compiler/rustc_ast_lowering/src/expr.rs

   1 use super::ResolverAstLoweringExt;
   2 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
   3 use crate::{FnDeclKind, ImplTraitPosition};
   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::source_map::{respan, DesugaringKind, Span, Spanned};
  15 use rustc_span::symbol::{sym, Ident};
  16 use rustc_span::DUMMY_SP;
  17
  18 impl<'hir> LoweringContext<'_, 'hir> {
  19     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  20         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  21     }
  22
  23     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  24         self.arena.alloc(self.lower_expr_mut(e))
  25     }
  26
  27     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  28         ensure_sufficient_stack(|| {
  29             let kind = match e.kind {
  30                 ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  31                 ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  32                 ExprKind::ConstBlock(ref anon_const) => {
  33                     let anon_const = self.lower_anon_const(anon_const);
  34                     hir::ExprKind::ConstBlock(anon_const)
  35                 }
  36                 ExprKind::Repeat(ref expr, ref count) => {
  37                     let expr = self.lower_expr(expr);
  38                     let count = self.lower_array_length(count);
  39                     hir::ExprKind::Repeat(expr, count)
  40                 }
  41                 ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  42                 ExprKind::Call(ref f, ref args) => {
  43                     if e.attrs.get(0).map_or(false, |a| a.has_name(sym::rustc_box)) {
  44                         if let [inner] = &args[..] && e.attrs.len() == 1 {
  45                             let kind = hir::ExprKind::Box(self.lower_expr(&inner));
  46                             let hir_id = self.lower_node_id(e.id);
  47                             return hir::Expr { hir_id, kind, span: self.lower_span(e.span) };
  48                         } else {
  49                             self.tcx.sess
  50                                 .struct_span_err(
  51                                     e.span,
  52                                     "#[rustc_box] requires precisely one argument \
  53                                     and no other attributes are allowed",
  54                                 )
  55                                 .emit();
  56                             hir::ExprKind::Err
  57                         }
  58                     } else if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
  59                         self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
  60                     } else {
  61                         let f = self.lower_expr(f);
  62                         hir::ExprKind::Call(f, self.lower_exprs(args))
  63                     }
  64                 }
  65                 ExprKind::MethodCall(ref seg, ref args, span) => {
  66                     let hir_seg = self.arena.alloc(self.lower_path_segment(
  67                         e.span,
  68                         seg,
  69                         ParamMode::Optional,
  70                         ParenthesizedGenericArgs::Err,
  71                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  72                     ));
  73                     let args = self.lower_exprs(args);
  74                     hir::ExprKind::MethodCall(hir_seg, args, self.lower_span(span))
  75                 }
  76                 ExprKind::Binary(binop, ref lhs, ref rhs) => {
  77                     let binop = self.lower_binop(binop);
  78                     let lhs = self.lower_expr(lhs);
  79                     let rhs = self.lower_expr(rhs);
  80                     hir::ExprKind::Binary(binop, lhs, rhs)
  81                 }
  82                 ExprKind::Unary(op, ref ohs) => {
  83                     let op = self.lower_unop(op);
  84                     let ohs = self.lower_expr(ohs);
  85                     hir::ExprKind::Unary(op, ohs)
  86                 }
  87                 ExprKind::Lit(ref l) => {
  88                     hir::ExprKind::Lit(respan(self.lower_span(l.span), l.kind.clone()))
  89                 }
  90                 ExprKind::Cast(ref expr, ref ty) => {
  91                     let expr = self.lower_expr(expr);
  92                     let ty =
  93                         self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
  94                     hir::ExprKind::Cast(expr, ty)
  95                 }
  96                 ExprKind::Type(ref expr, ref ty) => {
  97                     let expr = self.lower_expr(expr);
  98                     let ty =
  99                         self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 100                     hir::ExprKind::Type(expr, ty)
 101                 }
 102                 ExprKind::AddrOf(k, m, ref ohs) => {
 103                     let ohs = self.lower_expr(ohs);
 104                     hir::ExprKind::AddrOf(k, m, ohs)
 105                 }
 106                 ExprKind::Let(ref pat, ref scrutinee, span) => {
 107                     hir::ExprKind::Let(self.arena.alloc(hir::Let {
 108                         hir_id: self.next_id(),
 109                         span: self.lower_span(span),
 110                         pat: self.lower_pat(pat),
 111                         ty: None,
 112                         init: self.lower_expr(scrutinee),
 113                     }))
 114                 }
 115                 ExprKind::If(ref cond, ref then, ref else_opt) => {
 116                     self.lower_expr_if(cond, then, else_opt.as_deref())
 117                 }
 118                 ExprKind::While(ref cond, ref body, opt_label) => {
 119                     self.with_loop_scope(e.id, |this| {
 120                         let span =
 121                             this.mark_span_with_reason(DesugaringKind::WhileLoop, e.span, None);
 122                         this.lower_expr_while_in_loop_scope(span, cond, body, opt_label)
 123                     })
 124                 }
 125                 ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
 126                     hir::ExprKind::Loop(
 127                         this.lower_block(body, false),
 128                         this.lower_label(opt_label),
 129                         hir::LoopSource::Loop,
 130                         DUMMY_SP,
 131                     )
 132                 }),
 133                 ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
 134                 ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
 135                     self.lower_expr(expr),
 136                     self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
 137                     hir::MatchSource::Normal,
 138                 ),
 139                 ExprKind::Async(capture_clause, closure_node_id, ref block) => self
 140                     .make_async_expr(
 141                         capture_clause,
 142                         closure_node_id,
 143                         None,
 144                         block.span,
 145                         hir::AsyncGeneratorKind::Block,
 146                         |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
 147                     ),
 148                 ExprKind::Await(ref expr) => {
 149                     let span = if expr.span.hi() < e.span.hi() {
 150                         expr.span.shrink_to_hi().with_hi(e.span.hi())
 151                     } else {
 152                         // this is a recovered `await expr`
 153                         e.span
 154                     };
 155                     self.lower_expr_await(span, expr)
 156                 }
 157                 ExprKind::Closure(
 158                     capture_clause,
 159                     asyncness,
 160                     movability,
 161                     ref decl,
 162                     ref body,
 163                     fn_decl_span,
 164                 ) => {
 165                     if let Async::Yes { closure_id, .. } = asyncness {
 166                         self.lower_expr_async_closure(
 167                             capture_clause,
 168                             e.id,
 169                             closure_id,
 170                             decl,
 171                             body,
 172                             fn_decl_span,
 173                         )
 174                     } else {
 175                         self.lower_expr_closure(
 176                             capture_clause,
 177                             e.id,
 178                             movability,
 179                             decl,
 180                             body,
 181                             fn_decl_span,
 182                         )
 183                     }
 184                 }
 185                 ExprKind::Block(ref blk, opt_label) => {
 186                     let opt_label = self.lower_label(opt_label);
 187                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 188                 }
 189                 ExprKind::Assign(ref el, ref er, span) => {
 190                     self.lower_expr_assign(el, er, span, e.span)
 191                 }
 192                 ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 193                     self.lower_binop(op),
 194                     self.lower_expr(el),
 195                     self.lower_expr(er),
 196                 ),
 197                 ExprKind::Field(ref el, ident) => {
 198                     hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(ident))
 199                 }
 200                 ExprKind::Index(ref el, ref er) => {
 201                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 202                 }
 203                 ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 204                     self.lower_expr_range_closed(e.span, e1, e2)
 205                 }
 206                 ExprKind::Range(ref e1, ref e2, lims) => {
 207                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 208                 }
 209                 ExprKind::Underscore => {
 210                     self.tcx
 211                         .sess.struct_span_err(
 212                             e.span,
 213                             "in expressions, `_` can only be used on the left-hand side of an assignment",
 214                         )
 215                         .span_label(e.span, "`_` not allowed here")
 216                         .emit();
 217                     hir::ExprKind::Err
 218                 }
 219                 ExprKind::Path(ref qself, ref path) => {
 220                     let qpath = self.lower_qpath(
 221                         e.id,
 222                         qself,
 223                         path,
 224                         ParamMode::Optional,
 225                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 226                     );
 227                     hir::ExprKind::Path(qpath)
 228                 }
 229                 ExprKind::Break(opt_label, ref opt_expr) => {
 230                     let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 231                     hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
 232                 }
 233                 ExprKind::Continue(opt_label) => {
 234                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
 235                 }
 236                 ExprKind::Ret(ref e) => {
 237                     let e = e.as_ref().map(|x| self.lower_expr(x));
 238                     hir::ExprKind::Ret(e)
 239                 }
 240                 ExprKind::Yeet(ref sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
 241                 ExprKind::InlineAsm(ref asm) => {
 242                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
 243                 }
 244                 ExprKind::Struct(ref se) => {
 245                     let rest = match &se.rest {
 246                         StructRest::Base(e) => Some(self.lower_expr(e)),
 247                         StructRest::Rest(sp) => {
 248                             self.tcx
 249                                 .sess
 250                                 .struct_span_err(*sp, "base expression required after `..`")
 251                                 .span_label(*sp, "add a base expression here")
 252                                 .emit();
 253                             Some(&*self.arena.alloc(self.expr_err(*sp)))
 254                         }
 255                         StructRest::None => None,
 256                     };
 257                     hir::ExprKind::Struct(
 258                         self.arena.alloc(self.lower_qpath(
 259                             e.id,
 260                             &se.qself,
 261                             &se.path,
 262                             ParamMode::Optional,
 263                             ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 264                         )),
 265                         self.arena
 266                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
 267                         rest,
 268                     )
 269                 }
 270                 ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 271                 ExprKind::Err => hir::ExprKind::Err,
 272                 ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 273                 ExprKind::Paren(ref ex) => {
 274                     let mut ex = self.lower_expr_mut(ex);
 275                     // Include parens in span, but only if it is a super-span.
