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.tcx.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                     ref binder,
 159                     capture_clause,
 160                     asyncness,
 161                     movability,
 162                     ref decl,
 163                     ref body,
 164                     fn_decl_span,
 165                 ) => {
 166                     if let Async::Yes { closure_id, .. } = asyncness {
 167                         self.lower_expr_async_closure(
 168                             binder,
 169                             capture_clause,
 170                             e.id,
 171                             closure_id,
 172                             decl,
 173                             body,
 174                             fn_decl_span,
 175                         )
 176                     } else {
 177                         self.lower_expr_closure(
 178                             binder,
 179                             capture_clause,
 180                             e.id,
 181                             movability,
 182                             decl,
 183                             body,
 184                             fn_decl_span,
 185                         )
 186                     }
 187                 }
 188                 ExprKind::Block(ref blk, opt_label) => {
 189                     let opt_label = self.lower_label(opt_label);
 190                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 191                 }
 192                 ExprKind::Assign(ref el, ref er, span) => {
 193                     self.lower_expr_assign(el, er, span, e.span)
 194                 }
 195                 ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 196                     self.lower_binop(op),
 197                     self.lower_expr(el),
 198                     self.lower_expr(er),
 199                 ),
 200                 ExprKind::Field(ref el, ident) => {
 201                     hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(ident))
 202                 }
 203                 ExprKind::Index(ref el, ref er) => {
 204                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 205                 }
 206                 ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 207                     self.lower_expr_range_closed(e.span, e1, e2)
 208                 }
 209                 ExprKind::Range(ref e1, ref e2, lims) => {
 210                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 211                 }
 212                 ExprKind::Underscore => {
 213                     self.tcx
 214                         .sess.struct_span_err(
 215                             e.span,
 216                             "in expressions, `_` can only be used on the left-hand side of an assignment",
 217                         )
 218                         .span_label(e.span, "`_` not allowed here")
 219                         .emit();
 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                         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
 252                                 .sess
 253                                 .struct_span_err(*sp, "base expression required after `..`")
 254                                 .span_label(*sp, "add a base expression here")
 255                                 .emit();
 256                             Some(&*self.arena.alloc(self.expr_err(*sp)))
 257                         }
 258                         StructRest::None => None,
 259                     };
 260                     hir::ExprKind::Struct(
 261                         self.arena.alloc(self.lower_qpath(
 262                             e.id,
 263                             &se.qself,
 264                             &se.path,
 265                             ParamMode::Optional,
 266                             ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 267                         )),
 268                         self.arena
 269                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
 270                         rest,
 271                     )
 272                 }
 273                 ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 274                 ExprKind::Err => hir::ExprKind::Err,
 275                 ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 276                 ExprKind::Paren(ref ex) => {
 277                     let mut ex = self.lower_expr_mut(ex);
 278                     // Include parens in span, but only if it is a super-span.
 279                     if e.span.contains(ex.span) {
 280                         ex.span = self.lower_span(e.span);
 281                     }
 282                     // Merge attributes into the inner expression.
 283                     if !e.attrs.is_empty() {
 284                         let old_attrs =
 285                             self.attrs.get(&ex.hir_id.local_id).map(|la| *la).unwrap_or(&[]);
 286                         self.attrs.insert(
 287                             ex.hir_id.local_id,
 288                             &*self.arena.alloc_from_iter(
 289                                 e.attrs
 290                                     .iter()
 291                                     .map(|a| self.lower_attr(a))
 292                                     .chain(old_attrs.iter().cloned()),
 293                             ),
 294                         );
 295                     }
 296                     return ex;
 297                 }
 298
 299                 // Desugar `ExprForLoop`
 300                 // from: `[opt_ident]: for <pat> in <head> <body>`
 301                 ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 302                     return self.lower_expr_for(e, pat, head, body, opt_label);
 303                 }
 304                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 305             };
 306
 307             let hir_id = self.lower_node_id(e.id);
 308             self.lower_attrs(hir_id, &e.attrs);
 309             hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
 310         })
 311     }
 312
 313     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 314         match u {
 315             UnOp::Deref => hir::UnOp::Deref,
 316             UnOp::Not => hir::UnOp::Not,
 317             UnOp::Neg => hir::UnOp::Neg,
 318         }
 319     }
 320
 321     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 322         Spanned {
 323             node: match b.node {
 324                 BinOpKind::Add => hir::BinOpKind::Add,
 325                 BinOpKind::Sub => hir::BinOpKind::Sub,
 326                 BinOpKind::Mul => hir::BinOpKind::Mul,
 327                 BinOpKind::Div => hir::BinOpKind::Div,
 328                 BinOpKind::Rem => hir::BinOpKind::Rem,
 329                 BinOpKind::And => hir::BinOpKind::And,
 330                 BinOpKind::Or => hir::BinOpKind::Or,
 331                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 332                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 333                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 334                 BinOpKind::Shl => hir::BinOpKind::Shl,
 335                 BinOpKind::Shr => hir::BinOpKind::Shr,
 336                 BinOpKind::Eq => hir::BinOpKind::Eq,
 337                 BinOpKind::Lt => hir::BinOpKind::Lt,
 338                 BinOpKind::Le => hir::BinOpKind::Le,
 339                 BinOpKind::Ne => hir::BinOpKind::Ne,
 340                 BinOpKind::Ge => hir::BinOpKind::Ge,
 341                 BinOpKind::Gt => hir::BinOpKind::Gt,
 342             },
 343             span: self.lower_span(b.span),
 344         }
 345     }
 346
 347     fn lower_legacy_const_generics(
 348         &mut self,
 349         mut f: Expr,
 350         args: Vec<AstP<Expr>>,
 351         legacy_args_idx: &[usize],
 352     ) -> hir::ExprKind<'hir> {
 353         let ExprKind::Path(None, ref mut path) = f.kind else {
 354             unreachable!();
 355         };
 356
 357         // Split the arguments into const generics and normal arguments
 358         let mut real_args = vec![];
 359         let mut generic_args = vec![];
 360         for (idx, arg) in args.into_iter().enumerate() {
 361             if legacy_args_idx.contains(&idx) {
 362                 let parent_def_id = self.current_hir_id_owner;
 363                 let node_id = self.next_node_id();
 364
 365                 // Add a definition for the in-band const def.
 366                 self.create_def(parent_def_id, node_id, DefPathData::AnonConst);
 367
 368                 let anon_const = AnonConst { id: node_id, value: arg };
 369                 generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
 370             } else {
 371                 real_args.push(arg);
 372             }
 373         }
 374
 375         // Add generic args to the last element of the path.
 376         let last_segment = path.segments.last_mut().unwrap();
 377         assert!(last_segment.args.is_none());
 378         last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
 379             span: DUMMY_SP,
 380             args: generic_args,
 381         })));
 382
 383         // Now lower everything as normal.
