src/librustc_typeck/check/cast.rs

   1 //! Code for type-checking cast expressions.
   2 //!
   3 //! A cast `e as U` is valid if one of the following holds:
   4 //! * `e` has type `T` and `T` coerces to `U`; *coercion-cast*
   5 //! * `e` has type `*T`, `U` is `*U_0`, and either `U_0: Sized` or
   6 //!    pointer_kind(`T`) = pointer_kind(`U_0`); *ptr-ptr-cast*
   7 //! * `e` has type `*T` and `U` is a numeric type, while `T: Sized`; *ptr-addr-cast*
   8 //! * `e` is an integer and `U` is `*U_0`, while `U_0: Sized`; *addr-ptr-cast*
   9 //! * `e` has type `T` and `T` and `U` are any numeric types; *numeric-cast*
  10 //! * `e` is a C-like enum and `U` is an integer type; *enum-cast*
  11 //! * `e` has type `bool` or `char` and `U` is an integer; *prim-int-cast*
  12 //! * `e` has type `u8` and `U` is `char`; *u8-char-cast*
  13 //! * `e` has type `&[T; n]` and `U` is `*const T`; *array-ptr-cast*
  14 //! * `e` is a function pointer type and `U` has type `*T`,
  15 //!   while `T: Sized`; *fptr-ptr-cast*
  16 //! * `e` is a function pointer type and `U` is an integer; *fptr-addr-cast*
  17 //!
  18 //! where `&.T` and `*T` are references of either mutability,
  19 //! and where pointer_kind(`T`) is the kind of the unsize info
  20 //! in `T` - the vtable for a trait definition (e.g., `fmt::Display` or
  21 //! `Iterator`, not `Iterator<Item=u8>`) or a length (or `()` if `T: Sized`).
  22 //!
  23 //! Note that lengths are not adjusted when casting raw slices -
  24 //! `T: *const [u16] as *const [u8]` creates a slice that only includes
  25 //! half of the original memory.
  26 //!
  27 //! Casting is not transitive, that is, even if `e as U1 as U2` is a valid
  28 //! expression, `e as U2` is not necessarily so (in fact it will only be valid if
  29 //! `U1` coerces to `U2`).
  30
  31 use super::FnCtxt;
  32
  33 use crate::hir::def_id::DefId;
  34 use crate::type_error_struct;
  35 use rustc_ast::ast;
  36 use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder, ErrorReported};
  37 use rustc_hir as hir;
  38 use rustc_hir::lang_items;
  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 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                 if let ty::Ref(reg, _, mutbl) = self.cast_ty.kind {
 352                     if fcx
 353                         .try_coerce(
 354                             self.expr,
 355                             fcx.tcx.mk_ref(reg, TypeAndMut { ty: self.expr_ty, mutbl }),
 356                             self.cast_ty,
 357                             AllowTwoPhase::No,
 358                         )
 359                         .is_ok()
 360                     {
 361                         sugg = Some(format!("&{}", mutbl.prefix_str()));
 362                     }
 363                 }
 364                 if let Some(sugg) = sugg {
 365                     err.span_label(self.span, "invalid cast");
 366                     err.span_suggestion_verbose(
 367                         self.expr.span.shrink_to_lo(),
 368                         "borrow the value for the cast to be valid",
 369                         sugg,
 370                         Applicability::MachineApplicable,
 371                     );
 372                 } else if !matches!(
 373                     self.cast_ty.kind,
 374                     ty::FnDef(..) | ty::FnPtr(..) | ty::Closure(..)
 375                 ) {
 376                     let mut label = true;
 377                     // Check `impl From<self.expr_ty> for self.cast_ty {}` for accurate suggestion:
 378                     if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 379                         if let Some(from_trait) = fcx.tcx.get_diagnostic_item(sym::from_trait) {
 380                             let ty = fcx.resolve_vars_if_possible(&self.cast_ty);
 381                             // Erase regions to avoid panic in `prove_value` when calling
 382                             // `type_implements_trait`.
 383                             let ty = fcx.tcx.erase_regions(&ty);
 384                             let expr_ty = fcx.resolve_vars_if_possible(&self.expr_ty);
 385                             let expr_ty = fcx.tcx.erase_regions(&expr_ty);
 386                             let ty_params = fcx.tcx.mk_substs_trait(expr_ty, &[]);
 387                             // Check for infer types because cases like `Option<{integer}>` would
 388                             // panic otherwise.
