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 `README.md` for high-level documentation
13 use hir::def_id::{DefId, LOCAL_CRATE};
14 use syntax_pos::DUMMY_SP;
15 use traits::{self, Normalized, SelectionContext, Obligation, ObligationCause, Reveal};
16 use traits::IntercrateMode;
17 use traits::select::IntercrateAmbiguityCause;
18 use ty::{self, Ty, TyCtxt};
19 use ty::fold::TypeFoldable;
22 use infer::{InferCtxt, InferOk};
24 /// Whether we do the orphan check relative to this crate or
25 /// to some remote crate.
26 #[derive(Copy, Clone, Debug)]
32 #[derive(Debug, Copy, Clone)]
35 Downstream { used_to_be_broken: bool }
38 pub struct OverlapResult<'tcx> {
39 pub impl_header: ty::ImplHeader<'tcx>,
40 pub intercrate_ambiguity_causes: Vec<IntercrateAmbiguityCause>,
43 /// If there are types that satisfy both impls, invokes `on_overlap`
44 /// with a suitably-freshened `ImplHeader` with those types
45 /// substituted. Otherwise, invokes `no_overlap`.
46 pub fn overlapping_impls<F1, F2, R>(
47 infcx: &InferCtxt<'_, '_, '_>,
50 intercrate_mode: IntercrateMode,
55 F1: FnOnce(OverlapResult<'_>) -> R,
58 debug!("impl_can_satisfy(\
61 intercrate_mode={:?})",
66 let selcx = &mut SelectionContext::intercrate(infcx, intercrate_mode);
67 if let Some(r) = overlap(selcx, impl1_def_id, impl2_def_id) {
74 fn with_fresh_ty_vars<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
75 param_env: ty::ParamEnv<'tcx>,
77 -> ty::ImplHeader<'tcx>
79 let tcx = selcx.tcx();
80 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
82 let header = ty::ImplHeader {
84 self_ty: tcx.type_of(impl_def_id),
85 trait_ref: tcx.impl_trait_ref(impl_def_id),
86 predicates: tcx.predicates_of(impl_def_id).predicates
87 }.subst(tcx, impl_substs);
89 let Normalized { value: mut header, obligations } =
90 traits::normalize(selcx, param_env, ObligationCause::dummy(), &header);
92 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
96 /// Can both impl `a` and impl `b` be satisfied by a common type (including
97 /// `where` clauses)? If so, returns an `ImplHeader` that unifies the two impls.
98 fn overlap<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
101 -> Option<OverlapResult<'tcx>>
103 debug!("overlap(a_def_id={:?}, b_def_id={:?})",
107 // For the purposes of this check, we don't bring any skolemized
108 // types into scope; instead, we replace the generic types with
109 // fresh type variables, and hence we do our evaluations in an
110 // empty environment.
111 let param_env = ty::ParamEnv::empty(Reveal::UserFacing);
113 let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
114 let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
116 debug!("overlap: a_impl_header={:?}", a_impl_header);
117 debug!("overlap: b_impl_header={:?}", b_impl_header);
119 // Do `a` and `b` unify? If not, no overlap.
120 let obligations = match selcx.infcx().at(&ObligationCause::dummy(), param_env)
121 .eq_impl_headers(&a_impl_header, &b_impl_header) {
122 Ok(InferOk { obligations, value: () }) => {
125 Err(_) => return None
128 debug!("overlap: unification check succeeded");
130 // Are any of the obligations unsatisfiable? If so, no overlap.
131 let infcx = selcx.infcx();
132 let opt_failing_obligation =
133 a_impl_header.predicates
135 .chain(&b_impl_header.predicates)
136 .map(|p| infcx.resolve_type_vars_if_possible(p))
137 .map(|p| Obligation { cause: ObligationCause::dummy(),
142 .find(|o| !selcx.evaluate_obligation(o));
144 if let Some(failing_obligation) = opt_failing_obligation {
145 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
150 impl_header: selcx.infcx().resolve_type_vars_if_possible(&a_impl_header),
151 intercrate_ambiguity_causes: selcx.intercrate_ambiguity_causes().to_vec(),
155 pub fn trait_ref_is_knowable<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
156 trait_ref: ty::TraitRef<'tcx>)
159 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
160 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
161 // A downstream or cousin crate is allowed to implement some
162 // substitution of this trait-ref.
164 // A trait can be implementable for a trait ref by both the current
165 // crate and crates downstream of it. Older versions of rustc
166 // were not aware of this, causing incoherence (issue #43355).
