compiler/rustc_ast_lowering/src/expr.rs

   1 use crate::{FnDeclKind, ImplTraitPosition};
   2
   3 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
   4
   5 use rustc_ast::attr;
   6 use rustc_ast::ptr::P as AstP;
   7 use rustc_ast::*;
   8 use rustc_data_structures::stack::ensure_sufficient_stack;
   9 use rustc_data_structures::thin_vec::ThinVec;
  10 use rustc_errors::struct_span_err;
  11 use rustc_hir as hir;
  12 use rustc_hir::def::Res;
  13 use rustc_hir::definitions::DefPathData;
  14 use rustc_span::hygiene::ExpnId;
  15 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
  16 use rustc_span::symbol::{sym, Ident};
  17 use rustc_span::DUMMY_SP;
  18
  19 impl<'hir> LoweringContext<'_, 'hir> {
  20     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
  21         self.arena.alloc_from_iter(exprs.iter().map(|x| self.lower_expr_mut(x)))
  22     }
  23
  24     pub(super) fn lower_expr(&mut self, e: &Expr) -> &'hir hir::Expr<'hir> {
  25         self.arena.alloc(self.lower_expr_mut(e))
  26     }
  27
  28     pub(super) fn lower_expr_mut(&mut self, e: &Expr) -> hir::Expr<'hir> {
  29         ensure_sufficient_stack(|| {
  30             let kind = match e.kind {
  31                 ExprKind::Box(ref inner) => hir::ExprKind::Box(self.lower_expr(inner)),
  32                 ExprKind::Array(ref exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
  33                 ExprKind::ConstBlock(ref anon_const) => {
  34                     let anon_const = self.lower_anon_const(anon_const);
  35                     hir::ExprKind::ConstBlock(anon_const)
  36                 }
  37                 ExprKind::Repeat(ref expr, ref count) => {
  38                     let expr = self.lower_expr(expr);
  39                     let count = self.lower_array_length(count);
  40                     hir::ExprKind::Repeat(expr, count)
  41                 }
  42                 ExprKind::Tup(ref elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
  43                 ExprKind::Call(ref f, ref args) => {
  44                     if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
  45                         self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
  46                     } else {
  47                         let f = self.lower_expr(f);
  48                         hir::ExprKind::Call(f, self.lower_exprs(args))
  49                     }
  50                 }
  51                 ExprKind::MethodCall(ref seg, ref args, span) => {
  52                     let hir_seg = self.arena.alloc(self.lower_path_segment(
  53                         e.span,
  54                         seg,
  55                         ParamMode::Optional,
  56                         0,
  57                         ParenthesizedGenericArgs::Err,
  58                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
  59                     ));
  60                     let args = self.lower_exprs(args);
  61                     hir::ExprKind::MethodCall(hir_seg, args, self.lower_span(span))
  62                 }
  63                 ExprKind::Binary(binop, ref lhs, ref rhs) => {
  64                     let binop = self.lower_binop(binop);
  65                     let lhs = self.lower_expr(lhs);
  66                     let rhs = self.lower_expr(rhs);
  67                     hir::ExprKind::Binary(binop, lhs, rhs)
  68                 }
  69                 ExprKind::Unary(op, ref ohs) => {
  70                     let op = self.lower_unop(op);
  71                     let ohs = self.lower_expr(ohs);
  72                     hir::ExprKind::Unary(op, ohs)
  73                 }
  74                 ExprKind::Lit(ref l) => {
  75                     hir::ExprKind::Lit(respan(self.lower_span(l.span), l.kind.clone()))
  76                 }
  77                 ExprKind::Cast(ref expr, ref ty) => {
  78                     let expr = self.lower_expr(expr);
  79                     let ty =
  80                         self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
  81                     hir::ExprKind::Cast(expr, ty)
  82                 }
  83                 ExprKind::Type(ref expr, ref ty) => {
  84                     let expr = self.lower_expr(expr);
  85                     let ty =
  86                         self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Type));
  87                     hir::ExprKind::Type(expr, ty)
  88                 }
  89                 ExprKind::AddrOf(k, m, ref ohs) => {
  90                     let ohs = self.lower_expr(ohs);
  91                     hir::ExprKind::AddrOf(k, m, ohs)
  92                 }
  93                 ExprKind::Let(ref pat, ref scrutinee, span) => {
  94                     hir::ExprKind::Let(self.arena.alloc(hir::Let {
  95                         hir_id: self.next_id(),
  96                         span: self.lower_span(span),
  97                         pat: self.lower_pat(pat),
  98                         ty: None,
  99                         init: self.lower_expr(scrutinee),
 100                     }))
 101                 }
 102                 ExprKind::If(ref cond, ref then, ref else_opt) => {
 103                     self.lower_expr_if(cond, then, else_opt.as_deref())
 104                 }
 105                 ExprKind::While(ref cond, ref body, opt_label) => {
 106                     self.with_loop_scope(e.id, |this| {
 107                         let span =
 108                             this.mark_span_with_reason(DesugaringKind::WhileLoop, e.span, None);
 109                         this.lower_expr_while_in_loop_scope(span, cond, body, opt_label)
 110                     })
 111                 }
 112                 ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
 113                     hir::ExprKind::Loop(
 114                         this.lower_block(body, false),
 115                         this.lower_label(opt_label),
 116                         hir::LoopSource::Loop,
 117                         DUMMY_SP,
 118                     )
 119                 }),
 120                 ExprKind::TryBlock(ref body) => self.lower_expr_try_block(body),
 121                 ExprKind::Match(ref expr, ref arms) => hir::ExprKind::Match(
 122                     self.lower_expr(expr),
 123                     self.arena.alloc_from_iter(arms.iter().map(|x| self.lower_arm(x))),
 124                     hir::MatchSource::Normal,
 125                 ),
 126                 ExprKind::Async(capture_clause, closure_node_id, ref block) => self
 127                     .make_async_expr(
 128                         capture_clause,
 129                         closure_node_id,
 130                         None,
 131                         block.span,
 132                         hir::AsyncGeneratorKind::Block,
 133                         |this| this.with_new_scopes(|this| this.lower_block_expr(block)),
 134                     ),
 135                 ExprKind::Await(ref expr) => {
 136                     let span = if expr.span.hi() < e.span.hi() {
 137                         expr.span.shrink_to_hi().with_hi(e.span.hi())
 138                     } else {
 139                         // this is a recovered `await expr`
 140                         e.span
 141                     };
 142                     self.lower_expr_await(span, expr)
 143                 }
 144                 ExprKind::Closure(
 145                     capture_clause,
 146                     asyncness,
 147                     movability,
 148                     ref decl,
 149                     ref body,
 150                     fn_decl_span,
 151                 ) => {
 152                     if let Async::Yes { closure_id, .. } = asyncness {
 153                         self.lower_expr_async_closure(
 154                             capture_clause,
 155                             closure_id,
 156                             decl,
 157                             body,
 158                             fn_decl_span,
 159                         )
 160                     } else {
 161                         self.lower_expr_closure(
 162                             capture_clause,
 163                             movability,
 164                             decl,
 165                             body,
 166                             fn_decl_span,
 167                         )
 168                     }
 169                 }
 170                 ExprKind::Block(ref blk, opt_label) => {
 171                     let opt_label = self.lower_label(opt_label);
 172                     hir::ExprKind::Block(self.lower_block(blk, opt_label.is_some()), opt_label)
 173                 }
 174                 ExprKind::Assign(ref el, ref er, span) => {
 175                     self.lower_expr_assign(el, er, span, e.span)
 176                 }
 177                 ExprKind::AssignOp(op, ref el, ref er) => hir::ExprKind::AssignOp(
 178                     self.lower_binop(op),
 179                     self.lower_expr(el),
 180                     self.lower_expr(er),
 181                 ),
 182                 ExprKind::Field(ref el, ident) => {
 183                     hir::ExprKind::Field(self.lower_expr(el), self.lower_ident(ident))
 184                 }
 185                 ExprKind::Index(ref el, ref er) => {
 186                     hir::ExprKind::Index(self.lower_expr(el), self.lower_expr(er))
 187                 }
 188                 ExprKind::Range(Some(ref e1), Some(ref e2), RangeLimits::Closed) => {
 189                     self.lower_expr_range_closed(e.span, e1, e2)
 190                 }
 191                 ExprKind::Range(ref e1, ref e2, lims) => {
 192                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), lims)
 193                 }
 194                 ExprKind::Underscore => {
 195                     self.sess
 196                         .struct_span_err(
 197                             e.span,
 198                             "in expressions, `_` can only be used on the left-hand side of an assignment",
 199                         )
 200                         .span_label(e.span, "`_` not allowed here")
 201                         .emit();
 202                     hir::ExprKind::Err
 203                 }
 204                 ExprKind::Path(ref qself, ref path) => {
 205                     let qpath = self.lower_qpath(
 206                         e.id,
 207                         qself,
 208                         path,
 209                         ParamMode::Optional,
 210                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 211                     );
 212                     hir::ExprKind::Path(qpath)
 213                 }
 214                 ExprKind::Break(opt_label, ref opt_expr) => {
 215                     let opt_expr = opt_expr.as_ref().map(|x| self.lower_expr(x));
 216                     hir::ExprKind::Break(self.lower_jump_destination(e.id, opt_label), opt_expr)
 217                 }
 218                 ExprKind::Continue(opt_label) => {
 219                     hir::ExprKind::Continue(self.lower_jump_destination(e.id, opt_label))
 220                 }
 221                 ExprKind::Ret(ref e) => {
 222                     let e = e.as_ref().map(|x| self.lower_expr(x));
 223                     hir::ExprKind::Ret(e)
 224                 }
 225                 ExprKind::InlineAsm(ref asm) => {
 226                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
 227                 }
 228                 ExprKind::Struct(ref se) => {
 229                     let rest = match &se.rest {
 230                         StructRest::Base(e) => Some(self.lower_expr(e)),
 231                         StructRest::Rest(sp) => {
 232                             self.sess
 233                                 .struct_span_err(*sp, "base expression required after `..`")
 234                                 .span_label(*sp, "add a base expression here")
 235                                 .emit();
 236                             Some(&*self.arena.alloc(self.expr_err(*sp)))
 237                         }
 238                         StructRest::None => None,
 239                     };
 240                     hir::ExprKind::Struct(
 241                         self.arena.alloc(self.lower_qpath(
 242                             e.id,
 243                             &se.qself,
 244                             &se.path,
 245                             ParamMode::Optional,
 246                             ImplTraitContext::Disallowed(ImplTraitPosition::Path),
 247                         )),
 248                         self.arena
 249                             .alloc_from_iter(se.fields.iter().map(|x| self.lower_expr_field(x))),
 250                         rest,
 251                     )
 252                 }
 253                 ExprKind::Yield(ref opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
 254                 ExprKind::Err => hir::ExprKind::Err,
 255                 ExprKind::Try(ref sub_expr) => self.lower_expr_try(e.span, sub_expr),
 256                 ExprKind::Paren(ref ex) => {
 257                     let mut ex = self.lower_expr_mut(ex);
 258                     // Include parens in span, but only if it is a super-span.
