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::subst::Subst;
26 use rustc_middle::ty::visit::TypeVisitable;
27 use rustc_middle::ty::{self, ImplSubject, Ty, TyCtxt, TypeVisitor};
28 use rustc_span::symbol::sym;
29 use rustc_span::DUMMY_SP;
32 use std::ops::ControlFlow;
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, invokes `on_overlap`
65 /// with a suitably-freshened `ImplHeader` with those types
66 /// substituted. Otherwise, invokes `no_overlap`.
67 #[instrument(skip(tcx, skip_leak_check, on_overlap, no_overlap), level = "debug")]
68 pub fn overlapping_impls<F1, F2, R>(
72 skip_leak_check: SkipLeakCheck,
73 overlap_mode: OverlapMode,
78 F1: FnOnce(OverlapResult<'_>) -> R,
81 // Before doing expensive operations like entering an inference context, do
82 // a quick check via fast_reject to tell if the impl headers could possibly
84 let drcx = DeepRejectCtxt { treat_obligation_params: TreatParams::AsInfer };
85 let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
86 let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
87 let may_overlap = match (impl1_ref, impl2_ref) {
88 (Some(a), Some(b)) => iter::zip(a.substs, b.substs)
89 .all(|(arg1, arg2)| drcx.generic_args_may_unify(arg1, arg2)),
91 let self_ty1 = tcx.type_of(impl1_def_id);
92 let self_ty2 = tcx.type_of(impl2_def_id);
93 drcx.types_may_unify(self_ty1, self_ty2)
95 _ => bug!("unexpected impls: {impl1_def_id:?} {impl2_def_id:?}"),
99 // Some types involved are definitely different, so the impls couldn't possibly overlap.
100 debug!("overlapping_impls: fast_reject early-exit");
104 let overlaps = tcx.infer_ctxt().enter(|infcx| {
105 let selcx = &mut SelectionContext::intercrate(&infcx);
106 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).is_some()
113 // In the case where we detect an error, run the check again, but
114 // this time tracking intercrate ambiguity causes for better
115 // diagnostics. (These take time and can lead to false errors.)
116 tcx.infer_ctxt().enter(|infcx| {
117 let selcx = &mut SelectionContext::intercrate(&infcx);
118 selcx.enable_tracking_intercrate_ambiguity_causes();
120 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).unwrap(),
125 fn with_fresh_ty_vars<'cx, 'tcx>(
126 selcx: &mut SelectionContext<'cx, 'tcx>,
127 param_env: ty::ParamEnv<'tcx>,
129 ) -> ty::ImplHeader<'tcx> {
130 let tcx = selcx.tcx();
131 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
133 let header = ty::ImplHeader {
135 self_ty: tcx.bound_type_of(impl_def_id).subst(tcx, impl_substs),
136 trait_ref: tcx.bound_impl_trait_ref(impl_def_id).map(|i| i.subst(tcx, impl_substs)),
137 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
140 let Normalized { value: mut header, obligations } =
141 traits::normalize(selcx, param_env, ObligationCause::dummy(), header);
143 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
147 /// Can both impl `a` and impl `b` be satisfied by a common type (including
148 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
149 fn overlap<'cx, 'tcx>(
150 selcx: &mut SelectionContext<'cx, 'tcx>,
151 skip_leak_check: SkipLeakCheck,
154 overlap_mode: OverlapMode,
155 ) -> Option<OverlapResult<'tcx>> {
157 "overlap(impl1_def_id={:?}, impl2_def_id={:?}, overlap_mode={:?})",
158 impl1_def_id, impl2_def_id, overlap_mode
161 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
162 overlap_within_probe(selcx, impl1_def_id, impl2_def_id, overlap_mode, snapshot)
166 fn overlap_within_probe<'cx, 'tcx>(
167 selcx: &mut SelectionContext<'cx, 'tcx>,
170 overlap_mode: OverlapMode,
171 snapshot: &CombinedSnapshot<'_, 'tcx>,
172 ) -> Option<OverlapResult<'tcx>> {
173 let infcx = selcx.infcx();
175 if overlap_mode.use_negative_impl() {
176 if negative_impl(selcx, impl1_def_id, impl2_def_id)
177 || negative_impl(selcx, impl2_def_id, impl1_def_id)
183 // For the purposes of this check, we don't bring any placeholder
184 // types into scope; instead, we replace the generic types with
185 // fresh type variables, and hence we do our evaluations in an
186 // empty environment.
