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::{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, invokes `on_overlap`
64 /// with a suitably-freshened `ImplHeader` with those types
65 /// substituted. Otherwise, invokes `no_overlap`.
66 #[instrument(skip(tcx, skip_leak_check, on_overlap, no_overlap), level = "debug")]
67 pub fn overlapping_impls<F1, F2, R>(
71 skip_leak_check: SkipLeakCheck,
72 overlap_mode: OverlapMode,
77 F1: FnOnce(OverlapResult<'_>) -> R,
80 // Before doing expensive operations like entering an inference context, do
81 // a quick check via fast_reject to tell if the impl headers could possibly
83 let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsInfer };
84 let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
85 let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
86 let may_overlap = match (impl1_ref, impl2_ref) {
87 (Some(a), Some(b)) => iter::zip(a.substs, b.substs)
88 .all(|(arg1, arg2)| drcx.generic_args_may_unify(arg1, arg2)),
90 let self_ty1 = tcx.type_of(impl1_def_id);
91 let self_ty2 = tcx.type_of(impl2_def_id);
92 drcx.types_may_unify(self_ty1, self_ty2)
94 _ => bug!("unexpected impls: {impl1_def_id:?} {impl2_def_id:?}"),
98 // Some types involved are definitely different, so the impls couldn't possibly overlap.
99 debug!("overlapping_impls: fast_reject early-exit");
103 let overlaps = tcx.infer_ctxt().enter(|infcx| {
104 let selcx = &mut SelectionContext::intercrate(&infcx);
105 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).is_some()
112 // In the case where we detect an error, run the check again, but
113 // this time tracking intercrate ambiguity causes for better
114 // diagnostics. (These take time and can lead to false errors.)
115 tcx.infer_ctxt().enter(|infcx| {
116 let selcx = &mut SelectionContext::intercrate(&infcx);
117 selcx.enable_tracking_intercrate_ambiguity_causes();
119 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).unwrap(),
124 fn with_fresh_ty_vars<'cx, 'tcx>(
125 selcx: &mut SelectionContext<'cx, 'tcx>,
126 param_env: ty::ParamEnv<'tcx>,
128 ) -> ty::ImplHeader<'tcx> {
129 let tcx = selcx.tcx();
130 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
132 let header = ty::ImplHeader {
134 self_ty: tcx.bound_type_of(impl_def_id).subst(tcx, impl_substs),
135 trait_ref: tcx.bound_impl_trait_ref(impl_def_id).map(|i| i.subst(tcx, impl_substs)),
136 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
139 let Normalized { value: mut header, obligations } =
140 traits::normalize(selcx, param_env, ObligationCause::dummy(), header);
142 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
146 /// Can both impl `a` and impl `b` be satisfied by a common type (including
147 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
148 fn overlap<'cx, 'tcx>(
149 selcx: &mut SelectionContext<'cx, 'tcx>,
150 skip_leak_check: SkipLeakCheck,
153 overlap_mode: OverlapMode,
154 ) -> Option<OverlapResult<'tcx>> {
156 "overlap(impl1_def_id={:?}, impl2_def_id={:?}, overlap_mode={:?})",
157 impl1_def_id, impl2_def_id, overlap_mode
160 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
161 overlap_within_probe(selcx, impl1_def_id, impl2_def_id, overlap_mode, snapshot)
165 fn overlap_within_probe<'cx, 'tcx>(
166 selcx: &mut SelectionContext<'cx, 'tcx>,
169 overlap_mode: OverlapMode,
170 snapshot: &CombinedSnapshot<'_, 'tcx>,
171 ) -> Option<OverlapResult<'tcx>> {
172 let infcx = selcx.infcx();
174 if overlap_mode.use_negative_impl() {
175 if negative_impl(selcx, impl1_def_id, impl2_def_id)
176 || negative_impl(selcx, impl2_def_id, impl1_def_id)
182 // For the purposes of this check, we don't bring any placeholder
183 // types into scope; instead, we replace the generic types with
184 // fresh type variables, and hence we do our evaluations in an
185 // empty environment.
186 let param_env = ty::ParamEnv::empty();
188 let impl1_header = with_fresh_ty_vars(selcx, param_env, impl1_def_id);
189 let impl2_header = with_fresh_ty_vars(selcx, param_env, impl2_def_id);
191 let obligations = equate_impl_headers(selcx, &impl1_header, &impl2_header)?;
192 debug!("overlap: unification check succeeded");
194 if overlap_mode.use_implicit_negative() {
195 if implicit_negative(selcx, param_env, &impl1_header, impl2_header, obligations) {
200 // We disable the leak when when creating the `snapshot` by using
201 // `infcx.probe_maybe_disable_leak_check`.