 389                             if !expr_ty.has_infer_types()
 390                                 && fcx.tcx.type_implements_trait((
 391                                     from_trait,
 392                                     ty,
 393                                     ty_params,
 394                                     fcx.param_env,
 395                                 ))
 396                             {
 397                                 label = false;
 398                                 err.span_suggestion(
 399                                     self.span,
 400                                     "consider using the `From` trait instead",
 401                                     format!("{}::from({})", self.cast_ty, snippet),
 402                                     Applicability::MaybeIncorrect,
 403                                 );
 404                             }
 405                         }
 406                     }
 407                     let msg = "an `as` expression can only be used to convert between primitive \
 408                                types or to coerce to a specific trait object";
 409                     if label {
 410                         err.span_label(self.span, msg);
 411                     } else {
 412                         err.note(msg);
 413                     }
 414                 } else {
 415                     err.span_label(self.span, "invalid cast");
 416                 }
 417                 err.emit();
 418             }
 419             CastError::SizedUnsizedCast => {
 420                 use crate::structured_errors::{SizedUnsizedCastError, StructuredDiagnostic};
 421                 SizedUnsizedCastError::new(
 422                     &fcx.tcx.sess,
 423                     self.span,
 424                     self.expr_ty,
 425                     fcx.ty_to_string(self.cast_ty),
 426                 )
 427                 .diagnostic()
 428                 .emit();
 429             }
 430             CastError::UnknownCastPtrKind | CastError::UnknownExprPtrKind => {
 431                 let unknown_cast_to = match e {
 432                     CastError::UnknownCastPtrKind => true,
 433                     CastError::UnknownExprPtrKind => false,
 434                     _ => bug!(),
 435                 };
 436                 let mut err = struct_span_err!(
 437                     fcx.tcx.sess,
 438                     if unknown_cast_to { self.cast_span } else { self.span },
 439                     E0641,
 440                     "cannot cast {} a pointer of an unknown kind",
 441                     if unknown_cast_to { "to" } else { "from" }
 442                 );
 443                 if unknown_cast_to {
 444                     err.span_label(self.cast_span, "needs more type information");
 445                     err.note(
 446                         "the type information given here is insufficient to check whether \
 447                         the pointer cast is valid",
 448                     );
 449                 } else {
 450                     err.span_label(
 451                         self.span,
 452                         "the type information given here is insufficient to check whether \
 453                         the pointer cast is valid",
 454                     );
 455                 }
 456                 err.emit();
 457             }
 458         }
 459     }
 460
 461     fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'tcx>) {
 462         if self.cast_ty.references_error() || self.expr_ty.references_error() {
 463             return;
 464         }
 465
 466         let tstr = fcx.ty_to_string(self.cast_ty);
 467         let mut err = type_error_struct!(
 468             fcx.tcx.sess,
 469             self.span,
 470             self.expr_ty,
 471             E0620,
 472             "cast to unsized type: `{}` as `{}`",
 473             fcx.resolve_vars_if_possible(&self.expr_ty),
 474             tstr
 475         );
 476         match self.expr_ty.kind {
 477             ty::Ref(_, _, mt) => {
 478                 let mtstr = mt.prefix_str();
 479                 if self.cast_ty.is_trait() {
 480                     match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 481                         Ok(s) => {
 482                             err.span_suggestion(
 483                                 self.cast_span,
 484                                 "try casting to a reference instead",
 485                                 format!("&{}{}", mtstr, s),
 486                                 Applicability::MachineApplicable,
 487                             );
 488                         }
 489                         Err(_) => {
 490                             let msg = &format!("did you mean `&{}{}`?", mtstr, tstr);
 491                             err.span_help(self.cast_span, msg);
 492                         }
 493                     }
 494                 } else {
 495                     let msg = &format!(
 496                         "consider using an implicit coercion to `&{}{}` instead",
 497                         mtstr, tstr
 498                     );
 499                     err.span_help(self.span, msg);
 500                 }
 501             }
 502             ty::Adt(def, ..) if def.is_box() => {
 503                 match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 504                     Ok(s) => {
 505                         err.span_suggestion(
 506                             self.cast_span,
 507                             "you can cast to a `Box` instead",
 508                             format!("Box<{}>", s),
 509                             Applicability::MachineApplicable,
