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;
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<'gcx, F1, F2, R>(
47 tcx: TyCtxt<'_, 'gcx, 'gcx>,
50 intercrate_mode: IntercrateMode,
55 F1: FnOnce(OverlapResult<'_>) -> R,
58 debug!("impl_can_satisfy(\
61 intercrate_mode={:?})",
66 tcx.infer_ctxt().enter(|infcx| {
67 let selcx = &mut SelectionContext::intercrate(&infcx, intercrate_mode);
68 if let Some(r) = overlap(selcx, impl1_def_id, impl2_def_id) {
76 fn with_fresh_ty_vars<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
77 param_env: ty::ParamEnv<'tcx>,
79 -> ty::ImplHeader<'tcx>
81 let tcx = selcx.tcx();
82 let impl_substs = selcx.infcx().fresh_substs_for_item(DUMMY_SP, impl_def_id);
84 let header = ty::ImplHeader {
86 self_ty: tcx.type_of(impl_def_id),
87 trait_ref: tcx.impl_trait_ref(impl_def_id),
88 predicates: tcx.predicates_of(impl_def_id).predicates
89 }.subst(tcx, impl_substs);
91 let Normalized { value: mut header, obligations } =
92 traits::normalize(selcx, param_env, ObligationCause::dummy(), &header);
94 header.predicates.extend(obligations.into_iter().map(|o| o.predicate));
98 /// Can both impl `a` and impl `b` be satisfied by a common type (including
99 /// `where` clauses)? If so, returns an `ImplHeader` that unifies the two impls.
100 fn overlap<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
103 -> Option<OverlapResult<'tcx>>
105 debug!("overlap(a_def_id={:?}, b_def_id={:?})",
109 // For the purposes of this check, we don't bring any skolemized
110 // types into scope; instead, we replace the generic types with
111 // fresh type variables, and hence we do our evaluations in an
112 // empty environment.
113 let param_env = ty::ParamEnv::empty(Reveal::UserFacing);
115 let a_impl_header = with_fresh_ty_vars(selcx, param_env, a_def_id);
116 let b_impl_header = with_fresh_ty_vars(selcx, param_env, b_def_id);
118 debug!("overlap: a_impl_header={:?}", a_impl_header);
119 debug!("overlap: b_impl_header={:?}", b_impl_header);
121 // Do `a` and `b` unify? If not, no overlap.
122 let obligations = match selcx.infcx().at(&ObligationCause::dummy(), param_env)
123 .eq_impl_headers(&a_impl_header, &b_impl_header) {
124 Ok(InferOk { obligations, value: () }) => {
127 Err(_) => return None
130 debug!("overlap: unification check succeeded");
132 // Are any of the obligations unsatisfiable? If so, no overlap.
133 let infcx = selcx.infcx();
134 let opt_failing_obligation =
135 a_impl_header.predicates
137 .chain(&b_impl_header.predicates)
138 .map(|p| infcx.resolve_type_vars_if_possible(p))
139 .map(|p| Obligation { cause: ObligationCause::dummy(),
144 .find(|o| !selcx.evaluate_obligation(o));
146 if let Some(failing_obligation) = opt_failing_obligation {
147 debug!("overlap: obligation unsatisfiable {:?}", failing_obligation);
152 impl_header: selcx.infcx().resolve_type_vars_if_possible(&a_impl_header),
153 intercrate_ambiguity_causes: selcx.intercrate_ambiguity_causes().to_vec(),
157 pub fn trait_ref_is_knowable<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
158 trait_ref: ty::TraitRef<'tcx>)
161 debug!("trait_ref_is_knowable(trait_ref={:?})", trait_ref);
162 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Remote).is_ok() {
163 // A downstream or cousin crate is allowed to implement some
164 // substitution of this trait-ref.
166 // A trait can be implementable for a trait ref by both the current
167 // crate and crates downstream of it. Older versions of rustc
168 // were not aware of this, causing incoherence (issue #43355).
169 let used_to_be_broken =
170 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok();
171 if used_to_be_broken {
172 debug!("trait_ref_is_knowable({:?}) - USED TO BE BROKEN", trait_ref);
174 return Some(Conflict::Downstream { used_to_be_broken });
177 if trait_ref_is_local_or_fundamental(tcx, trait_ref) {
178 // This is a local or fundamental trait, so future-compatibility
179 // is no concern. We know that downstream/cousin crates are not
180 // allowed to implement a substitution of this trait ref, which
181 // means impls could only come from dependencies of this crate,
182 // which we already know about.
186 // This is a remote non-fundamental trait, so if another crate
187 // can be the "final owner" of a substitution of this trait-ref,
188 // they are allowed to implement it future-compatibly.
