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::select::IntercrateAmbiguityCause;
9 use crate::traits::SkipLeakCheck;
10 use crate::traits::{self, Normalized, Obligation, ObligationCause, SelectionContext};
11 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
12 use rustc_middle::ty::fold::TypeFoldable;
13 use rustc_middle::ty::subst::Subst;
14 use rustc_middle::ty::{self, Ty, TyCtxt};
15 use rustc_span::symbol::sym;
16 use rustc_span::DUMMY_SP;
19 /// Whether we do the orphan check relative to this crate or
20 /// to some remote crate.
21 #[derive(Copy, Clone, Debug)]
27 #[derive(Debug, Copy, Clone)]
33 pub struct OverlapResult<'tcx> {
34 pub impl_header: ty::ImplHeader<'tcx>,
35 pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
37 /// `true` if the overlap might've been permitted before the shift
39 pub involves_placeholder: bool,
42 pub fn add_placeholder_note(err: &mut rustc_errors::DiagnosticBuilder<'_>) {
44 "this behavior recently changed as a result of a bug fix; \
45 see rust-lang/rust#56105 for details",
49 /// If there are types that satisfy both impls, invokes `on_overlap`
50 /// with a suitably-freshened `ImplHeader` with those types
51 /// substituted. Otherwise, invokes `no_overlap`.
52 pub fn overlapping_impls<F1, F2, R>(
56 skip_leak_check: SkipLeakCheck,
61 F1: FnOnce(OverlapResult<'_>) -> R,
68 impl1_def_id, impl2_def_id,
71 let overlaps = tcx.infer_ctxt().enter(|infcx| {
72 let selcx = &mut SelectionContext::intercrate(&infcx);
73 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).is_some()
80 // In the case where we detect an error, run the check again, but
81 // this time tracking intercrate ambuiguity causes for better
82 // diagnostics. (These take time and can lead to false errors.)
83 tcx.infer_ctxt().enter(|infcx| {
84 let selcx = &mut SelectionContext::intercrate(&infcx);
85 selcx.enable_tracking_intercrate_ambiguity_causes();
86 on_overlap(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).unwrap())
90 fn with_fresh_ty_vars<'cx, 'tcx>(
91 selcx: &mut SelectionContext<'cx, 'tcx>,
92 param_env: ty::ParamEnv<'tcx>,
94 ) -> ty::ImplHeader<'tcx> {
95 let tcx = selcx.tcx();
96 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
98 let header = ty::ImplHeader {
100 self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
101 trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
102 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
105 let Normalized { value: mut header, obligations } =
106 traits::normalize(selcx, param_env, ObligationCause::dummy(), &header);
108 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
112 /// Can both impl `a` and impl `b` be satisfied by a common type (including
113 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
114 fn overlap<'cx, 'tcx>(
115 selcx: &mut SelectionContext<'cx, 'tcx>,
116 skip_leak_check: SkipLeakCheck,
119 ) -> Option<OverlapResult<'tcx>> {
120 debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
122 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
123 overlap_within_probe(selcx, a_def_id, b_def_id, snapshot)
127 fn overlap_within_probe(
128 selcx: &mut SelectionContext<'cx, 'tcx>,
131 snapshot: &CombinedSnapshot<'_, 'tcx>,
132 ) -> Option<OverlapResult<'tcx>> {
133 // For the purposes of this check, we don't bring any placeholder
134 // types into scope; instead, we replace the generic types with
135 // fresh type variables, and hence we do our evaluations in an
136 // empty environment.
137 let param_env = ty::ParamEnv::empty();
139 let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
140 let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
142 debug!("overlap: a_impl_header={:?}", a_impl_header);
143 debug!("overlap: b_impl_header={:?}", b_impl_header);
145 // Do `a` and `b` unify? If not, no overlap.
