src/librustc_ast_lowering/item.rs

   1 use super::{AnonymousLifetimeMode, LoweringContext, ParamMode};
   2 use super::{ImplTraitContext, ImplTraitPosition, ImplTraitTypeIdVisitor};
   3
   4 use rustc::arena::Arena;
   5 use rustc::bug;
   6 use rustc_errors::struct_span_err;
   7 use rustc_hir as hir;
   8 use rustc_hir::def::{DefKind, Res};
   9 use rustc_hir::def_id::DefId;
  10 use rustc_span::source_map::{respan, DesugaringKind};
  11 use rustc_span::symbol::{kw, sym};
  12 use rustc_span::Span;
  13 use rustc_target::spec::abi;
  14 use syntax::ast::*;
  15 use syntax::attr;
  16 use syntax::node_id::NodeMap;
  17 use syntax::visit::{self, AssocCtxt, Visitor};
  18
  19 use log::debug;
  20 use smallvec::{smallvec, SmallVec};
  21 use std::collections::BTreeSet;
  22
  23 pub(super) struct ItemLowerer<'a, 'lowering, 'hir> {
  24     pub(super) lctx: &'a mut LoweringContext<'lowering, 'hir>,
  25 }
  26
  27 impl ItemLowerer<'_, '_, '_> {
  28     fn with_trait_impl_ref(&mut self, impl_ref: &Option<TraitRef>, f: impl FnOnce(&mut Self)) {
  29         let old = self.lctx.is_in_trait_impl;
  30         self.lctx.is_in_trait_impl = if let &None = impl_ref { false } else { true };
  31         f(self);
  32         self.lctx.is_in_trait_impl = old;
  33     }
  34 }
  35
  36 impl<'a> Visitor<'a> for ItemLowerer<'a, '_, '_> {
  37     fn visit_mod(&mut self, m: &'a Mod, _s: Span, _attrs: &[Attribute], n: NodeId) {
  38         let hir_id = self.lctx.lower_node_id(n);
  39
  40         self.lctx.modules.insert(
  41             hir_id,
  42             hir::ModuleItems {
  43                 items: BTreeSet::new(),
  44                 trait_items: BTreeSet::new(),
  45                 impl_items: BTreeSet::new(),
  46             },
  47         );
  48
  49         let old = self.lctx.current_module;
  50         self.lctx.current_module = hir_id;
  51         visit::walk_mod(self, m);
  52         self.lctx.current_module = old;
  53     }
  54
  55     fn visit_item(&mut self, item: &'a Item) {
  56         let mut item_hir_id = None;
  57         self.lctx.with_hir_id_owner(item.id, |lctx| {
  58             lctx.without_in_scope_lifetime_defs(|lctx| {
  59                 if let Some(hir_item) = lctx.lower_item(item) {
  60                     item_hir_id = Some(hir_item.hir_id);
  61                     lctx.insert_item(hir_item);
  62                 }
  63             })
  64         });
  65
  66         if let Some(hir_id) = item_hir_id {
  67             self.lctx.with_parent_item_lifetime_defs(hir_id, |this| {
  68                 let this = &mut ItemLowerer { lctx: this };
  69                 if let ItemKind::Impl { ref of_trait, .. } = item.kind {
  70                     this.with_trait_impl_ref(of_trait, |this| visit::walk_item(this, item));
  71                 } else {
  72                     visit::walk_item(this, item);
  73                 }
  74             });
  75         }
  76     }
  77
  78     fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
  79         self.lctx.with_hir_id_owner(item.id, |lctx| match ctxt {
  80             AssocCtxt::Trait => {
  81                 let hir_item = lctx.lower_trait_item(item);
  82                 let id = hir::TraitItemId { hir_id: hir_item.hir_id };
  83                 lctx.trait_items.insert(id, hir_item);
  84                 lctx.modules.get_mut(&lctx.current_module).unwrap().trait_items.insert(id);
  85             }
  86             AssocCtxt::Impl => {
  87                 let hir_item = lctx.lower_impl_item(item);
  88                 let id = hir::ImplItemId { hir_id: hir_item.hir_id };
  89                 lctx.impl_items.insert(id, hir_item);
  90                 lctx.modules.get_mut(&lctx.current_module).unwrap().impl_items.insert(id);
  91             }
  92         });
  93
  94         visit::walk_assoc_item(self, item, ctxt);
  95     }
  96 }
  97
  98 impl<'hir> LoweringContext<'_, 'hir> {
  99     // Same as the method above, but accepts `hir::GenericParam`s
 100     // instead of `ast::GenericParam`s.
 101     // This should only be used with generics that have already had their
 102     // in-band lifetimes added. In practice, this means that this function is
 103     // only used when lowering a child item of a trait or impl.
 104     fn with_parent_item_lifetime_defs<T>(
 105         &mut self,
 106         parent_hir_id: hir::HirId,
 107         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 108     ) -> T {
 109         let old_len = self.in_scope_lifetimes.len();
 110
 111         let parent_generics = match self.items.get(&parent_hir_id).unwrap().kind {
 112             hir::ItemKind::Impl { ref generics, .. }
 113             | hir::ItemKind::Trait(_, _, ref generics, ..) => &generics.params[..],
 114             _ => &[],
 115         };
 116         let lt_def_names = parent_generics.iter().filter_map(|param| match param.kind {
 117             hir::GenericParamKind::Lifetime { .. } => Some(param.name.modern()),
 118             _ => None,
 119         });
 120         self.in_scope_lifetimes.extend(lt_def_names);
 121
 122         let res = f(self);
 123
 124         self.in_scope_lifetimes.truncate(old_len);
 125         res
 126     }
 127
 128     // Clears (and restores) the `in_scope_lifetimes` field. Used when
 129     // visiting nested items, which never inherit in-scope lifetimes
 130     // from their surrounding environment.
