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::mir::Mutability;
  39 use rustc_middle::ty::adjustment::AllowTwoPhase;
  40 use rustc_middle::ty::cast::{CastKind, CastTy};
  41 use rustc_middle::ty::error::TypeError;
  42 use rustc_middle::ty::subst::SubstsRef;
  43 use rustc_middle::ty::{self, Ty, TypeAndMut, TypeFoldable};
  44 use rustc_session::lint;
  45 use rustc_session::Session;
  46 use rustc_span::symbol::sym;
  47 use rustc_span::Span;
  48 use rustc_trait_selection::traits;
  49 use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
  50
  51 /// Reifies a cast check to be checked once we have full type information for
  52 /// a function context.
  53 pub struct CastCheck<'tcx> {
  54     expr: &'tcx hir::Expr<'tcx>,
  55     expr_ty: Ty<'tcx>,
  56     cast_ty: Ty<'tcx>,
  57     cast_span: Span,
  58     span: Span,
  59 }
  60
  61 /// The kind of pointer and associated metadata (thin, length or vtable) - we
  62 /// only allow casts between fat pointers if their metadata have the same
  63 /// kind.
  64 #[derive(Copy, Clone, PartialEq, Eq)]
  65 enum PointerKind<'tcx> {
  66     /// No metadata attached, ie pointer to sized type or foreign type
  67     Thin,
  68     /// A trait object
  69     Vtable(Option<DefId>),
  70     /// Slice
  71     Length,
  72     /// The unsize info of this projection
  73     OfProjection(&'tcx ty::ProjectionTy<'tcx>),
  74     /// The unsize info of this opaque ty
  75     OfOpaque(DefId, SubstsRef<'tcx>),
  76     /// The unsize info of this parameter
  77     OfParam(&'tcx ty::ParamTy),
  78 }
  79
  80 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  81     /// Returns the kind of unsize information of t, or None
  82     /// if t is unknown.
  83     fn pointer_kind(
  84         &self,
  85         t: Ty<'tcx>,
  86         span: Span,
  87     ) -> Result<Option<PointerKind<'tcx>>, ErrorReported> {
  88         debug!("pointer_kind({:?}, {:?})", t, span);
  89
  90         let t = self.resolve_vars_if_possible(t);
  91
  92         if t.references_error() {
  93             return Err(ErrorReported);
  94         }
  95
  96         if self.type_is_known_to_be_sized_modulo_regions(t, span) {
  97             return Ok(Some(PointerKind::Thin));
  98         }
  99
 100         Ok(match *t.kind() {
 101             ty::Slice(_) | ty::Str => Some(PointerKind::Length),
 102             ty::Dynamic(ref tty, ..) => Some(PointerKind::Vtable(tty.principal_def_id())),
 103             ty::Adt(def, substs) if def.is_struct() => match def.non_enum_variant().fields.last() {
 104                 None => Some(PointerKind::Thin),
 105                 Some(f) => {
 106                     let field_ty = self.field_ty(span, f, substs);
 107                     self.pointer_kind(field_ty, span)?
 108                 }
 109             },
 110             ty::Tuple(fields) => match fields.last() {
 111                 None => Some(PointerKind::Thin),
 112                 Some(f) => self.pointer_kind(f.expect_ty(), span)?,
 113             },
 114
 115             // Pointers to foreign types are thin, despite being unsized
 116             ty::Foreign(..) => Some(PointerKind::Thin),
 117             // We should really try to normalize here.
 118             ty::Projection(ref pi) => Some(PointerKind::OfProjection(pi)),
 119             ty::Opaque(def_id, substs) => Some(PointerKind::OfOpaque(def_id, substs)),
 120             ty::Param(ref p) => Some(PointerKind::OfParam(p)),
 121             // Insufficient type information.
 122             ty::Placeholder(..) | ty::Bound(..) | ty::Infer(_) => None,
 123
 124             ty::Bool
 125             | ty::Char
 126             | ty::Int(..)
 127             | ty::Uint(..)
 128             | ty::Float(_)
 129             | ty::Array(..)
 130             | ty::GeneratorWitness(..)
 131             | ty::RawPtr(_)
 132             | ty::Ref(..)
 133             | ty::FnDef(..)
 134             | ty::FnPtr(..)
