1 // Copyright 2012-2014 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.
13 use syntax::codemap::Span;
14 use traits::FulfillmentContext;
15 use ty::{self, Ty, TypeFoldable};
16 use ty::outlives::Component;
19 /// Outlives bounds are relationships between generic parameters,
20 /// whether they both be regions (`'a: 'b`) or whether types are
21 /// involved (`T: 'a`). These relationships can be extracted from the
22 /// full set of predicates we understand or also from types (in which
23 /// case they are called implied bounds). They are fed to the
24 /// `OutlivesEnv` which in turn is supplied to the region checker and
25 /// other parts of the inference system.
27 pub enum OutlivesBound<'tcx> {
28 RegionSubRegion(ty::Region<'tcx>, ty::Region<'tcx>),
29 RegionSubParam(ty::Region<'tcx>, ty::ParamTy),
30 RegionSubProjection(ty::Region<'tcx>, ty::ProjectionTy<'tcx>),
33 impl<'cx, 'gcx, 'tcx> InferCtxt<'cx, 'gcx, 'tcx> {
34 /// Implied bounds are region relationships that we deduce
35 /// automatically. The idea is that (e.g.) a caller must check that a
36 /// function's argument types are well-formed immediately before
37 /// calling that fn, and hence the *callee* can assume that its
38 /// argument types are well-formed. This may imply certain relationships
39 /// between generic parameters. For example:
41 /// fn foo<'a,T>(x: &'a T)
43 /// can only be called with a `'a` and `T` such that `&'a T` is WF.
44 /// For `&'a T` to be WF, `T: 'a` must hold. So we can assume `T: 'a`.
48 /// - `param_env`, the where-clauses in scope
49 /// - `body_id`, the body-id to use when normalizing assoc types.
50 /// Note that this may cause outlives obligations to be injected
51 /// into the inference context with this body-id.
52 /// - `ty`, the type that we are supposed to assume is WF.
53 /// - `span`, a span to use when normalizing, hopefully not important,
54 /// might be useful if a `bug!` occurs.
55 pub fn implied_outlives_bounds(
57 param_env: ty::ParamEnv<'tcx>,
61 ) -> Vec<OutlivesBound<'tcx>> {
64 // Sometimes when we ask what it takes for T: WF, we get back that
65 // U: WF is required; in that case, we push U onto this stack and
66 // process it next. Currently (at least) these resulting
67 // predicates are always guaranteed to be a subset of the original
68 // type, so we need not fear non-termination.
69 let mut wf_types = vec![ty];
71 let mut implied_bounds = vec![];
73 let mut fulfill_cx = FulfillmentContext::new();
75 while let Some(ty) = wf_types.pop() {
76 // Compute the obligations for `ty` to be well-formed. If `ty` is
77 // an unresolved inference variable, just substituted an empty set
78 // -- because the return type here is going to be things we *add*
79 // to the environment, it's always ok for this set to be smaller
80 // than the ultimate set. (Note: normally there won't be
81 // unresolved inference variables here anyway, but there might be
82 // during typeck under some circumstances.)
83 let obligations = wf::obligations(self, param_env, body_id, ty, span).unwrap_or(vec![]);
85 // NB: All of these predicates *ought* to be easily proven
86 // true. In fact, their correctness is (mostly) implied by
87 // other parts of the program. However, in #42552, we had
88 // an annoying scenario where:
90 // - Some `T::Foo` gets normalized, resulting in a
91 // variable `_1` and a `T: Trait<Foo=_1>` constraint
92 // (not sure why it couldn't immediately get
93 // solved). This result of `_1` got cached.
94 // - These obligations were dropped on the floor here,
95 // rather than being registered.
96 // - Then later we would get a request to normalize
97 // `T::Foo` which would result in `_1` being used from
98 // the cache, but hence without the `T: Trait<Foo=_1>`
99 // constraint. As a result, `_1` never gets resolved,
100 // and we get an ICE (in dropck).
