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, Symbol};
  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::FormatArgs(fmt) => self.lower_format_args(e.span, fmt),
 298                 ExprKind::Struct(se) => {
 299                     let rest = match &se.rest {
 300                         StructRest::Base(e) => Some(self.lower_expr(e)),
 301                         StructRest::Rest(sp) => {
 302                             self.tcx.sess.emit_err(BaseExpressionDoubleDot { span: *sp });
 303                             Some(&*self.arena.alloc(self.expr_err(*sp)))
 304                         }
 305                         StructRest::None => None,
 306                     };
 307                     hir::ExprKind::Struct(
 308                         self.arena.alloc(self.lower_qpath(
 309                             e.id,
 310                             &se.qself,
 311                             &se.path,
 312                             ParamMode::Optional,
 313                             &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 314                         )),
 315                         self.arena
 316                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
 317                         rest,
 318                     )
 319                 }
 320                 ExprKind::Yield(opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 321                 ExprKind::Err => hir::ExprKind::Err,
 322                 ExprKind::Try(sub_expr) => self.lower_expr_try(e.span, sub_expr),
 323
 324                 ExprKind::Paren(_) | ExprKind::ForLoop(..) => unreachable!("already handled"),
 325
 326                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 327             };
 328
 329             hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
 330         })
 331     }
 332
 333     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 334         match u {
 335             UnOp::Deref => hir::UnOp::Deref,
 336             UnOp::Not => hir::UnOp::Not,
 337             UnOp::Neg => hir::UnOp::Neg,
 338         }
 339     }
 340
 341     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 342         Spanned {
 343             node: match b.node {
 344                 BinOpKind::Add => hir::BinOpKind::Add,
 345                 BinOpKind::Sub => hir::BinOpKind::Sub,
 346                 BinOpKind::Mul => hir::BinOpKind::Mul,
 347                 BinOpKind::Div => hir::BinOpKind::Div,
 348                 BinOpKind::Rem => hir::BinOpKind::Rem,
 349                 BinOpKind::And => hir::BinOpKind::And,
 350                 BinOpKind::Or => hir::BinOpKind::Or,
 351                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 352                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 353                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 354                 BinOpKind::Shl => hir::BinOpKind::Shl,
 355                 BinOpKind::Shr => hir::BinOpKind::Shr,
 356                 BinOpKind::Eq => hir::BinOpKind::Eq,
 357                 BinOpKind::Lt => hir::BinOpKind::Lt,
 358                 BinOpKind::Le => hir::BinOpKind::Le,
 359                 BinOpKind::Ne => hir::BinOpKind::Ne,
 360                 BinOpKind::Ge => hir::BinOpKind::Ge,
 361                 BinOpKind::Gt => hir::BinOpKind::Gt,
 362             },
 363             span: self.lower_span(b.span),
 364         }
 365     }
 366
 367     fn lower_legacy_const_generics(
 368         &mut self,
 369         mut f: Expr,
 370         args: Vec<AstP<Expr>>,
 371         legacy_args_idx: &[usize],
 372     ) -> hir::ExprKind<'hir> {
 373         let ExprKind::Path(None, path) = &mut f.kind else {
 374             unreachable!();
 375         };
 376
 377         // Split the arguments into const generics and normal arguments
 378         let mut real_args = vec![];
 379         let mut generic_args = vec![];
 380         for (idx, arg) in args.into_iter().enumerate() {
 381             if legacy_args_idx.contains(&idx) {
 382                 let parent_def_id = self.current_hir_id_owner;
 383                 let node_id = self.next_node_id();
 384
 385                 // Add a definition for the in-band const def.
 386                 self.create_def(parent_def_id.def_id, node_id, DefPathData::AnonConst, f.span);
 387
 388                 let anon_const = AnonConst { id: node_id, value: arg };
 389                 generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
 390             } else {
 391                 real_args.push(arg);
 392             }
 393         }
 394
 395         // Add generic args to the last element of the path.
 396         let last_segment = path.segments.last_mut().unwrap();
 397         assert!(last_segment.args.is_none());
 398         last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
 399             span: DUMMY_SP,
 400             args: generic_args,
 401         })));
 402
 403         // Now lower everything as normal.
 404         let f = self.lower_expr(&f);
 405         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
 406     }
 407
 408     fn lower_expr_if(
 409         &mut self,
 410         cond: &Expr,
 411         then: &Block,
 412         else_opt: Option<&Expr>,
 413     ) -> hir::ExprKind<'hir> {
 414         let lowered_cond = self.lower_cond(cond);
 415         let then_expr = self.lower_block_expr(then);
 416         if let Some(rslt) = else_opt {
 417             hir::ExprKind::If(
 418                 lowered_cond,
 419                 self.arena.alloc(then_expr),
 420                 Some(self.lower_expr(rslt)),
 421             )
 422         } else {
 423             hir::ExprKind::If(lowered_cond, self.arena.alloc(then_expr), None)
 424         }
 425     }
 426
 427     // Lowers a condition (i.e. `cond` in `if cond` or `while cond`), wrapping it in a terminating scope
 428     // so that temporaries created in the condition don't live beyond it.
 429     fn lower_cond(&mut self, cond: &Expr) -> &'hir hir::Expr<'hir> {
 430         fn has_let_expr(expr: &Expr) -> bool {
 431             match &expr.kind {
 432                 ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
 433                 ExprKind::Let(..) => true,
 434                 _ => false,
 435             }
 436         }
 437
 438         // We have to take special care for `let` exprs in the condition, e.g. in
 439         // `if let pat = val` or `if foo && let pat = val`, as we _do_ want `val` to live beyond the
 440         // condition in this case.
 441         //
 442         // In order to mantain the drop behavior for the non `let` parts of the condition,
 443         // we still wrap them in terminating scopes, e.g. `if foo && let pat = val` essentially
 444         // gets transformed into `if { let _t = foo; _t } && let pat = val`
 445         match &cond.kind {
 446             ExprKind::Binary(op @ Spanned { node: ast::BinOpKind::And, .. }, lhs, rhs)
 447                 if has_let_expr(cond) =>
 448             {
 449                 let op = self.lower_binop(*op);
 450                 let lhs = self.lower_cond(lhs);
 451                 let rhs = self.lower_cond(rhs);
 452
 453                 self.arena.alloc(self.expr(cond.span, hir::ExprKind::Binary(op, lhs, rhs)))
 454             }
 455             ExprKind::Let(..) => self.lower_expr(cond),
 456             _ => {
 457                 let cond = self.lower_expr(cond);
 458                 let reason = DesugaringKind::CondTemporary;
 459                 let span_block = self.mark_span_with_reason(reason, cond.span, None);
 460                 self.expr_drop_temps(span_block, cond)
 461             }
 462         }
 463     }
 464
 465     // We desugar: `'label: while $cond $body` into:
 466     //
 467     // ```
 468     // 'label: loop {
 469     //   if { let _t = $cond; _t } {
 470     //     $body
 471     //   }
 472     //   else {
 473     //     break;
 474     //   }
 475     // }
 476     // ```
 477     //
 478     // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 479     // to preserve drop semantics since `while $cond { ... }` does not
 480     // let temporaries live outside of `cond`.
