src/librustc/hir/lowering/item.rs

   1 use super::LoweringContext;
   2 use super::ImplTraitContext;
   3 use super::ImplTraitPosition;
   4 use super::ImplTraitTypeIdVisitor;
   5 use super::AnonymousLifetimeMode;
   6 use super::ParamMode;
   7
   8 use crate::hir::{self, HirVec};
   9 use crate::hir::ptr::P;
  10 use crate::hir::def_id::DefId;
  11 use crate::hir::def::{Res, DefKind};
  12 use crate::util::nodemap::NodeMap;
  13
  14 use rustc_data_structures::thin_vec::ThinVec;
  15
  16 use std::collections::BTreeSet;
  17 use smallvec::SmallVec;
  18 use syntax::attr;
  19 use syntax::ast::*;
  20 use syntax::visit::{self, Visitor};
  21 use syntax::expand::SpecialDerives;
  22 use syntax::source_map::{respan, DesugaringKind, Spanned};
  23 use syntax::symbol::{kw, sym};
  24 use syntax_pos::Span;
  25
  26 pub(super) struct ItemLowerer<'tcx, 'interner> {
  27     pub(super) lctx: &'tcx mut LoweringContext<'interner>,
  28 }
  29
  30 impl<'tcx, 'interner> ItemLowerer<'tcx, 'interner> {
  31     fn with_trait_impl_ref<F>(&mut self, trait_impl_ref: &Option<TraitRef>, f: F)
  32     where
  33         F: FnOnce(&mut Self),
  34     {
  35         let old = self.lctx.is_in_trait_impl;
  36         self.lctx.is_in_trait_impl = if let &None = trait_impl_ref {
  37             false
  38         } else {
  39             true
  40         };
  41         f(self);
  42         self.lctx.is_in_trait_impl = old;
  43     }
  44 }
  45
  46 impl<'tcx, 'interner> Visitor<'tcx> for ItemLowerer<'tcx, 'interner> {
  47     fn visit_mod(&mut self, m: &'tcx Mod, _s: Span, _attrs: &[Attribute], n: NodeId) {
  48         let hir_id = self.lctx.lower_node_id(n);
  49
  50         self.lctx.modules.insert(hir_id, hir::ModuleItems {
  51             items: BTreeSet::new(),
  52             trait_items: BTreeSet::new(),
  53             impl_items: BTreeSet::new(),
  54         });
  55
  56         let old = self.lctx.current_module;
  57         self.lctx.current_module = hir_id;
  58         visit::walk_mod(self, m);
  59         self.lctx.current_module = old;
  60     }
  61
  62     fn visit_item(&mut self, item: &'tcx Item) {
  63         let mut item_hir_id = None;
  64         self.lctx.with_hir_id_owner(item.id, |lctx| {
  65             lctx.without_in_scope_lifetime_defs(|lctx| {
  66                 if let Some(hir_item) = lctx.lower_item(item) {
  67                     item_hir_id = Some(hir_item.hir_id);
  68                     lctx.insert_item(hir_item);
  69                 }
  70             })
  71         });
  72
  73         if let Some(hir_id) = item_hir_id {
  74             self.lctx.with_parent_item_lifetime_defs(hir_id, |this| {
  75                 let this = &mut ItemLowerer { lctx: this };
  76                 if let ItemKind::Impl(.., ref opt_trait_ref, _, _) = item.kind {
  77                     this.with_trait_impl_ref(opt_trait_ref, |this| {
  78                         visit::walk_item(this, item)
  79                     });
  80                 } else {
  81                     visit::walk_item(this, item);
  82                 }
  83             });
  84         }
  85     }
  86
  87     fn visit_trait_item(&mut self, item: &'tcx TraitItem) {
  88         self.lctx.with_hir_id_owner(item.id, |lctx| {
  89             let hir_item = lctx.lower_trait_item(item);
  90             let id = hir::TraitItemId { hir_id: hir_item.hir_id };
  91             lctx.trait_items.insert(id, hir_item);
  92             lctx.modules.get_mut(&lctx.current_module).unwrap().trait_items.insert(id);
  93         });
  94
  95         visit::walk_trait_item(self, item);
  96     }
  97
  98     fn visit_impl_item(&mut self, item: &'tcx ImplItem) {
  99         self.lctx.with_hir_id_owner(item.id, |lctx| {
 100             let hir_item = lctx.lower_impl_item(item);
 101             let id = hir::ImplItemId { hir_id: hir_item.hir_id };
 102             lctx.impl_items.insert(id, hir_item);
 103             lctx.modules.get_mut(&lctx.current_module).unwrap().impl_items.insert(id);
 104         });
 105         visit::walk_impl_item(self, item);
 106     }
 107 }
 108
 109 impl LoweringContext<'_> {
 110     // Same as the method above, but accepts `hir::GenericParam`s
 111     // instead of `ast::GenericParam`s.
 112     // This should only be used with generics that have already had their
 113     // in-band lifetimes added. In practice, this means that this function is
 114     // only used when lowering a child item of a trait or impl.
 115     fn with_parent_item_lifetime_defs<T>(
 116         &mut self,
 117         parent_hir_id: hir::HirId,
 118         f: impl FnOnce(&mut LoweringContext<'_>) -> T,
 119     ) -> T {
 120         let old_len = self.in_scope_lifetimes.len();
 121
 122         let parent_generics = match self.items.get(&parent_hir_id).unwrap().kind {
 123             hir::ItemKind::Impl(_, _, _, ref generics, ..)
 124             | hir::ItemKind::Trait(_, _, ref generics, ..) => {
 125                 &generics.params[..]
 126             }
 127             _ => &[],
 128         };
 129         let lt_def_names = parent_generics.iter().filter_map(|param| match param.kind {
 130             hir::GenericParamKind::Lifetime { .. } => Some(param.name.modern()),
 131             _ => None,
 132         });
 133         self.in_scope_lifetimes.extend(lt_def_names);
 134
 135         let res = f(self);
 136
 137         self.in_scope_lifetimes.truncate(old_len);
 138         res
 139     }
 140
 141     // Clears (and restores) the `in_scope_lifetimes` field. Used when
 142     // visiting nested items, which never inherit in-scope lifetimes
 143     // from their surrounding environment.
