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_session::errors::report_lit_error;
  18 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
  19 use rustc_span::symbol::{sym, Ident};
  20 use rustc_span::DUMMY_SP;
  21 use thin_vec::thin_vec;
  22
  23 impl<'hir> LoweringContext<'_, 'hir> {
  24     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  25         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  26     }
  27
  28     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  29         self.arena.alloc(self.lower_expr_mut(e))
  30     }
  31
  32     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  33         ensure_sufficient_stack(|| {
  34             let kind = match &e.kind {
  35                 ExprKind::Box(inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  36                 ExprKind::Array(exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  37                 ExprKind::ConstBlock(anon_const) => {
  38                     let anon_const = self.lower_anon_const(anon_const);
  39                     hir::ExprKind::ConstBlock(anon_const)
  40                 }
  41                 ExprKind::Repeat(expr, count) => {
  42                     let expr = self.lower_expr(expr);
  43                     let count = self.lower_array_length(count);
  44                     hir::ExprKind::Repeat(expr, count)
  45                 }
  46                 ExprKind::Tup(elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  47                 ExprKind::Call(f, args) => {
  48                     if e.attrs.get(0).map_or(false, |a| a.has_name(sym::rustc_box)) {
  49                         if let [inner] = &args[..] && e.attrs.len() == 1 {
  50                             let kind = hir::ExprKind::Box(self.lower_expr(&inner));
  51                             let hir_id = self.lower_node_id(e.id);
  52                             return hir::Expr { hir_id, kind, span: self.lower_span(e.span) };
  53                         } else {
  54                             self.tcx.sess.emit_err(RustcBoxAttributeError { span: e.span });
  55                             hir::ExprKind::Err
  56                         }
  57                     } else if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
  58                         self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
  59                     } else {
  60                         let f = self.lower_expr(f);
  61                         hir::ExprKind::Call(f, self.lower_exprs(args))
  62                     }
  63                 }
  64                 ExprKind::MethodCall(box MethodCall { seg, receiver, args, span }) => {
  65                     let hir_seg = self.arena.alloc(self.lower_path_segment(
  66                         e.span,
  67                         seg,
  68                         ParamMode::Optional,
  69                         ParenthesizedGenericArgs::Err,
  70                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  71                     ));
  72                     let receiver = self.lower_expr(receiver);
  73                     let args =
  74                         self.arena.alloc_from_iter(args.iter().map(|x| self.lower_expr_mut(x)));
  75                     hir::ExprKind::MethodCall(hir_seg, receiver, args, self.lower_span(*span))
  76                 }
  77                 ExprKind::Binary(binop, lhs, rhs) => {
  78                     let binop = self.lower_binop(*binop);
  79                     let lhs = self.lower_expr(lhs);
  80                     let rhs = self.lower_expr(rhs);
  81                     hir::ExprKind::Binary(binop, lhs, rhs)
  82                 }
  83                 ExprKind::Unary(op, ohs) => {
  84                     let op = self.lower_unop(*op);
  85                     let ohs = self.lower_expr(ohs);
  86                     hir::ExprKind::Unary(op, ohs)
  87                 }
  88                 ExprKind::Lit(token_lit) => {
  89                     let lit_kind = match LitKind::from_token_lit(*token_lit) {
  90                         Ok(lit_kind) => lit_kind,
  91                         Err(err) => {
  92                             report_lit_error(&self.tcx.sess.parse_sess, err, *token_lit, e.span);
  93                             LitKind::Err
  94                         }
  95                     };
  96                     hir::ExprKind::Lit(respan(self.lower_span(e.span), lit_kind))
  97                 }
  98                 ExprKind::IncludedBytes(bytes) => hir::ExprKind::Lit(respan(
  99                     self.lower_span(e.span),
 100                     LitKind::ByteStr(bytes.clone()),
 101                 )),
 102                 ExprKind::Cast(expr, ty) => {
 103                     let expr = self.lower_expr(expr);
 104                     let ty =
 105                         self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 106                     hir::ExprKind::Cast(expr, ty)
 107                 }
 108                 ExprKind::Type(expr, ty) => {
 109                     let expr = self.lower_expr(expr);
 110                     let ty =
 111                         self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 112                     hir::ExprKind::Type(expr, ty)
 113                 }
 114                 ExprKind::AddrOf(k, m, ohs) => {
 115                     let ohs = self.lower_expr(ohs);
 116                     hir::ExprKind::AddrOf(*k, *m, ohs)
 117                 }
 118                 ExprKind::Let(pat, scrutinee, span) => {
 119                     hir::ExprKind::Let(self.arena.alloc(hir::Let {
 120                         hir_id: self.next_id(),
 121                         span: self.lower_span(*span),
 122                         pat: self.lower_pat(pat),
 123                         ty: None,
 124                         init: self.lower_expr(scrutinee),
 125                     }))
 126                 }
 127                 ExprKind::If(cond, then, else_opt) => {
 128                     self.lower_expr_if(cond, then, else_opt.as_deref())
 129                 }
 130                 ExprKind::While(cond, body, opt_label) => self.with_loop_scope(e.id, |this| {
 131                     let span = this.mark_span_with_reason(DesugaringKind::WhileLoop, e.span, None);
 132                     this.lower_expr_while_in_loop_scope(span, cond, body, *opt_label)
 133                 }),
 134                 ExprKind::Loop(body, opt_label, span) => self.with_loop_scope(e.id, |this| {
 135                     hir::ExprKind::Loop(
 136                         this.lower_block(body, false),
 137                         this.lower_label(*opt_label),
 138                         hir::LoopSource::Loop,
 139                         this.lower_span(*span),
 140                     )
 141                 }),
 142                 ExprKind::TryBlock(body) => self.lower_expr_try_block(body),
 143                 ExprKind::Match(expr, arms) => hir::ExprKind::Match(
 144                     self.lower_expr(expr),
 145                     self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
 146                     hir::MatchSource::Normal,
 147                 ),
 148                 ExprKind::Async(capture_clause, closure_node_id, block) => self.make_async_expr(
 149                     *capture_clause,
 150                     *closure_node_id,
 151                     None,
 152                     e.span,
 153                     hir::AsyncGeneratorKind::Block,
 154                     |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
 155                 ),
 156                 ExprKind::Await(expr) => {
 157                     let dot_await_span = if expr.span.hi() < e.span.hi() {
 158                         let span_with_whitespace = self
 159                             .tcx
 160                             .sess
 161                             .source_map()
 162                             .span_extend_while(expr.span, char::is_whitespace)
 163                             .unwrap_or(expr.span);
 164                         span_with_whitespace.shrink_to_hi().with_hi(e.span.hi())
 165                     } else {
 166                         // this is a recovered `await expr`
 167                         e.span
 168                     };
 169                     self.lower_expr_await(dot_await_span, expr)
 170                 }
 171                 ExprKind::Closure(box Closure {
 172                     binder,
 173                     capture_clause,
 174                     asyncness,
 175                     movability,
 176                     fn_decl,
 177                     body,
 178                     fn_decl_span,
 179                 }) => {
 180                     if let Async::Yes { closure_id, .. } = asyncness {
 181                         self.lower_expr_async_closure(
 182                             binder,
 183                             *capture_clause,
 184                             e.id,
 185                             *closure_id,
 186                             fn_decl,
 187                             body,
 188                             *fn_decl_span,
 189                         )
 190                     } else {
 191                         self.lower_expr_closure(
 192                             binder,
 193                             *capture_clause,
 194                             e.id,
 195                             *movability,
 196                             fn_decl,
 197                             body,
 198                             *fn_decl_span,
 199                         )
 200                     }
 201                 }
 202                 ExprKind::Block(blk, opt_label) => {
 203                     let opt_label = self.lower_label(*opt_label);
 204                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 205                 }
 206                 ExprKind::Assign(el, er, span) => self.lower_expr_assign(el, er, *span, e.span),
 207                 ExprKind::AssignOp(op, el, er) => hir::ExprKind::AssignOp(
 208                     self.lower_binop(*op),
