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                     constness,
 213                     asyncness,
 214                     movability,
 215                     fn_decl,
 216                     body,
 217                     fn_decl_span,
 218                     fn_arg_span,
 219                 }) => {
 220                     if let Async::Yes { closure_id, .. } = asyncness {
 221                         self.lower_expr_async_closure(
 222                             binder,
 223                             *capture_clause,
 224                             e.id,
 225                             hir_id,
 226                             *closure_id,
 227                             fn_decl,
 228                             body,
 229                             *fn_decl_span,
 230                             *fn_arg_span,
 231                         )
 232                     } else {
 233                         self.lower_expr_closure(
 234                             binder,
 235                             *capture_clause,
 236                             e.id,
 237                             *constness,
 238                             *movability,
 239                             fn_decl,
 240                             body,
 241                             *fn_decl_span,
 242                             *fn_arg_span,
 243                         )
 244                     }
 245                 }
 246                 ExprKind::Block(blk, opt_label) => {
 247                     let opt_label = self.lower_label(*opt_label);
 248                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 249                 }
 250                 ExprKind::Assign(el, er, span) => self.lower_expr_assign(el, er, *span, e.span),
 251                 ExprKind::AssignOp(op, el, er) => hir::ExprKind::AssignOp(
 252                     self.lower_binop(*op),
 253                     self.lower_expr(el),
 254                     self.lower_expr(er),
 255                 ),
 256                 ExprKind::Field(el, ident) => {
 257                     hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(*ident))
 258                 }
 259                 ExprKind::Index(el, er) => {
 260                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 261                 }
 262                 ExprKind::Range(Some(e1), Some(e2), RangeLimits::Closed) => {
 263                     self.lower_expr_range_closed(e.span, e1, e2)
 264                 }
 265                 ExprKind::Range(e1, e2, lims) => {
 266                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), *lims)
 267                 }
 268                 ExprKind::Underscore => {
 269                     self.tcx.sess.emit_err(UnderscoreExprLhsAssign { span: e.span });
 270                     hir::ExprKind::Err
 271                 }
 272                 ExprKind::Path(qself, path) => {
 273                     let qpath = self.lower_qpath(
 274                         e.id,
 275                         qself,
 276                         path,
 277                         ParamMode::Optional,
 278                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 279                     );
 280                     hir::ExprKind::Path(qpath)
 281                 }
 282                 ExprKind::Break(opt_label, opt_expr) => {
 283                     let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 284                     hir::ExprKind::Break(self.lower_jump_destination(e.id, *opt_label), opt_expr)
 285                 }
 286                 ExprKind::Continue(opt_label) => {
 287                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, *opt_label))
 288                 }
 289                 ExprKind::Ret(e) => {
 290                     let e = e.as_ref().map(|x| self.lower_expr(x));
 291                     hir::ExprKind::Ret(e)
 292                 }
 293                 ExprKind::Yeet(sub_expr) => self.lower_expr_yeet(e.span, sub_expr.as_deref()),
 294                 ExprKind::InlineAsm(asm) => {
 295                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
 296                 }
 297                 ExprKind::Struct(se) => {
 298                     let rest = match &se.rest {
 299                         StructRest::Base(e) => Some(self.lower_expr(e)),
 300                         StructRest::Rest(sp) => {
 301                             self.tcx.sess.emit_err(BaseExpressionDoubleDot { span: *sp });
 302                             Some(&*self.arena.alloc(self.expr_err(*sp)))
 303                         }
 304                         StructRest::None => None,
 305                     };
 306                     hir::ExprKind::Struct(
 307                         self.arena.alloc(self.lower_qpath(
 308                             e.id,
 309                             &se.qself,
 310                             &se.path,
 311                             ParamMode::Optional,
 312                             &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 313                         )),
 314                         self.arena
 315                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
 316                         rest,
 317                     )
 318                 }
 319                 ExprKind::Yield(opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 320                 ExprKind::Err => hir::ExprKind::Err,
 321                 ExprKind::Try(sub_expr) => self.lower_expr_try(e.span, sub_expr),
 322
 323                 ExprKind::Paren(_) | ExprKind::ForLoop(..) => unreachable!("already handled"),
 324
 325                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 326             };
 327
 328             hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
 329         })
 330     }
 331
 332     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 333         match u {
 334             UnOp::Deref => hir::UnOp::Deref,
 335             UnOp::Not => hir::UnOp::Not,
 336             UnOp::Neg => hir::UnOp::Neg,
 337         }
 338     }
 339
 340     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 341         Spanned {
 342             node: match b.node {
 343                 BinOpKind::Add => hir::BinOpKind::Add,
 344                 BinOpKind::Sub => hir::BinOpKind::Sub,
 345                 BinOpKind::Mul => hir::BinOpKind::Mul,
 346                 BinOpKind::Div => hir::BinOpKind::Div,
 347                 BinOpKind::Rem => hir::BinOpKind::Rem,
 348                 BinOpKind::And => hir::BinOpKind::And,
 349                 BinOpKind::Or => hir::BinOpKind::Or,
 350                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 351                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 352                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 353                 BinOpKind::Shl => hir::BinOpKind::Shl,
 354                 BinOpKind::Shr => hir::BinOpKind::Shr,
 355                 BinOpKind::Eq => hir::BinOpKind::Eq,
 356                 BinOpKind::Lt => hir::BinOpKind::Lt,
 357                 BinOpKind::Le => hir::BinOpKind::Le,
 358                 BinOpKind::Ne => hir::BinOpKind::Ne,
 359                 BinOpKind::Ge => hir::BinOpKind::Ge,
 360                 BinOpKind::Gt => hir::BinOpKind::Gt,
 361             },
 362             span: self.lower_span(b.span),
 363         }
 364     }
 365
 366     fn lower_legacy_const_generics(
 367         &mut self,
 368         mut f: Expr,
 369         args: Vec<AstP<Expr>>,
 370         legacy_args_idx: &[usize],
 371     ) -> hir::ExprKind<'hir> {
 372         let ExprKind::Path(None, path) = &mut f.kind else {
 373             unreachable!();
 374         };
 375
 376         // Split the arguments into const generics and normal arguments
 377         let mut real_args = vec![];
 378         let mut generic_args = vec![];
 379         for (idx, arg) in args.into_iter().enumerate() {
 380             if legacy_args_idx.contains(&idx) {
 381                 let parent_def_id = self.current_hir_id_owner;
 382                 let node_id = self.next_node_id();
 383
 384                 // Add a definition for the in-band const def.
 385                 self.create_def(parent_def_id.def_id, node_id, DefPathData::AnonConst, f.span);
 386
 387                 let anon_const = AnonConst { id: node_id, value: arg };
 388                 generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
 389             } else {
 390                 real_args.push(arg);
 391             }
 392         }
 393
 394         // Add generic args to the last element of the path.
 395         let last_segment = path.segments.last_mut().unwrap();
 396         assert!(last_segment.args.is_none());
 397         last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
 398             span: DUMMY_SP,
 399             args: generic_args,
 400         })));
 401
 402         // Now lower everything as normal.
