src/librustc/hir/lowering/expr.rs

   1 use super::{LoweringContext, ParamMode, ParenthesizedGenericArgs, ImplTraitContext};
   2 use crate::hir::{self, HirVec};
   3 use crate::hir::ptr::P;
   4
   5 use rustc_data_structures::thin_vec::ThinVec;
   6
   7 use syntax::attr;
   8 use syntax::ptr::P as AstP;
   9 use syntax::ast::*;
  10 use syntax::source_map::{respan, DesugaringKind, Span};
  11 use syntax::symbol::{sym, Symbol};
  12
  13 impl LoweringContext<'_> {
  14     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> HirVec<hir::Expr> {
  15         exprs.iter().map(|x| self.lower_expr(x)).collect()
  16     }
  17
  18     pub(super) fn lower_expr(&mut self, e: &Expr) -> hir::Expr {
  19         let kind = match e.node {
  20             ExprKind::Box(ref inner) => hir::ExprKind::Box(P(self.lower_expr(inner))),
  21             ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  22             ExprKind::Repeat(ref expr, ref count) => {
  23                 let expr = P(self.lower_expr(expr));
  24                 let count = self.lower_anon_const(count);
  25                 hir::ExprKind::Repeat(expr, count)
  26             }
  27             ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  28             ExprKind::Call(ref f, ref args) => {
  29                 let f = P(self.lower_expr(f));
  30                 hir::ExprKind::Call(f, self.lower_exprs(args))
  31             }
  32             ExprKind::MethodCall(ref seg, ref args) => {
  33                 let hir_seg = P(self.lower_path_segment(
  34                     e.span,
  35                     seg,
  36                     ParamMode::Optional,
  37                     0,
  38                     ParenthesizedGenericArgs::Err,
  39                     ImplTraitContext::disallowed(),
  40                     None,
  41                 ));
  42                 let args = self.lower_exprs(args);
  43                 hir::ExprKind::MethodCall(hir_seg, seg.ident.span, args)
  44             }
  45             ExprKind::Binary(binop, ref lhs, ref rhs) => {
  46                 let binop = self.lower_binop(binop);
  47                 let lhs = P(self.lower_expr(lhs));
  48                 let rhs = P(self.lower_expr(rhs));
  49                 hir::ExprKind::Binary(binop, lhs, rhs)
  50             }
  51             ExprKind::Unary(op, ref ohs) => {
  52                 let op = self.lower_unop(op);
  53                 let ohs = P(self.lower_expr(ohs));
  54                 hir::ExprKind::Unary(op, ohs)
  55             }
  56             ExprKind::Lit(ref l) => hir::ExprKind::Lit(respan(l.span, l.node.clone())),
  57             ExprKind::Cast(ref expr, ref ty) => {
  58                 let expr = P(self.lower_expr(expr));
  59                 hir::ExprKind::Cast(expr, self.lower_ty(ty, ImplTraitContext::disallowed()))
  60             }
  61             ExprKind::Type(ref expr, ref ty) => {
  62                 let expr = P(self.lower_expr(expr));
  63                 hir::ExprKind::Type(expr, self.lower_ty(ty, ImplTraitContext::disallowed()))
  64             }
  65             ExprKind::AddrOf(m, ref ohs) => {
  66                 let m = self.lower_mutability(m);
  67                 let ohs = P(self.lower_expr(ohs));
  68                 hir::ExprKind::AddrOf(m, ohs)
  69             }
  70             ExprKind::Let(ref pats, ref scrutinee) => {
  71                 // If we got here, the `let` expression is not allowed.
  72                 self.sess
  73                     .struct_span_err(e.span, "`let` expressions are not supported here")
  74                     .note("only supported directly in conditions of `if`- and `while`-expressions")
  75                     .note("as well as when nested within `&&` and parenthesis in those conditions")
  76                     .emit();
  77
  78                 // For better recovery, we emit:
  79                 // ```
  80                 // match scrutinee { pats => true, _ => false }
  81                 // ```
  82                 // While this doesn't fully match the user's intent, it has key advantages:
  83                 // 1. We can avoid using `abort_if_errors`.
  84                 // 2. We can typeck both `pats` and `scrutinee`.
  85                 // 3. `pats` is allowed to be refutable.
  86                 // 4. The return type of the block is `bool` which seems like what the user wanted.
