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::{CombinedSnapshot, InferOk, TyCtxtInferExt};
8 use crate::traits::query::evaluate_obligation::InferCtxtExt;
9 use crate::traits::select::IntercrateAmbiguityCause;
10 use crate::traits::util::impl_trait_ref_and_oblig;
11 use crate::traits::SkipLeakCheck;
13 self, FulfillmentContext, Normalized, Obligation, ObligationCause, PredicateObligation,
14 PredicateObligations, SelectionContext,
16 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
17 use rustc_middle::traits::specialization_graph::OverlapMode;
18 use rustc_middle::ty::fast_reject::{self, SimplifyParams, StripReferences};
19 use rustc_middle::ty::fold::TypeFoldable;
20 use rustc_middle::ty::subst::Subst;
21 use rustc_middle::ty::{self, Ty, TyCtxt};
22 use rustc_span::symbol::sym;
23 use rustc_span::DUMMY_SP;
26 /// Whether we do the orphan check relative to this crate or
27 /// to some remote crate.
28 #[derive(Copy, Clone, Debug)]
34 #[derive(Debug, Copy, Clone)]
40 pub struct OverlapResult<'tcx> {
41 pub impl_header: ty::ImplHeader<'tcx>,
42 pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
44 /// `true` if the overlap might've been permitted before the shift
46 pub involves_placeholder: bool,
49 pub fn add_placeholder_note(err: &mut rustc_errors::DiagnosticBuilder<'_>) {
51 "this behavior recently changed as a result of a bug fix; \
52 see rust-lang/rust#56105 for details",
56 /// If there are types that satisfy both impls, invokes `on_overlap`
57 /// with a suitably-freshened `ImplHeader` with those types
58 /// substituted. Otherwise, invokes `no_overlap`.
59 #[instrument(skip(tcx, skip_leak_check, on_overlap, no_overlap), level = "debug")]
60 pub fn overlapping_impls<F1, F2, R>(
64 skip_leak_check: SkipLeakCheck,
65 overlap_mode: OverlapMode,
70 F1: FnOnce(OverlapResult<'_>) -> R,
73 // Before doing expensive operations like entering an inference context, do
74 // a quick check via fast_reject to tell if the impl headers could possibly
76 let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
77 let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
79 // Check if any of the input types definitely do not unify.
81 impl1_ref.iter().flat_map(|tref| tref.substs.types()),
82 impl2_ref.iter().flat_map(|tref| tref.substs.types()),
85 let t1 = fast_reject::simplify_type(tcx, ty1, SimplifyParams::No, StripReferences::No);
86 let t2 = fast_reject::simplify_type(tcx, ty2, SimplifyParams::No, StripReferences::No);
88 if let (Some(t1), Some(t2)) = (t1, t2) {
89 // Simplified successfully
96 // Some types involved are definitely different, so the impls couldn't possibly overlap.
97 debug!("overlapping_impls: fast_reject early-exit");
101 let overlaps = tcx.infer_ctxt().enter(|infcx| {
102 let selcx = &mut SelectionContext::intercrate(&infcx);
103 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).is_some()
110 // In the case where we detect an error, run the check again, but
111 // this time tracking intercrate ambuiguity causes for better
112 // diagnostics. (These take time and can lead to false errors.)
113 tcx.infer_ctxt().enter(|infcx| {
114 let selcx = &mut SelectionContext::intercrate(&infcx);
115 selcx.enable_tracking_intercrate_ambiguity_causes();
117 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id, overlap_mode).unwrap(),
122 fn with_fresh_ty_vars<'cx, 'tcx>(
123 selcx: &mut SelectionContext<'cx, 'tcx>,
124 param_env: ty::ParamEnv<'tcx>,
126 ) -> ty::ImplHeader<'tcx> {
127 let tcx = selcx.tcx();
128 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
130 let header = ty::ImplHeader {
132 self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
133 trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
134 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
137 let Normalized { value: mut header, obligations } =
138 traits::normalize(selcx, param_env, ObligationCause::dummy(), header);
140 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
144 /// Can both impl `a` and impl `b` be satisfied by a common type (including
145 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
146 fn overlap<'cx, 'tcx>(
147 selcx: &mut SelectionContext<'cx, 'tcx>,
148 skip_leak_check: SkipLeakCheck,
151 overlap_mode: OverlapMode,
152 ) -> Option<OverlapResult<'tcx>> {
153 debug!("overlap(impl1_def_id={:?}, impl2_def_id={:?})", impl1_def_id, impl2_def_id);
155 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
156 overlap_within_probe(
167 fn overlap_within_probe<'cx, 'tcx>(
168 selcx: &mut SelectionContext<'cx, 'tcx>,
169 skip_leak_check: SkipLeakCheck,
172 overlap_mode: OverlapMode,
173 snapshot: &CombinedSnapshot<'_, 'tcx>,
174 ) -> Option<OverlapResult<'tcx>> {
175 let infcx = selcx.infcx();
177 if overlap_mode.use_negative_impl() {
178 if negative_impl(selcx, impl1_def_id, impl2_def_id)
179 || negative_impl(selcx, impl2_def_id, impl1_def_id)
185 // For the purposes of this check, we don't bring any placeholder
186 // types into scope; instead, we replace the generic types with
187 // fresh type variables, and hence we do our evaluations in an
188 // empty environment.
