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::infer::InferCtxtExt;
  49 use rustc_trait_selection::traits;
  50 use rustc_trait_selection::traits::error_reporting::report_object_safety_error;
  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::Opaque(def_id, substs) => Some(PointerKind::OfOpaque(def_id, substs)),
 121             ty::Param(ref p) => Some(PointerKind::OfParam(p)),
 122             // Insufficient type information.
 123             ty::Placeholder(..) | ty::Bound(..) | ty::Infer(_) => None,
 124
 125             ty::Bool
 126             | ty::Char
 127             | ty::Int(..)
 128             | ty::Uint(..)
 129             | ty::Float(_)
 130             | ty::Array(..)
 131             | ty::GeneratorWitness(..)
 132             | ty::RawPtr(_)
 133             | ty::Ref(..)
 134             | ty::FnDef(..)
 135             | ty::FnPtr(..)
 136             | ty::Closure(..)
 137             | ty::Generator(..)
 138             | ty::Adt(..)
 139             | ty::Never
 140             | ty::Error(_) => {
 141                 self.tcx
 142                     .sess
 143                     .delay_span_bug(span, &format!("`{:?}` should be sized but is not?", t));
 144                 return Err(ErrorReported);
 145             }
 146         })
 147     }
 148 }
 149
 150 #[derive(Copy, Clone)]
 151 pub enum CastError {
 152     ErrorReported,
 153
 154     CastToBool,
 155     CastToChar,
 156     DifferingKinds,
 157     /// Cast of thin to fat raw ptr (e.g., `*const () as *const [u8]`).
 158     SizedUnsizedCast,
 159     IllegalCast,
 160     NeedDeref,
 161     NeedViaPtr,
 162     NeedViaThinPtr,
 163     NeedViaInt,
 164     NonScalar,
 165     UnknownExprPtrKind,
 166     UnknownCastPtrKind,
 167 }
 168
 169 impl From<ErrorReported> for CastError {
 170     fn from(ErrorReported: ErrorReported) -> Self {
 171         CastError::ErrorReported
 172     }
 173 }
 174
 175 fn make_invalid_casting_error<'a, 'tcx>(
 176     sess: &'a Session,
 177     span: Span,
 178     expr_ty: Ty<'tcx>,
 179     cast_ty: Ty<'tcx>,
 180     fcx: &FnCtxt<'a, 'tcx>,
 181 ) -> DiagnosticBuilder<'a> {
 182     type_error_struct!(
 183         sess,
 184         span,
 185         expr_ty,
 186         E0606,
 187         "casting `{}` as `{}` is invalid",
 188         fcx.ty_to_string(expr_ty),
 189         fcx.ty_to_string(cast_ty)
 190     )
 191 }
 192
 193 impl<'a, 'tcx> CastCheck<'tcx> {
 194     pub fn new(
 195         fcx: &FnCtxt<'a, 'tcx>,
 196         expr: &'tcx hir::Expr<'tcx>,
 197         expr_ty: Ty<'tcx>,
 198         cast_ty: Ty<'tcx>,
 199         cast_span: Span,
 200         span: Span,
 201     ) -> Result<CastCheck<'tcx>, ErrorReported> {
 202         let check = CastCheck { expr, expr_ty, cast_ty, cast_span, span };
 203
 204         // For better error messages, check for some obviously unsized
 205         // cases now. We do a more thorough check at the end, once
 206         // inference is more completely known.
