compiler/rustc_ast_lowering/src/expr.rs

   1 use super::errors::{
   2     AsyncGeneratorsNotSupported, AsyncNonMoveClosureNotSupported, AwaitOnlyInAsyncFnAndBlocks,
   3     BaseExpressionDoubleDot, ClosureCannotBeStatic, FunctionalRecordUpdateDestructuringAssignemnt,
   4     GeneratorTooManyParameters, InclusiveRangeWithNoEnd, NotSupportedForLifetimeBinderAsyncClosure,
   5     RustcBoxAttributeError, UnderscoreExprLhsAssign,
   6 };
   7 use super::ResolverAstLoweringExt;
   8 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
   9 use crate::{FnDeclKind, ImplTraitPosition};
  10 use rustc_ast::attr;
  11 use rustc_ast::ptr::P as AstP;
  12 use rustc_ast::*;
  13 use rustc_data_structures::stack::ensure_sufficient_stack;
  14 use rustc_hir as hir;
  15 use rustc_hir::def::Res;
  16 use rustc_hir::definitions::DefPathData;
  17 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
  18 use rustc_span::symbol::{sym, Ident};
  19 use rustc_span::DUMMY_SP;
  20 use thin_vec::thin_vec;
  21
  22 impl<'hir> LoweringContext<'_, 'hir> {
  23     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  24         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  25     }
  26
  27     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  28         self.arena.alloc(self.lower_expr_mut(e))
  29     }
  30
  31     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  32         ensure_sufficient_stack(|| {
  33             let kind = match e.kind {
  34                 ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  35                 ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  36                 ExprKind::ConstBlock(ref anon_const) => {
  37                     let anon_const = self.lower_anon_const(anon_const);
  38                     hir::ExprKind::ConstBlock(anon_const)
  39                 }
  40                 ExprKind::Repeat(ref expr, ref count) => {
  41                     let expr = self.lower_expr(expr);
  42                     let count = self.lower_array_length(count);
  43                     hir::ExprKind::Repeat(expr, count)
  44                 }
  45                 ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  46                 ExprKind::Call(ref f, ref args) => {
  47                     if e.attrs.get(0).map_or(false, |a| a.has_name(sym::rustc_box)) {
  48                         if let [inner] = &args[..] && e.attrs.len() == 1 {
  49                             let kind = hir::ExprKind::Box(self.lower_expr(&inner));
  50                             let hir_id = self.lower_node_id(e.id);
  51                             return hir::Expr { hir_id, kind, span: self.lower_span(e.span) };
  52                         } else {
  53                             self.tcx.sess.emit_err(RustcBoxAttributeError { span: e.span });
  54                             hir::ExprKind::Err
  55                         }
  56                     } else if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
  57                         self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
  58                     } else {
  59                         let f = self.lower_expr(f);
  60                         hir::ExprKind::Call(f, self.lower_exprs(args))
  61                     }
  62                 }
  63                 ExprKind::MethodCall(ref seg, ref receiver, ref args, span) => {
  64                     let hir_seg = self.arena.alloc(self.lower_path_segment(
  65                         e.span,
  66                         seg,
  67                         ParamMode::Optional,
  68                         ParenthesizedGenericArgs::Err,
  69                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  70                     ));
  71                     let receiver = self.lower_expr(receiver);
  72                     let args =
  73                         self.arena.alloc_from_iter(args.iter().map(|x| self.lower_expr_mut(x)));
  74                     hir::ExprKind::MethodCall(hir_seg, receiver, 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                     ref binder,
 159                     capture_clause,
 160                     asyncness,
 161                     movability,
 162                     ref decl,
 163                     ref body,
 164                     fn_decl_span,
 165                 ) => {
 166                     if let Async::Yes { closure_id, .. } = asyncness {
 167                         self.lower_expr_async_closure(
 168                             binder,
 169                             capture_clause,
 170                             e.id,
 171                             closure_id,
 172                             decl,
 173                             body,
 174                             fn_decl_span,
 175                         )
 176                     } else {
 177                         self.lower_expr_closure(
 178                             binder,
 179                             capture_clause,
 180                             e.id,
 181                             movability,
 182                             decl,
 183                             body,
 184                             fn_decl_span,
 185                         )
 186                     }
 187                 }
 188                 ExprKind::Block(ref blk, opt_label) => {
 189                     let opt_label = self.lower_label(opt_label);
 190                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 191                 }
 192                 ExprKind::Assign(ref el, ref er, span) => {
 193                     self.lower_expr_assign(el, er, span, e.span)
 194                 }
 195                 ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 196                     self.lower_binop(op),
 197                     self.lower_expr(el),
 198                     self.lower_expr(er),
 199                 ),
 200                 ExprKind::Field(ref el, ident) => {
 201                     hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(ident))
 202                 }
 203                 ExprKind::Index(ref el, ref er) => {
 204                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 205                 }
 206                 ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 207                     self.lower_expr_range_closed(e.span, e1, e2)
 208                 }
 209                 ExprKind::Range(ref e1, ref e2, lims) => {
 210                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 211                 }
 212                 ExprKind::Underscore => {
 213                     self.tcx.sess.emit_err(UnderscoreExprLhsAssign { span: e.span });
 214                     hir::ExprKind::Err
 215                 }
 216                 ExprKind::Path(ref qself, ref path) => {
 217                     let qpath = self.lower_qpath(
 218                         e.id,
 219                         qself,
 220                         path,
 221                         ParamMode::Optional,
 222                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 223                     );
 224                     hir::ExprKind::Path(qpath)
 225                 }
 226                 ExprKind::Break(opt_label, ref opt_expr) => {
 227                     let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 228                     hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
 229                 }
 230                 ExprKind::Continue(opt_label) => {
 231                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
 232                 }
 233                 ExprKind::Ret(ref e) => {
 234                     let e = e.as_ref().map(|x| self.lower_expr(x));
 235                     hir::ExprKind::Ret(e)
 236                 }
 237                 ExprKind::Yeet(ref sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
 238                 ExprKind::InlineAsm(ref asm) => {
 239                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
 240                 }
 241                 ExprKind::Struct(ref se) => {
 242                     let rest = match &se.rest {
 243                         StructRest::Base(e) => Some(self.lower_expr(e)),
 244                         StructRest::Rest(sp) => {
 245                             self.tcx.sess.emit_err(BaseExpressionDoubleDot { span: *sp });
 246                             Some(&*self.arena.alloc(self.expr_err(*sp)))
 247                         }
 248                         StructRest::None => None,
 249                     };
 250                     hir::ExprKind::Struct(
 251                         self.arena.alloc(self.lower_qpath(
 252                             e.id,
 253                             &se.qself,
 254                             &se.path,
 255                             ParamMode::Optional,
 256                             ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 257                         )),
 258                         self.arena
 259                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
 260                         rest,
 261                     )
 262                 }
 263                 ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 264                 ExprKind::Err => hir::ExprKind::Err,
 265                 ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 266                 ExprKind::Paren(ref ex) => {
 267                     let mut ex = self.lower_expr_mut(ex);
 268                     // Include parens in span, but only if it is a super-span.
 269                     if e.span.contains(ex.span) {
 270                         ex.span = self.lower_span(e.span);
 271                     }
 272                     // Merge attributes into the inner expression.