 384         let f = self.lower_expr(&f);
 385         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
 386     }
 387
 388     fn lower_expr_if(
 389         &mut self,
 390         cond: &Expr,
 391         then: &Block,
 392         else_opt: Option<&Expr>,
 393     ) -> hir::ExprKind<'hir> {
 394         let lowered_cond = self.lower_expr(cond);
 395         let new_cond = self.manage_let_cond(lowered_cond);
 396         let then_expr = self.lower_block_expr(then);
 397         if let Some(rslt) = else_opt {
 398             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), Some(self.lower_expr(rslt)))
 399         } else {
 400             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), None)
 401         }
 402     }
 403
 404     // If `cond` kind is `let`, returns `let`. Otherwise, wraps and returns `cond`
 405     // in a temporary block.
 406     fn manage_let_cond(&mut self, cond: &'hir hir::Expr<'hir>) -> &'hir hir::Expr<'hir> {
 407         fn has_let_expr<'hir>(expr: &'hir hir::Expr<'hir>) -> bool {
 408             match expr.kind {
 409                 hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
 410                 hir::ExprKind::Let(..) => true,
 411                 _ => false,
 412             }
 413         }
 414         if has_let_expr(cond) {
 415             cond
 416         } else {
 417             let reason = DesugaringKind::CondTemporary;
 418             let span_block = self.mark_span_with_reason(reason, cond.span, None);
 419             self.expr_drop_temps(span_block, cond, AttrVec::new())
 420         }
 421     }
 422
 423     // We desugar: `'label: while $cond $body` into:
 424     //
 425     // ```
 426     // 'label: loop {
 427     //   if { let _t = $cond; _t } {
 428     //     $body
 429     //   }
 430     //   else {
 431     //     break;
 432     //   }
 433     // }
 434     // ```
 435     //
 436     // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 437     // to preserve drop semantics since `while $cond { ... }` does not
 438     // let temporaries live outside of `cond`.
 439     fn lower_expr_while_in_loop_scope(
 440         &mut self,
 441         span: Span,
 442         cond: &Expr,
 443         body: &Block,
 444         opt_label: Option<Label>,
 445     ) -> hir::ExprKind<'hir> {
 446         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
 447         let new_cond = self.manage_let_cond(lowered_cond);
 448         let then = self.lower_block_expr(body);
 449         let expr_break = self.expr_break(span, ThinVec::new());
 450         let stmt_break = self.stmt_expr(span, expr_break);
 451         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
 452         let else_expr = self.arena.alloc(self.expr_block(else_blk, ThinVec::new()));
 453         let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
 454         let if_expr = self.expr(span, if_kind, ThinVec::new());
 455         let block = self.block_expr(self.arena.alloc(if_expr));
 456         let span = self.lower_span(span.with_hi(cond.span.hi()));
 457         let opt_label = self.lower_label(opt_label);
 458         hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
 459     }
 460
 461     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
 462     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
 463     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 464     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 465         self.with_catch_scope(body.id, |this| {
 466             let mut block = this.lower_block_noalloc(body, true);
 467
 468             // Final expression of the block (if present) or `()` with span at the end of block
 469             let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
 470                 (
 471                     this.mark_span_with_reason(
 472                         DesugaringKind::TryBlock,
 473                         expr.span,
 474                         this.allow_try_trait.clone(),
 475                     ),
 476                     expr,
 477                 )
 478             } else {
 479                 let try_span = this.mark_span_with_reason(
 480                     DesugaringKind::TryBlock,
 481                     this.tcx.sess.source_map().end_point(body.span),
 482                     this.allow_try_trait.clone(),
 483                 );
 484
 485                 (try_span, this.expr_unit(try_span))
 486             };
 487
 488             let ok_wrapped_span =
 489                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 490
 491             // `::std::ops::Try::from_output($tail_expr)`
 492             block.expr = Some(this.wrap_in_try_constructor(
 493                 hir::LangItem::TryTraitFromOutput,
 494                 try_span,
 495                 tail_expr,
 496                 ok_wrapped_span,
 497             ));
 498
 499             hir::ExprKind::Block(this.arena.alloc(block), None)
 500         })
 501     }
 502
 503     fn wrap_in_try_constructor(
 504         &mut self,
 505         lang_item: hir::LangItem,
 506         method_span: Span,
 507         expr: &'hir hir::Expr<'hir>,
 508         overall_span: Span,
 509     ) -> &'hir hir::Expr<'hir> {
 510         let constructor = self.arena.alloc(self.expr_lang_item_path(
 511             method_span,
 512             lang_item,
 513             ThinVec::new(),
 514             None,
 515         ));
 516         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 517     }
 518
 519     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 520         let pat = self.lower_pat(&arm.pat);
 521         let guard = arm.guard.as_ref().map(|cond| {
 522             if let ExprKind::Let(ref pat, ref scrutinee, span) = cond.kind {
 523                 hir::Guard::IfLet(self.arena.alloc(hir::Let {
 524                     hir_id: self.next_id(),
 525                     span: self.lower_span(span),
 526                     pat: self.lower_pat(pat),
 527                     ty: None,
 528                     init: self.lower_expr(scrutinee),
 529                 }))
 530             } else {
 531                 hir::Guard::If(self.lower_expr(cond))
 532             }
 533         });
 534         let hir_id = self.next_id();
 535         self.lower_attrs(hir_id, &arm.attrs);
 536         hir::Arm {
 537             hir_id,
 538             pat,
 539             guard,
 540             body: self.lower_expr(&arm.body),
 541             span: self.lower_span(arm.span),
 542         }
 543     }
 544
 545     /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
 546     ///
 547     /// This results in:
 548     ///
 549     /// ```text
 550     /// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
 551     ///     <body>
 552     /// })
 553     /// ```
 554     pub(super) fn make_async_expr(
 555         &mut self,
 556         capture_clause: CaptureBy,
 557         closure_node_id: NodeId,
 558         ret_ty: Option<AstP<Ty>>,
 559         span: Span,
 560         async_gen_kind: hir::AsyncGeneratorKind,
 561         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 562     ) -> hir::ExprKind<'hir> {
 563         let output = match ret_ty {
 564             Some(ty) => hir::FnRetTy::Return(
 565                 self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock)),
 566             ),
 567             None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
 568         };
 569
 570         // Resume argument type. We let the compiler infer this to simplify the lowering. It is
 571         // fully constrained by `future::from_generator`.
