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 hir::def_id::LOCAL_CRATE;
  36 use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed};
  37 use rustc_hir as hir;
  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, TypeVisitable, VariantDef};
  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::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 #[derive(Debug)]
  54 pub struct CastCheck<'tcx> {
  55     expr: &'tcx hir::Expr<'tcx>,
  56     expr_ty: Ty<'tcx>,
  57     expr_span: Span,
  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>>, ErrorGuaranteed> {
  90         debug!("pointer_kind({:?}, {:?})", t, span);
  91
  92         let t = self.resolve_vars_if_possible(t);
  93
  94         if let Some(reported) = t.error_reported() {
  95             return Err(reported);
  96         }
  97
  98         if self.type_is_sized_modulo_regions(self.param_env, 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                 let reported = self
 143                     .tcx
 144                     .sess
 145                     .delay_span_bug(span, &format!("`{:?}` should be sized but is not?", t));
 146                 return Err(reported);
 147             }
 148         })
 149     }
 150 }
 151
 152 #[derive(Copy, Clone)]
 153 pub enum CastError {
 154     ErrorGuaranteed,
 155
 156     CastToBool,
 157     CastToChar,
 158     DifferingKinds,
 159     /// Cast of thin to fat raw ptr (e.g., `*const () as *const [u8]`).
 160     SizedUnsizedCast,
 161     IllegalCast,
 162     NeedDeref,
 163     NeedViaPtr,
 164     NeedViaThinPtr,
 165     NeedViaInt,
 166     NonScalar,
 167     UnknownExprPtrKind,
 168     UnknownCastPtrKind,
 169     /// Cast of int to (possibly) fat raw pointer.
 170     ///
 171     /// Argument is the specific name of the metadata in plain words, such as "a vtable"
 172     /// or "a length". If this argument is None, then the metadata is unknown, for example,
 173     /// when we're typechecking a type parameter with a ?Sized bound.
 174     IntToFatCast(Option<&'static str>),
 175     ForeignNonExhaustiveAdt,
 176 }
 177
 178 impl From<ErrorGuaranteed> for CastError {
 179     fn from(_: ErrorGuaranteed) -> Self {
 180         CastError::ErrorGuaranteed
 181     }
 182 }
 183
 184 fn make_invalid_casting_error<'a, 'tcx>(
 185     sess: &'a Session,
 186     span: Span,
 187     expr_ty: Ty<'tcx>,
 188     cast_ty: Ty<'tcx>,
 189     fcx: &FnCtxt<'a, 'tcx>,
 190 ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
 191     type_error_struct!(
 192         sess,
 193         span,
 194         expr_ty,
 195         E0606,
 196         "casting `{}` as `{}` is invalid",
 197         fcx.ty_to_string(expr_ty),
 198         fcx.ty_to_string(cast_ty)
 199     )
 200 }
 201
 202 impl<'a, 'tcx> CastCheck<'tcx> {
 203     pub fn new(
 204         fcx: &FnCtxt<'a, 'tcx>,
 205         expr: &'tcx hir::Expr<'tcx>,
 206         expr_ty: Ty<'tcx>,
 207         cast_ty: Ty<'tcx>,
 208         cast_span: Span,
 209         span: Span,
 210     ) -> Result<CastCheck<'tcx>, ErrorGuaranteed> {
 211         let expr_span = expr.span.find_ancestor_inside(span).unwrap_or(expr.span);
 212         let check = CastCheck { expr, expr_ty, expr_span, cast_ty, cast_span, span };
 213
 214         // For better error messages, check for some obviously unsized
 215         // cases now. We do a more thorough check at the end, once
 216         // inference is more completely known.