 510                         );
 511                     }
 512                     Err(_) => {
 513                         err.span_help(
 514                             self.cast_span,
 515                             &format!("you might have meant `Box<{}>`", tstr),
 516                         );
 517                     }
 518                 }
 519             }
 520             _ => {
 521                 err.span_help(self.expr.span, "consider using a box or reference as appropriate");
 522             }
 523         }
 524         err.emit();
 525     }
 526
 527     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 528         let t_cast = self.cast_ty;
 529         let t_expr = self.expr_ty;
 530         let type_asc_or =
 531             if fcx.tcx.features().type_ascription { "type ascription or " } else { "" };
 532         let (adjective, lint) = if t_cast.is_numeric() && t_expr.is_numeric() {
 533             ("numeric ", lint::builtin::TRIVIAL_NUMERIC_CASTS)
 534         } else {
 535             ("", lint::builtin::TRIVIAL_CASTS)
 536         };
 537         fcx.tcx.struct_span_lint_hir(lint, self.expr.hir_id, self.span, |err| {
 538             err.build(&format!(
 539                 "trivial {}cast: `{}` as `{}`",
 540                 adjective,
 541                 fcx.ty_to_string(t_expr),
 542                 fcx.ty_to_string(t_cast)
 543             ))
 544             .help(&format!(
 545                 "cast can be replaced by coercion; this might \
 546                                    require {}a temporary variable",
 547                 type_asc_or
 548             ))
 549             .emit();
 550         });
 551     }
 552
 553     pub fn check(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 554         self.expr_ty = fcx.structurally_resolved_type(self.span, self.expr_ty);
 555         self.cast_ty = fcx.structurally_resolved_type(self.span, self.cast_ty);
 556
 557         debug!("check_cast({}, {:?} as {:?})", self.expr.hir_id, self.expr_ty, self.cast_ty);
 558
 559         if !fcx.type_is_known_to_be_sized_modulo_regions(self.cast_ty, self.span) {
 560             self.report_cast_to_unsized_type(fcx);
 561         } else if self.expr_ty.references_error() || self.cast_ty.references_error() {
 562             // No sense in giving duplicate error messages
 563         } else {
 564             match self.try_coercion_cast(fcx) {
 565                 Ok(()) => {
 566                     self.trivial_cast_lint(fcx);
 567                     debug!(" -> CoercionCast");
 568                     fcx.tables.borrow_mut().set_coercion_cast(self.expr.hir_id.local_id);
 569                 }
 570                 Err(ty::error::TypeError::ObjectUnsafeCoercion(did)) => {
 571                     self.report_object_unsafe_cast(&fcx, did);
 572                 }
 573                 Err(_) => {
 574                     match self.do_check(fcx) {
 575                         Ok(k) => {
 576                             debug!(" -> {:?}", k);
 577                         }
 578                         Err(e) => self.report_cast_error(fcx, e),
 579                     };
 580                 }
 581             };
 582         }
 583     }
 584
 585     fn report_object_unsafe_cast(&self, fcx: &FnCtxt<'a, 'tcx>, did: DefId) {
 586         let violations = fcx.tcx.object_safety_violations(did);
 587         let mut err = report_object_safety_error(fcx.tcx, self.cast_span, did, violations);
 588         err.note(&format!("required by cast to type '{}'", fcx.ty_to_string(self.cast_ty)));
 589         err.emit();
 590     }
 591
 592     /// Checks a cast, and report an error if one exists. In some cases, this
 593     /// can return Ok and create type errors in the fcx rather than returning
 594     /// directly. coercion-cast is handled in check instead of here.
 595     fn do_check(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
 596         use rustc_middle::ty::cast::CastTy::*;
 597         use rustc_middle::ty::cast::IntTy::*;
 598
 599         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty), CastTy::from_ty(self.cast_ty))
 600         {
 601             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 602             // Function item types may need to be reified before casts.
 603             (None, Some(t_cast)) => {
 604                 match self.expr_ty.kind {
 605                     ty::FnDef(..) => {
 606                         // Attempt a coercion to a fn pointer type.
 607                         let f = fcx.normalize_associated_types_in(
 608                             self.expr.span,
 609                             &self.expr_ty.fn_sig(fcx.tcx),
 610                         );
 611                         let res = fcx.try_coerce(
 612                             self.expr,
 613                             self.expr_ty,
 614                             fcx.tcx.mk_fn_ptr(f),
 615                             AllowTwoPhase::No,
 616                         );
 617                         if let Err(TypeError::IntrinsicCast) = res {
 618                             return Err(CastError::IllegalCast);
 619                         }
 620                         if res.is_err() {
 621                             return Err(CastError::NonScalar);
 622                         }
 623                         (FnPtr, t_cast)
 624                     }
 625                     // Special case some errors for references, and check for
 626                     // array-ptr-casts. `Ref` is not a CastTy because the cast
 627                     // is split into a coercion to a pointer type, followed by
 628                     // a cast.
