compiler/rustc_ast_lowering/src/expr.rs

   1 use super::errors::{
   2     AsyncGeneratorsNotSupported, AsyncNonMoveClosureNotSupported, AwaitOnlyInAsyncFnAndBlocks,
   3     BaseExpressionDoubleDot, ClosureCannotBeStatic, FunctionalRecordUpdateDestructuringAssignemnt,
   4     GeneratorTooManyParameters, InclusiveRangeWithNoEnd, NotSupportedForLifetimeBinderAsyncClosure,
   5     RustcBoxAttributeError, UnderscoreExprLhsAssign,
   6 };
   7 use super::ResolverAstLoweringExt;
   8 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
   9 use crate::{FnDeclKind, ImplTraitPosition};
  10 use rustc_ast::attr;
  11 use rustc_ast::ptr::P as AstP;
  12 use rustc_ast::*;
  13 use rustc_data_structures::stack::ensure_sufficient_stack;
  14 use rustc_hir as hir;
  15 use rustc_hir::def::Res;
  16 use rustc_hir::definitions::DefPathData;
  17 use rustc_session::errors::report_lit_error;
  18 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
  19 use rustc_span::symbol::{sym, Ident};
  20 use rustc_span::DUMMY_SP;
  21 use thin_vec::thin_vec;
  22
  23 impl<'hir> LoweringContext<'_, 'hir> {
  24     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  25         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  26     }
  27
  28     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  29         self.arena.alloc(self.lower_expr_mut(e))
  30     }
  31
  32     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  33         ensure_sufficient_stack(|| {
  34             match &e.kind {
  35                 // Paranthesis expression does not have a HirId and is handled specially.
  36                 ExprKind::Paren(ex) => {
  37                     let mut ex = self.lower_expr_mut(ex);
  38                     // Include parens in span, but only if it is a super-span.
  39                     if e.span.contains(ex.span) {
  40                         ex.span = self.lower_span(e.span);
  41                     }
  42                     // Merge attributes into the inner expression.
  43                     if !e.attrs.is_empty() {
  44                         let old_attrs =
  45                             self.attrs.get(&ex.hir_id.local_id).map(|la| *la).unwrap_or(&[]);
  46                         self.attrs.insert(
  47                             ex.hir_id.local_id,
  48                             &*self.arena.alloc_from_iter(
  49                                 e.attrs
  50                                     .iter()
  51                                     .map(|a| self.lower_attr(a))
  52                                     .chain(old_attrs.iter().cloned()),
  53                             ),
  54                         );
  55                     }
  56                     return ex;
  57                 }
  58                 // Desugar `ExprForLoop`
  59                 // from: `[opt_ident]: for <pat> in <head> <body>`
  60                 //
  61                 // This also needs special handling because the HirId of the returned `hir::Expr` will not
  62                 // correspond to the `e.id`, so `lower_expr_for` handles attribute lowering itself.
  63                 ExprKind::ForLoop(pat, head, body, opt_label) => {
  64                     return self.lower_expr_for(e, pat, head, body, *opt_label);
  65                 }
  66                 _ => (),
  67             }
  68
  69             let hir_id = self.lower_node_id(e.id);
  70             self.lower_attrs(hir_id, &e.attrs);
  71
  72             let kind = match &e.kind {
  73                 ExprKind::Box(inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  74                 ExprKind::Array(exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  75                 ExprKind::ConstBlock(anon_const) => {
  76                     let anon_const = self.lower_anon_const(anon_const);
  77                     hir::ExprKind::ConstBlock(anon_const)
  78                 }
  79                 ExprKind::Repeat(expr, count) => {
  80                     let expr = self.lower_expr(expr);
  81                     let count = self.lower_array_length(count);
  82                     hir::ExprKind::Repeat(expr, count)
  83                 }
  84                 ExprKind::Tup(elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  85                 ExprKind::Call(f, args) => {
  86                     if e.attrs.get(0).map_or(false, |a| a.has_name(sym::rustc_box)) {
  87                         if let [inner] = &args[..] && e.attrs.len() == 1 {
  88                             let kind = hir::ExprKind::Box(self.lower_expr(&inner));
  89                             return hir::Expr { hir_id, kind, span: self.lower_span(e.span) };
  90                         } else {
  91                             self.tcx.sess.emit_err(RustcBoxAttributeError { span: e.span });
  92                             hir::ExprKind::Err
  93                         }
  94                     } else if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
  95                         self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
  96                     } else {
  97                         let f = self.lower_expr(f);
  98                         hir::ExprKind::Call(f, self.lower_exprs(args))
  99                     }
 100                 }
 101                 ExprKind::MethodCall(box MethodCall { seg, receiver, args, span }) => {
 102                     let hir_seg = self.arena.alloc(self.lower_path_segment(
 103                         e.span,
 104                         seg,
 105                         ParamMode::Optional,
 106                         ParenthesizedGenericArgs::Err,
 107                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 108                     ));
 109                     let receiver = self.lower_expr(receiver);
 110                     let args =
 111                         self.arena.alloc_from_iter(args.iter().map(|x| self.lower_expr_mut(x)));
 112                     hir::ExprKind::MethodCall(hir_seg, receiver, args, self.lower_span(*span))
 113                 }
 114                 ExprKind::Binary(binop, lhs, rhs) => {
 115                     let binop = self.lower_binop(*binop);
 116                     let lhs = self.lower_expr(lhs);
 117                     let rhs = self.lower_expr(rhs);
 118                     hir::ExprKind::Binary(binop, lhs, rhs)
 119                 }
 120                 ExprKind::Unary(op, ohs) => {
 121                     let op = self.lower_unop(*op);
 122                     let ohs = self.lower_expr(ohs);
 123                     hir::ExprKind::Unary(op, ohs)
 124                 }
 125                 ExprKind::Lit(token_lit) => {
 126                     let lit_kind = match LitKind::from_token_lit(*token_lit) {
 127                         Ok(lit_kind) => lit_kind,
 128                         Err(err) => {
 129                             report_lit_error(&self.tcx.sess.parse_sess, err, *token_lit, e.span);
 130                             LitKind::Err
 131                         }
 132                     };
 133                     hir::ExprKind::Lit(respan(self.lower_span(e.span), lit_kind))
 134                 }
 135                 ExprKind::IncludedBytes(bytes) => hir::ExprKind::Lit(respan(
 136                     self.lower_span(e.span),
 137                     LitKind::ByteStr(bytes.clone(), StrStyle::Cooked),
 138                 )),
 139                 ExprKind::Cast(expr, ty) => {
 140                     let expr = self.lower_expr(expr);
 141                     let ty =
 142                         self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 143                     hir::ExprKind::Cast(expr, ty)
 144                 }
 145                 ExprKind::Type(expr, ty) => {
 146                     let expr = self.lower_expr(expr);
 147                     let ty =
 148                         self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
 149                     hir::ExprKind::Type(expr, ty)
 150                 }
 151                 ExprKind::AddrOf(k, m, ohs) => {
 152                     let ohs = self.lower_expr(ohs);
 153                     hir::ExprKind::AddrOf(*k, *m, ohs)
 154                 }
 155                 ExprKind::Let(pat, scrutinee, span) => {
 156                     hir::ExprKind::Let(self.arena.alloc(hir::Let {
 157                         hir_id: self.next_id(),
 158                         span: self.lower_span(*span),
 159                         pat: self.lower_pat(pat),
 160                         ty: None,
 161                         init: self.lower_expr(scrutinee),
 162                     }))
 163                 }
 164                 ExprKind::If(cond, then, else_opt) => {
 165                     self.lower_expr_if(cond, then, else_opt.as_deref())
 166                 }
 167                 ExprKind::While(cond, body, opt_label) => self.with_loop_scope(e.id, |this| {
 168                     let span = this.mark_span_with_reason(DesugaringKind::WhileLoop, e.span, None);
 169                     this.lower_expr_while_in_loop_scope(span, cond, body, *opt_label)
 170                 }),
 171                 ExprKind::Loop(body, opt_label, span) => self.with_loop_scope(e.id, |this| {
 172                     hir::ExprKind::Loop(
 173                         this.lower_block(body, false),
 174                         this.lower_label(*opt_label),
 175                         hir::LoopSource::Loop,
 176                         this.lower_span(*span),
 177                     )
 178                 }),
 179                 ExprKind::TryBlock(body) => self.lower_expr_try_block(body),
 180                 ExprKind::Match(expr, arms) => hir::ExprKind::Match(
 181                     self.lower_expr(expr),
 182                     self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
 183                     hir::MatchSource::Normal,
 184                 ),
 185                 ExprKind::Async(capture_clause, closure_node_id, block) => self.make_async_expr(
 186                     *capture_clause,
