src/librustc_typeck/coherence/orphan.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 //! Orphan checker: every impl either implements a trait defined in this
  12 //! crate or pertains to a type defined in this crate.
  13
  14 use rustc::traits;
  15 use rustc::ty::{self, TyCtxt};
  16 use rustc::hir::itemlikevisit::ItemLikeVisitor;
  17 use rustc::hir;
  18
  19 pub fn check<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
  20     let mut orphan = OrphanChecker { tcx: tcx };
  21     tcx.hir.krate().visit_all_item_likes(&mut orphan);
  22 }
  23
  24 struct OrphanChecker<'cx, 'tcx: 'cx> {
  25     tcx: TyCtxt<'cx, 'tcx, 'tcx>,
  26 }
  27
  28 impl<'cx, 'tcx, 'v> ItemLikeVisitor<'v> for OrphanChecker<'cx, 'tcx> {
  29     /// Checks exactly one impl for orphan rules and other such
  30     /// restrictions.  In this fn, it can happen that multiple errors
  31     /// apply to a specific impl, so just return after reporting one
  32     /// to prevent inundating the user with a bunch of similar error
  33     /// reports.
  34     fn visit_item(&mut self, item: &hir::Item) {
  35         let def_id = self.tcx.hir.local_def_id(item.id);
  36         match item.node {
  37             hir::ItemImpl(.., Some(_), _, _) => {
  38                 // "Trait" impl
  39                 debug!("coherence2::orphan check: trait impl {}",
  40                        self.tcx.hir.node_to_string(item.id));
  41                 let trait_ref = self.tcx.impl_trait_ref(def_id).unwrap();
  42                 let trait_def_id = trait_ref.def_id;
  43                 match traits::orphan_check(self.tcx, def_id) {
  44                     Ok(()) => {}
  45                     Err(traits::OrphanCheckErr::NoLocalInputType) => {
  46                         struct_span_err!(self.tcx.sess,
  47                                          item.span,
  48                                          E0117,
  49                                          "only traits defined in the current crate can be \
  50                                           implemented for arbitrary types")
  51                             .span_label(item.span, "impl doesn't use types inside crate")
  52                             .note(&format!("the impl does not reference any types defined in \
  53                                             this crate"))
  54                             .note("define and implement a trait or new type instead")
  55                             .emit();
  56                         return;
  57                     }
  58                     Err(traits::OrphanCheckErr::UncoveredTy(param_ty)) => {
  59                         span_err!(self.tcx.sess,
  60                                   item.span,
  61                                   E0210,
  62                                   "type parameter `{}` must be used as the type parameter for \
  63                                    some local type (e.g. `MyStruct<T>`); only traits defined in \
  64                                    the current crate can be implemented for a type parameter",
  65                                   param_ty);
  66                         return;
  67                     }
  68                 }
  69
  70                 // In addition to the above rules, we restrict impls of defaulted traits
  71                 // so that they can only be implemented on nominal types, such as structs,
  72                 // enums or foreign types. To see why this restriction exists, consider the
  73                 // following example (#22978). Imagine that crate A defines a defaulted trait
  74                 // `Foo` and a fn that operates on pairs of types:
  75                 //
  76                 // ```
  77                 // // Crate A
  78                 // trait Foo { }
  79                 // impl Foo for .. { }
  80                 // fn two_foos<A:Foo,B:Foo>(..) {
  81                 //     one_foo::<(A,B)>(..)
  82                 // }
  83                 // fn one_foo<T:Foo>(..) { .. }
  84                 // ```
  85                 //
  86                 // This type-checks fine; in particular the fn
  87                 // `two_foos` is able to conclude that `(A,B):Foo`
  88                 // because `A:Foo` and `B:Foo`.
  89                 //
  90                 // Now imagine that crate B comes along and does the following:
  91                 //
  92                 // ```
  93                 // struct A { }
  94                 // struct B { }
  95                 // impl Foo for A { }
  96                 // impl Foo for B { }
  97                 // impl !Send for (A, B) { }
  98                 // ```
  99                 //
 100                 // This final impl is legal according to the orpan
 101                 // rules, but it invalidates the reasoning from
 102                 // `two_foos` above.
 103                 debug!("trait_ref={:?} trait_def_id={:?} trait_is_auto={}",
 104                        trait_ref,
 105                        trait_def_id,
 106                        self.tcx.trait_is_auto(trait_def_id));
 107                 if self.tcx.trait_is_auto(trait_def_id) &&
 108                    !trait_def_id.is_local() {
 109                     let self_ty = trait_ref.self_ty();
 110                     let opt_self_def_id = match self_ty.sty {
 111                         ty::TyAdt(self_def, _) => Some(self_def.did),
 112                         ty::TyForeign(did) => Some(did),
 113                         _ => None,
 114                     };
 115
 116                     let msg = match opt_self_def_id {
 117                         // We only want to permit nominal types, but not *all* nominal types.
 118                         // They must be local to the current crate, so that people
 119                         // can't do `unsafe impl Send for Rc<SomethingLocal>` or
 120                         // `impl !Send for Box<SomethingLocalAndSend>`.
 121                         Some(self_def_id) => {
 122                             if self_def_id.is_local() {
 123                                 None
 124                             } else {
 125                                 Some(format!("cross-crate traits with a default impl, like `{}`, \
 126                                               can only be implemented for a struct/enum type \
 127                                               defined in the current crate",
 128                                              self.tcx.item_path_str(trait_def_id)))
 129                             }
 130                         }
 131                         _ => {
 132                             Some(format!("cross-crate traits with a default impl, like `{}`, can \
 133                                           only be implemented for a struct/enum type, not `{}`",
 134                                          self.tcx.item_path_str(trait_def_id),
 135                                          self_ty))
 136                         }
 137                     };
 138
 139                     if let Some(msg) = msg {
 140                         span_err!(self.tcx.sess, item.span, E0321, "{}", msg);
 141                         return;
 142                     }
 143                 }
 144             }
 145             hir::ItemAutoImpl(_, ref item_trait_ref) => {
 146                 // "Trait" impl
 147                 debug!("coherence2::orphan check: default trait impl {}",
 148                        self.tcx.hir.node_to_string(item.id));
 149                 let trait_ref = self.tcx.impl_trait_ref(def_id).unwrap();
 150                 if !trait_ref.def_id.is_local() {
 151                     struct_span_err!(self.tcx.sess,
 152                                      item_trait_ref.path.span,
 153                                      E0318,
 154                                      "cannot create default implementations for traits outside \
 155                                       the crate they're defined in; define a new trait instead")
 156                         .span_label(item_trait_ref.path.span,
 157                                     format!("`{}` trait not defined in this crate",
 158                             self.tcx.hir.node_to_pretty_string(item_trait_ref.ref_id)))
 159                         .emit();
 160                     return;
 161                 }
 162             }
 163             _ => {
 164                 // Not an impl
 165             }
 166         }
 167     }
 168
 169     fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem) {
 170     }
 171
 172     fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem) {
 173     }
 174 }