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, ErrorGuaranteed};
  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     expr_span: Span,
  59     cast_ty: Ty<'tcx>,
  60     cast_span: Span,
  61     span: Span,
  62 }
  63
  64 /// The kind of pointer and associated metadata (thin, length or vtable) - we
  65 /// only allow casts between fat pointers if their metadata have the same
  66 /// kind.
  67 #[derive(Copy, Clone, PartialEq, Eq)]
  68 enum PointerKind<'tcx> {
  69     /// No metadata attached, ie pointer to sized type or foreign type
  70     Thin,
  71     /// A trait object
  72     Vtable(Option<DefId>),
  73     /// Slice
  74     Length,
  75     /// The unsize info of this projection
  76     OfProjection(&'tcx ty::ProjectionTy<'tcx>),
  77     /// The unsize info of this opaque ty
  78     OfOpaque(DefId, SubstsRef<'tcx>),
  79     /// The unsize info of this parameter
  80     OfParam(&'tcx ty::ParamTy),
  81 }
  82
  83 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  84     /// Returns the kind of unsize information of t, or None
  85     /// if t is unknown.
  86     fn pointer_kind(
  87         &self,
  88         t: Ty<'tcx>,
  89         span: Span,
  90     ) -> Result<Option<PointerKind<'tcx>>, ErrorGuaranteed> {
  91         debug!("pointer_kind({:?}, {:?})", t, span);
  92
  93         let t = self.resolve_vars_if_possible(t);
  94
  95         if let Some(reported) = t.error_reported() {
  96             return Err(reported);
  97         }
  98
  99         if self.type_is_known_to_be_sized_modulo_regions(t, span) {
 100             return Ok(Some(PointerKind::Thin));
 101         }
 102
 103         Ok(match *t.kind() {
 104             ty::Slice(_) | ty::Str => Some(PointerKind::Length),
 105             ty::Dynamic(ref tty, ..) => Some(PointerKind::Vtable(tty.principal_def_id())),
 106             ty::Adt(def, substs) if def.is_struct() => match def.non_enum_variant().fields.last() {
 107                 None => Some(PointerKind::Thin),
 108                 Some(f) => {
 109                     let field_ty = self.field_ty(span, f, substs);
 110                     self.pointer_kind(field_ty, span)?
 111                 }
 112             },
 113             ty::Tuple(fields) => match fields.last() {
 114                 None => Some(PointerKind::Thin),
 115                 Some(&f) => self.pointer_kind(f, span)?,
 116             },
 117
 118             // Pointers to foreign types are thin, despite being unsized
 119             ty::Foreign(..) => Some(PointerKind::Thin),
 120             // We should really try to normalize here.
 121             ty::Projection(ref pi) => Some(PointerKind::OfProjection(pi)),
 122             ty::Opaque(def_id, substs) => Some(PointerKind::OfOpaque(def_id, substs)),
 123             ty::Param(ref p) => Some(PointerKind::OfParam(p)),
 124             // Insufficient type information.
 125             ty::Placeholder(..) | ty::Bound(..) | ty::Infer(_) => None,
 126
 127             ty::Bool
 128             | ty::Char
 129             | ty::Int(..)
 130             | ty::Uint(..)
 131             | ty::Float(_)
 132             | ty::Array(..)
 133             | ty::GeneratorWitness(..)
 134             | ty::RawPtr(_)
 135             | ty::Ref(..)
 136             | ty::FnDef(..)
 137             | ty::FnPtr(..)
 138             | ty::Closure(..)
 139             | ty::Generator(..)
 140             | ty::Adt(..)
 141             | ty::Never
 142             | ty::Error(_) => {
 143                 let reported = self
 144                     .tcx
 145                     .sess
 146                     .delay_span_bug(span, &format!("`{:?}` should be sized but is not?", t));
 147                 return Err(reported);
 148             }
 149         })
 150     }
 151 }
 152
 153 #[derive(Copy, Clone)]
 154 pub enum CastError {
 155     ErrorGuaranteed,
 156
 157     CastToBool,
 158     CastToChar,
 159     DifferingKinds,
 160     /// Cast of thin to fat raw ptr (e.g., `*const () as *const [u8]`).
