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 #[derive(Debug)]
  55 pub struct CastCheck<'tcx> {
  56     expr: &'tcx hir::Expr<'tcx>,
  57     expr_ty: Ty<'tcx>,
  58     cast_ty: Ty<'tcx>,
  59     cast_span: Span,
  60     span: Span,
  61 }
  62
  63 /// The kind of pointer and associated metadata (thin, length or vtable) - we
  64 /// only allow casts between fat pointers if their metadata have the same
  65 /// kind.
  66 #[derive(Copy, Clone, PartialEq, Eq)]
  67 enum PointerKind<'tcx> {
  68     /// No metadata attached, ie pointer to sized type or foreign type
  69     Thin,
  70     /// A trait object
  71     Vtable(Option<DefId>),
  72     /// Slice
  73     Length,
  74     /// The unsize info of this projection
  75     OfProjection(&'tcx ty::ProjectionTy<'tcx>),
  76     /// The unsize info of this opaque ty
  77     OfOpaque(DefId, SubstsRef<'tcx>),
  78     /// The unsize info of this parameter
  79     OfParam(&'tcx ty::ParamTy),
  80 }
  81
  82 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  83     /// Returns the kind of unsize information of t, or None
  84     /// if t is unknown.
  85     fn pointer_kind(
  86         &self,
  87         t: Ty<'tcx>,
  88         span: Span,
  89     ) -> Result<Option<PointerKind<'tcx>>, ErrorReported> {
  90         debug!("pointer_kind({:?}, {:?})", t, span);
  91
  92         let t = self.resolve_vars_if_possible(t);
  93
  94         if t.references_error() {
  95             return Err(ErrorReported);
  96         }
  97
  98         if self.type_is_known_to_be_sized_modulo_regions(t, span) {
  99             return Ok(Some(PointerKind::Thin));
 100         }
 101
 102         Ok(match *t.kind() {
 103             ty::Slice(_) | ty::Str => Some(PointerKind::Length),
 104             ty::Dynamic(ref tty, ..) => Some(PointerKind::Vtable(tty.principal_def_id())),
 105             ty::Adt(def, substs) if def.is_struct() => match def.non_enum_variant().fields.last() {
 106                 None => Some(PointerKind::Thin),
 107                 Some(f) => {
 108                     let field_ty = self.field_ty(span, f, substs);
 109                     self.pointer_kind(field_ty, span)?
 110                 }
 111             },
 112             ty::Tuple(fields) => match fields.last() {
 113                 None => Some(PointerKind::Thin),
 114                 Some(&f) => self.pointer_kind(f, span)?,
 115             },
 116
 117             // Pointers to foreign types are thin, despite being unsized
 118             ty::Foreign(..) => Some(PointerKind::Thin),
 119             // We should really try to normalize here.
 120             ty::Projection(ref pi) => Some(PointerKind::OfProjection(pi)),
 121             ty::Opaque(def_id, substs) => Some(PointerKind::OfOpaque(def_id, substs)),
 122             ty::Param(ref p) => Some(PointerKind::OfParam(p)),
 123             // Insufficient type information.
 124             ty::Placeholder(..) | ty::Bound(..) | ty::Infer(_) => None,
 125
 126             ty::Bool
 127             | ty::Char
 128             | ty::Int(..)
 129             | ty::Uint(..)
 130             | ty::Float(_)
 131             | ty::Array(..)
 132             | ty::GeneratorWitness(..)
 133             | ty::RawPtr(_)
 134             | ty::Ref(..)
 135             | ty::FnDef(..)
 136             | ty::FnPtr(..)
 137             | ty::Closure(..)
 138             | ty::Generator(..)
 139             | ty::Adt(..)
 140             | ty::Never
 141             | ty::Error(_) => {
 142                 self.tcx
 143                     .sess
 144                     .delay_span_bug(span, &format!("`{:?}` should be sized but is not?", t));
 145                 return Err(ErrorReported);
 146             }
 147         })
 148     }
 149 }
 150
 151 #[derive(Copy, Clone)]
 152 pub enum CastError {
 153     ErrorReported,
 154
 155     CastToBool,
 156     CastToChar,
 157     DifferingKinds,
 158     /// Cast of thin to fat raw ptr (e.g., `*const () as *const [u8]`).
