src/librustc_ast_lowering/expr.rs

   1 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
   2
   3 use rustc::bug;
   4 use rustc_data_structures::thin_vec::ThinVec;
   5 use rustc_errors::struct_span_err;
   6 use rustc_hir as hir;
   7 use rustc_hir::def::Res;
   8 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
   9 use rustc_span::symbol::{sym, Symbol};
  10 use syntax::ast::*;
  11 use syntax::attr;
  12 use syntax::ptr::P as AstP;
  13
  14 impl<'hir> LoweringContext<'_, 'hir> {
  15     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  16         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  17     }
  18
  19     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  20         self.arena.alloc(self.lower_expr_mut(e))
  21     }
  22
  23     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  24         let kind = match e.kind {
  25             ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  26             ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  27             ExprKind::Repeat(ref expr, ref count) => {
  28                 let expr = self.lower_expr(expr);
  29                 let count = self.lower_anon_const(count);
  30                 hir::ExprKind::Repeat(expr, count)
  31             }
  32             ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  33             ExprKind::Call(ref f, ref args) => {
  34                 let f = self.lower_expr(f);
  35                 hir::ExprKind::Call(f, self.lower_exprs(args))
  36             }
  37             ExprKind::MethodCall(ref seg, ref args) => {
  38                 let hir_seg = self.arena.alloc(self.lower_path_segment(
  39                     e.span,
  40                     seg,
  41                     ParamMode::Optional,
  42                     0,
  43                     ParenthesizedGenericArgs::Err,
  44                     ImplTraitContext::disallowed(),
  45                     None,
  46                 ));
  47                 let args = self.lower_exprs(args);
  48                 hir::ExprKind::MethodCall(hir_seg, seg.ident.span, args)
  49             }
  50             ExprKind::Binary(binop, ref lhs, ref rhs) => {
  51                 let binop = self.lower_binop(binop);
  52                 let lhs = self.lower_expr(lhs);
  53                 let rhs = self.lower_expr(rhs);
  54                 hir::ExprKind::Binary(binop, lhs, rhs)
  55             }
  56             ExprKind::Unary(op, ref ohs) => {
  57                 let op = self.lower_unop(op);
  58                 let ohs = self.lower_expr(ohs);
  59                 hir::ExprKind::Unary(op, ohs)
  60             }
  61             ExprKind::Lit(ref l) => hir::ExprKind::Lit(respan(l.span, l.kind.clone())),
  62             ExprKind::Cast(ref expr, ref ty) => {
  63                 let expr = self.lower_expr(expr);
  64                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
  65                 hir::ExprKind::Cast(expr, ty)
  66             }
  67             ExprKind::Type(ref expr, ref ty) => {
  68                 let expr = self.lower_expr(expr);
  69                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
  70                 hir::ExprKind::Type(expr, ty)
  71             }
  72             ExprKind::AddrOf(k, m, ref ohs) => {
  73                 let ohs = self.lower_expr(ohs);
  74                 hir::ExprKind::AddrOf(k, m, ohs)
  75             }
  76             ExprKind::Let(ref pat, ref scrutinee) => self.lower_expr_let(e.span, pat, scrutinee),
  77             ExprKind::If(ref cond, ref then, ref else_opt) => {
  78                 self.lower_expr_if(e.span, cond, then, else_opt.as_deref())
  79             }
  80             ExprKind::While(ref cond, ref body, opt_label) => self.with_loop_scope(e.id, |this| {
  81                 this.lower_expr_while_in_loop_scope(e.span, cond, body, opt_label)
  82             }),
  83             ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
  84                 hir::ExprKind::Loop(this.lower_block(body, false), opt_label, hir::LoopSource::Loop)
  85             }),
  86             ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
  87             ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
  88                 self.lower_expr(expr),
  89                 self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
  90                 hir::MatchSource::Normal,
  91             ),
  92             ExprKind::Async(capture_clause, closure_node_id, ref block) => self.make_async_expr(
  93                 capture_clause,
  94                 closure_node_id,
  95                 None,
  96                 block.span,
  97                 hir::AsyncGeneratorKind::Block,
  98                 |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
  99             ),
 100             ExprKind::Await(ref expr) => self.lower_expr_await(e.span, expr),
 101             ExprKind::Closure(
 102                 capture_clause,
 103                 asyncness,
 104                 movability,
 105                 ref decl,
 106                 ref body,
 107                 fn_decl_span,
 108             ) => {
 109                 if let IsAsync::Async { closure_id, .. } = asyncness {
 110                     self.lower_expr_async_closure(
 111                         capture_clause,
 112                         closure_id,
 113                         decl,
 114                         body,
 115                         fn_decl_span,
 116                     )
 117                 } else {
 118                     self.lower_expr_closure(capture_clause, movability, decl, body, fn_decl_span)
 119                 }
 120             }
 121             ExprKind::Block(ref blk, opt_label) => {
 122                 hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 123             }
 124             ExprKind::Assign(ref el, ref er, span) => {
 125                 hir::ExprKind::Assign(self.lower_expr(el), self.lower_expr(er), span)
 126             }
 127             ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 128                 self.lower_binop(op),
 129                 self.lower_expr(el),
 130                 self.lower_expr(er),
 131             ),
 132             ExprKind::Field(ref el, ident) => hir::ExprKind::Field(self.lower_expr(el), ident),
 133             ExprKind::Index(ref el, ref er) => {
 134                 hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 135             }
 136             ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 137                 self.lower_expr_range_closed(e.span, e1, e2)
 138             }
 139             ExprKind::Range(ref e1, ref e2, lims) => {
 140                 self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 141             }
 142             ExprKind::Path(ref qself, ref path) => {
 143                 let qpath = self.lower_qpath(
 144                     e.id,
 145                     qself,
 146                     path,
 147                     ParamMode::Optional,
 148                     ImplTraitContext::disallowed(),
 149                 );
 150                 hir::ExprKind::Path(qpath)
 151             }
 152             ExprKind::Break(opt_label, ref opt_expr) => {
 153                 let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 154                 hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
 155             }
 156             ExprKind::Continue(opt_label) => {
 157                 hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
 158             }
 159             ExprKind::Ret(ref e) => {
 160                 let e = e.as_ref().map(|x| self.lower_expr(x));
 161                 hir::ExprKind::Ret(e)
 162             }
 163             ExprKind::InlineAsm(ref asm) => self.lower_expr_asm(asm),
 164             ExprKind::Struct(ref path, ref fields, ref maybe_expr) => {
 165                 let maybe_expr = maybe_expr.as_ref().map(|x| self.lower_expr(x));
 166                 hir::ExprKind::Struct(
 167                     self.arena.alloc(self.lower_qpath(
 168                         e.id,
 169                         &None,
 170                         path,
 171                         ParamMode::Optional,
 172                         ImplTraitContext::disallowed(),
 173                     )),
 174                     self.arena.alloc_from_iter(fields.iter().map(|x| self.lower_field(x))),
 175                     maybe_expr,
 176                 )
 177             }
 178             ExprKind::Paren(ref ex) => {
 179                 let mut ex = self.lower_expr_mut(ex);
 180                 // Include parens in span, but only if it is a super-span.
