src/librustc_ast_lowering/expr.rs

   1 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
   2
   3 use rustc::bug;
   4 use rustc::hir;
   5 use rustc::hir::def::Res;
   6 use rustc_data_structures::thin_vec::ThinVec;
   7 use rustc_error_codes::*;
   8 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
   9 use rustc_span::symbol::{sym, Symbol};
  10 use syntax::ast::*;
  11 use syntax::attr;
  12 use syntax::ptr::P as AstP;
  13 use syntax::{span_err, struct_span_err};
  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::UnDeref,
 211             UnOp::Not => hir::UnNot,
 212             UnOp::Neg => hir::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                     span_err!(
 689                         self.sess,
 690                         fn_decl_span,
 691                         E0628,
 692                         "generators cannot have explicit parameters"
 693                     );
 694                 }
 695                 Some(movability)
 696             }
 697             Some(hir::GeneratorKind::Async(_)) => {
 698                 bug!("non-`async` closure body turned `async` during lowering");
 699             }
 700             None => {
 701                 if movability == Movability::Static {
 702                     span_err!(self.sess, fn_decl_span, E0697, "closures cannot be static");
 703                 }
 704                 None
 705             }
 706         }
 707     }
 708
 709     fn lower_expr_async_closure(
 710         &mut self,
 711         capture_clause: CaptureBy,
 712         closure_id: NodeId,
 713         decl: &FnDecl,
 714         body: &Expr,
 715         fn_decl_span: Span,
 716     ) -> hir::ExprKind<'hir> {
 717         let outer_decl =
 718             FnDecl { inputs: decl.inputs.clone(), output: FunctionRetTy::Default(fn_decl_span) };
 719         // We need to lower the declaration outside the new scope, because we
 720         // have to conserve the state of being inside a loop condition for the
 721         // closure argument types.
 722         let fn_decl = self.lower_fn_decl(&outer_decl, None, false, None);
 723
 724         self.with_new_scopes(move |this| {
 725             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 726             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 727                 struct_span_err!(
 728                     this.sess,
 729                     fn_decl_span,
 730                     E0708,
 731                     "`async` non-`move` closures with parameters are not currently supported",
 732                 )
 733                 .help(
 734                     "consider using `let` statements to manually capture \
 735                     variables by reference before entering an `async move` closure",
 736                 )
 737                 .emit();
 738             }
 739
 740             // Transform `async |x: u8| -> X { ... }` into
 741             // `|x: u8| future_from_generator(|| -> X { ... })`.
 742             let body_id = this.lower_fn_body(&outer_decl, |this| {
 743                 let async_ret_ty =
 744                     if let FunctionRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 745                 let async_body = this.make_async_expr(
 746                     capture_clause,
 747                     closure_id,
 748                     async_ret_ty,
 749                     body.span,
 750                     hir::AsyncGeneratorKind::Closure,
 751                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 752                 );
 753                 this.expr(fn_decl_span, async_body, ThinVec::new())
 754             });
 755             hir::ExprKind::Closure(capture_clause, fn_decl, body_id, fn_decl_span, None)
 756         })
 757     }
 758
 759     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
 760     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
 761         let id = self.next_id();
 762         let e1 = self.lower_expr_mut(e1);