 481     fn lower_expr_while_in_loop_scope(
 482         &mut self,
 483         span: Span,
 484         cond: &Expr,
 485         body: &Block,
 486         opt_label: Option<Label>,
 487     ) -> hir::ExprKind<'hir> {
 488         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_cond(cond));
 489         let then = self.lower_block_expr(body);
 490         let expr_break = self.expr_break(span);
 491         let stmt_break = self.stmt_expr(span, expr_break);
 492         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
 493         let else_expr = self.arena.alloc(self.expr_block(else_blk));
 494         let if_kind = hir::ExprKind::If(lowered_cond, self.arena.alloc(then), Some(else_expr));
 495         let if_expr = self.expr(span, if_kind);
 496         let block = self.block_expr(self.arena.alloc(if_expr));
 497         let span = self.lower_span(span.with_hi(cond.span.hi()));
 498         let opt_label = self.lower_label(opt_label);
 499         hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
 500     }
 501
 502     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
 503     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
 504     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 505     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 506         self.with_catch_scope(body.id, |this| {
 507             let mut block = this.lower_block_noalloc(body, true);
 508
 509             // Final expression of the block (if present) or `()` with span at the end of block
 510             let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
 511                 (
 512                     this.mark_span_with_reason(
 513                         DesugaringKind::TryBlock,
 514                         expr.span,
 515                         this.allow_try_trait.clone(),
 516                     ),
 517                     expr,
 518                 )
 519             } else {
 520                 let try_span = this.mark_span_with_reason(
 521                     DesugaringKind::TryBlock,
 522                     this.tcx.sess.source_map().end_point(body.span),
 523                     this.allow_try_trait.clone(),
 524                 );
 525
 526                 (try_span, this.expr_unit(try_span))
 527             };
 528
 529             let ok_wrapped_span =
 530                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 531
 532             // `::std::ops::Try::from_output($tail_expr)`
 533             block.expr = Some(this.wrap_in_try_constructor(
 534                 hir::LangItem::TryTraitFromOutput,
 535                 try_span,
 536                 tail_expr,
 537                 ok_wrapped_span,
 538             ));
 539
 540             hir::ExprKind::Block(this.arena.alloc(block), None)
 541         })
 542     }
 543
 544     fn wrap_in_try_constructor(
 545         &mut self,
 546         lang_item: hir::LangItem,
 547         method_span: Span,
 548         expr: &'hir hir::Expr<'hir>,
 549         overall_span: Span,
 550     ) -> &'hir hir::Expr<'hir> {
 551         let constructor = self.arena.alloc(self.expr_lang_item_path(method_span, lang_item, None));
 552         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 553     }
 554
 555     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 556         let pat = self.lower_pat(&arm.pat);
 557         let guard = arm.guard.as_ref().map(|cond| {
 558             if let ExprKind::Let(pat, scrutinee, span) = &cond.kind {
 559                 hir::Guard::IfLet(self.arena.alloc(hir::Let {
 560                     hir_id: self.next_id(),
 561                     span: self.lower_span(*span),
 562                     pat: self.lower_pat(pat),
 563                     ty: None,
 564                     init: self.lower_expr(scrutinee),
 565                 }))
 566             } else {
 567                 hir::Guard::If(self.lower_expr(cond))
 568             }
 569         });
 570         let hir_id = self.next_id();
 571         self.lower_attrs(hir_id, &arm.attrs);
 572         hir::Arm {
 573             hir_id,
 574             pat,
 575             guard,
 576             body: self.lower_expr(&arm.body),
 577             span: self.lower_span(arm.span),
 578         }
 579     }
 580
 581     /// Lower an `async` construct to a generator that implements `Future`.
 582     ///
 583     /// This results in:
 584     ///
 585     /// ```text
 586     /// std::future::identity_future(static move? |_task_context| -> <ret_ty> {
 587     ///     <body>
 588     /// })
 589     /// ```
 590     pub(super) fn make_async_expr(
 591         &mut self,
 592         capture_clause: CaptureBy,
 593         outer_hir_id: hir::HirId,
 594         closure_node_id: NodeId,
 595         ret_ty: Option<hir::FnRetTy<'hir>>,
 596         span: Span,
 597         async_gen_kind: hir::AsyncGeneratorKind,
 598         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 599     ) -> hir::ExprKind<'hir> {
 600         let output = ret_ty.unwrap_or_else(|| hir::FnRetTy::DefaultReturn(self.lower_span(span)));
 601
 602         // Resume argument type: `ResumeTy`
 603         let unstable_span =
 604             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 605         let resume_ty = hir::QPath::LangItem(hir::LangItem::ResumeTy, unstable_span, None);
 606         let input_ty = hir::Ty {
 607             hir_id: self.next_id(),
 608             kind: hir::TyKind::Path(resume_ty),
 609             span: unstable_span,
 610         };
 611
 612         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 613         let fn_decl = self.arena.alloc(hir::FnDecl {
 614             inputs: arena_vec![self; input_ty],
 615             output,
 616             c_variadic: false,
 617             implicit_self: hir::ImplicitSelfKind::None,
 618             lifetime_elision_allowed: false,
 619         });
 620
 621         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 622         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 623             span,
 624             Ident::with_dummy_span(sym::_task_context),
 625             hir::BindingAnnotation::MUT,
 626         );
 627         let param = hir::Param {
 628             hir_id: self.next_id(),
 629             pat,
 630             ty_span: self.lower_span(span),
 631             span: self.lower_span(span),
 632         };
 633         let params = arena_vec![self; param];
 634
 635         let body = self.lower_body(move |this| {
 636             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 637
 638             let old_ctx = this.task_context;
 639             this.task_context = Some(task_context_hid);
 640             let res = body(this);
 641             this.task_context = old_ctx;
 642             (params, res)
 643         });
 644
 645         // `static |_task_context| -> <ret_ty> { body }`:
 646         let generator_kind = {
 647             let c = self.arena.alloc(hir::Closure {
 648                 def_id: self.local_def_id(closure_node_id),
 649                 binder: hir::ClosureBinder::Default,
 650                 capture_clause,
 651                 bound_generic_params: &[],
 652                 fn_decl,
 653                 body,
 654                 fn_decl_span: self.lower_span(span),
 655                 fn_arg_span: None,
 656                 movability: Some(hir::Movability::Static),
 657                 constness: hir::Constness::NotConst,
 658             });
 659
 660             hir::ExprKind::Closure(c)
 661         };
 662
 663         let hir_id = self.lower_node_id(closure_node_id);
 664         let unstable_span =
 665             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 666
 667         if self.tcx.features().closure_track_caller
 668             && let Some(attrs) = self.attrs.get(&outer_hir_id.local_id)
 669             && attrs.into_iter().any(|attr| attr.has_name(sym::track_caller))
 670         {
 671             self.lower_attrs(
 672                 hir_id,
 673                 &[Attribute {
 674                     kind: AttrKind::Normal(ptr::P(NormalAttr {
 675                         item: AttrItem {
 676                             path: Path::from_ident(Ident::new(sym::track_caller, span)),
 677                             args: AttrArgs::Empty,
 678                             tokens: None,
 679                         },
 680                         tokens: None,
 681                     })),
 682                     id: self.tcx.sess.parse_sess.attr_id_generator.mk_attr_id(),
 683                     style: AttrStyle::Outer,
 684                     span: unstable_span,
 685                 }],
 686             );
 687         }
 688
 689         let generator = hir::Expr { hir_id, kind: generator_kind, span: self.lower_span(span) };
 690
 691         // FIXME(swatinem):
 692         // For some reason, the async block needs to flow through *any*
 693         // call (like the identity function), as otherwise type and lifetime
 694         // inference have a hard time figuring things out.