 181                 if e.span.contains(ex.span) {
 182                     ex.span = e.span;
 183                 }
 184                 // Merge attributes into the inner expression.
 185                 let mut attrs = e.attrs.clone();
 186                 attrs.extend::<Vec<_>>(ex.attrs.into());
 187                 ex.attrs = attrs;
 188                 return ex;
 189             }
 190
 191             ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 192
 193             ExprKind::Err => hir::ExprKind::Err,
 194
 195             // Desugar `ExprForLoop`
 196             // from: `[opt_ident]: for <pat> in <head> <body>`
 197             ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 198                 return self.lower_expr_for(e, pat, head, body, opt_label);
 199             }
 200             ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 201             ExprKind::Mac(_) => panic!("Shouldn't exist here"),
 202         };
 203
 204         hir::Expr {
 205             hir_id: self.lower_node_id(e.id),
 206             kind,
 207             span: e.span,
 208             attrs: e.attrs.iter().map(|a| self.lower_attr(a)).collect::<Vec<_>>().into(),
 209         }
 210     }
 211
 212     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 213         match u {
 214             UnOp::Deref => hir::UnOp::UnDeref,
 215             UnOp::Not => hir::UnOp::UnNot,
 216             UnOp::Neg => hir::UnOp::UnNeg,
 217         }
 218     }
 219
 220     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 221         Spanned {
 222             node: match b.node {
 223                 BinOpKind::Add => hir::BinOpKind::Add,
 224                 BinOpKind::Sub => hir::BinOpKind::Sub,
 225                 BinOpKind::Mul => hir::BinOpKind::Mul,
 226                 BinOpKind::Div => hir::BinOpKind::Div,
 227                 BinOpKind::Rem => hir::BinOpKind::Rem,
 228                 BinOpKind::And => hir::BinOpKind::And,
 229                 BinOpKind::Or => hir::BinOpKind::Or,
 230                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 231                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 232                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 233                 BinOpKind::Shl => hir::BinOpKind::Shl,
 234                 BinOpKind::Shr => hir::BinOpKind::Shr,
 235                 BinOpKind::Eq => hir::BinOpKind::Eq,
 236                 BinOpKind::Lt => hir::BinOpKind::Lt,
 237                 BinOpKind::Le => hir::BinOpKind::Le,
 238                 BinOpKind::Ne => hir::BinOpKind::Ne,
 239                 BinOpKind::Ge => hir::BinOpKind::Ge,
 240                 BinOpKind::Gt => hir::BinOpKind::Gt,
 241             },
 242             span: b.span,
 243         }
 244     }
 245
 246     /// Emit an error and lower `ast::ExprKind::Let(pat, scrutinee)` into:
 247     /// ```rust
 248     /// match scrutinee { pats => true, _ => false }
 249     /// ```
 250     fn lower_expr_let(&mut self, span: Span, pat: &Pat, scrutinee: &Expr) -> hir::ExprKind<'hir> {
 251         // If we got here, the `let` expression is not allowed.
 252
 253         if self.sess.opts.unstable_features.is_nightly_build() {
 254             self.sess
 255                 .struct_span_err(span, "`let` expressions are not supported here")
 256                 .note("only supported directly in conditions of `if`- and `while`-expressions")
 257                 .note("as well as when nested within `&&` and parenthesis in those conditions")
 258                 .emit();
 259         } else {
 260             self.sess
 261                 .struct_span_err(span, "expected expression, found statement (`let`)")
 262                 .note("variable declaration using `let` is a statement")
 263                 .emit();
 264         }
 265
 266         // For better recovery, we emit:
 267         // ```
 268         // match scrutinee { pat => true, _ => false }
 269         // ```
 270         // While this doesn't fully match the user's intent, it has key advantages:
 271         // 1. We can avoid using `abort_if_errors`.
 272         // 2. We can typeck both `pat` and `scrutinee`.
 273         // 3. `pat` is allowed to be refutable.
 274         // 4. The return type of the block is `bool` which seems like what the user wanted.
 275         let scrutinee = self.lower_expr(scrutinee);
 276         let then_arm = {
 277             let pat = self.lower_pat(pat);
 278             let expr = self.expr_bool(span, true);
 279             self.arm(pat, expr)
 280         };
 281         let else_arm = {
 282             let pat = self.pat_wild(span);
 283             let expr = self.expr_bool(span, false);
 284             self.arm(pat, expr)
 285         };
 286         hir::ExprKind::Match(
 287             scrutinee,
 288             arena_vec![self; then_arm, else_arm],
 289             hir::MatchSource::Normal,
 290         )
 291     }
 292
 293     fn lower_expr_if(
 294         &mut self,
 295         span: Span,
 296         cond: &Expr,
 297         then: &Block,
 298         else_opt: Option<&Expr>,
 299     ) -> hir::ExprKind<'hir> {
 300         // FIXME(#53667): handle lowering of && and parens.
 301
 302         // `_ => else_block` where `else_block` is `{}` if there's `None`:
 303         let else_pat = self.pat_wild(span);
 304         let (else_expr, contains_else_clause) = match else_opt {
 305             None => (self.expr_block_empty(span), false),
 306             Some(els) => (self.lower_expr(els), true),
 307         };
 308         let else_arm = self.arm(else_pat, else_expr);
 309
 310         // Handle then + scrutinee:
 311         let then_expr = self.lower_block_expr(then);
 312         let (then_pat, scrutinee, desugar) = match cond.kind {
 313             // `<pat> => <then>`:
 314             ExprKind::Let(ref pat, ref scrutinee) => {
 315                 let scrutinee = self.lower_expr(scrutinee);
 316                 let pat = self.lower_pat(pat);
 317                 (pat, scrutinee, hir::MatchSource::IfLetDesugar { contains_else_clause })
 318             }
 319             // `true => <then>`:
 320             _ => {
 321                 // Lower condition:
 322                 let cond = self.lower_expr(cond);
 323                 let span_block =
 324                     self.mark_span_with_reason(DesugaringKind::CondTemporary, cond.span, None);
 325                 // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 326                 // to preserve drop semantics since `if cond { ... }` does not
 327                 // let temporaries live outside of `cond`.