 572         let input_ty = hir::Ty {
 573             hir_id: self.next_id(),
 574             kind: hir::TyKind::Infer,
 575             span: self.lower_span(span),
 576         };
 577
 578         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 579         let fn_decl = self.arena.alloc(hir::FnDecl {
 580             inputs: arena_vec![self; input_ty],
 581             output,
 582             c_variadic: false,
 583             implicit_self: hir::ImplicitSelfKind::None,
 584         });
 585
 586         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 587         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 588             span,
 589             Ident::with_dummy_span(sym::_task_context),
 590             hir::BindingAnnotation::Mutable,
 591         );
 592         let param = hir::Param {
 593             hir_id: self.next_id(),
 594             pat,
 595             ty_span: self.lower_span(span),
 596             span: self.lower_span(span),
 597         };
 598         let params = arena_vec![self; param];
 599
 600         let body = self.lower_body(move |this| {
 601             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 602
 603             let old_ctx = this.task_context;
 604             this.task_context = Some(task_context_hid);
 605             let res = body(this);
 606             this.task_context = old_ctx;
 607             (params, res)
 608         });
 609
 610         // `static |_task_context| -> <ret_ty> { body }`:
 611         let generator_kind = {
 612             let c = self.arena.alloc(hir::Closure {
 613                 binder: hir::ClosureBinder::Default,
 614                 capture_clause,
 615                 bound_generic_params: &[],
 616                 fn_decl,
 617                 body,
 618                 fn_decl_span: self.lower_span(span),
 619                 movability: Some(hir::Movability::Static),
 620             });
 621
 622             hir::ExprKind::Closure(c)
 623         };
 624         let generator = hir::Expr {
 625             hir_id: self.lower_node_id(closure_node_id),
 626             kind: generator_kind,
 627             span: self.lower_span(span),
 628         };
 629
 630         // `future::from_generator`:
 631         let unstable_span =
 632             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 633         let gen_future = self.expr_lang_item_path(
 634             unstable_span,
 635             hir::LangItem::FromGenerator,
 636             ThinVec::new(),
 637             None,
 638         );
 639
 640         // `future::from_generator(generator)`:
 641         hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
 642     }
 643
 644     /// Desugar `<expr>.await` into:
 645     /// ```ignore (pseudo-rust)
 646     /// match ::std::future::IntoFuture::into_future(<expr>) {
 647     ///     mut __awaitee => loop {
 648     ///         match unsafe { ::std::future::Future::poll(
 649     ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
 650     ///             ::std::future::get_context(task_context),
 651     ///         ) } {
 652     ///             ::std::task::Poll::Ready(result) => break result,
 653     ///             ::std::task::Poll::Pending => {}
 654     ///         }
 655     ///         task_context = yield ();
 656     ///     }
 657     /// }
 658     /// ```
 659     fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 660         let full_span = expr.span.to(dot_await_span);
 661         match self.generator_kind {
 662             Some(hir::GeneratorKind::Async(_)) => {}
 663             Some(hir::GeneratorKind::Gen) | None => {
 664                 let mut err = struct_span_err!(
 665                     self.tcx.sess,
 666                     dot_await_span,
 667                     E0728,
 668                     "`await` is only allowed inside `async` functions and blocks"
 669                 );
 670                 err.span_label(dot_await_span, "only allowed inside `async` functions and blocks");
 671                 if let Some(item_sp) = self.current_item {
 672                     err.span_label(item_sp, "this is not `async`");
 673                 }
 674                 err.emit();
 675             }
 676         }
 677         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
 678         let gen_future_span = self.mark_span_with_reason(
 679             DesugaringKind::Await,
 680             full_span,
 681             self.allow_gen_future.clone(),
 682         );
 683         let expr = self.lower_expr_mut(expr);
 684         let expr_hir_id = expr.hir_id;
 685
 686         // Note that the name of this binding must not be changed to something else because
 687         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
 688         // this name to identify what is being awaited by a suspended async functions.
 689         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
 690         let (awaitee_pat, awaitee_pat_hid) =
 691             self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::Mutable);
 692
 693         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 694
 695         // unsafe {
 696         //     ::std::future::Future::poll(
 697         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
 698         //         ::std::future::get_context(task_context),
 699         //     )
 700         // }
 701         let poll_expr = {
 702             let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
 703             let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
 704             let task_context = if let Some(task_context_hid) = self.task_context {
 705                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 706             } else {
 707                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 708                 self.expr_err(span)
 709             };
 710             let new_unchecked = self.expr_call_lang_item_fn_mut(
 711                 span,
 712                 hir::LangItem::PinNewUnchecked,
 713                 arena_vec![self; ref_mut_awaitee],
 714                 Some(expr_hir_id),
 715             );
 716             let get_context = self.expr_call_lang_item_fn_mut(
 717                 gen_future_span,
 718                 hir::LangItem::GetContext,
 719                 arena_vec![self; task_context],
 720                 Some(expr_hir_id),
 721             );
 722             let call = self.expr_call_lang_item_fn(
 723                 span,
 724                 hir::LangItem::FuturePoll,
 725                 arena_vec![self; new_unchecked, get_context],
 726                 Some(expr_hir_id),
 727             );
 728             self.arena.alloc(self.expr_unsafe(call))
 729         };
 730
 731         // `::std::task::Poll::Ready(result) => break result`
 732         let loop_node_id = self.next_node_id();
 733         let loop_hir_id = self.lower_node_id(loop_node_id);
 734         let ready_arm = {
 735             let x_ident = Ident::with_dummy_span(sym::result);
 736             let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
 737             let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
 738             let ready_field = self.single_pat_field(gen_future_span, x_pat);
 739             let ready_pat = self.pat_lang_item_variant(
 740                 span,
 741                 hir::LangItem::PollReady,
 742                 ready_field,
 743                 Some(expr_hir_id),
 744             );
 745             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 746                 let expr_break =
 747                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 748                 this.arena.alloc(this.expr(gen_future_span, expr_break, ThinVec::new()))
 749             });
 750             self.arm(ready_pat, break_x)
 751         };
 752
 753         // `::std::task::Poll::Pending => {}`
 754         let pending_arm = {
 755             let pending_pat = self.pat_lang_item_variant(
 756                 span,
 757                 hir::LangItem::PollPending,
 758                 &[],
 759                 Some(expr_hir_id),
 760             );
 761             let empty_block = self.expr_block_empty(span);
 762             self.arm(pending_pat, empty_block)
 763         };
 764
 765         let inner_match_stmt = {
 766             let match_expr = self.expr_match(
 767                 span,
 768                 poll_expr,
 769                 arena_vec![self; ready_arm, pending_arm],
 770                 hir::MatchSource::AwaitDesugar,
 771             );
 772             self.stmt_expr(span, match_expr)
 773         };
 774
 775         // task_context = yield ();
 776         let yield_stmt = {
 777             let unit = self.expr_unit(span);
 778             let yield_expr = self.expr(
 779                 span,
 780                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
 781                 ThinVec::new(),
 782             );
 783             let yield_expr = self.arena.alloc(yield_expr);
 784
 785             if let Some(task_context_hid) = self.task_context {
 786                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 787                 let assign = self.expr(
 788                     span,
 789                     hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)),
 790                     AttrVec::new(),
 791                 );
 792                 self.stmt_expr(span, assign)
 793             } else {
 794                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 795                 // proceed with type checking.
