1 //! See Rustc Guide chapters on [trait-resolution] and [trait-specialization] for more info on how
4 //! [trait-resolution]: https://rust-lang.github.io/rustc-guide/traits/resolution.html
5 //! [trait-specialization]: https://rust-lang.github.io/rustc-guide/traits/specialization.html
7 use crate::hir::def_id::{DefId, LOCAL_CRATE};
8 use crate::infer::{CombinedSnapshot, InferOk};
9 use crate::traits::select::IntercrateAmbiguityCause;
10 use crate::traits::IntercrateMode;
11 use crate::traits::{self, Normalized, Obligation, ObligationCause, SelectionContext};
12 use crate::ty::fold::TypeFoldable;
13 use crate::ty::subst::Subst;
14 use crate::ty::{self, Ty, TyCtxt};
15 use syntax::symbol::sym;
16 use syntax_pos::DUMMY_SP;
18 /// Whether we do the orphan check relative to this crate or
19 /// to some remote crate.
20 #[derive(Copy, Clone, Debug)]
26 #[derive(Debug, Copy, Clone)]
29 Downstream { used_to_be_broken: bool },
32 pub struct OverlapResult<'tcx> {
33 pub impl_header: ty::ImplHeader<'tcx>,
34 pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
36 /// `true` if the overlap might've been permitted before the shift
38 pub involves_placeholder: bool,
41 pub fn add_placeholder_note(err: &mut errors::DiagnosticBuilder<'_>) {
43 "this behavior recently changed as a result of a bug fix; \
44 see rust-lang/rust#56105 for details"
48 /// If there are types that satisfy both impls, invokes `on_overlap`
49 /// with a suitably-freshened `ImplHeader` with those types
50 /// substituted. Otherwise, invokes `no_overlap`.
51 pub fn overlapping_impls<F1, F2, R>(
55 intercrate_mode: IntercrateMode,
60 F1: FnOnce(OverlapResult<'_>) -> R,
67 intercrate_mode={:?})",
68 impl1_def_id, impl2_def_id, intercrate_mode
71 let overlaps = tcx.infer_ctxt().enter(|infcx| {
72 let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
73 overlap(selcx, 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, intercrate_mode);
85 selcx.enable_tracking_intercrate_ambiguity_causes();
86 on_overlap(overlap(selcx, 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>,
118 ) -> Option<OverlapResult<'tcx>> {
119 debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
121 selcx.infcx().probe(|snapshot| overlap_within_probe(selcx, a_def_id, b_def_id, snapshot))
124 fn overlap_within_probe(
125 selcx: &mut SelectionContext<'cx, 'tcx>,
128 snapshot: &CombinedSnapshot<'_, 'tcx>,
129 ) -> Option<OverlapResult<'tcx>> {
130 // For the purposes of this check, we don't bring any placeholder
131 // types into scope; instead, we replace the generic types with
132 // fresh type variables, and hence we do our evaluations in an
133 // empty environment.
134 let param_env = ty::ParamEnv::empty();
136 let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
137 let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
139 debug!("overlap: a_impl_header={:?}", a_impl_header);
140 debug!("overlap: b_impl_header={:?}", b_impl_header);
142 // Do `a` and `b` unify? If not, no overlap.
143 let obligations = match selcx
145 .at(&ObligationCause::dummy(), param_env)
146 .eq_impl_headers(&a_impl_header, &b_impl_header)
148 Ok(InferOk { obligations, value: () }) => obligations,
149 Err(_) => return None,
152 debug!("overlap: unification check succeeded");
154 // Are any of the obligations unsatisfiable? If so, no overlap.
155 let infcx = selcx.infcx();
156 let opt_failing_obligation = a_impl_header
159 .chain(&b_impl_header.predicates)
160 .map(|p| infcx.resolve_vars_if_possible(p))
161 .map(|p| Obligation {
162 cause: ObligationCause::dummy(),
168 .find(|o| !selcx.predicate_may_hold_fatal(o));
169 // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
170 // to the canonical trait query form, `infcx.predicate_may_hold`, once
171 // the new system supports intercrate mode (which coherence needs).
173 if let Some(failing_obligation) = opt_failing_obligation {
174 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
178 let impl_header = selcx.infcx().resolve_vars_if_possible(&a_impl_header);
179 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
180 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
182 let involves_placeholder = match selcx.infcx().region_constraints_added_in_snapshot(snapshot) {
187 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
190 pub fn trait_ref_is_knowable<'tcx>(
192 trait_ref: ty::TraitRef<'tcx>,
193 ) -> Option<Conflict> {
194 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
195 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
196 // A downstream or cousin crate is allowed to implement some
197 // substitution of this trait-ref.
199 // A trait can be implementable for a trait ref by both the current
200 // crate and crates downstream of it. Older versions of rustc
201 // were not aware of this, causing incoherence (issue #43355).