 207         match cast_ty.kind() {
 208             ty::Dynamic(..) | ty::Slice(..) => {
 209                 check.report_cast_to_unsized_type(fcx);
 210                 Err(ErrorReported)
 211             }
 212             _ => Ok(check),
 213         }
 214     }
 215
 216     fn report_cast_error(&self, fcx: &FnCtxt<'a, 'tcx>, e: CastError) {
 217         match e {
 218             CastError::ErrorReported => {
 219                 // an error has already been reported
 220             }
 221             CastError::NeedDeref => {
 222                 let error_span = self.span;
 223                 let mut err = make_invalid_casting_error(
 224                     fcx.tcx.sess,
 225                     self.span,
 226                     self.expr_ty,
 227                     self.cast_ty,
 228                     fcx,
 229                 );
 230                 let cast_ty = fcx.ty_to_string(self.cast_ty);
 231                 err.span_label(
 232                     error_span,
 233                     format!("cannot cast `{}` as `{}`", fcx.ty_to_string(self.expr_ty), cast_ty),
 234                 );
 235                 if let Ok(snippet) = fcx.sess().source_map().span_to_snippet(self.expr.span) {
 236                     err.span_suggestion(
 237                         self.expr.span,
 238                         "dereference the expression",
 239                         format!("*{}", snippet),
 240                         Applicability::MaybeIncorrect,
 241                     );
 242                 } else {
 243                     err.span_help(self.expr.span, "dereference the expression with `*`");
 244                 }
 245                 err.emit();
 246             }
 247             CastError::NeedViaThinPtr | CastError::NeedViaPtr => {
 248                 let mut err = make_invalid_casting_error(
 249                     fcx.tcx.sess,
 250                     self.span,
 251                     self.expr_ty,
 252                     self.cast_ty,
 253                     fcx,
 254                 );
 255                 if self.cast_ty.is_integral() {
 256                     err.help(&format!(
 257                         "cast through {} first",
 258                         match e {
 259                             CastError::NeedViaPtr => "a raw pointer",
 260                             CastError::NeedViaThinPtr => "a thin pointer",
 261                             _ => bug!(),
 262                         }
 263                     ));
 264                 }
 265                 err.emit();
 266             }
 267             CastError::NeedViaInt => {
 268                 make_invalid_casting_error(
 269                     fcx.tcx.sess,
 270                     self.span,
 271                     self.expr_ty,
 272                     self.cast_ty,
 273                     fcx,
 274                 )
 275                 .help(&format!(
 276                     "cast through {} first",
 277                     match e {
 278                         CastError::NeedViaInt => "an integer",
 279                         _ => bug!(),
 280                     }
 281                 ))
 282                 .emit();
 283             }
 284             CastError::IllegalCast => {
 285                 make_invalid_casting_error(
 286                     fcx.tcx.sess,
 287                     self.span,
 288                     self.expr_ty,
 289                     self.cast_ty,
 290                     fcx,
 291                 )
 292                 .emit();
 293             }
 294             CastError::DifferingKinds => {
 295                 make_invalid_casting_error(
 296                     fcx.tcx.sess,
 297                     self.span,
 298                     self.expr_ty,
 299                     self.cast_ty,
 300                     fcx,
 301                 )
 302                 .note("vtable kinds may not match")
 303                 .emit();
 304             }
 305             CastError::CastToBool => {
 306                 let mut err =
 307                     struct_span_err!(fcx.tcx.sess, self.span, E0054, "cannot cast as `bool`");
 308
 309                 if self.expr_ty.is_numeric() {
 310                     match fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 311                         Ok(snippet) => {
 312                             err.span_suggestion(
 313                                 self.span,
 314                                 "compare with zero instead",
 315                                 format!("{} != 0", snippet),
 316                                 Applicability::MachineApplicable,
 317                             );
 318                         }
 319                         Err(_) => {
 320                             err.span_help(self.span, "compare with zero instead");
 321                         }
 322                     }
 323                 } else {
 324                     err.span_label(self.span, "unsupported cast");
 325                 }
 326
 327                 err.emit();
 328             }
 329             CastError::CastToChar => {
 330                 type_error_struct!(
 331                     fcx.tcx.sess,
 332                     self.span,