 161     SizedUnsizedCast,
 162     IllegalCast,
 163     NeedDeref,
 164     NeedViaPtr,
 165     NeedViaThinPtr,
 166     NeedViaInt,
 167     NonScalar,
 168     UnknownExprPtrKind,
 169     UnknownCastPtrKind,
 170     /// Cast of int to (possibly) fat raw pointer.
 171     ///
 172     /// Argument is the specific name of the metadata in plain words, such as "a vtable"
 173     /// or "a length". If this argument is None, then the metadata is unknown, for example,
 174     /// when we're typechecking a type parameter with a ?Sized bound.
 175     IntToFatCast(Option<&'static str>),
 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         }
 595     }
 596
 597     fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'tcx>) -> ErrorGuaranteed {
 598         if let Some(reported) =
 599             self.cast_ty.error_reported().or_else(|| self.expr_ty.error_reported())
 600         {
 601             return reported;
 602         }
 603
 604         let tstr = fcx.ty_to_string(self.cast_ty);
 605         let mut err = type_error_struct!(
 606             fcx.tcx.sess,
 607             self.span,
 608             self.expr_ty,
 609             E0620,
 610             "cast to unsized type: `{}` as `{}`",
 611             fcx.resolve_vars_if_possible(self.expr_ty),
 612             tstr
 613         );
 614         match self.expr_ty.kind() {
 615             ty::Ref(_, _, mt) => {
 616                 let mtstr = mt.prefix_str();
 617                 if self.cast_ty.is_trait() {
 618                     match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 619                         Ok(s) => {
 620                             err.span_suggestion(
 621                                 self.cast_span,
 622                                 "try casting to a reference instead",
 623                                 format!("&{}{}", mtstr, s),
 624                                 Applicability::MachineApplicable,
 625                             );
 626                         }
 627                         Err(_) => {
 628                             let msg = &format!("did you mean `&{}{}`?", mtstr, tstr);
 629                             err.span_help(self.cast_span, msg);
 630                         }
 631                     }
 632                 } else {
 633                     let msg =
 634                         &format!("consider using an implicit coercion to `&{mtstr}{tstr}` instead");
 635                     err.span_help(self.span, msg);
 636                 }
 637             }
 638             ty::Adt(def, ..) if def.is_box() => {
 639                 match fcx.tcx.sess.source_map().span_to_snippet(self.cast_span) {
 640                     Ok(s) => {
 641                         err.span_suggestion(
 642                             self.cast_span,
 643                             "you can cast to a `Box` instead",
 644                             format!("Box<{s}>"),
 645                             Applicability::MachineApplicable,
 646                         );
 647                     }
 648                     Err(_) => {
 649                         err.span_help(
 650                             self.cast_span,
 651                             &format!("you might have meant `Box<{tstr}>`"),
 652                         );
 653                     }
 654                 }
 655             }
 656             _ => {
 657                 err.span_help(self.expr_span, "consider using a box or reference as appropriate");
 658             }
 659         }
 660         err.emit()
 661     }
 662
 663     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 664         let t_cast = self.cast_ty;
 665         let t_expr = self.expr_ty;
 666         let type_asc_or =
 667             if fcx.tcx.features().type_ascription { "type ascription or " } else { "" };
 668         let (adjective, lint) = if t_cast.is_numeric() && t_expr.is_numeric() {
 669             ("numeric ", lint::builtin::TRIVIAL_NUMERIC_CASTS)
 670         } else {
 671             ("", lint::builtin::TRIVIAL_CASTS)
 672         };
 673         fcx.tcx.struct_span_lint_hir(lint, self.expr.hir_id, self.span, |err| {
 674             err.build(&format!(
 675                 "trivial {}cast: `{}` as `{}`",
 676                 adjective,
 677                 fcx.ty_to_string(t_expr),
 678                 fcx.ty_to_string(t_cast)
 679             ))
 680             .help(&format!(
 681                 "cast can be replaced by coercion; this might \
 682                                    require {type_asc_or}a temporary variable"
 683             ))
 684             .emit();
 685         });
 686     }
 687
 688     #[instrument(skip(fcx), level = "debug")]
 689     pub fn check(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 690         self.expr_ty = fcx.structurally_resolved_type(self.expr_span, self.expr_ty);
 691         self.cast_ty = fcx.structurally_resolved_type(self.cast_span, self.cast_ty);
 692
 693         debug!("check_cast({}, {:?} as {:?})", self.expr.hir_id, self.expr_ty, self.cast_ty);