 273                     if !e.attrs.is_empty() {
 274                         let old_attrs =
 275                             self.attrs.get(&ex.hir_id.local_id).map(|la| *la).unwrap_or(&[]);
 276                         self.attrs.insert(
 277                             ex.hir_id.local_id,
 278                             &*self.arena.alloc_from_iter(
 279                                 e.attrs
 280                                     .iter()
 281                                     .map(|a| self.lower_attr(a))
 282                                     .chain(old_attrs.iter().cloned()),
 283                             ),
 284                         );
 285                     }
 286                     return ex;
 287                 }
 288
 289                 // Desugar `ExprForLoop`
 290                 // from: `[opt_ident]: for <pat> in <head> <body>`
 291                 ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 292                     return self.lower_expr_for(e, pat, head, body, opt_label);
 293                 }
 294                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 295             };
 296
 297             let hir_id = self.lower_node_id(e.id);
 298             self.lower_attrs(hir_id, &e.attrs);
 299             hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
 300         })
 301     }
 302
 303     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 304         match u {
 305             UnOp::Deref => hir::UnOp::Deref,
 306             UnOp::Not => hir::UnOp::Not,
 307             UnOp::Neg => hir::UnOp::Neg,
 308         }
 309     }
 310
 311     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 312         Spanned {
 313             node: match b.node {
 314                 BinOpKind::Add => hir::BinOpKind::Add,
 315                 BinOpKind::Sub => hir::BinOpKind::Sub,
 316                 BinOpKind::Mul => hir::BinOpKind::Mul,
 317                 BinOpKind::Div => hir::BinOpKind::Div,
 318                 BinOpKind::Rem => hir::BinOpKind::Rem,
 319                 BinOpKind::And => hir::BinOpKind::And,
 320                 BinOpKind::Or => hir::BinOpKind::Or,
 321                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 322                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 323                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 324                 BinOpKind::Shl => hir::BinOpKind::Shl,
 325                 BinOpKind::Shr => hir::BinOpKind::Shr,
 326                 BinOpKind::Eq => hir::BinOpKind::Eq,
 327                 BinOpKind::Lt => hir::BinOpKind::Lt,
 328                 BinOpKind::Le => hir::BinOpKind::Le,
 329                 BinOpKind::Ne => hir::BinOpKind::Ne,
 330                 BinOpKind::Ge => hir::BinOpKind::Ge,
 331                 BinOpKind::Gt => hir::BinOpKind::Gt,
 332             },
 333             span: self.lower_span(b.span),
 334         }
 335     }
 336
 337     fn lower_legacy_const_generics(
 338         &mut self,
 339         mut f: Expr,
 340         args: Vec<AstP<Expr>>,
 341         legacy_args_idx: &[usize],
 342     ) -> hir::ExprKind<'hir> {
 343         let ExprKind::Path(None, ref mut path) = f.kind else {
 344             unreachable!();
 345         };
 346
 347         // Split the arguments into const generics and normal arguments
 348         let mut real_args = vec![];
 349         let mut generic_args = vec![];
 350         for (idx, arg) in args.into_iter().enumerate() {
 351             if legacy_args_idx.contains(&idx) {
 352                 let parent_def_id = self.current_hir_id_owner;
 353                 let node_id = self.next_node_id();
 354
 355                 // Add a definition for the in-band const def.
 356                 self.create_def(parent_def_id, node_id, DefPathData::AnonConst);
 357
 358                 let anon_const = AnonConst { id: node_id, value: arg };
 359                 generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
 360             } else {
 361                 real_args.push(arg);
 362             }
 363         }
 364
 365         // Add generic args to the last element of the path.
 366         let last_segment = path.segments.last_mut().unwrap();
 367         assert!(last_segment.args.is_none());
 368         last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
 369             span: DUMMY_SP,
 370             args: generic_args,
 371         })));
 372
 373         // Now lower everything as normal.
 374         let f = self.lower_expr(&f);
 375         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
 376     }
 377
 378     fn lower_expr_if(
 379         &mut self,
 380         cond: &Expr,
 381         then: &Block,
 382         else_opt: Option<&Expr>,
 383     ) -> hir::ExprKind<'hir> {
 384         let lowered_cond = self.lower_expr(cond);
 385         let new_cond = self.manage_let_cond(lowered_cond);
 386         let then_expr = self.lower_block_expr(then);
 387         if let Some(rslt) = else_opt {
 388             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), Some(self.lower_expr(rslt)))
 389         } else {
 390             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), None)
 391         }
 392     }
 393
 394     // If `cond` kind is `let`, returns `let`. Otherwise, wraps and returns `cond`
 395     // in a temporary block.
 396     fn manage_let_cond(&mut self, cond: &'hir hir::Expr<'hir>) -> &'hir hir::Expr<'hir> {
 397         fn has_let_expr<'hir>(expr: &'hir hir::Expr<'hir>) -> bool {
 398             match expr.kind {
 399                 hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
 400                 hir::ExprKind::Let(..) => true,
 401                 _ => false,
 402             }
 403         }
 404         if has_let_expr(cond) {
 405             cond
 406         } else {
 407             let reason = DesugaringKind::CondTemporary;
 408             let span_block = self.mark_span_with_reason(reason, cond.span, None);
 409             self.expr_drop_temps(span_block, cond, AttrVec::new())
 410         }
 411     }
 412
 413     // We desugar: `'label: while $cond $body` into:
 414     //
 415     // ```
 416     // 'label: loop {
 417     //   if { let _t = $cond; _t } {
 418     //     $body
 419     //   }
 420     //   else {
 421     //     break;
 422     //   }
 423     // }
 424     // ```
 425     //
 426     // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 427     // to preserve drop semantics since `while $cond { ... }` does not
 428     // let temporaries live outside of `cond`.
