1 //! See Rustc Dev Guide chapters on [trait-resolution] and [trait-specialization] for more info on
4 //! [trait-resolution]: https://rustc-dev-guide.rust-lang.org/traits/resolution.html
5 //! [trait-specialization]: https://rustc-dev-guide.rust-lang.org/traits/specialization.html
7 use crate::infer::outlives::env::OutlivesEnvironment;
8 use crate::infer::{CombinedSnapshot, InferOk};
9 use crate::traits::outlives_bounds::InferCtxtExt as _;
10 use crate::traits::select::IntercrateAmbiguityCause;
11 use crate::traits::util::impl_subject_and_oblig;
12 use crate::traits::SkipLeakCheck;
14 self, Obligation, ObligationCause, ObligationCtxt, PredicateObligation, PredicateObligations,
17 use rustc_data_structures::fx::FxIndexSet;
18 use rustc_errors::Diagnostic;
19 use rustc_hir::def_id::{DefId, CRATE_DEF_ID, LOCAL_CRATE};
20 use rustc_infer::infer::{DefiningAnchor, InferCtxt, TyCtxtInferExt};
21 use rustc_infer::traits::util;
22 use rustc_middle::traits::specialization_graph::OverlapMode;
23 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
24 use rustc_middle::ty::visit::TypeVisitable;
25 use rustc_middle::ty::{self, ImplSubject, Ty, TyCtxt, TypeVisitor};
26 use rustc_span::symbol::sym;
27 use rustc_span::DUMMY_SP;
30 use std::ops::ControlFlow;
32 use super::NormalizeExt;
34 /// Whether we do the orphan check relative to this crate or
35 /// to some remote crate.
36 #[derive(Copy, Clone, Debug)]
42 #[derive(Debug, Copy, Clone)]
48 pub struct OverlapResult<'tcx> {
49 pub impl_header: ty::ImplHeader<'tcx>,
50 pub intercrate_ambiguity_causes: FxIndexSet<IntercrateAmbiguityCause>,
52 /// `true` if the overlap might've been permitted before the shift
54 pub involves_placeholder: bool,
57 pub fn add_placeholder_note(err: &mut Diagnostic) {
59 "this behavior recently changed as a result of a bug fix; \
60 see rust-lang/rust#56105 for details",
64 /// If there are types that satisfy both impls, returns `Some`
65 /// with a suitably-freshened `ImplHeader` with those types
66 /// substituted. Otherwise, returns `None`.
67 #[instrument(skip(tcx, skip_leak_check), level = "debug")]
68 pub fn overlapping_impls(
72 skip_leak_check: SkipLeakCheck,
73 overlap_mode: OverlapMode,
74 ) -> Option<OverlapResult<'_>> {
75 // Before doing expensive operations like entering an inference context, do
76 // a quick check via fast_reject to tell if the impl headers could possibly
78 let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsInfer };
79 let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
80 let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
81 let may_overlap = match (impl1_ref, impl2_ref) {
82 (Some(a), Some(b)) => iter::zip(a.skip_binder().substs, b.skip_binder().substs)
83 .all(|(arg1, arg2)| drcx.generic_args_may_unify(arg1, arg2)),
85 let self_ty1 = tcx.type_of(impl1_def_id);
86 let self_ty2 = tcx.type_of(impl2_def_id);
87 drcx.types_may_unify(self_ty1, self_ty2)
89 _ => bug!("unexpected impls: {impl1_def_id:?} {impl2_def_id:?}"),
93 // Some types involved are definitely different, so the impls couldn't possibly overlap.
94 debug!("overlapping_impls: fast_reject early-exit");
99 tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).intercrate().build();
100 let selcx = &mut SelectionContext::new(&infcx);
102 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).is_some();
107 // In the case where we detect an error, run the check again, but
108 // this time tracking intercrate ambiguity causes for better
109 // diagnostics. (These take time and can lead to false errors.)
