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) => self.lower_expr_try_block(body),
 229             ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
 230                 P(self.lower_expr(expr)),
 231                 arms.iter().map(|x| self.lower_arm(x)).collect(),
 232                 hir::MatchSource::Normal,
 233             ),
 234             ExprKind::Async(capture_clause, closure_node_id, ref block) => {
 235                 self.make_async_expr(capture_clause, closure_node_id, None, block.span, |this| {
 236                     this.with_new_scopes(|this| {
 237                         let block = this.lower_block(block, false);
 238                         this.expr_block(block, ThinVec::new())
 239                     })
 240                 })
 241             }
 242             ExprKind::Await(ref expr) => self.lower_expr_await(e.span, expr),
 243             ExprKind::Closure(
 244                 capture_clause, asyncness, movability, ref decl, ref body, fn_decl_span
 245             ) => if let IsAsync::Async { closure_id, .. } = asyncness {
 246                 self.lower_expr_async_closure(capture_clause, closure_id, decl, body, fn_decl_span)
 247             } else {
 248                 self.lower_expr_closure(capture_clause, movability, decl, body, fn_decl_span)
 249             }
 250             ExprKind::Block(ref blk, opt_label) => {
 251                 hir::ExprKind::Block(self.lower_block(blk,
 252                                                       opt_label.is_some()),
 253                                                       self.lower_label(opt_label))
 254             }
 255             ExprKind::Assign(ref el, ref er) => {
 256                 hir::ExprKind::Assign(P(self.lower_expr(el)), P(self.lower_expr(er)))
 257             }
 258             ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 259                 self.lower_binop(op),
 260                 P(self.lower_expr(el)),
 261                 P(self.lower_expr(er)),
 262             ),
 263             ExprKind::Field(ref el, ident) => hir::ExprKind::Field(P(self.lower_expr(el)), ident),
 264             ExprKind::Index(ref el, ref er) => {
 265                 hir::ExprKind::Index(P(self.lower_expr(el)), P(self.lower_expr(er)))
 266             }
 267             ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 268                 self.lower_expr_range_closed(e.span, e1, e2)
 269             }
 270             ExprKind::Range(ref e1, ref e2, lims) => {
 271                 self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 272             }
 273             ExprKind::Path(ref qself, ref path) => {
 274                 let qpath = self.lower_qpath(
 275                     e.id,
 276                     qself,
 277                     path,
 278                     ParamMode::Optional,
 279                     ImplTraitContext::disallowed(),
 280                 );
 281                 hir::ExprKind::Path(qpath)
 282             }
 283             ExprKind::Break(opt_label, ref opt_expr) => {
 284                 let destination = if self.is_in_loop_condition && opt_label.is_none() {
 285                     hir::Destination {
 286                         label: None,
 287                         target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 288                     }
 289                 } else {
 290                     self.lower_loop_destination(opt_label.map(|label| (e.id, label)))
 291                 };
 292                 hir::ExprKind::Break(
 293                     destination,
 294                     opt_expr.as_ref().map(|x| P(self.lower_expr(x))),
 295                 )
 296             }
 297             ExprKind::Continue(opt_label) => {
 298                 hir::ExprKind::Continue(if self.is_in_loop_condition && opt_label.is_none() {
 299                     hir::Destination {
 300                         label: None,
 301                         target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 302                     }
 303                 } else {
 304                     self.lower_loop_destination(opt_label.map(|label| (e.id, label)))
 305                 })
 306             }
 307             ExprKind::Ret(ref e) => hir::ExprKind::Ret(e.as_ref().map(|x| P(self.lower_expr(x)))),
 308             ExprKind::InlineAsm(ref asm) => self.lower_expr_asm(asm),
 309             ExprKind::Struct(ref path, ref fields, ref maybe_expr) => hir::ExprKind::Struct(
 310                 P(self.lower_qpath(
 311                     e.id,
 312                     &None,
 313                     path,
 314                     ParamMode::Optional,
 315                     ImplTraitContext::disallowed(),
 316                 )),
 317                 fields.iter().map(|x| self.lower_field(x)).collect(),
 318                 maybe_expr.as_ref().map(|x| P(self.lower_expr(x))),
 319             ),
 320             ExprKind::Paren(ref ex) => {
 321                 let mut ex = self.lower_expr(ex);
 322                 // Include parens in span, but only if it is a super-span.
