src/librustc_typeck/check/expr.rs

   1 //! Type checking expressions.
   2 //!
   3 //! See `mod.rs` for more context on type checking in general.
   4
   5 use crate::check::BreakableCtxt;
   6 use crate::check::cast;
   7 use crate::check::coercion::CoerceMany;
   8 use crate::check::Diverges;
   9 use crate::check::FnCtxt;
  10 use crate::check::Expectation::{self, NoExpectation, ExpectHasType, ExpectCastableToType};
  11 use crate::check::fatally_break_rust;
  12 use crate::check::report_unexpected_variant_res;
  13 use crate::check::Needs;
  14 use crate::check::TupleArgumentsFlag::DontTupleArguments;
  15 use crate::check::method::SelfSource;
  16 use crate::middle::lang_items;
  17 use crate::util::common::ErrorReported;
  18
  19 use errors::Applicability;
  20 use syntax::ast;
  21 use syntax::ptr::P;
  22 use syntax::symbol::{kw, sym};
  23 use syntax::source_map::Span;
  24 use rustc::hir;
  25 use rustc::hir::{ExprKind, QPath};
  26 use rustc::hir::def::{CtorKind, Res, DefKind};
  27 use rustc::infer;
  28 use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  29 use rustc::mir::interpret::GlobalId;
  30 use rustc::ty;
  31 use rustc::ty::adjustment::{
  32     Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability,
  33 };
  34 use rustc::ty::Ty;
  35 use rustc::ty::TypeFoldable;
  36 use rustc::ty::subst::InternalSubsts;
  37 use rustc::traits::{self, ObligationCauseCode};
  38
  39 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  40     fn check_expr_eq_type(&self, expr: &'tcx hir::Expr, expected: Ty<'tcx>) {
  41         let ty = self.check_expr_with_hint(expr, expected);
  42         self.demand_eqtype(expr.span, expected, ty);
  43     }
  44
  45     pub fn check_expr_has_type_or_error(
  46         &self,
  47         expr: &'tcx hir::Expr,
  48         expected: Ty<'tcx>,
  49     ) -> Ty<'tcx> {
  50         self.check_expr_meets_expectation_or_error(expr, ExpectHasType(expected))
  51     }
  52
  53     fn check_expr_meets_expectation_or_error(
  54         &self,
  55         expr: &'tcx hir::Expr,
  56         expected: Expectation<'tcx>,
  57     ) -> Ty<'tcx> {
  58         let expected_ty = expected.to_option(&self).unwrap_or(self.tcx.types.bool);
  59         let mut ty = self.check_expr_with_expectation(expr, expected);
  60
  61         // While we don't allow *arbitrary* coercions here, we *do* allow
  62         // coercions from ! to `expected`.
  63         if ty.is_never() {
  64             assert!(!self.tables.borrow().adjustments().contains_key(expr.hir_id),
  65                     "expression with never type wound up being adjusted");
  66             let adj_ty = self.next_diverging_ty_var(
  67                 TypeVariableOrigin {
  68                     kind: TypeVariableOriginKind::AdjustmentType,
  69                     span: expr.span,
  70                 },
  71             );
  72             self.apply_adjustments(expr, vec![Adjustment {
  73                 kind: Adjust::NeverToAny,
  74                 target: adj_ty
  75             }]);
  76             ty = adj_ty;
  77         }
  78
  79         if let Some(mut err) = self.demand_suptype_diag(expr.span, expected_ty, ty) {
  80             let expr = match &expr.node {
  81                 ExprKind::DropTemps(expr) => expr,
  82                 _ => expr,
  83             };
  84             // Error possibly reported in `check_assign` so avoid emitting error again.
  85             err.emit_unless(self.is_assign_to_bool(expr, expected_ty));
  86         }
  87         ty
  88     }
  89
  90     pub(super) fn check_expr_coercable_to_type(
  91         &self,
  92         expr: &'tcx hir::Expr,
  93         expected: Ty<'tcx>
  94     ) -> Ty<'tcx> {
  95         let ty = self.check_expr_with_hint(expr, expected);
  96         // checks don't need two phase
  97         self.demand_coerce(expr, ty, expected, AllowTwoPhase::No)
  98     }
  99
 100     pub(super) fn check_expr_with_hint(
 101         &self,
 102         expr: &'tcx hir::Expr,
 103         expected: Ty<'tcx>
 104     ) -> Ty<'tcx> {
 105         self.check_expr_with_expectation(expr, ExpectHasType(expected))
 106     }
 107
 108     pub(super) fn check_expr_with_expectation(
 109         &self,
 110         expr: &'tcx hir::Expr,
 111         expected: Expectation<'tcx>,
 112     ) -> Ty<'tcx> {
 113         self.check_expr_with_expectation_and_needs(expr, expected, Needs::None)
 114     }
 115
 116     pub(super) fn check_expr(&self, expr: &'tcx hir::Expr) -> Ty<'tcx> {
 117         self.check_expr_with_expectation(expr, NoExpectation)
 118     }
 119
 120     pub(super) fn check_expr_with_needs(&self, expr: &'tcx hir::Expr, needs: Needs) -> Ty<'tcx> {
 121         self.check_expr_with_expectation_and_needs(expr, NoExpectation, needs)
 122     }
 123
 124     /// Invariant:
 125     /// If an expression has any sub-expressions that result in a type error,
 126     /// inspecting that expression's type with `ty.references_error()` will return
 127     /// true. Likewise, if an expression is known to diverge, inspecting its
 128     /// type with `ty::type_is_bot` will return true (n.b.: since Rust is
 129     /// strict, _|_ can appear in the type of an expression that does not,
 130     /// itself, diverge: for example, fn() -> _|_.)
 131     /// Note that inspecting a type's structure *directly* may expose the fact
 132     /// that there are actually multiple representations for `Error`, so avoid
 133     /// that when err needs to be handled differently.