 694
 695         if !fcx.type_is_known_to_be_sized_modulo_regions(self.cast_ty, self.span)
 696             && !self.cast_ty.has_infer_types()
 697         {
 698             self.report_cast_to_unsized_type(fcx);
 699         } else if self.expr_ty.references_error() || self.cast_ty.references_error() {
 700             // No sense in giving duplicate error messages
 701         } else {
 702             match self.try_coercion_cast(fcx) {
 703                 Ok(()) => {
 704                     self.trivial_cast_lint(fcx);
 705                     debug!(" -> CoercionCast");
 706                     fcx.typeck_results.borrow_mut().set_coercion_cast(self.expr.hir_id.local_id);
 707                 }
 708                 Err(ty::error::TypeError::ObjectUnsafeCoercion(did)) => {
 709                     self.report_object_unsafe_cast(&fcx, did);
 710                 }
 711                 Err(_) => {
 712                     match self.do_check(fcx) {
 713                         Ok(k) => {
 714                             debug!(" -> {:?}", k);
 715                         }
 716                         Err(e) => self.report_cast_error(fcx, e),
 717                     };
 718                 }
 719             };
 720         }
 721     }
 722
 723     fn report_object_unsafe_cast(&self, fcx: &FnCtxt<'a, 'tcx>, did: DefId) {
 724         let violations = fcx.tcx.object_safety_violations(did);
 725         let mut err = report_object_safety_error(fcx.tcx, self.cast_span, did, violations);
 726         err.note(&format!("required by cast to type '{}'", fcx.ty_to_string(self.cast_ty)));
 727         err.emit();
 728     }
 729
 730     /// Checks a cast, and report an error if one exists. In some cases, this
 731     /// can return Ok and create type errors in the fcx rather than returning
 732     /// directly. coercion-cast is handled in check instead of here.
 733     pub fn do_check(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
 734         use rustc_middle::ty::cast::CastTy::*;
 735         use rustc_middle::ty::cast::IntTy::*;
 736
 737         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty), CastTy::from_ty(self.cast_ty))
 738         {
 739             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 740             // Function item types may need to be reified before casts.
 741             (None, Some(t_cast)) => {
 742                 match *self.expr_ty.kind() {
 743                     ty::FnDef(..) => {
 744                         // Attempt a coercion to a fn pointer type.
 745                         let f = fcx.normalize_associated_types_in(
 746                             self.expr_span,
 747                             self.expr_ty.fn_sig(fcx.tcx),
 748                         );
 749                         let res = fcx.try_coerce(
 750                             self.expr,
 751                             self.expr_ty,
 752                             fcx.tcx.mk_fn_ptr(f),
 753                             AllowTwoPhase::No,
 754                             None,
 755                         );
 756                         if let Err(TypeError::IntrinsicCast) = res {
 757                             return Err(CastError::IllegalCast);
 758                         }
 759                         if res.is_err() {
 760                             return Err(CastError::NonScalar);
 761                         }
 762                         (FnPtr, t_cast)
 763                     }
 764                     // Special case some errors for references, and check for
 765                     // array-ptr-casts. `Ref` is not a CastTy because the cast
 766                     // is split into a coercion to a pointer type, followed by
 767                     // a cast.
 768                     ty::Ref(_, inner_ty, mutbl) => {
 769                         return match t_cast {
 770                             Int(_) | Float => match *inner_ty.kind() {
 771                                 ty::Int(_)
 772                                 | ty::Uint(_)
 773                                 | ty::Float(_)
 774                                 | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(_)) => {
 775                                     Err(CastError::NeedDeref)
 776                                 }
 777                                 _ => Err(CastError::NeedViaPtr),
 778                             },
 779                             // array-ptr-cast
 780                             Ptr(mt) => {
 781                                 self.check_ref_cast(fcx, TypeAndMut { mutbl, ty: inner_ty }, mt)
 782                             }
 783                             _ => Err(CastError::NonScalar),
 784                         };
 785                     }
 786                     _ => return Err(CastError::NonScalar),
 787                 }
 788             }
 789             _ => return Err(CastError::NonScalar),
 790         };
 791
 792         match (t_from, t_cast) {
 793             // These types have invariants! can't cast into them.
