compiler/rustc_ast_lowering/src/item.rs

   1 use super::{AnonymousLifetimeMode, LoweringContext, ParamMode};
   2 use super::{ImplTraitContext, ImplTraitPosition};
   3 use crate::{Arena, FnDeclKind};
   4
   5 use rustc_ast::ptr::P;
   6 use rustc_ast::visit::{self, AssocCtxt, FnCtxt, FnKind, Visitor};
   7 use rustc_ast::*;
   8 use rustc_data_structures::fx::FxHashSet;
   9 use rustc_errors::struct_span_err;
  10 use rustc_hir as hir;
  11 use rustc_hir::def::{DefKind, Res};
  12 use rustc_hir::def_id::LocalDefId;
  13 use rustc_index::vec::Idx;
  14 use rustc_span::source_map::{respan, DesugaringKind};
  15 use rustc_span::symbol::{kw, sym, Ident};
  16 use rustc_span::Span;
  17 use rustc_target::spec::abi;
  18 use smallvec::{smallvec, SmallVec};
  19 use tracing::debug;
  20
  21 use std::iter;
  22 use std::mem;
  23
  24 pub(super) struct ItemLowerer<'a, 'lowering, 'hir> {
  25     pub(super) lctx: &'a mut LoweringContext<'lowering, 'hir>,
  26 }
  27
  28 /// When we have a ty alias we *may* have two where clauses. To give the best diagnostics, we set the span
  29 /// to the where clause that is prefered, if it exists. Otherwise, it sets the span to the other where
  30 /// clause if it exists.
  31 fn add_ty_alias_where_clause(
  32     generics: &mut ast::Generics,
  33     mut where_clauses: (TyAliasWhereClause, TyAliasWhereClause),
  34     prefer_first: bool,
  35 ) {
  36     if !prefer_first {
  37         where_clauses = (where_clauses.1, where_clauses.0);
  38     }
  39     if where_clauses.0.0 || !where_clauses.1.0 {
  40         generics.where_clause.has_where_token = where_clauses.0.0;
  41         generics.where_clause.span = where_clauses.0.1;
  42     } else {
  43         generics.where_clause.has_where_token = where_clauses.1.0;
  44         generics.where_clause.span = where_clauses.1.1;
  45     }
  46 }
  47
  48 impl ItemLowerer<'_, '_, '_> {
  49     fn with_trait_impl_ref<T>(
  50         &mut self,
  51         impl_ref: &Option<TraitRef>,
  52         f: impl FnOnce(&mut Self) -> T,
  53     ) -> T {
  54         let old = self.lctx.is_in_trait_impl;
  55         self.lctx.is_in_trait_impl = impl_ref.is_some();
  56         let ret = f(self);
  57         self.lctx.is_in_trait_impl = old;
  58         ret
  59     }
  60 }
  61
  62 impl<'a> Visitor<'a> for ItemLowerer<'a, '_, '_> {
  63     fn visit_attribute(&mut self, _: &'a Attribute) {
  64         // We do not want to lower expressions that appear in attributes,
  65         // as they are not accessible to the rest of the HIR.
  66     }
  67
  68     fn visit_item(&mut self, item: &'a Item) {
  69         let hir_id = self.lctx.with_hir_id_owner(item.id, |lctx| {
  70             let node = lctx.without_in_scope_lifetime_defs(|lctx| lctx.lower_item(item));
  71             hir::OwnerNode::Item(node)
  72         });
  73
  74         self.lctx.with_parent_item_lifetime_defs(hir_id, |this| {
  75             let this = &mut ItemLowerer { lctx: this };
  76             match item.kind {
  77                 ItemKind::Impl(box Impl { ref of_trait, .. }) => {
  78                     this.with_trait_impl_ref(of_trait, |this| visit::walk_item(this, item));
  79                 }
  80                 _ => visit::walk_item(this, item),
  81             }
  82         });
  83     }
  84
  85     fn visit_fn(&mut self, fk: FnKind<'a>, sp: Span, _: NodeId) {
  86         match fk {
  87             FnKind::Fn(FnCtxt::Foreign, _, sig, _, _) => {
  88                 self.visit_fn_header(&sig.header);
  89                 visit::walk_fn_decl(self, &sig.decl);
  90                 // Don't visit the foreign function body even if it has one, since lowering the
  91                 // body would have no meaning and will have already been caught as a parse error.
  92             }
  93             _ => visit::walk_fn(self, fk, sp),
  94         }
  95     }
  96
  97     fn visit_assoc_item(&mut self, item: &'a AssocItem, ctxt: AssocCtxt) {
  98         self.lctx.with_hir_id_owner(item.id, |lctx| match ctxt {
  99             AssocCtxt::Trait => hir::OwnerNode::TraitItem(lctx.lower_trait_item(item)),
 100             AssocCtxt::Impl => hir::OwnerNode::ImplItem(lctx.lower_impl_item(item)),
 101         });
 102
 103         visit::walk_assoc_item(self, item, ctxt);
 104     }
 105
 106     fn visit_foreign_item(&mut self, item: &'a ForeignItem) {
 107         self.lctx.with_hir_id_owner(item.id, |lctx| {
 108             hir::OwnerNode::ForeignItem(lctx.lower_foreign_item(item))
 109         });
 110
 111         visit::walk_foreign_item(self, item);
 112     }
 113 }
 114
 115 impl<'hir> LoweringContext<'_, 'hir> {
 116     // Same as the method above, but accepts `hir::GenericParam`s
 117     // instead of `ast::GenericParam`s.
 118     // This should only be used with generics that have already had their
 119     // in-band lifetimes added. In practice, this means that this function is
 120     // only used when lowering a child item of a trait or impl.
 121     fn with_parent_item_lifetime_defs<T>(
 122         &mut self,
 123         parent_hir_id: LocalDefId,
 124         f: impl FnOnce(&mut Self) -> T,
 125     ) -> T {
 126         let old_len = self.in_scope_lifetimes.len();
 127
 128         let parent_generics = match self.owners[parent_hir_id].unwrap().node().expect_item().kind {
 129             hir::ItemKind::Impl(hir::Impl { ref generics, .. })
 130             | hir::ItemKind::Trait(_, _, ref generics, ..) => generics.params,
 131             _ => &[],
 132         };
 133         let lt_def_names = parent_generics.iter().filter_map(|param| match param.kind {
 134             hir::GenericParamKind::Lifetime { .. } => Some(param.name.normalize_to_macros_2_0()),
 135             _ => None,
 136         });
 137         self.in_scope_lifetimes.extend(lt_def_names);
 138
 139         let res = f(self);
 140
 141         self.in_scope_lifetimes.truncate(old_len);
 142         res
 143     }
 144
 145     // Clears (and restores) the `in_scope_lifetimes` field. Used when
 146     // visiting nested items, which never inherit in-scope lifetimes
 147     // from their surrounding environment.
