src/librustc_ast_lowering/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::fx::FxHashMap;
   7 use rustc_data_structures::stack::ensure_sufficient_stack;
   8 use rustc_data_structures::thin_vec::ThinVec;
   9 use rustc_errors::struct_span_err;
  10 use rustc_hir as hir;
  11 use rustc_hir::def::Res;
  12 use rustc_span::hygiene::ForLoopLoc;
  13 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
  14 use rustc_span::symbol::{sym, Ident, Symbol};
  15 use rustc_target::asm;
  16 use std::collections::hash_map::Entry;
  17 use std::fmt::Write;
  18
  19 impl<'hir> LoweringContext<'_, 'hir> {
  20     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  21         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  22     }
  23
  24     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  25         self.arena.alloc(self.lower_expr_mut(e))
  26     }
  27
  28     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  29         ensure_sufficient_stack(|| {
  30             let kind = match e.kind {
  31                 ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  32                 ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  33                 ExprKind::Repeat(ref expr, ref count) => {
  34                     let expr = self.lower_expr(expr);
  35                     let count = self.lower_anon_const(count);
  36                     hir::ExprKind::Repeat(expr, count)
  37                 }
  38                 ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  39                 ExprKind::Call(ref f, ref args) => {
  40                     let f = self.lower_expr(f);
  41                     hir::ExprKind::Call(f, self.lower_exprs(args))
  42                 }
  43                 ExprKind::MethodCall(ref seg, ref args, span) => {
  44                     let hir_seg = self.arena.alloc(self.lower_path_segment(
  45                         e.span,
  46                         seg,
  47                         ParamMode::Optional,
  48                         0,
  49                         ParenthesizedGenericArgs::Err,
  50                         ImplTraitContext::disallowed(),
  51                         None,
  52                     ));
  53                     let args = self.lower_exprs(args);
  54                     hir::ExprKind::MethodCall(hir_seg, seg.ident.span, args, span)
  55                 }
  56                 ExprKind::Binary(binop, ref lhs, ref rhs) => {
  57                     let binop = self.lower_binop(binop);
  58                     let lhs = self.lower_expr(lhs);
  59                     let rhs = self.lower_expr(rhs);
  60                     hir::ExprKind::Binary(binop, lhs, rhs)
  61                 }
  62                 ExprKind::Unary(op, ref ohs) => {
  63                     let op = self.lower_unop(op);
  64                     let ohs = self.lower_expr(ohs);
  65                     hir::ExprKind::Unary(op, ohs)
  66                 }
  67                 ExprKind::Lit(ref l) => hir::ExprKind::Lit(respan(l.span, l.kind.clone())),
  68                 ExprKind::Cast(ref expr, ref ty) => {
  69                     let expr = self.lower_expr(expr);
  70                     let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
  71                     hir::ExprKind::Cast(expr, ty)
  72                 }
  73                 ExprKind::Type(ref expr, ref ty) => {
  74                     let expr = self.lower_expr(expr);
  75                     let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
  76                     hir::ExprKind::Type(expr, ty)
  77                 }
  78                 ExprKind::AddrOf(k, m, ref ohs) => {
  79                     let ohs = self.lower_expr(ohs);
  80                     hir::ExprKind::AddrOf(k, m, ohs)
  81                 }
  82                 ExprKind::Let(ref pat, ref scrutinee) => {
  83                     self.lower_expr_let(e.span, pat, scrutinee)
  84                 }
  85                 ExprKind::If(ref cond, ref then, ref else_opt) => {
  86                     self.lower_expr_if(e.span, cond, then, else_opt.as_deref())
  87                 }
  88                 ExprKind::While(ref cond, ref body, opt_label) => self
  89                     .with_loop_scope(e.id, |this| {
  90                         this.lower_expr_while_in_loop_scope(e.span, cond, body, opt_label)
  91                     }),
  92                 ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
  93                     hir::ExprKind::Loop(
  94                         this.lower_block(body, false),
  95                         opt_label,
  96                         hir::LoopSource::Loop,
  97                     )
  98                 }),
  99                 ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
 100                 ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
 101                     self.lower_expr(expr),
 102                     self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
 103                     hir::MatchSource::Normal,
 104                 ),
 105                 ExprKind::Async(capture_clause, closure_node_id, ref block) => self
 106                     .make_async_expr(
 107                         capture_clause,
 108                         closure_node_id,
 109                         None,
 110                         block.span,
 111                         hir::AsyncGeneratorKind::Block,
 112                         |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
 113                     ),
 114                 ExprKind::Await(ref expr) => self.lower_expr_await(e.span, expr),
 115                 ExprKind::Closure(
 116                     capture_clause,
 117                     asyncness,
 118                     movability,
 119                     ref decl,
 120                     ref body,
 121                     fn_decl_span,
 122                 ) => {
 123                     if let Async::Yes { closure_id, .. } = asyncness {
 124                         self.lower_expr_async_closure(
 125                             capture_clause,
 126                             closure_id,
 127                             decl,
 128                             body,
 129                             fn_decl_span,
 130                         )
 131                     } else {
 132                         self.lower_expr_closure(
 133                             capture_clause,
 134                             movability,
 135                             decl,
 136                             body,
 137                             fn_decl_span,
 138                         )
 139                     }
 140                 }
 141                 ExprKind::Block(ref blk, opt_label) => {
 142                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 143                 }
 144                 ExprKind::Assign(ref el, ref er, span) => {
 145                     hir::ExprKind::Assign(self.lower_expr(el), self.lower_expr(er), span)
 146                 }
 147                 ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 148                     self.lower_binop(op),
 149                     self.lower_expr(el),
 150                     self.lower_expr(er),
 151                 ),
 152                 ExprKind::Field(ref el, ident) => hir::ExprKind::Field(self.lower_expr(el), ident),
 153                 ExprKind::Index(ref el, ref er) => {
 154                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 155                 }
 156                 ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 157                     self.lower_expr_range_closed(e.span, e1, e2)
 158                 }
 159                 ExprKind::Range(ref e1, ref e2, lims) => {
 160                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 161                 }
 162                 ExprKind::Path(ref qself, ref path) => {
 163                     let qpath = self.lower_qpath(
 164                         e.id,
 165                         qself,
 166                         path,
 167                         ParamMode::Optional,
 168                         ImplTraitContext::disallowed(),
 169                     );
 170                     hir::ExprKind::Path(qpath)
 171                 }
 172                 ExprKind::Break(opt_label, ref opt_expr) => {
 173                     let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 174                     hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
 175                 }
 176                 ExprKind::Continue(opt_label) => {
 177                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
 178                 }
 179                 ExprKind::Ret(ref e) => {
 180                     let e = e.as_ref().map(|x| self.lower_expr(x));
 181                     hir::ExprKind::Ret(e)
 182                 }
 183                 ExprKind::InlineAsm(ref asm) => self.lower_expr_asm(e.span, asm),
 184                 ExprKind::LlvmInlineAsm(ref asm) => self.lower_expr_llvm_asm(asm),
 185                 ExprKind::Struct(ref path, ref fields, ref maybe_expr) => {
 186                     let maybe_expr = maybe_expr.as_ref().map(|x| self.lower_expr(x));
 187                     hir::ExprKind::Struct(
 188                         self.arena.alloc(self.lower_qpath(
 189                             e.id,
 190                             &None,
 191                             path,
 192                             ParamMode::Optional,
 193                             ImplTraitContext::disallowed(),
 194                         )),
 195                         self.arena.alloc_from_iter(fields.iter().map(|x| self.lower_field(x))),
 196                         maybe_expr,
 197                     )
 198                 }
 199                 ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 200                 ExprKind::Err => hir::ExprKind::Err,
 201                 ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 202                 ExprKind::Paren(ref ex) => {
 203                     let mut ex = self.lower_expr_mut(ex);
 204                     // Include parens in span, but only if it is a super-span.
 205                     if e.span.contains(ex.span) {
 206                         ex.span = e.span;
 207                     }
 208                     // Merge attributes into the inner expression.
