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::SkipLeakCheck;
12 self, Normalized, Obligation, ObligationCause, PredicateObligation, SelectionContext,
14 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
15 use rustc_middle::ty::fold::TypeFoldable;
16 use rustc_middle::ty::subst::Subst;
17 use rustc_middle::ty::{self, fast_reject, Ty, TyCtxt};
18 use rustc_span::symbol::sym;
19 use rustc_span::DUMMY_SP;
22 /// Whether we do the orphan check relative to this crate or
23 /// to some remote crate.
24 #[derive(Copy, Clone, Debug)]
30 #[derive(Debug, Copy, Clone)]
36 pub struct OverlapResult<'tcx> {
37 pub impl_header: ty::ImplHeader<'tcx>,
38 pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
40 /// `true` if the overlap might've been permitted before the shift
42 pub involves_placeholder: bool,
45 pub fn add_placeholder_note(err: &mut rustc_errors::DiagnosticBuilder<'_>) {
47 "this behavior recently changed as a result of a bug fix; \
48 see rust-lang/rust#56105 for details",
52 /// If there are types that satisfy both impls, invokes `on_overlap`
53 /// with a suitably-freshened `ImplHeader` with those types
54 /// substituted. Otherwise, invokes `no_overlap`.
55 pub fn overlapping_impls<F1, F2, R>(
59 skip_leak_check: SkipLeakCheck,
64 F1: FnOnce(OverlapResult<'_>) -> R,
71 impl1_def_id, impl2_def_id,
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, false);
86 let t2 = fast_reject::simplify_type(tcx, ty2, false);
87 if let (Some(t1), Some(t2)) = (t1, t2) {
88 // Simplified successfully
89 // Types cannot unify if they differ in their reference mutability or simplify to different types
90 t1 != t2 || ty1.ref_mutability() != ty2.ref_mutability()
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).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();
116 on_overlap(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).unwrap())
120 fn with_fresh_ty_vars<'cx, 'tcx>(
121 selcx: &mut SelectionContext<'cx, 'tcx>,
122 param_env: ty::ParamEnv<'tcx>,
124 ) -> ty::ImplHeader<'tcx> {
125 let tcx = selcx.tcx();
126 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
128 let header = ty::ImplHeader {
130 self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
131 trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
132 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
135 let Normalized { value: mut header, obligations } =
136 traits::normalize(selcx, param_env, ObligationCause::dummy(), header);
138 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
142 /// Can both impl `a` and impl `b` be satisfied by a common type (including
143 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
144 fn overlap<'cx, 'tcx>(
145 selcx: &mut SelectionContext<'cx, 'tcx>,
146 skip_leak_check: SkipLeakCheck,
149 ) -> Option<OverlapResult<'tcx>> {
150 debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
152 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
153 overlap_within_probe(selcx, skip_leak_check, a_def_id, b_def_id, snapshot)
157 fn overlap_within_probe(
158 selcx: &mut SelectionContext<'cx, 'tcx>,
159 skip_leak_check: SkipLeakCheck,
162 snapshot: &CombinedSnapshot<'_, 'tcx>,
163 ) -> Option<OverlapResult<'tcx>> {
164 fn loose_check(selcx: &mut SelectionContext<'cx, 'tcx>, o: &PredicateObligation<'tcx>) -> bool {
165 !selcx.predicate_may_hold_fatal(o)
168 fn strict_check(selcx: &SelectionContext<'cx, 'tcx>, o: &PredicateObligation<'tcx>) -> bool {
169 let infcx = selcx.infcx();
173 .map(|o| selcx.infcx().predicate_must_hold_modulo_regions(o))
177 // For the purposes of this check, we don't bring any placeholder
178 // types into scope; instead, we replace the generic types with
179 // fresh type variables, and hence we do our evaluations in an
180 // empty environment.
181 let param_env = ty::ParamEnv::empty();
183 let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
184 let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
186 debug!("overlap: a_impl_header={:?}", a_impl_header);
187 debug!("overlap: b_impl_header={:?}", b_impl_header);
189 // Do `a` and `b` unify? If not, no overlap.
