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_hir::def_id::{DefId, LOCAL_CRATE};
12 use rustc_middle::ty::fold::TypeFoldable;
13 use rustc_middle::ty::subst::Subst;
14 use rustc_middle::ty::{self, fast_reject, Ty, TyCtxt};
15 use rustc_span::symbol::sym;
16 use rustc_span::DUMMY_SP;
19 /// Whether we do the orphan check relative to this crate or
20 /// to some remote crate.
21 #[derive(Copy, Clone, Debug)]
27 #[derive(Debug, Copy, Clone)]
33 pub struct OverlapResult<'tcx> {
34 pub impl_header: ty::ImplHeader<'tcx>,
35 pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
37 /// `true` if the overlap might've been permitted before the shift
39 pub involves_placeholder: bool,
42 pub fn add_placeholder_note(err: &mut rustc_errors::DiagnosticBuilder<'_>) {
44 "this behavior recently changed as a result of a bug fix; \
45 see rust-lang/rust#56105 for details",
49 /// If there are types that satisfy both impls, invokes `on_overlap`
50 /// with a suitably-freshened `ImplHeader` with those types
51 /// substituted. Otherwise, invokes `no_overlap`.
52 pub fn overlapping_impls<F1, F2, R>(
56 skip_leak_check: SkipLeakCheck,
61 F1: FnOnce(OverlapResult<'_>) -> R,
68 impl1_def_id, impl2_def_id,
70 // Before doing expensive operations like entering an inference context, do
71 // a quick check via fast_reject to tell if the impl headers could possibly
73 let impl1_ref = tcx.impl_trait_ref(impl1_def_id);
74 let impl2_ref = tcx.impl_trait_ref(impl2_def_id);
76 // Check if any of the input types definitely do not unify.
78 impl1_ref.iter().flat_map(|tref| tref.substs.types()),
79 impl2_ref.iter().flat_map(|tref| tref.substs.types()),
82 let t1 = fast_reject::simplify_type(tcx, ty1, false);
83 let t2 = fast_reject::simplify_type(tcx, ty2, false);
84 if let (Some(t1), Some(t2)) = (t1, t2) {
85 // Simplified successfully
86 // Types cannot unify if they differ in their reference mutability or simplify to different types
87 t1 != t2 || ty1.ref_mutability() != ty2.ref_mutability()
93 // Some types involved are definitely different, so the impls couldn't possibly overlap.
94 debug!("overlapping_impls: fast_reject early-exit");
98 let overlaps = tcx.infer_ctxt().enter(|infcx| {
99 let selcx = &mut SelectionContext::intercrate(&infcx);
100 overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).is_some()
107 // In the case where we detect an error, run the check again, but
108 // this time tracking intercrate ambuiguity causes for better
109 // diagnostics. (These take time and can lead to false errors.)
110 tcx.infer_ctxt().enter(|infcx| {
111 let selcx = &mut SelectionContext::intercrate(&infcx);
112 selcx.enable_tracking_intercrate_ambiguity_causes();
113 on_overlap(overlap(selcx, skip_leak_check, impl1_def_id, impl2_def_id).unwrap())
117 fn with_fresh_ty_vars<'cx, 'tcx>(
118 selcx: &mut SelectionContext<'cx, 'tcx>,
119 param_env: ty::ParamEnv<'tcx>,
121 ) -> ty::ImplHeader<'tcx> {
122 let tcx = selcx.tcx();
123 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
125 let header = ty::ImplHeader {
127 self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
128 trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
129 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
132 let Normalized { value: mut header, obligations } =
133 traits::normalize(selcx, param_env, ObligationCause::dummy(), header);
135 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
139 /// Can both impl `a` and impl `b` be satisfied by a common type (including
140 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
141 fn overlap<'cx, 'tcx>(
142 selcx: &mut SelectionContext<'cx, 'tcx>,
143 skip_leak_check: SkipLeakCheck,
146 ) -> Option<OverlapResult<'tcx>> {
147 debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
149 selcx.infcx().probe_maybe_skip_leak_check(skip_leak_check.is_yes(), |snapshot| {
150 overlap_within_probe(selcx, skip_leak_check, a_def_id, b_def_id, snapshot)
154 fn overlap_within_probe(
155 selcx: &mut SelectionContext<'cx, 'tcx>,
156 skip_leak_check: SkipLeakCheck,
159 snapshot: &CombinedSnapshot<'_, 'tcx>,
160 ) -> Option<OverlapResult<'tcx>> {
161 // For the purposes of this check, we don't bring any placeholder
162 // types into scope; instead, we replace the generic types with
163 // fresh type variables, and hence we do our evaluations in an
164 // empty environment.