 429     fn lower_expr_while_in_loop_scope(
 430         &mut self,
 431         span: Span,
 432         cond: &Expr,
 433         body: &Block,
 434         opt_label: Option<Label>,
 435     ) -> hir::ExprKind<'hir> {
 436         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
 437         let new_cond = self.manage_let_cond(lowered_cond);
 438         let then = self.lower_block_expr(body);
 439         let expr_break = self.expr_break(span, AttrVec::new());
 440         let stmt_break = self.stmt_expr(span, expr_break);
 441         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
 442         let else_expr = self.arena.alloc(self.expr_block(else_blk, AttrVec::new()));
 443         let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
 444         let if_expr = self.expr(span, if_kind, AttrVec::new());
 445         let block = self.block_expr(self.arena.alloc(if_expr));
 446         let span = self.lower_span(span.with_hi(cond.span.hi()));
 447         let opt_label = self.lower_label(opt_label);
 448         hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
 449     }
 450
 451     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
 452     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
 453     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 454     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 455         self.with_catch_scope(body.id, |this| {
 456             let mut block = this.lower_block_noalloc(body, true);
 457
 458             // Final expression of the block (if present) or `()` with span at the end of block
 459             let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
 460                 (
 461                     this.mark_span_with_reason(
 462                         DesugaringKind::TryBlock,
 463                         expr.span,
 464                         this.allow_try_trait.clone(),
 465                     ),
 466                     expr,
 467                 )
 468             } else {
 469                 let try_span = this.mark_span_with_reason(
 470                     DesugaringKind::TryBlock,
 471                     this.tcx.sess.source_map().end_point(body.span),
 472                     this.allow_try_trait.clone(),
 473                 );
 474
 475                 (try_span, this.expr_unit(try_span))
 476             };
 477
 478             let ok_wrapped_span =
 479                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 480
 481             // `::std::ops::Try::from_output($tail_expr)`
 482             block.expr = Some(this.wrap_in_try_constructor(
 483                 hir::LangItem::TryTraitFromOutput,
 484                 try_span,
 485                 tail_expr,
 486                 ok_wrapped_span,
 487             ));
 488
 489             hir::ExprKind::Block(this.arena.alloc(block), None)
 490         })
 491     }
 492
 493     fn wrap_in_try_constructor(
 494         &mut self,
 495         lang_item: hir::LangItem,
 496         method_span: Span,
 497         expr: &'hir hir::Expr<'hir>,
 498         overall_span: Span,
 499     ) -> &'hir hir::Expr<'hir> {
 500         let constructor = self.arena.alloc(self.expr_lang_item_path(
 501             method_span,
 502             lang_item,
 503             AttrVec::new(),
 504             None,
 505         ));
 506         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 507     }
 508
 509     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 510         let pat = self.lower_pat(&arm.pat);
 511         let guard = arm.guard.as_ref().map(|cond| {
 512             if let ExprKind::Let(ref pat, ref scrutinee, span) = cond.kind {
 513                 hir::Guard::IfLet(self.arena.alloc(hir::Let {
 514                     hir_id: self.next_id(),
 515                     span: self.lower_span(span),
 516                     pat: self.lower_pat(pat),
 517                     ty: None,
 518                     init: self.lower_expr(scrutinee),
 519                 }))
 520             } else {
 521                 hir::Guard::If(self.lower_expr(cond))
 522             }
 523         });
 524         let hir_id = self.next_id();
 525         self.lower_attrs(hir_id, &arm.attrs);
 526         hir::Arm {
 527             hir_id,
 528             pat,
 529             guard,
 530             body: self.lower_expr(&arm.body),
 531             span: self.lower_span(arm.span),
 532         }
 533     }
 534
 535     /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
 536     ///
 537     /// This results in:
 538     ///
 539     /// ```text
 540     /// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
 541     ///     <body>
 542     /// })
 543     /// ```
 544     pub(super) fn make_async_expr(
 545         &mut self,
 546         capture_clause: CaptureBy,
 547         closure_node_id: NodeId,
 548         ret_ty: Option<AstP<Ty>>,
 549         span: Span,
 550         async_gen_kind: hir::AsyncGeneratorKind,
 551         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 552     ) -> hir::ExprKind<'hir> {
 553         let output = match ret_ty {
 554             Some(ty) => hir::FnRetTy::Return(
 555                 self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock)),
 556             ),
 557             None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
 558         };
 559
 560         // Resume argument type. We let the compiler infer this to simplify the lowering. It is
 561         // fully constrained by `future::from_generator`.
 562         let input_ty = hir::Ty {
 563             hir_id: self.next_id(),
 564             kind: hir::TyKind::Infer,
 565             span: self.lower_span(span),
 566         };
 567
 568         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 569         let fn_decl = self.arena.alloc(hir::FnDecl {
 570             inputs: arena_vec![self; input_ty],
 571             output,
 572             c_variadic: false,
 573             implicit_self: hir::ImplicitSelfKind::None,
 574         });
 575
 576         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 577         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 578             span,
 579             Ident::with_dummy_span(sym::_task_context),
 580             hir::BindingAnnotation::MUT,
 581         );
 582         let param = hir::Param {
 583             hir_id: self.next_id(),
 584             pat,
 585             ty_span: self.lower_span(span),
 586             span: self.lower_span(span),
 587         };
 588         let params = arena_vec![self; param];
 589
 590         let body = self.lower_body(move |this| {
 591             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 592
 593             let old_ctx = this.task_context;
 594             this.task_context = Some(task_context_hid);
 595             let res = body(this);
 596             this.task_context = old_ctx;
 597             (params, res)
 598         });
 599
 600         // `static |_task_context| -> <ret_ty> { body }`:
 601         let generator_kind = {
 602             let c = self.arena.alloc(hir::Closure {
 603                 binder: hir::ClosureBinder::Default,
 604                 capture_clause,
 605                 bound_generic_params: &[],
 606                 fn_decl,
 607                 body,
 608                 fn_decl_span: self.lower_span(span),
 609                 movability: Some(hir::Movability::Static),
 610             });
 611
 612             hir::ExprKind::Closure(c)
 613         };
 614         let generator = hir::Expr {
 615             hir_id: self.lower_node_id(closure_node_id),
 616             kind: generator_kind,
 617             span: self.lower_span(span),
 618         };
 619
 620         // `future::from_generator`:
 621         let unstable_span =
 622             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 623         let gen_future = self.expr_lang_item_path(
 624             unstable_span,
 625             hir::LangItem::FromGenerator,
 626             AttrVec::new(),
 627             None,
 628         );
 629
 630         // `future::from_generator(generator)`:
 631         hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
 632     }
 633
 634     /// Desugar `<expr>.await` into:
 635     /// ```ignore (pseudo-rust)
 636     /// match ::std::future::IntoFuture::into_future(<expr>) {
 637     ///     mut __awaitee => loop {
 638     ///         match unsafe { ::std::future::Future::poll(
 639     ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
 640     ///             ::std::future::get_context(task_context),
 641     ///         ) } {
 642     ///             ::std::task::Poll::Ready(result) => break result,
 643     ///             ::std::task::Poll::Pending => {}
 644     ///         }
 645     ///         task_context = yield ();
 646     ///     }
 647     /// }
 648     /// ```
 649     fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 650         let full_span = expr.span.to(dot_await_span);
 651         match self.generator_kind {
 652             Some(hir::GeneratorKind::Async(_)) => {}
 653             Some(hir::GeneratorKind::Gen) | None => {
 654                 self.tcx.sess.emit_err(AwaitOnlyInAsyncFnAndBlocks {
 655                     dot_await_span,
 656                     item_span: self.current_item,
 657                 });
 658             }
 659         }
 660         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
 661         let gen_future_span = self.mark_span_with_reason(
 662             DesugaringKind::Await,
 663             full_span,
 664             self.allow_gen_future.clone(),
 665         );
 666         let expr = self.lower_expr_mut(expr);
 667         let expr_hir_id = expr.hir_id;
 668
 669         // Note that the name of this binding must not be changed to something else because
 670         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
 671         // this name to identify what is being awaited by a suspended async functions.
