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;
11 use crate::traits::{self, Normalized, Obligation, ObligationCause, SelectionContext};
12 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
13 use rustc_middle::ty::fold::TypeFoldable;
14 use rustc_middle::ty::subst::Subst;
15 use rustc_middle::ty::{self, fast_reject, Ty, TyCtxt};
16 use rustc_span::symbol::sym;
17 use rustc_span::DUMMY_SP;
20 /// Whether we do the orphan check relative to this crate or
21 /// to some remote crate.
22 #[derive(Copy, Clone, Debug)]
28 #[derive(Debug, Copy, Clone)]
34 pub struct OverlapResult<'tcx> {
35 pub impl_header: ty::ImplHeader<'tcx>,
36 pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
38 /// `true` if the overlap might've been permitted before the shift
40 pub involves_placeholder: bool,
43 pub fn add_placeholder_note(err: &mut rustc_errors::DiagnosticBuilder<'_>) {
45 "this behavior recently changed as a result of a bug fix; \
46 see rust-lang/rust#56105 for details",
50 /// If there are types that satisfy both impls, invokes `on_overlap`
51 /// with a suitably-freshened `ImplHeader` with those types
52 /// substituted. Otherwise, invokes `no_overlap`.
53 pub fn overlapping_impls<F1, F2, R>(
57 skip_leak_check: SkipLeakCheck,
62 F1: FnOnce(OverlapResult<'_>) -> R,
69 impl1_def_id, impl2_def_id,
71 // Before doing expensive operations like entering an inference context, do
72 // a quick check via fast_reject to tell if the impl headers could possibly
74 let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
75 let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
77 // Check if any of the input types definitely do not unify.
79 impl1_ref.iter().flat_map(|tref| tref.substs.types()),
80 impl2_ref.iter().flat_map(|tref| tref.substs.types()),
83 let t1 = fast_reject::simplify_type(tcx, ty1, false);
84 let t2 = fast_reject::simplify_type(tcx, ty2, false);
85 if let (Some(t1), Some(t2)) = (t1, t2) {
86 // Simplified successfully
87 // Types cannot unify if they differ in their reference mutability or simplify to different types
88 t1 != t2 || ty1.ref_mutability() != ty2.ref_mutability()
94 // Some types involved are definitely different, so the impls couldn't possibly overlap.
95 debug!("overlapping_impls: fast_reject early-exit");
99 let overlaps = tcx.infer_ctxt().enter(|infcx| {
100 let selcx = &mut SelectionContext::intercrate(&infcx);
101 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).is_some()
108 // In the case where we detect an error, run the check again, but
109 // this time tracking intercrate ambuiguity causes for better
110 // diagnostics. (These take time and can lead to false errors.)
111 tcx.infer_ctxt().enter(|infcx| {
112 let selcx = &mut SelectionContext::intercrate(&infcx);
113 selcx.enable_tracking_intercrate_ambiguity_causes();
114 on_overlap(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).unwrap())
118 fn with_fresh_ty_vars<'cx, 'tcx>(
119 selcx: &mut SelectionContext<'cx, 'tcx>,
120 param_env: ty::ParamEnv<'tcx>,
122 ) -> ty::ImplHeader<'tcx> {
123 let tcx = selcx.tcx();
124 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
126 let header = ty::ImplHeader {
128 self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
129 trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
130 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
133 let Normalized { value: mut header, obligations } =
134 traits::normalize(selcx, param_env, ObligationCause::dummy(), header);
136 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
140 /// Can both impl `a` and impl `b` be satisfied by a common type (including
141 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
142 fn overlap<'cx, 'tcx>(
143 selcx: &mut SelectionContext<'cx, 'tcx>,
144 skip_leak_check: SkipLeakCheck,
147 ) -> Option<OverlapResult<'tcx>> {
148 debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
150 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
151 overlap_within_probe(selcx, skip_leak_check, a_def_id, b_def_id, snapshot)
155 fn overlap_within_probe(
156 selcx: &mut SelectionContext<'cx, 'tcx>,
157 skip_leak_check: SkipLeakCheck,
160 snapshot: &CombinedSnapshot<'_, 'tcx>,
161 ) -> Option<OverlapResult<'tcx>> {
162 // For the purposes of this check, we don't bring any placeholder
163 // types into scope; instead, we replace the generic types with
164 // fresh type variables, and hence we do our evaluations in an
165 // empty environment.
166 let param_env = ty::ParamEnv::empty();
168 let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
169 let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
171 debug!("overlap: a_impl_header={:?}", a_impl_header);
172 debug!("overlap: b_impl_header={:?}", b_impl_header);
174 // Do `a` and `b` unify? If not, no overlap.