 135             | ty::Closure(..)
 136             | ty::Generator(..)
 137             | ty::Adt(..)
 138             | ty::Never
 139             | ty::Error(_) => {
 140                 self.tcx
 141                     .sess
 142                     .delay_span_bug(span, &format!("`{:?}` should be sized but is not?", t));
 143                 return Err(ErrorReported);
 144             }
 145         })
 146     }
 147 }
 148
 149 #[derive(Copy, Clone)]
 150 pub enum CastError {
 151     ErrorReported,
 152
 153     CastToBool,
 154     CastToChar,
 155     DifferingKinds,
 156     /// Cast of thin to fat raw ptr (e.g., `*const () as *const [u8]`).
 157     SizedUnsizedCast,
 158     IllegalCast,
 159     NeedDeref,
 160     NeedViaPtr,
 161     NeedViaThinPtr,
 162     NeedViaInt,
 163     NonScalar,
 164     UnknownExprPtrKind,
 165     UnknownCastPtrKind,
 166 }
 167
 168 impl From<ErrorReported> for CastError {
 169     fn from(ErrorReported: ErrorReported) -> Self {
 170         CastError::ErrorReported
 171     }
 172 }
 173
 174 fn make_invalid_casting_error<'a, 'tcx>(
 175     sess: &'a Session,
 176     span: Span,
 177     expr_ty: Ty<'tcx>,
 178     cast_ty: Ty<'tcx>,
 179     fcx: &FnCtxt<'a, 'tcx>,
 180 ) -> DiagnosticBuilder<'a> {
 181     type_error_struct!(
 182         sess,
 183         span,
 184         expr_ty,
 185         E0606,
 186         "casting `{}` as `{}` is invalid",
 187         fcx.ty_to_string(expr_ty),
 188         fcx.ty_to_string(cast_ty)
 189     )
 190 }
 191
 192 impl<'a, 'tcx> CastCheck<'tcx> {
 193     pub fn new(
 194         fcx: &FnCtxt<'a, 'tcx>,
 195         expr: &'tcx hir::Expr<'tcx>,
 196         expr_ty: Ty<'tcx>,
 197         cast_ty: Ty<'tcx>,
 198         cast_span: Span,
 199         span: Span,
 200     ) -> Result<CastCheck<'tcx>, ErrorReported> {
 201         let check = CastCheck { expr, expr_ty, cast_ty, cast_span, span };
 202
 203         // For better error messages, check for some obviously unsized
 204         // cases now. We do a more thorough check at the end, once
 205         // inference is more completely known.
 206         match cast_ty.kind() {
 207             ty::Dynamic(..) | ty::Slice(..) => {
 208                 check.report_cast_to_unsized_type(fcx);
 209                 Err(ErrorReported)
 210             }
 211             _ => Ok(check),
 212         }
 213     }
 214
 215     fn report_cast_error(&self, fcx: &FnCtxt<'a, 'tcx>, e: CastError) {
 216         match e {
 217             CastError::ErrorReported => {
 218                 // an error has already been reported
 219             }
 220             CastError::NeedDeref => {
 221                 let error_span = self.span;
 222                 let mut err = make_invalid_casting_error(
 223                     fcx.tcx.sess,
 224                     self.span,
 225                     self.expr_ty,
 226                     self.cast_ty,
 227                     fcx,
 228                 );
 229                 let cast_ty = fcx.ty_to_string(self.cast_ty);
 230                 err.span_label(
 231                     error_span,
 232                     format!("cannot cast `{}` as `{}`", fcx.ty_to_string(self.expr_ty), cast_ty),
 233                 );
 234                 if let Ok(snippet) = fcx.sess().source_map().span_to_snippet(self.expr.span) {
 235                     err.span_suggestion(
 236                         self.expr.span,
 237                         "dereference the expression",
 238                         format!("*{}", snippet),
 239                         Applicability::MaybeIncorrect,
 240                     );
 241                 } else {
 242                     err.span_help(self.expr.span, "dereference the expression with `*`");
 243                 }
 244                 err.emit();
 245             }
 246             CastError::NeedViaThinPtr | CastError::NeedViaPtr => {
 247                 let mut err = make_invalid_casting_error(
 248                     fcx.tcx.sess,
 249                     self.span,
 250                     self.expr_ty,
 251                     self.cast_ty,
 252                     fcx,
 253                 );
 254                 if self.cast_ty.is_integral() {
 255                     err.help(&format!(
 256                         "cast through {} first",
 257                         match e {
 258                             CastError::NeedViaPtr => "a raw pointer",
 259                             CastError::NeedViaThinPtr => "a thin pointer",
 260                             _ => bug!(),
 261                         }
 262                     ));
 263                 }
 264                 err.emit();
 265             }
 266             CastError::NeedViaInt => {
 267                 make_invalid_casting_error(
 268                     fcx.tcx.sess,
 269                     self.span,
 270                     self.expr_ty,
 271                     self.cast_ty,
 272                     fcx,
 273                 )
 274                 .help(&format!(
 275                     "cast through {} first",
 276                     match e {
 277                         CastError::NeedViaInt => "an integer",