 796                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 797             }
 798         };
 799
 800         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 801
 802         // loop { .. }
 803         let loop_expr = self.arena.alloc(hir::Expr {
 804             hir_id: loop_hir_id,
 805             kind: hir::ExprKind::Loop(
 806                 loop_block,
 807                 None,
 808                 hir::LoopSource::Loop,
 809                 self.lower_span(span),
 810             ),
 811             span: self.lower_span(span),
 812         });
 813
 814         // mut __awaitee => loop { ... }
 815         let awaitee_arm = self.arm(awaitee_pat, loop_expr);
 816
 817         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
 818         let into_future_span = self.mark_span_with_reason(
 819             DesugaringKind::Await,
 820             dot_await_span,
 821             self.allow_into_future.clone(),
 822         );
 823         let into_future_expr = self.expr_call_lang_item_fn(
 824             into_future_span,
 825             hir::LangItem::IntoFutureIntoFuture,
 826             arena_vec![self; expr],
 827             Some(expr_hir_id),
 828         );
 829
 830         // match <into_future_expr> {
 831         //     mut __awaitee => loop { .. }
 832         // }
 833         hir::ExprKind::Match(
 834             into_future_expr,
 835             arena_vec![self; awaitee_arm],
 836             hir::MatchSource::AwaitDesugar,
 837         )
 838     }
 839
 840     fn lower_expr_closure(
 841         &mut self,
 842         binder: &ClosureBinder,
 843         capture_clause: CaptureBy,
 844         closure_id: NodeId,
 845         movability: Movability,
 846         decl: &FnDecl,
 847         body: &Expr,
 848         fn_decl_span: Span,
 849     ) -> hir::ExprKind<'hir> {
 850         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 851
 852         let (body_id, generator_option) = self.with_new_scopes(move |this| {
 853             let prev = this.current_item;
 854             this.current_item = Some(fn_decl_span);
 855             let mut generator_kind = None;
 856             let body_id = this.lower_fn_body(decl, |this| {
 857                 let e = this.lower_expr_mut(body);
 858                 generator_kind = this.generator_kind;
 859                 e
 860             });
 861             let generator_option =
 862                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 863             this.current_item = prev;
 864             (body_id, generator_option)
 865         });
 866
 867         self.with_lifetime_binder(closure_id, generic_params, |this, bound_generic_params| {
 868             // Lower outside new scope to preserve `is_in_loop_condition`.
 869             let fn_decl = this.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
 870
 871             let c = self.arena.alloc(hir::Closure {
 872                 binder: binder_clause,
 873                 capture_clause,
 874                 bound_generic_params,
 875                 fn_decl,
 876                 body: body_id,
 877                 fn_decl_span: this.lower_span(fn_decl_span),
 878                 movability: generator_option,
 879             });
 880
 881             hir::ExprKind::Closure(c)
 882         })
 883     }
 884
 885     fn generator_movability_for_fn(
 886         &mut self,
 887         decl: &FnDecl,
 888         fn_decl_span: Span,
 889         generator_kind: Option<hir::GeneratorKind>,
 890         movability: Movability,
 891     ) -> Option<hir::Movability> {
 892         match generator_kind {
 893             Some(hir::GeneratorKind::Gen) => {
 894                 if decl.inputs.len() > 1 {
 895                     struct_span_err!(
 896                         self.tcx.sess,
 897                         fn_decl_span,
 898                         E0628,
 899                         "too many parameters for a generator (expected 0 or 1 parameters)"
 900                     )
 901                     .emit();
 902                 }
 903                 Some(movability)
 904             }
 905             Some(hir::GeneratorKind::Async(_)) => {
 906                 panic!("non-`async` closure body turned `async` during lowering");
 907             }
 908             None => {
 909                 if movability == Movability::Static {
 910                     struct_span_err!(
 911                         self.tcx.sess,
 912                         fn_decl_span,
 913                         E0697,
 914                         "closures cannot be static"
 915                     )
 916                     .emit();
 917                 }
 918                 None
 919             }
 920         }
 921     }
 922
 923     fn lower_closure_binder<'c>(
 924         &mut self,
 925         binder: &'c ClosureBinder,
 926     ) -> (hir::ClosureBinder, &'c [GenericParam]) {
 927         let (binder, params) = match binder {
 928             ClosureBinder::NotPresent => (hir::ClosureBinder::Default, &[][..]),
 929             &ClosureBinder::For { span, ref generic_params } => {
 930                 let span = self.lower_span(span);
 931                 (hir::ClosureBinder::For { span }, &**generic_params)
 932             }
 933         };
 934
 935         (binder, params)
 936     }
 937
 938     fn lower_expr_async_closure(
 939         &mut self,
 940         binder: &ClosureBinder,
 941         capture_clause: CaptureBy,
 942         closure_id: NodeId,
 943         inner_closure_id: NodeId,
 944         decl: &FnDecl,
 945         body: &Expr,
 946         fn_decl_span: Span,
 947     ) -> hir::ExprKind<'hir> {
 948         if let &ClosureBinder::For { span, .. } = binder {
 949             self.tcx.sess.span_err(
 950                 span,
 951                 "`for<...>` binders on `async` closures are not currently supported",
 952             );
 953         }
 954
 955         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 956
 957         let outer_decl =
 958             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 959
 960         let body = self.with_new_scopes(|this| {
 961             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 962             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 963                 struct_span_err!(
 964                     this.tcx.sess,
 965                     fn_decl_span,
 966                     E0708,
 967                     "`async` non-`move` closures with parameters are not currently supported",
 968                 )
 969                 .help(
 970                     "consider using `let` statements to manually capture \
 971                     variables by reference before entering an `async move` closure",
 972                 )
 973                 .emit();
 974             }
 975
 976             // Transform `async |x: u8| -> X { ... }` into
 977             // `|x: u8| future_from_generator(|| -> X { ... })`.
 978             let body_id = this.lower_fn_body(&outer_decl, |this| {
 979                 let async_ret_ty =
 980                     if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 981                 let async_body = this.make_async_expr(
 982                     capture_clause,
 983                     inner_closure_id,
 984                     async_ret_ty,
 985                     body.span,
 986                     hir::AsyncGeneratorKind::Closure,
 987                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 988                 );
 989                 this.expr(fn_decl_span, async_body, ThinVec::new())
 990             });
 991             body_id
 992         });
 993
 994         self.with_lifetime_binder(closure_id, generic_params, |this, bound_generic_params| {
 995             // We need to lower the declaration outside the new scope, because we
 996             // have to conserve the state of being inside a loop condition for the
 997             // closure argument types.
 998             let fn_decl = this.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
 999
1000             let c = self.arena.alloc(hir::Closure {
1001                 binder: binder_clause,
1002                 capture_clause,
1003                 bound_generic_params,
1004                 fn_decl,
1005                 body,
1006                 fn_decl_span: this.lower_span(fn_decl_span),
1007                 movability: None,
1008             });
1009             hir::ExprKind::Closure(c)
1010         })
1011     }
1012
1013     /// Destructure the LHS of complex assignments.