 403         let f = self.lower_expr(&f);
 404         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
 405     }
 406
 407     fn lower_expr_if(
 408         &mut self,
 409         cond: &Expr,
 410         then: &Block,
 411         else_opt: Option<&Expr>,
 412     ) -> hir::ExprKind<'hir> {
 413         let lowered_cond = self.lower_cond(cond);
 414         let then_expr = self.lower_block_expr(then);
 415         if let Some(rslt) = else_opt {
 416             hir::ExprKind::If(
 417                 lowered_cond,
 418                 self.arena.alloc(then_expr),
 419                 Some(self.lower_expr(rslt)),
 420             )
 421         } else {
 422             hir::ExprKind::If(lowered_cond, self.arena.alloc(then_expr), None)
 423         }
 424     }
 425
 426     // Lowers a condition (i.e. `cond` in `if cond` or `while cond`), wrapping it in a terminating scope
 427     // so that temporaries created in the condition don't live beyond it.
 428     fn lower_cond(&mut self, cond: &Expr) -> &'hir hir::Expr<'hir> {
 429         fn has_let_expr(expr: &Expr) -> bool {
 430             match &expr.kind {
 431                 ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
 432                 ExprKind::Let(..) => true,
 433                 _ => false,
 434             }
 435         }
 436
 437         // We have to take special care for `let` exprs in the condition, e.g. in
 438         // `if let pat = val` or `if foo && let pat = val`, as we _do_ want `val` to live beyond the
 439         // condition in this case.
 440         //
 441         // In order to mantain the drop behavior for the non `let` parts of the condition,
 442         // we still wrap them in terminating scopes, e.g. `if foo && let pat = val` essentially
 443         // gets transformed into `if { let _t = foo; _t } && let pat = val`
 444         match &cond.kind {
 445             ExprKind::Binary(op @ Spanned { node: ast::BinOpKind::And, .. }, lhs, rhs)
 446                 if has_let_expr(cond) =>
 447             {
 448                 let op = self.lower_binop(*op);
 449                 let lhs = self.lower_cond(lhs);
 450                 let rhs = self.lower_cond(rhs);
 451
 452                 self.arena.alloc(self.expr(cond.span, hir::ExprKind::Binary(op, lhs, rhs)))
 453             }
 454             ExprKind::Let(..) => self.lower_expr(cond),
 455             _ => {
 456                 let cond = self.lower_expr(cond);
 457                 let reason = DesugaringKind::CondTemporary;
 458                 let span_block = self.mark_span_with_reason(reason, cond.span, None);
 459                 self.expr_drop_temps(span_block, cond)
 460             }
 461         }
 462     }
 463
 464     // We desugar: `'label: while $cond $body` into:
 465     //
 466     // ```
 467     // 'label: loop {
 468     //   if { let _t = $cond; _t } {
 469     //     $body
 470     //   }
 471     //   else {
 472     //     break;
 473     //   }
 474     // }
 475     // ```
 476     //
 477     // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 478     // to preserve drop semantics since `while $cond { ... }` does not
 479     // let temporaries live outside of `cond`.
 480     fn lower_expr_while_in_loop_scope(
 481         &mut self,
 482         span: Span,
 483         cond: &Expr,
 484         body: &Block,
 485         opt_label: Option<Label>,
 486     ) -> hir::ExprKind<'hir> {
 487         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_cond(cond));
 488         let then = self.lower_block_expr(body);
 489         let expr_break = self.expr_break(span);
 490         let stmt_break = self.stmt_expr(span, expr_break);
 491         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
 492         let else_expr = self.arena.alloc(self.expr_block(else_blk));
 493         let if_kind = hir::ExprKind::If(lowered_cond, self.arena.alloc(then), Some(else_expr));
 494         let if_expr = self.expr(span, if_kind);
 495         let block = self.block_expr(self.arena.alloc(if_expr));
 496         let span = self.lower_span(span.with_hi(cond.span.hi()));
 497         let opt_label = self.lower_label(opt_label);
 498         hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
 499     }
 500
 501     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
 502     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
 503     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 504     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 505         self.with_catch_scope(body.id, |this| {
 506             let mut block = this.lower_block_noalloc(body, true);
 507
 508             // Final expression of the block (if present) or `()` with span at the end of block
 509             let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
 510                 (
 511                     this.mark_span_with_reason(
 512                         DesugaringKind::TryBlock,
 513                         expr.span,
 514                         this.allow_try_trait.clone(),
 515                     ),
 516                     expr,
 517                 )
 518             } else {
 519                 let try_span = this.mark_span_with_reason(
 520                     DesugaringKind::TryBlock,
 521                     this.tcx.sess.source_map().end_point(body.span),
 522                     this.allow_try_trait.clone(),
 523                 );
 524
 525                 (try_span, this.expr_unit(try_span))
 526             };
 527
 528             let ok_wrapped_span =
 529                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 530
 531             // `::std::ops::Try::from_output($tail_expr)`
 532             block.expr = Some(this.wrap_in_try_constructor(
 533                 hir::LangItem::TryTraitFromOutput,
 534                 try_span,
 535                 tail_expr,
 536                 ok_wrapped_span,
 537             ));
 538
 539             hir::ExprKind::Block(this.arena.alloc(block), None)
 540         })
 541     }
 542
 543     fn wrap_in_try_constructor(
 544         &mut self,
 545         lang_item: hir::LangItem,
 546         method_span: Span,
 547         expr: &'hir hir::Expr<'hir>,
 548         overall_span: Span,
 549     ) -> &'hir hir::Expr<'hir> {
 550         let constructor = self.arena.alloc(self.expr_lang_item_path(method_span, lang_item, None));
 551         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 552     }
 553
 554     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 555         let pat = self.lower_pat(&arm.pat);
 556         let guard = arm.guard.as_ref().map(|cond| {
 557             if let ExprKind::Let(pat, scrutinee, span) = &cond.kind {
 558                 hir::Guard::IfLet(self.arena.alloc(hir::Let {
 559                     hir_id: self.next_id(),
 560                     span: self.lower_span(*span),
 561                     pat: self.lower_pat(pat),
 562                     ty: None,
 563                     init: self.lower_expr(scrutinee),
 564                 }))
 565             } else {
 566                 hir::Guard::If(self.lower_expr(cond))
 567             }
 568         });
 569         let hir_id = self.next_id();
 570         self.lower_attrs(hir_id, &arm.attrs);
 571         hir::Arm {
 572             hir_id,
 573             pat,
 574             guard,
 575             body: self.lower_expr(&arm.body),
 576             span: self.lower_span(arm.span),
 577         }
 578     }
 579
 580     /// Lower an `async` construct to a generator that implements `Future`.