 209                     self.lower_expr(el),
 210                     self.lower_expr(er),
 211                 ),
 212                 ExprKind::Field(el, ident) => {
 213                     hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(*ident))
 214                 }
 215                 ExprKind::Index(el, er) => {
 216                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 217                 }
 218                 ExprKind::Range(Some(e1), Some(e2), RangeLimits::Closed) => {
 219                     self.lower_expr_range_closed(e.span, e1, e2)
 220                 }
 221                 ExprKind::Range(e1, e2, lims) => {
 222                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), *lims)
 223                 }
 224                 ExprKind::Underscore => {
 225                     self.tcx.sess.emit_err(UnderscoreExprLhsAssign { span: e.span });
 226                     hir::ExprKind::Err
 227                 }
 228                 ExprKind::Path(qself, path) => {
 229                     let qpath = self.lower_qpath(
 230                         e.id,
 231                         qself,
 232                         path,
 233                         ParamMode::Optional,
 234                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 235                     );
 236                     hir::ExprKind::Path(qpath)
 237                 }
 238                 ExprKind::Break(opt_label, opt_expr) => {
 239                     let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 240                     hir::ExprKind::Break(self.lower_jump_destination(e.id, *opt_label), opt_expr)
 241                 }
 242                 ExprKind::Continue(opt_label) => {
 243                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, *opt_label))
 244                 }
 245                 ExprKind::Ret(e) => {
 246                     let e = e.as_ref().map(|x| self.lower_expr(x));
 247                     hir::ExprKind::Ret(e)
 248                 }
 249                 ExprKind::Yeet(sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
 250                 ExprKind::InlineAsm(asm) => {
 251                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
 252                 }
 253                 ExprKind::Struct(se) => {
 254                     let rest = match &se.rest {
 255                         StructRest::Base(e) => Some(self.lower_expr(e)),
 256                         StructRest::Rest(sp) => {
 257                             self.tcx.sess.emit_err(BaseExpressionDoubleDot { span: *sp });
 258                             Some(&*self.arena.alloc(self.expr_err(*sp)))
 259                         }
 260                         StructRest::None => None,
 261                     };
 262                     hir::ExprKind::Struct(
 263                         self.arena.alloc(self.lower_qpath(
 264                             e.id,
 265                             &se.qself,
 266                             &se.path,
 267                             ParamMode::Optional,
 268                             &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 269                         )),
 270                         self.arena
 271                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
 272                         rest,
 273                     )
 274                 }
 275                 ExprKind::Yield(opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 276                 ExprKind::Err => hir::ExprKind::Err,
 277                 ExprKind::Try(sub_expr) => self.lower_expr_try(e.span, sub_expr),
 278                 ExprKind::Paren(ex) => {
 279                     let mut ex = self.lower_expr_mut(ex);
 280                     // Include parens in span, but only if it is a super-span.
 281                     if e.span.contains(ex.span) {
 282                         ex.span = self.lower_span(e.span);
 283                     }
 284                     // Merge attributes into the inner expression.
 285                     if !e.attrs.is_empty() {
 286                         let old_attrs =
 287                             self.attrs.get(&ex.hir_id.local_id).map(|la| *la).unwrap_or(&[]);
 288                         self.attrs.insert(
 289                             ex.hir_id.local_id,
 290                             &*self.arena.alloc_from_iter(
 291                                 e.attrs
 292                                     .iter()
 293                                     .map(|a| self.lower_attr(a))
 294                                     .chain(old_attrs.iter().cloned()),
 295                             ),
 296                         );
 297                     }
 298                     return ex;
 299                 }
 300
 301                 // Desugar `ExprForLoop`
 302                 // from: `[opt_ident]: for <pat> in <head> <body>`
 303                 ExprKind::ForLoop(pat, head, body, opt_label) => {
 304                     return self.lower_expr_for(e, pat, head, body, *opt_label);
 305                 }
 306                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 307             };
 308
 309             let hir_id = self.lower_node_id(e.id);
 310             self.lower_attrs(hir_id, &e.attrs);
 311             hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
 312         })
 313     }
 314
 315     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 316         match u {
 317             UnOp::Deref => hir::UnOp::Deref,
 318             UnOp::Not => hir::UnOp::Not,
 319             UnOp::Neg => hir::UnOp::Neg,
 320         }
 321     }
 322
 323     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 324         Spanned {
 325             node: match b.node {
 326                 BinOpKind::Add => hir::BinOpKind::Add,
 327                 BinOpKind::Sub => hir::BinOpKind::Sub,
 328                 BinOpKind::Mul => hir::BinOpKind::Mul,
 329                 BinOpKind::Div => hir::BinOpKind::Div,
 330                 BinOpKind::Rem => hir::BinOpKind::Rem,
 331                 BinOpKind::And => hir::BinOpKind::And,
 332                 BinOpKind::Or => hir::BinOpKind::Or,
 333                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 334                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 335                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 336                 BinOpKind::Shl => hir::BinOpKind::Shl,
 337                 BinOpKind::Shr => hir::BinOpKind::Shr,
 338                 BinOpKind::Eq => hir::BinOpKind::Eq,
 339                 BinOpKind::Lt => hir::BinOpKind::Lt,
 340                 BinOpKind::Le => hir::BinOpKind::Le,
 341                 BinOpKind::Ne => hir::BinOpKind::Ne,
 342                 BinOpKind::Ge => hir::BinOpKind::Ge,
 343                 BinOpKind::Gt => hir::BinOpKind::Gt,
 344             },
 345             span: self.lower_span(b.span),
 346         }
 347     }
 348
 349     fn lower_legacy_const_generics(
 350         &mut self,
 351         mut f: Expr,
 352         args: Vec<AstP<Expr>>,
 353         legacy_args_idx: &[usize],
 354     ) -> hir::ExprKind<'hir> {
 355         let ExprKind::Path(None, path) = &mut f.kind else {
 356             unreachable!();
 357         };
 358
 359         // Split the arguments into const generics and normal arguments
 360         let mut real_args = vec![];
 361         let mut generic_args = vec![];
 362         for (idx, arg) in args.into_iter().enumerate() {
 363             if legacy_args_idx.contains(&idx) {
 364                 let parent_def_id = self.current_hir_id_owner;
 365                 let node_id = self.next_node_id();
 366
 367                 // Add a definition for the in-band const def.
 368                 self.create_def(parent_def_id.def_id, node_id, DefPathData::AnonConst);
 369
 370                 let anon_const = AnonConst { id: node_id, value: arg };
 371                 generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
 372             } else {
 373                 real_args.push(arg);
 374             }
 375         }
 376
 377         // Add generic args to the last element of the path.
 378         let last_segment = path.segments.last_mut().unwrap();
 379         assert!(last_segment.args.is_none());
 380         last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
 381             span: DUMMY_SP,
 382             args: generic_args,
 383         })));
 384
 385         // Now lower everything as normal.
 386         let f = self.lower_expr(&f);
 387         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
 388     }
 389
 390     fn lower_expr_if(
 391         &mut self,
 392         cond: &Expr,
 393         then: &Block,
 394         else_opt: Option<&Expr>,
 395     ) -> hir::ExprKind<'hir> {
 396         let lowered_cond = self.lower_cond(cond);
 397         let then_expr = self.lower_block_expr(then);
 398         if let Some(rslt) = else_opt {
 399             hir::ExprKind::If(
 400                 lowered_cond,
 401                 self.arena.alloc(then_expr),
 402                 Some(self.lower_expr(rslt)),
 403             )
 404         } else {
 405             hir::ExprKind::If(lowered_cond, self.arena.alloc(then_expr), None)
 406         }
 407     }
 408
 409     // Lowers a condition (i.e. `cond` in `if cond` or `while cond`), wrapping it in a terminating scope
 410     // so that temporaries created in the condition don't live beyond it.
 411     fn lower_cond(&mut self, cond: &Expr) -> &'hir hir::Expr<'hir> {
 412         fn has_let_expr(expr: &Expr) -> bool {
 413             match &expr.kind {
 414                 ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
 415                 ExprKind::Let(..) => true,
 416                 _ => false,
 417             }
 418         }
 419
 420         // We have to take special care for `let` exprs in the condition, e.g. in
 421         // `if let pat = val` or `if foo && let pat = val`, as we _do_ want `val` to live beyond the
 422         // condition in this case.