 187                     hir_id,
 188                     *closure_node_id,
 189                     None,
 190                     e.span,
 191                     hir::AsyncGeneratorKind::Block,
 192                     |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
 193                 ),
 194                 ExprKind::Await(expr) => {
 195                     let dot_await_span = if expr.span.hi() < e.span.hi() {
 196                         let span_with_whitespace = self
 197                             .tcx
 198                             .sess
 199                             .source_map()
 200                             .span_extend_while(expr.span, char::is_whitespace)
 201                             .unwrap_or(expr.span);
 202                         span_with_whitespace.shrink_to_hi().with_hi(e.span.hi())
 203                     } else {
 204                         // this is a recovered `await expr`
 205                         e.span
 206                     };
 207                     self.lower_expr_await(dot_await_span, expr)
 208                 }
 209                 ExprKind::Closure(box Closure {
 210                     binder,
 211                     capture_clause,
 212                     asyncness,
 213                     movability,
 214                     fn_decl,
 215                     body,
 216                     fn_decl_span,
 217                     fn_arg_span,
 218                 }) => {
 219                     if let Async::Yes { closure_id, .. } = asyncness {
 220                         self.lower_expr_async_closure(
 221                             binder,
 222                             *capture_clause,
 223                             e.id,
 224                             hir_id,
 225                             *closure_id,
 226                             fn_decl,
 227                             body,
 228                             *fn_decl_span,
 229                             *fn_arg_span,
 230                         )
 231                     } else {
 232                         self.lower_expr_closure(
 233                             binder,
 234                             *capture_clause,
 235                             e.id,
 236                             *movability,
 237                             fn_decl,
 238                             body,
 239                             *fn_decl_span,
 240                             *fn_arg_span,
 241                         )
 242                     }
 243                 }
 244                 ExprKind::Block(blk, opt_label) => {
 245                     let opt_label = self.lower_label(*opt_label);
 246                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 247                 }
 248                 ExprKind::Assign(el, er, span) => self.lower_expr_assign(el, er, *span, e.span),
 249                 ExprKind::AssignOp(op, el, er) => hir::ExprKind::AssignOp(
 250                     self.lower_binop(*op),
 251                     self.lower_expr(el),
 252                     self.lower_expr(er),
 253                 ),
 254                 ExprKind::Field(el, ident) => {
 255                     hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(*ident))
 256                 }
 257                 ExprKind::Index(el, er) => {
 258                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 259                 }
 260                 ExprKind::Range(Some(e1), Some(e2), RangeLimits::Closed) => {
 261                     self.lower_expr_range_closed(e.span, e1, e2)
 262                 }
 263                 ExprKind::Range(e1, e2, lims) => {
 264                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), *lims)
 265                 }
 266                 ExprKind::Underscore => {
 267                     self.tcx.sess.emit_err(UnderscoreExprLhsAssign { span: e.span });
 268                     hir::ExprKind::Err
 269                 }
 270                 ExprKind::Path(qself, path) => {
 271                     let qpath = self.lower_qpath(
 272                         e.id,
 273                         qself,
 274                         path,
 275                         ParamMode::Optional,
 276                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 277                     );
 278                     hir::ExprKind::Path(qpath)
 279                 }
 280                 ExprKind::Break(opt_label, opt_expr) => {
 281                     let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 282                     hir::ExprKind::Break(self.lower_jump_destination(e.id, *opt_label), opt_expr)
 283                 }
 284                 ExprKind::Continue(opt_label) => {
 285                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, *opt_label))
 286                 }
 287                 ExprKind::Ret(e) => {
 288                     let e = e.as_ref().map(|x| self.lower_expr(x));
 289                     hir::ExprKind::Ret(e)
 290                 }
 291                 ExprKind::Yeet(sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
 292                 ExprKind::InlineAsm(asm) => {
 293                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
 294                 }
 295                 ExprKind::Struct(se) => {
 296                     let rest = match &se.rest {
 297                         StructRest::Base(e) => Some(self.lower_expr(e)),
 298                         StructRest::Rest(sp) => {
 299                             self.tcx.sess.emit_err(BaseExpressionDoubleDot { span: *sp });
 300                             Some(&*self.arena.alloc(self.expr_err(*sp)))
 301                         }
 302                         StructRest::None => None,
 303                     };
 304                     hir::ExprKind::Struct(
 305                         self.arena.alloc(self.lower_qpath(
 306                             e.id,
 307                             &se.qself,
 308                             &se.path,
 309                             ParamMode::Optional,
 310                             &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 311                         )),
 312                         self.arena
 313                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
 314                         rest,
 315                     )
 316                 }
 317                 ExprKind::Yield(opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 318                 ExprKind::Err => hir::ExprKind::Err,
 319                 ExprKind::Try(sub_expr) => self.lower_expr_try(e.span, sub_expr),
 320
 321                 ExprKind::Paren(_) | ExprKind::ForLoop(..) => unreachable!("already handled"),
 322
 323                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 324             };
 325
 326             hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
 327         })
 328     }
 329
 330     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 331         match u {
 332             UnOp::Deref => hir::UnOp::Deref,
 333             UnOp::Not => hir::UnOp::Not,
 334             UnOp::Neg => hir::UnOp::Neg,
 335         }
 336     }
 337
 338     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 339         Spanned {
 340             node: match b.node {
 341                 BinOpKind::Add => hir::BinOpKind::Add,
 342                 BinOpKind::Sub => hir::BinOpKind::Sub,
 343                 BinOpKind::Mul => hir::BinOpKind::Mul,
 344                 BinOpKind::Div => hir::BinOpKind::Div,
 345                 BinOpKind::Rem => hir::BinOpKind::Rem,
 346                 BinOpKind::And => hir::BinOpKind::And,
 347                 BinOpKind::Or => hir::BinOpKind::Or,
 348                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 349                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 350                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 351                 BinOpKind::Shl => hir::BinOpKind::Shl,
 352                 BinOpKind::Shr => hir::BinOpKind::Shr,
 353                 BinOpKind::Eq => hir::BinOpKind::Eq,
 354                 BinOpKind::Lt => hir::BinOpKind::Lt,
 355                 BinOpKind::Le => hir::BinOpKind::Le,
 356                 BinOpKind::Ne => hir::BinOpKind::Ne,
 357                 BinOpKind::Ge => hir::BinOpKind::Ge,
 358                 BinOpKind::Gt => hir::BinOpKind::Gt,
 359             },
 360             span: self.lower_span(b.span),
 361         }
 362     }
 363
 364     fn lower_legacy_const_generics(
 365         &mut self,
 366         mut f: Expr,
 367         args: Vec<AstP<Expr>>,
 368         legacy_args_idx: &[usize],
 369     ) -> hir::ExprKind<'hir> {
 370         let ExprKind::Path(None, path) = &mut f.kind else {
 371             unreachable!();
 372         };
 373
 374         // Split the arguments into const generics and normal arguments
 375         let mut real_args = vec![];
 376         let mut generic_args = vec![];
 377         for (idx, arg) in args.into_iter().enumerate() {
 378             if legacy_args_idx.contains(&idx) {
 379                 let parent_def_id = self.current_hir_id_owner;
 380                 let node_id = self.next_node_id();
 381
 382                 // Add a definition for the in-band const def.
 383                 self.create_def(parent_def_id.def_id, node_id, DefPathData::AnonConst, f.span);
 384
 385                 let anon_const = AnonConst { id: node_id, value: arg };
 386                 generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
 387             } else {
 388                 real_args.push(arg);
 389             }
 390         }
 391
 392         // Add generic args to the last element of the path.
 393         let last_segment = path.segments.last_mut().unwrap();
 394         assert!(last_segment.args.is_none());
 395         last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
 396             span: DUMMY_SP,
 397             args: generic_args,
 398         })));
 399
 400         // Now lower everything as normal.
