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, 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");
97 let infcx = tcx.infer_ctxt().build();
98 let selcx = &mut SelectionContext::intercrate(&infcx);
100 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).is_some();
105 // In the case where we detect an error, run the check again, but
106 // this time tracking intercrate ambiguity causes for better
107 // diagnostics. (These take time and can lead to false errors.)
108 let infcx = tcx.infer_ctxt().build();
109 let selcx = &mut SelectionContext::intercrate(&infcx);
110 selcx.enable_tracking_intercrate_ambiguity_causes();
111 Some(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).unwrap())
114 fn with_fresh_ty_vars<'cx, 'tcx>(
115 selcx: &mut SelectionContext<'cx, 'tcx>,
116 param_env: ty::ParamEnv<'tcx>,
118 ) -> ty::ImplHeader<'tcx> {
119 let tcx = selcx.tcx();
120 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
122 let header = ty::ImplHeader {
124 self_ty: tcx.bound_type_of(impl_def_id).subst(tcx, impl_substs),
125 trait_ref: tcx.bound_impl_trait_ref(impl_def_id).map(|i| i.subst(tcx, impl_substs)),
126 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
129 let Normalized { value: mut header, obligations } =
130 traits::normalize(selcx, param_env, ObligationCause::dummy(), header);
132 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
136 /// Can both impl `a` and impl `b` be satisfied by a common type (including
137 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
138 fn overlap<'cx, 'tcx>(
139 selcx: &mut SelectionContext<'cx, 'tcx>,
140 skip_leak_check: SkipLeakCheck,
143 overlap_mode: OverlapMode,
144 ) -> Option<OverlapResult<'tcx>> {
146 "overlap(impl1_def_id={:?}, impl2_def_id={:?}, overlap_mode={:?})",
147 impl1_def_id, impl2_def_id, overlap_mode
150 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
151 overlap_within_probe(selcx, impl1_def_id, impl2_def_id, overlap_mode, snapshot)
155 fn overlap_within_probe<'cx, 'tcx>(
156 selcx: &mut SelectionContext<'cx, 'tcx>,
159 overlap_mode: OverlapMode,
160 snapshot: &CombinedSnapshot<'tcx>,
161 ) -> Option<OverlapResult<'tcx>> {
162 let infcx = selcx.infcx();
164 if overlap_mode.use_negative_impl() {
165 if negative_impl(selcx, impl1_def_id, impl2_def_id)
166 || negative_impl(selcx, impl2_def_id, impl1_def_id)
172 // For the purposes of this check, we don't bring any placeholder
173 // types into scope; instead, we replace the generic types with
174 // fresh type variables, and hence we do our evaluations in an
175 // empty environment.
176 let param_env = ty::ParamEnv::empty();
178 let impl1_header = with_fresh_ty_vars(selcx, param_env, impl1_def_id);
179 let impl2_header = with_fresh_ty_vars(selcx, param_env, impl2_def_id);
181 let obligations = equate_impl_headers(selcx, &impl1_header, &impl2_header)?;
182 debug!("overlap: unification check succeeded");
184 if overlap_mode.use_implicit_negative() {
185 if implicit_negative(selcx, param_env, &impl1_header, impl2_header, obligations) {
190 // We disable the leak when creating the `snapshot` by using
191 // `infcx.probe_maybe_disable_leak_check`.
192 if infcx.leak_check(true, snapshot).is_err() {
193 debug!("overlap: leak check failed");
197 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
198 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
200 let involves_placeholder =
201 matches!(selcx.infcx().region_constraints_added_in_snapshot(snapshot), Some(true));
203 let impl_header = selcx.infcx().resolve_vars_if_possible(impl1_header);
204 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
207 fn equate_impl_headers<'cx, 'tcx>(
208 selcx: &mut SelectionContext<'cx, 'tcx>,
209 impl1_header: &ty::ImplHeader<'tcx>,
210 impl2_header: &ty::ImplHeader<'tcx>,
211 ) -> Option<PredicateObligations<'tcx>> {
212 // Do `a` and `b` unify? If not, no overlap.
213 debug!("equate_impl_headers(impl1_header={:?}, impl2_header={:?}", impl1_header, impl2_header);
216 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
217 .eq_impl_headers(impl1_header, impl2_header)
218 .map(|infer_ok| infer_ok.obligations)
222 /// Given impl1 and impl2 check if both impls can be satisfied by a common type (including
223 /// where-clauses) If so, return false, otherwise return true, they are disjoint.
