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_error_codes::*;
   6 use rustc_errors::struct_span_err;
   7 use rustc_hir as hir;
   8 use rustc_hir::def::Res;
   9 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
  10 use rustc_span::symbol::{sym, Symbol};
  11 use syntax::ast::*;
  12 use syntax::attr;
  13 use syntax::ptr::P as AstP;
  14
  15 impl<'hir> LoweringContext<'_, 'hir> {
  16     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  17         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  18     }
  19
  20     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  21         self.arena.alloc(self.lower_expr_mut(e))
  22     }
  23
  24     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  25         let kind = match e.kind {
  26             ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  27             ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  28             ExprKind::Repeat(ref expr, ref count) => {
  29                 let expr = self.lower_expr(expr);
  30                 let count = self.lower_anon_const(count);
  31                 hir::ExprKind::Repeat(expr, count)
  32             }
  33             ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  34             ExprKind::Call(ref f, ref args) => {
  35                 let f = self.lower_expr(f);
  36                 hir::ExprKind::Call(f, self.lower_exprs(args))
  37             }
  38             ExprKind::MethodCall(ref seg, ref args) => {
  39                 let hir_seg = self.arena.alloc(self.lower_path_segment(
  40                     e.span,
  41                     seg,
  42                     ParamMode::Optional,
  43                     0,
  44                     ParenthesizedGenericArgs::Err,
  45                     ImplTraitContext::disallowed(),
  46                     None,
  47                 ));
  48                 let args = self.lower_exprs(args);
  49                 hir::ExprKind::MethodCall(hir_seg, seg.ident.span, args)
  50             }
  51             ExprKind::Binary(binop, ref lhs, ref rhs) => {
  52                 let binop = self.lower_binop(binop);
  53                 let lhs = self.lower_expr(lhs);
  54                 let rhs = self.lower_expr(rhs);
  55                 hir::ExprKind::Binary(binop, lhs, rhs)
  56             }
  57             ExprKind::Unary(op, ref ohs) => {
  58                 let op = self.lower_unop(op);
  59                 let ohs = self.lower_expr(ohs);
  60                 hir::ExprKind::Unary(op, ohs)
  61             }
  62             ExprKind::Lit(ref l) => hir::ExprKind::Lit(respan(l.span, l.kind.clone())),
  63             ExprKind::Cast(ref expr, ref ty) => {
  64                 let expr = self.lower_expr(expr);
  65                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
  66                 hir::ExprKind::Cast(expr, ty)
  67             }
  68             ExprKind::Type(ref expr, ref ty) => {
  69                 let expr = self.lower_expr(expr);
  70                 let ty = self.lower_ty(ty, ImplTraitContext::disallowed());
  71                 hir::ExprKind::Type(expr, ty)
  72             }
  73             ExprKind::AddrOf(k, m, ref ohs) => {
  74                 let ohs = self.lower_expr(ohs);
  75                 hir::ExprKind::AddrOf(k, m, ohs)
  76             }
  77             ExprKind::Let(ref pat, ref scrutinee) => self.lower_expr_let(e.span, pat, scrutinee),
  78             ExprKind::If(ref cond, ref then, ref else_opt) => {
  79                 self.lower_expr_if(e.span, cond, then, else_opt.as_deref())
  80             }
  81             ExprKind::While(ref cond, ref body, opt_label) => self.with_loop_scope(e.id, |this| {
  82                 this.lower_expr_while_in_loop_scope(e.span, cond, body, opt_label)
  83             }),
  84             ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
  85                 hir::ExprKind::Loop(this.lower_block(body, false), opt_label, hir::LoopSource::Loop)
  86             }),
  87             ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
  88             ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
  89                 self.lower_expr(expr),
  90                 self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
  91                 hir::MatchSource::Normal,
  92             ),
  93             ExprKind::Async(capture_clause, closure_node_id, ref block) => self.make_async_expr(
  94                 capture_clause,
  95                 closure_node_id,
  96                 None,
  97                 block.span,
  98                 hir::AsyncGeneratorKind::Block,
  99                 |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
 100             ),
 101             ExprKind::Await(ref expr) => self.lower_expr_await(e.span, expr),
 102             ExprKind::Closure(
 103                 capture_clause,
 104                 asyncness,
 105                 movability,
 106                 ref decl,
 107                 ref body,
 108                 fn_decl_span,
 109             ) => {
 110                 if let IsAsync::Async { closure_id, .. } = asyncness {
 111                     self.lower_expr_async_closure(
 112                         capture_clause,
 113                         closure_id,
 114                         decl,
 115                         body,
 116                         fn_decl_span,
 117                     )
 118                 } else {
 119                     self.lower_expr_closure(capture_clause, movability, decl, body, fn_decl_span)
 120                 }
 121             }
 122             ExprKind::Block(ref blk, opt_label) => {
 123                 hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 124             }
 125             ExprKind::Assign(ref el, ref er, span) => {
 126                 hir::ExprKind::Assign(self.lower_expr(el), self.lower_expr(er), span)
 127             }
 128             ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 129                 self.lower_binop(op),
 130                 self.lower_expr(el),
 131                 self.lower_expr(er),
 132             ),
 133             ExprKind::Field(ref el, ident) => hir::ExprKind::Field(self.lower_expr(el), ident),
 134             ExprKind::Index(ref el, ref er) => {
 135                 hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 136             }
 137             ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 138                 self.lower_expr_range_closed(e.span, e1, e2)
 139             }
 140             ExprKind::Range(ref e1, ref e2, lims) => {
 141                 self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 142             }
 143             ExprKind::Path(ref qself, ref path) => {
 144                 let qpath = self.lower_qpath(
 145                     e.id,
 146                     qself,
 147                     path,
 148                     ParamMode::Optional,
 149                     ImplTraitContext::disallowed(),
 150                 );
 151                 hir::ExprKind::Path(qpath)
 152             }
 153             ExprKind::Break(opt_label, ref opt_expr) => {
 154                 let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 155                 hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
 156             }
 157             ExprKind::Continue(opt_label) => {
 158                 hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
 159             }
 160             ExprKind::Ret(ref e) => {
 161                 let e = e.as_ref().map(|x| self.lower_expr(x));
 162                 hir::ExprKind::Ret(e)
 163             }
 164             ExprKind::InlineAsm(ref asm) => self.lower_expr_asm(asm),
 165             ExprKind::Struct(ref path, ref fields, ref maybe_expr) => {
 166                 let maybe_expr = maybe_expr.as_ref().map(|x| self.lower_expr(x));
 167                 hir::ExprKind::Struct(
 168                     self.arena.alloc(self.lower_qpath(
 169                         e.id,
 170                         &None,
 171                         path,
 172                         ParamMode::Optional,
 173                         ImplTraitContext::disallowed(),
 174                     )),
 175                     self.arena.alloc_from_iter(fields.iter().map(|x| self.lower_field(x))),
 176                     maybe_expr,
 177                 )
 178             }
 179             ExprKind::Paren(ref ex) => {
 180                 let mut ex = self.lower_expr_mut(ex);
 181                 // Include parens in span, but only if it is a super-span.
