src/librustc/middle/intrinsicck.rs

   1 // Copyright 2012-2014 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 use ast_map::NodeForeignItem;
  12 use metadata::csearch;
  13 use middle::def::DefFn;
  14 use middle::subst::{Subst, Substs, EnumeratedItems};
  15 use middle::ty::{TransmuteRestriction, ctxt, TyBareFn};
  16 use middle::ty::{self, Ty};
  17
  18 use std::fmt;
  19
  20 use syntax::abi::RustIntrinsic;
  21 use syntax::ast::DefId;
  22 use syntax::ast;
  23 use syntax::codemap::Span;
  24 use syntax::parse::token;
  25 use syntax::visit::Visitor;
  26 use syntax::visit;
  27
  28 pub fn check_crate(tcx: &ctxt) {
  29     let mut visitor = IntrinsicCheckingVisitor {
  30         tcx: tcx,
  31         param_envs: Vec::new(),
  32         dummy_sized_ty: tcx.types.isize,
  33         dummy_unsized_ty: ty::mk_vec(tcx, tcx.types.isize, None),
  34     };
  35     visit::walk_crate(&mut visitor, tcx.map.krate());
  36 }
  37
  38 struct IntrinsicCheckingVisitor<'a, 'tcx: 'a> {
  39     tcx: &'a ctxt<'tcx>,
  40
  41     // As we traverse the AST, we keep a stack of the parameter
  42     // environments for each function we encounter. When we find a
  43     // call to `transmute`, we can check it in the context of the top
  44     // of the stack (which ought not to be empty).
  45     param_envs: Vec<ty::ParameterEnvironment<'a,'tcx>>,
  46
  47     // Dummy sized/unsized types that use to substitute for type
  48     // parameters in order to estimate how big a type will be for any
  49     // possible instantiation of the type parameters in scope.  See
  50     // `check_transmute` for more details.
  51     dummy_sized_ty: Ty<'tcx>,
  52     dummy_unsized_ty: Ty<'tcx>,
  53 }
  54
  55 impl<'a, 'tcx> IntrinsicCheckingVisitor<'a, 'tcx> {
  56     fn def_id_is_transmute(&self, def_id: DefId) -> bool {
  57         let intrinsic = match ty::lookup_item_type(self.tcx, def_id).ty.sty {
  58             ty::TyBareFn(_, ref bfty) => bfty.abi == RustIntrinsic,
  59             _ => return false
  60         };
  61         if def_id.krate == ast::LOCAL_CRATE {
  62             match self.tcx.map.get(def_id.node) {
  63                 NodeForeignItem(ref item) if intrinsic => {
  64                     token::get_ident(item.ident) ==
  65                         token::intern_and_get_ident("transmute")
  66                 }
  67                 _ => false,
  68             }
  69         } else {
  70             match csearch::get_item_path(self.tcx, def_id).last() {
  71                 Some(ref last) if intrinsic => {
  72                     token::get_name(last.name()) ==
  73                         token::intern_and_get_ident("transmute")
  74                 }
  75                 _ => false,
  76             }
  77         }
  78     }
  79
  80     fn check_transmute(&self, span: Span, from: Ty<'tcx>, to: Ty<'tcx>, id: ast::NodeId) {
  81         // Find the parameter environment for the most recent function that
  82         // we entered.
  83
  84         let param_env = match self.param_envs.last() {
  85             Some(p) => p,
  86             None => {
  87                 self.tcx.sess.span_bug(
  88                     span,
  89                     "transmute encountered outside of any fn");
  90             }
  91         };
  92
  93         // Simple case: no type parameters involved.
  94         if
  95             !ty::type_has_params(from) && !ty::type_has_self(from) &&
  96             !ty::type_has_params(to) && !ty::type_has_self(to)
  97         {
  98             let restriction = TransmuteRestriction {
  99                 span: span,
 100                 original_from: from,
 101                 original_to: to,
 102                 substituted_from: from,
 103                 substituted_to: to,
 104                 id: id,
 105             };
 106             self.push_transmute_restriction(restriction);
 107             return;
 108         }
 109
 110         // The rules around type parameters are a bit subtle. We are
 111         // checking these rules before monomorphization, so there may
 112         // be unsubstituted type parameters present in the
 113         // types. Obviously we cannot create LLVM types for those.
 114         // However, if a type parameter appears only indirectly (i.e.,
 115         // through a pointer), it does not necessarily affect the
 116         // size, so that should be allowed. The only catch is that we
 117         // DO want to be careful around unsized type parameters, since
 118         // fat pointers have a different size than a thin pointer, and
 119         // hence `&T` and `&U` have different sizes if `T : Sized` but
 120         // `U : Sized` does not hold.
 121         //
 122         // However, it's not as simple as checking whether `T :
 123         // Sized`, because even if `T : Sized` does not hold, that
 124         // just means that `T` *may* not be sized.  After all, even a
 125         // type parameter `T: ?Sized` could be bound to a sized
 126         // type. (Issue #20116)
 127         //
 128         // To handle this, we first check for "interior" type
 129         // parameters, which are always illegal. If there are none of
 130         // those, then we know that the only way that all type
 131         // parameters `T` are referenced indirectly, e.g. via a
 132         // pointer type like `&T`. In that case, we only care whether
 133         // `T` is sized or not, because that influences whether `&T`
 134         // is a thin or fat pointer.