 276                     if e.span.contains(ex.span) {
 277                         ex.span = self.lower_span(e.span);
 278                     }
 279                     // Merge attributes into the inner expression.
 280                     if !e.attrs.is_empty() {
 281                         let old_attrs =
 282                             self.attrs.get(&ex.hir_id.local_id).map(|la| *la).unwrap_or(&[]);
 283                         self.attrs.insert(
 284                             ex.hir_id.local_id,
 285                             &*self.arena.alloc_from_iter(
 286                                 e.attrs
 287                                     .iter()
 288                                     .map(|a| self.lower_attr(a))
 289                                     .chain(old_attrs.iter().cloned()),
 290                             ),
 291                         );
 292                     }
 293                     return ex;
 294                 }
 295
 296                 // Desugar `ExprForLoop`
 297                 // from: `[opt_ident]: for <pat> in <head> <body>`
 298                 ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 299                     return self.lower_expr_for(e, pat, head, body, opt_label);
 300                 }
 301                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 302             };
 303
 304             let hir_id = self.lower_node_id(e.id);
 305             self.lower_attrs(hir_id, &e.attrs);
 306             hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
 307         })
 308     }
 309
 310     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 311         match u {
 312             UnOp::Deref => hir::UnOp::Deref,
 313             UnOp::Not => hir::UnOp::Not,
 314             UnOp::Neg => hir::UnOp::Neg,
 315         }
 316     }
 317
 318     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 319         Spanned {
 320             node: match b.node {
 321                 BinOpKind::Add => hir::BinOpKind::Add,
 322                 BinOpKind::Sub => hir::BinOpKind::Sub,
 323                 BinOpKind::Mul => hir::BinOpKind::Mul,
 324                 BinOpKind::Div => hir::BinOpKind::Div,
 325                 BinOpKind::Rem => hir::BinOpKind::Rem,
 326                 BinOpKind::And => hir::BinOpKind::And,
 327                 BinOpKind::Or => hir::BinOpKind::Or,
 328                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 329                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 330                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 331                 BinOpKind::Shl => hir::BinOpKind::Shl,
 332                 BinOpKind::Shr => hir::BinOpKind::Shr,
 333                 BinOpKind::Eq => hir::BinOpKind::Eq,
 334                 BinOpKind::Lt => hir::BinOpKind::Lt,
 335                 BinOpKind::Le => hir::BinOpKind::Le,
 336                 BinOpKind::Ne => hir::BinOpKind::Ne,
 337                 BinOpKind::Ge => hir::BinOpKind::Ge,
 338                 BinOpKind::Gt => hir::BinOpKind::Gt,
 339             },
 340             span: self.lower_span(b.span),
 341         }
 342     }
 343
 344     fn lower_legacy_const_generics(
 345         &mut self,
 346         mut f: Expr,
 347         args: Vec<AstP<Expr>>,
 348         legacy_args_idx: &[usize],
 349     ) -> hir::ExprKind<'hir> {
 350         let ExprKind::Path(None, ref mut path) = f.kind else {
 351             unreachable!();
 352         };
 353
 354         // Split the arguments into const generics and normal arguments
 355         let mut real_args = vec![];
 356         let mut generic_args = vec![];
 357         for (idx, arg) in args.into_iter().enumerate() {
 358             if legacy_args_idx.contains(&idx) {
 359                 let parent_def_id = self.current_hir_id_owner;
 360                 let node_id = self.next_node_id();
 361
 362                 // Add a definition for the in-band const def.
 363                 self.create_def(parent_def_id, node_id, DefPathData::AnonConst);
 364
 365                 let anon_const = AnonConst { id: node_id, value: arg };
 366                 generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
 367             } else {
 368                 real_args.push(arg);
 369             }
 370         }
 371
 372         // Add generic args to the last element of the path.
 373         let last_segment = path.segments.last_mut().unwrap();
 374         assert!(last_segment.args.is_none());
 375         last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
 376             span: DUMMY_SP,
 377             args: generic_args,
 378         })));
 379
 380         // Now lower everything as normal.
 381         let f = self.lower_expr(&f);
 382         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
 383     }
 384
 385     fn lower_expr_if(
 386         &mut self,
 387         cond: &Expr,
 388         then: &Block,
 389         else_opt: Option<&Expr>,
 390     ) -> hir::ExprKind<'hir> {
 391         let lowered_cond = self.lower_expr(cond);
 392         let new_cond = self.manage_let_cond(lowered_cond);
 393         let then_expr = self.lower_block_expr(then);
 394         if let Some(rslt) = else_opt {
 395             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), Some(self.lower_expr(rslt)))
 396         } else {
 397             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), None)
 398         }
 399     }
 400
 401     // If `cond` kind is `let`, returns `let`. Otherwise, wraps and returns `cond`
 402     // in a temporary block.