  87                 let scrutinee = self.lower_expr(scrutinee);
  88                 let then_arm = {
  89                     let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
  90                     let expr = self.expr_bool(e.span, true);
  91                     self.arm(pats, P(expr))
  92                 };
  93                 let else_arm = {
  94                     let pats = hir_vec![self.pat_wild(e.span)];
  95                     let expr = self.expr_bool(e.span, false);
  96                     self.arm(pats, P(expr))
  97                 };
  98                 hir::ExprKind::Match(
  99                     P(scrutinee),
 100                     vec![then_arm, else_arm].into(),
 101                     hir::MatchSource::Normal,
 102                 )
 103             }
 104             // FIXME(#53667): handle lowering of && and parens.
 105             ExprKind::If(ref cond, ref then, ref else_opt) => {
 106                 // `_ => else_block` where `else_block` is `{}` if there's `None`:
 107                 let else_pat = self.pat_wild(e.span);
 108                 let (else_expr, contains_else_clause) = match else_opt {
 109                     None => (self.expr_block_empty(e.span), false),
 110                     Some(els) => (self.lower_expr(els), true),
 111                 };
 112                 let else_arm = self.arm(hir_vec![else_pat], P(else_expr));
 113
 114                 // Handle then + scrutinee:
 115                 let then_blk = self.lower_block(then, false);
 116                 let then_expr = self.expr_block(then_blk, ThinVec::new());
 117                 let (then_pats, scrutinee, desugar) = match cond.node {
 118                     // `<pat> => <then>`:
 119                     ExprKind::Let(ref pats, ref scrutinee) => {
 120                         let scrutinee = self.lower_expr(scrutinee);
 121                         let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
 122                         let desugar = hir::MatchSource::IfLetDesugar { contains_else_clause };
 123                         (pats, scrutinee, desugar)
 124                     }
 125                     // `true => <then>`:
 126                     _ => {
 127                         // Lower condition:
 128                         let cond = self.lower_expr(cond);
 129                         let span_block = self.mark_span_with_reason(
 130                             DesugaringKind::CondTemporary, cond.span, None
 131                         );
 132                         // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 133                         // to preserve drop semantics since `if cond { ... }` does not
 134                         // let temporaries live outside of `cond`.
 135                         let cond = self.expr_drop_temps(span_block, P(cond), ThinVec::new());
 136
 137                         let desugar = hir::MatchSource::IfDesugar { contains_else_clause };
 138                         let pats = hir_vec![self.pat_bool(e.span, true)];
 139                         (pats, cond, desugar)
 140                     }
 141                 };
 142                 let then_arm = self.arm(then_pats, P(then_expr));
 143
 144                 hir::ExprKind::Match(P(scrutinee), vec![then_arm, else_arm].into(), desugar)
 145             }
 146             // FIXME(#53667): handle lowering of && and parens.
 147             ExprKind::While(ref cond, ref body, opt_label) => self.with_loop_scope(e.id, |this| {
 148                 // Note that the block AND the condition are evaluated in the loop scope.
 149                 // This is done to allow `break` from inside the condition of the loop.
 150
 151                 // `_ => break`:
 152                 let else_arm = {
 153                     let else_pat = this.pat_wild(e.span);
 154                     let else_expr = this.expr_break(e.span, ThinVec::new());
 155                     this.arm(hir_vec![else_pat], else_expr)
 156                 };
 157
 158                 // Handle then + scrutinee:
 159                 let then_blk = this.lower_block(body, false);
 160                 let then_expr = this.expr_block(then_blk, ThinVec::new());
 161                 let (then_pats, scrutinee, desugar, source) = match cond.node {
 162                     ExprKind::Let(ref pats, ref scrutinee) => {
 163                         // to:
 164                         //
 165                         //   [opt_ident]: loop {
 166                         //     match <sub_expr> {
 167                         //       <pat> => <body>,
 168                         //       _ => break
 169                         //     }
 170                         //   }
 171                         let scrutinee = this.with_loop_condition_scope(|t| t.lower_expr(scrutinee));
 172                         let pats = pats.iter().map(|pat| this.lower_pat(pat)).collect();
 173                         let desugar = hir::MatchSource::WhileLetDesugar;
 174                         (pats, scrutinee, desugar, hir::LoopSource::WhileLet)
 175                     }
 176                     _ => {
 177                         // We desugar: `'label: while $cond $body` into:
 178                         //
 179                         // ```
 180                         // 'label: loop {
 181                         //     match DropTemps($cond) {
 182                         //         true => $body,
 183                         //         _ => break,
 184                         //     }
 185                         // }
 186                         // ```
 187
 188                         // Lower condition:
 189                         let cond = this.with_loop_condition_scope(|this| this.lower_expr(cond));
 190                         let span_block = this.mark_span_with_reason(
 191                             DesugaringKind::CondTemporary, cond.span, None
 192                         );
 193                         // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 194                         // to preserve drop semantics since `while cond { ... }` does not
 195                         // let temporaries live outside of `cond`.
