src/librustc_typeck/check/vtable.rs

   1 // Copyright 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 check::{FnCtxt, structurally_resolved_type};
  12 use middle::traits::{mod, ObjectSafetyViolation, MethodViolationCode};
  13 use middle::traits::{Obligation, ObligationCause};
  14 use middle::traits::report_fulfillment_errors;
  15 use middle::ty::{mod, Ty, AsPredicate};
  16 use middle::infer;
  17 use syntax::ast;
  18 use syntax::codemap::Span;
  19 use util::nodemap::FnvHashSet;
  20 use util::ppaux::{Repr, UserString};
  21
  22 pub fn check_object_cast<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
  23                                    cast_expr: &ast::Expr,
  24                                    source_expr: &ast::Expr,
  25                                    target_object_ty: Ty<'tcx>)
  26 {
  27     debug!("check_object_cast(cast_expr={}, target_object_ty={})",
  28            cast_expr.repr(fcx.tcx()),
  29            target_object_ty.repr(fcx.tcx()));
  30
  31     // Look up vtables for the type we're casting to,
  32     // passing in the source and target type.  The source
  33     // must be a pointer type suitable to the object sigil,
  34     // e.g.: `&x as &Trait` or `box x as Box<Trait>`
  35     let source_ty = fcx.expr_ty(source_expr);
  36     let source_ty = structurally_resolved_type(fcx, source_expr.span, source_ty);
  37     debug!("source_ty={}", source_ty.repr(fcx.tcx()));
  38     match (&source_ty.sty, &target_object_ty.sty) {
  39         (&ty::ty_uniq(referent_ty), &ty::ty_uniq(object_trait_ty)) => {
  40             let object_trait = object_trait(&object_trait_ty);
  41
  42             // Ensure that if ~T is cast to ~Trait, then T : Trait
  43             push_cast_obligation(fcx, cast_expr, object_trait, referent_ty);
  44             check_object_safety(fcx.tcx(), object_trait, source_expr.span);
  45         }
  46
  47         (&ty::ty_rptr(referent_region, ty::mt { ty: referent_ty,
  48                                                 mutbl: referent_mutbl }),
  49          &ty::ty_rptr(target_region, ty::mt { ty: object_trait_ty,
  50                                               mutbl: target_mutbl })) =>
  51         {
  52             let object_trait = object_trait(&object_trait_ty);
  53             if !mutability_allowed(referent_mutbl, target_mutbl) {
  54                 fcx.tcx().sess.span_err(source_expr.span,
  55                                         "types differ in mutability");
  56             } else {
  57                 // Ensure that if &'a T is cast to &'b Trait, then T : Trait
  58                 push_cast_obligation(fcx, cast_expr,
  59                                      object_trait,
  60                                      referent_ty);
  61
  62                 // Ensure that if &'a T is cast to &'b Trait, then 'b <= 'a
  63                 infer::mk_subr(fcx.infcx(),
  64                                infer::RelateObjectBound(source_expr.span),
  65                                *target_region,
  66                                *referent_region);
  67
  68                 check_object_safety(fcx.tcx(), object_trait, source_expr.span);
  69             }
  70         }
  71
  72         (_, &ty::ty_uniq(..)) => {
  73             fcx.ccx.tcx.sess.span_err(
  74                 source_expr.span,
  75                 format!("can only cast an boxed pointer \
  76                          to a boxed object, not a {}",
  77                         ty::ty_sort_string(fcx.tcx(), source_ty))[]);
  78         }
  79
  80         (_, &ty::ty_rptr(..)) => {
  81             fcx.ccx.tcx.sess.span_err(
  82                 source_expr.span,
  83                 format!("can only cast a &-pointer \
  84                          to an &-object, not a {}",
  85                         ty::ty_sort_string(fcx.tcx(), source_ty))[]);
  86         }
  87
  88         _ => {
  89             fcx.tcx().sess.span_bug(
  90                 source_expr.span,
  91                 "expected object type");
  92         }
  93     }
  94
  95     fn object_trait<'a, 'tcx>(t: &'a Ty<'tcx>) -> &'a ty::TyTrait<'tcx> {
  96         match t.sty {
  97             ty::ty_trait(ref ty_trait) => &**ty_trait,
  98             _ => panic!("expected ty_trait")
  99         }
 100     }
 101
 102     fn mutability_allowed(a_mutbl: ast::Mutability,
 103                           b_mutbl: ast::Mutability)
 104                           -> bool {
 105         a_mutbl == b_mutbl ||
 106             (a_mutbl == ast::MutMutable && b_mutbl == ast::MutImmutable)
 107     }
 108
 109     fn push_cast_obligation<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
 110                                       cast_expr: &ast::Expr,
 111                                       object_trait: &ty::TyTrait<'tcx>,
 112                                       referent_ty: Ty<'tcx>) {
 113         let object_trait_ref =
 114             register_object_cast_obligations(fcx,
 115                                              cast_expr.span,
 116                                              object_trait,
 117                                              referent_ty);
 118
 119         // Finally record the object_trait_ref for use during trans
 120         // (it would prob be better not to do this, but it's just kind
 121         // of a pain to have to reconstruct it).
