compiler/rustc_ast_lowering/src/expr.rs

   1 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
   2
   3 use rustc_ast::attr;
   4 use rustc_ast::ptr::P as AstP;
   5 use rustc_ast::*;
   6 use rustc_data_structures::stack::ensure_sufficient_stack;
   7 use rustc_data_structures::thin_vec::ThinVec;
   8 use rustc_errors::struct_span_err;
   9 use rustc_hir as hir;
  10 use rustc_hir::def::Res;
  11 use rustc_hir::definitions::DefPathData;
  12 use rustc_session::parse::feature_err;
  13 use rustc_span::hygiene::ExpnId;
  14 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
  15 use rustc_span::symbol::{sym, Ident, Symbol};
  16 use rustc_span::{hygiene::ForLoopLoc, DUMMY_SP};
  17
  18 impl<'hir> LoweringContext<'_, 'hir> {
  19     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  20         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  21     }
  22
  23     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  24         self.arena.alloc(self.lower_expr_mut(e))
  25     }
  26
  27     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  28         ensure_sufficient_stack(|| {
  29             let kind = match e.kind {
  30                 ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  31                 ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  32                 ExprKind::ConstBlock(ref anon_const) => {
  33                     let anon_const = self.lower_anon_const(anon_const);
  34                     hir::ExprKind::ConstBlock(anon_const)
  35                 }
  36                 ExprKind::Repeat(ref expr, ref count) => {
  37                     let expr = self.lower_expr(expr);
  38                     let count = self.lower_anon_const(count);
  39                     hir::ExprKind::Repeat(expr, count)
  40                 }
  41                 ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  42                 ExprKind::Call(ref f, ref args) => {
  43                     if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
  44                         self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
  45                     } else {
  46                         let f = self.lower_expr(f);
  47                         hir::ExprKind::Call(f, self.lower_exprs(args))
  48                     }
  49                 }
  50                 ExprKind::MethodCall(ref seg, ref args, span) => {
  51                     let hir_seg = self.arena.alloc(self.lower_path_segment(
  52                         e.span,
  53                         seg,
  54                         ParamMode::Optional,
  55                         0,
  56                         ParenthesizedGenericArgs::Err,
  57                         ImplTraitContext::disallowed(),
  58                     ));
  59                     let args = self.lower_exprs(args);
  60                     hir::ExprKind::MethodCall(
  61                         hir_seg,
  62                         self.lower_span(seg.ident.span),
  63                         args,
  64                         self.lower_span(span),
  65                     )
  66                 }
  67                 ExprKind::Binary(binop, ref lhs, ref rhs) => {
  68                     let binop = self.lower_binop(binop);
  69                     let lhs = self.lower_expr(lhs);
  70                     let rhs = self.lower_expr(rhs);
  71                     hir::ExprKind::Binary(binop, lhs, rhs)
  72                 }
  73                 ExprKind::Unary(op, ref ohs) => {
  74                     let op = self.lower_unop(op);
  75                     let ohs = self.lower_expr(ohs);
  76                     hir::ExprKind::Unary(op, ohs)
  77                 }
  78                 ExprKind::Lit(ref l) => {
  79                     hir::ExprKind::Lit(respan(self.lower_span(l.span), l.kind.clone()))
  80                 }
  81                 ExprKind::Cast(ref expr, ref ty) => {
  82                     let expr = self.lower_expr(expr);
  83                     let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
  84                     hir::ExprKind::Cast(expr, ty)
  85                 }
  86                 ExprKind::Type(ref expr, ref ty) => {
  87                     let expr = self.lower_expr(expr);
  88                     let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
  89                     hir::ExprKind::Type(expr, ty)
  90                 }
  91                 ExprKind::AddrOf(k, m, ref ohs) => {
  92                     let ohs = self.lower_expr(ohs);
  93                     hir::ExprKind::AddrOf(k, m, ohs)
  94                 }
  95                 ExprKind::Let(ref pat, ref scrutinee, span) => hir::ExprKind::Let(
  96                     self.lower_pat(pat),
  97                     self.lower_expr(scrutinee),
  98                     self.lower_span(span),
  99                 ),
 100                 ExprKind::If(ref cond, ref then, ref else_opt) => {
 101                     self.lower_expr_if(cond, then, else_opt.as_deref())
 102                 }
 103                 ExprKind::While(ref cond, ref body, opt_label) => {
 104                     self.with_loop_scope(e.id, |this| {
 105                         let span =
 106                             this.mark_span_with_reason(DesugaringKind::WhileLoop, e.span, None);
 107                         this.lower_expr_while_in_loop_scope(span, cond, body, opt_label)
 108                     })
 109                 }
 110                 ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
 111                     hir::ExprKind::Loop(
 112                         this.lower_block(body, false),
 113                         this.lower_label(opt_label),
 114                         hir::LoopSource::Loop,
 115                         DUMMY_SP,
 116                     )
 117                 }),
 118                 ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
 119                 ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
 120                     self.lower_expr(expr),
 121                     self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
 122                     hir::MatchSource::Normal,
 123                 ),
 124                 ExprKind::Async(capture_clause, closure_node_id, ref block) => self
 125                     .make_async_expr(
 126                         capture_clause,
 127                         closure_node_id,
 128                         None,
 129                         block.span,
 130                         hir::AsyncGeneratorKind::Block,
 131                         |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
 132                     ),
 133                 ExprKind::Await(ref expr) => self.lower_expr_await(e.span, expr),
 134                 ExprKind::Closure(
 135                     capture_clause,
 136                     asyncness,
 137                     movability,
 138                     ref decl,
 139                     ref body,
 140                     fn_decl_span,
 141                 ) => {
 142                     if let Async::Yes { closure_id, .. } = asyncness {
 143                         self.lower_expr_async_closure(
 144                             capture_clause,
 145                             closure_id,
 146                             decl,
 147                             body,
 148                             fn_decl_span,
 149                         )
 150                     } else {
 151                         self.lower_expr_closure(
 152                             capture_clause,
 153                             movability,
 154                             decl,
 155                             body,
 156                             fn_decl_span,
 157                         )
 158                     }
 159                 }
 160                 ExprKind::Block(ref blk, opt_label) => {
 161                     let opt_label = self.lower_label(opt_label);
 162                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 163                 }
 164                 ExprKind::Assign(ref el, ref er, span) => {
 165                     self.lower_expr_assign(el, er, span, e.span)
 166                 }
 167                 ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 168                     self.lower_binop(op),
 169                     self.lower_expr(el),
 170                     self.lower_expr(er),
 171                 ),
 172                 ExprKind::Field(ref el, ident) => {
 173                     hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(ident))
 174                 }
 175                 ExprKind::Index(ref el, ref er) => {
 176                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 177                 }
 178                 ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 179                     self.lower_expr_range_closed(e.span, e1, e2)
 180                 }
 181                 ExprKind::Range(ref e1, ref e2, lims) => {
 182                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 183                 }
 184                 ExprKind::Underscore => {
 185                     self.sess
 186                         .struct_span_err(
 187                             e.span,
 188                             "in expressions, `_` can only be used on the left-hand side of an assignment",
 189                         )
 190                         .span_label(e.span, "`_` not allowed here")
 191                         .emit();
 192                     hir::ExprKind::Err
 193                 }
 194                 ExprKind::Path(ref qself, ref path) => {
 195                     let qpath = self.lower_qpath(
 196                         e.id,
 197                         qself,
 198                         path,
 199                         ParamMode::Optional,
 200                         ImplTraitContext::disallowed(),
 201                     );
 202                     hir::ExprKind::Path(qpath)
 203                 }
 204                 ExprKind::Break(opt_label, ref opt_expr) => {
 205                     let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 206                     hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
 207                 }
 208                 ExprKind::Continue(opt_label) => {
 209                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
 210                 }
 211                 ExprKind::Ret(ref e) => {
 212                     let e = e.as_ref().map(|x| self.lower_expr(x));
 213                     hir::ExprKind::Ret(e)
 214                 }
 215                 ExprKind::InlineAsm(ref asm) => {
 216                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
 217                 }
 218                 ExprKind::LlvmInlineAsm(ref asm) => self.lower_expr_llvm_asm(asm),
 219                 ExprKind::Struct(ref se) => {
 220                     let rest = match &se.rest {
 221                         StructRest::Base(e) => Some(self.lower_expr(e)),
 222                         StructRest::Rest(sp) => {
 223                             self.sess
 224                                 .struct_span_err(*sp, "base expression required after `..`")
 225                                 .span_label(*sp, "add a base expression here")
 226                                 .emit();
 227                             Some(&*self.arena.alloc(self.expr_err(*sp)))
 228                         }
 229                         StructRest::None => None,
 230                     };
 231                     hir::ExprKind::Struct(
 232                         self.arena.alloc(self.lower_qpath(
 233                             e.id,
 234                             &se.qself,
 235                             &se.path,
 236                             ParamMode::Optional,
 237                             ImplTraitContext::disallowed(),
 238                         )),
 239                         self.arena
 240                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
 241                         rest,
 242                     )
 243                 }
 244                 ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 245                 ExprKind::Err => hir::ExprKind::Err,
 246                 ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 247                 ExprKind::Paren(ref ex) => {
 248                     let mut ex = self.lower_expr_mut(ex);
 249                     // Include parens in span, but only if it is a super-span.