 259                     if e.span.contains(ex.span) {
 260                         ex.span = self.lower_span(e.span);
 261                     }
 262                     // Merge attributes into the inner expression.
 263                     if !e.attrs.is_empty() {
 264                         let old_attrs =
 265                             self.attrs.get(&ex.hir_id.local_id).map(|la| *la).unwrap_or(&[]);
 266                         self.attrs.insert(
 267                             ex.hir_id.local_id,
 268                             &*self.arena.alloc_from_iter(
 269                                 e.attrs
 270                                     .iter()
 271                                     .map(|a| self.lower_attr(a))
 272                                     .chain(old_attrs.iter().cloned()),
 273                             ),
 274                         );
 275                     }
 276                     return ex;
 277                 }
 278
 279                 // Desugar `ExprForLoop`
 280                 // from: `[opt_ident]: for <pat> in <head> <body>`
 281                 ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {
 282                     return self.lower_expr_for(e, pat, head, body, opt_label);
 283                 }
 284                 ExprKind::MacCall(_) => panic!("{:?} shouldn't exist here", e.span),
 285             };
 286
 287             let hir_id = self.lower_node_id(e.id);
 288             self.lower_attrs(hir_id, &e.attrs);
 289             hir::Expr { hir_id, kind, span: self.lower_span(e.span) }
 290         })
 291     }
 292
 293     fn lower_unop(&mut self, u: UnOp) -> hir::UnOp {
 294         match u {
 295             UnOp::Deref => hir::UnOp::Deref,
 296             UnOp::Not => hir::UnOp::Not,
 297             UnOp::Neg => hir::UnOp::Neg,
 298         }
 299     }
 300
 301     fn lower_binop(&mut self, b: BinOp) -> hir::BinOp {
 302         Spanned {
 303             node: match b.node {
 304                 BinOpKind::Add => hir::BinOpKind::Add,
 305                 BinOpKind::Sub => hir::BinOpKind::Sub,
 306                 BinOpKind::Mul => hir::BinOpKind::Mul,
 307                 BinOpKind::Div => hir::BinOpKind::Div,
 308                 BinOpKind::Rem => hir::BinOpKind::Rem,
 309                 BinOpKind::And => hir::BinOpKind::And,
 310                 BinOpKind::Or => hir::BinOpKind::Or,
 311                 BinOpKind::BitXor => hir::BinOpKind::BitXor,
 312                 BinOpKind::BitAnd => hir::BinOpKind::BitAnd,
 313                 BinOpKind::BitOr => hir::BinOpKind::BitOr,
 314                 BinOpKind::Shl => hir::BinOpKind::Shl,
 315                 BinOpKind::Shr => hir::BinOpKind::Shr,
 316                 BinOpKind::Eq => hir::BinOpKind::Eq,
 317                 BinOpKind::Lt => hir::BinOpKind::Lt,
 318                 BinOpKind::Le => hir::BinOpKind::Le,
 319                 BinOpKind::Ne => hir::BinOpKind::Ne,
 320                 BinOpKind::Ge => hir::BinOpKind::Ge,
 321                 BinOpKind::Gt => hir::BinOpKind::Gt,
 322             },
 323             span: self.lower_span(b.span),
 324         }
 325     }
 326
 327     fn lower_legacy_const_generics(
 328         &mut self,
 329         mut f: Expr,
 330         args: Vec<AstP<Expr>>,
 331         legacy_args_idx: &[usize],
 332     ) -> hir::ExprKind<'hir> {
 333         let path = match f.kind {
 334             ExprKind::Path(None, ref mut path) => path,
 335             _ => unreachable!(),
 336         };
 337
 338         // Split the arguments into const generics and normal arguments
 339         let mut real_args = vec![];
 340         let mut generic_args = vec![];
 341         for (idx, arg) in args.into_iter().enumerate() {
 342             if legacy_args_idx.contains(&idx) {
 343                 let parent_def_id = self.current_hir_id_owner;
 344                 let node_id = self.resolver.next_node_id();
 345
 346                 // Add a definition for the in-band const def.
 347                 self.resolver.create_def(
 348                     parent_def_id,
 349                     node_id,
 350                     DefPathData::AnonConst,
 351                     ExpnId::root(),
 352                     arg.span,
 353                 );
 354
 355                 let anon_const = AnonConst { id: node_id, value: arg };
 356                 generic_args.push(AngleBracketedArg::Arg(GenericArg::Const(anon_const)));
 357             } else {
 358                 real_args.push(arg);
 359             }
 360         }
 361
 362         // Add generic args to the last element of the path.
 363         let last_segment = path.segments.last_mut().unwrap();
 364         assert!(last_segment.args.is_none());
 365         last_segment.args = Some(AstP(GenericArgs::AngleBracketed(AngleBracketedArgs {
 366             span: DUMMY_SP,
 367             args: generic_args,
 368         })));
 369
 370         // Now lower everything as normal.
 371         let f = self.lower_expr(&f);
 372         hir::ExprKind::Call(f, self.lower_exprs(&real_args))
 373     }
 374
 375     fn lower_expr_if(
 376         &mut self,
 377         cond: &Expr,
 378         then: &Block,
 379         else_opt: Option<&Expr>,
 380     ) -> hir::ExprKind<'hir> {
 381         let lowered_cond = self.lower_expr(cond);
 382         let new_cond = self.manage_let_cond(lowered_cond);
 383         let then_expr = self.lower_block_expr(then);
 384         if let Some(rslt) = else_opt {
 385             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), Some(self.lower_expr(rslt)))
 386         } else {
 387             hir::ExprKind::If(new_cond, self.arena.alloc(then_expr), None)
 388         }
 389     }
 390
 391     // If `cond` kind is `let`, returns `let`. Otherwise, wraps and returns `cond`
 392     // in a temporary block.
 393     fn manage_let_cond(&mut self, cond: &'hir hir::Expr<'hir>) -> &'hir hir::Expr<'hir> {
 394         fn has_let_expr<'hir>(expr: &'hir hir::Expr<'hir>) -> bool {
 395             match expr.kind {
 396                 hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
 397                 hir::ExprKind::Let(..) => true,
 398                 _ => false,
 399             }
 400         }
 401         if has_let_expr(cond) {
 402             cond
 403         } else {
 404             let reason = DesugaringKind::CondTemporary;
 405             let span_block = self.mark_span_with_reason(reason, cond.span, None);
 406             self.expr_drop_temps(span_block, cond, AttrVec::new())
 407         }
 408     }
 409
 410     // We desugar: `'label: while $cond $body` into:
 411     //
 412     // ```
 413     // 'label: loop {
 414     //   if { let _t = $cond; _t } {
 415     //     $body
 416     //   }
 417     //   else {
 418     //     break;
 419     //   }
 420     // }
 421     // ```
 422     //
 423     // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
 424     // to preserve drop semantics since `while $cond { ... }` does not
 425     // let temporaries live outside of `cond`.