 182                 if e.span.contains(ex.span) {
 183                     ex.span = e.span;
 184                 }
 185                 // Merge attributes into the inner expression.
 186                 let mut attrs = e.attrs.clone();
 187                 attrs.extend::<Vec<_>>(ex.attrs.into());
 188                 ex.attrs = attrs;
 189                 return ex;
 190             }
 191
 192             ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 193
 194             ExprKind::Err => hir::ExprKind::Err,
 195
 196             // Desugar `ExprForLoop`
 197             // from: `[opt_ident]: for <pat> in <head> <body>`
 198             ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 199                 return self.lower_expr_for(e, pat, head, body, opt_label);
 200             }
 201             ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 202             ExprKind::Mac(_) => panic!("Shouldn't exist here"),
 203         };
 204
 205         hir::Expr {
 206             hir_id: self.lower_node_id(e.id),
 207             kind,
 208             span: e.span,
 209             attrs: e.attrs.iter().map(|a| self.lower_attr(a)).collect::<Vec<_>>().into(),
 210         }
 211     }
 212
 213     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 214         match u {
 215             UnOp::Deref => hir::UnOp::UnDeref,
 216             UnOp::Not => hir::UnOp::UnNot,
 217             UnOp::Neg => hir::UnOp::UnNeg,
 218         }
 219     }
 220
 221     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 222         Spanned {
 223             node: match b.node {
 224                 BinOpKind::Add => hir::BinOpKind::Add,
 225                 BinOpKind::Sub => hir::BinOpKind::Sub,
 226                 BinOpKind::Mul => hir::BinOpKind::Mul,
 227                 BinOpKind::Div => hir::BinOpKind::Div,
 228                 BinOpKind::Rem => hir::BinOpKind::Rem,
 229                 BinOpKind::And => hir::BinOpKind::And,
 230                 BinOpKind::Or => hir::BinOpKind::Or,
 231                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 232                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 233                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 234                 BinOpKind::Shl => hir::BinOpKind::Shl,
 235                 BinOpKind::Shr => hir::BinOpKind::Shr,
 236                 BinOpKind::Eq => hir::BinOpKind::Eq,
 237                 BinOpKind::Lt => hir::BinOpKind::Lt,
 238                 BinOpKind::Le => hir::BinOpKind::Le,
 239                 BinOpKind::Ne => hir::BinOpKind::Ne,
 240                 BinOpKind::Ge => hir::BinOpKind::Ge,
 241                 BinOpKind::Gt => hir::BinOpKind::Gt,
 242             },
 243             span: b.span,
 244         }
 245     }
 246
 247     /// Emit an error and lower `ast::ExprKind::Let(pat, scrutinee)` into:
 248     /// ```rust
 249     /// match scrutinee { pats => true, _ => false }
 250     /// ```
 251     fn lower_expr_let(&mut self, span: Span, pat: &Pat, scrutinee: &Expr) -> hir::ExprKind<'hir> {
 252         // If we got here, the `let` expression is not allowed.
 253
 254         if self.sess.opts.unstable_features.is_nightly_build() {
 255             self.sess
 256                 .struct_span_err(span, "`let` expressions are not supported here")
 257                 .note("only supported directly in conditions of `if`- and `while`-expressions")
 258                 .note("as well as when nested within `&&` and parenthesis in those conditions")
 259                 .emit();
 260         } else {
 261             self.sess
 262                 .struct_span_err(span, "expected expression, found statement (`let`)")
 263                 .note("variable declaration using `let` is a statement")
 264                 .emit();
 265         }
 266
 267         // For better recovery, we emit:
 268         // ```
 269         // match scrutinee { pat => true, _ => false }
 270         // ```
 271         // While this doesn't fully match the user's intent, it has key advantages:
 272         // 1. We can avoid using `abort_if_errors`.
 273         // 2. We can typeck both `pat` and `scrutinee`.
 274         // 3. `pat` is allowed to be refutable.
 275         // 4. The return type of the block is `bool` which seems like what the user wanted.
 276         let scrutinee = self.lower_expr(scrutinee);
 277         let then_arm = {
 278             let pat = self.lower_pat(pat);
 279             let expr = self.expr_bool(span, true);
 280             self.arm(pat, expr)
 281         };
 282         let else_arm = {
 283             let pat = self.pat_wild(span);
 284             let expr = self.expr_bool(span, false);
 285             self.arm(pat, expr)
 286         };
 287         hir::ExprKind::Match(
 288             scrutinee,
 289             arena_vec![self; then_arm, else_arm],
 290             hir::MatchSource::Normal,
 291         )
 292     }
 293
 294     fn lower_expr_if(
 295         &mut self,
 296         span: Span,
 297         cond: &Expr,
 298         then: &Block,
 299         else_opt: Option<&Expr>,
 300     ) -> hir::ExprKind<'hir> {
 301         // FIXME(#53667): handle lowering of && and parens.
 302
 303         // `_ => else_block` where `else_block` is `{}` if there's `None`:
 304         let else_pat = self.pat_wild(span);
 305         let (else_expr, contains_else_clause) = match else_opt {
 306             None => (self.expr_block_empty(span), false),
 307             Some(els) => (self.lower_expr(els), true),
 308         };
 309         let else_arm = self.arm(else_pat, else_expr);
 310
 311         // Handle then + scrutinee:
 312         let then_expr = self.lower_block_expr(then);
 313         let (then_pat, scrutinee, desugar) = match cond.kind {
 314             // `<pat> => <then>`:
 315             ExprKind::Let(ref pat, ref scrutinee) => {
 316                 let scrutinee = self.lower_expr(scrutinee);
 317                 let pat = self.lower_pat(pat);
 318                 (pat, scrutinee, hir::MatchSource::IfLetDesugar { contains_else_clause })
 319             }
 320             // `true => <then>`:
 321             _ => {
 322                 // Lower condition:
 323                 let cond = self.lower_expr(cond);
 324                 let span_block =
 325                     self.mark_span_with_reason(DesugaringKind::CondTemporary, cond.span, None);
 326                 // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 327                 // to preserve drop semantics since `if cond { ... }` does not
 328                 // let temporaries live outside of `cond`.