 209                     let mut attrs = e.attrs.clone();
 210                     attrs.extend::<Vec<_>>(ex.attrs.into());
 211                     ex.attrs = attrs;
 212                     return ex;
 213                 }
 214
 215                 // Desugar `ExprForLoop`
 216                 // from: `[opt_ident]: for <pat> in <head> <body>`
 217                 ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 218                     return self.lower_expr_for(e, pat, head, body, opt_label);
 219                 }
 220                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 221             };
 222
 223             hir::Expr {
 224                 hir_id: self.lower_node_id(e.id),
 225                 kind,
 226                 span: e.span,
 227                 attrs: e.attrs.iter().map(|a| self.lower_attr(a)).collect::<Vec<_>>().into(),
 228             }
 229         })
 230     }
 231
 232     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 233         match u {
 234             UnOp::Deref => hir::UnOp::UnDeref,
 235             UnOp::Not => hir::UnOp::UnNot,
 236             UnOp::Neg => hir::UnOp::UnNeg,
 237         }
 238     }
 239
 240     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 241         Spanned {
 242             node: match b.node {
 243                 BinOpKind::Add => hir::BinOpKind::Add,
 244                 BinOpKind::Sub => hir::BinOpKind::Sub,
 245                 BinOpKind::Mul => hir::BinOpKind::Mul,
 246                 BinOpKind::Div => hir::BinOpKind::Div,
 247                 BinOpKind::Rem => hir::BinOpKind::Rem,
 248                 BinOpKind::And => hir::BinOpKind::And,
 249                 BinOpKind::Or => hir::BinOpKind::Or,
 250                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 251                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 252                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 253                 BinOpKind::Shl => hir::BinOpKind::Shl,
 254                 BinOpKind::Shr => hir::BinOpKind::Shr,
 255                 BinOpKind::Eq => hir::BinOpKind::Eq,
 256                 BinOpKind::Lt => hir::BinOpKind::Lt,
 257                 BinOpKind::Le => hir::BinOpKind::Le,
 258                 BinOpKind::Ne => hir::BinOpKind::Ne,
 259                 BinOpKind::Ge => hir::BinOpKind::Ge,
 260                 BinOpKind::Gt => hir::BinOpKind::Gt,
 261             },
 262             span: b.span,
 263         }
 264     }
 265
 266     /// Emit an error and lower `ast::ExprKind::Let(pat, scrutinee)` into:
 267     /// ```rust
 268     /// match scrutinee { pats => true, _ => false }
 269     /// ```
 270     fn lower_expr_let(&mut self, span: Span, pat: &Pat, scrutinee: &Expr) -> hir::ExprKind<'hir> {
 271         // If we got here, the `let` expression is not allowed.
 272
 273         if self.sess.opts.unstable_features.is_nightly_build() {
 274             self.sess
 275                 .struct_span_err(span, "`let` expressions are not supported here")
 276                 .note("only supported directly in conditions of `if`- and `while`-expressions")
 277                 .note("as well as when nested within `&&` and parenthesis in those conditions")
 278                 .emit();
 279         } else {
 280             self.sess
 281                 .struct_span_err(span, "expected expression, found statement (`let`)")
 282                 .note("variable declaration using `let` is a statement")
 283                 .emit();
 284         }
 285
 286         // For better recovery, we emit:
 287         // ```
 288         // match scrutinee { pat => true, _ => false }
 289         // ```
 290         // While this doesn't fully match the user's intent, it has key advantages:
 291         // 1. We can avoid using `abort_if_errors`.
 292         // 2. We can typeck both `pat` and `scrutinee`.
 293         // 3. `pat` is allowed to be refutable.
 294         // 4. The return type of the block is `bool` which seems like what the user wanted.
 295         let scrutinee = self.lower_expr(scrutinee);
 296         let then_arm = {
 297             let pat = self.lower_pat(pat);
 298             let expr = self.expr_bool(span, true);
 299             self.arm(pat, expr)
 300         };
 301         let else_arm = {
 302             let pat = self.pat_wild(span);
 303             let expr = self.expr_bool(span, false);
 304             self.arm(pat, expr)
 305         };
 306         hir::ExprKind::Match(
 307             scrutinee,
 308             arena_vec![self; then_arm, else_arm],
 309             hir::MatchSource::Normal,
 310         )
 311     }
 312
 313     fn lower_expr_if(
 314         &mut self,
 315         span: Span,
 316         cond: &Expr,
 317         then: &Block,
 318         else_opt: Option<&Expr>,
 319     ) -> hir::ExprKind<'hir> {
 320         // FIXME(#53667): handle lowering of && and parens.
 321
 322         // `_ => else_block` where `else_block` is `{}` if there's `None`:
 323         let else_pat = self.pat_wild(span);
 324         let (else_expr, contains_else_clause) = match else_opt {
 325             None => (self.expr_block_empty(span), false),
 326             Some(els) => (self.lower_expr(els), true),
 327         };
 328         let else_arm = self.arm(else_pat, else_expr);
 329
 330         // Handle then + scrutinee:
 331         let then_expr = self.lower_block_expr(then);
 332         let (then_pat, scrutinee, desugar) = match cond.kind {
 333             // `<pat> => <then>`:
 334             ExprKind::Let(ref pat, ref scrutinee) => {
 335                 let scrutinee = self.lower_expr(scrutinee);
 336                 let pat = self.lower_pat(pat);
 337                 (pat, scrutinee, hir::MatchSource::IfLetDesugar { contains_else_clause })
 338             }
 339             // `true => <then>`:
 340             _ => {
 341                 // Lower condition:
 342                 let cond = self.lower_expr(cond);
 343                 let span_block =
 344                     self.mark_span_with_reason(DesugaringKind::CondTemporary, cond.span, None);
 345                 // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 346                 // to preserve drop semantics since `if cond { ... }` does not
 347                 // let temporaries live outside of `cond`.
 348                 let cond = self.expr_drop_temps(span_block, cond, ThinVec::new());
 349                 let pat = self.pat_bool(span, true);
 350                 (pat, cond, hir::MatchSource::IfDesugar { contains_else_clause })
 351             }
 352         };
 353         let then_arm = self.arm(then_pat, self.arena.alloc(then_expr));
 354
 355         hir::ExprKind::Match(scrutinee, arena_vec![self; then_arm, else_arm], desugar)
 356     }
 357
 358     fn lower_expr_while_in_loop_scope(
 359         &mut self,
 360         span: Span,
 361         cond: &Expr,
 362         body: &Block,
 363         opt_label: Option<Label>,
 364     ) -> hir::ExprKind<'hir> {
 365         // FIXME(#53667): handle lowering of && and parens.
 366
 367         // Note that the block AND the condition are evaluated in the loop scope.
 368         // This is done to allow `break` from inside the condition of the loop.
 369
 370         // `_ => break`:
 371         let else_arm = {
 372             let else_pat = self.pat_wild(span);
 373             let else_expr = self.expr_break(span, ThinVec::new());
 374             self.arm(else_pat, else_expr)
 375         };
 376
 377         // Handle then + scrutinee:
 378         let then_expr = self.lower_block_expr(body);
 379         let (then_pat, scrutinee, desugar, source) = match cond.kind {
 380             ExprKind::Let(ref pat, ref scrutinee) => {
 381                 // to:
 382                 //
 383                 //   [opt_ident]: loop {
 384                 //     match <sub_expr> {
 385                 //       <pat> => <body>,
 386                 //       _ => break
 387                 //     }
 388                 //   }
 389                 let scrutinee = self.with_loop_condition_scope(|t| t.lower_expr(scrutinee));
 390                 let pat = self.lower_pat(pat);
 391                 (pat, scrutinee, hir::MatchSource::WhileLetDesugar, hir::LoopSource::WhileLet)
 392             }
 393             _ => {
 394                 // We desugar: `'label: while $cond $body` into:
 395                 //
 396                 // ```
 397                 // 'label: loop {
 398                 //     match drop-temps { $cond } {
 399                 //         true => $body,
 400                 //         _ => break,
 401                 //     }
 402                 // }
 403                 // ```
 404
 405                 // Lower condition:
 406                 let cond = self.with_loop_condition_scope(|this| this.lower_expr(cond));
 407                 let span_block =
 408                     self.mark_span_with_reason(DesugaringKind::CondTemporary, cond.span, None);
 409                 // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 410                 // to preserve drop semantics since `while cond { ... }` does not
 411                 // let temporaries live outside of `cond`.