 278                         _ => bug!(),
 279                     }
 280                 ))
 281                 .emit();
 282             }
 283             CastError::IllegalCast => {
 284                 make_invalid_casting_error(
 285                     fcx.tcx.sess,
 286                     self.span,
 287                     self.expr_ty,
 288                     self.cast_ty,
 289                     fcx,
 290                 )
 291                 .emit();
 292             }
 293             CastError::DifferingKinds => {
 294                 make_invalid_casting_error(
 295                     fcx.tcx.sess,
 296                     self.span,
 297                     self.expr_ty,
 298                     self.cast_ty,
 299                     fcx,
 300                 )
 301                 .note("vtable kinds may not match")
 302                 .emit();
 303             }
 304             CastError::CastToBool => {
 305                 let mut err =
 306                     struct_span_err!(fcx.tcx.sess, self.span, E0054, "cannot cast as `bool`");
 307
 308                 if self.expr_ty.is_numeric() {
 309                     match fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 310                         Ok(snippet) => {
 311                             err.span_suggestion(
 312                                 self.span,
 313                                 "compare with zero instead",
 314                                 format!("{} != 0", snippet),
 315                                 Applicability::MachineApplicable,
 316                             );
 317                         }
 318                         Err(_) => {
 319                             err.span_help(self.span, "compare with zero instead");
 320                         }
 321                     }
 322                 } else {
 323                     err.span_label(self.span, "unsupported cast");
 324                 }
 325
 326                 err.emit();
 327             }
 328             CastError::CastToChar => {
 329                 type_error_struct!(
 330                     fcx.tcx.sess,
 331                     self.span,
 332                     self.expr_ty,
 333                     E0604,
 334                     "only `u8` can be cast as `char`, not `{}`",
 335                     self.expr_ty
 336                 )
 337                 .span_label(self.span, "invalid cast")
 338                 .emit();
 339             }
 340             CastError::NonScalar => {
 341                 let mut err = type_error_struct!(
 342                     fcx.tcx.sess,
 343                     self.span,
 344                     self.expr_ty,
 345                     E0605,
 346                     "non-primitive cast: `{}` as `{}`",
 347                     self.expr_ty,
 348                     fcx.ty_to_string(self.cast_ty)
 349                 );
 350                 let mut sugg = None;
 351                 let mut sugg_mutref = false;
 352                 if let ty::Ref(reg, _, mutbl) = *self.cast_ty.kind() {
 353                     if let ty::RawPtr(TypeAndMut { ty: expr_ty, .. }) = *self.expr_ty.kind() {
 354                         if fcx
 355                             .try_coerce(
 356                                 self.expr,
 357                                 fcx.tcx.mk_ref(
 358                                     &ty::RegionKind::ReErased,
 359                                     TypeAndMut { ty: expr_ty, mutbl },
 360                                 ),
 361                                 self.cast_ty,
 362                                 AllowTwoPhase::No,
 363                             )
 364                             .is_ok()
 365                         {
 366                             sugg = Some(format!("&{}*", mutbl.prefix_str()));
 367                         }
 368                     } else if let ty::Ref(expr_reg, expr_ty, expr_mutbl) = *self.expr_ty.kind() {
 369                         if expr_mutbl == Mutability::Not
 370                             && mutbl == Mutability::Mut
 371                             && fcx
 372                                 .try_coerce(
 373                                     self.expr,
 374                                     fcx.tcx.mk_ref(
 375                                         expr_reg,
 376                                         TypeAndMut { ty: expr_ty, mutbl: Mutability::Mut },
 377                                     ),
 378                                     self.cast_ty,
 379                                     AllowTwoPhase::No,
 380                                 )
 381                                 .is_ok()
 382                         {
 383                             sugg_mutref = true;
 384                         }
 385                     }
 386
 387                     if !sugg_mutref
 388                         && sugg == None
 389                         && fcx
 390                             .try_coerce(
 391                                 self.expr,
 392                                 fcx.tcx.mk_ref(reg, TypeAndMut { ty: self.expr_ty, mutbl }),