 403     fn manage_let_cond(&mut self, cond: &'hir hir::Expr<'hir>) -> &'hir hir::Expr<'hir> {
 404         fn has_let_expr<'hir>(expr: &'hir hir::Expr<'hir>) -> bool {
 405             match expr.kind {
 406                 hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
 407                 hir::ExprKind::Let(..) => true,
 408                 _ => false,
 409             }
 410         }
 411         if has_let_expr(cond) {
 412             cond
 413         } else {
 414             let reason = DesugaringKind::CondTemporary;
 415             let span_block = self.mark_span_with_reason(reason, cond.span, None);
 416             self.expr_drop_temps(span_block, cond, AttrVec::new())
 417         }
 418     }
 419
 420     // We desugar: `'label: while $cond $body` into:
 421     //
 422     // ```
 423     // 'label: loop {
 424     //   if { let _t = $cond; _t } {
 425     //     $body
 426     //   }
 427     //   else {
 428     //     break;
 429     //   }
 430     // }
 431     // ```
 432     //
 433     // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 434     // to preserve drop semantics since `while $cond { ... }` does not
 435     // let temporaries live outside of `cond`.
 436     fn lower_expr_while_in_loop_scope(
 437         &mut self,
 438         span: Span,
 439         cond: &Expr,
 440         body: &Block,
 441         opt_label: Option<Label>,
 442     ) -> hir::ExprKind<'hir> {
 443         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
 444         let new_cond = self.manage_let_cond(lowered_cond);
 445         let then = self.lower_block_expr(body);
 446         let expr_break = self.expr_break(span, ThinVec::new());
 447         let stmt_break = self.stmt_expr(span, expr_break);
 448         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
 449         let else_expr = self.arena.alloc(self.expr_block(else_blk, ThinVec::new()));
 450         let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
 451         let if_expr = self.expr(span, if_kind, ThinVec::new());
 452         let block = self.block_expr(self.arena.alloc(if_expr));
 453         let span = self.lower_span(span.with_hi(cond.span.hi()));
 454         let opt_label = self.lower_label(opt_label);
 455         hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
 456     }
 457
 458     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
 459     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
 460     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 461     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 462         self.with_catch_scope(body.id, |this| {
 463             let mut block = this.lower_block_noalloc(body, true);
 464
 465             // Final expression of the block (if present) or `()` with span at the end of block
 466             let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
 467                 (
 468                     this.mark_span_with_reason(
 469                         DesugaringKind::TryBlock,
 470                         expr.span,
 471                         this.allow_try_trait.clone(),
 472                     ),
 473                     expr,
 474                 )
 475             } else {
 476                 let try_span = this.mark_span_with_reason(
 477                     DesugaringKind::TryBlock,
 478                     this.tcx.sess.source_map().end_point(body.span),
 479                     this.allow_try_trait.clone(),
 480                 );
 481
 482                 (try_span, this.expr_unit(try_span))
 483             };
 484
 485             let ok_wrapped_span =
 486                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 487
 488             // `::std::ops::Try::from_output($tail_expr)`
 489             block.expr = Some(this.wrap_in_try_constructor(
 490                 hir::LangItem::TryTraitFromOutput,
 491                 try_span,
 492                 tail_expr,
 493                 ok_wrapped_span,
 494             ));
 495
 496             hir::ExprKind::Block(this.arena.alloc(block), None)
 497         })
 498     }
 499
 500     fn wrap_in_try_constructor(
 501         &mut self,
 502         lang_item: hir::LangItem,
 503         method_span: Span,
 504         expr: &'hir hir::Expr<'hir>,
 505         overall_span: Span,
 506     ) -> &'hir hir::Expr<'hir> {
 507         let constructor = self.arena.alloc(self.expr_lang_item_path(
 508             method_span,
 509             lang_item,
 510             ThinVec::new(),
 511             None,
 512         ));
 513         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 514     }
 515
 516     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 517         let pat = self.lower_pat(&arm.pat);
 518         let guard = arm.guard.as_ref().map(|cond| {
 519             if let ExprKind::Let(ref pat, ref scrutinee, span) = cond.kind {
 520                 hir::Guard::IfLet(self.arena.alloc(hir::Let {
 521                     hir_id: self.next_id(),
 522                     span: self.lower_span(span),
 523                     pat: self.lower_pat(pat),
 524                     ty: None,
 525                     init: self.lower_expr(scrutinee),
 526                 }))
 527             } else {
 528                 hir::Guard::If(self.lower_expr(cond))
 529             }
 530         });
 531         let hir_id = self.next_id();
 532         self.lower_attrs(hir_id, &arm.attrs);
 533         hir::Arm {
 534             hir_id,
 535             pat,
 536             guard,
 537             body: self.lower_expr(&arm.body),
 538             span: self.lower_span(arm.span),
 539         }
 540     }
 541
 542     /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
 543     ///
 544     /// This results in:
 545     ///
 546     /// ```text
 547     /// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
 548     ///     <body>
 549     /// })
 550     /// ```
 551     pub(super) fn make_async_expr(
 552         &mut self,
 553         capture_clause: CaptureBy,
 554         closure_node_id: NodeId,
 555         ret_ty: Option<AstP<Ty>>,
 556         span: Span,
 557         async_gen_kind: hir::AsyncGeneratorKind,
 558         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 559     ) -> hir::ExprKind<'hir> {
 560         let output = match ret_ty {
 561             Some(ty) => hir::FnRetTy::Return(
 562                 self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock)),
 563             ),
 564             None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
 565         };
 566
 567         // Resume argument type. We let the compiler infer this to simplify the lowering. It is
 568         // fully constrained by `future::from_generator`.
 569         let input_ty = hir::Ty {
 570             hir_id: self.next_id(),
 571             kind: hir::TyKind::Infer,
 572             span: self.lower_span(span),
 573         };
 574
 575         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 576         let fn_decl = self.arena.alloc(hir::FnDecl {
 577             inputs: arena_vec![self; input_ty],
 578             output,
 579             c_variadic: false,
 580             implicit_self: hir::ImplicitSelfKind::None,
 581         });
 582
 583         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 584         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 585             span,
 586             Ident::with_dummy_span(sym::_task_context),
 587             hir::BindingAnnotation::Mutable,
 588         );
 589         let param = hir::Param {
 590             hir_id: self.next_id(),
 591             pat,
 592             ty_span: self.lower_span(span),
 593             span: self.lower_span(span),
 594         };
 595         let params = arena_vec![self; param];
 596
 597         let body = self.lower_body(move |this| {
 598             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 599
 600             let old_ctx = this.task_context;
 601             this.task_context = Some(task_context_hid);
 602             let res = body(this);
 603             this.task_context = old_ctx;
 604             (params, res)
 605         });
 606
 607         // `static |_task_context| -> <ret_ty> { body }`:
 608         let generator_kind = hir::ExprKind::Closure {
 609             capture_clause,
 610             bound_generic_params: &[],
 611             fn_decl,
 612             body,
 613             fn_decl_span: self.lower_span(span),
 614             movability: Some(hir::Movability::Static),
 615         };
 616         let generator = hir::Expr {
 617             hir_id: self.lower_node_id(closure_node_id),
 618             kind: generator_kind,
 619             span: self.lower_span(span),
 620         };
 621
 622         // `future::from_generator`:
 623         let unstable_span =
 624             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 625         let gen_future = self.expr_lang_item_path(
 626             unstable_span,
 627             hir::LangItem::FromGenerator,
 628             ThinVec::new(),
 629             None,
 630         );
 631
 632         // `future::from_generator(generator)`:
 633         hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
 634     }
 635
 636     /// Desugar `<expr>.await` into:
 637     /// ```ignore (pseudo-rust)
 638     /// match ::std::future::IntoFuture::into_future(<expr>) {
 639     ///     mut __awaitee => loop {
 640     ///         match unsafe { ::std::future::Future::poll(
 641     ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
 642     ///             ::std::future::get_context(task_context),
 643     ///         ) } {
 644     ///             ::std::task::Poll::Ready(result) => break result,
 645     ///             ::std::task::Poll::Pending => {}
 646     ///         }
 647     ///         task_context = yield ();
 648     ///     }
 649     /// }
 650     /// ```
 651     fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 652         let full_span = expr.span.to(dot_await_span);
 653         match self.generator_kind {
 654             Some(hir::GeneratorKind::Async(_)) => {}
 655             Some(hir::GeneratorKind::Gen) | None => {
 656                 let mut err = struct_span_err!(
 657                     self.tcx.sess,
 658                     dot_await_span,
 659                     E0728,
 660                     "`await` is only allowed inside `async` functions and blocks"
 661                 );
 662                 err.span_label(dot_await_span, "only allowed inside `async` functions and blocks");
 663                 if let Some(item_sp) = self.current_item {
 664                     err.span_label(item_sp, "this is not `async`");
 665                 }
 666                 err.emit();
 667             }
 668         }
 669         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
 670         let gen_future_span = self.mark_span_with_reason(
 671             DesugaringKind::Await,
 672             full_span,
 673             self.allow_gen_future.clone(),
 674         );
 675         let expr = self.lower_expr_mut(expr);
 676         let expr_hir_id = expr.hir_id;
 677
 678         // Note that the name of this binding must not be changed to something else because
 679         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
 680         // this name to identify what is being awaited by a suspended async functions.