 412                 let cond = self.expr_drop_temps(span_block, cond, ThinVec::new());
 413                 // `true => <then>`:
 414                 let pat = self.pat_bool(span, true);
 415                 (pat, cond, hir::MatchSource::WhileDesugar, hir::LoopSource::While)
 416             }
 417         };
 418         let then_arm = self.arm(then_pat, self.arena.alloc(then_expr));
 419
 420         // `match <scrutinee> { ... }`
 421         let match_expr =
 422             self.expr_match(span, scrutinee, arena_vec![self; then_arm, else_arm], desugar);
 423
 424         // `[opt_ident]: loop { ... }`
 425         hir::ExprKind::Loop(self.block_expr(self.arena.alloc(match_expr)), opt_label, source)
 426     }
 427
 428     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_ok(<expr>) }`,
 429     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_ok(()) }`
 430     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 431     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 432         self.with_catch_scope(body.id, |this| {
 433             let mut block = this.lower_block_noalloc(body, true);
 434
 435             let try_span = this.mark_span_with_reason(
 436                 DesugaringKind::TryBlock,
 437                 body.span,
 438                 this.allow_try_trait.clone(),
 439             );
 440
 441             // Final expression of the block (if present) or `()` with span at the end of block
 442             let tail_expr = block
 443                 .expr
 444                 .take()
 445                 .unwrap_or_else(|| this.expr_unit(this.sess.source_map().end_point(try_span)));
 446
 447             let ok_wrapped_span =
 448                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 449
 450             // `::std::ops::Try::from_ok($tail_expr)`
 451             block.expr = Some(this.wrap_in_try_constructor(
 452                 hir::LangItem::TryFromOk,
 453                 try_span,
 454                 tail_expr,
 455                 ok_wrapped_span,
 456             ));
 457
 458             hir::ExprKind::Block(this.arena.alloc(block), None)
 459         })
 460     }
 461
 462     fn wrap_in_try_constructor(
 463         &mut self,
 464         lang_item: hir::LangItem,
 465         method_span: Span,
 466         expr: &'hir hir::Expr<'hir>,
 467         overall_span: Span,
 468     ) -> &'hir hir::Expr<'hir> {
 469         let constructor =
 470             self.arena.alloc(self.expr_lang_item_path(method_span, lang_item, ThinVec::new()));
 471         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 472     }
 473
 474     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 475         hir::Arm {
 476             hir_id: self.next_id(),
 477             attrs: self.lower_attrs(&arm.attrs),
 478             pat: self.lower_pat(&arm.pat),
 479             guard: match arm.guard {
 480                 Some(ref x) => Some(hir::Guard::If(self.lower_expr(x))),
 481                 _ => None,
 482             },
 483             body: self.lower_expr(&arm.body),
 484             span: arm.span,
 485         }
 486     }
 487
 488     /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
 489     ///
 490     /// This results in:
 491     ///
 492     /// ```text
 493     /// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
 494     ///     <body>
 495     /// })
 496     /// ```
 497     pub(super) fn make_async_expr(
 498         &mut self,
 499         capture_clause: CaptureBy,
 500         closure_node_id: NodeId,
 501         ret_ty: Option<AstP<Ty>>,
 502         span: Span,
 503         async_gen_kind: hir::AsyncGeneratorKind,
 504         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 505     ) -> hir::ExprKind<'hir> {
 506         let output = match ret_ty {
 507             Some(ty) => hir::FnRetTy::Return(self.lower_ty(&ty, ImplTraitContext::disallowed())),
 508             None => hir::FnRetTy::DefaultReturn(span),
 509         };
 510
 511         // Resume argument type. We let the compiler infer this to simplify the lowering. It is
 512         // fully constrained by `future::from_generator`.
 513         let input_ty = hir::Ty { hir_id: self.next_id(), kind: hir::TyKind::Infer, span };
 514
 515         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 516         let decl = self.arena.alloc(hir::FnDecl {
 517             inputs: arena_vec![self; input_ty],
 518             output,
 519             c_variadic: false,
 520             implicit_self: hir::ImplicitSelfKind::None,
 521         });
 522
 523         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 524         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 525             span,
 526             Ident::with_dummy_span(sym::_task_context),
 527             hir::BindingAnnotation::Mutable,
 528         );
 529         let param = hir::Param { attrs: &[], hir_id: self.next_id(), pat, ty_span: span, span };
 530         let params = arena_vec![self; param];
 531
 532         let body_id = self.lower_body(move |this| {
 533             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 534
 535             let old_ctx = this.task_context;
 536             this.task_context = Some(task_context_hid);
 537             let res = body(this);
 538             this.task_context = old_ctx;
 539             (params, res)
 540         });
 541
 542         // `static |_task_context| -> <ret_ty> { body }`:
 543         let generator_kind = hir::ExprKind::Closure(
 544             capture_clause,
 545             decl,
 546             body_id,
 547             span,
 548             Some(hir::Movability::Static),
 549         );
 550         let generator = hir::Expr {
 551             hir_id: self.lower_node_id(closure_node_id),
 552             kind: generator_kind,
 553             span,
 554             attrs: ThinVec::new(),
 555         };
 556
 557         // `future::from_generator`:
 558         let unstable_span =
 559             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 560         let gen_future =
 561             self.expr_lang_item_path(unstable_span, hir::LangItem::FromGenerator, ThinVec::new());
 562
 563         // `future::from_generator(generator)`:
 564         hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
 565     }
 566
 567     /// Desugar `<expr>.await` into:
 568     /// ```rust
 569     /// match <expr> {
 570     ///     mut pinned => loop {
 571     ///         match unsafe { ::std::future::Future::poll(
 572     ///             <::std::pin::Pin>::new_unchecked(&mut pinned),
 573     ///             ::std::future::get_context(task_context),
 574     ///         ) } {
 575     ///             ::std::task::Poll::Ready(result) => break result,
 576     ///             ::std::task::Poll::Pending => {}
 577     ///         }
 578     ///         task_context = yield ();
 579     ///     }
 580     /// }
 581     /// ```
 582     fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 583         match self.generator_kind {
 584             Some(hir::GeneratorKind::Async(_)) => {}
 585             Some(hir::GeneratorKind::Gen) | None => {
 586                 let mut err = struct_span_err!(
 587                     self.sess,
 588                     await_span,
 589                     E0728,
 590                     "`await` is only allowed inside `async` functions and blocks"
 591                 );
 592                 err.span_label(await_span, "only allowed inside `async` functions and blocks");
 593                 if let Some(item_sp) = self.current_item {
 594                     err.span_label(item_sp, "this is not `async`");
 595                 }
 596                 err.emit();
 597             }
 598         }
 599         let span = self.mark_span_with_reason(DesugaringKind::Await, await_span, None);
 600         let gen_future_span = self.mark_span_with_reason(
 601             DesugaringKind::Await,
 602             await_span,
 603             self.allow_gen_future.clone(),
 604         );
 605         let expr = self.lower_expr(expr);
 606
 607         let pinned_ident = Ident::with_dummy_span(sym::pinned);
 608         let (pinned_pat, pinned_pat_hid) =
 609             self.pat_ident_binding_mode(span, pinned_ident, hir::BindingAnnotation::Mutable);
 610
 611         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 612
 613         // unsafe {
 614         //     ::std::future::Future::poll(
 615         //         ::std::pin::Pin::new_unchecked(&mut pinned),
 616         //         ::std::future::get_context(task_context),
 617         //     )
 618         // }
 619         let poll_expr = {
 620             let pinned = self.expr_ident(span, pinned_ident, pinned_pat_hid);
 621             let ref_mut_pinned = self.expr_mut_addr_of(span, pinned);
 622             let task_context = if let Some(task_context_hid) = self.task_context {
 623                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 624             } else {
 625                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 626                 self.expr_err(span)
 627             };
 628             let new_unchecked = self.expr_call_lang_item_fn_mut(
 629                 span,
 630                 hir::LangItem::PinNewUnchecked,
 631                 arena_vec![self; ref_mut_pinned],
 632             );
 633             let get_context = self.expr_call_lang_item_fn_mut(
 634                 gen_future_span,
 635                 hir::LangItem::GetContext,
 636                 arena_vec![self; task_context],
 637             );
 638             let call = self.expr_call_lang_item_fn(
 639                 span,
 640                 hir::LangItem::FuturePoll,
 641                 arena_vec![self; new_unchecked, get_context],
 642             );
 643             self.arena.alloc(self.expr_unsafe(call))
 644         };
 645
 646         // `::std::task::Poll::Ready(result) => break result`
 647         let loop_node_id = self.resolver.next_node_id();
 648         let loop_hir_id = self.lower_node_id(loop_node_id);
 649         let ready_arm = {
 650             let x_ident = Ident::with_dummy_span(sym::result);
 651             let (x_pat, x_pat_hid) = self.pat_ident(span, x_ident);
 652             let x_expr = self.expr_ident(span, x_ident, x_pat_hid);
 653             let ready_field = self.single_pat_field(span, x_pat);
 654             let ready_pat = self.pat_lang_item_variant(span, hir::LangItem::PollReady, ready_field);
 655             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 656                 let expr_break =
 657                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 658                 this.arena.alloc(this.expr(await_span, expr_break, ThinVec::new()))
 659             });
 660             self.arm(ready_pat, break_x)
 661         };
 662
 663         // `::std::task::Poll::Pending => {}`
 664         let pending_arm = {
 665             let pending_pat = self.pat_lang_item_variant(span, hir::LangItem::PollPending, &[]);
 666             let empty_block = self.expr_block_empty(span);
 667             self.arm(pending_pat, empty_block)
 668         };
 669
 670         let inner_match_stmt = {
 671             let match_expr = self.expr_match(
 672                 span,
 673                 poll_expr,
 674                 arena_vec![self; ready_arm, pending_arm],
 675                 hir::MatchSource::AwaitDesugar,
 676             );
 677             self.stmt_expr(span, match_expr)
 678         };
 679
 680         // task_context = yield ();
 681         let yield_stmt = {
 682             let unit = self.expr_unit(span);
 683             let yield_expr = self.expr(
 684                 span,
 685                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr.hir_id) }),
 686                 ThinVec::new(),
 687             );
 688             let yield_expr = self.arena.alloc(yield_expr);
 689
 690             if let Some(task_context_hid) = self.task_context {
 691                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 692                 let assign =
 693                     self.expr(span, hir::ExprKind::Assign(lhs, yield_expr, span), AttrVec::new());
 694                 self.stmt_expr(span, assign)
 695             } else {
 696                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 697                 // proceed with type checking.