165 let param_env = ty::ParamEnv::empty();
167 let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
168 let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
170 debug!("overlap: a_impl_header={:?}", a_impl_header);
171 debug!("overlap: b_impl_header={:?}", b_impl_header);
173 // Do `a` and `b` unify? If not, no overlap.
174 let obligations = match selcx
176 .at(&ObligationCause::dummy(), param_env)
177 .eq_impl_headers(&a_impl_header, &b_impl_header)
179 Ok(InferOk { obligations, value: () }) => obligations,
185 debug!("overlap: unification check succeeded");
187 // Are any of the obligations unsatisfiable? If so, no overlap.
188 let infcx = selcx.infcx();
189 let opt_failing_obligation = a_impl_header
193 .chain(b_impl_header.predicates)
194 .map(|p| infcx.resolve_vars_if_possible(p))
195 .map(|p| Obligation {
196 cause: ObligationCause::dummy(),
202 .find(|o| !selcx.predicate_may_hold_fatal(o));
203 // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
204 // to the canonical trait query form, `infcx.predicate_may_hold`, once
205 // the new system supports intercrate mode (which coherence needs).
207 if let Some(failing_obligation) = opt_failing_obligation {
208 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
212 if !skip_leak_check.is_yes() {
213 if infcx.leak_check(true, snapshot).is_err() {
214 debug!("overlap: leak check failed");
219 let impl_header = selcx.infcx().resolve_vars_if_possible(a_impl_header);
220 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
221 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
223 let involves_placeholder =
224 matches!(selcx.infcx().region_constraints_added_in_snapshot(snapshot), Some(true));
226 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
229 pub fn trait_ref_is_knowable<'tcx>(
231 trait_ref: ty::TraitRef<'tcx>,
232 ) -> Option<Conflict> {
233 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
234 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
235 // A downstream or cousin crate is allowed to implement some
236 // substitution of this trait-ref.
237 return Some(Conflict::Downstream);
240 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
241 // This is a local or fundamental trait, so future-compatibility
242 // is no concern. We know that downstream/cousin crates are not
243 // allowed to implement a substitution of this trait ref, which
244 // means impls could only come from dependencies of this crate,
245 // which we already know about.
249 // This is a remote non-fundamental trait, so if another crate
250 // can be the "final owner" of a substitution of this trait-ref,
251 // they are allowed to implement it future-compatibly.
253 // However, if we are a final owner, then nobody else can be,
254 // and if we are an intermediate owner, then we don't care
255 // about future-compatibility, which means that we're OK if
257 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
258 debug!("trait_ref_is_knowable: orphan check passed");
261 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
262 Some(Conflict::Upstream)
266 pub fn trait_ref_is_local_or_fundamental<'tcx>(
268 trait_ref: ty::TraitRef<'tcx>,
270 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, sym::fundamental)
273 pub enum OrphanCheckErr<'tcx> {
274 NonLocalInputType(Vec<(Ty<'tcx>, bool /* Is this the first input type? */)>),
275 UncoveredTy(Ty<'tcx>, Option<Ty<'tcx>>),
278 /// Checks the coherence orphan rules. `impl_def_id` should be the
279 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
280 /// two conditions must be satisfied:
282 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
283 /// 2. Some local type must appear in `Self`.
284 pub fn orphan_check(tcx: TyCtxt<'_>, impl_def_id: DefId) -> Result<(), OrphanCheckErr<'_>> {
285 debug!("orphan_check({:?})", impl_def_id);
287 // We only except this routine to be invoked on implementations
288 // of a trait, not inherent implementations.
289 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
290 debug!("orphan_check: trait_ref={:?}", trait_ref);
292 // If the *trait* is local to the crate, ok.
293 if trait_ref.def_id.is_local() {
294 debug!("trait {:?} is local to current crate", trait_ref.def_id);
298 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
301 /// Checks whether a trait-ref is potentially implementable by a crate.
303 /// The current rule is that a trait-ref orphan checks in a crate C:
305 /// 1. Order the parameters in the trait-ref in subst order - Self first,
306 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
307 /// 2. Of these type parameters, there is at least one type parameter
308 /// in which, walking the type as a tree, you can reach a type local
309 /// to C where all types in-between are fundamental types. Call the
310 /// first such parameter the "local key parameter".
311 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
312 /// going through `Box`, which is fundamental.