 250                     if e.span.contains(ex.span) {
 251                         ex.span = self.lower_span(e.span);
 252                     }
 253                     // Merge attributes into the inner expression.
 254                     if !e.attrs.is_empty() {
 255                         let old_attrs =
 256                             self.attrs.get(&ex.hir_id.local_id).map(|la| *la).unwrap_or(&[]);
 257                         self.attrs.insert(
 258                             ex.hir_id.local_id,
 259                             &*self.arena.alloc_from_iter(
 260                                 e.attrs
 261                                     .iter()
 262                                     .map(|a| self.lower_attr(a))
 263                                     .chain(old_attrs.iter().cloned()),
 264                             ),
 265                         );
 266                     }
 267                     return ex;
 268                 }
 269
 270                 // Desugar `ExprForLoop`
 271                 // from: `[opt_ident]: for <pat> in <head> <body>`
 272                 ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 273                     return self.lower_expr_for(e, pat, head, body, opt_label);
 274                 }
 275                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 276             };
 277
 278             let hir_id = self.lower_node_id(e.id);
 279             self.lower_attrs(hir_id, &e.attrs);
 280             hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
 281         })
 282     }
 283
 284     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 285         match u {
 286             UnOp::Deref => hir::UnOp::Deref,
 287             UnOp::Not => hir::UnOp::Not,
 288             UnOp::Neg => hir::UnOp::Neg,
 289         }
 290     }
 291
 292     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 293         Spanned {
 294             node: match b.node {
 295                 BinOpKind::Add => hir::BinOpKind::Add,
 296                 BinOpKind::Sub => hir::BinOpKind::Sub,
 297                 BinOpKind::Mul => hir::BinOpKind::Mul,
 298                 BinOpKind::Div => hir::BinOpKind::Div,
 299                 BinOpKind::Rem => hir::BinOpKind::Rem,
 300                 BinOpKind::And => hir::BinOpKind::And,
 301                 BinOpKind::Or => hir::BinOpKind::Or,
 302                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 303                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 304                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 305                 BinOpKind::Shl => hir::BinOpKind::Shl,
 306                 BinOpKind::Shr => hir::BinOpKind::Shr,
 307                 BinOpKind::Eq => hir::BinOpKind::Eq,
 308                 BinOpKind::Lt => hir::BinOpKind::Lt,
 309                 BinOpKind::Le => hir::BinOpKind::Le,
 310                 BinOpKind::Ne => hir::BinOpKind::Ne,
 311                 BinOpKind::Ge => hir::BinOpKind::Ge,
 312                 BinOpKind::Gt => hir::BinOpKind::Gt,
 313             },
 314             span: self.lower_span(b.span),
 315         }
 316     }
 317
 318     fn lower_legacy_const_generics(
 319         &mut self,
 320         mut f: Expr,
 321         args: Vec<AstP<Expr>>,
 322         legacy_args_idx: &[usize],
 323     ) -> hir::ExprKind<'hir> {
 324         let path = match f.kind {
 325             ExprKind::Path(None, ref mut path) => path,
 326             _ => unreachable!(),
 327         };
 328
 329         // Split the arguments into const generics and normal arguments
 330         let mut real_args = vec![];
 331         let mut generic_args = vec![];
 332         for (idx, arg) in args.into_iter().enumerate() {
 333             if legacy_args_idx.contains(&idx) {
 334                 let parent_def_id = self.current_hir_id_owner;
 335                 let node_id = self.resolver.next_node_id();
 336
 337                 // Add a definition for the in-band const def.
 338                 self.resolver.create_def(
 339                     parent_def_id,
 340                     node_id,
 341                     DefPathData::AnonConst,
 342                     ExpnId::root(),
 343                     arg.span,
 344                 );
 345
 346                 let anon_const = AnonConst { id: node_id, value: arg };
 347                 generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
 348             } else {
 349                 real_args.push(arg);
 350             }
 351         }
 352
 353         // Add generic args to the last element of the path.
 354         let last_segment = path.segments.last_mut().unwrap();
 355         assert!(last_segment.args.is_none());
 356         last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
 357             span: DUMMY_SP,
 358             args: generic_args,
 359         })));
 360
 361         // Now lower everything as normal.
 362         let f = self.lower_expr(&f);
 363         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
 364     }
 365
 366     fn lower_expr_if(
 367         &mut self,
 368         cond: &Expr,
 369         then: &Block,
 370         else_opt: Option<&Expr>,
 371     ) -> hir::ExprKind<'hir> {
 372         let lowered_cond = self.lower_expr(cond);
 373         let new_cond = self.manage_let_cond(lowered_cond);
 374         let then_expr = self.lower_block_expr(then);
 375         if let Some(rslt) = else_opt {
 376             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), Some(self.lower_expr(rslt)))
 377         } else {
 378             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), None)
 379         }
 380     }
 381
 382     // If `cond` kind is `let`, returns `let`. Otherwise, wraps and returns `cond`
 383     // in a temporary block.
 384     fn manage_let_cond(&mut self, cond: &'hir hir::Expr<'hir>) -> &'hir hir::Expr<'hir> {
 385         match cond.kind {
 386             hir::ExprKind::Let(..) => cond,
 387             _ => {
 388                 let span_block =
 389                     self.mark_span_with_reason(DesugaringKind::CondTemporary, cond.span, None);
 390                 self.expr_drop_temps(span_block, cond, AttrVec::new())
 391             }
 392         }
 393     }
 394
 395     // We desugar: `'label: while $cond $body` into:
 396     //
 397     // ```
 398     // 'label: loop {
 399     //   if { let _t = $cond; _t } {
 400     //     $body
 401     //   }
 402     //   else {
 403     //     break;
 404     //   }
 405     // }
 406     // ```
 407     //
 408     // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 409     // to preserve drop semantics since `while $cond { ... }` does not
 410     // let temporaries live outside of `cond`.
