compiler/rustc_ast_lowering/src/expr.rs

   1 use super::ResolverAstLoweringExt;
   2 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
   3 use crate::{FnDeclKind, ImplTraitPosition};
   4
   5 use rustc_ast::attr;
   6 use rustc_ast::ptr::P as AstP;
   7 use rustc_ast::*;
   8 use rustc_data_structures::stack::ensure_sufficient_stack;
   9 use rustc_data_structures::thin_vec::ThinVec;
  10 use rustc_errors::struct_span_err;
  11 use rustc_hir as hir;
  12 use rustc_hir::def::Res;
  13 use rustc_hir::definitions::DefPathData;
  14 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
  15 use rustc_span::symbol::{sym, Ident};
  16 use rustc_span::DUMMY_SP;
  17
  18 impl<'hir> LoweringContext<'_, 'hir> {
  19     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  20         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  21     }
  22
  23     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  24         self.arena.alloc(self.lower_expr_mut(e))
  25     }
  26
  27     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  28         ensure_sufficient_stack(|| {
  29             let kind = match e.kind {
  30                 ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  31                 ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  32                 ExprKind::ConstBlock(ref anon_const) => {
  33                     let anon_const = self.lower_anon_const(anon_const);
  34                     hir::ExprKind::ConstBlock(anon_const)
  35                 }
  36                 ExprKind::Repeat(ref expr, ref count) => {
  37                     let expr = self.lower_expr(expr);
  38                     let count = self.lower_array_length(count);
  39                     hir::ExprKind::Repeat(expr, count)
  40                 }
  41                 ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  42                 ExprKind::Call(ref f, ref args) => {
  43                     if e.attrs.get(0).map_or(false, |a| a.has_name(sym::rustc_box)) {
  44                         if let [inner] = &args[..] && e.attrs.len() == 1 {
  45                             let kind = hir::ExprKind::Box(self.lower_expr(&inner));
  46                             let hir_id = self.lower_node_id(e.id);
  47                             return hir::Expr { hir_id, kind, span: self.lower_span(e.span) };
  48                         } else {
  49                             self.sess
  50                                 .struct_span_err(
  51                                     e.span,
  52                                     "#[rustc_box] requires precisely one argument \
  53                                     and no other attributes are allowed",
  54                                 )
  55                                 .emit();
  56                             hir::ExprKind::Err
  57                         }
  58                     } else if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
  59                         self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
  60                     } else {
  61                         let f = self.lower_expr(f);
  62                         hir::ExprKind::Call(f, self.lower_exprs(args))
  63                     }
  64                 }
  65                 ExprKind::MethodCall(ref seg, ref args, span) => {
  66                     let hir_seg = self.arena.alloc(self.lower_path_segment(
  67                         e.span,
  68                         seg,
  69                         ParamMode::Optional,
  70                         ParenthesizedGenericArgs::Err,
  71                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  72                     ));
  73                     let args = self.lower_exprs(args);
  74                     hir::ExprKind::MethodCall(hir_seg, args, self.lower_span(span))
  75                 }
  76                 ExprKind::Binary(binop, ref lhs, ref rhs) => {
  77                     let binop = self.lower_binop(binop);
  78                     let lhs = self.lower_expr(lhs);
  79                     let rhs = self.lower_expr(rhs);
  80                     hir::ExprKind::Binary(binop, lhs, rhs)
  81                 }
  82                 ExprKind::Unary(op, ref ohs) => {
  83                     let op = self.lower_unop(op);
  84                     let ohs = self.lower_expr(ohs);
  85                     hir::ExprKind::Unary(op, ohs)
  86                 }
  87                 ExprKind::Lit(ref l) => {
  88                     hir::ExprKind::Lit(respan(self.lower_span(l.span), l.kind.clone()))
  89                 }
  90                 ExprKind::Cast(ref expr, ref ty) => {
  91                     let expr = self.lower_expr(expr);
  92                     let ty =
  93                         self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
  94                     hir::ExprKind::Cast(expr, ty)
  95                 }
  96                 ExprKind::Type(ref expr, ref ty) => {
  97                     let expr = self.lower_expr(expr);
  98                     let ty =
  99                         self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 100                     hir::ExprKind::Type(expr, ty)
 101                 }
 102                 ExprKind::AddrOf(k, m, ref ohs) => {
 103                     let ohs = self.lower_expr(ohs);
 104                     hir::ExprKind::AddrOf(k, m, ohs)
 105                 }
 106                 ExprKind::Let(ref pat, ref scrutinee, span) => {
 107                     hir::ExprKind::Let(self.arena.alloc(hir::Let {
 108                         hir_id: self.next_id(),
 109                         span: self.lower_span(span),
 110                         pat: self.lower_pat(pat),
 111                         ty: None,
 112                         init: self.lower_expr(scrutinee),
 113                     }))
 114                 }
 115                 ExprKind::If(ref cond, ref then, ref else_opt) => {
 116                     self.lower_expr_if(cond, then, else_opt.as_deref())
 117                 }
 118                 ExprKind::While(ref cond, ref body, opt_label) => {
 119                     self.with_loop_scope(e.id, |this| {
 120                         let span =
 121                             this.mark_span_with_reason(DesugaringKind::WhileLoop, e.span, None);
 122                         this.lower_expr_while_in_loop_scope(span, cond, body, opt_label)
 123                     })
 124                 }
 125                 ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
 126                     hir::ExprKind::Loop(
 127                         this.lower_block(body, false),
 128                         this.lower_label(opt_label),
 129                         hir::LoopSource::Loop,
 130                         DUMMY_SP,
 131                     )
 132                 }),
 133                 ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
 134                 ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
 135                     self.lower_expr(expr),
 136                     self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
 137                     hir::MatchSource::Normal,
 138                 ),
 139                 ExprKind::Async(capture_clause, closure_node_id, ref block) => self
 140                     .make_async_expr(
 141                         capture_clause,
 142                         closure_node_id,
 143                         None,
 144                         block.span,
 145                         hir::AsyncGeneratorKind::Block,
 146                         |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
 147                     ),
 148                 ExprKind::Await(ref expr) => {
 149                     let span = if expr.span.hi() < e.span.hi() {
 150                         expr.span.shrink_to_hi().with_hi(e.span.hi())
 151                     } else {
 152                         // this is a recovered `await expr`
 153                         e.span
 154                     };
 155                     self.lower_expr_await(span, expr)
 156                 }
 157                 ExprKind::Closure(
 158                     capture_clause,
 159                     asyncness,
 160                     movability,
 161                     ref decl,
 162                     ref body,
 163                     fn_decl_span,
 164                 ) => {
 165                     if let Async::Yes { closure_id, .. } = asyncness {
 166                         self.lower_expr_async_closure(
 167                             capture_clause,
 168                             e.id,
 169                             closure_id,
 170                             decl,
 171                             body,
 172                             fn_decl_span,
 173                         )
 174                     } else {
 175                         self.lower_expr_closure(
 176                             capture_clause,
 177                             e.id,
 178                             movability,
 179                             decl,
 180                             body,
 181                             fn_decl_span,
 182                         )
 183                     }
 184                 }
 185                 ExprKind::Block(ref blk, opt_label) => {
 186                     let opt_label = self.lower_label(opt_label);
 187                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 188                 }
 189                 ExprKind::Assign(ref el, ref er, span) => {
 190                     self.lower_expr_assign(el, er, span, e.span)
 191                 }
 192                 ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 193                     self.lower_binop(op),
 194                     self.lower_expr(el),