 794             (_, Int(CEnum) | FnPtr) => Err(CastError::NonScalar),
 795
 796             // * -> Bool
 797             (_, Int(Bool)) => Err(CastError::CastToBool),
 798
 799             // * -> Char
 800             (Int(U(ty::UintTy::U8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 801             (_, Int(Char)) => Err(CastError::CastToChar),
 802
 803             // prim -> float,ptr
 804             (Int(Bool) | Int(CEnum) | Int(Char), Float) => Err(CastError::NeedViaInt),
 805
 806             (Int(Bool) | Int(CEnum) | Int(Char) | Float, Ptr(_)) | (Ptr(_) | FnPtr, Float) => {
 807                 Err(CastError::IllegalCast)
 808             }
 809
 810             // ptr -> *
 811             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 812
 813             // ptr-addr-cast
 814             (Ptr(m_expr), Int(t_c)) => {
 815                 self.lossy_provenance_ptr2int_lint(fcx, t_c);
 816                 self.check_ptr_addr_cast(fcx, m_expr)
 817             }
 818             (FnPtr, Int(_)) => {
 819                 // FIXME(#95489): there should eventually be a lint for these casts
 820                 Ok(CastKind::FnPtrAddrCast)
 821             }
 822             // addr-ptr-cast
 823             (Int(_), Ptr(mt)) => {
 824                 self.fuzzy_provenance_int2ptr_lint(fcx);
 825                 self.check_addr_ptr_cast(fcx, mt)
 826             }
 827             // fn-ptr-cast
 828             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 829
 830             // prim -> prim
 831             (Int(CEnum), Int(_)) => {
 832                 self.cenum_impl_drop_lint(fcx);
 833                 Ok(CastKind::EnumCast)
 834             }
 835             (Int(Char) | Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 836
 837             (Int(_) | Float, Int(_) | Float) => Ok(CastKind::NumericCast),
 838         }
 839     }
 840
 841     fn check_ptr_ptr_cast(
 842         &self,
 843         fcx: &FnCtxt<'a, 'tcx>,
 844         m_expr: ty::TypeAndMut<'tcx>,
 845         m_cast: ty::TypeAndMut<'tcx>,
 846     ) -> Result<CastKind, CastError> {
 847         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_expr, m_cast);
 848         // ptr-ptr cast. vtables must match.
 849
 850         let expr_kind = fcx.pointer_kind(m_expr.ty, self.span)?;
 851         let cast_kind = fcx.pointer_kind(m_cast.ty, self.span)?;
 852
 853         let Some(cast_kind) = cast_kind else {
 854             // We can't cast if target pointer kind is unknown
 855             return Err(CastError::UnknownCastPtrKind);
 856         };
 857
 858         // Cast to thin pointer is OK
 859         if cast_kind == PointerKind::Thin {
 860             return Ok(CastKind::PtrPtrCast);
 861         }
 862
 863         let Some(expr_kind) = expr_kind else {
 864             // We can't cast to fat pointer if source pointer kind is unknown
 865             return Err(CastError::UnknownExprPtrKind);
 866         };
 867
 868         // thin -> fat? report invalid cast (don't complain about vtable kinds)
 869         if expr_kind == PointerKind::Thin {
 870             return Err(CastError::SizedUnsizedCast);
 871         }
 872
