1 // Copyright 2015 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 // Logic and data structures related to impl specialization, explained in
12 // greater detail below.
14 // At the moment, this implementation support only the simple "chain" rule:
15 // If any two impls overlap, one must be a strict subset of the other.
17 // See traits/README.md for a bit more detail on how specialization
18 // fits together with the rest of the trait machinery.
20 use super::{SelectionContext, FulfillmentContext};
21 use super::util::impl_trait_ref_and_oblig;
23 use rustc_data_structures::fx::FxHashMap;
24 use hir::def_id::DefId;
25 use infer::{InferCtxt, InferOk};
27 use ty::subst::{Subst, Substs};
28 use traits::{self, Reveal, ObligationCause};
29 use ty::{self, TyCtxt, TypeFoldable};
30 use syntax_pos::DUMMY_SP;
34 pub mod specialization_graph;
36 /// Information pertinent to an overlapping impl error.
37 pub struct OverlapError {
39 pub trait_desc: String,
40 pub self_desc: Option<String>
43 /// Given a subst for the requested impl, translate it to a subst
44 /// appropriate for the actual item definition (whether it be in that impl,
45 /// a parent impl, or the trait).
46 /// When we have selected one impl, but are actually using item definitions from
47 /// a parent impl providing a default, we need a way to translate between the
48 /// type parameters of the two impls. Here the `source_impl` is the one we've
49 /// selected, and `source_substs` is a substitution of its generics.
50 /// And `target_node` is the impl/trait we're actually going to get the
51 /// definition from. The resulting substitution will map from `target_node`'s
52 /// generics to `source_impl`'s generics as instantiated by `source_subst`.
54 /// For example, consider the following scenario:
58 /// impl<T, U> Foo for (T, U) { ... } // target impl
59 /// impl<V> Foo for (V, V) { ... } // source impl
62 /// Suppose we have selected "source impl" with `V` instantiated with `u32`.
63 /// This function will produce a substitution with `T` and `U` both mapping to `u32`.
65 /// Where clauses add some trickiness here, because they can be used to "define"
66 /// an argument indirectly:
69 /// impl<'a, I, T: 'a> Iterator for Cloned<I>
70 /// where I: Iterator<Item=&'a T>, T: Clone
73 /// In a case like this, the substitution for `T` is determined indirectly,
74 /// through associated type projection. We deal with such cases by using
75 /// *fulfillment* to relate the two impls, requiring that all projections are
77 pub fn translate_substs<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
79 source_substs: &'tcx Substs<'tcx>,
80 target_node: specialization_graph::Node)
81 -> &'tcx Substs<'tcx> {
82 let source_trait_ref = infcx.tcx
83 .impl_trait_ref(source_impl)
85 .subst(infcx.tcx, &source_substs);
87 // translate the Self and TyParam parts of the substitution, since those
89 let target_substs = match target_node {
90 specialization_graph::Node::Impl(target_impl) => {
91 // no need to translate if we're targetting the impl we started with
92 if source_impl == target_impl {
96 fulfill_implication(infcx, source_trait_ref, target_impl).unwrap_or_else(|_| {
97 bug!("When translating substitutions for specialization, the expected \
98 specializaiton failed to hold")
101 specialization_graph::Node::Trait(..) => source_trait_ref.substs,
104 // directly inherent the method generics, since those do not vary across impls
105 source_substs.rebase_onto(infcx.tcx, source_impl, target_substs)
108 /// Given a selected impl described by `impl_data`, returns the
109 /// definition and substitions for the method with the name `name`,
110 /// and trait method substitutions `substs`, in that impl, a less
111 /// specialized impl, or the trait default, whichever applies.
112 pub fn find_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
114 substs: &'tcx Substs<'tcx>,
115 impl_data: &super::VtableImplData<'tcx, ()>)
116 -> (DefId, &'tcx Substs<'tcx>)
118 assert!(!substs.needs_infer());
120 let trait_def_id = tcx.trait_id_of_impl(impl_data.impl_def_id).unwrap();
121 let trait_def = tcx.lookup_trait_def(trait_def_id);
123 let ancestors = trait_def.ancestors(impl_data.impl_def_id);
124 match ancestors.defs(tcx, name, ty::AssociatedKind::Method).next() {
126 let substs = tcx.infer_ctxt((), Reveal::All).enter(|infcx| {
127 let substs = substs.rebase_onto(tcx, trait_def_id, impl_data.substs);
128 let substs = translate_substs(&infcx, impl_data.impl_def_id,
129 substs, node_item.node);
130 let substs = infcx.tcx.erase_regions(&substs);
131 tcx.lift(&substs).unwrap_or_else(|| {
132 bug!("find_method: translate_substs \
133 returned {:?} which contains inference types/regions",
137 (node_item.item.def_id, substs)
140 bug!("method {:?} not found in {:?}", name, impl_data.impl_def_id)
145 /// Is impl1 a specialization of impl2?