 629                     ty::Ref(_, inner_ty, mutbl) => {
 630                         return match t_cast {
 631                             Int(_) | Float => match inner_ty.kind {
 632                                 ty::Int(_)
 633                                 | ty::Uint(_)
 634                                 | ty::Float(_)
 635                                 | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(_)) => {
 636                                     Err(CastError::NeedDeref)
 637                                 }
 638                                 _ => Err(CastError::NeedViaPtr),
 639                             },
 640                             // array-ptr-cast
 641                             Ptr(mt) => {
 642                                 self.check_ref_cast(fcx, TypeAndMut { mutbl, ty: inner_ty }, mt)
 643                             }
 644                             _ => Err(CastError::NonScalar),
 645                         };
 646                     }
 647                     _ => return Err(CastError::NonScalar),
 648                 }
 649             }
 650             _ => return Err(CastError::NonScalar),
 651         };
 652
 653         match (t_from, t_cast) {
 654             // These types have invariants! can't cast into them.
 655             (_, Int(CEnum) | FnPtr) => Err(CastError::NonScalar),
 656
 657             // * -> Bool
 658             (_, Int(Bool)) => Err(CastError::CastToBool),
 659
 660             // * -> Char
 661             (Int(U(ast::UintTy::U8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 662             (_, Int(Char)) => Err(CastError::CastToChar),
 663
 664             // prim -> float,ptr
 665             (Int(Bool) | Int(CEnum) | Int(Char), Float) => Err(CastError::NeedViaInt),
 666
 667             (Int(Bool) | Int(CEnum) | Int(Char) | Float, Ptr(_)) | (Ptr(_) | FnPtr, Float) => {
 668                 Err(CastError::IllegalCast)
 669             }
 670
 671             // ptr -> *
 672             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 673             (Ptr(m_expr), Int(_)) => self.check_ptr_addr_cast(fcx, m_expr), // ptr-addr-cast
 674             (FnPtr, Int(_)) => Ok(CastKind::FnPtrAddrCast),
 675
 676             // * -> ptr
 677             (Int(_), Ptr(mt)) => self.check_addr_ptr_cast(fcx, mt), // addr-ptr-cast
 678             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 679
 680             // prim -> prim
 681             (Int(CEnum), Int(_)) => {
 682                 self.cenum_impl_drop_lint(fcx);
 683                 Ok(CastKind::EnumCast)
 684             }
 685             (Int(Char) | Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 686
 687             (Int(_) | Float, Int(_) | Float) => Ok(CastKind::NumericCast),
 688         }
 689     }
 690
 691     fn check_ptr_ptr_cast(
 692         &self,
 693         fcx: &FnCtxt<'a, 'tcx>,
 694         m_expr: ty::TypeAndMut<'tcx>,
 695         m_cast: ty::TypeAndMut<'tcx>,
 696     ) -> Result<CastKind, CastError> {
 697         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_expr, m_cast);
 698         // ptr-ptr cast. vtables must match.
 699
 700         let expr_kind = fcx.pointer_kind(m_expr.ty, self.span)?;
 701         let cast_kind = fcx.pointer_kind(m_cast.ty, self.span)?;
 702
 703         let cast_kind = match cast_kind {
 704             // We can't cast if target pointer kind is unknown
 705             None => return Err(CastError::UnknownCastPtrKind),
 706             Some(cast_kind) => cast_kind,
 707         };
 708
 709         // Cast to thin pointer is OK
 710         if cast_kind == PointerKind::Thin {
 711             return Ok(CastKind::PtrPtrCast);
 712         }
 713
 714         let expr_kind = match expr_kind {
 715             // We can't cast to fat pointer if source pointer kind is unknown
 716             None => return Err(CastError::UnknownExprPtrKind),
 717             Some(expr_kind) => expr_kind,
 718         };
 719
 720         // thin -> fat? report invalid cast (don't complain about vtable kinds)
 721         if expr_kind == PointerKind::Thin {
 722             return Err(CastError::SizedUnsizedCast);
 723         }
 724
 725         // vtable kinds must match
 726         if cast_kind == expr_kind {
 727             Ok(CastKind::PtrPtrCast)
 728         } else {
 729             Err(CastError::DifferingKinds)
 730         }
 731     }
 732
 733     fn check_fptr_ptr_cast(
 734         &self,
 735         fcx: &FnCtxt<'a, 'tcx>,
 736         m_cast: ty::TypeAndMut<'tcx>,
