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::fast_reject::{self, SimplifyParams, StripReferences};
16 use rustc_middle::ty::fold::TypeFoldable;
17 use rustc_middle::ty::subst::Subst;
18 use rustc_middle::ty::{self, Ty, TyCtxt};
19 use rustc_span::symbol::sym;
20 use rustc_span::DUMMY_SP;
23 /// Whether we do the orphan check relative to this crate or
24 /// to some remote crate.
25 #[derive(Copy, Clone, Debug)]
31 #[derive(Debug, Copy, Clone)]
37 pub struct OverlapResult<'tcx> {
38 pub impl_header: ty::ImplHeader<'tcx>,
39 pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
41 /// `true` if the overlap might've been permitted before the shift
43 pub involves_placeholder: bool,
46 pub fn add_placeholder_note(err: &mut rustc_errors::DiagnosticBuilder<'_>) {
48 "this behavior recently changed as a result of a bug fix; \
49 see rust-lang/rust#56105 for details",
53 /// If there are types that satisfy both impls, invokes `on_overlap`
54 /// with a suitably-freshened `ImplHeader` with those types
55 /// substituted. Otherwise, invokes `no_overlap`.
56 #[instrument(skip(tcx, skip_leak_check, on_overlap, no_overlap), level = "debug")]
57 pub fn overlapping_impls<F1, F2, R>(
61 skip_leak_check: SkipLeakCheck,
66 F1: FnOnce(OverlapResult<'_>) -> R,
69 // Before doing expensive operations like entering an inference context, do
70 // a quick check via fast_reject to tell if the impl headers could possibly
72 let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
73 let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
75 // Check if any of the input types definitely do not unify.
77 impl1_ref.iter().flat_map(|tref| tref.substs.types()),
78 impl2_ref.iter().flat_map(|tref| tref.substs.types()),
81 let t1 = fast_reject::simplify_type(tcx, ty1, SimplifyParams::No, StripReferences::No);
82 let t2 = fast_reject::simplify_type(tcx, ty2, SimplifyParams::No, StripReferences::No);
84 if let (Some(t1), Some(t2)) = (t1, t2) {
85 // Simplified successfully
92 // Some types involved are definitely different, so the impls couldn't possibly overlap.
93 debug!("overlapping_impls: fast_reject early-exit");
97 let overlaps = tcx.infer_ctxt().enter(|infcx| {
98 let selcx = &mut SelectionContext::intercrate(&infcx);
99 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).is_some()
106 // In the case where we detect an error, run the check again, but
107 // this time tracking intercrate ambuiguity causes for better
108 // diagnostics. (These take time and can lead to false errors.)
109 tcx.infer_ctxt().enter(|infcx| {
110 let selcx = &mut SelectionContext::intercrate(&infcx);
111 selcx.enable_tracking_intercrate_ambiguity_causes();
112 on_overlap(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).unwrap())
116 fn with_fresh_ty_vars<'cx, 'tcx>(
117 selcx: &mut SelectionContext<'cx, 'tcx>,
118 param_env: ty::ParamEnv<'tcx>,
120 ) -> ty::ImplHeader<'tcx> {
121 let tcx = selcx.tcx();
122 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
124 let header = ty::ImplHeader {
126 self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
127 trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
128 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
131 let Normalized { value: mut header, obligations } =
132 traits::normalize(selcx, param_env, ObligationCause::dummy(), header);
134 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
138 /// Can both impl `a` and impl `b` be satisfied by a common type (including
139 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
140 fn overlap<'cx, 'tcx>(
141 selcx: &mut SelectionContext<'cx, 'tcx>,
142 skip_leak_check: SkipLeakCheck,
145 ) -> Option<OverlapResult<'tcx>> {
146 debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
148 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
149 overlap_within_probe(selcx, skip_leak_check, a_def_id, b_def_id, snapshot)
153 fn overlap_within_probe<'cx, 'tcx>(
154 selcx: &mut SelectionContext<'cx, 'tcx>,
155 skip_leak_check: SkipLeakCheck,
158 snapshot: &CombinedSnapshot<'_, 'tcx>,
159 ) -> Option<OverlapResult<'tcx>> {
160 fn loose_check<'cx, 'tcx>(
161 selcx: &mut SelectionContext<'cx, 'tcx>,
162 o: &PredicateObligation<'tcx>,
164 !selcx.predicate_may_hold_fatal(o)
167 fn strict_check<'cx, 'tcx>(
168 selcx: &SelectionContext<'cx, 'tcx>,
169 o: &PredicateObligation<'tcx>,
171 let infcx = selcx.infcx();
175 .map(|o| selcx.infcx().predicate_must_hold_modulo_regions(o))
179 // For the purposes of this check, we don't bring any placeholder
180 // types into scope; instead, we replace the generic types with
181 // fresh type variables, and hence we do our evaluations in an
182 // empty environment.