 681         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
 682         let (awaitee_pat, awaitee_pat_hid) =
 683             self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::Mutable);
 684
 685         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 686
 687         // unsafe {
 688         //     ::std::future::Future::poll(
 689         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
 690         //         ::std::future::get_context(task_context),
 691         //     )
 692         // }
 693         let poll_expr = {
 694             let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
 695             let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
 696             let task_context = if let Some(task_context_hid) = self.task_context {
 697                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 698             } else {
 699                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 700                 self.expr_err(span)
 701             };
 702             let new_unchecked = self.expr_call_lang_item_fn_mut(
 703                 span,
 704                 hir::LangItem::PinNewUnchecked,
 705                 arena_vec![self; ref_mut_awaitee],
 706                 Some(expr_hir_id),
 707             );
 708             let get_context = self.expr_call_lang_item_fn_mut(
 709                 gen_future_span,
 710                 hir::LangItem::GetContext,
 711                 arena_vec![self; task_context],
 712                 Some(expr_hir_id),
 713             );
 714             let call = self.expr_call_lang_item_fn(
 715                 span,
 716                 hir::LangItem::FuturePoll,
 717                 arena_vec![self; new_unchecked, get_context],
 718                 Some(expr_hir_id),
 719             );
 720             self.arena.alloc(self.expr_unsafe(call))
 721         };
 722
 723         // `::std::task::Poll::Ready(result) => break result`
 724         let loop_node_id = self.next_node_id();
 725         let loop_hir_id = self.lower_node_id(loop_node_id);
 726         let ready_arm = {
 727             let x_ident = Ident::with_dummy_span(sym::result);
 728             let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
 729             let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
 730             let ready_field = self.single_pat_field(gen_future_span, x_pat);
 731             let ready_pat = self.pat_lang_item_variant(
 732                 span,
 733                 hir::LangItem::PollReady,
 734                 ready_field,
 735                 Some(expr_hir_id),
 736             );
 737             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 738                 let expr_break =
 739                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 740                 this.arena.alloc(this.expr(gen_future_span, expr_break, ThinVec::new()))
 741             });
 742             self.arm(ready_pat, break_x)
 743         };
 744
 745         // `::std::task::Poll::Pending => {}`
 746         let pending_arm = {
 747             let pending_pat = self.pat_lang_item_variant(
 748                 span,
 749                 hir::LangItem::PollPending,
 750                 &[],
 751                 Some(expr_hir_id),
 752             );
 753             let empty_block = self.expr_block_empty(span);
 754             self.arm(pending_pat, empty_block)
 755         };
 756
 757         let inner_match_stmt = {
 758             let match_expr = self.expr_match(
 759                 span,
 760                 poll_expr,
 761                 arena_vec![self; ready_arm, pending_arm],
 762                 hir::MatchSource::AwaitDesugar,
 763             );
 764             self.stmt_expr(span, match_expr)
 765         };
 766
 767         // task_context = yield ();
 768         let yield_stmt = {
 769             let unit = self.expr_unit(span);
 770             let yield_expr = self.expr(
 771                 span,
 772                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
 773                 ThinVec::new(),
 774             );
 775             let yield_expr = self.arena.alloc(yield_expr);
 776
 777             if let Some(task_context_hid) = self.task_context {
 778                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 779                 let assign = self.expr(
 780                     span,
 781                     hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)),
 782                     AttrVec::new(),
 783                 );
 784                 self.stmt_expr(span, assign)
 785             } else {
 786                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 787                 // proceed with type checking.
 788                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 789             }
 790         };
 791
 792         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 793
 794         // loop { .. }
 795         let loop_expr = self.arena.alloc(hir::Expr {
 796             hir_id: loop_hir_id,
 797             kind: hir::ExprKind::Loop(
 798                 loop_block,
 799                 None,
 800                 hir::LoopSource::Loop,
 801                 self.lower_span(span),
 802             ),
 803             span: self.lower_span(span),
 804         });
 805
 806         // mut __awaitee => loop { ... }
 807         let awaitee_arm = self.arm(awaitee_pat, loop_expr);
 808
 809         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
 810         let into_future_span = self.mark_span_with_reason(
 811             DesugaringKind::Await,
 812             dot_await_span,
 813             self.allow_into_future.clone(),
 814         );
 815         let into_future_expr = self.expr_call_lang_item_fn(
 816             into_future_span,
 817             hir::LangItem::IntoFutureIntoFuture,
 818             arena_vec![self; expr],
 819             Some(expr_hir_id),
 820         );
 821
 822         // match <into_future_expr> {
 823         //     mut __awaitee => loop { .. }
 824         // }
 825         hir::ExprKind::Match(
 826             into_future_expr,
 827             arena_vec![self; awaitee_arm],
 828             hir::MatchSource::AwaitDesugar,
 829         )
 830     }
 831
 832     fn lower_expr_closure(
 833         &mut self,
 834         capture_clause: CaptureBy,
 835         closure_id: NodeId,
 836         movability: Movability,
 837         decl: &FnDecl,
 838         body: &Expr,
 839         fn_decl_span: Span,
 840     ) -> hir::ExprKind<'hir> {
 841         let (body, generator_option) = self.with_new_scopes(move |this| {
 842             let prev = this.current_item;
 843             this.current_item = Some(fn_decl_span);
 844             let mut generator_kind = None;
 845             let body_id = this.lower_fn_body(decl, |this| {
 846                 let e = this.lower_expr_mut(body);
 847                 generator_kind = this.generator_kind;
 848                 e
 849             });
 850             let generator_option =
 851                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 852             this.current_item = prev;
 853             (body_id, generator_option)
 854         });
 855
 856         self.with_lifetime_binder(closure_id, &[], |this, bound_generic_params| {
 857             // Lower outside new scope to preserve `is_in_loop_condition`.
