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 crate::hir::def_id::DefId;
  34 use crate::lint;
  35 use crate::type_error_struct;
  36 use crate::util::common::ErrorReported;
  37 use rustc::middle::lang_items;
  38 use rustc::session::Session;
  39 use rustc::traits;
  40 use rustc::traits::error_reporting::report_object_safety_error;
  41 use rustc::traits::object_safety_violations;
  42 use rustc::ty::adjustment::AllowTwoPhase;
  43 use rustc::ty::cast::{CastKind, CastTy};
  44 use rustc::ty::error::TypeError;
  45 use rustc::ty::subst::SubstsRef;
  46 use rustc::ty::{self, Ty, TypeAndMut, TypeFoldable};
  47 use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder};
  48 use rustc_hir as hir;
  49 use rustc_span::Span;
  50 use syntax::ast;
  51
  52 /// Reifies a cast check to be checked once we have full type information for
  53 /// a function context.
  54 pub struct CastCheck<'tcx> {
  55     expr: &'tcx hir::Expr<'tcx>,
  56     expr_ty: Ty<'tcx>,
  57     cast_ty: Ty<'tcx>,
  58     cast_span: Span,
  59     span: Span,
  60 }
  61
  62 /// The kind of pointer and associated metadata (thin, length or vtable) - we
  63 /// only allow casts between fat pointers if their metadata have the same
  64 /// kind.
  65 #[derive(Copy, Clone, PartialEq, Eq)]
  66 enum PointerKind<'tcx> {
  67     /// No metadata attached, ie pointer to sized type or foreign type
  68     Thin,
  69     /// A trait object
  70     Vtable(Option<DefId>),
  71     /// Slice
  72     Length,
  73     /// The unsize info of this projection
  74     OfProjection(&'tcx ty::ProjectionTy<'tcx>),
  75     /// The unsize info of this opaque ty
  76     OfOpaque(DefId, SubstsRef<'tcx>),
  77     /// The unsize info of this parameter
  78     OfParam(&'tcx ty::ParamTy),
  79 }
  80
  81 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  82     /// Returns the kind of unsize information of t, or None
  83     /// if t is unknown.
  84     fn pointer_kind(
  85         &self,
  86         t: Ty<'tcx>,
  87         span: Span,
  88     ) -> Result<Option<PointerKind<'tcx>>, ErrorReported> {
  89         debug!("pointer_kind({:?}, {:?})", t, span);
  90
  91         let t = self.resolve_vars_if_possible(&t);
  92
  93         if t.references_error() {
  94             return Err(ErrorReported);
  95         }
  96
  97         if self.type_is_known_to_be_sized_modulo_regions(t, span) {
  98             return Ok(Some(PointerKind::Thin));
  99         }
 100
 101         Ok(match t.kind {
 102             ty::Slice(_) | ty::Str => Some(PointerKind::Length),
 103             ty::Dynamic(ref tty, ..) => Some(PointerKind::Vtable(tty.principal_def_id())),
 104             ty::Adt(def, substs) if def.is_struct() => match def.non_enum_variant().fields.last() {
 105                 None => Some(PointerKind::Thin),
 106                 Some(f) => {
 107                     let field_ty = self.field_ty(span, f, substs);
 108                     self.pointer_kind(field_ty, span)?
 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
 127             | ty::Char
 128             | ty::Int(..)
 129             | ty::Uint(..)
 130             | ty::Float(_)
 131             | ty::Array(..)
 132             | ty::GeneratorWitness(..)
 133             | ty::RawPtr(_)
 134             | ty::Ref(..)
 135             | ty::FnDef(..)
 136             | ty::FnPtr(..)
 137             | ty::Closure(..)
 138             | ty::Generator(..)
 139             | ty::Adt(..)