 401         let f = self.lower_expr(&f);
 402         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
 403     }
 404
 405     fn lower_expr_if(
 406         &mut self,
 407         cond: &Expr,
 408         then: &Block,
 409         else_opt: Option<&Expr>,
 410     ) -> hir::ExprKind<'hir> {
 411         let lowered_cond = self.lower_cond(cond);
 412         let then_expr = self.lower_block_expr(then);
 413         if let Some(rslt) = else_opt {
 414             hir::ExprKind::If(
 415                 lowered_cond,
 416                 self.arena.alloc(then_expr),
 417                 Some(self.lower_expr(rslt)),
 418             )
 419         } else {
 420             hir::ExprKind::If(lowered_cond, self.arena.alloc(then_expr), None)
 421         }
 422     }
 423
 424     // Lowers a condition (i.e. `cond` in `if cond` or `while cond`), wrapping it in a terminating scope
 425     // so that temporaries created in the condition don't live beyond it.
 426     fn lower_cond(&mut self, cond: &Expr) -> &'hir hir::Expr<'hir> {
 427         fn has_let_expr(expr: &Expr) -> bool {
 428             match &expr.kind {
 429                 ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
 430                 ExprKind::Let(..) => true,
 431                 _ => false,
 432             }
 433         }
 434
 435         // We have to take special care for `let` exprs in the condition, e.g. in
 436         // `if let pat = val` or `if foo && let pat = val`, as we _do_ want `val` to live beyond the
 437         // condition in this case.
 438         //
 439         // In order to mantain the drop behavior for the non `let` parts of the condition,
 440         // we still wrap them in terminating scopes, e.g. `if foo && let pat = val` essentially
 441         // gets transformed into `if { let _t = foo; _t } && let pat = val`
 442         match &cond.kind {
 443             ExprKind::Binary(op @ Spanned { node: ast::BinOpKind::And, .. }, lhs, rhs)
 444                 if has_let_expr(cond) =>
 445             {
 446                 let op = self.lower_binop(*op);
 447                 let lhs = self.lower_cond(lhs);
 448                 let rhs = self.lower_cond(rhs);
 449
 450                 self.arena.alloc(self.expr(cond.span, hir::ExprKind::Binary(op, lhs, rhs)))
 451             }
 452             ExprKind::Let(..) => self.lower_expr(cond),
 453             _ => {
 454                 let cond = self.lower_expr(cond);
 455                 let reason = DesugaringKind::CondTemporary;
 456                 let span_block = self.mark_span_with_reason(reason, cond.span, None);
 457                 self.expr_drop_temps(span_block, cond)
 458             }
 459         }
 460     }
 461
 462     // We desugar: `'label: while $cond $body` into:
 463     //
 464     // ```
 465     // 'label: loop {
 466     //   if { let _t = $cond; _t } {
 467     //     $body
 468     //   }
 469     //   else {
 470     //     break;
 471     //   }
 472     // }
 473     // ```
 474     //
 475     // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 476     // to preserve drop semantics since `while $cond { ... }` does not
 477     // let temporaries live outside of `cond`.
 478     fn lower_expr_while_in_loop_scope(
 479         &mut self,
 480         span: Span,
 481         cond: &Expr,
 482         body: &Block,
 483         opt_label: Option<Label>,
 484     ) -> hir::ExprKind<'hir> {
 485         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_cond(cond));
 486         let then = self.lower_block_expr(body);
 487         let expr_break = self.expr_break(span);
 488         let stmt_break = self.stmt_expr(span, expr_break);
 489         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
 490         let else_expr = self.arena.alloc(self.expr_block(else_blk));
 491         let if_kind = hir::ExprKind::If(lowered_cond, self.arena.alloc(then), Some(else_expr));
 492         let if_expr = self.expr(span, if_kind);
 493         let block = self.block_expr(self.arena.alloc(if_expr));
 494         let span = self.lower_span(span.with_hi(cond.span.hi()));
 495         let opt_label = self.lower_label(opt_label);
 496         hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
 497     }
 498
 499     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
 500     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
 501     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 502     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 503         self.with_catch_scope(body.id, |this| {
 504             let mut block = this.lower_block_noalloc(body, true);
 505
 506             // Final expression of the block (if present) or `()` with span at the end of block
 507             let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
 508                 (
 509                     this.mark_span_with_reason(
 510                         DesugaringKind::TryBlock,
 511                         expr.span,
 512                         this.allow_try_trait.clone(),
 513                     ),
 514                     expr,
 515                 )
 516             } else {
 517                 let try_span = this.mark_span_with_reason(
 518                     DesugaringKind::TryBlock,
 519                     this.tcx.sess.source_map().end_point(body.span),
 520                     this.allow_try_trait.clone(),
 521                 );
 522
 523                 (try_span, this.expr_unit(try_span))
 524             };
 525
 526             let ok_wrapped_span =
 527                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 528
 529             // `::std::ops::Try::from_output($tail_expr)`
 530             block.expr = Some(this.wrap_in_try_constructor(
 531                 hir::LangItem::TryTraitFromOutput,
 532                 try_span,
 533                 tail_expr,
 534                 ok_wrapped_span,
 535             ));
 536
 537             hir::ExprKind::Block(this.arena.alloc(block), None)
 538         })
 539     }
 540
 541     fn wrap_in_try_constructor(
 542         &mut self,
 543         lang_item: hir::LangItem,
 544         method_span: Span,
 545         expr: &'hir hir::Expr<'hir>,
 546         overall_span: Span,
 547     ) -> &'hir hir::Expr<'hir> {
 548         let constructor = self.arena.alloc(self.expr_lang_item_path(method_span, lang_item, None));
 549         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 550     }
 551
 552     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 553         let pat = self.lower_pat(&arm.pat);
 554         let guard = arm.guard.as_ref().map(|cond| {
 555             if let ExprKind::Let(pat, scrutinee, span) = &cond.kind {
 556                 hir::Guard::IfLet(self.arena.alloc(hir::Let {
 557                     hir_id: self.next_id(),
 558                     span: self.lower_span(*span),
 559                     pat: self.lower_pat(pat),
 560                     ty: None,
 561                     init: self.lower_expr(scrutinee),
 562                 }))
 563             } else {
 564                 hir::Guard::If(self.lower_expr(cond))
 565             }
 566         });
 567         let hir_id = self.next_id();
 568         self.lower_attrs(hir_id, &arm.attrs);
 569         hir::Arm {
 570             hir_id,
 571             pat,
 572             guard,
 573             body: self.lower_expr(&arm.body),
 574             span: self.lower_span(arm.span),
 575         }
 576     }
 577
 578     /// Lower an `async` construct to a generator that implements `Future`.
