1 //! Orphan checker: every impl either implements a trait defined in this
2 //! crate or pertains to a type defined in this crate.
5 use rustc::ty::{self, TyCtxt};
6 use rustc::hir::itemlikevisit::ItemLikeVisitor;
9 pub fn check(tcx: TyCtxt<'_>) {
10 let mut orphan = OrphanChecker { tcx };
11 tcx.hir().krate().visit_all_item_likes(&mut orphan);
14 struct OrphanChecker<'tcx> {
18 impl ItemLikeVisitor<'v> for OrphanChecker<'tcx> {
19 /// Checks exactly one impl for orphan rules and other such
20 /// restrictions. In this fn, it can happen that multiple errors
21 /// apply to a specific impl, so just return after reporting one
22 /// to prevent inundating the user with a bunch of similar error
24 fn visit_item(&mut self, item: &hir::Item) {
25 let def_id = self.tcx.hir().local_def_id(item.hir_id);
27 if let hir::ItemKind::Impl(.., generics, Some(tr), impl_ty, _) = &item.kind {
28 debug!("coherence2::orphan check: trait impl {}",
29 self.tcx.hir().node_to_string(item.hir_id));
30 let trait_ref = self.tcx.impl_trait_ref(def_id).unwrap();
31 let trait_def_id = trait_ref.def_id;
32 let cm = self.tcx.sess.source_map();
33 let sp = cm.def_span(item.span);
34 match traits::orphan_check(self.tcx, def_id) {
36 Err(traits::OrphanCheckErr::NonLocalInputType(tys)) => {
37 let mut err = struct_span_err!(
41 "only traits defined in the current crate can be implemented for \
44 err.span_label(sp, "impl doesn't use only types from inside the current crate");
45 for (ty, is_target_ty) in &tys {
47 self.tcx.infer_ctxt().enter(|infcx| {
48 // Remove the lifetimes unnecessary for this error.
49 ty = infcx.freshen(ty);
52 // Remove the type arguments from the output, as they are not relevant.
53 // You can think of this as the reverse of `resolve_vars_if_possible`.
54 // That way if we had `Vec<MyType>`, we will properly attribute the
55 // problem to `Vec<T>` and avoid confusing the user if they were to see
56 // `MyType` in the error.
57 ty::Adt(def, _) => self.tcx.mk_adt(def, ty::List::empty()),
60 let this = "this".to_string();
61 let (ty, postfix) = match &ty.kind {
62 ty::Slice(_) => (this, " because slices are always foreign"),
63 ty::Array(..) => (this, " because arrays are always foreign"),
64 ty::Tuple(..) => (this, " because tuples are always foreign"),
65 _ => (format!("`{}`", ty), ""),
67 let msg = format!("{} is not defined in the current crate{}", ty, postfix);
69 // Point at `D<A>` in `impl<A, B> for C<B> in D<A>`
70 err.span_label(impl_ty.span, &msg);
72 // Point at `C<B>` in `impl<A, B> for C<B> in D<A>`
73 err.span_label(tr.path.span, &msg);
76 err.note("define and implement a trait or new type instead");
80 Err(traits::OrphanCheckErr::UncoveredTy(param_ty, local_type)) => {
82 for param in &generics.params {
83 if param.name.ident().to_string() == param_ty.to_string() {
94 "type parameter `{}` must be covered by another type \
95 when it appears before the first local type (`{}`)",
98 ).span_label(sp, format!(
99 "type parameter `{}` must be covered by another type \
100 when it appears before the first local type (`{}`)",
103 )).note("implementing a foreign trait is only possible if at \
104 least one of the types for which is it implemented is local, \
105 and no uncovered type parameters appear before that first \
107 ).note("in this case, 'before' refers to the following order: \
108 `impl<..> ForeignTrait<T1, ..., Tn> for T0`, \
109 where `T0` is the first and `Tn` is the last"
117 "type parameter `{}` must be used as the type parameter for some \
118 local type (e.g., `MyStruct<{}>`)",
121 ).span_label(sp, format!(
122 "type parameter `{}` must be used as the type parameter for some \
125 )).note("implementing a foreign trait is only possible if at \
126 least one of the types for which is it implemented is local"
127 ).note("only traits defined in the current crate can be \
128 implemented for a type parameter"
136 // In addition to the above rules, we restrict impls of auto traits
137 // so that they can only be implemented on nominal types, such as structs,
138 // enums or foreign types. To see why this restriction exists, consider the
139 // following example (#22978). Imagine that crate A defines an auto trait
140 // `Foo` and a fn that operates on pairs of types:
144 // auto trait Foo { }
145 // fn two_foos<A:Foo,B:Foo>(..) {
146 // one_foo::<(A,B)>(..)
148 // fn one_foo<T:Foo>(..) { .. }
151 // This type-checks fine; in particular the fn
152 // `two_foos` is able to conclude that `(A,B):Foo`
153 // because `A:Foo` and `B:Foo`.
155 // Now imagine that crate B comes along and does the following:
160 // impl Foo for A { }
161 // impl Foo for B { }
162 // impl !Send for (A, B) { }
165 // This final impl is legal according to the orpan
166 // rules, but it invalidates the reasoning from
168 debug!("trait_ref={:?} trait_def_id={:?} trait_is_auto={}",
171 self.tcx.trait_is_auto(trait_def_id));
172 if self.tcx.trait_is_auto(trait_def_id) &&
173 !trait_def_id.is_local() {
174 let self_ty = trait_ref.self_ty();
175 let opt_self_def_id = match self_ty.kind {
176 ty::Adt(self_def, _) => Some(self_def.did),
177 ty::Foreign(did) => Some(did),
181 let msg = match opt_self_def_id {
182 // We only want to permit nominal types, but not *all* nominal types.
183 // They must be local to the current crate, so that people
184 // can't do `unsafe impl Send for Rc<SomethingLocal>` or
185 // `impl !Send for Box<SomethingLocalAndSend>`.
186 Some(self_def_id) => {
187 if self_def_id.is_local() {
191 format!("cross-crate traits with a default impl, like `{}`, \
192 can only be implemented for a struct/enum type \
193 defined in the current crate",
194 self.tcx.def_path_str(trait_def_id)),
195 "can't implement cross-crate trait for type in another crate"
200 Some((format!("cross-crate traits with a default impl, like `{}`, can \
201 only be implemented for a struct/enum type, not `{}`",
202 self.tcx.def_path_str(trait_def_id),
204 "can't implement cross-crate trait with a default impl for \
205 non-struct/enum type"))
209 if let Some((msg, label)) = msg {
210 struct_span_err!(self.tcx.sess, sp, E0321, "{}", msg)
211 .span_label(sp, label)
219 fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem) {
222 fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem) {