 134     fn check_expr_with_expectation_and_needs(
 135         &self,
 136         expr: &'tcx hir::Expr,
 137         expected: Expectation<'tcx>,
 138         needs: Needs,
 139     ) -> Ty<'tcx> {
 140         debug!(">> type-checking: expr={:?} expected={:?}",
 141                expr, expected);
 142
 143         // Warn for expressions after diverging siblings.
 144         self.warn_if_unreachable(expr.hir_id, expr.span, "expression");
 145
 146         // Hide the outer diverging and has_errors flags.
 147         let old_diverges = self.diverges.get();
 148         let old_has_errors = self.has_errors.get();
 149         self.diverges.set(Diverges::Maybe);
 150         self.has_errors.set(false);
 151
 152         let ty = self.check_expr_kind(expr, expected, needs);
 153
 154         // Warn for non-block expressions with diverging children.
 155         match expr.node {
 156             ExprKind::Block(..) |
 157             ExprKind::Loop(..) | ExprKind::While(..) |
 158             ExprKind::Match(..) => {}
 159
 160             _ => self.warn_if_unreachable(expr.hir_id, expr.span, "expression")
 161         }
 162
 163         // Any expression that produces a value of type `!` must have diverged
 164         if ty.is_never() {
 165             self.diverges.set(self.diverges.get() | Diverges::Always);
 166         }
 167
 168         // Record the type, which applies it effects.
 169         // We need to do this after the warning above, so that
 170         // we don't warn for the diverging expression itself.
 171         self.write_ty(expr.hir_id, ty);
 172
 173         // Combine the diverging and has_error flags.
 174         self.diverges.set(self.diverges.get() | old_diverges);
 175         self.has_errors.set(self.has_errors.get() | old_has_errors);
 176
 177         debug!("type of {} is...", self.tcx.hir().hir_to_string(expr.hir_id));
 178         debug!("... {:?}, expected is {:?}", ty, expected);
 179
 180         ty
 181     }
 182
 183     fn check_expr_kind(
 184         &self,
 185         expr: &'tcx hir::Expr,
 186         expected: Expectation<'tcx>,
 187         needs: Needs,
 188     ) -> Ty<'tcx> {
 189         debug!(
 190             "check_expr_kind(expr={:?}, expected={:?}, needs={:?})",
 191             expr,
 192             expected,
 193             needs,
 194         );
 195
 196         let tcx = self.tcx;
 197         match expr.node {
 198             ExprKind::Box(ref subexpr) => {
 199                 self.check_expr_box(subexpr, expected)
 200             }
 201             ExprKind::Lit(ref lit) => {
 202                 self.check_lit(&lit, expected)
 203             }
 204             ExprKind::Binary(op, ref lhs, ref rhs) => {
 205                 self.check_binop(expr, op, lhs, rhs)
 206             }
 207             ExprKind::AssignOp(op, ref lhs, ref rhs) => {
 208                 self.check_binop_assign(expr, op, lhs, rhs)
 209             }
 210             ExprKind::Unary(unop, ref oprnd) => {
 211                 self.check_expr_unary(unop, oprnd, expected, needs, expr)
 212             }
 213             ExprKind::AddrOf(mutbl, ref oprnd) => {
 214                 self.check_expr_addr_of(mutbl, oprnd, expected, expr)
 215             }
 216             ExprKind::Path(ref qpath) => {
 217                 self.check_expr_path(qpath, expr)
 218             }
 219             ExprKind::InlineAsm(_, ref outputs, ref inputs) => {
 220                 for expr in outputs.iter().chain(inputs.iter()) {
 221                     self.check_expr(expr);
 222                 }
 223                 tcx.mk_unit()
 224             }
 225             ExprKind::Break(destination, ref expr_opt) => {
 226                 self.check_expr_break(destination, expr_opt.deref(), expr)
 227             }
 228             ExprKind::Continue(destination) => {
 229                 if destination.target_id.is_ok() {
 230                     tcx.types.never
 231                 } else {
 232                     // There was an error; make type-check fail.
 233                     tcx.types.err
 234                 }
 235             }
 236             ExprKind::Ret(ref expr_opt) => {
 237                 self.check_expr_return(expr_opt.deref(), expr)
 238             }
 239             ExprKind::Assign(ref lhs, ref rhs) => {
 240                 self.check_expr_assign(expr, expected, lhs, rhs)
 241             }
 242             ExprKind::While(ref cond, ref body, _) => {
 243                 self.check_expr_while(cond, body, expr)
 244             }
 245             ExprKind::Loop(ref body, _, source) => {
 246                 self.check_expr_loop(body, source, expected, expr)
 247             }
 248             ExprKind::Match(ref discrim, ref arms, match_src) => {
 249                 self.check_match(expr, &discrim, arms, expected, match_src)
 250             }
 251             ExprKind::Closure(capture, ref decl, body_id, _, gen) => {
 252                 self.check_expr_closure(expr, capture, &decl, body_id, gen, expected)
 253             }
 254             ExprKind::Block(ref body, _) => {
 255                 self.check_block_with_expected(&body, expected)
 256             }
 257             ExprKind::Call(ref callee, ref args) => {
 258                 self.check_call(expr, &callee, args, expected)
 259             }
 260             ExprKind::MethodCall(ref segment, span, ref args) => {
 261                 self.check_method_call(expr, segment, span, args, expected, needs)
 262             }
 263             ExprKind::Cast(ref e, ref t) => {
 264                 self.check_expr_cast(e, t, expr)
 265             }
 266             ExprKind::Type(ref e, ref t) => {
 267                 let ty = self.to_ty_saving_user_provided_ty(&t);
 268                 self.check_expr_eq_type(&e, ty);
 269                 ty
 270             }
 271             ExprKind::DropTemps(ref e) => {
 272                 self.check_expr_with_expectation(e, expected)
 273             }
 274             ExprKind::Array(ref args) => {
 275                 self.check_expr_array(args, expected, expr)
 276             }
 277             ExprKind::Repeat(ref element, ref count) => {
 278                 self.check_expr_repeat(element, count, expected, expr)
 279             }