 323                 if e.span.contains(ex.span) {
 324                     ex.span = e.span;
 325                 }
 326                 // Merge attributes into the inner expression.
 327                 let mut attrs = e.attrs.clone();
 328                 attrs.extend::<Vec<_>>(ex.attrs.into());
 329                 ex.attrs = attrs;
 330                 return ex;
 331             }
 332
 333             ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 334
 335             ExprKind::Err => hir::ExprKind::Err,
 336
 337             // Desugar `ExprForLoop`
 338             // from: `[opt_ident]: for <pat> in <head> <body>`
 339             ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 340                 return self.lower_expr_for(e, pat, head, body, opt_label);
 341             }
 342             ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 343             ExprKind::Mac(_) => panic!("Shouldn't exist here"),
 344         };
 345
 346         hir::Expr {
 347             hir_id: self.lower_node_id(e.id),
 348             node: kind,
 349             span: e.span,
 350             attrs: e.attrs.clone(),
 351         }
 352     }
 353
 354     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind {
 355         self.with_catch_scope(body.id, |this| {
 356             let unstable_span = this.mark_span_with_reason(
 357                 DesugaringKind::TryBlock,
 358                 body.span,
 359                 this.allow_try_trait.clone(),
 360             );
 361             let mut block = this.lower_block(body, true).into_inner();
 362             let tail = block.expr.take().map_or_else(
 363                 || this.expr_unit(this.sess.source_map().end_point(unstable_span)),
 364                 |x: P<hir::Expr>| x.into_inner(),
 365             );
 366             block.expr = Some(this.wrap_in_try_constructor(sym::from_ok, tail, unstable_span));
 367             hir::ExprKind::Block(P(block), None)
 368         })
 369     }
 370
 371     /// Desugar `<expr>.await` into:
 372     /// ```rust
 373     /// {
 374     ///     let mut pinned = <expr>;
 375     ///     loop {
 376     ///         match ::std::future::poll_with_tls_context(unsafe {
 377     ///             ::std::pin::Pin::new_unchecked(&mut pinned)
 378     ///         }) {
 379     ///             ::std::task::Poll::Ready(result) => break result,
 380     ///             ::std::task::Poll::Pending => {},
 381     ///         }
 382     ///         yield ();
 383     ///     }
 384     /// }
 385     /// ```
 386     fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind {
 387         match self.generator_kind {
 388             Some(hir::GeneratorKind::Async) => {},
 389             Some(hir::GeneratorKind::Gen) |
 390             None => {
 391                 let mut err = struct_span_err!(
 392                     self.sess,
 393                     await_span,
 394                     E0728,
 395                     "`await` is only allowed inside `async` functions and blocks"
 396                 );
 397                 err.span_label(await_span, "only allowed inside `async` functions and blocks");
 398                 if let Some(item_sp) = self.current_item {
 399                     err.span_label(item_sp, "this is not `async`");
 400                 }
 401                 err.emit();
 402             }
 403         }
 404         let span = self.mark_span_with_reason(
 405             DesugaringKind::Await,
 406             await_span,
 407             None,
 408         );
 409         let gen_future_span = self.mark_span_with_reason(
 410             DesugaringKind::Await,
 411             await_span,
 412             self.allow_gen_future.clone(),
 413         );
 414
 415         // let mut pinned = <expr>;
 416         let expr = P(self.lower_expr(expr));
 417         let pinned_ident = Ident::with_empty_ctxt(sym::pinned);
 418         let (pinned_pat, pinned_pat_hid) = self.pat_ident_binding_mode(
 419             span,
 420             pinned_ident,
 421             hir::BindingAnnotation::Mutable,
 422         );
 423         let pinned_let = self.stmt_let_pat(
 424             ThinVec::new(),
 425             span,
 426             Some(expr),
 427             pinned_pat,
 428             hir::LocalSource::AwaitDesugar,