 426     fn lower_expr_while_in_loop_scope(
 427         &mut self,
 428         span: Span,
 429         cond: &Expr,
 430         body: &Block,
 431         opt_label: Option<Label>,
 432     ) -> hir::ExprKind<'hir> {
 433         let lowered_cond = self.with_loop_condition_scope(|t| t.lower_expr(cond));
 434         let new_cond = self.manage_let_cond(lowered_cond);
 435         let then = self.lower_block_expr(body);
 436         let expr_break = self.expr_break(span, ThinVec::new());
 437         let stmt_break = self.stmt_expr(span, expr_break);
 438         let else_blk = self.block_all(span, arena_vec![self; stmt_break], None);
 439         let else_expr = self.arena.alloc(self.expr_block(else_blk, ThinVec::new()));
 440         let if_kind = hir::ExprKind::If(new_cond, self.arena.alloc(then), Some(else_expr));
 441         let if_expr = self.expr(span, if_kind, ThinVec::new());
 442         let block = self.block_expr(self.arena.alloc(if_expr));
 443         let span = self.lower_span(span.with_hi(cond.span.hi()));
 444         let opt_label = self.lower_label(opt_label);
 445         hir::ExprKind::Loop(block, opt_label, hir::LoopSource::While, span)
 446     }
 447
 448     /// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_output(<expr>) }`,
 449     /// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_output(()) }`
 450     /// and save the block id to use it as a break target for desugaring of the `?` operator.
 451     fn lower_expr_try_block(&mut self, body: &Block) -> hir::ExprKind<'hir> {
 452         self.with_catch_scope(body.id, |this| {
 453             let mut block = this.lower_block_noalloc(body, true);
 454
 455             // Final expression of the block (if present) or `()` with span at the end of block
 456             let (try_span, tail_expr) = if let Some(expr) = block.expr.take() {
 457                 (
 458                     this.mark_span_with_reason(
 459                         DesugaringKind::TryBlock,
 460                         expr.span,
 461                         this.allow_try_trait.clone(),
 462                     ),
 463                     expr,
 464                 )
 465             } else {
 466                 let try_span = this.mark_span_with_reason(
 467                     DesugaringKind::TryBlock,
 468                     this.sess.source_map().end_point(body.span),
 469                     this.allow_try_trait.clone(),
 470                 );
 471
 472                 (try_span, this.expr_unit(try_span))
 473             };
 474
 475             let ok_wrapped_span =
 476                 this.mark_span_with_reason(DesugaringKind::TryBlock, tail_expr.span, None);
 477
 478             // `::std::ops::Try::from_output($tail_expr)`
 479             block.expr = Some(this.wrap_in_try_constructor(
 480                 hir::LangItem::TryTraitFromOutput,
 481                 try_span,
 482                 tail_expr,
 483                 ok_wrapped_span,
 484             ));
 485
 486             hir::ExprKind::Block(this.arena.alloc(block), None)
 487         })
 488     }
 489
 490     fn wrap_in_try_constructor(
 491         &mut self,
 492         lang_item: hir::LangItem,
 493         method_span: Span,
 494         expr: &'hir hir::Expr<'hir>,
 495         overall_span: Span,
 496     ) -> &'hir hir::Expr<'hir> {
 497         let constructor = self.arena.alloc(self.expr_lang_item_path(
 498             method_span,
 499             lang_item,
 500             ThinVec::new(),
 501             None,
 502         ));
 503         self.expr_call(overall_span, constructor, std::slice::from_ref(expr))
 504     }
 505
 506     fn lower_arm(&mut self, arm: &Arm) -> hir::Arm<'hir> {
 507         let pat = self.lower_pat(&arm.pat);
 508         let guard = arm.guard.as_ref().map(|cond| {
 509             if let ExprKind::Let(ref pat, ref scrutinee, _) = cond.kind {
 510                 hir::Guard::IfLet(self.lower_pat(pat), self.lower_expr(scrutinee))
 511             } else {
 512                 hir::Guard::If(self.lower_expr(cond))
 513             }
 514         });
 515         let hir_id = self.next_id();
 516         self.lower_attrs(hir_id, &arm.attrs);
 517         hir::Arm {
 518             hir_id,
 519             pat,
 520             guard,
 521             body: self.lower_expr(&arm.body),
 522             span: self.lower_span(arm.span),
 523         }
 524     }
 525
 526     /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
 527     ///
 528     /// This results in:
 529     ///
 530     /// ```text
 531     /// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
 532     ///     <body>
 533     /// })
 534     /// ```
 535     pub(super) fn make_async_expr(
 536         &mut self,
 537         capture_clause: CaptureBy,
 538         closure_node_id: NodeId,
 539         ret_ty: Option<AstP<Ty>>,
 540         span: Span,
 541         async_gen_kind: hir::AsyncGeneratorKind,
 542         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
 543     ) -> hir::ExprKind<'hir> {
 544         let output = match ret_ty {
 545             Some(ty) => hir::FnRetTy::Return(
 546                 self.lower_ty(&ty, ImplTraitContext::Disallowed(ImplTraitPosition::AsyncBlock)),
 547             ),
 548             None => hir::FnRetTy::DefaultReturn(self.lower_span(span)),
 549         };
 550
 551         // Resume argument type. We let the compiler infer this to simplify the lowering. It is
 552         // fully constrained by `future::from_generator`.
 553         let input_ty = hir::Ty {
 554             hir_id: self.next_id(),
 555             kind: hir::TyKind::Infer,
 556             span: self.lower_span(span),
 557         };
 558
 559         // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
 560         let decl = self.arena.alloc(hir::FnDecl {
 561             inputs: arena_vec![self; input_ty],
 562             output,
 563             c_variadic: false,
 564             implicit_self: hir::ImplicitSelfKind::None,
 565         });
 566
 567         // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
 568         let (pat, task_context_hid) = self.pat_ident_binding_mode(
 569             span,
 570             Ident::with_dummy_span(sym::_task_context),
 571             hir::BindingAnnotation::Mutable,
 572         );
 573         let param = hir::Param {
 574             hir_id: self.next_id(),
 575             pat,
 576             ty_span: self.lower_span(span),
 577             span: self.lower_span(span),
 578         };
 579         let params = arena_vec![self; param];
 580
 581         let body_id = self.lower_body(move |this| {
 582             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
 583
 584             let old_ctx = this.task_context;
 585             this.task_context = Some(task_context_hid);
 586             let res = body(this);
 587             this.task_context = old_ctx;
 588             (params, res)
 589         });
 590
 591         // `static |_task_context| -> <ret_ty> { body }`:
 592         let generator_kind = hir::ExprKind::Closure(
 593             capture_clause,
 594             decl,
 595             body_id,
 596             self.lower_span(span),
 597             Some(hir::Movability::Static),
 598         );
 599         let generator = hir::Expr {
 600             hir_id: self.lower_node_id(closure_node_id),
 601             kind: generator_kind,
 602             span: self.lower_span(span),
 603         };
 604
 605         // `future::from_generator`:
 606         let unstable_span =
 607             self.mark_span_with_reason(DesugaringKind::Async, span, self.allow_gen_future.clone());
 608         let gen_future = self.expr_lang_item_path(
 609             unstable_span,
 610             hir::LangItem::FromGenerator,
 611             ThinVec::new(),
 612             None,
 613         );
 614
 615         // `future::from_generator(generator)`:
 616         hir::ExprKind::Call(self.arena.alloc(gen_future), arena_vec![self; generator])
 617     }
 618
 619     /// Desugar `<expr>.await` into:
 620     /// ```rust
 621     /// match ::std::future::IntoFuture::into_future(<expr>) {
 622     ///     mut pinned => loop {
 623     ///         match unsafe { ::std::future::Future::poll(
 624     ///             <::std::pin::Pin>::new_unchecked(&mut pinned),
 625     ///             ::std::future::get_context(task_context),
 626     ///         ) } {
 627     ///             ::std::task::Poll::Ready(result) => break result,
 628     ///             ::std::task::Poll::Pending => {}
 629     ///         }
 630     ///         task_context = yield ();
 631     ///     }
 632     /// }
 633     /// ```
 634     fn lower_expr_await(&mut self, dot_await_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
 635         let full_span = expr.span.to(dot_await_span);
 636         match self.generator_kind {
 637             Some(hir::GeneratorKind::Async(_)) => {}
 638             Some(hir::GeneratorKind::Gen) | None => {
 639                 let mut err = struct_span_err!(
 640                     self.sess,
 641                     dot_await_span,
 642                     E0728,
 643                     "`await` is only allowed inside `async` functions and blocks"
 644                 );
 645                 err.span_label(dot_await_span, "only allowed inside `async` functions and blocks");
 646                 if let Some(item_sp) = self.current_item {
 647                     err.span_label(item_sp, "this is not `async`");
 648                 }
 649                 err.emit();
 650             }
 651         }
 652         let span = self.mark_span_with_reason(DesugaringKind::Await, dot_await_span, None);
 653         let gen_future_span = self.mark_span_with_reason(
 654             DesugaringKind::Await,
 655             full_span,
 656             self.allow_gen_future.clone(),
 657         );
 658         let expr = self.lower_expr_mut(expr);
 659         let expr_hir_id = expr.hir_id;
 660
 661         let pinned_ident = Ident::with_dummy_span(sym::pinned);
 662         let (pinned_pat, pinned_pat_hid) =
 663             self.pat_ident_binding_mode(span, pinned_ident, hir::BindingAnnotation::Mutable);
 664
 665         let task_context_ident = Ident::with_dummy_span(sym::_task_context);
 666
 667         // unsafe {
 668         //     ::std::future::Future::poll(
 669         //         ::std::pin::Pin::new_unchecked(&mut pinned),
 670         //         ::std::future::get_context(task_context),
 671         //     )
 672         // }
 673         let poll_expr = {
 674             let pinned = self.expr_ident(span, pinned_ident, pinned_pat_hid);
 675             let ref_mut_pinned = self.expr_mut_addr_of(span, pinned);
 676             let task_context = if let Some(task_context_hid) = self.task_context {
 677                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
 678             } else {
 679                 // Use of `await` outside of an async context, we cannot use `task_context` here.