 217         match cast_ty.kind() {
 218             ty::Dynamic(..) | ty::Slice(..) => {
 219                 let reported = check.report_cast_to_unsized_type(fcx);
 220                 Err(reported)
 221             }
 222             _ => Ok(check),
 223         }
 224     }
 225
 226     fn report_cast_error(&self, fcx: &FnCtxt<'a, 'tcx>, e: CastError) {
 227         match e {
 228             CastError::ErrorGuaranteed => {
 229                 // an error has already been reported
 230             }
 231             CastError::NeedDeref => {
 232                 let error_span = self.span;
 233                 let mut err = make_invalid_casting_error(
 234                     fcx.tcx.sess,
 235                     self.span,
 236                     self.expr_ty,
 237                     self.cast_ty,
 238                     fcx,
 239                 );
 240                 let cast_ty = fcx.ty_to_string(self.cast_ty);
 241                 err.span_label(
 242                     error_span,
 243                     format!("cannot cast `{}` as `{}`", fcx.ty_to_string(self.expr_ty), cast_ty),
 244                 );
 245                 if let Ok(snippet) = fcx.sess().source_map().span_to_snippet(self.expr_span) {
 246                     err.span_suggestion(
 247                         self.expr_span,
 248                         "dereference the expression",
 249                         format!("*{}", snippet),
 250                         Applicability::MaybeIncorrect,
 251                     );
 252                 } else {
 253                     err.span_help(self.expr_span, "dereference the expression with `*`");
 254                 }
 255                 err.emit();
 256             }
 257             CastError::NeedViaThinPtr | CastError::NeedViaPtr => {
 258                 let mut err = make_invalid_casting_error(
 259                     fcx.tcx.sess,
 260                     self.span,
 261                     self.expr_ty,
 262                     self.cast_ty,
 263                     fcx,
 264                 );
 265                 if self.cast_ty.is_integral() {
 266                     err.help(&format!(
 267                         "cast through {} first",
 268                         match e {
 269                             CastError::NeedViaPtr => "a raw pointer",
 270                             CastError::NeedViaThinPtr => "a thin pointer",
 271                             _ => bug!(),
 272                         }
 273                     ));
 274                 }
 275                 err.emit();
 276             }
 277             CastError::NeedViaInt => {
 278                 make_invalid_casting_error(
 279                     fcx.tcx.sess,
 280                     self.span,
 281                     self.expr_ty,
 282                     self.cast_ty,
 283                     fcx,
 284                 )
 285                 .help(&format!(
 286                     "cast through {} first",
 287                     match e {
 288                         CastError::NeedViaInt => "an integer",
 289                         _ => bug!(),
 290                     }
 291                 ))
 292                 .emit();
 293             }
 294             CastError::IllegalCast => {
 295                 make_invalid_casting_error(
 296                     fcx.tcx.sess,
 297                     self.span,
 298                     self.expr_ty,
 299                     self.cast_ty,
 300                     fcx,
 301                 )
 302                 .emit();
 303             }
 304             CastError::DifferingKinds => {
 305                 make_invalid_casting_error(
 306                     fcx.tcx.sess,
 307                     self.span,
 308                     self.expr_ty,
 309                     self.cast_ty,
 310                     fcx,
 311                 )
 312                 .note("vtable kinds may not match")
 313                 .emit();
 314             }
 315             CastError::CastToBool => {
 316                 let mut err =
 317                     struct_span_err!(fcx.tcx.sess, self.span, E0054, "cannot cast as `bool`");
 318
 319                 if self.expr_ty.is_numeric() {
 320                     match fcx.tcx.sess.source_map().span_to_snippet(self.expr_span) {
 321                         Ok(snippet) => {
 322                             err.span_suggestion(
 323                                 self.span,
 324                                 "compare with zero instead",
 325                                 format!("{snippet} != 0"),
 326                                 Applicability::MachineApplicable,
 327                             );
 328                         }
 329                         Err(_) => {
 330                             err.span_help(self.span, "compare with zero instead");
 331                         }
 332                     }
 333                 } else {
 334                     err.span_label(self.span, "unsupported cast");
 335                 }
 336
 337                 err.emit();
 338             }
 339             CastError::CastToChar => {
 340                 let mut err = type_error_struct!(
 341                     fcx.tcx.sess,
 342                     self.span,
 343                     self.expr_ty,
 344                     E0604,
 345                     "only `u8` can be cast as `char`, not `{}`",
 346                     self.expr_ty
 347                 );
 348                 err.span_label(self.span, "invalid cast");
 349                 if self.expr_ty.is_numeric() {
 350                     if self.expr_ty == fcx.tcx.types.u32 {
 351                         match fcx.tcx.sess.source_map().span_to_snippet(self.expr.span) {
 352                             Ok(snippet) => err.span_suggestion(
 353                                 self.span,
 354                                 "try `char::from_u32` instead",
 355                                 format!("char::from_u32({snippet})"),
 356                                 Applicability::MachineApplicable,
 357                             ),
 358
 359                             Err(_) => err.span_help(self.span, "try `char::from_u32` instead"),
 360                         };
 361                     } else if self.expr_ty == fcx.tcx.types.i8 {
 362                         err.span_help(self.span, "try casting from `u8` instead");
 363                     } else {
 364                         err.span_help(self.span, "try `char::from_u32` instead (via a `u32`)");
 365                     };
 366                 }