 195                     self.lower_expr(er),
 196                 ),
 197                 ExprKind::Field(ref el, ident) => {
 198                     hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(ident))
 199                 }
 200                 ExprKind::Index(ref el, ref er) => {
 201                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 202                 }
 203                 ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 204                     self.lower_expr_range_closed(e.span, e1, e2)
 205                 }
 206                 ExprKind::Range(ref e1, ref e2, lims) => {
 207                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 208                 }
 209                 ExprKind::Underscore => {
 210                     self.sess
 211                         .struct_span_err(
 212                             e.span,
 213                             "in expressions, `_` can only be used on the left-hand side of an assignment",
 214                         )
 215                         .span_label(e.span, "`_` not allowed here")
 216                         .emit();
 217                     hir::ExprKind::Err
 218                 }
 219                 ExprKind::Path(ref qself, ref path) => {
 220                     let qpath = self.lower_qpath(
 221                         e.id,
 222                         qself,
 223                         path,
 224                         ParamMode::Optional,
 225                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 226                     );
 227                     hir::ExprKind::Path(qpath)
 228                 }
 229                 ExprKind::Break(opt_label, ref opt_expr) => {
 230                     let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 231                     hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
 232                 }
 233                 ExprKind::Continue(opt_label) => {
 234                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
 235                 }
 236                 ExprKind::Ret(ref e) => {
 237                     let e = e.as_ref().map(|x| self.lower_expr(x));
 238                     hir::ExprKind::Ret(e)
 239                 }
 240                 ExprKind::Yeet(ref sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
 241                 ExprKind::InlineAsm(ref asm) => {
 242                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
 243                 }
 244                 ExprKind::Struct(ref se) => {
 245                     let rest = match &se.rest {
 246                         StructRest::Base(e) => Some(self.lower_expr(e)),
 247                         StructRest::Rest(sp) => {
 248                             self.sess
 249                                 .struct_span_err(*sp, "base expression required after `..`")
 250                                 .span_label(*sp, "add a base expression here")
 251                                 .emit();
 252                             Some(&*self.arena.alloc(self.expr_err(*sp)))
 253                         }
 254                         StructRest::None => None,
 255                     };
 256                     hir::ExprKind::Struct(
 257                         self.arena.alloc(self.lower_qpath(
 258                             e.id,
 259                             &se.qself,
 260                             &se.path,
 261                             ParamMode::Optional,
 262                             ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 263                         )),
 264                         self.arena
 265                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
 266                         rest,
 267                     )
 268                 }
 269                 ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 270                 ExprKind::Err => hir::ExprKind::Err,
 271                 ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 272                 ExprKind::Paren(ref ex) => {
 273                     let mut ex = self.lower_expr_mut(ex);
 274                     // Include parens in span, but only if it is a super-span.
 275                     if e.span.contains(ex.span) {
 276                         ex.span = self.lower_span(e.span);
 277                     }
 278                     // Merge attributes into the inner expression.
 279                     if !e.attrs.is_empty() {
 280                         let old_attrs =
 281                             self.attrs.get(&ex.hir_id.local_id).map(|la| *la).unwrap_or(&[]);
 282                         self.attrs.insert(
 283                             ex.hir_id.local_id,
 284                             &*self.arena.alloc_from_iter(
 285                                 e.attrs
 286                                     .iter()
 287                                     .map(|a| self.lower_attr(a))
 288                                     .chain(old_attrs.iter().cloned()),
 289                             ),
 290                         );
 291                     }
 292                     return ex;
 293                 }
 294
 295                 // Desugar `ExprForLoop`
 296                 // from: `[opt_ident]: for <pat> in <head> <body>`
 297                 ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 298                     return self.lower_expr_for(e, pat, head, body, opt_label);
 299                 }
 300                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 301             };
 302
 303             let hir_id = self.lower_node_id(e.id);
 304             self.lower_attrs(hir_id, &e.attrs);
 305             hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
 306         })
 307     }
 308
 309     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 310         match u {
 311             UnOp::Deref => hir::UnOp::Deref,
 312             UnOp::Not => hir::UnOp::Not,
 313             UnOp::Neg => hir::UnOp::Neg,
 314         }
 315     }
 316
 317     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 318         Spanned {
 319             node: match b.node {
 320                 BinOpKind::Add => hir::BinOpKind::Add,
 321                 BinOpKind::Sub => hir::BinOpKind::Sub,
 322                 BinOpKind::Mul => hir::BinOpKind::Mul,
 323                 BinOpKind::Div => hir::BinOpKind::Div,
 324                 BinOpKind::Rem => hir::BinOpKind::Rem,
 325                 BinOpKind::And => hir::BinOpKind::And,
 326                 BinOpKind::Or => hir::BinOpKind::Or,
 327                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 328                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 329                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 330                 BinOpKind::Shl => hir::BinOpKind::Shl,
 331                 BinOpKind::Shr => hir::BinOpKind::Shr,
 332                 BinOpKind::Eq => hir::BinOpKind::Eq,
 333                 BinOpKind::Lt => hir::BinOpKind::Lt,
 334                 BinOpKind::Le => hir::BinOpKind::Le,
 335                 BinOpKind::Ne => hir::BinOpKind::Ne,
 336                 BinOpKind::Ge => hir::BinOpKind::Ge,
 337                 BinOpKind::Gt => hir::BinOpKind::Gt,
 338             },
 339             span: self.lower_span(b.span),
 340         }
 341     }
 342
 343     fn lower_legacy_const_generics(
 344         &mut self,
 345         mut f: Expr,
 346         args: Vec<AstP<Expr>>,
 347         legacy_args_idx: &[usize],
 348     ) -> hir::ExprKind<'hir> {
 349         let ExprKind::Path(None, ref mut path) = f.kind else {
 350             unreachable!();
 351         };
 352
 353         // Split the arguments into const generics and normal arguments
 354         let mut real_args = vec![];
 355         let mut generic_args = vec![];
 356         for (idx, arg) in args.into_iter().enumerate() {
 357             if legacy_args_idx.contains(&idx) {
 358                 let parent_def_id = self.current_hir_id_owner;
 359                 let node_id = self.next_node_id();
 360
 361                 // Add a definition for the in-band const def.
 362                 self.create_def(parent_def_id, node_id, DefPathData::AnonConst);
 363
 364                 let anon_const = AnonConst { id: node_id, value: arg };
 365                 generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
 366             } else {
 367                 real_args.push(arg);
 368             }
 369         }
 370
 371         // Add generic args to the last element of the path.
 372         let last_segment = path.segments.last_mut().unwrap();
 373         assert!(last_segment.args.is_none());
 374         last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
 375             span: DUMMY_SP,
 376             args: generic_args,
 377         })));
 378
 379         // Now lower everything as normal.
 380         let f = self.lower_expr(&f);
 381         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
 382     }
 383
 384     fn lower_expr_if(
 385         &mut self,
 386         cond: &Expr,
 387         then: &Block,
 388         else_opt: Option<&Expr>,
 389     ) -> hir::ExprKind<'hir> {
 390         let lowered_cond = self.lower_expr(cond);
 391         let new_cond = self.manage_let_cond(lowered_cond);
 392         let then_expr = self.lower_block_expr(then);
 393         if let Some(rslt) = else_opt {
 394             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), Some(self.lower_expr(rslt)))
 395         } else {
 396             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), None)
 397         }
 398     }
 399
 400     // If `cond` kind is `let`, returns `let`. Otherwise, wraps and returns `cond`
 401     // in a temporary block.
