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::coercion;
  42 use super::demand;
  43 use super::FnCtxt;
  44 use super::structurally_resolved_type;
  45
  46 use lint;
  47 use hir::def_id::DefId;
  48 use rustc::ty::{self, Ty, TypeFoldable};
  49 use rustc::ty::cast::{CastKind, CastTy};
  50 use syntax::codemap::Span;
  51 use rustc::hir;
  52 use syntax::ast;
  53
  54
  55 /// Reifies a cast check to be checked once we have full type information for
  56 /// a function context.
  57 pub struct CastCheck<'tcx> {
  58     expr: &'tcx hir::Expr,
  59     expr_ty: Ty<'tcx>,
  60     cast_ty: Ty<'tcx>,
  61     span: Span,
  62 }
  63
  64 /// The kind of the unsize info (length or vtable) - we only allow casts between
  65 /// fat pointers if their unsize-infos have the same kind.
  66 #[derive(Copy, Clone, PartialEq, Eq)]
  67 enum UnsizeKind<'tcx> {
  68     Vtable(DefId),
  69     Length,
  70     /// The unsize info of this projection
  71     OfProjection(&'tcx ty::ProjectionTy<'tcx>),
  72     /// The unsize info of this parameter
  73     OfParam(&'tcx ty::ParamTy)
  74 }
  75
  76 /// Returns the kind of unsize information of t, or None
  77 /// if t is sized or it is unknown.
  78 fn unsize_kind<'a,'tcx>(fcx: &FnCtxt<'a, 'tcx>,
  79                         t: Ty<'tcx>)
  80                         -> Option<UnsizeKind<'tcx>> {
  81     match t.sty {
  82         ty::TySlice(_) | ty::TyStr => Some(UnsizeKind::Length),
  83         ty::TyTrait(ref tty) => Some(UnsizeKind::Vtable(tty.principal_def_id())),
  84         ty::TyStruct(def, substs) => {
  85             // FIXME(arielb1): do some kind of normalization
  86             match def.struct_variant().fields.last() {
  87                 None => None,
  88                 Some(f) => unsize_kind(fcx, f.ty(fcx.tcx(), substs))
  89             }
  90         }
  91         // We should really try to normalize here.
  92         ty::TyProjection(ref pi) => Some(UnsizeKind::OfProjection(pi)),
  93         ty::TyParam(ref p) => Some(UnsizeKind::OfParam(p)),
  94         _ => None
  95     }
  96 }
  97
  98 #[derive(Copy, Clone)]
  99 enum CastError {
 100     CastToBool,
 101     CastToChar,
 102     DifferingKinds,
 103     /// Cast of thin to fat raw ptr (eg. `*const () as *const [u8]`)
 104     SizedUnsizedCast,
 105     IllegalCast,
 106     NeedViaPtr,
 107     NeedViaThinPtr,
 108     NeedViaInt,
 109     NeedViaUsize,
 110     NonScalar,
 111 }
 112
 113 impl<'tcx> CastCheck<'tcx> {
 114     pub fn new(expr: &'tcx hir::Expr, expr_ty: Ty<'tcx>, cast_ty: Ty<'tcx>, span: Span)
 115                -> CastCheck<'tcx> {
 116         CastCheck {
 117             expr: expr,
 118             expr_ty: expr_ty,
 119             cast_ty: cast_ty,
 120             span: span,
 121         }
 122     }
 123
 124     fn report_cast_error<'a>(&self, fcx: &FnCtxt<'a, 'tcx>,
 125                              e: CastError) {
 126         match e {
 127             CastError::NeedViaPtr |
 128             CastError::NeedViaThinPtr |
 129             CastError::NeedViaInt |
 130             CastError::NeedViaUsize => {
 131                 fcx.type_error_struct(self.span, |actual| {
 132                     format!("casting `{}` as `{}` is invalid",
 133                             actual,
 134                             fcx.infcx().ty_to_string(self.cast_ty))
 135                 }, self.expr_ty, None)
 136                     .fileline_help(self.span,
 137                         &format!("cast through {} first", match e {
 138                             CastError::NeedViaPtr => "a raw pointer",
 139                             CastError::NeedViaThinPtr => "a thin pointer",
 140                             CastError::NeedViaInt => "an integer",
 141                             CastError::NeedViaUsize => "a usize",
 142                             _ => bug!()
 143                         }))
 144                     .emit();
 145             }
 146             CastError::CastToBool => {
 147                 struct_span_err!(fcx.tcx().sess, self.span, E0054, "cannot cast as `bool`")
 148                     .fileline_help(self.span, "compare with zero instead")
 149                     .emit();
 150             }
 151             CastError::CastToChar => {
 152                 fcx.type_error_message(self.span, |actual| {
 153                     format!("only `u8` can be cast as `char`, not `{}`", actual)
 154                 }, self.expr_ty, None);
 155             }