 148     fn without_in_scope_lifetime_defs<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
 149         let old_in_scope_lifetimes = mem::replace(&mut self.in_scope_lifetimes, vec![]);
 150
 151         // this vector is only used when walking over impl headers,
 152         // input types, and the like, and should not be non-empty in
 153         // between items
 154         assert!(self.lifetimes_to_define.is_empty());
 155
 156         let res = f(self);
 157
 158         assert!(self.in_scope_lifetimes.is_empty());
 159         self.in_scope_lifetimes = old_in_scope_lifetimes;
 160
 161         res
 162     }
 163
 164     pub(super) fn lower_mod(&mut self, items: &[P<Item>], inner: Span) -> hir::Mod<'hir> {
 165         hir::Mod {
 166             inner: self.lower_span(inner),
 167             item_ids: self.arena.alloc_from_iter(items.iter().flat_map(|x| self.lower_item_ref(x))),
 168         }
 169     }
 170
 171     pub(super) fn lower_item_ref(&mut self, i: &Item) -> SmallVec<[hir::ItemId; 1]> {
 172         let mut node_ids = smallvec![hir::ItemId { def_id: self.resolver.local_def_id(i.id) }];
 173         if let ItemKind::Use(ref use_tree) = &i.kind {
 174             self.lower_item_id_use_tree(use_tree, i.id, &mut node_ids);
 175         }
 176         node_ids
 177     }
 178
 179     fn lower_item_id_use_tree(
 180         &mut self,
 181         tree: &UseTree,
 182         base_id: NodeId,
 183         vec: &mut SmallVec<[hir::ItemId; 1]>,
 184     ) {
 185         match tree.kind {
 186             UseTreeKind::Nested(ref nested_vec) => {
 187                 for &(ref nested, id) in nested_vec {
 188                     vec.push(hir::ItemId { def_id: self.resolver.local_def_id(id) });
 189                     self.lower_item_id_use_tree(nested, id, vec);
 190                 }
 191             }
 192             UseTreeKind::Glob => {}
 193             UseTreeKind::Simple(_, id1, id2) => {
 194                 for (_, &id) in
 195                     iter::zip(self.expect_full_res_from_use(base_id).skip(1), &[id1, id2])
 196                 {
 197                     vec.push(hir::ItemId { def_id: self.resolver.local_def_id(id) });
 198                 }
 199             }
 200         }
 201     }
 202
 203     fn lower_item(&mut self, i: &Item) -> &'hir hir::Item<'hir> {
 204         let mut ident = i.ident;
 205         let mut vis = self.lower_visibility(&i.vis);
 206         let hir_id = self.lower_node_id(i.id);
 207         let attrs = self.lower_attrs(hir_id, &i.attrs);
 208         let kind = self.lower_item_kind(i.span, i.id, hir_id, &mut ident, attrs, &mut vis, &i.kind);
 209         let item = hir::Item {
 210             def_id: hir_id.expect_owner(),
 211             ident: self.lower_ident(ident),
 212             kind,
 213             vis,
 214             span: self.lower_span(i.span),
 215         };
 216         self.arena.alloc(item)
 217     }
 218
 219     fn lower_item_kind(
 220         &mut self,
 221         span: Span,
 222         id: NodeId,
 223         hir_id: hir::HirId,
 224         ident: &mut Ident,
 225         attrs: Option<&'hir [Attribute]>,
 226         vis: &mut hir::Visibility<'hir>,
 227         i: &ItemKind,
 228     ) -> hir::ItemKind<'hir> {
 229         match *i {
 230             ItemKind::ExternCrate(orig_name) => hir::ItemKind::ExternCrate(orig_name),
 231             ItemKind::Use(ref use_tree) => {
 232                 // Start with an empty prefix.
 233                 let prefix = Path { segments: vec![], span: use_tree.span, tokens: None };
 234
 235                 self.lower_use_tree(use_tree, &prefix, id, vis, ident, attrs)
 236             }
 237             ItemKind::Static(ref t, m, ref e) => {
 238                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 239                 hir::ItemKind::Static(ty, m, body_id)
 240             }
 241             ItemKind::Const(_, ref t, ref e) => {
 242                 let (ty, body_id) = self.lower_const_item(t, span, e.as_deref());
 243                 hir::ItemKind::Const(ty, body_id)
 244             }
 245             ItemKind::Fn(box Fn {
 246                 sig: FnSig { ref decl, header, span: fn_sig_span },
 247                 ref generics,
 248                 ref body,
 249                 ..
 250             }) => {
 251                 let fn_def_id = self.resolver.local_def_id(id);
 252                 self.with_new_scopes(|this| {
 253                     this.current_item = Some(ident.span);
 254
 255                     // Note: we don't need to change the return type from `T` to
 256                     // `impl Future<Output = T>` here because lower_body
 257                     // only cares about the input argument patterns in the function
 258                     // declaration (decl), not the return types.
 259                     let asyncness = header.asyncness;
 260                     let body_id =
 261                         this.lower_maybe_async_body(span, &decl, asyncness, body.as_deref());
 262
 263                     let (generics, decl) = this.add_in_band_defs(
 264                         generics,
 265                         fn_def_id,
 266                         AnonymousLifetimeMode::PassThrough,
 267                         |this, idty| {
 268                             let ret_id = asyncness.opt_return_id();
 269                             this.lower_fn_decl(
 270                                 &decl,
 271                                 Some((fn_def_id, idty)),
 272                                 FnDeclKind::Fn,
 273                                 ret_id,
 274                             )
 275                         },
 276                     );
 277                     let sig = hir::FnSig {
 278                         decl,
 279                         header: this.lower_fn_header(header),
 280                         span: this.lower_span(fn_sig_span),
 281                     };
 282                     hir::ItemKind::Fn(sig, generics, body_id)
 283                 })
 284             }
 285             ItemKind::Mod(_, ref mod_kind) => match mod_kind {
 286                 ModKind::Loaded(items, _, inner_span) => {
 287                     hir::ItemKind::Mod(self.lower_mod(items, *inner_span))
 288                 }
 289                 ModKind::Unloaded => panic!("`mod` items should have been loaded by now"),
 290             },
 291             ItemKind::ForeignMod(ref fm) => hir::ItemKind::ForeignMod {
 292                 abi: fm.abi.map_or(abi::Abi::FALLBACK, |abi| self.lower_abi(abi)),
 293                 items: self
 294                     .arena
 295                     .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
 296             },
 297             ItemKind::GlobalAsm(ref asm) => {
 298                 hir::ItemKind::GlobalAsm(self.lower_inline_asm(span, asm))
 299             }
 300             ItemKind::TyAlias(box TyAlias {
 301                 ref generics,
 302                 where_clauses,
 303                 ty: Some(ref ty),
 304                 ..
 305             }) => {
 306                 // We lower
 307                 //
 308                 // type Foo = impl Trait
 309                 //
 310                 // to
 311                 //
 312                 // type Foo = Foo1
 313                 // opaque type Foo1: Trait
 314                 let ty = self.lower_ty(
 315                     ty,
 316                     ImplTraitContext::TypeAliasesOpaqueTy {
 317                         capturable_lifetimes: &mut FxHashSet::default(),
 318                     },
 319                 );
 320                 let mut generics = generics.clone();
 321                 add_ty_alias_where_clause(&mut generics, where_clauses, true);
 322                 let generics = self.lower_generics(
 323                     &generics,
 324                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 325                 );
 326                 hir::ItemKind::TyAlias(ty, generics)
 327             }
 328             ItemKind::TyAlias(box TyAlias {
 329                 ref generics, ref where_clauses, ty: None, ..
 330             }) => {
 331                 let ty = self.arena.alloc(self.ty(span, hir::TyKind::Err));
 332                 let mut generics = generics.clone();
 333                 add_ty_alias_where_clause(&mut generics, *where_clauses, true);
 334                 let generics = self.lower_generics(
 335                     &generics,
 336                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 337                 );
 338                 hir::ItemKind::TyAlias(ty, generics)
 339             }
 340             ItemKind::Enum(ref enum_definition, ref generics) => hir::ItemKind::Enum(
 341                 hir::EnumDef {
 342                     variants: self.arena.alloc_from_iter(
 343                         enum_definition.variants.iter().map(|x| self.lower_variant(x)),
 344                     ),
 345                 },
 346                 self.lower_generics(
 347                     generics,
 348                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 349                 ),
 350             ),
 351             ItemKind::Struct(ref struct_def, ref generics) => {
 352                 let struct_def = self.lower_variant_data(hir_id, struct_def);
 353                 hir::ItemKind::Struct(
 354                     struct_def,
 355                     self.lower_generics(
 356                         generics,
 357                         ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 358                     ),
 359                 )
 360             }
 361             ItemKind::Union(ref vdata, ref generics) => {
 362                 let vdata = self.lower_variant_data(hir_id, vdata);
 363                 hir::ItemKind::Union(
 364                     vdata,
 365                     self.lower_generics(
 366                         generics,
 367                         ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 368                     ),
 369                 )
 370             }
 371             ItemKind::Impl(box Impl {
 372                 unsafety,
 373                 polarity,
 374                 defaultness,
 375                 constness,
 376                 generics: ref ast_generics,
 377                 of_trait: ref trait_ref,
 378                 self_ty: ref ty,
 379                 items: ref impl_items,
 380             }) => {
 381                 // Lower the "impl header" first. This ordering is important
 382                 // for in-band lifetimes! Consider `'a` here:
 383                 //
 384                 //     impl Foo<'a> for u32 {
 385                 //         fn method(&'a self) { .. }
 386                 //     }
 387                 //
 388                 // Because we start by lowering the `Foo<'a> for u32`
 389                 // part, we will add `'a` to the list of generics on
 390                 // the impl. When we then encounter it later in the
 391                 // method, it will not be considered an in-band
 392                 // lifetime to be added, but rather a reference to a
 393                 // parent lifetime.
