compiler/rustc_ast_lowering/src/expr.rs

   1 use super::errors::{
   2     AsyncGeneratorsNotSupported, AsyncNonMoveClosureNotSupported, AwaitOnlyInAsyncFnAndBlocks,
   3     BaseExpressionDoubleDot, ClosureCannotBeStatic, FunctionalRecordUpdateDestructuringAssignemnt,
   4     GeneratorTooManyParameters, InclusiveRangeWithNoEnd, NotSupportedForLifetimeBinderAsyncClosure,
   5     RustcBoxAttributeError, UnderscoreExprLhsAssign,
   6 };
   7 use super::ResolverAstLoweringExt;
   8 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
   9 use crate::{FnDeclKind, ImplTraitPosition};
  10 use rustc_ast::attr;
  11 use rustc_ast::ptr::P as AstP;
  12 use rustc_ast::*;
  13 use rustc_data_structures::stack::ensure_sufficient_stack;
  14 use rustc_hir as hir;
  15 use rustc_hir::def::Res;
  16 use rustc_hir::definitions::DefPathData;
  17 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
  18 use rustc_span::symbol::{sym, Ident};
  19 use rustc_span::DUMMY_SP;
  20 use thin_vec::thin_vec;
  21
  22 impl<'hir> LoweringContext<'_, 'hir> {
  23     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  24         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  25     }
  26
  27     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  28         self.arena.alloc(self.lower_expr_mut(e))
  29     }
  30
  31     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  32         ensure_sufficient_stack(|| {
  33             let kind = match e.kind {
  34                 ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  35                 ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  36                 ExprKind::ConstBlock(ref anon_const) => {
  37                     let anon_const = self.lower_anon_const(anon_const);
  38                     hir::ExprKind::ConstBlock(anon_const)
  39                 }
  40                 ExprKind::Repeat(ref expr, ref count) => {
  41                     let expr = self.lower_expr(expr);
  42                     let count = self.lower_array_length(count);
  43                     hir::ExprKind::Repeat(expr, count)
  44                 }
  45                 ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  46                 ExprKind::Call(ref f, ref args) => {
  47                     if e.attrs.get(0).map_or(false, |a| a.has_name(sym::rustc_box)) {
  48                         if let [inner] = &args[..] && e.attrs.len() == 1 {
  49                             let kind = hir::ExprKind::Box(self.lower_expr(&inner));
  50                             let hir_id = self.lower_node_id(e.id);
  51                             return hir::Expr { hir_id, kind, span: self.lower_span(e.span) };
  52                         } else {
  53                             self.tcx.sess.emit_err(RustcBoxAttributeError { span: e.span });
  54                             hir::ExprKind::Err
  55                         }
  56                     } else if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
  57                         self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
  58                     } else {
  59                         let f = self.lower_expr(f);
  60                         hir::ExprKind::Call(f, self.lower_exprs(args))
  61                     }
  62                 }
  63                 ExprKind::MethodCall(ref seg, ref receiver, ref args, span) => {
  64                     let hir_seg = self.arena.alloc(self.lower_path_segment(
  65                         e.span,
  66                         seg,
  67                         ParamMode::Optional,
  68                         ParenthesizedGenericArgs::Err,
  69                         &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  70                     ));
  71                     let receiver = self.lower_expr(receiver);
  72                     let args =
  73                         self.arena.alloc_from_iter(args.iter().map(|x| self.lower_expr_mut(x)));
  74                     hir::ExprKind::MethodCall(hir_seg, receiver, args, self.lower_span(span))
  75                 }
  76                 ExprKind::Binary(binop, ref lhs, ref rhs) => {
  77                     let binop = self.lower_binop(binop);
  78                     let lhs = self.lower_expr(lhs);
  79                     let rhs = self.lower_expr(rhs);
  80                     hir::ExprKind::Binary(binop, lhs, rhs)
  81                 }
  82                 ExprKind::Unary(op, ref ohs) => {
  83                     let op = self.lower_unop(op);
  84                     let ohs = self.lower_expr(ohs);
  85                     hir::ExprKind::Unary(op, ohs)
  86                 }
  87                 ExprKind::Lit(ref l) => {
  88                     hir::ExprKind::Lit(respan(self.lower_span(l.span), l.kind.clone()))
  89                 }
  90                 ExprKind::Cast(ref expr, ref ty) => {
  91                     let expr = self.lower_expr(expr);
  92                     let ty = self
  93                         .lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
  94                     hir::ExprKind::Cast(expr, ty)
  95                 }
  96                 ExprKind::Type(ref expr, ref ty) => {
  97                     let expr = self.lower_expr(expr);
  98                     let ty = self
  99                         .lower_ty(ty, &mut ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 100                     hir::ExprKind::Type(expr, ty)
 101                 }
 102                 ExprKind::AddrOf(k, m, ref ohs) => {
 103                     let ohs = self.lower_expr(ohs);
 104                     hir::ExprKind::AddrOf(k, m, ohs)
 105                 }
 106                 ExprKind::Let(ref pat, ref scrutinee, span) => {
 107                     hir::ExprKind::Let(self.arena.alloc(hir::Let {
 108                         hir_id: self.next_id(),
 109                         span: self.lower_span(span),
 110                         pat: self.lower_pat(pat),
 111                         ty: None,
 112                         init: self.lower_expr(scrutinee),
 113                     }))
 114                 }
 115                 ExprKind::If(ref cond, ref then, ref else_opt) => {
 116                     self.lower_expr_if(cond, then, else_opt.as_deref())
 117                 }
 118                 ExprKind::While(ref cond, ref body, opt_label) => {
 119                     self.with_loop_scope(e.id, |this| {
 120                         let span =
 121                             this.mark_span_with_reason(DesugaringKind::WhileLoop, e.span, None);
 122                         this.lower_expr_while_in_loop_scope(span, cond, body, opt_label)
 123                     })
 124                 }
 125                 ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
 126                     hir::ExprKind::Loop(
 127                         this.lower_block(body, false),
 128                         this.lower_label(opt_label),
 129                         hir::LoopSource::Loop,
 130                         DUMMY_SP,