 159     SizedUnsizedCast,
 160     IllegalCast,
 161     NeedDeref,
 162     NeedViaPtr,
 163     NeedViaThinPtr,
 164     NeedViaInt,
 165     NonScalar,
 166     UnknownExprPtrKind,
 167     UnknownCastPtrKind,
 168 }
 169
 170 impl From<ErrorReported> for CastError {
 171     fn from(ErrorReported: ErrorReported) -> Self {
 172         CastError::ErrorReported
 173     }
 174 }
 175
 176 fn make_invalid_casting_error<'a, 'tcx>(
 177     sess: &'a Session,
 178     span: Span,
 179     expr_ty: Ty<'tcx>,
 180     cast_ty: Ty<'tcx>,
 181     fcx: &FnCtxt<'a, 'tcx>,
 182 ) -> DiagnosticBuilder<'a, ErrorReported> {
 183     type_error_struct!(
 184         sess,
 185         span,
 186         expr_ty,
 187         E0606,
 188         "casting `{}` as `{}` is invalid",
 189         fcx.ty_to_string(expr_ty),
 190         fcx.ty_to_string(cast_ty)
 191     )
 192 }
 193
 194 impl<'a, 'tcx> CastCheck<'tcx> {
 195     pub fn new(
 196         fcx: &FnCtxt<'a, 'tcx>,
 197         expr: &'tcx hir::Expr<'tcx>,
 198         expr_ty: Ty<'tcx>,
 199         cast_ty: Ty<'tcx>,
 200         cast_span: Span,
 201         span: Span,
 202     ) -> Result<CastCheck<'tcx>, ErrorReported> {
 203         let check = CastCheck { expr, expr_ty, cast_ty, cast_span, span };
 204
 205         // For better error messages, check for some obviously unsized
 206         // cases now. We do a more thorough check at the end, once
 207         // inference is more completely known.
 208         match cast_ty.kind() {
 209             ty::Dynamic(..) | ty::Slice(..) => {
 210                 check.report_cast_to_unsized_type(fcx);
 211                 Err(ErrorReported)
 212             }
 213             _ => Ok(check),
 214         }
 215     }
 216
 217     fn report_cast_error(&self, fcx: &FnCtxt<'a, 'tcx>, e: CastError) {
 218         match e {
 219             CastError::ErrorReported => {
 220                 // an error has already been reported
 221             }
 222             CastError::NeedDeref => {
 223                 let error_span = self.span;
 224                 let mut err = make_invalid_casting_error(
 225                     fcx.tcx.sess,
 226                     self.span,
 227                     self.expr_ty,
 228                     self.cast_ty,
 229                     fcx,
 230                 );
 231                 let cast_ty = fcx.ty_to_string(self.cast_ty);
 232                 err.span_label(
 233                     error_span,
 234                     format!("cannot cast `{}` as `{}`", fcx.ty_to_string(self.expr_ty), cast_ty),
 235                 );
 236                 if let Ok(snippet) = fcx.sess().source_map().span_to_snippet(self.expr.span) {
 237                     err.span_suggestion(
 238                         self.expr.span,
 239                         "dereference the expression",
 240                         format!("*{}", snippet),
 241                         Applicability::MaybeIncorrect,
 242                     );
 243                 } else {
 244                     err.span_help(self.expr.span, "dereference the expression with `*`");
 245                 }
 246                 err.emit();
 247             }
 248             CastError::NeedViaThinPtr | CastError::NeedViaPtr => {
 249                 let mut err = make_invalid_casting_error(
 250                     fcx.tcx.sess,
 251                     self.span,
 252                     self.expr_ty,
 253                     self.cast_ty,
 254                     fcx,
 255                 );
 256                 if self.cast_ty.is_integral() {
 257                     err.help(&format!(
 258                         "cast through {} first",
 259                         match e {
 260                             CastError::NeedViaPtr => "a raw pointer",
 261                             CastError::NeedViaThinPtr => "a thin pointer",
 262                             _ => bug!(),
 263                         }
 264                     ));
 265                 }
 266                 err.emit();
 267             }
 268             CastError::NeedViaInt => {
 269                 make_invalid_casting_error(
 270                     fcx.tcx.sess,
 271                     self.span,
 272                     self.expr_ty,
 273                     self.cast_ty,
 274                     fcx,
 275                 )
 276                 .help(&format!(
 277                     "cast through {} first",
 278                     match e {
 279                         CastError::NeedViaInt => "an integer",
 280                         _ => bug!(),
 281                     }
 282                 ))
 283                 .emit();
 284             }
 285             CastError::IllegalCast => {
 286                 make_invalid_casting_error(
