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                         &mut 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 = self
  93                         .lower_ty(ty, &mut 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 = self
  99                         .lower_ty(ty, &mut 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                         &mut 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                             &mut 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 =
 554             match ret_ty {
 555                 Some(ty) => hir::FnRetTy::Return(self.lower_ty(
 556                     &ty,
 557                     &mut ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock),
 558                 )),
 559                 None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
 560             };
 561
 562         // Resume argument type. We let the compiler infer this to simplify the lowering. It is
 563         // fully constrained by `future::from_generator`.
 564         let input_ty = hir::Ty {
 565             hir_id: self.next_id(),
 566             kind: hir::TyKind::Infer,
 567             span: self.lower_span(span),
 568         };
 569
 570         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 571         let fn_decl = self.arena.alloc(hir::FnDecl {
 572             inputs: arena_vec![self; input_ty],
 573             output,
 574             c_variadic: false,
 575             implicit_self: hir::ImplicitSelfKind::None,
 576         });
 577
 578         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 579         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 580             span,
 581             Ident::with_dummy_span(sym::_task_context),
 582             hir::BindingAnnotation::MUT,
 583         );
 584         let param = hir::Param {
 585             hir_id: self.next_id(),
 586             pat,
 587             ty_span: self.lower_span(span),
 588             span: self.lower_span(span),
 589         };
 590         let params = arena_vec![self; param];
 591
 592         let body = self.lower_body(move |this| {
 593             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 594
 595             let old_ctx = this.task_context;
 596             this.task_context = Some(task_context_hid);
 597             let res = body(this);
 598             this.task_context = old_ctx;
 599             (params, res)
 600         });
 601
 602         // `static |_task_context| -> <ret_ty> { body }`:
 603         let generator_kind = {
 604             let c = self.arena.alloc(hir::Closure {
 605                 binder: hir::ClosureBinder::Default,
 606                 capture_clause,
 607                 bound_generic_params: &[],
 608                 fn_decl,
 609                 body,
 610                 fn_decl_span: self.lower_span(span),
 611                 movability: Some(hir::Movability::Static),
 612             });
 613
 614             hir::ExprKind::Closure(c)
 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             AttrVec::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                 self.tcx.sess.emit_err(AwaitOnlyInAsyncFnAndBlocks {
 657                     dot_await_span,
 658                     item_span: self.current_item,
 659                 });
 660             }
 661         }
 662         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
 663         let gen_future_span = self.mark_span_with_reason(
 664             DesugaringKind::Await,
 665             full_span,
 666             self.allow_gen_future.clone(),
 667         );
 668         let expr = self.lower_expr_mut(expr);
 669         let expr_hir_id = expr.hir_id;
 670
 671         // Note that the name of this binding must not be changed to something else because
 672         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
 673         // this name to identify what is being awaited by a suspended async functions.
 674         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
 675         let (awaitee_pat, awaitee_pat_hid) =
 676             self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::MUT);
 677
 678         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 679
 680         // unsafe {
 681         //     ::std::future::Future::poll(
 682         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
 683         //         ::std::future::get_context(task_context),
 684         //     )
 685         // }
 686         let poll_expr = {
 687             let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
 688             let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
 689             let task_context = if let Some(task_context_hid) = self.task_context {
 690                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 691             } else {
 692                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 693                 self.expr_err(span)
 694             };
 695             let new_unchecked = self.expr_call_lang_item_fn_mut(
 696                 span,
 697                 hir::LangItem::PinNewUnchecked,
 698                 arena_vec![self; ref_mut_awaitee],
 699                 Some(expr_hir_id),
 700             );
 701             let get_context = self.expr_call_lang_item_fn_mut(
 702                 gen_future_span,
 703                 hir::LangItem::GetContext,
 704                 arena_vec![self; task_context],
 705                 Some(expr_hir_id),
 706             );
 707             let call = self.expr_call_lang_item_fn(
 708                 span,
 709                 hir::LangItem::FuturePoll,
 710                 arena_vec![self; new_unchecked, get_context],
 711                 Some(expr_hir_id),
 712             );
 713             self.arena.alloc(self.expr_unsafe(call))
 714         };
 715
 716         // `::std::task::Poll::Ready(result) => break result`
 717         let loop_node_id = self.next_node_id();
 718         let loop_hir_id = self.lower_node_id(loop_node_id);
 719         let ready_arm = {
 720             let x_ident = Ident::with_dummy_span(sym::result);
 721             let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
 722             let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
 723             let ready_field = self.single_pat_field(gen_future_span, x_pat);
 724             let ready_pat = self.pat_lang_item_variant(
 725                 span,
 726                 hir::LangItem::PollReady,
 727                 ready_field,
 728                 Some(expr_hir_id),
 729             );
 730             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 731                 let expr_break =
 732                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 733                 this.arena.alloc(this.expr(gen_future_span, expr_break, AttrVec::new()))
 734             });
 735             self.arm(ready_pat, break_x)
 736         };
 737
 738         // `::std::task::Poll::Pending => {}`
 739         let pending_arm = {
 740             let pending_pat = self.pat_lang_item_variant(
 741                 span,
 742                 hir::LangItem::PollPending,
 743                 &[],
 744                 Some(expr_hir_id),
 745             );
 746             let empty_block = self.expr_block_empty(span);
 747             self.arm(pending_pat, empty_block)
 748         };
 749
 750         let inner_match_stmt = {
 751             let match_expr = self.expr_match(
 752                 span,
 753                 poll_expr,
 754                 arena_vec![self; ready_arm, pending_arm],
 755                 hir::MatchSource::AwaitDesugar,
 756             );
 757             self.stmt_expr(span, match_expr)
 758         };
 759
 760         // task_context = yield ();
 761         let yield_stmt = {
 762             let unit = self.expr_unit(span);
 763             let yield_expr = self.expr(
 764                 span,
 765                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
 766                 AttrVec::new(),
 767             );
 768             let yield_expr = self.arena.alloc(yield_expr);
 769
 770             if let Some(task_context_hid) = self.task_context {
 771                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 772                 let assign = self.expr(
 773                     span,
 774                     hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)),
 775                     AttrVec::new(),
 776                 );
 777                 self.stmt_expr(span, assign)
 778             } else {
 779                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 780                 // proceed with type checking.
