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             ExprKind::If(ref cond, ref then, ref else_opt) => {
 105                 self.lower_expr_if(e.span, cond, then, else_opt.as_deref())
 106             }
 107             ExprKind::While(ref cond, ref body, opt_label) => self.with_loop_scope(e.id, |this| {
 108                 this.lower_expr_while_in_loop_scope(e.span, cond, body, opt_label)
 109             }),
 110             ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
 111                 hir::ExprKind::Loop(
 112                     this.lower_block(body, false),
 113                     this.lower_label(opt_label),
 114                     hir::LoopSource::Loop,
 115                 )
 116             }),
 117             ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
 118             ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
 119                 P(self.lower_expr(expr)),
 120                 arms.iter().map(|x| self.lower_arm(x)).collect(),
 121                 hir::MatchSource::Normal,
 122             ),
 123             ExprKind::Async(capture_clause, closure_node_id, ref block) => {
 124                 self.make_async_expr(capture_clause, closure_node_id, None, block.span, |this| {
 125                     this.with_new_scopes(|this| {
 126                         let block = this.lower_block(block, false);
 127                         this.expr_block(block, ThinVec::new())
 128                     })
 129                 })
 130             }
 131             ExprKind::Await(ref expr) => self.lower_expr_await(e.span, expr),
 132             ExprKind::Closure(
 133                 capture_clause, asyncness, movability, ref decl, ref body, fn_decl_span
 134             ) => if let IsAsync::Async { closure_id, .. } = asyncness {
 135                 self.lower_expr_async_closure(capture_clause, closure_id, decl, body, fn_decl_span)
 136             } else {
 137                 self.lower_expr_closure(capture_clause, movability, decl, body, fn_decl_span)
 138             }
 139             ExprKind::Block(ref blk, opt_label) => {
 140                 hir::ExprKind::Block(self.lower_block(blk,
 141                                                       opt_label.is_some()),
 142                                                       self.lower_label(opt_label))
 143             }
 144             ExprKind::Assign(ref el, ref er) => {
 145                 hir::ExprKind::Assign(P(self.lower_expr(el)), P(self.lower_expr(er)))
 146             }
 147             ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 148                 self.lower_binop(op),
 149                 P(self.lower_expr(el)),
 150                 P(self.lower_expr(er)),
 151             ),
 152             ExprKind::Field(ref el, ident) => hir::ExprKind::Field(P(self.lower_expr(el)), ident),
 153             ExprKind::Index(ref el, ref er) => {
 154                 hir::ExprKind::Index(P(self.lower_expr(el)), P(self.lower_expr(er)))
 155             }
 156             ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 157                 self.lower_expr_range_closed(e.span, e1, e2)
 158             }
 159             ExprKind::Range(ref e1, ref e2, lims) => {
 160                 self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 161             }
 162             ExprKind::Path(ref qself, ref path) => {
 163                 let qpath = self.lower_qpath(
 164                     e.id,
 165                     qself,
 166                     path,
 167                     ParamMode::Optional,
 168                     ImplTraitContext::disallowed(),
 169                 );
 170                 hir::ExprKind::Path(qpath)
 171             }
 172             ExprKind::Break(opt_label, ref opt_expr) => {
 173                 let destination = if self.is_in_loop_condition && opt_label.is_none() {
 174                     hir::Destination {
 175                         label: None,
 176                         target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 177                     }
 178                 } else {
 179                     self.lower_loop_destination(opt_label.map(|label| (e.id, label)))
 180                 };
 181                 hir::ExprKind::Break(
 182                     destination,
 183                     opt_expr.as_ref().map(|x| P(self.lower_expr(x))),
 184                 )
 185             }
 186             ExprKind::Continue(opt_label) => {
 187                 hir::ExprKind::Continue(if self.is_in_loop_condition && opt_label.is_none() {
 188                     hir::Destination {
 189                         label: None,
 190                         target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 191                     }
 192                 } else {
 193                     self.lower_loop_destination(opt_label.map(|label| (e.id, label)))
 194                 })
 195             }
 196             ExprKind::Ret(ref e) => hir::ExprKind::Ret(e.as_ref().map(|x| P(self.lower_expr(x)))),
 197             ExprKind::InlineAsm(ref asm) => self.lower_expr_asm(asm),
 198             ExprKind::Struct(ref path, ref fields, ref maybe_expr) => hir::ExprKind::Struct(
 199                 P(self.lower_qpath(
 200                     e.id,
 201                     &None,
 202                     path,
 203                     ParamMode::Optional,
 204                     ImplTraitContext::disallowed(),
 205                 )),
 206                 fields.iter().map(|x| self.lower_field(x)).collect(),
 207                 maybe_expr.as_ref().map(|x| P(self.lower_expr(x))),
 208             ),
 209             ExprKind::Paren(ref ex) => {
 210                 let mut ex = self.lower_expr(ex);
 211                 // Include parens in span, but only if it is a super-span.