 672         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
 673         let (awaitee_pat, awaitee_pat_hid) =
 674             self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::MUT);
 675
 676         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 677
 678         // unsafe {
 679         //     ::std::future::Future::poll(
 680         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
 681         //         ::std::future::get_context(task_context),
 682         //     )
 683         // }
 684         let poll_expr = {
 685             let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
 686             let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
 687             let task_context = if let Some(task_context_hid) = self.task_context {
 688                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 689             } else {
 690                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 691                 self.expr_err(span)
 692             };
 693             let new_unchecked = self.expr_call_lang_item_fn_mut(
 694                 span,
 695                 hir::LangItem::PinNewUnchecked,
 696                 arena_vec![self; ref_mut_awaitee],
 697                 Some(expr_hir_id),
 698             );
 699             let get_context = self.expr_call_lang_item_fn_mut(
 700                 gen_future_span,
 701                 hir::LangItem::GetContext,
 702                 arena_vec![self; task_context],
 703                 Some(expr_hir_id),
 704             );
 705             let call = self.expr_call_lang_item_fn(
 706                 span,
 707                 hir::LangItem::FuturePoll,
 708                 arena_vec![self; new_unchecked, get_context],
 709                 Some(expr_hir_id),
 710             );
 711             self.arena.alloc(self.expr_unsafe(call))
 712         };
 713
 714         // `::std::task::Poll::Ready(result) => break result`
 715         let loop_node_id = self.next_node_id();
 716         let loop_hir_id = self.lower_node_id(loop_node_id);
 717         let ready_arm = {
 718             let x_ident = Ident::with_dummy_span(sym::result);
 719             let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
 720             let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
 721             let ready_field = self.single_pat_field(gen_future_span, x_pat);
 722             let ready_pat = self.pat_lang_item_variant(
 723                 span,
 724                 hir::LangItem::PollReady,
 725                 ready_field,
 726                 Some(expr_hir_id),
 727             );
 728             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 729                 let expr_break =
 730                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 731                 this.arena.alloc(this.expr(gen_future_span, expr_break, AttrVec::new()))
 732             });
 733             self.arm(ready_pat, break_x)
 734         };
 735
 736         // `::std::task::Poll::Pending => {}`
 737         let pending_arm = {
 738             let pending_pat = self.pat_lang_item_variant(
 739                 span,
 740                 hir::LangItem::PollPending,
 741                 &[],
 742                 Some(expr_hir_id),
 743             );
 744             let empty_block = self.expr_block_empty(span);
 745             self.arm(pending_pat, empty_block)
 746         };
 747
 748         let inner_match_stmt = {
 749             let match_expr = self.expr_match(
 750                 span,
 751                 poll_expr,
 752                 arena_vec![self; ready_arm, pending_arm],
 753                 hir::MatchSource::AwaitDesugar,
 754             );
 755             self.stmt_expr(span, match_expr)
 756         };
 757
 758         // task_context = yield ();
 759         let yield_stmt = {
 760             let unit = self.expr_unit(span);
 761             let yield_expr = self.expr(
 762                 span,
 763                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
 764                 AttrVec::new(),
 765             );
 766             let yield_expr = self.arena.alloc(yield_expr);
 767
 768             if let Some(task_context_hid) = self.task_context {
 769                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 770                 let assign = self.expr(
 771                     span,
 772                     hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)),
 773                     AttrVec::new(),
 774                 );
 775                 self.stmt_expr(span, assign)
 776             } else {
 777                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 778                 // proceed with type checking.
 779                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 780             }
 781         };
 782
 783         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 784
 785         // loop { .. }
 786         let loop_expr = self.arena.alloc(hir::Expr {
 787             hir_id: loop_hir_id,
 788             kind: hir::ExprKind::Loop(
 789                 loop_block,
 790                 None,
 791                 hir::LoopSource::Loop,
 792                 self.lower_span(span),
 793             ),
 794             span: self.lower_span(span),
 795         });
 796
 797         // mut __awaitee => loop { ... }
 798         let awaitee_arm = self.arm(awaitee_pat, loop_expr);
 799
 800         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
 801         let into_future_span = self.mark_span_with_reason(
 802             DesugaringKind::Await,
 803             dot_await_span,
 804             self.allow_into_future.clone(),
 805         );
 806         let into_future_expr = self.expr_call_lang_item_fn(
 807             into_future_span,
 808             hir::LangItem::IntoFutureIntoFuture,
 809             arena_vec![self; expr],
 810             Some(expr_hir_id),
 811         );
 812
 813         // match <into_future_expr> {
 814         //     mut __awaitee => loop { .. }
 815         // }
 816         hir::ExprKind::Match(
 817             into_future_expr,
 818             arena_vec![self; awaitee_arm],
 819             hir::MatchSource::AwaitDesugar,
 820         )
 821     }
 822
 823     fn lower_expr_closure(
 824         &mut self,
 825         binder: &ClosureBinder,
 826         capture_clause: CaptureBy,
 827         closure_id: NodeId,
 828         movability: Movability,
 829         decl: &FnDecl,
 830         body: &Expr,
 831         fn_decl_span: Span,
 832     ) -> hir::ExprKind<'hir> {
 833         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 834
 835         let (body_id, generator_option) = self.with_new_scopes(move |this| {
 836             let prev = this.current_item;
 837             this.current_item = Some(fn_decl_span);
 838             let mut generator_kind = None;
 839             let body_id = this.lower_fn_body(decl, |this| {
 840                 let e = this.lower_expr_mut(body);
 841                 generator_kind = this.generator_kind;
 842                 e
 843             });
 844             let generator_option =
 845                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 846             this.current_item = prev;
 847             (body_id, generator_option)
 848         });
 849
 850         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
 851         // Lower outside new scope to preserve `is_in_loop_condition`.
 852         let fn_decl = self.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
 853
 854         let c = self.arena.alloc(hir::Closure {
 855             binder: binder_clause,
 856             capture_clause,
 857             bound_generic_params,
 858             fn_decl,
 859             body: body_id,
 860             fn_decl_span: self.lower_span(fn_decl_span),
 861             movability: generator_option,
 862         });
 863
 864         hir::ExprKind::Closure(c)
 865     }
 866
 867     fn generator_movability_for_fn(
 868         &mut self,
 869         decl: &FnDecl,
 870         fn_decl_span: Span,
 871         generator_kind: Option<hir::GeneratorKind>,
 872         movability: Movability,
 873     ) -> Option<hir::Movability> {
 874         match generator_kind {
 875             Some(hir::GeneratorKind::Gen) => {
 876                 if decl.inputs.len() > 1 {
 877                     self.tcx.sess.emit_err(GeneratorTooManyParameters { fn_decl_span });
 878                 }
 879                 Some(movability)
 880             }
 881             Some(hir::GeneratorKind::Async(_)) => {
 882                 panic!("non-`async` closure body turned `async` during lowering");
 883             }
 884             None => {
 885                 if movability == Movability::Static {
 886                     self.tcx.sess.emit_err(ClosureCannotBeStatic { fn_decl_span });
 887                 }
 888                 None
 889             }
 890         }
 891     }
 892
 893     fn lower_closure_binder<'c>(
 894         &mut self,
 895         binder: &'c ClosureBinder,
 896     ) -> (hir::ClosureBinder, &'c [GenericParam]) {
 897         let (binder, params) = match binder {
 898             ClosureBinder::NotPresent => (hir::ClosureBinder::Default, &[][..]),
 899             &ClosureBinder::For { span, ref generic_params } => {
 900                 let span = self.lower_span(span);
 901                 (hir::ClosureBinder::For { span }, &**generic_params)
 902             }
 903         };
 904
 905         (binder, params)
 906     }
 907
 908     fn lower_expr_async_closure(
 909         &mut self,
 910         binder: &ClosureBinder,
 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         if let &ClosureBinder::For { span, .. } = binder {
 919             self.tcx.sess.emit_err(NotSupportedForLifetimeBinderAsyncClosure { span });
 920         }
 921
 922         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 923
 924         let outer_decl =
 925             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 926
 927         let body = self.with_new_scopes(|this| {
 928             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 929             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 930                 this.tcx.sess.emit_err(AsyncNonMoveClosureNotSupported { fn_decl_span });
 931             }
 932
 933             // Transform `async |x: u8| -> X { ... }` into
 934             // `|x: u8| future_from_generator(|| -> X { ... })`.
