compiler/rustc_ast_lowering/src/expr.rs

   1 use crate::{FnDeclKind, ImplTraitPosition};
   2
   3 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
   4
   5 use rustc_ast::attr;
   6 use rustc_ast::ptr::P as AstP;
   7 use rustc_ast::*;
   8 use rustc_data_structures::stack::ensure_sufficient_stack;
   9 use rustc_data_structures::thin_vec::ThinVec;
  10 use rustc_errors::struct_span_err;
  11 use rustc_hir as hir;
  12 use rustc_hir::def::Res;
  13 use rustc_hir::definitions::DefPathData;
  14 use rustc_span::hygiene::ExpnId;
  15 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
  16 use rustc_span::symbol::{sym, Ident};
  17 use rustc_span::DUMMY_SP;
  18
  19 impl<'hir> LoweringContext<'_, 'hir> {
  20     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  21         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  22     }
  23
  24     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  25         self.arena.alloc(self.lower_expr_mut(e))
  26     }
  27
  28     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  29         ensure_sufficient_stack(|| {
  30             let kind = match e.kind {
  31                 ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  32                 ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  33                 ExprKind::ConstBlock(ref anon_const) => {
  34                     let anon_const = self.lower_anon_const(anon_const);
  35                     hir::ExprKind::ConstBlock(anon_const)
  36                 }
  37                 ExprKind::Repeat(ref expr, ref count) => {
  38                     let expr = self.lower_expr(expr);
  39                     let count = self.lower_array_length(count);
  40                     hir::ExprKind::Repeat(expr, count)
  41                 }
  42                 ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  43                 ExprKind::Call(ref f, ref args) => {
  44                     if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
  45                         self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
  46                     } else {
  47                         let f = self.lower_expr(f);
  48                         hir::ExprKind::Call(f, self.lower_exprs(args))
  49                     }
  50                 }
  51                 ExprKind::MethodCall(ref seg, ref args, span) => {
  52                     let hir_seg = self.arena.alloc(self.lower_path_segment(
  53                         e.span,
  54                         seg,
  55                         ParamMode::Optional,
  56                         ParenthesizedGenericArgs::Err,
  57                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  58                     ));
  59                     let args = self.lower_exprs(args);
  60                     hir::ExprKind::MethodCall(hir_seg, args, self.lower_span(span))
  61                 }
  62                 ExprKind::Binary(binop, ref lhs, ref rhs) => {
  63                     let binop = self.lower_binop(binop);
  64                     let lhs = self.lower_expr(lhs);
  65                     let rhs = self.lower_expr(rhs);
  66                     hir::ExprKind::Binary(binop, lhs, rhs)
  67                 }
  68                 ExprKind::Unary(op, ref ohs) => {
  69                     let op = self.lower_unop(op);
  70                     let ohs = self.lower_expr(ohs);
  71                     hir::ExprKind::Unary(op, ohs)
  72                 }
  73                 ExprKind::Lit(ref l) => {
  74                     hir::ExprKind::Lit(respan(self.lower_span(l.span), l.kind.clone()))
  75                 }
  76                 ExprKind::Cast(ref expr, ref ty) => {
  77                     let expr = self.lower_expr(expr);
  78                     let ty =
  79                         self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
  80                     hir::ExprKind::Cast(expr, ty)
  81                 }
  82                 ExprKind::Type(ref expr, ref ty) => {
  83                     let expr = self.lower_expr(expr);
  84                     let ty =
  85                         self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
  86                     hir::ExprKind::Type(expr, ty)
  87                 }
  88                 ExprKind::AddrOf(k, m, ref ohs) => {
  89                     let ohs = self.lower_expr(ohs);
  90                     hir::ExprKind::AddrOf(k, m, ohs)
  91                 }
  92                 ExprKind::Let(ref pat, ref scrutinee, span) => {
  93                     hir::ExprKind::Let(self.arena.alloc(hir::Let {
  94                         hir_id: self.next_id(),
  95                         span: self.lower_span(span),
  96                         pat: self.lower_pat(pat),
  97                         ty: None,
  98                         init: self.lower_expr(scrutinee),
  99                     }))
 100                 }
 101                 ExprKind::If(ref cond, ref then, ref else_opt) => {
 102                     self.lower_expr_if(cond, then, else_opt.as_deref())
 103                 }
 104                 ExprKind::While(ref cond, ref body, opt_label) => {
 105                     self.with_loop_scope(e.id, |this| {
 106                         let span =
 107                             this.mark_span_with_reason(DesugaringKind::WhileLoop, e.span, None);
 108                         this.lower_expr_while_in_loop_scope(span, cond, body, opt_label)
 109                     })
 110                 }
 111                 ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
 112                     hir::ExprKind::Loop(
 113                         this.lower_block(body, false),
 114                         this.lower_label(opt_label),
 115                         hir::LoopSource::Loop,
 116                         DUMMY_SP,
 117                     )
 118                 }),
 119                 ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
 120                 ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
 121                     self.lower_expr(expr),
 122                     self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
 123                     hir::MatchSource::Normal,
 124                 ),
 125                 ExprKind::Async(capture_clause, closure_node_id, ref block) => self
 126                     .make_async_expr(
 127                         capture_clause,
 128                         closure_node_id,
 129                         None,
 130                         block.span,
 131                         hir::AsyncGeneratorKind::Block,
 132                         |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
 133                     ),
 134                 ExprKind::Await(ref expr) => {
 135                     let span = if expr.span.hi() < e.span.hi() {
 136                         expr.span.shrink_to_hi().with_hi(e.span.hi())
 137                     } else {
 138                         // this is a recovered `await expr`
 139                         e.span
 140                     };
 141                     self.lower_expr_await(span, expr)
 142                 }
 143                 ExprKind::Closure(
 144                     capture_clause,
 145                     asyncness,
 146                     movability,
 147                     ref decl,
 148                     ref body,
 149                     fn_decl_span,
 150                 ) => {
 151                     if let Async::Yes { closure_id, .. } = asyncness {
 152                         self.lower_expr_async_closure(
 153                             capture_clause,
 154                             closure_id,
 155                             decl,
 156                             body,
 157                             fn_decl_span,
 158                         )
 159                     } else {
 160                         self.lower_expr_closure(
 161                             capture_clause,
 162                             movability,
 163                             decl,
 164                             body,
 165                             fn_decl_span,
 166                         )
 167                     }
 168                 }
 169                 ExprKind::Block(ref blk, opt_label) => {
 170                     let opt_label = self.lower_label(opt_label);
 171                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 172                 }
 173                 ExprKind::Assign(ref el, ref er, span) => {
 174                     self.lower_expr_assign(el, er, span, e.span)
 175                 }
 176                 ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 177                     self.lower_binop(op),
 178                     self.lower_expr(el),
 179                     self.lower_expr(er),
 180                 ),
 181                 ExprKind::Field(ref el, ident) => {
 182                     hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(ident))
 183                 }
 184                 ExprKind::Index(ref el, ref er) => {
 185                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 186                 }
 187                 ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 188                     self.lower_expr_range_closed(e.span, e1, e2)
 189                 }
