compiler/rustc_typeck/src/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::type_error_struct;
  35 use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder, ErrorReported};
  36 use rustc_hir as hir;
  37 use rustc_hir::lang_items::LangItem;
  38 use rustc_middle::ty::adjustment::AllowTwoPhase;
  39 use rustc_middle::ty::cast::{CastKind, CastTy};
  40 use rustc_middle::ty::error::TypeError;
  41 use rustc_middle::ty::subst::SubstsRef;
  42 use rustc_middle::ty::{self, Ty, TypeAndMut, TypeFoldable};
  43 use rustc_session::lint;
  44 use rustc_session::Session;
  45 use rustc_span::symbol::sym;
  46 use rustc_span::Span;
  47 use rustc_trait_selection::traits;
  48 use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
  49
  50 /// Reifies a cast check to be checked once we have full type information for
  51 /// a function context.
  52 pub struct CastCheck<'tcx> {
  53     expr: &'tcx hir::Expr<'tcx>,
  54     expr_ty: Ty<'tcx>,
  55     cast_ty: Ty<'tcx>,
  56     cast_span: Span,
  57     span: Span,
  58 }
  59
  60 /// The kind of pointer and associated metadata (thin, length or vtable) - we
  61 /// only allow casts between fat pointers if their metadata have the same
  62 /// kind.
  63 #[derive(Copy, Clone, PartialEq, Eq)]
  64 enum PointerKind<'tcx> {
  65     /// No metadata attached, ie pointer to sized type or foreign type
  66     Thin,
  67     /// A trait object
  68     Vtable(Option<DefId>),
  69     /// Slice
  70     Length,
  71     /// The unsize info of this projection
  72     OfProjection(&'tcx ty::ProjectionTy<'tcx>),
  73     /// The unsize info of this opaque ty
  74     OfOpaque(DefId, SubstsRef<'tcx>),
  75     /// The unsize info of this parameter
  76     OfParam(&'tcx ty::ParamTy),
  77 }
  78
  79 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  80     /// Returns the kind of unsize information of t, or None
  81     /// if t is unknown.
  82     fn pointer_kind(
  83         &self,
  84         t: Ty<'tcx>,
  85         span: Span,
  86     ) -> Result<Option<PointerKind<'tcx>>, ErrorReported> {
  87         debug!("pointer_kind({:?}, {:?})", t, span);
  88
  89         let t = self.resolve_vars_if_possible(t);
  90
  91         if t.references_error() {
  92             return Err(ErrorReported);
  93         }
  94
  95         if self.type_is_known_to_be_sized_modulo_regions(t, span) {
  96             return Ok(Some(PointerKind::Thin));
  97         }
  98
  99         Ok(match *t.kind() {
 100             ty::Slice(_) | ty::Str => Some(PointerKind::Length),
 101             ty::Dynamic(ref tty, ..) => Some(PointerKind::Vtable(tty.principal_def_id())),
 102             ty::Adt(def, substs) if def.is_struct() => match def.non_enum_variant().fields.last() {
 103                 None => Some(PointerKind::Thin),
 104                 Some(f) => {
 105                     let field_ty = self.field_ty(span, f, substs);
 106                     self.pointer_kind(field_ty, span)?
 107                 }
 108             },
 109             ty::Tuple(fields) => match fields.last() {
 110                 None => Some(PointerKind::Thin),
 111                 Some(f) => self.pointer_kind(f.expect_ty(), span)?,
 112             },
 113
 114             // Pointers to foreign types are thin, despite being unsized
 115             ty::Foreign(..) => Some(PointerKind::Thin),
 116             // We should really try to normalize here.
 117             ty::Projection(ref pi) => Some(PointerKind::OfProjection(pi)),
 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
 124             | ty::Char
 125             | ty::Int(..)
 126             | ty::Uint(..)
 127             | ty::Float(_)
 128             | ty::Array(..)
 129             | ty::GeneratorWitness(..)
 130             | ty::RawPtr(_)
 131             | ty::Ref(..)
 132             | ty::FnDef(..)