 131     fn without_in_scope_lifetime_defs<T>(
 132         &mut self,
 133         f: impl FnOnce(&mut LoweringContext<'_, '_>) -> T,
 134     ) -> T {
 135         let old_in_scope_lifetimes = std::mem::replace(&mut self.in_scope_lifetimes, vec![]);
 136
 137         // this vector is only used when walking over impl headers,
 138         // input types, and the like, and should not be non-empty in
 139         // between items
 140         assert!(self.lifetimes_to_define.is_empty());
 141
 142         let res = f(self);
 143
 144         assert!(self.in_scope_lifetimes.is_empty());
 145         self.in_scope_lifetimes = old_in_scope_lifetimes;
 146
 147         res
 148     }
 149
 150     pub(super) fn lower_mod(&mut self, m: &Mod) -> hir::Mod<'hir> {
 151         hir::Mod {
 152             inner: m.inner,
 153             item_ids: self
 154                 .arena
 155                 .alloc_from_iter(m.items.iter().flat_map(|x| self.lower_item_id(x))),
 156         }
 157     }
 158
 159     pub(super) fn lower_item_id(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
 160         let node_ids = match i.kind {
 161             ItemKind::Use(ref use_tree) => {
 162                 let mut vec = smallvec![i.id];
 163                 self.lower_item_id_use_tree(use_tree, i.id, &mut vec);
 164                 vec
 165             }
 166             ItemKind::MacroDef(..) => SmallVec::new(),
 167             ItemKind::Fn(..) | ItemKind::Impl { of_trait: None, .. } => smallvec![i.id],
 168             ItemKind::Static(ref ty, ..) => {
 169                 let mut ids = smallvec![i.id];
 170                 if self.sess.features_untracked().impl_trait_in_bindings {
 171                     let mut visitor = ImplTraitTypeIdVisitor { ids: &mut ids };
 172                     visitor.visit_ty(ty);
 173                 }
 174                 ids
 175             }
 176             ItemKind::Const(ref ty, ..) => {
 177                 let mut ids = smallvec![i.id];
 178                 if self.sess.features_untracked().impl_trait_in_bindings {
 179                     let mut visitor = ImplTraitTypeIdVisitor { ids: &mut ids };
 180                     visitor.visit_ty(ty);
 181                 }
 182                 ids
 183             }
 184             _ => smallvec![i.id],
 185         };
 186
 187         node_ids
 188             .into_iter()
 189             .map(|node_id| hir::ItemId { id: self.allocate_hir_id_counter(node_id) })
 190             .collect()
 191     }
 192
 193     fn lower_item_id_use_tree(
 194         &mut self,
 195         tree: &UseTree,
 196         base_id: NodeId,
 197         vec: &mut SmallVec<[NodeId; 1]>,
 198     ) {
 199         match tree.kind {
 200             UseTreeKind::Nested(ref nested_vec) => {
 201                 for &(ref nested, id) in nested_vec {
 202                     vec.push(id);
 203                     self.lower_item_id_use_tree(nested, id, vec);
 204                 }
 205             }
 206             UseTreeKind::Glob => {}
 207             UseTreeKind::Simple(_, id1, id2) => {
 208                 for (_, &id) in
 209                     self.expect_full_res_from_use(base_id).skip(1).zip([id1, id2].iter())
 210                 {
 211                     vec.push(id);
 212                 }
 213             }
 214         }
 215     }
 216
 217     pub fn lower_item(&mut self, i: &Item) -> Option<hir::Item<'hir>> {
 218         let mut ident = i.ident;
 219         let mut vis = self.lower_visibility(&i.vis, None);
 220         let attrs = self.lower_attrs(&i.attrs);
 221
 222         if let ItemKind::MacroDef(ref def) = i.kind {
 223             if !def.legacy || attr::contains_name(&i.attrs, sym::macro_export) {
 224                 let body = self.lower_token_stream(def.body.inner_tokens());
 225                 let hir_id = self.lower_node_id(i.id);
 226                 self.exported_macros.push(hir::MacroDef {
 227                     name: ident.name,
 228                     vis,
 229                     attrs,
 230                     hir_id,
 231                     span: i.span,
 232                     body,
 233                     legacy: def.legacy,
 234                 });
 235             } else {
 236                 self.non_exported_macro_attrs.extend(attrs.iter().cloned());
 237             }
 238             return None;
 239         }
 240
 241         let kind = self.lower_item_kind(i.span, i.id, &mut ident, attrs, &mut vis, &i.kind);
 242
 243         Some(hir::Item { hir_id: self.lower_node_id(i.id), ident, attrs, kind, vis, span: i.span })
 244     }
 245
 246     fn lower_item_kind(
 247         &mut self,
 248         span: Span,
 249         id: NodeId,
 250         ident: &mut Ident,
 251         attrs: &'hir [Attribute],
 252         vis: &mut hir::Visibility<'hir>,
 253         i: &ItemKind,
 254     ) -> hir::ItemKind<'hir> {
 255         match *i {
 256             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
 257             ItemKind::Use(ref use_tree) => {
 258                 // Start with an empty prefix.
 259                 let prefix = Path { segments: vec![], span: use_tree.span };
 260
 261                 self.lower_use_tree(use_tree, &prefix, id, vis, ident, attrs)
 262             }
 263             ItemKind::Static(ref t, m, ref e) => {
 264                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 265                 hir::ItemKind::Static(ty, m, body_id)
 266             }
 267             ItemKind::Const(ref t, ref e) => {
 268                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 269                 hir::ItemKind::Const(ty, body_id)
 270             }
 271             ItemKind::Fn(FnSig { ref decl, header }, ref generics, ref body) => {
 272                 let fn_def_id = self.resolver.definitions().local_def_id(id);
 273                 self.with_new_scopes(|this| {
 274                     this.current_item = Some(ident.span);
 275
 276                     // Note: we don't need to change the return type from `T` to
 277                     // `impl Future<Output = T>` here because lower_body
 278                     // only cares about the input argument patterns in the function
 279                     // declaration (decl), not the return types.
