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::ty::fold::TypeFoldable;
12 use rustc::ty::subst::Subst;
13 use rustc::ty::{self, Ty, TyCtxt};
14 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
15 use rustc_span::symbol::sym;
16 use rustc_span::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)]
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 rustc_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 skip_leak_check: SkipLeakCheck,
60 F1: FnOnce(OverlapResult<'_>) -> R,
67 impl1_def_id, impl2_def_id,
70 let overlaps = tcx.infer_ctxt().enter(|infcx| {
71 let selcx = &mut SelectionContext::intercrate(&infcx);
72 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).is_some()
79 // In the case where we detect an error, run the check again, but
80 // this time tracking intercrate ambuiguity causes for better
81 // diagnostics. (These take time and can lead to false errors.)
82 tcx.infer_ctxt().enter(|infcx| {
83 let selcx = &mut SelectionContext::intercrate(&infcx);
84 selcx.enable_tracking_intercrate_ambiguity_causes();
85 on_overlap(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).unwrap())
89 fn with_fresh_ty_vars<'cx, 'tcx>(
90 selcx: &mut SelectionContext<'cx, 'tcx>,
91 param_env: ty::ParamEnv<'tcx>,
93 ) -> ty::ImplHeader<'tcx> {
94 let tcx = selcx.tcx();
95 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
97 let header = ty::ImplHeader {
99 self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
100 trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
101 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
104 let Normalized { value: mut header, obligations } =
105 traits::normalize(selcx, param_env, ObligationCause::dummy(), &header);
107 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
111 /// Can both impl `a` and impl `b` be satisfied by a common type (including
112 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
113 fn overlap<'cx, 'tcx>(
114 selcx: &mut SelectionContext<'cx, 'tcx>,
115 skip_leak_check: SkipLeakCheck,
118 ) -> Option<OverlapResult<'tcx>> {
119 debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
121 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
122 overlap_within_probe(selcx, a_def_id, b_def_id, snapshot)
126 fn overlap_within_probe(
127 selcx: &mut SelectionContext<'cx, 'tcx>,
130 snapshot: &CombinedSnapshot<'_, 'tcx>,
131 ) -> Option<OverlapResult<'tcx>> {
132 // For the purposes of this check, we don't bring any placeholder
133 // types into scope; instead, we replace the generic types with
134 // fresh type variables, and hence we do our evaluations in an
135 // empty environment.
136 let param_env = ty::ParamEnv::empty();
138 let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
139 let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
141 debug!("overlap: a_impl_header={:?}", a_impl_header);
142 debug!("overlap: b_impl_header={:?}", b_impl_header);
144 // Do `a` and `b` unify? If not, no overlap.
145 let obligations = match selcx
147 .at(&ObligationCause::dummy(), param_env)
148 .eq_impl_headers(&a_impl_header, &b_impl_header)
150 Ok(InferOk { obligations, value: () }) => obligations,
156 debug!("overlap: unification check succeeded");
158 // Are any of the obligations unsatisfiable? If so, no overlap.
159 let infcx = selcx.infcx();
160 let opt_failing_obligation = a_impl_header
163 .chain(&b_impl_header.predicates)
164 .map(|p| infcx.resolve_vars_if_possible(p))
165 .map(|p| Obligation {
166 cause: ObligationCause::dummy(),
172 .find(|o| !selcx.predicate_may_hold_fatal(o));
173 // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
174 // to the canonical trait query form, `infcx.predicate_may_hold`, once
175 // the new system supports intercrate mode (which coherence needs).
177 if let Some(failing_obligation) = opt_failing_obligation {
178 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
182 let impl_header = selcx.infcx().resolve_vars_if_possible(&a_impl_header);
183 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
184 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
186 let involves_placeholder = match selcx.infcx().region_constraints_added_in_snapshot(snapshot) {
191 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
194 pub fn trait_ref_is_knowable<'tcx>(
196 trait_ref: ty::TraitRef<'tcx>,
197 ) -> Option<Conflict> {
198 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
199 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
200 // A downstream or cousin crate is allowed to implement some
201 // substitution of this trait-ref.
202 return Some(Conflict::Downstream);
205 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
206 // This is a local or fundamental trait, so future-compatibility
207 // is no concern. We know that downstream/cousin crates are not
208 // allowed to implement a substitution of this trait ref, which
209 // means impls could only come from dependencies of this crate,
210 // which we already know about.
214 // This is a remote non-fundamental trait, so if another crate
215 // can be the "final owner" of a substitution of this trait-ref,
216 // they are allowed to implement it future-compatibly.
