src/librustc_typeck/check/cast.rs

   1 //! Code for type-checking cast expressions.
   2 //!
   3 //! A cast `e as U` is valid if one of the following holds:
   4 //! * `e` has type `T` and `T` coerces to `U`; *coercion-cast*
   5 //! * `e` has type `*T`, `U` is `*U_0`, and either `U_0: Sized` or
   6 //!    pointer_kind(`T`) = pointer_kind(`U_0`); *ptr-ptr-cast*
   7 //! * `e` has type `*T` and `U` is a numeric type, while `T: Sized`; *ptr-addr-cast*
   8 //! * `e` is an integer and `U` is `*U_0`, while `U_0: Sized`; *addr-ptr-cast*
   9 //! * `e` has type `T` and `T` and `U` are any numeric types; *numeric-cast*
  10 //! * `e` is a C-like enum and `U` is an integer type; *enum-cast*
  11 //! * `e` has type `bool` or `char` and `U` is an integer; *prim-int-cast*
  12 //! * `e` has type `u8` and `U` is `char`; *u8-char-cast*
  13 //! * `e` has type `&[T; n]` and `U` is `*const T`; *array-ptr-cast*
  14 //! * `e` is a function pointer type and `U` has type `*T`,
  15 //!   while `T: Sized`; *fptr-ptr-cast*
  16 //! * `e` is a function pointer type and `U` is an integer; *fptr-addr-cast*
  17 //!
  18 //! where `&.T` and `*T` are references of either mutability,
  19 //! and where pointer_kind(`T`) is the kind of the unsize info
  20 //! in `T` - the vtable for a trait definition (e.g., `fmt::Display` or
  21 //! `Iterator`, not `Iterator<Item=u8>`) or a length (or `()` if `T: Sized`).
  22 //!
  23 //! Note that lengths are not adjusted when casting raw slices -
  24 //! `T: *const [u16] as *const [u8]` creates a slice that only includes
  25 //! half of the original memory.
  26 //!
  27 //! Casting is not transitive, that is, even if `e as U1 as U2` is a valid
  28 //! expression, `e as U2` is not necessarily so (in fact it will only be valid if
  29 //! `U1` coerces to `U2`).
  30
  31 use super::FnCtxt;
  32
  33 use errors::{DiagnosticBuilder,Applicability};
  34 use crate::hir::def_id::DefId;
  35 use crate::lint;
  36 use rustc::hir;
  37 use rustc::session::Session;
  38 use rustc::traits;
  39 use rustc::ty::{self, Ty, TypeFoldable, TypeAndMut};
  40 use rustc::ty::subst::SubstsRef;
  41 use rustc::ty::adjustment::AllowTwoPhase;
  42 use rustc::ty::cast::{CastKind, CastTy};
  43 use rustc::ty::error::TypeError;
  44 use rustc::middle::lang_items;
  45 use syntax::ast;
  46 use syntax_pos::Span;
  47 use crate::util::common::ErrorReported;
  48
  49 use rustc_error_codes::*;
  50
  51 /// Reifies a cast check to be checked once we have full type information for
  52 /// a function context.
  53 pub struct CastCheck<'tcx> {
  54     expr: &'tcx hir::Expr,
  55     expr_ty: Ty<'tcx>,
  56     cast_ty: Ty<'tcx>,
  57     cast_span: Span,
  58     span: Span,
  59 }
  60
  61 /// The kind of pointer and associated metadata (thin, length or vtable) - we
  62 /// only allow casts between fat pointers if their metadata have the same
  63 /// kind.
  64 #[derive(Copy, Clone, PartialEq, Eq)]
  65 enum PointerKind<'tcx> {
  66     /// No metadata attached, ie pointer to sized type or foreign type
  67     Thin,
  68     /// A trait object
  69     Vtable(Option<DefId>),
  70     /// Slice
  71     Length,
  72     /// The unsize info of this projection
  73     OfProjection(&'tcx ty::ProjectionTy<'tcx>),
  74     /// The unsize info of this opaque ty
  75     OfOpaque(DefId, SubstsRef<'tcx>),
  76     /// The unsize info of this parameter
  77     OfParam(&'tcx ty::ParamTy),
  78 }
  79
  80 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  81     /// Returns the kind of unsize information of t, or None
  82     /// if t is unknown.