 579     ///
 580     /// This results in:
 581     ///
 582     /// ```text
 583     /// std::future::identity_future(static move? |_task_context| -> <ret_ty> {
 584     ///     <body>
 585     /// })
 586     /// ```
 587     pub(super) fn make_async_expr(
 588         &mut self,
 589         capture_clause: CaptureBy,
 590         outer_hir_id: hir::HirId,
 591         closure_node_id: NodeId,
 592         ret_ty: Option<hir::FnRetTy<'hir>>,
 593         span: Span,
 594         async_gen_kind: hir::AsyncGeneratorKind,
 595         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 596     ) -> hir::ExprKind<'hir> {
 597         let output = ret_ty.unwrap_or_else(|| hir::FnRetTy::DefaultReturn(self.lower_span(span)));
 598
 599         // Resume argument type: `ResumeTy`
 600         let unstable_span =
 601             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 602         let resume_ty = hir::QPath::LangItem(hir::LangItem::ResumeTy, unstable_span, None);
 603         let input_ty = hir::Ty {
 604             hir_id: self.next_id(),
 605             kind: hir::TyKind::Path(resume_ty),
 606             span: unstable_span,
 607         };
 608
 609         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 610         let fn_decl = self.arena.alloc(hir::FnDecl {
 611             inputs: arena_vec![self; input_ty],
 612             output,
 613             c_variadic: false,
 614             implicit_self: hir::ImplicitSelfKind::None,
 615             lifetime_elision_allowed: false,
 616         });
 617
 618         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 619         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 620             span,
 621             Ident::with_dummy_span(sym::_task_context),
 622             hir::BindingAnnotation::MUT,
 623         );
 624         let param = hir::Param {
 625             hir_id: self.next_id(),
 626             pat,
 627             ty_span: self.lower_span(span),
 628             span: self.lower_span(span),
 629         };
 630         let params = arena_vec![self; param];
 631
 632         let body = self.lower_body(move |this| {
 633             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 634
 635             let old_ctx = this.task_context;
 636             this.task_context = Some(task_context_hid);
 637             let res = body(this);
 638             this.task_context = old_ctx;
 639             (params, res)
 640         });
 641
 642         // `static |_task_context| -> <ret_ty> { body }`:
 643         let generator_kind = {
 644             let c = self.arena.alloc(hir::Closure {
 645                 def_id: self.local_def_id(closure_node_id),
 646                 binder: hir::ClosureBinder::Default,
 647                 capture_clause,
 648                 bound_generic_params: &[],
 649                 fn_decl,
 650                 body,
 651                 fn_decl_span: self.lower_span(span),
 652                 fn_arg_span: None,
 653                 movability: Some(hir::Movability::Static),
 654             });
 655
 656             hir::ExprKind::Closure(c)
 657         };
 658
 659         let hir_id = self.lower_node_id(closure_node_id);
 660         let unstable_span =
 661             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 662
 663         if self.tcx.features().closure_track_caller
 664             && let Some(attrs) = self.attrs.get(&outer_hir_id.local_id)
 665             && attrs.into_iter().any(|attr| attr.has_name(sym::track_caller))
 666         {
 667             self.lower_attrs(
 668                 hir_id,
 669                 &[Attribute {
 670                     kind: AttrKind::Normal(ptr::P(NormalAttr {
 671                         item: AttrItem {
 672                             path: Path::from_ident(Ident::new(sym::track_caller, span)),
 673                             args: AttrArgs::Empty,
 674                             tokens: None,
 675                         },
 676                         tokens: None,
 677                     })),
 678                     id: self.tcx.sess.parse_sess.attr_id_generator.mk_attr_id(),
 679                     style: AttrStyle::Outer,
 680                     span: unstable_span,
 681                 }],
 682             );
 683         }
 684
 685         let generator = hir::Expr { hir_id, kind: generator_kind, span: self.lower_span(span) };
 686
 687         // FIXME(swatinem):
 688         // For some reason, the async block needs to flow through *any*
 689         // call (like the identity function), as otherwise type and lifetime
 690         // inference have a hard time figuring things out.
 691         // Without this, we would get:
 692         // E0720 in src/test/ui/impl-trait/in-trait/default-body-with-rpit.rs
 693         // E0700 in src/test/ui/self/self_lifetime-async.rs
 694
 695         // `future::identity_future`:
 696         let identity_future =
 697             self.expr_lang_item_path(unstable_span, hir::LangItem::IdentityFuture, None);
 698
 699         // `future::identity_future(generator)`:
 700         hir::ExprKind::Call(self.arena.alloc(identity_future), arena_vec![self; generator])
 701     }
 702
 703     /// Desugar `<expr>.await` into:
 704     /// ```ignore (pseudo-rust)
 705     /// match ::std::future::IntoFuture::into_future(<expr>) {
 706     ///     mut __awaitee => loop {
 707     ///         match unsafe { ::std::future::Future::poll(
 708     ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
 709     ///             ::std::future::get_context(task_context),
 710     ///         ) } {
 711     ///             ::std::task::Poll::Ready(result) => break result,
 712     ///             ::std::task::Poll::Pending => {}
 713     ///         }
 714     ///         task_context = yield ();
 715     ///     }
 716     /// }
 717     /// ```
 718     fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 719         let full_span = expr.span.to(dot_await_span);
 720         match self.generator_kind {
 721             Some(hir::GeneratorKind::Async(_)) => {}
 722             Some(hir::GeneratorKind::Gen) | None => {
 723                 self.tcx.sess.emit_err(AwaitOnlyInAsyncFnAndBlocks {
 724                     dot_await_span,
 725                     item_span: self.current_item,
 726                 });
 727             }
 728         }
 729         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
 730         let gen_future_span = self.mark_span_with_reason(
 731             DesugaringKind::Await,
 732             full_span,
 733             self.allow_gen_future.clone(),
 734         );
 735         let expr = self.lower_expr_mut(expr);
 736         let expr_hir_id = expr.hir_id;
 737
 738         // Note that the name of this binding must not be changed to something else because
 739         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
 740         // this name to identify what is being awaited by a suspended async functions.
 741         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
 742         let (awaitee_pat, awaitee_pat_hid) =
 743             self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::MUT);
 744
 745         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 746
 747         // unsafe {
 748         //     ::std::future::Future::poll(
 749         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
 750         //         ::std::future::get_context(task_context),
 751         //     )
 752         // }
 753         let poll_expr = {
 754             let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
 755             let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
 756             let task_context = if let Some(task_context_hid) = self.task_context {
 757                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 758             } else {
 759                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 760                 self.expr_err(span)
 761             };
 762             let new_unchecked = self.expr_call_lang_item_fn_mut(
 763                 span,
 764                 hir::LangItem::PinNewUnchecked,
 765                 arena_vec![self; ref_mut_awaitee],
 766                 Some(expr_hir_id),
 767             );
 768             let get_context = self.expr_call_lang_item_fn_mut(
 769                 gen_future_span,
 770                 hir::LangItem::GetContext,
 771                 arena_vec![self; task_context],
 772                 Some(expr_hir_id),
 773             );
 774             let call = self.expr_call_lang_item_fn(
 775                 span,
 776                 hir::LangItem::FuturePoll,
 777                 arena_vec![self; new_unchecked, get_context],
 778                 Some(expr_hir_id),
 779             );
 780             self.arena.alloc(self.expr_unsafe(call))
 781         };
 782
 783         // `::std::task::Poll::Ready(result) => break result`
 784         let loop_node_id = self.next_node_id();
 785         let loop_hir_id = self.lower_node_id(loop_node_id);
 786         let ready_arm = {
 787             let x_ident = Ident::with_dummy_span(sym::result);
 788             let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
 789             let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
 790             let ready_field = self.single_pat_field(gen_future_span, x_pat);
 791             let ready_pat = self.pat_lang_item_variant(
 792                 span,
 793                 hir::LangItem::PollReady,
 794                 ready_field,
 795                 Some(expr_hir_id),
 796             );
 797             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 798                 let expr_break =
 799                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 800                 this.arena.alloc(this.expr(gen_future_span, expr_break))
 801             });
 802             self.arm(ready_pat, break_x)
 803         };
 804
 805         // `::std::task::Poll::Pending => {}`
 806         let pending_arm = {
 807             let pending_pat = self.pat_lang_item_variant(
 808                 span,
 809                 hir::LangItem::PollPending,
 810                 &[],
 811                 Some(expr_hir_id),
 812             );
 813             let empty_block = self.expr_block_empty(span);
 814             self.arm(pending_pat, empty_block)
 815         };
 816
 817         let inner_match_stmt = {
 818             let match_expr = self.expr_match(
 819                 span,
 820                 poll_expr,
 821                 arena_vec![self; ready_arm, pending_arm],
 822                 hir::MatchSource::AwaitDesugar,
 823             );
 824             self.stmt_expr(span, match_expr)
 825         };
 826
 827         // task_context = yield ();
 828         let yield_stmt = {
 829             let unit = self.expr_unit(span);
 830             let yield_expr = self.expr(
 831                 span,
 832                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
 833             );
 834             let yield_expr = self.arena.alloc(yield_expr);
 835
 836             if let Some(task_context_hid) = self.task_context {
 837                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 838                 let assign =
 839                     self.expr(span, hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)));
 840                 self.stmt_expr(span, assign)
 841             } else {
 842                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 843                 // proceed with type checking.