190 let obligations = match selcx
192 .at(&ObligationCause::dummy(), param_env)
193 .eq_impl_headers(&a_impl_header, &b_impl_header)
195 Ok(InferOk { obligations, value: () }) => obligations,
201 debug!("overlap: unification check succeeded");
203 // There's no overlap if obligations are unsatisfiable or if the obligation negated is
206 // For example, given these two impl headers:
208 // `impl<'a> From<&'a str> for Box<dyn Error>`
209 // `impl<E> From<E> for Box<dyn Error> where E: Error`
213 // `Box<dyn Error>: From<&'?a str>`
214 // `Box<dyn Error>: From<?E>`
216 // After equating the two headers:
218 // `Box<dyn Error> = Box<dyn Error>`
219 // So, `?E = &'?a str` and then given the where clause `&'?a str: Error`.
221 // If the obligation `&'?a str: Error` holds, it means that there's overlap. If that doesn't
222 // hold we need to check if `&'?a str: !Error` holds, if doesn't hold there's overlap because
223 // at some point an impl for `&'?a str: Error` could be added.
224 let infcx = selcx.infcx();
226 let opt_failing_obligation = a_impl_header
230 .chain(b_impl_header.predicates)
231 .map(|p| infcx.resolve_vars_if_possible(p))
232 .map(|p| Obligation {
233 cause: ObligationCause::dummy(),
240 // if both impl headers are set to strict coherence it means that this will be accepted
241 // only if it's stated that T: !Trait. So only prove that the negated obligation holds.
242 if tcx.has_attr(a_def_id, sym::rustc_strict_coherence)
243 && tcx.has_attr(b_def_id, sym::rustc_strict_coherence)
245 strict_check(selcx, o)
247 loose_check(selcx, o) || tcx.features().negative_impls && strict_check(selcx, o)
250 // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
251 // to the canonical trait query form, `infcx.predicate_may_hold`, once
252 // the new system supports intercrate mode (which coherence needs).
254 if let Some(failing_obligation) = opt_failing_obligation {
255 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
259 if !skip_leak_check.is_yes() {
260 if infcx.leak_check(true, snapshot).is_err() {
261 debug!("overlap: leak check failed");
266 let impl_header = selcx.infcx().resolve_vars_if_possible(a_impl_header);
267 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
268 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
270 let involves_placeholder =
271 matches!(selcx.infcx().region_constraints_added_in_snapshot(snapshot), Some(true));
273 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
276 pub fn trait_ref_is_knowable<'tcx>(
278 trait_ref: ty::TraitRef<'tcx>,
279 ) -> Option<Conflict> {
280 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
281 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
282 // A downstream or cousin crate is allowed to implement some
283 // substitution of this trait-ref.
284 return Some(Conflict::Downstream);
287 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
288 // This is a local or fundamental trait, so future-compatibility
289 // is no concern. We know that downstream/cousin crates are not
290 // allowed to implement a substitution of this trait ref, which
291 // means impls could only come from dependencies of this crate,
292 // which we already know about.
296 // This is a remote non-fundamental trait, so if another crate
297 // can be the "final owner" of a substitution of this trait-ref,
298 // they are allowed to implement it future-compatibly.
300 // However, if we are a final owner, then nobody else can be,
301 // and if we are an intermediate owner, then we don't care
302 // about future-compatibility, which means that we're OK if
304 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
305 debug!("trait_ref_is_knowable: orphan check passed");
308 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
309 Some(Conflict::Upstream)
313 pub fn trait_ref_is_local_or_fundamental<'tcx>(
315 trait_ref: ty::TraitRef<'tcx>,
317 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
320 pub enum OrphanCheckErr<'tcx> {
321 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
322 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
325 /// Checks the coherence orphan rules. `impl_def_id` should be the
326 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
327 /// two conditions must be satisfied:
329 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
330 /// 2. Some local type must appear in `Self`.
331 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
332 debug!("orphan_check({:?})", impl_def_id);
334 // We only except this routine to be invoked on implementations
335 // of a trait, not inherent implementations.
336 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
337 debug!("orphan_check: trait_ref={:?}", trait_ref);
339 // If the *trait* is local to the crate, ok.
340 if trait_ref.def_id.is_local() {
341 debug!("trait {:?} is local to current crate", trait_ref.def_id);
345 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
348 /// Checks whether a trait-ref is potentially implementable by a crate.
350 /// The current rule is that a trait-ref orphan checks in a crate C:
352 /// 1. Order the parameters in the trait-ref in subst order - Self first,
353 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
354 /// 2. Of these type parameters, there is at least one type parameter
355 /// in which, walking the type as a tree, you can reach a type local
356 /// to C where all types in-between are fundamental types. Call the
357 /// first such parameter the "local key parameter".