 133             | ty::FnPtr(..)
 134             | ty::Closure(..)
 135             | ty::Generator(..)
 136             | ty::Adt(..)
 137             | ty::Never
 138             | ty::Error(_) => {
 139                 self.tcx
 140                     .sess
 141                     .delay_span_bug(span, &format!("`{:?}` should be sized but is not?", t));
 142                 return Err(ErrorReported);
 143             }
 144         })
 145     }
 146 }
 147
 148 #[derive(Copy, Clone)]
 149 pub enum CastError {
 150     ErrorReported,
 151
 152     CastToBool,
 153     CastToChar,
 154     DifferingKinds,
 155     /// Cast of thin to fat raw ptr (e.g., `*const () as *const [u8]`).
 156     SizedUnsizedCast,
 157     IllegalCast,
 158     NeedDeref,
 159     NeedViaPtr,
 160     NeedViaThinPtr,
 161     NeedViaInt,
 162     NonScalar,
 163     UnknownExprPtrKind,
 164     UnknownCastPtrKind,
 165 }
 166
 167 impl From<ErrorReported> for CastError {
 168     fn from(ErrorReported: ErrorReported) -> Self {
 169         CastError::ErrorReported
 170     }
 171 }
 172
 173 fn make_invalid_casting_error<'a, 'tcx>(
 174     sess: &'a Session,
 175     span: Span,
 176     expr_ty: Ty<'tcx>,
 177     cast_ty: Ty<'tcx>,
 178     fcx: &FnCtxt<'a, 'tcx>,
 179 ) -> DiagnosticBuilder<'a> {
 180     type_error_struct!(
 181         sess,
 182         span,
 183         expr_ty,
 184         E0606,
 185         "casting `{}` as `{}` is invalid",
 186         fcx.ty_to_string(expr_ty),
 187         fcx.ty_to_string(cast_ty)
 188     )
 189 }
 190
 191 impl<'a, 'tcx> CastCheck<'tcx> {
 192     pub fn new(
 193         fcx: &FnCtxt<'a, 'tcx>,
 194         expr: &'tcx hir::Expr<'tcx>,
 195         expr_ty: Ty<'tcx>,
 196         cast_ty: Ty<'tcx>,
 197         cast_span: Span,
 198         span: Span,
 199     ) -> Result<CastCheck<'tcx>, ErrorReported> {
 200         let check = CastCheck { expr, expr_ty, cast_ty, cast_span, span };
 201
 202         // For better error messages, check for some obviously unsized
 203         // cases now. We do a more thorough check at the end, once
 204         // inference is more completely known.
 205         match cast_ty.kind() {
 206             ty::Dynamic(..) | ty::Slice(..) => {
 207                 check.report_cast_to_unsized_type(fcx);
 208                 Err(ErrorReported)
 209             }
 210             _ => Ok(check),
 211         }
 212     }
 213
 214     fn report_cast_error(&self, fcx: &FnCtxt<'a, 'tcx>, e: CastError) {
 215         match e {
 216             CastError::ErrorReported => {
 217                 // an error has already been reported
 218             }
 219             CastError::NeedDeref => {
 220                 let error_span = self.span;
 221                 let mut err = make_invalid_casting_error(
 222                     fcx.tcx.sess,
 223                     self.span,
 224                     self.expr_ty,
 225                     self.cast_ty,
 226                     fcx,
 227                 );
 228                 let cast_ty = fcx.ty_to_string(self.cast_ty);
 229                 err.span_label(
 230                     error_span,
 231                     format!("cannot cast `{}` as `{}`", fcx.ty_to_string(self.expr_ty), cast_ty),
 232                 );
 233                 if let Ok(snippet) = fcx.sess().source_map().span_to_snippet(self.expr.span) {
 234                     err.span_suggestion(
 235                         self.expr.span,
 236                         "dereference the expression",
 237                         format!("*{}", snippet),
 238                         Applicability::MaybeIncorrect,
 239                     );
 240                 } else {
 241                     err.span_help(self.expr.span, "dereference the expression with `*`");
 242                 }
 243                 err.emit();
 244             }
 245             CastError::NeedViaThinPtr | CastError::NeedViaPtr => {
 246                 let mut err = make_invalid_casting_error(
 247                     fcx.tcx.sess,
 248                     self.span,
 249                     self.expr_ty,
 250                     self.cast_ty,
 251                     fcx,
 252                 );
 253                 if self.cast_ty.is_integral() {
 254                     err.help(&format!(
 255                         "cast through {} first",
 256                         match e {
 257                             CastError::NeedViaPtr => "a raw pointer",
 258                             CastError::NeedViaThinPtr => "a thin pointer",
 259                             _ => bug!(),
 260                         }
 261                     ));
 262                 }
 263                 err.emit();
 264             }
 265             CastError::NeedViaInt => {
 266                 make_invalid_casting_error(
 267                     fcx.tcx.sess,
 268                     self.span,
 269                     self.expr_ty,
 270                     self.cast_ty,
 271                     fcx,
 272                 )