 190                 ExprKind::Range(ref e1, ref e2, lims) => {
 191                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 192                 }
 193                 ExprKind::Underscore => {
 194                     self.sess
 195                         .struct_span_err(
 196                             e.span,
 197                             "in expressions, `_` can only be used on the left-hand side of an assignment",
 198                         )
 199                         .span_label(e.span, "`_` not allowed here")
 200                         .emit();
 201                     hir::ExprKind::Err
 202                 }
 203                 ExprKind::Path(ref qself, ref path) => {
 204                     let qpath = self.lower_qpath(
 205                         e.id,
 206                         qself,
 207                         path,
 208                         ParamMode::Optional,
 209                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 210                     );
 211                     hir::ExprKind::Path(qpath)
 212                 }
 213                 ExprKind::Break(opt_label, ref opt_expr) => {
 214                     let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 215                     hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
 216                 }
 217                 ExprKind::Continue(opt_label) => {
 218                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
 219                 }
 220                 ExprKind::Ret(ref e) => {
 221                     let e = e.as_ref().map(|x| self.lower_expr(x));
 222                     hir::ExprKind::Ret(e)
 223                 }
 224                 ExprKind::InlineAsm(ref asm) => {
 225                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
 226                 }
 227                 ExprKind::Struct(ref se) => {
 228                     let rest = match &se.rest {
 229                         StructRest::Base(e) => Some(self.lower_expr(e)),
 230                         StructRest::Rest(sp) => {
 231                             self.sess
 232                                 .struct_span_err(*sp, "base expression required after `..`")
 233                                 .span_label(*sp, "add a base expression here")
 234                                 .emit();
 235                             Some(&*self.arena.alloc(self.expr_err(*sp)))
 236                         }
 237                         StructRest::None => None,
 238                     };
 239                     hir::ExprKind::Struct(
 240                         self.arena.alloc(self.lower_qpath(
 241                             e.id,
 242                             &se.qself,
 243                             &se.path,
 244                             ParamMode::Optional,
 245                             ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 246                         )),
 247                         self.arena
 248                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
 249                         rest,
 250                     )
 251                 }
 252                 ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 253                 ExprKind::Err => hir::ExprKind::Err,
 254                 ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 255                 ExprKind::Paren(ref ex) => {
 256                     let mut ex = self.lower_expr_mut(ex);
 257                     // Include parens in span, but only if it is a super-span.
 258                     if e.span.contains(ex.span) {
 259                         ex.span = self.lower_span(e.span);
 260                     }
 261                     // Merge attributes into the inner expression.
 262                     if !e.attrs.is_empty() {
 263                         let old_attrs =
 264                             self.attrs.get(&ex.hir_id.local_id).map(|la| *la).unwrap_or(&[]);
 265                         self.attrs.insert(
 266                             ex.hir_id.local_id,
 267                             &*self.arena.alloc_from_iter(
 268                                 e.attrs
 269                                     .iter()
 270                                     .map(|a| self.lower_attr(a))
 271                                     .chain(old_attrs.iter().cloned()),
 272                             ),
 273                         );
 274                     }
 275                     return ex;
 276                 }
 277
 278                 // Desugar `ExprForLoop`
 279                 // from: `[opt_ident]: for <pat> in <head> <body>`
 280                 ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 281                     return self.lower_expr_for(e, pat, head, body, opt_label);
 282                 }
 283                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 284             };
 285
 286             let hir_id = self.lower_node_id(e.id);
 287             self.lower_attrs(hir_id, &e.attrs);
 288             hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
 289         })
 290     }
 291
 292     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 293         match u {
 294             UnOp::Deref => hir::UnOp::Deref,
 295             UnOp::Not => hir::UnOp::Not,
 296             UnOp::Neg => hir::UnOp::Neg,
 297         }
 298     }
 299
 300     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 301         Spanned {
 302             node: match b.node {
 303                 BinOpKind::Add => hir::BinOpKind::Add,
 304                 BinOpKind::Sub => hir::BinOpKind::Sub,
 305                 BinOpKind::Mul => hir::BinOpKind::Mul,
 306                 BinOpKind::Div => hir::BinOpKind::Div,
 307                 BinOpKind::Rem => hir::BinOpKind::Rem,
 308                 BinOpKind::And => hir::BinOpKind::And,
 309                 BinOpKind::Or => hir::BinOpKind::Or,
 310                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 311                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 312                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 313                 BinOpKind::Shl => hir::BinOpKind::Shl,
 314                 BinOpKind::Shr => hir::BinOpKind::Shr,
 315                 BinOpKind::Eq => hir::BinOpKind::Eq,
 316                 BinOpKind::Lt => hir::BinOpKind::Lt,
 317                 BinOpKind::Le => hir::BinOpKind::Le,
 318                 BinOpKind::Ne => hir::BinOpKind::Ne,
 319                 BinOpKind::Ge => hir::BinOpKind::Ge,
 320                 BinOpKind::Gt => hir::BinOpKind::Gt,
 321             },
 322             span: self.lower_span(b.span),
 323         }
 324     }
 325
 326     fn lower_legacy_const_generics(
 327         &mut self,
 328         mut f: Expr,
 329         args: Vec<AstP<Expr>>,
 330         legacy_args_idx: &[usize],
 331     ) -> hir::ExprKind<'hir> {
 332         let ExprKind::Path(None, ref mut path) = f.kind else {
 333             unreachable!();
 334         };
 335
 336         // Split the arguments into const generics and normal arguments
 337         let mut real_args = vec![];
 338         let mut generic_args = vec![];
 339         for (idx, arg) in args.into_iter().enumerate() {
 340             if legacy_args_idx.contains(&idx) {
 341                 let parent_def_id = self.current_hir_id_owner;
 342                 let node_id = self.resolver.next_node_id();
 343
 344                 // Add a definition for the in-band const def.
 345                 self.resolver.create_def(
 346                     parent_def_id,
 347                     node_id,
 348                     DefPathData::AnonConst,
 349                     ExpnId::root(),
 350                     arg.span,
 351                 );
 352
 353                 let anon_const = AnonConst { id: node_id, value: arg };
 354                 generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
 355             } else {
 356                 real_args.push(arg);
 357             }
 358         }
 359
 360         // Add generic args to the last element of the path.
 361         let last_segment = path.segments.last_mut().unwrap();
 362         assert!(last_segment.args.is_none());
 363         last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
 364             span: DUMMY_SP,
 365             args: generic_args,
 366         })));
 367
 368         // Now lower everything as normal.
 369         let f = self.lower_expr(&f);
 370         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
 371     }
 372
 373     fn lower_expr_if(
 374         &mut self,
 375         cond: &Expr,
 376         then: &Block,
 377         else_opt: Option<&Expr>,
 378     ) -> hir::ExprKind<'hir> {
 379         let lowered_cond = self.lower_expr(cond);
 380         let new_cond = self.manage_let_cond(lowered_cond);
 381         let then_expr = self.lower_block_expr(then);
 382         if let Some(rslt) = else_opt {
 383             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), Some(self.lower_expr(rslt)))
 384         } else {
 385             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), None)
 386         }
 387     }
 388
 389     // If `cond` kind is `let`, returns `let`. Otherwise, wraps and returns `cond`
 390     // in a temporary block.