 781                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 782             }
 783         };
 784
 785         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 786
 787         // loop { .. }
 788         let loop_expr = self.arena.alloc(hir::Expr {
 789             hir_id: loop_hir_id,
 790             kind: hir::ExprKind::Loop(
 791                 loop_block,
 792                 None,
 793                 hir::LoopSource::Loop,
 794                 self.lower_span(span),
 795             ),
 796             span: self.lower_span(span),
 797         });
 798
 799         // mut __awaitee => loop { ... }
 800         let awaitee_arm = self.arm(awaitee_pat, loop_expr);
 801
 802         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
 803         let into_future_span = self.mark_span_with_reason(
 804             DesugaringKind::Await,
 805             dot_await_span,
 806             self.allow_into_future.clone(),
 807         );
 808         let into_future_expr = self.expr_call_lang_item_fn(
 809             into_future_span,
 810             hir::LangItem::IntoFutureIntoFuture,
 811             arena_vec![self; expr],
 812             Some(expr_hir_id),
 813         );
 814
 815         // match <into_future_expr> {
 816         //     mut __awaitee => loop { .. }
 817         // }
 818         hir::ExprKind::Match(
 819             into_future_expr,
 820             arena_vec![self; awaitee_arm],
 821             hir::MatchSource::AwaitDesugar,
 822         )
 823     }
 824
 825     fn lower_expr_closure(
 826         &mut self,
 827         binder: &ClosureBinder,
 828         capture_clause: CaptureBy,
 829         closure_id: NodeId,
 830         movability: Movability,
 831         decl: &FnDecl,
 832         body: &Expr,
 833         fn_decl_span: Span,
 834     ) -> hir::ExprKind<'hir> {
 835         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 836
 837         let (body_id, generator_option) = self.with_new_scopes(move |this| {
 838             let prev = this.current_item;
 839             this.current_item = Some(fn_decl_span);
 840             let mut generator_kind = None;
 841             let body_id = this.lower_fn_body(decl, |this| {
 842                 let e = this.lower_expr_mut(body);
 843                 generator_kind = this.generator_kind;
 844                 e
 845             });
 846             let generator_option =
 847                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 848             this.current_item = prev;
 849             (body_id, generator_option)
 850         });
 851
 852         self.lower_lifetime_binder(closure_id, generic_params, |lctx, bound_generic_params| {
 853             // Lower outside new scope to preserve `is_in_loop_condition`.
 854             let fn_decl = lctx.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
 855
 856             let c = lctx.arena.alloc(hir::Closure {
 857                 binder: binder_clause,
 858                 capture_clause,
 859                 bound_generic_params,
 860                 fn_decl,
 861                 body: body_id,
 862                 fn_decl_span: lctx.lower_span(fn_decl_span),
 863                 movability: generator_option,
 864             });
 865
 866             hir::ExprKind::Closure(c)
 867         })
 868     }
 869
 870     fn generator_movability_for_fn(
 871         &mut self,
 872         decl: &FnDecl,
 873         fn_decl_span: Span,
 874         generator_kind: Option<hir::GeneratorKind>,
 875         movability: Movability,
 876     ) -> Option<hir::Movability> {
 877         match generator_kind {
 878             Some(hir::GeneratorKind::Gen) => {
 879                 if decl.inputs.len() > 1 {
 880                     self.tcx.sess.emit_err(GeneratorTooManyParameters { fn_decl_span });
 881                 }
 882                 Some(movability)
 883             }
 884             Some(hir::GeneratorKind::Async(_)) => {
 885                 panic!("non-`async` closure body turned `async` during lowering");
 886             }
 887             None => {
 888                 if movability == Movability::Static {
 889                     self.tcx.sess.emit_err(ClosureCannotBeStatic { fn_decl_span });
 890                 }
 891                 None
 892             }
 893         }
 894     }
 895
 896     fn lower_closure_binder<'c>(
 897         &mut self,
 898         binder: &'c ClosureBinder,
 899     ) -> (hir::ClosureBinder, &'c [GenericParam]) {
 900         let (binder, params) = match binder {
 901             ClosureBinder::NotPresent => (hir::ClosureBinder::Default, &[][..]),
 902             &ClosureBinder::For { span, ref generic_params } => {
 903                 let span = self.lower_span(span);
 904                 (hir::ClosureBinder::For { span }, &**generic_params)
 905             }
 906         };
 907
 908         (binder, params)
 909     }
 910
 911     fn lower_expr_async_closure(
 912         &mut self,
 913         binder: &ClosureBinder,
 914         capture_clause: CaptureBy,
 915         closure_id: NodeId,
 916         inner_closure_id: NodeId,
 917         decl: &FnDecl,
 918         body: &Expr,
 919         fn_decl_span: Span,
 920     ) -> hir::ExprKind<'hir> {
 921         if let &ClosureBinder::For { span, .. } = binder {
 922             self.tcx.sess.emit_err(NotSupportedForLifetimeBinderAsyncClosure { span });
 923         }
 924
 925         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 926
 927         let outer_decl =
 928             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 929
 930         let body = self.with_new_scopes(|this| {
 931             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 932             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 933                 this.tcx.sess.emit_err(AsyncNonMoveClosureNotSupported { fn_decl_span });
 934             }
 935
 936             // Transform `async |x: u8| -> X { ... }` into
 937             // `|x: u8| future_from_generator(|| -> X { ... })`.