 287                     fcx.tcx.sess,
 288                     self.span,
 289                     self.expr_ty,
 290                     self.cast_ty,
 291                     fcx,
 292                 )
 293                 .emit();
 294             }
 295             CastError::DifferingKinds => {
 296                 make_invalid_casting_error(
 297                     fcx.tcx.sess,
 298                     self.span,
 299                     self.expr_ty,
 300                     self.cast_ty,
 301                     fcx,
 302                 )
 303                 .note("vtable kinds may not match")
 304                 .emit();
 305             }
 306             CastError::CastToBool => {
 307                 let mut err =
 308                     struct_span_err!(fcx.tcx.sess, self.span, E0054, "cannot cast as `bool`");
 309
 310                 if self.expr_ty.is_numeric() {
 311                     match fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 312                         Ok(snippet) => {
 313                             err.span_suggestion(
 314                                 self.span,
 315                                 "compare with zero instead",
 316                                 format!("{} != 0", snippet),
 317                                 Applicability::MachineApplicable,
 318                             );
 319                         }
 320                         Err(_) => {
 321                             err.span_help(self.span, "compare with zero instead");
 322                         }
 323                     }
 324                 } else {
 325                     err.span_label(self.span, "unsupported cast");
 326                 }
 327
 328                 err.emit();
 329             }
 330             CastError::CastToChar => {
 331                 let mut err = type_error_struct!(
 332                     fcx.tcx.sess,
 333                     self.span,
 334                     self.expr_ty,
 335                     E0604,
 336                     "only `u8` can be cast as `char`, not `{}`",
 337                     self.expr_ty
 338                 );
 339                 err.span_label(self.span, "invalid cast");
 340                 if self.expr_ty.is_numeric() {
 341                     err.span_help(
 342                         self.span,
 343                         if self.expr_ty == fcx.tcx.types.i8 {
 344                             "try casting from `u8` instead"
 345                         } else if self.expr_ty == fcx.tcx.types.u32 {
 346                             "try `char::from_u32` instead"
 347                         } else {
 348                             "try `char::from_u32` instead (via a `u32`)"
 349                         },
 350                     );
 351                 }
 352                 err.emit();
 353             }
 354             CastError::NonScalar => {
 355                 let mut err = type_error_struct!(
 356                     fcx.tcx.sess,
 357                     self.span,
 358                     self.expr_ty,
 359                     E0605,
 360                     "non-primitive cast: `{}` as `{}`",
 361                     self.expr_ty,
 362                     fcx.ty_to_string(self.cast_ty)
 363                 );
 364                 let mut sugg = None;
 365                 let mut sugg_mutref = false;
 366                 if let ty::Ref(reg, cast_ty, mutbl) = *self.cast_ty.kind() {
 367                     if let ty::RawPtr(TypeAndMut { ty: expr_ty, .. }) = *self.expr_ty.kind() {
 368                         if fcx
 369                             .try_coerce(
 370                                 self.expr,
 371                                 fcx.tcx.mk_ref(
 372                                     fcx.tcx.lifetimes.re_erased,
 373                                     TypeAndMut { ty: expr_ty, mutbl },
 374                                 ),
 375                                 self.cast_ty,
 376                                 AllowTwoPhase::No,
 377                                 None,
 378                             )
 379                             .is_ok()
 380                         {
 381                             sugg = Some((format!("&{}*", mutbl.prefix_str()), cast_ty == expr_ty));
 382                         }
 383                     } else if let ty::Ref(expr_reg, expr_ty, expr_mutbl) = *self.expr_ty.kind() {
 384                         if expr_mutbl == Mutability::Not
 385                             && mutbl == Mutability::Mut
 386                             && fcx
 387                                 .try_coerce(
 388                                     self.expr,
 389                                     fcx.tcx.mk_ref(