 131                     )
 132                 }),
 133                 ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
 134                 ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
 135                     self.lower_expr(expr),
 136                     self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
 137                     hir::MatchSource::Normal,
 138                 ),
 139                 ExprKind::Async(capture_clause, closure_node_id, ref block) => self
 140                     .make_async_expr(
 141                         capture_clause,
 142                         closure_node_id,
 143                         None,
 144                         block.span,
 145                         hir::AsyncGeneratorKind::Block,
 146                         |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
 147                     ),
 148                 ExprKind::Await(ref expr) => {
 149                     let dot_await_span = if expr.span.hi() < e.span.hi() {
 150                         let span_with_whitespace = self
 151                             .tcx
 152                             .sess
 153                             .source_map()
 154                             .span_extend_while(expr.span, char::is_whitespace)
 155                             .unwrap_or(expr.span);
 156                         span_with_whitespace.shrink_to_hi().with_hi(e.span.hi())
 157                     } else {
 158                         // this is a recovered `await expr`
 159                         e.span
 160                     };
 161                     self.lower_expr_await(dot_await_span, expr)
 162                 }
 163                 ExprKind::Closure(
 164                     ref binder,
 165                     capture_clause,
 166                     asyncness,
 167                     movability,
 168                     ref decl,
 169                     ref body,
 170                     fn_decl_span,
 171                 ) => {
 172                     if let Async::Yes { closure_id, .. } = asyncness {
 173                         self.lower_expr_async_closure(
 174                             binder,
 175                             capture_clause,
 176                             e.id,
 177                             closure_id,
 178                             decl,
 179                             body,
 180                             fn_decl_span,
 181                         )
 182                     } else {
 183                         self.lower_expr_closure(
 184                             binder,
 185                             capture_clause,
 186                             e.id,
 187                             movability,
 188                             decl,
 189                             body,
 190                             fn_decl_span,
 191                         )
 192                     }
 193                 }
 194                 ExprKind::Block(ref blk, opt_label) => {
 195                     let opt_label = self.lower_label(opt_label);
 196                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 197                 }
 198                 ExprKind::Assign(ref el, ref er, span) => {
 199                     self.lower_expr_assign(el, er, span, e.span)
 200                 }
 201                 ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 202                     self.lower_binop(op),
 203                     self.lower_expr(el),
 204                     self.lower_expr(er),
 205                 ),
 206                 ExprKind::Field(ref el, ident) => {
 207                     hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(ident))
 208                 }
 209                 ExprKind::Index(ref el, ref er) => {
 210                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 211                 }
 212                 ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 213                     self.lower_expr_range_closed(e.span, e1, e2)
 214                 }
 215                 ExprKind::Range(ref e1, ref e2, lims) => {
 216                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 217                 }
 218                 ExprKind::Underscore => {
 219                     self.tcx.sess.emit_err(UnderscoreExprLhsAssign { span: e.span });
 220                     hir::ExprKind::Err
 221                 }
 222                 ExprKind::Path(ref qself, ref path) => {
 223                     let qpath = self.lower_qpath(
 224                         e.id,
 225                         qself,
 226                         path,
 227                         ParamMode::Optional,
 228                         &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 229                     );
 230                     hir::ExprKind::Path(qpath)
 231                 }
 232                 ExprKind::Break(opt_label, ref opt_expr) => {
 233                     let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 234                     hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
 235                 }
 236                 ExprKind::Continue(opt_label) => {
 237                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
 238                 }
 239                 ExprKind::Ret(ref e) => {
 240                     let e = e.as_ref().map(|x| self.lower_expr(x));
 241                     hir::ExprKind::Ret(e)
 242                 }
 243                 ExprKind::Yeet(ref sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
 244                 ExprKind::InlineAsm(ref asm) => {
 245                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
 246                 }
 247                 ExprKind::Struct(ref se) => {
 248                     let rest = match &se.rest {
 249                         StructRest::Base(e) => Some(self.lower_expr(e)),
 250                         StructRest::Rest(sp) => {
 251                             self.tcx.sess.emit_err(BaseExpressionDoubleDot { span: *sp });
 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                             &mut 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, AttrVec::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, AttrVec::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, AttrVec::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.tcx.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             AttrVec::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 =
 560             match ret_ty {
 561                 Some(ty) => hir::FnRetTy::Return(self.lower_ty(
 562                     &ty,
 563                     &mut ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock),
 564                 )),
 565                 None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
 566             };
 567
 568         // Resume argument type. We let the compiler infer this to simplify the lowering. It is
 569         // fully constrained by `future::from_generator`.