175 let obligations = match selcx
177 .at(&ObligationCause::dummy(), param_env)
178 .eq_impl_headers(&a_impl_header, &b_impl_header)
180 Ok(InferOk { obligations, value: () }) => obligations,
186 debug!("overlap: unification check succeeded");
188 // There's no overlap if obligations are unsatisfiable or if the obligation negated is
191 // For example, given these two impl headers:
193 // `impl<'a> From<&'a str> for Box<dyn Error>`
194 // `impl<E> From<E> for Box<dyn Error> where E: Error`
198 // `Box<dyn Error>: From<&'?a str>`
199 // `Box<dyn Error>: From<?E>`
201 // After equating the two headers:
203 // `Box<dyn Error> = Box<dyn Error>`
204 // So, `?E = &'?a str` and then given the where clause `&'?a str: Error`.
206 // If the obligation `&'?a str: Error` holds, it means that there's overlap. If that doesn't
207 // hold we need to check if `&'?a str: !Error` holds, if doesn't hold there's overlap because
208 // at some point an impl for `&'?a str: Error` could be added.
209 let infcx = selcx.infcx();
211 let opt_failing_obligation = a_impl_header
215 .chain(b_impl_header.predicates)
216 .map(|p| infcx.resolve_vars_if_possible(p))
217 .map(|p| Obligation {
218 cause: ObligationCause::dummy(),
225 // if both impl headers are set to strict coherence it means that this will be accepted
226 // only if it's stated that T: !Trait. So only prove that the negated obligation holds.
227 if tcx.has_attr(a_def_id, sym::rustc_strict_coherence)
228 && tcx.has_attr(b_def_id, sym::rustc_strict_coherence)
232 .map(|o| selcx.infcx().predicate_must_hold_modulo_regions(o))
235 !selcx.predicate_may_hold_fatal(o)
236 || o.flip_polarity(tcx)
238 .map(|o| selcx.infcx().predicate_must_hold_modulo_regions(o))
242 // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
243 // to the canonical trait query form, `infcx.predicate_may_hold`, once
244 // the new system supports intercrate mode (which coherence needs).
246 if let Some(failing_obligation) = opt_failing_obligation {
247 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
251 if !skip_leak_check.is_yes() {
252 if infcx.leak_check(true, snapshot).is_err() {
253 debug!("overlap: leak check failed");
258 let impl_header = selcx.infcx().resolve_vars_if_possible(a_impl_header);
259 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
260 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
262 let involves_placeholder =
263 matches!(selcx.infcx().region_constraints_added_in_snapshot(snapshot), Some(true));
265 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
268 pub fn trait_ref_is_knowable<'tcx>(
270 trait_ref: ty::TraitRef<'tcx>,
271 ) -> Option<Conflict> {
272 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
273 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
274 // A downstream or cousin crate is allowed to implement some
275 // substitution of this trait-ref.
276 return Some(Conflict::Downstream);
279 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
280 // This is a local or fundamental trait, so future-compatibility
281 // is no concern. We know that downstream/cousin crates are not
282 // allowed to implement a substitution of this trait ref, which
283 // means impls could only come from dependencies of this crate,
284 // which we already know about.
288 // This is a remote non-fundamental trait, so if another crate
289 // can be the "final owner" of a substitution of this trait-ref,
290 // they are allowed to implement it future-compatibly.
292 // However, if we are a final owner, then nobody else can be,
293 // and if we are an intermediate owner, then we don't care
294 // about future-compatibility, which means that we're OK if
296 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
297 debug!("trait_ref_is_knowable: orphan check passed");
300 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
301 Some(Conflict::Upstream)
305 pub fn trait_ref_is_local_or_fundamental<'tcx>(
307 trait_ref: ty::TraitRef<'tcx>,
309 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
312 pub enum OrphanCheckErr<'tcx> {
313 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
314 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
317 /// Checks the coherence orphan rules. `impl_def_id` should be the
318 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
319 /// two conditions must be satisfied:
321 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
322 /// 2. Some local type must appear in `Self`.
323 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
324 debug!("orphan_check({:?})", impl_def_id);
326 // We only except this routine to be invoked on implementations
327 // of a trait, not inherent implementations.
328 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
329 debug!("orphan_check: trait_ref={:?}", trait_ref);
331 // If the *trait* is local to the crate, ok.