 394                 let lowered_trait_def_id = hir_id.expect_owner();
 395                 let (generics, (trait_ref, lowered_ty)) = self.add_in_band_defs(
 396                     ast_generics,
 397                     lowered_trait_def_id,
 398                     AnonymousLifetimeMode::CreateParameter,
 399                     |this, _| {
 400                         let trait_ref = trait_ref.as_ref().map(|trait_ref| {
 401                             this.lower_trait_ref(
 402                                 trait_ref,
 403                                 ImplTraitContext::Disallowed(ImplTraitPosition::Trait),
 404                             )
 405                         });
 406
 407                         let lowered_ty = this
 408                             .lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 409
 410                         (trait_ref, lowered_ty)
 411                     },
 412                 );
 413
 414                 let new_impl_items =
 415                     self.with_in_scope_lifetime_defs(&ast_generics.params, |this| {
 416                         this.arena.alloc_from_iter(
 417                             impl_items.iter().map(|item| this.lower_impl_item_ref(item)),
 418                         )
 419                     });
 420
 421                 // `defaultness.has_value()` is never called for an `impl`, always `true` in order
 422                 // to not cause an assertion failure inside the `lower_defaultness` function.
 423                 let has_val = true;
 424                 let (defaultness, defaultness_span) = self.lower_defaultness(defaultness, has_val);
 425                 let polarity = match polarity {
 426                     ImplPolarity::Positive => ImplPolarity::Positive,
 427                     ImplPolarity::Negative(s) => ImplPolarity::Negative(self.lower_span(s)),
 428                 };
 429                 hir::ItemKind::Impl(hir::Impl {
 430                     unsafety: self.lower_unsafety(unsafety),
 431                     polarity,
 432                     defaultness,
 433                     defaultness_span,
 434                     constness: self.lower_constness(constness),
 435                     generics,
 436                     of_trait: trait_ref,
 437                     self_ty: lowered_ty,
 438                     items: new_impl_items,
 439                 })
 440             }
 441             ItemKind::Trait(box Trait {
 442                 is_auto,
 443                 unsafety,
 444                 ref generics,
 445                 ref bounds,
 446                 ref items,
 447             }) => {
 448                 let bounds = self.lower_param_bounds(
 449                     bounds,
 450                     ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
 451                 );
 452                 let items = self
 453                     .arena
 454                     .alloc_from_iter(items.iter().map(|item| self.lower_trait_item_ref(item)));
 455                 hir::ItemKind::Trait(
 456                     is_auto,
 457                     self.lower_unsafety(unsafety),
 458                     self.lower_generics(
 459                         generics,
 460                         ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 461                     ),
 462                     bounds,
 463                     items,
 464                 )
 465             }
 466             ItemKind::TraitAlias(ref generics, ref bounds) => hir::ItemKind::TraitAlias(
 467                 self.lower_generics(
 468                     generics,
 469                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 470                 ),
 471                 self.lower_param_bounds(
 472                     bounds,
 473                     ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
 474                 ),
 475             ),
 476             ItemKind::MacroDef(MacroDef { ref body, macro_rules }) => {
 477                 let body = P(self.lower_mac_args(body));
 478                 let macro_kind = self.resolver.decl_macro_kind(self.resolver.local_def_id(id));
 479                 hir::ItemKind::Macro(ast::MacroDef { body, macro_rules }, macro_kind)
 480             }
 481             ItemKind::MacCall(..) => {
 482                 panic!("`TyMac` should have been expanded by now")
 483             }
 484         }
 485     }
 486
 487     fn lower_const_item(
 488         &mut self,
 489         ty: &Ty,
 490         span: Span,
 491         body: Option<&Expr>,
 492     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
 493         let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 494         (ty, self.lower_const_body(span, body))
 495     }
 496
 497     fn lower_use_tree(
 498         &mut self,
 499         tree: &UseTree,
 500         prefix: &Path,
 501         id: NodeId,
 502         vis: &mut hir::Visibility<'hir>,
 503         ident: &mut Ident,
 504         attrs: Option<&'hir [Attribute]>,
 505     ) -> hir::ItemKind<'hir> {
 506         debug!("lower_use_tree(tree={:?})", tree);
 507         debug!("lower_use_tree: vis = {:?}", vis);
 508
 509         let path = &tree.prefix;
 510         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 511
 512         match tree.kind {
 513             UseTreeKind::Simple(rename, id1, id2) => {
 514                 *ident = tree.ident();
 515
 516                 // First, apply the prefix to the path.
 517                 let mut path = Path { segments, span: path.span, tokens: None };
 518
 519                 // Correctly resolve `self` imports.
 520                 if path.segments.len() > 1
 521                     && path.segments.last().unwrap().ident.name == kw::SelfLower
 522                 {
 523                     let _ = path.segments.pop();
 524                     if rename.is_none() {
 525                         *ident = path.segments.last().unwrap().ident;
 526                     }
 527                 }
 528
 529                 let mut resolutions = self.expect_full_res_from_use(id).fuse();
 530                 // We want to return *something* from this function, so hold onto the first item
 531                 // for later.
 532                 let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
 533
 534                 // Here, we are looping over namespaces, if they exist for the definition
 535                 // being imported. We only handle type and value namespaces because we
 536                 // won't be dealing with macros in the rest of the compiler.
 537                 // Essentially a single `use` which imports two names is desugared into
 538                 // two imports.
 539                 for new_node_id in [id1, id2] {
 540                     let new_id = self.resolver.local_def_id(new_node_id);
 541                     let Some(res) = resolutions.next() else {
 542                         // Associate an HirId to both ids even if there is no resolution.
 543                         self.owners.ensure_contains_elem(new_id, || hir::MaybeOwner::Phantom);
 544                         let _old = std::mem::replace(
 545                             &mut self.owners[new_id],
 546                             hir::MaybeOwner::NonOwner(hir::HirId::make_owner(new_id)),
 547                         );
 548                         debug_assert!(matches!(_old, hir::MaybeOwner::Phantom));
 549                         continue;
 550                     };
 551                     let ident = *ident;
 552                     let mut path = path.clone();
 553                     for seg in &mut path.segments {
 554                         seg.id = self.resolver.next_node_id();
 555                     }
 556                     let span = path.span;
 557
 558                     self.with_hir_id_owner(new_node_id, |this| {
 559                         let res = this.lower_res(res);
 560                         let path = this.lower_path_extra(res, &path, ParamMode::Explicit);
 561                         let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
 562                         let vis = this.rebuild_vis(&vis);
 563                         if let Some(attrs) = attrs {
 564                             this.attrs.insert(hir::ItemLocalId::new(0), attrs);
 565                         }
 566
 567                         let item = hir::Item {
 568                             def_id: new_id,
 569                             ident: this.lower_ident(ident),
 570                             kind,
 571                             vis,
 572                             span: this.lower_span(span),
 573                         };
 574                         hir::OwnerNode::Item(this.arena.alloc(item))
 575                     });
 576                 }
 577
 578                 let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit);
 579                 hir::ItemKind::Use(path, hir::UseKind::Single)
 580             }
 581             UseTreeKind::Glob => {
 582                 let path = self.lower_path(
 583                     id,
 584                     &Path { segments, span: path.span, tokens: None },
 585                     ParamMode::Explicit,
 586                 );
 587                 hir::ItemKind::Use(path, hir::UseKind::Glob)
 588             }
 589             UseTreeKind::Nested(ref trees) => {
 590                 // Nested imports are desugared into simple imports.