 858             let fn_decl = this.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
 859
 860             hir::ExprKind::Closure {
 861                 capture_clause,
 862                 bound_generic_params,
 863                 fn_decl,
 864                 body,
 865                 fn_decl_span: this.lower_span(fn_decl_span),
 866                 movability: generator_option,
 867             }
 868         })
 869     }
 870
 871     fn generator_movability_for_fn(
 872         &mut self,
 873         decl: &FnDecl,
 874         fn_decl_span: Span,
 875         generator_kind: Option<hir::GeneratorKind>,
 876         movability: Movability,
 877     ) -> Option<hir::Movability> {
 878         match generator_kind {
 879             Some(hir::GeneratorKind::Gen) => {
 880                 if decl.inputs.len() > 1 {
 881                     struct_span_err!(
 882                         self.tcx.sess,
 883                         fn_decl_span,
 884                         E0628,
 885                         "too many parameters for a generator (expected 0 or 1 parameters)"
 886                     )
 887                     .emit();
 888                 }
 889                 Some(movability)
 890             }
 891             Some(hir::GeneratorKind::Async(_)) => {
 892                 panic!("non-`async` closure body turned `async` during lowering");
 893             }
 894             None => {
 895                 if movability == Movability::Static {
 896                     struct_span_err!(
 897                         self.tcx.sess,
 898                         fn_decl_span,
 899                         E0697,
 900                         "closures cannot be static"
 901                     )
 902                     .emit();
 903                 }
 904                 None
 905             }
 906         }
 907     }
 908
 909     fn lower_expr_async_closure(
 910         &mut self,
 911         capture_clause: CaptureBy,
 912         closure_id: NodeId,
 913         inner_closure_id: NodeId,
 914         decl: &FnDecl,
 915         body: &Expr,
 916         fn_decl_span: Span,
 917     ) -> hir::ExprKind<'hir> {
 918         let outer_decl =
 919             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 920
 921         let body = self.with_new_scopes(|this| {
 922             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 923             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 924                 struct_span_err!(
 925                     this.tcx.sess,
 926                     fn_decl_span,
 927                     E0708,
 928                     "`async` non-`move` closures with parameters are not currently supported",
 929                 )
 930                 .help(
 931                     "consider using `let` statements to manually capture \
 932                     variables by reference before entering an `async move` closure",
 933                 )
 934                 .emit();
 935             }
 936
 937             // Transform `async |x: u8| -> X { ... }` into
 938             // `|x: u8| future_from_generator(|| -> X { ... })`.
 939             let body_id = this.lower_fn_body(&outer_decl, |this| {
 940                 let async_ret_ty =
 941                     if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 942                 let async_body = this.make_async_expr(
 943                     capture_clause,
 944                     inner_closure_id,
 945                     async_ret_ty,
 946                     body.span,
 947                     hir::AsyncGeneratorKind::Closure,
 948                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 949                 );
 950                 this.expr(fn_decl_span, async_body, ThinVec::new())
 951             });
 952             body_id
 953         });
 954
 955         self.with_lifetime_binder(closure_id, &[], |this, bound_generic_params| {
 956             // We need to lower the declaration outside the new scope, because we
 957             // have to conserve the state of being inside a loop condition for the
 958             // closure argument types.
 959             let fn_decl = this.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
 960
 961             hir::ExprKind::Closure {
 962                 capture_clause,
 963                 bound_generic_params,
 964                 fn_decl,
 965                 body,
 966                 fn_decl_span: this.lower_span(fn_decl_span),
 967                 movability: None,
 968             }
 969         })
 970     }
 971
 972     /// Destructure the LHS of complex assignments.
 973     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
 974     fn lower_expr_assign(
 975         &mut self,
 976         lhs: &Expr,
 977         rhs: &Expr,
 978         eq_sign_span: Span,
 979         whole_span: Span,
 980     ) -> hir::ExprKind<'hir> {
 981         // Return early in case of an ordinary assignment.
 982         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
 983             match &lhs.kind {
 984                 ExprKind::Array(..)
 985                 | ExprKind::Struct(..)
 986                 | ExprKind::Tup(..)
 987                 | ExprKind::Underscore => false,
 988                 // Check for tuple struct constructor.
 989                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
 990                 ExprKind::Paren(e) => {
 991                     match e.kind {
 992                         // We special-case `(..)` for consistency with patterns.
 993                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
 994                         _ => is_ordinary(lower_ctx, e),
 995                     }
 996                 }
 997                 _ => true,
 998             }
 999         }
1000         if is_ordinary(self, lhs) {
1001             return hir::ExprKind::Assign(
1002                 self.lower_expr(lhs),
1003                 self.lower_expr(rhs),
1004                 self.lower_span(eq_sign_span),
1005             );
1006         }
1007
1008         let mut assignments = vec![];
1009
1010         // The LHS becomes a pattern: `(lhs1, lhs2)`.
1011         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
1012         let rhs = self.lower_expr(rhs);
1013
1014         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
1015         let destructure_let = self.stmt_let_pat(
1016             None,
1017             whole_span,
1018             Some(rhs),
1019             pat,
1020             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
1021         );
1022
1023         // `a = lhs1; b = lhs2;`.
1024         let stmts = self
1025             .arena
1026             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
1027
1028         // Wrap everything in a block.
1029         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
1030     }
1031
1032     /// If the given expression is a path to a tuple struct, returns that path.
1033     /// It is not a complete check, but just tries to reject most paths early
1034     /// if they are not tuple structs.
1035     /// Type checking will take care of the full validation later.
1036     fn extract_tuple_struct_path<'a>(
1037         &mut self,
1038         expr: &'a Expr,
1039     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1040         if let ExprKind::Path(qself, path) = &expr.kind {
1041             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1042             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1043                 if partial_res.unresolved_segments() == 0
1044                     && !partial_res.base_res().expected_in_tuple_struct_pat()
1045                 {
1046                     return None;
1047                 }
1048             }
1049             return Some((qself, path));
1050         }
1051         None
1052     }
1053
1054     /// If the given expression is a path to a unit struct, returns that path.
1055     /// It is not a complete check, but just tries to reject most paths early
1056     /// if they are not unit structs.
1057     /// Type checking will take care of the full validation later.
1058     fn extract_unit_struct_path<'a>(
1059         &mut self,
1060         expr: &'a Expr,
1061     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1062         if let ExprKind::Path(qself, path) = &expr.kind {
1063             // Does the path resolve to something disallowed in a unit struct/variant pattern?
1064             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1065                 if partial_res.unresolved_segments() == 0
1066                     && !partial_res.base_res().expected_in_unit_struct_pat()
1067                 {
1068                     return None;
1069                 }
1070             }
1071             return Some((qself, path));
1072         }
1073         None
1074     }
1075
1076     /// Convert the LHS of a destructuring assignment to a pattern.
1077     /// Each sub-assignment is recorded in `assignments`.
1078     fn destructure_assign(
1079         &mut self,
1080         lhs: &Expr,
1081         eq_sign_span: Span,
1082         assignments: &mut Vec<hir::Stmt<'hir>>,
1083     ) -> &'hir hir::Pat<'hir> {
1084         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1085     }
1086
1087     fn destructure_assign_mut(
1088         &mut self,
1089         lhs: &Expr,
1090         eq_sign_span: Span,
1091         assignments: &mut Vec<hir::Stmt<'hir>>,
1092     ) -> hir::Pat<'hir> {
1093         match &lhs.kind {
1094             // Underscore pattern.
1095             ExprKind::Underscore => {
1096                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1097             }
1098             // Slice patterns.
1099             ExprKind::Array(elements) => {
1100                 let (pats, rest) =
1101                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1102                 let slice_pat = if let Some((i, span)) = rest {
1103                     let (before, after) = pats.split_at(i);
1104                     hir::PatKind::Slice(
1105                         before,
1106                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1107                         after,
1108                     )
1109                 } else {
1110                     hir::PatKind::Slice(pats, None, &[])
1111                 };
1112                 return self.pat_without_dbm(lhs.span, slice_pat);
1113             }
1114             // Tuple structs.