 680                 self.expr_err(span)
 681             };
 682             let new_unchecked = self.expr_call_lang_item_fn_mut(
 683                 span,
 684                 hir::LangItem::PinNewUnchecked,
 685                 arena_vec![self; ref_mut_pinned],
 686                 Some(expr_hir_id),
 687             );
 688             let get_context = self.expr_call_lang_item_fn_mut(
 689                 gen_future_span,
 690                 hir::LangItem::GetContext,
 691                 arena_vec![self; task_context],
 692                 Some(expr_hir_id),
 693             );
 694             let call = self.expr_call_lang_item_fn(
 695                 span,
 696                 hir::LangItem::FuturePoll,
 697                 arena_vec![self; new_unchecked, get_context],
 698                 Some(expr_hir_id),
 699             );
 700             self.arena.alloc(self.expr_unsafe(call))
 701         };
 702
 703         // `::std::task::Poll::Ready(result) => break result`
 704         let loop_node_id = self.resolver.next_node_id();
 705         let loop_hir_id = self.lower_node_id(loop_node_id);
 706         let ready_arm = {
 707             let x_ident = Ident::with_dummy_span(sym::result);
 708             let (x_pat, x_pat_hid) = self.pat_ident(gen_future_span, x_ident);
 709             let x_expr = self.expr_ident(gen_future_span, x_ident, x_pat_hid);
 710             let ready_field = self.single_pat_field(gen_future_span, x_pat);
 711             let ready_pat = self.pat_lang_item_variant(
 712                 span,
 713                 hir::LangItem::PollReady,
 714                 ready_field,
 715                 Some(expr_hir_id),
 716             );
 717             let break_x = self.with_loop_scope(loop_node_id, move |this| {
 718                 let expr_break =
 719                     hir::ExprKind::Break(this.lower_loop_destination(None), Some(x_expr));
 720                 this.arena.alloc(this.expr(gen_future_span, expr_break, ThinVec::new()))
 721             });
 722             self.arm(ready_pat, break_x)
 723         };
 724
 725         // `::std::task::Poll::Pending => {}`
 726         let pending_arm = {
 727             let pending_pat = self.pat_lang_item_variant(
 728                 span,
 729                 hir::LangItem::PollPending,
 730                 &[],
 731                 Some(expr_hir_id),
 732             );
 733             let empty_block = self.expr_block_empty(span);
 734             self.arm(pending_pat, empty_block)
 735         };
 736
 737         let inner_match_stmt = {
 738             let match_expr = self.expr_match(
 739                 span,
 740                 poll_expr,
 741                 arena_vec![self; ready_arm, pending_arm],
 742                 hir::MatchSource::AwaitDesugar,
 743             );
 744             self.stmt_expr(span, match_expr)
 745         };
 746
 747         // task_context = yield ();
 748         let yield_stmt = {
 749             let unit = self.expr_unit(span);
 750             let yield_expr = self.expr(
 751                 span,
 752                 hir::ExprKind::Yield(unit, hir::YieldSource::Await { expr: Some(expr_hir_id) }),
 753                 ThinVec::new(),
 754             );
 755             let yield_expr = self.arena.alloc(yield_expr);
 756
 757             if let Some(task_context_hid) = self.task_context {
 758                 let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
 759                 let assign = self.expr(
 760                     span,
 761                     hir::ExprKind::Assign(lhs, yield_expr, self.lower_span(span)),
 762                     AttrVec::new(),
 763                 );
 764                 self.stmt_expr(span, assign)
 765             } else {
 766                 // Use of `await` outside of an async context. Return `yield_expr` so that we can
 767                 // proceed with type checking.
 768                 self.stmt(span, hir::StmtKind::Semi(yield_expr))
 769             }
 770         };
 771
 772         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
 773
 774         // loop { .. }
 775         let loop_expr = self.arena.alloc(hir::Expr {
 776             hir_id: loop_hir_id,
 777             kind: hir::ExprKind::Loop(
 778                 loop_block,
 779                 None,
 780                 hir::LoopSource::Loop,
 781                 self.lower_span(span),
 782             ),
 783             span: self.lower_span(span),
 784         });
 785
 786         // mut pinned => loop { ... }
 787         let pinned_arm = self.arm(pinned_pat, loop_expr);
 788
 789         // `match ::std::future::IntoFuture::into_future(<expr>) { ... }`
 790         let into_future_span = self.mark_span_with_reason(
 791             DesugaringKind::Await,
 792             dot_await_span,
 793             self.allow_into_future.clone(),
 794         );
 795         let into_future_expr = self.expr_call_lang_item_fn(
 796             into_future_span,
 797             hir::LangItem::IntoFutureIntoFuture,
 798             arena_vec![self; expr],
 799             Some(expr_hir_id),
 800         );
 801
 802         // match <into_future_expr> {
 803         //     mut pinned => loop { .. }
 804         // }
 805         hir::ExprKind::Match(
 806             into_future_expr,
 807             arena_vec![self; pinned_arm],
 808             hir::MatchSource::AwaitDesugar,
 809         )
 810     }
 811
 812     fn lower_expr_closure(
 813         &mut self,
 814         capture_clause: CaptureBy,
 815         movability: Movability,
 816         decl: &FnDecl,
 817         body: &Expr,
 818         fn_decl_span: Span,
 819     ) -> hir::ExprKind<'hir> {
 820         let (body_id, generator_option) = self.with_new_scopes(move |this| {
 821             let prev = this.current_item;
 822             this.current_item = Some(fn_decl_span);
 823             let mut generator_kind = None;
 824             let body_id = this.lower_fn_body(decl, |this| {
 825                 let e = this.lower_expr_mut(body);
 826                 generator_kind = this.generator_kind;
 827                 e
 828             });
 829             let generator_option =
 830                 this.generator_movability_for_fn(&decl, fn_decl_span, generator_kind, movability);
 831             this.current_item = prev;
 832             (body_id, generator_option)
 833         });
 834
 835         // Lower outside new scope to preserve `is_in_loop_condition`.
 836         let fn_decl = self.lower_fn_decl(decl, None, FnDeclKind::Closure, None);
 837
 838         hir::ExprKind::Closure(
 839             capture_clause,
 840             fn_decl,
 841             body_id,
 842             self.lower_span(fn_decl_span),
 843             generator_option,
 844         )
 845     }
 846
 847     fn generator_movability_for_fn(
 848         &mut self,
 849         decl: &FnDecl,
 850         fn_decl_span: Span,
 851         generator_kind: Option<hir::GeneratorKind>,
 852         movability: Movability,
 853     ) -> Option<hir::Movability> {
 854         match generator_kind {
 855             Some(hir::GeneratorKind::Gen) => {
 856                 if decl.inputs.len() > 1 {
 857                     struct_span_err!(
 858                         self.sess,
 859                         fn_decl_span,
 860                         E0628,
 861                         "too many parameters for a generator (expected 0 or 1 parameters)"
 862                     )
 863                     .emit();
 864                 }
 865                 Some(movability)
 866             }
 867             Some(hir::GeneratorKind::Async(_)) => {
 868                 panic!("non-`async` closure body turned `async` during lowering");
 869             }
 870             None => {
 871                 if movability == Movability::Static {
 872                     struct_span_err!(self.sess, fn_decl_span, E0697, "closures cannot be static")
 873                         .emit();
 874                 }
 875                 None
 876             }
 877         }
 878     }
 879
 880     fn lower_expr_async_closure(
 881         &mut self,
 882         capture_clause: CaptureBy,
 883         closure_id: NodeId,
 884         decl: &FnDecl,
 885         body: &Expr,
 886         fn_decl_span: Span,
 887     ) -> hir::ExprKind<'hir> {
 888         let outer_decl =
 889             FnDecl { inputs: decl.inputs.clone(), output: FnRetTy::Default(fn_decl_span) };
 890
 891         let body_id = self.with_new_scopes(|this| {
 892             // FIXME(cramertj): allow `async` non-`move` closures with arguments.