 763         let e2 = self.lower_expr_mut(e2);
 764         self.expr_call_std_assoc_fn(
 765             id,
 766             span,
 767             &[sym::ops, sym::RangeInclusive],
 768             "new",
 769             arena_vec![self; e1, e2],
 770         )
 771     }
 772
 773     fn lower_expr_range(
 774         &mut self,
 775         span: Span,
 776         e1: Option<&Expr>,
 777         e2: Option<&Expr>,
 778         lims: RangeLimits,
 779     ) -> hir::ExprKind<'hir> {
 780         use syntax::ast::RangeLimits::*;
 781
 782         let path = match (e1, e2, lims) {
 783             (None, None, HalfOpen) => sym::RangeFull,
 784             (Some(..), None, HalfOpen) => sym::RangeFrom,
 785             (None, Some(..), HalfOpen) => sym::RangeTo,
 786             (Some(..), Some(..), HalfOpen) => sym::Range,
 787             (None, Some(..), Closed) => sym::RangeToInclusive,
 788             (Some(..), Some(..), Closed) => unreachable!(),
 789             (_, None, Closed) => {
 790                 self.diagnostic().span_fatal(span, "inclusive range with no end").raise()
 791             }
 792         };
 793
 794         let fields = self.arena.alloc_from_iter(
 795             e1.iter().map(|e| ("start", e)).chain(e2.iter().map(|e| ("end", e))).map(|(s, e)| {
 796                 let expr = self.lower_expr(&e);
 797                 let ident = Ident::new(Symbol::intern(s), e.span);
 798                 self.field(ident, expr, e.span)
 799             }),
 800         );
 801
 802         let is_unit = fields.is_empty();
 803         let struct_path = [sym::ops, path];
 804         let struct_path = self.std_path(span, &struct_path, None, is_unit);
 805         let struct_path = hir::QPath::Resolved(None, struct_path);
 806
 807         if is_unit {
 808             hir::ExprKind::Path(struct_path)
 809         } else {
 810             hir::ExprKind::Struct(self.arena.alloc(struct_path), fields, None)
 811         }
 812     }
 813
 814     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
 815         let target_id = match destination {
 816             Some((id, _)) => {
 817                 if let Some(loop_id) = self.resolver.get_label_res(id) {
 818                     Ok(self.lower_node_id(loop_id))
 819                 } else {
 820                     Err(hir::LoopIdError::UnresolvedLabel)
 821                 }
 822             }
 823             None => self
 824                 .loop_scopes
 825                 .last()
 826                 .cloned()
 827                 .map(|id| Ok(self.lower_node_id(id)))
 828                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope))
 829                 .into(),
 830         };
 831         hir::Destination { label: destination.map(|(_, label)| label), target_id }
 832     }
 833
 834     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
 835         if self.is_in_loop_condition && opt_label.is_none() {
 836             hir::Destination {
 837                 label: None,
 838                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition).into(),
 839             }
 840         } else {
 841             self.lower_loop_destination(opt_label.map(|label| (id, label)))
 842         }
 843     }
 844
 845     fn with_catch_scope<T, F>(&mut self, catch_id: NodeId, f: F) -> T
 846     where
 847         F: FnOnce(&mut Self) -> T,
 848     {
 849         let len = self.catch_scopes.len();
 850         self.catch_scopes.push(catch_id);
 851
 852         let result = f(self);
 853         assert_eq!(
 854             len + 1,
 855             self.catch_scopes.len(),
 856             "catch scopes should be added and removed in stack order"
 857         );
 858
 859         self.catch_scopes.pop().unwrap();
 860
 861         result
 862     }
 863
 864     fn with_loop_scope<T, F>(&mut self, loop_id: NodeId, f: F) -> T
 865     where
 866         F: FnOnce(&mut Self) -> T,
 867     {
 868         // We're no longer in the base loop's condition; we're in another loop.