 367                 err.emit();
 368             }
 369             CastError::NonScalar => {
 370                 let mut err = type_error_struct!(
 371                     fcx.tcx.sess,
 372                     self.span,
 373                     self.expr_ty,
 374                     E0605,
 375                     "non-primitive cast: `{}` as `{}`",
 376                     self.expr_ty,
 377                     fcx.ty_to_string(self.cast_ty)
 378                 );
 379                 let mut sugg = None;
 380                 let mut sugg_mutref = false;
 381                 if let ty::Ref(reg, cast_ty, mutbl) = *self.cast_ty.kind() {
 382                     if let ty::RawPtr(TypeAndMut { ty: expr_ty, .. }) = *self.expr_ty.kind()
 383                         && fcx
 384                             .try_coerce(
 385                                 self.expr,
 386                                 fcx.tcx.mk_ref(
 387                                     fcx.tcx.lifetimes.re_erased,
 388                                     TypeAndMut { ty: expr_ty, mutbl },
 389                                 ),
 390                                 self.cast_ty,
 391                                 AllowTwoPhase::No,
 392                                 None,
 393                             )
 394                             .is_ok()
 395                     {
 396                         sugg = Some((format!("&{}*", mutbl.prefix_str()), cast_ty == expr_ty));
 397                     } else if let ty::Ref(expr_reg, expr_ty, expr_mutbl) = *self.expr_ty.kind()
 398                         && expr_mutbl == Mutability::Not
 399                         && mutbl == Mutability::Mut
 400                         && fcx
 401                             .try_coerce(
 402                                 self.expr,
 403                                 fcx.tcx.mk_ref(
 404                                     expr_reg,
 405                                     TypeAndMut { ty: expr_ty, mutbl: Mutability::Mut },
 406                                 ),
 407                                 self.cast_ty,
 408                                 AllowTwoPhase::No,
 409                                 None,
 410                             )
 411                             .is_ok()
 412                     {
 413                         sugg_mutref = true;
 414                     }
 415
 416                     if !sugg_mutref
 417                         && sugg == None
 418                         && fcx
 419                             .try_coerce(
 420                                 self.expr,
 421                                 fcx.tcx.mk_ref(reg, TypeAndMut { ty: self.expr_ty, mutbl }),
 422                                 self.cast_ty,
 423                                 AllowTwoPhase::No,
 424                                 None,
 425                             )
 426                             .is_ok()
 427                     {
 428                         sugg = Some((format!("&{}", mutbl.prefix_str()), false));
 429                     }
 430                 } else if let ty::RawPtr(TypeAndMut { mutbl, .. }) = *self.cast_ty.kind()
 431                     && fcx
 432                         .try_coerce(
 433                             self.expr,
 434                             fcx.tcx.mk_ref(
 435                                 fcx.tcx.lifetimes.re_erased,
 436                                 TypeAndMut { ty: self.expr_ty, mutbl },
 437                             ),
 438                             self.cast_ty,
 439                             AllowTwoPhase::No,
 440                             None,
 441                         )
 442                         .is_ok()
 443                 {
 444                     sugg = Some((format!("&{}", mutbl.prefix_str()), false));
 445                 }
 446                 if sugg_mutref {
 447                     err.span_label(self.span, "invalid cast");
 448                     err.span_note(self.expr_span, "this reference is immutable");
 449                     err.span_note(self.cast_span, "trying to cast to a mutable reference type");
 450                 } else if let Some((sugg, remove_cast)) = sugg {
 451                     err.span_label(self.span, "invalid cast");
 452
 453                     let has_parens = fcx
 454                         .tcx
 455                         .sess
 456                         .source_map()
 457                         .span_to_snippet(self.expr_span)
 458                         .map_or(false, |snip| snip.starts_with('('));
 459
 460                     // Very crude check to see whether the expression must be wrapped
 461                     // in parentheses for the suggestion to work (issue #89497).
 462                     // Can/should be extended in the future.
 463                     let needs_parens =
 464                         !has_parens && matches!(self.expr.kind, hir::ExprKind::Cast(..));
 465
 466                     let mut suggestion = vec![(self.expr_span.shrink_to_lo(), sugg)];
 467                     if needs_parens {
 468                         suggestion[0].1 += "(";
 469                         suggestion.push((self.expr_span.shrink_to_hi(), ")".to_string()));
 470                     }
 471                     if remove_cast {
 472                         suggestion.push((
 473                             self.expr_span.shrink_to_hi().to(self.cast_span),
 474                             String::new(),
 475                         ));
 476                     }
 477
 478                     err.multipart_suggestion_verbose(
 479                         "consider borrowing the value",
 480                         suggestion,
 481                         Applicability::MachineApplicable,
 482                     );
 483                 } else if !matches!(
 484                     self.cast_ty.kind(),
 485                     ty::FnDef(..) | ty::FnPtr(..) | ty::Closure(..)
 486                 ) {
 487                     let mut label = true;
 488                     // Check `impl From<self.expr_ty> for self.cast_ty {}` for accurate suggestion:
 489                     if let Ok(snippet) = fcx.tcx.sess.source_map().span_to_snippet(self.expr_span)
 490                         && let Some(from_trait) = fcx.tcx.get_diagnostic_item(sym::From)
 491                     {
 492                         let ty = fcx.resolve_vars_if_possible(self.cast_ty);
 493                         // Erase regions to avoid panic in `prove_value` when calling
 494                         // `type_implements_trait`.