167 let used_to_be_broken =
168 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok();
169 if used_to_be_broken {
170 debug!("trait_ref_is_knowable({:?}) - USED TO BE BROKEN", trait_ref);
172 return Some(Conflict::Downstream { used_to_be_broken });
175 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
176 // This is a local or fundamental trait, so future-compatibility
177 // is no concern. We know that downstream/cousin crates are not
178 // allowed to implement a substitution of this trait ref, which
179 // means impls could only come from dependencies of this crate,
180 // which we already know about.
184 // This is a remote non-fundamental trait, so if another crate
185 // can be the "final owner" of a substitution of this trait-ref,
186 // they are allowed to implement it future-compatibly.
188 // However, if we are a final owner, then nobody else can be,
189 // and if we are an intermediate owner, then we don't care
190 // about future-compatibility, which means that we're OK if
192 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
193 debug!("trait_ref_is_knowable: orphan check passed");
196 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
197 return Some(Conflict::Upstream);
201 pub fn trait_ref_is_local_or_fundamental<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
202 trait_ref: ty::TraitRef<'tcx>)
204 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, "fundamental")
207 pub enum OrphanCheckErr<'tcx> {
209 UncoveredTy(Ty<'tcx>),
212 /// Checks the coherence orphan rules. `impl_def_id` should be the
213 /// def-id of a trait impl. To pass, either the trait must be local, or else
214 /// two conditions must be satisfied:
216 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
217 /// 2. Some local type must appear in `Self`.
218 pub fn orphan_check<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
220 -> Result<(), OrphanCheckErr<'tcx>>
222 debug!("orphan_check({:?})", impl_def_id);
224 // We only except this routine to be invoked on implementations
225 // of a trait, not inherent implementations.
226 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
227 debug!("orphan_check: trait_ref={:?}", trait_ref);
229 // If the *trait* is local to the crate, ok.
230 if trait_ref.def_id.is_local() {
231 debug!("trait {:?} is local to current crate",
236 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
239 /// Check whether a trait-ref is potentially implementable by a crate.
241 /// The current rule is that a trait-ref orphan checks in a crate C:
243 /// 1. Order the parameters in the trait-ref in subst order - Self first,
244 /// others linearly (e.g. `<U as Foo<V, W>>` is U < V < W).
245 /// 2. Of these type parameters, there is at least one type parameter
246 /// in which, walking the type as a tree, you can reach a type local
247 /// to C where all types in-between are fundamental types. Call the
248 /// first such parameter the "local key parameter".
249 /// - e.g. `Box<LocalType>` is OK, because you can visit LocalType
250 /// going through `Box`, which is fundamental.
251 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
253 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
254 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
255 /// the local type and the type parameter.
256 /// 3. Every type parameter before the local key parameter is fully known in C.
257 /// - e.g. `impl<T> T: Trait<LocalType>` is bad, because `T` might be
259 /// - but `impl<T> LocalType: Trait<T>` is OK, because `LocalType`
260 /// occurs before `T`.
261 /// 4. Every type in the local key parameter not known in C, going
262 /// through the parameter's type tree, must appear only as a subtree of
263 /// a type local to C, with only fundamental types between the type
264 /// local to C and the local key parameter.
265 /// - e.g. `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
266 /// is bad, because the only local type with `T` as a subtree is
267 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
268 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
269 /// the second occurence of `T` is not a subtree of *any* local type.
270 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
271 /// `LocalType<Vec<T>>`, which is local and has no types between it and
272 /// the type parameter.
274 /// The orphan rules actually serve several different purposes:
276 /// 1. They enable link-safety - i.e. 2 mutually-unknowing crates (where
277 /// every type local to one crate is unknown in the other) can't implement
278 /// the same trait-ref. This follows because it can be seen that no such
279 /// type can orphan-check in 2 such crates.
281 /// To check that a local impl follows the orphan rules, we check it in
282 /// InCrate::Local mode, using type parameters for the "generic" types.
284 /// 2. They ground negative reasoning for coherence. If a user wants to
285 /// write both a conditional blanket impl and a specific impl, we need to
286 /// make sure they do not overlap. For example, if we write
288 /// impl<T> IntoIterator for Vec<T>
289 /// impl<T: Iterator> IntoIterator for T
291 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
292 /// We can observe that this holds in the current crate, but we need to make
293 /// sure this will also hold in all unknown crates (both "independent" crates,
294 /// which we need for link-safety, and also child crates, because we don't want
295 /// child crates to get error for impl conflicts in a *dependency*).
297 /// For that, we only allow negative reasoning if, for every assignment to the
298 /// inference variables, every unknown crate would get an orphan error if they
299 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
300 /// mode. That is sound because we already know all the impls from known crates.
302 /// 3. For non-#[fundamental] traits, they guarantee that parent crates can
303 /// add "non-blanket" impls without breaking negative reasoning in dependent
304 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
306 /// For that, we only a allow crate to perform negative reasoning on
307 /// non-local-non-#[fundamental] only if there's a local key parameter as per (2).