 333                     self.expr_ty,
 334                     E0604,
 335                     "only `u8` can be cast as `char`, not `{}`",
 336                     self.expr_ty
 337                 )
 338                 .span_label(self.span, "invalid cast")
 339                 .emit();
 340             }
 341             CastError::NonScalar => {
 342                 let mut err = 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                 let mut sugg = None;
 352                 let mut sugg_mutref = false;
 353                 if let ty::Ref(reg, _, mutbl) = *self.cast_ty.kind() {
 354                     if let ty::RawPtr(TypeAndMut { ty: expr_ty, .. }) = *self.expr_ty.kind() {
 355                         if fcx
 356                             .try_coerce(
 357                                 self.expr,
 358                                 fcx.tcx.mk_ref(
 359                                     &ty::RegionKind::ReErased,
 360                                     TypeAndMut { ty: expr_ty, mutbl },
 361                                 ),
 362                                 self.cast_ty,
 363                                 AllowTwoPhase::No,
 364                             )
 365                             .is_ok()
 366                         {
 367                             sugg = Some(format!("&{}*", mutbl.prefix_str()));
 368                         }
 369                     } else if let ty::Ref(expr_reg, expr_ty, expr_mutbl) = *self.expr_ty.kind() {
 370                         if expr_mutbl == Mutability::Not
 371                             && mutbl == Mutability::Mut
 372                             && fcx
 373                                 .try_coerce(
 374                                     self.expr,
 375                                     fcx.tcx.mk_ref(
 376                                         expr_reg,
 377                                         TypeAndMut { ty: expr_ty, mutbl: Mutability::Mut },
 378                                     ),
 379                                     self.cast_ty,
 380                                     AllowTwoPhase::No,
 381                                 )
 382                                 .is_ok()
 383                         {
 384                             sugg_mutref = true;
 385                         }
 386                     }
 387
 388                     if !sugg_mutref
 389                         && sugg == None
 390                         && fcx
 391                             .try_coerce(
 392                                 self.expr,
 393                                 fcx.tcx.mk_ref(reg, TypeAndMut { ty: self.expr_ty, mutbl }),
 394                                 self.cast_ty,
 395                                 AllowTwoPhase::No,
 396                             )
 397                             .is_ok()
 398                     {
 399                         sugg = Some(format!("&{}", mutbl.prefix_str()));
 400                     }
 401                 } else if let ty::RawPtr(TypeAndMut { mutbl, .. }) = *self.cast_ty.kind() {
 402                     if fcx
 403                         .try_coerce(
 404                             self.expr,
 405                             fcx.tcx.mk_ref(
 406                                 &ty::RegionKind::ReErased,
 407                                 TypeAndMut { ty: self.expr_ty, mutbl },
 408                             ),
 409                             self.cast_ty,
 410                             AllowTwoPhase::No,
 411                         )
 412                         .is_ok()
 413                     {
 414                         sugg = Some(format!("&{}", mutbl.prefix_str()));
 415                     }
 416                 }
 417                 if sugg_mutref {
 418                     err.span_label(self.span, "invalid cast");
 419                     err.span_note(self.expr.span, "this reference is immutable");
 420                     err.span_note(self.cast_span, "trying to cast to a mutable reference type");
 421                 } else if let Some(sugg) = sugg {
 422                     err.span_label(self.span, "invalid cast");
 423                     err.span_suggestion_verbose(
 424                         self.expr.span.shrink_to_lo(),
 425                         "consider borrowing the value",
 426                         sugg,
 427                         Applicability::MachineApplicable,
 428                     );
 429                 } else if !matches!(
 430                     self.cast_ty.kind(),
 431                     ty::FnDef(..) | ty::FnPtr(..) | ty::Closure(..)
 432                 ) {
 433                     let mut label = true;
 434                     // Check `impl From<self.expr_ty> for self.cast_ty {}` for accurate suggestion:
 435                     if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 436                         if let Some(from_trait) = fcx.tcx.get_diagnostic_item(sym::from_trait) {
 437                             let ty = fcx.resolve_vars_if_possible(self.cast_ty);
 438                             // Erase regions to avoid panic in `prove_value` when calling
 439                             // `type_implements_trait`.