187 let param_env = ty::ParamEnv::empty();
189 let impl1_header = with_fresh_ty_vars(selcx, param_env, impl1_def_id);
190 let impl2_header = with_fresh_ty_vars(selcx, param_env, impl2_def_id);
192 let obligations = equate_impl_headers(selcx, &impl1_header, &impl2_header)?;
193 debug!("overlap: unification check succeeded");
195 if overlap_mode.use_implicit_negative() {
196 if implicit_negative(selcx, param_env, &impl1_header, impl2_header, obligations) {
201 // We disable the leak when when creating the `snapshot` by using
202 // `infcx.probe_maybe_disable_leak_check`.
203 if infcx.leak_check(true, snapshot).is_err() {
204 debug!("overlap: leak check failed");
208 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
209 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
211 let involves_placeholder =
212 matches!(selcx.infcx().region_constraints_added_in_snapshot(snapshot), Some(true));
214 let impl_header = selcx.infcx().resolve_vars_if_possible(impl1_header);
215 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
218 fn equate_impl_headers<'cx, 'tcx>(
219 selcx: &mut SelectionContext<'cx, 'tcx>,
220 impl1_header: &ty::ImplHeader<'tcx>,
221 impl2_header: &ty::ImplHeader<'tcx>,
222 ) -> Option<PredicateObligations<'tcx>> {
223 // Do `a` and `b` unify? If not, no overlap.
224 debug!("equate_impl_headers(impl1_header={:?}, impl2_header={:?}", impl1_header, impl2_header);
227 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
228 .eq_impl_headers(impl1_header, impl2_header)
229 .map(|infer_ok| infer_ok.obligations)
233 /// Given impl1 and impl2 check if both impls can be satisfied by a common type (including
234 /// where-clauses) If so, return false, otherwise return true, they are disjoint.
235 fn implicit_negative<'cx, 'tcx>(
236 selcx: &mut SelectionContext<'cx, 'tcx>,
237 param_env: ty::ParamEnv<'tcx>,
238 impl1_header: &ty::ImplHeader<'tcx>,
239 impl2_header: ty::ImplHeader<'tcx>,
240 obligations: PredicateObligations<'tcx>,
242 // There's no overlap if obligations are unsatisfiable or if the obligation negated is
245 // For example, given these two impl headers:
247 // `impl<'a> From<&'a str> for Box<dyn Error>`
248 // `impl<E> From<E> for Box<dyn Error> where E: Error`
252 // `Box<dyn Error>: From<&'?a str>`
253 // `Box<dyn Error>: From<?E>`
255 // After equating the two headers:
257 // `Box<dyn Error> = Box<dyn Error>`
258 // So, `?E = &'?a str` and then given the where clause `&'?a str: Error`.
260 // If the obligation `&'?a str: Error` holds, it means that there's overlap. If that doesn't
261 // hold we need to check if `&'?a str: !Error` holds, if doesn't hold there's overlap because
262 // at some point an impl for `&'?a str: Error` could be added.
264 "implicit_negative(impl1_header={:?}, impl2_header={:?}, obligations={:?})",
265 impl1_header, impl2_header, obligations
267 let infcx = selcx.infcx();
268 let opt_failing_obligation = impl1_header
272 .chain(impl2_header.predicates)
273 .map(|p| infcx.resolve_vars_if_possible(p))
274 .map(|p| Obligation {
275 cause: ObligationCause::dummy(),
281 .find(|o| !selcx.predicate_may_hold_fatal(o));
283 if let Some(failing_obligation) = opt_failing_obligation {
284 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
291 /// Given impl1 and impl2 check if both impls are never satisfied by a common type (including
292 /// where-clauses) If so, return true, they are disjoint and false otherwise.