 935             let body_id = this.lower_fn_body(&outer_decl, |this| {
 936                 let async_ret_ty =
 937                     if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 938                 let async_body = this.make_async_expr(
 939                     capture_clause,
 940                     inner_closure_id,
 941                     async_ret_ty,
 942                     body.span,
 943                     hir::AsyncGeneratorKind::Closure,
 944                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 945                 );
 946                 this.expr(fn_decl_span, async_body, AttrVec::new())
 947             });
 948             body_id
 949         });
 950
 951         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
 952
 953         // We need to lower the declaration outside the new scope, because we
 954         // have to conserve the state of being inside a loop condition for the
 955         // closure argument types.
 956         let fn_decl = self.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
 957
 958         let c = self.arena.alloc(hir::Closure {
 959             binder: binder_clause,
 960             capture_clause,
 961             bound_generic_params,
 962             fn_decl,
 963             body,
 964             fn_decl_span: self.lower_span(fn_decl_span),
 965             movability: None,
 966         });
 967         hir::ExprKind::Closure(c)
 968     }
 969
 970     /// Destructure the LHS of complex assignments.
 971     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
 972     fn lower_expr_assign(
 973         &mut self,
 974         lhs: &Expr,
 975         rhs: &Expr,
 976         eq_sign_span: Span,
 977         whole_span: Span,
 978     ) -> hir::ExprKind<'hir> {
 979         // Return early in case of an ordinary assignment.
 980         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
 981             match &lhs.kind {
 982                 ExprKind::Array(..)
 983                 | ExprKind::Struct(..)
 984                 | ExprKind::Tup(..)
 985                 | ExprKind::Underscore => false,
 986                 // Check for tuple struct constructor.
 987                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
 988                 ExprKind::Paren(e) => {
 989                     match e.kind {
 990                         // We special-case `(..)` for consistency with patterns.
 991                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
 992                         _ => is_ordinary(lower_ctx, e),
 993                     }
 994                 }
 995                 _ => true,
 996             }
 997         }
 998         if is_ordinary(self, lhs) {
 999             return hir::ExprKind::Assign(
1000                 self.lower_expr(lhs),
1001                 self.lower_expr(rhs),
1002                 self.lower_span(eq_sign_span),
1003             );
1004         }
1005
1006         let mut assignments = vec![];
1007
1008         // The LHS becomes a pattern: `(lhs1, lhs2)`.
1009         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
1010         let rhs = self.lower_expr(rhs);
1011
1012         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
1013         let destructure_let = self.stmt_let_pat(
1014             None,
1015             whole_span,
1016             Some(rhs),
1017             pat,
1018             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
1019         );
1020
1021         // `a = lhs1; b = lhs2;`.
1022         let stmts = self
1023             .arena
1024             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
1025
1026         // Wrap everything in a block.
1027         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
1028     }
1029
1030     /// If the given expression is a path to a tuple struct, returns that path.
1031     /// It is not a complete check, but just tries to reject most paths early
1032     /// if they are not tuple structs.
1033     /// Type checking will take care of the full validation later.
1034     fn extract_tuple_struct_path<'a>(
1035         &mut self,
1036         expr: &'a Expr,
1037     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1038         if let ExprKind::Path(qself, path) = &expr.kind {
1039             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1040             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1041                 if partial_res.unresolved_segments() == 0
1042                     && !partial_res.base_res().expected_in_tuple_struct_pat()
1043                 {
1044                     return None;
1045                 }
1046             }
1047             return Some((qself, path));
1048         }
1049         None
1050     }
1051
1052     /// If the given expression is a path to a unit struct, returns that path.
1053     /// It is not a complete check, but just tries to reject most paths early
1054     /// if they are not unit structs.
1055     /// Type checking will take care of the full validation later.
1056     fn extract_unit_struct_path<'a>(
1057         &mut self,
1058         expr: &'a Expr,
1059     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1060         if let ExprKind::Path(qself, path) = &expr.kind {
1061             // Does the path resolve to something disallowed in a unit struct/variant pattern?
1062             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1063                 if partial_res.unresolved_segments() == 0
1064                     && !partial_res.base_res().expected_in_unit_struct_pat()
1065                 {
1066                     return None;
1067                 }
1068             }
1069             return Some((qself, path));
1070         }
1071         None
1072     }
1073
1074     /// Convert the LHS of a destructuring assignment to a pattern.
1075     /// Each sub-assignment is recorded in `assignments`.
1076     fn destructure_assign(
1077         &mut self,
1078         lhs: &Expr,
1079         eq_sign_span: Span,
1080         assignments: &mut Vec<hir::Stmt<'hir>>,
1081     ) -> &'hir hir::Pat<'hir> {
1082         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1083     }
1084
1085     fn destructure_assign_mut(
1086         &mut self,
1087         lhs: &Expr,
1088         eq_sign_span: Span,
1089         assignments: &mut Vec<hir::Stmt<'hir>>,
1090     ) -> hir::Pat<'hir> {
1091         match &lhs.kind {
1092             // Underscore pattern.
1093             ExprKind::Underscore => {
1094                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1095             }
1096             // Slice patterns.
1097             ExprKind::Array(elements) => {
1098                 let (pats, rest) =
1099                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1100                 let slice_pat = if let Some((i, span)) = rest {
1101                     let (before, after) = pats.split_at(i);
1102                     hir::PatKind::Slice(
1103                         before,
1104                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1105                         after,
1106                     )
1107                 } else {
1108                     hir::PatKind::Slice(pats, None, &[])
1109                 };
1110                 return self.pat_without_dbm(lhs.span, slice_pat);
1111             }
1112             // Tuple structs.
1113             ExprKind::Call(callee, args) => {
1114                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1115                     let (pats, rest) = self.destructure_sequence(
1116                         args,
1117                         "tuple struct or variant",
1118                         eq_sign_span,
1119                         assignments,
1120                     );
1121                     let qpath = self.lower_qpath(
1122                         callee.id,
1123                         qself,
1124                         path,
1125                         ParamMode::Optional,
1126                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1127                     );
1128                     // Destructure like a tuple struct.
1129                     let tuple_struct_pat =
1130                         hir::PatKind::TupleStruct(qpath, pats, rest.map(|r| r.0));
1131                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1132                 }
1133             }
1134             // Unit structs and enum variants.
1135             ExprKind::Path(..) => {
1136                 if let Some((qself, path)) = self.extract_unit_struct_path(lhs) {
1137                     let qpath = self.lower_qpath(
1138                         lhs.id,
1139                         qself,
1140                         path,
1141                         ParamMode::Optional,
1142                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1143                     );
1144                     // Destructure like a unit struct.
1145                     let unit_struct_pat = hir::PatKind::Path(qpath);
1146                     return self.pat_without_dbm(lhs.span, unit_struct_pat);
1147                 }
1148             }
1149             // Structs.