 591                 // So, if we start with
 592                 //
 593                 // ```
 594                 // pub(x) use foo::{a, b};
 595                 // ```
 596                 //
 597                 // we will create three items:
 598                 //
 599                 // ```
 600                 // pub(x) use foo::a;
 601                 // pub(x) use foo::b;
 602                 // pub(x) use foo::{}; // <-- this is called the `ListStem`
 603                 // ```
 604                 //
 605                 // The first two are produced by recursively invoking
 606                 // `lower_use_tree` (and indeed there may be things
 607                 // like `use foo::{a::{b, c}}` and so forth).  They
 608                 // wind up being directly added to
 609                 // `self.items`. However, the structure of this
 610                 // function also requires us to return one item, and
 611                 // for that we return the `{}` import (called the
 612                 // `ListStem`).
 613
 614                 let prefix = Path { segments, span: prefix.span.to(path.span), tokens: None };
 615
 616                 // Add all the nested `PathListItem`s to the HIR.
 617                 for &(ref use_tree, id) in trees {
 618                     let new_hir_id = self.resolver.local_def_id(id);
 619
 620                     let mut prefix = prefix.clone();
 621
 622                     // Give the segments new node-ids since they are being cloned.
 623                     for seg in &mut prefix.segments {
 624                         seg.id = self.resolver.next_node_id();
 625                     }
 626
 627                     // Each `use` import is an item and thus are owners of the
 628                     // names in the path. Up to this point the nested import is
 629                     // the current owner, since we want each desugared import to
 630                     // own its own names, we have to adjust the owner before
 631                     // lowering the rest of the import.
 632                     self.with_hir_id_owner(id, |this| {
 633                         let mut vis = this.rebuild_vis(&vis);
 634                         let mut ident = *ident;
 635
 636                         let kind =
 637                             this.lower_use_tree(use_tree, &prefix, id, &mut vis, &mut ident, attrs);
 638                         if let Some(attrs) = attrs {
 639                             this.attrs.insert(hir::ItemLocalId::new(0), attrs);
 640                         }
 641
 642                         let item = hir::Item {
 643                             def_id: new_hir_id,
 644                             ident: this.lower_ident(ident),
 645                             kind,
 646                             vis,
 647                             span: this.lower_span(use_tree.span),
 648                         };
 649                         hir::OwnerNode::Item(this.arena.alloc(item))
 650                     });
 651                 }
 652
 653                 // Subtle and a bit hacky: we lower the privacy level
 654                 // of the list stem to "private" most of the time, but
 655                 // not for "restricted" paths. The key thing is that
 656                 // we don't want it to stay as `pub` (with no caveats)
 657                 // because that affects rustdoc and also the lints
 658                 // about `pub` items. But we can't *always* make it
 659                 // private -- particularly not for restricted paths --
 660                 // because it contains node-ids that would then be
 661                 // unused, failing the check that HirIds are "densely
 662                 // assigned".
 663                 match vis.node {
 664                     hir::VisibilityKind::Public
 665                     | hir::VisibilityKind::Crate(_)
 666                     | hir::VisibilityKind::Inherited => {
 667                         *vis = respan(
 668                             self.lower_span(prefix.span.shrink_to_lo()),
 669                             hir::VisibilityKind::Inherited,
 670                         );
 671                     }
 672                     hir::VisibilityKind::Restricted { .. } => {
 673                         // Do nothing here, as described in the comment on the match.
 674                     }
 675                 }
 676
 677                 let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
 678                 let res = self.lower_res(res);
 679                 let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit);
 680                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
 681             }
 682         }
 683     }
 684
 685     /// Paths like the visibility path in `pub(super) use foo::{bar, baz}` are repeated
 686     /// many times in the HIR tree; for each occurrence, we need to assign distinct
 687     /// `NodeId`s. (See, e.g., #56128.)
 688     fn rebuild_use_path(&mut self, path: &hir::Path<'hir>) -> &'hir hir::Path<'hir> {
 689         debug!("rebuild_use_path(path = {:?})", path);
 690         let segments =
 691             self.arena.alloc_from_iter(path.segments.iter().map(|seg| hir::PathSegment {
 692                 ident: seg.ident,
 693                 hir_id: seg.hir_id.map(|_| self.next_id()),
 694                 res: seg.res,
 695                 args: None,
 696                 infer_args: seg.infer_args,
 697             }));
 698         self.arena.alloc(hir::Path { span: path.span, res: path.res, segments })
 699     }
 700
 701     fn rebuild_vis(&mut self, vis: &hir::Visibility<'hir>) -> hir::Visibility<'hir> {
 702         let vis_kind = match vis.node {
 703             hir::VisibilityKind::Public => hir::VisibilityKind::Public,
 704             hir::VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
 705             hir::VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
 706             hir::VisibilityKind::Restricted { ref path, hir_id: _ } => {
 707                 hir::VisibilityKind::Restricted {
 708                     path: self.rebuild_use_path(path),
 709                     hir_id: self.next_id(),
 710                 }
 711             }
 712         };
 713         respan(self.lower_span(vis.span), vis_kind)
 714     }
 715
 716     fn lower_foreign_item(&mut self, i: &ForeignItem) -> &'hir hir::ForeignItem<'hir> {
 717         let hir_id = self.lower_node_id(i.id);
 718         let def_id = hir_id.expect_owner();
 719         self.lower_attrs(hir_id, &i.attrs);
 720         let item = hir::ForeignItem {
 721             def_id,
 722             ident: self.lower_ident(i.ident),
 723             kind: match i.kind {
 724                 ForeignItemKind::Fn(box Fn { ref sig, ref generics, .. }) => {
 725                     let fdec = &sig.decl;
 726                     let (generics, (fn_dec, fn_args)) = self.add_in_band_defs(
 727                         generics,
 728                         def_id,
 729                         AnonymousLifetimeMode::PassThrough,
 730                         |this, _| {
 731                             (
 732                                 // Disallow `impl Trait` in foreign items.