 893             if capture_clause == CaptureBy::Ref && !decl.inputs.is_empty() {
 894                 struct_span_err!(
 895                     this.sess,
 896                     fn_decl_span,
 897                     E0708,
 898                     "`async` non-`move` closures with parameters are not currently supported",
 899                 )
 900                 .help(
 901                     "consider using `let` statements to manually capture \
 902                     variables by reference before entering an `async move` closure",
 903                 )
 904                 .emit();
 905             }
 906
 907             // Transform `async |x: u8| -> X { ... }` into
 908             // `|x: u8| future_from_generator(|| -> X { ... })`.
 909             let body_id = this.lower_fn_body(&outer_decl, |this| {
 910                 let async_ret_ty =
 911                     if let FnRetTy::Ty(ty) = &decl.output { Some(ty.clone()) } else { None };
 912                 let async_body = this.make_async_expr(
 913                     capture_clause,
 914                     closure_id,
 915                     async_ret_ty,
 916                     body.span,
 917                     hir::AsyncGeneratorKind::Closure,
 918                     |this| this.with_new_scopes(|this| this.lower_expr_mut(body)),
 919                 );
 920                 this.expr(fn_decl_span, async_body, ThinVec::new())
 921             });
 922             body_id
 923         });
 924
 925         // We need to lower the declaration outside the new scope, because we
 926         // have to conserve the state of being inside a loop condition for the
 927         // closure argument types.
 928         let fn_decl = self.lower_fn_decl(&outer_decl, None, FnDeclKind::Closure, None);
 929
 930         hir::ExprKind::Closure(
 931             capture_clause,
 932             fn_decl,
 933             body_id,
 934             self.lower_span(fn_decl_span),
 935             None,
 936         )
 937     }
 938
 939     /// Destructure the LHS of complex assignments.
 940     /// For instance, lower `(a, b) = t` to `{ let (lhs1, lhs2) = t; a = lhs1; b = lhs2; }`.
 941     fn lower_expr_assign(
 942         &mut self,
 943         lhs: &Expr,
 944         rhs: &Expr,
 945         eq_sign_span: Span,
 946         whole_span: Span,
 947     ) -> hir::ExprKind<'hir> {
 948         // Return early in case of an ordinary assignment.
 949         fn is_ordinary(lower_ctx: &mut LoweringContext<'_, '_>, lhs: &Expr) -> bool {
 950             match &lhs.kind {
 951                 ExprKind::Array(..)
 952                 | ExprKind::Struct(..)
 953                 | ExprKind::Tup(..)
 954                 | ExprKind::Underscore => false,
 955                 // Check for tuple struct constructor.
 956                 ExprKind::Call(callee, ..) => lower_ctx.extract_tuple_struct_path(callee).is_none(),
 957                 ExprKind::Paren(e) => {
 958                     match e.kind {
 959                         // We special-case `(..)` for consistency with patterns.
 960                         ExprKind::Range(None, None, RangeLimits::HalfOpen) => false,
 961                         _ => is_ordinary(lower_ctx, e),
 962                     }
 963                 }
 964                 _ => true,
 965             }
 966         }
 967         if is_ordinary(self, lhs) {
 968             return hir::ExprKind::Assign(
 969                 self.lower_expr(lhs),
 970                 self.lower_expr(rhs),
 971                 self.lower_span(eq_sign_span),
 972             );
 973         }
 974
 975         let mut assignments = vec![];
 976
 977         // The LHS becomes a pattern: `(lhs1, lhs2)`.
 978         let pat = self.destructure_assign(lhs, eq_sign_span, &mut assignments);
 979         let rhs = self.lower_expr(rhs);
 980
 981         // Introduce a `let` for destructuring: `let (lhs1, lhs2) = t`.
 982         let destructure_let = self.stmt_let_pat(
 983             None,
 984             whole_span,
 985             Some(rhs),
 986             pat,
 987             hir::LocalSource::AssignDesugar(self.lower_span(eq_sign_span)),
 988         );
 989
 990         // `a = lhs1; b = lhs2;`.
 991         let stmts = self
 992             .arena
 993             .alloc_from_iter(std::iter::once(destructure_let).chain(assignments.into_iter()));
 994
 995         // Wrap everything in a block.
 996         hir::ExprKind::Block(&self.block_all(whole_span, stmts, None), None)
 997     }
 998
 999     /// If the given expression is a path to a tuple struct, returns that path.
1000     /// It is not a complete check, but just tries to reject most paths early
1001     /// if they are not tuple structs.
1002     /// Type checking will take care of the full validation later.
1003     fn extract_tuple_struct_path<'a>(
1004         &mut self,
1005         expr: &'a Expr,
1006     ) -> Option<(&'a Option<QSelf>, &'a Path)> {
1007         if let ExprKind::Path(qself, path) = &expr.kind {
1008             // Does the path resolve to something disallowed in a tuple struct/variant pattern?
1009             if let Some(partial_res) = self.resolver.get_partial_res(expr.id) {
1010                 if partial_res.unresolved_segments() == 0
1011                     && !partial_res.base_res().expected_in_tuple_struct_pat()
1012                 {
1013                     return None;
1014                 }
1015             }
1016             return Some((qself, path));
1017         }
1018         None
1019     }
1020
1021     /// Convert the LHS of a destructuring assignment to a pattern.
1022     /// Each sub-assignment is recorded in `assignments`.
1023     fn destructure_assign(
1024         &mut self,
1025         lhs: &Expr,
1026         eq_sign_span: Span,
1027         assignments: &mut Vec<hir::Stmt<'hir>>,
1028     ) -> &'hir hir::Pat<'hir> {
1029         self.arena.alloc(self.destructure_assign_mut(lhs, eq_sign_span, assignments))
1030     }
1031
1032     fn destructure_assign_mut(
1033         &mut self,
1034         lhs: &Expr,
1035         eq_sign_span: Span,
1036         assignments: &mut Vec<hir::Stmt<'hir>>,
1037     ) -> hir::Pat<'hir> {
1038         match &lhs.kind {
1039             // Underscore pattern.
1040             ExprKind::Underscore => {
1041                 return self.pat_without_dbm(lhs.span, hir::PatKind::Wild);
1042             }
1043             // Slice patterns.
1044             ExprKind::Array(elements) => {
1045                 let (pats, rest) =
1046                     self.destructure_sequence(elements, "slice", eq_sign_span, assignments);
1047                 let slice_pat = if let Some((i, span)) = rest {
1048                     let (before, after) = pats.split_at(i);
1049                     hir::PatKind::Slice(
1050                         before,
1051                         Some(self.arena.alloc(self.pat_without_dbm(span, hir::PatKind::Wild))),
1052                         after,
1053                     )
1054                 } else {
1055                     hir::PatKind::Slice(pats, None, &[])
1056                 };
1057                 return self.pat_without_dbm(lhs.span, slice_pat);
1058             }
1059             // Tuple structs.
1060             ExprKind::Call(callee, args) => {
1061                 if let Some((qself, path)) = self.extract_tuple_struct_path(callee) {
1062                     let (pats, rest) = self.destructure_sequence(
1063                         args,
1064                         "tuple struct or variant",
1065                         eq_sign_span,
1066                         assignments,
1067                     );
1068                     let qpath = self.lower_qpath(
1069                         callee.id,
1070                         qself,
1071                         path,
1072                         ParamMode::Optional,
1073                         ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1074                     );
1075                     // Destructure like a tuple struct.
1076                     let tuple_struct_pat =
1077                         hir::PatKind::TupleStruct(qpath, pats, rest.map(|r| r.0));
1078                     return self.pat_without_dbm(lhs.span, tuple_struct_pat);
1079                 }
1080             }
1081             // Structs.
