1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! See rustc guide chapters on [trait-resolution] and [trait-specialization] for more info on how
14 //! [trait-resolution]: https://rust-lang-nursery.github.io/rustc-guide/traits/resolution.html
15 //! [trait-specialization]: https://rust-lang-nursery.github.io/rustc-guide/traits/specialization.html
17 use hir::def_id::{DefId, LOCAL_CRATE};
18 use syntax_pos::DUMMY_SP;
19 use traits::{self, Normalized, SelectionContext, Obligation, ObligationCause};
20 use traits::IntercrateMode;
21 use traits::select::IntercrateAmbiguityCause;
22 use ty::{self, Ty, TyCtxt};
23 use ty::fold::TypeFoldable;
28 /// Whether we do the orphan check relative to this crate or
29 /// to some remote crate.
30 #[derive(Copy, Clone, Debug)]
36 #[derive(Debug, Copy, Clone)]
39 Downstream { used_to_be_broken: bool }
42 pub struct OverlapResult<'tcx> {
43 pub impl_header: ty::ImplHeader<'tcx>,
44 pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
47 /// If there are types that satisfy both impls, invokes `on_overlap`
48 /// with a suitably-freshened `ImplHeader` with those types
49 /// substituted. Otherwise, invokes `no_overlap`.
50 pub fn overlapping_impls<'gcx, F1, F2, R>(
51 tcx: TyCtxt<'_, 'gcx, 'gcx>,
54 intercrate_mode: IntercrateMode,
59 F1: FnOnce(OverlapResult<'_>) -> R,
62 debug!("overlapping_impls(\
65 intercrate_mode={:?})",
70 let overlaps = tcx.infer_ctxt().enter(|infcx| {
71 let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
72 overlap(selcx, 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, intercrate_mode);
84 selcx.enable_tracking_intercrate_ambiguity_causes();
85 on_overlap(overlap(selcx, impl1_def_id, impl2_def_id).unwrap())
89 fn with_fresh_ty_vars<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
90 param_env: ty::ParamEnv<'tcx>,
92 -> 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),
100 trait_ref: tcx.impl_trait_ref(impl_def_id),
101 predicates: tcx.predicates_of(impl_def_id).predicates
102 }.subst(tcx, impl_substs);
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, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
116 -> Option<OverlapResult<'tcx>>
118 debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
120 // For the purposes of this check, we don't bring any skolemized
121 // types into scope; instead, we replace the generic types with
122 // fresh type variables, and hence we do our evaluations in an
123 // empty environment.
124 let param_env = ty::ParamEnv::empty();
126 let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
127 let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
129 debug!("overlap: a_impl_header={:?}", a_impl_header);
130 debug!("overlap: b_impl_header={:?}", b_impl_header);
132 // Do `a` and `b` unify? If not, no overlap.
133 let obligations = match selcx.infcx().at(&ObligationCause::dummy(), param_env)
134 .eq_impl_headers(&a_impl_header, &b_impl_header)
136 Ok(InferOk { obligations, value: () }) => obligations,
137 Err(_) => return None
140 debug!("overlap: unification check succeeded");
142 // Are any of the obligations unsatisfiable? If so, no overlap.
143 let infcx = selcx.infcx();
144 let opt_failing_obligation =
145 a_impl_header.predicates
147 .chain(&b_impl_header.predicates)
148 .map(|p| infcx.resolve_type_vars_if_possible(p))
149 .map(|p| Obligation { cause: ObligationCause::dummy(),
154 .find(|o| !selcx.predicate_may_hold_fatal(o));
155 // FIXME: the call to `selcx.predicate_may_hold_fatal` above should be ported
156 // to the canonical trait query form, `infcx.predicate_may_hold`, once
157 // the new system supports intercrate mode (which coherence needs).
159 if let Some(failing_obligation) = opt_failing_obligation {
160 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
164 let impl_header = selcx.infcx().resolve_type_vars_if_possible(&a_impl_header);
165 let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
166 debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
167 Some(OverlapResult { impl_header, intercrate_ambiguity_causes })
170 pub fn trait_ref_is_knowable<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
171 trait_ref: ty::TraitRef<'tcx>)
174 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
175 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
176 // A downstream or cousin crate is allowed to implement some
177 // substitution of this trait-ref.
179 // A trait can be implementable for a trait ref by both the current
180 // crate and crates downstream of it. Older versions of rustc
181 // were not aware of this, causing incoherence (issue #43355).