 733                                 this.lower_fn_decl(fdec, None, FnDeclKind::ExternFn, None),
 734                                 this.lower_fn_params_to_names(fdec),
 735                             )
 736                         },
 737                     );
 738
 739                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
 740                 }
 741                 ForeignItemKind::Static(ref t, m, _) => {
 742                     let ty =
 743                         self.lower_ty(t, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 744                     hir::ForeignItemKind::Static(ty, m)
 745                 }
 746                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
 747                 ForeignItemKind::MacCall(_) => panic!("macro shouldn't exist here"),
 748             },
 749             vis: self.lower_visibility(&i.vis),
 750             span: self.lower_span(i.span),
 751         };
 752         self.arena.alloc(item)
 753     }
 754
 755     fn lower_foreign_item_ref(&mut self, i: &ForeignItem) -> hir::ForeignItemRef {
 756         hir::ForeignItemRef {
 757             id: hir::ForeignItemId { def_id: self.resolver.local_def_id(i.id) },
 758             ident: self.lower_ident(i.ident),
 759             span: self.lower_span(i.span),
 760         }
 761     }
 762
 763     fn lower_variant(&mut self, v: &Variant) -> hir::Variant<'hir> {
 764         let id = self.lower_node_id(v.id);
 765         self.lower_attrs(id, &v.attrs);
 766         hir::Variant {
 767             id,
 768             data: self.lower_variant_data(id, &v.data),
 769             disr_expr: v.disr_expr.as_ref().map(|e| self.lower_anon_const(e)),
 770             ident: self.lower_ident(v.ident),
 771             span: self.lower_span(v.span),
 772         }
 773     }
 774
 775     fn lower_variant_data(
 776         &mut self,
 777         parent_id: hir::HirId,
 778         vdata: &VariantData,
 779     ) -> hir::VariantData<'hir> {
 780         match *vdata {
 781             VariantData::Struct(ref fields, recovered) => hir::VariantData::Struct(
 782                 self.arena
 783                     .alloc_from_iter(fields.iter().enumerate().map(|f| self.lower_field_def(f))),
 784                 recovered,
 785             ),
 786             VariantData::Tuple(ref fields, id) => {
 787                 let ctor_id = self.lower_node_id(id);
 788                 self.alias_attrs(ctor_id, parent_id);
 789                 hir::VariantData::Tuple(
 790                     self.arena.alloc_from_iter(
 791                         fields.iter().enumerate().map(|f| self.lower_field_def(f)),
 792                     ),
 793                     ctor_id,
 794                 )
 795             }
 796             VariantData::Unit(id) => {
 797                 let ctor_id = self.lower_node_id(id);
 798                 self.alias_attrs(ctor_id, parent_id);
 799                 hir::VariantData::Unit(ctor_id)
 800             }
 801         }
 802     }
 803
 804     fn lower_field_def(&mut self, (index, f): (usize, &FieldDef)) -> hir::FieldDef<'hir> {
 805         let ty = if let TyKind::Path(ref qself, ref path) = f.ty.kind {
 806             let t = self.lower_path_ty(
 807                 &f.ty,
 808                 qself,
 809                 path,
 810                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
 811                 ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 812             );
 813             self.arena.alloc(t)
 814         } else {
 815             self.lower_ty(&f.ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type))
 816         };
 817         let hir_id = self.lower_node_id(f.id);
 818         self.lower_attrs(hir_id, &f.attrs);
 819         hir::FieldDef {
 820             span: self.lower_span(f.span),
 821             hir_id,
 822             ident: match f.ident {
 823                 Some(ident) => self.lower_ident(ident),
 824                 // FIXME(jseyfried): positional field hygiene.
 825                 None => Ident::new(sym::integer(index), self.lower_span(f.span)),
 826             },
 827             vis: self.lower_visibility(&f.vis),
 828             ty,
 829         }
 830     }
 831
 832     fn lower_trait_item(&mut self, i: &AssocItem) -> &'hir hir::TraitItem<'hir> {
 833         let hir_id = self.lower_node_id(i.id);
 834         let trait_item_def_id = hir_id.expect_owner();
 835
 836         let (generics, kind) = match i.kind {
 837             AssocItemKind::Const(_, ref ty, ref default) => {
 838                 let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 839                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
 840                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body))
 841             }
 842             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: None, .. }) => {
 843                 let names = self.lower_fn_params_to_names(&sig.decl);
 844                 let (generics, sig) = self.lower_method_sig(
 845                     generics,
 846                     sig,
 847                     trait_item_def_id,
 848                     FnDeclKind::Trait,
 849                     None,
 850                 );
 851                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Required(names)))
 852             }
 853             AssocItemKind::Fn(box Fn { ref sig, ref generics, body: Some(ref body), .. }) => {
 854                 let asyncness = sig.header.asyncness;
 855                 let body_id =
 856                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, Some(&body));
 857                 let (generics, sig) = self.lower_method_sig(
 858                     generics,
 859                     sig,
 860                     trait_item_def_id,
 861                     FnDeclKind::Trait,
 862                     asyncness.opt_return_id(),
 863                 );
 864                 (generics, hir::TraitItemKind::Fn(sig, hir::TraitFn::Provided(body_id)))
 865             }
 866             AssocItemKind::TyAlias(box TyAlias {
 867                 ref generics,
 868                 where_clauses,
 869                 ref bounds,
 870                 ref ty,
 871                 ..
 872             }) => {
 873                 let ty = ty.as_ref().map(|x| {
 874                     self.lower_ty(x, ImplTraitContext::Disallowed(ImplTraitPosition::Type))
 875                 });
 876                 let mut generics = generics.clone();
 877                 add_ty_alias_where_clause(&mut generics, where_clauses, false);
 878                 let generics = self.lower_generics(
 879                     &generics,
 880                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 881                 );
 882                 let kind = hir::TraitItemKind::Type(
 883                     self.lower_param_bounds(
 884                         bounds,
 885                         ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 886                     ),
 887                     ty,
 888                 );
 889
 890                 (generics, kind)
 891             }
 892             AssocItemKind::MacCall(..) => panic!("macro item shouldn't exist at this point"),
 893         };
 894
 895         self.lower_attrs(hir_id, &i.attrs);
 896         let item = hir::TraitItem {
 897             def_id: trait_item_def_id,
 898             ident: self.lower_ident(i.ident),
 899             generics,
 900             kind,
 901             span: self.lower_span(i.span),
 902         };
 903         self.arena.alloc(item)
 904     }
 905
 906     fn lower_trait_item_ref(&mut self, i: &AssocItem) -> hir::TraitItemRef {
 907         let (kind, has_default) = match &i.kind {
 908             AssocItemKind::Const(_, _, default) => (hir::AssocItemKind::Const, default.is_some()),
 909             AssocItemKind::TyAlias(box TyAlias { ty, .. }) => {
 910                 (hir::AssocItemKind::Type, ty.is_some())
 911             }
 912             AssocItemKind::Fn(box Fn { sig, body, .. }) => {
 913                 (hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }, body.is_some())
 914             }
 915             AssocItemKind::MacCall(..) => unimplemented!(),
 916         };
 917         let id = hir::TraitItemId { def_id: self.resolver.local_def_id(i.id) };
 918         let defaultness = hir::Defaultness::Default { has_value: has_default };
 919         hir::TraitItemRef {
 920             id,
 921             ident: self.lower_ident(i.ident),
 922             span: self.lower_span(i.span),
 923             defaultness,
 924             kind,
 925         }
 926     }
 927
 928     /// Construct `ExprKind::Err` for the given `span`.