111 tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).intercrate().build();
112 let selcx = &mut SelectionContext::new(&infcx);
113 selcx.enable_tracking_intercrate_ambiguity_causes();
114 Some(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).unwrap())
117 fn with_fresh_ty_vars<'cx, 'tcx>(
118 selcx: &mut SelectionContext<'cx, 'tcx>,
119 param_env: ty::ParamEnv<'tcx>,
121 ) -> ty::ImplHeader<'tcx> {
122 let tcx = selcx.tcx();
123 let impl_substs = selcx.infcx.fresh_substs_for_item(DUMMY_SP, impl_def_id);
125 let header = ty::ImplHeader {
127 self_ty: tcx.bound_type_of(impl_def_id).subst(tcx, impl_substs),
128 trait_ref: tcx.impl_trait_ref(impl_def_id).map(|i| i.subst(tcx, impl_substs)),
129 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
132 let InferOk { value: mut header, obligations } =
133 selcx.infcx.at(&ObligationCause::dummy(), param_env).normalize(header);
135 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
139 /// Can both impl `a` and impl `b` be satisfied by a common type (including
140 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
141 fn overlap<'cx, 'tcx>(
142 selcx: &mut SelectionContext<'cx, 'tcx>,
143 skip_leak_check: SkipLeakCheck,
146 overlap_mode: OverlapMode,
147 ) -> Option<OverlapResult<'tcx>> {
149 "overlap(impl1_def_id={:?}, impl2_def_id={:?}, overlap_mode={:?})",
150 impl1_def_id, impl2_def_id, overlap_mode
153 selcx.infcx.probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
154 overlap_within_probe(selcx, impl1_def_id, impl2_def_id, overlap_mode, snapshot)
158 fn overlap_within_probe<'cx, 'tcx>(
159 selcx: &mut SelectionContext<'cx, 'tcx>,
162 overlap_mode: OverlapMode,
163 snapshot: &CombinedSnapshot<'tcx>,
164 ) -> Option<OverlapResult<'tcx>> {
165 let infcx = selcx.infcx;
167 if overlap_mode.use_negative_impl() {
168 if negative_impl(infcx.tcx, impl1_def_id, impl2_def_id)
169 || negative_impl(infcx.tcx, impl2_def_id, impl1_def_id)
175 // For the purposes of this check, we don't bring any placeholder
176 // types into scope; instead, we replace the generic types with
177 // fresh type variables, and hence we do our evaluations in an
178 // empty environment.
179 let param_env = ty::ParamEnv::empty();
181 let impl1_header = with_fresh_ty_vars(selcx, param_env, impl1_def_id);
182 let impl2_header = with_fresh_ty_vars(selcx, param_env, impl2_def_id);
184 let obligations = equate_impl_headers(selcx, &impl1_header, &impl2_header)?;
185 debug!("overlap: unification check succeeded");
187 if overlap_mode.use_implicit_negative() {
188 if implicit_negative(selcx, param_env, &impl1_header, impl2_header, obligations) {
193 // We disable the leak when creating the `snapshot` by using
194 // `infcx.probe_maybe_disable_leak_check`.
195 if infcx.leak_check(true, snapshot).is_err() {
196 debug!("overlap: leak check failed");
200 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
201 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
203 let involves_placeholder =
204 matches!(selcx.infcx.region_constraints_added_in_snapshot(snapshot), Some(true));
206 let impl_header = selcx.infcx.resolve_vars_if_possible(impl1_header);
207 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
210 fn equate_impl_headers<'cx, 'tcx>(
211 selcx: &mut SelectionContext<'cx, 'tcx>,
212 impl1_header: &ty::ImplHeader<'tcx>,
213 impl2_header: &ty::ImplHeader<'tcx>,
214 ) -> Option<PredicateObligations<'tcx>> {
215 // Do `a` and `b` unify? If not, no overlap.
216 debug!("equate_impl_headers(impl1_header={:?}, impl2_header={:?}", impl1_header, impl2_header);
219 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
220 .eq_impl_headers(impl1_header, impl2_header)
221 .map(|infer_ok| infer_ok.obligations)
225 /// Given impl1 and impl2 check if both impls can be satisfied by a common type (including
226 /// where-clauses) If so, return false, otherwise return true, they are disjoint.