 844                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 845             }
 846         };
 847
 848         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 849
 850         // loop { .. }
 851         let loop_expr = self.arena.alloc(hir::Expr {
 852             hir_id: loop_hir_id,
 853             kind: hir::ExprKind::Loop(
 854                 loop_block,
 855                 None,
 856                 hir::LoopSource::Loop,
 857                 self.lower_span(span),
 858             ),
 859             span: self.lower_span(span),
 860         });
 861
 862         // mut __awaitee => loop { ... }
 863         let awaitee_arm = self.arm(awaitee_pat, loop_expr);
 864
 865         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
 866         let into_future_span = self.mark_span_with_reason(
 867             DesugaringKind::Await,
 868             dot_await_span,
 869             self.allow_into_future.clone(),
 870         );
 871         let into_future_expr = self.expr_call_lang_item_fn(
 872             into_future_span,
 873             hir::LangItem::IntoFutureIntoFuture,
 874             arena_vec![self; expr],
 875             Some(expr_hir_id),
 876         );
 877
 878         // match <into_future_expr> {
 879         //     mut __awaitee => loop { .. }
 880         // }
 881         hir::ExprKind::Match(
 882             into_future_expr,
 883             arena_vec![self; awaitee_arm],
 884             hir::MatchSource::AwaitDesugar,
 885         )
 886     }
 887
 888     fn lower_expr_closure(
 889         &mut self,
 890         binder: &ClosureBinder,
 891         capture_clause: CaptureBy,
 892         closure_id: NodeId,
 893         movability: Movability,
 894         decl: &FnDecl,
 895         body: &Expr,
 896         fn_decl_span: Span,
 897         fn_arg_span: Span,
 898     ) -> hir::ExprKind<'hir> {
 899         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 900
 901         let (body_id, generator_option) = self.with_new_scopes(move |this| {
 902             let prev = this.current_item;
 903             this.current_item = Some(fn_decl_span);
 904             let mut generator_kind = None;
 905             let body_id = this.lower_fn_body(decl, |this| {
 906                 let e = this.lower_expr_mut(body);
 907                 generator_kind = this.generator_kind;
 908                 e
 909             });
 910             let generator_option =
 911                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 912             this.current_item = prev;
 913             (body_id, generator_option)
 914         });
 915
 916         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
 917         // Lower outside new scope to preserve `is_in_loop_condition`.
 918         let fn_decl = self.lower_fn_decl(decl, closure_id, fn_decl_span, FnDeclKind::Closure, None);
 919
 920         let c = self.arena.alloc(hir::Closure {
 921             def_id: self.local_def_id(closure_id),
 922             binder: binder_clause,
 923             capture_clause,
 924             bound_generic_params,
 925             fn_decl,
 926             body: body_id,
 927             fn_decl_span: self.lower_span(fn_decl_span),
 928             fn_arg_span: Some(self.lower_span(fn_arg_span)),
 929             movability: generator_option,
 930         });
 931
 932         hir::ExprKind::Closure(c)
 933     }
 934
 935     fn generator_movability_for_fn(
 936         &mut self,
 937         decl: &FnDecl,
 938         fn_decl_span: Span,
 939         generator_kind: Option<hir::GeneratorKind>,
 940         movability: Movability,
 941     ) -> Option<hir::Movability> {
 942         match generator_kind {
 943             Some(hir::GeneratorKind::Gen) => {
 944                 if decl.inputs.len() > 1 {
 945                     self.tcx.sess.emit_err(GeneratorTooManyParameters { fn_decl_span });
 946                 }
 947                 Some(movability)
 948             }
 949             Some(hir::GeneratorKind::Async(_)) => {
 950                 panic!("non-`async` closure body turned `async` during lowering");
 951             }
 952             None => {
 953                 if movability == Movability::Static {
 954                     self.tcx.sess.emit_err(ClosureCannotBeStatic { fn_decl_span });
 955                 }
 956                 None
 957             }
 958         }
 959     }
 960
 961     fn lower_closure_binder<'c>(
 962         &mut self,
 963         binder: &'c ClosureBinder,
 964     ) -> (hir::ClosureBinder, &'c [GenericParam]) {
 965         let (binder, params) = match binder {
 966             ClosureBinder::NotPresent => (hir::ClosureBinder::Default, &[][..]),
 967             ClosureBinder::For { span, generic_params } => {
 968                 let span = self.lower_span(*span);
 969                 (hir::ClosureBinder::For { span }, &**generic_params)
 970             }
 971         };
 972
 973         (binder, params)
 974     }
 975
 976     fn lower_expr_async_closure(
 977         &mut self,
 978         binder: &ClosureBinder,
 979         capture_clause: CaptureBy,
 980         closure_id: NodeId,
 981         closure_hir_id: hir::HirId,
 982         inner_closure_id: NodeId,
 983         decl: &FnDecl,
 984         body: &Expr,
 985         fn_decl_span: Span,
 986         fn_arg_span: Span,
 987     ) -> hir::ExprKind<'hir> {
 988         if let &ClosureBinder::For { span, .. } = binder {
 989             self.tcx.sess.emit_err(NotSupportedForLifetimeBinderAsyncClosure { span });
 990         }
 991
 992         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 993
 994         let outer_decl =
 995             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 996
 997         let body = self.with_new_scopes(|this| {
 998             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 999             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
1000                 this.tcx.sess.emit_err(AsyncNonMoveClosureNotSupported { fn_decl_span });
1001             }
1002
1003             // Transform `async |x: u8| -> X { ... }` into
1004             // `|x: u8| identity_future(|| -> X { ... })`.
1005             let body_id = this.lower_fn_body(&outer_decl, |this| {
1006                 let async_ret_ty = if let FnRetTy::Ty(ty) = &decl.output {
1007                     let itctx = ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock);
1008                     Some(hir::FnRetTy::Return(this.lower_ty(&ty, &itctx)))
1009                 } else {
1010                     None
1011                 };
1012
1013                 let async_body = this.make_async_expr(
1014                     capture_clause,
1015                     closure_hir_id,
1016                     inner_closure_id,
1017                     async_ret_ty,
1018                     body.span,
1019                     hir::AsyncGeneratorKind::Closure,
1020                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
1021                 );
1022                 this.expr(fn_decl_span, async_body)
1023             });
1024             body_id
1025         });
1026
1027         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
1028         // We need to lower the declaration outside the new scope, because we
1029         // have to conserve the state of being inside a loop condition for the
1030         // closure argument types.
1031         let fn_decl =
1032             self.lower_fn_decl(&outer_decl, closure_id, fn_decl_span, FnDeclKind::Closure, None);
1033
1034         let c = self.arena.alloc(hir::Closure {
1035             def_id: self.local_def_id(closure_id),
1036             binder: binder_clause,
1037             capture_clause,
1038             bound_generic_params,
1039             fn_decl,
1040             body,
1041             fn_decl_span: self.lower_span(fn_decl_span),
1042             fn_arg_span: Some(self.lower_span(fn_arg_span)),
1043             movability: None,
1044         });
1045         hir::ExprKind::Closure(c)
1046     }
1047
1048     /// Destructure the LHS of complex assignments.
1049     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
1050     fn lower_expr_assign(
1051         &mut self,
1052         lhs: &Expr,
1053         rhs: &Expr,
1054         eq_sign_span: Span,
1055         whole_span: Span,
1056     ) -> hir::ExprKind<'hir> {
1057         // Return early in case of an ordinary assignment.
1058         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
1059             match &lhs.kind {
1060                 ExprKind::Array(..)
1061                 | ExprKind::Struct(..)
1062                 | ExprKind::Tup(..)
1063                 | ExprKind::Underscore => false,
1064                 // Check for tuple struct constructor.
1065                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
1066                 ExprKind::Paren(e) => {
1067                     match e.kind {
1068                         // We special-case `(..)` for consistency with patterns.
1069                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
1070                         _ => is_ordinary(lower_ctx, e),
1071                     }
1072                 }
1073                 _ => true,
1074             }
1075         }
1076         if is_ordinary(self, lhs) {
1077             return hir::ExprKind::Assign(
1078                 self.lower_expr(lhs),
1079                 self.lower_expr(rhs),
1080                 self.lower_span(eq_sign_span),
1081             );
1082         }
1083
1084         let mut assignments = vec![];
1085
1086         // The LHS becomes a pattern: `(lhs1, lhs2)`.
1087         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
1088         let rhs = self.lower_expr(rhs);
1089
1090         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
1091         let destructure_let = self.stmt_let_pat(
1092             None,
1093             whole_span,
1094             Some(rhs),
1095             pat,
1096             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
1097         );
1098
1099         // `a = lhs1; b = lhs2;`.
1100         let stmts = self
1101             .arena
1102             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
1103
1104         // Wrap everything in a block.
1105         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
1106     }
1107
1108     /// If the given expression is a path to a tuple struct, returns that path.
1109     /// It is not a complete check, but just tries to reject most paths early
1110     /// if they are not tuple structs.
1111     /// Type checking will take care of the full validation later.