 698                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 699             }
 700         };
 701
 702         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 703
 704         // loop { .. }
 705         let loop_expr = self.arena.alloc(hir::Expr {
 706             hir_id: loop_hir_id,
 707             kind: hir::ExprKind::Loop(loop_block, None, hir::LoopSource::Loop),
 708             span,
 709             attrs: ThinVec::new(),
 710         });
 711
 712         // mut pinned => loop { ... }
 713         let pinned_arm = self.arm(pinned_pat, loop_expr);
 714
 715         // match <expr> {
 716         //     mut pinned => loop { .. }
 717         // }
 718         hir::ExprKind::Match(expr, arena_vec![self; pinned_arm], hir::MatchSource::AwaitDesugar)
 719     }
 720
 721     fn lower_expr_closure(
 722         &mut self,
 723         capture_clause: CaptureBy,
 724         movability: Movability,
 725         decl: &FnDecl,
 726         body: &Expr,
 727         fn_decl_span: Span,
 728     ) -> hir::ExprKind<'hir> {
 729         // Lower outside new scope to preserve `is_in_loop_condition`.
 730         let fn_decl = self.lower_fn_decl(decl, None, false, None);
 731
 732         self.with_new_scopes(move |this| {
 733             let prev = this.current_item;
 734             this.current_item = Some(fn_decl_span);
 735             let mut generator_kind = None;
 736             let body_id = this.lower_fn_body(decl, |this| {
 737                 let e = this.lower_expr_mut(body);
 738                 generator_kind = this.generator_kind;
 739                 e
 740             });
 741             let generator_option =
 742                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 743             this.current_item = prev;
 744             hir::ExprKind::Closure(capture_clause, fn_decl, body_id, fn_decl_span, generator_option)
 745         })
 746     }
 747
 748     fn generator_movability_for_fn(
 749         &mut self,
 750         decl: &FnDecl,
 751         fn_decl_span: Span,
 752         generator_kind: Option<hir::GeneratorKind>,
 753         movability: Movability,
 754     ) -> Option<hir::Movability> {
 755         match generator_kind {
 756             Some(hir::GeneratorKind::Gen) => {
 757                 if decl.inputs.len() > 1 {
 758                     struct_span_err!(
 759                         self.sess,
 760                         fn_decl_span,
 761                         E0628,
 762                         "too many parameters for a generator (expected 0 or 1 parameters)"
 763                     )
 764                     .emit();
 765                 }
 766                 Some(movability)
 767             }
 768             Some(hir::GeneratorKind::Async(_)) => {
 769                 panic!("non-`async` closure body turned `async` during lowering");
 770             }
 771             None => {
 772                 if movability == Movability::Static {
 773                     struct_span_err!(self.sess, fn_decl_span, E0697, "closures cannot be static")
 774                         .emit();
 775                 }
 776                 None
 777             }
 778         }
 779     }
 780
 781     fn lower_expr_async_closure(
 782         &mut self,
 783         capture_clause: CaptureBy,
 784         closure_id: NodeId,
 785         decl: &FnDecl,
 786         body: &Expr,
 787         fn_decl_span: Span,
 788     ) -> hir::ExprKind<'hir> {
 789         let outer_decl =
 790             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 791         // We need to lower the declaration outside the new scope, because we
 792         // have to conserve the state of being inside a loop condition for the
 793         // closure argument types.
 794         let fn_decl = self.lower_fn_decl(&outer_decl, None, false, None);
 795
 796         self.with_new_scopes(move |this| {
 797             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 798             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 799                 struct_span_err!(
 800                     this.sess,
 801                     fn_decl_span,
 802                     E0708,
 803                     "`async` non-`move` closures with parameters are not currently supported",
 804                 )
 805                 .help(
 806                     "consider using `let` statements to manually capture \
 807                     variables by reference before entering an `async move` closure",
 808                 )
 809                 .emit();
 810             }
 811
 812             // Transform `async |x: u8| -> X { ... }` into
 813             // `|x: u8| future_from_generator(|| -> X { ... })`.
 814             let body_id = this.lower_fn_body(&outer_decl, |this| {
 815                 let async_ret_ty =
 816                     if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 817                 let async_body = this.make_async_expr(
 818                     capture_clause,
 819                     closure_id,
 820                     async_ret_ty,
 821                     body.span,
 822                     hir::AsyncGeneratorKind::Closure,
 823                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 824                 );
 825                 this.expr(fn_decl_span, async_body, ThinVec::new())
 826             });
 827             hir::ExprKind::Closure(capture_clause, fn_decl, body_id, fn_decl_span, None)
 828         })
 829     }
 830
 831     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
 832     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
 833         let e1 = self.lower_expr_mut(e1);
 834         let e2 = self.lower_expr_mut(e2);
 835         let fn_path = hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, span);
 836         let fn_expr =
 837             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
 838         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
 839     }
 840
 841     fn lower_expr_range(
 842         &mut self,
 843         span: Span,
 844         e1: Option<&Expr>,
 845         e2: Option<&Expr>,
 846         lims: RangeLimits,
 847     ) -> hir::ExprKind<'hir> {
 848         use rustc_ast::RangeLimits::*;
 849
 850         let lang_item = match (e1, e2, lims) {
 851             (None, None, HalfOpen) => hir::LangItem::RangeFull,
 852             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
 853             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
 854             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
 855             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
 856             (Some(..), Some(..), Closed) => unreachable!(),
 857             (_, None, Closed) => {
 858                 self.diagnostic().span_fatal(span, "inclusive range with no end").raise()
 859             }
 860         };
 861
 862         let fields = self.arena.alloc_from_iter(
 863             e1.iter().map(|e| ("start", e)).chain(e2.iter().map(|e| ("end", e))).map(|(s, e)| {
 864                 let expr = self.lower_expr(&e);
 865                 let ident = Ident::new(Symbol::intern(s), e.span);
 866                 self.field(ident, expr, e.span)
 867             }),
 868         );
 869
 870         hir::ExprKind::Struct(self.arena.alloc(hir::QPath::LangItem(lang_item, span)), fields, None)
 871     }
 872
 873     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
 874         let target_id = match destination {
 875             Some((id, _)) => {
 876                 if let Some(loop_id) = self.resolver.get_label_res(id) {
 877                     Ok(self.lower_node_id(loop_id))
 878                 } else {
 879                     Err(hir::LoopIdError::UnresolvedLabel)
 880                 }
 881             }
 882             None => self
 883                 .loop_scopes
 884                 .last()
 885                 .cloned()
 886                 .map(|id| Ok(self.lower_node_id(id)))
 887                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
 888         };
 889         hir::Destination { label: destination.map(|(_, label)| label), target_id }
 890     }
 891
 892     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
 893         if self.is_in_loop_condition && opt_label.is_none() {
 894             hir::Destination {
 895                 label: None,
 896                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
 897             }
 898         } else {
 899             self.lower_loop_destination(opt_label.map(|label| (id, label)))
 900         }
 901     }
 902
 903     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
 904         let len = self.catch_scopes.len();
 905         self.catch_scopes.push(catch_id);
 906
 907         let result = f(self);
 908         assert_eq!(
 909             len + 1,
 910             self.catch_scopes.len(),
 911             "catch scopes should be added and removed in stack order"
 912         );
 913
 914         self.catch_scopes.pop().unwrap();
 915
 916         result
 917     }
 918
 919     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
 920         // We're no longer in the base loop's condition; we're in another loop.