 423         //
 424         // In order to mantain the drop behavior for the non `let` parts of the condition,
 425         // we still wrap them in terminating scopes, e.g. `if foo && let pat = val` essentially
 426         // gets transformed into `if { let _t = foo; _t } && let pat = val`
 427         match &cond.kind {
 428             ExprKind::Binary(op @ Spanned { node: ast::BinOpKind::And, .. }, lhs, rhs)
 429                 if has_let_expr(cond) =>
 430             {
 431                 let op = self.lower_binop(*op);
 432                 let lhs = self.lower_cond(lhs);
 433                 let rhs = self.lower_cond(rhs);
 434
 435                 self.arena.alloc(self.expr(
 436                     cond.span,
 437                     hir::ExprKind::Binary(op, lhs, rhs),
 438                     AttrVec::new(),
 439                 ))
 440             }
 441             ExprKind::Let(..) => self.lower_expr(cond),
 442             _ => {
 443                 let cond = self.lower_expr(cond);
 444                 let reason = DesugaringKind::CondTemporary;
 445                 let span_block = self.mark_span_with_reason(reason, cond.span, None);
 446                 self.expr_drop_temps(span_block, cond, AttrVec::new())
 447             }
 448         }
 449     }
 450
 451     // We desugar: `'label: while $cond $body` into:
 452     //
 453     // ```
 454     // 'label: loop {
 455     //   if { let _t = $cond; _t } {
 456     //     $body
 457     //   }
 458     //   else {
 459     //     break;
 460     //   }
 461     // }
 462     // ```
 463     //
 464     // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 465     // to preserve drop semantics since `while $cond { ... }` does not
 466     // let temporaries live outside of `cond`.
 467     fn lower_expr_while_in_loop_scope(
 468         &mut self,
 469         span: Span,
 470         cond: &Expr,
 471         body: &Block,
 472         opt_label: Option<Label>,
 473     ) -> hir::ExprKind<'hir> {
 474         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_cond(cond));
 475         let then = self.lower_block_expr(body);
 476         let expr_break = self.expr_break(span, AttrVec::new());
 477         let stmt_break = self.stmt_expr(span, expr_break);
 478         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
 479         let else_expr = self.arena.alloc(self.expr_block(else_blk, AttrVec::new()));
 480         let if_kind = hir::ExprKind::If(lowered_cond, self.arena.alloc(then), Some(else_expr));
 481         let if_expr = self.expr(span, if_kind, AttrVec::new());
 482         let block = self.block_expr(self.arena.alloc(if_expr));
 483         let span = self.lower_span(span.with_hi(cond.span.hi()));
 484         let opt_label = self.lower_label(opt_label);
 485         hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
 486     }
 487
 488     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
 489     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
 490     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 491     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 492         self.with_catch_scope(body.id, |this| {
 493             let mut block = this.lower_block_noalloc(body, true);
 494
 495             // Final expression of the block (if present) or `()` with span at the end of block
 496             let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
 497                 (
 498                     this.mark_span_with_reason(
 499                         DesugaringKind::TryBlock,
 500                         expr.span,
 501                         this.allow_try_trait.clone(),
 502                     ),
 503                     expr,
 504                 )
 505             } else {
 506                 let try_span = this.mark_span_with_reason(
 507                     DesugaringKind::TryBlock,
 508                     this.tcx.sess.source_map().end_point(body.span),
 509                     this.allow_try_trait.clone(),
 510                 );
 511
 512                 (try_span, this.expr_unit(try_span))
 513             };
 514
 515             let ok_wrapped_span =
 516                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 517
 518             // `::std::ops::Try::from_output($tail_expr)`
 519             block.expr = Some(this.wrap_in_try_constructor(
 520                 hir::LangItem::TryTraitFromOutput,
 521                 try_span,
 522                 tail_expr,
 523                 ok_wrapped_span,
 524             ));
 525
 526             hir::ExprKind::Block(this.arena.alloc(block), None)
 527         })
 528     }
 529
 530     fn wrap_in_try_constructor(
 531         &mut self,
 532         lang_item: hir::LangItem,
 533         method_span: Span,
 534         expr: &'hir hir::Expr<'hir>,
 535         overall_span: Span,
 536     ) -> &'hir hir::Expr<'hir> {
 537         let constructor = self.arena.alloc(self.expr_lang_item_path(
 538             method_span,
 539             lang_item,
 540             AttrVec::new(),
 541             None,
 542         ));
 543         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 544     }
 545
 546     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 547         let pat = self.lower_pat(&arm.pat);
 548         let guard = arm.guard.as_ref().map(|cond| {
 549             if let ExprKind::Let(pat, scrutinee, span) = &cond.kind {
 550                 hir::Guard::IfLet(self.arena.alloc(hir::Let {
 551                     hir_id: self.next_id(),
 552                     span: self.lower_span(*span),
 553                     pat: self.lower_pat(pat),
 554                     ty: None,
 555                     init: self.lower_expr(scrutinee),
 556                 }))
 557             } else {
 558                 hir::Guard::If(self.lower_expr(cond))
 559             }
 560         });
 561         let hir_id = self.next_id();
 562         self.lower_attrs(hir_id, &arm.attrs);
 563         hir::Arm {
 564             hir_id,
 565             pat,
 566             guard,
 567             body: self.lower_expr(&arm.body),
 568             span: self.lower_span(arm.span),
 569         }
 570     }
 571
 572     /// Lower an `async` construct to a generator that implements `Future`.
 573     ///
 574     /// This results in:
 575     ///
 576     /// ```text
 577     /// std::future::identity_future(static move? |_task_context| -> <ret_ty> {
 578     ///     <body>
 579     /// })
 580     /// ```
 581     pub(super) fn make_async_expr(
 582         &mut self,
 583         capture_clause: CaptureBy,
 584         closure_node_id: NodeId,
 585         ret_ty: Option<hir::FnRetTy<'hir>>,
 586         span: Span,
 587         async_gen_kind: hir::AsyncGeneratorKind,
 588         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 589     ) -> hir::ExprKind<'hir> {
 590         let output = ret_ty.unwrap_or_else(|| hir::FnRetTy::DefaultReturn(self.lower_span(span)));
 591
 592         // Resume argument type: `ResumeTy`
 593         let unstable_span =
 594             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 595         let resume_ty = hir::QPath::LangItem(hir::LangItem::ResumeTy, unstable_span, None);
 596         let input_ty = hir::Ty {
 597             hir_id: self.next_id(),
 598             kind: hir::TyKind::Path(resume_ty),
 599             span: unstable_span,
 600         };
 601
 602         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 603         let fn_decl = self.arena.alloc(hir::FnDecl {
 604             inputs: arena_vec![self; input_ty],
 605             output,
 606             c_variadic: false,
 607             implicit_self: hir::ImplicitSelfKind::None,
 608         });
 609
 610         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 611         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 612             span,
 613             Ident::with_dummy_span(sym::_task_context),
 614             hir::BindingAnnotation::MUT,
 615         );
 616         let param = hir::Param {
 617             hir_id: self.next_id(),
 618             pat,
 619             ty_span: self.lower_span(span),
 620             span: self.lower_span(span),
 621         };
 622         let params = arena_vec![self; param];
 623
 624         let body = self.lower_body(move |this| {
 625             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 626
 627             let old_ctx = this.task_context;
 628             this.task_context = Some(task_context_hid);
 629             let res = body(this);
 630             this.task_context = old_ctx;
 631             (params, res)
 632         });
 633
 634         // `static |_task_context| -> <ret_ty> { body }`:
 635         let generator_kind = {
 636             let c = self.arena.alloc(hir::Closure {
 637                 def_id: self.local_def_id(closure_node_id),
 638                 binder: hir::ClosureBinder::Default,
 639                 capture_clause,
 640                 bound_generic_params: &[],
 641                 fn_decl,
 642                 body,
 643                 fn_decl_span: self.lower_span(span),
 644                 movability: Some(hir::Movability::Static),
 645             });
 646
 647             hir::ExprKind::Closure(c)
 648         };
 649         let parent_has_track_caller = self
 650             .attrs
 651             .values()
 652             .find(|attrs| attrs.into_iter().find(|attr| attr.has_name(sym::track_caller)).is_some())
 653             .is_some();
 654         let unstable_span =
 655             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 656
 657         let hir_id = if parent_has_track_caller {
 658             let generator_hir_id = self.lower_node_id(closure_node_id);
 659             self.lower_attrs(
 660                 generator_hir_id,
 661                 &[Attribute {
 662                     kind: AttrKind::Normal(ptr::P(NormalAttr {
 663                         item: AttrItem {
 664                             path: Path::from_ident(Ident::new(sym::track_caller, span)),
 665                             args: AttrArgs::Empty,
 666                             tokens: None,
 667                         },
 668                         tokens: None,
 669                     })),
 670                     id: self.tcx.sess.parse_sess.attr_id_generator.mk_attr_id(),
 671                     style: AttrStyle::Outer,
 672                     span: unstable_span,
 673                 }],
 674             );
 675             generator_hir_id
 676         } else {
 677             self.lower_node_id(closure_node_id)
 678         };
 679
 680         let generator = hir::Expr { hir_id, kind: generator_kind, span: self.lower_span(span) };
 681
 682         // FIXME(swatinem):
 683         // For some reason, the async block needs to flow through *any*
 684         // call (like the identity function), as otherwise type and lifetime
 685         // inference have a hard time figuring things out.