 196                         let cond = this.expr_drop_temps(span_block, P(cond), ThinVec::new());
 197
 198                         let desugar = hir::MatchSource::WhileDesugar;
 199                         // `true => <then>`:
 200                         let pats = hir_vec![this.pat_bool(e.span, true)];
 201                         (pats, cond, desugar, hir::LoopSource::While)
 202                     }
 203                 };
 204                 let then_arm = this.arm(then_pats, P(then_expr));
 205
 206                 // `match <scrutinee> { ... }`
 207                 let match_expr = this.expr_match(
 208                     scrutinee.span,
 209                     P(scrutinee),
 210                     hir_vec![then_arm, else_arm],
 211                     desugar,
 212                 );
 213
 214                 // `[opt_ident]: loop { ... }`
 215                 hir::ExprKind::Loop(
 216                     P(this.block_expr(P(match_expr))),
 217                     this.lower_label(opt_label),
 218                     source
 219                 )
 220             }),
 221             ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
 222                 hir::ExprKind::Loop(
 223                     this.lower_block(body, false),
 224                     this.lower_label(opt_label),
 225                     hir::LoopSource::Loop,
 226                 )
 227             }),
 228             ExprKind::TryBlock(ref body) => {
 229                 self.with_catch_scope(body.id, |this| {
 230                     let unstable_span = this.mark_span_with_reason(
 231                         DesugaringKind::TryBlock,
 232                         body.span,
 233                         this.allow_try_trait.clone(),
 234                     );
 235                     let mut block = this.lower_block(body, true).into_inner();
 236                     let tail = block.expr.take().map_or_else(
 237                         || {
 238                             let span = this.sess.source_map().end_point(unstable_span);
 239                             hir::Expr {
 240                                 span,
 241                                 node: hir::ExprKind::Tup(hir_vec![]),
 242                                 attrs: ThinVec::new(),
 243                                 hir_id: this.next_id(),
 244                             }
 245                         },
 246                         |x: P<hir::Expr>| x.into_inner(),
 247                     );
 248                     block.expr = Some(this.wrap_in_try_constructor(
 249                         sym::from_ok, tail, unstable_span));
 250                     hir::ExprKind::Block(P(block), None)
 251                 })
 252             }
 253             ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
 254                 P(self.lower_expr(expr)),
 255                 arms.iter().map(|x| self.lower_arm(x)).collect(),
 256                 hir::MatchSource::Normal,
 257             ),
 258             ExprKind::Async(capture_clause, closure_node_id, ref block) => {
 259                 self.make_async_expr(capture_clause, closure_node_id, None, block.span, |this| {
 260                     this.with_new_scopes(|this| {
 261                         let block = this.lower_block(block, false);
 262                         this.expr_block(block, ThinVec::new())
 263                     })
 264                 })
 265             }
 266             ExprKind::Await(ref expr) => self.lower_await(e.span, expr),
 267             ExprKind::Closure(
 268                 capture_clause, asyncness, movability, ref decl, ref body, fn_decl_span
 269             ) => {
 270                 if let IsAsync::Async { closure_id, .. } = asyncness {
 271                     self.lower_expr_async_closure(
 272                         capture_clause,
 273                         closure_id,
 274                         decl,
 275                         body,
 276                         fn_decl_span,
 277                     )
 278                 } else {
 279                     // Lower outside new scope to preserve `is_in_loop_condition`.