 695         // Without this, we would get:
 696         // E0720 in tests/ui/impl-trait/in-trait/default-body-with-rpit.rs
 697         // E0700 in tests/ui/self/self_lifetime-async.rs
 698
 699         // `future::identity_future`:
 700         let identity_future =
 701             self.expr_lang_item_path(unstable_span, hir::LangItem::IdentityFuture, None);
 702
 703         // `future::identity_future(generator)`:
 704         hir::ExprKind::Call(self.arena.alloc(identity_future), arena_vec![self; generator])
 705     }
 706
 707     /// Desugar `<expr>.await` into:
 708     /// ```ignore (pseudo-rust)
 709     /// match ::std::future::IntoFuture::into_future(<expr>) {
 710     ///     mut __awaitee => loop {
 711     ///         match unsafe { ::std::future::Future::poll(
 712     ///             <::std::pin::Pin>::new_unchecked(&mut __awaitee),
 713     ///             ::std::future::get_context(task_context),
 714     ///         ) } {
 715     ///             ::std::task::Poll::Ready(result) => break result,
 716     ///             ::std::task::Poll::Pending => {}
 717     ///         }
 718     ///         task_context = yield ();
 719     ///     }
 720     /// }
 721     /// ```
 722     fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 723         let full_span = expr.span.to(dot_await_span);
 724         match self.generator_kind {
 725             Some(hir::GeneratorKind::Async(_)) => {}
 726             Some(hir::GeneratorKind::Gen) | None => {
 727                 self.tcx.sess.emit_err(AwaitOnlyInAsyncFnAndBlocks {
 728                     dot_await_span,
 729                     item_span: self.current_item,
 730                 });
 731             }
 732         }
 733         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
 734         let gen_future_span = self.mark_span_with_reason(
 735             DesugaringKind::Await,
 736             full_span,
 737             self.allow_gen_future.clone(),
 738         );
 739         let expr = self.lower_expr_mut(expr);
 740         let expr_hir_id = expr.hir_id;
 741
 742         // Note that the name of this binding must not be changed to something else because
 743         // debuggers and debugger extensions expect it to be called `__awaitee`. They use
 744         // this name to identify what is being awaited by a suspended async functions.
 745         let awaitee_ident = Ident::with_dummy_span(sym::__awaitee);
 746         let (awaitee_pat, awaitee_pat_hid) =
 747             self.pat_ident_binding_mode(span, awaitee_ident, hir::BindingAnnotation::MUT);
 748
 749         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 750
 751         // unsafe {
 752         //     ::std::future::Future::poll(
 753         //         ::std::pin::Pin::new_unchecked(&mut __awaitee),
 754         //         ::std::future::get_context(task_context),
 755         //     )
 756         // }
 757         let poll_expr = {
 758             let awaitee = self.expr_ident(span, awaitee_ident, awaitee_pat_hid);
 759             let ref_mut_awaitee = self.expr_mut_addr_of(span, awaitee);
 760             let task_context = if let Some(task_context_hid) = self.task_context {
 761                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 762             } else {
 763                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 764                 self.expr_err(span)
 765             };
 766             let new_unchecked = self.expr_call_lang_item_fn_mut(
 767                 span,
 768                 hir::LangItem::PinNewUnchecked,
 769                 arena_vec![self; ref_mut_awaitee],
 770                 Some(expr_hir_id),
 771             );
 772             let get_context = self.expr_call_lang_item_fn_mut(
 773                 gen_future_span,
 774                 hir::LangItem::GetContext,
 775                 arena_vec![self; task_context],
 776                 Some(expr_hir_id),
 777             );
 778             let call = self.expr_call_lang_item_fn(
 779                 span,
 780                 hir::LangItem::FuturePoll,
 781                 arena_vec![self; new_unchecked, get_context],
 782                 Some(expr_hir_id),
 783             );
 784             self.arena.alloc(self.expr_unsafe(call))
 785         };
 786
 787         // `::std::task::Poll::Ready(result) => break result`
 788         let loop_node_id = self.next_node_id();
 789         let loop_hir_id = self.lower_node_id(loop_node_id);
 790         let ready_arm = {
 791             let x_ident = Ident::with_dummy_span(sym::result);
 792             let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
 793             let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
 794             let ready_field = self.single_pat_field(gen_future_span, x_pat);
 795             let ready_pat = self.pat_lang_item_variant(
 796                 span,
 797                 hir::LangItem::PollReady,
 798                 ready_field,
 799                 Some(expr_hir_id),
 800             );
 801             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 802                 let expr_break =
 803                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 804                 this.arena.alloc(this.expr(gen_future_span, expr_break))
 805             });
 806             self.arm(ready_pat, break_x)
 807         };
 808
 809         // `::std::task::Poll::Pending => {}`
 810         let pending_arm = {
 811             let pending_pat = self.pat_lang_item_variant(
 812                 span,
 813                 hir::LangItem::PollPending,
 814                 &[],
 815                 Some(expr_hir_id),
 816             );
 817             let empty_block = self.expr_block_empty(span);
 818             self.arm(pending_pat, empty_block)
 819         };
 820
 821         let inner_match_stmt = {
 822             let match_expr = self.expr_match(
 823                 span,
 824                 poll_expr,
 825                 arena_vec![self; ready_arm, pending_arm],
 826                 hir::MatchSource::AwaitDesugar,
 827             );
 828             self.stmt_expr(span, match_expr)
 829         };
 830
 831         // task_context = yield ();
 832         let yield_stmt = {
 833             let unit = self.expr_unit(span);
 834             let yield_expr = self.expr(
 835                 span,
 836                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
 837             );
 838             let yield_expr = self.arena.alloc(yield_expr);
 839
 840             if let Some(task_context_hid) = self.task_context {
 841                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 842                 let assign =
 843                     self.expr(span, hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)));
 844                 self.stmt_expr(span, assign)
 845             } else {
 846                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 847                 // proceed with type checking.
 848                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 849             }
 850         };
 851
 852         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 853
 854         // loop { .. }
 855         let loop_expr = self.arena.alloc(hir::Expr {
 856             hir_id: loop_hir_id,
 857             kind: hir::ExprKind::Loop(
 858                 loop_block,
 859                 None,
 860                 hir::LoopSource::Loop,
 861                 self.lower_span(span),
 862             ),
 863             span: self.lower_span(span),
 864         });
 865
 866         // mut __awaitee => loop { ... }
 867         let awaitee_arm = self.arm(awaitee_pat, loop_expr);
 868
 869         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
 870         let into_future_span = self.mark_span_with_reason(
 871             DesugaringKind::Await,
 872             dot_await_span,
 873             self.allow_into_future.clone(),
 874         );
 875         let into_future_expr = self.expr_call_lang_item_fn(
 876             into_future_span,
 877             hir::LangItem::IntoFutureIntoFuture,
 878             arena_vec![self; expr],
 879             Some(expr_hir_id),
 880         );
 881
 882         // match <into_future_expr> {
 883         //     mut __awaitee => loop { .. }
 884         // }
 885         hir::ExprKind::Match(
 886             into_future_expr,
 887             arena_vec![self; awaitee_arm],
 888             hir::MatchSource::AwaitDesugar,
 889         )
 890     }
 891
 892     fn lower_expr_closure(
 893         &mut self,
 894         binder: &ClosureBinder,
 895         capture_clause: CaptureBy,
 896         closure_id: NodeId,
 897         constness: Const,
 898         movability: Movability,
 899         decl: &FnDecl,
 900         body: &Expr,
 901         fn_decl_span: Span,
 902         fn_arg_span: Span,
 903     ) -> hir::ExprKind<'hir> {
 904         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 905
 906         let (body_id, generator_option) = self.with_new_scopes(move |this| {
 907             let prev = this.current_item;
 908             this.current_item = Some(fn_decl_span);
 909             let mut generator_kind = None;
 910             let body_id = this.lower_fn_body(decl, |this| {
 911                 let e = this.lower_expr_mut(body);
 912                 generator_kind = this.generator_kind;
 913                 e
 914             });
 915             let generator_option =
 916                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 917             this.current_item = prev;
 918             (body_id, generator_option)
 919         });
 920
 921         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
 922         // Lower outside new scope to preserve `is_in_loop_condition`.