 393                                 self.cast_ty,
 394                                 AllowTwoPhase::No,
 395                             )
 396                             .is_ok()
 397                     {
 398                         sugg = Some(format!("&{}", mutbl.prefix_str()));
 399                     }
 400                 } else if let ty::RawPtr(TypeAndMut { mutbl, .. }) = *self.cast_ty.kind() {
 401                     if fcx
 402                         .try_coerce(
 403                             self.expr,
 404                             fcx.tcx.mk_ref(
 405                                 &ty::RegionKind::ReErased,
 406                                 TypeAndMut { ty: self.expr_ty, mutbl },
 407                             ),
 408                             self.cast_ty,
 409                             AllowTwoPhase::No,
 410                         )
 411                         .is_ok()
 412                     {
 413                         sugg = Some(format!("&{}", mutbl.prefix_str()));
 414                     }
 415                 }
 416                 if sugg_mutref {
 417                     err.span_label(self.span, "invalid cast");
 418                     err.span_note(self.expr.span, "this reference is immutable");
 419                     err.span_note(self.cast_span, "trying to cast to a mutable reference type");
 420                 } else if let Some(sugg) = sugg {
 421                     err.span_label(self.span, "invalid cast");
 422                     err.span_suggestion_verbose(
 423                         self.expr.span.shrink_to_lo(),
 424                         "consider borrowing the value",
 425                         sugg,
 426                         Applicability::MachineApplicable,
 427                     );
 428                 } else if !matches!(
 429                     self.cast_ty.kind(),
 430                     ty::FnDef(..) | ty::FnPtr(..) | ty::Closure(..)
 431                 ) {
 432                     let mut label = true;
 433                     // Check `impl From<self.expr_ty> for self.cast_ty {}` for accurate suggestion:
 434                     if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 435                         if let Some(from_trait) = fcx.tcx.get_diagnostic_item(sym::from_trait) {
 436                             let ty = fcx.resolve_vars_if_possible(self.cast_ty);
 437                             // Erase regions to avoid panic in `prove_value` when calling
 438                             // `type_implements_trait`.
 439                             let ty = fcx.tcx.erase_regions(ty);
 440                             let expr_ty = fcx.resolve_vars_if_possible(self.expr_ty);
 441                             let expr_ty = fcx.tcx.erase_regions(expr_ty);
 442                             let ty_params = fcx.tcx.mk_substs_trait(expr_ty, &[]);
 443                             if fcx
 444                                 .tcx
 445                                 .type_implements_trait((from_trait, ty, ty_params, fcx.param_env))
 446                                 .must_apply_modulo_regions()
 447                             {
 448                                 label = false;
 449                                 err.span_suggestion(
 450                                     self.span,
 451                                     "consider using the `From` trait instead",
 452                                     format!("{}::from({})", self.cast_ty, snippet),
 453                                     Applicability::MaybeIncorrect,
 454                                 );
 455                             }
 456                         }
 457                     }
 458                     let msg = "an `as` expression can only be used to convert between primitive \
 459                                types or to coerce to a specific trait object";
 460                     if label {
 461                         err.span_label(self.span, msg);
 462                     } else {
 463                         err.note(msg);
 464                     }
 465                 } else {
 466                     err.span_label(self.span, "invalid cast");
 467                 }
 468                 err.emit();
 469             }
 470             CastError::SizedUnsizedCast => {
 471                 use crate::structured_errors::{SizedUnsizedCast, StructuredDiagnostic};
 472
 473                 SizedUnsizedCast {
 474                     sess: &fcx.tcx.sess,
 475                     span: self.span,
 476                     expr_ty: self.expr_ty,
 477                     cast_ty: fcx.ty_to_string(self.cast_ty),
 478                 }
 479                 .diagnostic()
 480                 .emit();
 481             }
 482             CastError::UnknownCastPtrKind | CastError::UnknownExprPtrKind => {
 483                 let unknown_cast_to = match e {
 484                     CastError::UnknownCastPtrKind => true,
 485                     CastError::UnknownExprPtrKind => false,
 486                     _ => bug!(),
 487                 };
 488                 let mut err = struct_span_err!(
 489                     fcx.tcx.sess,