202 let used_to_be_broken = orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok();
203 if used_to_be_broken {
204 debug!("trait_ref_is_knowable({:?}) - USED TO BE BROKEN", trait_ref);
206 return Some(Conflict::Downstream { used_to_be_broken });
209 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
210 // This is a local or fundamental trait, so future-compatibility
211 // is no concern. We know that downstream/cousin crates are not
212 // allowed to implement a substitution of this trait ref, which
213 // means impls could only come from dependencies of this crate,
214 // which we already know about.
218 // This is a remote non-fundamental trait, so if another crate
219 // can be the "final owner" of a substitution of this trait-ref,
220 // they are allowed to implement it future-compatibly.
222 // However, if we are a final owner, then nobody else can be,
223 // and if we are an intermediate owner, then we don't care
224 // about future-compatibility, which means that we're OK if
226 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
227 debug!("trait_ref_is_knowable: orphan check passed");
230 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
231 return Some(Conflict::Upstream);
235 pub fn trait_ref_is_local_or_fundamental<'tcx>(
237 trait_ref: ty::TraitRef<'tcx>,
239 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
242 pub enum OrphanCheckErr<'tcx> {
243 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
244 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
247 /// Checks the coherence orphan rules. `impl_def_id` should be the
248 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
249 /// two conditions must be satisfied:
251 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
252 /// 2. Some local type must appear in `Self`.
253 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
254 debug!("orphan_check({:?})", impl_def_id);
256 // We only except this routine to be invoked on implementations
257 // of a trait, not inherent implementations.
258 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
259 debug!("orphan_check: trait_ref={:?}", trait_ref);
261 // If the *trait* is local to the crate, ok.
262 if trait_ref.def_id.is_local() {
263 debug!("trait {:?} is local to current crate", trait_ref.def_id);
267 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
270 /// Checks whether a trait-ref is potentially implementable by a crate.
272 /// The current rule is that a trait-ref orphan checks in a crate C:
274 /// 1. Order the parameters in the trait-ref in subst order - Self first,
275 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
276 /// 2. Of these type parameters, there is at least one type parameter
277 /// in which, walking the type as a tree, you can reach a type local
278 /// to C where all types in-between are fundamental types. Call the
279 /// first such parameter the "local key parameter".
280 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
281 /// going through `Box`, which is fundamental.
282 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
284 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
285 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
286 /// the local type and the type parameter.
287 /// 3. Every type parameter before the local key parameter is fully known in C.
288 /// - e.g., `impl<T> T: Trait<LocalType>` is bad, because `T` might be
290 /// - but `impl<T> LocalType: Trait<T>` is OK, because `LocalType`
291 /// occurs before `T`.
292 /// 4. Every type in the local key parameter not known in C, going
293 /// through the parameter's type tree, must appear only as a subtree of
294 /// a type local to C, with only fundamental types between the type
295 /// local to C and the local key parameter.
296 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
297 /// is bad, because the only local type with `T` as a subtree is
298 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
299 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
300 /// the second occurrence of `T` is not a subtree of *any* local type.
301 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
302 /// `LocalType<Vec<T>>`, which is local and has no types between it and
303 /// the type parameter.
305 /// The orphan rules actually serve several different purposes:
307 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
308 /// every type local to one crate is unknown in the other) can't implement
309 /// the same trait-ref. This follows because it can be seen that no such
310 /// type can orphan-check in 2 such crates.
312 /// To check that a local impl follows the orphan rules, we check it in
313 /// InCrate::Local mode, using type parameters for the "generic" types.
315 /// 2. They ground negative reasoning for coherence. If a user wants to
316 /// write both a conditional blanket impl and a specific impl, we need to
317 /// make sure they do not overlap. For example, if we write
319 /// impl<T> IntoIterator for Vec<T>
320 /// impl<T: Iterator> IntoIterator for T
322 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
323 /// We can observe that this holds in the current crate, but we need to make
324 /// sure this will also hold in all unknown crates (both "independent" crates,
325 /// which we need for link-safety, and also child crates, because we don't want
326 /// child crates to get error for impl conflicts in a *dependency*).
328 /// For that, we only allow negative reasoning if, for every assignment to the
329 /// inference variables, every unknown crate would get an orphan error if they
330 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
331 /// mode. That is sound because we already know all the impls from known crates.
333 /// 3. For non-#[fundamental] traits, they guarantee that parent crates can
334 /// add "non-blanket" impls without breaking negative reasoning in dependent
335 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
337 /// For that, we only a allow crate to perform negative reasoning on
338 /// non-local-non-#[fundamental] only if there's a local key parameter as per (2).
340 /// Because we never perform negative reasoning generically (coherence does
341 /// not involve type parameters), this can be interpreted as doing the full
342 /// orphan check (using InCrate::Local mode), substituting non-local known
343 /// types for all inference variables.