 570         let input_ty = hir::Ty {
 571             hir_id: self.next_id(),
 572             kind: hir::TyKind::Infer,
 573             span: self.lower_span(span),
 574         };
 575
 576         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 577         let fn_decl = self.arena.alloc(hir::FnDecl {
 578             inputs: arena_vec![self; input_ty],
 579             output,
 580             c_variadic: false,
 581             implicit_self: hir::ImplicitSelfKind::None,
 582         });
 583
 584         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 585         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 586             span,
 587             Ident::with_dummy_span(sym::_task_context),
 588             hir::BindingAnnotation::MUT,
 589         );
 590         let param = hir::Param {
 591             hir_id: self.next_id(),
 592             pat,
 593             ty_span: self.lower_span(span),
 594             span: self.lower_span(span),
 595         };
 596         let params = arena_vec![self; param];
 597
 598         let body = self.lower_body(move |this| {
 599             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 600
 601             let old_ctx = this.task_context;
 602             this.task_context = Some(task_context_hid);
 603             let res = body(this);
 604             this.task_context = old_ctx;
 605             (params, res)
 606         });
 607
 608         // `static |_task_context| -> <ret_ty> { body }`:
 609         let generator_kind = {
 610             let c = self.arena.alloc(hir::Closure {
 611                 binder: hir::ClosureBinder::Default,
 612                 capture_clause,
 613                 bound_generic_params: &[],
 614                 fn_decl,
 615                 body,
 616                 fn_decl_span: self.lower_span(span),
 617                 movability: Some(hir::Movability::Static),
 618             });
 619
 620             hir::ExprKind::Closure(c)
 621         };
 622         let generator = hir::Expr {
 623             hir_id: self.lower_node_id(closure_node_id),
 624             kind: generator_kind,
 625             span: self.lower_span(span),
 626         };
 627
 628         // `future::from_generator`:
 629         let unstable_span =
 630             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 631         let gen_future = self.expr_lang_item_path(
 632             unstable_span,
 633             hir::LangItem::FromGenerator,
 634             AttrVec::new(),
 635             None,
 636         );
 637
 638         // `future::from_generator(generator)`:
 639         hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
 640     }
 641
 642     /// Desugar `<expr>.await` into:
 643     /// ```ignore (pseudo-rust)
 644     /// match ::std::future::IntoFuture::into_future(<expr>) {
 645     ///     mut __awaitee => loop {
 646     ///         match unsafe { ::std::future::Future::poll(
 647     ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
 648     ///             ::std::future::get_context(task_context),
 649     ///         ) } {
 650     ///             ::std::task::Poll::Ready(result) => break result,
 651     ///             ::std::task::Poll::Pending => {}
 652     ///         }
 653     ///         task_context = yield ();
 654     ///     }
 655     /// }
 656     /// ```
 657     fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 658         let full_span = expr.span.to(dot_await_span);
 659         match self.generator_kind {
 660             Some(hir::GeneratorKind::Async(_)) => {}
 661             Some(hir::GeneratorKind::Gen) | None => {
 662                 self.tcx.sess.emit_err(AwaitOnlyInAsyncFnAndBlocks {
 663                     dot_await_span,
 664                     item_span: self.current_item,
 665                 });
 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::MUT);
 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, AttrVec::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                 AttrVec::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         binder: &ClosureBinder,
 834         capture_clause: CaptureBy,
 835         closure_id: NodeId,
 836         movability: Movability,
 837         decl: &FnDecl,
 838         body: &Expr,
 839         fn_decl_span: Span,
 840     ) -> hir::ExprKind<'hir> {
 841         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 842
 843         let (body_id, generator_option) = self.with_new_scopes(move |this| {
 844             let prev = this.current_item;
 845             this.current_item = Some(fn_decl_span);
 846             let mut generator_kind = None;
 847             let body_id = this.lower_fn_body(decl, |this| {
 848                 let e = this.lower_expr_mut(body);
 849                 generator_kind = this.generator_kind;
 850                 e
 851             });
 852             let generator_option =
 853                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 854             this.current_item = prev;
 855             (body_id, generator_option)
 856         });
 857
 858         self.lower_lifetime_binder(closure_id, generic_params, |lctx, bound_generic_params| {
 859             // Lower outside new scope to preserve `is_in_loop_condition`.
 860             let fn_decl = lctx.lower_fn_decl(decl, None, fn_decl_span, FnDeclKind::Closure, None);
 861
 862             let c = lctx.arena.alloc(hir::Closure {
 863                 binder: binder_clause,
 864                 capture_clause,
 865                 bound_generic_params,
 866                 fn_decl,
 867                 body: body_id,
 868                 fn_decl_span: lctx.lower_span(fn_decl_span),
 869                 movability: generator_option,
 870             });
 871
 872             hir::ExprKind::Closure(c)
 873         })
 874     }
 875
 876     fn generator_movability_for_fn(
 877         &mut self,
 878         decl: &FnDecl,
 879         fn_decl_span: Span,
 880         generator_kind: Option<hir::GeneratorKind>,
 881         movability: Movability,
 882     ) -> Option<hir::Movability> {
 883         match generator_kind {
 884             Some(hir::GeneratorKind::Gen) => {
 885                 if decl.inputs.len() > 1 {
 886                     self.tcx.sess.emit_err(GeneratorTooManyParameters { fn_decl_span });
 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                     self.tcx.sess.emit_err(ClosureCannotBeStatic { fn_decl_span });
 896                 }
 897                 None
 898             }
 899         }
 900     }
 901
 902     fn lower_closure_binder<'c>(
 903         &mut self,
 904         binder: &'c ClosureBinder,
 905     ) -> (hir::ClosureBinder, &'c [GenericParam]) {
 906         let (binder, params) = match binder {
 907             ClosureBinder::NotPresent => (hir::ClosureBinder::Default, &[][..]),
 908             &ClosureBinder::For { span, ref generic_params } => {
 909                 let span = self.lower_span(span);
 910                 (hir::ClosureBinder::For { span }, &**generic_params)
 911             }
 912         };
 913
 914         (binder, params)
 915     }
 916
 917     fn lower_expr_async_closure(
 918         &mut self,
 919         binder: &ClosureBinder,
 920         capture_clause: CaptureBy,
 921         closure_id: NodeId,
 922         inner_closure_id: NodeId,
 923         decl: &FnDecl,
 924         body: &Expr,
 925         fn_decl_span: Span,
 926     ) -> hir::ExprKind<'hir> {
 927         if let &ClosureBinder::For { span, .. } = binder {
 928             self.tcx.sess.emit_err(NotSupportedForLifetimeBinderAsyncClosure { span });
 929         }
 930
 931         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 932
 933         let outer_decl =
 934             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 935
 936         let body = self.with_new_scopes(|this| {
 937             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 938             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 939                 this.tcx.sess.emit_err(AsyncNonMoveClosureNotSupported { fn_decl_span });
 940             }
 941
 942             // Transform `async |x: u8| -> X { ... }` into
 943             // `|x: u8| future_from_generator(|| -> X { ... })`.