1115             ExprKind::Call(callee, args) => {
1116                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1117                     let (pats, rest) = self.destructure_sequence(
1118                         args,
1119                         "tuple struct or variant",
1120                         eq_sign_span,
1121                         assignments,
1122                     );
1123                     let qpath = self.lower_qpath(
1124                         callee.id,
1125                         qself,
1126                         path,
1127                         ParamMode::Optional,
1128                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1129                     );
1130                     // Destructure like a tuple struct.
1131                     let tuple_struct_pat =
1132                         hir::PatKind::TupleStruct(qpath, pats, rest.map(|r| r.0));
1133                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1134                 }
1135             }
1136             // Unit structs and enum variants.
1137             ExprKind::Path(..) => {
1138                 if let Some((qself, path)) = self.extract_unit_struct_path(lhs) {
1139                     let qpath = self.lower_qpath(
1140                         lhs.id,
1141                         qself,
1142                         path,
1143                         ParamMode::Optional,
1144                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1145                     );
1146                     // Destructure like a unit struct.
1147                     let unit_struct_pat = hir::PatKind::Path(qpath);
1148                     return self.pat_without_dbm(lhs.span, unit_struct_pat);
1149                 }
1150             }
1151             // Structs.
1152             ExprKind::Struct(se) => {
1153                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1154                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1155                     hir::PatField {
1156                         hir_id: self.next_id(),
1157                         ident: self.lower_ident(f.ident),
1158                         pat,
1159                         is_shorthand: f.is_shorthand,
1160                         span: self.lower_span(f.span),
1161                     }
1162                 }));
1163                 let qpath = self.lower_qpath(
1164                     lhs.id,
1165                     &se.qself,
1166                     &se.path,
1167                     ParamMode::Optional,
1168                     ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1169                 );
1170                 let fields_omitted = match &se.rest {
1171                     StructRest::Base(e) => {
1172                         self.tcx
1173                             .sess
1174                             .struct_span_err(
1175                                 e.span,
1176                                 "functional record updates are not allowed in destructuring \
1177                                     assignments",
1178                             )
1179                             .span_suggestion(
1180                                 e.span,
1181                                 "consider removing the trailing pattern",
1182                                 "",
1183                                 rustc_errors::Applicability::MachineApplicable,
1184                             )
1185                             .emit();
1186                         true
1187                     }
1188                     StructRest::Rest(_) => true,
1189                     StructRest::None => false,
1190                 };
1191                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1192                 return self.pat_without_dbm(lhs.span, struct_pat);
1193             }
1194             // Tuples.
1195             ExprKind::Tup(elements) => {
1196                 let (pats, rest) =
1197                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1198                 let tuple_pat = hir::PatKind::Tuple(pats, rest.map(|r| r.0));
1199                 return self.pat_without_dbm(lhs.span, tuple_pat);
1200             }
1201             ExprKind::Paren(e) => {
1202                 // We special-case `(..)` for consistency with patterns.
1203                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1204                     let tuple_pat = hir::PatKind::Tuple(&[], Some(0));
1205                     return self.pat_without_dbm(lhs.span, tuple_pat);
1206                 } else {
1207                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1208                 }
1209             }
1210             _ => {}
1211         }
1212         // Treat all other cases as normal lvalue.
1213         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1214         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1215         let ident = self.expr_ident(lhs.span, ident, binding);
1216         let assign =
1217             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1218         let expr = self.expr(lhs.span, assign, ThinVec::new());
1219         assignments.push(self.stmt_expr(lhs.span, expr));
1220         pat
1221     }
1222
1223     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1224     /// Such a sequence occurs in a tuple (struct)/slice.
1225     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1226     /// exists.
1227     /// Each sub-assignment is recorded in `assignments`.
1228     fn destructure_sequence(
1229         &mut self,
1230         elements: &[AstP<Expr>],
1231         ctx: &str,
1232         eq_sign_span: Span,
1233         assignments: &mut Vec<hir::Stmt<'hir>>,
1234     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1235         let mut rest = None;
1236         let elements =
1237             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1238                 // Check for `..` pattern.
1239                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1240                     if let Some((_, prev_span)) = rest {
1241                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1242                     } else {
1243                         rest = Some((i, e.span));
1244                     }
1245                     None
1246                 } else {
1247                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1248                 }
1249             }));
1250         (elements, rest)
1251     }
1252
1253     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1254     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1255         let e1 = self.lower_expr_mut(e1);
1256         let e2 = self.lower_expr_mut(e2);
1257         let fn_path =
1258             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1259         let fn_expr =
1260             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
1261         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1262     }
1263
1264     fn lower_expr_range(
1265         &mut self,
1266         span: Span,
1267         e1: Option<&Expr>,
1268         e2: Option<&Expr>,
1269         lims: RangeLimits,
1270     ) -> hir::ExprKind<'hir> {
1271         use rustc_ast::RangeLimits::*;
1272
1273         let lang_item = match (e1, e2, lims) {
1274             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1275             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1276             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1277             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1278             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1279             (Some(..), Some(..), Closed) => unreachable!(),
1280             (_, None, Closed) => self.diagnostic().span_fatal(span, "inclusive range with no end"),
1281         };
1282
1283         let fields = self.arena.alloc_from_iter(
1284             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1285                 |(s, e)| {
1286                     let expr = self.lower_expr(&e);
1287                     let ident = Ident::new(s, self.lower_span(e.span));
1288                     self.expr_field(ident, expr, e.span)
1289                 },
1290             ),
1291         );
1292
1293         hir::ExprKind::Struct(
1294             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1295             fields,
1296             None,
1297         )
1298     }
1299
1300     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1301         let label = opt_label?;
1302         Some(Label { ident: self.lower_ident(label.ident) })
1303     }
1304
1305     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1306         let target_id = match destination {
1307             Some((id, _)) => {
1308                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1309                     Ok(self.lower_node_id(loop_id))
1310                 } else {
1311                     Err(hir::LoopIdError::UnresolvedLabel)
1312                 }
1313             }
1314             None => self
1315                 .loop_scope
1316                 .map(|id| Ok(self.lower_node_id(id)))
1317                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1318         };
1319         let label = self.lower_label(destination.map(|(_, label)| label));
1320         hir::Destination { label, target_id }
1321     }
1322
1323     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1324         if self.is_in_loop_condition && opt_label.is_none() {
1325             hir::Destination {
1326                 label: None,
1327                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1328             }
1329         } else {
1330             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1331         }
1332     }
1333
1334     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1335         let old_scope = self.catch_scope.replace(catch_id);
1336         let result = f(self);
1337         self.catch_scope = old_scope;
1338         result
1339     }
1340
1341     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1342         // We're no longer in the base loop's condition; we're in another loop.
