src/librustc_typeck/check/cast.rs

   1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Code for type-checking cast expressions.
  12 //!
  13 //! A cast `e as U` is valid if one of the following holds:
  14 //! * `e` has type `T` and `T` coerces to `U`; *coercion-cast*
  15 //! * `e` has type `*T`, `U` is `*U_0`, and either `U_0: Sized` or
  16 //!    unsize_kind(`T`) = unsize_kind(`U_0`); *ptr-ptr-cast*
  17 //! * `e` has type `*T` and `U` is a numeric type, while `T: Sized`; *ptr-addr-cast*
  18 //! * `e` is an integer and `U` is `*U_0`, while `U_0: Sized`; *addr-ptr-cast*
  19 //! * `e` has type `T` and `T` and `U` are any numeric types; *numeric-cast*
  20 //! * `e` is a C-like enum and `U` is an integer type; *enum-cast*
  21 //! * `e` has type `bool` or `char` and `U` is an integer; *prim-int-cast*
  22 //! * `e` has type `u8` and `U` is `char`; *u8-char-cast*
  23 //! * `e` has type `&[T; n]` and `U` is `*const T`; *array-ptr-cast*
  24 //! * `e` is a function pointer type and `U` has type `*T`,
  25 //!   while `T: Sized`; *fptr-ptr-cast*
  26 //! * `e` is a function pointer type and `U` is an integer; *fptr-addr-cast*
  27 //!
  28 //! where `&.T` and `*T` are references of either mutability,
  29 //! and where unsize_kind(`T`) is the kind of the unsize info
  30 //! in `T` - the vtable for a trait definition (e.g. `fmt::Display` or
  31 //! `Iterator`, not `Iterator<Item=u8>`) or a length (or `()` if `T: Sized`).
  32 //!
  33 //! Note that lengths are not adjusted when casting raw slices -
  34 //! `T: *const [u16] as *const [u8]` creates a slice that only includes
  35 //! half of the original memory.
  36 //!
  37 //! Casting is not transitive, that is, even if `e as U1 as U2` is a valid
  38 //! expression, `e as U2` is not necessarily so (in fact it will only be valid if
  39 //! `U1` coerces to `U2`).
  40
  41 use super::{Diverges, FnCtxt};
  42
  43 use lint;
  44 use hir::def_id::DefId;
  45 use rustc::hir;
  46 use rustc::traits;
  47 use rustc::ty::{self, Ty, TypeFoldable};
  48 use rustc::ty::cast::{CastKind, CastTy};
  49 use rustc::middle::lang_items;
  50 use syntax::ast;
  51 use syntax_pos::Span;
  52 use util::common::ErrorReported;
  53
  54 /// Reifies a cast check to be checked once we have full type information for
  55 /// a function context.
  56 pub struct CastCheck<'tcx> {
  57     expr: &'tcx hir::Expr,
  58     expr_ty: Ty<'tcx>,
  59     expr_diverges: Diverges,
  60     cast_ty: Ty<'tcx>,
  61     cast_span: Span,
  62     span: Span,
  63 }
  64
  65 /// The kind of the unsize info (length or vtable) - we only allow casts between
  66 /// fat pointers if their unsize-infos have the same kind.
  67 #[derive(Copy, Clone, PartialEq, Eq)]
  68 enum UnsizeKind<'tcx> {
  69     Vtable(Option<DefId>),
  70     Length,
  71     /// The unsize info of this projection
  72     OfProjection(&'tcx ty::ProjectionTy<'tcx>),
  73     /// The unsize info of this parameter
  74     OfParam(&'tcx ty::ParamTy),
  75 }
  76
  77 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
  78     /// Returns the kind of unsize information of t, or None
  79     /// if t is sized or it is unknown.