 212                 if e.span.contains(ex.span) {
 213                     ex.span = e.span;
 214                 }
 215                 // Merge attributes into the inner expression.
 216                 let mut attrs = e.attrs.clone();
 217                 attrs.extend::<Vec<_>>(ex.attrs.into());
 218                 ex.attrs = attrs;
 219                 return ex;
 220             }
 221
 222             ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 223
 224             ExprKind::Err => hir::ExprKind::Err,
 225
 226             // Desugar `ExprForLoop`
 227             // from: `[opt_ident]: for <pat> in <head> <body>`
 228             ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 229                 return self.lower_expr_for(e, pat, head, body, opt_label);
 230             }
 231             ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 232             ExprKind::Mac(_) => panic!("Shouldn't exist here"),
 233         };
 234
 235         hir::Expr {
 236             hir_id: self.lower_node_id(e.id),
 237             node: kind,
 238             span: e.span,
 239             attrs: e.attrs.clone(),
 240         }
 241     }
 242
 243     fn lower_expr_if(
 244         &mut self,
 245         span: Span,
 246         cond: &Expr,
 247         then: &Block,
 248         else_opt: Option<&Expr>,
 249     ) -> hir::ExprKind {
 250         // FIXME(#53667): handle lowering of && and parens.
 251
 252         // `_ => else_block` where `else_block` is `{}` if there's `None`:
 253         let else_pat = self.pat_wild(span);
 254         let (else_expr, contains_else_clause) = match else_opt {
 255             None => (self.expr_block_empty(span), false),
 256             Some(els) => (self.lower_expr(els), true),
 257         };
 258         let else_arm = self.arm(hir_vec![else_pat], P(else_expr));
 259
 260         // Handle then + scrutinee:
 261         let then_blk = self.lower_block(then, false);
 262         let then_expr = self.expr_block(then_blk, ThinVec::new());
 263         let (then_pats, scrutinee, desugar) = match cond.node {
 264             // `<pat> => <then>`:
 265             ExprKind::Let(ref pats, ref scrutinee) => {
 266                 let scrutinee = self.lower_expr(scrutinee);
 267                 let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
 268                 let desugar = hir::MatchSource::IfLetDesugar { contains_else_clause };
 269                 (pats, scrutinee, desugar)
 270             }
 271             // `true => <then>`:
 272             _ => {
 273                 // Lower condition:
 274                 let cond = self.lower_expr(cond);
 275                 let span_block = self.mark_span_with_reason(
 276                     DesugaringKind::CondTemporary,
 277                     cond.span,
 278                     None
 279                 );
 280                 // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 281                 // to preserve drop semantics since `if cond { ... }` does not
 282                 // let temporaries live outside of `cond`.
 283                 let cond = self.expr_drop_temps(span_block, P(cond), ThinVec::new());
 284
 285                 let desugar = hir::MatchSource::IfDesugar { contains_else_clause };
 286                 let pats = hir_vec![self.pat_bool(span, true)];
 287                 (pats, cond, desugar)
 288             }
 289         };
 290         let then_arm = self.arm(then_pats, P(then_expr));
 291
 292         hir::ExprKind::Match(P(scrutinee), vec![then_arm, else_arm].into(), desugar)
 293     }
 294
 295     fn lower_expr_while_in_loop_scope(
 296         &mut self,
 297         span: Span,
 298         cond: &Expr,
 299         body: &Block,
 300         opt_label: Option<Label>
 301     ) -> hir::ExprKind {
 302         // FIXME(#53667): handle lowering of && and parens.
 303
 304         // Note that the block AND the condition are evaluated in the loop scope.
 305         // This is done to allow `break` from inside the condition of the loop.