1343         let was_in_loop_condition = self.is_in_loop_condition;
1344         self.is_in_loop_condition = false;
1345
1346         let old_scope = self.loop_scope.replace(loop_id);
1347         let result = f(self);
1348         self.loop_scope = old_scope;
1349
1350         self.is_in_loop_condition = was_in_loop_condition;
1351
1352         result
1353     }
1354
1355     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1356         let was_in_loop_condition = self.is_in_loop_condition;
1357         self.is_in_loop_condition = true;
1358
1359         let result = f(self);
1360
1361         self.is_in_loop_condition = was_in_loop_condition;
1362
1363         result
1364     }
1365
1366     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1367         hir::ExprField {
1368             hir_id: self.next_id(),
1369             ident: self.lower_ident(f.ident),
1370             expr: self.lower_expr(&f.expr),
1371             span: self.lower_span(f.span),
1372             is_shorthand: f.is_shorthand,
1373         }
1374     }
1375
1376     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1377         match self.generator_kind {
1378             Some(hir::GeneratorKind::Gen) => {}
1379             Some(hir::GeneratorKind::Async(_)) => {
1380                 struct_span_err!(
1381                     self.tcx.sess,
1382                     span,
1383                     E0727,
1384                     "`async` generators are not yet supported"
1385                 )
1386                 .emit();
1387             }
1388             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1389         }
1390
1391         let expr =
1392             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1393
1394         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1395     }
1396
1397     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1398     /// ```ignore (pseudo-rust)
1399     /// {
1400     ///     let result = match IntoIterator::into_iter(<head>) {
1401     ///         mut iter => {
1402     ///             [opt_ident]: loop {
1403     ///                 match Iterator::next(&mut iter) {
1404     ///                     None => break,
1405     ///                     Some(<pat>) => <body>,
1406     ///                 };
1407     ///             }
1408     ///         }
1409     ///     };
1410     ///     result
1411     /// }
1412     /// ```
1413     fn lower_expr_for(
1414         &mut self,
1415         e: &Expr,
1416         pat: &Pat,
1417         head: &Expr,
1418         body: &Block,
1419         opt_label: Option<Label>,
1420     ) -> hir::Expr<'hir> {
1421         let head = self.lower_expr_mut(head);
1422         let pat = self.lower_pat(pat);
1423         let for_span =
1424             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1425         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1426         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1427
1428         // `None => break`
1429         let none_arm = {
1430             let break_expr =
1431                 self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, ThinVec::new()));
1432             let pat = self.pat_none(for_span);
1433             self.arm(pat, break_expr)
1434         };
1435
1436         // Some(<pat>) => <body>,
1437         let some_arm = {
1438             let some_pat = self.pat_some(pat_span, pat);
1439             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1440             let body_expr = self.arena.alloc(self.expr_block(body_block, ThinVec::new()));
1441             self.arm(some_pat, body_expr)
1442         };
1443
1444         // `mut iter`
1445         let iter = Ident::with_dummy_span(sym::iter);
1446         let (iter_pat, iter_pat_nid) =
1447             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::Mutable);
1448
1449         // `match Iterator::next(&mut iter) { ... }`
1450         let match_expr = {
1451             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1452             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1453             let next_expr = self.expr_call_lang_item_fn(
1454                 head_span,
1455                 hir::LangItem::IteratorNext,
1456                 arena_vec![self; ref_mut_iter],
1457                 None,
1458             );
1459             let arms = arena_vec![self; none_arm, some_arm];
1460
1461             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1462         };
1463         let match_stmt = self.stmt_expr(for_span, match_expr);
1464
1465         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1466
1467         // `[opt_ident]: loop { ... }`
1468         let kind = hir::ExprKind::Loop(
1469             loop_block,
1470             self.lower_label(opt_label),
1471             hir::LoopSource::ForLoop,
1472             self.lower_span(for_span.with_hi(head.span.hi())),
1473         );
1474         let loop_expr =
1475             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1476
1477         // `mut iter => { ... }`
1478         let iter_arm = self.arm(iter_pat, loop_expr);
1479
1480         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1481         let into_iter_expr = {
1482             self.expr_call_lang_item_fn(
1483                 head_span,
1484                 hir::LangItem::IntoIterIntoIter,
1485                 arena_vec![self; head],
1486                 None,
1487             )
1488         };
1489
1490         let match_expr = self.arena.alloc(self.expr_match(
1491             for_span,
1492             into_iter_expr,
1493             arena_vec![self; iter_arm],
1494             hir::MatchSource::ForLoopDesugar,
1495         ));
1496
1497         let attrs: Vec<_> = e.attrs.iter().map(|a| self.lower_attr(a)).collect();
1498
1499         // This is effectively `{ let _result = ...; _result }`.
1500         // The construct was introduced in #21984 and is necessary to make sure that
1501         // temporaries in the `head` expression are dropped and do not leak to the
1502         // surrounding scope of the `match` since the `match` is not a terminating scope.
1503         //
1504         // Also, add the attributes to the outer returned expr node.
1505         self.expr_drop_temps_mut(for_span, match_expr, attrs.into())
1506     }
1507
1508     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1509     /// ```ignore (pseudo-rust)
1510     /// match Try::branch(<expr>) {
1511     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1512     ///     ControlFlow::Break(residual) =>
1513     ///         #[allow(unreachable_code)]
1514     ///         // If there is an enclosing `try {...}`:
1515     ///         break 'catch_target Try::from_residual(residual),
1516     ///         // Otherwise:
1517     ///         return Try::from_residual(residual),
1518     /// }
1519     /// ```
1520     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1521         let unstable_span = self.mark_span_with_reason(
1522             DesugaringKind::QuestionMark,
1523             span,
1524             self.allow_try_trait.clone(),
1525         );
1526         let try_span = self.tcx.sess.source_map().end_point(span);
1527         let try_span = self.mark_span_with_reason(
1528             DesugaringKind::QuestionMark,
1529             try_span,
1530             self.allow_try_trait.clone(),
1531         );
1532
1533         // `Try::branch(<expr>)`
1534         let scrutinee = {
1535             // expand <expr>
1536             let sub_expr = self.lower_expr_mut(sub_expr);
1537
1538             self.expr_call_lang_item_fn(
1539                 unstable_span,
1540                 hir::LangItem::TryTraitBranch,
1541                 arena_vec![self; sub_expr],
1542                 None,
1543             )
1544         };
1545
1546         // `#[allow(unreachable_code)]`
1547         let attr = {
1548             // `allow(unreachable_code)`
1549             let allow = {
1550                 let allow_ident = Ident::new(sym::allow, self.lower_span(span));
1551                 let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
1552                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1553                 attr::mk_list_item(allow_ident, vec![uc_nested])
1554             };
1555             attr::mk_attr_outer(allow)
1556         };
1557         let attrs = vec![attr];
1558
1559         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1560         let continue_arm = {
1561             let val_ident = Ident::with_dummy_span(sym::val);
1562             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1563             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1564                 span,
1565                 val_ident,
1566                 val_pat_nid,
1567                 ThinVec::from(attrs.clone()),
1568             ));
1569             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1570             self.arm(continue_pat, val_expr)
1571         };
1572
1573         // `ControlFlow::Break(residual) =>
1574         //     #[allow(unreachable_code)]
1575         //     return Try::from_residual(residual),`
1576         let break_arm = {
1577             let residual_ident = Ident::with_dummy_span(sym::residual);
1578             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1579             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1580             let from_residual_expr = self.wrap_in_try_constructor(
1581                 hir::LangItem::TryTraitFromResidual,
1582                 try_span,
1583                 self.arena.alloc(residual_expr),
1584                 unstable_span,
1585             );
1586             let thin_attrs = ThinVec::from(attrs);
1587             let ret_expr = if let Some(catch_node) = self.catch_scope {
1588                 let target_id = Ok(self.lower_node_id(catch_node));
1589                 self.arena.alloc(self.expr(
1590                     try_span,
1591                     hir::ExprKind::Break(
1592                         hir::Destination { label: None, target_id },
1593                         Some(from_residual_expr),
1594                     ),
1595                     thin_attrs,
1596                 ))
1597             } else {
1598                 self.arena.alloc(self.expr(
1599                     try_span,
1600                     hir::ExprKind::Ret(Some(from_residual_expr)),
1601                     thin_attrs,
1602                 ))
1603             };
1604
1605             let break_pat = self.pat_cf_break(try_span, residual_local);
1606             self.arm(break_pat, ret_expr)
1607         };
1608
1609         hir::ExprKind::Match(
1610             scrutinee,
1611             arena_vec![self; break_arm, continue_arm],
1612             hir::MatchSource::TryDesugar,
1613         )
1614     }
1615
1616     /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
1617     /// ```rust
1618     /// // If there is an enclosing `try {...}`:
1619     /// break 'catch_target FromResidual::from_residual(Yeet(residual)),
1620     /// // Otherwise:
1621     /// return FromResidual::from_residual(Yeet(residual)),
1622     /// ```
1623     /// But to simplify this, there's a `from_yeet` lang item function which
1624     /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
1625     fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1626         // The expression (if present) or `()` otherwise.