1082             ExprKind::Struct(se) => {
1083                 let field_pats = self.arena.alloc_from_iter(se.fields.iter().map(|f| {
1084                     let pat = self.destructure_assign(&f.expr, eq_sign_span, assignments);
1085                     hir::PatField {
1086                         hir_id: self.next_id(),
1087                         ident: self.lower_ident(f.ident),
1088                         pat,
1089                         is_shorthand: f.is_shorthand,
1090                         span: self.lower_span(f.span),
1091                     }
1092                 }));
1093                 let qpath = self.lower_qpath(
1094                     lhs.id,
1095                     &se.qself,
1096                     &se.path,
1097                     ParamMode::Optional,
1098                     ImplTraitContext::Disallowed(ImplTraitPosition::Path),
1099                 );
1100                 let fields_omitted = match &se.rest {
1101                     StructRest::Base(e) => {
1102                         self.sess
1103                             .struct_span_err(
1104                                 e.span,
1105                                 "functional record updates are not allowed in destructuring \
1106                                     assignments",
1107                             )
1108                             .span_suggestion(
1109                                 e.span,
1110                                 "consider removing the trailing pattern",
1111                                 String::new(),
1112                                 rustc_errors::Applicability::MachineApplicable,
1113                             )
1114                             .emit();
1115                         true
1116                     }
1117                     StructRest::Rest(_) => true,
1118                     StructRest::None => false,
1119                 };
1120                 let struct_pat = hir::PatKind::Struct(qpath, field_pats, fields_omitted);
1121                 return self.pat_without_dbm(lhs.span, struct_pat);
1122             }
1123             // Tuples.
1124             ExprKind::Tup(elements) => {
1125                 let (pats, rest) =
1126                     self.destructure_sequence(elements, "tuple", eq_sign_span, assignments);
1127                 let tuple_pat = hir::PatKind::Tuple(pats, rest.map(|r| r.0));
1128                 return self.pat_without_dbm(lhs.span, tuple_pat);
1129             }
1130             ExprKind::Paren(e) => {
1131                 // We special-case `(..)` for consistency with patterns.
1132                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1133                     let tuple_pat = hir::PatKind::Tuple(&[], Some(0));
1134                     return self.pat_without_dbm(lhs.span, tuple_pat);
1135                 } else {
1136                     return self.destructure_assign_mut(e, eq_sign_span, assignments);
1137                 }
1138             }
1139             _ => {}
1140         }
1141         // Treat all other cases as normal lvalue.
1142         let ident = Ident::new(sym::lhs, self.lower_span(lhs.span));
1143         let (pat, binding) = self.pat_ident_mut(lhs.span, ident);
1144         let ident = self.expr_ident(lhs.span, ident, binding);
1145         let assign =
1146             hir::ExprKind::Assign(self.lower_expr(lhs), ident, self.lower_span(eq_sign_span));
1147         let expr = self.expr(lhs.span, assign, ThinVec::new());
1148         assignments.push(self.stmt_expr(lhs.span, expr));
1149         pat
1150     }
1151
1152     /// Destructure a sequence of expressions occurring on the LHS of an assignment.
1153     /// Such a sequence occurs in a tuple (struct)/slice.
1154     /// Return a sequence of corresponding patterns, and the index and the span of `..` if it
1155     /// exists.
1156     /// Each sub-assignment is recorded in `assignments`.
1157     fn destructure_sequence(
1158         &mut self,
1159         elements: &[AstP<Expr>],
1160         ctx: &str,
1161         eq_sign_span: Span,
1162         assignments: &mut Vec<hir::Stmt<'hir>>,
1163     ) -> (&'hir [hir::Pat<'hir>], Option<(usize, Span)>) {
1164         let mut rest = None;
1165         let elements =
1166             self.arena.alloc_from_iter(elements.iter().enumerate().filter_map(|(i, e)| {
1167                 // Check for `..` pattern.
1168                 if let ExprKind::Range(None, None, RangeLimits::HalfOpen) = e.kind {
1169                     if let Some((_, prev_span)) = rest {
1170                         self.ban_extra_rest_pat(e.span, prev_span, ctx);
1171                     } else {
1172                         rest = Some((i, e.span));
1173                     }
1174                     None
1175                 } else {
1176                     Some(self.destructure_assign_mut(e, eq_sign_span, assignments))
1177                 }
1178             }));
1179         (elements, rest)
1180     }
1181
1182     /// Desugar `<start>..=<end>` into `std::ops::RangeInclusive::new(<start>, <end>)`.
1183     fn lower_expr_range_closed(&mut self, span: Span, e1: &Expr, e2: &Expr) -> hir::ExprKind<'hir> {
1184         let e1 = self.lower_expr_mut(e1);
1185         let e2 = self.lower_expr_mut(e2);
1186         let fn_path =
1187             hir::QPath::LangItem(hir::LangItem::RangeInclusiveNew, self.lower_span(span), None);
1188         let fn_expr =
1189             self.arena.alloc(self.expr(span, hir::ExprKind::Path(fn_path), ThinVec::new()));
1190         hir::ExprKind::Call(fn_expr, arena_vec![self; e1, e2])
1191     }
1192
1193     fn lower_expr_range(
1194         &mut self,
1195         span: Span,
1196         e1: Option<&Expr>,
1197         e2: Option<&Expr>,
1198         lims: RangeLimits,
1199     ) -> hir::ExprKind<'hir> {
1200         use rustc_ast::RangeLimits::*;
1201
1202         let lang_item = match (e1, e2, lims) {
1203             (None, None, HalfOpen) => hir::LangItem::RangeFull,
1204             (Some(..), None, HalfOpen) => hir::LangItem::RangeFrom,
1205             (None, Some(..), HalfOpen) => hir::LangItem::RangeTo,
1206             (Some(..), Some(..), HalfOpen) => hir::LangItem::Range,
1207             (None, Some(..), Closed) => hir::LangItem::RangeToInclusive,
1208             (Some(..), Some(..), Closed) => unreachable!(),
1209             (_, None, Closed) => self.diagnostic().span_fatal(span, "inclusive range with no end"),
1210         };
1211
1212         let fields = self.arena.alloc_from_iter(
1213             e1.iter().map(|e| (sym::start, e)).chain(e2.iter().map(|e| (sym::end, e))).map(
1214                 |(s, e)| {
1215                     let expr = self.lower_expr(&e);
1216                     let ident = Ident::new(s, self.lower_span(e.span));
1217                     self.expr_field(ident, expr, e.span)
1218                 },
1219             ),
1220         );
1221
1222         hir::ExprKind::Struct(
1223             self.arena.alloc(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
1224             fields,
1225             None,
1226         )
1227     }
1228
1229     fn lower_label(&self, opt_label: Option<Label>) -> Option<Label> {
1230         let label = opt_label?;
1231         Some(Label { ident: self.lower_ident(label.ident) })
1232     }
1233
1234     fn lower_loop_destination(&mut self, destination: Option<(NodeId, Label)>) -> hir::Destination {
1235         let target_id = match destination {
1236             Some((id, _)) => {
1237                 if let Some(loop_id) = self.resolver.get_label_res(id) {
1238                     Ok(self.lower_node_id(loop_id))
1239                 } else {
1240                     Err(hir::LoopIdError::UnresolvedLabel)
1241                 }
1242             }
1243             None => self
1244                 .loop_scope
1245                 .map(|id| Ok(self.lower_node_id(id)))
1246                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
1247         };
1248         let label = self.lower_label(destination.map(|(_, label)| label));
1249         hir::Destination { label, target_id }
1250     }
1251
1252     fn lower_jump_destination(&mut self, id: NodeId, opt_label: Option<Label>) -> hir::Destination {
1253         if self.is_in_loop_condition && opt_label.is_none() {
1254             hir::Destination {
1255                 label: None,
1256                 target_id: Err(hir::LoopIdError::UnlabeledCfInWhileCondition),
1257             }
1258         } else {
1259             self.lower_loop_destination(opt_label.map(|label| (id, label)))
1260         }
1261     }
1262
1263     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1264         let old_scope = self.catch_scope.replace(catch_id);
1265         let result = f(self);
1266         self.catch_scope = old_scope;
1267         result
1268     }
1269
1270     fn with_loop_scope<T>(&mut self, loop_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
1271         // We're no longer in the base loop's condition; we're in another loop.