358 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
359 /// going through `Box`, which is fundamental.
360 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
362 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
363 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
364 /// the local type and the type parameter.
365 /// 3. Before this local type, no generic type parameter of the impl must
366 /// be reachable through fundamental types.
367 /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
368 /// - while `impl<T> Trait<LocalType for Box<T>` results in an error, as `T` is
369 /// reachable through the fundamental type `Box`.
370 /// 4. Every type in the local key parameter not known in C, going
371 /// through the parameter's type tree, must appear only as a subtree of
372 /// a type local to C, with only fundamental types between the type
373 /// local to C and the local key parameter.
374 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
375 /// is bad, because the only local type with `T` as a subtree is
376 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
377 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
378 /// the second occurrence of `T` is not a subtree of *any* local type.
379 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
380 /// `LocalType<Vec<T>>`, which is local and has no types between it and
381 /// the type parameter.
383 /// The orphan rules actually serve several different purposes:
385 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
386 /// every type local to one crate is unknown in the other) can't implement
387 /// the same trait-ref. This follows because it can be seen that no such
388 /// type can orphan-check in 2 such crates.
390 /// To check that a local impl follows the orphan rules, we check it in
391 /// InCrate::Local mode, using type parameters for the "generic" types.
393 /// 2. They ground negative reasoning for coherence. If a user wants to
394 /// write both a conditional blanket impl and a specific impl, we need to
395 /// make sure they do not overlap. For example, if we write
397 /// impl<T> IntoIterator for Vec<T>
398 /// impl<T: Iterator> IntoIterator for T
400 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
401 /// We can observe that this holds in the current crate, but we need to make
402 /// sure this will also hold in all unknown crates (both "independent" crates,
403 /// which we need for link-safety, and also child crates, because we don't want
404 /// child crates to get error for impl conflicts in a *dependency*).
406 /// For that, we only allow negative reasoning if, for every assignment to the
407 /// inference variables, every unknown crate would get an orphan error if they
408 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
409 /// mode. That is sound because we already know all the impls from known crates.
411 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
412 /// add "non-blanket" impls without breaking negative reasoning in dependent
413 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
415 /// For that, we only a allow crate to perform negative reasoning on
416 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
418 /// Because we never perform negative reasoning generically (coherence does
419 /// not involve type parameters), this can be interpreted as doing the full
420 /// orphan check (using InCrate::Local mode), substituting non-local known
421 /// types for all inference variables.
423 /// This allows for crates to future-compatibly add impls as long as they
424 /// can't apply to types with a key parameter in a child crate - applying
425 /// the rules, this basically means that every type parameter in the impl
426 /// must appear behind a non-fundamental type (because this is not a
427 /// type-system requirement, crate owners might also go for "semantic
428 /// future-compatibility" involving things such as sealed traits, but
429 /// the above requirement is sufficient, and is necessary in "open world"
432 /// Note that this function is never called for types that have both type
433 /// parameters and inference variables.
434 fn orphan_check_trait_ref<'tcx>(
436 trait_ref: ty::TraitRef<'tcx>,
438 ) -> Result<(), OrphanCheckErr<'tcx>> {
439 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
441 if trait_ref.needs_infer() && trait_ref.definitely_needs_subst(tcx) {
443 "can't orphan check a trait ref with both params and inference variables {:?}",
448 // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
449 // if at least one of the following is true:
451 // - Trait is a local trait
452 // (already checked in orphan_check prior to calling this function)
454 // - At least one of the types T0..=Tn must be a local type.
455 // Let Ti be the first such type.
456 // - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
458 fn uncover_fundamental_ty<'tcx>(
463 // FIXME: this is currently somewhat overly complicated,
464 // but fixing this requires a more complicated refactor.
465 if !contained_non_local_types(tcx, ty, in_crate).is_empty() {
466 if let Some(inner_tys) = fundamental_ty_inner_tys(tcx, ty) {
468 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
476 let mut non_local_spans = vec![];
477 for (i, input_ty) in trait_ref
480 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
483 debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
484 let non_local_tys = contained_non_local_types(tcx, input_ty, in_crate);
485 if non_local_tys.is_empty() {
486 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
488 } else if let ty::Param(_) = input_ty.kind() {
489 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
490 let local_type = trait_ref
493 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
494 .find(|ty| ty_is_local_constructor(ty, in_crate));
496 debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type);
498 return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type));
501 for input_ty in non_local_tys {
502 non_local_spans.push((input_ty, i == 0));
505 // If we exit above loop, never found a local type.