  80     fn unsize_kind(&self, t: Ty<'tcx>) -> Option<UnsizeKind<'tcx>> {
  81         match t.sty {
  82             ty::TySlice(_) | ty::TyStr => Some(UnsizeKind::Length),
  83             ty::TyDynamic(ref tty, ..) =>
  84                 Some(UnsizeKind::Vtable(tty.principal().map(|p| p.def_id()))),
  85             ty::TyAdt(def, substs) if def.is_struct() => {
  86                 // FIXME(arielb1): do some kind of normalization
  87                 match def.struct_variant().fields.last() {
  88                     None => None,
  89                     Some(f) => self.unsize_kind(f.ty(self.tcx, substs)),
  90                 }
  91             }
  92             // We should really try to normalize here.
  93             ty::TyProjection(ref pi) => Some(UnsizeKind::OfProjection(pi)),
  94             ty::TyParam(ref p) => Some(UnsizeKind::OfParam(p)),
  95             _ => None,
  96         }
  97     }
  98 }
  99
 100 #[derive(Copy, Clone)]
 101 enum CastError {
 102     CastToBool,
 103     CastToChar,
 104     DifferingKinds,
 105     /// Cast of thin to fat raw ptr (eg. `*const () as *const [u8]`)
 106     SizedUnsizedCast,
 107     IllegalCast,
 108     NeedDeref,
 109     NeedViaPtr,
 110     NeedViaThinPtr,
 111     NeedViaInt,
 112     NonScalar,
 113 }
 114
 115 impl<'a, 'gcx, 'tcx> CastCheck<'tcx> {
 116     pub fn new(fcx: &FnCtxt<'a, 'gcx, 'tcx>,
 117                expr: &'tcx hir::Expr,
 118                expr_ty: Ty<'tcx>,
 119                expr_diverges: Diverges,
 120                cast_ty: Ty<'tcx>,
 121                cast_span: Span,
 122                span: Span)
 123                -> Result<CastCheck<'tcx>, ErrorReported> {
 124         let check = CastCheck {
 125             expr: expr,
 126             expr_ty: expr_ty,
 127             expr_diverges: expr_diverges,
 128             cast_ty: cast_ty,
 129             cast_span: cast_span,
 130             span: span,
 131         };
 132
 133         // For better error messages, check for some obviously unsized
 134         // cases now. We do a more thorough check at the end, once
 135         // inference is more completely known.
 136         match cast_ty.sty {
 137             ty::TyDynamic(..) | ty::TySlice(..) => {
 138                 check.report_cast_to_unsized_type(fcx);
 139                 Err(ErrorReported)
 140             }
 141             _ => Ok(check),
 142         }
 143     }
 144
 145     fn report_cast_error(&self, fcx: &FnCtxt<'a, 'gcx, 'tcx>, e: CastError) {
 146         match e {
 147             CastError::NeedDeref => {
 148                 let error_span = self.span;
 149                 let cast_ty = fcx.ty_to_string(self.cast_ty);
 150                 let mut err = fcx.type_error_struct(error_span,
 151                                        |actual| {
 152                                            format!("casting `{}` as `{}` is invalid",
 153                                                    actual,
 154                                                    cast_ty)
 155                                        },
 156                                        self.expr_ty);
 157                 err.span_label(error_span,
 158                                format!("cannot cast `{}` as `{}`",
 159                                         fcx.ty_to_string(self.expr_ty),
 160                                         cast_ty));
 161                 if let Ok(snippet) = fcx.sess().codemap().span_to_snippet(self.expr.span) {
 162                     err.span_help(self.expr.span,
 163                                    &format!("did you mean `*{}`?", snippet));
 164                 }
 165                 err.emit();
 166             }
 167             CastError::NeedViaThinPtr |
 168             CastError::NeedViaPtr => {
 169                 let mut err = fcx.type_error_struct(self.span,
 170                                                     |actual| {
 171                                                         format!("casting `{}` as `{}` is invalid",
 172                                                                 actual,
 173                                                                 fcx.ty_to_string(self.cast_ty))
 174                                                     },
 175                                                     self.expr_ty);
 176                 if self.cast_ty.is_uint() {
 177                     err.help(&format!("cast through {} first",
 178                                       match e {
 179                                           CastError::NeedViaPtr => "a raw pointer",
 180                                           CastError::NeedViaThinPtr => "a thin pointer",
 181                                           _ => bug!(),
 182                                       }));
 183                 }
 184                 err.emit();
 185             }
 186             CastError::NeedViaInt => {
 187                 fcx.type_error_struct(self.span,
 188                                       |actual| {