 402     fn manage_let_cond(&mut self, cond: &'hir hir::Expr<'hir>) -> &'hir hir::Expr<'hir> {
 403         fn has_let_expr<'hir>(expr: &'hir hir::Expr<'hir>) -> bool {
 404             match expr.kind {
 405                 hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
 406                 hir::ExprKind::Let(..) => true,
 407                 _ => false,
 408             }
 409         }
 410         if has_let_expr(cond) {
 411             cond
 412         } else {
 413             let reason = DesugaringKind::CondTemporary;
 414             let span_block = self.mark_span_with_reason(reason, cond.span, None);
 415             self.expr_drop_temps(span_block, cond, AttrVec::new())
 416         }
 417     }
 418
 419     // We desugar: `'label: while $cond $body` into:
 420     //
 421     // ```
 422     // 'label: loop {
 423     //   if { let _t = $cond; _t } {
 424     //     $body
 425     //   }
 426     //   else {
 427     //     break;
 428     //   }
 429     // }
 430     // ```
 431     //
 432     // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 433     // to preserve drop semantics since `while $cond { ... }` does not
 434     // let temporaries live outside of `cond`.
 435     fn lower_expr_while_in_loop_scope(
 436         &mut self,
 437         span: Span,
 438         cond: &Expr,
 439         body: &Block,
 440         opt_label: Option<Label>,
 441     ) -> hir::ExprKind<'hir> {
 442         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
 443         let new_cond = self.manage_let_cond(lowered_cond);
 444         let then = self.lower_block_expr(body);
 445         let expr_break = self.expr_break(span, ThinVec::new());
 446         let stmt_break = self.stmt_expr(span, expr_break);
 447         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
 448         let else_expr = self.arena.alloc(self.expr_block(else_blk, ThinVec::new()));
 449         let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
 450         let if_expr = self.expr(span, if_kind, ThinVec::new());
 451         let block = self.block_expr(self.arena.alloc(if_expr));
 452         let span = self.lower_span(span.with_hi(cond.span.hi()));
 453         let opt_label = self.lower_label(opt_label);
 454         hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
 455     }
 456
 457     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
 458     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
 459     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 460     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 461         self.with_catch_scope(body.id, |this| {
 462             let mut block = this.lower_block_noalloc(body, true);
 463
 464             // Final expression of the block (if present) or `()` with span at the end of block
 465             let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
 466                 (
 467                     this.mark_span_with_reason(
 468                         DesugaringKind::TryBlock,
 469                         expr.span,
 470                         this.allow_try_trait.clone(),
 471                     ),
 472                     expr,
 473                 )
 474             } else {
 475                 let try_span = this.mark_span_with_reason(
 476                     DesugaringKind::TryBlock,
 477                     this.sess.source_map().end_point(body.span),
 478                     this.allow_try_trait.clone(),
 479                 );
 480
 481                 (try_span, this.expr_unit(try_span))
 482             };
 483
 484             let ok_wrapped_span =
 485                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 486
 487             // `::std::ops::Try::from_output($tail_expr)`
 488             block.expr = Some(this.wrap_in_try_constructor(
 489                 hir::LangItem::TryTraitFromOutput,
 490                 try_span,
 491                 tail_expr,
 492                 ok_wrapped_span,
 493             ));
 494
 495             hir::ExprKind::Block(this.arena.alloc(block), None)
 496         })
 497     }
 498
 499     fn wrap_in_try_constructor(
 500         &mut self,
 501         lang_item: hir::LangItem,
 502         method_span: Span,
 503         expr: &'hir hir::Expr<'hir>,
 504         overall_span: Span,
 505     ) -> &'hir hir::Expr<'hir> {
 506         let constructor = self.arena.alloc(self.expr_lang_item_path(
 507             method_span,
 508             lang_item,
 509             ThinVec::new(),
 510             None,
 511         ));
 512         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 513     }
 514
 515     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 516         let pat = self.lower_pat(&arm.pat);
 517         let guard = arm.guard.as_ref().map(|cond| {
 518             if let ExprKind::Let(ref pat, ref scrutinee, span) = cond.kind {
 519                 hir::Guard::IfLet(self.arena.alloc(hir::Let {
 520                     hir_id: self.next_id(),
 521                     span: self.lower_span(span),
 522                     pat: self.lower_pat(pat),
 523                     ty: None,
 524                     init: self.lower_expr(scrutinee),
 525                 }))
 526             } else {
 527                 hir::Guard::If(self.lower_expr(cond))
 528             }
 529         });
 530         let hir_id = self.next_id();
 531         self.lower_attrs(hir_id, &arm.attrs);
 532         hir::Arm {
 533             hir_id,
 534             pat,
 535             guard,
 536             body: self.lower_expr(&arm.body),
 537             span: self.lower_span(arm.span),
 538         }
 539     }
 540
 541     /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
 542     ///
 543     /// This results in:
 544     ///
 545     /// ```text
 546     /// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
 547     ///     <body>
 548     /// })
 549     /// ```
 550     pub(super) fn make_async_expr(
 551         &mut self,
 552         capture_clause: CaptureBy,
 553         closure_node_id: NodeId,
 554         ret_ty: Option<AstP<Ty>>,
 555         span: Span,
 556         async_gen_kind: hir::AsyncGeneratorKind,
 557         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 558     ) -> hir::ExprKind<'hir> {
 559         let output = match ret_ty {
 560             Some(ty) => hir::FnRetTy::Return(
 561                 self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock)),
 562             ),
 563             None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
 564         };
 565
 566         // Resume argument type. We let the compiler infer this to simplify the lowering. It is
 567         // fully constrained by `future::from_generator`.
 568         let input_ty = hir::Ty {
 569             hir_id: self.next_id(),
 570             kind: hir::TyKind::Infer,
 571             span: self.lower_span(span),
 572         };
 573
 574         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 575         let fn_decl = self.arena.alloc(hir::FnDecl {
 576             inputs: arena_vec![self; input_ty],
 577             output,
 578             c_variadic: false,
 579             implicit_self: hir::ImplicitSelfKind::None,
 580         });
 581
 582         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 583         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 584             span,
 585             Ident::with_dummy_span(sym::_task_context),
 586             hir::BindingAnnotation::Mutable,
 587         );
 588         let param = hir::Param {
 589             hir_id: self.next_id(),
 590             pat,
 591             ty_span: self.lower_span(span),
 592             span: self.lower_span(span),
 593         };
 594         let params = arena_vec![self; param];
 595
 596         let body = self.lower_body(move |this| {
 597             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 598
 599             let old_ctx = this.task_context;
 600             this.task_context = Some(task_context_hid);
 601             let res = body(this);
 602             this.task_context = old_ctx;
 603             (params, res)
 604         });
 605
 606         // `static |_task_context| -> <ret_ty> { body }`:
 607         let generator_kind = hir::ExprKind::Closure {
 608             capture_clause,
 609             bound_generic_params: &[],
 610             fn_decl,
 611             body,
 612             fn_decl_span: self.lower_span(span),
 613             movability: Some(hir::Movability::Static),
 614         };
 615         let generator = hir::Expr {
 616             hir_id: self.lower_node_id(closure_node_id),
 617             kind: generator_kind,
 618             span: self.lower_span(span),
 619         };
 620
 621         // `future::from_generator`:
 622         let unstable_span =
 623             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 624         let gen_future = self.expr_lang_item_path(
 625             unstable_span,
 626             hir::LangItem::FromGenerator,
 627             ThinVec::new(),
 628             None,
 629         );
 630
 631         // `future::from_generator(generator)`:
 632         hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
 633     }
 634
 635     /// Desugar `<expr>.await` into:
 636     /// ```ignore (pseudo-rust)
 637     /// match ::std::future::IntoFuture::into_future(<expr>) {
 638     ///     mut __awaitee => loop {
 639     ///         match unsafe { ::std::future::Future::poll(
 640     ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
 641     ///             ::std::future::get_context(task_context),
 642     ///         ) } {
 643     ///             ::std::task::Poll::Ready(result) => break result,
 644     ///             ::std::task::Poll::Pending => {}
 645     ///         }
 646     ///         task_context = yield ();
 647     ///     }
 648     /// }
 649     /// ```
 650     fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 651         let full_span = expr.span.to(dot_await_span);
 652         match self.generator_kind {
 653             Some(hir::GeneratorKind::Async(_)) => {}
 654             Some(hir::GeneratorKind::Gen) | None => {
 655                 let mut err = struct_span_err!(
 656                     self.sess,
 657                     dot_await_span,
 658                     E0728,
 659                     "`await` is only allowed inside `async` functions and blocks"
 660                 );
 661                 err.span_label(dot_await_span, "only allowed inside `async` functions and blocks");
 662                 if let Some(item_sp) = self.current_item {
 663                     err.span_label(item_sp, "this is not `async`");
 664                 }
 665                 err.emit();
 666             }
 667         }
 668         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
 669         let gen_future_span = self.mark_span_with_reason(
 670             DesugaringKind::Await,
 671             full_span,
 672             self.allow_gen_future.clone(),
 673         );
 674         let expr = self.lower_expr_mut(expr);
 675         let expr_hir_id = expr.hir_id;
 676
 677         // Note that the name of this binding must not be changed to something else because
 678         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
 679         // this name to identify what is being awaited by a suspended async functions.