227 fn implicit_negative<'cx, 'tcx>(
228 selcx: &mut SelectionContext<'cx, 'tcx>,
229 param_env: ty::ParamEnv<'tcx>,
230 impl1_header: &ty::ImplHeader<'tcx>,
231 impl2_header: ty::ImplHeader<'tcx>,
232 obligations: PredicateObligations<'tcx>,
234 // There's no overlap if obligations are unsatisfiable or if the obligation negated is
237 // For example, given these two impl headers:
239 // `impl<'a> From<&'a str> for Box<dyn Error>`
240 // `impl<E> From<E> for Box<dyn Error> where E: Error`
244 // `Box<dyn Error>: From<&'?a str>`
245 // `Box<dyn Error>: From<?E>`
247 // After equating the two headers:
249 // `Box<dyn Error> = Box<dyn Error>`
250 // So, `?E = &'?a str` and then given the where clause `&'?a str: Error`.
252 // If the obligation `&'?a str: Error` holds, it means that there's overlap. If that doesn't
253 // hold we need to check if `&'?a str: !Error` holds, if doesn't hold there's overlap because
254 // at some point an impl for `&'?a str: Error` could be added.
256 "implicit_negative(impl1_header={:?}, impl2_header={:?}, obligations={:?})",
257 impl1_header, impl2_header, obligations
259 let infcx = selcx.infcx;
260 let opt_failing_obligation = impl1_header
264 .chain(impl2_header.predicates)
265 .map(|p| infcx.resolve_vars_if_possible(p))
266 .map(|p| Obligation {
267 cause: ObligationCause::dummy(),
273 .find(|o| !selcx.predicate_may_hold_fatal(o));
275 if let Some(failing_obligation) = opt_failing_obligation {
276 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
283 /// Given impl1 and impl2 check if both impls are never satisfied by a common type (including
284 /// where-clauses) If so, return true, they are disjoint and false otherwise.
285 fn negative_impl(tcx: TyCtxt<'_>, impl1_def_id: DefId, impl2_def_id: DefId) -> bool {
286 debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
288 // Create an infcx, taking the predicates of impl1 as assumptions:
289 let infcx = tcx.infer_ctxt().build();
290 // create a parameter environment corresponding to a (placeholder) instantiation of impl1
291 let impl_env = tcx.param_env(impl1_def_id);
292 let subject1 = match traits::fully_normalize(
294 ObligationCause::dummy(),
296 tcx.impl_subject(impl1_def_id),
300 tcx.sess.delay_span_bug(
301 tcx.def_span(impl1_def_id),
302 format!("failed to fully normalize {:?}: {:?}", impl1_def_id, err),
308 // Attempt to prove that impl2 applies, given all of the above.
309 let selcx = &mut SelectionContext::new(&infcx);
310 let impl2_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl2_def_id);
311 let (subject2, obligations) =
312 impl_subject_and_oblig(selcx, impl_env, impl2_def_id, impl2_substs);
314 !equate(&infcx, impl_env, subject1, subject2, obligations, impl1_def_id)
318 infcx: &InferCtxt<'tcx>,
319 impl_env: ty::ParamEnv<'tcx>,
320 subject1: ImplSubject<'tcx>,
321 subject2: ImplSubject<'tcx>,
322 obligations: impl Iterator<Item = PredicateObligation<'tcx>>,
325 // do the impls unify? If not, not disjoint.
326 let Ok(InferOk { obligations: more_obligations, .. }) =
327 infcx.at(&ObligationCause::dummy(), impl_env).eq(subject1, subject2)
329 debug!("explicit_disjoint: {:?} does not unify with {:?}", subject1, subject2);
333 let opt_failing_obligation = obligations
335 .chain(more_obligations)
336 .find(|o| negative_impl_exists(infcx, o, body_def_id));
338 if let Some(failing_obligation) = opt_failing_obligation {
339 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
346 /// Try to prove that a negative impl exist for the given obligation and its super predicates.