  83     fn pointer_kind(&self, t: Ty<'tcx>, span: Span) ->
  84         Result<Option<PointerKind<'tcx>>, ErrorReported>
  85     {
  86         debug!("pointer_kind({:?}, {:?})", t, span);
  87
  88         let t = self.resolve_vars_if_possible(&t);
  89
  90         if t.references_error() {
  91             return Err(ErrorReported);
  92         }
  93
  94         if self.type_is_known_to_be_sized_modulo_regions(t, span) {
  95             return Ok(Some(PointerKind::Thin));
  96         }
  97
  98         Ok(match t.kind {
  99             ty::Slice(_) | ty::Str => Some(PointerKind::Length),
 100             ty::Dynamic(ref tty, ..) =>
 101                 Some(PointerKind::Vtable(tty.principal_def_id())),
 102             ty::Adt(def, substs) if def.is_struct() => {
 103                 match def.non_enum_variant().fields.last() {
 104                     None => Some(PointerKind::Thin),
 105                     Some(f) => {
 106                         let field_ty = self.field_ty(span, f, substs);
 107                         self.pointer_kind(field_ty, span)?
 108                     }
 109                 }
 110             }
 111             ty::Tuple(fields) => match fields.last() {
 112                 None => Some(PointerKind::Thin),
 113                 Some(f) => self.pointer_kind(f.expect_ty(), span)?
 114             },
 115
 116             // Pointers to foreign types are thin, despite being unsized
 117             ty::Foreign(..) => Some(PointerKind::Thin),
 118             // We should really try to normalize here.
 119             ty::Projection(ref pi) => Some(PointerKind::OfProjection(pi)),
 120             ty::UnnormalizedProjection(..) => bug!("only used with chalk-engine"),
 121             ty::Opaque(def_id, substs) => Some(PointerKind::OfOpaque(def_id, substs)),
 122             ty::Param(ref p) => Some(PointerKind::OfParam(p)),
 123             // Insufficient type information.
 124             ty::Placeholder(..) | ty::Bound(..) | ty::Infer(_) => None,
 125
 126             ty::Bool | ty::Char | ty::Int(..) | ty::Uint(..) |
 127             ty::Float(_) | ty::Array(..) | ty::GeneratorWitness(..) |
 128             ty::RawPtr(_) | ty::Ref(..) | ty::FnDef(..) |
 129             ty::FnPtr(..) | ty::Closure(..) | ty::Generator(..) |
 130             ty::Adt(..) | ty::Never | ty::Error => {
 131                 self.tcx.sess.delay_span_bug(
 132                     span, &format!("`{:?}` should be sized but is not?", t));
 133                 return Err(ErrorReported);
 134             }
 135         })
 136     }
 137 }
 138
 139 #[derive(Copy, Clone)]
 140 enum CastError {
 141     ErrorReported,
 142
 143     CastToBool,
 144     CastToChar,
 145     DifferingKinds,
 146     /// Cast of thin to fat raw ptr (e.g., `*const () as *const [u8]`).
 147     SizedUnsizedCast,
 148     IllegalCast,
 149     NeedDeref,
 150     NeedViaPtr,
 151     NeedViaThinPtr,
 152     NeedViaInt,
 153     NonScalar,
 154     UnknownExprPtrKind,
 155     UnknownCastPtrKind,
 156 }
 157
 158 impl From<ErrorReported> for CastError {
 159     fn from(ErrorReported: ErrorReported) -> Self {
 160         CastError::ErrorReported
 161     }
 162 }
 163
 164 fn make_invalid_casting_error<'a, 'tcx>(
 165     sess: &'a Session,
 166     span: Span,
 167     expr_ty: Ty<'tcx>,
 168     cast_ty: Ty<'tcx>,
 169     fcx: &FnCtxt<'a, 'tcx>,
 170 ) -> DiagnosticBuilder<'a> {
 171     type_error_struct!(sess, span, expr_ty, E0606,
 172                        "casting `{}` as `{}` is invalid",
 173                        fcx.ty_to_string(expr_ty),
 174                        fcx.ty_to_string(cast_ty))
 175 }
 176
 177 impl<'a, 'tcx> CastCheck<'tcx> {
 178     pub fn new(
 179         fcx: &FnCtxt<'a, 'tcx>,
 180         expr: &'tcx hir::Expr,
 181         expr_ty: Ty<'tcx>,
 182         cast_ty: Ty<'tcx>,
 183         cast_span: Span,
 184         span: Span,
 185     ) -> Result<CastCheck<'tcx>, ErrorReported> {
 186         let check = CastCheck {
 187             expr,
 188             expr_ty,
 189             cast_ty,
 190             cast_span,
 191             span,
 192         };
 193
 194         // For better error messages, check for some obviously unsized
 195         // cases now. We do a more thorough check at the end, once
 196         // inference is more completely known.