 440                             let ty = fcx.tcx.erase_regions(ty);
 441                             let expr_ty = fcx.resolve_vars_if_possible(self.expr_ty);
 442                             let expr_ty = fcx.tcx.erase_regions(expr_ty);
 443                             let ty_params = fcx.tcx.mk_substs_trait(expr_ty, &[]);
 444                             if fcx
 445                                 .infcx
 446                                 .type_implements_trait(from_trait, ty, ty_params, fcx.param_env)
 447                                 .must_apply_modulo_regions()
 448                             {
 449                                 label = false;
 450                                 err.span_suggestion(
 451                                     self.span,
 452                                     "consider using the `From` trait instead",
 453                                     format!("{}::from({})", self.cast_ty, snippet),
 454                                     Applicability::MaybeIncorrect,
 455                                 );
 456                             }
 457                         }
 458                     }
 459                     let msg = "an `as` expression can only be used to convert between primitive \
 460                                types or to coerce to a specific trait object";
 461                     if label {
 462                         err.span_label(self.span, msg);
 463                     } else {
 464                         err.note(msg);
 465                     }
 466                 } else {
 467                     err.span_label(self.span, "invalid cast");
 468                 }
 469                 err.emit();
 470             }
 471             CastError::SizedUnsizedCast => {
 472                 use crate::structured_errors::{SizedUnsizedCast, StructuredDiagnostic};
 473
 474                 SizedUnsizedCast {
 475                     sess: &fcx.tcx.sess,
 476                     span: self.span,
 477                     expr_ty: self.expr_ty,
 478                     cast_ty: fcx.ty_to_string(self.cast_ty),
 479                 }
 480                 .diagnostic()
 481                 .emit();
 482             }
 483             CastError::UnknownCastPtrKind | CastError::UnknownExprPtrKind => {
 484                 let unknown_cast_to = match e {
 485                     CastError::UnknownCastPtrKind => true,
 486                     CastError::UnknownExprPtrKind => false,
 487                     _ => bug!(),
 488                 };
 489                 let mut err = struct_span_err!(
 490                     fcx.tcx.sess,
 491                     if unknown_cast_to { self.cast_span } else { self.span },
 492                     E0641,
 493                     "cannot cast {} a pointer of an unknown kind",
 494                     if unknown_cast_to { "to" } else { "from" }
 495                 );
 496                 if unknown_cast_to {
 497                     err.span_label(self.cast_span, "needs more type information");
 498                     err.note(
 499                         "the type information given here is insufficient to check whether \
 500                         the pointer cast is valid",
 501                     );
 502                 } else {
 503                     err.span_label(
 504                         self.span,
 505                         "the type information given here is insufficient to check whether \
 506                         the pointer cast is valid",
 507                     );
 508                 }
 509                 err.emit();
 510             }
 511         }
 512     }
 513
 514     fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'tcx>) {
 515         if self.cast_ty.references_error() || self.expr_ty.references_error() {
 516             return;
 517         }
 518
 519         let tstr = fcx.ty_to_string(self.cast_ty);
 520         let mut err = type_error_struct!(
 521             fcx.tcx.sess,
 522             self.span,
 523             self.expr_ty,
 524             E0620,
 525             "cast to unsized type: `{}` as `{}`",
 526             fcx.resolve_vars_if_possible(self.expr_ty),
 527             tstr
 528         );
 529         match self.expr_ty.kind() {
 530             ty::Ref(_, _, mt) => {
 531                 let mtstr = mt.prefix_str();
 532                 if self.cast_ty.is_trait() {
 533                     match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 534                         Ok(s) => {
 535                             err.span_suggestion(
 536                                 self.cast_span,
 537                                 "try casting to a reference instead",
 538                                 format!("&{}{}", mtstr, s),
 539                                 Applicability::MachineApplicable,
 540                             );
 541                         }
 542                         Err(_) => {
 543                             let msg = &format!("did you mean `&{}{}`?", mtstr, tstr);
 544                             err.span_help(self.cast_span, msg);
 545                         }
 546                     }
 547                 } else {
 548                     let msg = &format!(
 549                         "consider using an implicit coercion to `&{}{}` instead",
 550                         mtstr, tstr
 551                     );
 552                     err.span_help(self.span, msg);
 553                 }
 554             }
 555             ty::Adt(def, ..) if def.is_box() => {
 556                 match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 557                     Ok(s) => {
 558                         err.span_suggestion(
 559                             self.cast_span,
 560                             "you can cast to a `Box` instead",