190 // However, if we are a final owner, then nobody else can be,
191 // and if we are an intermediate owner, then we don't care
192 // about future-compatibility, which means that we're OK if
194 if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
195 debug!("trait_ref_is_knowable: orphan check passed");
198 debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
199 return Some(Conflict::Upstream);
203 pub fn trait_ref_is_local_or_fundamental<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
204 trait_ref: ty::TraitRef<'tcx>)
206 trait_ref.def_id.krate == LOCAL_CRATE || tcx.has_attr(trait_ref.def_id, "fundamental")
209 pub enum OrphanCheckErr<'tcx> {
211 UncoveredTy(Ty<'tcx>),
214 /// Checks the coherence orphan rules. `impl_def_id` should be the
215 /// def-id of a trait impl. To pass, either the trait must be local, or else
216 /// two conditions must be satisfied:
218 /// 1. All type parameters in `Self` must be "covered" by some local type constructor.
219 /// 2. Some local type must appear in `Self`.
220 pub fn orphan_check<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
222 -> Result<(), OrphanCheckErr<'tcx>>
224 debug!("orphan_check({:?})", impl_def_id);
226 // We only except this routine to be invoked on implementations
227 // of a trait, not inherent implementations.
228 let trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap();
229 debug!("orphan_check: trait_ref={:?}", trait_ref);
231 // If the *trait* is local to the crate, ok.
232 if trait_ref.def_id.is_local() {
233 debug!("trait {:?} is local to current crate",
238 orphan_check_trait_ref(tcx, trait_ref, InCrate::Local)
241 /// Check whether a trait-ref is potentially implementable by a crate.
243 /// The current rule is that a trait-ref orphan checks in a crate C:
245 /// 1. Order the parameters in the trait-ref in subst order - Self first,
246 /// others linearly (e.g. `<U as Foo<V, W>>` is U < V < W).
247 /// 2. Of these type parameters, there is at least one type parameter
248 /// in which, walking the type as a tree, you can reach a type local
249 /// to C where all types in-between are fundamental types. Call the
250 /// first such parameter the "local key parameter".
251 /// - e.g. `Box<LocalType>` is OK, because you can visit LocalType
252 /// going through `Box`, which is fundamental.
253 /// - similarly, `FundamentalPair<Vec<()>, Box<LocalType>>` is OK for
255 /// - but (knowing that `Vec<T>` is non-fundamental, and assuming it's
256 /// not local), `Vec<LocalType>` is bad, because `Vec<->` is between
257 /// the local type and the type parameter.
258 /// 3. Every type parameter before the local key parameter is fully known in C.
259 /// - e.g. `impl<T> T: Trait<LocalType>` is bad, because `T` might be
261 /// - but `impl<T> LocalType: Trait<T>` is OK, because `LocalType`
262 /// occurs before `T`.
263 /// 4. Every type in the local key parameter not known in C, going
264 /// through the parameter's type tree, must appear only as a subtree of
265 /// a type local to C, with only fundamental types between the type
266 /// local to C and the local key parameter.
267 /// - e.g. `Vec<LocalType<T>>>` (or equivalently `Box<Vec<LocalType<T>>>`)
268 /// is bad, because the only local type with `T` as a subtree is
269 /// `LocalType<T>`, and `Vec<->` is between it and the type parameter.
270 /// - similarly, `FundamentalPair<LocalType<T>, T>` is bad, because
271 /// the second occurence of `T` is not a subtree of *any* local type.
272 /// - however, `LocalType<Vec<T>>` is OK, because `T` is a subtree of
273 /// `LocalType<Vec<T>>`, which is local and has no types between it and
274 /// the type parameter.
276 /// The orphan rules actually serve several different purposes:
278 /// 1. They enable link-safety - i.e. 2 mutually-unknowing crates (where
279 /// every type local to one crate is unknown in the other) can't implement
280 /// the same trait-ref. This follows because it can be seen that no such
281 /// type can orphan-check in 2 such crates.
283 /// To check that a local impl follows the orphan rules, we check it in
284 /// InCrate::Local mode, using type parameters for the "generic" types.
286 /// 2. They ground negative reasoning for coherence. If a user wants to
287 /// write both a conditional blanket impl and a specific impl, we need to
288 /// make sure they do not overlap. For example, if we write
290 /// impl<T> IntoIterator for Vec<T>
291 /// impl<T: Iterator> IntoIterator for T
293 /// We need to be able to prove that `Vec<$0>: !Iterator` for every type $0.
294 /// We can observe that this holds in the current crate, but we need to make
295 /// sure this will also hold in all unknown crates (both "independent" crates,
296 /// which we need for link-safety, and also child crates, because we don't want
297 /// child crates to get error for impl conflicts in a *dependency*).
299 /// For that, we only allow negative reasoning if, for every assignment to the
300 /// inference variables, every unknown crate would get an orphan error if they
301 /// try to implement this trait-ref. To check for this, we use InCrate::Remote
302 /// mode. That is sound because we already know all the impls from known crates.
304 /// 3. For non-#[fundamental] traits, they guarantee that parent crates can
305 /// add "non-blanket" impls without breaking negative reasoning in dependent
306 /// crates. This is the "rebalancing coherence" (RFC 1023) restriction.
