1 //! Orphan checker: every impl either implements a trait defined in this
2 //! crate or pertains to a type defined in this crate.
4 use rustc_data_structures::fx::FxHashSet;
5 use rustc_errors::{struct_span_err, DelayDm};
6 use rustc_errors::{Diagnostic, ErrorGuaranteed};
8 use rustc_middle::ty::subst::InternalSubsts;
9 use rustc_middle::ty::util::IgnoreRegions;
10 use rustc_middle::ty::{
11 self, ImplPolarity, Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor,
13 use rustc_session::lint;
14 use rustc_span::def_id::{DefId, LocalDefId};
16 use rustc_trait_selection::traits;
17 use std::ops::ControlFlow;
19 #[instrument(skip(tcx), level = "debug")]
20 pub(crate) fn orphan_check_impl(
22 impl_def_id: LocalDefId,
23 ) -> Result<(), ErrorGuaranteed> {
24 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
25 trait_ref.error_reported()?;
27 let ret = do_orphan_check_impl(tcx, trait_ref, impl_def_id);
28 if tcx.trait_is_auto(trait_ref.def_id) {
29 lint_auto_trait_impl(tcx, trait_ref, impl_def_id);
35 fn do_orphan_check_impl<'tcx>(
37 trait_ref: ty::TraitRef<'tcx>,
39 ) -> Result<(), ErrorGuaranteed> {
40 let trait_def_id = trait_ref.def_id;
42 let item = tcx.hir().expect_item(def_id);
43 let hir::ItemKind::Impl(ref impl_) = item.kind else {
44 bug!("{:?} is not an impl: {:?}", def_id, item);
46 let sp = tcx.def_span(def_id);
47 let tr = impl_.of_trait.as_ref().unwrap();
49 match traits::orphan_check(tcx, item.owner_id.to_def_id()) {
51 Err(err) => emit_orphan_check_error(
63 // In addition to the above rules, we restrict impls of auto traits
64 // so that they can only be implemented on nominal types, such as structs,
65 // enums or foreign types. To see why this restriction exists, consider the
66 // following example (#22978). Imagine that crate A defines an auto trait
67 // `Foo` and a fn that operates on pairs of types:
72 // fn two_foos<A:Foo,B:Foo>(..) {
73 // one_foo::<(A,B)>(..)
75 // fn one_foo<T:Foo>(..) { .. }
78 // This type-checks fine; in particular the fn
79 // `two_foos` is able to conclude that `(A,B):Foo`
80 // because `A:Foo` and `B:Foo`.
82 // Now imagine that crate B comes along and does the following:
89 // impl !Send for (A, B) { }
92 // This final impl is legal according to the orphan
93 // rules, but it invalidates the reasoning from
96 "trait_ref={:?} trait_def_id={:?} trait_is_auto={}",
99 tcx.trait_is_auto(trait_def_id)
102 if tcx.trait_is_auto(trait_def_id) && !trait_def_id.is_local() {
103 let self_ty = trait_ref.self_ty();
104 let opt_self_def_id = match *self_ty.kind() {
105 ty::Adt(self_def, _) => Some(self_def.did()),
106 ty::Foreign(did) => Some(did),
110 let msg = match opt_self_def_id {
111 // We only want to permit nominal types, but not *all* nominal types.
112 // They must be local to the current crate, so that people
113 // can't do `unsafe impl Send for Rc<SomethingLocal>` or
114 // `impl !Send for Box<SomethingLocalAndSend>`.