 490                     if unknown_cast_to { self.cast_span } else { self.span },
 491                     E0641,
 492                     "cannot cast {} a pointer of an unknown kind",
 493                     if unknown_cast_to { "to" } else { "from" }
 494                 );
 495                 if unknown_cast_to {
 496                     err.span_label(self.cast_span, "needs more type information");
 497                     err.note(
 498                         "the type information given here is insufficient to check whether \
 499                         the pointer cast is valid",
 500                     );
 501                 } else {
 502                     err.span_label(
 503                         self.span,
 504                         "the type information given here is insufficient to check whether \
 505                         the pointer cast is valid",
 506                     );
 507                 }
 508                 err.emit();
 509             }
 510         }
 511     }
 512
 513     fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'tcx>) {
 514         if self.cast_ty.references_error() || self.expr_ty.references_error() {
 515             return;
 516         }
 517
 518         let tstr = fcx.ty_to_string(self.cast_ty);
 519         let mut err = type_error_struct!(
 520             fcx.tcx.sess,
 521             self.span,
 522             self.expr_ty,
 523             E0620,
 524             "cast to unsized type: `{}` as `{}`",
 525             fcx.resolve_vars_if_possible(self.expr_ty),
 526             tstr
 527         );
 528         match self.expr_ty.kind() {
 529             ty::Ref(_, _, mt) => {
 530                 let mtstr = mt.prefix_str();
 531                 if self.cast_ty.is_trait() {
 532                     match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 533                         Ok(s) => {
 534                             err.span_suggestion(
 535                                 self.cast_span,
 536                                 "try casting to a reference instead",
 537                                 format!("&{}{}", mtstr, s),
 538                                 Applicability::MachineApplicable,
 539                             );
 540                         }
 541                         Err(_) => {
 542                             let msg = &format!("did you mean `&{}{}`?", mtstr, tstr);
 543                             err.span_help(self.cast_span, msg);
 544                         }
 545                     }
 546                 } else {
 547                     let msg = &format!(
 548                         "consider using an implicit coercion to `&{}{}` instead",
 549                         mtstr, tstr
 550                     );
 551                     err.span_help(self.span, msg);
 552                 }
 553             }
 554             ty::Adt(def, ..) if def.is_box() => {
 555                 match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 556                     Ok(s) => {
 557                         err.span_suggestion(
 558                             self.cast_span,
 559                             "you can cast to a `Box` instead",
 560                             format!("Box<{}>", s),
 561                             Applicability::MachineApplicable,
 562                         );
 563                     }
 564                     Err(_) => {
 565                         err.span_help(
 566                             self.cast_span,
 567                             &format!("you might have meant `Box<{}>`", tstr),
 568                         );
 569                     }
 570                 }
 571             }
 572             _ => {
 573                 err.span_help(self.expr.span, "consider using a box or reference as appropriate");
 574             }
 575         }
 576         err.emit();
 577     }
 578
 579     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 580         let t_cast = self.cast_ty;
 581         let t_expr = self.expr_ty;
 582         let type_asc_or =
 583             if fcx.tcx.features().type_ascription { "type ascription or " } else { "" };
 584         let (adjective, lint) = if t_cast.is_numeric() && t_expr.is_numeric() {
 585             ("numeric ", lint::builtin::TRIVIAL_NUMERIC_CASTS)
 586         } else {
 587             ("", lint::builtin::TRIVIAL_CASTS)
 588         };
 589         fcx.tcx.struct_span_lint_hir(lint, self.expr.hir_id, self.span, |err| {
 590             err.build(&format!(
 591                 "trivial {}cast: `{}` as `{}`",
 592                 adjective,
 593                 fcx.ty_to_string(t_expr),
 594                 fcx.ty_to_string(t_cast)
 595             ))
 596             .help(&format!(
 597                 "cast can be replaced by coercion; this might \
 598                                    require {}a temporary variable",
 599                 type_asc_or
 600             ))
 601             .emit();
 602         });
 603     }
 604
 605     pub fn check(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 606         self.expr_ty = fcx.structurally_resolved_type(self.span, self.expr_ty);