 495                         let ty = fcx.tcx.erase_regions(ty);
 496                         let expr_ty = fcx.resolve_vars_if_possible(self.expr_ty);
 497                         let expr_ty = fcx.tcx.erase_regions(expr_ty);
 498                         let ty_params = fcx.tcx.mk_substs_trait(expr_ty, &[]);
 499                         if fcx
 500                             .infcx
 501                             .type_implements_trait(from_trait, ty, ty_params, fcx.param_env)
 502                             .must_apply_modulo_regions()
 503                         {
 504                             label = false;
 505                             err.span_suggestion(
 506                                 self.span,
 507                                 "consider using the `From` trait instead",
 508                                 format!("{}::from({})", self.cast_ty, snippet),
 509                                 Applicability::MaybeIncorrect,
 510                             );
 511                         }
 512                     }
 513                     let msg = "an `as` expression can only be used to convert between primitive \
 514                                types or to coerce to a specific trait object";
 515                     if label {
 516                         err.span_label(self.span, msg);
 517                     } else {
 518                         err.note(msg);
 519                     }
 520                 } else {
 521                     err.span_label(self.span, "invalid cast");
 522                 }
 523                 err.emit();
 524             }
 525             CastError::SizedUnsizedCast => {
 526                 use crate::structured_errors::{SizedUnsizedCast, StructuredDiagnostic};
 527
 528                 SizedUnsizedCast {
 529                     sess: &fcx.tcx.sess,
 530                     span: self.span,
 531                     expr_ty: self.expr_ty,
 532                     cast_ty: fcx.ty_to_string(self.cast_ty),
 533                 }
 534                 .diagnostic()
 535                 .emit();
 536             }
 537             CastError::IntToFatCast(known_metadata) => {
 538                 let mut err = struct_span_err!(
 539                     fcx.tcx.sess,
 540                     self.cast_span,
 541                     E0606,
 542                     "cannot cast `{}` to a pointer that {} wide",
 543                     fcx.ty_to_string(self.expr_ty),
 544                     if known_metadata.is_some() { "is" } else { "may be" }
 545                 );
 546
 547                 err.span_label(
 548                     self.cast_span,
 549                     format!(
 550                         "creating a `{}` requires both an address and {}",
 551                         self.cast_ty,
 552                         known_metadata.unwrap_or("type-specific metadata"),
 553                     ),
 554                 );
 555
 556                 if fcx.tcx.sess.is_nightly_build() {
 557                     err.span_label(
 558                         self.expr_span,
 559                         "consider casting this expression to `*const ()`, \
 560                         then using `core::ptr::from_raw_parts`",
 561                     );
 562                 }
 563
 564                 err.emit();
 565             }
 566             CastError::UnknownCastPtrKind | CastError::UnknownExprPtrKind => {
 567                 let unknown_cast_to = match e {
 568                     CastError::UnknownCastPtrKind => true,
 569                     CastError::UnknownExprPtrKind => false,
 570                     _ => bug!(),
 571                 };
 572                 let mut err = struct_span_err!(
 573                     fcx.tcx.sess,
 574                     if unknown_cast_to { self.cast_span } else { self.span },
 575                     E0641,
 576                     "cannot cast {} a pointer of an unknown kind",
 577                     if unknown_cast_to { "to" } else { "from" }
 578                 );
 579                 if unknown_cast_to {
 580                     err.span_label(self.cast_span, "needs more type information");
 581                     err.note(
 582                         "the type information given here is insufficient to check whether \
 583                         the pointer cast is valid",
 584                     );
 585                 } else {
 586                     err.span_label(
 587                         self.span,
 588                         "the type information given here is insufficient to check whether \
 589                         the pointer cast is valid",
 590                     );
 591                 }
 592                 err.emit();
 593             }
 594             CastError::ForeignNonExhaustiveAdt => {
 595                 make_invalid_casting_error(
 596                     fcx.tcx.sess,
 597                     self.span,
 598                     self.expr_ty,
 599                     self.cast_ty,
 600                     fcx,
 601                 )
 602                 .note("cannot cast an enum with a non-exhaustive variant when it's defined in another crate")
 603                 .emit();
 604             }
 605         }
 606     }
 607
 608     fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'tcx>) -> ErrorGuaranteed {
 609         if let Some(reported) =
 610             self.cast_ty.error_reported().or_else(|| self.expr_ty.error_reported())
 611         {
 612             return reported;
 613         }
 614
 615         let tstr = fcx.ty_to_string(self.cast_ty);
 616         let mut err = type_error_struct!(
 617             fcx.tcx.sess,
 618             self.span,
 619             self.expr_ty,
 620             E0620,