 869         let was_in_loop_condition = self.is_in_loop_condition;
 870         self.is_in_loop_condition = false;
 871
 872         let len = self.loop_scopes.len();
 873         self.loop_scopes.push(loop_id);
 874
 875         let result = f(self);
 876         assert_eq!(
 877             len + 1,
 878             self.loop_scopes.len(),
 879             "loop scopes should be added and removed in stack order"
 880         );
 881
 882         self.loop_scopes.pop().unwrap();
 883
 884         self.is_in_loop_condition = was_in_loop_condition;
 885
 886         result
 887     }
 888
 889     fn with_loop_condition_scope<T, F>(&mut self, f: F) -> T
 890     where
 891         F: FnOnce(&mut Self) -> T,
 892     {
 893         let was_in_loop_condition = self.is_in_loop_condition;
 894         self.is_in_loop_condition = true;
 895
 896         let result = f(self);
 897
 898         self.is_in_loop_condition = was_in_loop_condition;
 899
 900         result
 901     }
 902
 903     fn lower_expr_asm(&mut self, asm: &InlineAsm) -> hir::ExprKind<'hir> {
 904         let inner = hir::InlineAsmInner {
 905             inputs: asm.inputs.iter().map(|&(ref c, _)| c.clone()).collect(),
 906             outputs: asm
 907                 .outputs
 908                 .iter()
 909                 .map(|out| hir::InlineAsmOutput {
 910                     constraint: out.constraint.clone(),
 911                     is_rw: out.is_rw,
 912                     is_indirect: out.is_indirect,
 913                     span: out.expr.span,
 914                 })
 915                 .collect(),
 916             asm: asm.asm.clone(),
 917             asm_str_style: asm.asm_str_style,
 918             clobbers: asm.clobbers.clone().into(),
 919             volatile: asm.volatile,
 920             alignstack: asm.alignstack,
 921             dialect: asm.dialect,
 922         };
 923         let hir_asm = hir::InlineAsm {
 924             inner,
 925             inputs_exprs: self.arena.alloc_from_iter(
 926                 asm.inputs.iter().map(|&(_, ref input)| self.lower_expr_mut(input)),
 927             ),
 928             outputs_exprs: self
 929                 .arena
 930                 .alloc_from_iter(asm.outputs.iter().map(|out| self.lower_expr_mut(&out.expr))),
 931         };
 932         hir::ExprKind::InlineAsm(self.arena.alloc(hir_asm))
 933     }
 934
 935     fn lower_field(&mut self, f: &Field) -> hir::Field<'hir> {
 936         hir::Field {
 937             hir_id: self.next_id(),
 938             ident: f.ident,
 939             expr: self.lower_expr(&f.expr),
 940             span: f.span,
 941             is_shorthand: f.is_shorthand,
 942         }
 943     }
 944
 945     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
 946         match self.generator_kind {
 947             Some(hir::GeneratorKind::Gen) => {}
 948             Some(hir::GeneratorKind::Async(_)) => {
 949                 span_err!(self.sess, span, E0727, "`async` generators are not yet supported",);
 950                 return hir::ExprKind::Err;
 951             }
 952             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
 953         }
 954
 955         let expr =
 956             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
 957
 958         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
 959     }
 960
 961     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
 962     /// ```rust
 963     /// {
 964     ///     let result = match ::std::iter::IntoIterator::into_iter(<head>) {
 965     ///         mut iter => {
 966     ///             [opt_ident]: loop {
 967     ///                 let mut __next;
 968     ///                 match ::std::iter::Iterator::next(&mut iter) {
 969     ///                     ::std::option::Option::Some(val) => __next = val,
 970     ///                     ::std::option::Option::None => break
 971     ///                 };
 972     ///                 let <pat> = __next;
 973     ///                 StmtKind::Expr(<body>);
 974     ///             }
 975     ///         }
 976     ///     };
 977     ///     result
 978     /// }
 979     /// ```
 980     fn lower_expr_for(
 981         &mut self,
 982         e: &Expr,
 983         pat: &Pat,
 984         head: &Expr,
 985         body: &Block,
 986         opt_label: Option<Label>,
 987     ) -> hir::Expr<'hir> {
 988         // expand <head>
 989         let mut head = self.lower_expr_mut(head);
 990         let desugared_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