182 let used_to_be_broken =
183 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok();
184 if used_to_be_broken {
185 debug!("trait_ref_is_knowable({:?}) - USED TO BE BROKEN", trait_ref);
187 return Some(Conflict::Downstream { used_to_be_broken });
190 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
191 // This is a local or fundamental trait, so future-compatibility
192 // is no concern. We know that downstream/cousin crates are not
193 // allowed to implement a substitution of this trait ref, which
194 // means impls could only come from dependencies of this crate,
195 // which we already know about.
199 // This is a remote non-fundamental trait, so if another crate
200 // can be the "final owner" of a substitution of this trait-ref,
201 // they are allowed to implement it future-compatibly.
203 // However, if we are a final owner, then nobody else can be,
204 // and if we are an intermediate owner, then we don't care
205 // about future-compatibility, which means that we're OK if
207 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
208 debug!("trait_ref_is_knowable: orphan check passed");
211 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
212 return Some(Conflict::Upstream);
216 pub fn trait_ref_is_local_or_fundamental<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
217 trait_ref: ty::TraitRef<'tcx>)
219 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, "fundamental")
222 pub enum OrphanCheckErr<'tcx> {
224 UncoveredTy(Ty<'tcx>),
227 /// Checks the coherence orphan rules. `impl_def_id` should be the
228 /// def-id of a trait impl. To pass, either the trait must be local, or else
229 /// two conditions must be satisfied:
231 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
232 /// 2. Some local type must appear in `Self`.
233 pub fn orphan_check<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
235 -> Result<(), OrphanCheckErr<'tcx>>
237 debug!("orphan_check({:?})", impl_def_id);
239 // We only except this routine to be invoked on implementations
240 // of a trait, not inherent implementations.
241 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
242 debug!("orphan_check: trait_ref={:?}", trait_ref);
244 // If the *trait* is local to the crate, ok.
245 if trait_ref.def_id.is_local() {
246 debug!("trait {:?} is local to current crate",
251 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
254 /// Check whether a trait-ref is potentially implementable by a crate.
256 /// The current rule is that a trait-ref orphan checks in a crate C:
258 /// 1. Order the parameters in the trait-ref in subst order - Self first,
259 /// others linearly (e.g. `<U as Foo<V, W>>` is U < V < W).
260 /// 2. Of these type parameters, there is at least one type parameter
261 /// in which, walking the type as a tree, you can reach a type local
262 /// to C where all types in-between are fundamental types. Call the
263 /// first such parameter the "local key parameter".
264 /// - e.g. `Box<LocalType>` is OK, because you can visit LocalType
265 /// going through `Box`, which is fundamental.
266 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
268 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
269 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
270 /// the local type and the type parameter.
271 /// 3. Every type parameter before the local key parameter is fully known in C.
272 /// - e.g. `impl<T> T: Trait<LocalType>` is bad, because `T` might be
274 /// - but `impl<T> LocalType: Trait<T>` is OK, because `LocalType`
275 /// occurs before `T`.
276 /// 4. Every type in the local key parameter not known in C, going
277 /// through the parameter's type tree, must appear only as a subtree of
278 /// a type local to C, with only fundamental types between the type
279 /// local to C and the local key parameter.
280 /// - e.g. `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
281 /// is bad, because the only local type with `T` as a subtree is
282 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
283 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
284 /// the second occurrence of `T` is not a subtree of *any* local type.
285 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
286 /// `LocalType<Vec<T>>`, which is local and has no types between it and
287 /// the type parameter.
289 /// The orphan rules actually serve several different purposes:
291 /// 1. They enable link-safety - i.e. 2 mutually-unknowing crates (where
292 /// every type local to one crate is unknown in the other) can't implement
293 /// the same trait-ref. This follows because it can be seen that no such
294 /// type can orphan-check in 2 such crates.
296 /// To check that a local impl follows the orphan rules, we check it in
297 /// InCrate::Local mode, using type parameters for the "generic" types.
299 /// 2. They ground negative reasoning for coherence. If a user wants to
300 /// write both a conditional blanket impl and a specific impl, we need to
301 /// make sure they do not overlap. For example, if we write
303 /// impl<T> IntoIterator for Vec<T>
304 /// impl<T: Iterator> IntoIterator for T
306 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
307 /// We can observe that this holds in the current crate, but we need to make
308 /// sure this will also hold in all unknown crates (both "independent" crates,
309 /// which we need for link-safety, and also child crates, because we don't want
310 /// child crates to get error for impl conflicts in a *dependency*).
312 /// For that, we only allow negative reasoning if, for every assignment to the
313 /// inference variables, every unknown crate would get an orphan error if they
314 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
315 /// mode. That is sound because we already know all the impls from known crates.