 621             "cast to unsized type: `{}` as `{}`",
 622             fcx.resolve_vars_if_possible(self.expr_ty),
 623             tstr
 624         );
 625         match self.expr_ty.kind() {
 626             ty::Ref(_, _, mt) => {
 627                 let mtstr = mt.prefix_str();
 628                 if self.cast_ty.is_trait() {
 629                     match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 630                         Ok(s) => {
 631                             err.span_suggestion(
 632                                 self.cast_span,
 633                                 "try casting to a reference instead",
 634                                 format!("&{}{}", mtstr, s),
 635                                 Applicability::MachineApplicable,
 636                             );
 637                         }
 638                         Err(_) => {
 639                             let msg = &format!("did you mean `&{}{}`?", mtstr, tstr);
 640                             err.span_help(self.cast_span, msg);
 641                         }
 642                     }
 643                 } else {
 644                     let msg =
 645                         &format!("consider using an implicit coercion to `&{mtstr}{tstr}` instead");
 646                     err.span_help(self.span, msg);
 647                 }
 648             }
 649             ty::Adt(def, ..) if def.is_box() => {
 650                 match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 651                     Ok(s) => {
 652                         err.span_suggestion(
 653                             self.cast_span,
 654                             "you can cast to a `Box` instead",
 655                             format!("Box<{s}>"),
 656                             Applicability::MachineApplicable,
 657                         );
 658                     }
 659                     Err(_) => {
 660                         err.span_help(
 661                             self.cast_span,
 662                             &format!("you might have meant `Box<{tstr}>`"),
 663                         );
 664                     }
 665                 }
 666             }
 667             _ => {
 668                 err.span_help(self.expr_span, "consider using a box or reference as appropriate");
 669             }
 670         }
 671         err.emit()
 672     }
 673
 674     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 675         let t_cast = self.cast_ty;
 676         let t_expr = self.expr_ty;
 677         let type_asc_or =
 678             if fcx.tcx.features().type_ascription { "type ascription or " } else { "" };
 679         let (adjective, lint) = if t_cast.is_numeric() && t_expr.is_numeric() {
 680             ("numeric ", lint::builtin::TRIVIAL_NUMERIC_CASTS)
 681         } else {
 682             ("", lint::builtin::TRIVIAL_CASTS)
 683         };
 684         fcx.tcx.struct_span_lint_hir(lint, self.expr.hir_id, self.span, |err| {
 685             err.build(&format!(
 686                 "trivial {}cast: `{}` as `{}`",
 687                 adjective,
 688                 fcx.ty_to_string(t_expr),
 689                 fcx.ty_to_string(t_cast)
 690             ))
 691             .help(&format!(
 692                 "cast can be replaced by coercion; this might \
 693                                    require {type_asc_or}a temporary variable"
 694             ))
 695             .emit();
 696         });
 697     }
 698
 699     #[instrument(skip(fcx), level = "debug")]
 700     pub fn check(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 701         self.expr_ty = fcx.structurally_resolved_type(self.expr_span, self.expr_ty);
 702         self.cast_ty = fcx.structurally_resolved_type(self.cast_span, self.cast_ty);
 703
 704         debug!("check_cast({}, {:?} as {:?})", self.expr.hir_id, self.expr_ty, self.cast_ty);
 705
 706         if !fcx.type_is_sized_modulo_regions(fcx.param_env, self.cast_ty, self.span)
 707             && !self.cast_ty.has_infer_types()
 708         {
 709             self.report_cast_to_unsized_type(fcx);
 710         } else if self.expr_ty.references_error() || self.cast_ty.references_error() {
 711             // No sense in giving duplicate error messages
 712         } else {
 713             match self.try_coercion_cast(fcx) {
 714                 Ok(()) => {
 715                     self.trivial_cast_lint(fcx);
 716                     debug!(" -> CoercionCast");
 717                     fcx.typeck_results.borrow_mut().set_coercion_cast(self.expr.hir_id.local_id);
 718                 }
 719                 Err(ty::error::TypeError::ObjectUnsafeCoercion(did)) => {
 720                     self.report_object_unsafe_cast(&fcx, did);
 721                 }
 722                 Err(_) => {
 723                     match self.do_check(fcx) {
 724                         Ok(k) => {
 725                             debug!(" -> {:?}", k);
 726                         }
 727                         Err(e) => self.report_cast_error(fcx, e),
 728                     };
 729                 }
 730             };
 731         }
 732     }
 733
 734     fn report_object_unsafe_cast(&self, fcx: &FnCtxt<'a, 'tcx>, did: DefId) {
 735         let violations = fcx.tcx.object_safety_violations(did);
 736         let mut err = report_object_safety_error(fcx.tcx, self.cast_span, did, violations);
 737         err.note(&format!("required by cast to type '{}'", fcx.ty_to_string(self.cast_ty)));
 738         err.emit();
 739     }
 740
 741     /// Checks a cast, and report an error if one exists. In some cases, this
 742     /// can return Ok and create type errors in the fcx rather than returning
 743     /// directly. coercion-cast is handled in check instead of here.