 944             let body_id = this.lower_fn_body(&outer_decl, |this| {
 945                 let async_ret_ty =
 946                     if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 947                 let async_body = this.make_async_expr(
 948                     capture_clause,
 949                     inner_closure_id,
 950                     async_ret_ty,
 951                     body.span,
 952                     hir::AsyncGeneratorKind::Closure,
 953                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 954                 );
 955                 this.expr(fn_decl_span, async_body, AttrVec::new())
 956             });
 957             body_id
 958         });
 959
 960         self.lower_lifetime_binder(closure_id, generic_params, |lctx, bound_generic_params| {
 961             // We need to lower the declaration outside the new scope, because we
 962             // have to conserve the state of being inside a loop condition for the
 963             // closure argument types.
 964             let fn_decl =
 965                 lctx.lower_fn_decl(&outer_decl, None, fn_decl_span, FnDeclKind::Closure, None);
 966
 967             let c = lctx.arena.alloc(hir::Closure {
 968                 binder: binder_clause,
 969                 capture_clause,
 970                 bound_generic_params,
 971                 fn_decl,
 972                 body,
 973                 fn_decl_span: lctx.lower_span(fn_decl_span),
 974                 movability: None,
 975             });
 976             hir::ExprKind::Closure(c)
 977         })
 978     }
 979
 980     /// Destructure the LHS of complex assignments.
 981     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
 982     fn lower_expr_assign(
 983         &mut self,
 984         lhs: &Expr,
 985         rhs: &Expr,
 986         eq_sign_span: Span,
 987         whole_span: Span,
 988     ) -> hir::ExprKind<'hir> {
 989         // Return early in case of an ordinary assignment.
 990         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
 991             match &lhs.kind {
 992                 ExprKind::Array(..)
 993                 | ExprKind::Struct(..)
 994                 | ExprKind::Tup(..)
 995                 | ExprKind::Underscore => false,
 996                 // Check for tuple struct constructor.
 997                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
 998                 ExprKind::Paren(e) => {
 999                     match e.kind {
1000                         // We special-case `(..)` for consistency with patterns.
1001                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
1002                         _ => is_ordinary(lower_ctx, e),
1003                     }
1004                 }
1005                 _ => true,
1006             }
1007         }
1008         if is_ordinary(self, lhs) {
1009             return hir::ExprKind::Assign(
1010                 self.lower_expr(lhs),
1011                 self.lower_expr(rhs),
1012                 self.lower_span(eq_sign_span),
1013             );
1014         }
1015
1016         let mut assignments = vec![];
1017
1018         // The LHS becomes a pattern: `(lhs1, lhs2)`.
1019         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
1020         let rhs = self.lower_expr(rhs);
1021
1022         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
1023         let destructure_let = self.stmt_let_pat(
1024             None,
1025             whole_span,
1026             Some(rhs),
1027             pat,
1028             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
1029         );
1030
1031         // `a = lhs1; b = lhs2;`.
1032         let stmts = self
1033             .arena
1034             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
1035
1036         // Wrap everything in a block.
1037         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
1038     }
1039
1040     /// If the given expression is a path to a tuple struct, returns that path.
1041     /// It is not a complete check, but just tries to reject most paths early
1042     /// if they are not tuple structs.
1043     /// Type checking will take care of the full validation later.
1044     fn extract_tuple_struct_path<'a>(
1045         &mut self,
1046         expr: &'a Expr,
1047     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1048         if let ExprKind::Path(qself, path) = &expr.kind {
1049             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1050             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1051                 if partial_res.unresolved_segments() == 0
1052                     && !partial_res.base_res().expected_in_tuple_struct_pat()
1053                 {
1054                     return None;
1055                 }
1056             }
1057             return Some((qself, path));
1058         }
1059         None
1060     }
1061
1062     /// If the given expression is a path to a unit struct, returns that path.
1063     /// It is not a complete check, but just tries to reject most paths early
1064     /// if they are not unit structs.
1065     /// Type checking will take care of the full validation later.
1066     fn extract_unit_struct_path<'a>(
1067         &mut self,
1068         expr: &'a Expr,
1069     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1070         if let ExprKind::Path(qself, path) = &expr.kind {
1071             // Does the path resolve to something disallowed in a unit struct/variant pattern?
1072             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1073                 if partial_res.unresolved_segments() == 0
1074                     && !partial_res.base_res().expected_in_unit_struct_pat()
1075                 {
1076                     return None;
1077                 }
1078             }
1079             return Some((qself, path));
1080         }
1081         None
1082     }
1083
1084     /// Convert the LHS of a destructuring assignment to a pattern.
1085     /// Each sub-assignment is recorded in `assignments`.
1086     fn destructure_assign(
1087         &mut self,
1088         lhs: &Expr,
1089         eq_sign_span: Span,
1090         assignments: &mut Vec<hir::Stmt<'hir>>,
1091     ) -> &'hir hir::Pat<'hir> {
1092         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1093     }
1094
1095     fn destructure_assign_mut(
1096         &mut self,
1097         lhs: &Expr,
1098         eq_sign_span: Span,
1099         assignments: &mut Vec<hir::Stmt<'hir>>,
1100     ) -> hir::Pat<'hir> {
1101         match &lhs.kind {
1102             // Underscore pattern.
1103             ExprKind::Underscore => {
1104                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1105             }
1106             // Slice patterns.
1107             ExprKind::Array(elements) => {
1108                 let (pats, rest) =
1109                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1110                 let slice_pat = if let Some((i, span)) = rest {
1111                     let (before, after) = pats.split_at(i);
1112                     hir::PatKind::Slice(
1113                         before,
1114                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1115                         after,
1116                     )
1117                 } else {
1118                     hir::PatKind::Slice(pats, None, &[])
1119                 };
1120                 return self.pat_without_dbm(lhs.span, slice_pat);
1121             }
1122             // Tuple structs.