 273                 .help(&format!(
 274                     "cast through {} first",
 275                     match e {
 276                         CastError::NeedViaInt => "an integer",
 277                         _ => bug!(),
 278                     }
 279                 ))
 280                 .emit();
 281             }
 282             CastError::IllegalCast => {
 283                 make_invalid_casting_error(
 284                     fcx.tcx.sess,
 285                     self.span,
 286                     self.expr_ty,
 287                     self.cast_ty,
 288                     fcx,
 289                 )
 290                 .emit();
 291             }
 292             CastError::DifferingKinds => {
 293                 make_invalid_casting_error(
 294                     fcx.tcx.sess,
 295                     self.span,
 296                     self.expr_ty,
 297                     self.cast_ty,
 298                     fcx,
 299                 )
 300                 .note("vtable kinds may not match")
 301                 .emit();
 302             }
 303             CastError::CastToBool => {
 304                 let mut err =
 305                     struct_span_err!(fcx.tcx.sess, self.span, E0054, "cannot cast as `bool`");
 306
 307                 if self.expr_ty.is_numeric() {
 308                     match fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 309                         Ok(snippet) => {
 310                             err.span_suggestion(
 311                                 self.span,
 312                                 "compare with zero instead",
 313                                 format!("{} != 0", snippet),
 314                                 Applicability::MachineApplicable,
 315                             );
 316                         }
 317                         Err(_) => {
 318                             err.span_help(self.span, "compare with zero instead");
 319                         }
 320                     }
 321                 } else {
 322                     err.span_label(self.span, "unsupported cast");
 323                 }
 324
 325                 err.emit();
 326             }
 327             CastError::CastToChar => {
 328                 type_error_struct!(
 329                     fcx.tcx.sess,
 330                     self.span,
 331                     self.expr_ty,
 332                     E0604,
 333                     "only `u8` can be cast as `char`, not `{}`",
 334                     self.expr_ty
 335                 )
 336                 .span_label(self.span, "invalid cast")
 337                 .emit();
 338             }
 339             CastError::NonScalar => {
 340                 let mut err = type_error_struct!(
 341                     fcx.tcx.sess,
 342                     self.span,
 343                     self.expr_ty,
 344                     E0605,
 345                     "non-primitive cast: `{}` as `{}`",
 346                     self.expr_ty,
 347                     fcx.ty_to_string(self.cast_ty)
 348                 );
 349                 let mut sugg = None;
 350                 if let ty::Ref(reg, _, mutbl) = *self.cast_ty.kind() {
 351                     if fcx
 352                         .try_coerce(
 353                             self.expr,
 354                             fcx.tcx.mk_ref(reg, TypeAndMut { ty: self.expr_ty, mutbl }),
 355                             self.cast_ty,
 356                             AllowTwoPhase::No,
 357                         )
 358                         .is_ok()
 359                     {
 360                         sugg = Some(format!("&{}", mutbl.prefix_str()));
 361                     }
 362                 } else if let ty::RawPtr(TypeAndMut { mutbl, .. }) = *self.cast_ty.kind() {
 363                     if fcx
 364                         .try_coerce(
 365                             self.expr,
 366                             fcx.tcx.mk_ref(
 367                                 &ty::RegionKind::ReErased,
 368                                 TypeAndMut { ty: self.expr_ty, mutbl },
 369                             ),
 370                             self.cast_ty,
 371                             AllowTwoPhase::No,
 372                         )
 373                         .is_ok()
 374                     {
 375                         sugg = Some(format!("&{}", mutbl.prefix_str()));
 376                     }
 377                 }
 378                 if let Some(sugg) = sugg {
 379                     err.span_label(self.span, "invalid cast");
 380                     err.span_suggestion_verbose(
 381                         self.expr.span.shrink_to_lo(),
 382                         "borrow the value for the cast to be valid",
 383                         sugg,
 384                         Applicability::MachineApplicable,
 385                     );
 386                 } else if !matches!(
 387                     self.cast_ty.kind(),
 388                     ty::FnDef(..) | ty::FnPtr(..) | ty::Closure(..)
