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                 use syntax::ast::RangeLimits::*;
 380
 381                 let path = match (e1, e2, lims) {
 382                     (&None, &None, HalfOpen) => sym::RangeFull,
 383                     (&Some(..), &None, HalfOpen) => sym::RangeFrom,
 384                     (&None, &Some(..), HalfOpen) => sym::RangeTo,
 385                     (&Some(..), &Some(..), HalfOpen) => sym::Range,
 386                     (&None, &Some(..), Closed) => sym::RangeToInclusive,
 387                     (&Some(..), &Some(..), Closed) => unreachable!(),
 388                     (_, &None, Closed) => self.diagnostic()
 389                         .span_fatal(e.span, "inclusive range with no end")
 390                         .raise(),
 391                 };
 392
 393                 let fields = e1.iter()
 394                     .map(|e| ("start", e))
 395                     .chain(e2.iter().map(|e| ("end", e)))
 396                     .map(|(s, e)| {
 397                         let expr = P(self.lower_expr(&e));
 398                         let ident = Ident::new(Symbol::intern(s), e.span);
 399                         self.field(ident, expr, e.span)
 400                     })
 401                     .collect::<P<[hir::Field]>>();
 402
 403                 let is_unit = fields.is_empty();
 404                 let struct_path = [sym::ops, path];
 405                 let struct_path = self.std_path(e.span, &struct_path, None, is_unit);
 406                 let struct_path = hir::QPath::Resolved(None, P(struct_path));
 407
 408                 return hir::Expr {
 409                     hir_id: self.lower_node_id(e.id),
 410                     node: if is_unit {
 411                         hir::ExprKind::Path(struct_path)
 412                     } else {
 413                         hir::ExprKind::Struct(P(struct_path), fields, None)
 414                     },
 415                     span: e.span,
 416                     attrs: e.attrs.clone(),
 417                 };
 418             }
 419             ExprKind::Path(ref qself, ref path) => {
 420                 let qpath = self.lower_qpath(
 421                     e.id,
 422                     qself,
 423                     path,
 424                     ParamMode::Optional,
 425                     ImplTraitContext::disallowed(),
 426                 );
 427                 hir::ExprKind::Path(qpath)
 428             }
 429             ExprKind::Break(opt_label, ref opt_expr) => {
 430                 let destination = if self.is_in_loop_condition && opt_label.is_none() {
 431                     hir::Destination {
 432                         label: None,
 433                         target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 434                     }
 435                 } else {
 436                     self.lower_loop_destination(opt_label.map(|label| (e.id, label)))
 437                 };
 438                 hir::ExprKind::Break(
 439                     destination,
 440                     opt_expr.as_ref().map(|x| P(self.lower_expr(x))),
 441                 )
 442             }
 443             ExprKind::Continue(opt_label) => {
 444                 hir::ExprKind::Continue(if self.is_in_loop_condition && opt_label.is_none() {
 445                     hir::Destination {
 446                         label: None,
 447                         target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 448                     }
 449                 } else {
 450                     self.lower_loop_destination(opt_label.map(|label| (e.id, label)))
 451                 })
 452             }
 453             ExprKind::Ret(ref e) => hir::ExprKind::Ret(e.as_ref().map(|x| P(self.lower_expr(x)))),
 454             ExprKind::InlineAsm(ref asm) => {
 455                 let hir_asm = hir::InlineAsm {
 456                     inputs: asm.inputs.iter().map(|&(ref c, _)| c.clone()).collect(),
 457                     outputs: asm.outputs
 458                         .iter()
 459                         .map(|out| hir::InlineAsmOutput {
 460                             constraint: out.constraint.clone(),
 461                             is_rw: out.is_rw,
 462                             is_indirect: out.is_indirect,
 463                             span: out.expr.span,
 464                         })
 465                         .collect(),
 466                     asm: asm.asm.clone(),
 467                     asm_str_style: asm.asm_str_style,
 468                     clobbers: asm.clobbers.clone().into(),
 469                     volatile: asm.volatile,
 470                     alignstack: asm.alignstack,
 471                     dialect: asm.dialect,
 472                     ctxt: asm.ctxt,
 473                 };
 474                 let outputs = asm.outputs
 475                     .iter()
 476                     .map(|out| self.lower_expr(&out.expr))
 477                     .collect();
 478                 let inputs = asm.inputs
 479                     .iter()
 480                     .map(|&(_, ref input)| self.lower_expr(input))
 481                     .collect();
 482                 hir::ExprKind::InlineAsm(P(hir_asm), outputs, inputs)
 483             }
 484             ExprKind::Struct(ref path, ref fields, ref maybe_expr) => hir::ExprKind::Struct(
 485                 P(self.lower_qpath(
 486                     e.id,
 487                     &None,
 488                     path,
 489                     ParamMode::Optional,
 490                     ImplTraitContext::disallowed(),
 491                 )),
 492                 fields.iter().map(|x| self.lower_field(x)).collect(),
 493                 maybe_expr.as_ref().map(|x| P(self.lower_expr(x))),
 494             ),
 495             ExprKind::Paren(ref ex) => {
 496                 let mut ex = self.lower_expr(ex);
 497                 // Include parens in span, but only if it is a super-span.