1014     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
1015     fn lower_expr_assign(
1016         &mut self,
1017         lhs: &Expr,
1018         rhs: &Expr,
1019         eq_sign_span: Span,
1020         whole_span: Span,
1021     ) -> hir::ExprKind<'hir> {
1022         // Return early in case of an ordinary assignment.
1023         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
1024             match &lhs.kind {
1025                 ExprKind::Array(..)
1026                 | ExprKind::Struct(..)
1027                 | ExprKind::Tup(..)
1028                 | ExprKind::Underscore => false,
1029                 // Check for tuple struct constructor.
1030                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
1031                 ExprKind::Paren(e) => {
1032                     match e.kind {
1033                         // We special-case `(..)` for consistency with patterns.
1034                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
1035                         _ => is_ordinary(lower_ctx, e),
1036                     }
1037                 }
1038                 _ => true,
1039             }
1040         }
1041         if is_ordinary(self, lhs) {
1042             return hir::ExprKind::Assign(
1043                 self.lower_expr(lhs),
1044                 self.lower_expr(rhs),
1045                 self.lower_span(eq_sign_span),
1046             );
1047         }
1048
1049         let mut assignments = vec![];
1050
1051         // The LHS becomes a pattern: `(lhs1, lhs2)`.
1052         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
1053         let rhs = self.lower_expr(rhs);
1054
1055         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
1056         let destructure_let = self.stmt_let_pat(
1057             None,
1058             whole_span,
1059             Some(rhs),
1060             pat,
1061             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
1062         );
1063
1064         // `a = lhs1; b = lhs2;`.
1065         let stmts = self
1066             .arena
1067             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
1068
1069         // Wrap everything in a block.
1070         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
1071     }
1072
1073     /// If the given expression is a path to a tuple struct, returns that path.
1074     /// It is not a complete check, but just tries to reject most paths early
1075     /// if they are not tuple structs.
1076     /// Type checking will take care of the full validation later.
1077     fn extract_tuple_struct_path<'a>(
1078         &mut self,
1079         expr: &'a Expr,
1080     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1081         if let ExprKind::Path(qself, path) = &expr.kind {
1082             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1083             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1084                 if partial_res.unresolved_segments() == 0
1085                     && !partial_res.base_res().expected_in_tuple_struct_pat()
1086                 {
1087                     return None;
1088                 }
1089             }
1090             return Some((qself, path));
1091         }
1092         None
1093     }
1094
1095     /// If the given expression is a path to a unit struct, returns that path.
1096     /// It is not a complete check, but just tries to reject most paths early
1097     /// if they are not unit structs.
1098     /// Type checking will take care of the full validation later.
1099     fn extract_unit_struct_path<'a>(
1100         &mut self,
1101         expr: &'a Expr,
1102     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1103         if let ExprKind::Path(qself, path) = &expr.kind {
1104             // Does the path resolve to something disallowed in a unit struct/variant pattern?
1105             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1106                 if partial_res.unresolved_segments() == 0
1107                     && !partial_res.base_res().expected_in_unit_struct_pat()
1108                 {
1109                     return None;
1110                 }
1111             }
1112             return Some((qself, path));
1113         }
1114         None
1115     }
1116
1117     /// Convert the LHS of a destructuring assignment to a pattern.
1118     /// Each sub-assignment is recorded in `assignments`.
1119     fn destructure_assign(
1120         &mut self,
1121         lhs: &Expr,
1122         eq_sign_span: Span,
1123         assignments: &mut Vec<hir::Stmt<'hir>>,
1124     ) -> &'hir hir::Pat<'hir> {
1125         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1126     }
1127
1128     fn destructure_assign_mut(
1129         &mut self,
1130         lhs: &Expr,
1131         eq_sign_span: Span,
1132         assignments: &mut Vec<hir::Stmt<'hir>>,
1133     ) -> hir::Pat<'hir> {
1134         match &lhs.kind {
1135             // Underscore pattern.
1136             ExprKind::Underscore => {
1137                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1138             }
1139             // Slice patterns.
1140             ExprKind::Array(elements) => {
1141                 let (pats, rest) =
1142                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1143                 let slice_pat = if let Some((i, span)) = rest {
1144                     let (before, after) = pats.split_at(i);
1145                     hir::PatKind::Slice(
1146                         before,
1147                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1148                         after,
1149                     )
1150                 } else {
1151                     hir::PatKind::Slice(pats, None, &[])
1152                 };
1153                 return self.pat_without_dbm(lhs.span, slice_pat);
1154             }
1155             // Tuple structs.
1156             ExprKind::Call(callee, args) => {
1157                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1158                     let (pats, rest) = self.destructure_sequence(
1159                         args,
1160                         "tuple struct or variant",
1161                         eq_sign_span,
1162                         assignments,
1163                     );
1164                     let qpath = self.lower_qpath(
1165                         callee.id,
1166                         qself,
1167                         path,
1168                         ParamMode::Optional,
1169                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1170                     );
1171                     // Destructure like a tuple struct.
1172                     let tuple_struct_pat =
1173                         hir::PatKind::TupleStruct(qpath, pats, rest.map(|r| r.0));
1174                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1175                 }
1176             }
1177             // Unit structs and enum variants.
1178             ExprKind::Path(..) => {
1179                 if let Some((qself, path)) = self.extract_unit_struct_path(lhs) {
1180                     let qpath = self.lower_qpath(
1181                         lhs.id,
1182                         qself,
1183                         path,
1184                         ParamMode::Optional,
1185                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1186                     );
1187                     // Destructure like a unit struct.
1188                     let unit_struct_pat = hir::PatKind::Path(qpath);
1189                     return self.pat_without_dbm(lhs.span, unit_struct_pat);
1190                 }
1191             }
1192             // Structs.
1193             ExprKind::Struct(se) => {
1194                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1195                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1196                     hir::PatField {
1197                         hir_id: self.next_id(),
1198                         ident: self.lower_ident(f.ident),
1199                         pat,
1200                         is_shorthand: f.is_shorthand,
1201                         span: self.lower_span(f.span),
1202                     }
1203                 }));
1204                 let qpath = self.lower_qpath(
1205                     lhs.id,
1206                     &se.qself,
1207                     &se.path,
1208                     ParamMode::Optional,
1209                     ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1210                 );
1211                 let fields_omitted = match &se.rest {
1212                     StructRest::Base(e) => {
1213                         self.tcx
1214                             .sess
1215                             .struct_span_err(
1216                                 e.span,
1217                                 "functional record updates are not allowed in destructuring \
1218                                     assignments",
1219                             )
1220                             .span_suggestion(
1221                                 e.span,
1222                                 "consider removing the trailing pattern",
1223                                 "",
1224                                 rustc_errors::Applicability::MachineApplicable,
1225                             )
1226                             .emit();
1227                         true
1228                     }
1229                     StructRest::Rest(_) => true,
1230                     StructRest::None => false,
1231                 };
1232                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1233                 return self.pat_without_dbm(lhs.span, struct_pat);
1234             }
1235             // Tuples.