 280             ExprKind::Tup(ref elts) => {
 281                 self.check_expr_tuple(elts, expected, expr)
 282             }
 283             ExprKind::Struct(ref qpath, ref fields, ref base_expr) => {
 284                 self.check_expr_struct(expr, expected, qpath, fields, base_expr)
 285             }
 286             ExprKind::Field(ref base, field) => {
 287                 self.check_field(expr, needs, &base, field)
 288             }
 289             ExprKind::Index(ref base, ref idx) => {
 290                 self.check_expr_index(base, idx, needs, expr)
 291             }
 292             ExprKind::Yield(ref value) => {
 293                 self.check_expr_yield(value, expr)
 294             }
 295             hir::ExprKind::Err => {
 296                 tcx.types.err
 297             }
 298         }
 299     }
 300
 301     fn check_expr_box(&self, expr: &'tcx hir::Expr, expected: Expectation<'tcx>) -> Ty<'tcx> {
 302         let expected_inner = expected.to_option(self).map_or(NoExpectation, |ty| {
 303             match ty.sty {
 304                 ty::Adt(def, _) if def.is_box()
 305                     => Expectation::rvalue_hint(self, ty.boxed_ty()),
 306                 _ => NoExpectation
 307             }
 308         });
 309         let referent_ty = self.check_expr_with_expectation(expr, expected_inner);
 310         self.tcx.mk_box(referent_ty)
 311     }
 312
 313     fn check_expr_unary(
 314         &self,
 315         unop: hir::UnOp,
 316         oprnd: &'tcx hir::Expr,
 317         expected: Expectation<'tcx>,
 318         needs: Needs,
 319         expr: &'tcx hir::Expr,
 320     ) -> Ty<'tcx> {
 321         let tcx = self.tcx;
 322         let expected_inner = match unop {
 323             hir::UnNot | hir::UnNeg => expected,
 324             hir::UnDeref => NoExpectation,
 325         };
 326         let needs = match unop {
 327             hir::UnDeref => needs,
 328             _ => Needs::None
 329         };
 330         let mut oprnd_t = self.check_expr_with_expectation_and_needs(&oprnd, expected_inner, needs);
 331
 332         if !oprnd_t.references_error() {
 333             oprnd_t = self.structurally_resolved_type(expr.span, oprnd_t);
 334             match unop {
 335                 hir::UnDeref => {
 336                     if let Some(mt) = oprnd_t.builtin_deref(true) {
 337                         oprnd_t = mt.ty;
 338                     } else if let Some(ok) = self.try_overloaded_deref(
 339                             expr.span, oprnd_t, needs) {
 340                         let method = self.register_infer_ok_obligations(ok);
 341                         if let ty::Ref(region, _, mutbl) = method.sig.inputs()[0].sty {
 342                             let mutbl = match mutbl {
 343                                 hir::MutImmutable => AutoBorrowMutability::Immutable,
 344                                 hir::MutMutable => AutoBorrowMutability::Mutable {
 345                                     // (It shouldn't actually matter for unary ops whether
 346                                     // we enable two-phase borrows or not, since a unary
 347                                     // op has no additional operands.)
 348                                     allow_two_phase_borrow: AllowTwoPhase::No,
 349                                 }
 350                             };
 351                             self.apply_adjustments(oprnd, vec![Adjustment {
 352                                 kind: Adjust::Borrow(AutoBorrow::Ref(region, mutbl)),
 353                                 target: method.sig.inputs()[0]
 354                             }]);
 355                         }
 356                         oprnd_t = self.make_overloaded_place_return_type(method).ty;
 357                         self.write_method_call(expr.hir_id, method);
 358                     } else {
 359                         let mut err = type_error_struct!(
 360                             tcx.sess,
 361                             expr.span,
 362                             oprnd_t,
 363                             E0614,
 364                             "type `{}` cannot be dereferenced",
 365                             oprnd_t,
 366                         );
 367                         let sp = tcx.sess.source_map().start_point(expr.span);
 368                         if let Some(sp) = tcx.sess.parse_sess.ambiguous_block_expr_parse
 369                             .borrow().get(&sp)
 370                         {
 371                             tcx.sess.parse_sess.expr_parentheses_needed(
 372                                 &mut err,
 373                                 *sp,
 374                                 None,
 375                             );
 376                         }
 377                         err.emit();
 378                         oprnd_t = tcx.types.err;
 379                     }
 380                 }
 381                 hir::UnNot => {
 382                     let result = self.check_user_unop(expr, oprnd_t, unop);
 383                     // If it's builtin, we can reuse the type, this helps inference.
 384                     if !(oprnd_t.is_integral() || oprnd_t.sty == ty::Bool) {
 385                         oprnd_t = result;
 386                     }
 387                 }
 388                 hir::UnNeg => {
 389                     let result = self.check_user_unop(expr, oprnd_t, unop);
 390                     // If it's builtin, we can reuse the type, this helps inference.
 391                     if !oprnd_t.is_numeric() {
 392                         oprnd_t = result;
 393                     }
 394                 }
 395             }
 396         }
 397         oprnd_t
 398     }
 399
 400     fn check_expr_addr_of(
 401         &self,
 402         mutbl: hir::Mutability,
 403         oprnd: &'tcx hir::Expr,
 404         expected: Expectation<'tcx>,
 405         expr: &'tcx hir::Expr,
 406     ) -> Ty<'tcx> {
 407         let hint = expected.only_has_type(self).map_or(NoExpectation, |ty| {
 408             match ty.sty {
 409                 ty::Ref(_, ty, _) | ty::RawPtr(ty::TypeAndMut { ty, .. }) => {
 410                     if oprnd.is_place_expr() {
 411                         // Places may legitimately have unsized types.