 680         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
 681         let (awaitee_pat, awaitee_pat_hid) =
 682             self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::Mutable);
 683
 684         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 685
 686         // unsafe {
 687         //     ::std::future::Future::poll(
 688         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
 689         //         ::std::future::get_context(task_context),
 690         //     )
 691         // }
 692         let poll_expr = {
 693             let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
 694             let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
 695             let task_context = if let Some(task_context_hid) = self.task_context {
 696                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 697             } else {
 698                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 699                 self.expr_err(span)
 700             };
 701             let new_unchecked = self.expr_call_lang_item_fn_mut(
 702                 span,
 703                 hir::LangItem::PinNewUnchecked,
 704                 arena_vec![self; ref_mut_awaitee],
 705                 Some(expr_hir_id),
 706             );
 707             let get_context = self.expr_call_lang_item_fn_mut(
 708                 gen_future_span,
 709                 hir::LangItem::GetContext,
 710                 arena_vec![self; task_context],
 711                 Some(expr_hir_id),
 712             );
 713             let call = self.expr_call_lang_item_fn(
 714                 span,
 715                 hir::LangItem::FuturePoll,
 716                 arena_vec![self; new_unchecked, get_context],
 717                 Some(expr_hir_id),
 718             );
 719             self.arena.alloc(self.expr_unsafe(call))
 720         };
 721
 722         // `::std::task::Poll::Ready(result) => break result`
 723         let loop_node_id = self.next_node_id();
 724         let loop_hir_id = self.lower_node_id(loop_node_id);
 725         let ready_arm = {
 726             let x_ident = Ident::with_dummy_span(sym::result);
 727             let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
 728             let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
 729             let ready_field = self.single_pat_field(gen_future_span, x_pat);
 730             let ready_pat = self.pat_lang_item_variant(
 731                 span,
 732                 hir::LangItem::PollReady,
 733                 ready_field,
 734                 Some(expr_hir_id),
 735             );
 736             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 737                 let expr_break =
 738                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 739                 this.arena.alloc(this.expr(gen_future_span, expr_break, ThinVec::new()))
 740             });
 741             self.arm(ready_pat, break_x)
 742         };
 743
 744         // `::std::task::Poll::Pending => {}`
 745         let pending_arm = {
 746             let pending_pat = self.pat_lang_item_variant(
 747                 span,
 748                 hir::LangItem::PollPending,
 749                 &[],
 750                 Some(expr_hir_id),
 751             );
 752             let empty_block = self.expr_block_empty(span);
 753             self.arm(pending_pat, empty_block)
 754         };
 755
 756         let inner_match_stmt = {
 757             let match_expr = self.expr_match(
 758                 span,
 759                 poll_expr,
 760                 arena_vec![self; ready_arm, pending_arm],
 761                 hir::MatchSource::AwaitDesugar,
 762             );
 763             self.stmt_expr(span, match_expr)
 764         };
 765
 766         // task_context = yield ();
 767         let yield_stmt = {
 768             let unit = self.expr_unit(span);
 769             let yield_expr = self.expr(
 770                 span,
 771                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
 772                 ThinVec::new(),
 773             );
 774             let yield_expr = self.arena.alloc(yield_expr);
 775
 776             if let Some(task_context_hid) = self.task_context {
 777                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 778                 let assign = self.expr(
 779                     span,
 780                     hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)),
 781                     AttrVec::new(),
 782                 );
 783                 self.stmt_expr(span, assign)
 784             } else {
 785                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 786                 // proceed with type checking.
 787                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 788             }
 789         };
 790
 791         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 792
 793         // loop { .. }
 794         let loop_expr = self.arena.alloc(hir::Expr {
 795             hir_id: loop_hir_id,
 796             kind: hir::ExprKind::Loop(
 797                 loop_block,
 798                 None,
 799                 hir::LoopSource::Loop,
 800                 self.lower_span(span),
 801             ),
 802             span: self.lower_span(span),
 803         });
 804
 805         // mut __awaitee => loop { ... }
 806         let awaitee_arm = self.arm(awaitee_pat, loop_expr);
 807
 808         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
 809         let into_future_span = self.mark_span_with_reason(
 810             DesugaringKind::Await,
 811             dot_await_span,
 812             self.allow_into_future.clone(),
 813         );
 814         let into_future_expr = self.expr_call_lang_item_fn(
 815             into_future_span,
 816             hir::LangItem::IntoFutureIntoFuture,
 817             arena_vec![self; expr],
 818             Some(expr_hir_id),
 819         );
 820
 821         // match <into_future_expr> {
 822         //     mut __awaitee => loop { .. }
 823         // }
 824         hir::ExprKind::Match(
 825             into_future_expr,
 826             arena_vec![self; awaitee_arm],
 827             hir::MatchSource::AwaitDesugar,
 828         )
 829     }
 830
 831     fn lower_expr_closure(
 832         &mut self,
 833         capture_clause: CaptureBy,
 834         closure_id: NodeId,
 835         movability: Movability,
 836         decl: &FnDecl,
 837         body: &Expr,
 838         fn_decl_span: Span,
 839     ) -> hir::ExprKind<'hir> {
 840         let (body, generator_option) = self.with_new_scopes(move |this| {
 841             let prev = this.current_item;
 842             this.current_item = Some(fn_decl_span);
 843             let mut generator_kind = None;
 844             let body_id = this.lower_fn_body(decl, |this| {
 845                 let e = this.lower_expr_mut(body);
 846                 generator_kind = this.generator_kind;
 847                 e
 848             });
 849             let generator_option =
 850                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 851             this.current_item = prev;
 852             (body_id, generator_option)
 853         });
 854
 855         self.with_lifetime_binder(closure_id, &[], |this, bound_generic_params| {
 856             // Lower outside new scope to preserve `is_in_loop_condition`.