202 if infcx.leak_check(true, snapshot).is_err() {
203 debug!("overlap: leak check failed");
207 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
208 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
210 let involves_placeholder =
211 matches!(selcx.infcx().region_constraints_added_in_snapshot(snapshot), Some(true));
213 let impl_header = selcx.infcx().resolve_vars_if_possible(impl1_header);
214 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
217 fn equate_impl_headers<'cx, 'tcx>(
218 selcx: &mut SelectionContext<'cx, 'tcx>,
219 impl1_header: &ty::ImplHeader<'tcx>,
220 impl2_header: &ty::ImplHeader<'tcx>,
221 ) -> Option<PredicateObligations<'tcx>> {
222 // Do `a` and `b` unify? If not, no overlap.
223 debug!("equate_impl_headers(impl1_header={:?}, impl2_header={:?}", impl1_header, impl2_header);
226 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
227 .eq_impl_headers(impl1_header, impl2_header)
228 .map(|infer_ok| infer_ok.obligations)
232 /// Given impl1 and impl2 check if both impls can be satisfied by a common type (including
233 /// where-clauses) If so, return false, otherwise return true, they are disjoint.
234 fn implicit_negative<'cx, 'tcx>(
235 selcx: &mut SelectionContext<'cx, 'tcx>,
236 param_env: ty::ParamEnv<'tcx>,
237 impl1_header: &ty::ImplHeader<'tcx>,
238 impl2_header: ty::ImplHeader<'tcx>,
239 obligations: PredicateObligations<'tcx>,
241 // There's no overlap if obligations are unsatisfiable or if the obligation negated is
244 // For example, given these two impl headers:
246 // `impl<'a> From<&'a str> for Box<dyn Error>`
247 // `impl<E> From<E> for Box<dyn Error> where E: Error`
251 // `Box<dyn Error>: From<&'?a str>`
252 // `Box<dyn Error>: From<?E>`
254 // After equating the two headers:
256 // `Box<dyn Error> = Box<dyn Error>`
257 // So, `?E = &'?a str` and then given the where clause `&'?a str: Error`.
259 // If the obligation `&'?a str: Error` holds, it means that there's overlap. If that doesn't
260 // hold we need to check if `&'?a str: !Error` holds, if doesn't hold there's overlap because
261 // at some point an impl for `&'?a str: Error` could be added.
263 "implicit_negative(impl1_header={:?}, impl2_header={:?}, obligations={:?})",
264 impl1_header, impl2_header, obligations
266 let infcx = selcx.infcx();
267 let opt_failing_obligation = impl1_header
271 .chain(impl2_header.predicates)
272 .map(|p| infcx.resolve_vars_if_possible(p))
273 .map(|p| Obligation {
274 cause: ObligationCause::dummy(),
280 .find(|o| !selcx.predicate_may_hold_fatal(o));
282 if let Some(failing_obligation) = opt_failing_obligation {
283 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
290 /// Given impl1 and impl2 check if both impls are never satisfied by a common type (including
291 /// where-clauses) If so, return true, they are disjoint and false otherwise.
292 fn negative_impl<'cx, 'tcx>(
293 selcx: &mut SelectionContext<'cx, 'tcx>,
297 debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
298 let tcx = selcx.infcx().tcx;
300 // Create an infcx, taking the predicates of impl1 as assumptions:
301 tcx.infer_ctxt().enter(|infcx| {
302 // create a parameter environment corresponding to a (placeholder) instantiation of impl1
303 let impl_env = tcx.param_env(impl1_def_id);
304 let subject1 = match traits::fully_normalize(
306 ObligationCause::dummy(),
308 tcx.impl_subject(impl1_def_id),
312 tcx.sess.delay_span_bug(
313 tcx.def_span(impl1_def_id),
314 format!("failed to fully normalize {:?}: {:?}", impl1_def_id, err),
320 // Attempt to prove that impl2 applies, given all of the above.
321 let selcx = &mut SelectionContext::new(&infcx);
322 let impl2_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl2_def_id);
323 let (subject2, obligations) =
324 impl_subject_and_oblig(selcx, impl_env, impl2_def_id, impl2_substs);
326 !equate(&infcx, impl_env, subject1, subject2, obligations, impl1_def_id)
330 fn equate<'cx, 'tcx>(
331 infcx: &InferCtxt<'cx, 'tcx>,
332 impl_env: ty::ParamEnv<'tcx>,
333 subject1: ImplSubject<'tcx>,
334 subject2: ImplSubject<'tcx>,
335 obligations: impl Iterator<Item = PredicateObligation<'tcx>>,
338 // do the impls unify? If not, not disjoint.