224 fn implicit_negative<'cx, 'tcx>(
225 selcx: &mut SelectionContext<'cx, 'tcx>,
226 param_env: ty::ParamEnv<'tcx>,
227 impl1_header: &ty::ImplHeader<'tcx>,
228 impl2_header: ty::ImplHeader<'tcx>,
229 obligations: PredicateObligations<'tcx>,
231 // There's no overlap if obligations are unsatisfiable or if the obligation negated is
234 // For example, given these two impl headers:
236 // `impl<'a> From<&'a str> for Box<dyn Error>`
237 // `impl<E> From<E> for Box<dyn Error> where E: Error`
241 // `Box<dyn Error>: From<&'?a str>`
242 // `Box<dyn Error>: From<?E>`
244 // After equating the two headers:
246 // `Box<dyn Error> = Box<dyn Error>`
247 // So, `?E = &'?a str` and then given the where clause `&'?a str: Error`.
249 // If the obligation `&'?a str: Error` holds, it means that there's overlap. If that doesn't
250 // hold we need to check if `&'?a str: !Error` holds, if doesn't hold there's overlap because
251 // at some point an impl for `&'?a str: Error` could be added.
253 "implicit_negative(impl1_header={:?}, impl2_header={:?}, obligations={:?})",
254 impl1_header, impl2_header, obligations
256 let infcx = selcx.infcx();
257 let opt_failing_obligation = impl1_header
261 .chain(impl2_header.predicates)
262 .map(|p| infcx.resolve_vars_if_possible(p))
263 .map(|p| Obligation {
264 cause: ObligationCause::dummy(),
270 .find(|o| !selcx.predicate_may_hold_fatal(o));
272 if let Some(failing_obligation) = opt_failing_obligation {
273 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
280 /// Given impl1 and impl2 check if both impls are never satisfied by a common type (including
281 /// where-clauses) If so, return true, they are disjoint and false otherwise.
282 fn negative_impl<'cx, 'tcx>(
283 selcx: &mut SelectionContext<'cx, 'tcx>,
287 debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
288 let tcx = selcx.infcx().tcx;
290 // Create an infcx, taking the predicates of impl1 as assumptions:
291 let infcx = tcx.infer_ctxt().build();
292 // create a parameter environment corresponding to a (placeholder) instantiation of impl1
293 let impl_env = tcx.param_env(impl1_def_id);
294 let subject1 = match traits::fully_normalize(
296 ObligationCause::dummy(),
298 tcx.impl_subject(impl1_def_id),
302 tcx.sess.delay_span_bug(
303 tcx.def_span(impl1_def_id),
304 format!("failed to fully normalize {:?}: {:?}", impl1_def_id, err),
310 // Attempt to prove that impl2 applies, given all of the above.
311 let selcx = &mut SelectionContext::new(&infcx);
312 let impl2_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl2_def_id);
313 let (subject2, obligations) =
314 impl_subject_and_oblig(selcx, impl_env, impl2_def_id, impl2_substs);
316 !equate(&infcx, impl_env, subject1, subject2, obligations, impl1_def_id)
320 infcx: &InferCtxt<'tcx>,
321 impl_env: ty::ParamEnv<'tcx>,
322 subject1: ImplSubject<'tcx>,
323 subject2: ImplSubject<'tcx>,
324 obligations: impl Iterator<Item = PredicateObligation<'tcx>>,
327 // do the impls unify? If not, not disjoint.
328 let Ok(InferOk { obligations: more_obligations, .. }) =
329 infcx.at(&ObligationCause::dummy(), impl_env).eq(subject1, subject2)
331 debug!("explicit_disjoint: {:?} does not unify with {:?}", subject1, subject2);
335 let selcx = &mut SelectionContext::new(&infcx);
336 let opt_failing_obligation = obligations
338 .chain(more_obligations)
339 .find(|o| negative_impl_exists(selcx, o, body_def_id));
341 if let Some(failing_obligation) = opt_failing_obligation {
342 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
349 /// Try to prove that a negative impl exist for the given obligation and its super predicates.