 737     ) -> Result<CastKind, CastError> {
 738         // fptr-ptr cast. must be to thin ptr
 739
 740         match fcx.pointer_kind(m_cast.ty, self.span)? {
 741             None => Err(CastError::UnknownCastPtrKind),
 742             Some(PointerKind::Thin) => Ok(CastKind::FnPtrPtrCast),
 743             _ => Err(CastError::IllegalCast),
 744         }
 745     }
 746
 747     fn check_ptr_addr_cast(
 748         &self,
 749         fcx: &FnCtxt<'a, 'tcx>,
 750         m_expr: ty::TypeAndMut<'tcx>,
 751     ) -> Result<CastKind, CastError> {
 752         // ptr-addr cast. must be from thin ptr
 753
 754         match fcx.pointer_kind(m_expr.ty, self.span)? {
 755             None => Err(CastError::UnknownExprPtrKind),
 756             Some(PointerKind::Thin) => Ok(CastKind::PtrAddrCast),
 757             _ => Err(CastError::NeedViaThinPtr),
 758         }
 759     }
 760
 761     fn check_ref_cast(
 762         &self,
 763         fcx: &FnCtxt<'a, 'tcx>,
 764         m_expr: ty::TypeAndMut<'tcx>,
 765         m_cast: ty::TypeAndMut<'tcx>,
 766     ) -> Result<CastKind, CastError> {
 767         // array-ptr-cast.
 768
 769         if m_expr.mutbl == hir::Mutability::Not && m_cast.mutbl == hir::Mutability::Not {
 770             if let ty::Array(ety, _) = m_expr.ty.kind {
 771                 // Due to the limitations of LLVM global constants,
 772                 // region pointers end up pointing at copies of
 773                 // vector elements instead of the original values.
 774                 // To allow raw pointers to work correctly, we
 775                 // need to special-case obtaining a raw pointer
 776                 // from a region pointer to a vector.
 777
 778                 // Coerce to a raw pointer so that we generate AddressOf in MIR.
 779                 let array_ptr_type = fcx.tcx.mk_ptr(m_expr);
 780                 fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No)
 781                     .unwrap_or_else(|_| {
 782                         bug!(
 783                         "could not cast from reference to array to pointer to array ({:?} to {:?})",
 784                         self.expr_ty,
 785                         array_ptr_type,
 786                     )
 787                     });
 788
 789                 // this will report a type mismatch if needed
 790                 fcx.demand_eqtype(self.span, ety, m_cast.ty);
 791                 return Ok(CastKind::ArrayPtrCast);
 792             }
 793         }
 794
 795         Err(CastError::IllegalCast)
 796     }
 797
 798     fn check_addr_ptr_cast(
 799         &self,
 800         fcx: &FnCtxt<'a, 'tcx>,
 801         m_cast: TypeAndMut<'tcx>,
 802     ) -> Result<CastKind, CastError> {
 803         // ptr-addr cast. pointer must be thin.
 804         match fcx.pointer_kind(m_cast.ty, self.span)? {
 805             None => Err(CastError::UnknownCastPtrKind),
 806             Some(PointerKind::Thin) => Ok(CastKind::AddrPtrCast),
 807             _ => Err(CastError::IllegalCast),
 808         }
 809     }
 810
 811     fn try_coercion_cast(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<(), ty::error::TypeError<'_>> {
 812         match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No) {
 813             Ok(_) => Ok(()),
 814             Err(err) => Err(err),
 815         }
 816     }
 817
 818     fn cenum_impl_drop_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 819         if let ty::Adt(d, _) = self.expr_ty.kind {
 820             if d.has_dtor(fcx.tcx) {
 821                 fcx.tcx.struct_span_lint_hir(
 822                     lint::builtin::CENUM_IMPL_DROP_CAST,
 823                     self.expr.hir_id,
 824                     self.span,
 825                     |err| {
 826                         err.build(&format!(
 827                             "cannot cast enum `{}` into integer `{}` because it implements `Drop`",
 828                             self.expr_ty, self.cast_ty
 829                         ))
 830                         .emit();
 831                     },
 832                 );
 833             }
 834         }
 835     }
 836 }
 837
 838 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 839     fn type_is_known_to_be_sized_modulo_regions(&self, ty: Ty<'tcx>, span: Span) -> bool {
 840         let lang_item = self.tcx.require_lang_item(lang_items::SizedTraitLangItem, None);
 841         traits::type_known_to_meet_bound_modulo_regions(self, self.param_env, ty, lang_item, span)
 842     }
 843 }