 429         );
 430
 431         // ::std::future::poll_with_tls_context(unsafe {
 432         //     ::std::pin::Pin::new_unchecked(&mut pinned)
 433         // })`
 434         let poll_expr = {
 435             let pinned = P(self.expr_ident(span, pinned_ident, pinned_pat_hid));
 436             let ref_mut_pinned = self.expr_mut_addr_of(span, pinned);
 437             let pin_ty_id = self.next_id();
 438             let new_unchecked_expr_kind = self.expr_call_std_assoc_fn(
 439                 pin_ty_id,
 440                 span,
 441                 &[sym::pin, sym::Pin],
 442                 "new_unchecked",
 443                 hir_vec![ref_mut_pinned],
 444             );
 445             let new_unchecked = P(self.expr(span, new_unchecked_expr_kind, ThinVec::new()));
 446             let unsafe_expr = self.expr_unsafe(new_unchecked);
 447             P(self.expr_call_std_path(
 448                 gen_future_span,
 449                 &[sym::future, sym::poll_with_tls_context],
 450                 hir_vec![unsafe_expr],
 451             ))
 452         };
 453
 454         // `::std::task::Poll::Ready(result) => break result`
 455         let loop_node_id = self.sess.next_node_id();
 456         let loop_hir_id = self.lower_node_id(loop_node_id);
 457         let ready_arm = {
 458             let x_ident = Ident::with_empty_ctxt(sym::result);
 459             let (x_pat, x_pat_hid) = self.pat_ident(span, x_ident);
 460             let x_expr = P(self.expr_ident(span, x_ident, x_pat_hid));
 461             let ready_pat = self.pat_std_enum(
 462                 span,
 463                 &[sym::task, sym::Poll, sym::Ready],
 464                 hir_vec![x_pat],
 465             );
 466             let break_x = self.with_loop_scope(loop_node_id, |this| {
 467                 let expr_break = hir::ExprKind::Break(
 468                     this.lower_loop_destination(None),
 469                     Some(x_expr),
 470                 );
 471                 P(this.expr(await_span, expr_break, ThinVec::new()))
 472             });
 473             self.arm(hir_vec![ready_pat], break_x)
 474         };
 475
 476         // `::std::task::Poll::Pending => {}`
 477         let pending_arm = {
 478             let pending_pat = self.pat_std_enum(
 479                 span,
 480                 &[sym::task, sym::Poll, sym::Pending],
 481                 hir_vec![],
 482             );
 483             let empty_block = P(self.expr_block_empty(span));
 484             self.arm(hir_vec![pending_pat], empty_block)
 485         };
 486
 487         let match_stmt = {
 488             let match_expr = self.expr_match(
 489                 span,
 490                 poll_expr,
 491                 hir_vec![ready_arm, pending_arm],
 492                 hir::MatchSource::AwaitDesugar,
 493             );
 494             self.stmt_expr(span, match_expr)
 495         };
 496
 497         let yield_stmt = {
 498             let unit = self.expr_unit(span);
 499             let yield_expr = self.expr(
 500                 span,
 501                 hir::ExprKind::Yield(P(unit), hir::YieldSource::Await),
 502                 ThinVec::new(),
 503             );
 504             self.stmt_expr(span, yield_expr)
 505         };
 506
 507         let loop_block = P(self.block_all(
 508             span,
 509             hir_vec![match_stmt, yield_stmt],
 510             None,
 511         ));
 512
 513         let loop_expr = P(hir::Expr {
 514             hir_id: loop_hir_id,
 515             node: hir::ExprKind::Loop(
 516                 loop_block,
 517                 None,
 518                 hir::LoopSource::Loop,
 519             ),
 520             span,
 521             attrs: ThinVec::new(),
 522         });
 523
 524         hir::ExprKind::Block(
 525             P(self.block_all(span, hir_vec![pinned_let], Some(loop_expr))),
 526             None,
 527         )
 528     }
 529
 530     fn lower_expr_closure(
 531         &mut self,
 532         capture_clause: CaptureBy,
 533         movability: Movability,
 534         decl: &FnDecl,
 535         body: &Expr,
 536         fn_decl_span: Span,
 537     ) -> hir::ExprKind {
 538         // Lower outside new scope to preserve `is_in_loop_condition`.