 991         head.span = desugared_span;
 992
 993         let iter = Ident::with_dummy_span(sym::iter);
 994
 995         let next_ident = Ident::with_dummy_span(sym::__next);
 996         let (next_pat, next_pat_hid) = self.pat_ident_binding_mode(
 997             desugared_span,
 998             next_ident,
 999             hir::BindingAnnotation::Mutable,
1000         );
1001
1002         // `::std::option::Option::Some(val) => __next = val`
1003         let pat_arm = {
1004             let val_ident = Ident::with_dummy_span(sym::val);
1005             let (val_pat, val_pat_hid) = self.pat_ident(pat.span, val_ident);
1006             let val_expr = self.expr_ident(pat.span, val_ident, val_pat_hid);
1007             let next_expr = self.expr_ident(pat.span, next_ident, next_pat_hid);
1008             let assign = self.arena.alloc(self.expr(
1009                 pat.span,
1010                 hir::ExprKind::Assign(next_expr, val_expr, pat.span),
1011                 ThinVec::new(),
1012             ));
1013             let some_pat = self.pat_some(pat.span, val_pat);
1014             self.arm(some_pat, assign)
1015         };
1016
1017         // `::std::option::Option::None => break`
1018         let break_arm = {
1019             let break_expr =
1020                 self.with_loop_scope(e.id, |this| this.expr_break(e.span, ThinVec::new()));
1021             let pat = self.pat_none(e.span);
1022             self.arm(pat, break_expr)
1023         };
1024
1025         // `mut iter`
1026         let (iter_pat, iter_pat_nid) =
1027             self.pat_ident_binding_mode(desugared_span, iter, hir::BindingAnnotation::Mutable);
1028
1029         // `match ::std::iter::Iterator::next(&mut iter) { ... }`
1030         let match_expr = {
1031             let iter = self.expr_ident(desugared_span, iter, iter_pat_nid);
1032             let ref_mut_iter = self.expr_mut_addr_of(desugared_span, iter);
1033             let next_path = &[sym::iter, sym::Iterator, sym::next];
1034             let next_expr =
1035                 self.expr_call_std_path(desugared_span, next_path, arena_vec![self; ref_mut_iter]);
1036             let arms = arena_vec![self; pat_arm, break_arm];
1037
1038             self.expr_match(desugared_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1039         };
1040         let match_stmt = self.stmt_expr(desugared_span, match_expr);
1041
1042         let next_expr = self.expr_ident(desugared_span, next_ident, next_pat_hid);
1043
1044         // `let mut __next`
1045         let next_let = self.stmt_let_pat(
1046             ThinVec::new(),
1047             desugared_span,
1048             None,
1049             next_pat,
1050             hir::LocalSource::ForLoopDesugar,
1051         );
1052
1053         // `let <pat> = __next`
1054         let pat = self.lower_pat(pat);
1055         let pat_let = self.stmt_let_pat(
1056             ThinVec::new(),
1057             desugared_span,
1058             Some(next_expr),
1059             pat,
1060             hir::LocalSource::ForLoopDesugar,
1061         );
1062
1063         let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1064         let body_expr = self.expr_block(body_block, ThinVec::new());
1065         let body_stmt = self.stmt_expr(body.span, body_expr);
1066
1067         let loop_block = self.block_all(
1068             e.span,
1069             arena_vec![self; next_let, match_stmt, pat_let, body_stmt],
1070             None,
1071         );
1072
1073         // `[opt_ident]: loop { ... }`
1074         let kind = hir::ExprKind::Loop(loop_block, opt_label, hir::LoopSource::ForLoop);
1075         let loop_expr = self.arena.alloc(hir::Expr {
1076             hir_id: self.lower_node_id(e.id),
1077             kind,
1078             span: e.span,
1079             attrs: ThinVec::new(),
1080         });
1081
1082         // `mut iter => { ... }`
1083         let iter_arm = self.arm(iter_pat, loop_expr);
1084
1085         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1086         let into_iter_expr = {
1087             let into_iter_path = &[sym::iter, sym::IntoIterator, sym::into_iter];
1088             self.expr_call_std_path(desugared_span, into_iter_path, arena_vec![self; head])
1089         };
1090
1091         let match_expr = self.arena.alloc(self.expr_match(
1092             desugared_span,
1093             into_iter_expr,
1094             arena_vec![self; iter_arm],
1095             hir::MatchSource::ForLoopDesugar,
1096         ));
1097
1098         // This is effectively `{ let _result = ...; _result }`.