 873         // vtable kinds must match
 874         if cast_kind == expr_kind {
 875             Ok(CastKind::PtrPtrCast)
 876         } else {
 877             Err(CastError::DifferingKinds)
 878         }
 879     }
 880
 881     fn check_fptr_ptr_cast(
 882         &self,
 883         fcx: &FnCtxt<'a, 'tcx>,
 884         m_cast: ty::TypeAndMut<'tcx>,
 885     ) -> Result<CastKind, CastError> {
 886         // fptr-ptr cast. must be to thin ptr
 887
 888         match fcx.pointer_kind(m_cast.ty, self.span)? {
 889             None => Err(CastError::UnknownCastPtrKind),
 890             Some(PointerKind::Thin) => Ok(CastKind::FnPtrPtrCast),
 891             _ => Err(CastError::IllegalCast),
 892         }
 893     }
 894
 895     fn check_ptr_addr_cast(
 896         &self,
 897         fcx: &FnCtxt<'a, 'tcx>,
 898         m_expr: ty::TypeAndMut<'tcx>,
 899     ) -> Result<CastKind, CastError> {
 900         // ptr-addr cast. must be from thin ptr
 901
 902         match fcx.pointer_kind(m_expr.ty, self.span)? {
 903             None => Err(CastError::UnknownExprPtrKind),
 904             Some(PointerKind::Thin) => Ok(CastKind::PtrAddrCast),
 905             _ => Err(CastError::NeedViaThinPtr),
 906         }
 907     }
 908
 909     fn check_ref_cast(
 910         &self,
 911         fcx: &FnCtxt<'a, 'tcx>,
 912         m_expr: ty::TypeAndMut<'tcx>,
 913         m_cast: ty::TypeAndMut<'tcx>,
 914     ) -> Result<CastKind, CastError> {
 915         // array-ptr-cast: allow mut-to-mut, mut-to-const, const-to-const
 916         if m_expr.mutbl == hir::Mutability::Mut || m_cast.mutbl == hir::Mutability::Not {
 917             if let ty::Array(ety, _) = m_expr.ty.kind() {
 918                 // Due to the limitations of LLVM global constants,
 919                 // region pointers end up pointing at copies of
 920                 // vector elements instead of the original values.
 921                 // To allow raw pointers to work correctly, we
 922                 // need to special-case obtaining a raw pointer
 923                 // from a region pointer to a vector.
 924
 925                 // Coerce to a raw pointer so that we generate AddressOf in MIR.
 926                 let array_ptr_type = fcx.tcx.mk_ptr(m_expr);
 927                 fcx.try_coerce(self.expr, self.expr_ty, array_ptr_type, AllowTwoPhase::No, None)
 928                     .unwrap_or_else(|_| {
 929                         bug!(
 930                         "could not cast from reference to array to pointer to array ({:?} to {:?})",
 931                         self.expr_ty,
 932                         array_ptr_type,
 933                     )
 934                     });
 935
 936                 // this will report a type mismatch if needed
 937                 fcx.demand_eqtype(self.span, *ety, m_cast.ty);
 938                 return Ok(CastKind::ArrayPtrCast);
 939             }
 940         }
 941
 942         Err(CastError::IllegalCast)
 943     }
 944
 945     fn check_addr_ptr_cast(
 946         &self,
 947         fcx: &FnCtxt<'a, 'tcx>,
 948         m_cast: TypeAndMut<'tcx>,
 949     ) -> Result<CastKind, CastError> {
 950         // ptr-addr cast. pointer must be thin.