 411     fn lower_expr_while_in_loop_scope(
 412         &mut self,
 413         span: Span,
 414         cond: &Expr,
 415         body: &Block,
 416         opt_label: Option<Label>,
 417     ) -> hir::ExprKind<'hir> {
 418         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
 419         let new_cond = self.manage_let_cond(lowered_cond);
 420         let then = self.lower_block_expr(body);
 421         let expr_break = self.expr_break(span, ThinVec::new());
 422         let stmt_break = self.stmt_expr(span, expr_break);
 423         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
 424         let else_expr = self.arena.alloc(self.expr_block(else_blk, ThinVec::new()));
 425         let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
 426         let if_expr = self.expr(span, if_kind, ThinVec::new());
 427         let block = self.block_expr(self.arena.alloc(if_expr));
 428         let span = self.lower_span(span.with_hi(cond.span.hi()));
 429         let opt_label = self.lower_label(opt_label);
 430         hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
 431     }
 432
 433     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
 434     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
 435     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 436     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 437         self.with_catch_scope(body.id, |this| {
 438             let mut block = this.lower_block_noalloc(body, true);
 439
 440             // Final expression of the block (if present) or `()` with span at the end of block
 441             let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
 442                 (
 443                     this.mark_span_with_reason(
 444                         DesugaringKind::TryBlock,
 445                         expr.span,
 446                         this.allow_try_trait.clone(),
 447                     ),
 448                     expr,
 449                 )
 450             } else {
 451                 let try_span = this.mark_span_with_reason(
 452                     DesugaringKind::TryBlock,
 453                     this.sess.source_map().end_point(body.span),
 454                     this.allow_try_trait.clone(),
 455                 );
 456
 457                 (try_span, this.expr_unit(try_span))
 458             };
 459
 460             let ok_wrapped_span =
 461                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 462
 463             // `::std::ops::Try::from_output($tail_expr)`
 464             block.expr = Some(this.wrap_in_try_constructor(
 465                 hir::LangItem::TryTraitFromOutput,
 466                 try_span,
 467                 tail_expr,
 468                 ok_wrapped_span,
 469             ));
 470
 471             hir::ExprKind::Block(this.arena.alloc(block), None)
 472         })
 473     }
 474
 475     fn wrap_in_try_constructor(
 476         &mut self,
 477         lang_item: hir::LangItem,
 478         method_span: Span,
 479         expr: &'hir hir::Expr<'hir>,
 480         overall_span: Span,
 481     ) -> &'hir hir::Expr<'hir> {
 482         let constructor =
 483             self.arena.alloc(self.expr_lang_item_path(method_span, lang_item, ThinVec::new()));
 484         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 485     }
 486
 487     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 488         let pat = self.lower_pat(&arm.pat);
 489         let guard = arm.guard.as_ref().map(|cond| {
 490             if let ExprKind::Let(ref pat, ref scrutinee, _) = cond.kind {
 491                 hir::Guard::IfLet(self.lower_pat(pat), self.lower_expr(scrutinee))
 492             } else {
 493                 hir::Guard::If(self.lower_expr(cond))
 494             }
 495         });
 496         let hir_id = self.next_id();
 497         self.lower_attrs(hir_id, &arm.attrs);
 498         hir::Arm {
 499             hir_id,
 500             pat,
 501             guard,
 502             body: self.lower_expr(&arm.body),
 503             span: self.lower_span(arm.span),
 504         }
 505     }
 506
 507     /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
 508     ///
 509     /// This results in:
 510     ///
 511     /// ```text
 512     /// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
 513     ///     <body>
 514     /// })
 515     /// ```
 516     pub(super) fn make_async_expr(
 517         &mut self,
 518         capture_clause: CaptureBy,
 519         closure_node_id: NodeId,
 520         ret_ty: Option<AstP<Ty>>,
 521         span: Span,
 522         async_gen_kind: hir::AsyncGeneratorKind,
 523         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 524     ) -> hir::ExprKind<'hir> {
 525         let output = match ret_ty {
 526             Some(ty) => hir::FnRetTy::Return(self.lower_ty(&ty, ImplTraitContext::disallowed())),
 527             None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
 528         };
 529
 530         // Resume argument type. We let the compiler infer this to simplify the lowering. It is
 531         // fully constrained by `future::from_generator`.
 532         let input_ty = hir::Ty {
 533             hir_id: self.next_id(),
 534             kind: hir::TyKind::Infer,
 535             span: self.lower_span(span),
 536         };
 537
 538         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 539         let decl = self.arena.alloc(hir::FnDecl {
 540             inputs: arena_vec![self; input_ty],
 541             output,
 542             c_variadic: false,
 543             implicit_self: hir::ImplicitSelfKind::None,
 544         });
 545
 546         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 547         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 548             span,
 549             Ident::with_dummy_span(sym::_task_context),
 550             hir::BindingAnnotation::Mutable,
 551         );
 552         let param = hir::Param {
 553             hir_id: self.next_id(),
 554             pat,
 555             ty_span: self.lower_span(span),
 556             span: self.lower_span(span),
 557         };
 558         let params = arena_vec![self; param];
 559
 560         let body_id = self.lower_body(move |this| {
 561             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 562
 563             let old_ctx = this.task_context;
 564             this.task_context = Some(task_context_hid);
 565             let res = body(this);
 566             this.task_context = old_ctx;
 567             (params, res)
 568         });
 569
 570         // `static |_task_context| -> <ret_ty> { body }`:
 571         let generator_kind = hir::ExprKind::Closure(
 572             capture_clause,
 573             decl,
 574             body_id,
 575             self.lower_span(span),
 576             Some(hir::Movability::Static),
 577         );
 578         let generator = hir::Expr {
 579             hir_id: self.lower_node_id(closure_node_id),
 580             kind: generator_kind,
 581             span: self.lower_span(span),
 582         };
 583
 584         // `future::from_generator`:
 585         let unstable_span =
 586             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 587         let gen_future =
 588             self.expr_lang_item_path(unstable_span, hir::LangItem::FromGenerator, ThinVec::new());
 589
 590         // `future::from_generator(generator)`:
 591         hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
 592     }
 593
 594     /// Desugar `<expr>.await` into:
 595     /// ```rust
 596     /// match <expr> {
 597     ///     mut pinned => loop {
 598     ///         match unsafe { ::std::future::Future::poll(
 599     ///             <::std::pin::Pin>::new_unchecked(&mut pinned),
 600     ///             ::std::future::get_context(task_context),
 601     ///         ) } {
 602     ///             ::std::task::Poll::Ready(result) => break result,
 603     ///             ::std::task::Poll::Pending => {}
 604     ///         }
 605     ///         task_context = yield ();
 606     ///     }
 607     /// }
 608     /// ```
 609     fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 610         match self.generator_kind {
 611             Some(hir::GeneratorKind::Async(_)) => {}
 612             Some(hir::GeneratorKind::Gen) | None => {
 613                 let mut err = struct_span_err!(
 614                     self.sess,
 615                     await_span,
 616                     E0728,
 617                     "`await` is only allowed inside `async` functions and blocks"
 618                 );
 619                 err.span_label(await_span, "only allowed inside `async` functions and blocks");
 620                 if let Some(item_sp) = self.current_item {
 621                     err.span_label(item_sp, "this is not `async`");
 622                 }
 623                 err.emit();
 624             }
 625         }
 626         let span = self.mark_span_with_reason(DesugaringKind::Await, await_span, None);
 627         let gen_future_span = self.mark_span_with_reason(
 628             DesugaringKind::Await,
 629             await_span,
 630             self.allow_gen_future.clone(),
 631         );
