(id, &v.data),
 731             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 732             ident: self.lower_ident(v.ident),
 733             span: self.lower_span(v.span),
 734         }
 735     }
 736
 737     fn lower_variant_data(
 738         &mut self,
 739         parent_id: hir::HirId,
 740         vdata: &VariantData,
 741     ) -> hir::VariantData<'hir> {
 742         match *vdata {
 743             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 744                 self.arena
 745                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f))),
 746                 recovered,
 747             ),
 748             VariantData::Tuple(ref fields, id) => {
 749                 let ctor_id = self.lower_node_id(id);
 750                 self.alias_attrs(ctor_id, parent_id);
 751                 hir::VariantData::Tuple(
 752                     self.arena.alloc_from_iter(
 753                         fields.iter().enumerate().map(|f| self.lower_field_def(f)),
 754                     ),
 755                     ctor_id,
 756                 )
 757             }
 758             VariantData::Unit(id) => {
 759                 let ctor_id = self.lower_node_id(id);
 760                 self.alias_attrs(ctor_id, parent_id);
 761                 hir::VariantData::Unit(ctor_id)
 762             }
 763         }
 764     }
 765
 766     pub(super) fn lower_field_def(
 767         &mut self,
 768         (index, f): (usize, &FieldDef),
 769     ) -> hir::FieldDef<'hir> {
 770         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 771             let t = self.lower_path_ty(
 772                 &f.ty,
 773                 qself,
 774                 path,
 775                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 776                 ImplTraitContext::disallowed(),
 777             );
 778             self.arena.alloc(t)
 779         } else {
 780             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 781         };
 782         let hir_id = self.lower_node_id(f.id);
 783         self.lower_attrs(hir_id, &f.attrs);
 784         hir::FieldDef {
 785             span: self.lower_span(f.span),
 786             hir_id,
 787             ident: match f.ident {
 788                 Some(ident) => self.lower_ident(ident),
 789                 // FIXME(jseyfried): positional field hygiene.
 790                 None => Ident::new(sym::integer(index), self.lower_span(f.span)),
 791             },
 792             vis: self.lower_visibility(&f.vis, None),
 793             ty,
 794         }
 795     }
 796
 797     fn lower_trait_item(&mut self, i: &AssocItem) -> hir::TraitItem<'hir> {
 798         let hir_id = self.lower_node_id(i.id);
 799         let trait_item_def_id = hir_id.expect_owner();
 800
 801         let (generics, kind) = match i.kind {
 802             AssocItemKind::Const(_, ref ty, ref default) => {
 803                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 804                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 805                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
 806             }
 807             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, None)) => {
 808                 let names = self.lower_fn_params_to_names(&sig.decl);
 809                 let (generics, sig) =
 810                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None, i.id);
 811                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)))
 812             }
 813             AssocItemKind::Fn(box FnKind(_, ref sig, ref generics, Some(ref body))) => {
 814                 let asyncness = sig.header.asyncness;
 815                 let body_id =
 816                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, Some(&body));
 817                 let (generics, sig) = self.lower_method_sig(
 818                     generics,
 819                     sig,
 820                     trait_item_def_id,
 821                     false,
 822                     asyncness.opt_return_id(),
 823                     i.id,
 824                 );
 825                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)))
 826             }
 827             AssocItemKind::TyAlias(box TyAliasKind(_, ref generics, ref bounds, ref default)) => {
 828                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
 829                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 830                 let kind = hir::TraitItemKind::Type(
 831                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 832                     ty,
 833                 );
 834
 835                 (generics, kind)
 836             }
 837             AssocItemKind::MacCall(..) => panic!("macro item shouldn't exist at this point"),
 838         };
 839
 840         self.lower_attrs(hir_id, &i.attrs);
 841         hir::TraitItem {
 842             def_id: trait_item_def_id,
 843             ident: self.lower_ident(i.ident),
 844             generics,
 845             kind,
 846             span: self.lower_span(i.span),
 847         }
 848     }
 849
 850     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 851         let (kind, has_default) = match &i.kind {
 852             AssocItemKind::Const(_, _, default) => (hir::AssocItemKind::Const, default.is_some()),
 853             AssocItemKind::TyAlias(box TyAliasKind(_, _, _, default)) => {
 854                 (hir::AssocItemKind::Type, default.is_some())
 855             }
 856             AssocItemKind::Fn(box FnKind(_, sig, _, default)) => {
 857                 (hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }, default.is_some())
 858             }
 859             AssocItemKind::MacCall(..) => unimplemented!(),
 860         };
 861         let id = hir::TraitItemId { def_id: self.lower_node_id(i.id).expect_owner() };
 862         let defaultness = hir::Defaultness::Default { has_value: has_default };
 863         hir::TraitItemRef {
 864             id,
 865             ident: self.lower_ident(i.ident),
 866             span: self.lower_span(i.span),
 867             defaultness,
 868             kind,
 869         }
 870     }
 871
 872     /// Construct `ExprKind::Err` for the given `span`.