 857             let fn_decl = this.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
 858
 859             hir::ExprKind::Closure {
 860                 capture_clause,
 861                 bound_generic_params,
 862                 fn_decl,
 863                 body,
 864                 fn_decl_span: this.lower_span(fn_decl_span),
 865                 movability: generator_option,
 866             }
 867         })
 868     }
 869
 870     fn generator_movability_for_fn(
 871         &mut self,
 872         decl: &FnDecl,
 873         fn_decl_span: Span,
 874         generator_kind: Option<hir::GeneratorKind>,
 875         movability: Movability,
 876     ) -> Option<hir::Movability> {
 877         match generator_kind {
 878             Some(hir::GeneratorKind::Gen) => {
 879                 if decl.inputs.len() > 1 {
 880                     struct_span_err!(
 881                         self.sess,
 882                         fn_decl_span,
 883                         E0628,
 884                         "too many parameters for a generator (expected 0 or 1 parameters)"
 885                     )
 886                     .emit();
 887                 }
 888                 Some(movability)
 889             }
 890             Some(hir::GeneratorKind::Async(_)) => {
 891                 panic!("non-`async` closure body turned `async` during lowering");
 892             }
 893             None => {
 894                 if movability == Movability::Static {
 895                     struct_span_err!(self.sess, fn_decl_span, E0697, "closures cannot be static")
 896                         .emit();
 897                 }
 898                 None
 899             }
 900         }
 901     }
 902
 903     fn lower_expr_async_closure(
 904         &mut self,
 905         capture_clause: CaptureBy,
 906         closure_id: NodeId,
 907         inner_closure_id: NodeId,
 908         decl: &FnDecl,
 909         body: &Expr,
 910         fn_decl_span: Span,
 911     ) -> hir::ExprKind<'hir> {
 912         let outer_decl =
 913             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 914
 915         let body = self.with_new_scopes(|this| {
 916             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 917             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 918                 struct_span_err!(
 919                     this.sess,
 920                     fn_decl_span,
 921                     E0708,
 922                     "`async` non-`move` closures with parameters are not currently supported",
 923                 )
 924                 .help(
 925                     "consider using `let` statements to manually capture \
 926                     variables by reference before entering an `async move` closure",
 927                 )
 928                 .emit();
 929             }
 930
 931             // Transform `async |x: u8| -> X { ... }` into
 932             // `|x: u8| future_from_generator(|| -> X { ... })`.
 933             let body_id = this.lower_fn_body(&outer_decl, |this| {
 934                 let async_ret_ty =
 935                     if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 936                 let async_body = this.make_async_expr(
 937                     capture_clause,
 938                     inner_closure_id,
 939                     async_ret_ty,
 940                     body.span,
 941                     hir::AsyncGeneratorKind::Closure,
 942                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 943                 );
 944                 this.expr(fn_decl_span, async_body, ThinVec::new())
 945             });
 946             body_id
 947         });
 948
 949         self.with_lifetime_binder(closure_id, &[], |this, bound_generic_params| {
 950             // We need to lower the declaration outside the new scope, because we
 951             // have to conserve the state of being inside a loop condition for the
 952             // closure argument types.
 953             let fn_decl = this.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
 954
 955             hir::ExprKind::Closure {
 956                 capture_clause,
 957                 bound_generic_params,
 958                 fn_decl,
 959                 body,
 960                 fn_decl_span: this.lower_span(fn_decl_span),
 961                 movability: None,
 962             }
 963         })
 964     }
 965
 966     /// Destructure the LHS of complex assignments.
 967     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
 968     fn lower_expr_assign(
 969         &mut self,
 970         lhs: &Expr,
 971         rhs: &Expr,
 972         eq_sign_span: Span,
 973         whole_span: Span,
 974     ) -> hir::ExprKind<'hir> {
 975         // Return early in case of an ordinary assignment.
 976         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
 977             match &lhs.kind {
 978                 ExprKind::Array(..)
 979                 | ExprKind::Struct(..)
 980                 | ExprKind::Tup(..)
 981                 | ExprKind::Underscore => false,
 982                 // Check for tuple struct constructor.
 983                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
 984                 ExprKind::Paren(e) => {
 985                     match e.kind {
 986                         // We special-case `(..)` for consistency with patterns.
 987                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
 988                         _ => is_ordinary(lower_ctx, e),
 989                     }
 990                 }
 991                 _ => true,
 992             }
 993         }
 994         if is_ordinary(self, lhs) {
 995             return hir::ExprKind::Assign(
 996                 self.lower_expr(lhs),
 997                 self.lower_expr(rhs),
 998                 self.lower_span(eq_sign_span),
 999             );
1000         }
1001
1002         let mut assignments = vec![];
1003
1004         // The LHS becomes a pattern: `(lhs1, lhs2)`.
1005         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
1006         let rhs = self.lower_expr(rhs);
1007
1008         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
1009         let destructure_let = self.stmt_let_pat(
1010             None,
1011             whole_span,
1012             Some(rhs),
1013             pat,
1014             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
1015         );
1016
1017         // `a = lhs1; b = lhs2;`.
1018         let stmts = self
1019             .arena
1020             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
1021
1022         // Wrap everything in a block.
1023         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
1024     }
1025
1026     /// If the given expression is a path to a tuple struct, returns that path.
1027     /// It is not a complete check, but just tries to reject most paths early
1028     /// if they are not tuple structs.
1029     /// Type checking will take care of the full validation later.
1030     fn extract_tuple_struct_path<'a>(
1031         &mut self,
1032         expr: &'a Expr,
1033     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1034         if let ExprKind::Path(qself, path) = &expr.kind {
1035             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1036             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1037                 if partial_res.unresolved_segments() == 0
1038                     && !partial_res.base_res().expected_in_tuple_struct_pat()
1039                 {
1040                     return None;
1041                 }
1042             }
1043             return Some((qself, path));
1044         }
1045         None
1046     }
1047
1048     /// If the given expression is a path to a unit struct, returns that path.
1049     /// It is not a complete check, but just tries to reject most paths early
1050     /// if they are not unit structs.
1051     /// Type checking will take care of the full validation later.
1052     fn extract_unit_struct_path<'a>(
1053         &mut self,
1054         expr: &'a Expr,
1055     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1056         if let ExprKind::Path(qself, path) = &expr.kind {
1057             // Does the path resolve to something disallowed in a unit struct/variant pattern?
1058             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1059                 if partial_res.unresolved_segments() == 0
1060                     && !partial_res.base_res().expected_in_unit_struct_pat()
1061                 {
1062                     return None;
1063                 }
1064             }
1065             return Some((qself, path));
1066         }
1067         None
1068     }
1069
1070     /// Convert the LHS of a destructuring assignment to a pattern.
1071     /// Each sub-assignment is recorded in `assignments`.
1072     fn destructure_assign(
1073         &mut self,
1074         lhs: &Expr,
1075         eq_sign_span: Span,
1076         assignments: &mut Vec<hir::Stmt<'hir>>,
1077     ) -> &'hir hir::Pat<'hir> {
1078         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1079     }
1080
1081     fn destructure_assign_mut(
1082         &mut self,
1083         lhs: &Expr,
1084         eq_sign_span: Span,
1085         assignments: &mut Vec<hir::Stmt<'hir>>,
1086     ) -> hir::Pat<'hir> {
1087         match &lhs.kind {
1088             // Underscore pattern.
1089             ExprKind::Underscore => {
1090                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1091             }
1092             // Slice patterns.
1093             ExprKind::Array(elements) => {
1094                 let (pats, rest) =
1095                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1096                 let slice_pat = if let Some((i, span)) = rest {
1097                     let (before, after) = pats.split_at(i);
1098                     hir::PatKind::Slice(
1099                         before,
1100                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1101                         after,
1102                     )
1103                 } else {
1104                     hir::PatKind::Slice(pats, None, &[])
1105                 };
1106                 return self.pat_without_dbm(lhs.span, slice_pat);
1107             }
1108             // Tuple structs.