 197         match cast_ty.kind {
 198             ty::Dynamic(..) | ty::Slice(..) => {
 199                 check.report_cast_to_unsized_type(fcx);
 200                 Err(ErrorReported)
 201             }
 202             _ => Ok(check),
 203         }
 204     }
 205
 206     fn report_cast_error(&self, fcx: &FnCtxt<'a, 'tcx>, e: CastError) {
 207         match e {
 208             CastError::ErrorReported => {
 209                 // an error has already been reported
 210             }
 211             CastError::NeedDeref => {
 212                 let error_span = self.span;
 213                 let mut err = make_invalid_casting_error(fcx.tcx.sess, self.span, self.expr_ty,
 214                                                          self.cast_ty, fcx);
 215                 let cast_ty = fcx.ty_to_string(self.cast_ty);
 216                 err.span_label(error_span,
 217                                format!("cannot cast `{}` as `{}`",
 218                                        fcx.ty_to_string(self.expr_ty),
 219                                        cast_ty));
 220                 if let Ok(snippet) = fcx.sess().source_map().span_to_snippet(self.expr.span) {
 221                     err.span_suggestion(
 222                         self.expr.span,
 223                         "dereference the expression",
 224                         format!("*{}", snippet),
 225                         Applicability::MaybeIncorrect,
 226                     );
 227                 } else {
 228                     err.span_help(self.expr.span, "dereference the expression with `*`");
 229                 }
 230                 err.emit();
 231             }
 232             CastError::NeedViaThinPtr |
 233             CastError::NeedViaPtr => {
 234                 let mut err = make_invalid_casting_error(fcx.tcx.sess, self.span, self.expr_ty,
 235                                                          self.cast_ty, fcx);
 236                 if self.cast_ty.is_integral() {
 237                     err.help(&format!("cast through {} first",
 238                                       match e {
 239                                           CastError::NeedViaPtr => "a raw pointer",
 240                                           CastError::NeedViaThinPtr => "a thin pointer",
 241                                           _ => bug!(),
 242                                       }));
 243                 }
 244                 err.emit();
 245             }
 246             CastError::NeedViaInt => {
 247                 make_invalid_casting_error(fcx.tcx.sess, self.span, self.expr_ty, self.cast_ty, fcx)
 248                    .help(&format!("cast through {} first",
 249                                   match e {
 250                                       CastError::NeedViaInt => "an integer",
 251                                       _ => bug!(),
 252                                   }))
 253                    .emit();
 254             }
 255             CastError::IllegalCast => {
 256                 make_invalid_casting_error(fcx.tcx.sess, self.span, self.expr_ty, self.cast_ty, fcx)
 257                     .emit();
 258             }
 259             CastError::DifferingKinds => {
 260                 make_invalid_casting_error(fcx.tcx.sess, self.span, self.expr_ty, self.cast_ty, fcx)
 261                     .note("vtable kinds may not match")
 262                     .emit();
 263             }
 264             CastError::CastToBool => {
 265                 let mut err =
 266                     struct_span_err!(fcx.tcx.sess, self.span, E0054, "cannot cast as `bool`");
 267
 268                 if self.expr_ty.is_numeric() {
 269                     match fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 270                         Ok(snippet) => {
 271                             err.span_suggestion(
 272                                 self.span,
 273                                 "compare with zero instead",
 274                                 format!("{} != 0", snippet),
 275                                 Applicability::MachineApplicable,
 276                             );
 277                         }
 278                         Err(_) => {
 279                             err.span_help(self.span, "compare with zero instead");
 280                         }
 281                     }
 282                 } else {
 283                     err.span_label(self.span, "unsupported cast");
 284                 }
 285
 286                 err.emit();
 287             }
 288             CastError::CastToChar => {
 289                 type_error_struct!(fcx.tcx.sess, self.span, self.expr_ty, E0604,
 290                     "only `u8` can be cast as `char`, not `{}`", self.expr_ty).emit();
 291             }
 292             CastError::NonScalar => {
 293                 type_error_struct!(fcx.tcx.sess, self.span, self.expr_ty, E0605,
