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