 156             CastError::NonScalar => {
 157                 fcx.type_error_message(self.span, |actual| {
 158                     format!("non-scalar cast: `{}` as `{}`",
 159                             actual,
 160                             fcx.infcx().ty_to_string(self.cast_ty))
 161                 }, self.expr_ty, None);
 162             }
 163             CastError::IllegalCast => {
 164                 fcx.type_error_message(self.span, |actual| {
 165                     format!("casting `{}` as `{}` is invalid",
 166                             actual,
 167                             fcx.infcx().ty_to_string(self.cast_ty))
 168                 }, self.expr_ty, None);
 169             }
 170             CastError::SizedUnsizedCast => {
 171                 fcx.type_error_message(self.span, |actual| {
 172                     format!("cannot cast thin pointer `{}` to fat pointer `{}`",
 173                             actual,
 174                             fcx.infcx().ty_to_string(self.cast_ty))
 175                 }, self.expr_ty, None)
 176             }
 177             CastError::DifferingKinds => {
 178                 fcx.type_error_struct(self.span, |actual| {
 179                     format!("casting `{}` as `{}` is invalid",
 180                             actual,
 181                             fcx.infcx().ty_to_string(self.cast_ty))
 182                 }, self.expr_ty, None)
 183                     .fileline_note(self.span, "vtable kinds may not match")
 184                     .emit();
 185             }
 186         }
 187     }
 188
 189     fn trivial_cast_lint<'a>(&self, fcx: &FnCtxt<'a, 'tcx>) {
 190         let t_cast = self.cast_ty;
 191         let t_expr = self.expr_ty;
 192         if t_cast.is_numeric() && t_expr.is_numeric() {
 193             fcx.tcx().sess.add_lint(lint::builtin::TRIVIAL_NUMERIC_CASTS,
 194                                     self.expr.id,
 195                                     self.span,
 196                                     format!("trivial numeric cast: `{}` as `{}`. Cast can be \
 197                                              replaced by coercion, this might require type \
 198                                              ascription or a temporary variable",
 199                                             fcx.infcx().ty_to_string(t_expr),
 200                                             fcx.infcx().ty_to_string(t_cast)));
 201         } else {
 202             fcx.tcx().sess.add_lint(lint::builtin::TRIVIAL_CASTS,
 203                                     self.expr.id,
 204                                     self.span,
 205                                     format!("trivial cast: `{}` as `{}`. Cast can be \
 206                                              replaced by coercion, this might require type \
 207                                              ascription or a temporary variable",
 208                                             fcx.infcx().ty_to_string(t_expr),
 209                                             fcx.infcx().ty_to_string(t_cast)));
 210         }
 211
 212     }
 213
 214     pub fn check<'a>(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 215         self.expr_ty = structurally_resolved_type(fcx, self.span, self.expr_ty);
 216         self.cast_ty = structurally_resolved_type(fcx, self.span, self.cast_ty);
 217
 218         debug!("check_cast({}, {:?} as {:?})", self.expr.id, self.expr_ty,
 219                self.cast_ty);
 220
 221         if self.expr_ty.references_error() || self.cast_ty.references_error() {
 222             // No sense in giving duplicate error messages
 223         } else if self.try_coercion_cast(fcx) {
 224             self.trivial_cast_lint(fcx);
 225             debug!(" -> CoercionCast");
 226             fcx.tcx().cast_kinds.borrow_mut().insert(self.expr.id,
 227                                                      CastKind::CoercionCast);
 228         } else { match self.do_check(fcx) {
 229             Ok(k) => {
 230                 debug!(" -> {:?}", k);
 231                 fcx.tcx().cast_kinds.borrow_mut().insert(self.expr.id, k);
 232             }
 233             Err(e) => self.report_cast_error(fcx, e)
 234         };}
 235     }
 236
 237     /// Check a cast, and report an error if one exists. In some cases, this
 238     /// can return Ok and create type errors in the fcx rather than returning
 239     /// directly. coercion-cast is handled in check instead of here.