 607         self.cast_ty = fcx.structurally_resolved_type(self.span, self.cast_ty);
 608
 609         debug!("check_cast({}, {:?} as {:?})", self.expr.hir_id, self.expr_ty, self.cast_ty);
 610
 611         if !fcx.type_is_known_to_be_sized_modulo_regions(self.cast_ty, self.span) {
 612             self.report_cast_to_unsized_type(fcx);
 613         } else if self.expr_ty.references_error() || self.cast_ty.references_error() {
 614             // No sense in giving duplicate error messages
 615         } else {
 616             match self.try_coercion_cast(fcx) {
 617                 Ok(()) => {
 618                     self.trivial_cast_lint(fcx);
 619                     debug!(" -> CoercionCast");
 620                     fcx.typeck_results.borrow_mut().set_coercion_cast(self.expr.hir_id.local_id);
 621                 }
 622                 Err(ty::error::TypeError::ObjectUnsafeCoercion(did)) => {
 623                     self.report_object_unsafe_cast(&fcx, did);
 624                 }
 625                 Err(_) => {
 626                     match self.do_check(fcx) {
 627                         Ok(k) => {
 628                             debug!(" -> {:?}", k);
 629                         }
 630                         Err(e) => self.report_cast_error(fcx, e),
 631                     };
 632                 }
 633             };
 634         }
 635     }
 636
 637     fn report_object_unsafe_cast(&self, fcx: &FnCtxt<'a, 'tcx>, did: DefId) {
 638         let violations = fcx.tcx.object_safety_violations(did);
 639         let mut err = report_object_safety_error(fcx.tcx, self.cast_span, did, violations);
 640         err.note(&format!("required by cast to type '{}'", fcx.ty_to_string(self.cast_ty)));
 641         err.emit();
 642     }
 643
 644     /// Checks a cast, and report an error if one exists. In some cases, this
 645     /// can return Ok and create type errors in the fcx rather than returning
 646     /// directly. coercion-cast is handled in check instead of here.
 647     pub fn do_check(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
 648         use rustc_middle::ty::cast::CastTy::*;
 649         use rustc_middle::ty::cast::IntTy::*;
 650
 651         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty), CastTy::from_ty(self.cast_ty))
 652         {
 653             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 654             // Function item types may need to be reified before casts.
 655             (None, Some(t_cast)) => {
 656                 match *self.expr_ty.kind() {
 657                     ty::FnDef(..) => {
 658                         // Attempt a coercion to a fn pointer type.
 659                         let f = fcx.normalize_associated_types_in(
 660                             self.expr.span,
 661                             self.expr_ty.fn_sig(fcx.tcx),
 662                         );
 663                         let res = fcx.try_coerce(
 664                             self.expr,
 665                             self.expr_ty,
 666                             fcx.tcx.mk_fn_ptr(f),
 667                             AllowTwoPhase::No,
 668                         );
 669                         if let Err(TypeError::IntrinsicCast) = res {
 670                             return Err(CastError::IllegalCast);
 671                         }
 672                         if res.is_err() {
 673                             return Err(CastError::NonScalar);
 674                         }
 675                         (FnPtr, t_cast)
 676                     }
 677                     // Special case some errors for references, and check for
 678                     // array-ptr-casts. `Ref` is not a CastTy because the cast
 679                     // is split into a coercion to a pointer type, followed by
 680                     // a cast.
 681                     ty::Ref(_, inner_ty, mutbl) => {
 682                         return match t_cast {
 683                             Int(_) | Float => match *inner_ty.kind() {
 684                                 ty::Int(_)
 685                                 | ty::Uint(_)
 686                                 | ty::Float(_)
 687                                 | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(_)) => {
 688                                     Err(CastError::NeedDeref)
 689                                 }
 690                                 _ => Err(CastError::NeedViaPtr),
 691                             },
 692                             // array-ptr-cast
 693                             Ptr(mt) => {
 694                                 self.check_ref_cast(fcx, TypeAndMut { mutbl, ty: inner_ty }, mt)
 695                             }
 696                             _ => Err(CastError::NonScalar),
 697                         };
 698                     }
 699                     _ => return Err(CastError::NonScalar),
 700                 }
 701             }
 702             _ => return Err(CastError::NonScalar),
 703         };
 704
 705         match (t_from, t_cast) {
 706             // These types have invariants! can't cast into them.