350 #[instrument(level = "debug", skip(selcx))]
351 fn negative_impl_exists<'cx, 'tcx>(
352 selcx: &SelectionContext<'cx, 'tcx>,
353 o: &PredicateObligation<'tcx>,
356 if resolve_negative_obligation(selcx.infcx().fork(), o, body_def_id) {
360 // Try to prove a negative obligation exists for super predicates
361 for o in util::elaborate_predicates(selcx.tcx(), iter::once(o.predicate)) {
362 if resolve_negative_obligation(selcx.infcx().fork(), &o, body_def_id) {
370 #[instrument(level = "debug", skip(infcx))]
371 fn resolve_negative_obligation<'tcx>(
372 infcx: InferCtxt<'tcx>,
373 o: &PredicateObligation<'tcx>,
378 let Some(o) = o.flip_polarity(tcx) else {
382 let param_env = o.param_env;
383 if !super::fully_solve_obligation(&infcx, o).is_empty() {
387 let (body_id, body_def_id) = if let Some(body_def_id) = body_def_id.as_local() {
388 (tcx.hir().local_def_id_to_hir_id(body_def_id), body_def_id)
390 (CRATE_HIR_ID, CRATE_DEF_ID)
393 let ocx = ObligationCtxt::new(&infcx);
394 let wf_tys = ocx.assumed_wf_types(param_env, DUMMY_SP, body_def_id);
395 let outlives_env = OutlivesEnvironment::with_bounds(
398 infcx.implied_bounds_tys(param_env, body_id, wf_tys),
401 infcx.process_registered_region_obligations(outlives_env.region_bound_pairs(), param_env);
403 infcx.resolve_regions(&outlives_env).is_empty()
406 pub fn trait_ref_is_knowable<'tcx>(
408 trait_ref: ty::TraitRef<'tcx>,
409 ) -> Result<(), Conflict> {
410 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
411 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
412 // A downstream or cousin crate is allowed to implement some
413 // substitution of this trait-ref.
414 return Err(Conflict::Downstream);
417 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
418 // This is a local or fundamental trait, so future-compatibility
419 // is no concern. We know that downstream/cousin crates are not
420 // allowed to implement a substitution of this trait ref, which
421 // means impls could only come from dependencies of this crate,
422 // which we already know about.
426 // This is a remote non-fundamental trait, so if another crate
427 // can be the "final owner" of a substitution of this trait-ref,
428 // they are allowed to implement it future-compatibly.
430 // However, if we are a final owner, then nobody else can be,
431 // and if we are an intermediate owner, then we don't care
432 // about future-compatibility, which means that we're OK if
434 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
435 debug!("trait_ref_is_knowable: orphan check passed");
438 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
439 Err(Conflict::Upstream)
443 pub fn trait_ref_is_local_or_fundamental<'tcx>(
445 trait_ref: ty::TraitRef<'tcx>,
447 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
450 pub enum OrphanCheckErr<'tcx> {
451 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
452 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
455 /// Checks the coherence orphan rules. `impl_def_id` should be the
456 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
457 /// two conditions must be satisfied:
459 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
460 /// 2. Some local type must appear in `Self`.
461 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
462 debug!("orphan_check({:?})", impl_def_id);
464 // We only except this routine to be invoked on implementations
465 // of a trait, not inherent implementations.
466 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
467 debug!("orphan_check: trait_ref={:?}", trait_ref);
469 // If the *trait* is local to the crate, ok.
470 if trait_ref.def_id.is_local() {
471 debug!("trait {:?} is local to current crate", trait_ref.def_id);
475 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
478 /// Checks whether a trait-ref is potentially implementable by a crate.
480 /// The current rule is that a trait-ref orphan checks in a crate C:
482 /// 1. Order the parameters in the trait-ref in subst order - Self first,
483 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
484 /// 2. Of these type parameters, there is at least one type parameter
485 /// in which, walking the type as a tree, you can reach a type local
486 /// to C where all types in-between are fundamental types. Call the
487 /// first such parameter the "local key parameter".
488 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
489 /// going through `Box`, which is fundamental.
490 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
492 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
493 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
494 /// the local type and the type parameter.
495 /// 3. Before this local type, no generic type parameter of the impl must
496 /// be reachable through fundamental types.