189 let param_env = ty::ParamEnv::empty();
191 let impl1_header = with_fresh_ty_vars(selcx, param_env, impl1_def_id);
192 let impl2_header = with_fresh_ty_vars(selcx, param_env, impl2_def_id);
194 debug!("overlap: impl1_header={:?}", impl1_header);
195 debug!("overlap: impl2_header={:?}", impl2_header);
197 let obligations = equate_impl_headers(selcx, &impl1_header, &impl2_header)?;
198 debug!("overlap: unification check succeeded");
200 if overlap_mode.use_implicit_negative() {
201 if implicit_negative(selcx, param_env, &impl1_header, impl2_header, obligations) {
206 if !skip_leak_check.is_yes() {
207 if infcx.leak_check(true, snapshot).is_err() {
208 debug!("overlap: leak check failed");
213 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
214 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
216 let involves_placeholder =
217 matches!(selcx.infcx().region_constraints_added_in_snapshot(snapshot), Some(true));
219 let impl_header = selcx.infcx().resolve_vars_if_possible(impl1_header);
220 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
223 fn equate_impl_headers<'cx, 'tcx>(
224 selcx: &mut SelectionContext<'cx, 'tcx>,
225 impl1_header: &ty::ImplHeader<'tcx>,
226 impl2_header: &ty::ImplHeader<'tcx>,
227 ) -> Option<PredicateObligations<'tcx>> {
228 // Do `a` and `b` unify? If not, no overlap.
231 .at(&ObligationCause::dummy(), ty::ParamEnv::empty())
232 .eq_impl_headers(impl1_header, impl2_header)
233 .map(|infer_ok| infer_ok.obligations)
237 /// Given impl1 and impl2 check if both impls can be satisfied by a common type (including
238 /// where-clauses) If so, return false, otherwise return true, they are disjoint.
239 fn implicit_negative<'cx, 'tcx>(
240 selcx: &mut SelectionContext<'cx, 'tcx>,
241 param_env: ty::ParamEnv<'tcx>,
242 impl1_header: &ty::ImplHeader<'tcx>,
243 impl2_header: ty::ImplHeader<'tcx>,
244 obligations: PredicateObligations<'tcx>,
246 // There's no overlap if obligations are unsatisfiable or if the obligation negated is
249 // For example, given these two impl headers:
251 // `impl<'a> From<&'a str> for Box<dyn Error>`
252 // `impl<E> From<E> for Box<dyn Error> where E: Error`
256 // `Box<dyn Error>: From<&'?a str>`
257 // `Box<dyn Error>: From<?E>`
259 // After equating the two headers:
261 // `Box<dyn Error> = Box<dyn Error>`
262 // So, `?E = &'?a str` and then given the where clause `&'?a str: Error`.
264 // If the obligation `&'?a str: Error` holds, it means that there's overlap. If that doesn't
265 // hold we need to check if `&'?a str: !Error` holds, if doesn't hold there's overlap because
266 // at some point an impl for `&'?a str: Error` could be added.
267 let infcx = selcx.infcx();
269 let opt_failing_obligation = impl1_header
273 .chain(impl2_header.predicates)
274 .map(|p| infcx.resolve_vars_if_possible(p))
275 .map(|p| Obligation {
276 cause: ObligationCause::dummy(),
283 loose_check(selcx, o) || tcx.features().negative_impls && negative_impl_exists(selcx, o)
285 // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
286 // to the canonical trait query form, `infcx.predicate_may_hold`, once
287 // the new system supports intercrate mode (which coherence needs).
289 if let Some(failing_obligation) = opt_failing_obligation {
290 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
297 /// Given impl1 and impl2 check if both impls are never satisfied by a common type (including
298 /// where-clauses) If so, return true, they are disjoint and false otherwise.
