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, Normalized, Obligation, ObligationCause, ObligationCtxt, PredicateObligation,
15 PredicateObligations, SelectionContext,
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_hir::CRATE_HIR_ID;
21 use rustc_infer::infer::{DefiningAnchor, InferCtxt, TyCtxtInferExt};
22 use rustc_infer::traits::util;
23 use rustc_middle::traits::specialization_graph::OverlapMode;
24 use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams};
25 use rustc_middle::ty::visit::TypeVisitable;
26 use rustc_middle::ty::{self, ImplSubject, Ty, TyCtxt, TypeVisitor};
27 use rustc_span::symbol::sym;
28 use rustc_span::DUMMY_SP;
31 use std::ops::ControlFlow;
33 /// Whether we do the orphan check relative to this crate or
34 /// to some remote crate.
35 #[derive(Copy, Clone, Debug)]
41 #[derive(Debug, Copy, Clone)]
47 pub struct OverlapResult<'tcx> {
48 pub impl_header: ty::ImplHeader<'tcx>,
49 pub intercrate_ambiguity_causes: FxIndexSet<IntercrateAmbiguityCause>,
51 /// `true` if the overlap might've been permitted before the shift
53 pub involves_placeholder: bool,
56 pub fn add_placeholder_note(err: &mut Diagnostic) {
58 "this behavior recently changed as a result of a bug fix; \
59 see rust-lang/rust#56105 for details",
63 /// If there are types that satisfy both impls, returns `Some`
64 /// with a suitably-freshened `ImplHeader` with those types
65 /// substituted. Otherwise, returns `None`.
66 #[instrument(skip(tcx, skip_leak_check), level = "debug")]
67 pub fn overlapping_impls<'tcx>(
71 skip_leak_check: SkipLeakCheck,
72 overlap_mode: OverlapMode,
73 ) -> Option<OverlapResult<'tcx>> {
74 // Before doing expensive operations like entering an inference context, do
75 // a quick check via fast_reject to tell if the impl headers could possibly
77 let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsInfer };
78 let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
79 let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
80 let may_overlap = match (impl1_ref, impl2_ref) {
81 (Some(a), Some(b)) => iter::zip(a.substs, b.substs)
82 .all(|(arg1, arg2)| drcx.generic_args_may_unify(arg1, arg2)),
84 let self_ty1 = tcx.type_of(impl1_def_id);
85 let self_ty2 = tcx.type_of(impl2_def_id);
86 drcx.types_may_unify(self_ty1, self_ty2)
88 _ => bug!("unexpected impls: {impl1_def_id:?} {impl2_def_id:?}"),
92 // Some types involved are definitely different, so the impls couldn't possibly overlap.
93 debug!("overlapping_impls: fast_reject early-exit");
98 tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).intercrate().build();
99 let selcx = &mut SelectionContext::new(&infcx);
101 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).is_some();
106 // In the case where we detect an error, run the check again, but
107 // this time tracking intercrate ambiguity causes for better
108 // diagnostics. (These take time and can lead to false errors.)