 390                                         expr_reg,
 391                                         TypeAndMut { ty: expr_ty, mutbl: Mutability::Mut },
 392                                     ),
 393                                     self.cast_ty,
 394                                     AllowTwoPhase::No,
 395                                     None,
 396                                 )
 397                                 .is_ok()
 398                         {
 399                             sugg_mutref = true;
 400                         }
 401                     }
 402
 403                     if !sugg_mutref
 404                         && sugg == None
 405                         && fcx
 406                             .try_coerce(
 407                                 self.expr,
 408                                 fcx.tcx.mk_ref(reg, TypeAndMut { ty: self.expr_ty, mutbl }),
 409                                 self.cast_ty,
 410                                 AllowTwoPhase::No,
 411                                 None,
 412                             )
 413                             .is_ok()
 414                     {
 415                         sugg = Some((format!("&{}", mutbl.prefix_str()), false));
 416                     }
 417                 } else if let ty::RawPtr(TypeAndMut { mutbl, .. }) = *self.cast_ty.kind() {
 418                     if fcx
 419                         .try_coerce(
 420                             self.expr,
 421                             fcx.tcx.mk_ref(
 422                                 fcx.tcx.lifetimes.re_erased,
 423                                 TypeAndMut { ty: self.expr_ty, mutbl },
 424                             ),
 425                             self.cast_ty,
 426                             AllowTwoPhase::No,
 427                             None,
 428                         )
 429                         .is_ok()
 430                     {
 431                         sugg = Some((format!("&{}", mutbl.prefix_str()), false));
 432                     }
 433                 }
 434                 if sugg_mutref {
 435                     err.span_label(self.span, "invalid cast");
 436                     err.span_note(self.expr.span, "this reference is immutable");
 437                     err.span_note(self.cast_span, "trying to cast to a mutable reference type");
 438                 } else if let Some((sugg, remove_cast)) = sugg {
 439                     err.span_label(self.span, "invalid cast");
 440
 441                     let has_parens = fcx
 442                         .tcx
 443                         .sess
 444                         .source_map()
 445                         .span_to_snippet(self.expr.span)
 446                         .map_or(false, |snip| snip.starts_with('('));
 447
 448                     // Very crude check to see whether the expression must be wrapped
 449                     // in parentheses for the suggestion to work (issue #89497).
 450                     // Can/should be extended in the future.
 451                     let needs_parens =
 452                         !has_parens && matches!(self.expr.kind, hir::ExprKind::Cast(..));
 453
 454                     let mut suggestion = vec![(self.expr.span.shrink_to_lo(), sugg)];
 455                     if needs_parens {
 456                         suggestion[0].1 += "(";
 457                         suggestion.push((self.expr.span.shrink_to_hi(), ")".to_string()));
 458                     }
 459                     if remove_cast {
 460                         suggestion.push((
 461                             self.expr.span.shrink_to_hi().to(self.cast_span),
 462                             String::new(),
 463                         ));
 464                     }
 465
 466                     err.multipart_suggestion_verbose(
 467                         "consider borrowing the value",
 468                         suggestion,
 469                         Applicability::MachineApplicable,
 470                     );
 471                 } else if !matches!(
 472                     self.cast_ty.kind(),
 473                     ty::FnDef(..) | ty::FnPtr(..) | ty::Closure(..)
 474                 ) {
 475                     let mut label = true;
 476                     // Check `impl From<self.expr_ty> for self.cast_ty {}` for accurate suggestion:
 477                     if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 478                         if let Some(from_trait) = fcx.tcx.get_diagnostic_item(sym::From) {
 479                             let ty = fcx.resolve_vars_if_possible(self.cast_ty);
 480                             // Erase regions to avoid panic in `prove_value` when calling
 481                             // `type_implements_trait`.