 938             let body_id = this.lower_fn_body(&outer_decl, |this| {
 939                 let async_ret_ty =
 940                     if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 941                 let async_body = this.make_async_expr(
 942                     capture_clause,
 943                     inner_closure_id,
 944                     async_ret_ty,
 945                     body.span,
 946                     hir::AsyncGeneratorKind::Closure,
 947                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 948                 );
 949                 this.expr(fn_decl_span, async_body, AttrVec::new())
 950             });
 951             body_id
 952         });
 953
 954         self.lower_lifetime_binder(closure_id, generic_params, |lctx, bound_generic_params| {
 955             // We need to lower the declaration outside the new scope, because we
 956             // have to conserve the state of being inside a loop condition for the
 957             // closure argument types.
 958             let fn_decl = lctx.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
 959
 960             let c = lctx.arena.alloc(hir::Closure {
 961                 binder: binder_clause,
 962                 capture_clause,
 963                 bound_generic_params,
 964                 fn_decl,
 965                 body,
 966                 fn_decl_span: lctx.lower_span(fn_decl_span),
 967                 movability: None,
 968             });
 969             hir::ExprKind::Closure(c)
 970         })
 971     }
 972
 973     /// Destructure the LHS of complex assignments.
 974     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
 975     fn lower_expr_assign(
 976         &mut self,
 977         lhs: &Expr,
 978         rhs: &Expr,
 979         eq_sign_span: Span,
 980         whole_span: Span,
 981     ) -> hir::ExprKind<'hir> {
 982         // Return early in case of an ordinary assignment.
 983         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
 984             match &lhs.kind {
 985                 ExprKind::Array(..)
 986                 | ExprKind::Struct(..)
 987                 | ExprKind::Tup(..)
 988                 | ExprKind::Underscore => false,
 989                 // Check for tuple struct constructor.
 990                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
 991                 ExprKind::Paren(e) => {
 992                     match e.kind {
 993                         // We special-case `(..)` for consistency with patterns.
 994                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
 995                         _ => is_ordinary(lower_ctx, e),
 996                     }
 997                 }
 998                 _ => true,
 999             }
1000         }
1001         if is_ordinary(self, lhs) {
1002             return hir::ExprKind::Assign(
1003                 self.lower_expr(lhs),
1004                 self.lower_expr(rhs),
1005                 self.lower_span(eq_sign_span),
1006             );
1007         }
1008
1009         let mut assignments = vec![];
1010
1011         // The LHS becomes a pattern: `(lhs1, lhs2)`.
1012         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
1013         let rhs = self.lower_expr(rhs);
1014
1015         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
1016         let destructure_let = self.stmt_let_pat(
1017             None,
1018             whole_span,
1019             Some(rhs),
1020             pat,
1021             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
1022         );
1023
1024         // `a = lhs1; b = lhs2;`.
1025         let stmts = self
1026             .arena
1027             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
1028
1029         // Wrap everything in a block.
1030         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
1031     }
1032
1033     /// If the given expression is a path to a tuple struct, returns that path.
1034     /// It is not a complete check, but just tries to reject most paths early
1035     /// if they are not tuple structs.
1036     /// Type checking will take care of the full validation later.
1037     fn extract_tuple_struct_path<'a>(
1038         &mut self,
1039         expr: &'a Expr,
1040     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1041         if let ExprKind::Path(qself, path) = &expr.kind {
1042             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1043             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1044                 if partial_res.unresolved_segments() == 0
1045                     && !partial_res.base_res().expected_in_tuple_struct_pat()
1046                 {
1047                     return None;
1048                 }
1049             }
1050             return Some((qself, path));
1051         }
1052         None
1053     }
1054
1055     /// If the given expression is a path to a unit struct, returns that path.