 561                             format!("Box<{}>", s),
 562                             Applicability::MachineApplicable,
 563                         );
 564                     }
 565                     Err(_) => {
 566                         err.span_help(
 567                             self.cast_span,
 568                             &format!("you might have meant `Box<{}>`", tstr),
 569                         );
 570                     }
 571                 }
 572             }
 573             _ => {
 574                 err.span_help(self.expr.span, "consider using a box or reference as appropriate");
 575             }
 576         }
 577         err.emit();
 578     }
 579
 580     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 581         let t_cast = self.cast_ty;
 582         let t_expr = self.expr_ty;
 583         let type_asc_or =
 584             if fcx.tcx.features().type_ascription { "type ascription or " } else { "" };
 585         let (adjective, lint) = if t_cast.is_numeric() && t_expr.is_numeric() {
 586             ("numeric ", lint::builtin::TRIVIAL_NUMERIC_CASTS)
 587         } else {
 588             ("", lint::builtin::TRIVIAL_CASTS)
 589         };
 590         fcx.tcx.struct_span_lint_hir(lint, self.expr.hir_id, self.span, |err| {
 591             err.build(&format!(
 592                 "trivial {}cast: `{}` as `{}`",
 593                 adjective,
 594                 fcx.ty_to_string(t_expr),
 595                 fcx.ty_to_string(t_cast)
 596             ))
 597             .help(&format!(
 598                 "cast can be replaced by coercion; this might \
 599                                    require {}a temporary variable",
 600                 type_asc_or
 601             ))
 602             .emit();
 603         });
 604     }
 605
 606     pub fn check(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 607         self.expr_ty = fcx.structurally_resolved_type(self.expr.span, self.expr_ty);
 608         self.cast_ty = fcx.structurally_resolved_type(self.cast_span, self.cast_ty);
 609
 610         debug!("check_cast({}, {:?} as {:?})", self.expr.hir_id, self.expr_ty, self.cast_ty);
 611
 612         if !fcx.type_is_known_to_be_sized_modulo_regions(self.cast_ty, self.span) {
 613             self.report_cast_to_unsized_type(fcx);
 614         } else if self.expr_ty.references_error() || self.cast_ty.references_error() {
 615             // No sense in giving duplicate error messages
 616         } else {
 617             match self.try_coercion_cast(fcx) {
 618                 Ok(()) => {
 619                     self.trivial_cast_lint(fcx);
 620                     debug!(" -> CoercionCast");
 621                     fcx.typeck_results.borrow_mut().set_coercion_cast(self.expr.hir_id.local_id);
 622                 }
 623                 Err(ty::error::TypeError::ObjectUnsafeCoercion(did)) => {
 624                     self.report_object_unsafe_cast(&fcx, did);
 625                 }
 626                 Err(_) => {
 627                     match self.do_check(fcx) {
 628                         Ok(k) => {
 629                             debug!(" -> {:?}", k);
 630                         }
 631                         Err(e) => self.report_cast_error(fcx, e),
 632                     };
 633                 }
 634             };
 635         }
 636     }
 637
 638     fn report_object_unsafe_cast(&self, fcx: &FnCtxt<'a, 'tcx>, did: DefId) {
 639         let violations = fcx.tcx.object_safety_violations(did);
 640         let mut err = report_object_safety_error(fcx.tcx, self.cast_span, did, violations);
 641         err.note(&format!("required by cast to type '{}'", fcx.ty_to_string(self.cast_ty)));
 642         err.emit();
 643     }
 644
 645     /// Checks a cast, and report an error if one exists. In some cases, this
 646     /// can return Ok and create type errors in the fcx rather than returning
 647     /// directly. coercion-cast is handled in check instead of here.
 648     pub fn do_check(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
 649         use rustc_middle::ty::cast::CastTy::*;
 650         use rustc_middle::ty::cast::IntTy::*;
 651
 652         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty), CastTy::from_ty(self.cast_ty))
 653         {
 654             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 655             // Function item types may need to be reified before casts.
 656             (None, Some(t_cast)) => {
 657                 match *self.expr_ty.kind() {
 658                     ty::FnDef(..) => {
 659                         // Attempt a coercion to a fn pointer type.
 660                         let f = fcx.normalize_associated_types_in(
 661                             self.expr.span,
 662                             self.expr_ty.fn_sig(fcx.tcx),
 663                         );
 664                         let res = fcx.try_coerce(
 665                             self.expr,
 666                             self.expr_ty,
 667                             fcx.tcx.mk_fn_ptr(f),
 668                             AllowTwoPhase::No,
 669                         );
 670                         if let Err(TypeError::IntrinsicCast) = res {
 671                             return Err(CastError::IllegalCast);
 672                         }
 673                         if res.is_err() {
 674                             return Err(CastError::NonScalar);
 675                         }
 676                         (FnPtr, t_cast)
 677                     }
 678                     // Special case some errors for references, and check for
 679                     // array-ptr-casts. `Ref` is not a CastTy because the cast
 680                     // is split into a coercion to a pointer type, followed by
 681                     // a cast.