 921         let was_in_loop_condition = self.is_in_loop_condition;
 922         self.is_in_loop_condition = false;
 923
 924         let len = self.loop_scopes.len();
 925         self.loop_scopes.push(loop_id);
 926
 927         let result = f(self);
 928         assert_eq!(
 929             len + 1,
 930             self.loop_scopes.len(),
 931             "loop scopes should be added and removed in stack order"
 932         );
 933
 934         self.loop_scopes.pop().unwrap();
 935
 936         self.is_in_loop_condition = was_in_loop_condition;
 937
 938         result
 939     }
 940
 941     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
 942         let was_in_loop_condition = self.is_in_loop_condition;
 943         self.is_in_loop_condition = true;
 944
 945         let result = f(self);
 946
 947         self.is_in_loop_condition = was_in_loop_condition;
 948
 949         result
 950     }
 951
 952     fn lower_expr_asm(&mut self, sp: Span, asm: &InlineAsm) -> hir::ExprKind<'hir> {
 953         if self.sess.asm_arch.is_none() {
 954             struct_span_err!(self.sess, sp, E0472, "asm! is unsupported on this target").emit();
 955         }
 956         if asm.options.contains(InlineAsmOptions::ATT_SYNTAX)
 957             && !matches!(
 958                 self.sess.asm_arch,
 959                 Some(asm::InlineAsmArch::X86 | asm::InlineAsmArch::X86_64)
 960             )
 961         {
 962             self.sess
 963                 .struct_span_err(sp, "the `att_syntax` option is only supported on x86")
 964                 .emit();
 965         }
 966
 967         // Lower operands to HIR, filter_map skips any operands with invalid
 968         // register classes.
 969         let sess = self.sess;
 970         let operands: Vec<_> = asm
 971             .operands
 972             .iter()
 973             .filter_map(|(op, op_sp)| {
 974                 let lower_reg = |reg| {
 975                     Some(match reg {
 976                         InlineAsmRegOrRegClass::Reg(s) => asm::InlineAsmRegOrRegClass::Reg(
 977                             asm::InlineAsmReg::parse(
 978                                 sess.asm_arch?,
 979                                 |feature| sess.target_features.contains(&Symbol::intern(feature)),
 980                                 &sess.target.target,
 981                                 s,
 982                             )
 983                             .map_err(|e| {
 984                                 let msg = format!("invalid register `{}`: {}", s.as_str(), e);
 985                                 sess.struct_span_err(*op_sp, &msg).emit();
 986                             })
 987                             .ok()?,
 988                         ),
 989                         InlineAsmRegOrRegClass::RegClass(s) => {
 990                             asm::InlineAsmRegOrRegClass::RegClass(
 991                                 asm::InlineAsmRegClass::parse(sess.asm_arch?, s)
 992                                     .map_err(|e| {
 993                                         let msg = format!(
 994                                             "invalid register class `{}`: {}",
 995                                             s.as_str(),
 996                                             e
 997                                         );
 998                                         sess.struct_span_err(*op_sp, &msg).emit();
 999                                     })
1000                                     .ok()?,
1001                             )
1002                         }
1003                     })
1004                 };
1005
1006                 // lower_reg is executed last because we need to lower all
1007                 // sub-expressions even if we throw them away later.
1008                 let op = match *op {
1009                     InlineAsmOperand::In { reg, ref expr } => hir::InlineAsmOperand::In {
1010                         expr: self.lower_expr_mut(expr),
1011                         reg: lower_reg(reg)?,
1012                     },
1013                     InlineAsmOperand::Out { reg, late, ref expr } => hir::InlineAsmOperand::Out {
1014                         late,
1015                         expr: expr.as_ref().map(|expr| self.lower_expr_mut(expr)),
1016                         reg: lower_reg(reg)?,
1017                     },
1018                     InlineAsmOperand::InOut { reg, late, ref expr } => {
1019                         hir::InlineAsmOperand::InOut {
1020                             late,
1021                             expr: self.lower_expr_mut(expr),
1022                             reg: lower_reg(reg)?,
1023                         }
1024                     }
1025                     InlineAsmOperand::SplitInOut { reg, late, ref in_expr, ref out_expr } => {
1026                         hir::InlineAsmOperand::SplitInOut {
1027                             late,
1028                             in_expr: self.lower_expr_mut(in_expr),
1029                             out_expr: out_expr.as_ref().map(|expr| self.lower_expr_mut(expr)),
1030                             reg: lower_reg(reg)?,
1031                         }
1032                     }
1033                     InlineAsmOperand::Const { ref expr } => {
1034                         hir::InlineAsmOperand::Const { expr: self.lower_expr_mut(expr) }
1035                     }
1036                     InlineAsmOperand::Sym { ref expr } => {
1037                         hir::InlineAsmOperand::Sym { expr: self.lower_expr_mut(expr) }
1038                     }
1039                 };
1040                 Some(op)
1041             })
1042             .collect();
1043
1044         // Stop if there were any errors when lowering the register classes
1045         if operands.len() != asm.operands.len() || sess.asm_arch.is_none() {
1046             return hir::ExprKind::Err;
1047         }
1048
1049         // Validate template modifiers against the register classes for the operands
1050         let asm_arch = sess.asm_arch.unwrap();
1051         for p in &asm.template {
1052             if let InlineAsmTemplatePiece::Placeholder {
1053                 operand_idx,
1054                 modifier: Some(modifier),
1055                 span: placeholder_span,
1056             } = *p
1057             {
1058                 let op_sp = asm.operands[operand_idx].1;
1059                 match &operands[operand_idx] {
1060                     hir::InlineAsmOperand::In { reg, .. }
1061                     | hir::InlineAsmOperand::Out { reg, .. }
1062                     | hir::InlineAsmOperand::InOut { reg, .. }
1063                     | hir::InlineAsmOperand::SplitInOut { reg, .. } => {
1064                         let class = reg.reg_class();
1065                         let valid_modifiers = class.valid_modifiers(asm_arch);
1066                         if !valid_modifiers.contains(&modifier) {
1067                             let mut err = sess.struct_span_err(
1068                                 placeholder_span,
1069                                 "invalid asm template modifier for this register class",
1070                             );
1071                             err.span_label(placeholder_span, "template modifier");
1072                             err.span_label(op_sp, "argument");
1073                             if !valid_modifiers.is_empty() {
1074                                 let mut mods = format!("`{}`", valid_modifiers[0]);
1075                                 for m in &valid_modifiers[1..] {
1076                                     let _ = write!(mods, ", `{}`", m);
1077                                 }
1078                                 err.note(&format!(
1079                                     "the `{}` register class supports \
1080                                      the following template modifiers: {}",
1081                                     class.name(),
1082                                     mods
1083                                 ));
1084                             } else {
1085                                 err.note(&format!(
1086                                     "the `{}` register class does not support template modifiers",
1087                                     class.name()
1088                                 ));
1089                             }
1090                             err.emit();
1091                         }
1092                     }
1093                     hir::InlineAsmOperand::Const { .. } => {
1094                         let mut err = sess.struct_span_err(
1095                             placeholder_span,
1096                             "asm template modifiers are not allowed for `const` arguments",
1097                         );
1098                         err.span_label(placeholder_span, "template modifier");
1099                         err.span_label(op_sp, "argument");
1100                         err.emit();
1101                     }
1102                     hir::InlineAsmOperand::Sym { .. } => {
1103                         let mut err = sess.struct_span_err(
1104                             placeholder_span,
1105                             "asm template modifiers are not allowed for `sym` arguments",
1106                         );
1107                         err.span_label(placeholder_span, "template modifier");
1108                         err.span_label(op_sp, "argument");
1109                         err.emit();
1110                     }
1111                 }
1112             }
1113         }
1114
1115         let mut used_input_regs = FxHashMap::default();
1116         let mut used_output_regs = FxHashMap::default();
1117         for (idx, op) in operands.iter().enumerate() {
1118             let op_sp = asm.operands[idx].1;
1119             if let Some(reg) = op.reg() {
1120                 // Validate register classes against currently enabled target
1121                 // features. We check that at least one type is available for
1122                 // the current target.