 328                 let cond = self.expr_drop_temps(span_block, cond, ThinVec::new());
 329                 let pat = self.pat_bool(span, true);
 330                 (pat, cond, hir::MatchSource::IfDesugar { contains_else_clause })
 331             }
 332         };
 333         let then_arm = self.arm(then_pat, self.arena.alloc(then_expr));
 334
 335         hir::ExprKind::Match(scrutinee, arena_vec![self; then_arm, else_arm], desugar)
 336     }
 337
 338     fn lower_expr_while_in_loop_scope(
 339         &mut self,
 340         span: Span,
 341         cond: &Expr,
 342         body: &Block,
 343         opt_label: Option<Label>,
 344     ) -> hir::ExprKind<'hir> {
 345         // FIXME(#53667): handle lowering of && and parens.
 346
 347         // Note that the block AND the condition are evaluated in the loop scope.
 348         // This is done to allow `break` from inside the condition of the loop.
 349
 350         // `_ => break`:
 351         let else_arm = {
 352             let else_pat = self.pat_wild(span);
 353             let else_expr = self.expr_break(span, ThinVec::new());
 354             self.arm(else_pat, else_expr)
 355         };
 356
 357         // Handle then + scrutinee:
 358         let then_expr = self.lower_block_expr(body);
 359         let (then_pat, scrutinee, desugar, source) = match cond.kind {
 360             ExprKind::Let(ref pat, ref scrutinee) => {
 361                 // to:
 362                 //
 363                 //   [opt_ident]: loop {
 364                 //     match <sub_expr> {
 365                 //       <pat> => <body>,
 366                 //       _ => break
 367                 //     }
 368                 //   }
 369                 let scrutinee = self.with_loop_condition_scope(|t| t.lower_expr(scrutinee));
 370                 let pat = self.lower_pat(pat);
 371                 (pat, scrutinee, hir::MatchSource::WhileLetDesugar, hir::LoopSource::WhileLet)
 372             }
 373             _ => {
 374                 // We desugar: `'label: while $cond $body` into:
 375                 //
 376                 // ```
 377                 // 'label: loop {
 378                 //     match drop-temps { $cond } {
 379                 //         true => $body,
 380                 //         _ => break,
 381                 //     }
 382                 // }
 383                 // ```
 384
 385                 // Lower condition:
 386                 let cond = self.with_loop_condition_scope(|this| this.lower_expr(cond));
 387                 let span_block =
 388                     self.mark_span_with_reason(DesugaringKind::CondTemporary, cond.span, None);
 389                 // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 390                 // to preserve drop semantics since `while cond { ... }` does not
 391                 // let temporaries live outside of `cond`.
 392                 let cond = self.expr_drop_temps(span_block, cond, ThinVec::new());
 393                 // `true => <then>`:
 394                 let pat = self.pat_bool(span, true);
 395                 (pat, cond, hir::MatchSource::WhileDesugar, hir::LoopSource::While)
 396             }
 397         };
 398         let then_arm = self.arm(then_pat, self.arena.alloc(then_expr));
 399
 400         // `match <scrutinee> { ... }`
 401         let match_expr = self.expr_match(
 402             scrutinee.span,
 403             scrutinee,
 404             arena_vec![self; then_arm, else_arm],
 405             desugar,
 406         );
 407
 408         // `[opt_ident]: loop { ... }`
 409         hir::ExprKind::Loop(self.block_expr(self.arena.alloc(match_expr)), opt_label, source)
 410     }
 411
 412     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_ok(<expr>) }`,
 413     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_ok(()) }`
 414     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 415     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 416         self.with_catch_scope(body.id, |this| {
 417             let mut block = this.lower_block_noalloc(body, true);
 418
 419             let try_span = this.mark_span_with_reason(
 420                 DesugaringKind::TryBlock,
 421                 body.span,
 422                 this.allow_try_trait.clone(),
 423             );
 424
 425             // Final expression of the block (if present) or `()` with span at the end of block
 426             let tail_expr = block
 427                 .expr
 428                 .take()
 429                 .unwrap_or_else(|| this.expr_unit(this.sess.source_map().end_point(try_span)));
 430
 431             let ok_wrapped_span =
 432                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 433
 434             // `::std::ops::Try::from_ok($tail_expr)`
 435             block.expr = Some(this.wrap_in_try_constructor(
 436                 sym::from_ok,
 437                 try_span,
 438                 tail_expr,
 439                 ok_wrapped_span,
 440             ));
 441
 442             hir::ExprKind::Block(this.arena.alloc(block), None)
 443         })
 444     }
 445
 446     fn wrap_in_try_constructor(
 447         &mut self,
 448         method: Symbol,
 449         method_span: Span,
 450         expr: &'hir hir::Expr<'hir>,
 451         overall_span: Span,
 452     ) -> &'hir hir::Expr<'hir> {
 453         let path = &[sym::ops, sym::Try, method];
 454         let constructor =
 455             self.arena.alloc(self.expr_std_path(method_span, path, None, ThinVec::new()));
 456         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 457     }
 458
 459     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 460         hir::Arm {
 461             hir_id: self.next_id(),
 462             attrs: self.lower_attrs(&arm.attrs),
 463             pat: self.lower_pat(&arm.pat),
 464             guard: match arm.guard {
 465                 Some(ref x) => Some(hir::Guard::If(self.lower_expr(x))),
 466                 _ => None,
 467             },
 468             body: self.lower_expr(&arm.body),
 469             span: arm.span,
 470         }
 471     }
 472
 473     pub(super) fn make_async_expr(
 474         &mut self,
 475         capture_clause: CaptureBy,
 476         closure_node_id: NodeId,
 477         ret_ty: Option<AstP<Ty>>,
 478         span: Span,
 479         async_gen_kind: hir::AsyncGeneratorKind,
 480         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 481     ) -> hir::ExprKind<'hir> {
 482         let output = match ret_ty {
 483             Some(ty) => FunctionRetTy::Ty(ty),
 484             None => FunctionRetTy::Default(span),
 485         };
 486         let ast_decl = FnDecl { inputs: vec![], output };
 487         let decl = self.lower_fn_decl(&ast_decl, None, /* impl trait allowed */ false, None);
 488         let body_id = self.lower_fn_body(&ast_decl, |this| {
 489             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 490             body(this)
 491         });
 492
 493         // `static || -> <ret_ty> { body }`:
 494         let generator_kind = hir::ExprKind::Closure(
 495             capture_clause,
 496             decl,
 497             body_id,
 498             span,
 499             Some(hir::Movability::Static),
 500         );
 501         let generator = hir::Expr {
 502             hir_id: self.lower_node_id(closure_node_id),
 503             kind: generator_kind,
 504             span,
 505             attrs: ThinVec::new(),
 506         };
 507
 508         // `future::from_generator`:
 509         let unstable_span =
 510             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 511         let gen_future = self.expr_std_path(
 512             unstable_span,
 513             &[sym::future, sym::from_generator],
 514             None,
 515             ThinVec::new(),
 516         );
 517
 518         // `future::from_generator(generator)`:
 519         hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
 520     }
 521
 522     /// Desugar `<expr>.await` into:
 523     /// ```rust
 524     /// match <expr> {
 525     ///     mut pinned => loop {
 526     ///         match ::std::future::poll_with_tls_context(unsafe {
 527     ///             <::std::pin::Pin>::new_unchecked(&mut pinned)
 528     ///         }) {
 529     ///             ::std::task::Poll::Ready(result) => break result,
 530     ///             ::std::task::Poll::Pending => {}
 531     ///         }
 532     ///         yield ();
 533     ///     }
 534     /// }
 535     /// ```
 536     fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 537         match self.generator_kind {
 538             Some(hir::GeneratorKind::Async(_)) => {}
 539             Some(hir::GeneratorKind::Gen) | None => {
 540                 let mut err = struct_span_err!(
 541                     self.sess,
 542                     await_span,
 543                     E0728,
 544                     "`await` is only allowed inside `async` functions and blocks"
 545                 );
 546                 err.span_label(await_span, "only allowed inside `async` functions and blocks");
 547                 if let Some(item_sp) = self.current_item {
 548                     err.span_label(item_sp, "this is not `async`");
 549                 }
 550                 err.emit();
 551             }
 552         }
 553         let span = self.mark_span_with_reason(DesugaringKind::Await, await_span, None);
 554         let gen_future_span = self.mark_span_with_reason(
 555             DesugaringKind::Await,
 556             await_span,
 557             self.allow_gen_future.clone(),
 558         );
 559
 560         let pinned_ident = Ident::with_dummy_span(sym::pinned);
 561         let (pinned_pat, pinned_pat_hid) =
 562             self.pat_ident_binding_mode(span, pinned_ident, hir::BindingAnnotation::Mutable);
 563
 564         // ::std::future::poll_with_tls_context(unsafe {
 565         //     ::std::pin::Pin::new_unchecked(&mut pinned)
 566         // })`
 567         let poll_expr = {
 568             let pinned = self.expr_ident(span, pinned_ident, pinned_pat_hid);
 569             let ref_mut_pinned = self.expr_mut_addr_of(span, pinned);
 570             let pin_ty_id = self.next_id();
 571             let new_unchecked_expr_kind = self.expr_call_std_assoc_fn(
 572                 pin_ty_id,
 573                 span,
 574                 &[sym::pin, sym::Pin],
 575                 "new_unchecked",
 576                 arena_vec![self; ref_mut_pinned],
 577             );
 578             let new_unchecked =
 579                 self.arena.alloc(self.expr(span, new_unchecked_expr_kind, ThinVec::new()));
 580             let unsafe_expr = self.expr_unsafe(new_unchecked);
 581             self.expr_call_std_path(
 582                 gen_future_span,
 583                 &[sym::future, sym::poll_with_tls_context],
 584                 arena_vec![self; unsafe_expr],
 585             )
 586         };
 587
 588         // `::std::task::Poll::Ready(result) => break result`
 589         let loop_node_id = self.resolver.next_node_id();
 590         let loop_hir_id = self.lower_node_id(loop_node_id);
 591         let ready_arm = {
 592             let x_ident = Ident::with_dummy_span(sym::result);
 593             let (x_pat, x_pat_hid) = self.pat_ident(span, x_ident);
 594             let x_expr = self.expr_ident(span, x_ident, x_pat_hid);
 595             let ready_pat = self.pat_std_enum(
 596                 span,
 597                 &[sym::task, sym::Poll, sym::Ready],
 598                 arena_vec![self; x_pat],
 599             );
 600             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 601                 let expr_break =
 602                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 603                 this.arena.alloc(this.expr(await_span, expr_break, ThinVec::new()))
 604             });
 605             self.arm(ready_pat, break_x)
 606         };
 607
 608         // `::std::task::Poll::Pending => {}`
 609         let pending_arm = {
 610             let pending_pat = self.pat_std_enum(span, &[sym::task, sym::Poll, sym::Pending], &[]);
 611             let empty_block = self.expr_block_empty(span);
 612             self.arm(pending_pat, empty_block)
 613         };
 614
 615         let inner_match_stmt = {
 616             let match_expr = self.expr_match(
 617                 span,
 618                 poll_expr,
 619                 arena_vec![self; ready_arm, pending_arm],
 620                 hir::MatchSource::AwaitDesugar,
 621             );
 622             self.stmt_expr(span, match_expr)
 623         };
 624
 625         let yield_stmt = {
 626             let unit = self.expr_unit(span);
 627             let yield_expr = self.expr(
 628                 span,
 629                 hir::ExprKind::Yield(unit, hir::YieldSource::Await),
 630                 ThinVec::new(),
 631             );
 632             self.stmt_expr(span, yield_expr)
 633         };
 634
 635         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 636
 637         // loop { .. }
 638         let loop_expr = self.arena.alloc(hir::Expr {
 639             hir_id: loop_hir_id,
 640             kind: hir::ExprKind::Loop(loop_block, None, hir::LoopSource::Loop),
 641             span,
 642             attrs: ThinVec::new(),
 643         });
 644
 645         // mut pinned => loop { ... }
 646         let pinned_arm = self.arm(pinned_pat, loop_expr);
 647
 648         // match <expr> {
 649         //     mut pinned => loop { .. }
 650         // }
 651         let expr = self.lower_expr(expr);
 652         hir::ExprKind::Match(expr, arena_vec![self; pinned_arm], hir::MatchSource::AwaitDesugar)
 653     }
 654
 655     fn lower_expr_closure(
 656         &mut self,
 657         capture_clause: CaptureBy,
 658         movability: Movability,
 659         decl: &FnDecl,
 660         body: &Expr,
 661         fn_decl_span: Span,
 662     ) -> hir::ExprKind<'hir> {
 663         // Lower outside new scope to preserve `is_in_loop_condition`.