 280                     let fn_decl = self.lower_fn_decl(decl, None, false, None);
 281
 282                     self.with_new_scopes(|this| {
 283                         this.current_item = Some(fn_decl_span);
 284                         let mut generator_kind = None;
 285                         let body_id = this.lower_fn_body(decl, |this| {
 286                             let e = this.lower_expr(body);
 287                             generator_kind = this.generator_kind;
 288                             e
 289                         });
 290                         let generator_option = this.generator_movability_for_fn(
 291                             &decl,
 292                             fn_decl_span,
 293                             generator_kind,
 294                             movability,
 295                         );
 296                         hir::ExprKind::Closure(
 297                             this.lower_capture_clause(capture_clause),
 298                             fn_decl,
 299                             body_id,
 300                             fn_decl_span,
 301                             generator_option,
 302                         )
 303                     })
 304                 }
 305             }
 306             ExprKind::Block(ref blk, opt_label) => {
 307                 hir::ExprKind::Block(self.lower_block(blk,
 308                                                       opt_label.is_some()),
 309                                                       self.lower_label(opt_label))
 310             }
 311             ExprKind::Assign(ref el, ref er) => {
 312                 hir::ExprKind::Assign(P(self.lower_expr(el)), P(self.lower_expr(er)))
 313             }
 314             ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 315                 self.lower_binop(op),
 316                 P(self.lower_expr(el)),
 317                 P(self.lower_expr(er)),
 318             ),
 319             ExprKind::Field(ref el, ident) => hir::ExprKind::Field(P(self.lower_expr(el)), ident),
 320             ExprKind::Index(ref el, ref er) => {
 321                 hir::ExprKind::Index(P(self.lower_expr(el)), P(self.lower_expr(er)))
 322             }
 323             ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 324                 self.lower_expr_range_closed(e.span, e1, e2)
 325             }
 326             ExprKind::Range(ref e1, ref e2, lims) => {
 327                 self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 328             }
 329             ExprKind::Path(ref qself, ref path) => {
 330                 let qpath = self.lower_qpath(
 331                     e.id,
 332                     qself,
 333                     path,
 334                     ParamMode::Optional,
 335                     ImplTraitContext::disallowed(),
 336                 );
 337                 hir::ExprKind::Path(qpath)
 338             }
 339             ExprKind::Break(opt_label, ref opt_expr) => {
 340                 let destination = if self.is_in_loop_condition && opt_label.is_none() {
 341                     hir::Destination {
 342                         label: None,
 343                         target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 344                     }
 345                 } else {
 346                     self.lower_loop_destination(opt_label.map(|label| (e.id, label)))
 347                 };
 348                 hir::ExprKind::Break(
 349                     destination,
 350                     opt_expr.as_ref().map(|x| P(self.lower_expr(x))),
 351                 )
 352             }
 353             ExprKind::Continue(opt_label) => {
 354                 hir::ExprKind::Continue(if self.is_in_loop_condition && opt_label.is_none() {
 355                     hir::Destination {
 356                         label: None,
 357                         target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 358                     }
 359                 } else {
 360                     self.lower_loop_destination(opt_label.map(|label| (e.id, label)))
 361                 })
 362             }
 363             ExprKind::Ret(ref e) => hir::ExprKind::Ret(e.as_ref().map(|x| P(self.lower_expr(x)))),
 364             ExprKind::InlineAsm(ref asm) => self.lower_expr_asm(asm),
 365             ExprKind::Struct(ref path, ref fields, ref maybe_expr) => hir::ExprKind::Struct(
 366                 P(self.lower_qpath(
 367                     e.id,
 368                     &None,
 369                     path,
 370                     ParamMode::Optional,
 371                     ImplTraitContext::disallowed(),
 372                 )),
 373                 fields.iter().map(|x| self.lower_field(x)).collect(),
 374                 maybe_expr.as_ref().map(|x| P(self.lower_expr(x))),
 375             ),
 376             ExprKind::Paren(ref ex) => {
 377                 let mut ex = self.lower_expr(ex);
 378                 // Include parens in span, but only if it is a super-span.
 379                 if e.span.contains(ex.span) {
 380                     ex.span = e.span;
 381                 }
 382                 // Merge attributes into the inner expression.