 744     pub fn do_check(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
 745         use rustc_middle::ty::cast::CastTy::*;
 746         use rustc_middle::ty::cast::IntTy::*;
 747
 748         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty), CastTy::from_ty(self.cast_ty))
 749         {
 750             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 751             // Function item types may need to be reified before casts.
 752             (None, Some(t_cast)) => {
 753                 match *self.expr_ty.kind() {
 754                     ty::FnDef(..) => {
 755                         // Attempt a coercion to a fn pointer type.
 756                         let f = fcx.normalize_associated_types_in(
 757                             self.expr_span,
 758                             self.expr_ty.fn_sig(fcx.tcx),
 759                         );
 760                         let res = fcx.try_coerce(
 761                             self.expr,
 762                             self.expr_ty,
 763                             fcx.tcx.mk_fn_ptr(f),
 764                             AllowTwoPhase::No,
 765                             None,
 766                         );
 767                         if let Err(TypeError::IntrinsicCast) = res {
 768                             return Err(CastError::IllegalCast);
 769                         }
 770                         if res.is_err() {
 771                             return Err(CastError::NonScalar);
 772                         }
 773                         (FnPtr, t_cast)
 774                     }
 775                     // Special case some errors for references, and check for
 776                     // array-ptr-casts. `Ref` is not a CastTy because the cast
 777                     // is split into a coercion to a pointer type, followed by
 778                     // a cast.
 779                     ty::Ref(_, inner_ty, mutbl) => {
 780                         return match t_cast {
 781                             Int(_) | Float => match *inner_ty.kind() {
 782                                 ty::Int(_)
 783                                 | ty::Uint(_)
 784                                 | ty::Float(_)
 785                                 | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(_)) => {
 786                                     Err(CastError::NeedDeref)
 787                                 }
 788                                 _ => Err(CastError::NeedViaPtr),
 789                             },
 790                             // array-ptr-cast
 791                             Ptr(mt) => {
 792                                 self.check_ref_cast(fcx, TypeAndMut { mutbl, ty: inner_ty }, mt)
 793                             }
 794                             _ => Err(CastError::NonScalar),
 795                         };
 796                     }
 797                     _ => return Err(CastError::NonScalar),
 798                 }
 799             }
 800             _ => return Err(CastError::NonScalar),
 801         };
 802
 803         if let ty::Adt(adt_def, _) = *self.expr_ty.kind() {
 804             if adt_def.did().krate != LOCAL_CRATE {
 805                 if adt_def.variants().iter().any(VariantDef::is_field_list_non_exhaustive) {
 806                     return Err(CastError::ForeignNonExhaustiveAdt);
 807                 }
 808             }
 809         }
 810
 811         match (t_from, t_cast) {
 812             // These types have invariants! can't cast into them.
 813             (_, Int(CEnum) | FnPtr) => Err(CastError::NonScalar),
 814
 815             // * -> Bool
 816             (_, Int(Bool)) => Err(CastError::CastToBool),
 817
 818             // * -> Char
 819             (Int(U(ty::UintTy::U8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 820             (_, Int(Char)) => Err(CastError::CastToChar),
 821
 822             // prim -> float,ptr
 823             (Int(Bool) | Int(CEnum) | Int(Char), Float) => Err(CastError::NeedViaInt),
 824
 825             (Int(Bool) | Int(CEnum) | Int(Char) | Float, Ptr(_)) | (Ptr(_) | FnPtr, Float) => {
 826                 Err(CastError::IllegalCast)
 827             }
 828
 829             // ptr -> *
 830             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 831
 832             // ptr-addr-cast
 833             (Ptr(m_expr), Int(t_c)) => {
 834                 self.lossy_provenance_ptr2int_lint(fcx, t_c);
 835                 self.check_ptr_addr_cast(fcx, m_expr)
 836             }
 837             (FnPtr, Int(_)) => {
 838                 // FIXME(#95489): there should eventually be a lint for these casts
 839                 Ok(CastKind::FnPtrAddrCast)
 840             }
 841             // addr-ptr-cast
 842             (Int(_), Ptr(mt)) => {
 843                 self.fuzzy_provenance_int2ptr_lint(fcx);
 844                 self.check_addr_ptr_cast(fcx, mt)
 845             }
 846             // fn-ptr-cast
 847             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 848
 849             // prim -> prim
 850             (Int(CEnum), Int(_)) => {
 851                 self.cenum_impl_drop_lint(fcx);
 852                 Ok(CastKind::EnumCast)
 853             }
 854             (Int(Char) | Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 855
 856             (Int(_) | Float, Int(_) | Float) => Ok(CastKind::NumericCast),
 857         }
 858     }
 859
 860     fn check_ptr_ptr_cast(
 861         &self,
 862         fcx: &FnCtxt<'a, 'tcx>,
 863         m_expr: ty::TypeAndMut<'tcx>,
 864         m_cast: ty::TypeAndMut<'tcx>,
 865     ) -> Result<CastKind, CastError> {
 866         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_expr, m_cast);
 867         // ptr-ptr cast. vtables must match.