 144     fn without_in_scope_lifetime_defs<T>(
 145         &mut self,
 146         f: impl FnOnce(&mut LoweringContext<'_>) -> T,
 147     ) -> T {
 148         let old_in_scope_lifetimes = std::mem::replace(&mut self.in_scope_lifetimes, vec![]);
 149
 150         // this vector is only used when walking over impl headers,
 151         // input types, and the like, and should not be non-empty in
 152         // between items
 153         assert!(self.lifetimes_to_define.is_empty());
 154
 155         let res = f(self);
 156
 157         assert!(self.in_scope_lifetimes.is_empty());
 158         self.in_scope_lifetimes = old_in_scope_lifetimes;
 159
 160         res
 161     }
 162
 163     pub(super) fn lower_mod(&mut self, m: &Mod) -> hir::Mod {
 164         hir::Mod {
 165             inner: m.inner,
 166             item_ids: m.items.iter().flat_map(|x| self.lower_item_id(x)).collect(),
 167         }
 168     }
 169
 170     pub(super) fn lower_item_id(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
 171         let node_ids = match i.kind {
 172             ItemKind::Use(ref use_tree) => {
 173                 let mut vec = smallvec![i.id];
 174                 self.lower_item_id_use_tree(use_tree, i.id, &mut vec);
 175                 vec
 176             }
 177             ItemKind::MacroDef(..) => SmallVec::new(),
 178             ItemKind::Fn(..) |
 179             ItemKind::Impl(.., None, _, _) => smallvec![i.id],
 180             ItemKind::Static(ref ty, ..) => {
 181                 let mut ids = smallvec![i.id];
 182                 if self.sess.features_untracked().impl_trait_in_bindings {
 183                     let mut visitor = ImplTraitTypeIdVisitor { ids: &mut ids };
 184                     visitor.visit_ty(ty);
 185                 }
 186                 ids
 187             },
 188             ItemKind::Const(ref ty, ..) => {
 189                 let mut ids = smallvec![i.id];
 190                 if self.sess.features_untracked().impl_trait_in_bindings {
 191                     let mut visitor = ImplTraitTypeIdVisitor { ids: &mut ids };
 192                     visitor.visit_ty(ty);
 193                 }
 194                 ids
 195             },
 196             _ => smallvec![i.id],
 197         };
 198
 199         node_ids.into_iter().map(|node_id| hir::ItemId {
 200             id: self.allocate_hir_id_counter(node_id)
 201         }).collect()
 202     }
 203
 204     fn lower_item_id_use_tree(
 205         &mut self,
 206         tree: &UseTree,
 207         base_id: NodeId,
 208         vec: &mut SmallVec<[NodeId; 1]>
 209     ) {
 210         match tree.kind {
 211             UseTreeKind::Nested(ref nested_vec) => for &(ref nested, id) in nested_vec {
 212                 vec.push(id);
 213                 self.lower_item_id_use_tree(nested, id, vec);
 214             },
 215             UseTreeKind::Glob => {}
 216             UseTreeKind::Simple(_, id1, id2) => {
 217                 for (_, &id) in self.expect_full_res_from_use(base_id)
 218                                     .skip(1)
 219                                     .zip([id1, id2].iter())
 220                 {
 221                     vec.push(id);
 222                 }
 223             },
 224         }
 225     }
 226
 227     pub fn lower_item(&mut self, i: &Item) -> Option<hir::Item> {
 228         let mut ident = i.ident;
 229         let mut vis = self.lower_visibility(&i.vis, None);
 230         let mut attrs = self.lower_attrs_extendable(&i.attrs);
 231         if self.resolver.has_derives(i.id, SpecialDerives::PARTIAL_EQ | SpecialDerives::EQ) {
 232             // Add `#[structural_match]` if the item derived both `PartialEq` and `Eq`.
 233             let ident = Ident::new(sym::structural_match, i.span);
 234             attrs.push(attr::mk_attr_outer(attr::mk_word_item(ident)));
 235         }
 236         let attrs = attrs.into();
 237
 238         if let ItemKind::MacroDef(ref def) = i.kind {
 239             if !def.legacy || attr::contains_name(&i.attrs, sym::macro_export) {
 240                 let body = self.lower_token_stream(def.stream());
 241                 let hir_id = self.lower_node_id(i.id);
 242                 self.exported_macros.push(hir::MacroDef {
 243                     name: ident.name,
 244                     vis,
 245                     attrs,
 246                     hir_id,
 247                     span: i.span,
 248                     body,
 249                     legacy: def.legacy,
 250                 });
 251             } else {
 252                 self.non_exported_macro_attrs.extend(attrs.into_iter());
 253             }
 254             return None;
 255         }
 256
 257         let kind = self.lower_item_kind(i.id, &mut ident, &attrs, &mut vis, &i.kind);
 258
 259         Some(hir::Item {
 260             hir_id: self.lower_node_id(i.id),
 261             ident,
 262             attrs,
 263             kind,
 264             vis,
 265             span: i.span,
 266         })
 267     }
 268
 269     fn lower_item_kind(
 270         &mut self,
 271         id: NodeId,
 272         ident: &mut Ident,
 273         attrs: &hir::HirVec<Attribute>,
 274         vis: &mut hir::Visibility,
 275         i: &ItemKind,
 276     ) -> hir::ItemKind {
 277         match *i {
 278             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
 279             ItemKind::Use(ref use_tree) => {
 280                 // Start with an empty prefix.
 281                 let prefix = Path {
 282                     segments: vec![],
 283                     span: use_tree.span,
 284                 };
 285
 286                 self.lower_use_tree(use_tree, &prefix, id, vis, ident, attrs)
 287             }
 288             ItemKind::Static(ref t, m, ref e) => {
 289                 hir::ItemKind::Static(
 290                     self.lower_ty(
 291                         t,
 292                         if self.sess.features_untracked().impl_trait_in_bindings {
 293                             ImplTraitContext::OpaqueTy(None)
 294                         } else {
 295                             ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
 296                         }
 297                     ),
 298                     self.lower_mutability(m),
 299                     self.lower_const_body(e),
 300                 )
 301             }
 302             ItemKind::Const(ref t, ref e) => {
 303                 hir::ItemKind::Const(
 304                     self.lower_ty(
 305                         t,
 306                         if self.sess.features_untracked().impl_trait_in_bindings {
 307                             ImplTraitContext::OpaqueTy(None)
 308                         } else {
 309                             ImplTraitContext::Disallowed(ImplTraitPosition::Binding)
 310                         }
 311                     ),
 312                     self.lower_const_body(e)
 313                 )
 314             }
 315             ItemKind::Fn(ref decl, header, ref generics, ref body) => {
 316                 let fn_def_id = self.resolver.definitions().local_def_id(id);
 317                 self.with_new_scopes(|this| {
 318                     this.current_item = Some(ident.span);
 319
 320                     // Note: we don't need to change the return type from `T` to
 321                     // `impl Future<Output = T>` here because lower_body
 322                     // only cares about the input argument patterns in the function
 323                     // declaration (decl), not the return types.
