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 middle::cast::{CastKind, CastTy};
  48 use middle::ty;
  49 use middle::ty::Ty;
  50 use syntax::ast;
  51 use syntax::ast::UintTy::{TyU8};
  52 use syntax::codemap::Span;
  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: ast::Expr,
  58     expr_ty: Ty<'tcx>,
  59     cast_ty: Ty<'tcx>,
  60     span: Span,
  61 }
  62
  63 /// The kind of the unsize info (length or vtable) - we only allow casts between
  64 /// fat pointers if their unsize-infos have the same kind.
  65 #[derive(Copy, Clone, PartialEq, Eq)]
  66 enum UnsizeKind<'tcx> {
  67     Vtable(ast::DefId),
  68     Length,
  69     /// The unsize info of this projection
  70     OfProjection(&'tcx ty::ProjectionTy<'tcx>),
  71     /// The unsize info of this parameter
  72     OfParam(&'tcx ty::ParamTy)
  73 }
  74
  75 /// Returns the kind of unsize information of t, or None
  76 /// if t is sized or it is unknown.
  77 fn unsize_kind<'a,'tcx>(fcx: &FnCtxt<'a, 'tcx>,
  78                         t: Ty<'tcx>)
  79                         -> Option<UnsizeKind<'tcx>> {
  80     match t.sty {
  81         ty::TySlice(_) | ty::TyStr => Some(UnsizeKind::Length),
  82         ty::TyTrait(ref tty) => Some(UnsizeKind::Vtable(tty.principal_def_id())),
  83         ty::TyStruct(did, substs) => {
  84             match ty::struct_fields(fcx.tcx(), did, substs).pop() {
  85                 None => None,
  86                 Some(f) => unsize_kind(fcx, f.mt.ty)
  87             }
  88         }
  89         // We should really try to normalize here.
  90         ty::TyProjection(ref pi) => Some(UnsizeKind::OfProjection(pi)),
  91         ty::TyParam(ref p) => Some(UnsizeKind::OfParam(p)),
  92         _ => None
  93     }
  94 }
  95
  96 #[derive(Copy, Clone)]
  97 enum CastError {
  98     CastToBool,
  99     CastToChar,
 100     DifferingKinds,
 101     IllegalCast,
 102     NeedViaPtr,
 103     NeedViaInt,
 104     NeedViaUsize,
 105     NonScalar,
 106 }
 107
 108 impl<'tcx> CastCheck<'tcx> {
 109     pub fn new(expr: ast::Expr, expr_ty: Ty<'tcx>, cast_ty: Ty<'tcx>, span: Span)
 110                -> CastCheck<'tcx> {
 111         CastCheck {
 112             expr: expr,
 113             expr_ty: expr_ty,
 114             cast_ty: cast_ty,
 115             span: span,
 116         }
 117     }
 118
 119     fn report_cast_error<'a>(&self, fcx: &FnCtxt<'a, 'tcx>,
 120                              e: CastError) {
 121         match e {
 122             CastError::NeedViaPtr |
 123             CastError::NeedViaInt |
 124             CastError::NeedViaUsize => {
 125                 fcx.type_error_message(self.span, |actual| {
 126                     format!("illegal cast; cast through {} first: `{}` as `{}`",
 127                             match e {
 128                                 CastError::NeedViaPtr => "a raw pointer",
 129                                 CastError::NeedViaInt => "an integer",
 130                                 CastError::NeedViaUsize => "a usize",
 131                                 _ => unreachable!()
 132                             },
 133                             actual,
 134                             fcx.infcx().ty_to_string(self.cast_ty))
 135                 }, self.expr_ty, None)
 136             }
 137             CastError::CastToBool => {
 138                 span_err!(fcx.tcx().sess, self.span, E0054,
 139                           "cannot cast as `bool`, compare with zero instead");
 140             }
 141             CastError::CastToChar => {
 142                 fcx.type_error_message(self.span, |actual| {
 143                     format!("only `u8` can be cast as `char`, not `{}`", actual)
 144                 }, self.expr_ty, None);
 145             }
 146             CastError::NonScalar => {
 147                 fcx.type_error_message(self.span, |actual| {
 148                     format!("non-scalar cast: `{}` as `{}`",
 149                             actual,
 150                             fcx.infcx().ty_to_string(self.cast_ty))
 151                 }, self.expr_ty, None);
 152             }
 153             CastError::IllegalCast => {
 154                 fcx.type_error_message(self.span, |actual| {
 155                     format!("illegal cast: `{}` as `{}`",
 156                             actual,
 157                             fcx.infcx().ty_to_string(self.cast_ty))
 158                 }, self.expr_ty, None);
 159             }
 160             CastError::DifferingKinds => {
 161                 fcx.type_error_message(self.span, |actual| {
 162                     format!("illegal cast: `{}` as `{}`; vtable kinds may not match",
 163                             actual,
 164                             fcx.infcx().ty_to_string(self.cast_ty))
 165                 }, self.expr_ty, None);
 166             }
 167         }
 168     }
 169
 170     fn trivial_cast_lint<'a>(&self, fcx: &FnCtxt<'a, 'tcx>) {
 171         let t_cast = self.cast_ty;
 172         let t_expr = self.expr_ty;
 173         if ty::type_is_numeric(t_cast) && ty::type_is_numeric(t_expr) {
 174             fcx.tcx().sess.add_lint(lint::builtin::TRIVIAL_NUMERIC_CASTS,
 175                                     self.expr.id,
 176                                     self.span,
 177                                     format!("trivial numeric cast: `{}` as `{}`. Cast can be \