1056     /// It is not a complete check, but just tries to reject most paths early
1057     /// if they are not unit structs.
1058     /// Type checking will take care of the full validation later.
1059     fn extract_unit_struct_path<'a>(
1060         &mut self,
1061         expr: &'a Expr,
1062     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1063         if let ExprKind::Path(qself, path) = &expr.kind {
1064             // Does the path resolve to something disallowed in a unit struct/variant pattern?
1065             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1066                 if partial_res.unresolved_segments() == 0
1067                     && !partial_res.base_res().expected_in_unit_struct_pat()
1068                 {
1069                     return None;
1070                 }
1071             }
1072             return Some((qself, path));
1073         }
1074         None
1075     }
1076
1077     /// Convert the LHS of a destructuring assignment to a pattern.
1078     /// Each sub-assignment is recorded in `assignments`.
1079     fn destructure_assign(
1080         &mut self,
1081         lhs: &Expr,
1082         eq_sign_span: Span,
1083         assignments: &mut Vec<hir::Stmt<'hir>>,
1084     ) -> &'hir hir::Pat<'hir> {
1085         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1086     }
1087
1088     fn destructure_assign_mut(
1089         &mut self,
1090         lhs: &Expr,
1091         eq_sign_span: Span,
1092         assignments: &mut Vec<hir::Stmt<'hir>>,
1093     ) -> hir::Pat<'hir> {
1094         match &lhs.kind {
1095             // Underscore pattern.
1096             ExprKind::Underscore => {
1097                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1098             }
1099             // Slice patterns.
1100             ExprKind::Array(elements) => {
1101                 let (pats, rest) =
1102                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1103                 let slice_pat = if let Some((i, span)) = rest {
1104                     let (before, after) = pats.split_at(i);
1105                     hir::PatKind::Slice(
1106                         before,
1107                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1108                         after,
1109                     )
1110                 } else {
1111                     hir::PatKind::Slice(pats, None, &[])
1112                 };
1113                 return self.pat_without_dbm(lhs.span, slice_pat);
1114             }
1115             // Tuple structs.
1116             ExprKind::Call(callee, args) => {
1117                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1118                     let (pats, rest) = self.destructure_sequence(
1119                         args,
1120                         "tuple struct or variant",
1121                         eq_sign_span,
1122                         assignments,
1123                     );
1124                     let qpath = self.lower_qpath(
1125                         callee.id,
1126                         qself,
1127                         path,
1128                         ParamMode::Optional,
1129                         &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1130                     );
1131                     // Destructure like a tuple struct.
1132                     let tuple_struct_pat = hir::PatKind::TupleStruct(
1133                         qpath,
1134                         pats,
1135                         hir::DotDotPos::new(rest.map(|r| r.0)),
1136                     );
1137                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1138                 }
1139             }
1140             // Unit structs and enum variants.
1141             ExprKind::Path(..) => {
1142                 if let Some((qself, path)) = self.extract_unit_struct_path(lhs) {
1143                     let qpath = self.lower_qpath(
1144                         lhs.id,
1145                         qself,
1146                         path,
1147                         ParamMode::Optional,
1148                         &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1149                     );
1150                     // Destructure like a unit struct.
1151                     let unit_struct_pat = hir::PatKind::Path(qpath);
1152                     return self.pat_without_dbm(lhs.span, unit_struct_pat);
1153                 }
1154             }
1155             // Structs.
1156             ExprKind::Struct(se) => {
1157                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1158                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1159                     hir::PatField {
1160                         hir_id: self.next_id(),
1161                         ident: self.lower_ident(f.ident),
1162                         pat,
1163                         is_shorthand: f.is_shorthand,
1164                         span: self.lower_span(f.span),
1165                     }
1166                 }));
1167                 let qpath = self.lower_qpath(
1168                     lhs.id,
1169                     &se.qself,
1170                     &se.path,
1171                     ParamMode::Optional,
1172                     &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1173                 );
1174                 let fields_omitted = match &se.rest {
1175                     StructRest::Base(e) => {
1176                         self.tcx.sess.emit_err(FunctionalRecordUpdateDestructuringAssignemnt {
1177                             span: e.span,
1178                         });
1179                         true
1180                     }
1181                     StructRest::Rest(_) => true,
1182                     StructRest::None => false,
1183                 };
1184                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1185                 return self.pat_without_dbm(lhs.span, struct_pat);
1186             }
1187             // Tuples.
1188             ExprKind::Tup(elements) => {
1189                 let (pats, rest) =
1190                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1191                 let tuple_pat = hir::PatKind::Tuple(pats, hir::DotDotPos::new(rest.map(|r| r.0)));
1192                 return self.pat_without_dbm(lhs.span, tuple_pat);
1193             }
1194             ExprKind::Paren(e) => {
1195                 // We special-case `(..)` for consistency with patterns.
1196                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1197                     let tuple_pat = hir::PatKind::Tuple(&[], hir::DotDotPos::new(Some(0)));
1198                     return self.pat_without_dbm(lhs.span, tuple_pat);
1199                 } else {
1200                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1201                 }
1202             }
1203             _ => {}
1204         }
1205         // Treat all other cases as normal lvalue.