146 let obligations = match selcx
148 .at(&ObligationCause::dummy(), param_env)
149 .eq_impl_headers(&a_impl_header, &b_impl_header)
151 Ok(InferOk { obligations, value: () }) => obligations,
157 debug!("overlap: unification check succeeded");
159 // Are any of the obligations unsatisfiable? If so, no overlap.
160 let infcx = selcx.infcx();
161 let opt_failing_obligation = a_impl_header
164 .chain(&b_impl_header.predicates)
165 .map(|p| infcx.resolve_vars_if_possible(p))
166 .map(|p| Obligation {
167 cause: ObligationCause::dummy(),
173 .find(|o| !selcx.predicate_may_hold_fatal(o));
174 // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
175 // to the canonical trait query form, `infcx.predicate_may_hold`, once
176 // the new system supports intercrate mode (which coherence needs).
178 if let Some(failing_obligation) = opt_failing_obligation {
179 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
183 let impl_header = selcx.infcx().resolve_vars_if_possible(&a_impl_header);
184 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
185 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
187 let involves_placeholder = match selcx.infcx().region_constraints_added_in_snapshot(snapshot) {
192 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
195 pub fn trait_ref_is_knowable<'tcx>(
197 trait_ref: ty::TraitRef<'tcx>,
198 ) -> Option<Conflict> {
199 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
200 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
201 // A downstream or cousin crate is allowed to implement some
202 // substitution of this trait-ref.
203 return Some(Conflict::Downstream);
206 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
207 // This is a local or fundamental trait, so future-compatibility
208 // is no concern. We know that downstream/cousin crates are not
209 // allowed to implement a substitution of this trait ref, which
210 // means impls could only come from dependencies of this crate,
211 // which we already know about.
215 // This is a remote non-fundamental trait, so if another crate
216 // can be the "final owner" of a substitution of this trait-ref,
217 // they are allowed to implement it future-compatibly.
219 // However, if we are a final owner, then nobody else can be,
220 // and if we are an intermediate owner, then we don't care
221 // about future-compatibility, which means that we're OK if
223 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
224 debug!("trait_ref_is_knowable: orphan check passed");
227 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
228 Some(Conflict::Upstream)
232 pub fn trait_ref_is_local_or_fundamental<'tcx>(
234 trait_ref: ty::TraitRef<'tcx>,
236 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
239 pub enum OrphanCheckErr<'tcx> {
240 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
241 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
244 /// Checks the coherence orphan rules. `impl_def_id` should be the
245 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
246 /// two conditions must be satisfied:
248 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
249 /// 2. Some local type must appear in `Self`.
250 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
251 debug!("orphan_check({:?})", impl_def_id);
253 // We only except this routine to be invoked on implementations
254 // of a trait, not inherent implementations.
255 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
256 debug!("orphan_check: trait_ref={:?}", trait_ref);
258 // If the *trait* is local to the crate, ok.
259 if trait_ref.def_id.is_local() {
260 debug!("trait {:?} is local to current crate", trait_ref.def_id);
264 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
267 /// Checks whether a trait-ref is potentially implementable by a crate.
269 /// The current rule is that a trait-ref orphan checks in a crate C:
271 /// 1. Order the parameters in the trait-ref in subst order - Self first,
272 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
273 /// 2. Of these type parameters, there is at least one type parameter
274 /// in which, walking the type as a tree, you can reach a type local
275 /// to C where all types in-between are fundamental types. Call the
276 /// first such parameter the "local key parameter".
277 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
278 /// going through `Box`, which is fundamental.
279 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
281 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
282 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
283 /// the local type and the type parameter.
284 /// 3. Every type parameter before the local key parameter is fully known in C.
285 /// - e.g., `impl<T> T: Trait<LocalType>` is bad, because `T` might be
287 /// - but `impl<T> LocalType: Trait<T>` is OK, because `LocalType`
288 /// occurs before `T`.
289 /// 4. Every type in the local key parameter not known in C, going
290 /// through the parameter's type tree, must appear only as a subtree of
291 /// a type local to C, with only fundamental types between the type
292 /// local to C and the local key parameter.
