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             ) => if let IsAsync::Async { closure_id, .. } = asyncness {
 270                 self.lower_expr_async_closure(capture_clause, closure_id, decl, body, fn_decl_span)
 271             } else {
 272                 self.lower_expr_closure(capture_clause, movability, decl, body, fn_decl_span)
 273             }
 274             ExprKind::Block(ref blk, opt_label) => {
 275                 hir::ExprKind::Block(self.lower_block(blk,
 276                                                       opt_label.is_some()),
 277                                                       self.lower_label(opt_label))
 278             }
 279             ExprKind::Assign(ref el, ref er) => {
 280                 hir::ExprKind::Assign(P(self.lower_expr(el)), P(self.lower_expr(er)))
 281             }
 282             ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 283                 self.lower_binop(op),
 284                 P(self.lower_expr(el)),
 285                 P(self.lower_expr(er)),
 286             ),
 287             ExprKind::Field(ref el, ident) => hir::ExprKind::Field(P(self.lower_expr(el)), ident),
 288             ExprKind::Index(ref el, ref er) => {
 289                 hir::ExprKind::Index(P(self.lower_expr(el)), P(self.lower_expr(er)))
 290             }
 291             ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 292                 self.lower_expr_range_closed(e.span, e1, e2)
 293             }
 294             ExprKind::Range(ref e1, ref e2, lims) => {
 295                 self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 296             }
 297             ExprKind::Path(ref qself, ref path) => {
 298                 let qpath = self.lower_qpath(
 299                     e.id,
 300                     qself,
 301                     path,
 302                     ParamMode::Optional,
 303                     ImplTraitContext::disallowed(),
 304                 );
 305                 hir::ExprKind::Path(qpath)
 306             }
 307             ExprKind::Break(opt_label, ref opt_expr) => {
 308                 let destination = if self.is_in_loop_condition && opt_label.is_none() {
 309                     hir::Destination {
 310                         label: None,
 311                         target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 312                     }
 313                 } else {
 314                     self.lower_loop_destination(opt_label.map(|label| (e.id, label)))
 315                 };
 316                 hir::ExprKind::Break(
 317                     destination,
 318                     opt_expr.as_ref().map(|x| P(self.lower_expr(x))),
 319                 )
 320             }
 321             ExprKind::Continue(opt_label) => {
 322                 hir::ExprKind::Continue(if self.is_in_loop_condition && opt_label.is_none() {
 323                     hir::Destination {
 324                         label: None,
 325                         target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 326                     }
 327                 } else {
 328                     self.lower_loop_destination(opt_label.map(|label| (e.id, label)))
 329                 })
 330             }
 331             ExprKind::Ret(ref e) => hir::ExprKind::Ret(e.as_ref().map(|x| P(self.lower_expr(x)))),
 332             ExprKind::InlineAsm(ref asm) => self.lower_expr_asm(asm),
 333             ExprKind::Struct(ref path, ref fields, ref maybe_expr) => hir::ExprKind::Struct(
 334                 P(self.lower_qpath(
 335                     e.id,
 336                     &None,
 337                     path,
 338                     ParamMode::Optional,
 339                     ImplTraitContext::disallowed(),
 340                 )),
 341                 fields.iter().map(|x| self.lower_field(x)).collect(),
 342                 maybe_expr.as_ref().map(|x| P(self.lower_expr(x))),
 343             ),
 344             ExprKind::Paren(ref ex) => {
 345                 let mut ex = self.lower_expr(ex);
 346                 // Include parens in span, but only if it is a super-span.
 347                 if e.span.contains(ex.span) {
 348                     ex.span = e.span;
 349                 }
 350                 // Merge attributes into the inner expression.
 351                 let mut attrs = e.attrs.clone();
 352                 attrs.extend::<Vec<_>>(ex.attrs.into());
 353                 ex.attrs = attrs;
 354                 return ex;
 355             }
 356
 357             ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 358
 359             ExprKind::Err => hir::ExprKind::Err,
 360
 361             // Desugar `ExprForLoop`
 362             // from: `[opt_ident]: for <pat> in <head> <body>`
 363             ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 364                 return self.lower_expr_for(e, pat, head, body, opt_label);
 365             }
 366             ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 367             ExprKind::Mac(_) => panic!("Shouldn't exist here"),
 368         };
 369
 370         hir::Expr {
 371             hir_id: self.lower_node_id(e.id),
 372             node: kind,
 373             span: e.span,
 374             attrs: e.attrs.clone(),
 375         }
 376     }
 377
 378     fn lower_expr_closure(
 379         &mut self,
 380         capture_clause: CaptureBy,
 381         movability: Movability,
 382         decl: &FnDecl,
 383         body: &Expr,
 384         fn_decl_span: Span,
 385     ) -> hir::ExprKind {
 386         // Lower outside new scope to preserve `is_in_loop_condition`.