 581     ///
 582     /// This results in:
 583     ///
 584     /// ```text
 585     /// std::future::identity_future(static move? |_task_context| -> <ret_ty> {
 586     ///     <body>
 587     /// })
 588     /// ```
 589     pub(super) fn make_async_expr(
 590         &mut self,
 591         capture_clause: CaptureBy,
 592         outer_hir_id: hir::HirId,
 593         closure_node_id: NodeId,
 594         ret_ty: Option<hir::FnRetTy<'hir>>,
 595         span: Span,
 596         async_gen_kind: hir::AsyncGeneratorKind,
 597         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 598     ) -> hir::ExprKind<'hir> {
 599         let output = ret_ty.unwrap_or_else(|| hir::FnRetTy::DefaultReturn(self.lower_span(span)));
 600
 601         // Resume argument type: `ResumeTy`
 602         let unstable_span =
 603             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 604         let resume_ty = hir::QPath::LangItem(hir::LangItem::ResumeTy, unstable_span, None);
 605         let input_ty = hir::Ty {
 606             hir_id: self.next_id(),
 607             kind: hir::TyKind::Path(resume_ty),
 608             span: unstable_span,
 609         };
 610
 611         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 612         let fn_decl = self.arena.alloc(hir::FnDecl {
 613             inputs: arena_vec![self; input_ty],
 614             output,
 615             c_variadic: false,
 616             implicit_self: hir::ImplicitSelfKind::None,
 617             lifetime_elision_allowed: false,
 618         });
 619
 620         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 621         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 622             span,
 623             Ident::with_dummy_span(sym::_task_context),
 624             hir::BindingAnnotation::MUT,
 625         );
 626         let param = hir::Param {
 627             hir_id: self.next_id(),
 628             pat,
 629             ty_span: self.lower_span(span),
 630             span: self.lower_span(span),
 631         };
 632         let params = arena_vec![self; param];
 633
 634         let body = self.lower_body(move |this| {
 635             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 636
 637             let old_ctx = this.task_context;
 638             this.task_context = Some(task_context_hid);
 639             let res = body(this);
 640             this.task_context = old_ctx;
 641             (params, res)
 642         });
 643
 644         // `static |_task_context| -> <ret_ty> { body }`:
 645         let generator_kind = {
 646             let c = self.arena.alloc(hir::Closure {
 647                 def_id: self.local_def_id(closure_node_id),
 648                 binder: hir::ClosureBinder::Default,
 649                 capture_clause,
 650                 bound_generic_params: &[],
 651                 fn_decl,
 652                 body,
 653                 fn_decl_span: self.lower_span(span),
 654                 fn_arg_span: None,
 655                 movability: Some(hir::Movability::Static),
 656                 constness: hir::Constness::NotConst,
 657             });
 658
 659             hir::ExprKind::Closure(c)
 660         };
 661
 662         let hir_id = self.lower_node_id(closure_node_id);
 663         let unstable_span =
 664             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 665
 666         if self.tcx.features().closure_track_caller
 667             && let Some(attrs) = self.attrs.get(&outer_hir_id.local_id)
 668             && attrs.into_iter().any(|attr| attr.has_name(sym::track_caller))
 669         {
 670             self.lower_attrs(
 671                 hir_id,
 672                 &[Attribute {
 673                     kind: AttrKind::Normal(ptr::P(NormalAttr {
 674                         item: AttrItem {
 675                             path: Path::from_ident(Ident::new(sym::track_caller, span)),
 676                             args: AttrArgs::Empty,
 677                             tokens: None,
 678                         },
 679                         tokens: None,
 680                     })),
 681                     id: self.tcx.sess.parse_sess.attr_id_generator.mk_attr_id(),
 682                     style: AttrStyle::Outer,
 683                     span: unstable_span,
 684                 }],
 685             );
 686         }
 687
 688         let generator = hir::Expr { hir_id, kind: generator_kind, span: self.lower_span(span) };
 689
 690         // FIXME(swatinem):
 691         // For some reason, the async block needs to flow through *any*
 692         // call (like the identity function), as otherwise type and lifetime
 693         // inference have a hard time figuring things out.
 694         // Without this, we would get:
 695         // E0720 in tests/ui/impl-trait/in-trait/default-body-with-rpit.rs
 696         // E0700 in tests/ui/self/self_lifetime-async.rs
 697
 698         // `future::identity_future`:
 699         let identity_future =
 700             self.expr_lang_item_path(unstable_span, hir::LangItem::IdentityFuture, None);
 701
 702         // `future::identity_future(generator)`:
 703         hir::ExprKind::Call(self.arena.alloc(identity_future), arena_vec![self; generator])
 704     }
 705
 706     /// Desugar `<expr>.await` into:
 707     /// ```ignore (pseudo-rust)
 708     /// match ::std::future::IntoFuture::into_future(<expr>) {
 709     ///     mut __awaitee => loop {
 710     ///         match unsafe { ::std::future::Future::poll(
 711     ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
 712     ///             ::std::future::get_context(task_context),
 713     ///         ) } {
 714     ///             ::std::task::Poll::Ready(result) => break result,
 715     ///             ::std::task::Poll::Pending => {}
 716     ///         }
 717     ///         task_context = yield ();
 718     ///     }
 719     /// }
 720     /// ```
 721     fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 722         let full_span = expr.span.to(dot_await_span);
 723         match self.generator_kind {
 724             Some(hir::GeneratorKind::Async(_)) => {}
 725             Some(hir::GeneratorKind::Gen) | None => {
 726                 self.tcx.sess.emit_err(AwaitOnlyInAsyncFnAndBlocks {
 727                     dot_await_span,
 728                     item_span: self.current_item,
 729                 });
 730             }
 731         }
 732         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
 733         let gen_future_span = self.mark_span_with_reason(
 734             DesugaringKind::Await,
 735             full_span,
 736             self.allow_gen_future.clone(),
 737         );
 738         let expr = self.lower_expr_mut(expr);
 739         let expr_hir_id = expr.hir_id;
 740
 741         // Note that the name of this binding must not be changed to something else because
 742         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
 743         // this name to identify what is being awaited by a suspended async functions.