1112     fn extract_tuple_struct_path<'a>(
1113         &mut self,
1114         expr: &'a Expr,
1115     ) -> Option<(&'a Option<AstP<QSelf>>, &'a Path)> {
1116         if let ExprKind::Path(qself, path) = &expr.kind {
1117             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1118             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1119                 if let Some(res) = partial_res.full_res() && !res.expected_in_tuple_struct_pat() {
1120                     return None;
1121                 }
1122             }
1123             return Some((qself, path));
1124         }
1125         None
1126     }
1127
1128     /// If the given expression is a path to a unit struct, returns that path.
1129     /// It is not a complete check, but just tries to reject most paths early
1130     /// if they are not unit structs.
1131     /// Type checking will take care of the full validation later.
1132     fn extract_unit_struct_path<'a>(
1133         &mut self,
1134         expr: &'a Expr,
1135     ) -> Option<(&'a Option<AstP<QSelf>>, &'a Path)> {
1136         if let ExprKind::Path(qself, path) = &expr.kind {
1137             // Does the path resolve to something disallowed in a unit struct/variant pattern?
1138             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1139                 if let Some(res) = partial_res.full_res() && !res.expected_in_unit_struct_pat() {
1140                     return None;
1141                 }
1142             }
1143             return Some((qself, path));
1144         }
1145         None
1146     }
1147
1148     /// Convert the LHS of a destructuring assignment to a pattern.
1149     /// Each sub-assignment is recorded in `assignments`.
1150     fn destructure_assign(
1151         &mut self,
1152         lhs: &Expr,
1153         eq_sign_span: Span,
1154         assignments: &mut Vec<hir::Stmt<'hir>>,
1155     ) -> &'hir hir::Pat<'hir> {
1156         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1157     }
1158
1159     fn destructure_assign_mut(
1160         &mut self,
1161         lhs: &Expr,
1162         eq_sign_span: Span,
1163         assignments: &mut Vec<hir::Stmt<'hir>>,
1164     ) -> hir::Pat<'hir> {
1165         match &lhs.kind {
1166             // Underscore pattern.
1167             ExprKind::Underscore => {
1168                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1169             }
1170             // Slice patterns.
1171             ExprKind::Array(elements) => {
1172                 let (pats, rest) =
1173                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1174                 let slice_pat = if let Some((i, span)) = rest {
1175                     let (before, after) = pats.split_at(i);
1176                     hir::PatKind::Slice(
1177                         before,
1178                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1179                         after,
1180                     )
1181                 } else {
1182                     hir::PatKind::Slice(pats, None, &[])
1183                 };
1184                 return self.pat_without_dbm(lhs.span, slice_pat);
1185             }
1186             // Tuple structs.
1187             ExprKind::Call(callee, args) => {
1188                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1189                     let (pats, rest) = self.destructure_sequence(
1190                         args,
1191                         "tuple struct or variant",
1192                         eq_sign_span,
1193                         assignments,
1194                     );
1195                     let qpath = self.lower_qpath(
1196                         callee.id,
1197                         qself,
1198                         path,
1199                         ParamMode::Optional,
1200                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1201                     );
1202                     // Destructure like a tuple struct.
1203                     let tuple_struct_pat = hir::PatKind::TupleStruct(
1204                         qpath,
1205                         pats,
1206                         hir::DotDotPos::new(rest.map(|r| r.0)),
1207                     );
1208                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1209                 }
1210             }
1211             // Unit structs and enum variants.
1212             ExprKind::Path(..) => {
1213                 if let Some((qself, path)) = self.extract_unit_struct_path(lhs) {
1214                     let qpath = self.lower_qpath(
1215                         lhs.id,
1216                         qself,
1217                         path,
1218                         ParamMode::Optional,
1219                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1220                     );
1221                     // Destructure like a unit struct.
1222                     let unit_struct_pat = hir::PatKind::Path(qpath);
1223                     return self.pat_without_dbm(lhs.span, unit_struct_pat);
1224                 }
1225             }
1226             // Structs.
1227             ExprKind::Struct(se) => {
1228                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1229                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1230                     hir::PatField {
1231                         hir_id: self.next_id(),
1232                         ident: self.lower_ident(f.ident),
1233                         pat,
1234                         is_shorthand: f.is_shorthand,
1235                         span: self.lower_span(f.span),
1236                     }
1237                 }));
1238                 let qpath = self.lower_qpath(
1239                     lhs.id,
1240                     &se.qself,
1241                     &se.path,
1242                     ParamMode::Optional,
1243                     &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1244                 );
1245                 let fields_omitted = match &se.rest {
1246                     StructRest::Base(e) => {
1247                         self.tcx.sess.emit_err(FunctionalRecordUpdateDestructuringAssignemnt {
1248                             span: e.span,
1249                         });
1250                         true
1251                     }
1252                     StructRest::Rest(_) => true,
1253                     StructRest::None => false,
1254                 };
1255                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1256                 return self.pat_without_dbm(lhs.span, struct_pat);
1257             }
1258             // Tuples.
1259             ExprKind::Tup(elements) => {
1260                 let (pats, rest) =
1261                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1262                 let tuple_pat = hir::PatKind::Tuple(pats, hir::DotDotPos::new(rest.map(|r| r.0)));
1263                 return self.pat_without_dbm(lhs.span, tuple_pat);
1264             }
1265             ExprKind::Paren(e) => {
1266                 // We special-case `(..)` for consistency with patterns.
1267                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1268                     let tuple_pat = hir::PatKind::Tuple(&[], hir::DotDotPos::new(Some(0)));
1269                     return self.pat_without_dbm(lhs.span, tuple_pat);
1270                 } else {
1271                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1272                 }
1273             }
1274             _ => {}
1275         }
1276         // Treat all other cases as normal lvalue.
1277         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1278         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1279         let ident = self.expr_ident(lhs.span, ident, binding);
1280         let assign =
1281             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1282         let expr = self.expr(lhs.span, assign);
1283         assignments.push(self.stmt_expr(lhs.span, expr));
1284         pat
1285     }
1286
1287     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1288     /// Such a sequence occurs in a tuple (struct)/slice.
1289     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1290     /// exists.
1291     /// Each sub-assignment is recorded in `assignments`.
1292     fn destructure_sequence(
1293         &mut self,
1294         elements: &[AstP<Expr>],
1295         ctx: &str,
1296         eq_sign_span: Span,
1297         assignments: &mut Vec<hir::Stmt<'hir>>,
1298     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1299         let mut rest = None;
1300         let elements =
1301             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1302                 // Check for `..` pattern.
1303                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1304                     if let Some((_, prev_span)) = rest {
1305                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1306                     } else {
1307                         rest = Some((i, e.span));
1308                     }
1309                     None
1310                 } else {
1311                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1312                 }
1313             }));
1314         (elements, rest)
1315     }
1316
1317     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1318     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1319         let e1 = self.lower_expr_mut(e1);
1320         let e2 = self.lower_expr_mut(e2);
1321         let fn_path =
1322             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1323         let fn_expr = self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path)));
1324         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1325     }
1326
1327     fn lower_expr_range(
1328         &mut self,
1329         span: Span,
1330         e1: Option<&Expr>,
1331         e2: Option<&Expr>,
1332         lims: RangeLimits,
1333     ) -> hir::ExprKind<'hir> {
1334         use rustc_ast::RangeLimits::*;
1335
1336         let lang_item = match (e1, e2, lims) {
1337             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1338             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1339             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1340             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1341             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1342             (Some(..), Some(..), Closed) => unreachable!(),
1343             (start, None, Closed) => {
1344                 self.tcx.sess.emit_err(InclusiveRangeWithNoEnd { span });
1345                 match start {
1346                     Some(..) => hir::LangItem::RangeFrom,
1347                     None => hir::LangItem::RangeFull,
1348                 }
1349             }
1350         };
1351
1352         let fields = self.arena.alloc_from_iter(
1353             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1354                 |(s, e)| {
1355                     let expr = self.lower_expr(&e);
1356                     let ident = Ident::new(s, self.lower_span(e.span));
1357                     self.expr_field(ident, expr, e.span)
1358                 },
1359             ),
1360         );
1361
1362         hir::ExprKind::Struct(
1363             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1364             fields,
1365             None,
1366         )
1367     }
1368
1369     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1370         let label = opt_label?;
1371         Some(Label { ident: self.lower_ident(label.ident) })
1372     }
1373
1374     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1375         let target_id = match destination {
1376             Some((id, _)) => {
1377                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1378                     Ok(self.lower_node_id(loop_id))
1379                 } else {
1380                     Err(hir::LoopIdError::UnresolvedLabel)
1381                 }
1382             }
1383             None => self
1384                 .loop_scope
1385                 .map(|id| Ok(self.lower_node_id(id)))
1386                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1387         };
1388         let label = self.lower_label(destination.map(|(_, label)| label));
1389         hir::Destination { label, target_id }
1390     }
1391
1392     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1393         if self.is_in_loop_condition && opt_label.is_none() {
1394             hir::Destination {
1395                 label: None,
1396                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1397             }
1398         } else {
1399             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1400         }
1401     }
1402
1403     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1404         let old_scope = self.catch_scope.replace(catch_id);
1405         let result = f(self);
1406         self.catch_scope = old_scope;
1407         result
1408     }
1409
1410     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1411         // We're no longer in the base loop's condition; we're in another loop.