332 if trait_ref.def_id.is_local() {
333 debug!("trait {:?} is local to current crate", trait_ref.def_id);
337 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
340 /// Checks whether a trait-ref is potentially implementable by a crate.
342 /// The current rule is that a trait-ref orphan checks in a crate C:
344 /// 1. Order the parameters in the trait-ref in subst order - Self first,
345 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
346 /// 2. Of these type parameters, there is at least one type parameter
347 /// in which, walking the type as a tree, you can reach a type local
348 /// to C where all types in-between are fundamental types. Call the
349 /// first such parameter the "local key parameter".
350 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
351 /// going through `Box`, which is fundamental.
352 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
354 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
355 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
356 /// the local type and the type parameter.
357 /// 3. Before this local type, no generic type parameter of the impl must
358 /// be reachable through fundamental types.
359 /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
360 /// - while `impl<T> Trait<LocalType for Box<T>` results in an error, as `T` is
361 /// reachable through the fundamental type `Box`.
362 /// 4. Every type in the local key parameter not known in C, going
363 /// through the parameter's type tree, must appear only as a subtree of
364 /// a type local to C, with only fundamental types between the type
365 /// local to C and the local key parameter.
366 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
367 /// is bad, because the only local type with `T` as a subtree is
368 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
369 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
370 /// the second occurrence of `T` is not a subtree of *any* local type.
371 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
372 /// `LocalType<Vec<T>>`, which is local and has no types between it and
373 /// the type parameter.
375 /// The orphan rules actually serve several different purposes:
377 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
378 /// every type local to one crate is unknown in the other) can't implement
379 /// the same trait-ref. This follows because it can be seen that no such
380 /// type can orphan-check in 2 such crates.
382 /// To check that a local impl follows the orphan rules, we check it in
383 /// InCrate::Local mode, using type parameters for the "generic" types.
385 /// 2. They ground negative reasoning for coherence. If a user wants to
386 /// write both a conditional blanket impl and a specific impl, we need to
387 /// make sure they do not overlap. For example, if we write
389 /// impl<T> IntoIterator for Vec<T>
390 /// impl<T: Iterator> IntoIterator for T
392 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
393 /// We can observe that this holds in the current crate, but we need to make
394 /// sure this will also hold in all unknown crates (both "independent" crates,
395 /// which we need for link-safety, and also child crates, because we don't want
396 /// child crates to get error for impl conflicts in a *dependency*).
398 /// For that, we only allow negative reasoning if, for every assignment to the
399 /// inference variables, every unknown crate would get an orphan error if they
400 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
401 /// mode. That is sound because we already know all the impls from known crates.
403 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
404 /// add "non-blanket" impls without breaking negative reasoning in dependent
405 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
407 /// For that, we only a allow crate to perform negative reasoning on
408 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
410 /// Because we never perform negative reasoning generically (coherence does
411 /// not involve type parameters), this can be interpreted as doing the full
412 /// orphan check (using InCrate::Local mode), substituting non-local known
413 /// types for all inference variables.
415 /// This allows for crates to future-compatibly add impls as long as they
416 /// can't apply to types with a key parameter in a child crate - applying
417 /// the rules, this basically means that every type parameter in the impl
418 /// must appear behind a non-fundamental type (because this is not a
419 /// type-system requirement, crate owners might also go for "semantic
420 /// future-compatibility" involving things such as sealed traits, but
421 /// the above requirement is sufficient, and is necessary in "open world"
424 /// Note that this function is never called for types that have both type
425 /// parameters and inference variables.
426 fn orphan_check_trait_ref<'tcx>(
428 trait_ref: ty::TraitRef<'tcx>,
430 ) -> Result<(), OrphanCheckErr<'tcx>> {
431 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
433 if trait_ref.needs_infer() && trait_ref.definitely_needs_subst(tcx) {
435 "can't orphan check a trait ref with both params and inference variables {:?}",
440 // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
441 // if at least one of the following is true:
443 // - Trait is a local trait
444 // (already checked in orphan_check prior to calling this function)
446 // - At least one of the types T0..=Tn must be a local type.
447 // Let Ti be the first such type.
448 // - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
450 fn uncover_fundamental_ty<'tcx>(
455 // FIXME: this is currently somewhat overly complicated,
456 // but fixing this requires a more complicated refactor.