 294                                    "non-primitive cast: `{}` as `{}`",
 295                                    self.expr_ty,
 296                                    fcx.ty_to_string(self.cast_ty))
 297                                   .note("an `as` expression can only be used to convert between \
 298                                          primitive types. Consider using the `From` trait")
 299                                   .emit();
 300             }
 301             CastError::SizedUnsizedCast => {
 302                 use crate::structured_errors::{SizedUnsizedCastError, StructuredDiagnostic};
 303                 SizedUnsizedCastError::new(&fcx.tcx.sess,
 304                                            self.span,
 305                                            self.expr_ty,
 306                                            fcx.ty_to_string(self.cast_ty))
 307                     .diagnostic().emit();
 308             }
 309             CastError::UnknownCastPtrKind |
 310             CastError::UnknownExprPtrKind => {
 311                 let unknown_cast_to = match e {
 312                     CastError::UnknownCastPtrKind => true,
 313                     CastError::UnknownExprPtrKind => false,
 314                     _ => bug!(),
 315                 };
 316                 let mut err = struct_span_err!(fcx.tcx.sess, self.span, E0641,
 317                                                "cannot cast {} a pointer of an unknown kind",
 318                                                if unknown_cast_to { "to" } else { "from" });
 319                 err.note("The type information given here is insufficient to check whether \
 320                           the pointer cast is valid");
 321                 if unknown_cast_to {
 322                     err.span_suggestion_short(
 323                         self.cast_span,
 324                         "consider giving more type information",
 325                         String::new(),
 326                         Applicability::Unspecified,
 327                     );
 328                 }
 329                 err.emit();
 330             }
 331         }
 332     }
 333
 334     fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'tcx>) {
 335         if self.cast_ty.references_error() || self.expr_ty.references_error() {
 336             return;
 337         }
 338
 339         let tstr = fcx.ty_to_string(self.cast_ty);
 340         let mut err = type_error_struct!(fcx.tcx.sess, self.span, self.expr_ty, E0620,
 341                                          "cast to unsized type: `{}` as `{}`",
 342                                          fcx.resolve_vars_if_possible(&self.expr_ty),
 343                                          tstr);
 344         match self.expr_ty.kind {
 345             ty::Ref(_, _, mt) => {
 346                 let mtstr = mt.prefix_str();
 347                 if self.cast_ty.is_trait() {
 348                     match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 349                         Ok(s) => {
 350                             err.span_suggestion(
 351                                 self.cast_span,
 352                                 "try casting to a reference instead",
 353                                 format!("&{}{}", mtstr, s),
 354                                 Applicability::MachineApplicable,
 355                             );
 356                         }
 357                         Err(_) => {
 358                             span_help!(err, self.cast_span, "did you mean `&{}{}`?", mtstr, tstr)
 359                         }
 360                     }
 361                 } else {
 362                     span_help!(err,
 363                                self.span,
 364                                "consider using an implicit coercion to `&{}{}` instead",
 365                                mtstr,
 366                                tstr);
 367                 }
 368             }
 369             ty::Adt(def, ..) if def.is_box() => {
 370                 match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 371                     Ok(s) => {
 372                         err.span_suggestion(
 373                             self.cast_span,
 374                             "try casting to a `Box` instead",
 375                             format!("Box<{}>", s),
 376                             Applicability::MachineApplicable,
 377                         );
 378                     }
 379                     Err(_) => span_help!(err, self.cast_span, "did you mean `Box<{}>`?", tstr),
 380                 }
 381             }
 382             _ => {
 383                 span_help!(err,
 384                            self.expr.span,
 385                            "consider using a box or reference as appropriate");
 386             }
 387         }
 388         err.emit();
 389     }
 390