 632         let expr = self.lower_expr(expr);
 633
 634         let pinned_ident = Ident::with_dummy_span(sym::pinned);
 635         let (pinned_pat, pinned_pat_hid) =
 636             self.pat_ident_binding_mode(span, pinned_ident, hir::BindingAnnotation::Mutable);
 637
 638         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 639
 640         // unsafe {
 641         //     ::std::future::Future::poll(
 642         //         ::std::pin::Pin::new_unchecked(&mut pinned),
 643         //         ::std::future::get_context(task_context),
 644         //     )
 645         // }
 646         let poll_expr = {
 647             let pinned = self.expr_ident(span, pinned_ident, pinned_pat_hid);
 648             let ref_mut_pinned = self.expr_mut_addr_of(span, pinned);
 649             let task_context = if let Some(task_context_hid) = self.task_context {
 650                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 651             } else {
 652                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 653                 self.expr_err(span)
 654             };
 655             let new_unchecked = self.expr_call_lang_item_fn_mut(
 656                 span,
 657                 hir::LangItem::PinNewUnchecked,
 658                 arena_vec![self; ref_mut_pinned],
 659             );
 660             let get_context = self.expr_call_lang_item_fn_mut(
 661                 gen_future_span,
 662                 hir::LangItem::GetContext,
 663                 arena_vec![self; task_context],
 664             );
 665             let call = self.expr_call_lang_item_fn(
 666                 span,
 667                 hir::LangItem::FuturePoll,
 668                 arena_vec![self; new_unchecked, get_context],
 669             );
 670             self.arena.alloc(self.expr_unsafe(call))
 671         };
 672
 673         // `::std::task::Poll::Ready(result) => break result`
 674         let loop_node_id = self.resolver.next_node_id();
 675         let loop_hir_id = self.lower_node_id(loop_node_id);
 676         let ready_arm = {
 677             let x_ident = Ident::with_dummy_span(sym::result);
 678             let (x_pat, x_pat_hid) = self.pat_ident(span, x_ident);
 679             let x_expr = self.expr_ident(span, x_ident, x_pat_hid);
 680             let ready_field = self.single_pat_field(span, x_pat);
 681             let ready_pat = self.pat_lang_item_variant(span, hir::LangItem::PollReady, ready_field);
 682             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 683                 let expr_break =
 684                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 685                 this.arena.alloc(this.expr(await_span, expr_break, ThinVec::new()))
 686             });
 687             self.arm(ready_pat, break_x)
 688         };
 689
 690         // `::std::task::Poll::Pending => {}`
 691         let pending_arm = {
 692             let pending_pat = self.pat_lang_item_variant(span, hir::LangItem::PollPending, &[]);
 693             let empty_block = self.expr_block_empty(span);
 694             self.arm(pending_pat, empty_block)
 695         };
 696
 697         let inner_match_stmt = {
 698             let match_expr = self.expr_match(
 699                 span,
 700                 poll_expr,
 701                 arena_vec![self; ready_arm, pending_arm],
 702                 hir::MatchSource::AwaitDesugar,
 703             );
 704             self.stmt_expr(span, match_expr)
 705         };
 706
 707         // task_context = yield ();
 708         let yield_stmt = {
 709             let unit = self.expr_unit(span);
 710             let yield_expr = self.expr(
 711                 span,
 712                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr.hir_id) }),
 713                 ThinVec::new(),
 714             );
 715             let yield_expr = self.arena.alloc(yield_expr);
 716
 717             if let Some(task_context_hid) = self.task_context {
 718                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 719                 let assign = self.expr(
 720                     span,
 721                     hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)),
 722                     AttrVec::new(),
 723                 );
 724                 self.stmt_expr(span, assign)
 725             } else {
 726                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 727                 // proceed with type checking.
 728                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 729             }
 730         };
 731
 732         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 733
 734         // loop { .. }
 735         let loop_expr = self.arena.alloc(hir::Expr {
 736             hir_id: loop_hir_id,
 737             kind: hir::ExprKind::Loop(
 738                 loop_block,
 739                 None,
 740                 hir::LoopSource::Loop,
 741                 self.lower_span(span),
 742             ),
 743             span: self.lower_span(span),
 744         });
 745
 746         // mut pinned => loop { ... }
 747         let pinned_arm = self.arm(pinned_pat, loop_expr);
 748
 749         // match <expr> {
 750         //     mut pinned => loop { .. }
 751         // }
 752         hir::ExprKind::Match(expr, arena_vec![self; pinned_arm], hir::MatchSource::AwaitDesugar)
 753     }
 754
 755     fn lower_expr_closure(
 756         &mut self,
 757         capture_clause: CaptureBy,
 758         movability: Movability,
 759         decl: &FnDecl,
 760         body: &Expr,
 761         fn_decl_span: Span,
 762     ) -> hir::ExprKind<'hir> {
 763         let (body_id, generator_option) = self.with_new_scopes(move |this| {
 764             let prev = this.current_item;
 765             this.current_item = Some(fn_decl_span);
 766             let mut generator_kind = None;
 767             let body_id = this.lower_fn_body(decl, |this| {
 768                 let e = this.lower_expr_mut(body);
 769                 generator_kind = this.generator_kind;
 770                 e
 771             });
 772             let generator_option =
 773                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 774             this.current_item = prev;
 775             (body_id, generator_option)
 776         });
 777
 778         // Lower outside new scope to preserve `is_in_loop_condition`.
 779         let fn_decl = self.lower_fn_decl(decl, None, false, None);
 780
 781         hir::ExprKind::Closure(
 782             capture_clause,
 783             fn_decl,
 784             body_id,
 785             self.lower_span(fn_decl_span),
 786             generator_option,
 787         )
 788     }
 789
 790     fn generator_movability_for_fn(
 791         &mut self,
 792         decl: &FnDecl,
 793         fn_decl_span: Span,
 794         generator_kind: Option<hir::GeneratorKind>,
 795         movability: Movability,
 796     ) -> Option<hir::Movability> {
 797         match generator_kind {
 798             Some(hir::GeneratorKind::Gen) => {
 799                 if decl.inputs.len() > 1 {
 800                     struct_span_err!(
 801                         self.sess,
 802                         fn_decl_span,
 803                         E0628,
 804                         "too many parameters for a generator (expected 0 or 1 parameters)"
 805                     )
 806                     .emit();
 807                 }
 808                 Some(movability)
 809             }
 810             Some(hir::GeneratorKind::Async(_)) => {
 811                 panic!("non-`async` closure body turned `async` during lowering");
 812             }
 813             None => {
 814                 if movability == Movability::Static {
 815                     struct_span_err!(self.sess, fn_decl_span, E0697, "closures cannot be static")
 816                         .emit();
 817                 }
 818                 None
 819             }
 820         }
 821     }
 822
 823     fn lower_expr_async_closure(
 824         &mut self,
 825         capture_clause: CaptureBy,
 826         closure_id: NodeId,
 827         decl: &FnDecl,
 828         body: &Expr,
 829         fn_decl_span: Span,
 830     ) -> hir::ExprKind<'hir> {
 831         let outer_decl =
 832             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 833
 834         let body_id = self.with_new_scopes(|this| {
 835             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 836             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 837                 struct_span_err!(
 838                     this.sess,
 839                     fn_decl_span,
 840                     E0708,
 841                     "`async` non-`move` closures with parameters are not currently supported",
 842                 )
 843                 .help(
 844                     "consider using `let` statements to manually capture \
 845                     variables by reference before entering an `async move` closure",
 846                 )
 847                 .emit();
 848             }
 849
 850             // Transform `async |x: u8| -> X { ... }` into
 851             // `|x: u8| future_from_generator(|| -> X { ... })`.