 391     fn manage_let_cond(&mut self, cond: &'hir hir::Expr<'hir>) -> &'hir hir::Expr<'hir> {
 392         fn has_let_expr<'hir>(expr: &'hir hir::Expr<'hir>) -> bool {
 393             match expr.kind {
 394                 hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
 395                 hir::ExprKind::Let(..) => true,
 396                 _ => false,
 397             }
 398         }
 399         if has_let_expr(cond) {
 400             cond
 401         } else {
 402             let reason = DesugaringKind::CondTemporary;
 403             let span_block = self.mark_span_with_reason(reason, cond.span, None);
 404             self.expr_drop_temps(span_block, cond, AttrVec::new())
 405         }
 406     }
 407
 408     // We desugar: `'label: while $cond $body` into:
 409     //
 410     // ```
 411     // 'label: loop {
 412     //   if { let _t = $cond; _t } {
 413     //     $body
 414     //   }
 415     //   else {
 416     //     break;
 417     //   }
 418     // }
 419     // ```
 420     //
 421     // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 422     // to preserve drop semantics since `while $cond { ... }` does not
 423     // let temporaries live outside of `cond`.
 424     fn lower_expr_while_in_loop_scope(
 425         &mut self,
 426         span: Span,
 427         cond: &Expr,
 428         body: &Block,
 429         opt_label: Option<Label>,
 430     ) -> hir::ExprKind<'hir> {
 431         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
 432         let new_cond = self.manage_let_cond(lowered_cond);
 433         let then = self.lower_block_expr(body);
 434         let expr_break = self.expr_break(span, ThinVec::new());
 435         let stmt_break = self.stmt_expr(span, expr_break);
 436         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
 437         let else_expr = self.arena.alloc(self.expr_block(else_blk, ThinVec::new()));
 438         let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
 439         let if_expr = self.expr(span, if_kind, ThinVec::new());
 440         let block = self.block_expr(self.arena.alloc(if_expr));
 441         let span = self.lower_span(span.with_hi(cond.span.hi()));
 442         let opt_label = self.lower_label(opt_label);
 443         hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
 444     }
 445
 446     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
 447     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
 448     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 449     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 450         self.with_catch_scope(body.id, |this| {
 451             let mut block = this.lower_block_noalloc(body, true);
 452
 453             // Final expression of the block (if present) or `()` with span at the end of block
 454             let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
 455                 (
 456                     this.mark_span_with_reason(
 457                         DesugaringKind::TryBlock,
 458                         expr.span,
 459                         this.allow_try_trait.clone(),
 460                     ),
 461                     expr,
 462                 )
 463             } else {
 464                 let try_span = this.mark_span_with_reason(
 465                     DesugaringKind::TryBlock,
 466                     this.sess.source_map().end_point(body.span),
 467                     this.allow_try_trait.clone(),
 468                 );
 469
 470                 (try_span, this.expr_unit(try_span))
 471             };
 472
 473             let ok_wrapped_span =
 474                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 475
 476             // `::std::ops::Try::from_output($tail_expr)`
 477             block.expr = Some(this.wrap_in_try_constructor(
 478                 hir::LangItem::TryTraitFromOutput,
 479                 try_span,
 480                 tail_expr,
 481                 ok_wrapped_span,
 482             ));
 483
 484             hir::ExprKind::Block(this.arena.alloc(block), None)
 485         })
 486     }
 487
 488     fn wrap_in_try_constructor(
 489         &mut self,
 490         lang_item: hir::LangItem,
 491         method_span: Span,
 492         expr: &'hir hir::Expr<'hir>,
 493         overall_span: Span,
 494     ) -> &'hir hir::Expr<'hir> {
 495         let constructor = self.arena.alloc(self.expr_lang_item_path(
 496             method_span,
 497             lang_item,
 498             ThinVec::new(),
 499             None,
 500         ));
 501         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 502     }
 503
 504     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 505         let pat = self.lower_pat(&arm.pat);
 506         let guard = arm.guard.as_ref().map(|cond| {
 507             if let ExprKind::Let(ref pat, ref scrutinee, _) = cond.kind {
 508                 hir::Guard::IfLet(self.lower_pat(pat), self.lower_expr(scrutinee))
 509             } else {
 510                 hir::Guard::If(self.lower_expr(cond))
 511             }
 512         });
 513         let hir_id = self.next_id();
 514         self.lower_attrs(hir_id, &arm.attrs);
 515         hir::Arm {
 516             hir_id,
 517             pat,
 518             guard,
 519             body: self.lower_expr(&arm.body),
 520             span: self.lower_span(arm.span),
 521         }
 522     }
 523
 524     /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
 525     ///
 526     /// This results in:
 527     ///
 528     /// ```text
 529     /// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
 530     ///     <body>
 531     /// })
 532     /// ```
 533     pub(super) fn make_async_expr(
 534         &mut self,
 535         capture_clause: CaptureBy,
 536         closure_node_id: NodeId,
 537         ret_ty: Option<AstP<Ty>>,
 538         span: Span,
 539         async_gen_kind: hir::AsyncGeneratorKind,
 540         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 541     ) -> hir::ExprKind<'hir> {
 542         let output = match ret_ty {
 543             Some(ty) => hir::FnRetTy::Return(
 544                 self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock)),
 545             ),
 546             None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
 547         };
 548
 549         // Resume argument type. We let the compiler infer this to simplify the lowering. It is
 550         // fully constrained by `future::from_generator`.