 873     crate fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 874         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 875     }
 876
 877     fn lower_impl_item(&mut self, i: &AssocItem) -> hir::ImplItem<'hir> {
 878         let impl_item_def_id = self.resolver.local_def_id(i.id);
 879
 880         let (generics, kind) = match &i.kind {
 881             AssocItemKind::Const(_, ty, expr) => {
 882                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 883                 (
 884                     hir::Generics::empty(),
 885                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 886                 )
 887             }
 888             AssocItemKind::Fn(box FnKind(_, sig, generics, body)) => {
 889                 self.current_item = Some(i.span);
 890                 let asyncness = sig.header.asyncness;
 891                 let body_id =
 892                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 893                 let impl_trait_return_allow = !self.is_in_trait_impl;
 894                 let (generics, sig) = self.lower_method_sig(
 895                     generics,
 896                     sig,
 897                     impl_item_def_id,
 898                     impl_trait_return_allow,
 899                     asyncness.opt_return_id(),
 900                     i.id,
 901                 );
 902
 903                 (generics, hir::ImplItemKind::Fn(sig, body_id))
 904             }
 905             AssocItemKind::TyAlias(box TyAliasKind(_, generics, _, ty)) => {
 906                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 907                 let kind = match ty {
 908                     None => {
 909                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 910                         hir::ImplItemKind::TyAlias(ty)
 911                     }
 912                     Some(ty) => {
 913                         let ty = self.lower_ty(
 914                             ty,
 915                             ImplTraitContext::TypeAliasesOpaqueTy {
 916                                 capturable_lifetimes: &mut FxHashSet::default(),
 917                             },
 918                         );
 919                         hir::ImplItemKind::TyAlias(ty)
 920                     }
 921                 };
 922                 (generics, kind)
 923             }
 924             AssocItemKind::MacCall(..) => panic!("`TyMac` should have been expanded by now"),
 925         };
 926
 927         // Since `default impl` is not yet implemented, this is always true in impls.
 928         let has_value = true;
 929         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 930         let hir_id = self.lower_node_id(i.id);
 931         self.lower_attrs(hir_id, &i.attrs);
 932         hir::ImplItem {
 933             def_id: hir_id.expect_owner(),
 934             ident: self.lower_ident(i.ident),
 935             generics,
 936             vis: self.lower_visibility(&i.vis, None),
 937             defaultness,
 938             kind,
 939             span: self.lower_span(i.span),
 940         }
 941     }
 942
 943     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef<'hir> {
 944         // Since `default impl` is not yet implemented, this is always true in impls.
 945         let has_value = true;
 946         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
 947         hir::ImplItemRef {
 948             id: hir::ImplItemId { def_id: self.lower_node_id(i.id).expect_owner() },
 949             ident: self.lower_ident(i.ident),
 950             span: self.lower_span(i.span),
 951             vis: self.lower_visibility(&i.vis, Some(i.id)),
 952             defaultness,
 953             kind: match &i.kind {
 954                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 955                 AssocItemKind::TyAlias(..) => hir::AssocItemKind::Type,
 956                 AssocItemKind::Fn(box FnKind(_, sig, ..)) => {
 957                     hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
 958                 }
 959                 AssocItemKind::MacCall(..) => unimplemented!(),
 960             },
 961         }
 962     }
 963
 964     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 965     /// the address space of that item instead of the item currently being
 966     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 967     /// lower a `Visibility` value although we haven't lowered the owning
 968     /// `ImplItem` in question yet.