345 /// This allows for crates to future-compatibly add impls as long as they
346 /// can't apply to types with a key parameter in a child crate - applying
347 /// the rules, this basically means that every type parameter in the impl
348 /// must appear behind a non-fundamental type (because this is not a
349 /// type-system requirement, crate owners might also go for "semantic
350 /// future-compatibility" involving things such as sealed traits, but
351 /// the above requirement is sufficient, and is necessary in "open world"
354 /// Note that this function is never called for types that have both type
355 /// parameters and inference variables.
356 fn orphan_check_trait_ref<'tcx>(
358 trait_ref: ty::TraitRef<'tcx>,
360 ) -> Result<(), OrphanCheckErr<'tcx>> {
361 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
363 if trait_ref.needs_infer() && trait_ref.needs_subst() {
365 "can't orphan check a trait ref with both params and inference variables {:?}",
370 // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
371 // if at least one of the following is true:
373 // - Trait is a local trait
374 // (already checked in orphan_check prior to calling this function)
376 // - At least one of the types T0..=Tn must be a local type.
377 // Let Ti be the first such type.
378 // - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
380 fn uncover_fundamental_ty<'tcx>(
385 if fundamental_ty(ty) && ty_is_non_local(ty, in_crate).is_some() {
386 ty.walk_shallow().flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate)).collect()
392 let mut non_local_spans = vec![];
394 trait_ref.input_types().flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate)).enumerate()
396 debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
397 let non_local_tys = ty_is_non_local(input_ty, in_crate);
398 if non_local_tys.is_none() {
399 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
401 } else if let ty::Param(_) = input_ty.kind {
402 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
403 let local_type = trait_ref
405 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
406 .filter(|ty| ty_is_non_local_constructor(ty, in_crate).is_none())
409 debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type);
411 return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type));
413 if let Some(non_local_tys) = non_local_tys {
414 for input_ty in non_local_tys {
415 non_local_spans.push((input_ty, i == 0));
419 // If we exit above loop, never found a local type.
420 debug!("orphan_check_trait_ref: no local type");
421 Err(OrphanCheckErr::NonLocalInputType(non_local_spans))
424 fn ty_is_non_local<'t>(ty: Ty<'t>, in_crate: InCrate) -> Option<Vec<Ty<'t>>> {
425 match ty_is_non_local_constructor(ty, in_crate) {
427 if !fundamental_ty(ty) {
432 .filter_map(|t| ty_is_non_local(t, in_crate))
435 if tys.is_empty() { None } else { Some(tys) }
442 fn fundamental_ty(ty: Ty<'_>) -> bool {
445 ty::Adt(def, _) => def.is_fundamental(),
450 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
452 // The type is local to *this* crate - it will not be
453 // local in any other crate.
454 InCrate::Remote => false,
455 InCrate::Local => def_id.is_local(),
459 fn ty_is_non_local_constructor<'tcx>(ty: Ty<'tcx>, in_crate: InCrate) -> Option<Ty<'tcx>> {
460 debug!("ty_is_non_local_constructor({:?})", ty);
478 | ty::Projection(..) => Some(ty),
480 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
481 InCrate::Local => Some(ty),
482 // The inference variable might be unified with a local
483 // type in that remote crate.
484 InCrate::Remote => None,
488 if def_id_is_local(def.did, in_crate) {
494 ty::Foreign(did) => {
495 if def_id_is_local(did, in_crate) {
502 // This merits some explanation.
503 // Normally, opaque types are not involed when performing
504 // coherence checking, since it is illegal to directly
505 // implement a trait on an opaque type. However, we might
506 // end up looking at an opaque type during coherence checking
507 // if an opaque type gets used within another type (e.g. as
508 // a type parameter). This requires us to decide whether or
509 // not an opaque type should be considered 'local' or not.
511 // We choose to treat all opaque types as non-local, even
512 // those that appear within the same crate. This seems
513 // somewhat suprising at first, but makes sense when
514 // you consider that opaque types are supposed to hide
515 // the underlying type *within the same crate*. When an
516 // opaque type is used from outside the module
517 // where it is declared, it should be impossible to observe
518 // anyything about it other than the traits that it implements.
520 // The alternative would be to look at the underlying type
521 // to determine whether or not the opaque type itself should
522 // be considered local. However, this could make it a breaking change
523 // to switch the underlying ('defining') type from a local type
524 // to a remote type. This would violate the rule that opaque
525 // types should be completely opaque apart from the traits
526 // that they implement, so we don't use this behavior.
530 ty::Dynamic(ref tt, ..) => {
531 if let Some(principal) = tt.principal() {
532 if def_id_is_local(principal.def_id(), in_crate) { None } else { Some(ty) }
540 ty::UnnormalizedProjection(..)
543 | ty::GeneratorWitness(..) => bug!("ty_is_local invoked on unexpected type: {:?}", ty),