1109             ExprKind::Call(callee, args) => {
1110                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1111                     let (pats, rest) = self.destructure_sequence(
1112                         args,
1113                         "tuple struct or variant",
1114                         eq_sign_span,
1115                         assignments,
1116                     );
1117                     let qpath = self.lower_qpath(
1118                         callee.id,
1119                         qself,
1120                         path,
1121                         ParamMode::Optional,
1122                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1123                     );
1124                     // Destructure like a tuple struct.
1125                     let tuple_struct_pat =
1126                         hir::PatKind::TupleStruct(qpath, pats, rest.map(|r| r.0));
1127                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1128                 }
1129             }
1130             // Unit structs and enum variants.
1131             ExprKind::Path(..) => {
1132                 if let Some((qself, path)) = self.extract_unit_struct_path(lhs) {
1133                     let qpath = self.lower_qpath(
1134                         lhs.id,
1135                         qself,
1136                         path,
1137                         ParamMode::Optional,
1138                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1139                     );
1140                     // Destructure like a unit struct.
1141                     let unit_struct_pat = hir::PatKind::Path(qpath);
1142                     return self.pat_without_dbm(lhs.span, unit_struct_pat);
1143                 }
1144             }
1145             // Structs.
1146             ExprKind::Struct(se) => {
1147                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1148                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1149                     hir::PatField {
1150                         hir_id: self.next_id(),
1151                         ident: self.lower_ident(f.ident),
1152                         pat,
1153                         is_shorthand: f.is_shorthand,
1154                         span: self.lower_span(f.span),
1155                     }
1156                 }));
1157                 let qpath = self.lower_qpath(
1158                     lhs.id,
1159                     &se.qself,
1160                     &se.path,
1161                     ParamMode::Optional,
1162                     ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1163                 );
1164                 let fields_omitted = match &se.rest {
1165                     StructRest::Base(e) => {
1166                         self.sess
1167                             .struct_span_err(
1168                                 e.span,
1169                                 "functional record updates are not allowed in destructuring \
1170                                     assignments",
1171                             )
1172                             .span_suggestion(
1173                                 e.span,
1174                                 "consider removing the trailing pattern",
1175                                 "",
1176                                 rustc_errors::Applicability::MachineApplicable,
1177                             )
1178                             .emit();
1179                         true
1180                     }
1181                     StructRest::Rest(_) => true,
1182                     StructRest::None => false,
1183                 };
1184                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1185                 return self.pat_without_dbm(lhs.span, struct_pat);
1186             }
1187             // Tuples.
1188             ExprKind::Tup(elements) => {
1189                 let (pats, rest) =
1190                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1191                 let tuple_pat = hir::PatKind::Tuple(pats, rest.map(|r| r.0));
1192                 return self.pat_without_dbm(lhs.span, tuple_pat);
1193             }
1194             ExprKind::Paren(e) => {
1195                 // We special-case `(..)` for consistency with patterns.
1196                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1197                     let tuple_pat = hir::PatKind::Tuple(&[], Some(0));
1198                     return self.pat_without_dbm(lhs.span, tuple_pat);
1199                 } else {
1200                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1201                 }
1202             }
1203             _ => {}
1204         }
1205         // Treat all other cases as normal lvalue.
1206         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1207         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1208         let ident = self.expr_ident(lhs.span, ident, binding);
1209         let assign =
1210             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1211         let expr = self.expr(lhs.span, assign, ThinVec::new());
1212         assignments.push(self.stmt_expr(lhs.span, expr));
1213         pat
1214     }
1215
1216     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1217     /// Such a sequence occurs in a tuple (struct)/slice.
1218     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1219     /// exists.
1220     /// Each sub-assignment is recorded in `assignments`.
1221     fn destructure_sequence(
1222         &mut self,
1223         elements: &[AstP<Expr>],
1224         ctx: &str,
1225         eq_sign_span: Span,
1226         assignments: &mut Vec<hir::Stmt<'hir>>,
1227     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1228         let mut rest = None;
1229         let elements =
1230             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1231                 // Check for `..` pattern.
1232                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1233                     if let Some((_, prev_span)) = rest {
1234                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1235                     } else {
1236                         rest = Some((i, e.span));
1237                     }
1238                     None
1239                 } else {
1240                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1241                 }
1242             }));
1243         (elements, rest)
1244     }
1245
1246     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1247     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1248         let e1 = self.lower_expr_mut(e1);
1249         let e2 = self.lower_expr_mut(e2);
1250         let fn_path =
1251             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1252         let fn_expr =
1253             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
1254         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1255     }
1256
1257     fn lower_expr_range(
1258         &mut self,
1259         span: Span,
1260         e1: Option<&Expr>,
1261         e2: Option<&Expr>,
1262         lims: RangeLimits,
1263     ) -> hir::ExprKind<'hir> {
1264         use rustc_ast::RangeLimits::*;
1265
1266         let lang_item = match (e1, e2, lims) {
1267             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1268             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1269             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1270             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1271             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1272             (Some(..), Some(..), Closed) => unreachable!(),
1273             (_, None, Closed) => self.diagnostic().span_fatal(span, "inclusive range with no end"),
1274         };
1275
1276         let fields = self.arena.alloc_from_iter(
1277             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1278                 |(s, e)| {
1279                     let expr = self.lower_expr(&e);
1280                     let ident = Ident::new(s, self.lower_span(e.span));
1281                     self.expr_field(ident, expr, e.span)
1282                 },
1283             ),
1284         );
1285
1286         hir::ExprKind::Struct(
1287             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1288             fields,
1289             None,
1290         )
1291     }
1292
1293     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1294         let label = opt_label?;
1295         Some(Label { ident: self.lower_ident(label.ident) })
1296     }
1297
1298     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1299         let target_id = match destination {
1300             Some((id, _)) => {
1301                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1302                     Ok(self.lower_node_id(loop_id))
1303                 } else {
1304                     Err(hir::LoopIdError::UnresolvedLabel)
1305                 }
1306             }
1307             None => self
1308                 .loop_scope
1309                 .map(|id| Ok(self.lower_node_id(id)))
1310                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1311         };
1312         let label = self.lower_label(destination.map(|(_, label)| label));
1313         hir::Destination { label, target_id }
1314     }
1315
1316     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1317         if self.is_in_loop_condition && opt_label.is_none() {
1318             hir::Destination {
1319                 label: None,
1320                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1321             }
1322         } else {
1323             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1324         }
1325     }
1326
1327     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1328         let old_scope = self.catch_scope.replace(catch_id);
1329         let result = f(self);
1330         self.catch_scope = old_scope;
1331         result
1332     }
1333
1334     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1335         // We're no longer in the base loop's condition; we're in another loop.