 498                 if e.span.contains(ex.span) {
 499                     ex.span = e.span;
 500                 }
 501                 // Merge attributes into the inner expression.
 502                 let mut attrs = e.attrs.clone();
 503                 attrs.extend::<Vec<_>>(ex.attrs.into());
 504                 ex.attrs = attrs;
 505                 return ex;
 506             }
 507
 508             ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 509
 510             ExprKind::Err => hir::ExprKind::Err,
 511
 512             // Desugar `ExprForLoop`
 513             // from: `[opt_ident]: for <pat> in <head> <body>`
 514             ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 515                 return self.lower_expr_for(e, pat, head, body, opt_label);
 516             }
 517             ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 518             ExprKind::Mac(_) => panic!("Shouldn't exist here"),
 519         };
 520
 521         hir::Expr {
 522             hir_id: self.lower_node_id(e.id),
 523             node: kind,
 524             span: e.span,
 525             attrs: e.attrs.clone(),
 526         }
 527     }
 528
 529     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind {
 530         match self.generator_kind {
 531             Some(hir::GeneratorKind::Gen) => {},
 532             Some(hir::GeneratorKind::Async) => {
 533                 span_err!(
 534                     self.sess,
 535                     span,
 536                     E0727,
 537                     "`async` generators are not yet supported",
 538                 );
 539                 self.sess.abort_if_errors();
 540             },
 541             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
 542         }
 543
 544         let expr = opt_expr
 545             .as_ref()
 546             .map(|x| self.lower_expr(x))
 547             .unwrap_or_else(|| self.expr_unit(span));
 548
 549         hir::ExprKind::Yield(P(expr), hir::YieldSource::Yield)
 550     }
 551
 552     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
 553     /// ```rust
 554     /// {
 555     ///     let result = match ::std::iter::IntoIterator::into_iter(<head>) {
 556     ///         mut iter => {
 557     ///             [opt_ident]: loop {
 558     ///                 let mut __next;
 559     ///                 match ::std::iter::Iterator::next(&mut iter) {
 560     ///                     ::std::option::Option::Some(val) => __next = val,
 561     ///                     ::std::option::Option::None => break
 562     ///                 };
 563     ///                 let <pat> = __next;
 564     ///                 StmtKind::Expr(<body>);
 565     ///             }
 566     ///         }
 567     ///     };
 568     ///     result
 569     /// }
 570     /// ```
 571     fn lower_expr_for(
 572         &mut self,
 573         e: &Expr,
 574         pat: &Pat,
 575         head: &Expr,
 576         body: &Block,
 577         opt_label: Option<Label>,
 578     ) -> hir::Expr {
 579         // expand <head>
 580         let mut head = self.lower_expr(head);
 581         let head_sp = head.span;
 582         let desugared_span = self.mark_span_with_reason(
 583             DesugaringKind::ForLoop,
 584             head_sp,
 585             None,
 586         );
 587         head.span = desugared_span;
 588
 589         let iter = Ident::with_empty_ctxt(sym::iter);
 590
 591         let next_ident = Ident::with_empty_ctxt(sym::__next);
 592         let (next_pat, next_pat_hid) = self.pat_ident_binding_mode(
 593             desugared_span,
 594             next_ident,
 595             hir::BindingAnnotation::Mutable,
 596         );
 597
 598         // `::std::option::Option::Some(val) => __next = val`
 599         let pat_arm = {
 600             let val_ident = Ident::with_empty_ctxt(sym::val);
 601             let (val_pat, val_pat_hid) = self.pat_ident(pat.span, val_ident);
 602             let val_expr = P(self.expr_ident(pat.span, val_ident, val_pat_hid));
 603             let next_expr = P(self.expr_ident(pat.span, next_ident, next_pat_hid));
 604             let assign = P(self.expr(
 605                 pat.span,