1236             ExprKind::Tup(elements) => {
1237                 let (pats, rest) =
1238                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1239                 let tuple_pat = hir::PatKind::Tuple(pats, rest.map(|r| r.0));
1240                 return self.pat_without_dbm(lhs.span, tuple_pat);
1241             }
1242             ExprKind::Paren(e) => {
1243                 // We special-case `(..)` for consistency with patterns.
1244                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1245                     let tuple_pat = hir::PatKind::Tuple(&[], Some(0));
1246                     return self.pat_without_dbm(lhs.span, tuple_pat);
1247                 } else {
1248                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1249                 }
1250             }
1251             _ => {}
1252         }
1253         // Treat all other cases as normal lvalue.
1254         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1255         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1256         let ident = self.expr_ident(lhs.span, ident, binding);
1257         let assign =
1258             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1259         let expr = self.expr(lhs.span, assign, ThinVec::new());
1260         assignments.push(self.stmt_expr(lhs.span, expr));
1261         pat
1262     }
1263
1264     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1265     /// Such a sequence occurs in a tuple (struct)/slice.
1266     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1267     /// exists.
1268     /// Each sub-assignment is recorded in `assignments`.
1269     fn destructure_sequence(
1270         &mut self,
1271         elements: &[AstP<Expr>],
1272         ctx: &str,
1273         eq_sign_span: Span,
1274         assignments: &mut Vec<hir::Stmt<'hir>>,
1275     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1276         let mut rest = None;
1277         let elements =
1278             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1279                 // Check for `..` pattern.
1280                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1281                     if let Some((_, prev_span)) = rest {
1282                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1283                     } else {
1284                         rest = Some((i, e.span));
1285                     }
1286                     None
1287                 } else {
1288                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1289                 }
1290             }));
1291         (elements, rest)
1292     }
1293
1294     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1295     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1296         let e1 = self.lower_expr_mut(e1);
1297         let e2 = self.lower_expr_mut(e2);
1298         let fn_path =
1299             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1300         let fn_expr =
1301             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
1302         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1303     }
1304
1305     fn lower_expr_range(
1306         &mut self,
1307         span: Span,
1308         e1: Option<&Expr>,
1309         e2: Option<&Expr>,
1310         lims: RangeLimits,
1311     ) -> hir::ExprKind<'hir> {
1312         use rustc_ast::RangeLimits::*;
1313
1314         let lang_item = match (e1, e2, lims) {
1315             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1316             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1317             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1318             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1319             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1320             (Some(..), Some(..), Closed) => unreachable!(),
1321             (_, None, Closed) => self.diagnostic().span_fatal(span, "inclusive range with no end"),
1322         };
1323
1324         let fields = self.arena.alloc_from_iter(
1325             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1326                 |(s, e)| {
1327                     let expr = self.lower_expr(&e);
1328                     let ident = Ident::new(s, self.lower_span(e.span));
1329                     self.expr_field(ident, expr, e.span)
1330                 },
1331             ),
1332         );
1333
1334         hir::ExprKind::Struct(
1335             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1336             fields,
1337             None,
1338         )
1339     }
1340
1341     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1342         let label = opt_label?;
1343         Some(Label { ident: self.lower_ident(label.ident) })
1344     }
1345
1346     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1347         let target_id = match destination {
1348             Some((id, _)) => {
1349                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1350                     Ok(self.lower_node_id(loop_id))
1351                 } else {
1352                     Err(hir::LoopIdError::UnresolvedLabel)
1353                 }
1354             }
1355             None => self
1356                 .loop_scope
1357                 .map(|id| Ok(self.lower_node_id(id)))
1358                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1359         };
1360         let label = self.lower_label(destination.map(|(_, label)| label));
1361         hir::Destination { label, target_id }
1362     }
1363
1364     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1365         if self.is_in_loop_condition && opt_label.is_none() {
1366             hir::Destination {
1367                 label: None,
1368                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1369             }
1370         } else {
1371             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1372         }
1373     }
1374
1375     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1376         let old_scope = self.catch_scope.replace(catch_id);
1377         let result = f(self);
1378         self.catch_scope = old_scope;
1379         result
1380     }
1381
1382     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1383         // We're no longer in the base loop's condition; we're in another loop.
1384         let was_in_loop_condition = self.is_in_loop_condition;
1385         self.is_in_loop_condition = false;
1386
1387         let old_scope = self.loop_scope.replace(loop_id);
1388         let result = f(self);
1389         self.loop_scope = old_scope;
1390
1391         self.is_in_loop_condition = was_in_loop_condition;
1392
1393         result
1394     }
1395
1396     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1397         let was_in_loop_condition = self.is_in_loop_condition;
1398         self.is_in_loop_condition = true;
1399
1400         let result = f(self);
1401
1402         self.is_in_loop_condition = was_in_loop_condition;
1403
1404         result
1405     }
1406
1407     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1408         hir::ExprField {
1409             hir_id: self.next_id(),
1410             ident: self.lower_ident(f.ident),
1411             expr: self.lower_expr(&f.expr),
1412             span: self.lower_span(f.span),
1413             is_shorthand: f.is_shorthand,
1414         }
1415     }
1416
1417     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1418         match self.generator_kind {
1419             Some(hir::GeneratorKind::Gen) => {}
1420             Some(hir::GeneratorKind::Async(_)) => {
1421                 struct_span_err!(
1422                     self.tcx.sess,
1423                     span,
1424                     E0727,
1425                     "`async` generators are not yet supported"
1426                 )
1427                 .emit();
1428             }
1429             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1430         }
1431
1432         let expr =
1433             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1434
1435         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1436     }
1437
1438     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1439     /// ```ignore (pseudo-rust)
1440     /// {
1441     ///     let result = match IntoIterator::into_iter(<head>) {
1442     ///         mut iter => {
1443     ///             [opt_ident]: loop {
1444     ///                 match Iterator::next(&mut iter) {
1445     ///                     None => break,
1446     ///                     Some(<pat>) => <body>,
1447     ///                 };
1448     ///             }
1449     ///         }
1450     ///     };
1451     ///     result
1452     /// }
1453     /// ```
1454     fn lower_expr_for(
1455         &mut self,
1456         e: &Expr,
1457         pat: &Pat,
1458         head: &Expr,
1459         body: &Block,