 852             let body_id = this.lower_fn_body(&outer_decl, |this| {
 853                 let async_ret_ty =
 854                     if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 855                 let async_body = this.make_async_expr(
 856                     capture_clause,
 857                     closure_id,
 858                     async_ret_ty,
 859                     body.span,
 860                     hir::AsyncGeneratorKind::Closure,
 861                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 862                 );
 863                 this.expr(fn_decl_span, async_body, ThinVec::new())
 864             });
 865             body_id
 866         });
 867
 868         // We need to lower the declaration outside the new scope, because we
 869         // have to conserve the state of being inside a loop condition for the
 870         // closure argument types.
 871         let fn_decl = self.lower_fn_decl(&outer_decl, None, false, None);
 872
 873         hir::ExprKind::Closure(
 874             capture_clause,
 875             fn_decl,
 876             body_id,
 877             self.lower_span(fn_decl_span),
 878             None,
 879         )
 880     }
 881
 882     /// Destructure the LHS of complex assignments.
 883     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
 884     fn lower_expr_assign(
 885         &mut self,
 886         lhs: &Expr,
 887         rhs: &Expr,
 888         eq_sign_span: Span,
 889         whole_span: Span,
 890     ) -> hir::ExprKind<'hir> {
 891         // Return early in case of an ordinary assignment.
 892         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
 893             match &lhs.kind {
 894                 ExprKind::Array(..)
 895                 | ExprKind::Struct(..)
 896                 | ExprKind::Tup(..)
 897                 | ExprKind::Underscore => false,
 898                 // Check for tuple struct constructor.
 899                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
 900                 ExprKind::Paren(e) => {
 901                     match e.kind {
 902                         // We special-case `(..)` for consistency with patterns.
 903                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
 904                         _ => is_ordinary(lower_ctx, e),
 905                     }
 906                 }
 907                 _ => true,
 908             }
 909         }
 910         if is_ordinary(self, lhs) {
 911             return hir::ExprKind::Assign(
 912                 self.lower_expr(lhs),
 913                 self.lower_expr(rhs),
 914                 self.lower_span(eq_sign_span),
 915             );
 916         }
 917         if !self.sess.features_untracked().destructuring_assignment {
 918             feature_err(
 919                 &self.sess.parse_sess,
 920                 sym::destructuring_assignment,
 921                 eq_sign_span,
 922                 "destructuring assignments are unstable",
 923             )
 924             .span_label(lhs.span, "cannot assign to this expression")
 925             .emit();
 926         }
 927
 928         let mut assignments = vec![];
 929
 930         // The LHS becomes a pattern: `(lhs1, lhs2)`.
 931         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
 932         let rhs = self.lower_expr(rhs);
 933
 934         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
 935         let destructure_let = self.stmt_let_pat(
 936             None,
 937             whole_span,
 938             Some(rhs),
 939             pat,
 940             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
 941         );
 942
 943         // `a = lhs1; b = lhs2;`.
 944         let stmts = self
 945             .arena
 946             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
 947
 948         // Wrap everything in a block.
 949         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
 950     }
 951
 952     /// If the given expression is a path to a tuple struct, returns that path.
 953     /// It is not a complete check, but just tries to reject most paths early
 954     /// if they are not tuple structs.
 955     /// Type checking will take care of the full validation later.
 956     fn extract_tuple_struct_path<'a>(
 957         &mut self,
 958         expr: &'a Expr,
 959     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
 960         if let ExprKind::Path(qself, path) = &expr.kind {
 961             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
 962             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
 963                 if partial_res.unresolved_segments() == 0
 964                     && !partial_res.base_res().expected_in_tuple_struct_pat()
 965                 {
 966                     return None;
 967                 }
 968             }
 969             return Some((qself, path));
 970         }
 971         None
 972     }
 973
 974     /// Convert the LHS of a destructuring assignment to a pattern.
 975     /// Each sub-assignment is recorded in `assignments`.
 976     fn destructure_assign(
 977         &mut self,
 978         lhs: &Expr,
 979         eq_sign_span: Span,
 980         assignments: &mut Vec<hir::Stmt<'hir>>,
 981     ) -> &'hir hir::Pat<'hir> {
 982         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
 983     }
 984
 985     fn destructure_assign_mut(
 986         &mut self,
 987         lhs: &Expr,
 988         eq_sign_span: Span,
 989         assignments: &mut Vec<hir::Stmt<'hir>>,
 990     ) -> hir::Pat<'hir> {
 991         match &lhs.kind {
 992             // Underscore pattern.
 993             ExprKind::Underscore => {
 994                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
 995             }
 996             // Slice patterns.
 997             ExprKind::Array(elements) => {
 998                 let (pats, rest) =
 999                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1000                 let slice_pat = if let Some((i, span)) = rest {
1001                     let (before, after) = pats.split_at(i);
1002                     hir::PatKind::Slice(
1003                         before,
1004                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1005                         after,
1006                     )
1007                 } else {
1008                     hir::PatKind::Slice(pats, None, &[])
1009                 };
1010                 return self.pat_without_dbm(lhs.span, slice_pat);
1011             }
1012             // Tuple structs.
1013             ExprKind::Call(callee, args) => {
1014                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1015                     let (pats, rest) = self.destructure_sequence(
1016                         args,
1017                         "tuple struct or variant",
1018                         eq_sign_span,
1019                         assignments,
1020                     );
1021                     let qpath = self.lower_qpath(
1022                         callee.id,
1023                         qself,
1024                         path,
1025                         ParamMode::Optional,
1026                         ImplTraitContext::disallowed(),
1027                     );
1028                     // Destructure like a tuple struct.
1029                     let tuple_struct_pat =
1030                         hir::PatKind::TupleStruct(qpath, pats, rest.map(|r| r.0));
1031                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1032                 }
1033             }
1034             // Structs.
1035             ExprKind::Struct(se) => {
1036                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1037                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1038                     hir::PatField {
1039                         hir_id: self.next_id(),
1040                         ident: self.lower_ident(f.ident),
1041                         pat,
1042                         is_shorthand: f.is_shorthand,
1043                         span: self.lower_span(f.span),
1044                     }
1045                 }));
1046                 let qpath = self.lower_qpath(
1047                     lhs.id,
1048                     &se.qself,
1049                     &se.path,
1050                     ParamMode::Optional,
1051                     ImplTraitContext::disallowed(),
1052                 );
1053                 let fields_omitted = match &se.rest {
1054                     StructRest::Base(e) => {
1055                         self.sess
1056                             .struct_span_err(
1057                                 e.span,
1058                                 "functional record updates are not allowed in destructuring \
1059                                     assignments",
1060                             )
1061                             .span_suggestion(
1062                                 e.span,
1063                                 "consider removing the trailing pattern",
1064                                 String::new(),
1065                                 rustc_errors::Applicability::MachineApplicable,
1066                             )
1067                             .emit();
1068                         true
1069                     }
1070                     StructRest::Rest(_) => true,
1071                     StructRest::None => false,
1072                 };
1073                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1074                 return self.pat_without_dbm(lhs.span, struct_pat);
1075             }
1076             // Tuples.
1077             ExprKind::Tup(elements) => {
1078                 let (pats, rest) =
1079                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1080                 let tuple_pat = hir::PatKind::Tuple(pats, rest.map(|r| r.0));
1081                 return self.pat_without_dbm(lhs.span, tuple_pat);
1082             }
1083             ExprKind::Paren(e) => {
1084                 // We special-case `(..)` for consistency with patterns.
1085                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1086                     let tuple_pat = hir::PatKind::Tuple(&[], Some(0));
1087                     return self.pat_without_dbm(lhs.span, tuple_pat);
1088                 } else {
1089                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1090                 }
1091             }
1092             _ => {}
1093         }
1094         // Treat all other cases as normal lvalue.
