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::infer::CombinedSnapshot;
8 use crate::hir::def_id::{DefId, LOCAL_CRATE};
9 use syntax_pos::DUMMY_SP;
10 use crate::traits::{self, Normalized, SelectionContext, Obligation, ObligationCause};
11 use crate::traits::IntercrateMode;
12 use crate::traits::select::IntercrateAmbiguityCause;
13 use crate::ty::{self, Ty, TyCtxt};
14 use crate::ty::fold::TypeFoldable;
15 use crate::ty::subst::Subst;
17 use crate::infer::{InferOk};
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)]
30 Downstream { used_to_be_broken: bool }
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 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<'gcx, F1, F2, R>(
53 tcx: TyCtxt<'_, 'gcx, 'gcx>,
56 intercrate_mode: IntercrateMode,
61 F1: FnOnce(OverlapResult<'_>) -> R,
64 debug!("overlapping_impls(\
67 intercrate_mode={:?})",
72 let overlaps = tcx.infer_ctxt().enter(|infcx| {
73 let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
74 overlap(selcx, impl1_def_id, impl2_def_id).is_some()
81 // In the case where we detect an error, run the check again, but
82 // this time tracking intercrate ambuiguity causes for better
83 // diagnostics. (These take time and can lead to false errors.)
84 tcx.infer_ctxt().enter(|infcx| {
85 let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
86 selcx.enable_tracking_intercrate_ambiguity_causes();
87 on_overlap(overlap(selcx, impl1_def_id, impl2_def_id).unwrap())
91 fn with_fresh_ty_vars<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
92 param_env: ty::ParamEnv<'tcx>,
94 -> ty::ImplHeader<'tcx>
96 let tcx = selcx.tcx();
97 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
99 let header = ty::ImplHeader {
101 self_ty: tcx.type_of(impl_def_id).subst(tcx, impl_substs),
102 trait_ref: tcx.impl_trait_ref(impl_def_id).subst(tcx, impl_substs),
103 predicates: tcx.predicates_of(impl_def_id).instantiate(tcx, impl_substs).predicates,
106 let Normalized { value: mut header, obligations } =
107 traits::normalize(selcx, param_env, ObligationCause::dummy(), &header);
109 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
113 /// Can both impl `a` and impl `b` be satisfied by a common type (including
114 /// where-clauses)? If so, returns an `ImplHeader` that unifies the two impls.
115 fn overlap<'cx, 'gcx, 'tcx>(
116 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
119 ) -> Option<OverlapResult<'tcx>> {
120 debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
122 selcx.infcx().probe(|snapshot| overlap_within_probe(selcx, a_def_id, b_def_id, snapshot))
125 fn overlap_within_probe(
126 selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
129 snapshot: &CombinedSnapshot<'_, 'tcx>,
130 ) -> Option<OverlapResult<'tcx>> {
131 // For the purposes of this check, we don't bring any placeholder
132 // types into scope; instead, we replace the generic types with
133 // fresh type variables, and hence we do our evaluations in an
134 // empty environment.
135 let param_env = ty::ParamEnv::empty();
137 let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
138 let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
140 debug!("overlap: a_impl_header={:?}", a_impl_header);
141 debug!("overlap: b_impl_header={:?}", b_impl_header);
143 // Do `a` and `b` unify? If not, no overlap.
144 let obligations = match selcx.infcx().at(&ObligationCause::dummy(), param_env)
145 .eq_impl_headers(&a_impl_header, &b_impl_header)
147 Ok(InferOk { obligations, value: () }) => obligations,
148 Err(_) => return None
151 debug!("overlap: unification check succeeded");
153 // Are any of the obligations unsatisfiable? If so, no overlap.
154 let infcx = selcx.infcx();
155 let opt_failing_obligation =
156 a_impl_header.predicates
158 .chain(&b_impl_header.predicates)
159 .map(|p| infcx.resolve_type_vars_if_possible(p))
160 .map(|p| Obligation { cause: ObligationCause::dummy(),
165 .find(|o| !selcx.predicate_may_hold_fatal(o));
166 // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
167 // to the canonical trait query form, `infcx.predicate_may_hold`, once
168 // the new system supports intercrate mode (which coherence needs).
170 if let Some(failing_obligation) = opt_failing_obligation {
171 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
175 let impl_header = selcx.infcx().resolve_type_vars_if_possible(&a_impl_header);
176 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
177 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
179 let involves_placeholder = match selcx.infcx().region_constraints_added_in_snapshot(snapshot) {
184 Some(OverlapResult { impl_header, intercrate_ambiguity_causes, involves_placeholder })
187 pub fn trait_ref_is_knowable<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
188 trait_ref: ty::TraitRef<'tcx>)
191 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
192 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
193 // A downstream or cousin crate is allowed to implement some
194 // substitution of this trait-ref.
196 // A trait can be implementable for a trait ref by both the current
197 // crate and crates downstream of it. Older versions of rustc
198 // were not aware of this, causing incoherence (issue #43355).