 969     fn lower_visibility(
 970         &mut self,
 971         v: &Visibility,
 972         explicit_owner: Option<NodeId>,
 973     ) -> hir::Visibility<'hir> {
 974         let node = match v.kind {
 975             VisibilityKind::Public => hir::VisibilityKind::Public,
 976             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 977             VisibilityKind::Restricted { ref path, id } => {
 978                 debug!("lower_visibility: restricted path id = {:?}", id);
 979                 let lowered_id = if let Some(owner) = explicit_owner {
 980                     self.lower_node_id_with_owner(id, owner)
 981                 } else {
 982                     self.lower_node_id(id)
 983                 };
 984                 let res = self.expect_full_res(id);
 985                 let res = self.lower_res(res);
 986                 hir::VisibilityKind::Restricted {
 987                     path: self.lower_path_extra(res, path, ParamMode::Explicit, explicit_owner),
 988                     hir_id: lowered_id,
 989                 }
 990             }
 991             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 992         };
 993         respan(self.lower_span(v.span), node)
 994     }
 995
 996     fn lower_defaultness(
 997         &self,
 998         d: Defaultness,
 999         has_value: bool,
1000     ) -> (hir::Defaultness, Option<Span>) {
1001         match d {
1002             Defaultness::Default(sp) => {
1003                 (hir::Defaultness::Default { has_value }, Some(self.lower_span(sp)))
1004             }
1005             Defaultness::Final => {
1006                 assert!(has_value);
1007                 (hir::Defaultness::Final, None)
1008             }
1009         }
1010     }
1011
1012     fn record_body(
1013         &mut self,
1014         params: &'hir [hir::Param<'hir>],
1015         value: hir::Expr<'hir>,
1016     ) -> hir::BodyId {
1017         let body = hir::Body { generator_kind: self.generator_kind, params, value };
1018         let id = body.id();
1019         self.bodies.insert(id, body);
1020         id
1021     }
1022
1023     pub(super) fn lower_body(
1024         &mut self,
1025         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
1026     ) -> hir::BodyId {
1027         let prev_gen_kind = self.generator_kind.take();
1028         let task_context = self.task_context.take();
1029         let (parameters, result) = f(self);
1030         let body_id = self.record_body(parameters, result);
1031         self.task_context = task_context;
1032         self.generator_kind = prev_gen_kind;
1033         body_id
1034     }
1035
1036     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
1037         let hir_id = self.lower_node_id(param.id);
1038         self.lower_attrs(hir_id, &param.attrs);
1039         hir::Param {
1040             hir_id,
1041             pat: self.lower_pat(&param.pat),
1042             ty_span: self.lower_span(param.ty.span),
1043             span: self.lower_span(param.span),
1044         }
1045     }
1046
1047     pub(super) fn lower_fn_body(
1048         &mut self,
1049         decl: &FnDecl,
1050         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
1051     ) -> hir::BodyId {
1052         self.lower_body(|this| {
1053             (
1054                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
1055                 body(this),
1056             )
1057         })
1058     }
1059
1060     fn lower_fn_body_block(
1061         &mut self,
1062         span: Span,
1063         decl: &FnDecl,
1064         body: Option<&Block>,
1065     ) -> hir::BodyId {
1066         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1067     }
1068
1069     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1070         match block {
1071             Some(block) => self.lower_block_expr(block),
1072             None => self.expr_err(span),
1073         }
1074     }
1075
1076     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1077         self.lower_body(|this| {
1078             (
1079                 &[],
1080                 match expr {
1081                     Some(expr) => this.lower_expr_mut(expr),
1082                     None => this.expr_err(span),
1083                 },
1084             )
1085         })
1086     }
1087
1088     fn lower_maybe_async_body(
1089         &mut self,
1090         span: Span,
1091         decl: &FnDecl,
1092         asyncness: Async,
1093         body: Option<&Block>,
1094     ) -> hir::BodyId {
1095         let closure_id = match asyncness {
1096             Async::Yes { closure_id, .. } => closure_id,
1097             Async::No => return self.lower_fn_body_block(span, decl, body),
1098         };
1099
1100         self.lower_body(|this| {
1101             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1102             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1103
1104             // Async function parameters are lowered into the closure body so that they are
1105             // captured and so that the drop order matches the equivalent non-async functions.