110 tcx.infer_ctxt().with_opaque_type_inference(DefiningAnchor::Bubble).intercrate().build();
111 let selcx = &mut SelectionContext::new(&infcx);
112 selcx.enable_tracking_intercrate_ambiguity_causes();
113 Some(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).unwrap())
116 fn with_fresh_ty_vars<'cx, 'tcx>(
117 selcx: &mut SelectionContext<'cx, 'tcx>,
118 param_env: ty::ParamEnv<'tcx>,
120 ) -> ty::ImplHeader<'tcx> {
121 let tcx = selcx.tcx();
122 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
124 let header = ty::ImplHeader {
126 self_ty: tcx.bound_type_of(impl_def_id).subst(tcx, impl_substs),
127 trait_ref: tcx.bound_impl_trait_ref(impl_def_id).map(|i| i.subst(tcx, impl_substs)),
128 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
131 let Normalized { value: mut header, obligations } =
132 traits::normalize(selcx, param_env, ObligationCause::dummy(), header);
134 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
138 /// Can both impl `a` and impl `b` be satisfied by a common type (including
139 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
140 fn overlap<'cx, 'tcx>(
141 selcx: &mut SelectionContext<'cx, 'tcx>,
142 skip_leak_check: SkipLeakCheck,
145 overlap_mode: OverlapMode,
146 ) -> Option<OverlapResult<'tcx>> {
148 "overlap(impl1_def_id={:?}, impl2_def_id={:?}, overlap_mode={:?})",
149 impl1_def_id, impl2_def_id, overlap_mode
152 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
153 overlap_within_probe(selcx, impl1_def_id, impl2_def_id, overlap_mode, snapshot)
157 fn overlap_within_probe<'cx, 'tcx>(
158 selcx: &mut SelectionContext<'cx, 'tcx>,
161 overlap_mode: OverlapMode,
162 snapshot: &CombinedSnapshot<'tcx>,
163 ) -> Option<OverlapResult<'tcx>> {
164 let infcx = selcx.infcx();
166 if overlap_mode.use_negative_impl() {
167 if negative_impl(infcx.tcx, impl1_def_id, impl2_def_id)
168 || negative_impl(infcx.tcx, impl2_def_id, impl1_def_id)
174 // For the purposes of this check, we don't bring any placeholder
175 // types into scope; instead, we replace the generic types with
176 // fresh type variables, and hence we do our evaluations in an
177 // empty environment.
178 let param_env = ty::ParamEnv::empty();
180 let impl1_header = with_fresh_ty_vars(selcx, param_env, impl1_def_id);
181 let impl2_header = with_fresh_ty_vars(selcx, param_env, impl2_def_id);
183 let obligations = equate_impl_headers(selcx, &impl1_header, &impl2_header)?;
184 debug!("overlap: unification check succeeded");
186 if overlap_mode.use_implicit_negative() {
187 if implicit_negative(selcx, param_env, &impl1_header, impl2_header, obligations) {
192 // We disable the leak when creating the `snapshot` by using
193 // `infcx.probe_maybe_disable_leak_check`.
194 if infcx.leak_check(true, snapshot).is_err() {
195 debug!("overlap: leak check failed");
199 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
200 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
202 let involves_placeholder =
203 matches!(selcx.infcx().region_constraints_added_in_snapshot(snapshot), Some(true));
205 let impl_header = selcx.infcx().resolve_vars_if_possible(impl1_header);
206 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
209 fn equate_impl_headers<'cx, 'tcx>(
210 selcx: &mut SelectionContext<'cx, 'tcx>,
211 impl1_header: &ty::ImplHeader<'tcx>,
212 impl2_header: &ty::ImplHeader<'tcx>,
213 ) -> Option<PredicateObligations<'tcx>> {
214 // Do `a` and `b` unify? If not, no overlap.
215 debug!("equate_impl_headers(impl1_header={:?}, impl2_header={:?}", impl1_header, impl2_header);
218 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
219 .eq_impl_headers(impl1_header, impl2_header)
220 .map(|infer_ok| infer_ok.obligations)
224 /// Given impl1 and impl2 check if both impls can be satisfied by a common type (including
225 /// where-clauses) If so, return false, otherwise return true, they are disjoint.
226 fn implicit_negative<'cx, 'tcx>(
227 selcx: &mut SelectionContext<'cx, 'tcx>,
228 param_env: ty::ParamEnv<'tcx>,
229 impl1_header: &ty::ImplHeader<'tcx>,
230 impl2_header: ty::ImplHeader<'tcx>,
231 obligations: PredicateObligations<'tcx>,
233 // There's no overlap if obligations are unsatisfiable or if the obligation negated is
236 // For example, given these two impl headers:
238 // `impl<'a> From<&'a str> for Box<dyn Error>`
239 // `impl<E> From<E> for Box<dyn Error> where E: Error`
243 // `Box<dyn Error>: From<&'?a str>`
244 // `Box<dyn Error>: From<?E>`
246 // After equating the two headers:
248 // `Box<dyn Error> = Box<dyn Error>`
249 // So, `?E = &'?a str` and then given the where clause `&'?a str: Error`.