 240     fn do_check<'a>(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
 241         use rustc::ty::cast::IntTy::*;
 242         use rustc::ty::cast::CastTy::*;
 243
 244         let (t_from, t_cast) = match (CastTy::from_ty(self.expr_ty),
 245                                       CastTy::from_ty(self.cast_ty)) {
 246             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 247             // Function item types may need to be reified before casts.
 248             (None, Some(t_cast)) => {
 249                 if let ty::TyFnDef(_, _, f) = self.expr_ty.sty {
 250                     // Attempt a coercion to a fn pointer type.
 251                     let res = coercion::try(fcx, self.expr,
 252                                             fcx.tcx().mk_ty(ty::TyFnPtr(f)));
 253                     if !res.is_ok() {
 254                         return Err(CastError::NonScalar);
 255                     }
 256                     (FnPtr, t_cast)
 257                 } else {
 258                     return Err(CastError::NonScalar);
 259                 }
 260             }
 261             _ => {
 262                 return Err(CastError::NonScalar)
 263             }
 264         };
 265
 266         match (t_from, t_cast) {
 267             // These types have invariants! can't cast into them.
 268             (_, RPtr(_)) | (_, Int(CEnum)) | (_, FnPtr) => Err(CastError::NonScalar),
 269
 270             // * -> Bool
 271             (_, Int(Bool)) => Err(CastError::CastToBool),
 272
 273             // * -> Char
 274             (Int(U(ast::UintTy::U8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 275             (_, Int(Char)) => Err(CastError::CastToChar),
 276
 277             // prim -> float,ptr
 278             (Int(Bool), Float) | (Int(CEnum), Float) | (Int(Char), Float)
 279                 => Err(CastError::NeedViaInt),
 280             (Int(Bool), Ptr(_)) | (Int(CEnum), Ptr(_)) | (Int(Char), Ptr(_))
 281                 => Err(CastError::NeedViaUsize),
 282
 283             // ptr -> *
 284             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 285             (Ptr(m_expr), Int(_)) => self.check_ptr_addr_cast(fcx, m_expr), // ptr-addr-cast
 286             (Ptr(_), Float) | (FnPtr, Float) => Err(CastError::NeedViaUsize),
 287             (FnPtr, Int(_)) => Ok(CastKind::FnPtrAddrCast),
 288             (RPtr(_), Int(_)) | (RPtr(_), Float) => Err(CastError::NeedViaPtr),
 289             // * -> ptr
 290             (Int(_), Ptr(mt)) => self.check_addr_ptr_cast(fcx, mt), // addr-ptr-cast
 291             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 292             (Float, Ptr(_)) => Err(CastError::NeedViaUsize),
 293             (RPtr(rmt), Ptr(mt)) => self.check_ref_cast(fcx, rmt, mt), // array-ptr-cast
 294
 295             // prim -> prim
 296             (Int(CEnum), Int(_)) => Ok(CastKind::EnumCast),
 297             (Int(Char), Int(_)) | (Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 298
 299             (Int(_), Int(_)) |
 300             (Int(_), Float) |
 301             (Float, Int(_)) |
 302             (Float, Float) => Ok(CastKind::NumericCast),
 303
 304         }
 305     }
 306
 307     fn check_ptr_ptr_cast<'a>(&self,
 308                               fcx: &FnCtxt<'a, 'tcx>,
 309                               m_expr: &'tcx ty::TypeAndMut<'tcx>,
 310                               m_cast: &'tcx ty::TypeAndMut<'tcx>)
 311                               -> Result<CastKind, CastError>
 312     {
 313         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}",
 314                m_expr, m_cast);
 315         // ptr-ptr cast. vtables must match.