 389                 ) {
 390                     let mut label = true;
 391                     // Check `impl From<self.expr_ty> for self.cast_ty {}` for accurate suggestion:
 392                     if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 393                         if let Some(from_trait) = fcx.tcx.get_diagnostic_item(sym::from_trait) {
 394                             let ty = fcx.resolve_vars_if_possible(self.cast_ty);
 395                             // Erase regions to avoid panic in `prove_value` when calling
 396                             // `type_implements_trait`.
 397                             let ty = fcx.tcx.erase_regions(ty);
 398                             let expr_ty = fcx.resolve_vars_if_possible(self.expr_ty);
 399                             let expr_ty = fcx.tcx.erase_regions(expr_ty);
 400                             let ty_params = fcx.tcx.mk_substs_trait(expr_ty, &[]);
 401                             // Check for infer types because cases like `Option<{integer}>` would
 402                             // panic otherwise.
 403                             if !expr_ty.has_infer_types()
 404                                 && !ty.has_infer_types()
 405                                 && fcx.tcx.type_implements_trait((
 406                                     from_trait,
 407                                     ty,
 408                                     ty_params,
 409                                     fcx.param_env,
 410                                 ))
 411                             {
 412                                 label = false;
 413                                 err.span_suggestion(
 414                                     self.span,
 415                                     "consider using the `From` trait instead",
 416                                     format!("{}::from({})", self.cast_ty, snippet),
 417                                     Applicability::MaybeIncorrect,
 418                                 );
 419                             }
 420                         }
 421                     }
 422                     let msg = "an `as` expression can only be used to convert between primitive \
 423                                types or to coerce to a specific trait object";
 424                     if label {
 425                         err.span_label(self.span, msg);
 426                     } else {
 427                         err.note(msg);
 428                     }
 429                 } else {
 430                     err.span_label(self.span, "invalid cast");
 431                 }
 432                 err.emit();
 433             }
 434             CastError::SizedUnsizedCast => {
 435                 use crate::structured_errors::{SizedUnsizedCast, StructuredDiagnostic};
 436
 437                 SizedUnsizedCast {
 438                     sess: &fcx.tcx.sess,
 439                     span: self.span,
 440                     expr_ty: self.expr_ty,
 441                     cast_ty: fcx.ty_to_string(self.cast_ty),
 442                 }
 443                 .diagnostic()
 444                 .emit();
 445             }
 446             CastError::UnknownCastPtrKind | CastError::UnknownExprPtrKind => {
 447                 let unknown_cast_to = match e {
 448                     CastError::UnknownCastPtrKind => true,
 449                     CastError::UnknownExprPtrKind => false,
 450                     _ => bug!(),
 451                 };
 452                 let mut err = struct_span_err!(
 453                     fcx.tcx.sess,
 454                     if unknown_cast_to { self.cast_span } else { self.span },
 455                     E0641,
 456                     "cannot cast {} a pointer of an unknown kind",
 457                     if unknown_cast_to { "to" } else { "from" }
 458                 );
 459                 if unknown_cast_to {
 460                     err.span_label(self.cast_span, "needs more type information");
 461                     err.note(
 462                         "the type information given here is insufficient to check whether \
 463                         the pointer cast is valid",
 464                     );
 465                 } else {
 466                     err.span_label(
 467                         self.span,
 468                         "the type information given here is insufficient to check whether \
 469                         the pointer cast is valid",
 470                     );
 471                 }
 472                 err.emit();
 473             }
 474         }
 475     }
 476