 551         let input_ty = hir::Ty {
 552             hir_id: self.next_id(),
 553             kind: hir::TyKind::Infer,
 554             span: self.lower_span(span),
 555         };
 556
 557         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 558         let decl = self.arena.alloc(hir::FnDecl {
 559             inputs: arena_vec![self; input_ty],
 560             output,
 561             c_variadic: false,
 562             implicit_self: hir::ImplicitSelfKind::None,
 563         });
 564
 565         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 566         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 567             span,
 568             Ident::with_dummy_span(sym::_task_context),
 569             hir::BindingAnnotation::Mutable,
 570         );
 571         let param = hir::Param {
 572             hir_id: self.next_id(),
 573             pat,
 574             ty_span: self.lower_span(span),
 575             span: self.lower_span(span),
 576         };
 577         let params = arena_vec![self; param];
 578
 579         let body_id = self.lower_body(move |this| {
 580             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 581
 582             let old_ctx = this.task_context;
 583             this.task_context = Some(task_context_hid);
 584             let res = body(this);
 585             this.task_context = old_ctx;
 586             (params, res)
 587         });
 588
 589         // `static |_task_context| -> <ret_ty> { body }`:
 590         let generator_kind = hir::ExprKind::Closure(
 591             capture_clause,
 592             decl,
 593             body_id,
 594             self.lower_span(span),
 595             Some(hir::Movability::Static),
 596         );
 597         let generator = hir::Expr {
 598             hir_id: self.lower_node_id(closure_node_id),
 599             kind: generator_kind,
 600             span: self.lower_span(span),
 601         };
 602
 603         // `future::from_generator`:
 604         let unstable_span =
 605             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 606         let gen_future = self.expr_lang_item_path(
 607             unstable_span,
 608             hir::LangItem::FromGenerator,
 609             ThinVec::new(),
 610             None,
 611         );
 612
 613         // `future::from_generator(generator)`:
 614         hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
 615     }
 616
 617     /// Desugar `<expr>.await` into:
 618     /// ```ignore (pseudo-rust)
 619     /// match ::std::future::IntoFuture::into_future(<expr>) {
 620     ///     mut __awaitee => loop {
 621     ///         match unsafe { ::std::future::Future::poll(
 622     ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
 623     ///             ::std::future::get_context(task_context),
 624     ///         ) } {
 625     ///             ::std::task::Poll::Ready(result) => break result,
 626     ///             ::std::task::Poll::Pending => {}
 627     ///         }
 628     ///         task_context = yield ();
 629     ///     }
 630     /// }
 631     /// ```
 632     fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 633         let full_span = expr.span.to(dot_await_span);
 634         match self.generator_kind {
 635             Some(hir::GeneratorKind::Async(_)) => {}
 636             Some(hir::GeneratorKind::Gen) | None => {
 637                 let mut err = struct_span_err!(
 638                     self.sess,
 639                     dot_await_span,
 640                     E0728,
 641                     "`await` is only allowed inside `async` functions and blocks"
 642                 );
 643                 err.span_label(dot_await_span, "only allowed inside `async` functions and blocks");
 644                 if let Some(item_sp) = self.current_item {
 645                     err.span_label(item_sp, "this is not `async`");
 646                 }
 647                 err.emit();
 648             }
 649         }
 650         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
 651         let gen_future_span = self.mark_span_with_reason(
 652             DesugaringKind::Await,
 653             full_span,
 654             self.allow_gen_future.clone(),
 655         );
 656         let expr = self.lower_expr_mut(expr);
 657         let expr_hir_id = expr.hir_id;
 658
 659         // Note that the name of this binding must not be changed to something else because
 660         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
 661         // this name to identify what is being awaited by a suspended async functions.
 662         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
 663         let (awaitee_pat, awaitee_pat_hid) =
 664             self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::Mutable);
 665
 666         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 667
 668         // unsafe {
 669         //     ::std::future::Future::poll(
 670         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
 671         //         ::std::future::get_context(task_context),
 672         //     )
 673         // }
 674         let poll_expr = {
 675             let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
 676             let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
 677             let task_context = if let Some(task_context_hid) = self.task_context {
 678                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 679             } else {
 680                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 681                 self.expr_err(span)
 682             };
 683             let new_unchecked = self.expr_call_lang_item_fn_mut(
 684                 span,
 685                 hir::LangItem::PinNewUnchecked,
 686                 arena_vec![self; ref_mut_awaitee],
 687                 Some(expr_hir_id),
 688             );
 689             let get_context = self.expr_call_lang_item_fn_mut(
 690                 gen_future_span,
 691                 hir::LangItem::GetContext,
 692                 arena_vec![self; task_context],
 693                 Some(expr_hir_id),
 694             );
 695             let call = self.expr_call_lang_item_fn(
 696                 span,
 697                 hir::LangItem::FuturePoll,
 698                 arena_vec![self; new_unchecked, get_context],
 699                 Some(expr_hir_id),
 700             );
 701             self.arena.alloc(self.expr_unsafe(call))
 702         };
 703
 704         // `::std::task::Poll::Ready(result) => break result`
 705         let loop_node_id = self.resolver.next_node_id();
 706         let loop_hir_id = self.lower_node_id(loop_node_id);
 707         let ready_arm = {
 708             let x_ident = Ident::with_dummy_span(sym::result);
 709             let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
 710             let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
 711             let ready_field = self.single_pat_field(gen_future_span, x_pat);
 712             let ready_pat = self.pat_lang_item_variant(
 713                 span,
 714                 hir::LangItem::PollReady,
 715                 ready_field,
 716                 Some(expr_hir_id),
 717             );
 718             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 719                 let expr_break =
 720                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 721                 this.arena.alloc(this.expr(gen_future_span, expr_break, ThinVec::new()))
 722             });
 723             self.arm(ready_pat, break_x)
 724         };
 725
 726         // `::std::task::Poll::Pending => {}`
 727         let pending_arm = {
 728             let pending_pat = self.pat_lang_item_variant(
 729                 span,
 730                 hir::LangItem::PollPending,
 731                 &[],
 732                 Some(expr_hir_id),
 733             );
 734             let empty_block = self.expr_block_empty(span);
 735             self.arm(pending_pat, empty_block)
 736         };
 737
 738         let inner_match_stmt = {
 739             let match_expr = self.expr_match(
 740                 span,
 741                 poll_expr,
 742                 arena_vec![self; ready_arm, pending_arm],
 743                 hir::MatchSource::AwaitDesugar,
 744             );
 745             self.stmt_expr(span, match_expr)
 746         };
 747
 748         // task_context = yield ();
 749         let yield_stmt = {
 750             let unit = self.expr_unit(span);
 751             let yield_expr = self.expr(
 752                 span,
 753                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
 754                 ThinVec::new(),
 755             );
 756             let yield_expr = self.arena.alloc(yield_expr);
 757
 758             if let Some(task_context_hid) = self.task_context {
 759                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 760                 let assign = self.expr(
 761                     span,
 762                     hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)),
 763                     AttrVec::new(),
 764                 );
 765                 self.stmt_expr(span, assign)
 766             } else {
 767                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 768                 // proceed with type checking.
 769                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 770             }
 771         };
 772
 773         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 774
 775         // loop { .. }
 776         let loop_expr = self.arena.alloc(hir::Expr {
 777             hir_id: loop_hir_id,
 778             kind: hir::ExprKind::Loop(
 779                 loop_block,
 780                 None,
 781                 hir::LoopSource::Loop,
 782                 self.lower_span(span),
 783             ),
 784             span: self.lower_span(span),
 785         });
 786
 787         // mut __awaitee => loop { ... }
 788         let awaitee_arm = self.arm(awaitee_pat, loop_expr);
 789
 790         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
 791         let into_future_span = self.mark_span_with_reason(
 792             DesugaringKind::Await,
 793             dot_await_span,
 794             self.allow_into_future.clone(),
 795         );
 796         let into_future_expr = self.expr_call_lang_item_fn(
 797             into_future_span,
 798             hir::LangItem::IntoFutureIntoFuture,
 799             arena_vec![self; expr],
 800             Some(expr_hir_id),
 801         );
 802
 803         // match <into_future_expr> {
 804         //     mut __awaitee => loop { .. }
 805         // }
 806         hir::ExprKind::Match(
 807             into_future_expr,
 808             arena_vec![self; awaitee_arm],
 809             hir::MatchSource::AwaitDesugar,
 810         )
 811     }
 812
 813     fn lower_expr_closure(
 814         &mut self,
 815         capture_clause: CaptureBy,
 816         movability: Movability,
 817         decl: &FnDecl,
 818         body: &Expr,
 819         fn_decl_span: Span,
 820     ) -> hir::ExprKind<'hir> {
 821         let (body_id, generator_option) = self.with_new_scopes(move |this| {
 822             let prev = this.current_item;
 823             this.current_item = Some(fn_decl_span);
 824             let mut generator_kind = None;
 825             let body_id = this.lower_fn_body(decl, |this| {
 826                 let e = this.lower_expr_mut(body);
 827                 generator_kind = this.generator_kind;
 828                 e
 829             });
 830             let generator_option =
 831                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 832             this.current_item = prev;
 833             (body_id, generator_option)
 834         });
 835
 836         // Lower outside new scope to preserve `is_in_loop_condition`.