1095         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1096         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1097         let ident = self.expr_ident(lhs.span, ident, binding);
1098         let assign =
1099             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1100         let expr = self.expr(lhs.span, assign, ThinVec::new());
1101         assignments.push(self.stmt_expr(lhs.span, expr));
1102         pat
1103     }
1104
1105     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1106     /// Such a sequence occurs in a tuple (struct)/slice.
1107     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1108     /// exists.
1109     /// Each sub-assignment is recorded in `assignments`.
1110     fn destructure_sequence(
1111         &mut self,
1112         elements: &[AstP<Expr>],
1113         ctx: &str,
1114         eq_sign_span: Span,
1115         assignments: &mut Vec<hir::Stmt<'hir>>,
1116     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1117         let mut rest = None;
1118         let elements =
1119             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1120                 // Check for `..` pattern.
1121                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1122                     if let Some((_, prev_span)) = rest {
1123                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1124                     } else {
1125                         rest = Some((i, e.span));
1126                     }
1127                     None
1128                 } else {
1129                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1130                 }
1131             }));
1132         (elements, rest)
1133     }
1134
1135     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1136     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1137         let e1 = self.lower_expr_mut(e1);
1138         let e2 = self.lower_expr_mut(e2);
1139         let fn_path = hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span));
1140         let fn_expr =
1141             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
1142         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1143     }
1144
1145     fn lower_expr_range(
1146         &mut self,
1147         span: Span,
1148         e1: Option<&Expr>,
1149         e2: Option<&Expr>,
1150         lims: RangeLimits,
1151     ) -> hir::ExprKind<'hir> {
1152         use rustc_ast::RangeLimits::*;
1153
1154         let lang_item = match (e1, e2, lims) {
1155             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1156             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1157             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1158             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1159             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1160             (Some(..), Some(..), Closed) => unreachable!(),
1161             (_, None, Closed) => self.diagnostic().span_fatal(span, "inclusive range with no end"),
1162         };
1163
1164         let fields = self.arena.alloc_from_iter(
1165             e1.iter().map(|e| ("start", e)).chain(e2.iter().map(|e| ("end", e))).map(|(s, e)| {
1166                 let expr = self.lower_expr(&e);
1167                 let ident = Ident::new(Symbol::intern(s), self.lower_span(e.span));
1168                 self.expr_field(ident, expr, e.span)
1169             }),
1170         );
1171
1172         hir::ExprKind::Struct(
1173             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span))),
1174             fields,
1175             None,
1176         )
1177     }
1178
1179     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1180         let label = opt_label?;
1181         Some(Label { ident: self.lower_ident(label.ident) })
1182     }
1183
1184     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1185         let target_id = match destination {
1186             Some((id, _)) => {
1187                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1188                     Ok(self.lower_node_id(loop_id))
1189                 } else {
1190                     Err(hir::LoopIdError::UnresolvedLabel)
1191                 }
1192             }
1193             None => self
1194                 .loop_scope
1195                 .map(|id| Ok(self.lower_node_id(id)))
1196                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1197         };
1198         let label = self.lower_label(destination.map(|(_, label)| label));
1199         hir::Destination { label, target_id }
1200     }
1201
1202     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1203         if self.is_in_loop_condition && opt_label.is_none() {
1204             hir::Destination {
1205                 label: None,
1206                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1207             }
1208         } else {
1209             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1210         }
1211     }
1212
1213     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1214         let old_scope = self.catch_scope.replace(catch_id);
1215         let result = f(self);
1216         self.catch_scope = old_scope;
1217         result
1218     }
1219
1220     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1221         // We're no longer in the base loop's condition; we're in another loop.
1222         let was_in_loop_condition = self.is_in_loop_condition;
1223         self.is_in_loop_condition = false;
1224
1225         let old_scope = self.loop_scope.replace(loop_id);
1226         let result = f(self);
1227         self.loop_scope = old_scope;
1228
1229         self.is_in_loop_condition = was_in_loop_condition;
1230
1231         result
1232     }
1233
1234     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1235         let was_in_loop_condition = self.is_in_loop_condition;
1236         self.is_in_loop_condition = true;
1237
1238         let result = f(self);
1239
1240         self.is_in_loop_condition = was_in_loop_condition;
1241
1242         result
1243     }
1244
1245     fn lower_expr_llvm_asm(&mut self, asm: &LlvmInlineAsm) -> hir::ExprKind<'hir> {
1246         let inner = hir::LlvmInlineAsmInner {
1247             inputs: asm.inputs.iter().map(|&(c, _)| c).collect(),
1248             outputs: asm
1249                 .outputs
1250                 .iter()
1251                 .map(|out| hir::LlvmInlineAsmOutput {
1252                     constraint: out.constraint,
1253                     is_rw: out.is_rw,
1254                     is_indirect: out.is_indirect,
1255                     span: self.lower_span(out.expr.span),
1256                 })
1257                 .collect(),
1258             asm: asm.asm,
1259             asm_str_style: asm.asm_str_style,
1260             clobbers: asm.clobbers.clone(),
1261             volatile: asm.volatile,
1262             alignstack: asm.alignstack,
1263             dialect: asm.dialect,
1264         };
1265         let hir_asm = hir::LlvmInlineAsm {
1266             inner,
1267             inputs_exprs: self.arena.alloc_from_iter(
1268                 asm.inputs.iter().map(|&(_, ref input)| self.lower_expr_mut(input)),
1269             ),
1270             outputs_exprs: self
1271                 .arena
1272                 .alloc_from_iter(asm.outputs.iter().map(|out| self.lower_expr_mut(&out.expr))),
1273         };
1274         hir::ExprKind::LlvmInlineAsm(self.arena.alloc(hir_asm))
1275     }
1276
1277     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1278         hir::ExprField {
1279             hir_id: self.next_id(),
1280             ident: self.lower_ident(f.ident),
1281             expr: self.lower_expr(&f.expr),
1282             span: self.lower_span(f.span),
1283             is_shorthand: f.is_shorthand,
1284         }
1285     }
1286
1287     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1288         match self.generator_kind {
1289             Some(hir::GeneratorKind::Gen) => {}
1290             Some(hir::GeneratorKind::Async(_)) => {
1291                 struct_span_err!(
1292                     self.sess,
1293                     span,
1294                     E0727,
1295                     "`async` generators are not yet supported"
1296                 )
1297                 .emit();
1298             }
1299             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1300         }
1301
1302         let expr =
1303             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1304
1305         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1306     }
1307
1308     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1309     /// ```rust
1310     /// {
1311     ///     let result = match ::std::iter::IntoIterator::into_iter(<head>) {
1312     ///         mut iter => {
1313     ///             [opt_ident]: loop {
1314     ///                 let mut __next;
1315     ///                 match ::std::iter::Iterator::next(&mut iter) {
1316     ///                     ::std::option::Option::Some(val) => __next = val,
1317     ///                     ::std::option::Option::None => break
1318     ///                 };
1319     ///                 let <pat> = __next;
1320     ///                 StmtKind::Expr(<body>);
1321     ///             }
1322     ///         }
1323     ///     };
1324     ///     result
1325     /// }
1326     /// ```
1327     fn lower_expr_for(
1328         &mut self,
1329         e: &Expr,
1330         pat: &Pat,
1331         head: &Expr,
1332         body: &Block,
1333         opt_label: Option<Label>,
1334     ) -> hir::Expr<'hir> {
1335         let orig_head_span = head.span;
1336         // expand <head>
1337         let mut head = self.lower_expr_mut(head);
1338         let desugared_span = self.mark_span_with_reason(
1339             DesugaringKind::ForLoop(ForLoopLoc::Head),
1340             orig_head_span,
1341             None,
1342         );
1343         head.span = self.lower_span(desugared_span);
1344
1345         let iter = Ident::with_dummy_span(sym::iter);
1346
1347         let next_ident = Ident::with_dummy_span(sym::__next);
1348         let (next_pat, next_pat_hid) = self.pat_ident_binding_mode(