309 /// Because we never perform negative reasoning generically (coherence does
310 /// not involve type parameters), this can be interpreted as doing the full
311 /// orphan check (using InCrate::Local mode), substituting non-local known
312 /// types for all inference variables.
314 /// This allows for crates to future-compatibly add impls as long as they
315 /// can't apply to types with a key parameter in a child crate - applying
316 /// the rules, this basically means that every type parameter in the impl
317 /// must appear behind a non-fundamental type (because this is not a
318 /// type-system requirement, crate owners might also go for "semantic
319 /// future-compatibility" involving things such as sealed traits, but
320 /// the above requirement is sufficient, and is necessary in "open world"
323 /// Note that this function is never called for types that have both type
324 /// parameters and inference variables.
325 fn orphan_check_trait_ref<'tcx>(tcx: TyCtxt,
326 trait_ref: ty::TraitRef<'tcx>,
328 -> Result<(), OrphanCheckErr<'tcx>>
330 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})",
331 trait_ref, in_crate);
333 if trait_ref.needs_infer() && trait_ref.needs_subst() {
334 bug!("can't orphan check a trait ref with both params and inference variables {:?}",
338 // First, create an ordered iterator over all the type parameters to the trait, with the self
339 // type appearing first.
340 // Find the first input type that either references a type parameter OR
342 for input_ty in trait_ref.input_types() {
343 if ty_is_local(tcx, input_ty, in_crate) {
344 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
346 // First local input type. Check that there are no
347 // uncovered type parameters.
348 let uncovered_tys = uncovered_tys(tcx, input_ty, in_crate);
349 for uncovered_ty in uncovered_tys {
350 if let Some(param) = uncovered_ty.walk()
351 .find(|t| is_possibly_remote_type(t, in_crate))
353 debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
354 return Err(OrphanCheckErr::UncoveredTy(param));
358 // OK, found local type, all prior types upheld invariant.
362 // Otherwise, enforce invariant that there are no type
363 // parameters reachable.
364 if let Some(param) = input_ty.walk()
365 .find(|t| is_possibly_remote_type(t, in_crate))
367 debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
368 return Err(OrphanCheckErr::UncoveredTy(param));
372 // If we exit above loop, never found a local type.
373 debug!("orphan_check_trait_ref: no local type");
374 return Err(OrphanCheckErr::NoLocalInputType);
377 fn uncovered_tys<'tcx>(tcx: TyCtxt, ty: Ty<'tcx>, in_crate: InCrate)
379 if ty_is_local_constructor(ty, in_crate) {
381 } else if fundamental_ty(tcx, ty) {
383 .flat_map(|t| uncovered_tys(tcx, t, in_crate))
390 fn is_possibly_remote_type(ty: Ty, _in_crate: InCrate) -> bool {
392 ty::TyProjection(..) | ty::TyParam(..) => true,
397 fn ty_is_local(tcx: TyCtxt, ty: Ty, in_crate: InCrate) -> bool {
398 ty_is_local_constructor(ty, in_crate) ||
399 fundamental_ty(tcx, ty) && ty.walk_shallow().any(|t| ty_is_local(tcx, t, in_crate))
402 fn fundamental_ty(tcx: TyCtxt, ty: Ty) -> bool {
404 ty::TyRef(..) => true,
405 ty::TyAdt(def, _) => def.is_fundamental(),
406 ty::TyDynamic(ref data, ..) => {
407 data.principal().map_or(false, |p| tcx.has_attr(p.def_id(), "fundamental"))
413 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
415 // The type is local to *this* crate - it will not be
416 // local in any other crate.
417 InCrate::Remote => false,
418 InCrate::Local => def_id.is_local()
422 fn ty_is_local_constructor(ty: Ty, in_crate: InCrate) -> bool {
423 debug!("ty_is_local_constructor({:?})", ty);
441 ty::TyProjection(..) => {
445 ty::TyInfer(..) => match in_crate {
446 InCrate::Local => false,
447 // The inference variable might be unified with a local
448 // type in that remote crate.
449 InCrate::Remote => true,
452 ty::TyAdt(def, _) => def_id_is_local(def.did, in_crate),
453 ty::TyForeign(did) => def_id_is_local(did, in_crate),
455 ty::TyDynamic(ref tt, ..) => {
456 tt.principal().map_or(false, |p| {
457 def_id_is_local(p.def_id(), in_crate)
465 ty::TyClosure(..) | ty::TyGenerator(..) | ty::TyAnon(..) => {
466 bug!("ty_is_local invoked on unexpected type: {:?}", ty)