 837         let fn_decl = self.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
 838
 839         hir::ExprKind::Closure(
 840             capture_clause,
 841             fn_decl,
 842             body_id,
 843             self.lower_span(fn_decl_span),
 844             generator_option,
 845         )
 846     }
 847
 848     fn generator_movability_for_fn(
 849         &mut self,
 850         decl: &FnDecl,
 851         fn_decl_span: Span,
 852         generator_kind: Option<hir::GeneratorKind>,
 853         movability: Movability,
 854     ) -> Option<hir::Movability> {
 855         match generator_kind {
 856             Some(hir::GeneratorKind::Gen) => {
 857                 if decl.inputs.len() > 1 {
 858                     struct_span_err!(
 859                         self.sess,
 860                         fn_decl_span,
 861                         E0628,
 862                         "too many parameters for a generator (expected 0 or 1 parameters)"
 863                     )
 864                     .emit();
 865                 }
 866                 Some(movability)
 867             }
 868             Some(hir::GeneratorKind::Async(_)) => {
 869                 panic!("non-`async` closure body turned `async` during lowering");
 870             }
 871             None => {
 872                 if movability == Movability::Static {
 873                     struct_span_err!(self.sess, fn_decl_span, E0697, "closures cannot be static")
 874                         .emit();
 875                 }
 876                 None
 877             }
 878         }
 879     }
 880
 881     fn lower_expr_async_closure(
 882         &mut self,
 883         capture_clause: CaptureBy,
 884         closure_id: NodeId,
 885         decl: &FnDecl,
 886         body: &Expr,
 887         fn_decl_span: Span,
 888     ) -> hir::ExprKind<'hir> {
 889         let outer_decl =
 890             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 891
 892         let body_id = self.with_new_scopes(|this| {
 893             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 894             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 895                 struct_span_err!(
 896                     this.sess,
 897                     fn_decl_span,
 898                     E0708,
 899                     "`async` non-`move` closures with parameters are not currently supported",
 900                 )
 901                 .help(
 902                     "consider using `let` statements to manually capture \
 903                     variables by reference before entering an `async move` closure",
 904                 )
 905                 .emit();
 906             }
 907
 908             // Transform `async |x: u8| -> X { ... }` into
 909             // `|x: u8| future_from_generator(|| -> X { ... })`.
 910             let body_id = this.lower_fn_body(&outer_decl, |this| {
 911                 let async_ret_ty =
 912                     if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 913                 let async_body = this.make_async_expr(
 914                     capture_clause,
 915                     closure_id,
 916                     async_ret_ty,
 917                     body.span,
 918                     hir::AsyncGeneratorKind::Closure,
 919                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 920                 );
 921                 this.expr(fn_decl_span, async_body, ThinVec::new())
 922             });
 923             body_id
 924         });
 925
 926         // We need to lower the declaration outside the new scope, because we
 927         // have to conserve the state of being inside a loop condition for the
 928         // closure argument types.
 929         let fn_decl = self.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
 930
 931         hir::ExprKind::Closure(
 932             capture_clause,
 933             fn_decl,
 934             body_id,
 935             self.lower_span(fn_decl_span),
 936             None,
 937         )
 938     }
 939
 940     /// Destructure the LHS of complex assignments.
 941     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
 942     fn lower_expr_assign(
 943         &mut self,
 944         lhs: &Expr,
 945         rhs: &Expr,
 946         eq_sign_span: Span,
 947         whole_span: Span,
 948     ) -> hir::ExprKind<'hir> {
 949         // Return early in case of an ordinary assignment.
 950         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
 951             match &lhs.kind {
 952                 ExprKind::Array(..)
 953                 | ExprKind::Struct(..)
 954                 | ExprKind::Tup(..)
 955                 | ExprKind::Underscore => false,
 956                 // Check for tuple struct constructor.
 957                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
 958                 ExprKind::Paren(e) => {
 959                     match e.kind {
 960                         // We special-case `(..)` for consistency with patterns.
 961                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
 962                         _ => is_ordinary(lower_ctx, e),
 963                     }
 964                 }
 965                 _ => true,
 966             }
 967         }
 968         if is_ordinary(self, lhs) {
 969             return hir::ExprKind::Assign(
 970                 self.lower_expr(lhs),
 971                 self.lower_expr(rhs),
 972                 self.lower_span(eq_sign_span),
 973             );
 974         }
 975
 976         let mut assignments = vec![];
 977
 978         // The LHS becomes a pattern: `(lhs1, lhs2)`.
 979         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
 980         let rhs = self.lower_expr(rhs);
 981
 982         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
 983         let destructure_let = self.stmt_let_pat(
 984             None,
 985             whole_span,
 986             Some(rhs),
 987             pat,
 988             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
 989         );
 990
 991         // `a = lhs1; b = lhs2;`.
 992         let stmts = self
 993             .arena
 994             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
 995
 996         // Wrap everything in a block.
 997         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
 998     }
 999
1000     /// If the given expression is a path to a tuple struct, returns that path.
1001     /// It is not a complete check, but just tries to reject most paths early
1002     /// if they are not tuple structs.
1003     /// Type checking will take care of the full validation later.
1004     fn extract_tuple_struct_path<'a>(
1005         &mut self,
1006         expr: &'a Expr,
1007     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1008         if let ExprKind::Path(qself, path) = &expr.kind {
1009             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1010             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1011                 if partial_res.unresolved_segments() == 0
1012                     && !partial_res.base_res().expected_in_tuple_struct_pat()
1013                 {
1014                     return None;
1015                 }
1016             }
1017             return Some((qself, path));
1018         }
1019         None
1020     }
1021
1022     /// Convert the LHS of a destructuring assignment to a pattern.
1023     /// Each sub-assignment is recorded in `assignments`.
1024     fn destructure_assign(
1025         &mut self,
1026         lhs: &Expr,
1027         eq_sign_span: Span,
1028         assignments: &mut Vec<hir::Stmt<'hir>>,
1029     ) -> &'hir hir::Pat<'hir> {
1030         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1031     }
1032
1033     fn destructure_assign_mut(
1034         &mut self,
1035         lhs: &Expr,
1036         eq_sign_span: Span,
1037         assignments: &mut Vec<hir::Stmt<'hir>>,
1038     ) -> hir::Pat<'hir> {
1039         match &lhs.kind {
1040             // Underscore pattern.
1041             ExprKind::Underscore => {
1042                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1043             }
1044             // Slice patterns.