 412                         // For example, dereferences of a fat pointer and
 413                         // the last field of a struct can be unsized.
 414                         ExpectHasType(ty)
 415                     } else {
 416                         Expectation::rvalue_hint(self, ty)
 417                     }
 418                 }
 419                 _ => NoExpectation
 420             }
 421         });
 422         let needs = Needs::maybe_mut_place(mutbl);
 423         let ty = self.check_expr_with_expectation_and_needs(&oprnd, hint, needs);
 424
 425         let tm = ty::TypeAndMut { ty: ty, mutbl: mutbl };
 426         if tm.ty.references_error() {
 427             self.tcx.types.err
 428         } else {
 429             // Note: at this point, we cannot say what the best lifetime
 430             // is to use for resulting pointer.  We want to use the
 431             // shortest lifetime possible so as to avoid spurious borrowck
 432             // errors.  Moreover, the longest lifetime will depend on the
 433             // precise details of the value whose address is being taken
 434             // (and how long it is valid), which we don't know yet until type
 435             // inference is complete.
 436             //
 437             // Therefore, here we simply generate a region variable.  The
 438             // region inferencer will then select the ultimate value.
 439             // Finally, borrowck is charged with guaranteeing that the
 440             // value whose address was taken can actually be made to live
 441             // as long as it needs to live.
 442             let region = self.next_region_var(infer::AddrOfRegion(expr.span));
 443             self.tcx.mk_ref(region, tm)
 444         }
 445     }
 446
 447     fn check_expr_path(&self, qpath: &hir::QPath, expr: &'tcx hir::Expr) -> Ty<'tcx> {
 448         let tcx = self.tcx;
 449         let (res, opt_ty, segs) = self.resolve_ty_and_res_ufcs(qpath, expr.hir_id, expr.span);
 450         let ty = match res {
 451             Res::Err => {
 452                 self.set_tainted_by_errors();
 453                 tcx.types.err
 454             }
 455             Res::Def(DefKind::Ctor(_, CtorKind::Fictive), _) => {
 456                 report_unexpected_variant_res(tcx, res, expr.span, qpath);
 457                 tcx.types.err
 458             }
 459             _ => self.instantiate_value_path(segs, opt_ty, res, expr.span, expr.hir_id).0,
 460         };
 461
 462         if let ty::FnDef(..) = ty.sty {
 463             let fn_sig = ty.fn_sig(tcx);
 464             if !tcx.features().unsized_locals {
 465                 // We want to remove some Sized bounds from std functions,
 466                 // but don't want to expose the removal to stable Rust.
 467                 // i.e., we don't want to allow
 468                 //
 469                 // ```rust
 470                 // drop as fn(str);
 471                 // ```
 472                 //
 473                 // to work in stable even if the Sized bound on `drop` is relaxed.
 474                 for i in 0..fn_sig.inputs().skip_binder().len() {
 475                     // We just want to check sizedness, so instead of introducing
 476                     // placeholder lifetimes with probing, we just replace higher lifetimes
 477                     // with fresh vars.
 478                     let input = self.replace_bound_vars_with_fresh_vars(
 479                         expr.span,
 480                         infer::LateBoundRegionConversionTime::FnCall,
 481                         &fn_sig.input(i)).0;
 482                     self.require_type_is_sized_deferred(input, expr.span,
 483                                                         traits::SizedArgumentType);
 484                 }
 485             }
 486             // Here we want to prevent struct constructors from returning unsized types.
 487             // There were two cases this happened: fn pointer coercion in stable
 488             // and usual function call in presense of unsized_locals.
 489             // Also, as we just want to check sizedness, instead of introducing
 490             // placeholder lifetimes with probing, we just replace higher lifetimes
 491             // with fresh vars.
 492             let output = self.replace_bound_vars_with_fresh_vars(
 493                 expr.span,
 494                 infer::LateBoundRegionConversionTime::FnCall,
 495                 &fn_sig.output()).0;
 496             self.require_type_is_sized_deferred(output, expr.span, traits::SizedReturnType);
 497         }
 498
 499         // We always require that the type provided as the value for
 500         // a type parameter outlives the moment of instantiation.
 501         let substs = self.tables.borrow().node_substs(expr.hir_id);
 502         self.add_wf_bounds(substs, expr);
 503
 504         ty
 505     }
 506
 507     fn check_expr_break(
 508         &self,
 509         destination: hir::Destination,
 510         expr_opt: Option<&'tcx hir::Expr>,
 511         expr: &'tcx hir::Expr,
 512     ) -> Ty<'tcx> {
 513         let tcx = self.tcx;
 514         if let Ok(target_id) = destination.target_id {
 515             let (e_ty, cause);
 516             if let Some(ref e) = expr_opt {
 517                 // If this is a break with a value, we need to type-check
 518                 // the expression. Get an expected type from the loop context.
 519                 let opt_coerce_to = {
 520                     let mut enclosing_breakables = self.enclosing_breakables.borrow_mut();
 521                     enclosing_breakables.find_breakable(target_id)
 522                                         .coerce
 523                                         .as_ref()
 524                                         .map(|coerce| coerce.expected_ty())
 525                 };
 526
 527                 // If the loop context is not a `loop { }`, then break with
 528                 // a value is illegal, and `opt_coerce_to` will be `None`.
 529                 // Just set expectation to error in that case.
 530                 let coerce_to = opt_coerce_to.unwrap_or(tcx.types.err);
 531
 532                 // Recurse without `enclosing_breakables` borrowed.
 533                 e_ty = self.check_expr_with_hint(e, coerce_to);
 534                 cause = self.misc(e.span);
 535             } else {
 536                 // Otherwise, this is a break *without* a value. That's
 537                 // always legal, and is equivalent to `break ()`.
 538                 e_ty = tcx.mk_unit();
 539                 cause = self.misc(expr.span);
 540             }
 541
 542             // Now that we have type-checked `expr_opt`, borrow
 543             // the `enclosing_loops` field and let's coerce the
 544             // type of `expr_opt` into what is expected.