 482                             let ty = fcx.tcx.erase_regions(ty);
 483                             let expr_ty = fcx.resolve_vars_if_possible(self.expr_ty);
 484                             let expr_ty = fcx.tcx.erase_regions(expr_ty);
 485                             let ty_params = fcx.tcx.mk_substs_trait(expr_ty, &[]);
 486                             if fcx
 487                                 .infcx
 488                                 .type_implements_trait(from_trait, ty, ty_params, fcx.param_env)
 489                                 .must_apply_modulo_regions()
 490                             {
 491                                 label = false;
 492                                 err.span_suggestion(
 493                                     self.span,
 494                                     "consider using the `From` trait instead",
 495                                     format!("{}::from({})", self.cast_ty, snippet),
 496                                     Applicability::MaybeIncorrect,
 497                                 );
 498                             }
 499                         }
 500                     }
 501                     let msg = "an `as` expression can only be used to convert between primitive \
 502                                types or to coerce to a specific trait object";
 503                     if label {
 504                         err.span_label(self.span, msg);
 505                     } else {
 506                         err.note(msg);
 507                     }
 508                 } else {
 509                     err.span_label(self.span, "invalid cast");
 510                 }
 511                 err.emit();
 512             }
 513             CastError::SizedUnsizedCast => {
 514                 use crate::structured_errors::{SizedUnsizedCast, StructuredDiagnostic};
 515
 516                 SizedUnsizedCast {
 517                     sess: &fcx.tcx.sess,
 518                     span: self.span,
 519                     expr_ty: self.expr_ty,
 520                     cast_ty: fcx.ty_to_string(self.cast_ty),
 521                 }
 522                 .diagnostic()
 523                 .emit();
 524             }
 525             CastError::UnknownCastPtrKind | CastError::UnknownExprPtrKind => {
 526                 let unknown_cast_to = match e {
 527                     CastError::UnknownCastPtrKind => true,
 528                     CastError::UnknownExprPtrKind => false,
 529                     _ => bug!(),
 530                 };
 531                 let mut err = struct_span_err!(
 532                     fcx.tcx.sess,
 533                     if unknown_cast_to { self.cast_span } else { self.span },
 534                     E0641,
 535                     "cannot cast {} a pointer of an unknown kind",
 536                     if unknown_cast_to { "to" } else { "from" }
 537                 );
 538                 if unknown_cast_to {
 539                     err.span_label(self.cast_span, "needs more type information");
 540                     err.note(
 541                         "the type information given here is insufficient to check whether \
 542                         the pointer cast is valid",
 543                     );
 544                 } else {
 545                     err.span_label(
 546                         self.span,
 547                         "the type information given here is insufficient to check whether \
 548                         the pointer cast is valid",
 549                     );
 550                 }
 551                 err.emit();
 552             }
 553         }
 554     }
 555
 556     fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'tcx>) {
 557         if self.cast_ty.references_error() || self.expr_ty.references_error() {
 558             return;
 559         }
 560
 561         let tstr = fcx.ty_to_string(self.cast_ty);
 562         let mut err = type_error_struct!(
 563             fcx.tcx.sess,
 564             self.span,
 565             self.expr_ty,
 566             E0620,
 567             "cast to unsized type: `{}` as `{}`",
 568             fcx.resolve_vars_if_possible(self.expr_ty),
 569             tstr
 570         );
 571         match self.expr_ty.kind() {
 572             ty::Ref(_, _, mt) => {
 573                 let mtstr = mt.prefix_str();
 574                 if self.cast_ty.is_trait() {
 575                     match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 576                         Ok(s) => {
 577                             err.span_suggestion(
 578                                 self.cast_span,
 579                                 "try casting to a reference instead",
 580                                 format!("&{}{}", mtstr, s),
 581                                 Applicability::MachineApplicable,
 582                             );
 583                         }
 584                         Err(_) => {
 585                             let msg = &format!("did you mean `&{}{}`?", mtstr, tstr);
 586                             err.span_help(self.cast_span, msg);
 587                         }
 588                     }
 589                 } else {
 590                     let msg = &format!(
 591                         "consider using an implicit coercion to `&{}{}` instead",
 592                         mtstr, tstr
 593                     );
 594                     err.span_help(self.span, msg);
 595                 }
 596             }
 597             ty::Adt(def, ..) if def.is_box() => {
 598                 match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 599                     Ok(s) => {
 600                         err.span_suggestion(
 601                             self.cast_span,
 602                             "you can cast to a `Box` instead",
 603                             format!("Box<{}>", s),