102 // Therefore, we register any predicates involving
103 // inference variables. We restrict ourselves to those
104 // involving inference variables both for efficiency and
105 // to avoids duplicate errors that otherwise show up.
106 fulfill_cx.register_predicate_obligations(
110 .filter(|o| o.predicate.has_infer_types())
114 // From the full set of obligations, just filter down to the
115 // region relationships.
116 implied_bounds.extend(obligations.into_iter().flat_map(|obligation| {
117 assert!(!obligation.has_escaping_regions());
118 match obligation.predicate {
119 ty::Predicate::Trait(..) |
120 ty::Predicate::Subtype(..) |
121 ty::Predicate::Projection(..) |
122 ty::Predicate::ClosureKind(..) |
123 ty::Predicate::ObjectSafe(..) |
124 ty::Predicate::ConstEvaluatable(..) => vec![],
126 ty::Predicate::WellFormed(subty) => {
127 wf_types.push(subty);
131 ty::Predicate::RegionOutlives(ref data) => match data.no_late_bound_regions() {
133 Some(ty::OutlivesPredicate(r_a, r_b)) => {
134 vec![OutlivesBound::RegionSubRegion(r_b, r_a)]
138 ty::Predicate::TypeOutlives(ref data) => match data.no_late_bound_regions() {
140 Some(ty::OutlivesPredicate(ty_a, r_b)) => {
141 let ty_a = self.resolve_type_vars_if_possible(&ty_a);
142 let components = tcx.outlives_components(ty_a);
143 Self::implied_bounds_from_components(r_b, components)
150 // Ensure that those obligations that we had to solve
151 // get solved *here*.
152 match fulfill_cx.select_all_or_error(self) {
154 Err(errors) => self.report_fulfillment_errors(&errors, None),
160 /// When we have an implied bound that `T: 'a`, we can further break
161 /// this down to determine what relationships would have to hold for
162 /// `T: 'a` to hold. We get to assume that the caller has validated
163 /// those relationships.
164 fn implied_bounds_from_components(
165 sub_region: ty::Region<'tcx>,
166 sup_components: Vec<Component<'tcx>>,
167 ) -> Vec<OutlivesBound<'tcx>> {
170 .flat_map(|component| {
172 Component::Region(r) =>
173 vec![OutlivesBound::RegionSubRegion(sub_region, r)],
174 Component::Param(p) =>
175 vec![OutlivesBound::RegionSubParam(sub_region, p)],
176 Component::Projection(p) =>
177 vec![OutlivesBound::RegionSubProjection(sub_region, p)],
178 Component::EscapingProjection(_) =>
179 // If the projection has escaping regions, don't
180 // try to infer any implied bounds even for its
181 // free components. This is conservative, because
182 // the caller will still have to prove that those
183 // free components outlive `sub_region`. But the
184 // idea is that the WAY that the caller proves
185 // that may change in the future and we want to
186 // give ourselves room to get smarter here.
188 Component::UnresolvedInferenceVariable(..) =>
196 pub fn explicit_outlives_bounds<'tcx>(
197 param_env: ty::ParamEnv<'tcx>,
198 ) -> impl Iterator<Item = OutlivesBound<'tcx>> + 'tcx {
199 debug!("explicit_outlives_bounds()");
203 .filter_map(move |predicate| match predicate {
204 ty::Predicate::Projection(..) |
205 ty::Predicate::Trait(..) |
206 ty::Predicate::Subtype(..) |
207 ty::Predicate::WellFormed(..) |
208 ty::Predicate::ObjectSafe(..) |
209 ty::Predicate::ClosureKind(..) |
210 ty::Predicate::TypeOutlives(..) |
211 ty::Predicate::ConstEvaluatable(..) => None,
212 ty::Predicate::RegionOutlives(ref data) => data.no_late_bound_regions().map(
213 |ty::OutlivesPredicate(r_a, r_b)| OutlivesBound::RegionSubRegion(r_b, r_a),