 744         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
 745         let (awaitee_pat, awaitee_pat_hid) =
 746             self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::MUT);
 747
 748         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 749
 750         // unsafe {
 751         //     ::std::future::Future::poll(
 752         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
 753         //         ::std::future::get_context(task_context),
 754         //     )
 755         // }
 756         let poll_expr = {
 757             let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
 758             let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
 759             let task_context = if let Some(task_context_hid) = self.task_context {
 760                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 761             } else {
 762                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 763                 self.expr_err(span)
 764             };
 765             let new_unchecked = self.expr_call_lang_item_fn_mut(
 766                 span,
 767                 hir::LangItem::PinNewUnchecked,
 768                 arena_vec![self; ref_mut_awaitee],
 769                 Some(expr_hir_id),
 770             );
 771             let get_context = self.expr_call_lang_item_fn_mut(
 772                 gen_future_span,
 773                 hir::LangItem::GetContext,
 774                 arena_vec![self; task_context],
 775                 Some(expr_hir_id),
 776             );
 777             let call = self.expr_call_lang_item_fn(
 778                 span,
 779                 hir::LangItem::FuturePoll,
 780                 arena_vec![self; new_unchecked, get_context],
 781                 Some(expr_hir_id),
 782             );
 783             self.arena.alloc(self.expr_unsafe(call))
 784         };
 785
 786         // `::std::task::Poll::Ready(result) => break result`
 787         let loop_node_id = self.next_node_id();
 788         let loop_hir_id = self.lower_node_id(loop_node_id);
 789         let ready_arm = {
 790             let x_ident = Ident::with_dummy_span(sym::result);
 791             let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
 792             let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
 793             let ready_field = self.single_pat_field(gen_future_span, x_pat);
 794             let ready_pat = self.pat_lang_item_variant(
 795                 span,
 796                 hir::LangItem::PollReady,
 797                 ready_field,
 798                 Some(expr_hir_id),
 799             );
 800             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 801                 let expr_break =
 802                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 803                 this.arena.alloc(this.expr(gen_future_span, expr_break))
 804             });
 805             self.arm(ready_pat, break_x)
 806         };
 807
 808         // `::std::task::Poll::Pending => {}`
 809         let pending_arm = {
 810             let pending_pat = self.pat_lang_item_variant(
 811                 span,
 812                 hir::LangItem::PollPending,
 813                 &[],
 814                 Some(expr_hir_id),
 815             );
 816             let empty_block = self.expr_block_empty(span);
 817             self.arm(pending_pat, empty_block)
 818         };
 819
 820         let inner_match_stmt = {
 821             let match_expr = self.expr_match(
 822                 span,
 823                 poll_expr,
 824                 arena_vec![self; ready_arm, pending_arm],
 825                 hir::MatchSource::AwaitDesugar,
 826             );
 827             self.stmt_expr(span, match_expr)
 828         };
 829
 830         // task_context = yield ();
 831         let yield_stmt = {
 832             let unit = self.expr_unit(span);
 833             let yield_expr = self.expr(
 834                 span,
 835                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
 836             );
 837             let yield_expr = self.arena.alloc(yield_expr);
 838
 839             if let Some(task_context_hid) = self.task_context {
 840                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 841                 let assign =
 842                     self.expr(span, hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)));
 843                 self.stmt_expr(span, assign)
 844             } else {
 845                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 846                 // proceed with type checking.
 847                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 848             }
 849         };
 850
 851         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 852
 853         // loop { .. }
 854         let loop_expr = self.arena.alloc(hir::Expr {
 855             hir_id: loop_hir_id,
 856             kind: hir::ExprKind::Loop(
 857                 loop_block,
 858                 None,
 859                 hir::LoopSource::Loop,
 860                 self.lower_span(span),
 861             ),
 862             span: self.lower_span(span),
 863         });
 864
 865         // mut __awaitee => loop { ... }
 866         let awaitee_arm = self.arm(awaitee_pat, loop_expr);
 867
 868         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
 869         let into_future_span = self.mark_span_with_reason(
 870             DesugaringKind::Await,
 871             dot_await_span,
 872             self.allow_into_future.clone(),
 873         );
 874         let into_future_expr = self.expr_call_lang_item_fn(
 875             into_future_span,
 876             hir::LangItem::IntoFutureIntoFuture,
 877             arena_vec![self; expr],
 878             Some(expr_hir_id),
 879         );
 880
 881         // match <into_future_expr> {
 882         //     mut __awaitee => loop { .. }
 883         // }
 884         hir::ExprKind::Match(
 885             into_future_expr,
 886             arena_vec![self; awaitee_arm],
 887             hir::MatchSource::AwaitDesugar,
 888         )
 889     }
 890
 891     fn lower_expr_closure(
 892         &mut self,
 893         binder: &ClosureBinder,
 894         capture_clause: CaptureBy,
 895         closure_id: NodeId,
 896         constness: Const,
 897         movability: Movability,
 898         decl: &FnDecl,
 899         body: &Expr,
 900         fn_decl_span: Span,
 901         fn_arg_span: Span,
 902     ) -> hir::ExprKind<'hir> {
 903         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 904
 905         let (body_id, generator_option) = self.with_new_scopes(move |this| {
 906             let prev = this.current_item;
 907             this.current_item = Some(fn_decl_span);
 908             let mut generator_kind = None;
 909             let body_id = this.lower_fn_body(decl, |this| {
 910                 let e = this.lower_expr_mut(body);
 911                 generator_kind = this.generator_kind;
 912                 e
 913             });
 914             let generator_option =
 915                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 916             this.current_item = prev;
 917             (body_id, generator_option)
 918         });
 919
 920         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
 921         // Lower outside new scope to preserve `is_in_loop_condition`.
 922         let fn_decl = self.lower_fn_decl(decl, closure_id, fn_decl_span, FnDeclKind::Closure, None);
 923
 924         let c = self.arena.alloc(hir::Closure {
 925             def_id: self.local_def_id(closure_id),
 926             binder: binder_clause,
 927             capture_clause,
 928             bound_generic_params,
 929             fn_decl,
 930             body: body_id,
 931             fn_decl_span: self.lower_span(fn_decl_span),
 932             fn_arg_span: Some(self.lower_span(fn_arg_span)),
 933             movability: generator_option,
 934             constness: self.lower_constness(constness),
 935         });
 936
 937         hir::ExprKind::Closure(c)
 938     }
 939
 940     fn generator_movability_for_fn(
 941         &mut self,
 942         decl: &FnDecl,
 943         fn_decl_span: Span,
 944         generator_kind: Option<hir::GeneratorKind>,
 945         movability: Movability,
 946     ) -> Option<hir::Movability> {
 947         match generator_kind {
 948             Some(hir::GeneratorKind::Gen) => {
 949                 if decl.inputs.len() > 1 {
 950                     self.tcx.sess.emit_err(GeneratorTooManyParameters { fn_decl_span });
 951                 }
 952                 Some(movability)
 953             }
 954             Some(hir::GeneratorKind::Async(_)) => {
 955                 panic!("non-`async` closure body turned `async` during lowering");
 956             }
 957             None => {
 958                 if movability == Movability::Static {
 959                     self.tcx.sess.emit_err(ClosureCannotBeStatic { fn_decl_span });
 960                 }
 961                 None
 962             }
 963         }
 964     }
 965
 966     fn lower_closure_binder<'c>(
 967         &mut self,
 968         binder: &'c ClosureBinder,
 969     ) -> (hir::ClosureBinder, &'c [GenericParam]) {
 970         let (binder, params) = match binder {
 971             ClosureBinder::NotPresent => (hir::ClosureBinder::Default, &[][..]),
 972             ClosureBinder::For { span, generic_params } => {
 973                 let span = self.lower_span(*span);
 974                 (hir::ClosureBinder::For { span }, &**generic_params)
 975             }
 976         };
 977
 978         (binder, params)
 979     }
 980
 981     fn lower_expr_async_closure(
 982         &mut self,
 983         binder: &ClosureBinder,
 984         capture_clause: CaptureBy,
 985         closure_id: NodeId,
 986         closure_hir_id: hir::HirId,
 987         inner_closure_id: NodeId,
 988         decl: &FnDecl,
 989         body: &Expr,
 990         fn_decl_span: Span,
 991         fn_arg_span: Span,
 992     ) -> hir::ExprKind<'hir> {
 993         if let &ClosureBinder::For { span, .. } = binder {
 994             self.tcx.sess.emit_err(NotSupportedForLifetimeBinderAsyncClosure { span });
 995         }
 996
 997         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 998
 999         let outer_decl =
1000             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
1001
1002         let body = self.with_new_scopes(|this| {
1003             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
1004             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
1005                 this.tcx.sess.emit_err(AsyncNonMoveClosureNotSupported { fn_decl_span });
1006             }
1007
1008             // Transform `async |x: u8| -> X { ... }` into
1009             // `|x: u8| identity_future(|| -> X { ... })`.