 280                     let asyncness = header.asyncness;
 281                     let body_id =
 282                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 283
 284                     let (generics, decl) = this.add_in_band_defs(
 285                         generics,
 286                         fn_def_id,
 287                         AnonymousLifetimeMode::PassThrough,
 288                         |this, idty| {
 289                             let ret_id = asyncness.opt_return_id();
 290                             this.lower_fn_decl(&decl, Some((fn_def_id, idty)), true, ret_id)
 291                         },
 292                     );
 293                     let sig = hir::FnSig { decl, header: this.lower_fn_header(header) };
 294                     hir::ItemKind::Fn(sig, generics, body_id)
 295                 })
 296             }
 297             ItemKind::Mod(ref m) => hir::ItemKind::Mod(self.lower_mod(m)),
 298             ItemKind::ForeignMod(ref nm) => hir::ItemKind::ForeignMod(self.lower_foreign_mod(nm)),
 299             ItemKind::GlobalAsm(ref ga) => hir::ItemKind::GlobalAsm(self.lower_global_asm(ga)),
 300             ItemKind::TyAlias(ref ty, ref generics) => match ty.kind.opaque_top_hack() {
 301                 None => {
 302                     let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 303                     let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 304                     hir::ItemKind::TyAlias(ty, generics)
 305                 }
 306                 Some(bounds) => {
 307                     let ty = hir::OpaqueTy {
 308                         generics: self.lower_generics(
 309                             generics,
 310                             ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc),
 311                         ),
 312                         bounds: self.lower_param_bounds(
 313                             bounds,
 314                             ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc),
 315                         ),
 316                         impl_trait_fn: None,
 317                         origin: hir::OpaqueTyOrigin::TypeAlias,
 318                     };
 319                     hir::ItemKind::OpaqueTy(ty)
 320                 }
 321             },
 322             ItemKind::Enum(ref enum_definition, ref generics) => hir::ItemKind::Enum(
 323                 hir::EnumDef {
 324                     variants: self.arena.alloc_from_iter(
 325                         enum_definition.variants.iter().map(|x| self.lower_variant(x)),
 326                     ),
 327                 },
 328                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 329             ),
 330             ItemKind::Struct(ref struct_def, ref generics) => {
 331                 let struct_def = self.lower_variant_data(struct_def);
 332                 hir::ItemKind::Struct(
 333                     struct_def,
 334                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 335                 )
 336             }
 337             ItemKind::Union(ref vdata, ref generics) => {
 338                 let vdata = self.lower_variant_data(vdata);
 339                 hir::ItemKind::Union(
 340                     vdata,
 341                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 342                 )
 343             }
 344             ItemKind::Impl {
 345                 unsafety,
 346                 polarity,
 347                 defaultness,
 348                 constness,
 349                 generics: ref ast_generics,
 350                 of_trait: ref trait_ref,
 351                 self_ty: ref ty,
 352                 items: ref impl_items,
 353             } => {
 354                 let def_id = self.resolver.definitions().local_def_id(id);
 355
 356                 // Lower the "impl header" first. This ordering is important
 357                 // for in-band lifetimes! Consider `'a` here:
 358                 //
 359                 //     impl Foo<'a> for u32 {
 360                 //         fn method(&'a self) { .. }
 361                 //     }
 362                 //
 363                 // Because we start by lowering the `Foo<'a> for u32`
 364                 // part, we will add `'a` to the list of generics on
 365                 // the impl. When we then encounter it later in the
 366                 // method, it will not be considered an in-band
 367                 // lifetime to be added, but rather a reference to a
 368                 // parent lifetime.
 369                 let lowered_trait_impl_id = self.lower_node_id(id);
 370                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 371                     ast_generics,
 372                     def_id,
 373                     AnonymousLifetimeMode::CreateParameter,
 374                     |this, _| {
 375                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 376                             this.lower_trait_ref(trait_ref, ImplTraitContext::disallowed())
 377                         });
 378
 379                         if let Some(ref trait_ref) = trait_ref {
 380                             if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res {
 381                                 this.trait_impls
 382                                     .entry(def_id)
 383                                     .or_default()
 384                                     .push(lowered_trait_impl_id);
 385                             }
 386                         }
 387
 388                         let lowered_ty = this.lower_ty(ty, ImplTraitContext::disallowed());
 389
 390                         (trait_ref, lowered_ty)
 391                     },
 392                 );
 393
 394                 let new_impl_items =
 395                     self.with_in_scope_lifetime_defs(&ast_generics.params, |this| {
 396                         this.arena.alloc_from_iter(
 397                             impl_items.iter().map(|item| this.lower_impl_item_ref(item)),
 398                         )
 399                     });
 400
 401                 hir::ItemKind::Impl {
 402                     unsafety: self.lower_unsafety(unsafety),
 403                     polarity,
 404                     defaultness: self.lower_defaultness(defaultness, true /* [1] */),
 405                     constness: self.lower_constness(constness),
 406                     generics,
 407                     of_trait: trait_ref,
 408                     self_ty: lowered_ty,
 409                     items: new_impl_items,
 410                 }
 411             }
 412             ItemKind::Trait(is_auto, unsafety, ref generics, ref bounds, ref items) => {
 413                 let bounds = self.lower_param_bounds(bounds, ImplTraitContext::disallowed());
 414                 let items = self
 415                     .arena
 416                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
 417                 hir::ItemKind::Trait(
 418                     is_auto,
 419                     self.lower_unsafety(unsafety),
 420                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 421                     bounds,
 422                     items,
 423                 )
 424             }
 425             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 426                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 427                 self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 428             ),
 429             ItemKind::MacroDef(..) | ItemKind::Mac(..) => {
 430                 bug!("`TyMac` should have been expanded by now")
 431             }
 432         }
 433
 434         // [1] `defaultness.has_value()` is never called for an `impl`, always `true` in order to
 435         //     not cause an assertion failure inside the `lower_defaultness` function.
 436     }
 437
 438     fn lower_const_item(
 439         &mut self,
 440         ty: &Ty,
 441         span: Span,
 442         body: Option<&Expr>,
 443     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 444         let itctx = if self.sess.features_untracked().impl_trait_in_bindings {
 445             ImplTraitContext::OpaqueTy(None, hir::OpaqueTyOrigin::Misc)
 446         } else {
 447             ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
 448         };
 449         let ty = self.lower_ty(ty, itctx);
 450         (ty, self.lower_const_body(span, body))
 451     }
 452
 453     fn lower_use_tree(
 454         &mut self,
 455         tree: &UseTree,
 456         prefix: &Path,
 457         id: NodeId,
 458         vis: &mut hir::Visibility<'hir>,
 459         ident: &mut Ident,
 460         attrs: &'hir [Attribute],
 461     ) -> hir::ItemKind<'hir> {
 462         debug!("lower_use_tree(tree={:?})", tree);
 463         debug!("lower_use_tree: vis = {:?}", vis);
 464
 465         let path = &tree.prefix;
 466         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 467
 468         match tree.kind {
 469             UseTreeKind::Simple(rename, id1, id2) => {
 470                 *ident = tree.ident();
 471
 472                 // First, apply the prefix to the path.
 473                 let mut path = Path { segments, span: path.span };
 474
 475                 // Correctly resolve `self` imports.
 476                 if path.segments.len() > 1
 477                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 478                 {
 479                     let _ = path.segments.pop();
 480                     if rename.is_none() {
 481                         *ident = path.segments.last().unwrap().ident;
 482                     }
 483                 }
 484
 485                 let mut resolutions = self.expect_full_res_from_use(id);
 486                 // We want to return *something* from this function, so hold onto the first item
 487                 // for later.