 539         let fn_decl = self.lower_fn_decl(decl, None, false, None);
 540
 541         self.with_new_scopes(|this| {
 542             this.current_item = Some(fn_decl_span);
 543             let mut generator_kind = None;
 544             let body_id = this.lower_fn_body(decl, |this| {
 545                 let e = this.lower_expr(body);
 546                 generator_kind = this.generator_kind;
 547                 e
 548             });
 549             let generator_option = this.generator_movability_for_fn(
 550                 &decl,
 551                 fn_decl_span,
 552                 generator_kind,
 553                 movability,
 554             );
 555             hir::ExprKind::Closure(
 556                 this.lower_capture_clause(capture_clause),
 557                 fn_decl,
 558                 body_id,
 559                 fn_decl_span,
 560                 generator_option,
 561             )
 562         })
 563     }
 564
 565     fn lower_expr_async_closure(
 566         &mut self,
 567         capture_clause: CaptureBy,
 568         closure_id: NodeId,
 569         decl: &FnDecl,
 570         body: &Expr,
 571         fn_decl_span: Span,
 572     ) -> hir::ExprKind {
 573         let outer_decl = FnDecl {
 574             inputs: decl.inputs.clone(),
 575             output: FunctionRetTy::Default(fn_decl_span),
 576             c_variadic: false,
 577         };
 578         // We need to lower the declaration outside the new scope, because we
 579         // have to conserve the state of being inside a loop condition for the
 580         // closure argument types.
 581         let fn_decl = self.lower_fn_decl(&outer_decl, None, false, None);
 582
 583         self.with_new_scopes(|this| {
 584             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 585             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 586                 struct_span_err!(
 587                     this.sess,
 588                     fn_decl_span,
 589                     E0708,
 590                     "`async` non-`move` closures with arguments are not currently supported",
 591                 )
 592                 .help(
 593                     "consider using `let` statements to manually capture \
 594                     variables by reference before entering an `async move` closure"
 595                 )
 596                 .emit();
 597             }
 598
 599             // Transform `async |x: u8| -> X { ... }` into
 600             // `|x: u8| future_from_generator(|| -> X { ... })`.
 601             let body_id = this.lower_fn_body(&outer_decl, |this| {
 602                 let async_ret_ty = if let FunctionRetTy::Ty(ty) = &decl.output {
 603                     Some(ty.clone())
 604                 } else {
 605                     None
 606                 };
 607                 let async_body = this.make_async_expr(
 608                     capture_clause, closure_id, async_ret_ty, body.span,
 609                     |this| {
 610                         this.with_new_scopes(|this| this.lower_expr(body))
 611                     }
 612                 );
 613                 this.expr(fn_decl_span, async_body, ThinVec::new())
 614             });
 615             hir::ExprKind::Closure(
 616                 this.lower_capture_clause(capture_clause),
 617                 fn_decl,
 618                 body_id,
 619                 fn_decl_span,
 620                 None,
 621             )
 622         })
 623     }
 624
 625     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
 626     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind {
 627         let id = self.next_id();
 628         let e1 = self.lower_expr(e1);
 629         let e2 = self.lower_expr(e2);
 630         self.expr_call_std_assoc_fn(
 631             id,
 632             span,
 633             &[sym::ops, sym::RangeInclusive],
 634             "new",
 635             hir_vec![e1, e2],
 636         )
 637     }
 638
 639     fn lower_expr_range(
 640         &mut self,
 641         span: Span,
 642         e1: Option<&Expr>,
 643         e2: Option<&Expr>,
 644         lims: RangeLimits,
 645     ) -> hir::ExprKind {
 646         use syntax::ast::RangeLimits::*;
 647
 648         let path = match (e1, e2, lims) {
 649             (None, None, HalfOpen) => sym::RangeFull,
 650             (Some(..), None, HalfOpen) => sym::RangeFrom,
 651             (None, Some(..), HalfOpen) => sym::RangeTo,
 652             (Some(..), Some(..), HalfOpen) => sym::Range,
 653             (None, Some(..), Closed) => sym::RangeToInclusive,
 654             (Some(..), Some(..), Closed) => unreachable!(),
 655             (_, None, Closed) => self.diagnostic()
 656                 .span_fatal(span, "inclusive range with no end")