 707             (_, Int(CEnum) | FnPtr) => Err(CastError::NonScalar),
 708
 709             // * -> Bool
 710             (_, Int(Bool)) => Err(CastError::CastToBool),
 711
 712             // * -> Char
 713             (Int(U(ty::UintTy::U8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 714             (_, Int(Char)) => Err(CastError::CastToChar),
 715
 716             // prim -> float,ptr
 717             (Int(Bool) | Int(CEnum) | Int(Char), Float) => Err(CastError::NeedViaInt),
 718
 719             (Int(Bool) | Int(CEnum) | Int(Char) | Float, Ptr(_)) | (Ptr(_) | FnPtr, Float) => {
 720                 Err(CastError::IllegalCast)
 721             }
 722
 723             // ptr -> *
 724             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 725             (Ptr(m_expr), Int(_)) => self.check_ptr_addr_cast(fcx, m_expr), // ptr-addr-cast
 726             (FnPtr, Int(_)) => Ok(CastKind::FnPtrAddrCast),
 727
 728             // * -> ptr
 729             (Int(_), Ptr(mt)) => self.check_addr_ptr_cast(fcx, mt), // addr-ptr-cast
 730             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 731
 732             // prim -> prim
 733             (Int(CEnum), Int(_)) => {
 734                 self.cenum_impl_drop_lint(fcx);
 735                 Ok(CastKind::EnumCast)
 736             }
 737             (Int(Char) | Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 738
 739             (Int(_) | Float, Int(_) | Float) => Ok(CastKind::NumericCast),
 740         }
 741     }
 742
 743     fn check_ptr_ptr_cast(
 744         &self,
 745         fcx: &FnCtxt<'a, 'tcx>,
 746         m_expr: ty::TypeAndMut<'tcx>,
 747         m_cast: ty::TypeAndMut<'tcx>,
 748     ) -> Result<CastKind, CastError> {
 749         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_expr, m_cast);
 750         // ptr-ptr cast. vtables must match.
 751
 752         let expr_kind = fcx.pointer_kind(m_expr.ty, self.span)?;
 753         let cast_kind = fcx.pointer_kind(m_cast.ty, self.span)?;
 754
 755         let cast_kind = match cast_kind {
 756             // We can't cast if target pointer kind is unknown
 757             None => return Err(CastError::UnknownCastPtrKind),
 758             Some(cast_kind) => cast_kind,
 759         };
 760
 761         // Cast to thin pointer is OK
 762         if cast_kind == PointerKind::Thin {
 763             return Ok(CastKind::PtrPtrCast);
 764         }
 765
 766         let expr_kind = match expr_kind {
 767             // We can't cast to fat pointer if source pointer kind is unknown
 768             None => return Err(CastError::UnknownExprPtrKind),
 769             Some(expr_kind) => expr_kind,
 770         };
 771
 772         // thin -> fat? report invalid cast (don't complain about vtable kinds)
 773         if expr_kind == PointerKind::Thin {
 774             return Err(CastError::SizedUnsizedCast);
 775         }
 776
 777         // vtable kinds must match
 778         if cast_kind == expr_kind {
 779             Ok(CastKind::PtrPtrCast)
 780         } else {
 781             Err(CastError::DifferingKinds)
 782         }
 783     }
 784
 785     fn check_fptr_ptr_cast(
 786         &self,
 787         fcx: &FnCtxt<'a, 'tcx>,
 788         m_cast: ty::TypeAndMut<'tcx>,
 789     ) -> Result<CastKind, CastError> {
 790         // fptr-ptr cast. must be to thin ptr
 791
 792         match fcx.pointer_kind(m_cast.ty, self.span)? {
 793             None => Err(CastError::UnknownCastPtrKind),
 794             Some(PointerKind::Thin) => Ok(CastKind::FnPtrPtrCast),
 795             _ => Err(CastError::IllegalCast),
 796         }
 797     }
 798
 799     fn check_ptr_addr_cast(
 800         &self,
 801         fcx: &FnCtxt<'a, 'tcx>,
 802         m_expr: ty::TypeAndMut<'tcx>,
 803     ) -> Result<CastKind, CastError> {
 804         // ptr-addr cast. must be from thin ptr
 805
 806         match fcx.pointer_kind(m_expr.ty, self.span)? {