 606                 hir::ExprKind::Assign(next_expr, val_expr),
 607                 ThinVec::new(),
 608             ));
 609             let some_pat = self.pat_some(pat.span, val_pat);
 610             self.arm(hir_vec![some_pat], assign)
 611         };
 612
 613         // `::std::option::Option::None => break`
 614         let break_arm = {
 615             let break_expr =
 616                 self.with_loop_scope(e.id, |this| this.expr_break(e.span, ThinVec::new()));
 617             let pat = self.pat_none(e.span);
 618             self.arm(hir_vec![pat], break_expr)
 619         };
 620
 621         // `mut iter`
 622         let (iter_pat, iter_pat_nid) = self.pat_ident_binding_mode(
 623             desugared_span,
 624             iter,
 625             hir::BindingAnnotation::Mutable
 626         );
 627
 628         // `match ::std::iter::Iterator::next(&mut iter) { ... }`
 629         let match_expr = {
 630             let iter = P(self.expr_ident(head_sp, iter, iter_pat_nid));
 631             let ref_mut_iter = self.expr_mut_addr_of(head_sp, iter);
 632             let next_path = &[sym::iter, sym::Iterator, sym::next];
 633             let next_expr = P(self.expr_call_std_path(
 634                 head_sp,
 635                 next_path,
 636                 hir_vec![ref_mut_iter],
 637             ));
 638             let arms = hir_vec![pat_arm, break_arm];
 639
 640             self.expr_match(head_sp, next_expr, arms, hir::MatchSource::ForLoopDesugar)
 641         };
 642         let match_stmt = self.stmt_expr(head_sp, match_expr);
 643
 644         let next_expr = P(self.expr_ident(head_sp, next_ident, next_pat_hid));
 645
 646         // `let mut __next`
 647         let next_let = self.stmt_let_pat(
 648             ThinVec::new(),
 649             desugared_span,
 650             None,
 651             next_pat,
 652             hir::LocalSource::ForLoopDesugar,
 653         );
 654
 655         // `let <pat> = __next`
 656         let pat = self.lower_pat(pat);
 657         let pat_let = self.stmt_let_pat(
 658             ThinVec::new(),
 659             head_sp,
 660             Some(next_expr),
 661             pat,
 662             hir::LocalSource::ForLoopDesugar,
 663         );
 664
 665         let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
 666         let body_expr = self.expr_block(body_block, ThinVec::new());
 667         let body_stmt = self.stmt_expr(body.span, body_expr);
 668
 669         let loop_block = P(self.block_all(
 670             e.span,
 671             hir_vec![next_let, match_stmt, pat_let, body_stmt],
 672             None,
 673         ));
 674
 675         // `[opt_ident]: loop { ... }`
 676         let loop_expr = hir::ExprKind::Loop(
 677             loop_block,
 678             self.lower_label(opt_label),
 679             hir::LoopSource::ForLoop,
 680         );
 681         let loop_expr = P(hir::Expr {
 682             hir_id: self.lower_node_id(e.id),
 683             node: loop_expr,
 684             span: e.span,
 685             attrs: ThinVec::new(),
 686         });
 687
 688         // `mut iter => { ... }`
 689         let iter_arm = self.arm(hir_vec![iter_pat], loop_expr);
 690
 691         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
 692         let into_iter_expr = {
 693             let into_iter_path =
 694                 &[sym::iter, sym::IntoIterator, sym::into_iter];
 695             P(self.expr_call_std_path(
 696                 head_sp,
 697                 into_iter_path,
 698                 hir_vec![head],
 699             ))
 700         };
 701
 702         let match_expr = P(self.expr_match(
 703             head_sp,
 704             into_iter_expr,
 705             hir_vec![iter_arm],
 706             hir::MatchSource::ForLoopDesugar,
 707         ));
 708
 709         // This is effectively `{ let _result = ...; _result }`.
 710         // The construct was introduced in #21984 and is necessary to make sure that
 711         // temporaries in the `head` expression are dropped and do not leak to the
 712         // surrounding scope of the `match` since the `match` is not a terminating scope.
 713         //
 714         // Also, add the attributes to the outer returned expr node.