147 /// Specialization is determined by the sets of types to which the impls apply;
148 /// impl1 specializes impl2 if it applies to a subset of the types impl2 applies
150 pub fn specializes<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
152 impl2_def_id: DefId) -> bool {
153 debug!("specializes({:?}, {:?})", impl1_def_id, impl2_def_id);
155 if let Some(r) = tcx.specializes_cache.borrow().check(impl1_def_id, impl2_def_id) {
159 // The feature gate should prevent introducing new specializations, but not
160 // taking advantage of upstream ones.
161 if !tcx.sess.features.borrow().specialization &&
162 (impl1_def_id.is_local() || impl2_def_id.is_local()) {
166 // We determine whether there's a subset relationship by:
168 // - skolemizing impl1,
169 // - assuming the where clauses for impl1,
170 // - instantiating impl2 with fresh inference variables,
172 // - attempting to prove the where clauses for impl2
174 // The last three steps are encapsulated in `fulfill_implication`.
176 // See RFC 1210 for more details and justification.
178 // Currently we do not allow e.g. a negative impl to specialize a positive one
179 if tcx.trait_impl_polarity(impl1_def_id) != tcx.trait_impl_polarity(impl2_def_id) {
183 // create a parameter environment corresponding to a (skolemized) instantiation of impl1
184 let penv = tcx.construct_parameter_environment(DUMMY_SP,
186 region::DUMMY_CODE_EXTENT);
187 let impl1_trait_ref = tcx.impl_trait_ref(impl1_def_id)
189 .subst(tcx, &penv.free_substs);
191 // Create a infcx, taking the predicates of impl1 as assumptions:
192 let result = tcx.infer_ctxt(penv, Reveal::UserFacing).enter(|infcx| {
193 // Normalize the trait reference. The WF rules ought to ensure
194 // that this always succeeds.
195 let impl1_trait_ref =
196 match traits::fully_normalize(&infcx, ObligationCause::dummy(), &impl1_trait_ref) {
197 Ok(impl1_trait_ref) => impl1_trait_ref,
199 bug!("failed to fully normalize {:?}: {:?}", impl1_trait_ref, err);
203 // Attempt to prove that impl2 applies, given all of the above.
204 fulfill_implication(&infcx, impl1_trait_ref, impl2_def_id).is_ok()
207 tcx.specializes_cache.borrow_mut().insert(impl1_def_id, impl2_def_id, result);
211 /// Attempt to fulfill all obligations of `target_impl` after unification with
212 /// `source_trait_ref`. If successful, returns a substitution for *all* the
213 /// generics of `target_impl`, including both those needed to unify with
214 /// `source_trait_ref` and those whose identity is determined via a where
215 /// clause in the impl.
216 fn fulfill_implication<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
217 source_trait_ref: ty::TraitRef<'tcx>,
219 -> Result<&'tcx Substs<'tcx>, ()> {
220 let selcx = &mut SelectionContext::new(&infcx);
221 let target_substs = infcx.fresh_substs_for_item(DUMMY_SP, target_impl);
222 let (target_trait_ref, obligations) = impl_trait_ref_and_oblig(selcx,
226 // do the impls unify? If not, no specialization.
227 match infcx.eq_trait_refs(true,
228 &ObligationCause::dummy(),
231 Ok(InferOk { obligations, .. }) => {
232 // FIXME(#32730) propagate obligations
233 assert!(obligations.is_empty())
236 debug!("fulfill_implication: {:?} does not unify with {:?}",
243 // attempt to prove all of the predicates for impl2 given those for impl1
244 // (which are packed up in penv)
246 infcx.save_and_restore_obligations_in_snapshot_flag(|infcx| {
247 let mut fulfill_cx = FulfillmentContext::new();
248 for oblig in obligations.into_iter() {
249 fulfill_cx.register_predicate_obligation(&infcx, oblig);
251 match fulfill_cx.select_all_or_error(infcx) {
254 debug!("fulfill_implication: for impls on {:?} and {:?}, \
255 could not fulfill: {:?} given {:?}",
259 infcx.parameter_environment.caller_bounds);
264 debug!("fulfill_implication: an impl for {:?} specializes {:?}",
268 // Now resolve the *substitution* we built for the target earlier, replacing
269 // the inference variables inside with whatever we got from fulfillment.
270 Ok(infcx.resolve_type_vars_if_possible(&target_substs))
276 pub struct SpecializesCache {
277 map: FxHashMap<(DefId, DefId), bool>
280 impl SpecializesCache {
281 pub fn new() -> Self {
287 pub fn check(&self, a: DefId, b: DefId) -> Option<bool> {
288 self.map.get(&(a, b)).cloned()
291 pub fn insert(&mut self, a: DefId, b: DefId, result: bool) {
292 self.map.insert((a, b), result);