 306
 307         // `_ => break`:
 308         let else_arm = {
 309             let else_pat = self.pat_wild(span);
 310             let else_expr = self.expr_break(span, ThinVec::new());
 311             self.arm(hir_vec![else_pat], else_expr)
 312         };
 313
 314         // Handle then + scrutinee:
 315         let then_blk = self.lower_block(body, false);
 316         let then_expr = self.expr_block(then_blk, ThinVec::new());
 317         let (then_pats, scrutinee, desugar, source) = match cond.node {
 318             ExprKind::Let(ref pats, ref scrutinee) => {
 319                 // to:
 320                 //
 321                 //   [opt_ident]: loop {
 322                 //     match <sub_expr> {
 323                 //       <pat> => <body>,
 324                 //       _ => break
 325                 //     }
 326                 //   }
 327                 let scrutinee = self.with_loop_condition_scope(|t| t.lower_expr(scrutinee));
 328                 let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
 329                 let desugar = hir::MatchSource::WhileLetDesugar;
 330                 (pats, scrutinee, desugar, hir::LoopSource::WhileLet)
 331             }
 332             _ => {
 333                 // We desugar: `'label: while $cond $body` into:
 334                 //
 335                 // ```
 336                 // 'label: loop {
 337                 //     match DropTemps($cond) {
 338                 //         true => $body,
 339                 //         _ => break,
 340                 //     }
 341                 // }
 342                 // ```
 343
 344                 // Lower condition:
 345                 let cond = self.with_loop_condition_scope(|this| this.lower_expr(cond));
 346                 let span_block = self.mark_span_with_reason(
 347                     DesugaringKind::CondTemporary,
 348                     cond.span,
 349                     None,
 350                 );
 351                 // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 352                 // to preserve drop semantics since `while cond { ... }` does not
 353                 // let temporaries live outside of `cond`.
 354                 let cond = self.expr_drop_temps(span_block, P(cond), ThinVec::new());
 355
 356                 let desugar = hir::MatchSource::WhileDesugar;
 357                 // `true => <then>`:
 358                 let pats = hir_vec![self.pat_bool(span, true)];
 359                 (pats, cond, desugar, hir::LoopSource::While)
 360             }
 361         };
 362         let then_arm = self.arm(then_pats, P(then_expr));
 363
 364         // `match <scrutinee> { ... }`
 365         let match_expr = self.expr_match(
 366             scrutinee.span,
 367             P(scrutinee),
 368             hir_vec![then_arm, else_arm],
 369             desugar,
 370         );
 371
 372         // `[opt_ident]: loop { ... }`
 373         hir::ExprKind::Loop(
 374             P(self.block_expr(P(match_expr))),
 375             self.lower_label(opt_label),
 376             source
 377         )
 378     }
 379
 380     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind {
 381         self.with_catch_scope(body.id, |this| {
 382             let unstable_span = this.mark_span_with_reason(
 383                 DesugaringKind::TryBlock,
 384                 body.span,
 385                 this.allow_try_trait.clone(),
 386             );
 387             let mut block = this.lower_block(body, true).into_inner();
 388             let tail = block.expr.take().map_or_else(
 389                 || this.expr_unit(this.sess.source_map().end_point(unstable_span)),
 390                 |x: P<hir::Expr>| x.into_inner(),
 391             );
 392             block.expr = Some(this.wrap_in_try_constructor(sym::from_ok, tail, unstable_span));
 393             hir::ExprKind::Block(P(block), None)
 394         })
 395     }
 396
 397     fn wrap_in_try_constructor(
 398         &mut self,
 399         method: Symbol,
 400         e: hir::Expr,
 401         unstable_span: Span,
 402     ) -> P<hir::Expr> {
 403         let path = &[sym::ops, sym::Try, method];
 404         let from_err = P(self.expr_std_path(unstable_span, path, None, ThinVec::new()));
 405         P(self.expr_call(e.span, from_err, hir_vec![e]))
 406     }
 407
 408     /// Desugar `<expr>.await` into:
 409     /// ```rust
 410     /// {
 411     ///     let mut pinned = <expr>;
 412     ///     loop {
 413     ///         match ::std::future::poll_with_tls_context(unsafe {
 414     ///             ::std::pin::Pin::new_unchecked(&mut pinned)
 415     ///         }) {
 416     ///             ::std::task::Poll::Ready(result) => break result,
 417     ///             ::std::task::Poll::Pending => {},
 418     ///         }
 419     ///         yield ();
 420     ///     }
 421     /// }
 422     /// ```
 423     fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind {
 424         match self.generator_kind {
 425             Some(hir::GeneratorKind::Async) => {},
 426             Some(hir::GeneratorKind::Gen) |
 427             None => {
 428                 let mut err = struct_span_err!(
 429                     self.sess,
 430                     await_span,
 431                     E0728,
 432                     "`await` is only allowed inside `async` functions and blocks"
 433                 );
 434                 err.span_label(await_span, "only allowed inside `async` functions and blocks");