299 fn negative_impl<'cx, 'tcx>(
300 selcx: &mut SelectionContext<'cx, 'tcx>,
304 let tcx = selcx.infcx().tcx;
306 // create a parameter environment corresponding to a (placeholder) instantiation of impl1
307 let impl1_env = tcx.param_env(impl1_def_id);
308 let impl1_trait_ref = tcx.impl_trait_ref(impl1_def_id).unwrap();
310 // Create an infcx, taking the predicates of impl1 as assumptions:
311 tcx.infer_ctxt().enter(|infcx| {
312 // Normalize the trait reference. The WF rules ought to ensure
313 // that this always succeeds.
314 let impl1_trait_ref = match traits::fully_normalize(
316 FulfillmentContext::new(),
317 ObligationCause::dummy(),
321 Ok(impl1_trait_ref) => impl1_trait_ref,
323 bug!("failed to fully normalize {:?}: {:?}", impl1_trait_ref, err);
327 // Attempt to prove that impl2 applies, given all of the above.
328 let selcx = &mut SelectionContext::new(&infcx);
329 let impl2_substs = infcx.fresh_substs_for_item(DUMMY_SP, impl2_def_id);
330 let (impl2_trait_ref, obligations) =
331 impl_trait_ref_and_oblig(selcx, impl1_env, impl2_def_id, impl2_substs);
333 // do the impls unify? If not, not disjoint.
334 let more_obligations = match infcx
335 .at(&ObligationCause::dummy(), impl1_env)
336 .eq(impl1_trait_ref, impl2_trait_ref)
338 Ok(InferOk { obligations, .. }) => obligations,
341 "explicit_disjoint: {:?} does not unify with {:?}",
342 impl1_trait_ref, impl2_trait_ref
348 let opt_failing_obligation = obligations
350 .chain(more_obligations)
351 .find(|o| negative_impl_exists(selcx, o));
353 if let Some(failing_obligation) = opt_failing_obligation {
354 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
362 fn loose_check<'cx, 'tcx>(
363 selcx: &mut SelectionContext<'cx, 'tcx>,
364 o: &PredicateObligation<'tcx>,
366 !selcx.predicate_may_hold_fatal(o)
369 fn negative_impl_exists<'cx, 'tcx>(
370 selcx: &SelectionContext<'cx, 'tcx>,
371 o: &PredicateObligation<'tcx>,
373 let infcx = selcx.infcx();
378 // FIXME This isn't quite correct, regions should be included
379 selcx.infcx().predicate_must_hold_modulo_regions(o)
384 pub fn trait_ref_is_knowable<'tcx>(
386 trait_ref: ty::TraitRef<'tcx>,
387 ) -> Option<Conflict> {
388 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
389 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
390 // A downstream or cousin crate is allowed to implement some
391 // substitution of this trait-ref.
392 return Some(Conflict::Downstream);
395 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
396 // This is a local or fundamental trait, so future-compatibility
397 // is no concern. We know that downstream/cousin crates are not
398 // allowed to implement a substitution of this trait ref, which
399 // means impls could only come from dependencies of this crate,
400 // which we already know about.
404 // This is a remote non-fundamental trait, so if another crate
405 // can be the "final owner" of a substitution of this trait-ref,
406 // they are allowed to implement it future-compatibly.
408 // However, if we are a final owner, then nobody else can be,
409 // and if we are an intermediate owner, then we don't care
410 // about future-compatibility, which means that we're OK if
412 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
413 debug!("trait_ref_is_knowable: orphan check passed");
416 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
417 Some(Conflict::Upstream)
421 pub fn trait_ref_is_local_or_fundamental<'tcx>(
423 trait_ref: ty::TraitRef<'tcx>,
425 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
428 pub enum OrphanCheckErr<'tcx> {
429 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
430 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
433 /// Checks the coherence orphan rules. `impl_def_id` should be the
434 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
435 /// two conditions must be satisfied:
437 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
438 /// 2. Some local type must appear in `Self`.
439 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
440 debug!("orphan_check({:?})", impl_def_id);
442 // We only except this routine to be invoked on implementations
443 // of a trait, not inherent implementations.
444 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
445 debug!("orphan_check: trait_ref={:?}", trait_ref);
447 // If the *trait* is local to the crate, ok.
448 if trait_ref.def_id.is_local() {
449 debug!("trait {:?} is local to current crate", trait_ref.def_id);
453 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
456 /// Checks whether a trait-ref is potentially implementable by a crate.