 604                             Applicability::MachineApplicable,
 605                         );
 606                     }
 607                     Err(_) => {
 608                         err.span_help(
 609                             self.cast_span,
 610                             &format!("you might have meant `Box<{}>`", tstr),
 611                         );
 612                     }
 613                 }
 614             }
 615             _ => {
 616                 err.span_help(self.expr.span, "consider using a box or reference as appropriate");
 617             }
 618         }
 619         err.emit();
 620     }
 621
 622     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 623         let t_cast = self.cast_ty;
 624         let t_expr = self.expr_ty;
 625         let type_asc_or =
 626             if fcx.tcx.features().type_ascription { "type ascription or " } else { "" };
 627         let (adjective, lint) = if t_cast.is_numeric() && t_expr.is_numeric() {
 628             ("numeric ", lint::builtin::TRIVIAL_NUMERIC_CASTS)
 629         } else {
 630             ("", lint::builtin::TRIVIAL_CASTS)
 631         };
 632         fcx.tcx.struct_span_lint_hir(lint, self.expr.hir_id, self.span, |err| {
 633             err.build(&format!(
 634                 "trivial {}cast: `{}` as `{}`",
 635                 adjective,
 636                 fcx.ty_to_string(t_expr),
 637                 fcx.ty_to_string(t_cast)
 638             ))
 639             .help(&format!(
 640                 "cast can be replaced by coercion; this might \
 641                                    require {}a temporary variable",
 642                 type_asc_or
 643             ))
 644             .emit();
 645         });
 646     }
 647
 648     #[instrument(skip(fcx), level = "debug")]
 649     pub fn check(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 650         self.expr_ty = fcx.structurally_resolved_type(self.expr.span, self.expr_ty);
 651         self.cast_ty = fcx.structurally_resolved_type(self.cast_span, self.cast_ty);
 652
 653         debug!("check_cast({}, {:?} as {:?})", self.expr.hir_id, self.expr_ty, self.cast_ty);
 654
 655         if !fcx.type_is_known_to_be_sized_modulo_regions(self.cast_ty, self.span)
 656             && !self.cast_ty.has_infer_types()
 657         {
 658             self.report_cast_to_unsized_type(fcx);
 659         } else if self.expr_ty.references_error() || self.cast_ty.references_error() {
 660             // No sense in giving duplicate error messages
 661         } else {
 662             match self.try_coercion_cast(fcx) {
 663                 Ok(()) => {
 664                     self.trivial_cast_lint(fcx);
 665                     debug!(" -> CoercionCast");
 666                     fcx.typeck_results.borrow_mut().set_coercion_cast(self.expr.hir_id.local_id);
 667                 }
 668                 Err(ty::error::TypeError::ObjectUnsafeCoercion(did)) => {
 669                     self.report_object_unsafe_cast(&fcx, did);
 670                 }
 671                 Err(_) => {
 672                     match self.do_check(fcx) {
 673                         Ok(k) => {
 674                             debug!(" -> {:?}", k);
 675                         }
 676                         Err(e) => self.report_cast_error(fcx, e),
 677                     };
 678                 }
 679             };
 680         }
 681     }
 682
 683     fn report_object_unsafe_cast(&self, fcx: &FnCtxt<'a, 'tcx>, did: DefId) {
 684         let violations = fcx.tcx.object_safety_violations(did);
 685         let mut err = report_object_safety_error(fcx.tcx, self.cast_span, did, violations);
 686         err.note(&format!("required by cast to type '{}'", fcx.ty_to_string(self.cast_ty)));
 687         err.emit();
 688     }
 689
 690     /// Checks a cast, and report an error if one exists. In some cases, this
 691     /// can return Ok and create type errors in the fcx rather than returning
 692     /// directly. coercion-cast is handled in check instead of here.
 693     pub fn do_check(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
 694         use rustc_middle::ty::cast::CastTy::*;
 695         use rustc_middle::ty::cast::IntTy::*;
 696
 697         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty), CastTy::from_ty(self.cast_ty))
 698         {
 699             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 700             // Function item types may need to be reified before casts.
 701             (None, Some(t_cast)) => {
 702                 match *self.expr_ty.kind() {
 703                     ty::FnDef(..) => {
 704                         // Attempt a coercion to a fn pointer type.
 705                         let f = fcx.normalize_associated_types_in(
 706                             self.expr.span,
 707                             self.expr_ty.fn_sig(fcx.tcx),
 708                         );
 709                         let res = fcx.try_coerce(
 710                             self.expr,
 711                             self.expr_ty,
 712                             fcx.tcx.mk_fn_ptr(f),
 713                             AllowTwoPhase::No,
 714                             None,
 715                         );
 716                         if let Err(TypeError::IntrinsicCast) = res {
 717                             return Err(CastError::IllegalCast);
 718                         }
 719                         if res.is_err() {
 720                             return Err(CastError::NonScalar);
 721                         }
 722                         (FnPtr, t_cast)
 723                     }
 724                     // Special case some errors for references, and check for
 725                     // array-ptr-casts. `Ref` is not a CastTy because the cast
 726                     // is split into a coercion to a pointer type, followed by
 727                     // a cast.