497 /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
498 /// - while `impl<T> Trait<LocalType> for Box<T>` results in an error, as `T` is
499 /// reachable through the fundamental type `Box`.
500 /// 4. Every type in the local key parameter not known in C, going
501 /// through the parameter's type tree, must appear only as a subtree of
502 /// a type local to C, with only fundamental types between the type
503 /// local to C and the local key parameter.
504 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
505 /// is bad, because the only local type with `T` as a subtree is
506 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
507 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
508 /// the second occurrence of `T` is not a subtree of *any* local type.
509 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
510 /// `LocalType<Vec<T>>`, which is local and has no types between it and
511 /// the type parameter.
513 /// The orphan rules actually serve several different purposes:
515 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
516 /// every type local to one crate is unknown in the other) can't implement
517 /// the same trait-ref. This follows because it can be seen that no such
518 /// type can orphan-check in 2 such crates.
520 /// To check that a local impl follows the orphan rules, we check it in
521 /// InCrate::Local mode, using type parameters for the "generic" types.
523 /// 2. They ground negative reasoning for coherence. If a user wants to
524 /// write both a conditional blanket impl and a specific impl, we need to
525 /// make sure they do not overlap. For example, if we write
526 /// ```ignore (illustrative)
527 /// impl<T> IntoIterator for Vec<T>
528 /// impl<T: Iterator> IntoIterator for T
530 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
531 /// We can observe that this holds in the current crate, but we need to make
532 /// sure this will also hold in all unknown crates (both "independent" crates,
533 /// which we need for link-safety, and also child crates, because we don't want
534 /// child crates to get error for impl conflicts in a *dependency*).
536 /// For that, we only allow negative reasoning if, for every assignment to the
537 /// inference variables, every unknown crate would get an orphan error if they
538 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
539 /// mode. That is sound because we already know all the impls from known crates.
541 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
542 /// add "non-blanket" impls without breaking negative reasoning in dependent
543 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
545 /// For that, we only a allow crate to perform negative reasoning on
546 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
548 /// Because we never perform negative reasoning generically (coherence does
549 /// not involve type parameters), this can be interpreted as doing the full
550 /// orphan check (using InCrate::Local mode), substituting non-local known
551 /// types for all inference variables.
553 /// This allows for crates to future-compatibly add impls as long as they
554 /// can't apply to types with a key parameter in a child crate - applying
555 /// the rules, this basically means that every type parameter in the impl
556 /// must appear behind a non-fundamental type (because this is not a
557 /// type-system requirement, crate owners might also go for "semantic
558 /// future-compatibility" involving things such as sealed traits, but
559 /// the above requirement is sufficient, and is necessary in "open world"
562 /// Note that this function is never called for types that have both type
563 /// parameters and inference variables.
564 fn orphan_check_trait_ref<'tcx>(
566 trait_ref: ty::TraitRef<'tcx>,
568 ) -> Result<(), OrphanCheckErr<'tcx>> {
569 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
571 if trait_ref.needs_infer() && trait_ref.needs_subst() {
573 "can't orphan check a trait ref with both params and inference variables {:?}",
578 let mut checker = OrphanChecker::new(tcx, in_crate);
579 match trait_ref.visit_with(&mut checker) {
580 ControlFlow::Continue(()) => Err(OrphanCheckErr::NonLocalInputType(checker.non_local_tys)),
581 ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(ty)) => {
582 // Does there exist some local type after the `ParamTy`.