 868
 869         let expr_kind = fcx.pointer_kind(m_expr.ty, self.span)?;
 870         let cast_kind = fcx.pointer_kind(m_cast.ty, self.span)?;
 871
 872         let Some(cast_kind) = cast_kind else {
 873             // We can't cast if target pointer kind is unknown
 874             return Err(CastError::UnknownCastPtrKind);
 875         };
 876
 877         // Cast to thin pointer is OK
 878         if cast_kind == PointerKind::Thin {
 879             return Ok(CastKind::PtrPtrCast);
 880         }
 881
 882         let Some(expr_kind) = expr_kind else {
 883             // We can't cast to fat pointer if source pointer kind is unknown
 884             return Err(CastError::UnknownExprPtrKind);
 885         };
 886
 887         // thin -> fat? report invalid cast (don't complain about vtable kinds)
 888         if expr_kind == PointerKind::Thin {
 889             return Err(CastError::SizedUnsizedCast);
 890         }
 891
 892         // vtable kinds must match
 893         if cast_kind == expr_kind {
 894             Ok(CastKind::PtrPtrCast)
 895         } else {
 896             Err(CastError::DifferingKinds)
 897         }
 898     }
 899
 900     fn check_fptr_ptr_cast(
 901         &self,
 902         fcx: &FnCtxt<'a, 'tcx>,
 903         m_cast: ty::TypeAndMut<'tcx>,
 904     ) -> Result<CastKind, CastError> {
 905         // fptr-ptr cast. must be to thin ptr
 906
 907         match fcx.pointer_kind(m_cast.ty, self.span)? {
 908             None => Err(CastError::UnknownCastPtrKind),
 909             Some(PointerKind::Thin) => Ok(CastKind::FnPtrPtrCast),
 910             _ => Err(CastError::IllegalCast),
 911         }
 912     }
 913
 914     fn check_ptr_addr_cast(
 915         &self,
 916         fcx: &FnCtxt<'a, 'tcx>,
 917         m_expr: ty::TypeAndMut<'tcx>,
 918     ) -> Result<CastKind, CastError> {
 919         // ptr-addr cast. must be from thin ptr
 920
 921         match fcx.pointer_kind(m_expr.ty, self.span)? {
 922             None => Err(CastError::UnknownExprPtrKind),
 923             Some(PointerKind::Thin) => Ok(CastKind::PtrAddrCast),
 924             _ => Err(CastError::NeedViaThinPtr),
 925         }
 926     }
 927
 928     fn check_ref_cast(
 929         &self,
 930         fcx: &FnCtxt<'a, 'tcx>,
 931         m_expr: ty::TypeAndMut<'tcx>,
 932         m_cast: ty::TypeAndMut<'tcx>,
 933     ) -> Result<CastKind, CastError> {
 934         // array-ptr-cast: allow mut-to-mut, mut-to-const, const-to-const
 935         if m_expr.mutbl == hir::Mutability::Mut || m_cast.mutbl == hir::Mutability::Not {
 936             if let ty::Array(ety, _) = m_expr.ty.kind() {
 937                 // Due to the limitations of LLVM global constants,
 938                 // region pointers end up pointing at copies of
 939                 // vector elements instead of the original values.
 940                 // To allow raw pointers to work correctly, we
 941                 // need to special-case obtaining a raw pointer
 942                 // from a region pointer to a vector.
 943
 944                 // Coerce to a raw pointer so that we generate AddressOf in MIR.
 945                 let array_ptr_type = fcx.tcx.mk_ptr(m_expr);
 946                 fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No, None)
 947                     .unwrap_or_else(|_| {
 948                         bug!(
 949                         "could not cast from reference to array to pointer to array ({:?} to {:?})",
 950                         self.expr_ty,
 951                         array_ptr_type,
 952                     )
 953                     });
 954
 955                 // this will report a type mismatch if needed
 956                 fcx.demand_eqtype(self.span, *ety, m_cast.ty);
 957                 return Ok(CastKind::ArrayPtrCast);
 958             }
 959         }
 960
 961         Err(CastError::IllegalCast)
 962     }
 963
 964     fn check_addr_ptr_cast(
 965         &self,
 966         fcx: &FnCtxt<'a, 'tcx>,
 967         m_cast: TypeAndMut<'tcx>,
 968     ) -> Result<CastKind, CastError> {
 969         // ptr-addr cast. pointer must be thin.