 929     crate fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
 930         self.expr(span, hir::ExprKind::Err, AttrVec::new())
 931     }
 932
 933     fn lower_impl_item(&mut self, i: &AssocItem) -> &'hir hir::ImplItem<'hir> {
 934         let impl_item_def_id = self.resolver.local_def_id(i.id);
 935
 936         let (generics, kind) = match &i.kind {
 937             AssocItemKind::Const(_, ty, expr) => {
 938                 let ty = self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 939                 (
 940                     hir::Generics::empty(),
 941                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
 942                 )
 943             }
 944             AssocItemKind::Fn(box Fn { sig, generics, body, .. }) => {
 945                 self.current_item = Some(i.span);
 946                 let asyncness = sig.header.asyncness;
 947                 let body_id =
 948                     self.lower_maybe_async_body(i.span, &sig.decl, asyncness, body.as_deref());
 949                 let (generics, sig) = self.lower_method_sig(
 950                     generics,
 951                     sig,
 952                     impl_item_def_id,
 953                     if self.is_in_trait_impl { FnDeclKind::Impl } else { FnDeclKind::Inherent },
 954                     asyncness.opt_return_id(),
 955                 );
 956
 957                 (generics, hir::ImplItemKind::Fn(sig, body_id))
 958             }
 959             AssocItemKind::TyAlias(box TyAlias { generics, where_clauses, ty, .. }) => {
 960                 let mut generics = generics.clone();
 961                 add_ty_alias_where_clause(&mut generics, *where_clauses, false);
 962                 let generics = self.lower_generics(
 963                     &generics,
 964                     ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
 965                 );
 966                 let kind = match ty {
 967                     None => {
 968                         let ty = self.arena.alloc(self.ty(i.span, hir::TyKind::Err));
 969                         hir::ImplItemKind::TyAlias(ty)
 970                     }
 971                     Some(ty) => {
 972                         let ty = self.lower_ty(
 973                             ty,
 974                             ImplTraitContext::TypeAliasesOpaqueTy {
 975                                 capturable_lifetimes: &mut FxHashSet::default(),
 976                             },
 977                         );
 978                         hir::ImplItemKind::TyAlias(ty)
 979                     }
 980                 };
 981                 (generics, kind)
 982             }
 983             AssocItemKind::MacCall(..) => panic!("`TyMac` should have been expanded by now"),
 984         };
 985
 986         let hir_id = self.lower_node_id(i.id);
 987         self.lower_attrs(hir_id, &i.attrs);
 988         let item = hir::ImplItem {
 989             def_id: hir_id.expect_owner(),
 990             ident: self.lower_ident(i.ident),
 991             generics,
 992             vis: self.lower_visibility(&i.vis),
 993             kind,
 994             span: self.lower_span(i.span),
 995         };
 996         self.arena.alloc(item)
 997     }
 998
 999     fn lower_impl_item_ref(&mut self, i: &AssocItem) -> hir::ImplItemRef {
1000         // Since `default impl` is not yet implemented, this is always true in impls.
1001         let has_value = true;
1002         let (defaultness, _) = self.lower_defaultness(i.kind.defaultness(), has_value);
1003         hir::ImplItemRef {
1004             id: hir::ImplItemId { def_id: self.resolver.local_def_id(i.id) },
1005             ident: self.lower_ident(i.ident),
1006             span: self.lower_span(i.span),
1007             defaultness,
1008             kind: match &i.kind {
1009                 AssocItemKind::Const(..) => hir::AssocItemKind::Const,
1010                 AssocItemKind::TyAlias(..) => hir::AssocItemKind::Type,
1011                 AssocItemKind::Fn(box Fn { sig, .. }) => {
1012                     hir::AssocItemKind::Fn { has_self: sig.decl.has_self() }
1013                 }
1014                 AssocItemKind::MacCall(..) => unimplemented!(),
1015             },
1016             trait_item_def_id: self.resolver.get_partial_res(i.id).map(|r| r.base_res().def_id()),
1017         }
1018     }
1019
1020     /// If an `explicit_owner` is given, this method allocates the `HirId` in
1021     /// the address space of that item instead of the item currently being
1022     /// lowered. This can happen during `lower_impl_item_ref()` where we need to
1023     /// lower a `Visibility` value although we haven't lowered the owning
1024     /// `ImplItem` in question yet.
1025     fn lower_visibility(&mut self, v: &Visibility) -> hir::Visibility<'hir> {
1026         let node = match v.kind {
1027             VisibilityKind::Public => hir::VisibilityKind::Public,
1028             VisibilityKind::Crate(sugar) => hir::VisibilityKind::Crate(sugar),
1029             VisibilityKind::Restricted { ref path, id } => {
1030                 debug!("lower_visibility: restricted path id = {:?}", id);
1031                 let lowered_id = self.lower_node_id(id);
1032                 hir::VisibilityKind::Restricted {
1033                     path: self.lower_path(id, path, ParamMode::Explicit),
1034                     hir_id: lowered_id,
1035                 }
1036             }
1037             VisibilityKind::Inherited => hir::VisibilityKind::Inherited,
1038         };
1039         respan(self.lower_span(v.span), node)
1040     }
1041
1042     fn lower_defaultness(
1043         &self,
1044         d: Defaultness,
1045         has_value: bool,
1046     ) -> (hir::Defaultness, Option<Span>) {
1047         match d {
1048             Defaultness::Default(sp) => {
1049                 (hir::Defaultness::Default { has_value }, Some(self.lower_span(sp)))
1050             }
1051             Defaultness::Final => {
1052                 assert!(has_value);
1053                 (hir::Defaultness::Final, None)
1054             }
1055         }
1056     }
1057
1058     fn record_body(
1059         &mut self,
1060         params: &'hir [hir::Param<'hir>],
1061         value: hir::Expr<'hir>,
1062     ) -> hir::BodyId {
1063         let body = hir::Body { generator_kind: self.generator_kind, params, value };
1064         let id = body.id();
1065         debug_assert_eq!(id.hir_id.owner, self.current_hir_id_owner);
1066         self.bodies.push((id.hir_id.local_id, self.arena.alloc(body)));
1067         id
1068     }
1069
1070     pub(super) fn lower_body(
1071         &mut self,
1072         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
1073     ) -> hir::BodyId {
1074         let prev_gen_kind = self.generator_kind.take();
1075         let task_context = self.task_context.take();
1076         let (parameters, result) = f(self);
1077         let body_id = self.record_body(parameters, result);
1078         self.task_context = task_context;
1079         self.generator_kind = prev_gen_kind;
1080         body_id
1081     }
1082
1083     fn lower_param(&mut self, param: &Param) -> hir::Param<'hir> {
1084         let hir_id = self.lower_node_id(param.id);
1085         self.lower_attrs(hir_id, &param.attrs);
1086         hir::Param {
1087             hir_id,
1088             pat: self.lower_pat(&param.pat),
1089             ty_span: self.lower_span(param.ty.span),
1090             span: self.lower_span(param.span),
1091         }
1092     }
1093
1094     pub(super) fn lower_fn_body(
1095         &mut self,
1096         decl: &FnDecl,
1097         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
1098     ) -> hir::BodyId {
1099         self.lower_body(|this| {
1100             (
1101                 this.arena.alloc_from_iter(decl.inputs.iter().map(|x| this.lower_param(x))),
1102                 body(this),
1103             )
1104         })
1105     }
1106
1107     fn lower_fn_body_block(
1108         &mut self,
1109         span: Span,
1110         decl: &FnDecl,
1111         body: Option<&Block>,
1112     ) -> hir::BodyId {
1113         self.lower_fn_body(decl, |this| this.lower_block_expr_opt(span, body))
1114     }
1115
1116     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
1117         match block {
1118             Some(block) => self.lower_block_expr(block),
1119             None => self.expr_err(span),
1120         }
1121     }
1122
1123     pub(super) fn lower_const_body(&mut self, span: Span, expr: Option<&Expr>) -> hir::BodyId {
1124         self.lower_body(|this| {
1125             (
1126                 &[],
1127                 match expr {
1128                     Some(expr) => this.lower_expr_mut(expr),
1129                     None => this.expr_err(span),
1130                 },
1131             )
1132         })
1133     }
1134
1135     fn lower_maybe_async_body(
1136         &mut self,
1137         span: Span,
1138         decl: &FnDecl,
1139         asyncness: Async,
1140         body: Option<&Block>,
1141     ) -> hir::BodyId {
1142         let closure_id = match asyncness {
1143             Async::Yes { closure_id, .. } => closure_id,
1144             Async::No => return self.lower_fn_body_block(span, decl, body),
1145         };
1146
1147         self.lower_body(|this| {
1148             let mut parameters: Vec<hir::Param<'_>> = Vec::new();
1149             let mut statements: Vec<hir::Stmt<'_>> = Vec::new();
1150
1151             // Async function parameters are lowered into the closure body so that they are
1152             // captured and so that the drop order matches the equivalent non-async functions.