 435                 if let Some(item_sp) = self.current_item {
 436                     err.span_label(item_sp, "this is not `async`");
 437                 }
 438                 err.emit();
 439             }
 440         }
 441         let span = self.mark_span_with_reason(
 442             DesugaringKind::Await,
 443             await_span,
 444             None,
 445         );
 446         let gen_future_span = self.mark_span_with_reason(
 447             DesugaringKind::Await,
 448             await_span,
 449             self.allow_gen_future.clone(),
 450         );
 451
 452         // let mut pinned = <expr>;
 453         let expr = P(self.lower_expr(expr));
 454         let pinned_ident = Ident::with_empty_ctxt(sym::pinned);
 455         let (pinned_pat, pinned_pat_hid) = self.pat_ident_binding_mode(
 456             span,
 457             pinned_ident,
 458             hir::BindingAnnotation::Mutable,
 459         );
 460         let pinned_let = self.stmt_let_pat(
 461             ThinVec::new(),
 462             span,
 463             Some(expr),
 464             pinned_pat,
 465             hir::LocalSource::AwaitDesugar,
 466         );
 467
 468         // ::std::future::poll_with_tls_context(unsafe {
 469         //     ::std::pin::Pin::new_unchecked(&mut pinned)
 470         // })`
 471         let poll_expr = {
 472             let pinned = P(self.expr_ident(span, pinned_ident, pinned_pat_hid));
 473             let ref_mut_pinned = self.expr_mut_addr_of(span, pinned);
 474             let pin_ty_id = self.next_id();
 475             let new_unchecked_expr_kind = self.expr_call_std_assoc_fn(
 476                 pin_ty_id,
 477                 span,
 478                 &[sym::pin, sym::Pin],
 479                 "new_unchecked",
 480                 hir_vec![ref_mut_pinned],
 481             );
 482             let new_unchecked = P(self.expr(span, new_unchecked_expr_kind, ThinVec::new()));
 483             let unsafe_expr = self.expr_unsafe(new_unchecked);
 484             P(self.expr_call_std_path(
 485                 gen_future_span,
 486                 &[sym::future, sym::poll_with_tls_context],
 487                 hir_vec![unsafe_expr],
 488             ))
 489         };
 490
 491         // `::std::task::Poll::Ready(result) => break result`
 492         let loop_node_id = self.sess.next_node_id();
 493         let loop_hir_id = self.lower_node_id(loop_node_id);
 494         let ready_arm = {
 495             let x_ident = Ident::with_empty_ctxt(sym::result);
 496             let (x_pat, x_pat_hid) = self.pat_ident(span, x_ident);
 497             let x_expr = P(self.expr_ident(span, x_ident, x_pat_hid));
 498             let ready_pat = self.pat_std_enum(
 499                 span,
 500                 &[sym::task, sym::Poll, sym::Ready],
 501                 hir_vec![x_pat],
 502             );
 503             let break_x = self.with_loop_scope(loop_node_id, |this| {
 504                 let expr_break = hir::ExprKind::Break(
 505                     this.lower_loop_destination(None),
 506                     Some(x_expr),
 507                 );
 508                 P(this.expr(await_span, expr_break, ThinVec::new()))
 509             });
 510             self.arm(hir_vec![ready_pat], break_x)
 511         };
 512
 513         // `::std::task::Poll::Pending => {}`
 514         let pending_arm = {
 515             let pending_pat = self.pat_std_enum(
 516                 span,
 517                 &[sym::task, sym::Poll, sym::Pending],
 518                 hir_vec![],
 519             );
 520             let empty_block = P(self.expr_block_empty(span));
 521             self.arm(hir_vec![pending_pat], empty_block)
 522         };
 523
 524         let match_stmt = {
 525             let match_expr = self.expr_match(
 526                 span,
 527                 poll_expr,
 528                 hir_vec![ready_arm, pending_arm],
 529                 hir::MatchSource::AwaitDesugar,
 530             );
 531             self.stmt_expr(span, match_expr)
 532         };
 533
 534         let yield_stmt = {
 535             let unit = self.expr_unit(span);
 536             let yield_expr = self.expr(
 537                 span,
 538                 hir::ExprKind::Yield(P(unit), hir::YieldSource::Await),
 539                 ThinVec::new(),
 540             );
 541             self.stmt_expr(span, yield_expr)
 542         };
 543
 544         let loop_block = P(self.block_all(
 545             span,
 546             hir_vec![match_stmt, yield_stmt],
 547             None,
 548         ));
 549
 550         let loop_expr = P(hir::Expr {
 551             hir_id: loop_hir_id,
 552             node: hir::ExprKind::Loop(
 553                 loop_block,
 554                 None,
 555                 hir::LoopSource::Loop,
 556             ),
 557             span,
 558             attrs: ThinVec::new(),
 559         });
 560
 561         hir::ExprKind::Block(
 562             P(self.block_all(span, hir_vec![pinned_let], Some(loop_expr))),
 563             None,
 564         )
 565     }
 566
 567     fn lower_expr_closure(
 568         &mut self,
 569         capture_clause: CaptureBy,
 570         movability: Movability,
 571         decl: &FnDecl,
 572         body: &Expr,
 573         fn_decl_span: Span,
 574     ) -> hir::ExprKind {
 575         // Lower outside new scope to preserve `is_in_loop_condition`.