1123             ExprKind::Call(callee, args) => {
1124                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1125                     let (pats, rest) = self.destructure_sequence(
1126                         args,
1127                         "tuple struct or variant",
1128                         eq_sign_span,
1129                         assignments,
1130                     );
1131                     let qpath = self.lower_qpath(
1132                         callee.id,
1133                         qself,
1134                         path,
1135                         ParamMode::Optional,
1136                         &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1137                     );
1138                     // Destructure like a tuple struct.
1139                     let tuple_struct_pat = hir::PatKind::TupleStruct(
1140                         qpath,
1141                         pats,
1142                         hir::DotDotPos::new(rest.map(|r| r.0)),
1143                     );
1144                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1145                 }
1146             }
1147             // Unit structs and enum variants.
1148             ExprKind::Path(..) => {
1149                 if let Some((qself, path)) = self.extract_unit_struct_path(lhs) {
1150                     let qpath = self.lower_qpath(
1151                         lhs.id,
1152                         qself,
1153                         path,
1154                         ParamMode::Optional,
1155                         &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1156                     );
1157                     // Destructure like a unit struct.
1158                     let unit_struct_pat = hir::PatKind::Path(qpath);
1159                     return self.pat_without_dbm(lhs.span, unit_struct_pat);
1160                 }
1161             }
1162             // Structs.
1163             ExprKind::Struct(se) => {
1164                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1165                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1166                     hir::PatField {
1167                         hir_id: self.next_id(),
1168                         ident: self.lower_ident(f.ident),
1169                         pat,
1170                         is_shorthand: f.is_shorthand,
1171                         span: self.lower_span(f.span),
1172                     }
1173                 }));
1174                 let qpath = self.lower_qpath(
1175                     lhs.id,
1176                     &se.qself,
1177                     &se.path,
1178                     ParamMode::Optional,
1179                     &mut ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1180                 );
1181                 let fields_omitted = match &se.rest {
1182                     StructRest::Base(e) => {
1183                         self.tcx.sess.emit_err(FunctionalRecordUpdateDestructuringAssignemnt {
1184                             span: e.span,
1185                         });
1186                         true
1187                     }
1188                     StructRest::Rest(_) => true,
1189                     StructRest::None => false,
1190                 };
1191                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1192                 return self.pat_without_dbm(lhs.span, struct_pat);
1193             }
1194             // Tuples.
1195             ExprKind::Tup(elements) => {
1196                 let (pats, rest) =
1197                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1198                 let tuple_pat = hir::PatKind::Tuple(pats, hir::DotDotPos::new(rest.map(|r| r.0)));
1199                 return self.pat_without_dbm(lhs.span, tuple_pat);
1200             }
1201             ExprKind::Paren(e) => {
1202                 // We special-case `(..)` for consistency with patterns.
1203                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1204                     let tuple_pat = hir::PatKind::Tuple(&[], hir::DotDotPos::new(Some(0)));
1205                     return self.pat_without_dbm(lhs.span, tuple_pat);
1206                 } else {
1207                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1208                 }
1209             }
1210             _ => {}
1211         }
1212         // Treat all other cases as normal lvalue.
1213         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1214         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1215         let ident = self.expr_ident(lhs.span, ident, binding);
1216         let assign =
1217             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1218         let expr = self.expr(lhs.span, assign, AttrVec::new());
1219         assignments.push(self.stmt_expr(lhs.span, expr));
1220         pat
1221     }
1222
1223     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1224     /// Such a sequence occurs in a tuple (struct)/slice.
1225     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1226     /// exists.
1227     /// Each sub-assignment is recorded in `assignments`.
1228     fn destructure_sequence(
1229         &mut self,
1230         elements: &[AstP<Expr>],
1231         ctx: &str,
1232         eq_sign_span: Span,
1233         assignments: &mut Vec<hir::Stmt<'hir>>,
1234     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1235         let mut rest = None;
1236         let elements =
1237             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1238                 // Check for `..` pattern.
1239                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1240                     if let Some((_, prev_span)) = rest {
1241                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1242                     } else {
1243                         rest = Some((i, e.span));
1244                     }
1245                     None
1246                 } else {
1247                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1248                 }
1249             }));
1250         (elements, rest)
1251     }
1252
1253     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1254     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1255         let e1 = self.lower_expr_mut(e1);
1256         let e2 = self.lower_expr_mut(e2);
1257         let fn_path =
1258             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1259         let fn_expr =
1260             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), AttrVec::new()));
1261         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1262     }
1263
1264     fn lower_expr_range(
1265         &mut self,
1266         span: Span,
1267         e1: Option<&Expr>,
1268         e2: Option<&Expr>,
1269         lims: RangeLimits,
1270     ) -> hir::ExprKind<'hir> {
1271         use rustc_ast::RangeLimits::*;
1272
1273         let lang_item = match (e1, e2, lims) {
1274             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1275             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1276             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1277             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1278             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1279             (Some(..), Some(..), Closed) => unreachable!(),
1280             (start, None, Closed) => {
1281                 self.tcx.sess.emit_err(InclusiveRangeWithNoEnd { span });
1282                 match start {
1283                     Some(..) => hir::LangItem::RangeFrom,
1284                     None => hir::LangItem::RangeFull,
1285                 }
1286             }
1287         };
1288
1289         let fields = self.arena.alloc_from_iter(
1290             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1291                 |(s, e)| {
1292                     let expr = self.lower_expr(&e);
1293                     let ident = Ident::new(s, self.lower_span(e.span));
1294                     self.expr_field(ident, expr, e.span)
1295                 },
1296             ),
1297         );
1298
1299         hir::ExprKind::Struct(
1300             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1301             fields,
1302             None,
1303         )
1304     }
1305
1306     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1307         let label = opt_label?;
1308         Some(Label { ident: self.lower_ident(label.ident) })
1309     }
1310
1311     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1312         let target_id = match destination {
1313             Some((id, _)) => {
1314                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1315                     Ok(self.lower_node_id(loop_id))
1316                 } else {
1317                     Err(hir::LoopIdError::UnresolvedLabel)
1318                 }
1319             }
1320             None => self
1321                 .loop_scope
1322                 .map(|id| Ok(self.lower_node_id(id)))
1323                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1324         };
1325         let label = self.lower_label(destination.map(|(_, label)| label));
1326         hir::Destination { label, target_id }
1327     }
1328
1329     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1330         if self.is_in_loop_condition && opt_label.is_none() {
1331             hir::Destination {
1332                 label: None,
1333                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1334             }
1335         } else {
1336             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1337         }
1338     }
1339
1340     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1341         let old_scope = self.catch_scope.replace(catch_id);
1342         let result = f(self);
1343         self.catch_scope = old_scope;
1344         result
1345     }
1346
1347     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1348         // We're no longer in the base loop's condition; we're in another loop.