1336         let was_in_loop_condition = self.is_in_loop_condition;
1337         self.is_in_loop_condition = false;
1338
1339         let old_scope = self.loop_scope.replace(loop_id);
1340         let result = f(self);
1341         self.loop_scope = old_scope;
1342
1343         self.is_in_loop_condition = was_in_loop_condition;
1344
1345         result
1346     }
1347
1348     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1349         let was_in_loop_condition = self.is_in_loop_condition;
1350         self.is_in_loop_condition = true;
1351
1352         let result = f(self);
1353
1354         self.is_in_loop_condition = was_in_loop_condition;
1355
1356         result
1357     }
1358
1359     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1360         hir::ExprField {
1361             hir_id: self.next_id(),
1362             ident: self.lower_ident(f.ident),
1363             expr: self.lower_expr(&f.expr),
1364             span: self.lower_span(f.span),
1365             is_shorthand: f.is_shorthand,
1366         }
1367     }
1368
1369     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1370         match self.generator_kind {
1371             Some(hir::GeneratorKind::Gen) => {}
1372             Some(hir::GeneratorKind::Async(_)) => {
1373                 struct_span_err!(
1374                     self.sess,
1375                     span,
1376                     E0727,
1377                     "`async` generators are not yet supported"
1378                 )
1379                 .emit();
1380             }
1381             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1382         }
1383
1384         let expr =
1385             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1386
1387         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1388     }
1389
1390     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1391     /// ```ignore (pseudo-rust)
1392     /// {
1393     ///     let result = match IntoIterator::into_iter(<head>) {
1394     ///         mut iter => {
1395     ///             [opt_ident]: loop {
1396     ///                 match Iterator::next(&mut iter) {
1397     ///                     None => break,
1398     ///                     Some(<pat>) => <body>,
1399     ///                 };
1400     ///             }
1401     ///         }
1402     ///     };
1403     ///     result
1404     /// }
1405     /// ```
1406     fn lower_expr_for(
1407         &mut self,
1408         e: &Expr,
1409         pat: &Pat,
1410         head: &Expr,
1411         body: &Block,
1412         opt_label: Option<Label>,
1413     ) -> hir::Expr<'hir> {
1414         let head = self.lower_expr_mut(head);
1415         let pat = self.lower_pat(pat);
1416         let for_span =
1417             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1418         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1419         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1420
1421         // `None => break`
1422         let none_arm = {
1423             let break_expr =
1424                 self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, ThinVec::new()));
1425             let pat = self.pat_none(for_span);
1426             self.arm(pat, break_expr)
1427         };
1428
1429         // Some(<pat>) => <body>,
1430         let some_arm = {
1431             let some_pat = self.pat_some(pat_span, pat);
1432             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1433             let body_expr = self.arena.alloc(self.expr_block(body_block, ThinVec::new()));
1434             self.arm(some_pat, body_expr)
1435         };
1436
1437         // `mut iter`
1438         let iter = Ident::with_dummy_span(sym::iter);
1439         let (iter_pat, iter_pat_nid) =
1440             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::Mutable);
1441
1442         // `match Iterator::next(&mut iter) { ... }`
1443         let match_expr = {
1444             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1445             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1446             let next_expr = self.expr_call_lang_item_fn(
1447                 head_span,
1448                 hir::LangItem::IteratorNext,
1449                 arena_vec![self; ref_mut_iter],
1450                 None,
1451             );
1452             let arms = arena_vec![self; none_arm, some_arm];
1453
1454             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1455         };
1456         let match_stmt = self.stmt_expr(for_span, match_expr);
1457
1458         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1459
1460         // `[opt_ident]: loop { ... }`
1461         let kind = hir::ExprKind::Loop(
1462             loop_block,
1463             self.lower_label(opt_label),
1464             hir::LoopSource::ForLoop,
1465             self.lower_span(for_span.with_hi(head.span.hi())),
1466         );
1467         let loop_expr =
1468             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1469
1470         // `mut iter => { ... }`
1471         let iter_arm = self.arm(iter_pat, loop_expr);
1472
1473         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1474         let into_iter_expr = {
1475             self.expr_call_lang_item_fn(
1476                 head_span,
1477                 hir::LangItem::IntoIterIntoIter,
1478                 arena_vec![self; head],
1479                 None,
1480             )
1481         };
1482
1483         let match_expr = self.arena.alloc(self.expr_match(
1484             for_span,
1485             into_iter_expr,
1486             arena_vec![self; iter_arm],
1487             hir::MatchSource::ForLoopDesugar,
1488         ));
1489
1490         let attrs: Vec<_> = e.attrs.iter().map(|a| self.lower_attr(a)).collect();
1491
1492         // This is effectively `{ let _result = ...; _result }`.
1493         // The construct was introduced in #21984 and is necessary to make sure that
1494         // temporaries in the `head` expression are dropped and do not leak to the
1495         // surrounding scope of the `match` since the `match` is not a terminating scope.
1496         //
1497         // Also, add the attributes to the outer returned expr node.
1498         self.expr_drop_temps_mut(for_span, match_expr, attrs.into())
1499     }
1500
1501     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1502     /// ```ignore (pseudo-rust)
1503     /// match Try::branch(<expr>) {
1504     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1505     ///     ControlFlow::Break(residual) =>
1506     ///         #[allow(unreachable_code)]
1507     ///         // If there is an enclosing `try {...}`:
1508     ///         break 'catch_target Try::from_residual(residual),
1509     ///         // Otherwise:
1510     ///         return Try::from_residual(residual),
1511     /// }
1512     /// ```
1513     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1514         let unstable_span = self.mark_span_with_reason(
1515             DesugaringKind::QuestionMark,
1516             span,
1517             self.allow_try_trait.clone(),
1518         );
1519         let try_span = self.sess.source_map().end_point(span);
1520         let try_span = self.mark_span_with_reason(
1521             DesugaringKind::QuestionMark,
1522             try_span,
1523             self.allow_try_trait.clone(),
1524         );
1525
1526         // `Try::branch(<expr>)`
1527         let scrutinee = {
1528             // expand <expr>
1529             let sub_expr = self.lower_expr_mut(sub_expr);
1530
1531             self.expr_call_lang_item_fn(
1532                 unstable_span,
1533                 hir::LangItem::TryTraitBranch,
1534                 arena_vec![self; sub_expr],
1535                 None,
1536             )
1537         };
1538
1539         // `#[allow(unreachable_code)]`
1540         let attr = {
1541             // `allow(unreachable_code)`
1542             let allow = {
1543                 let allow_ident = Ident::new(sym::allow, self.lower_span(span));
1544                 let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
1545                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1546                 attr::mk_list_item(allow_ident, vec![uc_nested])
1547             };
1548             attr::mk_attr_outer(allow)
1549         };
1550         let attrs = vec![attr];
1551
1552         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1553         let continue_arm = {
1554             let val_ident = Ident::with_dummy_span(sym::val);
1555             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1556             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1557                 span,
1558                 val_ident,
1559                 val_pat_nid,
1560                 ThinVec::from(attrs.clone()),
1561             ));
1562             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1563             self.arm(continue_pat, val_expr)
1564         };
1565
1566         // `ControlFlow::Break(residual) =>
1567         //     #[allow(unreachable_code)]
1568         //     return Try::from_residual(residual),`
1569         let break_arm = {
1570             let residual_ident = Ident::with_dummy_span(sym::residual);
1571             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1572             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1573             let from_residual_expr = self.wrap_in_try_constructor(
1574                 hir::LangItem::TryTraitFromResidual,
1575                 try_span,
1576                 self.arena.alloc(residual_expr),
1577                 unstable_span,
1578             );
1579             let thin_attrs = ThinVec::from(attrs);
1580             let ret_expr = if let Some(catch_node) = self.catch_scope {
1581                 let target_id = Ok(self.lower_node_id(catch_node));
1582                 self.arena.alloc(self.expr(
1583                     try_span,
1584                     hir::ExprKind::Break(
1585                         hir::Destination { label: None, target_id },
1586                         Some(from_residual_expr),
1587                     ),
1588                     thin_attrs,
1589                 ))
1590             } else {
1591                 self.arena.alloc(self.expr(
1592                     try_span,
1593                     hir::ExprKind::Ret(Some(from_residual_expr)),
1594                     thin_attrs,
1595                 ))
1596             };
1597
1598             let break_pat = self.pat_cf_break(try_span, residual_local);
1599             self.arm(break_pat, ret_expr)
1600         };
1601
1602         hir::ExprKind::Match(
1603             scrutinee,
1604             arena_vec![self; break_arm, continue_arm],
1605             hir::MatchSource::TryDesugar,
1606         )
1607     }
1608
1609     /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
1610     /// ```rust
1611     /// // If there is an enclosing `try {...}`:
1612     /// break 'catch_target FromResidual::from_residual(Yeet(residual)),
1613     /// // Otherwise:
1614     /// return FromResidual::from_residual(Yeet(residual)),
1615     /// ```
1616     /// But to simplify this, there's a `from_yeet` lang item function which
1617     /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
1618     fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1619         // The expression (if present) or `()` otherwise.