 383                 let mut attrs = e.attrs.clone();
 384                 attrs.extend::<Vec<_>>(ex.attrs.into());
 385                 ex.attrs = attrs;
 386                 return ex;
 387             }
 388
 389             ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 390
 391             ExprKind::Err => hir::ExprKind::Err,
 392
 393             // Desugar `ExprForLoop`
 394             // from: `[opt_ident]: for <pat> in <head> <body>`
 395             ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 396                 return self.lower_expr_for(e, pat, head, body, opt_label);
 397             }
 398             ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 399             ExprKind::Mac(_) => panic!("Shouldn't exist here"),
 400         };
 401
 402         hir::Expr {
 403             hir_id: self.lower_node_id(e.id),
 404             node: kind,
 405             span: e.span,
 406             attrs: e.attrs.clone(),
 407         }
 408     }
 409
 410     fn lower_expr_async_closure(
 411         &mut self,
 412         capture_clause: CaptureBy,
 413         closure_id: NodeId,
 414         decl: &FnDecl,
 415         body: &Expr,
 416         fn_decl_span: Span,
 417     ) -> hir::ExprKind {
 418         let outer_decl = FnDecl {
 419             inputs: decl.inputs.clone(),
 420             output: FunctionRetTy::Default(fn_decl_span),
 421             c_variadic: false,
 422         };
 423         // We need to lower the declaration outside the new scope, because we
 424         // have to conserve the state of being inside a loop condition for the
 425         // closure argument types.
 426         let fn_decl = self.lower_fn_decl(&outer_decl, None, false, None);
 427
 428         self.with_new_scopes(|this| {
 429             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 430             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 431                 struct_span_err!(
 432                     this.sess,
 433                     fn_decl_span,
 434                     E0708,
 435                     "`async` non-`move` closures with arguments are not currently supported",
 436                 )
 437                 .help(
 438                     "consider using `let` statements to manually capture \
 439                     variables by reference before entering an `async move` closure"
 440                 )
 441                 .emit();
 442             }
 443
 444             // Transform `async |x: u8| -> X { ... }` into
 445             // `|x: u8| future_from_generator(|| -> X { ... })`.
 446             let body_id = this.lower_fn_body(&outer_decl, |this| {
 447                 let async_ret_ty = if let FunctionRetTy::Ty(ty) = &decl.output {
 448                     Some(ty.clone())
 449                 } else {
 450                     None
 451                 };
 452                 let async_body = this.make_async_expr(
 453                     capture_clause, closure_id, async_ret_ty, body.span,
 454                     |this| {
 455                         this.with_new_scopes(|this| this.lower_expr(body))
 456                     }
 457                 );
 458                 this.expr(fn_decl_span, async_body, ThinVec::new())
 459             });
 460             hir::ExprKind::Closure(
 461                 this.lower_capture_clause(capture_clause),
 462                 fn_decl,
 463                 body_id,
 464                 fn_decl_span,
 465                 None,
 466             )
 467         })
 468     }
 469
 470     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
 471     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind {
 472         let id = self.next_id();
 473         let e1 = self.lower_expr(e1);
 474         let e2 = self.lower_expr(e2);
 475         self.expr_call_std_assoc_fn(
 476             id,
 477             span,
 478             &[sym::ops, sym::RangeInclusive],
 479             "new",
 480             hir_vec![e1, e2],
 481         )
 482     }
 483
 484     fn lower_expr_range(
 485         &mut self,
 486         span: Span,
 487         e1: Option<&Expr>,
 488         e2: Option<&Expr>,
 489         lims: RangeLimits,
 490     ) -> hir::ExprKind {
 491         use syntax::ast::RangeLimits::*;
 492
 493         let path = match (e1, e2, lims) {
 494             (None, None, HalfOpen) => sym::RangeFull,
 495             (Some(..), None, HalfOpen) => sym::RangeFrom,
 496             (None, Some(..), HalfOpen) => sym::RangeTo,
 497             (Some(..), Some(..), HalfOpen) => sym::Range,
 498             (None, Some(..), Closed) => sym::RangeToInclusive,
 499             (Some(..), Some(..), Closed) => unreachable!(),
 500             (_, None, Closed) => self.diagnostic()