1349         let was_in_loop_condition = self.is_in_loop_condition;
1350         self.is_in_loop_condition = false;
1351
1352         let old_scope = self.loop_scope.replace(loop_id);
1353         let result = f(self);
1354         self.loop_scope = old_scope;
1355
1356         self.is_in_loop_condition = was_in_loop_condition;
1357
1358         result
1359     }
1360
1361     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1362         let was_in_loop_condition = self.is_in_loop_condition;
1363         self.is_in_loop_condition = true;
1364
1365         let result = f(self);
1366
1367         self.is_in_loop_condition = was_in_loop_condition;
1368
1369         result
1370     }
1371
1372     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1373         let hir_id = self.lower_node_id(f.id);
1374         self.lower_attrs(hir_id, &f.attrs);
1375         hir::ExprField {
1376             hir_id,
1377             ident: self.lower_ident(f.ident),
1378             expr: self.lower_expr(&f.expr),
1379             span: self.lower_span(f.span),
1380             is_shorthand: f.is_shorthand,
1381         }
1382     }
1383
1384     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1385         match self.generator_kind {
1386             Some(hir::GeneratorKind::Gen) => {}
1387             Some(hir::GeneratorKind::Async(_)) => {
1388                 self.tcx.sess.emit_err(AsyncGeneratorsNotSupported { span });
1389             }
1390             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1391         }
1392
1393         let expr =
1394             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1395
1396         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1397     }
1398
1399     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1400     /// ```ignore (pseudo-rust)
1401     /// {
1402     ///     let result = match IntoIterator::into_iter(<head>) {
1403     ///         mut iter => {
1404     ///             [opt_ident]: loop {
1405     ///                 match Iterator::next(&mut iter) {
1406     ///                     None => break,
1407     ///                     Some(<pat>) => <body>,
1408     ///                 };
1409     ///             }
1410     ///         }
1411     ///     };
1412     ///     result
1413     /// }
1414     /// ```
1415     fn lower_expr_for(
1416         &mut self,
1417         e: &Expr,
1418         pat: &Pat,
1419         head: &Expr,
1420         body: &Block,
1421         opt_label: Option<Label>,
1422     ) -> hir::Expr<'hir> {
1423         let head = self.lower_expr_mut(head);
1424         let pat = self.lower_pat(pat);
1425         let for_span =
1426             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1427         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1428         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1429
1430         // `None => break`
1431         let none_arm = {
1432             let break_expr =
1433                 self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, AttrVec::new()));
1434             let pat = self.pat_none(for_span);
1435             self.arm(pat, break_expr)
1436         };
1437
1438         // Some(<pat>) => <body>,
1439         let some_arm = {
1440             let some_pat = self.pat_some(pat_span, pat);
1441             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1442             let body_expr = self.arena.alloc(self.expr_block(body_block, AttrVec::new()));
1443             self.arm(some_pat, body_expr)
1444         };
1445
1446         // `mut iter`
1447         let iter = Ident::with_dummy_span(sym::iter);
1448         let (iter_pat, iter_pat_nid) =
1449             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::MUT);
1450
1451         // `match Iterator::next(&mut iter) { ... }`
1452         let match_expr = {
1453             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1454             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1455             let next_expr = self.expr_call_lang_item_fn(
1456                 head_span,
1457                 hir::LangItem::IteratorNext,
1458                 arena_vec![self; ref_mut_iter],
1459                 None,
1460             );
1461             let arms = arena_vec![self; none_arm, some_arm];
1462
1463             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1464         };
1465         let match_stmt = self.stmt_expr(for_span, match_expr);
1466
1467         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1468
1469         // `[opt_ident]: loop { ... }`
1470         let kind = hir::ExprKind::Loop(
1471             loop_block,
1472             self.lower_label(opt_label),
1473             hir::LoopSource::ForLoop,
1474             self.lower_span(for_span.with_hi(head.span.hi())),
1475         );
1476         let loop_expr =
1477             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1478
1479         // `mut iter => { ... }`
1480         let iter_arm = self.arm(iter_pat, loop_expr);
1481
1482         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1483         let into_iter_expr = {
1484             self.expr_call_lang_item_fn(
1485                 head_span,
1486                 hir::LangItem::IntoIterIntoIter,
1487                 arena_vec![self; head],
1488                 None,
1489             )
1490         };
1491
1492         let match_expr = self.arena.alloc(self.expr_match(
1493             for_span,
1494             into_iter_expr,
1495             arena_vec![self; iter_arm],
1496             hir::MatchSource::ForLoopDesugar,
1497         ));
1498
1499         // This is effectively `{ let _result = ...; _result }`.
1500         // The construct was introduced in #21984 and is necessary to make sure that
1501         // temporaries in the `head` expression are dropped and do not leak to the
1502         // surrounding scope of the `match` since the `match` is not a terminating scope.