1099         // The construct was introduced in #21984 and is necessary to make sure that
1100         // temporaries in the `head` expression are dropped and do not leak to the
1101         // surrounding scope of the `match` since the `match` is not a terminating scope.
1102         //
1103         // Also, add the attributes to the outer returned expr node.
1104         self.expr_drop_temps_mut(desugared_span, match_expr, e.attrs.clone())
1105     }
1106
1107     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1108     /// ```rust
1109     /// match Try::into_result(<expr>) {
1110     ///     Ok(val) => #[allow(unreachable_code)] val,
1111     ///     Err(err) => #[allow(unreachable_code)]
1112     ///                 // If there is an enclosing `try {...}`:
1113     ///                 break 'catch_target Try::from_error(From::from(err)),
1114     ///                 // Otherwise:
1115     ///                 return Try::from_error(From::from(err)),
1116     /// }
1117     /// ```
1118     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1119         let unstable_span = self.mark_span_with_reason(
1120             DesugaringKind::QuestionMark,
1121             span,
1122             self.allow_try_trait.clone(),
1123         );
1124         let try_span = self.sess.source_map().end_point(span);
1125         let try_span = self.mark_span_with_reason(
1126             DesugaringKind::QuestionMark,
1127             try_span,
1128             self.allow_try_trait.clone(),
1129         );
1130
1131         // `Try::into_result(<expr>)`
1132         let scrutinee = {
1133             // expand <expr>
1134             let sub_expr = self.lower_expr_mut(sub_expr);
1135
1136             let path = &[sym::ops, sym::Try, sym::into_result];
1137             self.expr_call_std_path(unstable_span, path, arena_vec![self; sub_expr])
1138         };
1139
1140         // `#[allow(unreachable_code)]`
1141         let attr = {
1142             // `allow(unreachable_code)`
1143             let allow = {
1144                 let allow_ident = Ident::new(sym::allow, span);
1145                 let uc_ident = Ident::new(sym::unreachable_code, span);
1146                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1147                 attr::mk_list_item(allow_ident, vec![uc_nested])
1148             };
1149             attr::mk_attr_outer(allow)
1150         };
1151         let attrs = vec![attr];
1152
1153         // `Ok(val) => #[allow(unreachable_code)] val,`
1154         let ok_arm = {
1155             let val_ident = Ident::with_dummy_span(sym::val);
1156             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1157             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1158                 span,
1159                 val_ident,
1160                 val_pat_nid,
1161                 ThinVec::from(attrs.clone()),
1162             ));
1163             let ok_pat = self.pat_ok(span, val_pat);
1164             self.arm(ok_pat, val_expr)
1165         };
1166
1167         // `Err(err) => #[allow(unreachable_code)]
1168         //              return Try::from_error(From::from(err)),`
1169         let err_arm = {
1170             let err_ident = Ident::with_dummy_span(sym::err);
1171             let (err_local, err_local_nid) = self.pat_ident(try_span, err_ident);
1172             let from_expr = {
1173                 let from_path = &[sym::convert, sym::From, sym::from];
1174                 let err_expr = self.expr_ident_mut(try_span, err_ident, err_local_nid);
1175                 self.expr_call_std_path(try_span, from_path, arena_vec![self; err_expr])
1176             };
1177             let from_err_expr =
1178                 self.wrap_in_try_constructor(sym::from_error, unstable_span, from_expr, try_span);
1179             let thin_attrs = ThinVec::from(attrs);
1180             let catch_scope = self.catch_scopes.last().map(|x| *x);
1181             let ret_expr = if let Some(catch_node) = catch_scope {
1182                 let target_id = Ok(self.lower_node_id(catch_node));
1183                 self.arena.alloc(self.expr(
1184                     try_span,
1185                     hir::ExprKind::Break(
1186                         hir::Destination { label: None, target_id },
1187                         Some(from_err_expr),
1188                     ),
1189                     thin_attrs,
1190                 ))
1191             } else {
1192                 self.arena.alloc(self.expr(
1193                     try_span,
1194                     hir::ExprKind::Ret(Some(from_err_expr)),
1195                     thin_attrs,
1196                 ))
1197             };
1198
1199             let err_pat = self.pat_err(try_span, err_local);
1200             self.arm(err_pat, ret_expr)
1201         };
1202
1203         hir::ExprKind::Match(
1204             scrutinee,
1205             arena_vec![self; err_arm, ok_arm],
1206             hir::MatchSource::TryDesugar,
1207         )
1208     }
1209
1210     // =========================================================================
1211     // Helper methods for building HIR.