506 debug!("orphan_check_trait_ref: no local type");
507 Err(OrphanCheckErr::NonLocalInputType(non_local_spans))
510 /// Returns a list of relevant non-local types for `ty`.
512 /// This is just `ty` itself unless `ty` is `#[fundamental]`,
513 /// in which case we recursively look into this type.
515 /// If `ty` is local itself, this method returns an empty `Vec`.
519 /// - `u32` is not local, so this returns `[u32]`.
520 /// - for `Foo<u32>`, where `Foo` is a local type, this returns `[]`.
521 /// - `&mut u32` returns `[u32]`, as `&mut` is a fundamental type, similar to `Box`.
522 /// - `Box<Foo<u32>>` returns `[]`, as `Box` is a fundamental type and `Foo` is local.
523 fn contained_non_local_types(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, in_crate: InCrate) -> Vec<Ty<'tcx>> {
524 if ty_is_local_constructor(ty, in_crate) {
527 match fundamental_ty_inner_tys(tcx, ty) {
529 inner_tys.flat_map(|ty| contained_non_local_types(tcx, ty, in_crate)).collect()
536 /// For `#[fundamental]` ADTs and `&T` / `&mut T`, returns `Some` with the
537 /// type parameters of the ADT, or `T`, respectively. For non-fundamental
538 /// types, returns `None`.
539 fn fundamental_ty_inner_tys(
542 ) -> Option<impl Iterator<Item = Ty<'tcx>>> {
543 let (first_ty, rest_tys) = match *ty.kind() {
544 ty::Ref(_, ty, _) => (ty, ty::subst::InternalSubsts::empty().types()),
545 ty::Adt(def, substs) if def.is_fundamental() => {
546 let mut types = substs.types();
548 // FIXME(eddyb) actually validate `#[fundamental]` up-front.
552 tcx.def_span(def.did),
553 "`#[fundamental]` requires at least one type parameter",
559 Some(first_ty) => (first_ty, types),
565 Some(iter::once(first_ty).chain(rest_tys))
568 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
570 // The type is local to *this* crate - it will not be
571 // local in any other crate.
572 InCrate::Remote => false,
573 InCrate::Local => def_id.is_local(),
577 fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
578 debug!("ty_is_local_constructor({:?})", ty);
596 | ty::Projection(..) => false,
598 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
599 InCrate::Local => false,
600 // The inference variable might be unified with a local
601 // type in that remote crate.
602 InCrate::Remote => true,
605 ty::Adt(def, _) => def_id_is_local(def.did, in_crate),
606 ty::Foreign(did) => def_id_is_local(did, in_crate),
608 // This merits some explanation.
609 // Normally, opaque types are not involed when performing
610 // coherence checking, since it is illegal to directly
611 // implement a trait on an opaque type. However, we might
612 // end up looking at an opaque type during coherence checking
613 // if an opaque type gets used within another type (e.g. as
614 // a type parameter). This requires us to decide whether or
615 // not an opaque type should be considered 'local' or not.
617 // We choose to treat all opaque types as non-local, even
618 // those that appear within the same crate. This seems
619 // somewhat surprising at first, but makes sense when
620 // you consider that opaque types are supposed to hide
621 // the underlying type *within the same crate*. When an
622 // opaque type is used from outside the module
623 // where it is declared, it should be impossible to observe
624 // anything about it other than the traits that it implements.
626 // The alternative would be to look at the underlying type
627 // to determine whether or not the opaque type itself should
628 // be considered local. However, this could make it a breaking change
629 // to switch the underlying ('defining') type from a local type
630 // to a remote type. This would violate the rule that opaque
631 // types should be completely opaque apart from the traits
632 // that they implement, so we don't use this behavior.
637 // Similar to the `Opaque` case (#83613).
641 ty::Dynamic(ref tt, ..) => {
642 if let Some(principal) = tt.principal() {
643 def_id_is_local(principal.def_id(), in_crate)
649 ty::Error(_) => true,
651 ty::Generator(..) | ty::GeneratorWitness(..) => {
652 bug!("ty_is_local invoked on unexpected type: {:?}", ty)