 329                 let cond = self.expr_drop_temps(span_block, cond, ThinVec::new());
 330                 let pat = self.pat_bool(span, true);
 331                 (pat, cond, hir::MatchSource::IfDesugar { contains_else_clause })
 332             }
 333         };
 334         let then_arm = self.arm(then_pat, self.arena.alloc(then_expr));
 335
 336         hir::ExprKind::Match(scrutinee, arena_vec![self; then_arm, else_arm], desugar)
 337     }
 338
 339     fn lower_expr_while_in_loop_scope(
 340         &mut self,
 341         span: Span,
 342         cond: &Expr,
 343         body: &Block,
 344         opt_label: Option<Label>,
 345     ) -> hir::ExprKind<'hir> {
 346         // FIXME(#53667): handle lowering of && and parens.
 347
 348         // Note that the block AND the condition are evaluated in the loop scope.
 349         // This is done to allow `break` from inside the condition of the loop.
 350
 351         // `_ => break`:
 352         let else_arm = {
 353             let else_pat = self.pat_wild(span);
 354             let else_expr = self.expr_break(span, ThinVec::new());
 355             self.arm(else_pat, else_expr)
 356         };
 357
 358         // Handle then + scrutinee:
 359         let then_expr = self.lower_block_expr(body);
 360         let (then_pat, scrutinee, desugar, source) = match cond.kind {
 361             ExprKind::Let(ref pat, ref scrutinee) => {
 362                 // to:
 363                 //
 364                 //   [opt_ident]: loop {
 365                 //     match <sub_expr> {
 366                 //       <pat> => <body>,
 367                 //       _ => break
 368                 //     }
 369                 //   }
 370                 let scrutinee = self.with_loop_condition_scope(|t| t.lower_expr(scrutinee));
 371                 let pat = self.lower_pat(pat);
 372                 (pat, scrutinee, hir::MatchSource::WhileLetDesugar, hir::LoopSource::WhileLet)
 373             }
 374             _ => {
 375                 // We desugar: `'label: while $cond $body` into:
 376                 //
 377                 // ```
 378                 // 'label: loop {
 379                 //     match drop-temps { $cond } {
 380                 //         true => $body,
 381                 //         _ => break,
 382                 //     }
 383                 // }
 384                 // ```
 385
 386                 // Lower condition:
 387                 let cond = self.with_loop_condition_scope(|this| this.lower_expr(cond));
 388                 let span_block =
 389                     self.mark_span_with_reason(DesugaringKind::CondTemporary, cond.span, None);
 390                 // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 391                 // to preserve drop semantics since `while cond { ... }` does not
 392                 // let temporaries live outside of `cond`.
 393                 let cond = self.expr_drop_temps(span_block, cond, ThinVec::new());
 394                 // `true => <then>`:
 395                 let pat = self.pat_bool(span, true);
 396                 (pat, cond, hir::MatchSource::WhileDesugar, hir::LoopSource::While)
 397             }
 398         };
 399         let then_arm = self.arm(then_pat, self.arena.alloc(then_expr));
 400
 401         // `match <scrutinee> { ... }`
 402         let match_expr = self.expr_match(
 403             scrutinee.span,
 404             scrutinee,
 405             arena_vec![self; then_arm, else_arm],
 406             desugar,
 407         );
 408
 409         // `[opt_ident]: loop { ... }`
 410         hir::ExprKind::Loop(self.block_expr(self.arena.alloc(match_expr)), opt_label, source)
 411     }
 412
 413     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_ok(<expr>) }`,
 414     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_ok(()) }`
 415     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 416     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 417         self.with_catch_scope(body.id, |this| {
 418             let mut block = this.lower_block_noalloc(body, true);
 419
 420             let try_span = this.mark_span_with_reason(
 421                 DesugaringKind::TryBlock,
 422                 body.span,
 423                 this.allow_try_trait.clone(),
 424             );
 425
 426             // Final expression of the block (if present) or `()` with span at the end of block
 427             let tail_expr = block
 428                 .expr
 429                 .take()
 430                 .unwrap_or_else(|| this.expr_unit(this.sess.source_map().end_point(try_span)));
 431
 432             let ok_wrapped_span =
 433                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 434
 435             // `::std::ops::Try::from_ok($tail_expr)`
 436             block.expr = Some(this.wrap_in_try_constructor(
 437                 sym::from_ok,
 438                 try_span,
 439                 tail_expr,
 440                 ok_wrapped_span,
 441             ));
 442
 443             hir::ExprKind::Block(this.arena.alloc(block), None)
 444         })
 445     }
 446
 447     fn wrap_in_try_constructor(
 448         &mut self,
 449         method: Symbol,
 450         method_span: Span,
 451         expr: &'hir hir::Expr<'hir>,
 452         overall_span: Span,
 453     ) -> &'hir hir::Expr<'hir> {
 454         let path = &[sym::ops, sym::Try, method];
 455         let constructor =
 456             self.arena.alloc(self.expr_std_path(method_span, path, None, ThinVec::new()));
 457         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 458     }
 459
 460     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 461         hir::Arm {
 462             hir_id: self.next_id(),
 463             attrs: self.lower_attrs(&arm.attrs),
 464             pat: self.lower_pat(&arm.pat),
 465             guard: match arm.guard {
 466                 Some(ref x) => Some(hir::Guard::If(self.lower_expr(x))),
 467                 _ => None,
 468             },
 469             body: self.lower_expr(&arm.body),
 470             span: arm.span,
 471         }
 472     }
 473
 474     pub(super) fn make_async_expr(
 475         &mut self,
 476         capture_clause: CaptureBy,
 477         closure_node_id: NodeId,
 478         ret_ty: Option<AstP<Ty>>,
 479         span: Span,
 480         async_gen_kind: hir::AsyncGeneratorKind,
 481         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 482     ) -> hir::ExprKind<'hir> {
 483         let output = match ret_ty {
 484             Some(ty) => FunctionRetTy::Ty(ty),
 485             None => FunctionRetTy::Default(span),
 486         };
 487         let ast_decl = FnDecl { inputs: vec![], output };
 488         let decl = self.lower_fn_decl(&ast_decl, None, /* impl trait allowed */ false, None);
 489         let body_id = self.lower_fn_body(&ast_decl, |this| {
 490             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 491             body(this)
 492         });
 493
 494         // `static || -> <ret_ty> { body }`:
 495         let generator_kind = hir::ExprKind::Closure(
 496             capture_clause,
 497             decl,
 498             body_id,
 499             span,
 500             Some(hir::Movability::Static),
 501         );
 502         let generator = hir::Expr {
 503             hir_id: self.lower_node_id(closure_node_id),
 504             kind: generator_kind,
 505             span,
 506             attrs: ThinVec::new(),
 507         };
 508
 509         // `future::from_generator`:
 510         let unstable_span =
 511             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 512         let gen_future = self.expr_std_path(
 513             unstable_span,
 514             &[sym::future, sym::from_generator],
 515             None,
 516             ThinVec::new(),
 517         );
 518
 519         // `future::from_generator(generator)`:
 520         hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
 521     }
 522
 523     /// Desugar `<expr>.await` into:
 524     /// ```rust
 525     /// match <expr> {
 526     ///     mut pinned => loop {
 527     ///         match ::std::future::poll_with_tls_context(unsafe {
 528     ///             <::std::pin::Pin>::new_unchecked(&mut pinned)
 529     ///         }) {
 530     ///             ::std::task::Poll::Ready(result) => break result,
 531     ///             ::std::task::Poll::Pending => {}
 532     ///         }
 533     ///         yield ();
 534     ///     }
 535     /// }
 536     /// ```
 537     fn lower_expr_await(&mut self, await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 538         match self.generator_kind {
 539             Some(hir::GeneratorKind::Async(_)) => {}
 540             Some(hir::GeneratorKind::Gen) | None => {
 541                 let mut err = struct_span_err!(
 542                     self.sess,
 543                     await_span,
 544                     E0728,
 545                     "`await` is only allowed inside `async` functions and blocks"
 546                 );
 547                 err.span_label(await_span, "only allowed inside `async` functions and blocks");
 548                 if let Some(item_sp) = self.current_item {
 549                     err.span_label(item_sp, "this is not `async`");
 550                 }
 551                 err.emit();
 552             }
 553         }
 554         let span = self.mark_span_with_reason(DesugaringKind::Await, await_span, None);
 555         let gen_future_span = self.mark_span_with_reason(
 556             DesugaringKind::Await,
 557             await_span,
 558             self.allow_gen_future.clone(),
 559         );
 560
 561         let pinned_ident = Ident::with_dummy_span(sym::pinned);
 562         let (pinned_pat, pinned_pat_hid) =
 563             self.pat_ident_binding_mode(span, pinned_ident, hir::BindingAnnotation::Mutable);
 564
 565         // ::std::future::poll_with_tls_context(unsafe {
 566         //     ::std::pin::Pin::new_unchecked(&mut pinned)
 567         // })`
 568         let poll_expr = {
 569             let pinned = self.expr_ident(span, pinned_ident, pinned_pat_hid);
 570             let ref_mut_pinned = self.expr_mut_addr_of(span, pinned);
 571             let pin_ty_id = self.next_id();
 572             let new_unchecked_expr_kind = self.expr_call_std_assoc_fn(
 573                 pin_ty_id,
 574                 span,
 575                 &[sym::pin, sym::Pin],
 576                 "new_unchecked",
 577                 arena_vec![self; ref_mut_pinned],
 578             );
 579             let new_unchecked =
 580                 self.arena.alloc(self.expr(span, new_unchecked_expr_kind, ThinVec::new()));
 581             let unsafe_expr = self.expr_unsafe(new_unchecked);
 582             self.expr_call_std_path(
 583                 gen_future_span,
 584                 &[sym::future, sym::poll_with_tls_context],
 585                 arena_vec![self; unsafe_expr],
 586             )
 587         };
 588
 589         // `::std::task::Poll::Ready(result) => break result`
 590         let loop_node_id = self.resolver.next_node_id();
 591         let loop_hir_id = self.lower_node_id(loop_node_id);
 592         let ready_arm = {
 593             let x_ident = Ident::with_dummy_span(sym::result);
 594             let (x_pat, x_pat_hid) = self.pat_ident(span, x_ident);
 595             let x_expr = self.expr_ident(span, x_ident, x_pat_hid);
 596             let ready_pat = self.pat_std_enum(
 597                 span,
 598                 &[sym::task, sym::Poll, sym::Ready],
 599                 arena_vec![self; x_pat],
 600             );
 601             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 602                 let expr_break =
 603                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 604                 this.arena.alloc(this.expr(await_span, expr_break, ThinVec::new()))
 605             });
 606             self.arm(ready_pat, break_x)
 607         };
 608
 609         // `::std::task::Poll::Pending => {}`
 610         let pending_arm = {
 611             let pending_pat = self.pat_std_enum(span, &[sym::task, sym::Poll, sym::Pending], &[]);
 612             let empty_block = self.expr_block_empty(span);
 613             self.arm(pending_pat, empty_block)
 614         };
 615
 616         let inner_match_stmt = {
 617             let match_expr = self.expr_match(
 618                 span,
 619                 poll_expr,
 620                 arena_vec![self; ready_arm, pending_arm],
 621                 hir::MatchSource::AwaitDesugar,
 622             );
 623             self.stmt_expr(span, match_expr)
 624         };
 625
 626         let yield_stmt = {
 627             let unit = self.expr_unit(span);
 628             let yield_expr = self.expr(
 629                 span,
 630                 hir::ExprKind::Yield(unit, hir::YieldSource::Await),
 631                 ThinVec::new(),
 632             );
 633             self.stmt_expr(span, yield_expr)
 634         };
 635
 636         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 637
 638         // loop { .. }
 639         let loop_expr = self.arena.alloc(hir::Expr {
 640             hir_id: loop_hir_id,
 641             kind: hir::ExprKind::Loop(loop_block, None, hir::LoopSource::Loop),
 642             span,
 643             attrs: ThinVec::new(),
 644         });
 645
 646         // mut pinned => loop { ... }
 647         let pinned_arm = self.arm(pinned_pat, loop_expr);
 648
 649         // match <expr> {
 650         //     mut pinned => loop { .. }
 651         // }
 652         let expr = self.lower_expr(expr);
 653         hir::ExprKind::Match(expr, arena_vec![self; pinned_arm], hir::MatchSource::AwaitDesugar)
 654     }
 655
 656     fn lower_expr_closure(
 657         &mut self,
 658         capture_clause: CaptureBy,
 659         movability: Movability,
 660         decl: &FnDecl,
 661         body: &Expr,
 662         fn_decl_span: Span,
 663     ) -> hir::ExprKind<'hir> {
 664         // Lower outside new scope to preserve `is_in_loop_condition`.