 686         // Without this, we would get:
 687         // E0720 in src/test/ui/impl-trait/in-trait/default-body-with-rpit.rs
 688         // E0700 in src/test/ui/self/self_lifetime-async.rs
 689
 690         // `future::identity_future`:
 691         let identity_future = self.expr_lang_item_path(
 692             unstable_span,
 693             hir::LangItem::IdentityFuture,
 694             AttrVec::new(),
 695             None,
 696         );
 697
 698         // `future::identity_future(generator)`:
 699         hir::ExprKind::Call(self.arena.alloc(identity_future), arena_vec![self; generator])
 700     }
 701
 702     /// Desugar `<expr>.await` into:
 703     /// ```ignore (pseudo-rust)
 704     /// match ::std::future::IntoFuture::into_future(<expr>) {
 705     ///     mut __awaitee => loop {
 706     ///         match unsafe { ::std::future::Future::poll(
 707     ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
 708     ///             ::std::future::get_context(task_context),
 709     ///         ) } {
 710     ///             ::std::task::Poll::Ready(result) => break result,
 711     ///             ::std::task::Poll::Pending => {}
 712     ///         }
 713     ///         task_context = yield ();
 714     ///     }
 715     /// }
 716     /// ```
 717     fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 718         let full_span = expr.span.to(dot_await_span);
 719         match self.generator_kind {
 720             Some(hir::GeneratorKind::Async(_)) => {}
 721             Some(hir::GeneratorKind::Gen) | None => {
 722                 self.tcx.sess.emit_err(AwaitOnlyInAsyncFnAndBlocks {
 723                     dot_await_span,
 724                     item_span: self.current_item,
 725                 });
 726             }
 727         }
 728         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
 729         let gen_future_span = self.mark_span_with_reason(
 730             DesugaringKind::Await,
 731             full_span,
 732             self.allow_gen_future.clone(),
 733         );
 734         let expr = self.lower_expr_mut(expr);
 735         let expr_hir_id = expr.hir_id;
 736
 737         // Note that the name of this binding must not be changed to something else because
 738         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
 739         // this name to identify what is being awaited by a suspended async functions.
 740         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
 741         let (awaitee_pat, awaitee_pat_hid) =
 742             self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::MUT);
 743
 744         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 745
 746         // unsafe {
 747         //     ::std::future::Future::poll(
 748         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
 749         //         ::std::future::get_context(task_context),
 750         //     )
 751         // }
 752         let poll_expr = {
 753             let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
 754             let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
 755             let task_context = if let Some(task_context_hid) = self.task_context {
 756                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 757             } else {
 758                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 759                 self.expr_err(span)
 760             };
 761             let new_unchecked = self.expr_call_lang_item_fn_mut(
 762                 span,
 763                 hir::LangItem::PinNewUnchecked,
 764                 arena_vec![self; ref_mut_awaitee],
 765                 Some(expr_hir_id),
 766             );
 767             let get_context = self.expr_call_lang_item_fn_mut(
 768                 gen_future_span,
 769                 hir::LangItem::GetContext,
 770                 arena_vec![self; task_context],
 771                 Some(expr_hir_id),
 772             );
 773             let call = self.expr_call_lang_item_fn(
 774                 span,
 775                 hir::LangItem::FuturePoll,
 776                 arena_vec![self; new_unchecked, get_context],
 777                 Some(expr_hir_id),
 778             );
 779             self.arena.alloc(self.expr_unsafe(call))
 780         };
 781
 782         // `::std::task::Poll::Ready(result) => break result`
 783         let loop_node_id = self.next_node_id();
 784         let loop_hir_id = self.lower_node_id(loop_node_id);
 785         let ready_arm = {
 786             let x_ident = Ident::with_dummy_span(sym::result);
 787             let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
 788             let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
 789             let ready_field = self.single_pat_field(gen_future_span, x_pat);
 790             let ready_pat = self.pat_lang_item_variant(
 791                 span,
 792                 hir::LangItem::PollReady,
 793                 ready_field,
 794                 Some(expr_hir_id),
 795             );
 796             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 797                 let expr_break =
 798                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 799                 this.arena.alloc(this.expr(gen_future_span, expr_break, AttrVec::new()))
 800             });
 801             self.arm(ready_pat, break_x)
 802         };
 803
 804         // `::std::task::Poll::Pending => {}`
 805         let pending_arm = {
 806             let pending_pat = self.pat_lang_item_variant(
 807                 span,
 808                 hir::LangItem::PollPending,
 809                 &[],
 810                 Some(expr_hir_id),
 811             );
 812             let empty_block = self.expr_block_empty(span);
 813             self.arm(pending_pat, empty_block)
 814         };
 815
 816         let inner_match_stmt = {
 817             let match_expr = self.expr_match(
 818                 span,
 819                 poll_expr,
 820                 arena_vec![self; ready_arm, pending_arm],
 821                 hir::MatchSource::AwaitDesugar,
 822             );
 823             self.stmt_expr(span, match_expr)
 824         };
 825
 826         // task_context = yield ();
 827         let yield_stmt = {
 828             let unit = self.expr_unit(span);
 829             let yield_expr = self.expr(
 830                 span,
 831                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
 832                 AttrVec::new(),
 833             );
 834             let yield_expr = self.arena.alloc(yield_expr);
 835
 836             if let Some(task_context_hid) = self.task_context {
 837                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 838                 let assign = self.expr(
 839                     span,
 840                     hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)),
 841                     AttrVec::new(),
 842                 );
 843                 self.stmt_expr(span, assign)
 844             } else {
 845                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 846                 // proceed with type checking.
 847                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 848             }
 849         };
 850
 851         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 852
 853         // loop { .. }
 854         let loop_expr = self.arena.alloc(hir::Expr {
 855             hir_id: loop_hir_id,
 856             kind: hir::ExprKind::Loop(
 857                 loop_block,
 858                 None,
 859                 hir::LoopSource::Loop,
 860                 self.lower_span(span),
 861             ),
 862             span: self.lower_span(span),
 863         });
 864
 865         // mut __awaitee => loop { ... }
 866         let awaitee_arm = self.arm(awaitee_pat, loop_expr);
 867
 868         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
 869         let into_future_span = self.mark_span_with_reason(
 870             DesugaringKind::Await,
 871             dot_await_span,
 872             self.allow_into_future.clone(),
 873         );
 874         let into_future_expr = self.expr_call_lang_item_fn(
 875             into_future_span,
 876             hir::LangItem::IntoFutureIntoFuture,
 877             arena_vec![self; expr],
 878             Some(expr_hir_id),
 879         );
 880
 881         // match <into_future_expr> {
 882         //     mut __awaitee => loop { .. }
 883         // }
 884         hir::ExprKind::Match(
 885             into_future_expr,
 886             arena_vec![self; awaitee_arm],
 887             hir::MatchSource::AwaitDesugar,
 888         )
 889     }
 890
 891     fn lower_expr_closure(
 892         &mut self,
 893         binder: &ClosureBinder,
 894         capture_clause: CaptureBy,
 895         closure_id: NodeId,
 896         movability: Movability,
 897         decl: &FnDecl,
 898         body: &Expr,
 899         fn_decl_span: Span,
 900     ) -> hir::ExprKind<'hir> {
 901         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 902
 903         let (body_id, generator_option) = self.with_new_scopes(move |this| {
 904             let prev = this.current_item;
 905             this.current_item = Some(fn_decl_span);
 906             let mut generator_kind = None;
 907             let body_id = this.lower_fn_body(decl, |this| {
 908                 let e = this.lower_expr_mut(body);
 909                 generator_kind = this.generator_kind;
 910                 e
 911             });
 912             let generator_option =
 913                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 914             this.current_item = prev;
 915             (body_id, generator_option)
 916         });
 917
 918         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
 919         // Lower outside new scope to preserve `is_in_loop_condition`.