183 let param_env = ty::ParamEnv::empty();
185 let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
186 let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
188 debug!("overlap: a_impl_header={:?}", a_impl_header);
189 debug!("overlap: b_impl_header={:?}", b_impl_header);
191 // Do `a` and `b` unify? If not, no overlap.
192 let obligations = match selcx
194 .at(&ObligationCause::dummy(), param_env)
195 .eq_impl_headers(&a_impl_header, &b_impl_header)
197 Ok(InferOk { obligations, value: () }) => obligations,
203 debug!("overlap: unification check succeeded");
205 // There's no overlap if obligations are unsatisfiable or if the obligation negated is
208 // For example, given these two impl headers:
210 // `impl<'a> From<&'a str> for Box<dyn Error>`
211 // `impl<E> From<E> for Box<dyn Error> where E: Error`
215 // `Box<dyn Error>: From<&'?a str>`
216 // `Box<dyn Error>: From<?E>`
218 // After equating the two headers:
220 // `Box<dyn Error> = Box<dyn Error>`
221 // So, `?E = &'?a str` and then given the where clause `&'?a str: Error`.
223 // If the obligation `&'?a str: Error` holds, it means that there's overlap. If that doesn't
224 // hold we need to check if `&'?a str: !Error` holds, if doesn't hold there's overlap because
225 // at some point an impl for `&'?a str: Error` could be added.
226 let infcx = selcx.infcx();
228 let opt_failing_obligation = a_impl_header
232 .chain(b_impl_header.predicates)
233 .map(|p| infcx.resolve_vars_if_possible(p))
234 .map(|p| Obligation {
235 cause: ObligationCause::dummy(),
242 // if both impl headers are set to strict coherence it means that this will be accepted
243 // only if it's stated that T: !Trait. So only prove that the negated obligation holds.
244 if tcx.has_attr(a_def_id, sym::rustc_strict_coherence)
245 && tcx.has_attr(b_def_id, sym::rustc_strict_coherence)
247 strict_check(selcx, o)
249 loose_check(selcx, o) || tcx.features().negative_impls && strict_check(selcx, o)
252 // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
253 // to the canonical trait query form, `infcx.predicate_may_hold`, once
254 // the new system supports intercrate mode (which coherence needs).
256 if let Some(failing_obligation) = opt_failing_obligation {
257 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
261 if !skip_leak_check.is_yes() {
262 if infcx.leak_check(true, snapshot).is_err() {
263 debug!("overlap: leak check failed");
268 let impl_header = selcx.infcx().resolve_vars_if_possible(a_impl_header);
269 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
270 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
272 let involves_placeholder =
273 matches!(selcx.infcx().region_constraints_added_in_snapshot(snapshot), Some(true));
275 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
278 pub fn trait_ref_is_knowable<'tcx>(
280 trait_ref: ty::TraitRef<'tcx>,
281 ) -> Option<Conflict> {
282 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
283 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
284 // A downstream or cousin crate is allowed to implement some
285 // substitution of this trait-ref.
286 return Some(Conflict::Downstream);
289 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
290 // This is a local or fundamental trait, so future-compatibility
291 // is no concern. We know that downstream/cousin crates are not
292 // allowed to implement a substitution of this trait ref, which
293 // means impls could only come from dependencies of this crate,
294 // which we already know about.
298 // This is a remote non-fundamental trait, so if another crate
299 // can be the "final owner" of a substitution of this trait-ref,
300 // they are allowed to implement it future-compatibly.
302 // However, if we are a final owner, then nobody else can be,
303 // and if we are an intermediate owner, then we don't care
304 // about future-compatibility, which means that we're OK if
306 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
307 debug!("trait_ref_is_knowable: orphan check passed");
310 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
311 Some(Conflict::Upstream)
315 pub fn trait_ref_is_local_or_fundamental<'tcx>(
317 trait_ref: ty::TraitRef<'tcx>,
319 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
322 pub enum OrphanCheckErr<'tcx> {
323 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
324 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
327 /// Checks the coherence orphan rules. `impl_def_id` should be the
328 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
329 /// two conditions must be satisfied:
331 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
332 /// 2. Some local type must appear in `Self`.