1123                 let reg_class = reg.reg_class();
1124                 let mut required_features = vec![];
1125                 for &(_, feature) in reg_class.supported_types(asm_arch) {
1126                     if let Some(feature) = feature {
1127                         if self.sess.target_features.contains(&Symbol::intern(feature)) {
1128                             required_features.clear();
1129                             break;
1130                         } else {
1131                             required_features.push(feature);
1132                         }
1133                     } else {
1134                         required_features.clear();
1135                         break;
1136                     }
1137                 }
1138                 required_features.sort();
1139                 required_features.dedup();
1140                 match &required_features[..] {
1141                     [] => {}
1142                     [feature] => {
1143                         let msg = format!(
1144                             "register class `{}` requires the `{}` target feature",
1145                             reg_class.name(),
1146                             feature
1147                         );
1148                         sess.struct_span_err(op_sp, &msg).emit();
1149                     }
1150                     features => {
1151                         let msg = format!(
1152                             "register class `{}` requires at least one target feature: {}",
1153                             reg_class.name(),
1154                             features.join(", ")
1155                         );
1156                         sess.struct_span_err(op_sp, &msg).emit();
1157                     }
1158                 }
1159
1160                 // Check for conflicts between explicit register operands.
1161                 if let asm::InlineAsmRegOrRegClass::Reg(reg) = reg {
1162                     let (input, output) = match op {
1163                         hir::InlineAsmOperand::In { .. } => (true, false),
1164                         // Late output do not conflict with inputs, but normal outputs do
1165                         hir::InlineAsmOperand::Out { late, .. } => (!late, true),
1166                         hir::InlineAsmOperand::InOut { .. }
1167                         | hir::InlineAsmOperand::SplitInOut { .. } => (true, true),
1168                         hir::InlineAsmOperand::Const { .. } | hir::InlineAsmOperand::Sym { .. } => {
1169                             unreachable!()
1170                         }
1171                     };
1172
1173                     // Flag to output the error only once per operand
1174                     let mut skip = false;
1175                     reg.overlapping_regs(|r| {
1176                         let mut check = |used_regs: &mut FxHashMap<asm::InlineAsmReg, usize>,
1177                                          input| {
1178                             match used_regs.entry(r) {
1179                                 Entry::Occupied(o) => {
1180                                     if !skip {
1181                                         skip = true;
1182
1183                                         let idx2 = *o.get();
1184                                         let op2 = &operands[idx2];
1185                                         let op_sp2 = asm.operands[idx2].1;
1186                                         let reg2 = match op2.reg() {
1187                                             Some(asm::InlineAsmRegOrRegClass::Reg(r)) => r,
1188                                             _ => unreachable!(),
1189                                         };
1190
1191                                         let msg = format!(
1192                                             "register `{}` conflicts with register `{}`",
1193                                             reg.name(),
1194                                             reg2.name()
1195                                         );
1196                                         let mut err = sess.struct_span_err(op_sp, &msg);
1197                                         err.span_label(
1198                                             op_sp,
1199                                             &format!("register `{}`", reg.name()),
1200                                         );
1201                                         err.span_label(
1202                                             op_sp2,
1203                                             &format!("register `{}`", reg2.name()),
1204                                         );
1205
1206                                         match (op, op2) {
1207                                             (
1208                                                 hir::InlineAsmOperand::In { .. },
1209                                                 hir::InlineAsmOperand::Out { late, .. },
1210                                             )
1211                                             | (
1212                                                 hir::InlineAsmOperand::Out { late, .. },
1213                                                 hir::InlineAsmOperand::In { .. },
1214                                             ) => {
1215                                                 assert!(!*late);
1216                                                 let out_op_sp = if input { op_sp2 } else { op_sp };
1217                                                 let msg = "use `lateout` instead of \
1218                                                      `out` to avoid conflict";
1219                                                 err.span_help(out_op_sp, msg);
1220                                             }
1221                                             _ => {}
1222                                         }
1223
1224                                         err.emit();
1225                                     }
1226                                 }
1227                                 Entry::Vacant(v) => {
1228                                     v.insert(idx);
1229                                 }
1230                             }
1231                         };
1232                         if input {
1233                             check(&mut used_input_regs, true);
1234                         }
1235                         if output {
1236                             check(&mut used_output_regs, false);
1237                         }
1238                     });
1239                 }
1240             }
1241         }
1242
1243         let operands = self.arena.alloc_from_iter(operands);
1244         let template = self.arena.alloc_from_iter(asm.template.iter().cloned());
1245         let line_spans = self.arena.alloc_slice(&asm.line_spans[..]);
1246         let hir_asm = hir::InlineAsm { template, operands, options: asm.options, line_spans };
1247         hir::ExprKind::InlineAsm(self.arena.alloc(hir_asm))
1248     }
1249
1250     fn lower_expr_llvm_asm(&mut self, asm: &LlvmInlineAsm) -> hir::ExprKind<'hir> {
1251         let inner = hir::LlvmInlineAsmInner {
1252             inputs: asm.inputs.iter().map(|&(c, _)| c).collect(),
1253             outputs: asm
1254                 .outputs
1255                 .iter()
1256                 .map(|out| hir::LlvmInlineAsmOutput {
1257                     constraint: out.constraint,
1258                     is_rw: out.is_rw,
1259                     is_indirect: out.is_indirect,
1260                     span: out.expr.span,
1261                 })
1262                 .collect(),
1263             asm: asm.asm,
1264             asm_str_style: asm.asm_str_style,
1265             clobbers: asm.clobbers.clone(),
1266             volatile: asm.volatile,
1267             alignstack: asm.alignstack,
1268             dialect: asm.dialect,
1269         };
1270         let hir_asm = hir::LlvmInlineAsm {
1271             inner,
1272             inputs_exprs: self.arena.alloc_from_iter(