 545             let mut enclosing_breakables = self.enclosing_breakables.borrow_mut();
 546             let ctxt = enclosing_breakables.find_breakable(target_id);
 547             if let Some(ref mut coerce) = ctxt.coerce {
 548                 if let Some(ref e) = expr_opt {
 549                     coerce.coerce(self, &cause, e, e_ty);
 550                 } else {
 551                     assert!(e_ty.is_unit());
 552                     coerce.coerce_forced_unit(self, &cause, &mut |_| (), true);
 553                 }
 554             } else {
 555                 // If `ctxt.coerce` is `None`, we can just ignore
 556                 // the type of the expresison.  This is because
 557                 // either this was a break *without* a value, in
 558                 // which case it is always a legal type (`()`), or
 559                 // else an error would have been flagged by the
 560                 // `loops` pass for using break with an expression
 561                 // where you are not supposed to.
 562                 assert!(expr_opt.is_none() || self.tcx.sess.err_count() > 0);
 563             }
 564
 565             ctxt.may_break = true;
 566
 567             // the type of a `break` is always `!`, since it diverges
 568             tcx.types.never
 569         } else {
 570             // Otherwise, we failed to find the enclosing loop;
 571             // this can only happen if the `break` was not
 572             // inside a loop at all, which is caught by the
 573             // loop-checking pass.
 574             if self.tcx.sess.err_count() == 0 {
 575                 self.tcx.sess.delay_span_bug(expr.span,
 576                     "break was outside loop, but no error was emitted");
 577             }
 578
 579             // We still need to assign a type to the inner expression to
 580             // prevent the ICE in #43162.
 581             if let Some(ref e) = expr_opt {
 582                 self.check_expr_with_hint(e, tcx.types.err);
 583
 584                 // ... except when we try to 'break rust;'.
 585                 // ICE this expression in particular (see #43162).
 586                 if let ExprKind::Path(QPath::Resolved(_, ref path)) = e.node {
 587                     if path.segments.len() == 1 &&
 588                         path.segments[0].ident.name == sym::rust {
 589                         fatally_break_rust(self.tcx.sess);
 590                     }
 591                 }
 592             }
 593             // There was an error; make type-check fail.
 594             tcx.types.err
 595         }
 596     }
 597
 598     fn check_expr_return(
 599         &self,
 600         expr_opt: Option<&'tcx hir::Expr>,
 601         expr: &'tcx hir::Expr
 602     ) -> Ty<'tcx> {
 603         if self.ret_coercion.is_none() {
 604             struct_span_err!(self.tcx.sess, expr.span, E0572,
 605                                 "return statement outside of function body").emit();
 606         } else if let Some(ref e) = expr_opt {
 607             if self.ret_coercion_span.borrow().is_none() {
 608                 *self.ret_coercion_span.borrow_mut() = Some(e.span);
 609             }
 610             self.check_return_expr(e);
 611         } else {
 612             let mut coercion = self.ret_coercion.as_ref().unwrap().borrow_mut();
 613             if self.ret_coercion_span.borrow().is_none() {
 614                 *self.ret_coercion_span.borrow_mut() = Some(expr.span);
 615             }
 616             let cause = self.cause(expr.span, ObligationCauseCode::ReturnNoExpression);
 617             if let Some((fn_decl, _)) = self.get_fn_decl(expr.hir_id) {
 618                 coercion.coerce_forced_unit(
 619                     self,
 620                     &cause,
 621                     &mut |db| {
 622                         db.span_label(
 623                             fn_decl.output.span(),
 624                             format!(
 625                                 "expected `{}` because of this return type",
 626                                 fn_decl.output,
 627                             ),
 628                         );
 629                     },
 630                     true,
 631                 );
 632             } else {
 633                 coercion.coerce_forced_unit(self, &cause, &mut |_| (), true);
 634             }
 635         }
 636         self.tcx.types.never
 637     }
 638
 639     pub(super) fn check_return_expr(&self, return_expr: &'tcx hir::Expr) {
 640         let ret_coercion =
 641             self.ret_coercion
 642                 .as_ref()
 643                 .unwrap_or_else(|| span_bug!(return_expr.span,
 644                                              "check_return_expr called outside fn body"));
 645
 646         let ret_ty = ret_coercion.borrow().expected_ty();
 647         let return_expr_ty = self.check_expr_with_hint(return_expr, ret_ty.clone());
 648         ret_coercion.borrow_mut()
 649                     .coerce(self,
 650                             &self.cause(return_expr.span,
 651                                         ObligationCauseCode::ReturnType(return_expr.hir_id)),
 652                             return_expr,
 653                             return_expr_ty);
 654     }
 655
 656     /// Type check assignment expression `expr` of form `lhs = rhs`.
 657     /// The expected type is `()` and is passsed to the function for the purposes of diagnostics.
 658     fn check_expr_assign(
 659         &self,
 660         expr: &'tcx hir::Expr,
 661         expected: Expectation<'tcx>,
 662         lhs: &'tcx hir::Expr,
 663         rhs: &'tcx hir::Expr,
 664     ) -> Ty<'tcx> {
 665         let lhs_ty = self.check_expr_with_needs(&lhs, Needs::MutPlace);
 666         let rhs_ty = self.check_expr_coercable_to_type(&rhs, lhs_ty);
 667
 668         let expected_ty = expected.coercion_target_type(self, expr.span);
 669         if expected_ty == self.tcx.types.bool {
 670             // The expected type is `bool` but this will result in `()` so we can reasonably
 671             // say that the user intended to write `lhs == rhs` instead of `lhs = rhs`.
 672             // The likely cause of this is `if foo = bar { .. }`.