 664         let fn_decl = self.lower_fn_decl(decl, None, false, None);
 665
 666         self.with_new_scopes(move |this| {
 667             let prev = this.current_item;
 668             this.current_item = Some(fn_decl_span);
 669             let mut generator_kind = None;
 670             let body_id = this.lower_fn_body(decl, |this| {
 671                 let e = this.lower_expr_mut(body);
 672                 generator_kind = this.generator_kind;
 673                 e
 674             });
 675             let generator_option =
 676                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 677             this.current_item = prev;
 678             hir::ExprKind::Closure(capture_clause, fn_decl, body_id, fn_decl_span, generator_option)
 679         })
 680     }
 681
 682     fn generator_movability_for_fn(
 683         &mut self,
 684         decl: &FnDecl,
 685         fn_decl_span: Span,
 686         generator_kind: Option<hir::GeneratorKind>,
 687         movability: Movability,
 688     ) -> Option<hir::Movability> {
 689         match generator_kind {
 690             Some(hir::GeneratorKind::Gen) => {
 691                 if !decl.inputs.is_empty() {
 692                     struct_span_err!(
 693                         self.sess,
 694                         fn_decl_span,
 695                         E0628,
 696                         "generators cannot have explicit parameters"
 697                     )
 698                     .emit();
 699                 }
 700                 Some(movability)
 701             }
 702             Some(hir::GeneratorKind::Async(_)) => {
 703                 bug!("non-`async` closure body turned `async` during lowering");
 704             }
 705             None => {
 706                 if movability == Movability::Static {
 707                     struct_span_err!(self.sess, fn_decl_span, E0697, "closures cannot be static")
 708                         .emit();
 709                 }
 710                 None
 711             }
 712         }
 713     }
 714
 715     fn lower_expr_async_closure(
 716         &mut self,
 717         capture_clause: CaptureBy,
 718         closure_id: NodeId,
 719         decl: &FnDecl,
 720         body: &Expr,
 721         fn_decl_span: Span,
 722     ) -> hir::ExprKind<'hir> {
 723         let outer_decl =
 724             FnDecl { inputs: decl.inputs.clone(), output: FunctionRetTy::Default(fn_decl_span) };
 725         // We need to lower the declaration outside the new scope, because we
 726         // have to conserve the state of being inside a loop condition for the
 727         // closure argument types.
 728         let fn_decl = self.lower_fn_decl(&outer_decl, None, false, None);
 729
 730         self.with_new_scopes(move |this| {
 731             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 732             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 733                 struct_span_err!(
 734                     this.sess,
 735                     fn_decl_span,
 736                     E0708,
 737                     "`async` non-`move` closures with parameters are not currently supported",
 738                 )
 739                 .help(
 740                     "consider using `let` statements to manually capture \
 741                     variables by reference before entering an `async move` closure",
 742                 )
 743                 .emit();
 744             }
 745
 746             // Transform `async |x: u8| -> X { ... }` into
 747             // `|x: u8| future_from_generator(|| -> X { ... })`.
 748             let body_id = this.lower_fn_body(&outer_decl, |this| {
 749                 let async_ret_ty =
 750                     if let FunctionRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 751                 let async_body = this.make_async_expr(
 752                     capture_clause,
 753                     closure_id,
 754                     async_ret_ty,
 755                     body.span,
 756                     hir::AsyncGeneratorKind::Closure,
 757                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 758                 );
 759                 this.expr(fn_decl_span, async_body, ThinVec::new())
 760             });
 761             hir::ExprKind::Closure(capture_clause, fn_decl, body_id, fn_decl_span, None)
 762         })
 763     }
 764
 765     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
 766     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
 767         let id = self.next_id();
 768         let e1 = self.lower_expr_mut(e1);
 769         let e2 = self.lower_expr_mut(e2);
 770         self.expr_call_std_assoc_fn(
 771             id,
 772             span,
 773             &[sym::ops, sym::RangeInclusive],
 774             "new",
 775             arena_vec![self; e1, e2],
 776         )
 777     }
 778
 779     fn lower_expr_range(
 780         &mut self,
 781         span: Span,
 782         e1: Option<&Expr>,
 783         e2: Option<&Expr>,
 784         lims: RangeLimits,
 785     ) -> hir::ExprKind<'hir> {
 786         use syntax::ast::RangeLimits::*;
 787
 788         let path = match (e1, e2, lims) {
 789             (None, None, HalfOpen) => sym::RangeFull,
 790             (Some(..), None, HalfOpen) => sym::RangeFrom,
 791             (None, Some(..), HalfOpen) => sym::RangeTo,
 792             (Some(..), Some(..), HalfOpen) => sym::Range,
 793             (None, Some(..), Closed) => sym::RangeToInclusive,
 794             (Some(..), Some(..), Closed) => unreachable!(),
 795             (_, None, Closed) => {
 796                 self.diagnostic().span_fatal(span, "inclusive range with no end").raise()
 797             }
 798         };
 799
 800         let fields = self.arena.alloc_from_iter(
 801             e1.iter().map(|e| ("start", e)).chain(e2.iter().map(|e| ("end", e))).map(|(s, e)| {
 802                 let expr = self.lower_expr(&e);
 803                 let ident = Ident::new(Symbol::intern(s), e.span);
 804                 self.field(ident, expr, e.span)
 805             }),
 806         );
 807
 808         let is_unit = fields.is_empty();
 809         let struct_path = [sym::ops, path];
 810         let struct_path = self.std_path(span, &struct_path, None, is_unit);
 811         let struct_path = hir::QPath::Resolved(None, struct_path);
 812
 813         if is_unit {
 814             hir::ExprKind::Path(struct_path)
 815         } else {
 816             hir::ExprKind::Struct(self.arena.alloc(struct_path), fields, None)
 817         }
 818     }
 819
 820     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
 821         let target_id = match destination {
 822             Some((id, _)) => {
 823                 if let Some(loop_id) = self.resolver.get_label_res(id) {
 824                     Ok(self.lower_node_id(loop_id))
 825                 } else {
 826                     Err(hir::LoopIdError::UnresolvedLabel)
 827                 }
 828             }
 829             None => self
 830                 .loop_scopes
 831                 .last()
 832                 .cloned()
 833                 .map(|id| Ok(self.lower_node_id(id)))
 834                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope))
 835                 .into(),
 836         };
 837         hir::Destination { label: destination.map(|(_, label)| label), target_id }
 838     }
 839
 840     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
 841         if self.is_in_loop_condition && opt_label.is_none() {
 842             hir::Destination {
 843                 label: None,
 844                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 845             }
 846         } else {
 847             self.lower_loop_destination(opt_label.map(|label| (id, label)))
 848         }
 849     }
 850
 851     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
 852         let len = self.catch_scopes.len();
 853         self.catch_scopes.push(catch_id);
 854
 855         let result = f(self);
 856         assert_eq!(
 857             len + 1,
 858             self.catch_scopes.len(),
 859             "catch scopes should be added and removed in stack order"
 860         );
 861
 862         self.catch_scopes.pop().unwrap();
 863
 864         result
 865     }
 866
 867     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
 868         // We're no longer in the base loop's condition; we're in another loop.