1212     // =========================================================================
1213
1214     /// Constructs a `true` or `false` literal expression.
1215     pub(super) fn expr_bool(&mut self, span: Span, val: bool) -> &'hir hir::Expr<'hir> {
1216         let lit = Spanned { span, node: LitKind::Bool(val) };
1217         self.arena.alloc(self.expr(span, hir::ExprKind::Lit(lit), ThinVec::new()))
1218     }
1219
1220     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1221     ///
1222     /// In terms of drop order, it has the same effect as wrapping `expr` in
1223     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1224     ///
1225     /// The drop order can be important in e.g. `if expr { .. }`.
1226     pub(super) fn expr_drop_temps(
1227         &mut self,
1228         span: Span,
1229         expr: &'hir hir::Expr<'hir>,
1230         attrs: AttrVec,
1231     ) -> &'hir hir::Expr<'hir> {
1232         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1233     }
1234
1235     pub(super) fn expr_drop_temps_mut(
1236         &mut self,
1237         span: Span,
1238         expr: &'hir hir::Expr<'hir>,
1239         attrs: AttrVec,
1240     ) -> hir::Expr<'hir> {
1241         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1242     }
1243
1244     fn expr_match(
1245         &mut self,
1246         span: Span,
1247         arg: &'hir hir::Expr<'hir>,
1248         arms: &'hir [hir::Arm<'hir>],
1249         source: hir::MatchSource,
1250     ) -> hir::Expr<'hir> {
1251         self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
1252     }
1253
1254     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1255         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1256         self.arena.alloc(self.expr(span, expr_break, attrs))
1257     }
1258
1259     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1260         self.expr(
1261             span,
1262             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1263             ThinVec::new(),
1264         )
1265     }
1266
1267     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1268         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
1269     }
1270
1271     fn expr_call(
1272         &mut self,
1273         span: Span,
1274         e: &'hir hir::Expr<'hir>,
1275         args: &'hir [hir::Expr<'hir>],
1276     ) -> &'hir hir::Expr<'hir> {
1277         self.arena.alloc(self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new()))
1278     }
1279
1280     // Note: associated functions must use `expr_call_std_path`.
1281     fn expr_call_std_path(
1282         &mut self,
1283         span: Span,
1284         path_components: &[Symbol],
1285         args: &'hir [hir::Expr<'hir>],
1286     ) -> &'hir hir::Expr<'hir> {
1287         let path =
1288             self.arena.alloc(self.expr_std_path(span, path_components, None, ThinVec::new()));
1289         self.expr_call(span, path, args)
1290     }
1291
1292     // Create an expression calling an associated function of an std type.
1293     //
1294     // Associated functions cannot be resolved through the normal `std_path` function,
1295     // as they are resolved differently and so cannot use `expr_call_std_path`.
1296     //
1297     // This function accepts the path component (`ty_path_components`) separately from
1298     // the name of the associated function (`assoc_fn_name`) in order to facilitate
1299     // separate resolution of the type and creation of a path referring to its associated
1300     // function.