 189                                           format!("casting `{}` as `{}` is invalid",
 190                                                   actual,
 191                                                   fcx.ty_to_string(self.cast_ty))
 192                                       },
 193                                       self.expr_ty)
 194                    .help(&format!("cast through {} first",
 195                                   match e {
 196                                       CastError::NeedViaInt => "an integer",
 197                                       _ => bug!(),
 198                                   }))
 199                    .emit();
 200             }
 201             CastError::CastToBool => {
 202                 struct_span_err!(fcx.tcx.sess, self.span, E0054, "cannot cast as `bool`")
 203                     .span_label(self.span, "unsupported cast")
 204                     .help("compare with zero instead")
 205                     .emit();
 206             }
 207             CastError::CastToChar => {
 208                 fcx.type_error_message(self.span,
 209                                        |actual| {
 210                                            format!("only `u8` can be cast as `char`, not `{}`",
 211                                                    actual)
 212                                        },
 213                                        self.expr_ty);
 214             }
 215             CastError::NonScalar => {
 216                 fcx.type_error_message(self.span,
 217                                        |actual| {
 218                                            format!("non-scalar cast: `{}` as `{}`",
 219                                                    actual,
 220                                                    fcx.ty_to_string(self.cast_ty))
 221                                        },
 222                                        self.expr_ty);
 223             }
 224             CastError::IllegalCast => {
 225                 fcx.type_error_message(self.span,
 226                                        |actual| {
 227                                            format!("casting `{}` as `{}` is invalid",
 228                                                    actual,
 229                                                    fcx.ty_to_string(self.cast_ty))
 230                                        },
 231                                        self.expr_ty);
 232             }
 233             CastError::SizedUnsizedCast => {
 234                 fcx.type_error_message(self.span,
 235                                        |actual| {
 236                                            format!("cannot cast thin pointer `{}` to fat pointer \
 237                                                     `{}`",
 238                                                    actual,
 239                                                    fcx.ty_to_string(self.cast_ty))
 240                                        },
 241                                        self.expr_ty)
 242             }
 243             CastError::DifferingKinds => {
 244                 fcx.type_error_struct(self.span,
 245                                        |actual| {
 246                                            format!("casting `{}` as `{}` is invalid",
 247                                                    actual,
 248                                                    fcx.ty_to_string(self.cast_ty))
 249                                        },
 250                                        self.expr_ty)
 251                     .note("vtable kinds may not match")
 252                     .emit();
 253             }
 254         }
 255     }
 256
 257     fn report_cast_to_unsized_type(&self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) {
 258         if self.cast_ty.references_error() || self.expr_ty.references_error() {
 259             return;
 260         }
 261
 262         let tstr = fcx.ty_to_string(self.cast_ty);
 263         let mut err =
 264             fcx.type_error_struct(self.span,
 265                                   |actual| {
 266                                       format!("cast to unsized type: `{}` as `{}`", actual, tstr)
 267                                   },
 268                                   self.expr_ty);
 269         match self.expr_ty.sty {
 270             ty::TyRef(_, ty::TypeAndMut { mutbl: mt, .. }) => {
 271                 let mtstr = match mt {
 272                     hir::MutMutable => "mut ",
 273                     hir::MutImmutable => "",
 274                 };
 275                 if self.cast_ty.is_trait() {
 276                     match fcx.tcx.sess.codemap().span_to_snippet(self.cast_span) {
 277                         Ok(s) => {
 278                             err.span_suggestion(self.cast_span,
 279                                                 "try casting to a reference instead:",