 682                     ty::Ref(_, inner_ty, mutbl) => {
 683                         return match t_cast {
 684                             Int(_) | Float => match *inner_ty.kind() {
 685                                 ty::Int(_)
 686                                 | ty::Uint(_)
 687                                 | ty::Float(_)
 688                                 | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(_)) => {
 689                                     Err(CastError::NeedDeref)
 690                                 }
 691                                 _ => Err(CastError::NeedViaPtr),
 692                             },
 693                             // array-ptr-cast
 694                             Ptr(mt) => {
 695                                 self.check_ref_cast(fcx, TypeAndMut { mutbl, ty: inner_ty }, mt)
 696                             }
 697                             _ => Err(CastError::NonScalar),
 698                         };
 699                     }
 700                     _ => return Err(CastError::NonScalar),
 701                 }
 702             }
 703             _ => return Err(CastError::NonScalar),
 704         };
 705
 706         match (t_from, t_cast) {
 707             // These types have invariants! can't cast into them.
 708             (_, Int(CEnum) | FnPtr) => Err(CastError::NonScalar),
 709
 710             // * -> Bool
 711             (_, Int(Bool)) => Err(CastError::CastToBool),
 712
 713             // * -> Char
 714             (Int(U(ty::UintTy::U8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 715             (_, Int(Char)) => Err(CastError::CastToChar),
 716
 717             // prim -> float,ptr
 718             (Int(Bool) | Int(CEnum) | Int(Char), Float) => Err(CastError::NeedViaInt),
 719
 720             (Int(Bool) | Int(CEnum) | Int(Char) | Float, Ptr(_)) | (Ptr(_) | FnPtr, Float) => {
 721                 Err(CastError::IllegalCast)
 722             }
 723
 724             // ptr -> *
 725             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 726             (Ptr(m_expr), Int(_)) => self.check_ptr_addr_cast(fcx, m_expr), // ptr-addr-cast
 727             (FnPtr, Int(_)) => Ok(CastKind::FnPtrAddrCast),
 728
 729             // * -> ptr
 730             (Int(_), Ptr(mt)) => self.check_addr_ptr_cast(fcx, mt), // addr-ptr-cast
 731             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 732
 733             // prim -> prim
 734             (Int(CEnum), Int(_)) => {
 735                 self.cenum_impl_drop_lint(fcx);
 736                 Ok(CastKind::EnumCast)
 737             }
 738             (Int(Char) | Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 739
 740             (Int(_) | Float, Int(_) | Float) => Ok(CastKind::NumericCast),
 741         }
 742     }
 743
 744     fn check_ptr_ptr_cast(
 745         &self,
 746         fcx: &FnCtxt<'a, 'tcx>,
 747         m_expr: ty::TypeAndMut<'tcx>,
 748         m_cast: ty::TypeAndMut<'tcx>,
 749     ) -> Result<CastKind, CastError> {
 750         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_expr, m_cast);
 751         // ptr-ptr cast. vtables must match.
 752
 753         let expr_kind = fcx.pointer_kind(m_expr.ty, self.span)?;
 754         let cast_kind = fcx.pointer_kind(m_cast.ty, self.span)?;
 755
 756         let cast_kind = match cast_kind {
 757             // We can't cast if target pointer kind is unknown
 758             None => return Err(CastError::UnknownCastPtrKind),
 759             Some(cast_kind) => cast_kind,
 760         };
 761
 762         // Cast to thin pointer is OK
 763         if cast_kind == PointerKind::Thin {
 764             return Ok(CastKind::PtrPtrCast);
 765         }
 766
 767         let expr_kind = match expr_kind {
 768             // We can't cast to fat pointer if source pointer kind is unknown
 769             None => return Err(CastError::UnknownExprPtrKind),
 770             Some(expr_kind) => expr_kind,
 771         };
 772
 773         // thin -> fat? report invalid cast (don't complain about vtable kinds)
 774         if expr_kind == PointerKind::Thin {
 775             return Err(CastError::SizedUnsizedCast);
 776         }
 777
 778         // vtable kinds must match
 779         if cast_kind == expr_kind {
 780             Ok(CastKind::PtrPtrCast)
 781         } else {
 782             Err(CastError::DifferingKinds)
 783         }
 784     }
 785
 786     fn check_fptr_ptr_cast(
 787         &self,
 788         fcx: &FnCtxt<'a, 'tcx>,
 789         m_cast: ty::TypeAndMut<'tcx>,