 869         let was_in_loop_condition = self.is_in_loop_condition;
 870         self.is_in_loop_condition = false;
 871
 872         let len = self.loop_scopes.len();
 873         self.loop_scopes.push(loop_id);
 874
 875         let result = f(self);
 876         assert_eq!(
 877             len + 1,
 878             self.loop_scopes.len(),
 879             "loop scopes should be added and removed in stack order"
 880         );
 881
 882         self.loop_scopes.pop().unwrap();
 883
 884         self.is_in_loop_condition = was_in_loop_condition;
 885
 886         result
 887     }
 888
 889     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
 890         let was_in_loop_condition = self.is_in_loop_condition;
 891         self.is_in_loop_condition = true;
 892
 893         let result = f(self);
 894
 895         self.is_in_loop_condition = was_in_loop_condition;
 896
 897         result
 898     }
 899
 900     fn lower_expr_asm(&mut self, asm: &InlineAsm) -> hir::ExprKind<'hir> {
 901         let inner = hir::InlineAsmInner {
 902             inputs: asm.inputs.iter().map(|&(c, _)| c).collect(),
 903             outputs: asm
 904                 .outputs
 905                 .iter()
 906                 .map(|out| hir::InlineAsmOutput {
 907                     constraint: out.constraint,
 908                     is_rw: out.is_rw,
 909                     is_indirect: out.is_indirect,
 910                     span: out.expr.span,
 911                 })
 912                 .collect(),
 913             asm: asm.asm,
 914             asm_str_style: asm.asm_str_style,
 915             clobbers: asm.clobbers.clone().into(),
 916             volatile: asm.volatile,
 917             alignstack: asm.alignstack,
 918             dialect: asm.dialect,
 919         };
 920         let hir_asm = hir::InlineAsm {
 921             inner,
 922             inputs_exprs: self.arena.alloc_from_iter(
 923                 asm.inputs.iter().map(|&(_, ref input)| self.lower_expr_mut(input)),
 924             ),
 925             outputs_exprs: self
 926                 .arena
 927                 .alloc_from_iter(asm.outputs.iter().map(|out| self.lower_expr_mut(&out.expr))),
 928         };
 929         hir::ExprKind::InlineAsm(self.arena.alloc(hir_asm))
 930     }
 931
 932     fn lower_field(&mut self, f: &Field) -> hir::Field<'hir> {
 933         hir::Field {
 934             hir_id: self.next_id(),
 935             ident: f.ident,
 936             expr: self.lower_expr(&f.expr),
 937             span: f.span,
 938             is_shorthand: f.is_shorthand,
 939         }
 940     }
 941
 942     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
 943         match self.generator_kind {
 944             Some(hir::GeneratorKind::Gen) => {}
 945             Some(hir::GeneratorKind::Async(_)) => {
 946                 struct_span_err!(
 947                     self.sess,
 948                     span,
 949                     E0727,
 950                     "`async` generators are not yet supported"
 951                 )
 952                 .emit();
 953                 return hir::ExprKind::Err;
 954             }
 955             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
 956         }
 957
 958         let expr =
 959             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
 960
 961         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
 962     }
 963
 964     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
 965     /// ```rust
 966     /// {
 967     ///     let result = match ::std::iter::IntoIterator::into_iter(<head>) {
 968     ///         mut iter => {
 969     ///             [opt_ident]: loop {
 970     ///                 let mut __next;
 971     ///                 match ::std::iter::Iterator::next(&mut iter) {
 972     ///                     ::std::option::Option::Some(val) => __next = val,
 973     ///                     ::std::option::Option::None => break
 974     ///                 };
 975     ///                 let <pat> = __next;
 976     ///                 StmtKind::Expr(<body>);
 977     ///             }
 978     ///         }
 979     ///     };
 980     ///     result
 981     /// }
 982     /// ```
 983     fn lower_expr_for(
 984         &mut self,
 985         e: &Expr,
 986         pat: &Pat,
 987         head: &Expr,
 988         body: &Block,
 989         opt_label: Option<Label>,
 990     ) -> hir::Expr<'hir> {
 991         // expand <head>
 992         let mut head = self.lower_expr_mut(head);
 993         let desugared_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
 994         head.span = desugared_span;
 995
 996         let iter = Ident::with_dummy_span(sym::iter);
 997
 998         let next_ident = Ident::with_dummy_span(sym::__next);
 999         let (next_pat, next_pat_hid) = self.pat_ident_binding_mode(
1000             desugared_span,
1001             next_ident,
1002             hir::BindingAnnotation::Mutable,
1003         );
1004
1005         // `::std::option::Option::Some(val) => __next = val`
1006         let pat_arm = {
1007             let val_ident = Ident::with_dummy_span(sym::val);
1008             let (val_pat, val_pat_hid) = self.pat_ident(pat.span, val_ident);
1009             let val_expr = self.expr_ident(pat.span, val_ident, val_pat_hid);
1010             let next_expr = self.expr_ident(pat.span, next_ident, next_pat_hid);
1011             let assign = self.arena.alloc(self.expr(
1012                 pat.span,
1013                 hir::ExprKind::Assign(next_expr, val_expr, pat.span),
1014                 ThinVec::new(),
1015             ));
1016             let some_pat = self.pat_some(pat.span, val_pat);
1017             self.arm(some_pat, assign)
1018         };
1019
1020         // `::std::option::Option::None => break`
1021         let break_arm = {
1022             let break_expr =
1023                 self.with_loop_scope(e.id, |this| this.expr_break(e.span, ThinVec::new()));
1024             let pat = self.pat_none(e.span);
1025             self.arm(pat, break_expr)
1026         };
1027
1028         // `mut iter`
1029         let (iter_pat, iter_pat_nid) =
1030             self.pat_ident_binding_mode(desugared_span, iter, hir::BindingAnnotation::Mutable);
1031
1032         // `match ::std::iter::Iterator::next(&mut iter) { ... }`
1033         let match_expr = {
1034             let iter = self.expr_ident(desugared_span, iter, iter_pat_nid);
1035             let ref_mut_iter = self.expr_mut_addr_of(desugared_span, iter);
1036             let next_path = &[sym::iter, sym::Iterator, sym::next];
1037             let next_expr =
1038                 self.expr_call_std_path(desugared_span, next_path, arena_vec![self; ref_mut_iter]);
1039             let arms = arena_vec![self; pat_arm, break_arm];
1040
1041             self.expr_match(desugared_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1042         };
1043         let match_stmt = self.stmt_expr(desugared_span, match_expr);
1044
1045         let next_expr = self.expr_ident(desugared_span, next_ident, next_pat_hid);
1046
1047         // `let mut __next`
1048         let next_let = self.stmt_let_pat(
1049             ThinVec::new(),
1050             desugared_span,
1051             None,
1052             next_pat,
1053             hir::LocalSource::ForLoopDesugar,
1054         );
1055
1056         // `let <pat> = __next`
1057         let pat = self.lower_pat(pat);
1058         let pat_let = self.stmt_let_pat(
1059             ThinVec::new(),
1060             desugared_span,
1061             Some(next_expr),
1062             pat,
1063             hir::LocalSource::ForLoopDesugar,
1064         );
1065
1066         let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1067         let body_expr = self.expr_block(body_block, ThinVec::new());
1068         let body_stmt = self.stmt_expr(body.span, body_expr);
1069
1070         let loop_block = self.block_all(
1071             e.span,
1072             arena_vec![self; next_let, match_stmt, pat_let, body_stmt],
1073             None,
1074         );
1075
1076         // `[opt_ident]: loop { ... }`
1077         let kind = hir::ExprKind::Loop(loop_block, opt_label, hir::LoopSource::ForLoop);
1078         let loop_expr = self.arena.alloc(hir::Expr {
1079             hir_id: self.lower_node_id(e.id),
1080             kind,
1081             span: e.span,
1082             attrs: ThinVec::new(),
1083         });
1084
1085         // `mut iter => { ... }`
1086         let iter_arm = self.arm(iter_pat, loop_expr);
1087
1088         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1089         let into_iter_expr = {
1090             let into_iter_path = &[sym::iter, sym::IntoIterator, sym::into_iter];
1091             self.expr_call_std_path(desugared_span, into_iter_path, arena_vec![self; head])
1092         };
1093
1094         let match_expr = self.arena.alloc(self.expr_match(
1095             desugared_span,
1096             into_iter_expr,
1097             arena_vec![self; iter_arm],
1098             hir::MatchSource::ForLoopDesugar,
1099         ));
1100
1101         // This is effectively `{ let _result = ...; _result }`.