 923         let fn_decl = self.lower_fn_decl(decl, closure_id, fn_decl_span, FnDeclKind::Closure, None);
 924
 925         let c = self.arena.alloc(hir::Closure {
 926             def_id: self.local_def_id(closure_id),
 927             binder: binder_clause,
 928             capture_clause,
 929             bound_generic_params,
 930             fn_decl,
 931             body: body_id,
 932             fn_decl_span: self.lower_span(fn_decl_span),
 933             fn_arg_span: Some(self.lower_span(fn_arg_span)),
 934             movability: generator_option,
 935             constness: self.lower_constness(constness),
 936         });
 937
 938         hir::ExprKind::Closure(c)
 939     }
 940
 941     fn generator_movability_for_fn(
 942         &mut self,
 943         decl: &FnDecl,
 944         fn_decl_span: Span,
 945         generator_kind: Option<hir::GeneratorKind>,
 946         movability: Movability,
 947     ) -> Option<hir::Movability> {
 948         match generator_kind {
 949             Some(hir::GeneratorKind::Gen) => {
 950                 if decl.inputs.len() > 1 {
 951                     self.tcx.sess.emit_err(GeneratorTooManyParameters { fn_decl_span });
 952                 }
 953                 Some(movability)
 954             }
 955             Some(hir::GeneratorKind::Async(_)) => {
 956                 panic!("non-`async` closure body turned `async` during lowering");
 957             }
 958             None => {
 959                 if movability == Movability::Static {
 960                     self.tcx.sess.emit_err(ClosureCannotBeStatic { fn_decl_span });
 961                 }
 962                 None
 963             }
 964         }
 965     }
 966
 967     fn lower_closure_binder<'c>(
 968         &mut self,
 969         binder: &'c ClosureBinder,
 970     ) -> (hir::ClosureBinder, &'c [GenericParam]) {
 971         let (binder, params) = match binder {
 972             ClosureBinder::NotPresent => (hir::ClosureBinder::Default, &[][..]),
 973             ClosureBinder::For { span, generic_params } => {
 974                 let span = self.lower_span(*span);
 975                 (hir::ClosureBinder::For { span }, &**generic_params)
 976             }
 977         };
 978
 979         (binder, params)
 980     }
 981
 982     fn lower_expr_async_closure(
 983         &mut self,
 984         binder: &ClosureBinder,
 985         capture_clause: CaptureBy,
 986         closure_id: NodeId,
 987         closure_hir_id: hir::HirId,
 988         inner_closure_id: NodeId,
 989         decl: &FnDecl,
 990         body: &Expr,
 991         fn_decl_span: Span,
 992         fn_arg_span: Span,
 993     ) -> hir::ExprKind<'hir> {
 994         if let &ClosureBinder::For { span, .. } = binder {
 995             self.tcx.sess.emit_err(NotSupportedForLifetimeBinderAsyncClosure { span });
 996         }
 997
 998         let (binder_clause, generic_params) = self.lower_closure_binder(binder);
 999
1000         let outer_decl =
1001             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
1002
1003         let body = self.with_new_scopes(|this| {
1004             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
1005             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
1006                 this.tcx.sess.emit_err(AsyncNonMoveClosureNotSupported { fn_decl_span });
1007             }
1008
1009             // Transform `async |x: u8| -> X { ... }` into
1010             // `|x: u8| identity_future(|| -> X { ... })`.
1011             let body_id = this.lower_fn_body(&outer_decl, |this| {
1012                 let async_ret_ty = if let FnRetTy::Ty(ty) = &decl.output {
1013                     let itctx = ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock);
1014                     Some(hir::FnRetTy::Return(this.lower_ty(&ty, &itctx)))
1015                 } else {
1016                     None
1017                 };
1018
1019                 let async_body = this.make_async_expr(
1020                     capture_clause,
1021                     closure_hir_id,
1022                     inner_closure_id,
1023                     async_ret_ty,
1024                     body.span,
1025                     hir::AsyncGeneratorKind::Closure,
1026                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
1027                 );
1028                 this.expr(fn_decl_span, async_body)
1029             });
1030             body_id
1031         });
1032
1033         let bound_generic_params = self.lower_lifetime_binder(closure_id, generic_params);
1034         // We need to lower the declaration outside the new scope, because we
1035         // have to conserve the state of being inside a loop condition for the
1036         // closure argument types.
1037         let fn_decl =
1038             self.lower_fn_decl(&outer_decl, closure_id, fn_decl_span, FnDeclKind::Closure, None);
1039
1040         let c = self.arena.alloc(hir::Closure {
1041             def_id: self.local_def_id(closure_id),
1042             binder: binder_clause,
1043             capture_clause,
1044             bound_generic_params,
1045             fn_decl,
1046             body,
1047             fn_decl_span: self.lower_span(fn_decl_span),
1048             fn_arg_span: Some(self.lower_span(fn_arg_span)),
1049             movability: None,
1050             constness: hir::Constness::NotConst,
1051         });
1052         hir::ExprKind::Closure(c)
1053     }
1054
1055     /// Destructure the LHS of complex assignments.
1056     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
1057     fn lower_expr_assign(
1058         &mut self,
1059         lhs: &Expr,
1060         rhs: &Expr,
1061         eq_sign_span: Span,
1062         whole_span: Span,
1063     ) -> hir::ExprKind<'hir> {
1064         // Return early in case of an ordinary assignment.
1065         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
1066             match &lhs.kind {
1067                 ExprKind::Array(..)
1068                 | ExprKind::Struct(..)
1069                 | ExprKind::Tup(..)
1070                 | ExprKind::Underscore => false,
1071                 // Check for tuple struct constructor.
1072                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
1073                 ExprKind::Paren(e) => {
1074                     match e.kind {
1075                         // We special-case `(..)` for consistency with patterns.
1076                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
1077                         _ => is_ordinary(lower_ctx, e),
1078                     }
1079                 }
1080                 _ => true,
1081             }
1082         }
1083         if is_ordinary(self, lhs) {
1084             return hir::ExprKind::Assign(
1085                 self.lower_expr(lhs),
1086                 self.lower_expr(rhs),
1087                 self.lower_span(eq_sign_span),
1088             );
1089         }
1090
1091         let mut assignments = vec![];
1092
1093         // The LHS becomes a pattern: `(lhs1, lhs2)`.