 673             let actual_ty = self.tcx.mk_unit();
 674             let mut err = self.demand_suptype_diag(expr.span, expected_ty, actual_ty).unwrap();
 675             let msg = "try comparing for equality";
 676             let left = self.tcx.sess.source_map().span_to_snippet(lhs.span);
 677             let right = self.tcx.sess.source_map().span_to_snippet(rhs.span);
 678             if let (Ok(left), Ok(right)) = (left, right) {
 679                 let help = format!("{} == {}", left, right);
 680                 err.span_suggestion(expr.span, msg, help, Applicability::MaybeIncorrect);
 681             } else {
 682                 err.help(msg);
 683             }
 684             err.emit();
 685         } else if !lhs.is_place_expr() {
 686             struct_span_err!(self.tcx.sess, expr.span, E0070,
 687                                 "invalid left-hand side expression")
 688                 .span_label(expr.span, "left-hand of expression not valid")
 689                 .emit();
 690         }
 691
 692         self.require_type_is_sized(lhs_ty, lhs.span, traits::AssignmentLhsSized);
 693
 694         if lhs_ty.references_error() || rhs_ty.references_error() {
 695             self.tcx.types.err
 696         } else {
 697             self.tcx.mk_unit()
 698         }
 699     }
 700
 701     fn check_expr_while(
 702         &self,
 703         cond: &'tcx hir::Expr,
 704         body: &'tcx hir::Block,
 705         expr: &'tcx hir::Expr
 706     ) -> Ty<'tcx> {
 707         let ctxt = BreakableCtxt {
 708             // Cannot use break with a value from a while loop.
 709             coerce: None,
 710             may_break: false, // Will get updated if/when we find a `break`.
 711         };
 712
 713         let (ctxt, ()) = self.with_breakable_ctxt(expr.hir_id, ctxt, || {
 714             self.check_expr_has_type_or_error(&cond, self.tcx.types.bool);
 715             let cond_diverging = self.diverges.get();
 716             self.check_block_no_value(&body);
 717
 718             // We may never reach the body so it diverging means nothing.
 719             self.diverges.set(cond_diverging);
 720         });
 721
 722         if ctxt.may_break {
 723             // No way to know whether it's diverging because
 724             // of a `break` or an outer `break` or `return`.
 725             self.diverges.set(Diverges::Maybe);
 726         }
 727
 728         self.tcx.mk_unit()
 729     }
 730
 731     fn check_expr_loop(
 732         &self,
 733         body: &'tcx hir::Block,
 734         source: hir::LoopSource,
 735         expected: Expectation<'tcx>,
 736         expr: &'tcx hir::Expr,
 737     ) -> Ty<'tcx> {
 738         let coerce = match source {
 739             // you can only use break with a value from a normal `loop { }`
 740             hir::LoopSource::Loop => {
 741                 let coerce_to = expected.coercion_target_type(self, body.span);
 742                 Some(CoerceMany::new(coerce_to))
 743             }
 744
 745             hir::LoopSource::WhileLet |
 746             hir::LoopSource::ForLoop => {
 747                 None
 748             }
 749         };
 750
 751         let ctxt = BreakableCtxt {
 752             coerce,
 753             may_break: false, // Will get updated if/when we find a `break`.
 754         };
 755
 756         let (ctxt, ()) = self.with_breakable_ctxt(expr.hir_id, ctxt, || {
 757             self.check_block_no_value(&body);
 758         });
 759
 760         if ctxt.may_break {
 761             // No way to know whether it's diverging because
 762             // of a `break` or an outer `break` or `return`.
 763             self.diverges.set(Diverges::Maybe);
 764         }
 765
 766         // If we permit break with a value, then result type is
 767         // the LUB of the breaks (possibly ! if none); else, it
 768         // is nil. This makes sense because infinite loops
 769         // (which would have type !) are only possible iff we
 770         // permit break with a value [1].
 771         if ctxt.coerce.is_none() && !ctxt.may_break {
 772             // [1]
 773             self.tcx.sess.delay_span_bug(body.span, "no coercion, but loop may not break");
 774         }
 775         ctxt.coerce.map(|c| c.complete(self)).unwrap_or_else(|| self.tcx.mk_unit())
 776     }
 777
 778     /// Checks a method call.
 779     fn check_method_call(
 780         &self,
 781         expr: &'tcx hir::Expr,
 782         segment: &hir::PathSegment,
 783         span: Span,
 784         args: &'tcx [hir::Expr],
 785         expected: Expectation<'tcx>,
 786         needs: Needs,
 787     ) -> Ty<'tcx> {
 788         let rcvr = &args[0];
 789         let rcvr_t = self.check_expr_with_needs(&rcvr, needs);
 790         // no need to check for bot/err -- callee does that
 791         let rcvr_t = self.structurally_resolved_type(args[0].span, rcvr_t);
 792
 793         let method = match self.lookup_method(rcvr_t,
 794                                               segment,
 795                                               span,
 796                                               expr,
 797                                               rcvr) {
 798             Ok(method) => {
 799                 self.write_method_call(expr.hir_id, method);
 800                 Ok(method)
 801             }
 802             Err(error) => {
 803                 if segment.ident.name != kw::Invalid {
 804                     self.report_method_error(span,
 805                                              rcvr_t,
 806                                              segment.ident,
 807                                              SelfSource::MethodCall(rcvr),
 808                                              error,
 809                                              Some(args));
 810                 }
 811                 Err(())
 812             }
 813         };
 814
 815         // Call the generic checker.
 816         self.check_method_argument_types(span,
 817                                          expr.span,
 818                                          method,
 819                                          &args[1..],
 820                                          DontTupleArguments,
 821                                          expected)
 822     }
 823
 824     fn check_expr_cast(
 825         &self,
 826         e: &'tcx hir::Expr,
 827         t: &'tcx hir::Ty,
 828         expr: &'tcx hir::Expr,
 829     ) -> Ty<'tcx> {
 830         // Find the type of `e`. Supply hints based on the type we are casting to,
 831         // if appropriate.