 576         let fn_decl = self.lower_fn_decl(decl, None, false, None);
 577
 578         self.with_new_scopes(|this| {
 579             this.current_item = Some(fn_decl_span);
 580             let mut generator_kind = None;
 581             let body_id = this.lower_fn_body(decl, |this| {
 582                 let e = this.lower_expr(body);
 583                 generator_kind = this.generator_kind;
 584                 e
 585             });
 586             let generator_option = this.generator_movability_for_fn(
 587                 &decl,
 588                 fn_decl_span,
 589                 generator_kind,
 590                 movability,
 591             );
 592             hir::ExprKind::Closure(
 593                 this.lower_capture_clause(capture_clause),
 594                 fn_decl,
 595                 body_id,
 596                 fn_decl_span,
 597                 generator_option,
 598             )
 599         })
 600     }
 601
 602     fn lower_expr_async_closure(
 603         &mut self,
 604         capture_clause: CaptureBy,
 605         closure_id: NodeId,
 606         decl: &FnDecl,
 607         body: &Expr,
 608         fn_decl_span: Span,
 609     ) -> hir::ExprKind {
 610         let outer_decl = FnDecl {
 611             inputs: decl.inputs.clone(),
 612             output: FunctionRetTy::Default(fn_decl_span),
 613             c_variadic: false,
 614         };
 615         // We need to lower the declaration outside the new scope, because we
 616         // have to conserve the state of being inside a loop condition for the
 617         // closure argument types.
 618         let fn_decl = self.lower_fn_decl(&outer_decl, None, false, None);
 619
 620         self.with_new_scopes(|this| {
 621             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 622             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 623                 struct_span_err!(
 624                     this.sess,
 625                     fn_decl_span,
 626                     E0708,
 627                     "`async` non-`move` closures with arguments are not currently supported",
 628                 )
 629                 .help(
 630                     "consider using `let` statements to manually capture \
 631                     variables by reference before entering an `async move` closure"
 632                 )
 633                 .emit();
 634             }
 635
 636             // Transform `async |x: u8| -> X { ... }` into
 637             // `|x: u8| future_from_generator(|| -> X { ... })`.
 638             let body_id = this.lower_fn_body(&outer_decl, |this| {
 639                 let async_ret_ty = if let FunctionRetTy::Ty(ty) = &decl.output {
 640                     Some(ty.clone())
 641                 } else {
 642                     None
 643                 };
 644                 let async_body = this.make_async_expr(
 645                     capture_clause, closure_id, async_ret_ty, body.span,
 646                     |this| {
 647                         this.with_new_scopes(|this| this.lower_expr(body))
 648                     }
 649                 );
 650                 this.expr(fn_decl_span, async_body, ThinVec::new())
 651             });
 652             hir::ExprKind::Closure(
 653                 this.lower_capture_clause(capture_clause),
 654                 fn_decl,
 655                 body_id,
 656                 fn_decl_span,
 657                 None,
 658             )
 659         })
 660     }
 661
 662     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
 663     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind {
 664         let id = self.next_id();
 665         let e1 = self.lower_expr(e1);
 666         let e2 = self.lower_expr(e2);
 667         self.expr_call_std_assoc_fn(
 668             id,
 669             span,
 670             &[sym::ops, sym::RangeInclusive],
 671             "new",
 672             hir_vec![e1, e2],
 673         )
 674     }
 675
 676     fn lower_expr_range(
 677         &mut self,
 678         span: Span,
 679         e1: Option<&Expr>,
 680         e2: Option<&Expr>,
 681         lims: RangeLimits,
 682     ) -> hir::ExprKind {
 683         use syntax::ast::RangeLimits::*;
 684
 685         let path = match (e1, e2, lims) {
 686             (None, None, HalfOpen) => sym::RangeFull,
 687             (Some(..), None, HalfOpen) => sym::RangeFrom,
 688             (None, Some(..), HalfOpen) => sym::RangeTo,
 689             (Some(..), Some(..), HalfOpen) => sym::Range,
 690             (None, Some(..), Closed) => sym::RangeToInclusive,
 691             (Some(..), Some(..), Closed) => unreachable!(),