1272         let was_in_loop_condition = self.is_in_loop_condition;
1273         self.is_in_loop_condition = false;
1274
1275         let old_scope = self.loop_scope.replace(loop_id);
1276         let result = f(self);
1277         self.loop_scope = old_scope;
1278
1279         self.is_in_loop_condition = was_in_loop_condition;
1280
1281         result
1282     }
1283
1284     fn with_loop_condition_scope<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> T {
1285         let was_in_loop_condition = self.is_in_loop_condition;
1286         self.is_in_loop_condition = true;
1287
1288         let result = f(self);
1289
1290         self.is_in_loop_condition = was_in_loop_condition;
1291
1292         result
1293     }
1294
1295     fn lower_expr_field(&mut self, f: &ExprField) -> hir::ExprField<'hir> {
1296         hir::ExprField {
1297             hir_id: self.next_id(),
1298             ident: self.lower_ident(f.ident),
1299             expr: self.lower_expr(&f.expr),
1300             span: self.lower_span(f.span),
1301             is_shorthand: f.is_shorthand,
1302         }
1303     }
1304
1305     fn lower_expr_yield(&mut self, span: Span, opt_expr: Option<&Expr>) -> hir::ExprKind<'hir> {
1306         match self.generator_kind {
1307             Some(hir::GeneratorKind::Gen) => {}
1308             Some(hir::GeneratorKind::Async(_)) => {
1309                 struct_span_err!(
1310                     self.sess,
1311                     span,
1312                     E0727,
1313                     "`async` generators are not yet supported"
1314                 )
1315                 .emit();
1316             }
1317             None => self.generator_kind = Some(hir::GeneratorKind::Gen),
1318         }
1319
1320         let expr =
1321             opt_expr.as_ref().map(|x| self.lower_expr(x)).unwrap_or_else(|| self.expr_unit(span));
1322
1323         hir::ExprKind::Yield(expr, hir::YieldSource::Yield)
1324     }
1325
1326     /// Desugar `ExprForLoop` from: `[opt_ident]: for <pat> in <head> <body>` into:
1327     /// ```rust
1328     /// {
1329     ///     let result = match IntoIterator::into_iter(<head>) {
1330     ///         mut iter => {
1331     ///             [opt_ident]: loop {
1332     ///                 match Iterator::next(&mut iter) {
1333     ///                     None => break,
1334     ///                     Some(<pat>) => <body>,
1335     ///                 };
1336     ///             }
1337     ///         }
1338     ///     };
1339     ///     result
1340     /// }
1341     /// ```
1342     fn lower_expr_for(
1343         &mut self,
1344         e: &Expr,
1345         pat: &Pat,
1346         head: &Expr,
1347         body: &Block,
1348         opt_label: Option<Label>,
1349     ) -> hir::Expr<'hir> {
1350         let head = self.lower_expr_mut(head);
1351         let pat = self.lower_pat(pat);
1352         let for_span =
1353             self.mark_span_with_reason(DesugaringKind::ForLoop, self.lower_span(e.span), None);
1354         let head_span = self.mark_span_with_reason(DesugaringKind::ForLoop, head.span, None);
1355         let pat_span = self.mark_span_with_reason(DesugaringKind::ForLoop, pat.span, None);
1356
1357         // `None => break`
1358         let none_arm = {
1359             let break_expr =
1360                 self.with_loop_scope(e.id, |this| this.expr_break_alloc(for_span, ThinVec::new()));
1361             let pat = self.pat_none(for_span);
1362             self.arm(pat, break_expr)
1363         };
1364
1365         // Some(<pat>) => <body>,
1366         let some_arm = {
1367             let some_pat = self.pat_some(pat_span, pat);
1368             let body_block = self.with_loop_scope(e.id, |this| this.lower_block(body, false));
1369             let body_expr = self.arena.alloc(self.expr_block(body_block, ThinVec::new()));
1370             self.arm(some_pat, body_expr)
1371         };
1372
1373         // `mut iter`
1374         let iter = Ident::with_dummy_span(sym::iter);
1375         let (iter_pat, iter_pat_nid) =
1376             self.pat_ident_binding_mode(head_span, iter, hir::BindingAnnotation::Mutable);
1377
1378         // `match Iterator::next(&mut iter) { ... }`
1379         let match_expr = {
1380             let iter = self.expr_ident(head_span, iter, iter_pat_nid);
1381             let ref_mut_iter = self.expr_mut_addr_of(head_span, iter);
1382             let next_expr = self.expr_call_lang_item_fn(
1383                 head_span,
1384                 hir::LangItem::IteratorNext,
1385                 arena_vec![self; ref_mut_iter],
1386                 None,
1387             );
1388             let arms = arena_vec![self; none_arm, some_arm];
1389
1390             self.expr_match(head_span, next_expr, arms, hir::MatchSource::ForLoopDesugar)
1391         };
1392         let match_stmt = self.stmt_expr(for_span, match_expr);
1393
1394         let loop_block = self.block_all(for_span, arena_vec![self; match_stmt], None);
1395
1396         // `[opt_ident]: loop { ... }`
1397         let kind = hir::ExprKind::Loop(
1398             loop_block,
1399             self.lower_label(opt_label),
1400             hir::LoopSource::ForLoop,
1401             self.lower_span(for_span.with_hi(head.span.hi())),
1402         );
1403         let loop_expr =
1404             self.arena.alloc(hir::Expr { hir_id: self.lower_node_id(e.id), kind, span: for_span });
1405
1406         // `mut iter => { ... }`
1407         let iter_arm = self.arm(iter_pat, loop_expr);
1408
1409         // `match ::std::iter::IntoIterator::into_iter(<head>) { ... }`
1410         let into_iter_expr = {
1411             self.expr_call_lang_item_fn(
1412                 head_span,
1413                 hir::LangItem::IntoIterIntoIter,
1414                 arena_vec![self; head],
1415                 None,
1416             )
1417         };
1418
1419         let match_expr = self.arena.alloc(self.expr_match(
1420             for_span,
1421             into_iter_expr,
1422             arena_vec![self; iter_arm],
1423             hir::MatchSource::ForLoopDesugar,
1424         ));
1425
1426         let attrs: Vec<_> = e.attrs.iter().map(|a| self.lower_attr(a)).collect();
1427
1428         // This is effectively `{ let _result = ...; _result }`.
1429         // The construct was introduced in #21984 and is necessary to make sure that
1430         // temporaries in the `head` expression are dropped and do not leak to the
1431         // surrounding scope of the `match` since the `match` is not a terminating scope.
1432         //
1433         // Also, add the attributes to the outer returned expr node.
1434         self.expr_drop_temps_mut(for_span, match_expr, attrs.into())
1435     }
1436
1437     /// Desugar `ExprKind::Try` from: `<expr>?` into:
1438     /// ```rust
1439     /// match Try::branch(<expr>) {
1440     ///     ControlFlow::Continue(val) => #[allow(unreachable_code)] val,,
1441     ///     ControlFlow::Break(residual) =>
1442     ///         #[allow(unreachable_code)]
1443     ///         // If there is an enclosing `try {...}`:
1444     ///         break 'catch_target Try::from_residual(residual),
1445     ///         // Otherwise:
1446     ///         return Try::from_residual(residual),
1447     /// }
1448     /// ```
1449     fn lower_expr_try(&mut self, span: Span, sub_expr: &Expr) -> hir::ExprKind<'hir> {
1450         let unstable_span = self.mark_span_with_reason(
1451             DesugaringKind::QuestionMark,
1452             span,
1453             self.allow_try_trait.clone(),
1454         );
1455         let try_span = self.sess.source_map().end_point(span);
1456         let try_span = self.mark_span_with_reason(
1457             DesugaringKind::QuestionMark,
1458             try_span,
1459             self.allow_try_trait.clone(),
1460         );
1461
1462         // `Try::branch(<expr>)`
1463         let scrutinee = {
1464             // expand <expr>
1465             let sub_expr = self.lower_expr_mut(sub_expr);
1466
1467             self.expr_call_lang_item_fn(
1468                 unstable_span,
1469                 hir::LangItem::TryTraitBranch,
1470                 arena_vec![self; sub_expr],
1471                 None,
1472             )
1473         };
1474
1475         // `#[allow(unreachable_code)]`
1476         let attr = {
1477             // `allow(unreachable_code)`
1478             let allow = {
1479                 let allow_ident = Ident::new(sym::allow, self.lower_span(span));
1480                 let uc_ident = Ident::new(sym::unreachable_code, self.lower_span(span));
1481                 let uc_nested = attr::mk_nested_word_item(uc_ident);
1482                 attr::mk_list_item(allow_ident, vec![uc_nested])
1483             };
1484             attr::mk_attr_outer(allow)
1485         };
1486         let attrs = vec![attr];
1487
1488         // `ControlFlow::Continue(val) => #[allow(unreachable_code)] val,`
1489         let continue_arm = {
1490             let val_ident = Ident::with_dummy_span(sym::val);
1491             let (val_pat, val_pat_nid) = self.pat_ident(span, val_ident);
1492             let val_expr = self.arena.alloc(self.expr_ident_with_attrs(
1493                 span,
1494                 val_ident,
1495                 val_pat_nid,
1496                 ThinVec::from(attrs.clone()),
1497             ));
1498             let continue_pat = self.pat_cf_continue(unstable_span, val_pat);
1499             self.arm(continue_pat, val_expr)
1500         };
1501
1502         // `ControlFlow::Break(residual) =>
1503         //     #[allow(unreachable_code)]
1504         //     return Try::from_residual(residual),`
1505         let break_arm = {
1506             let residual_ident = Ident::with_dummy_span(sym::residual);
1507             let (residual_local, residual_local_nid) = self.pat_ident(try_span, residual_ident);
1508             let residual_expr = self.expr_ident_mut(try_span, residual_ident, residual_local_nid);
1509             let from_residual_expr = self.wrap_in_try_constructor(
1510                 hir::LangItem::TryTraitFromResidual,
1511                 try_span,
1512                 self.arena.alloc(residual_expr),
1513                 unstable_span,
1514             );
1515             let thin_attrs = ThinVec::from(attrs);
1516             let ret_expr = if let Some(catch_node) = self.catch_scope {
1517                 let target_id = Ok(self.lower_node_id(catch_node));
1518                 self.arena.alloc(self.expr(
1519                     try_span,
1520                     hir::ExprKind::Break(
1521                         hir::Destination { label: None, target_id },
1522                         Some(from_residual_expr),
1523                     ),
1524                     thin_attrs,
1525                 ))
1526             } else {
1527                 self.arena.alloc(self.expr(
1528                     try_span,
1529                     hir::ExprKind::Ret(Some(from_residual_expr)),
1530                     thin_attrs,
1531                 ))
1532             };
1533
1534             let break_pat = self.pat_cf_break(try_span, residual_local);
1535             self.arm(break_pat, ret_expr)
1536         };
1537
1538         hir::ExprKind::Match(
1539             scrutinee,
1540             arena_vec![self; break_arm, continue_arm],
1541             hir::MatchSource::TryDesugar,
1542         )
1543     }
1544
1545     // =========================================================================
1546     // Helper methods for building HIR.