1206         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1207         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1208         let ident = self.expr_ident(lhs.span, ident, binding);
1209         let assign =
1210             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1211         let expr = self.expr(lhs.span, assign, AttrVec::new());
1212         assignments.push(self.stmt_expr(lhs.span, expr));
1213         pat
1214     }
1215
1216     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1217     /// Such a sequence occurs in a tuple (struct)/slice.
1218     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1219     /// exists.
1220     /// Each sub-assignment is recorded in `assignments`.
1221     fn destructure_sequence(
1222         &mut self,
1223         elements: &[AstP<Expr>],
1224         ctx: &str,
1225         eq_sign_span: Span,
1226         assignments: &mut Vec<hir::Stmt<'hir>>,
1227     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1228         let mut rest = None;
1229         let elements =
1230             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1231                 // Check for `..` pattern.
1232                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1233                     if let Some((_, prev_span)) = rest {
1234                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1235                     } else {
1236                         rest = Some((i, e.span));
1237                     }
1238                     None
1239                 } else {
1240                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1241                 }
1242             }));
1243         (elements, rest)
1244     }
1245
1246     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1247     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1248         let e1 = self.lower_expr_mut(e1);
1249         let e2 = self.lower_expr_mut(e2);
1250         let fn_path =
1251             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1252         let fn_expr =
1253             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), AttrVec::new()));
1254         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1255     }
1256
1257     fn lower_expr_range(
1258         &mut self,
1259         span: Span,
1260         e1: Option<&Expr>,
1261         e2: Option<&Expr>,
1262         lims: RangeLimits,
1263     ) -> hir::ExprKind<'hir> {
1264         use rustc_ast::RangeLimits::*;
1265
1266         let lang_item = match (e1, e2, lims) {
1267             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1268             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1269             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1270             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1271             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1272             (Some(..), Some(..), Closed) => unreachable!(),
1273             (start, None, Closed) => {
1274                 self.tcx.sess.emit_err(InclusiveRangeWithNoEnd { span });
1275                 match start {
1276                     Some(..) => hir::LangItem::RangeFrom,
1277                     None => hir::LangItem::RangeFull,
1278                 }
1279             }
1280         };
1281
1282         let fields = self.arena.alloc_from_iter(
1283             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1284                 |(s, e)| {
1285                     let expr = self.lower_expr(&e);
1286                     let ident = Ident::new(s, self.lower_span(e.span));
1287                     self.expr_field(ident, expr, e.span)
1288                 },
1289             ),
1290         );
1291
1292         hir::ExprKind::Struct(
1293             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1294             fields,
1295             None,
1296         )
1297     }
1298
1299     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1300         let label = opt_label?;
1301         Some(Label { ident: self.lower_ident(label.ident) })
1302     }
1303
1304     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1305         let target_id = match destination {
1306             Some((id, _)) => {
1307                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1308                     Ok(self.lower_node_id(loop_id))
1309                 } else {
1310                     Err(hir::LoopIdError::UnresolvedLabel)
1311                 }
1312             }
1313             None => self
1314                 .loop_scope
1315                 .map(|id| Ok(self.lower_node_id(id)))
1316                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1317         };
1318         let label = self.lower_label(destination.map(|(_, label)| label));
1319         hir::Destination { label, target_id }
1320     }
1321
1322     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1323         if self.is_in_loop_condition && opt_label.is_none() {
1324             hir::Destination {
1325                 label: None,
1326                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1327             }
1328         } else {
1329             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1330         }
1331     }
1332
1333     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1334         let old_scope = self.catch_scope.replace(catch_id);
1335         let result = f(self);
1336         self.catch_scope = old_scope;
1337         result
1338     }
1339
1340     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1341         // We're no longer in the base loop's condition; we're in another loop.