 280                                                 format!("&{}{}", mtstr, s));
 281                         }
 282                         Err(_) => {
 283                             span_help!(err, self.cast_span, "did you mean `&{}{}`?", mtstr, tstr)
 284                         }
 285                     }
 286                 } else {
 287                     span_help!(err,
 288                                self.span,
 289                                "consider using an implicit coercion to `&{}{}` instead",
 290                                mtstr,
 291                                tstr);
 292                 }
 293             }
 294             ty::TyAdt(def, ..) if def.is_box() => {
 295                 match fcx.tcx.sess.codemap().span_to_snippet(self.cast_span) {
 296                     Ok(s) => {
 297                         err.span_suggestion(self.cast_span,
 298                                             "try casting to a `Box` instead:",
 299                                             format!("Box<{}>", s));
 300                     }
 301                     Err(_) => span_help!(err, self.cast_span, "did you mean `Box<{}>`?", tstr),
 302                 }
 303             }
 304             _ => {
 305                 span_help!(err,
 306                            self.expr.span,
 307                            "consider using a box or reference as appropriate");
 308             }
 309         }
 310         err.emit();
 311     }
 312
 313     fn trivial_cast_lint(&self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) {
 314         let t_cast = self.cast_ty;
 315         let t_expr = self.expr_ty;
 316         if t_cast.is_numeric() && t_expr.is_numeric() {
 317             fcx.tables.borrow_mut().lints.add_lint(
 318                 lint::builtin::TRIVIAL_NUMERIC_CASTS,
 319                 self.expr.id,
 320                 self.span,
 321                 format!("trivial numeric cast: `{}` as `{}`. Cast can be \
 322                          replaced by coercion, this might require type \
 323                          ascription or a temporary variable",
 324                         fcx.ty_to_string(t_expr),
 325                         fcx.ty_to_string(t_cast)));
 326         } else {
 327             fcx.tables.borrow_mut().lints.add_lint(
 328                 lint::builtin::TRIVIAL_CASTS,
 329                 self.expr.id,
 330                 self.span,
 331                 format!("trivial cast: `{}` as `{}`. Cast can be \
 332                          replaced by coercion, this might require type \
 333                          ascription or a temporary variable",
 334                         fcx.ty_to_string(t_expr),
 335                         fcx.ty_to_string(t_cast)));
 336         }
 337
 338     }
 339
 340     pub fn check(mut self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) {
 341         self.expr_ty = fcx.structurally_resolved_type(self.span, self.expr_ty);
 342         self.cast_ty = fcx.structurally_resolved_type(self.span, self.cast_ty);
 343
 344         debug!("check_cast({}, {:?} as {:?})",
 345                self.expr.id,
 346                self.expr_ty,
 347                self.cast_ty);
 348
 349         if !fcx.type_is_known_to_be_sized(self.cast_ty, self.span) {
 350             self.report_cast_to_unsized_type(fcx);
 351         } else if self.expr_ty.references_error() || self.cast_ty.references_error() {
 352             // No sense in giving duplicate error messages
 353         } else if self.try_coercion_cast(fcx) {
 354             self.trivial_cast_lint(fcx);
 355             debug!(" -> CoercionCast");
 356             fcx.tables.borrow_mut().cast_kinds.insert(self.expr.id, CastKind::CoercionCast);
 357         } else {
 358             match self.do_check(fcx) {
 359                 Ok(k) => {
 360                     debug!(" -> {:?}", k);
 361                     fcx.tables.borrow_mut().cast_kinds.insert(self.expr.id, k);
 362                 }
 363                 Err(e) => self.report_cast_error(fcx, e),
 364             };
 365         }
 366     }
 367
 368     /// Check a cast, and report an error if one exists. In some cases, this
 369     /// can return Ok and create type errors in the fcx rather than returning
 370     /// directly. coercion-cast is handled in check instead of here.
 371     fn do_check(&self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) -> Result<CastKind, CastError> {
 372         use rustc::ty::cast::IntTy::*;
 373         use rustc::ty::cast::CastTy::*;
 374
 375         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty),
 376                                       CastTy::from_ty(self.cast_ty)) {
 377             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 378             // Function item types may need to be reified before casts.
 379             (None, Some(t_cast)) => {
 380                 if let ty::TyFnDef(.., f) = self.expr_ty.sty {
 381                     // Attempt a coercion to a fn pointer type.