313 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
315 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
316 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
317 /// the local type and the type parameter.
318 /// 3. Before this local type, no generic type parameter of the impl must
319 /// be reachable through fundamental types.
320 /// - e.g. `impl<T> Trait<LocalType> for Vec<T>` is fine, as `Vec` is not fundamental.
321 /// - while `impl<T> Trait<LocalType for Box<T>` results in an error, as `T` is
322 /// reachable through the fundamental type `Box`.
323 /// 4. Every type in the local key parameter not known in C, going
324 /// through the parameter's type tree, must appear only as a subtree of
325 /// a type local to C, with only fundamental types between the type
326 /// local to C and the local key parameter.
327 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
328 /// is bad, because the only local type with `T` as a subtree is
329 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
330 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
331 /// the second occurrence of `T` is not a subtree of *any* local type.
332 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
333 /// `LocalType<Vec<T>>`, which is local and has no types between it and
334 /// the type parameter.
336 /// The orphan rules actually serve several different purposes:
338 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
339 /// every type local to one crate is unknown in the other) can't implement
340 /// the same trait-ref. This follows because it can be seen that no such
341 /// type can orphan-check in 2 such crates.
343 /// To check that a local impl follows the orphan rules, we check it in
344 /// InCrate::Local mode, using type parameters for the "generic" types.
346 /// 2. They ground negative reasoning for coherence. If a user wants to
347 /// write both a conditional blanket impl and a specific impl, we need to
348 /// make sure they do not overlap. For example, if we write
350 /// impl<T> IntoIterator for Vec<T>
351 /// impl<T: Iterator> IntoIterator for T
353 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
354 /// We can observe that this holds in the current crate, but we need to make
355 /// sure this will also hold in all unknown crates (both "independent" crates,
356 /// which we need for link-safety, and also child crates, because we don't want
357 /// child crates to get error for impl conflicts in a *dependency*).
359 /// For that, we only allow negative reasoning if, for every assignment to the
360 /// inference variables, every unknown crate would get an orphan error if they
361 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
362 /// mode. That is sound because we already know all the impls from known crates.
364 /// 3. For non-`#[fundamental]` traits, they guarantee that parent crates can
365 /// add "non-blanket" impls without breaking negative reasoning in dependent
366 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
368 /// For that, we only a allow crate to perform negative reasoning on
369 /// non-local-non-`#[fundamental]` only if there's a local key parameter as per (2).
371 /// Because we never perform negative reasoning generically (coherence does
372 /// not involve type parameters), this can be interpreted as doing the full
373 /// orphan check (using InCrate::Local mode), substituting non-local known
374 /// types for all inference variables.
376 /// This allows for crates to future-compatibly add impls as long as they
377 /// can't apply to types with a key parameter in a child crate - applying
378 /// the rules, this basically means that every type parameter in the impl
379 /// must appear behind a non-fundamental type (because this is not a
380 /// type-system requirement, crate owners might also go for "semantic
381 /// future-compatibility" involving things such as sealed traits, but
382 /// the above requirement is sufficient, and is necessary in "open world"
385 /// Note that this function is never called for types that have both type
386 /// parameters and inference variables.
387 fn orphan_check_trait_ref<'tcx>(
389 trait_ref: ty::TraitRef<'tcx>,
391 ) -> Result<(), OrphanCheckErr<'tcx>> {
392 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
394 if trait_ref.needs_infer() && trait_ref.definitely_needs_subst(tcx) {
396 "can't orphan check a trait ref with both params and inference variables {:?}",
401 // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
402 // if at least one of the following is true:
404 // - Trait is a local trait
405 // (already checked in orphan_check prior to calling this function)
407 // - At least one of the types T0..=Tn must be a local type.
408 // Let Ti be the first such type.
409 // - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
411 fn uncover_fundamental_ty<'tcx>(
416 // FIXME: this is currently somewhat overly complicated,
417 // but fixing this requires a more complicated refactor.
418 if !contained_non_local_types(tcx, ty, in_crate).is_empty() {
419 if let Some(inner_tys) = fundamental_ty_inner_tys(tcx, ty) {
421 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
429 let mut non_local_spans = vec![];
430 for (i, input_ty) in trait_ref
433 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
436 debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
437 let non_local_tys = contained_non_local_types(tcx, input_ty, in_crate);
438 if non_local_tys.is_empty() {
439 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
441 } else if let ty::Param(_) = input_ty.kind() {
442 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
443 let local_type = trait_ref
446 .flat_map(|ty| uncover_fundamental_ty(tcx, ty, in_crate))
447 .find(|ty| ty_is_local_constructor(ty, in_crate));
449 debug!("orphan_check_trait_ref: uncovered ty local_type: `{:?}`", local_type);
451 return Err(OrphanCheckErr::UncoveredTy(input_ty, local_type));
454 for input_ty in non_local_tys {
455 non_local_spans.push((input_ty, i == 0));
458 // If we exit above loop, never found a local type.