251 // If the obligation `&'?a str: Error` holds, it means that there's overlap. If that doesn't
252 // hold we need to check if `&'?a str: !Error` holds, if doesn't hold there's overlap because
253 // at some point an impl for `&'?a str: Error` could be added.
255 "implicit_negative(impl1_header={:?}, impl2_header={:?}, obligations={:?})",
256 impl1_header, impl2_header, obligations
258 let infcx = selcx.infcx();
259 let opt_failing_obligation = impl1_header
263 .chain(impl2_header.predicates)
264 .map(|p| infcx.resolve_vars_if_possible(p))
265 .map(|p| Obligation {
266 cause: ObligationCause::dummy(),
272 .find(|o| !selcx.predicate_may_hold_fatal(o));
274 if let Some(failing_obligation) = opt_failing_obligation {
275 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
282 /// Given impl1 and impl2 check if both impls are never satisfied by a common type (including
283 /// where-clauses) If so, return true, they are disjoint and false otherwise.
284 fn negative_impl<'tcx>(tcx: TyCtxt<'tcx>, impl1_def_id: DefId, impl2_def_id: DefId) -> bool {
285 debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
287 // Create an infcx, taking the predicates of impl1 as assumptions:
288 let infcx = tcx.infer_ctxt().build();
289 // create a parameter environment corresponding to a (placeholder) instantiation of impl1
290 let impl_env = tcx.param_env(impl1_def_id);
291 let subject1 = match traits::fully_normalize(
293 ObligationCause::dummy(),
295 tcx.impl_subject(impl1_def_id),
299 tcx.sess.delay_span_bug(
300 tcx.def_span(impl1_def_id),
301 format!("failed to fully normalize {:?}: {:?}", impl1_def_id, err),
307 // Attempt to prove that impl2 applies, given all of the above.
308 let selcx = &mut SelectionContext::new(&infcx);
309 let impl2_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl2_def_id);
310 let (subject2, obligations) =
311 impl_subject_and_oblig(selcx, impl_env, impl2_def_id, impl2_substs);
313 !equate(&infcx, impl_env, subject1, subject2, obligations, impl1_def_id)
317 infcx: &InferCtxt<'tcx>,
318 impl_env: ty::ParamEnv<'tcx>,
319 subject1: ImplSubject<'tcx>,
320 subject2: ImplSubject<'tcx>,
321 obligations: impl Iterator<Item = PredicateObligation<'tcx>>,
324 // do the impls unify? If not, not disjoint.
325 let Ok(InferOk { obligations: more_obligations, .. }) =
326 infcx.at(&ObligationCause::dummy(), impl_env).eq(subject1, subject2)
328 debug!("explicit_disjoint: {:?} does not unify with {:?}", subject1, subject2);
332 let opt_failing_obligation = obligations
334 .chain(more_obligations)
335 .find(|o| negative_impl_exists(infcx, o, body_def_id));
337 if let Some(failing_obligation) = opt_failing_obligation {
338 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
345 /// Try to prove that a negative impl exist for the given obligation and its super predicates.