 665         let fn_decl = self.lower_fn_decl(decl, None, false, None);
 666
 667         self.with_new_scopes(move |this| {
 668             let prev = this.current_item;
 669             this.current_item = Some(fn_decl_span);
 670             let mut generator_kind = None;
 671             let body_id = this.lower_fn_body(decl, |this| {
 672                 let e = this.lower_expr_mut(body);
 673                 generator_kind = this.generator_kind;
 674                 e
 675             });
 676             let generator_option =
 677                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 678             this.current_item = prev;
 679             hir::ExprKind::Closure(capture_clause, fn_decl, body_id, fn_decl_span, generator_option)
 680         })
 681     }
 682
 683     fn generator_movability_for_fn(
 684         &mut self,
 685         decl: &FnDecl,
 686         fn_decl_span: Span,
 687         generator_kind: Option<hir::GeneratorKind>,
 688         movability: Movability,
 689     ) -> Option<hir::Movability> {
 690         match generator_kind {
 691             Some(hir::GeneratorKind::Gen) => {
 692                 if !decl.inputs.is_empty() {
 693                     struct_span_err!(
 694                         self.sess,
 695                         fn_decl_span,
 696                         E0628,
 697                         "generators cannot have explicit parameters"
 698                     )
 699                     .emit();
 700                 }
 701                 Some(movability)
 702             }
 703             Some(hir::GeneratorKind::Async(_)) => {
 704                 bug!("non-`async` closure body turned `async` during lowering");
 705             }
 706             None => {
 707                 if movability == Movability::Static {
 708                     struct_span_err!(self.sess, fn_decl_span, E0697, "closures cannot be static")
 709                         .emit();
 710                 }
 711                 None
 712             }
 713         }
 714     }
 715
 716     fn lower_expr_async_closure(
 717         &mut self,
 718         capture_clause: CaptureBy,
 719         closure_id: NodeId,
 720         decl: &FnDecl,
 721         body: &Expr,
 722         fn_decl_span: Span,
 723     ) -> hir::ExprKind<'hir> {
 724         let outer_decl =
 725             FnDecl { inputs: decl.inputs.clone(), output: FunctionRetTy::Default(fn_decl_span) };
 726         // We need to lower the declaration outside the new scope, because we
 727         // have to conserve the state of being inside a loop condition for the
 728         // closure argument types.
 729         let fn_decl = self.lower_fn_decl(&outer_decl, None, false, None);
 730
 731         self.with_new_scopes(move |this| {
 732             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 733             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 734                 struct_span_err!(
 735                     this.sess,
 736                     fn_decl_span,
 737                     E0708,
 738                     "`async` non-`move` closures with parameters are not currently supported",
 739                 )
 740                 .help(
 741                     "consider using `let` statements to manually capture \
 742                     variables by reference before entering an `async move` closure",
 743                 )
 744                 .emit();
 745             }
 746
 747             // Transform `async |x: u8| -> X { ... }` into
 748             // `|x: u8| future_from_generator(|| -> X { ... })`.
 749             let body_id = this.lower_fn_body(&outer_decl, |this| {
 750                 let async_ret_ty =
 751                     if let FunctionRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 752                 let async_body = this.make_async_expr(
 753                     capture_clause,
 754                     closure_id,
 755                     async_ret_ty,
 756                     body.span,
 757                     hir::AsyncGeneratorKind::Closure,
 758                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 759                 );
 760                 this.expr(fn_decl_span, async_body, ThinVec::new())
 761             });
 762             hir::ExprKind::Closure(capture_clause, fn_decl, body_id, fn_decl_span, None)
 763         })
 764     }
 765
 766     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
 767     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
 768         let id = self.next_id();
 769         let e1 = self.lower_expr_mut(e1);
 770         let e2 = self.lower_expr_mut(e2);
 771         self.expr_call_std_assoc_fn(
 772             id,
 773             span,
 774             &[sym::ops, sym::RangeInclusive],
 775             "new",
 776             arena_vec![self; e1, e2],
 777         )
 778     }
 779
 780     fn lower_expr_range(
 781         &mut self,
 782         span: Span,
 783         e1: Option<&Expr>,
 784         e2: Option<&Expr>,
 785         lims: RangeLimits,
 786     ) -> hir::ExprKind<'hir> {
 787         use syntax::ast::RangeLimits::*;
 788
 789         let path = match (e1, e2, lims) {
 790             (None, None, HalfOpen) => sym::RangeFull,
 791             (Some(..), None, HalfOpen) => sym::RangeFrom,
 792             (None, Some(..), HalfOpen) => sym::RangeTo,
 793             (Some(..), Some(..), HalfOpen) => sym::Range,
 794             (None, Some(..), Closed) => sym::RangeToInclusive,
 795             (Some(..), Some(..), Closed) => unreachable!(),
 796             (_, None, Closed) => {
 797                 self.diagnostic().span_fatal(span, "inclusive range with no end").raise()
 798             }
 799         };
 800
 801         let fields = self.arena.alloc_from_iter(
 802             e1.iter().map(|e| ("start", e)).chain(e2.iter().map(|e| ("end", e))).map(|(s, e)| {
 803                 let expr = self.lower_expr(&e);
 804                 let ident = Ident::new(Symbol::intern(s), e.span);
 805                 self.field(ident, expr, e.span)
 806             }),
 807         );
 808
 809         let is_unit = fields.is_empty();
 810         let struct_path = [sym::ops, path];
 811         let struct_path = self.std_path(span, &struct_path, None, is_unit);
 812         let struct_path = hir::QPath::Resolved(None, struct_path);
 813
 814         if is_unit {
 815             hir::ExprKind::Path(struct_path)
 816         } else {
 817             hir::ExprKind::Struct(self.arena.alloc(struct_path), fields, None)
 818         }
 819     }
 820
 821     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
 822         let target_id = match destination {
 823             Some((id, _)) => {
 824                 if let Some(loop_id) = self.resolver.get_label_res(id) {
 825                     Ok(self.lower_node_id(loop_id))
 826                 } else {
 827                     Err(hir::LoopIdError::UnresolvedLabel)
 828                 }
 829             }
 830             None => self
 831                 .loop_scopes
 832                 .last()
 833                 .cloned()
 834                 .map(|id| Ok(self.lower_node_id(id)))
 835                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope))
 836                 .into(),
 837         };
 838         hir::Destination { label: destination.map(|(_, label)| label), target_id }
 839     }
 840
 841     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
 842         if self.is_in_loop_condition && opt_label.is_none() {
 843             hir::Destination {
 844                 label: None,
 845                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 846             }
 847         } else {
 848             self.lower_loop_destination(opt_label.map(|label| (id, label)))
 849         }
 850     }
 851
 852     fn with_catch_scope<T, F>(&mut self, catch_id: NodeId, f: F) -> T
 853     where
 854         F: FnOnce(&mut Self) -> T,
 855     {
 856         let len = self.catch_scopes.len();
 857         self.catch_scopes.push(catch_id);
 858
 859         let result = f(self);
 860         assert_eq!(
 861             len + 1,
 862             self.catch_scopes.len(),
 863             "catch scopes should be added and removed in stack order"
 864         );
 865
 866         self.catch_scopes.pop().unwrap();
 867
 868         result
 869     }
 870
 871     fn with_loop_scope<T, F>(&mut self, loop_id: NodeId, f: F) -> T
 872     where
 873         F: FnOnce(&mut Self) -> T,
 874     {
 875         // We're no longer in the base loop's condition; we're in another loop.