 920         let fn_decl = self.lower_fn_decl(decl, None, fn_decl_span, FnDeclKind::Closure, None);
 921
 922         let c = self.arena.alloc(hir::Closure {
 923             def_id: self.local_def_id(closure_id),
 924             binder: binder_clause,
 925             capture_clause,
 926             bound_generic_params,
 927             fn_decl,
 928             body: body_id,
 929             fn_decl_span: self.lower_span(fn_decl_span),
 930             movability: generator_option,
 931         });
 932
 933         hir::ExprKind::Closure(c)
 934     }
 935
 936     fn generator_movability_for_fn(
 937         &mut self,
 938         decl: &FnDecl,
 939         fn_decl_span: Span,
 940         generator_kind: Option<hir::GeneratorKind>,
 941         movability: Movability,
 942     ) -> Option<hir::Movability> {
 943         match generator_kind {
 944             Some(hir::GeneratorKind::Gen) => {
 945                 if decl.inputs.len() > 1 {
 946                     self.tcx.sess.emit_err(GeneratorTooManyParameters { fn_decl_span });
 947                 }
 948                 Some(movability)
 949             }
 950             Some(hir::GeneratorKind::Async(_)) => {
 951                 panic!("non-`async` closure body turned `async` during lowering");
 952             }
 953             None => {
 954                 if movability == Movability::Static {
 955                     self.tcx.sess.emit_err(ClosureCannotBeStatic { fn_decl_span });
 956                 }
 957                 None
 958             }
 959         }
 960     }
 961
 962     fn lower_closure_binder<'c>(
 963         &mut self,
 964         binder: &'c ClosureBinder,
 965     ) -> (hir::ClosureBinder, &'c [GenericParam]) {
 966         let (binder, params) = match binder {
 967             ClosureBinder::NotPresent => (hir::ClosureBinder::Default, &[][..]),
 968             ClosureBinder::For { span, generic_params } => {
 969                 let span = self.lower_span(*span);
 970                 (hir::ClosureBinder::For { span }, &**generic_params)
 971             }
 972         };
 973
 974         (binder, params)
 975     }
 976
 977     fn lower_expr_async_closure(
 978         &mut self,
 979         binder: &ClosureBinder,
 980         capture_clause: CaptureBy,
 981         closure_id: NodeId,
 982         inner_closure_id: NodeId,
 983         decl: &FnDecl,
 984         body: &Expr,
 985         fn_decl_span: Span,
 986     ) -> hir::ExprKind<'hir> {
 987         if let &ClosureBinder::For { span, .. } = binder {
 988             self.tcx.sess.emit_err(NotSupportedForLifetimeBinderAsyncClosure { span });
 989         }
 990
 991         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 992
 993         let outer_decl =
 994             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 995
 996         let body = self.with_new_scopes(|this| {
 997             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 998             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 999                 this.tcx.sess.emit_err(AsyncNonMoveClosureNotSupported { fn_decl_span });
1000             }
1001
1002             // Transform `async |x: u8| -> X { ... }` into
1003             // `|x: u8| identity_future(|| -> X { ... })`.
1004             let body_id = this.lower_fn_body(&outer_decl, |this| {
1005                 let async_ret_ty = if let FnRetTy::Ty(ty) = &decl.output {
1006                     let itctx = ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock);
1007                     Some(hir::FnRetTy::Return(this.lower_ty(&ty, &itctx)))
1008                 } else {
1009                     None
1010                 };
1011
1012                 let async_body = this.make_async_expr(
1013                     capture_clause,
1014                     inner_closure_id,
1015                     async_ret_ty,
1016                     body.span,
1017                     hir::AsyncGeneratorKind::Closure,
1018                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
1019                 );
1020                 this.expr(fn_decl_span, async_body, AttrVec::new())
1021             });
1022             body_id
1023         });
1024
1025         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
1026         // We need to lower the declaration outside the new scope, because we
1027         // have to conserve the state of being inside a loop condition for the
1028         // closure argument types.
1029         let fn_decl =
1030             self.lower_fn_decl(&outer_decl, None, fn_decl_span, FnDeclKind::Closure, None);
1031
1032         let c = self.arena.alloc(hir::Closure {
1033             def_id: self.local_def_id(closure_id),
1034             binder: binder_clause,
1035             capture_clause,
1036             bound_generic_params,
1037             fn_decl,
1038             body,
1039             fn_decl_span: self.lower_span(fn_decl_span),
1040             movability: None,
1041         });
1042         hir::ExprKind::Closure(c)
1043     }
1044
1045     /// Destructure the LHS of complex assignments.
1046     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
1047     fn lower_expr_assign(
1048         &mut self,
1049         lhs: &Expr,
1050         rhs: &Expr,
1051         eq_sign_span: Span,
1052         whole_span: Span,
1053     ) -> hir::ExprKind<'hir> {
1054         // Return early in case of an ordinary assignment.
1055         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
1056             match &lhs.kind {
1057                 ExprKind::Array(..)
1058                 | ExprKind::Struct(..)
1059                 | ExprKind::Tup(..)
1060                 | ExprKind::Underscore => false,
1061                 // Check for tuple struct constructor.
1062                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
1063                 ExprKind::Paren(e) => {
1064                     match e.kind {
1065                         // We special-case `(..)` for consistency with patterns.
1066                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
1067                         _ => is_ordinary(lower_ctx, e),
1068                     }
1069                 }
1070                 _ => true,
1071             }
1072         }
1073         if is_ordinary(self, lhs) {
1074             return hir::ExprKind::Assign(
1075                 self.lower_expr(lhs),
1076                 self.lower_expr(rhs),
1077                 self.lower_span(eq_sign_span),
1078             );
1079         }
1080
1081         let mut assignments = vec![];
1082
1083         // The LHS becomes a pattern: `(lhs1, lhs2)`.
1084         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
1085         let rhs = self.lower_expr(rhs);
1086
1087         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
1088         let destructure_let = self.stmt_let_pat(
1089             None,
1090             whole_span,
1091             Some(rhs),
1092             pat,
1093             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
1094         );
1095
1096         // `a = lhs1; b = lhs2;`.
1097         let stmts = self
1098             .arena
1099             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
1100
1101         // Wrap everything in a block.
1102         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
1103     }
1104
1105     /// If the given expression is a path to a tuple struct, returns that path.
1106     /// It is not a complete check, but just tries to reject most paths early
1107     /// if they are not tuple structs.
1108     /// Type checking will take care of the full validation later.
1109     fn extract_tuple_struct_path<'a>(
1110         &mut self,
1111         expr: &'a Expr,
1112     ) -> Option<(&'a Option<AstP<QSelf>>, &'a Path)> {
1113         if let ExprKind::Path(qself, path) = &expr.kind {
1114             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1115             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1116                 if let Some(res) = partial_res.full_res() && !res.expected_in_tuple_struct_pat() {
1117                     return None;
1118                 }
1119             }
1120             return Some((qself, path));
1121         }
1122         None
1123     }
1124
1125     /// If the given expression is a path to a unit struct, returns that path.
1126     /// It is not a complete check, but just tries to reject most paths early
1127     /// if they are not unit structs.
1128     /// Type checking will take care of the full validation later.
1129     fn extract_unit_struct_path<'a>(
1130         &mut self,
1131         expr: &'a Expr,
1132     ) -> Option<(&'a Option<AstP<QSelf>>, &'a Path)> {
1133         if let ExprKind::Path(qself, path) = &expr.kind {
1134             // Does the path resolve to something disallowed in a unit struct/variant pattern?