1094         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
1095         let rhs = self.lower_expr(rhs);
1096
1097         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
1098         let destructure_let = self.stmt_let_pat(
1099             None,
1100             whole_span,
1101             Some(rhs),
1102             pat,
1103             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
1104         );
1105
1106         // `a = lhs1; b = lhs2;`.
1107         let stmts = self
1108             .arena
1109             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
1110
1111         // Wrap everything in a block.
1112         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
1113     }
1114
1115     /// If the given expression is a path to a tuple struct, returns that path.
1116     /// It is not a complete check, but just tries to reject most paths early
1117     /// if they are not tuple structs.
1118     /// Type checking will take care of the full validation later.
1119     fn extract_tuple_struct_path<'a>(
1120         &mut self,
1121         expr: &'a Expr,
1122     ) -> Option<(&'a Option<AstP<QSelf>>, &'a Path)> {
1123         if let ExprKind::Path(qself, path) = &expr.kind {
1124             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1125             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1126                 if let Some(res) = partial_res.full_res() && !res.expected_in_tuple_struct_pat() {
1127                     return None;
1128                 }
1129             }
1130             return Some((qself, path));
1131         }
1132         None
1133     }
1134
1135     /// If the given expression is a path to a unit struct, returns that path.
1136     /// It is not a complete check, but just tries to reject most paths early
1137     /// if they are not unit structs.
1138     /// Type checking will take care of the full validation later.
1139     fn extract_unit_struct_path<'a>(
1140         &mut self,
1141         expr: &'a Expr,
1142     ) -> Option<(&'a Option<AstP<QSelf>>, &'a Path)> {
1143         if let ExprKind::Path(qself, path) = &expr.kind {
1144             // Does the path resolve to something disallowed in a unit struct/variant pattern?
1145             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1146                 if let Some(res) = partial_res.full_res() && !res.expected_in_unit_struct_pat() {
1147                     return None;
1148                 }
1149             }
1150             return Some((qself, path));
1151         }
1152         None
1153     }
1154
1155     /// Convert the LHS of a destructuring assignment to a pattern.
1156     /// Each sub-assignment is recorded in `assignments`.
1157     fn destructure_assign(
1158         &mut self,
1159         lhs: &Expr,
1160         eq_sign_span: Span,
1161         assignments: &mut Vec<hir::Stmt<'hir>>,
1162     ) -> &'hir hir::Pat<'hir> {
1163         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1164     }
1165
1166     fn destructure_assign_mut(
1167         &mut self,
1168         lhs: &Expr,
1169         eq_sign_span: Span,
1170         assignments: &mut Vec<hir::Stmt<'hir>>,
1171     ) -> hir::Pat<'hir> {
1172         match &lhs.kind {
1173             // Underscore pattern.
1174             ExprKind::Underscore => {
1175                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1176             }
1177             // Slice patterns.
1178             ExprKind::Array(elements) => {
1179                 let (pats, rest) =
1180                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1181                 let slice_pat = if let Some((i, span)) = rest {
1182                     let (before, after) = pats.split_at(i);
1183                     hir::PatKind::Slice(
1184                         before,
1185                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1186                         after,
1187                     )
1188                 } else {
1189                     hir::PatKind::Slice(pats, None, &[])
1190                 };
1191                 return self.pat_without_dbm(lhs.span, slice_pat);
1192             }
1193             // Tuple structs.
1194             ExprKind::Call(callee, args) => {
1195                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1196                     let (pats, rest) = self.destructure_sequence(
1197                         args,
1198                         "tuple struct or variant",
1199                         eq_sign_span,
1200                         assignments,
1201                     );
1202                     let qpath = self.lower_qpath(
1203                         callee.id,
1204                         qself,
1205                         path,
1206                         ParamMode::Optional,
1207                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1208                     );
1209                     // Destructure like a tuple struct.
1210                     let tuple_struct_pat = hir::PatKind::TupleStruct(
1211                         qpath,
1212                         pats,
1213                         hir::DotDotPos::new(rest.map(|r| r.0)),
1214                     );
1215                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1216                 }
1217             }
1218             // Unit structs and enum variants.
1219             ExprKind::Path(..) => {
1220                 if let Some((qself, path)) = self.extract_unit_struct_path(lhs) {
1221                     let qpath = self.lower_qpath(
1222                         lhs.id,
1223                         qself,
1224                         path,
1225                         ParamMode::Optional,
1226                         &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1227                     );
1228                     // Destructure like a unit struct.
1229                     let unit_struct_pat = hir::PatKind::Path(qpath);
1230                     return self.pat_without_dbm(lhs.span, unit_struct_pat);
1231                 }
1232             }
1233             // Structs.
1234             ExprKind::Struct(se) => {
1235                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1236                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1237                     hir::PatField {
1238                         hir_id: self.next_id(),
1239                         ident: self.lower_ident(f.ident),
1240                         pat,
1241                         is_shorthand: f.is_shorthand,
1242                         span: self.lower_span(f.span),
1243                     }
1244                 }));
1245                 let qpath = self.lower_qpath(
1246                     lhs.id,
1247                     &se.qself,
1248                     &se.path,
1249                     ParamMode::Optional,
1250                     &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1251                 );
1252                 let fields_omitted = match &se.rest {
1253                     StructRest::Base(e) => {
1254                         self.tcx.sess.emit_err(FunctionalRecordUpdateDestructuringAssignemnt {
1255                             span: e.span,
1256                         });
1257                         true
1258                     }
1259                     StructRest::Rest(_) => true,
1260                     StructRest::None => false,
1261                 };
1262                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1263                 return self.pat_without_dbm(lhs.span, struct_pat);
1264             }
1265             // Tuples.
1266             ExprKind::Tup(elements) => {
1267                 let (pats, rest) =
1268                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1269                 let tuple_pat = hir::PatKind::Tuple(pats, hir::DotDotPos::new(rest.map(|r| r.0)));
1270                 return self.pat_without_dbm(lhs.span, tuple_pat);
1271             }
1272             ExprKind::Paren(e) => {
1273                 // We special-case `(..)` for consistency with patterns.
1274                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1275                     let tuple_pat = hir::PatKind::Tuple(&[], hir::DotDotPos::new(Some(0)));
1276                     return self.pat_without_dbm(lhs.span, tuple_pat);
1277                 } else {
1278                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1279                 }
1280             }
1281             _ => {}
1282         }
1283         // Treat all other cases as normal lvalue.
1284         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1285         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1286         let ident = self.expr_ident(lhs.span, ident, binding);
1287         let assign =
1288             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1289         let expr = self.expr(lhs.span, assign);
1290         assignments.push(self.stmt_expr(lhs.span, expr));
1291         pat
1292     }
1293
1294     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1295     /// Such a sequence occurs in a tuple (struct)/slice.
1296     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1297     /// exists.
1298     /// Each sub-assignment is recorded in `assignments`.
1299     fn destructure_sequence(
1300         &mut self,
1301         elements: &[AstP<Expr>],
1302         ctx: &str,
1303         eq_sign_span: Span,
1304         assignments: &mut Vec<hir::Stmt<'hir>>,
1305     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1306         let mut rest = None;
1307         let elements =
1308             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1309                 // Check for `..` pattern.