1153             //
1154             // from:
1155             //
1156             //     async fn foo(<pattern>: <ty>, <pattern>: <ty>, <pattern>: <ty>) {
1157             //         <body>
1158             //     }
1159             //
1160             // into:
1161             //
1162             //     fn foo(__arg0: <ty>, __arg1: <ty>, __arg2: <ty>) {
1163             //       async move {
1164             //         let __arg2 = __arg2;
1165             //         let <pattern> = __arg2;
1166             //         let __arg1 = __arg1;
1167             //         let <pattern> = __arg1;
1168             //         let __arg0 = __arg0;
1169             //         let <pattern> = __arg0;
1170             //         drop-temps { <body> } // see comments later in fn for details
1171             //       }
1172             //     }
1173             //
1174             // If `<pattern>` is a simple ident, then it is lowered to a single
1175             // `let <pattern> = <pattern>;` statement as an optimization.
1176             //
1177             // Note that the body is embedded in `drop-temps`; an
1178             // equivalent desugaring would be `return { <body>
1179             // };`. The key point is that we wish to drop all the
1180             // let-bound variables and temporaries created in the body
1181             // (and its tail expression!) before we drop the
1182             // parameters (c.f. rust-lang/rust#64512).
1183             for (index, parameter) in decl.inputs.iter().enumerate() {
1184                 let parameter = this.lower_param(parameter);
1185                 let span = parameter.pat.span;
1186
1187                 // Check if this is a binding pattern, if so, we can optimize and avoid adding a
1188                 // `let <pat> = __argN;` statement. In this case, we do not rename the parameter.
1189                 let (ident, is_simple_parameter) = match parameter.pat.kind {
1190                     hir::PatKind::Binding(
1191                         hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable,
1192                         _,
1193                         ident,
1194                         _,
1195                     ) => (ident, true),
1196                     // For `ref mut` or wildcard arguments, we can't reuse the binding, but
1197                     // we can keep the same name for the parameter.
1198                     // This lets rustdoc render it correctly in documentation.
1199                     hir::PatKind::Binding(_, _, ident, _) => (ident, false),
1200                     hir::PatKind::Wild => {
1201                         (Ident::with_dummy_span(rustc_span::symbol::kw::Underscore), false)
1202                     }
1203                     _ => {
1204                         // Replace the ident for bindings that aren't simple.
1205                         let name = format!("__arg{}", index);
1206                         let ident = Ident::from_str(&name);
1207
1208                         (ident, false)
1209                     }
1210                 };
1211
1212                 let desugared_span = this.mark_span_with_reason(DesugaringKind::Async, span, None);
1213
1214                 // Construct a parameter representing `__argN: <ty>` to replace the parameter of the
1215                 // async function.
1216                 //
1217                 // If this is the simple case, this parameter will end up being the same as the
1218                 // original parameter, but with a different pattern id.
1219                 let stmt_attrs = this.attrs.get(&parameter.hir_id.local_id).copied();
1220                 let (new_parameter_pat, new_parameter_id) = this.pat_ident(desugared_span, ident);
1221                 let new_parameter = hir::Param {
1222                     hir_id: parameter.hir_id,
1223                     pat: new_parameter_pat,
1224                     ty_span: this.lower_span(parameter.ty_span),
1225                     span: this.lower_span(parameter.span),
1226                 };
1227
1228                 if is_simple_parameter {
1229                     // If this is the simple case, then we only insert one statement that is
1230                     // `let <pat> = <pat>;`. We re-use the original argument's pattern so that
1231                     // `HirId`s are densely assigned.
1232                     let expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1233                     let stmt = this.stmt_let_pat(
1234                         stmt_attrs,
1235                         desugared_span,
1236                         Some(expr),
1237                         parameter.pat,
1238                         hir::LocalSource::AsyncFn,
1239                     );
1240                     statements.push(stmt);
1241                 } else {
1242                     // If this is not the simple case, then we construct two statements:
1243                     //
1244                     // ```
1245                     // let __argN = __argN;
1246                     // let <pat> = __argN;
1247                     // ```
1248                     //
1249                     // The first statement moves the parameter into the closure and thus ensures
1250                     // that the drop order is correct.
1251                     //
1252                     // The second statement creates the bindings that the user wrote.
1253
1254                     // Construct the `let mut __argN = __argN;` statement. It must be a mut binding
1255                     // because the user may have specified a `ref mut` binding in the next
1256                     // statement.
1257                     let (move_pat, move_id) = this.pat_ident_binding_mode(
1258                         desugared_span,
1259                         ident,
1260                         hir::BindingAnnotation::Mutable,
1261                     );
1262                     let move_expr = this.expr_ident(desugared_span, ident, new_parameter_id);
1263                     let move_stmt = this.stmt_let_pat(
1264                         None,
1265                         desugared_span,
1266                         Some(move_expr),
1267                         move_pat,
1268                         hir::LocalSource::AsyncFn,
1269                     );
1270
1271                     // Construct the `let <pat> = __argN;` statement. We re-use the original
1272                     // parameter's pattern so that `HirId`s are densely assigned.
1273                     let pattern_expr = this.expr_ident(desugared_span, ident, move_id);
1274                     let pattern_stmt = this.stmt_let_pat(
1275                         stmt_attrs,
1276                         desugared_span,
1277                         Some(pattern_expr),
1278                         parameter.pat,
1279                         hir::LocalSource::AsyncFn,
1280                     );
1281
1282                     statements.push(move_stmt);
1283                     statements.push(pattern_stmt);
1284                 };
1285
1286                 parameters.push(new_parameter);
1287             }
1288
1289             let body_span = body.map_or(span, |b| b.span);
1290             let async_expr = this.make_async_expr(
1291                 CaptureBy::Value,
1292                 closure_id,
1293                 None,
1294                 body_span,
1295                 hir::AsyncGeneratorKind::Fn,
1296                 |this| {
1297                     // Create a block from the user's function body:
1298                     let user_body = this.lower_block_expr_opt(body_span, body);
1299
1300                     // Transform into `drop-temps { <user-body> }`, an expression:
1301                     let desugared_span =
1302                         this.mark_span_with_reason(DesugaringKind::Async, user_body.span, None);
1303                     let user_body = this.expr_drop_temps(
1304                         desugared_span,
1305                         this.arena.alloc(user_body),
1306                         AttrVec::new(),
1307                     );
1308
1309                     // As noted above, create the final block like
1310                     //
1311                     // ```
1312                     // {
1313                     //   let $param_pattern = $raw_param;
1314                     //   ...