1547     // =========================================================================
1548
1549     /// Wrap the given `expr` in a terminating scope using `hir::ExprKind::DropTemps`.
1550     ///
1551     /// In terms of drop order, it has the same effect as wrapping `expr` in
1552     /// `{ let _t = $expr; _t }` but should provide better compile-time performance.
1553     ///
1554     /// The drop order can be important in e.g. `if expr { .. }`.
1555     pub(super) fn expr_drop_temps(
1556         &mut self,
1557         span: Span,
1558         expr: &'hir hir::Expr<'hir>,
1559         attrs: AttrVec,
1560     ) -> &'hir hir::Expr<'hir> {
1561         self.arena.alloc(self.expr_drop_temps_mut(span, expr, attrs))
1562     }
1563
1564     pub(super) fn expr_drop_temps_mut(
1565         &mut self,
1566         span: Span,
1567         expr: &'hir hir::Expr<'hir>,
1568         attrs: AttrVec,
1569     ) -> hir::Expr<'hir> {
1570         self.expr(span, hir::ExprKind::DropTemps(expr), attrs)
1571     }
1572
1573     fn expr_match(
1574         &mut self,
1575         span: Span,
1576         arg: &'hir hir::Expr<'hir>,
1577         arms: &'hir [hir::Arm<'hir>],
1578         source: hir::MatchSource,
1579     ) -> hir::Expr<'hir> {
1580         self.expr(span, hir::ExprKind::Match(arg, arms, source), ThinVec::new())
1581     }
1582
1583     fn expr_break(&mut self, span: Span, attrs: AttrVec) -> hir::Expr<'hir> {
1584         let expr_break = hir::ExprKind::Break(self.lower_loop_destination(None), None);
1585         self.expr(span, expr_break, attrs)
1586     }
1587
1588     fn expr_break_alloc(&mut self, span: Span, attrs: AttrVec) -> &'hir hir::Expr<'hir> {
1589         let expr_break = self.expr_break(span, attrs);
1590         self.arena.alloc(expr_break)
1591     }
1592
1593     fn expr_mut_addr_of(&mut self, span: Span, e: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1594         self.expr(
1595             span,
1596             hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Mut, e),
1597             ThinVec::new(),
1598         )
1599     }
1600
1601     fn expr_unit(&mut self, sp: Span) -> &'hir hir::Expr<'hir> {
1602         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[]), ThinVec::new()))
1603     }
1604
1605     fn expr_call_mut(
1606         &mut self,
1607         span: Span,
1608         e: &'hir hir::Expr<'hir>,
1609         args: &'hir [hir::Expr<'hir>],
1610     ) -> hir::Expr<'hir> {
1611         self.expr(span, hir::ExprKind::Call(e, args), ThinVec::new())
1612     }
1613
1614     fn expr_call(
1615         &mut self,
1616         span: Span,
1617         e: &'hir hir::Expr<'hir>,
1618         args: &'hir [hir::Expr<'hir>],
1619     ) -> &'hir hir::Expr<'hir> {
1620         self.arena.alloc(self.expr_call_mut(span, e, args))
1621     }
1622
1623     fn expr_call_lang_item_fn_mut(
1624         &mut self,
1625         span: Span,
1626         lang_item: hir::LangItem,
1627         args: &'hir [hir::Expr<'hir>],
1628         hir_id: Option<hir::HirId>,
1629     ) -> hir::Expr<'hir> {
1630         let path =
1631             self.arena.alloc(self.expr_lang_item_path(span, lang_item, ThinVec::new(), hir_id));
1632         self.expr_call_mut(span, path, args)
1633     }
1634
1635     fn expr_call_lang_item_fn(
1636         &mut self,
1637         span: Span,
1638         lang_item: hir::LangItem,
1639         args: &'hir [hir::Expr<'hir>],
1640         hir_id: Option<hir::HirId>,
1641     ) -> &'hir hir::Expr<'hir> {
1642         self.arena.alloc(self.expr_call_lang_item_fn_mut(span, lang_item, args, hir_id))
1643     }
1644
1645     fn expr_lang_item_path(
1646         &mut self,
1647         span: Span,
1648         lang_item: hir::LangItem,
1649         attrs: AttrVec,
1650         hir_id: Option<hir::HirId>,
1651     ) -> hir::Expr<'hir> {
1652         self.expr(
1653             span,
1654             hir::ExprKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), hir_id)),
1655             attrs,
1656         )
1657     }
1658
1659     pub(super) fn expr_ident(
1660         &mut self,
1661         sp: Span,
1662         ident: Ident,
1663         binding: hir::HirId,
1664     ) -> &'hir hir::Expr<'hir> {
1665         self.arena.alloc(self.expr_ident_mut(sp, ident, binding))
1666     }
1667
1668     pub(super) fn expr_ident_mut(
1669         &mut self,
1670         sp: Span,
1671         ident: Ident,
1672         binding: hir::HirId,
1673     ) -> hir::Expr<'hir> {
1674         self.expr_ident_with_attrs(sp, ident, binding, ThinVec::new())
1675     }
1676
1677     fn expr_ident_with_attrs(
1678         &mut self,
1679         span: Span,
1680         ident: Ident,
1681         binding: hir::HirId,
1682         attrs: AttrVec,
1683     ) -> hir::Expr<'hir> {
1684         let expr_path = hir::ExprKind::Path(hir::QPath::Resolved(
1685             None,
1686             self.arena.alloc(hir::Path {
1687                 span: self.lower_span(span),
1688                 res: Res::Local(binding),
1689                 segments: arena_vec![self; hir::PathSegment::from_ident(ident)],
1690             }),
1691         ));
1692
1693         self.expr(span, expr_path, attrs)
1694     }
1695
1696     fn expr_unsafe(&mut self, expr: &'hir hir::Expr<'hir>) -> hir::Expr<'hir> {
1697         let hir_id = self.next_id();
1698         let span = expr.span;
1699         self.expr(
1700             span,
1701             hir::ExprKind::Block(
1702                 self.arena.alloc(hir::Block {
1703                     stmts: &[],
1704                     expr: Some(expr),
1705                     hir_id,
1706                     rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
1707                     span: self.lower_span(span),
1708                     targeted_by_break: false,
1709                 }),
1710                 None,
1711             ),
1712             ThinVec::new(),
1713         )
1714     }
1715
1716     fn expr_block_empty(&mut self, span: Span) -> &'hir hir::Expr<'hir> {
1717         let blk = self.block_all(span, &[], None);
1718         let expr = self.expr_block(blk, ThinVec::new());
1719         self.arena.alloc(expr)
1720     }
1721
1722     pub(super) fn expr_block(
1723         &mut self,
1724         b: &'hir hir::Block<'hir>,
1725         attrs: AttrVec,
1726     ) -> hir::Expr<'hir> {
1727         self.expr(b.span, hir::ExprKind::Block(b, None), attrs)
1728     }
1729
1730     pub(super) fn expr(
1731         &mut self,
1732         span: Span,
1733         kind: hir::ExprKind<'hir>,
1734         attrs: AttrVec,
1735     ) -> hir::Expr<'hir> {
1736         let hir_id = self.next_id();
1737         self.lower_attrs(hir_id, &attrs);
1738         hir::Expr { hir_id, kind, span: self.lower_span(span) }
1739     }
1740
1741     fn expr_field(
1742         &mut self,
1743         ident: Ident,
1744         expr: &'hir hir::Expr<'hir>,
1745         span: Span,
1746     ) -> hir::ExprField<'hir> {
1747         hir::ExprField {
1748             hir_id: self.next_id(),
1749             ident,
1750             span: self.lower_span(span),
1751             expr,
1752             is_shorthand: false,
1753         }
1754     }
1755
1756     fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
1757         hir::Arm {
1758             hir_id: self.next_id(),
1759             pat,
1760             guard: None,
1761             span: self.lower_span(expr.span),
1762             body: expr,
1763         }
1764     }
1765 }