 140             | ty::Never
 141             | ty::Error => {
 142                 self.tcx
 143                     .sess
 144                     .delay_span_bug(span, &format!("`{:?}` should be sized but is not?", t));
 145                 return Err(ErrorReported);
 146             }
 147         })
 148     }
 149 }
 150
 151 #[derive(Copy, Clone)]
 152 enum CastError {
 153     ErrorReported,
 154
 155     CastToBool,
 156     CastToChar,
 157     DifferingKinds,
 158     /// Cast of thin to fat raw ptr (e.g., `*const () as *const [u8]`).
 159     SizedUnsizedCast,
 160     IllegalCast,
 161     NeedDeref,
 162     NeedViaPtr,
 163     NeedViaThinPtr,
 164     NeedViaInt,
 165     NonScalar,
 166     UnknownExprPtrKind,
 167     UnknownCastPtrKind,
 168 }
 169
 170 impl From<ErrorReported> for CastError {
 171     fn from(ErrorReported: ErrorReported) -> Self {
 172         CastError::ErrorReported
 173     }
 174 }
 175
 176 fn make_invalid_casting_error<'a, 'tcx>(
 177     sess: &'a Session,
 178     span: Span,
 179     expr_ty: Ty<'tcx>,
 180     cast_ty: Ty<'tcx>,
 181     fcx: &FnCtxt<'a, 'tcx>,
 182 ) -> DiagnosticBuilder<'a> {
 183     type_error_struct!(
 184         sess,
 185         span,
 186         expr_ty,
 187         E0606,
 188         "casting `{}` as `{}` is invalid",
 189         fcx.ty_to_string(expr_ty),
 190         fcx.ty_to_string(cast_ty)
 191     )
 192 }
 193
 194 impl<'a, 'tcx> CastCheck<'tcx> {
 195     pub fn new(
 196         fcx: &FnCtxt<'a, 'tcx>,
 197         expr: &'tcx hir::Expr<'tcx>,
 198         expr_ty: Ty<'tcx>,
 199         cast_ty: Ty<'tcx>,
 200         cast_span: Span,
 201         span: Span,
 202     ) -> Result<CastCheck<'tcx>, ErrorReported> {
 203         let check = CastCheck { expr, expr_ty, cast_ty, cast_span, span };
 204
 205         // For better error messages, check for some obviously unsized
 206         // cases now. We do a more thorough check at the end, once
 207         // inference is more completely known.
 208         match cast_ty.kind {
 209             ty::Dynamic(..) | ty::Slice(..) => {
 210                 check.report_cast_to_unsized_type(fcx);
 211                 Err(ErrorReported)
 212             }
 213             _ => Ok(check),
 214         }
 215     }
 216
 217     fn report_cast_error(&self, fcx: &FnCtxt<'a, 'tcx>, e: CastError) {
 218         match e {
 219             CastError::ErrorReported => {
 220                 // an error has already been reported
 221             }
 222             CastError::NeedDeref => {
 223                 let error_span = self.span;
 224                 let mut err = make_invalid_casting_error(
 225                     fcx.tcx.sess,
 226                     self.span,
 227                     self.expr_ty,
 228                     self.cast_ty,
 229                     fcx,
 230                 );
 231                 let cast_ty = fcx.ty_to_string(self.cast_ty);
 232                 err.span_label(
 233                     error_span,
 234                     format!("cannot cast `{}` as `{}`", fcx.ty_to_string(self.expr_ty), cast_ty),
 235                 );
 236                 if let Ok(snippet) = fcx.sess().source_map().span_to_snippet(self.expr.span) {
 237                     err.span_suggestion(
 238                         self.expr.span,
 239                         "dereference the expression",
 240                         format!("*{}", snippet),
 241                         Applicability::MaybeIncorrect,
 242                     );
 243                 } else {
 244                     err.span_help(self.expr.span, "dereference the expression with `*`");
 245                 }
 246                 err.emit();
 247             }
 248             CastError::NeedViaThinPtr | CastError::NeedViaPtr => {
 249                 let mut err = make_invalid_casting_error(
 250                     fcx.tcx.sess,
 251                     self.span,
 252                     self.expr_ty,
 253                     self.cast_ty,
 254                     fcx,
 255                 );
 256                 if self.cast_ty.is_integral() {
 257                     err.help(&format!(
 258                         "cast through {} first",
 259                         match e {
 260                             CastError::NeedViaPtr => "a raw pointer",
 261                             CastError::NeedViaThinPtr => "a thin pointer",
 262                             _ => bug!(),
 263                         }
 264                     ));
 265                 }
 266                 err.emit();
 267             }
 268             CastError::NeedViaInt => {
 269                 make_invalid_casting_error(
 270                     fcx.tcx.sess,