 832         let t_cast = self.to_ty_saving_user_provided_ty(t);
 833         let t_cast = self.resolve_vars_if_possible(&t_cast);
 834         let t_expr = self.check_expr_with_expectation(e, ExpectCastableToType(t_cast));
 835         let t_cast = self.resolve_vars_if_possible(&t_cast);
 836
 837         // Eagerly check for some obvious errors.
 838         if t_expr.references_error() || t_cast.references_error() {
 839             self.tcx.types.err
 840         } else {
 841             // Defer other checks until we're done type checking.
 842             let mut deferred_cast_checks = self.deferred_cast_checks.borrow_mut();
 843             match cast::CastCheck::new(self, e, t_expr, t_cast, t.span, expr.span) {
 844                 Ok(cast_check) => {
 845                     deferred_cast_checks.push(cast_check);
 846                     t_cast
 847                 }
 848                 Err(ErrorReported) => {
 849                     self.tcx.types.err
 850                 }
 851             }
 852         }
 853     }
 854
 855     fn check_expr_array(
 856         &self,
 857         args: &'tcx [hir::Expr],
 858         expected: Expectation<'tcx>,
 859         expr: &'tcx hir::Expr
 860     ) -> Ty<'tcx> {
 861         let uty = expected.to_option(self).and_then(|uty| {
 862             match uty.sty {
 863                 ty::Array(ty, _) | ty::Slice(ty) => Some(ty),
 864                 _ => None
 865             }
 866         });
 867
 868         let element_ty = if !args.is_empty() {
 869             let coerce_to = uty.unwrap_or_else(|| {
 870                 self.next_ty_var(TypeVariableOrigin {
 871                     kind: TypeVariableOriginKind::TypeInference,
 872                     span: expr.span,
 873                 })
 874             });
 875             let mut coerce = CoerceMany::with_coercion_sites(coerce_to, args);
 876             assert_eq!(self.diverges.get(), Diverges::Maybe);
 877             for e in args {
 878                 let e_ty = self.check_expr_with_hint(e, coerce_to);
 879                 let cause = self.misc(e.span);
 880                 coerce.coerce(self, &cause, e, e_ty);
 881             }
 882             coerce.complete(self)
 883         } else {
 884             self.next_ty_var(TypeVariableOrigin {
 885                 kind: TypeVariableOriginKind::TypeInference,
 886                 span: expr.span,
 887             })
 888         };
 889         self.tcx.mk_array(element_ty, args.len() as u64)
 890     }
 891
 892     fn check_expr_repeat(
 893         &self,
 894         element: &'tcx hir::Expr,
 895         count: &'tcx hir::AnonConst,
 896         expected: Expectation<'tcx>,
 897         expr: &'tcx hir::Expr,
 898     ) -> Ty<'tcx> {
 899         let tcx = self.tcx;
 900         let count_def_id = tcx.hir().local_def_id_from_hir_id(count.hir_id);
 901         let count = if self.const_param_def_id(count).is_some() {
 902             Ok(self.to_const(count, tcx.type_of(count_def_id)))
 903         } else {
 904             let param_env = ty::ParamEnv::empty();
 905             let substs = InternalSubsts::identity_for_item(tcx.global_tcx(), count_def_id);
 906             let instance = ty::Instance::resolve(
 907                 tcx.global_tcx(),
 908                 param_env,
 909                 count_def_id,
 910                 substs,
 911             ).unwrap();
 912             let global_id = GlobalId {
 913                 instance,
 914                 promoted: None
 915             };
 916
 917             tcx.const_eval(param_env.and(global_id))
 918         };
 919
 920         let uty = match expected {
 921             ExpectHasType(uty) => {
 922                 match uty.sty {
 923                     ty::Array(ty, _) | ty::Slice(ty) => Some(ty),
 924                     _ => None
 925                 }
 926             }
 927             _ => None
 928         };
 929
 930         let (element_ty, t) = match uty {
 931             Some(uty) => {
 932                 self.check_expr_coercable_to_type(&element, uty);
 933                 (uty, uty)
 934             }
 935             None => {
 936                 let ty = self.next_ty_var(TypeVariableOrigin {
 937                     kind: TypeVariableOriginKind::MiscVariable,
 938                     span: element.span,
 939                 });
 940                 let element_ty = self.check_expr_has_type_or_error(&element, ty);
 941                 (element_ty, ty)
 942             }
 943         };
 944
 945         if let Ok(count) = count {
 946             let zero_or_one = count.assert_usize(tcx).map_or(false, |count| count <= 1);
 947             if !zero_or_one {
 948                 // For [foo, ..n] where n > 1, `foo` must have
 949                 // Copy type:
 950                 let lang_item = tcx.require_lang_item(lang_items::CopyTraitLangItem);
 951                 self.require_type_meets(t, expr.span, traits::RepeatVec, lang_item);
 952             }
 953         }
 954
 955         if element_ty.references_error() {
 956             tcx.types.err
 957         } else if let Ok(count) = count {
 958             tcx.mk_ty(ty::Array(t, count))
 959         } else {
 960             tcx.types.err
 961         }
 962     }
 963
 964     fn check_expr_tuple(
 965         &self,
 966         elts: &'tcx [hir::Expr],
 967         expected: Expectation<'tcx>,
 968         expr: &'tcx hir::Expr,
 969     ) -> Ty<'tcx> {
 970         let flds = expected.only_has_type(self).and_then(|ty| {
 971             let ty = self.resolve_type_vars_with_obligations(ty);
 972             match ty.sty {
 973                 ty::Tuple(ref flds) => Some(&flds[..]),
 974                 _ => None
 975             }
 976         });
 977
 978         let elt_ts_iter = elts.iter().enumerate().map(|(i, e)| {
 979             let t = match flds {
 980                 Some(ref fs) if i < fs.len() => {
 981                     let ety = fs[i].expect_ty();
 982                     self.check_expr_coercable_to_type(&e, ety);
 983                     ety
 984                 }
 985                 _ => {
 986                     self.check_expr_with_expectation(&e, NoExpectation)
 987                 }
 988             };
 989             t
 990         });
 991         let tuple = self.tcx.mk_tup(elt_ts_iter);
 992         if tuple.references_error() {
 993             self.tcx.types.err
 994         } else {
 995             self.require_type_is_sized(tuple, expr.span, traits::TupleInitializerSized);
 996             tuple
 997         }
 998     }
 999
1000     fn check_expr_struct(
1001         &self,
1002         expr: &hir::Expr,
1003         expected: Expectation<'tcx>,
1004         qpath: &QPath,
1005         fields: &'tcx [hir::Field],
1006         base_expr: &'tcx Option<P<hir::Expr>>,
1007     ) -> Ty<'tcx> {
1008         // Find the relevant variant
1009         let (variant, adt_ty) =
1010             if let Some(variant_ty) = self.check_struct_path(qpath, expr.hir_id) {
1011                 variant_ty
1012             } else {
1013                 self.check_struct_fields_on_error(fields, base_expr);
1014                 return self.tcx.types.err;
1015             };
1016
1017         let path_span = match *qpath {
1018             QPath::Resolved(_, ref path) => path.span,
1019             QPath::TypeRelative(ref qself, _) => qself.span
1020         };
1021
1022         // Prohibit struct expressions when non-exhaustive flag is set.