1102         // The construct was introduced in #21984 and is necessary to make sure that
1103         // temporaries in the `head` expression are dropped and do not leak to the
1104         // surrounding scope of the `match` since the `match` is not a terminating scope.
1105         //
1106         // Also, add the attributes to the outer returned expr node.
1107         self.expr_drop_temps_mut(desugared_span, match_expr, e.attrs.clone())
1108     }
1109
1110     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1111     /// ```rust
1112     /// match Try::into_result(<expr>) {
1113     ///     Ok(val) => #[allow(unreachable_code)] val,
1114     ///     Err(err) => #[allow(unreachable_code)]
1115     ///                 // If there is an enclosing `try {...}`:
1116     ///                 break 'catch_target Try::from_error(From::from(err)),
1117     ///                 // Otherwise:
1118     ///                 return Try::from_error(From::from(err)),
1119     /// }
1120     /// ```
1121     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1122         let unstable_span = self.mark_span_with_reason(
1123             DesugaringKind::QuestionMark,
1124             span,
1125             self.allow_try_trait.clone(),
1126         );
1127         let try_span = self.sess.source_map().end_point(span);
1128         let try_span = self.mark_span_with_reason(
1129             DesugaringKind::QuestionMark,
1130             try_span,
1131             self.allow_try_trait.clone(),
1132         );
1133
1134         // `Try::into_result(<expr>)`
1135         let scrutinee = {
1136             // expand <expr>
1137             let sub_expr = self.lower_expr_mut(sub_expr);
1138
1139             let path = &[sym::ops, sym::Try, sym::into_result];
1140             self.expr_call_std_path(unstable_span, path, arena_vec![self; sub_expr])
1141         };
1142
1143         // `#[allow(unreachable_code)]`
1144         let attr = {
1145             // `allow(unreachable_code)`
1146             let allow = {
1147                 let allow_ident = Ident::new(sym::allow, span);
1148                 let uc_ident = Ident::new(sym::unreachable_code, span);
1149                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1150                 attr::mk_list_item(allow_ident, vec![uc_nested])
1151             };
1152             attr::mk_attr_outer(allow)
1153         };
1154         let attrs = vec![attr];
1155
1156         // `Ok(val) => #[allow(unreachable_code)] val,`
1157         let ok_arm = {
1158             let val_ident = Ident::with_dummy_span(sym::val);
1159             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1160             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1161                 span,
1162                 val_ident,
1163                 val_pat_nid,
1164                 ThinVec::from(attrs.clone()),
1165             ));
1166             let ok_pat = self.pat_ok(span, val_pat);
1167             self.arm(ok_pat, val_expr)
1168         };
1169
1170         // `Err(err) => #[allow(unreachable_code)]
1171         //              return Try::from_error(From::from(err)),`
1172         let err_arm = {
1173             let err_ident = Ident::with_dummy_span(sym::err);
1174             let (err_local, err_local_nid) = self.pat_ident(try_span, err_ident);
1175             let from_expr = {
1176                 let from_path = &[sym::convert, sym::From, sym::from];
1177                 let err_expr = self.expr_ident_mut(try_span, err_ident, err_local_nid);
1178                 self.expr_call_std_path(try_span, from_path, arena_vec![self; err_expr])
1179             };
1180             let from_err_expr =
1181                 self.wrap_in_try_constructor(sym::from_error, unstable_span, from_expr, try_span);
1182             let thin_attrs = ThinVec::from(attrs);
1183             let catch_scope = self.catch_scopes.last().map(|x| *x);
1184             let ret_expr = if let Some(catch_node) = catch_scope {
1185                 let target_id = Ok(self.lower_node_id(catch_node));
1186                 self.arena.alloc(self.expr(
1187                     try_span,
1188                     hir::ExprKind::Break(
1189                         hir::Destination { label: None, target_id },
1190                         Some(from_err_expr),
1191                     ),
1192                     thin_attrs,
1193                 ))
1194             } else {
1195                 self.arena.alloc(self.expr(
1196                     try_span,
1197                     hir::ExprKind::Ret(Some(from_err_expr)),
1198                     thin_attrs,
1199                 ))
1200             };
1201
1202             let err_pat = self.pat_err(try_span, err_local);
1203             self.arm(err_pat, ret_expr)
1204         };
1205
1206         hir::ExprKind::Match(
1207             scrutinee,
1208             arena_vec![self; err_arm, ok_arm],
1209             hir::MatchSource::TryDesugar,
1210         )
1211     }
1212
1213     // =========================================================================
1214     // Helper methods for building HIR.
1215     // =========================================================================
1216
1217     /// Constructs a `true` or `false` literal expression.