 807             None => Err(CastError::UnknownExprPtrKind),
 808             Some(PointerKind::Thin) => Ok(CastKind::PtrAddrCast),
 809             _ => Err(CastError::NeedViaThinPtr),
 810         }
 811     }
 812
 813     fn check_ref_cast(
 814         &self,
 815         fcx: &FnCtxt<'a, 'tcx>,
 816         m_expr: ty::TypeAndMut<'tcx>,
 817         m_cast: ty::TypeAndMut<'tcx>,
 818     ) -> Result<CastKind, CastError> {
 819         // array-ptr-cast: allow mut-to-mut, mut-to-const, const-to-const
 820         if m_expr.mutbl == hir::Mutability::Mut || m_cast.mutbl == hir::Mutability::Not {
 821             if let ty::Array(ety, _) = m_expr.ty.kind() {
 822                 // Due to the limitations of LLVM global constants,
 823                 // region pointers end up pointing at copies of
 824                 // vector elements instead of the original values.
 825                 // To allow raw pointers to work correctly, we
 826                 // need to special-case obtaining a raw pointer
 827                 // from a region pointer to a vector.
 828
 829                 // Coerce to a raw pointer so that we generate AddressOf in MIR.
 830                 let array_ptr_type = fcx.tcx.mk_ptr(m_expr);
 831                 fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No)
 832                     .unwrap_or_else(|_| {
 833                         bug!(
 834                         "could not cast from reference to array to pointer to array ({:?} to {:?})",
 835                         self.expr_ty,
 836                         array_ptr_type,
 837                     )
 838                     });
 839
 840                 // this will report a type mismatch if needed
 841                 fcx.demand_eqtype(self.span, ety, m_cast.ty);
 842                 return Ok(CastKind::ArrayPtrCast);
 843             }
 844         }
 845
 846         Err(CastError::IllegalCast)
 847     }
 848
 849     fn check_addr_ptr_cast(
 850         &self,
 851         fcx: &FnCtxt<'a, 'tcx>,
 852         m_cast: TypeAndMut<'tcx>,
 853     ) -> Result<CastKind, CastError> {
 854         // ptr-addr cast. pointer must be thin.
 855         match fcx.pointer_kind(m_cast.ty, self.span)? {
 856             None => Err(CastError::UnknownCastPtrKind),
 857             Some(PointerKind::Thin) => Ok(CastKind::AddrPtrCast),
 858             _ => Err(CastError::IllegalCast),
 859         }
 860     }
 861
 862     fn try_coercion_cast(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<(), ty::error::TypeError<'_>> {
 863         match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No) {
 864             Ok(_) => Ok(()),
 865             Err(err) => Err(err),
 866         }
 867     }
 868
 869     fn cenum_impl_drop_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 870         if let ty::Adt(d, _) = self.expr_ty.kind() {
 871             if d.has_dtor(fcx.tcx) {
 872                 fcx.tcx.struct_span_lint_hir(
 873                     lint::builtin::CENUM_IMPL_DROP_CAST,
 874                     self.expr.hir_id,
 875                     self.span,
 876                     |err| {
 877                         err.build(&format!(
 878                             "cannot cast enum `{}` into integer `{}` because it implements `Drop`",
 879                             self.expr_ty, self.cast_ty
 880                         ))
 881                         .emit();
 882                     },
 883                 );
 884             }
 885         }
 886     }
 887 }
 888
 889 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 890     fn type_is_known_to_be_sized_modulo_regions(&self, ty: Ty<'tcx>, span: Span) -> bool {
 891         let lang_item = self.tcx.require_lang_item(LangItem::Sized, None);
 892         traits::type_known_to_meet_bound_modulo_regions(self, self.param_env, ty, lang_item, span)
 893     }
 894 }