 387         let fn_decl = self.lower_fn_decl(decl, None, false, None);
 388
 389         self.with_new_scopes(|this| {
 390             this.current_item = Some(fn_decl_span);
 391             let mut generator_kind = None;
 392             let body_id = this.lower_fn_body(decl, |this| {
 393                 let e = this.lower_expr(body);
 394                 generator_kind = this.generator_kind;
 395                 e
 396             });
 397             let generator_option = this.generator_movability_for_fn(
 398                 &decl,
 399                 fn_decl_span,
 400                 generator_kind,
 401                 movability,
 402             );
 403             hir::ExprKind::Closure(
 404                 this.lower_capture_clause(capture_clause),
 405                 fn_decl,
 406                 body_id,
 407                 fn_decl_span,
 408                 generator_option,
 409             )
 410         })
 411     }
 412
 413     fn lower_expr_async_closure(
 414         &mut self,
 415         capture_clause: CaptureBy,
 416         closure_id: NodeId,
 417         decl: &FnDecl,
 418         body: &Expr,
 419         fn_decl_span: Span,
 420     ) -> hir::ExprKind {
 421         let outer_decl = FnDecl {
 422             inputs: decl.inputs.clone(),
 423             output: FunctionRetTy::Default(fn_decl_span),
 424             c_variadic: false,
 425         };
 426         // We need to lower the declaration outside the new scope, because we
 427         // have to conserve the state of being inside a loop condition for the
 428         // closure argument types.
 429         let fn_decl = self.lower_fn_decl(&outer_decl, None, false, None);
 430
 431         self.with_new_scopes(|this| {
 432             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 433             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 434                 struct_span_err!(
 435                     this.sess,
 436                     fn_decl_span,
 437                     E0708,
 438                     "`async` non-`move` closures with arguments are not currently supported",
 439                 )
 440                 .help(
 441                     "consider using `let` statements to manually capture \
 442                     variables by reference before entering an `async move` closure"
 443                 )
 444                 .emit();
 445             }
 446
 447             // Transform `async |x: u8| -> X { ... }` into
 448             // `|x: u8| future_from_generator(|| -> X { ... })`.
 449             let body_id = this.lower_fn_body(&outer_decl, |this| {
 450                 let async_ret_ty = if let FunctionRetTy::Ty(ty) = &decl.output {
 451                     Some(ty.clone())
 452                 } else {
 453                     None
 454                 };
 455                 let async_body = this.make_async_expr(
 456                     capture_clause, closure_id, async_ret_ty, body.span,
 457                     |this| {
 458                         this.with_new_scopes(|this| this.lower_expr(body))
 459                     }
 460                 );
 461                 this.expr(fn_decl_span, async_body, ThinVec::new())
 462             });
 463             hir::ExprKind::Closure(
 464                 this.lower_capture_clause(capture_clause),
 465                 fn_decl,
 466                 body_id,
 467                 fn_decl_span,
 468                 None,
 469             )
 470         })
 471     }
 472
 473     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
 474     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind {
 475         let id = self.next_id();
 476         let e1 = self.lower_expr(e1);
 477         let e2 = self.lower_expr(e2);
 478         self.expr_call_std_assoc_fn(
 479             id,
 480             span,
 481             &[sym::ops, sym::RangeInclusive],
 482             "new",
 483             hir_vec![e1, e2],
 484         )
 485     }
 486
 487     fn lower_expr_range(
 488         &mut self,
 489         span: Span,
 490         e1: Option<&Expr>,
 491         e2: Option<&Expr>,
 492         lims: RangeLimits,
 493     ) -> hir::ExprKind {
 494         use syntax::ast::RangeLimits::*;
 495
 496         let path = match (e1, e2, lims) {
 497             (None, None, HalfOpen) => sym::RangeFull,
 498             (Some(..), None, HalfOpen) => sym::RangeFrom,
 499             (None, Some(..), HalfOpen) => sym::RangeTo,
 500             (Some(..), Some(..), HalfOpen) => sym::Range,
 501             (None, Some(..), Closed) => sym::RangeToInclusive,
 502             (Some(..), Some(..), Closed) => unreachable!(),
 503             (_, None, Closed) => self.diagnostic()