 728                     ty::Ref(_, inner_ty, mutbl) => {
 729                         return match t_cast {
 730                             Int(_) | Float => match *inner_ty.kind() {
 731                                 ty::Int(_)
 732                                 | ty::Uint(_)
 733                                 | ty::Float(_)
 734                                 | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(_)) => {
 735                                     Err(CastError::NeedDeref)
 736                                 }
 737                                 _ => Err(CastError::NeedViaPtr),
 738                             },
 739                             // array-ptr-cast
 740                             Ptr(mt) => {
 741                                 self.check_ref_cast(fcx, TypeAndMut { mutbl, ty: inner_ty }, mt)
 742                             }
 743                             _ => Err(CastError::NonScalar),
 744                         };
 745                     }
 746                     _ => return Err(CastError::NonScalar),
 747                 }
 748             }
 749             _ => return Err(CastError::NonScalar),
 750         };
 751
 752         match (t_from, t_cast) {
 753             // These types have invariants! can't cast into them.
 754             (_, Int(CEnum) | FnPtr) => Err(CastError::NonScalar),
 755
 756             // * -> Bool
 757             (_, Int(Bool)) => Err(CastError::CastToBool),
 758
 759             // * -> Char
 760             (Int(U(ty::UintTy::U8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 761             (_, Int(Char)) => Err(CastError::CastToChar),
 762
 763             // prim -> float,ptr
 764             (Int(Bool) | Int(CEnum) | Int(Char), Float) => Err(CastError::NeedViaInt),
 765
 766             (Int(Bool) | Int(CEnum) | Int(Char) | Float, Ptr(_)) | (Ptr(_) | FnPtr, Float) => {
 767                 Err(CastError::IllegalCast)
 768             }
 769
 770             // ptr -> *
 771             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 772             (Ptr(m_expr), Int(_)) => self.check_ptr_addr_cast(fcx, m_expr), // ptr-addr-cast
 773             (FnPtr, Int(_)) => Ok(CastKind::FnPtrAddrCast),
 774
 775             // * -> ptr
 776             (Int(_), Ptr(mt)) => self.check_addr_ptr_cast(fcx, mt), // addr-ptr-cast
 777             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 778
 779             // prim -> prim
 780             (Int(CEnum), Int(_)) => {
 781                 self.cenum_impl_drop_lint(fcx);
 782                 Ok(CastKind::EnumCast)
 783             }
 784             (Int(Char) | Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 785
 786             (Int(_) | Float, Int(_) | Float) => Ok(CastKind::NumericCast),
 787         }
 788     }
 789
 790     fn check_ptr_ptr_cast(
 791         &self,
 792         fcx: &FnCtxt<'a, 'tcx>,
 793         m_expr: ty::TypeAndMut<'tcx>,
 794         m_cast: ty::TypeAndMut<'tcx>,
 795     ) -> Result<CastKind, CastError> {
 796         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_expr, m_cast);
 797         // ptr-ptr cast. vtables must match.
 798
 799         let expr_kind = fcx.pointer_kind(m_expr.ty, self.span)?;
 800         let cast_kind = fcx.pointer_kind(m_cast.ty, self.span)?;
 801
 802         let Some(cast_kind) = cast_kind else {
 803             // We can't cast if target pointer kind is unknown
 804             return Err(CastError::UnknownCastPtrKind);
 805         };
 806
 807         // Cast to thin pointer is OK
 808         if cast_kind == PointerKind::Thin {
 809             return Ok(CastKind::PtrPtrCast);
 810         }
 811
 812         let Some(expr_kind) = expr_kind else {
 813             // We can't cast to fat pointer if source pointer kind is unknown
 814             return Err(CastError::UnknownExprPtrKind);
 815         };
 816
 817         // thin -> fat? report invalid cast (don't complain about vtable kinds)
 818         if expr_kind == PointerKind::Thin {
 819             return Err(CastError::SizedUnsizedCast);
 820         }
 821
 822         // vtable kinds must match
 823         if cast_kind == expr_kind {
 824             Ok(CastKind::PtrPtrCast)
 825         } else {
 826             Err(CastError::DifferingKinds)
 827         }
 828     }
 829
 830     fn check_fptr_ptr_cast(
 831         &self,
 832         fcx: &FnCtxt<'a, 'tcx>,
 833         m_cast: ty::TypeAndMut<'tcx>,
 834     ) -> Result<CastKind, CastError> {