1310                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1311                     if let Some((_, prev_span)) = rest {
1312                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1313                     } else {
1314                         rest = Some((i, e.span));
1315                     }
1316                     None
1317                 } else {
1318                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1319                 }
1320             }));
1321         (elements, rest)
1322     }
1323
1324     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1325     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1326         let e1 = self.lower_expr_mut(e1);
1327         let e2 = self.lower_expr_mut(e2);
1328         let fn_path =
1329             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1330         let fn_expr = self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path)));
1331         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1332     }
1333
1334     fn lower_expr_range(
1335         &mut self,
1336         span: Span,
1337         e1: Option<&Expr>,
1338         e2: Option<&Expr>,
1339         lims: RangeLimits,
1340     ) -> hir::ExprKind<'hir> {
1341         use rustc_ast::RangeLimits::*;
1342
1343         let lang_item = match (e1, e2, lims) {
1344             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1345             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1346             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1347             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1348             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1349             (Some(..), Some(..), Closed) => unreachable!(),
1350             (start, None, Closed) => {
1351                 self.tcx.sess.emit_err(InclusiveRangeWithNoEnd { span });
1352                 match start {
1353                     Some(..) => hir::LangItem::RangeFrom,
1354                     None => hir::LangItem::RangeFull,
1355                 }
1356             }
1357         };
1358
1359         let fields = self.arena.alloc_from_iter(
1360             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1361                 |(s, e)| {
1362                     let expr = self.lower_expr(&e);
1363                     let ident = Ident::new(s, self.lower_span(e.span));
1364                     self.expr_field(ident, expr, e.span)
1365                 },
1366             ),
1367         );
1368
1369         hir::ExprKind::Struct(
1370             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1371             fields,
1372             None,
1373         )
1374     }
1375
1376     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1377         let label = opt_label?;
1378         Some(Label { ident: self.lower_ident(label.ident) })
1379     }
1380
1381     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1382         let target_id = match destination {
1383             Some((id, _)) => {
1384                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1385                     Ok(self.lower_node_id(loop_id))
1386                 } else {
1387                     Err(hir::LoopIdError::UnresolvedLabel)
1388                 }
1389             }
1390             None => self
1391                 .loop_scope
1392                 .map(|id| Ok(self.lower_node_id(id)))
1393                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1394         };
1395         let label = self.lower_label(destination.map(|(_, label)| label));
1396         hir::Destination { label, target_id }
1397     }
1398
1399     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1400         if self.is_in_loop_condition && opt_label.is_none() {
1401             hir::Destination {
1402                 label: None,
1403                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1404             }
1405         } else {
1406             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1407         }
1408     }
1409
1410     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1411         let old_scope = self.catch_scope.replace(catch_id);
1412         let result = f(self);
1413         self.catch_scope = old_scope;
1414         result
1415     }
1416
1417     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1418         // We're no longer in the base loop's condition; we're in another loop.
1419         let was_in_loop_condition = self.is_in_loop_condition;
1420         self.is_in_loop_condition = false;
1421
1422         let old_scope = self.loop_scope.replace(loop_id);
1423         let result = f(self);
1424         self.loop_scope = old_scope;
1425
1426         self.is_in_loop_condition = was_in_loop_condition;
1427
1428         result
1429     }
1430
1431     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1432         let was_in_loop_condition = self.is_in_loop_condition;
1433         self.is_in_loop_condition = true;
1434
1435         let result = f(self);
1436
1437         self.is_in_loop_condition = was_in_loop_condition;
1438
1439         result
1440     }
1441
1442     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1443         let hir_id = self.lower_node_id(f.id);
1444         self.lower_attrs(hir_id, &f.attrs);
1445         hir::ExprField {
1446             hir_id,
1447             ident: self.lower_ident(f.ident),
1448             expr: self.lower_expr(&f.expr),
1449             span: self.lower_span(f.span),
1450             is_shorthand: f.is_shorthand,
1451         }
1452     }
1453
1454     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1455         match self.generator_kind {
1456             Some(hir::GeneratorKind::Gen) => {}
1457             Some(hir::GeneratorKind::Async(_)) => {
1458                 self.tcx.sess.emit_err(AsyncGeneratorsNotSupported { span });
1459             }
1460             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1461         }
1462
1463         let expr =
1464             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1465
1466         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1467     }
1468
1469     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1470     /// ```ignore (pseudo-rust)
1471     /// {
1472     ///     let result = match IntoIterator::into_iter(<head>) {
1473     ///         mut iter => {
1474     ///             [opt_ident]: loop {
1475     ///                 match Iterator::next(&mut iter) {
1476     ///                     None => break,
1477     ///                     Some(<pat>) => <body>,
1478     ///                 };
1479     ///             }
1480     ///         }
1481     ///     };
1482     ///     result
1483     /// }
1484     /// ```
1485     fn lower_expr_for(
1486         &mut self,
1487         e: &Expr,
1488         pat: &Pat,
1489         head: &Expr,
1490         body: &Block,
1491         opt_label: Option<Label>,
1492     ) -> hir::Expr<'hir> {
1493         let head = self.lower_expr_mut(head);
1494         let pat = self.lower_pat(pat);
1495         let for_span =
1496             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1497         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1498         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1499
1500         // `None => break`
1501         let none_arm = {
1502             let break_expr = self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span));
1503             let pat = self.pat_none(for_span);
1504             self.arm(pat, break_expr)
1505         };
1506
1507         // Some(<pat>) => <body>,
1508         let some_arm = {
1509             let some_pat = self.pat_some(pat_span, pat);
1510             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1511             let body_expr = self.arena.alloc(self.expr_block(body_block));
1512             self.arm(some_pat, body_expr)
1513         };
1514
1515         // `mut iter`
1516         let iter = Ident::with_dummy_span(sym::iter);
1517         let (iter_pat, iter_pat_nid) =
1518             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::MUT);
1519
1520         // `match Iterator::next(&mut iter) { ... }`
1521         let match_expr = {
1522             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1523             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1524             let next_expr = self.expr_call_lang_item_fn(
1525                 head_span,
1526                 hir::LangItem::IteratorNext,
1527                 arena_vec![self; ref_mut_iter],
1528                 None,
1529             );
1530             let arms = arena_vec![self; none_arm, some_arm];
1531
1532             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1533         };
1534         let match_stmt = self.stmt_expr(for_span, match_expr);
1535
1536         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1537
1538         // `[opt_ident]: loop { ... }`
1539         let kind = hir::ExprKind::Loop(
1540             loop_block,
1541             self.lower_label(opt_label),
1542             hir::LoopSource::ForLoop,
1543             self.lower_span(for_span.with_hi(head.span.hi())),
1544         );
1545         let loop_expr =
1546             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1547
1548         // `mut iter => { ... }`
1549         let iter_arm = self.arm(iter_pat, loop_expr);
1550
1551         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1552         let into_iter_expr = {
1553             self.expr_call_lang_item_fn(
1554                 head_span,
1555                 hir::LangItem::IntoIterIntoIter,
1556                 arena_vec![self; head],
1557                 None,
1558             )
1559         };
1560
1561         let match_expr = self.arena.alloc(self.expr_match(
1562             for_span,
1563             into_iter_expr,
1564             arena_vec![self; iter_arm],
1565             hir::MatchSource::ForLoopDesugar,
1566         ));
1567
1568         // This is effectively `{ let _result = ...; _result }`.