 504                 .span_fatal(span, "inclusive range with no end")
 505                 .raise(),
 506         };
 507
 508         let fields = e1.iter()
 509             .map(|e| ("start", e))
 510             .chain(e2.iter().map(|e| ("end", e)))
 511             .map(|(s, e)| {
 512                 let expr = P(self.lower_expr(&e));
 513                 let ident = Ident::new(Symbol::intern(s), e.span);
 514                 self.field(ident, expr, e.span)
 515             })
 516             .collect::<P<[hir::Field]>>();
 517
 518         let is_unit = fields.is_empty();
 519         let struct_path = [sym::ops, path];
 520         let struct_path = self.std_path(span, &struct_path, None, is_unit);
 521         let struct_path = hir::QPath::Resolved(None, P(struct_path));
 522
 523         if is_unit {
 524             hir::ExprKind::Path(struct_path)
 525         } else {
 526             hir::ExprKind::Struct(P(struct_path), fields, None)
 527         }
 528     }
 529
 530     fn lower_expr_asm(&mut self, asm: &InlineAsm) -> hir::ExprKind {
 531         let hir_asm = hir::InlineAsm {
 532             inputs: asm.inputs.iter().map(|&(ref c, _)| c.clone()).collect(),
 533             outputs: asm.outputs
 534                 .iter()
 535                 .map(|out| hir::InlineAsmOutput {
 536                     constraint: out.constraint.clone(),
 537                     is_rw: out.is_rw,
 538                     is_indirect: out.is_indirect,
 539                     span: out.expr.span,
 540                 })
 541                 .collect(),
 542             asm: asm.asm.clone(),
 543             asm_str_style: asm.asm_str_style,
 544             clobbers: asm.clobbers.clone().into(),
 545             volatile: asm.volatile,
 546             alignstack: asm.alignstack,
 547             dialect: asm.dialect,
 548             ctxt: asm.ctxt,
 549         };
 550
 551         let outputs = asm.outputs
 552             .iter()
 553             .map(|out| self.lower_expr(&out.expr))
 554             .collect();
 555
 556         let inputs = asm.inputs
 557             .iter()
 558             .map(|&(_, ref input)| self.lower_expr(input))
 559             .collect();
 560
 561         hir::ExprKind::InlineAsm(P(hir_asm), outputs, inputs)
 562     }
 563
 564     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind {
 565         match self.generator_kind {
 566             Some(hir::GeneratorKind::Gen) => {},
 567             Some(hir::GeneratorKind::Async) => {
 568                 span_err!(
 569                     self.sess,
 570                     span,
 571                     E0727,
 572                     "`async` generators are not yet supported",
 573                 );
 574                 self.sess.abort_if_errors();
 575             },
 576             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
 577         }
 578
 579         let expr = opt_expr
 580             .as_ref()
 581             .map(|x| self.lower_expr(x))
 582             .unwrap_or_else(|| self.expr_unit(span));
 583
 584         hir::ExprKind::Yield(P(expr), hir::YieldSource::Yield)
 585     }
 586
 587     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
 588     /// ```rust
 589     /// {
 590     ///     let result = match ::std::iter::IntoIterator::into_iter(<head>) {
 591     ///         mut iter => {
 592     ///             [opt_ident]: loop {
 593     ///                 let mut __next;
 594     ///                 match ::std::iter::Iterator::next(&mut iter) {
 595     ///                     ::std::option::Option::Some(val) => __next = val,
 596     ///                     ::std::option::Option::None => break
 597     ///                 };
 598     ///                 let <pat> = __next;
 599     ///                 StmtKind::Expr(<body>);
 600     ///             }
 601     ///         }
 602     ///     };
 603     ///     result
 604     /// }
 605     /// ```
 606     fn lower_expr_for(
 607         &mut self,
 608         e: &Expr,
 609         pat: &Pat,
 610         head: &Expr,
 611         body: &Block,
 612         opt_label: Option<Label>,
 613     ) -> hir::Expr {
 614         // expand <head>
 615         let mut head = self.lower_expr(head);
 616         let head_sp = head.span;
 617         let desugared_span = self.mark_span_with_reason(
 618             DesugaringKind::ForLoop,
 619             head_sp,
 620             None,
 621         );
 622         head.span = desugared_span;
 623
 624         let iter = Ident::with_empty_ctxt(sym::iter);
 625
 626         let next_ident = Ident::with_empty_ctxt(sym::__next);
 627         let (next_pat, next_pat_hid) = self.pat_ident_binding_mode(
 628             desugared_span,
 629             next_ident,