339 let Ok(InferOk { obligations: more_obligations, .. }) =
340 infcx.at(&ObligationCause::dummy(), impl_env).eq(subject1, subject2)
342 debug!("explicit_disjoint: {:?} does not unify with {:?}", subject1, subject2);
346 let selcx = &mut SelectionContext::new(&infcx);
347 let opt_failing_obligation = obligations
349 .chain(more_obligations)
350 .find(|o| negative_impl_exists(selcx, o, body_def_id));
352 if let Some(failing_obligation) = opt_failing_obligation {
353 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
360 /// Try to prove that a negative impl exist for the given obligation and its super predicates.
361 #[instrument(level = "debug", skip(selcx))]
362 fn negative_impl_exists<'cx, 'tcx>(
363 selcx: &SelectionContext<'cx, 'tcx>,
364 o: &PredicateObligation<'tcx>,
367 if resolve_negative_obligation(selcx.infcx().fork(), o, body_def_id) {
371 // Try to prove a negative obligation exists for super predicates
372 for o in util::elaborate_predicates(selcx.tcx(), iter::once(o.predicate)) {
373 if resolve_negative_obligation(selcx.infcx().fork(), &o, body_def_id) {
381 #[instrument(level = "debug", skip(infcx))]
382 fn resolve_negative_obligation<'cx, 'tcx>(
383 infcx: InferCtxt<'cx, 'tcx>,
384 o: &PredicateObligation<'tcx>,
389 let Some(o) = o.flip_polarity(tcx) else {
393 let param_env = o.param_env;
394 if !super::fully_solve_obligation(&infcx, o).is_empty() {
398 let (body_id, body_def_id) = if let Some(body_def_id) = body_def_id.as_local() {
399 (tcx.hir().local_def_id_to_hir_id(body_def_id), body_def_id)
401 (CRATE_HIR_ID, CRATE_DEF_ID)
404 let ocx = ObligationCtxt::new(&infcx);
405 let wf_tys = ocx.assumed_wf_types(param_env, DUMMY_SP, body_def_id);
406 let outlives_env = OutlivesEnvironment::with_bounds(
409 infcx.implied_bounds_tys(param_env, body_id, wf_tys),
412 infcx.process_registered_region_obligations(outlives_env.region_bound_pairs(), param_env);
414 infcx.resolve_regions(&outlives_env).is_empty()
417 pub fn trait_ref_is_knowable<'tcx>(
419 trait_ref: ty::TraitRef<'tcx>,
420 ) -> Result<(), Conflict> {
421 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
422 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
423 // A downstream or cousin crate is allowed to implement some
424 // substitution of this trait-ref.
425 return Err(Conflict::Downstream);
428 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
429 // This is a local or fundamental trait, so future-compatibility
430 // is no concern. We know that downstream/cousin crates are not
431 // allowed to implement a substitution of this trait ref, which
432 // means impls could only come from dependencies of this crate,
433 // which we already know about.
437 // This is a remote non-fundamental trait, so if another crate
438 // can be the "final owner" of a substitution of this trait-ref,
439 // they are allowed to implement it future-compatibly.
441 // However, if we are a final owner, then nobody else can be,
442 // and if we are an intermediate owner, then we don't care
443 // about future-compatibility, which means that we're OK if
445 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
446 debug!("trait_ref_is_knowable: orphan check passed");
449 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
450 Err(Conflict::Upstream)
454 pub fn trait_ref_is_local_or_fundamental<'tcx>(
456 trait_ref: ty::TraitRef<'tcx>,
458 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
461 pub enum OrphanCheckErr<'tcx> {
462 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
463 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
466 /// Checks the coherence orphan rules. `impl_def_id` should be the
467 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
468 /// two conditions must be satisfied:
470 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
471 /// 2. Some local type must appear in `Self`.
472 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
473 debug!("orphan_check({:?})", impl_def_id);
475 // We only except this routine to be invoked on implementations
476 // of a trait, not inherent implementations.
477 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
478 debug!("orphan_check: trait_ref={:?}", trait_ref);
480 // If the *trait* is local to the crate, ok.
481 if trait_ref.def_id.is_local() {
482 debug!("trait {:?} is local to current crate", trait_ref.def_id);
486 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
489 /// Checks whether a trait-ref is potentially implementable by a crate.