 271                     self.span,
 272                     self.expr_ty,
 273                     self.cast_ty,
 274                     fcx,
 275                 )
 276                 .help(&format!(
 277                     "cast through {} first",
 278                     match e {
 279                         CastError::NeedViaInt => "an integer",
 280                         _ => bug!(),
 281                     }
 282                 ))
 283                 .emit();
 284             }
 285             CastError::IllegalCast => {
 286                 make_invalid_casting_error(
 287                     fcx.tcx.sess,
 288                     self.span,
 289                     self.expr_ty,
 290                     self.cast_ty,
 291                     fcx,
 292                 )
 293                 .emit();
 294             }
 295             CastError::DifferingKinds => {
 296                 make_invalid_casting_error(
 297                     fcx.tcx.sess,
 298                     self.span,
 299                     self.expr_ty,
 300                     self.cast_ty,
 301                     fcx,
 302                 )
 303                 .note("vtable kinds may not match")
 304                 .emit();
 305             }
 306             CastError::CastToBool => {
 307                 let mut err =
 308                     struct_span_err!(fcx.tcx.sess, self.span, E0054, "cannot cast as `bool`");
 309
 310                 if self.expr_ty.is_numeric() {
 311                     match fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 312                         Ok(snippet) => {
 313                             err.span_suggestion(
 314                                 self.span,
 315                                 "compare with zero instead",
 316                                 format!("{} != 0", snippet),
 317                                 Applicability::MachineApplicable,
 318                             );
 319                         }
 320                         Err(_) => {
 321                             err.span_help(self.span, "compare with zero instead");
 322                         }
 323                     }
 324                 } else {
 325                     err.span_label(self.span, "unsupported cast");
 326                 }
 327
 328                 err.emit();
 329             }
 330             CastError::CastToChar => {
 331                 type_error_struct!(
 332                     fcx.tcx.sess,
 333                     self.span,
 334                     self.expr_ty,
 335                     E0604,
 336                     "only `u8` can be cast as `char`, not `{}`",
 337                     self.expr_ty
 338                 )
 339                 .emit();
 340             }
 341             CastError::NonScalar => {
 342                 type_error_struct!(
 343                     fcx.tcx.sess,
 344                     self.span,
 345                     self.expr_ty,
 346                     E0605,
 347                     "non-primitive cast: `{}` as `{}`",
 348                     self.expr_ty,
 349                     fcx.ty_to_string(self.cast_ty)
 350                 )
 351                 .note(
 352                     "an `as` expression can only be used to convert between \
 353                                          primitive types. Consider using the `From` trait",
 354                 )
 355                 .emit();
 356             }
 357             CastError::SizedUnsizedCast => {
 358                 use crate::structured_errors::{SizedUnsizedCastError, StructuredDiagnostic};
 359                 SizedUnsizedCastError::new(
 360                     &fcx.tcx.sess,
 361                     self.span,
 362                     self.expr_ty,
 363                     fcx.ty_to_string(self.cast_ty),
 364                 )
 365                 .diagnostic()
 366                 .emit();
 367             }
 368             CastError::UnknownCastPtrKind | CastError::UnknownExprPtrKind => {
 369                 let unknown_cast_to = match e {
 370                     CastError::UnknownCastPtrKind => true,
 371                     CastError::UnknownExprPtrKind => false,
 372                     _ => bug!(),
 373                 };
 374                 let mut err = struct_span_err!(
 375                     fcx.tcx.sess,
 376                     self.span,
 377                     E0641,
 378                     "cannot cast {} a pointer of an unknown kind",
 379                     if unknown_cast_to { "to" } else { "from" }
 380                 );
 381                 err.note(
 382                     "the type information given here is insufficient to check whether \
 383                           the pointer cast is valid",