 178                                              replaced by coercion, this might require type \
 179                                              ascription or a temporary variable",
 180                                             fcx.infcx().ty_to_string(t_expr),
 181                                             fcx.infcx().ty_to_string(t_cast)));
 182         } else {
 183             fcx.tcx().sess.add_lint(lint::builtin::TRIVIAL_CASTS,
 184                                     self.expr.id,
 185                                     self.span,
 186                                     format!("trivial cast: `{}` as `{}`. Cast can be \
 187                                              replaced by coercion, this might require type \
 188                                              ascription or a temporary variable",
 189                                             fcx.infcx().ty_to_string(t_expr),
 190                                             fcx.infcx().ty_to_string(t_cast)));
 191         }
 192
 193     }
 194
 195     pub fn check<'a>(mut self, fcx: &FnCtxt<'a, 'tcx>) {
 196         self.expr_ty = structurally_resolved_type(fcx, self.span, self.expr_ty);
 197         self.cast_ty = structurally_resolved_type(fcx, self.span, self.cast_ty);
 198
 199         debug!("check_cast({}, {:?} as {:?})", self.expr.id, self.expr_ty,
 200                self.cast_ty);
 201
 202         if ty::type_is_error(self.expr_ty) || ty::type_is_error(self.cast_ty) {
 203             // No sense in giving duplicate error messages
 204         } else if self.try_coercion_cast(fcx) {
 205             self.trivial_cast_lint(fcx);
 206             debug!(" -> CoercionCast");
 207             fcx.tcx().cast_kinds.borrow_mut().insert(self.expr.id,
 208                                                      CastKind::CoercionCast);
 209         } else { match self.do_check(fcx) {
 210             Ok(k) => {
 211                 debug!(" -> {:?}", k);
 212                 fcx.tcx().cast_kinds.borrow_mut().insert(self.expr.id, k);
 213             }
 214             Err(e) => self.report_cast_error(fcx, e)
 215         };}
 216     }
 217
 218     /// Check a cast, and report an error if one exists. In some cases, this
 219     /// can return Ok and create type errors in the fcx rather than returning
 220     /// directly. coercion-cast is handled in check instead of here.
 221     fn do_check<'a>(&self, fcx: &FnCtxt<'a, 'tcx>) -> Result<CastKind, CastError> {
 222         use middle::cast::IntTy::*;
 223         use middle::cast::CastTy::*;
 224
 225         let (t_from, t_cast) = match (CastTy::from_ty(fcx.tcx(), self.expr_ty),
 226                                       CastTy::from_ty(fcx.tcx(), self.cast_ty)) {
 227             (Some(t_from), Some(t_cast)) => (t_from, t_cast),
 228             _ => {
 229                 return Err(CastError::NonScalar)
 230             }
 231         };
 232
 233         match (t_from, t_cast) {
 234             // These types have invariants! can't cast into them.
 235             (_, RPtr(_)) | (_, Int(CEnum)) | (_, FnPtr) => Err(CastError::NonScalar),
 236
 237             // * -> Bool
 238             (_, Int(Bool)) => Err(CastError::CastToBool),
 239
 240             // * -> Char
 241             (Int(U(ast::TyU8)), Int(Char)) => Ok(CastKind::U8CharCast), // u8-char-cast
 242             (_, Int(Char)) => Err(CastError::CastToChar),
 243
 244             // prim -> float,ptr
 245             (Int(Bool), Float) | (Int(CEnum), Float) | (Int(Char), Float)
 246                 => Err(CastError::NeedViaInt),
 247             (Int(Bool), Ptr(_)) | (Int(CEnum), Ptr(_)) | (Int(Char), Ptr(_))
 248                 => Err(CastError::NeedViaUsize),
 249
 250             // ptr -> *
 251             (Ptr(m_e), Ptr(m_c)) => self.check_ptr_ptr_cast(fcx, m_e, m_c), // ptr-ptr-cast
 252             (Ptr(m_expr), Int(_)) => self.check_ptr_addr_cast(fcx, m_expr), // ptr-addr-cast
 253             (Ptr(_), Float) | (FnPtr, Float) => Err(CastError::NeedViaUsize),
 254             (FnPtr, Int(_)) => Ok(CastKind::FnPtrAddrCast),
 255             (RPtr(_), Int(_)) | (RPtr(_), Float) => Err(CastError::NeedViaPtr),
 256             // * -> ptr
 257             (Int(_), Ptr(mt)) => self.check_addr_ptr_cast(fcx, mt), // addr-ptr-cast
 258             (FnPtr, Ptr(mt)) => self.check_fptr_ptr_cast(fcx, mt),
 259             (Float, Ptr(_)) => Err(CastError::NeedViaUsize),
 260             (RPtr(rmt), Ptr(mt)) => self.check_ref_cast(fcx, rmt, mt), // array-ptr-cast
 261
 262             // prim -> prim
 263             (Int(CEnum), Int(_)) => Ok(CastKind::EnumCast),
 264             (Int(Char), Int(_)) | (Int(Bool), Int(_)) => Ok(CastKind::PrimIntCast),
 265
 266             (Int(_), Int(_)) |
 267             (Int(_), Float) |
 268             (Float, Int(_)) |
 269             (Float, Float) => Ok(CastKind::NumericCast),
 270
 271         }
 272     }
 273
 274     fn check_ptr_ptr_cast<'a>(&self,
 275                               fcx: &FnCtxt<'a, 'tcx>,
 276                               m_expr: &'tcx ty::mt<'tcx>,
 277                               m_cast: &'tcx ty::mt<'tcx>)
 278                               -> Result<CastKind, CastError>
 279     {
 280         debug!("check_ptr_ptr_cast m_expr={:?} m_cast={:?}",
 281                m_expr, m_cast);
 282         // ptr-ptr cast. vtables must match.