1569         // The construct was introduced in #21984 and is necessary to make sure that
1570         // temporaries in the `head` expression are dropped and do not leak to the
1571         // surrounding scope of the `match` since the `match` is not a terminating scope.
1572         //
1573         // Also, add the attributes to the outer returned expr node.
1574         let expr = self.expr_drop_temps_mut(for_span, match_expr);
1575         self.lower_attrs(expr.hir_id, &e.attrs);
1576         expr
1577     }
1578
1579     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1580     /// ```ignore (pseudo-rust)
1581     /// match Try::branch(<expr>) {
1582     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1583     ///     ControlFlow::Break(residual) =>
1584     ///         #[allow(unreachable_code)]
1585     ///         // If there is an enclosing `try {...}`:
1586     ///         break 'catch_target Try::from_residual(residual),
1587     ///         // Otherwise:
1588     ///         return Try::from_residual(residual),
1589     /// }
1590     /// ```
1591     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1592         let unstable_span = self.mark_span_with_reason(
1593             DesugaringKind::QuestionMark,
1594             span,
1595             self.allow_try_trait.clone(),
1596         );
1597         let try_span = self.tcx.sess.source_map().end_point(span);
1598         let try_span = self.mark_span_with_reason(
1599             DesugaringKind::QuestionMark,
1600             try_span,
1601             self.allow_try_trait.clone(),
1602         );
1603
1604         // `Try::branch(<expr>)`
1605         let scrutinee = {
1606             // expand <expr>
1607             let sub_expr = self.lower_expr_mut(sub_expr);
1608
1609             self.expr_call_lang_item_fn(
1610                 unstable_span,
1611                 hir::LangItem::TryTraitBranch,
1612                 arena_vec![self; sub_expr],
1613                 None,
1614             )
1615         };
1616
1617         // `#[allow(unreachable_code)]`
1618         let attr = attr::mk_attr_nested_word(
1619             &self.tcx.sess.parse_sess.attr_id_generator,
1620             AttrStyle::Outer,
1621             sym::allow,
1622             sym::unreachable_code,
1623             self.lower_span(span),
1624         );
1625         let attrs: AttrVec = thin_vec![attr];
1626
1627         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1628         let continue_arm = {
1629             let val_ident = Ident::with_dummy_span(sym::val);
1630             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1631             let val_expr = self.expr_ident(span, val_ident, val_pat_nid);
1632             self.lower_attrs(val_expr.hir_id, &attrs);
1633             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1634             self.arm(continue_pat, val_expr)
1635         };
1636
1637         // `ControlFlow::Break(residual) =>
1638         //     #[allow(unreachable_code)]
1639         //     return Try::from_residual(residual),`
1640         let break_arm = {
1641             let residual_ident = Ident::with_dummy_span(sym::residual);
1642             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1643             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1644             let from_residual_expr = self.wrap_in_try_constructor(
1645                 hir::LangItem::TryTraitFromResidual,
1646                 try_span,
1647                 self.arena.alloc(residual_expr),
1648                 unstable_span,
1649             );
1650             let ret_expr = if let Some(catch_node) = self.catch_scope {
1651                 let target_id = Ok(self.lower_node_id(catch_node));
1652                 self.arena.alloc(self.expr(
1653                     try_span,
1654                     hir::ExprKind::Break(
1655                         hir::Destination { label: None, target_id },
1656                         Some(from_residual_expr),
1657                     ),
1658                 ))
1659             } else {
1660                 self.arena.alloc(self.expr(try_span, hir::ExprKind::Ret(Some(from_residual_expr))))
1661             };
1662             self.lower_attrs(ret_expr.hir_id, &attrs);
1663
1664             let break_pat = self.pat_cf_break(try_span, residual_local);
1665             self.arm(break_pat, ret_expr)
1666         };
1667
1668         hir::ExprKind::Match(
1669             scrutinee,
1670             arena_vec![self; break_arm, continue_arm],
1671             hir::MatchSource::TryDesugar,
1672         )
1673     }
1674
1675     /// Desugar `ExprKind::Yeet` from: `do yeet <expr>` into:
1676     /// ```ignore(illustrative)
1677     /// // If there is an enclosing `try {...}`:
1678     /// break 'catch_target FromResidual::from_residual(Yeet(residual));
1679     /// // Otherwise:
1680     /// return FromResidual::from_residual(Yeet(residual));
1681     /// ```
1682     /// But to simplify this, there's a `from_yeet` lang item function which
1683     /// handles the combined `FromResidual::from_residual(Yeet(residual))`.
1684     fn lower_expr_yeet(&mut self, span: Span, sub_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1685         // The expression (if present) or `()` otherwise.
1686         let (yeeted_span, yeeted_expr) = if let Some(sub_expr) = sub_expr {
1687             (sub_expr.span, self.lower_expr(sub_expr))
1688         } else {
1689             (self.mark_span_with_reason(DesugaringKind::YeetExpr, span, None), self.expr_unit(span))
1690         };
1691
1692         let unstable_span = self.mark_span_with_reason(
1693             DesugaringKind::YeetExpr,
1694             span,
1695             self.allow_try_trait.clone(),
1696         );
1697
1698         let from_yeet_expr = self.wrap_in_try_constructor(
1699             hir::LangItem::TryTraitFromYeet,
1700             unstable_span,
1701             yeeted_expr,
1702             yeeted_span,
1703         );
1704
1705         if let Some(catch_node) = self.catch_scope {
1706             let target_id = Ok(self.lower_node_id(catch_node));
1707             hir::ExprKind::Break(hir::Destination { label: None, target_id }, Some(from_yeet_expr))
1708         } else {
1709             hir::ExprKind::Ret(Some(from_yeet_expr))
1710         }
1711     }
1712
1713     // =========================================================================
1714     // Helper methods for building HIR.
1715     // =========================================================================
1716
1717     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1718     ///
1719     /// In terms of drop order, it has the same effect as wrapping `expr` in
1720     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1721     ///
1722     /// The drop order can be important in e.g. `if expr { .. }`.