1150             ExprKind::Struct(se) => {
1151                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1152                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1153                     hir::PatField {
1154                         hir_id: self.next_id(),
1155                         ident: self.lower_ident(f.ident),
1156                         pat,
1157                         is_shorthand: f.is_shorthand,
1158                         span: self.lower_span(f.span),
1159                     }
1160                 }));
1161                 let qpath = self.lower_qpath(
1162                     lhs.id,
1163                     &se.qself,
1164                     &se.path,
1165                     ParamMode::Optional,
1166                     ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1167                 );
1168                 let fields_omitted = match &se.rest {
1169                     StructRest::Base(e) => {
1170                         self.tcx.sess.emit_err(FunctionalRecordUpdateDestructuringAssignemnt {
1171                             span: e.span,
1172                         });
1173                         true
1174                     }
1175                     StructRest::Rest(_) => true,
1176                     StructRest::None => false,
1177                 };
1178                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1179                 return self.pat_without_dbm(lhs.span, struct_pat);
1180             }
1181             // Tuples.
1182             ExprKind::Tup(elements) => {
1183                 let (pats, rest) =
1184                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1185                 let tuple_pat = hir::PatKind::Tuple(pats, rest.map(|r| r.0));
1186                 return self.pat_without_dbm(lhs.span, tuple_pat);
1187             }
1188             ExprKind::Paren(e) => {
1189                 // We special-case `(..)` for consistency with patterns.
1190                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1191                     let tuple_pat = hir::PatKind::Tuple(&[], Some(0));
1192                     return self.pat_without_dbm(lhs.span, tuple_pat);
1193                 } else {
1194                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1195                 }
1196             }
1197             _ => {}
1198         }
1199         // Treat all other cases as normal lvalue.
1200         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1201         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1202         let ident = self.expr_ident(lhs.span, ident, binding);
1203         let assign =
1204             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1205         let expr = self.expr(lhs.span, assign, AttrVec::new());
1206         assignments.push(self.stmt_expr(lhs.span, expr));
1207         pat
1208     }
1209
1210     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1211     /// Such a sequence occurs in a tuple (struct)/slice.
1212     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1213     /// exists.
1214     /// Each sub-assignment is recorded in `assignments`.
1215     fn destructure_sequence(
1216         &mut self,
1217         elements: &[AstP<Expr>],
1218         ctx: &str,
1219         eq_sign_span: Span,
1220         assignments: &mut Vec<hir::Stmt<'hir>>,
1221     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1222         let mut rest = None;
1223         let elements =
1224             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1225                 // Check for `..` pattern.
1226                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1227                     if let Some((_, prev_span)) = rest {
1228                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1229                     } else {
1230                         rest = Some((i, e.span));
1231                     }
1232                     None
1233                 } else {
1234                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1235                 }
1236             }));
1237         (elements, rest)
1238     }
1239
1240     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1241     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1242         let e1 = self.lower_expr_mut(e1);
1243         let e2 = self.lower_expr_mut(e2);
1244         let fn_path =
1245             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1246         let fn_expr =
1247             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), AttrVec::new()));
1248         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1249     }
1250
1251     fn lower_expr_range(
1252         &mut self,
1253         span: Span,
1254         e1: Option<&Expr>,
1255         e2: Option<&Expr>,
1256         lims: RangeLimits,
1257     ) -> hir::ExprKind<'hir> {
1258         use rustc_ast::RangeLimits::*;
1259
1260         let lang_item = match (e1, e2, lims) {
1261             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1262             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1263             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1264             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1265             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1266             (Some(..), Some(..), Closed) => unreachable!(),
1267             (start, None, Closed) => {
1268                 self.tcx.sess.emit_err(InclusiveRangeWithNoEnd { span });
1269                 match start {
1270                     Some(..) => hir::LangItem::RangeFrom,
1271                     None => hir::LangItem::RangeFull,
1272                 }
1273             }
1274         };
1275
1276         let fields = self.arena.alloc_from_iter(
1277             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1278                 |(s, e)| {
1279                     let expr = self.lower_expr(&e);
1280                     let ident = Ident::new(s, self.lower_span(e.span));
1281                     self.expr_field(ident, expr, e.span)
1282                 },
1283             ),
1284         );
1285
1286         hir::ExprKind::Struct(
1287             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1288             fields,
1289             None,
1290         )
1291     }
1292
1293     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1294         let label = opt_label?;
1295         Some(Label { ident: self.lower_ident(label.ident) })
1296     }
1297
1298     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1299         let target_id = match destination {
1300             Some((id, _)) => {
1301                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1302                     Ok(self.lower_node_id(loop_id))
1303                 } else {
1304                     Err(hir::LoopIdError::UnresolvedLabel)
1305                 }
1306             }
1307             None => self
1308                 .loop_scope
1309                 .map(|id| Ok(self.lower_node_id(id)))
1310                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1311         };
1312         let label = self.lower_label(destination.map(|(_, label)| label));
1313         hir::Destination { label, target_id }
1314     }
1315
1316     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1317         if self.is_in_loop_condition && opt_label.is_none() {
1318             hir::Destination {
1319                 label: None,
1320                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1321             }
1322         } else {
1323             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1324         }
1325     }
1326
1327     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1328         let old_scope = self.catch_scope.replace(catch_id);
1329         let result = f(self);
1330         self.catch_scope = old_scope;
1331         result
1332     }
1333
1334     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1335         // We're no longer in the base loop's condition; we're in another loop.