 316
 317         // Cast to sized is OK
 318         if fcx.type_is_known_to_be_sized(m_cast.ty, self.span) {
 319             return Ok(CastKind::PtrPtrCast);
 320         }
 321
 322         // sized -> unsized? report invalid cast (don't complain about vtable kinds)
 323         if fcx.type_is_known_to_be_sized(m_expr.ty, self.span) {
 324             return Err(CastError::SizedUnsizedCast);
 325         }
 326
 327         // vtable kinds must match
 328         match (unsize_kind(fcx, m_cast.ty), unsize_kind(fcx, m_expr.ty)) {
 329             (Some(a), Some(b)) if a == b => Ok(CastKind::PtrPtrCast),
 330             _ => Err(CastError::DifferingKinds)
 331         }
 332     }
 333
 334     fn check_fptr_ptr_cast<'a>(&self,
 335                                fcx: &FnCtxt<'a, 'tcx>,
 336                                m_cast: &'tcx ty::TypeAndMut<'tcx>)
 337                                -> Result<CastKind, CastError>
 338     {
 339         // fptr-ptr cast. must be to sized ptr
 340
 341         if fcx.type_is_known_to_be_sized(m_cast.ty, self.span) {
 342             Ok(CastKind::FnPtrPtrCast)
 343         } else {
 344             Err(CastError::IllegalCast)
 345         }
 346     }
 347
 348     fn check_ptr_addr_cast<'a>(&self,
 349                                fcx: &FnCtxt<'a, 'tcx>,
 350                                m_expr: &'tcx ty::TypeAndMut<'tcx>)
 351                                -> Result<CastKind, CastError>
 352     {
 353         // ptr-addr cast. must be from sized ptr
 354
 355         if fcx.type_is_known_to_be_sized(m_expr.ty, self.span) {
 356             Ok(CastKind::PtrAddrCast)
 357         } else {
 358             Err(CastError::NeedViaThinPtr)
 359         }
 360     }
 361
 362     fn check_ref_cast<'a>(&self,
 363                           fcx: &FnCtxt<'a, 'tcx>,
 364                           m_expr: &'tcx ty::TypeAndMut<'tcx>,
 365                           m_cast: &'tcx ty::TypeAndMut<'tcx>)
 366                           -> Result<CastKind, CastError>
 367     {
 368         // array-ptr-cast.
 369
 370         if m_expr.mutbl == hir::MutImmutable && m_cast.mutbl == hir::MutImmutable {
 371             if let ty::TyArray(ety, _) = m_expr.ty.sty {
 372                 // Due to the limitations of LLVM global constants,
 373                 // region pointers end up pointing at copies of
 374                 // vector elements instead of the original values.
 375                 // To allow raw pointers to work correctly, we
 376                 // need to special-case obtaining a raw pointer
 377                 // from a region pointer to a vector.
 378
 379                 // this will report a type mismatch if needed
 380                 demand::eqtype(fcx, self.span, ety, m_cast.ty);
 381                 return Ok(CastKind::ArrayPtrCast);
 382             }
 383         }
 384
 385         Err(CastError::IllegalCast)
 386     }
 387
 388     fn check_addr_ptr_cast<'a>(&self,
 389                                fcx: &FnCtxt<'a, 'tcx>,
 390                                m_cast: &'tcx ty::TypeAndMut<'tcx>)
 391                                -> Result<CastKind, CastError>
 392     {
 393         // ptr-addr cast. pointer must be thin.
 394         if fcx.type_is_known_to_be_sized(m_cast.ty, self.span) {
 395            Ok(CastKind::AddrPtrCast)
 396         } else {
 397            Err(CastError::IllegalCast)
 398         }
 399     }
 400
 401     fn try_coercion_cast<'a>(&self, fcx: &FnCtxt<'a, 'tcx>) -> bool {
 402         coercion::try(fcx, self.expr, self.cast_ty).is_ok()
 403     }
 404
 405 }