 384                 );
 385                 if unknown_cast_to {
 386                     err.span_suggestion_short(
 387                         self.cast_span,
 388                         "consider giving more type information",
 389                         String::new(),
 390                         Applicability::Unspecified,
 391                     );
 392                 }
 393                 err.emit();
 394             }
 395         }
 396     }
 397
 398     fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'tcx>) {
 399         if self.cast_ty.references_error() || self.expr_ty.references_error() {
 400             return;
 401         }
 402
 403         let tstr = fcx.ty_to_string(self.cast_ty);
 404         let mut err = type_error_struct!(
 405             fcx.tcx.sess,
 406             self.span,
 407             self.expr_ty,
 408             E0620,
 409             "cast to unsized type: `{}` as `{}`",
 410             fcx.resolve_vars_if_possible(&self.expr_ty),
 411             tstr
 412         );
 413         match self.expr_ty.kind {
 414             ty::Ref(_, _, mt) => {
 415                 let mtstr = mt.prefix_str();
 416                 if self.cast_ty.is_trait() {
 417                     match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 418                         Ok(s) => {
 419                             err.span_suggestion(
 420                                 self.cast_span,
 421                                 "try casting to a reference instead",
 422                                 format!("&{}{}", mtstr, s),
 423                                 Applicability::MachineApplicable,
 424                             );
 425                         }
 426                         Err(_) => {
 427                             let msg = &format!("did you mean `&{}{}`?", mtstr, tstr);
 428                             err.span_help(self.cast_span, msg);
 429                         }
 430                     }
 431                 } else {
 432                     let msg = &format!(
 433                         "consider using an implicit coercion to `&{}{}` instead",
 434                         mtstr, tstr
 435                     );
 436                     err.span_help(self.span, msg);
 437                 }
 438             }
 439             ty::Adt(def, ..) if def.is_box() => {
 440                 match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 441                     Ok(s) => {
 442                         err.span_suggestion(
 443                             self.cast_span,
 444                             "try casting to a `Box` instead",
 445                             format!("Box<{}>", s),
 446                             Applicability::MachineApplicable,
 447                         );
 448                     }
 449                     Err(_) => {
 450                         err.span_help(self.cast_span, &format!("did you mean `Box<{}>`?", tstr));
 451                     }
 452                 }
 453             }
 454             _ => {
 455                 err.span_help(self.expr.span, "consider using a box or reference as appropriate");
 456             }
 457         }
 458         err.emit();
 459     }
 460
 461     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 462         let t_cast = self.cast_ty;
 463         let t_expr = self.expr_ty;
 464         let type_asc_or =
 465             if fcx.tcx.features().type_ascription { "type ascription or " } else { "" };
 466         let (adjective, lint) = if t_cast.is_numeric() && t_expr.is_numeric() {
 467             ("numeric ", lint::builtin::TRIVIAL_NUMERIC_CASTS)
 468         } else {
 469             ("", lint::builtin::TRIVIAL_CASTS)
 470         };
 471         let mut err = fcx.tcx.struct_span_lint_hir(
 472             lint,
 473             self.expr.hir_id,
 474             self.span,
 475             &format!(
 476                 "trivial {}cast: `{}` as `{}`",
 477                 adjective,
 478                 fcx.ty_to_string(t_expr),
 479                 fcx.ty_to_string(t_cast)
 480             ),
 481         );
 482         err.help(&format!(
 483             "cast can be replaced by coercion; this might \
 484                            require {}a temporary variable",
 485             type_asc_or
 486         ));
 487         err.emit();
 488     }
 489
 490     pub fn check(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 491         self.expr_ty = fcx.structurally_resolved_type(self.span, self.expr_ty);
 492         self.cast_ty = fcx.structurally_resolved_type(self.span, self.cast_ty);
 493
 494         debug!("check_cast({}, {:?} as {:?})", self.expr.hir_id, self.expr_ty, self.cast_ty);
 495