1010             let body_id = this.lower_fn_body(&outer_decl, |this| {
1011                 let async_ret_ty = if let FnRetTy::Ty(ty) = &decl.output {
1012                     let itctx = ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock);
1013                     Some(hir::FnRetTy::Return(this.lower_ty(&ty, &itctx)))
1014                 } else {
1015                     None
1016                 };
1017
1018                 let async_body = this.make_async_expr(
1019                     capture_clause,
1020                     closure_hir_id,
1021                     inner_closure_id,
1022                     async_ret_ty,
1023                     body.span,
1024                     hir::AsyncGeneratorKind::Closure,
1025                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
1026                 );
1027                 this.expr(fn_decl_span, async_body)
1028             });
1029             body_id
1030         });
1031
1032         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
1033         // We need to lower the declaration outside the new scope, because we
1034         // have to conserve the state of being inside a loop condition for the
1035         // closure argument types.
1036         let fn_decl =
1037             self.lower_fn_decl(&outer_decl, closure_id, fn_decl_span, FnDeclKind::Closure, None);
1038
1039         let c = self.arena.alloc(hir::Closure {
1040             def_id: self.local_def_id(closure_id),
1041             binder: binder_clause,
1042             capture_clause,
1043             bound_generic_params,
1044             fn_decl,
1045             body,
1046             fn_decl_span: self.lower_span(fn_decl_span),
1047             fn_arg_span: Some(self.lower_span(fn_arg_span)),
1048             movability: None,
1049             constness: hir::Constness::NotConst,
1050         });
1051         hir::ExprKind::Closure(c)
1052     }
1053
1054     /// Destructure the LHS of complex assignments.
1055     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
1056     fn lower_expr_assign(
1057         &mut self,
1058         lhs: &Expr,
1059         rhs: &Expr,
1060         eq_sign_span: Span,
1061         whole_span: Span,
1062     ) -> hir::ExprKind<'hir> {
1063         // Return early in case of an ordinary assignment.
1064         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
1065             match &lhs.kind {
1066                 ExprKind::Array(..)
1067                 | ExprKind::Struct(..)
1068                 | ExprKind::Tup(..)
1069                 | ExprKind::Underscore => false,
1070                 // Check for tuple struct constructor.
1071                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
1072                 ExprKind::Paren(e) => {
1073                     match e.kind {
1074                         // We special-case `(..)` for consistency with patterns.
1075                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
1076                         _ => is_ordinary(lower_ctx, e),
1077                     }
1078                 }
1079                 _ => true,
1080             }
1081         }
1082         if is_ordinary(self, lhs) {
1083             return hir::ExprKind::Assign(
1084                 self.lower_expr(lhs),
1085                 self.lower_expr(rhs),
1086                 self.lower_span(eq_sign_span),
1087             );
1088         }
1089
1090         let mut assignments = vec![];
1091
1092         // The LHS becomes a pattern: `(lhs1, lhs2)`.
1093         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
1094         let rhs = self.lower_expr(rhs);
1095
1096         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
1097         let destructure_let = self.stmt_let_pat(
1098             None,
1099             whole_span,
1100             Some(rhs),
1101             pat,
1102             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
1103         );
1104
1105         // `a = lhs1; b = lhs2;`.
1106         let stmts = self
1107             .arena
1108             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
1109
1110         // Wrap everything in a block.
1111         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
1112     }
1113
1114     /// If the given expression is a path to a tuple struct, returns that path.
1115     /// It is not a complete check, but just tries to reject most paths early
1116     /// if they are not tuple structs.
1117     /// Type checking will take care of the full validation later.
1118     fn extract_tuple_struct_path<'a>(
1119         &mut self,
1120         expr: &'a Expr,
1121     ) -> Option<(&'a Option<AstP<QSelf>>, &'a Path)> {
1122         if let ExprKind::Path(qself, path) = &expr.kind {
1123             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1124             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1125                 if let Some(res) = partial_res.full_res() && !res.expected_in_tuple_struct_pat() {
1126                     return None;
1127                 }
1128             }
1129             return Some((qself, path));
1130         }
1131         None
1132     }
1133
1134     /// If the given expression is a path to a unit struct, returns that path.
1135     /// It is not a complete check, but just tries to reject most paths early
1136     /// if they are not unit structs.
1137     /// Type checking will take care of the full validation later.
1138     fn extract_unit_struct_path<'a>(
1139         &mut self,
1140         expr: &'a Expr,
1141     ) -> Option<(&'a Option<AstP<QSelf>>, &'a Path)> {
1142         if let ExprKind::Path(qself, path) = &expr.kind {
1143             // Does the path resolve to something disallowed in a unit struct/variant pattern?
1144             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1145                 if let Some(res) = partial_res.full_res() && !res.expected_in_unit_struct_pat() {
1146                     return None;
1147                 }
1148             }
1149             return Some((qself, path));
1150         }
1151         None
1152     }
1153
1154     /// Convert the LHS of a destructuring assignment to a pattern.
1155     /// Each sub-assignment is recorded in `assignments`.
1156     fn destructure_assign(
1157         &mut self,
1158         lhs: &Expr,
1159         eq_sign_span: Span,
1160         assignments: &mut Vec<hir::Stmt<'hir>>,
1161     ) -> &'hir hir::Pat<'hir> {
1162         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1163     }
1164
1165     fn destructure_assign_mut(
1166         &mut self,
1167         lhs: &Expr,
1168         eq_sign_span: Span,
1169         assignments: &mut Vec<hir::Stmt<'hir>>,
1170     ) -> hir::Pat<'hir> {
1171         match &lhs.kind {
1172             // Underscore pattern.
1173             ExprKind::Underscore => {
1174                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1175             }
1176             // Slice patterns.
1177             ExprKind::Array(elements) => {
1178                 let (pats, rest) =
1179                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1180                 let slice_pat = if let Some((i, span)) = rest {
1181                     let (before, after) = pats.split_at(i);
1182                     hir::PatKind::Slice(
1183                         before,
1184                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1185                         after,
1186                     )
1187                 } else {
1188                     hir::PatKind::Slice(pats, None, &[])
1189                 };
1190                 return self.pat_without_dbm(lhs.span, slice_pat);
1191             }
1192             // Tuple structs.
1193             ExprKind::Call(callee, args) => {
1194                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1195                     let (pats, rest) = self.destructure_sequence(
1196                         args,
1197                         "tuple struct or variant",
1198                         eq_sign_span,
1199                         assignments,
1200                     );
1201                     let qpath = self.lower_qpath(
1202                         callee.id,
1203                         qself,
1204                         path,
1205                         ParamMode::Optional,
1206                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1207                     );
1208                     // Destructure like a tuple struct.