 122         fcx.write_object_cast(cast_expr.id, object_trait_ref);
 123     }
 124 }
 125
 126 // Check that a trait is 'object-safe'. This should be checked whenever a trait object
 127 // is created (by casting or coercion, etc.). A trait is object-safe if all its
 128 // methods are object-safe. A trait method is object-safe if it does not take
 129 // self by value, has no type parameters and does not use the `Self` type, except
 130 // in self position.
 131 pub fn check_object_safety<'tcx>(tcx: &ty::ctxt<'tcx>,
 132                                  object_trait: &ty::TyTrait<'tcx>,
 133                                  span: Span)
 134 {
 135     let object_trait_ref =
 136         object_trait.principal_trait_ref_with_self_ty(tcx, tcx.types.err);
 137
 138     if traits::is_object_safe(tcx, object_trait_ref.clone()) {
 139         return;
 140     }
 141
 142     span_err!(tcx.sess, span, E0038,
 143               "cannot convert to a trait object because trait `{}` is not object-safe",
 144               ty::item_path_str(tcx, object_trait_ref.def_id()));
 145
 146     let violations = traits::object_safety_violations(tcx, object_trait_ref.clone());
 147     for violation in violations.into_iter() {
 148         match violation {
 149             ObjectSafetyViolation::SizedSelf => {
 150                 tcx.sess.span_note(
 151                     span,
 152                     "the trait cannot require that `Self : Sized`");
 153             }
 154
 155             ObjectSafetyViolation::Method(method, MethodViolationCode::ByValueSelf) => {
 156                 tcx.sess.span_note(
 157                     span,
 158                     format!("method `{}` has a receiver type of `Self`, \
 159                              which cannot be used with a trait object",
 160                             method.name.user_string(tcx)).as_slice());
 161             }
 162
 163             ObjectSafetyViolation::Method(method, MethodViolationCode::StaticMethod) => {
 164                 tcx.sess.span_note(
 165                     span,
 166                     format!("method `{}` has no receiver",
 167                             method.name.user_string(tcx)).as_slice());
 168             }
 169
 170             ObjectSafetyViolation::Method(method, MethodViolationCode::ReferencesSelf) => {
 171                 tcx.sess.span_note(
 172                     span,
 173                     format!("method `{}` references the `Self` type \
 174                              in its arguments or return type",
 175                             method.name.user_string(tcx)).as_slice());
 176             }
 177
 178             ObjectSafetyViolation::Method(method, MethodViolationCode::Generic) => {
 179                 tcx.sess.span_note(
 180                     span,
 181                     format!("method `{}` has generic type parameters",
 182                             method.name.user_string(tcx)).as_slice());
 183             }
 184         }
 185     }
 186 }
 187
 188 pub fn register_object_cast_obligations<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
 189                                                   span: Span,
 190                                                   object_trait: &ty::TyTrait<'tcx>,
 191                                                   referent_ty: Ty<'tcx>)
 192                                                   -> ty::PolyTraitRef<'tcx>
 193 {
 194     // We can only make objects from sized types.
 195     fcx.register_builtin_bound(
 196         referent_ty,
 197         ty::BoundSized,
 198         traits::ObligationCause::new(span, fcx.body_id, traits::ObjectSized));
 199
 200     // This is just for better error reporting. Kinda goofy. The object type stuff
 201     // needs some refactoring so there is a more convenient type to pass around.
 202     let object_trait_ty =
 203         ty::mk_trait(fcx.tcx(),
 204                      object_trait.principal.clone(),
 205                      object_trait.bounds.clone());
 206
 207     debug!("register_object_cast_obligations: referent_ty={} object_trait_ty={}",
 208            referent_ty.repr(fcx.tcx()),
 209            object_trait_ty.repr(fcx.tcx()));
 210
 211     let cause = ObligationCause::new(span,
 212                                      fcx.body_id,
 213                                      traits::ObjectCastObligation(object_trait_ty));
 214
 215     // Create the obligation for casting from T to Trait.