346 #[instrument(level = "debug", skip(infcx))]
347 fn negative_impl_exists<'tcx>(
348 infcx: &InferCtxt<'tcx>,
349 o: &PredicateObligation<'tcx>,
352 if resolve_negative_obligation(infcx.fork(), o, body_def_id) {
356 // Try to prove a negative obligation exists for super predicates
357 for o in util::elaborate_predicates(infcx.tcx, iter::once(o.predicate)) {
358 if resolve_negative_obligation(infcx.fork(), &o, body_def_id) {
366 #[instrument(level = "debug", skip(infcx))]
367 fn resolve_negative_obligation<'tcx>(
368 infcx: InferCtxt<'tcx>,
369 o: &PredicateObligation<'tcx>,
374 let Some(o) = o.flip_polarity(tcx) else {
378 let param_env = o.param_env;
379 if !super::fully_solve_obligation(&infcx, o).is_empty() {
383 let (body_id, body_def_id) = if let Some(body_def_id) = body_def_id.as_local() {
384 (tcx.hir().local_def_id_to_hir_id(body_def_id), body_def_id)
386 (CRATE_HIR_ID, CRATE_DEF_ID)
389 let ocx = ObligationCtxt::new(&infcx);
390 let wf_tys = ocx.assumed_wf_types(param_env, DUMMY_SP, body_def_id);
391 let outlives_env = OutlivesEnvironment::with_bounds(
394 infcx.implied_bounds_tys(param_env, body_id, wf_tys),
397 infcx.process_registered_region_obligations(outlives_env.region_bound_pairs(), param_env);
399 infcx.resolve_regions(&outlives_env).is_empty()
402 pub fn trait_ref_is_knowable<'tcx>(
404 trait_ref: ty::TraitRef<'tcx>,
405 ) -> Result<(), Conflict> {
406 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
407 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
408 // A downstream or cousin crate is allowed to implement some
409 // substitution of this trait-ref.
410 return Err(Conflict::Downstream);
413 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
414 // This is a local or fundamental trait, so future-compatibility
415 // is no concern. We know that downstream/cousin crates are not
416 // allowed to implement a substitution of this trait ref, which
417 // means impls could only come from dependencies of this crate,
418 // which we already know about.
422 // This is a remote non-fundamental trait, so if another crate
423 // can be the "final owner" of a substitution of this trait-ref,
424 // they are allowed to implement it future-compatibly.
426 // However, if we are a final owner, then nobody else can be,
427 // and if we are an intermediate owner, then we don't care
428 // about future-compatibility, which means that we're OK if
430 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
431 debug!("trait_ref_is_knowable: orphan check passed");
434 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
435 Err(Conflict::Upstream)
439 pub fn trait_ref_is_local_or_fundamental<'tcx>(
441 trait_ref: ty::TraitRef<'tcx>,
443 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
446 pub enum OrphanCheckErr<'tcx> {
447 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
448 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
451 /// Checks the coherence orphan rules. `impl_def_id` should be the
452 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
453 /// two conditions must be satisfied:
455 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
456 /// 2. Some local type must appear in `Self`.
457 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
458 debug!("orphan_check({:?})", impl_def_id);
460 // We only except this routine to be invoked on implementations
461 // of a trait, not inherent implementations.
462 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
463 debug!("orphan_check: trait_ref={:?}", trait_ref);
465 // If the *trait* is local to the crate, ok.
466 if trait_ref.def_id.is_local() {
467 debug!("trait {:?} is local to current crate", trait_ref.def_id);
471 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
474 /// Checks whether a trait-ref is potentially implementable by a crate.
476 /// The current rule is that a trait-ref orphan checks in a crate C:
478 /// 1. Order the parameters in the trait-ref in subst order - Self first,
479 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
480 /// 2. Of these type parameters, there is at least one type parameter
481 /// in which, walking the type as a tree, you can reach a type local
482 /// to C where all types in-between are fundamental types. Call the
483 /// first such parameter the "local key parameter".
484 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
485 /// going through `Box`, which is fundamental.
486 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
488 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
489 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
490 /// the local type and the type parameter.
491 /// 3. Before this local type, no generic type parameter of the impl must
492 /// be reachable through fundamental types.
493 /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
494 /// - while `impl<T> Trait<LocalType> for Box<T>` results in an error, as `T` is
495 /// reachable through the fundamental type `Box`.
496 /// 4. Every type in the local key parameter not known in C, going
497 /// through the parameter's type tree, must appear only as a subtree of
498 /// a type local to C, with only fundamental types between the type
499 /// local to C and the local key parameter.
500 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
501 /// is bad, because the only local type with `T` as a subtree is
502 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
503 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
504 /// the second occurrence of `T` is not a subtree of *any* local type.
505 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
506 /// `LocalType<Vec<T>>`, which is local and has no types between it and
507 /// the type parameter.