199 let used_to_be_broken =
200 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok();
201 if used_to_be_broken {
202 debug!("trait_ref_is_knowable({:?}) - USED TO BE BROKEN", trait_ref);
204 return Some(Conflict::Downstream { used_to_be_broken });
207 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
208 // This is a local or fundamental trait, so future-compatibility
209 // is no concern. We know that downstream/cousin crates are not
210 // allowed to implement a substitution of this trait ref, which
211 // means impls could only come from dependencies of this crate,
212 // which we already know about.
216 // This is a remote non-fundamental trait, so if another crate
217 // can be the "final owner" of a substitution of this trait-ref,
218 // they are allowed to implement it future-compatibly.
220 // However, if we are a final owner, then nobody else can be,
221 // and if we are an intermediate owner, then we don't care
222 // about future-compatibility, which means that we're OK if
224 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
225 debug!("trait_ref_is_knowable: orphan check passed");
228 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
229 return Some(Conflict::Upstream);
233 pub fn trait_ref_is_local_or_fundamental<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
234 trait_ref: ty::TraitRef<'tcx>)
236 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, "fundamental")
239 pub enum OrphanCheckErr<'tcx> {
241 UncoveredTy(Ty<'tcx>),
244 /// Checks the coherence orphan rules. `impl_def_id` should be the
245 /// `DefId` of a trait impl. To pass, either the trait must be local, or else
246 /// two conditions must be satisfied:
248 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
249 /// 2. Some local type must appear in `Self`.
250 pub fn orphan_check<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
252 -> Result<(), OrphanCheckErr<'tcx>>
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",
268 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
271 /// Checks whether a trait-ref is potentially implementable by a crate.
273 /// The current rule is that a trait-ref orphan checks in a crate C:
275 /// 1. Order the parameters in the trait-ref in subst order - Self first,
276 /// others linearly (e.g., `<U as Foo<V, W>>` is U < V < W).
277 /// 2. Of these type parameters, there is at least one type parameter
278 /// in which, walking the type as a tree, you can reach a type local
279 /// to C where all types in-between are fundamental types. Call the
280 /// first such parameter the "local key parameter".
281 /// - e.g., `Box<LocalType>` is OK, because you can visit LocalType
282 /// going through `Box`, which is fundamental.
283 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
285 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
286 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
287 /// the local type and the type parameter.
288 /// 3. Every type parameter before the local key parameter is fully known in C.
289 /// - e.g., `impl<T> T: Trait<LocalType>` is bad, because `T` might be
291 /// - but `impl<T> LocalType: Trait<T>` is OK, because `LocalType`
292 /// occurs before `T`.
293 /// 4. Every type in the local key parameter not known in C, going
294 /// through the parameter's type tree, must appear only as a subtree of
295 /// a type local to C, with only fundamental types between the type
296 /// local to C and the local key parameter.
297 /// - e.g., `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
298 /// is bad, because the only local type with `T` as a subtree is
299 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
300 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
301 /// the second occurrence of `T` is not a subtree of *any* local type.
302 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
303 /// `LocalType<Vec<T>>`, which is local and has no types between it and
304 /// the type parameter.
306 /// The orphan rules actually serve several different purposes:
308 /// 1. They enable link-safety - i.e., 2 mutually-unknowing crates (where
309 /// every type local to one crate is unknown in the other) can't implement
310 /// the same trait-ref. This follows because it can be seen that no such
311 /// type can orphan-check in 2 such crates.
313 /// To check that a local impl follows the orphan rules, we check it in
314 /// InCrate::Local mode, using type parameters for the "generic" types.
316 /// 2. They ground negative reasoning for coherence. If a user wants to
317 /// write both a conditional blanket impl and a specific impl, we need to
318 /// make sure they do not overlap. For example, if we write
320 /// impl<T> IntoIterator for Vec<T>
321 /// impl<T: Iterator> IntoIterator for T
323 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
324 /// We can observe that this holds in the current crate, but we need to make
325 /// sure this will also hold in all unknown crates (both "independent" crates,
326 /// which we need for link-safety, and also child crates, because we don't want
327 /// child crates to get error for impl conflicts in a *dependency*).
329 /// For that, we only allow negative reasoning if, for every assignment to the
330 /// inference variables, every unknown crate would get an orphan error if they
331 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
332 /// mode. That is sound because we already know all the impls from known crates.
334 /// 3. For non-#[fundamental] traits, they guarantee that parent crates can
335 /// add "non-blanket" impls without breaking negative reasoning in dependent
336 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
338 /// For that, we only a allow crate to perform negative reasoning on
339 /// non-local-non-#[fundamental] only if there's a local key parameter as per (2).
341 /// Because we never perform negative reasoning generically (coherence does
342 /// not involve type parameters), this can be interpreted as doing the full
343 /// orphan check (using InCrate::Local mode), substituting non-local known
344 /// types for all inference variables.