 324                     let body_id = this.lower_maybe_async_body(&decl, header.asyncness.node, body);
 325
 326                     let (generics, fn_decl) = this.add_in_band_defs(
 327                         generics,
 328                         fn_def_id,
 329                         AnonymousLifetimeMode::PassThrough,
 330                         |this, idty| this.lower_fn_decl(
 331                             &decl,
 332                             Some((fn_def_id, idty)),
 333                             true,
 334                             header.asyncness.node.opt_return_id()
 335                         ),
 336                     );
 337
 338                     hir::ItemKind::Fn(
 339                         fn_decl,
 340                         this.lower_fn_header(header),
 341                         generics,
 342                         body_id,
 343                     )
 344                 })
 345             }
 346             ItemKind::Mod(ref m) => hir::ItemKind::Mod(self.lower_mod(m)),
 347             ItemKind::ForeignMod(ref nm) => hir::ItemKind::ForeignMod(self.lower_foreign_mod(nm)),
 348             ItemKind::GlobalAsm(ref ga) => hir::ItemKind::GlobalAsm(self.lower_global_asm(ga)),
 349             ItemKind::TyAlias(ref t, ref generics) => hir::ItemKind::TyAlias(
 350                 self.lower_ty(t, ImplTraitContext::disallowed()),
 351                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 352             ),
 353             ItemKind::OpaqueTy(ref b, ref generics) => hir::ItemKind::OpaqueTy(
 354                 hir::OpaqueTy {
 355                     generics: self.lower_generics(generics,
 356                         ImplTraitContext::OpaqueTy(None)),
 357                     bounds: self.lower_param_bounds(b,
 358                         ImplTraitContext::OpaqueTy(None)),
 359                     impl_trait_fn: None,
 360                     origin: hir::OpaqueTyOrigin::TypeAlias,
 361                 },
 362             ),
 363             ItemKind::Enum(ref enum_definition, ref generics) => {
 364                 hir::ItemKind::Enum(
 365                     hir::EnumDef {
 366                         variants: enum_definition
 367                             .variants
 368                             .iter()
 369                             .map(|x| self.lower_variant(x))
 370                             .collect(),
 371                     },
 372                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 373                 )
 374             },
 375             ItemKind::Struct(ref struct_def, ref generics) => {
 376                 let struct_def = self.lower_variant_data(struct_def);
 377                 hir::ItemKind::Struct(
 378                     struct_def,
 379                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 380                 )
 381             }
 382             ItemKind::Union(ref vdata, ref generics) => {
 383                 let vdata = self.lower_variant_data(vdata);
 384                 hir::ItemKind::Union(
 385                     vdata,
 386                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 387                 )
 388             }
 389             ItemKind::Impl(
 390                 unsafety,
 391                 polarity,
 392                 defaultness,
 393                 ref ast_generics,
 394                 ref trait_ref,
 395                 ref ty,
 396                 ref impl_items,
 397             ) => {
 398                 let def_id = self.resolver.definitions().local_def_id(id);
 399
 400                 // Lower the "impl header" first. This ordering is important
 401                 // for in-band lifetimes! Consider `'a` here:
 402                 //
 403                 //     impl Foo<'a> for u32 {
 404                 //         fn method(&'a self) { .. }
 405                 //     }
 406                 //
 407                 // Because we start by lowering the `Foo<'a> for u32`
 408                 // part, we will add `'a` to the list of generics on
 409                 // the impl. When we then encounter it later in the
 410                 // method, it will not be considered an in-band
 411                 // lifetime to be added, but rather a reference to a
 412                 // parent lifetime.
 413                 let lowered_trait_impl_id = self.lower_node_id(id);
 414                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 415                     ast_generics,
 416                     def_id,
 417                     AnonymousLifetimeMode::CreateParameter,
 418                     |this, _| {
 419                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 420                             this.lower_trait_ref(trait_ref, ImplTraitContext::disallowed())
 421                         });
 422
 423                         if let Some(ref trait_ref) = trait_ref {
 424                             if let Res::Def(DefKind::Trait, def_id) = trait_ref.path.res {
 425                                 this.trait_impls.entry(def_id).or_default().push(
 426                                     lowered_trait_impl_id);
 427                             }
 428                         }
 429
 430                         let lowered_ty = this.lower_ty(ty, ImplTraitContext::disallowed());
 431
 432                         (trait_ref, lowered_ty)
 433                     },
 434                 );
 435
 436                 let new_impl_items = self.with_in_scope_lifetime_defs(
 437                     &ast_generics.params,
 438                     |this| {
 439                         impl_items
 440                             .iter()
 441                             .map(|item| this.lower_impl_item_ref(item))
 442                             .collect()
 443                     },
 444                 );
 445
 446                 hir::ItemKind::Impl(
 447                     self.lower_unsafety(unsafety),
 448                     self.lower_impl_polarity(polarity),
 449                     self.lower_defaultness(defaultness, true /* [1] */),
 450                     generics,
 451                     trait_ref,
 452                     lowered_ty,
 453                     new_impl_items,
 454                 )
 455             }
 456             ItemKind::Trait(is_auto, unsafety, ref generics, ref bounds, ref items) => {
 457                 let bounds = self.lower_param_bounds(bounds, ImplTraitContext::disallowed());
 458                 let items = items
 459                     .iter()
 460                     .map(|item| self.lower_trait_item_ref(item))
 461                     .collect();
 462                 hir::ItemKind::Trait(
 463                     self.lower_is_auto(is_auto),
 464                     self.lower_unsafety(unsafety),
 465                     self.lower_generics(generics, ImplTraitContext::disallowed()),
 466                     bounds,
 467                     items,
 468                 )
 469             }
 470             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 471                 self.lower_generics(generics, ImplTraitContext::disallowed()),
 472                 self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 473             ),
 474             ItemKind::MacroDef(..)
 475             | ItemKind::Mac(..) => bug!("`TyMac` should have been expanded by now"),
 476         }
 477
 478         // [1] `defaultness.has_value()` is never called for an `impl`, always `true` in order to
 479         //     not cause an assertion failure inside the `lower_defaultness` function.
 480     }
 481
 482     fn lower_use_tree(
 483         &mut self,
 484         tree: &UseTree,
 485         prefix: &Path,
 486         id: NodeId,
 487         vis: &mut hir::Visibility,
 488         ident: &mut Ident,
 489         attrs: &hir::HirVec<Attribute>,
 490     ) -> hir::ItemKind {
 491         debug!("lower_use_tree(tree={:?})", tree);
 492         debug!("lower_use_tree: vis = {:?}", vis);
 493
 494         let path = &tree.prefix;
 495         let segments = prefix
 496             .segments
 497             .iter()
 498             .chain(path.segments.iter())
 499             .cloned()
 500             .collect();
 501
 502         match tree.kind {
 503             UseTreeKind::Simple(rename, id1, id2) => {
 504                 *ident = tree.ident();
 505
 506                 // First, apply the prefix to the path.
 507                 let mut path = Path {
 508                     segments,
 509                     span: path.span,
 510                 };
 511
 512                 // Correctly resolve `self` imports.
 513                 if path.segments.len() > 1
 514                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 515                 {
 516                     let _ = path.segments.pop();
 517                     if rename.is_none() {
 518                         *ident = path.segments.last().unwrap().ident;
 519                     }
 520                 }
 521
 522                 let mut resolutions = self.expect_full_res_from_use(id);
 523                 // We want to return *something* from this function, so hold onto the first item
 524                 // for later.