509 /// The orphan rules actually serve several different purposes:
511 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
512 /// every type local to one crate is unknown in the other) can't implement
513 /// the same trait-ref. This follows because it can be seen that no such
514 /// type can orphan-check in 2 such crates.
516 /// To check that a local impl follows the orphan rules, we check it in
517 /// InCrate::Local mode, using type parameters for the "generic" types.
519 /// 2. They ground negative reasoning for coherence. If a user wants to
520 /// write both a conditional blanket impl and a specific impl, we need to
521 /// make sure they do not overlap. For example, if we write
522 /// ```ignore (illustrative)
523 /// impl<T> IntoIterator for Vec<T>
524 /// impl<T: Iterator> IntoIterator for T
526 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
527 /// We can observe that this holds in the current crate, but we need to make
528 /// sure this will also hold in all unknown crates (both "independent" crates,
529 /// which we need for link-safety, and also child crates, because we don't want
530 /// child crates to get error for impl conflicts in a *dependency*).
532 /// For that, we only allow negative reasoning if, for every assignment to the
533 /// inference variables, every unknown crate would get an orphan error if they
534 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
535 /// mode. That is sound because we already know all the impls from known crates.
537 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
538 /// add "non-blanket" impls without breaking negative reasoning in dependent
539 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
541 /// For that, we only a allow crate to perform negative reasoning on
542 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
544 /// Because we never perform negative reasoning generically (coherence does
545 /// not involve type parameters), this can be interpreted as doing the full
546 /// orphan check (using InCrate::Local mode), substituting non-local known
547 /// types for all inference variables.
549 /// This allows for crates to future-compatibly add impls as long as they
550 /// can't apply to types with a key parameter in a child crate - applying
551 /// the rules, this basically means that every type parameter in the impl
552 /// must appear behind a non-fundamental type (because this is not a
553 /// type-system requirement, crate owners might also go for "semantic
554 /// future-compatibility" involving things such as sealed traits, but
555 /// the above requirement is sufficient, and is necessary in "open world"
558 /// Note that this function is never called for types that have both type
559 /// parameters and inference variables.
560 fn orphan_check_trait_ref<'tcx>(
562 trait_ref: ty::TraitRef<'tcx>,
564 ) -> Result<(), OrphanCheckErr<'tcx>> {
565 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
567 if trait_ref.needs_infer() && trait_ref.needs_subst() {
569 "can't orphan check a trait ref with both params and inference variables {:?}",
574 let mut checker = OrphanChecker::new(tcx, in_crate);
575 match trait_ref.visit_with(&mut checker) {
576 ControlFlow::Continue(()) => Err(OrphanCheckErr::NonLocalInputType(checker.non_local_tys)),
577 ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(ty)) => {
578 // Does there exist some local type after the `ParamTy`.
579 checker.search_first_local_ty = true;
580 if let Some(OrphanCheckEarlyExit::LocalTy(local_ty)) =
581 trait_ref.visit_with(&mut checker).break_value()
583 Err(OrphanCheckErr::UncoveredTy(ty, Some(local_ty)))
585 Err(OrphanCheckErr::UncoveredTy(ty, None))
588 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(_)) => Ok(()),
592 struct OrphanChecker<'tcx> {
596 /// Ignore orphan check failures and exclusively search for the first
598 search_first_local_ty: bool,
599 non_local_tys: Vec<(Ty<'tcx>, bool)>,
602 impl<'tcx> OrphanChecker<'tcx> {
603 fn new(tcx: TyCtxt<'tcx>, in_crate: InCrate) -> Self {
608 search_first_local_ty: false,
609 non_local_tys: Vec::new(),
613 fn found_non_local_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx>> {
614 self.non_local_tys.push((t, self.in_self_ty));
615 ControlFlow::CONTINUE
618 fn found_param_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx>> {
619 if self.search_first_local_ty {
620 ControlFlow::CONTINUE
622 ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(t))
626 fn def_id_is_local(&mut self, def_id: DefId) -> bool {
627 match self.in_crate {
628 InCrate::Local => def_id.is_local(),
629 InCrate::Remote => false,
634 enum OrphanCheckEarlyExit<'tcx> {
639 impl<'tcx> TypeVisitor<'tcx> for OrphanChecker<'tcx> {
640 type BreakTy = OrphanCheckEarlyExit<'tcx>;
641 fn visit_region(&mut self, _r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
642 ControlFlow::CONTINUE
645 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
646 let result = match *ty.kind() {
660 | ty::Projection(..) => self.found_non_local_ty(ty),
662 ty::Param(..) => self.found_param_ty(ty),
664 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match self.in_crate {
665 InCrate::Local => self.found_non_local_ty(ty),
666 // The inference variable might be unified with a local
667 // type in that remote crate.