1723     pub(super) fn expr_drop_temps(
1724         &mut self,
1725         span: Span,
1726         expr: &'hir hir::Expr<'hir>,
1727     ) -> &'hir hir::Expr<'hir> {
1728         self.arena.alloc(self.expr_drop_temps_mut(span, expr))
1729     }
1730
1731     pub(super) fn expr_drop_temps_mut(
1732         &mut self,
1733         span: Span,
1734         expr: &'hir hir::Expr<'hir>,
1735     ) -> hir::Expr<'hir> {
1736         self.expr(span, hir::ExprKind::DropTemps(expr))
1737     }
1738
1739     pub(super) fn expr_match(
1740         &mut self,
1741         span: Span,
1742         arg: &'hir hir::Expr<'hir>,
1743         arms: &'hir [hir::Arm<'hir>],
1744         source: hir::MatchSource,
1745     ) -> hir::Expr<'hir> {
1746         self.expr(span, hir::ExprKind::Match(arg, arms, source))
1747     }
1748
1749     fn expr_break(&mut self, span: Span) -> hir::Expr<'hir> {
1750         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1751         self.expr(span, expr_break)
1752     }
1753
1754     fn expr_break_alloc(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1755         let expr_break = self.expr_break(span);
1756         self.arena.alloc(expr_break)
1757     }
1758
1759     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1760         self.expr(span, hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e))
1761     }
1762
1763     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1764         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[])))
1765     }
1766
1767     pub(super) fn expr_usize(&mut self, sp: Span, value: usize) -> hir::Expr<'hir> {
1768         self.expr(
1769             sp,
1770             hir::ExprKind::Lit(hir::Lit {
1771                 span: sp,
1772                 node: ast::LitKind::Int(
1773                     value as u128,
1774                     ast::LitIntType::Unsigned(ast::UintTy::Usize),
1775                 ),
1776             }),
1777         )
1778     }
1779
1780     pub(super) fn expr_u32(&mut self, sp: Span, value: u32) -> hir::Expr<'hir> {
1781         self.expr(
1782             sp,
1783             hir::ExprKind::Lit(hir::Lit {
1784                 span: sp,
1785                 node: ast::LitKind::Int(value.into(), ast::LitIntType::Unsigned(ast::UintTy::U32)),
1786             }),
1787         )
1788     }
1789
1790     pub(super) fn expr_char(&mut self, sp: Span, value: char) -> hir::Expr<'hir> {
1791         self.expr(sp, hir::ExprKind::Lit(hir::Lit { span: sp, node: ast::LitKind::Char(value) }))
1792     }
1793
1794     pub(super) fn expr_str(&mut self, sp: Span, value: Symbol) -> hir::Expr<'hir> {
1795         self.expr(
1796             sp,
1797             hir::ExprKind::Lit(hir::Lit {
1798                 span: sp,
1799                 node: ast::LitKind::Str(value, ast::StrStyle::Cooked),
1800             }),
1801         )
1802     }
1803
1804     pub(super) fn expr_call_mut(
1805         &mut self,
1806         span: Span,
1807         e: &'hir hir::Expr<'hir>,
1808         args: &'hir [hir::Expr<'hir>],
1809     ) -> hir::Expr<'hir> {
1810         self.expr(span, hir::ExprKind::Call(e, args))
1811     }
1812
1813     pub(super) fn expr_call(
1814         &mut self,
1815         span: Span,
1816         e: &'hir hir::Expr<'hir>,
1817         args: &'hir [hir::Expr<'hir>],
1818     ) -> &'hir hir::Expr<'hir> {
1819         self.arena.alloc(self.expr_call_mut(span, e, args))
1820     }
1821
1822     fn expr_call_lang_item_fn_mut(
1823         &mut self,
1824         span: Span,
1825         lang_item: hir::LangItem,
1826         args: &'hir [hir::Expr<'hir>],
1827         hir_id: Option<hir::HirId>,
1828     ) -> hir::Expr<'hir> {
1829         let path = self.arena.alloc(self.expr_lang_item_path(span, lang_item, hir_id));
1830         self.expr_call_mut(span, path, args)
1831     }
1832
1833     fn expr_call_lang_item_fn(
1834         &mut self,
1835         span: Span,
1836         lang_item: hir::LangItem,
1837         args: &'hir [hir::Expr<'hir>],
1838         hir_id: Option<hir::HirId>,
1839     ) -> &'hir hir::Expr<'hir> {
1840         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1841     }
1842
1843     fn expr_lang_item_path(
1844         &mut self,
1845         span: Span,
1846         lang_item: hir::LangItem,
1847         hir_id: Option<hir::HirId>,
1848     ) -> hir::Expr<'hir> {
1849         self.expr(
1850             span,
1851             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1852         )
1853     }
1854
1855     /// `<LangItem>::name`
1856     pub(super) fn expr_lang_item_type_relative(
1857         &mut self,
1858         span: Span,
1859         lang_item: hir::LangItem,
1860         name: Symbol,
1861     ) -> hir::Expr<'hir> {
1862         let path = hir::ExprKind::Path(hir::QPath::TypeRelative(
1863             self.arena.alloc(self.ty(
1864                 span,
1865                 hir::TyKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1866             )),
1867             self.arena.alloc(hir::PathSegment::new(
1868                 Ident::new(name, span),
1869                 self.next_id(),
1870                 Res::Err,
1871             )),
1872         ));
1873         self.expr(span, path)
1874     }
1875
1876     pub(super) fn expr_ident(
1877         &mut self,
1878         sp: Span,
1879         ident: Ident,
1880         binding: hir::HirId,
1881     ) -> &'hir hir::Expr<'hir> {
1882         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1883     }
1884
1885     pub(super) fn expr_ident_mut(
1886         &mut self,
1887         span: Span,
1888         ident: Ident,
1889         binding: hir::HirId,
1890     ) -> hir::Expr<'hir> {
1891         let hir_id = self.next_id();
1892         let res = Res::Local(binding);
1893         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1894             None,
1895             self.arena.alloc(hir::Path {
1896                 span: self.lower_span(span),
1897                 res,
1898                 segments: arena_vec![self; hir::PathSegment::new(ident, hir_id, res)],
1899             }),
1900         ));
1901
1902         self.expr(span, expr_path)
1903     }
1904
1905     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1906         let hir_id = self.next_id();
1907         let span = expr.span;
1908         self.expr(
1909             span,
1910             hir::ExprKind::Block(
1911                 self.arena.alloc(hir::Block {
1912                     stmts: &[],
1913                     expr: Some(expr),
1914                     hir_id,
1915                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1916                     span: self.lower_span(span),
1917                     targeted_by_break: false,
1918                 }),
1919                 None,
1920             ),
1921         )
1922     }
1923
1924     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1925         let blk = self.block_all(span, &[], None);
1926         let expr = self.expr_block(blk);
1927         self.arena.alloc(expr)
1928     }
1929
1930     pub(super) fn expr_block(&mut self, b: &'hir hir::Block<'hir>) -> hir::Expr<'hir> {
1931         self.expr(b.span, hir::ExprKind::Block(b, None))
1932     }
1933
1934     pub(super) fn expr_array_ref(
1935         &mut self,
1936         span: Span,
1937         elements: &'hir [hir::Expr<'hir>],
1938     ) -> hir::Expr<'hir> {
1939         let addrof = hir::ExprKind::AddrOf(
1940             hir::BorrowKind::Ref,
1941             hir::Mutability::Not,
1942             self.arena.alloc(self.expr(span, hir::ExprKind::Array(elements))),
1943         );
1944         self.expr(span, addrof)
1945     }
1946
1947     pub(super) fn expr(&mut self, span: Span, kind: hir::ExprKind<'hir>) -> hir::Expr<'hir> {
1948         let hir_id = self.next_id();
1949         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1950     }
1951
1952     pub(super) fn expr_field(
1953         &mut self,
1954         ident: Ident,
1955         expr: &'hir hir::Expr<'hir>,
1956         span: Span,
1957     ) -> hir::ExprField<'hir> {
1958         hir::ExprField {
1959             hir_id: self.next_id(),
1960             ident,
1961             span: self.lower_span(span),
1962             expr,
1963             is_shorthand: false,
1964         }
1965     }
1966
1967     pub(super) fn arm(
1968         &mut self,
1969         pat: &'hir hir::Pat<'hir>,
1970         expr: &'hir hir::Expr<'hir>,
1971     ) -> hir::Arm<'hir> {
1972         hir::Arm {
1973             hir_id: self.next_id(),
1974             pat,
1975             guard: None,
1976             span: self.lower_span(expr.span),
1977             body: expr,
1978         }
1979     }
1980 }