 657                 .raise(),
 658         };
 659
 660         let fields = e1.iter()
 661             .map(|e| ("start", e))
 662             .chain(e2.iter().map(|e| ("end", e)))
 663             .map(|(s, e)| {
 664                 let expr = P(self.lower_expr(&e));
 665                 let ident = Ident::new(Symbol::intern(s), e.span);
 666                 self.field(ident, expr, e.span)
 667             })
 668             .collect::<P<[hir::Field]>>();
 669
 670         let is_unit = fields.is_empty();
 671         let struct_path = [sym::ops, path];
 672         let struct_path = self.std_path(span, &struct_path, None, is_unit);
 673         let struct_path = hir::QPath::Resolved(None, P(struct_path));
 674
 675         if is_unit {
 676             hir::ExprKind::Path(struct_path)
 677         } else {
 678             hir::ExprKind::Struct(P(struct_path), fields, None)
 679         }
 680     }
 681
 682     fn lower_expr_asm(&mut self, asm: &InlineAsm) -> hir::ExprKind {
 683         let hir_asm = hir::InlineAsm {
 684             inputs: asm.inputs.iter().map(|&(ref c, _)| c.clone()).collect(),
 685             outputs: asm.outputs
 686                 .iter()
 687                 .map(|out| hir::InlineAsmOutput {
 688                     constraint: out.constraint.clone(),
 689                     is_rw: out.is_rw,
 690                     is_indirect: out.is_indirect,
 691                     span: out.expr.span,
 692                 })
 693                 .collect(),
 694             asm: asm.asm.clone(),
 695             asm_str_style: asm.asm_str_style,
 696             clobbers: asm.clobbers.clone().into(),
 697             volatile: asm.volatile,
 698             alignstack: asm.alignstack,
 699             dialect: asm.dialect,
 700             ctxt: asm.ctxt,
 701         };
 702
 703         let outputs = asm.outputs
 704             .iter()
 705             .map(|out| self.lower_expr(&out.expr))
 706             .collect();
 707
 708         let inputs = asm.inputs
 709             .iter()
 710             .map(|&(_, ref input)| self.lower_expr(input))
 711             .collect();
 712
 713         hir::ExprKind::InlineAsm(P(hir_asm), outputs, inputs)
 714     }
 715
 716     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind {
 717         match self.generator_kind {
 718             Some(hir::GeneratorKind::Gen) => {},
 719             Some(hir::GeneratorKind::Async) => {
 720                 span_err!(
 721                     self.sess,
 722                     span,
 723                     E0727,
 724                     "`async` generators are not yet supported",
 725                 );
 726                 self.sess.abort_if_errors();
 727             },
 728             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
 729         }
 730
 731         let expr = opt_expr
 732             .as_ref()
 733             .map(|x| self.lower_expr(x))
 734             .unwrap_or_else(|| self.expr_unit(span));
 735
 736         hir::ExprKind::Yield(P(expr), hir::YieldSource::Yield)
 737     }
 738
 739     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
 740     /// ```rust
 741     /// {
 742     ///     let result = match ::std::iter::IntoIterator::into_iter(<head>) {
 743     ///         mut iter => {
 744     ///             [opt_ident]: loop {
 745     ///                 let mut __next;
 746     ///                 match ::std::iter::Iterator::next(&mut iter) {
 747     ///                     ::std::option::Option::Some(val) => __next = val,
 748     ///                     ::std::option::Option::None => break
 749     ///                 };
 750     ///                 let <pat> = __next;
 751     ///                 StmtKind::Expr(<body>);
 752     ///             }
 753     ///         }
 754     ///     };
 755     ///     result
 756     /// }
 757     /// ```
 758     fn lower_expr_for(
 759         &mut self,
 760         e: &Expr,
 761         pat: &Pat,
 762         head: &Expr,
 763         body: &Block,
 764         opt_label: Option<Label>,
 765     ) -> hir::Expr {
 766         // expand <head>
 767         let mut head = self.lower_expr(head);
 768         let head_sp = head.span;
 769         let desugared_span = self.mark_span_with_reason(
 770             DesugaringKind::ForLoop,
 771             head_sp,
 772             None,
 773         );
 774         head.span = desugared_span;
 775
 776         let iter = Ident::with_empty_ctxt(sym::iter);
 777
 778         let next_ident = Ident::with_empty_ctxt(sym::__next);
 779         let (next_pat, next_pat_hid) = self.pat_ident_binding_mode(
 780             desugared_span,
 781             next_ident,
 782             hir::BindingAnnotation::Mutable,