 477     fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'tcx>) {
 478         if self.cast_ty.references_error() || self.expr_ty.references_error() {
 479             return;
 480         }
 481
 482         let tstr = fcx.ty_to_string(self.cast_ty);
 483         let mut err = type_error_struct!(
 484             fcx.tcx.sess,
 485             self.span,
 486             self.expr_ty,
 487             E0620,
 488             "cast to unsized type: `{}` as `{}`",
 489             fcx.resolve_vars_if_possible(self.expr_ty),
 490             tstr
 491         );
 492         match self.expr_ty.kind() {
 493             ty::Ref(_, _, mt) => {
 494                 let mtstr = mt.prefix_str();
 495                 if self.cast_ty.is_trait() {
 496                     match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 497                         Ok(s) => {
 498                             err.span_suggestion(
 499                                 self.cast_span,
 500                                 "try casting to a reference instead",
 501                                 format!("&{}{}", mtstr, s),
 502                                 Applicability::MachineApplicable,
 503                             );
 504                         }
 505                         Err(_) => {
 506                             let msg = &format!("did you mean `&{}{}`?", mtstr, tstr);
 507                             err.span_help(self.cast_span, msg);
 508                         }
 509                     }
 510                 } else {
 511                     let msg = &format!(
 512                         "consider using an implicit coercion to `&{}{}` instead",
 513                         mtstr, tstr
 514                     );
 515                     err.span_help(self.span, msg);
 516                 }
 517             }
 518             ty::Adt(def, ..) if def.is_box() => {
 519                 match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 520                     Ok(s) => {
 521                         err.span_suggestion(
 522                             self.cast_span,
 523                             "you can cast to a `Box` instead",
 524                             format!("Box<{}>", s),
 525                             Applicability::MachineApplicable,
 526                         );
 527                     }
 528                     Err(_) => {
 529                         err.span_help(
 530                             self.cast_span,
 531                             &format!("you might have meant `Box<{}>`", tstr),
 532                         );
 533                     }
 534                 }
 535             }
 536             _ => {
 537                 err.span_help(self.expr.span, "consider using a box or reference as appropriate");
 538             }
 539         }
 540         err.emit();
 541     }
 542
 543     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 544         let t_cast = self.cast_ty;
 545         let t_expr = self.expr_ty;
 546         let type_asc_or =
 547             if fcx.tcx.features().type_ascription { "type ascription or " } else { "" };
 548         let (adjective, lint) = if t_cast.is_numeric() && t_expr.is_numeric() {
 549             ("numeric ", lint::builtin::TRIVIAL_NUMERIC_CASTS)
 550         } else {
 551             ("", lint::builtin::TRIVIAL_CASTS)
 552         };
 553         fcx.tcx.struct_span_lint_hir(lint, self.expr.hir_id, self.span, |err| {
 554             err.build(&format!(
 555                 "trivial {}cast: `{}` as `{}`",
 556                 adjective,
 557                 fcx.ty_to_string(t_expr),
 558                 fcx.ty_to_string(t_cast)
 559             ))
 560             .help(&format!(
 561                 "cast can be replaced by coercion; this might \
 562                                    require {}a temporary variable",
 563                 type_asc_or
 564             ))
 565             .emit();
 566         });
 567     }
 568
 569     pub fn check(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 570         self.expr_ty = fcx.structurally_resolved_type(self.span, self.expr_ty);
 571         self.cast_ty = fcx.structurally_resolved_type(self.span, self.cast_ty);
 572
 573         debug!("check_cast({}, {:?} as {:?})", self.expr.hir_id, self.expr_ty, self.cast_ty);
 574
 575         if !fcx.type_is_known_to_be_sized_modulo_regions(self.cast_ty, self.span) {
 576             self.report_cast_to_unsized_type(fcx);
 577         } else if self.expr_ty.references_error() || self.cast_ty.references_error() {
 578             // No sense in giving duplicate error messages
 579         } else {