583 checker.search_first_local_ty = true;
584 if let Some(OrphanCheckEarlyExit::LocalTy(local_ty)) =
585 trait_ref.visit_with(&mut checker).break_value()
587 Err(OrphanCheckErr::UncoveredTy(ty, Some(local_ty)))
589 Err(OrphanCheckErr::UncoveredTy(ty, None))
592 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(_)) => Ok(()),
596 struct OrphanChecker<'tcx> {
600 /// Ignore orphan check failures and exclusively search for the first
602 search_first_local_ty: bool,
603 non_local_tys: Vec<(Ty<'tcx>, bool)>,
606 impl<'tcx> OrphanChecker<'tcx> {
607 fn new(tcx: TyCtxt<'tcx>, in_crate: InCrate) -> Self {
612 search_first_local_ty: false,
613 non_local_tys: Vec::new(),
617 fn found_non_local_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx>> {
618 self.non_local_tys.push((t, self.in_self_ty));
619 ControlFlow::CONTINUE
622 fn found_param_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx>> {
623 if self.search_first_local_ty {
624 ControlFlow::CONTINUE
626 ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(t))
630 fn def_id_is_local(&mut self, def_id: DefId) -> bool {
631 match self.in_crate {
632 InCrate::Local => def_id.is_local(),
633 InCrate::Remote => false,
638 enum OrphanCheckEarlyExit<'tcx> {
643 impl<'tcx> TypeVisitor<'tcx> for OrphanChecker<'tcx> {
644 type BreakTy = OrphanCheckEarlyExit<'tcx>;
645 fn visit_region(&mut self, _r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
646 ControlFlow::CONTINUE
649 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
650 let result = match *ty.kind() {
664 | ty::Projection(..) => self.found_non_local_ty(ty),
666 ty::Param(..) => self.found_param_ty(ty),
668 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match self.in_crate {
669 InCrate::Local => self.found_non_local_ty(ty),
670 // The inference variable might be unified with a local
671 // type in that remote crate.
672 InCrate::Remote => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
675 // For fundamental types, we just look inside of them.
676 ty::Ref(_, ty, _) => ty.visit_with(self),
677 ty::Adt(def, substs) => {
678 if self.def_id_is_local(def.did()) {
679 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
680 } else if def.is_fundamental() {
681 substs.visit_with(self)
683 self.found_non_local_ty(ty)
686 ty::Foreign(def_id) => {
687 if self.def_id_is_local(def_id) {
688 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
690 self.found_non_local_ty(ty)
693 ty::Dynamic(tt, ..) => {
694 let principal = tt.principal().map(|p| p.def_id());
695 if principal.map_or(false, |p| self.def_id_is_local(p)) {
696 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
698 self.found_non_local_ty(ty)
701 ty::Error(_) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
702 ty::Closure(..) | ty::Generator(..) | ty::GeneratorWitness(..) => {
703 self.tcx.sess.delay_span_bug(
705 format!("ty_is_local invoked on closure or generator: {:?}", ty),
707 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
710 // This merits some explanation.
711 // Normally, opaque types are not involved when performing
712 // coherence checking, since it is illegal to directly
713 // implement a trait on an opaque type. However, we might
714 // end up looking at an opaque type during coherence checking
715 // if an opaque type gets used within another type (e.g. as
716 // the type of a field) when checking for auto trait or `Sized`
717 // impls. This requires us to decide whether or not an opaque
718 // type should be considered 'local' or not.
720 // We choose to treat all opaque types as non-local, even
721 // those that appear within the same crate. This seems
722 // somewhat surprising at first, but makes sense when
723 // you consider that opaque types are supposed to hide
724 // the underlying type *within the same crate*. When an
725 // opaque type is used from outside the module
726 // where it is declared, it should be impossible to observe
727 // anything about it other than the traits that it implements.
729 // The alternative would be to look at the underlying type
730 // to determine whether or not the opaque type itself should
731 // be considered local. However, this could make it a breaking change
732 // to switch the underlying ('defining') type from a local type
733 // to a remote type. This would violate the rule that opaque
734 // types should be completely opaque apart from the traits
735 // that they implement, so we don't use this behavior.
736 self.found_non_local_ty(ty)
739 // A bit of a hack, the `OrphanChecker` is only used to visit a `TraitRef`, so
740 // the first type we visit is always the self type.
741 self.in_self_ty = false;
745 /// All possible values for a constant parameter already exist
746 /// in the crate defining the trait, so they are always non-local[^1].
748 /// Because there's no way to have an impl where the first local
749 /// generic argument is a constant, we also don't have to fail
750 /// the orphan check when encountering a parameter or a generic constant.
752 /// This means that we can completely ignore constants during the orphan check.
754 /// See `src/test/ui/coherence/const-generics-orphan-check-ok.rs` for examples.
756 /// [^1]: This might not hold for function pointers or trait objects in the future.
757 /// As these should be quite rare as const arguments and especially rare as impl
758 /// parameters, allowing uncovered const parameters in impls seems more useful
759 /// than allowing `impl<T> Trait<local_fn_ptr, T> for i32` to compile.
760 fn visit_const(&mut self, _c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
761 ControlFlow::CONTINUE