1620         let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
1621             (sub_expr.span, self.lower_expr(sub_expr))
1622         } else {
1623             (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
1624         };
1625
1626         let unstable_span = self.mark_span_with_reason(
1627             DesugaringKind::YeetExpr,
1628             span,
1629             self.allow_try_trait.clone(),
1630         );
1631
1632         let from_yeet_expr = self.wrap_in_try_constructor(
1633             hir::LangItem::TryTraitFromYeet,
1634             unstable_span,
1635             yeeted_expr,
1636             yeeted_span,
1637         );
1638
1639         if let Some(catch_node) = self.catch_scope {
1640             let target_id = Ok(self.lower_node_id(catch_node));
1641             hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
1642         } else {
1643             hir::ExprKind::Ret(Some(from_yeet_expr))
1644         }
1645     }
1646
1647     // =========================================================================
1648     // Helper methods for building HIR.
1649     // =========================================================================
1650
1651     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1652     ///
1653     /// In terms of drop order, it has the same effect as wrapping `expr` in
1654     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1655     ///
1656     /// The drop order can be important in e.g. `if expr { .. }`.
1657     pub(super) fn expr_drop_temps(
1658         &mut self,
1659         span: Span,
1660         expr: &'hir hir::Expr<'hir>,
1661         attrs: AttrVec,
1662     ) -> &'hir hir::Expr<'hir> {
1663         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1664     }
1665
1666     pub(super) fn expr_drop_temps_mut(
1667         &mut self,
1668         span: Span,
1669         expr: &'hir hir::Expr<'hir>,
1670         attrs: AttrVec,
1671     ) -> hir::Expr<'hir> {
1672         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1673     }
1674
1675     fn expr_match(
1676         &mut self,
1677         span: Span,
1678         arg: &'hir hir::Expr<'hir>,
1679         arms: &'hir [hir::Arm<'hir>],
1680         source: hir::MatchSource,
1681     ) -> hir::Expr<'hir> {
1682         self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
1683     }
1684
1685     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
1686         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1687         self.expr(span, expr_break, attrs)
1688     }
1689
1690     fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1691         let expr_break = self.expr_break(span, attrs);
1692         self.arena.alloc(expr_break)
1693     }
1694
1695     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1696         self.expr(
1697             span,
1698             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1699             ThinVec::new(),
1700         )
1701     }
1702
1703     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1704         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
1705     }
1706
1707     fn expr_call_mut(
1708         &mut self,
1709         span: Span,
1710         e: &'hir hir::Expr<'hir>,
1711         args: &'hir [hir::Expr<'hir>],
1712     ) -> hir::Expr<'hir> {
1713         self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new())
1714     }
1715
1716     fn expr_call(
1717         &mut self,
1718         span: Span,
1719         e: &'hir hir::Expr<'hir>,
1720         args: &'hir [hir::Expr<'hir>],
1721     ) -> &'hir hir::Expr<'hir> {
1722         self.arena.alloc(self.expr_call_mut(span, e, args))
1723     }
1724
1725     fn expr_call_lang_item_fn_mut(
1726         &mut self,
1727         span: Span,
1728         lang_item: hir::LangItem,
1729         args: &'hir [hir::Expr<'hir>],
1730         hir_id: Option<hir::HirId>,
1731     ) -> hir::Expr<'hir> {
1732         let path =
1733             self.arena.alloc(self.expr_lang_item_path(span, lang_item, ThinVec::new(), hir_id));
1734         self.expr_call_mut(span, path, args)
1735     }
1736
1737     fn expr_call_lang_item_fn(
1738         &mut self,
1739         span: Span,
1740         lang_item: hir::LangItem,
1741         args: &'hir [hir::Expr<'hir>],
1742         hir_id: Option<hir::HirId>,
1743     ) -> &'hir hir::Expr<'hir> {
1744         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1745     }
1746
1747     fn expr_lang_item_path(
1748         &mut self,
1749         span: Span,
1750         lang_item: hir::LangItem,
1751         attrs: AttrVec,
1752         hir_id: Option<hir::HirId>,
1753     ) -> hir::Expr<'hir> {
1754         self.expr(
1755             span,
1756             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1757             attrs,
1758         )
1759     }
1760
1761     pub(super) fn expr_ident(
1762         &mut self,
1763         sp: Span,
1764         ident: Ident,
1765         binding: hir::HirId,
1766     ) -> &'hir hir::Expr<'hir> {
1767         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1768     }
1769
1770     pub(super) fn expr_ident_mut(
1771         &mut self,
1772         sp: Span,
1773         ident: Ident,
1774         binding: hir::HirId,
1775     ) -> hir::Expr<'hir> {
1776         self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
1777     }
1778
1779     fn expr_ident_with_attrs(
1780         &mut self,
1781         span: Span,
1782         ident: Ident,
1783         binding: hir::HirId,
1784         attrs: AttrVec,
1785     ) -> hir::Expr<'hir> {
1786         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1787             None,
1788             self.arena.alloc(hir::Path {
1789                 span: self.lower_span(span),
1790                 res: Res::Local(binding),
1791                 segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
1792             }),
1793         ));
1794
1795         self.expr(span, expr_path, attrs)
1796     }
1797
1798     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1799         let hir_id = self.next_id();
1800         let span = expr.span;
1801         self.expr(
1802             span,
1803             hir::ExprKind::Block(
1804                 self.arena.alloc(hir::Block {
1805                     stmts: &[],
1806                     expr: Some(expr),
1807                     hir_id,
1808                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1809                     span: self.lower_span(span),
1810                     targeted_by_break: false,
1811                 }),
1812                 None,
1813             ),
1814             ThinVec::new(),
1815         )
1816     }
1817
1818     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1819         let blk = self.block_all(span, &[], None);
1820         let expr = self.expr_block(blk, ThinVec::new());
1821         self.arena.alloc(expr)
1822     }
1823
1824     pub(super) fn expr_block(
1825         &mut self,
1826         b: &'hir hir::Block<'hir>,
1827         attrs: AttrVec,
1828     ) -> hir::Expr<'hir> {
1829         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1830     }
1831
1832     pub(super) fn expr(
1833         &mut self,
1834         span: Span,
1835         kind: hir::ExprKind<'hir>,
1836         attrs: AttrVec,
1837     ) -> hir::Expr<'hir> {
1838         let hir_id = self.next_id();
1839         self.lower_attrs(hir_id, &attrs);
1840         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1841     }
1842
1843     fn expr_field(
1844         &mut self,
1845         ident: Ident,
1846         expr: &'hir hir::Expr<'hir>,
1847         span: Span,
1848     ) -> hir::ExprField<'hir> {
1849         hir::ExprField {
1850             hir_id: self.next_id(),
1851             ident,
1852             span: self.lower_span(span),
1853             expr,
1854             is_shorthand: false,
1855         }
1856     }
1857
1858     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1859         hir::Arm {
1860             hir_id: self.next_id(),
1861             pat,
1862             guard: None,
1863             span: self.lower_span(expr.span),
1864             body: expr,
1865         }
1866     }
1867 }