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 use rustc_error_codes::*;
11 pub fn check(tcx: TyCtxt<'_>) {
12 let mut orphan = OrphanChecker { tcx };
13 tcx.hir().krate().visit_all_item_likes(&mut orphan);
16 struct OrphanChecker<'tcx> {
20 impl ItemLikeVisitor<'v> for OrphanChecker<'tcx> {
21 /// Checks exactly one impl for orphan rules and other such
22 /// restrictions. In this fn, it can happen that multiple errors
23 /// apply to a specific impl, so just return after reporting one
24 /// to prevent inundating the user with a bunch of similar error
26 fn visit_item(&mut self, item: &hir::Item) {
27 let def_id = self.tcx.hir().local_def_id(item.hir_id);
29 if let hir::ItemKind::Impl(.., generics, Some(tr), impl_ty, _) = &item.kind {
30 debug!("coherence2::orphan check: trait impl {}",
31 self.tcx.hir().node_to_string(item.hir_id));
32 let trait_ref = self.tcx.impl_trait_ref(def_id).unwrap();
33 let trait_def_id = trait_ref.def_id;
34 let cm = self.tcx.sess.source_map();
35 let sp = cm.def_span(item.span);
36 match traits::orphan_check(self.tcx, def_id) {
38 Err(traits::OrphanCheckErr::NonLocalInputType(tys)) => {
39 let mut err = struct_span_err!(
43 "only traits defined in the current crate can be implemented for \
46 err.span_label(sp, "impl doesn't use only types from inside the current crate");
47 for (ty, is_target_ty) in &tys {
49 self.tcx.infer_ctxt().enter(|infcx| {
50 // Remove the lifetimes unnecessary for this error.
51 ty = infcx.freshen(ty);
54 // Remove the type arguments from the output, as they are not relevant.
55 // You can think of this as the reverse of `resolve_vars_if_possible`.
56 // That way if we had `Vec<MyType>`, we will properly attribute the
57 // problem to `Vec<T>` and avoid confusing the user if they were to see
58 // `MyType` in the error.
59 ty::Adt(def, _) => self.tcx.mk_adt(def, ty::List::empty()),
62 let this = "this".to_string();
63 let (ty, postfix) = match &ty.kind {
64 ty::Slice(_) => (this, " because slices are always foreign"),
65 ty::Array(..) => (this, " because arrays are always foreign"),
66 ty::Tuple(..) => (this, " because tuples are always foreign"),
67 _ => (format!("`{}`", ty), ""),
69 let msg = format!("{} is not defined in the current crate{}", ty, postfix);
71 // Point at `D<A>` in `impl<A, B> for C<B> in D<A>`
72 err.span_label(impl_ty.span, &msg);
74 // Point at `C<B>` in `impl<A, B> for C<B> in D<A>`
75 err.span_label(tr.path.span, &msg);
78 err.note("define and implement a trait or new type instead");
82 Err(traits::OrphanCheckErr::UncoveredTy(param_ty, local_type)) => {
84 for param in &generics.params {
85 if param.name.ident().to_string() == param_ty.to_string() {
96 "type parameter `{}` must be covered by another type \
97 when it appears before the first local type (`{}`)",
100 ).span_label(sp, format!(
101 "type parameter `{}` must be covered by another type \
102 when it appears before the first local type (`{}`)",
105 )).note("implementing a foreign trait is only possible if at \
106 least one of the types for which is it implemented is local, \
107 and no uncovered type parameters appear before that first \
109 ).note("in this case, 'before' refers to the following order: \
110 `impl<..> ForeignTrait<T1, ..., Tn> for T0`, \
111 where `T0` is the first and `Tn` is the last"
119 "type parameter `{}` must be used as the type parameter for some \
120 local type (e.g., `MyStruct<{}>`)",
123 ).span_label(sp, format!(
124 "type parameter `{}` must be used as the type parameter for some \
127 )).note("implementing a foreign trait is only possible if at \
128 least one of the types for which is it implemented is local"
129 ).note("only traits defined in the current crate can be \
130 implemented for a type parameter"
138 // In addition to the above rules, we restrict impls of auto traits
139 // so that they can only be implemented on nominal types, such as structs,
140 // enums or foreign types. To see why this restriction exists, consider the
141 // following example (#22978). Imagine that crate A defines an auto trait
142 // `Foo` and a fn that operates on pairs of types:
146 // auto trait Foo { }
147 // fn two_foos<A:Foo,B:Foo>(..) {
148 // one_foo::<(A,B)>(..)
150 // fn one_foo<T:Foo>(..) { .. }
153 // This type-checks fine; in particular the fn
154 // `two_foos` is able to conclude that `(A,B):Foo`
155 // because `A:Foo` and `B:Foo`.
157 // Now imagine that crate B comes along and does the following:
162 // impl Foo for A { }
163 // impl Foo for B { }
164 // impl !Send for (A, B) { }
167 // This final impl is legal according to the orpan
168 // rules, but it invalidates the reasoning from
170 debug!("trait_ref={:?} trait_def_id={:?} trait_is_auto={}",
173 self.tcx.trait_is_auto(trait_def_id));
174 if self.tcx.trait_is_auto(trait_def_id) &&
175 !trait_def_id.is_local() {
176 let self_ty = trait_ref.self_ty();
177 let opt_self_def_id = match self_ty.kind {
178 ty::Adt(self_def, _) => Some(self_def.did),
179 ty::Foreign(did) => Some(did),
183 let msg = match opt_self_def_id {
184 // We only want to permit nominal types, but not *all* nominal types.
185 // They must be local to the current crate, so that people
186 // can't do `unsafe impl Send for Rc<SomethingLocal>` or
187 // `impl !Send for Box<SomethingLocalAndSend>`.
188 Some(self_def_id) => {
189 if self_def_id.is_local() {
193 format!("cross-crate traits with a default impl, like `{}`, \
194 can only be implemented for a struct/enum type \
195 defined in the current crate",
196 self.tcx.def_path_str(trait_def_id)),
197 "can't implement cross-crate trait for type in another crate"
202 Some((format!("cross-crate traits with a default impl, like `{}`, can \
203 only be implemented for a struct/enum type, not `{}`",
204 self.tcx.def_path_str(trait_def_id),
206 "can't implement cross-crate trait with a default impl for \
207 non-struct/enum type"))
211 if let Some((msg, label)) = msg {
212 struct_span_err!(self.tcx.sess, sp, E0321, "{}", msg)
213 .span_label(sp, label)
221 fn visit_trait_item(&mut self, _trait_item: &hir::TraitItem) {
224 fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem) {