1135             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1136                 if let Some(res) = partial_res.full_res() && !res.expected_in_unit_struct_pat() {
1137                     return None;
1138                 }
1139             }
1140             return Some((qself, path));
1141         }
1142         None
1143     }
1144
1145     /// Convert the LHS of a destructuring assignment to a pattern.
1146     /// Each sub-assignment is recorded in `assignments`.
1147     fn destructure_assign(
1148         &mut self,
1149         lhs: &Expr,
1150         eq_sign_span: Span,
1151         assignments: &mut Vec<hir::Stmt<'hir>>,
1152     ) -> &'hir hir::Pat<'hir> {
1153         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1154     }
1155
1156     fn destructure_assign_mut(
1157         &mut self,
1158         lhs: &Expr,
1159         eq_sign_span: Span,
1160         assignments: &mut Vec<hir::Stmt<'hir>>,
1161     ) -> hir::Pat<'hir> {
1162         match &lhs.kind {
1163             // Underscore pattern.
1164             ExprKind::Underscore => {
1165                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1166             }
1167             // Slice patterns.
1168             ExprKind::Array(elements) => {
1169                 let (pats, rest) =
1170                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1171                 let slice_pat = if let Some((i, span)) = rest {
1172                     let (before, after) = pats.split_at(i);
1173                     hir::PatKind::Slice(
1174                         before,
1175                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1176                         after,
1177                     )
1178                 } else {
1179                     hir::PatKind::Slice(pats, None, &[])
1180                 };
1181                 return self.pat_without_dbm(lhs.span, slice_pat);
1182             }
1183             // Tuple structs.
1184             ExprKind::Call(callee, args) => {
1185                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1186                     let (pats, rest) = self.destructure_sequence(
1187                         args,
1188                         "tuple struct or variant",
1189                         eq_sign_span,
1190                         assignments,
1191                     );
1192                     let qpath = self.lower_qpath(
1193                         callee.id,
1194                         qself,
1195                         path,
1196                         ParamMode::Optional,
1197                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1198                     );
1199                     // Destructure like a tuple struct.
1200                     let tuple_struct_pat = hir::PatKind::TupleStruct(
1201                         qpath,
1202                         pats,
1203                         hir::DotDotPos::new(rest.map(|r| r.0)),
1204                     );
1205                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1206                 }
1207             }
1208             // Unit structs and enum variants.
1209             ExprKind::Path(..) => {
1210                 if let Some((qself, path)) = self.extract_unit_struct_path(lhs) {
1211                     let qpath = self.lower_qpath(
1212                         lhs.id,
1213                         qself,
1214                         path,
1215                         ParamMode::Optional,
1216                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1217                     );
1218                     // Destructure like a unit struct.
1219                     let unit_struct_pat = hir::PatKind::Path(qpath);
1220                     return self.pat_without_dbm(lhs.span, unit_struct_pat);
1221                 }
1222             }
1223             // Structs.
1224             ExprKind::Struct(se) => {
1225                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1226                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1227                     hir::PatField {
1228                         hir_id: self.next_id(),
1229                         ident: self.lower_ident(f.ident),
1230                         pat,
1231                         is_shorthand: f.is_shorthand,
1232                         span: self.lower_span(f.span),
1233                     }
1234                 }));
1235                 let qpath = self.lower_qpath(
1236                     lhs.id,
1237                     &se.qself,
1238                     &se.path,
1239                     ParamMode::Optional,
1240                     &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1241                 );
1242                 let fields_omitted = match &se.rest {
1243                     StructRest::Base(e) => {
1244                         self.tcx.sess.emit_err(FunctionalRecordUpdateDestructuringAssignemnt {
1245                             span: e.span,
1246                         });
1247                         true
1248                     }
1249                     StructRest::Rest(_) => true,
1250                     StructRest::None => false,
1251                 };
1252                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1253                 return self.pat_without_dbm(lhs.span, struct_pat);
1254             }
1255             // Tuples.
1256             ExprKind::Tup(elements) => {
1257                 let (pats, rest) =
1258                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1259                 let tuple_pat = hir::PatKind::Tuple(pats, hir::DotDotPos::new(rest.map(|r| r.0)));
1260                 return self.pat_without_dbm(lhs.span, tuple_pat);
1261             }
1262             ExprKind::Paren(e) => {
1263                 // We special-case `(..)` for consistency with patterns.
1264                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1265                     let tuple_pat = hir::PatKind::Tuple(&[], hir::DotDotPos::new(Some(0)));
1266                     return self.pat_without_dbm(lhs.span, tuple_pat);
1267                 } else {
1268                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1269                 }
1270             }
1271             _ => {}
1272         }
1273         // Treat all other cases as normal lvalue.
1274         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1275         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1276         let ident = self.expr_ident(lhs.span, ident, binding);
1277         let assign =
1278             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1279         let expr = self.expr(lhs.span, assign, AttrVec::new());
1280         assignments.push(self.stmt_expr(lhs.span, expr));
1281         pat
1282     }
1283
1284     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1285     /// Such a sequence occurs in a tuple (struct)/slice.
1286     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1287     /// exists.
1288     /// Each sub-assignment is recorded in `assignments`.
1289     fn destructure_sequence(
1290         &mut self,
1291         elements: &[AstP<Expr>],
1292         ctx: &str,
1293         eq_sign_span: Span,
1294         assignments: &mut Vec<hir::Stmt<'hir>>,
1295     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1296         let mut rest = None;
1297         let elements =
1298             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1299                 // Check for `..` pattern.
1300                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1301                     if let Some((_, prev_span)) = rest {
1302                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1303                     } else {
1304                         rest = Some((i, e.span));
1305                     }
1306                     None
1307                 } else {
1308                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1309                 }
1310             }));
1311         (elements, rest)
1312     }
1313
1314     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1315     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1316         let e1 = self.lower_expr_mut(e1);
1317         let e2 = self.lower_expr_mut(e2);
1318         let fn_path =
1319             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1320         let fn_expr =
1321             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), AttrVec::new()));
1322         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1323     }
1324
1325     fn lower_expr_range(
1326         &mut self,
1327         span: Span,
1328         e1: Option<&Expr>,
1329         e2: Option<&Expr>,
1330         lims: RangeLimits,
1331     ) -> hir::ExprKind<'hir> {
1332         use rustc_ast::RangeLimits::*;
1333
1334         let lang_item = match (e1, e2, lims) {
1335             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1336             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1337             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1338             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1339             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1340             (Some(..), Some(..), Closed) => unreachable!(),
1341             (start, None, Closed) => {
1342                 self.tcx.sess.emit_err(InclusiveRangeWithNoEnd { span });
1343                 match start {
1344                     Some(..) => hir::LangItem::RangeFrom,
1345                     None => hir::LangItem::RangeFull,
1346                 }
1347             }
1348         };
1349
1350         let fields = self.arena.alloc_from_iter(
1351             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1352                 |(s, e)| {
1353                     let expr = self.lower_expr(&e);
1354                     let ident = Ident::new(s, self.lower_span(e.span));
1355                     self.expr_field(ident, expr, e.span)
1356                 },
1357             ),
1358         );
1359
1360         hir::ExprKind::Struct(
1361             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1362             fields,
1363             None,
1364         )
1365     }
1366
1367     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1368         let label = opt_label?;
1369         Some(Label { ident: self.lower_ident(label.ident) })
1370     }
1371
1372     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1373         let target_id = match destination {
1374             Some((id, _)) => {
1375                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1376                     Ok(self.lower_node_id(loop_id))
1377                 } else {
1378                     Err(hir::LoopIdError::UnresolvedLabel)
1379                 }
1380             }
1381             None => self
1382                 .loop_scope
1383                 .map(|id| Ok(self.lower_node_id(id)))
1384                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1385         };
1386         let label = self.lower_label(destination.map(|(_, label)| label));
1387         hir::Destination { label, target_id }
1388     }
1389
1390     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1391         if self.is_in_loop_condition && opt_label.is_none() {
1392             hir::Destination {
1393                 label: None,
1394                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1395             }
1396         } else {
1397             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1398         }
1399     }
1400
1401     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1402         let old_scope = self.catch_scope.replace(catch_id);
1403         let result = f(self);
1404         self.catch_scope = old_scope;
1405         result
1406     }
1407
1408     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1409         // We're no longer in the base loop's condition; we're in another loop.