 283
 284         // Cast to sized is OK
 285         if fcx.type_is_known_to_be_sized(m_cast.ty, self.span) {
 286             return Ok(CastKind::PtrPtrCast);
 287         }
 288
 289         // sized -> unsized? report illegal cast (don't complain about vtable kinds)
 290         if fcx.type_is_known_to_be_sized(m_expr.ty, self.span) {
 291             return Err(CastError::IllegalCast);
 292         }
 293
 294         // vtable kinds must match
 295         match (unsize_kind(fcx, m_cast.ty), unsize_kind(fcx, m_expr.ty)) {
 296             (Some(a), Some(b)) if a == b => Ok(CastKind::PtrPtrCast),
 297             _ => Err(CastError::DifferingKinds)
 298         }
 299     }
 300
 301     fn check_fptr_ptr_cast<'a>(&self,
 302                                fcx: &FnCtxt<'a, 'tcx>,
 303                                m_cast: &'tcx ty::mt<'tcx>)
 304                                -> Result<CastKind, CastError>
 305     {
 306         // fptr-ptr cast. must be to sized ptr
 307
 308         if fcx.type_is_known_to_be_sized(m_cast.ty, self.span) {
 309             Ok(CastKind::FnPtrPtrCast)
 310         } else {
 311             Err(CastError::IllegalCast)
 312         }
 313     }
 314
 315     fn check_ptr_addr_cast<'a>(&self,
 316                                fcx: &FnCtxt<'a, 'tcx>,
 317                                m_expr: &'tcx ty::mt<'tcx>)
 318                                -> Result<CastKind, CastError>
 319     {
 320         // ptr-addr cast. must be from sized ptr
 321
 322         if fcx.type_is_known_to_be_sized(m_expr.ty, self.span) {
 323             Ok(CastKind::PtrAddrCast)
 324         } else {
 325             Err(CastError::NeedViaPtr)
 326         }
 327     }
 328
 329     fn check_ref_cast<'a>(&self,
 330                           fcx: &FnCtxt<'a, 'tcx>,
 331                           m_expr: &'tcx ty::mt<'tcx>,
 332                           m_cast: &'tcx ty::mt<'tcx>)
 333                           -> Result<CastKind, CastError>
 334     {
 335         // array-ptr-cast.
 336
 337         if m_expr.mutbl == ast::MutImmutable && m_cast.mutbl == ast::MutImmutable {
 338             if let ty::TyArray(ety, _) = m_expr.ty.sty {
 339                 // Due to the limitations of LLVM global constants,
 340                 // region pointers end up pointing at copies of
 341                 // vector elements instead of the original values.
 342                 // To allow raw pointers to work correctly, we
 343                 // need to special-case obtaining a raw pointer
 344                 // from a region pointer to a vector.
 345
 346                 // this will report a type mismatch if needed
 347                 demand::eqtype(fcx, self.span, ety, m_cast.ty);
 348                 return Ok(CastKind::ArrayPtrCast);
 349             }
 350         }
 351
 352         Err(CastError::IllegalCast)
 353     }
 354
 355     fn check_addr_ptr_cast<'a>(&self,
 356                                fcx: &FnCtxt<'a, 'tcx>,
 357                                m_cast: &'tcx ty::mt<'tcx>)
 358                                -> Result<CastKind, CastError>
 359     {
 360         // ptr-addr cast. pointer must be thin.
 361         if fcx.type_is_known_to_be_sized(m_cast.ty, self.span) {
 362            Ok(CastKind::AddrPtrCast)
 363         } else {
 364            Err(CastError::IllegalCast)
 365         }
 366     }
 367
 368     fn try_coercion_cast<'a>(&self, fcx: &FnCtxt<'a, 'tcx>) -> bool {
 369         if let Ok(()) = coercion::mk_assignty(fcx,
 370                                               &self.expr,
 371                                               self.expr_ty,
 372                                               self.cast_ty) {
 373             true
 374         } else {
 375             false
 376         }
 377     }
 378
 379 }