 876         let was_in_loop_condition = self.is_in_loop_condition;
 877         self.is_in_loop_condition = false;
 878
 879         let len = self.loop_scopes.len();
 880         self.loop_scopes.push(loop_id);
 881
 882         let result = f(self);
 883         assert_eq!(
 884             len + 1,
 885             self.loop_scopes.len(),
 886             "loop scopes should be added and removed in stack order"
 887         );
 888
 889         self.loop_scopes.pop().unwrap();
 890
 891         self.is_in_loop_condition = was_in_loop_condition;
 892
 893         result
 894     }
 895
 896     fn with_loop_condition_scope<T, F>(&mut self, f: F) -> T
 897     where
 898         F: FnOnce(&mut Self) -> T,
 899     {
 900         let was_in_loop_condition = self.is_in_loop_condition;
 901         self.is_in_loop_condition = true;
 902
 903         let result = f(self);
 904
 905         self.is_in_loop_condition = was_in_loop_condition;
 906
 907         result
 908     }
 909
 910     fn lower_expr_asm(&mut self, asm: &InlineAsm) -> hir::ExprKind<'hir> {
 911         let inner = hir::InlineAsmInner {
 912             inputs: asm.inputs.iter().map(|&(c, _)| c).collect(),
 913             outputs: asm
 914                 .outputs
 915                 .iter()
 916                 .map(|out| hir::InlineAsmOutput {
 917                     constraint: out.constraint,
 918                     is_rw: out.is_rw,
 919                     is_indirect: out.is_indirect,
 920                     span: out.expr.span,
 921                 })
 922                 .collect(),
 923             asm: asm.asm,
 924             asm_str_style: asm.asm_str_style,
 925             clobbers: asm.clobbers.clone().into(),
 926             volatile: asm.volatile,
 927             alignstack: asm.alignstack,
 928             dialect: asm.dialect,
 929         };
 930         let hir_asm = hir::InlineAsm {
 931             inner,
 932             inputs_exprs: self.arena.alloc_from_iter(
 933                 asm.inputs.iter().map(|&(_, ref input)| self.lower_expr_mut(input)),
 934             ),
 935             outputs_exprs: self
 936                 .arena
 937                 .alloc_from_iter(asm.outputs.iter().map(|out| self.lower_expr_mut(&out.expr))),
 938         };
 939         hir::ExprKind::InlineAsm(self.arena.alloc(hir_asm))
 940     }
 941
 942     fn lower_field(&mut self, f: &Field) -> hir::Field<'hir> {
 943         hir::Field {
 944             hir_id: self.next_id(),
 945             ident: f.ident,
 946             expr: self.lower_expr(&f.expr),
 947             span: f.span,
 948             is_shorthand: f.is_shorthand,
 949         }
 950     }
 951
 952     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
 953         match self.generator_kind {
 954             Some(hir::GeneratorKind::Gen) => {}
 955             Some(hir::GeneratorKind::Async(_)) => {
 956                 struct_span_err!(
 957                     self.sess,
 958                     span,
 959                     E0727,
 960                     "`async` generators are not yet supported"
 961                 )
 962                 .emit();
 963                 return hir::ExprKind::Err;
 964             }
 965             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
 966         }
 967
 968         let expr =
 969             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
 970
 971         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
 972     }
 973
 974     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
 975     /// ```rust
 976     /// {
 977     ///     let result = match ::std::iter::IntoIterator::into_iter(<head>) {
 978     ///         mut iter => {
 979     ///             [opt_ident]: loop {
 980     ///                 let mut __next;
 981     ///                 match ::std::iter::Iterator::next(&mut iter) {
 982     ///                     ::std::option::Option::Some(val) => __next = val,
 983     ///                     ::std::option::Option::None => break
 984     ///                 };
 985     ///                 let <pat> = __next;
 986     ///                 StmtKind::Expr(<body>);
 987     ///             }
 988     ///         }
 989     ///     };
 990     ///     result
 991     /// }
 992     /// ```
 993     fn lower_expr_for(
 994         &mut self,
 995         e: &Expr,
 996         pat: &Pat,
 997         head: &Expr,
 998         body: &Block,
 999         opt_label: Option<Label>,
1000     ) -> hir::Expr<'hir> {
1001         // expand <head>
1002         let mut head = self.lower_expr_mut(head);
1003         let desugared_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1004         head.span = desugared_span;
1005
1006         let iter = Ident::with_dummy_span(sym::iter);
1007
1008         let next_ident = Ident::with_dummy_span(sym::__next);
1009         let (next_pat, next_pat_hid) = self.pat_ident_binding_mode(
1010             desugared_span,
1011             next_ident,
1012             hir::BindingAnnotation::Mutable,
1013         );
1014
1015         // `::std::option::Option::Some(val) => __next = val`
1016         let pat_arm = {
1017             let val_ident = Ident::with_dummy_span(sym::val);
1018             let (val_pat, val_pat_hid) = self.pat_ident(pat.span, val_ident);
1019             let val_expr = self.expr_ident(pat.span, val_ident, val_pat_hid);
1020             let next_expr = self.expr_ident(pat.span, next_ident, next_pat_hid);
1021             let assign = self.arena.alloc(self.expr(
1022                 pat.span,
1023                 hir::ExprKind::Assign(next_expr, val_expr, pat.span),
1024                 ThinVec::new(),
1025             ));
1026             let some_pat = self.pat_some(pat.span, val_pat);
1027             self.arm(some_pat, assign)
1028         };
1029
1030         // `::std::option::Option::None => break`
1031         let break_arm = {
1032             let break_expr =
1033                 self.with_loop_scope(e.id, |this| this.expr_break(e.span, ThinVec::new()));
1034             let pat = self.pat_none(e.span);
1035             self.arm(pat, break_expr)
1036         };
1037
1038         // `mut iter`
1039         let (iter_pat, iter_pat_nid) =
1040             self.pat_ident_binding_mode(desugared_span, iter, hir::BindingAnnotation::Mutable);
1041
1042         // `match ::std::iter::Iterator::next(&mut iter) { ... }`
1043         let match_expr = {
1044             let iter = self.expr_ident(desugared_span, iter, iter_pat_nid);
1045             let ref_mut_iter = self.expr_mut_addr_of(desugared_span, iter);
1046             let next_path = &[sym::iter, sym::Iterator, sym::next];
1047             let next_expr =
1048                 self.expr_call_std_path(desugared_span, next_path, arena_vec![self; ref_mut_iter]);
1049             let arms = arena_vec![self; pat_arm, break_arm];
1050
1051             self.expr_match(desugared_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1052         };
1053         let match_stmt = self.stmt_expr(desugared_span, match_expr);
1054
1055         let next_expr = self.expr_ident(desugared_span, next_ident, next_pat_hid);
1056
1057         // `let mut __next`
1058         let next_let = self.stmt_let_pat(
1059             ThinVec::new(),
1060             desugared_span,
1061             None,
1062             next_pat,
1063             hir::LocalSource::ForLoopDesugar,
1064         );
1065
1066         // `let <pat> = __next`
1067         let pat = self.lower_pat(pat);
1068         let pat_let = self.stmt_let_pat(
1069             ThinVec::new(),
1070             desugared_span,
1071             Some(next_expr),
1072             pat,
1073             hir::LocalSource::ForLoopDesugar,
1074         );
1075
1076         let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1077         let body_expr = self.expr_block(body_block, ThinVec::new());
1078         let body_stmt = self.stmt_expr(body.span, body_expr);
1079
1080         let loop_block = self.block_all(
1081             e.span,
1082             arena_vec![self; next_let, match_stmt, pat_let, body_stmt],
1083             None,
1084         );
1085
1086         // `[opt_ident]: loop { ... }`
1087         let kind = hir::ExprKind::Loop(loop_block, opt_label, hir::LoopSource::ForLoop);
1088         let loop_expr = self.arena.alloc(hir::Expr {
1089             hir_id: self.lower_node_id(e.id),
1090             kind,
1091             span: e.span,
1092             attrs: ThinVec::new(),
1093         });
1094
1095         // `mut iter => { ... }`
1096         let iter_arm = self.arm(iter_pat, loop_expr);
1097
1098         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1099         let into_iter_expr = {
1100             let into_iter_path = &[sym::iter, sym::IntoIterator, sym::into_iter];
1101             self.expr_call_std_path(desugared_span, into_iter_path, arena_vec![self; head])
1102         };
1103
1104         let match_expr = self.arena.alloc(self.expr_match(
1105             desugared_span,
1106             into_iter_expr,
1107             arena_vec![self; iter_arm],
1108             hir::MatchSource::ForLoopDesugar,
1109         ));
1110
1111         // This is effectively `{ let _result = ...; _result }`.