 382                     let res = fcx.try_coerce(self.expr,
 383                                              self.expr_ty,
 384                                              self.expr_diverges,
 385                                              fcx.tcx.mk_fn_ptr(f));
 386                     if !res.is_ok() {
 387                         return Err(CastError::NonScalar);
 388                     }
 389                     (FnPtr, t_cast)
 390                 } else {
 391                     return Err(CastError::NonScalar);
 392                 }
 393             }
 394             _ => return Err(CastError::NonScalar),
 395         };
 396
 397         match (t_from, t_cast) {
 398             // These types have invariants! can't cast into them.
 399             (_, RPtr(_)) | (_, Int(CEnum)) | (_, FnPtr) => Err(CastError::NonScalar),
 400
 401             // * -> Bool
 402             (_, Int(Bool)) => Err(CastError::CastToBool),
 403
 404             // * -> Char
 405             (Int(U(ast::UintTy::U8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 406             (_, Int(Char)) => Err(CastError::CastToChar),
 407
 408             // prim -> float,ptr
 409             (Int(Bool), Float) |
 410             (Int(CEnum), Float) |
 411             (Int(Char), Float) => Err(CastError::NeedViaInt),
 412
 413             (Int(Bool), Ptr(_)) |
 414             (Int(CEnum), Ptr(_)) |
 415             (Int(Char), Ptr(_)) |
 416             (Ptr(_), Float) |
 417             (FnPtr, Float) |
 418             (Float, Ptr(_)) => Err(CastError::IllegalCast),
 419
 420             // ptr -> *
 421             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 422             (Ptr(m_expr), Int(_)) => self.check_ptr_addr_cast(fcx, m_expr), // ptr-addr-cast
 423             (FnPtr, Int(_)) => Ok(CastKind::FnPtrAddrCast),
 424             (RPtr(p), Int(_)) |
 425             (RPtr(p), Float) => {
 426                 match p.ty.sty {
 427                     ty::TypeVariants::TyInt(_) |
 428                     ty::TypeVariants::TyUint(_) |
 429                     ty::TypeVariants::TyFloat(_) => {
 430                         Err(CastError::NeedDeref)
 431                     }
 432                     ty::TypeVariants::TyInfer(t) => {
 433                         match t {
 434                             ty::InferTy::IntVar(_) |
 435                             ty::InferTy::FloatVar(_) |
 436                             ty::InferTy::FreshIntTy(_) |
 437                             ty::InferTy::FreshFloatTy(_) => {
 438                                 Err(CastError::NeedDeref)
 439                             }
 440                             _ => Err(CastError::NeedViaPtr),
 441                         }
 442                     }
 443                     _ => Err(CastError::NeedViaPtr),
 444                 }
 445             }
 446             // * -> ptr
 447             (Int(_), Ptr(mt)) => self.check_addr_ptr_cast(fcx, mt), // addr-ptr-cast
 448             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 449             (RPtr(rmt), Ptr(mt)) => self.check_ref_cast(fcx, rmt, mt), // array-ptr-cast
 450
 451             // prim -> prim
 452             (Int(CEnum), Int(_)) => Ok(CastKind::EnumCast),
 453             (Int(Char), Int(_)) |
 454             (Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 455
 456             (Int(_), Int(_)) | (Int(_), Float) | (Float, Int(_)) | (Float, Float) => {
 457                 Ok(CastKind::NumericCast)
 458             }
 459         }
 460     }
 461
 462     fn check_ptr_ptr_cast(&self,
 463                           fcx: &FnCtxt<'a, 'gcx, 'tcx>,
 464                           m_expr: &'tcx ty::TypeAndMut<'tcx>,
 465                           m_cast: &'tcx ty::TypeAndMut<'tcx>)
 466                           -> Result<CastKind, CastError> {
 467         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}", m_expr, m_cast);
 468         // ptr-ptr cast. vtables must match.