1336         let was_in_loop_condition = self.is_in_loop_condition;
1337         self.is_in_loop_condition = false;
1338
1339         let old_scope = self.loop_scope.replace(loop_id);
1340         let result = f(self);
1341         self.loop_scope = old_scope;
1342
1343         self.is_in_loop_condition = was_in_loop_condition;
1344
1345         result
1346     }
1347
1348     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1349         let was_in_loop_condition = self.is_in_loop_condition;
1350         self.is_in_loop_condition = true;
1351
1352         let result = f(self);
1353
1354         self.is_in_loop_condition = was_in_loop_condition;
1355
1356         result
1357     }
1358
1359     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1360         let hir_id = self.lower_node_id(f.id);
1361         self.lower_attrs(hir_id, &f.attrs);
1362         hir::ExprField {
1363             hir_id,
1364             ident: self.lower_ident(f.ident),
1365             expr: self.lower_expr(&f.expr),
1366             span: self.lower_span(f.span),
1367             is_shorthand: f.is_shorthand,
1368         }
1369     }
1370
1371     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1372         match self.generator_kind {
1373             Some(hir::GeneratorKind::Gen) => {}
1374             Some(hir::GeneratorKind::Async(_)) => {
1375                 self.tcx.sess.emit_err(AsyncGeneratorsNotSupported { span });
1376             }
1377             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1378         }
1379
1380         let expr =
1381             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1382
1383         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1384     }
1385
1386     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1387     /// ```ignore (pseudo-rust)
1388     /// {
1389     ///     let result = match IntoIterator::into_iter(<head>) {
1390     ///         mut iter => {
1391     ///             [opt_ident]: loop {
1392     ///                 match Iterator::next(&mut iter) {
1393     ///                     None => break,
1394     ///                     Some(<pat>) => <body>,
1395     ///                 };
1396     ///             }
1397     ///         }
1398     ///     };
1399     ///     result
1400     /// }
1401     /// ```
1402     fn lower_expr_for(
1403         &mut self,
1404         e: &Expr,
1405         pat: &Pat,
1406         head: &Expr,
1407         body: &Block,
1408         opt_label: Option<Label>,
1409     ) -> hir::Expr<'hir> {
1410         let head = self.lower_expr_mut(head);
1411         let pat = self.lower_pat(pat);
1412         let for_span =
1413             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1414         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1415         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1416
1417         // `None => break`
1418         let none_arm = {
1419             let break_expr =
1420                 self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, AttrVec::new()));
1421             let pat = self.pat_none(for_span);
1422             self.arm(pat, break_expr)
1423         };
1424
1425         // Some(<pat>) => <body>,
1426         let some_arm = {
1427             let some_pat = self.pat_some(pat_span, pat);
1428             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1429             let body_expr = self.arena.alloc(self.expr_block(body_block, AttrVec::new()));
1430             self.arm(some_pat, body_expr)
1431         };
1432
1433         // `mut iter`
1434         let iter = Ident::with_dummy_span(sym::iter);
1435         let (iter_pat, iter_pat_nid) =
1436             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::MUT);
1437
1438         // `match Iterator::next(&mut iter) { ... }`
1439         let match_expr = {
1440             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1441             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1442             let next_expr = self.expr_call_lang_item_fn(
1443                 head_span,
1444                 hir::LangItem::IteratorNext,
1445                 arena_vec![self; ref_mut_iter],
1446                 None,
1447             );
1448             let arms = arena_vec![self; none_arm, some_arm];
1449
1450             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1451         };
1452         let match_stmt = self.stmt_expr(for_span, match_expr);
1453
1454         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1455
1456         // `[opt_ident]: loop { ... }`
1457         let kind = hir::ExprKind::Loop(
1458             loop_block,
1459             self.lower_label(opt_label),
1460             hir::LoopSource::ForLoop,
1461             self.lower_span(for_span.with_hi(head.span.hi())),
1462         );
1463         let loop_expr =
1464             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1465
1466         // `mut iter => { ... }`
1467         let iter_arm = self.arm(iter_pat, loop_expr);
1468
1469         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1470         let into_iter_expr = {
1471             self.expr_call_lang_item_fn(
1472                 head_span,
1473                 hir::LangItem::IntoIterIntoIter,
1474                 arena_vec![self; head],
1475                 None,
1476             )
1477         };
1478
1479         let match_expr = self.arena.alloc(self.expr_match(
1480             for_span,
1481             into_iter_expr,
1482             arena_vec![self; iter_arm],
1483             hir::MatchSource::ForLoopDesugar,
1484         ));
1485
1486         // This is effectively `{ let _result = ...; _result }`.
1487         // The construct was introduced in #21984 and is necessary to make sure that
1488         // temporaries in the `head` expression are dropped and do not leak to the
1489         // surrounding scope of the `match` since the `match` is not a terminating scope.
1490         //
1491         // Also, add the attributes to the outer returned expr node.
1492         self.expr_drop_temps_mut(for_span, match_expr, e.attrs.clone())
1493     }
1494
1495     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1496     /// ```ignore (pseudo-rust)
1497     /// match Try::branch(<expr>) {
1498     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1499     ///     ControlFlow::Break(residual) =>
1500     ///         #[allow(unreachable_code)]
1501     ///         // If there is an enclosing `try {...}`:
1502     ///         break 'catch_target Try::from_residual(residual),
1503     ///         // Otherwise:
1504     ///         return Try::from_residual(residual),
1505     /// }
1506     /// ```
1507     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1508         let unstable_span = self.mark_span_with_reason(
1509             DesugaringKind::QuestionMark,
1510             span,
1511             self.allow_try_trait.clone(),
1512         );
1513         let try_span = self.tcx.sess.source_map().end_point(span);
1514         let try_span = self.mark_span_with_reason(
1515             DesugaringKind::QuestionMark,
1516             try_span,
1517             self.allow_try_trait.clone(),
1518         );
1519
1520         // `Try::branch(<expr>)`
1521         let scrutinee = {
1522             // expand <expr>
1523             let sub_expr = self.lower_expr_mut(sub_expr);
1524
1525             self.expr_call_lang_item_fn(
1526                 unstable_span,
1527                 hir::LangItem::TryTraitBranch,
1528                 arena_vec![self; sub_expr],
1529                 None,
1530             )
1531         };
1532
1533         // `#[allow(unreachable_code)]`
1534         let attr = {
1535             // `allow(unreachable_code)`
1536             let allow = {
1537                 let allow_ident = Ident::new(sym::allow, self.lower_span(span));
1538                 let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
1539                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1540                 attr::mk_list_item(allow_ident, vec![uc_nested])
1541             };
1542             attr::mk_attr_outer(allow)
1543         };
1544         let attrs: AttrVec = thin_vec![attr];
1545
1546         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1547         let continue_arm = {
1548             let val_ident = Ident::with_dummy_span(sym::val);
1549             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1550             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1551                 span,
1552                 val_ident,
1553                 val_pat_nid,
1554                 attrs.clone(),
1555             ));
1556             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1557             self.arm(continue_pat, val_expr)
1558         };
1559
1560         // `ControlFlow::Break(residual) =>
1561         //     #[allow(unreachable_code)]
1562         //     return Try::from_residual(residual),`
1563         let break_arm = {
1564             let residual_ident = Ident::with_dummy_span(sym::residual);
1565             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1566             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1567             let from_residual_expr = self.wrap_in_try_constructor(
1568                 hir::LangItem::TryTraitFromResidual,
1569                 try_span,
1570                 self.arena.alloc(residual_expr),
1571                 unstable_span,
1572             );
1573             let ret_expr = if let Some(catch_node) = self.catch_scope {
1574                 let target_id = Ok(self.lower_node_id(catch_node));
1575                 self.arena.alloc(self.expr(
1576                     try_span,
1577                     hir::ExprKind::Break(
1578                         hir::Destination { label: None, target_id },
1579                         Some(from_residual_expr),
1580                     ),
1581                     attrs,
1582                 ))
1583             } else {
1584                 self.arena.alloc(self.expr(
1585                     try_span,
1586                     hir::ExprKind::Ret(Some(from_residual_expr)),
1587                     attrs,
1588                 ))
1589             };
1590
1591             let break_pat = self.pat_cf_break(try_span, residual_local);
1592             self.arm(break_pat, ret_expr)
1593         };
1594
1595         hir::ExprKind::Match(
1596             scrutinee,
1597             arena_vec![self; break_arm, continue_arm],
1598             hir::MatchSource::TryDesugar,
1599         )
1600     }
1601
1602     /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
1603     /// ```rust
1604     /// // If there is an enclosing `try {...}`:
1605     /// break 'catch_target FromResidual::from_residual(Yeet(residual)),
1606     /// // Otherwise:
1607     /// return FromResidual::from_residual(Yeet(residual)),
1608     /// ```
1609     /// But to simplify this, there's a `from_yeet` lang item function which
1610     /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
1611     fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1612         // The expression (if present) or `()` otherwise.