 496         if !fcx.type_is_known_to_be_sized_modulo_regions(self.cast_ty, self.span) {
 497             self.report_cast_to_unsized_type(fcx);
 498         } else if self.expr_ty.references_error() || self.cast_ty.references_error() {
 499             // No sense in giving duplicate error messages
 500         } else {
 501             match self.try_coercion_cast(fcx) {
 502                 Ok(()) => {
 503                     self.trivial_cast_lint(fcx);
 504                     debug!(" -> CoercionCast");
 505                     fcx.tables.borrow_mut().set_coercion_cast(self.expr.hir_id.local_id);
 506                 }
 507                 Err(ty::error::TypeError::ObjectUnsafeCoercion(did)) => {
 508                     self.report_object_unsafe_cast(&fcx, did);
 509                 }
 510                 Err(_) => {
 511                     match self.do_check(fcx) {
 512                         Ok(k) => {
 513                             debug!(" -> {:?}", k);
 514                         }
 515                         Err(e) => self.report_cast_error(fcx, e),
 516                     };
 517                 }
 518             };
 519         }
 520     }
 521
 522     fn report_object_unsafe_cast(&self, fcx: &FnCtxt<'a, 'tcx>, did: DefId) {
 523         let violations = object_safety_violations(fcx.tcx, did);
 524         let mut err = report_object_safety_error(fcx.tcx, self.cast_span, did, violations);
 525         err.note(&format!("required by cast to type '{}'", fcx.ty_to_string(self.cast_ty)));
 526         err.emit();
 527     }
 528
 529     /// Checks a cast, and report an error if one exists. In some cases, this
 530     /// can return Ok and create type errors in the fcx rather than returning
 531     /// directly. coercion-cast is handled in check instead of here.
 532     fn do_check(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
 533         use rustc::ty::cast::CastTy::*;
 534         use rustc::ty::cast::IntTy::*;
 535
 536         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty), CastTy::from_ty(self.cast_ty))
 537         {
 538             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 539             // Function item types may need to be reified before casts.
 540             (None, Some(t_cast)) => {
 541                 match self.expr_ty.kind {
 542                     ty::FnDef(..) => {
 543                         // Attempt a coercion to a fn pointer type.
 544                         let f = self.expr_ty.fn_sig(fcx.tcx);
 545                         let res = fcx.try_coerce(
 546                             self.expr,
 547                             self.expr_ty,
 548                             fcx.tcx.mk_fn_ptr(f),
 549                             AllowTwoPhase::No,
 550                         );
 551                         if let Err(TypeError::IntrinsicCast) = res {
 552                             return Err(CastError::IllegalCast);
 553                         }
 554                         if res.is_err() {
 555                             return Err(CastError::NonScalar);
 556                         }
 557                         (FnPtr, t_cast)
 558                     }
 559                     // Special case some errors for references, and check for
 560                     // array-ptr-casts. `Ref` is not a CastTy because the cast
 561                     // is split into a coercion to a pointer type, followed by
 562                     // a cast.
 563                     ty::Ref(_, inner_ty, mutbl) => {
 564                         return match t_cast {
 565                             Int(_) | Float => match inner_ty.kind {
 566                                 ty::Int(_)
 567                                 | ty::Uint(_)
 568                                 | ty::Float(_)
 569                                 | ty::Infer(ty::InferTy::IntVar(_))
 570                                 | ty::Infer(ty::InferTy::FloatVar(_)) => Err(CastError::NeedDeref),
 571                                 _ => Err(CastError::NeedViaPtr),
 572                             },
 573                             // array-ptr-cast
 574                             Ptr(mt) => {
 575                                 self.check_ref_cast(fcx, TypeAndMut { mutbl, ty: inner_ty }, mt)
 576                             }
 577                             _ => Err(CastError::NonScalar),
 578                         };
 579                     }
 580                     _ => return Err(CastError::NonScalar),
 581                 }
 582             }
 583             _ => return Err(CastError::NonScalar),
 584         };
 585
 586         match (t_from, t_cast) {
 587             // These types have invariants! can't cast into them.