458 /// The current rule is that a trait-ref orphan checks in a crate C:
460 /// 1. Order the parameters in the trait-ref in subst order - Self first,
461 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
462 /// 2. Of these type parameters, there is at least one type parameter
463 /// in which, walking the type as a tree, you can reach a type local
464 /// to C where all types in-between are fundamental types. Call the
465 /// first such parameter the "local key parameter".
466 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
467 /// going through `Box`, which is fundamental.
468 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
470 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
471 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
472 /// the local type and the type parameter.
473 /// 3. Before this local type, no generic type parameter of the impl must
474 /// be reachable through fundamental types.
475 /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
476 /// - while `impl<T> Trait<LocalType for Box<T>` results in an error, as `T` is
477 /// reachable through the fundamental type `Box`.
478 /// 4. Every type in the local key parameter not known in C, going
479 /// through the parameter's type tree, must appear only as a subtree of
480 /// a type local to C, with only fundamental types between the type
481 /// local to C and the local key parameter.
482 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
483 /// is bad, because the only local type with `T` as a subtree is
484 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
485 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
486 /// the second occurrence of `T` is not a subtree of *any* local type.
487 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
488 /// `LocalType<Vec<T>>`, which is local and has no types between it and
489 /// the type parameter.
491 /// The orphan rules actually serve several different purposes:
493 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
494 /// every type local to one crate is unknown in the other) can't implement
495 /// the same trait-ref. This follows because it can be seen that no such
496 /// type can orphan-check in 2 such crates.
498 /// To check that a local impl follows the orphan rules, we check it in
499 /// InCrate::Local mode, using type parameters for the "generic" types.
501 /// 2. They ground negative reasoning for coherence. If a user wants to
502 /// write both a conditional blanket impl and a specific impl, we need to
503 /// make sure they do not overlap. For example, if we write
505 /// impl<T> IntoIterator for Vec<T>
506 /// impl<T: Iterator> IntoIterator for T
508 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
509 /// We can observe that this holds in the current crate, but we need to make
510 /// sure this will also hold in all unknown crates (both "independent" crates,
511 /// which we need for link-safety, and also child crates, because we don't want
512 /// child crates to get error for impl conflicts in a *dependency*).
514 /// For that, we only allow negative reasoning if, for every assignment to the
515 /// inference variables, every unknown crate would get an orphan error if they
516 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
517 /// mode. That is sound because we already know all the impls from known crates.
519 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
520 /// add "non-blanket" impls without breaking negative reasoning in dependent
521 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
523 /// For that, we only a allow crate to perform negative reasoning on
524 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
526 /// Because we never perform negative reasoning generically (coherence does
527 /// not involve type parameters), this can be interpreted as doing the full
528 /// orphan check (using InCrate::Local mode), substituting non-local known
529 /// types for all inference variables.
531 /// This allows for crates to future-compatibly add impls as long as they
532 /// can't apply to types with a key parameter in a child crate - applying
533 /// the rules, this basically means that every type parameter in the impl
534 /// must appear behind a non-fundamental type (because this is not a
535 /// type-system requirement, crate owners might also go for "semantic
536 /// future-compatibility" involving things such as sealed traits, but
537 /// the above requirement is sufficient, and is necessary in "open world"
540 /// Note that this function is never called for types that have both type
541 /// parameters and inference variables.
542 fn orphan_check_trait_ref<'tcx>(
544 trait_ref: ty::TraitRef<'tcx>,
546 ) -> Result<(), OrphanCheckErr<'tcx>> {
547 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
549 if trait_ref.needs_infer() && trait_ref.needs_subst() {
551 "can't orphan check a trait ref with both params and inference variables {:?}",
556 // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
557 // if at least one of the following is true:
559 // - Trait is a local trait
560 // (already checked in orphan_check prior to calling this function)
562 // - At least one of the types T0..=Tn must be a local type.
563 // Let Ti be the first such type.
564 // - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
566 fn uncover_fundamental_ty<'tcx>(
571 // FIXME: this is currently somewhat overly complicated,
572 // but fixing this requires a more complicated refactor.