 391     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 392         let t_cast = self.cast_ty;
 393         let t_expr = self.expr_ty;
 394         let type_asc_or = if fcx.tcx.features().type_ascription {
 395             "type ascription or "
 396         } else {
 397             ""
 398         };
 399         let (adjective, lint) = if t_cast.is_numeric() && t_expr.is_numeric() {
 400             ("numeric ", lint::builtin::TRIVIAL_NUMERIC_CASTS)
 401         } else {
 402             ("", lint::builtin::TRIVIAL_CASTS)
 403         };
 404         let mut err = fcx.tcx.struct_span_lint_hir(
 405             lint,
 406             self.expr.hir_id,
 407             self.span,
 408             &format!("trivial {}cast: `{}` as `{}`",
 409                      adjective,
 410                      fcx.ty_to_string(t_expr),
 411                      fcx.ty_to_string(t_cast)));
 412         err.help(&format!("cast can be replaced by coercion; this might \
 413                            require {}a temporary variable", type_asc_or));
 414         err.emit();
 415     }
 416
 417     pub fn check(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 418         self.expr_ty = fcx.structurally_resolved_type(self.span, self.expr_ty);
 419         self.cast_ty = fcx.structurally_resolved_type(self.span, self.cast_ty);
 420
 421         debug!("check_cast({}, {:?} as {:?})",
 422                self.expr.hir_id,
 423                self.expr_ty,
 424                self.cast_ty);
 425
 426         if !fcx.type_is_known_to_be_sized_modulo_regions(self.cast_ty, self.span) {
 427             self.report_cast_to_unsized_type(fcx);
 428         } else if self.expr_ty.references_error() || self.cast_ty.references_error() {
 429             // No sense in giving duplicate error messages
 430         } else {
 431             match self.try_coercion_cast(fcx) {
 432                 Ok(()) => {
 433                     self.trivial_cast_lint(fcx);
 434                     debug!(" -> CoercionCast");
 435                     fcx.tables.borrow_mut()
 436                         .set_coercion_cast(self.expr.hir_id.local_id);
 437                 }
 438                 Err(ty::error::TypeError::ObjectUnsafeCoercion(did)) => {
 439                     self.report_object_unsafe_cast(&fcx, did);
 440                 }
 441                 Err(_) => {
 442                     match self.do_check(fcx) {
 443                         Ok(k) => {
 444                             debug!(" -> {:?}", k);
 445                         }
 446                         Err(e) => self.report_cast_error(fcx, e),
 447                     };
 448                 }
 449             };
 450         }
 451     }
 452
 453     fn report_object_unsafe_cast(&self, fcx: &FnCtxt<'a, 'tcx>, did: DefId) {
 454         let violations = fcx.tcx.object_safety_violations(did);
 455         let mut err = fcx.tcx.report_object_safety_error(self.cast_span, did, violations);
 456         err.note(&format!("required by cast to type '{}'", fcx.ty_to_string(self.cast_ty)));
 457         err.emit();
 458     }
 459
 460     /// Checks a cast, and report an error if one exists. In some cases, this
 461     /// can return Ok and create type errors in the fcx rather than returning
 462     /// directly. coercion-cast is handled in check instead of here.
 463     fn do_check(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
 464         use rustc::ty::cast::IntTy::*;
 465         use rustc::ty::cast::CastTy::*;
 466
 467         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty),
 468                                       CastTy::from_ty(self.cast_ty)) {
 469             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 470             // Function item types may need to be reified before casts.
 471             (None, Some(t_cast)) => {
 472                 if let ty::FnDef(..) = self.expr_ty.kind {
 473                     // Attempt a coercion to a fn pointer type.
 474                     let f = self.expr_ty.fn_sig(fcx.tcx);
 475                     let res = fcx.try_coerce(self.expr,
 476                                              self.expr_ty,
 477                                              fcx.tcx.mk_fn_ptr(f),
 478                                              AllowTwoPhase::No);
 479                     if let Err(TypeError::IntrinsicCast) = res {
 480                         return Err(CastError::IllegalCast);
 481                     }
 482                     if res.is_err() {
 483                         return Err(CastError::NonScalar);
 484                     }
 485                     (FnPtr, t_cast)
 486                 } else {
 487                     return Err(CastError::NonScalar);
 488                 }
 489             }
 490             _ => return Err(CastError::NonScalar),
 491         };
 492
 493         match (t_from, t_cast) {
 494             // These types have invariants! can't cast into them.