1045             ExprKind::Array(elements) => {
1046                 let (pats, rest) =
1047                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1048                 let slice_pat = if let Some((i, span)) = rest {
1049                     let (before, after) = pats.split_at(i);
1050                     hir::PatKind::Slice(
1051                         before,
1052                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1053                         after,
1054                     )
1055                 } else {
1056                     hir::PatKind::Slice(pats, None, &[])
1057                 };
1058                 return self.pat_without_dbm(lhs.span, slice_pat);
1059             }
1060             // Tuple structs.
1061             ExprKind::Call(callee, args) => {
1062                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1063                     let (pats, rest) = self.destructure_sequence(
1064                         args,
1065                         "tuple struct or variant",
1066                         eq_sign_span,
1067                         assignments,
1068                     );
1069                     let qpath = self.lower_qpath(
1070                         callee.id,
1071                         qself,
1072                         path,
1073                         ParamMode::Optional,
1074                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1075                     );
1076                     // Destructure like a tuple struct.
1077                     let tuple_struct_pat =
1078                         hir::PatKind::TupleStruct(qpath, pats, rest.map(|r| r.0));
1079                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1080                 }
1081             }
1082             // Structs.
1083             ExprKind::Struct(se) => {
1084                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1085                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1086                     hir::PatField {
1087                         hir_id: self.next_id(),
1088                         ident: self.lower_ident(f.ident),
1089                         pat,
1090                         is_shorthand: f.is_shorthand,
1091                         span: self.lower_span(f.span),
1092                     }
1093                 }));
1094                 let qpath = self.lower_qpath(
1095                     lhs.id,
1096                     &se.qself,
1097                     &se.path,
1098                     ParamMode::Optional,
1099                     ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1100                 );
1101                 let fields_omitted = match &se.rest {
1102                     StructRest::Base(e) => {
1103                         self.sess
1104                             .struct_span_err(
1105                                 e.span,
1106                                 "functional record updates are not allowed in destructuring \
1107                                     assignments",
1108                             )
1109                             .span_suggestion(
1110                                 e.span,
1111                                 "consider removing the trailing pattern",
1112                                 String::new(),
1113                                 rustc_errors::Applicability::MachineApplicable,
1114                             )
1115                             .emit();
1116                         true
1117                     }
1118                     StructRest::Rest(_) => true,
1119                     StructRest::None => false,
1120                 };
1121                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1122                 return self.pat_without_dbm(lhs.span, struct_pat);
1123             }
1124             // Tuples.
1125             ExprKind::Tup(elements) => {
1126                 let (pats, rest) =
1127                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1128                 let tuple_pat = hir::PatKind::Tuple(pats, rest.map(|r| r.0));
1129                 return self.pat_without_dbm(lhs.span, tuple_pat);
1130             }
1131             ExprKind::Paren(e) => {
1132                 // We special-case `(..)` for consistency with patterns.
1133                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1134                     let tuple_pat = hir::PatKind::Tuple(&[], Some(0));
1135                     return self.pat_without_dbm(lhs.span, tuple_pat);
1136                 } else {
1137                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1138                 }
1139             }
1140             _ => {}
1141         }
1142         // Treat all other cases as normal lvalue.
1143         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1144         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1145         let ident = self.expr_ident(lhs.span, ident, binding);
1146         let assign =
1147             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1148         let expr = self.expr(lhs.span, assign, ThinVec::new());
1149         assignments.push(self.stmt_expr(lhs.span, expr));
1150         pat
1151     }
1152
1153     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1154     /// Such a sequence occurs in a tuple (struct)/slice.
1155     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1156     /// exists.
1157     /// Each sub-assignment is recorded in `assignments`.
1158     fn destructure_sequence(
1159         &mut self,
1160         elements: &[AstP<Expr>],
1161         ctx: &str,
1162         eq_sign_span: Span,
1163         assignments: &mut Vec<hir::Stmt<'hir>>,
1164     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1165         let mut rest = None;
1166         let elements =
1167             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1168                 // Check for `..` pattern.
1169                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1170                     if let Some((_, prev_span)) = rest {
1171                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1172                     } else {
1173                         rest = Some((i, e.span));
1174                     }
1175                     None
1176                 } else {
1177                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1178                 }
1179             }));
1180         (elements, rest)
1181     }
1182
1183     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1184     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1185         let e1 = self.lower_expr_mut(e1);
1186         let e2 = self.lower_expr_mut(e2);
1187         let fn_path =
1188             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1189         let fn_expr =
1190             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
1191         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1192     }
1193
1194     fn lower_expr_range(
1195         &mut self,
1196         span: Span,
1197         e1: Option<&Expr>,
1198         e2: Option<&Expr>,
1199         lims: RangeLimits,
1200     ) -> hir::ExprKind<'hir> {
1201         use rustc_ast::RangeLimits::*;
1202
1203         let lang_item = match (e1, e2, lims) {
1204             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1205             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1206             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1207             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1208             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1209             (Some(..), Some(..), Closed) => unreachable!(),
1210             (_, None, Closed) => self.diagnostic().span_fatal(span, "inclusive range with no end"),
1211         };
1212
1213         let fields = self.arena.alloc_from_iter(
1214             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1215                 |(s, e)| {
1216                     let expr = self.lower_expr(&e);
1217                     let ident = Ident::new(s, self.lower_span(e.span));
1218                     self.expr_field(ident, expr, e.span)
1219                 },
1220             ),
1221         );
1222
1223         hir::ExprKind::Struct(
1224             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1225             fields,
1226             None,
1227         )
1228     }
1229
1230     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1231         let label = opt_label?;
1232         Some(Label { ident: self.lower_ident(label.ident) })
1233     }
1234
1235     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1236         let target_id = match destination {
1237             Some((id, _)) => {
1238                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1239                     Ok(self.lower_node_id(loop_id))
1240                 } else {
1241                     Err(hir::LoopIdError::UnresolvedLabel)
1242                 }
1243             }
1244             None => self
1245                 .loop_scope
1246                 .map(|id| Ok(self.lower_node_id(id)))
1247                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1248         };
1249         let label = self.lower_label(destination.map(|(_, label)| label));
1250         hir::Destination { label, target_id }
1251     }
1252
1253     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1254         if self.is_in_loop_condition && opt_label.is_none() {
1255             hir::Destination {
1256                 label: None,
1257                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1258             }
1259         } else {
1260             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1261         }
1262     }
1263
1264     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1265         let old_scope = self.catch_scope.replace(catch_id);
1266         let result = f(self);
1267         self.catch_scope = old_scope;
1268         result
1269     }
1270
1271     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1272         // We're no longer in the base loop's condition; we're in another loop.