 488                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 489
 490                 // Here, we are looping over namespaces, if they exist for the definition
 491                 // being imported. We only handle type and value namespaces because we
 492                 // won't be dealing with macros in the rest of the compiler.
 493                 // Essentially a single `use` which imports two names is desugared into
 494                 // two imports.
 495                 for (res, &new_node_id) in resolutions.zip([id1, id2].iter()) {
 496                     let ident = *ident;
 497                     let mut path = path.clone();
 498                     for seg in &mut path.segments {
 499                         seg.id = self.resolver.next_node_id();
 500                     }
 501                     let span = path.span;
 502
 503                     self.with_hir_id_owner(new_node_id, |this| {
 504                         let new_id = this.lower_node_id(new_node_id);
 505                         let res = this.lower_res(res);
 506                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit, None);
 507                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 508                         let vis = this.rebuild_vis(&vis);
 509
 510                         this.insert_item(hir::Item {
 511                             hir_id: new_id,
 512                             ident,
 513                             attrs,
 514                             kind,
 515                             vis,
 516                             span,
 517                         });
 518                     });
 519                 }
 520
 521                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit, None);
 522                 hir::ItemKind::Use(path, hir::UseKind::Single)
 523             }
 524             UseTreeKind::Glob => {
 525                 let path =
 526                     self.lower_path(id, &Path { segments, span: path.span }, ParamMode::Explicit);
 527                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 528             }
 529             UseTreeKind::Nested(ref trees) => {
 530                 // Nested imports are desugared into simple imports.
 531                 // So, if we start with
 532                 //
 533                 // ```
 534                 // pub(x) use foo::{a, b};
 535                 // ```
 536                 //
 537                 // we will create three items:
 538                 //
 539                 // ```
 540                 // pub(x) use foo::a;
 541                 // pub(x) use foo::b;
 542                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 543                 // ```
 544                 //
 545                 // The first two are produced by recursively invoking
 546                 // `lower_use_tree` (and indeed there may be things
 547                 // like `use foo::{a::{b, c}}` and so forth).  They
 548                 // wind up being directly added to
 549                 // `self.items`. However, the structure of this
 550                 // function also requires us to return one item, and
 551                 // for that we return the `{}` import (called the
 552                 // `ListStem`).
 553
 554                 let prefix = Path { segments, span: prefix.span.to(path.span) };
 555
 556                 // Add all the nested `PathListItem`s to the HIR.
 557                 for &(ref use_tree, id) in trees {
 558                     let new_hir_id = self.lower_node_id(id);
 559
 560                     let mut prefix = prefix.clone();
 561
 562                     // Give the segments new node-ids since they are being cloned.
 563                     for seg in &mut prefix.segments {
 564                         seg.id = self.resolver.next_node_id();
 565                     }
 566
 567                     // Each `use` import is an item and thus are owners of the
 568                     // names in the path. Up to this point the nested import is
 569                     // the current owner, since we want each desugared import to
 570                     // own its own names, we have to adjust the owner before
 571                     // lowering the rest of the import.
 572                     self.with_hir_id_owner(id, |this| {
 573                         let mut vis = this.rebuild_vis(&vis);
 574                         let mut ident = *ident;
 575
 576                         let kind =
 577                             this.lower_use_tree(use_tree, &prefix, id, &mut vis, &mut ident, attrs);
 578
 579                         this.insert_item(hir::Item {
 580                             hir_id: new_hir_id,
 581                             ident,
 582                             attrs,
 583                             kind,
 584                             vis,
 585                             span: use_tree.span,
 586                         });
 587                     });
 588                 }
 589
 590                 // Subtle and a bit hacky: we lower the privacy level
 591                 // of the list stem to "private" most of the time, but
 592                 // not for "restricted" paths. The key thing is that
 593                 // we don't want it to stay as `pub` (with no caveats)
 594                 // because that affects rustdoc and also the lints
 595                 // about `pub` items. But we can't *always* make it
 596                 // private -- particularly not for restricted paths --
 597                 // because it contains node-ids that would then be
 598                 // unused, failing the check that HirIds are "densely
 599                 // assigned".
 600                 match vis.node {
 601                     hir::VisibilityKind::Public
 602                     | hir::VisibilityKind::Crate(_)
 603                     | hir::VisibilityKind::Inherited => {
 604                         *vis = respan(prefix.span.shrink_to_lo(), hir::VisibilityKind::Inherited);
 605                     }
 606                     hir::VisibilityKind::Restricted { .. } => {
 607                         // Do nothing here, as described in the comment on the match.
 608                     }
 609                 }
 610
 611                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 612                 let res = self.lower_res(res);
 613                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit, None);
 614                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 615             }
 616         }
 617     }
 618
 619     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 620     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 621     /// `NodeId`s. (See, e.g., #56128.)
 622     fn rebuild_use_path(&mut self, path: &hir::Path<'hir>) -> &'hir hir::Path<'hir> {
 623         debug!("rebuild_use_path(path = {:?})", path);
 624         let segments =
 625             self.arena.alloc_from_iter(path.segments.iter().map(|seg| hir::PathSegment {
 626                 ident: seg.ident,
 627                 hir_id: seg.hir_id.map(|_| self.next_id()),
 628                 res: seg.res,
 629                 args: None,
 630                 infer_args: seg.infer_args,
 631             }));
 632         self.arena.alloc(hir::Path { span: path.span, res: path.res, segments })
 633     }
 634
 635     fn rebuild_vis(&mut self, vis: &hir::Visibility<'hir>) -> hir::Visibility<'hir> {
 636         let vis_kind = match vis.node {
 637             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 638             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 639             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 640             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 641                 hir::VisibilityKind::Restricted {
 642                     path: self.rebuild_use_path(path),
 643                     hir_id: self.next_id(),
 644                 }
 645             }
 646         };
 647         respan(vis.span, vis_kind)
 648     }
 649
 650     fn lower_foreign_item(&mut self, i: &ForeignItem) -> hir::ForeignItem<'hir> {
 651         let def_id = self.resolver.definitions().local_def_id(i.id);
 652         hir::ForeignItem {
 653             hir_id: self.lower_node_id(i.id),
 654             ident: i.ident,
 655             attrs: self.lower_attrs(&i.attrs),
 656             kind: match i.kind {
 657                 ForeignItemKind::Fn(ref sig, ref generics, _) => {
 658                     let fdec = &sig.decl;
 659                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 660                         generics,
 661                         def_id,
 662                         AnonymousLifetimeMode::PassThrough,
 663                         |this, _| {
 664                             (
 665                                 // Disallow `impl Trait` in foreign items.