1273                 asm.inputs.iter().map(|&(_, ref input)| self.lower_expr_mut(input)),
1274             ),
1275             outputs_exprs: self
1276                 .arena
1277                 .alloc_from_iter(asm.outputs.iter().map(|out| self.lower_expr_mut(&out.expr))),
1278         };
1279         hir::ExprKind::LlvmInlineAsm(self.arena.alloc(hir_asm))
1280     }
1281
1282     fn lower_field(&mut self, f: &Field) -> hir::Field<'hir> {
1283         hir::Field {
1284             hir_id: self.next_id(),
1285             ident: f.ident,
1286             expr: self.lower_expr(&f.expr),
1287             span: f.span,
1288             is_shorthand: f.is_shorthand,
1289         }
1290     }
1291
1292     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1293         match self.generator_kind {
1294             Some(hir::GeneratorKind::Gen) => {}
1295             Some(hir::GeneratorKind::Async(_)) => {
1296                 struct_span_err!(
1297                     self.sess,
1298                     span,
1299                     E0727,
1300                     "`async` generators are not yet supported"
1301                 )
1302                 .emit();
1303             }
1304             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1305         }
1306
1307         let expr =
1308             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1309
1310         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1311     }
1312
1313     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1314     /// ```rust
1315     /// {
1316     ///     let result = match ::std::iter::IntoIterator::into_iter(<head>) {
1317     ///         mut iter => {
1318     ///             [opt_ident]: loop {
1319     ///                 let mut __next;
1320     ///                 match ::std::iter::Iterator::next(&mut iter) {
1321     ///                     ::std::option::Option::Some(val) => __next = val,
1322     ///                     ::std::option::Option::None => break
1323     ///                 };
1324     ///                 let <pat> = __next;
1325     ///                 StmtKind::Expr(<body>);
1326     ///             }
1327     ///         }
1328     ///     };
1329     ///     result
1330     /// }
1331     /// ```
1332     fn lower_expr_for(
1333         &mut self,
1334         e: &Expr,
1335         pat: &Pat,
1336         head: &Expr,
1337         body: &Block,
1338         opt_label: Option<Label>,
1339     ) -> hir::Expr<'hir> {
1340         let orig_head_span = head.span;
1341         // expand <head>
1342         let mut head = self.lower_expr_mut(head);
1343         let desugared_span = self.mark_span_with_reason(
1344             DesugaringKind::ForLoop(ForLoopLoc::Head),
1345             orig_head_span,
1346             None,
1347         );
1348         head.span = desugared_span;
1349
1350         let iter = Ident::with_dummy_span(sym::iter);
1351
1352         let next_ident = Ident::with_dummy_span(sym::__next);
1353         let (next_pat, next_pat_hid) = self.pat_ident_binding_mode(
1354             desugared_span,
1355             next_ident,
1356             hir::BindingAnnotation::Mutable,
1357         );
1358
1359         // `::std::option::Option::Some(val) => __next = val`
1360         let pat_arm = {
1361             let val_ident = Ident::with_dummy_span(sym::val);
1362             let (val_pat, val_pat_hid) = self.pat_ident(pat.span, val_ident);
1363             let val_expr = self.expr_ident(pat.span, val_ident, val_pat_hid);
1364             let next_expr = self.expr_ident(pat.span, next_ident, next_pat_hid);
1365             let assign = self.arena.alloc(self.expr(
1366                 pat.span,
1367                 hir::ExprKind::Assign(next_expr, val_expr, pat.span),
1368                 ThinVec::new(),
1369             ));
1370             let some_pat = self.pat_some(pat.span, val_pat);
1371             self.arm(some_pat, assign)
1372         };
1373
1374         // `::std::option::Option::None => break`
1375         let break_arm = {
1376             let break_expr =
1377                 self.with_loop_scope(e.id, |this| this.expr_break(e.span, ThinVec::new()));
1378             let pat = self.pat_none(e.span);
1379             self.arm(pat, break_expr)
1380         };
1381
1382         // `mut iter`
1383         let (iter_pat, iter_pat_nid) =
1384             self.pat_ident_binding_mode(desugared_span, iter, hir::BindingAnnotation::Mutable);
1385
1386         // `match ::std::iter::Iterator::next(&mut iter) { ... }`
1387         let match_expr = {
1388             let iter = self.expr_ident(desugared_span, iter, iter_pat_nid);
1389             let ref_mut_iter = self.expr_mut_addr_of(desugared_span, iter);
1390             let next_expr = self.expr_call_lang_item_fn(
1391                 desugared_span,
1392                 hir::LangItem::IteratorNext,
1393                 arena_vec![self; ref_mut_iter],
1394             );
1395             let arms = arena_vec![self; pat_arm, break_arm];
1396
1397             self.expr_match(desugared_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1398         };
1399         let match_stmt = self.stmt_expr(desugared_span, match_expr);
1400
1401         let next_expr = self.expr_ident(desugared_span, next_ident, next_pat_hid);
1402
1403         // `let mut __next`
1404         let next_let = self.stmt_let_pat(
1405             ThinVec::new(),
1406             desugared_span,
1407             None,
1408             next_pat,
1409             hir::LocalSource::ForLoopDesugar,
1410         );
1411
1412         // `let <pat> = __next`
1413         let pat = self.lower_pat(pat);
1414         let pat_let = self.stmt_let_pat(
1415             ThinVec::new(),
1416             desugared_span,
1417             Some(next_expr),
1418             pat,
1419             hir::LocalSource::ForLoopDesugar,
1420         );
1421
1422         let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1423         let body_expr = self.expr_block(body_block, ThinVec::new());
1424         let body_stmt = self.stmt_expr(body.span, body_expr);
1425
1426         let loop_block = self.block_all(
1427             e.span,
1428             arena_vec![self; next_let, match_stmt, pat_let, body_stmt],
1429             None,
1430         );
1431
1432         // `[opt_ident]: loop { ... }`
1433         let kind = hir::ExprKind::Loop(loop_block, opt_label, hir::LoopSource::ForLoop);
1434         let loop_expr = self.arena.alloc(hir::Expr {
1435             hir_id: self.lower_node_id(e.id),
1436             kind,
1437             span: e.span,
1438             attrs: ThinVec::new(),
1439         });
1440
1441         // `mut iter => { ... }`
1442         let iter_arm = self.arm(iter_pat, loop_expr);
1443
1444         let into_iter_span = self.mark_span_with_reason(
1445             DesugaringKind::ForLoop(ForLoopLoc::IntoIter),
1446             orig_head_span,
1447             None,
1448         );
1449
1450         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1451         let into_iter_expr = {
1452             self.expr_call_lang_item_fn(
1453                 into_iter_span,
1454                 hir::LangItem::IntoIterIntoIter,
1455                 arena_vec![self; head],
1456             )
1457         };
1458
1459         let match_expr = self.arena.alloc(self.expr_match(
1460             desugared_span,
1461             into_iter_expr,
1462             arena_vec![self; iter_arm],
1463             hir::MatchSource::ForLoopDesugar,
1464         ));
1465
1466         // This is effectively `{ let _result = ...; _result }`.
1467         // The construct was introduced in #21984 and is necessary to make sure that
1468         // temporaries in the `head` expression are dropped and do not leak to the
1469         // surrounding scope of the `match` since the `match` is not a terminating scope.
1470         //
1471         // Also, add the attributes to the outer returned expr node.