 525                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 526
 527                 // Here, we are looping over namespaces, if they exist for the definition
 528                 // being imported. We only handle type and value namespaces because we
 529                 // won't be dealing with macros in the rest of the compiler.
 530                 // Essentially a single `use` which imports two names is desugared into
 531                 // two imports.
 532                 for (res, &new_node_id) in resolutions.zip([id1, id2].iter()) {
 533                     let ident = *ident;
 534                     let mut path = path.clone();
 535                     for seg in &mut path.segments {
 536                         seg.id = self.sess.next_node_id();
 537                     }
 538                     let span = path.span;
 539
 540                     self.with_hir_id_owner(new_node_id, |this| {
 541                         let new_id = this.lower_node_id(new_node_id);
 542                         let res = this.lower_res(res);
 543                         let path =
 544                             this.lower_path_extra(res, &path, ParamMode::Explicit, None);
 545                         let kind = hir::ItemKind::Use(P(path), hir::UseKind::Single);
 546                         let vis = this.rebuild_vis(&vis);
 547
 548                         this.insert_item(
 549                             hir::Item {
 550                                 hir_id: new_id,
 551                                 ident,
 552                                 attrs: attrs.into_iter().cloned().collect(),
 553                                 kind,
 554                                 vis,
 555                                 span,
 556                             },
 557                         );
 558                     });
 559                 }
 560
 561                 let path = P(self.lower_path_extra(ret_res, &path, ParamMode::Explicit, None));
 562                 hir::ItemKind::Use(path, hir::UseKind::Single)
 563             }
 564             UseTreeKind::Glob => {
 565                 let path = P(self.lower_path(
 566                     id,
 567                     &Path {
 568                         segments,
 569                         span: path.span,
 570                     },
 571                     ParamMode::Explicit,
 572                 ));
 573                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 574             }
 575             UseTreeKind::Nested(ref trees) => {
 576                 // Nested imports are desugared into simple imports.
 577                 // So, if we start with
 578                 //
 579                 // ```
 580                 // pub(x) use foo::{a, b};
 581                 // ```
 582                 //
 583                 // we will create three items:
 584                 //
 585                 // ```
 586                 // pub(x) use foo::a;
 587                 // pub(x) use foo::b;
 588                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 589                 // ```
 590                 //
 591                 // The first two are produced by recursively invoking
 592                 // `lower_use_tree` (and indeed there may be things
 593                 // like `use foo::{a::{b, c}}` and so forth).  They
 594                 // wind up being directly added to
 595                 // `self.items`. However, the structure of this
 596                 // function also requires us to return one item, and
 597                 // for that we return the `{}` import (called the
 598                 // `ListStem`).
 599
 600                 let prefix = Path {
 601                     segments,
 602                     span: prefix.span.to(path.span),
 603                 };
 604
 605                 // Add all the nested `PathListItem`s to the HIR.
 606                 for &(ref use_tree, id) in trees {
 607                     let new_hir_id = self.lower_node_id(id);
 608
 609                     let mut prefix = prefix.clone();
 610
 611                     // Give the segments new node-ids since they are being cloned.
 612                     for seg in &mut prefix.segments {
 613                         seg.id = self.sess.next_node_id();
 614                     }
 615
 616                     // Each `use` import is an item and thus are owners of the
 617                     // names in the path. Up to this point the nested import is
 618                     // the current owner, since we want each desugared import to
 619                     // own its own names, we have to adjust the owner before
 620                     // lowering the rest of the import.
 621                     self.with_hir_id_owner(id, |this| {
 622                         let mut vis = this.rebuild_vis(&vis);
 623                         let mut ident = *ident;
 624
 625                         let kind = this.lower_use_tree(use_tree,
 626                                                        &prefix,
 627                                                        id,
 628                                                        &mut vis,
 629                                                        &mut ident,
 630                                                        attrs);
 631
 632                         this.insert_item(
 633                             hir::Item {
 634                                 hir_id: new_hir_id,
 635                                 ident,
 636                                 attrs: attrs.into_iter().cloned().collect(),
 637                                 kind,
 638                                 vis,
 639                                 span: use_tree.span,
 640                             },
 641                         );
 642                     });
 643                 }
 644
 645                 // Subtle and a bit hacky: we lower the privacy level
 646                 // of the list stem to "private" most of the time, but
 647                 // not for "restricted" paths. The key thing is that
 648                 // we don't want it to stay as `pub` (with no caveats)
 649                 // because that affects rustdoc and also the lints
 650                 // about `pub` items. But we can't *always* make it
 651                 // private -- particularly not for restricted paths --
 652                 // because it contains node-ids that would then be
 653                 // unused, failing the check that HirIds are "densely
 654                 // assigned".
 655                 match vis.node {
 656                     hir::VisibilityKind::Public |
 657                     hir::VisibilityKind::Crate(_) |
 658                     hir::VisibilityKind::Inherited => {
 659                         *vis = respan(prefix.span.shrink_to_lo(), hir::VisibilityKind::Inherited);
 660                     }
 661                     hir::VisibilityKind::Restricted { .. } => {
 662                         // Do nothing here, as described in the comment on the match.
 663                     }
 664                 }
 665
 666                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 667                 let res = self.lower_res(res);
 668                 let path = P(self.lower_path_extra(res, &prefix, ParamMode::Explicit, None));
 669                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 670             }
 671         }
 672     }
 673
 674     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 675     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 676     /// `NodeId`s. (See, e.g., #56128.)
 677     fn rebuild_use_path(&mut self, path: &hir::Path) -> hir::Path {
 678         debug!("rebuild_use_path(path = {:?})", path);
 679         let segments = path.segments.iter().map(|seg| hir::PathSegment {
 680             ident: seg.ident,
 681             hir_id: seg.hir_id.map(|_| self.next_id()),
 682             res: seg.res,
 683             args: None,
 684             infer_args: seg.infer_args,
 685         }).collect();
 686         hir::Path {
 687             span: path.span,
 688             res: path.res,
 689             segments,
 690         }
 691     }
 692
 693     fn rebuild_vis(&mut self, vis: &hir::Visibility) -> hir::Visibility {
 694         let vis_kind = match vis.node {
 695             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 696             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 697             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 698             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 699                 hir::VisibilityKind::Restricted {
 700                     path: P(self.rebuild_use_path(path)),
 701                     hir_id: self.next_id(),
 702                 }
 703             }
 704         };
 705         respan(vis.span, vis_kind)
 706     }
 707
 708     fn lower_foreign_item(&mut self, i: &ForeignItem) -> hir::ForeignItem {
 709         let def_id = self.resolver.definitions().local_def_id(i.id);
 710         hir::ForeignItem {
 711             hir_id: self.lower_node_id(i.id),
 712             ident: i.ident,
 713             attrs: self.lower_attrs(&i.attrs),
 714             kind: match i.kind {
 715                 ForeignItemKind::Fn(ref fdec, ref generics) => {
 716                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 717                         generics,
 718                         def_id,
 719                         AnonymousLifetimeMode::PassThrough,
 720                         |this, _| {
 721                             (
 722                                 // Disallow `impl Trait` in foreign items.