668 InCrate::Remote => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
671 // For fundamental types, we just look inside of them.
672 ty::Ref(_, ty, _) => ty.visit_with(self),
673 ty::Adt(def, substs) => {
674 if self.def_id_is_local(def.did()) {
675 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
676 } else if def.is_fundamental() {
677 substs.visit_with(self)
679 self.found_non_local_ty(ty)
682 ty::Foreign(def_id) => {
683 if self.def_id_is_local(def_id) {
684 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
686 self.found_non_local_ty(ty)
689 ty::Dynamic(tt, ..) => {
690 let principal = tt.principal().map(|p| p.def_id());
691 if principal.map_or(false, |p| self.def_id_is_local(p)) {
692 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
694 self.found_non_local_ty(ty)
697 ty::Error(_) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
698 ty::Closure(..) | ty::Generator(..) | ty::GeneratorWitness(..) => {
699 self.tcx.sess.delay_span_bug(
701 format!("ty_is_local invoked on closure or generator: {:?}", ty),
703 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
706 // This merits some explanation.
707 // Normally, opaque types are not involved when performing
708 // coherence checking, since it is illegal to directly
709 // implement a trait on an opaque type. However, we might
710 // end up looking at an opaque type during coherence checking
711 // if an opaque type gets used within another type (e.g. as
712 // the type of a field) when checking for auto trait or `Sized`
713 // impls. This requires us to decide whether or not an opaque
714 // type should be considered 'local' or not.
716 // We choose to treat all opaque types as non-local, even
717 // those that appear within the same crate. This seems
718 // somewhat surprising at first, but makes sense when
719 // you consider that opaque types are supposed to hide
720 // the underlying type *within the same crate*. When an
721 // opaque type is used from outside the module
722 // where it is declared, it should be impossible to observe
723 // anything about it other than the traits that it implements.
725 // The alternative would be to look at the underlying type
726 // to determine whether or not the opaque type itself should
727 // be considered local. However, this could make it a breaking change
728 // to switch the underlying ('defining') type from a local type
729 // to a remote type. This would violate the rule that opaque
730 // types should be completely opaque apart from the traits
731 // that they implement, so we don't use this behavior.
732 self.found_non_local_ty(ty)
735 // A bit of a hack, the `OrphanChecker` is only used to visit a `TraitRef`, so
736 // the first type we visit is always the self type.
737 self.in_self_ty = false;
741 /// All possible values for a constant parameter already exist
742 /// in the crate defining the trait, so they are always non-local[^1].
744 /// Because there's no way to have an impl where the first local
745 /// generic argument is a constant, we also don't have to fail
746 /// the orphan check when encountering a parameter or a generic constant.
748 /// This means that we can completely ignore constants during the orphan check.
750 /// See `src/test/ui/coherence/const-generics-orphan-check-ok.rs` for examples.
752 /// [^1]: This might not hold for function pointers or trait objects in the future.
753 /// As these should be quite rare as const arguments and especially rare as impl
754 /// parameters, allowing uncovered const parameters in impls seems more useful
755 /// than allowing `impl<T> Trait<local_fn_ptr, T> for i32` to compile.
756 fn visit_const(&mut self, _c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
757 ControlFlow::CONTINUE