346 /// This allows for crates to future-compatibly add impls as long as they
347 /// can't apply to types with a key parameter in a child crate - applying
348 /// the rules, this basically means that every type parameter in the impl
349 /// must appear behind a non-fundamental type (because this is not a
350 /// type-system requirement, crate owners might also go for "semantic
351 /// future-compatibility" involving things such as sealed traits, but
352 /// the above requirement is sufficient, and is necessary in "open world"
355 /// Note that this function is never called for types that have both type
356 /// parameters and inference variables.
357 fn orphan_check_trait_ref<'tcx>(tcx: TyCtxt<'_, '_, '_>,
358 trait_ref: ty::TraitRef<'tcx>,
360 -> Result<(), OrphanCheckErr<'tcx>>
362 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})",
363 trait_ref, in_crate);
365 if trait_ref.needs_infer() && trait_ref.needs_subst() {
366 bug!("can't orphan check a trait ref with both params and inference variables {:?}",
370 if tcx.features().re_rebalance_coherence {
371 // Given impl<P1..=Pn> Trait<T1..=Tn> for T0, an impl is valid only
372 // if at least one of the following is true:
374 // - Trait is a local trait
375 // (already checked in orphan_check prior to calling this function)
377 // - At least one of the types T0..=Tn must be a local type.
378 // Let Ti be the first such type.
379 // - No uncovered type parameters P1..=Pn may appear in T0..Ti (excluding Ti)
381 for input_ty in trait_ref.input_types() {
382 debug!("orphan_check_trait_ref: check ty `{:?}`", input_ty);
383 if ty_is_local(tcx, input_ty, in_crate) {
384 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
386 } else if let ty::Param(_) = input_ty.sty {
387 debug!("orphan_check_trait_ref: uncovered ty: `{:?}`", input_ty);
388 return Err(OrphanCheckErr::UncoveredTy(input_ty))
391 // If we exit above loop, never found a local type.
392 debug!("orphan_check_trait_ref: no local type");
393 Err(OrphanCheckErr::NoLocalInputType)
395 // First, create an ordered iterator over all the type
396 // parameters to the trait, with the self type appearing
397 // first. Find the first input type that either references a
398 // type parameter OR some local type.
399 for input_ty in trait_ref.input_types() {
400 if ty_is_local(tcx, input_ty, in_crate) {
401 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
403 // First local input type. Check that there are no
404 // uncovered type parameters.
405 let uncovered_tys = uncovered_tys(tcx, input_ty, in_crate);
406 for uncovered_ty in uncovered_tys {
407 if let Some(param) = uncovered_ty.walk()
408 .find(|t| is_possibly_remote_type(t, in_crate))
410 debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
411 return Err(OrphanCheckErr::UncoveredTy(param));
415 // OK, found local type, all prior types upheld invariant.
419 // Otherwise, enforce invariant that there are no type
420 // parameters reachable.
421 if let Some(param) = input_ty.walk()
422 .find(|t| is_possibly_remote_type(t, in_crate))
424 debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
425 return Err(OrphanCheckErr::UncoveredTy(param));
428 // If we exit above loop, never found a local type.
429 debug!("orphan_check_trait_ref: no local type");
430 Err(OrphanCheckErr::NoLocalInputType)
434 fn uncovered_tys<'tcx>(tcx: TyCtxt<'_, '_, '_>, ty: Ty<'tcx>, in_crate: InCrate)
436 if ty_is_local_constructor(ty, in_crate) {
438 } else if fundamental_ty(ty) {
440 .flat_map(|t| uncovered_tys(tcx, t, in_crate))
447 fn is_possibly_remote_type(ty: Ty<'_>, _in_crate: InCrate) -> bool {
449 ty::Projection(..) | ty::Param(..) => true,
454 fn ty_is_local(tcx: TyCtxt<'_, '_, '_>, ty: Ty<'_>, in_crate: InCrate) -> bool {
455 ty_is_local_constructor(ty, in_crate) ||
456 fundamental_ty(ty) && ty.walk_shallow().any(|t| ty_is_local(tcx, t, in_crate))
459 fn fundamental_ty(ty: Ty<'_>) -> bool {
462 ty::Adt(def, _) => def.is_fundamental(),
467 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
469 // The type is local to *this* crate - it will not be
470 // local in any other crate.
471 InCrate::Remote => false,
472 InCrate::Local => def_id.is_local()
476 fn ty_is_local_constructor(ty: Ty<'_>, in_crate: InCrate) -> bool {
477 debug!("ty_is_local_constructor({:?})", ty);
495 ty::Projection(..) => {
499 ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => match in_crate {
500 InCrate::Local => false,
501 // The inference variable might be unified with a local
502 // type in that remote crate.
503 InCrate::Remote => true,
506 ty::Adt(def, _) => def_id_is_local(def.did, in_crate),
507 ty::Foreign(did) => def_id_is_local(did, in_crate),
509 ty::Dynamic(ref tt, ..) => {
510 if let Some(principal) = tt.principal() {
511 def_id_is_local(principal.def_id(), in_crate)
519 ty::UnnormalizedProjection(..) |
522 ty::GeneratorWitness(..) |
524 bug!("ty_is_local invoked on unexpected type: {:?}", ty)