1209                     let tuple_struct_pat = hir::PatKind::TupleStruct(
1210                         qpath,
1211                         pats,
1212                         hir::DotDotPos::new(rest.map(|r| r.0)),
1213                     );
1214                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1215                 }
1216             }
1217             // Unit structs and enum variants.
1218             ExprKind::Path(..) => {
1219                 if let Some((qself, path)) = self.extract_unit_struct_path(lhs) {
1220                     let qpath = self.lower_qpath(
1221                         lhs.id,
1222                         qself,
1223                         path,
1224                         ParamMode::Optional,
1225                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1226                     );
1227                     // Destructure like a unit struct.
1228                     let unit_struct_pat = hir::PatKind::Path(qpath);
1229                     return self.pat_without_dbm(lhs.span, unit_struct_pat);
1230                 }
1231             }
1232             // Structs.
1233             ExprKind::Struct(se) => {
1234                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1235                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1236                     hir::PatField {
1237                         hir_id: self.next_id(),
1238                         ident: self.lower_ident(f.ident),
1239                         pat,
1240                         is_shorthand: f.is_shorthand,
1241                         span: self.lower_span(f.span),
1242                     }
1243                 }));
1244                 let qpath = self.lower_qpath(
1245                     lhs.id,
1246                     &se.qself,
1247                     &se.path,
1248                     ParamMode::Optional,
1249                     &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1250                 );
1251                 let fields_omitted = match &se.rest {
1252                     StructRest::Base(e) => {
1253                         self.tcx.sess.emit_err(FunctionalRecordUpdateDestructuringAssignemnt {
1254                             span: e.span,
1255                         });
1256                         true
1257                     }
1258                     StructRest::Rest(_) => true,
1259                     StructRest::None => false,
1260                 };
1261                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1262                 return self.pat_without_dbm(lhs.span, struct_pat);
1263             }
1264             // Tuples.
1265             ExprKind::Tup(elements) => {
1266                 let (pats, rest) =
1267                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1268                 let tuple_pat = hir::PatKind::Tuple(pats, hir::DotDotPos::new(rest.map(|r| r.0)));
1269                 return self.pat_without_dbm(lhs.span, tuple_pat);
1270             }
1271             ExprKind::Paren(e) => {
1272                 // We special-case `(..)` for consistency with patterns.
1273                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1274                     let tuple_pat = hir::PatKind::Tuple(&[], hir::DotDotPos::new(Some(0)));
1275                     return self.pat_without_dbm(lhs.span, tuple_pat);
1276                 } else {
1277                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1278                 }
1279             }
1280             _ => {}
1281         }
1282         // Treat all other cases as normal lvalue.
1283         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1284         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1285         let ident = self.expr_ident(lhs.span, ident, binding);
1286         let assign =
1287             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1288         let expr = self.expr(lhs.span, assign);
1289         assignments.push(self.stmt_expr(lhs.span, expr));
1290         pat
1291     }
1292
1293     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1294     /// Such a sequence occurs in a tuple (struct)/slice.
1295     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1296     /// exists.
1297     /// Each sub-assignment is recorded in `assignments`.
1298     fn destructure_sequence(
1299         &mut self,
1300         elements: &[AstP<Expr>],
1301         ctx: &str,
1302         eq_sign_span: Span,
1303         assignments: &mut Vec<hir::Stmt<'hir>>,
1304     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1305         let mut rest = None;
1306         let elements =
1307             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1308                 // Check for `..` pattern.
1309                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1310                     if let Some((_, prev_span)) = rest {
1311                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1312                     } else {
1313                         rest = Some((i, e.span));
1314                     }
1315                     None
1316                 } else {
1317                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1318                 }
1319             }));
1320         (elements, rest)
1321     }
1322
1323     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1324     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1325         let e1 = self.lower_expr_mut(e1);
1326         let e2 = self.lower_expr_mut(e2);
1327         let fn_path =
1328             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1329         let fn_expr = self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path)));
1330         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1331     }
1332
1333     fn lower_expr_range(
1334         &mut self,
1335         span: Span,
1336         e1: Option<&Expr>,
1337         e2: Option<&Expr>,
1338         lims: RangeLimits,
1339     ) -> hir::ExprKind<'hir> {
1340         use rustc_ast::RangeLimits::*;
1341
1342         let lang_item = match (e1, e2, lims) {
1343             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1344             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1345             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1346             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1347             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1348             (Some(..), Some(..), Closed) => unreachable!(),
1349             (start, None, Closed) => {
1350                 self.tcx.sess.emit_err(InclusiveRangeWithNoEnd { span });
1351                 match start {
1352                     Some(..) => hir::LangItem::RangeFrom,
1353                     None => hir::LangItem::RangeFull,
1354                 }
1355             }
1356         };
1357
1358         let fields = self.arena.alloc_from_iter(
1359             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1360                 |(s, e)| {
1361                     let expr = self.lower_expr(&e);
1362                     let ident = Ident::new(s, self.lower_span(e.span));
1363                     self.expr_field(ident, expr, e.span)
1364                 },
1365             ),
1366         );
1367
1368         hir::ExprKind::Struct(
1369             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1370             fields,
1371             None,
1372         )
1373     }
1374
1375     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1376         let label = opt_label?;
1377         Some(Label { ident: self.lower_ident(label.ident) })
1378     }
1379
1380     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1381         let target_id = match destination {
1382             Some((id, _)) => {
1383                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1384                     Ok(self.lower_node_id(loop_id))
1385                 } else {
1386                     Err(hir::LoopIdError::UnresolvedLabel)
1387                 }
1388             }
1389             None => self
1390                 .loop_scope
1391                 .map(|id| Ok(self.lower_node_id(id)))
1392                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1393         };
1394         let label = self.lower_label(destination.map(|(_, label)| label));
1395         hir::Destination { label, target_id }
1396     }
1397
1398     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1399         if self.is_in_loop_condition && opt_label.is_none() {
1400             hir::Destination {
1401                 label: None,
1402                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1403             }
1404         } else {
1405             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1406         }
1407     }
1408
1409     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1410         let old_scope = self.catch_scope.replace(catch_id);
1411         let result = f(self);
1412         self.catch_scope = old_scope;
1413         result
1414     }
1415
1416     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1417         // We're no longer in the base loop's condition; we're in another loop.