115 Some(self_def_id) => {
116 if self_def_id.is_local() {
121 "cross-crate traits with a default impl, like `{}`, \
122 can only be implemented for a struct/enum type \
123 defined in the current crate",
124 tcx.def_path_str(trait_def_id)
126 "can't implement cross-crate trait for type in another crate",
132 "cross-crate traits with a default impl, like `{}`, can \
133 only be implemented for a struct/enum type, not `{}`",
134 tcx.def_path_str(trait_def_id),
137 "can't implement cross-crate trait with a default impl for \
138 non-struct/enum type",
142 if let Some((msg, label)) = msg {
144 struct_span_err!(tcx.sess, sp, E0321, "{}", msg).span_label(sp, label).emit();
145 return Err(reported);
152 fn emit_orphan_check_error<'tcx>(
155 full_impl_span: Span,
159 generics: &hir::Generics<'tcx>,
160 err: traits::OrphanCheckErr<'tcx>,
161 ) -> Result<!, ErrorGuaranteed> {
163 traits::OrphanCheckErr::NonLocalInputType(tys) => {
164 let msg = match self_ty.kind() {
165 ty::Adt(..) => "can be implemented for types defined outside of the crate",
166 _ if self_ty.is_primitive() => "can be implemented for primitive types",
167 _ => "can be implemented for arbitrary types",
169 let mut err = struct_span_err!(
173 "only traits defined in the current crate {msg}"
175 err.span_label(sp, "impl doesn't use only types from inside the current crate");
176 for &(mut ty, is_target_ty) in &tys {
177 ty = tcx.erase_regions(ty);
178 ty = match ty.kind() {
179 // Remove the type arguments from the output, as they are not relevant.
180 // You can think of this as the reverse of `resolve_vars_if_possible`.
181 // That way if we had `Vec<MyType>`, we will properly attribute the
182 // problem to `Vec<T>` and avoid confusing the user if they were to see
183 // `MyType` in the error.
184 ty::Adt(def, _) => tcx.mk_adt(*def, ty::List::empty()),
187 let this = "this".to_string();
188 let (ty, postfix) = match &ty.kind() {
189 ty::Slice(_) => (this, " because slices are always foreign"),
190 ty::Array(..) => (this, " because arrays are always foreign"),
191 ty::Tuple(..) => (this, " because tuples are always foreign"),
192 ty::RawPtr(ptr_ty) => {
193 emit_newtype_suggestion_for_raw_ptr(
201 (format!("`{}`", ty), " because raw pointers are always foreign")
203 _ => (format!("`{}`", ty), ""),
206 let msg = format!("{} is not defined in the current crate{}", ty, postfix);
208 // Point at `D<A>` in `impl<A, B> for C<B> in D<A>`
209 err.span_label(self_ty_span, &msg);
211 // Point at `C<B>` in `impl<A, B> for C<B> in D<A>`
212 err.span_label(trait_span, &msg);
215 err.note("define and implement a trait or new type instead");
218 traits::OrphanCheckErr::UncoveredTy(param_ty, local_type) => {
220 for param in generics.params {
221 if param.name.ident().to_string() == param_ty.to_string() {
227 Some(local_type) => struct_span_err!(
231 "type parameter `{}` must be covered by another type \
232 when it appears before the first local type (`{}`)",
239 "type parameter `{}` must be covered by another type \
240 when it appears before the first local type (`{}`)",
245 "implementing a foreign trait is only possible if at \
246 least one of the types for which it is implemented is local, \
247 and no uncovered type parameters appear before that first \
251 "in this case, 'before' refers to the following order: \
252 `impl<..> ForeignTrait<T1, ..., Tn> for T0`, \
253 where `T0` is the first and `Tn` is the last",
256 None => struct_span_err!(
260 "type parameter `{}` must be used as the type parameter for some \
261 local type (e.g., `MyStruct<{}>`)",
268 "type parameter `{}` must be used as the type parameter for some \
274 "implementing a foreign trait is only possible if at \
275 least one of the types for which it is implemented is local",
278 "only traits defined in the current crate can be \
279 implemented for a type parameter",
287 fn emit_newtype_suggestion_for_raw_ptr(
288 full_impl_span: Span,
291 ptr_ty: &ty::TypeAndMut<'_>,
292 diag: &mut Diagnostic,
294 if !self_ty.needs_subst() {
295 let mut_key = ptr_ty.mutbl.prefix_str();
296 let msg_sugg = "consider introducing a new wrapper type".to_owned();
299 full_impl_span.shrink_to_lo(),
300 format!("struct WrapperType(*{}{});\n\n", mut_key, ptr_ty.ty),
302 (self_ty_span, "WrapperType".to_owned()),
304 diag.multipart_suggestion(msg_sugg, sugg, rustc_errors::Applicability::MaybeIncorrect);
308 /// Lint impls of auto traits if they are likely to have
309 /// unsound or surprising effects on auto impls.