293 fn negative_impl<'cx, 'tcx>(
294 selcx: &mut SelectionContext<'cx, 'tcx>,
298 debug!("negative_impl(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
299 let tcx = selcx.infcx().tcx;
301 // Create an infcx, taking the predicates of impl1 as assumptions:
302 tcx.infer_ctxt().enter(|infcx| {
303 // create a parameter environment corresponding to a (placeholder) instantiation of impl1
304 let impl_env = tcx.param_env(impl1_def_id);
305 let subject1 = match traits::fully_normalize(
307 ObligationCause::dummy(),
309 tcx.impl_subject(impl1_def_id),
313 tcx.sess.delay_span_bug(
314 tcx.def_span(impl1_def_id),
315 format!("failed to fully normalize {:?}: {:?}", impl1_def_id, err),
321 // Attempt to prove that impl2 applies, given all of the above.
322 let selcx = &mut SelectionContext::new(&infcx);
323 let impl2_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl2_def_id);
324 let (subject2, obligations) =
325 impl_subject_and_oblig(selcx, impl_env, impl2_def_id, impl2_substs);
327 !equate(&infcx, impl_env, subject1, subject2, obligations, impl1_def_id)
331 fn equate<'cx, 'tcx>(
332 infcx: &InferCtxt<'cx, 'tcx>,
333 impl_env: ty::ParamEnv<'tcx>,
334 subject1: ImplSubject<'tcx>,
335 subject2: ImplSubject<'tcx>,
336 obligations: impl Iterator<Item = PredicateObligation<'tcx>>,
339 // do the impls unify? If not, not disjoint.
340 let Ok(InferOk { obligations: more_obligations, .. }) =
341 infcx.at(&ObligationCause::dummy(), impl_env).eq(subject1, subject2)
343 debug!("explicit_disjoint: {:?} does not unify with {:?}", subject1, subject2);
347 let selcx = &mut SelectionContext::new(&infcx);
348 let opt_failing_obligation = obligations
350 .chain(more_obligations)
351 .find(|o| negative_impl_exists(selcx, o, body_def_id));
353 if let Some(failing_obligation) = opt_failing_obligation {
354 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
361 /// Try to prove that a negative impl exist for the given obligation and its super predicates.
362 #[instrument(level = "debug", skip(selcx))]
363 fn negative_impl_exists<'cx, 'tcx>(
364 selcx: &SelectionContext<'cx, 'tcx>,
365 o: &PredicateObligation<'tcx>,
368 if resolve_negative_obligation(selcx.infcx().fork(), o, body_def_id) {
372 // Try to prove a negative obligation exists for super predicates
373 for o in util::elaborate_predicates(selcx.tcx(), iter::once(o.predicate)) {
374 if resolve_negative_obligation(selcx.infcx().fork(), &o, body_def_id) {
382 #[instrument(level = "debug", skip(infcx))]
383 fn resolve_negative_obligation<'cx, 'tcx>(
384 infcx: InferCtxt<'cx, 'tcx>,
385 o: &PredicateObligation<'tcx>,
390 let Some(o) = o.flip_polarity(tcx) else {
394 let param_env = o.param_env;
395 if !super::fully_solve_obligation(&infcx, o).is_empty() {
399 let (body_id, body_def_id) = if let Some(body_def_id) = body_def_id.as_local() {
400 (tcx.hir().local_def_id_to_hir_id(body_def_id), body_def_id)
402 (CRATE_HIR_ID, CRATE_DEF_ID)
405 let ocx = ObligationCtxt::new(&infcx);
406 let wf_tys = ocx.assumed_wf_types(param_env, DUMMY_SP, body_def_id);
407 let outlives_env = OutlivesEnvironment::with_bounds(
410 infcx.implied_bounds_tys(param_env, body_id, wf_tys),
413 infcx.process_registered_region_obligations(outlives_env.region_bound_pairs(), param_env);
415 infcx.resolve_regions(&outlives_env).is_empty()
418 pub fn trait_ref_is_knowable<'tcx>(
420 trait_ref: ty::TraitRef<'tcx>,
421 ) -> Result<(), Conflict> {
422 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
423 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
424 // A downstream or cousin crate is allowed to implement some
425 // substitution of this trait-ref.