1418         let was_in_loop_condition = self.is_in_loop_condition;
1419         self.is_in_loop_condition = false;
1420
1421         let old_scope = self.loop_scope.replace(loop_id);
1422         let result = f(self);
1423         self.loop_scope = old_scope;
1424
1425         self.is_in_loop_condition = was_in_loop_condition;
1426
1427         result
1428     }
1429
1430     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1431         let was_in_loop_condition = self.is_in_loop_condition;
1432         self.is_in_loop_condition = true;
1433
1434         let result = f(self);
1435
1436         self.is_in_loop_condition = was_in_loop_condition;
1437
1438         result
1439     }
1440
1441     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1442         let hir_id = self.lower_node_id(f.id);
1443         self.lower_attrs(hir_id, &f.attrs);
1444         hir::ExprField {
1445             hir_id,
1446             ident: self.lower_ident(f.ident),
1447             expr: self.lower_expr(&f.expr),
1448             span: self.lower_span(f.span),
1449             is_shorthand: f.is_shorthand,
1450         }
1451     }
1452
1453     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1454         match self.generator_kind {
1455             Some(hir::GeneratorKind::Gen) => {}
1456             Some(hir::GeneratorKind::Async(_)) => {
1457                 self.tcx.sess.emit_err(AsyncGeneratorsNotSupported { span });
1458             }
1459             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1460         }
1461
1462         let expr =
1463             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1464
1465         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1466     }
1467
1468     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1469     /// ```ignore (pseudo-rust)
1470     /// {
1471     ///     let result = match IntoIterator::into_iter(<head>) {
1472     ///         mut iter => {
1473     ///             [opt_ident]: loop {
1474     ///                 match Iterator::next(&mut iter) {
1475     ///                     None => break,
1476     ///                     Some(<pat>) => <body>,
1477     ///                 };
1478     ///             }
1479     ///         }
1480     ///     };
1481     ///     result
1482     /// }
1483     /// ```
1484     fn lower_expr_for(
1485         &mut self,
1486         e: &Expr,
1487         pat: &Pat,
1488         head: &Expr,
1489         body: &Block,
1490         opt_label: Option<Label>,
1491     ) -> hir::Expr<'hir> {
1492         let head = self.lower_expr_mut(head);
1493         let pat = self.lower_pat(pat);
1494         let for_span =
1495             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1496         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1497         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1498
1499         // `None => break`
1500         let none_arm = {
1501             let break_expr = self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span));
1502             let pat = self.pat_none(for_span);
1503             self.arm(pat, break_expr)
1504         };
1505
1506         // Some(<pat>) => <body>,
1507         let some_arm = {
1508             let some_pat = self.pat_some(pat_span, pat);
1509             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1510             let body_expr = self.arena.alloc(self.expr_block(body_block));
1511             self.arm(some_pat, body_expr)
1512         };
1513
1514         // `mut iter`
1515         let iter = Ident::with_dummy_span(sym::iter);
1516         let (iter_pat, iter_pat_nid) =
1517             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::MUT);
1518
1519         // `match Iterator::next(&mut iter) { ... }`
1520         let match_expr = {
1521             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1522             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1523             let next_expr = self.expr_call_lang_item_fn(
1524                 head_span,
1525                 hir::LangItem::IteratorNext,
1526                 arena_vec![self; ref_mut_iter],
1527                 None,
1528             );
1529             let arms = arena_vec![self; none_arm, some_arm];
1530
1531             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1532         };
1533         let match_stmt = self.stmt_expr(for_span, match_expr);
1534
1535         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1536
1537         // `[opt_ident]: loop { ... }`
1538         let kind = hir::ExprKind::Loop(
1539             loop_block,
1540             self.lower_label(opt_label),
1541             hir::LoopSource::ForLoop,
1542             self.lower_span(for_span.with_hi(head.span.hi())),
1543         );
1544         let loop_expr =
1545             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1546
1547         // `mut iter => { ... }`
1548         let iter_arm = self.arm(iter_pat, loop_expr);
1549
1550         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1551         let into_iter_expr = {
1552             self.expr_call_lang_item_fn(
1553                 head_span,
1554                 hir::LangItem::IntoIterIntoIter,
1555                 arena_vec![self; head],
1556                 None,
1557             )
1558         };
1559
1560         let match_expr = self.arena.alloc(self.expr_match(
1561             for_span,
1562             into_iter_expr,
1563             arena_vec![self; iter_arm],
1564             hir::MatchSource::ForLoopDesugar,
1565         ));
1566
1567         // This is effectively `{ let _result = ...; _result }`.
1568         // The construct was introduced in #21984 and is necessary to make sure that
1569         // temporaries in the `head` expression are dropped and do not leak to the
1570         // surrounding scope of the `match` since the `match` is not a terminating scope.
1571         //
1572         // Also, add the attributes to the outer returned expr node.
1573         let expr = self.expr_drop_temps_mut(for_span, match_expr);
1574         self.lower_attrs(expr.hir_id, &e.attrs);
1575         expr
1576     }
1577
1578     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1579     /// ```ignore (pseudo-rust)
1580     /// match Try::branch(<expr>) {
1581     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1582     ///     ControlFlow::Break(residual) =>
1583     ///         #[allow(unreachable_code)]
1584     ///         // If there is an enclosing `try {...}`:
1585     ///         break 'catch_target Try::from_residual(residual),
1586     ///         // Otherwise:
1587     ///         return Try::from_residual(residual),
1588     /// }
1589     /// ```
1590     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1591         let unstable_span = self.mark_span_with_reason(
1592             DesugaringKind::QuestionMark,
1593             span,
1594             self.allow_try_trait.clone(),
1595         );
1596         let try_span = self.tcx.sess.source_map().end_point(span);
1597         let try_span = self.mark_span_with_reason(
1598             DesugaringKind::QuestionMark,
1599             try_span,
1600             self.allow_try_trait.clone(),
1601         );
1602
1603         // `Try::branch(<expr>)`
1604         let scrutinee = {
1605             // expand <expr>
1606             let sub_expr = self.lower_expr_mut(sub_expr);
1607
1608             self.expr_call_lang_item_fn(
1609                 unstable_span,
1610                 hir::LangItem::TryTraitBranch,
1611                 arena_vec![self; sub_expr],
1612                 None,
1613             )
1614         };
1615
1616         // `#[allow(unreachable_code)]`
1617         let attr = attr::mk_attr_nested_word(
1618             &self.tcx.sess.parse_sess.attr_id_generator,
1619             AttrStyle::Outer,
1620             sym::allow,
1621             sym::unreachable_code,
1622             self.lower_span(span),
1623         );
1624         let attrs: AttrVec = thin_vec![attr];
1625
1626         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1627         let continue_arm = {
1628             let val_ident = Ident::with_dummy_span(sym::val);
1629             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1630             let val_expr = self.expr_ident(span, val_ident, val_pat_nid);
1631             self.lower_attrs(val_expr.hir_id, &attrs);
1632             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1633             self.arm(continue_pat, val_expr)
1634         };
1635
1636         // `ControlFlow::Break(residual) =>
1637         //     #[allow(unreachable_code)]
1638         //     return Try::from_residual(residual),`
1639         let break_arm = {
1640             let residual_ident = Ident::with_dummy_span(sym::residual);
1641             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1642             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1643             let from_residual_expr = self.wrap_in_try_constructor(
1644                 hir::LangItem::TryTraitFromResidual,
1645                 try_span,
1646                 self.arena.alloc(residual_expr),
1647                 unstable_span,
1648             );
1649             let ret_expr = if let Some(catch_node) = self.catch_scope {
1650                 let target_id = Ok(self.lower_node_id(catch_node));
1651                 self.arena.alloc(self.expr(
1652                     try_span,
1653                     hir::ExprKind::Break(
1654                         hir::Destination { label: None, target_id },
1655                         Some(from_residual_expr),
1656                     ),
1657                 ))
1658             } else {
1659                 self.arena.alloc(self.expr(try_span, hir::ExprKind::Ret(Some(from_residual_expr))))
1660             };
1661             self.lower_attrs(ret_expr.hir_id, &attrs);
1662
1663             let break_pat = self.pat_cf_break(try_span, residual_local);
1664             self.arm(break_pat, ret_expr)
1665         };
1666
1667         hir::ExprKind::Match(
1668             scrutinee,
1669             arena_vec![self; break_arm, continue_arm],
1670             hir::MatchSource::TryDesugar,
1671         )
1672     }
1673
1674     /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
1675     /// ```ignore(illustrative)
1676     /// // If there is an enclosing `try {...}`:
1677     /// break 'catch_target FromResidual::from_residual(Yeet(residual));
1678     /// // Otherwise:
1679     /// return FromResidual::from_residual(Yeet(residual));
1680     /// ```
1681     /// But to simplify this, there's a `from_yeet` lang item function which
1682     /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
1683     fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1684         // The expression (if present) or `()` otherwise.