1342         let was_in_loop_condition = self.is_in_loop_condition;
1343         self.is_in_loop_condition = false;
1344
1345         let old_scope = self.loop_scope.replace(loop_id);
1346         let result = f(self);
1347         self.loop_scope = old_scope;
1348
1349         self.is_in_loop_condition = was_in_loop_condition;
1350
1351         result
1352     }
1353
1354     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1355         let was_in_loop_condition = self.is_in_loop_condition;
1356         self.is_in_loop_condition = true;
1357
1358         let result = f(self);
1359
1360         self.is_in_loop_condition = was_in_loop_condition;
1361
1362         result
1363     }
1364
1365     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1366         let hir_id = self.lower_node_id(f.id);
1367         self.lower_attrs(hir_id, &f.attrs);
1368         hir::ExprField {
1369             hir_id,
1370             ident: self.lower_ident(f.ident),
1371             expr: self.lower_expr(&f.expr),
1372             span: self.lower_span(f.span),
1373             is_shorthand: f.is_shorthand,
1374         }
1375     }
1376
1377     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1378         match self.generator_kind {
1379             Some(hir::GeneratorKind::Gen) => {}
1380             Some(hir::GeneratorKind::Async(_)) => {
1381                 self.tcx.sess.emit_err(AsyncGeneratorsNotSupported { span });
1382             }
1383             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1384         }
1385
1386         let expr =
1387             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1388
1389         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1390     }
1391
1392     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1393     /// ```ignore (pseudo-rust)
1394     /// {
1395     ///     let result = match IntoIterator::into_iter(<head>) {
1396     ///         mut iter => {
1397     ///             [opt_ident]: loop {
1398     ///                 match Iterator::next(&mut iter) {
1399     ///                     None => break,
1400     ///                     Some(<pat>) => <body>,
1401     ///                 };
1402     ///             }
1403     ///         }
1404     ///     };
1405     ///     result
1406     /// }
1407     /// ```
1408     fn lower_expr_for(
1409         &mut self,
1410         e: &Expr,
1411         pat: &Pat,
1412         head: &Expr,
1413         body: &Block,
1414         opt_label: Option<Label>,
1415     ) -> hir::Expr<'hir> {
1416         let head = self.lower_expr_mut(head);
1417         let pat = self.lower_pat(pat);
1418         let for_span =
1419             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1420         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1421         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1422
1423         // `None => break`
1424         let none_arm = {
1425             let break_expr =
1426                 self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, AttrVec::new()));
1427             let pat = self.pat_none(for_span);
1428             self.arm(pat, break_expr)
1429         };
1430
1431         // Some(<pat>) => <body>,
1432         let some_arm = {
1433             let some_pat = self.pat_some(pat_span, pat);
1434             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1435             let body_expr = self.arena.alloc(self.expr_block(body_block, AttrVec::new()));
1436             self.arm(some_pat, body_expr)
1437         };
1438
1439         // `mut iter`
1440         let iter = Ident::with_dummy_span(sym::iter);
1441         let (iter_pat, iter_pat_nid) =
1442             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::MUT);
1443
1444         // `match Iterator::next(&mut iter) { ... }`
1445         let match_expr = {
1446             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1447             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1448             let next_expr = self.expr_call_lang_item_fn(
1449                 head_span,
1450                 hir::LangItem::IteratorNext,
1451                 arena_vec![self; ref_mut_iter],
1452                 None,
1453             );
1454             let arms = arena_vec![self; none_arm, some_arm];
1455
1456             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1457         };
1458         let match_stmt = self.stmt_expr(for_span, match_expr);
1459
1460         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1461
1462         // `[opt_ident]: loop { ... }`
1463         let kind = hir::ExprKind::Loop(
1464             loop_block,
1465             self.lower_label(opt_label),
1466             hir::LoopSource::ForLoop,
1467             self.lower_span(for_span.with_hi(head.span.hi())),
1468         );
1469         let loop_expr =
1470             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1471
1472         // `mut iter => { ... }`
1473         let iter_arm = self.arm(iter_pat, loop_expr);
1474
1475         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1476         let into_iter_expr = {
1477             self.expr_call_lang_item_fn(
1478                 head_span,
1479                 hir::LangItem::IntoIterIntoIter,
1480                 arena_vec![self; head],
1481                 None,
1482             )
1483         };
1484
1485         let match_expr = self.arena.alloc(self.expr_match(
1486             for_span,
1487             into_iter_expr,
1488             arena_vec![self; iter_arm],
1489             hir::MatchSource::ForLoopDesugar,
1490         ));
1491
1492         // This is effectively `{ let _result = ...; _result }`.
1493         // The construct was introduced in #21984 and is necessary to make sure that
1494         // temporaries in the `head` expression are dropped and do not leak to the
1495         // surrounding scope of the `match` since the `match` is not a terminating scope.
1496         //
1497         // Also, add the attributes to the outer returned expr node.