 588             (_, Int(CEnum)) | (_, FnPtr) => Err(CastError::NonScalar),
 589
 590             // * -> Bool
 591             (_, Int(Bool)) => Err(CastError::CastToBool),
 592
 593             // * -> Char
 594             (Int(U(ast::UintTy::U8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 595             (_, Int(Char)) => Err(CastError::CastToChar),
 596
 597             // prim -> float,ptr
 598             (Int(Bool), Float) | (Int(CEnum), Float) | (Int(Char), Float) => {
 599                 Err(CastError::NeedViaInt)
 600             }
 601
 602             (Int(Bool), Ptr(_))
 603             | (Int(CEnum), Ptr(_))
 604             | (Int(Char), Ptr(_))
 605             | (Ptr(_), Float)
 606             | (FnPtr, Float)
 607             | (Float, Ptr(_)) => Err(CastError::IllegalCast),
 608
 609             // ptr -> *
 610             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 611             (Ptr(m_expr), Int(_)) => self.check_ptr_addr_cast(fcx, m_expr), // ptr-addr-cast
 612             (FnPtr, Int(_)) => Ok(CastKind::FnPtrAddrCast),
 613
 614             // * -> ptr
 615             (Int(_), Ptr(mt)) => self.check_addr_ptr_cast(fcx, mt), // addr-ptr-cast
 616             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 617
 618             // prim -> prim
 619             (Int(CEnum), Int(_)) => Ok(CastKind::EnumCast),
 620             (Int(Char), Int(_)) | (Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 621
 622             (Int(_), Int(_)) | (Int(_), Float) | (Float, Int(_)) | (Float, Float) => {
 623                 Ok(CastKind::NumericCast)
 624             }
 625         }
 626     }
 627
 628     fn check_ptr_ptr_cast(
 629         &self,
 630         fcx: &FnCtxt<'a, 'tcx>,
 631         m_expr: ty::TypeAndMut<'tcx>,
 632         m_cast: ty::TypeAndMut<'tcx>,
 633     ) -> Result<CastKind, CastError> {
 634         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_expr, m_cast);
 635         // ptr-ptr cast. vtables must match.
 636
 637         let expr_kind = fcx.pointer_kind(m_expr.ty, self.span)?;
 638         let cast_kind = fcx.pointer_kind(m_cast.ty, self.span)?;
 639
 640         let cast_kind = match cast_kind {
 641             // We can't cast if target pointer kind is unknown
 642             None => return Err(CastError::UnknownCastPtrKind),
 643             Some(cast_kind) => cast_kind,
 644         };
 645
 646         // Cast to thin pointer is OK
 647         if cast_kind == PointerKind::Thin {
 648             return Ok(CastKind::PtrPtrCast);
 649         }
 650
 651         let expr_kind = match expr_kind {
 652             // We can't cast to fat pointer if source pointer kind is unknown
 653             None => return Err(CastError::UnknownExprPtrKind),
 654             Some(expr_kind) => expr_kind,
 655         };
 656
 657         // thin -> fat? report invalid cast (don't complain about vtable kinds)
 658         if expr_kind == PointerKind::Thin {
 659             return Err(CastError::SizedUnsizedCast);
 660         }
 661
 662         // vtable kinds must match
 663         if cast_kind == expr_kind {
 664             Ok(CastKind::PtrPtrCast)
 665         } else {
 666             Err(CastError::DifferingKinds)
 667         }
 668     }
 669
 670     fn check_fptr_ptr_cast(
 671         &self,
 672         fcx: &FnCtxt<'a, 'tcx>,
 673         m_cast: ty::TypeAndMut<'tcx>,
 674     ) -> Result<CastKind, CastError> {
 675         // fptr-ptr cast. must be to thin ptr
 676
 677         match fcx.pointer_kind(m_cast.ty, self.span)? {