308 /// For that, we only a allow crate to perform negative reasoning on
309 /// non-local-non-#[fundamental] only if there's a local key parameter as per (2).
311 /// Because we never perform negative reasoning generically (coherence does
312 /// not involve type parameters), this can be interpreted as doing the full
313 /// orphan check (using InCrate::Local mode), substituting non-local known
314 /// types for all inference variables.
316 /// This allows for crates to future-compatibly add impls as long as they
317 /// can't apply to types with a key parameter in a child crate - applying
318 /// the rules, this basically means that every type parameter in the impl
319 /// must appear behind a non-fundamental type (because this is not a
320 /// type-system requirement, crate owners might also go for "semantic
321 /// future-compatibility" involving things such as sealed traits, but
322 /// the above requirement is sufficient, and is necessary in "open world"
325 /// Note that this function is never called for types that have both type
326 /// parameters and inference variables.
327 fn orphan_check_trait_ref<'tcx>(tcx: TyCtxt,
328 trait_ref: ty::TraitRef<'tcx>,
330 -> Result<(), OrphanCheckErr<'tcx>>
332 debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})",
333 trait_ref, in_crate);
335 if trait_ref.needs_infer() && trait_ref.needs_subst() {
336 bug!("can't orphan check a trait ref with both params and inference variables {:?}",
340 // First, create an ordered iterator over all the type parameters to the trait, with the self
341 // type appearing first.
342 // Find the first input type that either references a type parameter OR
344 for input_ty in trait_ref.input_types() {
345 if ty_is_local(tcx, input_ty, in_crate) {
346 debug!("orphan_check_trait_ref: ty_is_local `{:?}`", input_ty);
348 // First local input type. Check that there are no
349 // uncovered type parameters.
350 let uncovered_tys = uncovered_tys(tcx, input_ty, in_crate);
351 for uncovered_ty in uncovered_tys {
352 if let Some(param) = uncovered_ty.walk()
353 .find(|t| is_possibly_remote_type(t, in_crate))
355 debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
356 return Err(OrphanCheckErr::UncoveredTy(param));
360 // OK, found local type, all prior types upheld invariant.
364 // Otherwise, enforce invariant that there are no type
365 // parameters reachable.
366 if let Some(param) = input_ty.walk()
367 .find(|t| is_possibly_remote_type(t, in_crate))
369 debug!("orphan_check_trait_ref: uncovered type `{:?}`", param);
370 return Err(OrphanCheckErr::UncoveredTy(param));
374 // If we exit above loop, never found a local type.
375 debug!("orphan_check_trait_ref: no local type");
376 return Err(OrphanCheckErr::NoLocalInputType);
379 fn uncovered_tys<'tcx>(tcx: TyCtxt, ty: Ty<'tcx>, in_crate: InCrate)
381 if ty_is_local_constructor(ty, in_crate) {
383 } else if fundamental_ty(tcx, ty) {
385 .flat_map(|t| uncovered_tys(tcx, t, in_crate))
392 fn is_possibly_remote_type(ty: Ty, _in_crate: InCrate) -> bool {
394 ty::TyProjection(..) | ty::TyParam(..) => true,
399 fn ty_is_local(tcx: TyCtxt, ty: Ty, in_crate: InCrate) -> bool {
400 ty_is_local_constructor(ty, in_crate) ||
401 fundamental_ty(tcx, ty) && ty.walk_shallow().any(|t| ty_is_local(tcx, t, in_crate))
404 fn fundamental_ty(tcx: TyCtxt, ty: Ty) -> bool {
406 ty::TyRef(..) => true,
407 ty::TyAdt(def, _) => def.is_fundamental(),
408 ty::TyDynamic(ref data, ..) => {
409 data.principal().map_or(false, |p| tcx.has_attr(p.def_id(), "fundamental"))
415 fn def_id_is_local(def_id: DefId, in_crate: InCrate) -> bool {
417 // The type is local to *this* crate - it will not be
418 // local in any other crate.
419 InCrate::Remote => false,
420 InCrate::Local => def_id.is_local()
424 fn ty_is_local_constructor(ty: Ty, in_crate: InCrate) -> bool {
425 debug!("ty_is_local_constructor({:?})", ty);
443 ty::TyProjection(..) => {
447 ty::TyInfer(..) => match in_crate {
448 InCrate::Local => false,
449 // The inference variable might be unified with a local
450 // type in that remote crate.
451 InCrate::Remote => true,
454 ty::TyAdt(def, _) => def_id_is_local(def.did, in_crate),
455 ty::TyForeign(did) => def_id_is_local(did, in_crate),
457 ty::TyDynamic(ref tt, ..) => {
458 tt.principal().map_or(false, |p| {
459 def_id_is_local(p.def_id(), in_crate)
467 ty::TyClosure(..) | ty::TyGenerator(..) | ty::TyAnon(..) => {
468 bug!("ty_is_local invoked on unexpected type: {:?}", ty)