426 return Err(Conflict::Downstream);
429 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
430 // This is a local or fundamental trait, so future-compatibility
431 // is no concern. We know that downstream/cousin crates are not
432 // allowed to implement a substitution of this trait ref, which
433 // means impls could only come from dependencies of this crate,
434 // which we already know about.
438 // This is a remote non-fundamental trait, so if another crate
439 // can be the "final owner" of a substitution of this trait-ref,
440 // they are allowed to implement it future-compatibly.
442 // However, if we are a final owner, then nobody else can be,
443 // and if we are an intermediate owner, then we don't care
444 // about future-compatibility, which means that we're OK if
446 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
447 debug!("trait_ref_is_knowable: orphan check passed");
450 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
451 Err(Conflict::Upstream)
455 pub fn trait_ref_is_local_or_fundamental<'tcx>(
457 trait_ref: ty::TraitRef<'tcx>,
459 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
462 pub enum OrphanCheckErr<'tcx> {
463 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
464 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
467 /// Checks the coherence orphan rules. `impl_def_id` should be the
468 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
469 /// two conditions must be satisfied:
471 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
472 /// 2. Some local type must appear in `Self`.
473 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
474 debug!("orphan_check({:?})", impl_def_id);
476 // We only except this routine to be invoked on implementations
477 // of a trait, not inherent implementations.
478 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
479 debug!("orphan_check: trait_ref={:?}", trait_ref);
481 // If the *trait* is local to the crate, ok.
482 if trait_ref.def_id.is_local() {
483 debug!("trait {:?} is local to current crate", trait_ref.def_id);
487 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
490 /// Checks whether a trait-ref is potentially implementable by a crate.
492 /// The current rule is that a trait-ref orphan checks in a crate C:
494 /// 1. Order the parameters in the trait-ref in subst order - Self first,
495 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
496 /// 2. Of these type parameters, there is at least one type parameter
497 /// in which, walking the type as a tree, you can reach a type local
498 /// to C where all types in-between are fundamental types. Call the
499 /// first such parameter the "local key parameter".
500 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
501 /// going through `Box`, which is fundamental.
502 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
504 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
505 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
506 /// the local type and the type parameter.
507 /// 3. Before this local type, no generic type parameter of the impl must
508 /// be reachable through fundamental types.
509 /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
510 /// - while `impl<T> Trait<LocalType> for Box<T>` results in an error, as `T` is
511 /// reachable through the fundamental type `Box`.
512 /// 4. Every type in the local key parameter not known in C, going
513 /// through the parameter's type tree, must appear only as a subtree of
514 /// a type local to C, with only fundamental types between the type
515 /// local to C and the local key parameter.
516 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
517 /// is bad, because the only local type with `T` as a subtree is
518 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
519 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
520 /// the second occurrence of `T` is not a subtree of *any* local type.
521 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
522 /// `LocalType<Vec<T>>`, which is local and has no types between it and
523 /// the type parameter.
525 /// The orphan rules actually serve several different purposes:
527 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
528 /// every type local to one crate is unknown in the other) can't implement
529 /// the same trait-ref. This follows because it can be seen that no such
530 /// type can orphan-check in 2 such crates.
532 /// To check that a local impl follows the orphan rules, we check it in
533 /// InCrate::Local mode, using type parameters for the "generic" types.