457 if !contained_non_local_types(tcx, ty, in_crate).is_empty() {
458 if let Some(inner_tys) = fundamental_ty_inner_tys(tcx, ty) {
460 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
468 let mut non_local_spans = vec![];
469 for (i, input_ty) in trait_ref
472 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
475 debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
476 let non_local_tys = contained_non_local_types(tcx, input_ty, in_crate);
477 if non_local_tys.is_empty() {
478 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
480 } else if let ty::Param(_) = input_ty.kind() {
481 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
482 let local_type = trait_ref
485 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
486 .find(|ty| ty_is_local_constructor(ty, in_crate));
488 debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type);
490 return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type));
493 for input_ty in non_local_tys {
494 non_local_spans.push((input_ty, i == 0));
497 // If we exit above loop, never found a local type.
498 debug!("orphan_check_trait_ref: no local type");
499 Err(OrphanCheckErr::NonLocalInputType(non_local_spans))
502 /// Returns a list of relevant non-local types for `ty`.
504 /// This is just `ty` itself unless `ty` is `#[fundamental]`,
505 /// in which case we recursively look into this type.
507 /// If `ty` is local itself, this method returns an empty `Vec`.
511 /// - `u32` is not local, so this returns `[u32]`.
512 /// - for `Foo<u32>`, where `Foo` is a local type, this returns `[]`.
513 /// - `&mut u32` returns `[u32]`, as `&mut` is a fundamental type, similar to `Box`.
514 /// - `Box<Foo<u32>>` returns `[]`, as `Box` is a fundamental type and `Foo` is local.
515 fn contained_non_local_types(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, in_crate: InCrate) -> Vec<Ty<'tcx>> {
516 if ty_is_local_constructor(ty, in_crate) {
519 match fundamental_ty_inner_tys(tcx, ty) {
521 inner_tys.flat_map(|ty| contained_non_local_types(tcx, ty, in_crate)).collect()
528 /// For `#[fundamental]` ADTs and `&T` / `&mut T`, returns `Some` with the
529 /// type parameters of the ADT, or `T`, respectively. For non-fundamental
530 /// types, returns `None`.
531 fn fundamental_ty_inner_tys(
534 ) -> Option<impl Iterator<Item = Ty<'tcx>>> {
535 let (first_ty, rest_tys) = match *ty.kind() {
536 ty::Ref(_, ty, _) => (ty, ty::subst::InternalSubsts::empty().types()),
537 ty::Adt(def, substs) if def.is_fundamental() => {
538 let mut types = substs.types();
540 // FIXME(eddyb) actually validate `#[fundamental]` up-front.
544 tcx.def_span(def.did),
545 "`#[fundamental]` requires at least one type parameter",
551 Some(first_ty) => (first_ty, types),
557 Some(iter::once(first_ty).chain(rest_tys))
560 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
562 // The type is local to *this* crate - it will not be
563 // local in any other crate.
564 InCrate::Remote => false,
565 InCrate::Local => def_id.is_local(),
569 fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
570 debug!("ty_is_local_constructor({:?})", ty);
588 | ty::Projection(..) => false,
590 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
591 InCrate::Local => false,
592 // The inference variable might be unified with a local
593 // type in that remote crate.
594 InCrate::Remote => true,
597 ty::Adt(def, _) => def_id_is_local(def.did, in_crate),
598 ty::Foreign(did) => def_id_is_local(did, in_crate),
600 // This merits some explanation.
601 // Normally, opaque types are not involed when performing
602 // coherence checking, since it is illegal to directly
603 // implement a trait on an opaque type. However, we might
604 // end up looking at an opaque type during coherence checking
605 // if an opaque type gets used within another type (e.g. as
606 // a type parameter). This requires us to decide whether or
607 // not an opaque type should be considered 'local' or not.
609 // We choose to treat all opaque types as non-local, even
610 // those that appear within the same crate. This seems
611 // somewhat surprising at first, but makes sense when
612 // you consider that opaque types are supposed to hide
613 // the underlying type *within the same crate*. When an
614 // opaque type is used from outside the module
615 // where it is declared, it should be impossible to observe
616 // anything about it other than the traits that it implements.
618 // The alternative would be to look at the underlying type
619 // to determine whether or not the opaque type itself should
620 // be considered local. However, this could make it a breaking change
621 // to switch the underlying ('defining') type from a local type
622 // to a remote type. This would violate the rule that opaque
623 // types should be completely opaque apart from the traits
624 // that they implement, so we don't use this behavior.
629 // Similar to the `Opaque` case (#83613).
633 ty::Dynamic(ref tt, ..) => {
634 if let Some(principal) = tt.principal() {
635 def_id_is_local(principal.def_id(), in_crate)
641 ty::Error(_) => true,
643 ty::Generator(..) | ty::GeneratorWitness(..) => {
644 bug!("ty_is_local invoked on unexpected type: {:?}", ty)