 501                 .span_fatal(span, "inclusive range with no end")
 502                 .raise(),
 503         };
 504
 505         let fields = e1.iter()
 506             .map(|e| ("start", e))
 507             .chain(e2.iter().map(|e| ("end", e)))
 508             .map(|(s, e)| {
 509                 let expr = P(self.lower_expr(&e));
 510                 let ident = Ident::new(Symbol::intern(s), e.span);
 511                 self.field(ident, expr, e.span)
 512             })
 513             .collect::<P<[hir::Field]>>();
 514
 515         let is_unit = fields.is_empty();
 516         let struct_path = [sym::ops, path];
 517         let struct_path = self.std_path(span, &struct_path, None, is_unit);
 518         let struct_path = hir::QPath::Resolved(None, P(struct_path));
 519
 520         if is_unit {
 521             hir::ExprKind::Path(struct_path)
 522         } else {
 523             hir::ExprKind::Struct(P(struct_path), fields, None)
 524         }
 525     }
 526
 527     fn lower_expr_asm(&mut self, asm: &InlineAsm) -> hir::ExprKind {
 528         let hir_asm = hir::InlineAsm {
 529             inputs: asm.inputs.iter().map(|&(ref c, _)| c.clone()).collect(),
 530             outputs: asm.outputs
 531                 .iter()
 532                 .map(|out| hir::InlineAsmOutput {
 533                     constraint: out.constraint.clone(),
 534                     is_rw: out.is_rw,
 535                     is_indirect: out.is_indirect,
 536                     span: out.expr.span,
 537                 })
 538                 .collect(),
 539             asm: asm.asm.clone(),
 540             asm_str_style: asm.asm_str_style,
 541             clobbers: asm.clobbers.clone().into(),
 542             volatile: asm.volatile,
 543             alignstack: asm.alignstack,
 544             dialect: asm.dialect,
 545             ctxt: asm.ctxt,
 546         };
 547
 548         let outputs = asm.outputs
 549             .iter()
 550             .map(|out| self.lower_expr(&out.expr))
 551             .collect();
 552
 553         let inputs = asm.inputs
 554             .iter()
 555             .map(|&(_, ref input)| self.lower_expr(input))
 556             .collect();
 557
 558         hir::ExprKind::InlineAsm(P(hir_asm), outputs, inputs)
 559     }
 560
 561     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind {
 562         match self.generator_kind {
 563             Some(hir::GeneratorKind::Gen) => {},
 564             Some(hir::GeneratorKind::Async) => {
 565                 span_err!(
 566                     self.sess,
 567                     span,
 568                     E0727,
 569                     "`async` generators are not yet supported",
 570                 );
 571                 self.sess.abort_if_errors();
 572             },
 573             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
 574         }
 575
 576         let expr = opt_expr
 577             .as_ref()
 578             .map(|x| self.lower_expr(x))
 579             .unwrap_or_else(|| self.expr_unit(span));
 580
 581         hir::ExprKind::Yield(P(expr), hir::YieldSource::Yield)
 582     }
 583
 584     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
 585     /// ```rust
 586     /// {
 587     ///     let result = match ::std::iter::IntoIterator::into_iter(<head>) {
 588     ///         mut iter => {
 589     ///             [opt_ident]: loop {
 590     ///                 let mut __next;
 591     ///                 match ::std::iter::Iterator::next(&mut iter) {
 592     ///                     ::std::option::Option::Some(val) => __next = val,
 593     ///                     ::std::option::Option::None => break
 594     ///                 };
 595     ///                 let <pat> = __next;
 596     ///                 StmtKind::Expr(<body>);
 597     ///             }
 598     ///         }
 599     ///     };
 600     ///     result
 601     /// }
 602     /// ```
 603     fn lower_expr_for(
 604         &mut self,
 605         e: &Expr,
 606         pat: &Pat,
 607         head: &Expr,
 608         body: &Block,
 609         opt_label: Option<Label>,
 610     ) -> hir::Expr {
 611         // expand <head>
 612         let mut head = self.lower_expr(head);
 613         let head_sp = head.span;
 614         let desugared_span = self.mark_span_with_reason(
 615             DesugaringKind::ForLoop,
 616             head_sp,
 617             None,
 618         );
 619         head.span = desugared_span;
 620
 621         let iter = Ident::with_empty_ctxt(sym::iter);
 622
 623         let next_ident = Ident::with_empty_ctxt(sym::__next);
 624         let (next_pat, next_pat_hid) = self.pat_ident_binding_mode(
 625             desugared_span,
 626             next_ident,
 627             hir::BindingAnnotation::Mutable,