1498         self.expr_drop_temps_mut(for_span, match_expr, e.attrs.clone())
1499     }
1500
1501     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1502     /// ```ignore (pseudo-rust)
1503     /// match Try::branch(<expr>) {
1504     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1505     ///     ControlFlow::Break(residual) =>
1506     ///         #[allow(unreachable_code)]
1507     ///         // If there is an enclosing `try {...}`:
1508     ///         break 'catch_target Try::from_residual(residual),
1509     ///         // Otherwise:
1510     ///         return Try::from_residual(residual),
1511     /// }
1512     /// ```
1513     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1514         let unstable_span = self.mark_span_with_reason(
1515             DesugaringKind::QuestionMark,
1516             span,
1517             self.allow_try_trait.clone(),
1518         );
1519         let try_span = self.tcx.sess.source_map().end_point(span);
1520         let try_span = self.mark_span_with_reason(
1521             DesugaringKind::QuestionMark,
1522             try_span,
1523             self.allow_try_trait.clone(),
1524         );
1525
1526         // `Try::branch(<expr>)`
1527         let scrutinee = {
1528             // expand <expr>
1529             let sub_expr = self.lower_expr_mut(sub_expr);
1530
1531             self.expr_call_lang_item_fn(
1532                 unstable_span,
1533                 hir::LangItem::TryTraitBranch,
1534                 arena_vec![self; sub_expr],
1535                 None,
1536             )
1537         };
1538
1539         // `#[allow(unreachable_code)]`
1540         let attr = {
1541             // `allow(unreachable_code)`
1542             let allow = {
1543                 let allow_ident = Ident::new(sym::allow, self.lower_span(span));
1544                 let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
1545                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1546                 attr::mk_list_item(allow_ident, vec![uc_nested])
1547             };
1548             attr::mk_attr_outer(allow)
1549         };
1550         let attrs: AttrVec = thin_vec![attr];
1551
1552         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1553         let continue_arm = {
1554             let val_ident = Ident::with_dummy_span(sym::val);
1555             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1556             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1557                 span,
1558                 val_ident,
1559                 val_pat_nid,
1560                 attrs.clone(),
1561             ));
1562             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1563             self.arm(continue_pat, val_expr)
1564         };
1565
1566         // `ControlFlow::Break(residual) =>
1567         //     #[allow(unreachable_code)]
1568         //     return Try::from_residual(residual),`
1569         let break_arm = {
1570             let residual_ident = Ident::with_dummy_span(sym::residual);
1571             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1572             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1573             let from_residual_expr = self.wrap_in_try_constructor(
1574                 hir::LangItem::TryTraitFromResidual,
1575                 try_span,
1576                 self.arena.alloc(residual_expr),
1577                 unstable_span,
1578             );
1579             let ret_expr = if let Some(catch_node) = self.catch_scope {
1580                 let target_id = Ok(self.lower_node_id(catch_node));
1581                 self.arena.alloc(self.expr(
1582                     try_span,
1583                     hir::ExprKind::Break(
1584                         hir::Destination { label: None, target_id },
1585                         Some(from_residual_expr),
1586                     ),
1587                     attrs,
1588                 ))
1589             } else {
1590                 self.arena.alloc(self.expr(
1591                     try_span,
1592                     hir::ExprKind::Ret(Some(from_residual_expr)),
1593                     attrs,
1594                 ))
1595             };
1596
1597             let break_pat = self.pat_cf_break(try_span, residual_local);
1598             self.arm(break_pat, ret_expr)
1599         };
1600
1601         hir::ExprKind::Match(
1602             scrutinee,
1603             arena_vec![self; break_arm, continue_arm],
1604             hir::MatchSource::TryDesugar,
1605         )
1606     }
1607
1608     /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
1609     /// ```rust
1610     /// // If there is an enclosing `try {...}`:
1611     /// break 'catch_target FromResidual::from_residual(Yeet(residual)),
1612     /// // Otherwise:
1613     /// return FromResidual::from_residual(Yeet(residual)),
1614     /// ```
1615     /// But to simplify this, there's a `from_yeet` lang item function which
1616     /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
1617     fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1618         // The expression (if present) or `()` otherwise.
1619         let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
1620             (sub_expr.span, self.lower_expr(sub_expr))
1621         } else {
1622             (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
1623         };
1624
1625         let unstable_span = self.mark_span_with_reason(
1626             DesugaringKind::YeetExpr,
1627             span,
1628             self.allow_try_trait.clone(),
1629         );
1630
1631         let from_yeet_expr = self.wrap_in_try_constructor(
1632             hir::LangItem::TryTraitFromYeet,
1633             unstable_span,
1634             yeeted_expr,
1635             yeeted_span,
1636         );
1637
1638         if let Some(catch_node) = self.catch_scope {
1639             let target_id = Ok(self.lower_node_id(catch_node));
1640             hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
1641         } else {
1642             hir::ExprKind::Ret(Some(from_yeet_expr))
1643         }
1644     }
1645
1646     // =========================================================================
1647     // Helper methods for building HIR.
1648     // =========================================================================
1649
1650     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1651     ///
1652     /// In terms of drop order, it has the same effect as wrapping `expr` in
1653     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1654     ///
1655     /// The drop order can be important in e.g. `if expr { .. }`.