 783         );
 784
 785         // `::std::option::Option::Some(val) => __next = val`
 786         let pat_arm = {
 787             let val_ident = Ident::with_empty_ctxt(sym::val);
 788             let (val_pat, val_pat_hid) = self.pat_ident(pat.span, val_ident);
 789             let val_expr = P(self.expr_ident(pat.span, val_ident, val_pat_hid));
 790             let next_expr = P(self.expr_ident(pat.span, next_ident, next_pat_hid));
 791             let assign = P(self.expr(
 792                 pat.span,
 793                 hir::ExprKind::Assign(next_expr, val_expr),
 794                 ThinVec::new(),
 795             ));
 796             let some_pat = self.pat_some(pat.span, val_pat);
 797             self.arm(hir_vec![some_pat], assign)
 798         };
 799
 800         // `::std::option::Option::None => break`
 801         let break_arm = {
 802             let break_expr =
 803                 self.with_loop_scope(e.id, |this| this.expr_break(e.span, ThinVec::new()));
 804             let pat = self.pat_none(e.span);
 805             self.arm(hir_vec![pat], break_expr)
 806         };
 807
 808         // `mut iter`
 809         let (iter_pat, iter_pat_nid) = self.pat_ident_binding_mode(
 810             desugared_span,
 811             iter,
 812             hir::BindingAnnotation::Mutable
 813         );
 814
 815         // `match ::std::iter::Iterator::next(&mut iter) { ... }`
 816         let match_expr = {
 817             let iter = P(self.expr_ident(head_sp, iter, iter_pat_nid));
 818             let ref_mut_iter = self.expr_mut_addr_of(head_sp, iter);
 819             let next_path = &[sym::iter, sym::Iterator, sym::next];
 820             let next_expr = P(self.expr_call_std_path(
 821                 head_sp,
 822                 next_path,
 823                 hir_vec![ref_mut_iter],
 824             ));
 825             let arms = hir_vec![pat_arm, break_arm];
 826
 827             self.expr_match(head_sp, next_expr, arms, hir::MatchSource::ForLoopDesugar)
 828         };
 829         let match_stmt = self.stmt_expr(head_sp, match_expr);
 830
 831         let next_expr = P(self.expr_ident(head_sp, next_ident, next_pat_hid));
 832
 833         // `let mut __next`
 834         let next_let = self.stmt_let_pat(
 835             ThinVec::new(),
 836             desugared_span,
 837             None,
 838             next_pat,
 839             hir::LocalSource::ForLoopDesugar,
 840         );
 841
 842         // `let <pat> = __next`
 843         let pat = self.lower_pat(pat);
 844         let pat_let = self.stmt_let_pat(
 845             ThinVec::new(),
 846             head_sp,
 847             Some(next_expr),
 848             pat,
 849             hir::LocalSource::ForLoopDesugar,
 850         );
 851
 852         let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
 853         let body_expr = self.expr_block(body_block, ThinVec::new());
 854         let body_stmt = self.stmt_expr(body.span, body_expr);
 855
 856         let loop_block = P(self.block_all(
 857             e.span,
 858             hir_vec![next_let, match_stmt, pat_let, body_stmt],
 859             None,
 860         ));
 861
 862         // `[opt_ident]: loop { ... }`
 863         let loop_expr = hir::ExprKind::Loop(
 864             loop_block,
 865             self.lower_label(opt_label),
 866             hir::LoopSource::ForLoop,
 867         );
 868         let loop_expr = P(hir::Expr {
 869             hir_id: self.lower_node_id(e.id),
 870             node: loop_expr,
 871             span: e.span,
 872             attrs: ThinVec::new(),
 873         });
 874
 875         // `mut iter => { ... }`
 876         let iter_arm = self.arm(hir_vec![iter_pat], loop_expr);
 877
 878         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
 879         let into_iter_expr = {
 880             let into_iter_path =
 881                 &[sym::iter, sym::IntoIterator, sym::into_iter];
 882             P(self.expr_call_std_path(
 883                 head_sp,
 884                 into_iter_path,
 885                 hir_vec![head],
 886             ))
 887         };
 888
 889         let match_expr = P(self.expr_match(
 890             head_sp,
 891             into_iter_expr,
 892             hir_vec![iter_arm],
 893             hir::MatchSource::ForLoopDesugar,
 894         ));
 895
 896         // This is effectively `{ let _result = ...; _result }`.
 897         // The construct was introduced in #21984 and is necessary to make sure that
 898         // temporaries in the `head` expression are dropped and do not leak to the
 899         // surrounding scope of the `match` since the `match` is not a terminating scope.
 900         //
 901         // Also, add the attributes to the outer returned expr node.