 495             (_, RPtr(_)) | (_, Int(CEnum)) | (_, FnPtr) => Err(CastError::NonScalar),
 496
 497             // * -> Bool
 498             (_, Int(Bool)) => Err(CastError::CastToBool),
 499
 500             // * -> Char
 501             (Int(U(ast::UintTy::U8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 502             (_, Int(Char)) => Err(CastError::CastToChar),
 503
 504             // prim -> float,ptr
 505             (Int(Bool), Float) |
 506             (Int(CEnum), Float) |
 507             (Int(Char), Float) => Err(CastError::NeedViaInt),
 508
 509             (Int(Bool), Ptr(_)) |
 510             (Int(CEnum), Ptr(_)) |
 511             (Int(Char), Ptr(_)) |
 512             (Ptr(_), Float) |
 513             (FnPtr, Float) |
 514             (Float, Ptr(_)) => Err(CastError::IllegalCast),
 515
 516             // ptr -> *
 517             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 518             (Ptr(m_expr), Int(_)) => self.check_ptr_addr_cast(fcx, m_expr), // ptr-addr-cast
 519             (FnPtr, Int(_)) => Ok(CastKind::FnPtrAddrCast),
 520             (RPtr(p), Int(_)) |
 521             (RPtr(p), Float) => {
 522                 match p.ty.kind {
 523                     ty::Int(_) |
 524                     ty::Uint(_) |
 525                     ty::Float(_) => {
 526                         Err(CastError::NeedDeref)
 527                     }
 528                     ty::Infer(t) => {
 529                         match t {
 530                             ty::InferTy::IntVar(_) |
 531                             ty::InferTy::FloatVar(_) => Err(CastError::NeedDeref),
 532                             _ => Err(CastError::NeedViaPtr),
 533                         }
 534                     }
 535                     _ => Err(CastError::NeedViaPtr),
 536                 }
 537             }
 538             // * -> ptr
 539             (Int(_), Ptr(mt)) => self.check_addr_ptr_cast(fcx, mt), // addr-ptr-cast
 540             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 541             (RPtr(rmt), Ptr(mt)) => self.check_ref_cast(fcx, rmt, mt), // array-ptr-cast
 542
 543             // prim -> prim
 544             (Int(CEnum), Int(_)) => Ok(CastKind::EnumCast),
 545             (Int(Char), Int(_)) |
 546             (Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 547
 548             (Int(_), Int(_)) | (Int(_), Float) | (Float, Int(_)) | (Float, Float) => {
 549                 Ok(CastKind::NumericCast)
 550             }
 551         }
 552     }
 553
 554     fn check_ptr_ptr_cast(
 555         &self,
 556         fcx: &FnCtxt<'a, 'tcx>,
 557         m_expr: ty::TypeAndMut<'tcx>,
 558         m_cast: ty::TypeAndMut<'tcx>,
 559     ) -> Result<CastKind, CastError> {
 560         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_expr, m_cast);
 561         // ptr-ptr cast. vtables must match.