1412         let was_in_loop_condition = self.is_in_loop_condition;
1413         self.is_in_loop_condition = false;
1414
1415         let old_scope = self.loop_scope.replace(loop_id);
1416         let result = f(self);
1417         self.loop_scope = old_scope;
1418
1419         self.is_in_loop_condition = was_in_loop_condition;
1420
1421         result
1422     }
1423
1424     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1425         let was_in_loop_condition = self.is_in_loop_condition;
1426         self.is_in_loop_condition = true;
1427
1428         let result = f(self);
1429
1430         self.is_in_loop_condition = was_in_loop_condition;
1431
1432         result
1433     }
1434
1435     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1436         let hir_id = self.lower_node_id(f.id);
1437         self.lower_attrs(hir_id, &f.attrs);
1438         hir::ExprField {
1439             hir_id,
1440             ident: self.lower_ident(f.ident),
1441             expr: self.lower_expr(&f.expr),
1442             span: self.lower_span(f.span),
1443             is_shorthand: f.is_shorthand,
1444         }
1445     }
1446
1447     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1448         match self.generator_kind {
1449             Some(hir::GeneratorKind::Gen) => {}
1450             Some(hir::GeneratorKind::Async(_)) => {
1451                 self.tcx.sess.emit_err(AsyncGeneratorsNotSupported { span });
1452             }
1453             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1454         }
1455
1456         let expr =
1457             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1458
1459         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1460     }
1461
1462     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1463     /// ```ignore (pseudo-rust)
1464     /// {
1465     ///     let result = match IntoIterator::into_iter(<head>) {
1466     ///         mut iter => {
1467     ///             [opt_ident]: loop {
1468     ///                 match Iterator::next(&mut iter) {
1469     ///                     None => break,
1470     ///                     Some(<pat>) => <body>,
1471     ///                 };
1472     ///             }
1473     ///         }
1474     ///     };
1475     ///     result
1476     /// }
1477     /// ```
1478     fn lower_expr_for(
1479         &mut self,
1480         e: &Expr,
1481         pat: &Pat,
1482         head: &Expr,
1483         body: &Block,
1484         opt_label: Option<Label>,
1485     ) -> hir::Expr<'hir> {
1486         let head = self.lower_expr_mut(head);
1487         let pat = self.lower_pat(pat);
1488         let for_span =
1489             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1490         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1491         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1492
1493         // `None => break`
1494         let none_arm = {
1495             let break_expr = self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span));
1496             let pat = self.pat_none(for_span);
1497             self.arm(pat, break_expr)
1498         };
1499
1500         // Some(<pat>) => <body>,
1501         let some_arm = {
1502             let some_pat = self.pat_some(pat_span, pat);
1503             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1504             let body_expr = self.arena.alloc(self.expr_block(body_block));
1505             self.arm(some_pat, body_expr)
1506         };
1507
1508         // `mut iter`
1509         let iter = Ident::with_dummy_span(sym::iter);
1510         let (iter_pat, iter_pat_nid) =
1511             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::MUT);
1512
1513         // `match Iterator::next(&mut iter) { ... }`
1514         let match_expr = {
1515             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1516             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1517             let next_expr = self.expr_call_lang_item_fn(
1518                 head_span,
1519                 hir::LangItem::IteratorNext,
1520                 arena_vec![self; ref_mut_iter],
1521                 None,
1522             );
1523             let arms = arena_vec![self; none_arm, some_arm];
1524
1525             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1526         };
1527         let match_stmt = self.stmt_expr(for_span, match_expr);
1528
1529         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1530
1531         // `[opt_ident]: loop { ... }`
1532         let kind = hir::ExprKind::Loop(
1533             loop_block,
1534             self.lower_label(opt_label),
1535             hir::LoopSource::ForLoop,
1536             self.lower_span(for_span.with_hi(head.span.hi())),
1537         );
1538         let loop_expr =
1539             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1540
1541         // `mut iter => { ... }`
1542         let iter_arm = self.arm(iter_pat, loop_expr);
1543
1544         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1545         let into_iter_expr = {
1546             self.expr_call_lang_item_fn(
1547                 head_span,
1548                 hir::LangItem::IntoIterIntoIter,
1549                 arena_vec![self; head],
1550                 None,
1551             )
1552         };
1553
1554         let match_expr = self.arena.alloc(self.expr_match(
1555             for_span,
1556             into_iter_expr,
1557             arena_vec![self; iter_arm],
1558             hir::MatchSource::ForLoopDesugar,
1559         ));
1560
1561         // This is effectively `{ let _result = ...; _result }`.
1562         // The construct was introduced in #21984 and is necessary to make sure that
1563         // temporaries in the `head` expression are dropped and do not leak to the
1564         // surrounding scope of the `match` since the `match` is not a terminating scope.
1565         //
1566         // Also, add the attributes to the outer returned expr node.
1567         let expr = self.expr_drop_temps_mut(for_span, match_expr);
1568         self.lower_attrs(expr.hir_id, &e.attrs);
1569         expr
1570     }
1571
1572     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1573     /// ```ignore (pseudo-rust)
1574     /// match Try::branch(<expr>) {
1575     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1576     ///     ControlFlow::Break(residual) =>
1577     ///         #[allow(unreachable_code)]
1578     ///         // If there is an enclosing `try {...}`:
1579     ///         break 'catch_target Try::from_residual(residual),
1580     ///         // Otherwise:
1581     ///         return Try::from_residual(residual),
1582     /// }
1583     /// ```
1584     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1585         let unstable_span = self.mark_span_with_reason(
1586             DesugaringKind::QuestionMark,
1587             span,
1588             self.allow_try_trait.clone(),
1589         );
1590         let try_span = self.tcx.sess.source_map().end_point(span);
1591         let try_span = self.mark_span_with_reason(
1592             DesugaringKind::QuestionMark,
1593             try_span,
1594             self.allow_try_trait.clone(),
1595         );
1596
1597         // `Try::branch(<expr>)`
1598         let scrutinee = {
1599             // expand <expr>
1600             let sub_expr = self.lower_expr_mut(sub_expr);
1601
1602             self.expr_call_lang_item_fn(
1603                 unstable_span,
1604                 hir::LangItem::TryTraitBranch,
1605                 arena_vec![self; sub_expr],
1606                 None,
1607             )
1608         };
1609
1610         // `#[allow(unreachable_code)]`
1611         let attr = attr::mk_attr_nested_word(
1612             &self.tcx.sess.parse_sess.attr_id_generator,
1613             AttrStyle::Outer,
1614             sym::allow,
1615             sym::unreachable_code,
1616             self.lower_span(span),
1617         );
1618         let attrs: AttrVec = thin_vec![attr];
1619
1620         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1621         let continue_arm = {
1622             let val_ident = Ident::with_dummy_span(sym::val);
1623             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1624             let val_expr = self.expr_ident(span, val_ident, val_pat_nid);
1625             self.lower_attrs(val_expr.hir_id, &attrs);
1626             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1627             self.arm(continue_pat, val_expr)
1628         };
1629
1630         // `ControlFlow::Break(residual) =>
1631         //     #[allow(unreachable_code)]
1632         //     return Try::from_residual(residual),`
1633         let break_arm = {
1634             let residual_ident = Ident::with_dummy_span(sym::residual);
1635             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1636             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1637             let from_residual_expr = self.wrap_in_try_constructor(
1638                 hir::LangItem::TryTraitFromResidual,
1639                 try_span,
1640                 self.arena.alloc(residual_expr),
1641                 unstable_span,
1642             );
1643             let ret_expr = if let Some(catch_node) = self.catch_scope {
1644                 let target_id = Ok(self.lower_node_id(catch_node));
1645                 self.arena.alloc(self.expr(
1646                     try_span,
1647                     hir::ExprKind::Break(
1648                         hir::Destination { label: None, target_id },
1649                         Some(from_residual_expr),
1650                     ),
1651                 ))
1652             } else {
1653                 self.arena.alloc(self.expr(try_span, hir::ExprKind::Ret(Some(from_residual_expr))))
1654             };
1655             self.lower_attrs(ret_expr.hir_id, &attrs);
1656
1657             let break_pat = self.pat_cf_break(try_span, residual_local);
1658             self.arm(break_pat, ret_expr)
1659         };
1660
1661         hir::ExprKind::Match(
1662             scrutinee,
1663             arena_vec![self; break_arm, continue_arm],
1664             hir::MatchSource::TryDesugar,
1665         )
1666     }
1667
1668     /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
1669     /// ```ignore(illustrative)
1670     /// // If there is an enclosing `try {...}`:
1671     /// break 'catch_target FromResidual::from_residual(Yeet(residual));
1672     /// // Otherwise:
1673     /// return FromResidual::from_residual(Yeet(residual));
1674     /// ```
1675     /// But to simplify this, there's a `from_yeet` lang item function which
1676     /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
1677     fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1678         // The expression (if present) or `()` otherwise.
