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};
26 use ty::subst::{Subst, Substs};
27 use traits::{self, Reveal, ObligationCause};
28 use ty::{self, TyCtxt, TypeFoldable};
29 use syntax_pos::DUMMY_SP;
31 pub mod specialization_graph;
33 /// Information pertinent to an overlapping impl error.
34 pub struct OverlapError {
36 pub trait_desc: String,
37 pub self_desc: Option<String>,
40 /// Given a subst for the requested impl, translate it to a subst
41 /// appropriate for the actual item definition (whether it be in that impl,
42 /// a parent impl, or the trait).
43 /// When we have selected one impl, but are actually using item definitions from
44 /// a parent impl providing a default, we need a way to translate between the
45 /// type parameters of the two impls. Here the `source_impl` is the one we've
46 /// selected, and `source_substs` is a substitution of its generics.
47 /// And `target_node` is the impl/trait we're actually going to get the
48 /// definition from. The resulting substitution will map from `target_node`'s
49 /// generics to `source_impl`'s generics as instantiated by `source_subst`.
51 /// For example, consider the following scenario:
55 /// impl<T, U> Foo for (T, U) { ... } // target impl
56 /// impl<V> Foo for (V, V) { ... } // source impl
59 /// Suppose we have selected "source impl" with `V` instantiated with `u32`.
60 /// This function will produce a substitution with `T` and `U` both mapping to `u32`.
62 /// Where clauses add some trickiness here, because they can be used to "define"
63 /// an argument indirectly:
66 /// impl<'a, I, T: 'a> Iterator for Cloned<I>
67 /// where I: Iterator<Item=&'a T>, T: Clone
70 /// In a case like this, the substitution for `T` is determined indirectly,
71 /// through associated type projection. We deal with such cases by using
72 /// *fulfillment* to relate the two impls, requiring that all projections are
74 pub fn translate_substs<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
76 source_substs: &'tcx Substs<'tcx>,
77 target_node: specialization_graph::Node)
78 -> &'tcx Substs<'tcx> {
79 let source_trait_ref = infcx.tcx
80 .impl_trait_ref(source_impl)
82 .subst(infcx.tcx, &source_substs);
84 // translate the Self and TyParam parts of the substitution, since those
86 let target_substs = match target_node {
87 specialization_graph::Node::Impl(target_impl) => {
88 // no need to translate if we're targetting the impl we started with
89 if source_impl == target_impl {
93 fulfill_implication(infcx, source_trait_ref, target_impl).unwrap_or_else(|_| {
94 bug!("When translating substitutions for specialization, the expected \
95 specializaiton failed to hold")
98 specialization_graph::Node::Trait(..) => source_trait_ref.substs,
101 // directly inherent the method generics, since those do not vary across impls
102 source_substs.rebase_onto(infcx.tcx, source_impl, target_substs)
105 /// Given a selected impl described by `impl_data`, returns the
106 /// definition and substitions for the method with the name `name`
107 /// the kind `kind`, and trait method substitutions `substs`, in
108 /// that impl, a less specialized impl, or the trait default,
109 /// whichever applies.
110 pub fn find_associated_item<'a, 'tcx>(
111 tcx: TyCtxt<'a, 'tcx, 'tcx>,
112 item: &ty::AssociatedItem,
113 substs: &'tcx Substs<'tcx>,
114 impl_data: &super::VtableImplData<'tcx, ()>,
115 ) -> (DefId, &'tcx Substs<'tcx>) {
116 assert!(!substs.needs_infer());
118 let trait_def_id = tcx.trait_id_of_impl(impl_data.impl_def_id).unwrap();
119 let trait_def = tcx.trait_def(trait_def_id);
121 let ancestors = trait_def.ancestors(impl_data.impl_def_id);
122 match ancestors.defs(tcx, item.name, item.kind).next() {
124 let substs = tcx.infer_ctxt((), Reveal::All).enter(|infcx| {
125 let substs = substs.rebase_onto(tcx, trait_def_id, impl_data.substs);
126 let substs = translate_substs(&infcx, impl_data.impl_def_id,
127 substs, node_item.node);
128 let substs = infcx.tcx.erase_regions(&substs);
129 tcx.lift(&substs).unwrap_or_else(|| {
130 bug!("find_method: translate_substs \
131 returned {:?} which contains inference types/regions",
135 (node_item.item.def_id, substs)
138 bug!("{:?} not found in {:?}", item, impl_data.impl_def_id)
143 /// Is impl1 a specialization of impl2?