491 /// The current rule is that a trait-ref orphan checks in a crate C:
493 /// 1. Order the parameters in the trait-ref in subst order - Self first,
494 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
495 /// 2. Of these type parameters, there is at least one type parameter
496 /// in which, walking the type as a tree, you can reach a type local
497 /// to C where all types in-between are fundamental types. Call the
498 /// first such parameter the "local key parameter".
499 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
500 /// going through `Box`, which is fundamental.
501 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
503 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
504 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
505 /// the local type and the type parameter.
506 /// 3. Before this local type, no generic type parameter of the impl must
507 /// be reachable through fundamental types.
508 /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
509 /// - while `impl<T> Trait<LocalType> for Box<T>` results in an error, as `T` is
510 /// reachable through the fundamental type `Box`.
511 /// 4. Every type in the local key parameter not known in C, going
512 /// through the parameter's type tree, must appear only as a subtree of
513 /// a type local to C, with only fundamental types between the type
514 /// local to C and the local key parameter.
515 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
516 /// is bad, because the only local type with `T` as a subtree is
517 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
518 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
519 /// the second occurrence of `T` is not a subtree of *any* local type.
520 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
521 /// `LocalType<Vec<T>>`, which is local and has no types between it and
522 /// the type parameter.
524 /// The orphan rules actually serve several different purposes:
526 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
527 /// every type local to one crate is unknown in the other) can't implement
528 /// the same trait-ref. This follows because it can be seen that no such
529 /// type can orphan-check in 2 such crates.
531 /// To check that a local impl follows the orphan rules, we check it in
532 /// InCrate::Local mode, using type parameters for the "generic" types.
534 /// 2. They ground negative reasoning for coherence. If a user wants to
535 /// write both a conditional blanket impl and a specific impl, we need to
536 /// make sure they do not overlap. For example, if we write
537 /// ```ignore (illustrative)
538 /// impl<T> IntoIterator for Vec<T>
539 /// impl<T: Iterator> IntoIterator for T
541 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
542 /// We can observe that this holds in the current crate, but we need to make
543 /// sure this will also hold in all unknown crates (both "independent" crates,
544 /// which we need for link-safety, and also child crates, because we don't want
545 /// child crates to get error for impl conflicts in a *dependency*).
547 /// For that, we only allow negative reasoning if, for every assignment to the
548 /// inference variables, every unknown crate would get an orphan error if they
549 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
550 /// mode. That is sound because we already know all the impls from known crates.
552 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
553 /// add "non-blanket" impls without breaking negative reasoning in dependent
554 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
556 /// For that, we only a allow crate to perform negative reasoning on
557 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
559 /// Because we never perform negative reasoning generically (coherence does
560 /// not involve type parameters), this can be interpreted as doing the full
561 /// orphan check (using InCrate::Local mode), substituting non-local known
562 /// types for all inference variables.
564 /// This allows for crates to future-compatibly add impls as long as they
565 /// can't apply to types with a key parameter in a child crate - applying
566 /// the rules, this basically means that every type parameter in the impl
567 /// must appear behind a non-fundamental type (because this is not a
568 /// type-system requirement, crate owners might also go for "semantic
569 /// future-compatibility" involving things such as sealed traits, but
570 /// the above requirement is sufficient, and is necessary in "open world"
573 /// Note that this function is never called for types that have both type
574 /// parameters and inference variables.
575 fn orphan_check_trait_ref<'tcx>(
577 trait_ref: ty::TraitRef<'tcx>,
579 ) -> Result<(), OrphanCheckErr<'tcx>> {
580 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
582 if trait_ref.needs_infer() && trait_ref.needs_subst() {
584 "can't orphan check a trait ref with both params and inference variables {:?}",
589 let mut checker = OrphanChecker::new(tcx, in_crate);
590 match trait_ref.visit_with(&mut checker) {
591 ControlFlow::Continue(()) => Err(OrphanCheckErr::NonLocalInputType(checker.non_local_tys)),
592 ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(ty)) => {
593 // Does there exist some local type after the `ParamTy`.