 666                                 this.lower_fn_decl(fdec, None, false, None),
 667                                 this.lower_fn_params_to_names(fdec),
 668                             )
 669                         },
 670                     );
 671
 672                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 673                 }
 674                 ForeignItemKind::Static(ref t, m, _) => {
 675                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 676                     hir::ForeignItemKind::Static(ty, m)
 677                 }
 678                 ForeignItemKind::Const(ref t, _) => {
 679                     // For recovery purposes.
 680                     let ty = self.lower_ty(t, ImplTraitContext::disallowed());
 681                     hir::ForeignItemKind::Static(ty, Mutability::Not)
 682                 }
 683                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 684                 ForeignItemKind::Macro(_) => panic!("macro shouldn't exist here"),
 685             },
 686             vis: self.lower_visibility(&i.vis, None),
 687             span: i.span,
 688         }
 689     }
 690
 691     fn lower_foreign_mod(&mut self, fm: &ForeignMod) -> hir::ForeignMod<'hir> {
 692         hir::ForeignMod {
 693             abi: fm.abi.map_or(abi::Abi::C, |abi| self.lower_abi(abi)),
 694             items: self.arena.alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item(x))),
 695         }
 696     }
 697
 698     fn lower_global_asm(&mut self, ga: &GlobalAsm) -> &'hir hir::GlobalAsm {
 699         self.arena.alloc(hir::GlobalAsm { asm: ga.asm })
 700     }
 701
 702     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 703         hir::Variant {
 704             attrs: self.lower_attrs(&v.attrs),
 705             data: self.lower_variant_data(&v.data),
 706             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 707             id: self.lower_node_id(v.id),
 708             ident: v.ident,
 709             span: v.span,
 710         }
 711     }
 712
 713     fn lower_variant_data(&mut self, vdata: &VariantData) -> hir::VariantData<'hir> {
 714         match *vdata {
 715             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 716                 self.arena
 717                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 718                 recovered,
 719             ),
 720             VariantData::Tuple(ref fields, id) => hir::VariantData::Tuple(
 721                 self.arena
 722                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_struct_field(f))),
 723                 self.lower_node_id(id),
 724             ),
 725             VariantData::Unit(id) => hir::VariantData::Unit(self.lower_node_id(id)),
 726         }
 727     }
 728
 729     fn lower_struct_field(&mut self, (index, f): (usize, &StructField)) -> hir::StructField<'hir> {
 730         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 731             let t = self.lower_path_ty(
 732                 &f.ty,
 733                 qself,
 734                 path,
 735                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 736                 ImplTraitContext::disallowed(),
 737             );
 738             self.arena.alloc(t)
 739         } else {
 740             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 741         };
 742         hir::StructField {
 743             span: f.span,
 744             hir_id: self.lower_node_id(f.id),
 745             ident: match f.ident {
 746                 Some(ident) => ident,
 747                 // FIXME(jseyfried): positional field hygiene.
 748                 None => Ident::new(sym::integer(index), f.span),
 749             },
 750             vis: self.lower_visibility(&f.vis, None),
 751             ty,
 752             attrs: self.lower_attrs(&f.attrs),
 753         }
 754     }
 755
 756     fn lower_trait_item(&mut self, i: &AssocItem) -> hir::TraitItem<'hir> {
 757         let trait_item_def_id = self.resolver.definitions().local_def_id(i.id);
 758
 759         let (generics, kind) = match i.kind {
 760             AssocItemKind::Static(ref ty, _, ref default) // Let's pretend this is a `const`.
 761             | AssocItemKind::Const(ref ty, ref default) => {
 762                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 763                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 764                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
 765             }
 766             AssocItemKind::Fn(ref sig, ref generics, None) => {
 767                 let names = self.lower_fn_params_to_names(&sig.decl);
 768                 let (generics, sig) =
 769                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
 770                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Required(names)))
 771             }
 772             AssocItemKind::Fn(ref sig, ref generics, Some(ref body)) => {
 773                 let body_id = self.lower_fn_body_block(i.span, &sig.decl, Some(body));
 774                 let (generics, sig) =
 775                     self.lower_method_sig(generics, sig, trait_item_def_id, false, None);
 776                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Provided(body_id)))
 777             }
 778             AssocItemKind::TyAlias(ref generics, ref bounds, ref default) => {
 779                 let ty = default.as_ref().map(|x| self.lower_ty(x, ImplTraitContext::disallowed()));
 780                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 781                 let kind = hir::TraitItemKind::Type(
 782                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 783                     ty,
 784                 );
 785
 786                 (generics, kind)
 787             }
 788             AssocItemKind::Macro(..) => bug!("macro item shouldn't exist at this point"),
 789         };
 790
 791         hir::TraitItem {
 792             hir_id: self.lower_node_id(i.id),
 793             ident: i.ident,
 794             attrs: self.lower_attrs(&i.attrs),
 795             generics,
 796             kind,
 797             span: i.span,
 798         }
 799     }
 800
 801     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 802         let (kind, has_default) = match &i.kind {
 803             AssocItemKind::Static(_, _, default) // Let's pretend this is a `const` for recovery.
 804             | AssocItemKind::Const(_, default) => {
 805                 (hir::AssocItemKind::Const, default.is_some())
 806             }
 807             AssocItemKind::TyAlias(_, _, default) => (hir::AssocItemKind::Type, default.is_some()),
 808             AssocItemKind::Fn(sig, _, default) => {
 809                 (hir::AssocItemKind::Method { has_self: sig.decl.has_self() }, default.is_some())
 810             }
 811             AssocItemKind::Macro(..) => unimplemented!(),
 812         };
 813         hir::TraitItemRef {
 814             id: hir::TraitItemId { hir_id: self.lower_node_id(i.id) },
 815             ident: i.ident,
 816             span: i.span,
 817             defaultness: self.lower_defaultness(Defaultness::Default, has_default),
 818             kind,
 819         }
 820     }
 821
 822     /// Construct `ExprKind::Err` for the given `span`.