145 /// Specialization is determined by the sets of types to which the impls apply;
146 /// impl1 specializes impl2 if it applies to a subset of the types impl2 applies
148 pub fn specializes<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
150 impl2_def_id: DefId) -> bool {
151 debug!("specializes({:?}, {:?})", impl1_def_id, impl2_def_id);
153 if let Some(r) = tcx.specializes_cache.borrow().check(impl1_def_id, impl2_def_id) {
157 // The feature gate should prevent introducing new specializations, but not
158 // taking advantage of upstream ones.
159 if !tcx.sess.features.borrow().specialization &&
160 (impl1_def_id.is_local() || impl2_def_id.is_local()) {
164 // We determine whether there's a subset relationship by:
166 // - skolemizing impl1,
167 // - assuming the where clauses for impl1,
168 // - instantiating impl2 with fresh inference variables,
170 // - attempting to prove the where clauses for impl2
172 // The last three steps are encapsulated in `fulfill_implication`.
174 // See RFC 1210 for more details and justification.
176 // Currently we do not allow e.g. a negative impl to specialize a positive one
177 if tcx.impl_polarity(impl1_def_id) != tcx.impl_polarity(impl2_def_id) {
181 // create a parameter environment corresponding to a (skolemized) instantiation of impl1
182 let penv = tcx.construct_parameter_environment(DUMMY_SP,
185 let impl1_trait_ref = tcx.impl_trait_ref(impl1_def_id)
187 .subst(tcx, &penv.free_substs);
189 // Create a infcx, taking the predicates of impl1 as assumptions:
190 let result = tcx.infer_ctxt(penv, Reveal::UserFacing).enter(|infcx| {
191 // Normalize the trait reference. The WF rules ought to ensure
192 // that this always succeeds.
193 let impl1_trait_ref =
194 match traits::fully_normalize(&infcx, ObligationCause::dummy(), &impl1_trait_ref) {
195 Ok(impl1_trait_ref) => impl1_trait_ref,
197 bug!("failed to fully normalize {:?}: {:?}", impl1_trait_ref, err);
201 // Attempt to prove that impl2 applies, given all of the above.
202 fulfill_implication(&infcx, impl1_trait_ref, impl2_def_id).is_ok()
205 tcx.specializes_cache.borrow_mut().insert(impl1_def_id, impl2_def_id, result);
209 /// Attempt to fulfill all obligations of `target_impl` after unification with
210 /// `source_trait_ref`. If successful, returns a substitution for *all* the
211 /// generics of `target_impl`, including both those needed to unify with
212 /// `source_trait_ref` and those whose identity is determined via a where
213 /// clause in the impl.
214 fn fulfill_implication<'a, 'gcx, 'tcx>(infcx: &InferCtxt<'a, 'gcx, 'tcx>,
215 source_trait_ref: ty::TraitRef<'tcx>,
217 -> Result<&'tcx Substs<'tcx>, ()> {
218 let selcx = &mut SelectionContext::new(&infcx);
219 let target_substs = infcx.fresh_substs_for_item(DUMMY_SP, target_impl);
220 let (target_trait_ref, mut obligations) = impl_trait_ref_and_oblig(selcx,
224 // do the impls unify? If not, no specialization.
225 match infcx.eq_trait_refs(true,
226 &ObligationCause::dummy(),
229 Ok(InferOk { obligations: o, .. }) => {
230 obligations.extend(o);
233 debug!("fulfill_implication: {:?} does not unify with {:?}",
240 // attempt to prove all of the predicates for impl2 given those for impl1
241 // (which are packed up in penv)
243 infcx.save_and_restore_in_snapshot_flag(|infcx| {
244 let mut fulfill_cx = FulfillmentContext::new();
245 for oblig in obligations.into_iter() {
246 fulfill_cx.register_predicate_obligation(&infcx, oblig);
248 match fulfill_cx.select_all_or_error(infcx) {
251 debug!("fulfill_implication: for impls on {:?} and {:?}, \
252 could not fulfill: {:?} given {:?}",
256 infcx.parameter_environment.caller_bounds);
261 debug!("fulfill_implication: an impl for {:?} specializes {:?}",
265 // Now resolve the *substitution* we built for the target earlier, replacing
266 // the inference variables inside with whatever we got from fulfillment.
267 Ok(infcx.resolve_type_vars_if_possible(&target_substs))
273 pub struct SpecializesCache {
274 map: FxHashMap<(DefId, DefId), bool>,
277 impl SpecializesCache {
278 pub fn new() -> Self {
284 pub fn check(&self, a: DefId, b: DefId) -> Option<bool> {
285 self.map.get(&(a, b)).cloned()
288 pub fn insert(&mut self, a: DefId, b: DefId, result: bool) {
289 self.map.insert((a, b), result);