 678             None => Err(CastError::UnknownCastPtrKind),
 679             Some(PointerKind::Thin) => Ok(CastKind::FnPtrPtrCast),
 680             _ => Err(CastError::IllegalCast),
 681         }
 682     }
 683
 684     fn check_ptr_addr_cast(
 685         &self,
 686         fcx: &FnCtxt<'a, 'tcx>,
 687         m_expr: ty::TypeAndMut<'tcx>,
 688     ) -> Result<CastKind, CastError> {
 689         // ptr-addr cast. must be from thin ptr
 690
 691         match fcx.pointer_kind(m_expr.ty, self.span)? {
 692             None => Err(CastError::UnknownExprPtrKind),
 693             Some(PointerKind::Thin) => Ok(CastKind::PtrAddrCast),
 694             _ => Err(CastError::NeedViaThinPtr),
 695         }
 696     }
 697
 698     fn check_ref_cast(
 699         &self,
 700         fcx: &FnCtxt<'a, 'tcx>,
 701         m_expr: ty::TypeAndMut<'tcx>,
 702         m_cast: ty::TypeAndMut<'tcx>,
 703     ) -> Result<CastKind, CastError> {
 704         // array-ptr-cast.
 705
 706         if m_expr.mutbl == hir::Mutability::Not && m_cast.mutbl == hir::Mutability::Not {
 707             if let ty::Array(ety, _) = m_expr.ty.kind {
 708                 // Due to the limitations of LLVM global constants,
 709                 // region pointers end up pointing at copies of
 710                 // vector elements instead of the original values.
 711                 // To allow raw pointers to work correctly, we
 712                 // need to special-case obtaining a raw pointer
 713                 // from a region pointer to a vector.
 714
 715                 // Coerce to a raw pointer so that we generate AddressOf in MIR.
 716                 let array_ptr_type = fcx.tcx.mk_ptr(m_expr);
 717                 fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No)
 718                     .unwrap_or_else(|_| bug!(
 719                         "could not cast from reference to array to pointer to array ({:?} to {:?})",
 720                         self.expr_ty,
 721                         array_ptr_type,
 722                     ));
 723
 724                 // this will report a type mismatch if needed
 725                 fcx.demand_eqtype(self.span, ety, m_cast.ty);
 726                 return Ok(CastKind::ArrayPtrCast);
 727             }
 728         }
 729
 730         Err(CastError::IllegalCast)
 731     }
 732
 733     fn check_addr_ptr_cast(
 734         &self,
 735         fcx: &FnCtxt<'a, 'tcx>,
 736         m_cast: TypeAndMut<'tcx>,
 737     ) -> Result<CastKind, CastError> {
 738         // ptr-addr cast. pointer must be thin.
 739         match fcx.pointer_kind(m_cast.ty, self.span)? {
 740             None => Err(CastError::UnknownCastPtrKind),
 741             Some(PointerKind::Thin) => Ok(CastKind::AddrPtrCast),
 742             _ => Err(CastError::IllegalCast),
 743         }
 744     }
 745
 746     fn try_coercion_cast(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<(), ty::error::TypeError<'_>> {
 747         match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No) {
 748             Ok(_) => Ok(()),
 749             Err(err) => Err(err),
 750         }
 751     }
 752 }
 753
 754 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 755     fn type_is_known_to_be_sized_modulo_regions(&self, ty: Ty<'tcx>, span: Span) -> bool {
 756         let lang_item = self.tcx.require_lang_item(lang_items::SizedTraitLangItem, None);
 757         traits::type_known_to_meet_bound_modulo_regions(self, self.param_env, ty, lang_item, span)
 758     }
 759 }