1349             desugared_span,
1350             next_ident,
1351             hir::BindingAnnotation::Mutable,
1352         );
1353
1354         // `::std::option::Option::Some(val) => __next = val`
1355         let pat_arm = {
1356             let val_ident = Ident::with_dummy_span(sym::val);
1357             let (val_pat, val_pat_hid) = self.pat_ident(pat.span, val_ident);
1358             let val_expr = self.expr_ident(pat.span, val_ident, val_pat_hid);
1359             let next_expr = self.expr_ident(pat.span, next_ident, next_pat_hid);
1360             let assign = self.arena.alloc(self.expr(
1361                 pat.span,
1362                 hir::ExprKind::Assign(next_expr, val_expr, self.lower_span(pat.span)),
1363                 ThinVec::new(),
1364             ));
1365             let some_pat = self.pat_some(pat.span, val_pat);
1366             self.arm(some_pat, assign)
1367         };
1368
1369         // `::std::option::Option::None => break`
1370         let break_arm = {
1371             let break_expr =
1372                 self.with_loop_scope(e.id, |this| this.expr_break_alloc(e.span, ThinVec::new()));
1373             let pat = self.pat_none(e.span);
1374             self.arm(pat, break_expr)
1375         };
1376
1377         // `mut iter`
1378         let (iter_pat, iter_pat_nid) =
1379             self.pat_ident_binding_mode(desugared_span, iter, hir::BindingAnnotation::Mutable);
1380
1381         // `match ::std::iter::Iterator::next(&mut iter) { ... }`
1382         let match_expr = {
1383             let iter = self.expr_ident(desugared_span, iter, iter_pat_nid);
1384             let ref_mut_iter = self.expr_mut_addr_of(desugared_span, iter);
1385             let next_expr = self.expr_call_lang_item_fn(
1386                 desugared_span,
1387                 hir::LangItem::IteratorNext,
1388                 arena_vec![self; ref_mut_iter],
1389             );
1390             let arms = arena_vec![self; pat_arm, break_arm];
1391
1392             self.expr_match(desugared_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1393         };
1394         let match_stmt = self.stmt_expr(desugared_span, match_expr);
1395
1396         let next_expr = self.expr_ident(desugared_span, next_ident, next_pat_hid);
1397
1398         // `let mut __next`
1399         let next_let = self.stmt_let_pat(
1400             None,
1401             desugared_span,
1402             None,
1403             next_pat,
1404             hir::LocalSource::ForLoopDesugar,
1405         );
1406
1407         // `let <pat> = __next`
1408         let pat = self.lower_pat(pat);
1409         let pat_let = self.stmt_let_pat(
1410             None,
1411             desugared_span,
1412             Some(next_expr),
1413             pat,
1414             hir::LocalSource::ForLoopDesugar,
1415         );
1416
1417         let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1418         let body_expr = self.expr_block(body_block, ThinVec::new());
1419         let body_stmt = self.stmt_expr(body.span, body_expr);
1420
1421         let loop_block = self.block_all(
1422             e.span,
1423             arena_vec![self; next_let, match_stmt, pat_let, body_stmt],
1424             None,
1425         );
1426
1427         // `[opt_ident]: loop { ... }`
1428         let kind = hir::ExprKind::Loop(
1429             loop_block,
1430             self.lower_label(opt_label),
1431             hir::LoopSource::ForLoop,
1432             self.lower_span(e.span.with_hi(orig_head_span.hi())),
1433         );
1434         let loop_expr = self.arena.alloc(hir::Expr {
1435             hir_id: self.lower_node_id(e.id),
1436             kind,
1437             span: self.lower_span(e.span),
1438         });
1439
1440         // `mut iter => { ... }`
1441         let iter_arm = self.arm(iter_pat, loop_expr);
1442
1443         let into_iter_span = self.mark_span_with_reason(
1444             DesugaringKind::ForLoop(ForLoopLoc::IntoIter),
1445             orig_head_span,
1446             None,
1447         );
1448
1449         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1450         let into_iter_expr = {
1451             self.expr_call_lang_item_fn(
1452                 into_iter_span,
1453                 hir::LangItem::IntoIterIntoIter,
1454                 arena_vec![self; head],
1455             )
1456         };
1457
1458         // #82462: to correctly diagnose borrow errors, the block that contains
1459         // the iter expr needs to have a span that covers the loop body.
1460         let desugared_full_span =
1461             self.mark_span_with_reason(DesugaringKind::ForLoop(ForLoopLoc::Head), e.span, None);
1462
1463         let match_expr = self.arena.alloc(self.expr_match(
1464             desugared_full_span,
1465             into_iter_expr,
1466             arena_vec![self; iter_arm],
1467             hir::MatchSource::ForLoopDesugar,
1468         ));
1469
1470         let attrs: Vec<_> = e.attrs.iter().map(|a| self.lower_attr(a)).collect();
1471
1472         // This is effectively `{ let _result = ...; _result }`.
1473         // The construct was introduced in #21984 and is necessary to make sure that
1474         // temporaries in the `head` expression are dropped and do not leak to the
1475         // surrounding scope of the `match` since the `match` is not a terminating scope.
1476         //
1477         // Also, add the attributes to the outer returned expr node.
1478         self.expr_drop_temps_mut(desugared_full_span, match_expr, attrs.into())
1479     }
1480
1481     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1482     /// ```rust
1483     /// match Try::branch(<expr>) {
1484     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1485     ///     ControlFlow::Break(residual) =>
1486     ///         #[allow(unreachable_code)]
1487     ///         // If there is an enclosing `try {...}`:
1488     ///         break 'catch_target Try::from_residual(residual),
1489     ///         // Otherwise:
1490     ///         return Try::from_residual(residual),
1491     /// }
1492     /// ```
1493     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1494         let unstable_span = self.mark_span_with_reason(
1495             DesugaringKind::QuestionMark,
1496             span,
1497             self.allow_try_trait.clone(),
1498         );
1499         let try_span = self.sess.source_map().end_point(span);
1500         let try_span = self.mark_span_with_reason(
1501             DesugaringKind::QuestionMark,
1502             try_span,
1503             self.allow_try_trait.clone(),
1504         );
1505
1506         // `Try::branch(<expr>)`
1507         let scrutinee = {
1508             // expand <expr>
1509             let sub_expr = self.lower_expr_mut(sub_expr);
1510
1511             self.expr_call_lang_item_fn(
1512                 unstable_span,
1513                 hir::LangItem::TryTraitBranch,
1514                 arena_vec![self; sub_expr],
1515             )
1516         };
1517
1518         // `#[allow(unreachable_code)]`
1519         let attr = {
1520             // `allow(unreachable_code)`
1521             let allow = {
1522                 let allow_ident = Ident::new(sym::allow, self.lower_span(span));
1523                 let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
1524                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1525                 attr::mk_list_item(allow_ident, vec![uc_nested])
1526             };
1527             attr::mk_attr_outer(allow)
1528         };
1529         let attrs = vec![attr];
1530
1531         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1532         let continue_arm = {
1533             let val_ident = Ident::with_dummy_span(sym::val);
1534             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1535             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1536                 span,
1537                 val_ident,
1538                 val_pat_nid,
1539                 ThinVec::from(attrs.clone()),
1540             ));
1541             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1542             self.arm(continue_pat, val_expr)
1543         };
1544
1545         // `ControlFlow::Break(residual) =>
1546         //     #[allow(unreachable_code)]
1547         //     return Try::from_residual(residual),`
1548         let break_arm = {
1549             let residual_ident = Ident::with_dummy_span(sym::residual);
1550             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1551             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1552             let from_residual_expr = self.wrap_in_try_constructor(
1553                 hir::LangItem::TryTraitFromResidual,
1554                 try_span,
1555                 self.arena.alloc(residual_expr),
1556                 unstable_span,
1557             );
1558             let thin_attrs = ThinVec::from(attrs);
1559             let ret_expr = if let Some(catch_node) = self.catch_scope {
1560                 let target_id = Ok(self.lower_node_id(catch_node));
1561                 self.arena.alloc(self.expr(
1562                     try_span,
1563                     hir::ExprKind::Break(
1564                         hir::Destination { label: None, target_id },
1565                         Some(from_residual_expr),
1566                     ),
1567                     thin_attrs,
1568                 ))
1569             } else {
1570                 self.arena.alloc(self.expr(
1571                     try_span,
1572                     hir::ExprKind::Ret(Some(from_residual_expr)),
1573                     thin_attrs,
1574                 ))
1575             };
1576
1577             let break_pat = self.pat_cf_break(try_span, residual_local);
1578             self.arm(break_pat, ret_expr)
1579         };
1580
1581         hir::ExprKind::Match(
1582             scrutinee,
1583             arena_vec![self; break_arm, continue_arm],
1584             hir::MatchSource::TryDesugar,
1585         )
1586     }
1587
1588     // =========================================================================
1589     // Helper methods for building HIR.
