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