 790     ) -> Result<CastKind, CastError> {
 791         // fptr-ptr cast. must be to thin ptr
 792
 793         match fcx.pointer_kind(m_cast.ty, self.span)? {
 794             None => Err(CastError::UnknownCastPtrKind),
 795             Some(PointerKind::Thin) => Ok(CastKind::FnPtrPtrCast),
 796             _ => Err(CastError::IllegalCast),
 797         }
 798     }
 799
 800     fn check_ptr_addr_cast(
 801         &self,
 802         fcx: &FnCtxt<'a, 'tcx>,
 803         m_expr: ty::TypeAndMut<'tcx>,
 804     ) -> Result<CastKind, CastError> {
 805         // ptr-addr cast. must be from thin ptr
 806
 807         match fcx.pointer_kind(m_expr.ty, self.span)? {
 808             None => Err(CastError::UnknownExprPtrKind),
 809             Some(PointerKind::Thin) => Ok(CastKind::PtrAddrCast),
 810             _ => Err(CastError::NeedViaThinPtr),
 811         }
 812     }
 813
 814     fn check_ref_cast(
 815         &self,
 816         fcx: &FnCtxt<'a, 'tcx>,
 817         m_expr: ty::TypeAndMut<'tcx>,
 818         m_cast: ty::TypeAndMut<'tcx>,
 819     ) -> Result<CastKind, CastError> {
 820         // array-ptr-cast: allow mut-to-mut, mut-to-const, const-to-const
 821         if m_expr.mutbl == hir::Mutability::Mut || m_cast.mutbl == hir::Mutability::Not {
 822             if let ty::Array(ety, _) = m_expr.ty.kind() {
 823                 // Due to the limitations of LLVM global constants,
 824                 // region pointers end up pointing at copies of
 825                 // vector elements instead of the original values.
 826                 // To allow raw pointers to work correctly, we
 827                 // need to special-case obtaining a raw pointer
 828                 // from a region pointer to a vector.
 829
 830                 // Coerce to a raw pointer so that we generate AddressOf in MIR.
 831                 let array_ptr_type = fcx.tcx.mk_ptr(m_expr);
 832                 fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No)
 833                     .unwrap_or_else(|_| {
 834                         bug!(
 835                         "could not cast from reference to array to pointer to array ({:?} to {:?})",
 836                         self.expr_ty,
 837                         array_ptr_type,
 838                     )
 839                     });
 840
 841                 // this will report a type mismatch if needed
 842                 fcx.demand_eqtype(self.span, ety, m_cast.ty);
 843                 return Ok(CastKind::ArrayPtrCast);
 844             }
 845         }
 846
 847         Err(CastError::IllegalCast)
 848     }
 849
 850     fn check_addr_ptr_cast(
 851         &self,
 852         fcx: &FnCtxt<'a, 'tcx>,
 853         m_cast: TypeAndMut<'tcx>,
 854     ) -> Result<CastKind, CastError> {
 855         // ptr-addr cast. pointer must be thin.
 856         match fcx.pointer_kind(m_cast.ty, self.span)? {
 857             None => Err(CastError::UnknownCastPtrKind),
 858             Some(PointerKind::Thin) => Ok(CastKind::AddrPtrCast),
 859             _ => Err(CastError::IllegalCast),
 860         }
 861     }
 862
 863     fn try_coercion_cast(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<(), ty::error::TypeError<'_>> {
 864         match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No) {
 865             Ok(_) => Ok(()),
 866             Err(err) => Err(err),
 867         }
 868     }
 869
 870     fn cenum_impl_drop_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 871         if let ty::Adt(d, _) = self.expr_ty.kind() {
 872             if d.has_dtor(fcx.tcx) {
 873                 fcx.tcx.struct_span_lint_hir(
 874                     lint::builtin::CENUM_IMPL_DROP_CAST,
 875                     self.expr.hir_id,
 876                     self.span,
 877                     |err| {
 878                         err.build(&format!(
 879                             "cannot cast enum `{}` into integer `{}` because it implements `Drop`",
 880                             self.expr_ty, self.cast_ty
 881                         ))
 882                         .emit();
 883                     },
 884                 );
 885             }
 886         }
 887     }
 888 }
 889
 890 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 891     fn type_is_known_to_be_sized_modulo_regions(&self, ty: Ty<'tcx>, span: Span) -> bool {
 892         let lang_item = self.tcx.require_lang_item(LangItem::Sized, None);
 893         traits::type_known_to_meet_bound_modulo_regions(self, self.param_env, ty, lang_item, span)
 894     }
 895 }