1613         let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
1614             (sub_expr.span, self.lower_expr(sub_expr))
1615         } else {
1616             (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
1617         };
1618
1619         let unstable_span = self.mark_span_with_reason(
1620             DesugaringKind::YeetExpr,
1621             span,
1622             self.allow_try_trait.clone(),
1623         );
1624
1625         let from_yeet_expr = self.wrap_in_try_constructor(
1626             hir::LangItem::TryTraitFromYeet,
1627             unstable_span,
1628             yeeted_expr,
1629             yeeted_span,
1630         );
1631
1632         if let Some(catch_node) = self.catch_scope {
1633             let target_id = Ok(self.lower_node_id(catch_node));
1634             hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
1635         } else {
1636             hir::ExprKind::Ret(Some(from_yeet_expr))
1637         }
1638     }
1639
1640     // =========================================================================
1641     // Helper methods for building HIR.
1642     // =========================================================================
1643
1644     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1645     ///
1646     /// In terms of drop order, it has the same effect as wrapping `expr` in
1647     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1648     ///
1649     /// The drop order can be important in e.g. `if expr { .. }`.
1650     pub(super) fn expr_drop_temps(
1651         &mut self,
1652         span: Span,
1653         expr: &'hir hir::Expr<'hir>,
1654         attrs: AttrVec,
1655     ) -> &'hir hir::Expr<'hir> {
1656         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1657     }
1658
1659     pub(super) fn expr_drop_temps_mut(
1660         &mut self,
1661         span: Span,
1662         expr: &'hir hir::Expr<'hir>,
1663         attrs: AttrVec,
1664     ) -> hir::Expr<'hir> {
1665         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1666     }
1667
1668     fn expr_match(
1669         &mut self,
1670         span: Span,
1671         arg: &'hir hir::Expr<'hir>,
1672         arms: &'hir [hir::Arm<'hir>],
1673         source: hir::MatchSource,
1674     ) -> hir::Expr<'hir> {
1675         self.expr(span, hir::ExprKind::Match(arg, arms, source), AttrVec::new())
1676     }
1677
1678     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
1679         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1680         self.expr(span, expr_break, attrs)
1681     }
1682
1683     fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1684         let expr_break = self.expr_break(span, attrs);
1685         self.arena.alloc(expr_break)
1686     }
1687
1688     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1689         self.expr(
1690             span,
1691             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1692             AttrVec::new(),
1693         )
1694     }
1695
1696     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1697         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), AttrVec::new()))
1698     }
1699
1700     fn expr_call_mut(
1701         &mut self,
1702         span: Span,
1703         e: &'hir hir::Expr<'hir>,
1704         args: &'hir [hir::Expr<'hir>],
1705     ) -> hir::Expr<'hir> {
1706         self.expr(span, hir::ExprKind::Call(e, args), AttrVec::new())
1707     }
1708
1709     fn expr_call(
1710         &mut self,
1711         span: Span,
1712         e: &'hir hir::Expr<'hir>,
1713         args: &'hir [hir::Expr<'hir>],
1714     ) -> &'hir hir::Expr<'hir> {
1715         self.arena.alloc(self.expr_call_mut(span, e, args))
1716     }
1717
1718     fn expr_call_lang_item_fn_mut(
1719         &mut self,
1720         span: Span,
1721         lang_item: hir::LangItem,
1722         args: &'hir [hir::Expr<'hir>],
1723         hir_id: Option<hir::HirId>,
1724     ) -> hir::Expr<'hir> {
1725         let path =
1726             self.arena.alloc(self.expr_lang_item_path(span, lang_item, AttrVec::new(), hir_id));
1727         self.expr_call_mut(span, path, args)
1728     }
1729
1730     fn expr_call_lang_item_fn(
1731         &mut self,
1732         span: Span,
1733         lang_item: hir::LangItem,
1734         args: &'hir [hir::Expr<'hir>],
1735         hir_id: Option<hir::HirId>,
1736     ) -> &'hir hir::Expr<'hir> {
1737         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1738     }
1739
1740     fn expr_lang_item_path(
1741         &mut self,
1742         span: Span,
1743         lang_item: hir::LangItem,
1744         attrs: AttrVec,
1745         hir_id: Option<hir::HirId>,
1746     ) -> hir::Expr<'hir> {
1747         self.expr(
1748             span,
1749             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1750             attrs,
1751         )
1752     }
1753
1754     pub(super) fn expr_ident(
1755         &mut self,
1756         sp: Span,
1757         ident: Ident,
1758         binding: hir::HirId,
1759     ) -> &'hir hir::Expr<'hir> {
1760         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1761     }
1762
1763     pub(super) fn expr_ident_mut(
1764         &mut self,
1765         sp: Span,
1766         ident: Ident,
1767         binding: hir::HirId,
1768     ) -> hir::Expr<'hir> {
1769         self.expr_ident_with_attrs(sp, ident, binding, AttrVec::new())
1770     }
1771
1772     fn expr_ident_with_attrs(
1773         &mut self,
1774         span: Span,
1775         ident: Ident,
1776         binding: hir::HirId,
1777         attrs: AttrVec,
1778     ) -> hir::Expr<'hir> {
1779         let hir_id = self.next_id();
1780         let res = Res::Local(binding);
1781         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1782             None,
1783             self.arena.alloc(hir::Path {
1784                 span: self.lower_span(span),
1785                 res,
1786                 segments: arena_vec![self; hir::PathSegment::new(ident, hir_id, res)],
1787             }),
1788         ));
1789
1790         self.expr(span, expr_path, attrs)
1791     }
1792
1793     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1794         let hir_id = self.next_id();
1795         let span = expr.span;
1796         self.expr(
1797             span,
1798             hir::ExprKind::Block(
1799                 self.arena.alloc(hir::Block {
1800                     stmts: &[],
1801                     expr: Some(expr),
1802                     hir_id,
1803                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1804                     span: self.lower_span(span),
1805                     targeted_by_break: false,
1806                 }),
1807                 None,
1808             ),
1809             AttrVec::new(),
1810         )
1811     }
1812
1813     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1814         let blk = self.block_all(span, &[], None);
1815         let expr = self.expr_block(blk, AttrVec::new());
1816         self.arena.alloc(expr)
1817     }
1818
1819     pub(super) fn expr_block(
1820         &mut self,
1821         b: &'hir hir::Block<'hir>,
1822         attrs: AttrVec,
1823     ) -> hir::Expr<'hir> {
1824         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1825     }
1826
1827     pub(super) fn expr(
1828         &mut self,
1829         span: Span,
1830         kind: hir::ExprKind<'hir>,
1831         attrs: AttrVec,
1832     ) -> hir::Expr<'hir> {
1833         let hir_id = self.next_id();
1834         self.lower_attrs(hir_id, &attrs);
1835         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1836     }
1837
1838     fn expr_field(
1839         &mut self,
1840         ident: Ident,
1841         expr: &'hir hir::Expr<'hir>,
1842         span: Span,
1843     ) -> hir::ExprField<'hir> {
1844         hir::ExprField {
1845             hir_id: self.next_id(),
1846             ident,
1847             span: self.lower_span(span),
1848             expr,
1849             is_shorthand: false,
1850         }
1851     }
1852
1853     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1854         hir::Arm {
1855             hir_id: self.next_id(),
1856             pat,
1857             guard: None,
1858             span: self.lower_span(expr.span),
1859             body: expr,
1860         }
1861     }
1862 }