459 debug!("orphan_check_trait_ref: no local type");
460 Err(OrphanCheckErr::NonLocalInputType(non_local_spans))
463 /// Returns a list of relevant non-local types for `ty`.
465 /// This is just `ty` itself unless `ty` is `#[fundamental]`,
466 /// in which case we recursively look into this type.
468 /// If `ty` is local itself, this method returns an empty `Vec`.
472 /// - `u32` is not local, so this returns `[u32]`.
473 /// - for `Foo<u32>`, where `Foo` is a local type, this returns `[]`.
474 /// - `&mut u32` returns `[u32]`, as `&mut` is a fundamental type, similar to `Box`.
475 /// - `Box<Foo<u32>>` returns `[]`, as `Box` is a fundamental type and `Foo` is local.
476 fn contained_non_local_types(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>, in_crate: InCrate) -> Vec<Ty<'tcx>> {
477 if ty_is_local_constructor(ty, in_crate) {
480 match fundamental_ty_inner_tys(tcx, ty) {
482 inner_tys.flat_map(|ty| contained_non_local_types(tcx, ty, in_crate)).collect()
489 /// For `#[fundamental]` ADTs and `&T` / `&mut T`, returns `Some` with the
490 /// type parameters of the ADT, or `T`, respectively. For non-fundamental
491 /// types, returns `None`.
492 fn fundamental_ty_inner_tys(
495 ) -> Option<impl Iterator<Item = Ty<'tcx>>> {
496 let (first_ty, rest_tys) = match *ty.kind() {
497 ty::Ref(_, ty, _) => (ty, ty::subst::InternalSubsts::empty().types()),
498 ty::Adt(def, substs) if def.is_fundamental() => {
499 let mut types = substs.types();
501 // FIXME(eddyb) actually validate `#[fundamental]` up-front.
505 tcx.def_span(def.did),
506 "`#[fundamental]` requires at least one type parameter",
512 Some(first_ty) => (first_ty, types),
518 Some(iter::once(first_ty).chain(rest_tys))
521 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
523 // The type is local to *this* crate - it will not be
524 // local in any other crate.
525 InCrate::Remote => false,
526 InCrate::Local => def_id.is_local(),
530 fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
531 debug!("ty_is_local_constructor({:?})", ty);
549 | ty::Projection(..) => false,
551 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
552 InCrate::Local => false,
553 // The inference variable might be unified with a local
554 // type in that remote crate.
555 InCrate::Remote => true,
558 ty::Adt(def, _) => def_id_is_local(def.did, in_crate),
559 ty::Foreign(did) => def_id_is_local(did, in_crate),
561 // This merits some explanation.
562 // Normally, opaque types are not involed when performing
563 // coherence checking, since it is illegal to directly
564 // implement a trait on an opaque type. However, we might
565 // end up looking at an opaque type during coherence checking
566 // if an opaque type gets used within another type (e.g. as
567 // a type parameter). This requires us to decide whether or
568 // not an opaque type should be considered 'local' or not.
570 // We choose to treat all opaque types as non-local, even
571 // those that appear within the same crate. This seems
572 // somewhat surprising at first, but makes sense when
573 // you consider that opaque types are supposed to hide
574 // the underlying type *within the same crate*. When an
575 // opaque type is used from outside the module
576 // where it is declared, it should be impossible to observe
577 // anything about it other than the traits that it implements.
579 // The alternative would be to look at the underlying type
580 // to determine whether or not the opaque type itself should
581 // be considered local. However, this could make it a breaking change
582 // to switch the underlying ('defining') type from a local type
583 // to a remote type. This would violate the rule that opaque
584 // types should be completely opaque apart from the traits
585 // that they implement, so we don't use this behavior.
590 // Similar to the `Opaque` case (#83613).
594 ty::Dynamic(ref tt, ..) => {
595 if let Some(principal) = tt.principal() {
596 def_id_is_local(principal.def_id(), in_crate)
602 ty::Error(_) => true,
604 ty::Generator(..) | ty::GeneratorWitness(..) => {
605 bug!("ty_is_local invoked on unexpected type: {:?}", ty)