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