 823     fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 824         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 825     }
 826
 827     fn lower_impl_item(&mut self, i: &AssocItem) -> hir::ImplItem<'hir> {
 828         let impl_item_def_id = self.resolver.definitions().local_def_id(i.id);
 829
 830         let (generics, kind) = match i.kind {
 831             AssocItemKind::Static(ref ty, _, ref expr) | AssocItemKind::Const(ref ty, ref expr) => {
 832                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 833                 (
 834                     hir::Generics::empty(),
 835                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 836                 )
 837             }
 838             AssocItemKind::Fn(ref sig, ref generics, ref body) => {
 839                 self.current_item = Some(i.span);
 840                 let asyncness = sig.header.asyncness;
 841                 let body_id =
 842                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 843                 let impl_trait_return_allow = !self.is_in_trait_impl;
 844                 let (generics, sig) = self.lower_method_sig(
 845                     generics,
 846                     sig,
 847                     impl_item_def_id,
 848                     impl_trait_return_allow,
 849                     asyncness.opt_return_id(),
 850                 );
 851
 852                 (generics, hir::ImplItemKind::Method(sig, body_id))
 853             }
 854             AssocItemKind::TyAlias(ref generics, _, ref ty) => {
 855                 let generics = self.lower_generics(generics, ImplTraitContext::disallowed());
 856                 let kind = match ty {
 857                     None => {
 858                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 859                         hir::ImplItemKind::TyAlias(ty)
 860                     }
 861                     Some(ty) => match ty.kind.opaque_top_hack() {
 862                         None => {
 863                             let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
 864                             hir::ImplItemKind::TyAlias(ty)
 865                         }
 866                         Some(bs) => {
 867                             let bs = self.lower_param_bounds(bs, ImplTraitContext::disallowed());
 868                             hir::ImplItemKind::OpaqueTy(bs)
 869                         }
 870                     },
 871                 };
 872                 (generics, kind)
 873             }
 874             AssocItemKind::Macro(..) => bug!("`TyMac` should have been expanded by now"),
 875         };
 876
 877         hir::ImplItem {
 878             hir_id: self.lower_node_id(i.id),
 879             ident: i.ident,
 880             attrs: self.lower_attrs(&i.attrs),
 881             generics,
 882             vis: self.lower_visibility(&i.vis, None),
 883             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
 884             kind,
 885             span: i.span,
 886         }
 887
 888         // [1] since `default impl` is not yet implemented, this is always true in impls
 889     }
 890
 891     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef<'hir> {
 892         hir::ImplItemRef {
 893             id: hir::ImplItemId { hir_id: self.lower_node_id(i.id) },
 894             ident: i.ident,
 895             span: i.span,
 896             vis: self.lower_visibility(&i.vis, Some(i.id)),
 897             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
 898             kind: match &i.kind {
 899                 AssocItemKind::Static(..) // Let's pretend this is a `const` for recovery.
 900                 | AssocItemKind::Const(..) => hir::AssocItemKind::Const,
 901                 AssocItemKind::TyAlias(_, _, ty) => {
 902                     match ty.as_deref().and_then(|ty| ty.kind.opaque_top_hack()) {
 903                         None => hir::AssocItemKind::Type,
 904                         Some(_) => hir::AssocItemKind::OpaqueTy,
 905                     }
 906                 }
 907                 AssocItemKind::Fn(sig, _, _) => {
 908                     hir::AssocItemKind::Method { has_self: sig.decl.has_self() }
 909                 }
 910                 AssocItemKind::Macro(..) => unimplemented!(),
 911             },
 912         }
 913
 914         // [1] since `default impl` is not yet implemented, this is always true in impls
 915     }
 916
 917     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 918     /// the address space of that item instead of the item currently being
 919     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 920     /// lower a `Visibility` value although we haven't lowered the owning
 921     /// `ImplItem` in question yet.
 922     fn lower_visibility(
 923         &mut self,
 924         v: &Visibility,
 925         explicit_owner: Option<NodeId>,
 926     ) -> hir::Visibility<'hir> {
 927         let node = match v.node {
 928             VisibilityKind::Public => hir::VisibilityKind::Public,
 929             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 930             VisibilityKind::Restricted { ref path, id } => {
 931                 debug!("lower_visibility: restricted path id = {:?}", id);
 932                 let lowered_id = if let Some(owner) = explicit_owner {
 933                     self.lower_node_id_with_owner(id, owner)
 934                 } else {
 935                     self.lower_node_id(id)
 936                 };
 937                 let res = self.expect_full_res(id);
 938                 let res = self.lower_res(res);
 939                 hir::VisibilityKind::Restricted {
 940                     path: self.lower_path_extra(res, path, ParamMode::Explicit, explicit_owner),
 941                     hir_id: lowered_id,
 942                 }
 943             }
 944             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 945         };
 946         respan(v.span, node)
 947     }
 948
 949     fn lower_defaultness(&self, d: Defaultness, has_value: bool) -> hir::Defaultness {
 950         match d {
 951             Defaultness::Default => hir::Defaultness::Default { has_value: has_value },
 952             Defaultness::Final => {
 953                 assert!(has_value);
 954                 hir::Defaultness::Final
 955             }
 956         }
 957     }
 958
 959     fn record_body(
 960         &mut self,
 961         params: &'hir [hir::Param<'hir>],
 962         value: hir::Expr<'hir>,
 963     ) -> hir::BodyId {
 964         let body = hir::Body { generator_kind: self.generator_kind, params, value };
 965         let id = body.id();
 966         self.bodies.insert(id, body);
 967         id
 968     }
 969
 970     fn lower_body(
 971         &mut self,
 972         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
 973     ) -> hir::BodyId {
 974         let prev_gen_kind = self.generator_kind.take();
 975         let (parameters, result) = f(self);
 976         let body_id = self.record_body(parameters, result);
 977         self.generator_kind = prev_gen_kind;
 978         body_id
 979     }
 980
 981     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
 982         hir::Param {
 983             attrs: self.lower_attrs(&param.attrs),
 984             hir_id: self.lower_node_id(param.id),
 985             pat: self.lower_pat(&param.pat),
 986             span: param.span,
 987         }
 988     }
 989
 990     pub(super) fn lower_fn_body(
 991         &mut self,
 992         decl: &FnDecl,
 993         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 994     ) -> hir::BodyId {
 995         self.lower_body(|this| {
 996             (
 997                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
 998                 body(this),
 999             )
1000         })
1001     }
1002
1003     fn lower_fn_body_block(
1004         &mut self,
1005         span: Span,
1006         decl: &FnDecl,
1007         body: Option<&Block>,
1008     ) -> hir::BodyId {
1009         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1010     }
1011
1012     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1013         match block {
1014             Some(block) => self.lower_block_expr(block),
1015             None => self.expr_err(span),
1016         }
1017     }
1018
1019     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1020         self.lower_body(|this| {
1021             (
1022                 &[],
1023                 match expr {
1024                     Some(expr) => this.lower_expr_mut(expr),
1025                     None => this.expr_err(span),
1026                 },
1027             )
1028         })
1029     }
1030
1031     fn lower_maybe_async_body(
1032         &mut self,
1033         span: Span,
1034         decl: &FnDecl,
1035         asyncness: Async,
1036         body: Option<&Block>,
1037     ) -> hir::BodyId {
1038         let closure_id = match asyncness {
1039             Async::Yes { closure_id, .. } => closure_id,
1040             Async::No => return self.lower_fn_body_block(span, decl, body),
1041         };
1042
1043         self.lower_body(|this| {
1044             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1045             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1046
1047             // Async function parameters are lowered into the closure body so that they are
1048             // captured and so that the drop order matches the equivalent non-async functions.