1106             //
1107             // from:
1108             //
1109             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1110             //         <body>
1111             //     }
1112             //
1113             // into:
1114             //
1115             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1116             //       async move {
1117             //         let __arg2 = __arg2;
1118             //         let <pattern> = __arg2;
1119             //         let __arg1 = __arg1;
1120             //         let <pattern> = __arg1;
1121             //         let __arg0 = __arg0;
1122             //         let <pattern> = __arg0;
1123             //         drop-temps { <body> } // see comments later in fn for details
1124             //       }
1125             //     }
1126             //
1127             // If `<pattern>` is a simple ident, then it is lowered to a single
1128             // `let <pattern> = <pattern>;` statement as an optimization.
1129             //
1130             // Note that the body is embedded in `drop-temps`; an
1131             // equivalent desugaring would be `return { <body>
1132             // };`. The key point is that we wish to drop all the
1133             // let-bound variables and temporaries created in the body
1134             // (and its tail expression!) before we drop the
1135             // parameters (c.f. rust-lang/rust#64512).
1136             for (index, parameter) in decl.inputs.iter().enumerate() {
1137                 let parameter = this.lower_param(parameter);
1138                 let span = parameter.pat.span;
1139
1140                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1141                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1142                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1143                     hir::PatKind::Binding(
1144                         hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable,
1145                         _,
1146                         ident,
1147                         _,
1148                     ) => (ident, true),
1149                     // For `ref mut` or wildcard arguments, we can't reuse the binding, but
1150                     // we can keep the same name for the parameter.
1151                     // This lets rustdoc render it correctly in documentation.
1152                     hir::PatKind::Binding(_, _, ident, _) => (ident, false),
1153                     hir::PatKind::Wild => {
1154                         (Ident::with_dummy_span(rustc_span::symbol::kw::Underscore), false)
1155                     }
1156                     _ => {
1157                         // Replace the ident for bindings that aren't simple.
1158                         let name = format!("__arg{}", index);
1159                         let ident = Ident::from_str(&name);
1160
1161                         (ident, false)
1162                     }
1163                 };
1164
1165                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1166
1167                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1168                 // async function.
1169                 //
1170                 // If this is the simple case, this parameter will end up being the same as the
1171                 // original parameter, but with a different pattern id.
1172                 let stmt_attrs = this.attrs.get(&parameter.hir_id).copied();
1173                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1174                 let new_parameter = hir::Param {
1175                     hir_id: parameter.hir_id,
1176                     pat: new_parameter_pat,
1177                     ty_span: this.lower_span(parameter.ty_span),
1178                     span: this.lower_span(parameter.span),
1179                 };
1180
1181                 if is_simple_parameter {
1182                     // If this is the simple case, then we only insert one statement that is
1183                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1184                     // `HirId`s are densely assigned.
1185                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1186                     let stmt = this.stmt_let_pat(
1187                         stmt_attrs,
1188                         desugared_span,
1189                         Some(expr),
1190                         parameter.pat,
1191                         hir::LocalSource::AsyncFn,
1192                     );
1193                     statements.push(stmt);
1194                 } else {
1195                     // If this is not the simple case, then we construct two statements:
1196                     //
1197                     // ```
1198                     // let __argN = __argN;
1199                     // let <pat> = __argN;
1200                     // ```
1201                     //
1202                     // The first statement moves the parameter into the closure and thus ensures
1203                     // that the drop order is correct.
1204                     //
1205                     // The second statement creates the bindings that the user wrote.
1206
1207                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1208                     // because the user may have specified a `ref mut` binding in the next
1209                     // statement.
1210                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1211                         desugared_span,
1212                         ident,
1213                         hir::BindingAnnotation::Mutable,
1214                     );
1215                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1216                     let move_stmt = this.stmt_let_pat(
1217                         None,
1218                         desugared_span,
1219                         Some(move_expr),
1220                         move_pat,
1221                         hir::LocalSource::AsyncFn,
1222                     );
1223
1224                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1225                     // parameter's pattern so that `HirId`s are densely assigned.