594 checker.search_first_local_ty = true;
595 if let Some(OrphanCheckEarlyExit::LocalTy(local_ty)) =
596 trait_ref.visit_with(&mut checker).break_value()
598 Err(OrphanCheckErr::UncoveredTy(ty, Some(local_ty)))
600 Err(OrphanCheckErr::UncoveredTy(ty, None))
603 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(_)) => Ok(()),
607 struct OrphanChecker<'tcx> {
611 /// Ignore orphan check failures and exclusively search for the first
613 search_first_local_ty: bool,
614 non_local_tys: Vec<(Ty<'tcx>, bool)>,
617 impl<'tcx> OrphanChecker<'tcx> {
618 fn new(tcx: TyCtxt<'tcx>, in_crate: InCrate) -> Self {
623 search_first_local_ty: false,
624 non_local_tys: Vec::new(),
628 fn found_non_local_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx>> {
629 self.non_local_tys.push((t, self.in_self_ty));
630 ControlFlow::CONTINUE
633 fn found_param_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx>> {
634 if self.search_first_local_ty {
635 ControlFlow::CONTINUE
637 ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(t))
641 fn def_id_is_local(&mut self, def_id: DefId) -> bool {
642 match self.in_crate {
643 InCrate::Local => def_id.is_local(),
644 InCrate::Remote => false,
649 enum OrphanCheckEarlyExit<'tcx> {
654 impl<'tcx> TypeVisitor<'tcx> for OrphanChecker<'tcx> {
655 type BreakTy = OrphanCheckEarlyExit<'tcx>;
656 fn visit_region(&mut self, _r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
657 ControlFlow::CONTINUE
660 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
661 let result = match *ty.kind() {
675 | ty::Projection(..) => self.found_non_local_ty(ty),
677 ty::Param(..) => self.found_param_ty(ty),
679 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match self.in_crate {
680 InCrate::Local => self.found_non_local_ty(ty),
681 // The inference variable might be unified with a local
682 // type in that remote crate.
683 InCrate::Remote => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
686 // For fundamental types, we just look inside of them.
687 ty::Ref(_, ty, _) => ty.visit_with(self),
688 ty::Adt(def, substs) => {
689 if self.def_id_is_local(def.did()) {
690 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
691 } else if def.is_fundamental() {
692 substs.visit_with(self)
694 self.found_non_local_ty(ty)
697 ty::Foreign(def_id) => {
698 if self.def_id_is_local(def_id) {
699 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
701 self.found_non_local_ty(ty)
704 ty::Dynamic(tt, ..) => {
705 let principal = tt.principal().map(|p| p.def_id());
706 if principal.map_or(false, |p| self.def_id_is_local(p)) {
707 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
709 self.found_non_local_ty(ty)
712 ty::Error(_) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
713 ty::Closure(..) | ty::Generator(..) | ty::GeneratorWitness(..) => {
714 self.tcx.sess.delay_span_bug(
716 format!("ty_is_local invoked on closure or generator: {:?}", ty),
718 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
721 // This merits some explanation.
722 // Normally, opaque types are not involved when performing
723 // coherence checking, since it is illegal to directly
724 // implement a trait on an opaque type. However, we might
725 // end up looking at an opaque type during coherence checking
726 // if an opaque type gets used within another type (e.g. as
727 // the type of a field) when checking for auto trait or `Sized`
728 // impls. This requires us to decide whether or not an opaque
729 // type should be considered 'local' or not.
731 // We choose to treat all opaque types as non-local, even
732 // those that appear within the same crate. This seems
733 // somewhat surprising at first, but makes sense when
734 // you consider that opaque types are supposed to hide
735 // the underlying type *within the same crate*. When an
736 // opaque type is used from outside the module
737 // where it is declared, it should be impossible to observe
738 // anything about it other than the traits that it implements.
740 // The alternative would be to look at the underlying type
741 // to determine whether or not the opaque type itself should
742 // be considered local. However, this could make it a breaking change
743 // to switch the underlying ('defining') type from a local type
744 // to a remote type. This would violate the rule that opaque
745 // types should be completely opaque apart from the traits
746 // that they implement, so we don't use this behavior.
747 self.found_non_local_ty(ty)
750 // A bit of a hack, the `OrphanChecker` is only used to visit a `TraitRef`, so
751 // the first type we visit is always the self type.
752 self.in_self_ty = false;
756 /// All possible values for a constant parameter already exist
757 /// in the crate defining the trait, so they are always non-local[^1].
759 /// Because there's no way to have an impl where the first local
760 /// generic argument is a constant, we also don't have to fail
761 /// the orphan check when encountering a parameter or a generic constant.
763 /// This means that we can completely ignore constants during the orphan check.
765 /// See `src/test/ui/coherence/const-generics-orphan-check-ok.rs` for examples.
767 /// [^1]: This might not hold for function pointers or trait objects in the future.
768 /// As these should be quite rare as const arguments and especially rare as impl
769 /// parameters, allowing uncovered const parameters in impls seems more useful
770 /// than allowing `impl<T> Trait<local_fn_ptr, T> for i32` to compile.
771 fn visit_const(&mut self, _c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
772 ControlFlow::CONTINUE