1049             //
1050             // from:
1051             //
1052             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1053             //         <body>
1054             //     }
1055             //
1056             // into:
1057             //
1058             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1059             //       async move {
1060             //         let __arg2 = __arg2;
1061             //         let <pattern> = __arg2;
1062             //         let __arg1 = __arg1;
1063             //         let <pattern> = __arg1;
1064             //         let __arg0 = __arg0;
1065             //         let <pattern> = __arg0;
1066             //         drop-temps { <body> } // see comments later in fn for details
1067             //       }
1068             //     }
1069             //
1070             // If `<pattern>` is a simple ident, then it is lowered to a single
1071             // `let <pattern> = <pattern>;` statement as an optimization.
1072             //
1073             // Note that the body is embedded in `drop-temps`; an
1074             // equivalent desugaring would be `return { <body>
1075             // };`. The key point is that we wish to drop all the
1076             // let-bound variables and temporaries created in the body
1077             // (and its tail expression!) before we drop the
1078             // parameters (c.f. rust-lang/rust#64512).
1079             for (index, parameter) in decl.inputs.iter().enumerate() {
1080                 let parameter = this.lower_param(parameter);
1081                 let span = parameter.pat.span;
1082
1083                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1084                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1085                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1086                     hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, ident, _) => {
1087                         (ident, true)
1088                     }
1089                     _ => {
1090                         // Replace the ident for bindings that aren't simple.
1091                         let name = format!("__arg{}", index);
1092                         let ident = Ident::from_str(&name);
1093
1094                         (ident, false)
1095                     }
1096                 };
1097
1098                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1099
1100                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1101                 // async function.
1102                 //
1103                 // If this is the simple case, this parameter will end up being the same as the
1104                 // original parameter, but with a different pattern id.
1105                 let mut stmt_attrs = AttrVec::new();
1106                 stmt_attrs.extend(parameter.attrs.iter().cloned());
1107                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1108                 let new_parameter = hir::Param {
1109                     attrs: parameter.attrs,
1110                     hir_id: parameter.hir_id,
1111                     pat: new_parameter_pat,
1112                     span: parameter.span,
1113                 };
1114
1115                 if is_simple_parameter {
1116                     // If this is the simple case, then we only insert one statement that is
1117                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1118                     // `HirId`s are densely assigned.
1119                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1120                     let stmt = this.stmt_let_pat(
1121                         stmt_attrs,
1122                         desugared_span,
1123                         Some(expr),
1124                         parameter.pat,
1125                         hir::LocalSource::AsyncFn,
1126                     );
1127                     statements.push(stmt);
1128                 } else {
1129                     // If this is not the simple case, then we construct two statements:
1130                     //
1131                     // ```
1132                     // let __argN = __argN;
1133                     // let <pat> = __argN;
1134                     // ```
1135                     //
1136                     // The first statement moves the parameter into the closure and thus ensures
1137                     // that the drop order is correct.
1138                     //
1139                     // The second statement creates the bindings that the user wrote.
1140
1141                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1142                     // because the user may have specified a `ref mut` binding in the next
1143                     // statement.
1144                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1145                         desugared_span,
1146                         ident,
1147                         hir::BindingAnnotation::Mutable,
1148                     );
1149                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1150                     let move_stmt = this.stmt_let_pat(
1151                         AttrVec::new(),
1152                         desugared_span,
1153                         Some(move_expr),
1154                         move_pat,
1155                         hir::LocalSource::AsyncFn,
1156                     );
1157
1158                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1159                     // parameter's pattern so that `HirId`s are densely assigned.
1160                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1161                     let pattern_stmt = this.stmt_let_pat(
1162                         stmt_attrs,
1163                         desugared_span,
1164                         Some(pattern_expr),
1165                         parameter.pat,
1166                         hir::LocalSource::AsyncFn,
1167                     );
1168
1169                     statements.push(move_stmt);
1170                     statements.push(pattern_stmt);
1171                 };
1172
1173                 parameters.push(new_parameter);
1174             }
1175
1176             let body_span = body.map_or(span, |b| b.span);
1177             let async_expr = this.make_async_expr(
1178                 CaptureBy::Value,
1179                 closure_id,
1180                 None,
1181                 body_span,
1182                 hir::AsyncGeneratorKind::Fn,
1183                 |this| {
1184                     // Create a block from the user's function body:
1185                     let user_body = this.lower_block_expr_opt(body_span, body);
1186
1187                     // Transform into `drop-temps { <user-body> }`, an expression:
1188                     let desugared_span =
1189                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1190                     let user_body = this.expr_drop_temps(
1191                         desugared_span,
1192                         this.arena.alloc(user_body),
1193                         AttrVec::new(),
1194                     );
1195
1196                     // As noted above, create the final block like
1197                     //
1198                     // ```
1199                     // {
1200                     //   let $param_pattern = $raw_param;
1201                     //   ...
