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