1273         let was_in_loop_condition = self.is_in_loop_condition;
1274         self.is_in_loop_condition = false;
1275
1276         let old_scope = self.loop_scope.replace(loop_id);
1277         let result = f(self);
1278         self.loop_scope = old_scope;
1279
1280         self.is_in_loop_condition = was_in_loop_condition;
1281
1282         result
1283     }
1284
1285     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1286         let was_in_loop_condition = self.is_in_loop_condition;
1287         self.is_in_loop_condition = true;
1288
1289         let result = f(self);
1290
1291         self.is_in_loop_condition = was_in_loop_condition;
1292
1293         result
1294     }
1295
1296     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1297         hir::ExprField {
1298             hir_id: self.next_id(),
1299             ident: self.lower_ident(f.ident),
1300             expr: self.lower_expr(&f.expr),
1301             span: self.lower_span(f.span),
1302             is_shorthand: f.is_shorthand,
1303         }
1304     }
1305
1306     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1307         match self.generator_kind {
1308             Some(hir::GeneratorKind::Gen) => {}
1309             Some(hir::GeneratorKind::Async(_)) => {
1310                 struct_span_err!(
1311                     self.sess,
1312                     span,
1313                     E0727,
1314                     "`async` generators are not yet supported"
1315                 )
1316                 .emit();
1317             }
1318             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1319         }
1320
1321         let expr =
1322             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1323
1324         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1325     }
1326
1327     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1328     /// ```ignore (pseudo-rust)
1329     /// {
1330     ///     let result = match IntoIterator::into_iter(<head>) {
1331     ///         mut iter => {
1332     ///             [opt_ident]: loop {
1333     ///                 match Iterator::next(&mut iter) {
1334     ///                     None => break,
1335     ///                     Some(<pat>) => <body>,
1336     ///                 };
1337     ///             }
1338     ///         }
1339     ///     };
1340     ///     result
1341     /// }
1342     /// ```
1343     fn lower_expr_for(
1344         &mut self,
1345         e: &Expr,
1346         pat: &Pat,
1347         head: &Expr,
1348         body: &Block,
1349         opt_label: Option<Label>,
1350     ) -> hir::Expr<'hir> {
1351         let head = self.lower_expr_mut(head);
1352         let pat = self.lower_pat(pat);
1353         let for_span =
1354             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1355         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1356         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1357
1358         // `None => break`
1359         let none_arm = {
1360             let break_expr =
1361                 self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, ThinVec::new()));
1362             let pat = self.pat_none(for_span);
1363             self.arm(pat, break_expr)
1364         };
1365
1366         // Some(<pat>) => <body>,
1367         let some_arm = {
1368             let some_pat = self.pat_some(pat_span, pat);
1369             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1370             let body_expr = self.arena.alloc(self.expr_block(body_block, ThinVec::new()));
1371             self.arm(some_pat, body_expr)
1372         };
1373
1374         // `mut iter`
1375         let iter = Ident::with_dummy_span(sym::iter);
1376         let (iter_pat, iter_pat_nid) =
1377             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::Mutable);
1378
1379         // `match Iterator::next(&mut iter) { ... }`
1380         let match_expr = {
1381             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1382             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1383             let next_expr = self.expr_call_lang_item_fn(
1384                 head_span,
1385                 hir::LangItem::IteratorNext,
1386                 arena_vec![self; ref_mut_iter],
1387                 None,
1388             );
1389             let arms = arena_vec![self; none_arm, some_arm];
1390
1391             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1392         };
1393         let match_stmt = self.stmt_expr(for_span, match_expr);
1394
1395         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1396
1397         // `[opt_ident]: loop { ... }`
1398         let kind = hir::ExprKind::Loop(
1399             loop_block,
1400             self.lower_label(opt_label),
1401             hir::LoopSource::ForLoop,
1402             self.lower_span(for_span.with_hi(head.span.hi())),
1403         );
1404         let loop_expr =
1405             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1406
1407         // `mut iter => { ... }`
1408         let iter_arm = self.arm(iter_pat, loop_expr);
1409
1410         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1411         let into_iter_expr = {
1412             self.expr_call_lang_item_fn(
1413                 head_span,
1414                 hir::LangItem::IntoIterIntoIter,
1415                 arena_vec![self; head],
1416                 None,
1417             )
1418         };
1419
1420         let match_expr = self.arena.alloc(self.expr_match(
1421             for_span,
1422             into_iter_expr,
1423             arena_vec![self; iter_arm],
1424             hir::MatchSource::ForLoopDesugar,
1425         ));
1426
1427         let attrs: Vec<_> = e.attrs.iter().map(|a| self.lower_attr(a)).collect();
1428
1429         // This is effectively `{ let _result = ...; _result }`.
1430         // The construct was introduced in #21984 and is necessary to make sure that
1431         // temporaries in the `head` expression are dropped and do not leak to the
1432         // surrounding scope of the `match` since the `match` is not a terminating scope.
1433         //
1434         // Also, add the attributes to the outer returned expr node.
1435         self.expr_drop_temps_mut(for_span, match_expr, attrs.into())
1436     }
1437
1438     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1439     /// ```ignore (pseudo-rust)
1440     /// match Try::branch(<expr>) {
1441     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1442     ///     ControlFlow::Break(residual) =>
1443     ///         #[allow(unreachable_code)]
1444     ///         // If there is an enclosing `try {...}`:
1445     ///         break 'catch_target Try::from_residual(residual),
1446     ///         // Otherwise:
1447     ///         return Try::from_residual(residual),
1448     /// }
1449     /// ```
1450     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1451         let unstable_span = self.mark_span_with_reason(
1452             DesugaringKind::QuestionMark,
1453             span,
1454             self.allow_try_trait.clone(),
1455         );
1456         let try_span = self.sess.source_map().end_point(span);
1457         let try_span = self.mark_span_with_reason(
1458             DesugaringKind::QuestionMark,
1459             try_span,
1460             self.allow_try_trait.clone(),
1461         );
1462
1463         // `Try::branch(<expr>)`
1464         let scrutinee = {
1465             // expand <expr>
1466             let sub_expr = self.lower_expr_mut(sub_expr);
1467
1468             self.expr_call_lang_item_fn(
1469                 unstable_span,
1470                 hir::LangItem::TryTraitBranch,
1471                 arena_vec![self; sub_expr],
1472                 None,
1473             )
1474         };
1475
1476         // `#[allow(unreachable_code)]`
1477         let attr = {
1478             // `allow(unreachable_code)`
1479             let allow = {
1480                 let allow_ident = Ident::new(sym::allow, self.lower_span(span));
1481                 let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
1482                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1483                 attr::mk_list_item(allow_ident, vec![uc_nested])
1484             };
1485             attr::mk_attr_outer(allow)
1486         };
1487         let attrs = vec![attr];
1488
1489         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1490         let continue_arm = {
1491             let val_ident = Ident::with_dummy_span(sym::val);
1492             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1493             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1494                 span,
1495                 val_ident,
1496                 val_pat_nid,
1497                 ThinVec::from(attrs.clone()),
1498             ));
1499             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1500             self.arm(continue_pat, val_expr)
1501         };
1502
1503         // `ControlFlow::Break(residual) =>
1504         //     #[allow(unreachable_code)]
1505         //     return Try::from_residual(residual),`
1506         let break_arm = {
1507             let residual_ident = Ident::with_dummy_span(sym::residual);
1508             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1509             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1510             let from_residual_expr = self.wrap_in_try_constructor(
1511                 hir::LangItem::TryTraitFromResidual,
1512                 try_span,
1513                 self.arena.alloc(residual_expr),
1514                 unstable_span,
1515             );
1516             let thin_attrs = ThinVec::from(attrs);
1517             let ret_expr = if let Some(catch_node) = self.catch_scope {
1518                 let target_id = Ok(self.lower_node_id(catch_node));
1519                 self.arena.alloc(self.expr(
1520                     try_span,
1521                     hir::ExprKind::Break(
1522                         hir::Destination { label: None, target_id },
1523                         Some(from_residual_expr),
1524                     ),
1525                     thin_attrs,
1526                 ))
1527             } else {
1528                 self.arena.alloc(self.expr(
1529                     try_span,
1530                     hir::ExprKind::Ret(Some(from_residual_expr)),
1531                     thin_attrs,
1532                 ))
1533             };
1534
1535             let break_pat = self.pat_cf_break(try_span, residual_local);
1536             self.arm(break_pat, ret_expr)
1537         };
1538
1539         hir::ExprKind::Match(
1540             scrutinee,
1541             arena_vec![self; break_arm, continue_arm],
1542             hir::MatchSource::TryDesugar,
1543         )
1544     }
1545
1546     // =========================================================================
1547     // Helper methods for building HIR.