1202                     //   drop-temps { <user-body> }
1203                     // }
1204                     // ```
1205                     let body = this.block_all(
1206                         desugared_span,
1207                         this.arena.alloc_from_iter(statements),
1208                         Some(user_body),
1209                     );
1210
1211                     this.expr_block(body, AttrVec::new())
1212                 },
1213             );
1214
1215             (
1216                 this.arena.alloc_from_iter(parameters),
1217                 this.expr(body_span, async_expr, AttrVec::new()),
1218             )
1219         })
1220     }
1221
1222     fn lower_method_sig(
1223         &mut self,
1224         generics: &Generics,
1225         sig: &FnSig,
1226         fn_def_id: DefId,
1227         impl_trait_return_allow: bool,
1228         is_async: Option<NodeId>,
1229     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1230         let header = self.lower_fn_header(sig.header);
1231         let (generics, decl) = self.add_in_band_defs(
1232             generics,
1233             fn_def_id,
1234             AnonymousLifetimeMode::PassThrough,
1235             |this, idty| {
1236                 this.lower_fn_decl(
1237                     &sig.decl,
1238                     Some((fn_def_id, idty)),
1239                     impl_trait_return_allow,
1240                     is_async,
1241                 )
1242             },
1243         );
1244         (generics, hir::FnSig { header, decl })
1245     }
1246
1247     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1248         hir::FnHeader {
1249             unsafety: self.lower_unsafety(h.unsafety),
1250             asyncness: self.lower_asyncness(h.asyncness),
1251             constness: self.lower_constness(h.constness),
1252             abi: self.lower_extern(h.ext),
1253         }
1254     }
1255
1256     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1257         abi::lookup(&abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1258             self.error_on_invalid_abi(abi);
1259             abi::Abi::Rust
1260         })
1261     }
1262
1263     pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
1264         match ext {
1265             Extern::None => abi::Abi::Rust,
1266             Extern::Implicit => abi::Abi::C,
1267             Extern::Explicit(abi) => self.lower_abi(abi),
1268         }
1269     }
1270
1271     fn error_on_invalid_abi(&self, abi: StrLit) {
1272         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1273             .span_label(abi.span, "invalid ABI")
1274             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1275             .emit();
1276     }
1277
1278     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1279         match a {
1280             Async::Yes { .. } => hir::IsAsync::Async,
1281             Async::No => hir::IsAsync::NotAsync,
1282         }
1283     }
1284
1285     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1286         match c {
1287             Const::Yes(_) => hir::Constness::Const,
1288             Const::No => hir::Constness::NotConst,
1289         }
1290     }
1291
1292     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1293         match u {
1294             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1295             Unsafe::No => hir::Unsafety::Normal,
1296         }
1297     }
1298
1299     pub(super) fn lower_generics_mut(
1300         &mut self,
1301         generics: &Generics,
1302         itctx: ImplTraitContext<'_, 'hir>,
1303     ) -> GenericsCtor<'hir> {
1304         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1305         // FIXME: this could probably be done with less rightward drift. It also looks like two
1306         // control paths where `report_error` is called are the only paths that advance to after the
1307         // match statement, so the error reporting could probably just be moved there.
1308         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1309         for pred in &generics.where_clause.predicates {
1310             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1311                 'next_bound: for bound in &bound_pred.bounds {
1312                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1313                         let report_error = |this: &mut Self| {
1314                             this.diagnostic().span_err(
1315                                 bound_pred.bounded_ty.span,
1316                                 "`?Trait` bounds are only permitted at the \
1317                                  point where a type parameter is declared",
1318                             );
1319                         };
1320                         // Check if the where clause type is a plain type parameter.
1321                         match bound_pred.bounded_ty.kind {
1322                             TyKind::Path(None, ref path)
1323                                 if path.segments.len() == 1
1324                                     && bound_pred.bound_generic_params.is_empty() =>
1325                             {
1326                                 if let Some(Res::Def(DefKind::TyParam, def_id)) = self
1327                                     .resolver
1328                                     .get_partial_res(bound_pred.bounded_ty.id)
1329                                     .map(|d| d.base_res())
1330                                 {
1331                                     if let Some(node_id) =
1332                                         self.resolver.definitions().as_local_node_id(def_id)
1333                                     {
1334                                         for param in &generics.params {
1335                                             match param.kind {
1336                                                 GenericParamKind::Type { .. } => {
1337                                                     if node_id == param.id {
1338                                                         add_bounds
1339                                                             .entry(param.id)
1340                                                             .or_default()
1341                                                             .push(bound.clone());
1342                                                         continue 'next_bound;
1343                                                     }
1344                                                 }
1345                                                 _ => {}
1346                                             }
1347                                         }
1348                                     }
1349                                 }
1350                                 report_error(self)
1351                             }
1352                             _ => report_error(self),
1353                         }
1354                     }
1355                 }
1356             }
1357         }
1358
1359         GenericsCtor {
1360             params: self.lower_generic_params_mut(&generics.params, &add_bounds, itctx).collect(),
1361             where_clause: self.lower_where_clause(&generics.where_clause),
1362             span: generics.span,
1363         }
1364     }
1365
1366     pub(super) fn lower_generics(
1367         &mut self,
1368         generics: &Generics,
1369         itctx: ImplTraitContext<'_, 'hir>,
1370     ) -> hir::Generics<'hir> {
1371         let generics_ctor = self.lower_generics_mut(generics, itctx);
1372         generics_ctor.into_generics(self.arena)
1373     }
1374
1375     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1376         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1377             hir::WhereClause {
1378                 predicates: this.arena.alloc_from_iter(
1379                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1380                 ),
1381                 span: wc.span,
1382             }
1383         })
1384     }
1385
1386     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1387         match *pred {
1388             WherePredicate::BoundPredicate(WhereBoundPredicate {
1389                 ref bound_generic_params,
1390                 ref bounded_ty,
1391                 ref bounds,
1392                 span,
1393             }) => {
1394                 self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1395                     hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1396                         bound_generic_params: this.lower_generic_params(
1397                             bound_generic_params,
1398                             &NodeMap::default(),
1399                             ImplTraitContext::disallowed(),
1400                         ),
1401                         bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1402                         bounds: this.arena.alloc_from_iter(bounds.iter().filter_map(|bound| {
1403                             match *bound {
1404                                 // Ignore `?Trait` bounds.
1405                                 // They were copied into type parameters already.
1406                                 GenericBound::Trait(_, TraitBoundModifier::Maybe) => None,
1407                                 _ => Some(
1408                                     this.lower_param_bound(bound, ImplTraitContext::disallowed()),
1409                                 ),
1410                             }
1411                         })),
1412                         span,
1413                     })
1414                 })
1415             }
1416             WherePredicate::RegionPredicate(WhereRegionPredicate {
1417                 ref lifetime,
1418                 ref bounds,
1419                 span,
1420             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1421                 span,
1422                 lifetime: self.lower_lifetime(lifetime),
1423                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1424             }),
1425             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1426                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1427                     hir_id: self.lower_node_id(id),
1428                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1429                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1430                     span,
1431                 })
1432             }
1433         }
1434     }
1435 }
1436
1437 /// Helper struct for delayed construction of Generics.
1438 pub(super) struct GenericsCtor<'hir> {
1439     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1440     where_clause: hir::WhereClause<'hir>,
1441     span: Span,
1442 }
1443
1444 impl<'hir> GenericsCtor<'hir> {
1445     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1446         hir::Generics {
1447             params: arena.alloc_from_iter(self.params),
1448             where_clause: self.where_clause,
1449             span: self.span,
1450         }
1451     }
1452 }