 970         match fcx.pointer_kind(m_cast.ty, self.span)? {
 971             None => Err(CastError::UnknownCastPtrKind),
 972             Some(PointerKind::Thin) => Ok(CastKind::AddrPtrCast),
 973             Some(PointerKind::VTable(_)) => Err(CastError::IntToFatCast(Some("a vtable"))),
 974             Some(PointerKind::Length) => Err(CastError::IntToFatCast(Some("a length"))),
 975             Some(
 976                 PointerKind::OfProjection(_)
 977                 | PointerKind::OfOpaque(_, _)
 978                 | PointerKind::OfParam(_),
 979             ) => Err(CastError::IntToFatCast(None)),
 980         }
 981     }
 982
 983     fn try_coercion_cast(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<(), ty::error::TypeError<'tcx>> {
 984         match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No, None) {
 985             Ok(_) => Ok(()),
 986             Err(err) => Err(err),
 987         }
 988     }
 989
 990     fn cenum_impl_drop_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 991         if let ty::Adt(d, _) = self.expr_ty.kind()
 992             && d.has_dtor(fcx.tcx)
 993         {
 994             fcx.tcx.struct_span_lint_hir(
 995                 lint::builtin::CENUM_IMPL_DROP_CAST,
 996                 self.expr.hir_id,
 997                 self.span,
 998                 |err| {
 999                     err.build(&format!(
1000                         "cannot cast enum `{}` into integer `{}` because it implements `Drop`",
1001                         self.expr_ty, self.cast_ty
1002                     ))
1003                     .emit();
1004                 },
1005             );
1006         }
1007     }
1008
1009     fn lossy_provenance_ptr2int_lint(&self, fcx: &FnCtxt<'a, 'tcx>, t_c: ty::cast::IntTy) {
1010         fcx.tcx.struct_span_lint_hir(
1011             lint::builtin::LOSSY_PROVENANCE_CASTS,
1012             self.expr.hir_id,
1013             self.span,
1014             |err| {
1015                 let mut err = err.build(&format!(
1016                     "under strict provenance it is considered bad style to cast pointer `{}` to integer `{}`",
1017                     self.expr_ty, self.cast_ty
1018                 ));
1019
1020                 let msg = "use `.addr()` to obtain the address of a pointer";
1021
1022                 let expr_prec = self.expr.precedence().order();
1023                 let needs_parens = expr_prec < rustc_ast::util::parser::PREC_POSTFIX;
1024
1025                 let scalar_cast = match t_c {
1026                     ty::cast::IntTy::U(ty::UintTy::Usize) => String::new(),
1027                     _ => format!(" as {}", self.cast_ty),
1028                 };
1029
1030                 let cast_span = self.expr_span.shrink_to_hi().to(self.cast_span);
1031
1032                 if needs_parens {
1033                     let suggestions = vec![
1034                         (self.expr_span.shrink_to_lo(), String::from("(")),
1035                         (cast_span, format!(").addr(){scalar_cast}")),
1036                     ];
1037
1038                     err.multipart_suggestion(msg, suggestions, Applicability::MaybeIncorrect);
1039                 } else {
1040                     err.span_suggestion(
1041                         cast_span,
1042                         msg,
1043                         format!(".addr(){scalar_cast}"),
1044                         Applicability::MaybeIncorrect,
1045                     );
1046                 }
1047
1048                 err.help(
1049                     "if you can't comply with strict provenance and need to expose the pointer \
1050                     provenance you can use `.expose_addr()` instead"
1051                 );
1052
1053                 err.emit();
1054             },
1055         );
1056     }
1057
1058     fn fuzzy_provenance_int2ptr_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
1059         fcx.tcx.struct_span_lint_hir(
1060             lint::builtin::FUZZY_PROVENANCE_CASTS,
1061             self.expr.hir_id,
1062             self.span,
1063             |err| {
1064                 let mut err = err.build(&format!(
1065                     "strict provenance disallows casting integer `{}` to pointer `{}`",
1066                     self.expr_ty, self.cast_ty
1067                 ));
1068                 let msg = "use `.with_addr()` to adjust a valid pointer in the same allocation, to this address";
1069                 let suggestions = vec![
1070                     (self.expr_span.shrink_to_lo(), String::from("(...).with_addr(")),
1071                     (self.expr_span.shrink_to_hi().to(self.cast_span), String::from(")")),
1072                 ];
1073
1074                 err.multipart_suggestion(msg, suggestions, Applicability::MaybeIncorrect);
1075                 err.help(
1076                     "if you can't comply with strict provenance and don't have a pointer with \
1077                     the correct provenance you can use `std::ptr::from_exposed_addr()` instead"
1078                  );
1079
1080                 err.emit();
1081             },
1082         );
1083     }
1084 }