 562
 563         let expr_kind = fcx.pointer_kind(m_expr.ty, self.span)?;
 564         let cast_kind = fcx.pointer_kind(m_cast.ty, self.span)?;
 565
 566         let cast_kind = match cast_kind {
 567             // We can't cast if target pointer kind is unknown
 568             None => return Err(CastError::UnknownCastPtrKind),
 569             Some(cast_kind) => cast_kind,
 570         };
 571
 572         // Cast to thin pointer is OK
 573         if cast_kind == PointerKind::Thin {
 574             return Ok(CastKind::PtrPtrCast);
 575         }
 576
 577         let expr_kind = match expr_kind {
 578             // We can't cast to fat pointer if source pointer kind is unknown
 579             None => return Err(CastError::UnknownExprPtrKind),
 580             Some(expr_kind) => expr_kind,
 581         };
 582
 583         // thin -> fat? report invalid cast (don't complain about vtable kinds)
 584         if expr_kind == PointerKind::Thin {
 585             return Err(CastError::SizedUnsizedCast);
 586         }
 587
 588         // vtable kinds must match
 589         if cast_kind == expr_kind {
 590             Ok(CastKind::PtrPtrCast)
 591         } else {
 592             Err(CastError::DifferingKinds)
 593         }
 594     }
 595
 596     fn check_fptr_ptr_cast(
 597         &self,
 598         fcx: &FnCtxt<'a, 'tcx>,
 599         m_cast: ty::TypeAndMut<'tcx>,
 600     ) -> Result<CastKind, CastError> {
 601         // fptr-ptr cast. must be to thin ptr
 602
 603         match fcx.pointer_kind(m_cast.ty, self.span)? {
 604             None => Err(CastError::UnknownCastPtrKind),
 605             Some(PointerKind::Thin) => Ok(CastKind::FnPtrPtrCast),
 606             _ => Err(CastError::IllegalCast),
 607         }
 608     }
 609
 610     fn check_ptr_addr_cast(
 611         &self,
 612         fcx: &FnCtxt<'a, 'tcx>,
 613         m_expr: ty::TypeAndMut<'tcx>,
 614     ) -> Result<CastKind, CastError> {
 615         // ptr-addr cast. must be from thin ptr
 616
 617         match fcx.pointer_kind(m_expr.ty, self.span)? {
 618             None => Err(CastError::UnknownExprPtrKind),
 619             Some(PointerKind::Thin) => Ok(CastKind::PtrAddrCast),
 620             _ => Err(CastError::NeedViaThinPtr),
 621         }
 622     }
 623
 624     fn check_ref_cast(
 625         &self,
 626         fcx: &FnCtxt<'a, 'tcx>,
 627         m_expr: ty::TypeAndMut<'tcx>,
 628         m_cast: ty::TypeAndMut<'tcx>,
 629     ) -> Result<CastKind, CastError> {
 630         // array-ptr-cast.
 631
 632         if m_expr.mutbl == hir::Mutability::Immutable &&
 633             m_cast.mutbl == hir::Mutability::Immutable {
 634             if let ty::Array(ety, _) = m_expr.ty.kind {
 635                 // Due to the limitations of LLVM global constants,
 636                 // region pointers end up pointing at copies of
 637                 // vector elements instead of the original values.
 638                 // To allow raw pointers to work correctly, we
 639                 // need to special-case obtaining a raw pointer
 640                 // from a region pointer to a vector.
 641
 642                 // Coerce to a raw pointer so that we generate AddressOf in MIR.
 643                 let array_ptr_type = fcx.tcx.mk_ptr(m_expr);
 644                 fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No)
 645                     .unwrap_or_else(|_| bug!(
 646                         "could not cast from reference to array to pointer to array ({:?} to {:?})",
 647                         self.expr_ty,
 648                         array_ptr_type,
 649                     ));
 650
 651                 // this will report a type mismatch if needed
 652                 fcx.demand_eqtype(self.span, ety, m_cast.ty);
 653                 return Ok(CastKind::ArrayPtrCast);
 654             }
 655         }
 656
 657         Err(CastError::IllegalCast)
 658     }
 659
 660     fn check_addr_ptr_cast(
 661         &self,
 662         fcx: &FnCtxt<'a, 'tcx>,
 663         m_cast: TypeAndMut<'tcx>,
 664     ) -> Result<CastKind, CastError> {
 665         // ptr-addr cast. pointer must be thin.
 666         match fcx.pointer_kind(m_cast.ty, self.span)? {
 667             None => Err(CastError::UnknownCastPtrKind),
 668             Some(PointerKind::Thin) => Ok(CastKind::AddrPtrCast),
 669             _ => Err(CastError::IllegalCast),
 670         }
 671     }
 672
 673     fn try_coercion_cast(
 674         &self,
 675         fcx: &FnCtxt<'a, 'tcx>,
 676     ) -> Result<(), ty::error::TypeError<'_>> {
 677         match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No) {
 678             Ok(_) => Ok(()),
 679             Err(err) => Err(err),
 680         }
 681     }
 682 }
 683
 684 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 685     fn type_is_known_to_be_sized_modulo_regions(&self, ty: Ty<'tcx>, span: Span) -> bool {
 686         let lang_item = self.tcx.require_lang_item(lang_items::SizedTraitLangItem, None);
 687         traits::type_known_to_meet_bound_modulo_regions(self, self.param_env, ty, lang_item, span)
 688     }
 689 }