 723                                 this.lower_fn_decl(fdec, None, false, None),
 724                                 this.lower_fn_params_to_names(fdec),
 725                             )
 726                         },
 727                     );
 728
 729                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 730                 }
 731                 ForeignItemKind::Static(ref t, m) => {
 732                     hir::ForeignItemKind::Static(
 733                         self.lower_ty(t, ImplTraitContext::disallowed()), self.lower_mutability(m))
 734                 }
 735                 ForeignItemKind::Ty => hir::ForeignItemKind::Type,
 736                 ForeignItemKind::Macro(_) => panic!("macro shouldn't exist here"),
 737             },
 738             vis: self.lower_visibility(&i.vis, None),
 739             span: i.span,
 740         }
 741     }
 742
 743     fn lower_foreign_mod(&mut self, fm: &ForeignMod) -> hir::ForeignMod {
 744         hir::ForeignMod {
 745             abi: fm.abi,
 746             items: fm.items
 747                 .iter()
 748                 .map(|x| self.lower_foreign_item(x))
 749                 .collect(),
 750         }
 751     }
 752
 753     fn lower_global_asm(&mut self, ga: &GlobalAsm) -> P<hir::GlobalAsm> {
 754         P(hir::GlobalAsm { asm: ga.asm })
 755     }
 756
 757     fn lower_variant(&mut self, v: &Variant) -> hir::Variant {
 758         hir::Variant {
 759             attrs: self.lower_attrs(&v.attrs),
 760             data: self.lower_variant_data(&v.data),
 761             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 762             id: self.lower_node_id(v.id),
 763             ident: v.ident,
 764             span: v.span,
 765         }
 766     }
 767
 768     fn lower_variant_data(&mut self, vdata: &VariantData) -> hir::VariantData {
 769         match *vdata {
 770             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 771                 fields.iter().enumerate().map(|f| self.lower_struct_field(f)).collect(),
 772                 recovered,
 773             ),
 774             VariantData::Tuple(ref fields, id) => {
 775                 hir::VariantData::Tuple(
 776                     fields
 777                         .iter()
 778                         .enumerate()
 779                         .map(|f| self.lower_struct_field(f))
 780                         .collect(),
 781                     self.lower_node_id(id),
 782                 )
 783             },
 784             VariantData::Unit(id) => {
 785                 hir::VariantData::Unit(self.lower_node_id(id))
 786             },
 787         }
 788     }
 789
 790     fn lower_struct_field(&mut self, (index, f): (usize, &StructField)) -> hir::StructField {
 791         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 792             let t = self.lower_path_ty(
 793                 &f.ty,
 794                 qself,
 795                 path,
 796                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 797                 ImplTraitContext::disallowed()
 798             );
 799             P(t)
 800         } else {
 801             self.lower_ty(&f.ty, ImplTraitContext::disallowed())
 802         };
 803         hir::StructField {
 804             span: f.span,
 805             hir_id: self.lower_node_id(f.id),
 806             ident: match f.ident {
 807                 Some(ident) => ident,
 808                 // FIXME(jseyfried): positional field hygiene.
 809                 None => Ident::new(sym::integer(index), f.span),
 810             },
 811             vis: self.lower_visibility(&f.vis, None),
 812             ty,
 813             attrs: self.lower_attrs(&f.attrs),
 814         }
 815     }
 816
 817     fn lower_trait_item(&mut self, i: &TraitItem) -> hir::TraitItem {
 818         let trait_item_def_id = self.resolver.definitions().local_def_id(i.id);
 819
 820         let (generics, kind) = match i.kind {
 821             TraitItemKind::Const(ref ty, ref default) => (
 822                 self.lower_generics(&i.generics, ImplTraitContext::disallowed()),
 823                 hir::TraitItemKind::Const(
 824                     self.lower_ty(ty, ImplTraitContext::disallowed()),
 825                     default
 826                         .as_ref()
 827                         .map(|x| self.lower_const_body(x)),
 828                 ),
 829             ),
 830             TraitItemKind::Method(ref sig, None) => {
 831                 let names = self.lower_fn_params_to_names(&sig.decl);
 832                 let (generics, sig) = self.lower_method_sig(
 833                     &i.generics,
 834                     sig,
 835                     trait_item_def_id,
 836                     false,
 837                     None,
 838                 );
 839                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Required(names)))
 840             }
 841             TraitItemKind::Method(ref sig, Some(ref body)) => {
 842                 let body_id = self.lower_fn_body_block(&sig.decl, body);
 843                 let (generics, sig) = self.lower_method_sig(
 844                     &i.generics,
 845                     sig,
 846                     trait_item_def_id,
 847                     false,
 848                     None,
 849                 );
 850                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Provided(body_id)))
 851             }
 852             TraitItemKind::Type(ref bounds, ref default) => {
 853                 let generics = self.lower_generics(&i.generics, ImplTraitContext::disallowed());
 854                 let kind = hir::TraitItemKind::Type(
 855                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 856                     default
 857                         .as_ref()
 858                         .map(|x| self.lower_ty(x, ImplTraitContext::disallowed())),
 859                 );
 860
 861                 (generics, kind)
 862             },
 863             TraitItemKind::Macro(..) => bug!("macro item shouldn't exist at this point"),
 864         };
 865
 866         hir::TraitItem {
 867             hir_id: self.lower_node_id(i.id),
 868             ident: i.ident,
 869             attrs: self.lower_attrs(&i.attrs),
 870             generics,
 871             kind,
 872             span: i.span,
 873         }
 874     }
 875
 876     fn lower_trait_item_ref(&mut self, i: &TraitItem) -> hir::TraitItemRef {
 877         let (kind, has_default) = match i.kind {
 878             TraitItemKind::Const(_, ref default) => {
 879                 (hir::AssocItemKind::Const, default.is_some())
 880             }
 881             TraitItemKind::Type(_, ref default) => {
 882                 (hir::AssocItemKind::Type, default.is_some())
 883             }
 884             TraitItemKind::Method(ref sig, ref default) => (
 885                 hir::AssocItemKind::Method {
 886                     has_self: sig.decl.has_self(),
 887                 },
 888                 default.is_some(),
 889             ),
 890             TraitItemKind::Macro(..) => unimplemented!(),
 891         };
 892         hir::TraitItemRef {
 893             id: hir::TraitItemId { hir_id: self.lower_node_id(i.id) },
 894             ident: i.ident,
 895             span: i.span,
 896             defaultness: self.lower_defaultness(Defaultness::Default, has_default),
 897             kind,
 898         }
 899     }
 900
 901     fn lower_impl_item(&mut self, i: &ImplItem) -> hir::ImplItem {
 902         let impl_item_def_id = self.resolver.definitions().local_def_id(i.id);
 903
 904         let (generics, kind) = match i.kind {
 905             ImplItemKind::Const(ref ty, ref expr) => (
 906                 self.lower_generics(&i.generics, ImplTraitContext::disallowed()),
 907                 hir::ImplItemKind::Const(
 908                     self.lower_ty(ty, ImplTraitContext::disallowed()),
 909                     self.lower_const_body(expr),
 910                 ),