 692             (_, None, Closed) => self.diagnostic()
 693                 .span_fatal(span, "inclusive range with no end")
 694                 .raise(),
 695         };
 696
 697         let fields = e1.iter()
 698             .map(|e| ("start", e))
 699             .chain(e2.iter().map(|e| ("end", e)))
 700             .map(|(s, e)| {
 701                 let expr = P(self.lower_expr(&e));
 702                 let ident = Ident::new(Symbol::intern(s), e.span);
 703                 self.field(ident, expr, e.span)
 704             })
 705             .collect::<P<[hir::Field]>>();
 706
 707         let is_unit = fields.is_empty();
 708         let struct_path = [sym::ops, path];
 709         let struct_path = self.std_path(span, &struct_path, None, is_unit);
 710         let struct_path = hir::QPath::Resolved(None, P(struct_path));
 711
 712         if is_unit {
 713             hir::ExprKind::Path(struct_path)
 714         } else {
 715             hir::ExprKind::Struct(P(struct_path), fields, None)
 716         }
 717     }
 718
 719     fn lower_expr_asm(&mut self, asm: &InlineAsm) -> hir::ExprKind {
 720         let hir_asm = hir::InlineAsm {
 721             inputs: asm.inputs.iter().map(|&(ref c, _)| c.clone()).collect(),
 722             outputs: asm.outputs
 723                 .iter()
 724                 .map(|out| hir::InlineAsmOutput {
 725                     constraint: out.constraint.clone(),
 726                     is_rw: out.is_rw,
 727                     is_indirect: out.is_indirect,
 728                     span: out.expr.span,
 729                 })
 730                 .collect(),
 731             asm: asm.asm.clone(),
 732             asm_str_style: asm.asm_str_style,
 733             clobbers: asm.clobbers.clone().into(),
 734             volatile: asm.volatile,
 735             alignstack: asm.alignstack,
 736             dialect: asm.dialect,
 737             ctxt: asm.ctxt,
 738         };
 739
 740         let outputs = asm.outputs
 741             .iter()
 742             .map(|out| self.lower_expr(&out.expr))
 743             .collect();
 744
 745         let inputs = asm.inputs
 746             .iter()
 747             .map(|&(_, ref input)| self.lower_expr(input))
 748             .collect();
 749
 750         hir::ExprKind::InlineAsm(P(hir_asm), outputs, inputs)
 751     }
 752
 753     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind {
 754         match self.generator_kind {
 755             Some(hir::GeneratorKind::Gen) => {},
 756             Some(hir::GeneratorKind::Async) => {
 757                 span_err!(
 758                     self.sess,
 759                     span,
 760                     E0727,
 761                     "`async` generators are not yet supported",
 762                 );
 763                 self.sess.abort_if_errors();
 764             },
 765             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
 766         }
 767
 768         let expr = opt_expr
 769             .as_ref()
 770             .map(|x| self.lower_expr(x))
 771             .unwrap_or_else(|| self.expr_unit(span));
 772
 773         hir::ExprKind::Yield(P(expr), hir::YieldSource::Yield)
 774     }
 775
 776     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
 777     /// ```rust
 778     /// {
 779     ///     let result = match ::std::iter::IntoIterator::into_iter(<head>) {
 780     ///         mut iter => {
 781     ///             [opt_ident]: loop {
 782     ///                 let mut __next;
 783     ///                 match ::std::iter::Iterator::next(&mut iter) {
 784     ///                     ::std::option::Option::Some(val) => __next = val,
 785     ///                     ::std::option::Option::None => break
 786     ///                 };
 787     ///                 let <pat> = __next;
 788     ///                 StmtKind::Expr(<body>);
 789     ///             }
 790     ///         }
 791     ///     };
 792     ///     result
 793     /// }
 794     /// ```
 795     fn lower_expr_for(
 796         &mut self,
 797         e: &Expr,
 798         pat: &Pat,
 799         head: &Expr,
 800         body: &Block,
 801         opt_label: Option<Label>,
 802     ) -> hir::Expr {
 803         // expand <head>
 804         let mut head = self.lower_expr(head);
 805         let head_sp = head.span;
 806         let desugared_span = self.mark_span_with_reason(
 807             DesugaringKind::ForLoop,
 808             head_sp,
 809             None,
 810         );
 811         head.span = desugared_span;
 812
 813         let iter = Ident::with_empty_ctxt(sym::iter);
 814