310 fn lint_auto_trait_impl<'tcx>(
312 trait_ref: ty::TraitRef<'tcx>,
313 impl_def_id: LocalDefId,
315 if tcx.impl_polarity(impl_def_id) != ImplPolarity::Positive {
319 assert_eq!(trait_ref.substs.len(), 1);
320 let self_ty = trait_ref.self_ty();
321 let (self_type_did, substs) = match self_ty.kind() {
322 ty::Adt(def, substs) => (def.did(), substs),
324 // FIXME: should also lint for stuff like `&i32` but
325 // considering that auto traits are unstable, that
326 // isn't too important for now as this only affects
327 // crates using `nightly`, and std.
332 // Impls which completely cover a given root type are fine as they
333 // disable auto impls entirely. So only lint if the substs
334 // are not a permutation of the identity substs.
335 let Err(arg) = tcx.uses_unique_generic_params(substs, IgnoreRegions::Yes) else {
342 // - compute the requirements for the auto impl candidate
343 // - check whether these are implied by the non covering impls
344 // - if not, emit the lint
346 // What we do here is a bit simpler:
348 // - badly check if an auto impl candidate definitely does not apply
349 // for the given simplified type
350 // - if so, do not lint
351 if fast_reject_auto_impl(tcx, trait_ref.def_id, self_ty) {
356 tcx.struct_span_lint_hir(
357 lint::builtin::SUSPICIOUS_AUTO_TRAIT_IMPLS,
358 tcx.hir().local_def_id_to_hir_id(impl_def_id),
359 tcx.def_span(impl_def_id),
362 "cross-crate traits with a default impl, like `{}`, \
363 should not be specialized",
364 tcx.def_path_str(trait_ref.def_id),
368 let item_span = tcx.def_span(self_type_did);
369 let self_descr = tcx.def_kind(self_type_did).descr(self_type_did);
371 ty::util::NotUniqueParam::DuplicateParam(arg) => {
372 lint.note(&format!("`{}` is mentioned multiple times", arg));
374 ty::util::NotUniqueParam::NotParam(arg) => {
375 lint.note(&format!("`{}` is not a generic parameter", arg));
381 "try using the same sequence of generic parameters as the {} definition",
389 fn fast_reject_auto_impl<'tcx>(tcx: TyCtxt<'tcx>, trait_def_id: DefId, self_ty: Ty<'tcx>) -> bool {
390 struct DisableAutoTraitVisitor<'tcx> {
393 self_ty_root: Ty<'tcx>,
394 seen: FxHashSet<DefId>,
397 impl<'tcx> TypeVisitor<'tcx> for DisableAutoTraitVisitor<'tcx> {
399 fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
401 if t != self.self_ty_root {
402 for impl_def_id in tcx.non_blanket_impls_for_ty(self.trait_def_id, t) {
403 match tcx.impl_polarity(impl_def_id) {
404 ImplPolarity::Negative => return ControlFlow::BREAK,
405 ImplPolarity::Reservation => {}
406 // FIXME(@lcnr): That's probably not good enough, idk
408 // We might just want to take the rustdoc code and somehow avoid
409 // explicit impls for `Self`.
410 ImplPolarity::Positive => return ControlFlow::CONTINUE,
416 ty::Adt(def, substs) if def.is_phantom_data() => substs.visit_with(self),
417 ty::Adt(def, substs) => {
418 // @lcnr: This is the only place where cycles can happen. We avoid this
419 // by only visiting each `DefId` once.
421 // This will be is incorrect in subtle cases, but I don't care :)
422 if self.seen.insert(def.did()) {
423 for ty in def.all_fields().map(|field| field.ty(tcx, substs)) {
424 ty.visit_with(self)?;
428 ControlFlow::CONTINUE
430 _ => t.super_visit_with(self),
435 let self_ty_root = match self_ty.kind() {
436 ty::Adt(def, _) => tcx.mk_adt(*def, InternalSubsts::identity_for_item(tcx, def.did())),
437 _ => unimplemented!("unexpected self ty {:?}", self_ty),
441 .visit_with(&mut DisableAutoTraitVisitor {
445 seen: FxHashSet::default(),