535 /// 2. They ground negative reasoning for coherence. If a user wants to
536 /// write both a conditional blanket impl and a specific impl, we need to
537 /// make sure they do not overlap. For example, if we write
538 /// ```ignore (illustrative)
539 /// impl<T> IntoIterator for Vec<T>
540 /// impl<T: Iterator> IntoIterator for T
542 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
543 /// We can observe that this holds in the current crate, but we need to make
544 /// sure this will also hold in all unknown crates (both "independent" crates,
545 /// which we need for link-safety, and also child crates, because we don't want
546 /// child crates to get error for impl conflicts in a *dependency*).
548 /// For that, we only allow negative reasoning if, for every assignment to the
549 /// inference variables, every unknown crate would get an orphan error if they
550 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
551 /// mode. That is sound because we already know all the impls from known crates.
553 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
554 /// add "non-blanket" impls without breaking negative reasoning in dependent
555 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
557 /// For that, we only a allow crate to perform negative reasoning on
558 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
560 /// Because we never perform negative reasoning generically (coherence does
561 /// not involve type parameters), this can be interpreted as doing the full
562 /// orphan check (using InCrate::Local mode), substituting non-local known
563 /// types for all inference variables.
565 /// This allows for crates to future-compatibly add impls as long as they
566 /// can't apply to types with a key parameter in a child crate - applying
567 /// the rules, this basically means that every type parameter in the impl
568 /// must appear behind a non-fundamental type (because this is not a
569 /// type-system requirement, crate owners might also go for "semantic
570 /// future-compatibility" involving things such as sealed traits, but
571 /// the above requirement is sufficient, and is necessary in "open world"
574 /// Note that this function is never called for types that have both type
575 /// parameters and inference variables.
576 fn orphan_check_trait_ref<'tcx>(
578 trait_ref: ty::TraitRef<'tcx>,
580 ) -> Result<(), OrphanCheckErr<'tcx>> {
581 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
583 if trait_ref.needs_infer() && trait_ref.needs_subst() {
585 "can't orphan check a trait ref with both params and inference variables {:?}",
590 let mut checker = OrphanChecker::new(tcx, in_crate);
591 match trait_ref.visit_with(&mut checker) {
592 ControlFlow::Continue(()) => Err(OrphanCheckErr::NonLocalInputType(checker.non_local_tys)),
593 ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(ty)) => {
594 // Does there exist some local type after the `ParamTy`.
595 checker.search_first_local_ty = true;
596 if let Some(OrphanCheckEarlyExit::LocalTy(local_ty)) =
597 trait_ref.visit_with(&mut checker).break_value()
599 Err(OrphanCheckErr::UncoveredTy(ty, Some(local_ty)))
601 Err(OrphanCheckErr::UncoveredTy(ty, None))
604 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(_)) => Ok(()),
608 struct OrphanChecker<'tcx> {
612 /// Ignore orphan check failures and exclusively search for the first
614 search_first_local_ty: bool,
615 non_local_tys: Vec<(Ty<'tcx>, bool)>,
618 impl<'tcx> OrphanChecker<'tcx> {
619 fn new(tcx: TyCtxt<'tcx>, in_crate: InCrate) -> Self {
624 search_first_local_ty: false,
625 non_local_tys: Vec::new(),
629 fn found_non_local_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx>> {
630 self.non_local_tys.push((t, self.in_self_ty));
631 ControlFlow::CONTINUE
634 fn found_param_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<OrphanCheckEarlyExit<'tcx>> {
635 if self.search_first_local_ty {
636 ControlFlow::CONTINUE
638 ControlFlow::Break(OrphanCheckEarlyExit::ParamTy(t))
642 fn def_id_is_local(&mut self, def_id: DefId) -> bool {
643 match self.in_crate {
644 InCrate::Local => def_id.is_local(),
645 InCrate::Remote => false,
650 enum OrphanCheckEarlyExit<'tcx> {
655 impl<'tcx> TypeVisitor<'tcx> for OrphanChecker<'tcx> {
656 type BreakTy = OrphanCheckEarlyExit<'tcx>;
657 fn visit_region(&mut self, _r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
658 ControlFlow::CONTINUE
661 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
662 let result = match *ty.kind() {
676 | ty::Projection(..) => self.found_non_local_ty(ty),
678 ty::Param(..) => self.found_param_ty(ty),
680 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match self.in_crate {
681 InCrate::Local => self.found_non_local_ty(ty),
682 // The inference variable might be unified with a local
683 // type in that remote crate.