1685         let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
1686             (sub_expr.span, self.lower_expr(sub_expr))
1687         } else {
1688             (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
1689         };
1690
1691         let unstable_span = self.mark_span_with_reason(
1692             DesugaringKind::YeetExpr,
1693             span,
1694             self.allow_try_trait.clone(),
1695         );
1696
1697         let from_yeet_expr = self.wrap_in_try_constructor(
1698             hir::LangItem::TryTraitFromYeet,
1699             unstable_span,
1700             yeeted_expr,
1701             yeeted_span,
1702         );
1703
1704         if let Some(catch_node) = self.catch_scope {
1705             let target_id = Ok(self.lower_node_id(catch_node));
1706             hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
1707         } else {
1708             hir::ExprKind::Ret(Some(from_yeet_expr))
1709         }
1710     }
1711
1712     // =========================================================================
1713     // Helper methods for building HIR.
1714     // =========================================================================
1715
1716     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1717     ///
1718     /// In terms of drop order, it has the same effect as wrapping `expr` in
1719     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1720     ///
1721     /// The drop order can be important in e.g. `if expr { .. }`.
1722     pub(super) fn expr_drop_temps(
1723         &mut self,
1724         span: Span,
1725         expr: &'hir hir::Expr<'hir>,
1726     ) -> &'hir hir::Expr<'hir> {
1727         self.arena.alloc(self.expr_drop_temps_mut(span, expr))
1728     }
1729
1730     pub(super) fn expr_drop_temps_mut(
1731         &mut self,
1732         span: Span,
1733         expr: &'hir hir::Expr<'hir>,
1734     ) -> hir::Expr<'hir> {
1735         self.expr(span, hir::ExprKind::DropTemps(expr))
1736     }
1737
1738     fn expr_match(
1739         &mut self,
1740         span: Span,
1741         arg: &'hir hir::Expr<'hir>,
1742         arms: &'hir [hir::Arm<'hir>],
1743         source: hir::MatchSource,
1744     ) -> hir::Expr<'hir> {
1745         self.expr(span, hir::ExprKind::Match(arg, arms, source))
1746     }
1747
1748     fn expr_break(&mut self, span: Span) -> hir::Expr<'hir> {
1749         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1750         self.expr(span, expr_break)
1751     }
1752
1753     fn expr_break_alloc(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1754         let expr_break = self.expr_break(span);
1755         self.arena.alloc(expr_break)
1756     }
1757
1758     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1759         self.expr(span, hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e))
1760     }
1761
1762     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1763         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[])))
1764     }
1765
1766     fn expr_call_mut(
1767         &mut self,
1768         span: Span,
1769         e: &'hir hir::Expr<'hir>,
1770         args: &'hir [hir::Expr<'hir>],
1771     ) -> hir::Expr<'hir> {
1772         self.expr(span, hir::ExprKind::Call(e, args))
1773     }
1774
1775     fn expr_call(
1776         &mut self,
1777         span: Span,
1778         e: &'hir hir::Expr<'hir>,
1779         args: &'hir [hir::Expr<'hir>],
1780     ) -> &'hir hir::Expr<'hir> {
1781         self.arena.alloc(self.expr_call_mut(span, e, args))
1782     }
1783
1784     fn expr_call_lang_item_fn_mut(
1785         &mut self,
1786         span: Span,
1787         lang_item: hir::LangItem,
1788         args: &'hir [hir::Expr<'hir>],
1789         hir_id: Option<hir::HirId>,
1790     ) -> hir::Expr<'hir> {
1791         let path = self.arena.alloc(self.expr_lang_item_path(span, lang_item, hir_id));
1792         self.expr_call_mut(span, path, args)
1793     }
1794
1795     fn expr_call_lang_item_fn(
1796         &mut self,
1797         span: Span,
1798         lang_item: hir::LangItem,
1799         args: &'hir [hir::Expr<'hir>],
1800         hir_id: Option<hir::HirId>,
1801     ) -> &'hir hir::Expr<'hir> {
1802         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1803     }
1804
1805     fn expr_lang_item_path(
1806         &mut self,
1807         span: Span,
1808         lang_item: hir::LangItem,
1809         hir_id: Option<hir::HirId>,
1810     ) -> hir::Expr<'hir> {
1811         self.expr(
1812             span,
1813             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1814         )
1815     }
1816
1817     pub(super) fn expr_ident(
1818         &mut self,
1819         sp: Span,
1820         ident: Ident,
1821         binding: hir::HirId,
1822     ) -> &'hir hir::Expr<'hir> {
1823         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1824     }
1825
1826     pub(super) fn expr_ident_mut(
1827         &mut self,
1828         span: Span,
1829         ident: Ident,
1830         binding: hir::HirId,
1831     ) -> hir::Expr<'hir> {
1832         let hir_id = self.next_id();
1833         let res = Res::Local(binding);
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,
1839                 segments: arena_vec![self; hir::PathSegment::new(ident, hir_id, res)],
1840             }),
1841         ));
1842
1843         self.expr(span, expr_path)
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         )
1863     }
1864
1865     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1866         let blk = self.block_all(span, &[], None);
1867         let expr = self.expr_block(blk);
1868         self.arena.alloc(expr)
1869     }
1870
1871     pub(super) fn expr_block(&mut self, b: &'hir hir::Block<'hir>) -> hir::Expr<'hir> {
1872         self.expr(b.span, hir::ExprKind::Block(b, None))
1873     }
1874
1875     pub(super) fn expr(&mut self, span: Span, kind: hir::ExprKind<'hir>) -> hir::Expr<'hir> {
1876         let hir_id = self.next_id();
1877         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1878     }
1879
1880     fn expr_field(
1881         &mut self,
1882         ident: Ident,
1883         expr: &'hir hir::Expr<'hir>,
1884         span: Span,
1885     ) -> hir::ExprField<'hir> {
1886         hir::ExprField {
1887             hir_id: self.next_id(),
1888             ident,
1889             span: self.lower_span(span),
1890             expr,
1891             is_shorthand: false,
1892         }
1893     }
1894
1895     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1896         hir::Arm {
1897             hir_id: self.next_id(),
1898             pat,
1899             guard: None,
1900             span: self.lower_span(expr.span),
1901             body: expr,
1902         }
1903     }
1904 }