347 #[instrument(level = "debug", skip(infcx))]
348 fn negative_impl_exists<'tcx>(
349 infcx: &InferCtxt<'tcx>,
350 o: &PredicateObligation<'tcx>,
353 if resolve_negative_obligation(infcx.fork(), o, body_def_id) {
357 // Try to prove a negative obligation exists for super predicates
358 for o in util::elaborate_predicates(infcx.tcx, iter::once(o.predicate)) {
359 if resolve_negative_obligation(infcx.fork(), &o, body_def_id) {
367 #[instrument(level = "debug", skip(infcx))]
368 fn resolve_negative_obligation<'tcx>(
369 infcx: InferCtxt<'tcx>,
370 o: &PredicateObligation<'tcx>,
375 let Some(o) = o.flip_polarity(tcx) else {
379 let param_env = o.param_env;
380 if !super::fully_solve_obligation(&infcx, o).is_empty() {
384 let body_def_id = body_def_id.as_local().unwrap_or(CRATE_DEF_ID);
386 let ocx = ObligationCtxt::new(&infcx);
387 let wf_tys = ocx.assumed_wf_types(param_env, DUMMY_SP, body_def_id);
388 let outlives_env = OutlivesEnvironment::with_bounds(
391 infcx.implied_bounds_tys(param_env, body_def_id, wf_tys),
394 infcx.process_registered_region_obligations(outlives_env.region_bound_pairs(), param_env);
396 infcx.resolve_regions(&outlives_env).is_empty()
399 #[instrument(level = "debug", skip(tcx), ret)]
400 pub fn trait_ref_is_knowable<'tcx>(
402 trait_ref: ty::TraitRef<'tcx>,
403 ) -> Result<(), Conflict> {
404 if orphan_check_trait_ref(trait_ref, InCrate::Remote).is_ok() {
405 // A downstream or cousin crate is allowed to implement some
406 // substitution of this trait-ref.
407 return Err(Conflict::Downstream);
410 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
411 // This is a local or fundamental trait, so future-compatibility
412 // is no concern. We know that downstream/cousin crates are not
413 // allowed to implement a substitution of this trait ref, which
414 // means impls could only come from dependencies of this crate,
415 // which we already know about.
419 // This is a remote non-fundamental trait, so if another crate
420 // can be the "final owner" of a substitution of this trait-ref,
421 // they are allowed to implement it future-compatibly.
423 // However, if we are a final owner, then nobody else can be,
424 // and if we are an intermediate owner, then we don't care
425 // about future-compatibility, which means that we're OK if
427 if orphan_check_trait_ref(trait_ref, InCrate::Local).is_ok() {
430 Err(Conflict::Upstream)
434 pub fn trait_ref_is_local_or_fundamental<'tcx>(
436 trait_ref: ty::TraitRef<'tcx>,
438 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
442 pub enum OrphanCheckErr<'tcx> {
443 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
444 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
447 /// Checks the coherence orphan rules. `impl_def_id` should be the
448 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
449 /// two conditions must be satisfied:
451 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
452 /// 2. Some local type must appear in `Self`.
453 #[instrument(level = "debug", skip(tcx), ret)]
454 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
455 // We only except this routine to be invoked on implementations
456 // of a trait, not inherent implementations.
457 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap().subst_identity();
460 // If the *trait* is local to the crate, ok.
461 if trait_ref.def_id.is_local() {
462 debug!("trait {:?} is local to current crate", trait_ref.def_id);
466 orphan_check_trait_ref(trait_ref, InCrate::Local)
469 /// Checks whether a trait-ref is potentially implementable by a crate.
471 /// The current rule is that a trait-ref orphan checks in a crate C:
473 /// 1. Order the parameters in the trait-ref in subst order - Self first,
474 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
475 /// 2. Of these type parameters, there is at least one type parameter
476 /// in which, walking the type as a tree, you can reach a type local
477 /// to C where all types in-between are fundamental types. Call the
478 /// first such parameter the "local key parameter".
479 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
480 /// going through `Box`, which is fundamental.
481 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
483 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
484 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
485 /// the local type and the type parameter.
486 /// 3. Before this local type, no generic type parameter of the impl must
487 /// be reachable through fundamental types.
488 /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
489 /// - while `impl<T> Trait<LocalType> for Box<T>` results in an error, as `T` is
490 /// reachable through the fundamental type `Box`.
491 /// 4. Every type in the local key parameter not known in C, going
492 /// through the parameter's type tree, must appear only as a subtree of
493 /// a type local to C, with only fundamental types between the type
494 /// local to C and the local key parameter.
495 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
496 /// is bad, because the only local type with `T` as a subtree is
497 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
498 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
499 /// the second occurrence of `T` is not a subtree of *any* local type.