 951         match fcx.pointer_kind(m_cast.ty, self.span)? {
 952             None => Err(CastError::UnknownCastPtrKind),
 953             Some(PointerKind::Thin) => Ok(CastKind::AddrPtrCast),
 954             Some(PointerKind::Vtable(_)) => Err(CastError::IntToFatCast(Some("a vtable"))),
 955             Some(PointerKind::Length) => Err(CastError::IntToFatCast(Some("a length"))),
 956             Some(
 957                 PointerKind::OfProjection(_)
 958                 | PointerKind::OfOpaque(_, _)
 959                 | PointerKind::OfParam(_),
 960             ) => Err(CastError::IntToFatCast(None)),
 961         }
 962     }
 963
 964     fn try_coercion_cast(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<(), ty::error::TypeError<'tcx>> {
 965         match fcx.try_coerce(self.expr, self.expr_ty, self.cast_ty, AllowTwoPhase::No, None) {
 966             Ok(_) => Ok(()),
 967             Err(err) => Err(err),
 968         }
 969     }
 970
 971     fn cenum_impl_drop_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
 972         if let ty::Adt(d, _) = self.expr_ty.kind()
 973             && d.has_dtor(fcx.tcx)
 974         {
 975             fcx.tcx.struct_span_lint_hir(
 976                 lint::builtin::CENUM_IMPL_DROP_CAST,
 977                 self.expr.hir_id,
 978                 self.span,
 979                 |err| {
 980                     err.build(&format!(
 981                         "cannot cast enum `{}` into integer `{}` because it implements `Drop`",
 982                         self.expr_ty, self.cast_ty
 983                     ))
 984                     .emit();
 985                 },
 986             );
 987         }
 988     }
 989
 990     fn lossy_provenance_ptr2int_lint(&self, fcx: &FnCtxt<'a, 'tcx>, t_c: ty::cast::IntTy) {
 991         fcx.tcx.struct_span_lint_hir(
 992             lint::builtin::LOSSY_PROVENANCE_CASTS,
 993             self.expr.hir_id,
 994             self.span,
 995             |err| {
 996                 let mut err = err.build(&format!(
 997                     "under strict provenance it is considered bad style to cast pointer `{}` to integer `{}`",
 998                     self.expr_ty, self.cast_ty
 999                 ));
1000
1001                 let msg = "use `.addr()` to obtain the address of a pointer";
1002
1003                 let expr_prec = self.expr.precedence().order();
1004                 let needs_parens = expr_prec < rustc_ast::util::parser::PREC_POSTFIX;
1005
1006                 let scalar_cast = match t_c {
1007                     ty::cast::IntTy::U(ty::UintTy::Usize) => String::new(),
1008                     _ => format!(" as {}", self.cast_ty),
1009                 };
1010
1011                 let cast_span = self.expr_span.shrink_to_hi().to(self.cast_span);
1012
1013                 if needs_parens {
1014                     let suggestions = vec![
1015                         (self.expr_span.shrink_to_lo(), String::from("(")),
1016                         (cast_span, format!(").addr(){scalar_cast}")),
1017                     ];
1018
1019                     err.multipart_suggestion(msg, suggestions, Applicability::MaybeIncorrect);
1020                 } else {
1021                     err.span_suggestion(
1022                         cast_span,
1023                         msg,
1024                         format!(".addr(){scalar_cast}"),
1025                         Applicability::MaybeIncorrect,
1026                     );
1027                 }
1028
1029                 err.help(
1030                     "if you can't comply with strict provenance and need to expose the pointer \
1031                     provenance you can use `.expose_addr()` instead"
1032                 );
1033
1034                 err.emit();
1035             },
1036         );
1037     }
1038
1039     fn fuzzy_provenance_int2ptr_lint(&self, fcx: &FnCtxt<'a, 'tcx>) {
1040         fcx.tcx.struct_span_lint_hir(
1041             lint::builtin::FUZZY_PROVENANCE_CASTS,
1042             self.expr.hir_id,
1043             self.span,
1044             |err| {
1045                 let mut err = err.build(&format!(
1046                     "strict provenance disallows casting integer `{}` to pointer `{}`",
1047                     self.expr_ty, self.cast_ty
1048                 ));
1049                 let msg = "use `.with_addr()` to adjust a valid pointer in the same allocation, to this address";
1050                 let suggestions = vec![
1051                     (self.expr_span.shrink_to_lo(), String::from("(...).with_addr(")),
1052                     (self.expr_span.shrink_to_hi().to(self.cast_span), String::from(")")),
1053                 ];
1054
1055                 err.multipart_suggestion(msg, suggestions, Applicability::MaybeIncorrect);
1056                 err.help(
1057                     "if you can't comply with strict provenance and don't have a pointer with \
1058                     the correct provenance you can use `std::ptr::from_exposed_addr()` instead"
1059                  );
1060
1061                 err.emit();
1062             },
1063         );
1064     }
1065 }
1066
1067 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
1068     fn type_is_known_to_be_sized_modulo_regions(&self, ty: Ty<'tcx>, span: Span) -> bool {
1069         let lang_item = self.tcx.require_lang_item(LangItem::Sized, None);
1070         traits::type_known_to_meet_bound_modulo_regions(self, self.param_env, ty, lang_item, span)
1071     }
1072 }