 911             ),
 912             ImplItemKind::Method(ref sig, ref body) => {
 913                 self.current_item = Some(i.span);
 914                 let body_id = self.lower_maybe_async_body(
 915                     &sig.decl, sig.header.asyncness.node, body
 916                 );
 917                 let impl_trait_return_allow = !self.is_in_trait_impl;
 918                 let (generics, sig) = self.lower_method_sig(
 919                     &i.generics,
 920                     sig,
 921                     impl_item_def_id,
 922                     impl_trait_return_allow,
 923                     sig.header.asyncness.node.opt_return_id(),
 924                 );
 925
 926                 (generics, hir::ImplItemKind::Method(sig, body_id))
 927             }
 928             ImplItemKind::TyAlias(ref ty) => (
 929                 self.lower_generics(&i.generics, ImplTraitContext::disallowed()),
 930                 hir::ImplItemKind::TyAlias(self.lower_ty(ty, ImplTraitContext::disallowed())),
 931             ),
 932             ImplItemKind::OpaqueTy(ref bounds) => (
 933                 self.lower_generics(&i.generics, ImplTraitContext::disallowed()),
 934                 hir::ImplItemKind::OpaqueTy(
 935                     self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
 936                 ),
 937             ),
 938             ImplItemKind::Macro(..) => bug!("`TyMac` should have been expanded by now"),
 939         };
 940
 941         hir::ImplItem {
 942             hir_id: self.lower_node_id(i.id),
 943             ident: i.ident,
 944             attrs: self.lower_attrs(&i.attrs),
 945             generics,
 946             vis: self.lower_visibility(&i.vis, None),
 947             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
 948             kind,
 949             span: i.span,
 950         }
 951
 952         // [1] since `default impl` is not yet implemented, this is always true in impls
 953     }
 954
 955     fn lower_impl_item_ref(&mut self, i: &ImplItem) -> hir::ImplItemRef {
 956         hir::ImplItemRef {
 957             id: hir::ImplItemId { hir_id: self.lower_node_id(i.id) },
 958             ident: i.ident,
 959             span: i.span,
 960             vis: self.lower_visibility(&i.vis, Some(i.id)),
 961             defaultness: self.lower_defaultness(i.defaultness, true /* [1] */),
 962             kind: match i.kind {
 963                 ImplItemKind::Const(..) => hir::AssocItemKind::Const,
 964                 ImplItemKind::TyAlias(..) => hir::AssocItemKind::Type,
 965                 ImplItemKind::OpaqueTy(..) => hir::AssocItemKind::OpaqueTy,
 966                 ImplItemKind::Method(ref sig, _) => hir::AssocItemKind::Method {
 967                     has_self: sig.decl.has_self(),
 968                 },
 969                 ImplItemKind::Macro(..) => unimplemented!(),
 970             },
 971         }
 972
 973         // [1] since `default impl` is not yet implemented, this is always true in impls
 974     }
 975
 976     /// If an `explicit_owner` is given, this method allocates the `HirId` in
 977     /// the address space of that item instead of the item currently being
 978     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
 979     /// lower a `Visibility` value although we haven't lowered the owning
 980     /// `ImplItem` in question yet.
 981     fn lower_visibility(
 982         &mut self,
 983         v: &Visibility,
 984         explicit_owner: Option<NodeId>,
 985     ) -> hir::Visibility {
 986         let node = match v.node {
 987             VisibilityKind::Public => hir::VisibilityKind::Public,
 988             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 989             VisibilityKind::Restricted { ref path, id } => {
 990                 debug!("lower_visibility: restricted path id = {:?}", id);
 991                 let lowered_id = if let Some(owner) = explicit_owner {
 992                     self.lower_node_id_with_owner(id, owner)
 993                 } else {
 994                     self.lower_node_id(id)
 995                 };
 996                 let res = self.expect_full_res(id);
 997                 let res = self.lower_res(res);
 998                 hir::VisibilityKind::Restricted {
 999                     path: P(self.lower_path_extra(
1000                         res,
1001                         path,
1002                         ParamMode::Explicit,
1003                         explicit_owner,
1004                     )),
1005                     hir_id: lowered_id,
1006                 }
1007             },
1008             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
1009         };
1010         respan(v.span, node)
1011     }
1012
1013     fn lower_defaultness(&self, d: Defaultness, has_value: bool) -> hir::Defaultness {
1014         match d {
1015             Defaultness::Default => hir::Defaultness::Default {
1016                 has_value: has_value,
1017             },
1018             Defaultness::Final => {
1019                 assert!(has_value);
1020                 hir::Defaultness::Final
1021             }
1022         }
1023     }
1024
1025     fn lower_impl_polarity(&mut self, i: ImplPolarity) -> hir::ImplPolarity {
1026         match i {
1027             ImplPolarity::Positive => hir::ImplPolarity::Positive,
1028             ImplPolarity::Negative => hir::ImplPolarity::Negative,
1029         }
1030     }
1031
1032     fn record_body(&mut self, params: HirVec<hir::Param>, value: hir::Expr) -> hir::BodyId {
1033         let body = hir::Body {
1034             generator_kind: self.generator_kind,
1035             params,
1036             value,
1037         };
1038         let id = body.id();
1039         self.bodies.insert(id, body);
1040         id
1041     }
1042
1043     fn lower_body(
1044         &mut self,
1045         f: impl FnOnce(&mut LoweringContext<'_>) -> (HirVec<hir::Param>, hir::Expr),
1046     ) -> hir::BodyId {
1047         let prev_gen_kind = self.generator_kind.take();
1048         let (parameters, result) = f(self);
1049         let body_id = self.record_body(parameters, result);
1050         self.generator_kind = prev_gen_kind;
1051         body_id
1052     }
1053
1054     fn lower_param(&mut self, param: &Param) -> hir::Param {
1055         hir::Param {
1056             attrs: self.lower_attrs(&param.attrs),
1057             hir_id: self.lower_node_id(param.id),
1058             pat: self.lower_pat(&param.pat),
1059             span: param.span,
1060         }
1061     }
1062
1063     pub(super) fn lower_fn_body(
1064         &mut self,
1065         decl: &FnDecl,
1066         body: impl FnOnce(&mut LoweringContext<'_>) -> hir::Expr,
1067     ) -> hir::BodyId {
1068         self.lower_body(|this| (
1069             decl.inputs.iter().map(|x| this.lower_param(x)).collect(),
1070             body(this),
1071         ))
1072     }
1073
1074     fn lower_fn_body_block(&mut self, decl: &FnDecl, body: &Block) -> hir::BodyId {
1075         self.lower_fn_body(decl, |this| this.lower_block_expr(body))
1076     }
1077
1078     pub(super) fn lower_const_body(&mut self, expr: &Expr) -> hir::BodyId {
1079         self.lower_body(|this| (hir_vec![], this.lower_expr(expr)))
1080     }
1081
1082     fn lower_maybe_async_body(
1083         &mut self,
1084         decl: &FnDecl,
1085         asyncness: IsAsync,
1086         body: &Block,
1087     ) -> hir::BodyId {
1088         let closure_id = match asyncness {
1089             IsAsync::Async { closure_id, .. } => closure_id,
1090             IsAsync::NotAsync => return self.lower_fn_body_block(decl, body),
1091         };
1092
1093         self.lower_body(|this| {
1094             let mut parameters: Vec<hir::Param> = Vec::new();
1095             let mut statements: Vec<hir::Stmt> = Vec::new();
1096
1097             // Async function parameters are lowered into the closure body so that they are
1098             // captured and so that the drop order matches the equivalent non-async functions.