 715         self.expr_drop_temps(head_sp, match_expr, e.attrs.clone())
 716     }
 717
 718     /// Desugar `ExprKind::Try` from: `<expr>?` into:
 719     /// ```rust
 720     /// match Try::into_result(<expr>) {
 721     ///     Ok(val) => #[allow(unreachable_code)] val,
 722     ///     Err(err) => #[allow(unreachable_code)]
 723     ///                 // If there is an enclosing `try {...}`:
 724     ///                 break 'catch_target Try::from_error(From::from(err)),
 725     ///                 // Otherwise:
 726     ///                 return Try::from_error(From::from(err)),
 727     /// }
 728     /// ```
 729     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind {
 730         let unstable_span = self.mark_span_with_reason(
 731             DesugaringKind::QuestionMark,
 732             span,
 733             self.allow_try_trait.clone(),
 734         );
 735         let try_span = self.sess.source_map().end_point(span);
 736         let try_span = self.mark_span_with_reason(
 737             DesugaringKind::QuestionMark,
 738             try_span,
 739             self.allow_try_trait.clone(),
 740         );
 741
 742         // `Try::into_result(<expr>)`
 743         let scrutinee = {
 744             // expand <expr>
 745             let sub_expr = self.lower_expr(sub_expr);
 746
 747             let path = &[sym::ops, sym::Try, sym::into_result];
 748             P(self.expr_call_std_path(unstable_span, path, hir_vec![sub_expr]))
 749         };
 750
 751         // `#[allow(unreachable_code)]`
 752         let attr = {
 753             // `allow(unreachable_code)`
 754             let allow = {
 755                 let allow_ident = Ident::new(sym::allow, span);
 756                 let uc_ident = Ident::new(sym::unreachable_code, span);
 757                 let uc_nested = attr::mk_nested_word_item(uc_ident);
 758                 attr::mk_list_item(allow_ident, vec![uc_nested])
 759             };
 760             attr::mk_attr_outer(allow)
 761         };
 762         let attrs = vec![attr];
 763
 764         // `Ok(val) => #[allow(unreachable_code)] val,`
 765         let ok_arm = {
 766             let val_ident = Ident::with_empty_ctxt(sym::val);
 767             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
 768             let val_expr = P(self.expr_ident_with_attrs(
 769                 span,
 770                 val_ident,
 771                 val_pat_nid,
 772                 ThinVec::from(attrs.clone()),
 773             ));
 774             let ok_pat = self.pat_ok(span, val_pat);
 775
 776             self.arm(hir_vec![ok_pat], val_expr)
 777         };
 778
 779         // `Err(err) => #[allow(unreachable_code)]
 780         //              return Try::from_error(From::from(err)),`
 781         let err_arm = {
 782             let err_ident = Ident::with_empty_ctxt(sym::err);
 783             let (err_local, err_local_nid) = self.pat_ident(try_span, err_ident);
 784             let from_expr = {
 785                 let from_path = &[sym::convert, sym::From, sym::from];
 786                 let err_expr = self.expr_ident(try_span, err_ident, err_local_nid);
 787                 self.expr_call_std_path(try_span, from_path, hir_vec![err_expr])
 788             };
 789             let from_err_expr =
 790                 self.wrap_in_try_constructor(sym::from_error, from_expr, unstable_span);
 791             let thin_attrs = ThinVec::from(attrs);
 792             let catch_scope = self.catch_scopes.last().map(|x| *x);
 793             let ret_expr = if let Some(catch_node) = catch_scope {
 794                 let target_id = Ok(self.lower_node_id(catch_node));
 795                 P(self.expr(
 796                     try_span,
 797                     hir::ExprKind::Break(
 798                         hir::Destination {
 799                             label: None,
 800                             target_id,
 801                         },
 802                         Some(from_err_expr),
 803                     ),
 804                     thin_attrs,
 805                 ))
 806             } else {
 807                 P(self.expr(try_span, hir::ExprKind::Ret(Some(from_err_expr)), thin_attrs))
 808             };
 809
 810             let err_pat = self.pat_err(try_span, err_local);
 811             self.arm(hir_vec![err_pat], ret_expr)
 812         };
 813
 814         hir::ExprKind::Match(
 815             scrutinee,
 816             hir_vec![err_arm, ok_arm],
 817             hir::MatchSource::TryDesugar,
 818         )
 819     }
 820 }