 815         let next_ident = Ident::with_empty_ctxt(sym::__next);
 816         let (next_pat, next_pat_hid) = self.pat_ident_binding_mode(
 817             desugared_span,
 818             next_ident,
 819             hir::BindingAnnotation::Mutable,
 820         );
 821
 822         // `::std::option::Option::Some(val) => __next = val`
 823         let pat_arm = {
 824             let val_ident = Ident::with_empty_ctxt(sym::val);
 825             let (val_pat, val_pat_hid) = self.pat_ident(pat.span, val_ident);
 826             let val_expr = P(self.expr_ident(pat.span, val_ident, val_pat_hid));
 827             let next_expr = P(self.expr_ident(pat.span, next_ident, next_pat_hid));
 828             let assign = P(self.expr(
 829                 pat.span,
 830                 hir::ExprKind::Assign(next_expr, val_expr),
 831                 ThinVec::new(),
 832             ));
 833             let some_pat = self.pat_some(pat.span, val_pat);
 834             self.arm(hir_vec![some_pat], assign)
 835         };
 836
 837         // `::std::option::Option::None => break`
 838         let break_arm = {
 839             let break_expr =
 840                 self.with_loop_scope(e.id, |this| this.expr_break(e.span, ThinVec::new()));
 841             let pat = self.pat_none(e.span);
 842             self.arm(hir_vec![pat], break_expr)
 843         };
 844
 845         // `mut iter`
 846         let (iter_pat, iter_pat_nid) = self.pat_ident_binding_mode(
 847             desugared_span,
 848             iter,
 849             hir::BindingAnnotation::Mutable
 850         );
 851
 852         // `match ::std::iter::Iterator::next(&mut iter) { ... }`
 853         let match_expr = {
 854             let iter = P(self.expr_ident(head_sp, iter, iter_pat_nid));
 855             let ref_mut_iter = self.expr_mut_addr_of(head_sp, iter);
 856             let next_path = &[sym::iter, sym::Iterator, sym::next];
 857             let next_expr = P(self.expr_call_std_path(
 858                 head_sp,
 859                 next_path,
 860                 hir_vec![ref_mut_iter],
 861             ));
 862             let arms = hir_vec![pat_arm, break_arm];
 863
 864             self.expr_match(head_sp, next_expr, arms, hir::MatchSource::ForLoopDesugar)
 865         };
 866         let match_stmt = self.stmt_expr(head_sp, match_expr);
 867
 868         let next_expr = P(self.expr_ident(head_sp, next_ident, next_pat_hid));
 869
 870         // `let mut __next`
 871         let next_let = self.stmt_let_pat(
 872             ThinVec::new(),
 873             desugared_span,
 874             None,
 875             next_pat,
 876             hir::LocalSource::ForLoopDesugar,
 877         );
 878
 879         // `let <pat> = __next`
 880         let pat = self.lower_pat(pat);
 881         let pat_let = self.stmt_let_pat(
 882             ThinVec::new(),
 883             head_sp,
 884             Some(next_expr),
 885             pat,
 886             hir::LocalSource::ForLoopDesugar,
 887         );
 888
 889         let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
 890         let body_expr = self.expr_block(body_block, ThinVec::new());
 891         let body_stmt = self.stmt_expr(body.span, body_expr);
 892
 893         let loop_block = P(self.block_all(
 894             e.span,
 895             hir_vec![next_let, match_stmt, pat_let, body_stmt],
 896             None,
 897         ));
 898
 899         // `[opt_ident]: loop { ... }`
 900         let loop_expr = hir::ExprKind::Loop(
 901             loop_block,
 902             self.lower_label(opt_label),
 903             hir::LoopSource::ForLoop,
 904         );
 905         let loop_expr = P(hir::Expr {
 906             hir_id: self.lower_node_id(e.id),
 907             node: loop_expr,
 908             span: e.span,
 909             attrs: ThinVec::new(),
 910         });
 911
 912         // `mut iter => { ... }`
 913         let iter_arm = self.arm(hir_vec![iter_pat], loop_expr);
 914
 915         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
 916         let into_iter_expr = {
 917             let into_iter_path =
 918                 &[sym::iter, sym::IntoIterator, sym::into_iter];
 919             P(self.expr_call_std_path(
 920                 head_sp,
 921                 into_iter_path,
 922                 hir_vec![head],
 923             ))
 924         };
 925
 926         let match_expr = P(self.expr_match(
 927             head_sp,
 928             into_iter_expr,
 929             hir_vec![iter_arm],
 930             hir::MatchSource::ForLoopDesugar,
 931         ));
 932
 933         // This is effectively `{ let _result = ...; _result }`.