1099             //
1100             // from:
1101             //
1102             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1103             //         <body>
1104             //     }
1105             //
1106             // into:
1107             //
1108             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1109             //       async move {
1110             //         let __arg2 = __arg2;
1111             //         let <pattern> = __arg2;
1112             //         let __arg1 = __arg1;
1113             //         let <pattern> = __arg1;
1114             //         let __arg0 = __arg0;
1115             //         let <pattern> = __arg0;
1116             //         drop-temps { <body> } // see comments later in fn for details
1117             //       }
1118             //     }
1119             //
1120             // If `<pattern>` is a simple ident, then it is lowered to a single
1121             // `let <pattern> = <pattern>;` statement as an optimization.
1122             //
1123             // Note that the body is embedded in `drop-temps`; an
1124             // equivalent desugaring would be `return { <body>
1125             // };`. The key point is that we wish to drop all the
1126             // let-bound variables and temporaries created in the body
1127             // (and its tail expression!) before we drop the
1128             // parameters (c.f. rust-lang/rust#64512).
1129             for (index, parameter) in decl.inputs.iter().enumerate() {
1130                 let parameter = this.lower_param(parameter);
1131                 let span = parameter.pat.span;
1132
1133                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1134                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1135                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1136                     hir::PatKind::Binding(hir::BindingAnnotation::Unannotated, _, ident, _) =>
1137                         (ident, true),
1138                     _ => {
1139                         // Replace the ident for bindings that aren't simple.
1140                         let name = format!("__arg{}", index);
1141                         let ident = Ident::from_str(&name);
1142
1143                         (ident, false)
1144                     },
1145                 };
1146
1147                 let desugared_span =
1148                     this.mark_span_with_reason(DesugaringKind::Async, span, None);
1149
1150                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1151                 // async function.
1152                 //
1153                 // If this is the simple case, this parameter will end up being the same as the
1154                 // original parameter, but with a different pattern id.
1155                 let mut stmt_attrs = ThinVec::new();
1156                 stmt_attrs.extend(parameter.attrs.iter().cloned());
1157                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1158                 let new_parameter = hir::Param {
1159                     attrs: parameter.attrs,
1160                     hir_id: parameter.hir_id,
1161                     pat: new_parameter_pat,
1162                     span: parameter.span,
1163                 };
1164
1165
1166                 if is_simple_parameter {
1167                     // If this is the simple case, then we only insert one statement that is
1168                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1169                     // `HirId`s are densely assigned.
1170                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1171                     let stmt = this.stmt_let_pat(
1172                         stmt_attrs,
1173                         desugared_span,
1174                         Some(P(expr)),
1175                         parameter.pat,
1176                         hir::LocalSource::AsyncFn
1177                     );
1178                     statements.push(stmt);
1179                 } else {
1180                     // If this is not the simple case, then we construct two statements:
1181                     //
1182                     // ```
1183                     // let __argN = __argN;
1184                     // let <pat> = __argN;
1185                     // ```
1186                     //
1187                     // The first statement moves the parameter into the closure and thus ensures
1188                     // that the drop order is correct.
1189                     //
1190                     // The second statement creates the bindings that the user wrote.
1191
1192                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1193                     // because the user may have specified a `ref mut` binding in the next
1194                     // statement.
1195                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1196                         desugared_span, ident, hir::BindingAnnotation::Mutable);
1197                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1198                     let move_stmt = this.stmt_let_pat(
1199                         ThinVec::new(),
1200                         desugared_span,
1201                         Some(P(move_expr)),
1202                         move_pat,
1203                         hir::LocalSource::AsyncFn
1204                     );
1205
1206                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1207                     // parameter's pattern so that `HirId`s are densely assigned.
1208                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1209                     let pattern_stmt = this.stmt_let_pat(
1210                         stmt_attrs,
1211                         desugared_span,
1212                         Some(P(pattern_expr)),
1213                         parameter.pat,
1214                         hir::LocalSource::AsyncFn
1215                     );
1216
1217                     statements.push(move_stmt);
1218                     statements.push(pattern_stmt);
1219                 };
1220
1221                 parameters.push(new_parameter);
1222             }
1223
1224             let async_expr = this.make_async_expr(
1225                 CaptureBy::Value,
1226                 closure_id,
1227                 None,
1228                 body.span,
1229                 hir::AsyncGeneratorKind::Fn,
1230                 |this| {
1231                     // Create a block from the user's function body:
1232                     let user_body = this.lower_block_expr(body);
1233
1234                     // Transform into `drop-temps { <user-body> }`, an expression:
1235                     let desugared_span = this.mark_span_with_reason(
1236                         DesugaringKind::Async,
1237                         user_body.span,
1238                         None,
1239                     );
1240                     let user_body = this.expr_drop_temps(
1241                         desugared_span,
1242                         P(user_body),
1243                         ThinVec::new(),
1244                     );
1245
1246                     // As noted above, create the final block like
1247                     //
1248                     // ```
1249                     // {
1250                     //   let $param_pattern = $raw_param;
1251                     //   ...