 835         // fptr-ptr cast. must be to thin ptr
 836
 837         match fcx.pointer_kind(m_cast.ty, self.span)? {
 838             None => Err(CastError::UnknownCastPtrKind),
 839             Some(PointerKind::Thin) => Ok(CastKind::FnPtrPtrCast),
 840             _ => Err(CastError::IllegalCast),
 841         }
 842     }
 843
 844     fn check_ptr_addr_cast(
 845         &self,
 846         fcx: &FnCtxt<'a, 'tcx>,
 847         m_expr: ty::TypeAndMut<'tcx>,
 848     ) -> Result<CastKind, CastError> {
 849         // ptr-addr cast. must be from thin ptr
 850
 851         match fcx.pointer_kind(m_expr.ty, self.span)? {
 852             None => Err(CastError::UnknownExprPtrKind),
 853             Some(PointerKind::Thin) => Ok(CastKind::PtrAddrCast),
 854             _ => Err(CastError::NeedViaThinPtr),
 855         }
 856     }
 857
 858     fn check_ref_cast(
 859         &self,
 860         fcx: &FnCtxt<'a, 'tcx>,
 861         m_expr: ty::TypeAndMut<'tcx>,
 862         m_cast: ty::TypeAndMut<'tcx>,
 863     ) -> Result<CastKind, CastError> {
 864         // array-ptr-cast: allow mut-to-mut, mut-to-const, const-to-const
 865         if m_expr.mutbl == hir::Mutability::Mut || m_cast.mutbl == hir::Mutability::Not {
 866             if let ty::Array(ety, _) = m_expr.ty.kind() {
 867                 // Due to the limitations of LLVM global constants,
 868                 // region pointers end up pointing at copies of
 869                 // vector elements instead of the original values.
 870                 // To allow raw pointers to work correctly, we
 871                 // need to special-case obtaining a raw pointer
 872                 // from a region pointer to a vector.
 873
 874                 // Coerce to a raw pointer so that we generate AddressOf in MIR.
 875                 let array_ptr_type = fcx.tcx.mk_ptr(m_expr);
 876                 fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No, None)
 877                     .unwrap_or_else(|_| {
 878                         bug!(
 879                         "could not cast from reference to array to pointer to array ({:?} to {:?})",
 880                         self.expr_ty,
 881                         array_ptr_type,
 882                     )
 883                     });
 884
 885                 // this will report a type mismatch if needed
 886                 fcx.demand_eqtype(self.span, *ety, m_cast.ty);
 887                 return Ok(CastKind::ArrayPtrCast);
 888             }
 889         }
 890
 891         Err(CastError::IllegalCast)
 892     }
 893
 894     fn check_addr_ptr_cast(
 895         &self,
 896         fcx: &FnCtxt<'a, 'tcx>,
 897         m_cast: TypeAndMut<'tcx>,
 898     ) -> Result<CastKind, CastError> {
 899         // ptr-addr cast. pointer must be thin.
 900         match fcx.pointer_kind(m_cast.ty, self.span)? {
 901             None => Err(CastError::UnknownCastPtrKind),
 902             Some(PointerKind::Thin) => Ok(CastKind::AddrPtrCast),
 903             _ => Err(CastError::IllegalCast),
 904         }
 905     }
 906
 907     fn try_coercion_cast(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<(), ty::error::TypeError<'_>> {
 908         match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No, None) {
 909             Ok(_) => Ok(()),
 910             Err(err) => Err(err),
 911         }
 912     }
 913
 914     fn cenum_impl_drop_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 915         if let ty::Adt(d, _) = self.expr_ty.kind() {
 916             if d.has_dtor(fcx.tcx) {
 917                 fcx.tcx.struct_span_lint_hir(
 918                     lint::builtin::CENUM_IMPL_DROP_CAST,
 919                     self.expr.hir_id,
 920                     self.span,
 921                     |err| {
 922                         err.build(&format!(
 923                             "cannot cast enum `{}` into integer `{}` because it implements `Drop`",
 924                             self.expr_ty, self.cast_ty
 925                         ))
 926                         .emit();
 927                     },
 928                 );
 929             }
 930         }
 931     }
 932 }
 933
 934 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 935     fn type_is_known_to_be_sized_modulo_regions(&self, ty: Ty<'tcx>, span: Span) -> bool {
 936         let lang_item = self.tcx.require_lang_item(LangItem::Sized, None);
 937         traits::type_known_to_meet_bound_modulo_regions(self, self.param_env, ty, lang_item, span)
 938     }
 939 }