 580             match self.try_coercion_cast(fcx) {
 581                 Ok(()) => {
 582                     self.trivial_cast_lint(fcx);
 583                     debug!(" -> CoercionCast");
 584                     fcx.typeck_results.borrow_mut().set_coercion_cast(self.expr.hir_id.local_id);
 585                 }
 586                 Err(ty::error::TypeError::ObjectUnsafeCoercion(did)) => {
 587                     self.report_object_unsafe_cast(&fcx, did);
 588                 }
 589                 Err(_) => {
 590                     match self.do_check(fcx) {
 591                         Ok(k) => {
 592                             debug!(" -> {:?}", k);
 593                         }
 594                         Err(e) => self.report_cast_error(fcx, e),
 595                     };
 596                 }
 597             };
 598         }
 599     }
 600
 601     fn report_object_unsafe_cast(&self, fcx: &FnCtxt<'a, 'tcx>, did: DefId) {
 602         let violations = fcx.tcx.object_safety_violations(did);
 603         let mut err = report_object_safety_error(fcx.tcx, self.cast_span, did, violations);
 604         err.note(&format!("required by cast to type '{}'", fcx.ty_to_string(self.cast_ty)));
 605         err.emit();
 606     }
 607
 608     /// Checks a cast, and report an error if one exists. In some cases, this
 609     /// can return Ok and create type errors in the fcx rather than returning
 610     /// directly. coercion-cast is handled in check instead of here.
 611     pub fn do_check(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
 612         use rustc_middle::ty::cast::CastTy::*;
 613         use rustc_middle::ty::cast::IntTy::*;
 614
 615         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty), CastTy::from_ty(self.cast_ty))
 616         {
 617             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 618             // Function item types may need to be reified before casts.
 619             (None, Some(t_cast)) => {
 620                 match *self.expr_ty.kind() {
 621                     ty::FnDef(..) => {
 622                         // Attempt a coercion to a fn pointer type.
 623                         let f = fcx.normalize_associated_types_in(
 624                             self.expr.span,
 625                             self.expr_ty.fn_sig(fcx.tcx),
 626                         );
 627                         let res = fcx.try_coerce(
 628                             self.expr,
 629                             self.expr_ty,
 630                             fcx.tcx.mk_fn_ptr(f),
 631                             AllowTwoPhase::No,
 632                         );
 633                         if let Err(TypeError::IntrinsicCast) = res {
 634                             return Err(CastError::IllegalCast);
 635                         }
 636                         if res.is_err() {
 637                             return Err(CastError::NonScalar);
 638                         }
 639                         (FnPtr, t_cast)
 640                     }
 641                     // Special case some errors for references, and check for
 642                     // array-ptr-casts. `Ref` is not a CastTy because the cast
 643                     // is split into a coercion to a pointer type, followed by
 644                     // a cast.
 645                     ty::Ref(_, inner_ty, mutbl) => {
 646                         return match t_cast {
 647                             Int(_) | Float => match *inner_ty.kind() {
 648                                 ty::Int(_)
 649                                 | ty::Uint(_)
 650                                 | ty::Float(_)
 651                                 | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(_)) => {
 652                                     Err(CastError::NeedDeref)
 653                                 }
 654                                 _ => Err(CastError::NeedViaPtr),
 655                             },
 656                             // array-ptr-cast
 657                             Ptr(mt) => {
 658                                 self.check_ref_cast(fcx, TypeAndMut { mutbl, ty: inner_ty }, mt)
 659                             }
 660                             _ => Err(CastError::NonScalar),
 661                         };
 662                     }
 663                     _ => return Err(CastError::NonScalar),
 664                 }
 665             }
 666             _ => return Err(CastError::NonScalar),
 667         };
 668
 669         match (t_from, t_cast) {
 670             // These types have invariants! can't cast into them.