1627         let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
1628             (sub_expr.span, self.lower_expr(sub_expr))
1629         } else {
1630             (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
1631         };
1632
1633         let unstable_span = self.mark_span_with_reason(
1634             DesugaringKind::YeetExpr,
1635             span,
1636             self.allow_try_trait.clone(),
1637         );
1638
1639         let from_yeet_expr = self.wrap_in_try_constructor(
1640             hir::LangItem::TryTraitFromYeet,
1641             unstable_span,
1642             yeeted_expr,
1643             yeeted_span,
1644         );
1645
1646         if let Some(catch_node) = self.catch_scope {
1647             let target_id = Ok(self.lower_node_id(catch_node));
1648             hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
1649         } else {
1650             hir::ExprKind::Ret(Some(from_yeet_expr))
1651         }
1652     }
1653
1654     // =========================================================================
1655     // Helper methods for building HIR.
1656     // =========================================================================
1657
1658     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1659     ///
1660     /// In terms of drop order, it has the same effect as wrapping `expr` in
1661     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1662     ///
1663     /// The drop order can be important in e.g. `if expr { .. }`.
1664     pub(super) fn expr_drop_temps(
1665         &mut self,
1666         span: Span,
1667         expr: &'hir hir::Expr<'hir>,
1668         attrs: AttrVec,
1669     ) -> &'hir hir::Expr<'hir> {
1670         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1671     }
1672
1673     pub(super) fn expr_drop_temps_mut(
1674         &mut self,
1675         span: Span,
1676         expr: &'hir hir::Expr<'hir>,
1677         attrs: AttrVec,
1678     ) -> hir::Expr<'hir> {
1679         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1680     }
1681
1682     fn expr_match(
1683         &mut self,
1684         span: Span,
1685         arg: &'hir hir::Expr<'hir>,
1686         arms: &'hir [hir::Arm<'hir>],
1687         source: hir::MatchSource,
1688     ) -> hir::Expr<'hir> {
1689         self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
1690     }
1691
1692     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
1693         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1694         self.expr(span, expr_break, attrs)
1695     }
1696
1697     fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1698         let expr_break = self.expr_break(span, attrs);
1699         self.arena.alloc(expr_break)
1700     }
1701
1702     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1703         self.expr(
1704             span,
1705             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1706             ThinVec::new(),
1707         )
1708     }
1709
1710     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1711         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
1712     }
1713
1714     fn expr_call_mut(
1715         &mut self,
1716         span: Span,
1717         e: &'hir hir::Expr<'hir>,
1718         args: &'hir [hir::Expr<'hir>],
1719     ) -> hir::Expr<'hir> {
1720         self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new())
1721     }
1722
1723     fn expr_call(
1724         &mut self,
1725         span: Span,
1726         e: &'hir hir::Expr<'hir>,
1727         args: &'hir [hir::Expr<'hir>],
1728     ) -> &'hir hir::Expr<'hir> {
1729         self.arena.alloc(self.expr_call_mut(span, e, args))
1730     }
1731
1732     fn expr_call_lang_item_fn_mut(
1733         &mut self,
1734         span: Span,
1735         lang_item: hir::LangItem,
1736         args: &'hir [hir::Expr<'hir>],
1737         hir_id: Option<hir::HirId>,
1738     ) -> hir::Expr<'hir> {
1739         let path =
1740             self.arena.alloc(self.expr_lang_item_path(span, lang_item, ThinVec::new(), hir_id));
1741         self.expr_call_mut(span, path, args)
1742     }
1743
1744     fn expr_call_lang_item_fn(
1745         &mut self,
1746         span: Span,
1747         lang_item: hir::LangItem,
1748         args: &'hir [hir::Expr<'hir>],
1749         hir_id: Option<hir::HirId>,
1750     ) -> &'hir hir::Expr<'hir> {
1751         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1752     }
1753
1754     fn expr_lang_item_path(
1755         &mut self,
1756         span: Span,
1757         lang_item: hir::LangItem,
1758         attrs: AttrVec,
1759         hir_id: Option<hir::HirId>,
1760     ) -> hir::Expr<'hir> {
1761         self.expr(
1762             span,
1763             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1764             attrs,
1765         )
1766     }
1767
1768     pub(super) fn expr_ident(
1769         &mut self,
1770         sp: Span,
1771         ident: Ident,
1772         binding: hir::HirId,
1773     ) -> &'hir hir::Expr<'hir> {
1774         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1775     }
1776
1777     pub(super) fn expr_ident_mut(
1778         &mut self,
1779         sp: Span,
1780         ident: Ident,
1781         binding: hir::HirId,
1782     ) -> hir::Expr<'hir> {
1783         self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
1784     }
1785
1786     fn expr_ident_with_attrs(
1787         &mut self,
1788         span: Span,
1789         ident: Ident,
1790         binding: hir::HirId,
1791         attrs: AttrVec,
1792     ) -> hir::Expr<'hir> {
1793         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1794             None,
1795             self.arena.alloc(hir::Path {
1796                 span: self.lower_span(span),
1797                 res: Res::Local(binding),
1798                 segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
1799             }),
1800         ));
1801
1802         self.expr(span, expr_path, attrs)
1803     }
1804
1805     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1806         let hir_id = self.next_id();
1807         let span = expr.span;
1808         self.expr(
1809             span,
1810             hir::ExprKind::Block(
1811                 self.arena.alloc(hir::Block {
1812                     stmts: &[],
1813                     expr: Some(expr),
1814                     hir_id,
1815                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1816                     span: self.lower_span(span),
1817                     targeted_by_break: false,
1818                 }),
1819                 None,
1820             ),
1821             ThinVec::new(),
1822         )
1823     }
1824
1825     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1826         let blk = self.block_all(span, &[], None);
1827         let expr = self.expr_block(blk, ThinVec::new());
1828         self.arena.alloc(expr)
1829     }
1830
1831     pub(super) fn expr_block(
1832         &mut self,
1833         b: &'hir hir::Block<'hir>,
1834         attrs: AttrVec,
1835     ) -> hir::Expr<'hir> {
1836         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1837     }
1838
1839     pub(super) fn expr(
1840         &mut self,
1841         span: Span,
1842         kind: hir::ExprKind<'hir>,
1843         attrs: AttrVec,
1844     ) -> hir::Expr<'hir> {
1845         let hir_id = self.next_id();
1846         self.lower_attrs(hir_id, &attrs);
1847         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1848     }
1849
1850     fn expr_field(
1851         &mut self,
1852         ident: Ident,
1853         expr: &'hir hir::Expr<'hir>,
1854         span: Span,
1855     ) -> hir::ExprField<'hir> {
1856         hir::ExprField {
1857             hir_id: self.next_id(),
1858             ident,
1859             span: self.lower_span(span),
1860             expr,
1861             is_shorthand: false,
1862         }
1863     }
1864
1865     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1866         hir::Arm {
1867             hir_id: self.next_id(),
1868             pat,
1869             guard: None,
1870             span: self.lower_span(expr.span),
1871             body: expr,
1872         }
1873     }
1874 }