1590     // =========================================================================
1591
1592     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1593     ///
1594     /// In terms of drop order, it has the same effect as wrapping `expr` in
1595     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1596     ///
1597     /// The drop order can be important in e.g. `if expr { .. }`.
1598     pub(super) fn expr_drop_temps(
1599         &mut self,
1600         span: Span,
1601         expr: &'hir hir::Expr<'hir>,
1602         attrs: AttrVec,
1603     ) -> &'hir hir::Expr<'hir> {
1604         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1605     }
1606
1607     pub(super) fn expr_drop_temps_mut(
1608         &mut self,
1609         span: Span,
1610         expr: &'hir hir::Expr<'hir>,
1611         attrs: AttrVec,
1612     ) -> hir::Expr<'hir> {
1613         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1614     }
1615
1616     fn expr_match(
1617         &mut self,
1618         span: Span,
1619         arg: &'hir hir::Expr<'hir>,
1620         arms: &'hir [hir::Arm<'hir>],
1621         source: hir::MatchSource,
1622     ) -> hir::Expr<'hir> {
1623         self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
1624     }
1625
1626     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
1627         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1628         self.expr(span, expr_break, attrs)
1629     }
1630
1631     fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1632         let expr_break = self.expr_break(span, attrs);
1633         self.arena.alloc(expr_break)
1634     }
1635
1636     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1637         self.expr(
1638             span,
1639             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1640             ThinVec::new(),
1641         )
1642     }
1643
1644     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1645         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
1646     }
1647
1648     fn expr_call_mut(
1649         &mut self,
1650         span: Span,
1651         e: &'hir hir::Expr<'hir>,
1652         args: &'hir [hir::Expr<'hir>],
1653     ) -> hir::Expr<'hir> {
1654         self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new())
1655     }
1656
1657     fn expr_call(
1658         &mut self,
1659         span: Span,
1660         e: &'hir hir::Expr<'hir>,
1661         args: &'hir [hir::Expr<'hir>],
1662     ) -> &'hir hir::Expr<'hir> {
1663         self.arena.alloc(self.expr_call_mut(span, e, args))
1664     }
1665
1666     fn expr_call_lang_item_fn_mut(
1667         &mut self,
1668         span: Span,
1669         lang_item: hir::LangItem,
1670         args: &'hir [hir::Expr<'hir>],
1671     ) -> hir::Expr<'hir> {
1672         let path = self.arena.alloc(self.expr_lang_item_path(span, lang_item, ThinVec::new()));
1673         self.expr_call_mut(span, path, args)
1674     }
1675
1676     fn expr_call_lang_item_fn(
1677         &mut self,
1678         span: Span,
1679         lang_item: hir::LangItem,
1680         args: &'hir [hir::Expr<'hir>],
1681     ) -> &'hir hir::Expr<'hir> {
1682         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args))
1683     }
1684
1685     fn expr_lang_item_path(
1686         &mut self,
1687         span: Span,
1688         lang_item: hir::LangItem,
1689         attrs: AttrVec,
1690     ) -> hir::Expr<'hir> {
1691         self.expr(
1692             span,
1693             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span))),
1694             attrs,
1695         )
1696     }
1697
1698     pub(super) fn expr_ident(
1699         &mut self,
1700         sp: Span,
1701         ident: Ident,
1702         binding: hir::HirId,
1703     ) -> &'hir hir::Expr<'hir> {
1704         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1705     }
1706
1707     pub(super) fn expr_ident_mut(
1708         &mut self,
1709         sp: Span,
1710         ident: Ident,
1711         binding: hir::HirId,
1712     ) -> hir::Expr<'hir> {
1713         self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
1714     }
1715
1716     fn expr_ident_with_attrs(
1717         &mut self,
1718         span: Span,
1719         ident: Ident,
1720         binding: hir::HirId,
1721         attrs: AttrVec,
1722     ) -> hir::Expr<'hir> {
1723         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1724             None,
1725             self.arena.alloc(hir::Path {
1726                 span: self.lower_span(span),
1727                 res: Res::Local(binding),
1728                 segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
1729             }),
1730         ));
1731
1732         self.expr(span, expr_path, attrs)
1733     }
1734
1735     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1736         let hir_id = self.next_id();
1737         let span = expr.span;
1738         self.expr(
1739             span,
1740             hir::ExprKind::Block(
1741                 self.arena.alloc(hir::Block {
1742                     stmts: &[],
1743                     expr: Some(expr),
1744                     hir_id,
1745                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1746                     span: self.lower_span(span),
1747                     targeted_by_break: false,
1748                 }),
1749                 None,
1750             ),
1751             ThinVec::new(),
1752         )
1753     }
1754
1755     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1756         let blk = self.block_all(span, &[], None);
1757         let expr = self.expr_block(blk, ThinVec::new());
1758         self.arena.alloc(expr)
1759     }
1760
1761     pub(super) fn expr_block(
1762         &mut self,
1763         b: &'hir hir::Block<'hir>,
1764         attrs: AttrVec,
1765     ) -> hir::Expr<'hir> {
1766         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1767     }
1768
1769     pub(super) fn expr(
1770         &mut self,
1771         span: Span,
1772         kind: hir::ExprKind<'hir>,
1773         attrs: AttrVec,
1774     ) -> hir::Expr<'hir> {
1775         let hir_id = self.next_id();
1776         self.lower_attrs(hir_id, &attrs);
1777         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1778     }
1779
1780     fn expr_field(
1781         &mut self,
1782         ident: Ident,
1783         expr: &'hir hir::Expr<'hir>,
1784         span: Span,
1785     ) -> hir::ExprField<'hir> {
1786         hir::ExprField {
1787             hir_id: self.next_id(),
1788             ident,
1789             span: self.lower_span(span),
1790             expr,
1791             is_shorthand: false,
1792         }
1793     }
1794
1795     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1796         hir::Arm {
1797             hir_id: self.next_id(),
1798             pat,
1799             guard: None,
1800             span: self.lower_span(expr.span),
1801             body: expr,
1802         }
1803     }
1804 }