1112         // The construct was introduced in #21984 and is necessary to make sure that
1113         // temporaries in the `head` expression are dropped and do not leak to the
1114         // surrounding scope of the `match` since the `match` is not a terminating scope.
1115         //
1116         // Also, add the attributes to the outer returned expr node.
1117         self.expr_drop_temps_mut(desugared_span, match_expr, e.attrs.clone())
1118     }
1119
1120     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1121     /// ```rust
1122     /// match Try::into_result(<expr>) {
1123     ///     Ok(val) => #[allow(unreachable_code)] val,
1124     ///     Err(err) => #[allow(unreachable_code)]
1125     ///                 // If there is an enclosing `try {...}`:
1126     ///                 break 'catch_target Try::from_error(From::from(err)),
1127     ///                 // Otherwise:
1128     ///                 return Try::from_error(From::from(err)),
1129     /// }
1130     /// ```
1131     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1132         let unstable_span = self.mark_span_with_reason(
1133             DesugaringKind::QuestionMark,
1134             span,
1135             self.allow_try_trait.clone(),
1136         );
1137         let try_span = self.sess.source_map().end_point(span);
1138         let try_span = self.mark_span_with_reason(
1139             DesugaringKind::QuestionMark,
1140             try_span,
1141             self.allow_try_trait.clone(),
1142         );
1143
1144         // `Try::into_result(<expr>)`
1145         let scrutinee = {
1146             // expand <expr>
1147             let sub_expr = self.lower_expr_mut(sub_expr);
1148
1149             let path = &[sym::ops, sym::Try, sym::into_result];
1150             self.expr_call_std_path(unstable_span, path, arena_vec![self; sub_expr])
1151         };
1152
1153         // `#[allow(unreachable_code)]`
1154         let attr = {
1155             // `allow(unreachable_code)`
1156             let allow = {
1157                 let allow_ident = Ident::new(sym::allow, span);
1158                 let uc_ident = Ident::new(sym::unreachable_code, span);
1159                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1160                 attr::mk_list_item(allow_ident, vec![uc_nested])
1161             };
1162             attr::mk_attr_outer(allow)
1163         };
1164         let attrs = vec![attr];
1165
1166         // `Ok(val) => #[allow(unreachable_code)] val,`
1167         let ok_arm = {
1168             let val_ident = Ident::with_dummy_span(sym::val);
1169             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1170             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1171                 span,
1172                 val_ident,
1173                 val_pat_nid,
1174                 ThinVec::from(attrs.clone()),
1175             ));
1176             let ok_pat = self.pat_ok(span, val_pat);
1177             self.arm(ok_pat, val_expr)
1178         };
1179
1180         // `Err(err) => #[allow(unreachable_code)]
1181         //              return Try::from_error(From::from(err)),`
1182         let err_arm = {
1183             let err_ident = Ident::with_dummy_span(sym::err);
1184             let (err_local, err_local_nid) = self.pat_ident(try_span, err_ident);
1185             let from_expr = {
1186                 let from_path = &[sym::convert, sym::From, sym::from];
1187                 let err_expr = self.expr_ident_mut(try_span, err_ident, err_local_nid);
1188                 self.expr_call_std_path(try_span, from_path, arena_vec![self; err_expr])
1189             };
1190             let from_err_expr =
1191                 self.wrap_in_try_constructor(sym::from_error, unstable_span, from_expr, try_span);
1192             let thin_attrs = ThinVec::from(attrs);
1193             let catch_scope = self.catch_scopes.last().map(|x| *x);
1194             let ret_expr = if let Some(catch_node) = catch_scope {
1195                 let target_id = Ok(self.lower_node_id(catch_node));
1196                 self.arena.alloc(self.expr(
1197                     try_span,
1198                     hir::ExprKind::Break(
1199                         hir::Destination { label: None, target_id },
1200                         Some(from_err_expr),
1201                     ),
1202                     thin_attrs,
1203                 ))
1204             } else {
1205                 self.arena.alloc(self.expr(
1206                     try_span,
1207                     hir::ExprKind::Ret(Some(from_err_expr)),
1208                     thin_attrs,
1209                 ))
1210             };
1211
1212             let err_pat = self.pat_err(try_span, err_local);
1213             self.arm(err_pat, ret_expr)
1214         };
1215
1216         hir::ExprKind::Match(
1217             scrutinee,
1218             arena_vec![self; err_arm, ok_arm],
1219             hir::MatchSource::TryDesugar,
1220         )
1221     }
1222
1223     // =========================================================================
1224     // Helper methods for building HIR.
1225     // =========================================================================
1226
1227     /// Constructs a `true` or `false` literal expression.