1460         opt_label: Option<Label>,
1461     ) -> hir::Expr<'hir> {
1462         let head = self.lower_expr_mut(head);
1463         let pat = self.lower_pat(pat);
1464         let for_span =
1465             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1466         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1467         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1468
1469         // `None => break`
1470         let none_arm = {
1471             let break_expr =
1472                 self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, ThinVec::new()));
1473             let pat = self.pat_none(for_span);
1474             self.arm(pat, break_expr)
1475         };
1476
1477         // Some(<pat>) => <body>,
1478         let some_arm = {
1479             let some_pat = self.pat_some(pat_span, pat);
1480             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1481             let body_expr = self.arena.alloc(self.expr_block(body_block, ThinVec::new()));
1482             self.arm(some_pat, body_expr)
1483         };
1484
1485         // `mut iter`
1486         let iter = Ident::with_dummy_span(sym::iter);
1487         let (iter_pat, iter_pat_nid) =
1488             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::Mutable);
1489
1490         // `match Iterator::next(&mut iter) { ... }`
1491         let match_expr = {
1492             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1493             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1494             let next_expr = self.expr_call_lang_item_fn(
1495                 head_span,
1496                 hir::LangItem::IteratorNext,
1497                 arena_vec![self; ref_mut_iter],
1498                 None,
1499             );
1500             let arms = arena_vec![self; none_arm, some_arm];
1501
1502             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1503         };
1504         let match_stmt = self.stmt_expr(for_span, match_expr);
1505
1506         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1507
1508         // `[opt_ident]: loop { ... }`
1509         let kind = hir::ExprKind::Loop(
1510             loop_block,
1511             self.lower_label(opt_label),
1512             hir::LoopSource::ForLoop,
1513             self.lower_span(for_span.with_hi(head.span.hi())),
1514         );
1515         let loop_expr =
1516             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1517
1518         // `mut iter => { ... }`
1519         let iter_arm = self.arm(iter_pat, loop_expr);
1520
1521         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1522         let into_iter_expr = {
1523             self.expr_call_lang_item_fn(
1524                 head_span,
1525                 hir::LangItem::IntoIterIntoIter,
1526                 arena_vec![self; head],
1527                 None,
1528             )
1529         };
1530
1531         let match_expr = self.arena.alloc(self.expr_match(
1532             for_span,
1533             into_iter_expr,
1534             arena_vec![self; iter_arm],
1535             hir::MatchSource::ForLoopDesugar,
1536         ));
1537
1538         let attrs: Vec<_> = e.attrs.iter().map(|a| self.lower_attr(a)).collect();
1539
1540         // This is effectively `{ let _result = ...; _result }`.
1541         // The construct was introduced in #21984 and is necessary to make sure that
1542         // temporaries in the `head` expression are dropped and do not leak to the
1543         // surrounding scope of the `match` since the `match` is not a terminating scope.
1544         //
1545         // Also, add the attributes to the outer returned expr node.
1546         self.expr_drop_temps_mut(for_span, match_expr, attrs.into())
1547     }
1548
1549     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1550     /// ```ignore (pseudo-rust)
1551     /// match Try::branch(<expr>) {
1552     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1553     ///     ControlFlow::Break(residual) =>
1554     ///         #[allow(unreachable_code)]
1555     ///         // If there is an enclosing `try {...}`:
1556     ///         break 'catch_target Try::from_residual(residual),
1557     ///         // Otherwise:
1558     ///         return Try::from_residual(residual),
1559     /// }
1560     /// ```
1561     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1562         let unstable_span = self.mark_span_with_reason(
1563             DesugaringKind::QuestionMark,
1564             span,
1565             self.allow_try_trait.clone(),
1566         );
1567         let try_span = self.tcx.sess.source_map().end_point(span);
1568         let try_span = self.mark_span_with_reason(
1569             DesugaringKind::QuestionMark,
1570             try_span,
1571             self.allow_try_trait.clone(),
1572         );
1573
1574         // `Try::branch(<expr>)`
1575         let scrutinee = {
1576             // expand <expr>
1577             let sub_expr = self.lower_expr_mut(sub_expr);
1578
1579             self.expr_call_lang_item_fn(
1580                 unstable_span,
1581                 hir::LangItem::TryTraitBranch,
1582                 arena_vec![self; sub_expr],
1583                 None,
1584             )
1585         };
1586
1587         // `#[allow(unreachable_code)]`
1588         let attr = {
1589             // `allow(unreachable_code)`
1590             let allow = {
1591                 let allow_ident = Ident::new(sym::allow, self.lower_span(span));
1592                 let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
1593                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1594                 attr::mk_list_item(allow_ident, vec![uc_nested])
1595             };
1596             attr::mk_attr_outer(allow)
1597         };
1598         let attrs = vec![attr];
1599
1600         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1601         let continue_arm = {
1602             let val_ident = Ident::with_dummy_span(sym::val);
1603             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1604             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1605                 span,
1606                 val_ident,
1607                 val_pat_nid,
1608                 ThinVec::from(attrs.clone()),
1609             ));
1610             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1611             self.arm(continue_pat, val_expr)
1612         };
1613
1614         // `ControlFlow::Break(residual) =>
1615         //     #[allow(unreachable_code)]
1616         //     return Try::from_residual(residual),`
1617         let break_arm = {
1618             let residual_ident = Ident::with_dummy_span(sym::residual);
1619             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1620             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1621             let from_residual_expr = self.wrap_in_try_constructor(
1622                 hir::LangItem::TryTraitFromResidual,
1623                 try_span,
1624                 self.arena.alloc(residual_expr),
1625                 unstable_span,
1626             );
1627             let thin_attrs = ThinVec::from(attrs);
1628             let ret_expr = if let Some(catch_node) = self.catch_scope {
1629                 let target_id = Ok(self.lower_node_id(catch_node));
1630                 self.arena.alloc(self.expr(
1631                     try_span,
1632                     hir::ExprKind::Break(
1633                         hir::Destination { label: None, target_id },
1634                         Some(from_residual_expr),
1635                     ),
1636                     thin_attrs,
1637                 ))
1638             } else {
1639                 self.arena.alloc(self.expr(
1640                     try_span,
1641                     hir::ExprKind::Ret(Some(from_residual_expr)),
1642                     thin_attrs,
1643                 ))
1644             };
1645
1646             let break_pat = self.pat_cf_break(try_span, residual_local);
1647             self.arm(break_pat, ret_expr)
1648         };
1649
1650         hir::ExprKind::Match(
1651             scrutinee,
1652             arena_vec![self; break_arm, continue_arm],
1653             hir::MatchSource::TryDesugar,
1654         )
1655     }
1656
1657     /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
1658     /// ```rust
1659     /// // If there is an enclosing `try {...}`:
1660     /// break 'catch_target FromResidual::from_residual(Yeet(residual)),
1661     /// // Otherwise:
1662     /// return FromResidual::from_residual(Yeet(residual)),
1663     /// ```
1664     /// But to simplify this, there's a `from_yeet` lang item function which
1665     /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
1666     fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1667         // The expression (if present) or `()` otherwise.