1679         let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
1680             (sub_expr.span, self.lower_expr(sub_expr))
1681         } else {
1682             (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
1683         };
1684
1685         let unstable_span = self.mark_span_with_reason(
1686             DesugaringKind::YeetExpr,
1687             span,
1688             self.allow_try_trait.clone(),
1689         );
1690
1691         let from_yeet_expr = self.wrap_in_try_constructor(
1692             hir::LangItem::TryTraitFromYeet,
1693             unstable_span,
1694             yeeted_expr,
1695             yeeted_span,
1696         );
1697
1698         if let Some(catch_node) = self.catch_scope {
1699             let target_id = Ok(self.lower_node_id(catch_node));
1700             hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
1701         } else {
1702             hir::ExprKind::Ret(Some(from_yeet_expr))
1703         }
1704     }
1705
1706     // =========================================================================
1707     // Helper methods for building HIR.
1708     // =========================================================================
1709
1710     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1711     ///
1712     /// In terms of drop order, it has the same effect as wrapping `expr` in
1713     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1714     ///
1715     /// The drop order can be important in e.g. `if expr { .. }`.
1716     pub(super) fn expr_drop_temps(
1717         &mut self,
1718         span: Span,
1719         expr: &'hir hir::Expr<'hir>,
1720     ) -> &'hir hir::Expr<'hir> {
1721         self.arena.alloc(self.expr_drop_temps_mut(span, expr))
1722     }
1723
1724     pub(super) fn expr_drop_temps_mut(
1725         &mut self,
1726         span: Span,
1727         expr: &'hir hir::Expr<'hir>,
1728     ) -> hir::Expr<'hir> {
1729         self.expr(span, hir::ExprKind::DropTemps(expr))
1730     }
1731
1732     fn expr_match(
1733         &mut self,
1734         span: Span,
1735         arg: &'hir hir::Expr<'hir>,
1736         arms: &'hir [hir::Arm<'hir>],
1737         source: hir::MatchSource,
1738     ) -> hir::Expr<'hir> {
1739         self.expr(span, hir::ExprKind::Match(arg, arms, source))
1740     }
1741
1742     fn expr_break(&mut self, span: Span) -> hir::Expr<'hir> {
1743         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1744         self.expr(span, expr_break)
1745     }
1746
1747     fn expr_break_alloc(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1748         let expr_break = self.expr_break(span);
1749         self.arena.alloc(expr_break)
1750     }
1751
1752     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1753         self.expr(span, hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e))
1754     }
1755
1756     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1757         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[])))
1758     }
1759
1760     fn expr_call_mut(
1761         &mut self,
1762         span: Span,
1763         e: &'hir hir::Expr<'hir>,
1764         args: &'hir [hir::Expr<'hir>],
1765     ) -> hir::Expr<'hir> {
1766         self.expr(span, hir::ExprKind::Call(e, args))
1767     }
1768
1769     fn expr_call(
1770         &mut self,
1771         span: Span,
1772         e: &'hir hir::Expr<'hir>,
1773         args: &'hir [hir::Expr<'hir>],
1774     ) -> &'hir hir::Expr<'hir> {
1775         self.arena.alloc(self.expr_call_mut(span, e, args))
1776     }
1777
1778     fn expr_call_lang_item_fn_mut(
1779         &mut self,
1780         span: Span,
1781         lang_item: hir::LangItem,
1782         args: &'hir [hir::Expr<'hir>],
1783         hir_id: Option<hir::HirId>,
1784     ) -> hir::Expr<'hir> {
1785         let path = self.arena.alloc(self.expr_lang_item_path(span, lang_item, hir_id));
1786         self.expr_call_mut(span, path, args)
1787     }
1788
1789     fn expr_call_lang_item_fn(
1790         &mut self,
1791         span: Span,
1792         lang_item: hir::LangItem,
1793         args: &'hir [hir::Expr<'hir>],
1794         hir_id: Option<hir::HirId>,
1795     ) -> &'hir hir::Expr<'hir> {
1796         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1797     }
1798
1799     fn expr_lang_item_path(
1800         &mut self,
1801         span: Span,
1802         lang_item: hir::LangItem,
1803         hir_id: Option<hir::HirId>,
1804     ) -> hir::Expr<'hir> {
1805         self.expr(
1806             span,
1807             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1808         )
1809     }
1810
1811     pub(super) fn expr_ident(
1812         &mut self,
1813         sp: Span,
1814         ident: Ident,
1815         binding: hir::HirId,
1816     ) -> &'hir hir::Expr<'hir> {
1817         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1818     }
1819
1820     pub(super) fn expr_ident_mut(
1821         &mut self,
1822         span: Span,
1823         ident: Ident,
1824         binding: hir::HirId,
1825     ) -> hir::Expr<'hir> {
1826         let hir_id = self.next_id();
1827         let res = Res::Local(binding);
1828         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1829             None,
1830             self.arena.alloc(hir::Path {
1831                 span: self.lower_span(span),
1832                 res,
1833                 segments: arena_vec![self; hir::PathSegment::new(ident, hir_id, res)],
1834             }),
1835         ));
1836
1837         self.expr(span, expr_path)
1838     }
1839
1840     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1841         let hir_id = self.next_id();
1842         let span = expr.span;
1843         self.expr(
1844             span,
1845             hir::ExprKind::Block(
1846                 self.arena.alloc(hir::Block {
1847                     stmts: &[],
1848                     expr: Some(expr),
1849                     hir_id,
1850                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1851                     span: self.lower_span(span),
1852                     targeted_by_break: false,
1853                 }),
1854                 None,
1855             ),
1856         )
1857     }
1858
1859     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1860         let blk = self.block_all(span, &[], None);
1861         let expr = self.expr_block(blk);
1862         self.arena.alloc(expr)
1863     }
1864
1865     pub(super) fn expr_block(&mut self, b: &'hir hir::Block<'hir>) -> hir::Expr<'hir> {
1866         self.expr(b.span, hir::ExprKind::Block(b, None))
1867     }
1868
1869     pub(super) fn expr(&mut self, span: Span, kind: hir::ExprKind<'hir>) -> hir::Expr<'hir> {
1870         let hir_id = self.next_id();
1871         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1872     }
1873
1874     fn expr_field(
1875         &mut self,
1876         ident: Ident,
1877         expr: &'hir hir::Expr<'hir>,
1878         span: Span,
1879     ) -> hir::ExprField<'hir> {
1880         hir::ExprField {
1881             hir_id: self.next_id(),
1882             ident,
1883             span: self.lower_span(span),
1884             expr,
1885             is_shorthand: false,
1886         }
1887     }
1888
1889     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1890         hir::Arm {
1891             hir_id: self.next_id(),
1892             pat,
1893             guard: None,
1894             span: self.lower_span(expr.span),
1895             body: expr,
1896         }
1897     }
1898 }