 628         );
 629
 630         // `::std::option::Option::Some(val) => __next = val`
 631         let pat_arm = {
 632             let val_ident = Ident::with_empty_ctxt(sym::val);
 633             let (val_pat, val_pat_hid) = self.pat_ident(pat.span, val_ident);
 634             let val_expr = P(self.expr_ident(pat.span, val_ident, val_pat_hid));
 635             let next_expr = P(self.expr_ident(pat.span, next_ident, next_pat_hid));
 636             let assign = P(self.expr(
 637                 pat.span,
 638                 hir::ExprKind::Assign(next_expr, val_expr),
 639                 ThinVec::new(),
 640             ));
 641             let some_pat = self.pat_some(pat.span, val_pat);
 642             self.arm(hir_vec![some_pat], assign)
 643         };
 644
 645         // `::std::option::Option::None => break`
 646         let break_arm = {
 647             let break_expr =
 648                 self.with_loop_scope(e.id, |this| this.expr_break(e.span, ThinVec::new()));
 649             let pat = self.pat_none(e.span);
 650             self.arm(hir_vec![pat], break_expr)
 651         };
 652
 653         // `mut iter`
 654         let (iter_pat, iter_pat_nid) = self.pat_ident_binding_mode(
 655             desugared_span,
 656             iter,
 657             hir::BindingAnnotation::Mutable
 658         );
 659
 660         // `match ::std::iter::Iterator::next(&mut iter) { ... }`
 661         let match_expr = {
 662             let iter = P(self.expr_ident(head_sp, iter, iter_pat_nid));
 663             let ref_mut_iter = self.expr_mut_addr_of(head_sp, iter);
 664             let next_path = &[sym::iter, sym::Iterator, sym::next];
 665             let next_expr = P(self.expr_call_std_path(
 666                 head_sp,
 667                 next_path,
 668                 hir_vec![ref_mut_iter],
 669             ));
 670             let arms = hir_vec![pat_arm, break_arm];
 671
 672             self.expr_match(head_sp, next_expr, arms, hir::MatchSource::ForLoopDesugar)
 673         };
 674         let match_stmt = self.stmt_expr(head_sp, match_expr);
 675
 676         let next_expr = P(self.expr_ident(head_sp, next_ident, next_pat_hid));
 677
 678         // `let mut __next`
 679         let next_let = self.stmt_let_pat(
 680             ThinVec::new(),
 681             desugared_span,
 682             None,
 683             next_pat,
 684             hir::LocalSource::ForLoopDesugar,
 685         );
 686
 687         // `let <pat> = __next`
 688         let pat = self.lower_pat(pat);
 689         let pat_let = self.stmt_let_pat(
 690             ThinVec::new(),
 691             head_sp,
 692             Some(next_expr),
 693             pat,
 694             hir::LocalSource::ForLoopDesugar,
 695         );
 696
 697         let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
 698         let body_expr = self.expr_block(body_block, ThinVec::new());
 699         let body_stmt = self.stmt_expr(body.span, body_expr);
 700
 701         let loop_block = P(self.block_all(
 702             e.span,
 703             hir_vec![next_let, match_stmt, pat_let, body_stmt],
 704             None,
 705         ));
 706
 707         // `[opt_ident]: loop { ... }`
 708         let loop_expr = hir::ExprKind::Loop(
 709             loop_block,
 710             self.lower_label(opt_label),
 711             hir::LoopSource::ForLoop,
 712         );
 713         let loop_expr = P(hir::Expr {
 714             hir_id: self.lower_node_id(e.id),
 715             node: loop_expr,
 716             span: e.span,
 717             attrs: ThinVec::new(),
 718         });
 719
 720         // `mut iter => { ... }`
 721         let iter_arm = self.arm(hir_vec![iter_pat], loop_expr);
 722
 723         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
 724         let into_iter_expr = {
 725             let into_iter_path =
 726                 &[sym::iter, sym::IntoIterator, sym::into_iter];
 727             P(self.expr_call_std_path(
 728                 head_sp,
 729                 into_iter_path,
 730                 hir_vec![head],
 731             ))
 732         };
 733
 734         let match_expr = P(self.expr_match(
 735             head_sp,
 736             into_iter_expr,
 737             hir_vec![iter_arm],
 738             hir::MatchSource::ForLoopDesugar,
 739         ));
 740
 741         // This is effectively `{ let _result = ...; _result }`.
 742         // The construct was introduced in #21984 and is necessary to make sure that
 743         // temporaries in the `head` expression are dropped and do not leak to the
 744         // surrounding scope of the `match` since the `match` is not a terminating scope.
 745         //
 746         // Also, add the attributes to the outer returned expr node.