1410         let was_in_loop_condition = self.is_in_loop_condition;
1411         self.is_in_loop_condition = false;
1412
1413         let old_scope = self.loop_scope.replace(loop_id);
1414         let result = f(self);
1415         self.loop_scope = old_scope;
1416
1417         self.is_in_loop_condition = was_in_loop_condition;
1418
1419         result
1420     }
1421
1422     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1423         let was_in_loop_condition = self.is_in_loop_condition;
1424         self.is_in_loop_condition = true;
1425
1426         let result = f(self);
1427
1428         self.is_in_loop_condition = was_in_loop_condition;
1429
1430         result
1431     }
1432
1433     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1434         let hir_id = self.lower_node_id(f.id);
1435         self.lower_attrs(hir_id, &f.attrs);
1436         hir::ExprField {
1437             hir_id,
1438             ident: self.lower_ident(f.ident),
1439             expr: self.lower_expr(&f.expr),
1440             span: self.lower_span(f.span),
1441             is_shorthand: f.is_shorthand,
1442         }
1443     }
1444
1445     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1446         match self.generator_kind {
1447             Some(hir::GeneratorKind::Gen) => {}
1448             Some(hir::GeneratorKind::Async(_)) => {
1449                 self.tcx.sess.emit_err(AsyncGeneratorsNotSupported { span });
1450             }
1451             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1452         }
1453
1454         let expr =
1455             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1456
1457         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1458     }
1459
1460     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1461     /// ```ignore (pseudo-rust)
1462     /// {
1463     ///     let result = match IntoIterator::into_iter(<head>) {
1464     ///         mut iter => {
1465     ///             [opt_ident]: loop {
1466     ///                 match Iterator::next(&mut iter) {
1467     ///                     None => break,
1468     ///                     Some(<pat>) => <body>,
1469     ///                 };
1470     ///             }
1471     ///         }
1472     ///     };
1473     ///     result
1474     /// }
1475     /// ```
1476     fn lower_expr_for(
1477         &mut self,
1478         e: &Expr,
1479         pat: &Pat,
1480         head: &Expr,
1481         body: &Block,
1482         opt_label: Option<Label>,
1483     ) -> hir::Expr<'hir> {
1484         let head = self.lower_expr_mut(head);
1485         let pat = self.lower_pat(pat);
1486         let for_span =
1487             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1488         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1489         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1490
1491         // `None => break`
1492         let none_arm = {
1493             let break_expr =
1494                 self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, AttrVec::new()));
1495             let pat = self.pat_none(for_span);
1496             self.arm(pat, break_expr)
1497         };
1498
1499         // Some(<pat>) => <body>,
1500         let some_arm = {
1501             let some_pat = self.pat_some(pat_span, pat);
1502             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1503             let body_expr = self.arena.alloc(self.expr_block(body_block, AttrVec::new()));
1504             self.arm(some_pat, body_expr)
1505         };
1506
1507         // `mut iter`
1508         let iter = Ident::with_dummy_span(sym::iter);
1509         let (iter_pat, iter_pat_nid) =
1510             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::MUT);
1511
1512         // `match Iterator::next(&mut iter) { ... }`
1513         let match_expr = {
1514             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1515             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1516             let next_expr = self.expr_call_lang_item_fn(
1517                 head_span,
1518                 hir::LangItem::IteratorNext,
1519                 arena_vec![self; ref_mut_iter],
1520                 None,
1521             );
1522             let arms = arena_vec![self; none_arm, some_arm];
1523
1524             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1525         };
1526         let match_stmt = self.stmt_expr(for_span, match_expr);
1527
1528         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1529
1530         // `[opt_ident]: loop { ... }`
1531         let kind = hir::ExprKind::Loop(
1532             loop_block,
1533             self.lower_label(opt_label),
1534             hir::LoopSource::ForLoop,
1535             self.lower_span(for_span.with_hi(head.span.hi())),
1536         );
1537         let loop_expr =
1538             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1539
1540         // `mut iter => { ... }`
1541         let iter_arm = self.arm(iter_pat, loop_expr);
1542
1543         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1544         let into_iter_expr = {
1545             self.expr_call_lang_item_fn(
1546                 head_span,
1547                 hir::LangItem::IntoIterIntoIter,
1548                 arena_vec![self; head],
1549                 None,
1550             )
1551         };
1552
1553         let match_expr = self.arena.alloc(self.expr_match(
1554             for_span,
1555             into_iter_expr,
1556             arena_vec![self; iter_arm],
1557             hir::MatchSource::ForLoopDesugar,
1558         ));
1559
1560         // This is effectively `{ let _result = ...; _result }`.
1561         // The construct was introduced in #21984 and is necessary to make sure that
1562         // temporaries in the `head` expression are dropped and do not leak to the
1563         // surrounding scope of the `match` since the `match` is not a terminating scope.
1564         //
1565         // Also, add the attributes to the outer returned expr node.
1566         self.expr_drop_temps_mut(for_span, match_expr, e.attrs.clone())
1567     }
1568
1569     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1570     /// ```ignore (pseudo-rust)
1571     /// match Try::branch(<expr>) {
1572     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1573     ///     ControlFlow::Break(residual) =>
1574     ///         #[allow(unreachable_code)]
1575     ///         // If there is an enclosing `try {...}`:
1576     ///         break 'catch_target Try::from_residual(residual),
1577     ///         // Otherwise:
1578     ///         return Try::from_residual(residual),
1579     /// }
1580     /// ```
1581     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1582         let unstable_span = self.mark_span_with_reason(
1583             DesugaringKind::QuestionMark,
1584             span,
1585             self.allow_try_trait.clone(),
1586         );
1587         let try_span = self.tcx.sess.source_map().end_point(span);
1588         let try_span = self.mark_span_with_reason(
1589             DesugaringKind::QuestionMark,
1590             try_span,
1591             self.allow_try_trait.clone(),
1592         );
1593
1594         // `Try::branch(<expr>)`
1595         let scrutinee = {
1596             // expand <expr>
1597             let sub_expr = self.lower_expr_mut(sub_expr);
1598
1599             self.expr_call_lang_item_fn(
1600                 unstable_span,
1601                 hir::LangItem::TryTraitBranch,
1602                 arena_vec![self; sub_expr],
1603                 None,
1604             )
1605         };
1606
1607         // `#[allow(unreachable_code)]`
1608         let attr = {
1609             // `allow(unreachable_code)`
1610             let allow = {
1611                 let allow_ident = Ident::new(sym::allow, self.lower_span(span));
1612                 let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
1613                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1614                 attr::mk_list_item(allow_ident, vec![uc_nested])
1615             };
1616             attr::mk_attr_outer(&self.tcx.sess.parse_sess.attr_id_generator, allow)
1617         };
1618         let attrs: AttrVec = thin_vec![attr];
1619
1620         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1621         let continue_arm = {
1622             let val_ident = Ident::with_dummy_span(sym::val);
1623             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1624             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1625                 span,
1626                 val_ident,
1627                 val_pat_nid,
1628                 attrs.clone(),
1629             ));
1630             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1631             self.arm(continue_pat, val_expr)
1632         };
1633
1634         // `ControlFlow::Break(residual) =>
1635         //     #[allow(unreachable_code)]
1636         //     return Try::from_residual(residual),`
1637         let break_arm = {
1638             let residual_ident = Ident::with_dummy_span(sym::residual);
1639             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1640             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1641             let from_residual_expr = self.wrap_in_try_constructor(
1642                 hir::LangItem::TryTraitFromResidual,
1643                 try_span,
1644                 self.arena.alloc(residual_expr),
1645                 unstable_span,
1646             );
1647             let ret_expr = if let Some(catch_node) = self.catch_scope {
1648                 let target_id = Ok(self.lower_node_id(catch_node));
1649                 self.arena.alloc(self.expr(
1650                     try_span,
1651                     hir::ExprKind::Break(
1652                         hir::Destination { label: None, target_id },
1653                         Some(from_residual_expr),
1654                     ),
1655                     attrs,
1656                 ))
1657             } else {
1658                 self.arena.alloc(self.expr(
1659                     try_span,
1660                     hir::ExprKind::Ret(Some(from_residual_expr)),
1661                     attrs,
1662                 ))
1663             };
1664
1665             let break_pat = self.pat_cf_break(try_span, residual_local);
1666             self.arm(break_pat, ret_expr)
1667         };
1668
1669         hir::ExprKind::Match(
1670             scrutinee,
1671             arena_vec![self; break_arm, continue_arm],
1672             hir::MatchSource::TryDesugar,
1673         )
1674     }
1675
1676     /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
1677     /// ```ignore(illustrative)
1678     /// // If there is an enclosing `try {...}`:
1679     /// break 'catch_target FromResidual::from_residual(Yeet(residual));
1680     /// // Otherwise:
1681     /// return FromResidual::from_residual(Yeet(residual));
1682     /// ```
1683     /// But to simplify this, there's a `from_yeet` lang item function which
1684     /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
1685     fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1686         // The expression (if present) or `()` otherwise.