293 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
294 /// is bad, because the only local type with `T` as a subtree is
295 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
296 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
297 /// the second occurrence of `T` is not a subtree of *any* local type.
298 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
299 /// `LocalType<Vec<T>>`, which is local and has no types between it and
300 /// the type parameter.
302 /// The orphan rules actually serve several different purposes:
304 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
305 /// every type local to one crate is unknown in the other) can't implement
306 /// the same trait-ref. This follows because it can be seen that no such
307 /// type can orphan-check in 2 such crates.
309 /// To check that a local impl follows the orphan rules, we check it in
310 /// InCrate::Local mode, using type parameters for the "generic" types.
312 /// 2. They ground negative reasoning for coherence. If a user wants to
313 /// write both a conditional blanket impl and a specific impl, we need to
314 /// make sure they do not overlap. For example, if we write
316 /// impl<T> IntoIterator for Vec<T>
317 /// impl<T: Iterator> IntoIterator for T
319 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
320 /// We can observe that this holds in the current crate, but we need to make
321 /// sure this will also hold in all unknown crates (both "independent" crates,
322 /// which we need for link-safety, and also child crates, because we don't want
323 /// child crates to get error for impl conflicts in a *dependency*).
325 /// For that, we only allow negative reasoning if, for every assignment to the
326 /// inference variables, every unknown crate would get an orphan error if they
327 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
328 /// mode. That is sound because we already know all the impls from known crates.
330 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
331 /// add "non-blanket" impls without breaking negative reasoning in dependent
332 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
334 /// For that, we only a allow crate to perform negative reasoning on
335 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
337 /// Because we never perform negative reasoning generically (coherence does
338 /// not involve type parameters), this can be interpreted as doing the full
339 /// orphan check (using InCrate::Local mode), substituting non-local known
340 /// types for all inference variables.
342 /// This allows for crates to future-compatibly add impls as long as they
343 /// can't apply to types with a key parameter in a child crate - applying
344 /// the rules, this basically means that every type parameter in the impl
345 /// must appear behind a non-fundamental type (because this is not a
346 /// type-system requirement, crate owners might also go for "semantic
347 /// future-compatibility" involving things such as sealed traits, but
348 /// the above requirement is sufficient, and is necessary in "open world"
351 /// Note that this function is never called for types that have both type
352 /// parameters and inference variables.
353 fn orphan_check_trait_ref<'tcx>(
355 trait_ref: ty::TraitRef<'tcx>,
357 ) -> Result<(), OrphanCheckErr<'tcx>> {
358 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
360 if trait_ref.needs_infer() && trait_ref.needs_subst() {
362 "can't orphan check a trait ref with both params and inference variables {:?}",
367 // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
368 // if at least one of the following is true:
370 // - Trait is a local trait
371 // (already checked in orphan_check prior to calling this function)
373 // - At least one of the types T0..=Tn must be a local type.
374 // Let Ti be the first such type.
375 // - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
377 fn uncover_fundamental_ty<'tcx>(
382 // FIXME(eddyb) figure out if this is redundant with `ty_is_non_local`,
383 // or maybe if this should be calling `ty_is_non_local_constructor`.
384 if ty_is_non_local(tcx, ty, in_crate).is_some() {
385 if let Some(inner_tys) = fundamental_ty_inner_tys(tcx, ty) {
387 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
395 let mut non_local_spans = vec![];
396 for (i, input_ty) in trait_ref
399 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
402 debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
403 let non_local_tys = ty_is_non_local(tcx, input_ty, in_crate);
404 if non_local_tys.is_none() {
405 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
407 } else if let ty::Param(_) = input_ty.kind {
408 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
409 let local_type = trait_ref
412 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
413 .find(|ty| ty_is_non_local_constructor(ty, in_crate).is_none());
415 debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type);
417 return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type));
419 if let Some(non_local_tys) = non_local_tys {
420 for input_ty in non_local_tys {
421 non_local_spans.push((input_ty, i == 0));
425 // If we exit above loop, never found a local type.