1548     // =========================================================================
1549
1550     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1551     ///
1552     /// In terms of drop order, it has the same effect as wrapping `expr` in
1553     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1554     ///
1555     /// The drop order can be important in e.g. `if expr { .. }`.
1556     pub(super) fn expr_drop_temps(
1557         &mut self,
1558         span: Span,
1559         expr: &'hir hir::Expr<'hir>,
1560         attrs: AttrVec,
1561     ) -> &'hir hir::Expr<'hir> {
1562         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1563     }
1564
1565     pub(super) fn expr_drop_temps_mut(
1566         &mut self,
1567         span: Span,
1568         expr: &'hir hir::Expr<'hir>,
1569         attrs: AttrVec,
1570     ) -> hir::Expr<'hir> {
1571         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1572     }
1573
1574     fn expr_match(
1575         &mut self,
1576         span: Span,
1577         arg: &'hir hir::Expr<'hir>,
1578         arms: &'hir [hir::Arm<'hir>],
1579         source: hir::MatchSource,
1580     ) -> hir::Expr<'hir> {
1581         self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
1582     }
1583
1584     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
1585         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1586         self.expr(span, expr_break, attrs)
1587     }
1588
1589     fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1590         let expr_break = self.expr_break(span, attrs);
1591         self.arena.alloc(expr_break)
1592     }
1593
1594     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1595         self.expr(
1596             span,
1597             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1598             ThinVec::new(),
1599         )
1600     }
1601
1602     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1603         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
1604     }
1605
1606     fn expr_call_mut(
1607         &mut self,
1608         span: Span,
1609         e: &'hir hir::Expr<'hir>,
1610         args: &'hir [hir::Expr<'hir>],
1611     ) -> hir::Expr<'hir> {
1612         self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new())
1613     }
1614
1615     fn expr_call(
1616         &mut self,
1617         span: Span,
1618         e: &'hir hir::Expr<'hir>,
1619         args: &'hir [hir::Expr<'hir>],
1620     ) -> &'hir hir::Expr<'hir> {
1621         self.arena.alloc(self.expr_call_mut(span, e, args))
1622     }
1623
1624     fn expr_call_lang_item_fn_mut(
1625         &mut self,
1626         span: Span,
1627         lang_item: hir::LangItem,
1628         args: &'hir [hir::Expr<'hir>],
1629         hir_id: Option<hir::HirId>,
1630     ) -> hir::Expr<'hir> {
1631         let path =
1632             self.arena.alloc(self.expr_lang_item_path(span, lang_item, ThinVec::new(), hir_id));
1633         self.expr_call_mut(span, path, args)
1634     }
1635
1636     fn expr_call_lang_item_fn(
1637         &mut self,
1638         span: Span,
1639         lang_item: hir::LangItem,
1640         args: &'hir [hir::Expr<'hir>],
1641         hir_id: Option<hir::HirId>,
1642     ) -> &'hir hir::Expr<'hir> {
1643         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1644     }
1645
1646     fn expr_lang_item_path(
1647         &mut self,
1648         span: Span,
1649         lang_item: hir::LangItem,
1650         attrs: AttrVec,
1651         hir_id: Option<hir::HirId>,
1652     ) -> hir::Expr<'hir> {
1653         self.expr(
1654             span,
1655             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1656             attrs,
1657         )
1658     }
1659
1660     pub(super) fn expr_ident(
1661         &mut self,
1662         sp: Span,
1663         ident: Ident,
1664         binding: hir::HirId,
1665     ) -> &'hir hir::Expr<'hir> {
1666         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1667     }
1668
1669     pub(super) fn expr_ident_mut(
1670         &mut self,
1671         sp: Span,
1672         ident: Ident,
1673         binding: hir::HirId,
1674     ) -> hir::Expr<'hir> {
1675         self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
1676     }
1677
1678     fn expr_ident_with_attrs(
1679         &mut self,
1680         span: Span,
1681         ident: Ident,
1682         binding: hir::HirId,
1683         attrs: AttrVec,
1684     ) -> hir::Expr<'hir> {
1685         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1686             None,
1687             self.arena.alloc(hir::Path {
1688                 span: self.lower_span(span),
1689                 res: Res::Local(binding),
1690                 segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
1691             }),
1692         ));
1693
1694         self.expr(span, expr_path, attrs)
1695     }
1696
1697     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1698         let hir_id = self.next_id();
1699         let span = expr.span;
1700         self.expr(
1701             span,
1702             hir::ExprKind::Block(
1703                 self.arena.alloc(hir::Block {
1704                     stmts: &[],
1705                     expr: Some(expr),
1706                     hir_id,
1707                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1708                     span: self.lower_span(span),
1709                     targeted_by_break: false,
1710                 }),
1711                 None,
1712             ),
1713             ThinVec::new(),
1714         )
1715     }
1716
1717     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1718         let blk = self.block_all(span, &[], None);
1719         let expr = self.expr_block(blk, ThinVec::new());
1720         self.arena.alloc(expr)
1721     }
1722
1723     pub(super) fn expr_block(
1724         &mut self,
1725         b: &'hir hir::Block<'hir>,
1726         attrs: AttrVec,
1727     ) -> hir::Expr<'hir> {
1728         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1729     }
1730
1731     pub(super) fn expr(
1732         &mut self,
1733         span: Span,
1734         kind: hir::ExprKind<'hir>,
1735         attrs: AttrVec,
1736     ) -> hir::Expr<'hir> {
1737         let hir_id = self.next_id();
1738         self.lower_attrs(hir_id, &attrs);
1739         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1740     }
1741
1742     fn expr_field(
1743         &mut self,
1744         ident: Ident,
1745         expr: &'hir hir::Expr<'hir>,
1746         span: Span,
1747     ) -> hir::ExprField<'hir> {
1748         hir::ExprField {
1749             hir_id: self.next_id(),
1750             ident,
1751             span: self.lower_span(span),
1752             expr,
1753             is_shorthand: false,
1754         }
1755     }
1756
1757     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1758         hir::Arm {
1759             hir_id: self.next_id(),
1760             pat,
1761             guard: None,
1762             span: self.lower_span(expr.span),
1763             body: expr,
1764         }
1765     }
1766 }