 671             (_, Int(CEnum) | FnPtr) => Err(CastError::NonScalar),
 672
 673             // * -> Bool
 674             (_, Int(Bool)) => Err(CastError::CastToBool),
 675
 676             // * -> Char
 677             (Int(U(ty::UintTy::U8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 678             (_, Int(Char)) => Err(CastError::CastToChar),
 679
 680             // prim -> float,ptr
 681             (Int(Bool) | Int(CEnum) | Int(Char), Float) => Err(CastError::NeedViaInt),
 682
 683             (Int(Bool) | Int(CEnum) | Int(Char) | Float, Ptr(_)) | (Ptr(_) | FnPtr, Float) => {
 684                 Err(CastError::IllegalCast)
 685             }
 686
 687             // ptr -> *
 688             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 689             (Ptr(m_expr), Int(_)) => self.check_ptr_addr_cast(fcx, m_expr), // ptr-addr-cast
 690             (FnPtr, Int(_)) => Ok(CastKind::FnPtrAddrCast),
 691
 692             // * -> ptr
 693             (Int(_), Ptr(mt)) => self.check_addr_ptr_cast(fcx, mt), // addr-ptr-cast
 694             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 695
 696             // prim -> prim
 697             (Int(CEnum), Int(_)) => {
 698                 self.cenum_impl_drop_lint(fcx);
 699                 Ok(CastKind::EnumCast)
 700             }
 701             (Int(Char) | Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 702
 703             (Int(_) | Float, Int(_) | Float) => Ok(CastKind::NumericCast),
 704         }
 705     }
 706
 707     fn check_ptr_ptr_cast(
 708         &self,
 709         fcx: &FnCtxt<'a, 'tcx>,
 710         m_expr: ty::TypeAndMut<'tcx>,
 711         m_cast: ty::TypeAndMut<'tcx>,
 712     ) -> Result<CastKind, CastError> {
 713         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_expr, m_cast);
 714         // ptr-ptr cast. vtables must match.
 715
 716         let expr_kind = fcx.pointer_kind(m_expr.ty, self.span)?;
 717         let cast_kind = fcx.pointer_kind(m_cast.ty, self.span)?;
 718
 719         let cast_kind = match cast_kind {
 720             // We can't cast if target pointer kind is unknown
 721             None => return Err(CastError::UnknownCastPtrKind),
 722             Some(cast_kind) => cast_kind,
 723         };
 724
 725         // Cast to thin pointer is OK
 726         if cast_kind == PointerKind::Thin {
 727             return Ok(CastKind::PtrPtrCast);
 728         }
 729
 730         let expr_kind = match expr_kind {
 731             // We can't cast to fat pointer if source pointer kind is unknown
 732             None => return Err(CastError::UnknownExprPtrKind),
 733             Some(expr_kind) => expr_kind,
 734         };
 735
 736         // thin -> fat? report invalid cast (don't complain about vtable kinds)
 737         if expr_kind == PointerKind::Thin {
 738             return Err(CastError::SizedUnsizedCast);
 739         }
 740
 741         // vtable kinds must match
 742         if cast_kind == expr_kind {
 743             Ok(CastKind::PtrPtrCast)
 744         } else {
 745             Err(CastError::DifferingKinds)
 746         }
 747     }
 748
 749     fn check_fptr_ptr_cast(
 750         &self,
 751         fcx: &FnCtxt<'a, 'tcx>,
 752         m_cast: ty::TypeAndMut<'tcx>,
 753     ) -> Result<CastKind, CastError> {
 754         // fptr-ptr cast. must be to thin ptr
 755
 756         match fcx.pointer_kind(m_cast.ty, self.span)? {
 757             None => Err(CastError::UnknownCastPtrKind),
 758             Some(PointerKind::Thin) => Ok(CastKind::FnPtrPtrCast),
 759             _ => Err(CastError::IllegalCast),
 760         }
 761     }
 762
 763     fn check_ptr_addr_cast(
 764         &self,
 765         fcx: &FnCtxt<'a, 'tcx>,
 766         m_expr: ty::TypeAndMut<'tcx>,
 767     ) -> Result<CastKind, CastError> {
 768         // ptr-addr cast. must be from thin ptr
 769
 770         match fcx.pointer_kind(m_expr.ty, self.span)? {