 934         // The construct was introduced in #21984 and is necessary to make sure that
 935         // temporaries in the `head` expression are dropped and do not leak to the
 936         // surrounding scope of the `match` since the `match` is not a terminating scope.
 937         //
 938         // Also, add the attributes to the outer returned expr node.
 939         self.expr_drop_temps(head_sp, match_expr, e.attrs.clone())
 940     }
 941
 942     /// Desugar `ExprKind::Try` from: `<expr>?` into:
 943     /// ```rust
 944     /// match Try::into_result(<expr>) {
 945     ///     Ok(val) => #[allow(unreachable_code)] val,
 946     ///     Err(err) => #[allow(unreachable_code)]
 947     ///                 // If there is an enclosing `try {...}`:
 948     ///                 break 'catch_target Try::from_error(From::from(err)),
 949     ///                 // Otherwise:
 950     ///                 return Try::from_error(From::from(err)),
 951     /// }
 952     /// ```
 953     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind {
 954         let unstable_span = self.mark_span_with_reason(
 955             DesugaringKind::QuestionMark,
 956             span,
 957             self.allow_try_trait.clone(),
 958         );
 959         let try_span = self.sess.source_map().end_point(span);
 960         let try_span = self.mark_span_with_reason(
 961             DesugaringKind::QuestionMark,
 962             try_span,
 963             self.allow_try_trait.clone(),
 964         );
 965
 966         // `Try::into_result(<expr>)`
 967         let scrutinee = {
 968             // expand <expr>
 969             let sub_expr = self.lower_expr(sub_expr);
 970
 971             let path = &[sym::ops, sym::Try, sym::into_result];
 972             P(self.expr_call_std_path(unstable_span, path, hir_vec![sub_expr]))
 973         };
 974
 975         // `#[allow(unreachable_code)]`
 976         let attr = {
 977             // `allow(unreachable_code)`
 978             let allow = {
 979                 let allow_ident = Ident::new(sym::allow, span);
 980                 let uc_ident = Ident::new(sym::unreachable_code, span);
 981                 let uc_nested = attr::mk_nested_word_item(uc_ident);
 982                 attr::mk_list_item(allow_ident, vec![uc_nested])
 983             };
 984             attr::mk_attr_outer(allow)
 985         };
 986         let attrs = vec![attr];
 987
 988         // `Ok(val) => #[allow(unreachable_code)] val,`
 989         let ok_arm = {
 990             let val_ident = Ident::with_empty_ctxt(sym::val);
 991             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
 992             let val_expr = P(self.expr_ident_with_attrs(
 993                 span,
 994                 val_ident,
 995                 val_pat_nid,
 996                 ThinVec::from(attrs.clone()),
 997             ));
 998             let ok_pat = self.pat_ok(span, val_pat);
 999
1000             self.arm(hir_vec![ok_pat], val_expr)
1001         };
1002
1003         // `Err(err) => #[allow(unreachable_code)]
1004         //              return Try::from_error(From::from(err)),`
1005         let err_arm = {
1006             let err_ident = Ident::with_empty_ctxt(sym::err);
1007             let (err_local, err_local_nid) = self.pat_ident(try_span, err_ident);
1008             let from_expr = {
1009                 let from_path = &[sym::convert, sym::From, sym::from];
1010                 let err_expr = self.expr_ident(try_span, err_ident, err_local_nid);
1011                 self.expr_call_std_path(try_span, from_path, hir_vec![err_expr])
1012             };
1013             let from_err_expr =
1014                 self.wrap_in_try_constructor(sym::from_error, from_expr, unstable_span);
1015             let thin_attrs = ThinVec::from(attrs);
1016             let catch_scope = self.catch_scopes.last().map(|x| *x);
1017             let ret_expr = if let Some(catch_node) = catch_scope {
1018                 let target_id = Ok(self.lower_node_id(catch_node));
1019                 P(self.expr(
1020                     try_span,
1021                     hir::ExprKind::Break(
1022                         hir::Destination {
1023                             label: None,
1024                             target_id,
1025                         },
1026                         Some(from_err_expr),
1027                     ),
1028                     thin_attrs,
1029                 ))
1030             } else {
1031                 P(self.expr(try_span, hir::ExprKind::Ret(Some(from_err_expr)), thin_attrs))
1032             };
1033
1034             let err_pat = self.pat_err(try_span, err_local);
1035             self.arm(hir_vec![err_pat], ret_expr)
1036         };
1037
1038         hir::ExprKind::Match(
1039             scrutinee,
1040             hir_vec![err_arm, ok_arm],
1041             hir::MatchSource::TryDesugar,
1042         )
1043     }
1044 }