1656     pub(super) fn expr_drop_temps(
1657         &mut self,
1658         span: Span,
1659         expr: &'hir hir::Expr<'hir>,
1660         attrs: AttrVec,
1661     ) -> &'hir hir::Expr<'hir> {
1662         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1663     }
1664
1665     pub(super) fn expr_drop_temps_mut(
1666         &mut self,
1667         span: Span,
1668         expr: &'hir hir::Expr<'hir>,
1669         attrs: AttrVec,
1670     ) -> hir::Expr<'hir> {
1671         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1672     }
1673
1674     fn expr_match(
1675         &mut self,
1676         span: Span,
1677         arg: &'hir hir::Expr<'hir>,
1678         arms: &'hir [hir::Arm<'hir>],
1679         source: hir::MatchSource,
1680     ) -> hir::Expr<'hir> {
1681         self.expr(span, hir::ExprKind::Match(arg, arms, source), AttrVec::new())
1682     }
1683
1684     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
1685         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1686         self.expr(span, expr_break, attrs)
1687     }
1688
1689     fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1690         let expr_break = self.expr_break(span, attrs);
1691         self.arena.alloc(expr_break)
1692     }
1693
1694     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1695         self.expr(
1696             span,
1697             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1698             AttrVec::new(),
1699         )
1700     }
1701
1702     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1703         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), AttrVec::new()))
1704     }
1705
1706     fn expr_call_mut(
1707         &mut self,
1708         span: Span,
1709         e: &'hir hir::Expr<'hir>,
1710         args: &'hir [hir::Expr<'hir>],
1711     ) -> hir::Expr<'hir> {
1712         self.expr(span, hir::ExprKind::Call(e, args), AttrVec::new())
1713     }
1714
1715     fn expr_call(
1716         &mut self,
1717         span: Span,
1718         e: &'hir hir::Expr<'hir>,
1719         args: &'hir [hir::Expr<'hir>],
1720     ) -> &'hir hir::Expr<'hir> {
1721         self.arena.alloc(self.expr_call_mut(span, e, args))
1722     }
1723
1724     fn expr_call_lang_item_fn_mut(
1725         &mut self,
1726         span: Span,
1727         lang_item: hir::LangItem,
1728         args: &'hir [hir::Expr<'hir>],
1729         hir_id: Option<hir::HirId>,
1730     ) -> hir::Expr<'hir> {
1731         let path =
1732             self.arena.alloc(self.expr_lang_item_path(span, lang_item, AttrVec::new(), hir_id));
1733         self.expr_call_mut(span, path, args)
1734     }
1735
1736     fn expr_call_lang_item_fn(
1737         &mut self,
1738         span: Span,
1739         lang_item: hir::LangItem,
1740         args: &'hir [hir::Expr<'hir>],
1741         hir_id: Option<hir::HirId>,
1742     ) -> &'hir hir::Expr<'hir> {
1743         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1744     }
1745
1746     fn expr_lang_item_path(
1747         &mut self,
1748         span: Span,
1749         lang_item: hir::LangItem,
1750         attrs: AttrVec,
1751         hir_id: Option<hir::HirId>,
1752     ) -> hir::Expr<'hir> {
1753         self.expr(
1754             span,
1755             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1756             attrs,
1757         )
1758     }
1759
1760     pub(super) fn expr_ident(
1761         &mut self,
1762         sp: Span,
1763         ident: Ident,
1764         binding: hir::HirId,
1765     ) -> &'hir hir::Expr<'hir> {
1766         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1767     }
1768
1769     pub(super) fn expr_ident_mut(
1770         &mut self,
1771         sp: Span,
1772         ident: Ident,
1773         binding: hir::HirId,
1774     ) -> hir::Expr<'hir> {
1775         self.expr_ident_with_attrs(sp, ident, binding, AttrVec::new())
1776     }
1777
1778     fn expr_ident_with_attrs(
1779         &mut self,
1780         span: Span,
1781         ident: Ident,
1782         binding: hir::HirId,
1783         attrs: AttrVec,
1784     ) -> hir::Expr<'hir> {
1785         let hir_id = self.next_id();
1786         let res = Res::Local(binding);
1787         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1788             None,
1789             self.arena.alloc(hir::Path {
1790                 span: self.lower_span(span),
1791                 res,
1792                 segments: arena_vec![self; hir::PathSegment::new(ident, hir_id, res)],
1793             }),
1794         ));
1795
1796         self.expr(span, expr_path, attrs)
1797     }
1798
1799     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1800         let hir_id = self.next_id();
1801         let span = expr.span;
1802         self.expr(
1803             span,
1804             hir::ExprKind::Block(
1805                 self.arena.alloc(hir::Block {
1806                     stmts: &[],
1807                     expr: Some(expr),
1808                     hir_id,
1809                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1810                     span: self.lower_span(span),
1811                     targeted_by_break: false,
1812                 }),
1813                 None,
1814             ),
1815             AttrVec::new(),
1816         )
1817     }
1818
1819     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1820         let blk = self.block_all(span, &[], None);
1821         let expr = self.expr_block(blk, AttrVec::new());
1822         self.arena.alloc(expr)
1823     }
1824
1825     pub(super) fn expr_block(
1826         &mut self,
1827         b: &'hir hir::Block<'hir>,
1828         attrs: AttrVec,
1829     ) -> hir::Expr<'hir> {
1830         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1831     }
1832
1833     pub(super) fn expr(
1834         &mut self,
1835         span: Span,
1836         kind: hir::ExprKind<'hir>,
1837         attrs: AttrVec,
1838     ) -> hir::Expr<'hir> {
1839         let hir_id = self.next_id();
1840         self.lower_attrs(hir_id, &attrs);
1841         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1842     }
1843
1844     fn expr_field(
1845         &mut self,
1846         ident: Ident,
1847         expr: &'hir hir::Expr<'hir>,
1848         span: Span,
1849     ) -> hir::ExprField<'hir> {
1850         hir::ExprField {
1851             hir_id: self.next_id(),
1852             ident,
1853             span: self.lower_span(span),
1854             expr,
1855             is_shorthand: false,
1856         }
1857     }
1858
1859     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1860         hir::Arm {
1861             hir_id: self.next_id(),
1862             pat,
1863             guard: None,
1864             span: self.lower_span(expr.span),
1865             body: expr,
1866         }
1867     }
1868 }