1023         let adt = adt_ty.ty_adt_def().expect("`check_struct_path` returned non-ADT type");
1024         if !adt.did.is_local() && variant.is_field_list_non_exhaustive() {
1025             span_err!(self.tcx.sess, expr.span, E0639,
1026                       "cannot create non-exhaustive {} using struct expression",
1027                       adt.variant_descr());
1028         }
1029
1030         let error_happened = self.check_expr_struct_fields(adt_ty, expected, expr.hir_id, path_span,
1031                                                            variant, fields, base_expr.is_none());
1032         if let &Some(ref base_expr) = base_expr {
1033             // If check_expr_struct_fields hit an error, do not attempt to populate
1034             // the fields with the base_expr. This could cause us to hit errors later
1035             // when certain fields are assumed to exist that in fact do not.
1036             if !error_happened {
1037                 self.check_expr_has_type_or_error(base_expr, adt_ty);
1038                 match adt_ty.sty {
1039                     ty::Adt(adt, substs) if adt.is_struct() => {
1040                         let fru_field_types = adt.non_enum_variant().fields.iter().map(|f| {
1041                             self.normalize_associated_types_in(expr.span, &f.ty(self.tcx, substs))
1042                         }).collect();
1043
1044                         self.tables
1045                             .borrow_mut()
1046                             .fru_field_types_mut()
1047                             .insert(expr.hir_id, fru_field_types);
1048                     }
1049                     _ => {
1050                         span_err!(self.tcx.sess, base_expr.span, E0436,
1051                                   "functional record update syntax requires a struct");
1052                     }
1053                 }
1054             }
1055         }
1056         self.require_type_is_sized(adt_ty, expr.span, traits::StructInitializerSized);
1057         adt_ty
1058     }
1059
1060     fn check_expr_index(
1061         &self,
1062         base: &'tcx hir::Expr,
1063         idx: &'tcx hir::Expr,
1064         needs: Needs,
1065         expr: &'tcx hir::Expr,
1066     ) -> Ty<'tcx> {
1067         let base_t = self.check_expr_with_needs(&base, needs);
1068         let idx_t = self.check_expr(&idx);
1069
1070         if base_t.references_error() {
1071             base_t
1072         } else if idx_t.references_error() {
1073             idx_t
1074         } else {
1075             let base_t = self.structurally_resolved_type(base.span, base_t);
1076             match self.lookup_indexing(expr, base, base_t, idx_t, needs) {
1077                 Some((index_ty, element_ty)) => {
1078                     // two-phase not needed because index_ty is never mutable
1079                     self.demand_coerce(idx, idx_t, index_ty, AllowTwoPhase::No);
1080                     element_ty
1081                 }
1082                 None => {
1083                     let mut err =
1084                         type_error_struct!(self.tcx.sess, expr.span, base_t, E0608,
1085                                             "cannot index into a value of type `{}`",
1086                                             base_t);
1087                     // Try to give some advice about indexing tuples.
1088                     if let ty::Tuple(..) = base_t.sty {
1089                         let mut needs_note = true;
1090                         // If the index is an integer, we can show the actual
1091                         // fixed expression:
1092                         if let ExprKind::Lit(ref lit) = idx.node {
1093                             if let ast::LitKind::Int(i, ast::LitIntType::Unsuffixed) = lit.node {
1094                                 let snip = self.tcx.sess.source_map().span_to_snippet(base.span);
1095                                 if let Ok(snip) = snip {
1096                                     err.span_suggestion(
1097                                         expr.span,
1098                                         "to access tuple elements, use",
1099                                         format!("{}.{}", snip, i),
1100                                         Applicability::MachineApplicable,
1101                                     );
1102                                     needs_note = false;
1103                                 }
1104                             }
1105                         }
1106                         if needs_note {
1107                             err.help("to access tuple elements, use tuple indexing \
1108                                         syntax (e.g., `tuple.0`)");
1109                         }
1110                     }
1111                     err.emit();
1112                     self.tcx.types.err
1113                 }
1114             }
1115         }
1116     }
1117
1118     fn check_expr_yield(&self, value: &'tcx hir::Expr, expr: &'tcx hir::Expr) -> Ty<'tcx> {
1119         match self.yield_ty {
1120             Some(ty) => {
1121                 self.check_expr_coercable_to_type(&value, ty);
1122             }
1123             None => {
1124                 struct_span_err!(self.tcx.sess, expr.span, E0627,
1125                                     "yield statement outside of generator literal").emit();
1126             }
1127         }
1128         self.tcx.mk_unit()
1129     }
1130 }