573 if !contained_non_local_types(tcx, ty, in_crate).is_empty() {
574 if let Some(inner_tys) = fundamental_ty_inner_tys(tcx, ty) {
576 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
584 let mut non_local_spans = vec![];
585 for (i, input_ty) in trait_ref
588 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
591 debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
592 let non_local_tys = contained_non_local_types(tcx, input_ty, in_crate);
593 if non_local_tys.is_empty() {
594 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
596 } else if let ty::Param(_) = input_ty.kind() {
597 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
598 let local_type = trait_ref
601 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
602 .find(|ty| ty_is_local_constructor(ty, in_crate));
604 debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type);
606 return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type));
609 non_local_spans.extend(non_local_tys.into_iter().map(|input_ty| (input_ty, i == 0)));
611 // If we exit above loop, never found a local type.
612 debug!("orphan_check_trait_ref: no local type");
613 Err(OrphanCheckErr::NonLocalInputType(non_local_spans))
616 /// Returns a list of relevant non-local types for `ty`.
618 /// This is just `ty` itself unless `ty` is `#[fundamental]`,
619 /// in which case we recursively look into this type.
621 /// If `ty` is local itself, this method returns an empty `Vec`.
625 /// - `u32` is not local, so this returns `[u32]`.
626 /// - for `Foo<u32>`, where `Foo` is a local type, this returns `[]`.
627 /// - `&mut u32` returns `[u32]`, as `&mut` is a fundamental type, similar to `Box`.
628 /// - `Box<Foo<u32>>` returns `[]`, as `Box` is a fundamental type and `Foo` is local.
629 fn contained_non_local_types<'tcx>(
634 if ty_is_local_constructor(ty, in_crate) {
637 match fundamental_ty_inner_tys(tcx, ty) {
639 inner_tys.flat_map(|ty| contained_non_local_types(tcx, ty, in_crate)).collect()
646 /// For `#[fundamental]` ADTs and `&T` / `&mut T`, returns `Some` with the
647 /// type parameters of the ADT, or `T`, respectively. For non-fundamental
648 /// types, returns `None`.
649 fn fundamental_ty_inner_tys<'tcx>(
652 ) -> Option<impl Iterator<Item = Ty<'tcx>>> {
653 let (first_ty, rest_tys) = match *ty.kind() {
654 ty::Ref(_, ty, _) => (ty, ty::subst::InternalSubsts::empty().types()),
655 ty::Adt(def, substs) if def.is_fundamental() => {
656 let mut types = substs.types();
658 // FIXME(eddyb) actually validate `#[fundamental]` up-front.
662 tcx.def_span(def.did),
663 "`#[fundamental]` requires at least one type parameter",
669 Some(first_ty) => (first_ty, types),
675 Some(iter::once(first_ty).chain(rest_tys))
678 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
680 // The type is local to *this* crate - it will not be
681 // local in any other crate.
682 InCrate::Remote => false,
683 InCrate::Local => def_id.is_local(),
687 fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
688 debug!("ty_is_local_constructor({:?})", ty);
706 | ty::Projection(..) => false,
708 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
709 InCrate::Local => false,
710 // The inference variable might be unified with a local
711 // type in that remote crate.
712 InCrate::Remote => true,
715 ty::Adt(def, _) => def_id_is_local(def.did, in_crate),
716 ty::Foreign(did) => def_id_is_local(did, in_crate),
718 // This merits some explanation.
719 // Normally, opaque types are not involed when performing
720 // coherence checking, since it is illegal to directly
721 // implement a trait on an opaque type. However, we might
722 // end up looking at an opaque type during coherence checking
723 // if an opaque type gets used within another type (e.g. as
724 // a type parameter). This requires us to decide whether or
725 // not an opaque type should be considered 'local' or not.
727 // We choose to treat all opaque types as non-local, even
728 // those that appear within the same crate. This seems
729 // somewhat surprising at first, but makes sense when
730 // you consider that opaque types are supposed to hide
731 // the underlying type *within the same crate*. When an
732 // opaque type is used from outside the module
733 // where it is declared, it should be impossible to observe
734 // anything about it other than the traits that it implements.
736 // The alternative would be to look at the underlying type
737 // to determine whether or not the opaque type itself should
738 // be considered local. However, this could make it a breaking change
739 // to switch the underlying ('defining') type from a local type
740 // to a remote type. This would violate the rule that opaque
741 // types should be completely opaque apart from the traits
742 // that they implement, so we don't use this behavior.
747 // Similar to the `Opaque` case (#83613).
751 ty::Dynamic(ref tt, ..) => {
752 if let Some(principal) = tt.principal() {
753 def_id_is_local(principal.def_id(), in_crate)
759 ty::Error(_) => true,
761 ty::Generator(..) | ty::GeneratorWitness(..) => {
762 bug!("ty_is_local invoked on unexpected type: {:?}", ty)