1687         let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
1688             (sub_expr.span, self.lower_expr(sub_expr))
1689         } else {
1690             (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
1691         };
1692
1693         let unstable_span = self.mark_span_with_reason(
1694             DesugaringKind::YeetExpr,
1695             span,
1696             self.allow_try_trait.clone(),
1697         );
1698
1699         let from_yeet_expr = self.wrap_in_try_constructor(
1700             hir::LangItem::TryTraitFromYeet,
1701             unstable_span,
1702             yeeted_expr,
1703             yeeted_span,
1704         );
1705
1706         if let Some(catch_node) = self.catch_scope {
1707             let target_id = Ok(self.lower_node_id(catch_node));
1708             hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
1709         } else {
1710             hir::ExprKind::Ret(Some(from_yeet_expr))
1711         }
1712     }
1713
1714     // =========================================================================
1715     // Helper methods for building HIR.
1716     // =========================================================================
1717
1718     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1719     ///
1720     /// In terms of drop order, it has the same effect as wrapping `expr` in
1721     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1722     ///
1723     /// The drop order can be important in e.g. `if expr { .. }`.
1724     pub(super) fn expr_drop_temps(
1725         &mut self,
1726         span: Span,
1727         expr: &'hir hir::Expr<'hir>,
1728         attrs: AttrVec,
1729     ) -> &'hir hir::Expr<'hir> {
1730         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1731     }
1732
1733     pub(super) fn expr_drop_temps_mut(
1734         &mut self,
1735         span: Span,
1736         expr: &'hir hir::Expr<'hir>,
1737         attrs: AttrVec,
1738     ) -> hir::Expr<'hir> {
1739         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1740     }
1741
1742     fn expr_match(
1743         &mut self,
1744         span: Span,
1745         arg: &'hir hir::Expr<'hir>,
1746         arms: &'hir [hir::Arm<'hir>],
1747         source: hir::MatchSource,
1748     ) -> hir::Expr<'hir> {
1749         self.expr(span, hir::ExprKind::Match(arg, arms, source), AttrVec::new())
1750     }
1751
1752     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
1753         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1754         self.expr(span, expr_break, attrs)
1755     }
1756
1757     fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1758         let expr_break = self.expr_break(span, attrs);
1759         self.arena.alloc(expr_break)
1760     }
1761
1762     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1763         self.expr(
1764             span,
1765             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1766             AttrVec::new(),
1767         )
1768     }
1769
1770     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1771         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), AttrVec::new()))
1772     }
1773
1774     fn expr_call_mut(
1775         &mut self,
1776         span: Span,
1777         e: &'hir hir::Expr<'hir>,
1778         args: &'hir [hir::Expr<'hir>],
1779     ) -> hir::Expr<'hir> {
1780         self.expr(span, hir::ExprKind::Call(e, args), AttrVec::new())
1781     }
1782
1783     fn expr_call(
1784         &mut self,
1785         span: Span,
1786         e: &'hir hir::Expr<'hir>,
1787         args: &'hir [hir::Expr<'hir>],
1788     ) -> &'hir hir::Expr<'hir> {
1789         self.arena.alloc(self.expr_call_mut(span, e, args))
1790     }
1791
1792     fn expr_call_lang_item_fn_mut(
1793         &mut self,
1794         span: Span,
1795         lang_item: hir::LangItem,
1796         args: &'hir [hir::Expr<'hir>],
1797         hir_id: Option<hir::HirId>,
1798     ) -> hir::Expr<'hir> {
1799         let path =
1800             self.arena.alloc(self.expr_lang_item_path(span, lang_item, AttrVec::new(), hir_id));
1801         self.expr_call_mut(span, path, args)
1802     }
1803
1804     fn expr_call_lang_item_fn(
1805         &mut self,
1806         span: Span,
1807         lang_item: hir::LangItem,
1808         args: &'hir [hir::Expr<'hir>],
1809         hir_id: Option<hir::HirId>,
1810     ) -> &'hir hir::Expr<'hir> {
1811         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1812     }
1813
1814     fn expr_lang_item_path(
1815         &mut self,
1816         span: Span,
1817         lang_item: hir::LangItem,
1818         attrs: AttrVec,
1819         hir_id: Option<hir::HirId>,
1820     ) -> hir::Expr<'hir> {
1821         self.expr(
1822             span,
1823             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1824             attrs,
1825         )
1826     }
1827
1828     pub(super) fn expr_ident(
1829         &mut self,
1830         sp: Span,
1831         ident: Ident,
1832         binding: hir::HirId,
1833     ) -> &'hir hir::Expr<'hir> {
1834         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1835     }
1836
1837     pub(super) fn expr_ident_mut(
1838         &mut self,
1839         sp: Span,
1840         ident: Ident,
1841         binding: hir::HirId,
1842     ) -> hir::Expr<'hir> {
1843         self.expr_ident_with_attrs(sp, ident, binding, AttrVec::new())
1844     }
1845
1846     fn expr_ident_with_attrs(
1847         &mut self,
1848         span: Span,
1849         ident: Ident,
1850         binding: hir::HirId,
1851         attrs: AttrVec,
1852     ) -> hir::Expr<'hir> {
1853         let hir_id = self.next_id();
1854         let res = Res::Local(binding);
1855         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1856             None,
1857             self.arena.alloc(hir::Path {
1858                 span: self.lower_span(span),
1859                 res,
1860                 segments: arena_vec![self; hir::PathSegment::new(ident, hir_id, res)],
1861             }),
1862         ));
1863
1864         self.expr(span, expr_path, attrs)
1865     }
1866
1867     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1868         let hir_id = self.next_id();
1869         let span = expr.span;
1870         self.expr(
1871             span,
1872             hir::ExprKind::Block(
1873                 self.arena.alloc(hir::Block {
1874                     stmts: &[],
1875                     expr: Some(expr),
1876                     hir_id,
1877                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1878                     span: self.lower_span(span),
1879                     targeted_by_break: false,
1880                 }),
1881                 None,
1882             ),
1883             AttrVec::new(),
1884         )
1885     }
1886
1887     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1888         let blk = self.block_all(span, &[], None);
1889         let expr = self.expr_block(blk, AttrVec::new());
1890         self.arena.alloc(expr)
1891     }
1892
1893     pub(super) fn expr_block(
1894         &mut self,
1895         b: &'hir hir::Block<'hir>,
1896         attrs: AttrVec,
1897     ) -> hir::Expr<'hir> {
1898         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1899     }
1900
1901     pub(super) fn expr(
1902         &mut self,
1903         span: Span,
1904         kind: hir::ExprKind<'hir>,
1905         attrs: AttrVec,
1906     ) -> hir::Expr<'hir> {
1907         let hir_id = self.next_id();
1908         self.lower_attrs(hir_id, &attrs);
1909         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1910     }
1911
1912     fn expr_field(
1913         &mut self,
1914         ident: Ident,
1915         expr: &'hir hir::Expr<'hir>,
1916         span: Span,
1917     ) -> hir::ExprField<'hir> {
1918         hir::ExprField {
1919             hir_id: self.next_id(),
1920             ident,
1921             span: self.lower_span(span),
1922             expr,
1923             is_shorthand: false,
1924         }
1925     }
1926
1927     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1928         hir::Arm {
1929             hir_id: self.next_id(),
1930             pat,
1931             guard: None,
1932             span: self.lower_span(expr.span),
1933             body: expr,
1934         }
1935     }
1936 }