 630             hir::BindingAnnotation::Mutable,
 631         );
 632
 633         // `::std::option::Option::Some(val) => __next = val`
 634         let pat_arm = {
 635             let val_ident = Ident::with_empty_ctxt(sym::val);
 636             let (val_pat, val_pat_hid) = self.pat_ident(pat.span, val_ident);
 637             let val_expr = P(self.expr_ident(pat.span, val_ident, val_pat_hid));
 638             let next_expr = P(self.expr_ident(pat.span, next_ident, next_pat_hid));
 639             let assign = P(self.expr(
 640                 pat.span,
 641                 hir::ExprKind::Assign(next_expr, val_expr),
 642                 ThinVec::new(),
 643             ));
 644             let some_pat = self.pat_some(pat.span, val_pat);
 645             self.arm(hir_vec![some_pat], assign)
 646         };
 647
 648         // `::std::option::Option::None => break`
 649         let break_arm = {
 650             let break_expr =
 651                 self.with_loop_scope(e.id, |this| this.expr_break(e.span, ThinVec::new()));
 652             let pat = self.pat_none(e.span);
 653             self.arm(hir_vec![pat], break_expr)
 654         };
 655
 656         // `mut iter`
 657         let (iter_pat, iter_pat_nid) = self.pat_ident_binding_mode(
 658             desugared_span,
 659             iter,
 660             hir::BindingAnnotation::Mutable
 661         );
 662
 663         // `match ::std::iter::Iterator::next(&mut iter) { ... }`
 664         let match_expr = {
 665             let iter = P(self.expr_ident(head_sp, iter, iter_pat_nid));
 666             let ref_mut_iter = self.expr_mut_addr_of(head_sp, iter);
 667             let next_path = &[sym::iter, sym::Iterator, sym::next];
 668             let next_expr = P(self.expr_call_std_path(
 669                 head_sp,
 670                 next_path,
 671                 hir_vec![ref_mut_iter],
 672             ));
 673             let arms = hir_vec![pat_arm, break_arm];
 674
 675             self.expr_match(head_sp, next_expr, arms, hir::MatchSource::ForLoopDesugar)
 676         };
 677         let match_stmt = self.stmt_expr(head_sp, match_expr);
 678
 679         let next_expr = P(self.expr_ident(head_sp, next_ident, next_pat_hid));
 680
 681         // `let mut __next`
 682         let next_let = self.stmt_let_pat(
 683             ThinVec::new(),
 684             desugared_span,
 685             None,
 686             next_pat,
 687             hir::LocalSource::ForLoopDesugar,
 688         );
 689
 690         // `let <pat> = __next`
 691         let pat = self.lower_pat(pat);
 692         let pat_let = self.stmt_let_pat(
 693             ThinVec::new(),
 694             head_sp,
 695             Some(next_expr),
 696             pat,
 697             hir::LocalSource::ForLoopDesugar,
 698         );
 699
 700         let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
 701         let body_expr = self.expr_block(body_block, ThinVec::new());
 702         let body_stmt = self.stmt_expr(body.span, body_expr);
 703
 704         let loop_block = P(self.block_all(
 705             e.span,
 706             hir_vec![next_let, match_stmt, pat_let, body_stmt],
 707             None,
 708         ));
 709
 710         // `[opt_ident]: loop { ... }`
 711         let loop_expr = hir::ExprKind::Loop(
 712             loop_block,
 713             self.lower_label(opt_label),
 714             hir::LoopSource::ForLoop,
 715         );
 716         let loop_expr = P(hir::Expr {
 717             hir_id: self.lower_node_id(e.id),
 718             node: loop_expr,
 719             span: e.span,
 720             attrs: ThinVec::new(),
 721         });
 722
 723         // `mut iter => { ... }`
 724         let iter_arm = self.arm(hir_vec![iter_pat], loop_expr);
 725
 726         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
 727         let into_iter_expr = {
 728             let into_iter_path =
 729                 &[sym::iter, sym::IntoIterator, sym::into_iter];
 730             P(self.expr_call_std_path(
 731                 head_sp,
 732                 into_iter_path,
 733                 hir_vec![head],
 734             ))
 735         };
 736
 737         let match_expr = P(self.expr_match(
 738             head_sp,
 739             into_iter_expr,
 740             hir_vec![iter_arm],
 741             hir::MatchSource::ForLoopDesugar,
 742         ));
 743
 744         // This is effectively `{ let _result = ...; _result }`.
 745         // The construct was introduced in #21984 and is necessary to make sure that
 746         // temporaries in the `head` expression are dropped and do not leak to the
 747         // surrounding scope of the `match` since the `match` is not a terminating scope.