333 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
334 debug!("orphan_check({:?})", impl_def_id);
336 // We only except this routine to be invoked on implementations
337 // of a trait, not inherent implementations.
338 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
339 debug!("orphan_check: trait_ref={:?}", trait_ref);
341 // If the *trait* is local to the crate, ok.
342 if trait_ref.def_id.is_local() {
343 debug!("trait {:?} is local to current crate", trait_ref.def_id);
347 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
350 /// Checks whether a trait-ref is potentially implementable by a crate.
352 /// The current rule is that a trait-ref orphan checks in a crate C:
354 /// 1. Order the parameters in the trait-ref in subst order - Self first,
355 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
356 /// 2. Of these type parameters, there is at least one type parameter
357 /// in which, walking the type as a tree, you can reach a type local
358 /// to C where all types in-between are fundamental types. Call the
359 /// first such parameter the "local key parameter".
360 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
361 /// going through `Box`, which is fundamental.
362 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
364 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
365 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
366 /// the local type and the type parameter.
367 /// 3. Before this local type, no generic type parameter of the impl must
368 /// be reachable through fundamental types.
369 /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
370 /// - while `impl<T> Trait<LocalType for Box<T>` results in an error, as `T` is
371 /// reachable through the fundamental type `Box`.
372 /// 4. Every type in the local key parameter not known in C, going
373 /// through the parameter's type tree, must appear only as a subtree of
374 /// a type local to C, with only fundamental types between the type
375 /// local to C and the local key parameter.
376 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
377 /// is bad, because the only local type with `T` as a subtree is
378 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
379 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
380 /// the second occurrence of `T` is not a subtree of *any* local type.
381 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
382 /// `LocalType<Vec<T>>`, which is local and has no types between it and
383 /// the type parameter.
385 /// The orphan rules actually serve several different purposes:
387 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
388 /// every type local to one crate is unknown in the other) can't implement
389 /// the same trait-ref. This follows because it can be seen that no such
390 /// type can orphan-check in 2 such crates.
392 /// To check that a local impl follows the orphan rules, we check it in
393 /// InCrate::Local mode, using type parameters for the "generic" types.
395 /// 2. They ground negative reasoning for coherence. If a user wants to
396 /// write both a conditional blanket impl and a specific impl, we need to
397 /// make sure they do not overlap. For example, if we write
399 /// impl<T> IntoIterator for Vec<T>
400 /// impl<T: Iterator> IntoIterator for T
402 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
403 /// We can observe that this holds in the current crate, but we need to make
404 /// sure this will also hold in all unknown crates (both "independent" crates,
405 /// which we need for link-safety, and also child crates, because we don't want
406 /// child crates to get error for impl conflicts in a *dependency*).
408 /// For that, we only allow negative reasoning if, for every assignment to the
409 /// inference variables, every unknown crate would get an orphan error if they
410 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
411 /// mode. That is sound because we already know all the impls from known crates.
413 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
414 /// add "non-blanket" impls without breaking negative reasoning in dependent
415 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
417 /// For that, we only a allow crate to perform negative reasoning on
418 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
420 /// Because we never perform negative reasoning generically (coherence does
421 /// not involve type parameters), this can be interpreted as doing the full
422 /// orphan check (using InCrate::Local mode), substituting non-local known
423 /// types for all inference variables.
425 /// This allows for crates to future-compatibly add impls as long as they
426 /// can't apply to types with a key parameter in a child crate - applying
427 /// the rules, this basically means that every type parameter in the impl
428 /// must appear behind a non-fundamental type (because this is not a
429 /// type-system requirement, crate owners might also go for "semantic
430 /// future-compatibility" involving things such as sealed traits, but
431 /// the above requirement is sufficient, and is necessary in "open world"
434 /// Note that this function is never called for types that have both type
435 /// parameters and inference variables.
436 fn orphan_check_trait_ref<'tcx>(
438 trait_ref: ty::TraitRef<'tcx>,
440 ) -> Result<(), OrphanCheckErr<'tcx>> {
441 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
443 if trait_ref.needs_infer() && trait_ref.needs_subst() {
445 "can't orphan check a trait ref with both params and inference variables {:?}",
450 // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
451 // if at least one of the following is true:
453 // - Trait is a local trait
454 // (already checked in orphan_check prior to calling this function)
456 // - At least one of the types T0..=Tn must be a local type.