1668         let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
1669             (sub_expr.span, self.lower_expr(sub_expr))
1670         } else {
1671             (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
1672         };
1673
1674         let unstable_span = self.mark_span_with_reason(
1675             DesugaringKind::YeetExpr,
1676             span,
1677             self.allow_try_trait.clone(),
1678         );
1679
1680         let from_yeet_expr = self.wrap_in_try_constructor(
1681             hir::LangItem::TryTraitFromYeet,
1682             unstable_span,
1683             yeeted_expr,
1684             yeeted_span,
1685         );
1686
1687         if let Some(catch_node) = self.catch_scope {
1688             let target_id = Ok(self.lower_node_id(catch_node));
1689             hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
1690         } else {
1691             hir::ExprKind::Ret(Some(from_yeet_expr))
1692         }
1693     }
1694
1695     // =========================================================================
1696     // Helper methods for building HIR.
1697     // =========================================================================
1698
1699     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1700     ///
1701     /// In terms of drop order, it has the same effect as wrapping `expr` in
1702     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1703     ///
1704     /// The drop order can be important in e.g. `if expr { .. }`.
1705     pub(super) fn expr_drop_temps(
1706         &mut self,
1707         span: Span,
1708         expr: &'hir hir::Expr<'hir>,
1709         attrs: AttrVec,
1710     ) -> &'hir hir::Expr<'hir> {
1711         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1712     }
1713
1714     pub(super) fn expr_drop_temps_mut(
1715         &mut self,
1716         span: Span,
1717         expr: &'hir hir::Expr<'hir>,
1718         attrs: AttrVec,
1719     ) -> hir::Expr<'hir> {
1720         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1721     }
1722
1723     fn expr_match(
1724         &mut self,
1725         span: Span,
1726         arg: &'hir hir::Expr<'hir>,
1727         arms: &'hir [hir::Arm<'hir>],
1728         source: hir::MatchSource,
1729     ) -> hir::Expr<'hir> {
1730         self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
1731     }
1732
1733     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
1734         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1735         self.expr(span, expr_break, attrs)
1736     }
1737
1738     fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1739         let expr_break = self.expr_break(span, attrs);
1740         self.arena.alloc(expr_break)
1741     }
1742
1743     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1744         self.expr(
1745             span,
1746             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1747             ThinVec::new(),
1748         )
1749     }
1750
1751     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1752         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
1753     }
1754
1755     fn expr_call_mut(
1756         &mut self,
1757         span: Span,
1758         e: &'hir hir::Expr<'hir>,
1759         args: &'hir [hir::Expr<'hir>],
1760     ) -> hir::Expr<'hir> {
1761         self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new())
1762     }
1763
1764     fn expr_call(
1765         &mut self,
1766         span: Span,
1767         e: &'hir hir::Expr<'hir>,
1768         args: &'hir [hir::Expr<'hir>],
1769     ) -> &'hir hir::Expr<'hir> {
1770         self.arena.alloc(self.expr_call_mut(span, e, args))
1771     }
1772
1773     fn expr_call_lang_item_fn_mut(
1774         &mut self,
1775         span: Span,
1776         lang_item: hir::LangItem,
1777         args: &'hir [hir::Expr<'hir>],
1778         hir_id: Option<hir::HirId>,
1779     ) -> hir::Expr<'hir> {
1780         let path =
1781             self.arena.alloc(self.expr_lang_item_path(span, lang_item, ThinVec::new(), hir_id));
1782         self.expr_call_mut(span, path, args)
1783     }
1784
1785     fn expr_call_lang_item_fn(
1786         &mut self,
1787         span: Span,
1788         lang_item: hir::LangItem,
1789         args: &'hir [hir::Expr<'hir>],
1790         hir_id: Option<hir::HirId>,
1791     ) -> &'hir hir::Expr<'hir> {
1792         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1793     }
1794
1795     fn expr_lang_item_path(
1796         &mut self,
1797         span: Span,
1798         lang_item: hir::LangItem,
1799         attrs: AttrVec,
1800         hir_id: Option<hir::HirId>,
1801     ) -> hir::Expr<'hir> {
1802         self.expr(
1803             span,
1804             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1805             attrs,
1806         )
1807     }
1808
1809     pub(super) fn expr_ident(
1810         &mut self,
1811         sp: Span,
1812         ident: Ident,
1813         binding: hir::HirId,
1814     ) -> &'hir hir::Expr<'hir> {
1815         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1816     }
1817
1818     pub(super) fn expr_ident_mut(
1819         &mut self,
1820         sp: Span,
1821         ident: Ident,
1822         binding: hir::HirId,
1823     ) -> hir::Expr<'hir> {
1824         self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
1825     }
1826
1827     fn expr_ident_with_attrs(
1828         &mut self,
1829         span: Span,
1830         ident: Ident,
1831         binding: hir::HirId,
1832         attrs: AttrVec,
1833     ) -> hir::Expr<'hir> {
1834         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1835             None,
1836             self.arena.alloc(hir::Path {
1837                 span: self.lower_span(span),
1838                 res: Res::Local(binding),
1839                 segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
1840             }),
1841         ));
1842
1843         self.expr(span, expr_path, attrs)
1844     }
1845
1846     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1847         let hir_id = self.next_id();
1848         let span = expr.span;
1849         self.expr(
1850             span,
1851             hir::ExprKind::Block(
1852                 self.arena.alloc(hir::Block {
1853                     stmts: &[],
1854                     expr: Some(expr),
1855                     hir_id,
1856                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1857                     span: self.lower_span(span),
1858                     targeted_by_break: false,
1859                 }),
1860                 None,
1861             ),
1862             ThinVec::new(),
1863         )
1864     }
1865
1866     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1867         let blk = self.block_all(span, &[], None);
1868         let expr = self.expr_block(blk, ThinVec::new());
1869         self.arena.alloc(expr)
1870     }
1871
1872     pub(super) fn expr_block(
1873         &mut self,
1874         b: &'hir hir::Block<'hir>,
1875         attrs: AttrVec,
1876     ) -> hir::Expr<'hir> {
1877         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1878     }
1879
1880     pub(super) fn expr(
1881         &mut self,
1882         span: Span,
1883         kind: hir::ExprKind<'hir>,
1884         attrs: AttrVec,
1885     ) -> hir::Expr<'hir> {
1886         let hir_id = self.next_id();
1887         self.lower_attrs(hir_id, &attrs);
1888         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1889     }
1890
1891     fn expr_field(
1892         &mut self,
1893         ident: Ident,
1894         expr: &'hir hir::Expr<'hir>,
1895         span: Span,
1896     ) -> hir::ExprField<'hir> {
1897         hir::ExprField {
1898             hir_id: self.next_id(),
1899             ident,
1900             span: self.lower_span(span),
1901             expr,
1902             is_shorthand: false,
1903         }
1904     }
1905
1906     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1907         hir::Arm {
1908             hir_id: self.next_id(),
1909             pat,
1910             guard: None,
1911             span: self.lower_span(expr.span),
1912             body: expr,
1913         }
1914     }
1915 }