 748         //
 749         // Also, add the attributes to the outer returned expr node.
 750         self.expr_drop_temps(head_sp, match_expr, e.attrs.clone())
 751     }
 752
 753     /// Desugar `ExprKind::Try` from: `<expr>?` into:
 754     /// ```rust
 755     /// match Try::into_result(<expr>) {
 756     ///     Ok(val) => #[allow(unreachable_code)] val,
 757     ///     Err(err) => #[allow(unreachable_code)]
 758     ///                 // If there is an enclosing `try {...}`:
 759     ///                 break 'catch_target Try::from_error(From::from(err)),
 760     ///                 // Otherwise:
 761     ///                 return Try::from_error(From::from(err)),
 762     /// }
 763     /// ```
 764     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind {
 765         let unstable_span = self.mark_span_with_reason(
 766             DesugaringKind::QuestionMark,
 767             span,
 768             self.allow_try_trait.clone(),
 769         );
 770         let try_span = self.sess.source_map().end_point(span);
 771         let try_span = self.mark_span_with_reason(
 772             DesugaringKind::QuestionMark,
 773             try_span,
 774             self.allow_try_trait.clone(),
 775         );
 776
 777         // `Try::into_result(<expr>)`
 778         let scrutinee = {
 779             // expand <expr>
 780             let sub_expr = self.lower_expr(sub_expr);
 781
 782             let path = &[sym::ops, sym::Try, sym::into_result];
 783             P(self.expr_call_std_path(unstable_span, path, hir_vec![sub_expr]))
 784         };
 785
 786         // `#[allow(unreachable_code)]`
 787         let attr = {
 788             // `allow(unreachable_code)`
 789             let allow = {
 790                 let allow_ident = Ident::new(sym::allow, span);
 791                 let uc_ident = Ident::new(sym::unreachable_code, span);
 792                 let uc_nested = attr::mk_nested_word_item(uc_ident);
 793                 attr::mk_list_item(allow_ident, vec![uc_nested])
 794             };
 795             attr::mk_attr_outer(allow)
 796         };
 797         let attrs = vec![attr];
 798
 799         // `Ok(val) => #[allow(unreachable_code)] val,`
 800         let ok_arm = {
 801             let val_ident = Ident::with_empty_ctxt(sym::val);
 802             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
 803             let val_expr = P(self.expr_ident_with_attrs(
 804                 span,
 805                 val_ident,
 806                 val_pat_nid,
 807                 ThinVec::from(attrs.clone()),
 808             ));
 809             let ok_pat = self.pat_ok(span, val_pat);
 810
 811             self.arm(hir_vec![ok_pat], val_expr)
 812         };
 813
 814         // `Err(err) => #[allow(unreachable_code)]
 815         //              return Try::from_error(From::from(err)),`
 816         let err_arm = {
 817             let err_ident = Ident::with_empty_ctxt(sym::err);
 818             let (err_local, err_local_nid) = self.pat_ident(try_span, err_ident);
 819             let from_expr = {
 820                 let from_path = &[sym::convert, sym::From, sym::from];
 821                 let err_expr = self.expr_ident(try_span, err_ident, err_local_nid);
 822                 self.expr_call_std_path(try_span, from_path, hir_vec![err_expr])
 823             };
 824             let from_err_expr =
 825                 self.wrap_in_try_constructor(sym::from_error, from_expr, unstable_span);
 826             let thin_attrs = ThinVec::from(attrs);
 827             let catch_scope = self.catch_scopes.last().map(|x| *x);
 828             let ret_expr = if let Some(catch_node) = catch_scope {
 829                 let target_id = Ok(self.lower_node_id(catch_node));
 830                 P(self.expr(
 831                     try_span,
 832                     hir::ExprKind::Break(
 833                         hir::Destination {
 834                             label: None,
 835                             target_id,
 836                         },
 837                         Some(from_err_expr),
 838                     ),
 839                     thin_attrs,
 840                 ))
 841             } else {
 842                 P(self.expr(try_span, hir::ExprKind::Ret(Some(from_err_expr)), thin_attrs))
 843             };
 844
 845             let err_pat = self.pat_err(try_span, err_local);
 846             self.arm(hir_vec![err_pat], ret_expr)
 847         };
 848
 849         hir::ExprKind::Match(
 850             scrutinee,
 851             hir_vec![err_arm, ok_arm],
 852             hir::MatchSource::TryDesugar,
 853         )
 854     }
 855 }