1472         self.expr_drop_temps_mut(desugared_span, match_expr, e.attrs.clone())
1473     }
1474
1475     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1476     /// ```rust
1477     /// match Try::into_result(<expr>) {
1478     ///     Ok(val) => #[allow(unreachable_code)] val,
1479     ///     Err(err) => #[allow(unreachable_code)]
1480     ///                 // If there is an enclosing `try {...}`:
1481     ///                 break 'catch_target Try::from_error(From::from(err)),
1482     ///                 // Otherwise:
1483     ///                 return Try::from_error(From::from(err)),
1484     /// }
1485     /// ```
1486     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1487         let unstable_span = self.mark_span_with_reason(
1488             DesugaringKind::QuestionMark,
1489             span,
1490             self.allow_try_trait.clone(),
1491         );
1492         let try_span = self.sess.source_map().end_point(span);
1493         let try_span = self.mark_span_with_reason(
1494             DesugaringKind::QuestionMark,
1495             try_span,
1496             self.allow_try_trait.clone(),
1497         );
1498
1499         // `Try::into_result(<expr>)`
1500         let scrutinee = {
1501             // expand <expr>
1502             let sub_expr = self.lower_expr_mut(sub_expr);
1503
1504             self.expr_call_lang_item_fn(
1505                 unstable_span,
1506                 hir::LangItem::TryIntoResult,
1507                 arena_vec![self; sub_expr],
1508             )
1509         };
1510
1511         // `#[allow(unreachable_code)]`
1512         let attr = {
1513             // `allow(unreachable_code)`
1514             let allow = {
1515                 let allow_ident = Ident::new(sym::allow, span);
1516                 let uc_ident = Ident::new(sym::unreachable_code, span);
1517                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1518                 attr::mk_list_item(allow_ident, vec![uc_nested])
1519             };
1520             attr::mk_attr_outer(allow)
1521         };
1522         let attrs = vec![attr];
1523
1524         // `Ok(val) => #[allow(unreachable_code)] val,`
1525         let ok_arm = {
1526             let val_ident = Ident::with_dummy_span(sym::val);
1527             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1528             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1529                 span,
1530                 val_ident,
1531                 val_pat_nid,
1532                 ThinVec::from(attrs.clone()),
1533             ));
1534             let ok_pat = self.pat_ok(span, val_pat);
1535             self.arm(ok_pat, val_expr)
1536         };
1537
1538         // `Err(err) => #[allow(unreachable_code)]
1539         //              return Try::from_error(From::from(err)),`
1540         let err_arm = {
1541             let err_ident = Ident::with_dummy_span(sym::err);
1542             let (err_local, err_local_nid) = self.pat_ident(try_span, err_ident);
1543             let from_expr = {
1544                 let err_expr = self.expr_ident_mut(try_span, err_ident, err_local_nid);
1545                 self.expr_call_lang_item_fn(
1546                     try_span,
1547                     hir::LangItem::FromFrom,
1548                     arena_vec![self; err_expr],
1549                 )
1550             };
1551             let from_err_expr = self.wrap_in_try_constructor(
1552                 hir::LangItem::TryFromError,
1553                 unstable_span,
1554                 from_expr,
1555                 try_span,
1556             );
1557             let thin_attrs = ThinVec::from(attrs);
1558             let catch_scope = self.catch_scopes.last().copied();
1559             let ret_expr = if let Some(catch_node) = 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_err_expr),
1566                     ),
1567                     thin_attrs,
1568                 ))
1569             } else {
1570                 self.arena.alloc(self.expr(
1571                     try_span,
1572                     hir::ExprKind::Ret(Some(from_err_expr)),
1573                     thin_attrs,
1574                 ))
1575             };
1576
1577             let err_pat = self.pat_err(try_span, err_local);
1578             self.arm(err_pat, ret_expr)
1579         };
1580
1581         hir::ExprKind::Match(
1582             scrutinee,
1583             arena_vec![self; err_arm, ok_arm],
1584             hir::MatchSource::TryDesugar,
1585         )
1586     }
1587
1588     // =========================================================================
1589     // Helper methods for building HIR.
1590     // =========================================================================
1591
1592     /// Constructs a `true` or `false` literal expression.
1593     pub(super) fn expr_bool(&mut self, span: Span, val: bool) -> &'hir hir::Expr<'hir> {
1594         let lit = Spanned { span, node: LitKind::Bool(val) };
1595         self.arena.alloc(self.expr(span, hir::ExprKind::Lit(lit), ThinVec::new()))
1596     }
1597
1598     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1599     ///
1600     /// In terms of drop order, it has the same effect as wrapping `expr` in
1601     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1602     ///
1603     /// The drop order can be important in e.g. `if expr { .. }`.
1604     pub(super) fn expr_drop_temps(
1605         &mut self,
1606         span: Span,
1607         expr: &'hir hir::Expr<'hir>,
1608         attrs: AttrVec,
1609     ) -> &'hir hir::Expr<'hir> {
1610         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1611     }
1612
1613     pub(super) fn expr_drop_temps_mut(
1614         &mut self,
1615         span: Span,
1616         expr: &'hir hir::Expr<'hir>,
1617         attrs: AttrVec,
1618     ) -> hir::Expr<'hir> {
1619         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1620     }
1621
1622     fn expr_match(
1623         &mut self,
1624         span: Span,
1625         arg: &'hir hir::Expr<'hir>,
1626         arms: &'hir [hir::Arm<'hir>],
1627         source: hir::MatchSource,
1628     ) -> hir::Expr<'hir> {
1629         self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
1630     }
1631
1632     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1633         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1634         self.arena.alloc(self.expr(span, expr_break, attrs))
1635     }
1636
1637     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1638         self.expr(
1639             span,
1640             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1641             ThinVec::new(),
1642         )
1643     }
1644
1645     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1646         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
1647     }
1648
1649     fn expr_call_mut(
1650         &mut self,
1651         span: Span,
1652         e: &'hir hir::Expr<'hir>,
1653         args: &'hir [hir::Expr<'hir>],
1654     ) -> hir::Expr<'hir> {
1655         self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new())
1656     }
1657
1658     fn expr_call(
1659         &mut self,
1660         span: Span,
1661         e: &'hir hir::Expr<'hir>,
1662         args: &'hir [hir::Expr<'hir>],
1663     ) -> &'hir hir::Expr<'hir> {
1664         self.arena.alloc(self.expr_call_mut(span, e, args))
1665     }
1666
1667     fn expr_call_lang_item_fn_mut(
1668         &mut self,
1669         span: Span,
1670         lang_item: hir::LangItem,
1671         args: &'hir [hir::Expr<'hir>],
1672     ) -> hir::Expr<'hir> {
1673         let path = self.arena.alloc(self.expr_lang_item_path(span, lang_item, ThinVec::new()));
1674         self.expr_call_mut(span, path, args)
1675     }
1676
1677     fn expr_call_lang_item_fn(
1678         &mut self,
1679         span: Span,
1680         lang_item: hir::LangItem,
1681         args: &'hir [hir::Expr<'hir>],
1682     ) -> &'hir hir::Expr<'hir> {
1683         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args))
1684     }
1685
1686     fn expr_lang_item_path(
1687         &mut self,
1688         span: Span,
1689         lang_item: hir::LangItem,
1690         attrs: AttrVec,
1691     ) -> hir::Expr<'hir> {
1692         self.expr(span, hir::ExprKind::Path(hir::QPath::LangItem(lang_item, span)), attrs)
1693     }
1694
1695     pub(super) fn expr_ident(
1696         &mut self,
1697         sp: Span,
1698         ident: Ident,
1699         binding: hir::HirId,
1700     ) -> &'hir hir::Expr<'hir> {
1701         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1702     }
1703
1704     pub(super) fn expr_ident_mut(
1705         &mut self,
1706         sp: Span,
1707         ident: Ident,
1708         binding: hir::HirId,
1709     ) -> hir::Expr<'hir> {
1710         self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
1711     }
1712
1713     fn expr_ident_with_attrs(
1714         &mut self,
1715         span: Span,
1716         ident: Ident,
1717         binding: hir::HirId,
1718         attrs: AttrVec,
1719     ) -> hir::Expr<'hir> {
1720         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1721             None,
1722             self.arena.alloc(hir::Path {
1723                 span,
1724                 res: Res::Local(binding),
1725                 segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
1726             }),
1727         ));
1728
1729         self.expr(span, expr_path, attrs)
1730     }
1731
1732     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1733         let hir_id = self.next_id();
1734         let span = expr.span;
1735         self.expr(
1736             span,
1737             hir::ExprKind::Block(
1738                 self.arena.alloc(hir::Block {
1739                     stmts: &[],
1740                     expr: Some(expr),
1741                     hir_id,
1742                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1743                     span,
1744                     targeted_by_break: false,
1745                 }),
1746                 None,
1747             ),
1748             ThinVec::new(),
1749         )
1750     }
1751
1752     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1753         let blk = self.block_all(span, &[], None);
1754         let expr = self.expr_block(blk, ThinVec::new());
1755         self.arena.alloc(expr)
1756     }
1757
1758     pub(super) fn expr_block(
1759         &mut self,
1760         b: &'hir hir::Block<'hir>,
1761         attrs: AttrVec,
1762     ) -> hir::Expr<'hir> {
1763         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1764     }
1765
1766     pub(super) fn expr(
1767         &mut self,
1768         span: Span,
1769         kind: hir::ExprKind<'hir>,
1770         attrs: AttrVec,
1771     ) -> hir::Expr<'hir> {
1772         hir::Expr { hir_id: self.next_id(), kind, span, attrs }
1773     }
1774
1775     fn field(&mut self, ident: Ident, expr: &'hir hir::Expr<'hir>, span: Span) -> hir::Field<'hir> {
1776         hir::Field { hir_id: self.next_id(), ident, span, expr, is_shorthand: false }
1777     }
1778
1779     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1780         hir::Arm {
1781             hir_id: self.next_id(),
1782             attrs: &[],
1783             pat,
1784             guard: None,
1785             span: expr.span,
1786             body: expr,
1787         }
1788     }
1789 }