1503         //
1504         // Also, add the attributes to the outer returned expr node.
1505         self.expr_drop_temps_mut(for_span, match_expr, e.attrs.clone())
1506     }
1507
1508     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1509     /// ```ignore (pseudo-rust)
1510     /// match Try::branch(<expr>) {
1511     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1512     ///     ControlFlow::Break(residual) =>
1513     ///         #[allow(unreachable_code)]
1514     ///         // If there is an enclosing `try {...}`:
1515     ///         break 'catch_target Try::from_residual(residual),
1516     ///         // Otherwise:
1517     ///         return Try::from_residual(residual),
1518     /// }
1519     /// ```
1520     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1521         let unstable_span = self.mark_span_with_reason(
1522             DesugaringKind::QuestionMark,
1523             span,
1524             self.allow_try_trait.clone(),
1525         );
1526         let try_span = self.tcx.sess.source_map().end_point(span);
1527         let try_span = self.mark_span_with_reason(
1528             DesugaringKind::QuestionMark,
1529             try_span,
1530             self.allow_try_trait.clone(),
1531         );
1532
1533         // `Try::branch(<expr>)`
1534         let scrutinee = {
1535             // expand <expr>
1536             let sub_expr = self.lower_expr_mut(sub_expr);
1537
1538             self.expr_call_lang_item_fn(
1539                 unstable_span,
1540                 hir::LangItem::TryTraitBranch,
1541                 arena_vec![self; sub_expr],
1542                 None,
1543             )
1544         };
1545
1546         // `#[allow(unreachable_code)]`
1547         let attr = {
1548             // `allow(unreachable_code)`
1549             let allow = {
1550                 let allow_ident = Ident::new(sym::allow, self.lower_span(span));
1551                 let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
1552                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1553                 attr::mk_list_item(allow_ident, vec![uc_nested])
1554             };
1555             attr::mk_attr_outer(&self.tcx.sess.parse_sess.attr_id_generator, allow)
1556         };
1557         let attrs: AttrVec = thin_vec![attr];
1558
1559         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1560         let continue_arm = {
1561             let val_ident = Ident::with_dummy_span(sym::val);
1562             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1563             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1564                 span,
1565                 val_ident,
1566                 val_pat_nid,
1567                 attrs.clone(),
1568             ));
1569             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1570             self.arm(continue_pat, val_expr)
1571         };
1572
1573         // `ControlFlow::Break(residual) =>
1574         //     #[allow(unreachable_code)]
1575         //     return Try::from_residual(residual),`
1576         let break_arm = {
1577             let residual_ident = Ident::with_dummy_span(sym::residual);
1578             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1579             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1580             let from_residual_expr = self.wrap_in_try_constructor(
1581                 hir::LangItem::TryTraitFromResidual,
1582                 try_span,
1583                 self.arena.alloc(residual_expr),
1584                 unstable_span,
1585             );
1586             let ret_expr = if let Some(catch_node) = self.catch_scope {
1587                 let target_id = Ok(self.lower_node_id(catch_node));
1588                 self.arena.alloc(self.expr(
1589                     try_span,
1590                     hir::ExprKind::Break(
1591                         hir::Destination { label: None, target_id },
1592                         Some(from_residual_expr),
1593                     ),
1594                     attrs,
1595                 ))
1596             } else {
1597                 self.arena.alloc(self.expr(
1598                     try_span,
1599                     hir::ExprKind::Ret(Some(from_residual_expr)),
1600                     attrs,
1601                 ))
1602             };
1603
1604             let break_pat = self.pat_cf_break(try_span, residual_local);
1605             self.arm(break_pat, ret_expr)
1606         };
1607
1608         hir::ExprKind::Match(
1609             scrutinee,
1610             arena_vec![self; break_arm, continue_arm],
1611             hir::MatchSource::TryDesugar,
1612         )
1613     }
1614
1615     /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
1616     /// ```rust
1617     /// // If there is an enclosing `try {...}`:
1618     /// break 'catch_target FromResidual::from_residual(Yeet(residual)),
1619     /// // Otherwise:
1620     /// return FromResidual::from_residual(Yeet(residual)),
1621     /// ```
1622     /// But to simplify this, there's a `from_yeet` lang item function which
1623     /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
1624     fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1625         // The expression (if present) or `()` otherwise.
1626         let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
1627             (sub_expr.span, self.lower_expr(sub_expr))
1628         } else {
1629             (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
1630         };
1631
1632         let unstable_span = self.mark_span_with_reason(
1633             DesugaringKind::YeetExpr,
1634             span,
1635             self.allow_try_trait.clone(),
1636         );
1637
1638         let from_yeet_expr = self.wrap_in_try_constructor(
1639             hir::LangItem::TryTraitFromYeet,
1640             unstable_span,
1641             yeeted_expr,
1642             yeeted_span,
1643         );
1644
1645         if let Some(catch_node) = self.catch_scope {
1646             let target_id = Ok(self.lower_node_id(catch_node));
1647             hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
1648         } else {
1649             hir::ExprKind::Ret(Some(from_yeet_expr))
1650         }
1651     }
1652
1653     // =========================================================================
1654     // Helper methods for building HIR.
1655     // =========================================================================
1656
1657     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1658     ///
1659     /// In terms of drop order, it has the same effect as wrapping `expr` in
1660     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1661     ///
1662     /// The drop order can be important in e.g. `if expr { .. }`.
