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