1252                     //   drop-temps { <user-body> }
1253                     // }
1254                     // ```
1255                     let body = this.block_all(
1256                         desugared_span,
1257                         statements.into(),
1258                         Some(P(user_body)),
1259                     );
1260                     this.expr_block(P(body), ThinVec::new())
1261                 });
1262             (HirVec::from(parameters), this.expr(body.span, async_expr, ThinVec::new()))
1263         })
1264     }
1265
1266     fn lower_method_sig(
1267         &mut self,
1268         generics: &Generics,
1269         sig: &MethodSig,
1270         fn_def_id: DefId,
1271         impl_trait_return_allow: bool,
1272         is_async: Option<NodeId>,
1273     ) -> (hir::Generics, hir::MethodSig) {
1274         let header = self.lower_fn_header(sig.header);
1275         let (generics, decl) = self.add_in_band_defs(
1276             generics,
1277             fn_def_id,
1278             AnonymousLifetimeMode::PassThrough,
1279             |this, idty| this.lower_fn_decl(
1280                 &sig.decl,
1281                 Some((fn_def_id, idty)),
1282                 impl_trait_return_allow,
1283                 is_async,
1284             ),
1285         );
1286         (generics, hir::MethodSig { header, decl })
1287     }
1288
1289     fn lower_is_auto(&mut self, a: IsAuto) -> hir::IsAuto {
1290         match a {
1291             IsAuto::Yes => hir::IsAuto::Yes,
1292             IsAuto::No => hir::IsAuto::No,
1293         }
1294     }
1295
1296     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1297         hir::FnHeader {
1298             unsafety: self.lower_unsafety(h.unsafety),
1299             asyncness: self.lower_asyncness(h.asyncness.node),
1300             constness: self.lower_constness(h.constness),
1301             abi: h.abi,
1302         }
1303     }
1304
1305     pub(super) fn lower_unsafety(&mut self, u: Unsafety) -> hir::Unsafety {
1306         match u {
1307             Unsafety::Unsafe => hir::Unsafety::Unsafe,
1308             Unsafety::Normal => hir::Unsafety::Normal,
1309         }
1310     }
1311
1312     fn lower_constness(&mut self, c: Spanned<Constness>) -> hir::Constness {
1313         match c.node {
1314             Constness::Const => hir::Constness::Const,
1315             Constness::NotConst => hir::Constness::NotConst,
1316         }
1317     }
1318
1319     fn lower_asyncness(&mut self, a: IsAsync) -> hir::IsAsync {
1320         match a {
1321             IsAsync::Async { .. } => hir::IsAsync::Async,
1322             IsAsync::NotAsync => hir::IsAsync::NotAsync,
1323         }
1324     }
1325
1326     pub(super) fn lower_generics(
1327         &mut self,
1328         generics: &Generics,
1329         itctx: ImplTraitContext<'_>)
1330         -> hir::Generics
1331     {
1332         // Collect `?Trait` bounds in where clause and move them to parameter definitions.
1333         // FIXME: this could probably be done with less rightward drift. It also looks like two
1334         // control paths where `report_error` is called are the only paths that advance to after the
1335         // match statement, so the error reporting could probably just be moved there.
1336         let mut add_bounds: NodeMap<Vec<_>> = Default::default();
1337         for pred in &generics.where_clause.predicates {
1338             if let WherePredicate::BoundPredicate(ref bound_pred) = *pred {
1339                 'next_bound: for bound in &bound_pred.bounds {
1340                     if let GenericBound::Trait(_, TraitBoundModifier::Maybe) = *bound {
1341                         let report_error = |this: &mut Self| {
1342                             this.diagnostic().span_err(
1343                                 bound_pred.bounded_ty.span,
1344                                 "`?Trait` bounds are only permitted at the \
1345                                  point where a type parameter is declared",
1346                             );
1347                         };
1348                         // Check if the where clause type is a plain type parameter.
1349                         match bound_pred.bounded_ty.kind {
1350                             TyKind::Path(None, ref path)
1351                                 if path.segments.len() == 1
1352                                     && bound_pred.bound_generic_params.is_empty() =>
1353                             {
1354                                 if let Some(Res::Def(DefKind::TyParam, def_id)) = self.resolver
1355                                     .get_partial_res(bound_pred.bounded_ty.id)
1356                                     .map(|d| d.base_res())
1357                                 {
1358                                     if let Some(node_id) =
1359                                         self.resolver.definitions().as_local_node_id(def_id)
1360                                     {
1361                                         for param in &generics.params {
1362                                             match param.kind {
1363                                                 GenericParamKind::Type { .. } => {
1364                                                     if node_id == param.id {
1365                                                         add_bounds.entry(param.id)
1366                                                             .or_default()
1367                                                             .push(bound.clone());
1368                                                         continue 'next_bound;
1369                                                     }
1370                                                 }
1371                                                 _ => {}
1372                                             }
1373                                         }
1374                                     }
1375                                 }
1376                                 report_error(self)
1377                             }
1378                             _ => report_error(self),
1379                         }
1380                     }
1381                 }
1382             }
1383         }
1384
1385         hir::Generics {
1386             params: self.lower_generic_params(&generics.params, &add_bounds, itctx),
1387             where_clause: self.lower_where_clause(&generics.where_clause),
1388             span: generics.span,
1389         }
1390     }
1391
1392     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause {
1393         self.with_anonymous_lifetime_mode(
1394             AnonymousLifetimeMode::ReportError,
1395             |this| {
1396                 hir::WhereClause {
1397                     predicates: wc.predicates
1398                         .iter()
1399                         .map(|predicate| this.lower_where_predicate(predicate))
1400                         .collect(),
1401                     span: wc.span,
1402                 }
1403             },
1404         )
1405     }
1406
1407     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate {
1408         match *pred {
1409             WherePredicate::BoundPredicate(WhereBoundPredicate {
1410                 ref bound_generic_params,
1411                 ref bounded_ty,
1412                 ref bounds,
1413                 span,
1414             }) => {
1415                 self.with_in_scope_lifetime_defs(
1416                     &bound_generic_params,
1417                     |this| {
1418                         hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1419                             bound_generic_params: this.lower_generic_params(
1420                                 bound_generic_params,
1421                                 &NodeMap::default(),
1422                                 ImplTraitContext::disallowed(),
1423                             ),
1424                             bounded_ty: this.lower_ty(bounded_ty, ImplTraitContext::disallowed()),
1425                             bounds: bounds
1426                                 .iter()
1427                                 .filter_map(|bound| match *bound {
1428                                     // Ignore `?Trait` bounds.
1429                                     // They were copied into type parameters already.
1430                                     GenericBound::Trait(_, TraitBoundModifier::Maybe) => None,
1431                                     _ => Some(this.lower_param_bound(
1432                                         bound,
1433                                         ImplTraitContext::disallowed(),
1434                                     )),
1435                                 })
1436                                 .collect(),
1437                             span,
1438                         })
1439                     },
1440                 )
1441             }
1442             WherePredicate::RegionPredicate(WhereRegionPredicate {
1443                 ref lifetime,
1444                 ref bounds,
1445                 span,
1446             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1447                 span,
1448                 lifetime: self.lower_lifetime(lifetime),
1449                 bounds: self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
1450             }),
1451             WherePredicate::EqPredicate(WhereEqPredicate {
1452                 id,
1453                 ref lhs_ty,
1454                 ref rhs_ty,
1455                 span,
1456             }) => {
1457                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1458                     hir_id: self.lower_node_id(id),
1459                     lhs_ty: self.lower_ty(lhs_ty, ImplTraitContext::disallowed()),
1460                     rhs_ty: self.lower_ty(rhs_ty, ImplTraitContext::disallowed()),
1461                     span,
1462                 })
1463             },
1464         }
1465     }
1466 }