1663     pub(super) fn expr_drop_temps(
1664         &mut self,
1665         span: Span,
1666         expr: &'hir hir::Expr<'hir>,
1667         attrs: AttrVec,
1668     ) -> &'hir hir::Expr<'hir> {
1669         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1670     }
1671
1672     pub(super) fn expr_drop_temps_mut(
1673         &mut self,
1674         span: Span,
1675         expr: &'hir hir::Expr<'hir>,
1676         attrs: AttrVec,
1677     ) -> hir::Expr<'hir> {
1678         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1679     }
1680
1681     fn expr_match(
1682         &mut self,
1683         span: Span,
1684         arg: &'hir hir::Expr<'hir>,
1685         arms: &'hir [hir::Arm<'hir>],
1686         source: hir::MatchSource,
1687     ) -> hir::Expr<'hir> {
1688         self.expr(span, hir::ExprKind::Match(arg, arms, source), AttrVec::new())
1689     }
1690
1691     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
1692         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1693         self.expr(span, expr_break, attrs)
1694     }
1695
1696     fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1697         let expr_break = self.expr_break(span, attrs);
1698         self.arena.alloc(expr_break)
1699     }
1700
1701     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1702         self.expr(
1703             span,
1704             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1705             AttrVec::new(),
1706         )
1707     }
1708
1709     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1710         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), AttrVec::new()))
1711     }
1712
1713     fn expr_call_mut(
1714         &mut self,
1715         span: Span,
1716         e: &'hir hir::Expr<'hir>,
1717         args: &'hir [hir::Expr<'hir>],
1718     ) -> hir::Expr<'hir> {
1719         self.expr(span, hir::ExprKind::Call(e, args), AttrVec::new())
1720     }
1721
1722     fn expr_call(
1723         &mut self,
1724         span: Span,
1725         e: &'hir hir::Expr<'hir>,
1726         args: &'hir [hir::Expr<'hir>],
1727     ) -> &'hir hir::Expr<'hir> {
1728         self.arena.alloc(self.expr_call_mut(span, e, args))
1729     }
1730
1731     fn expr_call_lang_item_fn_mut(
1732         &mut self,
1733         span: Span,
1734         lang_item: hir::LangItem,
1735         args: &'hir [hir::Expr<'hir>],
1736         hir_id: Option<hir::HirId>,
1737     ) -> hir::Expr<'hir> {
1738         let path =
1739             self.arena.alloc(self.expr_lang_item_path(span, lang_item, AttrVec::new(), hir_id));
1740         self.expr_call_mut(span, path, args)
1741     }
1742
1743     fn expr_call_lang_item_fn(
1744         &mut self,
1745         span: Span,
1746         lang_item: hir::LangItem,
1747         args: &'hir [hir::Expr<'hir>],
1748         hir_id: Option<hir::HirId>,
1749     ) -> &'hir hir::Expr<'hir> {
1750         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1751     }
1752
1753     fn expr_lang_item_path(
1754         &mut self,
1755         span: Span,
1756         lang_item: hir::LangItem,
1757         attrs: AttrVec,
1758         hir_id: Option<hir::HirId>,
1759     ) -> hir::Expr<'hir> {
1760         self.expr(
1761             span,
1762             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1763             attrs,
1764         )
1765     }
1766
1767     pub(super) fn expr_ident(
1768         &mut self,
1769         sp: Span,
1770         ident: Ident,
1771         binding: hir::HirId,
1772     ) -> &'hir hir::Expr<'hir> {
1773         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1774     }
1775
1776     pub(super) fn expr_ident_mut(
1777         &mut self,
1778         sp: Span,
1779         ident: Ident,
1780         binding: hir::HirId,
1781     ) -> hir::Expr<'hir> {
1782         self.expr_ident_with_attrs(sp, ident, binding, AttrVec::new())
1783     }
1784
1785     fn expr_ident_with_attrs(
1786         &mut self,
1787         span: Span,
1788         ident: Ident,
1789         binding: hir::HirId,
1790         attrs: AttrVec,
1791     ) -> hir::Expr<'hir> {
1792         let hir_id = self.next_id();
1793         let res = Res::Local(binding);
1794         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1795             None,
1796             self.arena.alloc(hir::Path {
1797                 span: self.lower_span(span),
1798                 res,
1799                 segments: arena_vec![self; hir::PathSegment::new(ident, hir_id, res)],
1800             }),
1801         ));
1802
1803         self.expr(span, expr_path, attrs)
1804     }
1805
1806     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1807         let hir_id = self.next_id();
1808         let span = expr.span;
1809         self.expr(
1810             span,
1811             hir::ExprKind::Block(
1812                 self.arena.alloc(hir::Block {
1813                     stmts: &[],
1814                     expr: Some(expr),
1815                     hir_id,
1816                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1817                     span: self.lower_span(span),
1818                     targeted_by_break: false,
1819                 }),
1820                 None,
1821             ),
1822             AttrVec::new(),
1823         )
1824     }
1825
1826     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1827         let blk = self.block_all(span, &[], None);
1828         let expr = self.expr_block(blk, AttrVec::new());
1829         self.arena.alloc(expr)
1830     }
1831
1832     pub(super) fn expr_block(
1833         &mut self,
1834         b: &'hir hir::Block<'hir>,
1835         attrs: AttrVec,
1836     ) -> hir::Expr<'hir> {
1837         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1838     }
1839
1840     pub(super) fn expr(
1841         &mut self,
1842         span: Span,
1843         kind: hir::ExprKind<'hir>,
1844         attrs: AttrVec,
1845     ) -> hir::Expr<'hir> {
1846         let hir_id = self.next_id();
1847         self.lower_attrs(hir_id, &attrs);
1848         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1849     }
1850
1851     fn expr_field(
1852         &mut self,
1853         ident: Ident,
1854         expr: &'hir hir::Expr<'hir>,
1855         span: Span,
1856     ) -> hir::ExprField<'hir> {
1857         hir::ExprField {
1858             hir_id: self.next_id(),
1859             ident,
1860             span: self.lower_span(span),
1861             expr,
1862             is_shorthand: false,
1863         }
1864     }
1865
1866     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1867         hir::Arm {
1868             hir_id: self.next_id(),
1869             pat,
1870             guard: None,
1871             span: self.lower_span(expr.span),
1872             body: expr,
1873         }
1874     }
1875 }