 771             None => Err(CastError::UnknownExprPtrKind),
 772             Some(PointerKind::Thin) => Ok(CastKind::PtrAddrCast),
 773             _ => Err(CastError::NeedViaThinPtr),
 774         }
 775     }
 776
 777     fn check_ref_cast(
 778         &self,
 779         fcx: &FnCtxt<'a, 'tcx>,
 780         m_expr: ty::TypeAndMut<'tcx>,
 781         m_cast: ty::TypeAndMut<'tcx>,
 782     ) -> Result<CastKind, CastError> {
 783         // array-ptr-cast: allow mut-to-mut, mut-to-const, const-to-const
 784         if m_expr.mutbl == hir::Mutability::Mut || m_cast.mutbl == hir::Mutability::Not {
 785             if let ty::Array(ety, _) = m_expr.ty.kind() {
 786                 // Due to the limitations of LLVM global constants,
 787                 // region pointers end up pointing at copies of
 788                 // vector elements instead of the original values.
 789                 // To allow raw pointers to work correctly, we
 790                 // need to special-case obtaining a raw pointer
 791                 // from a region pointer to a vector.
 792
 793                 // Coerce to a raw pointer so that we generate AddressOf in MIR.
 794                 let array_ptr_type = fcx.tcx.mk_ptr(m_expr);
 795                 fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No)
 796                     .unwrap_or_else(|_| {
 797                         bug!(
 798                         "could not cast from reference to array to pointer to array ({:?} to {:?})",
 799                         self.expr_ty,
 800                         array_ptr_type,
 801                     )
 802                     });
 803
 804                 // this will report a type mismatch if needed
 805                 fcx.demand_eqtype(self.span, ety, m_cast.ty);
 806                 return Ok(CastKind::ArrayPtrCast);
 807             }
 808         }
 809
 810         Err(CastError::IllegalCast)
 811     }
 812
 813     fn check_addr_ptr_cast(
 814         &self,
 815         fcx: &FnCtxt<'a, 'tcx>,
 816         m_cast: TypeAndMut<'tcx>,
 817     ) -> Result<CastKind, CastError> {
 818         // ptr-addr cast. pointer must be thin.
 819         match fcx.pointer_kind(m_cast.ty, self.span)? {
 820             None => Err(CastError::UnknownCastPtrKind),
 821             Some(PointerKind::Thin) => Ok(CastKind::AddrPtrCast),
 822             _ => Err(CastError::IllegalCast),
 823         }
 824     }
 825
 826     fn try_coercion_cast(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<(), ty::error::TypeError<'_>> {
 827         match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No) {
 828             Ok(_) => Ok(()),
 829             Err(err) => Err(err),
 830         }
 831     }
 832
 833     fn cenum_impl_drop_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 834         if let ty::Adt(d, _) = self.expr_ty.kind() {
 835             if d.has_dtor(fcx.tcx) {
 836                 fcx.tcx.struct_span_lint_hir(
 837                     lint::builtin::CENUM_IMPL_DROP_CAST,
 838                     self.expr.hir_id,
 839                     self.span,
 840                     |err| {
 841                         err.build(&format!(
 842                             "cannot cast enum `{}` into integer `{}` because it implements `Drop`",
 843                             self.expr_ty, self.cast_ty
 844                         ))
 845                         .emit();
 846                     },
 847                 );
 848             }
 849         }
 850     }
 851 }
 852
 853 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 854     fn type_is_known_to_be_sized_modulo_regions(&self, ty: Ty<'tcx>, span: Span) -> bool {
 855         let lang_item = self.tcx.require_lang_item(LangItem::Sized, None);
 856         traits::type_known_to_meet_bound_modulo_regions(self, self.param_env, ty, lang_item, span)
 857     }
 858 }