684 InCrate::Remote => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
687 // For fundamental types, we just look inside of them.
688 ty::Ref(_, ty, _) => ty.visit_with(self),
689 ty::Adt(def, substs) => {
690 if self.def_id_is_local(def.did()) {
691 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
692 } else if def.is_fundamental() {
693 substs.visit_with(self)
695 self.found_non_local_ty(ty)
698 ty::Foreign(def_id) => {
699 if self.def_id_is_local(def_id) {
700 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
702 self.found_non_local_ty(ty)
705 ty::Dynamic(tt, ..) => {
706 let principal = tt.principal().map(|p| p.def_id());
707 if principal.map_or(false, |p| self.def_id_is_local(p)) {
708 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
710 self.found_non_local_ty(ty)
713 ty::Error(_) => ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty)),
714 ty::Closure(..) | ty::Generator(..) | ty::GeneratorWitness(..) => {
715 self.tcx.sess.delay_span_bug(
717 format!("ty_is_local invoked on closure or generator: {:?}", ty),
719 ControlFlow::Break(OrphanCheckEarlyExit::LocalTy(ty))
722 // This merits some explanation.
723 // Normally, opaque types are not involved when performing
724 // coherence checking, since it is illegal to directly
725 // implement a trait on an opaque type. However, we might
726 // end up looking at an opaque type during coherence checking
727 // if an opaque type gets used within another type (e.g. as
728 // the type of a field) when checking for auto trait or `Sized`
729 // impls. This requires us to decide whether or not an opaque
730 // type should be considered 'local' or not.
732 // We choose to treat all opaque types as non-local, even
733 // those that appear within the same crate. This seems
734 // somewhat surprising at first, but makes sense when
735 // you consider that opaque types are supposed to hide
736 // the underlying type *within the same crate*. When an
737 // opaque type is used from outside the module
738 // where it is declared, it should be impossible to observe
739 // anything about it other than the traits that it implements.
741 // The alternative would be to look at the underlying type
742 // to determine whether or not the opaque type itself should
743 // be considered local. However, this could make it a breaking change
744 // to switch the underlying ('defining') type from a local type
745 // to a remote type. This would violate the rule that opaque
746 // types should be completely opaque apart from the traits
747 // that they implement, so we don't use this behavior.
748 self.found_non_local_ty(ty)
751 // A bit of a hack, the `OrphanChecker` is only used to visit a `TraitRef`, so
752 // the first type we visit is always the self type.
753 self.in_self_ty = false;
757 /// All possible values for a constant parameter already exist
758 /// in the crate defining the trait, so they are always non-local[^1].
760 /// Because there's no way to have an impl where the first local
761 /// generic argument is a constant, we also don't have to fail
762 /// the orphan check when encountering a parameter or a generic constant.
764 /// This means that we can completely ignore constants during the orphan check.
766 /// See `src/test/ui/coherence/const-generics-orphan-check-ok.rs` for examples.
768 /// [^1]: This might not hold for function pointers or trait objects in the future.
769 /// As these should be quite rare as const arguments and especially rare as impl
770 /// parameters, allowing uncovered const parameters in impls seems more useful
771 /// than allowing `impl<T> Trait<local_fn_ptr, T> for i32` to compile.
772 fn visit_const(&mut self, _c: ty::Const<'tcx>) -> ControlFlow<Self::BreakTy> {
773 ControlFlow::CONTINUE