1218     pub(super) fn expr_bool(&mut self, span: Span, val: bool) -> &'hir hir::Expr<'hir> {
1219         let lit = Spanned { span, node: LitKind::Bool(val) };
1220         self.arena.alloc(self.expr(span, hir::ExprKind::Lit(lit), ThinVec::new()))
1221     }
1222
1223     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1224     ///
1225     /// In terms of drop order, it has the same effect as wrapping `expr` in
1226     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1227     ///
1228     /// The drop order can be important in e.g. `if expr { .. }`.
1229     pub(super) fn expr_drop_temps(
1230         &mut self,
1231         span: Span,
1232         expr: &'hir hir::Expr<'hir>,
1233         attrs: AttrVec,
1234     ) -> &'hir hir::Expr<'hir> {
1235         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1236     }
1237
1238     pub(super) fn expr_drop_temps_mut(
1239         &mut self,
1240         span: Span,
1241         expr: &'hir hir::Expr<'hir>,
1242         attrs: AttrVec,
1243     ) -> hir::Expr<'hir> {
1244         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1245     }
1246
1247     fn expr_match(
1248         &mut self,
1249         span: Span,
1250         arg: &'hir hir::Expr<'hir>,
1251         arms: &'hir [hir::Arm<'hir>],
1252         source: hir::MatchSource,
1253     ) -> hir::Expr<'hir> {
1254         self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
1255     }
1256
1257     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1258         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1259         self.arena.alloc(self.expr(span, expr_break, attrs))
1260     }
1261
1262     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1263         self.expr(
1264             span,
1265             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1266             ThinVec::new(),
1267         )
1268     }
1269
1270     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1271         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
1272     }
1273
1274     fn expr_call(
1275         &mut self,
1276         span: Span,
1277         e: &'hir hir::Expr<'hir>,
1278         args: &'hir [hir::Expr<'hir>],
1279     ) -> &'hir hir::Expr<'hir> {
1280         self.arena.alloc(self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new()))
1281     }
1282
1283     // Note: associated functions must use `expr_call_std_path`.
1284     fn expr_call_std_path(
1285         &mut self,
1286         span: Span,
1287         path_components: &[Symbol],
1288         args: &'hir [hir::Expr<'hir>],
1289     ) -> &'hir hir::Expr<'hir> {
1290         let path =
1291             self.arena.alloc(self.expr_std_path(span, path_components, None, ThinVec::new()));
1292         self.expr_call(span, path, args)
1293     }
1294
1295     // Create an expression calling an associated function of an std type.
1296     //
1297     // Associated functions cannot be resolved through the normal `std_path` function,
1298     // as they are resolved differently and so cannot use `expr_call_std_path`.
1299     //
1300     // This function accepts the path component (`ty_path_components`) separately from
1301     // the name of the associated function (`assoc_fn_name`) in order to facilitate
1302     // separate resolution of the type and creation of a path referring to its associated
1303     // function.
1304     fn expr_call_std_assoc_fn(
1305         &mut self,
1306         ty_path_id: hir::HirId,
1307         span: Span,
1308         ty_path_components: &[Symbol],
1309         assoc_fn_name: &str,
1310         args: &'hir [hir::Expr<'hir>],
1311     ) -> hir::ExprKind<'hir> {
1312         let ty_path = self.std_path(span, ty_path_components, None, false);
1313         let ty =
1314             self.arena.alloc(self.ty_path(ty_path_id, span, hir::QPath::Resolved(None, ty_path)));
1315         let fn_seg = self.arena.alloc(hir::PathSegment::from_ident(Ident::from_str(assoc_fn_name)));
1316         let fn_path = hir::QPath::TypeRelative(ty, fn_seg);
1317         let fn_expr =
1318             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
1319         hir::ExprKind::Call(fn_expr, args)
1320     }
1321
1322     fn expr_std_path(
1323         &mut self,
1324         span: Span,
1325         components: &[Symbol],
1326         params: Option<&'hir hir::GenericArgs<'hir>>,
1327         attrs: AttrVec,
1328     ) -> hir::Expr<'hir> {
1329         let path = self.std_path(span, components, params, true);
1330         self.expr(span, hir::ExprKind::Path(hir::QPath::Resolved(None, path)), attrs)
1331     }
1332
1333     pub(super) fn expr_ident(
1334         &mut self,
1335         sp: Span,
1336         ident: Ident,
1337         binding: hir::HirId,
1338     ) -> &'hir hir::Expr<'hir> {
1339         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1340     }
1341
1342     pub(super) fn expr_ident_mut(
1343         &mut self,
1344         sp: Span,
1345         ident: Ident,
1346         binding: hir::HirId,
1347     ) -> hir::Expr<'hir> {
1348         self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
1349     }
1350
1351     fn expr_ident_with_attrs(
1352         &mut self,
1353         span: Span,
1354         ident: Ident,
1355         binding: hir::HirId,
1356         attrs: AttrVec,
1357     ) -> hir::Expr<'hir> {
1358         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1359             None,
1360             self.arena.alloc(hir::Path {
1361                 span,
1362                 res: Res::Local(binding),
1363                 segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
1364             }),
1365         ));
1366
1367         self.expr(span, expr_path, attrs)
1368     }
1369
1370     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1371         let hir_id = self.next_id();
1372         let span = expr.span;
1373         self.expr(
1374             span,
1375             hir::ExprKind::Block(
1376                 self.arena.alloc(hir::Block {
1377                     stmts: &[],
1378                     expr: Some(expr),
1379                     hir_id,
1380                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1381                     span,
1382                     targeted_by_break: false,
1383                 }),
1384                 None,
1385             ),
1386             ThinVec::new(),
1387         )
1388     }
1389
1390     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1391         let blk = self.block_all(span, &[], None);
1392         let expr = self.expr_block(blk, ThinVec::new());
1393         self.arena.alloc(expr)
1394     }
1395
1396     pub(super) fn expr_block(
1397         &mut self,
1398         b: &'hir hir::Block<'hir>,
1399         attrs: AttrVec,
1400     ) -> hir::Expr<'hir> {
1401         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1402     }
1403
1404     pub(super) fn expr(
1405         &mut self,
1406         span: Span,
1407         kind: hir::ExprKind<'hir>,
1408         attrs: AttrVec,
1409     ) -> hir::Expr<'hir> {
1410         hir::Expr { hir_id: self.next_id(), kind, span, attrs }
1411     }
1412
1413     fn field(&mut self, ident: Ident, expr: &'hir hir::Expr<'hir>, span: Span) -> hir::Field<'hir> {
1414         hir::Field { hir_id: self.next_id(), ident, span, expr, is_shorthand: false }
1415     }
1416
1417     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1418         hir::Arm {
1419             hir_id: self.next_id(),
1420             attrs: &[],
1421             pat,
1422             guard: None,
1423             span: expr.span,
1424             body: expr,
1425         }
1426     }
1427 }