 216     let object_trait_ref =
 217         object_trait.principal_trait_ref_with_self_ty(fcx.tcx(), referent_ty);
 218     let object_obligation =
 219         Obligation::new(cause.clone(), object_trait_ref.as_predicate());
 220     fcx.register_predicate(object_obligation);
 221
 222     // Create additional obligations for all the various builtin
 223     // bounds attached to the object cast. (In other words, if the
 224     // object type is Foo+Send, this would create an obligation
 225     // for the Send check.)
 226     for builtin_bound in object_trait.bounds.builtin_bounds.iter() {
 227         fcx.register_builtin_bound(
 228             referent_ty,
 229             builtin_bound,
 230             cause.clone());
 231     }
 232
 233     // Create obligations for the projection predicates.
 234     let projection_bounds =
 235         object_trait.projection_bounds_with_self_ty(fcx.tcx(), referent_ty);
 236     for projection_bound in projection_bounds.iter() {
 237         let projection_obligation =
 238             Obligation::new(cause.clone(), projection_bound.as_predicate());
 239         fcx.register_predicate(projection_obligation);
 240     }
 241
 242     // Finally, check that there IS a projection predicate for every associated type.
 243     check_object_type_binds_all_associated_types(fcx.tcx(),
 244                                                  span,
 245                                                  object_trait);
 246
 247     object_trait_ref
 248 }
 249
 250 fn check_object_type_binds_all_associated_types<'tcx>(tcx: &ty::ctxt<'tcx>,
 251                                                       span: Span,
 252                                                       object_trait: &ty::TyTrait<'tcx>)
 253 {
 254     let object_trait_ref =
 255         object_trait.principal_trait_ref_with_self_ty(tcx, tcx.types.err);
 256
 257     let mut associated_types: FnvHashSet<(ast::DefId, ast::Name)> =
 258         traits::supertraits(tcx, object_trait_ref.clone())
 259         .flat_map(|tr| {
 260             let trait_def = ty::lookup_trait_def(tcx, tr.def_id());
 261             trait_def.associated_type_names
 262                 .clone()
 263                 .into_iter()
 264                 .map(move |associated_type_name| (tr.def_id(), associated_type_name))
 265         })
 266         .collect();
 267
 268     for projection_bound in object_trait.bounds.projection_bounds.iter() {
 269         let pair = (projection_bound.0.projection_ty.trait_ref.def_id,
 270                     projection_bound.0.projection_ty.item_name);
 271         associated_types.remove(&pair);
 272     }
 273
 274     for (trait_def_id, name) in associated_types.into_iter() {
 275         tcx.sess.span_err(
 276             span,
 277             format!("the value of the associated type `{}` (from the trait `{}`) must be specified",
 278                     name.user_string(tcx),
 279                     ty::item_path_str(tcx, trait_def_id)).as_slice());
 280     }
 281 }
 282
 283 pub fn select_all_fcx_obligations_or_error(fcx: &FnCtxt) {
 284     debug!("select_all_fcx_obligations_or_error");
 285
 286     select_fcx_obligations_where_possible(fcx);
 287     fcx.default_type_parameters();
 288
 289     let mut fulfillment_cx = fcx.inh.fulfillment_cx.borrow_mut();
 290     let r = fulfillment_cx.select_all_or_error(fcx.infcx(), fcx);
 291     match r {
 292         Ok(()) => { }
 293         Err(errors) => { report_fulfillment_errors(fcx.infcx(), &errors); }
 294     }
 295 }
 296
 297 /// Select as many obligations as we can at present.
 298 pub fn select_fcx_obligations_where_possible(fcx: &FnCtxt)
 299 {
 300     match
 301         fcx.inh.fulfillment_cx
 302         .borrow_mut()
 303         .select_where_possible(fcx.infcx(), fcx)
 304     {
 305         Ok(()) => { }
 306         Err(errors) => { report_fulfillment_errors(fcx.infcx(), &errors); }
 307     }
 308 }
 309
 310 /// Try to select any fcx obligation that we haven't tried yet, in an effort to improve inference.
 311 /// You could just call `select_fcx_obligations_where_possible` except that it leads to repeated
 312 /// work.
 313 pub fn select_new_fcx_obligations(fcx: &FnCtxt) {
 314     match
 315         fcx.inh.fulfillment_cx
 316         .borrow_mut()
 317         .select_new_obligations(fcx.infcx(), fcx)
 318     {
 319         Ok(()) => { }
 320         Err(errors) => { report_fulfillment_errors(fcx.infcx(), &errors); }
 321     }
 322 }
 323