457 // Let Ti be the first such type.
458 // - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
460 fn uncover_fundamental_ty<'tcx>(
465 // FIXME: this is currently somewhat overly complicated,
466 // but fixing this requires a more complicated refactor.
467 if !contained_non_local_types(tcx, ty, in_crate).is_empty() {
468 if let Some(inner_tys) = fundamental_ty_inner_tys(tcx, ty) {
470 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
478 let mut non_local_spans = vec![];
479 for (i, input_ty) in trait_ref
482 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
485 debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
486 let non_local_tys = contained_non_local_types(tcx, input_ty, in_crate);
487 if non_local_tys.is_empty() {
488 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
490 } else if let ty::Param(_) = input_ty.kind() {
491 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
492 let local_type = trait_ref
495 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
496 .find(|ty| ty_is_local_constructor(ty, in_crate));
498 debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type);
500 return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type));
503 non_local_spans.extend(non_local_tys.into_iter().map(|input_ty| (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>(
528 if ty_is_local_constructor(ty, in_crate) {
531 match fundamental_ty_inner_tys(tcx, ty) {
533 inner_tys.flat_map(|ty| contained_non_local_types(tcx, ty, in_crate)).collect()
540 /// For `#[fundamental]` ADTs and `&T` / `&mut T`, returns `Some` with the
541 /// type parameters of the ADT, or `T`, respectively. For non-fundamental
542 /// types, returns `None`.
543 fn fundamental_ty_inner_tys<'tcx>(
546 ) -> Option<impl Iterator<Item = Ty<'tcx>>> {
547 let (first_ty, rest_tys) = match *ty.kind() {
548 ty::Ref(_, ty, _) => (ty, ty::subst::InternalSubsts::empty().types()),
549 ty::Adt(def, substs) if def.is_fundamental() => {
550 let mut types = substs.types();
552 // FIXME(eddyb) actually validate `#[fundamental]` up-front.
556 tcx.def_span(def.did),
557 "`#[fundamental]` requires at least one type parameter",
563 Some(first_ty) => (first_ty, types),
569 Some(iter::once(first_ty).chain(rest_tys))
572 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
574 // The type is local to *this* crate - it will not be
575 // local in any other crate.
576 InCrate::Remote => false,
577 InCrate::Local => def_id.is_local(),
581 fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
582 debug!("ty_is_local_constructor({:?})", ty);
600 | ty::Projection(..) => false,
602 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
603 InCrate::Local => false,
604 // The inference variable might be unified with a local
605 // type in that remote crate.
606 InCrate::Remote => true,
609 ty::Adt(def, _) => def_id_is_local(def.did, in_crate),
610 ty::Foreign(did) => def_id_is_local(did, in_crate),
612 // This merits some explanation.
613 // Normally, opaque types are not involed when performing
614 // coherence checking, since it is illegal to directly
615 // implement a trait on an opaque type. However, we might
616 // end up looking at an opaque type during coherence checking
617 // if an opaque type gets used within another type (e.g. as
618 // a type parameter). This requires us to decide whether or
619 // not an opaque type should be considered 'local' or not.
621 // We choose to treat all opaque types as non-local, even
622 // those that appear within the same crate. This seems
623 // somewhat surprising at first, but makes sense when
624 // you consider that opaque types are supposed to hide
625 // the underlying type *within the same crate*. When an
626 // opaque type is used from outside the module
627 // where it is declared, it should be impossible to observe
628 // anything about it other than the traits that it implements.
630 // The alternative would be to look at the underlying type
631 // to determine whether or not the opaque type itself should
632 // be considered local. However, this could make it a breaking change
633 // to switch the underlying ('defining') type from a local type
634 // to a remote type. This would violate the rule that opaque
635 // types should be completely opaque apart from the traits
636 // that they implement, so we don't use this behavior.
641 // Similar to the `Opaque` case (#83613).
645 ty::Dynamic(ref tt, ..) => {
646 if let Some(principal) = tt.principal() {
647 def_id_is_local(principal.def_id(), in_crate)
653 ty::Error(_) => true,
655 ty::Generator(..) | ty::GeneratorWitness(..) => {
656 bug!("ty_is_local invoked on unexpected type: {:?}", ty)