426 debug!("orphan_check_trait_ref: no local type");
427 Err(OrphanCheckErr::NonLocalInputType(non_local_spans))
430 fn ty_is_non_local(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, in_crate: InCrate) -> Option<Vec<Ty<'tcx>>> {
431 match ty_is_non_local_constructor(ty, in_crate) {
433 if let Some(inner_tys) = fundamental_ty_inner_tys(tcx, ty) {
434 let tys: Vec<_> = inner_tys
435 .filter_map(|ty| ty_is_non_local(tcx, ty, in_crate))
438 if tys.is_empty() { None } else { Some(tys) }
447 /// For `#[fundamental]` ADTs and `&T` / `&mut T`, returns `Some` with the
448 /// type parameters of the ADT, or `T`, respectively. For non-fundamental
449 /// types, returns `None`.
450 fn fundamental_ty_inner_tys(
453 ) -> Option<impl Iterator<Item = Ty<'tcx>>> {
454 let (first_ty, rest_tys) = match ty.kind {
455 ty::Ref(_, ty, _) => (ty, ty::subst::InternalSubsts::empty().types()),
456 ty::Adt(def, substs) if def.is_fundamental() => {
457 let mut types = substs.types();
459 // FIXME(eddyb) actually validate `#[fundamental]` up-front.
463 tcx.def_span(def.did),
464 "`#[fundamental]` requires at least one type parameter",
470 Some(first_ty) => (first_ty, types),
476 Some(iter::once(first_ty).chain(rest_tys))
479 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
481 // The type is local to *this* crate - it will not be
482 // local in any other crate.
483 InCrate::Remote => false,
484 InCrate::Local => def_id.is_local(),
488 // FIXME(eddyb) this can just return `bool` as it always returns `Some(ty)` or `None`.
489 fn ty_is_non_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> Option<Ty<'_>> {
490 debug!("ty_is_non_local_constructor({:?})", ty);
508 | ty::Projection(..) => Some(ty),
510 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
511 InCrate::Local => Some(ty),
512 // The inference variable might be unified with a local
513 // type in that remote crate.
514 InCrate::Remote => None,
518 if def_id_is_local(def.did, in_crate) {
524 ty::Foreign(did) => {
525 if def_id_is_local(did, in_crate) {
532 // This merits some explanation.
533 // Normally, opaque types are not involed when performing
534 // coherence checking, since it is illegal to directly
535 // implement a trait on an opaque type. However, we might
536 // end up looking at an opaque type during coherence checking
537 // if an opaque type gets used within another type (e.g. as
538 // a type parameter). This requires us to decide whether or
539 // not an opaque type should be considered 'local' or not.
541 // We choose to treat all opaque types as non-local, even
542 // those that appear within the same crate. This seems
543 // somewhat surprising at first, but makes sense when
544 // you consider that opaque types are supposed to hide
545 // the underlying type *within the same crate*. When an
546 // opaque type is used from outside the module
547 // where it is declared, it should be impossible to observe
548 // anyything about it other than the traits that it implements.
550 // The alternative would be to look at the underlying type
551 // to determine whether or not the opaque type itself should
552 // be considered local. However, this could make it a breaking change
553 // to switch the underlying ('defining') type from a local type
554 // to a remote type. This would violate the rule that opaque
555 // types should be completely opaque apart from the traits
556 // that they implement, so we don't use this behavior.
560 ty::Dynamic(ref tt, ..) => {
561 if let Some(principal) = tt.principal() {
562 if def_id_is_local(principal.def_id(), in_crate) { None } else { Some(ty) }
568 ty::Error(_) => None,
570 ty::Closure(..) | ty::Generator(..) | ty::GeneratorWitness(..) => {
571 bug!("ty_is_local invoked on unexpected type: {:?}", ty)