500 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
501 /// `LocalType<Vec<T>>`, which is local and has no types between it and
502 /// the type parameter.
504 /// The orphan rules actually serve several different purposes:
506 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
507 /// every type local to one crate is unknown in the other) can't implement
508 /// the same trait-ref. This follows because it can be seen that no such
509 /// type can orphan-check in 2 such crates.
511 /// To check that a local impl follows the orphan rules, we check it in
512 /// InCrate::Local mode, using type parameters for the "generic" types.
514 /// 2. They ground negative reasoning for coherence. If a user wants to
515 /// write both a conditional blanket impl and a specific impl, we need to
516 /// make sure they do not overlap. For example, if we write
517 /// ```ignore (illustrative)
518 /// impl<T> IntoIterator for Vec<T>
519 /// impl<T: Iterator> IntoIterator for T
521 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
522 /// We can observe that this holds in the current crate, but we need to make
523 /// sure this will also hold in all unknown crates (both "independent" crates,
524 /// which we need for link-safety, and also child crates, because we don't want
525 /// child crates to get error for impl conflicts in a *dependency*).
527 /// For that, we only allow negative reasoning if, for every assignment to the
528 /// inference variables, every unknown crate would get an orphan error if they
529 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
530 /// mode. That is sound because we already know all the impls from known crates.
532 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
533 /// add "non-blanket" impls without breaking negative reasoning in dependent
534 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
536 /// For that, we only a allow crate to perform negative reasoning on
537 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
539 /// Because we never perform negative reasoning generically (coherence does
540 /// not involve type parameters), this can be interpreted as doing the full
541 /// orphan check (using InCrate::Local mode), substituting non-local known
542 /// types for all inference variables.
544 /// This allows for crates to future-compatibly add impls as long as they
545 /// can't apply to types with a key parameter in a child crate - applying
546 /// the rules, this basically means that every type parameter in the impl
547 /// must appear behind a non-fundamental type (because this is not a
548 /// type-system requirement, crate owners might also go for "semantic
549 /// future-compatibility" involving things such as sealed traits, but
550 /// the above requirement is sufficient, and is necessary in "open world"
553 /// Note that this function is never called for types that have both type
554 /// parameters and inference variables.
555 #[instrument(level = "trace", ret)]
556 fn orphan_check_trait_ref<'tcx>(
557 trait_ref: ty::TraitRef<'tcx>,
559 ) -> Result<(), OrphanCheckErr<'tcx>> {
560 if trait_ref.needs_infer() && trait_ref.needs_subst() {
562 "can't orphan check a trait ref with both params and inference variables {:?}",
567 let mut checker = OrphanChecker::new(in_crate);
568 match trait_ref.visit_with(&mut checker) {
569 ControlFlow::Continue(()) => Err(OrphanCheckErr::NonLocalInputType(checker.non_local_tys)),
570 ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(ty)) => {
571 // Does there exist some local type after the `ParamTy`.
572 checker.search_first_local_ty = true;
573 if let Some(OrphanCheckEarlyExit::LocalTy(local_ty)) =
574 trait_ref.visit_with(&mut checker).break_value()
576 Err(OrphanCheckErr::UncoveredTy(ty, Some(local_ty)))
578 Err(OrphanCheckErr::UncoveredTy(ty, None))
581 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(_)) => Ok(()),
585 struct OrphanChecker<'tcx> {
588 /// Ignore orphan check failures and exclusively search for the first
590 search_first_local_ty: bool,
591 non_local_tys: Vec<(Ty<'tcx>, bool)>,
594 impl<'tcx> OrphanChecker<'tcx> {
595 fn new(in_crate: InCrate) -> Self {
599 search_first_local_ty: false,
600 non_local_tys: Vec::new(),
604 fn found_non_local_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx>> {
605 self.non_local_tys.push((t, self.in_self_ty));
606 ControlFlow::Continue(())
609 fn found_param_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx>> {
610 if self.search_first_local_ty {
611 ControlFlow::Continue(())
613 ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(t))
617 fn def_id_is_local(&mut self, def_id: DefId) -> bool {
618 match self.in_crate {
619 InCrate::Local => def_id.is_local(),
620 InCrate::Remote => false,
625 enum OrphanCheckEarlyExit<'tcx> {
630 impl<'tcx> TypeVisitor<'tcx> for OrphanChecker<'tcx> {
631 type BreakTy = OrphanCheckEarlyExit<'tcx>;
632 fn visit_region(&mut self, _r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
633 ControlFlow::Continue(())
636 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
637 let result = match *ty.kind() {
651 | ty::Alias(ty::Projection, ..) => self.found_non_local_ty(ty),
653 ty::Param(..) => self.found_param_ty(ty),
655 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match self.in_crate {
656 InCrate::Local => self.found_non_local_ty(ty),
657 // The inference variable might be unified with a local
658 // type in that remote crate.