 469
 470         // Cast to sized is OK
 471         if fcx.type_is_known_to_be_sized(m_cast.ty, self.span) {
 472             return Ok(CastKind::PtrPtrCast);
 473         }
 474
 475         // sized -> unsized? report invalid cast (don't complain about vtable kinds)
 476         if fcx.type_is_known_to_be_sized(m_expr.ty, self.span) {
 477             return Err(CastError::SizedUnsizedCast);
 478         }
 479
 480         // vtable kinds must match
 481         match (fcx.unsize_kind(m_cast.ty), fcx.unsize_kind(m_expr.ty)) {
 482             (Some(a), Some(b)) if a == b => Ok(CastKind::PtrPtrCast),
 483             _ => Err(CastError::DifferingKinds),
 484         }
 485     }
 486
 487     fn check_fptr_ptr_cast(&self,
 488                            fcx: &FnCtxt<'a, 'gcx, 'tcx>,
 489                            m_cast: &'tcx ty::TypeAndMut<'tcx>)
 490                            -> Result<CastKind, CastError> {
 491         // fptr-ptr cast. must be to sized ptr
 492
 493         if fcx.type_is_known_to_be_sized(m_cast.ty, self.span) {
 494             Ok(CastKind::FnPtrPtrCast)
 495         } else {
 496             Err(CastError::IllegalCast)
 497         }
 498     }
 499
 500     fn check_ptr_addr_cast(&self,
 501                            fcx: &FnCtxt<'a, 'gcx, 'tcx>,
 502                            m_expr: &'tcx ty::TypeAndMut<'tcx>)
 503                            -> Result<CastKind, CastError> {
 504         // ptr-addr cast. must be from sized ptr
 505
 506         if fcx.type_is_known_to_be_sized(m_expr.ty, self.span) {
 507             Ok(CastKind::PtrAddrCast)
 508         } else {
 509             Err(CastError::NeedViaThinPtr)
 510         }
 511     }
 512
 513     fn check_ref_cast(&self,
 514                       fcx: &FnCtxt<'a, 'gcx, 'tcx>,
 515                       m_expr: &'tcx ty::TypeAndMut<'tcx>,
 516                       m_cast: &'tcx ty::TypeAndMut<'tcx>)
 517                       -> Result<CastKind, CastError> {
 518         // array-ptr-cast.
 519
 520         if m_expr.mutbl == hir::MutImmutable && m_cast.mutbl == hir::MutImmutable {
 521             if let ty::TyArray(ety, _) = m_expr.ty.sty {
 522                 // Due to the limitations of LLVM global constants,
 523                 // region pointers end up pointing at copies of
 524                 // vector elements instead of the original values.
 525                 // To allow raw pointers to work correctly, we
 526                 // need to special-case obtaining a raw pointer
 527                 // from a region pointer to a vector.
 528
 529                 // this will report a type mismatch if needed
 530                 fcx.demand_eqtype(self.span, ety, m_cast.ty);
 531                 return Ok(CastKind::ArrayPtrCast);
 532             }
 533         }
 534
 535         Err(CastError::IllegalCast)
 536     }
 537
 538     fn check_addr_ptr_cast(&self,
 539                            fcx: &FnCtxt<'a, 'gcx, 'tcx>,
 540                            m_cast: &'tcx ty::TypeAndMut<'tcx>)
 541                            -> Result<CastKind, CastError> {
 542         // ptr-addr cast. pointer must be thin.
 543         if fcx.type_is_known_to_be_sized(m_cast.ty, self.span) {
 544             Ok(CastKind::AddrPtrCast)
 545         } else {
 546             Err(CastError::IllegalCast)
 547         }
 548     }
 549
 550     fn try_coercion_cast(&self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) -> bool {
 551         fcx.try_coerce(self.expr, self.expr_ty, self.expr_diverges, self.cast_ty).is_ok()
 552     }
 553 }
 554
 555 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 556     fn type_is_known_to_be_sized(&self, ty: Ty<'tcx>, span: Span) -> bool {
 557         let lang_item = self.tcx.require_lang_item(lang_items::SizedTraitLangItem);
 558         traits::type_known_to_meet_bound(self, ty, lang_item, span)
 559     }
 560 }