1315                     //   drop-temps { <user-body> }
1316                     // }
1317                     // ```
1318                     let body = this.block_all(
1319                         desugared_span,
1320                         this.arena.alloc_from_iter(statements),
1321                         Some(user_body),
1322                     );
1323
1324                     this.expr_block(body, AttrVec::new())
1325                 },
1326             );
1327
1328             (
1329                 this.arena.alloc_from_iter(parameters),
1330                 this.expr(body_span, async_expr, AttrVec::new()),
1331             )
1332         })
1333     }
1334
1335     fn lower_method_sig(
1336         &mut self,
1337         generics: &Generics,
1338         sig: &FnSig,
1339         fn_def_id: LocalDefId,
1340         kind: FnDeclKind,
1341         is_async: Option<NodeId>,
1342     ) -> (hir::Generics<'hir>, hir::FnSig<'hir>) {
1343         let header = self.lower_fn_header(sig.header);
1344         let (generics, decl) = self.add_in_band_defs(
1345             generics,
1346             fn_def_id,
1347             AnonymousLifetimeMode::PassThrough,
1348             |this, idty| this.lower_fn_decl(&sig.decl, Some((fn_def_id, idty)), kind, is_async),
1349         );
1350         (generics, hir::FnSig { header, decl, span: self.lower_span(sig.span) })
1351     }
1352
1353     fn lower_fn_header(&mut self, h: FnHeader) -> hir::FnHeader {
1354         hir::FnHeader {
1355             unsafety: self.lower_unsafety(h.unsafety),
1356             asyncness: self.lower_asyncness(h.asyncness),
1357             constness: self.lower_constness(h.constness),
1358             abi: self.lower_extern(h.ext),
1359         }
1360     }
1361
1362     pub(super) fn lower_abi(&mut self, abi: StrLit) -> abi::Abi {
1363         abi::lookup(abi.symbol_unescaped.as_str()).unwrap_or_else(|| {
1364             self.error_on_invalid_abi(abi);
1365             abi::Abi::Rust
1366         })
1367     }
1368
1369     pub(super) fn lower_extern(&mut self, ext: Extern) -> abi::Abi {
1370         match ext {
1371             Extern::None => abi::Abi::Rust,
1372             Extern::Implicit => abi::Abi::FALLBACK,
1373             Extern::Explicit(abi) => self.lower_abi(abi),
1374         }
1375     }
1376
1377     fn error_on_invalid_abi(&self, abi: StrLit) {
1378         struct_span_err!(self.sess, abi.span, E0703, "invalid ABI: found `{}`", abi.symbol)
1379             .span_label(abi.span, "invalid ABI")
1380             .help(&format!("valid ABIs: {}", abi::all_names().join(", ")))
1381             .emit();
1382     }
1383
1384     fn lower_asyncness(&mut self, a: Async) -> hir::IsAsync {
1385         match a {
1386             Async::Yes { .. } => hir::IsAsync::Async,
1387             Async::No => hir::IsAsync::NotAsync,
1388         }
1389     }
1390
1391     fn lower_constness(&mut self, c: Const) -> hir::Constness {
1392         match c {
1393             Const::Yes(_) => hir::Constness::Const,
1394             Const::No => hir::Constness::NotConst,
1395         }
1396     }
1397
1398     pub(super) fn lower_unsafety(&mut self, u: Unsafe) -> hir::Unsafety {
1399         match u {
1400             Unsafe::Yes(_) => hir::Unsafety::Unsafe,
1401             Unsafe::No => hir::Unsafety::Normal,
1402         }
1403     }
1404
1405     pub(super) fn lower_generics_mut(
1406         &mut self,
1407         generics: &Generics,
1408         itctx: ImplTraitContext<'_, 'hir>,
1409     ) -> GenericsCtor<'hir> {
1410         // Error if `?Trait` bounds in where clauses don't refer directly to type parameters.
1411         // Note: we used to clone these bounds directly onto the type parameter (and avoid lowering
1412         // these into hir when we lower thee where clauses), but this makes it quite difficult to
1413         // keep track of the Span info. Now, `add_implicitly_sized` in `AstConv` checks both param bounds and
1414         // where clauses for `?Sized`.
1415         for pred in &generics.where_clause.predicates {
1416             let WherePredicate::BoundPredicate(ref bound_pred) = *pred else {
1417                 continue;
1418             };
1419             let compute_is_param = || {
1420                 // Check if the where clause type is a plain type parameter.
1421                 match self
1422                     .resolver
1423                     .get_partial_res(bound_pred.bounded_ty.id)
1424                     .map(|d| (d.base_res(), d.unresolved_segments()))
1425                 {
1426                     Some((Res::Def(DefKind::TyParam, def_id), 0))
1427                         if bound_pred.bound_generic_params.is_empty() =>
1428                     {
1429                         generics
1430                             .params
1431                             .iter()
1432                             .any(|p| def_id == self.resolver.local_def_id(p.id).to_def_id())
1433                     }
1434                     // Either the `bounded_ty` is not a plain type parameter, or
1435                     // it's not found in the generic type parameters list.
1436                     _ => false,
1437                 }
1438             };
1439             // We only need to compute this once per `WherePredicate`, but don't
1440             // need to compute this at all unless there is a Maybe bound.
1441             let mut is_param: Option<bool> = None;
1442             for bound in &bound_pred.bounds {
1443                 if !matches!(*bound, GenericBound::Trait(_, TraitBoundModifier::Maybe)) {
1444                     continue;
1445                 }
1446                 let is_param = *is_param.get_or_insert_with(compute_is_param);
1447                 if !is_param {
1448                     self.diagnostic().span_err(
1449                         bound.span(),
1450                         "`?Trait` bounds are only permitted at the \
1451                         point where a type parameter is declared",
1452                     );
1453                 }
1454             }
1455         }
1456
1457         GenericsCtor {
1458             params: self.lower_generic_params_mut(&generics.params, itctx).collect(),
1459             where_clause: self.lower_where_clause(&generics.where_clause),
1460             span: self.lower_span(generics.span),
1461         }
1462     }
1463
1464     pub(super) fn lower_generics(
1465         &mut self,
1466         generics: &Generics,
1467         itctx: ImplTraitContext<'_, 'hir>,
1468     ) -> hir::Generics<'hir> {
1469         let generics_ctor = self.lower_generics_mut(generics, itctx);
1470         generics_ctor.into_generics(self.arena)
1471     }
1472
1473     fn lower_where_clause(&mut self, wc: &WhereClause) -> hir::WhereClause<'hir> {
1474         self.with_anonymous_lifetime_mode(AnonymousLifetimeMode::ReportError, |this| {
1475             hir::WhereClause {
1476                 predicates: this.arena.alloc_from_iter(
1477                     wc.predicates.iter().map(|predicate| this.lower_where_predicate(predicate)),
1478                 ),
1479                 span: this.lower_span(wc.span),
1480             }
1481         })
1482     }
1483
1484     fn lower_where_predicate(&mut self, pred: &WherePredicate) -> hir::WherePredicate<'hir> {
1485         match *pred {
1486             WherePredicate::BoundPredicate(WhereBoundPredicate {
1487                 ref bound_generic_params,
1488                 ref bounded_ty,
1489                 ref bounds,
1490                 span,
1491             }) => self.with_in_scope_lifetime_defs(&bound_generic_params, |this| {
1492                 hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
1493                     bound_generic_params: this.lower_generic_params(
1494                         bound_generic_params,
1495                         ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
1496                     ),
1497                     bounded_ty: this.lower_ty(
1498                         bounded_ty,
1499                         ImplTraitContext::Disallowed(ImplTraitPosition::Type),
1500                     ),
1501                     bounds: this.arena.alloc_from_iter(bounds.iter().map(|bound| {
1502                         this.lower_param_bound(
1503                             bound,
1504                             ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
1505                         )
1506                     })),
1507                     span: this.lower_span(span),
1508                 })
1509             }),
1510             WherePredicate::RegionPredicate(WhereRegionPredicate {
1511                 ref lifetime,
1512                 ref bounds,
1513                 span,
1514             }) => hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
1515                 span: self.lower_span(span),
1516                 lifetime: self.lower_lifetime(lifetime),
1517                 bounds: self.lower_param_bounds(
1518                     bounds,
1519                     ImplTraitContext::Disallowed(ImplTraitPosition::Bound),
1520                 ),
1521             }),
1522             WherePredicate::EqPredicate(WhereEqPredicate { id, ref lhs_ty, ref rhs_ty, span }) => {
1523                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1524                     hir_id: self.lower_node_id(id),
1525                     lhs_ty: self
1526                         .lower_ty(lhs_ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
1527                     rhs_ty: self
1528                         .lower_ty(rhs_ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
1529                     span: self.lower_span(span),
1530                 })
1531             }
1532         }
1533     }
1534 }
1535
1536 /// Helper struct for delayed construction of Generics.
1537 pub(super) struct GenericsCtor<'hir> {
1538     pub(super) params: SmallVec<[hir::GenericParam<'hir>; 4]>,
1539     where_clause: hir::WhereClause<'hir>,
1540     span: Span,
1541 }
1542
1543 impl<'hir> GenericsCtor<'hir> {
1544     pub(super) fn into_generics(self, arena: &'hir Arena<'hir>) -> hir::Generics<'hir> {
1545         hir::Generics {
1546             params: arena.alloc_from_iter(self.params),
1547             where_clause: self.where_clause,
1548             span: self.span,
1549         }
1550     }
1551 }