 747         self.expr_drop_temps(head_sp, match_expr, e.attrs.clone())
 748     }
 749
 750     /// Desugar `ExprKind::Try` from: `<expr>?` into:
 751     /// ```rust
 752     /// match Try::into_result(<expr>) {
 753     ///     Ok(val) => #[allow(unreachable_code)] val,
 754     ///     Err(err) => #[allow(unreachable_code)]
 755     ///                 // If there is an enclosing `try {...}`:
 756     ///                 break 'catch_target Try::from_error(From::from(err)),
 757     ///                 // Otherwise:
 758     ///                 return Try::from_error(From::from(err)),
 759     /// }
 760     /// ```
 761     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind {
 762         let unstable_span = self.mark_span_with_reason(
 763             DesugaringKind::QuestionMark,
 764             span,
 765             self.allow_try_trait.clone(),
 766         );
 767         let try_span = self.sess.source_map().end_point(span);
 768         let try_span = self.mark_span_with_reason(
 769             DesugaringKind::QuestionMark,
 770             try_span,
 771             self.allow_try_trait.clone(),
 772         );
 773
 774         // `Try::into_result(<expr>)`
 775         let scrutinee = {
 776             // expand <expr>
 777             let sub_expr = self.lower_expr(sub_expr);
 778
 779             let path = &[sym::ops, sym::Try, sym::into_result];
 780             P(self.expr_call_std_path(unstable_span, path, hir_vec![sub_expr]))
 781         };
 782
 783         // `#[allow(unreachable_code)]`
 784         let attr = {
 785             // `allow(unreachable_code)`
 786             let allow = {
 787                 let allow_ident = Ident::new(sym::allow, span);
 788                 let uc_ident = Ident::new(sym::unreachable_code, span);
 789                 let uc_nested = attr::mk_nested_word_item(uc_ident);
 790                 attr::mk_list_item(allow_ident, vec![uc_nested])
 791             };
 792             attr::mk_attr_outer(allow)
 793         };
 794         let attrs = vec![attr];
 795
 796         // `Ok(val) => #[allow(unreachable_code)] val,`
 797         let ok_arm = {
 798             let val_ident = Ident::with_empty_ctxt(sym::val);
 799             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
 800             let val_expr = P(self.expr_ident_with_attrs(
 801                 span,
 802                 val_ident,
 803                 val_pat_nid,
 804                 ThinVec::from(attrs.clone()),
 805             ));
 806             let ok_pat = self.pat_ok(span, val_pat);
 807
 808             self.arm(hir_vec![ok_pat], val_expr)
 809         };
 810
 811         // `Err(err) => #[allow(unreachable_code)]
 812         //              return Try::from_error(From::from(err)),`
 813         let err_arm = {
 814             let err_ident = Ident::with_empty_ctxt(sym::err);
 815             let (err_local, err_local_nid) = self.pat_ident(try_span, err_ident);
 816             let from_expr = {
 817                 let from_path = &[sym::convert, sym::From, sym::from];
 818                 let err_expr = self.expr_ident(try_span, err_ident, err_local_nid);
 819                 self.expr_call_std_path(try_span, from_path, hir_vec![err_expr])
 820             };
 821             let from_err_expr =
 822                 self.wrap_in_try_constructor(sym::from_error, from_expr, unstable_span);
 823             let thin_attrs = ThinVec::from(attrs);
 824             let catch_scope = self.catch_scopes.last().map(|x| *x);
 825             let ret_expr = if let Some(catch_node) = catch_scope {
 826                 let target_id = Ok(self.lower_node_id(catch_node));
 827                 P(self.expr(
 828                     try_span,
 829                     hir::ExprKind::Break(
 830                         hir::Destination {
 831                             label: None,
 832                             target_id,
 833                         },
 834                         Some(from_err_expr),
 835                     ),
 836                     thin_attrs,
 837                 ))
 838             } else {
 839                 P(self.expr(try_span, hir::ExprKind::Ret(Some(from_err_expr)), thin_attrs))
 840             };
 841
 842             let err_pat = self.pat_err(try_span, err_local);
 843             self.arm(hir_vec![err_pat], ret_expr)
 844         };
 845
 846         hir::ExprKind::Match(
 847             scrutinee,
 848             hir_vec![err_arm, ok_arm],
 849             hir::MatchSource::TryDesugar,
 850         )
 851     }
 852 }