1228     pub(super) fn expr_bool(&mut self, span: Span, val: bool) -> &'hir hir::Expr<'hir> {
1229         let lit = Spanned { span, node: LitKind::Bool(val) };
1230         self.arena.alloc(self.expr(span, hir::ExprKind::Lit(lit), ThinVec::new()))
1231     }
1232
1233     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1234     ///
1235     /// In terms of drop order, it has the same effect as wrapping `expr` in
1236     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1237     ///
1238     /// The drop order can be important in e.g. `if expr { .. }`.
1239     pub(super) fn expr_drop_temps(
1240         &mut self,
1241         span: Span,
1242         expr: &'hir hir::Expr<'hir>,
1243         attrs: AttrVec,
1244     ) -> &'hir hir::Expr<'hir> {
1245         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1246     }
1247
1248     pub(super) fn expr_drop_temps_mut(
1249         &mut self,
1250         span: Span,
1251         expr: &'hir hir::Expr<'hir>,
1252         attrs: AttrVec,
1253     ) -> hir::Expr<'hir> {
1254         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1255     }
1256
1257     fn expr_match(
1258         &mut self,
1259         span: Span,
1260         arg: &'hir hir::Expr<'hir>,
1261         arms: &'hir [hir::Arm<'hir>],
1262         source: hir::MatchSource,
1263     ) -> hir::Expr<'hir> {
1264         self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
1265     }
1266
1267     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1268         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1269         self.arena.alloc(self.expr(span, expr_break, attrs))
1270     }
1271
1272     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1273         self.expr(
1274             span,
1275             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1276             ThinVec::new(),
1277         )
1278     }
1279
1280     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1281         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
1282     }
1283
1284     fn expr_call(
1285         &mut self,
1286         span: Span,
1287         e: &'hir hir::Expr<'hir>,
1288         args: &'hir [hir::Expr<'hir>],
1289     ) -> &'hir hir::Expr<'hir> {
1290         self.arena.alloc(self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new()))
1291     }
1292
1293     // Note: associated functions must use `expr_call_std_path`.
1294     fn expr_call_std_path(
1295         &mut self,
1296         span: Span,
1297         path_components: &[Symbol],
1298         args: &'hir [hir::Expr<'hir>],
1299     ) -> &'hir hir::Expr<'hir> {
1300         let path =
1301             self.arena.alloc(self.expr_std_path(span, path_components, None, ThinVec::new()));
1302         self.expr_call(span, path, args)
1303     }
1304
1305     // Create an expression calling an associated function of an std type.
1306     //
1307     // Associated functions cannot be resolved through the normal `std_path` function,
1308     // as they are resolved differently and so cannot use `expr_call_std_path`.
1309     //
1310     // This function accepts the path component (`ty_path_components`) separately from
1311     // the name of the associated function (`assoc_fn_name`) in order to facilitate
1312     // separate resolution of the type and creation of a path referring to its associated
1313     // function.
1314     fn expr_call_std_assoc_fn(
1315         &mut self,
1316         ty_path_id: hir::HirId,
1317         span: Span,
1318         ty_path_components: &[Symbol],
1319         assoc_fn_name: &str,
1320         args: &'hir [hir::Expr<'hir>],
1321     ) -> hir::ExprKind<'hir> {
1322         let ty_path = self.std_path(span, ty_path_components, None, false);
1323         let ty =
1324             self.arena.alloc(self.ty_path(ty_path_id, span, hir::QPath::Resolved(None, ty_path)));
1325         let fn_seg = self.arena.alloc(hir::PathSegment::from_ident(Ident::from_str(assoc_fn_name)));
1326         let fn_path = hir::QPath::TypeRelative(ty, fn_seg);
1327         let fn_expr =
1328             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
1329         hir::ExprKind::Call(fn_expr, args)
1330     }
1331
1332     fn expr_std_path(
1333         &mut self,
1334         span: Span,
1335         components: &[Symbol],
1336         params: Option<&'hir hir::GenericArgs<'hir>>,
1337         attrs: AttrVec,
1338     ) -> hir::Expr<'hir> {
1339         let path = self.std_path(span, components, params, true);
1340         self.expr(span, hir::ExprKind::Path(hir::QPath::Resolved(None, path)), attrs)
1341     }
1342
1343     pub(super) fn expr_ident(
1344         &mut self,
1345         sp: Span,
1346         ident: Ident,
1347         binding: hir::HirId,
1348     ) -> &'hir hir::Expr<'hir> {
1349         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1350     }
1351
1352     pub(super) fn expr_ident_mut(
1353         &mut self,
1354         sp: Span,
1355         ident: Ident,
1356         binding: hir::HirId,
1357     ) -> hir::Expr<'hir> {
1358         self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
1359     }
1360
1361     fn expr_ident_with_attrs(
1362         &mut self,
1363         span: Span,
1364         ident: Ident,
1365         binding: hir::HirId,
1366         attrs: AttrVec,
1367     ) -> hir::Expr<'hir> {
1368         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1369             None,
1370             self.arena.alloc(hir::Path {
1371                 span,
1372                 res: Res::Local(binding),
1373                 segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
1374             }),
1375         ));
1376
1377         self.expr(span, expr_path, attrs)
1378     }
1379
1380     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1381         let hir_id = self.next_id();
1382         let span = expr.span;
1383         self.expr(
1384             span,
1385             hir::ExprKind::Block(
1386                 self.arena.alloc(hir::Block {
1387                     stmts: &[],
1388                     expr: Some(expr),
1389                     hir_id,
1390                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1391                     span,
1392                     targeted_by_break: false,
1393                 }),
1394                 None,
1395             ),
1396             ThinVec::new(),
1397         )
1398     }
1399
1400     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1401         let blk = self.block_all(span, &[], None);
1402         let expr = self.expr_block(blk, ThinVec::new());
1403         self.arena.alloc(expr)
1404     }
1405
1406     pub(super) fn expr_block(
1407         &mut self,
1408         b: &'hir hir::Block<'hir>,
1409         attrs: AttrVec,
1410     ) -> hir::Expr<'hir> {
1411         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1412     }
1413
1414     pub(super) fn expr(
1415         &mut self,
1416         span: Span,
1417         kind: hir::ExprKind<'hir>,
1418         attrs: AttrVec,
1419     ) -> hir::Expr<'hir> {
1420         hir::Expr { hir_id: self.next_id(), kind, span, attrs }
1421     }
1422
1423     fn field(&mut self, ident: Ident, expr: &'hir hir::Expr<'hir>, span: Span) -> hir::Field<'hir> {
1424         hir::Field { hir_id: self.next_id(), ident, span, expr, is_shorthand: false }
1425     }
1426
1427     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1428         hir::Arm {
1429             hir_id: self.next_id(),
1430             attrs: &[],
1431             pat,
1432             guard: None,
1433             span: expr.span,
1434             body: expr,
1435         }
1436     }
1437 }