1301     fn expr_call_std_assoc_fn(
1302         &mut self,
1303         ty_path_id: hir::HirId,
1304         span: Span,
1305         ty_path_components: &[Symbol],
1306         assoc_fn_name: &str,
1307         args: &'hir [hir::Expr<'hir>],
1308     ) -> hir::ExprKind<'hir> {
1309         let ty_path = self.std_path(span, ty_path_components, None, false);
1310         let ty =
1311             self.arena.alloc(self.ty_path(ty_path_id, span, hir::QPath::Resolved(None, ty_path)));
1312         let fn_seg = self.arena.alloc(hir::PathSegment::from_ident(Ident::from_str(assoc_fn_name)));
1313         let fn_path = hir::QPath::TypeRelative(ty, fn_seg);
1314         let fn_expr =
1315             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
1316         hir::ExprKind::Call(fn_expr, args)
1317     }
1318
1319     fn expr_std_path(
1320         &mut self,
1321         span: Span,
1322         components: &[Symbol],
1323         params: Option<&'hir hir::GenericArgs<'hir>>,
1324         attrs: AttrVec,
1325     ) -> hir::Expr<'hir> {
1326         let path = self.std_path(span, components, params, true);
1327         self.expr(span, hir::ExprKind::Path(hir::QPath::Resolved(None, path)), attrs)
1328     }
1329
1330     pub(super) fn expr_ident(
1331         &mut self,
1332         sp: Span,
1333         ident: Ident,
1334         binding: hir::HirId,
1335     ) -> &'hir hir::Expr<'hir> {
1336         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1337     }
1338
1339     pub(super) fn expr_ident_mut(
1340         &mut self,
1341         sp: Span,
1342         ident: Ident,
1343         binding: hir::HirId,
1344     ) -> hir::Expr<'hir> {
1345         self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
1346     }
1347
1348     fn expr_ident_with_attrs(
1349         &mut self,
1350         span: Span,
1351         ident: Ident,
1352         binding: hir::HirId,
1353         attrs: AttrVec,
1354     ) -> hir::Expr<'hir> {
1355         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1356             None,
1357             self.arena.alloc(hir::Path {
1358                 span,
1359                 res: Res::Local(binding),
1360                 segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
1361             }),
1362         ));
1363
1364         self.expr(span, expr_path, attrs)
1365     }
1366
1367     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1368         let hir_id = self.next_id();
1369         let span = expr.span;
1370         self.expr(
1371             span,
1372             hir::ExprKind::Block(
1373                 self.arena.alloc(hir::Block {
1374                     stmts: &[],
1375                     expr: Some(expr),
1376                     hir_id,
1377                     rules: hir::UnsafeBlock(hir::CompilerGenerated),
1378                     span,
1379                     targeted_by_break: false,
1380                 }),
1381                 None,
1382             ),
1383             ThinVec::new(),
1384         )
1385     }
1386
1387     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1388         let blk = self.block_all(span, &[], None);
1389         let expr = self.expr_block(blk, ThinVec::new());
1390         self.arena.alloc(expr)
1391     }
1392
1393     pub(super) fn expr_block(
1394         &mut self,
1395         b: &'hir hir::Block<'hir>,
1396         attrs: AttrVec,
1397     ) -> hir::Expr<'hir> {
1398         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1399     }
1400
1401     pub(super) fn expr(
1402         &mut self,
1403         span: Span,
1404         kind: hir::ExprKind<'hir>,
1405         attrs: AttrVec,
1406     ) -> hir::Expr<'hir> {
1407         hir::Expr { hir_id: self.next_id(), kind, span, attrs }
1408     }
1409
1410     fn field(&mut self, ident: Ident, expr: &'hir hir::Expr<'hir>, span: Span) -> hir::Field<'hir> {
1411         hir::Field { hir_id: self.next_id(), ident, span, expr, is_shorthand: false }
1412     }
1413
1414     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1415         hir::Arm {
1416             hir_id: self.next_id(),
1417             attrs: &[],
1418             pat,
1419             guard: None,
1420             span: expr.span,
1421             body: expr,
1422         }
1423     }
1424 }