659 InCrate::Remote => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
662 // For fundamental types, we just look inside of them.
663 ty::Ref(_, ty, _) => ty.visit_with(self),
664 ty::Adt(def, substs) => {
665 if self.def_id_is_local(def.did()) {
666 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
667 } else if def.is_fundamental() {
668 substs.visit_with(self)
670 self.found_non_local_ty(ty)
673 ty::Foreign(def_id) => {
674 if self.def_id_is_local(def_id) {
675 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
677 self.found_non_local_ty(ty)
680 ty::Dynamic(tt, ..) => {
681 let principal = tt.principal().map(|p| p.def_id());
682 if principal.map_or(false, |p| self.def_id_is_local(p)) {
683 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
685 self.found_non_local_ty(ty)
688 ty::Error(_) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
689 ty::Closure(did, ..) | ty::Generator(did, ..) => {
690 if self.def_id_is_local(did) {
691 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
693 self.found_non_local_ty(ty)
696 // This should only be created when checking whether we have to check whether some
697 // auto trait impl applies. There will never be multiple impls, so we can just
698 // act as if it were a local type here.
699 ty::GeneratorWitness(_) | ty::GeneratorWitnessMIR(..) => {
700 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
702 ty::Alias(ty::Opaque, ..) => {
703 // This merits some explanation.
704 // Normally, opaque types are not involved when performing
705 // coherence checking, since it is illegal to directly
706 // implement a trait on an opaque type. However, we might
707 // end up looking at an opaque type during coherence checking
708 // if an opaque type gets used within another type (e.g. as
709 // the type of a field) when checking for auto trait or `Sized`
710 // impls. This requires us to decide whether or not an opaque
711 // type should be considered 'local' or not.
713 // We choose to treat all opaque types as non-local, even
714 // those that appear within the same crate. This seems
715 // somewhat surprising at first, but makes sense when
716 // you consider that opaque types are supposed to hide
717 // the underlying type *within the same crate*. When an
718 // opaque type is used from outside the module
719 // where it is declared, it should be impossible to observe
720 // anything about it other than the traits that it implements.
722 // The alternative would be to look at the underlying type
723 // to determine whether or not the opaque type itself should
724 // be considered local. However, this could make it a breaking change
725 // to switch the underlying ('defining') type from a local type
726 // to a remote type. This would violate the rule that opaque
727 // types should be completely opaque apart from the traits
728 // that they implement, so we don't use this behavior.
729 self.found_non_local_ty(ty)
732 // A bit of a hack, the `OrphanChecker` is only used to visit a `TraitRef`, so
733 // the first type we visit is always the self type.
734 self.in_self_ty = false;
738 /// All possible values for a constant parameter already exist
739 /// in the crate defining the trait, so they are always non-local[^1].
741 /// Because there's no way to have an impl where the first local
742 /// generic argument is a constant, we also don't have to fail
743 /// the orphan check when encountering a parameter or a generic constant.
745 /// This means that we can completely ignore constants during the orphan check.
747 /// See `tests/ui/coherence/const-generics-orphan-check-ok.rs` for examples.
749 /// [^1]: This might not hold for function pointers or trait objects in the future.
750 /// As these should be quite rare as const arguments and especially rare as impl
751 /// parameters, allowing uncovered const parameters in impls seems more useful
752 /// than allowing `impl<T> Trait<local_fn_ptr, T> for i32` to compile.
753 fn visit_const(&mut self, _c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
754 ControlFlow::Continue(())