218 // However, if we are a final owner, then nobody else can be,
219 // and if we are an intermediate owner, then we don't care
220 // about future-compatibility, which means that we're OK if
222 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
223 debug!("trait_ref_is_knowable: orphan check passed");
226 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
227 return Some(Conflict::Upstream);
231 pub fn trait_ref_is_local_or_fundamental<'tcx>(
233 trait_ref: ty::TraitRef<'tcx>,
235 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
238 pub enum OrphanCheckErr<'tcx> {
239 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
240 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
243 /// Checks the coherence orphan rules. `impl_def_id` should be the
244 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
245 /// two conditions must be satisfied:
247 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
248 /// 2. Some local type must appear in `Self`.
249 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
250 debug!("orphan_check({:?})", impl_def_id);
252 // We only except this routine to be invoked on implementations
253 // of a trait, not inherent implementations.
254 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
255 debug!("orphan_check: trait_ref={:?}", trait_ref);
257 // If the *trait* is local to the crate, ok.
258 if trait_ref.def_id.is_local() {
259 debug!("trait {:?} is local to current crate", trait_ref.def_id);
263 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
266 /// Checks whether a trait-ref is potentially implementable by a crate.
268 /// The current rule is that a trait-ref orphan checks in a crate C:
270 /// 1. Order the parameters in the trait-ref in subst order - Self first,
271 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
272 /// 2. Of these type parameters, there is at least one type parameter
273 /// in which, walking the type as a tree, you can reach a type local
274 /// to C where all types in-between are fundamental types. Call the
275 /// first such parameter the "local key parameter".
276 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
277 /// going through `Box`, which is fundamental.
278 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
280 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
281 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
282 /// the local type and the type parameter.
283 /// 3. Every type parameter before the local key parameter is fully known in C.
284 /// - e.g., `impl<T> T: Trait<LocalType>` is bad, because `T` might be
286 /// - but `impl<T> LocalType: Trait<T>` is OK, because `LocalType`
287 /// occurs before `T`.
288 /// 4. Every type in the local key parameter not known in C, going
289 /// through the parameter's type tree, must appear only as a subtree of
290 /// a type local to C, with only fundamental types between the type
291 /// local to C and the local key parameter.
292 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
293 /// is bad, because the only local type with `T` as a subtree is
294 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
295 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
296 /// the second occurrence of `T` is not a subtree of *any* local type.
297 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
298 /// `LocalType<Vec<T>>`, which is local and has no types between it and
299 /// the type parameter.
301 /// The orphan rules actually serve several different purposes:
303 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
304 /// every type local to one crate is unknown in the other) can't implement
305 /// the same trait-ref. This follows because it can be seen that no such
306 /// type can orphan-check in 2 such crates.
308 /// To check that a local impl follows the orphan rules, we check it in
309 /// InCrate::Local mode, using type parameters for the "generic" types.
311 /// 2. They ground negative reasoning for coherence. If a user wants to
312 /// write both a conditional blanket impl and a specific impl, we need to
313 /// make sure they do not overlap. For example, if we write
315 /// impl<T> IntoIterator for Vec<T>
316 /// impl<T: Iterator> IntoIterator for T
318 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
319 /// We can observe that this holds in the current crate, but we need to make
320 /// sure this will also hold in all unknown crates (both "independent" crates,
321 /// which we need for link-safety, and also child crates, because we don't want
322 /// child crates to get error for impl conflicts in a *dependency*).
324 /// For that, we only allow negative reasoning if, for every assignment to the
325 /// inference variables, every unknown crate would get an orphan error if they
326 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
327 /// mode. That is sound because we already know all the impls from known crates.
329 /// 3. For non-#[fundamental] traits, they guarantee that parent crates can
330 /// add "non-blanket" impls without breaking negative reasoning in dependent
331 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
333 /// For that, we only a allow crate to perform negative reasoning on
334 /// non-local-non-#[fundamental] only if there's a local key parameter as per (2).
336 /// Because we never perform negative reasoning generically (coherence does
337 /// not involve type parameters), this can be interpreted as doing the full
338 /// orphan check (using InCrate::Local mode), substituting non-local known
339 /// types for all inference variables.
341 /// This allows for crates to future-compatibly add impls as long as they
342 /// can't apply to types with a key parameter in a child crate - applying
343 /// the rules, this basically means that every type parameter in the impl
344 /// must appear behind a non-fundamental type (because this is not a
345 /// type-system requirement, crate owners might also go for "semantic
346 /// future-compatibility" involving things such as sealed traits, but
347 /// the above requirement is sufficient, and is necessary in "open world"
350 /// Note that this function is never called for types that have both type
351 /// parameters and inference variables.