1226                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1227                     let pattern_stmt = this.stmt_let_pat(
1228                         stmt_attrs,
1229                         desugared_span,
1230                         Some(pattern_expr),
1231                         parameter.pat,
1232                         hir::LocalSource::AsyncFn,
1233                     );
1234
1235                     statements.push(move_stmt);
1236                     statements.push(pattern_stmt);
1237                 };
1238
1239                 parameters.push(new_parameter);
1240             }
1241
1242             let body_span = body.map_or(span, |b| b.span);
1243             let async_expr = this.make_async_expr(
1244                 CaptureBy::Value,
1245                 closure_id,
1246                 None,
1247                 body_span,
1248                 hir::AsyncGeneratorKind::Fn,
1249                 |this| {
1250                     // Create a block from the user's function body:
1251                     let user_body = this.lower_block_expr_opt(body_span, body);
1252
1253                     // Transform into `drop-temps { <user-body> }`, an expression:
1254                     let desugared_span =
1255                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1256                     let user_body = this.expr_drop_temps(
1257                         desugared_span,
1258                         this.arena.alloc(user_body),
1259                         AttrVec::new(),
1260                     );
1261
1262                     // As noted above, create the final block like
1263                     //
1264                     // ```
1265                     // {
1266                     //   let $param_pattern = $raw_param;
1267                     //   ...
1268                     //   drop-temps { <user-body> }
1269                     // }
1270                     // ```
1271                     let body = this.block_all(
1272                         desugared_span,
1273                         this.arena.alloc_from_iter(statements),
1274                         Some(user_body),
1275                     );
1276
1277                     this.expr_block(body, AttrVec::new())
1278                 },
1279             );
1280
1281             (
1282                 this.arena.alloc_from_iter(parameters),
1283                 this.expr(body_span, async_expr, AttrVec::new()),
1284             )
1285         })
1286     }
1287
1288     fn lower_method_sig(
1289         &mut self,
1290         generics: &Generics,
1291         sig: &FnSig,
1292         fn_def_id: LocalDefId,
1293         impl_trait_return_allow: bool,
1294         is_async: Option<NodeId>,
1295         id: NodeId,
1296     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1297         let header = self.lower_fn_header(sig.header, sig.span, id);
1298         let (generics, decl) = self.add_in_band_defs(
1299             generics,
1300             fn_def_id,
1301             AnonymousLifetimeMode::PassThrough,
1302             |this, idty| {
1303                 this.lower_fn_decl(
1304                     &sig.decl,
1305                     Some((fn_def_id.to_def_id(), idty)),
1306                     impl_trait_return_allow,
1307                     is_async,
1308                 )
1309             },
1310         );
1311         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
1312     }
1313
1314     fn lower_fn_header(&mut self, h: FnHeader, span: Span, id: NodeId) -> hir::FnHeader {
1315         hir::FnHeader {
1316             unsafety: self.lower_unsafety(h.unsafety),
1317             asyncness: self.lower_asyncness(h.asyncness),
1318             constness: self.lower_constness(h.constness),
1319             abi: self.lower_extern(h.ext, span, id),
1320         }
1321     }
1322
1323     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1324         abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1325             self.error_on_invalid_abi(abi);
1326             abi::Abi::Rust
1327         })
1328     }
1329
1330     pub(super) fn lower_extern(&mut self, ext: Extern, span: Span, id: NodeId) -> abi::Abi {
1331         match ext {
1332             Extern::None => abi::Abi::Rust,
1333             Extern::Implicit => {
1334                 self.maybe_lint_missing_abi(span, id, abi::Abi::C { unwind: false });
1335                 abi::Abi::C { unwind: false }
1336             }
1337             Extern::Explicit(abi) => self.lower_abi(abi),
1338         }
1339     }
1340
1341     fn error_on_invalid_abi(&self, abi: StrLit) {
1342         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1343             .span_label(abi.span, "invalid ABI")
1344             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1345             .emit();
1346     }
1347
1348     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1349         match a {
1350             Async::Yes { .. } => hir::IsAsync::Async,
1351             Async::No => hir::IsAsync::NotAsync,
1352         }
1353     }
1354
1355     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1356         match c {
1357             Const::Yes(_) => hir::Constness::Const,
1358             Const::No => hir::Constness::NotConst,
1359         }
1360     }
1361
1362     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1363         match u {
1364             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1365             Unsafe::No => hir::Unsafety::Normal,
1366         }
1367     }
1368
1369     pub(super) fn lower_generics_mut(
1370         &mut self,
1371         generics: &Generics,
1372         itctx: ImplTraitContext<'_, 'hir>,
1373     ) -> GenericsCtor<'hir> {
1374         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1375         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1376         for pred in &generics.where_clause.predicates {
1377             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1378                 'next_bound: for bound in &bound_pred.bounds {
1379                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1380                         // Check if the where clause type is a plain type parameter.