317 /// 3. For non-#[fundamental] traits, they guarantee that parent crates can
318 /// add "non-blanket" impls without breaking negative reasoning in dependent
319 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
321 /// For that, we only a allow crate to perform negative reasoning on
322 /// non-local-non-#[fundamental] only if there's a local key parameter as per (2).
324 /// Because we never perform negative reasoning generically (coherence does
325 /// not involve type parameters), this can be interpreted as doing the full
326 /// orphan check (using InCrate::Local mode), substituting non-local known
327 /// types for all inference variables.
329 /// This allows for crates to future-compatibly add impls as long as they
330 /// can't apply to types with a key parameter in a child crate - applying
331 /// the rules, this basically means that every type parameter in the impl
332 /// must appear behind a non-fundamental type (because this is not a
333 /// type-system requirement, crate owners might also go for "semantic
334 /// future-compatibility" involving things such as sealed traits, but
335 /// the above requirement is sufficient, and is necessary in "open world"
338 /// Note that this function is never called for types that have both type
339 /// parameters and inference variables.
340 fn orphan_check_trait_ref<'tcx>(tcx: TyCtxt,
341 trait_ref: ty::TraitRef<'tcx>,
343 -> Result<(), OrphanCheckErr<'tcx>>
345 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})",
346 trait_ref, in_crate);
348 if trait_ref.needs_infer() && trait_ref.needs_subst() {
349 bug!("can't orphan check a trait ref with both params and inference variables {:?}",
353 // First, create an ordered iterator over all the type parameters to the trait, with the self
354 // type appearing first.
355 // Find the first input type that either references a type parameter OR
357 for input_ty in trait_ref.input_types() {
358 if ty_is_local(tcx, input_ty, in_crate) {
359 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
361 // First local input type. Check that there are no
362 // uncovered type parameters.
363 let uncovered_tys = uncovered_tys(tcx, input_ty, in_crate);
364 for uncovered_ty in uncovered_tys {
365 if let Some(param) = uncovered_ty.walk()
366 .find(|t| is_possibly_remote_type(t, in_crate))
368 debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
369 return Err(OrphanCheckErr::UncoveredTy(param));
373 // OK, found local type, all prior types upheld invariant.
377 // Otherwise, enforce invariant that there are no type
378 // parameters reachable.
379 if let Some(param) = input_ty.walk()
380 .find(|t| is_possibly_remote_type(t, in_crate))
382 debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
383 return Err(OrphanCheckErr::UncoveredTy(param));
387 // If we exit above loop, never found a local type.
388 debug!("orphan_check_trait_ref: no local type");
389 return Err(OrphanCheckErr::NoLocalInputType);
392 fn uncovered_tys<'tcx>(tcx: TyCtxt, ty: Ty<'tcx>, in_crate: InCrate)
394 if ty_is_local_constructor(ty, in_crate) {
396 } else if fundamental_ty(tcx, ty) {
398 .flat_map(|t| uncovered_tys(tcx, t, in_crate))
405 fn is_possibly_remote_type(ty: Ty, _in_crate: InCrate) -> bool {
407 ty::Projection(..) | ty::Param(..) => true,
412 fn ty_is_local(tcx: TyCtxt, ty: Ty, in_crate: InCrate) -> bool {
413 ty_is_local_constructor(ty, in_crate) ||
414 fundamental_ty(tcx, ty) && ty.walk_shallow().any(|t| ty_is_local(tcx, t, in_crate))
417 fn fundamental_ty(tcx: TyCtxt, ty: Ty) -> bool {
420 ty::Adt(def, _) => def.is_fundamental(),
421 ty::Dynamic(ref data, ..) => {
422 data.principal().map_or(false, |p| tcx.has_attr(p.def_id(), "fundamental"))
428 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
430 // The type is local to *this* crate - it will not be
431 // local in any other crate.
432 InCrate::Remote => false,
433 InCrate::Local => def_id.is_local()
437 fn ty_is_local_constructor(ty: Ty, in_crate: InCrate) -> bool {
438 debug!("ty_is_local_constructor({:?})", ty);
456 ty::Projection(..) => {
460 ty::Infer(..) => match in_crate {
461 InCrate::Local => false,
462 // The inference variable might be unified with a local
463 // type in that remote crate.
464 InCrate::Remote => true,
467 ty::Adt(def, _) => def_id_is_local(def.did, in_crate),
468 ty::Foreign(did) => def_id_is_local(did, in_crate),
470 ty::Dynamic(ref tt, ..) => {
471 tt.principal().map_or(false, |p|
472 def_id_is_local(p.def_id(), in_crate)
480 ty::GeneratorWitness(..) |
482 bug!("ty_is_local invoked on unexpected type: {:?}", ty)