352 fn orphan_check_trait_ref<'tcx>(
354 trait_ref: ty::TraitRef<'tcx>,
356 ) -> Result<(), OrphanCheckErr<'tcx>> {
357 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
359 if trait_ref.needs_infer() && trait_ref.needs_subst() {
361 "can't orphan check a trait ref with both params and inference variables {:?}",
366 // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
367 // if at least one of the following is true:
369 // - Trait is a local trait
370 // (already checked in orphan_check prior to calling this function)
372 // - At least one of the types T0..=Tn must be a local type.
373 // Let Ti be the first such type.
374 // - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
376 fn uncover_fundamental_ty<'tcx>(
381 if fundamental_ty(ty) && ty_is_non_local(ty, in_crate).is_some() {
382 ty.walk_shallow().flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate)).collect()
388 let mut non_local_spans = vec![];
390 trait_ref.input_types().flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate)).enumerate()
392 debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
393 let non_local_tys = ty_is_non_local(input_ty, in_crate);
394 if non_local_tys.is_none() {
395 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
397 } else if let ty::Param(_) = input_ty.kind {
398 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
399 let local_type = trait_ref
401 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
402 .find(|ty| ty_is_non_local_constructor(ty, in_crate).is_none());
404 debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type);
406 return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type));
408 if let Some(non_local_tys) = non_local_tys {
409 for input_ty in non_local_tys {
410 non_local_spans.push((input_ty, i == 0));
414 // If we exit above loop, never found a local type.
415 debug!("orphan_check_trait_ref: no local type");
416 Err(OrphanCheckErr::NonLocalInputType(non_local_spans))
419 fn ty_is_non_local<'t>(ty: Ty<'t>, in_crate: InCrate) -> Option<Vec<Ty<'t>>> {
420 match ty_is_non_local_constructor(ty, in_crate) {
422 if !fundamental_ty(ty) {
427 .filter_map(|t| ty_is_non_local(t, in_crate))
430 if tys.is_empty() { None } else { Some(tys) }
437 fn fundamental_ty(ty: Ty<'_>) -> bool {
440 ty::Adt(def, _) => def.is_fundamental(),
445 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
447 // The type is local to *this* crate - it will not be
448 // local in any other crate.
449 InCrate::Remote => false,
450 InCrate::Local => def_id.is_local(),
454 fn ty_is_non_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> Option<Ty<'_>> {
455 debug!("ty_is_non_local_constructor({:?})", ty);
473 | ty::Projection(..) => Some(ty),
475 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
476 InCrate::Local => Some(ty),
477 // The inference variable might be unified with a local
478 // type in that remote crate.
479 InCrate::Remote => None,
483 if def_id_is_local(def.did, in_crate) {
489 ty::Foreign(did) => {
490 if def_id_is_local(did, in_crate) {
497 // This merits some explanation.
498 // Normally, opaque types are not involed when performing
499 // coherence checking, since it is illegal to directly
500 // implement a trait on an opaque type. However, we might
501 // end up looking at an opaque type during coherence checking
502 // if an opaque type gets used within another type (e.g. as
503 // a type parameter). This requires us to decide whether or
504 // not an opaque type should be considered 'local' or not.
506 // We choose to treat all opaque types as non-local, even
507 // those that appear within the same crate. This seems
508 // somewhat surprising at first, but makes sense when
509 // you consider that opaque types are supposed to hide
510 // the underlying type *within the same crate*. When an
511 // opaque type is used from outside the module
512 // where it is declared, it should be impossible to observe
513 // anyything about it other than the traits that it implements.
515 // The alternative would be to look at the underlying type
516 // to determine whether or not the opaque type itself should
517 // be considered local. However, this could make it a breaking change
518 // to switch the underlying ('defining') type from a local type
519 // to a remote type. This would violate the rule that opaque
520 // types should be completely opaque apart from the traits
521 // that they implement, so we don't use this behavior.
525 ty::Dynamic(ref tt, ..) => {
526 if let Some(principal) = tt.principal() {
527 if def_id_is_local(principal.def_id(), in_crate) { None } else { Some(ty) }
535 ty::UnnormalizedProjection(..)
538 | ty::GeneratorWitness(..) => bug!("ty_is_local invoked on unexpected type: {:?}", ty),