 135         //
 136         // One could imagine establishing a sophisticated constraint
 137         // system to ensure that the transmute is legal, but instead
 138         // we do something brutally dumb. We just substitute dummy
 139         // sized or unsized types for every type parameter in scope,
 140         // exhaustively checking all possible combinations. Here are some examples:
 141         //
 142         // ```
 143         // fn foo<T, U>() {
 144         //     // T=int, U=int
 145         // }
 146         //
 147         // fn bar<T: ?Sized, U>() {
 148         //     // T=int, U=int
 149         //     // T=[int], U=int
 150         // }
 151         //
 152         // fn baz<T: ?Sized, U: ?Sized>() {
 153         //     // T=int, U=int
 154         //     // T=[int], U=int
 155         //     // T=int, U=[int]
 156         //     // T=[int], U=[int]
 157         // }
 158         // ```
 159         //
 160         // In all cases, we keep the original unsubstituted types
 161         // around for error reporting.
 162
 163         let from_tc = ty::type_contents(self.tcx, from);
 164         let to_tc = ty::type_contents(self.tcx, to);
 165         if from_tc.interior_param() || to_tc.interior_param() {
 166             span_err!(self.tcx.sess, span, E0139,
 167                       "cannot transmute to or from a type that contains \
 168                        type parameters in its interior");
 169             return;
 170         }
 171
 172         let mut substs = param_env.free_substs.clone();
 173         self.with_each_combination(
 174             span,
 175             param_env,
 176             param_env.free_substs.types.iter_enumerated(),
 177             &mut substs,
 178             &mut |substs| {
 179                 let restriction = TransmuteRestriction {
 180                     span: span,
 181                     original_from: from,
 182                     original_to: to,
 183                     substituted_from: from.subst(self.tcx, substs),
 184                     substituted_to: to.subst(self.tcx, substs),
 185                     id: id,
 186                 };
 187                 self.push_transmute_restriction(restriction);
 188             });
 189     }
 190
 191     fn with_each_combination(&self,
 192                              span: Span,
 193                              param_env: &ty::ParameterEnvironment<'a,'tcx>,
 194                              mut types_in_scope: EnumeratedItems<Ty<'tcx>>,
 195                              substs: &mut Substs<'tcx>,
 196                              callback: &mut FnMut(&Substs<'tcx>))
 197     {
 198         // This parameter invokes `callback` many times with different
 199         // substitutions that replace all the parameters in scope with
 200         // either `int` or `[int]`, depending on whether the type
 201         // parameter is known to be sized. See big comment above for
 202         // an explanation of why this is a reasonable thing to do.
 203
 204         match types_in_scope.next() {
 205             None => {
 206                 debug!("with_each_combination(substs={:?})",
 207                        substs);
 208
 209                 callback(substs);
 210             }
 211
 212             Some((space, index, &param_ty)) => {
 213                 debug!("with_each_combination: space={:?}, index={}, param_ty={:?}",
 214                        space, index, param_ty);
 215
 216                 if !ty::type_is_sized(Some(param_env), self.tcx, span, param_ty) {
 217                     debug!("with_each_combination: param_ty is not known to be sized");
 218
 219                     substs.types.get_mut_slice(space)[index] = self.dummy_unsized_ty;
 220                     self.with_each_combination(span, param_env, types_in_scope.clone(),
 221                                                substs, callback);
 222                 }
 223
 224                 substs.types.get_mut_slice(space)[index] = self.dummy_sized_ty;
 225                 self.with_each_combination(span, param_env, types_in_scope,
 226                                            substs, callback);
 227             }
 228         }
 229     }
 230
 231     fn push_transmute_restriction(&self, restriction: TransmuteRestriction<'tcx>) {
 232         debug!("Pushing transmute restriction: {:?}", restriction);
 233         self.tcx.transmute_restrictions.borrow_mut().push(restriction);
 234     }
 235 }
 236
 237 impl<'a, 'tcx, 'v> Visitor<'v> for IntrinsicCheckingVisitor<'a, 'tcx> {
 238     fn visit_fn(&mut self, fk: visit::FnKind<'v>, fd: &'v ast::FnDecl,
 239                 b: &'v ast::Block, s: Span, id: ast::NodeId) {
 240         match fk {
 241             visit::FkItemFn(..) | visit::FkMethod(..) => {
 242                 let param_env = ty::ParameterEnvironment::for_item(self.tcx, id);
 243                 self.param_envs.push(param_env);
 244                 visit::walk_fn(self, fk, fd, b, s);
 245                 self.param_envs.pop();
 246             }
 247             visit::FkFnBlock(..) => {
 248                 visit::walk_fn(self, fk, fd, b, s);
 249             }
 250         }
 251
 252     }
 253
 254     fn visit_expr(&mut self, expr: &ast::Expr) {
 255         if let ast::ExprPath(..) = expr.node {
 256             match ty::resolve_expr(self.tcx, expr) {
 257                 DefFn(did, _) if self.def_id_is_transmute(did) => {
 258                     let typ = ty::node_id_to_type(self.tcx, expr.id);
 259                     match typ.sty {
 260                         TyBareFn(_, ref bare_fn_ty) if bare_fn_ty.abi == RustIntrinsic => {
 261                             if let ty::FnConverging(to) = bare_fn_ty.sig.0.output {
 262                                 let from = bare_fn_ty.sig.0.inputs[0];
 263                                 self.check_transmute(expr.span, from, to, expr.id);
 264                             }
 265                         }
 266                         _ => {
 267                             self.tcx
 268                                 .sess
 269                                 .span_bug(expr.span, "transmute wasn't a bare fn?!");
 270                         }
 271                     }
 272                 }
 273                 _ => {}
 274             }
 275         }
 276
 277         visit::walk_expr(self, expr);
 278     }
 279 }
 280
 281 impl<'tcx> fmt::Debug for TransmuteRestriction<'tcx> {
 282     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 283         write!(f, "TransmuteRestriction(id={}, original=({:?},{:?}), substituted=({:?},{:?}))",
 284                self.id,
 285                self.original_from,
 286                self.original_to,
 287                self.substituted_from,
 288                self.substituted_to)
 289     }
 290 }