1381                         match self
1382                             .resolver
1383                             .get_partial_res(bound_pred.bounded_ty.id)
1384                             .map(|d| (d.base_res(), d.unresolved_segments()))
1385                         {
1386                             Some((Res::Def(DefKind::TyParam, def_id), 0))
1387                                 if bound_pred.bound_generic_params.is_empty() =>
1388                             {
1389                                 for param in &generics.params {
1390                                     if def_id == self.resolver.local_def_id(param.id).to_def_id() {
1391                                         add_bounds.entry(param.id).or_default().push(bound.clone());
1392                                         continue 'next_bound;
1393                                     }
1394                                 }
1395                             }
1396                             _ => {}
1397                         }
1398                         self.diagnostic().span_err(
1399                             bound_pred.bounded_ty.span,
1400                             "`?Trait` bounds are only permitted at the \
1401                                  point where a type parameter is declared",
1402                         );
1403                     }
1404                 }
1405             }
1406         }
1407
1408         GenericsCtor {
1409             params: self.lower_generic_params_mut(&generics.params, &add_bounds, itctx).collect(),
1410             where_clause: self.lower_where_clause(&generics.where_clause),
1411             span: self.lower_span(generics.span),
1412         }
1413     }
1414
1415     pub(super) fn lower_generics(
1416         &mut self,
1417         generics: &Generics,
1418         itctx: ImplTraitContext<'_, 'hir>,
1419     ) -> hir::Generics<'hir> {
1420         let generics_ctor = self.lower_generics_mut(generics, itctx);
1421         generics_ctor.into_generics(self.arena)
1422     }
1423
1424     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1425         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1426             hir::WhereClause {
1427                 predicates: this.arena.alloc_from_iter(
1428                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1429                 ),
1430                 span: this.lower_span(wc.span),
1431             }
1432         })
1433     }
1434
1435     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1436         match *pred {
1437             WherePredicate::BoundPredicate(WhereBoundPredicate {
1438                 ref bound_generic_params,
1439                 ref bounded_ty,
1440                 ref bounds,
1441                 span,
1442             }) => {
1443                 self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1444                     hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1445                         bound_generic_params: this.lower_generic_params(
1446                             bound_generic_params,
1447                             &NodeMap::default(),
1448                             ImplTraitContext::disallowed(),
1449                         ),
1450                         bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1451                         bounds: this.arena.alloc_from_iter(bounds.iter().map(
1452                             |bound| match bound {
1453                                 // We used to ignore `?Trait` bounds, as they were copied into type
1454                                 // parameters already, but we need to keep them around only for
1455                                 // diagnostics when we suggest removal of `?Sized` bounds. See
1456                                 // `suggest_constraining_type_param`. This will need to change if
1457                                 // we ever allow something *other* than `?Sized`.
1458                                 GenericBound::Trait(p, TraitBoundModifier::Maybe) => {
1459                                     hir::GenericBound::Unsized(this.lower_span(p.span))
1460                                 }
1461                                 _ => this.lower_param_bound(bound, ImplTraitContext::disallowed()),
1462                             },
1463                         )),
1464                         span: this.lower_span(span),
1465                     })
1466                 })
1467             }
1468             WherePredicate::RegionPredicate(WhereRegionPredicate {
1469                 ref lifetime,
1470                 ref bounds,
1471                 span,
1472             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1473                 span: self.lower_span(span),
1474                 lifetime: self.lower_lifetime(lifetime),
1475                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1476             }),
1477             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1478                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1479                     hir_id: self.lower_node_id(id),
1480                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1481                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1482                     span: self.lower_span(span),
1483                 })
1484             }
1485         }
1486     }
1487 }
1488
1489 /// Helper struct for delayed construction of Generics.
1490 pub(super) struct GenericsCtor<'hir> {
1491     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1492     where_clause: hir::WhereClause<'hir>,
1493     span: Span,
1494 }
1495
1496 impl<'hir> GenericsCtor<'hir> {
1497     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1498         hir::Generics {
1499             params: arena.alloc_from_iter(self.params),
1500             where_clause: self.where_clause,
1501             span: self.span,
1502         }
1503     }
1504 }