 902         self.expr_drop_temps(head_sp, match_expr, e.attrs.clone())
 903     }
 904
 905     /// Desugar `ExprKind::Try` from: `<expr>?` into:
 906     /// ```rust
 907     /// match Try::into_result(<expr>) {
 908     ///     Ok(val) => #[allow(unreachable_code)] val,
 909     ///     Err(err) => #[allow(unreachable_code)]
 910     ///                 // If there is an enclosing `try {...}`:
 911     ///                 break 'catch_target Try::from_error(From::from(err)),
 912     ///                 // Otherwise:
 913     ///                 return Try::from_error(From::from(err)),
 914     /// }
 915     /// ```
 916     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind {
 917         let unstable_span = self.mark_span_with_reason(
 918             DesugaringKind::QuestionMark,
 919             span,
 920             self.allow_try_trait.clone(),
 921         );
 922         let try_span = self.sess.source_map().end_point(span);
 923         let try_span = self.mark_span_with_reason(
 924             DesugaringKind::QuestionMark,
 925             try_span,
 926             self.allow_try_trait.clone(),
 927         );
 928
 929         // `Try::into_result(<expr>)`
 930         let scrutinee = {
 931             // expand <expr>
 932             let sub_expr = self.lower_expr(sub_expr);
 933
 934             let path = &[sym::ops, sym::Try, sym::into_result];
 935             P(self.expr_call_std_path(unstable_span, path, hir_vec![sub_expr]))
 936         };
 937
 938         // `#[allow(unreachable_code)]`
 939         let attr = {
 940             // `allow(unreachable_code)`
 941             let allow = {
 942                 let allow_ident = Ident::new(sym::allow, span);
 943                 let uc_ident = Ident::new(sym::unreachable_code, span);
 944                 let uc_nested = attr::mk_nested_word_item(uc_ident);
 945                 attr::mk_list_item(allow_ident, vec![uc_nested])
 946             };
 947             attr::mk_attr_outer(allow)
 948         };
 949         let attrs = vec![attr];
 950
 951         // `Ok(val) => #[allow(unreachable_code)] val,`
 952         let ok_arm = {
 953             let val_ident = Ident::with_empty_ctxt(sym::val);
 954             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
 955             let val_expr = P(self.expr_ident_with_attrs(
 956                 span,
 957                 val_ident,
 958                 val_pat_nid,
 959                 ThinVec::from(attrs.clone()),
 960             ));
 961             let ok_pat = self.pat_ok(span, val_pat);
 962
 963             self.arm(hir_vec![ok_pat], val_expr)
 964         };
 965
 966         // `Err(err) => #[allow(unreachable_code)]
 967         //              return Try::from_error(From::from(err)),`
 968         let err_arm = {
 969             let err_ident = Ident::with_empty_ctxt(sym::err);
 970             let (err_local, err_local_nid) = self.pat_ident(try_span, err_ident);
 971             let from_expr = {
 972                 let from_path = &[sym::convert, sym::From, sym::from];
 973                 let err_expr = self.expr_ident(try_span, err_ident, err_local_nid);
 974                 self.expr_call_std_path(try_span, from_path, hir_vec![err_expr])
 975             };
 976             let from_err_expr =
 977                 self.wrap_in_try_constructor(sym::from_error, from_expr, unstable_span);
 978             let thin_attrs = ThinVec::from(attrs);
 979             let catch_scope = self.catch_scopes.last().map(|x| *x);
 980             let ret_expr = if let Some(catch_node) = catch_scope {
 981                 let target_id = Ok(self.lower_node_id(catch_node));
 982                 P(self.expr(
 983                     try_span,
 984                     hir::ExprKind::Break(
 985                         hir::Destination {
 986                             label: None,
 987                             target_id,
 988                         },
 989                         Some(from_err_expr),
 990                     ),
 991                     thin_attrs,
 992                 ))
 993             } else {
 994                 P(self.expr(try_span, hir::ExprKind::Ret(Some(from_err_expr)), thin_attrs))
 995             };
 996
 997             let err_pat = self.pat_err(try_span, err_local);
 998             self.arm(hir_vec![err_pat], ret_expr)
 999         };
1000
1001         hir::ExprKind::Match(
1002             scrutinee,
1003             hir_vec![err_arm, ok_arm],
1004             hir::MatchSource::TryDesugar,
1005         )
1006     }
1007 }